Polymers heating rate effects

The problems of monomer recovery, reaction medium viscosity, and control of reaction heat are effectively dealt with by the process design of Montedison Fibre (53). This process produces polymer of exceptionally high density, so although the polymer is stiU swollen with monomer, the medium viscosity remains low because the amount of monomer absorbed in the porous areas of the polymer particles is greatly reduced. The process is carried out in a CSTR with a residence time, such that the product k jd x. Q is greater than or equal to 1. is the initiator decomposition rate constant. This condition controls the autocatalytic nature of the reaction because the catalyst and residence time combination assures that the catalyst is almost totally expended in the reactor. 

Cl in conjunction with a direct exposure probe is known as desorption chemical ionization (DCI). [30,89,90] In DCI, the analyte is applied from solution or suspension to the outside of a thin resistively heated wire loop or coil. Then, the analyte is directly exposed to the reagent gas plasma while being rapidly heated at rates of several hundred °C s and to temperatures up to about 1500 °C (Chap. 5.3.2 and Fig. 5.16). The actual shape of the wire, the method how exactly the sample is applied to it, and the heating rate are of importance for the analytical result. [91,92] The rapid heating of the sample plays an important role in promoting molecular species rather than pyrolysis products. [93] A laser can be used to effect extremely fast evaporation from the probe prior to CL [94] In case of nonavailability of a dedicated DCI probe, a field emitter on a field desorption probe (Chap. 8) might serve as a replacement. [30,95] Different from desorption electron ionization (DEI), DCI plays an important role. [92] DCI can be employed to detect arsenic compounds present in the marine and terrestrial environment [96], to determine the sequence distribution of P-hydroxyalkanoate units in bacterial copolyesters [97], to identify additives in polymer extracts [98] and more. [99] Provided appropriate experimental setup, high resolution and accurate mass measurements can also be achieved in DCI mode. [100]... 

In some cases, several of these processes occur simultaneously, depending on the sample size, the heating rate, the pyrolysis temperature, the environment, and the presence of any additives. Although polymer degradation schemes can be greatly altered by the presence of comonomers, side-chain substituents, and other chemical constituent factors, the ultimate thermal stability is determined by the relative strengths of the main-chain bonds. Many additives and comonomers employed as flame retardants are thermally labile and as a result the thermal stability of the polymer system is reduced. In order to reduce the observed effects of the flame-retardant additives on the thermal stability of the polymeric materials, more thermally stable and hence inherently flame-resistant polymers are of increasing interest. 

Figure 6. Kinetic effects appearing in DSC thermograms for bis-A-polycarbonate (16,000/17,000) block copolymer. Heating rate, 40 K/min. Range, 5 mcal/sec. (A) Dried polymer powder no thermal pretreatment. (B) Annealed 1 min at 493 K and quenched to 320 K.

Because of diffusion-controlled termination and propagation with concomitant at a particular conversion, it is possible to operate a CSTR at considerably higher production rates. Because of the additional heoeficial effects of cold monomer and water feeds on heat removal, much higher production rates are possible than with a batch reactor of the same volume. It should he remembered that polymer production rates are usually limited by heat removal capacity. 

An interesting comparison regarding the effect of different substituents on pyrolysis results for a polymer can be obtained from the Py-GC/MS analysis of a poly(2-vinylpyridine) and that of a poly(4-vinylpyridine) sample. The sample of poly(2-vinylpyridine) has a M = 5,000. The pyrolysis was done for both samples from 0.4 mg material, at 600° C in He, at a heating rate of 20° C/ms with 10 s THT, and with the separation on a Carbowax column (see Table 4.2.2). The pyrogram for poly(2-vinylpyridine) is shown in Figure 6.5.11 with the peak identification in Table 6.5.11 (see Figure 6.5.13 for the pyrogram of poly(4-vinylpyridine)). 

Figure 10.10 Scanning rate effects on DSC curves of a polyethylene terephthalate) (PET) sample (a) heating and (b) cooling. I, lOKmin-1 II, 20Kmin-1 III, 40 K min 1. (Reproduced with permission from T. Hatakeyama and F.X. Quinn, Thermal Analysis Fundamentals and Applications to Polymer Science, 2nd ed., John Wiley Sons Ltd, Chichester. 1999 John Wiley Sons Ltd.)...

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