Customized morphologies of self-condensed multisegment polymer nanowires

The direct formation of multisegment nanowires consisting of polymer domains by ion beam irradiation is investigated. Cross-linking reactions in the ion tracks result in localized gelation, giving isolated nanowires on substrates. It is demonstrated that the morphology of the final nanostructure is customized by appropriate selection of the ion fluence, combination of polymers, and the solvent employed for development. Octopus-like nanostructures consisting of a tangled hydrophilic polymer core and splayed hydrophobic polymer segments are successfully produced as an example of the process. The present technique provides universal feasibility for the formation of nanostructures based on "any" polymer materials in which radiation induces cross-linking reactions.     

AFM approach toward understanding morphologies in toughened thermosetting matrices

Atomic force microscopy, AFM, has been used for determining the microstructure of thermosetting matrices toughened by incorporation of core-shell particles and high-performance thermoplastics. A variety of systems has been considered in this work: one group is based on diglycidyl ether of bisphenol-A (DGEBA) epoxy matrix, and the other group is based on bisphenol-A dicyanate (DCBA) matrix. The studied epoxy systems were: DGEBA cured with an aromatic hardener, diamino diphenyl sulfone (DDS), and modified with polymethylmethacrylate (PMMA), or cured with a cycloaliphatic hardener, diamino dimethyl cyclohexylmethane (3DCM), and modified with core-shell particles of polystyrene-co-butylacrylate (PS-co-Bu). The DCBA-based matrices have been modified with polysulfone of bisphenol-A (PSU) or with polyetherimide (PEI). The influence of the modifiers and the curing conditions on the generated morphologies is reported as analysed by AFM in contact and tapping modes.     

Compatibilizer-aided toughening in polymer blends consisting of brittle polymer particles dispersed in a ductile polymer matrix

Blending brittle polymer particles in a ductile polymer matrix is a new way to obtain toughened plastics. Although the nylon-6/poly(acrylonitrile-co-styrene) (SAN) system is a ductile/brittle combination, the blend does not result in a toughened plastic. We have investigated the effect of adding a small amount of a third component, poly(styrene-co-maleic anhydride) (SMA), to the nylon/SAN system. SMA significantly improves the tensile and impact strength of the blend. Morphological observations indicate a finer dispersion of the SAN particles when SMA is present in the blend. The improved dispersion is attributed to the formation of nylon-SMA graft copolymer, and infrared analysis supports this supposition. That is, a “compatibilizer” seems to be produced during melt mixing of the ternary system. The role that the compatibilizer plays in improving the stress transfer in the two-phase system and its potential to induce a brittle-ductile transition of the glassy SAN particles are considered to explain the toughening mechanism.     

Epoxide polymer matrices for high-strength and heat-resistant composites

Results of an investigation of the strength and thermophysical characteristics of epoxide materials based on epoxidated amines, resorcin, para-oxybenzoic and isocyanuric acids, and cycloaliphatic resins, i.e., vinylcyclohexene dioxide, in comparison with diane resin ED-20 and epoxidated phenol-formaldehyde novolak are presented. It is shown that the investigated epoxides contain 29–45% of epoxy groups, and cured polymers on their basis have maximal values of physicomechanical characteristics for the materials of this class described in the literature.     

Controllable preparation of high-yield magnetic polymer latex

In order to overcome the low conversion and complex post-treatment, four different polymerization procedures were adopted to prepare the magnetic polymer latexes. The results clearly show that the strategy using magnetic emulsion template-dosage is the most effective and feasible. Based on the optimized procedure, various factors including the type of initiators such as oil soluble initiator, water soluble initiator, redox initiator system, crosslinking agent, functional monomers etc. were systematically studied. Magnetic polymer latex with high monomer conversion of 83% and high magnet content of 31.8% was successfully obtained. Besides, core-shell structured magnetic polymer latex with good film forming property was also prepared, which is promising for potential applications such as magnetic coatings and modification of cementitious materials with controlled polymer location.     

Interdiffusion of polymer chains during latex coating

The fluorescence method was used to study the interdiffusion of polymer chains during annealing of latex above its glass transition temperature (T_g). The latex film was prepared from mixture of naphthalene (N) (donor) and pyrene (P) (acceptor)-labeled poly(methyl methacrylate) (PMMA) latex particles. Heptane was used as the coalescent agent. A steady-state fluorescence technique was employed to measure the amount of direct nonradiative energy transfer from N to P during the interdiffusion of polymer chains across the particle–particle junction. Various latex films with different latex contents were used to measure the percentage critical occupation for the reliable steady-state fluorescence measurements. Diffusions activation energies in these latex films were measured and found to be around 30 kcal/mol, which was attributed to the backbone motion of PMMA chains.     

Elastification of heat-resistant epoxy polymer matrices

The behavior of fracture toughness parameters in comparison with other physicomechanical parameters of failure was examined with the aim to evaluate the efficiency of modification (elastification) of heat-resistant binders, with four epoxy-amine compounds taken as examples.     

