Co-continuous Structures Research Articles

Effect of the silica content on the physico-chemical and relaxation properties of hybrid polymer/silica nanocomposites of P(EMA- co-HEA)

Poly(ethyl methacrylate- co-hydroxyethyl acrylate) 70/30 %wt/silica, P(EMA- co-HEA)/SiO 2, nanocomposites, with silica contents ranging from 0 to 30 %wt, were synthesized and studied as promising candidate materials for the synthetic matrix of scaffolds for bone substitutes or dentin regeneration. The physico-chemical properties of the hybrids were studied by calorimetry and by contact angle measurements on the surfaces. The dynamic-mechanical and compression properties were analysed. Intermediate silica contents in the range from 10 to 20 %wt of silica rendered co-continuous interpenetrated structures, in which silica produced a reinforcing effect in the polymeric matrix and at the same time conferred bioactivity to the surfaces by improving surface wettability, making these hybrids appropriate for the proposed application. On the contrary, silica percentages below 10 %wt formed disconnected inorganic aggregates at the nanoscale dispersed in the copolymer matrix, which did not modify significantly the copolymer properties. Silica contents above 20 %wt formed denser inorganic networks with few terminal silanol groups available at the surfaces, much more rigid and hardly manageable samples.

Co-continuous Nanostructured Blend by Reactive Blending: Incorporation of High Molecular Weight Polymers

One of the most challenging issues remaining in the design and the synthesis of robust nanostructured polymers blends is incorporating long crystallizable chains while preserving the co-continuitv of the phases. Here, we demonstrate that by reactive blending of functionalized polyolefin and polyamides such co-continuous structures can be obtained provided that a bimodal mixture of short and long polyamides is used. The short chains react more readily to form graft copolymers that facilitate both grafting of long chains and formation of nanostructures. We discuss different mixing strategies, characterize, and compare reacted products and resulting blend structures.

Enhanced Young’s Modulus of Al-Si Alloys and Reinforced Matrices by Co-continuous Structures

In the present work, the elastic behavior of different hypoeutectic and hypereutectic Al-Si alloys and different Al2O3 short fiber and SiC particle reinforced materials (SFRM and PRM, respectively) is studied. The effective Young’s modulus (E) of materials was experimentally measured and compared with the different theoretical predictions of Hashin-Strikman, Tuchiniskii, shear lag, and the rule of mixtures (ROM). The unreinforced alloys present an interconnected lamellar Si structure after fast solidification, which increases the Young’s modulus up to that of the Tuchiniskii prediction for interpenetrating skeletal structures. On the other hand, alloys presenting isolated and coarse Si particles (after spheroidization treatment at 540°C) are well described by the lower bound of the ROMs. Similarly, the interconnected Si-SiC structure observed in 10 and 70 vol% SiC reinforced AlSi7Mg and AlSi7 matrices in the as cast condition is responsible for the higher stiffness of the composite, if compared with that of Al99.5 or spheroidized AlSi7 matrices. An analogous behavior is observed in the SFRMs in the as cast condition, where the Si lamellae bridge the Al2O3 fibers, increasing the Young’s modulus of the composites, if compared with the conditions of spheroidized Si. Furthermore, the primary Si particles produce an improvement in the Young’s modulus by connecting several fibers in the case of a short fiber-reinforced hypereutectic AlSi18 matrix.

Preparation and characterisation of silica monoliths using a triblock copolymer (F68) as porogen

Silica-based monoliths with co-continuous structure were successfully prepared through a sol–gel process in the presence of a poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (F68). The triblock copolymer was compared to the classical PEG, in the formation of silica monoliths and was demonstrated to lead to co-continuous structures in a wider composition range, presenting smaller through pores. Moreover, mesoporous structures templated at the sol–gel transition were assumed to occur at the surface of the silica skeleton while PEG exhibited no mesopore templating.

