Preparation of aluminophosphate/polyethylene nanocomposite membranes and their gas permeation properties

Abstract

Composite membranes were prepared by the incorporation of a porous layered aluminophosphate (ALPO) into polyethylene (PE) matrix, through both in situ polymerization with metallocene catalyst and melt compounding methods. ALPO was previously swollen with protonated octadecylammine (ODA) and cetyltrimethylammonium (CTA), which generated intercalated particles with different morphologies and dimensions. ODA–ALPO particles incorporated by in situ polymerization were found to be completely exfoliated in the composite, although probably with partial damage of their microporous structure. Composites prepared by melt compounding with CTA–ALPO particles, were found to contain particles predominantly in intercalated form, and apparently with good preservation of the aluminophosphate structure. Permeability results demonstrate that the incorporation of swollen-ALPO particles significantly improves the gas permeation properties of the semicrystalline polyolefin membrane. Gas permeability increase is attributed to novel transport pathways provided by the presence of the microporous ALPO layers as well as to the reduction of PE crystal size and the disruption of polymer-chain packing. Permeability of smaller gas molecules tended to be favoured, suggesting a molecular sieve effect through the microporous ALPO structure. ALPO/PE membranes exhibited particular H 2/CO permselectivity. Highest H 2/CO selectivity values were achieved at 5 wt.% loading of swollen-ALPO, particularly for membranes prepared by melt compounding using CTA–ALPO particles (H 2/CO = 9.5–17.0 as compared to H 2/CO = 1.0–2.6 for pure PE). The better performance of CTA–ALPO/PE membranes is attributed to a better preservation of the microporous structure of CTA–ALPO particles as well as to a better dispersion degree achieved through melt mixing technique. The results of this work open the possibility of the use of polyolefin composites prepared with the porous layered ALPO as a novel membrane material. Particular H 2/CO permselectivity exhibited by these membranes makes of it promising for hydrogen purification applications.