Figure 1
Peter C. Rieke and Liang Liang
This project focuses on developing scientific knowledge of the preparation and properties of multi-functional surfaces as applied to microchemical reactor devices. Specific emphasis is on reversible control of surface adhesion, wettability, physisorption and other properties through external thermal, optical, electrical, magnetic or electrochemical actuation. The objective is to develop a suite of surfaces with various property responses controlled by various actuation methods. These highly versatile surfaces can then be applied as desired in microchemical reactors.
Work was directed at developing various responsive surfaces using the n-isopropyacyrlamide (NIPAAM) and related polymer. These materials show a Lower Critical Solution Temperature LCST near 35 C. Below this temperature the materials are wetting and above this temperature the materials non-wetting. Thus by a change in temperature the material can transition from hydrophilic to hydrophobic in a reversible manner. The materials also respond to other physical and chemical stimuli and the work was directed at obtaining the hydrophilic/hydrophobic transition using these non-thermal stimuli.
NIPAAM was prepared in bulk samples in the form of thin sheets and placed in the appropriate water solution. Initially the swelling response of the polymer was measured by weight of water up-take in the temperature region of 25-50 C. Between 33 and 38 C the polymers showed a substantial loss of water. The advancing contact angle on these films was also measured in the same temperature region and showed a relatively abrupt change in contact angle from 45 degrees to 90 degrees over the same 33-38 C temperature region.
A co-polymer of polyacrylicacid and polyacrylamide was tested for response to electrical field stimuli. The temperature was poised near the transition temperature noted above and an electric field applied across the sample using two platinum mesh electrodes. Upon application of greater than 2 Volts the polymer was observed to bend with the concave side towards the negative electrode. Reversal of the polarity caused reversal of the bend direction.
This experiment demonstrated the general scheme of using external stimuli to induce physical response in the polymers. The sample is poised at a temperature near the LCST and application of the external stimuli causes a shift of the LCST and the thermal response curve to lower or higher temperatures. The choice of initial temperature depends on whether the external stimuli is expected to shift the response to higher or lower temperatures.
Similar experiments were preformed using a polyvinylacrylate and a polyacryllicacid co-polymer but with solution pH used as the external stimuli. Bulk water uptake was measured in these experiments. The materials showed a gradual increase in water uptake from pH 6 to pH 11.
For use in microtechnology especially in microchannel chemical reactors it is necessary to attach thin films of these polymer to the surfaces of the microchannels. Much of the effort was directed at developing attachment methods and testing the hydrophilic/hydrophobic thermal response. Ultimately these materials will be adapted to respond to electrical of photo stimulation.
NIPAAM was attached to glass, silica and silicon wafer substrates by direct photopolymerization of the monomers on an active surface. The substrates were first derivatized with a photoactive free radical generator with a silane surface linkage. Illumination in the near UV generated radicals at the surface and initiate polymerization of the vinyl groups in the monomers. A cross-linking agent in 1-10% mole ratio was also used. The resulting films were thick and poorly adhered to the surface but were sufficient for further testing. It should be noted that the film is a highly cross-linked gel.
Films were formed inside 2mm ID silica capillary tubes and the change in capillarity noted during change in temperature. Two variations on the experiment were performed. In the first the sample was immersed in a 25 C solution and the height of the capillary rise noted. The sample was then transferred to a 45 C solution. An approximately 7mm capillary rise was noted at 25 C and zero capillary rise noted at 45 C. This was precisely the expected result. In the second experimental variation the sample was left immersed in the same solution and the temperature of the solution varied from 25 to 45 C. The capillary rise was about 7mm at 25 C but did not change upon raising the temperature to 45 C or even higher. This was not the expected result and the difference between the two experiments became the subject of further exploration.
Using flat silicon wafers derivatized with the same NIPAAM gel both the advancing and receding contact angles were measured versus temperature. The results are shown in Figure 1. While the advancing contact angle showed the expected rise from 50 degree to 90 degree angle; the receding contact angle was nearly independent of temperature and slightly less than that observed for the advancing contact angle at 25 C. Apparently the film is unable to undergo dehydration and hence dewetting when left in contact with water.
Figure 1
This explained the difference between the two experimental variations. Physically moving the capillary from a cold to a warm bath allowed dehydration to occur and the expected change in capillary rise was observed. Changing the solution bath temperature did not allow dehydration to occur and no change was observed.
In further experiments a drop of water was captured in the middle of the capillary and heat applied to one end of the drop using a small hot nichrome wire wrapped around the outside of the capillary. It was expected based upon the advancing contact angles data that the drop would move away from the heated portion because of the increase in contact angle. It was not possible to do so unless gravity was used to help move the drop down the capillary. In essence, it was possible to get drop movement by change of capillary action but that the relevant difference was the advancing contact angle at the front or bottom of the drop and the receding contact angle at the top to the drop. The difference between these two values requires that gravity be used to poise the drop so that it is nearly but not quite able to move under the influence of gravity alone. The added heat at one end unbalanced the capillary forces and allowed the drop to move downward.
We attributed the retention of water in the NIPAAM gel to the extensive degree of cross-linking used in the preparation. In the most recent work on this project we have developed methods to attach single chains of polymer to the surface that are not cross-linked. We expect this to increase the mobility of the polymer chains and increase the ability to restructure to a non-wetting configuration. We have developed methods for attaching pre-polymerized oligomers to the surface using amide linkages to silane surface groups. We have also improved the efficiency of the surface initiated photopolymerization to eliminate the need for a cross-linking agent. The end result of these two approaches is attachment of single chains to the surface. The advancing and receding contact angles of these will be measured in the near future and capillary drop experiments repeated.
Journal Papers:
Reversible surface properties by photo polymerization of N-isopropylacrylamide on glass surface (accepted by Macromolecules).
Reversible change of hydrophilic/hydrophobic surface properties of crosslinked polyN-isopropylacrylamide network layers prepared by free radical polymerization (accepted by J. Appl. Polym. Sci.).
Meeting Paper:
Temperature-responsive surface from photografing crosslinked poly (N-isopropylacrylamide) network layers. Process Miniaturization: 2nd international conference on microreaction technology, New Orleans, LA March 9-12, 1998.
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