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Swollen PMMA networks show significant deviations from the theory of rubber elasticity. The unusual mechanical, optical and swelling properties have been explained by the existence of associations, well known for PMMA solutions. Unswollen PMMA networks show stress optical properties which are very similar to the networks swollen in carbon tetrachloride which is known as a strong complexing solvent. This indicates that associations exist in the bulk material. Swelling in carbon tetrachloride does not change the content of associations, only the glass transition temperature is reduced. Increasing the amount of crosslinking agent proves that the stress optical coefficient depends on the crosslinking density. This is contrary to the theoretical predictions. An explanation given by the effect of short network chains is not satisfactory. However, the consideration of a crosslinking chain instead of a crosslinking point gives a sufficient explanation. An oriented crosslinking chain gives a positive contribution to the total optical anisotropy while a PMMA chain shows a change from negative values (hindered rotation) to positive values (free rotation).

The optical anisotropyΔα (α=polarizability) of uniaxial deformed swollen and unswollen PMMA networks depends on the temperature. This is not in agreement with the Kuhn-Grun-theory. The temperature dependence and the negative values even above the glass transition temperatureTg can be explained by the existence of associations which cause a hindrance of the ester side group mobility. The swelling of the network in the optical isotropic solvent carbon tetrachloride does not degrade the associations and therefore not change the network structure; onlyTg decreases. The situation is more complicated if optical anisotropic solvents are used. Polar aliphatic and aromatic solvents degrade the associations to some extend and the birefringence is shifted to more positive values. An additional change is caused by specific polymer solvent interactions. Aromatic solvents form complexes with the ester side group and the aliphatic solvent chloroform completely degrades the associations, which can be explained by the formation of hydrogen bonds. Sign and magnitude of the optical anisotropy allow statements on the orientation of the solvent molecules with respect to the polymer chain or the ester side group.

The subject of this work is to determine the dynamic and static elastic properties of polymer networks and to compare the experimental results with theoretical predictions. It is found that poly(dimethylsiloxane) (PDMS) networks on the one hand and polyisoprene (IR) and natural rubber (NR) networks on the other hand show different behaviour in frequency dependence of the storage modulusG′ is observed in the range 10−3 − 1 Hz, contrary to IR or NR. The reason for this is the absence of entanglements in PDMS. Comparing the measured static moduli with those calculated by rubber elasticity theory, we found that the front factorA· /0 is near to 0.5 in case of PDMS, but near to I for IR/NR networks. Both parts of the front factor may possibly cause this difference. As the PDMS chains possess a relatively high mobility we can assume that fluctuations of the junction points are less restricted than in IR/NR. This causes a structure factor A smaller than 1 in agreement with Flory's recent theory. According to James and Guth, the network chains tend to contract during the crosslinking process. This will be more likely in the networks of entanglement-free, highly mobile PDMS chains than in IR or NR rubbers. Hence the memory term /0 is smaller for PDMS than for IR/NR. Both alternative explanations are based on the different mobility of the chains considered. It may be assumed that the front factor is influenced by both effects and not only by the fluctuations of crosslinks.

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...

Thermally stimulated depolarization currents (TSDC) of atactic (a), syndiotactic (s) and isotactic (i) PMMA and mixtures ofs- andi-PMMA were measured. In pure tactic PMMA, TSDC peak was obtained at a temperature corresponding to the glass transition temperature of each PMMA. TSDC of mixtures isolated as precipitates from 1/1 and 1/2 (i/sweight ratio) mixed solutions showed a peak which can be attributed to the glass transition temperature of the stereocomplex. TSDC of original l/1 mixture showed two peaks which are ascribed to the glass transition temperature of residual i-PMMA and the stereocomplex. The results suggests that stereocomplex formation occurs ini/s weight ratio of 1/2 independent of the way of preparation.

Es wird ein Uberblick gegeben uber die thermodynamischen Eigenschaften von Polymergelen, den gegenwartigen Stand der experimentellen Untersuchungen und die Anwendbarkeit der statistischen Theorien. Polymergele sind hochmolekulare Mischphasen mit elastischen Eigenschaften. Die Elastizitat beruht auf einer chemischen oder physikalischen Vernetzung der Makromolekule. Die thermodynamischen Eigenschaften von chemisch vernetzten Gelen konnen durch Kombination von Quellungsgleichgewichtsmessungen mit Messungen der Entquellung oder des Quellungsdrucks ermittelt werden. Die in den statistischen Theorien auftretenden Strukturparameter; Netzwerkdichte v, Funktionalitat f, memory-Term η Strukturfaktor A und Volumenfaktor B, werden diskutiert, und es wird gezeigt, welche physikalischen Methoden zu ihrer Bestimmung angewendet werden mussen. Auf die grose Bedeutung der Herstellung von Modellnetzwerken wird hingewiesen. Einige Mesergebnisse an Polystyrol-Gelen werden mitgeteilt. A survey on the thermodynamic properties of polymer gels, the actual state of the experimental investigations and the applicability of the statistical theories will be given. Polymer gels are elastic liquids. The elasticity is caused by a chemical or physical crosslinking of the macro-molecules. The thermodynamic properties of chemically crosslinked gels can be determined by combining swelling equilibrium measurements with measurements of deswelling or swelling pressure. The following structural parameters which appear in the statistical theories are discussed: crosslinking density v, functionality f, memory-term η, structure factor A and volume factor B. The physical methods for their determination will be stated. The importance of the formation of model networks will be pointed out. Some experimental results on gels of crosslinked polystyrene are reported.

Ever since the fundamental investigations by Tamman it has been known that liquids can be transformed into the glassy state if crystallisation can be suppressed.1 The glassy state is therefore a state of matter which in many substances can be observed in addition to the classical states, i.e. gaseous, liquid and crystalline. In particular, in high molecular weight substances a glassy solidification takes place frequently in combination with partial crystallisation. The rapid development of this technologically important class of materials has therefore called attention to the glassy state.