Polyethylenimine (PEI) polymers have become increasingly utilized for a myriad of applications including self-decontaminating materials and nonviral gene transfection. While the bulk properties of PEIs have been studied in detail, their surfacespecific structure/behavior remain unexplored. Here, we report the effects of relative humidity on the surface structure of linear polyethylenimine (LPEI), as investigated by vibrational sum frequency generation (VSFG) spectroscopy. Results show that the surface structure of (as prepared) anhydrate LPEI is highly dependent on relative humidity. As the relative humidity is varied from 0% to 75%, surface spectra of LPEI in the C−H and N−H stretching regions reveal multiple crystalline and amorphous states, including the gel-phase amorphous state that has previously only been observed in appreciable quantities above LPEI's upper critical solution temperature (64°C). Utilizing DFT calculations, we have assigned large characteristic frequency shifts (∼50 cm −1 ) of LPEI anhydrate crystalline methylene modes to the Bohlmann effect, which is the delocalization of the nitrogen lone electron pair causing a weakening of the C−H bonds of methylene moieties adjacent to the amine functionality. Similar frequency shifts (∼20 cm −1 ) observed in the hydrated crystalline forms are likely due to intermolecular interactions mediated by hydrogen bonding within the LPEI/water matrix.
■ INTRODUCTIONPolyethylenimines (PEIs) have found widespread utility in numerous fields including chemical and biological sensing, 1 carbon dioxide capture, 2 and electrical energy storage. 3 Recently, some of the more interesting and impactful applications are based around PEI's biocompatibility, where it is being investigated for its use as a nonviral vector for gene transfection, 4−8 enzyme or protein stabilization, 9,10 and cell culture. 11 Additionally, due to the generally high concentration of reactive amine moieties present in PEI materials, they are also of interest for reactive/self-decontaminating technologies for protection against nerve-type chemical warfare agents (CWAs), wherein amine groups hydrolyze CWAs to produce less lethal or nonlethal byproducts. Although surface reorganization of various polymeric materials due to environmental adsorbates (water and organics) has been established, 12−16 the characterization of self-decontaminating materials' environmentally mediated surface reorganization and its effects on surface functionality and reactivity is largely absent. To these ends, developing a surface-specific, molecular level understanding of the behavior of polyethylenimine films in varied environmental conditions will provide valuable insights into the molecular-level details that mediate the functionality of these materials for numerous applications.In the present study, we employ linear polyethylenimine (LPEI) as a model system for understanding the effects of relative humidity, and therefore hydrogen bonding (amine− amine, water−amine, and water−water hydrogen bonding), on the surface stru...