Increasing Grafting Density Research Articles

Dynamics and glass transition temperature of polystyrene films supported on hydroxyl terminated substrates

All atom molecular simulations are employed to systematically study the properties of polystyrene (PS) films supported on hydroxyl (–OH) terminated Si substrates. We modulate the –OH grafting density ϕG to probe its effect on polymer diffusion, local segmental relaxation, and glass transition temperature Tg. First, our results indicate a monotonic facilitation of polymer dynamics with increasing ϕG. The underlying mechanism is investigated in terms of energy and PS free volume change in detail, and a consistent scenario is obtained. The facilitated diffusion dynamics can be attributed to the –OH induced reduction of interaction energy between PS and the substrate. In addition, –OH grafting leads to the augment of the polymer free volume, which is responsible for the faster local segmental relaxation process. Second, we uncover a very intriguing increase–decrease non-monotonic tendency of glass transition temperature with increasing grafting density, where the maximum Tg occurs in a moderate grafting case. A wetting-friction transition is proposed to rationalize such an unusual behavior. As ϕG increases from a native case to a half grafting situation, polymer films undergo a sharply enhanced interpenetration (or wetting) into the substrate, resulting in the pronounced increment of Tg. As ϕG further increases, the polymer–substrate interaction energy experiences a heavy drop, which implies a greatly reduced friction effect, leading to the decreasing trend of Tg as observed in the range from half grafting to full grafting.

Structure formation in pH-sensitive micro porous membrane from well-defined ethyl cellulose-g-PDEAEMA via non-solvent-induced phase separation process

To clarify the structure formation in pH-sensitive cellulose based micro porous membrane in non-solvent-induced phase separation process, first, a series of ethyl cellulose-graft-poly(2-(diethylamino)) ethyl methacrylate) (EC-g-PDEAEMA) with different graft density and graft length were synthesized via atom transfer radical polymerization (ATRP). The results of the micelles characterized by UV, Laser particle analysis and TEM indicated that the copolymer had pH responsive behavior and the increase of grafting density and grafting length led to the decrease of critical pH value. Furthermore, the morphology study of micro porous membrane via the NIPS process from mixture of THF and DMF indicated that the pores became smaller and more uniform with the increase of THF content and “open time”, and the most optimal preparation conditions was using EC0.2-g-PDEAEMA32 as the film-forming material and 75%THF/25%DMF (v/v) as solvent with the “open time” 1 min. What’s more, the membrane was responsive to pH stimulus in terms of water flux and demonstrated by scanning electron microscopy.

Polymer Grafting Inside Wood Cellulose Fibers by Improved Hydroxyl Accessibility from Fiber Swelling.

Chemical modification of wood cellulose fibers is important for tailored wood-polymer interfaces, reduced moisture sorption, and novel grades of chemical wood pulp. The present study shows how the reaction solvent system influences hydroxyl accessibility during chemical fiber modification. Surface initiated ring-opening polymerization of ε-caprolactone from wood cellulose fibers was investigated in a wide range of solvent systems. The hydrogen bond donor strength of the solvent increased graft density and the amount of grafted polycaprolactone (PCL) on the fiber surface, and on nanoscale fibrils inside the fiber. Specifically, the reaction system with acetic acid as a new, green solvent for cellulose grafting increased graft density 24 times compared to bulk polymerization conditions. The results show relationships between solvent properties, hydroxyl accessibility, and grafting results in cellulosic plant fibers. The study clarifies the opportunities provided by controlling the interior of the cellulosic plant fiber cell wall during chemical modification so that the fiber becomes a swollen cellulose nanofibril gel.

Bioactive Peptide Brush Polymers via Photoinduced Reversible‐Deactivation Radical Polymerization

AbstractHarnessing metal‐free photoinduced reversible‐deactivation radical polymerization (photo‐RDRP) in organic and aqueous phases, we report a synthetic approach to enzyme‐responsive and pro‐apoptotic peptide brush polymers. Thermolysin‐responsive peptide‐based polymeric amphiphiles assembled into spherical micellar nanoparticles that undergo a morphology transition to worm‐like micelles upon enzyme‐triggered cleavage of coronal peptide sidechains. Moreover, pro‐apoptotic polypeptide brushes show enhanced cell uptake over individual peptide chains of the same sequence, resulting in a significant increase in cytotoxicity to cancer cells. Critically, increased grafting density of pro‐apoptotic peptides on brush polymers correlates with increased uptake efficiency and concurrently, cytotoxicity. The mild synthetic conditions afforded by photo‐RDRP, make it possible to access well‐defined peptide‐based polymer bioconjugate structures with tunable bioactivity.

