Porous composite structures derived from multiphase polymer blends

Porous biopolymer structures have attracted a lot of attention in the recent years because of their potential applications in tissue engineering. In this work, porous structures of poly(lactic‐co‐glycolic acid) (PLGA) reinforced with organically modified montmorillonite (Cl15A) were fabricated. A ternary, co‐continuous blend consisting of PLGA/PS/Cl15A (PS: Polystyrene) was prepared by melt extrusion. Then, a porous PLGA composite was created by the sacrificial extraction of the PS phase. The morphological characterization revealed the creation of a well‐formed 3D porous network consisting of Cl15A‐reinforced PLGA. Quantitative results obtained from the scanning electron microscopy (SEM) micrographs of the fabricated porous structures show that small variations in the clay loading affect the geometrical characteristics (% porosity and pores average diameter) of these porous structures. The results suggest that these porous PLGA/clay structures may be promising candidates for mechanically strong scaffolds in tissue engineering applications, but this remains upon testing. POLYM. ENG. SCI., 55:1856–1863, 2015. © 2014 Society of Plastics Engineers     

Mie scattering of electromagnetic radiation by a sphere: A computer program

A computer program for calculating the Mie scattering of electromagnetic radiation by a sphere is described. The program will evaluate, amongst other parameters, the Mie scattering functions a_m and b_m, the extinction, scattering, back-scattering, absorption and radiation pressure efficiencies, the intensity functions, I₁ and I₂ and the polarisation ratios

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. The program can be made available to other users and computations can be commissioned on a repayment basis. A library of results is being established.     

The interfacial properties and porous structures of polymer blends characterized by synchrotron small-angle X-ray scattering

Microvoids are induced upon uniaxial drawing of films made from immiscible polypropylene (PP)/polystyrene (PS) binary blends and ternary blends of PP, PS, and a block copolymer SEEPS. The shape of the uniaxially oriented microvoids is rod- or slit-like with a high aspect ratio. Synchrotron small-angle X-ray scattering (SAXS) is used to characterize the dimensions of these microvoids. Their scattering image is an intense azimuthally narrow equatorial streak on a two-dimensional SAXS pattern. This streak is analyzed to obtain the diameter, length and misorientation of the microvoids. The microvoids length is identified as an effective measure of the interfacial adhesion and strength between phase domains. Drawn films of binary blends of PP/PS are found to have the longest microvoids. The initial addition of the block copolymer SEEPS as a compatibilizer enhances the interfacial adhesion and shortens the length of microvoids. Further addition of compatibilizer induces the formation of aggregates of a composite PS/SEEPS dispersed phase, and this leads to reduced interfacial adhesion and a longer microvoids. Interfacial properties are also dependent on the mixing protocol used to produce the blends.

The transport property of the films is determined by porosity and the degree of interconnectivity. A convenient measure of the degree of interconnectivity is proposed. The degrees of interconnectivity of these films are in accordance with the interfacial adhesion and strength. Non-equatorial streaks are observed and attributed to the microvoids with a complex orientation and geometry, which are responsible for the interconnectivity among microvoids.     

Small angle neutron scattering of partially segregated polymer blends

Small angle neutron scattering (SANS) has been used extensively to investigate the conformation of macromolecules. However, the scattered intensity has been found to be extremely sensitive to the segregation of the isotopic labelled species. The segregation has resulted in enormous increases in both the radius of gyration and the molecular weight. A theoretical treatment based on the Zernicke-Prins equation with a modified pair correlation function has been developed in this work; the size and the degree of segregation can be calculated from the scattering data using the equations obtained herein.     

