Graft Molecular Weight Research Articles

Synthesis of Poly(styrene)-g-Poly(oleic acid) Graft Copolymers via Reversible Addition/Fragmentation Transfer (RAFT) Polymerization Using a Poly Oleic Acid Macro-RAFT Agent

In this study, a new polymeric oleic acid-derived macro addition/fragmentation transfer agent was utilized to produce a poly(styrene)-g-poly(oleic acid) graft copolymer. The double bond of oleic acid was initially saturated with bromine and the condensation polymerization between the carboxylic acid and the bromide resulted in polyoleic acid with pendant bromide groups. Xanthate groups were exchanged with the bromide groups to obtain the poly(oleic acid) macro RAFT agent (Pole-Xa). Poly(styrene)-g-poly(oleic acid) (PS-g-Pole) graft copolymers were synthesized via reversible addition fragmentation transfer (RAFT) polymerization of styrene and the reaction was evaluated in view of the polymerization kinetics. The effects of polymerization temperature and reaction time on graft copolymer yield, conversion and molecular weight were investigated. In the RAFT polymerization of styrene, the rate constant (k) was found to be 1.83 × 10–3 L/mol/dk and 7.27 × 10–4 L/mol/dk for the polymerization temperatures of 80 and 90 °C, respectively. The structural characteristics and thermal properties of the obtained products were characterized using FT-IR, 1H-NMR, GPC, TGA, DSC and SEM–EDX.

Comparative Study of Polymer-Grafted BaTiO3 Nanoparticles Synthesized Using Normal ATRP as Well as ATRP and ARGET-ATRP with Sacrificial Initiator with a Focus on Controlling the Polymer Graft Density and Molecular Weight.

Structurally well-defined polymer-grafted nanoparticle hybrids are highly sought after for a variety of applications, such as antifouling, mechanical reinforcement, separations, and sensing. Herein, we report the synthesis of poly(methyl methacrylate) grafted- and poly(styrene) grafted-BaTiO3 nanoparticles using activator regeneration via electron transfer (ARGET ATRP) with a sacrificial initiator, atom transfer radical polymerization (normal ATRP), and ATRP with sacrificial initiator, to understand the role of the polymerization procedure in influencing the structure of nanoparticle hybrids. Irrespective of the polymerization procedure adopted for the synthesis of nanoparticle hybrids, we noticed PS grafted on the nanoparticles showed moderation in molecular weight and graft density (ranging from 30,400 to 83,900 g/mol and 0.122 to 0.067 chain/nm2) compared to PMMA-grafted nanoparticles (ranging from 44,620 to 230,000 g/mol and 0.071 to 0.015 chain/nm2). Reducing the polymerization time during ATRP has a significant impact on the molecular weight of polymer brushes grafted on the nanoparticles. PMMA-grafted nanoparticles synthesized using ATRP had lower graft density and considerably higher molecular weight compared to PS-grafted nanoparticles. However, the addition of a sacrificial initiator during ATRP resulted in moderation of the molecular weight and graft density of PMMA-grafted nanoparticles. The use of a sacrificial initiator along with ARGET offered the best control in achieving lower molecular weight and narrow dispersity for both PS (37,870 g/mol and PDI of 1.259) and PMMA (44,620 g/mol and PDI of 1.263) nanoparticle hybrid systems.

Precise Modulation of Surface Layer Dynamics for Tunable Flowability and Gas Absorption Properties of Molecular Porous Liquids

AbstractPorous liquids (PLs) have improved the understanding of the porous properties of matter; however, rational tuning of the viscosities and gas absorption properties of PLs lag far behind the explosive growth of PL applications. Herein, neat molecular PLs are constructed by grafting polymers onto the surface of 2 nm molecular nanocages with precisely tuned graft densities and molecular weights Mws. The flowability of the PLs showed a non‐monotonic dependence on the Mw of the polymers due to the interplay between the surface confinement effect and polymer chain entanglements, enabling optimization of their viscosities, similar to those of conventional liquids. The surface layer dynamics and gas absorption kinetics are quantified, correlation between the two properties is confirmed, and gas permeation process is regulated by surface layer dynamics. From the understanding of the structure–property relationship, the surface layer grafting densities are tuned to promote surface layer dynamics to achieve improved gas sorption performance.

