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Polypropylene Phase Transitions
Studied by DSC
By: Ryne Sternberg
Sean McCrea, Adam Bielski
Pennsylvania State University
Due October 23rd 2014
Abstract:
Isotactic and Amorphous polypropylene were studied through Differential Scanning Calorimeter
(DSC). When analyzing amorphous polypropylene, both first order and second order transition
phases are observed with an enthalpy of crystallization of 13.22 J/g. This shows the sample was
not 100% pure. The glass transition temperature was found to be -12.68°C. The degree of
crystallinity for the amorphous sample was calculated for a value of 6.83%. The isotactic sample
had a degree of crystallinity at 41.2% as well as an enthalpy of crystallization of 85.32 J/g. The
entropy of the glass transition phase was calculated through a series of integrals ranging from
±20°C, this value was 1.58 J/mol-K. The entropy of crystallization was 0.20 J/mol-K, this was
calculated through utilization of the first law of thermodynamics. This experiment demonstrated
differences between stereochemistry in molecules, which showed differences in thermodynamic
properties of two isomers of polypropylene.
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INTRODUCTION:
Differential Scanning Calorimetry (DSC) is used to determine and compare the
phase transitions of both amorphous and isotactic polypropylene. 2Differential
scanning calorimeters are a widely used thermoanalyitcal instruments due to their
ease of use, relatively fast data collection times and ability to use small sample
sizes.3DSC is used to measure thermal properties of nanocomposite thermal-plastics
widely used in the engineering of polymers. Polymer engineering is important due
to the demand on innovative ways to create plastic. Using DSC can give
thermodynamic properties, which help engineers produce plastics that can resist
high or low temperatures. DSC works by measuring the heat flow to or from a
sample when chemical or physical changes occur during applied temperature
increase. When there is no chemical or physical change total heat is calculated by:
𝑞𝑝 = 𝐶𝑝 ∆𝑇
(1)
Cp is independent of temperature and ΔT is the temperature change. If there is any
excess heat transferred in the system equation 1 turns into:
𝐶𝑝,𝑒𝑥 =
2The
𝑞𝑒𝑥
∆𝑇
(2)
excess heat can be viewed, as the change in heat capacity therefore the DSC
thermogram is essentially a chart of Cp,ex vs. T. When the system undergoes any
chemical or physical changes a peak forms on the graph. These peaks can be
integrated to find enthalpy of transition. The integration is shown in Equation 3.
𝑇2
∆𝐻𝑝 = ∫𝑇1 𝐶𝑝,𝑒𝑥 𝑑𝑇
(3)
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T1 and T2 represent the beginning and ending temperatures of transition. There are
two different types of transition being studied in this lab. First order and second
order transition. A steep peak in the high temperature range represents first order
transition, this demonstrates crystallization. Slow rising peaks in the lower
temperature range represent second order. It is hypothesized before lab that the
amorphous polypropylene would only show second order transition. This is due to
its tacitcity, which is related to the stereochemistry of a molecule. 1Tacticity is a
term used to describe the way pendent groups on a polymer chain are arranged on a
polymer backbone. 1The tacticity of a polymer is determined by what side of the
polymer chain the pendant groups are on. Amorphous Polypropylene is a long
carbon chain where each center carbon is connected to a methyl group and
hydrogen; these are situated in random order. This random order of side groups
forms an amorphous solid that does not crystalize. Isotactic Polypropylene has all
methyl groups in the same position from the center carbon, which allows it to
crystalize. These phenomena’s are studied through the graphs produced by DSC. 1st
order transition occurs when the molecule crystallizes with an applied heat flow,
thus represent Isotactic Polypropylene. Amorphous Polypropylene was predicted to
only form second order transition because it does not crystallize. After observing
the different peaks of transition the entropy will be calculated through the integral
shown in Equation 4.
𝑇2 𝐶𝑝
∆𝑆 = ∫𝑇1
𝑇
𝑑𝑇
(4)
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The first law of thermodynamics shown in equation 5 helps shape an understanding
of the system and is used for calculations of entropy when the Gibbs free energy is
zero.
∆𝐺 = ∆𝐻 − 𝑇∆𝑆
(5)
EXPERIMENTAL:
The pressure of the nitrogen (CAS 7727-37-9) feed was set to 20 psi, exceeding this
pressure would damage the instrument. Q200-1724 was then opened on the
computer monitor. The cell was set to room temperature with a sample purge flow
rate of 50mL/min. Amorphous Polypropylene; Aldrich (CAS 9003-07-0) was then
placed in DSC Q2000 differential scanning calorimeter. There are three Hermetic
Tzero aluminum sample pans in which the polymer samples sat on for the
experiment. It was essential to use tweezers when handling the samples; this
prevented any skin oil interaction, which would affect results. A simplistic diagram
of the system is shown below.
Figure 3: Diagram of Experiment
Attained from http://pslc.ws/macrog/dsc.htm
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The sample and reference pan are both placed on top of two thermocouples inside
the cell. The computer was set to 9.3 mg for PPA_56, which was the sample being
tested. After all experimental constraints were confirmed the experiment started.
DSC graphs were made from the Q200-1724 program and integrated to find
enthalpy of transition. The Data for the PPC was attained from other sources after
completion.
RESULTS:
Figure 4: PPA_54 DSC thermogram
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Figure 4 represents the graph created from the DSC. It is observed that both 1 st
order and 2nd order transitions take place in the “supposed” amorphous
polypropylene. An average of the heating and cooling peaks gave a glass transition
temperature of -12.68°C. The first order crystallization peak had an enthalpy of
13.22 J/g with a melting temperature of 137.69°C. The degree of cystallinity was
reported at 6.83%. Figure 5 represents the thermogram for Isotactic Polypropylene.