Alignment of nanoparticles in polymer matrices

This paper briefly summarizes the state of the art in the field of designing composites containing semiconductor nanoparticles distributed in a polymer matrix. Special attention is focused on (i) nanocomposites based on block copolymers and (ii) LC polymer matrices capable of controlling the localization and alignment of nanoparticles.     

Thiol-terminated hydroxy-functional polymer as a transtab toward polymer latex particles

Hydroxy-functional macrodisulfides have been synthesized by atom transfer radical polymerization of 2-hydroxyethyl methacrylate or 2-hydroxypropyl methacrylate in 2-propanol. Mean degrees of polymerization of the polymer chains beside the disulfide were fixed at 30, 60, and 90; since ATRP has reasonably good living character, the molecular weight distribution is relatively narrow. Furthermore, the macrodisulfides were reduced to synthesize corresponding thiol-terminated polymers with relatively narrow molecular weight distributions. ¹H nuclear magnetic resonance and gel permeation chromatography were used to characterize the macrodisulfides and thiol-terminated polymers in terms of their chemical structure, molecular weight, and polydispersity, respectively. Dispersion polymerizations of styrene using the thiol-terminated hydroxy-functional polymers as a transtab (chain transfer agent + colloidal stabilizer) in ethanol resulted in colloidally stable submicrometer-sized polystyrene latex particles. Scanning electron microscopy, Fourier transform infrared spectroscopy, and elemental microanalysis were used to characterize the particles in terms of their morphologies, particle sizes and their distributions, and chemical compositions.     

Effect of polymer entanglement on the toughening of double network hydrogels

The mechanical strength of double network (DN) gels consisting of highly cross-linked poly(2-acrylamido-2-methylpropanesulfonic acid) (PAMPS) as the first component and linear polyacrylamide (PAAm) as the second component has been investigated by varying the molecular weight of the second polymer PAAm, M(w). The experimental results reveal that, for toughening of the DN gels, (1) M(w) is one of the dominant parameters; (2) there is a critical value of M(w) = 10(6) for a remarkable enhancement; (3) the fracture energy of DN gels with a M(w) larger than 10(6) reaches a value as high as 10(3) J/m(2). By plotting the strength of DN gels (fracture stress sigma and fracture energy G) against a characteristic parameter of c[eta], where c is the average concentration of PAAm in the DN gels and [eta] is the intrinsic viscosity of PAAm, it is found that the dramatic increase in the mechanical strength of the DN gels occurs above the region where linear PAAm chains are entangled with each other. Thus, we conclude that the entanglement between the second component PAAm plays an important role of the toughening mechanism of DN gels. This result supports the heterogeneous model, which predicts the presence of "voids" of the first network PAMPS with a size much larger than the radius of the second polymer PAAm.     

Reduced viscosity polymer matrices for microchip electrophoresis of double-stranded DNA

On a polymethylmethacrylate (PMMA) microchip, double-stranded DNA fragments with a wide size range from 50 bp to 20 kbp were separated by two polymer solutions. One was a hydroxypropylmethylcellulose-4000 (HPMC-4000) solution of 1.3% (w/v) to separate fragments below 590 bp, and another was a mixed four molecular weight poly(ethylene oxide) solution at a total concentration of 0.1% to separate fragments above 520 bp. The widths at half height (wh) of the fragments had a good relationship with their migration times (tR) in both polymer solutions. Such a relationship was suitable for obtaining the wh values of unresolved peaks, calculating the resolution of two adjacent fragments, and optimizing microchip separation matrices. Based on the relativity, a low viscosity medium containing 2% HPMC-50 and 8% glucose was optimized for high-performance separation of a phiX174 HaeIII restriction fragment digest.     

Site percolation model for latex film formation in soft polymer matrix

The UV-visible (UVV) technique was used to monitor latex film formation in a soft polymer matrix. Various film samples were prepared by increasing the amount of poly(methyl methacrylate) (PMMA) particles in a poly(isobutylene) (PIB) matrix. These samples were then annealed above the glass transition temperature to promote latex film formation. Transmitted photon intensities, Itr, were measured for each film. It is observed that Itr decrease as the latex content is increased, which was explained by the increase in scattered light intensity, Isc. The drastic increase in Isc above a certain latex content is attributed to the site percolation of latex particles in the PIB matrix. The percolation threshold and the critical exponent were measured and found to be 0.3 and 0.4, respectively. The increase in Itr by annealing of film samples above Tg was explained with the void closure process below 0.8 occupation probability. When the film is occupied completely with the latex particles, interdiffusion of polymer chains was observed. Viscous flow and chain diffusion activation energies were determined and found to be 8 and 51 kcal/mol, respectively.     

Oxciplex emission of naphthalenes in polymer matrices

In this communication we report the observation of oxciplex emission, (T₁ + O₂(¹Δ)) → (S₀ + O₂(³Σ)) + hν from naphthalene and octafluoronaphthalene in polystyrene fluffs. We detect this emission as an oxygen induced luminescence burst upon admission of oxygen to a phosphorescing sample. The assignment is based on a mirror image relationship of the emission to the absorption, and energetic, kinetic, and intensity considerations.     

