Ján Biroš

Dependence of the glass transition temperature of poly (methyl methacrylates) on their tacticity

The specific isobaric heat capacities of poly (methylmethacrylates) (PMMA) having various tacticities were measured by the DSC method within a broad range of temperatures including the glass transition. Glasses with uniform thermal history were used in the measurements and the data were treated by employing a procedure which provided the “thermodynamic” Tg independent of the experimental conditions. The semiquantitative validity of Boyers empirical relationTg×Δcp=const. was confirmed; also it was found that within the limits of experimental accuracy the cp,g values at 298 K andCp,l values at 400 K are independent of the tacticity of the sample.Using the data thus measured and linearized equations representing the dependence ofTg on the content of iso-, syndio- and heterotriads, the Tg values of pure isotactic PMMA and pure syndiotactic PMMA were found to be respectively 315 K and 397 K.

The kinetics of model reactions of curing epoxy resins with amines

Abstract The isothermal course of the reaction of phenylglycidyl ether and N,N-methylglycidylaniline with dibutyl amine at various temperatures was investigated DSC. The data were treated on the basis of a reaction scheme with two processes in parallel, one of them auto-catalyzed. A good fit with the experiment was reached only when the order of both processes with respect to amine had been reduced to half its original value. An assumption that the kinetics of the amine-epoxy resin rection are considerably affected by the formation of various complexes through hydrogen bonds may be an explanation. A simple mathematical model has been suggested to estimate this influence. Because of the relatively high heats of interaction for the formation of complexes, the dependence of the measured heat on the degree of conversion is not linear. The magnitude of the error caused by neglecting this fact in investigation of the kinetics by DSC has been estimated.

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.

Ordered structures in aqueous poly(vinyl alcohol) gels

Abstract The formation of ordered structures in swollen aqueous gels of poly(vinyl alcohol) was investigated by DSC and polarizing microscopy. It was found that the stability of these structures depended on the temperature at which the swelling proceeded and was affected by the water content inside the gel.

Thermochemistry of Model Curing Reactions of Epoxy Resins

SummaryThe heats of reaction of phenylglycidyl ether and N,N-methylglycidylaniline as model systems for two different types of epoxy resins with primary and secondary amine were determined. In all cases the heat of reaction was found to be dependent on the ratio of the reactant components. This dependence is probably due to the formation of various complexes via a hydrogen bond. It was also found that, while the heats of reaction of phenylglycidyl ether with primary and secondary amine are the same, the heats of reaction of N,N-methylglycidylaniline with primary and secondary amine differ significantly from each other.

Comments on the interpretation of thermodynamic data on swollen hydrogels

SummaryAs a reply to the paper of Bowstra et al. (1) the authors summarize their earlier conclusions (2,3,4) concerning the interpretation of data on sorption equilibrium, heats of mixing and heat capacities of water-swollen gels. Two unavoidable premises of correct interpretation are emphasized: (1) accounting for the physical states of the polymer before and after mixing with water, (2) correct choice of the thermodynamic criterion for the stability of water structure and of its interaction with the polymer.

The solid-liquid equilibria in the system polyoxyethylene/phenol

Abstract The solid-liquid (S-L) phase diagram and the X-ray diffractogram of a mixture of polyoxyethylene ( M = 2000) with phenol suggest the existence of a crystalline compound consisting of one molecule of phenol and two monomeric units of polyoxyethylene. The congruent melting point of this compound lies at 302.6 K. From an analysis of the phase diagram based on the theory of association equilibria, we have found that in the liquid phase: (1) the bond between phenol and polymer is very strong; (2) there exists a whole spectrum of association complexes containing different numbers of phenol molecules bound onto the macromolecule; and (3) the maximal number of bound phenol molecules is higher than in the crystalline compound.

A calorimetric study of the glass transition kinetics in polyisoprene

The connection between the dielectric and calorimetric relaxation behaviours of synthetic polyisoprene Cariflex IR 305 is studied. A similar comparison of dielectric and dilatometric results was described in [1]. The heat capacity was measured during heating of samples prepared with different thermal history. Experimental results were compared with the heat capacity curves calculated for a model based on the multiparameter theory of Kovacs et al. [4]. The model considers the relaxation system as being composed of a set of subsystems characterized by different relaxation times. The distribution of relaxation times and their temperature dependence were taken from the diclectric measurement. The relaxation time of a subsystem from posed to depend, not only on the actual, temperature of the sample, but also on the deviation of this subsystem from equilibrium, or alternatively, on the deviation of the system as a whole. The comparison between the measured and modeled curves shows that both influences must be taken into account in order to explain the experimental results.

A test of electrostatic and reaction-field theories: equilibria in isodesmic reactions of some chlorobenzenes

The enthalpy of solvation of chlorobenzene and 1,2-, 1,3-, and 1,4-dichlorobenzene has been measured in two solvents of different polarity. The results have allowed us to construct several isodesmic reactions and to evaluate their reaction enthalpies in solution. These values agree at least semiquantitatively with the reaction-field theory but the results are very sensitive to the choice of parameter and modification of the basic equations. It follows that the success of the reaction-field theory has been essentially fortuitous and almost every experimental finding can be reproduced by choosing a suitable modification from a number of those recommended in the literature. The alternative electrostatic calculation (the ‘Kirkwood–Westheimer’ model) yields still worse results and cannot be extended outside ionic equilibria.