摘要:

AbstractThe self‐consistent approach to free‐energy optimization is applied to regular star polymers in bad solvents (T<Θ); the Gaussian approximation and the Zimm‐Kilb description of Fourier modes are both warranted under these conditions leading to chain collapse. As currently done in this approach, the interatomic‐contact free energy is taken to be a single‐valued function of the mean‐square radius of gyration 〈S2〉, whence the universal plots of the mean‐square distances between two atoms and between the atoms and the center of mass are obtained as a function of the contraction ratio α 2S= 〈S2〉/〈S2〉0(the zero subscript denotes the unperturbed state atT= Θ) and of the degree of branching. The density as a function of the distance from the center of mass as well as the equilibrium structure factor are likewise evaluated. The decrease of αswith an increase of the undercooling (Θ −T) at a fixed molecular weight is sharper with a smaller number of arms, being sharpest with the linear chain. The relative fluctuation of the radius of gyration, expressed as (〈S4〉 − 〈S2〉2)/〈S2〉2, increases at first with the undercooling, showing a maximum close to the transition temperature which is more pronounced the smaller is the degree of branching, then decreases sharply at strong contration. The average location of the free ends of the collapsed molecule is close to, or even outside, the surface of the resulting globule, their mean‐square distance from the center of mass being 2 〈S2〉. Conversely, the mean‐square distance of the branch point from

ISSN:1022-1344    

DOI:10.1002/mats.1993.040020601

出版商:Hüthig&Wepf Verlag    

年代:1993

数据来源: WILEY

摘要:

AbstractA review is given on the use of the bend elastic chain model for the calculation of physical properties of polymers.

ISSN:1022-1344    

DOI:10.1002/mats.1993.040020602

出版商:Hüthig&Wepf Verlag    

年代:1993

数据来源: WILEY

摘要:

AbstractThe dynamic transition of nematics in (slow) shear from flow‐aligning to tumbling behaviour is here predicted by using the rigid rod‐like molecular model together with an extension of the Maier‐Saupe mean‐field potential. Such an extension, already available in the literature with reference to equilibrium data, includes interactions of rank higher than the basic quadrupolar one considered by Maier and Saupe. The theoretical predictions are then compared with the few existing sets of dynamic data. Although an acceptable qualitative agreement is found, the theory generally overpredicts the value of the relevant ratio α3/α2of Leslie coe

ISSN:1022-1344    

DOI:10.1002/mats.1993.040020603

出版商:Hüthig&Wepf Verlag    

年代:1993

数据来源: WILEY

摘要:

AbstractThe conformational distribution of unperturbed polyethylene (PE) has been studied utilizing the Monte Carlo approach. The results are in excellent agreement with experiments on the average dimensions of PE chains, as well as with the molecular scattering function obtained in mixtures of deuterated and hydrogenous PE at 400 K. Maps of the free energy as a function of two consecutive torsional angles confirm that an approximate description of the conformational distribution of PE can be given in terms of three rotational isomeric (RIS) states. The location of the states and the corresponding energy and entropy parameters, which can be separately evaluated from the Monte Carlo results, are compared with literature results obtained by internal energy calculations for butane and pentane.

ISSN:1022-1344    

DOI:10.1002/mats.1993.040020604

出版商:Hüthig&Wepf Verlag    

年代:1993

数据来源: WILEY

摘要:

AbstractLinear chain surfactants in a densely packed arrangement (such as alkane chains in lipid monolayers in the “uniform tilt” structures) are described by a crude coarse‐grained model where the endgroups grafted on the interface form a regular lattice and the chains are described by the bond fluctuation model with chains containingN= 4 effective monomers only. Square‐well interactions between the monomers are studied for both the attractive and repulsive case for three choices of the interaction range. None of these models exhibits a structure with uniform tilt. For attractive interactions the last bond has a strong tendency to fold back thus leading to a very high density close to the interface. Only when an intrachain‐potential favoring stiff chain configurations also is included one can obtain configurations with uniform tilt order. Although related models (with much longer chain lengths and lower grafting densities) are very useful for the study of polymer brushes, the present case of very short chains in a high‐density state clearly is plagued by various lattice artefacts and it is concluded that for modelling linear chain surfactants one should use an off‐lattice model even on a coarse

ISSN:1022-1344    

DOI:10.1002/mats.1993.040020605

出版商:Hüthig&Wepf Verlag    

年代:1993

数据来源: WILEY

摘要:

AbstractPolyolefins made with heterogeneous Ziegler‐Natta catalysts contain multiple components as a result of the different catalytic sites present in the catalyst. If a series of similar catalysts are made where only one experimental variable is varied, then a family of similar polymers can be prepared where the polymer microstructure (as studied by NMR) reflects the effects of that variable. An analytical methodology is developed in this work that permits a systematic analysis to be carried out on the NMR data of such polymers. Thus, from the NMR spectra one can deduce the influence of an isolated variable on the polymer components and (indirectly) on the catalytic sites. Illustrated in this work are the effects of Lewis bases on the catalytic sites, catalyst pretreatment with a higher olefin, and catalyst grinding. The approach can be applied to study both homopolymer tacticity and copolymer sequence

ISSN:1022-1344    

DOI:10.1002/mats.1993.040020606

出版商:Hüthig&Wepf Verlag    

年代:1993

数据来源: WILEY

摘要:

AbstractThe number of isomersir,tfor stacked polymeric phthalocyanines in which each phthalocyanine ring is tetrasubstituted or octasubstituted with two different substituents (each benzene moiety substituted with both substitutents) is determined to be\documentclass{article}\pagestyle{empty}\begin{document}$$ \mathop {\lim }\limits_{x \to \infty } i_{{\rm r,t}} = 2^m /16 $$\end{document}wheremis the number of substituted phthalonitrile molecules incorporated into the polymer andxis the number of phthalocyanine macrocycles (degree of polymerization;m= 4x). Specifically, for a stacked polymeric phthalocyanine withx= 2; there exist 298 isomers. The stacked polymeric phthalocyanines with substituted phthalocyanine rings possess different symmetry groups (D2h, D2d, C4, C2Vor CSE as sole symmetry operation). The results are valid for stacking of other macrocycles similar to phthalocyanine such as prophyrins.

ISSN:1022-1344    

DOI:10.1002/mats.1993.040020607

出版商:Hüthig&Wepf Verlag    

年代:1993

数据来源: WILEY

摘要:

AbstractThe main factors determining molar mass characteristics of copolymers formed in nonterminating copolymerization under the conditions of chain transfer to solvent are studied theoretically. The dependences of the mean polymerization degrees on conversion and monomer feed composition for various values of reactivity ratios are obtained. The results obtained greately differ from those for homopolymerization. This is explained by the contribution of cross‐propagation reactions. In particular, it is shown that at ≪ and ≪ the azeotropic copolymerization proceeds like the living one even if homopolymerization of each monomer is accompained by extensive chain transfer to so

ISSN:1022-1344    

DOI:10.1002/mats.1993.040020608

出版商:Hüthig&Wepf Verlag    

年代:1993

数据来源: WILEY