Techniques and mechanisms of polymer degradation

In order to fully characterize the long-term properties of polymers a variety of techniques must be used. Several of them may give similar results but often they complement each other. The most important thing is to relate the observations obtained by analysis with postulated degradation mechanisms, whereby it should be possible to describe the possible failure of the new materials. The interaction of the polymers with the environment will give valuable information concerning the use of plastics, e.g. in the human body or the possibility to compost them. 

Degradable Polymers. Edited by Gerald Scott and Dan Gilead. Published in 1995 by Chapman Hall, London. ISBN 0412 59010 7 

Several interesting solutions have been proposed to make filled polyethylene degrade faster than previously observed. Incorporation of a chromophoric group, which increases the susceptibility of polyethylene towards photooxidation, is one choice. Metal salts are also useful and potent additives, and they give room for radical formation within the polymer chain and again increase the photo-oxidative behaviour of these materials. 

It is now widely accepted that the conservation of resources is important. This demand has led to the use of renewable materials. The knowledge that biopolymers usually degrade quickly by microbiological action has led to the use of agricultural products as additives for synthetic polymers with the intention of producing biodegradable polyethylene. 

Polymers with hydrolyzable linkages in the backbone can also be used as degradable materials. So far, most of them are too expensive and do not have the desirable combination of mechanical and chemical properties. Well-known synthetic hydrolyzable polymers are polyesters [1], polycarbonates [2], polyanhydrides [2], polyamides [2] and poly(amino acids) [2]. Hydrolyzable natural polymers may be cheaper and are believed to be representative of the future development in degradable polymers. Many scientists today are looking for new possibilities using such traditional natural polymers as polysaccharides, proteins and lipids. Special interest is focused on poly(j8-hydroxybutyrate) and its copolymers [3,4]. Well-known natural products 

---

Karlsson S and Albertsson A-C, Techniques and mechanisms of polymer degradation in Scott G and Gilead D, Degradable Polymers Principles and Applications, London, Chapman HaU, 29—42, 1995. 

In random degradation molecular mass decreases early, while in chain degradation the molecular mass of the polymer remains almost constant. Characterisation methods for molecular mass are thus very sensitive methods to follow random degradation. In contrast, as monomer is produced in chain depolymerisation, weight loss measurement techniques are the best methods to follow this kind of degradation. (Chapters 10-12, in Section IV, of this book focus on the methods used in the molecular characterisation and analysis of polymer degradation and polymer degradation mechanisms.)... 

The prediction of the useful lifetime of polymer materials is a major challenge for the industry. New research efforts have been focused to understand the complex mechanism of polymer degradation by analyzing the chemiluminescence emission, due to its capabilities as well as commercially developed instrumentation. In the next few years, it would be expected that chemiluminescence will earn a place as a well-established research technique in many laboratories, and will contribute to the better understanding of the molecular structural level and the relationship with the macroscopic properties and the behavior in the use of the polymeric materials. 

Well before the advent of modern analytical instruments, it was demonstrated by chemical techniques that shear-induced polymer degradation occurred by homoly-tic bond scission. The presence of free radicals was detected photometrically after chemical reaction with a strong UV-absorbing radical scavenger like DPPH, or by analysis of the stable products formed from subsequent reactions of the generated radicals. The apparition of time-resolved ESR spectroscopy in the 1950s permitted identification of the structure of the macroradicals and elucidation of the kinetics and mechanisms of its formation and decay [15]. 

Thus, macroradicals have been obtained by stretching fibers (20), deforming plastics by compression (37), ball mill grinding (11), freezing and grinding of polymer solutions (10), ultrasonic irradiation (I), mastication (19), dispersion in a microblender (25), and other mechanical techniques (36). Many reviews on the formation of macroradicals by degradative processes have also been published (5, 12,13,16, 33). 

This somewhat critical situation may be resolved by the determination of specific and general physical and chemical regularities governing the formation and behavior of polymeric foams, which requires the use of a wide range of ideas and techniques developed in other sciences physical and colloidal chemistry, physicochemical mechanics, rheology, thermodynamics, physics of polymers, physics and mechanics of non-continuous media, physics of surface and transfer phenomena, chemical physics of oxidation and degradation processes, etc. 

Although photodegradation cannot proceed only by the radical mechanism, free radicals are nevertheless important intermediates in the photodegradation of polymers. The ESR technique is a most powerful method for detecting free radicals so that it is appropriate to use this technique to investigate the structure and behavior of the free radicals produced after UV irradiation in the hope that the results will elucidate the mechanism of the degradation of polymers. 

In the following, we will discuss some examples, which better illustrate the applications of the DPMS and the power of this technique in die assessment of the thermal degradation mechanisms of polymers. 

Thermogravimetric analysis (TGA) is one of the most commonly used techniques to study the primary reactions of the decomposition of polymers and other materials. TGA is also useful for the characterization and evaluation of polymer thermal stability. Although synthetic mbber nano composites have excellent mechanical properties, these properties may interfere with its low thermal stability. This may cause the polymer chain to be more susceptible to degradation. Degradation usually starts from a head-to-head stmcture, a site of unsaturation or a tertiary carbon atom [77]. 

The capabilities of the ESR technique for providing fundamental information about the mechanism of radiation degradation of polymers are shown in observations on gamma-irradiated poly(methyl methacrylate), polystyrene and their random copolymers. 

Controlled pyrolysis of the polymer followed by MS, possibly with a GC step in between, to resolve mixtures and simplify interpretation of mass spectrometric data, has been used extensively in polymer composition studies. These are the elucidation of details of polymer structure, and the use of the technique in studies of the thermal degradation mechanisms of polymers. It is only the latter that we are concerned with in this chapter. Polymer structure and composition studies have been extensively discussed in the author s earlier books [1,2]. 

Overall, inspection of the literature reveals that among the MS techniques (Figure 10.12) pyrolysis combined with GC/MS continues to be an important tool for the studies of thermal degradation mechanisms of polymers and to investigate the influence of additives on thermal stability. DPMS is also extensively applied since it is... 

---