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Mechanical Properties of Star Block Polymer Thermoplastic Elastomers with Glassy and Crystalline End Blocks
journal contribution
posted on 2016-12-09, 18:24 authored by Adam B. Burns, Richard A. RegisterThe mechanical properties
of thermoplastic elastomers (TPEs) consisting
of 6-arm star block polymers with glassy, crystalline, or composite
crystalline–glassy physical cross-linking (hard) domains were
investigated and compared to the analogous linear triblock or pentablock
polymers. The 6-arm stars exhibited qualitatively similar solid-state
morphologies and phase behavior to their linear counterparts, as demonstrated
by small-angle X-ray scattering and differential scanning calorimetry.
Consequently, the architecture had minimal impact on the small-strain
behavior in uniaxial extension at room temperature. As the applied
strain increased, the star polymers exhibited more pronounced strain
hardening than the corresponding linear TPEs, resulting in an increase
in the ultimate strength of 20% for the polymers with crystalline
end blocks and 30% when the end blocks were glassy. Each of the three
star polymers exhibited superior recovery (i.e., lower residual strain)
and lower hysteresis than the corresponding linear TPEs when subjected
to repeated strain cycles. The enhancement in the recovery was most
significant for the polymers with glassy hard domains. The TPEs with
crystalline or crystalline–glassy domains recovered more rapidly
than the corresponding linear block polymers but showed only modest
improvements in the recovery measured after the specimens were allowed
to rest for 5 min. These results indicate that the covalent junction
at the core of the star strengthens and accelerates the recovery of
the network but does not greatly suppress plastic deformation of the
crystallites. Overall, this work demonstrates that the mechanical
performance of block polymer TPEs can be improved by using a star
macromolecular architecture.