Methyl methacrylate behavior

With T as the independent variable, the transition between glassy and rubbery behavior can be read directly at Tg. Note that Tg is about 100° lower for poly(methyl acrylate) than for poly(methyl methacrylate). 

In addition, Bamford, Jenkins and coworkers (19) previously reported on the behavior of occluded radicals in the heterogeneous polymerizations of acrylonitrile, methyl acrylate, methyl methacrylate and vinylidene chloride. From their electron spin resonance studies, they concluded that the degree of occlusion was ... 

In thermal polymerization where the rate of initiation may also vary with composition, an abnormal cross initiation rate may introduce a further contribution to nonadditive behavior. The only system investigated quantitatively is styrene-methyl methacrylate, rates of thermal copolymerization of which were measured by Walling. The rate ratios appearing in Eq. (26) are known for this system from studies on the individual monomers, from copolymer composition studies, and from the copolymerization rate at fixed initiation rate. Hence a single measurement of the thermal copolymerization rate yields a value for Ri. Knowing hm and ki22 from the thermal initiation rates for either monomer alone (Chap. IV), the bimolecular cross initiation rate constant kii2 may be calculated. At 60°C it was found to be 2.8 times that... 

The most stable resin for many of our purposes has proven to be a copolymer of ethyl methacrylate and methyl acrylate. This comes as little surprise the Rohm and Haas Company has for years sold a durable resin based on these two monomers, Acryloid B-72 (6,28). We have also prepared polymers of similar physical properties based on methyl methacrylate and ethyl acrylate and have found that their behavior is practically the same - the methyl and ethyl groups apparently do not become seriously involved in crosslinking. As reported elsewhere( 23), rather than crosslink, Acryloid B-72 tends to chain break under visible and near-ultraviolet radiation, although at a very slow rate. Polyvinylacetate is another polymer used in the care of museum objects that tends more to chain break than crosslink under these conditions(23), but it is not our purpose to discuss its properties at this time. 

Characteristic initiation behavior of rare earth metals was also found in the polymerization of polar and nonpolar monomers. In spite of the accelarated development of living isotactic [15] and syndiotactic [16] polymerizations of methyl methacrylate (MMA), the lowest polydispersity indices obtained remain in the region of Mw/Mn = 1.08 for an Mn of only 21 200. Thus, the synthesis of high molecular weight polymers (Mn > 100 x 103) with Mw/Mn < 1.05 is still an important target in both polar and nonpolar polymer chemistry. Undoubtedly, the availability of compositionally pure materials is a must for the accurate physical and chemical characterization of polymeric materials. 

FIGURE 14.9 Influence of temperature on the stress-strain behavior of a sample of poly(methyl methacrylate). (Modeled after Carswell, T.S. and Nason, H.K. Effects of Environmental Conditions on the Mechanical Properties of Organic Plastics, 1944. Copyright, ASTM, Philadelphia, PA. With permission.)... 

Radical chain polymerizations are characterized by the presence of an autoacceleration in the polymerization rate as the reaction proceeds [North, 1974], One would normally expect a reaction rate to fall with time (i.e., the extent of conversion), since the monomer and initiator concentrations decrease with time. However, the exact opposite behavior is observed in many polymerizations—the reaction rate increases with conversion. A typical example is shown in Fig. 3-15 for the polymerization of methyl methacrylate in benzene solution [Schulz and Haborth, 1948]. The plot for the 10% methyl methacrylate solution shows the behavior that would generally be expected. The plot for neat (pure) monomer shows a dramatic autoacceleration in the polymerization rate. Such behavior is referred to as the gel effect. (The term gel as used here is different from its usage in Sec. 2-10 it does not refer to the formation of a crosslinked polymer.) The terms Trommsdorff effect and Norrish-Smith effect are also used in recognition of the early workers in the field. Similar behavior has been observed for a variety of monomers, including styrene, vinyl acetate, and methyl methacrylate [Balke and Hamielec, 1973 Cardenas and O Driscoll, 1976, 1977 Small, 1975 Turner, 1977 Yamamoto and Sugimoto, 1979]. It turns out that the gel effect is the normal ... 

Case 3 behavior occurs when the particle size is sufficiently large (about 0.1-1 pm) relative to kt such that two or more radicals can coexist in a polymer particle without instantaneous termination. This effect is more pronounced as the particle size and percent conversion increase. At high conversion the particle size increases and k, decreases, leading to an increase in h. The increase in h occurs at lower conversions for the larger-sized particles. Thus for styrene polymerization it increases from 0.5 to only 0.6 at 90% conversion for 0.7-pm particles. On the other hand, for 1.4-pm particles, n increases to about 1 at 80% conversion and more than 2 at 90% conversion [Chatterjee et al., 1979 Gerrens, 1959]. Much higher values of h have been reported in other emulsion polymerizations [Ballard et al., 1986 Mallya and Plamthottam, 1989]. Methyl methacrylate has a more pronounced Trommsdorff effect than styrene and vinyl acetate, and this results in a more exaggerated tendency toward case 3 behavior for methyl methacrylate. 

A few examples will illustrate the utility of Eqs. 6-33 and 6-34. Figure 6-3 shows the behavior observed in the radical copolymerization of styrene and methyl methacrylate. F and h i are the instantaneous copolymer compositions for the instantaneous feed... 

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