Co-continuous Polymer Blends Research Articles

A morphological study on the dispersion and selective localization behavior of graphene nanoplatelets in immiscible polymer blends of PC and SAN

In this morphological study the dispersion and localization behavior of 1 wt.% graphene nanoplatelets (GnPs) in melt mixed co-continuous polymer blends of polycarbonate (PC, 59 wt.%) and poly(styrene-acrylonitrile) (SAN, 40 wt.%) were investigated. Through varying the mixing sequence as well as the melt mixing parameters, different states of dispersion and different filler localizations were achieved. Melt mixing in a one-step process resulted in the poorest dispersion of the GnPs in the polymer blend. In this case, the filler could hardly be found localized selectively in one of the polymer components but formed its own component. In two-step mixing processes the GnPs were either premixed in PC or SAN to investigate the assumed filler transfer from the SAN into the PC component. Four different premixtures were prepared which showed that longer mixing time and higher rotation speed resulted in better dispersion of the GnPs in the polymer matrix. If the GnPs were predispersed in PC they could still be found in the PC component after blending with SAN. In the case that the GnPs were premixed in SAN, the filler was detected partially in the SAN or at the blend interface, as well as smaller sized particles were found in the PC component. It could be shown that the size and aspect ratio of the filler play a significant role on the localization of GnPs in melt mixed polymer blends.

Hierarchical Structures Composed of Confined Carbon Nanotubes in Cocontinuous Ternary Polymer Blends

An hierarchical structure, composed of a ternary cocontinuous polymer blend, where carbon nanotubes are mostly localized in one of the phases through π–π interactions, is fabricated by direct melt mixing of polyamide 12 and polypropylene, as the two major components of the ternary blend, together with pyridine-modified poly(ethylene-co-methacrylic acid) as the minor component that can form strong interactions with the CNTs via π–π interactions and confined the percolated network at the polyamide/polypropylene interface. The hierarchical structure was designed by means of surface energies, and the obtained morphology was verified using electron microscopy. This ternary structure has lower electrical resistivity as compared to cocontinuous binary composites. Different polymer viscosities were used in this study in order to emphasize the importance of kinetics during cocontinuous morphology formation.

Rheological and morphological study of cocontinuous polymer blends during coarsening

One of the challenges in the study of cocontinuous blends is to relate their rheological behavior with their morphology. This is due to the inherent instability of cocontinuous structures. We have studied the morphological and rheological time evolution of cocontinuous blends with different interfacial tension and viscosity ratio during annealing. In general, the presence of an interface generates an extra contribution to the elastic modulus, G′int. We have found that both the initial value of G′int and its rate of evolution during coarsening are proportional to the ratio between interfacial tension and blend viscosity. We have related time evolution of the elastic modulus to that of the interfacial area via a simplification of Doi–Ohta model for the case of small amplitude oscillatory shear flow. This simplification is based on the experimental evidence that the degree of anisotropy generated during small amplitude oscillations is negligible [López-Barrón and Macosko, Langmuir 26, 14284–14293 (2010b)]. The simplification gives a linear relation between the characteristic size and time and an asymptotic decrease of the elastic modulus with a limit behavior at long times: G′|t→∞∝t−1. This behavior was observed on blends with low interfacial tension. However, blends with high interfacial tension underwent a transition toward a decrease in the rate of coarsening, which is accompanied by a slower decrease of the elastic modulus. Doi–Ohta model is not sensitive to this transition and fails in predicting the time evolution of the characteristic length and elasticity of high interfacial tension blends.

A Conductive Carbon Nanotube-Polymer Composite Based on a Co-continuous Blend

A conductive carbon nanotube (CNT) based polymer composite was constructed in which the conductive network utilized the selective distribution of CNTs in a co-continuous phase polymer blend. CNTs were first uniformly coated on the surface of polyethylene (PE) fine particles through alcohol assisted dispersion under ultrasonication, and then the CNTs coated PE particles were melt compounded with polycarbonate (PC). The theoretical prediction from both thermodynamics and kinetics indicated the CNTs mainly stayed at the interface between PC and PE. Rheological measurements and scanning electron microscopy (SEM) and optical microscopy (OM) observations provided the evidences for the selective distribution of CNTs and revealed the transition from the dispersed/matrix phase structure to the co-continuous phase structure at 50 wt% of PC. As PE and PC formed co-continuous phases, the CNTs formed a perfect conductive network on the phase interfaces, which resulted in a percolation threshold between 0.3 and 0.5 vol%, a relatively low value for CNTs filled thermoplastic composites reported in the literature.

