dscpete.pdf

7/25/2019 DSCPETE.pdf

1/10

Differential Scanning Calorimetry;

First and Second OrderTransitions in PETE

Purpose: Determine the heat capacity, glass transition temperature, change in heat capacity forthe glass transition, enthalpy of crystallization, enthalpy of melting (fusion), and percentcrystallinity of a sample of polyethylene terephthalate, PETE.

Introduction

Differential scanning calorimetry (DSC) monitors heat effects associated with phasetransitions and chemical reactions as a function of temperature. In a DSC the difference in heatflow to the sample and a reference at the same temperature, is recorded as a function oftemperature. The sample is sealed in an aluminum pan. The reference is an inert material such asalumina, or just an empty aluminum pan. The temperature of both the sample and reference are

increased at a constant rate. Since the DSC is at constant pressure, heat flow is equivalent toenthalpy changes:

qpdt

=dH

dt 1

Here dH/dt is the heat flow measured in mW or equivalently mJ s -1. The heat flow differencebetween the sample and the reference:

dH

dt=

dH

dt sample

dH

dt reference 2

and can be either positive or negative. In an endothermic process, such as most phase transitions,

heat is absorbed and, therefore, heat flow to the sample is higher than that to the reference. HencedH/dt is positive. Other endothermic processes include helix-coil transitions in DNA, proteindenaturation, dehydrations, reduction reactions, and some decomposition reactions. In anexothermic process, such as crystallization, some cross-linking processes, oxidation reactions,

and some decomposition reactions, the opposite is true and dH/dt is negative.

SampleResistanceHeater

Reference

ResistanceHeater

Temperature

Sensors

Sample Reference

Sample Reference

Furnace Block

N2Inlet

Side View (without furnace block) Top View (cover off)

Figure 1. Differential scanning calorimeter sample and reference holder.

The calorimeter consists of a sample holder and a reference holder as shown in Figure 1. Bothare constructed of platinum to allow high temperature operation. Under each holder is aresistance heater and a temperature sensor. Currents are applied to the two heaters to increase the


2/10

DSC of PETE 2

temperature at the selected rate. The difference in the power to the two holders, necessary to

maintain the holders at the same temperature, is used to calculate dH/dt . A schematic diagramof a DSC is shown in Figure 2. A flow of nitrogen gas is maintained over the samples to create areproducible and dry atmosphere. The nitrogen atmosphere also eliminates air oxidation of thesamples at high temperatures. The sample is sealed into a small aluminum pan. The reference isusually an empty pan and cover. The pans hold up to about 10 mg of material.

Figure 2. Schematic of a DSC. You choose the linear temperature scan rate. The triangles are

amplifiers that determine the difference in the two input signals. The sample heater power isadjusted to keep the sample and reference at the same temperature during the scan.

time and Temperature (C)

heat flux(mW) Cp

Crystallizationpeak

Meltingpeak

Cp0

startingtransient

glasstransition

endothermic

exothermic

endingtransient

start 400 500 stop

Figure 3. Typical DSC scan. The heat capacity of the sample is calculated from the shift inthe baseline at the starting transient. Glass transitions cause a baseline shift. Crystallization is

a typical exothermic process and melting a typical endothermic process, trH is calculatedfrom the area under the peaks. Few samples show all the features shown in this thermogram.

+

Heater Heater ScanControl

T

time

+

trCp

samplepower

monitor

referencepowermonitor

T

dq

dt

Linear Tem erature Scan

endotherm

exotherm

time or temperature

mJ s-

dT

dt= 20C min-1

sample reference

Tsample Tref


3/10

DSC of PETE 3

During the heating of a sample, from room temperature to its decomposition temperature,

peaks with positive and negative dH/dt may be recorded. Each peak corresponds to a heat effectassociated with a specific process, such as crystallization or melting (Fig. 3).

What kind of information is obtained from a DSC thermogram? The first and most directinformation is the temperature at which a process occurs, for example, the melting point of apolymer. The temperature at which a reaction, such as decomposition, may start is anotherimportant parameter. For decompositions, the peak temperature is associated with thetemperature at which maximum reaction rate occurs.

