Preparation of macroporous biodegradable poly(L-lactide-co-epsilon-caprolactone) foams and characterization by mercury intrusion porosimetry, image analysis, and impedance spectroscopy.

Two poly(L-lactide-co-epsilon-caprolactone) random copolymers containing 5 and 40 mol % of epsilon-CL, namely P(LA-co-CL(5)) and P(LA-co-CL(40)), respectively, have been made macroporous by freeze-drying solutions in dimethylcarbonate. Most of the freeze-dried foams, prepared by varying polymer concentration and cooling rate, exhibited two main pore populations: (1). longitudinally oriented tube-like macropores with diameters >or=100 microm, and (2). interconnected micropores (10-100 microm). Pore characteristics, including macropore density, mean diameter, and interdistance, as well as micropore density, area, and shape, were determined by image analysis of scanning electron micrographs in order to study the influence of processing and formulation parameters on foam structure and properties. The pore orientation and the 3-D texture also were studied by image analysis and impedance spectroscopy. In the case of the P(LA-co-CL(5)), the macropore diameter increased with the cooling rate while the micropore diameter decreased. The micropores also became more circular when the cooling rate was increased. The pore size and morphology of the P(LA-co-CL(40)) were quite unchanged by varying the cooling rate. All the other conditions being the same, the P(LA-co-CL(5)) foams were better organized than the P(LA-co-CL(40)) foams, and pore orientation was improved at the higher cooling rate. Pore size and morphology also can be controlled by changing the polymer concentration (Cp), as we showed by studying P(LA-co-CL(5)) foams prepared by freeze-drying solutions in the 1-10 w/v % Cp range. Macropore density, average diameter, and interdistance of P(LA-co-CL(5)) foams increased with Cp, but the micropore characteristics remained almost unchanged no matter the Cp. The reliability of the characterization methods has been discussed, with special attention to mercury intrusion porosimetry, which is used primarily for measurement of pore volume and pore size distribution. However, this technique is reported here as a destructive and unreliable method for the characterization of fragile P(LA-co-CL(40)) foams. This study shows that image analysis and impedance spectroscopy can give reliable information relative to the pore morphology and anisotropy of freeze-dried foams.

Fractal Characterization of Wide Pore Range Catalysts: Application to Pd-Ag/SiO(2) Xerogels.

Fractal analysis has been applied to characterize the structure of Pd-Ag/SiO(2) catalysts dried under vacuum (150 degrees C and 12 hPa) with different concentrations of Pd-Ag. Nitrogen adsorption-desorption, mercury porosimetry, and small-angle X-ray scattering measurements have been used. The different approaches to fractal analysis and their conditions of applicability are briefly described. Results are discussed in relation to those derived previously from classical interpretations. This analysis shows that Pd-Ag/SiO(2) xerogels exhibit a very open self-similar pore structure analogous to the structure of supercritically dried aerogels and that the micropore texture can be tailored by varying the silver content. Copyright 2000 Academic Press.

Sequentially Compressive and Intrusive Mechanisms in Mercury Porosimetry of Carbon Blacks.

The mechanism of mercury penetration in two different commercial carbon blacks is studied. We show that the volume variation measured by mercury porosimetry in these systems is due to three successive mechanisms, which occur as the pressure increases: (1) mercury invasion of voids between macroscopic grains, (2) compaction and elastic compression of macroscopic grains, (3) intrusion in the voids of aggregates formed by primary particles. The cumulative surface calculated by the Rootare-Prenzlow equation (S(RP)) in the intrusion part and the BET surface area measured by nitrogen adsorption (S(BET)) are compared. Copyright 1999 Academic Press.

Fractal Analysis of Mercury Porosimetry Data in the Framework of the Thermodynamic Method.

The thermodynamic method for fractal analysis is applied to mercury intrusion data. The results for representative commercial carbon black samples and a series of resorcinol-formaldehyde (RF) freeze-dried gels for different values of the molar ratio of resorcinol (R) to catalyst (C) are discussed in relation with the type of behavior exhibited by the samples during mercury porosimetry measurements. The obtained surface fractal dimensions are compared with those derived previously from the small-angle X ray scattering (SAXS) and the nitrogen adsorption. Copyright 1999 Academic Press.