Transparency of polymer blends

Polymer blending is useful for improving physical properties. Blends of transparent polymers are generally hazy. However, transparency is required in many products such as packages (especially PET bottles). The miscible blends, PET (polyethylene terephthalate)/PBT (polybuthylene terephthalate), maintain transparency in almost all cases regardless of the blending ratio, whereas the immiscible blends, PET/MxNYLON (MXD6 nylon, i.e. polymethaxylirene adipamide) and PET/PMAI (polymethacril imide, i.e. N‐methyl dimethyl glutarimide) become hazy as the blending ratio increases. The reason for this haze is the number and size of the dispersed particles. Differences in the refractive indices of various polymers also have a large influence on haze. Stretching makes even the transparent blends hazy in the case of PET/MxNYLON. One reason for this phenomenon is that stretching increases the size of the dispersed particles in the sheet plane. A second reason is that the difference in the anisotropic refractive indices of the matrix and the dispersed phase is increased by stretching. These effects are very consistent with light scattering theory.     

The optical properties of two-phase polymer systems: Single scattering in monodisperse,non-absorbing systems

Two-phase polymer systems have achieved commercial importance due mainly to the improvement in impact strength brought about by the addition of dispersed rubber particles to a normally brittle glassy polymer. Rubber-reinforced polystyrene and ABS plastics are two familiar examples. An important drawback of this class of materials is their lack of transparency, caused by the scattering of light at the interface between the phases. The theory of light scattering by spherical particles indicates that the degree of scattering (turbidity) is a function of the amount of dispersed phase present, its particle size, the ratio of refractive indices of the phases, and the wavelength of light. Quantitative predictions of the effects of the above parameters on the transparency of two-phase systems can be made, providing answers to the questions “How close must the refractive indices be?” and “What size must the dispersed-phase particles be?” for a given level of transparency. Calculations for typical polymer pairs reveal that at a given dispersed-phase level, a maximum in turbidity is obtained roughly in the range of particle sizes thought to be necessary for good impact strength. Also, if the refractive indices are matched at a particular temperature, small particle sizes greatly increase the temperature range over which scattering is minimized.     

Mie scattering from sphere structures in polymer blends

Polymer blends of transparent poly(methyl methacrylate) and polystyrene become opaque due to light scattering at the boundaries of the two polymers. The polymer blend is light brown when it is illuminated by white light. The coloring depends on the spherical domain structures existing in the polymer blend. The coloring was analyzed by using the rigorous Mie theory. The Mie results were compared with the semiempirical results previously reported by the authors. The wavelength dependence of theoretical scattering efficiencies on radii of scattering spheres from 0.05 to 1.2 μm was obtained for polystyrene spheres in poly(methyl methacrylate) matrix, and vice versa. The scattering at the short wavelength region is stronger than at the long wavelength region. The scattering efficiencies become almost constant in the visible wavelength region for sufficiently large spheres.     

Transparent multiphasic polystyrene/epoxy blends

Blends of polystyrene (PS) with an epoxy monomer (DGEBA) and a tertiary amine (BDMA), were initially miscible at 120°C but phase‐separated at very low conversions in the course of polymerization. Although there was a significant difference between the refractive indices of polystyrene and the DGEBA/BDMA solution, the refractive index of the epoxy network increased in the course of polymerization, attaining a value close to that of PS at complete conversion. A sharp decrease of the light transmittance was observed at the cloud‐point, observed at very low conversions. However, the continuous increase of the refractive index of the epoxy phase with conversion produced an approximate matching of both refractive indices, leading to transparent materials at complete conversion. Morphologies generated by reaction‐induced phase separation depended on the molar mass distribution of polystyrene and its mass fraction in the blend. For a PS with a high value of the mass‐average molar mass (M_w), it was possible to generate a dispersion of PS particles in the epoxy matrix (blends containing 5 wt% PS), phase‐inverted morphologies (blends containing 15 wt% PS) and double‐phase morphologies (blends with 10 wt% PS). Therefore, PS/DGEBA/BDMA blends could be used to obtain transparent epoxy coatings toughened by polystyrene particles or transparent polystyrene parts reinforced by a dispersion of epoxy particles.     

