Effect of Confinement on the Thermal Expansion, Crystallization, and Density of Poly(lactic- co -glycolic acid) Copolymer Thin Film Coatings

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 ( T g ) 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/SiO 2 ) 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 T g 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.