Entropy-driven segregation of polymer-grafted nanoparticles under confinement

Abstract

The modification of nanoparticles with polymer ligands has emerged as a versatile approach to control the interactions and organization of nanoparticles in polymer nanocomposite materials. Besides their technological significance, polymer-grafted nanoparticle (PGNP) dispersions have attracted interest as model systems to understand the role of entropy as a driving force for microstructure formation. For instance, densely and sparsely grafted nanoparticles show distinct dispersion and assembly behaviors within polymer matrices due to the entropy variation associated with conformational changes in brush and matrix chains. Here we demonstrate how this entropy change can be harnessed to drive PGNPs into spatially organized domain structures on submicrometer scale within topographically patterned thin films. This selective segregation of PGNPs is induced by the conformational entropy penalty arising from local perturbations of grafted and matrix chains under confinement. The efficiency of this particle segregation process within patterned mesa−trench films can be tuned by changing the relative entropic confinement effects on grafted and matrix chains. The versatility of topographic patterning, combined with the compatibility with a wide range of nanoparticle and polymeric materials, renders SCPINS (soft-confinement pattern-induced nanoparticle segregation) an attractive method for fabricating nanostructured hybrid films with potential applications in nanomaterial-based technologies.polymer thin film | polymer-grafted nanoparticle | entropy | confinement | topographic pattern T he dispersion and organization of nanoparticles in polymeric matrices are important to improve the properties of polymer nanocomposite materials (1, 2). Polymer-grafted nanoparticles (PGNPs) have attracted much attention in this regard due to their enhanced miscibility and versatility of assembled structures in polymeric materials via interacting grafted polymer layers (3, 4). The cloaking of nanoparticles by densely grafted polymer layers with the same chemical composition as the matrix leads to an "athermal" particle−polymer blend where entropic interactions dominate the thermodynamics of the system (5). Selective segregation of PGNPs at the interface or center of phase-separated block copolymer microdomains depends on the interplay between conformational and translational entropy of the system (6, 7). Considering the significant contribution of entropy in directed nanostructure formation (8, 9), entropic interactions have the potential to tailor the organization of PGNPs within a chemically identical polymer matrix, which is not achievable by conventional techniques (10, 11).The entropic effects can be magnified by confining PGNPs in thin-film geometries. Under confinement, the conformation of the grafted chains is altered, and the associated entropic penalty could drive directional migration of PGNPs and potentially generate visually ordered microstructures. To illustrate this concept, we introduce lateral confinement contrast by topographi...

Control of Phase Morphology of Binary Polymer Grafted Nanoparticle Blend Films via Direct Immersion Annealing

“…Rather, the polymer chains in these layers are closer in form to the “random coil” shapes that individual flexible polymers adopt in solution, apart from a region near the grafting substrate. It is just this random coil aspect of the grafted polymer chains that gives rise to their entropy-driven segregation under confinement studied previously by our group. , The highly fluctuating nature of the grafted polymer layer , for normally encountered grafting densities also has significant implications for molecular packing of this kind of nanoparticle in the melt . The presence of a “soft” deformable layer on the nanoparticle (see Figure of Barnett and Kumar’s paper for a helpful illustration of a polymer-grafted nanoparticle melt generated through molecular dynamics simulation) gives rise to a strong interaction between the nanoparticle cores, leading to a “jammed” state where the degree of jamming is quantified by the hyperuniformity index, H . ,, …”

“…found that the phase separation process of the PGNPs in the homopolymer matrix is similar to ordinary homopolymer mixtures, which shows a comparable coarsening exponent (≈ 1/3 due to diffusive pattern growth and a late stage exponent near 1 caused by hydrodynamic instability). They also used polydimethylsiloxane (PDMS) as a soft pattern to guide the alignment of PGNPs. , The authors explained that the entropic forces created by applying the PDMS on the polymer nanocomposites could help guide the PGNPs into well-defined aligned nanostructures . Wang et al .…”

“…They also used polydimethylsiloxane (PDMS) as a soft pattern to guide the alignment of PGNPs. 29,31 The authors explained that the entropic forces created by applying the PDMS on the polymer nanocomposites could help guide the PGNPs into well-defined aligned nanostructures. 31 Wang et al 27 extended this research into a directed assembly of "clusters of PGNPs" (PGNPCs) and revealed their entropic localization within topographic printed patterns.…”

Well-defined metal-polymer nanocomposites: The interplay of structure, thermoplasmonics, and elastic mechanical properties

“…The as-prepared films display homogeneously distributed structures, but the thermodynamic stability is not warranted. Entropic contributions can strongly alter the nanoparticle distribution in composite films as shown recently for AuPS nanoparticles [36]. Thermal treatment also led to significant structural changes in the pure (no additional matrix polymer) nanocomposite films.…”

Crystal nucleation along an entropic pathway: Teaching liquids how to transition

“…As such, entropy-driven processes have been the focus of intense research in recent years, and have led to many insightful discoveries. Such phenomena include the entropydriven self-assembly in charged lock-key particles [17], entropy-driven segregation processes of polymer-grafted nanoparticles [18], the entropy-driven crystallization of DNA grafted nanoparticles [19], polymers [20], proteins [21], as well as of atomic [22] and molecular crystals [23]. Furthermore, such studies have shed light on the interplay between competing processes, e.g.…”