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Showing papers in "Macromolecules in 1998"


Journal ArticleDOI
TL;DR: The authors proposed a reversible additive-fragmentation chain transfer (RAFT) method for living free-radical polymerization, which can be used with a wide range of monomers and reaction conditions and in each case it provides controlled molecular weight polymers with very narrow polydispersities.
Abstract: mechanism involves Reversible Addition-Fragmentation chain Transfer, and we have designated the process the RAFT polymerization. What distinguishes RAFT polymerization from all other methods of controlled/living free-radical polymerization is that it can be used with a wide range of monomers and reaction conditions and in each case it provides controlled molecular weight polymers with very narrow polydispersities (usually <1.2; sometimes <1.1). Living polymerization processes offer many benefits. These include the ability to control molecular weight and polydispersity and to prepare block copolymers and other polymers of complex architecturesmaterials which are not readily synthesized using other methodologies. Therefore, one can understand the current drive to develop a truly effective process which would combine the virtues of living polymerization with versatility and convenience of free-radical polymerization.2-4 However, existing processes described under the banner “living free-radical polymerization” suffer from a number of disadvantages. In particular, they may be applicable to only a limited range of monomers, require reagents that are expensive or difficult to remove, require special polymerization conditions (e.g. high reaction temperatures), and/or show sensitivity to acid or protic monomers. These factors have provided the impetus to search for new and better methods. There are three principal mechanisms that have been put forward to achieve living free-radical polymerization.2,5 The first is polymerization with reversible termination by coupling. Currently, the best example in this class is alkoxyamine-initiated or nitroxidemediated polymerization as first described by Rizzardo et al.6,7 and recently exploited by a number of groups in syntheses of narrow polydispersity polystyrene and related materials.4,8 The second mechanism is radical polymerization with reversible termination by ligand transfer to a metal complex (usually abbreviated as ATRP).9,10 This method has been successfully applied to the polymerization of various acrylic and styrenic monomers. The third mechanism for achieving living character is free-radical polymerization with reversible chain transfer (also termed degenerative chain transfer2). A simplified mechanism for this process is shown in

4,561 citations


Journal ArticleDOI
TL;DR: Variations in the linear charge density of a weak polyacid brought about by controlling solution pH in a layer-by-layer sequential adsorption process were used to systematically control the layer thickness, level of layer interpenetration, and surface wettability of sequentially adsorbed layers of poly(acrylic acid) (PAA) and poly(allylamine) (PAH) as mentioned in this paper.
Abstract: Variations in the linear charge density of a weak polyacid brought about by controlling solution pH in a layer-by-layer sequential adsorption process were used to systematically control the layer thickness, level of layer interpenetration, and surface wettability of sequentially adsorbed layers of poly(acrylic acid) (PAA) and poly(allylamine) (PAH). The thickness contributed by an individual polyion layer was found to depend primarily on the pH of the polymer's dipping solution and, within the pH range examined, was not influenced by the thickness or level of interpenetration of the previously adsorbed layer. Contact angle and methylene blue adsorption measurements revealed that the deposited layers are typically highly interpenetrated and that the deposition process is a surface charge dominated adsorption process. Using this simple molecular-level blending approach, it is possible to create surfaces with advancing water contact angles that vary from essentially zero (completely wettable surfaces) to as ...

850 citations


Journal ArticleDOI
TL;DR: In this paper, various mathematical models derived to explain and predict solute diffusion in hydrogels are reviewed and tested against literature data, and it was determined that a scaling hydrodynamic model provided the best explanation for solutes diffusion.
Abstract: Solute diffusion in hydrogels is important in many biotechnology fields. Solute behavior in hydrogels has been explained in terms of reduction in hydrogel free volume, enhanced hydrodynamic drag on the solute, increased path length due to obstruction, and a combination of hydrodynamic drag and obstruction effects. In this article the various mathematical models derived to explain and predict solute diffusion in hydrogels are reviewed and tested against literature data. These models can be divided into those applicable to hydrogels composed of flexible polymer chains (i.e., homogeneous hydrogels) and those composed of rigid polymer chains (i.e., heterogeneous hydrogels). For homogeneous hydrogels it was determined that a scaling hydrodynamic model provided the best explanation for solute diffusion, while for heterogeneous hydrogels obstruction models were more consistent with the experimental data. Both the scaling hydrodynamic model and the most appropriate obstruction model contain undefined parameters w...

838 citations


Journal ArticleDOI
TL;DR: In this paper, the conformation of individual PNIPAM chains change from a coil to a fully collapsed thermodynamically stable single chain globule and then back to a coil in an extremely dilute aqueous solution (6.7 10-7 g/mL).
Abstract: Using a newly prepared nearly monodisperse (Mw/Mn < 1.05) high molar mass (Mw = 1.3 107 g/mol) poly(N-isopropylacrylamide) (PNIPAM) sample, we successfully, for the first time, made the conformation of individual PNIPAM chains change from a coil to a fully collapsed thermodynamically stable single chain globule and then back to a coil in an extremely dilute aqueous solution (6.7 10-7 g/mL). The average chain density in the globule state is 0.34 g/mL, close to 0.40 g/cm3 predicted on the basis of a space-filling model, indicating that the globule still contains 66% water even in its fully collapsed state. At a given temperature around the lower critical solution temperature, the chains are smaller in the globule-to-coil transition than in the coil-to-globule transition, revealing that the coil-to-globule transition is an irreversible process. The hysteresis can be attributed to the formation of intrachain structures, presumably the intrachain hydrogen bonding, in the globule state. We confirmed the existen...

