Characterization of Pore Structure of Membranes

1. Characterization of Pore Structure of Membranes Akshaya Jena and Krishna Gupta Porous Materials, Inc. 20 Dutch Mill Road, Ithaca, NY 14850

2. Outline • Characterization Techniques • Examples of Applications • Conclusions • Important Pore Structure Characteristics

3. Important Pore Structure Characteristics • The largest through pore diameter • Mean flow through pore diameter • Through pore distribution • Breakthrough pressure • Gas permeability • Liquid permeability • Water vapor transmission rate • Through pore throat diameter

4. Characterization Techniques • A pressurized gas extrudes liquid from pores Capillary Flow Porometry Principle • Pores are filled with a wetting liquid (gs/g > gs/l)

5. Characterization Techniques Capillary Flow Porometry Principle • Pressure & gas flow rates of wet & dry samples measured

6. Characterization Techniques • p = 4 g cos q (dS/dV) p = differential pressure q = contact angleg = surface tension S = pore surfaceV = volume of gas in pore area Capillary Flow Porometry Principle • Differential pressure related to pore size Work done by gas = Increase in surface free energy

7. Pore diameter and permeability Pore distribution Characterization Techniques • Many Characteristics computed Capillary Flow Porometry Principle

8. The Capillary Flow Porometer used in this study Characterization Techniques • Test execution • Data acquisition • Data storage • Data reduction Equipment • Fully automated:

9. Pore diameter, m Sample SEM Micrograph SEM PMI Porometer Etched stainless steel disc 81.7  5.2 86.7 4.1 Polycarbonate membrane 4.5  0.5 4.6  0.1 Characterization Techniques • Accuracy

10. Water Intrusion Porosimetry • Pressure on water is increased - Water intrudeshydrophobic pores Principle • Water is allowed to surround membranes - Water spontaneously enters all hydrophilic pore

11. Water Intrusion Porosimetry p = - g cos q (dS/dV) V = volume of liquid in pore • Intrusion volume gives pore volume Principle • Pressure yields pore size

12. PMI Aquapore (Water Intrusion Porosimeter) Water Intrusion Porosimetry • Equipment

13. Water Vapor Transmission Analyzer • Instrument evacuated • Vapor is introduced on one side & maintained at constant pressure Principle • Sample loaded

14. Principle of vapor transmission analyzer Water Vapor Transmission Analyzer • Increase in pressure on other side measured Principle

15. The PMI Water Vapor Transmission Analyzer Water Vapor Transmission Analyzer Equipment • Capable of detecting = 10-4 cm3/s

16. Examples of Applications Pore Diameter What is a pore diameter? • Most pore cross-sections irregular

17. Examples of Applications • D = +/- 4 g cos q/p Pore Diameter What is a pore diameter? • Definition of pore diameter, D: (dS/dV) pore= (dS/dV)circular opening of diameter, D= 4/D

18. Examples of Application • Each technique measures certain diameters of the pore Multiple diameters of Each Pore • Pore diameter varies along pore path • Each pore has many diameters

19. Pore Diameter Measured by Flow Porometry • Variations of flow rate with pressure for membranes #3 • Measured pressures and flow rates

20. Pore Diameter Measured by Flow Porometry Which diameter of pore is measured? • Flow porometry detects the most constricted pore diameter

21. Variations of flow rate with pressure for membrane #3 Pore Diameter Measured by Flow Porometry The largest pore diameter (Bubble Point) • Computed from pressure to start flow through wet sample.

22. Pore Diameter Measured by Flow Porometry The mean flow pore diameter • Computed from mean flow pressure

23. Pore Diameter Measured by Flow Porometry • Wide range of diameters measurable

24. Pore Diameter Measured by Flow Porometry Pore size distribution (Flow Distribution) • Distribution function, f: f = -d(Fw/Fd)x100)/dD

25. Pore Diameter Measured by Flow Porometry • Narrow bimodal distribution • Most of the pores: 5 to 11 mm constricted diameter Pore size distribution (Flow Distribution) • Area under the curve gives percentage flow

26. Pore Diameter Measured by Flow Porometry • Define the measurable parameter fi:fi = [1/(4 g cos q/pi)4]x [(fw,i+1/fd,i+1)-(fw,i/Fd,i)]p = differential pressurei & i+1 = two successive readings Pore Fraction Distribution • Fraction of pores of diameter Di = Ni/iNi

27. Example of fractional pore number distribution Pore Diameter Measured by Flow Porometry • It has been shown that:Ni/iNi= fi/ ifi Pore Fraction Distribution

28. Other Characteristics Measurable by Flow Porosmetry • In any desired unit: Darcy, Fazier, Gurley and Rayle Liquid permeability • Computed from liquid flow rate through sample Gas Permeability • Computed from gas flow rate through dry sample

29. Other Characteristics Measurable by Flow Porosmetry • Cyclic compression • Temperature • Chemical environment Effects of Service variables on pore structure • Compressive stress

30. Pore size of separator determined using KOH solution Other Characteristics Measurable by Flow Porometry • Sample orientation (x,y & z directions)

31. Other Characteristics Measurable by Flow Porometry • Pore diameters in various directions

32. Other Characteristics Measurable by Flow Porometry • In-situ pore structure of individual layers of a layered or graded material

33. Cumulative pore volume & pore diameter of a hydrophobic membrane determined in the PMI Water Intrusion Porosimeter Water Intrusion Porosimetry Pore Volume

34. Water Intrusion Porosimetry • Pressure required was only about 1000 psi for pore diameters down to about 0.01 microns. Pore Volume • Water rater than mercury was used for intrusion

35. Pore diameter • Pore diameter & volume of each part of pore measured Which diameter of pore measured?

36. Pore distribution • Area under the curve is the volume of pores • Bimodal board distribution • Distribution function, F:F = -[dV/ d log D]

37. Pore volume distribution by water intrusion porosimetry Pore distribution • Maximum contribution to distribution at 0.22 microns

38. Cumulative pore volume measured in mercury intrusion porosimetry Comparison with Mercury Intrusion

39. Pore volume distribution obtained using mercury porosimetry Comparison with Mercury Intrusion

40. Comparison with Mercury Intrusion • Volume distributions are similar • Required pressure 20000 psi compared with Hg 1000 psi for water • Mercury is toxic • Intrusion volumes similar

41. Change of pressure on the outlet side of two samples of the membrane in the PMI Water Vapor Transmission Analyzer Water Vapor Transmission Rate • Vapor transmission

42. Water Vapor Transmission Rate n = number of moles of vapor transferred across the membrane F= vapor transmission rate through the sample per unit time in volume of gas at STP (dp/dt)  (dn/dt)F  (dn/dt)F  (dp/dt)

43. Water Vapor Transmission Rate • Incubation period - Transmission rate zero • Small transient zone • Pressure increases with a decreasing rate[F (pi - p)] • The variation of pressure with time is almost sigmoial

44. Conclusion • Application of these techniques for characterization of membranes have been described with examples. • Principle of three characterization techniques, Capillary Flow Porometry, Water Intrusion Porosimetry & Water vapor transmission Analyzer have been explained.

45. Conclusions • The techniques: • Capillary Flow Porometry • Water Intrusion Porosimetry • Water Vapor transmission Analyzer are appropriate for pore structure characterization of membranes. • Capability of each technique has been discussed.

46. Thank You