Study on structure and vacuum membrane distillation performance of PVDF membranes: II. Influence of molecular weight

• PVDF molecular weights can significant effects on membrane properties and vacuum membrane distillation (VMD) performance.
• Membrane properties and VMD performances are affected by membrane casting solutions’ thermodynamic & rheological properties.
• The interplay of thermodynamic and kinetic effects influences membrane morphology.
• The formation of network pore and aggregate pore explains the pore size distribution at the membrane surface layer.

Membranes were prepared from three poly(vinylidene fluoride) (PVDF) of different molecular weights by the phase inversion process. The membranes were characterized by scanning electron microscopy, gas permeation tests, contact angle (CA) and liquid entry pressure of water (LEPw) measurements, and further subjected to the test of flux for vacuum membrane distillation (VMD) in a scenario that is applicable for cooling processes. The results showed that the increase in PVDF molecular weight increased the viscosity and thermodynamic instability of the casting solution significantly. Regarding characterization and performance testing, the membrane prepared from the intermediate molecular weight of Kynar® MG 15 polymer showed the highest VMD flux (325 g/m2 h) at the feed temperature of 27 °C and the lowest LEPw (622 kPa) due to the largest pore size (49.8 nm) observed among all the tested membranes. The highest flux of this particular membrane seems also due to the thinnest finger-like and sponge-like layer.

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