Partial Miscibility-Driven Nucleation of Poly(3-hydroxybutyrate- <i>co</i> -3-hydroxyhexanoate) with Indigo Dye

Melt-miscible nucleating agents for poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHB-co-HHx) can enhance the processability of the polymer for thin film and fiber applications. However, most melt-miscible nucleators have poor nucleation efficacy compared to nonmiscible nucleating agents such as orotic acid. In this work, indigo was blended into PHB-co-HHx, and its performance as a melt-miscible nucleating agent was evaluated. Indigo was found to dye the polymer at all loadings, but it was observed that indigo nucleates the polymer at loadings >0.075% w/w, with a 0.1% indigo sample having a Tc and %Xc of 92.2 °C and 37.8%, respectively. These values were found to be comparable to 1% orotic acid (Tc – 94.9 °C, %Xc – 36.7%), despite the substantially lower indigo loading. Subsequent analysis by polarized optical microscopy (POM) revealed that at low loadings, indigo was melt-miscible with PHB-co-HHx, which contributed to better nucleating agent dispersion throughout the polymer matrix. Additionally, the 1/t1/2,Theo. value of 0.1% indigo was observed to be 0.03 min–1 lower than 1% orotic acid at 100 °C, exhibiting nearly identical crystallization kinetics. Interestingly, the lattice parameter matching and potential H-bond interactions between indigo and PHB-co-HHx were found to be much worse than the matching between orotic acid and PHB-co-HHx. Instead, the excellent nucleation performance of indigo was attributed to partial miscibility at 0.1% loading, which allowed for thorough dispersion and even self-assembly of the nucleating agent in the polymer.