Latex film formation studied with the atomic force microscope: Influence of aging and annealing

The surface structure of latex dispersion films was examined with an atomic force microscope. All measurements were done in air on latex films having a minimum film formation temperature of 12°C and a glass transition temperature of 18°C. One aim of this study was to follow structural changes during film formation. Three minutes after spreading the film, its surface layer dried. Afterwards, the structure of the film did not change anymore. Only after 4 months could structural changes be observed: Though individual latex particles could be identified, the particles partly melted into one another.After annealing films at 50° or 60°C for 4 h, the latex particles partly melted into one another, but individual particles could still be identified. When annealing at or above 80°C, no individual latex particles were visible anymore. With increasing temperature the film roughness decreased from 3 nm without annealing to 0.8 nm at 100°C annealing temperature. In addition, islands of 2–4 nm thickness appeared on the film surface. These islands could be scraped off the film by increasing the force between tip and sample, indicating that they are composed of surfactant which was squeezed out of the film.     

Preparation and properties of a thermo-sensitive latex film

Polymer particles with hydrophobic core and hydrophilic shell were prepared via a three-step method. First, poly(butyl methacrylate-co-methyl methacrylate) (p-(BMA-MMA)) latex was prepared through emulsion polymerization. Then, a shell of poly(glycidyl methacrylate) (p-GMA) was introduced around the p-(BMA-MMA) particles by using a redox initiation system under kinetically controlled conditions. Finally, part of the epoxy groups existing in the shell were converted into quaternary ammonium salts, resulting in an ionic hydrophilic shell. The core-shell particles could be redispersed in water to form a stable emulsion. The contact angle of the core-shell latex film with water was around 16° at 25 °C, which became larger than 90° after the film was heated at 150 °C for a short period of time. This showed that the latex film was completely switched from hydrophilicity to hydrophobicity by the action of heat. Additionally, the latex film before heat treatment could be easily washed away from the substrate with neutral water, but it could no longer be removed after the heat treatment. When an IR dye with the maximum absorption at 830 nm was incorporated into the film, it became sensitive to LD laser emitting at 830 nm and gave negative image after exposed by LD laser and developed with neutral water. This showed that the latex film might find uses in chemical-free thermal laser imaging applications.     

荧光探针法研究核壳结构有机硅-丙烯酸酯乳液的聚合行为

以芘为荧光探针,探讨了有机硅-丙烯酸酯核壳乳液聚合过程中,芘的第一振动峰(373 nm处)与第三振动峰(384 nm处)荧光强度的比值I1/I3与乳化剂、有机硅单体(D4)和引发剂(KPS)用量之间的关系,并结合聚合过程中探针芘的I1/I3峰值与单体转化率及乳胶粒形态演变之间的关系,研究了核壳结构有机硅-丙烯酸酯乳液的聚合行为.研究结果表明,探针芘的I1/I3峰值随乳化剂用量,D4用量,KPS用量不同发生相应的变化,随单体转化率的增加而增大.当乳化剂用量、D4用量、KPS与总单体的质量比依次为2 g、8 g、0.7%时,得到的乳液具有优良的综合性能.聚合反应过程中,当种子乳胶粒转变为核壳乳胶粒时,芘的I1/I3峰值仍呈现出明显的转变,说明有机硅-丙烯酸酯核壳乳液具有互穿聚合物网络结构.因此,荧光探针可用于研究有机硅-丙烯酸酯核壳乳液聚合反应进程.     

Structured latex particles as impact modifiers for poly(styrene–co-acrylonitrile) blends

This work was focused on the influence of the morphology of composite natural rubber (NR)-based particles on the toughness of poly(styrene–co-acrylonitrile) (PSAN) blends. In order to be suitable for the reinforcement of PSAN blends, the NR-based particles were coated with a shell of crosslinked poly(methylmethacrylate) (PMMA). Furthermore, polystyrene (PS) subinclusions were introduced into the NR rubber core. PSAN blends were prepared by adding the wet latex directly into a twin screw-extruder. This new method allowed even tacky pure rubber particles to be dispersed as shown by transmission electron photomicrographs which confirmed the integrity of the soft particles after mixing. Solid NR particles or NR-based latex particles containing rigid PS subinclusions and no hard shell did not offer any impact improvement to PSAN. Only NR-based core–shell particles containing at least 25% PMMA in the shell toughened the brittle matrix. Prevulcanized NR-based latex particles which do not cavitate easily were less effective. Core–shell particles containing PS subinclusions within a natural rubber core allowed more effective use of the rubber phase. From the fracture surface morphology the failure mechanisms of PSAN blends containing the different composite NR particles could be deduced. Monodisperse poly(n-butylacrylate)-based core–shell particles were too small to toughen PSAN. However, a similar dependence of the fracture mechanisms on the morphology of the incorporated toughening agent could be established by scanning electron microscopy.     

