Composition effects on the mechanical properties of microemulsion-made core/shell polymers

The synthesis of hard-core/soft-shell and soft-core/hard-shell polymers by a two-stage semi-continuous microemulsion polymerization process is reported here. In the first stage, high-solid polymer seeds (>30 wt%) of slightly crosslinked polystyrene or poly(butyl acrylate) were obtained; then, the other monomer was added semi-continuously to form the shell. The effects on the mechanical properties (Young's modulus, ultimate properties, hardness and impact energy) of the ratio of rigid-to-soft and soft-to-rigid polymers were studied. It was found that the material becomes stiffer and presents a lower elongation at break as the amount of the rigid polymer increases. The mechanical properties also depend on the location of the hard and soft polymers. Experimental mechanical properties were compared with the predictions of the Kerner and the equivalent box models. Comparison with the predictions of the Kerner model suggests that phase inversion occurred in the case of hard-core/soft-shell materials. Phase inversion was corroborated by transmission electron microscopy. The thermodynamically preferred morphology, according to theory, is that of soft-core/hard-shell, regardless of the order of addition of monomers. Experimental data follow closely the predictions of the equivalent box model only for soft-core/hard-shell polymers.     

Fabrication and evaluation of one-component core/shell structured latex adhesives containing poly(styrene) cores and poly(acrylate) shells

A one-component, non-crosslinked, and ambient temperature-cured core/shell (CS) structured waterborne poly(styrene-acrylate) (PStA) latex adhesive was prepared by two-stage seeded emulsion polymerization. The first-stage polymers were non-polar styrene-based copolymers with T_gs ranging from 86 °C to 41 °C and the second-stage polymer was polar acrylate-based copolymer having a T_g of 8 °C. The effect of latex particle morphology and film microstructure on the CS structured PStA latex adhesive was examined by evaluating film thermal properties and mechanical performance. The results have shown that incorporation of soft shell polymers into hard core polymers can improve adhesive properties without compromising film-forming ability. Wood adhesive performances of latex adhesive were measured by shear strength and boiling water resistance of glued wood blocks. The utilization performances of the one-component CS structured PStA latex adhesive were found to largely depend on CS morphological character and the interfacial compatibility between various polymeric constituents, demonstrating that fabrication of CS latex particles provides a promising approach to realize high-performance latex adhesives.     

Effect of the core/shell latex particle interphase on the mechanical behavior of rubber-toughened poly(methyl methacrylate)

Two- and three-layer composite latex particles were used to prepare rubber-toughened poly(methyl methacrylate) (RT-PMMA). The interfacial thicknesses of the multi-layered particles were varied by using different emulsion polymerization synthesis techniques. The resulting interphases were previously characterized by ¹³C nuclear magnetic resonance techniques. The poly(divinyl benzene)/poly(butyl acrylate) (PDVB/PBA) interphase thickness was found to be in the range of 5–7 nm. It was also found that the PBA/PMMA interphase thickness could be varied from 5 to 7 nm (batch addition of MMA) to 15 to 17 nm (interphase compatibilized with PMMA macromonomer). The interphase thickness was expected to play an important role in the mechanical behavior of PMMA. The effect of the interphase of two- and three-layer particles on the tensile and fracture behavior of PMMA composites was evaluated. The fracture surfaces were examined by scanning electron microscopy. The two-layer PBA/PMMA particles with a thicker interphase (15–17 nm) exhibited higher K_IC values with the PMMA composites compared with PBA/PMMA particles with a thinner interphase (5–7 nm). The three-layer particles were found to be more effective in toughening PMMA compared with the two-layer particles. The differences in toughening behavior are speculated to arise from the morphological effects caused by a thicker interphase, which in turn results in better coverage by the PMMA shell and a more uniform distribution of the toughening particles in the PMMA matrix. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65:581–593, 1997     

Effect of Ti particle size on mechanical and tribological properties of TiCN coatings prepared by reactive plasma spraying

Titanium carbonitride (TiCN) coatings were successfully fabricated by reactive plasma spraying (RPS) from agglomerated Ti-graphite feedstock. The effect of Ti particle size on the microstructure and phase composition of plasma sprayed TiCN coatings was investigated. The Vickers microhardness of coatings was measured by a Microhardness Test and the corresponding Weibull distribution were also analyzed. In addition, a pin-on-disk tribometer was employed to determine the trobological properties of coatings. Results show that all the coatings consist of TiC_xN_1−x (0 ≤ x ≤1) and minor Ti₂O phases, and the amount of Ti₂O increases with the increase of Ti particle size. The Weibull distribution of Vickers microhardness of all the coatings shows apparent scattering, while the coating sprayed with Ti particle size of 28 µm exhibits a relatively even distribution. Compared with the coating sprayed with Ti particle size of 14 µm or 48 µm, the coating sprayed with Ti particle size of 28 µm exhibits improved mechanical and tribological properties, which are attributed to the high microhardness and strong bonding strength.     

