Controlling of Chain-Level Structure of Polymer via Freeze-Drying

Freeze-drying (FD) method may potentially control chain-level structures of polymer with varying entanglement densities. In this work, we investigated phase structure, chain-to-chain distance, <R> and self-assembled chain structure of ¹³C labeled poly(L-Lactic Acid) (PLLA) formed via FD from both good and poor solvents, by solid-state (ss) NMR. It is found that the FD-PLLA from the good solvent adopts an amorphous state while the FD one from the poor solvent forms a semicrystalline state with a crystallinity of ~ 40 %. By comparisons of the ¹³C-¹³C Double Quantum (DQ) experimental and simulated curves, it is demonstrated that i) FD-PLLA chains from both poor and good solvents adopt the same <R> value of 6.2 Å. ii) FD-PLLA chains in the latter adopts much larger nanoclusters via folding than those in the former, and is slightly smaller than those in the single crystals. While the FD-PLLA chains obtained from good solvent exhibited similar structure with the bulk glassy state. It is concluded that different solvent-polymer affinities do not affect <R> and significantly affect self-assembly structure of individual chains.