Organic/inorganic hybrid composites prepared by polymerization compounding and controlled free radical polymerization

A new method to produce highly filled and well dispersed polymer solid composites using controlled free radical polymerization has been developed. Grafting of polymers onto ultrafine silica was done in bulk polymerization at 120 °C in presence of N-tert-butyl-1-diethylphosphono2,2-dimethyl propyl nitroxide (DEPN) as a nitroxide stable free radical. Optimum conditions for tert-butyl hydroperoxide grafting onto fumed silica were first determined. The percentage of grafting, the architecture of grafted polymers, the length of chains, and the polydispersity index can be controlled at will using this approach. The effect of the number of grafted polymer chains combined with its molecular weight on the processing of these materials was investigated. The syntheses performed in this work gave grafting percentages of polymers and copolymers ranging from 12 to 88 wt%. All ‘synthesized’ composites gave stable suspensions in toluene and tetrahydrofuran.     

Synthesis of hybrid free and nanoporous silica aerogel‐anchored polystyrene chains via in situ atom transfer radical polymerization

Polystyrene nanocomposite with mixed free and anchored chains was synthesized by atom transfer radical polymerization. Attachment of 3‐(trimethoxysilyl)propyl methacrylate with a double bond on the nanoporous silica aerogel surface results in a double bond grafted silica aerogel which could be incorporated into the polystyrene chains by a grafting‐through process. Conversion and molecular weight evaluation was carried out using gas chromatography and gel permeation chromatography, respectively. Double bond containing silica aerogel has an inconsiderable effect on conversion. There is no considerable discrepancy between the molecular weights of the free and anchored chains. Addition of silica aerogel with pendant CC bonds leads to increase of apparent rate constant of polymerization and also molecular weights. This is mainly because of initiator trapping in silica aerogel pores. Every percent of double bond containing silica aerogel with respect to styrene results in trapping of about 0.08 mol of ethyl alpha‐bromoisobutyrate among the silica pores. POLYM. COMPOS., 34:1648–1654, 2013. © 2013 Society of Plastics Engineers     

Preparation of high PMMA grafted particle SiO2 using surface initiated free radical polymerization

In this work, a convenient surface-initiated free radical graft-polymerization method, by which polymethacrylic acid (PMAA) with a high grafting density was grafted on silica gel particles, was put forward, and it was feasible and effective. The coupling agent γ-aminopropyltrimethoxysilane (AMPS) was first bound onto the surfaces of silica gel particles, obtaining the modified particles AMPS-SiO₂. So a redox initiation system was constituted with the amino groups on the surfaces of AMPS-SiO₂ particles and ammonium persulphate in the solution. A great deal of primary free radicals on the surfaces of AMPS-SiO₂ particles is produced via the redox initiating reaction, so that the surface-initiated free radical graft- polymerization of methacrylic acid (MAA) on the silica gel particles was realized, giving the grafted particles PMAA/SiO₂ with a high grafting degree (about 30 g/100 g) of PMAA. The effects of the main factors on the surface initiated graft polymerization were examined and the corresponding mechanism was investigated in depth. The experimental results show that for this surface-initiated free radical graft-polymerization of MAA, the suitable temperature is 40 °C. If the temperature is over 40 °C, the graft polymerization will be affected negatively, and the grafting degree of PMAA will decline because of the intense heat decomposition of ammonium persulphate. During the graft polymerization, the grafted polymer layer that has formed is a hindrance to the subsequent graft polymerization. The used amount of initiator and the monomer concentration affect the graft polymerization greatly. The appropriate reaction conditions are as follows: reaction time of 10 h, initiator persulphate amount of 1.1% (it implies the mass percent of the monomer), and monomer MAA concentration of about 5% (it drives at the mass percent of the solution).     

