Polymer Engineering Faculty Research


Directed Assembly of Block Copolymer Films: Effects of Rough Substrates and Thermal Fields

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Contribution to Book

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Polymer coatings are used extensively as passive masking layers or active materials in diverse fields such as consumer products, microelectronics, and biomedical implants. A high packing density of functional elements is required for a wide variety of next-generation technologies, including high resolution displays, fast microprocessors, and lightweight batteries with high power densities. Photolithographic patterning of photosensitive polymers (i.e., photoresists) is currently the most popular industrial option to create such small scale structures. This technology has been so far successful in increasing the packing density of functional elements and decreasing pattern sizes to about 64 nm by the use of smaller irradiation wavelengths1 and highly optimized optical projection systems. However, with each new generation of patterning technology, not only does the cost of manufacturing get progressively higher, but the range of materials that can be used for creating the nanostructures becomes severely limited owing to the different operating energies of each lithographic device.2 Greater control of a polymer coating’s microstructure, and of the surface expression of chemical components, could enable a wide range of novel applications far beyond nanopatterning. Industrial applications typically require simple, low cost, robust, and defect-tolerant assembly strategies. Hence, there is an immediate need for developing alternate low cost and generic patterning methods that are also capable of producing defect-free nanoscale structures over large areas. Self-assembly of block copolymers (BCP)3 is one of the most promising of the emerging technologies4 for soft nanopatterning. BCPs are composed of two or more chemically immiscible polymer chains that are covalently bonded together. A diblock copolymer consists of two distinct polymers bonded to each other at one end. The range of phase structures exhibited by diblock copolymers makes it an attractive system to create nanoscale features of different shapes such as spheres, rods (cylinders), channels, and gyroid.5 This microphase separation can produce a variety of morphologies, with periodicities on the order of 5–100 nm.3,6–8 Owing to the size-scale of the structures, these systems have been proposed as the solution for next-generation patterning needs. Controlling the order and orientation of BCPs is crucial to any application. A variety of techniques have been developed to achieve this, i.e., shear alignment,9,10 electric field alignment,11 – 13 solvent annealing,14,15 mixing with nanoparticles,16,17 thermal techniques (temperature gradients,18 directional solidification,19,20 and zone refinement,21 – 27 including “cold” zone annealing28,29), topographic30 – 35 and chemical patterning,33,34 and adjusting substrate surface energy35,36 or roughness.37 – 43 The variety of available techniques emphasizes the delicate balance among the many factors competing for BCP self-assembly. Some of the topographic, chemical, and thermal treatments used to manipulate the orientation of BCPs are described in this chapter.

Publication Title

Soft Matter Gradient Surfaces: Methods and Applications



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