Differential Scanning Calorimetry; First and Second OrderTransitions in PETE

Transcription

1 Differential Scanning Calorimetry; First and Second OrderTransitions in PETE Purpose: Determine the heat capacity, glass transition temperature, change in heat capacity for the glass transition, enthalpy of crystallization, enthalpy of melting (fusion), and percent crystallinity of a sample of polyethylene terephthalate, PETE. Introduction Differential scanning calorimetry (DSC) monitors heat effects associated with phase transitions and chemical reactions as a function of temperature. In a DSC the difference in heat flow to the sample and a reference at the same temperature, is recorded as a function of temperature. The sample is sealed in an aluminum pan. The reference is an inert material such as alumina, 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 to enthalpy changes: đq p = dh Here dh/ is the heat flow measured in mw or equivalently mj s -1. The heat flow difference between the sample and the reference: dh = dh sample dh reference 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. Hence dh/ is positive. Other endothermic processes include helix-coil transitions in DNA, protein denaturation, dehydrations, reduction reactions, and some decomposition reactions. In an exothermic process, such as crystallization, some cross-linking processes, oxidation reactions, and some decomposition reactions, the opposite is true and dh/ is negative. N 2 Inlet 1 2 Sample Resistance Heater Temperature Sensors Reference Resistance Heater Sample Reference Sample Reference Side View (without furnace block) 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. Both are constructed of platinum to allow high temperature operation. Under each holder is a resistance heater and a temperature sensor. Currents are applied to the two heaters to increase the

2 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/. A schematic diagram of a DSC is shown in Figure 2. A flow of nitrogen gas is maintained over the samples to create a reproducible and dry atmosphere. The nitrogen atmosphere also eliminates air oxidation of the samples at high temperatures. The sample is sealed into a small aluminum pan. The reference is usually an empty pan and cover. The pans hold up to about 10 mg of material. sample reference Linear Temperature Scan T sample + T ref T dt = 20 C min-1 Heater sample power monitor T reference power monitor Heater + Scan Control dq mj s -1 tr C p endotherm time 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 is adjusted to keep the sample and reference at the same temperature during the scan. h e a t f l u x ( mw) 0 e n d o t h e r m i c s t a r t i n g t r a n s i e n t C p g l a s s t r a n s i t i o n C p C r y s t a l l i z a t i o n p e a k M e l t i n g p e a k exotherm time or temperature e n d i n g t r a n s i e n t s t a r t s t o p e x o t h e r m i c t i m e a n d T e m p e r a t u r e ( C ) Figure 3. Typical DSC scan. The heat capacity of the sample is calculated from the shift in the baseline at the starting transient. Glass transitions cause a baseline shift. Crystallization is a typical exothermic process and melting a typical endothermic process, tr H is calculated from the area under the peaks. Few samples show all the features shown in this thermogram.

3 DSC of PETE 3 During the heating of a sample, from room temperature to its decomposition temperature, peaks with positive and negative dh/ may be recorded. Each peak corresponds to a heat effect associated 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 direct information is the temperature at which a process occurs, for example, the melting point of a polymer. The temperature at which a reaction, such as decomposition, may start is another important parameter. For decompositions, the peak temperature is associated with the temperature at which maximum reaction rate occurs. The glass transition in polymers is an important type of phase transition. The glass transition temperature, T g, is the temperature at which amorphous (noncrystalline) polymers are converted from a brittle, glasslike form to a rubbery, flexible form. The glass transition involves a change in the local degrees of freedom. Above the glass transition temperature segmental motions of the polymer are comparatively unhindered by the interaction with neighboring chains. Below the glass transition temperature, such motions are hindered greatly, and the relaxation times associated with such hindered motions are usually long compared to the duration of the experiment. The motions are primarily torsional degrees of freedom around freely rotating bonds in the long chains of the polymer. The operative definition of glass transition temperature is that at this temperature, or within a few degrees, the specific heat, the coefficient of thermal expansion, the free volume, and the dielectric constant (in the case of a polar polymer) all change rapidly. Since the mechanical behavior of polymers changes markedly at the glass transition temperature, T g is an important characteristic of every polymer. In the DSC experiment, T g is manifested by a change in the base line, indicating a change in the heat capacity of the polymer (Fig.4). The baselines before and after the transition are extrapolated to the temperature where the change in heat capacity is 50% complete. The change in heat capacity is measured at the 50% point. Then T g is often reported as the temperature at the intersection of the baseline and the line extrapolated from the linear portion during the phase transition. First order phase transitions have an enthalpy and a heat capacity change for the phase transition. Second order transitions are manifested by a change in heat capacity, but with no accompanying change in enthalpy. No enthalpy is associated with the glass transition, so the glass transition is second order. The effect on a DSC curve is slight and is observable only if the instrument is sufficiently sensitive. endothermic Heat flux (mw) C p exothermic 0 90 Glass transition T g 100 Time and temperature ( C) Figure 4. The glass transition. If there are sloping baselines before and after the glass transition, the baseline before the transition is extrapolated forwards and the baseline after the transition is extrapolated backwards (as shown by dotted lines). The baseline shift is measured when the transition is about 50% complete (as shown by arrows).

