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Environmentally sensitive hydrogels: N-isopropyl acrylamide/ acrylamide/ mono-, di-, tricarboxylic acid crosslinked polymers.

INTRODUCTION

Hydrogels undergo a volume phase transition in response to a change in their environment such as temperature, pH, ionic strength, solvent and electric fields, and so on. Therefore, hydrogel can be used in many fields such as advanced material for biomedical, environmental, catalysis, and sensor applications. A very well known temperature sensitive hydrogel is crosslinked poly(/V-isopropyl acrylamide) which has a lower critical solution temperature (LCST) at 31-33[degrees]C in water [1, 2). Below that temperature, the hydrogel is swollen in water due to the hydrogen bonding (hydrated state), and above that LCST, the hydrogel collapsed (dehydrated) and becomes hydrophobic due to the breakage of hydrogen bonding. Importantly, the LCST of p(NIPAM) hydrogels can be controlled by incorporating more hydrophilic or hydrophobic monomer into the hydrogel structure. Additionally, the introduction of new functional groups such as acidic groups into the network make them responsive to the other stimuli such as pH, ionic strength, etc. depending on the nature of these new functional groups [3, 4).

In this study, a new smart polymers p(NIPAM-co-AAm)/XA were prepared using temperature sensitivity of N-isopropyl acrylamide, pH, and ionic strength sensitivity of carboxylic acid derivative monomers and mechanical strength of acrylamide monomer. These carboxylic acid such as mono-, di,- and tricarboxylic acid groups containing monomers were CA, IA, and ACA, respectively. The preparation of hydrogels was carried out via free radical polymerization reaction in aqueous solution. P(NIPAAm-co-AAm) and p(NIPAAm-co-AAm)/XA hydrogels that contain crotonic acid (CA) exhibit a lover critical solution temperature (LCST) at 28[degrees]C, whereas p(NIPAAm-ce;-AAm)/IA, and P(NIPAAm-co-AAm)/ACA hydrogels exhibit a lover critical solution temperature at 30.7[degrees]C and 34.4[degrees]C, respectively. As the number of carboxylic acid group was also increased LCST values. Especially, the LCST value of P(NIPAAm-co-AAm)/ ACA hydrogel is near body temperature. The structural characterizations and the thermal properties of these multiresponsive hydrogels were done by Fourier Transform Infrared Spectroscopy (FTIR), thermogravimetric (TG), and differential scanning calorimetric (DSC) analyses. Also, the availability of these polymers was investigated biomolecule adsorption.

EXPERIMENTAL

Chemicals

N-isopropyl acrylamide (NIPAAm) (Aldrich, Milwaukee, USA), acrylamide (AAm) (Merck Darmstadt, Germany), crotonic acid (CA) (Sigma, St. Louis, USA), itaconic acid (IA) (Sigma, St. Louis, USA), and aconitic acid (ACA) (Aldrich, Milwaukee, USA) as monomer, N,N'-methylenebisacrylamide (Bis) (Merck, Schuchardt, Germany) as crosslinkers, ammonium persulfate (APS) (Merck, Schuchardt, Germany) as redox initiator and N, N, N', N'-tetramethyl ethylene diamine (TEMED) (Sigma, St. Louis, USA) as catalyst were analytical grade and were used as received. Double-distilled water was used for all the experiments.

Preparation of Hydrogels

Hydrogels composed of NIPAAm, AAm and Carboxylic acid group containing monomers were prepared by free radical solution polymerization in the presence of a crosslinking agent, Bis (5.0 mol% based on total monomer amounts). An aqueous solution of monomers containing Bis with 0.1 mol APS and 0.1 mol TEMED were mixed and placed in PVC straws of 3 mm in diameter. p(NIPAAm)-based hydrogels were prepared in thermostated water bath at 70[degrees]C for 24 h.

The resulting hydrogel rods were cut into pieces of 3-4 mm length and washed with distilled water and dried in air and vacuum, and stored. The polymers were named as p(NIPAAm-co-AAm)/CA, p(NIPAAm-co-AAm)/IA and p(NIPAAm-co-AAm)/ ACA, respectively.

