Microscopic melting temperatures

AIBN (5 mg) added as initiator for all runs except the second. 6 Molar ratio of AN to CPT-S02. e Microscopic melting temperature. 

Isothermal crystallization was carried out at some range of degree of supercooling (AT = 3.3-14 K). AT was defined by AT = T - Tc, where Tj is the equilibrium melting temperature and Tc is the crystallization temperature. T s was estimated by applying the Gibbs-Thomson equation. It was confirmed that the crystals were isolated from each other by means of a polarizing optical microscope (POM). 

Figure 7.1 Illustration of different aggregation states obtained (from left to right) by increasing temperature crystal (K), smectic C (SmC), nematic (N) and isotropic (I). Row a shows macroscopic appearance of samples in row b, short-range microscopic ordering is represented (each bar represents a molecule) thermotropic phase diagram of row c illustrates relevant transition temperatures (Tm melting temperature Tsmc-N transition temperature between SmC and N Tc clearing temperature) row d shows different texture of different states as seen through polarizing microscope (with crossed polars, isotropic phase appears black).

A hot-stage-equipped polarizing microscope was used for measurement of these parameters. The anisotropic melting temperature (Tn) was determined as the onset temperature of stir-opalescence observed on the hot-stage. The liquid crystalline-isotropic transition temperature (71) was also determined by the use of the hot-stage-equipped microscope. 

Although the balance equations are linear, in the absence of bulk convection, the unknown shape of the melt-crystal interface and the dependence of the melting temperature on the energy and curvature of the surface make the model for microscopic interface shape rich in nonlinear structure. For a particular value of the spatial wavelength, a family of cellular interfaces evolves from the critical growth rate VC(X) when the velocity is increased. 

Enantiotropic polymorphs exhibit this transition temperature below their melting temperature (Figure 19.5), which means that the stable modiLcation depends on the temperature of reference This temperature represents the point of equal solubility for the two polymorphs, and one will have greater solubility above this temperature, and one below it. These transitions are often reversible, but may be kinetically limited or outside the temperature range studied. Haleblian and McCrone (1969) have cautioned that an enantiotropic relationship cannot be discounted because of the lack of an observed transition point. The transition can be examined microscopically if the crystal habits differ, or by solubility-temperature curves if the metastable form solubility can be measured before transformation. 

The parameters of the JT distortions were calculated by the X -method for a series of crystals in good agreement with experimental melting temperatures [14]. The details of the theory and specific calculations seemingly require additional refinements, but the main idea of the JT origin of the liquid-crystal phase transition seems to be quite reasonable. This work thus makes an important next step toward a better understanding of the relation between the macroscopic property of SB and the microscopic electronic structure, the JT effect parameters. 

Another observation from Table 1 show the disagreement between the calculated and the observed melting temperatures that exceed 25%, all are in heavy atoms. For example, Pt (at. wt. = 197), Nb (at. wt. = 92.9), Ta (at.wt. = 181), and T1 (at.wt. = 204). It is my view that they may be related to the effective mass. In our derivation of Eq. (3), the atomic mass, m was simply considered as equal on both sides of the equation and thus was cancelled out. However, this may not be true in the microscopic configuration being considered here. While it is beyond the scope of our present discussion, I thought it is necessary to point that out here. 

It was shown that the stress-induced orientational order is larger in a filled network than in an unfilled one [78]. Two effects explain this observation first, adsorption of network chains on filler particles leads to an increase of the effective crosslink density, and secondly, the microscopic deformation ratio differs from the macroscopic one, since part of the volume is occupied by solid filler particles. An important question for understanding the elastic properties of filled elastomeric systems, is to know to what extent the adsorption layer is affected by an external stress. Tong-time elastic relaxation and/or non-linearity in the elastic behaviour (Mullins effect, Payne effect) may be related to this question [79]. Just above the melting temperature Tm, it has been shown that local chain mobility in the adsorption layer decreases under stress, which may allow some elastic energy to be dissipated, (i.e., to relax). This may provide a mechanism for the reinforcement of filled PDMS networks [78]. 

Microscopic crystalline regions found within a solid polymer below the crystalline melting temperature, (p. 1237)... 

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