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

Outline F Characterization Techniques F Examples of Applications F Conclusions F Characterization Techniques F Examples of Applications F Conclusions F Important Pore Structure Characteristics

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

Characterization Techniques F A pressurized gas extrudes liquid from pores Capillary Flow Porometry Principle  Pores are filled with a wetting liquid (g s/g > g s/l ) Capillary Flow Porometry Principle  Pores are filled with a wetting liquid (g s/g > g s/l )

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

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

Characterization Techniques F Many Characteristics computed Capillary Flow Porometry Principle Capillary Flow Porometry Principle Pore diameter and permeability Pore distribution

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

Characterization Techniques 4.6   0.5 Polycarbonate membrane 86.7   5.2 Etched stainless steel disc PMI Porometer SEM SEM Micrograph Sample Pore diameter,  m F Accuracy

Water Intrusion Porosimetry F Pressure on water is increased - Water intrudes hydrophobic pores Principle F Water is allowed to surround membranes - Water spontaneously enters all hydrophilic pore Principle F Water is allowed to surround membranes - Water spontaneously enters all hydrophilic pore

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

Water Intrusion Porosimetry F Equipment PMI Aquapore (Water Intrusion Porosimeter)

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

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

Water Vapor Transmission Analyzer Equipment The PMI Water Vapor Transmission Analyzer F Capable of detecting = cm 3 /s

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

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

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

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

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

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

Pore Diameter Measured by Flow Porometry The mean flow pore diameter F Computed from mean flow pressure The mean flow pore diameter F Computed from mean flow pressure

Pore Diameter Measured by Flow Porometry F Wide range of diameters measurable

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

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

Pore Diameter Measured by Flow Porometry  Define the measurable parameter f i : fi = [1/(4 g cos q/p i ) 4 ]x [(f w,i+1 /f d,i+1 )-(f w,i /F d,i )] p = differential pressure i & i+1 = two successive readings Pore Fraction Distribution F Fraction of pores of diameter D i = N i /   i N i Pore Fraction Distribution F Fraction of pores of diameter D i = N i /   i N i

Pore Diameter Measured by Flow Porometry F It has been shown that: N i /   i N i = fi/   i f i Pore Fraction Distribution Example of fractional pore number distribution

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

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

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

Other Characteristics Measurable by Flow Porometry F Pore diameters in various directions

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

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

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

Pore diameter F Pore diameter & volume of each part of pore measured Which diameter of pore measured?

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

Pore distribution F Maximum contribution to distribution at 0.22 microns Pore volume distribution by water intrusion porosimetry

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

Comparison with Mercury Intrusion Pore volume distribution obtained using mercury porosimetry

Comparison with Mercury Intrusion F Volume distributions are similar F Required pressure psi compared with Hg 1000 psi for water F Mercury is toxic F Volume distributions are similar F Required pressure psi compared with Hg 1000 psi for water F Mercury is toxic F Intrusion volumes similar

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

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 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)

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

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

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

Thank You