materials Article Polyvinyl Alcohol Reinforced Flame-Retardant Polyacrylonitrile Composite Fiber Prepared by Boric Acid Cross-Linking and Phosphorylation Yuanlin Ren 1,2,*, Tian Tian 2, Lina Jiang 2, Xiaohui Liu 3 and Zhenbang Han 2 1 School of Textiles, Tianjin Polytechnic University, Tianjin 300387...