Amphiphilic derivatives of carboxymethylcellulose: Evidence for intra- and intermolecular hydrophobic associations in aqueous solutions.INTRODUCTION
Hydrophobically modified water-soluble polymers (both synthetic and natural) or amphiphilic am·phi·phil·ic
Of or relating to a molecule having a polar, water-soluble group attached to a nonpolar, water-insoluble hydrocarbon chain. polymers have received great attention because of their potential use in various industrial applications such as cosmetics, paints, drilling fluids, and oil recovery and owing to owing to
Because of; on account of: I couldn't attend, owing to illness.
owing to prep → debido a, por causa de the advantage of both the hydrophilic hydrophilic /hy·dro·phil·ic/ (-fil´ik) readily absorbing moisture; hygroscopic; having strongly polar groups that readily interact with water.
adj. and hydrophobic hydrophobic /hy·dro·pho·bic/ (-fo´bik)
1. pertaining to hydrophobia (rabies).
2. not readily absorbing water, or being adversely affected by water.
3. groups (1-9). However, from the practical point of view, the hydrophobically modified polysaccharides are important in many applications because of the nontoxicity of the polysaccharides as well as their biocompatibility biocompatibility
the quality of not having toxic or injurious effects on biological systems.
biocompatibility 1. The extent to which a foreign, usually implanted, material elicits an immune or other response in a recipient 2. and biodegradability (9-13). The associating polymers form a major part of the hydrophilic backbone and a small proportion of the hydrophilic backbone and small proportion of the hydrophobic groups (1), (4), (6-8), (13-16). In aqueous solution, the hydrophobic groups aggregate to minimize their exposure to water and thereby form intra or intermolecular Adj. 1. intermolecular - existing or acting between molecules; "intermolecular forces"; "intermolecular condensation" association resulting in hydrophobic microdomains (1), (7), (8), (16). In the semidilute regime, where intermolecular aggregates prevail over intramolecular in·tra·mo·lec·u·lar
Within a molecule.
intra·mo·lec ones, clusters of hydrophobic domains are formed (1), (8). The addition of salt enhances this interaction because of the increased in the solvent polarity, which results in good salt-tolerance (3), (8), (16). However, in the diluted regime the added salt can screen out the polyelectrolyte pol·y·e·lec·tro·lyte
An electrolyte, such as a protein or polysaccharide, having a high molecular weight. effect, while, at the same time, enhance the hydrophobic intramolecular association (7).
Amphiphilic polymers can be prepared either by chemical modification In biochemistry, chemical modification is the technique of chemically reacting a protein or nucleic acid with chemical reagents. Chemical modification can have several goals, such as
Surfactant is a complex naturally occurring substance made of six lipids (fats) and four proteins that is produced in the lungs. It can also be manufactured synthetically. micelles and the copolymerization occurring in the continuous water medium (5), (6), (8), (16), (19), (21).
The presence of hydrophobic groups greatly influences the physicochemical properties of amphiphilic polymers in aqueous solution and their properties are directly related to the involvement of intra or intermolecular associations between the hydrophobic moieties, depending on the investigated range of concentration (16), (22), (23). In an amphiphilic polyelectrolyte, if the attractive hydrophobic interaction between the hydrophobic groups prevails over the electrostatic repulsion repulsion /re·pul·sion/ (re-pul´shun)
1. the act of driving apart or away; a force that tends to drive two bodies apart.
2. between the charged groups, the hydrophobic groups can self-aggregate intramolecularly to form a micelle-like structure in aqueous solution. This compact conformation con·for·ma·tion
One of the spatial arrangements of atoms in a molecule that can come about through free rotation of the atoms about a single chemical bond. is reflected by a sharp decrease in solution viscosity. Such intramolecular micellization largely depends on chemical structure and composition (7). This effect has been observed at low polymer concentrations. In this case, macromolecular mac·ro·mol·e·cule
A very large molecule, such as a polymer or protein, consisting of many smaller structural units linked together. Also called supermolecule. coils behave as individual species and intramolecular interactions are almost exclusively favored. These lead to more compact, shrunken shrunk·en
A past participle of shrink.
a past participle of shrink
reduced in size
Adj. 1. conformations. Furthermore, the effects become more marked as ionic strength The ionic strength, I, of a solution is a function of the concentration of all ions present in a solution. increases because progressive screening of the repulsive charges overcomes their opposite expansion-generating effect (7), (23). However, at high concentration solutions, the coils are entangled, favoring intermolecular hydrophobic interactions, which lead to the formation of a three-dimensional network of polymer chains resulting in a rapid increase in apparent viscosity (1), (8), (16), (23). Moreover, the balance between the intra or intermolecular interactions depends on the structural parameters of the polymer, such as the nature, length and content of the hydrophobic groups, their distribution along the backbone, hydrophilic segment length and hydration hydration /hy·dra·tion/ (hi-dra´shun) the absorption of or combination with water.
1. The addition of water to a chemical molecule without hydrolysis.
2. , charge density, degree of polymerization The degree of polymerization, or DP, is the number of repeat units in an average polymer chain at time t in a polymerization reaction . The length is in monomer units. The degree of polymerization is a measure of molecular weight. and polymer concentration (2-5), (7-9), (11), (18-20) and on environmental parameters such as salinity (3), (8), (11), (16), pH (1), (18), organic solvent (11), and temperature (3), (8), (18). For this reason, unmodified and hydrophobically modified polymers are very different.
One of the aims of the present study was to examine the formation of the intra or intermolecular interactions of a series of hydrophobically modified carboxymethylcelluloses (CMC (Common Messaging Calls) A programming interface specified by the XAPIA as the standard messaging API for X.400 and other messaging systems. CMC is intended to provide a common API for applications that want to become mail enabled.
1. ) containing different amounts of the N,N-dihexyl-acrylamide (1 to 6 mol %) in aqueous solutions. CMC is a chemically modified cellulose derivative with high water solubility Water is a bent, polar compound and possesses the ability to Hydrogen bond. As a result, it has unique solubility characteristics as a solvent and functions differently at different temperatures. Polarity
Water Solubility, US Geological Survey , widely used for its low cost, lack of toxicity, and biodegradability (9), (18). The solution behavior of the modified polymers is expected to result from a balance between intra and intermolecular interactions due mainly to electrostatic repulsions and hydrophobic attractions. The relative contribution of these interactions was investigated as a function of the content of the hydrophobic group (N,N-dihexylacrilamide), polymer concentration, salt, and temperature. Another aim of this study was to evaluate the effect of introducing hydrophobic groups of various lengths on the solution behavior of copolymers and on temperature tolerance.
The commercial sample of sodium carboxymethylcellulose carboxymethylcellulose /car·boxy·meth·yl·cel·lu·lose/ (-meth?il-sel´u-los) a substituted cellulose polymer of variable size, used as the sodium or calcium salt as a pharmaceutical suspending agent, tablet excipient, and (CMC) was provided by Petrobras S/A S/A System Administrator
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S/A Sub-Assembly , Brazil. Its weight-average molar mass Molar mass, symbol M, is the mass of one mole of a substance (chemical element or chemical compound). It is a physical property which is characteristic of each pure substance. of 6.8 X [10.sup.5] g/mol was determined by static light scattering Static light scattering is a technique in physical chemistry that uses the intensity traces at a number of angles to derive information about the radius of gyration , molecular mass in 0.5 M NaC1 aqueous solution at 25[degrees]C. The content of carboxyl carboxyl /car·box·yl/ (kahr-bok´sil) the monovalent radical —COOH, occurring in those organic acids termed carboxylic acids.
n. groups was determined by (1)H NMR NMR: see magnetic resonance. (24) and was found to be 1.05 carboxyl groups per anydroglycose unit (DS = 1.05). Prior to use, the CMC was purified by polymer dissolution in [10.sup.-2] mol/L (0.58 g/L) NaCl aqueous solution under constant stirring at room temperature for at least two days. After complete dissolution of the polymer, the solution was centrifuged (15 min, 15,000 rpm, 25[degrees]C), filtered through a 3-[micro]m Millipore membrane, and lyophilized. Acryloyl chloride Acryloyl chloride, also known as 2-propenoyl chloride or acrylic acid chloride, is a clear, light yellow, flammable liquid with an acrid smell. It belongs to the acid chlorides group of compounds and is therefore a derivative of acrylic acid. (Merck), N,N-dihexylamine (Merck), tetrahydrofuran tetrahydrofuran: see furfural. (THF THF tetrahydrofolic acid.
tetrahydrofolic acid. ) (Merck), sodium dodecyl sulphate (SDS 1. (company) SDS - Scientific Data Systems.
2. (tool) SDS - Schema Definition Set. ) (Labsynth, 90% purity), potassium persulphate ([K.sub.2][S.sub.2][O.sub.8]) (Vetec), and acetone acetone (ăs`ĭtōn), dimethyl ketone (dīmĕth`əl kē`tōn), or 2-propanone (prō`pənōn), CH3COCH3 (AQEEL) were used without further purification.
Synthesis of N,N-Dihexylacrylamide Monomer
The N,N-dihexylacrylamide was prepared by a reaction of acryloyl chloride with N,N-dihexylamine, according to according to
1. As stated or indicated by; on the authority of: according to historians.
2. In keeping with: according to instructions.
3. the procedure described in the literature for N-alkylacryla-mide monomers (4), (25). In a 250-mL three-necked round-bottom flask Noun 1. round-bottom flask - a spherical flask with a narrow neck
flask - bottle that has a narrow neck equipped with a thermometer, a condenser, a magnetic stirrer A magnetic stirrer is a type of laboratory equipment consisting of a rotating magnet or stationary electomagnets creating a rotating magnetic field. The stirrer is used to cause a stir bar, immersed in a liquid to be stirred, to spin very quickly, stirring it. , and addition funnel, the N,N-dihexylamine (0.199 mol) was dissolved in 100 mL of tetrahydrofuran (THF) and placed in the flask. The solution was then cooled to 0[degrees]C. Acryloyl chloride (0.099 mol) dissolved in 40 mL of THF was added slowly to the reaction flask over a period of 2.5 h at such a rate that the temperature did not exceed 5[degrees]C. The resulting mixture was then stirred at 10[degrees]C for 2 h. The resulting product was filtered to remove the N,N-dihexylamide hydrochloride hydrochloride /hy·dro·chlo·ride/ (-klor´id) a salt of hydrochloric acid.