Experimental study of short- and long-time diffusion regimes of spherical particles in carboxymethylcellulose solutions

The behavior of latex particles in carboxymethylcellulose (CMC) solutions with different probe radii and solution concentrations is investigated. The diffusion processes of probe particles are studied by means of dynamic light scattering experiments. The existence of two diffusion regimes (a short-time regime and a long-time regime) is established for concentrations higher than the threshold value which changes for each probe dimension. Information is also provided about the relative weight of the probe population undergoing the two diffusion processes. This experimental evidence is discussed in connection with the existence of a defined volume (cage) in which short-time diffusion is the characteristic motion, while the long-time diffusion process dominates the motion over all the cages. Furthermore, there was found to be some deviation from Stokes-Einstein behavior.     

MMA/BA种子半连续乳液聚合成核过程及机理研究

引入一种不溶于水的染色剂(BL-S)作示踪剂,研究甲基丙烯酸甲酯(MMA)/丙烯酸丁酯(BA)种子半连续乳液聚合中各变量对成核过程及成核机理的影响,运用最终乳胶粒中染色剂的含量(Pdye)、最终乳胶粒子数(Npc)、胶束成核和均相成核所形成的粒子数目(Nm和Nh)等参数对聚合过程中的成核情况进行定量分析.结果发现,当引发剂浓度[I]增大时,Pdye、Nh、Nm和Nh/Npc均随之增大,同时Nm/Npc相应地减小,且Nm/Npc=-0.0262[I]+0.8833,表明均相成核随[I]的增大而增加,但胶束成核的比例减小.乳化剂浓度[E]在不同范围内对成核机理的影响不同,在[E]=0.7216×10-2mol.L-1时,体系中胶束成核和均相成核比例相等,各为50%;当[E]>0.7216×10-2mol.L-1时,随[E]增大,成核时间t1,2逐渐缩短,Nm/Npc增加,Nh/Npc减小,胶束成核所占比例大于均相成核,胶束成核逐渐上升为主要成核方式;反之当[E]<0.7216×10-2mol.L-1时,体系中胶束成核所占比例小于均相成核,均相成核为主要成核方式.当MMA的摩尔分率fMMA由0增至0.6时,Nm/Npc从80.93%下降至50%,体系中以胶束成核为主,均相成核为辅;当fMMA由0.6增至1时,Nm/Npc已降至40%,而Nh/Npc增至60%,体系中已转变成均相成核为主,胶束成核为辅.在常规和种子半连续乳液聚合中,t1,2分别为12min和6min,而Pdye变化较小,表明聚合方式只影响粒子的形成过程和成核时间的长短,对成核方式影响甚微.     

