Suppressing Autophobic Dewetting by Using a Bimodal Brush

Shaping the Structure and Response of Surface-Grafted Polymer Brushes via the Molecular Weight Distribution

Breadth in the molecular weight distribution is an inherent feature of synthetic polymer systems. While in the past this was typically considered as an unavoidable consequence of polymer synthesis, multiple recent studies have shown that tailoring the molecular weight distribution can alter the properties of polymer brushes grafted to surfaces. In this Perspective, we describe recent advances in synthetic methods to control the molecular weight distribution of surface-grafted polymers and highlight studies that reveal how shaping this distribution can generate novel or enhanced functionality in these materials.

Polymer-guided assembly of inorganic nanoparticles

The self-assembly of inorganic nanoparticles is of great importance in realizing their enormous potentials for broad applications due to the advanced collective properties of nanoparticle ensembles. Various molecular ligands (e.g., small molecules, DNAs, proteins, and polymers) have been used to assist the organization of inorganic nanoparticles into functional structures at different hierarchical levels. Among others, polymers are particularly attractive for use in nanoparticle assembly, because of the complex architectures and rich functionalities of assembled structures enabled by polymers. Polymer-guided assembly of nanoparticles has emerged as a powerful route to fabricate functional materials with desired mechanical, optical, electronic or magnetic properties for a broad range of applications such as sensing, nanomedicine, catalysis, energy storage/conversion, data storage, electronics and photonics. In this review article, we summarize recent advances in the polymer-guided self-assembly of inorganic nanoparticles in both bulk thin films and solution, with an emphasis on the role of polymers in the assembly process and functions of resulting nanostructures. Precise control over the location/arrangement, interparticle interaction, and packing of inorganic nanoparticles at various scales are highlighted.

Interactions between brush-grafted nanoparticles within chemically identical homopolymers: the effect of brush polydispersity

We systematically examined the polymer-mediated interparticle interactions between polymer-grafted nanoparticles (NPs) within chemically identical homopolymer matrices through experimental and computational efforts. In experiments, we prepared thermally stable gold NPs grafted with polystyrene (PS) or poly(methyl methacrylate) (PMMA), and they were mixed with corresponding homopolymers. The nanocomposites are well dispersed when the molecular weight ratio of free to grafted polymers, α, is small. For α above 10, NPs are partially aggregated or clumped within the polymer matrix. Such aggregation of NPs at large α has been understood as an autophobic dewetting behavior of free homopolymers on brushes. In order to theoretically investigate this phenomenon, we calculated two particle interaction using self-consistent field theory (SCFT) with our newly developed numerical scheme, adopting two-dimensional finite volume method (FVM) and multi-coordinate-system (MCS) scheme which makes use of the reflection symmetry between the two NPs. By calculating the polymer density profile and interparticle potential, we identified the effects of several parameters such as brush thickness, particle radius, α, brush chain polydispersity, and chain end mobility. It was found that increasing α is the most efficient method for promoting autophobic dewetting phenomenon, and the attraction keeps increasing up to α = 20. At small α values, high polydispersity in brush may completely nullify the autophobic dewetting, while at intermediate α values, its effect is still significant in that the interparticle attractions are heavily reduced. Our calculation also revealed that the grafting type is not a significant factor affecting the NP aggregation behavior. The simulation result qualitatively agrees with the dispersion/aggregation transition of NPs found in our experiments.