A compound resulting from the reaction of hydrochloric acid with an organic base. , and the THF was removed by vacuum distillation Vacuum distillation is a method of distillation whereby the pressure above the liquid mixture to be distilled is reduced to less than its vapor pressure (usually less than atmospheric) causing evaporation of the least volatile liquid(s) (those with the highest boiling points). at 60[degrees]C. At the end of this process, a yellow viscous liquid hydrophobic monomer was obtained.
Synthesis of CMC-g-Poly(N,N-Dihexylacrylamide) Copolymers
The CMC-g-poly(N,N-dihexylacrylamide) copolymers were prepared by an aqueous micellar copolymerization technique between CMC and N,N-dihexylacrylamide. This copolymerization reaction was adapted from the copolymerization method used for acrylamide acrylamide /acryl·a·mide/ (ah-kril´ah-mid) a vinyl monomer used in the production of polymers with many industrial and research uses; the monomeric form is a neurotoxin. monomers described in the literature (4), (5), (19), (20). The associating copolymers were obtained by inserting a low amount (1 to 6 mol %) of N,N-dihexylacrylamide into the CMC chain. They were obtained through a micellar radical copolymerization in water, with sodium dodecyl sulfate Sodium dodecyl sulfate (or sulphate) (SDS or NaDS) (C12H25NaO4S),is an anionic surfactant that is used in household products such as toothpastes, shampoos, shaving foams and bubble baths for its thickening effect and its ability to (SDS) as the surfactant and potassium persulfate Potassium persulfate (K2S2O8) is a chemical compound. It is a food additive and it is used in organic chemistry as an oxidizing agent for instance in the Elbs persulfate oxidation ([K.sub.2][S.sub.2][O.sub.8]) as the initiator. The presence of the surfactant allows the solubilization of the hydrophobic monomer within the micelles dispersed in the water. The initial concentration of CMC and N,N-dihexylacrylamide in water (total polymer and hydrophobic monomer) was 3% (w/w) (1.2 X [10.sup.-1] mol/L) and the initiator concentration was 0.03% (w/w) (1.12 X [10.sup.-3] mol/L) relative to the monomer feed. The surfactant concentration was 1.5% (w/v) (5.2 X [10.sup.-2] mol/L) in the feed, based on the water volume. The temperature was fixed at 50[degrees]C. The copolymerization was carried out in various amounts of hydrophobic monomer ranging from 1 to 6 mol %.
The sequence distribution of the hydrophobic monomer in the polymer chain depends mainly on the initial number of hydrophobic monomers per micelle micelle (mīsel´),
n a space formed by the brush structure of fibrils in colloidal gels. The spaces are occupied by water in hydrocolloid impressions. , [N.sub.H] , calculated as follows:
[N.sub.H] = [[M.sub.H]]/[SDS] - cmc/[N.sub.agg] (1)
where [[M.sub.H] is the initial molar concentration Noun 1. molar concentration - concentration measured by the number of moles of solute per liter of solution
concentration - the strength of a solution; number of molecules of a substance in a given volume of N,N-dihexylacrylamide, [SDS] the molar surfactant concentration, cmc its critical micellar concentration, and [N.sub.agg] its aggregation number Is the number of molecules that are associated to form a micelle once the surface in a solution is full of molecules of surfactant, this occurs when Critical micelle concentration is reached. ([cmc.sub.SDS] = 9.2 X [10.sup.-3] mol/L and [N.sub.agg] = 60 at the polymerization polymerization
Any process in which monomers combine chemically to produce a polymer. The monomer molecules—which in the polymer usually number from at least 100 to many thousands—may or may not all be the same. temperature of 50[degrees]C) (20), (26). [N.sub.H] values can be modified by varying the initial concentration of the hydrophobic monomer or the concentration of SDS (4). Keeping the hydrophobic concentration constant and decreasing the surfactant concentration leads to an increase in [N.sub.H], which results in an increase in the length of the hydrophobic pendant group A pendant group or side group is a group of molecules attached to a backbone chain of a long molecule. Usually, this molecule would be a polymer. on the polymer chain (20).
Polymer characteristics are given in Table 1. The sample code of the copolymers refers to the polymer precursor and to the initial concentration of the hydrophobic monomer (HM1) used in the copolymerization reaction. For example, CMCHM1 refers to a CMC copolymer copolymer: see polymer. containing approximately 1 mol % of N,N-dihexylacrylamide.
TABLE 1. Initial molar concentration of hydrophobic monomer N,N-dihexylacrylamide ([M.sub.H]), initial number of hydrophobic monomers per micelle ([N.sub.H]), intrinsic viscosity, and Huggins constant of the CMC and CMC-g-poly(N,N-dihexylacrylamide) in 0.5 NaCl aqueous solution and synthetic formation water (SFW), at 25[degrees]C. 0.5 M NaCI [[M.sub.H]] Sample (mol %) [[N.sub.H]] [[eta]] (mL/g) K' CMC -- -- 336 0.20 CMCHM1 1.12 1.96 333 0.67 CMCHM3 3.36 5.90 334 0.47 CMCHM6 5.94 10.41 323 0.45 SFW (a) Sample [[eta]] (mL/g) K' CMC 577 0.19 CMCHM1 797 0.33 CMCHM3 724 0.34 CMCHM6 622 0.33 (a) SFW, Synthetic formation water (5.4 X [10.sup.-3] M NaCl, 4.9 X [10.sup.-4] M Ca[Cl.sub.2] and 7.3 X [10.sup.-5] M Mg[Cl.sub.2]).
Polymer solutions were prepared by dissolving the desired polymer concentration in distilled water Noun 1. distilled water - water that has been purified by distillation
H2O, water - binary compound that occurs at room temperature as a clear colorless odorless tasteless liquid; freezes into ice below 0 degrees centigrade and boils above 100 degrees centigrade; , Milli-Q water, and synthetic formation water (5.4 X [10.sup.-3] M NaCl, 4.9 X [10.sup.-4] aqueous solution under gentle stirring at room temperature for at least three days. The ionic strength of the solutions is estimated to be 6.83 X [10.sup.-3] for synthetic formation water and 0.5 for 0.5 M NaCl aqueous solution.
Viscosities of the unmodified CMC and the hydrophobic-modified CMC solutions were measured using an Ubbelohde capillary viscometer viscometer
Instrument for measuring the viscosity (resistance to internal flow) of a fluid. In one type, the time taken for a given volume of fluid to flow through an opening is recorded. at a temperature of 25 [+ or -] 0.05[degrees]C maintained with a thermostatic bath. Stock solutions were prepared by dissolving the polymers in distilled water, synthetic formation water, and in 0.5 M NaCl solutions. In distilled water, the concentration of the stock solution used was equal to 1.0 g/L for the copolymer and 2.5 g/L for the unmodified CMC, while in synthetic formation water and 0.5 M NaCl the polymer concentration was 2.5 g/L for all the polymers. The solvents and the stock solutions were then filtered through a 0.22- and 0.45-[micro]m Millipore type membrane for light-scattering measurements, respectively.
The reduced viscosity was calculated as the ratio (t--t.sub.0])/([t.sub.0]c), where [t.sub.0] and t are the flow times of the solvent and of the polymer solution at concentration c, respectively. The intrinsic viscosity Intrinsic viscosity is a measure of a solute's contribution to the viscosity of a solution. [[eta]] was determined from the extrapolation (mathematics, algorithm) extrapolation - A mathematical procedure which estimates values of a function for certain desired inputs given values for known inputs.
If the desired input is outside the range of the known values this is called extrapolation, if it is inside then of the reduced viscosity curve to zero concentration using the Flory-Huggins equation as shown below:
[[eta].sub.red] = [[eta].sub.sp]/c = [[eta]] + k'[[eta]].sup.2]c (2)
where k' is the Huggins coefficient, a constant for a series of polymers of different molecular weights in a given solvent and temperature.
Light Scattering Measurement
Static light scattering (SLS (Selective Laser Sintering) See laser sintering and 3D printing. ) and dynamic light scattering Dynamic light scattering (also known as Photon Correlation Spectroscopy) is a powerful technique in physics, which can be used to determine the size distribution profile of small s in solution. (DLS DLS
Doctor of Library Science ) experiments were performed by ALV ALV Arvonlisävero (Finnish: value added tax)
ALV Avian Leukosis Virus
ALV Andorra La Vella (capital of Andorra)
ALV Autonomous Land Vehicle
ALV Asta La Vista
ALV Alvin, Texas
ALV Air Launched Vehicle laser goniometer goniometer /go·ni·om·e·ter/ (go?ne-om´e-ter)
1. an instrument for measuring angles.
2. a plank that can be tilted at one end to any height, used in testing for labyrinthine disease. , which consists of a 22-mW HeNe linear polarized laser operating at a wavelength of 632.8 nm and an ALV-5000/EPP multiple [tau] digital correlator with 125 ns initial sampling time. Data were acquired using ALV Correlator Control software. For the SLS experiments, the polymer solutions were maintained at a constant temperature of 25 [+ or -] 0.1[degrees]C and for the DLS experiments the temperature of the polymer solutions varied between 25 and 65[degrees]C. The minimum sample volume of 2 mL was used in the SLS and DLS experiments and the volume of the solution was placed in 10 mm diameter glass cells. Before the SLS and DLS measurements, all solvents and polymer solutions were filtered through 0.22-[micro]m and 0.45-[micro]m Millipore filters, respectively, to remove dust and any non-dissolved particles.
In the SLS experiments, the time average light scattering intensity by the sample (I) is recorded as a function of the scattering wave vector A wave vector is a vector that specifies the wavenumber and direction of propagation for a wave. The magnitude of the wave vector indicates the wavenumber. The orientation of the wave vector indicates the direction of wave propagation.
