1993 рік

Differential scanning calorimetry (d.s.c.), X-ray, dynamic mechanical and microscopy studies were carried out using a series of blends of natural rubber (NR) and ethylene-vinyl acetate copolymer (EVA) in the crosslinked and uncrosslinked states. The crosslinking system (sulfur, peroxide and mixed) and blend ratio do not influence significantly the melting temperature (Tin) of EVA. The decrease in percentage crystallinity of EVA with NR content is related to the incomplete crystallization, as indicated by the lower heat of fusion (AH). The glass transition temperature (Tg) of NR in the uncrosslinked blend did not change substantially and this indicated incompatibility. However, in the case of crosslinked systems, depending on the type of crosslinking agents used, the Tg of the NR phase in the blend was shifted to higher temperatures. This has been explained with reference to the predominant crosslinking of the NR phase. From X-ray diffraction patterns, the interplanar distances (d values) were calculated. With the addition of NR there was a tendency for the d values to increase in all systems. The crystallinity was measured by X-ray and the results were in line with those of d.s.c, measurements. The dynamic mechanical properties such as tan 6, storage modulus and loss modulus of the blends were evaluated. The existence of separate Tg values in dynamic mechanical thermal analyser studies indicates that the blends are incompatible. The morphology of the uncrosslinked blends has been studied by scanning electron microscopy.

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The toughening mechanism in a ternary alloy of poly(butylene terephthalate) (PBT), polycarbonate (PC) and rubber is discussed. Transmission electron microscopy (TEM) shows that the ternary alloy has a three-phase structure; fine rubber particles covered with PC shells are dispersed in a PBT matrix. Constructing the corresponding three-phase model for analysis by the finite element method (FEM), we carried out a two-dimensional elastic-plastic analysis of the deformation mechanism. When the model was uniaxially stretched, the rubber particles induced yielding of the PC shell and the PBT matrix and the yielding zone expanded over the whole space at larger strains. This massive yielding of the plastic phases is expected to result in a large amount of energy dissipation and hence the material would be toughened. Using similar analysis of a PBT/PC/void system, the void was shown to play the same role in the toughening mechanism. As expected from the FEM analysis, the three-phase alloy exhibited ductile behaviour in the falling-dart impact test. Both tensile dilatometry and light scattering experiments suggested void formation or cavitation. TEM observation after the impact test clearly supported internal cavitation of the rubber particles. The results suggest that the ternary alloy is transformed to the PBT/PC/void system during deformation, and even after the transformation, the toughening mechanism may persist.

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A new method for analysing and modelling anisothermal crystallization from a polymer melt is proposed in a way allowing comparison with isothermal conditions, and supposing that no variation of the process occurs in the whole temperature range. The model is based on a modified Avrami equation: 1-U=exp{--K(T)[(T~--T)/~]"}, where the starting point T~, highly dependent on the cooling rate c~, appears to be of major importance. Some methods for its determination are derived. Application to crystallization kinetics of poly(ethylene terephthalate) by differential scanning calorimetry is discussed and a comparison of both methods shows good accuracy. Moreover, anisothermal studies allow a more extended temperature range for K(T) determination and seem more closely connected with the current value of apparent n, interpreted in terms of a dual mechanism. Finally an equation describing K(T) variations is proposed and allows one to describe quantitatively any anisothermal process, particularly at the beginning, which is suitable for setting conditions.

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The nucleophilic cleavage of differently activated aryl ethers by a fluoride anion is studied. Two sulphone groups in para positions to the ether link appear to activate this sufficiently to allow its nucleophilic substitution by F- to occur from 280°C upwards. No reaction is observed up to 300°C when two ketone links are para to the ether, whilst the reaction occurs from 300°C upwards in the case of a mixed sulphone-ketone activation

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Relations between structure and equilibrium mechanical properties of polyurethane networks based on poly(butadiene) diol, 4,4'-diphenylmethane diisocyanate (MDI) and poly(oxypropylene) triol (POPT) prepared at various initial ratios of reactive groups were studied. An inhomogeneous structure of the networks was revealed by small-angle X-ray scattering. The microdomains consist of POPT molecules crosslinked with MDI and they have a diameter of about 4-5 nm. The characteristic distance of microdomains is about 9 nm. The poor mutual miscibility of the reaction components and the existence of hydroxyl group association in the reaction mixture are the probable reasons for microdomain formation. The strain dependence of the equilibrium elastic force obeyed the Mooney-Rivlin equation. The experimental moduli of dry and equilibrium swollen samples were higher than those calculated for the affine limit of the Flory junction fluctuation model on the assumption of a homogeneous network formation.