Modification of Silica Nanoparticles with Miktoarm Polymer Brushes via ATRP

Silica nanoparticles were successfully modified with miktoarm brushes via atom transfer radical polymerization (ATRP) using three different approaches. In the first approach: “graft onto and from”, a poly(tert-butyl acrylate) (PtBA) macroinitiator was grafted onto the surface of a monomer-modified silica nanoparticle. Then, polystyrene (PSt) brush was grafted from the surface-tethered reactive chain end. In the second approach: “two-step reverse ATRP”, the PtBA and poly(n-butyl acrylate) (PBA) brushes were consecutively grafted from initiator-modified silica particles via ATRP. The polymerization was initiated from the silica surface via a two-step controlled thermal decomposition of surface-tethered diazo initiator moieties. In the third method: “diblock first”, a diblock copolymer of poly(tert-butyl acrylate) and poly(glycidyl methacrylate) (PtBA-b-PGMA) was grafted onto amine-modified silica particles. The diblock copolymer was covalently attached to the silica surface via interaction between surface-tethered amine groups and the short reactive block containing glycidyl groups. Next, the polystyrene brushes were grafted from surface-tethered reactive chain end. The materials prepared by three different approaches were characterized using gel permeation chromatography (GPC) and thermogravimetric analysis (TGA). The PtBA brushes were hydrolyzed under acidic conditions to form poly(acrylic acid) (PAA) brushes. The resulting materials were imaged using atomic force microscopy (AFM) and transmission electron microscopy (TEM).     

Block copolymer grafted-silica particles: a core/double shell hybrid inorganic/organic material

Hybrid inorganic/organic materials consisting of a poly(n-butyl acrylate)-b-poly(styrene) diblock copolymer anchored to silica particles were synthesized via ‘grafting from’ technique using a controlled/living free radical polymerization named stable free radical polymerization. XPS and FTIR analysis were used to control the effectiveness of the chemical modification of the silica particles. Thermal characterizations were performed by thermal gravimetric analysis (TGA) and by differential scattering calorimetry (DSC). The TGA permitted the determination of the quantity of grafted polymer and thus the grafting density; DSC was used to study the influence of the silica and blocks of the copolymer on their thermal behaviors. The glass transition temperature of the grafted copolymers was compared to these of free polymers or copolymers homologues.     

聚合方法对硅胶表面接枝聚合物过程的影响研究

分别用溶液聚合与分散聚合两种方法,在硅胶载体表面实施了苯乙烯的接枝聚合,重点考察了聚合方法对接枝聚合过程的影响。结果表明,在基本相同的反应条件下,分散聚合体系的接枝率明显高于溶液聚合体系的接枝率,最高可达47%(即47g/100g);除聚合方法之外,影响聚合物在硅胶表面接枝率的因素还有硅胶表面的双键含量、单体浓度、引发剂浓度、温度等。     

Poly(N‐isopropylacrylamide)‐grafted silica as a support of platinum colloids: Preparation method,characterization, and catalytic properties in hydrogenation

We investigated the composition, properties, and utility of a novel hybrid material of poly(N‐isopropylacrylamide)‐grafted silica. The modified silica was prepared in two steps: (1) grafting silane layers of active precursors, such as initiator, transfer agent or comonomer onto the silica surface by solution deposition and (2) radical polymerization of N‐isopropylacrylamide (NIPAAm) on the grafted silane layers. Thermogravimetric analysis (TGA), elemental analysis (EA), FTIR spectroscopy, and TEM were used to characterize the layer composition, structure, and chemistry, respectively. Well‐dispersed platinum colloids have been prepared on the PNIPAAm‐grafted silica via the reduction of PtCl by ethanol. The immobilized Pt colloids were found to be active and stable heterogeneous catalysts for the hydrogenation of ally alcohol in ethanol. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2678–2684, 2000     

Surface‐initiated nitroxide‐mediated radical polymerization of 4‐vinylpyridine on magnetite nanoparticles

