1994 рік

A new route for the production of high modulus and strength, crosslinked ultra-high molecular weight polyethylene (UHMW-PE) structures was explored, trans-l,4-Polybutadiene (t-I,4-PB) is added to UHMW-PE, prior to dissolution and ultra-drawing, to enhance the crosslinking efficiency of drawn tapes upon electron-beam irradiation. It was found that undrawn solution-cast films exhibit a two-phase structure consisting of a continuous UHMW-PE phase and dispersed t-I,4-PB domains. Upon drawing, the t-I,4-PB phase is deformed to elongated domains with a width of a few nanometres, and both the UHMW-PE and t-I,4-PB phase are highly oriented. The addition of t-I,4-PB indeed enhances the crosslinking efficiency of drawn, oriented UHMW-PE tapes. Tapes with a high gel fraction (>95°/'0) and improved high-temperature resistance could be produced.

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Blends of poly(D(-)3-hydroxybutyrate) (PHB) and atactic poly(methyl methacrylate) (aPMMA) were isothermally cold-crystallized from homogeneous amorphous samples obtained by quenching. X-ray techniques were employed to investigate the phase structure of the cold-crystallized blends. The long period distance and the amorphous interlamellar distance increased with increasing aPM MA content in the blends, while the thickness of the crystalline lamellae was quite constant for the blends, and higher than for pure PHB. These results indicate that during isothermal cold-crystallization the aPMMA component is segregated in the amorphous PHB interlamellar regions

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The structures, solution properties and solid state properties of complexes of poly(benzobisimidazobenzophenanthroline) ladder (BBL) and its semiladder analogue (BBB) with the Lewis acid aluminium(III) chloride (A1C13) and gallium(Ill) chloride (GaC13) are reported. At a 4:1 mole ratio of Lewis acid to polymer (BBB or BBL) repeat unit, electron donor-acceptor complexes of Lewis acid-base type, which are highly soluble in organic solvents, are formed. A detailed spectroscopic investigation shows that complexation of both polymers involves coordination of the Lewis acids to the electron-rich carbonyi oxygens and the imine nitrogens in the polymer backbones. The complexes of BBL exhibit the characteristic rigid-rod behaviour of the pristine ladder polymer. BBL complexes have rigid, rod-like chain conformations in dilute solution as evidenced by their solvent independent intrinsic viscosities. The complexes form liquid crystalline solutions above 8 wt% BBL in A1C13/nitromethane and above 9-9.5 wt% in GaC13/nitromethane. The solid complexes of BBL have slightly smaller optical absorption edges than BBL and ~ 50% larger absorption coefficients at 2ma,. D.S.C. revealed a glass transition (T~) of the 4:1 GaCI3:BBL complex at 15°C. Dynamic mechanical experiments indicated the Ts, measured at the loss tangent peak, was in the range 29-37°C depending on frequency, with an activation energy of 448kJmo1-1. The complexes of BBB by contrast exhibit flexible-coil behaviour due to the polymer topology. In solution, BBB complexes have intrinsic viscosities which depend on the solvent and Lewis acid used, due to differing degrees of coil expansion in different solvent media. Coil expansion in solution leads to improved conjugation of BBB complexes as evidenced by comparison of the electronic absorption spectra with those of BBL. BBB complexes, like the pure polymer, are apparently not liquid crystalline at high concentrations in solution. The solid 4:1 GaC13:BBB complex has a Tg at 30°C, and a dynamic mechanical analysis Tg in the range 12-24°C. The Tg activation energy, at 293 kJ mol-1, is lower than that of the BBL/GaCI3 complex. The overall results of the present studies provide a basis for understanding the structure and properties of these polymers in terms of intermolecular interactions and also have implications for the processing of the polymers for diverse applications.

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Tensile stress and birefringence in polystyrene and polycarbonate melts have been measured at various temperatures during constant strain rate elongational flow. Deviations from the linear stress-optical rule appear at temperatures close to the Tg and are discussed by considering the total stress as the sum of an entropic and a non-entropic contribution. Anisotropy of properties below the Tg of samples where the chain orientation has been frozen-in by quenching, was characterized by thermal expansion measurements. A significant elastic recovery is found to occur at temperatures as low as Tg-30°C, and is discussed as a function of the thermomechanical history.

