Molecular Dynamics of Polymyrcene: Rheology and Broadband Dielectric Spectroscopy on a Stockmayer Type A Polymer

Molecular dynamics
of polymyrcene (PM) covering a wide range of
molecular weights from 6.7 × 103 ≤ Mw ≤ 3.47 × 105 g/mol
(weight-average molecular weight Mw) with
a molar mass dispersity Đ ≤ 1.16 were
investigated by oscillatory shear rheology and broadband dielectric
spectroscopy. The analysis of the zero-shear viscosity (η0) versus molecular weight in a double-logarithmic plot reveals
a scaling exponent of ∼1.2 below the critical molecular weight
and ∼3.7 above a critical molecular weight of Mc ≅ 4.4 × 104 g/mol. However, at
molecular weights greater than the reptation molecular weight Mr, a molecular weight substantially larger than
the entanglement molecular weight Me,
the zero shear viscosity scaling decreases. A scaling of η0 ∝ M3.0, which might be
attributed to a pure tube-reptation behavior, was found. The exponent
of 1.2 is attributed to a Rouse-like dynamic and is interpreted in
terms of PM’s bottlebrush-like nature arising from tightly
packed C6/C8 pendant groups. Dielectric spectroscopy measurements
revealed two relaxation processes typical for fully amorphous glassy
polymers: The first is a segmental mode process, with a dynamic glass
transition process (α), due to the segmental motions and the
second one is a local relaxation process in the glassy state (β).
Additionally, above the glass transition temperature at low frequency,
another relaxation process was observed, which appears to be the normal
mode process (n) due to the fluctuation of the end-to-end vector.
This polymer is assigned to be a new Stockmayer type A polymer, whose
normal mode relaxation is strongly dependent on its molecular weight.
These findings allow for the rare possibility to explore the global
dynamics of PM by using both oscillatory shear rheology and broadband
dielectric spectroscopy methods over a wide range of temperatures
to investigate molecular dynamics using the time–temperature
superposition principle.