Graft Chains Research Articles

Coupled matrix kinetic Monte Carlo simulations applied for advanced understanding of polymer grafting kinetics

An innovative coupled matrix-based Monte Carlo (CMMC) concept has been applied to successfully assess the detailed description of the molecular build-up of linear and non-linear chains in the free-radical induced grafting of linear precursors chains.

Copolymers of Methyl Acrylate and Vinylazoles: Synthesis, Thermolabile Properties, and Grafting of Polyamine Chains

New copolymers of methyl acrylate with 1-vinylimidazole, 1-vinyl-1,2,4-triazole, and 4-vinyl-1,2,3-triazole are synthesized by radical copolymerization. It is shown that the reactivity ratios for methyl acrylate and 1-vinylimidazole are 0.83 ± 0.09 and 0.25 ± 0.02, for methyl acrylate and 1-vinyl-1,2,4-triazole are 1.0 ± 0.04 and 0.27 ± 0.01, and for methyl acrylate and (4-vinyl-1Н-1,2,3-triazolyl)-methyl pivalate are 0.56 ± 0.03 and 2.90 ± 0.20, respectively. The behavior of the copolymers in aqueous media and their thermoresponsive properties are studied by potentiometric titration and dynamic light scattering. Copolymer macromolecules occur in aqueous solutions in the form of large associates the fraction of which increases upon heating; in some cases, an insoluble phase is formed. It is found that the copolymers with the grafted oligopropylamine moieties containing N-vinylazole units can interact with the DNA oligonucleotide, stimulating their research as agents for the delivery of nucleic acids to living cells.

Spatially regulated activation of membrane fusogenic peptides with chaperone-like ionic copolymers

Controlled or targeted membrane lysis induced by cascades of assembly and activation of biomolecules on membrane surfaces is important in programmed cell death and host defense systems. In a previous study, we reported that an ionic graft copolymer with a polycation backbone and water-soluble graft chains, poly(allylamine)-graft-dextran (PAA-g-Dex) chaperoned folding and assembly of E5, a membrane-destructive peptide derived from influenza hemagglutinin, to its increase membrane-disruptive activity. In this study, we modified the copolymer with long acyl chains, which resulted in delivery of the copolymer to membrane surfaces of liposomes and living cells. The liposomes with PAA-g-Dex functionalized with stearic acid (PAA-g-Dex-SA) on their surfaces underwent vesicle-to-sheet conversion upon addition of E5, whereas control liposomes did not. E5 also induced selective lysis of cells incubated with PAA-g-Dex-SA. The spatially specific activation of E5 on target membrane surfaces driven by self-assembly of copolymer and activation of E5 should find application in lipid-based delivery devices and cell-based therapeutics.

Preparation of poly‐d,l‐lactide based nanocomposites with polymer‐grafted silica by melt blending: Study of molecular, morphological, and mechanical properties

AbstractPolylactide was first designed for the pharmaceutical and medical fields. Unfortunately, polylactide, especially poly‐d,l‐lactide (PDLLA), presents too low mechanical properties for a wide range of applications. The addition of silica nanoparticles should reinforce the mechanical strength of PDLLA. In order to improve silica/polymer interactions, this study focused on the preparation of polymer chain grafted silica using two grafting methods. The first strategy relied upon the ring opening polymerization ofd,l‐lactide on initiating silica surface (ie, “grafting from”). The second approach considered a sol‐gel synthesis process in presence of Si‐OR terminated polylactide chains to promote grafting of these chains on new silica particles (ie, “grafting to” method). Only “grafting from” silica successfully led to a nanoscale dispersion into the polyester matrix for silica content up to 5 wt%. A 3 wt% content of this silica allowed improving Young modulus of 106.0% and ultimate tensile stress of 63.7% compared to the PDLLA control.

The Structure of Dipolar Polymer Brushes and Their Interaction in the Melt. Impact of Chain Stiffness.

