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Surface modification of microporous polyurethane membrane with poly(ethylene glycol) to develop a novel membrane
Journal of Membrane Science 274 (2006) 150–158
Surface modification of microporous polyurethane membrane with poly(ethylene glycol) to develop a novel membrane Kuitian Tan, S. Kay Obendorf ∗ Department of Textiles and Apparel, Cornell University, Ithaca, NY 14853, USA Received 17 May 2005; received in revised form 29 July 2005; accepted 4 August 2005 Available online 19 September 2005
Abstract To develop a novel microporous membrane that responds to moisture/liquid content for use in protective clothing, the surface of microporous polyurethane membrane was modified by graft polymerization with different molecular weights of poly(ethylene glycol) (PEG). Surface grafting was confirmed, and appropriate grafting time and temperature were determined. The hydrophilicity of microporous polyurethane membranes was improved after surface modification with PEG. Both the surface pore sizes of the PU membranes and the constricted parts of through pore sizes of the bulk PU membranes were reduced, as revealed by the pore size distribution using image analysis and capillary flow porometer, respectively. The reduced pore sizes are expected to enhance the barrier properties by reducing the possibility of harmful particles and liquid borne pathogen penetration. Water vapor transmission rate (WVTR) measurements indicated that the pores of the modified PU membranes were responsive to the moisture content. © 2005 Elsevier B.V. All rights reserved. Keywords: Microporous polyurethane membrane; Poly(ethylene glycol); Surface modification; Image analysis; Pore size distribution
1. Introduction Surgical gowns and drapes are used as barriers to protect patients and to help healthcare professionals reduce the risk of occupational exposure to bloodborne pathogens [1]. Surgical gowns currently available provide protection, but they are known to be uncomfortable due to the low water vapor transmission rate (WVTR) [2,3]. It was estimated that a resting body produces about 250 g of moisture per square meter of body surface in 24 h, while an active person in sports would produce moisture (perspiration) about 400 g per square meter of body surface, or even more over a 24 h period [3]. To combine the liquid barrier property with high WVTR, Gore designed a microporous PTFE membrane, which allows the vapor to pass through the tiny holes while being resistant to the strikethrough of water droplets [4]. The water repellent mechanism of microporous membrane is based on the size exclusion mechanism since the average size of a water droplet is around 100 m, while the water vapor from
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0376-7388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2005.08.004
perspiration is about 0.0004 m. This allows the transmission of moisture vapor away from the body, which enhances the thermal comfort of the wearer. Although a microporous membrane has an excellent WVTR, the water-entry pressure is very low and capillary wicking of the challenging liquids cannot be avoided totally. Studies show a direct correlation between wetting and microorganism penetration [5]. However, it is possible that a new microporous membrane can be developed in order to further optimize the barrier protection with little or no sacrifice of its higher WVTR property. Practical application of polymeric membranes has been dramatically extended by membrane fabrications and post-casting modifications that control the pore size and functionalize the membrane surfaces. A variety of membrane modification methods such as plasma treatment, wet chemical treatment and corona discharge have been used. A convenient way for such modification is to graft monomers/macromers with functional groups onto the membrane surface to form a permanent covalent bonding with desired properties [6–10]. In this study, microporous polyurethane membranes were used as the host membranes with potential applications as moisture responsive barrier membranes in protective clothing for medical and chemical workers. To balance the comfort and pro-
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tection, a new membrane with pores that are responsive to moisture/liquid content was developed by grafting different molecular weights of poly(ethylene glycol) onto the polyurethane membrane. Porous membranes have diverse, complex and irregular structures with a wide range of pore sizes that are crucial to both liquid and particle penetration but are difficult to quantitatively characterize [11,12]. Therefore, a combination of techniques was employed to characterize the heterogeneous membrane structures and the structure–property–performance relationships. 2. Experimental 2.1. Materials Microporous polyurethane (PU) membranes were obtained from Porvair Com., Norfolk, UK. These membranes have different surface morphologies on each side (Fig. 1). Monolithic PU membranes were solution cast in our lab using commercial polyurethane pellets (2103-80AE, Dow Chemical Com., Midland, MI), and were treated using the same method only for the purpose of comparison with the surface morphology of modified microporous PU membranes. Hexamethylene diisocyanate (HMDI), poly(ethylene glycol) (PEG) with a range of molecular weights (600–20,000) and tin(II) 2-ethylhexanoate were purchased from Sigma–Aldrich Com., St. Louis, MO. Toluene (Mallinckrodt Baker Inc., Phillipsburg, NJ) was dried ˚ molecular sieves (EM Science, Gibbstown, NJ) for over 4 A 48 h before use. Polyurethane membranes were extracted using ethanol twice each time 4 h and then extracted with toluene for 4 h to remove any chemical residues left on the membrane surface. Extracted membranes were first air dried under the fume hood and then dried under vacuum at 50 ◦ C overnight. PEG was vacuum dried at 50 ◦ C overnight before use.
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three swatches (2 cm × 2 cm) of polyurethane membranes were immersed into 100 ml toluene containing 5% (v/v) HMDI and 0.25% (v/v) tin(II). The PU membrane surfaces were functionalized with HMDI at 70 ◦ C for 1 h. Then, the nascent PU–HMDI membranes were extracted three times, 4 h each in toluene to remove unreacted reagents. One swatch was taken out to characterize the effect of HMDI modifications. In the second step, 10% (w/v) different molecular weights of PEG were grafted onto the membrane surface by the reaction of hydroxyl groups with the isocyanate end groups of HMDI in toluene solution at 45 ◦ C for 24 h. In preliminary experimentation at 45 ◦ C, we observed homogeneous and clear solutions upon stirring. Lower molecular weight PEG dissolved very quickly, while higher molecular weight PEG needed longer time to dissolve. Also, at this temperature, the grafting reaction was easier to handle than at higher temperatures. A similar extraction process as in the first step was used to remove unreacted reagents. All the membranes were vacuum dried at 45 ◦ C overnight and weighed before further characterization. The degree of grafting (Dg ) was calculated as Dg (%) =
Wg − Wo × 100 Wo
(1)
where Wg is the extracted, vacuum dried membrane weight after grafting with PEG, Wo is the original membrane weight before modification. Dg depends upon both the number of reacted sites, Nrs , and the molecular weights of PEG grafted. Assuming that all the grafted HMDI end groups reacted with the end group of PEG, Nrs can be estimated by Nrs =
Wg − W o MPEG + MHMDI
(2)
where Wg and Wo are as defined above, MPEG and MHMDI are the molecular weight of grafted PEG and HMDI, respectively.
2.2. Procedures of surface grafting
2.3. Characterization
A two-step grafting procedure was used similar to that of Han et al. [13] and Ukpabi and Obendorf [14]. Both reactions were performed under nitrogen purge, using tin(II) as the catalyst and toluene as the solvent and swelling agent. In the first step,
2.3.1. Spectral analysis Attenuated total reflectance mode Fourier Transform Infrared Spectroscopy (ATR–FT-IR, MAGNA-IR 560 spectrometer, Thermo Nicolet, Madison, WI) was used to study the chemi-
Fig. 1. Surface morphologies of the microporous control PU membrane.