The glass transition in polymers is an important type of phase transition. The glass transitiontemperature, Tg, is the temperature at which amorphous (noncrystalline) polymers are convertedfrom a brittle, glasslike form to a rubbery, flexible form. The glass transition involves a change inthe local degrees of freedom. Above the glass transition temperature segmental motions of thepolymer are comparatively unhindered by the interaction with neighboring chains. Below theglass transition temperature, such motions are hindered greatly, and the relaxation times

associated with such hindered motions are usually long compared to the duration of theexperiment. The motions are primarily torsional degrees of freedom around freely rotating bondsin the long chains of the polymer. The operative definition of glass transition temperature is thatat this temperature, or within a few degrees, the specific heat, the coefficient of thermalexpansion, the free volume, and the dielectric constant (in the case of a polar polymer) all changerapidly. Since the mechanical behavior of polymers changes markedly at the glass transitiontemperature, Tgis an important characteristic of every polymer.

In the DSC experiment, Tgis manifested by a change in the base line, indicating a change in theheat capacity of the polymer (Fig.4). The baselines before and after the transition are extrapolatedto the temperature where the change in heat capacity is 50% complete. The change in heatcapacity is measured at the 50% point. Then Tgis often reported as the temperature at the

intersection of the baseline and the line extrapolated from the linear portion during the phasetransition. First order phase transitions have an enthalpy and a heat capacity change for the phasetransition. Second order transitions are manifested by a change in heat capacity, but with noaccompanying change in enthalpy. No enthalpy is associated with the glass transition, so theglass transition is second order. The effect on a DSC curve is slight and is observable only if theinstrument is sufficiently sensitive.

Figure 4. The glass transition. If there are sloping baselines before and after the glasstransition, the baseline before the transition is extrapolated forwards and the baseline after thetransition is extrapolated backwards (as shown by dotted lines). The baseline shift ismeasured when the transition is about 50% complete (as shown by arrows).

Cp

0

10090

Glass transition Tg

Time and temperature (C)

endothermic

exothermic

Heat flux

(mW)


4/10

DSC of PETE 4

The second direct information obtainable from DSC thermograms is the enthalpy associated withfirst order processes.

Polyethylene terephthalate or PETE, is a commonly used plastic in food packaging, including

beverage bottles:

PETE is a semi-crystalline polymer. After molding, the plastic has crystalline and amorphousregions. In semi-crystalline polymers the glass transition and crystallization transitions occur overa broad temperature range. Crystallization of the small amount of amorphous polymer beginswith the glass phase transition. Rapid cooling of plastic melts produces an amorphous solid. The

glass transition and crystallization transition are readily apparent and often occur at distinctlydifferent temperatures in amorphous solids. The crystallization temperature is intermediatebetween the glass transition and the melting transition, at which temperature the polymermolecules gain sufficient translational and torsional energy to reorganize into the crystallinestructure. If the crystallization exothermic peak can be discerned in the semi-crystalline state, thepercent crystallinity of the original sample can be estimated by dividing the enthalpy change ofthe crystallization peak in the original sample by the enthalpy of crystallization for theamorphous sample, which is obtained after rapid quenching after the first DSC determination.

Theory

The integral under the DSC peak, above the baseline, gives the total enthalpy change for theprocess:

dH

dtsample

dt = trHsample 3

Assuming that the heat capacity of the reference is constant over the temperature range covered

by the peak, Hreferencewill cancel out because the integral above the baseline is taken. Therefore,Eq. 3 is also valid when the integral is taken from the DCS plot of dH/dt.

Heat capacities and changes in heat capacity can be determined from the shift in the baseline ofthe thermogram. The heat capacity is defined as:

Cp= qpdt

=

dHdT p

4

The temperature scan rate is:

= scan rate =dT

dt 5

Using the chain rule:

Cp=

dH

dT p=

dH

dt

dt

dT 6


5/10

DSC of PETE 5

where dH/dt is the shift from the baseline of the thermogram (Figure 3-4) and the last derivativeis just the inverse of the scan rate. For differential measurements, we determine the difference inthe heat capacity of the sample and the reference:

Cp= Cp(sample) Cp(reference) 7

Cp=

dH

dT p=

dHdt

dtdT 8

The units of the heat flow are mJ s-1

and the temperature scan rate is usually expressed asC min

-1. So to be consistent with units you must multiply by 60 s min

-1:

Cp=

mJ

s

min

C

60 s

min 9

Procedure

Use a #2 cork borer to cut a thin disk from a sheet of PETE. A soda bottle is a good source.The sample should weigh between 7 and 10 mg. Weigh an empty sample pan and cover. Add the

sample and reweigh. Use a micro-balance with an accuracy of at least 0.02 mg. Crimp the panusing the special pan crimper. If any aluminum is lost during crimping, reweigh the crimpedsample-pan-lid. An empty pan and lid are always kept in the reference holder. Sometime duringthe lab, also weigh the reference pan and lid.

