A study on thermal effect on structure and transport properties of a composite lignosulfonated-polyamide/polysulfone membrane

Changes in morphology and transport parameters for a composite lignosulfonate–polyamide/polysulfone membrane associated with temperature effects were studied. Pore size, pore density and porosity dependence with temperature (25 ≤ t (ºC) ≤ 93) was determined analyzing SEM micrographs, while diffusional permeability and electrical parameters (resistance and capacitance) were obtained from salt diffusion and impedance spectroscopy measurements carried out with the samples in contact with 0.01 M HCl solutions. Temperature seems to modify the structure of both the denser lignosulfonate contained top layer and the opener polysulfone support, but different temperature–membrane parameters relationships were obtained depending on the sublayer material, with changes around 40ºC for the lignosulfonated top layer and 80ºC for the polysulfonebased support. Permeability asymmetric degree for the different temperatures was also obtained.

Morphological characterization of a composite lignosulfonate–polyamide/polysulfone membrane, which was carried out by analyzing SEM micrographs cross-section with the IFME® software, shows a structure with three different zones (similar to NF/RO membranes): a denser top layer on the top of a gradually increases porous sublayer and a clearly differentiated much more open support with wide pores and macrovoids (comparable to paper/unwoven support of RO membranes). Membrane annealing seems to modified both denser and opener structures, which was also estimated from electrical and diffusional measurements. However, different membraneproperty/temperature relationships were obtained depending on the sublayer material, with changes around 40ºC for the lignosulfonated top layer and 80ºC for the polysulfone based support, which clearly show the different effect of temperature on the material forming each sublayer.