Evolution of Crystal Orientation in One-Dimensionally Confined Space Templated by Lamellae-Forming Block Copolymers

Polymer crystallites may exhibit preferred orientation when the crystallization is allowed to occur under the influence of spatial confinement. Using time-resolved wide-angle X-ray scattering (WAXS), we explore the time evolution of the preferred crystal orientation within one-dimensionally confined space constructed by the lamellar microdomains of two crystalline block copolymers, polyethylene-<i>block</i>-poly­(dl-lactide) (PE-<i>b</i>-PDLLA) and poly­(l-lactide)-<i>block</i>-polyethylene (PLLA-<i>b</i>-PE), where the developments of the parallel and the perpendicular orientation of PE and PLLA crystallites, respectively, were monitored from the early stage of crystallization. Both types of crystallites were randomly oriented at the early stage of formation. As crystallization proceeded further, the ensemble-average orientation progressively improved toward the preferred orientation type, and the rate of establishing the orientation exhibited the same dependence on crystallization temperature (<i>T</i>_c) as the crystallization kinetics. Further examination of the effectiveness of enhancing the average orientation with respect to the increase of crystallinity supported the postulate that the perpendicular orientation of PLLA crystallites arises from the tendency to attain long-range crystal growth, while the parallel crystal orientation of PE is driven by the excluded volume interaction between the crystallites as a result of the intrinsically high nucleating power of PE

Crystallization of Isotactic Polypropylene under the Spatial Confinement Templated by Block Copolymer Microdomains

We investigate the crystallization behavior of isotactic polypropylene (iPP) under the influence of nanoscale confinement templated by the microphase-separated structure of an iPP-based diblock copolymer system, isotactic polypropylene-<i>block</i>-atactic polystyrene (iPP-<i>b</i>-aPS). Three types of iPP microdomains, i.e., lamellae, cylinder, and sphere, were generated by controlling the composition of the diblock. The effect of microdomain morphology on the nucleation mechanism, crystallization kinetics, self-nucleation behavior, the population of the helical sequence of iPP block in the melt state, and crystal orientation have been systematically studied. It was found that the crystallization rate of iPP was predominantly controlled by homogeneous nucleation when the crystallization process was largely confined within the individual cylindrical and spherical microdomains. Such a nucleation mechanism and the highly frustrated crystal growth in the isolated microdomains led to the absence of Domain II and atypical crystallization kinetics in Domain III in the self-nucleation study. The population of the longer helical sequence of iPP block revealed by infrared spectroscopy was found to be affected by temperature but not by the spatial confinement, chain stretching, and junction point constraint imposed by the microdomains. Finally, the orientation of α-form iPP crystals in the lamellae-forming iPP-<i>b</i>-aPS was identified over a broad range of crystallization temperatures (<i>T</i>_c). Different from other crystalline–amorphous diblocks, a lamellar branching of α-form iPP was observed in the lamellar microdomains at <i>T</i>_c lying between 15 and 80 °C, where the daughter lamellae developed from the perpendicularly orientated parent iPP crystals with a specific angle of 80° or 100°. Once the sample was crystallized at <i>T</i>_c ≤ 10 °C, the iPP crystals became randomly oriented

Interactive Crystallization Kinetics in Double-Crystalline Block Copolymer

The crystallization kinetics and crystallization-induced morphological formation of an asymmetric double-crystalline block copolymer, syndiotactic polypropylene-<i>block</i>-poly­(ε-caprolactone) (sPP-<i>b</i>-PCL), have been investigated by time-resolved simultaneous small-angle and wide-angle X-ray scattering (SAXS/WAXS). The sPP-<i>b</i>-PCL under study exhibited hexagonally packed cylinder morphology in the melt state, where the minority sPP block formed the cylindrical microdomains dispersed in the PCL matrix. The crystallization behavior was studied by imposing two types of crystallization histories: (1) two-stage crystallization, where the diblock was first cooled to the temperature <i>T</i>_c^sPP situating between the melting points of the two components (<i>T</i>_m^PCL < <i>T</i>_c^sPP < <i>T</i>_m^sPP) to allow sPP crystallization to saturation followed by cooling to <i>T</i>_c^PCL < <i>T</i>_m^PCL to induce PCL crystallization; (2) one-stage crystallization, where the system was cooled directly to <i>T</i>_c < <i>T</i>_m^PCL to allow the two components to crystallize competitively. In both cases, the crystallization of sPP block was in general able to disrupt the melt structure and transformed it into a crystalline lamellar morphology. For the two-stage crystallization process, the PCL block was found to exhibit a faster crystallization at a given <i>T</i>_c^PCL when the sPP block was precrystallized at higher <i>T</i>_c^sPP. This “interactive crystallization kinetics” was attributed to the mediation of the stretching of PCL blocks by the thickness of sPP crystalline domains which depended on <i>T</i>_c^sPP. In the one-stage process, the crystallization events of the two blocks became more competitive with decreasing <i>T</i>_c. The morphological perturbation induced by crystallization was also more hindered at lower <i>T</i>_c, such that a significant portion of sPP blocks remained confined within the cylindrical microdomains so as to suppress the sPP crystallinity