Nanoimprinting-directed Assembly of Polymer-grafted Nanoparticles in Polymer Thin Films

Download or read book Nanoimprinting-directed Assembly of Polymer-grafted Nanoparticles in Polymer Thin Films written by Xiaoteng Wang and published by . This book was released on 2019 with total page 172 pages. Available in PDF, EPUB and Kindle. Book excerpt: Controlled dispersion and distribution of functional nanoparticles (NPs) in polymer matrix is prerequisite for improved properties of the composite materials. How to control the distribution of NPs in a facile manner remains to be a recurring challenge in the applications of polymer nanocomposites (PNCs). Surface functionalization of NPs with polymer brushes has emerged as an effective and versatile platform of tuning the interactions between the nanoparticles and the polymer hosts, allowing their integration into polymer nanocomposites. The current work aims to understand the phase behaviors of polymer-grafted nanoparticles (PGNPs) in polymer thin films and further control the spatial distribution of PGNPs through the interactions between the grafted and matrix polymer chains. In particular, polystyrene-grafted titanium dioxide nanoparticles (PS-TiO2) embedded in polystyrene (PS) thin film matrices having an initial film thickness h0 » 90 nm were investigated, where fluctuations in the grafting brush layer enables the formation of self-assembled PGNP clustering structures. Nanoimprinting directed lateral organization of the PGNP clusters in polymer thin films via topographically soft-pattern confinement was demonstrated. The PGNP clusters segregate to thicker film regions where they are less confined during thermal annealing. The partitioning of the PGNP clusters to the patterned regions was quantified by introducing the cluster partition coefficient Kc. It shows that the highly selective segregation of the clusters was driven by entropic driving forces while the film surface homogenization and shape transition of the clusters were induced by geometrical confinement of the nanopatterning. Simultaneously, the stability of the low molecular weight PS thin films is greatly enhanced against dewetting by the addition of PGNPs. The extent of the dewetting suppression depends on the PGNP concentration and can also be altered by nanopatterning. This form of soft pattern-directed self-assembly may boost colligative properties and provide enhanced and anisotropic optical such as UV-Vis, electronic and other material properties associated with organized NP clusters into precise large-scale patterns. With better understanding of the chemically identical blend systems, we further extend our model study to other PGNP/polymer blends where enthalpic interactions also participate in the phase behavior. The hybrid blend system composed of polystyrene-grafted silica nanoparticles in a poly (vinyl methyl ether) (PS-SiO2/PVME) blend thin film (≈100 nm) was studied where the brush and matrix polymers exhibit LCST type of phase behavior. Phase separation between the polymer-grafted nanoparticles (PGNPs) and matrix polymer occurs at a temperature ≈ 40° C lower than the LCST of classic binary linear PS/PVME polymer blends. Spatially organized PGNP domain structures on submicrometer scale were illustrated by introducing the symmetry-breaking soft elastomer pattern. Selective partition of the nanoparticles in both one-phase and two-phase regions can be obtained via nanoimprinting. Thermal cycling of the composite film through the critical temperature allows for thermodynamically reversible formation and dissolution of PGNP-rich domain structures. This nanoimprinting guided assembly of PGNPs in polymer nanocomposites would open pathways of novel hybrid materials for many technological applications such as responsive materials.