Session: 12-29-01: Mechanics of Soft Materials

Paper Number: 148021

148021 - An Elastomer With Ultrahigh Strain-Induced Crystallization 

Strain-induced crystallization (SIC) prevalently strengthens, toughens, and enables an elastocaloric effect in elastomers. Aligned crystalline domains can form in the amorphous network of elastomers when subject to large elongations. The increased crystallinity can effectively pin and blunt cracks, giving enhanced mechanical strength and fracture toughness. This SIC is also reversible since ordered chains can disassemble during bulk retraction. However, the crystallinity induced by mechanical stretching in common elastomers (e.g., natural rubber) is typically below 20%, and the stretchability plateaus due to trapped entanglements, impeding performance for advanced applications. We present a new class of elastomers formed by end-linking then deswelling star polymers with a low fraction of defects and no trapped entanglements. Structural characterization of the deswollen end-linked star elastomer (DELSE) through small-angle X-ray scattering and wide-angle X-ray scattering (SAXS and WAXS) unveils its capacity to achieve strain-induced crystallinity of up to 50% under large deformations. We attribute the ultrahigh SIC in the DELSE to two characteristics that differ for conventional elastomers: a uniform network structure and a high stretchability due to the lack of trapped entanglements. The DELSE reaches an ultra-high stretchability of 12.4-33.3, scaling beyond the limit of common elastomers. The theoretical ultimate stretch for common elastomers scales with the square root of the number of monomers per chain and saturates at the entanglement molecular weight. Molecular dynamics simulations show that polymer chains in the DELSE network display shorter initial end-to-end lengths, producing a higher elongation ratio when they straighten and break. We experimentally validate this by uniaxially extending DELSE samples—reinforced by SIC—with different chain lengths to failure. The stretchability of the DELSE notably scales higher and displays no upper limit due to the dearth of trapped chain entanglements. Mechanically, these features of the DELSE produce a high fracture energy of 4.2-4.5 kJ/m/m yet low mechanical hysteresis. Heightened SIC and stretchability synergistically make the DELSE a strong candidate for advanced solid-state cooling technologies since they can attain higher configurational entropy and crystallinity changes during stretching than common elastomeric alternatives. The DELSE exhibits a 9.3 °C adiabatic temperature change when unloaded at 54.5 °C, while natural rubber cools by 3.5 °C when unloaded at 53.5 °C. Generally, the elastomer fabrication approach harnessed here—end-linking a highly regular gel and then fully deswelling the network—provides a platform with promise across a breadth of polymer chemistries. Elastomers fabricated in this manner exhibit the capacity to outperform conventional counterparts. These results also suggest the potential for precisely engineering SIC in soft materials by controlling their network architecture. This class of deswollen elastomers can play a key role in the future of aerospace structures, medical devices, and elastocaloric refrigeration design.

Presenting Author: Chase Hartquist Massachusetts Institute of Technology

Presenting Author Biography: Chase Hartquist is a Ph.D. Candidate in Mechanical Engineering at Massachusetts Institute of Technology. Chase was awarded the MathWorks and NSF Graduate Research Fellowships to pursue his doctoral studies under Professor Xuanhe Zhao. He earned his B.S. and M.S. in Mechanical Engineering from Washington University in St. Louis.

Authors:

Chase Hartquist Massachusetts Institute of Technology
Shaoting Lin Michigan State University
James Zhang Massachusetts Institute of Technology
Shu Wang Massachusetts Institute of Technology
Michael Rubinstein Duke University
Xuanhe Zhao Massachusetts Institute of Technology