A novel approach to the tailoring of polymers for advanced composites and optical applications, involving the synthesis of liquid crystalline epoxy resins.INTRODUCTION
In recent years, intense research and patent activities have addressed the field of liquid crystalline epoxy resins (LCER LCER Long-Term Certified Emission Reduction ) (1-5). This class of materials seems to be very promising for the following reasons:
* The presence of chemical crosslinks between adjacent molecular segments can improve the relatively poor mechanical properties in the direction transverse to the molecular orientation, an inherent characteristic of thermoplastic A polymer material that turns to liquid when heated and becomes solid when cooled. There are more than 40 types of thermoplastics, including acrylic, polypropylene, polycarbonate and polyethylene. liquid crystalline polymers. In fact, the strong anisotropy anisotropy /an·isot·ro·py/ (an?i-sot´rah-pe) the quality of being anisotropic.
anisotropy (an´āsôt´r of the mechanical properties of uni-directionally oriented liquid crystalline thermoplastic polymers determines outstanding properties along the molecular direction, but only poor properties in directions orthogonal to the average molecular orientation.
* The low viscosity of the mixture of reagents before the crosslinking reaction permits easy flow during filling of suitable molds and a prompt orientation of the molecules in a preferred direction, if an external field is applied. Conversely, the high viscosity of polymers in the molten state prevents achievement of a uniform molecular orientation due to applied electromagnetic fields. Intense elongational and/or shearing flows are necessary to orient polymeric samples in the liquid crystalline state.
Moreover, because of the peculiar molecular structure of the liquid crystalline phase, they can offer improved toughness with respect to conventional thermosets thermosets, materials that can not be softened on heating. In thermosetting polymers, the polymer chains are joined (or cross-linked) by intermolecular bonding. Thermosets are usually supplied as partially polymerized or as monomer-polymer mixtures. used for advanced composites (3, 6)
New emphasis has been recently given to research in the field of liquid crystalline thermosets because of their potential optical and electro-optical applications (7). It is possible to take advantage of the anisotropy of liquid crystalline phases and exploit the unique molecular features of these phases in the production of optical and electro-optical devices. This can be achieved by orienting the material under the influence of an electric, magnetic, or mechanical field, or as an effect of surface interactions (4, 5, 8). With regard to this, LCER have been found to be very promising in this field: they can be oriented more easily before the crosslinking is performed than both ordinary and liquid crystalline thermoplastic polymers. Moreover, they can retain the orientation above [T.sub.g] because of the existence of chemical crosslinks between the chains. Many experimental and theoretical investigations concerning the orientation and relaxation processes of these materials with different procedures have been reported in the scientific and patent literature (9-11).
In previous papers (3, 6, 8, 12-14) we reported the synthesis and the properties of several liquid crystalline thermosetting thermosetting,
adj having the property of becoming irreversibly rigid or hardened with the application of heat. In dentistry the term is used in connection with resins. epoxies whose mesogenic portions lie in the main chain. For this reason the overall mechanical properties of the final thermosets are overwhelmed by strong chemical bonds constituting the rigid primary chemical structure.
Where optical and electro-optical applications are concerned, main chain LCER exhibit some advantages: first, because of their low monomeric monomeric /mono·mer·ic/ (mon?o-mer´ik)
1. pertaining to, composed of, or affecting a single segment.
2. in genetics, determined by a gene or genes at a single locus. viscosity, the curing resin can be easily oriented before gelation gelation /ge·la·tion/ (je-la´shun) conversion of a sol into a gel.
1. Solidification by cooling or freezing.
2. The process of forming a gel.
3. ; then the reduced system mobility prevents the chain relaxation. This procedure causes good control of the system morphology and structure before completion of the curing process. Of course, in order to obtain a uniformly oriented system without defects, as it is required for the previously proposed applications, crosslinking should not perturb the orientation achieved prior to the gel point.
The material must exhibit good chemical and physical stability up to high temperatures. This can be obtained by means of a proper selection of molecular parameters such as molecular shape and rigidity, nature of chemical bonds, absence of pendant reactive groups, etc.