Controllable Growth of Gradient Porous Structures

Cocontinuous phase structures of immiscible polymers can be developed under appropriate melt-blending conditions. Because of the presence of interfacial tension, such cocontinuous structures start to coarsen when heated to a temperature higher than the melting/softening temperature of both phases. In this study, a method for controllable growth of gradient porous structures utilizing variable coarsening rates in a gradient temperature field was investigated. The phase structure coarsens at a higher rate in higher temperature regions but at a slower rate in lower temperature regions, resulting in the generation of a gradient phase morphology. Subsequent dissolution of one phase in the binary blend yields a gradient porous structure made of the remaining polymer component. A polystyrene/poly(lactic acid) (PLA) blend was used as a model system. By designing proper thermal boundary conditions and introducing different thermal gradients during annealing, different types of gradient porous structures of PLA were created.

Structural Control of Co-Continuous Poly(L-lactide)/Poly(butylene succinate)/Clay Nanocomposites

Poly(L-lactide) (PLLA)/poly(butylene succinate) (PBS) (55/45 w/w) blends with different amounts of nanoclay loadings were prepared using a specially designed high-shear extruder, HSE3000mini, which can reach a maximum shear rate of 4400 sec(-1). The resulted co-continuous structural morphologies were observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM observation revealed that through the combination of various amounts of nanoclay loadings and processing under various shear conditions, the phase size of co-continuous structures of PLLA/PBS blends can be controlled over a wide range from several tens of micrometers to submicrometers. TEM observation shows that all the nanoclays are selectively dispersed in the PBS phase. We also found that clays in low-shear processed sample were mainly located at the interface of PBS phase, while in high-shear sample, the clays were mainly located inside of the PBS phase. It was considered that the dependence of nanoclay location in the PBS phase on the shear conditions, as well as the changing of the viscosity ratio of PBS and PLLA phase with different amounts of clay loading, play important roles in controlling the phase size of the co-continuous structures of PLLA/PBS blends.

Shear-induced change of phase morphology and tensile property in injection-molded bars of high-density polyethylene/polyoxymethylene blends

It is feasible to control the phase morphology and phase inversion for immiscible polymer blends to manipulate their properties. In this work, the blend of high-density polyethylene (HDPE)/polyoxymethylene (POM) was used as an example, to demonstrate the effect of shear on the phase morphology and resultant mechanical properties in immiscible polymer blends. To do so, a well defined “in-process morphology control” process during injection molding was conducted. That was: after making the blends via melt mixing, the injection-molded bars were prepared via a so-called dynamic packing injection molding equipment to impose a prolonged shearing on the melts during the solidification stage. Phase morphologies and crystal structures of the blends were estimated mainly through scanning electron microscopy, differential scanning calorimetry and 2D wide-angle X-ray scattering, respectively. For in-process morphology controlled samples, co-continuous structures, especially subinclusions inside another continuous phase induced by shear, were observed when the HDPE content was between 30 wt% and 50 wt%, leading to much early occurrence of phase inversion and also the lowest degree of orientation for both HDPE and POM. However, for samples obtained via conventional injection molding, a droplet morphology was always observed with HDPE dispersed in POM as the content of HDPE was up to 30 wt%, but with POM dispersed in HDPE as the content of HDPE was 50 wt%. The performances of injection-molded bars were mainly respect to the phase morphologies for samples obtained via conventional injection molding in which tensile properties continuously decreased with increasing of HDPE content up to 30 wt% and then increased with further increasing of HDPE content. For the in-process morphology controlled samples, the tensile properties depended not only on the phase morphology, but more importantly on the degree of orientation. One observed only a slight decrease of tensile property as the content of HDPE was less than 15 wt%, while an abrupt decrease when the content of HDPE was between 30 wt% and 50 wt%, probably due to the lowest degree of orientation in this composition range.

