Thermal stability of PMMA

The formation of macro-radicals created by non-thermally stable structures is observed prior to statistical chain scission. The thermal behavior of PMMA at temperatures below major degradation depends on many factors that have to be considered, in particular during certain stabilizer-free polymerization processes with high levels of radical generation. 

Vinylidene terminal groups can be formed by disproportioning they cause degradation by a zipper mechanism that reaches its maximum degradation rate at 280 °C [620]. Small amounts of statistically incorporated methyl acrylate disrupt this mechanism and increase thermal stability. 

Head-head links, which can be created by the interruption of recombination, cause considerable reduction in molecular weight at temperatures as low as 170 °C. This most unstable polymer species will degrade during processing or annealing. 

Under extrusion conditions these special degradation mechanisms play a secondary role. Even five consecutive extrusion processes do not cause a level of degradation that can be measured by thermogravimetry in air at 240 °C. Visible degradation sets in at 280 °C. At extruder residence times of 10 min, the degradation rate of 0.2%/ min. determined by TGA measurement at a heating rate of 5 K/min corresponds to an interpolated material loss of max. 0.5%. 

In collaboration with Dr. Thomas Arndt, Dr. Norbert Deusch, Steffen Richter, Dr. Klaus Schultes, Evonik Industries AG 

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To avoid these stability problems, it is necessary to minimize the proportion of chains that terminate by radical-radical reaction. One way of achieving this is to conduct the polymerization in the presence of an appropriate chain transfer agent. For example, if polymerization is performed in the presence of a H-donor chain transfer agent, conditions can be chosen such that most chains terminate by hydrogen-atom transfer. Bagby et al.iA examined the thermal stability of PMMA formed with dodecanethiol. These polymer chains will then possess, more... 

Ottewill RH, Schofield AB, Waters JA, Williams NSJ (1997) Preparation of core-shell polymer colloid particles by encapsirlation. Colloid Polym Sd 275 274-283 Okubo M, Lu Y, Wang Z (1999) Analysis of stepwise heterocoagulation for the preparation of soft core/hard shell composite polymer particles. Colloid Polym Sd 277 77-82 Xu Y, Brittain WJ, Xue C, Eby RK (2004) Effect of clay type on morphology and thermal stability of PMMA-clay nanocomposites prepared by heterocoagulation method. Polymer 45(ll) 3735-3746... 

In order to improve the thermal stability of PMMA, Roberts [32] has described a two-component electron-beam resist. 

The higher the initial molecular mass of PMMA, the more sharply its decrease takes place. This is well illustrated by the data presented in Table 3.2 on the dependence of the thermal stability of PMMA on the initial molecular mass (up to 50% decomposition at 320 °C). 

Table 3.2 Dependence of thermal stability of PMMA on initial relative molar mass (RMM) ...

However, inconsistent results had been reported by Puglia et al., where they found the thermal degradation rate increased and the onset temperature of thermal degradation decreased for epoxy resin after the addition of CNTs. It was attributed to the high thermal conductivity and specific surface areas of CNTs which can accelerate the transfer of heat. This interpretation is not completely substantiated. The improved thermal stability of PMMA was also proved in thermal degradation experiment of CNT/PMMA." A study by Troitskii et al. indicated that C60 can combine with free radicals produced in the thermal degradation process of PMMA, and form relative steady macro-free radicals. 

The molecular weights of PMMA extracted from PCN materials and bulk PMMA were determined by gel permeation chromatography (GPC) with tetrahydrofuran as an eluant. Effects of the material composition on the gas and water permeability, optical clarity and thermal stability of PMMA and PCN materials, in the form of both free-standing films and fine powders, were also studied by molecular permeability analysis, ultraviolet-visible transmission spectra, differential scaiming calorimetry and thermogravimetric analysis, respectively. 

