Laboratory Experiment Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX
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Effect of Chemical and Physical Modifications on the Wettability of Polydimethylsiloxane Surfaces Carolyn L. Wanamaker,‡ Brittany S. Neff, Azieta Nejati-Namin, Erin R. Spatenka, and Mong-Lin Yang* Science Department, Concordia UniversitySt. Paul, St. Paul, Minnesota 55104, United States
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S Supporting Information *
ABSTRACT: The importance of understanding factors that contribute to surface wettability is highlighted in this new organic chemistry laboratory experiment, which aims to introduce the application of organic chemistry at the interface of polymer chemistry and material science. Polydimethylsiloxane (PDMS), a hydrophobic silicon-based rubber, is polymerized and molded onto templates of varying roughnesses and textures to introduce topology that changes surface hydrophilicity. The topological changes are assessed through optical microscopy imaging, and the wettability is analyzed by measuring the contact angle using an inexpensive digital camera. The molds are treated with boiling water which generates hydroxyl groups on the surface. Successful chemical modification of the polymer surface is confirmed using attenuated total reflection infrared (ATR-IR) analysis, and wettability is assessed through contact angle measurement. Postlab questions focus on the wettability of the physically and chemically modified surfaces and their potential use as scaffolding for tissue engineering. This adaptable experiment is designed to reinforce concepts such as intermolecular forces, introduce polymer structure and synthesis, showcase polymer applications in biological research, and provide an introduction to surface wettability in a visually interesting manner. KEYWORDS: Second-Year Undergraduate, Organic Chemistry, Physical Chemistry, Laboratory Instruction, Interdisciplinary/Multidisciplinary, Hands-On Learning/Manipulatives, IR Spectroscopy, Materials Science, Polymerization, Surface Science
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INTRODUCTION Surface wettability, or the interaction between solids and liquids at an interface, is considered one of the most important surface properties. Surface wettability is controlled by the chemical composition as well as the topography of the solid surface.1 In the chemistry classroom, surface wettability is usually explained in terms of chemical composition, functional groups, and intermolecular forces. Visually interesting demonstrations on surface wettability have been published in this journal. However, these demonstrations focused only on the chemical composition of the surface or solution to explain wettability.2−6 The topography of surfaces is addressed in another laboratory article that utilizes biomimetic replication of superhydrophobic leaf surfaces to create superhydrophobic polymer molds.7 These laboratories and demonstrations explore either the effect of chemical modifications or the topological change of the surface on its wettability. To our knowledge, there is not a comprehensive laboratory that introduces holistically both the effect of chemical and topological modifications on surface wettability. Moreover, very little emphasis has been placed on the applications of these surfaces with varying wettability. A novel interdisciplinary experiment was sought to give students a more integrated view of thinking about surfaces in © XXXX American Chemical Society and Division of Chemical Education, Inc.
which they synthesize a hydrophobic polymer that is biocompatible with human tissue and determine a way to manipulate the hydrophobicity through chemical and physical modifications in discussion of utilizing the polymer as a scaffold for tissue engineering. This experiment has been intentionally designed for use in a second-year undergraduate organic chemistry laboratory to contrast its current emphasis on organic synthesis. However, this experiment covers a broad range of concepts and can be adopted as a general chemistry laboratory focusing on intermolecular forces or as a physical chemistry laboratory focusing on surface properties. The aims of this experiment are for students to be introduced to polymer structure and synthesis, the significance of chemical composition and topography of a solid surface, the wettability of surfaces, and potential biological applications of these surfaces. Students have the freedom to template their own polymer surfaces, and the experiment incorporates techniques and instrumentation already familiar to organic chemistry and introductory biology laboratories along with an introduction to a simple and unique analytical technique to quantitatively Received: October 23, 2018 Revised: April 4, 2019
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DOI: 10.1021/acs.jchemed.8b00814 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Laboratory Experiment
determine the hydrophilicity/hydrophobicity of a surface. This experiment aids in the understanding of intermolecular forces, surface structure, and surface wettability through a hands-on introduction to contact angle analysis and encourages enthusiasm on interdisciplinary topics. Furthermore, this laboratory serves to fill the 2015 ACS curriculum content requirements to cover the preparation, characterization, and physical properties of a macromolecular system.8 Polydimethylsiloxane (PDMS), a silicone rubber, can be synthesized from a two-part mixture of a PDMS prepolymer and curing agent (Scheme 1). PDMS has a wide range of uses
Figure 1. Illustration of a water droplet on a surface showing the location of the contact angle for (a) a hydrophobic surface and (b) a hydrophilic surface.
