Entropy melting

The conformational characteristics of PVF are the subject of several studies (53,65). The rotational isomeric state (RIS) model has been used to calculate mean square end-to-end distance, dipole moments, and conformational entropies. C-nmr chemical shifts are in agreement with these predictions (66). The stiffness parameter (5) has been calculated (67) using the relationship between chain stiffness and cross-sectional area (68). In comparison to polyethylene, PVF has greater chain stiffness which decreases melting entropy, ie, (AS ) = 8.58 J/(molK) [2.05 cal/(molK)] versus... 

This equation shows that a high melting temperature is obtained by raising the melt enthalpy AH and/or a lowering of the melt entropy AS. Since and... 

A low melting entropy is experienced if the intramolecular motions of the polymer are hindered, for example, the free rotation of chain segments. This is preferentially be done by stiffening of the polymer chain. Table 2.12 shows a selection of building blocks for temperature-resistant polymers. 

No general rules about the entropy of transitions, as were found for liquid and plastic crystal transitions, can be set up for condis crystals. Two typical examples may illustrate this point. Polytetrafluoroethylene has a relatively small room-temperature transition-entropy on its change to the condis state and a larger transition entropy for final melting. Polyethylene has, in contrast, a higher condis crystal transition entropy than melting entropy (see Sect. 5.3.2). 

At this temperature, the entropy change for dissolution of liquid hydrocarbons in water is zero. However, the entropy of protein denaturation is far from zero at this temperature but amounts to 17.6 J - K l per mole of amino acid residues (Privalov, 1979), a value that corresponds to an 8-fold increase of the number of possible configurations and is close to the value expected for the helix-coil transition of polypeptides (Schellman, 1955). This difference shows that an oil drop is an inadequate model for a globular protein. A more suitable model resembles that of a small crystal with a quite definite positive melting entropy (see also Bellow, 1977, 1978). 

A simple method to determine is based on the dependence of the melting temperature on the crystallization temperature, T. (14). Tm increases linearly with T. as Figure 2.16 shows (15). Since can never be lower than F., the line F = F represents the limit, i.e., the situation of perfect reversibility in which melting and crystallization occur at the same temperature. The extrapolation of F to F = F provides T%. This procedure is the one most widely used because it is applicable to all samples, not just to single crystals. Table 2.2 shows the values of 7%, melting enthalpy, and melting entropy of some polymers (16). The values of which... 

B is correct. The added energy goes into breaking bonds, and as is demonstrated by the heat curve above, the temperature remains constant until all the ice is melted. Entropy increases moving to the right on the heat curve. 

Inserting the measured values into Eq. (3.2) yields As = 0.39kg. As discussed in Section II.C, this value of the melting entropy is similar to the values measured for other simulated 2D systems. 

Table 2.1.1. Melting entropy of molecular crystals (in entropy units,...

Under thermodynamic considerations, high melting temperatures are the result of high melting enthalpies and low melting entropies. Consequently, the structural modification concepts are based either on a de-... 

Naoki, M., and Tomomatsu, T., Analysis of melting entropy and effect of volume on the conformational entropy of frawi-polyisoprene. Macromolecules, 13, 322—327 (1980). 

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