Helical synthetic polypeptides such as poly(γ-benzyl-L-glutamate) (PBLG) have created a unique opportunity to independently study the α-helical secondary structure. In thin films of PBLG, the choice of the casting solvent has been reported to influence the molecular packing configuration of α-helices and side chain interactions, resulting in distinct viscoelastic properties. In solution and melt, when chain length is increased or when PBLG is grafted as chains in brush polymers, the α-helical structure has been reported to consist of interrupted helices characterized by segments of random-coil and orientation defects as opposed to continuous free helices moreover, these segments could potentially lead to some form of entanglement observed in flexible polymers when chain length is increased. Motivated by these observations, we use thin films of PBLG to explore the effect of intermolecular interactions and interruptions in α-helices on viscoelastic properties when chain length is increased. PBLG films were cast from 1,2-dichloroethane (EDC) and N,N-dimethylformamide (DMF) using commercial PBLG homopolymers with 28 kDa, 66 kDa and 181 kDa number average molecular weight. Films cast from DMF and EDC were similar to known structural form B and form C respectively. The initial storage tensile modulus and the ultimate strength were higher in form C probably due to better orientation of the α-helices. However, in form B, chain length dependent molecular interactions associated with stacking of benzene rings are favored and restrict the side chain motion in the glass transition region, resulting in a higher storage tensile modulus in the rubbery plateau-like region. Within solvent series data, storage tensile modulus and ultimate strength increased with increase in chain length, suggesting a presence of broken rods, which have been observed in solutions studies, with sections of randomly coiled residues which partake in intermolecular chain entanglement and increase with increase in chain length<br/>

Influence of chain length and polymer-solvent interactions on the structure and viscoelastic properties of poly(γ-benzyl L-glutamate) films

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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