For example consider a plane wave. q = (4[pi]n/[lambda]sin([theta Theta
A measure of the rate of decline in the value of an option due to the passage of time. Theta can also be referred to as the time decay on the value of an option. If everything is held constant, then the option will lose value as time moves closer to the maturity of the option. ]/2) (27), where n is the refractive index A property of a material that changes the speed of light, computed as the ratio of the speed of light in a vacuum to the speed of light through the material. When light travels at an angle between two different materials, their refractive indices determine the angle of transmission of the solvent (1.33 for the water at 25[degrees]C), [lambda] is the wavelength of the incident light in the vacuum, and [theta] is the scattering angle. In the SLS experiments one measures the excess of scattering intensity I(q) with respect to the solvent. In our experiments, the scattering angle [theta] varied from 45 to 150[degrees] with a 5[degrees] stepwise stepwise
incremental; additional information is added at each step.
stepwise multiple regression
used when a large number of possible explanatory variables are available and there is difficulty interpreting the partial regression increase. Toluene toluene (tōl`yēn') or methylbenzene (mĕth'əlbĕn`zēn), C7H8 was used as a calibration standard. Weight-average molar mass [M.sub.w], z-average radius of gyration Radius of gyration
A relation of the area or mass of a figure to its moment of inertia. If I is the moment of inertia about a line of a figure whose area is A, the figure's radius of gyration with respect to that line is. [R.sub.g], and second virial coefficient Virial coefficients appear as coefficients in the virial expansion of the pressure of a many-particle system in powers of the density. [A.sub.2] were determined by double extrapolation using the Zimm method, based on the following relation:
Kc/[DELTA][R.sub.[theta] = 1/[M.sub.w][1 + [q.sub.2][R.sub.g.sup.2]/3] + 2[A.sub.2]c (3)
where c is the polymer concentration, [DELTA][R.sub.q] is the excess absolute time-average light scattering intensity (excess Rayleigh ratio The Rayleigh ratio is a quantity used to characterize the scattered intensity as a function of scattering angle , and is defined as
Molecular parameters [M.sub.w], [R.sub.g], and [A.sub.2] were determined in 0.5 M NaCl aqueous solution. The SLS measurements were carried out in a dilute regime and the concentration of the polymer solution varied from 0.5 to 2.0 g/L. These concentrations were chosen in such a way that the overlap concentration was not exceeded, but large enough to obtain a measurable light scattering intensity.
In the DLS experiment, the fluctuation of the scattering light because of the Brownian motion Brownian motion
Any of various physical phenomena in which some quantity is constantly undergoing small, random fluctuations. It was named for Robert Brown, who was investigating the fertilization process of flowers in 1827 when he noticed a “rapid oscillatory of the particles is analyzed in terms of an autocorrelation Autocorrelation
The correlation of a variable with itself over successive time intervals. Sometimes called serial correlation. function, which contains the distribution of relaxation times, [tau], and scattering amplitudes of the examined components (27). In the DLS experiments, the homodyne intensity autocorrelation function [G.sup.2] (t) was measured within the range of delay times from 1 X [10.sup.-4] to 10 X [10.sup.4] ms. The homodyne intensity autocorrelation function [g.sup.2](t) is related to the electric field time autocorrelation function [g.sup.1] (t), according to the Siegert relation (29):
[g.sup.2](t) = 1 + B|[[g.sup.1](t)|.sup.2] (4)
where [g.sup.2] (t) = [G.sup.2] (t)/[G.sup.2] ([infinity]), [G.sup.2] ([infinity]) is an experimentally determined baseline, B ([less than or equal to]1) is a coherence factor depending on the geometry of the detection and the ratio of the intensity scattered by the polymer to that scattered by the solvent. In a dilute solution of monodisperse A collection of objects are called monodisperse if they have the same size - i.e. their size distribution is effectively a delta function. A sample of objects with a broader size distribution is called polydisperse. In practice, exactly monodisperse collections rarely exist. particles, the field autocorrelation function is connected with the translation diffusion coefficient D as follows:
[g.sup.1](t) = exp(-t/[tau]) = exp(-[GAMMA]t) = exp(-D[q.sup.2]t) (5)
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII ASCII or American Standard Code for Information Interchange, a set of codes used to represent letters, numbers, a few symbols, and control characters. Originally designed for teletype operations, it has found wide application in computers. ] (6)
where [tau] is the decay time of the time autocorrelation function of the scattered electric field [g.sup.1] (t), [GAMMA] = 1/[tau] is the decay rate, and q is the wave vector. If there is a large number of independent decay processes in the system, [g.sup.1] (t) is a weighed sum of individual contributions. Generally, [g.sup.1] (t) may be expressed by a continuous distribution of decays:
[g.sup.1](t) = [[integral].sub.0.sup.[infinity]]A([tau])exp(-t)/[tau])d[tau] (7)
where A([tau]) is a distribution of scattered light over decay times. The CONTIN program (30) based on the inverse Laplace transform Laplace transform
In mathematics, an integral transform useful in solving differential equations. The Laplace transform of a function is found by integrating the product of that function and the exponential function e−pt was used to determine this distribution function. From the relaxation time relaxation time
The time required for an exponential variable to decrease to 1/e (0.368) of its initial value.
Noun 1. , diffusion coefficient D and hydrodynamic hy·dro·dy·nam·ic also hy·dro·dy·nam·i·cal
1. Of or relating to hydrodynamics.
2. Of, relating to, or operated by the force of liquid in motion. radius [R.sub.H] were calculated using relations D = 1/[q.sup.2][tau], D = kT/6[pi][eta][R.sub.H] (Stockes-Einstein Equation), where k is the Boltzmann constant Boltzmann constant
Ratio of the universal gas constant (see gas laws) to Avogadro's number. It has a value of 1.380662 × 10−23 joules per kelvin. , T is the absolute temperature, and [eta] is the viscosity of solvent at the same temperature. The true values of D and [R.sub.H] were obtained by extrapolation to q = 0 and c = 0.
The hydrodynamic radius [R.sub.H] values of the CMC and CMC copolymers were determined in Milli-Q water, synthetic formation water or aqueous 0.5 NaCl solutions at different polymer concentrations (1, 2, 3 and 4 g/L) and at different temperatures (25, 45, 55 and 65[degrees]C). The measurements in the DLS experiments were performed at 60, 90, and 120[degrees]. To avoid excessive information, we only show the results at 90[degrees], since they are representative of all the light scattering angles analyzed.
In the low concentration range, the autocorrelation function of scattered electric field [g.sup.1] (t) is a single exponential, whereas at higher concentrations, [g.sup.1] (t) can be described by a sum of two relaxations widely separated in time, with [A.sub.f] + [A.sub.s] = 1.
[g.sup.1](t) = [A.sub.f]exp(-1/[[tau].sub.f]) + [A.sub.s]exp[[ - (t/[[tau].sub.se]).sup.[beta]]] (8)
The parameters [A.sub.f] and [A.sub.s] are the amplitudes for the "fast" and "slow" relaxation modes, respectively. Analyses of the time correlation functions in the semidilute concentration regime (where q[R.sub.H] < 1) have shown that the first term (short-time behavior) on the right-hand side of Eq. 8 is related to a collective diffusion Collective diffusion is the diffusion of a large number of particles, most often within a solvent.
Contrarly to brownian motion, which is the diffusion of a single particle, interactions between particles may have to be considered, unless the particles form an ideal mix with coefficient [D.sub.c] ([tau].sub.r.sup.-1] = [D.sub.c][q.sup.2], where [[tau].sub.f] is the "fast" relaxation time), which reflects a concerted motion of the polymer chains relative to the solvents. The second term (long-time behavior) is expected to be associated with disengagement disengagement /dis·en·gage·ment/ (dis?en-gaj´ment) emergence of the fetus from the vaginal canal.
n. relaxation of individual chains (1), (31) or cluster relaxation (31), (32). The variable [[tau].sub.se] is some effective slow relaxation time, and the stretched exponent [beta](0 < [beta] [less than or equal to] 1) is an indication of the width of the distribution of relaxation times. In this work, the value of [beta] depends on the hydrophobicity of the polymer. The mean slow relaxation time is given by
[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (9)
where [GAMMA] ([beta].sup.-1] is the gamma function In mathematics, the Gamma function (represented by the capitalized Greek letter Γ) is an extension of the factorial function to real and complex numbers. For a complex number z with positive real part it is defined by
Rheological experiments were carried out using a Haake RheoStress RS150 rheometer rhe·om·e·ter
An instrument for measuring the flow of viscous liquids, such as blood. equipped with DG41 coaxial cylinder sensor. Solution viscosities of the unmodified and modified polymers with 1 and 3 mol % of the hydrophobic groups (CMCHM1 and CMCHM3) at 10 g/L polymer concentration were measured at shear rates ranging from 0.1 to 100 [s.sup.-1] at temperature T = 55[degrees]C.
RESULTS AND DISCUSSION
Synthesis and Characterization of the Hydrophobic Monomer N,N-Dihexylacrylamide
The hydrophobic monomer N,N-dihexylacrylamide used in this study was chosen based on literature data (19), which present evidence that N,N-disubstituted acrylamide derivatives used in micellar copolymerization favor the obtaining of copolymers with a more homogeneous chemical structure than that obtained for N-monosubstituted acrylamide monomers. The homogeneity of the copolymer structure is related to the fact that the N,N-disubstituted acrylamide monomers do not form mtermolecular hydrogen bonding due to the absence of the N--H amino group amino group, in chemistry, functional group that consists of a nitrogen atom attached by single bonds to hydrogen atoms, alkyl groups, aryl groups, or a combination of these three. An organic compound that contains an amino group is called an
amine. . Furthermore, at the lower copolymer concentration, it is expected that the presence of the two hexyl hex·yl
The univalent hydrocarbon radical, C6H13. chains, in the case of N,N-dihexylacrylamide monomer, with six carbon atoms in each chain and two terminal methyl groups, will make the polymer more hydrophobic, favoring strong associative properties and a significant viscosity increase of the copolymer solutions in water. In this study, the characterization of N,N-dihexylacrylamide monomer microstructure mi·cro·struc·ture
The structure of an organism or object as revealed through microscopic examination.
a structure on a microscopic scale, such as that of a metal or a cell by [.sup.1] NMR in CDC See Control Data, century date change and Back Orifice.