Characterization

FTIR spectra of the hydrogels were recorded with FTIR Nicolet-520 spectrofotometer in the 4000-400 [cm.sup.-1] range, on grinded hydrogel pelled with KBr, and 30 scans were taken at 4 [cm.sup.-1] resolution.

Thermogravimetric analysis was carried out using TG and DSC (Shimadzu-50 model Thermal Analyzer). Thermogravimetric analyses were performed employing 10 mg samples in a platinum pan heating up to 600[degrees]C under nitrogen gas How rate of 25 mL [min.sup.-1] with a heating rate of 10[degrees]C [min.sup.-1].

Swelling Measurement

Swelling studies of hydrogels were done gravimetrically as equilibrium, and cycled equilibrium swellings in different aqueous media (temperature, pH, and ionic strength). For the temperature dependent swelling studies, first, hydrogels were swollen in NaOH solution at pH 8.0 and heated to various temperatures to determine the range of the LCST phase transition. For the pH-dependent swelling studies, hydrogels were swollen in the different pH of solutions using HCl and NaOH ranging from 2 to 9 at 40[degrees]C. The total ionic strength of solutions was fixed at 0.05 M NaCl. The effect of ionic strength on swelling behavior was investigated using various NaCl concentrations (I = 0.05-1.0 M) at 25[degrees]C. The influences of various ions for the hydrogels swelling were examined at equal ionic strength using NaCl, NaN[O.sub.3], and Ca[Cl.sub.2] solutions. The effects of different organic solvents such as benzene, tetrahydrofuran, acetone, I-butanol, I-propanol, ethanol, methanol, ethanolamine, dimethylsulfoxide, and 2-propanol on hydrogel selling were inspected. The incubation time of 24 h at 25[degrees]C were chosen. The amount weight increase was calculated by removing hydrogel from the swelling medium and weighing.

The effects of temperature and pH on swelling behavior of hydrogels were investigated by cycled equilibrium swelling experiments in which hydrogels were swollen to their maximum swelling values consecutively between pH 3 and 8 at 25[degrees]C for 24, respectively, before weighing. The same procedure was repeated at 40[degrees]C that is above LCST.

RESULTS AND DISCUSSION

Synthesis of Hydrogels

Hydrogels composed of NIPAAm, AAm, and carboxylic acid group containing monomers were prepared by free radical solution polymerization using Bis as crosslinking agent. The polymerization mechanism is very well established and schematic representation is given in Scheme 1. It is obvious that the polymerization and crosslinking mechanism took place simultaneously. Although a gelatin was occur in about 1 h, the reaction let it proceed for 24 h, and finally the hydogel rods obtained in the plastic straws were cut into pieces of 3-4 mm length and washed with distilled water and dried in air and vacuum. Crosslinked copolynteric hydrogels were colorless and transparent.

Characterization

FTIR Analysis. Typical spectra of p(NIPAAm-co-AAm) and p(NIPAAm-co-AAm)/XA polymers is shown in Fig. I. In the FTIR spectra of the hydrogels, the typical absorption bands for NIPAAm, AAm and XA units can be seen about 3600-3100 [cm.sup.-1] as broad bands for secondary NH amide, between 2980 and 2878 [cm.sup.-1] for CH stretching frequencies for isopropyl groups, and at 1670 [cm.sup.-1] a strong C=0 amide 1 band, and at 1550 [cm.sup.-1] for other strong amide II bands. The peak around 1400-1300 [cm.sup.-1] belong to--CH[(C[H.sub.3]).sub.2] group in isopropyl groups, and the peaks about 1268-1000 [cm.sup.-1] can be attributed to C--N bands in amide groups [5-8]. From this spectral analysis, it can be concluded that a polymeric network is formed, because of the functional groups of the each component of the hydrogel units: NIPAAm, AAm, and XA monomers with Bis.

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TGA Analysis. The thermal degradation values such as the initial degradation temperatures ([T.sub.i]), the temperature of maximum speed ([T.sub.max]), the degradation final temperature ([T.sub.f]), the half-life temperature (7h), and the maximum decomposition rate ([r.sub.max]), and the maximum speed the amount of the substance ([C.sub.max]) values were found the thermograms of hydrogels (Figs. 2 and 3) and were given Table 1.

