Polyethylene Thermal expansion coefficient

Figure 9.2 shows the volume versus temperature for polyethylene. Thermal expansion coefficients are derived from the slopes of such plots by using... 

Composite-based PTC thermistors are potentially more economical. These devices are based on a combination of a conductor in a semicrystalline polymer—for example, carbon black in polyethylene. Other fillers include copper, iron, and silver. Important filler parameters in addition to conductivity include particle size, distribution, morphology, surface energy, oxidation state, and thermal expansion coefficient. Important polymer matrix characteristics in addition to conductivity include the glass transition temperature, Tg, and thermal expansion coefficient. Interfacial effects are extremely important in these materials and can influence the ultimate electrical properties of the composite. 

The state of polarization, and hence the electrical properties, responds to changes in temperature in several ways. Within the Bom-Oppenheimer approximation, the motion of electrons and atoms can be decoupled, and the atomic motions in the crystalline solid treated as thermally activated vibrations. These atomic vibrations give rise to the thermal expansion of the lattice itself, which can be measured independendy. The electronic motions are assumed to be rapidly equilibrated in the state defined by the temperature and electric field. At lower temperatures, the quantization of vibrational states can be significant, as manifested in such properties as thermal expansion and heat capacity. In polymer crystals quantum mechanical effects can be important even at room temperature. For example, the magnitude of the negative axial thermal expansion coefficient in polyethylene is a direct result of the quantum mechanical nature of the heat capacity at room temperature." At still higher temperatures, near a phase transition, e.g., the assumption of stricdy vibrational dynamics of atoms is no... 

The performance of wood-polyethylene composites was found to improve by an appropriate compatibilization with different MAs and the additional incorporation of organo-clay particles [47] consisting of natural mont-morillonite modified with quaternary ammonium salt. The thermal expansion coefficient and the heat of deflection values indicated that these materials displayed an improved interfacial adhesion. 

At (ft—> 0 equation [7.2.37] gives a linear dependence of the relative vapor pressure, P°v(/Pvo on the solvent volume concentration with the angle coefficient exp(l+x). At 1 solution obeys the Raul s law. Note that the value of < )i in [7.2.37] is temperature-dependent due to difference in thermal expansion coefficients of components. The % value for a given solvent depends on the concentration and molar mass of a polymer as well as on temperature. However, to a first approximation, these features may be ignored. Usually % varies within the range 0.2 - 0.5. For example, for solutions of polyethylene, natural rubber, and polystyrene in toluene % = 0.28,0.393 and 0.456, correspondingly. 

Figure 9.2 The specific volume v (cm g ) of polyethylene versus temperature T. According to Equation (9.1 7), the thermal expansion coefficient a is proportional to the slope dv/dT. At low temperature, polyethylene is a hard crystalline plastic material. On melting at around 130 °C, the specific volume v increases sharply and a is large. Source. JE Mark,...

Equation (9.17). The slope of the curve below about 130 °C gives the thermal expansion coefficient for crystalline polyethylene, which is a hard plastic material. The volume expands sharply at the melting temperature. Above about 140 °C, the slope gives the thermal expansion coefficient of the plastic liquid. Thermal expansion coefficients are usually positive because increasing temperature causes a loosening up of the intermolecular bonds in the material. 

The volume anomalies are also reflected in Figure 29.13(c), which shows the thermal expansion coefficient = (l/V)OV/3T)p, the temperature derivative of the partial molar volume (Figure 29.13(b)). Water is anomalous for a < 0, which occurs in the cold liquid state. For most materials a is positive at all temperatures. For example, Figures 9.2 and 9.5 show that the molar volumes of polyethylene, benzene, and other liquids increase monotonicaUy with temperature. 

The thermal expansion behaviour of ultra high modulus polyethylene is very anisotropic. Transverse to the draw direction the thermal expansion coefficient is positive and comparable to that for isotropic polymer. In the draw direction the coefficient is negative and very small ( v 10" / ). For low molecular weight polymers the value... 

Reasonable ab initio results have also been obtained for the thermal expansion coefficient and isothermal compressibility [S(/c = 0)] of Polyethylene melts. The latter was computed using the experimental T-dependent density and the assumed dominance of soft repulsive forces. The resulting S(0) was found to be roughly 20% larger than the experimental values, although excellent agreement was obtained for the relative temperature dependence over the entire experimental range of 7 = 380-525 K. 

The addition of inorganic compounds to polyethylene is carried out for a variety of reasons. Fillers are sometimes only low-cost particulate materials used to extend the polyethylene and, therefore, lower the cost of the fabricated item. However, fillers usually affect some other aspect of the finished product such as the stiffness (modulus) or the processability of the polyethylene to increase rates of fabrication. Fillers may also reduce mold shrinkage and the thermal expansion coefficient Filler such as mica may increase heat resistance. Some examples of these compounds are discussed below. 

The thermal expansion coefficient in the c-axis direction is in fact negative for polyethylene and also for many other polymers (Fig. 7.4). The thermal expansivities along the a and b axes are, however, positive. Polymer crystals are also highly birefringent, with the higher refractive index in the c direction. An = An — Ana(by ranging from 0.10 upwards. 

Figure 24.6 shows the reproduction of the measured temperature dependence of surface tension of (a) liquid Hg [52] and Ni [51], (b) Co [67] and H2O [68], and (c) hexadecane and polyethylene [69]. With the Debye temperature and the thermal expansion coefficient for the corresponding specimens as input parameters, the b(0) is derived. No other parameters are involved. Tables 24.1a and 24.1b summarize information of the estimated results for several specimens. 

The thermal expansion coefficient of a material is the increase in length that it undergoes when its temperature is raised by a given increment. The thermal expansion of a polyethylene sample depends on two factors the relative proper-... 

Table 10 Thermal Expansion Coefficients of Various Types of Polyethylene, Selected Polymers, and Some Common Nonpolymeric Materials...

Crosslinked low-density polyethylene foams with a closedcell structure were investigated using differential scanning calorimetry, scanning electron microscopy, density, and thermal expansion measurements. At room temperature, the coefficient of thermal expansion decreased as the density increased. This was attributed to the influence of gas expansion within the cells. At a given material density, the expansion increased as the cell size became smaller. At higher temperatures, the relationship between thermal expansion and density was more complex, due to physical transitions in the matrix polymer. Materials with high density and thick cell walls were concluded to be the best for low expansion applications. 16 refs. 

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