Application of two phase model to linear dynamic rheology of filled polymer melts

The linear dynamic rheology for a series of filled polymer melts is investigated to take account for the respective contributions of the bulky polymer phase away from the filler inclusions and the “filler phase” composed of filler particles coated with polymer layer. Time-concentration superposition principles are introduced to the linear viscoelasticities of both the bulky polymer phase and the “filler phase”. The result highlights the importance of the polymer dynamics to both the “filler phase” and the composite melt. Polymer mediated filler jamming towards glass formation is revealed for the “filler phase”, which accounts for origin of the fluid-to-solid transition upon filling.     

Linear viscoelasticity of polymer blends with co-continuous morphology

The co-continuous morphology of polymer blends has received much attention not only because of its potential promotion of mechanical or electrical properties of polymer blends, but also due to its importance in phase separation by spinodal decomposition. Compared to the recent advances in the characterization of co-continuous structure, the rheology of co-continuous blends has not been understood clearly. In this work, a rheological model is suggested to correlate the linear viscoelasticity and the structural information of co-continuous blends. The dynamic modulus of co-continuous blends is composed of the contribution from components and the interface. The interfacial contribution, which is most important in the rheology of blends, is calculated from a simplified co-continuous structure. This model has been compared satisfactorily with available experimental results, which proves a reasonable connection between the co-continuous structure and linear viscoelasticity of blends.     

Morphology and rheology of immiscible polymer blends filled with silica nanoparticles

The effect of silica nanoparticles on the morphology and the rheological properties of an immiscible polymer blend (polypropylene/polystyrene, PP/PS 70/30) was investigated. Two types of pyrogenic nanosilica were used: a hydrophilic silica with a specific surface area of 200 m²/g and a hydrophobic silica having a specific surface area of 150 m²/g. First, a significant reduction in the PS droplet volume radius, from 3.25 to nearly 1 μm for filled blends with 3 wt% silica, was observed. More interestingly, image analysis of the micrographs proved that the hydrophilic silica tends to confine in the PS phase whereas hydrophobic one was located in the PP phase and at the PP/PS interface (interphase thickness ≈ 100-200 nm). Furthermore, a migration of hydrophilic silica from PP phase toward PS domains was observed.An analysis of the rheological experimental data was based on the framework of the Palierne model, extended to filled immiscible blends. Due to the partition of silica particles in the two phases and its influence on the viscosity ratio, limited cases have been investigated. The rheological data obtained with the hydrophobic silica were more difficult to model since the existence of a thick interphase cannot be taken into account by the model. Finally, the hypothesis that hydrophilic silica is homogeneously dispersed in PS droplets and that hydrophobic silica is dispersed in PP matrix was much closer to the actual situation. It can be then concluded that stabilization mechanism of PP/PS blend by hydrophilic silica is the reduction in the interfacial tension whereas hydrophobic silica acts as a rigid layer preventing the coalescence of PS droplets.     

Rheological behavior of highly filled polymer melts

The rheological behavior of highly filled polymer melts has been examined. At concentrations near the maximum packing fraction, strain-dependent behavior was observed at strains as low as 1 percent. Selected surface treatments were shown to reduce particle agglomeration. This produced composite melts with lower viscosities and higher maximum loadings. While η* – ω plots provide information on the shear strength of the interparticle network, G′ – ω plots show evidence of phase separation.     

