Beyond Lattice Matching: The Role of Hydrogen Bonding in Epitaxial Nucleation of Poly(hydroxyalkanoates) by Methylxanthines

Xanthine and a series
of its methylated derivatives were evaluated
as nucleating agents for poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx), and the kinetics of isothermal and
nonisothermal crystallization of neat and compounded samples were
evaluated via differential scanning calorimetry. Xanthine and theobromine
were effective as nucleating agents, whereas caffeine and theophylline
were not. Xanthine and theobromine shortened the crystallization half-lives
for isothermal crystallization and decreased the ΔEa of the combined crystallization and nucleation process
from −51 to −160 kJ mol–1. The efficacy
of methylxanthine derivatives in poly(hydroxyalkanoate) (PHA) nucleation
correlates to the presence or absence of a hydrogen bond-donating
imide nitrogen along specific crystal surface planes that may template
periodic interactions within the melt despite marginal crystalline
lattice parameter matching between nucleating agent and polymer crystal.
The results indicate that the nucleation of PHBHHx by these imide-containing
compounds is governed by periodic hydrogen bonding motifs, which do
not necessarily correspond with crystalline lattice parameters.