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

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

SuFEx Postpolymerization Modification Kinetics and Reactivity in Polymer Brushes

Since its introduction in 2014, the sulfur­(VI) fluoride exchange (SuFEx) reaction has emerged as a promising new reaction in the field of polymer chemistry, in both polymerization of diverse polymer backbones and postpolymerization modification (PPM). Previously, we successfully reported the use of SuFEx chemistry as a method for surface derivatization through the PPM of sulfonyl fluoride containing polymer brushes. However, with the diversity of conditions, substrate scope, and catalyst selection afforded by this reaction, it is advantageous to expand the use of SuFEx for PPM on polymer brushes to discern the advantages and limitations of this reaction in surface conjugation. In this work, we used three different polymer brush systemsalkyl sulfonyl fluorides, aromatic sulfonyl fluorides, and aromatic fluorosulfonatesand each was reacted with three different silyl ether derivatives (aryl, alkyl, and benzyl). Each of these reactions was subjected to different catalysts, and herein, we present rates, conditions, and side products for PPM of polymer brushes using SuFEx chemistry. In addition, we explored the use of TBDMS brushes and their reaction with fluorosulfonate derivatives, where surprisingly no surface reaction occurs. With these studies, we are able to better understand the rates and limitations of this click reaction in the context of surface derivatization