Polypropylene Phase Transitions Studied by Differential Scanning Calorimetry (DSC)
Michael Buchanan
Partners: Derek Rowe, Alex Schneider, and Justin Miller
Chem 457 Sec. 006
March 6, 2015
Abstract
Differential Scanning Calorimetry is a process that measured the thermodynamic
properties of products. It works by placing a reference sample and the sample you want data
from into the calorimeter, it then changes the temperature at a constant rate to measure and
collect the amount of heat that flows into both the reference and the sample. At certain
temperatures each sample will change from one phase to another causing more heat to be
absorbed. Once this occurs, the data can be used to find the entropy and enthalpy of the sample.
In this experiment, enthalpy and entropy was calculated by determining the heat flow through
amorphous and crystalline polypropylene. The data was also able to determine the purity and the
temperature for the glass transition for the amorphous phase.
Introduction
Differential Scanning Calorimetry or DSC is known for determining certain
properties of a compound such as phase transitions, enthalpy, and entropy depending on the
temperature of the transition. DSC is a very unique machine because of its ability to detect heat
flow at many different ranges of temperatures. Heat flow can be used to find phase transitions,
heat capacity, and calculate both enthalpies of phase transitions and entropy that occurs during
phase transitions. When it comes to studying certain compounds, DSC is necessary. For
example, crystals have phase changes from a solid to a liquid crystal, and then to an isotropic
liquid.1
DSC can determine the purity of a sample. Using a known phase transitions of a
compound can be analyzed for its purity using DSC. When looking at the heat flow of the
compound and the transition state of each step, the purity of the compound can be calculated by
calculating the deviation from the known information. If the impurity compound and its
transition enthalpies are known then the percentage of impurity can be found and noted for
further testing. An example of this is that the “amorphous” polypropylene is not completely
amorphous and that was determined through DSC. Not knowing that information could cause
millions of dollars in failed product if not notified by the supplier. Finally, DSC can also be
applied to biological chemistry by looking at the change in structure for proteins. Proteins are
known to be large molecules with a specific structure and folding. At certain temperatures
proteins can start to unfold causing them to be useless and not do the job they were meant for.
Finally, the temperature of that the proteins unfold at can be determined by the steady increase in
temperature and heat flow of the DSC.
Experimental Method
The samples that were used for experiment were found to be in small aluminum
capsules that were put together for us prior to the experiment. The samples that were being tested
were an amorphous polypropylene, crystalline polypropylene, and a reference sample of
polypropylene. The lid was opened and the reference and crystalline polypropylene sample were
put into the calorimeter and then shut. The calorimeter then had the purity nitrogen tank turned
on to 20 psi and set the starting temperature to room temp. (25°C). The calorimeter scanning
procedure was set to equilibrate at 25°C then the data storage was turned on. Next it increased in
temperature with a Ramp of 10.000°C/min to 180°C and then Isotherm for 0.25min. Then it
dropped in temperature with a Ramp of 10.000°C/min to -50.0°C and the Isotherm for 1.00min.
Finally, it went back to room temperature with a Ramp of 10.000°C/min to 25.00°C and then
Isotherm for 0.50min. The data was collected and the procedure was then done for the
amorphous polypropylene. The data was then saved and analyzed find the melting and
solidification peaks and to find temperatures of glass transitions.
Results
The calorimeter created the graph below which represents the heat flow vs. temperature.
The data for the crystalline polypropylene was collected and saved and then integrated to find the
given temperatures of the melting and crystallization point.
Figure 1: Temperature vs. heat flow for polypropylene crystalline. Melting and crystallization peaks have been
integrated in TA Universal Analysis
Figure 1: Heat Flow vs. Temperature of Crystalline Polypropylene
After conversing with Dr. M, the peak at room temperature (25°C) should not be taken
into account. The point of integrating the two peaks on the graph is to find the entropy and
enthalpy of the compound. The melting and crystallization enthalpies equal to: ∆𝐻𝑓 =
𝐽
𝐽
90.49 𝑔 , ∆𝐻𝑓 = 91.47 𝑔, respectively. The entropy can be found by dividing the given
𝐽
enthalpies by their respected temperature. For the melting transition, ∆𝑆𝑓 = 0.2090 𝑔∙𝐾, and for
𝐽
the crystallization transition, ∆𝑆𝑓 = 0.2372 𝑔∙𝐾. As you can see on the graph, the melting and
crystallization points are not at the same temperature. This was due to the parameters that were
set for the data taken. When changing the process to a slower rate this would cause the data
collection to take a lot longer but be more accurate with the temperatures recorded for the two
transitions.
The experiment was then done for the amorphous polypropylene and the graph below
was created by plotting heat flow vs. temperature.
Figure 2: Heat Flow vs. Temperature of Amorphous Polypropylene
Using prior knowledge of what the amorphous polypropylene should look like, the two
peaks that would represent the melting and crystallization transitions should not be there which
means that it is not completely amorphous and has some percentage of crystalline in it. The
amount of crystalline can be found by dividing one of the enthalpies of the amorphous phase by
the corresponding peak of the crystalline polypropylene:
∆𝐻𝑓,𝐴𝑚𝑜𝑟𝑝ℎ𝑜𝑢𝑠
∆𝐻𝑓,𝑐𝑟𝑦𝑠𝑡𝑎𝑙𝑙𝑖𝑛𝑒
× 100% =
𝐽
𝑔
𝐽
90.49
𝑔
13.54
×
100% = 11.05%. The data shows that a glass transition occurs within the sample and can be
calculated by dividing the heat absorbed by the heating rate to get the heat capacity, CP. Using
CP and dividing by T, a graph of CP/T vs. T was made of the above and below the glass transition
to find the entropy.