The Relevance of Multi‐Injection and Temperature Profiles to Design Multi‐Phase Reactive Processing of Polyolefins

AbstractFor industrial‐scale continuous free radical induced grafting of polyethylene, which involves the contact of a monomer‐ and polymer‐rich phase, it is demonstrated that the functionalization selectivity and grafting density can be significantly improved (e.g., 100%) by considering multiple injection of monomer and/or a temperature profile. With parameters taken from literature, it is highlighted through simulations that step‐wise addition of monomer allows to reduce the homopolymerization rate, and an increased temperature at the start of the reactive processing allows to increase the hydrogen abstraction rate, so that more potential grafting points can be generated. As this increased grafting density goes along with a lower reaction extent, a better process efficiency is obtained albeit at a lowering of the average chain length of the grafts and an increase of the crosslinking density. In any case, a broader span of polymer characteristics (e.g., ranges of grafting/crosslinking densities and functionalization yields) can be achieved by including variations in reactant addition and temperature profiles.

Theoretical prediction of an isotropic to nematic phase transition in bottlebrush homopolymer melts.

Bottlebrushes are an emerging class of polymers, characterized by a high density of side chains grafted to a linear backbone that offer promise in creating materials with unusual combinations of mechanical, chemical, and optoelectronic properties. Understanding the role of molecular architecture in the organization and assembly of bottlebrushes is of fundamental importance in polymer physics, but also enabling in applications. Here, we apply field-theoretic simulations to study the effect of grafting density, backbone length, and side-chain (SC) length on the structure and thermodynamics of bottlebrush homopolymer melts. Our results provide evidence for a phase transition from an isotropic to a nematic state with increasing grafting density and side-chain length. Variation in the backbone length is also observed to influence the location of the transition, primarily for short polymers just above the star to bottlebrush transition.

Co-Nonsolvency Transition of Poly(N-isopropylacrylamide) Brushes in a Series of Binary Mixtures

Co-nonsolvency occurs if a mixture of two good solvents causes the collapse or demixing of polymers into a polymer-rich phase in a certain range of compositions of these two solvents. In this work, we systematically study the co-nonsolvency behavior of poly(N-isopropylacrylamide) brushes of different grafting densities in a series of alcohol–water binary mixtures with increasing hydrophobic parts ranging from methanol to 1-butanol by using ellipsometry. We report a strong collapse transition by increasing the alcohol concentration in the water-rich phase, which is enhanced for longer-chain alcohols. The analysis of the thermodynamic properties of the alcohol–water series displays that an increasing demixing tendency between alcohol and water is correlated with an enhancement of the collapse transition of the brush. The increase of grafting density weakens the transition behavior but does not shift the solvent composition point of maximum brush collapse, which is in agreement with the predictions of a rece...

Symmetry Transitions of Polymer-Grafted Nanoparticles: Grafting Density Effect

We examined the packing structure of polystyrene-coated gold nanoparticles (Au@PS) as a function of grafting density. A series of Au@PS nanoparticles with grafting densities in the range of 0.51–1.94 chains nm–2 were prepared by a ligand exchange process using thiol-terminated PS and then self-assembled at a liquid–air interface. We observed a transition from disordered to body-centered cubic (bcc) to face-centered cubic (fcc) arrangements with increasing grafting density, even though the ligand length-to-core radius ratio (λ) was as high as 3.0, a condition that typically favors nonclose-packed bcc symmetry in the self-assembly of hard nanoparticles. To explain this phenomenon, we define λeff to include the concentrated polymer brush regime as part of the “hard core”, which predicts that the softness of Au@PS nanoparticles is reduced from 1.53 to 0.14 in a theta solvent as the grafting density increases from 0.51 to 1.94 chains nm–2. This new definition of λ can also predict the effective radii of nanopa...

Effects of graft architecture on cellulose-based ordered porous film prepared by breath figures

It is well known that molecular structure has great influence on the porous structure prepared by breath figures. However, the effect of graft architecture on porous structure from graft copolymer is unclear. Here, the cellulose-based graft copolymers with different graft architectures were synthesized and the highly ordered porous films were facilely prepared by breath figure method. The effects of graft architecture with four different structural parameters on porous structure were systematically investigated. It was found that the pore size increased with the increasing graft chain length, main chain length or molecular weight, while the pore size decreased with the increasing graft density under other similar structural parameters. The shorter main chain and the higher graft density resulted in the smaller pore size. Moreover, the shorter main chain was more beneficial for the formation of highly ordered porous structure than the longer main chain. These results will help to understand the effect of graft architecture on porous structure and regulate pore size by adjusting the graft architecture.

