High Performance Polymers

A new liquid crystalline thermoset (LCT) based on 1,3,4-oxadiazole was prepared from liquid crystalline epoxy resin containing oxadiazole and diaminodiphenyl methane. The LCT was characterized and compared with the conventional thermoset. The Time—Temperature Transformation diagram was constructed to determine an appropriate set of cure cycles. The goal of this paper was centered on the evaluation of the mechanical performances of this new class of thermoset. In the case of LC thermoset, a higher T _g and good fracture resistance were noticed than that of the conventional thermoset. The LC thermoset exhibited higher thermal stability when compared with the conventional non-LC thermosets. The results tempted to consider the LCT materials as potential candidates for advanced composites.

A series of new aromatic poly(amideimide)s is synthesized via one-step high- temperature polycondensation from prepared diamidedianhydrides and various diamines: 4,4'-(hexafluoro-isopropylidene)dianiline, 4,4'-(9-fluorenylidene)dianiline, 2,7-diaminofluorene and 2,4-diamino-6-phenyl-1,3,5-triazine. Molecular and structural characterization of the resulting polymers is carried out using Fourier transform infrared and nuclear magnetic resonance spectroscopies. The resulting polymers exhibit a good level of thermal stability. The temperature of 10% weight loss is in the range of 420—525 °C (in N₂) associated with relatively high glass transition temperatures (249—299 °C). The polymers' residual weight loss at 800 °C is higher than 40%. The optical and photoluminescence characteristics of synthesized poly(amideimide)s were investigated in solution and in blend with poly(methylmethacrylate) (PMMA). The photoluminescence spectra of the polymers in N-methyl-2-pyrrolidinone solution exhibit maximum emission wavelength in the range of 405—495 nm depending upon the polymer structure.

Polyimide (PI) derived from 3,3',4,4'-biphenyltetracarboxylic dianhydride (s-BPDA) with trans-1,4-cyclohexanediamine (CHDA), i.e., s-BPDA/CHDA was investigated from the viewpoint of ordered structure and intermolecular interaction. Thermodynamic parameters of the model compounds for melting behavior suggested significantly restricted conformational changes in the trans-1,4-cyclohexylene unit and the presence of strong BPDI—BPDI interaction in s-BPDA/CHDA. The effect of diamine structure on the fluorescence yield also supported the presence of the BPDI—BPDI interaction or the BPDI dimer in s-BPDA-based semi-cycloaliphatic PIs. The results of the fluorescence depolarization measurements can be rationalized by a proposed mechanism assuming the presence of the BPDI dimer sites, where the fluorescence of s-BPDA/CHDA occurs by excitation of the lower energy trap sites consisting of the BPDI dimer via excitation energy migration or direct excitation of the dimer. The structure-sensitive infrared band around 550 cm^-1 gradually shifted toward higher frequency with simultaneous narrowing with increasing cure temperature, suggesting gradual ordered structure formation in s-BPDA/CHDA. Thermal imidization at 400 °C caused splitting of the C—H stretching band around 2940 cm^-1, corresponding to the disappearance of distinct glass transition for the s-BPDA/CHDA system.

Novel hyperbranched conductive copolymers containing 4,4',4'' trisubstituted triphenylamine and 2,7-(9,9'-dioctyl)fluorenyl or 2,5-thienyl moieties were synthesized by palladium catalyzed cross-coupling reaction of bis boronic acids of fluorene or thiophene with tris(4-iodophenyl)amine, in an one-step process. The copolymers were obtained as fully or partially soluble materials in chlorinated or aprotic polar solvents, depending on the monomer structure and concentration and polycondensation time, and were characterized by Fourier transform infrared, ¹H-NMR, UV-vis and fluorescence spectroscopy.

In the present study the effect that controlled structural modifications have on the physical and conformational properties of a series of isomeric aromatic polyamides synthesized with different bulky pendant lateral groups was investigated. Physical properties of the aromatic polyamides such as thermal properties were determined by differential scanning calorimetry. Structural changes were determined by Fourier transform infrared spectroscopy, density and fractional free volume (FFV). Light scattering and dynamic-mechanical analysis were used to characterize the thermodynamic and kinetic rigidity. Vogel's model was applied to evaluate kinetic rigidity in polyamide chains and the results were compared with physical properties data. It was found that there was a relationship between T _g and the conformational entropy determined by the structure which indicates that the higher the level of rigidity in the polyamide the higher the value of T _g. For kinetic rigidity, it was found that the relaxation time at infinite temperature B_α , was very close to the value 5 x 10^-12 s, reported for other amorphous glassy materials. There was also a relationship between the activation energy of the glass transition U _∞ and FFV, which is determined by the presence or absence of the bulky functional groups, indicating that the smaller the FFV, the higher the energy required for the glass transition to take place.

New azopolysiloxanes modified with nucleobases with potential application in biology were synthesized and their thermal stability was evaluated. These types of materials can present two possible applications: obtaining of films with nanostructured surface that may be used in cell directional development control and in biomolecular binding and nanohandling. Both applications involve, however, material interaction with laser beams, which require a series of precautions with regard to polymer thermal stability. We considered it essential to perform very thorough thermal stability studies that would provide accurate information concerning the lowest temperature at which thermal degradation occurred. Some correlations between structure—thermal stability were derived.

Polyurethanes (PUs) prepared by varying the hard-to-soft segments ratio, and the chain extender size and nature were studied. Polyurethane cross-linked copolymers composed of 4,4'-methylenebis(phenylisocyanate), poly(ethylene adipate)diol, and glycerine, 1,3 propanediol, 1,4-butanediol, 1,6-hexanediol as a chain extender were synthesized by a two-step process. In the present study, cross-linked poly(ester-urethanes) were prepared with hard segment contents from 22 to 44% by weight, by systematically varying the ratio of the hard-segment extender. Hard segment content has a significant influence on the mechanical and thermo-mechanical properties of PUs, such as maximum stress, tensile modulus, and elongation at break. In particular, maximum stress, tensile modulus, and elongation at break increased significantly at 24 wt.% of hard segment content. Interchain crosslinking improves thermal stability, which was measured by thermogravimetric analysis and differential scanning calorimetry. In contrast with glassy shape memory polymers, chemically cross-linked polyurethane behaves as an elastomer capable of shape memory.