Probing silver nanoparticles during catalytic H2 evolution

Employing silver nanoparticles from a recently developed synthesis [Evanoff, D. D.; Chumanov, G. J. Phys. Chem. B 2004, 108, 13948] and a well-studied probe molecule, p-aminothiophenol, we follow changes at the surface of the particles during the conditioning and eventually the catalytic production of hydrogen from water using strongly reducing radicals. Injection of electrons into the particles causes pronounced variations in the intensity of the surface enhanced Raman scattering (SERS) spectrum of the probe molecule. These spectral changes are caused by changes in the Fermi-level energy that are in turn caused by changes in the silver ion concentrations or in the pH, or by changes in electron density in the particle. This correlation highlights the effect of the chemical potential on the SERS enhancement at the end of the particles synthesis. The intensity of the SERS spectra increases in the presence of the silver ions when excitation at 514 nm is utilized. When the Ag(+) ions in the colloidal suspension are completely reduced by the radicals and the particles operate in the catalytic mode, the SERS spectrum is too weak to record, but it can reversibly be recovered upon the addition of Ag(+). The effect of pH on the SERS intensity is similar in nature to that of the silver ions but is complicated by the pKa of the aminothiol and the point of zero charge (pzc) of the particles. It is hypothesized that as the particles cross the pzc (around neutral pH) the electrostatic interaction between the protonated amine headgroup of the probe and the positively charged surface increases the density of probe molecules in the perpendicular orientation at the expense of a competing species. This conversion results in enhanced SERS signals and is observable during the preconditioning stage of the particles. Indeed, adsorption isotherms of the probe indicate the presence of two species. In analogous previous observations these two species have been attributed to perpendicular and flat adsorption orientations of the deprotonated probe molecule relative to the particle surface. However, preliminary density functional calculations on relevant prototypes raise the possibility that the two species may be the probe molecule and a cationic form produced by charge transfer in the ground state from the chemisorbed probe to the metal. These two forms of the probe have differing electronic structures and vibrational frequencies, with perhaps differing orientations relative to the surface. Whichever is the correct interpretation, a neutral molecule in a flat orientation or a radical cation, this species is easier to replace than the other in competitive adsorption by ethanethiol.     

The role of surfactant in controlling particle size and stability in the miniemulsion polymerization of polymeric nanocapsules

The influence of surfactant concentration on particle size and stability of nanocapsules with liquid cores, synthesized by an in situ miniemulsion polymerization process, was investigated. Although the role of surfactant in the synthesis of particles in the nanometer range has frequently been documented, the transition to structured particles, which almost consist of a 1:1 weight ratio of encapsulated liquid hydrophobe to polymeric shell, has not received much attention. Capillary hydrodynamic fractionation (CHDF) analyses were used to evaluate particle size. Results were subsequently used to stoichiometrically calculate the area which is occupied per surfactant molecule on the particle surface. These results were compared with “classical” miniemulsion data, i.e. data generated from the synthesis of polymeric latexes in the presence of a hydrophobe, but at a much lower hydrophobe:monomer ratio as was used here. The surface coverage per surfactant molecule could be related to the surface tension of the latex, thus providing a relationship between particle size and stability. CHDF was furthermore used to investigate particle size after grafting of a secondary PMMA shell. Data obtained from CHDF experiments were in all cases confirmed by TEM analysis of the synthesized particles. To conclude, the synthesis of nanocapsules with liquid cores could be successfully scaled-up, with retention of all the characteristics of the final latex.     

Structural changes of latex particles of ethyl acrylate—methacrylic acid copolymers during neutralization in the presence of methanol

Using a combination of static light scattering, potentiometry, and viscometry, structural changes of latex particles of ethyl acrylate—methacrylic acid (20–70 wt%) copolymers during neutralization in the presence of methanol were investigated. It was found that the latex particles disintegrate into small subparticles, swell, and at a high content of methacrylic acid units dissolve. Disintegration of latex particles is explained by considering the particles as agglomerates formed during polymerization by coalescence of smaller particles.     

TEMPERATURE-SENSITIVE LATEX PARTICLES FOR IMMOBILIZATION OF α-AMYLASE

Poly(methyl methacrylate-co-N-isopropylacrylamide-co-N-acryloxysuccin-imide) (poly(MMA/NIPAAni/NAS)) latex particles were prepared by emulsifier-free emulsion polymerization. The latex particles could reversibly flocculate and disperse when the solution temperature was kept above and below the characteristic lower critical solution temperature (LCST). The LCST of the latex dispersion could be adjusted lower or higher by increasing or decreasing, respectively, the ionic strength of the solution.α-Amylase was immobilized onto the latex particles by reacting with the succinimide groups on NAS. It was demonstrated that the ct-amylase immobilized latex particles could be separated by thermoflocculation. After 5 repeated thermocycles, 83 % of the latex particles could be recovered, and the apparent enzymatic activity could be retained as high as 78percnt;.     

Behavior of pH-sensitive core shell particles at the air-water interface

In this article, the adsorption of latex core-responsive polymer-shell nanoparticles at the air-water interface is investigated using a Langmuir trough. Phase transition isotherms are used to explore their responsive behavior at the interface as a function of changes in the pH of the subphase. By adjusting the pH of the water prior to particle deposition, we probe the effect of the stabilizing polymer wetting by the water subphase on the stability of these particles at the air-water interface. In addition, by initially compressing a stable film of adsorbed particles and then subsequently changing the pH of the subphase we study desorption of these particles into the water phase.     

