摘要:

AbstractA theoretical analysis of the conformational energies of poly(dichlorophosphazene) (PDCP) is presented. The results indicate that the bond pair PNP possesses a considerable conformational freedom, whereas the bond pair NPN is relatively rigid. This difference explains the low glass transition temperaturesTgand large end‐to‐end distances measured for polyphosphazenes. A statistical model containing four rotational isomers (ie.,trans,gauche,cis, and negativegauche) is developed and used to calculate unperturbed dimensions and dipole moments of PDCP. The results, obtained at 25°C withn= 400 skeletal bonds (200 repeating units), wereCn= 〈r2〉0/nl2= 13.5;CT= 103d(ln〈r2〉0)/dT= −3.0 K−1;Dn= 〈μ2〉/nμ 02= 0.35;DT= 103d(ln〈μ2〉)/dT= −3.4 K−1. All the calculated magnitudes are extremely sensitive to the energyEσthat controls the statistical weights of the conformations tg, tc, tg−, gt, ct, and g−t relative to tt for the bond pair PNP. A qualita

ISSN:0887-6266    

DOI:10.1002/polb.1987.090250801

出版商:John Wiley&Sons, Inc.    

年代:1987

数据来源: WILEY

摘要:

AbstractOligomers end‐capped by groups capable of thermally induced polymerization have found wide applications in advanced structural composites and adhesives. The styrene‐terminated polysulfone reactive oligomers described in this paper represent an example of such thermosetting systems. The advantage of these compounds, over high molecular weight polysulfone resin, lies in their easier processing and improved solvent resistance. This paper addresses the properties of the neat oligomer as a function of initial molecular weight and those of the resulting cured resin as well as a complete characterization of the curing reaction. Furthermore, a comparison will be made between these oligomers and the better known acetylene‐terminated sulfone olig

ISSN:0887-6266    

DOI:10.1002/polb.1987.090250802

出版商:John Wiley&Sons, Inc.    

年代:1987

数据来源: WILEY

摘要:

AbstractThe thermal properties of solution‐prepared blends of poly(butylene terephthalate) (PBT) and liquid crystalline poly(biphenyl‐4,4′‐ylene sebacate) (PB8) have been studied by differential scanning calorimetry (DSC). The smectic‐to‐isotropic (s → i) transition of the mesomorphic component is observed at temperatures slightly increasing with the PB8 content, in the 270–280°C range; on cooling, the mesophase formation (i → s transition) takes place at temperatures that decrease markedly with decreasing PB8 content. The temperatures of the crystal‐to‐smectic and smectic‐to‐crystal transitions of PB8 are almost invariant with blend composition. The melting and crystallization temperatures of the PBT phase decrease on increasing the content of the liquid crystalline component.These results, together with those of isothermal calorimetry of both the crystallization of the PBT component and the mesophase formation of the PB8 component of the blends, indicate that the two polymers are not immiscible in the isotropic state. In this latter state, however, the two polyesters undergo transesterification, which can be followed through changes in the DSC scans. The effect of the thermal history on the properties of the blends has been studied with particular attention. Both the dynamic and the isothermal calorimetric measurements provide evidence of an increase of the degree of crystallinity of PBT on addition of the liquid crystalline component up

ISSN:0887-6266    

DOI:10.1002/polb.1987.090250803

出版商:John Wiley&Sons, Inc.    

年代:1987

数据来源: WILEY

摘要:

AbstractA method is outlined for the simulation of the scattering pattern from systems of oriented fibrils using the radial distribution function of a hard‐disc fluid obtained under the Percus–Yevick approximation. In this manner both the diameter and the volume fraction of the fibrils may be estimated directly from the scattering pattern. The effect of polydispersity in the distribution of fibril diameters on the position of the scattering peak is discussed, suggesting that an estimate of the volume fraction from the peak position alone, such as in a modified Bragg's law, may be inaccur

ISSN:0887-6266    

DOI:10.1002/polb.1987.090250804

出版商:John Wiley&Sons, Inc.    

年代:1987

数据来源: WILEY

摘要:

AbstractThe conformational response of an associating‐type random coil macromolecule in solution was investigated utilizing an opposing jet device. This device, capable of generating a well‐defined elongational flow field, is quite useful for probing intra‐ and intermolecular interactions of lightly sulfonated polystyrene ionomers in both nonpolar and polar solvent systems. Below a critical concentration in nonpolar media, such ionomers qualitatively follow trends predicted by dilute solution theory, although intramolecular ionic associations markedly increase the critical elongational shear rate. With further increases in concentration, the extensional behavior is determined by the initial formation of relatively strong intermolecular associations. At even higher polymer concentrations, a third regime is observed where the conformational relaxation process becomes even more facile. On the contrary, in a polar solvent, the conformational relaxation process is markedly enhanced (i.e., critical elongational shear rate is reduced) due to the polyelectrolyte effect, i.e., dissociation of a significant level of the counterions. The effect of this dissociation process influences the relaxation process over the entire concentration region examined. These findings are compared directly with solution rheology, where in low polarity solvents the reduced viscosity is markedly diminished by ion pair‐type interactions, and in more polar environments the reduced viscosity is enhanced due to the dissociation of the counterions from the vicinity of the chain b

ISSN:0887-6266    

DOI:10.1002/polb.1987.090250805

出版商:John Wiley&Sons, Inc.    

