Degradation of polymer

When polymers are subjected to elevated temperature in an inert atmosphere they degrade by different mechanisms, which are described as follows. 

The structure of polypropylene is similar to PE, except that it possesses branched methyl groups along its backbone, making every other carbon atom in the chain tertiary as it holds a methyl group. This leads to scission of the carbon chain, predominantly between secondary and tertiary carbon atoms. 

In PMMA there are different kinds of bonds since C-H and C=0 possess high values of bond dissociation energy, whereas the bond dissociation energies of C-O and C-C 

Apart from these three degradation mechanisms, rearrangements of the fractions formed may take place. A polymer does not undergo only one pyrolysis route always, but multiple routes may be taken simultaneously. The type of reaction is totally governed by the strength of bonds in the molecules. The lowest energy path will be favored. 

The lifetime of many polymer products in use is limited by oxidative degradation. Exposed samples are usually non-uniformly oxidised. At the macroscopic level, the heterogeneities can result from oxygen-diffusion-limited effects. If the rate of oxygen consumption exceeds the rate of oxygen permeation, oxidation occurs in the surface layers, whereas the core remains practically unoxidised. The importance of this effect depends on several parameters. First, intrinsic parameters are linked to material geometry (e.g., sample thickness) coupled with the oxygen consumption rate, which depends on the reactivity of 

Oxidation is accelerated if the temperature is elevated. Exposure to optical and UV radiation can also accelerate the degradation. Additionally, the thermooxidation and photooxidation are accelerated by generating chemical effects by the application of mechanical forces (i.e., mechano-chemical degradation). Vibrational spectroscopy is particularly usefiil for studying the degradation processes as the measurements can be made in real time. 

We are now in a position to discuss the effects of low frequency ( 400 kHz) high intensity ( 3 W cm ) ultrasonic waves on macromolecules (polymers) and examine how the parameters such as frequency, intensity, hydrostatic pressure etc affect both the polymerisation and also the depolymerisation processes. We will also consider in this chapter the use of ultrasound in polymer processing. 

Schmid for his part considered two ideal cases for the degradation of the polymer in solution. The first considered the macromolecules to be immobile in solution and the solvent molecules were swept past them under the action of the applied acoustic field the second involved an allowance for macromolecule movement, the macromolecule moving with the solvent under the action of the applied ultrasonic field. 

In the case of an immobile macromolecule in solution it is possible to estimate a value of the frictional force (F) developed between the solvent and polymer molecules by assuming the macromolecule to consist of a number (n) of solid spherical entities and applying the Stokes formula in a modified form (Eq. 5.8). 

The second ideal case identified by Schmid assumed that the macromolecule moved with the solvent and in this case Eq. 5.9 must be employed  

Here m is the molar mass (R.M.M.) of the monomer, is the Avagadro number and 

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Amirudin A and Thierry D 1995 Applioation of eleotroohemioal impedanoe speotrosoopy to the study and degradation of polymer-ooated metals Prog. Org. Coat. 26 1... 

B. Gilg, H. MuUer, and K. Schwarzenbach. Paper presented Advances in Stabili tion and Controlled Degradation of Polymers, New Paltz, N.Y., June... 

R. Simha, "Degradation of Polymers," in Polymerisation andPoljcondensation Processes, No. 34, Advances in Chemistrj Series, American Chemical Society, Washington, D.C., 1962, p. 157. 

When the polymers are exposed to ultraviolet radiation, the activated ketone functionahties can fragment by two different mechanisms, known as Norrish types I and II. The degradation of polymers with the carbonyl functionahty in the backbone of the polymer results in chain cleavage by both mechanisms, but when the carbonyl is in the polymer side chain, only Norrish type II degradation produces main-chain scission (37,49). A Norrish type I reaction for backbone carbonyl functionahty is shown by equation 5, and a Norrish type II reaction for backbone carbonyl functionahty is equation 6. 