Porous Films via PE/PEO Cocontinuous Blends

The design of a porous membrane support layer derived from cocontinuous polymer blends is presented. We investigate the effect of blend composition, shear rate, residence time, and annealing time on the cocontinuous morphology of polyethylene (PE)/poly(ethylene oxide) (PEO) blends. Porous PE sheets were generated by water extraction of PEO and used as a support layer for gas separation membranes. The PE/PEO blends using nonfunctional and maleic anhydride functional PE (PE-g-MA) were mixed in a batch microcompounder and in a pilot plant scale corotating twin-screw extruder. Using PE-g-MA resulted in pore size reduction from 10 to 2 μm and suppression of coarsening of the morphology during further annealing of the blends due to formation of PE–PEO graft copolymers. Equilibrium interfacial tension, estimated by fitting the rheology of droplet blends to the Palierne viscoelastic droplet model, was 3 and 0.4 mN/m for PE/PEO and PE-g-MA/PEO systems, respectively. The specific interfacial area and phase size dis...

Enhanced Electrically Conductive Polypropylene/Nano Carbon Black Composite

In this work, a porous polypropylene (PP)/nano carbon black (CB) composite was facilely fabricated via immiscible co-continuous polymer blend and subsequent dissolution process. The porous structure was generated from co-continuous polymer blend, which was exploited as the substrate for depositing nano CB. The interconnected micro pores of the co-continuous polymer blend and nano pores derived from agglomerated CB resulted in a significant enhancement of conductivity. Comparing with the conventional carbon composite obtained through dual-percolation method, the electrical conductivity of PP/CB composite increased 10 orders of magnitude with CB loading ranged from 1 wt% to 5 wt%. Moreover, it was found that the percolation threshold of PP/CB composite decreased nearly 80% compared with that of as-mixed sample. The enhanced conductivity and much lower percolation make this novel method a potential way for fabricating porous conductive materials for advanced application.

Fabrication of Hierarchical Microporous Structures with Controlled Morphology and Topography

Co-continuous polymer blend has attracted broad interest in many technique fields due to its unique ability to be transferred into porous material with interconnected micro-channels. And combined with some surface patterning, it can provide material of hierarchical porous structure. A key challenge in the fabrication of material of desired morphology is the ability to control the interface movement. Here, we incorporate the thermo-geometrically controlled annealing and hot embossing to fabricate hierarchical microporous structures with well defined morphology and topography. Various novel structures demanded for many emerging applications can be created by judiciously using this newly developed technique.

Tailoring of Co-Continuous Polymer Blend Morphology: Joint Action of Nanoclays and Compatibilizers

The effect of both organoclay (Cloisite ® 30B) and compatibilizers (EVA copolymer or MAgPE) on the morphology and mechanical properties is investigated with a special focus on co-continuous structures. For that purpose, LDPE and PBAT are selected as an inert/reactive polymer pair model. The influence of the nature of the nanoclay/compatibilizer pair on the morphology of the nanocomposite is studied by means of SEM and TEM. Depending on the compatibilizer, it comes out that the organoclay can be selectively located in either the polyolefinic or polyester phase while the interface is invariably stabilized by clay platelets. Interestingly enough, it appears that such a combination (organoclay plus compatibilizer) allows for tailoring the final properties of polymer blends.

Annealing of Cocontinuous Polymer Blends: Effect of Block Copolymer Molecular Weight and Architecture

Cocontinuous morphologies of polymer blends are thermodynamically unstable: they will coarsen when held above their glass or melting transition temperature. We have found that properly chosen diblock copolymers (bcp) can arrest coarsening during quiescent annealing. The effects of bcp on the cocontinuous morphologies of polystyrene (PS)/polyethylene (PE), PS/poly(methyl methacrylate) (PMMA) and PS/styrene-ran-acrylonitrile copolymer (SAN) blends were studied using scanning electron microscopy (SEM) and laser scanning confocal microscopy (LSCM) with image analysis. Bcp effectiveness was dependent on copolymer molecular weight, concentration, and asymmetry. Our interpretation emphasizes the role of bcp micelle creation and destruction as potential bottlenecks in the kinetics of interfacial adsorption of copolymer during mixing and of interfacial desorption during coarsening. In cases where adsorption and desorption appear to be facile, our results for the rate of coarsening are consistent with equilibrium predictions from self-consistent field theory for the dependence of interfacial tension upon copolymer asymmetry. We show that the coarsening of cocontinuous blends can provide a method to quantify the reduction in interfacial tension due to block copolymer addition, which is difficult to measure by conventional methods.

