Origin of the Self-Reinforcement in PDMS Bimodal Networks

Abstract

Abstract For three series of bimodal networks, the stress-relaxation rates and the tearing energies were shown to exhibit maxima at about the same concentrations of short chains as those at which the tensile strengths, reported earlier, also exhibit maxima. These findings clearly show that the reinforcement produced by the addition of short chains results because chain mobility is reduced substantially, especially when the concentration of short chains is 30 to 40% by weight (Figures 1 and 5). For each series of bimodal networks prepared in our laboratory, the maximum tensile strength also occurs when the concentration of short chains is about 30% by weight. When a bimodal network is deformed progressively, deformation occurs in the long chains moving through the matrix of short chains. Also, the network junctions and the attached short chains must be dragged along with the deforming long chains. It is probable that the low mobility of the chains (high dissipation of energy), which accounts for the high tensile strengths, especially in networks having 30% or so by weight of short chains, results primarily from the energy dissipation associated with such complex molecular rearrangements. It was also shown that the maximum tensile strength in a series of networks increases with an increase in the difference between the molecular weights of the long and short chains, or as shown in Figure 7, with a decrease in M S /M L .