Determining the Molecular Weight of !Petri Dish Derived Polystyrene through!Thin Film Analysis!

!!Taicheng Song, Albert Tung, Arun Soni, and the rest of Group 5

!ABSTRACT

Molecular weight is frequently determined by expensive and costly methods, such as Gel Permeation Chromatography, that are often limited to certain size ranges. Approximating molecular weight through an efficient and inexpensive process can allow scientists to understand the properties of certain polymers as molecular weight is known to affect variables such as temperature and viscosity. Here, we spin coat thin polystyrene films onto silicon wafers with [1,0,0] orientation. The polystyrene from a standard petri dish is dissolved in toluene with different concentrations resulting in different thicknesses of film. Optical contact angle measurements and FTIR analysis were also taken of polystyrene to study its hydrophobic properties and infrared spectrum. The thickness of the film was determined by an ellipsometer which determined the average thickness of the polystyrene film on the silicon wafers. Our experiment, at a concentration of 10 mg/mL, created a film of average thickness 450.4Angstroms or 45.04 nanometers. Using our own polystyrene solution concentration versus thickness, the concentration at 300 nm was extrapolated using the trend line for the linear plotand calculated to be 50.79 mg/mL. The known relationship between polystyrene concentration to molecular weight values at 300 nm allowed us to calculate that the unknown polystyrene had amolecular weight of 156,040 daltons.!INTRODUCTION!

Molecular WeightMolecular weight represents the sum of the mass of each atom in a molecule. In polymers, it is known that molecular weight influences glass temperature, bulk viscosity, the viscosity temperature coefficient, and specific volume. Thus, knowing molecular weight is key to understanding

the properties of polystyrene.

One known method of evaluating molecular weight

Figure 1. Molecular Structure of Styrene http://en.wikipedia.org/wiki/Polystyrene

is gel permeation chromatography, an expensive process with limited capabilities that can only be used on soluble compounds. In this process, a specially polished expensive glass cuvette is required and the beam has to be tightly focused to hit just a tiny region of a cell. Furthermore, the mobile solvent, used to distribute the molecules according to size, is expensive.Another method known as viscometry measures the viscosity and flow parameters of a fluid but is very inaccurate as seen with results from a Colorado University study on the molecular weight of cellulose acetate ranging from 0 to 34,900g/mol.Polymer

The polymer being investigated is polystyrene obtained from a standard petri dish. Polystyrene from petri dishes are usually formed by injection molding to create a hard and durable plastic and typically sterilized in various manners, such as radiation. Though polystyrene possesses many intrinsic and extrinsic qualities, the most important ones we take into consideration are its melting point (about 100 degrees Celsius), its molecular weight, which isbeing determined in this experiment, and its chemical makeup which consists of monomers of styrene (C8H8).Major Techniques Used

Spin Coating

Spin coating is a process used to create uniform thin films onto a flat surface by means of spinner that spreads the material, usually a solution with a volatile solvent, by centrifugal force. Spin speed, spin duration, viscosity and concentration of the solution all affect the thickness of the final film.

Figure 2. Spin coating model derived by Emslie, Bonner, and Peck

Ellipsometry

Ellipsometry is an optical technique for investigating properties of thin films. It can be used to present composition, roughness, thickness, crystalline nature, doping concentration, electrical conductivity and other material properties. Generally, the measured stimulus is the change in polarization as the incident radiation interacts with the material structure of interest. Fourier Transform Infrared SpectroscopyFourier transform infrared spectroscopy (FTIR) is a technique which is used to obtain aninfrared spectrum of absorption or emission of a solid, liquid or gas. The machine collects data and converts the raw data into an actual spectrum. This spectrum can be analyzed to determine the property and identify of the material.!Compression Molding

Compression molding is the process in which pellets or pieces of a polymer are squeezed and heated into a mold by a hydraulic press.Optical Contact Angle Meter

Optical contact angle meter allowed an estimation of the hydrophobicity or hydrophilicity of a material based on the angle a 5L drop of deionized water formed with the surface.Purpose

This topic was researched to teach us how to process silicon wafer surfaces and understand its differing crystal structures, spin-coat polymer thin films, measure thin film thickness, and determine an unknown molecular weight. Each experiment conducted determined the thickness of a thin film at a particular concentration and collectively, the whole experiment was used to calculate the unknown molecular weight of the polystyrene in the petri dish. Our concentration of 10mg/mL, in particular, formed a 450.4 angstrom layer of polystyrene on the surface of the silicon wafer. Using this data point and other data points from other group members, the calculated value for the molecular weight of polystyrene is 156,040 daltons. METHODS AND MATERIALSSilicon Wafer Preparation

Single crystalline silicon wafers with Miller index of [1,0,0] orientation were cut into four approximately 1cm x 1cm squares using a diamond cutter and metal tweezers. Orientation of crystals inthe silicon wafers was verified using an Olympus Figure 3. The various planes of silicon. http://www.yale.edu/eas996/smart/lect_15.htmlBH-2-UMA Optical Microscope at 500x magnification and images were taken.