Fractionated Crystallization Behavior of Low-Molecular-Weight Polyethylene Grafted onto SiO2Nanoparticles

The synthesis of polymer-grafted nanoparticles is crucial for the development of polymer nanocomposites with excellent properties, but the synthesis of polyethylene (PE)-grafted SiO2 nanoparticles (PE-g-SiO2 NPs) remains a significant challenge due to the chemical inertia of PE. In this study, PE-g-SiO2 NPs with different grafting densities and molecular weights were successfully synthesized by a simple “grafting to” method, and differential scanning calorimetry was used to study the crystallization behavior of grafted PE. Some PE-g-SiO2 samples exhibited three crystallization peaks in the cooling curve (i.e., fractionated crystallization phenomena). Self-nucleation (SN) and successive SN and annealing (SSA) results reveal that the nanoparticles in PE-g-SiO2 samples provoke both confinement and nucleation effects on the grafted PE chains. For PE-g-SiO2 NPs with higher grafting density and molecular weight, the PE chains are more stretched and the interfacial chain segmental dynamics are less suppressed, which enhance the nucleation effect and weaken the confinement effect.

Controlled grafting of multi-block copolymers for improving membrane properties for CO2 separation

Linear poly(ether-urea-imide)s (PUIs) are attractive multi-block segmented copolymers well-known for high selectivity for CO2 separations. Their CO2 permeability generally increases but their selectivity decreases with their polyether soft content limited to 70 wt% to preserve their mechanical properties. In this work, the grafting of a PUI copolymer with PEO-based soft grafts is reported for strongly increasing the membrane properties. The design of the grafted copolymers involved step-growth polymerization, controlled radical polymerization, and “click” chemistry. This strategy ensured the control of grafting rate, graft molecular weight and soft contents varying from 57 to 85 wt%. The membrane properties for CO2 and N2 permeation were correlated to the PUI chemical structure, morphology and soft content. The best membrane properties (PCO2 = 196 Barrer; αCO2/N2 = 39 at 2 bar and 35 °C) were obtained for PUI-g-1PEDEGA5000 corresponding to the highest grafting rate and graft length. Compared to the non-grafted PUI, the best grafted copolymer had much higher CO2 permeability ( × 17) while the ideal separation factor αCO2/N2 was maintained at high level, thus leading to separation properties very close to the Robeson 2008 upper-bound. By allowing very high contents of amorphous soft phase and specific morphology, the new grafting strategy offered high-performance membranes for CO2 capture.

Local Structure of Polymer-Grafted Nanoparticle Melts.

Polymer-grafted nanoparticle (GNP) membranes show unexpected gas transport enhancements relative to the neat polymer, with a maximum as a function of graft molecular weight (MWg ≈ 100 kDa) for sufficiently high grafting densities. The structural origins of this behavior are unclear. Simulations suggest that polymer segments are stretched near the nanoparticle (NP) surface and form a dry layer, while more distal chain fragments are in their undeformed Gaussian states and interpenetrate with segments from neighboring NPs. This theoretical basis is derived by considering the behavior of two adjacent NPs; how this behavior is modified by multi-NP effects relevant to gas separation membranes is unexplored. Here, we measure and interpret SAXS data for poly(methyl acrylate)-grafted silica NPs and find that for very low MWgs, contact between GNPs obeys the two-NP theory─namely that the NPs act like hard spheres, with radii that are linear combinations of the NP core sizes and the dry zone dimensions; thus, the interpenetration zones relax into the interstitial spaces. For chains with MWg > 100 kDa, the interpenetration zones are in the contact regions between two NPs. These results suggest that for MWgs below the transition, gas primarily moves through a series of dry zones with favorable transport, with the interpenetration zone with less favorable transport properties in parallel. For higher MWgs, the dry and interpenetration zones are in series, resulting in a decrease in transport enhancement. The MWg at the transport maximum then corresponds to the chain length with the largest, unfavorable stretching free energy.

Effect of Graft Molecular Weight and Density on the Mechanical Properties of Polystyrene-Grafted Cellulose Nanocrystal Films

Polymer-grafted nanoparticle (PGN) films were prepared from polystyrene (PS) grafted to rodlike cellulose nanocrystals (MxG-CNC-g-PS) with a controllable grafting density (0.03–0.25 chains/nm2) and...

