1999 рік

In this article, we present the first ab initio calculations on the novel rigid-rod polymer PIPD using density functional techniques. The behaviour of the molecular chain under strain is studied and the chain modulus agrees excellently with experiment. Two crystal structures are considered and hydrogen bonding networks as proposed from X-ray diffraction measurements are demonstrated to exist. Negligible energy differences were found and both structures could exist at room temperature. The electronic structure reveals a p – p interaction like in graphite, resulting in a weak bonding. The interaction due to the hydrogen bonding network is smaller, however: it leads to a significantly larger bonding energy. As the interchain bonding in PIPD is approximately 3 times the bonding without the network, the improvement in the compressive strength of PIPD compared to PBO and PBT can be attributed to the hydrogen bonding network.

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The toughness and morphology of poly(butylene terephthalate), PBT, blends with acrylonitrile–butadiene–styrene, ABS, materials are very sensitive to the PBT molecular weight or melt viscosity. High PBT melt viscosity is shown to lead to improved ABS (and styrene– acrylonitrile copolymer, SAN) dispersion which improves the low temperature fracture toughness of PBT/ABS blends. Compatibilization by methyl methacrylate, glycidyl methacrylate, ethyl acrylate terpolymers, or MGE, was shown to further improve ABS and SAN dispersion and reduce the ductile–brittle transition temperature. Addition of the compatibilizer broadens the window of acceptable processing temperatures. The minimum quantity of ABS required to toughen PBT is lower the higher the PBT melt viscosity. The tensile properties of PBT/ ABS/MGE blends were shown to be relatively insensitive to PBT melt viscosity or reactive compatibilization.

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A new experimental setup designed to monitor by atomic force microscopy (AFM) the time evolution of a single micrometer-sized polymer spherulite during crystallization at elevated temperatures is presented. For the case of blends of poly(aryl-ether–ether–ketone), PEEK, with poly(ether–imide), PEI, it is shown how AFM can help visualizing the time sequence and space location of events occurring during polymer crystallization, specifically growth, branching and thickening of lamellar stacks (fibrils). This method complements other non-destructive techniques, e.g. optical microscopy, as it is able to examine the growth of very small crystals occurring for high degrees of supercooling. In addition, not only the overall spherulitic growth rate but also the growth rates of individual fibrils could be evaluated. The potential of the technique to evaluate crystallization near the surface by following the evolution of the surface roughness is briefly discussed.

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The mechanism by which dye solutes specifically interact with a variety of polymer structures has been investigated. FT-i.r. spectroscopy has been used to determine the type and strength of interactions that occur between dyes containing two or three hydroxyl groups and polymers containing a range of functional groups including phenyl, chloro, cyano, carboxyl, amide and amine. Dye–polymer intermolecular interactions and dye–dye self-association interactions occur in all polymers. The relative magnitudes and strengths of these interactions depend on the types of functional group present in the polymer matrix and the concentration of the dye solute in the polymer. Solvents with analogous structures to polymer repeating units were found to give very similar dye–matrix interactions and were used to assist the identification of dye–polymer and dye–dye interactions.

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The photo-polymerization of tetrachloroethylacrylate (TeCEA), pentafluorophenylacrylate (PFPA) and pentafluorophenylmethacrylate (PFPMA) was examined by quantitative FT-IR and FT-Raman spectroscopy and by thermal analysis. The agreement between the FT-Raman results and thermal analysis was good for all monomers. The FT-IR measurement yielded higher percentages of unsaturation than the two other techniques in the slowly reacting PFPMA polymer, which can be attributed to oxygen quenching effects. Calculations of unsaturation based on spectroscopic measurements were carried out using the relative band ratio method. The influence of the choice of the internal reference band on the unsaturation calculation was examined. The photo-polymerization of the acrylates reached conversions of 95%–99% after irradiation of only a few minutes, samples which contained PFPMA reached conversions of about 85% after 1.5 h of irradiation. Volatile compounds in the resulting polymers were quantified and identified by thermogravimetic analysis (TGA) and thermal desorption followed by gas chromatography with mass spectrometric detection (TD-GC-MS), respectively. Glass transition temperatures of the resulting polymers as determined by differential scanning calorimetry (DSC) vary from 70°C to 125°C.