Obtain the thermogram over the temperature range 30-275C, with a 20C min-1scan rate. Theinstrument instructions are listed in the appendix. After the first run is complete wait about 10mins for the sample to attain equilibrium and then run a second scan.

A maximum may be observed for the glass transition, which is much diminished on successiveruns (that end below the crystallization point). This maximum is probably due to the release ofstrains, frozen into the sample during rapid quenching.

Heat a water bath to the glass transition temperature that you determined. Immerse a sample ofPETE in the bath and play around a bit. What do you expect to happen to the properties of theplastic above the glass transition temperature? Record your observations.

Calculations

Use the second run for the following calculations.

Heat Capacity Determination: To calculate the heat capacity of the sample use equation 8. This

heat capacity includes the heat capacity of the polymer and the heat capacity given by thedifference in the mass of the sample pan and cover and the reference pan and cover. That is:

Cp(total) = Cp(polymer) + Cs(m(sample pan) m(ref pan)) 10

where Csis the specific heat of aluminum, m(sample pan) is the mass of the sample pan andcover and m(ref pan) is the mass of the reference pan and cover. The specific heat of aluminum isavailable in standard references or can be calculated from the molar heat capacity. Use equation10 to calculate the heat capacity and specific heat of PETE.


6/10

DSC of PETE 6

Glass Transition: Determine Tg. Extrapolate the baselines as shown in Figure 4. Take the

baseline shift when the transition is about 50% complete (as shown by arrows). Use equation 8 tocalculate the change in heat capacity and specific heat for the transition.

Enthalpy of Crystallization and Enthalpy of Fusion: Determine the approximate melting pointof your polymer from the maximum in the melting peak. Extrapolate the baseline under the peakby connecting the flat baseline before and after the melting peak. Determine the enthalpy of thephase transition by integrating the peak above or below the baseline, as indicated in equation 2.

Report the enthalpy change that corresponds to your integral value. Calculate the enthalpychanges per gram and the enthalpy changes per mole of monomer. In calculating the molar massof the monomer, neglect the ends of the polymer chain, just use the repeating unit.

Use the first run for the following calculations.

Enthalpy of Crystallization for the Semi-Crystalline Polymer and the Percent Crystallinity: The

crystallization peak is significantly smaller in the semi-crystalline sample compared to theamorphous form, since the original sample is mostly crystalline. In addition the crystallizationpeak begins near the glass transition temperature for semi-crystalline polymers, so the transitionis difficult to spot. Expand the y-axis and focus on the temperature interval near the glasstransition temperature. You should find a shallow, broad exothermic peak. As before, determinethe enthalpy of the crystallization phase transition by integrating the peak below the baseline.Report the enthalpy change that corresponds to your integral value. Calculate the percentcrystallinity using the two enthalpy of crystallization values, one for the original semi-crystallinesample and one for the quenched amorphous sample.

Report

In your report, give all the above results. Make sure to supply all the necessary information torepeat your calculations (e.g., sample and pan/lid weights, aluminum heat capacity, scan rate,baseline shifts, integrals). Report the heat capacity, glass transition temperature, change in heatcapacity for the glass transition, enthalpies of crystallization, enthalpy of melting (fusion), andthe percent crystallinity. Report the enthalpy of fusion per gram and per mole of monomer. The

ordinate values are good to 0.5%. Use significant figure rules to estimate the uncertainties inyour final results. Compare your results to the literature values for the heat capacity and the glasstransition temperature of PETE (Wikipedia is OK). Answer the following questions:

1. Why are the glass transition and crystallization transition obscured in the first run?

2. What is the state of the solid plastic at the beginning of the second run and how is this statedifferent from the first run?3. Discuss your observations of the PETE sample in the hot water bath, including a molecularinterpretation.4. Which transitions are first order and which are second order?

Discuss the chemical significance of your observations. For example: Why is this experimentcommonly used in the characterization of commercial PETE samples? What information do yougain from knowledge of the glass transition temperature? What other important uses does DSChave?