Barrier Properties of Blends Based on Liquid Crystalline Polymers and Poly(ethylene terephthalate)

Abstract

Blends of poly(ethylene terephthalate) (PETP) and two different thermotropic liquid crystalline (LC) polymers of the Vectra-type were prepared by melt mixing. Oxygen and water vapor permeability, light transmission and welding properties were measured on compression-molded and film-blown specimens. SEM showed that the LC polymers were the disperse phase with a good phase adhesion to the PETP matrix in the majority of the compression-molded blends. The 50/50 blend based on the low melting point LC polymer showed possibly a continuous LC polymer phase. The film-blown specimens showed LC polymer spheres at low LC polymer content. Above a certain LC polymer content (10-30% LC polymer), fibrous and ellipsoidal LC polymer particles was the dominant morphological feature of the blends. Density measurements showed that the void content in the blends was low. The compression-molded blends based on the high melting point LC polymer showed permeabilities conforming to the Maxwell equation assuming low permeability (LC polymer) spheres in a high permeability (PETP) matrix. The compression-molded blends based on the low melting point Vectra showed lower permeabilities than predicted by the Maxwell equation, particularly at high LC polymer content. The film-blown blends showed extensive scattering in the permeability data. The blend with 30% low melting LC polymer exhibited a 96% lower oxygen permeability than PETP. This was due to a reduction in both oxygen diffusivity and solubility. Ellipsoidal and fibrous LC polymer particles increased the diffusional path and lowered the diffusivity. The transparency of the compression-molded samples was lost already at 1% LCP. The blends showed welding properties superior to those of PETP.     

Characterization of polymer conformation and morphology through small-angle neutron scattering—A literature review

Small-angle neutron scattering, SANS, stands forth as one of the most important of the new tools for evaluating polymer chain conformation and morphology. This paper reviews the SANS literature through 1982, with a few early 1983 references added. The theory of SANS is outlined and compared to light scattering. SANS values of polymer molecular weights and radii of gyration obtained in the bulk state were found to be in agreement with values obtained from dilute solutions by light scattering. In each case, deuterated fractions of polymer were inserted into the hydrogeneous matrix, or vice versa, to provide contrast. Several new research areas are then discussed, including unidirectional stretching of elastomers, stress-relaxation, polymer-polymer miscibility, crystallization from the melt compared with crystallization from dilute solutions, nonclassical aggregation during bulk polymerization of thermoset systems, morphology of polymer blends, block copolymers and ionomers, the core-shell structure of latexes and polymer blocks, and grafts as surfactants in emulsions and latexes. Much new and sometimes unexpected information is being provided by the SANS research now in progress.     

Optical properties of dense and porous spheroids consisting of primary silica nanoparticles

The light scattering by granular and macroporous silica spheroids consisting of nanometer-sized primary particles was systematically investigated using a laser particle counter coupled with a pulse height analyzer. The shape- and porosity-controlled spheroids as model particles were prepared using spray drying method by changing the particle size of colloidal suspension. The effect of shape and porosity of dense and porous spheroidal particles on electrical mobility was also studied using a differential mobility analyzer and an electron microscope. The electrical mobility equivalent diameter of particles classified by the differential mobility analyzer was estimated by measuring Feret diameter and the projected area equivalent diameter from the SEM micrographs. The electrical mobility diameter of the spheroids was in good agreement with the projected area equivalent diameter regardless of the primary particle size and porosity. The measured partial scattering cross section of dense and porous silica particles with same mobility diameter showed significant differences. As the primary particle size of granules and the porosity of porous particles increased at parity of electrical mobility diameter, the scattering intensity decreased. The effective refractive indices of dense and porous particles were computed by best fitting of the scattering intensity measurements. The porosities of dense and porous spheroids were calculated using the effective refractive indices as obtained by the effective medium theory. The porosities were also measured by a comparison of particle size before and after annealing at 1700°C. By comparing these porosities, the effective refractive indices of the spheroidal particles were confirmed.     

Highly transparent thermoplastic elastomer from isotactic polypropylene and styrene/ethylene‐butylene/styrene triblock copolymer: Structure‐property correlations

