Qualitative and Quantitative Analysis of PS/PMMA Spin-coated Thin Films through Optical Microscopy and Atomic Force Microscopy to Explore Immiscible Film Behavior

Abstract:Our group of two learned essential lab

skills and determined the ratio of polystyrene to poly(methyl methacrylate) at which the least phase separation was evident. We prepared a solution of approximately 5 mg/mL of polystyrene and approximately 15 mg/mL of poly(methyl methacrylate) in toluene, such that the ratio of polystyrene to poly(methyl methacrylate) was 1 to 3. We then spin-casted this solution onto cleaved [1,0,0] silicon wafers. We examined these wafers under an optical microscope and found that spun-cast polymer molecules were not in their lowest energy positions. We also examined these wafers under an atomic force microscope and found that the surface of the spun-cast film exhibited both holes and bumps. Comparing data across groups, we concluded that the ratio of polystyrene to poly(methyl methacrylate) at which the least phase separation was evident was 1:1.

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I. Introduction

Recycling has become a major green trend in past years. The success of recycling depends on ease of use and cost-effectiveness. It is easiest, on the consumer end, and most cost-effective to recycle similar materials mixed together rather than separated by polymer composition. Thus, an important question to answer is whether certain polymers, such as polystyrene (PS) and poly(methyl methacrylate) (PMMA), may be melted down and recycled together without consequence.

To answer this question, we first spin-coated silicon wafers with toluene solutions of various ratios of PS/PMMA. Though both PS and PMMA are soluble in toluene, they are immiscible with each other. We then examined these samples through light microscopy and atomic force microscopy to determine how the immiscibility and solute ratios affected the films' topography.

In short, the purpose of this experiment was to analyze the topography of PS/PMMA thin films in varying PS:PMMA ratios to determine if a mixture of these polymers could be successfully recycled.

A. GoalsThe goals of this experiment are

twofold: first, to gain necessary skills to prepare us for our upcoming research projects; and second, to determine if it is possible to recycle a mixture of PS and PMMA. In this experiment, we learned how to use a pipette, make solutions in a fume hood, cleave silicon wafers, and spin-coat solutions onto silicon wafers. Importantly, we learned the proper safety procedures to employ during lab work.

II. Materials and Methods

A. MaterialsStony Brook University provided us with

PS and PMMA, along with toluene to make a solution. The structure of PS is as follows:

and the structure of PMMA is as follows:

We were also given a stock [1,0,0] silicon wafer, tweezers, a diamond-tipped cutter, two Petri dishes, two graduated Fischer Scientific pipettes, one ungraduated Fisher Scientific pipette, one glass vial, Parafilm, and numerous pairs of Kimberly-Clark Purple Nitrile Gloves.

1. Machines UsedThe machines used in this experiment

were a Headway Research Inc. PWM32 Spin-Caster, an Asylum Atomic Force Microscope, an Olympus BH2-UMA Optical Microscope, and a Fred S. Carver Inc. Model C Hot Press.

B. Methods

1. Preparing PS/PMMA SolutionWe started with a pre-prepared vial

containing 26.45 mg of PS and 76.36 mg of PMMA. Under a fume hood, we used the graduated pipette to add exactly 5.00 mL of toluene to the vial, thus making a solution.

2. Cleaving the Silicon WafersIn the next phase of the experiment, we

first observed the cleaving of a [1,1,1] silicon wafer. We were then given a circular stock wafer of [1,0,0] silicon with one shiny side and one dull side. (the shiny side was face-up). Placing the tweezers in the middle of the wafer to hold it in place, we cleaved the wafer by applying pressure on the edge of the wafer with the diamond-tipped cutter. We repeated this process until we had four 1 cm x 1 cm silicon wafers.

3. Spin-CoatingThe next step in the experiment involved

spin-coating our PS/PMMA solution onto two of our silicon wafers. First, we used the tweezers to gently place one of the silicon wafers onto the spin caster, making sure to cover the entire area. Next, we used an ungraduated pipette to set two drops of our solution onto the wafer, enough solution to cover the entire surface. We then turned on the vacuum to keep the wafer in place during the spin-coating process. After that, we spun the wafer for approximately 30 seconds. We repeated this process until all four wafers were spin-coated.

4. Optical MicroscopyOnce we created our spin-coated wafers,

we examined one of them under an optical microscope. We then annealed a sample and re-examined it under optical microscope.

