Low Grafting Density Research Articles

Brushes, Graft Copolymers, or Bottlebrushes? The Effect of Polymer Architecture on the Nanotribological Properties of Grafted-from Assemblies

Surface-grafted polyzwitterions (PZW) have gained a foothold in the design of synthetic materials that closely mimic the lubricious properties of articular joints in mammals. Besides their chemical composition, the architecture of PZW brushes strongly determines their morphological, nanomechanical, and nanotribological characteristics. This emerges while comparing the properties of linear poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) brushes with those displayed by graft copolymer and bottlebrush brushes, either featuring a low or a high content of PMPC side chains. Surface-initiated atom transfer radical polymerization (SI-ATRP) enabled the synthesis of different branched-brush architectures from multifunctional macroinitiators via multiple grafting steps, and allowed us to modulate their structure by tuning the polymerization conditions. At relatively low grafting densities (σ), long PMPC side segments extend at the interface of bottlebrush and graft copolymer brushes, providing both morphology and lubrication properties comparable to those shown by loosely grafted, linear PMPC brushes. When σ > 0.1 chains nm-2 the effect of the branched-brush architecture on the nanotribological properties of the films became evident. Linear PMPC brushes showed the lowest friction among the studied brush structures, with a coefficient of friction (μ) that reached 1 × 10-4, as measured by atomic force microscopy (AFM). Bottlebrush brushes showed comparatively higher friction, although the high content of hydrophilic PMPC side chains along their backbone substantially improved lubrication compared to that displayed by the more sparsely substituted graft copolymer brushes.

Evaluation of Surface-initiated Polymer brush as Anti-scaling Coating for Plate Heat Exchangers

Inorganic fouling is one of the challenging problems in heat exchanger applications. One approach to mitigate fouling is to employ surface coatings. In this study, we evaluated the feasibility of surface-initiated polymerization (SIP) as thin coating technology to mitigate CaCO3 formation for heat transfer applications. The extent of formation of CaCO3 on different types of poly(oligoethyleneglycol) methacrylate brushes (POEGMA) was investigated under stagnant and flow heat-exchanging conditions. Polymer brushes of high graft density reduced the surface coverage of CaCO3more effectively than the low graft density brushes. By contrast, the thickness of the brush did not correlate with the surface coverage of CaCO3. The comparison of stagnant and flow experiments revealed that the antiscaling property of- POEGMA brushes was due to low adhesion CaCO3 deposits though the brushes themselves do not prevent the nucleation of CaCO3. b. Finally, the SIP process was successfully scaled-up to coat commercial heat exchanger plates with thickness and homogeneity comparable to lab-scale surfaces. Under industrial testing, the POEGMA brushes extended the performance by 50 h before the commencement of complete blockage.

Structural and Physicochemical Properties and Biocompatibility of Linear and Looped Polymer-Capped Gold Nanoparticles.

Various polymer brushes with linear and looped conformations have gained considerable attention in the application of biomaterials and nanotechnology. In this work, the linear and looped polymer brush shells based on PEG-SH and SH-PEG-SH chains binding to gold nanoparticles (AuNPs) are synthesized. The structure and topology of the PEGylated AuNPs are systematically investigated. The basic physicochemical parameters of these PEGylated AuNPs such as hydrodynamic size, grafting density, hydrophilicity, colloidal stability, and biocompatibility are determined intensively. The looped polymer topology can remarkably alter physicochemical properties of polymer brushes compared with the linear counterparts. When the molecular weight of PEG is low (1 and 5 kDa), the looped polymer shells have smaller hydrodynamic size and lower grafting density than their linear analogues; when the molecular weight of PEG is high (10 kDa), the looped shells are much thicker and denser than the linear ones. Interestingly, the looped PEGs on AuNPs are more stable in a high-salt environment and have better hydrophilicity, which endow excellent biocompatibility, including protein resistance and cell viability. These results provide a simple approach to design polymer brushes with different topologies on AuNPs, improve the biocompatibility of hybrid AuNPs, and acquire the potential application in the biomedical field.