Calotropis gigantea fibers: A potential reinforcement for polymer matrices

Natural fibers are identified as one of the effective alternatives for reinforcing the polymer matrices on account of their sustainability and renewable characteristics by replacing the synthetic fibers. This study is intended to apprehend the properties of the fibers derived from the stem of Calotropis gigantea plant. The functional groups of biopolymers were recognized by Fourier transform infrared spectrum. The crystalline nature of the cellulose that represents the mechanical strength and integrity of the fibers was found from the X-ray diffraction, whereas the thermal behavior was studied by thermogravimetric analysis. Scanning electron microscope was used to study the morphology of the fibers. The results of these analytical studies have shown that the crystallinity index of the fibers was 56.08% and the fibers were able to withstand a temperature of about 220°C proving that the fibers can be used as effective reinforcements for polymer matrices similar to the commonly used bio-fibers.     

Supermicron polymer particles with core‐shell type morphologies

Polymer particles with controlled morphologies and having diameters from about 1–20 μ can be prepared using a new suspension polymerization‐based procedure. In contrast to existing procedures using emulsion polymerization, this process allows efficient preparation of supermicron particles that can be easily isolated as a dry powder. Control of the particle morphology is obtained by manipulating the monomer conversion at the beginning of the second stage of the reaction (when the second monomer is added). Two systems are studied. The first system uses styrene added to a partially polymerized MMA host particle, whereas the second system uses styrene added to a partially polymerized 45 wt % styrene to 55 wt % butyl methacrylate host particle. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 345–351, 2000     

Phase separation in polystyrene latex interpenetrating polymer networks

Polystyrene/polystyrene latex interpenetrating polymer networks (IPNs) were prepared by seeded emulsion polymerzation of styrene–divinylbenzene mixtures in crosslinked monodisperse polystyrene seed latexes. The resulting latexes comprised uniform nonspherical particles, which were formed by separation of the second-stage monomer from the crosslinked seed network during swelling and polymerization. The kinetics of phase separation were investigated by examining the changes in particle morphology using optical microscopy, which revealed that the phase separation was induced by the relaxation of the polymer chains before polymerization began and was enhanced by increased conversion. The thermodynamics of phase separation were investigated by analysis of the free-energy changes during swelling and polymerization, and the phase separation was described by a nucleation-and-growth mechanism. The results of this study have been applied to the design and synthesis of a series of uniform nonspherical particles of different morphology.     

An overview of polymer latex film formation and properties

The literature on polymer latex film formation has grown enormously in recent times--driven by the need to find alternatives for solvent-based systems with their adverse environmental impacts. Although greater insight has been shown by the use of modern instrumental techniques such as small angle neutron scattering, direct non-radiative energy transfer and atomic force microscopy, the actual mechanisms involved in deforming spherical particles into void-free films are still the subject of controversy and debate. Surfactant-free homopolymer model colloid latices, favoured in academic studies, together with latices containing surfactants whose redistribution can influence film properties, and the more complex copolymer, blended, core-shell and pigmented systems needed to satisfy a full range of film properties are all considered.     

An overview of the preparation and use of emulsion polymer particles for the toughening of plastics

The rationale for rubber-toughening of brittle plastics is presented together with a discussion of the advantages of using pre-formed toughening particles prepared by emulsion polymerisation. Rubber-toughened acrylic and epoxy resin materials are used to illustrate the preparation and use of pre-formed emulsion polymer particles for toughening. For both types of material, tensile properties are controlled by the volume fraction of rubbery polymer, essentially independent of the size and morphology of the toughening particles. However, under the additional constraint ahead of propagating cracks, toughening particle size and morphology are shown to be important, and to have different effects in the two types of material due to inherent differences in the mechanism of yielding of the matrix polymer.     

Characterization of advanced morphologies in polymer dispersions by AUC and HDC

Polymer latex particles are used in a wide spectrum of applications that are directly influenced by the surface characteristics as well as particle size and size distribution of these polymer particles. Accurate analysis of such characteristics is required to efficiently control the behavior of such particles. Analytical ultracentrifugation (AUC) and hydrodynamic chromatography (HDC) are the particle characterization methods of high relevance owing to their statistical ability, and the combination of these two techniques also allows to generate information about the surface morphology of the particles. Such a comparison is facilitated owing to different principles of particle characterization in these two methods. AUC relies on the density difference between the particles and the suspending medium to correlate to the particle size, whereas HDC is based on the measurement of hydrodynamic diameter by UV absorption. When particles functionalized by a thin layer of hydrophilic polymer are analyzed, these two methods by the virtues of their characterization principles allow to detect the presence of such a functionalizing polymer layer on the surface. Subsequently, these methods also provide accurate estimation of the thickness of such a layer. The comparisons can also be carried out as a function of time or amount of surface functionalization to tune the properties according to the requirement. In the case where a thick polymer layer is present in the surface of the particles, the comparisons are more qualitative in nature owing to the bridging and aggregation of the particles especially noticed in HDC. However, even in such a case, the combination of these methods is the only way to characterize the special morphology of the particles.