摘要:

This work is a part of a program aimed at understanding the response of polymer chains to external mechanical forces. The response may be nondestructive or destructive, and competition between the two types exists. Molecular dynamics was used to simulate systems of polymer chains on a computer. A constant strain was imposed, as in experimental stress relaxation tests. Free volume was varied, and its effect on relaxation rate and on bond breaking was studied. A crossover exists between the region in which the chain relaxation capability is sufficient and the region where the breaking of a small number of bonds leads to crack propagation. The results are significant for polymers, conventional polymer‐based composites, and liquid‐crystalline molecular composites.

ISSN:0148-6055    

DOI:10.1122/1.549868

出版商:The Society of Rheology    

年代:1986

数据来源: AIP

摘要:

A new constitutive equation has been developed for describing the nonlinear viscoelastic properties, including yield, of polymer solids. The nonlinear viscoelastic constitutive equation is the logical extension of the standard three‐dimensional linear viscoelastic constitutive equation to include the effects of deformation‐induced changes in the thermodynamic state of the polymer on the rate of viscoelastic relaxation. The rate of viscoelastic relaxation depends upon the fraction of unoccupied lattice sites (i.e., holes) in the Simha‐Somcynsky statistical thermodynamic equation of state, and the hole fraction is a known function of the temperature, pressure, and specific volume. The nonlinear viscoelastic yield and postyield behavior are a direct consequence of the deformation‐induced dilation. All material constants contained in the constitutive equation can be determined from independent elastic, linear viscoelastic, and pressure‐volume‐temperature measurements—there are no adjustable constants. The constitutive equation has been solved for model viscoelastic materials in uniaxial extension and shear deformations where the applied strain rate is constant. It is shown that an experimentally realizable “shear” deformation is only isochoric in the linear viscoelastic limit. Yield and postyield softening are predicted in both uniaxial extension and experimental shear, and the predicted yield stress exhibits the appropriate strain rate and temperature dependence. The effects of using various finite strains measures in the nonlinear viscoelastic constitutive equation are analyzed.

ISSN:0148-6055    

DOI:10.1122/1.549869

出版商:The Society of Rheology    

年代:1986

数据来源: AIP

摘要:

Simha's statistical thermodynamics theory, based on the cell/hole model, provides an excellent description of liquid properties in the full range of reduced variablesP̃,Ṽ,andT̃.In addition, it provides the occupied site fractionyas a function of these reduced variables. Recently, the theory was rigorously extended to homogeneous liquid mixtures. During the last few years, an attempt was made to correlate the zero‐shear viscosity η withY≡1/(1−y).TheYas a function of temperatureTand pressurePwas computed from Simha's theory forn‐paraffins (C5toC18), oligomers (polybutene‐1 and silicone oils), and polymers (poly(dimethyl siloxane), polystyrene, poly(methyl methacrylate), polyethylenes, polypropylene, etc.). It was found that the temperature‐ and pressure‐dependent zero‐shear viscosity η of these liquids is a function ofY(T,P).In the present work the method is being extended to mixtures ofn‐paraffins.

ISSN:0148-6055    

DOI:10.1122/1.549911

出版商:The Society of Rheology    

年代:1986

数据来源: AIP

摘要:

Experimental data from the literature on the pressure and temperature dependence of electrical conductivity in a poly(epoxidecosulfide) is analyzed. From the increase in resistivity (reciprocal of conductivity) with pressure, it is shown that electrical conduction, in this case, proceeds via an ionic diffusion process and should therefore be correlated with free volume. Using a recently proposed PVT equation of state to calculate free volume with the usual definition, fair correlation is found between log resistivity and reciprocal free volume fraction. The deviations from perfect correlation are not random, but vary in a systematic manner, showing that pressure dependence is not adequately described. Based on an observation by Utracki, much better correlation is found using a pressure‐reducing parameter in the equation of state which is larger than that used to fit the PVT data. This change is equivalent to changing the definition of free volume.

ISSN:0148-6055    

DOI:10.1122/1.549870

出版商:The Society of Rheology    

年代:1986

数据来源: AIP

摘要:

Free volume concepts in polymers, originally suggested by Bueche, have been modified and extended and now generate a good equation of state for polymers aboveTg.This paper is concerned with the extension of the concepts to describe gaseous diffusion. In order to do this, the notion ofactivated diffusionwas introduced. A gas molecule in a matrix can jump under three conditions. (1) A hole opens next to it (from segmental oscillations of the polymer) large enough to allow the molecule to jump (passive diffusion). (2) A hole opens which is smaller than the gas molecule, but the kinetic energy of the gas molecule allows it to open the hole and jump by compressing neighboring segments (activation). Once compressed, the segments cannot relax on the segmental oscillation time scale, and thus can assist the gas molecule to jump into other small holes (activated diffusion). The segments at the old site relax, but the segments around the new site are now compressed. This continues until a hole as large or larger than the gas molecule opens. The jump into such a hole allows all segments to relax (deactivation). This approach was successful in correlating diffusivity with activation energy for a wide variety of penetrants in polyethylene and poly(vinyl acetate). It also reproduced the non‐Arrhenius change in diffusivity with temperature for diffusion in rubber using the same constants. This paper gives the initial formulation for diffusion, but much work still needs to be done.

ISSN:0148-6055    

DOI:10.1122/1.549912

出版商:The Society of Rheology    

年代:1986

数据来源: AIP

摘要:

The Vogel‐Fulcher equation,ln τ∝H/R(T−T2),and the WLF equation,ln τ∝−C1(T−T0)/[C2+(T−T0)],can be expressed in the same form. They are known to fit well the relaxation data of liquids in equilibrium. Doolittle's free volume equation,ln τ∝1/f,and Adam‐Gibbs's entropy equation,ln τ∝C/RTS,can be reduced to the Vogel‐Fulcher equation with reasonable assumptions on the temperature dependence of the free volume fractionfand the configurational entropyS. However, in predicting the relaxation behavior in the nonequilibrium state, the Adam‐Gibbs equation predicts an Arrhenius‐type temperature dependence for a fixed structural parameterS, while the Doolittle equation predicts temperature‐independent behavior for a fixed structural parameterf. On the basis of experimental evidence, the Adam‐Gibbs equation is shown to be clearly a better theory than the Doolittle equation. Moreover, with the Adam‐Gibbs equation, it is shown that the kinetic parameters required to describe physical aging are the same as those necessary to describe dielectric relaxation behavior.

ISSN:0148-6055    

DOI:10.1122/1.549871

出版商:The Society of Rheology    

年代:1986

数据来源: AIP

摘要:

Given the convenience of scaling parameters leading to superimposed or master curves, we study scaling pressures for equilibrium and nonequilibrium properties. By relating those pressures to interparticle forces and pair radial distribution functions, we connect them to the average interparticle distanceR̄.We thus explain the experimental findings of several authors that the scaling pressures pertaining to nonequilibrium situations (stress relaxation, shear viscosity, electrical conductivity of polymers) are larger than those calculated from the pressure‐volume‐temperature relationships.

ISSN:0148-6055    

DOI:10.1122/1.549872

出版商:The Society of Rheology    

年代:1986

数据来源: AIP

ISSN:0148-6055    

DOI:10.1122/1.549873

出版商:The Society of Rheology    

年代:1986

数据来源: AIP