The effect of particle size and composition on the performance of styrene/butyl acrylate miniemulsion-based PSAs

Styrene/butyl acrylate batch miniemulsion copolymerizations were performed in a 1.2 L stainless steel reactor. Conversions were monitored off-line using gravimetry and in-line using ATR-FTIR spectroscopy. The final latexes were coated on a polyethylene terephthalate carrier and dried at room temperature for 2 days. Their performance as pressure-sensitive adhesives (PSAs) was evaluated by measuring their tackiness, peel strength and shear strength. By using a constrained mixture design, the influence of particle size and copolymer composition was investigated. Particle size was found to be the most influential factor for both tack and peel strength models. Tack showed a concave upward trend whereas peel strength decreased with increasing particle size. Shear strength decreased with increasing particle size but was also significantly influenced by copolymer composition. The final forms of the models allowed 3D response surfaces to be built and an optimal adhesive performance region (highest combined tack, peel strength and shear strength) was located near the smallest particle diameter investigated with the highest styrene composition. The positive effect of smaller particles on every adhesive property relates to the tighter packing provided by smaller particles during the drying process, thus increasing the area of contact between the adhesive and the substrate.     

Synthesis of polystyrene/poly(butyl acrylate) core–shell latex and its surface morphology

A polystyrene (PS)/poly(butyl acrylate) (PBA) composite emulsion was produced by seeded emulsion polymerization of butyl acrylate (BA) with PS seed particles which were prepared by emulsifier‐free polymerization of styrene with potassium persulfate (KPS) under a nitrogen atmosphere at 70°C for 24 h with stirring at 60 rpm and swelled with the BA monomer in an ethanol/water medium. The structure of the PS/PBA composite particles was confirmed by the presence of the characteristic absorption band attributed to PS and PBA from FTIR spectra. The particles for pure PS and PS/PBA with a low content of the BA monomer were almost spherical and regular. As the BA monomer content was increased, the particle size of the PS/PBA composite particles became larger, and more golf ball‐like particles were produced. The surface morphology of the PS/PBA composite particles was investigated by AFM and SEM. The T_g's attributed to PS and PBA in the PS/PBA composite particles were found at 110 and ?49°C, respectively. The thermal degradation of the pure PS and PS/PBA composite particles occurred in one and two steps, respectively. With an increasing amount of PBA, the initial thermal decomposition temperature increased. On the contrary the residual weight at 450°C decreased with an increasing amount of PBA. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 595–601, 2003     

Synthesis of polystyrene/polythiophene core/shell nanoparticles by dual initiation

Polystyrene/polythiophene (PSt/PTh) core/shell nanoparticles were successfully synthesized via a one-pot Fe³⁺-catalyzed oxidative and soap-free emulsion polymerization process. A small amount of sodium styrene sulfonate (NaSS) was used to maintain the colloidal stability of the PSt/PTh nanoparticles. Hydrogen peroxide (H₂O₂) and a trace of iron chloride (FeCl₃) were used to carry out the free-radical polymerization of styrene and the oxidative polymerization of thiophene. The dual initiation characteristics of H₂O₂/FeCl₃ in the PSt/PTh core/shell nanoparticle formation were investigated by observing the time-evolution of the particle morphology. In addition, photoluminescent property, particle size distribution, core/shell morphology and the formation mechanism of the PSt/PTh nanoparticles were studied by spectrofluorophotometery, dynamic light scattering (DLS), in-situ IR, zeta-potential, and time-evolution field-emission scanning electron microscope (FE-SEM) analyses.     

Aligned core/shell electrospinning of poly(glycerol sebacate)/poly(l‐lactic acid) with tuneable structural and mechanical properties

In an earlier study, scaffolds of biodegradable poly(glycerol sebacate) (PGS)/poly(l ‐lactic acid) (PLLA) core/shell fibres had been fabricated using a core/shell electrospinning method, and the scaffolds were found to have mechanical properties similar to those of natural soft tissues, excellent cytocompatibility and slow degradation rate. In this paper, PGS/PLLA core/shell fibre mats with tuneable degrees of fibre alignment were fabricated using core/shell electrospinning with a rotating fibre collection mandrel. An increase in the rotational speed raised the degree of fibre alignment in the fibre mats. Single and cyclic tensile testing of the mats showed that an increase in the fibre alignment raised the modulus, resilience, ultimate tensile strength (UTS) and elongation up to a maximum at 1000 or 1500 rpm, but the resilience, UTS and elongation decreased when the rotational speed was further raised to 2000 rpm. Nonlinearly elastic biomaterials with a large range of mechanical properties were successfully fabricated using this method and the aligned fibre structure may be capable of guiding the growth of attached cells. © 2016 Society of Chemical Industry     