Polymer Solution Creation

5.3 mg of virgin polystyrene from a standard petri dish was measured in a My Weigh iBalance 211 with standard weigh paper. It was subsequently dissolved in 5.3 mL of toulene from Sigma Aldrich that was measured using a measured pipet to achieve the desired concentration of 10mg/mL; it remained under a Safeaire fume hood for about 3 hours.Thin Film SpincoatingThe solution was then pipetted spincoated using Photo-Resis Spinner from Headway Research Inc. in open air at 2500 revolutions per minute (rpm) for 45 seconds onto the reflective half of the silicon wafer, attached to the spinner by double-sided tape. This process was repeated 4-6 times per concentration with concentrations ranging from 5 mg/mL to 25 mg/mL in 5

even intervals. The square silicon wafers with thin film coating were put into a petri dish and the thickness of the film was taken the next day.Silicon Wafer Characterization

Figure 4. Spincoating process. http://www.smartcoater.com/spin-coating- theory.html

Pure silicon wafers with [1,0,0] orientation and silicon wafers with a polystyrene thin film had their surfaces characterized by a CAM 200 Optical Contact Angle Meter to determine its hydrophobicity and hydrophilicity. The silicon squares were placed in the center of theplatform and 5L deionized water was dropped on the silicon. The contact angle was obtained by the software in the machine and analyzed.Molding and Polymer Analysis with FTIR

Carver Model C hot press compressed several pieces of polystyrene from the petri dish into a metal mold covered by non-stick, orange flame retardant Kapton paper and an additional thin copper cylinder. The temperature was set to be 356F or 180C for two minutes and then compressed at 6 psi for 5 minutes. The temperature of the sample was then reduced by blowing pressurized air onto it and removing it from the mold with a razor blade. The sample was then put into the Fourier Transform Infrared Spectroscopy: Thermo Scientific Nicolet 6700. The machine displayed an infrared spectrum, which was then compared with the graph in a database to determine what polymer it was.

Figure 5. This diagram illustrates how the FTIR works to obtain the infrared spectrum of a sample. http://sunedisonsemi.com/index.php?view=metrology

Ellipsometer Testing

One silicon square was randomly selected from the petri dish to be tested by a Rudolph Research Auto EL ellipsometer. The angle of incidence was 60 degree and 5 measurements of the thickness of the thin film on the silicon square were taken. Three readings were obtained fromthe ellipsometer because the ellipsometer reads the 0th, 1st and 2nd order. Qualitative analysis

using samples with known thicknesses were able to eliminate false values.

Figure 6. This diagram illustrates how the ellipsometer works to obtain the thickness of the film on a sample http://en.wikipedia.org/wiki/Ellipsometry#mediaviewer/

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!RESULTS AND DISCUSSION Silicon Wafer Images!

Figure 7. The left image is an image of [1,1,1] silicon and the right image is an image of [1,0,0] silicon taken by an optical microscope

Microscope images of two different cleavages of single crystalline silicon show the difference between [1,1,1] oriented silicon on the left and [1,0,0] oriented silicon on the right. The images show the properties of silicon cleaved in different ways, but the different properties do not affect the molecular weight of the polystyrene being analyzed.

!Optical Contact Microscope

Figure 8. The above image is an image of a water drop on pure silicon without any thin films.

Silicon Wafer OnlySilicon Wafer with Polystyrene Film

Trial 130.28 Left and 31.01 Right85.34 Left and 84.39 Right

Trial 222.25 Left and 20.46 Right83.72 Left and 83.45 Right

!Table 1. The lower angle of the water droplets on only the silicon wafer indicates that the silicon wafer is hydrophilic while the higher angle of the water droplets on the silicon wafer indicates the hydrophobicity of polystyrene.

FTIR Data

Figure 9. The FTIR spectrum was compared to that of polystyrene and found to be nearly an 80% match with few discrepancies.

!A graph of absorbance vs. wavenumbers (cm-1) was obtained from the FTIR and comparison with the standard spectrum from the Thermofisher spectrum library indicates that the substance is polystyrene. Furthermore, peaks in the 2800-3000 range indicate aromatic and aliphatic C-H stretch. Peaks in the 1400-1600 range are also indications of aromatic ring stretching, characteristic of the benzene ring within the styrene monomer.