The influences of grafting density and polymer–nanoparticle interaction on crystallisation of polymer composites

ABSTRACTCrystallisation of polymer chains grafted onto a nanoparticle and the effects of grafting density and polymer–nanoparticle interaction were investigated by Monte Carlo simulations. The increase of grafting density leads to the improvements of nucleation ability and the increase in both melting point and final crystallinity. Moreover, the increase of interfacial interaction can lead to the changes of nucleation mode from homogenous nucleation to heterogeneous nucleation, accelerating the nucleation process. To explain the increase of melting point induced by grafting, a prediction of theoretical formula is established, which indicates that the melting point is related to the deformation and concentration of grafted polymer chains. Our findings develop a thermodynamic background for the crystallisation of grafted polymer systems and explain the corresponding experimental results, which has a certain guiding implication for the design, preparation and application of polymer nanocomposites.

The Effect of Grafting Density on the Crystallization Behaviors of Polymer Chains Grafted onto One-Dimensional Nanorod

The crystallization behaviors of five polymer chain systems grafted on a nanorod and the corresponding effect of grafting density were investigated by dynamic Monte Carlo simulations. The segment density near the interfacial regions, the number of crystallites, and the mean square radius of gyration (<Rg2>) increase with increasing grafting density, which are beneficial to the enhancement of crystallizability. Meanwhile, the crystalline morphology is greatly influenced by grafting density and polymer-nanorod interaction. For the grafted system with 52 chains, a nanohybrid shish-kebab (NHSK) structure is formed, when the polymer-nanorod interaction (Eb/Ec) is -0.4. For the system with 128 chains, a NHSK structure is formed, when Eb/Ec is -1.0. For the system with 252 chains, NHSK structure cannot be formed. The findings in this work can supply important theoretical reference for the design, preparation, and application of polymer nanocomposites.

Insight into the role of grafting density in the self-assembly of acrylic acid-grafted-collagen.

Side chain modification of collagen provides an attractive way to enhance their structure and functions, which is highly desirable for the development of promising biomaterials. However, the impact of structural change of side chains on the intrinsic self-assembly property of collagen was always ignored. Here, a series of acrylic acid-grafted-collagen (AA-g-Col) with different grafting density were prepared to explore the impact of side chain structural variation on the self-assembly of collagen. The results showed that excessive grafting density would weaken or even disappear the self-assembly property of AA-g-Col, but only affects the triple helix to a minor extent. Compared to pristine collagen, the mechanical property and cytocompatibility of AA-g-Col based matrices also deteriorated, along with the increase of grafting density. Therefore, this work contributed a new insight into the importance of grafting density for the study of modified collagen, which would be helpful for the design of optimized formulate collagen-based hybrid materials with both additional novel functions and tissue-mimicking fibrillary structures.

Optical Properties of Silver Nanoclusters Synthesized in Surface-Grafted Polyacrylic Acid at Different Grafting Densities

Fluorescent silver nanoclusters immobilized on a plastic substrate are obtained by photoinduced template synthesis using polyacrylic acid chemically grafted to the substrate surface as a template. The electronic absorption and fluorescence spectra of nanoclusters are studied depending on the polyacrylic acid grafting density. It is found that the optical absorption and fluorescence intensities of nanoclusters monotonically increase with increasing grafting density. The position and shape of the absorption and emission spectra almost do not change. The higher the polymer template grafting density, the higher the fluorescence stability of samples during storage at room temperature.

Dispersion of Nanoparticles in Lubricating Oil: A Critical Review

Nanolubricants have attracted great interest due to the promise of friction and wear reduction by introducing nanoparticles. To date, the foremost challenge for developing a new nanolubricant is particle suspension. To understand the mechanisms of nanoparticle dispersion and identify bottlenecks, we conducted a comprehensive review of published literature and carried out an analysis of dispersion based on available data from the past 20 years. This research has led to three findings. First, there are two primary methods in dispersion: formulation with dispersant and surface modification. Second, surfactant and alkoxysilanes are primary chemical groups used for surface modification. Third, functionalization using surfactant is found to be suitable for nanoparticles smaller than 50 nm. For larger particles (>50 nm), alkoxysilanes are the best. The existence of a critical size has not been previously known. To better understand these three findings, we conducted an analysis using a numerical calculation based on colloidal theory. It revealed that a minimal thickness of the grafted layer in surfactant-modified nanoparticles was responsible for suspending small nanoparticles. For larger nanoparticles (>50 nm), they were suitable for silanization of alkoxysilane due to increased grafting density. This research provides new understanding and guidelines to disperse nanoparticle in a lubricating oil.