CDC - Control Data Corporation [l.sub.3] (not shown here), as previously described (33), was similar to that reported by Volpert et al. (19).
Synthesis and Characterization of the Copolymers CMC-g-poly(N,N-dihexylacrylamide)
The polymers were obtained by micellar copolymerization of carboxymethylcellulose with the length of hydrophobic monomer N,N-dihexylacrylamide ranging from 1.1 to 5.9 mol %, as shown in Table 1. The sequence distribution of the hydrophobic monomers in the copolymers was controlled by varying the number of hydrophobes per micelles, [N.sub.H], and it was assumed that the hydrophobe content corresponds to the initial feed composition. In this study, the length of the hydrophobic chain grafted onto the CMC backbone was increased by increasing the hydrophobic monomer concentration, making the surfactant concentration constant in the medium, i.e., the [N.sub.H] values varied by modifying the initial concentration of hydrophobic monomer, [[M.sub.H]]. Polymerization was carried out in an [N.sub.H] range from 1.96 to 10.41 (Table 1). For associating polyacrylamides, the micellar copolymerization process allows one to control the microstructure of the resulting copolymer by varying the hydrophobe to micelle ratio going from a very blocky structure (high [N.sub.H] values) to an almost random copolymer ([N.sub.H] = 1) . It must be emphasized that, in the case of CMC copolymers, the hydrophobic groups are grafted onto the CMC backbone. In this case, it was observed that the [N.sub.H] values were approximately equal to or greater than 2, so that the N,N-dihexylacrylamide monomers are grafted onto the CMC of the hydrophobic groups having different lengths. In this condition, the hydrophobic association in the hydrophobically modified polymers increase as [N.sub.H] increases. On the other hand, high [[M.sub.H]] and consequently high [N.sub.H] values can make the copolymers either partially or completely insoluble in water. All copolymer solutions used in this study were turbid tur·bid
Having sediment or foreign particles stirred up or suspended; muddy; cloudy.
tur·bidi·ty n. and this is due to the presence of the hydrophobic groups, and it was observed that this turbidity turbidity /tur·bid·i·ty/ (ter-bid´i-te) cloudiness; disturbance of solids (sediment) in a solution, so that it is not clear.tur´bid
The cloudiness or lack of transparency of a solution. increased with the length of the hydrophobic groups and polymer concentration in the medium. In spite of this, after several months under equilibrium conditions, no precipitation was observed in the polymer solutions. This result seems to indicate that all the polymers studied have good solubility in aqueous medium.
NMR spectroscopy is a commonly used method for determining copolymer structure. Nevertheless, in the present case, the technique is not sensitive enough owing to the very low hydrophobe content. However, information on copolymer microstructure was indirectly obtained from a study on their solution properties of viscosity, static and dynamic light scattering and rheological measurements, as will subsequently be shown. For hydrophobically modified polyacrylamide copolymers, the very low percentage of hydrophobic monomers leads to difficulties in evaluating its incorporation level in the polymer. For this reason, the hydrophobic monomer level is assumed to be equal to the initial feed composition (5), (19), (25). This probably leads to some error because the composition of hydrophobic copolymers is prepared from the micellar process. Some studies using N-disubstituted hydrophobic acrylamides as hydrophobic comonomers in the synthesis of the associating polyacrylamides prepared by micellar copolymerization indicate that the copolymers with hydrophobe content lower than 1 mol % do not allow the determination of composition by [H.sup.1] NMR, since the end methyl signal is too low, making this method insufficiently sensitive, or by UV spectroscopy because these copolymer samples do not contain phenyl phenyl (fĕn`əl), C6H5, organic free radical or alkyl group derived from benzene by removing one hydrogen atom. or other aromatic groups (2), (5). In this study, hydrophobe content was larger than that usually used in the synthesis of hydrophobically modified polyacrylamides; however, this content was much lower than that generally used in the chemical modification of polysaccharides (9). For this reason, we were unable to determine the copolymer microstructure by NMR.
The variation of the reduced viscosity of the CMC and CMC-g-poly(N,N-dihexylacrylamide) in distilled water at 25[degrees]C against the polymer concentration is plotted in Fig. 1. In distilled water, the reduced viscosity reached a maximum for all systems, but this maximum was greater for hydrophobically modified polymers than that for the polymer precursor, suggesting an increase in viscosity as the length of the hydrophobic chain grafted onto the CMC backbone increases. For the unmodified CMC, this behavior can be explained by the electrostatic repulsions between charged carboxylic car·box·yl
The univalent radical, COOH, the functional group characteristic of all organic acids.
[carb(o)- + ox(y)- + -yl. groups located along the CMC backbone, showing typical polyelectrolyte behavior (34). For the hydrophobically modified polymers, this phenomenon can be explained either by the flexibility of the chain or by the decrease in the solvent quality, which contributes to the increase or decrease of the reduced viscosity of these polymers in solution. In the case of flexible polymer chains, such as hydrophobically modified polyacrylamides, the decrease in the reduced viscosity occurs because of the strong intramolecular hydrophobic interactions, which promote the polymer-polymer interactions in aqueous medium due to the decrease in the solvent quality. This behavior suggests that the flexibility of the polymer backbone facilitates the entanglement of the polymeric chain. However, in the case of semi-rigid polymer chains such as CMC, the increase in the reduced viscosity for the hydrophobically modified CMC copolymers occurs because aggregate formation was favored by the decrease in the solvent quality (35). In this case, the rigidity of the polymer chain hinders the entanglement of the polymer chain, contributing to aggregate formation and consequently to the increase in reduced viscosity of these polymers in solution. Moreover, the electrostatic repulsions between charged carboxylic groups (CO[O.sup-]) located along the CMC hydrophilic chain also contribute to the increased hydrodynamic volume in distilled water. However, the presence of hydrophobic groups seems to contribute more significantly to the increase in the reduced viscosity of the copolymer solutions. This effect can be observed with the increased length of the hydrophobic group grafted onto the CMC backbone, as shown in Fig. 1.
[FIGURE 1 OMITTED]
Table 1 shows the values of intrinsic viscosity and Huggins constant for the CMC and CMC-g-poly(N,N-dihexylacrylamide) copolymers, in 0.5 M NaCl aqueous solution and synthetic formation water (SFW SFW Science Fiction Weekly
SFW Safe for Work (website links)
SFW Solaris Freeware (open source software delivered in Solaris and supported by Sun)
SFW Sensor Fuzed Weapon
SFW Suitable for Work ), at 25[degrees]C. In the 0.5 M NaCl, the copolymers had a tendency to lower intrinsic viscosity compared to CMC. In the presence of salt, two effects can contribute to the contraction of the polyelectrolyte associative coil in solution: (i) screening of electrostatic interactions and (ii) intramolecular hydrophobic interactions. The latter is promoted by decreasing the polymer-solvent interactions, favored by hydrophobic groups, and explaining the increase in the Huggins constant values. However, only a slight increase was observed in these values for the copolymers. The Huggins constant values for copolymers remained in the 0.3-0.7 range expected for non-associating polymers. This behavior can be explained by the very small content of the hydrophobic groups grafted onto the semi-rigid CMC backbone, favoring a weak reduction in the polymer-solvent interactions. In SFW, the values of intrinsic viscosity for copolymers were higher than that for unmodified CMC. This is due to intra and/or intermolecular hydrophobic interactions promoted by the salts (NaCl, Mg[Cl.sub.2], and Ca[Cl.sub.2]) present in the synthetic formation water (SFW) even at lower concentrations and carboxilate groups ([COO.sup.-]) in the CMC chain. Moreover, for copolymers, the slightly contraction of the polymer coil became more evident as the length of the hydrophobic groups grafted onto the CMC backbone increased. Thus, the copolymer with the largest size of the hydrophobic groups (CMCHM6) had the lowest value of intrinsic viscosity, suggesting strong intramolecular hydrophobic interactions. This effect was also observed for this copolymer in SFW. This behavior is in agreement with the results reported for CMCHMs with a small content of the hydrophobic groups (9).
The values of intrinsic viscosity of the copolymers in SFW were greater than that of the polymer precursor and in 0.5 M NaCl. The larger hydrodynamic volume can be explained by the interactions between the carboxylic groups (CO[O.sup.-]) in the hydrophilic CMC backbone and the divalent divalent /di·va·lent/ (di-va´lent) bivalent; carrying a valence of two.
di·va cations (C[a.sup.2+] and M[g.sup.2+]) present in the synthetic formation water. In this condition, the negatively charged groups in the CMC backbone are available to interact with the divalent cations present in the synthetic formation water. Moreover, this result suggests that in this solvent the interactions between charged groups of opposite charges are more significant than the intramolecular hydrophobic interactions. However, the contraction of the polymer coil, likely promoted by hydrophobic intramolecular associations, was also observed in the copolymers by their intrinsic viscosity reduction. Furthermore, the Huggins constant values were maintained around 0.3, suggesting good interaction between the macromolecules Macromolecules
A large molecule composed of thousands of atoms.
Mentioned in: Gene Therapy
macromolecules and the solvent. This behavior could explain the stability of the solutions, i.e., the absence of the precipitate as a function of time.
The difference between the values of intrinsic viscosity for unmodified CMC in 0.5 M NaCl and SFW can be explained by the ionic strength of the solvents. The hydrodynamic volume of the unmodified CMC decreases as ionic strength increases owing to the reduction of electrostatic interactions and consequently intrinsic viscosity. This effect was more significant in the solvent of higher ionic strength (0.5 M NaCl). However, the values of the Huggins constant not changed in both solvents, indicating good interaction between the unmodified CMC chain and the solvents.
