Supporting Information to A Sugar-Template ... - IOPscience

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A Sugar-Template Manufacturing Method for. Microsystem Ion-Exchange Membranes ... University, Kingston, ON, Canada. E-mail: [email protected]. 1 ...
Supporting Information to A Sugar-Template Manufacturing Method for Microsystem Ion-Exchange Membranes Rio V. Festarini, Minh-Hao Pham, Xinyue Liu, and Dominik P.J. Barz ∗ Department of Chemical Engineering, Queen’s University, Kingston, ON, Canada E-mail: [email protected]

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Supplemental Methods and Materials

In this section, we specify the materials, fabrication, and characterization techniques that are used to investigate the wettability of Nafion TM dispersion on PDMS surfaces.

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Materials

˙ Composite membrane consists of a PDMS scaffold filled with ionomer Nafion TM . Contact angle measurements are performed to infer the wettability characteristics of Nafion

TM

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persion on PDMS films. For the sake of comparison, the wettability of deionised (DI) water and 1-propanol (Sigma-Aldrich Canada Company, Oakville, ON) is tested as well. PDMS films for wettability characterizations are fabricated on plain Fisherbrand glass microscope slides (Fisher Scientific) of area 1 ×3 square inches. The glass slides are first cleaned according to an in-house cleaning protocol using acetone, isopropanol, both with a purity ≥ 99.5 % (ACP Chemicals), and DI water.

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Instruments

Contact angle measurements are performed with a VCA goniometer (AST Products Inc, Billerica, MA, USA). Preparation of PDMS films for contact angle measurements are made with a Laurell WS-650-23 spin coater (Laurell Technologies Corporation, PA, USA). Curing of PDMS is performed at higher temperature using a Corning PC-420D hot plate (Corning, New York, USA) or a Cole Parmer StableTemp Programmable Oven (Cole Parmer, Vernon Hills, IL, USA).

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Contact Angle Measurements

Physical entrapment of Nafion TM inside of a PDMS scaffold overcomes the lack of adhesion between these two materials. However, it is still important to consider the surface interactions since NafionTM dispersion needs to penetrate into the small pores of the PDMS scaffold during the fabrication process.

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Influence of droplet volume

Figure S1: Contact angle of water droplets on plain PDMS as a function of droplet volume. Error bars represent a standard deviation for three replicate measurements using left and right triple point. Lines between the data points are for the sake of a better visualization. At first, we examine the effect of droplet size on the contact angle (CA) using the syringecontrol of the VCA goniometer. The motivation for this investigation is twofold: 1) The instrument has limitations in terms of picture size, lighting adjustment, and focus ability. Hence, it is necessary to determine what range of droplet size measurements can be taken. 2) Measured CA can be influenced by gravitational effects as expressed by the Bond number Bo = (ΔρgL2 )/γ indicating the importance of surface tension force compared to the gravitational force. Here Δρ is the density difference liquid-gas, g is the acceleration due to gravity, L is the typical length scale of the droplet and γ is the surface tension of the 3

gas-liquid interface.

Figure S1 shows DI water droplets, with volumes of 0.5, 1, 2, 3, or 4 μL placed onto a flat film of PDMS supported on a microscopy glass slide. The corresponding Bond numbers of the droplets are Bo = 0.27, 0.43, 0.62, 0.78, and 0.97 respectively. A Bond number of ' 1 indicates that the droplet shapes are impacted by gravity. In regimes where the forces due to surface tension are dominant, the droplet features a semi-spherical cap shape which facilitates measuring CAs. We see that the CA initially increases with increasing droplet size. This trend changes when a droplet size of around 3 μL is exceeded. The decline in CA for the largest droplet can be explained with the influence of gravity as expressed by the Bond number. Another important observation is that there is a higher standard deviation for the lower droplet volumes. This is probably related to the absolute dispensing accuracy of the instrument which results in higher relative errors for small droplet volumes. Nevertheless, we choose 0.5 μL droplets for all water and 1-propanol CA measurements to minimize the influence of gravity. When using Nafion TM dispersion, having a higher viscosity compared to water or 1-propanol, it is not possible to dispense the desired volume with the equipment of the VCA goniometer. An Eppendorf pipette with a lowest nominal dispensable droplet volume of 2 μL is used instead.

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The influence of corona treatment on the wettability of Nafion TM dispersion, water and 1-propanol on PDMS

PDMS is a hydrophobic and non-polar polymer with repeating units of -OSi(CH 3 )2 -groups which are responsible for a hydrophobic surface with an advancing water contact angle (WCA) of 108 ◦ . 1 The NafionTM dispersion consists of the hydrophilic polymer dispersed in a water and 1-propanol mixture; hence, the dispersion is more compatible with hydrophilic surfaces. Exposure to air plasma, so-called corona treatment, converts the methyl groups 4

Si−CH3 into silanol Si−OH groups which makes the PDMS surface more hydrophilic. We study the CA of NafionTM dispersion, and for the sake of comparison, of water and 1propanol, on PDMS surfaces for different corona treatment times. In detail, we investigate the influence of 0, 0.5, 1, 3, 6 and 10 minutes corona treatment time where 0 minutes corresponds to an untreated PDMS film. The CA is measured immediately after the corona treatment.

