New Polymeric Materials

Chapter 25

Polymer Permeability Measurements via TGA 1,2

Brandi D. Holcomb and Harvey E. Bair

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Downloaded by UNIV OF NORTH CAROLINA on July 20, 2013 | http://pubs.acs.org Publication Date: September 29, 2005 | doi: 10.1021/bk-2005-0916.ch025

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Lucent Technologies, Bell Laboratories, 600 Mountain Avenue, Murray Hill, NJ 07974 Current address: North Carolina State University, Raleigh, NC 27607 Current address: HEB Enterprises, 17 Seminole Court, Newton, NJ 07860 2

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A patented metal capsule small enough to fit inside a commercial TGA was fashioned with an opening that was more than 200 times smaller in area than used in the standard ASTM test method for water vapor transmission of materials. Unfortunately, in the latter method the weight determinations are not made in the controlled environment and the size of the opening is recommended to be at least 3000 mm (4.65 in ) or greater. In an attempt to gain water vapor transmission rates in-situ and rapidly as a function of temperature and humidity a new thermogravimetric approach was created. 'Wet' and dry gas streams were mixed to control the humidity inside the TGA chamber to any value between 0 and 90%. In this manner a number of measurements could be made reproducibly and quickly at a variety of temperatures and humidities. Water vapor transmission measurements, WVT, were made on a variety of polymer films ranging from polyethylene terephthalate to a permeable, Goretex membrane. 2

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®

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In New Polymeric Materials; Korugic-Karasz, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

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Introduction If the amount of a new film for testing is quite small, it may be impractical to use one of the standard ÂSTM methods to track the egress of water vapor from the water rich side of a film to its drier or lower humidity side (1). Furthermore, in our work we wanted, if possible, to monitor the movement of moisture out of a tiny optoelectronic device directly. Hence, we started to think of using our TGA instrument to carry out these types of measurements. Immediately, we discovered that if one attempts to design a new container to conduct water vapor transmission (WVT) studies in a TGA environment a leakproof sealing technique must be developed to hold the film in-place since none of the ways ASTM suggested for sealing a film to a test dish appeared to work on any liquid container that was small enough to fit inside a TGA instrument. Once dus hurdle was cleared die TGA method proved to be superior in terms of time to any of the prior ASTM ways to determine the water vapor permeability of polymer films as a function of temperature and humidity. In this paper our patented WVT capsule is described and used in a TGA to determine the water permeability of polymer films (2).

Experimental Our invention relates to methods and apparatus for securing a solid film or a liquid sample for TGA or photo-dsc thermal analysis (2). In accordance with the invention a film of die polymer to be tested is positioned inside a circular lid and held in place via an o-ring that is typically a deformable material such as a rubber. The lid with film in-place is positioned atop an impermeable, metal receptacle that has a bottom and side walls as shown in Figure 1. The top edges of the side walls are bent towards the center of the receptacle. The capsule's lid is placed across the top edges of the container's side walls and the two parts are squeezed together. The lid compresses the o-ring onto the lower half of the capsule, sealing it against the bent top edges of the receptacle and the polymer film above it. In the case, of water transmission tests about 50 mg of water are sealed inside our WVT capsule. Another use for this capsule is in the confinement of volatile liquids that undergo photo-induced polymerization inside a photo-dsc. In the latter application a U V transparent polymer such as a copolymer of polyhexafluoropropylene/polytetrafluoroethylene or poly (vinylidene fluoride), PVF, can be employed. In the photo-dsc work the film


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12 Figure 1 Schematic of WVT capsule showing: (12) receptacle, (14) lid, (30) top edges of side wall, (55) polymer film, (50) liquid permeant or water and (60) o-ring.

