Temperature, critical ceiling

For negative AHP and ASP, (most commonly the case encountered in addition polymerization) AFP becomes positive above a certain critical temperature, Te = A HpjA Sp, known as the ceiling temperature" of the system. The value of Te depends on the concentrations of the monomer and of the polymer as well as on the nature of the solvent, if the latter is present in the system. Of course, the high polymer cannot be formed above the ceiling temperature. For any monomer-polymer system, the process which converts pure liquid monomer into a crystalline polymer has the maximum ceiling temperature. 

Equation 3 shows that for a given monomer concentration [M]eq at temperatures above a critical value Tc the rate of the depolymerization step becomes greater than the rate of the polymerization step and dominates the reaction. The critical temperature Tc is called ceiling temperature (22, 23). (AH is the enthalpy of polymerization, and AS° is the entropy of polymerization at the monomer concentration [M] = 1 mole/liter.) The concentration of the monomer at equilibrium [M]eq is identical to the equilibrium constant K, which is defined by the rate constants kp and kd. 

Poly(2-methyl-1-pentene sulfone) (PMPS) is an alternating copolymer of 2-methyl-l-pentene (2MP) and sulfur dioxide. The formation of PMPS occurs only by a free radical polymerization mechanism and is complicated to a degree by ceiling temperature considerations. For all exothermic addition polymerization reactions there is a critical temperature called the ceiling temperature (Tc) above which no reaction occurs. The precise Tc depends upon the monomer concentration according to the expression (i)... 

The most important feature of a growing room is the ability to maintain a constant temperature. In this respect, insulation is critical. The walls should be insulated with R = 11 or R = 19 and the ceiling with R = 30 insulating materials. Fiberglass or styrofoam work well but should be protected from the high humidities of the growing room to prevent water from saturating them. For This purpose, a 2-4 mil. plastic vapor barrier is placed between The insulation and the interior wall. 

In the mnaway of a polymerization, if a temperature in the range of the ceiling temperature may be reached, or in other words the MTSR may be close to the ceiling temperature MTSR st Tc), then the safety analysis must account for the contribution of the depolymerization reaction, which produces low molecular species and may result in a pressure increase. This can easily be realized with the criticality classes presented above, by choosing the temperature To24 on the basis of the depolymerization reaction. 

For thermodynamic reasons, some adhesives may not cure at all above a critical temperature, which is known as the ceiling temperature, and may only partially cure at lower temperatures. This particularly applies to adhesives which harden by polymerization of monomers with C=C bonds, such as acrylates, methacrylates, and cyanoacrylates. 

Values of thermodynamic parameters characterizing the polymerization ability of the most important cyclic and heterocycUc monomers are compared in Table 1.1. Equation 1.6 indicates that, at standard conditions, monomers for which AHp < 0 and ASp > 0 can be polymerized at any temperature, whereas those with AHp > 0 and ASp < 0 cannot be converted into Unear macromolecules. In the most typical case-that is, when AHp < 0 and AS[] < 0-an increase in the polymerization temperature leads to an increase in [M]eq (Equation 1.7b). Eventually, at or above the so-called ceiling temperature (T Equation 1.9a), at which [M]eq = [M]o, formation of the high polymer does not occur. In contrast, for AHp > 0 and ASp > 0, [Mje, decreases with increasing temperature (Equation 1.7b) and there is another critical temperature-called the floor temperature (Tf, Equation 1.9b), at or below which polymerization is thermodynamicaUy forbidden. 

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