Formation and properties of multivariant assemblies of surface-tethered diblock and triblock copolymers

Charge Density Gradients of Polymer Thin Film by Gaseous Phase Quaternization

We report on the rapid formation of charge density gradients in polymer films by exposing poly([2-dimethylaminoethyl] methacrylate) (PDMAEMA) films resting on flat silica substrates to methyl iodide (i.e., MI, also known as iodomethane) vapors. We adjust the charge gradient by varying the MI concentration in solution and the process time. The thickness of the parent PDMAEMA film does not affect the diffusion of MI through and the reaction kinetics in the films. Instead, the diffusion of MI through the gaseous phase constitutes the limiting step in the overall process.

Electrospray deposition tool: Creating compositionally gradient libraries of nanomaterials

Electrospray deposition (ESD) enables the growth of solution deposited thin films in a precise and continuous manner by the delivery of submicron droplets of dilute solutions to a heated substrate. By combining ESD with programmable motor control and gradient solution pumping in a first-of-its-kind user tool at the Center for Functional Nanomaterials at Brookhaven National Laboratory, we show the ability to create one or two dimensional compositional gradient nanoscale films via ESD. These capabilities make it possible to construct thin film multicomponent "libraries" on a single substrate to rapidly and systematically characterize composition-dependent properties in a variety of material systems such as thin films involving homopolymer and block copolymer blends. We report the design, construction, and validation of a gradient ESD tool that allows users to carefully control the jet stability, flow composition, spray position, and substrate temperature. Calibrated thin films range in thickness from tens to hundreds of nanometers. We demonstrate gradient thin films using a ternary dye triangle as well as a gradual blending of polystyrene homopolymer with poly(styrene-block-methyl methacrylate) on a single substrate. Paired with the rapid measurement capabilities of synchrotron small angle X-ray scattering, this tool forms an integral part of a new platform for high-throughput, autonomous characterization and design of nanomaterial thin films and soft materials more generally.

Tethering solvophilic blocks to the ends of polymer brushes: an effective method for adjusting surface patterns

The effect of different lengths of solvophilic A and C blocks on the assembled configuration of intermediate solvophobic B-blocks in both ABA and ABC polymer brush systems is investigated via dissipative particle dynamics simulations. For the AB diblock copolymer brush with solvophilic A-blocks being grafted to the surface, B-blocks self-assemble into spherical micelle structures that are immersed in a layer formed by the A-blocks. Tethering a very small solvophilic block A(C) at the free end of the polymer brush pulls the B-blocks toward the polymer brush/solvent interface and increases their local density which can significantly change the B-block self-assembled structure from spherical micelles to ripples. By increasing the length of the outermost solvophilic blocks, the lateral density distribution of B-blocks can be further changed, resulting in the domain size of the ripple structure first decreasing and then increasing. Compared to the ABA system, the incompatibility between the A and C blocks can effectively reduce the vertical domain separation caused by the fusion of the upper and lower A blocks. Then, based on an AB diblock copolymer brush system with self-assembled spherical micelles, we introduce extremely short free solvophilic blocks A(C) in dilute solution that can be tethered to the free ends of the polymer brush by using a reaction model [Liu et al., J. Chem. Phys., 2007, 127, 144903]. We find that the micelles' coalescence is mainly affected by the content of tethered reactive solvophilic blocks, and only weakly affected by the reaction rate of the reversible reactions. This strategy of tethering solvophilic blocks to the ends of polymer brushes can be an effective way for the fabrication of stimuli-responsive surfaces and for adjusting nanoscopic surface patterns.

Swelling of chemical and physical planar brushes of gradient copolymers in a selective solvent

We propose a mean-field theory of chemical and physical planar brushes of linear gradient copolymers swollen in a selective solvent. The polymer chains are grafted to the substrate by the ends with the excess of insoluble monomer units, and the majority of the soluble units are located near the free ends of the chains. The grafting points are considered to be immobile (chemical brush) and mobile in-plane (physical brush). In the latter case the grafting density is determined from the equilibrium conditions (minimum of the free energy). A common peculiarity of the brushes of both types is that the polymer concentration gradually changes from a relatively high value near the substrate (collapsed region of the brush) to a small value near the free surface (swollen region of the brush). In the case of the chemical brush, a polymer depletion zone can appear in the middle of the brush if incompatibility between insoluble and soluble (A and B) units is high enough. Here the polymer density is even lower than near the free surface of the brush. The grafting density of the physical brush is inversely proportional to the chain length and increases with the decrease of the solvent quality for the insoluble (A) units. The latter can be accompanied by shrinkage of the brush thickness due to broad distribution of the insoluble units through the chain: a minor fraction of insoluble units near the free ends can aggregate with a major fraction of them near the substrate. As a result, the concentration of the soluble (B) units can have a maximum in the middle of the brush rather than near the free surface.

