Polymethylmethacrylate relaxation

Triplet—triplet energy transfer from benzophenone to phenanthrene in polymethylmethacrylate at 77 and 298 K was studied by steady-state phosphorescence depolarisation techniques [182], They were unable to see any clear evidence for the orientational dependence of the transfer probability [eqn. (92)]. This may be due to the relative magnitude of the phosphorescence lifetime of benzophenone ( 5 ms) and the much shorter rotational relaxation time of benzophenone implied by the observation by Rice and Kenney-Wallace [250] that coumarin-2 and pyrene have rotational times of < 1 ns, and rhodamine 6G of 5.7 ns in polymethyl methacrylate at room temperature. Indeed, the latter system of rhodamine 6G in polymethyl methacrylate could provide an interesting donor (to rose bengal or some such acceptor) where the rotational time is comparable with the fluorescence time and hence to the dipole—dipole energy transfer time. In this case, the definition of R0 in eqn. (77) is incorrect, since k cannot now be averaged over all orientations. 

Figure 19 The terminal relaxation domain of diluted polymethylmethacrylate (a) and diluted polyisoprene (b) can easily be distinguished from the matrix one (dashed lines) in a Cole-Cole representation of complex viscosities [19]-...

Obtain DSC thermal curves of several semicrystaUine polymers such as polymethylmethacrylate (PMMA), polystyrene, polycarbonate, high-density polyethylene, low-density polyethylene and look for the glass transition in these polymers. The DSC run may need to be repeated twice with rapid cooling between runs. Many as received polymers will show a small peak on top of the glass transition on the first mn due to relaxation effects in the polymer. The second run should not show this peak , but only a step change in the baseline. Compare your values of Tg to literature values. Deviations may indicate the presence of plasticizers or other additives in the polymer. 

Blends of atactic polymethylmethacrylate with polyethyleneglycol, PMMA/ PEG, were reported miscible (Colby 1989). Their rheology, PMMA/ PEG = 50/50 and 80/20 at T = 160-210 °C, was studied in a dynamic shear field (Booij and Palmen 1992). By contrast with homopolymers, the blends did not follow the time-temperature superposition. The deviation was particularly poor at low temperatures. The reason for the deviation is most likely based on the different temperature dependence of the relaxation functions. The authors concluded that in miscible blends, the temperature dependence of the relaxation times of individual macromolecules depends on composition. This leads to different degree of mutual entanglement and hence the rubber plateau moduli. 

Konak, et al report dynanuc light scattering spectra of mixtures of polystyrene and polymethylmethacrylate in toluene(77). Neither polymer was dilute. Comparison was made for a limited number of concentrations with theoretical models arising from work of Benmouna, et al(6T). Treating spectra as bimodal, the ratio of relaxation times was predicted theoretically to better than 50%, but predictions of the mode amplitude ratio were often inexact by factors of 2 or 3. 

Sun and Wang report a series of studies of polystyrene polymethylmethacrylate mixtures (in benzene, dioxane, and toluene, respectively) using light scattering spectroscopy as the major experimental technique(78-80). Both polymers were in general nondilute. Neither polymer is isorefiractive with any of the solvents. The objective was to study the bimodal spectra that arise under these conditions and to show that the two relaxation times and the mode ampUtude ratio can be used to infer diffusion and cross-diffusion coefiBcients of the two components. Experimental series varied both the total polymer concentration and the concentration ratio of the two components. The theoretical model predicts a biexponential spectrum. The experimental data were fitted by a bimodal distribution of relaxation rates or by a sum of two Williams-Watts functions. The inferred self-diffusion coefiBcients of both species fall with increasing polymer concentration. 

Dependence of relaxation modulus upon time for polymethylmethacrylate, at temperatures between 40°C and 135°C (Comyn 1990, but original data from McLoughlin and Tobolsky 1952.)... 

INVESTIGATION OF PHYSICAL AGEING IN POLYMETHYLMETHACRYLATE USING POSITRON ANNIHILATION, DIELECTRIC RELAXATION AND DYNAMIC MECHANICAL THERMAL ANALYSIS Davis W J Pethrick R A Strathclyde,University... 

The temperatures inherent to forward lighting require the use of materials with glass transition temperatures (Tg) in excess of most commercial polymers. Whereas rear reflectors can be made of polycarbonate (PC) or even polymethylmethacrylate (PMMA), forward reflectors reach temperatures in excess of 150 °C. At these temperatures many polymers relax causing the metallic mirror smlace to buckle and haze . Some exhibit the formation of blisters on the metal surface. Described below are various materials used in the production of reflectors and some of the factors that affect their use temperatures. An explanation for the formation of blisters rather than haze is also proposed. A new class of high heat polycarbonates (Lexan XHT grade resins) is compared to other commercial thermoplastics of similar thermal capabilities. 

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