Polymer chains, thermal destruction

The industrial production of copolymers of trioxane with ethylene oxide or di-oxolane (1-5%) is conducted as a bulk polymerization in special equipment. The incorporation of small amounts of C-C bonds into the C-0 chain has a remarkable effect on the thermal and chemical stability. In homopolymers the thermal decomposition starts at the semiacetal end groups ( unzipping ) and leads to a complete destruction of the polymer chain, whereas this reaction stops in copolymers already at the first C-C bond. A thermally stable OH end group is thus formed which, in addition, contributes to a much better alkali resistance compared to ester group-terminated homopolymers (see Examples 3-40 and 5-13). 

Pyrolysis is simple thermal destruction of the molecular chain of the base polymer in the adhesive or sealant formulation. Pyrolysis causes chain scission and decreased molecular weight of the bulk polymer. This results in reduced cohesive strength and increased brittleness. Resistance to pyrolysis is predominantly a function of the intrinsic heat resistance of the polymers used in the adhesive formulation. As a result, many of the aromatic and multifunctional epoxy resins that are used as base resins in high-temperature adhesives are rigidly crosslinked or are made of a molecular backbone referred to as a ladder structure, as shown in Fig. 15.4. 

As a working hypothesis we may suggest the following. At 400-700°C two main processes occur simultaneously. One of them is a thermal destruction of three-dimensional polymer structures formed at wood hydrolysis. Another process is a polycondensation of polymer chain residues provided by the labile side propane chains and phenol lings with reactive groiqis. 

With the aim to study PA transformations Makarov [40] used method of free radicals initiation by thermal and photochemical decomposition of peroxides. The author succeeded in finding high efficiency of PA maeromolecules breakages under the action of free radicals at the temperature 20-98 ° C (within the limits of operational temperatures). He also succeeded in determination of reactions sequence and revealing the phase directly responsible for the acts of polymer chains destruction. It is shown that in the conditions of thermal initiation transformations of peroxides are caused by macroradicals and photochemical - by own radicals of peroxides. 

Time of temperature effect on the sample influences the process of PETP destruction. During short effect sharp decrease of polymer specific viscosity takes place while polymer mass does not change. Drop of viscosity value slows down during the increase of temperature effect time and polymer mass loss sharply increases. Viscosity change is caused by thermal destruction of macromolecule, and mass change - by the destruction of chain end groups [207]. 

So, all works on investigation of the mechanism of thermal and thermooxidative destruction suppose that these processes have radical nature and run by the way of formation and decay of peroxide radicals and hydroperoxides. Together with oxidative decay of aliphatic part, as a result of which polymer chain decomposes with volatile products liberation and formation of new end groups, there are also changes in aromatic part-jointing of polymer takes place. 

Kinetics of radical chain process of polymer thermal destruction includes stages of initiation, growth of reaction chain, chain transmission, its break. Reaction of chain transmission occurs mainly at the expence of hydrogen break Irom polymer chain. 

During addition of HC-3 and HC-4 such effect is not observed, probably these additives have inhibiting effect on thermal and thermooxidative destruction, as these additives do not have developed chain of conjugation. Taking into account the fact that polymer materials must work in narrow temperature ranges for a long time, we have studied also kinetics of thermal destruction at isothermal heating. 

Destruction of macromolecules as a result of chemical interactions with the environment leads to the fracture of polymer materials with the formation of active components. Participation of individual macromolecules in destruction processes is dependent upon the amount of the constituent reactive groups. Introduction of substituents of different compositions into the polymer chain changes their stability in thermal oxidation reactions and in response to chemical reagents and moisture effects. 

In principal, to describe the thermal destruction of polymer chains the mathematical models [16], which take into account the differences between the rates of decomposition of complexed (N,) and free (Nf) chain units, can be used ... 

