Influence of macromolecular architecture on the crystallization of (PCL₂)-b-(PS₂) 4-Miktoarm star block copolymers in comparison to linear PCL-b-PS diblock copolymer analogues

Miktoarm block copolymers (A₂B₂) composed of two poly(ε-caprolactone) (PCL) arms and two polystyrene arms (PS) were synthesized by a combination of ring-opening polymerization (ROP) and atom transfer radical polymerization (ATRP). Linear analogue PCL-b-PS diblock copolymer samples were also synthesized in almost identical composition regarding the content of each component. Almost all samples were found to be weakly segregated in the melt according to small angle X-ray scattering (SAXS) experiments. While the expected morphology (revealed by transmission electron microscopy, TEM) was found for the linear diblock copolymers, the miktoarm block copolymer samples exhibited different morphologies that indicated more entropie restrictions for chain stretching. For example, when a linear diblock copolymer with 41 % PCL formed lamellae, the analogue miktoarm star copolymer with 39% PCL formed hexagonally packed PCL cylinders in a PS matrix. These results may imply changes in the phase diagram between miktoarm and linear block copolymers that have been previously predicted theoretically, however a larger number of samples should be used to corroborate this hypothesis. Additionally, the effect of polydispersity of the samples as a possible source of phase boundary variations should also be considered. The enhanced topological restrictions in the miktoarm star copolymers were also strongly reflected in the overall crystallization kinetics of the PCL component within the copolymers, as determined by differential scanning calorimetry (DSC). The supercooling needed for crystallization of the PCL component was much larger for the miktoarm star copolymers than for the linear analogue block copolymer samples of similar composition or even of similar morphology, while the crystallization rate was also depressed. The degree of crystallinity of the micro- or nanodomains was also a function of composition (decrease with PS content in the copolymers) or molecular architecture (lower in the stars than in linear copolymers) and as a general rule decreased as the level of confinement for the PCL component increased. Several kinetic theories of crystallization were applied to the overall isothermal crystallization data and regardless of the theory employed, the parameters proportional to the energy barriers for overall crystallization also increased with the confinement of the PCL component. Both the confinement degree and the influence of molecular architecture on the nucleation and crystallization of the PCL component generally increased with the content of covalently bonded glassy PS.