Because of their optical properties, different applications can be planned for oriented liquid crystalline main chain thermosets in the field of optical and electro-optical displays. They can be used as waveguides, filters, second harmonic generation Second harmonic generation (SHG; also called frequency doubling) is a nonlinear optical process, in which photons interacting with a nonlinear material are effectively "combined" to form new photons with twice the energy, and therefore twice the frequency and half the (SHG SHG Second Harmonic Generation
SHG Short-Handed Goals (hockey)
SHG Self Help Group
SHG State History Guide (formerly State House Girls)
SHG Sacred Heart Griffin ) devices, matrices for Polymer Dispersed Liquid Crystals (PDLC PDLC Polymer Dispersed Liquid Crystal (display)
PDLC Premium Downloadable Content
PDLC Programming Development Life Cycle (steps to build computer programs) ), as well as optical switches (7, 11). Of course, for these purposes some requirements must be satisfied. The materials under investigation must exhibit high optical and electrical anisotropy. The achieved orientation in the fully cured material must be very homogeneous, i.e., the presence of defects must be carefully minimized; the LC thermosets need to have a very high dimensional stability dimensional stability,
n See stability, dimensional. in order to prevent internal stresses and increase durability; and, since the macroscopic macroscopic /mac·ro·scop·ic/ (mak?ro-skop´ik) gross (2).
mac·ro·scop·ic or mac·ro·scop·i·cal
1. Large enough to be perceived or examined by the unaided eye.
2. anisotropy of optical and electrical properties is the essential feature required for such applications, it is crucial that the molecular orientation is also very stable over a wide temperature range.
In order to attain this target, several points must be considered. First, the epoxy monomers should be properly selected if a liquid crystalline thermoset A polymer-based liquid or powder that becomes solid when heated, placed under pressure, treated with a chemical or via radiation. The curing process creates a chemical bond that, unlike a thermoplastic, prevents the material from being remelted. See thermoplastic. is to be obtained. The stability of a mesophase is in fact the result of different factors, such as the monomeric shape, its polarizability, the presence of highly polarizable po·lar·ize
v. po·lar·ized, po·lar·iz·ing, po·lar·iz·es
1. To induce polarization in; impart polarity to.
2. To cause to concentrate about two conflicting or contrasting positions. groups, and the existence of noncovalent interactions such as hydrogen bonds. All these parameters must be taken into account if a liquid crystalline phase is to develop and be stabilized during the cure.
As already pointed out, the minimization of defects is absolutely necessary in order to get a uniformly aligned network that is suitable for optical and electro-optical applications. On the other hand, it is also necessary that chain mobility in the cured network is hindered enough for the mesogen orientation to be retained. In order to balance both the objectives, it is crucial to control the crosslink density carefully. In fact, if this structural parameter has too high a value, this promotes the presence of defects in the mesophase because of density change during the reaction and because of the distortion of molecular arrangement in the liquid crystalline phase. However, too low a crosslink density cannot guarantee that the final material will retain its orientation prior to the gel point.
Furthermore, it must be possible to get a good orientation of the liquid crystalline resin before gelation drastically reduces the chain mobility. Therefore, it is also necessary to control the reaction rate by a proper selection of parameters such as temperature, purity of reagents, presence of inhibitors, or photoreactive groups. It was reported by the authors (12, 13) that the onset of the liquid crystalline phase during the curing reaction determines a change of curing kinetics with respect to commercial isotropic Refers to properties that do not differ no matter which direction is measured. For example, an isotropic antenna radiates almost the same power in all directions. In practice, antennas cannot be 100% isotropic. epoxy resins. This is another aspect that must be taken into account when careful control of the reaction rate is desired for such systems.
For the detailed syntheses of monomers the authors refer to the previously published papers (3, 6, 8, 12-14). In a typical functionalization with epoxy groups, the compounds were reacted with epichlorohydrin ep·i·chlo·ro·hy·drin
A colorless liquid, C3H5OCl, used as a solvent in making resins. in the presence of sodium hydroxide sodium hydroxide, chemical compound, NaOH, a white crystalline substance that readily absorbs carbon dioxide and moisture from the air. It is very soluble in water, alcohol, and glycerin. It is a caustic and a strong base (see acids and bases). 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 (3). DAF, DDS (1) (Digital Data Storage) See DAT.
(2) (Data Dictionary System) See QuickBuild and OpenDDS.
(3) (Dataphone Digital S , t-CN, DAT (1) (Dynamic Address Translator) A hardware circuit that converts a virtual memory address into a real address. See also DAT file.
(2) (Digital Audio Tape) A magnetic tape technology used for backing up data. , o-tolidine and the aliphatic acids aliphatic acids
The acids of nonaromatic hydrocarbons, such as acetic, propionic, and butyric acids. were purchased from Aldrich and their purity was checked by NMR NMR: see magnetic resonance. .