Rheograms of filled polymer melts from melt-flow index

Polymers filled with extending fillers, such as calcium carbonate or talc, or with reinforcing fillers, such as glass fibers or mica, are increasingly being used in a number of applications. The addition of fillers to a polymer changes the melt rheological behavior of the polymer. A knowledge of the viscosity vs. shear-rate flow curves of the filled system at various temperatures and as a function of filler parameters (such as filler type, shape, and amount) is necessary for process design, optimization, and trouble shooting. The generation of such rheological data is ezpensive and cumbersome in view of the broad range of fillers and the large numbers of filler parameters. In the present article, a unifying approach is proposed that coalesces the flow curves of filled systems of a polymer at various temperatures into a master curve that is independent of the filler parameters. An effective methods is presented to estimate the rheograms of filled systems with the use of a master curve, characteristic of a genetic resin type, knowing the melt-flow index and glass-transition temperature of the system. Master curves have been reported for filled system of low-density polyethylene, high-density polyethylene, poly-propylene polystyrene, nylons, poly(ethylene terephthalate), and polycarbonate.     

Effect of physical ageing on the viscoelasticity of interpenetrating polymer networks

The effect of physical ageing on viscoelastic properties was studied for semi‐IPNs based on crosslinked polyurethane and poly(butyl methacrylate) taken in the ratio 75/25 by mass. The viscoelastic properties of IPNs were studied after physical ageing and after heat treatment at 60 °C. Significant changes in viscoelastic behaviour after ageing were observed. It was found that heterogeneous systems such as IPNs have their own specific features of physical ageing which are related to the existence of two glass transition to temperatures. Relaxation processes to establish the equilibrium state may need a long period of time. © 2000 Society of Chemical Industry     

Porous wollastonite-hydroxyapatite bioceramics from a preceramic polymer and micro- or nano-sized fillers

Wollastonite-hydroxyapatite ceramics have been successfully prepared by a novel method, corresponding to the thermal treatment in air of a silicone embedding micro- and nano-sized fillers. CaCO₃ nano-sized particles, providing CaO upon decomposition, acted as “active” filler, whereas different commercially available or synthesised hydroxyapatite particles were used as “passive” filler. The homogeneous distribution of CaO, at a quasi-molecular level, favoured the reaction with silica derived from the polymer, at only 900 °C, preventing extensive decomposition of hydroxyapatite. Open-celled porous ceramics suitable for scaffolds for bone-tissue engineering applications were easily prepared from filler-containing silicone resin mixed with sacrificial PMMA microbeads as templates. The pore size (in the range of 80-400 μm) and the open porosity percentage (40-50%) were evaluated by means of micro-computerized tomographic analysis. A preliminary assessment of the biocompatibility and cell activity of the produced ceramics was performed successfully by in vitro tests using human osteoblast cells.     

Monitoring the quality of mix of polymer melts with particulate fillers using fluorescence spectroscopy

Fluorescence spectroscopy has been employed to monitor the mixing of polymer melts with filler material. The polymer melts were low molecular weight polybutadiene and PBAN, a terpolymer consisting of 85% butadiene, 11% acrylonitrile, 4% acrylic acid, and the filler material was aluminum oxide. To carry out the fluorescence observations, a dopant chromophore was mixed into the polymer melt at very low concentrations, 10^?4 to 10^?6 molar or 11 to 0.11 ppm by weight. The mixing experiments were carried out using a small laboratory mixer which had glass walls for viewing the fluorescence spectra from the dopant chromophore. Fluctuations in fluorescence intensity were observed to decrease as a function of mixing time indicating that the spatial distribution of the fluorescent chromophores was becoming more uniform. Concerning the mixing of polymer melt and filler, we hypothesize that uniform mixing of ingredients is achieved when fluorescence intensity as a function of time is constant. In order to obtain quantitative support for this hypothesis, we used a fluorescence microscope to measure fluorescence intensity and optical transmittance from microscopic regions of well-mixed and poorly mixed specimens.     

