## 4 Properties
### Biodegradation Properties
#### 4.2.3 Factors Influencing Hydrolysis  
There are many factors that can influence AP hydrolysis, which depend on material properties, like chemical composition and molecular architecture, molar mass, polydispersity index, stereochemical structure, chain orientation, polymer morphology, size and shape of specimen, presence of residual monomers, oligomers or other low molecular weight compounds, and on the environment, e.g., temperature, pH, moisture and oxygen level, presence of microorganisms or enzymes, and so on. Among others, crystallinity is undoubtedly the most important factor to be taken into consideration when evaluating polymer degradation rate. Both hydrolysis and enzymatic degradation preferably proceeds in the less packed amorphous regions, which allow for water and enzymes to more easily penetrate in the substrate. Due to the faster degradation of amorphous with respect to crystalline domains, the overall degree of crystallinity increases [88]. Crystallites are also eventually eroded from the edges inward [90]. Recent studies on PBS-based and PBCE-based copolymeric systems containing ether and thioether linkages have clearly demonstrated this phenomenon both for enzymatic [93; 94; 95; 96; 97; 98] as well as for hydrolytic degradation [99; 39] by DSC measurements. The presence of defects and imperfect crystalline domains plays a role in the degradation rate, as they are usually preferentially degraded together with the amorphous regions [93]. Chain orientation seems to have an effect as well. It has been reported that a higher orientation degree along the fiber axis of PGA sutures slows down the hydrolysis rate [100]. Indeed, when PGA fibers were annealed under clamped conditions, the lateral surface of crystalline regions on the fiber external section, which are very resistant to hydrolysis, remained parallel to the fiber axis thus forming a protective layer preventing water penetration [100]. The porosity of polymer matrix is also a factor. A higher degradation rate has been detected in nonporous films as compared with porous constructs. This effect can be explained taking into account that for porous structures there is no autocatalysis because the ionic exchange is facilitated by the porous structure [101; 102]. On the other hand, the enzymatic degradation rate of porous surfaces is faster because of the higher surface/volume ratio. As regards the molar mass is concerned, the lower the polymer molecular weight, the higher the degradation rate, as due to a higher content of carboxylic acid catalyzing groups [89]. Also the influence of pH on degradation rate has been carefully evaluated. Indeed, ester hydrolysis is highly influenced by pH variations [103]. In enzymatic degradation, pH plays an even more important role, due to the close dependence of enzymatic activity on pH [93]. In this respect, APs biodegradation rate can be tuned, i.e., quickened or reduced, by adding different nanoparticlescapable of influencing the pH of the medium, as it has been observed for PCL/layered silicate nanocomposites [103]. Usually, the higher the polymer surface hydrophilicity, the higher the hydrolysis rate due to easier water access. However, in the case of enzymatic degradation, it has been reported that the degradation rate increases when a balanced hydrophobic/hydrophilic ratio is present, due to the enzyme interactions with the polymer surface [94]. In copolymeric systems, there might be a preferential degradation of one copolymeric sequence over the other(s) because of its easier solubilization in water due to a more hydrophilic nature [104]. In this case, changes in molar composition over time, can be easily monitored by \(1\)H-NMR. The addition of nanofillers to APs can significantly alter their degradation profile, as due to the modification of the physical and morphological characteristics of the pristine polymer matrix. Composite materials based on inorganic nanoparticles have demonstrated a significant degradation rate increase with respect to the neat polyester [11].