# A Systematic Experimental and Computational Analysis of Commercially Available Aliphatic Polyesters
## 4 Discussion  
The observed trends are consistent with experimental data presented in the scientific literature and can be explained and supported by taking into account the synergistic effects of the phenomena behind degradation and erosion. Degradation kinetics are slower for PDLG 7507 than for PLGA 50/50 materials, especially considering RG505, whose molecular weight is close to that of PDLG 7507. This is due to the higher content in lactic units, which slows down the degradation rate because of the reduced hydrophilicity of the material. In this regard, the model results can also give some quantitative insights by comparing values of the kinetic constants. The degradation constant of PDLG 7507 is about 79% of the value related to RG505, confirming the impact of the enhanced hydrophobicity on hydrolysis kinetics. On the other hand, the addition of hydroxyapatite has a relevant impact on the hydrolysis rate. Indeed, HA is basic and dampens the impact of autocatalysis, locally increasing upH. This is confirmed by experimental data, since the degradation of polymer/HA compound is slower if compared to a pure polymer sample. Again, degradation kinetics can be quantitatively compared by means of the model results. As reported in the previous section, the hydrolysis of PDLG 5004/HA is about 50% slower than that of the pure polymer sample. RG505/HA is only 20% slower if compared to the pure polymer sample but still confirms the expected impact of the HA addition. The glass transition temperature has an analogous trend for all investigated materials. It decreases and then suddenly increases during the late degradation stage. The initial decrease is strictly due to hydrolysis, since degradation enhances chain mobility. This subsequent increase can be related to a change in the composition of the polymer, which becomes richer in lactic units towards the end of hydrolysis process and the non-uniform degradation of the sample. Indeed, during the late degradation stage, many water-soluble oligomers accumulated in the bulk of the sample, which is hydrolyzed faster than the surface because of autocatalysis. The resulting low molecular weight fragments that constitute the bulk are removed when the disk is rinsed and dried. Because of this, the measured \(T_{g}\) is mainly influenced by the less degraded polymer on the surface. Figure 6: Number (**a**) and weight (**b**) average molecular weights as a function of degradation time: comparison between the experimental data and model results (filled symbols: experimental data; dashed lines: model results). The copolymer composition exhibits the same trend for all considered formulations as well. The lactic content remains constant in time and increases during the late degradation stage. After the hydrolysis onset, the chains of the random copolymers are long enough that a random chain scission statistically creates two shorter chains with the same composition. Since GA units degrade faster than LA ones because of their higher hydrophilicity, this could also indicate the absence of large blocks composed by glycolide units in the macromolecular chains, which should be expected from a random copolymer. In addition, at the beginning, water is the limiting reactant that is slowly added to the system and, although the time scale of water diffusion is faster than that of consumption, it needs a given amount of time to be fully dispersed and accumulate into the matrix, as also shown by the water uptake data. In this situation, selectivity can be also be determined by the probability to find a GA or a LA unit, rather than by intrinsic kinetics [27]. Erosion is negligible during the initial stage of degradation, because the resulting fragments are still not mobile enough to appreciably diffuse out of the matrix. This sharply increases at the end of hydrolysis process because, at that stage, the chain scission results in high amounts of short water soluble oligomers that can easily diffuse in the degraded matrix towards the surrounding medium.