Effect of the Self-Assembled Structures of Hydrated Polyzwitterionic and Polyanionic Brushes on Their Self-Cleaning Capabilities.

The capability of polyelectrolyte brushes to spontaneously clean oil fouling via water is determined by factors including water wettability and the self-assembled structures of hydrated polyelectrolytes. Although the charged groups of polyelectrolytes provide the original source of water wettability, the self-assembled structures play a significant role in the self-cleaning performances. Here, we employ coarse-grained molecular dynamics simulations to study the general self-cleaning characteristics of two types of surface-grafted polyelectrolyte brushes (i.e., zwitterionic and anionic polyelectrolytes). It has been found that the high grafting density is favorable to fouling reduction for both polyzwitterions and polyanions. To be specific, the hydrated polyzwitterions form an intermolecular cross-linked network via zwitterionic complexes, resulting in better self-cleaning capabilities than the polyanions at lower grafting densities. However, polyanions form bundles with each consisting of several chains via hydrophobic interactions and electrostatic repulsions presenting better self-cleaning performances than the polyzwitterions at higher grafting densities.

Polymer brush relaxation during and after polymerization – Monte Carlo simulation study

The polymer chain growth in brushes takes place in a non-uniform environment which is one of the reasons of higher dispersity. Some active chain ends grow in monomer-reach regions while others in polymer-rich regions where the monomer concentration is lower. This effect depends on grafting density but also on the polymerization and diffusion rates. If chains grow fast, they adopt non-equilibrium conformations which change the polymer density profile and the growth conditions. In this paper we study this effect and chain relaxation using Monte Carlo simulation method. The growth of polymer brushes by living polymerization at various conditions was simulated until some average molecular weight was attained. Then, the polymerization was stopped and the changes of polymer parameters in time were monitored. The chain end-to-end distance increases and consequently the brush thickness increases in time. Polymer concentration decreases, especially near the grafted surface. The effect is stronger for higher polymerization rate and for higher grafting density. For low polymerization rates and lower grafting density, chain relaxation takes place during polymerization and the brush structure essentially does not change in time whenthe polymerization is finished. Such conditions assure also lower dispersity.

Designing Superlattice Structure via Self-Assembly of One-Component Polymer-Grafted Nanoparticles.

The control of the self-assembly of the nanocrystals into ordered structures has been extensively investigated, but fewer efforts have been devoted to studying one-component polymer-grafted nanoparticles (OPNPs). Herein, through coarse-grained molecular dynamics simulation, we design a novel nanoparticle (NP) grafted with polymer chains, focusing on its self-assembled structures. First, we examine the effects of length and density of grafted polymer chains by calculating the radial distribution function between NPs, as well as through direct visualization. We observe a monotonic change of the arranged morphology of grafted-NPs as a function of the density of grafted polymer chains, which indicates that the increase of the grafting density contributes to the order of the morphology. Meanwhile, we find that much longer grafted polymer chains worsen the regularity of the morphology. Then, we probe the influence of the stiffness of grafted polymer chains (denoted by K ranging from 0 to 500) on the order of grafted-NPs, finding that the order of the structure exhibits a nonmonotonic behavior as a function of K at moderate grafting density. For high grafting density, the order of the morphology is initially enhanced and becomes saturated as a function of K. For the effect of K on the stress-strain behavior, the system with the lowest order demonstrates the most remarkable reinforced mechanical behavior for both low and high grafting density. Last, we establish the phase diagram by varying the stiffness and density of the grafted polymer chains, which contains the amorphous, ordered, and superlattice structures, respectively. In general, our simulated results provide guidelines to tailor the self-assembly of the OPNPs by taking advantage of the length, density, and stiffness of grafted polymer chains.

