Thermal Analysis Discussion Examples for Plastic Materials | How to Interpret Melting Point and Glass Transition Temperature

What Is Thermal Analysis of Plastic Materials?

Thermal analysis of plastic materials is an analytical method used to investigate what kinds of thermal changes a material exhibits when its temperature is changed.
Plastics are polymeric materials and exhibit various temperature-dependent changes, including molecular-chain motion, melting of crystalline regions, glass transition of amorphous regions, cold crystallization, and thermal decomposition.

Low-molecular-weight compounds often show relatively sharp melting points, whereas thermal changes in plastic materials may be observed over a broad range because of effects such as molecular-weight distribution, crystallinity, additives, and thermal history.
Therefore, in a discussion of thermal analysis, it is necessary to interpret not only peak temperatures but also peak shape, area, baseline changes, and measurement conditions.

Main Items to Include in the Results

In the results of thermal analysis, organize the sample name, measurement method, sample amount, heating rate, measurement atmosphere, temperature range, Tg, Tm, crystallization temperature, decomposition onset temperature, peak area, heat change, and other information.
For DSC, record the shape and temperature of the curve, while for TGA, record the onset temperature and percentage of mass loss.

Main Items to Include in the Results

• Sample name
• Measurement method
• Sample amount
• Heating rate
• Cooling rate
• Measurement temperature range
• Measurement atmosphere
• Glass transition temperature Tg
• Melting point Tm
• Crystallization temperature Tc
• Cold crystallization temperature Tcc
• Heat of fusion
• Heat of crystallization
• Thermal decomposition onset temperature
• Mass-loss percentage
• Peak shape and width
• Baseline changes
• Comparison with literature values and standard samples
• Sources of error and points for improvement

Reference Experimental Values and Analysis Examples for Thermal Analysis of Plastic Materials

Here, reference experimental values are organized for discussing the melting point, glass transition temperature, crystallization temperature, heat of fusion, crystallinity, and thermal decomposition temperature of plastic materials using DSC and TG.

Plastic materials undergo changes such as glass transition, crystallization, melting, and thermal decomposition upon heating.
In DSC, melting points and crystallization temperatures can be read from endothermic and exothermic peaks, while in TG, thermal decomposition and the presence or absence of volatile components can be evaluated from mass loss.
By relating thermal-analysis results to material structure and crystallinity, the properties of plastics can be discussed.

Reference Experimental Conditions

Main Changes Observed in Thermal Analysis

Representative DSC Measurement Examples for Plastics

Reference examples of thermal changes observed by DSC for representative plastic materials are shown below.

Crystalline plastics show melting peaks, whereas amorphous plastics do not show a clear melting point and instead exhibit glass transition as their main thermal change.

Example DSC Measurement of Polyethylene

The following reference example shows the melting point and heat of fusion read from DSC curves of polyethylene samples.

HDPE has a higher melting point and heat of fusion than LDPE, indicating higher crystallinity.
Because LDPE has many branches and crystallizes less readily, its melting point and heat of fusion are considered to be lower.

Example Calculation of Crystallinity

For crystalline polymers, crystallinity can be approximately estimated from the heat of fusion.
Consider the case where the heat of fusion of completely crystalline polyethylene is 293 J/g.

Crystallinity (%) = Measured heat of fusion ÷ Heat of fusion of completely crystalline material × 100

If the measured heat of fusion of HDPE is 180 J/g,

Crystallinity = 180 ÷ 293 × 100 = 61.4%

If the measured heat of fusion of LDPE is 95 J/g,

Crystallinity = 95 ÷ 293 × 100 = 32.4%

From this calculation, HDPE can be judged to have a higher degree of crystallinity than LDPE.

Example Comparison of Crystallinity

Materials with high crystallinity generally tend to be harder and more heat resistant.
On the other hand, transparency and extensibility may decrease.

Example of Glass Transition, Cold Crystallization, and Melting of PET

Depending on its thermal history, PET may show glass transition, cold crystallization, and melting in sequence during heating.

In rapidly quenched PET, a large amount of amorphous material is initially present, and as the molecular chains become more mobile during heating, crystallization proceeds, so a cold-crystallization peak may be observed.

