Probing 3D Molecular Orientation and Crystallization Behavior in PLLA/PEG Spherulites Using O-PTIR

Infrared imaging (IR), in addition to chemical specificity, has recently gained the capability to retrieve 3D molecular orientation information. When combined with IR super-resolution optical photothermal infrared (O-PTIR) spectroscopy, it enables unprecedented tracking of the three-dimensional macromolecular organization of complex layer systems in PLLA spherulites. The use of linearly polarized IR excitation, combined with a rapid computational algorithm applied to two nonparallel absorption bands, allowed tracing different paths of spherulite formation in PLLA/PEG films depending on the crystallization conditions. The plasticizer facilitated polymer crystallization but also caused disruption of molecular organization and a second crystallization, leading to lamellae branching. High-resolution O-PTIR microscopy, independent of the IR diffraction limit, does not require time-consuming sample preparation and, in combination with 3D orientation determination, is an excellent tool in studies on structural changes and morphology-property relationships in biology and materials science.