1. Microstructure and Mechanical Enhancement in Randomly End-linked Bimodal Networks

2. 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

3. 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, (2008).

4. 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

5. 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