Ethylene/Propylene/Diene Terpolymers Grafted with Poly(methyl acrylate) by Reverse Atom Transfer Radical Polymerization

AbstractThis article describes the synthesis of ethylene‐propylene‐diene terpolymers grafted with poly(methyl acrylate) (PMA) by reverse atom transfer radical polymerization. The PMA graft molecular weights and densities are controlled by varying the concentration and conversion of methyl acrylate, as well as the concentration of initiator. The grafted copolymers are characterized by Fourier‐transform infrared spectroscopy, proton NMR, and gel permeation chromatography. Copolymers with graft molecular weights varying from 12 to 179 kDa and grafting densities from 11% to 53% are made following this procedure.

Flocculation and dewatering of oil sands tailings with a novel functionalized polyolefin flocculant

• Use of novel partially-hydrophobic graft polymer flocculant for mineral tailings treatment. • Grafts of PMA with different molecular weights and densities were attached to EPDM backbones. • Use of a central composite experimental design to assess the effect of polymer architecture on flocculation and dewatering performance. • The performance of the graft copolymers was quantified by initial settling rate (ISR), capillary suction time (CST), supernatant turbidity, and sediment solids content. This work investigates the impact of the molecular weight and grafting density of partially hydrolyzed poly(methyl acrylate) grafted onto ethylene-propylene-diene copolymer backbones (EPDM -g- HPMA) on the flocculation and dewatering of oil sands mature fine tailings through a central composite experimental design by measuring initial settling rate, capillary suction time, supernatant turbidity, and sediment solids content. Flocculants with high molecular weight grafts and low grafting densities reduced the initial settling rates, decreased supernatant turbidities, and increased the solids content of the flocculated sediments. The capillary suction time, however, depended mostly on the charge density and on the hydrophobicity of the grafted copolymer. The effect of the EPDM backbone molecular weight was also investigated. Shorter backbones resulted in flocculants that dispersed more easily in water and settled the sediments faster. In addition, they decreased the supernatant turbidity and increased the solids content in the sediments. EPDM -g- HPMA flocculants with graft molecular weight of 179 kDa and grafting density of 13% are outstanding filtration aids, helping generate sediments with 43% solids content and outperforming a reference commercial polyacrylamide flocculant.

Preparation of acrylamide and carboxymethyl cellulose graft copolymers and the effect of molecular weight on the flocculation properties in simulated dyeing wastewater under different pH conditions

In this study, graft copolymers with different graft ratios (GRs) and molecular weights (MWs) were prepared by water bath polymerization using acrylamide (AM) and carboxymethyl cellulose (CMC) as substrates; additionally, the best preparation parameters were provided. The MW of the copolymer was inversely correlated with the GR (R = −0.82). The obtained graft copolymer was characterized by FTIR, 1H NMR, XPS and DSC. The results showed that CMC and AM were successfully copolymerized and the thermal stability of the product is improved. The graft copolymers with different MWs were used to flocculate a simulated dyeing wastewater at pH values of 3, 5, 7, 9 and 11. The results showed that as the molecular weight of the graft copolymers increased, the average flocculation settling ratio decreased drastically from 61.2% to 19.4% at pH 3, and the higher the MW, the better the flocculation settling performance. But the average settling ratio increased sharply at pH 11, and the increase grew from 21.0% to 50.3%. The larger the MW, the more obvious the decrease in flocculation sedimentation performance. The supernatant turbidity was reduced from 13324 NTU to less than 150 NTU, and the turbidity removal ratio exceeded 99%. The flocculation mechanism was discussed.

Polyethylene Grafted Silica Nanoparticles Prepared via Surface-Initiated ROMP.

Polyethylene and nanosilica represent the most ubiquitous commodity plastic and nanocomposite filler, respectively. Despite their potential utility, few examples exist in the literature of successfully combining these two materials to form polyethylene nanocomposites. Synthesizing well-defined polyethylene grafted to a surface is a significant challenge in the nanocomposites community. Presented here is a synthetic approach toward polyethylene grafted nanoparticles with controllable graft density and molecular weight of the grafted polymer. The variably grafted nanoparticles were then incorporated into a commercial high density polyethylene matrix. The synthesis, characterization, and challenges in making these materials are discussed.