should have a leak rate of less than about 1.5 weight percent/hr. Also, in order to provide a uniformly thick sample on the bottom of the receptacle, the inside surface of the container is coated with a material not wetted by the sample such as a flurorcarbon. An example of the use of this capsule for photocalorimetric work can be found elsewhere (3,4). The WVT capsule was assembled with a 0.05 mm thick PVF film and 67 mg of water and held isothermally at temperatures up to 80°C. The capsule's lid had a hole with a diameter of 4.21 mm. The sealed system lost 0.0001, 0.0087, 0.024, 0.083 and 0.315 wt% per 10 minutes at 23, 36, 46, 60 and 80°C, respectively. In the prior experiment the level of humidity inside the TGA was held close to 0% with nitrogen flowing at 100 cc/min. If the humidity inside the TGA is raised from 0 to 62 % the rate of water loss at 23°C is reduced by a factor of five over the dry state rate. Typically in a photo-dsc free radical polymerizations are completed in less than 10 minutes (5,6). Hence, this capsule could be used in a photo-dsc to study the reaction of aqueous solutions at temperatures approaching 80°C. Recently this capsule was used to contain a blend of an epoxy and alcohol at 100°C for an 8 minute irradiation period with only the loss of a few micrograms of the volatile blend (4). In these studies the capsules were used inside a PerkinElmer TGA7 that was equipped with a high temperature furnace. The humidity was controlled inside the TGA unit by mixing two streams of wet (>95% humidity) and dry nitrogen gas. Dynamic scans of materials of interest were performed on a power compensated Perkinelmer DSC7 in a manner described elsewhere (7).


342

Results and Discussion


A polycarbonate film (Lexan®, General Electric Co.) with a thickness of 0.00305 cm was seated in the lid of a WVT capsule. The latter had a 4.25 mm diameter hole. The capsule was sealed with 53 mg of water present. The TGA

:=0.061mg

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TIME (min)

Figure 2 Weight loss of water vapor through a polycarbonate film at 23°C (weight changes are in one hour intervals). was maintained at 23°C for the duration of the weight loss measurement while the vapor pressure difference across the PC film was held at 21.1 mm of Hg. In Figure 2 the graph of die weight change of the water in the sealed receptacle is plotted as a function of time. From the data in Figure 2 it follows that water vapor is transmitted through the amorphous Lexan polycarbonate film at a constant rate of 0.062±0.001 mg/hr or 0.45 mg/cm hr for a period of 275 minutes. Based on this water vapor transmission rate, GE Lexan polycarbonate has a water permeability at 23°C of approximately 2.2 χ 10" cm -mm/cm -sec-cmHg at STP. In 1963 Norton measured a value of 1.4 χ 10* cm (STP)mm/cm seccmHg on polycarbonate using a mass spectrometer as an analyzer (8). Permeability (P) is calculated as 2

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P = AWd/tA(p p ) r

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(1)

343 where AW is the weight change for the sealed receptacle, d is the thickness of the polymer film, t is the time, A is the area of the hole in the receptacle lid, pi is the vapor pressure in the sealed capsule, and p is the vapor pressure inside the TGA chamber. A drop of water was placed in a stainless steel WVT capsule via a syringe. Then a 0.32 mm thick film of cured GE silicone RTV-615 was fitted over a 4.25 mm diameter opening in the container's lid. The sealed receptacle was positioned atop the Pt sample cup inside the TGA chamber. The rate of water vapor transmission through die silicone film was determined to be 1.2, 9.3, 4.5 and 16.7 mg/cm hr at 23, 48, 39 and 57°C, respectively (Figure 3). Note how quickly the sample adjusts to temperature changes and steady state water loss values are obtained. The partial pressure of the water in this experiment was 21.1,52.4, 83.7 and 129.8 mmHg at 23,39,48 and 57°C, respectively. 2


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120 TIME (min)

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Figure 3 Water vapor loss through a silicone film at 23,48,39 and 57°C.

The permeability of the GE 615RTV silicone is plotted in Figure 4 as function of temperature and is shown to increase linearly with increasing temperature. Note that when temperature rosefrom23 to 48°C permeability doubled.