Poly (4‐vinylpyridine) (P4VP) brushes had been prepared by the surface‐initiated nitroxide‐mediated radical polymerization of 4‐VP on the surface of 3‐methacryloxyproyltrimethoxysilane (3‐MPS)‐modified magnetite nanoparticles with an average diameter of 30 nm. The grafting polymerization was accomplished by nitroxide‐mediated polymerization of 4‐VP, using 4‐hydroxyl‐2,2,6,6‐tetramethyl‐1‐piperidinyl‐oxy (HTEMPO·) free radical as capping agent and benzoyl peroxide (BPO) as initiator. X‐ray photoelectron spectra (XPS) measurement demonstrated that the alkoxysilane initiator layer had formed on the magnetite surface. Gel permeation chromatograph analysis and XPS measurement suggested that the amount of grafted P4VP increases with increasing grafting time. The amount of P4VP grafted on the surface could be determined to be 0.09 chains/nm² by thermogravimetric analysis. The P4VP‐grafted magnetite particles exhibited the characteristics of multidomain system, distinct from the single domain attributes of the pure magnetite particles. Atomic force‐microscopy analysis revealed the diameter of the grafted P4VP magnetic latex particles is in the range of 120 nm to 150 nm. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Synthesis of lactose‐containing glycopolymer‐grafted silica gel particles

The surface free‐radical graft polymerization of 2‐O‐meth‐acryloyloxyethoxyl‐(2,3,4,6‐tetra‐O‐acetyl‐β‐D ‐galactopyranosyl)‐(1‐4)‐2,3,6‐tri‐O‐acetyl‐β‐D ‐glucopyranoside onto silica gel particles has been carried out with azobis (isobutyronitrile) as initiator. The grafting reaction conditions and the glycopolymer‐grafted silica gel particles have been investigated in detail. Chromatographic experiments have been attempted on glycopolymer‐modified silica gel particles as a stationary phase under normal phase conditions, and it was found that a certain separation effect of the quercetin and its derivant isorhamnetin was achieved. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

A comparative study on effects of two kinds of polymerization methods on grafting of polymer onto silica surface

In this article we present the result of a comparative study of two kinds of polymerization methods—solution polymerization (sol. poly.) and dispersion polymerization (dis. poly.) for grafting polymer onto silica. As a model for the grafting polymerization reaction, styrene was chosen as the monomer and azo diisobutyronitrile (AIBN) as the initiator. The study aims at supplying theoretical reference for better selecting polymerization method to graft polymer on the silica particle surface. First, monolayers of 3‐methacryloylpropyl trimethoxysilane were chemically bonded onto the surfaces of micrometer‐sized silica gel particles, and so double bonds were immobilized onto the silica surface. Second, the copolymerizations between the immobilized double bonds and the monomer styrene were carried out, homopolymerizations of styrene followed, and finally polystyrene was grafted to the silica surfaces. Two kinds of polymerization methods, sol. poly. and dis. poly., were adopted respectively, and the effects of polymerization methods on grafting process were examined mainly. At the same time, the effects of different polymerization conditions on the grafting degree were researched. It was found that in the dis. poly. system the grafting degree is obviously higher than that in the sol. poly. system under the same polymerization conditions, and the grafting degree can go up to 47%, i.e. 47g/100g. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 5808–5817, 2006     

Atom transfer radical polymerization from silica nanoparticles using the ‘grafting from’ method and structural study via small-angle neutron scattering

Polymer chains were grafted from silica beads (colloidal sol in dimethylacetamide) by atom transfer radical polymerization (ATRP), via the ‘grafting from’ method. The grafting of the initiator onto the silica surface was done in two steps. First, thiol-functionalization of the surface was achieved via silanization with a mercaptopropyl triethoxysilane. Second, we performed an over-grafting of the surface by reacting the thiol with 2-bromoisobutyryl bromide to generate the halogen-functional ATRP initiator. The nanoparticles were kept in solution (in the same solvent) at each stage of the functionalization (even during the purification steps), as this is the only way to avoid irreversible aggregation. Then, the polymerization of styrene was conducted. Control of both the molecular weight and the density of grafted chains can be achieved by this method. Careful characterization such as gel permeation chromatography, ²⁹Si CP/MAS NMR, elemental analysis, infrared spectroscopy and thermo-gravimetric analysis is performed. The state of dispersion of the grafted nanoparticles is followed in details by small angle neutron scattering and results obtained from this technique are presented here as well as the way the SANS data can be treated. Connection is systematically done between the information provided by this technique and the improvement of the synthetic procedure.     