Effects of Premixing Wood Flour with Compatibilizer with Different Characteristics on the Physical and Mechanical Properties of Wood/Plastic Composites

Abstract The addition of maleic anhydride (MA)-modified polypropylene (MAPP) as a compatibilizer increases mechanical and physical properties of wood/plastic composites (WPCs). It is thought that MA graft ratio (Gr), molecular weight (weight-average molecular weight [Mw]), and mixing methods of MAPP affect physical and mechanical properties of WPCs. However, evaluation, especially for mixing order, is not enough. The objective of this study was to investigate the effect of Gr and Mw of MAPP and its mixing methods on the mechanical and physical properties of WPCs. Two types of mixing methods were used: mixing the MAPP with the filler using a Henschel mixer before kneading with PP (premixing method) and mixing all of the materials at one time (one-step mixing method). The WPCs made with the premixing method had lower shear viscosity and higher flexural modulus than those made with the one-step mixing method. The particle size, moisture content, and surface morphology of fillers are thought to be more important in determining the shear viscosity of WPCs than the MAPP itself. The highest tensile, impact, and flexural strengths of WPCs were obtained in the WPC with premixing with low-Gr and high-Mw MAPP. It may be due to the effective reaction of the wood flour with MAPP and higher entanglement between the MAPP and PP. These results suggest that the premixing method could improve productivity and mechanical properties of WPCs.

Location of Imbibed Solvent in Polymer-Grafted Nanoparticle Membranes.

Membranes made purely from nanoparticles (NPs) grafted with polymer chains show increased gas permeability relative to the analogous neat polymer films, with this effect apparently being tunable with systematic variations in polymer graft density and molecular weight. To explore the structural origins of these unusual transport results, we use small angle scattering (neutron, X-ray) on the dry nanocomposite film and to critically examine in situ the structural effects of absorbed solvent. The relatively low diffusion coefficients of typical solvents (∼10-12 m2/s) restricts us to thin films (≈1 μm in thickness) if solute concentration profiles are to equilibrate on the 1 s time scale. The use of such thin films, however, renders them as weak scatterers. Inspired by our nearly two decades old previous work, we address these conflicting requirements through the use of a custom designed flow cell, where stacks of 10 individual ≈1 μm thick supported films are used, while ensuring that each film is individually exposed to solvent vapor. By using isotopically labeled solvents, we study the solvent distribution within the film and show surprisingly that the solvent homogeneously swells the polymer under all conditions that we examined. These results are not anticipated by current theories, but they suggest that, at least under some conditions, the free volume increases due to the grafting of chains to nanoparticles is apparently distributed isotropically in these materials.

Mechanics of polymer brush based soft active materials– theory and experiments

A brush-like structure emerges from stretching of long polymer chains, densely grafted on to the surface of an impermeable substrate. The structure arises due to the competition between conformational entropic elasticity of polymer chains and excluded volume interactions leading to intra and interchain monomer repulsions. Recently, soft materials based on stimuli responsive polymer brushes have been developed to produce controllable and reversible large bending deformation of the host substrates. To understand such systems and improve their functional properties, we study the stress distribution in a brush, and develop surface stress-curvature relation for an elastic beam of a soft material grafted with a neutral polymer brush. In the strongly stretched brush regime, we combine mean field theory from polymer physics with a continuum mechanics model and show that the residual stress variation in a brush is a quartic function of distance from the grafting surface, with a maximum occurring at the grafting surface. By idealizing a brush as a continuum elastic surface layer with residual stress, we derive a closed form expression for surface stress and surface elasticity of the layer as a function of brush parameters, such as graft density and molecular weight. A generalized continuum beam model accounts for the Young–Laplace and Steigmann–Ogden curvature elasticity correction terms, and yields a surface stress-curvature relation, that contains existing relations in the literature as special cases. Further, we report experiments on a thermoresponsive random copolymer brush, Poly(N- isopropylacrylamide)-co-Poly(N,N-Dimethylacrylamide) (PNIPAm-co-PDMA) brush, grafted on one side of a plasticized poly(vinyl chloride) (pPVC) thin film. Estimated surface stress from measured curvature is on the order of −10N/m, and its magnitude decreases gradually, and reversibly, on increasing ambient temperature from 15 °C to 55 °C.

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...

Thermoresponsive Surface-Grafted Gels: Controlling the Bulk Volume Change Properties by Surface-Localized Polymer Grafting with Various Densities.