Anisotropic Self-Assembly of Hairy Inorganic Nanoparticles

Current interest in functional assemblies of inorganic nanoparticles (NPs) stems from their collective properties and diverse applications ranging from nanomedicines to optically active metamaterials. Coating the surface of NPs with polymers allows for tailoring of the interactions between NPs to assemble them into hybrid nanocomposites with targeted architectures. This class of building blocks is termed "hairy" inorganic NPs (HINPs). Regiospecific attachment of polymers has been used to achieve directional interactions for HINP assembly. However, to date anisotropic surface functionalization of NPs still remains a challenge. This Account provides a review of the recent progress in the self-assembly of isotropically functionalized HINPs in both the condensed state and aqueous solution as well as the applications of assembled structures in such areas as biomedical imaging and therapy. It aims to provide fundamental mechanistic insights into the correlation between structural characteristics and self-assembly behaviors of HINPs, with an emphasis on HINPs made from NPs grafted with linear block copolymer (BCP) brushes. The key to the anisotropic self-assembly of these HINPs is the generation of directional interactions between HINPs by designing the surrounding medium (e.g., polymer matrix) or engineering the surface chemistry of the HINPs. First, HINPs can self-assemble into a variety of 1D, 2D, or 3D nanostructures with a nonisotropic local arrangement of NPs in films. Although a template is not always required, a polymer matrix (BCPs or supramolecules) can be used to assist the assembly of HINPs to form hybrid architectures. The interactions between brushes of neighboring HINPs or between HINPs and the polymer matrix can be modulated by varying the grafting density and length of one or multiple types of polymers on the surface of the NPs. Second, the rational design of deformable brushes of BCP or mixed homopolymer tethers on HINPs enables the anisotropic assembly of HINPs (in analogy to molecular self-assembly) into complex functional structures in selective solvents. It is evidenced that the directional interactions between BCP-grafted NPs arise from the redistribution and conformation change of the long, flexible polymer tethers, while the lateral phase separation of brushes on NP surfaces is responsible for the assembly of HINPs carrying binary immiscible homopolymers. For HINPs decorated with amphiphilic BCP brushes, their self-assembly can produce a variety of hybrid structures, such as vesicles with a monolayer of densely packed NPs in the membranes and with controlled sizes, shapes (e.g., spherical, hemispherical, disklike), and morphologies (e.g., patchy, Janus-like). This strategy allows fine-tuning of the NP organization and collective properties of HINP assemblies, thus facilitating their application in effective cancer imaging, therapy, and drug delivery. We expect that the design and assembly of such HINPs with isotropic functionalization is likely to open up new avenues for the fabrication of new functional nanocomposites and devices because of its simplicity, low cost, and ease of scale-up.

Bimodal “matrix-free” polymer nanocomposites

“Matrix-free” nanocomposites with a bimodal population of polymer brushes for optimizing filler loading while maintaining controlled dispersion.

γ-Alumina modification with long chain carboxylic acid surface nanocrystals for biocompatible polysulfone nanocomposites

High performance polysulfone/γ-alumina biocompatible nanocomposites are reported for the first time and the effects of γ-alumina surface modification are explored. We show that some fatty acids chemisorb over the surface of γ-alumina forming nanosized self-assembled structures. These structures present thermal transitions at high temperatures, 100 °C higher than the melting temperatures of the pure acids, and are further shifted about 50 °C in the presence of polysulfone. The chemistry involved in the chemisorption is mild and green meeting the stringent bio sanitary protocols for biocompatible devices. It has been found that the self-assembled structures increase mechanical strength by about 20% despite the foreseeable lack of strong particle-matrix interactions, which manifests as small variations in both the glass transition temperature and the Young's modulus. Electron microscopy observation of fractured surfaces has revealed that some acids induce an extended region of influence around the nanoparticles and this fact has been used to explain the enhancement of mechanical strength.

Ligand engineering of polymer nanocomposites: from the simple to the complex

One key to optimizing the performance of polymer nanocomposites for high-tech applications is surface ligand engineering of the nanofiller, which has been used to either tune the nanofiller morphology or introduce additional functionalities. Ligand engineering can be relatively simple such as a single population of short molecules on the nanoparticle surface designed for matrix compatibility. It can also have complexity that includes bimodal (or multimodal) populations of ligands that enable relatively independent control of enthalpic and entropic interactions between the nanofiller and matrix as well as introduce additional functionality and dynamic control. In this Spotlight on Applications, we provide a brief review into the use of brush ligands to tune the thermodynamic interactions between nanofiller and matrix and then focus on the potential for surface ligand engineering to create exciting nanocomposites properties for optoelectronic and dielectric applications.