0.0055
Cp/T (J/(g•K2))
0.0053
0.0051
0.0049
0.0047
0.0045
0.0043
0.0041
-34.0
-29.0
-24.0
-19.0
-14.0
-9.0
-4.0
1.0
6.0
T (°C)
Figure 3: Cp/T vs. Temperature for Cooling of the Glass transition
This plot was then integrated to calculate the entropy, ΔS, by using the equation the
equation below.
𝑇 𝐶𝑃
∆𝑆 = ∫𝑇 2
1
𝑇
𝑑𝑇.
Equation 1: Entropy from Heat Capacity and Temperature.
Using this equation, the change in entropy of the amorphous polypropylene can be
calculated during its glass transition.
Discussion
Polypropylene goes through two phase transitions (melting and crystallization) within the
temperature range of the experiment. The crystalline polypropylene would be considered to have
a first-order phase transition for the melting a crystallization transitions. The change in enthalpy
of the two transition points is the enthalpy of fusion, ΔHf, and it is found by integrating the area
under each of the peaks at the melting and crystallization points. The integrated peaks
correspond to the total amount of energy that is absorbed or released while either melting or
crystallizing. The heat of fusion for the melting and crystallization were calculated to be ∆𝐻𝑓 =
𝐽
𝐽
90.49 𝑔 , ∆𝐻𝑓 = 91.47 𝑔, respectively. The experiment was then done with the amorphous
polypropylene; the compound went through a second order phase transition. The amorphous
polypropylene had both melting and crystallization peaks which it should not have because of
there being crystalline in the sample. The enthalpy was calculated by integrating the melting and
crystallization peaks. By dividing one of the enthalpies by the corresponding peak from the
crystalline polypropylene the percentage of impurity was calculated. There was 11.05% of
crystalline polypropylene within the amorphous sample.
Information from the DSC could give insight to the glass transition from the amorphous
polypropylene. To find out the heat capacity of the sample the heat absorbed had to be divided
by the heating rate. The heat capacity was then divided by temperature and plotted against
temperature to get the change in entropy. By using the Figure 3 and Equation 1, the change in
entropy was calculated using the data collected. Using the data collected, the glass phase
transition occurred around -13.74°C in the crystallization transition of the compound. Referring
to Figure 2, an inflection point in the graph can be seen at this temperature
The change in entropy is due to the change in heat capacity and the change of the sample
itself from one phase to another during the glass transition. The heat capacity is a function of
temperature and can be considered constant in ideal settings which means as the temperature of a
compound increases, more energy has to be put into the system to increase the temperature. The
change in heat capacity in the graph shows a small change over a large range of temperature.
This means that the heat capacity can be considered to be constant over the change in
temperature. Though it can be considered constant, the heat capacity had a huge change at the
glass transition. This is due to the change in phases of the sample which makes the system have
to put in more energy to increase the temperature. This is due to the change in entropy that
occurs at the glass transition. Overall, the heat capacity can be considered constant until it
reaches the physical change of the sample.
Due to the accuracy of the calorimeter, error was very small during the taking of data for
the heat flow of the samples. The one error was that of the crystalline polypropylene which had
the peak at room temperature which was ignored when looking at the data. The other error was
the amorphous polypropylene that had crystalline within it instead of a pure amorphous sample.
Conclusion
The experiment determined the change in entropy and the temperature of the glass
transition of amorphous polypropylene. It also determined the percentage of impurity within the
amorphous polypropylene. The use of DSC helped understand how temperature affects the heat
flow within a given sample. It was also interesting to see and understand how changes in
temperature with respect to time can also measure the heat absorbed by the sample. Once the
data was recorded the change in enthalpy, entropy, and the heat capacity was found for the
crystalline and amorphous polypropylene. Seeing how the data was collected and used to
calculate the necessary components, DSC is a useful and important instrument in determining the
thermodynamics of a sample.
Acknowledgements
Chemical and Mechanical support from Dr. Milosavljevic and the whole Department of
Chemistry. Assistance with experiment and accurately assessing questions from Codey
Henderson.
Literature Cited
1. Pungo, E. A Practical Guide to Instrument Analysis, CRC Press: Florida, 1995; pp 181-191.
Appendix
Sample Calculations
𝑊
ℎ𝑒𝑎𝑡 𝑓𝑙𝑜𝑤 ( 𝑔 )
𝑝𝑜𝑤𝑒𝑟 𝑓𝑙𝑜𝑤 (𝑚𝑊)
𝑊
𝐻𝑒𝑎𝑡 𝐶𝑎𝑝𝑎𝑐𝑖𝑡𝑦, 𝐶𝑝 :
= ℎ𝑒𝑎𝑡 𝑓𝑙𝑜𝑤 ( ) →
= 𝐶𝑃
℃
𝑠𝑎𝑚𝑝𝑙𝑒 𝑚𝑎𝑠𝑠 (𝑚𝑔)
𝑔
ℎ𝑒𝑎𝑡𝑖𝑛𝑔 𝑟𝑎𝑡𝑒 ( 𝑠 )
𝑇 𝐶𝑃
𝐸𝑛𝑡𝑟𝑜𝑝𝑦, ∆𝑆 = ∫𝑇 2
1
𝑇
𝑑𝑇