647 citations


Journal ArticleDOI
TL;DR: A physical model of the surface helped explain these protein adsorption results in terms of the spacing and degree of overlap of grafted PEO chains.
Abstract: Poly(ethylene oxide) (PEO) polymer, in linear and star form, was covalently grafted to silicon surfaces, and the surfaces were tested for their ability to adsorb proteins. Linear PEG of molecular weight 3400, 10 000, and 20 000 g/mol and star PEO molecules were coupled via their terminal hydroxyl groups activated by tresyl chloride to aminosilane-treated silicon wafers. The amount of PEO coupled to the surface was varied by changing the concentration of the tresyl-PEO solution. The dry PEO thickness on the surface was measured using X-ray photoelectron spectroscopy (XPS) and ellipsometry, from which the grafting density was calculated. The PEO surfaces were exposed to solutions of each of three proteins: cytochrome-c, albumin, and fibronectin. The degree of adsorption of each protein was determined by XPS and ellipsometry and recorded as a function of PEO grafting density. All three proteins were found to reach zero adsorption at the highest grafting densities on all three PEG surfaces, which for all three PEG surfaces was a PEO content of 100 +/- 10 ng/cm2. On both star PEO surfaces, albumin and fibronectin decreased to zero adsorption at intermediate values of grafting density, whereas cytochrome-c continued to adsorb at all grafting densities, although with a decreasing trend. A physical model of the surface helped explain these protein adsorption results in terms of the spacing and degree of overlap of grafted PEO chains.

614 citations


Journal ArticleDOI
TL;DR: In this article, the radical-chain polymerization of styrene using self-assembled monolayers of azo initiators covalently bound to high surface area silica gels is described.
Abstract: We report on the radical-chain polymerization of styrene using self-assembled monolayers of azo initiators covalently bound to high surface area silica gels. In this process monolayers of poly(styrene) molecules terminally attached to the surface of the inorganic substrate are obtained. As the initiator molecules are immobilized at the surfaces in a one-step reaction, well-reproducible layers can be prepared and the surface concentration of the initiator can be adjusted in a wide range between the limit of detection and full surface coverage. In the subsequent polymerization reactions polymer monolayers with high, controlled graft density can be obtained. The synthesis of the attached layers and the characterization by X-ray photoelectron spectroscopy, diffuse reflectance infrared spectroscopy (DRIFT), and elemental analysis are described. After cleavage of an ester group that connects the polymers to the surface, the molecular weights of the polymers were determined. The results of the study show that th...

598 citations


Journal ArticleDOI
TL;DR: In this article, the internal structure of self-assembled polyelectrolyte multilayer films was resolved using neutron reflectometry, and a detailed molecular picture of such systems was developed by analyzing the data with a composition-space refinement technique.
Abstract: Using neutron reflectometry we have resolvedto high resolutionthe internal structure of self-assembled polyelectrolyte multilayer films and have developed a detailed molecular picture of such systems by analyzing the data with a composition-space refinement technique. We show that such surface films consist of stratified structures in which polyanions and polycations of individual layers interdigitate one another intimately. Nevertheless, the deposition technique leads to results that are predictable, if well-defined and constant environmental conditions are maintained during the preparation. For alternating layers of poly(styrenesulfonate) (PSS) and poly(allylamine hydrochloride) (PAH), adsorbed onto atomically flat surfaces, a roughening of successively deposited layers leads to a progressively larger number of adsorption sites for consecutive generations of adsorbed polymer, and thus to an increase in layer thicknesses with an increasing number of deposited layers. Because of the interpenetration of ad...

