polymers Article Waterproof-breathable PTFE nano- and Microfiber Membrane as High Efficiency PM2.5 Filter Xiao Li 1,†, Xiao-Xiong Wang 1,†, Tian-Tian Yue 1, Yuan Xu 1, Ming-Liang Zhao 2, Miao Yu 1,3, Seeram Ramakrishna 1,4 and Yun-Ze Long 1,* 1 Collaborative Innovation Center for Nanomaterials & Devices, College of Physics, Qingdao University,