Ceiling temperature polymer thermodynamics

The thermodynamic ceiling temperature (26) T for a polymerization is computed by dividing the AfTp by the standard entropy of polymerization, The T is the temperature at which monomer and polymer are in equHibrium in their standard states at 25°C (298.15 K) and 101.3... 

The heat of reaction for vinyl polymers affects the thermal stability of the polymer during extrusion, and the thermal stability is related to the ceiling temperature. The ceiling temperature is the temperature where the polymerization reaction equilibrium is shifted so that the monomer will not polymerize, or if kept at this temperature all the polymer will be converted back to monomer. From thermodynamics the equilibrium constant for any reaction is a function of the heat of reaction and the entropy of the reaction. For PS resin, the exothermic heat of reaction for polymerization is 70 kj/gmol, and the ceiling temperature is 310 °C. Ceiling temperatures for select polymers are shown in Table 2.5. 

The thermodynamic ceiling temperature (26) T for a polymerization is computed by dividing the Afi°polym by the standard entropy of polymerization, A+°polym. The T is the temperature at which monomer and polymer are in equilibrium in their standard states at 25°C (298.15 K) and 101.3 kPa (1 atm). (In the case of p-xylylene, such a state is, of course, purely hypothetical.) The T quantifies the binding forces between monomer units in a polymer and measures the tendency of the polymer to revert back to monomer. In other systems, the T indicates a temperature above which the polymer is unstable with respect to its monomer, but in the case of parylene it serves rather as a means of comparing the relative stability of the polymer with... 

Because of the general lack of quantitative thermodynamic (ceiling temperature Tc) and kinetic (kp, kp/k[J 5) data for the polymerization of the captodative olefins, it is impossible to draw firm conclusions about the importance of electronic factors on their polymerizability. If we compare them with other 1,1-disubstituted olefins by replacing the heteroatom O, S, or N by a CH2 and check the polymerizability of the resulting olefins, we find that the latter are in fact also difficult to polymerize as shown in Table 8 [77], Only methyl acrylates and methacrylates give high polymers easily. The polymerizability decreases rapidly with the steric hindrance of the substituent. 

For every vinyl monomer there exists a ceiling temperature above which it is thermodynamically impossible to convert monomer into high polymer because of the depropagation reaction. If two vinyl monomers are copolymerized under conditions such that one or both may depropagate, the resultant polymer will have an unusual composition and sequence distribution. Existing theoretical and experimental works are reviewed which treat of copolymer composition, rate of copolymerization, and degree of copolymerization. 

In view of thermodynamics the ceiling temperature looks like a melting point. Below and above this temperature the system consists of 100% polymer and of 100% monomer, respectively. This is shown in Fig. 20.2. If, however, the polymer is soluble in its monomer, then the free enthalpy of mixing also plays a role and the result will be that the "melting point" is not as sharp as shown there will be a gradual change (i.e. the dashed line) from 100% polymer to 100% monomer. The ceiling temperature is in this case defined as the temperature where the amount of monomer equals the amount of polymer (i.e. at 50%) and equal to (see, e.g. Ivin, 2000) ... 

In most cases, the growth of polymeric chains is accompanied by volume contraction. Therefore external pressure tends to shift the monomer polymer equilibrium in favour of the polymer or, in other words, it increases the ceiling temperature of polymerization (lowers 7 ). This analysis can be refined by means of the known thermodynamic relations. The change in enthalpy with pressure is described by the thermodynamic equation of state... 

From the materials viewpoint, it is of Interest to examine the thermodynamics of radlolysls of polymers. Since gamma radlolysls data is readily available, polymers can be compared as in Table X. Note that materials having a large heat of polymerization tend to crosslink under radlolysls. The same polymers are thermally resistant to degradation. Degrading polymers have low ceiling temperatures ( 150°C) and low heats of polymerization. 

The ceiling temperature (Tc) of THF cationic polymerisation is around 83 °C [3, 7, 35, 36, 38]. Above the ceiling temperature the transformation of THF in PTHF is practically impossible from the thermodynamic point of view. Tc is the temperature at which the variation of the monomer to polymer transformation free energy is zero (it is well known that a transformation takes place only at a negative variation of free energy, AG < 0). The value of Tc in polymerisation is given by relationship 7.4. 

In the above equation, T. is the ceiling temperature for the equilibrium monomer concentration. It is a function of the temperature of the reaction. Because the heat content is a negative quantity, the concentration of the monomer (in equilibrium with polymer) increases with increasing temperatures. There are a series of ceiling temperatures that correspond to different equilibrium monomer concentrations. For any given concentration of a monomer in solution, there is also some upper temperature at which polymerization will not proceed. This, however, is a thermodynamic approach. When there are no active centers present in the polymer structure, the material will appear stable even above the ceiling temperature in a state of metastable equilibrium. 

The glass-transition temperature depends on the mobility of the chain segments and can therefore be raised by stiffening the chain (see Section 10.5.3). Thus, a-methyl styrene forms a polymer that, in contrast to poly-(styrene), does not deform at lOC C, because of a glass-transition temperature of 170°C. However, since the thermodynamic ceiling temperature for for the polymerization/depolymerization equilibrium is also simultaneously lowered (see Section 16.3), poly(a-methyl styrene) degrades more easily than poly(styrene), so that it is not so easy to work by injection molding. 

The depolymerization can be prevented by incorporating monomeric units with higher thermodynamic ceiling temperatures into the polymer. Thus, a-methyl styrene/methyl methacrylate copolymers have achieved a certain commercial importance as heat-stable, transparent polymers for special applications. 

At the temperature for which AG = 0, polymerisation is in equilibrium with depolymerisation. Because this equilibrium generally occurs in conditions above room temperature, this temperature is called the ceiling temperature (Tc). If the temperature of the polymer is below T, then the thermodynamically stable species is the polymer. Conversely, if the temperature of the polymer increases above T, then the monomer becomes the most stable species, i.e. depolymerisation is possible. However, depolymerisation can occur only in the presence of active species, e.g. radicals. 

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