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In this paper, the chain transfer method was adopted to study the graft copolymerization of butyl acrylate onto poly(vinyl chloride) (PVC) in emulsion. This approach to the synthesis of graft copolymers was first suggested by Flory, and was experimentally demonstrated by Hayes, Blumczynski and Mayo. The work reported here shows that for the chain transfer method, the grafting efficiency does not increase, as Flory's chain transfer theory predicted, but decreases with increasing copolymerization temperature. An explanation of these observations is proposed. In addition, FTi.r. was used to study the resultant graft copolymer and the parent homopolymers in order to investigate the structural changes in PVC consequent on grafting. Thermogravimetry results showed that the thermal stability of the graft copolymer is better than those of PVC and PVC/poly(butyl acrylate) blend.

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The variation in torsional angles for a model linear epoxy system was studied as a function of applied stress and temperature. It was found that all of the torsional angles demonstrate a population migration from a lower to a higher energy state upon the application of an external stress. Subtle changes in torsional angle values under these conditions result in large energy differences.

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Small-strain moduli reported in the literature for polydimethylsiloxane (PDMS) model networks have been critically re-examined, with account being taken of recent studies which have demonstrated the occurrence of significant side reactions in hydrosilylations. When the effects of such side reactions on network structure parameters are properly taken into account, the elasticity results can be well explained using the recent molecular theory of Flory and Erman. The revised calculations show a transition in the modulus from the affine to the phantom limit of deformation as the degree of chemical crosslinking increases. This is to be expected when the constraints on the fluctuation of junctions vanish (because of the decreased interspersion of chains), and such constraints are expected to vanish even in the small-strain region when the chains are sufficiently short. In lieu of carefully controlled reactions with well-defined stoichiometries, it appears that the procedure best suited for testing the various theories of rubber-like elasticity is a plot of the modulus G (as approximated by the sum 2C1 + 2C2 of the Mooney-Rivlin constants) against the phantom modulus [f*]ph (as approximated by 2C0. Also of importance is the difference between the 'effective' number of network chains v (relevant to theory) and the commonly used 'active' number of chains va.

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The enthalpy of mixing (AMH) for polystyrene with poly(ct-methylstyrene) has been estimated by a semidirect calorimetric method. This involved measuring heats of solution of blends and of mechanical mixtures having the same composition to obtain, by difference, the heat of mixing of the two polymers in their glassy states. By assuming a model for the relaxation of a glass to its corresponding hypothetical liquid state, these were then converted to give a AMH value for the liquid polymers. A small negative value for the heat of mixing was obtained over part of the composition range, contrary to expectations

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Reactive polymers each bearing large numbers of reactive groups are important for new compatibilized blends with the graft copolymers generated in situ. Potential of gelation and phase homogeneity are two important issues. A case study on cellulose acetate and poly(styrene-co-maleic anhydride) (SMA) was reported here to demonstrate the two issues. It was found that phase homogeneity appears when the grafting conversion of SMA exceeds around 50% for the dispersed SMA phase. There is a relatively narrow range of SMA grafting conversion for the formation of compatibilized ternary mixtures with a homogeneous phase size and a reasonable weight-average molecular weight. The study shows the importance of polydispersity in promoting phase homogeneity due to the weighting towards more grafting of highermolecular- weight fraction of the reactive polymers.

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Mechanical properties and fracture tests were conducted on the thermoset diethylene glycol bis(allyl carbonate). It was found that both % and E increase as the testing rate increases. Crack propagation which is unstable (stick-slip) at a low loading rate changes to stable propagation as the loading rate is raised. Linear elastic fracture mechanics concepts were employed to analyse the stress intensity factors versus the yield stress. It is suggested that the stick-slip behaviour is due to blunting of the crack, which is controlled by the yield behaviour of the resin. It is shown that a critical stress of the order of five times the yield stress must be achieved at a critical distance ahead of the crack tip. Fracture surfaces were analysed using scanning electron microscopy and optical microscopy. No indication of crazing was detected

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