Surface Treatment on Cellular Structures and Mechanical Properties of Polypropylene/CNF Nanocomposite Foams via Core-Back Foam Injection Molding Long Wang 1,* , Kiyomi Okada 1, Yuta Hikima 1, Masahiro Ohshima 1,*, Takafumi Sekiguchi 2 and Hiroyuki Yano 3 1 Department of Chemical Engineering, Kyoto...