We prepared poly(N-isopropylacrylamide-r-N-3-(aminopropyl)methacrylamide) (poly(NIPAAm-r-NAPMAm)) gels with poly NIPAAm (PNIPAAm) grafted only in the surface region (so-called thermoresponsive surface-grafted gels) with various graft densities and investigated the effect of the graft density on the bulk volume change properties, shrinking and swelling, in response to temperature changes. Initiators for atom-transfer radical polymerization (ATRP) and structurally analogous compounds were introduced at certain ratios onto the surface regions of the gels, and a subsequent activator regeneration by electron transfer ATRP of NIPAAm was conducted in aqueous media. The graft densities and molecular weights of the grafted polymers were evaluated from the increment in the dry mass of the gels and the amount of introduced ATRP initiators, which was measured by elemental analyses. Three-dimensional measuring laser microscopy revealed that the prepared gels had graft-density-dependent fine wrinkle structures on their surfaces. The surface-grafted gels induced the formation of skin layers during the shrinking process in response to a temperature increase, and their permeability strongly depended on the graft density. The graft density also controlled the kinetics of the swelling behavior in response to a temperature decrease. These physical properties were discussed on the basis of Young's modulus of the surface determined by an atomic force microscopy force curve measurement and the homogeneity of the surface polymer network observed by cryo-scanning electron microscopy. This makes it possible to arbitrarily control the characteristics of gels as open or semiclosed systems, which was uniquely determined by the designs of the surface gel networks.

Porous membranes based on poly(ether imide)-graft-poly(vinyl acetate) as a scaffold for cell growth

A series of poly(ether imide)-graft-poly(vinyl acetate) copolymers with different molecular weights were synthesized successfully and characterized using Fourier transform infrared spectroscopy, ultraviolet–visible spectroscopy, proton nuclear magnetic resonance, gel permeation chromatography, differential scanning calorimeter, thermogravimetric analysis, and X-ray photoelectron spectroscopy analyses. These copolymers were used to fabricate honeycomb-structured porous films using the breath figure templating technique. The surface topology and composition of the highly ordered pattern film were further characterized using a scanning electron microscopy. The results indicated that the poly(ether imide)-graft-poly(vinyl acetate) graft molecular weight ratio influenced the breath figure film surface topology. A model was proposed to elucidate the stabilization process of the poly(ether imide)-graft-poly(vinyl acetate)-aggregated architecture on the water droplet–based templates. In addition, cell viability has been investigated via 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide test, and the cell morphology on the honeycomb-structured poly(ether imide)-graft-poly(vinyl acetate) porous film has been evaluated using a fluorescence microscope. This porous film is shown to be suitable as a matrix for cell growth.

Marine anti-biofouling efficacy of amphiphilic poly(coacrylate) grafted PDMSe: effect of graft molecular weight

There is currently strong motivation due to ecological concerns to develop effective anti-biofouling coatings that are environmentally benign, durable, and stable for use by the maritime industry. The antifouling (AF) and fouling-release (FR) efficacy of amphiphilic, charged copolymers composed of ~52% acrylamide, ~34% acrylic acid, and ~14% methyl acrylate grafted to poly(dimethyl siloxane) (PDMSe) surfaces were tested against zoospores of the green alga Ulva linza and the diatom Navicula incerta. The biofouling response to molecular weight variation was analyzed for grafts ranging from ~100 to 1,400 kg mol−1, The amphiphilic coatings showed a marked improvement in the FR response, with a 55% increase in the percentage removal of diatoms and increased AF efficacy, with 92% reduction in initial attachment density of zoospores, compared to PDMSe controls. However, graft molecular weight, in the range tested, was statistically insignificant. Grafting copolymers to PDMSe embossed with the Sharklet™ microtopography did not produce enhanced AF efficacy.

Grafting density dominant glass transition of dry polystyrene brushes.

The effects of the grafting densities (σp), molecular weights (Mn) and thicknesses of dry polystyrene (PS) brushes on their glass transition temperature (T) were investigated by ellipsometry. The results show that T strongly depends on the grafting density of the PS brushes. The T of the PS brushes with σp > 0.30 increases with decreasing Mn (or brush thickness) and is mainly dominated by entropic effects, in which the grafted chains are highly extended along the film thickness direction resulting in a sharp reduction in configurational entropy. The T of PS brushes with σp < 0.30 decreases with decreasing Mn (or brush thickness) which is mainly dominated by surface effects. For intermediate-density brushes (σp = 0.30), T becomes independent of Mn or brush thickness. The reason for this grafting density dependence of T is attributed to the transition of the PS brush conformation from mushroom-to-brush.

Grafting well-defined CO2-responsive polymers to cellulose nanocrystals via nitroxide-mediated polymerisation: effect of graft density and molecular weight on dispersion behaviour

Tertiary polyamines and polyamides were grafted to cellulose nanocrystals (CNC) via Nitroxide-Mediated Polymerisation to yield well-defined CNC, which were examined for their CO2-responsive dispersibility in water and oil.