1993 рік

The equilibrium association of urethane groups in a series of 4,4'-diphenylmethane diisocyanate/poly- (propylene oxide)-based poly(ether urethane) elastomers was studied on the basis of infra-red spectroscopy data. Thermodynamic parameters of the urethane-group association were estimated. The hard-segment equilibrium state was shown to fit a model of monomer solution association

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The molecular orientation of poly(styrene-block-butadiene-block-styrene) triblock copolymer with cylindrical microdomains of glassy polystyrene (PS) is investigated by i.r. spectroscopy and is discussed in relation to the structural changes of the microdomains caused by the uniaxial strain. For evaluation of the molecular orientation, the dichroic orientation factor (FD) was calculated from the dichroic ratio (D) of the absorption bands at 1493 and 966 cm-1 for PS and polybutadiene (PB), respectively. It was found that PB chains exhibit molecular orientation parallel to the stretching direction whereas there was no orientation for PS chains. The results indicated that PB chains are oriented parallel to the stretching direction but did not imply that the glassy PS microdomains are not deformed by the strain. The reason why the PS orientation was not clearly detected may be ascribed to a bad signal-to-noise ratio for the FD of the PS band used to evaluate PS orientation. It was also found that the orientation of PB chains effectively proceeded in an extremely stretched state, which is in good agreement with the results from wide-angle X-ray scattering.

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The dynamic Young's modulus, E*, and birefringence of bisphenol A polycarbonate were measured over the glassy to rubbery plateau zone. Measurements were performed over the frequency range of 1 to 130 Hz at various temperatures between 148 and 170°C. A modified stress-optical rule was applied in characteristic analysis of the glassy and glass transition regions. It was earlier proposed to replace the stress-optical rule which, generally, was valid only at relatively low frequencies corresponding to rubbery plateau and rubbery flow regions. E* could be separated into two component functions, E* and E~. The stress-optical coefficients,CR and C G, associated with the respective components were 5.0 x 10 9 and 3.5 x 10-11 Pa-1. In constructing master curves of E* and E* with the method of reduced variables, an unlike temperature dependence was found. This result could account for the breakdown of the time-temperature superposition principle for E* reported by several investigators.

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Monomer reactivity ratios in free-radical copolymerization of styrene and 4-vinylpyridine monomers at 80°C in toluene solution under reduced pressure were determined for low and high conversions. Finemann-Ross, Kelen-TiJd6s, extended Kelen-Tfid6s and Mayo-Lewis methods were used for this purpose. The most reliable method was found to be the extended Kelen-Tfid6s method. By the use of this method, for conversions below 20%, monomer reactivity values of 0.700+0.005 and 0.335 +0.001 were calculated for 4-vinylpyridine and styrene respectively. For conversions greater than 20%, these values were found to reduce to 0.538 +0.004 and 0.277 + 0.001 respectively.

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In this paper we investigate the effect on melting stability of blending linear with branched polyethylene. Linear polyethylene (LPE) was blended in solution with branched polyethylene (BPE) at a concentration of 10% LPE. Both homopolymers and the blend were held at 126°C, for 80 h. At this temperature solid-liquid phase separation takes place in pure LPE and in the blend; pure BPE remains molten. The melting point of the isothermally crystallized blend lamellae was lower than the melting point of pure LPE lamellae crystallized under the same conditions. This depression in melting temperature has previously been explained in two ways: by the inclusion of BPE into the LPE crystals, and because of the different environment of the crystals. We assess these two factors and find them insufficient to explain the decrease in melting temperature observed. We discuss another important factor, the dimension of the crystals in the chain direction. By transmission electron microscopy we show that crystals isothermally grown in the blend are thinner than LPE crystals grown under the same conditions. This difference in crystal thickness is sufficient to explain the observed depression in melting temperature. We believe that the lamellae in the blend fail to thicken during the initial stages of crystallization. We envisage three mechanisms that explain the suppression of thickening present in the blend, and one of these is eliminated.