Polypropylene (PP) is one of the most useful general purpose plastics. However, the poor transparency and brittleness of PP restricts its applications in the field of medical and personal care where silicone and polyvinyl chloride (PVC) are presently used. This work concentrates on developing highly transparent elastomeric PP blends and also thermoplastic elastomer by blending isotactic polypropylene (I‐PP) with styrene/ethylene‐butylene/styrene (SEBS) triblock copolymer. PP/SEBS blend derived from high melt flow index (MFI) PP and high MFI SEBS exhibit remarkable transparency (haze value as low as 6%) along with good percentage of elongation and processability. The reduction in difference of refractive index (RI) between PP and SEBS has been observed by blending SEBS with PP. The wide angle X‐ray diffraction studies show that there is significant reduction in the percentage crystallinity of PP by the addition of SEBS block copolymer. Temperature‐dependent polarized light microscopy studies reveal the reduction in spherulites size by the addition of SEBS block copolymer. Transmission electron micrographs show that the SEBS polymer forms a fine lamellar structure throughout the PP matrix with phase inversion at higher SEBS concentration. Development of phase morphology, crystalline morphology, and crystallinity in different blends has been analyzed and microstructure‐haze correlations have been developed. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Core-shell structured latex particles. III. Structure–properties relationship in toughening of polycarbonate with poly(n-butyl acrylate)/poly(benzyl methacrylate–styrene) structured latex particles

The performance of the designed structured core-shell latex particles in toughening polycarbonate (PC) matrix was examined. Izod impact testing of the PC-core-shell latex blends were used to evaluate the influence of parameters related to the core-shell latex particles on toughening polycarbonate. Among these parameters are the particle size and levels of crosslinking of the core rubber particles, composition and molecular weight of the shell polymer, and weight ratio of shell to core polymers as well as the particle morphology. In this work, core-shell structured latex particles with thinner shells of higher molecular weight polymers were found to improve the impact resistance of polycarbonate. The role of chain entanglements in increased adhesion between the discrete rubbery phase and the continuous glassy matrix and the importance of surface-to-surface interparticle distance for toughening at various temperatures are discussed. © 1995 John Wiley & Sons, Inc.     

Artificial opals prepared by melt compression

In recent years, colloidal crystals with a refractive index varying periodically on the scale of the light wavelengths have been prepared by various methods. These photonic crystals reflect light and exhibit, at sufficiently strong contrast, even a complete band-gap in which light cannot propagate in any direction. Most studies published so far were aimed at such high-contrast photonic crystals with a complete band-gap or their precursors. Frequently, a face-centered cubic (fcc) lattice was built up from monodisperse polymer or silica spheres with diameters in the submicron range. Methods as sedimentation and drying of dispersions led to usually small and thin specimens. This report deals with films that were produced by a novel technique based on shear flow in the melts of polymer core-shell latex spheres. The process is fast and yields large area films, thin or thick, in which the latex spheres are crystallized in fcc order. The refractive index contrast of these purely polymeric films is too small for a complete band-gap photonic crystal, but the films are attractive color materials showing wavelength and angle dependent reflection colors.     

Stereocomplex Crystallization Induced Significant Improvement in Transparency and Stiffness–Toughness Performance of Core-Shell Rubber Nanoparticles Toughened Poly(l-lactide) Blends

Toughening modification of poly(l -lactide) (PLLA) with rubber particles is often realized at the cost of transparency, mechanical strength, and modulus because high rubber loadings are generally required for toughening. In this work, a promising strategy to simultaneously improve the transparency and stiffness–toughness performance of poly(butyl acrylate)-poly(methyl methacrylate) (BAMMA) core-shell rubber nanoparticles toughened PLLA blends by utilizing the stereocomplex (SC) crystallization between PLLA and poly(d -lactide) (PDLA) is devised. The results reveal that the construction of SC crystallites in PLLA matrix via melt-mixing PLLA/BAMMA blends with PDLA can prevent BAMMA nanoparticles from aggregation and promote them to form network-like structure at lower contents. As a result, not only higher toughening efficiency with less rubber contents but also superior transparency is achieved in the PLLA/PDLA/BAMMA blends as compared with the PLLA/BAMMA ones where large aggregated BAMMA clusters are formed. Moreover, the outstanding reinforcement of SC crystallites network for PLLA can impart an enhanced tensile strength and modulus to PLLA/PDLA/BAMMA blends, thus improving the stiffness–toughness performance of PLLA/PDLA/BAMMA blends to a higher degree. This work demonstrates that SC crystallization is a promising solution to solve the contradiction between transparency and mechanical properties and then obtain superior comprehensive performances in rubber toughened PLLA blends.     

