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This paper studies a polymer network in which crosslinks are degradable but polymer chains are not. We show that entanglements markedly enhance the mechanical properties of the polymer network before degradation and slow down degradation. We synthesize polyacrylamide hydrogels with disulfide crosslinks. In a precursor of a low water-to-monomer molar ratio and low crosslinker-to-monomer molar ratio, the monomers are crowded and the resulting polymer chains are long, so that the entanglements greatly outnumber crosslinks. The as-synthesized hydrogels are submerged in pure water to swell to equilibrium. We show that entanglements enhance the swell resistance of the hydrogel, as well as stiffen and toughen the hydrogel. We further show that entanglements slow down degradation when the hydrogel is submerged in an aqueous solution of cysteine. This work demonstrates that entanglements substantially expand the properties space of degradable polymers.
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Polyacrylamide hydrogels. VI. Synthesis-property relation
ynthesis-property relation is fundamental to materials science, but many aspects of the relation are not well understood for many materials. Impetus for this paper comes from our recent appreciation for the distinct roles of entanglements and crosslinks in a polymer network. Here we study the synthesis-property relation of polyacrylamide hydrogels prepared by free radical polymerization. Some of the as-prepared hydrogels are further submerged in water to swell either to equilibrium or to a certain polymer content. The synthesis parameters include the composition of a precursor, as well as the polymer content of a hydrogel. Series of hydrogels are prepared along several paths in the space of synthesis parameters. For each hydrogel, the stress-stretch curve is measured, giving four properties: modulus, strength, stretchability, and work of fracture. We interpret the experimentally measured synthesis-property relation in terms of entropic polymer networks of covalent bonds. When the precursor has a low crosslinker-to-monomer molar ratio, the resulting polymer network has on average long polymer segments. When the precursor has a low water-to-monomer molar ratio, the resulting polymer network has on average many entanglements per polymer segment. We show that crosslinks lower strength, but entanglements do not. By contrast, both crosslinks and entanglements increase modulus. A network of highly entangled long polymer segments exhibits high swell resistance, modulus, and strength.
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- Award ID(s):
- 2011754
- PAR ID:
- 10500481
- Publisher / Repository:
- Elsevier
- Date Published:
- Journal Name:
- Journal of the Mechanics and Physics of Solids
- Volume:
- 170
- Issue:
- C
- ISSN:
- 0022-5096
- Page Range / eLocation ID:
- 105099
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1016/j.jmps.2022.105099