576 citations



Journal ArticleDOI
TL;DR: In this article, a macroinitiator with a grafting site at each repeat unit was used to obtain a broad molecular weight distribution of brush-like macromolecules using atom transfer radical polymerization (ATRP).
Abstract: Recent progress in the field of densely grafted, or “brush” (co)polymers has prompted a need to develop efficient methods to synthesize a wider variety of materials with the same basic architectural design. These brushlike macromolecules have been prepared previously using the macromonomer method.1-6 Macromonomers, usually prepared by anionic polymerization, were homopolymerized using conventional radical methods to maximize the number of branches possible from a linear backbone based on vinyl monomers. Upon fractionation of these materials using size exclusion chromatography, samples of narrow polydispersities were obtained which could then be cast on surfaces to form highly ordered thin films. To avoid the rigorous methods necessary for ionic polymerizations and sample fractionation, and to extend the variety of compositional content of these types of materials, atom transfer radical polymerization (ATRP) has been used to prepare similar macromolecular architectures. The approach described here involves grafting from a macroinitiator and can offer greater versatility in terms of both the length and the composition of the backbone and/or the side chains than previous methods which employed the synthesis of high molecular weight macromonomers and their subsequent polymerization by uncontrolled radical techniques; to obtain welldefined polymeric brushes required their fractionation, generally by SEC. To our knowledge, there are no known examples of using a macroinitaitor with a grafting site at each repeat unit to make well-defined polymeric brushes. Combinations of nitroxide-mediated, conventional free radical polymerization and ATRP to prepare graft copolymers from macroinitiators have been used previously.7,8 ATRP has also been combined with conventional radical polymerization to prepare amphiphilic graft copolymers9 and thermoplastic elastomers,10 as well. In each of these cases, however, the materials are loosely grafted, having been prepared from a macroinitiator which is a copolymer containing both initiation/ branch sites and spacing repeat units. Controlled radical polymerization and ATRP in particular afford access to materials of controlled molecular weight, predicted by the ratio of consumed monomer to initiation sites.11,12 This method also yields polymer segments of narrow molecular weight distributions13 in addition to being applicable to a host of vinyl monomers such as styrene, (meth)acrylates, acrylonitrile, etc.14 Thus, there are many possibilities which make its application to the area of brush (co)polymers appealing. Included here are preliminary synthetic data and AFM images which show that it is possible to prepare densely grafted copolymers using ATRP. Two approaches were used to prepare the macroinitiators, Scheme 1. The first involved conventional free radical homopolymerization of 2-(2-bromopropionyloxy)ethyl acrylate (BPEA)15 using AIBN in the presence of carbon tetrabromide to attenuate the molecular weight (Mn ) 27 300, Mw/Mn ) 2.3). By use of AIBN as an intiator to prepare the ATRP macroinitiator, a polymer with a broad molecular weight distribution was obtained. Such a macroinitiator would consequently result in the formation of brush polymers with broad molecular weight distributions, no matter how well controlled the polymerization of the side chains. Thus, the preparation of a well-defined macroinitiator was undertaken. In the second approach, trimethylsilylprotected 2-hydroxyethyl methacrylate (HEMA-TMS)16 was polymerized via ATRP and subsequently esterified with 2-bromoisobutyryl bromide (BriBuBr) in the presence of a catalytic amount of tetrabutylammonium fluoride (TBAF) to yield a different macroinitiator, poly(2-(2-bromoisobutyryloxy)ethyl methacrylate) (pBIEM)16 with controlled molecular weight and low polydispersity (Mn ) 55 500, Mw/Mn ) 1.3), Table 1. It should be noted that the macroinitiator prepared using ATRP was composed of a stiffer methacrylate structure and with a 2-bromoisobutyryl initiation site while the free radically prepared pBPEA contained an acrylate backbone and 2-bromopropionyl initiation sites. However, both types of initiating species have been shown to initiate styrene polymerization well.14 Both polymers were then used as macroinitiators for ATRP of styrene (S) and butyl acrylate (BA). Side chains with a degree of polymerization of about 40 from a macroinitiator of pBIEM with a Mn of approximately 50 000 (which contained about 200 initiation sites per Scheme 1 9413 Macromolecules 1998, 31, 9413-9415

520 citations


Journal ArticleDOI
TL;DR: In this paper, six polylactides, polymerized with Salen−Al−OCH3 initiator and having optical purities between 43% and 100%, were analyzed by differential scanning calorimetry, X-ray diffraction, and optical microscopy, following various crystallization conditions.
Abstract: Six polylactides, polymerized with Salen−Al−OCH3 initiator and having optical purities between 43% and 100%, were analyzed by differential scanning calorimetry, X-ray diffraction, and optical microscopy, following various crystallization conditions. It was found that each of those polylactides can crystallize, even those with low optical purities; their crystallization rate is, however, slower than those for high optical purity polyesters. Moreover, the low optical purity polymers tend to form stereocomplexes between the l and d sequences of the same polylactide, which behavior is ascribed to their multiblock microstructure. A correlation was found between the measured melting temperature of optically active polylactides and their average sequence length.

502 citations


Journal ArticleDOI
TL;DR: In this paper, an equilibrium mean-field theory for reversible network formation in solutions of associative polymers is presented, which is consistent with the classical gelation picture developed by Flory and Stockmayer.
Abstract: An equilibrium mean−field theory for reversible network formation in solutions of associative polymers is presented. We study polymer solutions with many associating groups per chain and consider pairwise association of these groups. A simple analytical expression for the free energy of these systems is derived and is shown to be consistent with the classical gelation picture developed by Flory and Stockmayer. It is shown that association and formation of a reversible network is always accompanied by a tendency for phase separation which might occur even under marginal solvent conditions. The mean-field theory is also generalized to take into account the effect of local intrachain loops as well as excluded volume interactions (partial swelling of polymer chains). It is shown that phase separation might be suppressed by the excluded volume interactions.

Journal ArticleDOI
TL;DR: A core−shell-type supramolecular assembly, a polyion complex micelle, was prepared from chicken egg white lysozyme and poly(ethylene glycol)−poly(aspartic acid) block copolymer through electrostatic interaction in aqueous medium and no precipitate formation was observed even after 1 month standing at ambient temperature, suggesting that the system is in a thermodynamic equilibrium state.
Abstract: A core−shell-type supramolecular assembly, a polyion complex micelle, was prepared in this study from chicken egg white lysozyme and poly(ethylene glycol)−poly(aspartic acid) block copolymer (PEG-P(Asp)) through electrostatic interaction in aqueous medium. Lysozyme/PEG-P(Asp) micelles thus prepared had an extremely narrow distribution (μ2/Γ2 < 0.04) with an average diameter of 47 nm in dynamic light scattering measurements. No precipitate formation was observed even after 1 month standing at ambient temperature, suggesting that the system is in a thermodynamic equilibrium state. The stoichiometry in terms of the molar ratio of Lys and Arg residues in lysozyme and Asp residues in PEG-P(Asp) was confirmed by dynamic and static light scattering as well as by laser-Doppler electrophoresis measurements. A change in the apparent molar mass of the micelle with varying PEG-P(Asp)/lysozyme ratio in the region with excess lysozyme agreed well with calculated values if a cooperative association mechanism is assumed ...

