Formation and Structure of End-Linked Elastomer Networks

The network structure of end-linked polymer networks prepared from telechelic polymers is determined mainly by the functionality distribution of the telechelic polymer, the relative reactivity of the functional groups, and the conditions of network formation. The main feature of network formation by end-linking (by a step reaction) is a relatively high critical conversion at the gel point and a relatively narrow range of conversions available for the build-up of the network structure. Therefore, the final equilibrium properties (modulus and degree of swelling) are rather sensitive to cyclization, incompleteness of the reaction, and possible errors in the determination of the content of functional groups or functionality. The wastage of bonds in elastically inactive cycles is rather low, amounting to several per cent, but its effect on the concentration of EANC's is not negligible. In multicomponent systems, where two or more structurally differing components contain groups of the same kind, chemical clustering (e.g., formation of hard clusters) occurs and affects the concentration of EANC's as well as a number of physical properties. For polyurethane networks, mechanical, dielectric and optical measurements indicate that networks of poly(oxypropylene)triol and diisocyanate are more homogeneous than those of poly(oxypropylene)diols, 1,1,1-trimethylolpropane, and diisocyanate, and that the two-stage process yields more homogeneous networks than the one-stage one. The correlations between the concentration of EANC's and the gel fraction can be generalized and employed in examining the rubber elasticity theories or the degree of crosslinking of industrially important elastomer networks. In addition to the common features, the endlinking processes have their own specificities given by differences in the crosslinking mechanisms and, sometimes, in physical interactions. An example of a more complex network formation (and a correspondingly more difficult theoretical treatment) is crosslinking of carboxyl-terminated rubbers with diepoxides.