Melting point entropy change

It is reasonable to consider that in an ester group the in-chain ether link —C—O—C— increases the chain flexibility compared with a polymethylene chain to decrease the heat of fusion. At the same time there will be some increase in interchain attraction via the carbonyl group which will decrease the entropy of fusion. Since these two effects almost cancel each other out there is almost no change in melting point with change in ester group concentration. 

Further information on the effect of polymer structure on melting points has been obtained by considering the heats and entropies of fusion. The relationship between free energy change AF with change in heat content A// and entropy change A5 at constant temperature is given by the equation... 

We can expect the entropy to increase when a solid melts and its molecules become more disordered. Similarly, we can expect an even greater increase in entropy when a liquid vaporizes, because its molecules then occupy a much greater volume and their motion is highly chaotic. In this section, we develop expressions for the change in entropy at the transition temperature for the prevailing pressure. For instance, if the pressure is 1 atm, then these expressions are applicable only at the normal melting point, Tf (the f stands for fusion), the temperature at which a solid melts when the pressure is 1 atm, or the normal boiling point, Th, the temperature at which a liquid boils when the pressure is 1 atm. 

Notes Tm, melting point AS, entropy change on melting K, elecholytic conductivity T, shear viscosity. Chemical scale of element proposed by Pettifor. ... 

Ammonium salts such as NH4C1 are sometimes observed to undergo an abrupt change in heat capacity (and hence entropy) at some temperature below the melting point. Describe the processes likely responsible for these observations. 

The graph should be similar to that for 2 Mg(s) + 02(g) —> 2 MgO(s). We expect a positive slope with slight changes in the slope after the melting point(839 °C) and boiling points(1484 °C). owing mainly to changes in entropy. The plot will always below the AG° line for 2 Mg(s) + 02(g) — 2 MgO(s) at all temperatures. 

B—The system is insulated and no work can be done on or by the system (rigid container) thus, the energy is constant. At the melting point, some of the gallium will melt and some will freeze, the entropy change of these two processes cancel each other so there is no net entropy change. 

Figure 6.4. (a) Variation of the entropy of fusion with melting point for different crystal structures (from Saunders el at. 1988) and (b) schematic illustration of the possibility of a change in value and sign for the entropy of transformation if the metastable structure has a low melting point (from Miodownik 1992). 

Both kinetic and thermodynamic approaches have been used to measure and explain the abrupt change in properties as a polymer changes from a glassy to a leathery state. These involve the coefficient of expansion, the compressibility, the index of refraction, and the specific heat values. In the thermodynamic approach used by Gibbs and DiMarzio, the process is considered to be related to conformational entropy changes with temperature and is related to a second-order transition. There is also an abrupt change from the solid crystalline to the liquid state at the first-order transition or melting point Tm. 

The addition of a diluent to a crystalline polymer depresses its melting point, as is shown schematically in Figure 2.43. The upper sketch again shows the standard reference case. In the lower sketch, solvent molecules are available to mix with the polymer chains once they separate from the crystalline lattice. The final state is now a polymer solution, instead of a molten polymer. This additional disordering greatly increases the entropy change for the process and therefore decreases the melting point, frequently to the extent of 40-50 °C. 

From the relation (3.2), and the observed change of volume on melting, we can compute the change of entropy. In Table XVI-2 we give values of volume of the solid per mole (extrapolated from room temperature to the melting point by use of the thermal expansion), volume of the liquid... 

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