Synthesis of poly(acrylate-styrene)/poly(acrylate-styrene) core/shell latex and TOPEM-DSC characterization

Poly(acrylate-styrene)/poly(acrylate-styrene) core/shell latex particles were synthesized via seeded semi-continuous emulsion polymerization. Both core polymer (CP) and shell polymer (SP) were copolymerized by using three identical monomers of methyl methacrylate (MMA), butyl acrylate (BA) and styrene (St) with different composition ratios. The synthesized core/shell latex particle presents a phase separated state with the interfacial layer between CP and SP. In this study, the weight fractions and the corresponding thickness of this interfacial layer, CP and SP phase in the core/shell latex particle has be successfully calculated by using multi-frequency temperature-modulated differential scanning calorimetry (TOPEM-DSC). The results indicate that the interfacial layer thickness of the core/shell latex particle is determined by the core/shell structure, such as hard core/soft shell (defined as HC/SS) and soft core/hard shell (defined as SC/HS), the glass transition temperature (T_g) of the “hard” phase (correspondingly core or shell for HC/SS or SC/HS structure, respectively), and the existence of hydrophilic monomer during the copolymerization process such as acrylic acid. Meanwhile, the influence of film-formation-temperature on the microstructure of the latex films was systematically explored in this work.     

聚苯乙烯/聚丙烯酸丁酯自交联胶乳互穿聚合物网络的力学性能和动态力学性能

采用种子乳液聚合制备了聚苯乙烯/聚丙烯酸丁酯自交联胶乳互穿聚合物网络。拉伸实验结果表明,用双丙酮丙烯酰胺和己二酸二酰肼自交联的胶乳互穿聚合物网络(LIPN)比相应的胶乳互穿聚合物网络具有较高的拉伸强度,并且随双丙酮丙烯酰胺用量的增加,拉伸强度增加以及永久变形降低,但离子键和氢键交联的LIPN具有很高的扯断伸长率和很大的永久变形,这些说明用双丙酮丙烯酰胺和己二酸二酰肼界面共价键交联能很好地改善力学性能和抗蠕变性能。动态力学谱结果表明,用双丙酮丙烯酰胺和己二酸二酰肼交联的LIPN比相应的LIPN以及离子键和氢键交联的LIPN具有较好的组分相容性和阻尼性能。     

Effect of chain topology on the self-organization and the mechanical properties of poly(n-butyl acrylate)-b-polystyrene block copolymers

A series of well defined ABA, 3-arm star and bottle brush type copolymers, containing soft poly(n-butyl acrylate) (PBA) blocks and hard blocks of polystyrene (PS) were synthesized by atom transfer radical polymerization (ATRP). Small angle X-ray scattering was used to study the phase separation in these systems and dynamic mechanical analysis and tensile tests were performed to characterize their thermo-mechanical properties. The specific molecular architecture has a major effect on the copolymers self-organization and material properties. The linear ABA type copolymers showed micro phase separation and thermoplastic elastomer (TPE) behavior only at very high PS content. The change of molecular architecture from linear to 3-arm star type resulted in an improved phase separation at lower PS content and better thermoplastic elastomer properties. In contrast the specific brush type molecular architecture seems to prevent the micro phase separation of the PBA and PS components, resulting in amorphous bulk material with single glass transition temperature.     

Structural studies of poly(butyl acrylate) - poly(ethylene oxide) miktoarm star polymers

Structural behavior of miktoarm star polymers comprising poly(butyl acrylate) (PBA) and poly(ethylene oxide) (PEO) arms was studied by means of Differential Scanning Calorimetry (DSC), Wide Angle X-Ray Scattering (WAXS), Polarized Optical Microscopy (POM) and Fourier Transform Infrared Spectroscopy (FTIR) methods. The aim of this study was to correlate changes in the composition of the arms of the PBA/PEO miktoarm star polymers with their structures. As a consequence of increasing PBA content, the decrease in crystallinity of the studied PBA/PEO heteroarm star copolymers was observed. Regardless of the copolymer composition, fraction of oxyethylene units in the crystalline PEO phase was similar in all investigated systems. The POM images showed spherulitic morphology of the materials having low PBA content, while an increase in PBA arms fraction leads to the formation of less ordered structures. The analysis of FTIR vibrational spectrum indicates helical conformation of PEO chains in the crystalline phase. Isothermal crystallization studies carried out using the FTIR technique suggest the existence of isolated domains in the nanoscopic scale of investigated materials.     