Differences in DSC Results of PET Due to Thermal History

In PET, the degree of crystallization changes depending on cooling rate and stretching history, so the DSC curve also changes greatly.

Examples of Glass Transition in Amorphous Plastics

In amorphous plastics such as polystyrene, no clear melting peak is observed, and the glass transition becomes an important thermal change.

Materials with glass transition temperatures higher than room temperature tend to be hard, whereas materials with glass transition temperatures lower than room temperature tend to be soft.

Changes in Glass Transition Temperature Caused by Plasticizers

Adding a plasticizer may make polymer chains more mobile and lower the glass transition temperature.

As the amount of plasticizer increases, Tg decreases and molecular-chain motion at room temperature increases, making the material softer.

Comparison of Thermal Decomposition Temperatures by TG Measurement

TG measurement can be used to evaluate thermal decomposition temperatures and the presence or absence of volatile components from mass loss during heating.

In PVC, mass loss begins at a relatively low temperature and multiple stages of decomposition are observed.
This is considered to occur because dehydrochlorination first takes place, followed by main-chain decomposition.

Difference Between Nitrogen and Air Atmospheres

Thermal-decomposition behavior changes depending on the measurement atmosphere.
In air, oxidative decomposition also occurs, so mass loss may proceed at a lower temperature than in nitrogen.

In air, oxidation by oxygen proceeds, so the decomposition onset temperature tends to become lower, and the residue also burns and becomes smaller.

Changes in Peak Temperature With Heating Rate

In DSC and TG, the observed peak temperature and decomposition temperature change depending on the heating rate.
At a faster heating rate, thermal changes may appear shifted toward higher temperatures.

The higher the heating rate, the more easily the peaks shift toward higher temperatures because of delays in temperature response and heat transfer inside the sample.

Examples of Thermal Analysis of Recycled and Mixed Materials

Recycled plastics and mixed materials may show multiple melting peaks or decomposition stages.

If multiple melting peaks are present, multiple polymers may be mixed rather than the sample consisting of a single material.

Effects of Additives and Moisture

Plastic materials may contain plasticizers, fillers, flame retardants, moisture, and other substances that affect thermal-analysis results.

By paying attention not only to the thermal decomposition temperature and residual percentage but also to small changes on the low-temperature side, the drying condition and presence or absence of additives can be discussed.

Differences in Thermal History Upon Repeated Measurement

In DSC, results may differ between the first and second heating runs.
The first heating reflects the molding and cooling history experienced by the sample, while the second heating makes it easier to observe properties after the thermal history has been standardized under the same conditions inside the instrument.

If the cold-crystallization peak or heat of fusion differs between the first and second heating runs, the thermal history or crystallization state of the sample is considered to have changed.

Example of How to Write the Results

In DSC measurements of polyethylene samples, melting peaks were observed at 112°C for LDPE and 132°C for HDPE.
The heat of fusion was 95 J/g for LDPE and 180 J/g for HDPE.
When crystallinity was calculated using 293 J/g as the heat of fusion of completely crystalline polyethylene, LDPE gave 32.4% and HDPE gave 61.4%.
From these results, HDPE is considered to have higher crystallinity than LDPE.

In PET, a glass transition was observed near 78°C, an exothermic peak near 125°C, and an endothermic peak near 252°C.
The step at 78°C is considered to correspond to the glass transition of the amorphous region, the exothermic peak at 125°C to cold crystallization, and the endothermic peak at 252°C to melting of the crystalline region.
From this, it can be judged that the sample contained an amorphous region and that crystallization proceeded during heating.

In TG measurement, PE began to lose mass near 390°C and underwent its main decomposition near 465°C.
In contrast, PVC began to lose mass near 240°C and showed two-stage decomposition near 300°C and 460°C.
The low-temperature mass loss in PVC is considered to have been caused by dehydrochlorination, followed by main-chain decomposition.

Points for Connecting the Results to the Discussion

In thermal analysis of plastic materials, it is important to combine DSC and TG results to discuss the crystallinity, amorphous character, thermal history, and decomposition behavior of the material.

