An official website of the United States government
PhotoCAN silicone elastomers, based on the thiol–ene reaction, exhibit rapid and reversible changes in dynamic modulus at room temperature when illuminated by UV. By combining results from magic angle spinning solid-state NMR as well as EPR and rheometry measurements, both under UV, it is concluded that the mechanical response can be attributed to a combination of dissociative, associative, and oxidation reactions. The cleavage of the C–S bonds under UV in the presence of an excess of thiyl radicals is identified as the reversible dissociative reaction responsible for abrupt drops in the storage modulus. A slower but concurrent reaction is a termination process involving thiyl radicals to form disulfide bonds. A kinetic model is developed that successfully relates the rates of the underlying reaction mechanisms to changes in the storage modulus. The results provide a basis for designing new, ambient temperature photoresponsive covalently adaptive network materials.
more »
« less
Photoswitchable Covalent Adaptive Networks Based on Thiol–Ene Elastomers
ABSTRACT: Covalent adaptive networks combine the advantages of cross-linked elastomers and dynamic bonding in a single system. In this work, we demonstrate a simple one-pot method to prepare thiol−ene elastomers that exhibit reversible photoinduced switching from an elastomeric gel to fluid state. This behavior can be generalized to thiol−ene cross-linked elastomers composed of different backbone chemistries (e.g., polydimethylsiloxane, polyethylene glycol, and polyurethane) and vinyl groups (e.g., allyl, vinyl ether, and acrylate). Photoswitching from the gel to fluid state occurs in seconds upon exposure to UV light and can be repeated over at least 180 cycles. These thiol−ene elastomers also exhibit the ability to heal, remold, and serve as reversible adhesives. KEYWORDS: covalent adaptive network, elastomer chemistry, click chemistry, self-healing, photoresponsive materials, adhesives
more »
« less
- Award ID(s):
- 2011754
- PAR ID:
- 10500013
- Publisher / Repository:
- ACS Applied Material Interfaces
- Date Published:
- Journal Name:
- ACS Applied Materials & Interfaces
- Volume:
- 14
- Issue:
- 3
- ISSN:
- 1944-8244
- Page Range / eLocation ID:
- 4552 to 4561
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Cholesteric liquid crystals (CLCs) exhibit Bragg reflection due to their spontaneous self-assembly into a one-dimensional photonic structure. Retaining this cholesteric order in a polymer network requires functionalizing liquid crystals with reactive end groups. However, conventional chemistries for synthesizing cholesteric liquid crystalline polymers often result in poor surface alignment and reduced optical quality. In this work, we investigate a thiol−ene step-growth polymerization approach to fabricate cholesteric liquid crystalline elastomers (CLCEs) with tunable mechanical properties and improved optical quality. By varying the cross-link density, we systematically study the effects on haze, cross-linking degree, and mechanical response. Compared to existing cholesteric liquid crystalline polymers, the thiol−ene-based CLCEs exhibit enhanced surface alignment, reduced haze, and greater mechanical tunability. These materials are further benchmarked against CLCEs synthesized via thiol−acrylate chain transfer polymerization, highlighting the advantages of the thiol−ene reaction for achieving precisely controlled properties in cholesteric polymer networks.more » « less
-
Abstract Dynamic microcapsules are reported that exhibit shell membranes with fast and reversible changes in permeability in response to external stimuli. A hydrophobic anhydride monomer is employed in the thiol–ene polymerization as a disguised precursor for the acid‐containing shells; this enables the direct encapsulation of aqueous cargo in the liquid core using microfluidic fabrication of water‐in‐oil‐in‐water double emulsion drops. The poly(anhydride) shells hydrolyze in their aqueous environment without further chemical treatment, yielding cross‐linked poly(acid) microcapsules that exhibit trigger‐responsive and reversible property changes. The microcapsule shell can actively be switched numerous times between impermeable and permeable due to the exceptional mechanical properties of the thiol–ene network that prevent rupture or failure of the membrane, allowing it to withstand the mechanical stresses imposed on the capsule during the dynamic property changes. The permeability and molecular weight cutoff of the microcapsules can dynamically be controlled with triggers such as pH and ionic environment. The reversibly triggered changes in permeability of the shell exhibit a response time of seconds, enabling actively adjustable release profiles, as well as on‐demand capture, trapping, and release of cargo molecules with molecular selectivity and fast on‐off rates.more » « less
-
Adhesives typically fall into two categories: those that have high but irreversible adhesion strength due to the formation of covalent bonds at the interface and are slow to deploy, and others that are fast to deploy and the adhesion is reversible but weak in strength due to formation of noncovalent bonds. Synergizing the advantages from both categories remains challenging but pivotal for the development of the next generation of wound dressing adhesives. Here, we report a fast and reversible adhesive consisting of dynamic boronic ester covalent bonds, formed between poly(vinyl alcohol) (PVA) and boric acid (BA) for potential use as a wound dressing adhesive. Mechanical testing shows that the adhesive film has strength in shear of 61 N/cm 2 and transcutaneous adhesive strength of 511 N/cm 2 , generated within 2 min of application. Yet the film can be effortlessly debonded when exposed to excess water. The mechanical properties of PVA/BA adhesives are tunable by varying the cross-linking density. Within seconds of activation by water, the surface boronic ester bonds in the PVA/BA film undergo fast debonding and instant softening, leading to conformal contact with the adherends and reformation of the boronic ester bonds at the interface. Meanwhile, the bulk film remains dehydrated to offer efficient load transmission, which is important to achieve strong adhesion without delamination at the interface. Whether the substrate surface is smooth (e.g., glass) or rough (e.g., hairy mouse skin), PVA/BA adhesives demonstrate superior adhesion compared to the most widely used topical skin adhesive in clinical medicine, Dermabond.more » « less
-
Thermoset networks are chemically cross-linked materials that exhibit high heat resistance and mechanical strength; however, the permanently cross-linked system makes end-of-life degradation difficult. Thermosets that are inherently degradable and made from renewably derived starting materials are an underexplored area in sustainable polymer chemistry. Here, we report the synthesis of novel sugar- and terpene-based monomers as the enes in thiol–ene network formation. The resulting networks showed varied mechanical properties depending on the thiol used during cross-linking, ranging from strain-at-breaks of 12 to 200%. Networks with carveol or an isosorbide-based thiol incorporated showed plastic deformation under tensile stress testing, while geraniol-containing networks demonstrated linear stress–strain behavior. The storage modulus at the rubbery plateau was highly dependent on the thiol cross-linker, showing an order of magnitude difference between commercial PETMP, DTT, and synthesized Iso2MC. Thermal degradation temperatures were low for the networks, primarily below 200 °C, and the Tg values ranged from −17 to 31 °C. Networks were rapidly degraded under basic conditions, showing complete degradation after 2 days for nearly all synthesized thermosets. This library demonstrates the range of thermal and mechanical properties that can be targeted using monomers from sugars and terpenes and expands the field of renewably derived and degradable thermoset network materials.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1021/acsami.1c22287