Synthesis and surface properties of PDMS–acrylate emulsion with gemini surfactant as co-emulsifier

Composite latex particles of acrylate and polydimethylsiloxane (PDMS) with high PDMS content was prepared by emulsion copolymerization and characterized by particle size analyzer, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR). With gemini surfactant as the co-emulsifier in the system, the PDMS content in the system reached 50%, which was far higher than the other reported values. Through the characterization of the particle size analyzer, the particle size augmented with the increase of the amount of PDMS, which could be said that the polysiloxane had participated into the reaction and had been introduced into the colloid particle. The results of FTIR indicated that almost all the monomer had been exhausted in the reaction because there was no C=C and D₄ characteristic peaks in the spectrum. Besides the surface properties also were measured by surface tension analysis, water absorption, and the static contact angle, it could be found that with the increase of polysiloxane content, the excellent properties acquired by PDMS were clearly revealed by the findings, such as the decrease of surface tension and water absorption, and the increase of static contact angle. All the measurements were consistent with the conclusion that the composite latex particles of polysiloxane and acrylate with high siloxane content had been prepared successfully.     

Interaction between poly(styrene-acrylic acid) latex nanoparticles and zinc oxide surfaces

Latex particles with an average diameter of 70 nm, functionalized at the surface with carboxylic groups, are chemically coated by layer-by-layer deposition onto a spherical probe attached on an atomic force microscope cantilever. The forces between poly(styrene-acrylic acid) latex nanoparticles and differently terminated zinc oxide surfaces are studied by a homemade atomic force microscope based apparatus. The results confirmed a preferred adhesion of the latex particles to zinc-terminated ZnO faces, 0001, compared to oxygen-terminated and apolar faces. The method proposed allows the measurement of the interaction between nanometric particles and planar surfaces, which may be of interest for different applications in surface and colloid sciences.     

Tracer diffusion properties of core–shell latex films studied by photoinduced grating relaxation

This article reports the application of the Photo‐Induced Grating Relaxation technique (also known as Forced Rayleigh Scattering) to investigate the dynamics of films prepared from structured core–shell latex particles via the transport property of the photochromic tracer molecule Aberchrome 540®. The core–shell particles were prepared with a fluoropolymer core (immiscible and impenetrable to the tracer) and a poly(butyl methacrylate) shell. The incompletely dried films (with residual water) manifest their spatial heterogeneity via non‐Fickian behavior (spatial scale‐ dependent apparent diffusion coefficient). The diffusion data was interpreted using the two‐state diffusion model, previously developed to describe the tracer diffusion in latex films without any core–shell structure. In contrast to dry latex films made from homogeneous particles, where one observes Fickian diffusion indicative of a homogeneous polymer film, we find that the lattice of fluoropolymer cores leads to a length scale dependent diffusion coefficient for the tracer. This effect can be interpreted as microscopic evidence for a strain hardening effect due to the presence of a hardened layer of matrix polymer (= shell) surrounding the core, which act as nanofillers. This strain hardening effect could be quantified within the two‐state diffusion model in terms of tracer diffusion coefficients and root mean squared displacements. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2823–2834, 2007     

Preparation of Bowl-Like Polymer Particles via Multi-Step Emulsion Polymerization and Alkali Post-Treatment

Summary : Monodisperse P(BA-MMA-MAA-EGDMA)/P(St-MAA-DVB) core/shell latex particles were first synthesized by a four-step emulsion polymerization, and a new kind of latex particles with “bowl-like” morphology were obtained by post-treating the resultant core/shell particles under alkali condition. Results indicated that the feeding rates of the monomer mixture and initiator aqueous solution were the key parameters to obtain monodisperse core/shell latex particles in the emulsion polymerization process, and the latex particles with “bowl-like” morphology could be generated only when the treatment temperature was equal or higher than 70 °C.     

