Hydrolytic Degradation of Comb-Like Graft Poly (Lactide-co-Trimethylene Carbonate): The Role of Comonomer Compositions and Sequences

The effect of sequence on copolymer properties is rarely studied, especially the degradation behavior of the biomaterials. A series of linear-comb block, gradient, random copolymers were successfully achieved using hydroxylated polybutadiene as the macroinitiator by simple ring-opening polymerization of l-lactide (l-LA) and 1,3-trimethylene carbonate (TMC). The hydrolytic degradation behaviors of the copolymers were systemically evaluated by using nuclear magnetic resonance (NMR), gel permeation chromatography (GPC), differential scanning calorimeter (DSC), and scanning electron microscopy (SEM) to illustrate the influences of comonomer compositions and sequence structures. The linear-comb block copolymers (lcP(TMC-b-LLA)) with different compositions had different degradation rates, which increased with l-LA content. Thermal property changes were observed with decreased Tm and increased ΔHm in all block copolymers during the degradation. To combine different sequence structures, unique degradation behaviors were observed for the linear-comb block, gradient and random copolymers even with similar comonomer composition. The degradation rates of linear-comb PLLA-gradient-PTMC (lcP(LLA-grad-TMC)) and linear-comb PLLA-random-PTMC (lcP(LLA-ran-TMC)) were accelerated due to the loss of regularity and crystallinity, resulting in a remarkable decrease on weight retention and molar mass. The hydrolysis degradation rate increased in the order lcP(TMC-b-LLA), lcP(LLA-ran-TMC), lcP(LLA-grad-TMC). Therefore, the hydrolytic degradation behavior of comb-like graft copolymers depends on both the compositions and the sequences dramatically.


Introduction
Synthetic biodegradable polymers, such as polylactide (PLA), polyglycolide (PGA), polycaprolactone (PCL) as well as their copolymers, have been proverbially studied and widely used in biomedical applications [1][2][3]. Among them, PLA is a very promising material because it combines biodegradability, biocompatibility, and excellent processability, while it is derived from natural resources [4]. Hence, PLA has been considered as an ideal biomaterial for biomedical and pharmaceutical applications, especially in tissue engineering and controlled drug delivery.
To be used in the biomedical field, polymers must generally meet strictly property requirements. Consequently, the improvement of properties appears necessary for most application fields. It has been

Synthesis of Linear-Comb Copolymers with Different Sequence Structures
The linear-comb copolymers of TMC and LA were designed with block, gradient, and random side chains. These copolymers were all successfully synthesized as described in our previous study [26], as shown in Scheme 1.

Hydrolytic Degradation Procedures
Sample disks were cut into 10 mm × 10 mm × 0.4 mm size and weighted (w0). Then samples were immersed in vials containing 10 mL of 0.05 M PBS solution (pH = 7.4), which were refreshed every 72 h. The vials were placed under 37 °C. At regular time intervals, the samples were rinsed thoroughly with distilled water and weighed immediately after wiping the surface with a tissue to obtain the wet weight (ww). Next, samples were vacuum-dried at 37 °C to a constant weight (wd) before analysis. Degradation studies were performed in quadruplicate, with given data corresponding to the average values. The weight retention (%WR) and water absorption (%WA) were calculated via the following equations: %WR = × 100 (1) %WA = − × 100 (2)

Measurements
The average molar mass (Mn,GPC) and dispersity (PDI) values of the polymers were determined by gel permeation chromatography (GPC) using a Waters 1515 HPLC pump, a Waters 2414 refractive index detector, and PS columns (one PL gel 5 µm 10E4A and one Shodex KF805, Shodex, Shanghai, China) in THF as eluent at a flow rate of 0.6 mL min −1 at 35 °C. The calibration was based on PS standards (Shodex PS STD SM-105). 1 H NMR and 13 C NMR spectra were recorded on a Bruker Avance 400 MHz spectrometer at 25-30 °C in CDCl3 with a concentration of 4% w/v. Chemical shifts (δ) are reported in ppm and were referenced internally relative to tetramethylsilane (δ 0 ppm) using the residual 1 H (δ 7.26 ppm) and 13 C (δ 77. 16 ppm) solvent resonances.
The thermal behavior of the polymers was measured on differential scanning calorimeter (DSC, TA, Q20). Each sample was heated to 200 °C at a heating rate of 10 °C min −1 in aluminum pans under a nitrogen atmosphere. Thermal history was removed by keeping the samples at 200 °C for 3 min. Then samples were cooled to −40 °C at 10 °C min −1 , followed by heating to 200 °C at 10 °C min −1 .
The morphological observation of the butanediol films before and after degradation was performed using a scanning electron microscope (SEM) under an acceleration of 20 kV. All the specimens were covered with a thin layer of gold before testing.

