Peering at a buried polymer-crystal interface: probing heterogeneous nucleation by sum frequency generation...

Sum frequency generation vibrational spectroscopy (SFG-VS) has been applied to investigate the selective crystallization of two forms of acetaminophen (ACM) on polymer surfaces. To our knowledge, this is the first account of SFG-VS being applied to study a polymer-crystal interface. SFG elucidates the molecular-level interactions governing phase selection at this buried interface, providing insight into the process of polymer-induced heteronucleation (PIHn) in solution as well as from the vapor phase. ACM heteronucleates from supersaturated aqueous solution in the metastable orthorhombic crystal form on poly(methyl methacrylate) (PMMA) surfaces, whereas the thermodynamically stable monoclinic crystal form is observed to form on poly(n-butyl methacrylate) (PBMA) surfaces. When the ACM crystals were grown by sublimation, only the monoclinic form was observed on both PMMA and PBMA. SFG-VS results indicate that hydrogen bonds are formed between PMMA C═O groups and the orthorhombic ACM crystals at the PMMA-ACM interface. At PBMA-monoclinic ACM interfaces, no hydrogen bond formation was observed. This research demonstrates that SFG-VS can be used to probe molecular interactions at polymer-crystal interfaces. Understanding the interfacial molecular interactions will ultimately provide a rational basis for improving methods for polymorph discovery and selection based on heteronucleation on polymer surfaces.