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cal compositions of PU membrane surfaces, both qualitatively confirming the grafting of isocyanate groups and quantitatively analyzing the degree of grafting at various reaction conditions. ATR spectra were collected in the frequency region of 3800–1000 cm−1 at a 4 cm−1 resolution. To obtain a high signal/noise ratio, 128 scans were performed for each sample, and three replicates were used. 2.3.2. Measurement of contact angle and surface free energy Surface free energy with its polar and dispersive portions of the PU membrane was determined using the sessile drop method with contact angles (cos θ) measured by the contact angle analyzer (CAA2, IMASS Inc., Accord, MA). Deionized and distilled water, ethylene glycol, and formamide were used as the probe liquids, with surface energy values obtained from the literature [15]. Before each measurement, all samples were conditioned overnight at 21 ± 1 ◦ C and a relative humidity of 65 ± 2%. For each measurement, three to six random locations of each sample were measured, and average values were reported. To measure the polar and dispersive surface free energy, the Owens–Wendt method using the extended Fowkes’ equation was employed [16]: p 1/2 γLV γL 1/2 1/2 p (1 + cos θ) = (γSd ) + (γS ) (3) 1/2 d γLd 2(γ ) L
p where γS and γSd are the polar and dispersive surface free energy, p γ LV , γLd and γL are the surface tensions of the total, dispersive
and polar parts of the probe liquid, respectively. 2.3.3. Membrane morphology Surface morphologies of PU membranes were examined by scanning electron microscopy (SEM, Cambridge/Leica Stereoscan 440, Leica Cambridge Ltd., Cambridge, UK) after sputter coating with Au–Pd for 30 s. 2.3.4. Pore size distribution analysis Surface pore size distributions of PU membranes before and after modification were analyzed by image analysis of SEM micrographs using the public domain software ImageJ 1.32 (National Institute of Health, http://www.rsb.info.nih.gov/ij). Ten images were taken across at least three replicates for each reaction condition; and for the same specimen, images were taken from random locations. Image analysis of the SEM micrographs only provides information about the surface porosity. The bulk membrane structure is known to be quite complicated due to the membrane casting method and membrane formation mechanism. As described by Jena and Gupta [17], there are basically three kinds of pores in a porous material: closed pore, blind pore, and through pore. The open end of blind pores and through pores can be detected by surface techniques, such as SEM, and the mercury intrusion technique, while closed pores can be detected by none of these techniques. Since through pores permit fluid flow, they can be detected by extrusion porosimetry and flow porometry. To under-
stand the bulk membrane structure, the constricted part of the through pore size distributions of the bulk PU membranes before and after modifications were determined with the capillary flow porometer (Model CFP1500AEX, Porous Materials Inc., Ithaca, NY), which gives the constricted part of the through pore diameters in the 0.013–500 m range. The analysis is based upon a three-curve graph: dry curve, wet curve and half-dry curve [17]. To get the dry curve, a dry PU membrane was put into the test chamber, gas pressure was increased on one side of the sample, and the flow rate and gas pressure were measured. To generate the wet curve, the same PU membrane was saturated with a wetting liquid with known surface tension, gas pressure was increased on one side of the sample. At a certain pressure, the largest pore was emptied and the gas flow started, which gave the bubble point pressure. On further increase of the pressure, smaller pores were emptied and gas flow increased. In order to compute the mean flow pore diameter, the half-dry curve was first computed to yield half of the flow rate through dry curve at a differential pressure. 2.3.5. Water vapor transmission rate (WVTR) Water vapor transmission rates of PU membranes before and after modifications were measured using the upright wet cup method as described by ASTM E96 [18], with a reduced air gap (6 mm) and membrane area (D = 3 mm) according to Hu et al. [19]. Together with the control and modified membranes, the WVTR of an open cover wet cup was also measured for comparison to exclude any environmental instability. 2.3.6. Mechanical properties Mechanical properties were measured according to ASTM D882 [20] using the universal testing machine (Instron 5566, Instron Com., Canton, MA). All samples were cut to the standard shape and conditioned overnight in the conditioning room (21 ± 1 ◦ C, relative humidity of 65 ± 2%) before testing. For each test, five samples were used, and average values with standard deviations were reported for modulus, tensile strength, tensile strain and energy at break, respectively. 2.3.7. Statistical analyses Statistical analyses were performed to determine the effect of surface modification with different molecular weights of PEG on PU membranes. Two-way analysis of variance (ANOVA) was performed to analyze the contact angle measurements, and one-way ANOVA was performed using contrast statements to construct an F test for the null hypothesis that different treatments have no effect on the mechanical properties. Both analyses were conducted at a confidence level of 95% using the SAS® system (SAS Institute Inc., Cary, NC). 3. Results and discussions 3.1. Spectral characterization of surface chemical compositions The extent of reaction with HMDI was analyzed by ATR–FTIR. After reaction with HMDI, the characteristic peak of iso-
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Fig. 3. Effect of temperature on the extent of isocyanate groups grafted onto the PU membrane. Fig. 2. ATR–FT-IR spectra of PU membrane surfaces before and after each step of modification (PU control, PU–HMDI, and PU–PEG).
ing decreases. This could be due to self condensation of the isocyanate groups.
cyanate group presents at the wavenumber of 2250 cm−1 , which indicates that the surface of the PU membrane was successfully functionalized with isocyanate groups, as shown in Fig. 2. Since functionalization of the PU membrane surface with isocyanate groups is the first step of the surface grafting, it is also the key step to get the maximum reactive isocyanate groups onto the membrane surface for the subsequent grafting with PEG. Therefore, it is crucial to determine the appropriate reaction conditions. To evaluate the effects of grafting temperatures, PU membranes were reacted with fixed mole ratio of HMDI at different temperatures (30–80 ◦ C) for 60 min in the presence of tin(II). The relative concentration of isocyanate groups grafted onto the surfaces of the PU membranes was estimated by the ratio of the peak intensities in an absorbance spectrum: A2250 Cisocyanate ∝ (4) A1600
3.2. Grafting yield of PU membrane
where Cisocyanate is the relative concentration of isocyanate groups grafted onto the PU membranes, A2250 and A1600 are the absorption bands of isocyanate groups and aromatic groups at the wavelength of 2250 and 1600 cm−1 , respectively. Since the aromatic band around 1600 cm−1 was relatively inert during the reaction, it was used for the normalization. The grafting of the isocyanate groups increases with the increase in temperature in our experimental range, as shown in Fig. 3. Although the grafting extent is higher at 80 ◦ C, the mechanical properties of the PU membrane were observed visually to be deteriorated. In addition, higher reaction temperature will possibly result in more side reactions. Therefore, 70 ◦ C was selected as the grafting temperature for the functionalization of PU membrane surfaces with HMDI. We also investigated the effects of grafting time on the extent of isocyanate groups grafted onto PU membrane surfaces. As shown in Fig. 4, the extent of grafted isocyanate groups increases with the increase in reaction time, reaching a maximum at 60 min. At reaction times longer than 60 min, the extent of graft-
After the PU membrane surfaces were functionalized by HMDI at 70 ◦ C for 60 min and treated as described in Section 2.2, different molecular weights of PEG were grafted onto PU membrane surfaces by reacting with the grafted isocyanate groups at 45 ◦ C for 24 h under N2 . As shown in Fig. 5, with the increase in PEG molecular weight, the degree of grafting (Dg ) increases gradually, and then levels off at higher molecular weights. This indicates that the number of reacted sites on the HMDI modified PU membrane surfaces decreases with the increase in the molecular weight of PEG. This behavior was expected since with the increase in PEG molecular weight, the possibility of masking the reactive sites increases due to the steric effect of entanglement and the increased possibility of crosslinking. Here, the weight increase of the PEG modified PU membrane was mainly attributed to the surface grafting yield. However, it is quite possible that physical adsorption, chain entanglement and crosslinking can also occur, especially at higher PEG molecular
Fig. 4. Effect of reaction time on the extent of isocyanate groups grafted onto the PU membrane at 70 ◦ C.
