PMMA, intensity effect

In PMMA the effects of molecular weight and of molecular weight distribution on craze dimensions and on crack opening have been intensively studied In Fig. 17 the results of the maximum craze width are represented as function... 

After irradiation of AC in an atmosphere of NO, a very weak ESR signal is observed. After evacuation, the intensity of the ESR spectrum increases sharply, at that the quantity of radicals increases with elevation in temperature and duration of pumping [60]. As in the case of PMMA, the effect observed is conditional on the decomposition of non-paramagnetic complexes of NO with radicals. The ESR spectrum is interpreted as an anisotropic triplet with the axial symmetry of g-tensor... 

An effective thickness of the layer where the fluorescence is observed is assumed to be the depth where the excitation light intensity is 1/e of the initial value. The thickness was calculated to be 1.4 im from an absorption coefficient of the film at 295 nm (excitation wavelength). Therefore, the observed fluorescence spectral change is due to that of aggregate states of EPy in the depth region of 1.4 fin from the ablated surface. Actually, it is well known in a PMMA matrix that the excimer band is due to the ground state dimer of the dopant (23). 

Table 6 summarizes the spectral data that have been recorded with various compositions of TMPTA/PMMA. It is apparent that the magnitude of the rigi-dochromic shift (AEem) is significantly reduced when the starting mixture has a higher polymer content. Similarly, the ratio (If/I0) of the final (If) and initial (I0) intensities of the MLCT phosphorescence is considerably reduced with a higher PMMA content in the unirradiated resin. Both these effects appear to relate to... 

Inhibition or the reduction in the o-Ps yield is most probaly due to the inhibiting species scavaning precursors of Ps such as electrons and/or hot Ps atoms in the terminal positron spur [80], It is also found that the inhibition effect is dependent on the concentration of the inhibiting species. Figure 10.7 shows the dependence of the o-Ps intensity (I3) versus the concentration of a dopping chromophore in a PMMA polymer at two temperatures. With increasing amounts of the chormophore there is a decrease in the I3 value. This data can be fitted with the following relationship [42] ... 

As reported in Ref. , the spread rate of a flame moving up a vertical surface of a sufficiently thick PMMA sheet increases under the effect of an external heat radiation. Depending on the heat radiation intensity and exposure time, various effects on the flame spread rate are observed. Additional heating of the polymer surface by a radiative flux results, first of all, in a decrease of the temperature dilTerence (T — Tp) and, in accordance with Eq. (2.19), in an increase of v. The experimental relationship v (T — To)" at T = 363 °C is close to that predicted by theory. According to Femandez-Pello , an increase of the initial polymer surface temperature, Tp, cause a parallel enhancement of the natural convection in the boundary heat layer and heat radiation by the surface, leading to its partial cooling. Therefore, when the intensity of the external radiative heat flux is low, the flame spread rate increases with time, but only up to a certain constant value. 

While chlorine might be expected to enhance the sensitivity to x-rays more than either gamma radiation or electrons, the present work does not demonstrate this effect. The most probable reason is that the synchrotron beam was operating under conditions where the intensity was not high at the absorption edge for chlorine (4.4A). The dissolution behavior of CO-PMMA blends was examined for two molecular weights of PMMA (Table 1). 

A large number of papers has appeared on the subject of excimer laser exposure of polymer films (16-21). Most of these have dealt with the phenomenon of photoablation. A few have observed intensity dependent photochemistry (22,23). The latter authors were concerned with the effect of exposure intensity on resist development characteristics. The utility of nonlinear photochemistry for image modification has not been explored except in our earlier communication, in which strongly nonlinear irreversible bleaching was observed for KrF laser irradiation of acridine/PMMA films with lOnsec pulses (5). 

Table 11. Effect of pulse repetition time on relative peak intensity and tacticity determination for radically prepared PMMA by 500-MHz 1H NMR. ...

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