Unraveling Hydration- and Temperature-Induced Structural Transformations of Linear Poly(ethylenimine) Using Raman Microscopy and Numerical Unmixing

Linear poly(ethylenimine) (LPEI) is a hydration-sensitive polymer of interest for its breadth of application in areas that include medicine, energy, and materials science. To facilitate advances, this work demonstrates in situ Raman microscopy and self-modeling curve resolution (SMCR) analysis as an approach for investigating changes in LPEI structure during melting and crystallization under controlled heating and cooling conditions. With focus on neutral (freebase) LPEI, SMCR analysis of Raman data sets showed that the melting transition of anhydrous LPEI was well described by a pair of spectral vectors representing anhydrous crystalline and amorphous states. However, the transition from the melted to the crystalline phase was more complex, requiring an additional spectral vector. This analysis revealed insights into the evolution of hydrogen bonding interactions as a sample progresses from an amorphous to a strongly hydrogen-bonded, helical-structured anhydrous crystalline state. Building on the understanding gained through measurements on the neutral anhydrous polymer, the approach was applied to identify LPEI structures that are prevalent during the heating and cooling of a neutral hydrated sample, where hydrogen bonding with water adds complexity to the thermal transitions. These findings lay a foundation for investigating structure within more complex LPEI formulations, particularly protonated forms that have wide-ranging practical significance.