The Impact of Polymerization Chemistry on the Mechanical Properties of Poly(dimethylsiloxane) Bottlebrush Elastomers

Wet/Dry Bottlebrush Pressure Sensitive Adhesives via a Dangling Defect-Driven Design

A series of bottlebrush pressure sensitive adhesives (PSAs) containing progressively greater numbers of large dangling ends were synthesized using ring-opening metathesis polymerization. These defects were engineered into PSAs by promoting the formation of loop defects and by manipulating the kinetic chain length such that large dangling defects are covalently bound to the bulk. The unique structure of these samples is shown to promote adhesion at interfaces by maximizing surface area contact and increasing local van der Waals forces. This design philosophy (termed defect-driven design, D3) provides an easy route to synthesize bottlebrush PSAs ∼6 times stronger than commercial VHB1000 tape. Furthermore, the inherent tendency of water to dewet on poly(dimethylsiloxane) PSAs is exacerbated in these samples, resulting in PSAs capable of indefinite cycles of wet-dry-wet adhesion (tested up to 50 cycles, 7 months apart). Further development of the D3 concept is expected to result in increased applications (e.g., dielectric actuators, wearable electronics, and especially soft robotics) as the unique design parameters are further explored.

Antifouling Activity of Bottlebrush Network Hydrogels

Mitigating the attachment of microorganisms to polymer biomaterials is critical for preventing hospital-acquired infections. Two chemical strategies to mitigate fouling include fabricating fouling-resistant surfaces, which typically present hydrophilic polymers, such as polyethylene glycol (PEG), or creating fouling-release surfaces, which are generally hydrophobic featuring polydimethylsiloxane (PDMS). Despite the demonstrated promise of employing PEG or PDMS, amphiphilic PEG/PDMS copolymer materials remain understudied. Here, for the first time, we investigated if phase-separated amphiphilic copolymers confounded microbial adhesion. We used bottlebrush amphiphilic PEG/PDMS co-networks and homopolymer networks to study bacterial adhesion across a library of gels (ϕPEG = 0.00, 0.21, 0.40, 0.55, 0.80, and 1.00). Hydrated atomic force microscopy measurements revealed that most of the gels had low surface roughness, less than 5 nm, and an elastic modulus of ∼80 kPa. Interestingly, the surface roughness and elastic modulus of the ϕPEG = 0.40 gel were twice as high as those of the other gels due to the presence of crystalline domains, as confirmed using polarized optical microscopy on the hydrated gel. The interactions of these six well-characterized gels with bacteria were determined using Escherichia coli K12 MG1655 and Staphylococcus aureus SH1000. The attachment of both microbes decreased by at least 60% on all polymer gels versus the glass controls. S. aureus adhesion peaked on the ϕPEG = 0.40, likely due to its increased elastic modulus, consistent with previous literature demonstrating that modulus impacts microbial adhesion. These findings suggest that hydrophilic, hydrophobic, and amphiphilic biomaterials effectively resist the early attachment of Gram-negative and Gram-positive microorganisms, providing guidance for the design of next-generation antifouling surfaces.

Programming Mechanical Properties through Encoded Network Topologies

Polymer networks remain an essential class of soft materials. Despite their use in everyday materials, connecting the molecular structure of the network to its macroscopic properties remains an active area of research. Much current research is enabled by advances in modern polymer chemistry providing an unprecedented level of control over macromolecular structure. At the same time, renewed interest in self-healing, dynamic, and/or adaptable materials continues to drive substantial interest in polymer network design. As part of a special issue focused on research performed in the Polymer Science and Engineering Department at the University of Massachusetts, Amherst, this review highlights connections between macromolecular structure of networks and observed mechanical properties as investigated by the Tew research group.

Bottlebrush Networks: A Primer for Advanced Architectures

Bottlebrush networks (BBNs) are an exciting new class of materials with interesting physical properties derived from their unique architecture. While great strides have been made in our fundamental understanding of bottlebrush polymers and networks, an interdisciplinary approach is necessary for the field to accelerate advancements. This review aims to act as a primer to BBN chemistry and physics for both new and current members of the community. In addition to providing an overview of contemporary BBN synthetic methods, we developed a workflow and desktop application (LengthScale), enabling bottlebrush physics to be more approachable. We conclude by addressing several topical issues and asking a series of pointed questions to stimulate conversation within the community.

Network Constitutional Isomers

Bottlebrush networks designed to be constitutional isomers of each other were synthesized for the first time. These network constitutional isomers (NCIs) have significantly different mechanical properties depending on their kinetic chain lengths (RK), which are controlled by the monomer-to-initiator ratio. Specifically, the low frequency moduli, yield behavior, elongation at break, and adhesive strength of these NCIs are different at the same cross-link densities. The NCI concept is extended to include RKs' dispersity through the choice of the catalyst. These NCIs highlight the impact of living polymerization chemistry on network formation. The use of living polymerization chemistry to synthesize new networks, including NCIs, is expected to significantly advance the development of next-generation materials.