Hydrolytic Degradation Procedures
Sample disks were cut into 10 mm × 10 mm × 0.4 mm size and weighted (w 0 ). Then samples were immersed in vials containing 10 mL of 0.05 M PBS solution (pH = 7.4), which were refreshed every 72 h. The vials were placed under 37 • C. At regular time intervals, the samples were rinsed thoroughly with distilled water and weighed immediately after wiping the surface with a tissue to obtain the wet weight (w w ). Next, samples were vacuum-dried at 37 • C to a constant weight (w d ) before analysis. Degradation studies were performed in quadruplicate, with given data corresponding to the average values. The weight retention (%WR) and water absorption (%WA) were calculated via the following equations:

Measurements
The average molar mass (M n , GPC ) and dispersity (PDI) values of the polymers were determined by gel permeation chromatography (GPC) using a Waters 1515 HPLC pump, a Waters 2414 refractive index detector, and PS columns (one PL gel 5 µm 10E4A and one Shodex KF805, Shodex, Shanghai, China) in THF as eluent at a flow rate of 0.6 mL min −1 at 35 • C. The calibration was based on PS standards (Shodex PS STD SM-105). 1 H NMR and 13 C NMR spectra were recorded on a Bruker Avance 400 MHz spectrometer at 25-30 • C in CDCl 3 with a concentration of 4% w/v. Chemical shifts (δ) are reported in ppm and were referenced internally relative to tetramethylsilane (δ 0 ppm) using the residual 1 H (δ 7.26 ppm) and 13 C (δ 77.16 ppm) solvent resonances.
The thermal behavior of the polymers was measured on differential scanning calorimeter (DSC, TA, Q20). Each sample was heated to 200 • C at a heating rate of 10 • C min −1 in aluminum pans under a nitrogen atmosphere. Thermal history was removed by keeping the samples at 200 • C for 3 min. Then samples were cooled to −40 • C at 10 • C min −1 , followed by heating to 200 • C at 10 • C min −1 . The morphological observation of the butanediol films before and after degradation was performed using a scanning electron microscope (SEM) under an acceleration of 20 kV. All the specimens were covered with a thin layer of gold before testing.

