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

Abstract

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 ΔE a 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.

Balancing Melt Solubility and Morphology in Epitaxial Nucleation: The Case of Nicotinic Acid and Poly(hydroxybutyrate-co-hydroxyhexanoate)

“…Imide-containing compounds have been repeatedly shown to nucleate PHA, in addition to molecules containing multiple alcohol functional groups. − A series of other nucleators have been reported with varying degrees of nucleation efficiency including cellulose, lignin, saccharin, and ammonium chloride. − Previous work by Jacquel et al hypothesized that different crystalline lattice parameters may be responsible for periodic alignment of hydrogen bond donating groups in the nucleating agent and hydrogen bond accepting carbonyl groups of the PHA backbone . Additionally, our recent work suggests the importance of crystal faces and hydrogen bonding motifs in the nucleation of PHAs …”

“…Moreover, the internal angles of these fittings are within 10° of the internal b , c crystal angle of the PHB crystal (90°), increasing the likelihood of molecular alignment over multiple adjacent unit cells. The fittings found on the (110) and (120) faces exhibit suboptimal unit cell integer multiples (at least one integer of 3 in each case), which may explain the relatively low recrystallization temperature of PHBHHx in the presence of NA compared to other reported nucleating agents which successfully recrystallize PHBHHx at higher temperatures . The (12) and (11) faces are also considered unlikely candidates for nucleation surfaces.…”

“…The dimensions of the (100) PHB face are 13.18 and 5.93 Å, corresponding to the b and c unit cell parameters, respectively . The proposed molecular alignments of the nucleating crystal face are compared with multiples of the c and b dimensions of the PHB crystal, and the plausibility of the fitting is evaluated by (1) the degree of mismatch between the two values, which ideally should fall below 10% and which should be rejected if exceeding 15%, (2) the size of the integer multiple required to make such a fitting, of which 1 or 2 is desirable and 3 or greater is considered rare or less probable, and (3) the internal angle matching of the proposed face with that of (100) PHB (90°). − The crystal structure of NA is monoclinic, with unit cell dimensions of a = 7.186, b = 11.688, and c = 7.231 Å; however, these molecular distances correspond to the dimensions of the (100), (010), and (001) faces and will not necessarily be explicitly used in molecular fitting analysis. , Our lab has recently reported that PHBHHx nucleation by heterogeneous nucleating agents corresponds to hydrogen bonding motifs on the nucleating agents’ crystal faces that can align with the (100) PHB face . Here, we aim to rationalize NA nucleation by determining hydrogen bonding projections and distances on the (110) NA and (120) NA faces.…”

Directional Epitaxy Intensifying Intermolecular =C–H···O=C Hydrogen Bonding to Expedite Bulk Crystallization of Biobased Poly(butylene 2,5-furandicarboxylate)

“…Moreover, the important effect of secondary nucleation upon epitaxial crystallization between polymers was also highlighted . Very recently, Tong and co-workers have approved that lattice matching is indeed required but insufficient for successful epitaxy and the critical nuclei size dominates the epitaxial growth. , Despite the extensive studies with diverse systems over the past decades, regulating polymer crystallization via epitaxy is nowadays hardly desirable. ,,− More importantly, due to the very short range of interfacial epitaxial interaction, the past works involve polymer epitaxy focused on very thin films with the main evidence of crystal morphologies and orientation relationships obtained by atomic force microscopy, transmission electron microscopy, and electron diffraction, ,− while polymer epitaxy from bulk crystallization is seldom concerned and the featured evidence is essentially missing . Meanwhile, a molecular-level comprehension of the epitaxial mechanism remains controversial and is still far from conclusive. ,− ,, Therefore, understanding and controlling polymer crystallization via epitaxy especially in bulk materials on foreign surfaces have been and remain to be a serious challenge in polymer physics. − , …”

“…In general, NA provides surface that can decline the nucleation barriers to efficiently activate crystallization via interfacial adsorption, , chemical nucleation, , or epitaxial nucleation. − Great efforts have been devoted to elucidating surface-induced epitaxy as the heterogeneous nucleation theory upon foreign substrates. − Epitaxy is generally defined as the guest crystals growing on the surface of the host crystals with one or more strictly defined crystallographic orientations; that is, the lattice matching plays a decisive role . Wittmann et al have proposed the upper limit of 15% lattice mismatch between the deposit and the substrate for triggering epitaxial crystallization, , but some researchers have emphasized the importance of good interactions between polymers and NAs on both the epitaxial nucleation and polymorph selection, which decreases the interfacial free energy. ,, For example, beyond lattice matching, the epitaxial nucleation of poly­(3-hydroxybutyrate- co -3-hydroxyhexanoate) by methylxanthines has been reported to be governed by periodic hydrogen bonding . Lattice matching was also found to be inadequate to unravel the β-nucleation of polypropylene (PP) .…”