Samples were prepared by mixing the monomers in stoichiometric stoi·chi·om·e·try
1. Calculation of the quantities of reactants and products in a chemical reaction.
2. The quantitative relationship between reactants and products in a chemical reaction. ratio and by subsequently curing them at the desired temperature in a test tube, DSC (1) (Digital Signal Controller) A microcontroller and DSP combined on the same chip. It adds the interrupt-driven capabilities normally associated with a microcontroller to a DSP, which typically functions as a continuous process. See microcontroller and DSP. pan or between two glass slides previously treated with a surfactant Surfactant Definition
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. agent (Surfasil[R], Pierce).
The phase transition of the specimens and the chemical reactions were monitored with a differential scanning calorimeter calorimeter: see calorimetry.
Device for measuring heat produced during a mechanical, electrical, or chemical reaction and for calculating the heat capacity of materials. (DSC) DuPont Mod. 2910. Nitrogen was used as purge gas. In dynamic experiments a heating rate of 10 [degrees] C/min was adopted.
The textures of the monomers and cured resins were observed with an optical microscope Reichert-Jung, mod. Polyvar in transmitted light, equipped with crossed polarizers. The temperature was controlled by means of a Linkam mod.TH600 hot stage.
The X-ray diffraction spectra were obtained by the photographic method using a Rigaku mod. III/D max generator. A Ni-filtered Cu-[K.sub.alpha] radiation was used. Fracture toughness was evaluated according to ASTM ASTM
American Society for Testing and Materials E-399 test procedure using 1[inch] x 1[inch] x 1/8[inch] compact tension specimens.
pertaining to the flexure of a joint.
fixation of joints in flexion. In the newborn called contracted calves or foals. properties of the cured resins were measured according to ASTM D 790. Coefficients of thermal expansion (CTE (Coefficient of Thermal Expansion) The difference between the way two materials expand when heat is applied. This is very critical when chips are mounted to printed circuit boards, because the silicon chip expands at a different rate than the plastic board. ) were evaluated by means of a thermomechanical analyzer (TMA TMA Turnaround Management Association
TMA Texas Medical Association
TMA Transportation Management Association
TMA Training and Management Assistance (a component of OHRD, which is a component of OWR)
TMA Tooling & Manufacturing Association ) DuPont Mod. 2940.
The length of the rigid rod molecules was calculated after minimizing energy by means of the program Desktop Molecular Modeller.
Sorption sorption /sorp·tion/ (sorp´shun) the process or state of being sorbed; absorption or adsorption.
Adsorption or absorption. tests were performed by immersion of the specimens respectively in methylene chloride at room temperature for 17 days and in distilled water at 100 [degrees] C for 17 days and then by monitoring the weight increase.
Physical Properties of Resins Cured With Different Curing Agents
In our previous investigations (3) we demonstrated that the use of a bifunctional bi·func·tion·al
1. Having two functions: bifunctional neurons.
2. Chemistry Having or involving two functional groups or binding sites: epoxy molecule with a stiff and elongated e·lon·gate
tr. & intr.v. e·lon·gat·ed, e·lon·gat·ing, e·lon·gates
To make or grow longer.
adj. or elongated
1. Made longer; extended.
2. Having more length than width; slender. structure is the main factor in determining the features of the resulting liquid crystalline network. The epoxy compounds do not necessarily need to exhibit a liquid crystalline phase as a pure material. In fact, the lack of long and flexible tails attached to the rigid core leads to a high melting temperature for the epoxy compounds, thus hindering the liquid crystalline phase. The liquid crystalline phase appears at an early stage during the curing process. The mechanism of this transition for the curing thermosets can be attributed to the development of weak interactions which contribute to the stabilization of the liquid crystalline phase. For completely cured systems, the liquid crystalline order of the molecules is stabilized over a temperature range depending on the overall molecular mobility, which in turn is mainly affected by the choice of the crosslinking agent. In the case of a low crosslinking density, the transition from a liquid crystalline to isotropic state can be observed as the temperature increases. When [TABULAR DATA FOR TABLE 1 OMITTED] the crosslinking density is higher, the mesophase develops during the initial curing step when the epoxy monomers and the curing agent react and gives rise to a linear prepolymer not yet crosslinked. The liquid crystalline phase appears and is furtherly stabilized during the gelation of the thermoset and can be destroyed only upon thermal decomposition of the network. High crosslinking density thermosets can be obtained by reacting the rigid-rod epoxy-terminated molecules with polyfunctional amines amines (mēnz´),
n.pl organic compounds that contain nitrogen. . In this case, the selection of the curing temperature turns out to be crucial. In fact, if the curing reaction is carried out at a temperature higher than the isotropization of the growing prepolymer, crosslinking takes place before the chain extension has developed enough to stabilize the liquid crystalline structure and the system cures in the isotropic state.
