DIFFERENTIAL SCANNING CALORIMETRY OF POLYETHYLENE TEREPHTHALATE
Please read the whole annual and fill the blank in Introduction. (The first letter is given)
In this experiment, differential scanning calorimetry (DSC) is used to determine some properties of polyethylene terephthalate (PET) sample cut from a beverage bottle. The melting, or fusion, temperature of the crystalline fraction (Tf), crystalline fraction (fc), the fraction of diethylene glycol (DEG) incorporated in the polymer during polymerization as an impurity (fDEG), the glass transition temperature (Tg) of the amorphous fraction of the PET are to be determined.
2.1 Differential scanning calorimetry (DSC)
Differential scanning calorimetry (DSC) monitors heat effects associated with phase transitions and chemical reactions as a function of t . In a DSC the difference in heat flow to the sample and a r 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 p , heat flow is equivalent to enthalpy changes:
Here dH/dt is the heat flow measured in mW or equivalently mJ s-1. The heat flow difference between the sample and the reference:
∆ dd𝐻𝑡 = (dd𝐻𝑡 )𝑠𝑎𝑚𝑝𝑙𝑒 − (dd𝐻𝑡 )𝑟𝑒𝑓𝑒𝑟𝑒𝑛𝑐𝑒 (2)
and can be either positive or negative. In an endothermic process, such as most phase transitions, heat is a and, therefore, heat flow to the sample is higher than that to the reference. Hence dH/dt 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/dt is n .
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 t s . Currents are applied to the two heaters to increase the 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 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.
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 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, Tg, is the temperature at which a (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 m behavior of polymers changes markedly at the glass transition temperature, Tg is an important characteristic of every polymer.
In the DSC experiment, Tg 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 Tg 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.
The second direct information obtainable from DSC thermograms is the enthalpy associated with first order processes. The integral under the DSC peak, above the baseline, gives the total enthalpy change for the process:
d𝑡 𝑠𝑎𝑚𝑝𝑙𝑒 𝑑𝑡 = ∆𝑡𝑟𝑠𝐻𝑠𝑎𝑚𝑝𝑙𝑒 (3)
Assuming that the heat capacity of the reference is constant over the temperature range covered by the peak, Hreference 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/dt.
2.2 Polyethylene terephthalate (PET)
Polyethylene terephthalate or PET, is a commonly used plastic in food packaging, including beverage bottles:
PET is a semi-crystalline polymer. After molding, the plastic has crystalline and
a region. 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 c 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.
The properties of polymers, such as PET, are significantly influenced by their degree of c . The higher the degree of crystallization, the stiffer and stronger, but also more brittle a molded part is. The degree of crystallization is influenced by the chemical structure and thermal history, such as the cooling conditions during processing or post-thermal treatment. For determination of the degree of crystallization, the measured enthalpy of melting, Hfus, is set in relation to the literature value for completely crystalline material.
3. Safety Practice
• Safety goggles must be worn at all times during this experiment.
4. Experimental Procedure
This experiment is modified for online data analysis. Thus, the following 4 scans are attached from an experimental analysis. (Please note: endothermic peaks are pointing upward in the Figures of “Introduction”, while those in the attached 4 DSC scans are pointing downward).
In differential scanning calorimetry (DSC), temperature and enthalpy are typically calibrated using the melting temperature (Tm) and the heat of fusion (∆Hf) of standard materials such as pure metals. In this experiment, indium is used as a standard sample.
- A standard indium sample DSC analysis (run 1 or Figure 1)
- DSC analysis of a sample of the PET as obtained from the beverage bottle. The sample was scanned from 40 °C to 299 °C at 10 °C /min. (run 2 or Figure 2)
- DSC analysis for the quenched sample, which was cooled very rapidly after run 2 (run 3 or Figure 3)
- DSC analysis of the amorphous region for the run 3 (run 3 or Figure 4)
5. Data Analysis
You should use the resulted numerical data labeled on the scans (see figures 1-4) to determine Tf , Hfus, Sfus, fDEG and fc where applicable for indium sample, the original PET and the quenched PET samples.
- Identify the states of the original PET and the quenched PET samples and the corresponding transitions observed as the temperature is increased from 40 to 290 oC as indicated by the DSC scans. Please note that indium DSC scan (Fig. 1) is used for calibration purpose. Thus, make sure you obtained literature values of indium for Tf and Hfus and compare these with your DSC scan 1 and correct your numerical data for the Figures 24, respectively for your calculations.