Mixed morphology by the extrusion of phase‐separated blends of a melt‐processed polymer and polymer solution

AbstractThe phase behavior and structure development in immiscible blends of melt‐extruded polycaprolactone and a viscous aqueous poly(ethylene oxide) solution were investigated. The coextrusion of an aqueous polymer solution and a molten polymer is a largely unexplored technique and offers exciting potential for creating new materials via the execution of chemical reactions in the aqueous phase. Samples were prepared both with and without a block copolymer acting as a surface‐active agent. The resulting morphology was characterized with scanning electron microscopy after the removal of the water‐soluble phase. Spheres, rods, fibers, and cocontinuous gyroidal structures were observed, yet the exact phase inversion was not observed, and the changes in the feature shape depended on which component composed the major phase. Significant orientation in the flow direction was observed when the less viscous poly(ethylene oxide) was the major phase, whereas orientation was minimal when polycaprolactone was the major phase. These observations indicate that control of the feature shape and orientation may be accomplished through the control of the viscosity. The spontaneous formation of an outer layer of polycaprolactone in all samples was observed, suggesting that the morphology could be induced by control of the material interaction with the extruder die wall. The inclusion of a diblock copolymer significantly reduced the feature size but did not alter the morphology type. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Conductive PVDF/PA6/CNTs Nanocomposites Fabricated by Dual Formation of Cocontinuous and Nanodispersion Structures

A conductive poly(vinylidene fluoride) (PVDF)/polyamide 6 (PA6)/carbon nanotubes (CNTs) composite with a unique hierarchical morphology has been fabricated by the dual formation of cocontinuous and nanodispersion structures using high-shear processing. PVDF and PA6 basically forms cocontinuous structure. The CNTs are exclusively located in the PA6 phase, and numerous PA6 domains with the size ranging from 10 to 150 nm are dispersed in the PVDF phase. The relationships between the physical properties (e.g., electrical conductivity and mechanical properties) and morphology of the composite have been investigated. The results indicate that the formation of PA6 nanodomains in PVDF phase by the high-shear processing not only increases the electrical conductivity but also improves the ductility of the obtained blend nanocomposites.

Structural analysis of multicomponent nanoclay-containing polymer blends through simple model systems

A systematic approach was adopted to study multicomponent clay-containing nanocomposites using compatibilized and non-compatibilized blends of polyamide 6 (PA6)/acrylonitrile–butadiene–styrene terpolymer (ABS) and their organoclay (OMMT) nanocomposites. For this purpose PA6/styrene–acrylonitrile copolymer (SAN) based blends and nanocomposites were selected as simple model systems. In this way the role of each component of the systems, especially the clay, compatibilizer, and polybutadiene fraction on the formation of intercalated or exfoliated OMMT structures as well as resulting dynamic mechanical properties (DMA) could be elucidated. Structural analysis of the model systems using theoretical approach, and X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and DMA revealed that the most crucial factor in controlling the morphology and achieving different levels of dispersion is the extent of interaction between clay and the polymer matrix. Morphological analysis revealed that the OMMT layers were dispersed and exfoliated largely in the PA6 phase but, some were also accumulated at the rubber particle surface which remained non-intercalated. The effect of a compatibilizer on the dispersion of OMMT was not completely clear. The SAN based nanocomposites containing PA6 showed fully exfoliated OMMT structures, whereas the ABS based nanocomposites, having an additional rubber fraction, showed a mixed exfoliated and also partly non-intercalated morphology. The OMMT did not change the general occurrence of the co-continuous structures but refined the structures and led to mechanical stiffening as indicated by the DMA results. A correlation was established between the changes in the morphological states and the DMA properties.