• Can the endothermic peak in DSC be interpreted as melting and the exothermic peak as crystallization?
• Can glass transition be explained as a baseline step rather than a peak?
• Can crystallinity be calculated from the heat of fusion and explained in relation to material structure?
• Can the difference between crystalline and amorphous plastics be explained?
• Can the effect of thermal history, such as cold crystallization in PET, be discussed?
• Can thermal decomposition temperature and the effects of additives and moisture be considered from TG mass loss?
• Can the reason decomposition temperature and residual percentage change between nitrogen and air atmospheres be explained?
• Can the effect of heating rate on peak temperature be explained?
• If multiple peaks are present, can the possibility of mixed or recycled materials be discussed?

Example Discussion

In this experiment, the thermal properties of plastic materials were evaluated using DSC and TG.
In polyethylene samples, LDPE showed a melting peak at 112°C and HDPE at 132°C.
Because HDPE had a higher melting point and a larger heat of fusion, it is considered to have a higher degree of crystallinity than LDPE.
In fact, the crystallinity calculated from the heat of fusion was 32.4% for LDPE and 61.4% for HDPE.

This difference can be explained by the difference in molecular structure.
Because LDPE has many branches, its molecular chains have difficulty arranging regularly, so its crystallinity tends to be low.
In contrast, HDPE has a highly linear structure and its molecular chains can pack closely, resulting in higher crystallinity and therefore a higher melting point and heat of fusion.

In PET, three thermal changes were observed: glass transition, cold crystallization, and melting.
The glass transition near 78°C indicates that molecular-chain motion in the amorphous region became more active.
The exothermic peak near 125°C is considered to have resulted from uncrystallized regions becoming crystalline as molecular chains became more mobile during heating.
Subsequently, the crystalline regions melted near 252°C and were observed as an endothermic peak.

In TG measurements, differences in thermal decomposition temperature were observed among materials.
PE and PP mainly showed one-stage mass loss at high temperatures, whereas PVC began to lose mass at a relatively low temperature and showed two-stage decomposition.
This is considered to occur because PVC first undergoes dehydrochlorination, followed by main-chain decomposition.
In addition, because oxidative decomposition also occurs in an air atmosphere, the decomposition onset temperature tended to be lower than in a nitrogen atmosphere.

Possible sources of error include differences in sample amount, the state of contact between the sample and the pan, heating rate, atmosphere, and thermal history of the sample.
At a high heating rate, the temperature inside the sample may not easily follow the set temperature of the instrument, causing the peak temperature to shift toward higher temperatures.
In addition, if the DSC curves differ between the first and second heating runs, the thermal history during molding or cooling may have affected the result.

Summary

In thermal analysis of plastic materials, DSC can be used to evaluate glass transition temperature, crystallization temperature, melting point, and heat of fusion, while TG can be used to evaluate thermal decomposition temperature, mass loss, and residual percentage.

This reference example used PE, PP, PET, PS, and PVC to examine melting point, glass transition temperature, cold crystallization, crystallinity, thermal decomposition temperature, atmosphere, heating rate, thermal history, and the effects of additives.
In a report, the measured temperatures and heat quantities should be discussed in relation to the crystallinity, amorphous character, molecular structure, and thermal history of the material.

What Is DSC?

DSC is a thermal-analysis method called differential scanning calorimetry.
The sample and a reference material are heated or cooled using the same temperature program, and the difference in the amount of heat required is measured.
When melting, crystallization, glass transition, or other changes occur in the sample, they appear as peaks or baseline changes in the DSC curve.

Melting generally appears as an endothermic change, while crystallization generally appears as an exothermic change.
Glass transition is commonly observed not as a distinct peak but as a step in the baseline.
DSC is widely used to evaluate Tg, Tm, Tc, heat of fusion, and crystallinity of plastic materials.

What Is TGA?

TGA is a thermal-analysis method called thermogravimetric analysis.
Mass changes are measured while the sample is heated, and mass losses caused by evaporation, dehydration, decomposition, oxidation, and similar processes are investigated.
For plastic materials, TGA is used to evaluate thermal decomposition onset temperature, heat resistance, residual amount, and the presence or absence of additives and moisture.

In TGA, the temperature at which the mass begins to decrease greatly provides an indication of the onset of thermal decomposition.
However, if small amounts of moisture or solvent are present, a small mass loss may be observed on the low-temperature side.
It is important to distinguish thermal decomposition from drying and volatilization in the discussion.

