Name: cooling rate dependent ellipsometry measurements to determine the dynamics of thin glassy films
Uploaded: 2016-01-26T18:00:00
Description: Watch this Scientific Journal Video about Cooling Rate Dependent Ellipsometry Measurements to Determine the Dynamics of Thin Glassy Films at JoVE.com

The Authors would like to acknowledge James A. Forrest for help in the initial idea for this technique.26&#160;This work was supported by funding from the University of Pennsylvania and was partially supported by the MRSEC program of the National Science Foundation under award no. DMR-11- 20901 at the University of Pennsylvania.

1. Film Preparation
<ol>
	<li>Weigh 0.04 g of polystyrene, and place into a 30 ml vial.</li>
	<li>Weigh 2 g of toluene into the vial. A 2% by weight solution of polystyrene in toluene yields a film of approximately 100 nm.</li>
	<li>Let the solution sit O/N to fully dissolve the polystyrene and let the solutions settle.</li>
	<li>Place a 1 cm x 1 cm Silicon (Si) wafer onto a Spin Coater.</li>
	<li>Spin the wafer at 8,000 rpm for 45 sec. While it is spinning, drop approximately 1 ml of toluene on the spinning wafer.<br ...

<ol>
	<li>Keddie, J. L., Jones, R. A. L., Cory, R. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Size-Dependent+depression+of+the+glass+transition+temperature+in+polymer+films.">Size-Dependent depression of the glass transition temperature in polymer films.</a> Europhys. Lett. 27 (1), 59-64 (1994).</li><li>Forrest, J. A., Veress, K. D., Dutcher, J. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Interface+and+chain+confinement+effects+on+the+glass+transition+temperature+of+thin+polymer+films.">Interface and chain confinement effects on the glass transition temperature of thin polymer films.</a> Phys. Rev.E. 56 (5), 5705-5716 (1997).</li><li>Forrest, J. A., Mattsson, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Reductions+of+the+glass+transition+temperature+in+thin+polymer+films:+Probing+the+length+scale+of+cooperative+dynamics.">Reductions of the glass transition temperature in thin polymer films: Probing the length scale of cooperative dynamics.</a> Phys. Rev.E. 61 (1), R53-R56 (2000).</li><li>Sharp, J. S., Forrest, J. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Free+surfaces+cause+reductions+in+the+glass+transition+temperature+of+thin+polystyrene+films.">Free surfaces cause reductions in the glass transition temperature of thin polystyrene films.</a> PRL. 91 (23), 235701 (2003).</li><li>Ellison, C. J., Torkelson, J. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+distribution+of+glass-transition+temperatures+in+nanoscopically+confined+glass+formers.">The distribution of glass-transition temperatures in nanoscopically confined glass formers.</a> Nat. Mat. 2 (10), 695-700 (2003).</li><li>Priestley, R. D., Ellison, C. J., Broadbelt, L. J., Torkelson, J. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Structural+relaxation+of+polymer+glasses+at+surfaces,+interfaces,+and+in+between.">Structural relaxation of polymer glasses at surfaces, interfaces, and in between.</a> Science. 309 (5733), 456-459 (2005).</li><li>Ellison, C. J., Kim, S. D., Hall, D. B., Torkelson, J. 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M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=T+g+depression+and+invariant+segmental+dynamics+in+polystyrene+thin+films.">T g depression and invariant segmental dynamics in polystyrene thin films.