Morphological characterization of simulated polymer blends by light scattering

The morphology and the properties of polymer blends are closely related to processing conditions. A skin/core morphology, with a minor phase undergoing variations in orientation and aspect ratio from the surface to the core of the material is observed in processes such as the injection molding of blends. The use of optical inspection to control the stability and the quality of the product on-line is an interesting tool. In this paper, polymer composites made from glass fibers and glass microspheres embedded in a matrix of PS are used to simulate two-phase polymer blend morphology with a skin/core configuration. The relationships between the morphology of two-phase systems and the diffuse pattern scattered from an incident light beam are investigated through an analysis of the iso-intensity plots. Information can be obtained from the ratio of the axes of the ellipsoidal iso-intensity domains for depth-invariant blend morphology, and from the variation of this ratio as a function of the distance from the center of the pattern for depth-varying morphology samples. It is shown that one can differentiate the skin morphology and thickness effects.     

Structural characterization of semicrystalline polymer blends by small-angle neutron scattering

A series of phase-separated polyethylene-polypropylene blends, which have undergone different thermal treatments, have been analysed by small-angle neutron scattering (SANS). The coherent scattering from normal hydrogenated blends is virtually zero, but strong contrast may be induced by partial or complete deuteration (labelling) of either phase. Here, the scattering from blends with complete labelling of the polyethylene phase was analysed to provide the domain dimensions by means of a theory due to Debye using an exponential correlation function. By this means the mean chord intercept lengths of both phases were shown to be in the range 1000–10000Å. The scattering from blends with partial labelling of the polyethylene was analysed to give the radius of gyration of the molecules in the polyethylene domains, which was found to be close to that measured in the homopolymer. For melt-quenched blends the deuterated polyethylene was shown to be statistically distributed in the polyethylene phase, whereas for slow-cooled blends, partial segregation of the labelled molecules occurred.     

Reptation in polymer blends

Forward recoil spectrometry (FRES) was used to measure the tracer diffusion coefficients D*_PS and D*_PXE of deuterated polystyrene (d-PS) and deuterated poly(xylenyl ether) (d-PXE) chains in high molecular weight protonated blends of these polymers. The D*s were shown to be independent of matrix molecular weights and to decrease as M⁻², where M is the tracer molecular weight, suggesting that the tracer diffusion of both species occurs by reptation. These D*s were used to determine the monomeric friction coefficients ζ_0,PS and ζ_0,PXE of the individual PS and PXE macromolecules as a function of ф, the volume fraction of PS in the PS:PXE blend. Since ζ_0,PSζ_0,PXE at each ф, the rate at which a PS molecule reptates is much greater than that of a PXE molecule, even though both chains are diffusing in identical surroundings. Part of this difference may be due to the difficulty of backbone bond rotation of the PXE molecule. However, even when measured at a constant temperature increment above the glass transition temperature, ζ_0,PS and ζ_0,PXE were observed to be markedly composition dependent. In addition the ratio ζ_0,PS/ζ_0,PXE varied from a maximum of 4 × 10⁻² near ф=0.85 to a minimum of 5 × 10⁻⁵ for ф=0.0. These results show that intramolecular barriers do not solely determine the ζ₀s of the components in this blend. Clearly, the interactions between the diffusing chains and the matrix chains also influence ζ₀.     

Hydrogen-bonding in polymer blends

Hydrogen bonding in polymer blends is a topic of great interest to polymer scientists because such systems have many potential applications. Introducing functional groups to one component to make it capable of forming hydrogen bonds to another, thereby enhancing the miscibility of otherwise immiscible blends, is one of the major achievements during the past 20 years of polymer science. The Painter–Coleman association model generally describes these interactions accurately. This Review discusses in detail the effects of hydrogen bonding on the miscibility and thermal properties of polymer blend systems.     

A polarized light scattering approach for dispersed phase morphology characterization in simulated polymer blends

A light scattering technique using a normal-incidence polarized light beam for the characterization of skin/core simulated polymer blend samples is described. The patterns of reflected, polarized scattered light from an inhomogeneous blend were captured using a video camera. The blend was illuminated from a focused laser source. The simulated samples were constructed by incorporating glass fibers (skin) and glass microspheres (core) in a polymer matrix. Asymmetrical patterns were obtained. They reflect the anisotropic nature of the near-surface morphology. Moreover, the change of the anisotropy ratio of the iso-intensity curves plotted, as a function of distance from the position of the incident laser beam on the sample, gives information about the skin and the core content as well as the skin thickness.     

