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).