Isothermal Crystallization Kinetics of Poly(4-hydroxybutyrate) Biopolymer

Abstract

Thermal properties and crystallization kinetics of poly(4-hydroxybutyrate) (P4HB) have been studied. The polymer shows the typical complex melting behavior associated to different lamellar populations. Annealing processes had great repercussions on properties and the morphology of constitutive lamellae as verified by X-ray scattering data. Kinetics of isothermal crystallization was evaluated by both polarizing optical microscopy (POM) and calorimetric (DSC) measurements, which indicated a single crystallization regime. P4HB rendered banded spherulites with a negative birefringence when crystallized from the melt. Infrared microspectroscopy was applied to determine differences on the molecular orientation inside a specific ring according to the spherulite sectorization or between different rings along a determined spherulitic radius. Primary nucleation was increased during crystallization and when temperature decreased. Similar crystallization parameters were deduced from DSC and POM analyses (e.g., secondary nucleation parameters of 1.69 × 10⁵ K² and 1.58 × 10⁵ K², respectively). The effect of a sporadic nucleation was therefore minimized in the experimental crystallization temperature range and a good proportionality between overall crystallization rate (k) and crystal growth rate (G) was inferred. Similar bell-shaped curves were postulated to express the temperature dependence of both k and G rates, corresponding to the maximum of these curves close to a crystallization temperature of 14-15 °C.

Keywords: Poly(4-hydroxybutyrate); biodegradable polyesters; infrared microspectroscopy; isothermal crystallization kinetics; secondary nucleation; spherulitic morphology; synchrotron radiation.

Figure 1

Sequence of calorimetric and heating runs performed with the initial commercial suture of poly(4-hydroxybutyrate (P4HB): (a) First heating run performed at 10 °C/min; (b) Cooling run at 10 °C/min after keeping the sample in the melt state for one min; (c) Heating run at 10 °C/min of the above melt crystallized samples and (d) Heating run at 10 °C/min of a sample quenched from the melt at the maximum rate allowed by the equipment.

Figure 2

Calorimetric (DSC) heating runs performed at the indicated rates with the commercial P4HB (Poly(4-hydroxybutyrate)) suture.

Figure 3

Wide angle X-ray diffraction (WAXD) profiles (a); Small angle X-ray scattering (SAXS) profiles (b) and correlation functions (c) of the initial commercial P4HB suture (red line) and a P4HB melt crystallized sample (blue line). Deconvoluted WAXD peaks are only indicated for the melt crystallized sample.

Figure 4

DSC heating runs of P4HB previously crystallized at the indicated temperatures.

Figure 5

Hoffman-Weeks plot for P4HB considering the temperatures of its first melting peak.

Figure 6

Exothermic DSC peaks corresponding to the isothermal crystallization from the melt state performed between 24 °C and 38 °C (a) and the cold crystallization performed between −26 °C and −20 °C (b).

Figure 7

Evolution of the relative crystallinity over time for isothermal crystallizations of P4HB at the indicated temperatures. (a) Samples coming from the melt state; (b) Glassy sample obtained from a fast cooling from the melt.

Figure 8

Avrami plots obtained from isothermal melt (a) and cold (b) crystallizations.

Figure 9

(a) Experimental (×) and simulated (●) temperature dependence of the overall crystallization rate of P4HB for isothermal melt crystallization. For the sake of completeness experimental cold crystallization data are also plotted (×); (b) Temperature dependence of the overall crystallization rates (◊, ◊) and the reciprocal crystallization half-times (о, о) of P4HB for melt and cold crystallizations.

Figure 10

Optical micrographs showing P4HB spherulites crystallized at 36 °C (a) and 47 °C (b). The inset of (b) corresponds to a micrograph taken with a red tint plate.to determine the spherulite sign.

Figure 11

Temperature dependence of the primary nucleation density for crystallization performed from the melt state.

Figure 12

Optical micrographs showing P4HB spherulites crystallized at 47 °C for (a) 100 min, (b) 150 min, (c) 180 min, (d) 250 min, (e) 350 min and (f) 350 min. In the case of (f), the micrograph has been taken with a red tint plate. Comparison of micrographs taken after 47 min (g) and 80 min (h) for the crystallization performed at 46 °C. The dashed red circle points out the region where the apparition of a new spherulite is clear.

Figure 13

Variation of the spherulitic radius with crystallization time for temperatures between 37 °C and 42 °C (a) and 43 °C and 49 °C (b).

Figure 14

(a) FTIR spectra of THE P4HB suture with labelling of main bands; (b) Optical polarized micrographs showing spherulites developed in a solvent casting film. Insets show the chemical image obtained from C=O and CH₂ (shoulder) bands; (c) Micrographs showing a representative banded spherulite and the specific microdomains were FTIR spectra were recorded; (d) Microinfrared spectra taken from the indicated microdomains (keeping the colour code). Different magnifications and wavenumber regions are showed.

Figure 15

Lauritzen-Hoffman plots for the crystal growth rate (a) and the overall crystallization rate (b) for the isothermal crystallization of P4HB.

Figure 16

Temperature dependence of the crystal growth (●, ◊) and overall crystallization (●, ◊) rate. Simulated data is represented by circles and experimental data by rhombus.