Thermoelastic elastomers

Hence, we are still at the very beginning of thermoelastic and thermomechanical investigations of liquid crystalline elastomers. 

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... 

These experimental results give only a first insight into the fascinating properties of these very new l.c. elastomers. Further detailed studies have to come to a more profound understanding of the interactions of polymer networks and l.c. order of the mesogenic side chains. The exceptional photoelastic and thermoelastic properties promise to open up a new scientific and technological field of interest. 

Using poly(oxytetramethylene)glycol as diol component, the subsequent polymerization of styrene led to a thermoplastic elastomer with properties similar to commercial thermoelastics . 

Static mechanical measurements to evaluate the stress-strain relationship in cholesteric sidechain LCEs have been described [71, 72]. In [72] it has been found, for example, thatfor0.94nominal stress Cn is nearly zero as the poly domain structure must be converted first into a monodomain structure. For deformations A < 0.94, the nominal stress increases steeply. Similar results have also been reported elsewhere [71]. The nominal mechanical stress as a function of temperature for fixed compression has also been studied for cholesteric sidechain elastomers [71]. It turns out that the thermoelastic behavior is rather similar as that of the corresponding nematic LCE [2, 5]. 

Thermoelastics (more of a special case) and the significant group of thermoplastic elastomers (TPE) are shown in Fig. 1 above thermoplastics with rubber elements... 

The important point is that the original Flory-Rehner equation contains no thermoelastic front factor term, yet simple analogy with the mechanical equations shows that one must exist, even for unfilled elastomers. The need becomes more obvious on considering reinforced elastomers, where equilibrium swelling is greatly reduced. In an empirical manner, equation (10.6) may be modified to... 

Figure 10.20. Variation of degree of swelling restriction with fj/. Values of fj f were computed from the thermoelastic data. The quantity v stands for the volume fraction of elastomer in the swollen gum, and t 2 stands for the volume fraction of elastomer in the swollen filled polymer after correcting for the volume of the filler. (O) HiSil ( ) Quso (A) TK (X) Celite (O) gum. (Galanti and Sperling, 1970c.)...

In Fig. 3 i/e will demonstrate the thermoelastic behavior of the LC-elastomer with m = 3 (x=0) at constant load (mean relative deformation X=0.78). Above T in the isotropic state cr and the modulus arespectively increase linearly with increasing temperature corresponding to the behavior of common elastomers. 

Talroze, R. V. Gubina, T. L Shibaev, V. P. Plate, N. A., Peculiarities of the Thermoelastic Behavior of Liquid-Crystalline Elastomers. Makromol. Rapid Commun. 1990,11, 67-71. 

In this chapter, we first describe the stmcture of networks, followed by the discussion of the simple classical models of elasticity and the more advanced theories such as the constraint and the tube models. We also give the molecular interpretation of coefficients obtained from the phenomenological theories. Some simulations relevant to mbberlike elasticity are then described, followed by a discussion of responsive gels because of their increasing interest to many groups. We then discuss the thermoelastic (force-temperature) behavior of networks, followed by the information on multimodal networks, liquid-crystalline (LG) elastomers, novel reinforcing fillers, and characterization methods. 

The thermoelastic behavior of LC elastomers resolving the nematic to isotropic transition into entropic and enthalpic contributions has been reported. These experiments provide values of the energetic and entropic parts of the force, /e and... 

The E-modulus in the isotropic phase can be determined from both stress-strain and thermoelastic measurements and Me can be calculated according to Me = 3 when the density p of the elastomer is known. The degree of... 

Thermoelastic measurements on such samples reveal a spontaneous elongation along n at the transition to the smectic phase, indicating a prolate polymer backbone conformation in the smectic elastomer [137]. On another hand, SANS results for end-on side-chain polymers in the smectic phase indicate an oblate chain conformation, with the backbone preferentially confined in the plane of the layers (Sect. 2.2). Thus, the chain distribution and macroscopic shape of the smectic elastomer change their sign if crosslinking is made under uniaxial mechanical stress in the isotropic and/or nematic phase. This result is remarkable and indicates that the oblate chain conformation of a smectic end-on polymer can be easily turned into prolate by a low uniaxial extension during solvent evaporation. 

Very little work has been done on elastomers subjected to torsion. There are, however, some results on stress-strain behavior and network thermoelasticity [2]. More results are presumably forthcoming, particularly on the unusual bimodal networks and on networks containing some of the unusual fillers described in Section 1.11. 

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