Оптические свойства нанокластеров серебра, синтезированных в поверхностно-привитой полиакриловой кислоте при разных плотностях прививки

AbstractFluorescent silver nanoclusters immobilized on a plastic substrate are obtained by photoinduced template synthesis using polyacrylic acid chemically grafted to the substrate surface as a template. The electronic absorption and fluorescence spectra of nanoclusters are studied depending on the polyacrylic acid grafting density. It is found that the optical absorption and fluorescence intensities of nanoclusters monotonically increase with increasing grafting density. The position and shape of the absorption and emission spectra almost do not change. The higher the polymer template grafting density, the higher the fluorescence stability of samples during storage at room temperature.

Polymers on nanoparticles: structure & dynamics.

Grafting polymers to nanoparticle surfaces influences properties from the conformation of the polymer chains to the dispersion and assembly of nanoparticles within a polymeric material. Recently, a small body of work has begun to address the question of how grafting polymers to a nanoparticle surface impacts chain dynamics, and the resulting physical properties of a material. This Review discusses recent work that characterizes the structure and dynamics of polymers that are grafted to nanoparticles and opportunities for future research. Starting from the case of a single polymer chain attached to a nanoparticle core, this Review follows the structure of the chains as grafting density increases, and how this structure slows relaxation of polymer chains and affects macroscopic material properties.

Computer simulation study on the self-assembly of tethered nanoparticles with tunable shapes.

We built a tethered nanoparticle (TNP) model that is composed of a nanoparticle with a hydrophobic tethered polymer chain. The shape of the nanoparticle can be tuned from a pure rigid cube to a soft sphere, mimicking the increase of grafting density on the nanocube surfaces. With this model, we study the self-assembly of TNPs in dilute solution using a dissipative particle dynamics simulation technique, and especially focus on the influence of particle shape, tethered chain length, and grafting density on the self-assembly structures. Some intriguing aggregates such as spherical micelles, pearl-necklace-like structures, cubic columnar structures, handshake structures, core–shell–corona micelles, and four-patch micelles have been observed when varying the interactions between cubes and solvents and the lengths of tethered chain. Modifying the nanocube surface with some hydrophilic grafted chains helps the TNPs form small micelles. Increased steric repulsion due to chain overlapping at larger grafting densities results in shape transformation of the nanoparticle from a rigid cube to a soft sphere. In these cases, the self-assembled structures are characterized by the packing of nanoparticles on the micelle surface, and the typical packing mode turns from rectangular (typical for cubes) to hexagonal (typical for spheres).

Dynamic Monte Carlo simulations of competition in crystallization of mixed polymers grafted on a substrate

ABSTRACTNowadays, preparation of poly(lactide) (PLA) with high content of stereocomplex crystallites (SCs) is receiving more and more attentions. The stereocomplex formation between enantiomeric poly(l‐lactide) and poly(d‐lactide) can efficiently improve the corresponding mechanical properties and thermal stability. In the current work, using dynamic Monte Carlo simulations, the microscopic mechanism of SC formation in grafted polymers was investigated. The increase in grafting density can lead to the enhancement of SC formation. On one hand, the miscibility between the chains of different types can be improved due to the grafting. On the other hand, the increase in grafting density can result in the higher degree of chain extension and the change in crystal nucleation and growth modes. The changes facilitate the stereocomplex formation. These findings not only provide an effective way to prepare PLAs with high content of SCs, but also reveal the underlying mechanisms controlling the SC formation. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019, 57, 89–97

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...

Tuning interfacial strength of silicone resin composites by varying the grafting density of octamaleamic acid-POSS modified onto carbon fiber

Octamaleamic acid-polyhedral oligomeric silsesquioxanes (OMA-POSS) were chemically modified onto the surface of carbon fibers (CFs) by a facile and high efficient two-step method. CF was grafted with hydroxyl groups via aryl diazonium reaction, and then covalently functionalized with OMA-POSS. The grafting density could be tuned through the concentration of OMA-POSS. Fiber surface structures were characterized by confirming the covalent bonding nature between OMA-POSS and CF. OMA-POSS was scattered on CF surface uniformly, and fiber polarity and roughness increased with the increased grafting density of OMA-POSS. Interfacial shear strength (IFSS) and interlaminar shear strength (ILSS) of the modified CFs composites enhanced as the grafting density grew, which could be ascribed to OMA-POSS interface with providing different degrees of chemical bonding and mechanical interlocking. Moreover, the hydrothermal aging resistance of composites is highly dependent on the amounts of the introduced OMA-POSS. The storage modulus and the glass transition temperature could increase by 8 GPa and 14 °C based on dynamic mechanical analysis testing. In addition, the interfacial reinforcing mechanisms have been also analyzed, and surface modification maintained fiber tensile strength.