As can be clearly seen, the thermograms of hydrogels exhibit three distinct stages. The first one is in the range of 25-250[degrees]C due to loss of water, and the other one is in the range of 250-400[degrees]C that can be ascribed to a complex process including dehydration and decarboxylation of the polymer chains. The third one that starts above 400[degrees]C corresponds mainly to the degradation of polymer chains [9].

T values for all p(NIPAAm-coAAm)/XA polymers, higher than the T values of p(NIPAAm-co-AAm) polymer. By the inclusion of new functional groups, carboxylic acid monomers, p(NIPAAm-co-AAm)/XA have extra side branches, and these side branches makes the hard polymeric structure, the flexibility of the polymer chains is reduced. Therefore, the degradation temperature increases. And with tricarboxylic acid > dicarboxylic acid > monocarboxylic acid sequence is due to the increase in the number of functionality cause an increase for all the T values. The amount of the substance at the maximum speed ([C.sub.max]) values, the range from 0.38 to 0.45.

The Thermal Decomposition Kinetics of the Polymers. To determine the thermal decomposition of the samples, Freeman-Carroll and Jeres (J) methods were used [10].

Freeman-Carroll equation

[DELTA]ln dc/dt/[DELTA]ln(1 - c) = n [E.sub.a]/R [DELTA]1/T/[DELTA]ln(1 - c) (1)

y = n - [E.sub.a]/R x (2)

can be derived, where y = [DELTA]ln dc/dt/[DELTA]ln(1 - c) and x = [DELTA]1/T/[DELTA]ln(1 - c)

In the Jeres method, the Freeman-Carroll method, the x and y data corrected with the mathematical form of an application (Fig. 4).

According to the Jeres method, the decomposition kinetics parameters such as thermal decomposition reaction degree (n), activation energy ([E.sub.a]) and frequency factor of reaction (A) of the polymers are calculated for the following equations and given in Table 2.

n = [bar.y]/1 - Q[bar.x] (3)

[E.sub.a] = RQ[bar.y]/1 - Q[bar.x] (4)

Where [bar.x] and [bar.y] are mean values x and y, and Q is given as

Q = [[c'[T.sup.2.sub.mak]/1 - c].sub.c'=mak] = [E.sub.a]/nR (5)

Here, c' = dc/dt and DTG peak maximum transformation rate peak is calculated by reading.

While the [n.sub.FC] values vary between about 0.03 and 4.52, the ii] values are zero for all hydrogels. The zero order is reasonable for the degradation of hydrogels, because the rate of degradation is independent of the amount of unreacted solid material according to the transition state theory given below.

The Thermal Decomposition Thermodynamics of the Polymers. According to the transition state theory the degradation reactions generally can be represented as

Solid reactants [left and right arrow] intermediates * [right arrow] products

The reaction rate is governed by the rate of decomposition of the intermediates, and the rate of formation of the intermediates is assumed to be so rapid that they are present in equilibrium concentration at all times.

-dC/dt = [kC.sup.n] (6)

where C; the amount of the substance still to decompose at time t, n; the order of reaction, and k; the temperature-dependent rate constant. The temperature-dependence of k is given by the Arrhenius equation:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (7)

where Z; pre-exponential factor, [E.sub.a]; activation energy of reaction, R; ideal gas constant and T; absolute temperature.

The k and Z values are calculated by using the [n.sub.J] and [E.sub.aJ] values at the maximum decomposition rate temperatures, [T.sub.max], with Eqs. 6 and 7, respectively.

The enthalpy of activation ([DELTA]H*), the entropy of activation (AS*) and the Gibbs function of activation ([DELTA]G*) values are calculated at [T.sub.max] by using the [E.sub.aJ] values with Eq. 8. The [T.sub.max], [DELTA]H*, [DELTA]S*, and [DELTA]G* values of the hydrogels are given in Table 3.