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Segmented copolyureas comprising soft segments based on a polyoxypropylene polyamine and hard segments based on 4,4'-diphenylmethane diisocyanate (MDI) reacting with either 3,5-diethyltoluenediamine (DETDA) or methylene-bis-2,6-diisopropylaniline (MDIPA) have been formed by reaction injection moulding (RIM). Novel RIM materials with MDI/DETDA hard segments and poly(dimethylsiloxane) soft segments have also been studied. RIM materials were characterized by d.s.c, and d.m.t.a, to obtain soft- and hard-segment glass transition temperatures, T~ and T~, and the degree of microphase separation. Correlations between the chemical structure, thermal properties and microphase separation have been established using interaction parameters, Z, estimated from solubility parameters, and critical values of Z estimated from block copolymer theory. Hard-segment sequence length increases with hard-segment content, and values of T~ are shown to increase according to the Flory-Fox relation. The effects of process and thermal histories on microphase separation are interpreted with reference to a phase diagram

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new linear method of calculation of monomer reactivity ratios is presented for copolymerizations up to very high conversion. It is a linear least-squares method involving several iterations. The basis of calculation is the differential copolymerization equation and the Kelen Tfid6s (K-T) method. The principle and the calculation of computer-simulated data indicate that this new method can correct completely the systematic errors due to high conversion. Calculation using data in the literature shows that this method can be applied in any real systems. The comparison of K-T and extended K-T methods is also discussed.

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Poly(ary1 ether ketone)s were synthesized by the reaction of 1,3-(bischlorobenzoyI)benzene or the corresponding fluoro analogue with bisphenoxides derived from either hydroquinone or 4,4’- isopropylidenebiphenol (bisphenol-A). With the stronger nucleophile, obtained from bisphenol-A, and either of the dihalides, high molecular weight polymers are formed exclusively via a nucleophilic aromatic substitution (S,AR) reaction. A similar reaction leads to the formation of a high molecular weight polymer when the weaker nucleophile, derived from hydroquinone, is allowed to react with the bisfluoride. On the other hand, oligomeric products are obtained when the bischloride is treated with this weaker nucleophile. In this case, both &AR and SRNl (substitution, radical-nucleophilic, unimolecular) mechanisms are operative for the replacement of the chlorine atoms. The E&,1 pathway, which is responsible for the formation of oligomeric products, can be eliminated by the addition of a suitable radical scavenger. High molecular weight poly(ary1 ether ketone) is then formed via the S,AR mechanism.

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The fracture toughness and failure mechanisms of epoxy resin composites filled with silica particulates have been investigated in the temperature range -50°C to 80°C and at two loading rates. There are significant effects of temperature and loading rate on impact fracture toughness, which shows a peak at ambient temperature and decreases as temperature is reduced or raised. Fracture toughness under static loading is slightly lower than that of impact loading with similar dependence on temperature. Failure mechanisms are characterized based on SEM examination, which is correlated with the measured fracture toughness, the damage zone size developed at the advancing crack tip, and the residual stresses arising from differential thermal contraction between matrix and particles upon cooling from the cure temperature

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Poly(diarylsiloxane)s are highly crystalline, high-melting polymers with excellent high-temperature properties. As a consequence of the rigid polymer backbone, they show crystal-liquid-crystal transitions, and both nematic and smectic states have been observed by optical microscopy. The polymers of greatest interest in this investigation have been those containing phenyl and p-tolyl substituents on the siloxane backbone. We have prepared a series of polymers with various combinations of these substituents, as well as the end members of the series, poly(diphenylsiloxane) and poly(di(p-tolyl)siloxane). The polymers are prepared with controlled molecular weights and distribution by ring-opening anionic polymerization of the cyclic trimers in solution. The crystal-liquid-crystal transition temperatures, Tic, for poly(diphenylsiloxane) and poly(di(p-tolyl)siloxane) are very high (265 and 300°C, respectively) and the polymers are only soluble in a few solvents at temperatures above 150°C. However, the T~c values are reduced for the mixed poly((phenyl/p-tolyl)siloxane)s and the polymers become soluble at lower temperatures. The symmetric poly((phenyl/p-tolyl)siloxane) is of particular interest since T~c is reduced to 150-160°C, and the polymer is soluble at room temperature in common solvents such as toluene, tetrahydrofuran and chloroform. In contrast to the retention of the crystalline and liquid-crystalline character in the mixed diarylsiloxanes, the replacement of a single phenyl group by a methyl group in the repeat hexaphenyl triad sequence of poly(diphenylsiloxane) is sufficient to destroy both the crystalline and liquid crystalline character of the polymer.

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