Synthesis and characterization of poly(butyl acrylate)/silica and poly(butyl acrylate)/silica/poly(methyl methacrylate) composite particles

Poly(butyl acrylate)/poly(methyl methacrylate) (PBA/PMMA) core–shell particles embedded with nanometer‐sized silica particles were prepared by emulsion polymerization of butyl acrylate (BA) in the presence of silica particles preabsorbed with 2,2′‐azobis(2‐amidinopropane)dihydrochloride (AIBA) initiator and subsequent MMA emulsion polymerization in the presence of PBA/silica composite particles. The morphologies of the resulting PBA/silica and PBA/silica/PMMA composite particles were characterized, which showed that AIBA could be absorbed effectively onto silica particles when the pH of the dispersion medium was greater than the isoelectric potential point of silica. The critical amount of AIBA added to have stable dispersion of silica particles increased as the pH of the dispersion medium increased. PBA/silica composite particles prepared by in situ emulsion polymerization using silica preabsorbed with AIBA showed higher silica absorption efficiency than did the PBA/silica composite particles prepared by direct mixing of PBA latex and silica dispersion or by emulsion polymerization in which AIBA was added after the mixing of BA and silica. The PBA/silica composite particles exhibited a raspberrylike morphology, with silica particles “adhered” to the surfaces of the PBA particles, whereas the PBA/silica/PMMA composite latex particles exhibited a sandwich morphology, with silica particles mainly at the interface between the PBA core and the PMMA shell. Subsequently, the PBA/silica/PMMA composite latex obtained had a narrow particle size distribution and good dispersion stability. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 99: 3425–3432, 2006     

Studies with gradient polymers of polystyrene and poly(methyl acrylate)

A series of [net-polystyrene]-ipn-[net-poly(methyl acrylate)] systems with gradients of poly(methyl acrylate) in a polystyrene matrix are prepared and tested mechanically. The results showed no distinct differences from that of corresponding interpenetrating polymer network (IPN) samples with similar compositions, in contrast to the substantially improved fracture strains obtained for [net-poly(methyl methacrylate)]-ipn-[-net poly(methyl acrylate)] gradient systems and their nongradient IPN's in our previous work. Hence, it is concluded that the toughening effect of gradient structure observed is not universally applicable for all gradient systems but it is unique for the acrylic system studied. Moduli-temperature responses and dynamic mechanical spectra of gradients and IPN's prepared in the current study are also presented. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1721–1725, 1999     

Effect of wet screening on particle size distribution and coal properties

Wet screening is one of the methods used to remove fine material from the coal feed to gasification. Sasol Synfuels in South Africa undertook an investigation to quantify fine coal generation in the coal supply to gasification. Coal samples were wet screened in the laboratory and results compared to the normal dry screening procedure. It was found that the fines (−0.5 mm) increased almost five times when the coal was wet screened compared to dry screening. This study was subsequently initiated by Sasol Technology R&D to establish the mechanism of fine coal generation during wet screening, as well as the effect of wet screening on particle size distribution (PSD) and chemical properties of coal. Changes in the PSD and chemical properties of coal from individual coal sources used at Sasol Synfuels were compared. Composite coal samples with a predetermined PSD of all individual coal sources used at Sasol Synfuels were screened under wet and dry conditions. The PSD was again determined after screening, as well as the mineral composition (by X-ray diffraction) of the fines. Results indicated that wet screening caused clay minerals to be removed from the coal structure leading to an increase in the fines. This removal of minerals weakened the coal structure causing further size degradation of coarser fractions.     

Effect of particle size and grafting density on the mechanical properties of polymer nanocomposites

End-grafting polymer chains to nanoparticles in polymer nanocomposite is a widely used method to disperse inorganic particles in a polymeric matrix in order to improve the material properties. While many fundamental studies have investigated how various factors influence the dispersion or aggregation of the nanoparticles, the effect of grafting on the resulting material properties has received considerably less attention. In particular, the effect of nanoparticle curvature and grafting density on the mechanical properties in polymer nanocomposites remains elusive. In this study, we develop a coarse-grained model of a polymer glass containing grafted nanoparticles and examine the resulting effects on the mechanical properties. By carefully designing the parameters of our polymer nanocomposites model, we can maintain dispersion of the nanoparticles whether they are grafted with polymer chains or not, which allows us to isolate the effect of end-grafting on the resulting mechanical properties. We examine how the nanoparticle size and grafting density affect the elastic constants, strain hardening modulus, as well as the mobility of the polymer segments during deformation. We find that the elastic constants and yield properties are enhanced nearly uniformly for all of our nanocomposite systems, while the strain hardening modulus depends weakly on the grafting density and the nanoparticle size.     

