Polymer network characterization by means of swelling pressure and unilateral compression data

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Abstract
The characterization of a polymer network entails at least the determination of three parameters, viz. the number of elastically effective chains v, the polymer diluent interaction parameter χ and the reference degree of swelling qo, at which the chains do not exert elastic forces on the crosslinks. In this study networks are described which are prepared by crosslinking poly‐(p‐nitrophenyl methacrylate) in solution with hexamethylene diamine. This yields gels of accurately known content of chemical crosslinks. Swelling pressure measurements (0–3 atm) coupled with unilateral compression data have been performed in order to determine the network parameters and to check the various forms of the theory of swollen rubber elastic networks. It is found that the functional relationships prescribed by the theory are indeed obeyed over a wide range of swelling from volume ratios of 5 to 20. The parameter v is constant over the whole range. If the number of chemically crosslinked chains is assumed to be equal to the number of elastically effective chains in the network, the data support the Flory‐Wall theory, although a modified form of the Hermans theory is also applicable. The reference degree of swelling qo is very large in these gels because they are formed by crosslinking in dimethylformamide in which the molecular chains are aggregated, while the swelling is measured in nitrobenzene in which the aggregates dissociate. This feature is also responsible for the interesting fact that it is the elastic free energy rather than the free energy of dilution which is negative and thus promotes the swelling of these gels.