Weight Retention and Water Absorption
For further biomedical applications, the studies would perform with physiological-like fluids [27]. Here, the hydrolytic degradation of the polymers was performed in pH 7.4 PBS at 37 • C. Figure 1a shows the evolution of the remaining weight of the linear-comb diblock copolymers as well as their homopolymers during 84 day-degradation. Homopolymer lcPTMC degrades extremely slow, as its remaining weight changes little during the 12 weeks. This is in agreement with the linear PTMC almost not degrade by pure hydrolysis in pH 7.4 phosphate-buffered saline (PBS) [28]. In contrast, the remaining weight of the block copolymers show some differences. Sample lcP(TMC-b-LLA)30 appears the most resistant to hydrolysis degradation among the block copolymers with 91.8% weight remaining after 12 weeks due to its low lactide content. Weight loss of the copolymers increases with LA content. Even though longer PLLA chain results in higher crystallinity, which would lead to slower degradation. PLLA block degraded much faster than PTMC block in the graft copolymer, no matter the PLLA parts crystallized or not. Homopolymer lcPLLA shows the fastest degradation rate with about 84.5% remaining weight at last. The higher LA content in the lcP(TMC-b-LLA) copolymer, the faster weight decreases, since ester bonds are more susceptible to hydrolysis than carbonate bonds [29].
Water uptake plays an important role during hydrolytic degradation, which can be caused and reflected by water absorption. Figure 1b presents the water absorption profile of the linear-comb diblock copolymers as well as their homopolymers during the degradation time. Homopolymer lcPTMC appears very hydrophobic with only 2.1% water absorption from the beginning to the end of the process. Higher water uptake is observed for diblock copolymer lcP(TMC-b-LLA)74 about 4.8% after 12 weeks. The other two copolymers with lower LA content show a similar low absorption level as compared to lcPTMC. Sample lcPLLA presents the highest water absorption, which under 6.9% after 12 weeks. These findings indicate that the linear-comb block polyesters are highly hydrophobic and slowly degrade during the hydrolytic degradation. Water uptake plays an important role during hydrolytic degradation, which can be caused and reflected by water absorption. Figure 1b presents the water absorption profile of the linear-comb diblock copolymers as well as their homopolymers during the degradation time. Homopolymer lcPTMC appears very hydrophobic with only 2.1% water absorption from the beginning to the end of the process. Higher water uptake is observed for diblock copolymer lcP(TMC-b-LLA)74 about 4.8% after 12 weeks. The other two copolymers with lower LA content show a similar low absorption level as compared to lcPTMC. Sample lcPLLA presents the highest water absorption, which under 6.9% after 12 weeks. These findings indicate that the linear-comb block polyesters are highly hydrophobic and slowly degrade during the hydrolytic degradation.
The linear-comb copolymers with block, gradient, and random sequences are studied to investigate the effect of side-chain sequence on the hydrolysis degradation behaviors of graft copolymers. As shown in Figure 2a,b the remaining weight and the water absorption of these graft copolymers were quite different. Unlike the linear-comb block copolymer, the random copolymer degrades constantly to reach 85.1% weight left at last. Due to the semi-crystalline morphology of the block copolymer, which will be shown below, lcP(TMC-b-LLA)51 degrade slower than lcP(LLA-ran-TMC)50. It is well known that water molecules can only penetrate amorphous zones of PLLA instead of compact crystalline ones. The highest weight loss was obtained by the gradient copolymer lcP(LLA-grad-TMC)52, which showed the most rapid weight loss and became too fragile to weight after 57 days. The highest weight loss and water absorption can be attributed to the gradient sequence and, more probably, to its highest PDI (2.2 versus 1.3) of the copolymers. The acceleration of water uptake observed at the later stages of degradation, in particular in the case of lcP(LLA-grad-TMC)52, can be described as releasing of soluble degradation byproducts. The soluble species leave pores or cavities in the bulk, both facilitating water absorption. The linear-comb copolymers with block, gradient, and random sequences are studied to investigate the effect of side-chain sequence on the hydrolysis degradation behaviors of graft copolymers. As shown in Figure 2a,b the remaining weight and the water absorption of these graft copolymers were quite different. Unlike the linear-comb block copolymer, the random copolymer degrades constantly to reach 85.1% weight left at last. Due to the semi-crystalline morphology of the block copolymer, which will be shown below, lcP(TMC-b-LLA)51 degrade slower than lcP(LLA-ran-TMC)50. It is well known that water molecules can only penetrate amorphous zones of PLLA instead of compact crystalline ones. The highest weight loss was obtained by the gradient copolymer lcP(LLA-grad-TMC)52, which showed the most rapid weight loss and became too fragile to weight after 57 days. The highest weight loss and water absorption can be attributed to the gradient sequence and, more probably, to its highest PDI (2.2 versus 1.3) of the copolymers. The acceleration of water uptake observed at the later stages of degradation, in particular in the case of lcP(LLA-grad-TMC)52, can be described as releasing of soluble degradation byproducts. The soluble species leave pores or cavities in the bulk, both facilitating water absorption.

Molecular Weight and Composition Evolution
The information of number-average molar mass (Mn) and the polydispersity index (PDI) during degradation were obtained via GPC monitoring. Figure 3 shows the Mn/Mn0 decrease during

