1993 рік

It has been shown by a number of authors that supercritical carbon dioxide is an extremely efficient plasticizer for amorphous, carbonyl-containing polymers. In this paper, we have examined the plasticization of poly(methyl methacrylate) (PMMA) networks by CO2 as a function of pressure using dielectric measurements. Results show that the degree of plasticization increases rapidly at pressures up to approximately 1200 psig (8.274 MPa), levelling off (and possibly decreasing) at higher pressures. This phenomenon is possibly due to the counteracting effects of pressure on free volume and CO2 sorption. The experimental trends are consistent with predictions of free volume in CO2/PMMA mixtures derived from a modified lattice model

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Polyaniline (PANI), fully protonated with camphor sulfonic acid (CSA), was found to form liquid-crystalline solutions in m-cresol. The existence of this interesting state of matter was inferred from observations of opalescence, depolarization of plane-polarized light and the existence of a critical concentration for the onset of the formation of the liquid-crystalline phase. This critical concentration was found, in accord with existing theory, to decrease monotonically as the viscosity or molecular weight of the polyaniline increased. By shearing the liquid-crystalline solutions onto glass substrates and subsequent precipitation in acetone, oriented polyaniline films were obtained. The existence of a liquid-crystalline phase implies a rather rigid conformation of the fully protonated polyaniline in solution. Importantly, the emeraldine base form of PANI remained isotropic in solutions in m-cresol up to very high concentrations. Thus it is suggested that protonation induces a chain stiffening and extension of the polyaniline macromolecules (relative to the insulating emeraldine base) resulting from a combination of electronic delocalization and counter-ion 'crowding'.

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The nanometre-scale resolution atomic force microscopy images of the internal structure of Kevlar C~ fibres made it possible to reconcile differences between existing proposed structural models. Detailed real-space structures of the 'pleats' in these fibres and evidence of chain-end segregation in fractured fibres revealed previously inaccessible structure-property relations

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The thermal shrinkage of poly(ethylene terephthalate) (PET) films drawn uniaxially to different draw ratios over a wide range of temperatures was measured and analysed primarily by differential scanning calorimetry (d.s.c.) studies and density measurements, and by recording the load-extension characteristics of the samples associated with the drawing process. The overall orientation of drawn samples was assessed from birefringence measurements. The d.s.c, analysis was found to be an accurate predictor of shrinkage behaviour, particularly with respect to the changes in the exothermic crystallization peak. Furthermore, the analysis has shown that shrinkage is completely suppressed when a degree of crystallinity equal to 43% is reached, which can only be achieved through annealing treatments. A reduction in shrinkage is always manifested by a down-shift and a concomitant flattening of the exothermic crystallization peak.

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Thermotropic liquid-crystalline (LC) behaviour of lignin-like model compounds, lignin, (commercial) petroleum pitch and blends of lignin and pitch was studied using hot-stage optical microscopy. The results indicate that, while all the dimeric lignin-like model compounds representing fl-O-4, #-5 and 5-5 intermonomer linkages exhibit thermotropic LC phenomena in some temperature range, the LC behaviour remains incomplete for lignin. At the temperature at which a maximum amount of LC behaviour is observed for lignin, large carbonized particles are observed as well. This prevents the formation of a molecularly organized fluid phase from ever developing. The inability to fluidize totally at elevated temperatures is likely to be responsible for lignin's (lignin derivative's) inability to produce high-strength, high-modulus carbon fibres.

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We report the application of thermal mechanical analysis (t.m.a.) to studying the crystallization behaviour of poly(ether ether ketone) (PEEK). We are able to show that the two crystallization stages of PEEK are clearly distinguished by measuring the variation of film thickness with time during isothermal crystallization. These two crystallization stages cannot be readily distinguished by d.s.c. The distinction by t.m.a, becomes less clear with increasing crystallization temperature due to overlap of the two crystallization stages

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Using a wafer bending technique, residual stress and its relaxation were investigated for four different kinds of high temperature polyimides: rodlike PMDA-PDA, semi-rigid BPDA-PDA, semi-flexible PMDA-ODA, and flexible BTDA-ODA. Residual stress was measured in situ on silicon wafers during thermal imidization of the polyimide precursors and subsequent cooling of the resulting polyimides as a function of temperature over the range of 25-400°C. The stress of the cured films at room temperature was significantly relaxed by the moisture uptake of the films rather than their creep behaviour. Both residual stress and moisture-induced stress relaxation in the cured polyimide films were strongly dependent upon the molecular nature (that is, molecular chain rigidity, degree of molecular orientation and packing), as well as the precursor origin. In addition, the diffusion coefficients of water in the cured polyimide films were estimated by best-fitting stress relaxation against time profiles.

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Recent theories have related the molecular-weight dependence of the conductivity of a conjugated polymer to the relative importance of interchain and intrachain charge transport processes. This prompted an experimental search for this effect using a series of fractionated poly(3-octylthiophenes). No significant molecular-weight dependence was found over the range 30 000 ~< Mw ~< 400 000, indicating that interchain transport does not limit the macroscopic conductivity of these samples. However, even small differences in the degree of crystallinity among the polymer samples were determined to have a profound effect on the conductivity.

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Inverse gas chromatography (i.g.c.) using a fused-silica open tubular column has been developed and evaluated for measuring diffusion coefficients of small molecules in polymers; this diffusivity data can be used, for example, to model reactive extrusion processes. We have shown the applicability of this technique by measuring the diffusion and partition coefficients of various small molecules in polystyrene, poly(methyl methacrylate) (PMMA), and poly(methyl methacrylimide). The results for polystyrene and PMMA were in good agreement with existing data by gravimetric sorption/desorption, packed column and glass capillary column i.g.c, techniques. The diffusion data for the polyimide were generated for the first time.

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The thermal degradation of poly(p-methylstyrene) (P-pMS) was studied in several solvents, such as 1-methylnaphthalene, tetralin and phenol, in the temperature range 3(Kt-400°C. No undesirable by-products, such as gels and cross-linked high molecular weight products, were formed, in contrast to the significant yield of these products with conventional polymer pyrolyses without solvents. The conversion of P-pMS to low molecular weight products, and the molecular weight of the residual polymer after pyrolysis, depended markedly on the kind of solvent used. Such degradation behaviour is explained well by a proposed mechanism including hydrogen transfer from solvents to intermediate polymer radicals.

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