Molecular Spectroscopy: Polymer...
Transcript of Molecular Spectroscopy: Polymer...
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Molecular Spectroscopy: Polymer Analysis Using FTIR, NIR and Raman
Dr. Todd Strother
August 9, 2012
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Outline of Polymer Analysis
• Review of vibrational spectroscopy
• Infrared
• Raman
• Instrumentation
• Is50 with diamond ATR
• Raman accessory
• TGA and NIR modules
• Polymer applications
• ATR into the Far IR
• Raman for analysis and polymorphs
• Near IR for Industrial Solutions
• Formulation studies with TGA
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Transitions Guide to the EM Spectrum
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• NIR • Common in industrial applications
• Deeply penetrating light
• Mid-IR • Most common type of molecular spectroscopy
• Useful for most organic compounds
• Far-IR • Provides more information that mid-IR
• Useful for inorganics and some organicc
• Raman • Complementary to infrared
• Provides information when IR isn’t suitable
Modes of Analysis with Molecular Spectroscopy
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When Does a Material Absorb Infrared Light ?
• The frequency of the light must be identical to the frequency of the
vibration (resonance)
• Higher Frequency Lower Frequency
• The dipole of the molecule must change during the vibration
• Strong Absorbance No Absorbance
Cl H N N
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Raman Spectroscopy – The Raman Effect
Rayleigh scattering (filtered out)
Raman scattering (Stokes shift)
LASER
200 400 600 800 1000 1200 1400 1600 1800 2000 Raman shift (cm-1)
Blo
ckin
g F
ilter
0
Rayle
igh
scatt
eri
ng
Ra
man
sc
att
eri
ng
Excit
ati
on
fre
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en
cy
V = virtual state
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Raman Compared with Infrared
• Complementary information
• Functional groups dominant in Infrared spectrum
• Molecular backbone dominant in Raman spectrum
• Raman often useful for characterizing morphology
• Weak IR absorbers often strong Raman emitters and vice versa
• Aqueous solutions pose fewer challenges with Raman
FT-IR Transmission Spectrum
20
40
60
80
%T
ran
sm
itta
nc
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Raman Spectrum
1
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Ram
an
In
ten
sit
y
1000 2000 3000 4000 Wavenumbers (cm-1)
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Instrumentation
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Instrumentation
• Nicolet iS50
• Diamond ATR
• Raman in accessory compartment
• Automatic beam changer
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Instrumentation
• Nicolet iS50
• NIR module
• TGA module
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Polymer Orientation Studies
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Polymer Orientation Studies
• Polymers are often stretched along an
axis • IR absorption is sensitive to the orientation of
the molecules
• Different spectra are obtained depending on molecular bond orientation
• FT-IR is used in the polymer industry • To do polarization studies
• Determine crystallinity
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• Stretched Polyethylene film placed in instrument
• Polarizer automatically moved into beam path and rotated during data
collection
• Series of spectra show changes in absorption as polarizer is rotated
Polymer Orientation Studies
0°
170° 10° In
cre
ments
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Polymer Orientation Studies (FT-IR)
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Far-IR: Extended Range
When Mid-IR isn’t enough
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Far-IR: Extended Range
Infrared Spectroscopy • Generally encompasses 4000cm-1 to around 600cm-1
(wavelength 2.5 microns to 20 microns)
• Limited by the detector, lens and window material
Far Infrared • Diamond allows longer wavelengths to pass through
• Down to ~200cm-1 (50 micron wavelength)
• Diamond ATR crystals and windows allow Far IR
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Far-IR: Extended Range
Polymer additives and pigments are scrutinized for safety • Some pigments are banned because they are heavy metals (lead, cadmium)
• Often these pigments have no mid-IR signature
• Can be detected with Far-IR
Yellow pigmented polymer was obtained from a supplier • It was suspected to contain the banned pigment CdS
• Mid-IR analysis using an ATR didn’t show anything unusual
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• Mid IR analysis
– The spectrum shows spectral features of polyethylene and carbonated
