1992 рік

The miscibility and domain structure of poly(vinyl alcohol)/poly(vinylpyrrolidone) (PVA/PVP) blends are investigated by high-resolution solid-state 13C nuclear magnetic resonance methods. The observed 13C spectra and the intermolecular cross-polarization of the blends suggest a hydrogen-bonding interaction between the two polymers, aH T1 and T1o results indicate that the blends are miscible at all compositions on a scale of 200-300 A. On a scale of 20-30 A, however, the miscibility of the blends depends significantly on the composition. When the PVA composition is more than 46 wt%, the blends are composed of two phases, an amorphous miscible phase of PVP plus PVA and a pure PVA phase. The crystallinity of the PVA phase decreases rapidly with decreasing PVA composition. When the PVA composition is less than 46 wt%, the blend is completely miscible. The composition of PVA phase in the blends was inferred from the results of ~H Tip.

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We have studied the melting behaviour of poly (phenylene sulphide), PPS, that has been crystallized from the melt over a wide range of undercooling conditions. Two grades of PPS were used in the study : low molecular weight Ryton V-1 and an experimental medium molecular weight film grade. In this work we report the melting behaviour after a single stage of isothermal melt crystallization. In nearly all cases, dual or triple melting endotherms are seen, but the location and shape of the uppermost endotherm depend upon the degree of undercooling from the melt and on the prior thermal history. At a low degree of undercooling, for both materials, dual endotherms are observed and both melting points increase with the crystallization temperature. Using the immediate rescan technique, we show that the dual endotherms exist together very early in the crystallization process, when only a small fraction of crystals have formed. These results suggest that multiple crystal perfections form early on in the crystallization at low undercooling. As the degree of undercooling increases, the melting point of the uppermost endotherm becomes independent of the crystallization temperature. This result, and the appearance of a triple endothermic response for Ryton at the highest undercooling, indicate that now reorganization of imperfect crystals is dominating the observed endothermic response. We present a model in which the distribution of crystal perfections created during melt crystallization controls the multiple melting behaviour of PPS. At low undercooling conditions, a bimodal distribution of crystals can form which eventually may become two morphologies. At high undercooling conditions, a broad distribution of crystals can form, a part of which may melt and reorganize during the normal d.s.c, scan.

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Photo block copolymerization of styrene with methyl methacrylate was studied using polymers obtained from the polymerization of styrene with a polymer of pentamethylene disulphide. The block copolymer was synthesized with a high block efficiency

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Phase separation and chemical structure changes occurring during thermal degradation are studied by high-resolution solid-state 13C n.m.r, methods for poly(vinyl alcohol)/poly(methacrylic acid) (PVA/PMAA) = 1/1 complex and poly(vinyl alcohol)/poly(acrylic acid) (PVA/PAA)= 1/l blend. 13C cross-polarization/magic-angle spinning spectra and 1H Tap are measured for samples subjected to various heat treatments. The results indicate that when the systems are heated to higher temperatures, phase separation occurs first because of the dissociation of intermolecular hydrogen bonding between the two different polymers. Further increases in temperature bring about thermal degradation. Degradation products of the complex and the blend are similar to those of the corresponding single-polymer systems. The thermal degradation temperatures for the complex and the blend are lower than for the single-polymer systems. This is ascribed to the loss of the crystalline phase of the PVA component and a lowering of glass transition temperatures of the PMAA and PAA components in the miscible system

Детальніше...  

