# A Systematic Experimental and Computational Analysis of Commercially Available Aliphatic Polyesters
## 3 Results
However, since experimental data at weeks 5 and 8 are consistent with the expected trend, such a point can be considered an outlier. PDLG 7507 takes about 12 weeks to experience full degradation. This time span is significantly longer than the one needed by its closest 50/50 counterparts here analyzed (PDLG 5004 and RG505) and comparable with the data obtained for PDLG 5010, whose initial molecular weight is about 30% higher. The experimental point at 8 weeks, which shows an increase in molecular weight, can be considered an outlier since the following point at 12 weeks is consistent with the expected experimental trend and thus does not affect the conclusions. Focusing on the effect of hydroxyapatite, the degradation rate is slower for PDLG 5004 with HA, which is fully degraded after about 5 weeks, whereas the pure polymer sample took 3 weeks, and the degradation of the compound RG505 did not slow down significantly. Polydispersity (Figure 2c) shows the same trend for all investigated materials. It remains approximatively constant during the early degradation stage; it subsequently increases and eventually decreases when degradation becomes significant. The observed trend can be supported taking into account the fundamental phenomena behind the degradation of aliphatic polyesters (Discussion section, _vide infra_). The glass transition temperature (\(T_{g}\)) as a function of degradation time is shown in Figure 3a and exhibits the same trend for each polymer formulation. The \(T_{g}\) value decreases as degradation proceeds and suddenly increases during the late degradation stage. Such a trend has been presented in the literature by other authors [23; 24] and is consistent with the degradation mechanism (vide infra). In addition, the obtained values for undegraded samples are comparable with the literature findings [25]. The dependence of the glass transition temperature on molecular weight is usually accounted for by means of the Flory-Fox equation:  
\[T_{g}=T_{g}^{\infty}-\frac{K}{MW_{n}} \tag{14}\]  
where \(T_{g}^{\infty}\) is a glass transition temperature at in infinite molecular weight, and \(K\) is a constant related to the excess free volume of the end groups of polymer chains. A linear fit of \(T_{g}\) vs. \(MW_{n}{}^{-1}\) of the undegraded PLGA 50/50 polymers, shown in Figure 3b, allows one to obtain \(T_{g}^{\infty}\) and \(K\) values equal to 50.3 \({}^{\circ}\)C and \(1.26\times 10^{5}\) g \({}^{\circ}\)C mol\({}^{-1}\), respectively (\(R^{2}\) = 0.95). In particular, the \(T_{g}^{\infty}\) value agrees well with the upper bound \(T_{g}\) reported in the literature for PLGA 50/50 materials, equal to 50 \({}^{\circ}\)C [25]. The Flory-Fox equation could not be employed for PDLG 7507, since only one PLGA 75/25 formulation was investigated here. Lactide content in the copolymer as a function of degradation time is depicted in Figure 4.  
Figure 3: Glass transition temperature as a function of degradation time; values are expressed as the arithmetic average of three independent measurements \(\pm\) standard deviation (**a**). Glass transition temperature as a function of \(MW_{n}{}^{-1}\) before degradation onset for PLGA 50/50 polymers (**b**). Dashed line represents the linear fit according to Flory–Fox equation. Copolymer composition exhibits an analogous trend for all investigated polymers. Lactic content remains approximatively constant until the late degradation stage, where the amount of lactic units starts increasing. This is consistent with previous literature findings [24; 26]. The trend exhibited by mass loss (Figure 5a) is consistent with that expected from bulk eroding aliphatic polyesters. The erosion is negligible during the early degradation stage and sharply increases when the molecular weight decay becomes significant. As expected, the attainment of a relevant mass loss is coupled with a significant water uptake (Figure 5b), since water can fill the pores resulting from matrix erosion, and with a pH decrease in the surrounding medium (Figure 5c).