By using the numerical lattice Scheutjens–Fleer self-consistent field (SF-SCF) method we have studied the effect of the restricted flexibility of grafted chains on the structure and mutual interaction of two opposing planar conventional and A-type dipolar brushes. Brushes are immersed in the solvent consisting of chains similar to the grafted ones. The increase of the chain rigidity enhances the segregation of grafted chains in a A-type dipolar brush into two populations: backfolded chains with terminal monomers near the grafting surface and chains with the ends at the brush periphery. The fraction of backfolded chains grows by an increase of the Kuhn segment length. It is shown that two opposite A-type dipolar brushes from semi-rigid chains are attracted to each other at short distances. The attraction becomes more pronounced and begins at larger distances for more rigid chains with the same brush characteristics: polymerization degree, grafting density, and dipole moments of monomer units. This attraction is connected with the dipole-dipole interactions between chains of oncoming brushes with oppositely directed dipoles penetrating deeply into each other upon contact. This effect of the chain rigidity is opposite to that for conventional brushes without dipoles in the chains. For such brushes, an increase in the chain rigidity leads to the enhanced repulsion between them.

Tuning Selectivities in Gas Separation Membranes Based on Polymer-Grafted Nanoparticles.

Polymer membranes are critical to many sustainability applications that require the size-based separation of gas mixtures. Despite their ubiquity, there is a continuing need to selectively affect the transport of different mixture components while enhancing mechanical strength and hindering aging. Polymer-grafted nanoparticles (GNPs) have recently been explored in the context of gas separations. Membranes made from pure GNPs have higher gas permeability and lower selectivity relative to the neat polymer because they have increased mean free volume. Going beyond this ability to manipulate the mean free volume by grafting chains to a nanoparticle, the conceptual advance of the present work is our finding that GNPs are spatially heterogeneous transport media, with this free volume distribution being easily manipulated by the addition of free polymer. In particular, adding a small amount of appropriately chosen free polymer can increase the membrane gas selectivity by up to two orders of magnitude while only moderately reducing small gas permeability. Added short free chains, which are homogeneously distributed in the polymer layer of the GNP, reduce the permeability of all gases but yield no dramatic increases in selectivity. In contrast, free chains with length comparable to the grafts, which populate the interstitial pockets between GNPs, preferentially hinder the transport of the larger gas and thus result in large selectivity increases. This work thus establishes that we can favorably manipulate the selective gas transport properties of GNP membranes through the entropic effects associated with the addition of free chains.

Enzymatic graft polymerization from cellulose acetoacetate: a versatile strategy for cellulose functionalization

A novel approach was developed to prepare cellulose acetoacetate-graft-copolymers via horseradish peroxidase (HRP)-mediated polymerization. Cellulose acetoacetate (CAA), the macroinitiator, was obtained by transesterification between tert-butyl acetoacetate (t-BAA) and cellulose in ionic liquid. The CAA-graft-polyacrylamide (CAA-g-PAM) was synthesized using a ternary initiator system consisting of CAA, HRP and hydrogen peroxide (H2O2). The results of surface-initiated polymerization showed that the formation of free polymers could be inhibited in this process. The kinetics exhibited that this method is a powerful tool for obtaining graft copolymers. The effects of H2O2 concentration and reaction temperature were investigated to explore the optimal conditions for the process of enzymatic initiation. The synthesized copolymers were characterized by FT IR, NMR, TGA and elemental analysis to confirm the successful grafting of polyacrylamide chains onto the cellulose backbones. Furthermore, generality of the HRP-mediated graft polymerization approach was demonstrated using various monomers, including 2-hydroxyethyl methacrylate (HEMA), methyl methacrylate (MMA) and sulfobetaine methacrylate (SBMA). This efficient and robust strategy has great potential in fabrication of cellulose-based functional polymers. Enzymatic grafting polymerization from cellulose acetoacetate

Assembly of Polymer-Grafted Nanoparticles in Polymer Matrices.

