1. Partnership in Technology Forum
Water Permeation Through
Polymers
Daniel Logan Howell
Russell Hallman, Jr.
Ashley Stowe

2. Outline
Permeation Overview
– Mechanism
– Terminology
– Implications
– Consequences of Measurement Inaccuracies
Current Methods
Extrans Method
– Method Overview
– Studies
Polymer Structural Changes and Implications
Conclusions
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3. Permeation Overview
Permeation is a standard measure of the quality of the membrane
h
Permeation = Solubility x Diffusion
𝑑𝑞 𝑝1 − 𝑝2
= 𝑄𝐴
𝑑𝑡 ℎ
Solubility is measure of the permeate’s ability to dissolve and form a homogenous
solution with the polymer
Desorption phase is essentially a second adsorption process
– Difference in adsorption levels is concentration gradient, the driving force for diffusion
Combination of Fick’s Law and Henry’s Law gives general permeation equation
– Concentration gradient represented by vapor pressure difference, p1-p2
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4. Permeation Overview
Permeation: penetration of a permeate through a solid; 2 step process
of adsorption and diffusion
Permeance: degree to which a material transmits another substance;
normally rate through unit area of material
Permeability: permeance normalized for thickness of the material
Transmission rate: steady flow in unit time through unit area of a
body under specific temperature and concentration gradient
Transfer flux: amount of material that passes through an area in a
given period of time; overall flux of material that will traverse the film
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5. Permeation Overview
Factors affecting permeation
– Temperature
– Pressure (for vapors)
– Concentration gradient (driving force)
– Material thickness
– Surface area
– Chemical structure and crystallinity
Generally accepted factors
Amorphous
– Moisture diffusion occurs through amorphous region
Interfacial
– Permeation is constant property of polymer in absence of
Crystalline
degradation for given permeate
Processing profile plays large role in observed permeation rates
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6. Implications of Permeation
Barrier materials are important in many industries and markets
– Pharmaceutical packaging ($54.8B world)
– Food and beverage packaging ($329B world)
– Protective Clothing ($250.1M US)
– Governmental applications ($792M US)
Polymers are required to provide adequate protection level
Protection times must be accurately calculated
– Shelf life guarantees for food and pharmaceuticals
– Protection level ratings for protective clothing
– Consequences vary for different protected materials
Current measurement methods have accuracy and repeatability
issues
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7. Polymer Selection
Selection between polymers based on multiple factors
– Barrier properties
– Tensile strength
– Hardness
– Processability
– Cost
Several polymers exist for different barrier applications
– Mylar
– Various polyethylenes (PE)
– Polypropylene
– Polystyrene
– Several others
Accurate barrier property measurement helps determine optimal
polymer
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8. Consequences of Inaccuracies
Accurate measurement of barrier properties valuable to polymer
manufacturers and customers
Inaccurate barrier property measurements can lead to several
problems for industry customers
– Inadvertent false shelf-life claims
– Unnecessary increased costs
 Suboptimal polymer selection
 Overuse of material
– Inventory obsolescence
– Disposal of non-obsolete inventory
Polymer manufacturers also face consequences
– Quality issues
– Lost sales
– Increased manufacturing cost
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9. Current Methods
Current measurement techniques and equipment are industry-
specific
– Standards set forth by groups such as ASTM, DIN, and ISO
– Equipment manufactured to test according to standards
Measurement techniques include “weight-gain” methods and
“transient measurement” methods
Weight-Gain Method
Transient Measurement Method
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10. Current Methods
Weight gain methods utilize a desiccant
– Desiccant covered by polymer in controlled atmosphere (i.e., temperature
and humidity)
– High-precision microbalance used to monitor weight as moisture permeates
the barrier
Transient methods monitor moisture changes on dry side of polymer
– Utilize sweep gas or monitor surrounding dry gas in controlled atmosphere
– Wet side of polymer contacted by vapor or liquid
Weight-Gain Transient
Advantages • Cheap, easy set • More accurate
up • Observe dynamic
changes in rate
Disadvantages • Inaccurate • Can be expensive
