摘要:

AbstractThe mean‐square radius of gyration for polysiloxanes has been derived according to the exact definition. Taking account of the examples of symmetrically substituted poly(dimethylsiloxane) and unsymmetrically substituted poly(methylphenylsiloxane), we find that the dependence of 〈S2〉 on the molecular weight follows the general formula 〈S2〉 =aMbwithb= 1 ± 0.016, which is analogous to the theoretical outcomes for vinyl or vinylidene polymers even though the skeletal bone of polysiloxanes consists of alternating heteroatoms. A numerical comparison of the rigorous expression of the mean‐square radius of gyration given in this paper with that reported by Flory shows that the difference is obvious for low‐molecular‐weight polymer and it gradually declines with increasing degree o

ISSN:1022-1344    

DOI:10.1002/mats.1997.040060111

出版商:Hüthig&Wepf Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractWe investigate the effect of shear and uniaxial extensional flows on the rate of the cyclization reaction in polymers. We find that for such anintramoleculardiffusion‐controlled reaction the flow always reduces the reaction rate, in contrast to the previously studied case ofintermolecularreactions, where linear flows enhance the reaction rat

ISSN:1022-1344    

DOI:10.1002/mats.1997.040060112

出版商:Hüthig&Wepf Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractOur past paper reported that the postyielding deformation of crystalline polymers such as polyethylene (PE), poly(oxymethylene) (POM), poly(propylene) (PP) and nylon 6 (Ny 6) was expressed by master curves with a characteristic constant for each polymer when normalized true stress and true strain are plotted in both logarithmic graphs and suitably shifted. For a molecular understanding of the postyielding process, we present a tie molecule model which assumes that the tie molecules are pulled out from the lamellar fragment at a constant number of tie molecules. The limit in applicability of the model is in the final stage of fiber formation. Fundamental equations of the model are solved to give a critical point at which all the molecular parameters can be uniquely determined from the characteristic constant for each polymer. At the critical point, the tie molecule length monotonously increases at a fixed number of tie molecules. The limit of the tie molecule length increases in the order of PE, POM, PP and Ny 6. By using an empirical relationship between the area fraction of tie molecules and the degree of crystallinity of these polymers, it is found that the order of area fraction of the tie molecules in Ny 6, PP, POM and PE is entirely reversed to the order of weight fraction of the tie molecules of the above polymers.

ISSN:1022-1344    

DOI:10.1002/mats.1997.040060113

出版商:Hüthig&Wepf Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractThe phase behavior and scattering properties of polymer blends, copolymers and networks in solution and in bulk are examined. The theoretical framework used here is based upon the extension of the random phase approximation to polymers proposed by de Gennes and its application to chains with various architectures. The case of blends containing stiff chains is considered and the effect of nematic interaction on the phase behavior and scattering properties is discussed. The compressibility problem is reviewed in connection with the free volume theory models. The coupling between density and composition fluctuations is examined together with the effects of pressure on the structural and thermodynamic properties of blends. The dynamics of copolymers are also examined in the light of the new developments both from the theoretical and experimental fronts. The extent to which the chain architecture affects phase behavior, static scattering and dynamic behavior is discussed. A particular emphasis is put on the case of cyclichomopolymers and copolymers. Free chains in a network and crosslinked blends are also a subject of particular interest in this paper. The interplay between macrophase and microphase transitions due to the crosslinks and the electrostatic forces for charged polymers is also considered. De Gennes' analogy betwen the elastic restoring forces in the network and the Coulomb forces in the dielectric medium is generalized by including the screening phenomenon. This generalization is required in order to account for the experimental observations in the low momentum transfer range. Following Briber et al., it is argued that the new screening length can be related to the initial fluctuations at the temperature of crosslinking.

ISSN:1022-1344    

DOI:10.1002/mats.1997.040060114

出版商:Hüthig&Wepf Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractThe molecular geometries of neutral and dicationic oligomers comprising 7, 10, 13 and 16 rings of thiophene, furan and pyrrole were determined using quantum mechanical calculations. The usual benzenoid structure was characterized for neutral oligomers, whereas a quinoid structural defect which extends over six rings was found for dications. Analysis of the transition region, where the quinoid structure changes to the benzenoid one, revealed structural differences between thiophene, furan and pyrrole derivatives.

ISSN:1022-1344    

DOI:10.1002/mats.1997.040060115

出版商:Hüthig&Wepf Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractA theory describing slow macromolecular reaction and interdiffuion in a compatible polymer blend is suggested based on the linear non‐equilibrium thermodynamic principles. A simple model system is considered. In a blend consisting initially of homopolymers A and B, the transformation A → B proceeds with the B units accelerating the reaction. A system of diffusive‐reaction equations for relevant macroscopic variables is derived. The randomness of the reacting chains' structure gives rise to a new interdiffusion mode in addition to the reacting polymer‐homopolymer B interdiffusion. Numerical calculations reveal that the diffusive intermixing of reacting chains of different composition may significantly affect the reaction rate and the local compositional heterogeneity as well. It is possible to discriminate the fast‐ and the slow‐mode theories of interdiffusion using reaction kinetics data. Under certain conditions, the reaction may proceed in a non‐trivial autowav

ISSN:1022-1344    

DOI:10.1002/mats.1997.040060116

出版商:Hüthig&Wepf Verlag    

年代:1997

数据来源: WILEY

摘要:

AbstractWe have explored the performance of a simulation model for Gaussian chains at different concentrations in a good solvent. The Gaussian statistics for the distances between contiguous beads in the model is directly implemented in the individual moves of a Monte Carlo algorithm. When the results of conformational properties for the Gaussian model are compared with those provided by a freely jointed model in the same conditions, significant differences arise at finite concentrations. The modeled Gaussian chain yields incorrect results for the quadratic average dimensions 〈R2〉 and 〈S2〉 at high concentrations, but correctly reproduces the results for the scaled end‐to‐end distance distribution function at any concentration, showing the effects of the screening of excluded volume when concentration increases. The reason for the wrong behavior of the simulated Gaussian model comes from a strong distortion of the “bond distance” distribution as a result of the concentration increase. We conclude that this model can only be safely applied to infinitely d

ISSN:1022-1344    

DOI:10.1002/mats.1997.040060117

出版商:Hüthig&Wepf Verlag    

年代:1997

数据来源: WILEY

ISSN:1022-1344    

DOI:10.1002/mats.1997.040060118

出版商:Hüthig&Wepf Verlag    

年代:1997

数据来源: WILEY