Thermal diffusivity and conductivity of crystalline polymers

The thermal diffusivity and conductivity of the isotropic and drawn samples of five semicrystalline polymers—nylon 6, poly(ethylene terephthalate), poly(butylene terephthalate), polybutene-1, and poly(4-methylpentene-1)—were measured by the flash method over the temperature range 100–350 K. The temperature dependence of the thermal diffusivity was found to follow a simple phenomenological pattern, while a more detailed understanding of the temperature dependence and the effect of orientation on thermal conductivity was obtained by using the modified Maxwell model.     

Thermal diffusivity of polymers by the flash method

We have adopted the flash method to the measurement of thermal diffusivity α of polymers in the temperature range 100–400K. The pulsed radiant energy from a flash tube is applied to the ‘front’ side of a suspended sample disc, and α is deduced from the exponential decay time constant of the subsequent transient temperature difference between the ‘front’ and the ‘back’ side, while correction against radiation loss is made by measuring the much longer decay time of the back-side temperature. Calibration runs on polycarbonate (PC) samples of several thicknesses show that the method is quick, precise and fairly accurate, and the results obtained are in reasonable agreement with previous determinations. We have also carried out measurements on polyoxymethylene (POM), poly (vinylidene fluoride) (PVF₂) and poly (ethylene terephthalate) (PET) and computed their thermal conductivities. Results on POM and PVF₂, which are semicrystalline, are analysed in the framework of several two-phase models, and the effect of crystallization (produced by annealing) on the glass transition behaviour of PET has also been studied.     

Orientation studies in polyacrylonitrile films uniaxially drawn in the solid state

Polyacrylonitrile (PAN) films were made by casting a dimethyl sulphoxide (DMSO) solution. After drying, the films were uniaxially drawn by an essentially solid-state drawing procedure. Using polarised-infrared (IR) spectroscopy, the dichroism of the nitrile-stretching vibration was monitored as a function of draw ratio. The dichroism results were converted to plots of chain-orientation factor versus draw ratio. For this, α, the transition-moment angle of the vibration, has to be known. Though a value of 68–73° has been suggested in the literature, for the present samples α cannot be 70° or less, and 73° is the most appropriate. The meaning and nature of the transition-moment angle in the context of a flexible-chain polymer molecule is discussed. The IR spectrum also showed dichroism in the S?O stretch and the ? CH₃ rocking vibrations from the (residual) DMSO present in the film, indicating that the solvent molecules were also orientated. It is proposed that this occurred due to dipolar interactions between the S?O and the C?N groups, which led to solvent molecules being bound to the polymer chains. X-ray diffraction studies indicated ‘lateral-ordering’ consisting of hexagonally packed, rod-like chains with no c-axis order. The diffraction patterns obtained here were the ‘standard’ ones for PAN, in contrast to those observed previously in solvent-containing PAN gels.     

The role of crystallite dimensions in mechanical and diffusion characteristics of uniaxially oriented polymers

Electron microscopy studies indicate that the microfibrils are a well defined element of many uniaxially oriented polymers. On the basis of these results it was inferred that the mechanical responses of uniaxially oriented polymers are similar to those of a parallel ensemble of the microfibrils. An important consequence of this microfibrillar model is that the type of load transfer is independent of crystallite aspect (length/diameter) ratio. The analysis of anisotropy in modulus of uniaxially oriented nylon 6, however, indicates that the load transfer characteristics of these polymers are dependent on crystallite dimension. A series response is observed only with fibers or films of low and intermediate draw ratio. Highly oriented structures, however, exhibit a nearly isotropic response in load transfer. These results show that uniaxial polymers must be analyzed in terms of a model where the crystallites are embedded in an “amorphous” matrix. The equations are derived for modulus, strength, and coefficient of diffusion in terms of crystallite dimensions, and the results compared with the experimental data.     

Thermal expansion of oriented glassy polymers

A phenomenological model of entanglement networks is developed and applied to deformed glassy polymers. Thermal expansivities and birefringence are analyzed. The model assumes a stable network of entanglements up to draw ratios of about 5. Beyond that range the entanglement structure breaks down to maintain a nearly constant drawing stress. The results are used to determine the ultimate linear expansivities of atactic polystyrene.     

An equation of state for molten polymers

A simple, generalized equation of state has been developed for molten polymers. The equation yielded calculated pressure–volume–temperature data that deviated by 1% or less from experimentally determined data. The use of the equation requires only that the polymer's glass temperature and density at 25°C and 1 atm be known. Polymer glass temperatures can also be estimated with the equation.     