5. Atomic Force MicroscopyThe AFM ran a cantilevered needle over

the surface of the coating while tracking the amount of deflection of a laser trained on the cantilever. Thus, the AFM was able to map the topography and friction of the surface of the coating.

III. Results and Discussion

A. Preparing PS/PMMA Solution

In creating our PS/PMMA solution, we learned how to properly use a pipette to prepare a solution under a fume hood. We also determined that both PS and PMMA are soluble in toluene.

B. Cleaving the Silicon WafersWe learned the correct way to cleave a

silicon wafer using tweezers and a diamond-tipped cutter. We also observed the difference between [1,0,0] and [1,1,1] silicon wafers. The [1,0,0] wafers cleaved into quadrangles with 90 degree interior angles, while the [1,1,1] wafers cleaved into triangles with 60 degree interior angles. This discrepancy occurs because the [1,0,0] wafer was manufactured such that it runs parallel to one plane of the silicon crystals, whereas the [1,1,1] wafer runs “diagonal” to all three planes of the crystals, as seen in the below image (blue is [1,0,0] and red is [1,1,1]).

C. Spin-coatingWe learned how to use the spin-caster to

cover a silicon wafer with a polymer thin film. We also observed that spinning a silicon wafer with polymer solution on top created an extremely thin film of polymer on the wafer. This phenomenon is caused by the confluence of centripetal force, evaporation rate of solvent, viscosity of solution, and molar mass of solutes (see Fig. 1).

D. Optical Microscopy ResultsThe results from the optical microscope

showed that the spin-coated polymer thin film was not at the lowest possible energy level.

The upper picture is an optical microscope image of the un-annealed silicon wafer, and the lower picture is of the same wafer at the same magnification, annealed. The

change in topography between these pictures suggests that the polymer molecules had not settled into the lowest possible energy level after spin-casting. Instead, they were “frozen” in place once the solvent evaporated. Annealing the wafer melted the film, allowing the polymer molecules to move about and settle into a lower energy configuration, thus changing the appearance of the film.

E. Atomic Force Microscopy ResultsThe AFM results showed us how PS and

PMMA, which are immiscible with each other, interact when in a 1:3 ratio.

Interestingly, when the ratio of PS to PMMA is 1:3, the surface of the polymer thin film exhibited both bumps (dark spots) and holes (bright spots). The depth of one hole was determined to be 9.567 nm, and the height of one bump was approximately the same value. The holes and bumps arise because PS and PMMA phase separate during spin-coating, despite both being soluble in toluene.

Even more interestingly, the presence of holes and bumps varied depending on the ratio

of PS to PMMA.

The ratios of PS to PMMA, from left to right and from top to bottom, were 1:9, 1:3 (shown previously), 1:1, 3:1, and 9:1. The 1:9 and 3:1 wafers both exhibited bumps of whichever polymer was in the minority. The 9:1 wafer contained a remarkable amount of holes, possibly containing PMMA, of apparently uniform depth. The 1:1 wafer was the only one to exhibit a “blanketing” effect, in which underlying holes and bumps were covered by a filling layer of either PS or PMMA.

IV. ConclusionThrough preforming this lab, we

acquired numerous skills that will serve us well in the near future. The machines and procedures we learned to use are involved in many different areas of research—we will likely be involved in at least some of them—so these skills are

essential to learn.The recyclability of a PS/PMMA blend

is impeded by the immiscibility of the two polymers with each other. However, there seems to be an optimum ratio of PS to PMMA at which the two show the least phase separation, visible as a lack of bumps and holes in the polymer film. We concluded that the blend of PS and PMMA that demonstrated the smoothest film, and thus most recyclability, was the 1:1 blend.

V. ReferencesCho, Chun-Hyung. "Characterization of

Young’s Modulus of Silicon versus Temperature Using a “beam Deflection” Method with a Four-point Bending Fixture." ScienceDirect.com. Elsevier B.V., 15 May 2008. Web. 11 July 2012.

Holmes, Michael J., and Carl E. Mungan. "Photobleaching of Cresyl Violet in Poly(methyl Methacrylate)." Journal of Young Investigators. The National Science Foundation, May 2004. Web. 11 July 2012.

"Polystyrene." Kids' Macrogalleria. Polymer Science Learning Center, 2003. Web. 11 July 2012.

Schubert, Dirk W., and Thomas Dunkel. "Spin Coating from a Molecular Point of View: Its Concentration Regimes, Influence of Molar Mass and Distribution." Materials Research Innovations 7.5 (2003): 314-21. Print.