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In Figure 4 at room temperature Ρ equals 5.1 χ 10" g-cm/cm sec-mmHg or 6.35 χ 10" cm (STP)cm/cm sec-mm Hg. At 23°C the solubility of water in RTV615 is 0.1 wr%. Since the product of the diffusion coefficient D and the solubility coefficient, S equals P, die estimated value of D is 9.0 χ 10" cm /sec Permeability of a polymer depends not only on its chemical structure but also on its morphology, crystallinity and orientation. In an attempt to examine the effect of crystallinity on die rate of water vapor transmission though a film of polyethylene terephthalate (PET), two PET samples of were prepared. One was made amorphous by quenching and the other was annealed at 175°C for 24 minutes to induce partial crystallinity. The DSC scan from 20 to 275°C at 15°C/min of the quenched PET film is shown in Figure 5 (dash-dot line). Note that a broad glass transition occurred beginning at about 45° and ending near 75°C with the magnitude of change in Cp associated with the glass transition, ACp, equal to 0.39 Jg'^C" . Tg occurred at !4ACp or 69 °C. Above Tg and near 100°C crystallization began and proceeded to about 212°C with the evolution of 36.5 J/g. Above this temperature crystals of PET melted with the absorption of 36.5 J/g. Since the heat evolved during crystallization equaled die subsequent heat lost during melting, one can conclude that the initial PET film was amorphous. In contrast to this behavior the film that was annealed for 24 minutes at 175°C showed no sign of crystallization 7

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when heated above Tg (solid line, Figure 5). In die latter case, ACp is diminished to 0.13 J/g°C and Tg is increased to 81°C due to restrictions placed on the glass by the crystal phase (9). In this way the polymer chains are kept from attaining their liquid-like amorphous configurations and the accompanying liquid like Cp above Tg. A small group of crystals associated with the annealing melted near 185°C while the more perfect and thicker crystals melted between 212 and 256°C with an apparent heat of fusion of 36.5 J/g. Hence, crystallinity equals 36.5/122 or 30% for the annealed PET film. A 0.241 mm thick film of the amorphous PET was sealed in a WVT capsule that contained a 4.25 mm diameter hole in its lid and placed in the TGA instrument. The interior of the TGA closure was maintained near 0% relative humidity and the WVT rate was monitored at a series of temperatures ranging from 20 to about 86°C. This data is displayed in Figure 6. Note that two regimes exit above and below 47°C. The latter temperature coincides with the onset of the glass transition that was observed by DSC for the amorphous PET as shown in Figure 5. Note that the rate of water vapor movement through PET increased sharply above the glass transition temperature as indicated in Figure 6.



Below Tg water movement through the partially crystalline PET was reduced by a factor of about three from the rate that was found for the amorphous PET. For example near room temperature the WVT rate was about 0.15 and 0.05 mg/cm hr for the amorphous and semi-crystalline films, respectively. A lot of clothing and footwear that is designed for outdoor activity allows water vapor to escape from the article but at the same time repels liquid water from the membrane's outer surface. In certain situations these same kinds of materials can be used to allow electronic devices to breathe while keeping liquid water out of the hardware. One manufacturer's product line using this special type of microporous membrane is Goretex®. The supplier reports the membrane is made from a fluorocarbon material. A 5 mg sample of this membrane was scanned in a DSC as shown in Figure 7 (solid line). The material exhibited a melting endotherm with a peak at 328°C and fusion required 19.9 J/g. A second sample of polytetrafluoroethylene (PTFE), Teflon, is shown to have nearly an identical melting pattern (dash-dot line, Figure 7). The latter polymer melting peak is at 328.7°C with the absorption of 19.4 J/g of heat Note that endothermic areas under each melting peak are defined by the solid lines with circles that stretch from about 275 to 340°C for each scan. In addition, both the PTFE and membrane samples were found to undergo two first order transitions near 19 and 31°C with a heat of transition of 2



347

T(°C) Figure 7 Two DSC scans of an unknown and polytetrafluoroethylene samples at 15°C/min.

about 5 J/g. These solid-solid phase transitions are associated with the onset of internal motion in the polymer's crystal phase (10,11). From the similarities of the two material's melting temperatures and apparent heats of fusion it is concluded that die Goretex material is nearly pure PTFE. The scanning electron micrographs of the Goretex membrane in Figure 8 reveal the secret behind the product's ability to repel liquid water but allow water vapor to move through it quickly. The lower half of Figure 8 shows a PTFE structure that appears to be composed of islands of solid PTFE measuring about 10 microns across and showing nearly infinite length in the opposite direction. Between the islands, many PTFE strands are stretched out into thin fibrils. These fibrils are about 1 μη in diameter and 20 μη in length. The spacing between strands of PTFE is clearly defined to be about 2μη as shown in the upper half of Figure 8. Hence, liquid water droplets will simply sit on the hydrophobic PTFE's surface but the vapor canfreelypass through the grating created by the fibrillar PTFE structure.