Figure 5: PPC_54 DSC Thermogram
The enthalpy of crystallization of isotactic polypropylene is 85.32 J/g at a
temperature of 151.65°C. The degree of crystallinity is 41.2%. The graph is
symmetrical showing no glass transition. The entropy of crystallization is calculated
using the first law of thermodynamics and has a value of 0.201J/g-K. Figure 6
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represents Entropy vs. Temperature of glass transition during the scanning of
Amorphous Polypropylene.
Entropy vs Temperature
1.8
1.6
S(J/molK)
1.4
1.2
1
0.8
0.6
0.4
0.2
0
250
260
270
280
Temperature (K)
290
300
Figure 6: Entropy vs. Temperature.
This figure represents the change in Entropy dependent on temperature. Each point
corresponds to its individual integrals with a range of -20°C to 20°C. The max value
gives the sum of all individual integrals as a value 1.58 J/g-K, this is the entropy
associated with glass transition.
DISCUSSION:
Figure 4 demonstrated the “supposed” pure amorphous polypropylene. After
witnessing the crystallization peak at 137.69°C, it is proved that the sample used
was not 100% pure. The degree of crystallinity was calculated using a reference
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enthalpy of crystallization as 207 J/g and a value of 6.83% was reported. Since the
sample was not 100% pure it could not dissolve properly which caused a small
portion crystallized. In future if this sample was used, believing it was 100% pure,
there would be a significant amount of error when creating products. This is can
cause failure to future reactions and their products and would lower the percent
yield of a target chemical. 2The melting temperature of amorphous polypropylene is
reported to be 160-165°C and a transition temperature of -10°C, these values were
obtained from Sigma Aldrich. Figure 4 gives values slightly off from literature
values. On a molecular level the bonds released at lower temperatures, a melting
temperature is observed at 152.57°C. This could be caused by inconsistency of
pressure during the experiment. The pressure of the cell could have fluctuated, since
the melting temperature was reported lower then ideal, it is assumed that the
pressure decreased. 4This assumption is made due to the direct relationship
between pressure and temperature, when pressure is decreased so does the melting
point. Also the discrepancy could have been caused by too rapid of increase in
temperature. If temperature increases faster then the DSC can read, the melting
temperature will be recorded lower then ideal.
Isotactic polypropylene reports a higher melting temperature of 189°C, this value is
found in the chemical book website.2Since this melting point is slightly higher then
amorphous this proves that a consistent stereochemistry within a molecule gives a
stronger bond between carbon and its corresponding R group. Using this
phenomena when comparing both Isotactic and Amorphous polypropylene, it shows
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the bonds are either closer together or attracted to each other more when
stereochemistry is the same for all carbons, the isotactic polypropylene.
The temperature range for the peak of crystallization varied when comparing the
amorphous to isotactic polypropylene. The wider the temperature range and
ultimately wider the peak corresponds to more crystallization enthalpy of the
molecule. In the amorphous sample, the crystallization peak was much thinner than
the isotactic sample. This agrees with the values of enthalpy of each, the isotactic
had a much higher enthalpy compared to amorphous. This shows the isotactic
sample involved a greater temperature range during the crystallization physical
change.
The entropies of both amorphous and isotactic sample differed significantly.
Entropy is defined as the disorder or randomness of a system. This is demonstrated
in both Figure 4 and 5 and it corresponds with their entropy values. When looking
at the glass transition phase the temperature changes at many different points of
heat flow. This is the reason for the calculation of the integral ranging from -20 to
20°C. The sum of those gives a value of 1.58 J/g-K. As the temperature changes so
does the disorder of the system. The entropy of the isotactic sample gave a value of
0.201 J/g-K, this is significantly lower then the amorphous sample because
temperature stays at a constant value, the melting temperature. The first law of
thermodynamics can be applied and the total energy in the system is equal to the
enthalpy of crystallization, which gives lower disorder thus lower entropy.
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CONCLUSION:
Differential scanning calorimeter was used to test both amorphous and isotactic
polypropylene. The DSC thermogram showed that the amorphous polypropylene
was not 100% pure; this was evaluated through both its predicted glass transition
phase as well as its 6.83% crystallization 1st order peak with an enthalpy value of
13.22J/g. The temperature of glass transition was an average of both hot and cold
temperatures and was a value of -12.68 °C. Pure amorphous polypropylene would
not exhibit a 1st order peak. The entropy of the glass transition was calculated
through a series of integrals, which gave a value of 1.58 J/g-K. The entropy of
crystallization, observed in the isotactic thermogram was a value of 0.20 J/g-K. The
degree of crystallization was 41.2 %. The variances between both the isotactic and
amorphous sample are due to its stereochemistry and strength of bonds. The more
consistent a structure the stronger its bonds are. Differences from ideal values could
have occurred from rapid temperature increase or decrease in cell pressure. DSC
proves to be a powerful tool when studying thermodynamic properties of isomers
and helps compare and contrast small differences within each compound.
ACKNOWLEDGMENTS:
I would like to thank Sean McCrea, Adam Bielski and Professor M for their help with
configuration and understanding of this experiment.
REFERENCES:
1) O'Lenick, Anthony. Comparatively Speaking: Isotactic vs. Syndiotactic vs. Atactic in
Polymers. 2009.
2) Milosavljevic, Bratoljub. Lab Packet for Experimental Physical Chemistry. 2013,
9(1)-9(6).
3) Polymer Engineering Science. Vol 54.,pp 2292-2300
4) Ewen, John. J. Am. Chem. Soc., 1984, 106 (21), pp 6355–6364
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