Segregated structures in carbon black-containing immiscible polymer blends: HIPS/LLDPE systems

The structure/electrical resistivity relationship in CB-loaded immiscible HIPS/LLDPE blends was studied. Effects of CB content and location, dispersed polymer phase size and shape, dispersed phase viscosity, and processing procedures were examined. The elongated dispersed phase in CB-containing blends is essential for promoting conductivity in formulations prepared by melt mixing and compression molding. However, the same formulations proved highly resistive when injection-molded, due to orientation and excessive shearing. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 1097–1106, 1997     

Computations of the Mie scattering coefficient corrected for the forward scattering

Computations of the actual Mie scattering coefficient (K_a) have been performed by taking into account the acceptance angle (₀) of the optical receiving system. Results show that in problems concerning the determination of the aerosol particle parameters through scattering methods, the total scattering coefficient (K_r) computed by taking ₀ = 0 has to be substituted by K_a. Tables of K_a have been arranged for the following values of the particle refractive index (m), size parameter (x) and ₀: M = 1·33, X = 0·1 (0·1) 200 and m = 1·55, X = 0·1 (0·1) 100, for ₀ = 0·1° (0·1°) 1·0°.     

Orientation suppression in fibers spun from polymer melt blends

The addition of very small amounts of carefully selected polymers by melt blending has been found to radically change the flow properties at spinning of conventional polymers such as polyethylene terephthalate, nylon 66, and polypropylene. This has a large effect on spun properties, especially at high wind-up speeds. The additive polymers used have included liquid crystal polymers, polyethlene, polyethylene glycol, and nylon 66. A major phenomen is a considerable lowering of spun orientation, or wind-up speed suppression. In order to activate the non-liquid crystal polymers to achieve this effect special spinning conditions are necessary. The mechanism is viewed as being nonuniform extensional flow in a two-phase system and not molecular interaction.     

Electroactive polymer blends

The rapid development of two new classes of electrically active polymer materials, electronically conducting and electroactive polymers and ion-conducting polymers respectively, offers new possibilities for application of both classes of material, especially in combination with each other. While some of these combinations have been attempted before, they all met serious problems due to poor interpenetration of the two polymers. The recent availability of solubilized and soluble electroactive and conductive polymers has greatly advanced the possibilities of reducing the interpenetration problem. Some experimental studies using the combination of solubilized electroactive polypyrrole with poly(ethylene oxide) in an electroactive polymer blend electrode for solid-state polymer batteries are discussed. The opportunities for using polymer blends for solid-state electrochemical polymeric devices, and avenues for the development of materials for such devices, are also reviewed.     

Interfacial coarsening of ternary polymer blends with partial and complete wetting structures

The coarsening of polymer mixtures is an important route towards major morphology modification in multiphase polymer systems. To date however the coarsening of ternary systems has not been significantly examined. In this study the phase coarsening mechanism via annealing for partial wetting, and complete wetting morphologies in ternary polymer blends is characterized. This is a route towards the examination of interfacial coarsening in polymer blends since ternary partially wet systems involve the presence of interfacial droplets while completely wet ternary systems are comprised of a complete interfacial layer. A partial wetting type of morphology is obtained for polybutylene succinate (PBS)/poly(lactic acid) (PLA)/polycaprolactone (PCL). Three different compositions for that system with composition ratios of ?_(PBS/PLA) = 1.5; ?_(PBS/PLA) = 3; and ?_(PBS/PLA) = 10 are prepared to show the effect of the concentration of the self-assembled PLA droplets located at the interface of PBS/PCL. As the concentration of PLA decreases, the growth rate of the PLA phase during the annealing process sharply decreases due to a significant increase of the “surface to volume ratio” of the PLA droplets required in order to cover the interface. In this case, due to the short inter-droplet distances between PLA droplets at the interface, coalescence is controlled by the drainage time. This mechanism is confirmed by the observation of a linear relationship between the third power of droplet size and annealing time. For the 37.5%PBS/12.5%PLA/50%PCL blend, the conservation of interfacial-angles confirms that the annealing time has no effect on the angle values between phases, as predicted by Harkins spreading theory.     

