1994 рік

The phase behaviour of binary mixtures of cyclic poly(dimethylsiloxane) (PDMS) and linear poly(methylphenylsiloxane) (PMPS) was investigated using a static wide-angle light-scattering technique. Evidence for the effect of component molecular architecture/topology (cyclic or linear) on the phase separation of blends was sought through cloud point measurements and by examining the composition dependence of the resulting phase diagram. The cyclic-linear blend exhibited an upper critical solution temperature (UCST) and showed a dependence on the molecular architecture/topology in that the critical temperature Tc for the cyclic-linear blend was lower than expected when compared with PDMS-PMPS blends in which both components had a linear polymeric structure. The interaction energy density parameter A 12 for the polymers was evaluated via the Flory-Huggins treatment for the thermodynamics of mixing of two polymers. The blending of cyclic PDMS led to a lower A12 when compared with the corresponding linear linear binary blend, again because of the chain architecture/topology effect. For this UCST system, it thus appears that PDMS cyclics are more soluble/compatible than their linear analogues of the same degree of polymerization. These findings are consistent with the current, but very limited, experimental data in the literature for large ring systems.

Детальніше...  

The three crystalline phases of isotactic poly(1-butene), which differ by their helix conformation and unit cell parameters, are obtained by bulk crystallization via epitaxy on appropriate organic substrates, in particular aromatic acids or salts. In this paper, the epitaxial relationships between forms III and II are established, based on composite electron diffraction patterns where possible. Form II1 epitaxy rests mainly on the matching of the chain axis repeat distance; form II is an original illustration of an epitaxy of irrational helices in which the interturn distance plays the main role

Детальніше...  

An intriguing filler-induced softening effect was observed in filled polydimethylsiloxanes that aged at high temperatures. It was found that the Young's modulus and hardness of the aged filled elastomers first decreased and then increased with filler fraction, remarkably different from those of the fresh samples which always increased monotonically. This softening effect cannot be explained by the original phase structure of rigid particles embedded in a soft matrix. However, due to the notable chain scissions and oxidative crosslinks detected in the aged neat resin, the softening was attributed to a heterogeneous oxidative hardening during the thermal ageing. It is proposed that filler particles behaved as secondary antioxidants to block the local reactions of free-radical crosslinks, effectively producing a soft elastomeric layer around each particle and thus this softening effect. This proposal is consistent with the fact that s-iron oxide, a strong free-radical scavenger, demonstrated the most pronounced softening effect. The proposal was further examined by comparing the mechanical strength at the filler/elastomer interfaces extracted from the wear rate data among the various fillers studied here. It was found that ~t-iron oxide/elastomer interfaces possess the lowest interfacial strength, in good agreement with the proposed mechanism

Детальніше...  

The incorporation of a cationic polymer, such as poly(4-vinylpyridine), to the outer leaflet of negatively charged phospholipid vesicles of dimyristoylphosphatidic acid has been investigated by viscometry and intrinsic fluorescence. Viscosity changes of dilute solutions follow the size of the polymer and the polymer-liposome complex. Variations in the emission spectrum of poly(4-vinylpyridine) upon the addition of lipid vesicles under different conditions give the amount of polymer bound per mole of accessible lipid. These experimental results have been interpreted by means of the Gouy Chapman formalism, which calculates the effective number of charges per polymer chain at the interface. The size of the adsorbed polyion has been computed from a discrete charge virial expansion that takes into account the mobility of charged groups at the interface

Детальніше...  

The hexagonal form I' of poly(1-butene) is epitaxially crystallized on 4-bromo and 4-chlorobenzoic acids, on the potassium salts of these acids, and on the hemiacid, potassium hydrogen 4-chlorobenzoate. The contact plane in all cases is (11.0). In form I', the successive (11.0) planes contain, alternately, only left-hand or only right-hand helices. The observed epitaxies can therefore be differentiated according to the chirality of the helices in interaction with the substrate. Epitaxies are governed by a one-dimensional match which involves the interturn distance, i.e. the distance between successive outer helical paths. For less specific interactions, appropriate tilts of the helix axis helps achieve matching with the substrate lattice periodicities.

