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NE 320 L Characterization of Materials

University of Waterloo

Nanotechnology Engineering

DEPARTMENT OF CHEMISTRY


Names:

Rajesh Swaminathan – 20194189

Ryan Iutzi – 20202504

Group Number:

3C

Experiment Name and Number:

#2 FTIR/DSC Characterization of MWNTs/PMMA Nanocomposites

Experiment Date:

17-Jul-2008

Report Submission Date:

30-Jul-2008

Report Submitted to T.A.:

Neeshma Dave

Table of Contents


1.Introduction 3

Background 3

Objective 3

Uses in Research 4

2.Theoretical Principles 4

3.Experimental 6

Summary of the Procedure 8

Deviations from the Manual 8

4.Observations and Results 9

Qualitative Observations 9

Quantitative Observations 9

5.Discussion 9

Error Analysis 9

6.Conclusions 9

7. References 9

Appendices 10

Appendix A – Original Observations 10

Appendix B – Sample Calculations 10

Appendix C – Questions 10

Appendix D FTIR/DSC Spectrum 11



  1. Introduction

Background



Fourier Transform Infrared Spectroscopy (FTIR) is a useful way of non-destructively analyzing the kinds of chemical bonds present in a material. Different bonds respond differently to incoming radiation due to variety in their molecular vibrations of stretching and bending. This response may be characterized by observing the percent transmission of infrared radiation and comparing it with known standards to identify the type and nature of chemical bonds. FTIR is applicable not only to solid materials, but liquids and solids as well. One limitation of IR spectroscopy is that molecule being studied must have a permanent dipole to be IR active.

Objective



The objective of FTIR is to determine the type of bonds present in a material by obtaining the percent transmission as a function of incident radiation wavelength. The ratio of the intensities of the transmitted and incident light are plotted against the incident light’s wavelength (or frequency/wavenumber) to obtain a fingerprint spectrum that is unique to that material. Interpretation of the spectrum with the help of known standards helps us identify the nature of the chemical bonds such as the functional groups present in the material (qualitative). It also helps us determine, with the help of Beer-Lambert’s law, the amount of material present in a sample by analyzing the relative size of the absorption peaks (quantitative). Further, an unknown sample can be identified by comparing its spectrum with spectrums of common materials available in computer databases. This application requires the use of modern software algorithms.
The key goals of this laboratory experiment are:


  1. To understand the working of an infrared spectrometer.

  2. To understand sample preparation of a polymethyl methacrylate (PMMA) nanocomposite filled with multi-walled carbon nanotubes (MWNTs).

  3. To learn to characterize this MWNT/PMMA nanocomposite with FTIR techniques.

  4. To determine quantitative and qualitative information from a DSC spectrum such as the glass transition temperature Tg.

Uses in Research



Measurements made by FTIR are extremely accurate and reproducible. Thus, it is a very reliable technique for positive identification of virtually any sample. The sensitivity benefits enable identification of even the smallest of contaminants. This makes FTIR an invaluable tool for quality control or quality assurance applications whether it be batch-to-batch comparisons to quality standards or analysis of an unknown contaminant. In addition, the sensitivity and accuracy of FTIR detectors, along with a wide variety of software algorithms, have dramatically increased the practical use of infrared for quantitative analysis. Quantitative methods can be easily developed and calibrated and can be incorporated into simple procedures for routine analysis.
Thus, the Fourier Transform Infrared (FTIR) technique has brought significant practical advantages to infrared spectroscopy. It has made possible the development of many new sampling techniques which were designed to tackle challenging problems which were impossible by older technology. It has made the use of infrared analysis virtually limitless.
In summary, even though FTIR can only observe the vibrational spectrum of a molecule, it a high useful technique since it is fast, easy to obtain, and non-destructive. It is also a fairly inexpensive instrument that is available in virtually all chemistry labs.
  1. Theoretical Principles