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The isothermal crystallization kinetics, semi-crystalline morphology and thermal behavior of poly(ethylene oxide) (PEO) in miscible binary blends with an amorphous component, poly(ether sulphone) (PES), were investigated below LCST by means of optical microscopy, differential scanning calorimetry and small angle X-ray scattering. Addition of PES to PEO reduces both the overall crystallization rate and the spherulite growth rate of PEO. The change of the nucleation regime and an Avrami analysis of the crystallization kinetics are discussed. The double melting behavior of PEO in blends with PES is attributed to both secondary crystallization and recrystallization during heating. A change in the supermolecular structure from intermediate spherulite-hedrite to spherulite appears on blending PEO with PES. The segregation behavior of amorphous PES during crystallization of PEO is investigated.

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An epoxy-imide adhesive has been exposed to tensile creep at various elevated temperatures in air. Using viscoelastic techniques, the glass-transition temperature has been monitored both for the polymer having undergone creep and the material simply aged at the same temperature. Defining DT g as the T g of the loaded material minus that of the aged material, for conditions otherwise equivalent, unusual behaviour has been observed. Initially, DT g increases for short times but later decreases and becomes slightly negative. This behaviour has been attributed to two phenomena, occurring concomitantly: that of macromolecular chain orientation causing a reduction of free volume and a (relative) increase of T g, and chain scission caused both by chain elongation and thermal degradation, leading to a drop of T g. A simple model is proposed to explain semi-quantitatively the observed behaviour.

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A b-PP sample was stretched to necking at room temperature and the morphologies of the deformed material over the necking region were examined with SEM and TEM. Before yielding, the strain was mainly accommodated between horizontal lamellae. Nevertheless, some vertical lamellae were stretched to break when the material approached yielding. At the yield point, many melt spots and deformation bands across vertical lamellae emerged. Some vertical lamellae became highly stretched and their thickness reduced significantly while the neighboring horizontal lamellae rotated towards the loading direction. At the upper shoulder of neck, the melt spots near the poles of spherulites enlarged and elongated in the stress direction, meanwhile, strain-induced crystals appeared in the melt spots. Furthermore, many vertical lamellae were broken into short fragments and some horizontal lamellae were sheared in the loading direction when the adjacent vertical lamellae were stretched heavily. In the tapered section of the neck, the original b-spherulites were shattered due to excessive crazing and deformation bands and some highly drawn material domains were formed. In turn, more shear bands were generated roughly along the loading direction. The horizontal lamella fragments rotated towards the loading direction and fed into the material flow continually. Finally, the lamella–spherulite structure was converted into an oriented fibril structure in the cold drawn material. Based on the observed results the deformation mechanisms of necking and cold drawing were discussed.

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Porous, membrane-like structures with different supramolecular organisation are prepared from solutions of syndiotactic polystyrene in cyclohexanol. Liquid–liquid (L–L) demixing leads to the formation of amorphous materials. They transform into the helix or g -phase on heating above Tg. Further heating leads to a melting–recrystallisation process that transforms the g -phase into the zigzag or b -phase. This structure formation prevents re-dissolution of the polymer and maintains the demixed state at temperatures far above the Q-temperature of the polymer–solvent system. Elimination of the solvent at different temperatures leads to amorphous porous materials or porous materials with the polymer in the g - or b -phase.

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A technique is described for the synthesis of polystyrene-block-poly(vinylidene fluoride)-block-polystyrene (St-VDF-St) triblock copolymers through a poly(vinylidene fluoride) macroinitiator. The bromine terminated poly(vinylidene fluoride), which was prepared by radical telomerization of vinylidene fluoride with 1,2-dibromotetrafluoroethane, initiated polymerization of styrene in presence of copper(I) bromide and a,a-bispyridine complexes. It was found that molecular weights increased linearly in styrene conversion and polydispersities were low. The macroinitiator and block copolymers were characterized by g.p.c. and n.m.r.

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