Thin films of poly(d,l-lactic-co-glycolic acid), or PLGA, supported on silicon were characterized with ellipsometry, grazing incidence X-ray diffraction, and AFM to assess the effect of confinement on thermal expansion and crystallization. Films were spin-cast between 20 and 200 nm and were thermally ramped from room temperature to 150 °C followed by a subsequent ramp back down to room temperature. The associated change in the refractive index and thickness as determined with ellipsometry was consistent with one-dimensional expansion. In the absence of crystallization, the glass transition temperature (Tg) of the coatings was statistically equivalent to the bulk and did not show any trends as a function of thickness down to 20 nm. If the ramp back down to room temperature was interrupted by a temperature soak at 100 °C, a clear reduction in coating thickness was observed due to crystallization. Here, crystallization proceeded an order of magnitude faster than in the bulk, most likely propagating from the free (air/PLGA) interface to the buried (PLGA/SiO2) interface, in which the apparent Avrami exponent decreased. Two features especially stood out. First, despite the reduction in thickness upon crystallization at 100 °C, subsequent cooling of the coating back to room temperature resulted in a similar thickness (and density) to a coating with no temperature soak, presumably due to the competing effects between the mismatch in the thermal contraction of the crystalline and amorphous regions and confinement. The Tg of such crystallized coatings was also reduced by 2-5 °C. Second, the relative amount of crystallization decreased with a reduction in coating thickness, which may be attributed to suppressed crystallization at the buried interface.
Ikoba et al. (2026) studied this question.