Tuning the Size of Cylindrical Micelles from Poly(l-lactide)-'b'-poly(acrylic acid) Diblock Copolymers Based on Crystallization-Driven Self-Assembly

Abstract

A series of poly(l-lactide)-'b'-poly(acrylic acid) (PLLA-'b'-PAA) diblock copolymers with a range of hydrophobic or hydrophilic block lengths were designed in order to tune the size of the resultant cylindrical micelles using a crystallization-driven self-assembly (CDSA) approach. The precursor poly(l-lactide)-'b'-poly(tetrahydropyran acrylate) (PLLA-'b'-PTHPA) was synthesized by a combination of ring-opening polymerization (ROP) and reversible addition-fragmentation chain transfer (RAFT) polymerization. The CDSA process was carried out in a tetrahydrofuran/water (THF/H₂O) mixture during the hydrolysis of PTHPA block at 65 °C using an evaporation method. A majority of PLLA-'b'-PAA diblock copolymers resulted in the formation of cylindrical micelles with narrow size distributions (Lw/Ln < 1.30) as determined by transmission electron microscopy (TEM) and dynamic light scattering (DLS). Furthermore, the length of PLLA block was found to control the length of the resultant cylindrical micelles while the length of PAA block governed their widths. Synchrotron small-angle X-ray scattering (SAXS) further proved that the length increase of these cylinders was a consequence of the decreasing PLLA block lengths. The crystalline core nature of these cylinders was characterized by wide-angle X-ray diffraction (WAXD), and the relative core crystallinity was calculated to compare different samples. Both the hydrophobic weight fraction and the relative core crystallinity were found to determine the geometry of the formed PLLA-'b'-PAA cylindrical micelles. Finally, changing the pH conditions of the CDSA process was found to have no significant effect on tuning the resultant dimensions of the cylinders.