1 research outputs found
CO<sub>2</sub> Selective PolyActive Membrane: Thermal Transitions and Gas Permeance as a Function of Thickness
It
is generally accepted that the melting point of a semicrystalline
polymer is associated with the thickness of the crystalline lamellae
(Gibbs–Thomson equation). In this study, a commercially available
multiblock copolymer PolyActive composed of 77 wt % of polyÂ(ethylene
glycol terephthalate) and 23 wt % of polyÂ(butylene terephthalate)
was dip-coated on top of a multilayer microporous support. The thickness
was changed between 0.2 and 8 μm using coating solutions containing
0.75–7.5 wt % PolyActive. The surface temperature of the membrane
during dip-coating was monitored using an infrared camera. Single
gas permeances of N<sub>2</sub>, H<sub>2</sub>, CH<sub>4</sub>, and
CO<sub>2</sub> were measured between 20 and 80 °C at temperature
steps of 2 °C. Spherulitic superstructures composed of radially
directed lamellae were observed in the polarized light microscope
in the prepared membranes. Atomic force microscopy studies showed
that the thickness of the crystalline lamellae was in the order of
10 nm or 0.01 μm at the surface of the membrane. Therefore,
according to the Gibbs–Thomson equation, the melting point
should not change in the thickness range 0.2–8 μm. However,
the gas permeance data showed that the melting point of the polyether
domains of the 0.2 μm PolyActive layer was 10 °C lower
compared to that of the 8 μm layer. The results can be explained
by considering that the width of many crystalline lamellae significantly
reduces as a function of film thickness, thereby reducing the average
fold surface free energy/lateral surface free energy ratio