Geometrical Confining Effects in Compression Molding of Co-continuous Polymer Blends

Polymer blending is a versatile method for production of co-continuous porous materials. While numerous studies have been performed to elucidate the thermal annealing effects on the bulk structure, the effect of geometrical confinement is little understood. In the present work, possible effects from geometrical confinement during in-mold annealing were explored. A 50/50 wt.% poly (lactic acid)/polystyrene (PLA/PS) blend was compression molded and annealed between two parallel plates without being confined circumferentially during the compression molding process. Different conditions for geometrical confinement, including varied gap size and compression ratio (initial to final thickness ratio), as well as modified surface properties, were employed. The experimental results indicated that the gap size played a profound role in affecting the structural development; the phase size near the mold surface was smaller than away from the surface. The actual phase structure and the resulting gradient in pore size were further affected by the mold surface properties. Additionally, the compression ratio was found to affect the morphological development especially near the mold surface. At a high compression ratio, a thin layer of PLA was formed immediately during compression. The thickness of this layer either grew or reduced in size depending on the mold surface properties. Understanding of these geometrical confining effects and implementing them in processing may lead to the development of innovative porous materials.

A knotted and interconnected skeleton structural model for predicting Young's modulus of binary phase polymer blends

A knotted and interconnected skeleton structural (KISS) model is proposed for predicting Young's modulus of polymer blends. The KISS model accounts for the structure variations of polymer blends using the percolation theory and emphasizes the structural symmetry of co-continuous polymer blends. It reduces to the Maxwell-Eucken models below the percolation threshold and a symmetric interconnected skeleton structure at an intermediate component volume fraction. The KISS model satisfactorily predicts the Young's modulus of polymer blends for the entire component volume fraction range. Due to mathematical analogy, the model might equally be applied to predictions of electrical and thermal conductivity of binary mixtures. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers

Controlling Burst and Final Drug Release Times from Porous Polylactide Devices Derived from Co-continuous Polymer Blends

This work has demonstrated that it is possible to exercise a wide range of control over both the initial burst release and the final drug release times from porous polylactide (PLA) devices derived from cocontinuous polymer blends. Two strategies were used: a layer-by-layer polyelectrolyte surface deposition approach on the porous PLA surface and the application of a partially closed-cell protocol. A PLA porous substrate with a pore size of 1.5 microm, derived from a blend of PLA and polystyrene (PS) via selective solvent extraction of the PS phase, was used as the drug delivery device. The surface area and pore dimensions were examined via BET nitrogen adsorption and image analysis. Porous PLA substrates with 0, 3, and 5 layers of polyelectrolytes and with open areas of 100, 12, and 2% were studied both separately and in combination. In vitro release tests were performed to study the release profile of bovine serum albumin (BSA) from the devices via UV spectrophotometry. It is shown that, while both are important, surface modification is more dominant in controlling the release rate than the partially closed cell approach. When a five layer surface modification of the PLA and a partially closed cell approach (2% open area) are combined, denoted as the L5C sample, the synergy is dramatic with a 5x reduction in the first two hour burst release amount and a total release time that is extended by 123x as compared to the 100% open cell, surface unmodified, reference sample. The L5C sample ultimately releases 89% of the total BSA loaded, demonstrating the high level of interconnectivity of the microchannels in the porous PLA. The mechanism of release in this system is clearly diffusion controlled with well-defined concentration gradients, as measured by X-ray photoelectron spectroscopy (XPS), observed in the direction of release. These results point toward a diffusion mechanism combined with a sorption/desorption interaction of the BSA with the modified PLA surface.