Figure S2: Contact angles of water, 1-propanol, and Nafion TM dispersion on plain PDMS versus different corona treatment times. The results are summarized in Figure S2 where the insets show pictures of water droplets with 0, 1, 3, and 6 minutes of corona treatment. We observe that any corona treatment lowers the CA compared to the untreated PDMS. It seems that there is a maximum time where any further corona treatment does not significantly improve the hydrophilicity anymore. For water and 1-propanol, the maximum treatment time is between 3 and 6 minutes. For higher times, the contact angle is too low to be accurately measured with the VCA goniometer as can be seen in the insets. For the Nafion TM dispersion, the minimum contact angle of 29 ◦ is already reached after around 1 minute. We notice that the WCA can be more influenced through corona treatment than the CA of 1-propanol and Nafion TM dispersion. The NafionTM dispersion also qualitatively behaves more like 1-propanol since both liquids 5

on PDMS are hydrophilic w/o any corona treatment. This might be related to the fact that 1-propanol has a somewhat higher concentration than water in the continuous phase of the dispersion. To summarize, we note that for our subsequent Nafion TM dispersion application, the PDMS scaffold should be treated no longer than around 1 minute.

Another important feature for the composite membrane fabrication process is the loss of wettability over time. Hence, we study the CA of the different liquids on PDMS films over a series of days after corona treatment. The motivation comes from study 1 in which the authors noted that after any kind of plasma treatment, the PDMS surface groups rearrange and new hydrophobic surface groups are formed that lower the surface free energy.

Figure S3a) shows the change of the WCA over time for PDMS surfaces that have been stored in air after corona treatment. In case of untreated PDMS (0 min), we find that the WCA remains stable at around 110 ◦ over 8 days stored in air. For treated PDMS surface, it is generally observed that the enhanced wettability vanishes over the storage time. The strongest increase in hydrophobicity is in the first 2 days of air storage for all treatment times but the 6 min. one. For longer air storage than 2 days, the further increase is small and it seems that the CAs approach asymptotic values which are always below the contact angle of the untreated PDMS. In other words, the hydrophobic features of an untreated PDMS surface are not restored within the studied storage time. It is also notable that the PDMS which has been treated for 6 minutes, always features the lowest CA for a given storage time. In terms of water on PDMS, a corona time of 6 minutes ensures a high wettability.

Figure S3b) shows the change of the 1-propanol CA for a PDMS surface stored in air. Similar characteristics as for water are observed. The CA of the untreated PDMS (0 min) does not significantly change over storage time and remains at around 35 ◦ . For all coronatreated surfaces, the CA increases with the storage time but remains always under the CA

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Figure S3: Contact angles of a) water, b) 1-propanol and c) Nafion TM dispersion on PDMS, grouped in terms of corona treatment time, versus time stored in air since treatment.

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of the untreated PDMS. The best performance is found for a treatment time of 0.5 min. Despite the fact that this short treatment results in a little higher immediate CA compared to the other treatment times, it hardly increases and becomes the lowest when stored in air.

Finally, Figure S3c) shows the change over air storage time of the Nafion TM dispersion CA on PDMS. Again, the CA of the PDMS surface without any corona treatment (0 min) does not significantly change over storage time and remains at around 53 ◦ . Generally, we find that the NafionTM dispersion CA changes to a lesser extent over the air storage time compared to the other liquids. All CAs approach after 8 days a value of roughly 40 ◦ . The only exception is the PDMS surface with the longest treatment time of 10 min, which features after 8 days a CA similar to the the untreated PDMS surface. Generally, we note again that the Nafion TM dispersion behaves rather like 1-propanol than water. While the WCA tends to reach a hydrophobic value after several days, the 1-proponal and the NafionTM dispersion clearly remain hydrophilic over the entire storage time. In terms of scaffold filling of Nafion TM , it appears that storing in air makes the surface more hydrophobic but even after 8 days, there is a slight benefit for corona treatments less than 6 min.

To summarize: With corona discharge, it is possible to lower the contact angle of all investigated liquids. It turns out that for filling with Nafion TM dispersion, the PDMS scaffold should be treated no longer than 1 min. Further investigations show that the influence of the corona treatment vanishes after a time of 1 to 2 days and the characteristics of the untreated surface are more or less restored.

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References (1) Lee, J.; Park, C.; Whitesides, G. Solvent compatibility of poly (dimethylsiloxane)-based microfluidic devices. Anal. Chem. 2003, 75, 6544–6554.

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