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Y.C. Jean 1,2 1 Department of Chemistry, University of Missouri-Kansas City

Characterization of Free volumes in Nano-scale Polymeric and Composite Membrane  Systems Using Positron Annihilation Spectroscopy *. Y.C. Jean 1,2 1 Department of Chemistry, University of Missouri-Kansas City

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Y.C. Jean 1,2 1 Department of Chemistry, University of Missouri-Kansas City

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  1. Characterization of Free volumes in Nano-scale Polymeric and Composite Membrane  Systems Using Positron Annihilation Spectroscopy* Y.C. Jean1,2 1Department of Chemistry, University of Missouri-Kansas City 2R&D Center for Membrane Technology and Department of Chemical Engineering, Chung-Yuan Christian University, Taiwan Collaborators: NIST:T. Nguyen, X. Gu; AIST: R. Suzuki, T. Ohdaira; CMT: J.I.Lai, Y.M. Sun, R. Lee, C.C. Hu *Supports: NSF and NIST, Ministry of Education (Taiwan)

  2. Outline • Positron and Positronium Annihilation • I. Positrons in Polymeric Nano-Scale Films (1) Multi-Layer Structures (2) Tg-depth dependence • II. Polymeric Composite Membranes (1) Surface layer structures (2)Permeability and selectivity

  3. The Positron 1930: Anti-electron (Positron) predicted by P.A.M. Dirac, Quantum Electrodynamics Theory. 1932: The Positron (positive electron), detected by C.D. Anderson in the cloud chamber from cosmic radiation. 1946: The Positronium atom (positron and electron bound state) detected by M. Deutch from positron annihilation in gases. 1960: Solid state physics: positron is localized in defects; positron is delocalized in lattices-Fermi surface. 1970: Nuclear medicine: positron emission tomography (PET) 1975: Surface science: positron has a negative work function. 1980-presence: Positron chemistry, material defect and surface tools.

  4. Positron Annihilation Processes • When positron and electron meet, they can form Positronium (Ps). Positronium exists in two states, p-Ps (spins anti-aligned) and o-Ps (spins aligned): 2 photons (for p-Ps with 125 ps, or a few ns for pick-off with electrons with molecules) or 3 photon (for o-Ps 142 ns) produced by annihilation. • Positrons can freely annihilate with electrons without forming Ps with lifetime ~10-9 s to ∞ (in UHV 10-11 torr, live one hour). • The Feynman diagram shows that the annihilation distance starts 10-12.5 m (Δx~ħ/mc) and the time 10-21 s (t~ħ/mc2)-delta function and sudden approximation. • 4. Annihilation characteristics depend on electron properties of matter.

  5. Positron Annihilation Spectroscopy (PAS) • PAS monitors annihilation γ-rays properties, which are related to materials and electronic properties of systems studied. Four experimental techniques are currently used in PAS: • Positron annihilation lifetime spectroscopy (PAL): atomic and molecular free volumes and holes in polymers, solids. • Doppler broadening of annihilation energy spectroscopy (DBES): atomic defects in semiconductors, polymers. • Angular correlation of annihilation radiation (ACAR): Anisotropy structures of free volumes, defects, Fermi surface. • Variable mono-energetic positron beam: surface and interfaces.

  6. PAS contains the most fundamental properties of molecules (chemistry):wave function and electron density  • PAL measure positron annihilation lifetime τ: τ= n|xyz n(x,y,z)* +(x,y,z) dxdydz |2 • DBES measures electrons’ momentum distributions at the longitudinal direction (z): N(pz)=n pxpy|xy zn(r)* +(r) e-iP•rd r|2dpxdpy • 2D-ACAR measures electrons’ momentum distribution at the transverse directions (x,y): N(px, py)=n pz|xy zn(r)* +(r) e-iP•rd r|2dpz

  7. Localization of positron and Ps Calibrated defect radius(Å)

  8. Doppler broadening S parameter S is a measure of defect property: (1) Large hole  Large S p•x  /2 (2) Large defect concentration large S Positron Annihilation Lifetime

  9. STM/AFM 10 STM/AFM 1 cm 1000 100 10 1 1000 100 10 1Å Mech TEM OM 1% OM Defect Concentration (ppm) X-ray Scattering 1000 Resolved Defect Size X-ray Scattering 100 Mech TEM Positron Spectroscopy Positron Spectroscopy 1 ppm 1Å 10 100 1000 1  10 100 1000 1 Å 10 100 1000 1  10 100 1000 Depth Depth PAS can resolve size, concentration and distribution of atomic scale defects Comparison of PAS and other techniques

  10. Magnetic coils 75G Lifetime detector attachment 22Na e+ source 50mCi ExB e+ filter Data acquisition system Moderator S.S. Detector Accelerator 0- 30kV Sample chamber A sow positron beam (0- 30 keV) for depth profile (0-10 µm) of free volume

  11. Depth and dispersion of depth

  12. Free-volume Concept in Macromolecules (polymers) Free volume is the free moving (very fast, ps-ns) open space (very small, sub-nm) inside a molecule or system. A simple expression of the free volume (Vf) can be written as the total volume (Vt) minus the “occupied volume” (V0): Vf = Vt – V0 The existence of free volume (1-10%) makes polymers as the most widely used materials in our human life today.