Direct insertion probe pyrolysis mass spectrometry (DPMS) utilises a device for introducing a single sample of a solid or liquid, usually contained in a quartz or other non-reactive sample holder, into a mass spectrometer ion source. A direct insertion probe consists of a shaft having a sample holder at one end [70] the probe is inserted through a vacuum lock to place the sample holder near to the ion source of the mass spectrometer. The sample is vaporized by heat from the ion source or by heat from a separate heater that surrounds the sample holder. Sample molecules are evaporated into the ion source where they are then ionized as gas-phase molecules. In a recent study, Uyar et al. [74] used such a device for studying the thermal stability of coalesced polymers of polycarbonate, PMMA and polylvinyl acetate) (PVAc) [75] and their binary and ternary blends [74] obtained from their preparation as inclusion compounds in cyclodextrins. 

The thermal stability of PNs also shows a dependence on the surface treatment, as noted in Figure 11.2595 in which TGA curves of PMMA and its clay nanocomposites (with the same MMT... 

R. Matsushima, A. Kaneko, A. Tomoda, M. Ishizuka, and H. Suzuki, Photochemical fatigue resistances and thermal stabilities of heterocyclic fulgides in PMMA film, Bull. Chem. Soc. Jpn., 61(10), 3569-3573 (1988). 

Addition of CNT to PMMA has also been found to increase the thermal stability of these composites (22,24-26,70). These studies have shown an increase in the temperature for the onset of thermal degradation of the polymer matrix by incorporation of different concentrations of CNTs in PMMA to enable it for high temperature applications (Figure 7.18). 

The reactions which are induced by pairs of different degradation agencies may also be clarified by studying the influence of one on the other. For example, the decrease In the thermal stability of poly(methyl methacrylate) (PMMA), as a result of pre-irradiation, has been clearly shown to be due to photo-induced chain scission which results in chain terminal unsaturation at which subsequent thermal decomposition is readily initiated (10). 

Pure PP and blends with increasing proportions of PMMA were irradiated in vacuum at 20°C for 20 hours. The weight loss on subsequent heating for 3 hours at 354°C was used as a measure of the Influence of blend composition on stability and compared with the behavior of un-lrradlated materials. The results, which are illustrated in figure 9, demonstrate that while the thermal stability of pure PP is greatly reduced by pre-irradiation, the additional effect of the presence of PMMA is relatively minor. 

Figure 10.8. Relative thermal stability of polymers by TGA (6). PVC = polyvinylchloride PMMA = polymethylmethacrylate PI = polyimide...

The H NMR spectrum of PMMA prepared with AIBN at 60°C showed vinylidene proton signals at 5.4 and 6.14ppm38 (Fig. 14). No inner olefin proton signal was observed. The intensity measurement indicated that 36% of the polymer molecules contained the unsaturated terminal. The thermal stability of radically prepared PMMA was discussed in relation to the... 

The thermal stability of holes burned at low temperatures has been investigated by temperature cycling experiments. As an example, we present (Table 2.13 ) the changes in hole profiles of the TPP/phenoxy resin system during successive increase in excursion temperature and subsequent cooling of the hole burned initially at 4 K.27) The detection of a hole for TPP at an excursion temperature is limited to 50 K in PMMA and most conventional polymers, but it can reach up to 80 K in a phenoxy resin and in poly(vinyl alcohol). In this temperature domain, hydrogen bonding of TPP to these polymer matrices, is believed to help suppress structural relaxations. 

In another TG study, Chiu (45) compared the relative thermal stabilities of five polymers, as shown in Figure 4.40. These polymers, poly(vinyl chloride) (PVC), poly(methyl methacrylate) (PMMA), high-pressure polyethylene (HPPE), polytetrafluoroethylene (PTFE), and an aromatic poly-pyromellitimide (PI), were all heated under identical conditions in the... 

Somewhat similar results were obtained with cellulose graft/blend systems with PMA, PMMA and poly(2-hydroxyethyl methacrylate) [Nishioka and Yoshida, 1992] and could be correlated with the degree of compatibility. In one case the thermal stability of blends containing grafted cellulose was 100°C less than those without grafted product [Nishioka et al., 1993]. 

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