View
27
Download
3
Category
Tags:
Preview:
DESCRIPTION
Microstructure and Mechanical Enhancement in Randomly End-linked Bimodal Networks. Bimodal Elastomer Networks. Short Chains. Long Chains. Cross-links. s m. Bimodal. Unimodal Short Chains. Unimodal Long Chains. a m. - PowerPoint PPT Presentation
Citation preview
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
Recommended