  13. Diagram Of Nano-scale Polymeric Film On Substrate

  14. 1. Free-volume S-parameter for different thickness of polystyrene films on Si

  15. S parameter spectrum and profilometry measurement on 80 nm PS film

  16. VEPFIT fitting result from S parameter spectrum

  17. 2. Tg of thick polystyrene film measured by conventional PAL spectroscopy

  18. Free volume distribution of thick PS film at different temperature, Spectchim Acta A, 61,1681, 2005

  19. FWHM of free volume distribution of thick PS film as a function of temperature

  20. Raw PAL spectrum measured on thin 80 nm PS film on Si

  21. o-Ps lifetime and intensity as a function of depth in 80 nm PS film

  22. Depth profile of Free-volume distribution A larger hole size and broader free-volume distribution near the surface are observed. J. Phys. Cond. Matt. 10, 10429 (1998)—a lower Tg near the polymer surface.

  23. Hole size distribution of 80 nm supported PS film at room temperature

  24. Glass transition temperature determined from o-Ps lifetime result at different depth of the supported PS film

  25. Free volume distribution of 80nm PS film at different temperature

  26. 3. FWHM of Free volume distribution of 80nm PS film as a function of temperature

  27. Free-volume thermal expansion coefficients and FWHM of distribution in an 80-nm polystyrene film.

  28. Interpretation: Tg-depth dependence (suppression) in nano-scale polymeric films: • Free-volume is distributed at a different degree as a function of the depth • Near the surface, the free-volume is distributed widest, therefore the Tg is the lowest • At interface, the free-volume is distributed next widest, Tg is also suppressed. • Loosely packing, end chains, incomplete entangling of chains, etc. lead to broad distribution.

  29. I. PAS for polymers and nano-films • PAS is a novel spectroscopic method in determining free-volume physical properties of polymers. • PAS could provide sub-nano and nano-scopic free-volume size, fraction, distribution and structures. • PAS is monitoring glass transition of polymer film as a function of depth from surface, films and interfaces. • Tg is found to be 17 K lower near the surface and 11 K lower in interface than the center of the film. • Tg suppression can be interpreted as the broadening of free-volume distribution in the surface and interfaces.

  30. II. Membrane Composites: Permselectivity (A/B) of a polymer film is a ratio of permeability PA/PB P: Permeability S: Solubility D: Diffusivity Coefficient D=A exp(-B/FFV) FFV: Fractional Free Volume P’s unit is Barrer=1.0{10-10 cm3 (STP) cm}/{cm Hg cm2 s} Polymer o o o o Molecule A o o o Molecule B o o

  31. Polyamide Composite membrane: Pervaporation

  32. polyamide composite membranes Interfacial polymerization 1 wt% TMC/ toluene solution 2 wt% aqueous TETA solution Polyamide thin-film Hydrolyzed PAN (dried at R. T.) Doping temperature: 50 oC Doping time: 1, 5,10, 30 min R. T. and 3 min

  33. Free-volume distributions of base polymers

  34. Doppler Broadening Free-volume S (free-volume) and R (pore) parameters in PA and m-PAN membranes

  35. 3-Layer structure of m-PAN from PAS

  36. SEM Cross-section image of m-PAN 402 ± 70 nm

  37. S free volume parameters for interfacial polymerized polyamide on m-PAN at different TETA doping time

  38. R (Pore) parameter of PA/m-PAN at different TETA doping time

  39. SEM skin thickness 100-500 µm 1 min 249 ± 16 nm PA 402 ± 16 nm m-PAN skin 5 min 326 ± 10 nm 10 min 203 ± 29 nm 30 min 186 ± 10 nm

  40. TETA doping time deceases PA film thickness: Both PAS and SEM data

  41. Three-layer structure of PA/m-PAN;1:PA (50-300 nm); 2:Transition skin to porous PAN (.5-4 µm); 3: porous PAN

  42. Pervaporation performance of polyamide/PAN membranes Polyamide skin layer of composite membranes reduced the permeation rate, but enhanced the selectivity of water effectively.

  43. PA layer (1): Correlations 1. PA thickness increases selectivity 2. Free-volume S decreases selectivity

  44. PA layer (2): Correlations • Transition layer thickness increases selectivity • Less relationship between S (transition) and selectivity

  45. Temperature of TETA doping (25, 50, 70 o)Temperature increases PA thickness

  46. Temp increases both layer 1 and layer 2 thickness but decreases free-volume S parameters

  47. PA layer (1) correlations • Flux (permeability) and S1 free-volume parameter follows D=A exp(-B/ffv) • Selectivity decreases as S1 and PA thickness increases

  48. Transition layer (2) correlations • Flux (permeability) and S2 free-volume parameter follows D=A exp(-B/ffv) • Selectivity decreases as S2 and PA thickness increases

  49. II. Conclusions based on PA/m-PAN of pervaporation of membrane separation 1. A 3-layer structure of the PAN membranes is determined: (1) Skin m-PAN (300-400 nm); (2) Transition larer from dense to porous PAN (2 µm) (3) Porous m-PAN 2. A 3-layer polyamide thin-film composite PAN membrane is (1) polyamide layer: a very near surface layer (50-300 nm) (2) Transition layer from dense to porous m-PAN (0.5-4 µm) (3) Porous m-PAN layer 3. Correlation between free volume S parameter and flux in the free-volume theory: Flux=A(-B/S) 4. Selectivity is mainly controlled by thickness of skin polyamide layer, and secondary affected by the transition layer. 5. Effect of free volume size and selectivity can be investigated using PAS 6. Future applications of PAS to membrane technology for RO and NF are promising.

  50. Summary • Nano-scale polymeric films: (1) Depth and interfacial structures for layers and nano-composite systems. (2) Tg-depth dependence of polymeric systems on different substrates with different interfacial interactions and UV irradiations. • Membranes and coatings: • Free-volume depth profile of membranes and coatings • Early detection of polymeric degradations. (3) Effects of free volume size and struture on membrane performances (permeselectivities)

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