Journal ArticleDOI
TL;DR: In this article, the morphological development in solvent-cast polystyrene thin films is described as a function of solvent evaporation, and a morphological morphological model for polybutadiene (PB) is proposed.
Abstract: This paper describes morphological development in solvent-cast polystyrene (PS)−polybutadiene (PB)−polystyrene (SBS) triblock copolymer thin films (30 wt % PS) as a function of solvent evaporation ...

Journal ArticleDOI
Kui Yu1, Adi Eisenberg1
TL;DR: In this article, a wide range of bilayer aggregates, among them tubules, vesicles, large compound Vesicles (LCVs), and lamellae, were prepared from various polystyrene-b-poly(ethylene oxide) (PS-b -PEO) diblock copolymers, and studied by transmission electron microscopy (TEM).
Abstract: A wide range of bilayer aggregates, among them tubules, vesicles, large compound vesicles (LCVs), and lamellae, were prepared from various polystyrene-b-poly(ethylene oxide) (PS-b-PEO) diblock copolymers, and studied by transmission electron microscopy (TEM). The preparation method involved copolymer dissolution in DMF at room temperature, followed by the addition of water. In addition, it was found that aggregates of various morphologies can be prepared from an identical block copolymer by changing the solvent from DMF to a water−DMF mixture, by the addition of electrolytes, or by the use of subambient temperatures. All of these methods can be used to facilitate the formation of specific bilayer aggregates. When the preparation method involved copolymer dissolution in water−DMF mixtures, it was found that the morphologies of aggregates under certain conditions also depended on the annealing time. For example, the ratio of tubules to vesicles is related to the annealing time; only tubules appear at long a...

Journal ArticleDOI
Gerrit Klaerner1, Robert D. Miller1
TL;DR: In this article, the authors used the nonintrusive end-capping reagent 2-bromofluorene to control molecular weights and to generate well-defined oligomers that can subsequently be separated and characterized.
Abstract: Efficient, thermally stable blue light emitting materials are needed both to complete the color spectrum and to serve as energy transfer media for incorporated fluorophores. The extended delocalization lengths of most fully conjugated polymers, however, result in small electronic band gaps and red-shifted emissions. Poly(p-phenylene) derivatives (PPP) show large band gaps since the aromatic rings are twisted to relieve unfavorable steric interactions in the backbone, which limits the effective conjugation length.1 The dihedral angle between the aromatic rings in unsubstituted PPP is ∼23°.2 Unsubstituted PPP is, however, highly insoluble, limiting the molecular weights and processability, and films are often generated via soluble precursor polymers.3 Large, solubilizing substituents may also be incorporated, resulting in improved processability, but this usually exacerbates the steric interactions in the polymer main chain.4 Fluorene derivatives are interesting since they contain a rigidly planar biphenyl unit and the facile substitution at the remote C9 site provides the possibility of improving the solubility and processability of polymers without significantly increasing the steric interactions in the polymer backbone. Remote substitution also offers the possibility of controlling interchain interactions, which can lead to excimer formation in the excited state.5 Polyfluorene derivatives also have extremely high luminescence efficiencies in solution (rivaling those of many laser dyes), which are largely maintained in the polymer films.6c,d,7 As a result, polyfluorenes have been intensively studied recently as blue light emitting materials.6,7 Despite this, the effective conjugation length, which determines such important electronic properties as band gaps, absorption coefficients, emission quantum yields, etc., has not been reported for these polymers. The effective conjugation length may be defined as the minimum number of bonded aromatic rings necessary to produce saturation of the optical and electronic properties.8 Substituted polyfluorene derivatives were first prepared by oxidative polymerization of 9-alkyl-substituted monomers.9 These polymers, however, were of low molecular weight9c and gave poor quality films. It is also difficult to remove residual metallic impurities; a potential problem for electroluminescent devices. Recently, however, we7 and others6c have described the successful preparation of high molecular weight polyfluorene homoand copolymers by the nickel(0)-mediated polymerization of 9,9-disubstituted-2,7-dibromofluorene monomers (Yamamoto polymerization).10 We have used this procedure in concert with polymer end capping to prepare well-defined fluorene oligomers and have used these to estimate the effective conjugation length11 for poly(9,9-di-n-hexylfluorene-2,7-diyl) (3). The synthetic procedure is illustrated in Scheme 1. We have used the nonintrusive end-capping reagent 2-bromofluorene to control molecular weights and to generate well-defined oligomers that can subsequently be separated and characterized. A typical procedure is described below. Into a Schlenk tube was placed 2.6 mmol of bis(1,5-cyclooctadiene)nickel(0), 2.6 mmol of 2,2′-bipyridyl, 0.2 mL of 1,5-cyclooctadiene, 4 mL of DMF, and 8 mL of toluene, and the mixture was stirred at 80 °C (0.5 h). The 2,7-dibromo-9,9-di-n-hexylfluorene (1.125 mmol) and 2-bromofluorene (0.375 mmol) in 6 mL of toluene were added to the blue solution, and the mixture was heated at 80 °C for 3 days. The capped oligomers were precipitated into an equivolume mixture of concentrated HCl, methanol, and acetone. The solid was redissolved in chloroform and precipitated from acetone/methanol. The addition of 2 at the onset of the reaction leads to the formation of both soluble oligomers and low molecular weight linear polymer. Higher molar ratios of 2/1 lead to lower molecular weight polymer and an enhancement of the oligomeric fraction. These data are reported in Table 1. End group analysis of the unfractionated low molecular weight materials isolated by precipitation after 3 days by 1H NMR (integration of the C9 methylene protons of the end groups at δ ∼4.00 relative to the methylene signals of the n-hexyl substituents at δ ∼2.0) provides a value for Mn to be compared with that determined by GPC analysis. The latter value, based on polystyrene standards, is always somewhat greater than that determined by 1H NMR. The crude reaction product is composed of a mixture of oligomers and low molecular weight polymer, as determined by GPC analysis. Better separation of the oligomers from the polymer fraction was achieved by HPLC on Kromasil C18-10 μm using a gradient elution with acetonitrile/ chloroform (Figure 1), and the constituents of the oligomeric fraction were separated in this manner. Using this technique, oligomers ranging from n ) 3 to n ) 10 were separated and characterized. The spectroscopic properties of these isolated materials are reported in Table 2. The parent masses for each of the oligomers, as determined by field desorption mass spectroscopy, are also included in the table. It is significant that the observed parent masses are consistent with the presence of a fluorene capping unit at each end of the oligomeric chain. Although the spectral absorption maxima continue to move to the red through n ) 10, the emission maxima remain virtually unchanged beyond n ) 6. This would suggest that the backbone geometry changes significantly in going from the ground state (S0) to the vibronically relaxed excitedstate (S1). For reference, the absorption maximum of high molecular weight poly(di-n-hexylfluorene-2,7-diyl)(3) (PDHF) prepared by nickel-mediated coupling in the absence of the end-capping reagent occurs at 388 nm for material with a degree of polymerization of 54 (Mn determined by GPC analysis using polystyrene standards).7 This value is very close to that reported by Fukuda et al.9c (387 nm) for the polymer where n ) 14. Figure 2 shows the absorption spectra of the separated oligomers (n ) 3-10) together with that of 2007 Macromolecules 1998, 31, 2007-2009

