1993 рік

Heat-of-mixing data, obtained on blends of poly(ethylene oxide) (PEn) with whole and fractionated poly(vinyl acetate) (PVAc), were used to feed Patterson's theory of polymer-polymer miscibility. Negative values of mixing enthalpy, contact-energy term, interaction parameter and excess volume were obtained only for blends with the lowest molecular weight PVAc fraction. These results show that miscibility of PVAc with PEn strongly depends on its molecular weight. The calculated unfavourable excess volume term of the Patterson equation is small in comparison with the absolute value of the interaction term. Therefore, miscibility of PEn and low-molecular-weight PVAc is dictated by the weak specific interactions between different repeat units and by the entropic gain in the mixing process.

Детальніше...  

The effect of reprocessing on the structural order of a poly(vinyl chloride) compound is studied using differential scanning calorimetry (d.s.c.). Reprocessing was simulated in the d.s.c, instrument by subjecting the polymer to multiple heating sequences leading to various maximum temperatures. The traces of the processed material contain information about its previous processing history. The development of a network of entanglements and of primary and more recent order re-formed in each of the cooling sequences is postulated.

Детальніше...  

The effect of high pressure on the main chain motion near the glass transition temperature (Tg) in amorphous chain-deuterated polystyrene is studied using deuteron n.m.r, spin alignment echo and quadrupole echo spectra. Comparison with simulated spectra yields information on the time-scale and the type of motion. In the vicinity of Tg the main chain performs ultra-slow small angle diffusive motion. The temperature dependence of the mean correlation time shows a strongly non-Arrhenius behaviour, typical for the so-called ~-process. The width of the correlation time distribution rapidly increases on approaching T~. The effect of pressure on the motion is found to be much weaker than could be expected from equation-ofstate data.

Детальніше...  

Sixteen nematogenic copolyesters were synthesized from four to seven different aromatic reactants, with the objective of suppressing crystallization. The constituent units contained rod-like 1,4-phenylene and 4,4'-biphenylene groups, crankshaft 2,6-naphthylene group and rigid angular 1,3-phenylene group. Wide-angle X-ray diffraction, differential scanning calorimetry and hot-stage optical microscopy showed that five of the copolyesters could be classified as lacking three-dimensional order. Within the range of compositions studied, a large number of different units of different types is conducive to suppressing crystallization.

Детальніше...  

A family of high-temperature, high-modulus aromatic polyimide fibres has been dry-jet wet spun from either its gel state or isotropic solution, followed by high-temperature drawing. In this report, thermal and dynamic mechanical properties of one of the family members, a segmented rigid-rod polyimide synthesized from 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) and 2,2'-bis(trifluoromethyl)-4,4'- diaminobiphenyl (PFMB), are presented in detail. Mechanical properties of these BPDA-PFMB fibres can be improved remarkably by drawing due to drastic increases in overall orientation, crystal orientation and crystallinity. These three structural parameters, however, do not show parallel changes with increasing draw ratio. It has been observed that the linear coefficient of thermal expansion (CTE) of BPDA-PFMB fibres after drawing generally show negative values in the solid state when low stresses are applied during measurements. For as-spun fibres, the CTEs are constant over a certain applied stress region, which is on the same order of magnitude as CTEs of in-plane oriented BPDA-PFMB films along the film surface. This may be an indication that within this region the stress applied is at the same level as the internal stress frozen into the fibres during spinning and drawing. Glass transition temperatures (T~) of as-spun fibres show a linear decrease at low applied stress region, then level off when the applied stress becomes high. Dynamic mechanical data indicate two relaxation processes in as-spun fibres above room temperature: an ~ relaxation corresponding to the glass transition and a fl relaxation which is a subglass transition. In the fibres with a draw ratio of above three times, the ~ relaxation is totally suppressed. This reveals a rigid fraction (above T o dependence of this relaxation in the fibres. The /3 relaxation is, on the other hand, crystallinity dependent. The Arrhenius activation energy (about 160 kJ mol- 1) of the/3 relaxation in as-spun fibres is about 50 kJ mol- 1 lower than that of drawn fibres, indicating that the cooperativity of molecular motion in the fibre changes with orientation and crystallinity.