Leibler pioneered the idea that long enough matrix polymers of length P will spontaneously dewet a chemically identical polymer layer, comprising chains of length N, densely end-grafted to a flat surface ("brush"). This entropically driven idea is routinely used to explain experiments in which 10-20 nm diameter nanoparticles (NPs) densely grafted with polymer chains are found to phase separate from chemically identical melts for P/N ≳4. At lower grafting densities, these effects are also thought to underpin the self-assembly of grafted NPs into a variety of structures. To explore the validity of this picture, we conducted large-scale molecular dynamics simulations of grafted NPs in a chemically identical polymer melt. For the NPs we consider, in the ≈5 nm diameter range, we find no phase separation even for P/N = 10 in the absence of attractions. Instead, we find behavior that more closely parallels experiments when all of the chain monomers are equally attractive to each other but repel the NPs. Our results thus imply that experimental situations investigated to date are dominated by the surfactancy of the NPs, which is driven by the chemical mismatch between the inorganic core and the organic ligands (the graft and free chains are chemically identical). Entropic effects, that is, the translational entropy of the NPs and the matrix, the entropy of mixing of the grafts and the matrix, and the conformational entropy of the chains appear to thus have a second-order effect even in the context of these model systems.

Effect of fine structure of chitosan-based flocculants on the flocculation of bentonite and humic acid: Evaluation and modeling

The fine molecular structure of a flocculant fundamentally determines the internal flocculation mechanism and the final application property. In this work, three series of chitosan-based polymers (CTS-g-PAMD) with divergent charge densities and graft chain distribution were synthesized by graft copolymerization using acrylamide (AM) and acryloyloxyethyltrimethylammonium chloride (DAC). Meanwhile, flocculant with linear chain structure (CTS-CTA) was prepared by etherification using 3-chloro-2-hydroxypropyltrimethylammonium chloride (CTA). The characterization results confirmed that various monomers had been successfully introduced into chitosan. The reaction basically happened on —NH2 at C2 of chitosan, and the ring structure of chitosan was destroyed by free radical reaction. The obtained flocculants were used to flocculate bentonite and humic acid solution. Besides dose, the effects of chain structure, charge density and chain distribution on flocculation performance were systematically studied. Based on the fractal theory and flocculation kinetics, the effects of structural factors on floc characteristics were also investigated. The results showed that, flocculant with abundant graft chains exerts better flocculation performance and floc characteristic due to enhanced adsorption electrical neutralization and adsorption bridging effect. The effects of charge density and chain distribution on the flocculation performance were disparate in the range of insufficient and excessive doses. Furthermore, on the basis of the quadratic polynomial model, quantitative structure-effect relationships were established, which has guiding significance for the development and utilization of flocculants.

Effects of graft locations on dispersion behavior of polymer-grafted nanorods: A molecular dynamics simulation study

In this paper, we present the effects of graft locations on the dispersion behavior of polymer-grafted nanorods using molecular dynamics simulations. Four graft locations of chains on nanorods, i.e., end-grafted, quarter-grafted, middle-grafted and uniformly grafted, are specially designed, and the effects of graft locations are examined in both sparsely grafted case and densely grafted case. Simulation results show that the end-grafted nanorods exhibit the poorest dispersion in the matrix, whilst the body-grafted (quarter-grafted and middle-grafted) ones show the best dispersion. At low grafting density in the mushroom regime, the middle-grafted disperse slightly better than the quarter-grafted, whereas at high grafting density in the brush regime, the situation is reversed. By analyzing the conformations of graft chains (mushroom height and brush height) and the behavior of grafted nanorods, it is inferred that the inter-rod cohesive energy provides a great contribution to the dispersion, as reflected by the significant role of the center region of nanorods. In general, a dense and wide coverage of the center/middle region of nanorods by graft chains can effectively improve the nanorod dispersion. This work is expected to provide some guidance on controlling and improving the dispersion of nanorods in polymer nanocomposites.

Tightly-coated and easily recyclable Ag@AgBr-cotton hybrid photocatalyst for organic dye degradation under visible light

Immobilization of photocatalysts on substrates is an important factor for water purification in practical applications. In this study, an efficient Ag@AgBr photocatalytic coating was attached onto the surface of cotton fabric via radiation-induced graft polymerization and subsequent chemical reactions, and the resultant product was denoted as Cot-Ag@AgBr. The graft chains acted not only as a source of bromine, but also as an intermediate layer to anchor the Ag@AgBr photocatalytic coating via strong electrostatic interactions. The immobilized Ag@AgBr photocatalytic coating exhibited outstanding photocatalytic properties for organic dye degradation under visible light irradiation. Furthermore, Cot-Ag@AgBr presented high stability and robust durability, which conferred its excellent visible-light-driven photocatalytic activity after five rounds of photodegradation. After 10 min of ultrasonication, the amount of Ag@AgBr precipitate that leached from the Cot-Ag@AgBr was < 2%. Therefore, the as-synthesized cotton fabric offered good insight into the functionalization of polymer substrates with photocatalysts and could be useful for advanced wastewater treatment under the cyclic operations.