</a> Soft Matter. 8 (19), 5119-5122 (2012).</li><li>Yu, M., Olson, E. A., Zhang, M., Zhang, Z., Allen, L. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Glass+transition+in+ultrathin+polymer+films:+Calorimetric+study.">Glass transition in ultrathin polymer films: Calorimetric study.</a> PRL. 91 (8), 085703 (2003).</li><li>Kremer, F., Tress, M., Mapesa, E. U. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Glassy+dynamics+and+glass+transition+in+nanometric+layers+and+films:+A+silver+lining+on+the+horizon.">Glassy dynamics and glass transition in nanometric layers and films: A silver lining on the horizon.</a> J. Non-Crys. Solids. 407, 277-283 (2015).</li><li>Qi, D., Ilton, M., Forrest, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Measuring+surface+and+bulk+relaxation+in+glassy+polymers.">Measuring surface and bulk relaxation in glassy polymers.</a> Euro. Phys. J. E. 34 (6), 1-7 (2011).</li><li>Teichroeb, J. H., Forrest, J. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Direct+imaging+of+nanoparticle+embedding+to+probe+viscoelasticity+of+polymer+surfaces.">Direct imaging of nanoparticle embedding to probe viscoelasticity of polymer surfaces.</a> PRL. 91 (1), 016104 (2003).</li><li>Fakhraai, Z., Forrest, J. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Measuring+the+surface+dynamics+of+glassy+polymers.">Measuring the surface dynamics of glassy polymers.</a> Science. 319 (5863), 600-604 (2008).</li><li>Paeng, K., Swallen, S. F., Ediger, M. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Direct+measurement+of+molecular+motion+in+freestanding+polystyrene+thin+films.">Direct measurement of molecular motion in freestanding polystyrene thin films.</a> J. Am. Chem. Soc. 133 (22), 8444-8447 (2011).</li><li>Glor, E. C., Fakhraai, Z. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Facilitation+of+interfacial+dynamics+in+entagled+polymer+films.">Facilitation of interfacial dynamics in entagled polymer films.</a> JCP. 141 (9), 194505 (2014).</li><li>Fakhraai, Z., Forrest, J. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Probing+slow+dynamics+in+supported+thin+polymer+films.">Probing slow dynamics in supported thin polymer films.</a> PRL. 95 (2), 025701 (2005).</li><li>Roth, C. B., McNerny, K. L., Jager, W. F., Torkelson, J. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Eliminating+the+enhanced+mobility+at+the+free+surface+of+polystyrene:+fluorescence+studies+of+the+glass+transition+temperature+in+thin+bilayer+films+of+immiscible+polymers.">Eliminating the enhanced mobility at the free surface of polystyrene: fluorescence studies of the glass transition temperature in thin bilayer films of immiscible polymers.</a> Macromolecules. 40 (7), 2568-2574 (2007).</li><li>Priestley, R. D., Ellison, C. J., Broadbelt, L. J., Torkelson, J. 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Cooling-Rate dependent Tg measurements are high throughput characterization experiments that can determine the Tg, the expansion coefficient of the glass and the super-cooled liquid, the temperature dependence of the average dynamics, and the fragility of a particular glassy material in a single experiment. Furthermore, unlike fluorescence, embedding, or nanohole relaxation experiments, CR-Tg experiments are relatively quick and simple because they do not utilize fluorophores or other com...

