CHEM 331L

Physical Chemistry I Laboratory

Revision 2.0

Differential Scanning Calorimetry of Polystyrene

In this laboratory exercise we will measure the glass transition temperature of Polystyrene. The

glass transition temperature (Tg) is the temperature at which an amorphous solid formed from a

polymeric substance transitions from a brittle to a rubbery material. Formally the glass transition

is not a true phase transition, but it is instead a transition involving an increased freedom of

movement of local "segments" within the polymer chain. We will measure the glass transition

temperature using differential scanning calorimetry.

Polystyrene Pellets

(http://www.diytrade.com/china/pd/6056264/PS_Polystyrene.html)

Differential scanning calorimetry (DSC) measures the amount of heat (QP) absorbed or released

by a sample during a chemical or physical process.

The basic principle underlying this technique is that when the sample undergoes a physical

transformation such as phase transitions, more or less heat will need to flow to it than the reference

to maintain both at the same temperature. Whether less or more heat must flow to the sample

depends on whether the process is exothermic or endothermic. For example, as a solid sample

melts to a liquid it will require more heat flowing to the sample to increase its temperature at the

same rate as the reference. This is due to the absorption of heat by the sample as it undergoes the

endothermic phase transition from solid to liquid. Likewise, as the sample undergoes exothermic

processes (such as crystallization) less heat is required to raise the sample temperature. By

observing the difference in heat flow between the sample and reference, differential scanning

calorimeters are able to measure the amount of heat absorbed or released during such transitions.

http://en.wikipedia.org/wiki/Differential_scanning_calorimetry

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In a DSC experiment, a sample and reference are heated equally into the temperature regime (T)

where the sample transformation is expected:

T = To + t (Eq. 1)

To is the initial temperature and is the DSC's scan rate (K/sec).

As the sample temperature deviates infinitesimally from the reference temperature, the heat input

to one cell is increased while it is reduced in the other, establishing a "null-balance."

The quantity of electrical energy per unit time, or power (P), supplied to the heating elements is

assumed to be proportional to the heat released by the sample per unit time. So, the heating rate

can be determined from:

~ P (Eq. 2)

And, a graph of the heating rate versus temperature yields a DSC trace called a thermogram.

Further, this mode of operation for the DSC is referred to as a power-compensation mode.

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The rate of heating QP/dt versus the increase in temperature may be translated into a rate of

heating per unit time Qp/dt using the heating rate :

(Eq. 3)

The heat released or absorbed by the sample is then simply the area under the thermogram's

peak:

QP =

(Eq. 4)

Instead of a peak in the thermogram, glass transitions cause a baseline shift instead (see below).

This shift is related to a difference in the heat capacities of the sample and the reference; Cp.

Under constant pressure conditions:

dH = Qp (Eq. 5)

Therefore,

=

(Eq. 6)

where dH/dt is the heating rate measured in mW = mJ/s.

The heating rate difference between the reference and the sample is then:

=

-

(Eq. 7)

Finally, the heat capacity at constant pressure is defined as:

Cp =

(Eq. 8)

Then, by Equation 3, noting the scan rate = dT/dt, we can express the difference in heat

capacities between the sample and the reference in terms of the difference in heating rates:

Cp = Cp,sample - Cp,ref

=

=

(Eq. 9)

On the thermogram, dH/dt is the shift of the baseline.

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When taking into account that the reference sample pan may have a different mass than the

sample pan itself, and assuming the DSC pans are constructed of Aluminum, Equation 9

becomes:

(Eq. 10)

Phase transitions in materials can be characterized as first order or higher order according to

whether molar parameters such as the Enthalpy (H) at the transformation temperature exhibit a

discontinuity. In a first order transition, such as melting, the latent heat of fusion gives rise to a

discontinuity in the enthalpy. In higher order transitions, it is the derivatives of these molar

parameters that are discontinuous. In the case of a second order transition, such as the Glass

Transition for glasses and amorphous polymers, it is the heat capacity, defined as the constant

temperature derivative of the enthalpy, which exhibits a discontinuity.

