Transcript sfd - LSU Macromolecular Studies Group

Analysis of Materials (Polymers)
by Thermal Methods:
DSC, TG/DTA
Instructor: Ioan I. Negulescu
CHEM 4010
Tuesday,
October 29, 2002
Thermal Methods:
Thermal methods are based upon
the measurement of the dynamic
relationship between temperature
and some property of the system
such as mass and heat absorbed
or evolved by/from it.
• Differential Scanning Calorimetry,
DSC
• Differential Thermal Analysis,
DTA
• Thermogravimetry,
TG
are the most important thermal methods
used in characterization of polymers.
The temperature increase, T, of a
body which is heated is directly
proportional to the amount of heat
absorbed,
Q,
and
inversely
proportional to its mass, m, and its
capacity C to store heat:
T = Q/m C
Eq. 1
Consider the temperature increase of two
different samples of the same mass
m1 = m2
for which the same amount of heat was given
Q1 = Q2
If their heat capacities are different
C1  C2
they do not experience the same temperature
increase, i.e.,
T1  T2
A greater heat flow (dQ/dt, where t is time)
will always flow into the sample whose
heat capacity is higher, in order that the
steady-state heating rate be maintained.
The heat capacity at constant pressure (Cp)
of a material is defined as the temperature
increase of a unit of substance (mass) as a
result of the supply of a unit of heat at
constant pressure.
For the same substance, Cp is
dependent upon its aggregation
state, i.e., it is different for the
liquid state as compared to loose
gaseous or to more compact solid
state.
A polymeric material has different heat
capacities for amorphous or crystalline
morphologies. For amorphous polymers,
the heat capacity for the glassy state (i.e.,
below glass transition, Tg, where only
vibrations of atomic groups occur) is
different from that characterizing the
leathery (short range diffusional motion, i.e.,
of chain segments), rubbery (retarded longrange motions), rubbery flow (slippage of
long-range entanglements) or liquid state.
Figure 1. Temperature - molecular mass diagram
for amorphous polymers: (1) Glass transition (Tg); (2)
Diffuse transition zone; and (3) Thermal decomposition.
Figure 2. Temperature - Molecular Mass diagram
for (semi) -crystalline polymers: (1) Glass transition (Tg);
(2) Melting point (Tm); (3) Diffuse transition zone; and (4)
Thermal decomposition.
For amorphous polymers, the
glass-rubber transition temperature is of
considerable importance technologically.
It (Tg) determines the lower use limit of a
rubber (e.g., polydienes, Tg  -50°C) and
the upper limit use of an amorphous
thermoplastic material(e.g., polystyrene,
Tg  100 ° C).
In the case of (semicrystalline) linear
polymers it is possible to identify a melting
temperature (Tm). Above this temperature
the polymer may be liquid, viscoelastic or
rubbery according to its molecular mass,
but below it, at least in the high molecular
mass range, it will tend to be leathery and
tough down to the glass transition
temperature.
POLYHYDROXYALKANOATES
R can be hydrogen or hydrocarbon chains of up to
around C13 in length, and x can range from 1 to 3 or
more. Varying x and R effect hydrophobicity, Tg, Tm,
and level of crystallinity
POLYHYDROXYALKANOATES
Fig 4: The Family of PHAs has Physical Characteristics that
Allow it to be Used Across a Wide Spectra of Applications
Detailed information on glass transition,
crystallization, and melting, is therefore
critically important in formation, processing
and utilization of polymers.
Differential thermal methods (DTA and
DSC) have been widely applied to the
study and characterization of polymeric
materials.
In DTA the heat absorbed or emitted
by a system is observed by measuring the
temperature difference (T) between that
system (the sample) and an inert reference
material (often alumina), as the temperature
of both is increased at a constant rate
(usually 5-10 C/min).
Figure 3. Schematic diagram of a typical
DTA apparatus.
• In DSC, the sample and the reference are
also subjected to a continuously increasing
or decreasing temperature.
• In the scanning operation the sample and
the reference show different temperature
independent heat capacities.
• Heat (dQ/dt) is added to the sample or to
the reference as necessary to maintain the
two identical temperatures.
Bottom: (b)
measured
curve. m is
the measured
heat flux. bl
is the heat flux
corresponding
to the base
line and t is
time.
• The ordinate is usually represented by the
heat flux (denominated as  or dQ/dt) or by
the variation of the heat capacity.
• The glass-to-rubber transition, or shortly the
glass transition (Tg) is a phase change
reminiscent of a thermodynamic second order
transition (melting and crystallization being first
order transitions) for which a plot of specific
heat versus temperature shows a sudden
jump.
• The first order transitions appear
as peaks.
DSC curve of a polymeric sample: (1), (3) and (5) are base
lines; (2) is glass-to-rubber transition, Tg; (4) is the
interpolated base line; and (6) is the first order transition
peak.
The glass transition region in cooling (a) and
subsequent heating (b) mode showing some commonly
used definitions of glass transition, Tg.