As an example, the aromatization of naphthene rings, thermal dealkylation, thermal dehydro-cyclization of alkyl chains, and subsequent aromatization, as well as some destruction of (oxygen) heterocyclic systems to name only four possibilities. Nevertheless, the data have been interpreted as indicating that the molecular weight of a coal molecule increases with rank of the coal but that of the unit structures remains constant up to approximately 82% carbon coal after which an increase is noted (Figure 10.22) (Makabe and Ouchi, 1979). There is no evidence to indicate that the unit structures are linked linearly (i.e., in the manner of a polymer chain) or that the unit structures are identical. 

In the case of vulcanizates with polysulfide bonds, when the process of stress relaxation is due to the decomposition of thermally unstable transverse bonds, and not to thermooxidative destruction of the polymer chain, the difference in the rates of chemical stress relaxation of the filled and unfilled cured rubbers is comparatively small (Fig. 187). 

Fig. 26. Sdiematic ilhistratioii of diotolysis of dianthracene units in polymer main chain and subsequent reactions. 1 photolysis of dianthracene main chain, 2 remaking of dianthiacence units, 3 thermal destruction of sandwich dimer structure...

The TGA data lend support to the assumption as to the high thermal stabillity of polymers with fragments Tg in the main chain (Fig. 7). The incipient decomposition temperature in a gel medium was 460 °C, and the wieght loss at 1000 °C was 14%. Simultaneously, thermal oxidative destruction in air causes a rapid structural degradation of the polymer starting at 300 °C. Here the weight loss was several times that observed in thermal destruction and could be as high as 60%. 

The mechanisms of PVC fusion have been studied in detail by several researchers and a number of articles have been written (5,6). However, Rabinovitch and Summers were the first to describe in detail how the morphology of PVC changes during hot processing and provided clear mechanisms of PVC fusion (5,6). As described in their work PVC fusion is considered as the thermal reduction of particle boundary surface. In other terms, the process of PVC fusion is essentially a destruction of the original coarse powder structure (100-200 pm in diameter) to submicroparticles (approximately 10-30 nm in diameter) so that ey can be compacted. During further interdiffusion of fhe PVC, the boundaries between the submicroparticles disappear and a three dimensional network of polymer chain is formed. 

It occurs during the chain cracking and radiolysis of hydrocarbons [38], radical polymerization and oligomerization of monomers [39], thermal and thermooxidative destruction of polymers (see Chapter 19) and hydrocarbon oxidation at low dioxygen pressure. 

Polyorganosiloxanes are very stable chemically the siloxane chain is preserved in many chemical reactions, whereas the thermal oxidative destruction of the molecule is generally connected only with the detachment of lateral radicals. It is essential that the decomposition product is polymer (SiC>2)x, which keeps all its dielectric properties and some strength, unlike the decomposion products of organic polymers. E.g., at 200 °C the dielec-... 

Thus, all used additives may be devided into two groups 1) increasing polymer resistance to thermal and thermooxidative destruction 2) decreasing polymer thermal stability. Moreover, effect of additive on thermo- and thermooxidative stability will depend on the length of conjugation chain in modifier s molecule. 

Scissions of main-chains by the mechanical destruction of the polymers are experimentally proved by the analyses of the observed ESR spectra for the various pdy-mers PE, PTFE, PB, PP and PMMA. A pair formation of the radicals, (mechano-radicals), after the milling is clearly demonstrated and this pair formation is believed to be the direct evidence for tl mechano-radicals formed primarily by the medianical actions. A model for chain rupture in an amorphous pdymer was proposed. Excess electrons produced by the triboelectricity due to the friction, diich is always accompanied with the mechanical fracture, play an important role, with coexistence of oxygen, in the thermal conversion of the mechano-radicals. The characteristic behaviors of the mechano-radicals, the hi er reactivity with oxygen, complete photoconversion of the peroxy radical, indicate that the mechano-radicak are formed and trapped on the fresh surfaces produced by cleavage in the solid polymer. The polymerizations initiated at the low temperatures by the PTFE mechano-radicals were reported and the copol5mierization is experimentally evidenced. 

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