CHEMISTRY 308
IDENTIFICATION OF POLYMERS BY INFRARED SPECTROSCOPY
Polymer films may be identified by allowing infrared radiation to pass through the material and
recording the spectrum of the radiation that emerges. Certain chemical bonds absorb radiation of
specific wavelengths and give rise to absorption bands in the emergent radiation as follows:
Stretching Modes for various functional groups:
O-H Stretch at 3400 cm-1, usually a broad peak
(cm-1 = wave numbers)
N-H stretch at 3300 cm-1, usually sharper than an O-H
C-H stretch is found near 3000 cm-1 (sp2 =C-H > 3000; sp3 C-H < 3000 cm-1)
C≡ stretch at 2250 cm-1
C=O stretch at 1735 cm-1 (ester) and 1660 cm-1 (amide)
The C=O appears at a lower value of about 1720 cm-1 if it is next to a benzene ring
C=C stretch at 1650 cm-1 (not part of benzene ring!)
The benzene ring is observed at 1600 and 1450 cm-1 (often two bands at
each position)
C-F stretch at 1240 to 1100 cm-1
C-O stretch appears between 1300 and 1000 cm-1 in alcohols, ethers, and esters
C-Cl stretch in poly(vinyl chloride) occurs as a pair of bands between 700-600 cm-1
Bending Modes
C-O-H bending and R-N-H bending modes are often observed at about 1600 cm-1
CH2 bending occurs near 1450 cm-1
CH3 bending occurs near 1375 cm-1
Aromatic ring out-of-plane bendings (oops):
monosubstituted ring; pair of strong peaks at 700 and 750 cm-1
para-disubstituted ring; one strong peak at about 750 cm-1
It should be noted that plasticizers often have C=O absorption at 1735 cm-1 or lower. In some
cases, their presence does not alter the remainder of the spectrum, and the peak at 1735 cm-1 may
simply be ignored when analyzing the spectrum. PVC is the most common example of plastics
that contain plasticizers.
Page 3 and 4 show some common infrared bands in organic compounds and polymers. You
would expect that a polyester should have all of the features expected for a non-polymeric ester.
Likewise, a polyamide should look like a non-polymeric amide!
Work in groups of three for this experiment
Obtain one clear packaging or other plastic sample and bring it to the laboratory (opaque
materials may also be analyzed). Examples of possible materials include: sandwich bags,
wrapping materials (Glad wrap, Saran wrap, etc.), oven bags, meat wrap from the supermarket,
transparent tape, packaging materials of various kinds, windows used in business envelopes,
cookie tray from inside package, credit card insert from wallet, gift wrap from fruit basket, and
thin films of plastic made by pressing the plastic on a hot press in the Plastics laboratory in the
Engineering Technology Building. Samples should be as thin as possible in order to obtain a
good spectrum. In addition to the sample you bring, I want you to analyze two of my samples.
First run a background scan. Then, secure your sample with a piece of tape on one of our holders
in such a way that it will hang in the infrared beam in the sample compartment without the light
beam hitting the tape, itself. Alternatively, mount the material on a card and insert the card into
the beam of the infrared spectrometer. Now determine the spectrum of the polymer. Label the
prominent peaks with their frequencies. Try to identify the material by using the spectral values
given on page 1, 3 and 4. Look for prominent features such as the carbonyl group (C=O). Be
sure to write the number of the sample on your spectrum for identification purposes.
It is difficult to make an absolute identity of the polymer sample without making reference to a
library of spectra. In some cases, the best we can do is to place the sample into a certain class,
for example, polyester, polyamide, polyethylene, polypropylene, or polyurethane. We will
attempt to identify the sample by comparing your spectrum to those in the Lampman polymer
library or by comparison to spectra in the books by Dieter O. Hummel. The Hummel books
contain hundreds of spectra of various polymer samples. Wilson library has the books in its
collection. Library call number: QD139.P6 H813 vol.1 and vol. 2 located in the oversize section
of Wilson Library, 4th floor west. These volumes are the 1971 edition of his books.
Library call number: QD139.P6 H8 1991 v.1 Bd.1 Pt.A and Pt. B located in oversize section of
Wilson Library, 4th floor west. This is the 1991 edition of his books.