Static Light Scattering
Physicochemical characteristics such as weight-average molar mass ([M.sub.w]), radius of gyration ([R.sub.g]) and the second virial coefficient ([A.sub.2]) of unmodified CMC and CMCHMs with different amounts of the hydrophobic group (1, 3 and 6 mol % of the initial feed composition), obtained from static light scattering by Zimm plot in 0.5 M NaCl, at 25[degrees]C, are presented in Table 2. The weight-average molar mass of the CMCHM copolymers is a parameter of utmost importance since it controls the viscosity of the polymer solutions, mainly in the semidilute solutions. In this study, a 0.5 M NaCl aqueous solution was used as solvent for the polymers to screen the electrostatic interactions between the carboxylate carboxylate,
n a carboxylic acid salt, ester, or ion. groups. Polymer concentrations of 0.5, 0.8, 1.2, 1.6, and 2.0 g/L were chosen to ensure a measurement of light scattering intensity. Although we could not determine critical viscosity values, i.e., the transition from the dilute to semi-dilute regime, owing to the transition from the dilute to semi-dilute regime, owing to the low polymer concentrations used, we relied on literature data to ensure that our study was carried out in a dilute regime, not exceeding the overlap concentration for the unmodified CMC of similar degree of substitution (DS = 0.9) and molar mass ([M.sub.w] = 3 X [10.sup.5] g/mol) investigated by Charpentier et al. (9). In the copolymer case, some concentrations were slightly higher than the critical hydrophobe concentration of the CMCHM grafted with 6% hexadecylamine (HDA) ([C.sub.16]) = ([C.sub.cr] = 1.5 g/L)  and of the bigrafted CMC with 30% [C.sub.4] and 3% [C.sub.12-18] ([C.sub.cr] = 1.4 g/L). Nevertheless, they were lower than those of the monografted CMC sample containing 9% [C.sub.12-18] ([C.sub.cr] = 2.5 g/L) . In the two latter studies, the copolymer samples had larger molar mass (4.8 X [10.sup.6] and 4.9 X [10.sup.6] g/mol, respectively) than those of the CMCHM samples studied in this work (Table 2). It is likely for this reason that the CMCHM copolymers studied here had higher critical concentrations. Therefore, it is believed that all copolymer concentrations used should be below the critical concentration values observed by Charpentier-Valenza et al. . It must be emphasized that, in the case of CMC copolymers, the transition from a dilute to semidilute regime occurs not only because of the close packing, but also because of the specific association between the coil and/or aggregates. Hence, it is more appropriate to use the term critical hydrophobe concentration than overlap concentration for the hydrophobically modified copolymers. A similar behavior was observed by a number of researchers [5, 10, 21]. In addition, the static light scattering study must be carried out in dilute polymer concentrations to guarantee the macromolecule macromolecule, term that may refer either to a crystal such as a diamond, in which the atoms are identical and held by covalent bonds (see chemical bond) of equal strength, or to one of the units that compose a polymer. disassociate dis·as·so·ci·ate
tr.v. dis·as·so·ci·at·ed, dis·as·so·ci·at·ing, dis·as·so·ci·ates
To remove from association; dissociate.
dis state in solution. Thus, the [M.sub.w] obtained is just a single molecule. However, to determine the [M.sub.w] of the aggregates, it is suggested that higher polymer concentrations be used .
TABLE 2. Molar mass ([M.sub.w]), radius of gyration ([R.sub.g]), and the second virial coefficient ([A.sub.2]) of unmodified CMC and CMC-g-poly(N,N-diheylacrylamide) copolymers with different lengths of the hydrophobic groups obtained from static light scattering by Zimm plot (dn/dc = 0.163 mL/g) in 0.5 M NaCl at 25[degrees]C. Sample [M.sub.w] (g/mol) [R.sub.g] (nm) [A.sub.2] (mol.mL/[g.sup.2]) CMC 6.8 x [10.sup.5] 125.4 5.2 x [10.sup.-4] CMCHM1 7.3 x [10.sup.5] 139.8 2.4 x [10.sup.-4] CMCHM3 8.9 x [10.sup.5] 82.88 -2.5 x [10.sup.-4] CMCHM6 5.9 x [10.sup.5] 96.66 7.1 x [10.sup.-5]
It can be seen from Table 2 that the molar mass of the hydrophobically modified CMC with 1 or 3 mol % of N, N-dihexyacrylamide was slightly larger than that of the unmodified CMC; however, the [M.sub.w] of the copolymer with 6 mol % of hydrophobic monomer was slightly lower. This result seems to indicate the existence of aggregating structures. This behavior agrees with the results reported by Charpentier-Valenza (21). The increase of [M.sub.w] is confirmed by the increasing light scattering intensity. It occurs because light scattering intensity is proportional to polymer molar mass (37). The greater the copolymer molar mass, the greater the light scattering intensity. Moreover, an increase in [M.sub.w] is observed as the hydrophobic. group length increases. However, the lowest [M.sub.w] value was observed for the copolymer of the hydrophobic groups with the largest length (CHCHM6), indicating strong intramolecular associations that induce macromolecular compactness. This occurred because the solvatation of the polymer chain decreased because of the increasing hydrophobe content. These results clearly show the presence of intramolecular hydrophobic interactions resulting from the largest content of the N,N-dihexylacrylamide groups in 0.5 M NaCl. This behavior agrees with results reported in the literature (37). In contrast, for all copolymers, the [R.sub.g] values decrease as the length of hydrophobic groups increases. Furthermore, they are lower than that of the precursor polymer, except for the one with 1 mol % of hydrophobic groups. This result seems to indicate that the hydrophobic interactions are favored by a reduction in the solvent quality and this effect becomes more significant with the increase in hydrophobic group length grafted onto the CMC backbone. This behavior suggests that the greater the hydrophobic group length, the stronger will be the intramolecular hydrophobic interactions and consequently the greater will be the tendency to contraction of the polymeric chain into a more compact coil when compared to unmodified CMC. It is also shown in Table 2 that all the copolymers had lower [A.sub.2] values than that of unmodified CMC, suggesting an increase in polymer-polymer interactions because of the reduction of the solvent quality caused by the presence of the hydrophobic groups grafted onto the CMC backbone in salt solution, which, as previously suggested, favor aggregate formation. However, only CMCHM3 had negative [A.sub.2] value, suggesting easier hydrophobic aggregate formation as a result of the reduction in polymer-solvent interactions in the presence of salt. Moreover, this copolymer also had the lowest [R.sub.g] and highest [M.sub.w] values. This result clearly indicates that this behavior is related to hydrophobic chain length, considering the solvent effect in the hydrophilic CMC backbone, which will be able to either facilitate or hinder the hydrophobic aggregate formation.
Dynamic Light Scattering
The DLS experiments were carried out at different polymer concentrations (1, 2, 3, and 4 g/L) and scattering angles (60[degrees], 90[degrees], and 120[degrees]) in Milli-Q water, synthetic formation water (SFW) and 0.5 M NaCl aqueous solution, at 25, 45, 55, and 65[degrees]C. The ionic strength of the two latter solutions was 6.83 X [10.sup.-3] and 0.5, respectively. However, we will show the results of typical autocorrelation function and distribution function of the hydrodynamic radius obtained by CONTIN analysis for two polymer concentrations (1 and 3 g/L) in 0.5 M NaCl, at a scattering angle of 90[degrees], at temperatures of 25 and 65[degrees]C, because these conditions represent the overall behavior observed for the copolymers studied.
Effect of Polymer Concentration and Solvent. Figure 2 shows the effect of polymer concentration and solvent on the typical autocorrelation function (see Fig. 2a and b) and the distribution function of the hydrodynamic radius (see Fig. 2c and d) for unmodified CMC and hydrophobically modified CMC with different lengths of the N,N-dihexylacrylamide groups in 0.5 M NaCl. The most important feature of the decays is the presence of long-time tails of the autocorrelation function for all polymers under the conditions analyzed (Fig. 2a and b). This suggests the existence of aggregates. This behavior agrees with the results reported by Kj[empty set]niksen et al. for hydrophobically modified EHEC EHEC
enterohemorrhagic Escherichia coli.
EHEC Enterohemorrhagic Escherichia coli, see there at different levels of SDS addition (31). It is important to emphasize that the aggregates observed in the unmodified CMC backbone are hydrophilic and produced by hydrogen bonding. In addition, the size of these aggregates decreases as ionic strength increases owing to the reduction of electrostatic interactions (Table 3).
[FIGURE 2 OMITTED]
TABLE 3. Hydrodynamic radii for the CMC and CMCHM copolymers obtained at polymer concentrations of 1 and 3 g/L, scattering angle of 90[degrees], and temperature of 25[degrees]C. [R.sub.H polymer] [R.sub.H aggregate] (nm) (fast mode) (nm) (slow mode) Solvent Sample [C.sub.p] [C.sub.p] [C.sub.p] [C.sub.p] = 1g/L = 3g/L = 1/L = 3 g/L Milli-Q Water CMC -- -- 179 620 CMCHM1 66 75 620 796 CMCHM3 85 35 902 1310 CMCHM6 40 85 484 548 SFW CMC 17 -- 294 213 CMCHM1 35 85 377 901 CMCHM3 21 31 484 703 CMCHM6 45 24 294 548 0.5 M NaCl CMC 35 21 -- 294 CMCHM1 24 24 549 294 CMCHM3 17 40 703 377 CMCHM6 17 19 333 202
An inspection of the autocorrelation function for the CMC and CMCHM systems shows that the relaxation process for the copolymers occurred at longer times than that of the unmodified CMC, i.e., the copolymers take longer to relax than the precursor polymer does. This probably occurs because of the hydrophobic aggregate on the copolymer chain, making the molecule denser and relaxation difficult. However, the shift towards a more prolonged relaxation process is more pronounced for a copolymer solution containing 3 mol % of the hydrophobic groups (CMCHM3), indicating that this copolymer needs more time to relax than the copolymers with 1 mol % (CMCHM1) or 6 mol % (CMCHM6) of N,N-dixehylacrylamide groups. This behavior may be related to the size of the aggregate formed. The greater the aggregate size, the longer time the polymer chain needs to relax. This result also clearly indicates that the copolymer of the hydrophobic groups with the largest length (CMCHM6) forms the smallest hydrophobic aggregates and consequently needs the shortest relaxation process. This behavior suggests the presence of strong intra and/or intermolecular hydrophobic interactions for this copolymer considering both regimes analyzed: dilute regime (Cp = 1 g/L) and semidilute regime (Cp = 3 g/L).
