1992 рік

Neutron reflectometry has been used to study the formation of thin polyphenylene films produced by the aromatization of poly(5,6-cis-diacetocyclohexa-l,3-diene). It has been found that the process results in the production of films with non-uniform density in a direction normal to the surface. In addition, the aromatized films are found to have a much higher surface roughness than the precursor polymer film. This has been attributed to the disrupting effect of the volatile products of aromatization.

Детальніше...  

The lamellar thickness of polyethylene crystals measured by electron microscopy following chlorosulphonic acid fixation is often lower than the expected value. Experiments indicate that this is the result of insufficient acid treatment : the electron beam interacts with poorly fixed crystals to reduce lamellar thickness. Thickness reduction is not recorded when polymer has been fixed for longer times

Детальніше...  

Films of poly (v-benzyl-L-glutamate) (PBLG) and poly(hexylisocyanate) (PHIC) were cast from solution whilst under electric fields generated between parallel strip electrodes placed at the surface. Because of their chain rigidity and large macromolecular dipole moments, this procedure induces some parallel chain alignments in the field direction, even if the fields are low ( ~< 300 V mm- ~ ). The field induces a degree of non-centrosymmetric order in the films which is retained after the field is removed. In these ordered states such films were found to have second-harmonic generating (SHG) ability under 1.06 #m irradiation and this ability did not age over a period of 6 months. The SHG activity varied with the angle of polarized radiation relative to the poling direction, showing a single maximum and minimum for PHIC (uniaxial symmetry) and double maxima and minima for PBLG (biaxial symmetry) over n radians. Optical microscopy of the PBLG films formed under electric fields showed regions of nematic as well as cholesteric order.

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A novel sol gel processed silica: poly(2-bromo-5-methoxy-p-phenylene vinylene)/silica composite was prepared using a soluble precursor. The optical properties of this material have been investigated using u.v.-visible and FTi.r. spectroscopy. The densification process was monitored by d.s.c, and t.g.a. The third-order non-linear susceptibility Z 13) has been investigated for both the pure polymer and its sol,el composite at 602 nm using femtosecond degenerate four-wave mixing. A relatively large ~(3) value with a subpicosecond response is observed. The electrical conductivities of AsF 5 doped polymer and composite films were studied using four-probe measurement

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The mechanical behaviour and morphology of films prepared from poly(ethylene terephthalate), poly (butylene terephthalate) and polyamide-6 blends subjected to zone drawing and annealing with fixed ends are studied. It is established that as a result of simultaneous physical (crystallization, relaxation, partial melting) and chemical (solid state exchange reactions and condensation) processes substantial structural changes take place in the blends which affect their mechanical properties.

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Crystallization of polygonal polyethylene (PE) crystals (lozenge, diamond-shaped or with rectangular 2 0 0 sectors) is well accounted for by the Seto and Frank theory (SF theory). Crystallization of polyethylene with rounded crystal habit can be explained, according to Sadler, by the study of a two-dimensional lattice model with addition and removal of small square units (Sa approach). In such an approach, the outline of the crystal is rough. It is not reasonable to explain the growth of polygonal crystals by the Sa approach and nor can the SF theory justify the shape of curved crystals. We have used, therefore, the equation of Point and Villers, which generalizes and mediates between these two approaches. This equation reproduces accurately the observed morphology and simultaneously furnishes the values of three important physical parameters (the kinetic length, the mean rate of spreading of a nucleus and the reflection coefficient of a step at the end of a substrate). It gives an insight into the structure of the crystals

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The phenomenon of melting-recrystallization during annealing below an equilibrium melting temperature (141.6°C) of gelation crystallized ultrahigh molecular weight polyethylene (UHMWPE) films has been investigated by means of time-resolved small angle X-ray scattering (SAXS) and wide angle X-ray diffraction (WAXD). Isothermal annealing below 130°C shows that the SAXS long period (periodic distance of successive lamellae) increases, then subsequently levels off with increasing annealing time. At 140°C, the long period increases from ! 18 to 690 A without any sign of levelling off. This type of increase in long period has been customarily attributed to lamellar thickening associated with solid state diffusion of intracrystalline chains. Recent synchrotron radiation studies on annealing of single crystal mats of conventional linear polyethylene demonstrated that melting-recrystallization is responsible for the increase in long period. The present time-dependent WAXD studies show a gradual decrease in crystallinity for some initial periods of annealing at 137°C, which slows down with elapsed time. At I40°C, the terminal melting occurs after 15 min. We propose that the increase in long period during annealing of this gelation crystallized UHMWPE near its melting temperature is due to the melting of successive lamellae rather than lamellar thickening.

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A decay of free radicals in y-irradiated rigid main chain thermotropic polymer BRY 856 was studied. The dependence of the rate constant for free radical decay over the pressure region between 1 M Pa and 800 M Pa was determined in the temperature range between 80°C and 230°C. Activation volumes of free radical decay were established. A decrease in activation volume at temperatures above the glass transition temperature of the polymer was observed and attributed to rotational structural order along the long axis of the macromolecules.

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Wide-angle and small-angle X-ray scattering and differential scanning calorimetry are used to elucidate the lamellar morphology of an aromatic polyketone (designated PK99). PK99 has three unusual crystallization and melting characteristics. First, there is a coupled melting and recrystallization process at around 260°C, well below the final melting point of 306°C; this process suggests two crystal forms. Secondly, there are two maxima in the growth rate versus temperature characteristics. Thirdly, the melting point is relatively low compared with the Tg (158°C). WAXS shows that above and below the 260°C process there is no essential difference in the crystal packing. However, WAXS and SAXS data do show small step differences in crystal perfection and in lamellar thickness. The lamellar thicknesses are comparable to the periodic repeat length. This produces a new situation not normally observed in other crystalline polymers, where more than one type of lamellae can be formed containing different chain sequences. The unusual characteristics can be explained in terms of these different types of lamellae.

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In this paper, the influence of deformation and thermomechanical drawing conditions on the development of birefringence and orientation in high-density polyethylene during zone drawing is described. The orientation process of the crystalline phase does not proceed uniformly with deformation and is complete at lower draw ratios, while orientation of the amorphous phase increases continuously up to the highest draw ratios. The maximum birefringence considerably exceeds that of a polyethylene crystal (as high as 0.081) and the amorphous orientation almost approaches that of the crystalline phase. This indicates that the zone-drawing technique is highly effective in producing orientation. The birefringence is affected simultaneously by drawing stress and temperature; at higher temperatures and lower stresses, softening effects predominate, while in the opposite case stiffening effects become operative. The occurrence of both factors is regulated by the time of thermal exposure in the heating zone.

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