Molecular Weight and Composition Evolution
The information of number-average molar mass (M n ) and the polydispersity index (PDI) during degradation were obtained via GPC monitoring. Figure 3 shows the M n /M n0 decrease during hydrolytic degradation. In order to compare the degradation rate of the graft copolymers, the exponential relationship between molecular weight and degradation time for biodegradable polyesters was used [30].  Correspondingly, the PDI of all the copolymers in Table 2 shows similar increase trend during the 84 days degradation except the gradient copolymer. After water permeation of the copolymer, ester bonds and carbonate bones in the polymer chains break into hydroxyl and carboxyl end groups. These internal autocatalytic effects accelerate scission and disentanglement of polymer chains to increase PDI as previous reported [23]. Among them, the PDI of lcP(LLA-ran-TMC)50 exhibits a dramatical growth with the most rapid degradation rate. The PDI values of the sample lcP(TMC-b-LLA)51 and lcP (TMC-b-LLA)30 increases slightly. It is for the low degradation rate of block copolymers with lower LLA content. As the weight retention shows in Figure 1a, the degradation time in this study is not long enough to observe an obvious PDI change of block graft copolymers with lower LLA content. In contrast, the gradient copolymer shows a decreased trend from the beginning. The initial PDI of lcP(LLA-grad-TMC)52 is the largest (PDI = 2.2), and it decreases to 1.9 after 57 days, and to 1.8 after 84 days. The decrease of PDI could be assigned to the release of soluble species and oligomers.
The Detailed analysis of compositional changes in the copolymers during degradation has been monitored by 1 H NMR. The LA contents of linear-comb PLLA/PTMC copolymers with different sequence structures remain constant during the degradation period, as shown in Figure 4. This finding could be assigned to the loss of TMC components together with the degradation of LA moieties, even PTMC itself does not degrade in phosphate-buffered saline without enzyme. Similar  The values of k are calculated from the slope of the fitting curve during the 84 days of study (R 2 > 0.99). The lcPTMC appears non-degradable as barely any molecular weight decrease is observed after 84 days (k = 0.0003 days −1 ). In contrast, the linear-comb copolymers exhibit various degradation rates, gathered in Table 2. Obtained k for lcP(TMC-b-LLA)74, lcP(TMC-b-LLA) 51 and lcP(TMC-b-LLA)30 are 0.0040, 0.0024 and 0.0015 days −1 , respectively. The higher content of TMC in the lcP(TMC-b-LLA) copolymer, the slower the M n decreases, in agreement with the non-degradability of PTMC. The M n of block copolymers decrease less than 32% of the original value after 84 days. The M n of lcP(LLA-grad-TMC)52 and lcP(LLA-ran-TMC)50 decrease 73% and 64% of the initial molar mass, respectively. The gradient copolymer (k = 0.0171 days −1 ) and random copolymers (k = 0.0125 days −1 ) with similar composition exhibit a higher M n decrease rate than all the block copolymers. The rapid degradation can be attributed to the decrease of chain regularity and crystallinity. This finding shows that the sequence structure of the copolymer may have a stronger effect on the degradation behaviors than the composition of the copolymer has.
Correspondingly, the PDI of all the copolymers in Table 2 shows similar increase trend during the 84 days degradation except the gradient copolymer. After water permeation of the copolymer, ester bonds and carbonate bones in the polymer chains break into hydroxyl and carboxyl end groups. These internal autocatalytic effects accelerate scission and disentanglement of polymer chains to increase PDI as previous reported [23]. Among them, the PDI of lcP(LLA-ran-TMC)50 exhibits a dramatical growth with the most rapid degradation rate. The PDI values of the sample lcP(TMC-b-LLA)51 and lcP (TMC-b-LLA)30 increases slightly. It is for the low degradation rate of block copolymers with lower LLA content. As the weight retention shows in Figure 1a, the degradation time in this study is not long enough to observe an obvious PDI change of block graft copolymers with lower LLA content. In contrast, the gradient copolymer shows a decreased trend from the beginning. The initial PDI of lcP(LLA-grad-TMC)52 is the largest (PDI = 2.2), and it decreases to 1.9 after 57 days, and to 1.8 after 84 days. The decrease of PDI could be assigned to the release of soluble species and oligomers. The Detailed analysis of compositional changes in the copolymers during degradation has been monitored by 1 H NMR. The LA contents of linear-comb PLLA/PTMC copolymers with different sequence structures remain constant during the degradation period, as shown in Figure 4. This finding could be assigned to the loss of TMC components together with the degradation of LA moieties, even PTMC itself does not degrade in phosphate-buffered saline without enzyme. Similar findings have also been reported in the case of other polylactide copolymers [22,31] Polymers 2019, 11, x FOR PEER REVIEW 8 of 14   Table 3.    Table 3.