- Find the literature value for the glass transition temperature of PET and compare with your result obtained from Figs. 3 and 4. In your report discuss why glass transition temperature study is important for a polymer. What type of thermophysical changes occurs at this state? For more information about glass transition, refer to the “Useful Links” section below (Polymer Chemistry: The Glass Transition).
- The heat of fusion, Hfus, for completely crystalline PET is 140 J/g. Using the value of Hfus obtained experimentally for the original PET sample (Fig. 2), calculate the crystalline fraction, fc, in the sample as used in the bottle. This interpretation of the result assumes that no recrystallization occurred during the scan. A recrystallization exotherm should be absent in the DSC scan of the original PET sample. Percent Crystallinity Calculator form Differential Scanning Calorimetry (DSC).
- Consider the DSC scan of the quenched sample (Fig. 3). Obtain the crystalline fraction of the sample at its melting temperature. Obtain the difference between the Hfus and the Hrecryst and estimate the crystalline fraction of the quenched sample before it was recrystallized during the DSC scan between the glass transition temperature (see Fig. 4). Comment on how much crystallinity reduced by quenching PET sample.
- Dimers of ethylene glycol (EG), i.e., diethylene glycol (DEG), are incorporated in the polymer as a copolymer unit when an excess of ethylene glycol is use in the formation of PET. The presence of DEG lowers the fusion temperature (Tf) and decreases the crystalline fraction from the values obtained if no DEG were included in the polymer. The weight percent DEG, based on the weight of EG and DEG, in the polymer can be estimated from the lowering of Tf. Use the empirical equation;
𝑇𝑓(℃) = 271 − 5.5(𝑤𝑡%𝐷𝐸𝐺) (4)
to estimate the weight percent of DEG, fDEG, present in the polymer.
- To reduce amount of work involved in this lab, a table is provided for you. Just fill in the table with the appropriate data along with your calculations using Figs. 1-4. Note that some of the cells will be empty. Attach this table into your report and cite within your text throughout your discussion. You may need to search for some calculations online or remember Physical Chemistry concepts regarding relations between enthalpy and entropy.
6. Useful Links
(1) This portion is adapted from “Differential Scanning Calorimetry; First and Second Order Transitions in Polymers”, Physical Chemistry Manual, Colby College.
Table 1. Experimental Data and Results for Indium, the original and the quenched PET samples
Glass transition temperature
Enthalpy of fusion
Entropy of fusion
Crystalline fraction of PET
Crystalline fraction before recrystallization
Weight percentage of DEG
- Title page and Abstract: The front page of the report should display the title of the experiment, your name, the name of any experimental partners, and the date on which the report is submitted, and a brief abstract. An abstract is typically 50 to 100 words, starting with the purpose of the study, then summarizing the main results of the study with the corresponding data included, and also stating any significant conclusions.
- Introduction: This should start at the top of the second page. The introduction presents the theory and motivation of the experiment briefly. The object of the experiment should be clearly stated. Theoretical equation used in treating the data should be included in this section.
- Experimental Method: The section provides a brief description (in your own words) of the experimental method used to obtain the data. Indicate any significant deviations from the prescribed procedures. Do not include detailed procedures copied from the laboratory manual. Include all equations needed to calculate the data from the experimental measurements (i.e. Calculations of concentration from absorbance, molar enthalpy from temperature change).
- Results: The section on results should present experimental results in an orderly fashion using table and graphs. Tabulated output of data recorded in the laboratory notebook and/or hardcopy or graphic of tabular data obtained from the instrument or computer interfaced to the instrument. Be certain to include units and uncertainties for any measured values. Include sample calculations when appropriate (i.e. whenever a calculation was performed). Unless specifically requested, do not derive the equations but use references to indicate the source. Include a brief summary of the error analysis.
- Discussion/Conclusion: This section should interpret the results in terms of the theory presented in the introduction and known molecular properties where possible — be certain to indicate whether the objective(s) of the experiment was accomplished. Compare results with literature values when possible (you may need to locate these values — they will not always be given to you). Answer all questions given additionally for some experiments, with each question clearly labeled. If appropriate, suggest modification to the experimental procedure that could improve the precision and/or error.6. References: This section includes citations of all sources to which YOU referred, including this laboratory manual and/or other sources of literature values.