7/10

DSC of PETE 7

Perkin Elmer DSC 8000 Instructions

Instrument Operation: Start the DSC 8000 data acquisition window. Turn on the nitrogen gas

flow at the regulator on the wall (yellow handle). Check the blue status bar at the top of the datadisplay area to verify the purge gas flow rate and the current temperature. Set the purge gas flowrate at 20 mL min-1if not already set:

Start the Perkin Elmer IntraCooler for the DSC by clicking the Cooler On/Off icon in the Control

Panel (see the Control Panel diagram, below). Enter 30.0C in the temperature dialog box in theControl Panel at the right of the screen and click the Go To Temperature button:

After the chiller has reached the set temperature, you are ready to load the sample. Click on theOpen Cover icon in the Control Panel. Use the vacuum wand to remove the sample cell lid andplace your sample in the sample cell. Make sure your sample is centered in the cell. Replace thesample cell lid and click on the Open Cover icon to close the cover.

To set up the DSC temperature program method, click on the Method Editor button, , in thecontrol buttons group:

Start/Stop

Go to Temperature:

Open Cover

Cooler On/Off

Purge Gas Controls

Set Heat Flowto Zero


8/10

DSC of PETE 8

Instrument Viewer Method Editor Data Analysis

Baseline correction Baseline offset Full Scale x&y Full Scale y-axis

Rescale y-axis

Control buttons group

In the Method dialog under the Sample Info tab, set the Sample ID, file name, and sampleweight. Make sure the file name ends in .ds8d. If you include a -# in the file name, eachsaved file will increment the file number automatically (for example, PETE-#.ds8d, the firstsaved file under this method will be PETE-1.ds8d, the second will be PETE-2.ds8d). For laterreference, note the name of the Directory where your files will be saved. Click on the Programtab.

Set the Initial Temp: to 30.0C the To: temperature to 275.0C and the Rate to 20.0C/min.

Set the Data Sampling Options to Seconds between Points and the Select Value to 1.0 sec/point.This last choice keeps the data file from getting too large to handle when exported into a text file.

Minimize the Method Editor window. Click on the Instrument Viewer button, , in the Controlbuttons group. Check that the green Contol LED is lit on the instrument front panel, to make surethe instrument has reached equilibrium. Click on the Set Heat Flow to Zero button in the ControlPanel, so that the initial baseline before heating is set to zero. Click the Start/Stop button in theControl Panel. Click on the Full Scale y-axis button in the Control buttons group. When thesample has reached the baseline after the melting transition, you can click the Start/Stop button tostop before the final programmed temperature is reached or you can just wait until the Methodstops automatically. The sample requires about 10 minutes to cool to the initial temperature andto regain equilibrium. Set up a new run with a different file name and do a repeat identical run.


9/10

DSC of PETE 9

Data Analysis: After a run is completed, the data is automatically opened in the Data Analysiswindow. Alternately, to load in a previous data set, click on the Data Analysis button in the

Control buttons group and select your data set. Click on the Full Scale y-axis button toautomatically expand the y-axis. The first step is to correct the slope of the baseline. Click on theSlope button in the Control buttons group. Drag the mouse under the temperature interval thatyou wish to flatten. For the PETE runs, select the linear region that follows the starting transient,ending before the glass transition starts, as shown below. Click on the Align Endpoints option toset a flat slope. Click on OK.

Place the cursor over the flat baseline immediately following the starting transient and record the

initial heat flux. The position of the cursor is listed in the lower left corner of the Data Analysiswindow. You will use this value to determine the heat capacity of the sample.

To determine the baseline shift for the glass transition, first rescale the y-axis to better see theglass transition, as shown below. Then pull down the Calc menu and choose Step. A new dialogbox will appear. Drag the mouse from a temperature in the linear region well before the transitionto the linear region well after the glass transition. You start the analysis interval well before thetransition and end well after the transition, so that accurate baselines may be determined forbaseline extrapolation to the transition temperature. Set the Transition criterion to Half Heightand click on Calculate.


10/10

DSC of PETE 10

In the next dialog you can adjust the baseline slops if necessary. Click on Calculate. Record theresults and print the thermogram.

Click on the Full Scale x&y button so that you can easily see the crystallization and meltingtransitions. To find an exothermic or endothermic transition peak area, pull down the Calc menuand choose Peak Area. A new dialog box will appear. Drag the mouse from a temperature in thelinear region well before the transition to the linear region well after the transition. Set theBaseline method to Standard and click Calculate.

Repeat this process for the remaining transition and then record the results and print thethermogram.

When your analysis is complete, close the Data Analysis window. Set the sample temperature

to 30C and remove your sample. Replace the sample cell lid. Make sure the samplecompartment cover is closed. If no one is scheduled to use the instrument next, turn off thenitrogen flow at the regulator on the wall (yellow handle). Check with your instructor todetermine if the chiller should be turned off. Enter your name in the DSC8000 instrument usebook.

dscpete.pdf

Documents