Journal ArticleDOI
TL;DR: Optimize systems were found to form adhesive bonds that rival in strength those formed by natural marine adhesive proteins, which are readily prepared in large quantities and require no enzymes or other biological components.
Abstract: Water soluble copolypeptides containing l-dihydroxyphenylalanine (DOPA) and l-lysine were prepared by ring-opening polymerization of α-amino acid N-carboxyanhydride (NCA) monomers. We have prepared a range of different copolymers to probe the effects of functional group composition on adhesive and cross-linking behavior. Aqueous solutions of these copolymers, when mixed with a suitable oxidizing agent (e.g., O2, mushroom tyrosinase, Fe3+, H2O2, or IO4-), formed cross-linked networks that were found to form moisture-resistant adhesive bonds to a variety of substrates (e.g., aluminum, steel, glass, and plastics). It was found that successful adhesive formation was dependent on oxidation conditions, with chemical oxidants giving the best results. Optimized systems were found to form adhesive bonds that rival in strength those formed by natural marine adhesive proteins. Our synthetic systems are readily prepared in large quantities and require no enzymes or other biological components.

Journal ArticleDOI
TL;DR: In this paper, the rate constants of polymerization of l,l-lactide (LA) initiated with aluminum isopropoxide (Al(OiPr)3) trimer (A3) or tetramer (A4) were determined at 20, 50, 80 (THF solvent), and 120 °C (dioxane-1,4 solvent), respectively.
Abstract: Kinetics of polymerization of l,l-lactide (LA) initiated with aluminum isopropoxide (Al(OiPr)3) trimer (A3) or tetramer (A4) was followed by polarimetry and by gel permeation chromatography (GPC). Results of the kinetic measurements show that A3 and A4 react with LA with different rates; namely, the ki(A3)/ki(A4) ratios (where ki(A3) and ki(A4) denote the rate constants of initiation with A3 and A4, respectively) determined at 20, 50, 80 (THF solvent), and 120 °C (dioxane-1,4 solvent), are equal to 2.8 × 103, 8.0 × 102, 2.9 × 102, and 1.1 × 102, respectively. Direct observations of the A3/LA and A4/LA reacting mixtures by means of 13C NMR spectroscopy confirm this large difference of A3 and A4 reactivities in their reactions with LA. Initiation with A4 is slow enough to give polymerization that is less under control, in comparison with that initiated by A3 alone. However, due to the relatively low rate of propagation, in comparison with that of the A4 → A3 transformation, the apparent rates of LA polymeri...

Journal ArticleDOI
TL;DR: In this paper, the authors investigated the kinetics and mechanism of a radical chain polymerization reaction initiated from a self-assembled monolayer of an azo initiator attached to the surfaces of silica particles.
Abstract: The kinetics and mechanism of a radical chain polymerization reaction initiated from a self-assembled monolayer of an azo initiator attached to the surfaces of silica particles are investigated. The rate of the decomposition of the surface-attached initiator is followed by differential scanning calorimetry and volumetry. The kinetics of formation of the terminally attached polymer is studied by dilatometry. After polymerization the polymer is removed from the surfaces and the molecular weight averages and molecular weight distribution of the degrafted polymer are studied as a function of reaction parameters during polymerization. From the molecular weight, the mass of the attached polymer, and the specific surface area, the number of polymer molecules per area (or the distance between the anchoring sites) can be calculated and compared to the corresponding values of the initiator monolayers. The mechanism of a polymerization with surface-attached radicals is compared to that of conventional radical chain ...

