Positron studies of the temperature-dependence of free volumes in Polydimethylsiloxane/poly(propylene oxide) urethane/urea membranes

Abstract

Free-volume parameters in polyurethane/urea membranes have been studied by positron annihilation lifetime and Doppler broadening measurements. The bi-soft segment membranes were obtained by varying the ratio of the structural constituents, polypropylene oxide and polydimethylsiloxane (PU/PDMS), with PDMS content from 25 to 75 wt%. The free-volume holes determined by PALS are correlated with gas permeation features. The phase separation of the various soft and hard segments in the membranes is mirrored in both lifetime and Doppler results.

Free-Volume Studies in Polycaprolactone/Poly(propylene oxide) Urethane/Urea Membranes by Positron Lifetime Spectroscopy

“…The smaller free volumes, R 3 , may correspond to holes in the crystalline regions connected with the aggregates of urethane urea groups, whereas the larger ones, R 4 , might be ascribed to holes in amorphous regions. As compared to previous results in [16,17], R 3 has about the same value as the other polyurethane membranes for the same content of the soft segment. The decrease in τ 4 with increasing PCL may be ascribed to an increase in cross-linking, resulting in a decrease in hole sizes.…”

“…The latter components could indicate the coexistence of two phases, corresponding to separated domains of the two soft segments (polypropylene oxide and polycaprolactone) present in the membranes. Such results were also observed in other two polyurethanes/urea membranes obtained through the introduction of two soft segments -polypropylene oxide/polybutadiene and polypropylene oxide/polydimetylsiloxane [16,17].…”

“…For the same content of PU, these membranes have lower CO 2 permeability when compared to the membranes with soft segments PBDO [16] or PDMS [17]. The types of soft segments can also affect the permeation behaviour, because they could change the degree of microphase separation and the crystallinity of the membranes, questions which can still not be answered here but will be addressed later.…”

“…Our fast-fast coincidence PALS setup with Pilot-U scintillators and XP 2020Q photomultipliers [17] had a time resolution of 270 ps. Sample-source sandwiches with 22 Na of ca.…”

“…The present work is an extension of studies of polyurethanes/urea membranes with two soft segments [14,16,17]. Our aim was to study how the freevolume properties are influenced by phase separation and how the free volume correlates with gas permeability.…”