The polyurethane/SiO 2 nano-hybrid membrane with temperature sensitivity for water vapor permeation

Abstract

A novel thermal-sensitive polyurethane (TSPU)/SiO 2 nano-hybrid membrane was prepared via in situ nano-hybrid method in wet phase inversion process, and used for water vapor permeation. Atom force microscope (AFM) and scanning electron microscope (SEM) were used to observe the size scale and the distribution of nano-SiO 2 throughout the TSPU matrix, and Fourier transform infrared (FTIR) spectroscopy was employed to reveal the nano-hybrid mechanism between TSPU and nano-SiO 2. Simultaneously, differential scanning calorimetry (DSC) and X-ray diffraction (XRD) analysis were used to investigate the phase separation behavior of the nano-composites and the influence of nano-SiO 2 on the crystallization of the TSPU. Results from AFM and SEM analysis show that the mean size and the distribution of nano-SiO 2 particles in the TSPU matrix is relied on the SiO 2 content, i.e. SiO 2 content lower than 5.0 wt% result in homogeneously dispersed nano-SiO 2 particles with a mean size below 100 nm approximately throughout the TSPU matrix, while obvious SiO 2 agglomerates (>1000 nm) are in the overwhelming majority with excessive addition of SiO 2 content into the TSPU resin. FTIR study indicates that two chemical bonds of Si–C and Si–O–C occur between the organic and the inorganic phase; one seems to be the reaction between nano-SiO 2 and H–Ar (aromatic ring), and another is the condensation reaction between Si–(OH) 4 produced by the hydrolysis of the nano-precursors (tetraethyoxyl silane) and the terminal HO–CH 2– groups of the TSPU. Due to these interfacial interactions, the tensile strength and the broken elongation of nano-hybrid samples in the measurement of mechanical properties increase by 40.1% and 23.2%, respectively, as the SiO 2 content rises from 0.00 wt% to 5.0 wt%. DSC study reveals that nano-hybridization does not disrupt the intrinsic phase-separated structure of the TSPU, but the phase transition temperature of soft segment (defined as switch temperature, T s) in TSPU increases slightly with the increase of nano-SiO 2 content. XRD analysis indicates that appropriate amount of nano-SiO 2 filled in the TSPU matrix is prone to induce the soft segment crystallization of the polymer. In addition, the water vapor permeability (WVP) of TSPU nano-hybrid membrane is found to depend on the size scale of SiO 2 particles, which is relied on SiO 2 content. To be specified, when nano-SiO 2 particles with an average size of 100 nm approximately are present in the TSPU matrix, the nano-hybrid samples show lower WVP at low temperatures and higher WVP at high temperatures than pure TSPU. Especially, when the temperature exceeds the T s, for example from 50 °C to 60 °C, the WVP of pure TSPU changes from 1026 g/m 2 day to 2200 g/m 2 day, whilst the WVP of the nano-hybrid TSPU with 5.0 wt% SiO 2 content changes from 995 g/m 2 day to 2927 g/m 2 day, the later shows more sensitive to thermal stimuli.