Autoacceleration indexes

The bulk polymerization of acrylonitrile in this range of temperatures exhibits kinetic features very similar to those observed with acrylic acid (cf. Table I). The very low over-all activation energies (11.3 and 12.5 Kj.mole-l) found in both systems suggest a high temperature coefficient for the termination step such as would be expected for a diffusion controlled bimolecular reaction involving two polymeric radicals. It follows that for these systems, in which radicals disappear rapidly and where the post-polymerization is strongly reduced, the concepts of nonsteady-state and of occluded polymer chains can hardly explain the observed auto-acceleration. Hence the auto-acceleration of acrylonitrile which persists above 60°C and exhibits the same "autoacceleration index" as at lower temperatures has to be accounted for by another cause. 

Similar experiments were performed with 0.05-mm. thick films. The log-log plot of the conversion curves is shown in Figure 4. Here the autoacceleration index is / = 1.15. The break on the curves occurs at lower grafting ratios than for the 0.1-mm. films. The dose-rate exponent for the instantaneous rate at 10% weight increase is a = 0.45 as shown in Figure 3. 

For 0.1-mm. thick PTFE films, the autoacceleration index—i.e., the slope of the initial portions of the conversion curves in log-log coordinates—remains approximately the same at all temperatures and dose rates. Figure 5 shows the data obtained at 350 rads/min. The autoacceleration index is 1.25-1.29. The Arrhenius plot of the instantaneous rates at 10% conversion is shown in Figure 6. This plot is based on the ratios Rt/R2o°c. of the rates measured at different temperatures and dose rates with respect to the corresponding rates at 20 °C. It exhibits a break at a temperature of ca. 33 °C. A transition point of PTFE has been reported at about the same temperature (II, 16). The activation energies are respectively 15.0 and 5.1 kcal./mole below and above 33 °C. 

Above 40 °C. the autoacceleration index increases. The conversion curves obtained at 40°, 58°, and 73°C. cross, and the yields for a given dose are comparable. At 73° and 95 °C. the autoacceleration index is 3.2 and the over-all activation energy determined on the basis of these two curves is 5.5 kcal./mole. 

The various data obtained for the kinetics of graft copolymerization onto PTFE films demonstrate that this reaction is complicated by the fact that the rate of diffusion of the monomer may become the controlling factor. It seems interesting at this point to compare and discuss together the results obtained with the different monomers. Table I summarizes the data obtained for autoacceleration indexes (/ ), dose-rate exponents (a), and over-all activation energies E, with styrene, acrylic acid, and vinylpyridine. Several conclusions can be derived from an examination of these data. 

Autoacceleration Indexes. The values of /3 differ widely for the different systems, and moreover, their variation with temperature has a different character depending on the monomer under consideration. 

Table I. Autoacceleration Indexes, / , Dose-Rate Exponents, a, and Over-all Activation Energies, , for the Direct Radiation Grafting of Various Monomers into PTFE Films...

With both styrene and vinylpyridine, the autoacceleration index decreases as the reaction temperature rises. This effect can be considered normal behavior of polymerizing systems in which the gel effect is operative. As the temperature rises, the termination step, which involves the interaction of two polymeric chains in a highly viscous medium, increases in rate, and the over-all reaction tends to become normal. Ultimately, the stationary-state conditions may eventually apply. 

With acrylic acid the situation is very different. For 0.1-mm. thick films /3 is independent of temperature, whereas for 0.05-mm. thick films /3 rises above 40°C. At low temperatures ft is unusually small. This effect may be ascribed to the use of a selective inhibitor to prevent homopolymerization. It was shown above that in grafting vinylpyridine the autoaccelerated character of the reaction at 20°C. vanishes in the presence of inhibitors. Moreover, with acrylic acid the autoacceleration index is lower if a more efficient inhibitor (CuCl2) is used, and f decreases as the amount of inhibitor increases (Figure 8). 

With styrene, the autoacceleration index gradually changes with temperature which makes it impossible to define an activation energy since its apparent value depends strongly on the grafting ratio at which the rates are measured. 

In both solution and FRRPP systems, conversions never reached 100%. The solution system reached an asymptote after four initiator half-lives, indicating the termination of radicals. The FRRPP system still had its conversion increasing almost linearly in the log-log plot with a slope less than 1. Note that this slope in the log-log scale is the so-called autoacceleration index, which would have values less than 1 for a well-controlled system. For future reference, the FRRPP system was named SAAl and the solution system was named SAA2. 

The Tromsdorf effect, also called the Norrish effect or gel effect, is associated with exothermic reactions during bulk polymerization. Autoacceleration of the polymerization rate can occur with medium to high polymerization conversions. This phenomenon inhibits termination. Strength of the Tromsdorf effect is calculated as the gel effect index [12]... 

Lewis and Volpert continue the discussion of the isothermal form of frontal polymerization in Chapter 5. Isothermal frontal polymerization is also a localized reaction zone that propagates but because of the autoacceleration of the rate of free-radical polymerization with conversion. A seed of poly(methyl methacrylate) is placed in contact with a solution of a peroxide or nitrile initiator, and a front propagates from the seed. The monomer diffuses into the seed, creating a viscous zone in which the rate of polymerization is faster than in the bulk solution. The result is a front that propagates but not with a constant velocity because the reaction is proceeding in the bulk solution at a slower rate. This process is used to create gradient refractive index materials by adding the appropriate dopant. 

The photo-oxidation of polypropylene is characterized by an induction period in which oxygen uptake (whether measured directly or by carbonyl index) occurs at a very low rate, followed by autoacceleration to a steady rate of oxidation. Measurements of mechanical properties and polymer molecular weight [2187] have shown that extensive degradation occurs in the induction period and the useful life extends little beyond this point [1443]. The surface of photo-oxidized polypropylene is highly degraded to a depth of 0.5 ym during the induction period, leading to microcracks and loss of mechanical properties [390]. 

Oligocarbonate dimethacrylates (OCM-2, Table 2) and organosilicon dimethacrylates [55, 61] have also been used as eross-linking agents for polystyrene. They simultaneously played the role of modifiers of the meehanieal properties of rigid-chain polystyrene. From this viewpoint, optimal results are obtained, when these eompounds are used in copolymerization with styrene in amount not more than 20% [55], There is no point in increasing the proportion of dimethacrylates, beeause, first of all, the refraetive index of the appropriate polymeric matrix decreases and, seeondly, the rate of eopolymerization autoacceleration at comparatively low eonversion of the monomer increases, which makes production of a shape-stable gel-matrix with required fraetion composition less regulated. 

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