!Data Table and GraphsThe data obtained from the ellipsometer was obtained and recorded below. Standard deviation and other errors were calculated (Appendix A) and error analysis was conducted.

5 mg/mL10 mg/mL15 mg/mL20 mg/mL25 mg/mL1213 427 695 1058 1412 2209 453 691 1059 1315 3220 469 656 1026 1403 4264 454 686 1033 1446 5196 449 670 1051 1506 Standard Deviation25.8915.1316.2615.0469.62Average Thickness(Angstroms)220.4450.4679.61045.41416.4Natural log (ln) thickness5.406.116.526.957.26ln Error of thickness0.117470.033580.023920.014390.04915Standard Deviation of theMean11.586.767.276.7331.13Mass (mg)24537898126Error in Mass (mg)11111Volume (mL)5.205.305.205.05.00Error in Volume (mL)0.050.050.050.050.05Error in concentration0.042760.021090.016030.014290.01277!

Table 2. Ellipsometry data with instrumental error calculations.

The equation obtained from the graph of concentration on thickness of polystyrene film is y = 1.1445x + 3.511 and the graph's axis minimum and maximum values have been altered to show the distribution of error bars and the points near the linear regression as close as possible. A

R2 value of 0.9924 also indicates that the linear regression is a extremely close fit and accurately represents the data, thus extrapolation is a viable technique to estimate what the concentration ofthe solution would be to achieve a thickness of 300 nm.

The Effect of Concentration on Thickness of PolystyreneFilm

8 y = 1.1445x + 3.5111R = 0.9924

7.25

ln(thickness)6.5

5.75

51.5 2 2.5 3 3.5

ln(concentration)Figure 10. The graph shows how different concentrations can affect thickness

Using the linear regression equation (calculations shown in Appendix A), the concentration was

determined to be 50.79 mg/mL and the molecular weight was calculated to be 156,040 Daltons.!Error Analysis

One error of the contact angle measurement is that because the silicon wafer was not cleaned with nitrogen gas or acid so there is a very high chance that dust particles settled onto the wafer or oils from fingertips before the drop of water was added and disrupted the results. Errors involved with FTIR analysis are similar to that of the contact angle measurement as dust, humidity and oils from the air and on the sample could affect the result. Furthermore, parallax in reading the syringe could get a different value for the liquid, spin casting time differences could

create variations in thickness and presence of dirt on the silicon wafer could affect the film. Human error could also result as different interpretations of analog values could create discrepancies within the data.CONCLUSION

In the experiment, confirmation of the type of single crystalline silicon being used was examined through a optical microscope. The square crystals of the [1,0,0] silicon image prove that it has the crystal planar structure it was believed to have. With the optical contact angle meter, the hydrophobicity of polystyrene was observed as the water droplet on regular silicon had anywhere from 20 to 30 angle while the water droplet on silicon coated with polystyrenehad between 80 and 90. The higher the contact angle, the more hydrophobic the material is and polystyrene is known for being hydrophobic. Furthermore, FTIR readings, when compared to other spectra, matched that of polystyrene, confirming the identity of the polymer. From the data that we gathered and using extrapolation from known molecular weight, polystyrene was determined to have a molecular weight of about 156,000 Daltons which is consistent with solid polymers. Furthermore, understanding the molecular weight of polystyrene using a simple extrapolation method is cheaper and is relatively simple compared to the methods discussed previously, an important part of science. Using cleaner methods to handle samples would resolve many of the errors discussed by improving the accuracy of our results and avoidingcontamination.!!

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APPENDIX A!

Standard Deviation

Standard Deviation was calculated by taking the square root of the summation of the squared difference between values of thickness and the average thickness, divided by 4 (1 less than the number of samples: 5).

Standard Deviation of the Mean

Standard deviation of the mean is a value used to approximate the accuracy to which another individual can obtain the same results and calculated with this equation in which standarddeviateon is divided by the square root of the number of samples: 5.

Error of Thickness

The error of thickness was calculated using the propagation error thickness equation from the lab protocol: dW=dx/X where X=thickness and dX=standard deviation of the thickness.Error in Concentration!

Error in concentration was calculated by taking the square root of the sum of the squares of dA/A and dB/B where dA and dB were the instrumental errors of mass and volume respectively and A and B were the values of A and B respectively. Calculation of Final Value

Regression equation from experiment, y = 1.1445x + 3.511, in which y was thickness in angstroms and x was concentration was extrapolated to 3000 angstroms of which a concentration of 50.79 mg/mL resulted.The equation from known polystyrene concentration vs molecular weight at 3000 ,

log(concentration) = 3.378 - 0.322log(molecular mass), was calculated from the figure below

and the material was of a size 156,040 daltons.!!!!!!!!!!!

REFERENCES

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