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Fig. 5. Effect of PEG molecular weights on degree of grafting and the amount of reacted sites on the PU membrane surface. The dashed arrow defines the most suitable molecular weight of PEG at which gives both the highest grafting yield and the most reacted sites.
Fig. 6. Surface free energy changes of the PU membrane after modified with different molecular weights of PEG. Table 2 ANOVA results for the fixed effect of contact angle measurements
weight, which can also attribute to the weight increase. Fig. 5 also suggests that the most suitable molecular weight of PEG with higher grafting yield is in the range of 2500–3500. 3.3. Contact angle and membrane surface free energy Contact angles were measured before and after modifications using three probe liquids to evaluate the changes in the hydrophilicity of microporous PU membrane surfaces due to grafting, and results are listed in Table 1. Both surface roughness and surface chemistry are known to impact the wettability of a surface. The two surfaces of the microporous PU membrane used in this study have quite different morphologies, as shown in Fig. 1. A direct correlation between the surface morphology and contact angles is observed with side 2 of the membrane having a lower contact angle than side 1. To analyze the influence of different factors on the measured contact angles, a two-way ANOVA analysis was performed, and results are given in Table 2. PEG molecular weights, surface morphologies of the membrane, and the interaction of these two factors, all significantly affect the contact angles. Fig. 6 shows the surface free energy of the PU membranes modified with different molecular weights of PEG. The addition of PEG onto the PU membrane surface increases the hydrophilicity over that of the control PU membrane. All PU membranes after PEG modifications have higher surface energies than the
Effect
# Degrees of freedom
F
Pr > F
Side Molecular weight Liquid Side × molecular weight
1 5 2 5
293.81 214.76 161.02 175.81
<0.0001 <0.0001 <0.0001 <0.0001
control membrane with the highest value observed for PEG1500 modified PU membrane. At PEG molecular weights higher than 1500, surface free energy is lower. This trend was observed for all three probe liquids during the experiment although it is still not clear whether this is due to the changes in surface roughness, or the differences in surface chemistry, or both. No allowance was made for any differences in the time related PEG binding efficiency. 3.4. Morphology analysis of the PU membrane SEM micrographs in Fig. 7 illustrate the surface morphology changes of microporous PU membranes before and after each step of treatment. Monolithic PU membrane micrographs under the same treatments are also shown for comparison. Both the microporous and monolithic PU membrane surfaces appear to be smoother before modification. After the first step of reaction with HMDI, both the membrane surfaces become quite
Table 1 Contact angles of differently treated PU membranes using water (W), formamide (F), and ethylene glycol (E) Molecular weight
Contact angle of side 1 θw
0 600 1500 4600 8000 20000
(◦ )
120.6 74.3 83.9 88.3 91.4 100.2
Contact angle of side 2 θF
± ± ± ± ± ±
0.9 1.3 3.9 2.0 1.8 2.9
(◦ )
116.6 64.2 73.6 81.2 85.4 90.1
θE ± ± ± ± ± ±
0.9 2.9 2.7 5.0 1.5 6.1
(◦ )
113.0 61.2 68.1 77.5 83.3 84.3
± ± ± ± ± ±
0.9 1.6 2.6 0.8 1.4 0.8
θ w (◦ )
θ F (◦ )
θ E (◦ )
82.6 ± 1.6 71.8 ± 3.3 77.1 ± 1.9 79.2 ± 2.5 81.3 ± 1.3 81.5 ± 0.8
72.5 ± 0.3 60.7 ± 2.8 66.9 ± 1.7 67.7 ± 0.8 69.8 ± 0.4 74.7 ± 1.8
69.9 ± 3.5 58.6 ± 1.6 66.8 ± 2.2 65.2 ± 0.1 67.0 ± 0.5 69.9 ± 3.0
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Fig. 7. SEM micrographs for the control and modified polyurethane membranes. (a), (c) and (e) are the microporous PU membranes.
rough; these changes in surface roughness were interpreted by Han et al. [13] as adduct formation during the reaction. After the second step of reaction with PEG1500, the membrane surfaces become smoother with finer textures. Similar changes also were observed for modifications with different molecular weights of PEG. Although the exact mechanism of what happened to surface morphology during the two-step grafting is not clear, changes of the surface roughness certainly affected the membrane wettability and related properties. The rougher PU membranes modified by HMDI had the lowest contact angles (62–65◦ compared to 72–74◦ after reaction with PEG1500, and 83–120◦ for the control).
3.5. Membrane pore statistics analysis Since the structures of most porous media are quite irregular and complicated, a combination of characterization techniques was employed in this study. SEM image analysis was used to characterize the surface pore size changes, and flow porometry was used to characterize the constricted parts of the through pore sizes of the bulk PU membranes [17]. SEM micrographs were converted into well-contrasted binary images using a threshold that was appropriate for the contrast of each micrograph. The membrane is represented as a two-phase texture, i.e. the polymeric matrix and pores, as shown
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Fig. 8. Illustration of SEM images before and after binary segmentation.
Fig. 9. Image analysis of pore size distribution of the PU membrane before and after surface modification with PEG1500.
in Fig. 8. Black objects in the binary image are considered as estimates of the cross section of the pores, as appear in the SEM micrographs. Average statistics of pore size distribution before and after modifications were derived by averaging the results of 10 image analyses. Fig. 9 shows the pore size distribution before and after the surface modification with PEG1500. The surface modification greatly reduces both the polydispersity of the pore sizes and the number of larger surface pores. Flow porometry has been used to study the pore structure of the bulk materials. The liquid flow rate through the control PU membrane versus differential pressure was measured, as shown in Fig. 10. The differential pressure at which the wet curve intersects the half-dry curve is used to compute the mean flow pore diameter of the constricted part of the through pores. Fig. 11 shows the through pore size distribution of the control PU membrane derived from the data in Fig. 10 using flow distribution equation [17]. Compared with the image analysis result in Fig. 8(a), the through pore sizes from flow porometry are much smaller than those of the open pores on the membrane surface determined by image analysis of SEM micrographs. This indicates that most of the through-pores for liquid flow are not cylindrical, but have quite constricted paths, i.e. flow porometry measures the constricted part of the though pore sizes of the bulk PU membrane. However, for the modified PU membranes, we could generate only the dry curve by flow porometer. This probably indicates
that either all of the constricted parts of through pores were closed due to the swelling of the grafted hydrophilic PEG layer when saturated with the wetting liquid, or the pore sizes were greatly reduced with the result being that the pressure necessary to expel the wetting liquid was beyond the instrumental range and the pore sizes were smaller than 13 nm, which is the detection limit of the flow porometer. Results from image analysis and flow porometry analysis indicate that the grafting of PEG onto the PU membrane occurred
Fig. 10. Flow rates as functions of differential pressure through control PU membrane.
