1992 рік

Novel poly (arytene ether benzoxazole)s were prepared by the aromatic nucleophilic displacement reaction of two new bis (hydroxyphenyl benzoxazole)s with activated aromatic difluorides. The bis (hydroxyphenyl benzoxazole)s were prepared from either 3,3'-dihydroxy-4,4'-diaminobiphenyl or 2,2-bis(3-amino-4- hydroxyphenyl)hexafluoropropane and phenyl-4-hydroxybenzoate. The polymerizations were carried out in either N,N-dimethylacetamide or diphenylsulphone, depending on the bis (hydroxyphenyl benzoxazole). The polymers had glass transition temperatures ranging from 204 to 275°C and inherent viscosities from 0.25 to 1.04 dl g- i. Several of the polymers prepared from the 3,3'-dihydroxy-4,4'-diaminobiphenyl-derived bis(hydroxyphenyl benzoxazole) were semi-crystalline. Thermogravimetric analysis showed a 5% weight loss occurring at ~450-500°C in air and ~ 475-530°C in nitrogen. Thin films of the hexafluoroisopropylidene- containing polymers exhibited 23°C tensile strengths and moduli of 83-103 MPa and 1.9-2.4 GPa respectively, and high elongations. The chemistry and the physical and mechanical properties of these polymers are discussed.

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A comparison of miscibility is presented for blends containing a fluorinated and a non-fluorinated polyimide sulphone, of otherwise comparable structure, with polybenzimidazole [ PBI, poly (2,2'-(m-phenylene)-5,5'- bibenzimidazole]. The miscibility behaviour was studied using Fourier transform infra-red (FTi.r.) spectroscopy and thermal analysis. The N-H and C=O absorbance regions in the FTi.r. spectra show that the fluorine-containing group in the hexafluoroisopropylidene-containing polyimide (PI1) decreases the miscibility of this polyimide with PBI, whereas the carbonyl group in the benzophenone moiety in the second polyimide (PI2) enhances miscibility. In blends with low PBI content, PI2 forms uniform hydrogen bonds with PBI, and the strength of the hydrogen bonding is maximized at a blend composition of 50/50 wt%. FTi.r. spectra of the blends were also obtained as a function of temperature. Above the glass transition temperature the PBI/PI2 blends contain two phases : pure PI2 (present in small amounts) and the predominant miscible phase. Analysis of the carbonyl band could be related to glass transition behaviour in the blend. Finally, an enhanced degradation of PI2 in the presence of PBI was observed and is discussed.

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In the present study, a unified mathematical framework is developed for modelling molecular and structural changes in free-radical homopolymerization reactions. Three modelling methods - namely, the instantaneous property method (IPM), the method of moments (MM) and a new hybrid method, the so-called property moments method (PMM)- are derived using a unified approach. A critical analysis is carried out to determine the conditions under which each of the above methods can be applied to a given polymerization system. This is achieved through the application of the three modelling methods to several experimentally investigated free-radical polymerization systems leading to linear and branched polymers. Polymerizations are examined in bulk and in solution, both in the presence and in the absence of strong diffusional limitations in the termination and propagation reactions.

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A series of new poly(oxadiazole-amide)s that. are soluble in polar solvents has been prepared by incorporating the hexafluoroisopropylidene unit and other flexible linkages into the backbone. Films cast of these polymers from solution are colourless to light tan and have ultimate tensile strengths ranging from 38.1 to 44.7 MPa. Weight loss does not occur below 450°C in air by thermogravimetric analysis and the inherent viscosities ranged from 0.31 to 0.42 dl g-1 in dimethylacetamide.

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Dichloroketene, generated by the ultrasound-promoted dechlorination of trichloroacetyl chloride with zinc, has been added to the carbon-carbon double bonds of poly (1,1-dimethyl-l-sila-cis-pent-3-ene) (poly-I). The molecular-weight distribution of the adduct polymer in which all of the carbon-carbon double bonds have reacted with dichloroketene, poly[2,2-dichloro-cis-3,4-bis(methylene)cyclobutanone dimethylsilylene], 100% poly (I-CI2C=C~O), has been characterized by gel permeation chromatography. The microstructure of 100% poly(I-C12C=C=O ) has been elucidated by 1H, 13C and 29Si nuclear magnetic resonance as well as by infra-red spectroscopy. Similarly, a series of random copolymers have been prepared in which only some of the carbon-carbon double bonds of poly-I have reacted with dichloroketene. These have been characterized as above. The glass transition temperatures (Tg) of these copolymers, as well as that of 100% poly (I--C12C=C=O), have been determined by differential scanning calorimetry. The T 8 values are found to increase linearly with the percentage of the carbon-carbon double bonds of poly-I that have reacted with dichloroketene..