年代:1987

数据来源: WILEY

摘要:

AbstractWe found 10 miscible pairs of dissimilar polymers that rendered birefringence‐free polymer blends. Each pair consists of a positive birefringence polymer and a negative one. Zero birefringence in the blend is attained as the result of compensation of positive and negative contributions to the overall birefringence. This concept was confirmed by characterizing the chain orientation of individual chains in the blend using polarized Fourier‐transform infrared spectroscopy and by estimating theoretically the intrinsic optical anisotropy of the chain molecu

ISSN:0887-6266    

DOI:10.1002/polb.1987.090250806

出版商:John Wiley&Sons, Inc.    

年代:1987

数据来源: WILEY

摘要:

AbstractThe partitioning of methyl and vinyl ends between the crystalline and noncrystalline regions of polyethylene has been investigated using13C‐NMR in the solid state. The polyethylene samples, which were crystallized from the melt, varied in molecular weight, polydispersity, crystallization rate, and comonomer content. For the limited set of samples considered, the ratio of crystal to overall end concentration is independent of those variables. This ratio takes the values of 0.75 and 0.60 for the methyl and vinyl ends, respectively. When the crystalline fraction of these samples is taken into account, 50–75% of the total saturated ends and 42–63% of the total vinyls reside in the crystal. For an ethylene/propylene copolymer, 21–27% of the methyl branches were determined to be in the crystal. This level of incorporation puts methyl branches in a position intermediate between chain ends and ethyl b

ISSN:0887-6266    

DOI:10.1002/polb.1987.090250807

出版商:John Wiley&Sons, Inc.    

年代:1987

数据来源: WILEY

摘要:

AbstractResults of crystallization kinetics study and x‐ray diffraction are in good agreement with our previous rheological and mechanical results. Poly(arylene ether ketones), which are modified by sulfur at concentrations below approximately 2% by weight, have higher crystallization rates owing to the decrease in molecular weight caused by chain scission and/or the presence of sulfur that acts as a nucleating agent. However, all the observed differences in mechanical properties can be rationalized only by the nucleation effect of the sulfur. Upon aging of the sulfur‐modified samples, cross linking occurs, leading to decreases in crystallization ra

ISSN:0887-6266    

DOI:10.1002/polb.1987.090250808

出版商:John Wiley&Sons, Inc.    

年代:1987

数据来源: WILEY

摘要:

AbstractCreep and creep rupture of PMMA at high stresses have been characterized and found to be relatable by use of reduced variables. It is shown that when the creep compliances can be correlated by a superposition principle for which the vertical shift is the ratio of the applied stress to a reference stress and when strain at failure is a constant, a commonly used failure criterion (that the product of the strain rate at failure and the time to failure is constant) becomes valid. The reduced variables approach is found to apply to two greatly different thermal histories. Consistent with the concept of physical aging, the response of a quenched sample is simply shifted along the log time axis to shorter times relative to the response of the aged sample.

ISSN:0887-6266    

DOI:10.1002/polb.1987.090250809

出版商:John Wiley&Sons, Inc.    

年代:1987

数据来源: WILEY

摘要:

AbstractLow‐angle electron diffraction (LAED) was used to study the microstructure of crazes produced at different temperaturesTand strain rates in thin films of monodisperse polystyrene (PS). At a slow strain rate of 4.1 × 10−6s−1both the fibril diameterDand the fibril spacingD0of crazes in 1800k molecular weight PS remained constant with temperature up toT≈ 70°C and then sharply increased asTapproachesTg. At a higher strain rate of ∼ 10−2s−1, bothDandD0increase only slightly withT. The values ofDandD0over a range of temperature are in very good agreement with those values obtained in bulk samples using small‐angle x‐ray scattering. The crazing stress was measured as a function of temperature in the thin films of the 1800k molecular weight PS strained at the same slow strain rate used for the LAED measurements. These measurements were analyzed using a simple model of craze growth to reveal the temperature and strain rate dependence of the craze surface energy Γ. At room temperature Γ ≈ 0.076 J/m2(versus Γ ≈ 0.087 J/m2predicted) and was observed to remain constant up toT≈ 70°C and then decrease by approximately a factor of two atT= 90°C. This decrease in Γ is believed to result from chain disentanglement to form fibril surfaces at sufficiently high temperatures and occurs in the same temperature range in which the craze fibril extension r

ISSN:0887-6266    

DOI:10.1002/polb.1987.090250810

出版商:John Wiley&Sons, Inc.    

年代:1987

数据来源: WILEY