Nanostructures Derived from Phase-Separated Polymers


Edwin L. Thomas Department of Materials Science and Engineering, Massachusetts Institute of Technology

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The field of polymer-based materials continues its enormous growth covering a wide range of products from disposable coffee cups to car bumpers to biomedical devices. The increased emphasis on enhancement of properties via materials with structures of components engineered on the nanoscale has opened up many new opportunities. For example, blending different polymers while retaining their individual properties in the composite is an effective way of engineering new nano- and microstructural materials from a limited palette of commodity polymers. At present five major reasons for the technological importance of polymer multicomponent systems can be identified: 1) improvements in material performance via synergistic interactions (e.g., temperature resistance, modulus, adhesion) 2) realizing desired processing conditions (e.g., melt viscosity, softening point, solvent resistance) 3) recycling industrial or municipal scrap polymers; 4) adjusting product composition to customer specifications by mixing of different batches; and 5) dilution of high-performance polymers for cost reduction.

Despite their great industrial relevance, there exists no formally accepted nomenclature for multicomponent polymer systems. Understanding and controlling the mechanisms of phase separation and nanostructure formation in polymer systems allows one to tailor the performance of these materials to a manifold of applications. For example, co-continuous blends of high- and low-melting point polymers where the low-melting point component is the majority component facilitate to dramatically increase thermal and mechanical properties such as toughness, stress at break, or high-temperature creep resistance while retaining ease of processability. Recent research suggests possible future applications of multicomponent polymer systems that are more far-reaching. Nanostructures based on block copolymer–homopolymer blends are currently studied as a platform for photonic materials with possible use in integrated optics or in thin films as nonlithographic route toward controlled patterning of 100-nm feature sizes. The future technological impact of the latter type of applications will crucially depend on the ability to control structure formation on multiple length scales by strategic design of chemical groups as well as integrating synthetic design with specific processing pathways that increase the likelihood of attaining a targeted structure.

This article reviews polymer phase behavior and nanostructure formation beginning with a discussion of molecular architecture, equilibrium thermodynamics, and phase separation dynamics. The second part describes recent achievements to control the structure formation processes over macroscopic dimensions. The interplay of relevant balancing forces in self-organization processes is discussed aiming to give the reader some intuition about how molecular details and processing conditions can be used in order to control structure formation. In the third part, new research areas will be presented in which polymer-based nanostructures are likely to have major technological impact. Throughout, examples will focus on synthetic polymers that either are of high industrial interest or that suitably represent characteristics of a broad range of macromolecules but leaving out the complex structure formation processes found in natural biopolymers.