Surface functionalization of Si3N4 nanoparticles by graft polymerization of glycidyl methacrylate and styrene

To improve dispersibility and interfacial interaction of nano‐Si₃N₄ particles in epoxy‐based composites, graft of glycidyl methacrylate (GMA) and styrene (St)/GMA onto the nanoparticles' surface was carried out in terms of emulsion polymerization method. The grafting polymers proved to be chemically attached to the nanoparticles via the double bonds introduced during the coupling agent pretreatment. The factors affecting the graft parameters, such as monomer concentration, initiator consumption, reaction time, etc., were investigated. It was shown that higher concentrations of monomer and initiator are favorable for the graft polymerization. When St/GMA was employed as the grafting monomer, the nanoparticles were found to play the role of polymerization loci. The grafted nanoparticles exhibit greatly improved dispersibility in cured epoxy. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 992–999, 2006     

Covalent grafting of polyacrylamide-based hydrogels to a polypropylene surface activated with functional polyperoxide

Polyacrylamide (PAAm)-based hydrogels covalently bonded to a polypropylene (PP) substrate have been synthesized using modification of the PP with a new functional polyperoxide (FPP). The FPP initiates radical polymerization from the PP’s surface by decomposing the peroxide groups. Two different approaches for forming the hydrogel network have been used. In the first, a grafted hydrogel network is formed by radical polymerization of the acrylamide on the peroxidized PP substrate (“grafting from”) in the presence of N,N′-methylene-bis-acrylamide (a cross-linking agent) and potassium persulfate (an additional initiator in bulk). In the second approach, a grafted hydrogel network is synthesized by: (i) grafting the PAAm macromolecules from the peroxidized PP substrate and (ii) following intermolecular condensation of the surface-grafted PAAm and adding to the reactive bulk PAAm and poly-N-(hydroxymethyl)acrylamide (used as a cross-linking macromolecule), resulting in the formation of a hydrogel network grafted to the PP substrate.The covalent attachment of hydrogel networks has been verified for both approaches by contact angle measurements, dye adsorption by the hydrogel residues after network removal and extraction, and mechanical failure tests. The intermolecular condensation method has been found to be more advantageous for the formation of covalently bonded hydrogels on the PP substrate, while grafted network formation by radical polymerization was detected only for a limited range of monomer and cross-linking agent concentrations.     

Synthesis and characterization of poly(methyl methacrylate)‐grafted silica microparticles

A procedure to synthesize poly(methyl methacrylate)‐grafted silica microparticles was developed by using radical photopolymerization of methyl methacrylate (MMA) initiated from N,N‐diethyldithiocarbamate (DEDT) groups previously bound to the silica surface (grafting “from”). The functionalization of silica microparticles with DEDT groups was performed in two steps: introduction of chlorinated functions onto the surface of silica particles, and then nucleophilic substitution of chlorines by DEDT functions via a S_N2 mechanism. The study was performed with a Kieselgel® S silica which was initially chlorinated in surface, either by direct chlorination of silanols with thionyl chloride, or by using a condensation reaction between silanols and a chlorofunctional trialkoxysilane reagent, 4‐(chloromethyl)phenyltrimethoxysilane and chloromethyltriethoxysilane, respectively. Three types of DEDT‐functionalized silica microparticles were prepared with a good control of the reactions, and then characterized by solid‐state ¹³C and ²⁹Si CP/MAS NMR. Their ability to initiate MMA photopolymerization was studied. The kinetics of MMA photopolymerization was followed by HPLC and ¹H‐NMR. Whatever the silica used the grafting progresses very slowly. On the other hand, the conversion of MMA in PMMA grafts is depending on the structure of the DEDT‐functionalized Kieselgel® S used. Poly(methyl methacrylate)‐grafted silica microparticles bearing high length grafts ( about 100) were synthesized. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Graft polymerization of vinyl acetate onto silica

