Molecular Dynamics of Poly(L-Lactic Acid) at around Glass Transition Temperature Elucidated by Solid-state NMR

Chain dynamics in so-called $\alpha_{\mathrm{c}}$ mobile crystals obey Arrhenius type behaviors at temperatures well above the glass transition temperature ($T_{\mathrm{g}})$ and below the melting temperatures ($T_{\mathrm{m}})$, while segmental motions of amorphous components above $T_{\mathrm{g}}$ follows WLF behaviors. If polymer chains in the crystalline regions perform overall chain dynamics at temperature around $T_{\mathrm{g}}$, how does dynamic correlation time \textless $\tau _{\mathrm{c}}$\textgreater change as a function of temperature? PLLA possessing a relatively high $T_{\mathrm{g}} \approx $ 60 $^{\circ}$C will provide an opportunity to challenge such a general question in polymer dynamics. Here molecular dynamics of PLLA chain in homo- ($\alpha \alpha $', and glassy states) and stereocomplex (SC) systems are investigated by Solid--State NMR. Results verify that the chains within crystalline region in $\alpha $ and SC begin molecular dynamics at temperatures well above $T_{\mathrm{g}}$ and temperature dependence of \textless $\tau _{\mathrm{c}}$\textgreater in both systems follows Arrhenius behavior. In the disordered $\alpha $' phase, the molecular dynamics of the backbone continues even at temperatures below $\sim$ $T_{\mathrm{g}}+$ 10 $^{\circ}$C. The temperature dependence of \textless $\tau_{\mathrm{c}}$\textgreater shows a non-Arrhenius behavior. The unique temperature dependence of molecular dynamics of PLLA in glassy state, disordered crystals, and stable crystals will be elucidated.