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Syndiotactic polystyrene was found to form a molecular compound with iodine as well as with a variety of organic compounds. The iodine molecular compound with uniaxial orientation was obtained by immersing amorphous polystyrene (obtained by quenching the melt into ice water followed by stretching) into molten iodine at 120-130°C. The crystal structure of the iodine molecular compound is isomorphous to that of the molecular compound with toluene. The crystal data are as follows: monoclinic, space group P2~/a, a= 17.29, b= 12.85, c (chain axis)= 7.79/~, 7' = 120.3° (c-unique), and there are eight monomer units (two chains) in the unit cell. The polymer chains take a (TTGG-) 2 twofold helical structure and it is expected, from the viewpoint of the crystal symmetry and the size of the hole, that two iodine molecules occupy one isolated hole formed between the polymer chains. However, in spite of several attempts to prepare the molecular compound, not all the holes contained iodine. The sample used for X-ray structure analysis was estimated to contain about 70% of the expected iodine content. On annealing in the range 130-150°C, the X-ray reflection intensities changed, indicating escape of iodine to some extent. However, the pure helical form free from the guest molecule was not obtained, in contrast to the case of molecular compounds with organic compounds; on annealing at temperatures higher than 160°C the iodine molecular compound was directly transformed to the planar form I.

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Rubber-modified polyamides were obtained directly by blending modified poly(styrene-b-butadiene-styrene) (SBS) triblock copolymer used as the rubbery component and nylon 6. The rubber was modified with various amounts of maleic anhydride (MA) with the aid of dicumyl peroxide. In this study, the influence of MA concentration on the crosslinking level of the rubber and its effect on the blend properties were investigated. Crosslinking level of MA-modified SBS was measured using isooctane, the calculation being done following the Flory-Rehner equation. Impact testing of the blends revealed a maximum peak in impact strength versus rubber crosslinking level. However, the maximum was not so eminent as we expected. Explanations about the impact behaviour of the blends related to toughening mechanisms are presented.

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A newly designed viscometer was used to measure the viscosity of polymer solutions under high-temperature and low-shear-rate conditions. Rheograms of dilute and concentrated scleroglucan solutions were determined in the range of temperatures between 30 and 150°C. Rheological behaviour was correlated with macromolecule structural parameters. Intrinsic viscosity values versus temperature were analysed on the basis of Yamakawa's theory of semirigid chains. As the temperature increased, local molecular rigidity decreased. An Arrhenius equation form was found for the persistence length variation versus temperature. An apparent activation energy of 7.5 kJ mol-1 (1.8 kcal mol-1) was determined. This value was of the same order of magnitude as the hydrogen bond energy that stabilizes the scleroglucan triple-helix structure. On considering a model of rod particles, the scleroglucan molecule behaves like a rod-like polymer up to 130°C.

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The surface composition of blends of poly(ethylene oxide) (PEO) with polystyrene (PS), poly(methyl methacrylate) (PMMA) and random copolymers of styrene and methyl methacrylate (MMAS) has been studied as a function of blend and copolymer compositions. In the case of homopolymer blends, only the incompatible blend PEO/PS showed significant surface enrichment in PS following annealing at temperatures above the T~ of the two constituents and the Tm of PEO. The compatible system PEO/PMMA presented a mixed surface. Surface enrichment in the MMAS copolymers was observed in all PEO/MMAS blends with styrene contents in excess of 2% w/w. This has been attributed to the incompatibility between the PEO and the MMAS copolymers as shown by the ternary phase diagram PEO/MMAS/CHCI3 and the melting-point depression of the PEO-rich phases with varying copolymer composition and concentration. Angle-dependent X-ray photoelectron spectroscopy provided an indication of the depth-concentration profile of the blend. The results of these measurements demonstrate that increased styrene content in the MMAS copolymer produces deeper concentration profiles, which reached down to 69 A from the surface. Mean-field calculations of the depth-concentration profile for a selected system gave very good agreement with the experimental data.

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The development of microphase separation after temperature jumps in poly(N,N-diethylacrylamide) hydrogels was studied by means of small-angle neutron (SANS) and X-ray (SAXS) scattering. The curves of excess SANS and SAXS for temperature jumps from 25 to 30°C were different in character from those for jumps from 30 to 35°C. After the temperature jump from 25 to 30°C two polymer phases with different water content are formed (excess scattering intensity lex ~ q- 2, where q is the length of the scattering vector); however, after the jump from 30 to 35°C the dilute phase is formed by water droplets (pronounced two-phase structure, Ie~q-4). Photoelastic measurements on the equilibrium swollen samples indicate that correlations exist between side groups at temperatures greater than or equal to 33°C. The association of side groups leads to microsyneresis and to the differences observed in scattering behaviour.

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