The seminal work of Keddie Jones and Corey1 showed that the glass transition temperature (Tg) of ultra-thin polystyrene films decreases with respect to the bulk value at thicknesses lower than 60 nm. Ever since, many experimental studies2-11 have supported the hypothesis that the observed reductions in Tg are caused by a layer of enhanced mobility near the free surface of these films. However, these experiments are indirect measures of a single relaxation time, and thus there is a debate12-18 centered on a direct correlation between average thin film dynamics and the dynamics at the air/polymer interface.
To answer this debate, many studies have directly measured the dynamics of the free surface (&#964;surface). Nanoparticle embedding,19,20 nanohole relaxation,21 and fluorescence22 studies show that the air/polymer interface has dynamics orders of magnitude faster than the bulk alpha relaxation time (&#964;&#945;) with a much weaker temperature dependence than that of &#964;&#945;. Because of its weak temperature dependence, the &#964;surface of these films,19-22 and enhanced dynamics of thin polystyrene films,23,24 intersects the bulk alpha relaxation (&#964;&#945;) at a single point T*, which is a few degrees above Tg, and at a &#964;&#945; of &#8776; 1 sec. The presence of T* could explain why experiments which probe relaxation times faster than * fail to see any thickness dependence on the Tg of ultra-thin Polystyrene films.13-18 Lastly, while direct measurements of the enhanced mobile layer show that it has a thickness of 4-8 nm,20-22 there is evidence that the propagation length of the dynamics at the air/polymer interface is much larger than the thickness of the mobile surface layer.5,25,26
This report aims to fully describe a protocol for using ellipsometry for cooling rate dependent Tg (CR-Tg) experiments. CR-Tg have been previously used to describe the average dynamics of ultra-thin films of polystyrene.23,24,27,28 Furthermore, This technique was recently used to show a direct correlation between the average dynamics in ultra-thin polystyrene films, and the dynamics at the free surface.23 The advantage of CR-Tg measurements over other types of measurements such as fluorescence, nanoparticle embedding, nanohole relaxation, nanocalorimetry, dielectric spectroscopy, and Brillouin light scattering, studies is that they are relatively quick and simple experiments that do not utilize fluorophores or other complicated experimental techniques. Recent advances in spectroscopic ellipsometry allow this technique to be used to efficiently determine the optical properties of ultra-thin films of polymers and other types of hybrid materials with exceptional accuracy. As such, this technique probes the average dynamics of technologically applicable thin films in temperature and time regimes relevant to the glass transition (T &#8804; Tg, &#964;&#945; &#8805; 100 sec). Furthermore, this technique will provide information on the expansion coefficients of the glassy and supper cooled liquid states as well as the fragility of the system, which can then be compared with data for bulk films. Lastly, CR- Tg experiments can be used for any glassy system so long as the integrity of the film remains throughout the experiment.

<table border="0" cellpadding="0" cellspacing="0" width="918">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Toluene</td>
			<td style="width:111px;">Sigma Aldrich</td>
			<td style="width:119px;">179418-1L</td>
			<td style="width:472px;">This can be purchased from any chemical company.</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Atactic Polystyrene</td>
			<td style="width:111px;">Polymer Source Inc.</td>
			<td style="width:119px;">P-4092-S</td>
			<td>This can be purchased from any chemical company.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">THMS 600 temperature stage</td>
			<td style="width:111px;">Linkam</td>
			<td style="width:119px;">THMS 600</td>
			<td>any temperature stage that can be fit to an ellipsometer could be used.</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">M2000V Spectroscopic Ellipsometer</td>
			<td style="width:111px;">J.A. Woollam</td>
			<td style="width:119px;">M200V</td>
			<td>This procedure should be applicable for any spectroscopic ellipsometer.</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Spin Coater</td>
			<td style="width:111px;">Laurell Technologies</td>
			<td style="width:119px;">WS-650-23B</td>
			<td>This Procedure is possible with any spin coater</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Sample vials</td>
			<td style="width:111px;">Fisher Scientific</td>
			<td style="width:119px;">02-912-379</td>
			<td>Any sample vials will do</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Silicon wafers</td>
			<td style="width:111px;">Virginia semi conductors</td>
			<td style="width:119px;">325S1410694D</td>
			<td></td>
		</tr>
	</tbody>
</table>

cooling rate dependent ellipsometry measurements to determine the dynamics of thin glassy films

