Nonlinear Elasticity and Swelling of Comb and Bottlebrush Networks

Abstract

While chain branching generally promotes swelling of polymer networks, it also leads to nonlinear modulus increase with network expansion. To understand the effect of branched architecture on swelling, we study comb-like and bottlebrush networks using a combination of theoretical analysis, computer simulations, and experiments. The equilibrium swelling ratio of such networks is shown to be larger than that of conventional linear chain networks as a result of two effects: architectural disentanglement of network strands and amplification of polymer–solvent interactions by side chains. For networks of brush-like strands with poly­(dimethylsiloxane) side chains in toluene, we achieve a swelling ratio of Q = 30, which is larger than that of linear chain networks with the same strand length. All of the studied systems, including linear chain, comb, and bottlebrush networks, follow a universal scaling relation, G(Q) ∝ Q –δ, between the deformation-dependent shear modulus G(Q) and swelling ratio Q with scaling exponents δ = 2.6 ± 0.08 (simulations) and δ = 2.6 ± 0.12 (experiments). These values agree with the theoretically predicted exponent δ = 8/3, confirming dominant contribution of three-body interactions to the osmotic pressure which drives network swelling. The established correlations between network strand architecture, nonlinear elastic modulus, and equilibrium swelling ratio provide a general framework for architectural control of swelling capacity and the design of superabsorbent materials.

Injectable bottlebrush hydrogels with tissue-mimetic mechanical properties

“…Furthermore, the 10 to 30 wt % LBL hydrogels do not exhibit any sign of syneresis, which is attributed to a relatively high equilibrium swelling ratio ranging from 12 to 24 (table S7). This behavior is consistent with the swellability enhancement observed in chemical brush networks caused by disentanglement of bottlebrush networks strands and soft elasticity of brush networks ( 42 ).…”

“…Fourth, both chemical and architectural dissimilarity of linear and bottlebrush blocks results in strong microphase separation ( 40 , 41 ), which sustain up to 700% deformation as discussed later. Fifth, the softness and disentangled nature of brush networks enhance their swellability, allowing equilibrium swelling ratios up to V gel / V dry = 40 ( 42 ), which eliminates the after-gelation syneresis.…”

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

“…However in the majority of experimental systems the side chains and backbones of the strands are still relatively short that makes the behavior of these polymers closer to that of linear chains. The DPs of grafts in currently simulated bottlebrush networks 33 remain also B10-30 monomers, which makes it difficult to compare the predictions of the presented scaling model (dealing with asymptotically long side chains and backbones) with the results of available experiments and simulations. Notably, a similar situation took place upon publication of the first scaling models of polymer brushes formed by linear chains, 36,37 for which the theoretical predictions preceded the experimental realizations.…”

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

“…6 Linear elastomers (green) cannot reach tissue relevant softness due to linear entanglements. 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.…”