We selected and synthesized a number of rigid-rod epoxy-terminated compounds that could give rise to a liquid crystalline phase upon curing. The molecular formulas of such compounds are reported in Fig. 1. These compounds were reacted with different polyfunctional amines [ILLUSTRATION FOR FIGURE 2 OMITTED] or with carboxylic acids of general formula HOOC-[(C[H.sub.2]).sub.n]-COOH, with n ranging from 2 to 8. In the case of reaction with amines, all the cured systems but the ones obtained with 2,6(2,3-epoxypropoxy)-naphtalene (NAFT NAFT Names Are For Tombstones (Danish metal band)
NAFT Names Are For Tombstones (Manila, Philippines solo act)
NAFT Natural Fuels Technology
NAFT Not A Freakin' Thing (polite form) ) gave a liquid crystalline structure upon curing. In the case of NAFT, the different structure of the cured thermoset can be ascribed to the lower length of the rigid part of this epoxy molecule, which is too short (4.7 [Angstrom angstrom (ăng`strəm), abbr. Å, unit of length equal to 10−10 meter (0.0000000001 meter); it is used to measure the wavelengths of visible light and of other forms of electromagnetic radiation, such as ultraviolet ]) with respect to the other considered compounds, whose rigid block lengths range between 7 and 13.5 [Angstrom]. This feature reduces the nematogenic character of the epoxy monomer monomer (mŏn`əmər): see polymer.
Molecule of any of a class of mostly organic compounds that can react with other molecules of the same or other compounds to form very large molecules (polymers). , as predicted by Flory (15), and allows only isotropic networks to be obtained.
In order to assess the effect of the relative amount of the curing agent on the properties of the crosslinked thermosets, p-(2,3-epoxypropoxy)-diphenyl (DIF (1) (Data Interchange Format) A standard file format for spreadsheet and other data structured in row and column form. Originally developed for VisiCalc, DIF is now under Lotus' jurisdiction. ) was cured with different amounts of p-aminoacetophenone azine azine (ăz`ēn), IUPAC name for pyridine. (NA2). Table 1 reports the properties of the cured resins obtained by reacting DIF with 85%, 100%, and 115% of the stoichiometric amount of NA2 respectively at different temperatures. All the materials obtained have liquid crystalline character. Regarding the fracture toughness, the best results were obtained for the resin cured with an excess of amine amine (əmēn`, ăm`ēn): see under amino group.
Any of a class of nitrogen-containing organic compounds derived, either in principle or in practice, from ammonia (NH3). ; however, this positive result is negatively balanced by the lower modulus and the lower [T.sub.g]. Higher [T.sub.g]s were reached for resins cured with less than stoichiometric amount of amine. However, a lightly more brittle material resulted in this ease. For each stoichiometric composition studied, a higher curing temperature causes a lower fracture toughness, while [T.sub.g] and modulus are not strongly affected by curing temperature. Since the most critical parameter to be investigated is fracture toughness, the effect of the state of order of the cured system was related to this parameter (3). The lower fracture toughness at higher temperatures is due to the progressive reduction of the extent of nematic The stage between a crystal and a liquid that has a threadlike nature; for example, a liquid crystal. See crystalline and LCD. portions and the progressive increase of the isotropic parts of the cured resin, as revealed by optical microscopy. The density of the cured resin progressively decreases with increasing curing temperature, indicating that at higher temperatures the extent of ordered domains, acting as nodules Nodules
A small mass of tissue in the form of a protuberance or a knot that is solid and can be detected by touch.
Mentioned in: Leprosy around which crack can deviate, decreases. In other terms, the 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 of LCER is predominant but consists of anisotropic Refers to properties that differ based on the direction that is measured. For example, an anisotropic antenna is a directional antenna; the power level is not the same in all directions. Contrast with isotropic. domains with properties, such as strength. that are different along and across their molecular orientations. This results in the deviation of crack propagation from a straight line and suggests that inhomogeneities and localized anisotropy of the nematic structure is the main reason for the fracture toughness increase in the liquid crystalline material.