predict and explain the way a surface interacts with water. The Young model is applicable to smooth surfaces and calculates a spreading parameter based on the interfacial tensions between all three phases (the solid surface, the water droplet, and the air) to determine if the liquid wets the surface completely. The Wenzel equation is applicable to rough surfaces and provides a way of predicting the effect of surface roughness on the contact angle behavior of a surface. The Wenzel model states that roughening a hydrophobic surface can enhance its hydrophobicity due to the increase in the solid−liquid interface. Further discussion of the Young and Wenzel models can be found in publications by Quéré.16 Physical and chemical altering of the surface of PDMS has been found to have significant effects on the hydrophobicity of PDMS. Biomimetic replicas of surfaces can be made using PDMS and result in a change to the PDMS surface’s hydrophobicity. For example, researchers have been able to increase the hydrophobicity of PDMS through biomimetic replication of superhydrophobic leaf surfaces.17 This replication is a relatively straightforward process that has been adapted as a laboratory experiment for undergraduates.7 Much research has also been dedicated to chemical modifications of the PDMS surface in efforts to decrease the hydrophobicity.9,18−22 One such modification involves treating PDMS with boiling water.22 In this treatment, the water reacts with the excess SiH groups in the cured PDMS to form more polar SiOH groups.23 ATR-IR analysis, an analysis tool common to organic chemistry undergraduate laboratories, showed evidence of SiOH groups from the boiled PDMS samples. These SiOH groups provide a more hydrophilic PDMS surface and have been shown to promote better cell adhesion through increasing cell attachment sites called focal adhesions onto the PDMS surface when compared to a nontreated counterpart as illustrated in Figure 2.22 In this experiment, students explore the effects of topological and chemical composition changes on the wettability of a PDMS surface. Students first cast negative molds of PDMS on the glass and a template of their choosing. The surfaces on the opposite side of the negative molds are used as controls (Figure 3). They then examine the surface topology of the molds utilizing an optical microscope and measure the contact angle of the surfaces using the digital microscope camera. Simultaneously, students treat the molds with boiling water and observe the change in surface chemistry using ATR-IR. Students are then guided to discuss how the observed results affect cell adherence to the treated surfaces followed by drawing conclusions to the hydrophilic/hydrophobic nature of the surfaces and the potential for these materials to be used as scaffolds in tissue engineering. This experiment is a unique departure from a classic organic chemistry laboratory due to its focus on surface modification and wettability.
Scheme 1. Structure of PDMS Prepolymer and Curing Agent and Schematic of Hydrosilylation Crosslinking Reaction to Cure SYLGARD 184 PDMS
from the housing for outdoor insulation9 to contact lenses,10 breast implants,11 and molds for soft lithography with patterns on the nanometer scale.12 The reason for its extensive use is because PDMS is transparent, inexpensive, permeable to gases, and pliable to surface modification.13 Moreover, PDMS is biocompatible with mechanical properties similar to human tissue14 and has been FDA approved for body implants. Together, these characteristics make PDMS a great candidate for tissue engineering. However, one of the major drawbacks to PDMS is that it is inherently hydrophobic. The hydrophobic character is due to the presence of the nonpolar methyl groups. Because of this, PDMS is not ideal for the adhesion of cells due to the low surface energy between its surface and water.13 However, alterations to the roughness and the hydrophobicity of PDMS surfaces have been shown to greatly influence cellular adhesion.15 Contact angle measurement is the ideal method to characterize surface hydrophilicity/hydrophobicity and is the most used technique in studies of PDMS surfaces.9 This measurement is taken by placing a small drop of water (a few microliters) on the surface of the sample, taking an image of the droplet, and measuring the angle at which the droplet is resting on the surface (Figure 1). A contact angle below 90° indicates a hydrophilic surface (wetting), and a contact angle above 90° indicates a hydrophobic surface (nonwetting). The Young and Wenzel models are surface models proposed to B
DOI: 10.1021/acs.jchemed.8b00814 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Laboratory Experiment
Figure 3, with a detailed procedure in the Supporting Information. In the second laboratory period, the PDMS negative molds are separated from the aluminum dish and cut out. One of the molds is cut in half with one half placed into a beaker of boiling water for an hour and a half. After the boiling water treatment, samples are dried with paper towels. ATR-IR is utilized to analyze the change in the chemical structure of the boiled sample compared to an untreated sample. A number of techniques are used to characterize the surface of the PDMS negative molds. Contact angle measurements are taken using the digital microscope camera and stage setup (Figure 4a) and free computer software (drop analysis plug-in
Figure 2. A cell on (a) a nontreated hydrophobic PDMS surface and (b) a chemically modified PDMS surface containing SiOH groups.
