Polyesters degradation mechanisms

In order to control polyester degradation, we must understand the processes involved in how polyesters degrade. The following section presents a brief summary of degradation mechanisms that are involved. 

More recently, polyesters with beneficial degradation products (salicylic acid) have been produced to promote healing through enhanced regeneration of tissue [10]. Degradation mechanisms relevant to medical applications include... 

Polyesters offer multiple options to meet the complex world of degradable polymers. All polyesters degrade eventually, with hydrolysis being the dominant mechanism. Degradation rates range from weeks for aliphatic polyesters (e.g. polyhydroxyalkanoates) to decades for aromatic polyesters (e.g. PET). Specific local environmental factors such as humidity, pH and temperature significantly influence the rate of degradation. 

Aliphatic polyesters degrade chemically by hydrolytic cleavage of the backbone ester bonds [38,92,93,143-145] and by enzymatic promotion [35,146]. Hydrolysis can be catalyzed by either acids or bases [38]. Polyester hydrolysis is schematically illustrated and exemplified for PLA in Fig. 5. Carboxylic end groups are formed during chain scission, and this may enhance the rate of further hydrolysis. This mechanism is denoted autocatalysis [147] and makes polyester matrices truly bulk eroding [38,43]. Degradation products are resorbed by the body with a minimal reaction of the tissues [8,15,95,148]. 

Based on the above reasons, polymers possessing a variety of degradation rates and mechanisms have been developed however, hydrolysis still remains the predominant degradation mechanism for polymers that are most commonly used in drug delivery applications. Many polymers that are susceptible to hydrolysis, for example, the polyesters PLA and PLG, degrade by random hydrolysis that takes place homogeneously throughout the bulk of the polymer device. In contrast, other classes of polymers, such as the polyanhydrides and polyorthoesters, have been developed in an attempt to yield hydrolysis only at the outer surface of the device that is exposed directly... 

Studies have been made of PEST/PO/styrene copolymer blend compatibilization in which a copolymer may be formed between polyester alcohol end-groups and pendent anhydride functionality on a styrene copolymer (Table 5.27). Because the alcohol-anhydride reaction is reversible (with the equilibrium lying on the side of unreacted anhydride), only a relatively small amount of copolymer may be formed. In consequence, the dispersed phase polymer may not be well stabilized against coalescence upon further thermal treatment [Sun et al., 1996]. Alternatively, at least some copolymer may be formed by a degradative mechanism through transesterification between PEST main-chain linkages and a low concentration of pendent acid groups in anhydride-functionalized styrene copolymer (see Section 5.7.12.5). 

Thermal Degradation Mechanisms of Flame-Retarded Polyesters... 

In a similar maimer, thermal degradation mechanisms of fully aromatic polyesters were investigated in detail by Py-GC/MS using a partially deuterated polymer sample. Moreover, the mechanisms of reactive pyrolysis of the same aromatic polyester in the presence of tetramethylammonium hydroxide (TMAH) was also studied by Py-GC/MS. In this case, the contribution of solvent (methanol) to the reaction was confirmed by use of a deuterated methanol solution of TMAH. 

Sueoka, K., Nagata, M., Ohtani, H., Nagai, N., and Tsuge, S., Thermal Degradation Mechanisms of Liquid Crystalline Aromatic Polyester Studied by Pyrolysis-Gas Chromatography/Mass Spectrometry, /. Polym. Sci Part A, 29,1903,1991. 

C Degradation Mechanisms of Condensation Polymers Polyesters and Polyamides... 

FIGURE 5.7 Thermal degradation mechanisms of an aromatic polyester [47], (K. Sueoka et... 

Table 8.5 Critical thickness (Lcritiail) of biodegradable polyesters where hydrolytic degradation mechanism changes from bulk erosion to surface erosion [200], (Reprinted from Biomaterials, vol. 23, von Burkersroda et al., Why degradation polymers undergo surface erosion or bulk erosion, pp. 4221-4231. Copyright Elsevier, 2002.)...

---