Using FT-IR Spectroscopy to Elucidate the … FT-IR Spectroscopy to Elucidate the Structures of...
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Using FT-IR Spectroscopy to Elucidate the Structures of Ablative Polymers
Wendy Fan The composition and structure of an ablative polymer has a multifaceted influence on its thermal,
mechanical and ablative properties. Understanding the molecular level information is critical to
the optimization of material performance because it helps to establish correlations with the
macroscopic properties of the material, the so-called structure-property relationship. Moreover,
accurate information of molecular structures is also essential to predict the thermal decomposition
pathways as well as to identify decomposition species that are fundamentally important to
modeling work. In this presentation, I will describe the use of infrared transmission
spectroscopy (FT-IR) as a convenient tool to aid the discovery and development of thermal
protection system materials.
https://ntrs.nasa.gov/search.jsp?R=20110012153 2018-07-15T23:29:17+00:00Z
Using FT-IR Spectroscopy to Elucidate the Structures of Ablative Polymers
Wendy Fan ERC Inc.
NASA Ames Research Center
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Outline
1. Introduction
• Importance of polymer structure (Motivation)
• FT-IR principle and technique
2. Applications
• Detailed structural analysis of a phenolic polymer
• Correlation among cure condition - crosslinking - thermal stability/char yield
• Correlation between crosslinking - thermal oxidative decomposition mechanism
• Diagnostic of a thermal decomposition product
3. Conclusion 2
Unique sequence of amino acids
Held together by hydrogen bonding
Held together by hydrogen, ionic, sulfide bonding
Several peptide chains joined together
Structure of Hemoglobin – How Nature Designs Its Materials
3 http://alevelnotes.com
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Polymer Structure and Morphology
Polymer matrix
Reinforcing fiber
Inter-face
Polymer structure and morphology (shape of a polymer)
...influence key thermal, mechanical and ablative properties
How the polymer distributes around the fiber
Good morphology
Poor morphology
Techniques to Identify Polymer Structures
1. Nuclear Magnetic Resonance (NMR, both liquid and solid state) • Precise information of how atoms are bonded
2. FT-IR (Fourier Transform Infrared) transmission spectroscopy • Identify polymer structures, in particular polar functional groups,
crosslink degree, crystalline region
3. Raman spectroscopy • Identify non-polar groups, such as olefinic groups (as in graphitic
species)
4. Mass spectrometry • Information about fragments/groups, fragmentation modes
5. XPS (x-ray Photoelectron Spectroscopy) • Information about elements and functional groups on a surface 5
FT-IR – An Introduction
andersonmaterials.com
http://www.biophysik.uni-freiburg.de/Spectroscopy/FTIR/spectroscopy.html
Main advantages of FT-IR • Speed • Sensitivity • High spectral accuracy and resolution • Internally calibrated • Mechanical simplicity • Easy sample preparation and operation
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FT-IR spectroscopy: Theory
Vibrational transitions absorb discrete energy levels unique to a molecule
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Symmetrical stretching
Twisting Wagging
Scissoring Rocking Asymmetric stretching
Vibrational modes of a -CH2 group
http://en.wikipedia.org/wiki/Fourier_transform_infrared_spectroscopy
FT-IR Spectrum of Formaldehyde
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• To chemists, a FT-IR spectrum is like a pair of eyes looking into a molecule • The intensity, shape, width and location of a peak all reflect the specific local environment the group is in
Wavenumber (cm-1)
C
Phenolic resin (resole) Phenolic Network polymer
Curing Chemistry of Phenolic Resin
A typical reaction in network formation
Cure
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FT-IR of a Fully Crosslinked Phenolic Polymer
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14
16
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20
22
24
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500 1000 1500 2000 2500 3000 3500 4000
Tran
smitt
ance
(%)
Wavenumber (cm-1)
2800-3100cm-1
3500cm-1
1640-1650cm-1 1160cm-1
1120cm-1 750-880cm-1
1475cm-1
1210cm-1
1260cm-1
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• Textbook example and extremely informative • Absorption peaks can be easily assigned to corresponding groups • Quantitative estimation of the degree of crosslinking is possible.