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Fig. 11. Constricted part of through pore size distribution of control PU membrane. Table 3 Physical properties of PU membranes related to WVTR Mw of PEG
Thickness (m)
Response time (min)
WVTR (g/m2 /24 h)
600 1500 4600
50.0 ± 0.1 52.5 ± 0.1 52.5 ± 0.1
0–60 60–120 120–180
630.5 ± 8.1 654.7 ± 3.3 693.6 ± 4.5
not only on the membrane surface, but also within the pores on the pore surfaces. The reduction in the pore sizes can not only restrict the liquid penetration, but also prevent the pathogen penetration according to ASTM F1671 [21], where the size of the pathogen surrogate (Phi-X 174) used is 27 nm, which is much larger than 13 nm, thus enhancing the membrane barrier properties. 3.6. Property analysis 3.6.1. WVTR analysis In practical application, a membrane needs to be a barrier to bloodborne pathogens, or to challenging liquids and particles, while remaining permeable to the water vapor. WVTR is used to evaluate the material performance relative to the wearer comfort. Fig. 12 shows the water weight loss as a function of water vapor transmission time for the control PU membrane and PU membrane modified with PEG1500. There is no difference in WVTR between the modified and the control membrane for the first 180 min. However, after 180 min, the control membrane transports moisture from the wet cup faster than the modified
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Fig. 12. Weight loss of the PEG1500 modified PU membrane as a function of time. The inlay is a partial enlargement of the circled part.
membrane. This is consistent with prolonged exposure to the high humidity causing the modified membranes to swell, further reducing the pore sizes of the membrane. Averaged over 1440 min, the modified membranes had WVTR in the range of 623–694 g/m2 /24 h, while the control membranes were in the range of 802–892 g/m2 /24 h. During the experiment, we also observed that the response time of the modified PU membrane to the moisture was dependent on the molecular weights of PEG, as shown in Table 3. When the membrane was modified with PEG600, the control membrane and modified membrane have different WVTR from the very beginning, i.e. the response time is in the 0–60 min range since we have a 60 min data collecting interval; when modified with PEG1500, the reduction of WVTR is observed at a 60–120 min range as compared to 180–240 min for PEG4600. This response behavior can be macroscopically attributed to swelling of the hydrophilic portion of the membrane as water is adsorbed resulting in the reduction in the pore size. Differences between grafting with PEG600, 1500 and 4600 can be attributed to the mobility of different chain lengths of PEG or possible differences in the add-ons of PEG. The reduction in the pore size with increased moisture is expected to increase the protection from bloodborne pathogens, challenging liquids and harmful particles, although the response time might be a little longer than that might be desired in the application. 3.6.2. Mechanical properties of the membranes Modulus, tensile strength, tensile strain and energy at break are presented in Table 4. From the results, PEG1500 modified
Table 4 Mechanical properties of PU membranes Mw of PEG
Modulus (MPa)
Tensile strength (MPa)
Tensile strain (%)
Energy at break (J)
0 (control) 600 1500 4600 8000 20000
4.84 ± 0.39 4.93 ± 0.43 6.01 ± 0.21 4.99 ± 0.37 4.24 ± 0.19 3.71 ± 0.53
12.60 ± 4.07 15.86 ± 2.36 23.19 ± 2.27 18.08 ± 3.04 13.63 ± 1.19 10.99 ± 1.94
331.67 ± 47.54 433.25 ± 45.41 435.23 ± 31.48 425.00 ± 32.69 411.20 ± 30.87 361.60 ± 37.69
0.29 ± 0.16 0.40 ± 0.08 0.66 ± 0.13 0.48 ± 0.10 0.43 ± 0.06 0.30 ± 0.07
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Table 5 Mechanical property contrast of PEG1500 with the control PU membrane Property
# Degrees of freedom
F
Pr > F
Modulus Tensile strength Tensile strain Energy at break
1 1 1 1
21.85 14.13 6.34 8.52
<0.0001 <0.0001 0.0003 <0.0001
membranes give the highest mean mechanical properties among all other treatments using different molecular weights of PEG. To determine whether some of the surface modifications significantly influenced the mechanical properties, one-way ANOVA was performed using contrast statements to give the closer evaluation. As expected, statistical analysis shows that PEG1500 modified PU membranes do significantly increase the mechanical properties of the original PU membranes, as shown in Table 5. This might be due to the higher grafting yield of PEG on the PU membrane surface, which reduced the pore sizes and increased the mechanical properties of the membrane. 4. Conclusions Protection and comfort are two competing performance characteristics for protective clothing. In this study, novel moisture responsive membranes were developed by surface grafting of different molecular weights of PEG onto microporous PU membranes. ATR–FT-IR and SEM characterizations confirmed the graft polymerization on the membrane surface. Surface hydrophilicity of the PU membrane was enhanced by the grafting of the hydrophilic PEG layer. Results from image analysis and flow porometry showed that the PEG grafting occurred not only on the membrane surface, but also within the pores on the pore surfaces, thus reducing the pore sizes which are expected to reduce the penetration of challenge liquids and bloodborne pathogens. Results from the flow porometry analysis also indicated that after the surface modification, the membrane barrier properties were greatly enhanced by reducing the constricted part of the through pores to be smaller than the size of the pathogen surrogate which is 27 nm. WVTR tests showed that the modified membranes were responsive to moisture stimuli; and both the response time and the final water vapor transmission rates were dependent on the molecular weights of the grafted PEG. Based upon the grafting yield and the mechanical properties analysis, this study also suggests that in order to get the better modification results, the molecular weight of PEG should lie between 1500 and 3500. Acknowledgements This research was supported in part by the Cornell University Human Ecology College Grant and by the National Textile Center (Project C05-CR01). The authors also wish to thank the Cornell Center for Materials Research (CCMR) for the use of
the electron microscope and Porous Materials Inc. (PMI), Ithaca, NY, for the flow porosity measurements. References [1] H. Laufman, N.L. Belkin, K.K. Meyer, A critical review of a century’s progress in surgical apparel: How far have we come? J. Am. Coll. Surg. 191 (2000) 554. [2] P.P. Shadduck, D.S. Tyler, H.K. Lyerly, M.W. Sebastian, C. Parnitano, K.T. Fitzpatrick, Commercially available surgical gowns do not prevent penetration by HIV-1, Surg. Forum 41 (1990) 77. [3] J. Martz, Protective healthcare garments, Nonwovens Industry 25 (1994) 30. [4] R.W. Gore, Process for producing porous products, US Patent 3,953,566 (1976). [5] J.A. Moylan, B.V. Kennedy, The importance of gown and drape barriers in the penetration of wound infection, Surg. Gynecol. Obstet. 151 (1980) 465. [6] F. Nie, Z. Xu, Y. Qian, W. Jian, L. Wan, Surface modification of poly(acrylonitrile-co-maleic acid) membranes by the immobilization of poly(ethylene glycol), J. Membr. Sci. 235 (2004) 147. [7] J.X. Tang, N.Y. He, M.J. Tan, Q.G. He, H. Chen, A novel substrate for in situ synthesis of oligonucleotide: plasma-treated polypropylene microporous membrane, Colloids Surf. A: Physicochem. Eng. Aspects 242 (2004) 53. [8] M. Taniguchi, G. Belfort, Low protein fouling synthetic membranes by UV-assisted surface grafting modification: varying monomer type, J. Membr. Sci. 231 (2004) 147. [9] A.M. Mika, R.F. Childs, J.M. Dickson, B.E. McCarry, D.R. Gagnon, Porous, polyelectrolyte-filled membranes: effect of crosslinking on flux and separation, J. Membr. Sci. 135 (1997) 81. [10] Z. Liu, Z. Xu, J. Wang, Q. Yang, J. Wu, P. Seta, Surface modification of microporous polypropylene membranes by the grafting of poly(gammastearyl-l-glutamate), Eur. Polym. J. 39 (2003) 2291. [11] F.A.L. Dullien, V.K. Batra, Determination of the structure of porous media, Ind. Eng. Chem. 62 (1970) 25. [12] C. Charcosset, A. Cherfi, J.C. Bernengo, Characterization of microporous membrane morphology using confocal scanning laser microscopy, Chem. Eng. Sci. 55 (2000) 5351. [13] D.K. Han, K.D. Park, K.D. Han, S.Y. Jeong, Y.H. Kim, Preparation and surface characterization of PEO-grafted and heparin-immobilized polyurethanes, J. Biomed. Mater. Res. 23 (1989) 87. [14] P.O. Ukpabi, S.K. Obendorf, Polyurethane membranes for surgical gown applications, in: C.N. Nelson, N.W. Henry (Eds.), Performance of Protective Clothing: Issues and Priorities for the 21st Century: Seventh Volume, ASTM STP 1386, American Society for Testing and Materials, West Conshohocken, PA, 2000, pp. 190–199. [15] C.J. Van Oss, Interfacial Forces in Aqueous Media, Marcel Dekker, New York, 1994. [16] D. Owens, R. Wendt, Estimation of surface free energy of polymers, J. Appl. Polym. Sci. 13 (1969) 1741. [17] A. Jena, K. Gupta, Flow Porometry: What Can Flow Porometry Do for Us? Porous Materials, Inc., Ithaca, NY, USA, 2002. [18] ASTM E96-00e1: Standard test methods for water vapor transmission of materials. [19] Y. Hu, V. Topolkaraev, A. Hiltner, E. Baer, Measurement of water vapor transmission rate in highly permeable films J. Appl. Polym. Sci. 81 (2001) 1624. [20] ASTM D882-02: Test method for tensile properties of thin plastic sheeting. [21] ASTM F1671-03: Standard test method for resistance of materials used in protective clothing to penetration by bloodborne pathogens using PhiX174 bacteriophage penetration as a tests system.