Viscoelastic properties of acrylonitrile-butadiene-styrene (ABS) polymers in the molten state

The viscoelastic behavior of acrylonitrile-butadiene-styrene (ABS) terpolymers is studied in the molten state. First, the behavior of the styrene-acrylonitrile (SAN) matrix is determined. Then, the effect of the degree of grafting on ABS rheological properties is emphasized. A critical degree of grafting (DGc), corresponding to a minimum of the viscoelastic functions in the low frequency region, is determined. It defines a critical thickness of the grafted layer, and is related to the conformation of the grafted chains at the surface of the rubber particle. The DGc affects the morphology of the ABS and also affects the appearance of a secondary plateau at low frequencies. The effects of the rubber particles, of the grafted chains, and of the morphology on the appearance of the secondary plateau at low frequencies are clearly dissociated.     

Properties of nylon-1 polymers and copolymers in the solid state

Polymers and copolymers of alkyl isocyanates have been prepared by the method of Shashoua and their properties have been investigated. All the polymers have a significant decomposition rate at 150°C but the copolymers of butyl isocyanate with ethyl isocyanate are somewhat more stable than is the butyl homopolymer. The copolymers generally show reduced crystallinity and, with high concentrations of ethyl isocyanate, substantially amorphous polymers have been prepared. Dynamic mechanical measurements give damping peaks with a fall in modulus in the temperature range of 50°–150°C but the peaks are broader and the decline in modulus less steep than for conventional vinyl polymers. Over the same temperature range the differential scanning calorimetry (d.s.c.) curves are nearly linear and show no observable discontinuity. In mechanical tests both modulus and yield stress correlate with the NCO/hydrocarbon composition ratio. The form of the stress-strain curve in tension is similar to those of cellulose derivatives. They do not show a maximum and uniform extension occurs.     

Fluorescence from soluble polythiophenes in uniaxially stretched polymers

Fluorescence behavior of poly(3-alkylthiophenes) (P3ATs) in poly(methyl methacrylate) (PMMA) films was studied under tensile loadings. The elastic deformation of the matrix induced by small strains up to 0.2% had a greater influence on time-resolved fluorescence rather than the steady-state one from P3ATs. The decay time of fluorescence decreased linearly as the applied stress was increased, and reached a minimum. We maintained that the attractive phenomenon for the time-resolved fluorescence under the tensile loadings was associated with distortion or deformation of the π-conjugated structure in P3AT chains having large molecular weight. When the applied strain exceeded the elastic limit of the matrix, the time-resolved or steady-state fluorescence behavior was unchanged. However, large elongation over 10% led to the red-shift of the steady-state fluorescence maximum and an increase in the decay time. These fluorescence properties, indicating the growth of the π-conjugation, were mainly governed by strain. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67: 259–266, 1998     

High modulus semi-crystalline polymers by solid state rolling

Solid state rolling of semi-crystalline polymers is shown to be an effective method of producing high strength, high modulus tape at acceptable production rates. High density polyethylene tape was produced having a tensile strength exceeding 300 MPa and a tensile modulus of 8.7 GPa at production rates exceeding 8 m/min. A significant factor in producing highly oriented tape by the rolling process is roll temperature. Increasing the roll temperature from 25°C to 125°C not only increases the maximum extent of orientation achievable, but increases the mechanical properties at a given degree of thickness reduction. Internal frictional heat development limited the maximum thickness reduction ratio of polypropylene to 6.6:1. This reduction was reached by rolling at 150°C. The resultant tape had a tensile modulus of 5.1 GPa and a tensile strength of 300 MPa.     

Payne effect and shear elasticity of silica-filled polymers in concentrated solutions and in molten state

The viscoelastic properties of silica-filled copolymer in concentrated solutions and melts were investigated. A wide variety of rheological observations, such as Payne effect, percolation threshold and elastic effects, of non-porous silica dispersed in a ethylene vinyl acetate copolymer (EVA, 60 wt% of vinyl acetate) were studied in the molten state or diluted solutions in xylene. The concept of the filler network breakdown seems to be adequate in describing the strain-dependence of dynamic mechanical properties. Nevertheless, the degree of non-linearity was found to be the highest for the highest dilution. It appears for silica concentration lower than the percolation threshold of the filler suggesting another mechanism associated with trapped entanglement. On the other hand, we observe a silica content (Φ_c=(3.3±0.1)×10⁻²) for the onset of a gel-like behaviour in the terminal zone. This critical level, defined as the percolation threshold, appears to be independent of EVA dilution in xylene. Furthermore, this liquid-solid transition can be expressed in the same general relaxation patterns as chemical gelation systems with a self similar behaviour at the rheological gel point. However, the universal properties of percolation are not properly described and elastic effect are better depicted by the cluster-cluster aggregation model. On the other hand, the concentration-dependence of the equilibrium storage modulus of EVA at different dilutions in xylene yields evidence of a screened effect of copolymer chains in the entangled regime (?≥?_c) of elastic bending-twisting interaction between filler particles.     