What Is the Glass Transition Temperature Tg?

The glass transition temperature Tg is the temperature at which the amorphous regions of a polymer change from a hard glassy state to a soft rubbery state.
Below Tg, molecular-chain motion is restricted, and the material tends to become hard and brittle.
Above Tg, some molecular chains become more mobile and the material becomes softer.

Tg is not melting, so it is not the temperature at which crystals melt.
It is the temperature at which molecular-chain motion in the amorphous region changes.
Therefore, in DSC, it often appears not as a sharp peak but as a step in the baseline.

What Is the Melting Point Tm?

The melting point Tm is the temperature at which crystalline regions in a polymer material melt.
In crystalline plastics, heating causes the crystalline regions to melt, producing an endothermic peak.
The melting point is read based on the peak-top temperature or onset temperature of this endothermic peak.

The melting point of a polymer does not necessarily appear as a single sharp point as it does for a low-molecular-weight compound.
Because crystal size and perfection, molecular weight, copolymer components, and thermal history have an influence, the melting peak may have a broad width.
In a report, clearly state whether the peak top, onset, or peak width was used to evaluate the melting point.

Difference Between Tg and Tm

Tg and Tm are both important temperature-related values, but their meanings are completely different.
Tg is the temperature at which molecular-chain motion in the amorphous region changes and is not melting.
Tm is the temperature at which the crystalline region melts and is observed in crystalline materials.

Amorphous plastics may not show a clear melting point Tm, and Tg may be the only important thermal indicator.
On the other hand, both Tg and Tm may be observed in crystalline or semicrystalline plastics.
However, because Tg appears as a small baseline change, it may be more difficult to read than a melting peak.

Thermal Analysis of Crystalline Plastics

Crystalline plastics are materials containing crystalline regions in which some molecular chains are regularly arranged.
In practice, they are often semicrystalline materials in which crystalline and amorphous regions coexist rather than being completely crystalline.
In DSC, Tg of the amorphous region, crystallization, and melting of crystalline regions may be observed.

Polyethylene, polypropylene, nylon, PET, and similar materials show crystallinity depending on the conditions.
The higher the crystallinity, the larger the melting peak and heat of fusion tend to become.
Crystallinity affects strength, heat resistance, transparency, and processability.

Thermal Analysis of Amorphous Plastics

Amorphous plastics are materials in which the molecular chains form very little regular crystalline structure.
Therefore, they do not show a clear melting peak, and Tg is observed as the main thermal change.
Polystyrene, PMMA, polycarbonate, and similar materials may be treated as representative amorphous plastics.

In amorphous materials, hardness and softness change greatly across Tg.
Below Tg, the material is glassy and hard, while above Tg, molecular-chain motion becomes active and the material becomes rubbery or softened.
In DSC, Tg is read as a baseline change rather than as a peak.

How to Read an Endothermic Peak

When an endothermic peak appears in a DSC curve, a change in which the sample absorbs heat from the surroundings is occurring.
In plastic materials, melting of crystalline regions is a representative endothermic change.
The melting point and heat of fusion can be evaluated by reading the peak-top temperature, onset temperature, and peak area of the endothermic peak.

If the peak is broad, there may be a distribution in crystal size or crystal perfection.
If multiple endothermic peaks are present, the effects of different crystal structures, recrystallization, multiple components, additives, and thermal history are considered.

How to Read an Exothermic Peak

When an exothermic peak appears in a DSC curve, a change in which the sample releases heat is occurring.
In plastic materials, crystallization during cooling or cold crystallization during heating may appear as an exothermic peak.
When molecular chains become regularly arranged and form crystals from an amorphous or insufficiently crystallized state, heat is released.

The temperature and area of the exothermic peak are affected by ease of crystallization, mobility of molecular chains, cooling rate, and thermal history.
If a cold-crystallization peak is observed, the sample may not have been sufficiently crystallized before measurement.

How to Read Baseline Changes

If the baseline changes in a step-like manner in a DSC curve, a glass transition may be occurring.
Glass transition is not a melting process accompanied by a distinct latent heat, but rather involves a change in heat capacity.
Therefore, it appears as a shift or step in the baseline rather than as a sharp peak.

