Effects of Relative Humidity and Nanoparticle Incorporation on Nanostructures of PS-b-PEO Diblock Copolymer
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Effects of Relative Humidity and Nanoparticle Incorporation on Nanostructures of PS-b-PEO Diblock Copolymer Juntao Wu, Minhua Zhao, Tinh Nguyen, and Xiaohong Gu BFRL, NIST, 100 Bureau Drive, Stop 8615, Gaithersburg, MD, 20899 ABSTRACT This paper is comprised of two major sections: (1) effects of relative humidity (RH) on nanostructures of polystyrene-block-poly (ethylene oxide) (PS-b-PEO) diblock copolymer, and (2) the structures of PS-b-PEO after incorporation of nanoparticles. Thin films of PS-b-PEO were prepared by spin coating under different RHs. Effects of RH on the nanostructures of selfassembled PS-b-PEO were investigated using atomic force microscopy (AFM). The stability of the formed structures under different RHs at ambient temperature was studied in-situ by AFM with the aid of a small volume AFM environmental chamber. The results show that the orientation and the ordering of the copolymer nanostructures are very sensitive to the humidity of the environments during film formation. Well-defined cylindrical nanodomains oriented perpendicular to the surface are induced by the high humidity during spin coating. These perpendicular cylindrical structures are stable at different levels of RHs, except that at very high RH (90 %), the PEO domains are swollen to form mushroom-like structures. Upon drying, the original nanostructures of the copolymer re-form, indicating that these structural changes are reversible. Moreover, the AFM phase contrast between the cylindrical PEO domains and the PS matrix is significantly increased at the elevated RHs. To prepare inorganic-organic nanocomposites, the sol-gel precursor of titinia was introduced to the PS-b-PEO system. The overall morphological structure of the PS-b-PEO is altered after nanoparticle incorporation. The cylindrical nanodomains are still observed, but are not as well-defined as those in the pure PS-bPEO system. For the materials and the conditions studied, the aggregation of the nanoparticles appears to interrupt the long-range ordering of the copolymer nanostructure.
INTRODUCTION Block copolymers have attracted increasing interest because they can self-assemble into various nanostructures due to microphase separation between immiscible blocks [1, 2], and can be used as templates for fabrication of unique inorganic-organic composites with structural features in the nanometer scale [3-5]. However, to produce a well-defined structure, the selfassembly processes alone are not sufficient [6-8]. Directed self-assembly is necessary where an external field is used to direct the orientation of structures formed. External fields include electric fields, shear, temperature gradients or chemically patterned substrates [8-10]. Solvent evaporation is also a strong, robust route to a high degree of long-range lateral ordering [6, 7, 11]. In this approach, the ordering is initiated at the surface of the film with the directionality of solvent evaporation, and is then propagated through the entire film. It has been reported [6] that an asymmetric PS-b-PEO blo
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