ISSN:0022-2348    

DOI:10.1080/00222348008243718

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

ISSN:0022-2348    

DOI:10.1080/00222348008243719

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

The behavior of polymers in solution depends on both temperature and concentration. At least four different regions of the concentration-temperature plane exist in which the physical properties are fundamentally different. These regions are known as the dilute good solvent, theta solvent, semidilute, and concentrated regions. In this investigation, Monte Carlo simulations were performed in order to examine how properties change in going from one region to another. Two series of simulations were performed. In the first series, properties were studied as a function of concentration so that crossover from dilute, to semidilute, and then to concentrated was obtained. In the concentrated or bulk region, it was found that the second and fourth moments of the end-to-end distance were characteristic of ideal chains (without excluded volume), consistent with neutron scattering results. In the semidilute region, the concentration dependence of the mean square end-to-end distance was not in agreement with scaling theory. In the second series of simulations, the temperature was changed for an isolated chain (zero concentration limit), so that crossover from good solvent to theta solvent behavior was obtained. Over the chain length range studies (10–300), no evidence was seen for the existence of “thermal blobs.” In addition, expansion of the average internal conformation over the expected result was observed and found to be increasingly important as the temperature increases from the theta temperature.

ISSN:0022-2348    

DOI:10.1080/00222348008243720

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

The compatibility of polymer mixtures is discussed from a thermodynamic standpoint. Deviations from the simple lattice theory (Flory-Huggins) show up sensitively in phase diagrams. Possible molecular origins for such deviations are discussed. Experimental techniques that have recently been developed to deal with highly viscous mixtures are described.

ISSN:0022-2348    

DOI:10.1080/00222348008243721

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

The methodology of a molecular theory encompassing the three states of melt, crystal and glass, and results are reviewed. The distinctions between monomeric-type and chain molecular ensembles reside in the obvious geometric restraints and in a dynamic factor, i.e., the spectrum of “soft” and “hard” modes in a flexible chain. The introduction of a flexibility parameter which is to be independent of temperature and pressure, consistent with the earlier proposal of Prigogine et al. reduces the polymer to a monomer problem. Here cell theory, intended originally by Lennard-Jones and Devonshire as a description of the liquid state, yields the equation of state for the crystal. The distinction between the two states resides in a vacancy fraction in the quasilattice which serves as a structural disorder function. At equilibrium it is derived by the minimization of a configurational free energy. The quantitative success of the liquid state theory is briefly illustrated by comparisons with low and high pressure data on oligomers over an extended temperature range, analogous results for high polymers having been discussed in detail previously. Regarding the description of the glassy state under quasi-equilibrium conditions, modifications of the equilibrium theory at low and high temperatures are explored. In the former, a freeze-in of the structure function, i.e., a cell theory description, yields a satisfactory thermal expansion up to about 50–70°K. Above this range not only a relaxation of the minimum restraint on the structure function, but moreover a coupling between the free volume fraction and a variable flexibility ratio is to be explored.

ISSN:0022-2348    

DOI:10.1080/00222348008243722

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

A quantum mechanical version of the cell theory including approximately the effect of the first anharmonic term in the Hamiltonian was developed earlier. On the basis of this theory it has been found possible to explain the P-V-T behavior of several polymeric crystals sufficiently below their melting temperature. In this paper we have not only refined the theoretical procedure but have also taken account of the next higher order anharmonic term in the Hamiltonian. The comparison of the resulting equation of state with experimental results for polyethylene shows considerable extension of its range of validity both with respect to temperature and pressure as compared to the earlier case.

ISSN:0022-2348    

DOI:10.1080/00222348008243723

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

In previous work [I], the relationship between the conformational entropy Sc, associated with rotational isomerization about main-chain bonds, and T0, the constant in the Vogel equation for polymer liquid mobility, was derived for a three-state rotational model with independent bond rotations. The present study takes into account the exclusion of transitions between gauche states of opposite sign (the “pentane interference”).

ISSN:0022-2348    

DOI:10.1080/00222348008243724

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

It is assumed that a real liquid, composed of simple molecules, behaves thermodynamically like a hypothetical liquid in which the contacts between neighboring molecules (at a given temperature) all have the same “contact energy” and the same “contact distance.” Intramolecular energies and the energies of interaction between noncontacting molecules are neglected. The molal energy is related to the contact energy, the contact distance, and the (average) contact number by an equation in which the attraction energy is of the London type and the repulsion energy is of the exponential Born-Mayer-Huggins form. The (four constant and one temperature dependent) parameters in the assumed equation have been evaluated for benzene from experimental measurements (from the literature) of the enthalpy of vaporization and the density. Using entropy data from the literature, Gibbs energies and other properties derivable therefrom can be calculated. The calculated results are for the whole liquid range of temperature. Compressibility data from the literature can be used to calculate the parameters and properties at higher pressures.

ISSN:0022-2348    

DOI:10.1080/00222348008243725

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

After a short discussion of the nature of the glass transition, a concept of ordering parameters is given which allows a thermodynamic treatment of the glass transition and the glassy state. PVT-measurements on atactic polystyrene show that the Ehrenfest equations which describe a thermodynamic transformation of the second order are not applicable to the glass transition. The introduction of one ordering parameter ζ in addition to the conventional variables T and P is sufficient to describe the behavior of PS in the glassy state nearly quantitatively. The Ehrenfest equations must be replaced by other equations which in the simplest case contain only one ζ parameter (freezing-in model). It will be shown that the nonvalidity of the Ehrenfest equations results in a path dependence of the thermodynamic properties in the glassy state. Recent PVT measurements on a polymer liquid crystal show that the physical properties in the nematic phase do not depend on the path and are not time-dependent. At lower temperatures a glass transition of the nematic phase will be observed which can only be described by the assumption of at least two ζ parameters according to two different states of order in the polymer liquid crystal. The pressure dependence of the clearing temperature obeys the Clausius-Clapeyron equation.

ISSN:0022-2348    

DOI:10.1080/00222348008243726

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor

摘要:

Gordon has shown that the Landau theory of second-order phase transitions when applied to the glass transition provides a plausible explanation of the apparent coincidence of the temperature Toof viscosity divergence and the T2of the Kauzmann paradox. It can be shown however that two current statistical mechanical theories of the transition, the Gibbs-DiMarzio theory and one proposed by Goldstein which takes into account nonconfigurational contributions to the entropy, do not follow the Landau theory. We are thus presented with a dilemma: if the Landau theory is applicable to the glass transition, neither of the two statistical mechanical theories can be, and vice versa.

ISSN:0022-2348    

DOI:10.1080/00222348008243727

出版商:Taylor & Francis Group    

年代:1980

数据来源: Taylor