The Influence of Side-chain Identity and Tacticity on Structural, Thermal, and Mechanical Properties of Syndiotactic Polyhydroxyalkanoates

Polyhydroxyalkanoates (PHAs) are a class of bioplastic polyesters whose properties can be tuned by polymer composition (i.e., side-chain identity) and microstructure (i.e., tacticity). Although key polymer structure–function relationships have emerged for isotactic PHAs (it-PHAs), similar relationships remain limited for syndiotactic PHAs (st-PHAs). Herein, we report a family of st-PHAs that vary in both side-chain identity (R = CH₃, C₂H₅, C₄H₇, C₄H₉, and C₆H₁₃) and tacticity (P_r ≈ 0.8, 0.9, and 0.99) and characterize their thermal, scattering, morphological, and mechanical properties. Generally, st-PHAs displayed comparable or higher melting temperatures (T_m) than it-PHAs, where T_m increased with (i) increasing syndiotacticity (ΔT_m ≈ 30 °C) and (ii) decreasing side-chain length (ΔT_m ≈ 100 °C). Similar to it-PHAs, scattering measurements revealed systematic increases in b-dimension (∼15.1–22.7 Å), cell volume (874–1170 ų), and long-period (12.9–14.9 nm) with increasing side-chain length. Unlike synthetic and bacterial it-PHAs, isothermal crystallization of st-PHAs generated spherulitic microstructures without banding, and suggested hierarchical structural differences between the two microstructures. Finally, tensile measurements of solvent-cast and melt-pressed dog-bone specimens revealed mechanical properties that were sensitive to side-chain identity. Similar to it-PHAs, Young’s moduli of st-PHAs increased with decreasing side-chain length (E: 52–831 MPa). In contrast, elongation to break (ε; 3.7–183%) and toughness (U_T: 0.2–54.6 MJ/m³) of st-PHAs varied nonmonotonically with respect to side-chain length. Our studies highlight key similarities and differences between it- and st-PHAs, and suggest unique and complementary opportunities to tune polymer properties with control over polymer microstructure.