Journal ArticleDOI
TL;DR: Starch is a natural, renewable, biodegradable polysaccharide produced by many plants as a storage polymer and has received considerable attention during the past two decades.
Abstract: Starch is a natural, renewable, biodegradable polysaccharide produced by many plants as a storage polymer. It usually has two major components and appears as a mixture of two glucosidic macromolecules very different in structure and properties: largely linear amylose of molecular weight between one thousand and one million, consisting of R-(1f4)-linked D-glucose, and amylopectin, having the same backbone as amylose but with a myriad of R-(1f6)-linked branch points. The most commercially important starch comes from corn, wheat, rice, potatoes, tapioca, and peas. Native starch occurs in the form of discrete and partially crystalline microscopic granules that are held together by an extended micellar network of associated molecules. Starch has received considerable attention during the past two decades as a biodegradable thermoplastic polymer and as a biodegradable particulate filler. Indeed, products from agricultural sources, such as starch, offer an attractive and cheap alternative in developing degradable materials. Starch is not truly thermoplastic as most synthetic polymers. However, it can be melted and made to flow at high temperatures under pressure and shear. If the mechanical shear becomes too high, then starch will degrade to form products with low molecular weight. Addition of water or other plasticizers enables starch to flow under milder conditions and reduces degradation considerably. However, the thermomechanical stability is strongly reduced by the addition of plasticizers. By itself, starch is a poor choice as a replacement for any plastic. It is mostly water soluble, difficult to process, and brittle. In principle, some of the properties of starch can be significantly improved by blending it with synthetic polymers. Physical incorporation of granular starch or starch derivatives as a functional additive and filler into synthetic polymers during processing has been largely used since the first announcements of using starch in combination with synthetic polymer either as starch gel blends with ethylene acrylic acid copolymers by Westhoff et al.1 or as particulate starch dispersions in polyolefins by Griffin.2 More recently, increasing interest in developing new and inexpensive biodegradable materials has led to a substantive amount of research in polymer blends containing starch.3-8 However, the mechanical properties of films are generally reduced by incorporation of starch. Like most of the polymers, starch is immiscible with most of the synthetic polymers at the molecular level. Grafting of synthetic polymers on starch is known to improve some of its properties, but although the grafting of starch with synthetic polymers has been known for 30 years, very few processes have led to full-scale commercialization. Another way of using starch in the material field is the processing of starch microcrystals, which can be obtained as an aqueous suspension. This filler brings a great reinforcing effect to a polymeric matrix, as shown previously.9 In this work, an attempt was made to improve the thermomechanical properties and to decrease the water sensitivity of starch-based systems, preserving the biodegradability of the material. An alternative way to palliate these restrictions is the use of cellulose as a natural and biodegradable filler. Natural cellulose fibers are gaining attention as a reinforcing phase in thermoplastic matrixes. Its low density, a highly reduced wear of the processing machinery, and a relatively reactive surface may be mentioned as attractive properties, together with their abundance and low price. In addition, the recycling by combustion of cellulose composites is easier in comparison with inorganic filler systems. Nevertheless, such fibers are used only to a limited extent in industrial practice, which may be explained by difficulties in achieving acceptable dispersion levels. Cellulose fibers are constituted of long threadlike bundles, called microfibrils, of cellulose molecules stabilized laterally by hydrogen bonds between hydroxyl groups of adjacent molecules. Cellulose microfibrils can also be found as intertwined microfibrils in parenchyma cell wall, in particular from potato pulp. They can be extracted from this biomass by a chemical treatment, leading to purified cellulose, followed by a mechanical treatment in order to obtain a homogeneous suspension due to the individualization of the microfibrils. This suspension has been used afterward to process composite materials with a high level of dispersion, by mixing with an aqueous suspension of gelatinized starch as matrix. Potato pulp was purified according to the treatment displayed in Figure 1. After the removal of starch granules, the remaining pulp is traditionally pressed and dried to be marketed as cattle feed. This byproduct was provided, as pellets, by Avebe Co. (Haussimont, France). The pellets were hydrated into water and ground in a Waring blender apparatus for 10 min. The potato slurry was then poured on a 0.25 mm sieve and washed with water, to remove most of the remaining starch granules. The alkali extraction with sodium hydroxide (NaOH) solution allowed the solubilization of pectins, residual starch, and hemicelluloses, which were then removed by filtration and finally rinsed with distilled water. The resulting insoluble residue was bleached with a sodium chlorite (NaClO2) solution, as already described for sugar beet.10,11 At this stage, the different cell walls are individualized, as shown in Figure 2A, but the microfibrils are still associated within the cell wall. To extract and individualize the microfibrils from the cell walls, a mechanical treatment is required. The insoluble bleached cellulosic pulp was suspended in distilled water (2 wt %) and disintegrated for 15 min in a Waring blender. The suspension was then homogenized by 15 passes through a Manton-Gaulin laboratory homogenizer, described elsewhere by Dufresne et * To whom correspondence should be addressed (e-mail: dufresne@cermav.cnrs.fr). 2693 Macromolecules 1998, 31, 2693-2696

