Formation and size distribution of pores in poly(ɛ-caprolactone) foams prepared by pressure quenching using supercritical CO2

Many biomaterial-based regenerative therapies require foam structures, which temporarily mimic the extracellular matrix. The pore structure of these scaffolds needs to be tailored to the specific requirements of the clinical application. Poly(ɛ-caprolactone) (PCL) is a semi-crystalline, aliphatic polyester, which is applied as degradable implant material. In this paper we explored how thermodynamic and kinetic conditions in a supercritical CO2 (scCO2) supported foaming process influence the final morphology of the foam. With help of a view cell, we have systematically investigated the foaming with scCO2 in the pressure range from 78 to 200 bar at temperatures between 25 and 50 °C. Foams were obtained both above the pressure dependent melting temperature (Tm) but also below this temperature, i.e. from supercooled melt states. Foams were characterized by μCT X-ray computed tomography and scanning electron microscopy. The pore size distributions of the obtained foams show characteristic properties (widths, maxima) depending on the initial thermodynamic state of the CO2/PCL system before pressure quenching, the rate of the pressure decay, and the thermal history of the system. We try to rationalize the dependency of foam morphology and quench conditions with thermodynamic model calculations. The initial amount of CO2 in the PCL melt was calculated with the Sanchez-Lacombe equation of state. Pressure quenching with a slow pressure decay rate is considered an isothermal process where for a fast rate an adiabatic process is assumed. Both processes differ in their phase separation mechanism. It turned out that the CO2-induced melting point depression of the semi-crystalline polymer is an important factor. The variation of foaming conditions allows the preparation of scaffolds with specific morphological parameters (mean pore size, pore distribution, and pore connectivity).

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► PCL-foaming with scCO2 systematically investigated at P = 78–200 bar and T = 25–50 °C.
► (T, P)-range involved supercooled melts at T < Tm(P), below P-dependent melting point.
► Adiabatic effects were considered during fast quenching.
► Dependency of foam morphology on quenching conditions is rationalized with models.
► Different phase separation mechanisms were identified for slow and fast pressure decay.