The interest in confined crystallization has greatly increased with the development and progress of nanotechnology applications. Investigating confined crystallization can also advance our understanding of the whole crystallization process.
Crystalline-amorphous di-block copolymers, owing to their self-assembled microdomains appear to be the most preferable approaches to achieve nanoscale confinement. Di-block copolymers have been shown to display several microphase separation structures, such as cylinders, spheres, gyroids, and lamellae. The orientation, shape, and size of the microdomains can be tuned by changing the attributes of the block.
Tuning the orientation of the microdomains within a selected localized region, remains an integral part in the development of nanometer structures and in various applications including, computer memory, data storage, lithography, and nanometer scale templating. Atomic force microscopy is among the many methods that have been utilized to manipulate the microdomains orientation.
Atomic force microscopy can be a powerful method in gaining a better understanding of the crystallization of polymers. It also provides nanometer-scale information with an ability to gather data in real time in the course of crystallization process. Recently a team of researchers led by Professor Jamie K. Hobbs at the University of Sheffield in United Kingdom implemented the atomic force microscopy tip to tune the orientation of the block copolymer micro-domains before they crystallized. This enabled them to define the direction the crystals grew relative to the block’s interface. Their research work is published in Macromolecules.
The authors observed that the number of dislocations, which represented defects, dropped with an increase in the number of orienting scans. Depending on the thermal conditions, there was a change from fully confined crystallization to template and breakout crystallization. Confined characteristic of the crystals was noted at high supercooling, templated crystallization was observed at intermediate temperatures, and finally, a mixture of breakout and templated crystallization was observed at the highest temperature. The authors explored the observed difference in morphology as well as behavior of crystals that grew parallel to the axes of the pre-existing cylinders, and crystals that grew perpendicular. They found that the growth rates of the templated crystals were higher as compared to the rates of crystals that grew against the melt structure.
The study found that when the crystallization temperature was increased, the rate of crystallization as well as the difference between the rates along the cylinder domains and against them was observed to decrease indicating that small variations in crystallization temperature could pose pivotal effects on the attributes and morphology of the product.
Prof Jamie Hobbs says “AFM gives us the ability to both interact with the sample to orient it and to follow the subsequent re-organisation and crystallization of the material. This provides unique insights into the mechanism behind the process, as well as producing a material with potentially interesting physical properties”.
Lamiaa G. Alharbe, Richard A. Register, and Jamie K. Hobbs. Orientation Control and Crystallization in a Soft Confined Phase Separated Block Copolymer. Macromolecules 2017, 50, 987−996.Go To Macromolecules