Cyanuric Acid-Induced Rapid Crystallization of Poly(3-hydroxybutyrate-co-4-hydroxybutyrate) under Fast Cooling via Lattice Matching and Hydrogen Bonding

Poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P34HB) is a promising biodegradable polyester, but its slow crystallization rate often leads to insufficient crystallization during fast-cooling processing. Although nucleating agents can usually accelerate polymer crystallization under conventional cooling, their nucleation efficiency often decreases markedly at high cooling rates because the time available for chain rearrangement becomes limited. Herein, cyanuric acid (CA) is proposed as an organic small-molecule nucleating agent for P34HB under fast-cooling conditions, and its crystallization-regulating effect, nucleation mechanism, and overall properties are systematically investigated. The results show that, with 0.75 wt % CA, the crystallization temperature of P34HB increases from 88.0 to 109.1 °C at a cooling rate of 10 °C/min, while the half-crystallization time decreases from 1.77 to 0.45 min. More importantly, under fast cooling at 80 °C/min, P34HB/CA0.75 and P34HB/CA1 still show pronounced crystallization during cooling, and no obvious cold-crystallization peak is observed during the subsequent heating scan. When directly cooled to 90 °C without isothermal holding, P34HB/CA0.75 still achieves a crystallinity of approximately 35%. Mechanistic analysis indicates that the lattice mismatch between twice the b-axis length of CA and the b-axis of P34HB is only 1.33%, suggesting a crystallographic condition favorable for lattice-matched heterogeneous nucleation. In addition, CA induces conformational preordering of P34HB chains through hydrogen bonding, thereby promoting the transition of disordered chain segments into stable crystalline regions. The synergistic effect of lattice matching and hydrogen-bond-assisted conformational preordering enables CA to maintain high nucleation efficiency during fast cooling. With the enhanced crystallization and possible contributions from crystalline morphology and CA-P34HB interactions, the heat deflection temperature (HDT) and water vapor barrier performance of P34HB/CA blends are simultaneously enhanced; specifically, the HDT increases from 104.6 to 135.4 °C, and the water vapor permeability decreases by approximately 42%, while the mechanical properties remain essentially unchanged. This study demonstrates that the cooperative design of structural matching and intermolecular interactions is an effective strategy for improving the fast-cooling crystallization efficiency of P34HB.