Degradation polymer classification

Degradable polymer a polymer designed to undergo a significant change in its chemical structure under specific environmental conditions, resulting in a loss of properties that may vary as measured by standard test methods appropriate to the pol)rmer and the application in a p eriod of time that determines its classification. 

Hocking, P.J. (1992) The classification, preparation, and utility of degradable polymers. Journal of... 

Polymers can be classified in various ways. The most obvious is to think of them as either being natural or synthetic. This is the classification used in this book. Alternative ways of classification are based on their use (structural and non-stmctural polymers) and their characteristics (degradable and non-degradable polymers). These aspects are discussed in the first chapter. This chapter will give an overview of the synthetic and natural degradable polymers. However, overall, the book focuses on the synthetic polymers used for biomedical appfications. Amongst the class of synthetic polymers, this chapter will discuss polyesters, polycarbonates, and polyurethanes, the most commonly used synthetics polymers for biomedical appfications. 

The classification scheme illustrated in Figure 13.1 contains many possible connections, not shown. For example, the latex core/shell materials and the latex IPNs differ principally by the presence of cross-links in the latter, but significant morphological and mechanical differences may also occur. The mechanochemical blends are synthesized when a polymer 1/monomer 2 mix is masticated with sufficient shear to degrade polymer 1 to a certain extent. The various free radicals, anions and cations formed then initiate the... 

The most significant factor in determining the material properties of a polymer is the structure of the monomer(s) selected for polymerization. Monomer choice dictates both side chain and backbone structure, the latter which determines the polymer classification. Even minor changes to the chemical modification of a monomer (and the resulting polymer) can have drastic effects on polymer solubility, mechanical strength, crystallinity, and sensitivity to degradation. Consequently, the choice of monomer also often dictates the manner in which the material can be processed for biomedical applications. 

Various schemes to classify polymer degradation subsist. Because of its complexity, with regard to both the causes and the response of the polymer, classification is usually performed on the basis of the dominating features. Thermal degradation, i.e. the decomposition induced by heat in an inert atmosphere or under vacuum, is involved in all life phase of polymers (Figure 10.1). 

The chemical and physical properties of the polymers obtained by these alternate methods are identical, except insofar as they are affected by differences in molecular weight. In order to avoid the confusion which would result if classification of the products were to be based on the method of synthesis actually employed in each case, it has been proposed that the substance be referred to as a condensation polymer in such instances, irrespective of whether a condensation or an addition polymerization process was used in its preparation. The cyclic compound is after all a condensation product of one or more bifunctional compounds, and in this sense the linear polymer obtained from the cyclic intermediate can be regarded as the polymeric derivative of the bifunctional monomer(s). Furthermore, each of the polymers listed in Table III may be degraded to bifunctional monomers differing in composition from the structural unit, although such degradation of polyethylene oxide and the polythioether may be difficult. Apart from the demands of any particular definition, it is clearly desirable to include all of these substances among the condensation... 

This sophisticated picture is reflected by the many test procedures dealing with degradation or biodegradation that are published by different national, international, or industry-driven organisations (e.g. ISO, ASTM). The aim of all these efforts is to obtain comparable data on the behaviour of the polymer under consideration, but a driving force is also the marketing need to present an attractive classification and labelling for the polymer product. 

From the above examples it is apparent that a pertinent classification of thermal stabilities is not obvious. however, some clear structure-stability relationships can be derived from experimental results. The most important characteristic to be found is the mechanism by which thermal aging proceeds a chain process or a step process. The most unstable polymers are those that undergo degradation chain processes, for example ... 

Any polymer degradation during pyrolysis consists of chemical reactions of the types described in Section 2.1 to Section 2.5. However, for a better understanding of the expected pyrolysis products of a polymer, a specific classification can be made allowing the correlation of the nature of the reaction products with the structure of the polymer. It is possible to categorize polymer degradation reactions as follows ... 

Polymer degradation reactions are frequently categorized based on the site in the macromolecule structure where the reaction occurs. This leads to the following classification of scission reactions a) polymeric chain scission, b) side group reactions, c) combined reactions [5, 3]. These reactions follow one of the mechanisms described previously, but this different classification allows a better correlation of the nature of the reaction products with the structure of the polymer and provides more understanding regarding the expected pyrolysis products. 

Despite their absence from the current sequence classification, much is known about the mechanisms of A -glycoside formation, hydrolysis and phos-phorolysis at the anomeric centre of o-ribofuranose. The enzymes physiologically are involved in nucleoside and nucleotide biosynthesis and degradation some work at the nucleic acid polymer level to remove or add modified nucleic acid bases. 

ZAGORSKI, Z.P. Modification, degradation and stabilization of polymers in view of the classification of radiation spurs Radiat.Phys.Chem. 63 (2002) 9-19. 

Polymers are commonly classified according to two main criteria thermal behaviour and polymerization mechanism. As explained further below, these classifications are important from the point of view of polymer recycling, because the most suitable method for the degradation of a given polymer is closely related to both its thermal properties and its polymerization mechanism. 

The overall science around polyanhydrides is summarised in Figure 5.1. The main focus of this chapter is to introduce and provide an extensive review of the various promising aspects of one specific class of synthetic biodegradable medical polymer — poly anhydride. In the first part of the chapter the classification, chemical stmctures, and synthesis methods of various polyanhydrides are discussed. This is followed by a discussion of the in vitro and in vivo behaviour and degradation mechanism of these materials. Also, the various processing techniques that are employed are introduced and explained. Finally, medical applications of polyanhydride systems are presented, highlighting their role and their potential to be used as a family of medical polymers of the future generation . 

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