Surface modification of microporous polyurethane membrane with poly(ethylene glycol) to develop a novel membrane Kuitian Tan, S. Kay Obendorf ∗ Department of Textiles and Apparel, Cornell University, Ithaca, NY 14853, USA Received 17 May 2005; received in revised form 29 July 2005; accepted 4 August 2005 Available online 19 September 2005
Abstract To develop a novel microporous membrane that responds to moisture/liquid content for use in protective clothing, the surface of microporous polyurethane membrane was modified by graft polymerization with different molecular weights of poly(ethylene glycol) (PEG). Surface grafting was confirmed, and appropriate grafting time and temperature were determined. The hydrophilicity of microporous polyurethane membranes was improved after surface modification with PEG. Both the surface pore sizes of the PU membranes and the constricted parts of through pore sizes of the bulk PU membranes were reduced, as revealed by the pore size distribution using image analysis and capillary flow porometer, respectively. The reduced pore sizes are expected to enhance the barrier properties by reducing the possibility of harmful particles and liquid borne pathogen penetration. Water vapor transmission rate (WVTR) measurements indicated that the pores of the modified PU membranes were responsive to the moisture content. © 2005 Elsevier B.V. All rights reserved. Keywords: Microporous polyurethane membrane; Poly(ethylene glycol); Surface modification; Image analysis; Pore size distribution
1. Introduction Surgical gowns and drapes are used as barriers to protect patients and to help healthcare professionals reduce the risk of occupational exposure to bloodborne pathogens [1]. Surgical gowns currently available provide protection, but they are known to be uncomfortable due to the low water vapor transmission rate (WVTR) [2,3]. It was estimated that a resting body produces about 250 g of moisture per square meter of body surface in 24 h, while an active person in sports would produce moisture (perspiration) about 400 g per square meter of body surface, or even more over a 24 h period [3]. To combine the liquid barrier property with high WVTR, Gore designed a microporous PTFE membrane, which allows the vapor to pass through the tiny holes while being resistant to the strikethrough of water droplets [4]. The water repellent mechanism of microporous membrane is based on the size exclusion mechanism since the average size of a water droplet is around 100 m, while the water vapor from
∗
Corresponding author. Tel.: +1 607 255 4719; fax: +1 607 255 1093. E-mail address: [email protected] (S.K. Obendorf).
0376-7388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2005.08.004
perspiration is about 0.0004 m. This allows the transmission of moisture vapor away from the body, which enhances the thermal comfort of the wearer. Although a microporous membrane has an excellent WVTR, the water-entry pressure is very low and capillary wicking of the challenging liquids cannot be avoided totally. Studies show a direct correlation between wetting and microorganism penetration [5]. However, it is possible that a new microporous membrane can be developed in order to further optimize the barrier protection with little or no sacrifice of its higher WVTR property. Practical application of polymeric membranes has been dramatically extended by membrane fabrications and post-casting modifications that control the pore size and functionalize the membrane surfaces. A variety of membrane modification methods such as plasma treatment, wet chemical treatment and corona discharge have been used. A convenient way for such modification is to graft monomers/macromers with functional groups onto the membrane surface to form a permanent covalent bonding with desired properties [6–10]. In this study, microporous polyurethane membranes were used as the host membranes with potential applications as moisture responsive barrier membranes in protective clothing for medical and chemical workers. To balance the comfort and pro-
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tection, a new membrane with pores that are responsive to moisture/liquid content was developed by grafting different molecular weights of poly(ethylene glycol) onto the polyurethane membrane. Porous membranes have diverse, complex and irregular structures with a wide range of pore sizes that are crucial to both liquid and particle penetration but are difficult to quantitatively characterize [11,12]. Therefore, a combination of techniques was employed to characterize the heterogeneous membrane structures and the structure–property–performance relationships. 2. Experimental 2.1. Materials Microporous polyurethane (PU) membranes were obtained from Porvair Com., Norfolk, UK. These membranes have different surface morphologies on each side (Fig. 1). Monolithic PU membranes were solution cast in our lab using commercial polyurethane pellets (2103-80AE, Dow Chemical Com., Midland, MI), and were treated using the same method only for the purpose of comparison with the surface morphology of modified microporous PU membranes. Hexamethylene diisocyanate (HMDI), poly(ethylene glycol) (PEG) with a range of molecular weights (600–20,000) and tin(II) 2-ethylhexanoate were purchased from Sigma–Aldrich Com., St. Louis, MO. Toluene (Mallinckrodt Baker Inc., Phillipsburg, NJ) was dried ˚ molecular sieves (EM Science, Gibbstown, NJ) for over 4 A 48 h before use. Polyurethane membranes were extracted using ethanol twice each time 4 h and then extracted with toluene for 4 h to remove any chemical residues left on the membrane surface. Extracted membranes were first air dried under the fume hood and then dried under vacuum at 50 ◦ C overnight. PEG was vacuum dried at 50 ◦ C overnight before use.