Applications of ISS to polymer analysis can provide some extremely useful and unique information that cannot be obtained by other means. This makes it extremely complementary to use ISS with other techniques, such as XPS and static SIMS. Some particularly important applications include the analysis of oxidation or degradation of polymers, adhesive failures, delaminations, silicone contamination, discolorations, and contamination by both organic or inorganic materials within the very outer layers of a sample. XPS and static SIMS are extremely comple-mentar when used in these studies, although these contaminants often are undetected by XPS and too complex because of interferences in SIMS. The concentration, and especially the thickness, of these thin surfiice layers has been found to have profound affects on adhesion. Besides problems in adhesion, ISS has proven very useful in studies related to printing operations, which are extremely sensitive to surface chemistry in the very outer layers. 

Oxidative stability is highly important because it deals with the degradation of polymers under actual performance conditions. Oxidative stability, as applied to urethanes, refers to the combination of oxygen and heat or oxygen and light that causes degradation of urethanes. 

FIGURE 22.8 Flow rate effect on the elution of polyacrylamide. Degradation of polymer during the analysis occurs in each case at flow rates just above those shown. Unmodified MW = 12-15 x I0 carboxyl substitution = 9.5%. , unmodified A, sheared for 30.0 min , sheared for 12 hr O, sonicated for 1.0 min O, sonicated for 5.0 min. (From Ref 23, Copyright 1988. Reprinted by permission of John Wiley Sons, Inc.)... 

The degradation of polymers may be prevented by the addition of sodium nitrile, thiourea, etc. 

Thermoxidative degradation of polymers can occur at all stages of their lifecycle (polymerization, storage, fabrication, weathering) but its effect is most pronounced... 

In contrast to the extensive work of the pure thermal degradation of polymers, less fundamental chemical information is available on the mechanism of oxidative degradation of polymeric materials. As another point of... 

Muller AJ, Odell JA, Carrington S (April 1991) In Degradation of polymer solutions in extensional flow, Proceedings of the polymer physics a Conference to mark the retirement of A Keller, Bristol UK 3-5... 

Volume 14 Degradation of Polymers Volume 14A Free-radical Polymerisation... 

Plutonium(IV) polymer is a product of Pu(IV) hydrolysis and is formed in aqueous solutions at low acid concentrations. Depolymerization generally is accomplished by acid reaction to form ionic Pu(IV), but acid degradation of polymer is strongly dependent on the age of the polymer and the conditions under which the polymer was formed (12). Photoenhancement of Pu(IV) depolymerization was first observed with a freshly prepared polymer material in 0.5 HClOh, Fig. 3 (3 ). Depolymerization proceeded in dark conditions until after 140 h, 18% of the polymer remained. Four rather mild 1-h illuminations of identical samples at 5, 25, 52, and 76 h enhanced the depolymerization rates so that only 1% polymer remained after the fourth light exposure (Fig. 3). 

Polymers have inherently high hydrocarbon ratios, making liquefaction of waste plastics into liquid fuel feedstocks a potentially viable commercial process. The objective is to characterise the thermal degradation of polymers during hydrogenation. LDPE is studied due to its simple strueture. Isothermal and non-isothermal TGA were used to obtain degradation kinetics. Systems of homopolymer, polymer mixtures, and solvent-swollen polymer are studied. The significant variables for... 

Over the last few decades, the use of radiation sources for industrial applications has been widespread. The areas of radiation applications are as follows (i) Wires and cables (ii) heat shrinkable tubes and films (iii) polymeric foam (iv) coating on wooden panels (v) coating on thin film-video/audio tapes (vi) printing and lithography (vii) degradation of polymers (viii) irradiation of diamonds (ix) vulcanization of mbber and rubber latex (x) grain irradiation. 

Kyker GS, Valaitis JK (1978) In Allara DL, Hawkins WL (eds) Stabilization and degradation of polymers. Advances in chemistry series, USA, chap 24,169 293... 

Kyker, G. S. and Valaitis, J. K., in "Stabilization and Degradation of Polymers," Advances in Chemistry Series 169, American Chemistry Society, Washington, D.C., 1978. 

J. C. Dawson and H. V. Le. Controlled degradation of polymer based aqueous gels. Patent US 5447199,1995. 

R.L. Gray and C. Neri, Proceedings 19th Ann. Inti. Conference on Advances in the Stabilization and Degradation of Polymers, Luzern (1997), pp. 63-79. 

Accelerate chemical, photochemical, biochemical reactions or processes, e.g. cross-linking or degradation of polymers. Also called promoters, co-catalysts. Refer usually to the cure process in thermosetting resins. 

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