The relationship between polyurethane foam microstructure and foam aging

Correlations between polyurethane foam formulation and foam aging are shown to simplify when sufficient data are available to eliminate casual relationships. The numerous factors that are purported to result in faster or slower foam aging are shown to primarily reflect how those factors influence polyurethane hard segment structure. Specifically, changes in formulation that results in better phase separated (but co-continuous) structures exhibit less humid and heat aging than foams with poorer phase separation (broader interfacial phase mixing). This is demonstrated for aging tests that probe the polymer phase structure at higher stress or strain levels than that sometimes used. Tests that do not adequately stress the foam do not truly probe differences in foam structure that result in property decrements. Humid aged compression set and heat aged load loss data for a large number of foams are related to their formulation, catalysis, foaming rheology, and their hard segment structure determined by AFM and X-ray analyses.

THE EFFECT OF BLENDING SEQUENCE ON PHASE MORPHOLOGY OF NYLON 6/ABS/SMA BLENDS

The preparation process-dependent phase morphology of blends composed of nylon 6 and acrylonitrile-butadiene-styrene (ABS) over a composition range of 30-70 wt% using a styrene-maleic anhydride (SMA) copolymer as the compatibilizing agent with a constant content (5phr) was investigated. The results of the scanning electron microscope (SEM) observation revealed that compared with the binary blends of nylon 6 and ABS, the existence of SMA caused a composition shift of phase inversion to a higher weight fraction of nylon 6 when ABS was blended with the preblended nylon 6/SMA blend, while the co-continuous structures could be observed over a considerably narrower composition range when nylon 6 was blended with the pre-blended ABS/SMA blend. An examination through dynamic mechanical analysis (DMA) tests confirmed the results obtained with SEM. It is found that near the phase inversion region a remarkable change in the dynamic storage modulus (G′) and the loss tangent (tanδ) appears. Moreover, the influence of blending sequence on the size of dispersed particles has been probed for uncompatibilized and compatibilized blends of nylon 6 and ABS over a wide range of compositions below or beyond the phase inversion points. For the blends of ABS dispersed in a nylon 6 matrix, little discernible effects of blending sequence on particle size could be observed. Furthermore, there exists a significant difference in morphologies of the blends prepared by nylon 6 particles dispersing in a ABS matrix in cases of different blending sequences used. Some possible factors responsible for the above asymmetric behaviors have been proposed.

Poly(methyl methacrylate)‐modified vinyl ester thermosets: Morphology, volume shrinkage, and mechanical properties

AbstractThermoset materials obtained from styrene/vinyl ester resins of different molecular weights modified with poly(methyl methacrylate) (PMMA) were prepared and studied. Scanning electron microscopy and transmission electron microscopy micrographs of the fracture surfaces allowed the determination of a two‐phase morphology of the modified networks. Depending on the molecular weight of the vinyl ester oligomer, the initial content of the PMMA additive, and the selected curing temperature, different morphologies were obtained, including the dispersion of thermoplastic‐rich particles in a thermoset‐rich matrix, cocontinuous structures, and the dispersion of thermoset‐rich particles in a thermoplastic‐rich matrix (phase‐inverted structure). Density measurements were performed to determine the effect of the PMMA‐modifier concentration and curing temperature on the volume shrinkage of the final materials. The development of cocontinuous or thermoplastic‐rich matrices was not too effective in controlling the volume shrinkage of the studied vinyl ester systems. The evaluation of the dynamic mechanical behavior, flexural modulus, compressive yield stress, and fracture toughness showed that the addition of PMMA increased the fracture resistance without significantly compromising the thermal or mechanical properties of the vinyl ester networks. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007

Sheared aqueous two-phase biopolymer–surfactant mixtures

Aqueous two-phase biopolymer–surfactant mixtures have so far received only little attention. Here, the behaviour of such mixtures was investigated adopting methods well established for aqueous two-phase biopolymer–biopolymer mixtures. The pullulan–sodium dodecyl sulphate (SDS) mixture, in which phase separation can be induced, and then enhanced by the addition of sodium chloride (NaCl), was chosen as a model system and its morphology and rheology under shear was analysed. Rheological measurements on phase-separated mixtures revealed that the viscosity depends on the chemical composition of the single phases, their viscosity ratio and volume fraction. Optical observations showed droplet-like morphology over a wide range of shear rates at low volume fractions of one phase, and co-continuous structures including string phases at high volume fractions and shear rates. For more concentrated systems, co-continuous structures were observed at all shear rates investigated, whereby in some cases string phases developed at high shear rates.

