Preparation of polymer foams with a gradient of cell size: Further exploring the nucleation effect of porous inorganic materials in polymer foaming

Polymeric foams with gradient of cell size have shown significant potential in many applications, but, the fabrication of such foams is difficult. By producing blowing agent concentration or temperature gradient inside the sample, some recently developed methods have successfully prepared gradient polymer foams, whereas, precise control system and/or delicate operation are highly needed. Herein, we reported a simple method to obtain cell size gradient in polymer foams by a batch foaming of the polymers on a substrate with nanopore structure. Anodized aluminum oxide (AAO) film modified with a fluorinated silane was used as the substrate. Polymers including polystyrene, polymethyl methacrylate, and polylactic acid were compressed onto the modified AAO film, and then foamed by a batch foaming process using scCO(2) as blowing agent. SEM characterizations on the foam morphology revealed that for all the samples, the size of cell near the substrate was smaller and gradually increased along the distance from AAO surface. It is also found that the gradient structure varied with the polymer type. Specifically, polylactic acid foams showed a perfect gradient of cell size from similar to 13 mu m to similar to 40 mu m in a thickness of more than 1.0 mm, providing significant potentials in many applications. Moreover, by choosing the substrates with nanopores of different size and/or changing the foaming condition, the foam morphologies including cell size, cell density, and cell size gradient range can be readily tuned. (C) 2016 Elsevier Ltd. All rights reserved.