hermogravimetric analysis (TGA) has come into wide T use for rapidly determining the thermal stability of substances. Essentially, by this method one measures weight loss as a function of temperature under controlled environmental conditions. Modern thermobalances are designed to produce a continuous record of weight as a function of temperature. Of course, information can be obtained by carrying out weighing operations manually. However, in order to r SPRING TRANSDUCER AKMATURE WEIGHT CALIBRATOR DAMPER VACUUM OL R CRUCIBLE CONTROLLING THERMOCOUPLE SAMPLE THERMOCOUPLE Figure I . Diagram of the design of the thermo-grav.POLYMER ENGINEERING A N D SCIENCE, JULY, 1965 insure that no features of the weight loss-temperature curve are overlooked, automatic continuous recording of weight and temperature are recommended.A commercial instrument that has been used by the present authors is shown schematically in Figure 1 ( 2 , 3 ) . The method used in this case is based upon an enclosed spring as the weight sensor, the extension of the spring being sensed electrically through the effect this has on the position of an iron core within a differential transformer. Several linear heating rates, as well as provision for maintenance of constant temperature, are incorporated in the furnace controls.Oftentimes pyrolysis occurs by a multi-step mechanism, with overlapping of temperature ranges for the various steps, so that the resulting irregular weight-temperature curves are very difficult to analyze. However, in many cases, including many polymer decompositions, the trace follows a simpler path which is characteristic of many decompositions. The sample weight decreases slowly as reaction begins, then drops rapidly over a relatively narrow temperature range and finally levels off as the reactant is exhausted. Figure 2 shows a primary thermogram for Teflon.The shapes of the curves in cases such as illustrated in Figure 2 are determined by the kinetic parameters of the pyrolysis, such as reaction order, frequency factor, and energy of activation. These values can be of importance in the elucidation of the mechanisms involved in polymer degradation (4) . TGA MethodsSome reported methods for obtaining kinetic parameters from such TGA traces involved lengthy equations and simplifying assumptions (5-7). Recently several exact methods have been proposed. These methods are based upon the assumption that the Arrhenius equation is valid for the degradation process.The relation of Freeman (8) may be written A log R, = n A log w, -(E/2,303R) A (1/T),where Rt is the rate of reaction, n is reaction order, E is 135
Due to abnormally small root‐mean‐square end‐to‐end distances for poly(n‐lauryl methacrylate) fractions in an ideal solvent reported previously, investigation of this polymer in another ideal solvent was carried out. Phase separation studies indicated that in n‐amyl alcohol the θ‐temperature is 29.5°C., and viscosity and light‐scattering measurements were made in this temperature. Dimensions in this solvent were found to be in the expected range. K values in the two ideal solvents are compared.
Section, Picatinny Arsenal, Dover, New Jersey 471 472 S. N. CHINAI, A. T,. RESNICK, H. T. LEE and then xith \Yuter. hours.The polymer tvas finally drird at G 0 C . for 24 The yield of poly-n-octyl methacrylatc polymer ivas 87%. FractionationThe fractional precipitation procedure, combined with lowering the temperature from 30' to 20°C., was used in the fractioiiatioii of poly-n-octyl methacrylate from a 3yo benzene solution, with the use of methanol as a precipitant. The original heterogeneous sample ~vab divided into five fractions a t the end of the first fractionation. Of these, only three fractions were subjected to the first refractionation from a o.5y0 benzene solution.This resulted in 12 subfractions. Of these 12 subfractions, 11 were subjected to the second refractionation from a 0.1% benzene solution. Only the first fraction from each of these 11 subfractions was recovered. This procedure resulted in 11 fractions which were more homogeneous with respect to molecular might than the original material. The fractions were rendered free from solvent by the frozen-benzene technique.
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