Dispersion of fillers and the electrical conductivity of polymer blends filled with carbon black

Dispersion state of carbon black(CB) was studied in polymer blends which are incompatible with each other. It was found that CB distributes unevenly in each component of the polymer blend. There are two types of distribution. (1) One is almost predominantly distributed in one phase of the blend matrix, and in this phase fillers are relatively homogeneously distributed in the same manner as a single polymer composite. (2) In the second, the filler distribution concentrates at interface of two polymers. As long as the viscosities of two polymers are comparable, interfacial energy is the main factor determining uneven distribution of fillers in polymer blend matrices. This heterogeneous dispersion of conductive fillers has much effect on the electrical conductivity of CB filled polymer blends. The electrical conductivity of CB filled polymer blends is determined by two factors. One is concentration of CB in the filler rich phase and the other is phase continuity of this phase. These double percolations affect conductivity of conductive particle filled polymer blends.     

Improvement of thermo-electrical properties of polymer composites filled with multiwall carbon nanotubes decorated carbon fillers

To enhance the thermo-electrical properties of liquid silicone rubber (LSR) in applications, the carbon fibres (CFs) modified by multiwall carbon nanotubes (MWCNT) on the surfaces were used as the fillers. The MWCNT-modified CFs (MPCFs) were analysed by Fourier transform infrared spectra, thermogravimetric analysis, scanning electron micrograph and energy dispersive X-ray spectroscopy. It was found that MWCNT were successfully adsorbed onto the surface of CFs. The MPCFs functioned as conductive fillers in LSR for thermal and electrical conductivity application and exhibited significant enhancement. The effects of MPCFs loading on thermal conductivity and volume resistivity of LSR composites were investigated in detail. Results of this work revealed that the MPCFs/LSR composites possessed a thermal conductivity of 0.73?W?m^?1?K^?1 with 14?vol.-% filler loading, approximately 3.48-fold higher than that of pure LSR substrate. And with the increase of MPCFs loading, the least volume resistivity of MPCFs/LSR composites is 10?Ω?cm. Besides, compared with that of neat LSR, the tensile strength of MPCFs/LSR composites increased 0.913?MPa.     

Chain dynamics in polymer melts near tg studied by depolarized rayleigh scattering and viscoelastic measurements

In terms of the freely jointed chain model, the depolarized Rayleigh scattering time-correlation function has been analysed. The reorientation motion of a Kuhn segment represents the process in a polymer melt as often observed by depolarized photon-correlation spectroscopy at temperatures close to the glass transition point. With respect to the coordinate frame associated with the Kuhn segment, it is shown that the local segmental motions (corresponding to the so-called (ß process) should be reflected in the short time region of the photon-correlation spectroscopy as implied by a previous experimental observation. The a process as probed by the depolarized light scattering and that as extracted from the viscoelastic data in terms of a previously developed molecular theory are in good agreement in both the magnitude and the temperature dependence of the relaxation time (from 110 to 130°C).     

聚阴离子纤维素(PAC)与羟丙基甲基纤维素(HPMC)混合体系动态粘弹性研究

考察了溶液浓度对PAC以及PAC/HPMC水溶液动态粘弹性的影响。结果表明,溶液储能模量(G’),耗能模量(G")均随溶液浓度的增大而增大,但储能模量随频率增大的幅度大于耗能模量随频率增大的幅度;溶液的损耗角正切(tanδ)值随浓度增大而逐渐减小;G’~f与G"~f两曲线交点随溶液浓度增大向低频方向移动;由于PAC与HPMC分子间存在氢键作用,因此,与纯PAC溶液相比,相同浓度下PAC/HPMC水溶液的G’~f与G"~f交点对应的频率更高;ω=0相对应的初始tanδ值比同浓度下纯PAC溶液更高。     

尼龙6动态黏弹性研究

利用熔融指数仪模拟尼龙6的加工工艺,通过改变挤出温度和挤出压力,使尼龙6熔融塑化挤出成丝状样条,热拉伸后所得纤维状样品采用动态力学分析仪测定其储能模量(E')、损耗模量(E")及损耗因子(tanδ)与温度和压力的关系。结果表明:在一定压力下,不同加工温度对尼龙6的储能模量、损耗模量无线性影响,然而加工温度越高,其损耗因子越大;同一加工温度下,储能模量、损耗模量与加工压力的线性关系不明显,而损耗因子对加工压力不敏感,几乎无变化。     