Journal ArticleDOI
TL;DR: In this article, it was shown that the morphologies and other characteristics of crew-cut aggregates of polystyrene-b-poly(acrylic acid) (PSb-PAA) diblock copolymers are related to the nature of the initial common solvent in which the micelle-like aggregates are prepared.
Abstract: It is shown that the morphologies and other characteristics of crew-cut aggregates of polystyrene-b-poly(acrylic acid) (PS-b-PAA) diblock copolymers are related to the nature of the initial common solvent in which the micelle-like aggregates are prepared. Polymer−solvent interactions determine the dimensions of both the core and the corona of the aggregates. Solubility parameters and dielectric constants of the solvents can be used to estimate the strength of the PS−solvent interaction (which influences the solvent content in the core) and the strength of PAA−solvent interaction (which influences the repulsion among corona chains). The closer the match between the solubility parameter of the solvent and that of the core forming block, the higher the solvent content of the core and the higher the degree of stretching of the core chains. The lower the polarity of the solvent, the weaker the PAA−solvent interaction and the weaker the repulsive interactions among the corona chains; this increases the aggregat...

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TL;DR: The mechanical relaxation behavior and microstructure of a series of novel norbornyl−POSS organic−inorganic copolymers have been investigated in this article, where it was observed that POSS copolymerization enhances the α-relaxation temperature, Tα, in proportion to the weight fraction of the POSS−norbornyl comonomer.
Abstract: The mechanical relaxation behavior and microstructure of a series of novel norbornyl−POSS organic−inorganic copolymers have been investigated. We have examined the influence on physical properties of both the weight fraction of POSS−norbornyl monomer and the corner group composition. POSS refers to the polyhedral oligomeric silsesquioxane inorganic/organic macromer, which is composed of an inorganic Si8O12 spherical core surrounded by seven inert organic corner groups and one reactive norbornyl moiety. It was observed that POSS copolymerization enhances the α-relaxation temperature, Tα, in proportion to the weight fraction of the POSS−norbornyl comonomer. Interestingly, however, the magnitude of this dependence is larger for the POSS−norbornyl comonomer possessing cyclohexyl corner groups (CyPOSS) than for the copolymer with cyclopentyl corner groups (CpPOSS). Although POSS copolymerization yields only slight enhancement of the room temperature storage modulus, at temperatures lower than a strong mechanic...

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TL;DR: In this article, a variety of high molecular weight, thermally stable, blue-light-emitting random copolymers of 9,9-di-n-hexylfluorene by nickel(0)-mediated polymerization are presented.
Abstract: We have prepared a variety of high molecular weight, thermally stable, blue-light-emitting random copolymers of 9,9-di-n-hexylfluorene by nickel(0)-mediated polymerization. The copolymers are readily soluble and easily processable from organic solvents. Both the polymer and electronic properties may be tuned by selection of comonomer structure. The electronic properties also vary with composition and film morphology. A blue-light-emitting device has been prepared using ionic salts for electrochemical doping.

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TL;DR: In this article, the authors considered the effect of multiple dissociation and recombination of the same pair of stickers and showed that this effect gives rise to an increase of the apparent activation energy which is predicted to be substantially larger than the priming activation energy for dissociation of two stickers.
Abstract: Viscoelastic properties of reversible networks formed in solutions of associating polymers are considered theoretically in the Rouse−Zimm (unentangled) regime. It is shown that the dynamics is governed primarily by the network strand size and by the effective lifetime of reversible junctions. Both frequency and concentration dependencies of viscosity and dynamical moduli are considered. A novel model taking into account the possibility of multiple dissociation and recombination of the same pair of stickers is developed. It is shown that this effect gives rise to an increase of the apparent activation energy which is predicted to be substantially larger than the priming activation energy for dissociation of two stickers.

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TL;DR: In this paper, the diffusion coefficients of He, O2, N2, CO2, and CH4 at 300 K in four silicone polymers, namely, poly(dimethylsiloxane) (PDMS), poly(propylmethylmethyloxane), poly((trifluoropropyl)methylsiloxideane), and poly(phenylmethyl methyloxane)) were estimated by molecular dynamics simulations.
Abstract: Diffusion coefficients of He, O2, N2, CO2, and CH4 at 300 K in four silicone polymers, namely, poly(dimethylsiloxane) (PDMS), poly(propylmethylsiloxane), poly((trifluoropropyl)methylsiloxane), and poly(phenylmethylsiloxane), have been estimated by molecular dynamics (MD) simulations. The estimated diffusion coefficients decrease with increasing size of the polymer side chains and of the penetrant molecules, as was also found experimentally. The estimated diffusion coefficient for He in PDMS is consistent with its experimental value. The values of the estimated diffusion coefficients for the other gas/silicone systems considered in this study are within ± 40−60% of the corresponding experimental values. The MD simulations revealed two types of motions of the penetrant molecules: (1) “jumps” from one cavity in a silicone matrix to another, and (2) “oscillating motions” inside cavities. The lengths of the jumps are of the order of 8−15 A, whereas the oscillating motions are of the order of ≤5 A. The total t...

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TL;DR: The foaming of polystyrene using supercritical (SC) CO2 has been studied to better understand the microcellular foaming process, as well as to plan future studies that involve the creation of composite microcellule foams as discussed by the authors.
Abstract: The foaming of polystyrene using supercritical (SC) CO2 has been studied to better understand the microcellular foaming process, as we plan future studies that involve the creation of composite microcellular foams. Rapid decompression of SC CO2-saturated polystyrene at sufficiently high temperatures (above the depressed Tg) yields expanded microcellular foams. The resulting foam structures can be controlled by manipulating processing conditions. Experiments varying the foaming temperature while holding other variables constant show that higher temperatures produce larger cells and reduced densities. Structures range from isotropic cells in samples retaining their initial geometry to highly expanded foams recovered in the shape of the foaming vessel and having oriented, anisotropic cells and limited density reduction. Higher saturation pressures lead to higher nucleation densities and hence smaller cells. Decreasing the rate of depressurization permits a longer period of cell growth and therefore larger ce...