[DELTA]S* = R[ln (hA/kT)- 1] [DELTA]H* = [E.sub.a] - RT [DELTA]G* = [DELTA]H* - T[DELTA]S* (8)

The [DELTA]S* values are negative and this suggests a high ordering of the transition state. The [DELTA]G* values are positive and this suggests that the formation of activated complex molecules does not proceed spontaneously. The values of [DELTA]H* and [DELTA]S* of the polymers containing carboxyl groups were found low than neutral p(NIPAAm-co-AAm) polymer. [DELTA]G* values of the all hydrogels approximately were found the same values.

DSC Analysis

Glass transition temperature of the hydrogels were determined from the DSC thermograms as demonstrated in Figs. 5 and 6 and their results are given in Table 4.

As can be seen from Table 4, the glass transition temperature for hydrogels decrease as the number of the carboxyl groups in hydrogel structure is increased. So, it is clear that the decrease in the glass transition is inversely related with number of carboxyl group in hydrogels due to plastizing effect of the carboxyl groups in hydrogels. The obtained results for [T.sub.g] can be denoted as [T.sub.g,no acid] > [T.sub.g,monoprotic] > [T.sub.g,diprotic] > [T.sub.g,triprotic].

Swelling Studies

To determine the environmental sensitivity (i.e., temperature, pH, ionic strength, solvent etc.) of hydrogels, swelling, and alternating equilibrium swelling ratio ([S.sub.eq]%) were calculated using Eq. 9.

[S.sub.eq] % = [m.sub.s] - [m.sub.d]/[m.sub.d] x 100 (9)

where [m.sub. s] and [m.sub.d] were the fully swollen gel and dried gel weights, respectively.

Temperature-Dependent Swelling Behavior

Temperature dependence on the swelling ratios of p(NIPAAm-co-AAm)/XA hydrogels at pH = 8 and I = 0.05 M is shown in Fig. 7.

The values of the LCST of the hydrogels were found by the first derivatives of the curves in Fig. 7, and are presented in Table 5.

These hydrogels swell upon cooling below LCST, and they collapse when heated above the LCST. With the increase in the number of ionizable groups, the volume change at the transition increases because of the increasing electrostatic interaction between the similarly charged groups and the transition temperature increases accordingly [11-15],

pH-Dependent Swelling Behavior

The temperature-sensitive networks containing ionizable functional groups exhibit pH sensitivity. The effect medium pH on the swelling ratios of p(NIPAAm-co-AAm)/XA hydrogels at 25[degrees]C between pH 2-9 with ionic strength of I = 0.05 M is shown in Fig. 8.

The swelling behavior of the copolymer hydrogels differs from that of p(NIPAAm-co-AAm) at 25[degrees]C. At 25[degrees]C, the nonionic p(NIPAAm-co-AAm) gel is below its LCST and it swells, but there was not a big change in the equilibrium degree of swelling with pH, as expected. In contrast, the swelling of the copolymer hydrogels p(NIPAAm-co-AAm)/XA were strongly dependent on the pH value of the external medium. At low pH values, the degree of swelling was low because the carboxylic groups in the side chains were not ionized and intermolecular complexation via hydrogen bonds may occur (physical crosslinking). As the degree of ionization increases above the nominal [pK.sub.a] values of XA (3.89-4.69), the increased hydrophilicity resulted in greater degrees of swelling as reported in the literature for the similar structures [16-18].

Ionic Strength-Dependent Swelling Behavior

Ionic strength can play an important role in the swelling behavior. The ionic strength's influence on swelling behavior was investigated at various NaCl concentrations at 25[degrees]C as illustrated in Fig. 9a. The swelling ratio of p(NIPAAm-co-AAm)/ XA hydrogels decreased as the ionic strength is increased. These phenomena can be attributed to the electrostatic repulsion between charged groups on the network chain, and the concentration difference between mobile ions inside the hydrogel and external solution [19, 20].

To compare the influence of ion and counter ion for the p(NIPAAm-co-AAm)/XA hydrogels swelling ratios at equal ionic strength are shown in Fig. 9b. The swelling ratios decreased according to the following sequence water, NaN[O.sub.3], NaCl, Ca[Cl.sub.2]. As the p(NIPAAm-co-AAm)/XA hydrogels was swelled in saline solutions, the -COOH groups were neutralized by the cations in the external solution, and the swelling ratios were decreased. When the fixed charges on polymeric side chains were fully neutralized, p(NIPAAm-co-AAm)/XA hydrogels showed nonionic behavior like p(NIPAAm-co-AAm). In various saline solutions, hydrogels showed a Donan effect when the charges on the polymeric side chain were neutralized, and then showed a salting-out effect with the gels going to a nonionic state [21-23].