4 DSC of PETE 4 The second direct information obtainable from DSC thermograms is the enthalpy associated with first 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 amorphous regions. In semi-crystalline polymers the glass transition and crystallization transitions occur over a broad temperature range. Crystallization of the small amount of amorphous polymer begins with 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 distinctly different temperatures in amorphous solids. The crystallization temperature is intermediate between the glass transition and the melting transition, at which temperature the polymer molecules gain sufficient translational and torsional energy to reorganize into the crystalline structure. If the crystallization exothermic peak can be discerned in the semi-crystalline state, the percent crystallinity of the original sample can be estimated by dividing the enthalpy change of the crystallization peak in the original sample by the enthalpy of crystallization for the amorphous 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 the process: dh sample = tr H sample 3 Assuming that the heat capacity of the reference is constant over the temperature range covered by the peak, H reference will 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/. Heat capacities and changes in heat capacity can be determined from the shift in the baseline of the thermogram. The heat capacity is defined as: C p = đq p = dh dt 4 p The temperature scan rate is: α = scan rate = dt Using the chain rule: C p = dh dt = dh p dt 5 6

5 DSC of PETE 5 where dh/ is the shift from the baseline of the thermogram (Figure 3-4) and the last derivative is just the inverse of the scan rate. For differential measurements, we determine the difference in the heat capacity of the sample and the reference: C p = C p (sample) C p (reference) 7 C p = dh dt = dh p dt The units of the heat flow are mj s -1 and the temperature scan rate is usually expressed as C min -1. So to be consistent with units you must multiply by 60 s min -1 : 8 C p = 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 pan using the special pan crimper. If any aluminum is lost during crimping, reweigh the crimped sample-pan-lid. An empty pan and lid are always kept in the reference holder. Sometime during the lab, also weigh the reference pan and lid. Obtain the thermogram over the temperature range C, with a 20 C min -1 scan rate. The instrument instructions are listed in the appendix. After the first run is complete wait about 10 mins 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 successive runs (that end below the crystallization point). This maximum is probably due to the release of strains, frozen into the sample during rapid quenching. Heat a water bath to the glass transition temperature that you determined. Immerse a sample of PETE in the bath and play around a bit. What do you expect to happen to the properties of the plastic 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 the difference in the mass of the sample pan and cover and the reference pan and cover. That is: C p (total) = C p (polymer) + C s (m(sample pan) m(ref pan)) 10 where C s is the specific heat of aluminum, m(sample pan) is the mass of the sample pan and cover and m(ref pan) is the mass of the reference pan and cover. The specific heat of aluminum is available in standard references or can be calculated from the molar heat capacity. Use equation 10 to calculate the heat capacity and specific heat of PETE.