Diffusion of polymer-grafted nanoparticles in a homopolymer matrix.

Molecular dynamics simulations are used to study the diffusion of polymer-grafted nanoparticles (PGNPs) in polymer. The diffusivity of PGNPs in the homopolymer matrix is investigated as a function of graft length and grafting density, and it is compared to that of bare nanoparticles with comparable effective size. Our results indicate that, in addition to the increase in the effective size of PGNPs due to grafting, the interpenetration of matrix polymers into the grafted layer also plays an important role in the mobility of PGNPs. In systems consisting of both PGNPs and bare particles, the spatial arrangement of the bare particles was found to be having a significant influence on the mobility of PGNPs. At low graft length and high grafting density, the matrix chains dewets the grafted layer, due to autophobic dewetting, creating a sharper interface between the matrix and the grafted layer. The bare particles then migrate to the interface creating a barrier around the PGNPs that hinders the matrix-graft interpenetration and results in the higher mobility of PGNPs. Our results emphasize the importance of polymer-particle interface on the dynamic properties of polymer nanocomposites.

Quantifying Mg2+ Binding to ssDNA Oligomers: A Self-Consistent Field Theory Study at Varying Ionic Strengths and Grafting Densities.

The performance of aptamer-based biosensors is crucially impacted by their interactions with physiological metal ions, which can alter their structures and chemical properties. Therefore, elucidating the nature of these interactions carries the utmost importance in the robust design of highly efficient biosensors. We investigated Mg binding to varying sequences of polymers to capture the effects of ionic strength and grafting density on ion binding and molecular reorganization of the polymer layer. The polymers are modeled as ssDNA aptamers using a self-consistent field theory, which accounts for non-covalent ion binding by integrating experimentally-derived binding constants. Our model captures the typical polyelectrolyte behavior of chain collapse with increased ionic strength for the ssDNA chains at low grafting density and exhibits the well-known re-entrant phenomena of stretched chains with increased ionic strength at high grafting density. The binding results suggest that electrostatic attraction between the monomers and Mg plays the dominant role in defining the ion cloud around the ssDNA chains and generates a nearly-uniform ion distribution along the chains containing varying monomer sequences. These findings are in qualitative agreement with recent experimental results for Mg binding to surface-bound ssDNA.

Selective location and migration of poly(methyl methacrylate)- grafted clay nanosheets with low grafting density in poly(methyl methacrylate)/poly(styrene-co-acrylonitrile) blends

The location and migration of poly (methyl methacrylate)-grafted clay (clay-g-PMMA) nanosheets with low grafting density in a phase-separated polymer blend of poly (methyl methacrylate)/poly (styrene-co-acrylonitrile) (PMMA/SAN) was investigated. The grafting density and molecular weight for grafted PMMA chains on the surface of clay nanosheets was about 0.04 chains/nm2 and 7.8 × 104, respectively, indicating that the grafted chains might exhibit the mushroom-like distribution. With phase separation of PMMA/SAN blend matrix, the well-dispersed clay-g-PMMA nanosheets in the blend matrix were first located in the SAN-rich phase and then gradually migrated to the interface between PMMA and SAN. The first abnormal location of modified clay in the blend matrix might be attributed to the favorable interaction between clay-g-PMMA and SAN induced by the miscibility of grafted PMMA/surrounding SAN chains and the unfavorable interaction between clay-g-PMMA and PMMA without any interpenetration of grafted and matrix chains. The enhanced miscibility between grafted PMMA and surrounding SAN just began to be destroyed after being annealed for a long time, resulting in the final migration of clay-g-PMMA to the interface of PMMA/SAN (thermodynamic equilibrium state).

Tunable Interfacial Properties in Silk Ionomer Microcapsules with Tailored Multilayer Interactions.