The free‐radical graft polymerization of vinyl acetate onto nonporous silica particles was studied experimentally. The grafting procedure consisted of surface activation with vinyltrimethoxysilane, followed by free‐radical graft polymerization of vinyl acetate in ethyl acetate with 2,2′‐azobis(2,4‐dimethylpentanenitrile) initiator. Initial monomer concentration was varied from 10 to 40% by volume and the reaction was spanned from 50 to 70°C. The resulting grafted polymer, which was stable over a wide range of pH levels, consisted of polymer chains that are terminally and covalently bonded to the silica substrate. The experimental polymerization rate order, with respect to monomer concentration, ranged from 1.61 to 2.00, consistent with the kinetic order for the high polymerization regime. The corresponding rate order for polymer grafting varied from 1.24 to 1.43. The polymer graft yield increased with both initial monomer concentration and reaction temperature, and the polymer‐grafted surface became more hydrophobic with increasing polymer graft yield. The present study suggests that a denser grafted polymer phase of shorter chains was created upon increasing temperature. On the other hand, both polymer chain length and polymer graft density increased with initial monomer concentration. Atomic force microscopy–determined topology of the polymer‐grafted surface revealed a distribution of surface clusters and surface elevations consistent with the expected broad molecular‐weight distribution for free‐radical polymerization. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 300–310, 2003     

Functionalization of poly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate) films via surface‐initiated atom transfer radical polymerization: Comparison with the conventional free‐radical grafting procedure

We modified hydrophobic poly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate) (PHBHV) films with hydrophilic chains to control their surface properties. We designed and investigated surface‐initiated atom transfer radical polymerization (SI‐ATRP) to modify the PHBHV films by grafting poly(2‐hydroxyethyl methacrylate) (PHEMA) from the surface. This method consisted of two steps. In the first step, amino functions were formed on the surface by aminolysis; this was followed by the immobilization of an atom transfer radical polymerization initiator, 2‐bromoisobutyryl bromide. In the second step, the PHEMA chains were grafted to the substrate by a polymerization process initiated by the surface‐bound initiator. The SI‐ATRP technique was expected to favor a polymerization process with a controlled manner. The experimental results demonstrate that the grafting density was controlled by the reaction conditions in the first step. The grafted films were analyzed by Fourier transform infrared spectroscopy, contact angle testing, scanning electron microscopy, and energy‐dispersive X‐ray spectroscopy. The results show that grafted chains under the SI‐ATRP method were preferentially located on the surface for surface grafting and in the bulk for conventional free‐radical polymerization initiated by benzoyl peroxide. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Synthesis of controlled thermoresponsive PET track‐etched membranes by ATRP method

Controlled thermoresponsive PET track‐etched membranes were synthesized by grafting N‐isopropylacrylamide (NIPAAm) onto the membrane surface via atom transfer radical polymerization (ATRP). The initial measurements were made to determine the anchoring of ATRP initiator on PET membrane surface. Thereafter, polymerization was carried out to control the mass of polymer by controlling reaction time grafted from the membrane surface and, ATR‐FTIR, grafting degree measurements, water contact angle measurements, TGA, and SEM were used to characterize changes in the chemical functionality, surface and pore morphology of membranes as a result of modification. Water flux measurements were used to evaluate the thermoresponsive capacity of grafted membranes. The results show the grafted PET track‐etched membranes exhibit rapid and reversible response of permeability to environmental temperature, and its permeability could be controlled by controlling polymerization time using ATRP method. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.     

Synthesis and characterization of well-defined hydrophilic block copolymer brushes by aqueous ATRP

