Birefringent properties of 3D printed and other polymers

Birefringence describes the optical properties of optically anisotropic or chiral materials. Linear birefringence is defined as the difference between the refractive indices of the extraordinary and ordinary ray direction, i.e., Δn = n_e – n_o, and proportional to the optical path difference. Circular birefringence, on the other hand, occurs in chiral materials and describes the rotation of the plane of polarization.

As linear birefringence is a measure of anisotropy in the structure of a glass or a polymer, stress induced in glasses or polymers can be measured by stress birefringence. Interestingly, this effect can also be found in 3D printed materials, such as fused deposition modeling (FDM)-printed models of different layer orientation and thickness [1], or stereolithography (SLA)-printed objects containing liquid crystals or nanofibers [2,3].

Here we report birefringence measurements on 3D printed translucent objects after different curing / thermal post-treatments, performed using a microscope with crossed polarizers as well as large-scale crossed polarizers, to measure linear and circular birefringence. These measurements can be extended to other polymeric objects, such as foils in solar panels or bulletproof glass, to investigate material aging in addition to residual stress.

[1] Roper, D. M.; Kwon, K.-A.; Malone, L. P.; Best, S. M.; Cameron, R. E. Induced Birefringence in 3D Printing: Concealing Information Optically within Printed Objects. Adv. Mater. Technol. 2022, 7, 2200139.

[2] Jeong, H. Y.; Lee, E.; An, S.-C.; Lim, Y. S.; Jun, Y. C. 3D and 4D printing for optics and metaphotonics. Nanophotonics 2020, 9, 1139-1160.

[3] Tang, A. M.; Li, J.; Li, J.; Zhao, S.; Liu, W. Y.; Liu, T. T.; Wang, J. F.; Liu, Y. Y. Nanocellulose/PEGDA aerogel scaffolds with tunable modulus prepared by stereolithography for three-dimensional cell culture. J. Biomater. Sci. 2019, 30, 797-814.