SYNTHESIS, CHARACTERIZATION, AND RUBBER ELASTICITY OF END-LINKED POLY(TETRAHYDROFURAN) ELASTOMER

For physical studies of network formation, inhomogeneity, rubber elasticity, and stress-induced crystallization in a model elastomer, an end-linked poly(tetrahydrofuran) network has been synthesized and characterized. Diallyl-terminated poly(tetrahydrofuran) was synthesized by cationic ring-opening polymerization and terminated with allyl alcohol. The functionality, molecular weight, and polydispersity were characterized with GPC, chemical titration, and NMR. The results indicate the average functionality is 2.0 +/- 0.1. End-linked elastomer can then be synthesized by reacting the stoichiometric amount of diallylpoly(tetrahydrofuran) with pentaerythritol tetrakis(3-mercaptopropionate), a tetrafunctional thiol. The resultant elastomers were characterized in terms of sol fraction and equilibrium degree of swelling. The average sol fraction of these elastomers is 2%. The thiol-ene addition reaction involved in the network formation was studied by using the small molecules that have similar chemical structures as that of prepolymer and cross-linker. NMR studies of the reaction product indicate that the thiol-ene addition reaction is well-defined, with no observed side reactions. Therefore, it is concluded that the chemical structure of this end-linked elastomer is known. This system is therefore adequate for our investigation of elastomeric properties. Stress-strain behavior of a swollen network was measured and compared with a constrained junction model. The results indicate the molecular weight between junctions by theoretical fitting of experimental data is much smaller than the true M(c) between junction points. This implies that the contribution of trapped entanglements as an effective junction points to the modulus of these elastomers.