Molecular Dynamics and Viscoelastic Properties of the Biobased 1,4-Polymyrcene
Ioannis Tzourtzouklis, Christoph Hahn, Holger Frey, George Floudas
Abstract
We report the synthesis and dynamics of a series of polymyrcene homopolymers, all with identical microstructures (95% 1,4-units and 5% of 3,4-units), the latter by dielectric spectroscopy and rheology. Polymyrcene belongs to an important class of bio-based polymers (polyterpenes) with known members, the cis-1,4-polyisoprene and the cis-1,4-polyfarnesene. Polyterpenes constitute a class of type-A polymers where by architectural design, one can control the thickness of chains and henceforth the segmental and chain dynamics. The dielectric, rheology, and thermodynamic results showed a lower glass temperature in cis-1,4-polymyrcene as compared to cis-1,4-polyisoprene. A weak dependence of the segmental and longest normal mode on pressure revealed the dominant effect of the backbone. Furthermore, comparing polymyrcene with available literature data of polyisoprene and polyfarnesene, we report the effect of chain thickening on the viscoelastic properties. The plateau modulus, GN0, decreased, the entanglement molar mass increased (from 5 kg·mol–1 in cis-1,4-polyisoprene to 22 kg·mol–1 in cis-1,4-polymyrcene), and the packing length, p, increased (from 3.1 Å in cis-1,4-polyisoprene to 4.7 Å in cis-1,4-polymyrcene) as anticipated by chain thickening. The plateau modulus, GN0, followed the proposed empirical relation: GN0 = 0.00226kBT/p3, further reflecting the proportionality between the tube diameter and the packing length.