Formation of Polymer Brushes with Diblock Copolymers on a Planar Surface

We use molecular dynamics simulations method to investigate the behavior characteristics of AB diblock copolymers that are adsorbed on a planar surface. Adsorption density has been distinguished, depending on the adsorption manner of A-block on the (100) surface and formation of brushes. It is examined in detail that conformational behavior of the brushes affects the adsorption density. In addition, we make a comparison of linear brush with length ratio of the A-block to the chain, in the cases of the fixed length of chain and the fixed length of A-block, respectively. The result shows that the adsorption density is strongly affected by the length ratio of the A-block to the chain. And our findings can be used as a guide for fabrication and preparation of actual synthetic polymer brushes on a solid surface by the approach of physical adsorption.

Block copolymer modified surfaces for conjugation of biomacromolecules with control of quantity and activity

Polymer brush layers based on block copolymers of poly(oligo(ethylene glycol) methacrylate) (POEGMA) and poly(glycidyl methacrylate) (PGMA) were formed on silicon wafers by activators generated by electron transfer atom transfer radical polymerization (AGET ATRP). Different types of biomolecule can be conjugated to these brush layers by reaction of PGMA epoxide groups with amino groups in the biomolecule, while POEGMA, which resists nonspecific protein adsorption, provides an antifouling environment. Surfaces were characterized by water contact angle, ellipsometry, and Fourier transform infrared spectroscopy (FTIR) to confirm the modification reactions. Phase segregation of the copolymer blocks in the layers was observed by AFM. The effect of surface properties on protein conjugation was investigated using radiolabeling methods. It was shown that surfaces with POEGMA layers were protein resistant, while the quantity of protein conjugated to the diblock copolymer modified surfaces increased with increasing PGMA layer thickness. The activity of lysozyme conjugated on the surface could also be controlled by varying the thickness of the copolymer layer. When biotin was conjugated to the block copolymer grafts, the surface remained resistant to nonspecific protein adsorption but showed specific binding of avidin. These properties, that is, well-controlled quantity and activity of conjugated biomolecules and specificity of interaction with target biomolecules may be exploited for the improvement of signal-to-noise ratio in sensor applications. More generally, such surfaces may be useful as biological recognition elements of high specificity for functional biomaterials.

Formation of polyampholyte brushes via controlled radical polymerization and their assembly in solution

We describe the formation of polyampholytic block copolymer brushes and their assembly in solution. Specifically, we employ "surface-initiated" activators regenerated by electron transfer atom transfer radical polymerization (ARGET-ATRP) sequentially to form diblock copolymer grafts comprising blocks of poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) and poly(sodium methacrylate) (PNaMA) on flat impenetrable silica surfaces, i.e., SiO(x)/PNaMA-b-PDMAEMA and SiO(x)/PDMAEMA-b-PNaMA. Protonation of the PNaMA block results in formation of poly(methacrylic acid) (PMAA). We demonstrate that ARGET-ATRP of NaMA provides a convenient route to preparation of PMAA, which is an alternative method to the more traditional approach based on preparing PMAA by polymerizing tert-butyl methacrylate (tBMA) followed by cleavage of the tert-butyl group. We also discuss conformational changes of the individual polyelectrolyte blocks in solution as a function of solution pH by monitoring adsorption behavior of functionalized polystyrene spheres.

Responsive PET nano/microfibers via surface-initiated polymerization

Poly(ethylene terephthalate) (PET) is one of the most important thermoplastics in ubiquitous use today because of its mechanical properties, clarity, solvent resistance, and recyclability. In this work, we functionalize the surface of electrospun PET microfibers by growing poly(N-isopropylacrylamide) (PNIPAAm) brushes through a chemical sequence that avoids PET degradation to generate thermoresponsive microfibers that remain mechanically robust. Amidation of deposited 3-aminopropyltriethoxysilane, followed by hydrolysis, yields silanol groups that permit surface attachment of initiator molecules, which can be used to grow PNIPAAm via "grafting from" atom-transfer radical polymerization. Spectroscopic analyses performed after each step confirm the expected reaction and the ultimate growth of PNIPAAm brushes. Water contact-angle measurements conducted at temperatures below and above the lower critical solution temperature of PNIPAAm, coupled with adsorption of Au nanoparticles from aqueous suspension, demonstrate that the brushes retain their reversible thermoresponsive nature, thereby making PNIPAAm-functionalized PET microfibers suitable for filtration media, tissue scaffolds, delivery vehicles, and sensors requiring robust microfibers.