Effect of the particle size distribution of spinel on the mechanical properties and thermal shock performance of MgO–spinel composites

The influence of varying the amounts of spinel with a similar median particle size, but with different distribution, on the mechanical properties and thermal shock performance of MgO–spinel composites was investigated. Mechanical properties of composites decreased significantly with increasing spinel content due to the thermal expansion mismatch. However, γ_WOF values of composites increased markedly, because of a significant change in the fracture mode from transgranular to intergranular fracture. A narrow distributed spinel A (Alcoa MR66) particles resulted in shorter initial crack propagation distances from the spinel particles, but spinel B (Britmag 67) particles with a significantly broader distribution were the origins of longer interlinked cracks. The improved resistance to thermal shock in MgO–spinel composites can therefore be attributed to the microcrack networks developed around the spinel particles, associated with the high values of γ_WOF, and not to an increased K_1c. On the basis of theoretically calculated R values and experimentally found γ_WOF/γ_i ratios, resistance to thermal shock damage would be more strongly favoured with materials containing spinel B particles, rather than spinel A, for which a much larger volume% was required to achieve a similar improvement.     

Compatibilizing effects of poly(styrene-graft-ethylene oxide) in blends of polystyrene and butyl acrylate polymers

The compatibilizing effect of poly(styrene-graft-ethylene oxide) in polystyrene (PS) blends with poly(n-butyl acrylate) (PBA) and poly(n-butyl acrylate-co-acrylic acid) (PBAAA) was investigated. No significant effects of the graft copolymer on the domain size were found in the PBA blends. By functionalizing PBA with acrylic acid, the average size of the polyacrylate domains was reduced considerably by the graft copolymer. Thermal and dynamic mechanical analysis of the PS/PBAAA blends revealed that the PBAAA glass transition temperature (T_g) decreased with increasing graft copolymer content. The effect of the graft copolymer in the PS/PBAAA blends can be explained by interactions across the interface due to the formation of hydrogen bonds between the poly(ethylene oxide) (PEO) side chains in the graft copolymer and the acrylic acid segments in the PBAAA phase. Hydrogen bonding was confirmed by IR analysis of binary blends of PEO and PBAAA. Partial miscibility in the PEO/PBAAA blends was indicated by a PEO melting point depression and by a T_g reduction of the PBAAA phase. The thermal properties of the PEO/PBA blends indicated only very limited miscibility. © 1996 John Wiley & Sons, Inc.     

Effect of crosslinking on the mechanical and thermal properties of poly(vinyl alcohol)

Poly(vinyl alcohol) was crosslinked with hexamethylene diisocyanate in solution. A broad range of degrees of crosslinking, from 1.7 up to 74 mol% of reacted hydroxyl groups, was achieved. The variation of the thermal and mechanical properties of PVA with the crosslinking density show an initial decrease due to the diminution of the crystallinity of the system, caused by the crosslinking. After an abrupt rise at about 20%, the properties tend to level off independently on the increase of the crosslinking. This behaviour is explained as a result of the competitive action of at least three factors during the crosslinking: (i) weakening of the existing physical network due to hydrogen bonding; (ii) formation of a chemical network; and (iii) introduction of flexible moieties. The last factor is closely connected with the specific chemical structure of the crosslinker itself.     

Grafting poly(t‐butyl acrylate) to poly(allylamine) by inverse microemulsion radical polymerization: From comb‐polymer to amphiphilic shell crosslinked polymer nanocapsule

An interfacial grafting radical polymerization method for amphiphilic comb copolymer and shell crosslinked polymer nanocapsules was reported. Macropolyradicals on a water soluble long chain polyamine were generated with hydrogen peroxide in the water phase and subsequent grafting radical polymerization of a vinylic monomer at the water/oil interface proceeded at 65°C. In the presence of a crosslinker, the resulting graft copolymer formed a defined core‐shell structure with hydrophilic aqueous core functionalized by the polyamine and a hydrophobic crosslinked polymer shell. The structure of the core‐shell material was characterized by NMR, FTIR, DSC, TGA, SEM, TEM, and the mechanism of the graft polymerization is proposed. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 1905–1911, 2007