Foaming strategies for bioabsorbable polymers in supercritical fluid mixtures. Part II. Foaming of poly(ɛ-caprolactone-co-lactide) in carbon dioxide and carbon dioxide + acetone fluid mixtures and formation of tubular foams via solution extrusio

This paper reports on the foaming of poly(ɛ-caprolactone-co-lactide) in carbon dioxide and carbon dioxide + acetone mixtures. Experiments were carried out in specially designed molds with porous metal surfaces and fluid circulation features to generate foams with uniform dimensions at 60, 70 and 80 °C at pressures in the range 7–28 MPa. Depending upon the conditions, foams with pores in the range from 5 to 200 μm were generated. Adding acetone to carbon dioxide improved the uniformity of the pores compared to foams formed by carbon dioxide alone. In addition, a unique high-pressure solution extrusion system was designed and used to form porous tubular constructs by piston-extrusion of a solution from a high-pressure dissolution chamber through an annular die into a second chamber maintained at controlled pressure/temperature and fluid conditions. Long uniform porous tubular constructs with 6 mm ID and 1 mm wall thickness were generated with glassy polymers like poly(methyl methacrylate) by extruding solutions composed of 50 wt% polymer + 50 wt% acetone, or 25 wt% polymer + 10% acetone + 65% carbon dioxide at 70 °C and 28 MPa. Pores were in the 50 μm range. The feasibility of forming similar tubular constructs were demonstrated with poly(ɛ-caprolactone-co-lactide) as well. Tubular foams of the copolymer with interconnected pores with pore sizes in the 50 μm range were generated by extrusion of the copolymer solution composed of 25 wt% polymer + 10 wt% acetone + 65 wt% carbon dioxide at 70 °C and 28 MPa. Reducing the acetone content in the solution led to a reduction of pore sizes. Comparisons with the foaming behavior of the homopolymer poly(ɛ-caprolactone) that were carried out in the molds with porous metal plates show that the foaming behavior of the copolymer is more akin to the foaming behavior of the caprolactone homopolymer component.