Tuning the dispersion of multiwall carbon nanotubes in co-continuous polymer blends: ageneric approach

Melt-mixed blends of polyamide 6 and acrylonitrile–butadiene–styrene (PA6/ABS) withmultiwall carbon nanotubes (MWNTs) were prepared with the intention to developconducting composites. A generic strategy, namely specific interactions combined withreactive coupling, was adopted to facilitate and to retain the ‘network-like’ structure ofMWNTs during melt-mixing. This was facilitated by the sodium salt of 6-aminohexanoic acid (Na-AHA) and certain phosphonium based modifiers, where itwas envisaged that these modifiers would establish specific interactions (either‘cation–π’ or ‘π–π’ ) withthe ‘π-electron’ clouds of MWNTs, as well as restricting them in the PA6 phase of theblends via reactive coupling. This route eventually led to a remarkable increase inthe electrical conductivity and dielectric constant in the blends with MWNTs.Raman, FTIR and TEM investigations further supported these observations.

Efficiency of graft copolymers at stabilizing co-continuous polymer blends during quiescent annealing

This work was aimed at studying the efficiency of graft copolymers at stabilizing the co-continuous morphology of polystyrene (PS)/polyamide 6 (PA6) blends during quiescent annealing. A series of graft copolymers with PS as backbone and PA6 as grafts, denoted as PS-g-PA6, with different molecular structures and compositions were used as compatibilizers. The co-continuous domain size of the blends without PS-g-PA6 increased almost linearly with annealing time. The addition of the PS-g-PA6 not only narrowed down the composition range of co-continuity of PS/PA6 blend but also slowed down and even stopped completely the coarsening of the co-continuous morphology during the quiescent annealing. Moreover, the efficiency of PS-g-PA6 depended very much on its molecular structure and/or composition. For graft copolymers with similar backbone and graft chain number, the longer the grafts, the higher their stabilizing efficiency. For a given backbone/graft composition, graft copolymers having fewer and longer grafts were more efficient at compatibilizing and stabilizing the co-continuous morphology.

Specific Interactions Induced Dispersion and Confinement of Multi-Walled Carbon Nanotubes in Co-continuous Polymer Blends

Multiwall carbon nanotubes (MWNT) were melt-mixed with 50/50 co-continuous blends of polyamide 6 (PA6) and acrylonitrile-butadiene-styrene (ABS). Blending sequence and moulding processes were found to have a strong impact on the conductivity of the blends with MWNT. Aggregated nature of the tubes, migration during processing and skin-core morphology generated during mould cooling step were found to be crucial parameters affecting the electrical conductivity of the blends. We report here the role of a reactive modifier: sodium salt of 6-amino hexanoic acid (Na-AHA) aiding in uniform dispersion of the MWNT in the 50/50 PA6/ABS blends and restricting the tubes utilizing specific interactions during melt-mixing in the PA6 phase in the blends. We further varied the MWNT to Na-AHA ratio from 1:1 to 1:15 to optimize the concentration of MWNT required in achieving lower electrical percolation threshold in co-continuous PA6/ABS blends. The associated percolation threshold was observed at approximately 0.5 wt% MWNT with high dielectric constant.

Porous Devices Derived from Co-Continuous Polymer Blends as a Route for Controlled Drug Release

In this study we examine the release profile of bovine serum albumin (BSA) from a porous polymer matrix derived from a co-continuous polymer blend. The porosity is generated through the selective extraction of one of the continuous phases. This is the first study to examine the approach of using morphologically tailored co-continuous polymer blends as a template for generating porous polymer materials for use in controlled release. A method for the preparation of polymeric capsules is introduced, and the effect of matrix pore size and surface area on the BSA release profile is investigated. Furthermore, the effect of surface charge on release is examined by surface modification of the porous substrate using layer-by-layer deposition techniques. Synthetic, nonerodible polymer, high-density polyethylene (HDPE), was used as a model substrate prepared by melt blending with two different styrene-ethylene-butylene copolymers. Blends with HDPE allow for the preparation of porous substrates with small pore sizes (300 and 600 nm). A blend of polylactide (PLA) and polystyrene was also used to prepare porous PLA with a larger pore size (1.5 microm). The extents of interconnectivity, surface area, and pore dimension of the prepared porous substrates were examined via gravimetric solvent extraction, BET nitrogen adsorption, mercury porosimetry, and image analysis of scanning electron microscopy micrographs. With a loading protocol into the porous HDPE and PLA involving the alternate application of pressure and vacuum, it is shown that virtually the entire porous network was accessible to BSA loading, and loading efficiencies of between 80% and 96% were obtained depending on the pore size of the carrier and the applied pressure. The release profile of BSA from the microporous structure was monitored by UV spectrophotometry. The influence of pore size, surface area, surface charge, and number of deposited layers is demonstrated. It is shown that an effective closed-cell structure in porous PLA can be prepared, effectively eliminating all short-term BSA release.