Microencapsulation techniques represent a critical step in realizing highly controlled transport of functional materials in multiphase systems. The first demonstration of microcapsules prepared from minimally grafted silk ionomers (silk fibroin modified with cationic/anionic charge groups) are presented here. These tailored biomacromolecules have shown significantly increased biocompatibility over traditional polyelectrolytes and heavily grafted silk ionomers, but the low grafting density had previously limited attempts to fabricate stable microcapsules. In addition, the first microcapsules from polyethylene-glycol-grafted silk ionomers are fabricated and the corresponding impact on microcapsule behavior is demonstrated. The materials are shown to exhibit pH-responsive properties, with the microcapsules demonstrating an approx. tenfold decrease in stiffness and an approx. threefold change in diffusion coefficient when moving from acidic to basic buffer. Finally, the effect of assembly conditions of the microcapsules are shown to play a large role in determining final properties, with microcapsules prepared in acidic buffers showing lower roughness, stiffness, and an inversion in transport behavior (i.e., permeability decreases at higher pH).

Multivalent Presentation of Ice Recrystallization Inhibiting Polymers on Nanoparticles Retains Activity.

Poly(vinyl alcohol) (PVA) has emerged as the most potent mimic of antifreeze (glyco)proteins ice recrystallization inhibition (IRI) activity, despite its lack of structural similarities and flexible, rather than rigid, backbone. The precise spacing of hydroxyl groups is hypothesized to enable PVA to recognize the prism planes of ice but not the basal plane, due to hydroxyl pattern matching of the ice surface giving rise to the macroscopic activity. Here, well-defined PVA derived from reversible addition–fragmentation chain-transfer (RAFT) polymerization is immobilized onto gold nanoparticles to enable the impact of nanoscale assembly and confinement on the observed IRI activity. Unlike previous reports using star-branched or bottle-brush PVAs, the nanoparticle–PVA retains all IRI activity compared to polymers in solution. Evidence is presented to show that this is due to the low grafting densities on the particle surface meaning the chains are free to explore the ice faces, rather than being constrained as in star-branched polymers. These results demonstrate a route to develop more functional IRI’s and inclusion of metallic particle cores for imaging and associated applications in cryobiology.

Hydration of Spherical PEO-Grafted Gold Nanoparticles: Curvature and Grafting Density Effect

Nanoparticle modification by water-soluble polymers, such as poly(ethylene oxide) (PEO), relies on polymer hydration to ensure nanoparticle solubility, dispersion, and protection from undesirable interactions. The state of polymer hydration in grafted polymer layers is not easily assessible experimentally but can be ascertained from computer simulations. Using atomistic molecular dynamics simulations, we studied the equilibrium and dynamic properties of spherical PEO brushes grafted to gold nanoparticles of different radii (1, 2, and 3 nm). We obtained the volume fraction of PEO (Φ(r)) as a function of radial distance r (counted from micelle core center) which is found to follow the Daoud–Cotton model, Φ(r) ∼ r–4/3, except for low grafting density when PEO adsorption onto the gold surface is observed, in agreement with experimental observations. With an increase of grafting density σ (up to 4.17 nm–2), the PEO chains become more stretched and oriented along the radial direction leading to an increase of t...

Consequences of Grafting Density on the Linear Viscoelastic Behavior of Graft Polymers.

The linear viscoelastic behavior of poly(norbornene)-graft-poly(±-lactide) was investigated as a function of grafting density and overall molar mass. Eight sets of polymers with grafting densities ranging from 0 to 100% were synthesized by living ring-opening metathesis copolymerization. Within each set, the graft chain molar mass and spacing between grafts were fixed, while the total backbone length was varied. Dynamic master curves reveal that these polymers display Rouse and reptation dynamics with a sharp transition in the zero-shear viscosity data, demonstrating that grafting density strongly impacts the entanglement molar mass. The entanglement modulus (Ge) scales with inverse grafting density (ng) as Ge ∼ ng1.2 and Ge ∼ ng0 in accordance with scaling theory in the high and low grafting density limits, respectively. However, a sharp transition between these limiting behaviors occurs, which does not conform to existing theoretical models for graft polymers. A molecular interpretation based on thin flexible chains at low grafting density and thick semiflexible chains at high grafting density anticipates the sharp transition between the limiting dynamical regimes.