Thermal Analysis and Visual Examination
During the hydrolytic degradation process, the corresponding melting temperature (T m ) of lcP(TMC-b-LLA)74 moves obviously toward lower temperatures 154.2 • C at day 0 to 150.8 • C at day 84. The melting enthalpy (∆H m ) increases from the initial value of 29.2 to 44.8 J/g at the end of the degradation. As the degradation occurs, the polymer chains become shorter, and the chain mobility is favored to rise crystallinity. Although the ∆H m increases, the created crystallites may become more disordered and imperfect, which leading to a lower T m . Regarding glass transition temperature (T g ) behavior, lcP(TMC-b-LLA)74 exhibits a single T g which decreases as the degradation time increases. The disassemble oligomers act as internal plasticizers, resulting in a decline of T g in agreement with   Figure 5 shows the DSC curves of the first and the second scans of lcP(TMC-b-LLA)74 at different degradation times. The thermal property changes of linear-comb PLLA/PTMC copolymers are summarized in Table 3.   The thermal properties during degradation are strongly affected by LA content, and also the sequence structures. The random copolymer lcP(LLA-ran-TMC)50 doesn t show any melting peak during the degradation. Its T g decreases from 14.3 • C to 6.1 • C as the degradation time increases from the beginning to day 84. The disassemble oligomers act plasticizing effect, resulting in a decline of T g in agreement with the PDI increase.