Ionic conductivity and electrochemical properties of nanocomposite polymer electrolytes based on electrospun poly(vinylidene fluoride-co-hexafluoropropylene) with nano-sized ceramic fillers

A series of nanocomposite polymer electrolytes (NCPEs) comprising nanoparticles of BaTiO₃, Al₂O₃ or SiO₂ were prepared by electrospinning technique. The nano-sized ceramic fillers were incorporated into poly(vinylidene fluoride-co-hexafluoropropylene) [P(VdF-HEP)] membranes during the electrospinning process. The resultant porous membranes are good absorbent of the liquid electrolyte and exhibit high electrolyte retention capacity. The presence of the ceramic nanoparticles has positive effect on the mechanical properties of the membranes. The ionic conductivity and the electrochemical stability window of the electrospun P(VdF-HFP)-based polymer are enhanced by the presence of the fillers. The cell Li/LiFePO₄ based on the NCPE containing BaTiO₃ delivers a discharge capacity of 164 mAh/g, which corresponds to 96.5% utilization of the active material. In comparison, the performance of Li/LiFePO₄ cells with NCPEs containing Al₂O₃ and SiO₂ was observed to be lower with respective discharge capacities of 153 and 156 mAh/g. The enhanced performance of the BaTiO₃-based-NCPE is attributed mainly to its better interaction with the host polymer and compatibility with lithium metal.     

Linear viscoelasticity of discotic mesophases

The small-amplitude oscillatory capillary Poiseuille flow of uniaxial incompressible discotic nematic liquid crystals, representative of discotic mesophases, is characterized using analytical, computational, and scaling methods. Linear viscoelastic material functions are calculated and discussed in terms of fundamental anisotropic viscoelastic processes. The role of orientation to generate flow and store elastic energy is discussed. Viscoelastic behavior is found only at frequencies of similar magnitude to the single orientation relaxation time. In the terminal small-frequency zone the storage modulus scale as G^′∼ω², and the loss modulus as G^″∼ω, typical of viscous fluids. Comparisons between steady and oscillatory Poiseuille flow shows that the Cox-Merz rule is not obeyed, but that as expected the steady and complex viscosities in the terminal zone are identical. A remarkable and useful correspondence between the stored elastic energy under steady flow and the storage modulus G^′ has been discovered.     

Contact-angle development of polymer melts

In powder coatings, the quality of the final film is often associated with its smoothness. This is affected by the wetting and levelling process which occurs during film formation and is mainly determined by surface tension (driving force) and viscosity (resistance).

Especially for the surface tension the requirements are complex. On the one hand a lower surface tension facilitates the substrate wetting process, but if it is too low levelling is poor, resulting in wavy surfaces (“orange peel”). On the other hand, a higher surface tension promotes levelling, but if it is too high the wetting is poor, resulting in crater defects. Therefore, the surface tension of a powder coating has to be optimised carefully. It can be adjusted by addition of “levelling” or “surface flow” compounds to a formulation as well as changing the composition of the resin. Most of the current knowledge however is based on trial and error and a more systematic approach should provide better fundamental insight into the interrelation between the different parameters.

Since there is no generally accepted method of measuring wetting and levelling, a new method has been devised by which the actual melting and flow process can be monitored. The method provides information about the wetting and flow process of polymers by measuring the contact-angle of polymer melts as a function of temperature and gives significant and consistent results while data analysis provides quantitative information.

In this presentation, the method will be explained and examples will be given, showing the effects of structural parameters of coating binders on their wetting and flow behaviour.     