SEC analysis of copolymers of different topologies with branched and linear copolyimides with PMMA blocks as a model system

For grafted copolyimides with variable length and grafting density of polymethylmethacrylate side chains, the molecular masses were determined using three modes of SEC analysis, which differ in the detectors set. The results were compared with the values obtained (i) using 1H NMR spectra, (ii) those calculated by summing the molecular masses of homopolymer linear blocks that make up the copolymer, as well as (iii) with the absolute values of molecular masses determined by sedimentation-diffusion analysis. It is shown that for branched grafted copolyimides the use of only a refractometric detector or its combination with a viscometric detector in the SEC analysis did not allow for obtaining reliable molecular masses. On the contrary, the combination of refractometric and viscometric detectors with a light scattering detector provided for the determination of the molecular masses close to the respective absolute values. It is also shown that for linear block copolymers, unlike branched copolymers, the molecular masses close to the absolute values can be obtained using any of the three modes of SEC analysis. For the first time, a concept of the critical molecular mass of the side chains of grafted copolyimides is introduced as a quantitative criterion that takes into account the total length and also the density of grafting of the side chains.

Synthesis and characterization of poly(TRIM/VPA) functionalized graphene oxide nanoribbons aerogel for highly efficient capture of thorium(IV) from aqueous solutions

An ultralight three-dimensional porous network phosphonic acid functionalized polymer/graphene oxide nanoribbons aerogel (PAFP/GONRs-A) was prepared using trimethylolpropane trimethacrylate (TRIM) and vinylphosphonic acid (VPA) as monomers via solvothermal polymerization method for thorium capture from aqueous solutions. The synthesized aerogel presented a low density (10.6 mg cm−3), large specific surface area (433.2 m2 g−1) and high phosphorus content (18.2%). The adsorption process of Th(IV) on PAFP/GONRs-A was pH-dependent, spontaneous and endothermic, and well described by the pseudo-second-order kinetic and Langmuir isotherm model. Under optimal experimental conditions (10 mg of adsorbent dosage, 50 mL of solution volume, 240 min of contact time, 298 K of temperature), PAFP/GONRs-A presented an excellent high adsorption capacity of thorium, with maximum capacity of 457.9 mg g−1 at a pH of 3.0, and maximum thorium-selectivity of 87.1% at a pH of 2.0. The values of ΔG for Th(IV) adsorption on PAFP/GONRs-A were calculated to be −21.78, –23.32, −24.87 and −26.41 kJ mol−1 at 283, 298, 313 and 328 K, respectively. Density functional theory (DFT) calculations and X-ray photoelectron spectroscopy (XPS) revealed that Th(IV) ions were fixed in aerogel by coordinating with the P = O groups of PAFP/GONRs-A, and both 1:2 ratio of Th(IV) with the P = O ligands on the same graft chain and 1:4 ratio of Th(IV) with P = O ligands on two distinct graft chains could jointly contribute to the adsorption of Th(IV) on PAFP/GONRs-A. This work offers a facile approach for synthesizing the phosphonic acid functionalized graphene oxide nanoribbons aerogel and demonstrates that PAFP/GONRs-A has high potential as a candidate adsorbent for the capture of thorium from aqueous solution.

Gold Nanoparticle Promoted Formation and Biological Properties of Injectable Hydrogels.

