Ivan Havlíček

Phase transition in swollen gels: 7. Effect of charge concentration on the temperature collapse of poly(N,N-diethylacrylamide) networks in water

The swelling and mechanical behaviour of gels of the copolymer of diethylacrylamide (DEAAm) with a small quantity of sodium methacrylate (mole fraction xMNa = 0–0.067) swollen in water was investigated in the temperature range 1–80°C. For networks in the range xMNa > 0.0095 phase transition was observed; both the critical transition temperature Tc,c and the extent of the collapse Δc increase with increasing xMNa. The formation of associations in the collapsed state contributes to the overall extent of the transition; these structures give rise to stable turbid gels at elevated temperatures. Evidence for the formation of ‘associated’ structures is also supported by the observed independence of cloud temperature of the concentration of DEAAm, c, in the polymerization DEAAm-water mixture without the crosslinking agent in the range c = 0.5–80 vol%. While PDEAAm solutions are formed in the range c < 4.5–6 vol%, physical gels arise at higher concentrations.

Diffusion controlled kinetics of crosslinking

Reactions of polymer formation and crosslinking become diffusion controlled when, during the reaction, the increasing glass transition temperature Tg comes close to the reaction temperature, the reaction still goes on below Tg but the reaction rate decreases steeply. A theory is presented relating the apparent rate constant to the difference between the reaction temperature and Tg based on the free volume or the Adam-Gibbs theory of glass transition. The theory is correlated with experiments on curing of diglycidyl ether of bisphenol A with 1,3-propanediamine. The implications for formation of protective films chemically crosslinked are discussed. The presence of a solvent and its evaporation affects the reaction rate through a change in concentrations of reactants as well as in Tg.

Effect of pressure on the cooperative relaxation properties and glass transition temperature of amorphous polymers

Abstract The Adam and Gibbs (AG) theory which describes the cooperative relaxation phenomena in the main transition region in terms of the configurational entropy of the system has been extended to describe also the dependence of both the relaxation times and the glass transition temperature Tg on the external pressure P. The resulting relations contain quantities accessible both by thermo-dynamic (discontinuities in the isobaric heat capacity and in the thermal expansion coefficient) and by viscoelastic (parameters of the temperature dependence of relaxation times) measurements. Theoretical relations adequately describe pressure experimental data on polystyrene, poly(vinyl acetate), and poly(vinyl chloride). The expression dTg /dP derived from the AG theory is at variance with Ehrenfests relation; the connection between the AG theory and the order parameter description of the glass transition is discussed.

The effect of sequence distribution on glass transition in the theory of the cooperative Behaviour of random copolymers

SummaryThe description of the cooperative behaviour of polymers in the glass transition region according to ADAM and GIBBS (1965) has been extended to comprise the dependence of glass transition temperature, Tg, on the chemical composition of random copolymers if the distribution of dyads of comonomer units is included. The compositional dependence of Tg is expressed in terms of (i) differences between the isobaric heat capacities in the liquid and glassy states, and of (ii) the equilibrium T2 temperatures of the parent homopolymers and of the alternating copolymer. Validity of this theory was verified by comparison with data on styrene/methyl methacrylate and styrene/n-octyl methacrylate copolymers; the results indicate that the relations derived agree quantitatively with the experiment similarly to the equations suggested earlier by JOHNSTON (1976) and BARTON (1970). Moreover, these relations are based on a physically better justified approach to the glass transition phenomenon.

The effect of composition on cooperative relaxation properties of random copolymers

SummaryThe theory of ADAM and GIBBS (1965) has been extended to include a description of the composition dependences of relaxation times and of the glass transition temperature of a random copolymer using the known thermodynamic and viscoelastic parameters of homopolymers. Comparison with a number of copolymer systems shows that theoretical equations adequately correspond to experimental data if the sequence distribution effect is not operative.

The thermorheological complexity of photoelastic behaviour of 2-hydroxyethyl methacrylate — dodecyl methacrylate copolymers

The viscoelastic photoelastic behaviour of networks of 2-hydroxyethyl methacrylate — dodecyl methacrylate (DMA) copolymers in the main transition and rubberlike region was investigated. With increasing DMA content, photoelastic functions are quickly shifted to lower temperatures or shorter times; a detailed course of the functions suggests heterogeneity of the copolymers. Due to the existence of long side chains, optical function of all samples change the sign from positive to negative with increasing temperature. While the temperature dependences of the moduli of copolymers can be described by the two-phase Takayanagi model, the temperature dependences of optical functions cannot be described by using this model. It has been found, however, that the tempeature and time dependences of photoelastic functions can be described semiquantitatively by a three-phase model with a hypothetical statistical copolymer as the third component. The high values of the volume fraction of the hypothetical statistical copolymer found for the individual samples, suggest a considerable miscibility and a strong influence of the interphase boundary on the photoelastic behaviour of the copolymers.

A simple phenomenological model for the order parameter description of glass transition

SummaryGlass transition has been investigated as a freezing-in process by using a simple form of the Gibbs free energy with two order parameters. Two cases are examined: that of a single freezing-in line (transition line) and that involving two different lines on which the individual order parameter freezein gradually. The latter case is a simple, but adequate description of the diffusivity of glass transition. The effect is discussed of the mutual position of freezing-in lines and of the Prigogine-Defay ratio on the dependence of the behaviour of material on the thermodynamic history of glass transition.

The connection between the measured glass transition temperature and the time dependent order parameter description of the glass transition

SummaryThe time dependent order parameter concept of the glass transition is used in a description of the relaxation behaviour of the glassforming material. The connection between the macroscopic theory and the measured glass transition temperature has been determined. It has been found that in a case when the Prigogine-Defay ratio is greater than unity, different values of the relaxation functions of entropy and volume obtained by different experiments (e.g., isobaric dilatometry and isothermal compression) should be used in order to obtain the identical Tg. The Prigogine-Defay ratio is found to be a decreasing function of the observation time reaching values between unity in the limit of infinite time and a limit which corresponds to extremely short experiments.