Dangling-End Double Networks: Tapping Hidden Toughness in Highly Swollen Thermoplastic Elastomer Hydrogels

Chen Guo, Jackson T. Lewis, Vincent F. Scalfani, Miriah M. Schwartz, Travis S. Bailey

Research output: Contribution to journalArticlepeer-review

6 Scopus citations

Abstract

This report introduces a unique method of significantly improving toughness in highly swollen block copolymer-based thermoplastic elastomer (TPE) hydrogels by converting an intrinsically large population of dangling chain ends into a mechanically active second network. In one form, the TPE hydrogels developed by our group are based on swelling of a vitrified melt-blend of two amphiphilic block copolymer species, sphere-forming polystyrene-poly(ethylene oxide) (SO-H) diblock and triblock (SOS) copolymers. Here, the PEO midblock in the SOS triblock copolymer serves to tether adjacent PS spherical aggregates, producing hydrogel networks that are incredibly elastic and mechanically robust, preserving their shape even at the very high intrinsic swelling ratios produced at low SOS concentrations (e.g, 37 g of H2O/(g of polymer) at 3.3 mol % SOS). In this report, we advance the utility of this framework by exploiting the hundreds of dangling PEO chain ends per spherical aggregate to form a second, mechanically active network. The approach is based on a stepwise installation of two tethering SOS triblock copolymer populations. The first is present directly during melt-state self-assembly of the original diblock/triblock copolymer blend and inherently determines the equilibrium swelling ratio of resulting hydrogel. The second population is then introduced postswelling, by simply coupling the dangling SO diblock copolymer chain ends under conditions largely free of the mechanical stress osmotically imposed on the primary network. Notably, this action simply shifts the ratio of diblock and triblock copolymer without compromising the thermoplasticity of the network. Here, we use the facile water-based coupling of PEO-terminal azide and alkyne groups to demonstrate the scale of toughness enhancements possible through conversion of dangling ends into a second network. The dangling-end double networks produced exhibit remarkable improvements in tensile properties (tensile modulus, toughness, strain at break, and stress at break), including a 58-fold increase in mean toughness (to 361 kJ/m3) and a 19-fold increase in mean stress to break (to 169 kPa) in highly swollen samples containing up to 95% (g/g) water. Importantly, these improvements could be realized without altering water content, shape, small-strain dynamic shear, and unconfined compressive properties of the original TPE hydrogels.

Original languageEnglish (US)
Pages (from-to)1678-1690
Number of pages13
JournalChemistry of Materials
Volume28
Issue number6
DOIs
StatePublished - Mar 22 2016
Externally publishedYes

Bibliographical note

Funding Information:
This material is based upon work supported by the National Science Foundation under Grant Nos. CBET-1160026 and DMR-0645781.

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