Hydrolytic degradation mechanisms

Figure 8.17 Hydrolytic degradation mechanisms of bulky PLA materials (a) surface erosion, (b) bulk erosion, (c) core-accelerated bulk erosion [8, 199], (Reproduced with permission from ref. [8] 2002, Wiley-VCH Veriag GmbH Co. KCaA and from ref. 1199], with permission from Corona Publishing Co., Ltd., Tokyo.)...

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.)...

In parallel, the physical structure of PLA has been reported to affect its hydrolytic degradation mechanism. For example, it has been found that the hydrolytic chain cleavage proceeds preferentially in the amorphous regions, leading to the increase in polymer crystallinity. The rate of the degradation reaction seems also to be affected by the shape of the specimen and by the chemical structure of the polymer, as well as by the conditions under which the hydrolysis is performed, including pH and temperature. ... 

The hydrolytic degradation mechanism of these copolymers was qualitatively similar and can be described by the preferential hydrolysis of ester bonds fliroughout... 

The hydrolytic degradation mechanism of P(TMC-co-GA) copolymers can be described by the preferential hydrolysis of GA component in the case of random or segmented copolymers. Morphological and compositional changes are observed. The copolymers can also be degraded by certain enzymes, in particular, lipases which are able to degrade PTMC. 

TABLE 21.4 Critical Thickness (Lcriticai) of Biodegradable Polyesters, Above which Hydrolytic Degradation Mechanism Changes from Bulk Erosion to Surface Erosion [150]... 

FIGURE 21.4 Hydrolytic degradation mechanisms of bulky PLA materials [1, 268]. 

The hydrolytic degradation mechanism, rate, and behavior are affected by the parameters of materials and media, as summarized in Table 21.2. The material parameters include molecular and highly ordered structures, additives including other polymers and fibers, and the material morphology. The medium parameters include the type and concentrations of catalytic species and other solutes, pH, temperature, and the type and density of microbes. 

By varying the material parameters, the hydrolytic degradation mechanism, behavior, and rate can be controlled. In other words, these parameters are carefully manipulated when PLA-based materials are biomedically and environmentally applied by using the hydrolyzability function. In the following section, the hydrolytic degradation conditions are temperature of 37°C and pH of about 7, unless otherwise specified. 

Photochemical degradation papers have been limited to studies on photo oxidation and photostabilization of nylon polymers. - Hydrolytic degradation mechanisms have been reviewed and utilized for nylon 6 and for selective degradation of commercially drawn nylon 6,6 fibres. ... 

Table 10.1 gives an overview of different aliphatic-aromatic copolyesters synthesised as degradable materials during the last few years. Part of the work reported in the literature dealt with hydrolytic degradation mechanisms which do not involve enzymic catalysis (chemical hydrolysis). This kind of degradation is often present in medical applications of polyesters, e.g., as implants in living tissues. Enzymic catalysed hydrolysis, in contrast, is usually connected to microbial degradation in the environment. 

The observed in vitro release pattern of TAM nitroxyl radicals from the TAM-PGA biomaterial and the close similarity between the TAM release pattern and the media pH reduction profiles suggested that pH must play a major role. Based on the well-known hydrolytic degradation mechanism of PGA biomaterial, we postulated the hydrolytic degradation mechanism of TAM-PGA biomaterial as illustrated in Figure 7. TAM-PGA... 

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