Preparation of monodisperse, fluorescent PMMA-latex colloids by dispersion polymerization

We report a single step procedure to prepare monodisperse colloidal poly(methyl)methacrylate (PMMA) particles where fluorescent dyes are incorporated into the polymer network. The particles are sterically stabilized against flocculation by poly(12-hydroxystearic acid). The fluorescent dyes are RITC (rhodamine isothiocyanate)-aminostyrene (RAS) and 4-methylaminoethylmethacrylate-7-nitrobenzo-2-oxa-1,3-diazol (NBD-MAEM), which are prepared from (commercially available) RITC and 4-chloro-7-nitrobenzo-2-oxa-1,3-diazol (NBD-Cl), respectively, as starting materials. The particles can be grown larger with nonfluorescent PMMA. Examples of the usefulness of these model particles in colloid science are given.     

Preparation and properties of cross-linked fluorescent poly(methyl methacrylate) latex colloids

We report a single step preparation of monodisperse fluorescent poly(methyl)methacrylate (PMMA) lattices cross-linked with ethylene glycol dimethacrylate with radii in the range 150-1000 nm using dispersion polymerization. The particles are applied as fluorescent cores in core-shell PMMA particles for confocal microscopy (Dullens et al. Langmuir 2003, 19, 5963). Contrary to un-cross-linked particles, these cross-linked colloids are stable in good solvents for PMMA as well. Therefore we studied the properties of the cross-linked PMMA particles in the good solvents tetrahydrofuran (THF), chloroform, and toluene using light scattering and confocal scanning laser microscopy. We show that the particles swell instantaneously and that their volume can increase up to more than seven times their volume in poor solvents. Further, it is very likely that the particles are charged in THF.     

Preparation of fluorescent particles with long excitation and emission wavelengths dispersible in organic solvents

We introduce a fast and simple one-step method, a variation of the methods of Barrett and Campbell and Bartlett, to synthesize monodisperse fluorescent particles that can be dispersed in organic solvents and have long excitation (649 nm) and emission wavelengths (679 nm). A lipophilic fluorescent dye, 1,1'-dioctadecyl-3,3,3',3'-tetramethylindodicarbocyanine perchlorate, is directly incorporated into PMMA particles through dispersion polymerization. A poly(hydroxystearic acid) graft (poly) methyl methacrylate (MM) and methacrylic acid (MA) copolymer is used as a stabilizer to prevent the particles from aggregating and flocculating in the nonaqueous solvents. The fluorescent PMMA particles are very uniform in size, bleach at very low rate, and behave like hard spheres in their ordering on substrates. One important achievement in our synthesis protocol is that we are able to produce particles of a desired size by choosing the composition of the reactants according to a predetermined relationship between particle size and composition of reactants. In addition, the effects of fluorescent dye and polar solvent (ethanol) on the formation and size of particles are discussed.     

Synthesis and characteristics of snowman-like fluorescent PMMA microbeads

This paper reports the fabrication of fluorescent snowman-like poly(methyl methacrylate) (PMMA) colloidal microbeads using a two-step method. The fluorescent cross-linked PMMA seeds were first synthesized by dispersion polymerization and then swelled in a monomer solution containing the methyl methacrylate monomer, a secondary cross-linker and initiator, followed by heat-induced polymerization. Scanning electron microscopy, confocal laser scanning microscopy, and Fourier transform infrared spectroscopy confirmed the synthesis of fluorescent snowman-like particles.     

Small-angle neutron scattering on a core-shell colloidal system: a contrast-variation study

Small-angle neutron scattering (SANS) measurements are reported on a sterically stabilized, core-shell colloidal system using contrast variation. Aqueous dispersions of polystyrene particles bearing grafted poly(ethylene glycol) (PEG) have been studied over a large range of particle concentrations and two different solvent conditions for the PEG polymer. SANS data are analyzed quantitatively by modeling the particles as core-shell colloids. In a good solvent and under particle contrast conditions, an effective hard-sphere interaction captures excluded-volume interactions up to high concentrations. Contrast variation, through isotopic substitution of both the core and solvent, expedite a detailed study of the PEG layer, both in the dilute limit and as a function of the particle concentration. Upon diminishing the solvent quality, subtle changes in the PEG layer translate into attractions among particles of moderate magnitude.     