Table 2 reports the mechanical properties of DOMS DOMS Director of Military Support
DoMS Department of Management Studies
DOMS Delayed Onset Muscular Soreness
DOMS Directorate Of Military Support
DOMS Digital Objects Management System
DOMS Diploma in Ophthalmic Medicine & Surgery cured with NA2 at two different temperatures. The system cured at lower temperature appeared to be opaque and exhibited a LC structure, while the one obtained in the other case was transparent and showed an isotropic structure. The nematic resins exhibited a fracture toughness nearly 50% higher without affecting any value of flexural mechanical properties. The water and methylene chloride uptake also remained unchanged. In the same Table, data are reported for nematic samples containing glass fibers as a reinforcing agent. The cured samples were opaque and had the same nematic textures as the unreinforced samples. The addition of glass fibers resulted in a significant improvement of modulus without reduction of toughness. The combination of high modulus and high toughness demonstrates property balance which can be attractive for applications in the area of high performance composites. The DOMS-NA2 resin exhibited excellent solvent resistance. Weight gain after immersion in methylene chloride for two weeks was only 1.0%. However, this system had a low water resistance - its weight gain after immersion in water at 100 [degrees] C for two weeks was 7.5%. Such high water uptake could be caused by the particular chemical structure of the curing agent containing nitrogen atoms, which are very sensitive to water. For this reason, NA2 was replaced by a different curing agent, o-tolidine [ILLUSTRATION FOR FIGURE 2 OMITTED]. The DOMS/o-tolidine system was cured [TABULAR DATA FOR TABLE 2 OMITTED] at 150 [degrees] C for one hour then at 175 [degrees] C for two hours. The cured panel was transparent and therefore exhibited no liquid crystalline character. It had both low water (2.5%) and low methylene chloride (1%) uptake, suggesting that with appropriate choice of chemical structure, the rod-like thermosets can have both good solvent and water resistance.
Another unusual property that sets apart rod-like thermosets from traditional flexible chain thermosets is their ability to maintain a high modulus up to very high temperatures. For example, the DOMS o-tolidine resin exhibits no significant drop in modulus up to 300 [degrees] C. Therefore, rod-like thermosets have possible applications where high temperature resins are required especially for areas such as surface mount technology. In conclusion, this example demonstrates the validity of the concept that a rigid rod epoxy thermoset material can have properties or property balance beyond the scope of existing flexible chain epoxy systems, and therefore, these materials can have numerous market opportunities.
In the case of thermosets obtained by reacting the same epoxy molecules with acids, liquid crystalline behavior was obtained in all cases. In the case of NAFT, an extra driving force, apart from the anisotropy of the molecule, must be claimed to explain this point owing to the low value of the aspect ratio of this molecule. It is known that hydrogen bonds play a positive role as driving force for liquid crystalline ordering and several authors have reposed on systems where hydrogen bonding play the major part in mesophase formation (16-19). Moreover, a recently appearing theory predicts that hydrogen bonds alone may cause liquid crystalline ordering in otherwise isotropic liquids (20). In our case, it may be supposed that the presence of a long and flexible curing agent, such as an acid, can provide enough conformational freedom to the polymeric chains which can then align under the influence of hydrogen bonding between the hydroxyl groups, formed as an effect of the epoxy ring opening during the cure, and the carboxyl groups of the chains. This could be, therefore, the driving force for the onset of a mesophase in the case of a very short rigid core in the epoxy molecule, such as NAFT.
The reaction with acids seems to favor a more ordered liquid crystalline phase compared to the amines. In the former case, with the exception of succinic acid succinic acid: see Krebs cycle. HOOC-[(C[H.sub.2]).sub.2]COOH COOH Carboxylic Acid (functional group) , thermosets with a smectic smec·tic
Of or relating to the mesomorphic phase of a liquid crystal in which molecules are closely aligned in a distinct series of layers, with the axes of the molecules lying perpendicular to the plane of the layers. structure were obtained, while in the latter case only networks having a nematic structure could be obtained. It was shown (21) that in the case of side chain LC polymers, the flexible segments mainly act as spacers between the main chain and the side mesogenic groups, thus allowing the assembling of the material into an anisotropic structure. A similar role is also played by the flexible segments connecting the mesogenic groups in the case of a main chain LC polymer. Theoretical calculations (22, 23) showed that in low molecular weight liquid crystals the flexible segments are not simply space fillers, but take part in the anisotropy of the phase. Therefore, it is not surprising that also in the case of a poorly crosslinked LC network, the aliphatic aliphatic /al·i·phat·ic/ (al?i-fat´ik) pertaining to any member of one of the two major groups of organic compounds, those with a straight or branched chain structure.
adj. portion plays an active role in the development and stabilization of the mesophase. In the case of reaction with amines, the high crosslinking density produces too many constraints and does not allow the mesogenic portions to organic in a mesophase more ordered than the nematic.