The relaxation process for the copolymer solutions in Milli-Q water and SFW (not shown here) occurred over slightly longer periods, with the increase of polymer concentration. This result seems to indicate that the extension of the hydrophobic aggregates increases as polymer concentration increases, making the macromolecule more denser. In the solvent of highest ionic strength (0.5 M NaCl) (Fig. 2a and b), the copolymer solutions exhibited the inverse behavior. In this case, the relaxation process for copolymers occurred at shorter times, indicating the smallest size of the hydrophobic aggregate formed. This could be explained by the reduction of electrostatic interactions, which favors the closer polymer chain, or of solvent quality, which promotes strong intramolecular and/or intermolecular associations, suggesting the contraction of the polymeric coil due to the presence of the hydrophobic groups. This result indicates that the size of the hydrophobic aggregates is reduced with the increase of ionic strength in the medium.
The autocorrelation functions are better described by the distribution function of the hydrodynamic radius obtained from CONTIN analysis for CMC and CMCHMs with different lengths of the hydrophobic groups (1 to 6 mol %). The analysis conditions are shown in Fig. 2c and d. It can be observed from these figures that all copolymers had two relaxation modes. This comparison can be made here because these curves are similar to those obtained by distribution of the relaxation times (not shown here). The fast mode of the relaxation process occurs in a shorter time while the slow mode takes place over a longer time. The former may reflect a motion of the polymer chains relative to the solvent (1) and the latter may indicate the existence of aggregates (31). The unmodified CMC exhibited monomodal relaxation time distribution in Milli-Q water (ionic strength zero, free salt) (not shown here); nevertheless, in the presence of the salts (SFW (not shown here) and 0.5 M NaCl (Fig. 2c and d), there is a tendency to bimodal distribution bimodal distribution
a distribution with two peaks separated by a region of low frequency of observations. , whereas the copolymers are bimodally distributed in all the solvents and polymer concentrations studied here. This behavior suggests that the polymer solutions have structures with different sizes. Moreover, polymer chain length and aggregate size depend strongly on the polymer concentration and ionic strength of the solvent, as shown in Table 3.
Table 3 also shows that the [R.sub.H] values attributed to the fast mode of relaxation (polymer chain) varied from 17 to 35 nm for CMC and from 17 to 85 nm for copolymers and that the [R.sub.H] values of the slow mode (aggregates) ranged from 179 to 620 for CMC and from 202 to 1480 nm for copolymers. The [R.sub.H] variation for the polymer chain likely occurred because the macromolecules are polydisperse, i.e., they have chains of different sizes. Moreover, the [R.sub.H] of the aggregates depends on polymer concentration, ionic strength and hydrophobic group length. The [R.sub.H] values lower than 10 nm (see Fig. 2d) may be due to the oligomers generally present in the polymer samples and the [R.sub.H] values around [10.sup.4] nm may be attributed to the analysis conditions used in the DLS experiments, where q.[R.sub.H] < 1. In this case, we can observe the relaxation of all the polymeric chain. However, when observing the system at q.[R.sub.H] > 1, one observes the relaxation of macromolecular and internal rotations.
The [R.sub.H] values shown in Table 3 also indicate that, in general, for the fast relaxation mode (polymer chain), [R.sub.H] increases with increasing polymer concentration and decreases with increasing ionic strength in the medium. Nevertheless, for the low relaxation mode (aggregates), [R.sub.H] increases with higher polymer concentration in Milli-Q water and SFW, but decreases in 0.5 M NaCl. This occurs as a result of the reduction in electrostatic interactions and solubility promoted by solvent of high ionic strength, suggesting strong intra and/or intermolecular associations. Furthermore, this effect was more significant with the increased polymer concentration (3 g/L), owing to the highest concentration of polymer chains in the medium, promoting closeness between the molecules and the formation of larger aggregates.
An interesting feature can be observed from the results of [R.sub.H], shown in Table 3. The CMCHM3 copolymer had the largest [R.sub.H] of the aggregates in all the polymer concentrations and solvents studied here, except in SFW at 3 g/L. However, the lowest [R.sub.H] of the aggregates was observed for the CMCHM6 copolymer, with the longest hydrophobic chain length of all the conditions analyzed. These results seem to indicate that up to 3 mol % of hydrophobic groups show equilibrium between enhanced hydrophobic aggregate size and reduced solvent quality. The polymer-polymer interactions do not seem to interfere in the aggregate formation, but above a certain concentration of hydrophobic groups, the polymer-polymer interactions become more significant. In this case, the intramolecular associations prevail over the intermolecular ones.
Temperature Effect. Figures 3 and 4 illustrate the effect on hydrophobic groups in the copolymers with the increase in temperature obtained by the typical autocorrelation function (see Figs. 3a and b, 4a and b) and the distribution function of the hydrodynamic radius (see Figs. 3c and d, 4c and d) in 0.5 M NaCl for polymer concentrations of 1 g/L (see Fig. 3) and 3 g/L (see Fig. 4). Figures 3a and b show that an increase of temperature reduces the relaxation process time. This behavior indicates that the size of aggregates decreases as the temperature increases; this takes place because of increased molecular agitation, favoring the polymer-solvent interactions and consequently polymer solubilization in the medium, but making the formation of more compact structures more difficult. For this reason, the molecule relaxes more easily. Moreover, even with the increased temperature, which favors the reduction of aggregates, the formation of major hydrophobic aggregates for the CMCHM3 copolymer was still observed. This result indicates that intermolecular associations are more significant than intramolecular ones. It also suggests that the copolymer with the largest aggregate size is more stable at higher temperatures than the other copolymers studied here, because this behavior persisted even at higher temperatures.
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
Figure 4a and b show that an increase in polymer concentration (3 g/L) and temperature reduced the relaxation process time in 0.5 M NaCl. However, the hydrophobic aggregates formed are larger than those at 1 g/L polymer concentration owing to the higher polymer concentration in the medium (see Table 4). This result seems to indicate that hydrophobic interactions are favored with an increase in temperature and polymer concentration in the presence of salt (0.5 M NaCl). It is well known that hydrophobic hydration is an exothermic exothermic /exo·ther·mic/ (-ther´mik) marked or accompanied by evolution of heat; liberating heat or energy.
ex·o·ther·mic or ex·o·ther·mal
1. process whereas the interaction between hydrophobic chains is an endothermic endothermic /en·do·ther·mic/ (-ther´mik) characterized by or accompanied by the absorption of heat.
en·do·ther·mic or en·do·ther·mal
1. process (8). Thus, hydrophobic aggregate formation is favored by increased temperatures. This effect, together with hydrophilic chain hydration, may contribute to increasing the [R.sub.H] of the aggregates in the copolymer chains. However, at very high temperatures, molecular motion can destroy the intermolecular hydrophobic interactions (3).
An interesting effect was observed for the CMCHM1 copolymer at temperatures of 45 and 55[degrees]C (Table 4). This copolymer had a slightly higher relaxation time (or hydrodynamic radius) than that of the larger copolymer of the hydrophobic groups (CMCHM3). This effect seems to indicate that with an increase in polymer concentration and temperature, hydrophobic aggregate formation is also favored for the copolymer with the lowest content of the hydrophobic groups. This may have occurred because of increased polymer solubility in the medium and intermolecular hydrophobic interactions. However, for all polymers at 65[degrees]C (see Fig. 3d and Table 4), a shift in the relaxation process for very short times was still observed. This may have occurred because of the rupture of intermolecular associations that takes place at very high temperatures.
TABLE 4. Hydrodynamic radii for the CMC and CMCHMs copolymers obtained in 0.5 M NaCl at different temperatures,polymer concentrations of 1 and 3 g/L and scattering angle of 90[degrees] Temperature ([degrees]C) 25 45 c(g/L) Sample [R.sub.H] [R.sub.H] [R.sub.H] [R.sub.H] polymer aggregate polymer aggregate 1 CMC 35 -- 35 -- CMCHM1 24 549 21 200 CMCHM3 17 703 21 227 CMCHM6 17 333 17 156 3 CMC 21 294 21 289 CMCHM1 24 294 24 478 CMCHM3 40 377 27 423 CMCHM6 19 202 11 291 Temperature ([degrees]C) 55 65 c(g/L) Sample [R.sub.H] [R.sub.H] [R.sub.H] [R.sub.H] polymer aggregate polymer aggregate 1 CMC 30 -- 33 -- CMCHM1 27 195 26 188 CMCHM3 23 250 42 309 CMCHM6 21 138 18 114 3 CMC 21 249 26 213 CMCHM1 11 527 23 213 CMCHM3 23 466 42 396 CMCHM6 13 186 18 166
Nevertheless, with increased polymer concentration (3 g/L), the unmodified CMC had two relaxation modes (Fig. 4c and d). The fast relaxation mode of the polymer chains and the slow mode of the hydrophobic aggregates. This behavior shows once again that CMC chemical modification took place. This result was also observed in Fig. 2d. Furthermore, we can observe more clearly that with increasing temperature, the shift in the relaxation process for aggregates occurs in less time, indicating a reduction in aggregate size.
Effect of Polymer Concentration on [R.sub.H]. Figure 5 illustrates the typical [q.sup.2] dependence on the relaxation frequency [GAMMA] for CMC and CMCHM copolymers in SFW, at polymer concentration of 1 g/L and temperature of 25[degrees]C. The [q.sup.2] values were obtained by q = (4[pi]n/[lambda]) sin ([theta]/2), where n is the refractive index of the solvent, [lambda] is the wavelength of the incident light in the vacuum and [theta] is the scattering angle. The relaxation frequency was calculated by [GAMMA] = 1/[tau], where [tau] is the maximum slow mode relaxation time obtained by G(t) curves as a function of relaxation time (not shown here), which in turn, was obtained during data treatment by decay time spectra. The coefficient angle of the relaxation frequencies for the CMC and CMCHM copolymer corresponds to the diffusion coefficient value for each system when [q.sup.2] tends to zero (Equation 6). The true [R.sub.H] values of the aggregates were obtained by D = kT/6[phi][eat][R.sub.H] (Stockes-Einstein Equation). This same route was used to determine true [R.sub.H] values for CMC and CMCHM copolymers in all polymer concentrations and temperatures used here.
