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Latex blend films were prepared from mixtures of two types of particles in dispersion, one composed of a high-Tg polymer [poly(methyl methacrylate), PMMA]; the other a copolymer of butyl methacrylate and butyl acrylate [P(BMA-co-BA)] with Tg < 10 °C. Transparent films were obtained under air-drying conditions if the PMMA particles had diameters less than ~250 nm and if the volume fraction of low-Tg latex polymer exceeded a certain critical fraction Oc. Values of 4>c varied over a narrow range (0.40-0.50) with P(BMA-co-BA) particle size, and were independent of the Tg of the soft latex, ranging from ca. -35 to +10 °C. Film morphologies were examined by scanning electron microscopy and by freeze-fracture transmission electron microscopy. In all films, the hard particles retain their original size and spherical shape. In the transparent films, they are uniformly distributed in a polymer matrix generated from deformed soft particles, whereas clustering of PMMA microspheres is observed in turbid films. Various factors, such as increasing the size ratio between the two types of particles, removing of surfactant in the systems, and annealing of the films after drying, disrupt the uniform particle packing required for transparent films.

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To provide an in vitro system for studies of brain capillary function we developed a method for culture of brain capillary endothelial cells. Capillaries were isolated from rat brain and enzymatically treated to remove the basement membrane and contaminating pericytes. Subsequent Percoll gradient centrifugation resulted in a homogeneous population of capillary endothelial cells that attached to a collagen substrate and incorporated [3H]thymidine. Evidence for the endothelial nature of these cells was provided by the presence of Factor VIII antigen and angiotensin converting enzyme activity and by the failure of platelets to adhere to the cell surface. In addition, the cells were joined together by tight junctions. Thus, primary cultures of these cells retained both endothelial and blood-brain barrier features.