Model networks of end‐linked polydimethylsiloxane chains. XI. Use of very short network chains to improve ultimate properties

Abstract

Elastomeric networks were prepared by tetrafunctionally end‐linking mixtures of various proportions of relatively long and very short polydimethylsiloxane (PDMS) chains. The former had a number‐average molecular weight of 18,500 and the latter either 660 or 220 g mole−1. The series of (unfilled) bimodal networks thus prepared were studied in elongation to the rupture point at 25°C, and in swelling equilibrium in benzene at room temperature. Elasticity constants characterizing the Gaussian regions of the stress–strain isotherms, and values of the degree of equilibrium swelling were used to evaluate the most recent molecular theories of rubberlike elasticity. The isotherms also gave values of the elongation at which the modulus begins to increase anomalously because of limited chain extensibility, and values of the elongation and nominal stress at the point of rupture. These results were interpreted in terms of the known configurational characteristics of the constituent PDMS chains. Values of the energy or work required for rupture were used as an overall measure of the “toughness” of the networks. The very short chains were found to give a marked increase in toughness, through an increase in ultimate strength without the usual corresponding decrease in maximum extensibility. A variety of additional experiments will be required in order to elucidate the molecular origins of this important effect.

Role of strain induced crystallization and oxidative crosslinking in fracture properties of rubbers

“…For instance Yanyo observed that in polydimethyl siloxane networks, the fracture behaviour depends also on the length distribution of EACs [9]. This was confirmed by Mark and co-workers [1,2] who studied a series of polydimethyl siloxane networks based on long chains to which variable quantities of short chains were added. It appeared that the modulus increases regularly with the average crosslink density.…”

“…However, the plot displays a positive curvature which could be linked to a specificity of the crosslinking process and can be explained as follows: random crosslinking creates a new population of chains shorter than initial ones. It is interesting to compare this trend with the one of model networks having a bimodal distribution of chain lengths [1,2] (Fig. 9a).…”

“…It can be noted that, in the case of Llorente's model for bimodal networks [2], the strain energy density at rupture in tension W r is also a decreasing function of M c . Assuming that W r and G IC are closely related, we reach the following conclusion: Oxidative crosslinking, due to its bimodal character, does not affect (at low conversions) ultimate elongations but induces an increase of stresses as expected from the theory of rubber elasticity.…”

“…There is a relatively abundant literature on the effect of crosslink density on fracture properties of elastomers. These studies have been performed essentially on network families of well controlled architecture such as crosslinked polydimethyl siloxanes [1,2] or polyurethane elastomers [3], in which all the additives susceptible to modify the mechanical behaviour, especially fillers, were avoided. A peculiar attention has been paid to the quasi equilibrium properties which could be deduced from the network theory and the theory of entropic elasticity applied to quasi ideal networks in which the length distribution of elastically active chains (EAC) would be unimodal.…”

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…”