Microstructure and Mechanical Enhancement in Randomly End-linked Bimodal Networks

Bimodal Elastomer Networks Bimodal Networks: formed by end-linking two sets of chemically identical linear chains with different molar mass Long Chains Cross-links Short Chains Mechanical Enhancement: When sizes differ by a factor greater than 10, enhanced mechanical properties can result. Unimodal Short Chains Unimodal Long Chains Bimodal sm am Large ultimate stress and strain Stress upturn Increased toughness

Slightly heterogeneous, quasi-homogeneous Slightly heterogeneous Mechanical Enhancement in Bimodal Networks The two main hypotheses for the mechanical enhancement are: Short chains Highly heterogeneous Slightly heterogeneous, quasi-homogeneous Limited extensibility of short chains at large deformation1 Heterogeneous domains of short chains can act as reinforcing agents2 Limited extensibility of short chains seems to be the correct interpretation3 Relationship between toughening mechanism and microstructure: not well understood Highly heterogeneous Slightly heterogeneous Polym. Sci., Polym. Phys., 22, 1849–1855 (1984). Macromolecules, 23, 351–353 (1990). Macromolecules, 41, 8231-8241 (2008).

Pshort chain= fraction of short chains in largest cluster Mechanical Enhancement and Topology A percolation transition occurs for short chains1,2 90 mol%: Slightly heterogeneous, rather extensible and high modulus. Optimum! ~ Elastic coupling of short chains Short chains 60 mol%: Highly heterogeneous, very extensible, but with poor modulus 95 mol%: quasi-homogeneous, high modulus, but brittle 95 mol% 60 mol% 90 mol% 90 mol% 60 mol% 95 mol% 95 mol% 95 mol% SANS Measurements2 90 mol% 90 mol% 60 mol% Snapshot: 60 mol% Macromolecules, 41, 8231 (2008). Macromolecules, in preparation. Pshort chain= fraction of short chains in largest cluster

Summary (Bimodal Networks) Optimal tensile properties occurs when most of short chains are elastically coupled with the greatest amount of long chains Short-chain elastic coupling ocurrs near the percolation transition for the short chains Percolated short chains form a hard skeleton with flexibility due to softer regions of long chains that join different parts of the skeleton Ongoing work A more quantitative analysis of elastic coupling of short chains through network connectivity order parameters