Michael-Joachim Brekner

Thermodynamic aspects of the glass-transition in the compatible blend poly(styrene)-poly(vinylmethylether)

SummaryIt is shown that composition dependence of glass-transition in the compatible poly(styrene)-poly(vinylmethylether) blend exhibits deviations from additivity rules derived in the supposition of continuity of the thermodynamic excess functions of mixing. Only the acceptance of an additional adjusting parameter which accounts for interaction, enables the interpretation of experimental Tg-data. This adjusting parameter is quite different for the blends of PVME with oligomeric and high moleculare PS, respectively.

Interactions in compatible polymer systems: 1. Viscoelasticity and glass transition of polystyrene-poly(vinylmethylether) blends

SummaryDynamic mechanical measurements on polystyrene — poly(vinylmethylether) blends are demonstrating that the relaxation processes in the blends are mainly connected with the motions of the poly(vinylmethylether) chain.Concerning the effect of mixing on topological properties of the blends, an increase of the polydispersity of the relaxation processes is detected in blends with high molecular weight polystyrene while low molecular weight polystyrene exerts an effect of dilution upon the relaxation of the high molecular poly(vinylmethylether) chains.From these measurements as well as from thermoanalytical data it results that the energetic interaction is more pronounced in the blends with oligomeric than with high molecular weight polystyrene. The glass transition temperature shows a larger deviation from additivity for blends with high molecular polystyrene than for those with oligomeric polystyrene.

Some remarks concerning the WLF-relation

SummaryThe limitations of the WLF-relation are discussed with respect to their inherent demands concerning universality. The problem of the reference temperature is discussed in some detail, and their constants are related to the temperature T∞ where the apparent activation energy of flow gets infinite, as well to the characteristic activation energy of viscous flow at infinitely high temperature, E∞.It is concluded that the use of experimentally derived temperature dependent apparent activation energy of flow, E(T), from the slopes of crossing isochrone and isotherm viscoelastic curves substitutes the WLF-relation favourably. Thus, the problematic choice of a reference temperature is avoided. The two kinetically specific constants, E∞ and T∞, may be admitted to characterize the energetic and entropic contribution, respectively, to the viscous flow. Experimental data are presented for some polymers.

Interrelations Between Shift Factors of Viscoelastic Functions and Apparent Activation Energy of Flow

SummaryThe applicability of an approach is tested experimentally, which interrelates the frequency as well as the temperature shift of isotherm and isochrone viscoelastic data with the apparent activation energy of the viscous flow. The procedure is based on the evaluation of the slopes of isotherms in the log viscoelastic function — log frequency plot and of isochrones in the log function — reciprocal temperature plot. Experimental data demonstrate that the approach applies over the entire temperature range from the high-temperature melt to the glass transition, where the WLF equation fails oftenly. The temperature range of variable activation energy turns out to be substantially smaller than supposed. Above Tg + 40 K constant activation energy suffices, suggesting that the range of validity of the EYRING model is quite larger than usually admitted.

Master surfaces of viscoelastic functions in the reciprocal temperature-frequency space

SummaryStarting from EYRINGs transition state theory it is demonstrated for systems obeying the superposition principle that the frequency shift factors of isotherm mastercurves of the viscoelastic functions as well as the temperature shift factors of the isochrone ones are unequivocally interrelated with the apparent activation energy of flow. The method for the evaluation of the activation energy from viscoelastic data is discussed, considering the moduli surfaces in the reciprocal temperature — frequency space. Experimental data prove the reliability of the approach. It is demonstrated that it is much more suitable to construct mastercurves as well as master surfaces in this way, because it is not necessary to apply measurements at low frequencies, which are unavoidable when the curves are constructed by empirical shift in the usual way.

Interrelations between relaxation distribution functions and apparent activation energy of flow

SummaryStarting from both the three-dimensional diagram of viscoelastic properties in the reciprocal temperature — log frequency — space and the time-temperature interrelation via apparent activation energy of flow, the possibility of theoretical interpretation of the isochronal behaviour is demonstrated, in analogy to the isothermal one. The determinative apparent activation energy of local flow is related with the relaxation spectra over the entire range of validity of the time-temperature superposition principle. Thus a theoretical background is offered for the method recommended before for the evaluation of the apparent activation energy of flow, which is applicable to calculate both isotherm and isochrone shift factors.

Theoretical approach to the temperature dependence of local flow in polymer systems

SummaryStarting with the concept of temperature dependent apparent activation energy of flow as a specific temperature function of the considered polymer system a general appraoch is presented, which includes both the Arrhenius and the WLF behaviour. It is demonstrated how local flow can be characterized directly from mechanical measurements via temperature dependent apparent activation energy of flow, using two parameters only, the apparent activation energy of flow extrapolated to infinite temperature, and the Vogel temperature as obtained by extrapolation to infinite activation energy.Examples are given in order to confirm the usefullness of these parameters for molecular interpretation of changes in the local flow mechanism as well as for clarification of the dependences between the polymer structure and the local flow process. As an unique effect it has been found that the energetic interaction of oligomeric polystyrene with poly(vinylmethylether) is much more pronounced than that with high molecular weight polystyrene. This findings may exhibit technical relevance.