2. Characterization of Polymers
Polymers are most commonly characterized by basic
methods:
i)
Spectroscopic Methods
Infrared Spectra
Ultraviolet spectra
Nuclear Magnetic Resonance
X-Ray diffraction
ii)
Thermal Analysis
Thermogravimetric analysis(TGA)
Differential Thermal Analysis(DTA)
Differential scanning calorimetry(DSC)
Pyrolysis gas chromatography
iii) Mechanical analysis
Tensile
Compressive
Flexural strength
Impact resistanse
Shore A and Shore D
Abrasion
i)
Spectroscopic Methods
Infrared Spectra
Figure : FTIR spectrum of carbon black EVA and EVA/Carbon black
0,07
0,06
y (Abs)
0,05
0,04
0,03
0,02
0,01
0
-0,01
1550
1600
1650
1700
1750
1800
1850
EVACB250
Y Actual
Y Predicted
Seriler 3
Seriler 4
Seriler 5
Seriler 6
Ultraviolet spectra
Ultraviolet spectra are not used as "fingerprints" the way
infrared spectra are, since the adsorption bands are generally
broad owing to superimposed vibrational transitions ; instead,
they provide information on conjugation in compounds. For
this reason, ultraviolet spectroscopy does not have the broad
applicability in the polymer field that infrared has. I has
however, proved to be particularly useful in the identifying
and analyzing materials in polymer such as residual monomer,
antioxidant, or inhibitors.
Figure : Uv spectrum of styrene in cyclohexane in different
concentrations (1) : 1 mg/L, (2) : 0.1mg/L, (3) 0.01 mg/L, (4) :
0.001mg/L.
1900
Nuclear Magnetic Resonance
Nuclear Magnetic resonance(NMR) spectroscopy, developed
ater infrared and ultraviolet, is probably the most useful of the
spectroscopic methods, not only for determining chemical
structure but also for providing information ob configuration
or conformations in molecules and on reaction rates. In the
polymer field, NMR spectroscopy can be used to "fingerprint"
polymer typesin much the same way that infrared is used, and
also to obtain structural information by reference to known
chemical shift data.
Considerable work in recent years has been directed toward
elucidating the configuration of polymers of varying tacticity.
Figure for example, illustrates variations in spectra for atactic,
sindiotactic and isotactic, poly(methyl metacrylate). The
resonance 10  is that of internal standard, tetramethylsilane.
The signal for the ester methyl (COOCH3) is the one occurring
furthest downfield at about 6.5 ; the CH2 group signal occurs
at about 8.2 , and the -Ch3 signal at about 9 . It should be
noted that the resonance signals are somewhat broader than
those for low-molecular -weight compounds, a phenomenon
arising from the lower mobility of polymer molecules.
ii)
Thermal Analysis
Thermogravimetric analysis(TGA)
Measurent of weight loss as a function of temperature is called
Thermogravimetry or Thermogravimetric analyses(TGA).
Like DTA, Tga has been known for a long time(the first
thermobalance was described in 1915) but has only been
applied to polymer studies in recent years. Weight loss arises
from polymer decomposition; hence TGA provides
information directly on thermal stability. It is also useful in
characterizing polymer structure, particularly where
decomposition occurs through loss of a known entity, as for
example, the elimination of HCl from polymer or copolymers
of vinyl chloride. Here, weight loss can be correlated with
percent vinyl chloride in the polymer.
400
250
140
116
96
48
0
Figure : TGA thermograms of aged ethylene/vinyl acetate(EVA)
copolymer
Differential Thermal Analysis(DTA)
Differential thermal analyses(DTA) is not a new technique; in
fact, it dates back to work by Le Chatelier in the last century.
But the application of DTA to the study of thermal behaviour
of polymers is relatively new. The principles of DTA is based
on temperature differences between a sample and an inert
reference material which are both heated up at the same rate.
The temperature difference can be arise from such
phenomena as change in specific heat such as occur at the
glass
transition
temperature,
from
endothermic
phase
transition, as at the melting point, or chemical reaction.
Typical reaction that might occur in polymeric materials are
cross-linked and thermal degradation.
GLASS TRANSITION TEMPERATURE : As the
temperature is raised, a point is reached where the
properties of the polymer change to those of a rubber. This
temperature is called the Glass transition temperature Tg
A schematic representation of the DTA apparatus is shown in
below.
Endothermic
T
DTA THERMOGRAM
Differential scanning calorimetry(DSC)
More recently, the technique of Differential scanning
calorimetry (DSC) has been developed. DSC is related to
DGA, the major difference being that energy is supplied to
sample and reference to keep their temperatures equal. The
recorded thus plots energy supplied against temperature. Peak
areas in DSC are therefore directly related to enthalpy
changes in the sample.
DSC THERMOGRAM
Pyrolysis gas chromatography
Scientists have been studying polymer pyrolysis almost as long
as they have been studying polymers, but it was not until 1954,
shortly after the technique of gas chromatography was first
reported, that pyrolysis products from polymers were
separated by this method. In 1959, the first report of the direct
coupling of a pyrolysis devise to the column inlet of a gas
chromatography appeared. Since then the combination of
pyrolysis and gas chromatography has to be extremely useful
tool for polymer characterization.
iii) Mechanical analysis
Among the most important mechanical properties of polymers
are
Tensile,
resistanse,
Compressive,
Flexural
strength,
Impact
Shore A and Shore D. For any test of such
properties to be meaningful it must be reproducible; and to
serve this end, a large number of standard tests have been
developed. In the united stated, the American society for
Testing Materials (ASTM) sets the standards.
Tensile strength
To measure tensile strength, a test specimen of uniform
cross section is clamped at each end and stretched until it
breaks. Tensile strength is defined as the stress or force
necessary to break the sample at a constant rate of
stretching.
Compressive Strength
Compressive strength is determined by subjecting a
sample to compression until it breaks. It usually varies
from about 2000 to 40.000 psi. Flexural strength is
measured by supporting at each end, in the horizontal
position, a test bar of uniform cross section, then
subjecting it to vertical stress until it yield or breaks.
Most common polymers have flexural strength ranging
from 3000 to 20.000 psi, with the more rigid, cross-linked
polymers usually having higher strengths than linear
polymers.
Impact Resistance
Impact resistance is a measure of toughness of a polymer.
It is commonly determined by allowing a weighted
pendulum to strike a vertically champed specimen and
measure the distance the pendulum travels after the
specimen breaks. Sample are often notched to give more
reproducible results. Values for common polymers vary
between about 0.5 and 10 foot-pounds Per inch. In all
testing of mechanical properties, a fairly wide spread in
measured values is obtained for the same batch of
polymer; hence it is necessary to test several samples and
record average values.
ZWICK/ROELL
MECHANICAL TESTING INSTRUMENTS