Compressing and Swelling To Study the Structure of Extremely Soft Bottlebrush Networks Prepared by ROMP

Abstract

To fully explore bottlebrush polymer networks as potential model materials, a robust and versatile synthetic platform is required. Ring-opening metathesis polymerization is a highly controlled, rapid, and functional group tolerant polymerization technique that has been used extensively for bottlebrush polymer generation but to this point has not been used to synthesize bottlebrush polymer networks. We polymerized a mononorbornene macromonomer and dinorbornene cross-linker (both poly­(n-butyl acrylate)) with Grubbs’ third-generation catalyst to achieve bottlebrush networks and in turn demonstrated control over network properties as the ratio of macromonomer and cross-linker was varied. Macromonomer to cross-linker ratios ([MM]/[XL]) of 10 to 100 were investigated, of which all derivative networks yielded gel fractions over 90%. Because of its amenability toward small samples, contact adhesion testing was used to quantify dry-state shear modulus G, which ranged from 1 to 10 kPa, reinforcing that bottlebrush polymer networks can achieve low moduli in the dry state compared to other polymer network materials through the mitigation of entanglements. A scaling relationship was found such that G ∼ ([MM]/[XL])−0.81, indicating that macromonomer to cross-linker ratio is a good estimator of cross-linking density. The swelling ratio in toluene, Q, was compared to dry-state modulus to test the universal scaling relationship for linear networks G ∼ Q –1.75, and a measured exponent of −1.71 indicated good agreement. The synthetic platform outlined here represents a highly flexible route to a myriad of different bottlebrush networks and will increase the accessibility of materials critical to applications ranging from fundamental to biomedical.

Bottlebrush polymer gels: architectural control over swelling and osmotic bulk modulus

“…As it follows from Table 1, in the subregimes C Ã a , C Ã b , and C ÃÃ a , at fixed strand architecture, n/m = const, swelling ratio V/V dr is predicted to vary with the strand backbone length M as V/V dr B M 3nÀ1 . Recent experiments on chemically crosslinked gels 35 implementing mononorbornene macromonomer for ROMP polymerization and dinorbornene cross-linker (both composed of poly(n-butyl acrylate)) demonstrated the increase in V/V dr upon increasing macromonomer to cross-linker ratio, [MM]/[XL]. The latter correlates with the density of gel crosslinking 35 to approximate the average DP of the strand backbone as M B [MM]/[XL].…”

“…Recent experiments on chemically crosslinked gels 35 implementing mononorbornene macromonomer for ROMP polymerization and dinorbornene cross-linker (both composed of poly(n-butyl acrylate)) demonstrated the increase in V/V dr upon increasing macromonomer to cross-linker ratio, [MM]/[XL]. The latter correlates with the density of gel crosslinking 35 to approximate the average DP of the strand backbone as M B [MM]/[XL]. Each macromonomer contained approximately 30 n-butyl acrylate monomer units, and according to the scaling estimates, sizes of the peripheral concentration blob in the bottlebrush strand (modelled as cylindrical brush) and of the thermal blob are close.…”

Bottlebrush polymers in the melt and polyelectrolytes in solution share common structural features

“…Previous studies have established that bottlebrush polymers exhibit a lower propensity to entangle, a property derived from the relatively large size of the sidechains in comparison to the overall molecular dimensions (7)(8)(9)(10). The discovery of this aspect of bottlebrush polymers has catalyzed the development of an entire field centered on investigating ultrasoft, entanglement-free elastomers for soft robotics and biological tissue mimics (3,(11)(12)(13). However, the majority of the brush polymer literature focuses on solution properties and conformations, with interest in bulk properties only gaining traction in the last 5 years (14)(15)(16)(17)(18)(19)(20)(21)(22)(23).…”

Independently Tuning Elastomer Softness and Firmness by Incorporating Side Chain Mixtures into Bottlebrush Network Strands

“…10 (B) [ 0 , ] map depicting distinct classes of materials including linear elastomers (), 1,10,11 gels (), 6 brush-like elastomers (), 17,18 and tissue (). 6 In this regard, brush-like polymers 6,[17][18][19][20][21][22][23][24][25][26][27] offer a promising path forward by exploiting the oxymoronic duality of side chains as mechanical softeners and stiffeners. At a given network strand length, introducing side chains has two concurrent effects: (i) increased strand volume resulting in decreased 0 and (ii) strand extension resulting in enhanced .…”