1994 рік

Real-time in situ observations of isolated spherulitic growth in poly(ethylene oxide) (PEO, M.= 100000) and s-phase isotactic polypropylene (iPP, M,=250000) have been carried out by using video microscopy coupled with a computer image-processing system. A macroscopic latent heat transfer model, which is consistent with the linear growth velocity measured experimentally during spherulitic growth, has been developed and tested in conjunction with Hoffman's nucleation theory. It was found that the latent heat release during spherulitic solidification is compatible with the linear growth velocity.

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Partially deuterated (DPE) and conventional (PE) polyethylene are used to study chain conformations in melt-crystallized blends. The deuterium fraction y ranged from 0.19 to 0.59, resulting in melting temperature differences ATm= 1-2°C between PE and DPE. Small-angle neutron scattering (SANS) from molten or quench-crystallized blends yields single-chain radii of gyration Rgkwhich apparently depend on y and are as much as 15% larger than values reported from experiments with fully labelled (y= 1) polyethylene. Quenched PE/DPE blends give Gaussian-like scattering patterns out to q =0.1 A-1, the limit of these studies. This implies considerably more intramolecular correlations at distances of 10-50 A than derived from a similar experiment employing fully deuterated chains. The present result is thought to be more representative of semicrystalline polyethylene since DPE and PE are nearly perfectly matched. Crystallization by slow cooling creates non-statistical concentration fluctuations which result in excess forward SANS, so a melting point difference of I°C is still sufficient to trigger 'clustering' or 'segregation' of PE and DPE. Intensity decays at q >0.05 A-x to the same value as in quenched samples, regardless of crystallinity changing from 0.52 to 0.74. These results are shown to be consistent with dominant 'near-adjacent re-entry' during crystallization of polyethylene from the melt.

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We have measured the small-angle neutron scattering from semidilute solutions of poly(vinyl methyl ether) in water and in methanol-water mixtures in a poor solvent regime. The binary and ternary cluster integrals of the polymer segments were determined from the concentration dependence of the correlation length at various temperatures just below the lower critical solution temperature. We have calculated the contributions of the segment-segment interactions to the entropy and enthalpy from the measured temperature dependences of these interaction parameters and found that both quantities are positive and decrease with increasing methanol content. The implication of these findings is discussed.

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Fluorescence spectra and decay curves of pyrene in poly(alkyl methacrylate) (methyl, n-propyl, isobutyl and cyclohexyl) films cast on sapphire substrates were measured under total internal reflection and normal excitation conditions, and were compared with each other. On the basis of the intensity ratio of the excimer to the monomer fluorescence and the fluorescence decay curve under the normal condition, the degree of pyrene aggregation in these films is discussed in terms of the solubility parameter, chain flexibility and hydrophobicity of the host polymers. Both the difference in the degree of pyrene aggregation and the difference in micropolarity around pyrene molecules between the vicinity of the sapphire/polymer interface and the bulk were found to be quite common for these polymer films cast on sapphire plates, and are discussed in terms of the interaction between substrate surface and polymers. The effect of tacticity of poly(methyl methacrylate) on the degree of the pyrene aggregation and the effect of labelling with pyrene were also investigated

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The peculiarity of a series of side-chain liquid crystalline polymers with mesogenic units attached directly in a side-on mode was studied by using nuclear magnetic resonance, wide-angle X-ray diffraction, polarizing microscopy and molecular simulation techniques. It was found that in special cases molecules of side-chain liquid crystalline polymers may become rather rigid as a whole and take the more extended conformation, like a rod with the main chain approximately parallel to the rod axis. The side-chain mesogens are packed around the main chain with their axes almost perpendicular to the main chain. These side-chain polymers may exhibit behaviour similar to that of main-chain polymers. The effect of interaction between main-chain and side-chain mesogens on the mesophase formation of side-chain polymers and the influence of mesogen location in molecules on the behaviour of liquid crystalline polymers are discussed