Thermal Analysis and Visual Examination
Interestingly, the gradient copolymer lcP(LLA-grad-TMC)52 doesn't have T m before degradation procedure. During the degradation, DSC curves show T m of 152.3 • C on day 27 and decrease to 149.1 • C at day 84. A peak of ∆H m also shows up on day 27 and then decreases from the initial value of 10.2 J/g to 3.4 J/g. It indicates the increased regularity of PLLA segments happened in the gradient copolymer along with degradation. This could be attributed to the rapid rupture of PTMC and amorphous region of PLLA segments, which promote additional crystalline regions formation. During this period, the decrease of PDI (Table 2) means low molecular weight oligomers are released outside. As a result, T g increases a little bit from the initial 17.0 • C to 17.5 • C on day 57 as the plasticizing effect becomes diminished.
SEM is used to monitor the film surface morphology changes during hydrolytic degradation. Figure 6 shows the SEM photographs of lcP(TMC-b-LLA)30 specimens after 0, 27, 47 and 84 days degradation. The copolymer presents a perfectly smooth surface before degradation. After 27 days of degradation, the surface appears slightly eroded to leave some cracks. At day 47, surface erosion continues, and the surface becomes more rugged. After 84 days, cracks grow deeper and larger, porous structures are also observed around cracks on the surface. the gradient copolymer along with degradation. This could be attributed to the rapid rupture of PTMC and amorphous region of PLLA segments, which promote additional crystalline regions formation. During this period, the decrease of PDI (Table 2) means low molecular weight oligomers are released outside. As a result, Tg increases a little bit from the initial 17.0 °C to 17.5 °C on day 57 as the plasticizing effect becomes diminished. SEM is used to monitor the film surface morphology changes during hydrolytic degradation. Figure 6 shows the SEM photographs of lcP(TMC-b-LLA)30 specimens after 0, 27, 47 and 84 days degradation. The copolymer presents a perfectly smooth surface before degradation. After 27 days of degradation, the surface appears slightly eroded to leave some cracks. At day 47, surface erosion continues, and the surface becomes more rugged. After 84 days, cracks grow deeper and larger, porous structures are also observed around cracks on the surface.   (Figure 1). Therefore, linear-comb PLLA/PTMC block copolymers are degradable in phosphate-buffered saline without enzyme, especially for those with high LA contents.   Figure 8 shows the SEM photographs of linear-comb copolymers with block, gradient and random sequences after 84 days degradation. Unlike the linear-comb block copolymers, the linearcomb gradient copolymer is greatly degraded with large holes observed after 84 days of degradation. The surface of lcP(LLA-grad-TMC)52 appears highly porous with numerous tiny spherulites of ca. 2 µm (Figure 8b). The spherulites are PLLA crystals, which are formed during degradation in agreement with the thermal properties of lcP(LLA-grad-TMC)52 (Table 3). It was reported that initially amorphous copolymers containing larger amounts of LA units were able to crystallize during degradation because of the presence of relatively long LLA blocks [25]. The boundaries between spherulites become clearly distinguishable. Since boundaries are mainly composed of amorphous material or crystallite defects, which are already degraded and removed after 84 days. The linearcomb random copolymer lcP(LLA-ran-TMC)50 was strongly eroded to leave some pores and spongelike structures (Figure 8c). No apparent spherulites were observed due to its random sequence structure. These findings well corroborate the DSC results of lcP(LLA-ran-TMC)50. After 84 days of degradation, the surface appeared largely eroded with some microdomains. Comparison on the superficial morphology of these three copolymers with different sequence structures shows that the surface characteristics of lcP(LLA-grad-TMC)52 changes most obviously, with the largest holes and cracks. The film surface corrodes variously with similar comonomer composition, indicating the deformation correlates with the sequence structures.  Figure 8 shows the SEM photographs of linear-comb copolymers with block, gradient and random sequences after 84 days degradation. Unlike the linear-comb block copolymers, the linear-comb gradient copolymer is greatly degraded with large holes observed after 84 days of degradation. The surface of lcP(LLA-grad-TMC)52 appears highly porous with numerous tiny spherulites of ca. 2 µm (Figure 8b). The spherulites are PLLA crystals, which are formed during degradation in agreement with the thermal properties of lcP(LLA-grad-TMC)52 (Table 3). It was reported that initially amorphous copolymers containing larger amounts of LA units were able to crystallize during degradation because of the presence of relatively long LLA blocks [25]. The boundaries between spherulites become clearly distinguishable. Since boundaries are mainly composed of amorphous material or crystallite defects, which are already degraded and removed after 84 days. The linear-comb random copolymer lcP(LLA-ran-TMC)50 was strongly eroded to leave some pores and sponge-like structures (Figure 8c). No apparent spherulites were observed due to its random sequence structure. These findings well corroborate the DSC results of lcP(LLA-ran-TMC)50. After 84 days of degradation, the surface appeared largely eroded with some microdomains.  Figure 8 shows the SEM photographs of linear-comb copolymers with block, gradient and random sequences after 84 days degradation. Unlike the linear-comb block copolymers, the linearcomb gradient copolymer is greatly degraded with large holes observed after 84 days of degradation. The surface of lcP(LLA-grad-TMC)52 appears highly porous with numerous tiny spherulites of ca. 2 µm (Figure 8b). The spherulites are PLLA crystals, which are formed during degradation in agreement with the thermal properties of lcP(LLA-grad-TMC)52 (Table 3). It was reported that initially amorphous copolymers containing larger amounts of LA units were able to crystallize during degradation because of the presence of relatively long LLA blocks [25]. The boundaries between spherulites become clearly distinguishable. Since boundaries are mainly composed of amorphous material or crystallite defects, which are already degraded and removed after 84 days. The linearcomb random copolymer lcP(LLA-ran-TMC)50 was strongly eroded to leave some pores and spongelike structures (Figure 8c). No apparent spherulites were observed due to its random sequence structure. These findings well corroborate the DSC results of lcP(LLA-ran-TMC)50. After 84 days of degradation, the surface appeared largely eroded with some microdomains. Comparison on the superficial morphology of these three copolymers with different sequence structures shows that the surface characteristics of lcP(LLA-grad-TMC)52 changes most obviously, with the largest holes and cracks. The film surface corrodes variously with similar comonomer composition, indicating the deformation correlates with the sequence structures. Comparison on the superficial morphology of these three copolymers with different sequence structures shows that the surface characteristics of lcP(LLA-grad-TMC)52 changes most obviously, with the largest holes and cracks. The film surface corrodes variously with similar comonomer composition, indicating the deformation correlates with the sequence structures.
In the case of linear-comb block copolymers, the degradation rate increases with the PLLA content. Since LA units are preferentially degraded during hydrolytic degradation. The weight retention and molar mass decrease due to the hydrolytic chain cleavage in the bulk. Little compositional changes are obtained during degradation, which could be assigned to the loss of TMC components together with the degradation of LA moieties. Thermal property changes are observed with decreased T m and increased ∆H m in all cases, which strongly supports a bulk erosion mechanism, in agreement with SEM observations. Different degradation behaviors are observed for the linear-comb block, gradient and random copolymers with similar LA content. The degradation rates of lcP(LLA-grad-TMC)52 and lcP(LLA-ran-TMC)50 are accelerated due to the loss of regularity and crystallinity, resulting in a remarkable decrease on weight retention and molar mass. The gradient copolymer lcP(LLA-grad-TMC)52 exhibits the most rapid degradation rate, which is attributable to the highest PDI. SEM observation shows the film surface corroded variously for the three kinds of linear-comb copolymers, indicating the deformation correlated with the sequence structures.
It is concluded that the degradation rate can be justified through the variation of comonomers compositions and sequence structures. Understanding the degradation mechanisms of graft copolyesters will permit the prediction and the adjustment of their degradation rate, enabling the adaptation of the polymers to the requirements of a specific biomedical application.