Polymethylmethacrylate thermal degradation

The thermal degradation of polymethylmethacrylate was investigated many years ago by isothermal methods [88, 89]. The two mechanisms of chain depolymerization were already identified at that time and analysed. At temperatures below 270°C, the reaction is initiated at the double bonds situated at chain ends and formed by radical disproportionation... 

Fig. 31. Dependence of the initial rate of thermal degradation of polymethylmethacrylate on molecular weight [89].

Thermal decomposition of polymers can be considered as a depolymerization process in only a few cases. Thus, polystyrene and polymethylmethacrylate are examples of polymers that can be thermally degraded with the formation of high yields of the corresponding monomer. However, for most polymers thermal decomposition leads to a complex mixture of products, containing low monomer concentrations. The type and distribution of products derived from the thermal degradation of each polymer depends on a number of factors the polymer itself, the reaction conditions, the type and operation mode of the reactor, etc. 

The use of DMTA to differentiate between plastic samples that have a similar chemical composition, but very different physical properties is shown in Figure 4. Both of these samples are polymethylmethacrylate (PMMA), but the top one is crosslinked having been prepared with a small amount of difunctional monomer and the bottom sample is the standard thermoplastic material. It can be seen that the crosslinked sample does not melt and the modulus does not fall away significantly until thermal degradation occurs. 

Contrary to what many people think, PLSNs are not a recent discovery. One of the earliest systematic studies of the interaction between a clay mineral and a macromolecule dates backs to 1949, when Bower described the absorption of DNA by montmorillonite. Even in the absence of X-ray diffraction (XRD) evidence, this finding implied insertion of the macromolecule in the lamellar structure of the silicate. In the case of synthetic polymers, Uskov found in 1960 that the softening point of polymethylmethacrylate derived by polymerisation of methylmethacrylate was raised by montmorillonite modified with octadecyl-ammonium, while in the following year Blumstein obtained a polymer inserted in the structure of a montmorillonite by polymerising a previously inserted vinyl monomer. In 1965 Blumstein first reported the improved thermal stability of a PMMA/clay nanocomposite. He showed that PMMA inserted between the lamellae of montmorillonite clay resisted thermal degradation under conditions that would otherwise completely degrade pure PMMA." ... 

The degradation behaviour of polymethylmethacrylate is easily characterized by thermal volatilization analysis [87] (Fig. 29). Monomer is obtained in very high yield in all cases. A polymer sample prepared by a free radical reaction undergoes a rapid depolymerization at about 275°C as indicated by the first peak. The second peak, situated between 350 and 400°C, corresponds to a second mode of initiation of chain depolymerization. For samples prepared by anionic polymerization, the first peak is not observed. Depolymerization of the whole sample occurs above 350°C. 

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