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Mixtures of nylon-6 and poly (m-xylene adipamide) (MXD6) undergo sufficient interchange reactions when melt blended at 290°C to give a homogeneous melt phase and a material with a single T~. However, the extent of reaction is sufficiently low that the product retains a high level of ability to crystallize. These phase-homogenized mixtures become supertough when blended with a maleic anhydride (MA) grafted elastomeric styrene- (ethytene/butene)-styrene (SEBS) triblock copolymer, i.e. SEBS-g-MA. Pure nylon-6, pure MXD6, or mixtures of the two prepared under low-temperature extrusion conditions (260°C) that do not lead to phase homogenization are not similarly supertoughened by blending with SEBS-g-MA. The reasons appear to relate to differences in elastomer particle morphology and inherent ductility for the various matrices. SEBS-g-MA blends with nylon-6 are not supertough because the rubber particles are too small. Addition of MXD6 causes the particles to become larger and of optimum size for toughening, but it is postulated that MXD6 is difficult to toughen because of low inherent ductility. The polyamide mixtures that were not phase homogenized are also inherently incompatible. The differences in rubber particle geometry are explained in terms of the functionality of the two polyamides towards reactions with anhydride groups.

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Despite the general knowledge that near-critical and supercritical CO2 will swell elastomers extensively, there has been little work done to date on modelling the phenomenon. We have used the mean-field-lattice-gas model of Kleintjens et al. for the free energy of mixing with an additional elastic term due to Flory, in order to model the swelling of crosslinked silicones by supercritical COz. Whereas the model reproduces the pressure trends in both volume change and weight fraction fluid absorbed well for the poly(dimethylsiloxane)-CO2 system, the absolute predictions of weight fraction COz absorbed are too high in the vicinity of critical pressure.

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The synthesis of copolymers of vinyl p-tert-butylbenzoate (PTBBA) with 3-methacryloxypropyltris- (trimethylsiloxy)silane (Si-1) and their properties including oxygen permeability are presented. All copolymers had molecular weights above 105 and were easily processable by casting from benzene solution. The Si-1 comonomer content in the obtained copolymer was estimated from the change in the characteristic infra-red absorption due to the carbonyl group of PTBBA and Si-1 units and the benzene ring of the PTBBA unit. The oxygen permeability Po2 increases from 9.1 x 10 1o cm 3 (STP)cm(cm2 s cmHg)-1 for the PTBBA homopolymer to 1.6?10-8cm3(STP)cm(cm2scmHg) I for the Si-1 homopolymer, whereas the corresponding ratio of PoJPN~ decreases from 2.7 to 2.2. Po~ and Po2/PN2 for the copolymer reach a plateau around 25-65 Si-1 mol oV o,w here the values are 0.9 x 10- 8 -1 ? 10- 8 cm 3 (STP) cm(cm2 s cmHg) and 2.4-2.5, respectively. The logarithmic value of Po2 linearly increases with decreasing glass transition temperature.

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A synthetic methodology is described for unequivocally establishing asymmetric induction in the backbone of copolymers based on 1,2-disubstituted olefins and 1-substituted olefins. For this purpose functional styrene monomers (e.g. 4-vinyl benzaldehyde and 4-vinyl boronic acid) were derivatized with chiral auxiliaries, such as R (+)-l-phenyl ethyl amine and diethyl-L-tartrate,which act as protecting groups for the functional groups of the styrene monomers. Copolymerization with maleimide derivatives followed by the removal of the chiral protecting groups produced the appropriate functional copolymers. Spectroscopic techniques were used to confirm quantitative deprotection had occurred and chiroptical techniques were used to ascertain asymmetric inductions in the polymer backbone

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Evaluation of energies for crack initiation and propagation is performed on isotropic low density polyethylene (LDPE) and polypropylene (PP). Loading-unloading loops were used to calculate the energies involved during cracking. For PP, an uncracked sample shows necking upon stretching and exhibits high toughness. It was found that, when loaded, the cracked PP sample also showed necking from the crack tip. As a consequence, a crack propagated inside the fully plastic deformed domain. While uncracked LDPE undergoes uniform deformation with strain hardening upon loading, no necking phenomenon was observed at the crack tip of the cracked sample. As the crack propagated, it had to go through the ligament. We suggest that the ligament area is an important fracture parameter in evaluating the fracture energy. In both cases, energies were divided by the ligament area before being related to the crack length increment. Thus, the potential necessary to create unit crack propagation is represented as energy per unit ligament area. As our results show, the fracture resistance of PP is higher than that of LDPE and this is related to the difference in toughness of these two materials.

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