Детальніше...  

The thermal properties of chitin and hydroxypropyl chitin (HPCH) were examined. Hydroxypropyl chitin was prepared from chitin and propylene oxide. The structure of HPCH was confirmed by solid-state 13C nuclear magnetic resonance and Fourier-transform infra-red spectroscopy. From the results of elemental analysis, the degree of substitution was revealed to be about 0.8. A s hydroxypropyl moieties were introduced on the C6 position in chitin, the solubility of HPCH in many organic solvents was enhanced compared with that of chitin. The dynamic mechanical analysis (d.m.a.) technique was employed to ascertain the glass transition temperature (Tg) of chitin and HPCH. The ct transition peak of cast chitin and HPCH film by d.m.a, was observed at 236 and 252°C, respectively. Considering that rigid-rod-type polymers show Tg followed by rapid thermal degradation, it was proposed that these temperatures should be Tg. Moreover, fl relaxation peaks were detected at ca. 143°C attributed to the presence of acetamide groups in chitin. In the d.m.a, spectra of HPCH, a new tan 6 peak, f12 relaxation behaviour, was found at 105°C, probably due to the relaxation of the hydroxypropyl moiety in HPCH. Weight loss by thermogravimetric analysis implied that the relaxation above 250°C resulted from the decrease in modulus and was caused by thermal decomposition.

Детальніше...  

Herein we report experimental results on a wide variety of common, linear flexible polymers in solution in an attempt to confirm prior reports of abrupt 'conformational transitions' for these materials. Such transitions have been claimed by numerous authors, and they have been reported to reflect sudden conformational changes brought about, in most cases, by varying temperature. Methods used both in our work and earlier work include viscometry, differential refractometry and size exclusion chromatography. We observed no evidence of such conformational transitions and conclude that in reality such events are probably rare. We believe that many prior reports of such sudden changes in shape/size are probably mistaken because of the failure of some authors to take into account realistic experimental errors during data analysis and interpretation.

Детальніше...  

In this paper we show that there exists a linear relationship between the size of a cavity where an ortho-positronium (o-Ps) atom annihilates by the 'pick-off" mechanism and the total free volume of the molecular liquid or polymer under consideration. This relationship is used to determine the free volume of electron irradiated poly(aryl-ether-ether-ketone) (PEEK) samples and the result is compared with that obtained from gas diffusivity measurements.

Детальніше...  

Measurements of poly(vinyl chloride) thermal shrinkage and shrinkage force permit the measurement of a yield flow temperature between elastoplastic behaviour and viscoplastic behaviour. Then, an analogy with mechanical modulus allowed an intrinsic shrinkage modulus to be defined. This concept illustrates the influence of crosslink density on thermal shrinkage.

Детальніше...  

The synthesis of isotactic, syndiotactic and atactic poly(allyltrimethylsilane) (PATMS) is described, along with the characterization of these polymers using thermal methods, X-ray diffraction, solution and solid-state nuclear magnetic resonance (n.m.r.). Isotactic PATMS was produced from rac-Me2Silnd2ZrC12/MAO (methylalumoxane) and displayed chains with an s(3/1) symmetry in the crystalline domains. Syndiotactic PATMS, produced from Me2C(Cp)(FIu)ZrCI2/MAO, was a semicrystalline polymer with chains in the crystalline phase exhibiting s(2/1)2 helical symmetry. A combined X-ray diffraction and solid-state n.m.r. analysis on poorly crystallized samples of syndiotactic PATMS indicates the occurrence in the crystalline phase, for given crystallization conditions, of disorder in the packing of chains, which preserve the helical conformation. Atactic PATMS, an amorphous polymer, was produced using an Ind2ZrCI2/MAO catalyst.

Детальніше...