Microphase separation and crystallization behaviors of bi-phased triblock terpolymers with a competitively dissolved middle block

A series of poly(ε-caprolactone)-b-poly(n-butyl acrylate)-b-polystyrene (PCL-b-PnBA-b-PS) triblock terpolymers with fixed PCL and PnBA block lengths but different PS block lengths were prepared. In all of these triblock terpolymers the PnBA block can be competitively dissolved in both the PCL and PS phases to form bi-phase structures in the melt, as revealed by atomic force microscopy (AFM). Due to the “competitive dissolution effect” of the PnBA middle block, the microphase separation and crystallization behaviors of these triblock terpolymers are different from those of common diblock copolymers to some extent. As revealed by the Flory-Huggins parameters, more PnBA segments tend to dissolved in the PS phase. Therefore, the volume fraction of the PS-rich phase (f_PS-rich) is evidently larger than the calculated volume fraction of the PS block (f_PS), and the phase boundaries between two different structures shift to lower f_PS. There also exists a thick interphase layer between the PS-rich and PCL-rich phases due to competitive dissolution of the PnBA block. However, confined crystallization can only occur at a larger f_PS-rich, though the measured glass transition temperature (T_g) is high for the PS block. This can be attributed to a high fraction of the soft zone surrounding the PCL-rich phase and the lower volume fraction of the hard zone.