Scientists from the CIRI beamline are developing a method to calculate the orientation of macromolecules in three-dimensional space based on microscopic measurements using mid-infrared light polarized linearly at different angles. Reconstructing the experimental shape of the absorption dependency on polarization by fitting a nonlinear function to the data points obtained from the measurements allows for determining the angles that define the orientation of macromolecules in three-dimensional space, as well as a parameter describing the degree of sample organization. The measurements were conducted using an FPA array detector available at the FT-IR microscopy end-station on the CIRI beamline.

This method is non-destructive and does not require labeling, making it particularly useful for imaging and quantitatively determining the order parameters in various types of anisotropic polymer and biological samples.

The study focuses on the structural characterization of poly(lactic acid) (PLLA), a biodegradable polymer known for its polymorphism and significant potential in various applications due to its environmentally friendly nature. Scientists prepared thin PLLA films and, through thermal treatment, obtained a sample with varied morphology, including an amorphous phase, isolated spherulites, and larger clusters of the semi-crystalline phase. The reconstruction of the three angles of macromolecule orientation revealed that the morphological organization in the amorphous phase is random, and the molecular orientation differs from the semi-crystalline phase also in the third dimension in thin films.

The findings show that the 4P-IR method can provide detailed insights into the molecular organization of materials. The parameter of the degree of organization <P2> enables the differentiation and localization of amorphous and crystalline regions based, offering new insights into polymer morphology. Scientists emphasize the potential of this method in better understanding polymer structures and contributing to the development of high-quality materials with tailored properties. The "four polarizations" 4P-IR method could become a valuable tool for scientists studying complex polymer systems.

Figure 1. Optical microscopic image of the PLA film (a). 4P-3D orientation results for a pair of 1088–1041 cm^–1. Visualization of the primary transition moment (1088 cm–1): centers of nucleation (b), borders between spherulites and amorphous phase (c), and the <P2> image of the same region (d). 

Author: dr inż. Karolina Kosowska

Link to the publication: K. Kosowska, Paulina Kozioł-Bohatkiewicz, Maciej Roman, Tomasz P. Wrobel, Insight into Poly(L‑lactic acid) Coexisting Amorphous and Semicrystalline Phases Using FT-IR Imaging and Four Polarization-Three Dimensional Macromolecular Orientation Calculation, Macromolecules, 57, 9, 4335-4344 doi: 10.1021/acs.macromol.3c01593 

This research was supported by the National Science Centre, Poland (MINIATURA 6, “Optimization and development of the method of imaging the orientation of 3D macromolecules - crystallization of polymers under various conditions”, Grant. No. 2022/06/X/ST4/01454).