Viscoelastic properties and morphological characterization of silica/polystyrene nanocomposites synthesized by nitroxide-mediated polymerization

Polystyrene (PS) chains with molecular weights comprised between 15,000 and 60,000 g/mol and narrow polydispersities were successfully grown from the surface of silica nanoparticles by nitroxide-mediated polymerization (NMP). Small angle X-ray scattering was used to characterize the structure of the interface layer formed around the silica particles, and at a larger scale, dynamic light scattering was used to determine the hydrodynamic diameter of the functionalized silica suspension. In a second part, blends of PS-grafted silica particles and pure polystyrene were prepared to evaluate the influence of the length of the grafted PS segments on the viscoelastic behavior of the so-produced nanocomposites in the linear viscoelasticity domain.Combination of all these techniques shows that the morphology of the nanocomposite materials is controlled by grafting. The steric hindrance generated by the grafted polymer chains enables partial destruction of the agglomerates that compose the original silica particles when the latter are dispersed either in a solvent or in a polymeric matrix.     

Effect of core-shell particles dispersed morphology on the toughening behavior of PBT/PC blends

Methyl methacrylate-co-styrene-co-glycidyl methacrylate grafted polybutadiene (PB-g-MSG) and styrene-co-glycidyl methacrylate grafted polybutadiene (PB-g-SG) core-shell particles were prepared to toughen poly (butylene terephthalate) (PBT) and polycarbonate (PC) blends. The compatibilization reaction between the epoxy groups of glycidyl methacrylate and the carboxyl groups of PBT induced the PB-g-SG particles dispersed in the PBT phase. On the other hand, the good miscibility between PMMA (the shell phase of PB-g-MSG) and PC induced the PB-g-MSG particles dispersed in the PC phase. The different phase morphology led to different toughening behavior. The PBT/PC/PB-g-MSG blends with the PC encapsulated morphology showed much lower brittle-ductile transition core-shell particles content (10-15 wt% or 15-20 wt%) compared with the PBT/PC/PB-g-SG blends (20-25 wt%). The difference between the toughening efficiency of the core-shell particles was due to the change of deformation mechanisms. In PBT/PC/PB-g-MSG blends, the cavitation of PB rubber phase led to the occurrence of shear yielding of the matrix. While in the PBT/PC/PB-g-SG blends, the debonding between PBT and PC interface induced the shear yielding of the matrix. The variation of the core-shell particles dispersed phase morphology also affected the crystallization properties and DMA results of the PBT/PC blends. Modification of the phase morphology provided an useful strategy to prepare PBT/PC blends with higher toughening efficiency.     

Rayleigh散射研究非相容聚苯乙烯／顺丁橡胶合金相结构

用光散射在线采集与分析方法完成了熔融混炼过程中非相容高分子共混物的形态结构分析。选择了聚苯乙烯／顺丁橡胶合金体系。使用了小角前向光散射和小角背散射（在线）技术，用Debye-Bueche光散射理论的结构参数如相关蹁ac、平均弦长l、平均旋转半径Rg和积分不变量Q表征了共混物中分散相尺度和均匀性。用扫描电子显微镜（SEM）测定了共混物中分散相尺寸，并与光散射的结果进行了比较。ac,l和Rg的变化规律与显微镜的结果是一致的。用积分不变量Q研究了共混物的均匀性。     

Polymer compatibility III. The system poly(methyl methacrylate)–poly(vinyl acetate). Characterization studies

Blends of poly(methyl methacrylate) (PMMA) and poly(vinyl acetate) (PVAc) were prepared by mixing the polymers in the melt and in the absence of a solvent. PMMA was the major constituent of the blend. The polymer blends were tested, using various methods, to determine if they are compatible as solids. Data obtained from dynamic mechanical and DSC measurements show that, when they are mixed under given Brabender mix conditions, the blends exhibit properties characteristic of polymer pairs compatible as solids. If the mix conditions are altered, a two-phase system is evidenced. Using micrographs obtained by light microscopy in phase contrast as criteria, two companion blends containing PMMA/PVAc 80/20 would be classified as incompatible as solids because of the differences in refractive index of PMMA and PVAc. The micrographs also show that, in the system that would otherwise be listed as compatible, the PVAc domains appear to be relatively uniform in size and distribution through the PMMA matrix. In its companion blend, large, irregularly shaped particles of PVAc which are poorly dispersed in the PMMA matrix are evident.     

Transparent YAG:Ce ceramic with designed low light scattering for high-power blue LED and LD applications

Yellow-emitting YAG:Ce transparent ceramic is recognized as an ideal color converter in high-power blue LEDs and LDs, but the absence of scattering centers in its microstructure leads to a low light extraction efficiency and poor light uniformity. Here, taking advantage of the scattering effect and the transparency of YAG:Ce ceramics, Ce-free YAG phase was used as a second component to form a composite with YAG:Ce phosphor. The sintered YAG:Ce-YAG ceramic possessed a high transparency of ～63 % and a thermal conductivity of 8.9 Wm^?1 K^?1. Due to its beneficial thermal properties and high external quantum efficiency of 70.2 %, the YAG:Ce-YAG ceramic could be excited under a high blue-laser flux density of up to 9.60 W/mm² and showed a luminous emittance of 1220 lm/mm². Due to light scattering arising from the slightly different refractive indices of the two phases, the designed YAG:Ce-YAG ceramic showed better lighting effects than a single-phase transparent YAG:Ce ceramic.     