Effect of the Crosslinking Density
In order to investigate the effect of crosslinking density on the presence of defects, and therefore on the optical homogeneity of oriented samples, the DOMS monomer was selected and reacted with different curing agents whose structures are reported in Fig. 2.
In our first investigation, reaction was performed with stoichiometric amounts of 2, 4-diaminotoluene (DAT) at 80 [degrees] C after melting of the mixture at 130 [degrees] C. The molten mixture is in the isotropic state at the beginning, when the fractional conversion is virtually zero. As the reaction starts and proceeds further on, the fractional conversion and the average molecular weight of the epoxy resin increases. The reaction of the primary amine (Chem.) an amine containing the amido group, or a derivative of ammonia in which only one atom of hydrogen has been replaced by a basic radical; - distinguished from
See also: Primary
The splitting which a wavefront experiences when a wave disturbance is propagated in an anisotropic material; also called double refraction. In anisotropic substances the velocity of a wave is a function of displacement direction. by optical microscopy between crossed polarizers. As the curing reaction proceeds, crosslinking irreversibly locks in the nematic structure. However, the low viscosity of the epoxy/curing agent mixture during the early stage of the reaction allows the molecules to be unidirectionally oriented before the chemical crosslinks hinder the molecular realignment re·a·lign
tr.v. re·a·ligned, re·a·lign·ing, re·a·ligns
1. To put back into proper order or alignment.
2. To make new groupings of or working arrangements between. process and finally lock the structure. This procedure can be used to produce highly aligned samples whose orientation is stable over a wide temperature range. In our case, a uniform molecular alignment in the liquid crystalline material was achieved by liquid crystal-surface interactions (8). The DOMS-DAT mixture was reacted between glass substrates whose surface had been previously spin-coated with polyimide Pronounced "poly-ih-mid." A type of plastic (a synthetic polymeric resin) originally developed by DuPont that is very durable, easy to machine and can handle very high temperatures. Polyimide is also highly insulative and does not contaminate its surroundings (does not outgas). and then unidirectionally robbed. When the conversion of the monomers reached a threshold value, the sample became liquid crystalline. Because of the surface-liquid crystal interactions, the molecules in the nematic phase became uniformly oriented. The fully cured sample behaved as a uniaxially U`ni`ax´i`al`ly
adv. 1. In a uniaxial manner. optical plate if rotated between crossed polarizers. However, the final texture of the epoxy network was found to be not completely uniform due to the presence of a number of defects. In fact, the reaction of the secondary amine and the formation of crosslinks create distortions of the molecular order in the nematic phase, thus generating such defects.
In order to reduce the formation of defects in the cured sample, a less densely crosslinked network was prepared by considering that the distance between contiguous crosslinks in the network is related to the length of the monomers. Therefore, DOMS molecules were replaced by linear oligomers obtained by reacting DOMS with aniline aniline (ăn`əlĭn), C6H5NH2, colorless, oily, basic liquid organic compound; chemically, a primary aromatic amine whose molecule is formed by replacing one hydrogen atom of a benzene molecule with an amino (ANi) with a stoichiometric imbalance of 2 to 1 mole. In this way, an epoxy-terminated dimer dimer /di·mer/ (di´mer)
1. a compound formed by combination of two identical molecules.
2. a capsomer having two structural subunits.
1. was obtained. In order to minimize secondary reactions which lead to the formation of branching and crosslinking using difunctional amines, the reaction was not driven to completion. By back titration of epoxy groups, a value of 550 g/mol was determined for the molecular weight of the so-obtained oligomers (oli-DOMS). The synthesis of the epoxy terminated oligomers and the preparation of the reacting mixture by addition of the stoichiometric equivalent of DAT in a second step ensured a control of crosslink density along the chemical backbone. The oli-DOMS/DAT mixture was melted at 130 [degrees] C and subsequently cured at 100 [degrees] C between glass substrates coated with unidirectionally aligned layers.
The curing reaction and formation of the nematic phase for oli-DOMS/DAT is analogous to what has been already described for the DOMS/DAT system, as it can be easily predicted from the close chemical similarities of the two systems.
