3.2 Characterization of membrane.
The surface and cross section morphology of membranes were observed using a field-emission scanning electron microscope (FE-SEM, NOVA NanoSEM 450, FEI, USA). The surface roughness was measured with an atomic force microscope (AFM; Dimension ICON, Bruker, USA). The porosity of the membranes was determined by means of the software called Image Pro Plus. Solution contact angle (CA) of membrane was evaluated using the JC2000D machine (POWEREACH®, Shanghai zhongchen, China). The thermal conductivity of the PTFE and the PES hollow fiber membrane at different temperatures was measured using a thermal conductivity meter (TC3200, XIATECH, China). Since the hydrophilicities of the two membranes were different, the porosity of PTFE and PES hollow fiber membrane were measured by density measurement and water absorption measurement, respectively.
The microstructure of the two membranes are shown in Figure 2. PTFE hollow fiber membrane is a microporous membrane with very small surface porosity, which can be considered as a symmetric membrane with uniform cross section structure. PES hollow fiber membrane does not have apparent pore with nano scale on the surface and the cross section of the membrane indicates its asymmetric structure with a thin skin layer. The characteristics of the membrane and homemade membrane module are listed in Table 1 and Table 2, respectively. Membrane modules with different packaging density φ were assembled to evaluate their nucleation inducing performance.