Homopolymer brushes of poly(N,N-dimethylacrylamide) (PDMA), poly(methoxyethylacrylamide) (PMEA) and poly(N-isopropylacrylamide)(PNIPAM) grown on atom transfer radical polymerization (ATRP) initiator functionalized latex particles were used as macroinitiators for the synthesis of PDMA-b-PNIPAM/PMEA, PMEA-b-PDMA/PNIPAM and PNIPAM-b-PDMA block copolymer brushes by surface initiated aqueous ATRP. The grafted homopolymer and block copolymer brushes were analyzed for molecular weight, molecular weight distribution, chain grafting density, composition and hydrodynamic thickness (HT) using gel permeation chromatography-multi-angle laser light scattering, ¹H NMR, particle size analysis and atomic force microscopy (AFM) techniques. The measured graft molecular weight increased following the second ATRP reaction in all cases, indicating the second block had been added. Chain growth depended on the nature of the monomer used for block copolymerization and its concentration. Unimodal distribution of polymer chains in GPC with non-overlap of molar mass-elution volume curves implied an efficient block copolymerization. This was supported by the increase in HT measured by particle size analysis, equilibrium thickness observed by AFM and the composition of the block copolymer layer by ¹H NMR analysis, both in situ and on cleaved chains in solution. ¹H NMR analysis of the grafted latex and cleaved polymers from the surface demonstrated that accurate determination of the copolymer composition by this method is possible without detaching polymer chains from surface. Block copolymer brushes obey the same power law dependence of HT on molecular weight as homopolymer brushes in good solvent conditions. The NIPAM-containing block copolymer brushes were sensitive to changes in the environment as shown by a decrease in HT with increase in the temperature of the medium.     

Grafting of polystyrene and poly(p‐chlorostyrene) from the surface of ramie fiber via RAFT polymerization

Reversible addition‐fragmentation chain transfer (RAFT) radical polymerization was used for controlled grafting of styrene and p‐chlorostyrene from the surface of ramie fiber. The hydroxyl groups in ramie fiber were first converted to thiocarbonylthio groups as the RAFT chain transfer agent then used to mediate the RAFT polymerization of styrene, and p‐chlorostyrene in tetrahydrofuran using azobis (isobutyronitrile) as initiator at 60°C. The grafted copolymers were analyzed by Fourier transform infrared spectroscopy, scanning electron microscopy, thermogravimetry, and contact angle measurements. The results confirmed that grafting occurred on the surface of the ramie fiber, substantially increasing the water contact angle from 75.31° to 147° for polystyrene grafted ramie fiber (cell‐PS) and 135° for poly(p‐chlorostyrene) (cell‐PSCl), and improving the hydrophobicity of the grafted fiber. The homopolymers formed in the polymerization were analyzed by size exclusion chromatography to estimate the molecular weights and polydispersity indices (PDIs) of chains grafted from the surface of the ramie fiber, as well as showed narrow chain length distributions with a PDI of 1.32–1.70. These materials possess potential applications in many advanced technologies. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Synthesis,characterization, and catalytic performance of highly dispersed vanadium grafted SBA-15 catalyst

Vanadium oxide grafted on mesoporous silica SBA-15 has been synthesized using a controlled grafting process. Its structure has been thoroughly investigated using different characterization techniques, including N₂-physisorption, X-ray diffraction, transmission electron microscopy (TEM), Raman spectroscopy, H₂ temperature-programmed reduction, X-ray absorption near-edge structure (XANES), and extended X-ray absorption fine structure (EXAFS). The spectroscopic results revealed that under dehydrated conditions, the grafted vanadium domains are highly dispersed on the SBA-15 surface, composed predominately of isolated VO₄ units with distorted tetrahedral coordination. The suggested (SiO)₃VO sites on the silica surface include one short bond (～1.54 Å) and three long bonds (1.74 Å). Methanol oxidation was used as a chemical probe reaction to examine the catalytic properties of these catalysts. At low vanadium loading, the vanadium species grafted on the surface show structural properties similar to those of vanadium-incorporated MCM-41 catalyst. However, the present mesoporous V-SBA-15 catalysts in the oxidation of methanol to formaldehyde show remarkable catalytic performance compared with that of VO_x/SBA-15 catalysts synthesized through a conventional wet impregnation method, which has been attributed to the homogeneous dispersion and uniformity of the catalytic vanadium species achieved on the SBA-15 support with large pore diameter and surface area. The acidic properties of V-SBA-15 was investigated by pyridine temperature-programmed desorption, which indicated the existence of both Lewis and Brönsted acid sites of the surface.