Tg is read from the onset, end point, midpoint, or other position of the baseline change.
Because the reading method differs depending on the laboratory manual or analysis software, it is important to state which definition was used to determine Tg.

Discussion of Cold Crystallization

Cold crystallization is a phenomenon in which a polymer that was not sufficiently crystallized because of rapid cooling or similar conditions gains molecular-chain mobility during heating and crystallizes.
In DSC, it may be observed as an exothermic peak after glass transition.
Subsequently, the crystalline regions may melt at a higher temperature and produce an endothermic peak.

Observation of cold crystallization indicates that the thermal history of the sample affected the result.
If the cooling rate during molding was high, the molecular chains may solidify before they can arrange sufficiently, leaving a large amorphous fraction.
This region crystallizes during heating measurement and produces a cold-crystallization peak.

Discussion of Crystallization Temperature Tc

The crystallization temperature Tc is the temperature at which a polymer crystallizes during cooling.
When a polymer is cooled from the molten state, molecular chains arrange regularly and crystals form.
At this time, an exothermic peak caused by crystallization appears in the DSC curve.

Tc is affected by cooling rate, regularity of molecular chains, molecular weight, additives, nucleating agents, and thermal history.
If the cooling rate is high, the temperature may drop before crystallization progresses sufficiently, causing Tc to shift toward lower temperatures or the degree of crystallinity to decrease.

Discussion of Crystallinity

In DSC, crystallinity may be estimated by determining the heat of fusion from the area of the melting peak and comparing it with the theoretical heat of fusion.
Crystallinity represents how much crystalline region is contained in the material.
Materials with high crystallinity require a greater amount of heat for melting.

Crystallinity (%) = Measured heat of fusion ÷ Heat of fusion of completely crystalline material × 100

If cold crystallization occurs, crystallinity may be determined by subtracting the heat of cold crystallization from the heat of fusion.
It is important to follow the equation in the laboratory manual and clearly state which heat quantities were used.

Effect of Thermal History

In thermal analysis of plastic materials, the previous heating and cooling experienced by the sample, that is, its thermal history, greatly affects the results.
Samples rapidly cooled during molding may have insufficient crystallization and show a cold-crystallization peak.
Samples cooled slowly may undergo more crystallization and show a larger melting peak.

In DSC, the effect of thermal history appears strongly in the first heating.
In the second heating after the sample has once been melted and cooled, it becomes easier to compare material properties under a thermal history standardized by the measurement conditions.
In a report, it is useful to distinguish the meanings of the first heating and second heating.

Effect of Heating Rate

Heating rate affects Tg, Tm, and crystallization peaks read from DSC.
If the heating rate is high, the temperature inside the sample may not keep up with the instrument setting, causing the peaks to shift toward higher temperatures or broaden.
It also affects the progress of cold crystallization and melting.

When comparing data measured under different conditions, it is necessary to check whether the heating rates are the same.
Differences in heating rate may also cause deviations from literature values.

Effect of Sample Amount

In DSC measurement, if the sample amount is too large, heat cannot easily be transferred uniformly throughout the sample, and peaks may broaden or temperatures may shift.
If the sample amount is too small, thermal changes become small, making peaks and baseline changes difficult to read.

In TGA, if the sample amount is too large, the movement of decomposition gases and heat may become nonuniform and affect the decomposition temperature or mass-loss curve.
The sample amount must be set while considering the balance between measurement sensitivity and thermal response.

Effect of Sample Shape and Contact Condition

In DSC, the state of contact between the sample and pan also affects the results.
If the sample does not make uniform contact with the bottom of the pan, heat conduction becomes nonuniform, and peaks may broaden or temperatures may shift.
Even for the same material, heat transfer may differ among films, powders, and granular samples.

Operations such as cutting the sample into small pieces, placing it uniformly on the bottom of the pan, and keeping the sample amount consistent are important.
Particularly when measuring pieces of plastic, differences in thickness and shape affect the thermal response.

Effects of Additives and Impurities

Actual plastic materials may contain additives such as plasticizers, stabilizers, fillers, pigments, flame retardants, and lubricants.
These additives may affect Tg, Tm, crystallization behavior, and thermal decomposition temperature.
For example, plasticizers may increase molecular-chain mobility and lower Tg.

