Influence of Hydrolysis on Non-Isothermal Crystallization of Poly(Butylene Succinate-Co-Adipate) (PBSA)

Abstract

This study investigates the impact of hydrolysis on the crystallization behavior of poly(butylene succinate-co-adipate) (PBSA), a biodegradable polyester. Hydrolysis was conducted in a controlled environment using phosphate-buffered saline at 70 °C to isolate the impact of hydrolytic degradation on the polymer's properties. The consequent changes in molecular weight characteristics were tracked using gel permeation chromatography (GPC), revealing a decrease in both weight average molecular weight (M_w) and an increase in polydispersity index (PDI) as hydrolysis progressed. The thermal behavior of PBSA during hydrolysis was thoroughly investigated using differential scanning calorimetry (DSC), which demonstrated significant changes in melting temperature (T_m), glass transition temperature (T_g), and crystallinity (X). These changes in T_m and T_g suggest a change in copolymer composition, likely due to the greater susceptibility of the adipic acid unit to hydrolysis compared to the succinic acid unit. Furthermore, polarized optical microscopy (POM) was employed to observe the morphological evolution of PBSA, showing a transition from spherulitic structures in the early stages of hydrolysis to dendritic structures with prolonged hydrolysis time. The decrease in nucleation activity led to a reduction in the number of spherulites, which in turn allowed the remaining spherulites to grow larger.

Keywords: biodegradable polyesters; crystallization kinetics; crystallization morphology; degradation; hydrolysis; molecular weight distribution; non-isothermal crystallization; poly(butylene succinate-co-adipate) (PBSA); spherulites.

Figure 1

Degree of hydrolysis of PBSA during hydrolysis at various temperatures.

Figure 2

(a) Molecular weight distribution of PBSA after hydrolysis at 70 °C, (b) decrease in molecular weight during hydrolysis at 70 °C.

Figure 3

Normalized heat flow curves obtained by differential scanning calorimetry (DSC) during a 20 °C/min cooling rate for various hydrolysis times. (a) PBSA at 37 °C, (b) PBSA at 58 °C.

Figure 4

Normalized heat flow curves obtained by differential scanning calorimetry (DSC) during a 5 °C/min cooling ramp following hydrolysis at 70 °C.

Figure 5

Crystallization peak temperature T_c as a function of (a) hydrolysis time at 70 °C and (b) cooling rate, as determined by differential scanning calorimetry (DSC).

Figure 6

Crystallinity X from DSC cooling scan as a function of (a) hydrolysis time, (b) cooling rate.

Figure 7

Crystallization kinetics from DSC. (a) relative crystallinity vs. time, (b) slope at the inflection point vs. time of hydrolysis.

Figure 8

Avrami plot from DSC. (a) for various hydrolysis times at cooling rate 15 °C/min, (b) PBSA after 32 days of hydrolysis at 70 °C for various cooling rates (5–20 °C/min).

Figure 9

Plot of Jeziorny’s parameter $Z_t$ from Avrami plot. (a) as a function of hydrolysis times at cooling rate 15 °C/min, (b) PBSA after 32 days of hydrolysis at 70 °C as a function of cooling rate.

Figure 10

DSC results: (a) Relative crystallinity as a function of temperature at three cooling rates for PBSA-32 (32 days of hydrolysis at 70 °C), (b) Ozawa plot.

Figure 11

Detailed analysis of Ozawa parameters obtained from Figure 10b: (a) Ozawa exponent $m$ for PBSA, (b) Ozawa cooling function.

Figure 12

Non-isothermal crystallization by polarized optical microscopy at 2 °C/min cooling rate: (a) PBSA-00 at 59.7 °C, (b) PBSA-04 at 60.6 °C, (c) PBSA-08 at 59.4 °C, (d) PBSA-16 at 63.2 °C, (e) PBSA-32 at 63.8 °C, (f) PBSA-64 at 68.7 °C. The pink color represents amorphous phase, the spherulites are the blue and yellow circles and the black line is a boarder of cavity (vacuum) created during the cooling.

Figure 13

Number of spherulites from polarized optical microscopy: (a) comparison of the number of spherulites for PBSA-04 and PBSA-32 during non-isothermal crystallization at 2 °C cooling rate, (b) final number of spherulites vs. hydrolysis time.

Figure 14

Spherulite growth rate as a function of temperature during non-isothermal crystallization at a cooling rate of 2 °C/min.

Figure 15

Change in two melting points T_m and cold crystallization temperatures by DSC at heating rate 20 °C/min.

Figure 16

Change in glass transition temperature T_g by DSC at heating rate 10 °C/min: (a) original heat flow curves, (b) first derivation of heat flow curves.

Scheme 1

Chemical structure of succinic acid, adipic acid, 1,4-butanediol and poly(butylene succinate-co-butylene adipate) copolymer (PBSA).