In the polymer chain, monovalent [Na.sup.+] ions interact with -CO[O.sup.-] unit, a divalent [Ca.sup.2+] ions interact with the two -CO[O.sup.-] units. NaCl solution and Ca[Cl.sub.2] solution than in the polymeric chain results in the increased neutralization will reduce the loads on. Consequently uncharged polymer chains to act as the polymer chains and the degree of swelling decreases.

Also according to [Cl.sup.-] ions N[O.sup.-.sub.3] ions stronger salting precipitant (salting out) due to the inducing action of water molecules around the polymer out of the degree of swelling of the polymer NaN[O.sub.3] > NaCl was in the form.

The Effect of Solvent on Hydrogel Swellings

Solvent-dependent swelling studies were done using organic solvent-water mixtures and pure organic solvents. The swelling measurements were also carried out in dimethylsulfoxide-water, and 2-propanol-water mixtures at 25[degrees]C. The results are demonstrated in Fig. 10, as the dependence of equilibrium swelling ratio in the external solution vol%.

It is seen that all hydrogels exhibit reentrant conformational transitions in these solvent mixtures. This demonstrates that the isopropyl groups on the network chains are responsible for the observed reentrant phenomena of the hydrogel in DMSO-water mixtures. Thus, water and DMSO taken separately are good solvents for the network. However, in mixtures, the attractive water-DMSO interactions seem to dominate over water-hydrogel or DMSO-hydrogel interactions so that the gel deswells in DMSO-water mixtures. While in the shrunken state, there appears a minimum on swelling curves at Fdmso % = 0.50 and [V.sub.PA]% = 0.20. Figure 10b also shows that both the collapse and recollapse transitions occur earlier, if DMSO is replaced with 2-propanol. The variation of the reentrant phase transition regions depending on the type of the solvent can be explained with higher extent of hydrogen bonding interactions between 2propanol and water compared to those between DMSO and water. In the presence of a hydrophobic polymer such as p(NIPAAm-co-AAm)/XA hydrogels, water molecules form clusters around the hydrogel due to the hydrophobic interaction. When 2-propanol is added to water, 2-propanol molecules like to stay in the solution due to the strong interaction between 2-propanol and water, which reduces the gel swelling. The decrease in the gel swelling will increase the intramolecular hydrophobic interactions of the isopropyl groups, which may promote further shrinkage of the hydrogels. At high 2-propanol concentration, attractive polymer-propanol interactions dominate over the 2-propanol-water interactions due to the increased number of contacts between 2-propanol molecules and NIPAAm segments. As a result, 2-propanol enters into the gel phase and results in gel swelling. As the 2-propanol concentration is decreased, water-propanol interactions start to dominate so that the gel deswells. Thus, the competing attractive interactions between water-propanol (DMSO) and polymer-propanol (DMSO) result in the reentrant conformational transitions in the network [24-26].

The hydrogels were immersed in an excess of the solvents (tetrahydrofuran, acetone, 1-butanol, 1-propanol, ethanol, methanol ve ethanolamine) at 25[degrees]C until equilibrium was attained. Swelling ratio of hydrogels Q(=[V.sub.s]/[V.sub.d]) were calculated by assuming additively of volumes, where Fs and are the volumes of swollen and dry gel samples, respectively.

The relation between the swelling ratio of the hydrogels and solubility parameter of various solvents are given in Fig. 11.

The solubility parameters of hydrogels which were found maximum of curves in Fig. 11 are given in Table 6. The solubility parameter of hydrogels are found to be in the range 13.48- 13.80 [(cal [cm.sup.-3]).sup.1/2] [27, 28].

