1999 рік

We present a method for analyzing the molecular scale orientation and structure of processed polymers using wide angle X-ray scattering data (WAXS). The technique is applied to the analysis of solution-spun fibers of a liquid crystalline polyester comprised of 1,4-hydroxybenzoic acid, isophthalic acid, and hydroquinone. The orientation distribution function (ODF) of the non-crystalline component of the polyester is constructed using a Legendre polynomial series expansion. To construct a consistent molecular scale description, a Monte Carlo sampling scheme which incorporates the standard Metropolis sampling was employed, coupled with a weighting factor that favors structures which are in closer accord with the experimental WAXS data. The members of the ensemble sampled by the simulation consist of rhombic lattices on which oligomers are placed in an aligned state. The conformation of each oligomer was obtained using a rotational isomeric state description. By direct comparison of the experimental and calculated structure factor coefficients, the ODF for the structural ensemble was deduced. For the polyester fiber considered here, a two-component system consisting of oriented and unoriented non-crystalline components is required for complete characterization of the scattering properties of the sample. The sample contains a negligible amount of crystalline material, but 85% of the sample is aligned locally (i.e. with respect to nearest neighbor chains). However, as the fibers are spun from an isotropic solution, the global orientation (i.e. orientation with respect to the sample axis) remains low.

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This article concerns the protonation of chitosan in acetic acid aqueous solution with variable concentration. Comparison with HCl is also drawn. Potentiometry allows the determination of the degree of protonation and the pK0. Conductimetry and viscometry are combined to follow the role of protonation on solubilization of chitosan. Solubilization occurs for a degree of protonation around 0.5 as previously found for HCl.

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A guest-host system of a nonlinear optical (NLO) arylphosphine oxide and polyhydroxystyrene (PSOH) has been investigated for secondorder nonlinear optics. The synthesized NLO-active arylphosphine oxides show spectroscopic properties in accordance with the proposed structures using n.m.r. spectrometry. These NO-active arylphosphine oxides and polymer matrix exhibit optical transparency down to 400 nm, and excellent thermal stability (Td > 345°C). An NLO-active chromophore with short absorption wavelength is desirable for application in frequency doubling. Moreover, the excellent thermal stability of the chromophore will prevent its decomposition during the poling process at elevated temperatures. The homogeneity of these guest-host NLO polymers are confirmed using scanning electron microscopy @EM). The second-order nonlinearity and temporal stability of these NLO polymers were also reported. The relationship between molecular motion and temporal stability are studied by dielectric analysis

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The differing glass transitions between deuterated polystyrene and poly(a-methylstyrene) are shown to have a profound effect on their interdiffusion coefficient. We have measured the interdiffusion coefficient as a function of concentration and temperature for two blends. The results can be interpreted in terms of the 'fast’ theory of interdiffusion with the temperature dependence obtained from a Williams–Landel– Ferry (WLF) treatment. The intradiffusion coefficients of the two polymers have also been deduced by this analysis. It is found that the deuterated polystyrene has a much more concentration-dependent intradiffusion coefficient than the poly(a-methylstyrene). Thermodynamic slowing down is also observed in one blend.

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We characterize the orientation distribution function (ODF) of molecular-scale structure in a set of solution- and melt-spun fibers of a liquid crystalline polyester in order to assess the role of processing in these fibers. The polyester is composed of 40% 1,4-hydroxybenzoic acid, 30% isophthalic acid, and 30% hydroquinone. Crystallinity is measured by both wide angle X-ray scattering (WAXS) and differential scanning calorimetry (DSC). A three-component model, consisting of crystalline, oriented non-crystalline, and unoriented non-crystalline components, is required to describe completely the X-ray scattering properties of the fibers. Monte Carlo simulations are used to define the ensemble structure on the atomic scale, for which the resulting ODF is determined. Our results indicate that a single ensemble of dimers on a 3 × 3 rhombic lattice can be used to characterize the molecular-level non-crystalline structure in all the fibers considered. Fibers spun from the nematic melt produce samples with higher degrees of global orientation as well as a higher degree of crystallinity than those spun from solution. Our results also show that the mechanical bias of the fiber spinning process induces an initial degree of orientation which is then enhanced and perfected, especially in the directions lateral to the chain, by subsequent annealing.

