Abstract
A series of semi-crystalline block copolymers, polystyrene-b-syndiotactic polypropylene (PS-sPP), with lamellar nanostructures have been synthesized. The crystallization of the sPP blocks in PS-sPP was carried out under hard confinement (i.e., the crystallization temperature of sPP (Tc,sPP) < the glass transition temperature of PS (Tg,PS)) and soft confinement (i.e., Tc,sPP > Tg,PS), where interesting morphological evolution was studied by transmission electron microscopy (TEM) and time-resolved small angle X-ray scattering (SAXS). A confined morphology for crystallized PS/sPP 71/212 (high-molecular-weight PS-sPP) can be observed when Tc,sPP > Tg,PS. However, the microphase-separated lamellar nanostructure becomes a templated morphology by crystallization once Tc,sPP approaches Tg,PS. Consequently, the breakout of nanostructure occurs when Tc,sPP > Tg,PS. This study reveals the Tg effect on the final morphologies of block copolymers after crystallization. At Tc,sPP < Tg,PS (hard confinement) or Tc,sPP ~ Tg,PS (soft confinement), no appreciable displacement in the first-order peak (i.e., long period of microphase-separated lamellae) during crystallization can be found. By contrast, the intensity of the first-order peak decreases while Tc,sPP > Tg,PS, and then is gradually replaced by crystalline lamellar diffraction as the progress of crystallization. Interestingly, a peculiar crystalline texture under confinement, a trilayer sPP crystalline morphology in-between vitrified PS confinement in the PS/sPP 71/212, was observed by TEM. The morphological evolution of the double-length-scale multilayered morphology was examined by time-resolved SAXS. The sPP crystals under confinement form perpendicular morphology having crystalline chains normal to the microphase-separated lamellae, as evidenced by two-dimensional SAXS and wide-angle X-ray diffraction (WAXD). On the basis of simple geometric argument and the crystallinity value, the sPP molecular chains in crystalline phase roughly appear as three-chain-traverse crystalline lamellae near the edge of sPP microdomain and one and a half chain-traverse crystalline lamellae in the middle of sPP microdomain for one single sPP chain. For comparison, the crystallization kinetics of sPP under soft and hard confinement (namely block copolymers) and of sPP homopolymers were studied by Avrami treatment for the change of crystallinity with time. Similar to the sPP homopolymers, the crystallization of sPP chains under confinement exhibits a heterogeneous nucleation process. To further examine the effect of spatially confined size on crystallization behavior, a low-molecular-weight PS-sPP (PS/sPP 58/174) was synthesized. Even with the decrease of segregation strength in the PS-sPP, breakout morphology similar to PS/sPP 71/212 was observed once Tc,sPP > Tg,PS in the crystalline morphology of PS/sPP 58/174; suggesting that Tg effect is dominant to determine the final crystalline morphology. Nevertheless, the crystalline lamellae of PS/sPP 58/174 under hard confinement appear as randomly oriented entities within the confined microdomains; indicating that the formation of multilayer sPP crystalline morphology is strongly dependent upon the confined dimension. Further decreasing the confined size, no significant crystallinity can be identified in the whole crystallization window; suggesting that the crystallizability is heavily depressed by the change of confined size. To truly examine the confined crystallization in the whole temperature range, a series of semi-crystalline block copolymers, poly(4-vinylpyridine)-b-poly(epsilon-caprolactone) (P4VP-PCL), with lamellar and cylindrical nanostructures have also been synthesized. Owing to the vitrified P4VP microdomains and strongly segregated microphase separation, the crystallization of the PCL blocks in P4VP-PCL was carried out within the nanoscale confinement (i.e., under strongly segregated and hard confinement). Simply by varying the molecular weight of the block copolymer, namely the confined size, polymeric crystallization can be tailored under confinement. A distinct nucleation mechanism in the case of lamellar nanostructure (namely, one-dimensional (1D) confinement), altering from heterogeneous to homogeneous nucleation can be obtained once the confined size becomes smaller than a critical dimension, equivalent to the regular thickness of heterogeneously nucleated crystalline lamellae (ca. 8 nm). Consequently, discrete crystalline granules can be generated through homogeneous nucleation, namely a single nucleus within one granule. Also, crystal growth is altered from specific to random orientation with respect to the interface between the crystalline and amorphous domains in the copolymers, as evidenced by simultaneous SAXS and WAXD experiments. Similar to the crystallization kinetics under 1D confinement, nucleation from heterogeneous to homogeneous can be identified once the confined size becomes smaller than 10 nm under cylinder nanostructure (i.e., two-dimensional (2D) confinement). Also, under 2D confinement, the crystalline chains of PCL appear as perpendicular orientation under confinement while the confine size > 10 nm. A preferred growth direction is along b-axis parallel to central axes of the cylinders. Nevertheless, while the confine size ~ 10 nm, the texture of crystalline granules (namely, typical morphology for homogeneously nucleated lamellae) can be found. Interestingly, the preferred growth direction is along [110]-axis parallel to central axes of the cylinders. As a result, the crystallization behavior including crystalline orientation and crystallization kinetics are indeed significantly affected by the change of confined size. This system thus serves as a model to analyze the impact of confined size in 1D and 2D spatial confinement on the crystallization of polymeric materials.