If impurities or residual solvents are present, small endothermic peaks or mass losses may be observed on the low-temperature side.
When measuring commercial plastics, differences between pure polymers and materials containing additives must be considered.

Effects of Moisture and Residual Solvent

If plastic materials contain moisture or residual solvent, the thermal-analysis results are affected.
In DSC, endothermic changes associated with evaporation may be observed on the low-temperature side.
In TGA, they may appear as a small mass loss in the low-temperature region.

In addition, if moisture or solvent acts like a plasticizer, Tg may appear lower.
In hygroscopic materials such as nylon, the presence or absence of moisture greatly affects thermal-analysis results, so it is important to confirm the drying condition.

Discussion of Thermal Decomposition Temperature

Thermal decomposition temperature is the temperature at which a plastic material begins to decompose chemically at high temperature.
In TGA, it may be read as the temperature at which the mass begins to decrease significantly.
In DSC, exothermic or endothermic changes associated with decomposition may also be observed.

Thermal decomposition temperature is affected by polymer structure, additives, the presence or absence of oxygen, measurement atmosphere, and heating rate.
Decomposition behavior differs between nitrogen and air, and oxidative decomposition may proceed more readily in air.

Causes of Differences From Literature Values

Causes of measured Tg, Tm, Tc, and decomposition temperatures differing from literature values include differences in sample grade, molecular weight, crystallinity, additives, thermal history, sample amount, heating rate, measurement atmosphere, instrument calibration, and reading method.
Because the properties of polymer materials readily change depending on manufacturing and molding conditions, complete agreement with literature values is not unusual.

Tg in particular readily changes depending on the reading method, and differences arise depending on whether the onset, midpoint, or endpoint is used.
Tm also differs depending on whether the peak top or onset is used.
In a report, the reading method should be clearly stated when making comparisons.

Discussion When a Peak Is Broad

If a DSC peak is broad, possible causes include a distribution in crystal size or crystal perfection within the sample, a broad molecular-weight distribution, a large sample amount, nonuniform heat conduction, the presence of multiple components, or a high heating rate.
In polymer materials, the crystalline state is not uniform, so melting peaks may have a certain width.

A broad peak does not necessarily indicate a failed measurement.
It may reflect the inherent nonuniformity of the material or the distribution of crystals.
However, measurement errors caused by sample amount or contact condition must also be checked.

Discussion When Peaks Are Difficult to Observe

If peaks or Tg are difficult to observe in DSC, possible causes include a small sample amount, a small heat change, low crystallinity, an amorphous material, an unstable baseline, an inappropriate measurement range, or an unsuitable heating rate.
Tg is inherently a small baseline change and may therefore be more difficult to read than a melting peak.

In addition, if additives or multiple components are present, peaks may overlap and become difficult to distinguish.
The measurement range and analysis method should be checked, and second-heating measurements or measurements under different conditions should be considered when necessary.

Discussion When Multiple Peaks Appear

If multiple peaks appear in a DSC curve, possible causes include multiple crystal structures, recrystallization, melting after cold crystallization, copolymers, blended materials, additives, and decomposition reactions.
If the plastic material is not a single component, thermal changes originating from the individual components may overlap.

If multiple melting peaks are present, incomplete crystals on the low-temperature side may melt, recrystallize, and then melt again at a higher temperature.
The temperature sequence of the peaks and whether they are endothermic or exothermic should be checked in the discussion.

When Thermal-Analysis Results Can Be Considered Good

Thermal-analysis results can be considered good when the DSC or TGA curve is stable and values such as Tg, Tm, Tc, and decomposition temperature can be read without contradicting the properties of the material.
In crystalline materials, if a clear melting peak is observed, and in amorphous materials, if the baseline change corresponding to Tg can be confirmed, it becomes easier to relate the material structure to the thermal-analysis results.

If the values also do not greatly contradict literature values or known-material values, the measurement conditions and interpretation can be considered generally appropriate.
However, rather than requiring perfect agreement, differences in thermal history, sample amount, heating rate, and additives must be considered.