151
three swatches (2 cm × 2 cm) of polyurethane membranes were immersed into 100 ml toluene containing 5% (v/v) HMDI and 0.25% (v/v) tin(II). The PU membrane surfaces were functionalized with HMDI at 70 ◦ C for 1 h. Then, the nascent PU–HMDI membranes were extracted three times, 4 h each in toluene to remove unreacted reagents. One swatch was taken out to characterize the effect of HMDI modifications. In the second step, 10% (w/v) different molecular weights of PEG were grafted onto the membrane surface by the reaction of hydroxyl groups with the isocyanate end groups of HMDI in toluene solution at 45 ◦ C for 24 h. In preliminary experimentation at 45 ◦ C, we observed homogeneous and clear solutions upon stirring. Lower molecular weight PEG dissolved very quickly, while higher molecular weight PEG needed longer time to dissolve. Also, at this temperature, the grafting reaction was easier to handle than at higher temperatures. A similar extraction process as in the first step was used to remove unreacted reagents. All the membranes were vacuum dried at 45 ◦ C overnight and weighed before further characterization. The degree of grafting (Dg ) was calculated as Dg (%) =
Wg − Wo × 100 Wo
(1)
where Wg is the extracted, vacuum dried membrane weight after grafting with PEG, Wo is the original membrane weight before modification. Dg depends upon both the number of reacted sites, Nrs , and the molecular weights of PEG grafted. Assuming that all the grafted HMDI end groups reacted with the end group of PEG, Nrs can be estimated by Nrs =
Wg − W o MPEG + MHMDI
(2)
where Wg and Wo are as defined above, MPEG and MHMDI are the molecular weight of grafted PEG and HMDI, respectively.
2.2. Procedures of surface grafting
2.3. Characterization
A two-step grafting procedure was used similar to that of Han et al. [13] and Ukpabi and Obendorf [14]. Both reactions were performed under nitrogen purge, using tin(II) as the catalyst and toluene as the solvent and swelling agent. In the first step,
2.3.1. Spectral analysis Attenuated total reflectance mode Fourier Transform Infrared Spectroscopy (ATR–FT-IR, MAGNA-IR 560 spectrometer, Thermo Nicolet, Madison, WI) was used to study the chemi-
Fig. 1. Surface morphologies of the microporous control PU membrane.
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cal compositions of PU membrane surfaces, both qualitatively confirming the grafting of isocyanate groups and quantitatively analyzing the degree of grafting at various reaction conditions. ATR spectra were collected in the frequency region of 3800–1000 cm−1 at a 4 cm−1 resolution. To obtain a high signal/noise ratio, 128 scans were performed for each sample, and three replicates were used. 2.3.2. Measurement of contact angle and surface free energy Surface free energy with its polar and dispersive portions of the PU membrane was determined using the sessile drop method with contact angles (cos θ) measured by the contact angle analyzer (CAA2, IMASS Inc., Accord, MA). Deionized and distilled water, ethylene glycol, and formamide were used as the probe liquids, with surface energy values obtained from the literature [15]. Before each measurement, all samples were conditioned overnight at 21 ± 1 ◦ C and a relative humidity of 65 ± 2%. For each measurement, three to six random locations of each sample were measured, and average values were reported. To measure the polar and dispersive surface free energy, the Owens–Wendt method using the extended Fowkes’ equation was employed [16]: p 1/2 γLV γL 1/2 1/2 p (1 + cos θ) = (γSd ) + (γS ) (3) 1/2 d γLd 2(γ ) L
p where γS and γSd are the polar and dispersive surface free energy, p γ LV , γLd and γL are the surface tensions of the total, dispersive
and polar parts of the probe liquid, respectively. 2.3.3. Membrane morphology Surface morphologies of PU membranes were examined by scanning electron microscopy (SEM, Cambridge/Leica Stereoscan 440, Leica Cambridge Ltd., Cambridge, UK) after sputter coating with Au–Pd for 30 s. 2.3.4. Pore size distribution analysis Surface pore size distributions of PU membranes before and after modification were analyzed by image analysis of SEM micrographs using the public domain software ImageJ 1.32 (National Institute of Health, http://www.rsb.info.nih.gov/ij). Ten images were taken across at least three replicates for each reaction condition; and for the same specimen, images were taken from random locations. Image analysis of the SEM micrographs only provides information about the surface porosity. The bulk membrane structure is known to be quite complicated due to the membrane casting method and membrane formation mechanism. As described by Jena and Gupta [17], there are basically three kinds of pores in a porous material: closed pore, blind pore, and through pore. The open end of blind pores and through pores can be detected by surface techniques, such as SEM, and the mercury intrusion technique, while closed pores can be detected by none of these techniques. Since through pores permit fluid flow, they can be detected by extrusion porosimetry and flow porometry. To under-
stand the bulk membrane structure, the constricted part of the through pore size distributions of the bulk PU membranes before and after modifications were determined with the capillary flow porometer (Model CFP1500AEX, Porous Materials Inc., Ithaca, NY), which gives the constricted part of the through pore diameters in the 0.013–500 m range. The analysis is based upon a three-curve graph: dry curve, wet curve and half-dry curve [17]. To get the dry curve, a dry PU membrane was put into the test chamber, gas pressure was increased on one side of the sample, and the flow rate and gas pressure were measured. To generate the wet curve, the same PU membrane was saturated with a wetting liquid with known surface tension, gas pressure was increased on one side of the sample. At a certain pressure, the largest pore was emptied and the gas flow started, which gave the bubble point pressure. On further increase of the pressure, smaller pores were emptied and gas flow increased. In order to compute the mean flow pore diameter, the half-dry curve was first computed to yield half of the flow rate through dry curve at a differential pressure. 2.3.5. Water vapor transmission rate (WVTR) Water vapor transmission rates of PU membranes before and after modifications were measured using the upright wet cup method as described by ASTM E96 [18], with a reduced air gap (6 mm) and membrane area (D = 3 mm) according to Hu et al. [19]. Together with the control and modified membranes, the WVTR of an open cover wet cup was also measured for comparison to exclude any environmental instability. 2.3.6. Mechanical properties Mechanical properties were measured according to ASTM D882 [20] using the universal testing machine (Instron 5566, Instron Com., Canton, MA). All samples were cut to the standard shape and conditioned overnight in the conditioning room (21 ± 1 ◦ C, relative humidity of 65 ± 2%) before testing. For each test, five samples were used, and average values with standard deviations were reported for modulus, tensile strength, tensile strain and energy at break, respectively. 2.3.7. Statistical analyses Statistical analyses were performed to determine the effect of surface modification with different molecular weights of PEG on PU membranes. Two-way analysis of variance (ANOVA) was performed to analyze the contact angle measurements, and one-way ANOVA was performed using contrast statements to construct an F test for the null hypothesis that different treatments have no effect on the mechanical properties. Both analyses were conducted at a confidence level of 95% using the SAS® system (SAS Institute Inc., Cary, NC). 3. Results and discussions 3.1. Spectral characterization of surface chemical compositions The extent of reaction with HMDI was analyzed by ATR–FTIR. After reaction with HMDI, the characteristic peak of iso-
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Fig. 3. Effect of temperature on the extent of isocyanate groups grafted onto the PU membrane. Fig. 2. ATR–FT-IR spectra of PU membrane surfaces before and after each step of modification (PU control, PU–HMDI, and PU–PEG).
ing decreases. This could be due to self condensation of the isocyanate groups.