6 DSC of PETE 6 Glass Transition: Determine T g. 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 to calculate the change in heat capacity and specific heat for the transition. Enthalpy of Crystallization and Enthalpy of Fusion: Determine the approximate melting point of your polymer from the maximum in the melting peak. Extrapolate the baseline under the peak by connecting the flat baseline before and after the melting peak. Determine the enthalpy of the phase 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 enthalpy changes per gram and the enthalpy changes per mole of monomer. In calculating the molar mass of 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 the amorphous form, since the original sample is mostly crystalline. In addition the crystallization peak begins near the glass transition temperature for semi-crystalline polymers, so the transition is difficult to spot. Expand the y-axis and focus on the temperature interval near the glass transition temperature. You should find a shallow, broad exothermic peak. As before, determine the 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 percent crystallinity using the two enthalpy of crystallization values, one for the original semi-crystalline sample and one for the quenched amorphous sample. Report In your report, give all the above results. Make sure to supply all the necessary information to repeat 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 heat capacity for the glass transition, enthalpies of crystallization, enthalpy of melting (fusion), and the 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 in your final results. Compare your results to the literature values for the heat capacity and the glass transition 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 state different from the first run? 3. Discuss your observations of the PETE sample in the hot water bath, including a molecular interpretation. 4. Which transitions are first order and which are second order? Discuss the chemical significance of your observations. For example: Why is this experiment commonly used in the characterization of commercial PETE samples? What information do you gain from knowledge of the glass transition temperature? What other important uses does DSC have?

7 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 data display area to verify the purge gas flow rate and the current temperature. Set the purge gas flow rate at 20 ml min -1 if 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 30.0 C in the temperature dialog box in the Control Panel at the right of the screen and click the Go To Temperature button: Start/Stop Go to Temperature: Set Heat Flow to Zero Open Cover Cooler On/Off Purge Gas Controls After the chiller has reached the set temperature, you are ready to load the sample. Click on the Open Cover icon in the Control Panel. Use the vacuum wand to remove the sample cell lid and place your sample in the sample cell. Make sure your sample is centered in the cell. Replace the sample 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 the control buttons group:

8 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 sample weight. Make sure the file name ends in.ds8d. If you include a -# in the file name, each saved file will increment the file number automatically (for example, PETE-#.ds8d, the first saved file under this method will be PETE-1.ds8d, the second will be PETE-2.ds8d). For later reference, note the name of the Directory where your files will be saved. Click on the Program tab. Set the Initial Temp: to 30.0 C the To: temperature to C and the Rate to 20.0 C/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 Control buttons group. Check that the green Contol LED is lit on the instrument front panel, to make sure the instrument has reached equilibrium. Click on the Set Heat Flow to Zero button in the Control Panel, so that the initial baseline before heating is set to zero. Click the Start/Stop button in the Control Panel. Click on the Full Scale y-axis button in the Control buttons group. When the sample has reached the baseline after the melting transition, you can click the Start/Stop button to stop before the final programmed temperature is reached or you can just wait until the Method stops automatically. The sample requires about 10 minutes to cool to the initial temperature and to regain equilibrium. Set up a new run with a different file name and do a repeat identical run.

9 DSC of PETE 9 Data Analysis: After a run is completed, the data is automatically opened in the Data Analysis window. 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 to automatically expand the y-axis. The first step is to correct the slope of the baseline. Click on the Slope button in the Control buttons group. Drag the mouse under the temperature interval that you 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 to set 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 Analysis window. 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 the glass transition, as shown below. Then pull down the Calc menu and choose Step. A new dialog box will appear. Drag the mouse from a temperature in the linear region well before the transition to the linear region well after the glass transition. You start the analysis interval well before the transition and end well after the transition, so that accurate baselines may be determined for baseline extrapolation to the transition temperature. Set the Transition criterion to Half Height and click on Calculate.

10 DSC of PETE 10 In the next dialog you can adjust the baseline slops if necessary. Click on Calculate. Record the results and print the thermogram. Click on the Full Scale x&y button so that you can easily see the crystallization and melting transitions. To find an exothermic or endothermic transition peak area, pull down the Calc menu and choose Peak Area. A new dialog box will appear. Drag the mouse from a temperature in the linear region well before the transition to the linear region well after the transition. Set the Baseline method to Standard and click Calculate. Repeat this process for the remaining transition and then record the results and print the thermogram. When your analysis is complete, close the Data Analysis window. Set the sample temperature to 30 C and remove your sample. Replace the sample cell lid. Make sure the sample compartment cover is closed. If no one is scheduled to use the instrument next, turn off the nitrogen flow at the regulator on the wall (yellow handle). Check with your instructor to determine if the chiller should be turned off. Enter your name in the DSC8000 instrument use book.