Effect of Nanoconfinement on the isodimorphic crystallization of Poly(butylene succinate-ran-caprolactone) Random Copolymers

Abstract

In this paper, we study for the first time the effect of hard confinement on the isodimorphic crystallization of random copolyesters. A series of poly(butylene succinate-ran-caprolactone) samples encompassing the entire composition range were successfully infiltrated into nanoporous alumina templates (AAO) with a fixed diameter of 100 nm. Samples were characterized using scanning electron microscopy (SEM), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC) and grazing-incidence wide-angle X-ray diffraction (GIWAXS). FTIR proved that PBS-rich nanofibers showed interactions between the copolyester chains and the walls of the AAO templates; however, the PCL-rich nanofibers did not show any interaction with the AAO walls. All infiltrated samples experienced a very large decrease in crystallization temperature, as expected, since the level of confinement is large enough to suppress heterogeneous nucleation. In spite of this, all copolymers were able to crystallize, regardless of composition. Additionally, when Tc or Tm are plotted versus copolymer composition a clear pseudo-eutectic point is observed. These results indicate that the isodimorphic behavior of the copolyesters is maintained under hard confinement. Infiltrated PCL undergoes homogeneous nucleation (inside the nanopore volume) as expected by the lack of interaction with the AAO walls. On the other hand, PBS exhibited a surface nucleation mechanism triggered by the interactions with the AAO walls. In the case of random copolymers, all PCL-rich copolymers nucleated homogeneously. However, in the case of PBS-rich copolymers, the nucleation gradually changed from surface-induced nucleation for neat PBS to homogeneous nucleation, as PCL concentration in the copolymers increased and the interactions of PBS chains with the AAO walls are diluted. The confinement under 100 nm nanopores did not change the orientation of the PBS or PCL phase crystals, which kept their chain direction perpendicular to the pore axis, as demonstrated by GIWAXS.