Elastic Properties of Crosslinked Polymers

At first a survey is given on the thermodynamics of rubber-elasticity and the results of the statistical theory of molecular networks. Experimental data of torsional vibrations, stress—strain measurements (simple extension and uniaxial compression) and photoelastic properties in dependence of temperature are reported. The measurements have been carried out on dry and swollen crosslinked polymers, especially poly(methyl methacrylate) (crosslinked in bulk or in solution), natural rubber, trans-polypentenamer and poly(butadiene). The modulus—temperature curves were always linear in the rubbery region when the samples had been carefully extracted. For poly(methyl methacrylate) the curve of retractive lorce against temperature showed a bend at a distinct temperature. Below this temperature the slope of the curve is lower than above. This behaviour can be explained by the existence of long living associations in poly(methyl methacrylate)—solvent systems already known by measurements on the corresponding solutions. The associations are able to form physical crosslinks. Most elastomers do not behave ideally in the sense of the theory: they are not purely entropy-elastic but show also an energy elasticity term of the modulus. The energy elasticity part is not constant but increases with the network density and the degree of swelling. From this it is concluded that the energy elasticity contribution is not only an intramolecular effect caused by rotational isomerism. The energy elasticity may also depend on other intramolecular effects and intermolecular sources. The existence of short chains in the network seems to be one essential reason for the nonconstancy of the energy elasticity contribution. Photoelastic measurements in dependence of the degree of swelling show that we must assume a short-range order in amorphous polymers. Following a newer theoretical consideration, the second term of the MooneyRivlin equation for the stress—strain behaviour of unswollen elastomers can be explained by the co-operation of the short-range order and the chain-length distribution in the random polymer network. INTRODUCTION Here we will deal only with the elastic properties above the glassor melting-temperature; that is, we will consider the behaviour of polymers in the rubbery state. In the highly elastic state the substance consists of a three-dimensional macromolecular network. The crosslinks can be formed by primary bonds, secondary bonds, entanglements or domains in copolymers, which are due to phase-separation1' 2, The primary bonds are chemical