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TL;DR: The molecular characteristics of the first five generations of poly(propyleneimine) dendrimers with two different types of end groups have been investigated using small-angle neutron scattering (SA....
Abstract: The molecular characteristics of the first five generations of poly(propyleneimine) dendrimers with two different types of end groups have been investigated using small-angle neutron scattering (SA...

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TL;DR: This paper presents an investigation of the thermal and viscoelastic property enhancements in commonly used model epoxy resins reinforced with monofunctional POSS-epoxy macromers and suggests that while POSS cages influence polymer chain motions, including the motion of the molecular junctions, these nanoreinforcements did not participate in the overall deformation of the chains.
Abstract: The properties of nanostructured plastics are determined by complex relationships between the type and size of the nanoreinforcement, the interface, and the chemical interaction between the nanoreinforcement and the polymeric chain, along with macroscopic processing and microstructural effects. Recently, families of mono- and difunctionalized polyhedral oligomeric silsesquioxane (POSS) macromers bearing epoxide groups have been developed. This paper presents an investigation of the thermal and viscoelastic property enhancements in commonly used model epoxy resins reinforced with monofunctional POSS-epoxy macromers. The glass transitions of these POSS-epoxy nanocomposites were studied using differential scanning calorimetry. Small-strain stress relaxations under uniaxial deformation were examined to provide insight into the time-dependent viscoelastic behavior of these nanocomposites. The POSS-epoxy macromers utilized in this study were monofunctional and hence occupied chain terminus points within the net...

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TL;DR: The Doi−Edwards (DE) theory for the rheological properties of entangled polymer melts and solutions successfully predicts the response to large step-shear strains but fails to predict other nonlinear shear properties, such as the steady-state viscosity or the relaxation of stress after cessation of steady shearing.
Abstract: The Doi−Edwards (DE) theory for the rheological properties of entangled polymer melts and solutions successfully predicts the response to large step-shear strains but fails to predict other nonlinear shear properties, such as the steady-state viscosity or the relaxation of stress after cessation of steady shearing. Many of these failures remain even in the extension of the theory by Marrucci and Grizzuti (Gazz. Chim. Ital. 1988, 118, 179)1 to allow deformation-induced “tube stretch”. Here, we find that a much more successful theory can be obtained by also accounting for “convective constraint release”, i.e., the loss of entanglement constraints caused by the retraction of surrounding chains in their tubes. (Marrucci, G. J. Non-Newtonian Fluid Mech. 1996, 62, 279 and Ianniruberto, G.; Marrucci, G. J. Non-Newtonian Fluid Mech. 1996, 65, 241).2,3 In the molecular model developed here, convective constraint release can both shorten the reptation tube and allow reorientation of interior tube segments. The revi...

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TL;DR: In this article, the authors investigated halide exchange during atom transfer radical polymerization (ATRP) using mixed halide initiation systems, R−X/Cu−Y (X, Y = Cl or Br).
Abstract: Halide exchange during atom transfer radical polymerization (ATRP) using mixed halide initiation systems, R−X/Cu−Y (X, Y = Cl or Br), was investigated. Model studies of mixed halide initiation systems (i.e., R−X/Cu−Y, X ≠ Y) demonstrated that exchange occurs rapidly at 90 °C, and there is a clear preference for alkyl chlorides to be formed in over alkyl bromides. This was attributed mainly to the carbon−chlorine bond being stronger than the corresponding carbon−bromine bond. This implies that, in ATRP with a mixed halide initiator/catalyst system, the bulk of the polymer chain ends are terminated by chlorine if [CuCl]0 ≥ [RBr]0. Examples of using this information to improve the control in ATRP of methyl methacrylate (MMA) are presented. It was shown that, when benzyl halides were used as the initiator in the ATRP of MMA, the rate of initiation was increased relative to the rate of propagation, thus providing better control by using the benzyl bromide/copper chloride mixed halide system. Better molecular w...

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TL;DR: Using numerical self-consistent field (SCF) calculations, Wang et al. as mentioned in this paper investigated the interactions between two closely spaced surfaces and the surrounding polymer melt, and developed an analytical SCF theory to model the interactions among the functionalized chains, nonfunctionalized polymers and the clay sheets.
Abstract: Using numerical self-consistent field (SCF) calculations, we investigate the interactions between two closely spaced surfaces and the surrounding polymer melt. Short chains (surfactants) are terminally anchored to each of the surfaces. The coated substrates model organically modified clay crystallites (sheets). Through the calculations, we vary the characteristics of the surfactants and polymers to isolate conditions that drive the polymer to penetrate the gap between the surfaces. We also consider the effect of employing end-functionalized chains to promote the dispersion of bare clay sheets within the polymer matrix. We find that this scheme provides a robust method for exfoliating the sheets. To consider this case in greater depth, we develop an analytical SCF theory to model the interactions among the functionalized chains, nonfunctionalized polymers, and the clay sheets. The results from the numerical and analytical SCF models show good agreement on the behavior of the system. The results indicate th...