Conductive polymer composites with double percolated architecture of carbonnanoparticles and ceramic microparticles for high heat dissipation and sharp PTCswitching

In classical self-limiting heating devices where conductive polymer composites(CPC) are used, one of the main problems to solve is the stability of propertieswith time. Different strategies are proposed to stabilize the morphologies duringthe structuring of these heterogeneous materials. Some of them are well knownin the use of co-continuous polymer blends or confinement but the interest ofthis work is to combine different structuring methods such as volume exclusion,adsorption and multiple percolations to achieve original properties. In fact theCPC developed exhibit enhanced heat dissipation and thermal stability (up to180 °C),independent adjustability of electrical and thermal conductivity, and a sharp and large amplitudePTC effect. These original results were obtained with a co-continuous structure associating athermally conductive polymer phase (syndiotactic poly(styrene) (sPS) filled with aluminum oxide(Al2O3) or boron nitride (BN)) with an electrically conductive polymer phase (high-densitypoly(ethylene)) (hdPE) filled with carbon nanoparticles (CNP) in appropriate proportions.

Ultralow Percolation Thresholds in Ternary Cocontinuous Polymer Blends

Ultralow Percolation Thresholds in Ternary Cocontinuous Polymer Blends

A patchwork approach to stochastic simulation: A route towards the analysis of morphology in multiphase systems

We propose a new sequential stochastic simulation approach for black and white images in which we focus on the accurate reproduction of the small scale geometry. Our approach aims at reproducing correctly the connectivity properties and the geometry of clusters which are small with respect to a given length scale called block size. Our method is based on the analysis of statistical relationships between adjacent square pieces of image called blocks. We estimate the transition probabilities between adjacent blocks of pixels in a training image. The simulations are constructed by juxtaposing one by one square blocks of pixels, hence the term patchwork simulations. We compare the performance of patchwork simulations with Strebelle’s multipoint simulation algorithm on several types of images of increasing complexity. For images composed of clusters which are small with respect to the block size (e.g. squares, discs and sticks), our patchwork approach produces better results than Strebelle’s method. The most noticeable improvement is that the cluster geometry is usually reproduced accurately. The accuracy of the patchwork approach is limited primarily by the block size. Clusters which are significantly larger than the block size are usually not reproduced accurately. As an example, we applied this approach to the analysis of a co-continuous polymer blend morphology as derived from an electron microscope micrograph.

Preparation of interconnected poly(ε-caprolactone) porous scaffolds by a combination of polymer and salt particulate leaching

This paper examines a new technique for the preparation of porous scaffolds by combining selective polymer leaching in a co-continuous blend and salt particulate leaching. In the first step of this technique, a co-continuous blend of two biodegradable polymers, poly(ε-caprolactone) (PCL) and polyethylene oxide (PEO), and a certain amount of sodium chloride salt particles are melt blended using a twin screw extruder. Subsequently, extraction of the continuous PEO and mineral salts using water as a selective solvent yields a highly porous PCL scaffold with fully interconnected pores. Since, the salt particles and the co-continuous polymer blend morphology lead to very different pore sizes, a particular feature of this technique is the creation of a bimodal pore size distribution. Scanning electron microscopy, mercury intrusion porosimetry and laser diffraction particle size analysis were carried out to characterize the pore morphology. The prepared scaffolds have relatively homogeneous pore structure throughout the matrix and the porosity can be controlled between 75% and about 88% by altering the initial volume fraction of salt particles and to a lesser extent by changing the PCL/PEO composition ratio. Compared to the conventional salt leaching technique and to its different variants, the proposed process allows a better interconnection between the large pores left by the salt leaching and a fully interconnected porous structure resulting from the selective polymer leaching. The average compressive modulus of the different porous scaffolds was found to decrease from 5.2MPa to about 1MPa with increasing porosity, according to a power–law relationship. Since, the blending and molding of the scaffold (prior to leaching) can be made using conventional polymer processing equipment, this process seems very promising for a large scale production of porous scaffold of many sizes and in an economic way.