Morphologies and properties of cured epoxy/brominated‐phenoxy blends

AbstractMorphologies of cured epoxy/brominated‐phenoxy blends were observed by scanning transmission electron microscopy (STEM) and energy dispersive X‐ray fluorescence spectroscopy (EDX). When brominated‐phenoxy content was 30 wt %, cocontinuous phase structures between cured epoxy and brominated‐phenoxy were found. Since every loss tangent (tan δ) curve as a function of temperature on dynamic mechanical analysis (DMA) showed 2 peaks at 128°C and 155°C respectively, cured epoxy phases and brominated‐phenoxy phases were incompatible together and Tgs of cured epoxy phases were not decreased. Tensile strength and tensile elongation of the cured blends were increased together. T‐peel adhesion strength and the lap‐shear adhesion strength were also increased together. These phenomena could be due to the cocontinuous structures consisted by the rigid cured epoxy phases of thermosets and ductile the brominated‐phenoxy phases of thermoplastics. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 1702–1713, 2007

Morphology and contact angle studies of poly(styrene-co-acrylonitrile) modified epoxy resin blends and their glass fibre reinforced composites

In this study, the surface characteristics of blends and composites of epoxy resin were investigated. Poly(styrene- co-acylonitrile) (SAN) was used to modify diglycidyl ether of bisphenol-A (DGEBA) type epoxy resin cured with diamino diphenyl sulfone (DDS) and the modified epoxy resin was used as the matrix for fibre reinforced composites (FRP's). E-glass fibre was used as the fibre reinforcement. The scanning electron micrographs of the fractured surfaces of the blends and composites were analyzed. Morphological analysis revealed different morphologies such as dispersed, cocontinuous and phase-inverted structures for the blends. Contact angle studies were carried out using water and methylene iodide at room temperature. The solid surface energy was calculated using harmonic mean equations. Blending of epoxy resin increases its contact angle. The surface free energy, work of adhesion, interfacial free energy, spreading coefficient and Girifalco-Good's interaction parameter were changed significantly in the case of blends and composites. The incorporation of thermoplastic and glass fibre reduces the wetting and hydrophilicity of epoxy resin.

Performance of Monolithic Silica Capillary Columns with Increased Phase Ratios and Small-Sized Domains

Monolithic silica capillary columns for HPLC were prepared from tetramethoxysilane to have smaller sized domains and increased phase ratios as compared to previous materials, and their performance was evaluated. The monolithic silica columns possessed an external porosity of 0.65-0.76 and a total porosity of 0.92-0.95 and showed considerably higher performance and greater retention factors in a reversed-phase mode after chemical modification than columns previously reported. An octadecylsilylated monolithic silica column with the smallest domain size (through-pores of approximately 1.3 microm and silica skeletons of approximately 0.9 microm) showed a plate height of less than 5 microm at optimum linear velocities (u) of 2-3 mm/s in 80% acetonitrile for a solute having retention factors of approximately 1, and approximately 7 microm at u = 8 mm/s. With a permeability similar to that of a column packed with 5-microm particles, the monolithic silica columns were able to attain column efficiencies comparable to that of particulate columns packed with 2-2.5-microm particles, and showed performance in the "forbidden region" for the previous columns. The performance of the monolithic column can be compared favorably with that of a particle-packed column when 15,000-30,000 or more theoretical plates are desired at a pressure drop of 20-40 MPa or lower. The increased homogeneity of the co-continuous structures, in addition to the small-sized domains, contributed to the higher performance as compared to previous monolithic silica columns.