Comparison of the DOMS/DAT and oli-DOMS/DAT samples between crossed polars at the end of the curing reaction showed remarkable differences between the highly crosslinked and the less densely crosslinked networks. Decreasing the crosslink density resulted in a reduction of defects in the sample. Disclinations and a focal conic-like texture were present in DOMS/DAT film. X-ray diffraction measurements performed on bulk samples were in agreement with a nematic order of the structure, nevertheless the high periodicity periodicity /pe·ri·o·dic·i·ty/ (per?e-ah-dis´i-te) recurrence at regular intervals of time.
1. of the crosslinks may have induced a smectic-like ordering of the phase. This periodicity was disrupted in the sample prepared with oli-DOMS/DAT mixture. As a consequence, the nematic texture was distorted to a lesser extent during the formation of the tridimensional tri·di·men·sion·al
Of, relating to, or having three dimensions. network. This procedure yields a sample which is uniformly aligned along the robbing direction of the polyimide aligning layers. The uniform molecular alignment is thermally and mechanically stable - it can be destroyed only upon a decomposition process at high temperatures. This makes this preparation procedure very attractive for optical and electro-optical applications.
Another attempt to obtain a less densely crosslinked liquid crystalline network was done by reacting DOMS with aliphatic acids. In this case, as it was explained before, a smectic phase was obtained in the cured thermoset; the cured systems still display a transition from LC to isotropic phase. The glass transition temperatures of these systems were quite low, i.e. they typically ranged between 26 [degrees] C and 49 [degrees] C. This result led us to investigate the possibility of orienting such systems in bulk under the influence of mechanical stress. Our attention was focused on the system obtained upon reaction of DOMS with decanedioic acid (SA) at 180 [degrees] C. This sample exhibits a [T.sub.g] of 27 [degrees] C and a smectic-isotropic transition temperature of 81 [degrees] C, as inferred from differential scanning calorimetry Differential scanning calorimetry or DSC is a thermoanalytical technique in which the difference in the amount of heat required to increase the temperature of a sample and reference are measured as a function of temperature. analyse (DSC) and optical microscopic observation.
A DOMS-SA film obtained by curing the monomers under the previously described conditions was stretched at 50 [degrees] C, while it was in its rubbery state, up to 160% strain at 0.2 mm/min, then quenched quench
tr.v. quenched, quench·ing, quench·es
1. To put out (a fire, for example); extinguish.
2. To suppress; squelch: down to the glassy state at room temperature. The X-ray patterns of the unstretched and stretched sample are reposed in Fig. 3. From a macroscopic point of view, the sample turned from turbid tur·bid
Having sediment or foreign particles stirred up or suspended; muddy; cloudy.
tur·bidi·ty n. and white to almost completely transparent. The X-ray pattern at room temperature of the stretched sample shows a broadened wide-angle reflection at the equator. The smectic layer reflections can be observed at the meridian, thus indicating a perpendicular orientation of the smectic layers to the stress direction. The order parameter Order Parameter
In a nonlinear dynamic system, a variable-acting link a macrovariable, or combination of variables-that summarizes the individual variables that can affect a system. S was calculated from the X-ray pattern at room temperature for this oriented system and from IR circular dichroism measurements performed at 60 [degrees] C during the stretching experiment. In both cases calculations gave a value of S equal to 0.78 for the stretched sample. No remarkable variation in the S value was observed after annealing annealing (ənēl`ĭng), process in which glass, metals, and other materials are treated to render them less brittle and more workable. the sample at room temperature for several months. This evidence indicates that very good orientation can be achieved for this material, and makes it very promising for optical applications. However, a serious limit to its applicability resides in the low value of [T.sub.g], which suggests that a quite fast relaxation to the unoriented state will occur if the operating temperature of the material is slightly higher than room temperature.
Effect of the Liquid Crystalline Phase on the Crosslinking Process
The possibility to predict the overall chemical and physical behavior of potentially mesogenic reacting systems is crucial. The extent of reaction achieved, the tridimensional network structure, the orientation of molecular segments along preferred directions all can affect the macroscopically mac·ro·scop·ic also mac·ro·scop·i·cal
1. Large enough to be perceived or examined by the unaided eye.
2. Relating to observations made by the unaided eye. measured properties. From the practical point of view, it is quite clear how the thorough understanding of the curing mechanism of reactive resins and the prediction of bulk properties are necessary for the implementation of TLCP's in industrial applications. If the mesogens exhibit monotropic liquid crystalline behavior, or if the temperature stability range of the mesophase is narrow, the final desired molecular arrangement can be achieved only by modeling the phase transition coupled with the extent of reaction. However, the microscopic molecular features are governed to different extents by the curing conditions and the physical state developed during the curing reaction.