[FIGURE 5 OMITTED]
The CMCHM3 copolymer had the lowest diffusion coefficient. This behavior indicates that CMCHM3 has a higher [R.sub.H] than that of its polymer precursor and other copolymers. Furthermore, the lower diffusion coefficient suggests aggregate growth (38), (39). Therefore, the lower the diffusion coefficient, the larger the hydrodynamic radius. In Fig. 5, we can observe that the other copolymers also have a diffusion coefficient lower than that of unmodified CMC, indicating that the hydrophobic aggregates are larger than the hydrophilic ones on the CMC backbone. This effect depends strongly on solvent quality and on hydrophobic group length. However, Fig. 5 shows that the [R.sub.H] value of the CMC aggregates is slightly lower than that of the CMCHM6 copolymer, while in Table 3 (1 g/L polymer concentration, 90[degrees] scattering angle, 25[degrees]C) equal values can be observed. It is important to remember that the [R.sub.H] obtained at 90[degrees] depicted in Figs. 2c and d, 3c and d, 4c and d showed only the apparent [R.sub.H] value of polymer backbone length in solution, while the [R.sub.H] value obtained by the diffusion coefficient from Stockes-Einstein Equation characterizes the real [R.sub.H] of the polymer in solution.
Figures 6a--c show the variation of the real hydrodynamic radius of the aggregates with polymer concentrations in different solvents at 25[degrees]C. The real [R.sub.H] of the aggregates increases with the polymer concentration in Milli-Q water (Fig. 6a) and SFW (Fig. 6b), but decreases in 0.5 M NaCl (Fig. 6c). This behavior also was observed in Table 3 and in Fig. 2c and d. In Milli-Q water, the increase in the real [R.sub.H] of the aggregates for the CMC solution with increasing polymer concentration is a result of two effects: the electrostatic repulsions between the carboxylate groups and the formation of intermolecular hydrophilic aggregates, which promotes the increase of the hydrodynamic volume in solution. The former is more significant at lower polymer concentrations and the latter at higher ones. In the first case, the chains are further apart from one another, in such a way that the presence of the charges hinders approximation between the chains and between the chain segments, making the molecules more stretched and more rigid. In the second case, the chains are closer to one another, in such a way that electrostatic repulsion becomes less significant. In this situation, intermolecular interactions are favored. In the case of copolymers, the increase of real [R.sub.H] of the aggregates takes place because of the electrostatic repulsion and hydrophobic aggregate formation. For this reason, the largest [R.sub.H] values are observed in Milli-Q water. Moreover, the hydrophobic aggregates are generating in this solvent owing to the strong interactions that occur between the hydrophobic groups to minimize their exposure to water.
[FIGURE 6 OMITTED]
In SFW (Fig. 6b), for all the polymer concentration ranges used here, it was observed that the copolymers had lower [R.sub.H] values than those obtained in the absence of salt. This is due to the reduction of electrostatic interactions promoted by the salts (NaCl, MgC[l.sub.2] and CaC[l.sub.2]) present in the synthetic formation water (SFW) even at lower concentrations. However, the [R.sub.H] of the aggregates increases as polymer concentration increases. This behavior was observed for CMC and CMCHM copolymers. For copolymers, this occurs because of intra and/or intermolecular hydrophobic aggregate formation that depends on solvent quality and interactions between the divalent cations and carboxylate groups. For CMC, hydrophilic aggregate formation takes place mainly owing to the reduction of electrostatic repulsions, which promotes greater interaction between the OH groups present on the CMC backbone. However, intermolecular hydrophobic and/or hydrophilic interactions are favored with increased polymer concentration.
In 0.5 M NaCl (Fig. 6c), the [R.sub.H] of the copolymer aggregates decreases with increased polymer concentration whereas that of CMC increases. For the copolymers, the salt, in addition to reducing electrostatic repulsions, may favor closeness between the polymer chains and reduce copolymer solubility, generating strong intra and/or intermolecular interactions. However, in this case, the intramolecular hydrophobic interactions seem to be more significant than the intermolecular hydrophobic ones, given the reduction of hydrodynamic volume observed. Charpentier-Valenza et al. (21) also observed reduced CMCHM copolymer solubility with increased ionic strength in the medium. They considered that this behavior was more significant for CMCHM than for amphiphilic polymers based on acrylamides, because CMCHM copolymers have lower charge density than that of associating synthetic polymers. For unmodified CMC, reduced electrostatic interactions promote the formation of intermolecular hydrophilic interactions. Thus, hydrophilic aggregate formation is more evident with higher polymer concentrations.
Another interesting factor to be observed is the effect of hydrophobic groups on the polymer backbone. With an increase in hydrophobic groups, intra and/or intermolecular hydrophobic interactions may become more significant owing to reduced solvent quality, which favors increased polymer--polymer interactions and consequently hydrophobic aggregate formation. Therefore, hydrophobic aggregate size depends strongly on the groups grafted onto the hydrophilic CMC backbone. This behavior was discussed in section Effect of Polymer Concentration and Solvent.
Temperature Effect on the [R.sub.H] of the Aggregates. Figure 7 shows the variation of the real hydrodynamic radius of the aggregates with temperature in 0.5 M NaCl at polymer concentrations of 1 g/L (Fig. 7a) and 3 g/L (Fig. 7b). An interesting effect was observed for the [R.sub.H] values of all the copolymer aggregates in the highest ionic strength solvent (0.5 M NaCl) with an increase in temperature. For polymer concentration of 1 g/L, all copolymers showed reduced [R.sub.H] of the aggregates with an increase in temperature. This result indicates that the NaCl salt in the concentration used here causes reduced copolymer solubility due to the increase of the polymer-polymer interaction favored by the hydrophobic groups. An opposite effect occurs for unmodified CMC, indicating that in a diluted regime greater polymeric chain solubility caused by increased polymer-solvent interactions seems to prevail over the reduction of electrostatic interactions favored by salt. Nevertheless, even with increasing temperature, the CMCHM3 copolymer had the highest [R.sub.H] values while CMCHM6 had the lowest. This result is shown in Fig. 6 and remained unchanged as temperature increased. In these two cases, intramolecular hydrophobic interactions prevail over intermolecular hydrophobic interactions prevail over intermolecular hydrophobic ones, but this effect was more significant for the CMCHM6 copolymer owing to its longer length of hydrophobic groups.
[FIGURE 7 OMITTED]
However, in Fig. 7b, the [R.sub.H] of the copolymer aggregates increases with higher temperatures, suggesting that intermolecular hydrophobic interactions are favored by the presence of high ionic strength salt solutions, high polymer concentrations and high temperatures. This effect was more significant for the copolymer with the shortest hydrophobic group length (CMCHM1). For this copolymer, an increase in [R.sub.H] was observed with an increase in temperature from 25 to 45[degrees]C. [R.sub.H] decreased slightly at 55[degrees]C, where intermolecular hydrophobic interactions are promoted by an endothermic process, and dropped significantly after 55[degrees]C, indicating the break of intermolecular associations due to fast molecular movement. For CMCHM3, the [R.sub.H] increases slightly with increasing temperature, suggesting only intermolecular associations and for CMCHM6 an increase of [R.sub.H] from 25 to 55[degrees]C was observed, indicating intermolecular associations. This was followed by falling [R.sub.H] after 55[degrees]C, suggesting a rupture of intermolecular hydrophobic interactions. This result suggests that the CMCHM3 copolymer is more stable thermally, since the rupture temperature had not yet been reached. Even though intermolecular hydrophobic interactions are favored by the presence of high ionic strength salt solutions, high temperatures and high polymer concentrations, in the presence of salt, the [R.sub.H] values of the aggregates were still lower than those observed in Milli-Q water and SFW (not shown here). However, [R.sub.H] decreases with an increase in temperature for the CMC, which indicates that in a semidilute regime the effect of reduced electrostatic repulsions is more significant than that of the increased polymer solubility favored by increasing temperatures.
The influence of shear rate on the apparent viscosity of unmodified CMC and hydrophobically modified CMC with 1 and 3 mol % of hydrophobic groups at constant polymer concentration (Cp = 10 g/L) in SFW, at 55[degrees]C, is shown in Fig. 8. All polymers exhibited pseudoplastic behavior, i.e., a reduction of apparent viscosity with an increase of shear rate. This is due to two factors: (1) orientation of the polymer chains in the flow direction as the stress rate increases, the viscosity (resistance to flow) decreases; (2) disruption of the entanglement present in the polymeric solutions as the shear rate increases (40). Moreover, the copolymers had higher viscosities than those of the unmodified CMC. This behavior indicates that intermolecular hydrophobic interactions can occur even in light saline solutions, but it is important to remember that this effect can also be caused by the interactions between divalent cations and carboxylate groups. However, the sum of the two effects was insufficient to increase the size of the aggregates or lead to a significant viscosity increase. Furthermore, the increase in copolymer viscosities in solution confirms the efficacy of incorporating the hydrophobic groups onto the CMC backbone. However, no significant difference in the viscosities of the aqueous copolymer solutions (CMCHM1 and CMCHM3) was observed.
[FIGURE 8 OMITTED]
The synthesis of CMC-g-poly(N,N-dihexylacrylamide) copolymers with 1 to 6 mol % of hydrophobic groups was not confirmed by [H.sub.1] NMR, likely because this technique is not sensitive enough to detect the presence of very low hydrophobe content in a medium. However, the results obtained by viscosity, static and dynamic light scattering and rheological measurements for all copolymers showed significant changes in their solution properties, indicating that a chemical modification of the cellulose derivative actually occurred.
