Molecular bottlebrushes are branched polymers with very high graft density which results in very rigid backbones. This conformation results in very large entanglement molecular weight (Me) and very low elastic modulus, which could be used to produce super-soft elastomers. Despite the growing interest in bottlebrush polymers, very few studies have been devoted to their linear viscoelastic response, whereas, to our knowledge, no study has been reported on their extensional rheology or their response to cold-drawing. We synthesized a series of ultra-high molecular weight (UHMW) a-olefin molecular bottlebrushes by organometallic coordinative insertion polymerization of 1-alkenes with lengths ranging from 8 to 18 carbons. The molecular weight of these polymers are in the order of a few million g/mol, which allows accurate measurement of their rubbery plateau modulus (GN0) and their Mevalues. The latter is an increasing function of the side chain length (Nsc) and takes values ranging from 25 kg/mol (for poly(1-octene)) to 115 kg/mol (for poly(1-octadecene)). These UHMW bottlebrush polymers have sufficient melt strength to perform extensional rheology measurements using commercial extensional rheometers. Bottlebrush chain alignment was measured using a novel method that combines the use of extensional rheometry with time-resolved SANS. Those measurements were used to confirm the direct correlation between strain hardening and bottlebrush chain alignment, both in the melt and in the semi-crystalline state. In situ WAXS and SAXS reveal that chain alignment is concomitant of self-assembly of the bottlebrush molecules into hexagonal packed cylinder phases induced by uniaxial extension. This work reports the first direct evidence of strain-induced alignment and packing of molecular bottlebrushes and their relation with the macroscopic rheological and mechanical responses.

Crystallizable α-olefin molecular bottlebrushes: Microstructure evolution during extensional deformation

2019 Spring National Meeting

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