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

1

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

4

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

6

FT-IR spectroscopy: Theory

Vibrational transitions absorb discrete energy levels unique to a molecule

7

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

8

•  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

9

FT-IR of a Fully Crosslinked Phenolic Polymer

12

14

16

18

20

22

24

26

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

10

•  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

15

16

17

18

19

20

21

22

23

24

25

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

11

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

12

14

16

18

20

22

24

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

12

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

13 Most highly crosslinked

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

14

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

15

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

40

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%

16

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

18

Application 2: Identifying a Decomposition Process

•  –CH3 group formed •  Lower degree of crosslinking

Yellow “cake” below

Dark surface “crust”

19

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

20

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)

23

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

25