Figure 4. (a) Image of the in-house setup for measuring contact angle. Instruments shown include the computer running a microscope and 3D analyzer program and a digital microscope camera pointing to a stage with the PDMS mold and a 3 μL droplet of water. (b) Screen shot of the ImageJ program using the drop analysis plugin to determine the contact angle of a 3 μL droplet of water.
Figure 3. Schematic of topological mold casting procedure.
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EXPERIMENTAL DETAILS One partial laboratory period and one 3 or 4 h laboratory period are required with a minimum of 48 h between the two laboratory sessions to allow for curing of the PDMS (at room temperature). The first partial laboratory period involves casting the polymer negative molds. Mold casting can be performed simultaneously with another laboratory experiment as it requires minimal hands-on lab time. Students were instructed to bring templates that were relatively flat with a textured surface (examples: leaf, sandpaper). Approximately 10 g of the PDMS prepolymer and 1 g of the curing agent reagent are combined, stirred for 3−4 min, degassed in a vacuum oven at room temperature, and poured onto a glass slide and a template of the students’ choosing in an aluminum weigh dish. The aluminum dishes are stored in a desk drawer until the next laboratory period. The basic mold procedure is depicted in
for ImageJ; Figure 4b). One at a time, each PDMS sample is placed on the stage with the templated or control side pointing up. Using a micropipette, a three microliter drop of distilled water is placed on the surface of PDMS. The digital microscope camera is then focused on the drop. An image is captured and saved then opened in the drop analysis program to measure the contact angle. Contact angle measurements are taken for three surfaces: the control side, the student template negative mold, and the glass slide negative mold. Optical microscopy is utilized to visualize the texture of the student template negative mold surface. Pictures of the surface are taken with a microscope camera using the 10× and 40× lenses. Additional experimental details are provided in the Supporting Information. C
DOI: 10.1021/acs.jchemed.8b00814 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
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Laboratory Experiment
Surface Wettability
HAZARDS Wear safety goggles and gloves when preparing the PDMS samples. The uncured PDMS mixture should be poured in a well-ventilated area. Cured PDMS is not a known contact hazard. Extreme care is required when using razor blades to cut PDMS molds. Extra caution is required when working with hot plates and boiling water.
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Contact angle measurements allow students to observe the effects the surface structure has on the wettability of the surface. Measuring contact angles usually requires costly hightech contact angle goniometers. For this experiment, we do not require the high level of precision of a goniometer (costing around $10,000) and have assembled a user-friendly digital microscope camera (purchased for $100) and a stage to fashion our own homemade device similar to those previously reported24 for measuring contact angles (Figure 4a). Students were able to focus the microscope on a 3 μL droplet, take images, and use free software (drop analysis plug-in for ImageJ; Figure 4b) to accurately measure the contact angle of the droplet on their molds (as shown in the right column of Figure 5). The PDMS surface opposite to the mold-facing side was used as a control. The control PDMS contact angles (109.9 ± 5.3°) were consistent with those previously reported.7,13 The glass negative molds of PDMS yielded lower contact angles (102.4 ± 6.5°) as predicted by the Wenzel equation that a smoother hydrophobic surface will become less hydrophobic.16 The majority of student pairs utilized a plant or tree leaf as a template for their own molds. Contact angle measurements for the leaf molds ranged from 74°−130°, with the majority of the contact angle measurements above 109°. The PDMS molds with contact angles below 109° could provide interesting empirical results showing how certain microstructured patterns can increase surface hydrophilicity. Of note is that three student groups had inaccurately low contact angles below 70° due to the presence of leaf particles on their PDMS molds (this was caught through examining microscope images). Additional templates such as an aluminum dish (Figure 5b) and a ceramic coaster (Figure 5c) also produced PDMS molds with a range of contact angles.
RESULTS AND DISCUSSION
PDMS Molds
This experiment was successfully performed by three students working together in a research setting followed by 92 students working in pairs in a second-year undergraduate organic chemistry laboratory over the course of three years. Examples of student templates include a variety of leaves from plants and trees on campus, sandpaper, plastic wrappers, a ceramic coaster, aluminum foil, fabric, and coins. All students were successful in creating their PDMS molds. Mold Surface Visualization
Utilizing a conventional optical microscope, students were able to observe the structural characteristics of the PDMS negative mold templated surface. The surfaces were viewed with 40×, 100×, and 400× total magnification. Adjusting the focus affords views of horizontal cross sections of their surface and affords a sense of the three-dimensional structure of the surface. Select images of student templated PDMS surfaces are shown in the left column of Figure 5.
Chemical Modification
Modified chemistry of the PDMS surface was attained by boiling a PDMS sample for at least an hour and a half. After the boiling treatment, students then quickly (
