Molecular dynamics in nanophase-separated comb-like poly( $\upalpha$- n-alkyl $\upbeta$-L-aspartate)s

Abstract.

A series of poly($\upalpha$-n-alkyl $\upbeta$-L-aspartates) which are nanophase self-assembled comb-like polymers has been studied by dielectric spectroscopy in a broad frequency range ( 10^-2≤ν≤3×10⁶ Hz), with n-alkyls side chains of various lengths, 10≤n≤18. In every member of the series the same relaxations were identified after the decomposition of the experimental isothermal trace in up to three peaks with relaxation times distributions. The strength, width and average relaxation time for all the relaxation modes were determined for each material. Besides the local low temperature, Arrhenius modes, two relaxation modes, $\upalpha$ and $\upalpha_{{\rm PE}}^{}$, present a cooperative character whose dynamics are not affected by the side chains melting. The $\upalpha_{{\rm PE}}^{}$ relaxation is a polyethylene-like glass transition of the amorphous side chains and its dynamics is strongly dependent on the n value due to the increasing restrictions imposed by the self-assembled confinement. The strength of the $\upalpha_{{\rm PE}}^{}$ relaxation mode increases as the lateral chains loose their 2D order. The restricted chopstick motion of the rigid rods is thought to be the origin of the $\upalpha$ mode; this motion is hindered at temperatures where the cage size decreases as a result of the increasing disorder with temperature.