Melt strength of calcium carbonate filled polypropylene melts

This paper investigates the extensional rheology (through melt strength measurement) of calcium carbonate (CaCO₃) filled polypropylene (PP) melts. Different concentrations of CaCO₃ filled PP were produced by mixing two master batches of pure PP and 70 wt% CaCO₃ filled PP in required proportions in a counter‐rotating twin‐screw extruder. It was found that the melt strength of the CaCO₃–PP melts was independent of CaCO₃ concentrations up to 25 vol%. Further increase in CaCO₃ concentration led to a severe reduction of melt strength. © 2002 Society of Chemical Industry     

Linear viscoelasticity and structure of polypropylene-montmorillonite nanocomposites

Polypropylene (PP)/clay composites were prepared by melt mixing in a thermoplastic mixer using a polypropylene grafted with maleic anhydride (PPg) as the compatibilizer. Concentrations of an organophilic montmorillonite (MMT) between 2 and 15 wt% and concentration ratios of PPg/clay between 1:3 and 3:1 were employed to investigate the relationship between the structural characteristics of the hybrids and their rheological properties. The structure was analyzed with electron microscopy, X-ray diffraction and melt rheology. Thermogravimetric analysis and infrared spectroscopy were also used. The clay interlayer spacing increases after mixing with PP while the addition of PPg only facilitates the partial exfoliation of the clay platelets without changing that spacing. When clay loadings of 8 wt% or larger were used, an important fraction of the original clay particles was found to remain unmodified. The dynamic moduli show little effect of the presence of the inorganic material when no compatibilizer is added or the amount of PPg or clay is too small. As the extent of exfoliation increases, the linear viscoelastic behavior of the composites gradually changes with time while in the molten state, mainly at low frequencies. Evidence of solid-like behavior appears as the concentration of clay increases, for a given PPg/clay or PP/PPg concentration, or as the PPg concentration increases (for a given clay concentration). The concentrations of PPg and clay that induce percolation were observed to have an inverse relation. Evidence of regions with large concentration of MMT was obtained in the annealed samples of composites with solid-like rheological behavior. Additionally, infrared spectra of these materials suggest the simultaneous occurrence of chemical reactions between the PPg and the surfactant or products derived from its thermal decomposition during the annealing process.     

Model filled polymers. XIII: Mixing and time-dependent rheological behavior of polymer melts containing crosslinked polymeric particles

The rheological properties of polystyrene (PS) and poly(methyl methacrylate) (PMMA) composites filled with monodisperse sized crosslinked polymeric particles are sensitive to processing history and chemical composition. Particles compatible with the matrix, such as PMMA or copolystyrene-vinylphenol in a PMMA matrix, are randomly dispersed on mixing, yielding (almost) Newtonian fluids. Particles incompatible with the matrix, such as PS or copolystyrene-acetoxystyrene in PMMA, produce composites whose steady shear viscosities depend on shear rate and whose dynamic moduli are elevated at low frequency. Particles in incompatible composites tend to cluster, producing a structure that is destroyed at high shear rates and that reforms on aging at elevated temperatures.     

Effects of polymer-filler adhesion on the properties of polychloroprene elastomers filled with surface-treated fillers

A new analytical method has been employed to determine values for the surface-energy properties, such as the solid/vapor surface energy and the equilibrium work of adhesion, of several polychloroprene compositions filled with organosilane-treated wollastonite fillers. The equilibrium work of adhesion, which represents the amount of energy stored in the interface formed between polymer and filler, has been used as a key material parameter to correlate changes in the mechanical and rheological properties of wollastonite-filled polychloroprene compositions. Experimental results show that some properties such as tear strength, tensile modulus, shear viscosity, and compression set, which depend on polymer-filler adhesion to varying degrees, increase as the equilibrium work of adhesion increases. On the other hand, properties such as tensile strength and ultimate elongation, which largely depend on the degree of mixing of filler particles and such defect structures as microvoids, decrease with the increase of the equilibrium work of adhesion. A power-law relationship between the tensile modulus and the equilibrium work of adhesion has also been established. This relationship can be used for selecting organosilane-treated fillers in order to achieve optimum properties.