materials
– There is not enough information to identify the yellow pigment composition
Polyethylene
hidden by
CaCO3
CaCO3
hidden by
polyethylene
Far-IR: Extended Range
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Far IR using the diamond ATR • Shows that it contains cadmium pigment
Far-IR: Extended Range
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Raman in Conjunction with FT-IR
Raman complements infrared information with
crystallinity, density and inorganics ID
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Polymer Orientation Studies
Amorphous
Linearly
stretched
• Polyethylene terephthalate (PET)
used in making bottles
• Thick walled blanks are heated and
blown into a mold
• The PET molecules in the sides of
the bottle become stretched and
linearly aligned
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Bottle side: Crystalline (PET)
Bottle Top: Amorphous (PET)
Polymer Orientation Studies (Raman)
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Raman complements FT-IR
• Mid infrared ATR
• …Reveals the material being high density
polyethylene, with “inorganic filler”
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• Same sample place in Raman module
• Unique peaks show information on additives
• Data combined with FT-IR gives a more
complete picture of the material
Raman complements FT-IR
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Bottle
Rutile (TiO2)
Anatase (TiO2)
CaCO3
Raman complements FT-IR
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NIR Raw Material Identification
Method development in process
environments
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Polyethylene is often classified according to density, which in turn
depends on numbers and size of side branches.
Linear Low Density (LLDPE)
•Large numbers of short branches
• 0.915 – 0.925 g/cm3
Medium density (MDPE)
•0.926 – 0.940 g/cm3
High density (HDPE)
•Very few branches
• greater than 0.941 g/cm3
Catalyzed
Polymerization
NIR for Raw Material Identification
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Density of Polyethylene by NIR
• NIR used for chemometric method development
• No sample preparation required
• Place sample on integrating sphere and get answer
NIR for Raw Material Identification
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• Ethylene/Polypropylene Copolymers
• Ethylene may be added in various amounts to propylene to form
copolymer material
• These copolymer materials exhibit high stiffness and impact strength
• Additionally they have better clarity and lower melting points than
polypropylene alone.
NIR for Raw Material Identification
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Ethylene in PP content range 2%-16%
NIR for Raw Material Identification
• Quantitative results based on
analyzing standard materials
• Method deployed into process
plants using process instruments
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TGA-IR
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# # #
weig
ht
time
• TGA • Thermo Gravimetric Analysis (TGA)
• The sample is precisely heated in a furnace
• As the sample releases vapors, the weight loss is recorded
• Provides Quantitative data
TGA IR
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• TGA-IR • The vapor output of the TGA is coupled to an FT-IR gas cell
• This allows the IR spectra of the vapors to be obtained
• Gives Qualitative Data
TGA IR
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TGA IR
• Deformulation and Failure Analysis • Rubber gaskets were found to be failing in the field
• TGA analysis of failing gaskets shows incorrect formulation
Good
Bad
Good
Bad
TGA IR of vapor at 67 min
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Missing material from Bad Gasket
Library Search Results
Isopropylidenediphenol
TGA IR
• Deformulation and Failure Analysis • Library search on the IR spectra
• Shows incorrect formulation
• Missing ingredient
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Troubleshooting in the Analytical Laboratory
• Deformulation of a polymer resin
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Conclusions
• Molecular vibrational spectroscopy is more than just mid-infrared
• Molecular vibrations are useful from Near IR (10000 cm-1) to Far IR (200cm-1)
• Raman is a bit unique but is still based on vibrations
• Applications benefiting from vibrational spectroscopy
• Orientation and crystallinity
• Pigments and additives
• Density and copolymers
• Deformulation and failure analysis
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Please Contact Us for More Information
Learn more about the instruments used:
Thermo Scientific Nicolet iS50
Please feel free to email me