Cp

dH

dt

dt

dT

p

Cp (Al)(msample.pan mref .pan )

msample

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For every amorphous polymer there exists a narrow temperature region in which it changes from a

viscous or rubbery condition at temperatures above this region, to a hard and relatively brittle one

below it. This transformation is equivalent to the solidification of a liquid to a glass; it is not a

phase transition. Thus, above about 100oC polystyrene is a rubbery material if its molecular

weight is high, or a very viscous liquid if it is low; it quickly becomes a hard glass below this

temperature in either case (although if the molecular weight is very low the critical temperature for

the transition may be lowered somewhat). Not only do hardness and brittleness undergo rapid

changes in the vicinity of the glass temperature, Tg, but other properties such as the thermal

expansion coefficient, the heat capacity, and the dielectric constant (in the case of a polar polymer)

also change markedly over an interval of a few degrees. Tg is regarded variously as the brittle

temperature, the critical temperature for the glassy state, or the second-order transition

temperature, although as mentioned above no phase transition is involved and reference to a

transition may therefore be misleading.

… values of Tg [indicate] the remperature region in which the polymer characteristically changes

from a hard, more or less brittle glass to a rubbery or viscous polymer within which motions of

portions of the chains, usually called segments, are comparatively unhampered by the interactions

between neighboring chains. It will be noted that structural units having higher van der Waals

interactions generally confer a higher Tg on the polymer.

Paul J. Flory

Principles of Polymer Chemistry

Our polymer under study will be polystyrene:

Polystyrene is one of the most widely used industrial plastics. It is normally a clear solid at

Room Temperature, but can be blown into a foam during processing. Its glass transition

temperature has been found to depend on its Number Average Moleculare Weight (Mn)

according to:

Tg[K] = 373 -

(Eq. 11)

Commercially produced polystyrene is atactic; the phenyl groups are located randomly on either

side of the polymer's backbone. This keeps the polymer from forming any crystalline regions

and ensures the material will exhibit a glass transition rather than melting.

We will be using the Perkin Elmer Pyris Diamond DSC:

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As mentioned above, this is a power-compensation DSC with high sensitivity and resolution.

According Perkin Elmer's technical literature, " The power-compensation technique is well

suited for situations where the material being analyzed is not well characterized. Research,

development and competitive analysis all benefit from the power-compensation approach. It is

also a superior way to simulate production conditions, providing valuable insights into how

heating and cooling affects product properties."

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DSC Operation

Our Pyris Diamond DSC is a top-of-the-line research instrument not designed for

student abuse. We trust that our students are able to operate such a sensitive

instrument with care. Please help to retain the precision and sensitivity of this

instrument by handling it cautiously – as if it were yours!

Preparation of the Instrument

Open the valves for both nitrogen tanks, switch the DSC on, and switch the refrigerator on. Then

log in and select the Pyris software. The default set temperature is 50°C. Click on ‘set

temperature’ to get the DSC probe equilibrated.

Sample Loading

For preparation of samples, please refer to the section below. When opening the DSC, you see a

black turntable located in a larger black block. The turntable contains the two sample

compartments. Rotate it so that the handle is pointing towards you, that will open the

compartments. The reference sample (usually an empty aluminum pan) is always on the right

hand side, the sample on the left. Both sample compartments are covered with a platinum lid.

The one for the reference sample has a tiny dimple in the center. Don’t mix those up. To remove

these lids, always use the suction pen. Never use a sharp device such as tweezers. Also remove

the samples with the suction pen. The contact surface for the sample pens is made out of

platinum. Never scratch these surfaces with a sharp device, always use the suction pen to access

the interior of the sample compartment. A single scratch will ruin the calibration of the

instrument. Repair means exchange of the entire probe head assembly at a cost of $10 k.

When placing new samples in the sample compartment using the suction pen, try to position

them in the center of the compartment. Close the compartment with the platinum lids (using the

suction pen). Drop them down carefully in the center. If they get wedged in at an angle, take

them out and try again. Never use force to push them in position.

Rotate the central turntable 90 degrees counterclockwise which closes the sample compartments.

Close the upper cover of the instrument. You are now ready for data acquisition.