cyanate group presents at the wavenumber of 2250 cm−1 , which indicates that the surface of the PU membrane was successfully functionalized with isocyanate groups, as shown in Fig. 2. Since functionalization of the PU membrane surface with isocyanate groups is the first step of the surface grafting, it is also the key step to get the maximum reactive isocyanate groups onto the membrane surface for the subsequent grafting with PEG. Therefore, it is crucial to determine the appropriate reaction conditions. To evaluate the effects of grafting temperatures, PU membranes were reacted with fixed mole ratio of HMDI at different temperatures (30–80 ◦ C) for 60 min in the presence of tin(II). The relative concentration of isocyanate groups grafted onto the surfaces of the PU membranes was estimated by the ratio of the peak intensities in an absorbance spectrum: A2250 Cisocyanate ∝ (4) A1600
3.2. Grafting yield of PU membrane
where Cisocyanate is the relative concentration of isocyanate groups grafted onto the PU membranes, A2250 and A1600 are the absorption bands of isocyanate groups and aromatic groups at the wavelength of 2250 and 1600 cm−1 , respectively. Since the aromatic band around 1600 cm−1 was relatively inert during the reaction, it was used for the normalization. The grafting of the isocyanate groups increases with the increase in temperature in our experimental range, as shown in Fig. 3. Although the grafting extent is higher at 80 ◦ C, the mechanical properties of the PU membrane were observed visually to be deteriorated. In addition, higher reaction temperature will possibly result in more side reactions. Therefore, 70 ◦ C was selected as the grafting temperature for the functionalization of PU membrane surfaces with HMDI. We also investigated the effects of grafting time on the extent of isocyanate groups grafted onto PU membrane surfaces. As shown in Fig. 4, the extent of grafted isocyanate groups increases with the increase in reaction time, reaching a maximum at 60 min. At reaction times longer than 60 min, the extent of graft-
After the PU membrane surfaces were functionalized by HMDI at 70 ◦ C for 60 min and treated as described in Section 2.2, different molecular weights of PEG were grafted onto PU membrane surfaces by reacting with the grafted isocyanate groups at 45 ◦ C for 24 h under N2 . As shown in Fig. 5, with the increase in PEG molecular weight, the degree of grafting (Dg ) increases gradually, and then levels off at higher molecular weights. This indicates that the number of reacted sites on the HMDI modified PU membrane surfaces decreases with the increase in the molecular weight of PEG. This behavior was expected since with the increase in PEG molecular weight, the possibility of masking the reactive sites increases due to the steric effect of entanglement and the increased possibility of crosslinking. Here, the weight increase of the PEG modified PU membrane was mainly attributed to the surface grafting yield. However, it is quite possible that physical adsorption, chain entanglement and crosslinking can also occur, especially at higher PEG molecular
Fig. 4. Effect of reaction time on the extent of isocyanate groups grafted onto the PU membrane at 70 ◦ C.
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Fig. 5. Effect of PEG molecular weights on degree of grafting and the amount of reacted sites on the PU membrane surface. The dashed arrow defines the most suitable molecular weight of PEG at which gives both the highest grafting yield and the most reacted sites.
Fig. 6. Surface free energy changes of the PU membrane after modified with different molecular weights of PEG. Table 2 ANOVA results for the fixed effect of contact angle measurements
weight, which can also attribute to the weight increase. Fig. 5 also suggests that the most suitable molecular weight of PEG with higher grafting yield is in the range of 2500–3500. 3.3. Contact angle and membrane surface free energy Contact angles were measured before and after modifications using three probe liquids to evaluate the changes in the hydrophilicity of microporous PU membrane surfaces due to grafting, and results are listed in Table 1. Both surface roughness and surface chemistry are known to impact the wettability of a surface. The two surfaces of the microporous PU membrane used in this study have quite different morphologies, as shown in Fig. 1. A direct correlation between the surface morphology and contact angles is observed with side 2 of the membrane having a lower contact angle than side 1. To analyze the influence of different factors on the measured contact angles, a two-way ANOVA analysis was performed, and results are given in Table 2. PEG molecular weights, surface morphologies of the membrane, and the interaction of these two factors, all significantly affect the contact angles. Fig. 6 shows the surface free energy of the PU membranes modified with different molecular weights of PEG. The addition of PEG onto the PU membrane surface increases the hydrophilicity over that of the control PU membrane. All PU membranes after PEG modifications have higher surface energies than the
Effect
# Degrees of freedom
F
Pr > F
Side Molecular weight Liquid Side × molecular weight
1 5 2 5
293.81 214.76 161.02 175.81
<0.0001 <0.0001 <0.0001 <0.0001
control membrane with the highest value observed for PEG1500 modified PU membrane. At PEG molecular weights higher than 1500, surface free energy is lower. This trend was observed for all three probe liquids during the experiment although it is still not clear whether this is due to the changes in surface roughness, or the differences in surface chemistry, or both. No allowance was made for any differences in the time related PEG binding efficiency. 3.4. Morphology analysis of the PU membrane SEM micrographs in Fig. 7 illustrate the surface morphology changes of microporous PU membranes before and after each step of treatment. Monolithic PU membrane micrographs under the same treatments are also shown for comparison. Both the microporous and monolithic PU membrane surfaces appear to be smoother before modification. After the first step of reaction with HMDI, both the membrane surfaces become quite
Table 1 Contact angles of differently treated PU membranes using water (W), formamide (F), and ethylene glycol (E) Molecular weight
Contact angle of side 1 θw
0 600 1500 4600 8000 20000
(◦ )
120.6 74.3 83.9 88.3 91.4 100.2
Contact angle of side 2 θF
± ± ± ± ± ±
0.9 1.3 3.9 2.0 1.8 2.9
(◦ )
116.6 64.2 73.6 81.2 85.4 90.1
θE ± ± ± ± ± ±
0.9 2.9 2.7 5.0 1.5 6.1
(◦ )
113.0 61.2 68.1 77.5 83.3 84.3
± ± ± ± ± ±
0.9 1.6 2.6 0.8 1.4 0.8
θ w (◦ )
θ F (◦ )
θ E (◦ )
82.6 ± 1.6 71.8 ± 3.3 77.1 ± 1.9 79.2 ± 2.5 81.3 ± 1.3 81.5 ± 0.8
72.5 ± 0.3 60.7 ± 2.8 66.9 ± 1.7 67.7 ± 0.8 69.8 ± 0.4 74.7 ± 1.8
69.9 ± 3.5 58.6 ± 1.6 66.8 ± 2.2 65.2 ± 0.1 67.0 ± 0.5 69.9 ± 3.0
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Fig. 7. SEM micrographs for the control and modified polyurethane membranes. (a), (c) and (e) are the microporous PU membranes.
rough; these changes in surface roughness were interpreted by Han et al. [13] as adduct formation during the reaction. After the second step of reaction with PEG1500, the membrane surfaces become smoother with finer textures. Similar changes also were observed for modifications with different molecular weights of PEG. Although the exact mechanism of what happened to surface morphology during the two-step grafting is not clear, changes of the surface roughness certainly affected the membrane wettability and related properties. The rougher PU membranes modified by HMDI had the lowest contact angles (62–65◦ compared to 72–74◦ after reaction with PEG1500, and 83–120◦ for the control).
3.5. Membrane pore statistics analysis Since the structures of most porous media are quite irregular and complicated, a combination of characterization techniques was employed in this study. SEM image analysis was used to characterize the surface pore size changes, and flow porometry was used to characterize the constricted parts of the through pore sizes of the bulk PU membranes [17]. SEM micrographs were converted into well-contrasted binary images using a threshold that was appropriate for the contrast of each micrograph. The membrane is represented as a two-phase texture, i.e. the polymeric matrix and pores, as shown
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Fig. 8. Illustration of SEM images before and after binary segmentation.
