Improved Elastomers through Control of Network Chain-Length Distributions

Abstract

Methods are described for obtaining elastomers of controlled network chain-length distributions by restricting the reactivity of the polymer chains to their ends, and then end linking these chains with a multi-functional reactant. The networks of this type that have proved to be of greatest interest consist of short chains end linked with long chains to yield a bimodal distribution of network chain lengths. These bimodal elastomers have unusually high extensibility for their values of the modulus and ultimate strength, and thus considerable toughness, even in the unfilled state. Most such elastomers have been prepared from chains of poly(dimethylsiloxane), by carrying out either a condensation reaction between hydroxyl-terminated chains and tetraethoxysilane, or an addition reaction between vinyl-terminated chains and a poly(methylhydrogen siloxane) oligomer. The material presented in this review discusses the preparation of such elastomers, the characterization of some of their properties, and the interpretation of some of these properties in terms of the molecular theories of rubber-like elasticity.

Heterogeneous networks of polyisoprene/polyvinylethylene

“…By taking advantage of the disparate crosslinking reactivity of the PI and PVE, potentially tougher rubbers might be obtained, in the manner of bidisperse PDMS networks [4] and hydrogel blends [5]. The present study, intended to permit analysis of the blend morphology, relied on linear polymers, available in limited quantity.…”

“…An example of this is bimodal networks, in which short (even oligomeric) chains are mixed with high molecular weight polymer. Forming a network by end-linking such a blend yields bimodal elastomers, which have novel mechanical properties, notably exceptional toughness [4]. Another example is hydrogel blends, formed by sequential crosslinking of the components.…”

Nonlinear Mechanical Behavior of Scarcely Crosslinked Poly(dimethyl siloxane) Gel: Effect of Strand Length Polydispersity

“…[18][19][20][21][22][23][24] In particular, Mark et al [19][20][21][22][23] prepared the model rubbers having a known, bimodal distribution of the strand length (through end-linking of bimodal prepolymers) to examine the relationship between this distribution and modulus/deformation of the rubbers. However, their model rubbers…”

Unconventional Rubber Networks: Circumventing the Compromise Between Stiffness and Strength

“…Such behavior is contrary to classic theories of rubber elasticity, which assume failure occurs due to rupture of the shortest network chains 65 ; this implies that the short chains would weaken the network. The improved properties of bimodal networks have been ascribed to a synergy between the high modulus of the short chains and the extensibility of the longer ones (a so-called delegation of responsibilities 54,66 ). However, infrared dichroism measurements on deuterium-labeled polytetramethylene oxide bimodal networks reveal the orientation of the long chains to be not much different than that of the short chains (Figure 9), 67 which is at odds with the putative mechanism for enhanced performance.…”