The literature concerned with the modeling of reactive systems suitable to form liquid crystalline phases is quite recent, and only few attempts have been made to develop a phenomenological model suitable to take into account the formation of the liquid crystalline phase during the curing reaction (12, 24).
Despite the complexity of the reaction between epoxy and amine groups, the overall reaction rate can be expressed by means of relatively simple kinetic equations (25, 26). A practical method for characterizing the reaction rate of exothermal exothermal /exo·ther·mal/ (ek?so-ther´mal) exothermic.
marked or accompanied by the evolution of heat; liberating heat or energy. reaction is to make use of calorimetric cal·o·rim·e·ter
1. An apparatus for measuring the heat generated by a chemical reaction, change of state, or formation of a solution.
Under the basic assumption that the heat flow relative to the DSC instrumental baseline is proportional to the reaction rate, fractional conversion [Alpha] is expressed by the following equation:
[[Alpha].sub.(t)] = [integral of] [Theta]dt between limit t and 0/[Delta]H
where [Theta] is the DSC heat flux, and [Delta]H = [Theta]dt is the enthalpy enthalpy (ĕn`thălpē), measure of the heat content of a chemical or physical system; it is a quantity derived from the heat and work relations studied in thermodynamics. of complete reaction.
Typical DSC signal vs. time plots are given in Fig. 4. Increasing the temperature results in an increased reaction rate. This affects the DSC data, reducing the time for completion of the reaction and increasing the value of peak maxima. An interesting feature is the presence of double peaks in the DSC curves for these resins. Similar behavior was previously reported for liquid crystalline thermosets, but to our knowledge it has never been observed for conventional crosslinking reactions of simple mixtures. Double peaks have been related to the formation of liquid crystalline phases during the crosslinking reaction. Formation of a liquid crystalline phase during the crosslinking reaction can be monitored by optical microscopy. An isotropic phase results after melting of the two components. At some intermediate level of crosslinking, the liquid crystalline phase transition is observed. It is interesting that the second peaks have never been observed before birefringence occurs, suggesting that such second peaks are related to different reaction kinetics in the liquid crystalline state.
A phenomenological model has been proposed to account for the reaction kinetics in the isotropic and in the liquid crystalline phases (12, 24, 27). Different parameters can be obtained by fitting the experimental curves in the stable regions of the different phases. The mechanism of phase transition as a function of the fractional conversion measured during the reaction is also implemented in the overall kinetic model.
The agreement of the experimental results with the proposed model is excellent over a wide range of experimental conditions, confirming the assumption that different kinetic models do apply in the isotropic and in the liquid crystalline phases. A full treatment of the model can be found in reference (27), as its complete explanation is beyond the scope of this paper. Research is still ongoing in order to extend the results to liquid crystalline systems with afferent afferent /af·fer·ent/ (af´er-ent)
1. conveying toward a center.
2. something that so conducts, such as a fiber or nerve.
adj. chemical compositions.
In conclusion, new thermosets can be obtained by curing rigid-rod epoxy monomers. The properties of the resulting resin depend on the length of the epoxy molecule, as well as on the nature of the curing agent. It was demonstrated that amines produce thermoses having high [T.sub.g]s and high fracture toughness. In particular, the enhancement of this last parameter is very promising if a new generation of matrices is to be sought for manufacturing of composites. As a comparison, if a toughness improvement of 50% is attempted by blending conventional epoxies with rubbers, a considerable depression of [T.sub.g] and modulus result. In the case of liquid crystalline epoxy resins, the higher fracture toughness is achieved without sacrificing [T.sub.g] or modulus. Moreover, if a modulus increase is also desired, composites with glass fibers can be prepared.
If the curing agent is an acid, thermosets having lower [T.sub.g]s and more ordered mesophases can be obtained. In this case, LC elastomers can be prepared that can find applications in the optical industry as waveguides or electro-optical devices. In this case, extra effort must be devoted to the synthesis of monomers having chiral chi·ral
Of or relating to the structural characteristic of a molecule that makes it impossible to superimpose it on its mirror image.
chi·ral properties, which can therefore give rise to sinectic [C.sup.*] phase. In fact, only in this case is the polarizability of the molecules great enough to have high values of electrical susceptibility, which is one of the requirements to produce polymers for electro-optical applications.
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