Phenanthrene-fused boron-dipyrromethenes as bright long-wavelength fluorophores

A new class of boron-dipyrromethene (BDP or BODIPY) dyes was obtained by phenanthrene fusion to the beta-pyrrole positions, absorbing in the wavelength range of important laser sources. Despite a 'propeller-like' distorted structure in the crystalline state, the chromophore absorbs (log epsilon > or = 5) and fluoresces (Phif > or = 0.8) strongly and can be easily turned into a fluorescence light-up probe. Incorporation into latex beads produces bright and photostable single-dye and F?rster Resonance Energy Transfer (FRET) particles for microscopy applications.     

Viscosity measurements of water at high temperatures and pressures using dynamic light scattering

The application of dynamic light scattering to measure viscosity of water at high temperatures and pressures is demonstrated. Viscosity was obtained from the translational diffusion coefficient of probe particles dispersed in the medium by the Einstein-Stokes relationship. Measurements were carried out with polystyrene latex, colloidal silica, and colloidal gold. Under a constant pressure of 25 MPa, good agreement was found between the measured and calculated viscosities up to 275 degrees C with the polystyrene latex, 200 degrees C with the colloidal silica, and 297 degrees C with the colloidal gold. It was found that failure of the measurements at high temperatures is ascribed to change in either the dispersion stability or chemical stability of the probe particles. The present results indicate that the technique could also be used for other supercritical fluids having high critical temperature and pressure, such as methanol (T(c) = 239.4 degrees C, P(c) = 8.1 MPa) and ethanol (T(c) = 243.1 degrees C, P(c) = 6.4 MPa).     

Morphology of peroxide-prevulcanised natural rubber latex: effect of reaction time and deproteinisation

A non-uniform mesh structure, i.e. a dense network near the surface of peroxide-prevulcanised natural rubber latex particles, was observed under transmission electron microscopy. In the initial period of prevulcanisation, the swelling ratio of the latex sheet decreased with longer reaction time while an increase in crosslink density of rubber particles containing polystyrene, prepared using the phase transfer/bulk polymerisation process, was noticed. The modulus of the rubber sheet increased up to maximum crosslinking and thereafter decreased. After removal of proteins from the latex membrane layer, derived from protein-lipid originally existing at the rubber particle surface, could not be detected. The absence of proteins, which act as free radical scavengers, resulted in a rapid diffusion of alkoxy radicals into the rubber phase of deproteinised latex and, therefore, a uniform crosslink distribution inside each particle was obtained.     

Adsorption of different amphiphilic molecules onto polystyrene latices

In order to know the influence of the surface characteristics and the chain properties on the adsorption of amphiphilic molecules onto polystyrene latex, a set of experiments to study the adsorption of ionic surfactants, nonionic surfactants and an amphiphilic synthetic peptide on different latex dispersions was performed. The adsorbed amount versus the equilibrium surfactant concentration was determined. The main adsorption mechanism was the hydrophobic attraction between the nonpolar tail of the molecule and the hydrophobic regions of the latex surface. This attraction overcame the electrostatic repulsion between chains and latex surface with identical charge sign. However, the electrostatic interactions chain-surface and chain-chain also played a role. General patterns for the adsorption of ionic chains on charged latex surfaces could be established. Regarding the shape, the isotherms presented different plateaus corresponding to electrostatic effects and conformational changes. The surfactant size also affects the adsorption results: the higher the hydrophilic moiety in the surfactant molecule the lower the adsorbed amount.     

Preparation and characterization of fluorinated acrylate copolymer latexes by miniemulsion polymerization under microwave irradiation

Fluoroacrylate copolymer miniemulsion was prepared by miniemulsion polymerization under microwave irradiation. The composition of the copolymer was determined by FTIR, DSC, ¹H NMR and ¹⁹F NMR. The morphology, size, and size distribution of the latex particles as well as changes in the size during polymerization were characterized by TEM and photon correlation spectroscopy (PCS). The effects of kinetic parameters on the polymerization were evaluated. The particle size of latex underwent almost no change during microwave irradiation polymerization. The diameters of latex particles prepared by microwave irradiation were smaller and more monodispersed than those prepared by conventional heating and the latex had good centrifugal stability. Polymerization under microwave irradiation had a higher reaction rate and higher conversion than traditional heating. By using 10 wt% fluoromonomer, the surface energy of the latex film could be reduced from 27.24 mJ/m² (latex film of fluorine-free) to 17.59 mJ/m² and the decomposition temperature increased by 25 °C.