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The temperature induced conformational transition of stereoregular PMMA in diluted or semi-diluted solution was studied by viscometry, NMR and FT-IR. The conformational energies of polymer chains were calculated from infrared spectral subtraction by following the method described by O’Reilly and Mosher [O’Reilly JM, Mosher RA. Macromolecules, 14 (1981) 602; O’Reilly JM, Teegarden DM, Mosher RA. Macromolecules, 14 (1981) 1693]. The fraction of sequences containing gauche conformation that can be deduced shows a large increase during the conformational transition. This increase in the probability of gauche conformations accounts for an increase in the chain flexibility deduced from the decrease of the calculated characteristic ratio. The increment in the hydrodynamic volume of the chain in chloroform that occurs at the transition temperature can be correlated with an increase in the density of the polymer–solvent specific interactions. The range of temperatures of the conformational transition, namely, 20–408C, was found to be independent of the quality of the solvent (theta or athermic solvent). However, the cooperativity of the conformational changes s is shown to depend on the solvent for the i-PMMA. Further, the aggregating character of the solvent (o-dichlorobenzene) shifts the observed transition for the i-PMMA solution towards higher temperatures which is ascribed to the disassociation of the aggregated structures.

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The formation of a polyethylene/polyurethane blend polymer via in situ polymerization of diols and diisocyanate monomers dispersed in a molten polyethylene matrix was investigated in an internal mixer. Two urethane systems were used in this study. The evolution of the blend morphology with increasing molecular weight of the polyurethane phase was discussed. The difference in viscosity between the minor and major phases has a controlling influence on the qualitative compounding behaviour exhibited. Furthermore, the viscosity ratio appears to be the physical parameter which governs the blend morphology development. The phase inversion was observed at ηd/ηm = 0.05. Beyond the phase inversion point, Wu’s model (Polym. Eng. Sci., 1987, 27, 335) quantitatively predicts the decrease of the size morphology with increasing molecular weight of the polyurethane phase. Nevertheless, high solubility of isocyanate monomers and low solubility of alcohols in molten polyethylene induce an imbalance stoichiometry which limits the degree of polymerization (molecular weight) during the blending process. A pre-polymerizing

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The mechanical response of rubber-modified high density polyethylene (HDPE) was investigated. The rubbers were either ethylene– propylene copolymers (EPDM) or ethylene–octene copolymers (EOR), blended into HDPE at volume fractions of up to 0.22. These rubbers were in the form of finely dispersed spherical inclusions with sizes well below 1 mm. The incorporation of rubber into HDPE does not substantially change its crystallinity, but produces special forms of preferential crystallization around the rubber particles. The notch toughness of the rubber-modified HDPE increases by more than 16-fold as a result. The single parameter, controlling the notch toughness of these blends was found to be the matrix ligament thickness between rubber inclusions. When this thickness is above a certain critical value, the notch toughness of the material remains as low as that of the unmodified HDPE. When the average ligament thickness is less than the critical value a dramatic toughness jump results. The critical ligament thickness for the HDPE–rubber systems was found to be around 0.6 mm, independent of the type of the rubber used. The sharp toughness threshold in the rubber-modified HDPEs results from a specific micro-morphology of the crystalline component of HDPE surrounding the rubber particles. The PE crystallites of approximately 0.3 mm length perpendicular to the interface are primarily oriented with their (100) planes parallel to the particle interfaces. Material of this constitution has an anisotropic plastic resistance of only about half that of randomly oriented crystallites. Thus, when the interparticle ligaments of PE are less than 0.6 mm in thickness the specially oriented crystalline layers overlap, and percolate through the blend, resulting in overall plastic resistance levels well under that which results in notch brittle behaviour, once rubbery particles cavitate in response to the deformation-induced internal negative pressure. This renders ineffective the usual strength-limiting microstructural flaws and results in superior toughness at impact strain rates.

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The crystal structure of polytetrafluoroethylene, –(CF2)n –, is unusual in having a number of crystal forms and also possessing substantial molecular motion within the crystal well below the melting point. This article deals with the unusual first-order transition occurring at about 19°C, changing the crystal structure from form II to form I. This article also updates current literature on the crystal structures of these two forms. X-ray and electron diffraction data from forms II and IV are interpreted as helical structures. Form II is defined mathematically as a non-commensurable helix close to a 13/6 conformation of six turns per 13 (CF2) groups. Form IV is interpreted as a commensurable 15/7 helix which is consistent with space group P31. Fiber diffraction patterns of these forms are described in detail.

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Recently, the interpretation of the reactivity of polymer radicals has been developed by means of some improvements to the so-called Patterns Scheme, itself an advance over the Alfrey-Price Q-e Scheme. Using the reactivity of a radical with styrene as a measure of its “general” (i.e. thermodynamic) reactivity, it has been shown that a parameter denoting radical polarity can easily be deduced from polymerisation data, and that these 2 parameters can be used quantitatively to characterise the behaviour of that radical in copolymerisations in general. It thus becomes possible to forecast monomer reactivity ratios with much greater precision than heretofore. The present article extends this treatment to the reactions of initiator radicals with monomers.

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