Application of Fourier's law in thermally induced phase separation (TIPS) process for porous poly(L-lactide) films

Porous films were fabricated from poly(L-lactide) (PLLA)-1,4-dioxane-dimethylacetamide ternary systems via thermally induced phase separation (TIPS) followed by using freeze extraction. The effect of processing parameters including freezing temperature, quenching time, and the film thickness was investigated on the physical, mechanical, and morphological properties of the porous films. The primary objective of increase in the film thickness is to cause a decreased temperature gradient between external layer and internal layers at the same temperature (according to Fourier's law) and to decrease the growth rate of the solvent-rich phase. The results revealed that increasing thickness from 1200 to 1600 mu m at - 20 degrees C for 6 h resulted in producing pore sizes 200 nm and nanofibrous (> 200 nm fiber diameter) instead of 400 nm and nanofibrous (< 100 nm fiber diameter). The nanofibrous (< 100 nm) exhibit higher porosity and water absorption, and lower tensile strength than the nanofibrous (> 200 nm). As a result of the high film thickness, decreasing temperature from - 20 to - 45 degrees C (increasing temperature gradient), and increasing time from 6 to 24 h, it is indicated that the phase separation happens via the coarsening process and forms nanospindle structure with the pore size 800 nm.