Acceleration of gelation in the biological environment and improvement of overall biological properties of a hydrogel is of enormous importance. Biopolymer stabilized gold (Au) nanoparticles (NPs) exhibit cytocompatibility and therapeutic activity. Hence, in situ gelation and subsequent improvement in the property of a hydrogel by employing Au NPs is an attractive approach. We report that stable Au NPs accelerate the conventional nucleophilic substitution reaction of activated halide-terminated poly(ethylene glycol) and tertiary amine functional macromolecules, leading to the rapid formation of injectable nanocomposite hydrogels in vivo and ex vivo with improved modulus, cell adhesion, cell proliferation, and cytocompatibility than that of a pristine hydrogel. NP surfaces with low chain grafting density and good colloidal stability are crucial requirements for the use of these NPs in the hydrogel formation. Influence of the structure of the amine functional prepolymer, the spacer connecting the halide leaving groups of the substrate, and the structure of the stabilizer on the rate promoting activity of the NPs have been evaluated with model low-molecular-weight substrates and macromolecules by 1H NMR spectroscopy, rheological experiments, and density functional theory. Results indicate a significant effect of the spacer connecting the halide leaving group with the macromolecule. The Au nanocomposite hydrogels show sustained co-release of methotrexate, an anti-rheumatic drug, and the Au NPs. This work provides insights for designing an injectable nanocomposite hydrogel system with multifunctional property. The strategy of the use of cytocompatible Au NPs as a promoter provides new opportunity to obtain an injectable hydrogel system for biological applications.

Surface PEGylation of polyacrylonitrile membrane via thiol-ene click chemistry for efficient separation of oil-in-water emulsions

Because of their low hydrophilic properties, the polymeric membranes are always subjected to fouling problems, which have become a major obstruction in the path of the practical application of membrane technology in oil–water separation. The present work describes a highly hydrophilic polyacrylonitrile (PAN) membrane successfully developed via thiol-ene click chemistry. To fabricate the membrane, the pristine PAN membrane was first hydrolyzed and then reacted with cysteamine hydrochloride to create a thiolated surface. Afterward, poly(ethylene glycol) methyl ether methacrylate (PEGMA) was covalently grafted onto the thiolated surface to obtain the PEGylated membrane. The effects of the number-average molecular weight (Mn) of PEGMA monomers on the structure, surface wettability, anti-fouling abilities and separation performances of the PEGylated membrane were investigated systematically. The grafting density of PEGMA chains increases with the increase in PEGMA concentration in the reaction solution, whereas it diminishes with the increasing Mn of PEGMA monomers as a result of the steric hindrance. The PEGylated PAN membrane has the superhydrophilic and underwater superoleophobic characteristics when it is grafted with the low-Mn PEGMA monomers. It is observed that the surface grafting of PEGMA chains results in the reduction of membrane pore size, which endows the PEGylated PAN membrane with a low flux. The separation efficiency of PEGylated membrane for several kinds of oil-in-water emulsions is as high as 99.9%, indicative of an excellent separation performance. The cycle filtrations of oil-in-water emulsions demonstrate that the flux decline of PEGylated membrane caused by both the irreversible and reversible fouling is suppressed effectively, as compared with the pure PAN membrane. Even after the three-cycle filtration of the oil-in-water emulsion, the recovery ratio of pure water flux of PEGylated membrane remains greater than 95%, suggesting a high fouling-resistant ability in the permeation process of emulsions.

Studies of zwitterionic sulfobetaine functionalized polypropylene surface with or without polyethylene glycol spacer: surface characterization, antibacterial adhesion, and platelet compatibility evaluation

Microbial adhesion reduction as well as platelet compatibility improvement have been suggested as the key requirements for developing blood-contacting synthetic biomaterials. Surface grafting of hydrophilic polyethylene glycol chains or alkyl chains with zwitterionic terminal ends has been proposed for reducing microbial or platelet adhesion. Nonetheless, none has been reported to incorporate both polyethylene glycol and zwitterionic terminal functionality on the same surface-grafted alkyl chain. In this investigation, a novel surface modification scheme was reported for grafting zwitterionic alkyl chains with or without polyethylene glycol as the spacer. It was noted the bacterial adhesion reduction capability on the zwitterionic modified surface was dependent upon the use of polyethylene glycol spacer or not and the strain of microbe tested. Besides, the zwitterionic modified ones all showed greater antimicrobial adhesion capability than the surface modified with polyethylene glycol alone. On the other hand, significantly reduced platelet adhesion and activation were found, but with no statistical differences noted among the polyethylene glycol-modified surface and zwitterionic ones, with or without polyethylene glycol spacer. These suggested that the use of polyethylene glycol spacer on the zwitterionic terminated surface could further enhance the antimicrobial adhesion against gram-negative bacterial while still keeping its platelet compatibility.