• Long test times • Long test times
• More accurate test method needed to meet
industry requirements
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11. Alternative Transient Technique: Extrans
Transient method employing sweep gas
and utilizing liquid on wet side of
polymer (bottom cell)
Advantages
Constant temperature operation
Low pressure differential across film
Lack of hydraulic pressure driving force
Constant contact between film and fluid Process Schematic
Highly accurate chilled-mirror hygrometer
Maintains nearly constant concentration gradient
Test
Cell
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12. Measurement Discrepancies
Mylar bag tested using Extrans technique found to have permeation
rate that did not agree with vendor report
Permeation rate (20 C) =
3.6 x 10-8 g H2O/min*in2
Vendor reports much higher
permeation = 3 x 10-7 g H2O/min*in2
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13. Liquid vs. Vapor Interface
Schroeder’s Paradox: uptake of solvent in polymer depends on
interaction with the boundary phase
– Well-known principle but not well understood
– Permeation assumed to be primarily diffusion-limited, especially for liquid interfaces
Need to determine comparability of liquid and vapor contact
permeation values
Concentration gradient (diffusion driving force) more constant for
liquid interface vs. vapor interface
Adsorption affects could play larger role in permeation rate for vapor
interfaces
Could have end-use application impact for certain polymer systems
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14. Liquid vs. Vapor Interface
LLDPE – Quick Cooled
sample put in contact
with liquid and 100%
RH vapor
Similar rates observed
No evidence of
Schroeder’s Paradox for
this system
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15. LLDPE
• Quenched LLDPE sample run using Extrans technique for several thermal
cycles
• Equilibrium permeation rate almost
immediately realized
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16. HDPE
• Slow cooled and annealed HDPE sample (more crystalline version of PE) run
using Extrans technique for several thermal cycles
• Equilibrium permeation rate approached over several
thermal cycles
• Phenomenon observation possible due to accuracy of
Extrans technology
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17. Structural Polymer Changes
• Permeation rate of a polymer changes as a result of thermal cycling
• Greater permeation rate conditioning was observed for more highly
crystalline polyethylene
• Cycling causes the crystal lattice to change, altering the diffusion path
tortuosity
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18. Structural Polymer Changes
Permeation rate changed as a function of time while exposed to
constant temperature and concentration gradient
Possible explanation is structural changes within polymer
– Swelling due to interaction between water and polymer
– Water clustering within amorphous regions or void spaces
Structural changes can affect crystallinity
– Suggests concentration-dependent permeation coefficient
– Concentration within polymer can affect diffusion rate
– Tortuosity of path changes with crystallinity
= H2O
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19. Implications of Structural Changes
Polymer permeation rates equilibrate as a function of thermal cycling
Suggests that permeation coefficient is dependent on the concentration
within the membrane
– Crystallinity changes due to swelling and clustering
– Tortuosity of diffusion path changes
Greater permeation rate conditioning was observed for more highly
crystalline polyethylenes
Thermal conditioning is reversible
– Drying polymer will remove inter-lamella water clusters
Understanding this mechanism can allow the fabrication of membranes
with precisely controlled barrier properties
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20. Conclusions
Permeation depends on many factors
– Temperature, pressure, driving force
– Crystallinity, diffusion path tortuosity
– Thermal history of polymer
– Interactions between polymer lattice structure and permeate
Extrans method is superior method that allows changes in these
properties to be dynamically observed through permeation rate
measurements
Further investigation into mechanism of thermal conditioning can lead
to better prediction of polymer barrier properties
– Use of spectroscopy to understand changes in crystallinity
Greater understanding of mechanisms affecting permeation rates can
allow for the engineering of polymers with precise barrier properties
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21. Acknowledgements
Y-12 Office of Technology Maturation
Y-12 Career Advantage Program
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