Polymerization - curves irradiation

Rp)max w ich stays nearly constant up to 40% conversion. It decreases later on because of mobility restriction brought upon by gelation and solidification of the UV-irradiated material. This behavior is best illustrated in Figure 4 where the instant rate of polymerization (Rp), calculated from the slope of the curve recorded by RTIR spectroscopy, was plotted as a function of the exposure time. 

The auto-acceleration observed under such conditions is reduced ( = 1.15) and could partially result from non-steady-conditions but also from a "matrix effect" operating on the surface of unswollen polymer particles. It should be noted in this respect that the post-polymerization which is induced by the growing chains occluded in the precipitated polymer exhibits an initial rate very much lower than the rate observed during irradiation (Curve 1 in Figure 91 which suggests that the contribution of the growth of occluded chains to the over-all rate is small. 

Figure 10. Polymerization of acrylonitrile at 20°C in an intimate mixture with a highly divided polyacrylonitrile obtained by pre-irradiation of the crystalline monomer at 95°C (20). Doses of pre-irradiation of 0.11 Mrad (curve 2) to 3.14 Mrad (curve 12). The broken curve 1 pertains to the polymerization of pure acrylonitrile curve 13 is obtained in the presence of polyacrylonitrile pre-poly-...

The two intermediate curves each represent a single dilatometer, each prepared according to Procedure I and irradiated at three dose rates. The slopes of these lines are 0.60 and 0.71, and the absolute values of the rates of polymerization are markedly higher than for the previous samples. Within experimental error, the slopes of these two lines are essentially the same and are less than the value given for die previously quoted, lower rates of polymerization. 

An important quantity that can be deduced from the reaction profile is the rate of the cross-linking polymerization (Rp), i.e., the number of double bonds polymerized or of cross-links formed per second. Rp values were determined from the maximum slope of the kinetic curves (usually reached for conversion degrees between 20 and 40%). Table I summarizes the Rp values for the two photoresists tested under various conditions, namely conventional UV and continuous or pulsed laser irradiation at different light intensities. According to these kinetic data, Rp increases almost as fast as the light-intensity the ratio Io/Rp which is directly related to the product of the light-intensity and the required exposure time was found to vary only in the range 10-8 to... 

In order to get further insight into the reaction mechanism for the degradation of PMMA, we have studied the nature and behavior of radical entities in irradiated PMMA by using the ESR and ESE techniques complementarily [37]. Two PMMA samples, a commerical PMMA and an initiator-free PMMA prepared by the radiation-polymerization of bulk monomer, were used, but no difference was found in the results. Residual monomer was carefully removed from the PMMA samples, because the monomer molecule readily modifies the radicals derived from the polymer. The samples were irradiated in vaccum. Figure 9 demonstrates the dose-yield curve we obtained by irradiating PMMA in vacuum at 273 K. The G value for the radical formation is determined to be 3.0 from the slope of the linear portion below 12 kGy. 

The influence of the monomer to water ratio on the polymerization rate was studied with sodium lauryl sulfate as the emulsifier. The conversion curves for the case of 3% emulsifier are shown in Figure 5. In Figure 6 the linear conversions for ten minutes of irradiation at 0.175 Mrads per hour are plotted against the water-monomer ratio for 1, 3, and 5% emulsifier. All three sets of data show a linear dependence of the rate on the ratio, in other words, the rate per cubic centimeter of water phase is independent of the monomer-water ratio. 

Additives, e.g. initiator, for any type of chain reaction are not involved in the crystalline state polymerization. An intermittant irradiation has no appreciable effect on kinetic curves as far as the total irradiation time is the same. An induction period has not been reported except in one case (see Sect. IV.b.)28). 

The continuous change of the thermodiagram in polymerization has mainly been studied for DSP by means of differentially scanning calorimetry (DSC)10). DSC curves obtained in the course of photopolymerization of DSP crystals at the irradiation times of 20, 40, 50, and 70 min with a xenon lamp are shown in Fig, 15 together with those of DSP, as-polymerized, recrystallized, and amorphous poly-DSP s. 

In Fig. 16 the heat-treated sampie(l) is obtained by heating poly-DSP crystals (a) up to 330°C at a scanning speed of 15°C/min. Then, the sample is cooled immediately to room temperature. The intrinsic viscosity of the original as-polymerized poly-DSP (2.1-2.9) is reduced (0.55-0.59) in sample (1). An X-ray pattern of sample (1) shows slight but definite differences when compared to that erf the original as-polymerized polymer and, in addition, the pattern agrees exactly with that ot the medium-sized polymer crystals (c), which are obtained by photopolymerization of DSP crystals upon irradiation with a xenon lamp for 50 min. DSC curves of sample (1) and polymer crystals (c) are also very similar to each other. From these results, it is concluded that the high... 

There is an early report in the literature claiming absence of the autocatalytic reaction enhancement in TS if the reaction is induced by UV-excitation of the monomer crystal. The implication would be that thermal and UV-polymerization involve different mechanisms. Later on, however. Chance and Patel found this to be an artifact caused by the neglect of spatially inhomogeneous absorption by polymer molecules which effectively competes with monomer excitation at increasing conversion and prematurely terminates the reaction. Although it is difficult to correct X(t)-curves obtained under UV-excitation for polymer absorption quantitatively, particularly if irradiation is done with unpolarized non-monochromatic light, it turns out that there is a qualitative agreement between X(t)-curves obtained under y-and UV-irradiation. Application of this correction, however, does not solve the puzzle why in case of y- or UV-polymerization of TS, the reaction rate increases less dramatically with conversion, than observed upon thermal conversion. 

The generation sequence of the most intense photoproduct series is shown in Fig. 5 by the integral absorptions of the individual photoproducts as a function of the irradiation time. Only the photoproduct A is generated without any delay. In the sequence B, C, D,. .. the induction period increases continuously. This corresponds to the expected polymerization reaction starting with the formation of the dimer A followed by subsequent addition reaction steps to the trimer B and tetramer C molecules etc. The curves are calculated using the kinetic expressions described below. 

In all the above-described experiments the irradiated mixtures were allowed to warm to room temperature under vacuum. In one series of experiments, the ampoules were opened at —196 °C., and their contents melted in the presence of acetone (an inhibitor of ionic polymerizations) according to the procedure described above. Under such conditions no polymer was obtained after irradiation at —196 °C. Hence, the conversion curve for —196°C. in Figure 1 and the rate curves in Figure 2 pertain to polymer formed as a result of an anionic after-effect which occurs during the warming of the irradiated mixture under vacuum. 

Figure 4. Thermogravimetric analysis curves in N2 and air for monomer III polymerized by S seconds irradiation using 0.5 mole % (4-octyloxyphenyl)phenyliodonium hexafluoro-antimonate as photoinitiator.

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