Melting and crystallization temperatures of semi-crystalline polymer thin films studied via spectral reflectance

For semi-crystalline polymers, the melting and crystallization temperatures play critical role in understanding the importance of supercooling on the kinetics of crystallization. Thus, being able to measure changes in the melting and crystallization temperature as a function of thin film confinement is incredibly important. Until recently, these measurements required the use of highly sophisticated and expensive equipment such as nano-DSC, ellipsometry, or grazing-incidence wide angle X-ray scattering (GIWAXS). We have shown that that spectral reflectance, a widely used and inexpensive technique for measuring film thickness, can be used to identify the melting and crystallization transitions in thin films (500 nm down to 20 nm thickness) with single degree resolution. This approach is based on measuring characteristic expansion and contraction profiles during the melting and crystallization transitions, respectively. We have validated this technique against DSC, GIWAXS, and atomic force microscopy. Additionally, we show that in poly(caprolactone) thin films, the melting temperature is not affected by film thickness whereas poly(ethylene glycol) thin films show a strong melting point depression with decreasing film thickness.