Materials
Methane with high purity (> 99.99%) was purchased from Heli Gas Co., Ltd. (Qingdao, China). The laboratory-made deionized water with a conductivity of 1.1 ± 0.1 μs/cm at 298.15 K was used. Styrene (St, A.R.), sodium p-styrene sulfonate (SS, A.R.) and acrylic acid (AA, A.R.) were purchased from www.aladdin-e.com. Potassium persulfate (KPS, A.R.) was provided by Shanghai Aibi Chemical Reagent Co., Ltd (Shanghai, China).