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The thickening of polymer lamellae which occurs when a melt-crystallized sample is heated within its melting range has been investigated for the morphologically simple polymer poly(4-methylpentene-1) using transmission electron microscopy following permanganic etching. It is shown that, even for an isothermally crystallized sample, the lamellar melting point varies repetitively and systematically with position in the spherulitic texture, being highest for dominant lamellae then progressively decreasing because, it is suggested, of increasing constraints during crystallization. Two thickening processes have been distinguished experimentally. At the beginning of the melting range, the least stable lamellae melt and, given time, recrystallize at greater thicknesses. However, the usual interpretation of a doubly peaked melting endotherm in terms of original and transformed lamellae is shown to be oversimplified. At a higher annealing temperature, between the two peaks of the melting endotherm, a population of lamellae forms which is never melted yet increases its average thickness in the solid state logarithmically with time by a second process operative for all lamellae

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Polymorphism in poly(aryl ether ketone)s (PEKs) has been studied by X-ray diffraction as a function of the chemical structure and the crystallization conditions, Two types of chemical variations were investigated: (1) the molar ratio of the keto/ether linkages (expressed as % ketone linkages) was varied from 33 to 67% using the wholly 1,4-substituted forms of poly(aryl ether ether ketone) (PEEK), PEK, poly(aryl ether ketone ether ketone ketone) (PEKEKK) and poly(aryl ether ketone ketone) (PEKK) and (2) the partial replacement of 1,4-substituted phenyl residues with their 1,3-substituted counterparts of PEKK and PEKEKK. As-polymerized polymers and materials prepared by melt crystallization, cold crystallization and crystallization induced by exposure to methylene chloride (solvent crystallization) were used. For all of the polymers investigated, the conventional structure, known as form 1, was observed in samples prepared by melt crystallization. In contrast, for polymers crystallized under one of the other conditions, the second polymorph, known as form 2, was sometimes observed. The occurrence of form 2 was found to be dependent on chain stiffness, which was systematically modified by changing the 1,3-substituted/1,4-substituted isomer ratio and/or by changing the ether/keto linkage ratio and molecular mobility during crystallization. With the increase of chain stiffness and the decrease of crystallization mobility, the ability to form the second polymorph was significantly enhanced. Polymorphism in poly(aryl ether ketone)s may be due to the nucleation control: the stable, form 1 phase has a slower nucleation rate, and thus a higher crystal surface free energy than the metastable form 2 phase

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Isotactic polypropylene sheets deformed by rolling to draw ratios between 6.6 and 15.8 were studied by wide-angle X-ray diffraction. Even order c coefficients of the Legendre polynomial series describing the orientation distribution of the molecular chains were determined from measurements made on nine hkO and hkl crystal planes, for n up to 100. The correlation between the results obtained from these different X-ray reflections is excellent. All the coefficients were used to calculate the orientation distribution of the molecular chains, which cannot be measured directly since there are no reflections parallel to the c-axis for the monoclinic unit cell of isotactic polypropylene. The (P2(cos g)>c and (P4(cos Z)>c values are close to unity for the sheets rolled to 2 >/7.6, indicating that the polymer chains are already well oriented in the deformation direction at these draw ratios. However, the evolution of the crystal phase orientation beyond 2 = 8 can be characterized using the high-order (P,(cos Z)>c coefficients, which still increase up to the highest rolling ratio measured in this work

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The degree of peroxide induced chain extension of polyethylene (PE) as a result of free radical silane grafting proves to be of vital importance to the rate of formation and also the final degree of crosslinking of the PE network structure upon moisture curing. The differences in solid state structure (which determines the moisture diffusion rate into the polymer) and silane grafting efficiency of various PE types have been shown to play an important but a secondary role in the crosslinking of the resins.

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Poly(p-oxybenzoate) (PpOBA) has been polymerized in the bulk at temperatures below the melting point of the monomer. Polymerization involves, isothermally, sublimation recrystallization-melting (chemical reaction)-polymerization-liquid crystal domain formation-crystallization processes. Lamellae, about 100/k thick for PpOBA, are observed along with particulate material. D.s.c. and FTi.r. scans indicate that the sublimate is identical to the monomer, but that the 'melt' has undergone some esterification. The polymer produced is of relatively low molecular weight, but the degree of polymerization can be rapidly increased by heating to elevated temperatures

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