Nanocavitation in silica filled styrene‐butadiene rubber regulated by varying silica‐rubber interfacial bonding

The influence of filler‐matrix interfacial bonding on the nanocavitation of rubber materials has not been explored clearly. We herein report the nanocavitation modes and geometrical features impacted by varying the silica‐styrene‐butadiene rubber interfacial bonding. The interfacial bonding is tuned by grafting different amount of multi‐functional silane coupling agents on to the silica nanoparticles. By using synchrotron radiation small angle X‐ray scattering measurements, 2 major classes of cavitation were detected. When the interfacial strength was weak, fibrillar nanocavitation, with lengths ranging from 120 to 217 nm and diameter of the same order of the particle size, was generated by interfacial decohesion at the particle pores along the stretching direction. After grafting modification, the fibril‐like nanocavitation almost disappears and nanocavitation content decreases first at lower grafting density; then nanocavitation content increases at higher grafting density, where nanocavitation in confined rubber regions dominates. It is finally proposed that nanoparticle interfacial strengthening can change the cavitation mode from interfacial decohesion to confined rubber matrix nanocavitation, which exhibits more favorable prevention of the macroscopic failure by cracking.

Molecular structure and multi-body potential of mean force in silica-polystyrene nanocomposites.

We perform a systematic application of the hybrid particle-field molecular dynamics technique [Milano, et al., J. Chem. Phys., 2009, 130, 214106] to study interfacial properties and potential of mean force (PMF) for separating nanoparticles (NPs) in a melt. Specifically, we consider Silica NPs bare or grafted with Polystyrene chains, aiming to shed light on the interactions among free and grafted chains affecting the dispersion of NPs in the nanocomposite. The proposed hybrid models show good performances in catching the local structure of the chains, and in particular their density profiles, documenting the existence of the "wet-brush-to-dry-brush" transition. By using these models, the PMF between pairs of ungrafted and grafted NPs in Polystyrene matrix are calculated. Moreover, we estimate the three-particle contribution to the total PMF and its role in regulating the phase separation on the nanometer scale. In particular, the multi-particle contribution to the PMF is able to give an explanation of the complex experimental morphologies observed at low grafting densities. More in general, we propose this approach and the models utilized here for a molecular understanding of specific systems and the impact of the chemical nature of the systems on the composite final properties.

Modeling individual and pairs of adsorbed polymer-grafted nanoparticles: structure and entanglements.

We analyze the canopy structure and entanglement network of isolated polymer-grafted nanoparticles (PGNs) adsorbed on a surface. As expected, increasing the monomer-surface adsorption strength causes the polymer chains to spread out to increase contact with the surface, leading to a canopy shape that is in qualitative agreement with recent experimental results. We compare height profiles and other structural features of four PGN systems to show the separate and combined effects of increasing chain length and graft density. At moderate graft density and low surface attraction strength, nearby PGN canopies interpenetrate substantially and their combined shape is similar to that of a single PGN canopy. At high graft density or surface interaction, the interparticle spacing increases significantly. We use a geometrical primitive path analysis to calculate average entanglement properties including canopy-canopy entanglements between pairs of PGNs. The longer chain systems are well entangled at both graft densities considered, and we find that as the monomer-surface interaction strength is increased (and the interparticle distance increases), entanglements between the two PGNs are reduced. We find that the number of inter-PGN entanglements per chain is slightly larger at the lower graft density, likely because steric constraints at high graft density tend to reduce interparticle entanglements.

