Douglas N. Winslow

Preparation and characterization of poly(l-lactic acid) foams

Abstract A particulate-leaching method was developed to prepare highly porous biodegradable polymer membranes. It involves the casting of polymer/salt composite membranes followed by the dissolution of the salt. Poly( l -lactic acid) porous membranes of controlled porosity, surface/volume ratio, and crystallinity were prepared with sodium chloride, sodium tartrate or sodium citrate sieved particles. For salt weight fractions of 50 and 60 wt%, asymmetric membranes were formed, independent of salt particle size. When 70–90 wt% salt was used, the membranes were homogeneous with interconnected pores. The membrane properties were independent of the salt type and were only related to the salt weight fraction and particle size. The porosity increased with the salt weight fraction, and the median pore diameter increased as the salt particle size increased. The polymer/salt composite membranes could be quenched or annealed to yield amorphous or semicrystalline foams with desired crystallinity. All foams were 99.9 wt% salt free and had porosities as high as 0.93 and median pore diameters up to 150 μm.

Percolation and pore structure in mortars and concrete

Abstract The cement paste in concrete and mortar has been shown to have a pore size distribution different than that of plain paste hydrated without aggregate. For mortar and concrete, additional porosity occurs in pore sizes larger than the plain pastes threshold diameter as measured by mercury intrusion. Based on the assumption that these larger pores are essentially present only in the interfacial zones surrounding each aggregate, an experimental program was designed in which the volume fraction of sand in a mortar was varied in a systematic fashion and the resultant pore system probed using mercury intrusion porosimetry. The intrusion characteristics were observed to change drastically at a critical sand content. Similar results are observed for a series of mortar specimens in which the cement paste contains 10% silica fume. To better interpret the experimental results, a hard core/soft shell computer model has been developed to examine the percolation characteristics of these interfacial zone pores. Using the model, interfacial zone percolation in concretes is also examined. Finally, the implications of interfacial zone percolation for transport properties and durability of mortar and concrete are discussed.

THE PORE STRUCTURE OF PASTE IN CONCRETE

Abstract The pore size distribution of the cement paste that develops in concrete and mortar was measured by mercury intrusion. The intruded pore volumes were expressed on a per unit mass of paste basis and compared with the pore structure of plain paste that hydrated without aggregate. The paste in concrete is more porous, and the difference increases with increasing hydration. The additional porosity occurs mainly in pore sizes that are larger than the plain pastes threshold pore diameter.

The fractal nature of the surface of cement paste

Abstract The basic nature and properties of a class of geometric forms known as fractals are presented. An X-ray scattering technique for measuring the dimension of a fractal surface is described. The technique is used to demonstrate that the surface of hydrated cement paste is fractal in character, and has a large fractal dimension. This latter fact indicates that the surface of cement paste is extremely irregular. Further, the degree of irregularity is found to depend somewhat on the water: cement ratio. The fractal character of the surface of paste provides a logical explanation for the different surface areas detected by different vapors in a sorption experiment.

Contact angle and damage during mercury intrusion into cement paste

Abstract The operant contact angle to be used for mercury intrusion porosimetry was measured directly by intrusion into specially prepared, cylindrical pores. It was found to vary for cement pastes of different ages, and for pastes containing flyash. It also changed when the paste was previously intruded and the mercury subsequently removed. Values of the contact angle ranged from 123° to 135°. The high pressures used during intrusion were found to reduce the pore volume of cement paste with the reduction being greater in the smaller pore (higher pressure) region. Distilling mercury from a paste was also found to alter the pore structure and this effect must be considered separately before alterations due only to intrusion are discussed.

The fractal arrangement of hydrated cement paste

Abstract Data from small-angle X-ray scattering experiments are used to determine the fractal types and dimensions of hydrated portland cement pastes over a range of length scales. The larger-scale, 200–1500 A, geometry at most degrees of water saturation is found to be that of a rough surface fractal. The smaller-scale, 30–200 A, geometry is found to be a mass fractal in saturated pastes. This geometry changes gradually with drying, and becomes a rough surface fractal at saturations less than about 50%. Completely oven dried pastes are found to have a considerably altered geometry.

Morphological changes of ethylene/vinyl acetate-based controlled delivery systems during release of water-soluble solutes

Abstract Thin slabs of theophylline and monomer albumin release systems were prepared by dispersing 212-300 μm and 300-25 μm particles respectively, of these bioactive agents in a methylene chloride solution of ethylene/vinyl acetate (EVAc) copolymer (40 wt% vinyl acetate), and evaporating the solvent at low temperatures according to the Langer—Folkman technique. Compositions containing 21.41 wt%, 31.04 wt% and 40.0 wt% albumin, and 19.32 wt% theophylline were prepared. Solute release experiments were performed in deionized water at 37 ± 0.1°C under perfect-sink conditions. The concentration of released solute was determined by measuring the absorbance of the UV spectra at 276 nm for albumin and 272 nm for theophylline. Both solutes could be released for long periods of time at controlled rates. The main mechanism of release was established to be solute dissolution and diffusion through the generated, waterfilled pore structure. Photomicrographs present the main features of this pore network. Mercury porosimetry was used to determine the pore volume and size of pores for freezedried slabs before, during and after the dissolution/diffusion/release process. Considerable pore collapse was observed and pore diameters of 8-650 μm were detected. In addition to solution diffusion through large pores, diffusion might occur through small constrictions between large pores or through a pore network of much smaller pores created in the matrix.

The validity of high pressure mercury intrusion porosimetry

Abstract The validity of pore size measurements by mercury intrusion was investigated on porous alumina. Mercury intrusion measurements were compared for virgin samples and reintruded samples after removal of mercury. Comparison of these and a distribution measured by capillary condensation showed no evidence of sample damage at high pressures. Mercury intrusion appears valid at high pressures but sample outgassing is important.

The relationship between an aggregate's pore size distribution and its freeze thaw durability in concrete

Abstract The relationship between the freeze/thaw durability of coarse aggregates and their pore structure was investigated. The pore size distribution of 14 aggregates was determined by mercury intrusion and compared to the durability factor derived from standard laboratory freeze/thaw tests. The results show that both the total pore volume and the median pore diameter influence the durability. Specifically, a lesser volume and a larger median diameter are associated with more durable aggregates and vice versa. An equation was developed that allows a prediction of the freeze/thaw durability from a measurement of the pore size distribution. The predictive equation was applied to aggregates removed from several Indiana highways and gave results that correlated well with the observed field performance of the concrete.

Advances in Experimental Techniques for Mercury Intrusion Porosimetry

In 1921 Edward Washburn published an elegantly brief note in the Proceedings of the National Academy of Sciences.(1) In the space of less than one small page he laid all of the groundwork for the experimental technique that has grown, over the ensuing 60 years, into the field of mercury intrusion porosimetry. Washburn pointed out that mercury will not voluntarily enter the pores in most materials and that pressure is required to induce it to do so. He derived a simple equation, the Washburn equation, that relates the required pressure and the size of pore being entered.