Stimuli-responsive hydrogel hollow capsules by material efficient and robust cross-linking-precipitation synthesis revisited

Monodisperse stimuli-responsive hydrogel capsules were synthesized in the 100-nm-diameter to 10-μm-diameter range from poly(4-vinylpyridine) (P4VP) and poly(ethyleneimine) (PEI) through a simple, efficient two-step cross-linking-precipitation template method under conditions of a good solvent. In this method, the core-shell particles were obtained by the deposition (heterocoagulation mechanism) of the cross-linked P4VP, PEI, or their mixtures on the surfaces of the template colloidal silica particles in nitromethane (for PEI) or a nitromethane-acetone mixture (for P4VP and P4VP-PEI mixtures) in the presence of cross-linker 1,4-diiodobutane. The cross-linked polymeric shell swollen in a good solvent stabilized the core-shell colloids. This mechanism provided the conditions for the synthesis of core-shell colloids in a submicrometer range of dimensions with an easily adjusted shell thickness (wall of the capsules) ranging from a few to hundreds of nanometers. The chemical composition of the shell was tuned by varying the ratio of co-cross-linked shell-forming polymers (P4VP and PEI). In the second step, the hollow capsules were obtained by etching the silica core in HF solutions. In this step, the colloidal stability of the hollow capsules was provided by ionized P4VP and PEI cross-linked shells. The hollow capsules demonstrate that the pH- and ionic-strength-triggered swelling and shrinking result in size-selective uptake and release properties. Cross-linked via quaternized functional groups, P4VP capsules undergo swelling and shrinking transitions at a physiologically relevant pH of around 6. The 200-nm-diameter hollow capsule with 25-nm-thick walls demonstrated a factor of 2 greater capacity to accommodate cargo molecules than the core-shell particles of the same dimensions because of an internal compartment and a combination of radial and a circumferential swelling modes in the capsules.     

Correlation of infinite dilution activity coefficients of solvents in polymer solutions using connectivity indices

In this work, correlations for the estimation of the infinite dilution activity coefficients of non-polar solvents in polystyrene (PS), poly(vinyl acetate) (PVAc), poly(n-butyl methacrylate) (PBMA), poly(dimethyl siloxane), poly(methyl methacrylate) (PMMA), poly(ethylene oxide) (PEO), poly(vinyl chloride) (PVC), polyisobutylene and polyethylene (PE), and that of polar solvents in PS, PVAc, PBMA, PMMA, PEO, PVC and PE are proposed. A total of 205 polymer/non-polar solvent systems with 1708 data points, and 118 polymer/polar solvent systems with 695 data points were used to develop the correlations. The overall average errors were 9.6% and 11.3%, respectively, significantly lower than other predictive models. Since the new correlations require only the connectivity indices of the solvents in the calculations, and the connectivity indices can be calculated easily once the molecular structure of the substance in question is known, they are easy to apply, and are useful for process design and development.     

Viscous solvent colloidal system for direct visualization of suspension structure, dynamics and rheology

We introduce a model colloid system comprised of particles dispersed in a viscous solvent that can be applied to 3D direct visualization studies of suspension structure, dynamics and rheology. The colloids are poly(methyl methacrylate) (PMMA) spheres sterically stabilized by a copolymer of poly(diphenyl-dimethyl) (DPDM) siloxane that matches the refractive index of PMMA. The monodisperse particles, synthesized with mean diameter varying from 0.7 to 1.1 microm, are stably dispersed in a DPDM siloxane solvent, with viscosity varying from 2.2 to 4.3 Pa s at 20 degrees C. As opposed to other classes of PMMA colloids dispersed in organic solvents, this system displays minimal charge interactions. At room temperature, pair potential interactions (measured by extrapolation of pair correlation functions to infinite dilution) are well modeled by a generalized Lennard-Jones alpha-2alpha potential (alpha=10) with dimensionless interaction energy, epsilon/k(B)T=0.6. We use the DPDM-PMMA colloidal system in conjunction with confocal microscopy studies to measure: (i) the radial distribution function in 3D at dilute concentrations and (ii) the colloid self-diffusivity in 3D at dilute concentrations. Both measurements, neither previously reported in uncharged systems, are facilitated by the slow, viscous dynamics of the system. We also show that the viscosity and particle size of the system are such that the high-volume fraction shear thickening transition can be accessed at shear rates amenable to direct visualization.     