Fig. 9. Image analysis of pore size distribution of the PU membrane before and after surface modification with PEG1500.
in Fig. 8. Black objects in the binary image are considered as estimates of the cross section of the pores, as appear in the SEM micrographs. Average statistics of pore size distribution before and after modifications were derived by averaging the results of 10 image analyses. Fig. 9 shows the pore size distribution before and after the surface modification with PEG1500. The surface modification greatly reduces both the polydispersity of the pore sizes and the number of larger surface pores. Flow porometry has been used to study the pore structure of the bulk materials. The liquid flow rate through the control PU membrane versus differential pressure was measured, as shown in Fig. 10. The differential pressure at which the wet curve intersects the half-dry curve is used to compute the mean flow pore diameter of the constricted part of the through pores. Fig. 11 shows the through pore size distribution of the control PU membrane derived from the data in Fig. 10 using flow distribution equation [17]. Compared with the image analysis result in Fig. 8(a), the through pore sizes from flow porometry are much smaller than those of the open pores on the membrane surface determined by image analysis of SEM micrographs. This indicates that most of the through-pores for liquid flow are not cylindrical, but have quite constricted paths, i.e. flow porometry measures the constricted part of the though pore sizes of the bulk PU membrane. However, for the modified PU membranes, we could generate only the dry curve by flow porometer. This probably indicates
that either all of the constricted parts of through pores were closed due to the swelling of the grafted hydrophilic PEG layer when saturated with the wetting liquid, or the pore sizes were greatly reduced with the result being that the pressure necessary to expel the wetting liquid was beyond the instrumental range and the pore sizes were smaller than 13 nm, which is the detection limit of the flow porometer. Results from image analysis and flow porometry analysis indicate that the grafting of PEG onto the PU membrane occurred
Fig. 10. Flow rates as functions of differential pressure through control PU membrane.
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Fig. 11. Constricted part of through pore size distribution of control PU membrane. Table 3 Physical properties of PU membranes related to WVTR Mw of PEG
Thickness (m)
Response time (min)
WVTR (g/m2 /24 h)
600 1500 4600
50.0 ± 0.1 52.5 ± 0.1 52.5 ± 0.1
0–60 60–120 120–180
630.5 ± 8.1 654.7 ± 3.3 693.6 ± 4.5
not only on the membrane surface, but also within the pores on the pore surfaces. The reduction in the pore sizes can not only restrict the liquid penetration, but also prevent the pathogen penetration according to ASTM F1671 [21], where the size of the pathogen surrogate (Phi-X 174) used is 27 nm, which is much larger than 13 nm, thus enhancing the membrane barrier properties. 3.6. Property analysis 3.6.1. WVTR analysis In practical application, a membrane needs to be a barrier to bloodborne pathogens, or to challenging liquids and particles, while remaining permeable to the water vapor. WVTR is used to evaluate the material performance relative to the wearer comfort. Fig. 12 shows the water weight loss as a function of water vapor transmission time for the control PU membrane and PU membrane modified with PEG1500. There is no difference in WVTR between the modified and the control membrane for the first 180 min. However, after 180 min, the control membrane transports moisture from the wet cup faster than the modified
157
Fig. 12. Weight loss of the PEG1500 modified PU membrane as a function of time. The inlay is a partial enlargement of the circled part.
membrane. This is consistent with prolonged exposure to the high humidity causing the modified membranes to swell, further reducing the pore sizes of the membrane. Averaged over 1440 min, the modified membranes had WVTR in the range of 623–694 g/m2 /24 h, while the control membranes were in the range of 802–892 g/m2 /24 h. During the experiment, we also observed that the response time of the modified PU membrane to the moisture was dependent on the molecular weights of PEG, as shown in Table 3. When the membrane was modified with PEG600, the control membrane and modified membrane have different WVTR from the very beginning, i.e. the response time is in the 0–60 min range since we have a 60 min data collecting interval; when modified with PEG1500, the reduction of WVTR is observed at a 60–120 min range as compared to 180–240 min for PEG4600. This response behavior can be macroscopically attributed to swelling of the hydrophilic portion of the membrane as water is adsorbed resulting in the reduction in the pore size. Differences between grafting with PEG600, 1500 and 4600 can be attributed to the mobility of different chain lengths of PEG or possible differences in the add-ons of PEG. The reduction in the pore size with increased moisture is expected to increase the protection from bloodborne pathogens, challenging liquids and harmful particles, although the response time might be a little longer than that might be desired in the application. 3.6.2. Mechanical properties of the membranes Modulus, tensile strength, tensile strain and energy at break are presented in Table 4. From the results, PEG1500 modified
Table 4 Mechanical properties of PU membranes Mw of PEG
Modulus (MPa)
Tensile strength (MPa)
Tensile strain (%)
Energy at break (J)
0 (control) 600 1500 4600 8000 20000
4.84 ± 0.39 4.93 ± 0.43 6.01 ± 0.21 4.99 ± 0.37 4.24 ± 0.19 3.71 ± 0.53
12.60 ± 4.07 15.86 ± 2.36 23.19 ± 2.27 18.08 ± 3.04 13.63 ± 1.19 10.99 ± 1.94
331.67 ± 47.54 433.25 ± 45.41 435.23 ± 31.48 425.00 ± 32.69 411.20 ± 30.87 361.60 ± 37.69
0.29 ± 0.16 0.40 ± 0.08 0.66 ± 0.13 0.48 ± 0.10 0.43 ± 0.06 0.30 ± 0.07
K. Tan, S.K. Obendorf / Journal of Membrane Science 274 (2006) 150–158
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Table 5 Mechanical property contrast of PEG1500 with the control PU membrane Property
# Degrees of freedom
F
Pr > F
Modulus Tensile strength Tensile strain Energy at break
1 1 1 1
21.85 14.13 6.34 8.52
<0.0001 <0.0001 0.0003 <0.0001
membranes give the highest mean mechanical properties among all other treatments using different molecular weights of PEG. To determine whether some of the surface modifications significantly influenced the mechanical properties, one-way ANOVA was performed using contrast statements to give the closer evaluation. As expected, statistical analysis shows that PEG1500 modified PU membranes do significantly increase the mechanical properties of the original PU membranes, as shown in Table 5. This might be due to the higher grafting yield of PEG on the PU membrane surface, which reduced the pore sizes and increased the mechanical properties of the membrane. 4. Conclusions Protection and comfort are two competing performance characteristics for protective clothing. In this study, novel moisture responsive membranes were developed by surface grafting of different molecular weights of PEG onto microporous PU membranes. ATR–FT-IR and SEM characterizations confirmed the graft polymerization on the membrane surface. Surface hydrophilicity of the PU membrane was enhanced by the grafting of the hydrophilic PEG layer. Results from image analysis and flow porometry showed that the PEG grafting occurred not only on the membrane surface, but also within the pores on the pore surfaces, thus reducing the pore sizes which are expected to reduce the penetration of challenge liquids and bloodborne pathogens. Results from the flow porometry analysis also indicated that after the surface modification, the membrane barrier properties were greatly enhanced by reducing the constricted part of the through pores to be smaller than the size of the pathogen surrogate which is 27 nm. WVTR tests showed that the modified membranes were responsive to moisture stimuli; and both the response time and the final water vapor transmission rates were dependent on the molecular weights of the grafted PEG. Based upon the grafting yield and the mechanical properties analysis, this study also suggests that in order to get the better modification results, the molecular weight of PEG should lie between 1500 and 3500. Acknowledgements This research was supported in part by the Cornell University Human Ecology College Grant and by the National Textile Center (Project C05-CR01). The authors also wish to thank the Cornell Center for Materials Research (CCMR) for the use of
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