Example Discussion When the Experiment Did Not Go Well

When thermal analysis does not go well, possible causes are considered from results such as unclear peaks, a disturbed baseline, inability to read Tg, large deviations from literature values, excessively broad peaks, difficulty interpreting multiple peaks, or unnatural mass changes in TGA.
Organizing the factors separately into sample amount, heating rate, sample contact condition, insufficient drying, additives, thermal history, and instrument calibration makes the discussion easier.

How to Write Points for Improvement

In a discussion of thermal analysis, including not only sources of error but also points for improvement makes the report easier to organize.
Improvements can be organized by dividing them into sample preparation, measurement conditions, instrument management, and analysis methods.

Improvements to Sample Preparation

• Use an appropriate and consistent sample amount
• Cut the sample into small pieces and improve contact with the pan
• Dry the sample sufficiently
• Avoid contamination by foreign matter or dirt
• Record the thermal history of the sample

Improvements to Measurement Conditions

• Keep the heating rate constant
• Use the same measurement atmosphere
• Set an appropriate measurement temperature range
• Calibrate the instrument using a standard substance
• Distinguish the first and second heating runs when necessary

Improvements to Analysis

• State the position used to read Tg
• State whether Tm was determined from the peak top or onset
• Perform baseline correction appropriately
• When determining heat quantity from peak area, use the same integration range
• When comparing with literature values, also check the measurement conditions

Difference Between a Superficial Discussion and a Good Discussion

In thermal analysis of plastic materials, simply writing that “a peak appeared,” “Tg was determined,” or “the melting point was found” results in a superficial discussion.
Relating thermal changes to polymer structure, crystallinity, amorphous character, thermal history, and measurement conditions produces a more persuasive discussion.

Examples of Expressions That Can Be Used in Reports

The following expressions can be used when writing the results and discussion of thermal analysis of plastic materials.
Adjust the necessary parts according to your own experimental results.

• The endothermic peak in the DSC curve is considered to correspond to melting of the crystalline region.
• Glass transition is observed not as a distinct peak but as a baseline step.
• Tg is the temperature that indicates a change in molecular-chain motion in the amorphous region.
• Tm is the temperature at which the crystalline region melts and is observed in crystalline materials.
• In amorphous plastics, a clear melting peak is difficult to observe, and Tg becomes the main thermal indicator.
• In crystalline plastics, crystallinity may be estimated from the area of the melting peak.
• A cold-crystallization peak indicates that the amorphous region crystallized during heating.
• Thermal history affects Tg, Tm, crystallization temperature, and peak shape.
• A high heating rate may cause shifts in peak temperature and increased peak width.
• Moisture and residual solvent may appear as low-temperature endothermic changes or mass losses in TGA.

Points to Check When Discussing Thermal Analysis

Checking the following points before writing the report makes the discussion easier to write.

• Is it clearly stated whether the measurement method is DSC or TGA?
• Is the difference between Tg and Tm explained?
• Is Tg read as a baseline change?
• Is Tm read as an endothermic peak?
• Are differences between crystalline and amorphous materials discussed?
• Are exothermic peaks related to crystallization or cold crystallization?
• Are heat quantity and crystallinity considered from peak area?
• Is the effect of thermal history considered?
• Are the effects of heating rate and sample amount considered?
• Are the effects of additives and moisture considered?
• Are causes of differences from literature values described?
• Do the points for improvement correspond to the sources of error?

Summary

In thermal analysis of plastic materials, DSC and TGA are used to evaluate melting point, glass transition temperature, crystallization temperature, thermal decomposition temperature, and other properties.
In DSC, melting is commonly observed as an endothermic peak, crystallization as an exothermic peak, and glass transition as a baseline step.
Tg indicates a change in molecular-chain motion in the amorphous region, while Tm indicates melting of the crystalline region.

In crystalline plastics, melting peaks are readily observed, whereas amorphous plastics do not show a clear melting peak and Tg may become the main thermal indicator.
In addition, cold crystallization, crystallization temperature, heat of fusion, and crystallinity are important for considering the thermal history of the material and the arrangement of molecular chains.

In a report, rather than simply writing that “a peak appeared,” explain whether the peak is endothermic or exothermic and whether it corresponds to melting, crystallization, glass transition, or decomposition.
Furthermore, considering the effects of sample amount, heating rate, thermal history, additives, moisture, and measurement atmosphere produces a more persuasive discussion.