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Figure 8 Scanning electron micrograph of a Goretex membrane



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Figure 9 Water vapor loss through films of a RTV silicone (upper curve) and a Goretex membrane (lower curve) at room temperature. Figure 9 demonstrates the effectiveness of the Goretex membrane in allowing water vapor to pass through its fibrillar structure as compared to the rubbery GE silicone, RTV-615. At 23°C the rate of water movement through the silicon RTV is 1.2 mg/cm hr whereas the rate through the permeable PTFE membrane is 65.7 mg/cm hr. From this data Ρ equals 1.94 χ 10" cm (STP) cm/ cm sec-cmHg for the membrane. In a separate experiment the microporous membrane was aged for 3 weeks in an accelerated dust chamber. Reportedly this treatment is roughly equivalent to 10 years of aging in a city environment such as Newark, NJ. The aging only reduced the rate of water transmission through the membrane slightly or about 7% lower than found for the imaged, as received membrane. 2

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Conclusions A new hermetically sealeable capsule was created for use as a container for making water vapor transmission rate measurements via a TGA instrument. In this way WVT data can be collected accurately and quickly as a function of temperature and humidity. Permeability data was measured not only on glassy, rubbery and semi-crystalline polymer films but also on a microporous Goretex® membrane. With a slight modification of the WVT capsule it is also possible to confine volatile reactive liquids such as used in photo-dsc work at elevated temperatures.


350

Acknowledgement th


This work is dedicated to Prof. Frank E. Karasz on the occasion of his 70 birthday for his many contributions to the field of polymer science, One of us, namely Harvey Bair, was fortunate to have Frank as his mentor when he was a research training fellow at the General Electric Research Laboratory in Schenectady, N Y during the early 1960s. The authors would also like to acknowledge helpful discussions with Dr. Lee Blyler, of Chromis Fiberoptics, on permeability issues. In addition, we thank Dr. Arturo Hale for the SEM photos.

References 1. 2. 3. 4.

5. 6. 7.

8. 9. 10. 11.

Annual Book of ASTM Standards, Standard Ε 96-94, American Society for Testing and Materials: Philadelphia, PA, 1994; Vol 04.06, pp.696-1055. Bair, Η. E.; Hale, Α.; Popielarski; S. R. US Patent 6,586,258 B1, 2003, July 1. Olsson, R. T.; Bair, H. E.; Kuck, V.; Hale A. Polymer Preprints 2000, 42 (2), 797.2 Olsson, R. T.; Bair H. E.; Kuck, V.; Hale, A. "Thermomechanical Studies of Photoinitiated Cationic Polymerization of a Cycloalipathitc Epox Resin", In Photinitiated Polymerization K . D. Belfield and J. V. Crivello, Eds.; ACS Symposium Series 2003, 847, p. 317. Bair, H. E.; Blyler, L. L. Proc. 14 NATAS Conference, September 1985, p. 392. Bair, H. E.; Blyler, L. L.; Simoff, D. A. ACS Polymeric Materials Sci.&Eng. 1993, 69, 268. Bair, H. E. "Glass Transition Measurements by DSC", In Assignment of the Glass Transition, ASTM STP, Seyler, R. J., Ed.; American Soc. For Testing and Materials, Philadelphia, PA 1991, p. 50. Norton, F., J. Appl. Polymer Sci. 1963, 7, 1649. DiMarzio, Ε. Α.; Dowell, F. J. App.Phys. 1979, 50, 6061. Bunn, C. W.; Howells, E. R. Nature 1954, 174,549. Kimmig, M . ; Strobl, G. ; Stuhn, B. Macromolecules 1994, 27,2481. th


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