摘要:

When a polymer is dissolved in a solvent, the heat measured is a sum of a polymer-solvent interaction term and a term related to the structure that existed in the solid polymer relative to its amorphous liquid state. This latter contribution, termed the “residual” heat, can have an endothermic contribution due to the fusion of crystalline regions and an exothermic contribution due to the disruption of structure in noncrystalline amorphous regions. For atactic polystyrene between 30 and 110°C, it is shown that the “residual” heat is exothermic, decreases linearly with temperature differences below Tg, and extrapolates to zero in the vicinity of Tg. The existence of an exothermic heat above Tgis probably related to a 160°C transition in polystyrene. This “residual” heat was further observed to be independent of the pressure at which the polystyrene was glassified.

ISSN:0022-2348    

DOI:10.1080/00222347408215160

出版商:Taylor & Francis Group    

年代:1974

数据来源: Taylor

摘要:

The chain conformation and molecular packing of isotactic poly(p-fluorostyrene) have been examined using calculations made with semiempirical potential energy functions. Isolated chain conformational energies indicate no difference in conformation for the fluoropolymer from the conformation for isotactic polystyrene. The energy for packing poly(p-fluorostyrene) chains into a crystalline array as 31or 41helices was also compared with the energies for packing polystyrene in both of these helical forms. While not being the lowest energy mode of packing for poly(p-fluorostyrene), the packing of 41-helices does yield a local energy minimum. Such packing of 41helical polystrene chains is considerably less energetically feasible. The results indicate the causes for the experimentally observed difference in the crystalline conformations of the two isotactic polymers as being due to intermolecular influences.

ISSN:0022-2348    

DOI:10.1080/00222347408215161

出版商:Taylor & Francis Group    

年代:1974

数据来源: Taylor

摘要:

Using as a starting point the atomic positions determined by potential energy calculations, the crystal structure of isotactic poly(p-fluorostyrene) has been determined from X-ray diffraction data. It is found that two chains having 4, helical conformations pack into a metrically orthorhombic unit cell with a = 17.6 Å, b = 12.1 Å, and c = 8.25 Å (chain axis). Satisfactory agreement between the calculated and observed structure factors was obtained only when statistical variation of left- and right-handed parallel helices was considered.

ISSN:0022-2348    

DOI:10.1080/00222347408215162

出版商:Taylor & Francis Group    

年代:1974

数据来源: Taylor

摘要:

The metal decoration technique was applied to solution grown crystals and cast thin films of (tetramethyl-p-silphenylene) siloxane and dimethylsiloxane (DMS) block copolymers in order to elucidate their surface textures as a function of copolymer composition. Electron diffraction and microscopy studies were used. Metal-free zones are observed at surface steps of single crystals, and decorating particles occasionally form spiral growths in the inner regions of crystals. However, the decorating particles form regular patterns, particularly on the copolymer crystal surfaces. In the case of solution-cast thin films, deposition of metallic particles occurs along lamellar crystal edges or where a fibrillar morphology occurs. The nucleation density of metallic particles depends on the copolymer composition and increases with increasing crystallinity of the samples. Probable models are suggested to account for this surface texture. The propensity for screw-dislocations crystal growths on the crystal surfaces decreases as the noncrystallizable DMS component increases.

ISSN:0022-2348    

DOI:10.1080/00222347408215163

出版商:Taylor & Francis Group    

年代:1974

数据来源: Taylor

摘要:

The effects of Arochlor 5442 (a chlorinated terphenyl) dibutyl phthalate, butyl stearate, a high molecular weight polyester plasticizer, acetone, methyl ethyl ketone, isopropyl alcohol, carbon tetrachloride, and water on the dynamic mechanical properties of polycarbonate have been studied. Results show that all liquids except water and carbon tetrachloride suppress the low-temperature β-transition loss peak and shift the temperature of maximum loss to lower temperatures. Water causes the β loss peak to broaden and be more asymmetrical. Nonpolar carbon tetrachloride causes a splitting of the β loss peak which may indicate a selective suppression of local segmental motions in nonpolar regions of the polymer. Polycarbonate liquid blends may also exhibit additional loss peaks at temperatures above or below the β loss peak. Volume losses are observed for all blends whose specific volume has been measured.

ISSN:0022-2348    

DOI:10.1080/00222347408215164

出版商:Taylor & Francis Group    

年代:1974

数据来源: Taylor

摘要:

Least squares programs were used to evaluate the correlation between recent experimental results and theoretical, semi-theoretical, or empirical relations between liquid viscosity and temperature. It was found that none of these describes the experimental dependence in the whole range of temperature. A new dependence based on the free volume concept and cell-hole liquid theory is proposed. The theory permits linearization of the viscosity-temperature data in the range (Ti, Ti+1), where the Ti's are the liquid-liquid transition temperatures. It was demonstrated that these transitions, both in small molecular and in polymeric liquids, occur in discrete steps: Ti= aiTg, where Tgis the glass transition temperature and the ai's are numerical parameters. Not all the transitions Tiwere apparent in all liquids. Transition T2= 1.26Tgwas observed for most polymeric liquids. On the basis of thermal analysis it was demonstrated that the Ti's coincide with the temperatures at which small changes in the apparent specific heat were detected. These temperatures can be assigned to the maximum rate of crystallization and melting temperatures of the metastable and stable crystalline forms.

ISSN:0022-2348    

DOI:10.1080/00222347408215165

出版商:Taylor & Francis Group    

年代:1974

数据来源: Taylor

摘要:

Significant differences have been observed in the steady-state permeation of gases through symmetric and asymmetric (Loeb-type) cellulose acetate membranes. The studies were made with O2, N2, Ar, Kr, Xe, and CO2in the temperature range from -5 to 85°C and at subatmospheric pressures. The differences in permeation behavior may reflect structural differences between the symmetric membranes and the dense surface layer (“skin”) of the asymmetric membranes. The overall mechanism of gas permeation through the symmetric membranes appears to be one of “solution-diffusion,” similar to that observed with many other nonporous polymeric membranes. In the case of the asymmetric membranes, this mechanism is probably modified by the presence of micropores or other imperfections in the dense surface layer. Cellulose acetate exhibits two second-order transitions in the presence of the penetrant gases, one between 60 and 70°C and the other near 15°C. The transitions were observed with both types of membranes.

ISSN:0022-2348    

DOI:10.1080/00222347408215166

出版商:Taylor & Francis Group    

年代:1974

数据来源: Taylor

摘要:

The properties of water in various cellulose acetate (CA) membranes have been investigated by magnetic resonance techniques. It has been observed that the longitudinal (T1) relaxation time of17O in CA membranes containing H217O is considerably shorter (by a factor of 5 to 6) than the corresponding value in pure water. The T1values of23Na ions contained in these membranes were, however, shorter than the corresponding values observed in measurements of the aqueous mother solution by a factor smaller than 2. The EPR spectra of Mn2+and of Cr3+contained in CA membranes were practically identical to the spectra of the pure aqueous solution.

ISSN:0022-2348    

DOI:10.1080/00222347408215167

出版商:Taylor & Francis Group    

年代:1974

数据来源: Taylor