Interactions between polymer brushes in solvents of variable quality: a density functional theory study

We present a density functional theory study of interactions between sterically stabilized colloidal particles in solvents of variable quality. Both flat and spherical polymer brushes are considered, as well as both monatomic and polymeric solvents. It is shown that the interaction between sterically stabilized particles can be tuned from repulsive to attractive by varying the solvent quality, the relative length of free and grafted chains, and by employing a mixed brush consisting of both well and poorly solvated chains.     

Swelling-based method for preparing stable, functionalized polymer colloids

We describe a swelling-based method to prepare sterically stabilized polymer colloids with different functional groups or biomolecules attached to their surface. It should be applicable to a variety of polymeric colloids, including magnetic particles, fluorescent particles, polystyrene particles, PMMA particles, and so forth. The resulting particles are more stable in the presence of monovalent and divalent salt than existing functionalized colloids, even in the absence of any surfactant or protein blocker. While we use a PEG polymer brush here, the method should enable the use of a variety of polymer chemistries and molecular weights.     

Electron transport in solution-grown TIPS-pentacene single crystals: Effects of gate dielectrics and polar impurities

The n-channel behavior has been occasionally reported in the organic field-effect transistors (OFETs) that usually exhibit p-channel transport only. Reconfirmation and further examination of these unusual device performances should deepen the understanding on the electron transport in organic semiconductors. 6,13-bis(triisopropyl-silylethynyl) pentacene (TIPS-pentacene), a widely examined p-channel material as Au is used for source-drain electrodes, has recently been reported to exhibit electron transport when grown from non-polar solvent on divinyltetramethyldisiloxanebis (benzocyclobutene) (BCB) dielectric, spurring the study on this unusual electron transport. This paper describes FET characteristics of solution-grown TIPS-pentacene single crystals on five polymer gate dielectrics including polystyrene (PS), poly(methyl methacrylate) (PMMA), poly(4-vinyl phenol) (PVP), poly(vinyl alcohol) (PVA) and poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (P(VDF-TrFE-CFE)). In addition to the p-channel behavior, electron transport occurs in the crystals on PMMA, PS, thick PVA (40 nm) and a bilayer dielectric of PMMA on P(VDF-TrFE-CFE), while does not on PVP and thin PVA (2 nm). The two distinct FET characteristics are consistent with the previous reported trap effect of hydroxyl groups (in PVP and PVA) and reduced injection barrier by Na⁺ ions (as impurity in PVA). The highest electron mobility of 0.48 cm² V^-1 s^-1 has been achieved in the crystals on PMMA. Furthermore, the electron transport is greatly attenuated after the crystals are exposed to the vapor of a variety of polar solvents and the attenuated electron transport partially recovers if the crystals are heated, indicating the adverse effect of polar impurities on electron transport. By reconfirming the n-channel behavior in the OFETs based on TIPS-pentacene, this work has implications for the design of n-channel and ambipolar OFETs.     

Synthesis of monodisperse fluorescent core-shell silica particles using a modified Stober method for imaging individual particles in dense colloidal suspensions

Core-shell silica particles, with a diameter of 1.5 mum, containing a dye fluorescein isothiocyanate (FITC), are synthesized by the hydrolysis and condensation of tetraethylorthosilicate (TEOS). Sodium dodecyl sulfate (SDS) is added to synthesize fluorescent core particles with the diameter of approximately 1 mum. In the addition of SDS, the surface charge reduced by counterions (Na+) of the surfactant leads to a higher degree of aggregation of the primary particles and the formation of larger secondary particles. The particle growth kinetics confirms the aggregation growth model for the synthesis of monodisperse silica particles, and also shows the dependence of final particle size on colloidal stability resulting from the addition of SDS. Light and X-ray scattering data reveal that the final particles have compactly packed structures with smooth surfaces. The seeded growth technique is then used to form a silica shell layer on the fluorescent core. The added amount of water and NH4OH has significant effects on shell formation. Finally, the final core-shell silica particles are modified by chemisorption of octadecanol at the surface to be dispersed in organic solvents. Octadecyl-coated silica particles are sterically stabilized in silica index-matching solvents such as chloroform and hexadecane to directly image separate particles using confocal microscopy. In chloroform, the organophilic silica particles disperse well, whereas in hexadecane they form a volume-filling gel structure at room temperature.     