This report aims to fully describe the experimental technique of using ellipsometry for cooling rate dependent Tg (CR-Tg) experiments. These measurements are simple high-throughput characterization experiments, which can determine the glass transition temperature (Tg), average dynamics, fragility and the expansion coefficient of the super-cooled liquid and glassy states for a variety of glassy materials. This technique allows for these parameters to be measured in a single experiment, while other methods must combine a variety of different techniques to investigate all of these properties. Measurements of dynamics close to Tg are particularly challenging. The advantage of cooling rate dependent Tg measurements over other methods which directly probe bulk and surface relaxation dynamics is that they are relatively quick and simple experiments, which do not utilize fluorophores or other complicated experimental techniques. Furthermore, this technique probes the average dynamics of technologically relevant thin films in temperature and relaxation time (&#964;&#945;) regimes relevant to the glass transition (&#964;&#945; &#62; 100 sec). The limitation to using ellipsometry for cooling rate dependent Tg experiments is that it cannot probe relaxation times relevant to measurements of viscosity (&#964;&#945; &#60;&#60; 1 sec). Other cooling rate dependent Tg measurement techniques, however, can extend the CR-Tg method to faster relaxation times. Furthermore, this technique can be used for any glassy system so long as the integrity of the film remains throughout the experiment.

Fitting Raw Ellipsometry Data
Polystyrene films are transparent in the wavelength range of the ellipsometer (500-1,600 nm). Thus a Cauchy model is a good model for describing the index of refraction of polystyrene films. Figure 1A shows an example of &#936;(&#955;) and &#916;(&#955;) for a thick (274 nm) film of polystyrene, and the resulting fit to the Cauchy model <img alt="Equation 1" src="/f...

Watch this Scientific Journal Video about Cooling Rate Dependent Ellipsometry Measurements to Determine the Dynamics of Thin Glassy Films at JoVE.com

Cooling Rate Dependent Ellipsometry Measurements to Determine the Dynamics of Thin Glassy Films

Here, we present a protocol for cooling rate dependent ellipsometry experiments, which can determine the glass transition temperature (Tg), average dynamics, fragility and the expansion coefficient of the super-cooled liquid and glass for a variety of glassy materials.

This report aims to fully describe the experimental technique of using ellipsometry for cooling rate dependent Tg (CR-Tg) experiments. These measurements ...

