Effects of Relative Humidity on the Surface and Bulk Structures of Linear Polyethylenimine Thin Films

Abstract

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 surface-specific 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.