Data Acquisition

Select ‘method’ to create your temperature program. Under ‘sample’ enter pertinent information

such as filename, sample ID, and sample weight. Then switch to ‘program’ to enter your

temperature profile. Clicking on the start/stop button will enable your program. Clicking on

‘instrument viewer’ brings up a window with the thermogram. Before starting your program,

make sure that the initial temperature defined in the program has been reached. This can be done

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by using the ‘set temperature’ feature as described above. This is particularly important for the

heat capacity measurement, since you will need a good baseline for its determination.

Data analysis

The status line will show you when the temperature program has been completed. The data are

automatically saved under the filename that you entered earlier. You can zoom in on sections of

your thermogram by drawing a rectangle with the mouse and clicking into it for enlargement.

Alternatively, under ‘display’ you can use the ‘rescale x,y’ options to manually enter your limits.

You can also use the icons at the top of the window.

For the heat capacity and glass transition temperature, you can use the ‘Tg’ option under ‘calc’.

The program prompts you with two pivot points for the two tangents that you can adjust with the

mouse. Pressing ‘calculate’ will enter those tangents into the program, and it will calculate both

the transition point and the Cp.

To determine the heat of fusion, you need to define the integration limits under the ‘peak area’

option. The program will then automatically calculate the heat of fusion.

The onset of a melting peak in a thermogram is generally used as the melting temperature.

Choose ‘onset’ from ‘calc’ and determine the tangents of the baseline and the rising part of the

peak as described previously. The intersection point between the two tangents is the onset

temperature. Further details will be provided by the instructor, or use the ‘help’ menu for

detailed information on the software.

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Sample preparation

On the micro-analytical balance, weigh out the samples in the aluminum pans. Weigh the pan

and lid first separately, then together (to check on reproducibility), then with the sample. This

allows you to determine the sample weight. Crimp the pan and lid together using the universal

crimper press, and weigh again. Knowing the mass of the sample will thus allow you to

determine the mass of aluminum encasing the sample.

Crimper press operation

Place the die (black disk) on the ejector (stub on the front end of the crimper press) and let the

pan slide into the opening of the die. Place the sample in the pan. Make sure that the sample is

centered and does not extend over the rim of the pan. Center the lid on top and remove the entire

assembly (die, pan, sample, lid) from the ejector. Place it on the die table below the plunger.

Move the lever forward until it hits the stopper, and then slowly back to its initial position.

Remove the die from the die table and place it on the ejector. Gently pushing down on the black

portion of the die will pop the sample out.

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Procedure

1. Prepare a sample of about 10 mg polystyrene in a 50 L pan and run a temperature scan

from 20°C to 60°C at a rate of 20°C/min with a holding time of 3 min at the starting point.

Weigh your sample and reference as described under ‘sample preparation’. Make sure to

use the 50 L reference.

2. Take the same polystyrene sample and run a temperature scan twice from 30°C to 130°C at

a rate of 30°C/min.

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Data Analysis

1. Determine the heat capacity for polystyrene using the Pyris software. Print your

thermogram showing the appropriate tangents and result.

2. Recalculate the heat capacity taking into account the difference in mass for the pans.

Estimate the graphical error in determining the tangents. Estimate the weighing error and

perform a complete error analysis of the heat capacity.

3. Find the literature value for the heat capacity of polystyrene and compare with your result.

Discuss.

4. Determine the glass transition temperature from polystyrene from your second run using

the Pyris software. Print your thermogram showing the appropriate tangents and result.

5. Find the literature value for the glass transition temperature of polystyrene and compare

with your result. Discuss.

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References

Flory, Paul J. (1953) "Principles of Polymer Chemisty," Cornell University Press, Ithaca and

London.

Kroschwitz, Jacqueline I., ed. (1985) "Encyclopedia of Polymer Science and Engineering, 2nd

Ed." John Wiley and Sons, New York.

Tanford, Charles (1961) "Physical Chemistry of Macromolecules," John Wiley and Sons, Inc.,

New York.

http://www-omcs.materials.ox.ac.uk/uploads/diamond%20dsc%20tech%20sheet.pdf," accessed

September 20, 2012.