Thermal gradient solid state forming of polypropylene

The near surface region of polypropylene was hardened in a forming process, which involved a large shear strain in a strong thermal gradient. The optimal processing conditions were estimated by modeling the temperature distribution in the deformation zone. Microindentation testing was used to characterize the corresponding depth‐dependent mechanical properties. The mechanical gradient, which developed after forming, was consistent with the observed variation in microstrucfture. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Transesterification reactions in molten polymers

The transesterification reaction of molten ethylene and vinyl alcohol copolymers (EVA), in presence of paraffinic alcohols and basic catalysts, leads to high conversion of the ester groups to secondary alcohol in both discontinuous and continuous processing equipment. Various kinds of alcohols and two different catalysts were used. Sodium methoxide is a powerful catalyst for the equilibrated transesterification reaction, but we also observed side reactions, such as cross-linking with low-molecular-weight alcohols and hydrolysis of the catalyst followed by partial saponification of the EVA. Kinetic studies were performed in the presence of dibutyltin dilaurate, an efficient catalyst without any side reactions. The solubility of the main alcohol reagents was verified by diffusion measurements. The general reaction scheme and the related kinetics, corresponding to a homogeneous system, lead to a fair evaluation of the rate constants.     

熔盐辅助固相反应合成NiO纳米片

通过聚乙二醇表面活性剂辅助低热固相反应首先制备了薄片状Ni(OH)2前驱体,再将所得前驱体置于马弗炉中于KNO3熔盐存在的条件下,600℃热分解5 h得到NiO纳米片。进一步借助X-射线衍射测试和透射电镜测试对所制备的纳米材料的晶型和形貌大小进行了表征。X-射线衍射测试结果证实了所得样品分别为单一的立方相NiO。透射电镜测试结果表明,所得NiO为片状结构,且大部分呈不规则六边形结构,相邻两边夹角约为120°。NiO纳米片的平均粒径约60 nm。     

Thermal reactions between MII sulphates and acid orthophosphates in the solid state

Two general types of high temperature reactions between M^II sulphates and acid orthophosphates have been found and a survey of some representative examples has been made. The reactions are typified by two examples using Ca²⁺ salts: and . Similar reactions have been found for other alkaline earth sulphates and phosphates and for corresponding mixed alkaline earth systems. The products have been found to be largely determined by the phase relationships in the respective M^IIO–P₂O₅ systems.     

SAXS study on structure formation from the uniaxially oriented glass in isotactic polypropylene

The structure formation of isotactic polypropylene from the uniaxially oriented glass into the smectic mesophase has been studied by small-angle X-ray scattering. Remarkable anisotropy between the directions parallel and perpendicular to the chain axis is found in the development of structure formation. Below −20 °C is observed a structure with density variation having the wave vector, perpendicular to the chain axis with a period of 5 to 6 nm. Between −20 and +25 °C, the density variations with both parallel and perpendicular to the chain axis develop. Above +25 °C, the density variation with perpendicular to the chain axis develops but disappears eventually, while that parallel to the chain axis keeps on growing.     

Measurements of the thermal conductivity and thermal diffusivity of polymers

In this paper, the thermal conductivity and thermal diffusivity of nine polymers were measured by using the transient short‐hot‐wire method. The corresponding specific heat was measured with a commercial Differential Scanning Calorimeter (DSC). The effects of temperature on the thermal conductivity, thermal diffusivity, and the product of density and specific heat are further discussed. The results show that the transient short‐hot‐wire method can be used to measure the thermal conductivity, thermal diffusivity, and the product of density and specific heat of polymers within uncertainties of 3%, 6%, and 9%, respectively.     

Thermal diffusivity in diamond,SiCxNy and BCxNy

Thermal diffusivity (α) of free standing diamond, amorphous silicon carbon nitride (a-SiC_xN_y) and boron carbon nitride (a-BC_xN_y) thin films on crystalline silicon, has been studied using the travelling wave technique. Thermal diffusivity in all of them was found to depend on the microstructure. For a-SiC_xN_y and a-BC_xN_y thin films two distinct regimes of high and low carbon contents were observed in which the microstructure changed considerably and that has a profound effect on the thermal diffusivity. The defective C(sp)N phase plays a key role in determining the film properties.