Contrast enhanced microscopy digital image correlation: a general method to contact-free coefficient of thermal expansion measurement of polymer films.

Thermal expansion represents a vital indicator of the processing history and dimensional stability of materials. Solvent-sensitive, thin, and compliant samples are particularly challenging to test. Here we describe how textures highlighted by contrast enhanced optical microscopy modes (i.e., polarized light (PL), phase contrast (PC)) and bright field (BF) can be used to determine the thermal expansion of polymer films in a contact-free way using digital image correlation (DIC). Three different films were explored: polyetherimide (PEI), polyimide (PI), and polyethylene naphthalate (PEN). Image textural features (e.g., intensity, size, speckle pattern characteristics) obtained by BF, PC, and PL were analyzed by two-dimensional Fourier transform and autocorrelations. The measured in-plane CTEs of PEI, PI, and PEN films, 51.8, 20.5, and 10.2 ppm/K, respectively, closely approached those previously reported using DIC with artificially applied speckle patterns.