Thermo- and pH-sensitive glycosaminoglycans derivatives obtained by controlled grafting of poly(N-isopropylacrylamide)

Poly(N-isopropyl acrylamide) grafted heparin and chondroitin sulfate were synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization. The copolymers were characterized by NMR, IR, SEC, DLS, SLS and NTA methods. High grafting densities were reached for both glycosaminoglycans. The temperature, pH and polymer concentration affected the low critical solution temperatures values. The increased pNIPAAm chain length, grafting density and concentration led to the sharp phase transition at 35 °C. Spherical nanogels were formed around this temperature. Terminal dodecyl trithiocarbonate groups of the copolymers were reduced to thiols that allowed formation of sensitive nanogels with sharp phase transitions induced by pNIPAAm chains. The copolymers showed no toxicity to the ocular cells and they provided the prolonged release of dexamethasone phosphate at 37 °C. These copolymers are interesting alternatives for ocular drug delivery.

Molecular Description of Grafted Supramolecular Assemblies on Gold Surfaces: Effect of Grafting Points and Chain Lengths.

The association of 4 aminoazobenzene (4AA) with two different water-soluble hosts, β-cyclodextrins (β-CD) and calixarenesulfonates (CnS), was studied in heterogeneous conditions using molecular simulations. This situation is achieved by immobilization of macrocycles onto a gold Au(111) surface. Several factors that can influence the binding properties are investigated here through the chain length of alkylthiols spacer of the surface-immobilized host and the number of attachment points to the surface. A conformational change of β-CD as a function of the chain length is evidenced upon grafting on the gold surface, whereas CnS does not show any changes. It is then possible to tune the thermodynamic properties of β-CD by changing the grafted chain length and forming a larger hydrophobic region. The mechanisms of insertion of guests into the cavities are similar to those obtained in a homogeneous system. 4AA is included longitudinally in the β-CD cavity, and it interacts rather with the sulfonate groups of the CnS located at the outer edge of the cavity.

Investigating the structure and self-assembly behavior of starch-g-VAc in starch-based adhesive by combining NMR analysis and multi-scale simulation

This work investigated the structure and self-assembly behavior of grafted starch (GS) prepared by grafting vinyl acetate (VAc) on the starch molecule. Our preliminary structure characterization, NMR, and quantum mechanical simulation demonstrated the C2 of the glucose unit as the main grafting site. The grafting frequency and chain length (starch, VAc) were calculated based on the result of gel permeation chromatography. Molecular dynamics simulation showed that, when compared with native starch, GS had less hydrogen bonding interaction, lower orderness, and higher extensibility, which were supported by the experimental results. In dissipative particle dynamics simulation, GS was shown to self-assemble into a core-shell structure (latex) and form a bridge structure with cross-linking interaction. The overall results indicate that chain entanglement and hydrogen bonding interaction of starch play a significant role in adhesive curing. This research provides a novel insight into the grafting and molecular interaction mechanism in the GS adhesive system.

Roughness-enhanced hydrophobic graphene oxide membrane for water desalination via membrane distillation

Novel design of hydrophobic membrane could improve the efficiency of membrane distillation (MD) technology. Owning to being easy to realize hydrophobic modification, fast radial heat transfer rate and limiting the passage of larger contaminants, hydrophobic graphene oxide (GO) membrane shows potential for MD process. In this work, we reported on a new type of nanoparticles intercalated GO membrane with hydrophobic surface for MD application fabricated by vacuum-filtration method. The introduction of nanoparticles (SiO2) by physical mixing enhanced the surface roughness of GO membrane. A subsequent grafting of long alkyl chains (hexadecyltrimethoxysilane) further increased the membrane hydrophobicity. The effects of treating temperature and nanoparticles content on the membrane nanostructures, surface hydrophobicity and desalination performance were systematically investigated. The optimized membrane annealed at 80 °C with hydrophobic surface (water contact angle: 120.8°) exhibited water flux of 13.59 kg m-2 h-1 and salt rejection of 99.99% under 60 °C for 35 g L-1 NaCl in water during vacuum MD process. Besides of the expected gradually higher water flux at elevated temperature, such excellent desalination performance remains stable by varying the salt concentration or adding model foulants such as surfactant sodium dodecyl sulfate and organic humid acid in the feed.