Free Volume in Polyimides:  Positron Annihilation Experiments and Molecular Modeling

Kruse, Jan Philipp; Kanzow, J.; Rätzke, Klaus; Faupel, Franz; Heuchel, Matthias; Frahn, Jörg; Hofmann, Dieter

doi:10.1021/ma0473521

Cited by 84 publications

(52 citation statements)

References 40 publications

(68 reference statements)

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“…[19][20][21][22][23] Concerning membrane characterization, PAS has mainly been used in the past to study membranes for gas separation. [24][25][26][27][28] Only a few manuscripts report on the use of PAS applied to reverse osmosis membranes [9,29] or pervaporation membranes. [15] To the best knowledge of the authors, PAS has not yet been applied to NF membranes.…”

Section: Introductionmentioning

confidence: 99%

Physico‐Chemical Characterization of Nanofiltration Membranes

et al. 2007

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This study presents a methodology for an in-depth characterization of six representative commercial nanofiltration membranes. Laboratory-made polyethersulfone membranes are included for reference. Besides the physical characterization [molecular weight cut-off (MWCO), surface charge, roughness and hydrophobicity], the membranes are also studied for their chemical composition [attenuated total reflectance Fourier spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS)] and porosity [positron annihilation spectroscopy (PAS)]. The chemical characterization indicates that all membranes are composed of at least two different layers. The presence of an additional third layer is proved and studied for membranes with a polyamide top layer. PAS experiments, in combination with FIB (focused ion beam) images, show that these membranes also have a thinner and a less porous skin layer (upper part of the top layer). In the skin layer, two different pore sizes are observed for all commercial membranes: a pore size of 1.25-1.55 angstroms as well as a pore size of 3.20-3.95 angstroms (both depending on the membrane type). Thus, the pore size distribution in nanofiltration membranes is bimodal, in contrast to the generally accepted log-normal distribution. Although the pore sizes are rather similar for all commercial membranes, their pore volume fraction and hence their porosity differ significantly.

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Section: Introductionmentioning

confidence: 99%

Physico‐Chemical Characterization of Nanofiltration Membranes

et al. 2007

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“…Positron annihilation experiments were performed in a fast-fast coincidence setup with a home-made temperaturecontrollable sample holder under high-vacuum conditions as described elsewhere 20 . In a Cu-pan, the powdery samples were sandwiched around a 1-MBq Na 22 source, with approximately 1 mm of material on either side of the radioactive source to ensure complete absorption of the positrons in the sample.…”

Section: Positron Annihilation Lifetime Spectroscopy (Pals)mentioning

confidence: 99%

“…20 °C and 33 % rh [days] Development of the hydroperoxide and propanal content of fish oil microencapsulated into matrices with different molecular weight profile upon storage at 20°C and 33% relative humidity…”

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confidence: 99%

Differences in Free Volume Elements of the Carrier Matrix Affect the Stability of Microencapsulated Lipophilic Food Ingredients

et al. 2009

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Fish oil was encapsulated by spray-drying into different matrices based on n-octenylsuccinate-derivatised starch (nOSA starch) and carbohydrate blends varying in dextrose equivalent and molecular weight profile. Based on the development of the hydroperoxide and propanal content upon storage significant differences in the stability of the microencapsulated oil were observed. With 40 mmol/kg oil the hydroperoxide content after eight weeks of storage at 20°C and 33% relative humidity was lowest in fish oil encapsulated in a matrix containing nOSA starch and maltose. In contrast, the lowest stability was observed in fish oil encapsulated in a matrix based on nOSA starch and maltodextrin with a dextrose equivalent of 18. Physical characteristics like viscosity of the feed emulsion and oil droplet size, which influence drying behaviour as well as particle characteristics like particle size, density or surface area did not differ and thus cannot explain the differences in the rate of autoxidation. Positron annihilation lifetime spectroscopy clearly showed distinct differences in the ortho-positronium lifetime, and thus in the size of free volume elements between the carrier matrices. It is suggested that as a consequence, the matrices differed in oxygen diffusivity, which must be considered to be a key determinant in autoxidation of fish oil encapsulated in glassy carbohydrate matrices.

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“…It should be noted that the overall FFVs encompass all volume that not occupied by polymer atoms. However, small free volume voids do not make considerable contribution to transport property of the membrane (Kruse et al, 2005). Therefore, the accessible free volume voids for penetrant molecules should be the major concern.…”

Section: Free Volume In Hybrid Membranesmentioning

confidence: 99%