Bimodal surface ligand engineering: the key to tunable nanocomposites

Tuning the dispersion of inorganic nanoparticles within organic matrices is critical to optimizing polymer nanocomposite properties and is intrinsically difficult due to their strong enthalpic incompatibility. Conventional attempts to use polymer brushes to control nanoparticle dispersion are challenged by the need for high graft density to reduce particle core-core attractions and the need for low graft density to reduce the entropic penalty for matrix penetration into the brush. We validated a parametric phase diagram previously reported by Pryamtisyn et al. (Pryamtisyn, V.; Ganesan, V.; Panagiotopoulos, A. Z.; Liu, H.; Kumar, S. K. Modeling the Anisotropic Self-Assembly of Spherical Polymer-Grafted Nanoparticles. J. Chem. Phys.2009, 131, 221102) for predicting dispersion of monomodal-polymer-brush-modified nanoparticles in polymer matrices. The theoretical calculation successfully predicted the experimental observation that the monomodal-poly(dimethyl siloxane) (PDMS)-brush-grafted TiO(2) nanoparticles can only be well dispersed within a small molecular weight silicone matrix. We further extended the parametric phase diagram to analyze the dispersion behavior of bimodal-PDMS-brush-grafted particles, which is also in good agreement with experimental results. Utilizing a bimodal grafted polymer brush design, with densely grafted short brushes to shield particle surfaces and sparsely grafted long brushes that favor the entanglement with matrix chains, we dispersed TiO(2) nanoparticles in high molecular weight commercial silicone matrices and successfully prepared thick (about 5 mm) transparent high-refractive-index TiO(2)/silicone nanocomposites.

Polymer Layered Silicate Nanocomposites: A Review

This review aims to present recent advances in the synthesis and structure characterization as well as the properties of polymer layered silicate nanocomposites. The advent of polymer layered silicate nanocomposites has revolutionized research into polymer composite materials. Nanocomposites are organic-inorganic hybrid materials in which at least one dimension of the filler is less than 100 nm. A number of synthesis routes have been developed in the recent years to prepare these materials, which include intercalation of polymers or pre-polymers from solution, in-situ polymerization, melt intercalation etc. The nanocomposites where the filler platelets can be dispersed in the polymer at the nanometer scale owing to the specific filler surface modifications, exhibit significant improvement in the composite properties, which include enhanced mechanical strength, gas barrier, thermal stability, flame retardancy etc. Only a small amount of filler is generally required for the enhancement in the properties, which helps the composite materials retain transparency and low density.

Polymer chains grafted “to” and “from” layered silicate clay platelets

Polymerization of lauryl methacrylate "to" and "from" the surface of montmorillonite platelets was studied under a range of different reaction conditions. The polymerization was performed in order to achieve better organic coverage of the platelets, thus facilitating their exfoliation in the polymer matrices. For polymerization "to" the surface, a methacrylic functionality was first generated on the clay surface which was subsequently polymerized with the external lauryl methacrylate monomer. Substantial amounts of the polymer could be attached to the surface when lower polymerization temperatures and longer reaction times were used. Bulk polymerization was more effective in increasing the amount of polymer mass on the surface. In order to achieve polymerization "from" the surface, a bicationic initiator was first ionically bound on the surface followed by polymerization with lauryl methacrylate. Under the nonliving conditions, however, no significant amount of polymer could be grown from the surface. Nitroxide-mediated living polymerization was successful in eliminating suspected termination reactions leading to substantial gains in the organic mass bound to clay surfaces. Care was taken to avoid the presence of excess of unbound ammonium ions which can interfere in the grafting of polymer chains on the surface. X-ray diffraction and transmission electron microscopy in conjunction with thermogravimetric analysis confirmed the grafting of the polymer chains on the surface.

Block copolymers in tomorrow's plastics

In this era of portability and rapid technological advances, polymers are more than ever under pressure to be cheap and offer tailored property profiles. Often, the key lies in designing blends and alloys carefully structured at the appropriate scale (preferably less than a micrometre) from existing polymers. Block copolymers - two or more different polymer chains linked together - have long been thought to offer the solution. Local segregation of the different polymer blocks yields molecular-scale aggregates of nanometre size. Recent progress in synthetic chemistry has unveiled unprecedented opportunities to prepare tailored block copolymers at reasonable cost. Over twenty years of intense academic research and the advent of powerful statistical theories and computational methods should help predict the equilibrium and even non-equilibrium behaviour of copolymers and their blends with other polymers. The gap between block copolymer self-assembly and affordable nanostructured plastics endowed with still-unexplored combinations of properties is getting narrower.