1992 рік

The structure of poly(di-n-hexyl germane) has been investigated using X-ray and conformational energy analysis. Wide-angle X-ray diffraction analysis shows that the polymer undergoes an order/disorder transition at about 15°C. The polymer is known to exhibit thermochromism associated with this phase transition. Below the transition temperature, the backbone conformation is all-trans and the molecules are arranged in an orthorhombic unit cell. Above the transition temperature, the backbone disorders and the polymer loses long-range ordering. Fixed bonding conformational energy calculations found the lowest energy structure to be a 7/3 helix, very similar to that found for poly(di-n-hexyl silane). Most probably, the observed all-tram backbone conformation is due to intermolecular interactions which force the germanium backbone to an all-trans conformation. In order to carry out the molecular mechanics calculations, non-bonded energy parameters for germanium atoms were derived.

Детальніше...  

Anisotropic polymerization shrinkage of liquid-crystalline acrylates has been studied. In order to investigate the changes occurring on the microscopic scale, X-ray diffraction was combined with density measurements. As a result it was shown that at high polymerization temperatures the polymerization shrinkage in the direction of the molecular orientation was higher than the shrinkage in the directions perpendicular to the molecular orientation. Subsequently the shrinkage obtained on the microscopic scale was compared with the results obtained on the macroscopic scale. The behaviour was interpreted in terms of the molecular structure and the packing density within the systems.

Детальніше...  

Polymer-modified aerogel samples were prepared by vacuum impregnation of silica aerogels with dimethylsiloxane-vinylmethylsiloxane copolymers. The specimens were characterized by ‘H nuclear magnetic resonance (n.m.r.) relaxation measurements. The effects of the polymer molecular weight, irradiation and aerogel density upon the spin-lattice ( Tl ) and spin-spin ( T2 ) relaxation times were studied. Significant reduction was observed in the T2 values owing to increases in both aerogel and crosslink densities. ‘H n.m.r. images reveal a high degree of heterogeneity in the samples. Distribution maps of the polymer-silica interactions and crosslink density are obtained.

Детальніше...  

A ruthenium tetroxide (RuO4) staining method has been evaluated for segmented polyisobutylene-bpolybutylene terephthalate (PIB-b-PBT). Solution cast films and melt pressed samples have been studied. For comparison PBT has also been studied. PBT and PIB-b-PBT could be stained with RuO4 at room temperature. The observed structures on the PBT and PIB-b-PBT films were spherulitic with lamellae. In some regions in the PIB-b-PBT films large scale phase separation was observed. In the melt pressed PIB-b-PBT samples the spherulitic and lamellar structures were less well developed. The PBT segments appeared to have crystallized out in bundles and the PIB phase was present as microspheres with diameters of 3-6 nm.

Детальніше...  

Low energy crystal structures of poly(di-methyl silane), which is known to have a trans backbone conformation, have been investigated. Energy calculations on isolated molecules show that, like its hexyl counterpart, the lowest energy backbone conformation of the molecule is not all-trans but a helix with the backbone bonds rotated 15 ° away from trans. This corresponds approximately to a 15/7 helix. To determine if the observed trans conformation results from intermolecular interactions, various packing modes of the trans and the 15/7 helix were analysed and their energies compared. Best packing was found for the trans conformation. For the all-trans molecules arranged in an orthorhombic unit cell, the lowest intermolecular energy was found for a = 1.19 and b = 0.78 rim, in good agreement with dimensions found experimentally at room temperature.

Детальніше...  

A study has been made on the post-radiation grafting of aqueous acrylamide (AAm) onto polypropylene films (PP). A good hydrogel supported on the polymer substrate was obtained by grafting of AAm onto preirradiated PP films. The effect of grafting conditions such as solvent and inhibitor concentration, preirradiation dose, monomer concentration and reaction temperature on the initial rate and grafting yield was determined. It was found that the addition of 0.05 wt % Mohr's salt into the reaction medium effectively reduced the homopolymerization of AAm and the grafting process was successfully achieved. The dependence of the initial rate of grafting on the preirradiation dose and AAm concentration was found to be of the order 1.3 and 1.7, respectively. Two overall activation energies for this grafting system were found: 31.1 and 13,5 kcal mol-' below and above 35°C, respectively. Some selected properties of the prepared graft copolymer were investigated and the possibility of its practical use discussed.

Детальніше...  

13C spin-lattice relaxation times and nuclear Overhauser effect factors were measured as a function of temperature in two magnetic fields for poly(/&hydroxybutyrate) (PHB) in chloroform-d. The relaxation data were interpreted in terms of chain segmental motion and methyl internal rotation by using various dynamic models. Among these, the Dejean-LauprCtre-Monnerie model offered the best description of the segmental motion along the PHB chain. However, all models used in this study resulted in similar activation energy (17 kJ mol-’ ) for cooperative segmental motion. An activation energy of 1 l- 12 kJ mol- ’ was found for methyl internal rotation. Further relaxation experiments on PHB in chloroform-d as a function of concentration and molecular weights were carried out as well

Детальніше...  

Monolayers containing pentacosa-10,12-diynoic acid and copper tetracynoquinodimethane (CuTCNQ) were formed on a pure water surface. Polymerization of the monolayers rendered polydiacetylene as a matrix in which CuTCNQ molecules were embedded. This result was proved by u.v. absorption spectra and scanning tunnelling microscopy studies of the films.

Детальніше...  

Thermally stimulated current depolarization (t.s.c.) was used to study the relaxations in amorphous polymers including poly (ethyl methacrylate ) (PEMA), poly (methyl methacrylate ) (PMMA), polystyrene (PS), polycarbonate (PC) and polyarylate (PAR) over temperature ranges covering the B and ct (glass transition) regions. A.c. dielectric was used to obtain activation energies (Ea) for PS and PC to verify the accuracy of those values determined by the t.s.c, thermal sampling method. At temperatures below the glass transition (Tg) the values of Ea were found to agree with those predicted using an activated states equation with a zero activation entropy. This is evidence of the localized, non-cooperative nature of the low temperature secondary fl relaxations which are found to be characterized by a continuous variation of activation energies as a function of temperature. The measured values of Ea depart from the zero activation entropy curve and exhibit a prominent maximum at Tg. This behaviour is known to be due to an enhanced degree of cooperativity of segmental relaxations near Tg. The results indicate that the main advantage of the thermal sampling method is the high sensitivity and high temperature resolution for cooperative relaxations. For the polymers studied here, only PEMA and PMMA show a substantial population of cooperative relaxations more than 60°C below Tg. This is tentatively explained in terms of structural heterogeneity due to variable tacticity in the methacrylates. Compensation of the t.s.c, relaxation spectra plotted in Arrhenius or Eyring plots was found for all polymers to differing degrees. Some discussion of compensation is made in terms of independently measured values of the coefficient of thermal expansion.

Детальніше...  

The existence of stable fl form crystallites in poly(butylene terephthalate) has been investigated. Wide angle X-ray measurements suggest that such crystallites are only stable provided that they are of comparatively small dimensions and that they are then poorly connected to the bulk polymer. A simple theoretical model is presented to explain the stability of the crystals in terms of the contributions to the free energy from both the crystallites and the surrounding amorphous material.

Детальніше...