It is predicted that the net repulsion between the segments of a polymer network and a poor solvent can cause a phase transition marked by a sudden change in the degree of swelling. This is analogous to the “coil–globule” transition recently predicted by Ptitsyn to occur for a macromolecule in solution. The critical conditions for the transition. as well as phase diagrams, are calculated for the gel in free swelling and under uniaxial tension, which facilitates the transition. The transition depends on the gel being formed of chains crosslinked while greatly swollen by a diluent and also having a high degree of crosslinking. It is concluded that it would be difficult to attain the conditions necessary for the transition in the free‐swelling case, but that it should be possible for gel under tension.

Cloud-point curves by DSC measurements at different scanning rates on solutions of two samples of poly(vinyl methyl ether) in water show two minima in a temperature−composition plot. The significance of the data was confirmed by static measurements of the coexisting-phase compositions and phase-volume ratios. A thermodynamic analysis in terms of a strongly concentration-dependent interaction parameter leads to the conclusion that the system H2O/PVME exhibits so-called type III behavior. For polymers of infinite molar mass, such behavior is characterized by the occurrence of two off-zero critical concentrations, in addition to the usual zero critical concentration marking the ϑ state.

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Polyurethane networks based on vegetable oils have very heterogeneous composition, and it is difficult to find a close correlation between their structure and properties. To establish benchmark structure-properties relationships, we have prepared model polyurethane networks based on triolein and 4,4'-diphenylmethane diisocyanate (MDI). Cross-linking in the middle of fatty acid chains leaves significant parts of the triglyceride as dangling chains. To examine their effect on properties, we have synthesized another polyurethane network using triolein without dangling chains (removed by metathesis). The structure of polyols was studied in detail since it affects the structure of polyurethane networks. The network structure was analyzed from swelling and mechanical measurements and by applying network and rubber elasticity theories. The cross-linking density in both networks was found to be close to theoretical. The triolein-based model network displayed modulus (around 6 MPa), tensile strength (8.7 MPa), and elongation at break (136%), characteristic of hard rubbers. Glass transition temperatures of the networks from triolein and its metathesis analogue were 25 and 31.5 degrees C, respectively.

Equilibrium swelling of networks capable of separating into a highly swollen and a collapsed phase is discussed on the basis of a concentration-dependent pair interaction parameter. Classic rules, to be obeyed by the various binodals and their metastable and unstable extensions, rationalize the experimentally accessible portions of the phase diagram and are covered by a simple model yielding acceptable parameter values when fitted to swelling data on the system water/polyW-isopropylacrylamide). The description of the solvent/network binodal is good, but the predicted location of the LCST miscibility gap for linear chains deviates considerably from the measured cloud points. It is probable that linear and branched chains do not differ much in enthalpic contributions to the interaction parameter but deviate appreciably in the entropic terms. This feature offers an acceptable explanation for the discrepancy.

It is demonstrated that the collapse of gels of radiation cross-linked poly(vinyl methyl ether) swollen in water is a truly discontinuous process obeying classic thermodynamic principles. The phenomenon can further be shown to be related to the relevant type of limiting critical demixing of solutions of the corresponding non-cross-linked polymer. There are three such types, including “classic” ϑ behavior, and they can also be recognized in the swelling behavior of the cross-linked analogue.