Thermoelastic range

It is understandable that there is more freedom for the creation of intricate shapes in the thermoplastic temperature range than in the thermoelastic range. However, as a result of biaxial orientation, processing in the thermoelastic range yields products with higher strength, and with better barrier characteristics, transparency, and brilliance. 

The thermoelastic law, valid only within the elastic range of isotropic and homogeneus materials, relates the peak to peak temperature changes to the peak to peak amplitude of the periodic change in the sum of principal stresses. 

The SPATE technique is based on measurement of the thermoelastic effect. Within the elastic range, a body subjected to tensile or compressive stresses experiences a reversible conversion between mechanical and thermal energy. Provided adiabatic conditions are maintained, the relationship between the reversible temperature change and the corresponding change in the sum of the principal stresses is linear and indipendent of the load frequency. 

Values of the mean-square dipole moment, , of PDEI are determined as a function of temperature. The value of the dipole moment ratio is 0.697 at 303 K. Trifunctional model networks are prepared. From thermoelastic experiments performed on the networks over a temperature range 293 - 353 K, it is found that the value of the temperature coefficient of the unperturbed dimensions amounts to 1.05 0.17 K-1. The dipole moments and the temperature coefficients of both the dipole moments and the unperturbed dimensions are critically interpreted in terms of the RIS model, and are found to be in a reasonable agreement. 

The measurements of the temperature dependence of the restoring force are usually carried out in the temperature range 350 100 K. Determination of f requires a rather far extrapolation of experimental results and it restricts the accuracy of this method. This type of thermoelastic measurements requires equilibrium conditions. Most widely, this method is used for simple elongation and seldom for compression 67) and torsion 68,69). 

From the dynamic mechanical investigations we have derived a discontinuous jump of G and G" at the phase transformation isotropic to l.c. Additional information about the mechanical properties of the elastomers can be obtained by measurements of the retractive force of a strained sample. In Fig. 40 the retractive force divided by the cross-sectional area of the unstrained sample at the corresponding temperature, a° is measured at constant length of the sample as function of temperature. In the upper temperature range, T > T0 (Tc is indicated by the dashed line), the typical behavior of rubbers is observed, where the (nominal) stress depends linearly on temperature. Because of the small elongation of the sample, however, a decrease of ct° with increasing temperature is observed for X < 1.1. This indicates that the thermal expansion of the material predominates the retractive force due to entropy elasticity. Fork = 1.1 the nominal stress o° is independent on T, which is the so-called thermoelastic inversion point. In contrast to this normal behavior of the l.c. elastomer... 

It is well known that the martensitic transformation of Al-deficient NiAl (see Sec. 4.3.2) is thermoelastic and produces the shape memory effect. Consequently materials developments have been started which aim at applications as shape memory alloys (Furukawa et al., 1988 Kainuma et al., 1992b, c). The martensitic transformation temperature can be varied within a broad temperature range up to 900 °C, and thus the shape memory efftct can be produced at high temperatures which allows the development of high-temperature shape memory alloys. The problem of low room temperature ductility of NiAl has been overcome by alloying with a third element - in particular Fe - to produce a ductile second phase with an f.c.c. structure. 

The temperature range between the glass transition range and the flow or fusion (melt) range, in which the shear modulus remains nearly constant, becomes wider as the molar mass grows. The plastic is then described as entropy or mbber elastic, or even thermoelastic (as opposed to thermoplastic). 

Hot secondary forming Used for amorphous thermoplastics in the thermoelastic (entropy elastic) range above Tg or in semicrystalline thermoplastics 30 °C below (see cup experiment below). 

At present most PVC films, both HT and LT, are stretched on this type of equipment. For the improvement of the mechanical properties by molecular orientation, the stretching temperature is in the range of 100-130°C in order to use the thermoelastic properties of the material. 

Once the glass transition temperature (7 ) has been exceeded, the intermolecular forces have become so weak that the influence of external forces can cause the macromolecules to slip apart from one another. The strength declines steeply, while the elongation leaps upward. In this temperature range, the plastic exists in a rubber-elastic or thermoelastic state. 

As the temperature continues to increase, the intermolecular forces are almost completely eliminated. The polymer proceeds in a continuous manner from the thermoelastic state to the thermoplastic or molten state. This transition is described as the range of the flow temperature Tf). This temperature cannot be specified precisely. Primary processing methods such as injection molding are carried out within the thermoplastic range. 

Young s modulus E Thermoelastic coefficient TKE Compensation range of E Shear modulus G... 

Stretch-blowing in the stretchable thermoelastic temperature range. 

Last but not least, models which assume ideal behavior do not account for the always present energetic effects. Therefore, with these models alone, a comprehensive representation of thermoelastic data encompassing a larger temperature range, like the ones displayed in Fig. 7.4, cannot be achieved. 

With regard to differences in polymer behavior in solution versus the bulk state, several points must be made. Clearly, it is now well-established that the choice of theta solvent can affect chain dimensions to some extent [42-44, 46, 47]. Hence, only the chain in an amorphous melt of identical neighbors can be considered to be in the unperturbed state. Particularly striking are some of the differences noted in temperature coefficients measured by different techniques. Is it possible that the thermal expansion of a polymer molecule is fundamentally different in the bulk and in solution Can specific solvent effects exist and vary in a systematic way within a series of chemically similar theta solvents Does the different range of temperatures usually employed in bulk versus solution studies affect K Are chains in the bulk (during SANS and thermoelastic experiments) allowed adequate time to completely relax to equilibrium All of these issues need further attention. Other topics perhaps worthy of consideration include the study of the impact of deuterium labelling on chain conformation (H has lower vibrational energy than does H ) and the potential temperature dependence of the Flory hydrodynamic parameter . 

---