Symmetric star poly(substituted glycolide) homopolymers and their surface properties

Well-defined star poly(substituted glycolide) (s-PSG) homopolymers with predetermined lengths and numbers of arms, which are alternatives to polylactides and polyglycolides, may offer great opportunity for the modulation of their physical properties, such as glass transition temperature (T_g), crystallinity, hydrophobicity, and surface characteristics due to their geometric and structural differences. Herein, we report the synthesis of s-PSG homopolymers, including a four-armed symmetrical poly(L-diisopropyl glycolide) (4s-PLDIPG) and poly(L-diisobutyl glycolide) (4s-PLDIBG) from the ring opening polymerization (ROP) of their monomers in the presence of tin(II) 2-ethylhexanoate [Sn(Oct)₂] as a catalyst and pentaerythritol as an initiator via a core-first approach under melt conditions. 4s-PLDIPG 8 exhibits lower T_g, melting temperature (T_m) and crystallinity % than 4s-PLDIPG 10 (T_g: 33.7 °C vs. 35.9 °C; T_m: 143.9 °C vs. 183.4 °C; X_c: 16.7% vs. 19.1%) due to its lower M_n per arm. 4s-PLDIPG 8 also has a dramatically lower T_m and crystallinity % than its linear counterpart PLDIPG 17 (T_m: 143.9 °C vs. 190.6 °C; X_c: 16.7% vs. 26.7%) due to its short arm length. As the side chain length of s-PSG homopolymers increased, there was a corresponding increase in the water contact angles and surface roughness values of the thin films, while the surface free energy decreased. This correlation between side chain length and surface properties was further validated by SEM and AFM profiles, which confirmed the impact of extended side chains on the polymer's surface characteristics.