Preparation and application of cross-linked core-shell PBA/PS and PBA/PMMA nanoparticles

This paper reports the preparation of cross-linked core-shell poly(butyl acrylate)/polystyrene (PBA/PS) and poly(butyl acrylate)/poly(methyl methacrylate) (PBA/PMMA) nanoparticles via seeded emulsion polymerization and their application in nylon-based composites. A highly cross-linked structure was formed in both the cores and the shells by using a cross-linking agent, which could prevent the migration of hydrophobic PS shells to the inside of particles. There were covalent bonds on the interfaces between the cores and the shells of both particles. The average particle sizes were 40–50 nm, and the size distributions were narrow. The kinetics of polymerization was investigated. Well-defined core-shell structure and narrow particle size distribution could be achieved under starved conditions of monomer feeding. Furthermore, PBA/PMMA particles were used to fill nylon 6, good dispersion was obtained because of the strong interfacial interaction between the nanoparticles and the nylon 6 matrix and the good deformation ability of nanoparticles, and the toughness and rigidity of the composites were improved evidently. __________ Translated from Acta Polymerica Sinica, 2005, (6) (in Chinese)     

Synthesis of micron-sized poly(styrene-<Emphasis Type="Italic">co</Emphasis>-divinylbenzene) hollow particles from seeded emulsions by using swelling solvents

Hollow micron-sized poly(styrene-co-divinylbenzene) particles were produced in seeded emulsions in the presence of swelling solvents. The size and morphology of the resulting polymer particles can be altered by varying swelling solvent in seeded polymerization at elevated temperature. The effects of swelling agents, including hydrophobic solvents, hydrophilic solvents and solvent mixtures, on the microstructure of particles were investigated. The formation of hollow poly(styrene-co-divinylbenzene) particles depended significantly on a fast and effective phase separation between the cross-linked shell and the swollen core, that occurred only in the presence of an appropriate swelling solvent.     

Interface hydrogen-bonded core-shell nanofibers by coaxial electrospinning

Core-shell nanofibers were prepared by coaxial electrospinning technology,with poly(ethylene oxide) (PEO) as the core while poly(acrylic acid) (PAA) as the shell.PEO and PAA can form polymer complexes based on hydrogen bonding.In order to avoid forming strong hydrogen bonding complexes at nozzle and blocking spinning process,a polar aprotic solvent,N,N-dimethylformamide (DMF),was selected to dissolve PEO and PAA respectively.SEM,TEM and DSC were utilized to characterize the morphology and structure of PEO-PAA core-shell nanofibers.FTIR spectra demonstrated that hydrogen bonding was formed at the core-shell interface.In addition,the PAA shell of the nanofibers can be cross-linked by ethylene glycol (EG) under heat treatment,which increases the stability and extends the potential applications in aqueous environment.     

MICRON CORE-SHELL PARTICLES PREPARED BY GRAFTING POLYMERIZATION OF METHYL METHACRYLATE FROM NARROW DISPERSE SURFACE OF CHLOROMETHYLATED POLYDIVINYLBENZENE VIA ATRP

Grafting of poly(methyl methacrylate) from narrow disperse polymer particles by surface-initiated atom transfer radical polymerization (ATRP) was investigated. Polydivinylbenzene (PDVB) particles were prepared by dispersion polymerization with poly(N-vinyl pyrrolidone) (PVP) as the stabilizer. Chloromethylated PDVB was used as initiating core sites for subsequent ATRP of methyl methacrylate with CuBr/bpy as catalyst system. It was found that poly(methyl methacrylate) was grafted not only from the particle surfaces but also from within a thin shell layer, leading to particles size increases from 2.38-3.00μm with a core-shell structure particles. The grafted core-shell particles were characterized with FTIR, SEM, DSC.     