Quasielastic scattering from dilute polymer solutions

The initial slope $\text{Ω(q)}$ of the time-dependent scattering function and the static structure factor S(q) of macromolecules in good solvents are calculated without using the Gaussian assumption. For S(q), the scaling relation S(q)～q^?1.7 is confirmed in the intermediate q range, whereas no q dependent cross-over behaviour can be observed. The calculation of $\text{Ω(q)}$ reveals a q³ dependence in this range and no evidence is seen for an exponent apart from 3. The time-dependent scattering function S(q,t) is treated within the framework of the Gaussian assumption. Hydrodynamic as well as excluded volume interactions are incorporated. It has been found that excluded volume effects decrease the decay of the scattering function in comparison with the Gaussian chain.     

Gel electrolyte membranes derived from co-continuous polymer blends

Polymer gel electrolyte membranes were prepared by first casting films of poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt, and poly(ethylene glycol) (PEG) monomethacrylate and dimethacrylate macromonomers. Polymerization of the macromonomers initiated by UV-irradiation then generated solid films having phase-separated morphologies with a microporous PVDF-HFP phase embedded in PEG-grafted polymethacrylates. Gel electrolyte membranes were finally prepared by allowing the films to take up solutions of LiTFSI in γ-butyrolactone (γ-BL). The PEG-grafted polymethacrylate in the membranes was found to host the largest part of the liquid electrolyte, giving rise to a highly swollen ionic conductive phase. Results by FTIR spectroscopy showed that the Li⁺ ions preferentially interacted with the ether oxygens of the PEG chains. The properties of the membranes were studied as a function of the ratio of PVDF-HFP to PEG-grafted polymethacrylate, as well as the degree of crosslinking, LiTFSI concentration, and liquid electrolyte content. The self-supporting and elastic gel membranes had ionic conductivities of 10⁻³ S cm⁻¹ and a mechanical storage modulus in the range of 2.5 MPa in the tension mode at room temperature. Variation of the salt concentration showed the greatest effect on the membrane properties.     

Hierarchically porous polymeric materials from ternary polymer blends

Hierarchically porous polymers with controllable pore size were generated through a novel polymer blending strategy in an A/B/C–B–C ternary blend system. Polylactide/high-density polyethylene/poly(styrene-ethylene/butylene-styrene) triblock copolymer (PLA/HDPE/SEBS) was used as a model system to demonstrate this technique. During melt blending, the SEBS was driven into the HDPE phase owing to the presence of the PE block in the copolymer. With proper volume fractions of PLA/HDPE/SEBS (e.g., 50/25/25), a bi-modal, dual co-continuous morphology was obtained and hierarchically porous polymeric materials were further generated by selectively removing the PLA and SEBS phases. Annealing and compositional variation were further employed to control the pore size and it is shown that the length scales of the two co-continuous morphologies can be controlled independently.     

Morphology development in polymer blends

The major morphological changes during polymer blending occur during the initial softening stage. This work explains the evolution of phase morphology of polymer blends from pellets to submicron particles in a co-rotating twin-screw extruder. The extruder was opened and blend samples were taken along its length. The major phase component was extracted by means of a selective solvent so that the dispersed phase morphology could be viewed directly by using scanning electron microscopy. The two systems studied were 80:20 polystyrene/amorphous polyamide and 80:20 polystyrene/polypropylene. In both systems, the initial morphology consisted of sheets of dispersed phase. Holes form in the sheets, and these holes grow as a result of interfacial tension forces until they coalesce with each other, forming thin ligaments. These fluid ligaments are unstable and break up via mixer shear forces. Very large changes in dispersed phase size are observed during the softening stage. The particle size changes less after the polymers are completely melted. The extruder results are compared to results from a batch mixer. The same dispersed phase sheeting mechanism is seen in the initial morphology in the batch mixer and the breakup of the dispersed phase domains parallels the breakup seen in the extruder.