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

Mechanical Enhancement: When sizes differ by a factor greater than 10, enhanced mechanical properties can result.

a

s

Unimodal Short Chains

Unimodal Long Chains

Bimodal

• Stress upturn

• Increased toughness

sm

am

• Large ultimate stress and strain

Long Chains

Cross-links

Short Chains

Short chains

Mechanical Enhancement in Bimodal Networks

Limited extensibility of short chains seems to be the correct interpretation3

The two main hypotheses for the mechanical enhancement are:

• Limited extensibility of short chains at large deformation1

• Heterogeneous domains of short chains can act as reinforcing agents2

(1) Polym. Sci., Polym. Phys., 22, 1849–1855 (1984).

(2) Macromolecules, 23, 351–353 (1990).

(3) Macromolecules, 41, 8231-8241 (2008).

Relationship between toughening mechanism and microstructure: not well understood

Highly heterogeneous

Slightly heterogeneous, quasi-homogeneous

Highly heterogeneous

Slightly heterogeneous

Short chains

Mechanical Enhancement and Topology A percolation transition occurs for short chains1,2

0.00

0.25

0.50

0.75

1.00

25 50 75 100

mol % of short chains

Ps

ho

rt c

ha

in

SANS Measurements2

60 mol%: Highly heterogeneous, very extensible,

but with poor modulus

Pshort chain= fraction of short chains in largest cluster

60 mol%

60 mol%60 mol%

60 mol%

90 mol%

90 mol%90 mol%

90 mol%

90 mol%: Slightly

heterogeneous, rather extensible

and high modulus.

Optimum! ~ Elastic coupling of short chains

95 mol%

95 mol%

95 mol%95 mol%

95 mol%: quasi-

homogeneous, high modulus,

but brittle

(1) Macromolecules, 41, 8231 (2008).

(2) Macromolecules, in preparation.

Snapshot:

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

Ongoing work

• A more quantitative analysis of elastic coupling of short chains through network connectivity order parameters

• Percolated short chains form a hard skeleton with flexibility due to softer regions of long chains that join different parts of the skeleton