The major idea behind FTIR is that all molecules above absolute zero temperatures are in constant stretching and bending vibration. These molecules absorb incident radiation if its own vibrational frequency equals the frequency of the incident radiation. Most importantly, molecules can absorb frequencies only corresponding to discrete energy levels due to energy quantization.
FTIR is an advancement over dispersive instruments like prisms and diffraction graters in that it allows us to examine all frequencies simultaneously. This helps us obtain spectrums much more quickly (in about one second per scan) and results in greater optical throughput. The FTIR is much more sensitive than its dispersive counterparts resulting in much lower noise levels. These combined advantages make FTIR measurements extremely accurate and reproducible.
FTIR can be performed using one of three techniques: Attenuated Total Reflectance (ATR) for very thin and highly viscous samples, KBr pellets for solid samples, or NaCl liquid cell for both solids as well as liquids. Both KBr and NaCl are transparent in the IR region of the electromagnetic spectrum.
The Beer Lambert’s Law can be used to calculate the concentration of a sample given its absorbance using the following formula:

The absorbance A of the sample can itself be calculated as a function of the relative absorbance:

Using these theoretical principles, one can calculate the various functional groups present in the composite sample. The differential scanning calorimeter (DSC) will help us find the glass transition temperature Tg of the prepared polymer composite. We expect the Tg of the PMMA/MWNT composite to be higher than just the pure PMMA. This is mostly due to the fact that the PMMA/MWNT composite has a larger number of intermolecular forces—primarily van der Waals but some minimal hydrogen bonding as well—as the PMMA molecules get adsorbed onto the MWNT surface. These increased intermolecular forces result in a higher Tg for the composite.
It is critical to understand what happens at the molecular level when the PMMA solution and the MWNT suspension are mixed. Physisorption or physical adsorption is a type of adsorption in which the adsorbate adheres to the surface only through Van der Waals (weak intermolecular) interactions [1]. Chemisorption, on the other hand, is a type of adsorption whereby a molecule adheres to a surface through the formation of a chemical bond, as opposed to the Van der Waals intermolecular forces which cause physisorption [2]. In our case, only physisorption occurs when we mix the PMMA and MWNT solutions due to the moderately low temperatures involved. Chemisorption would require a much higher temperature than the 55 °C of the water bath.

The key point to note is that intermolecular forces do not show up in an IR spectrum since they are quite weak and show up at much lower vibrational frequencies. Therefore we can confirm the fact that only physisorption and no chemisorption occurs by noticing the absence of the corresponding peaks in the FTIR spectrum. In general, peaks for chemisorbed and physisorbed molecules occur at different frequencies not only due to modifications in surface chemical composition, but also due to variances in the strengths of the bonds formed, as substantiated by [3]. In this journal article, ATR spectroscopy was used and it was observed that the asymmetric and symmetric stretch bands for chemisorbed 3-APTS occurs at 1096 cm–1 and 1022 cm–1, whereas the bands for physisorbed 3-APTS are found at 1115 cm–1 and 1038 cm–1.
  1. Experimental



The instrument used in this experiment is the Tensor 27 FT-IR manufactured by Bruker Optics GmbH. The experimental procedure used for this experiment was outlined in the lab manual of Characterization of Materials [4], under experiment #2. The two films prepared were PMMA and a PMMA/MWNT composite.
The basic components of an FTIR system are: the radiation source, the interferometer and the DTGS detector (Figure 1). A background spectrum must be obtained each time the


Figure 1 Basic Components of an FTIR System: The interferometer serves as the replacement counterpart for prisms (or diffraction gratings) used in traditional dispersion optics.
A background is used in order to remove all instrumental characteristics, thereby ensuring that the resulting spectrum is strictly a function of the sample only. The instrumentation process outlining the sequence of steps involved in using an FTIR is summarized in Figure 2.



Figure 2 FTIR Instrumentation Process: The sequence of steps
involved in obtaining an IR spectrum using an FTIR.

