Structural Evolution of Poly(L-lactide) Block Upon Heating of the Glassy ABA Triblock Copolymers Containing Poly(L-lactide) A Blocks

Abstract

Two triblock ABA copolymers poly(l-lactide-b-dimethylsiloxane-b-l-lactide) (PLLA-b-PDMS-b-PLLA) and poly(l-lactide-b-ethylene glycol-b-l-lactide) (PLLA-b-PEG-b-PLLA) containing poly(l-lactide) were synthesized by ring opening polymerization of l-lactide using bis(hydroxyalkyl) terminated PDMS and hydroxyl end-capped polyethylene glycol as macroinitiators, respectively. Both the triblock copolymers were melt-quenched to −40 °C to prepare the amorphous samples. The melt morphology of the triblock copolymers was preserved upon cooling the melt to −40 °C. PLLA and PDMS blocks are immiscible in the melt, and the amorphous triblock copolymer shows two distinct glass transition temperatures (Tg) on heating. On the other hand, PLLA and PEG segments are miscible and the triblock copolymer shows the single Tg. Structural evolution of PLLA during heating of the amorphous ABA triblock copolymers has been investigated by measuring the variable temperature small-angle and wide-angle X-ray scattering (SAXS and WAXS) and FTIR spectra. In the case of PLLA-b-PDMS-b-PLLA triblock copolymer (immiscible system), the mesophase of PLLA was found to appear just above the Tg of PLLA block (∼45 °C), and on further heating the mesophase changed to the regular α form at around 90 °C. On the other hand, in PLLA-b-PEG-b-PLLA (miscible system), the mesophase of PLLA was found to appear at lower temperature i.e. −20 °C, because of the enhanced molecular mobility of PLLA chains in the presence of PEG. However, irrespective of the mesophase formation temperature, the ordered α form has appeared at around 90 °C. In this way, during heating of the amorphous triblock copolymers, the PLLA block was found to crystallize into the ordered α form always through the mesophase just above the Tg of PLLA. These results are helpful in understanding the regularization process of semicrystalline polymers in different environments.