The overall goal of this procedure is to provide a simple method to quickly and accurately measure the glass transition temperature, apparent expansion coefficient, and average dynamics of ultra-thin glassy films. This method can help answer key questions in a polymer glass field, such as how the dynamics of thin-films relate to those of the bulk. The main advantage of this technique is that it can calculate the glass transition temperature, apparent activation coefficients, and fragilities of polymer ultra-thin films in a single high-throughput experiment.While the example provided here is mostly focused on polymer glasses, this method can be applied more broadly to study the properties of thick and thin films of other types of glasses, such as organic molecule glasses and nanocomposites. Start preparing the film a day before it is needed for glass transition temperature measurements. Have a scale ready to accurately weigh materials for the film.This experiment will use polystyrene and toluene to create the film. Begin by measuring 40 milligrams of polystyrene. Add two grams of toluene to produce an approximately 100 nanometer film.After the solution has sat overnight, use it to create a film. Move the vial to a spin coater that is in a fume hood. At the fume hood, put the solution aside for later use.Also, ensure that there is toluene ready for use with the spin coater. Next, get a silicon wafer to create the film on by spin coating. The silicon wafer for this protocol is one centimeter by one centimeter.Place the wafer in the spin coater, and spin it at 8, 000 rpm for 45 seconds. As the wafer spins, drop approximately one milliliter of toluene onto it. Stop the spin coater after 45 seconds of spinning.Use the prepared polystyrene and toluene solution, and begin to add it drop-wise to the silicon surface. Stop when the entire surface is covered. Before the solution dries, spin the wafer at 4, 000 rpm for 20 seconds.Stop the spin coater, and remove the wafer to measure the film thickness. Determine the film thickness using an ellipsometer. Begin by placing the film on the ellipsometer stage and securing it.Continue by checking the angle of incidence, acquisition time, and other settings. Then begin the scan. After collecting data, use the ellipsometer software to fit the ellipsometric angles.Use a three-layer model. The first layer is the substrate, in this case, silicon. The second layer, layer number one, is a native oxide.The layer has a thickness of 1.5 nanometers. The third layer, layer number two, is a Cauchy Model, corresponding to the optical properties of polystyrene film. The Cauchy Model for the index of refraction is given by this formula.Lambda is the wavelength, and A and B are constants to be determined from data. K equals zero for a transparent material. The model returns a determination of the film thickness, in this case, about 105 nanometers.Remove the wafer from the ellipsometer and proceed to the next step. If the film is of the desired thickness, take the wafer to a vacuum oven. Place the wafer in the oven and anneal it for 15 hours at 393 Kelvin.Return to the ellipsometer with the annealed film. Put the film aside while preparing the sample stage. The ellipsometer should be equipped with a variable temperature stage.Use a thermal paste to coat the surface of its heating element. Next, place the annealed polystyrene film onto the heating element. Then clamp it tightly into place.Start a flow of 100%dry nitrogen through the temperature stage. Turn to the computer and the temperature stage software to create a temperature profile. This figure provides an idea of the temperature profile.Temperature is along the vertical axis. Time is along the horizontal axis. The sample is alternately heated to 393 Kelvin and cooled to 293 Kelvin.The temperature ramp-up grade is always a constant 150 Kelvin per minute, as indicated by the constant steep upward slope. The ramp-down varies with each cycle. It starts fast, then slows down, as indicated by the changing downward slopes in the figure.The sample is held for 20 minutes after it first reaches 393 Kelvin. All later temperature holding times, at both 393 Kelvin and 293 Kelvin, are for five minutes. Stay at the computer to complete setting up the experiment.Use the ellipsometer software to create a temperature-dependent ellipsometry model. The substrate layer is a temperature-dependent silicon model. Layer number one is a 1.5 nanometer native oxide layer.Layer number two, the third layer, is the Cauchy Model for polystyrene film. Work with the temperature-dependent silicon layer settings. Turn on the Use Ext Temp from Parm Log"to allow use of the temperature stage temperature.Now navigate to be able to edit the hardware configurations. Set the fast acquisition time to one second. Also, select high-accuracy zone averaging.Set the normal acquisition time to three seconds. Once again, use the high-accuracy zone averaging. Click the In Situ"tab of the ellipsometer software.Check the Fast Acquisition Time Mode"box. Press Start Acquisition"to begin collecting data. Monitor the data collection as the temperature profile is followed.Just before the three Kelvin per minute cooling ramp, uncheck the Fast Acquisition"time box. Use measurements of the thickness versus temperature for a given cooling ramp to calculate the glass transition temperature. In this sample curve, the region highlighted in red corresponds to the supercooled liquid.The region highlighted in blue corresponds to the glassy regime. The point where linear fits to these regions intersect defines the glass transition temperature. This sample is 110 nanometer polystyrene film of 342 kilogram per mole polystyrene.The cooling rate is 10 Kelvin per minute. Measurements at one Kelvin per minute give a temperature of about 372 Kelvin. Here is data of the glass transition temperature versus the cooling rate for a 110 nanometer film of polystyrene.This same data is plotted with black circles using the log of the cooling rate on the left vertical axis. The horizontal axis is 1000 over the transition temperature, as suggested by an empirical relation. For comparison, the red squares are the bulk dynamics of polystyrene, as determined by dielectric spectroscopy.For this data, refer to the right axis of the log of the bulk alpha relaxation time and the horizontal axis, 1000 over temperature. Once mastered, this technique will take five hours if performed properly. While attempting this procedure, it's important to remember to use an appropriate material model, so that you accurately fit the ellipsometry values for the thickness and the index of refraction of the film.I first tried this method of studying polymer thin-films in James Forrest's lab in the University of Waterloo. However, with recent advances in ellipsometry technology, we are now able to use this as a high-throughput method to study broad systems, such as organic thin-films and newly-synthesized molecules. After this video, you should have a good understanding of how to make a polymer thin-film and calculate the glass transition temperature, expansion coefficient, and apparent activation barrier to the relaxation time near the glass transition.

Research

JoVE Journal

Chemistry

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