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

Abstract

We compare the low-strain mechanical properties of bottlebrush elastomers (BBEs) synthesized using ring-opening metathesis and free radical polymerization. Through comparison of experimentally measured elastic moduli and those predicted by an ideal, affine model, we evaluate the efficiency of our networks in forming stress-supporting strands. This comparison allowed us to develop a structural efficiency ratio that facilitates the prediction of mechanical properties relative to polymerization chemistry (e.g., softer BBEs when polymerizing under dilute conditions). This work highlights the impact that polymerization chemistry has on the structural efficiency ratio and the resultant mechanical properties of BBEs with identical side chains, providing another “knob” by which to control polymer network properties.

Network Constitutional Isomers

“…Due to the living nature of ROMP and the absence of any significant chain transfer, the R K of these ROMP polymers is controlled via variation of the M / I ratio. Within this study, M / I is varied from M / I = 10 to M / I = 1000 to synthesize BBNs with significantly different R K s. The gel fraction for all BBNs is >90% (Figure S5) and ssNMR confirms full conversion of closed-ring norbornene into open-ring norbornene (Figure ), reiterating the high conversion of ROMP BBNs …”

“…Fortunately, the easy-to-manipulate architecture of bottlebrush polymers and bottlebrush networks (BBNs) present an elegant solution to this problem. − BBNs are a class of polymer networks where the high grafting density and length of side chains stretch the polymeric backbone and its side-chains due to steric repulsion, leading to a marked lack of physical entanglements. − Polymerization of BBNs via a living polymerization method would provide direct control over R K in a material in which the effects of entanglements can be disregarded. The highly popular ring-opening metathesis polymerization (ROMP) is uniquely positioned to study the role of R K in BBNs due to its high conversion, living nature, lack of chain transfer, and established use in synthesizing BBNs. ,− Using ROMP, R K can be easily controlled by varying the monomer-to-initiator ratio ( M / I ), with large M / I s producing longer R K s.…”

Adhesion and mechanical properties of poly(dimethylsiloxane) bottlebrush elastomers

“…Though trapped entanglements in polymer networks can significantly impact mechanical properties, [18][19][20] the extent of entanglements in the BBEs studied here was assumed to be negligible based on previous experimental and scaling analysis. 17 Furthermore according to the multi-angle laser light scattering (MALLS) results (Fig. S4, ESI †), the molar mass of the uncrosslinked PDMS bottlebrush polymer was 2.92 Â 10 5 gmol À1 and PDI of it was 1.92, which is much lower than the previously reported molar mass of the entanglement strands, M e , of PDMS bottlebrush polymer with the same n g and n sc .…”

“…Based on the volume fraction of backbone monomers and the length of side chains (Table S2, ESI †), these BBEs belong to the stretched side chain (SSC) regime in the diagram of states of graft copolymers. 6,8,17 In this regime, all side chains stretch to satisfy the monomer packing condition. The softness of BBEs is well-controlled with n x and the elastic modulus has a linear relationship with the inverse of n x like previous studies 4,10,17 (Fig.…”

“…6,8,17 In this regime, all side chains stretch to satisfy the monomer packing condition. The softness of BBEs is well-controlled with n x and the elastic modulus has a linear relationship with the inverse of n x like previous studies 4,10,17 (Fig. S5(a), ESI †).…”