Structure and Stability of PEG‐ and Mixed PEG‐Layer‐Coated Nanoparticles at High Particle Concentrations Studied In Situ by Small‐Angle X‐Ray Scattering

AbstractLigand‐layer structure and stability of gold nanoparticles (AuNP) coated with α‐methoxypoly(ethylene glycol)‐ω‐(11‐mercaptoundecanoate) (PEGMUA) layers and mixed layers of PEGMUA and 11‐mercaptoundecanoic acid (MUA) at high AuNP concentrations are studied in situ by small‐angle X‐ray scattering (SAXS). The thickness of the ligand layer is modified by the molecular weight of the PEG‐ligands (2 and 5 kDa), and the PEG‐grafting density is decreased by coadsorption of MUA. The response of the conjugates to a pressure of up to 4 kbar is probed. The results indicate strongly hydrated PEG layers at high grafting densities. The stability of the mixed ligand‐layer conjugates is lower. This is most probably due to enhanced interparticle PEG–PEG interactions at lower grafting densities. The presented study demonstrates that a detailed structural characterization of polymer ligand layers in situ and in response to external stimuli is possible with SAXS.

The Order–Disorder Transition in Graft Block Copolymers

The order–disorder transitions of diblock copolymers grafted to a common backbone were examined by oscillatory shear rheology and small-angle X-ray scattering. The effect of grafting density, graft molecular weight, number of grafts, and backbone dispersity were studied using poly[(styrene-alt-N-hydroxyethylmaleimide)-random-(styrene-alt-N-ethylmaleimide)]-graft-poly(4-methylcaprolactone-block-d,l-lactide) [PSHE-g-(P4MCL-PLA)] as a model graft block copolymer. At high grafting densities (25–50%), the order–disorder temperature (TODT) of the graft polymers was nearly identical to the analogous linear diblock. At lower grafting densities (<25%), the TODT was found to systematically decrease. The number of grafts did not significantly change the value of the TODT; however, increasing the number of grafts resulted in broad, ill-defined transitions. Backbone dispersity was found to have little impact on the TODT. Ordered morphologies were imaged by transmission electron microscopy. Long-range order was observe...

High-Performance Self-Cross-Linked PGP–POEM Comb Copolymer Membranes for CO2 Capture

We report a high performance CO2 capture membrane based on the copolymerization and self-cross-linking of poly(glycidyl methacrylate-g-poly(propylene glycol))-co-poly(oxyethylene methacrylate) (PGP–POEM) comb copolymer. The epoxide–amine reaction is responsible for the self-cross-linking reaction, which takes place under mild conditions without any additional cross-linkers or catalysts. The effects of self-cross-linking on the membrane properties are investigated by comparing the copolymers with those containing a low PPG grafting density (l-PGP–POEM). Furthermore, the gas separation performance of the membranes is systematically investigated as a function of POEM content in the comb copolymer. Both the permeance and selectivity of the PGP–POEM membranes are enhanced simultaneously with increase in the POEM content up to 51.2 wt % (PGP–POEM13), at which the best performance was achieved among the membranes. The high performance results from the reduced diffusion of N2 due to the self-cross-linked structur...

Conformation of Methylcellulose as a Function of Poly(ethylene glycol) Graft Density.

Low molecular weight thiol-terminated poly(ethylene glycol) (PEG) (M ≈ 800) has been grafted onto a high molecular weight methylcellulose (MC, Mw ≈ 150000) by a facile thiol-ene click reaction; graft densities varied from 0.7% to 33% (grafts per anhydroglucose unit). Static and dynamic light scattering reveals that the overall radius of the chain increases systematically with graft density, in a manner in excellent agreement with theory. As the contour length remains unchanged, it is apparent that grafting leads to an increase in the persistence length of this semiflexible copolymer, by as much as a factor of 4. These results represent the first experimental verification of the excluded volume theory at low grafting densities, and demonstrate a promising synthetic platform for systematically increasing the persistence length of a model semiflexible, water-soluble polymer.