Interplay between poly(ethylene oxide) and poly(l-lactide) blocks during diblock copolymer crystallization

Abstract

The influence of composition and crystallization conditions on the behavior of double crystalline poly (ethylene oxide-b-L-lactide) (PEO-b-PLLA) diblock copolymers is investigated. Poly(L-lactide) contents in the synthesized copolymers vary from 50 to 91%, and the molecular weight of the PLLA block ranges from 2 to 20 kg mol⁻¹, while that of the PEO block is kept constant at 2 kg mol⁻¹. In bulk samples, DSC results show a synergistic interaction between the crystallization processes of the two blocks. The PEO block provides heterogeneities and exerts a plasticizing action which favors the crystallization of the PLLA block with a nucleation efficiency of 30%. In contrast, the subsequent crystallization of the PEO block is subject to two opposing effects: (a) the nucleating action of PLLA crystals and (b) the topological and geometrical constraints imposed by PLLA crystals, especially when the PEO content is 20 wt% or less. In the case of ultra thin films, block copolymers with PEO contents equal or smaller than 20 wt% form distorted PLLA single crystals when crystallized from the melt. However, upon increasing the PEO content in the system to 33 wt% (by blending or copolymerization), the distortions disappear and the angle between the {110} growth faces changes from 140° to 121°, since the PEO block acts as a solvent or plasticizer for the PLLA block during the crystallization process. PEO incorporation can therefore tailor the rate and morphology of PLLA block crystallization. TEM and AFM studies allowed direct observation of the PEO block dendritic crystals on the surface of lozenge-shaped PLLA crystals previously formed during cooling from the melt.