Synthesis and self-assembly of Poly(N-octyl benzamide)-μ-poly(ε-caprolactone) miktoarm star copolymers displaying uniform nanofibril morphology

Abstract

In this study, we employed an efficient strategy by the combinations of chain-growth condensation polymerization (CGCP), styrenics-assisted atom transfer radical coupling (SA ATRC), and ring-opening polymerization (ROP) to synthesize novel miktoarm (μ) star copolymers. Accordingly, poly(N-octyl benzamide) (PBA) and poly(ε-caprolactone) (PCL) arms were conjugated to form μ-(PBA)2(PCL)m star (m = ca. 4; Mn = ca. 13400; Đ = 1.17; abbr.: μ-(PBA)(PCL)). Thermal properties of homopolymers and μ-(PBA)(PCL) were examined by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). In μ-(PBA)(PCL) star copolymer, notably, an increased Tg of PBA chain (26.3 °C) was detected as well as the melting behavior in Tm (44.8 °C) and the peak profile were significantly changed. It might be due to topological influence on the restriction of both chain mobility and crystallinity. The behaviors of slow crystallization and low crystallinity of μ-(PBA)(PCL) star can be revealed by polarized optical microscopy (POM) and Fourier-transform infrared spectroscopy (FT-IR). We then analyzed morphology of drop-casting thin films of PCL, PBA, and μ-(PBA)(PCL) (co)polymers on Si wafer by atomic force microscopy (AFM). Casting by 1 wt% μ-star solution, we obtained uniform and clear nanofibers with an average diameter of ca. 20 nm. Using grazing-incidence X-ray diffractometer (GI XRD), orthorhombic (110) and (200) diffraction peaks were clear observed. In addition, grazing-incidence small-to-wide angle X-ray diffractometer (GI SWAXS) of μ-(PBA)(PCL) thin film displayed diffraction peaks composed of q/q* peaks of 1:√3 with a periodic size of approximately 16.1 nm that revealed the diameter of the nanofibril morphology.