Alternating Equilibrium Swelling Behavior

Alternating swelling experiments for hydrogels were also performed between two extreme pH values, 3.0 and 8.0. Each hydrogel was placed in a buffered solution at different pHs for 24 h, and its swelling equilibrium values were determined. Figure 12 shows the plot obtained from such swelling experiments.

In the swelling of neutral p(NIPAAm-co-AAm) hydrogel with different pH and temperature, there is no change in the swelling behavior. Because of carboxylic acid groups on p(NIPAAm-co-AAm)/XA, they show the swelling behavior of pH-dependence. At pH = 8.0, negatively charged hydrogel increases electrostatic repulsion between the chains, and at pH = 3.0, uncharging/discharging decrease the swelling. So, the hydrogels swell high at high pH, show relatively shrinkage at lower pH of the medium. It is important to note that hydrogels retained its shape and integrity during the whole period of the experiments (8 days). From this result, it can be presumed that hydrogels are mechanically the durable.

By taking the advantage of temperature sensitivity of N-isopropyl acrylamide monomer, and pH and ionic strength sensitivity of carboxylic acid containing monomers, and the mechanical strength of acrylamide monomer, a model multi stimuli responsive hydrogels can be prepared as durable, homogenous appearance materials in cylindrical geometry as a smart polymer, p(NIPAAm-co-AAm)/XA that is assumed as a terpolymer.

Adsorption of Albumin

Commercial HSA and human blood serum were used in the albumin adsorption experiments. The amount of albumin in human blood serum was found as 3.8 g [dL.sup.-1]. Commercial HSA and albumin in human blood serum concentration of 10 mg [L.sup.-1] were prepared and in universal buffer solution, pH = 3.0. Into these solutions, 0.1 g of dry polymer were placed and incubated in a water bath for 24 h at 40[degrees]C. The amount of adsorbed albumin from human blood serum and commercial HSA solution (Ads %) were calculated and compared in Fig. 13 are presented in.

HSA adsorption from human blood serum was less than that of commercial available one. Because of the complex structure of human blood serum, albumin adsorption of this solution is lower. The adsorption of albumin in human blood serum, and all the p(NIPAAm-co-AAm)/XA polymers are almost identical, i.e., do not change too much with the type of carboxylic acid monomer in the hydrogel structure. However, hydrogels that environmentally sensitive can be used as chromatographic separations and can be prefer in different formulations e.g.. p(NIPAAm-co-AAm)/ACA for commercial HSA, and p(NIPAAm-co-AAm) human blood serum. In any case these types of material can be used in biomedical fields for various purposes.

CONCLUSIONS

A new series of environmentally multi stimuli responsive hydrogels was synthesized in aqueous solution by the free radical polymerization method and investigated in terms of swelling. Spectroscopic and thermal analyses were performed for the structural and thermal characterizations of these hydrogels. The properties of the environmentally sensitive hydrogels depend on the number of carboxyl acid units and isopropyl groups in the polymer network. The temperature dependencies of the hydrogels are dependent on carboxylic acid monomer. It is seen that LCST values of hydrogels increased with increasing the number of carboxyl acid units in the hydrogel. Especially, the LCST value of P(NIPAAm-co-AAm)/ACA hydrogel is near body temperature. Additionally, alternating equilibrium swelling studies have shown that hydrogels mechanically stable for a long time. These hydrogels can be used in applications such as immobilization of biologically active molecules, drug delivery, and the environments.

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Yasemin Isikver, Dursun Saraydin

Chemistry Department, Cumhuriyet University, Hydrogel Research Laboratory, 58140 Sivas, Turkey

Correspondence to: Y. Isikver; e-mail: caldiran@cumhuriyet.edu.tr Contract grant sponsor: Cumhuriyet University Scientific Research Unit; contract grant number: F-133.

DOI 10.1002/pen.23950

Published online in Wiley Online Library (wileyonlinelibrary.com).

TABLE 1. Thermogravimetric parameters of the prepared polymers.

