Thermal expansion coefficients, molecular

Several material properties exhibit a distinct change over the range of Tg. These properties can be classified into three major categories—thermodynamic quantities (i.e., enthalpy, heat capacity, volume, and thermal expansion coefficient), molecular dynamics quantities (i.e., rotational and translational mobility), and physicochemical properties (i.e., viscosity, viscoelastic proprieties, dielectric constant). Figure 34 schematically illustrates changes in selected material properties (free volume, thermal expansion coefficient, enthalpy, heat capacity, viscosity, and dielectric constant) as functions of temperature over the range of Tg. A number of analytical methods can be used to monitor these and other property changes and... 

Aj3th is the jump of the linear thermal expansion coefficient at the glass transition temperature. As Fig. 1 lb shows, da is proportional to the rate (1 — a) of the flowing units, that is to the molecular arrangements, which can form some confer-... 

Here oth is the fraction of thermal expansion connected with changes in hole concentration (free-volume expansion), ), is the energy of hole formation, r=M/p0 Vn Na, where NA is the Avogadro number,M molecular weight, p0 the density of a liquid without holes at absolute zero, and Vn the hole volume. For polymeric systems r is very small, and then ahTis the function of E IRTalone. The value Of, is identified with experimentally observed changes in the thermal expansion coefficients A a at Tg, i.e. 

As known [7,8], the thermal expansion coefficient is reduced in the direction of the molecular orientation obtained by stretching of a thermoplastic polymer during or directly after its processing. In special cases thermotropic polyesters are applied to facilitate the process of molecular orientation [9]. However, in all these cases solidification must proceed either by cooling down from the melt or by evaporation of the solvent. These relatively slow processes are not suited for on-line optical fiber coating. 

It has been demonstrated that molecular orientation can be achieved starting with a low molecular weight species which is oriented in an elongational flow and subsequently cured under UV-irradiation. The orientation of the monomer is frozen-in by the ultra-fast process of polymerization and crosslinking. Both extrusion and stretching can be carried out at relatively low temperatures and pressures. Polymer filaments produced in this way are definitely anisotropic as is evidenced by their birefringence and by a strong increase of the tensile modulus and a decrease of the thermal expansion coefficient in the axial direction. 

Selected mechanical properties of liquid COFj have been estimated from molecular data [1683]. The calculated values for the molar volume, thermal expansion coefficient (a), and isothermal compressibility (3), are recorded as a function of temperature in Table 13.16. 

When heat transfer is involved, the density and viscosity can be functions of temperature, and the thermal properties of the fluid have to be prescribed as well. These properties include heat capacity, thermal conductivity, molecular weight (for gases), and thermal expansion coefficient. For problems involving chemical species, all physical properties have to be specified for all of the species along with a method to calculate the average property for mixtures in each cell using the properties of the component species. 

The powder method is capable of very high precision, often more than usable. Many substances have linear thermal expansion coefficients of 3 X 1(TS K-1 or more (often very much more for molecular crystals). It is of no use to quote a lattice constant with a relative estimated standard deviation of, say, 3 X 10"5 unless the temperature is specified. Similarly, it is important that diffractometer data be taken at a reasonably uniform temperature. 

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