Morphology and Properties of Soy Protein and Polylactide Blends

Blends of soy protein (SP) and a semicrystalline polylactide (PLA) were prepared using a twin-screw extruder. The melt rheology, phase morphology, mechanical properties, water resistance, and thermal and dynamic mechanical properties were investigated on specimens prepared by injection molding of these blends. The melt flowability of soy-based plastics was improved through blending with PLA. Scanning electron microscopy revealed that a co-continuous phase structure existed in the blends with soy protein concentrate (SPC) to PLA ratios ranging from 30:70 to 70:30. SPC/PLA blends showed fine co-continuous phase structures, while soy protein isolate (SPI)/PLA blends presented severe phase coarsening. At the same SP to PLA ratios, SPC/PLA blends demonstrated a higher tensile strength than SPI/PLA blends. The water absorption of soy plastics was greatly reduced by blending with PLA. The compatibility was improved by adding 1-5 phr poly(2-ethyl-2-oxazoline) (PEOX) in the blends, and the resulting blends showed an obvious increase in tensile strength and a reduction in water absorption for SPI/PLA blends. The compatibility between SP and PLA was evaluated by mechanical testing, dynamic mechanical analysis (DMA), water absorption, and scanning electron microscopy (SEM) experiments. Differential scanning calorimetry (DSC) revealed that PLA in the blends was mostly amorphous in the injection molded articles, and SP accelerated the cold crystallization and could increase the final crystallinity of PLA in the blends.

Autocatalytic phase separation and graded co-continuous morphology generated by photocuring.

Interpenetrating polymer networks (IPNs) with spatially graded co-continuous structures were constructed by photo-cross-linking a homogeneous mixture of photo-reactive polystyrene and methyl methacrylate monomer. For a given thickness, irradiation with weak ultraviolet (UV) light results in a co-continuous morphology uniform throughout the sample. However, as the irradiation intensity increases to some certain extent, spatially graded co-continuous morphology in the micrometre scales emerges due to the significant effect of the gradient of the light intensity along the irradiation direction. These 3-dimensional graded structures were observed by using laser scanning confocal microscopy (LSCM) and were analyzed by digital image analysis. The depth dependence of these graded structures can be well expressed by a power law with an exponent depending strongly on the irradiation intensity. The time-evolution of the graded morphology was monitored at different depths of the same irradiated sample. It was found that the phase separation does not follow conventional laws of kinetics, but instead exhibits the autocatalytic behavior, reflecting the effects of the heat produced by the photopolymerization of MMA monomer on the phase separation process. The experimental data obtained in this study suggest a method of producing polymeric materials with spatially graded structures in the micrometer scales by solely changing the irradiation intensity.

Multiphase blends from poly(L-lactide) and poly(methyl mathacrylate)

Melt processing of poly(L-lactide) (PLLA) and poly(methyl methacrylate) (PMMA) was conducted over a targeted range of compositions with PLLAs of 118 and 316 kDa in molecular mass to identify morphologies and the phase relationships in these blends. These blends are of interest for use in biomaterials and the morphologies are critical for tissue-engineering studies where biodegradability, pore connectivity and surface texture control tissue viability and adhesion. Simple extrusion of the two polymers produced multiphase blends with an average domain size near 25 μm. Scanning electron microscopy and dynamic mechanical analysis demonstrated that these blends are immiscible, at least in a metastable sense, and regions of co-continuous structures were identified. Such co-continuous, which occurred generally in accordance with rheology prediction models, exhibit a fine interconnected structure that appears effective for fabricating certain biomaterials. A broad and unexpected transition appears in these blends, as measured by modulated differential scanning calorimetry between 70 and 100°C, which may be the glass transition temperature of an alloy phase. The magnitude of this transition is greatest in the fine-structured co-continuous composition region of blends, suggesting the presence of a complex or other derivative of the two primary phases.