                       [T.sub.i]/   [T.sub.max]/   [T.sub.f]/
Polymers               [degrees]C    [degrees]C    [degrees]C

pP(NIPAAm-co-AAm)         330           382           415
p(NIPAAm-co-AAm)/CA       336           395           426
p(NIPAAm-co-AAm)/IA       342           395           424
p(NIPAAm-co-AAm)/ACA      347           402           438

                       [T.sub.h]/    [r.sub.max]/
Polymers               [degrees]C   mg [dk.sup.-1]   [C.sub.max]

pP(NIPAAm-co-AAm)         374            0.85           0.44
p(NIPAAm-co-AAm)/CA       380            0.76           0.38
p(NIPAAm-co-AAm)/IA       385            0.74           0.41
p(NIPAAm-co-AAm)/ACA      394            0.80           0.45

TABLE 2. Kinetic parameters of thermal degradation of
p(NIPAAm-co-AAm)/XA hydrogels.

Hydrogels              1DT/[degrees]C   [n.sub.FC]   [n.sub.J]

p(NIPAAm-co-AAm)            330            4.52        0.17
p(NIPAAm-co-AAm)/CA         336            1.38        0.11
p(NIPAAm-co-AAm)/IA         342            0.03        0.14
p(NIPAAm-co-AAm)/ACA        347            0.79        0.11

                        [E.sub.aFC]/       [E.sub.aJ]/
Hydrogels              kJ [mol.sup.-1]   kJ [mol.sup.-1]

p(NIPAAm-co-AAm)            225.9             118.3
p(NIPAAm-co-AAm)/CA         109.4              81.8
p(NIPAAm-co-AAm)/IA          93.6              95.1
p(NIPAAm-co-AAm)/ACA         87.4              74.5

FC, Freeman-Carroll; J, Jeres.

TABLE 3. Thermodynamic properties of the hydrogels, calculated at
[T.sub.max].

                       [T.sub.max]/   [DELTA][H.sup.*]/kJ
Hydrogels               [degrees]C       [mol.sup.-1]

p(NIPAAm-co-AAm)           382               112.8
p(NIPAAm-co-AAm)/CA        395                76.3
p(NIPAAm-co-AAm)/IA        395                89.6
p(NIPAAm-co-AAm)/ACA       402                68.9

                         [DELTA][S.sup.*]/J      [DELTA][G.sup.*]/kJ
Hydrogels              [mol.sup.-1] [K.sup.-1]      [mol.sup.-1]

p(NIPAAm-co-AAm)               -113.5                   187.2
p(NIPAAm-co-AAm)/CA            -173.0                   191.8
p(NIPAAm-co-AAm)/IA            -153.0                   191.8
p(NIPAAm-co-AAm)/ACA           -185.0                   193.7

TABLE 4. Glass transition temperature ([T.sub.g]) of the hydrogels.

Hydrogels              Number of carboxyl group   [T.sub.g]/[degrees]C

p(NIPAAm-co-AAm)                  0                        85
p(NIPAAm-co-AAm)/CA               1                        76
p(NIPAAm-co-AAm)/IA               2                        70
p(NIPAAm-co-AAm)/ACA              3                        69

TABLE 5. The lower critical solution temperatures of the prepared
polymers.

Polymers               LCST/[degrees]C

p(NIPAAm-co-AAm)            27.7
p(NIPAAm-co-AAm)/CA         28.0
p(NIPAAm-co-AAm)/IA         30.7
p(NIPAAm-co-AAm)/ACA        34.4

TABLE 6. Solubility parameters of used organic solvent and hydrogels.

                          [[delta].sub.solvent]
Solvent                [(cal [cm.sup.-3]).sup.1/2]

Benzene                            9.02
Tetrahydrofuran                    9.52
Acetone                            9.75
1-Butanol                         11.32
1-Propanol                        11.97
Ethanol                           12.98
Methanol                          14.48
Ethanolamine                      15.44
                          [[delta].sub.polymer]
Hydrogel               [(cal [cm.sup.-3]).sup.1/2]

p(NIPAAm-co-AAm)                  13.48
p(NiPAAm-co-AAm)/CA               13.64
p(N1PAAm-co-AAm)/IA               13.68
p(NIPAAm-co-AAm)/ACA              13.80
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Author:Isikver, Yasemin; Saraydin, Dursun
Publication:Polymer Engineering and Science
Article Type:Report
Date:Apr 1, 2015
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