Детальніше...  

The effect of high pressure on the phenyl group motion in amorphous polystyrene (PS-ds) has been studied using 2H n.m.r, quadrupole echo spectroscopy. The spectra are superpositions of two contributions from phenyl groups performing relatively slow and fast 180 ° flipping motion, respectively. The motion of the phenyl groups is characterized by a distribution of correlation times. The mean correlation times of the slowly flipping groups vary from milliseconds to microseconds and are in good agreement with data from other relaxation techniques. The effect of pressure on the molecular motion is strong in the glassy state. The reorientational activation volume is of the same order as the volume required for the rotation of a phenyl group. Above the glass transition temperature the effect of pressure is much weaker. We have no evidence that the phenyl group motion is dependent on the main chain motion.

Детальніше...  

Conformational energies, electronic band gaps1- and second-order hyperpolarizabilities were calculated as a function of torsion angle for structures designed to model the rigid rod polymers poly(pphenylenebenzobisoxazole), poly(p-phenylenebenzobisimidazole) and poly(p-phenylenebenzobisthiazole), including examples in which the phenylene group is mono- and dimethylated, using AM1 semi-empirical molecular orbital techniques. An increase in torsion angle decreases ~ electron delocalization, which is the dominant factor in electronic band gap and second-order hyperpolarizability changes. Methylation of the phenyl group destabilizes the fiat structures in all cases, and the resulting calculated barrier heights show the result of the opposing steric and conjugative effects. The methyl substituents have negligible direct effect on opto-electronic properties

Детальніше...  

An unsaturated polyester (UP) resin based on maleic anhydride, isophthalic acid, propylene glycol and dipropylene glycol was cured with variable amounts of styrene, using benzoyl peroxide as initiator. Differential scanning calorimetry was used to determine heats of copolymerization, thermal mechanical analysis was employed to measure glass transition temperatures and Fourier-transform infra-red spectroscopy was used to follow the individual conversions of styrene (S) and UP double bonds (E). A heat of copolymerization equal to 66 kJ mol- 1 was obtained, independently of the initial S/E ratio. Taking the overall conversion into account, the theoretical value for complete conversion of double bonds was estimated as 69 kJ mol- 1. These values are in good agreement with literature values for the copolymerization of diethyl fumarate and styrene (65-70 kJ mol- 1) and styrene homopolymerization (68.5-73 kJ mol - 1). The maximum glass transition temperature (T~) was obtained for formulations with S/E between 2 and 3, undergoing a near-azeotropic copolymerization. A single relationship between T~ and overall conversion was obtained for different cure temperatures

Детальніше...  

The effect of high pressure on 2H n.m.r, spin relaxation in polystyrene is studied. Deuteron spin-lattice relaxation behaves non-exponentially both in chain- and ring-deuterated polystyrene at temperatures up to 20 K above the glass transition. The non-exponentiality is caused by the broad distribution of correlation times and the slowness of the spin diffusion process. The fast flipping phenyl rings are an effective relaxation mechanism both for the chain and for the phenyl deuterons. The distribution of correlation times is either due to a heterogeneity of local environments or to a change in time-scale of the motion over long time intervals. Denser packing of the polymer caused by high pressure results in a decrease in the spin-lattice relaxation time. The apparent transverse relaxation time can be used as a monitor of the glass transition.

Детальніше...  

Crosslinked poly(glycidyl methacrylate) has been functionalized by ethanolamine and subsequently by xanthation. The xanthate based beads were used for anchoring Fe 2 + ions, after which they have been used in conjunction with H202 as a redox catalyst system for aqueous polymerization of acrylamide. The polymer beads were characterized for percentage modification at subsequent stages by elemental analysis, Fourier-transform infra-red spectroscopy and pore size distribution. The efficiencies of polymerization with the polymer-supported catalyst and with a conventional system were compared with respect to the yield and molecular weight.

Детальніше...