In a dilute regime, intramolecular hydrophobic associations promoted by hydrophobic groups became evident with a slight reduction of the intrinsic viscosity of the copolymers generated by the increase of ionic strength in the medium. This effect was more pronounced with the increased hydrophobic group length grafted onto the CMC backbone. Copolymer formation was also confirmed by SLS experiments, by increasing the [M.sub.w] and decreasing the [R.sub.g] and [A.sub.2] when compared to unmodified CMC, indicating the presence of hydrophobic structures on the CMC backbone that favor the polymer-polymer interactions caused by reduced solvent quality.
The results obtained by DLS experiments illustrates that the [R.sub.H] of the copolymer aggregates increases with increased polymer concentrations in low ionic strength systems (Milli-Q water and SFW), which suggests the formation of intermolecular hydrophobic associations. However, in 0.5 M NaCl (the highest ionic strength solvent) [R.sub.H] decreased as polymer concentration increased, indicating the formation of intramolecular hydrophobic interactions. The copolymer with the greatest hydrophobic group length had the lowest [R.sub.H] values in three solvents and in all polymer concentrations used, showing that the intramolecular hydrophobic associations became more significant as hydrophobic group length increased. The results held true at higher temperatures. Although, the intermolecular hydrophobic interactions in the copolymers were observed in the presence of salt of high ionic strength, high temperatures and high polymer concentrations, the [R.sub.H] values of the aggregates were still lower than those observed in Milli-Q water and SFW. The intermolecular hydrophobic interactions were also shown by the rheological behavior of copolymers in SFW at 55[degrees]C in a semidilute, since the two copolymers studied (CMCHM1 and CMCHM3) had higher viscosities than that of the unmodified CMC. However, the aggregates formed by the intermolecular hydrophobic interactions promoted by divalent cations and carboxilate groups were not enough to significantly increase the size of the aggregates.
The authors are grateful to the PRH PRH Prolactin-Releasing Hormone (also seen as PRLH)
PRH Personnalité et Relations Humaines (Edmonton, Alberta, Canada)
PRH Pacific Region Headquarters
PRH People's Republic of Haven 30-ANP/MCT and Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq) for the financial support toward the attainment of a Ph.D undertaken partly in Brazil and partly in France. The Experiments in this study were carried out in Brazil and in LCPO, Bordeaux, France.
(1.) B. Nystrom, A.-L. Kjoniksen, and C. Iversen, Adv. Colloid colloid (kŏl`oid) [Gr.,=gluelike], a mixture in which one substance is divided into minute particles (called colloidal particles) and dispersed throughout a second substance. Interface Sci., 79, 81 (1999).
(2.) W. Xue, I.W. Hamley, V. Castelletto, and P.D. Olmsted, Eur. Polym. J., 40, 47 (2004).
(3.) J. Zhang, L-M. Zhang, and Z-M. Li, J. Appl. Polym. Sci., 78, 537 (2000).
(4.) M. Camail, A. Margaillan, I. Martin, A.L. Papailhou, and J.L. Vernet, Eur. Polym. J., 36, 1853 (2000).
(5.) F. Candau and J. Selb, Adv. Colloid Interface Sci., 79, 149 (1999).
(6.) Y.A. Shashkina, Y.D. Zaroslov, V.A. Smirnov, O.E. Philippova, A.R. Khokhlov, T.A. Pryakhina, and N.A. Churochkina, Polymer, 44, 2289 (2003).
(7.) Y. Feng, L. Billon bil·lon
1. An alloy of gold or silver with a greater proportion of another metal, such as copper, used in making coins.
2. An alloy of silver with a high percentage of copper, used in making medals and tokens. , B. Grassl, A. Khoukh, and J. Francois, Polymer, 43, 2055 (2002).
(8.) J. Ma, P. Cui, L. Zhao, and R. Huang, Eur. Polym. J., 38, 1627 (2002).
(9.) D. Charpentier, G. Mocanu, A. Carpov, S. Chappelle, L. Merle merle
a pattern of coat color pigmentation with dark, irregular blotches on a lighter background. Seen in some Collies and Welsh corgis. In shorthaired dogs, e.g. Great Danes and Dachshunds, the similar pattern is called dapple. , and G. Muller, Carbohydr. Polym., 33, 177 (1997).
(10.) S. Simon, J.Y. Dugast, D. Le Cerf, L. Picton, and G. Muller, Polymer, 44, 7917 (2003).
(11.) I. Bataille, J. Huguet, G. Muller, G. Mocanu, and A. Carpov, Int. J. Biol. Macromol., 20, 179 (1997).
(12.) E. Rotureau, E. Dellacherie, and A. Durand, Eur. Polym. J., 42, 1086 (2006).
(13.) Y. Yamanaka and K. Esumi, Colloids Surf. A Physicochem. Eng. Aspects, 122, 121 (1997).
(14.) S. Nilsson, K. Thuresson, P. Hansson, and B. Lindman, J. Phys. Chem. B, 102, 7099 (1998).
(15.) T. Aubry, F. Bossard, and M. Moan, Polymer, 43, 3375 (2002).
(16.) G.A. Stahl and D.N. Schulz, Water-Soluble Polymer for Petroleum Recovery, Plenum Press, New York New York, state, United States
New York, Middle Atlantic state of the United States. It is bordered by Vermont, Massachusetts, Connecticut, and the Atlantic Ocean (E), New Jersey and Pennsylvania (S), Lakes Erie and Ontario and the Canadian province of , 147 (1988).
(17.) K. Podhajecka, K. Prochazka, and D. Hourdet, Polymer, 48, 1586 (2007).
(18.) G. Bokias, Y. Mylonas, G. Staikos, G.G. Bumbu, and C. Vasile, Macromolecules, 34, 4958 (2001).
(19.) E. Volpert, J. Selb, and F. Candau, Macromolecules, 29, 1452 (1996).
(20.) E. Volpert, J. Selb, and F. Candau, Polymer, 39, 1025 (1998).
(21.) D. Charpentier-Valenza, L. Merle, G. Mocanu, L. Picton, and G. Muler, Carb. Polym., 60, 87 (2005).
(22.) A. Sinquin, P. Hubert, and E. Dellacherie, Langmuir, 9, 3334 (1993).
(23.) A. Sinquin, P. Hubert, P. Marchal, L. Choplin, and E. Dellacherie, Colloids Surf. A Physicochem. Eng. Aspects, 112, 193 (1996).
(24.) F.F.-L. Ho and D.W. Kloslewlcz, Anal. Chem., 52,913 (1980).
(25.) C.L. McCormick, T. Nonaka, and C.B. Johnson, Polymer, 29, 731 (1988).
(26.) A. Hill, F. Candau, and J. Selb, Macromolecules, 26, 4521 (1993).
(27.) C. Follmer, F.V. Pereira, N.P. da Silveira, and C.R. Carlini, Biophys. Chem., 111, 79 (2004).
(28.) C.W. Hoogendam, A. de Keizer, M.A.C. Stuart, B.H. Bijsterbosch, J.A.M. Smit, J.A.P.P. van Dijk, P.M. van der Horst, and J.G. Batelaan, Macromolecules, 31, 6297 (1998).
(29.) A.J.F, Siegert, Corretation Functions in Dynamic Light Scattering, MIT MIT - Massachusetts Institute of Technology Rad. Lab. Rep. no. 465 (1943).
(30.) S.W. Provencher, J. Hendrix, L. de Maeyer, and N.L.J. Paulussen, J. Chem. Phys., 69, 4273 (1978).
(31.) A-L. Kjoniksen, B. Nystrom, and B. Lindman, Langmuir, 14, 5039 (1998).
(32.) A-L. Kjoniksen, S. Nilsson, S. Nilsson, K. Thuresson, B. Lindam, and B. Nystrom, Macromolecules, 33, 877 (2000).
(33.) A.M.S Maia, M. Costa, R. Borsali, and R.B. Garcia, Macromolecular Symposia, 229, 217 (2005).
(34.) M. Rinaudo, M. Milas, N. Jouon, and R. Borsali, Polymer, 34, 3710 (1993).
(35.) A. Tager; Physical Chemistry of Polymers, Cap. 15, Mir. Publishers, 2nd ed., Moscow (1978).
(36.) E.F. Lucas, B.G. Soares, and E. Monteiro, Serie Instituto de Macromoleculas: Caracterizacao de Polimeros - Determinacao de peso molecular e analise termica, Cap. 5, e-papers. Rio de Janeiro (2001).
(37.) W. Fred and JR. Billemeyer, Measurement of Molecular Weight and Size (Light Scattering), Textbook of Polymer Science, 3rd ed., A Wiley-Interscience Publication, New York: John Wiley & Sons (1984).
(38.) Z-G. Wang, Langmuir, 6, 928 (1990).
(39.) S Guillot, M. Delsanti, S. Desert, and D. Langevin, Langmuir, 19, 230 (2003).
(40.) O.H. Lin, R.N. Kumar, H.D. Rozman, M. Azemi, and M. Noor, Carbohydr. Polym., 59, 57 (2005).
Correspondence to: Rosangela Balaban; e-mail: firstname.lastname@example.org DOI (Digital Object Identifier) A method of applying a persistent name to documents, publications and other resources on the Internet rather than using a URL, which can change over time. 10.1002/pen.21180
Published online in Wiley InterScience (www.interscience.wiley.com).
[c] 2008 Society of Plastics Engineers
Rosangela Regia Lima Vidal, (1) Rosangela Balaban, (1) Redouane Borsali (2), (3)
(1) LAPET, Chemistry Department, Federal University of Rio Grande The University of Rio Grande and Rio Grande Community College are twin colleges in Rio Grande, Ohio.
The University of Rio Grande offers a range of courses and majors and is known in the region for its Education and Nursing programs. do Norte, 59078-970, Natal/ RN- Brazil
(2) LCPO-CNRS- ENSCPB ENSCPB Ecole Nationale Supérieure de Chimie et de Physique de Bordeaux (French) & Universite Bordeaux 1, 16 Avenue Pey Berland 33607 PESSAC Cedex, France
(3) CERMAV, CNRS CNRS Centre National de la Recherche Scientifique (National Center for Scientific Research, France)
CNRS Centro Nacional de Referencia Para El Sida (Argentinean National Reference Center for Aids) and Joseph Fourier University, BP53, 38041 Grenoble cedex 9, France