Nanocomposites of ZnS and poly-(dimethyl)-block-(phenyl)siloxane as a new high-refractive-index polymer media

ABSTRACT: In the present paper, we describe a new and original method to obtain transparent, siloxane-based composites, with high refractive index (up to 1.68). The method is based on the decomposition of Zn-siloxane, mixed with a poly-(dimethyl)-block-(phenyl)siloxane matrix in different ratios. It was found that after treatment of such mixed metal-containing polymer blend with H2S, the nanoparticles of ZnS are formed, with the size in a 1- to 5-nm range, which allow effective increase of the refractive index of the nanocomposite mixture with poly-(dimethyl)-block-(phenyl)siloxane without loss of film transparency. We succeded to increase the refractive index from 1.54 (pure matrix) up to 1.68 (composite with a ZnS content of 4.6 vol.%). The siloxane-based compositions are optically transparent, which makes it possible to use them as light-emitting diodes or solar cell sealants or adhesives.     

Analysis of contributing factors to production of highly transparent isotactic polypropylene extrusion sheets. Part I

To obtain highly transparent isotactic polypropylene sheet by an industrial process, various factors contributing to transparency were analyzed. Two kinds of sheets were prepared for analysis. The one was extruded by a direct single belt process as the quick quenching system. And the other was extruded by a conventional air knife process as the slow cooling system. The higher order structure of each sample sheet was investigated by phase‐contrast microscopy, polarizing microscopy, light scattering, wide‐angle X‐ray diffraction (WAXD), small angle X‐ray scattering (SAXS), and transmission electron microscope (TEM). In the case of the quick quenching sheet, many spherical substances were generated near sheet surfaces. Comparing with static cooling experiments, it was thought that the main cause was the orientation crystallization by the residual stress. The spherical substances in a phase‐contrast micrograph were extremely coincident with the maltese cross pattern in a polarizing micrograph. Other analysis results also showed that these spherical substances were spherulites. And after the heat treatment, the matrix was transformed from smectic structure to α‐monoclinic crystal phase. The density and refractive index differences between spherulite and matrix were decreased, and the transparency was drastically improved. In addition, some guideline for the improvement of transparency was indicated. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers     

Design of particulate composites for optical applications

This paper reviews the basic theoretical approach to describing light scattering in filled materials with nearly matching refractive indices (Rayleigh-Gans-Debye Theory). The modifications necessary to handle high filler concentrations (interparticle interference and multiple scattering) are included empirically. The primary result of this analysis is an expression for the optical transmission of a polymer composite as a function of particle size, volume fraction, composite thickness, and refractive index difference between the components. The angular dependence of the scattering is included so that scattered light which falls within the aperture of the transmission detector can be accounted for, Particular attention is given to including the temperature dependence of the refractive index and the effect of fillers on the thermal expansion properties of the polymer matrix in order to predict the temperature range over which the composite will be optically useful. Similarity to analyses of the Christiansen filter is discussed.     

Drying dynamics of polymers films by an interferometric technique

We analyze a simple laser reflectivity measurement as a tool to monitor the drying kinetics of transparent polymer films. The reflectivity signal of a laser beam at normal incidence shows oscillations due to interference arising from multiple reflection of the laser light within the drying film. We develop a model to interpret the reflectivity curves in terms of time evolving refractive indices at the top and bottom of the film. We present results of the drying kinetics of transparent alkyd films on a glass substrate of high refractive index. Data shows a clear hallmark indicating the evolution of the crosslinking process. From the reflectivity curves, the time evolution of the refractive indices at the top and bottom is obtained. Assuming a linear-gradient of the refractive index along the depth of the film the average refractive index and consequently the film thickness as a function of time are estimated. Clear features in the time evolution of the refractive indices and thickness, correlate well with qualitative “dust”, “touch” and “fingerprint” drying times. Additionally, we present some preliminary results for water based latex binders, where scattering of light is present, showing that this simple optical technique could be extended for studying latex film formation.