Polymerization - curves thermal

Both defect absorption and electroabsorption grow during the thermal polymerization process though differently. The absorption band saturates after about 6 h at 80°C. The electroabsorption peak increases rapidly when the autocatalytic range is reached, together with a red shift by about 70 meV similar as the excitons (Fig. 2, PTS-7). As shown in Fig. 3 the growth of Aa follows closely the polymerization curve (14). When most of the material is polymerized, however, Aa decreases again to about half its peak value quite in contrast to the absorption spectrum. 

Melting temperatures of as-polymerized powders are high, ie, 198—205°C as measured by differential thermal analysis (dta) or hot-stage microscopy (76). Two peaks are usually observed in dta curves a small lower temperature peak and the main melting peak. The small peak seems to be related to polymer crystallized by precipitation rather than during polymerization. 

Peaking and Non-isothermal Polymerizations. Biesenberger a (3) have studied the theory of "thermal ignition" applied to chain addition polymerization and worked out computational and experimental cases for batch styrene polymerization with various catalysts. They define thermal ignition as the condition where the reaction temperature increases rapidly with time and the rate of increase in temperature also increases with time (concave upward curve). Their theory, computations, and experiments were for well stirred batch reactors with constant heat transfer coefficients. Their work is of interest for understanding the boundaries of stability for abnormal situations like catalyst mischarge or control malfunctions. In practice, however, the criterion for stability in low conversion... 

Fig. 21.—A comparison of the effects of 0.1 percent of benzo-quinone (curve II), 0.5 percent of nitrobenzene (curve III), and 0.2 percent of nitrosobenzene (curve IV) on the thermal polymerization of styrene at 100°C. Curve I represents the polymerization of pure styrene. (Results of Schulz. )...

Fig. 7. Time-conversion curves of thermally initiated emulsion polymerization of 1,4-DVB at 0.1 (I) 0.65 (II) and 0.85 (III) M SDS concentrations. Polymerization temperature = 90 °C water/monomer volume ratio = 12.5. [Reproduced from Ref.84 with permission,Hiithig Wepf Publ., Zug, Switzerland].

In Figure 9, we have also represented the variation of (Rp)max with the temperature at which this quantity was measured (curve b). At first sight, this nearly straight line seems to indicate that an increase of the temperature leads to a faster polymerization, a typical behavior observed in thermal runaway processes. Actually, the polymerization rate is growing here because the light intensity was increased, which leads in turn to a greater rise in temperature. 

One of the more recently exploited forms of thermal analysis is the group of techniques known as thermomechanical analysis (TMA). These techniques are based on the measurement of mechanical properties such as expansion, contraction, extension or penetration of materials as a function of temperature. TMA curves obtained in this way are characteristic of the sample. The technique has obvious practical value in the study and assessment of the mechanical properties of materials. Measurements over the temperature range - 100°C to 1000°C may be made. Figure 11.19 shows a study of a polymeric material based upon linear expansion measurements. 

Second example was obtained from the copolymerization initiated with starch. The results were shown in Fig. 12. The copolymer isolated from the monomer phase was produced by the thermal polymerization and the composition curve was completely similar to the ordinary curve of the radical copolymerization product. The copolymer isolated from the water phase differed from the usual copolymer. The upper curve indicated that the HA formed by starch were soft, and soft MMA was much more easily incorporated than hard St. 

At Van Sickle s conditions of low temperatures and low conversions, branching routes A and B appear to be dominant since there is little alkenyl hydroperoxide decomposition. In our work above 100°C., the branching routes are supported by the nearly linear initial portions at low conversions for alkenyl hydroperoxide and polymeric dialkyl peroxide curves (see Figures 2, 3, and 4). The polymeric dialkyl peroxides formed under our reaction conditions include those formed by the branching mechanism postulated by Van Sickle (routes A and B) and those formed by the reaction of the alkenoxy and hydroxy radicals from alkenyl hydroperoxide thermal decomposition reacting further and alternately with olefin and oxygen (step C). The importance and kinetic fit of the sequential route A to C appears to increase with temperature and extent of olefin conversion owing to the extensive thermal decomposition of the alkenyl hydroperoxides above 100°C. 

The dynamic mechanical thermal analyzer (DMTA) is an important tool for studying the structure-property relationships in polymer nanocomposites. DMTA essentially probes the relaxations in polymers, thereby providing a method to understand the mechanical behavior and the molecular structure of these materials under various conditions of stress and temperature. The dynamics of polymer chain relaxation or molecular mobility of polymer main chains and side chains is one of the factors that determine the viscoelastic properties of polymeric macromolecules. The temperature dependence of molecular mobility is characterized by different transitions in which a certain mode of chain motion occurs. A reduction of the tan 8 peak height, a shift of the peak position to higher temperatures, an extra hump or peak in the tan 8 curve above the glass transition temperature (Tg), and a relatively high value of the storage modulus often are reported in support of the dispersion process of the layered silicate. 

Experimentally, fN is determined as a function of temperature T, solvent composition x, and degree of polymerization N fN = F xp(T, x, N) here Fexp stands for the experimentally obtained functional form. On the other hand, statistical-mechanical formulations allow fN to be expressed in terms of s, a, and N fN = Flhcor(s, a, N), where Fth denotes a theoretical function. Then it should be possible from a comparison of F p and Flheor to determine s and a as functions of T and x. How can this be achieved Since the pioneering work of Zimm et al. (17) in 1959 various methods have been proposed. Typical approaches are outlined below for the experimental situation in which a thermally induced helix-coil transition is observed. For most of the proposed methods such transition curves must be available for a series of samples of different N. Preferably, these samples ought to be sharp in molecular weight distribution and cover as wide a range of N as possible. 

Figure 11 shows a compilation of the compositions of the polymers which have been polymerized from different monomer mixtures as a function of polymerization temperature. The curves plotted next to the measured points were calculated at temperatures below 100 °C by Equation 33 and at temperatures above 100 °C by Equation 17. The dotted curves for temperatures above 100 °C were calculated by Equation 33. In addition to the measured values taken from Tables III and V, Figure 11 also contains some measured points at 0°C. Polymerization was done in flasks which were stored in a thermally controlled room for a long time (e.g.y with 30 mole % a-methylstyrene, 34 days, with 50 and 70 mole %, 166 days). It is apparent that the curve derived by Equation 17 agrees well with the measured points. However, the dotted curve at higher temperatures, calculated by Equation 33 shows significant deviations. 

The curve obtained with PVC prepared by suspension polymerization generally indicated autocatalytic thermal dehydrochlorination, and the time required for 0.1 mole % decomposition was usually less than 35 minutes (Figure 1). When PVC was prepared by bulk polymerization, the dehydrochlorination plot was generally slightly more linear, showed little autocatalytic character, and the time for 0.1 mole % decomposition was approximately 40 minutes (Figure 2). 

The addition of small amounts of radical scavenger (such as benzoquinone and diphenylpicrylhydrazyl) led to the appearance of induction periods in the kinetic curves. The duration of the induction periods are proportional to the concentration of the radical scavenger. The presence of atmospheric oxygen slightly slowed the polymerization. These observations indicate that the polymerization proceeds by a radical mechanism. The radicals are formed from the y-radiolysis of the monomers. By comparison to the ESR spectrum of the radicals formed by thermal initiation with azobisisobutyroni-trile in the presence of a spin trap, the radical formed is... 

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