Most copolymers and homopolymers can be identified by looking at the Hummel books or my
collection of spectra. Don't expect to differentiate conclusively between various "grades" of
polymers (i.e. manufacturer). It may also be difficult to differentiate between similar materials.
For example, you will have a hard time differentiating between nylon 6,6 or 6,10 or 6. All of
these nylons look pretty much the same! Fillers and colorants, etc. aren't easy to spot in the
spectrum because they may not be present in sufficient quantities. Remember that infrared
spectroscopy is really only good for identifying functional groups. Also, remember that the
thickness of the sample can make identification difficult. The Hummel samples may be thinner
than your samples. The spectrum for an identical sample may look different. Thick samples will
"bottom-out" on the spectrum obtained.
Now repeat the process with the two samples of polymers that I supply to you. Label the peaks
with frequency values on each spectrum. Print copies of each spectrum. Look for the major
peaks to see if you can use the information in the experimental writeup to make a tentative
identification. Now look at the Hummel library to see if you can make a positive identification.
Common Infrared Peaks in Organic Compounds and Polymers
Type of Group
Types of Compounds
OH
R-OH
alcohol
R-C-OH carboxylic acid
O
Infrared Value
3400 cm-1
3400-2400 cm-1 very broad
RNH2 1O amine
H
R-N-R
H
N
H
2O amine
3300 cm-1
NH
R-C-NH 2
O
H
R-C-N- R
O
C-H
sp3 carbon
=C-H sp2 carbon
amide
may overlap with OH
amide
-CH3 -CH2
-CH
alkene C-H
about 2950 cm-1
about 3050 cm-1
benzene C-H
R C N
C N
nitrile
2250 cm-1
R
C
O
C
O
aldehyde
1725 cm-1
C
O
ketone
1715 cm-1
C
O
ester
1735 cm--1
O
amide
1660 cm-1
H
carbonyl group
R
R
RO
R
HN
R
C
R
about 1715 if next to
benzene ring
Type of Group
C C
Types of Compounds
Infrared Value
1650cm-1
alkene
Carbon-Carbon
double bond
aromatic rings, including
substituted benzene ring
1600 - 1450 cm-1
two or more weak peaks
in range
alcohols
ethers
esters
carboxylic acids
C-O
-Cl -Br
-F
R-Cl
R-Br
R-F
halides
1300 - 1000 cm-1
These peaks are very strong!
It is very difficult to identify
these halides, except for
fluorides
R-F 1240 to 1100 cm-1
R-Cl and R-Br are often out
of the range; below 700 cm-1
______________________________________________________________________________
LAB REPORT
1.
Indicate the source of the sample you brought. Write the sample number printed on the
two mounted samples that I give you, on your spectrum.
2.
Identify on each spectrum the bands of diagnostic value. Draw the functional group on
the spectra. Examples include: C-H, O-H, benzene ring, C=O, C-O, etc. If possible,
identify the polymer samples using only the infrared spectrum.
3.
Make a "positive" identification of each of the plastic samples by using Hummel books.
Draw the chemical structure for the polymer on each spectrum.
4.
Submit the three labelled spectra with your report. Attach the plastic sample that you
brought to the lab onto the spectrum.
5.
What infrared bands are likely to be found in the following polymers? Be sure to assign
C-H aromatic, C-H aliphatic, C=O, C-O, O-H, N-H, C≡ and C-Cl with expected cm-1
values under each polymer structure:
O
CH 2
O
(CH 2)4
CH 2
CH 2
CH
CH 2
CH 2
CH 2
(CH 2)6
N
H
CH 2
CH 2
CH 2
CH 2
linear polyethylene
Polystyrene
O
N
H
Polyamide (Nylon 6,6)
Polyester (PET)
CH
O
O
O
O
CH 2
Cl
CH
CH 2
Cl
CH
Poly(vinyl chloride) PVC
without plasticizeer
polyoxymethylene (Delrin)
O
O
CH 2
OH
CH
CH 2
OH
CH
Poly(vinyl alcohol)
Revised 2 April 2007
CH 2
N
C
CH
CH 2
N
C
CH
H3C
CH 2
H3C
O
CH
CH 2
O
CH
Poly(acrylonitrile)
Poly(vinyl acetate)