Temperature control, birefringence

Accuracy in determining the melting temperature, Tm, is difficult to specify as the melting point of any sample depends on its thermal history. The accuracy of the birefringence experiment in determining the sample thickness is probably no better than 5% due to the limited control over the variables of the experiment. 

It will be shown that the level of uniaxial orientation and the variation of local mechanical properties generated by controlling the injection temperature can be conveniently characterized by microhardness measurement in combination with the measurement of optical birefringence An and DSC. In Section 2.7 we saw that microhardness is a very useful mechanical property, which can provide direct information about the anisotropy developed within highly oriented polymers. 

Serrano and coworkers studied the photopolymerization of mixtures of mono- and difunctional LC monomers to yield polymer materials with different degrees of cross-linking (Sanchez et al., 1999). Oriented films can be prepared by controlling the alignment of the monomers. The birefringence of photopolymer-izable samples and polymer films, as well as its temperature dependence, was evaluated. The birefringence of the films increased as the content of the monofunctional LC monomer increased. 

Contrary to expectations, on the millisecond time scale the initial drop //< in the luminescence studies of Mays and Ilgenfritz [24] did not increase significantly with the temperature-induced cluster growth but remained constant even when an infinite percolation cluster was present. Furthermore, the observed decays were always exponential (Fig. 9). Evidently, the initial drop no longer reflects the cluster size. The process responsible for it is over within 50 //s and should perhaps rather be looked upon similarly to the transient active sphere part of normal diffusion-controlled decay. The diffusion in this case is a random walk performed by the quencher on a (percolation) cluster. A stretched exponential decay would be expected for a random walk deactivation on a static cluster, as was observed close to the percolation threshold in earlier studies [23,24]. Those measurements were performed over a time window of about 50 //s, which is close to the reported value of the cluster lifetime from electrical birefringence measurements [60]. It is very likely that... 

The n and nj values as functions of ten jerature were determined with the aid of a Leitz-Jelley-microrefractometer, using a sodium lamp as light source. The birefringence was also determined by a conoscopic method using a polarizing microscope. The temperature in both experiments was adjusted by a Mettler hot stage FP-52 with the control unit FP-5. 

Thermal phase transitions have also been revealed in liquid crystal obtained from various tissues (Table 1). Not surprisingly, with thermal stage temperature increase, the birefringent liquid crystal droplets transit to non-refracting isotropic droplets. With temperature decrease, the liquid crystal droplets transit into crystal. However, when the isotropic droplets cool, two different results, controllable by rate of temperature decrease, were possible. If the rate of temperature drop is fast (the slide is placed on a 4°C plate), then the isotropic droplets will transition into liquid-crystals. However, if the rate of temperature decrease is slow (temperature is allowed to drop in step with the slowly cooling cop>per thermo-controller) then the isotropic droplets will transit to crystal (Figure 3). This finding... 

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