The morphology of composite polymer particles produced by multistage soapless seeded emulsion polymerization

 Composite polymer particles which contain poly(methyl methacrylate) (PMMA) and polystyrene (PS) components (PMMA/PS composite particle) were synthesized by the method of multistage soapless seeded emulsion polymerization. In this study, the process of multistage soapless seeded emulsion polymerization included two-stage polymerization, three-stage polymerization or four-stage polymerization. The morphologies of the PMMA/PS composite particles were studied. The kinetic factor was the main force to control the morphology of the linear PMMA–PS composite particles which were synthesized by the method of two-stage reaction. Both the kinetic factor and the thermodynamic factor decide the morphology of the linear composite particles which were synthesized by the method of either three-stage or four-stage reaction. However, the thermodynamic factor cannot influence the morphology of the PMMA/PS composite particles with a cross-linked structure which were synthesized by the method of three-stage reaction. The cross-linked composite polymer particles had the morphology of a multilayer structure, which showed that the polymer layers accumulated in their order of production. Received: 9 January 2001 Accepted: 14 June 2001     

Synthesis and characterization of fluorescently labelled poly(vinyl acetate) particles

Nonaqueous dispersions of poly(vinyl acetate) particles stabilized with poly(2-ethylhexyl methacrylate) and labelled with fluorescent dyes in the core and stabilizer polymer phases were prepared. The materials were characterized using a broad range of techniques (NMR, UV, and fluorescence spectroscopy. GPC, particle sizing, and DSC). The data show that dispersion polymerization of vinyl acetate in the presence of stabilizers containing certain fluorescent dye derivatives, instead of unlabelled stabilizer, results in changes in the mean particle size, size distribution, composition, and molecular weights of the colloidal polymer particles. It was found that increasing the amounts of fluorescent groups in the stabilizer resulted in smaller mean particle sizes and larger amounts of irreversibly attached stabilizer. When the average number of labels in stabilizer exceeded one per chain, a new ultra-high molecular weight polymer fraction appeared. The fluorescence spectra for several of these materials are reported. We describe how fluorescence decay curves can be used to assess block formation and local phase separation in particles labelled with phenanthrene groups. These materials are intended to be used for more detailed studies by fluorescence spectroscopy. Those results will be published in due course.     

Swelling behavior of PMMA-g-PEO microgel particles by organic solvents

Aqueous dispersions of cross-linked poly(methylmethacrylate)-g-poly(ethylene oxide) [PMMA-g-PEO] microgel particles have been prepared from mixtures of methylmethacrylate [MMA] and MMA-PEO macromonomer, with ethylene glycol dimethacrylate [EGDM] as the cross-linking monomer (0.2-0.5% wt%). The hydrodynamic radius of these (unswollen) microgel particles ranged from 73 to 85 nm, and the particles were essentially monodisperse with regard to their size distribution. Their swelling behavior has been investigated in the presence of both water-miscible and water-immiscible organic solvents. In general, with the addition of a water-miscible solvent, deswelling behavior was observed. However, the microgel particles were swollen on addition of 1,4-dioxan, which is a good solvent for PMMA. With water-immiscible organic solvents, the extent of swelling depended on the solvency properties of the organic liquid for PMMA. In the presence of benzene, the somewhat large increases in particle size have been attributed to weak flocculation. This has been assumed from an estimate of the van der Waals attraction energy between the swollen microgel particles.     

Sterically stabilized silica colloids: Radical grafting of poly(methyl methacrylate) and hydrosilylative grafting of silicones to functionalized silica

Colloidal silica sols having a narrow dispersity, prepared by the ammonia-catalyzed hydrolysis of Si(OEt)₄, were functionalized by reaction with vinyltrimethoxysilane (H₂C?CHSi(OMe)₃) or methacryloxypropyltri-methoxysilane (H₂C?CMeCO₂(CH₂)₃Si(OMe)₃. The electrostatically stabilized colloids were stable in acetone and dimethylformamide. Radical polymerization of methyl methacrylate in the presence of either type of functionalized particle led to particles with surfacegrafted poly(methyl methacrylate) (PMMA). The efficiency of polymer grafting was shown to be related to the nature of the functional groups. The PMMA-modified, sterically stabilized particles were colloidally stable in solvents ranging from acetone to toluene but unstable in water or hexane. The vinyl functionalized silica was alternatively reacted with HSiMe₂-terminated silicones in a platinum-catalyzed hydrosilylation. The resultant sterically stabilized particles were stable in hexane. It was thus possible to convert the unmodified silica to organo-functionalized silica and finally to polymer-grafted silica while maintaining colloidal stability. During the course of these modifications, the mechanism for colloidal stability changed from electrostatic to steric stabilization.