Summary of the Procedure



We first prepared a PMMA film by evaporating in a petri dish a dissolved mixture of PMMA in acetone. We then prepared the MWNT/PMMA nanocomposite by mixing acetone-dissolved PMMA with a dilute suspension of MWNT, also in acetone. The MWNT had to be sonicated using a low power ultrasonic bath. This was to form a cohesive suspension by preventing the nanotubes from agglomerating. The mixture was then poured on to a petri dish and allowed to evaporate to form a film.
The two films are then characterized using the FTIR instrument and the provided OPUS software. Any thin films are analyzed using the ATR-FTIR technique. Further, a DSC analysis of both the PMMA and MWNT/PMMA nanocomposite is performed. This is in order to determine the glass transition temperature of the prepared polymers.

Deviations from the Manual



Because both the films to be characterized were fairly thin, we simply used the ATR-FTIR characterization technique to analyze both films. Consequently, Part C involving the preparation of a KBr pellet was skipped. The ATR technique was much quicker since it required virtually no sample preparation.
For Part E, we performed a DSC run of only the PMMA/MWNT film, and not the pure PMMA film. This was in order to save time. The DSC run for the PMMA solid was already pre-run by the TA, and we were given a printout of the results for analysis and determination of the glass transition temperature.
  1. Observations and Results

Qualitative Observations




Quantitative Observations




  1. Discussion



Error Analysis




  1. Conclusions


7. References



[1] "IUPAC Compendium of Chemical Terminology, Electronic version,

http://goldbook.iupac.org/P04667.html. Retrieved on 2008-07-29.

[2] “IUPAC on properties of physisorption”, Chemisorption and physisorption,

http://old.iupac.org/reports/2001/colloid_2001/manual_of_s_and_t/node16.html.

Retrieved on 2008-07-29.

[3] “FTIR-ATR-spectroscopic investigation of the silanization of germanium surfaces

with 3-aminopropyltriethoxysilane”, Fresenius' Journal of Analytical Chemistry,

Springer Berlin, pp. 663-668, Volume 335, Number 7, December, 1989.

[4] Q. Xie, F. McCourt, Nanotechnology Engineering NE 320L Lab Manual,

University of Waterloo, Waterloo, pp 31-34 (2008).

[5] Principal IR Absorptions for Certain Functional Groups, IR Literature Values, NE

226 FTIR Supplement.

[6] “FTIR Spectroscopy—Attenuated Total Reflectance (ATR)”. Perkin Elmer Life

and Analytical Sciences (2005). Retrieved on 2008-07-28.

Appendices

Appendix A – Original Observations

Appendix B – Sample Calculations

Appendix C – Questions

Question 1



What are the advantages of the ATR-FTIR technique?

ATR stands for attenuated total reflectance. ATR is a sampling technique used in conjunction with infrared spectroscopy which enables samples to be examined directly in the solid or liquid state without further preparation [6]. ATR makes it possible to amplify the signal, thus facilitating surface analysis and analysis of very thin and highly viscous films. This is normally done by pressing a flexible sample against the surface of a large selenium crystal which has a high index of refraction.

The amplification is done using the optical principle of total internal reflection (TIR) where light incident upon a surface beyond a certain critical angle is completely reflected off the surface with very little, if any, transmittance. TIR is the very phenomenon that causes a diamond stone to glitter, and is precisely the idea behind the working of fibre optic cables.

The key features and major advantages of the ATR-FTIR technique are [6]:


  • Faster sampling

  • Improving sample-to-sample reproducibility

  • Minimizing user-to-user spectral variation

  • Higher quality spectral databases for more precise material verification and identification

Question 2



Why is it important to collect a background spectrum first?
A background spectrum is normally a spectrum with no sample in the beam. There are two good reasons for why it is important to collect background spectrum before each measurement:


  1. A background is used to obtain a spectrum in which all instrumental characteristics are removed, thereby ensuring that all spectral features present in the resulting spectrum of the sample is strictly a function of the sample only.




  1. Furthermore, the background spectrum ensures a relative scale for the absorption intensity. This background can then be compared to the measurement with the sample in the beam to determine the “percent transmittance”.



Appendix D FTIR/DSC Spectrum




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