Poly(Lactic Acid)
-[-O-CH(CH3)-CO-]n
Two of the most attractive features of
poly(lactic acid), PLA, polymers are:
• they are easily synthesized from
renewable resources (corn!)
• they are both hydro- and biodegradable
Poly (Lactic Acid)
Glass transition temperature, Tg.
-200
Glass
Transition,
Tg
DSC, W
DSC
-400
DDSC, W/min
240
200
160
120
80
-600
40
DDSC
0
-800
-40
30
40
50
60
70
o
Temperature, C
80
90
DSC traces
for melting
and
crystallization
of a polymer
sample.
Melting of polyoxymethylene with superheating.
DSC analysis of a poly(ethylene terephthalate)
sample quenched from the melt.
DSC traces of Low Crystallinity PLA treated in Water at
70C and 100C. The higher the crystallinity achieved
at 100 C, the higher and the less defined the Tg
2000
Same Melting
Pattern
Crystallization
Before Melting
W eak Cold
Crystallization
Strong Tg
DSC 70C  W
0
Melting
DSC 100C  W
W eak Tg
1000
0
-2000
o
1hr@ 100 C
o
1hr@ 70 C
0
50
-1000
100
150
o
Temperature, C
200
Melting of two semicrystalline HDPE samples.
DSC,  W
-5.0k
ENDO
0.0
 H: 132 mj/mg
-10.0k
o
132 C
 H: 165 mj/mg
-15.0k
HDPE Detergent Bottles
HDPE Milk Bottles
o
134 C
-20.0k
110
120
130
140
o
Temperature, C
150
Considering H = 200 mJ/mg as the
enthalpic change for the melting of a 100%
crystalline HDPE sample, from DSC data of
these two recyclable HDPE it can be found
that:
• the polymer derived from detergent bottles
was (132/200)x100 = 66% crystalline
• the polymer used for milk bottles was
(165/200)x100 = 82.5% crystalline.
Determination of the HDPE content in a blend with
inorganic filler from DSC data.
HDPE Detergent Bottles
ENDO
Heat Flow (m W )
0
 H = 106 mJ/mg
-5
% HDPE=(  H/  H 100 )x100
 H 100 =132 mj/mg
-10
% HDPE=(106/132)x100
HDPE = 80.3%
40
60
80
100
o
132.8 C
120
140
o
Temperature, C
160
180
Polyhydroxylated Nylons
Similarity of Nylon 6,6 and the poly hydroxylated
counterpart:
DSC Thermal Transitions in Polyhydroxylated
Nylon 6,6
3.5k
6k
Decomposition
5k
H
4k
DSC,  W
2.5k
3k
2.0k
Tm
1.5k
2k
Tg
1k
DDSC
1.0k
0
500.0
DDSC,  W /m in
3.0k
-1k
DSC
0.0
-2k
0
20
40
60
80
100
120
140
o
Temperature, C
160
180
200
Thermogravimetric (TG) analysis is concerned
with the change in weight of a material as its
temperature changes. This indicates:
• the temperature at which the material loses
weight through evaporation or decomposition
• the temperature at which no weight loss
takes place is revealed, which indicates
stability of the material.
TG Measurement Principle of Seiko TG/DTA
Thermobalance
Thermal Degradation of Polyhydroxylated
Nylon 6,6
15.0k
W eight Loss (TG), %
o
100 C
o
150 C
o
200 C
o
235 C
-20
-6.3%
-6.9%
-19.0%
-50.0%
-40
o
10.0k
DTG
TG
0
205 C
o
425 C
-60
5.0k
-80
DTG
0.0
-100
0
100
200
300
400
o
Temperature, C
500
600
Poly (4-dodecyl-1-4-aza heptamethylene-D-glucaramide).
Thermal decomposition.
2
o
166 C
0
o
372 C
o
1.3%/ C
-20
o
DTG (%/ C)
TG (% Weight Loss)
o
-1.3%@150 C
o
188 C
o
0.6%/ C
-40
-60
0
-80
o
-97.5%@400 C
-100
TGpercent
0
100
200
300
400
o
Temperature, C
500
600
Initial
decomposition of
linear polymers.
Initial sample
weight: 10 mg.
Heating rate:
5C/min.
Thermogravimetric analysis of a polymeric blend
containing HDPE and an inorganic filler
(phosphogypsum)
0
% weight loss
-10
-20
-30
-40
-50
-60
TGpercentL
0
-62.8%
100 200 300 400 500 600 700 800
o
Temperature, C
Almost any measurement that can be
done at different temperatures can be
expanded into thermal analysis; and any
series of thermal analysis techniques can
be combined with other non-thermal
technique for valuable multiple-parameter
information.
Coupled
techniques,
such
as
Thermogravimetry, Differential Thermal
Analysis and Mass Spectrometry (TGDTA-MS) or evolved gas analysis of
polymers by coupled Thermogravimetry,
Gas Chromatography, Fourier Transform
Infrared and Mass Spectrometry (TG-GCFTIR-MS) are just two examples often
used in industrial laboratories.