Ming-Champ Lin

Mediating polymer crystal orientation using nanotemplates from block copolymer microdomains and anodic aluminium oxide nanochannels

Crystals formed by polymers are typically nanoscale in at least one dimension. The directional control of properties via precise control of the orientations of polymer nanocrystals has strong relevance to technical applications in various areas. Polymer crystals may exhibit preferential orientation when they are formed inside nanoscale domains or pores. Such a confinement-mediated orientation behavior has been a subject of extensive investigation over the past two decades, where a block copolymer in which two or more chemically different sub-chains form a single molecule template system that has received the most attention. In this article, we focus on an overview of the orientation behavior of polymer crystals under the influence of one-dimensional (1-D) and two-dimensional (2-D) confinement templated by the lamellar and cylindrical microdomains of block copolymers, respectively. In the case of lamellae-forming diblock copolymers, both the crystalline–amorphous system (which is composed of one type of nanocrystals) and the more complex double-crystalline diblock (which consists of two types of nanocrystals) are considered. In addition to the templates offered by block copolymers, the preferential orientation of polymer crystals confined in the inorganic anodic aluminium oxide (AAO) nanochannels has also been critically reviewed due to strong relevance to the 2-D confinement effect. Moreover, the significant thermodynamic and kinetic factors governing the crystal orientation behavior have been summarized, which may allow one to understand the strategy for tuning the preferential orientation of polymer nanocrystals under the spatial confinement of different dimensionalities.

Critical analysis of the crystal orientation behavior in polyethylene-based crystalline-amorphous diblock copolymer.

Orientation of polyethylene (PE) crystals formed over a broad range of undercooling in a polyethylene-block-poly(D,L-lactide) (PE-b-PDLLA) diblock copolymer has been critically examined. Due to the large segregation strength and approximate 50/50 composition in this system, the crystallization took place within one-dimensionally confined lamellar microdomains without forming spherulites. A homogeneous crystal orientation with the PE crystalline stems orienting parallel to the lamellar interface was observed at crystallization temperatures (T(c)) between 45 and 102 °C. Once the sample was crystallized at T(c) ≤ 40 °C or directly quenched into liquid nitrogen from the melt, the isotropic WAXD pattern indicated that the PE crystals became randomly oriented. Considering that homeotropic orientation was not identified for the present system and other PE-based crystalline-amorphous diblocks, we concluded that PE crystals in the lamellar microdomains always show homogeneous orientation when there is a preferred orientational order. We further organized the thermodynamic and kinetic factors that may govern the preferred crystal orientation in diblock copolymers and concluded that the orientational order should be controlled by the competition between nucleation and crystal growth kinetics. The persistence of homogeneous orientation of PE was attributed to its excellent nucleating power.

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-block-atactic polystyrene (iPP-b-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-b-aPS was identified over a broad range of crystallization temperatures (T(c)). Different from other crystalline-amorphous diblocks, a lamellar branching of α-form iPP was observed in the lamellar microdomains at T(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 T(c) ≤ 10 °C, the iPP crystals became randomly oriented.