Multiple stages of melt crystallization have been used in this study to investigate the origin of the multiple melting behaviour of poly(phenylene sulphide), PPS. In a companion study we reported results of single-stage melt crystallization of PPS and presented a model to explain the observation of multiple melting and crystallization temperature dependency of the melting peaks. In our model, the degree of undercooling determines whether the existence of preformed crystals or reorganization of imperfect crystals dominates the observed endothermic response. In this work, multiple sequences of crystallization and annealing from the melt have been used to vary the perfection of the crystals. At low crystallization temperature (high degree of undercooling), very imperfect crystals are formed which can reorganize during the d.s.c, scan. In Ryton V-l, triple melting peaks are observed, and the endothermic response depends strongly on annealing time and scan rate. Annealing at elevated temperature, which perfects the crystals, or increasing the scan rate, which reduces the chance for reorganization, both result in a decrease in the size of the uppermost endotherm. However, at high crystallization temperature (low degree of undercooling), the pre-existing crystals are much more perfect and less susceptible to reorganization. Studies of multiple stage crystallization indicate that it is possible to generate many distinct crystal populations by step-wise crystallization from the melt. The pre-existing crystals display their own melting points, each in response to a separate stage of crystallization. As many as seven melting points can be seen after five stages of melt crystallization, indicating that morphology (pre-existing crystals) dominates the appearance of the endothermic response after multiple stages of melt crystallization

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Preferential solvation parameters for the ternary system poly(dimethylsiloxane)-benzene-methanol have been determined by comparing the areas of the solvated and non-solvated polymer g.p.c, peaks. Good agreement is found between the present results and those from other methods. The present method has the advantages of simplicity, speed and accuracy.

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Mechanical properties of semicrystalline polymers are determined mainly by the proportion of crystalline phase and its ordered microstructure. Owing to its inherent planar zigzag backbone conformation, syndiotactic polystyrene (sPS) is expected to have a crystalline modulus exceeding that of many polymers, and with potential applications as a low-cost, high-performance material. Molecular modelling has been used to simulate the deformation behaviour of a perfect sPS crystal lattice with the molecular chains oriented parallel to the applied stress, thus deriving the tensile modulus. Intrachain interactions including bond stretching and bending, are found to be the primary forces influencing tensile modulus, whereas the interchain van der Waals contribution is not significant. The calculated crystal modulus for sPS is comparable with the literature value utilizing X-ray diffraction.

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The swelling and mechanical properties of poly(vinyl alcohol) (PVA) hydrogel, and PVA gels obtained by swelling precursors in various solvents were investigated. On the basis of the experimental results, the structure of the gels in various solvents was estimated. PVA gels have a uniform structure with flexible PVA chains in a mixed solvent of dimethyl sulphoxide (DMSO) and water. On the other hand, those swollen in methanol, ethanol and formamide have a two-phase structure, which is composed of PVA-rich and solvent-rich phases. The PVA chains in the PVA-rich phase are crosslinked by hydrogen bonding.

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Nylon 6 is doped with iodine to form a complex. The doubly oriented specimen of the complex crystal is investigated by X-ray diffraction. The lattice constants of the complex crystal are determined; it has a monoclinic unit cell of the space group P21/a or P21 : a = 0.913 nm, b = 1.62 nm (chain axis), c = 1.79 nm and fl = 59.7 °. The hydrostatic compression of the crystal shows that the compressibility has become isotropic around the chain axis; doped iodines weaken the hydrogen bonds formed in the s-form of nylon 6. The structure of the complex crystal is discussed on the basis of the packing and orientation of polyiodine ions, I s and I[, in nylon 6.

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A set of polypropylene (PP) fractions with similar molecular masses and molecular mass distributions but different isotacticities have been investigated and their overall crystallization and crystal melting behaviours determined by differential scanning calorimetry and time resolved small-angle X-ray scattering experiments. The observations indicate that when the supercooling is high (AT> 48 K), only poor and imperfect crystals can form, which are continuously annealed to more perfect crystals upon heating after isothermal crystallization. On the other hand, below AT = 48 K, two different crystal morphologies are recognized with their own crystallization kinetics and thermal stabilities. The Avrami treatment for the overall crystallization kinetics of these PP fractions generally reveals a low Avrami exponent (n) compared with the literature data. The overall crystallization and crystal melting behaviour can be correlated to the crystal morphology change with supercooling, in particular to the 'cross-hatching' lamellar phenomenon uniquely observed in the case of PP. The isotacticity effect on the overall crystallization processes and melting behaviour are also focused upon.

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