Region 1: 4000cm-1 - 2700cm-1
-OH
-C=C-H
H 2 C
O H O H
O H
O H
O H
O H
H H
H 2 C
O H O H
O H
O H
O H
O H
H H
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16
17
18
19
20
21
22
23
24
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2800 3000 3200 3400 3600 3800
Tran
smitt
ance
(%)
Wavenumber (cm-1)
3040cm-1
2940cm-1
2860cm-1
3500cm-1: Phenolic OH 3040cm-1: aromatic sp2 C-H 2940cm-1: Aliphatic sp3 CH2
asym. Stretch 2860cm-1: Aliphatic sp3 CH2
sym. stretch
3500cm-1
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ICH2 / IC=C can be used to semi-quantitatively measure the crosslinking degree
Region 2: 1700cm-1 - 1000cm-1
H 2 C
O H O H
O H
O H
O H
O H
H H
H 2 C
O H O H
O H
O H
O H
O H
H H
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14
16
18
20
22
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1000 1100 1200 1300 1400 1500 1600 1700
Tran
smitt
ance
(%)
Wavenumber (cm-1)
1475cm-1
1210cm-1
1260cm-1
1610- 1650cm-1
1160cm-1
1120cm-1
1610-1650cm-1: Benzene C=C 1475cm-1: CH2 scissor 1260cm-1: CH2 rocking 1210cm-1: sp2 C-O 1160cm-1: CH2 wagging 1120cm-1: sp2 C-H
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Region 3: 900cm-1 - 700cm-1
22.5
22.7
22.9
23.1
23.3
23.5
23.7
23.9
24.1
24.3
24.5
700 750 800 850 900 950
Tran
smitt
ance
(%)
Wavenumber (cm-1)
790cm-1 750cm-1
880cm-1
820cm-1
1,2-ortho-
1,2,4-ortho, para, 1,2,6-ortho, ortho,
1,2,4,6-ortho, para, ortho-
1,6-para
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Application
Cure conditions Phenolic network density & structure
Thermal and oxidative stability
Experiments: phenolic resin was cured under three different conditions, each with increasing temperature and cure time.
Condition 1, Condition 2, Condition 3
Temperature and cure time increases
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0
20
40
60
80
100
120
140
500 700 900 1100 1300 1500 1700 1900
Tran
s. (%
)
Wavenumber (cm-1)
Condition 1 Condition 2 Condition 3
FT-IR of Cured Phenolic Polymers
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Most crosslinked
Medium crosslinked
Least crosslinked
CH2 rocking
sp2 C-H
Change of intensity in multiple peaks indicates the progress of curing (more crosslinking)
Thermogravimetric Analysis (TGA) (in Nitrogen, 20°C/min.)
Char yield correlates with degree of polymer crosslinking
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50
60
70
80
90
100
110
50 150 250 350 450 550 650 750 850 950
Wei
ght %
Temperature (°C)
Cure condition A Cure condition B Cure condition C
61.8%
53.3% 50.1%
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80
82
84
86
88
90
92
94
96
98
100
0 100 200 300 400 500 600
Wei
ght%
Temperature (°C)
Condition A Condition B Condition C
+1.26%
+1.68%
+4.82%
TGA in Air 20°C/min.
Weight% increases with increased crosslinking 17
Decomposition of Phenolic – Char Forming Mechanism
Substructures of graphitic/carbon char
Amorphous carbon char
Char structures were characterized by solid state 13C NMR
Virgin polymer structure dictates the decomposition pathways and char yield
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Application 2: Identifying a Decomposition Process
• –CH3 group formed • Lower degree of crosslinking
Yellow “cake” below
Dark surface “crust”
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2960cm-1 (-CH3)
1375cm-1
(-CH3)
820cm-1
(-CH3)
Thermal Decomposition in Vacuum
G. E. Maciel and I.-S. Chuang, Macromolecules, 1984, 17, 1081-1087 Fyfe, C. A. et al, Macromolecules, 1983, 16, 1216-1219 Zinke, A. J. Appl. Chem., 1951, 1, 257.
Likely Pathway
• Lower degree of crosslinking • –CH3 group formed
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FT-IR Spectrum of a Model Polymer
Center portion
Surface portion
Model polymer
Model polymer was cured in nitrogen and at an elevated temperature (close to the oven condition) 21
A Bottom Up Approach to TPS Materials Discovery
Molecular level
Microscopic level
Macroscopic level
System level
Ablative, thermal structural
properties
Thermal properties
Chemical structures, processing
Morphology, polymer-fiber
interface
Mechanical properties
Char properties
• A thorough understanding of chemistry/molecular structure is key • Most of these correlations have been obtained by experiments
Advantages and Limitations
Limitations: 1. Mostly qualitative, does not provide precise molecular
structures 2. Can be semi-quantitative with the right software 3. Not all polymers have well-resolved absorption peaks in the
IR region (overlaps, weak absorptions)
Advantages: 1. Convenient, low cost, speedy 2. Excellent tool for first screening of polymer structures and
identification of unknowns 3. Useful for predicting reactivity and physicochemical
properties, as well as interpreting decomposition mechanisms and products
4. Routinely used at Ames for qualitative determination of new polymer structures as well as analyzing curing products
5. Can be coupled with TGA to study thermal decomposition products (TGA-IR)
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abso
rban
ce
Wavenumber (cm-1)
http://cnx.org/content/m23038/latest/
http://www.uclan.ac.uk/schools/forensic_investigative/fire_hazards_science/equipment_tests/tga_ftir_magna.php
Using TGA-FT IR to Monitor Gas Phase Products
• TGA-IR allows the gas phase products to be collected continuously as a function of temperature. • Gas phase infrared spectra are much sharper than condensed phase spectra, and recognition of the individual molecules is possible.
Contributors
Acknowledgement
Hypersonics group of Thermal Protection Materials branch,
NASA Ames Research Center
Funding
Hypersonics EDL-TDP
Tane Boghozian, Ehson Ghandehari, Firouzeh Mohadjerani, Jeremy Thornton
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