Determination of glass transition temperature
Glass transitions occur in all non-crystalline or semicrystalline materials and lead to significant changes in material properties such as thermal expansion, the specific heat capacity or modulus. Because the glass transition is very sensitive to chemical and physical structure, it can be used to characterize materials and is therefore important in most industries.
Thermal Analysis provides different methods to measure the glass transition and the glass transition temperature.
In this Webinar, we will discuss the basic theory of the glass transition and the different thermal analysis techniques and methods used to measure the glass transition and the associated glass transition temperature.
The Webinar covers the following topics:
- Basic theory of the glass transition
- Thermal analysis techniques and methods
- Industries and applications
- Practical applications
The determination of glass transition temperature (Tg) is critical for understanding a material's properties. It indicates the temperature at which a substance transforms from a glassy state to a rubbery state or vice versa. The formation of such amorphous glasses is a universal phenomenon observed in practically all materials.
For practical applications, it is important to realize that the properties of glasses differ significantly from those of other solids. For example, the solubility of glasses is higher than for crystals; this influences the biological activity of pharmaceutical substances.
Why is the determination of glass transition temperature so important?
The glass transition provides information about molecular dynamics in the supercooled melt. It defines the upper temperature limit for the use of solid amorphous materials; for rubbery materials, it is the lower temperature limit.
Knowledge of the glass transition is also important for optimizing production parameters and the properties of products. In addition, the glass transition can be used to identify and compare materials and is therefore important for quality assurance and failure analysis
Determination of the Glass Transition Temperature
Slide 0: Determination of the Glass Transition Temperature
Ladies and Gentlemen,
Welcome to the METTLER TOLEDO webinar on the Determination of the Glass Transition Temperature.
In this presentation, I would like to focus on the measurement of the glass transition using thermal analysis techniques. In addition, I will discuss how we can obtain valuable information about the structure and composition of materials from the glass transition.
Slide 1: Contents
This slide lists the main topics covered in the seminar.
I will begin with some comments about the formation, structure, and properties of glasses as well as about differences between glassy and crystalline solids. I then want to discuss the characteristic changes that occur at the glass transition, and consider the most important techniques used for measuring the glass transition.
I will then go on to discuss the importance of the glass transition for the analysis and characterization of materials using practical examples from different application fields.
The presentation will end with a short summary.
Slide 2: Why is the Glass Transition Important?
The formation of so-called glasses is a universal phenomenon observed in practically all materials. Besides the generally well-known inorganic glasses based on silicon oxide, glasses also occur with other inorganic compounds, metal alloys, low-molecular weight organic substances, and polymers. As far as their molecular structure is concerned, glasses have no long-range order; they are amorphous like a liquid.
For practical applications, it is important to realize that the properties of glasses differ significantly from those of other solids. For example, the solubility of glasses is higher than for crystals; this influences the biological activity of pharmaceutical substances.
The glass transition is the transformation of a material from the glassy solid state to the liquid state, or vice versa. The characteristic temperature is the glass transition temperature, tee gee (Tg). With crosslinked polymer systems, the term rubbery state is often used instead of the liquid state.
The glass transition provides information about molecular dynamics in the supercooled melt. It defines the upper temperature limit for the use of solid amorphous materials; for rubbery materials it is the lower temperature limit.
The glass transition depends on the molecular structure and composition of materials. That’s why, for example, measurements of the glass transition provide information about the degree of crosslinking or cure of adhesives. Knowledge of the glass transition is also important for optimizing production parameters and the properties of products. In addition, the glass transition can be used to identify and compare materials and is therefore important for quality assurance and failure analysis.
Slide 3: Introduction: The Glassy State
The diagram illustrates the formation of liquid melts, crystalline solids and glassy solids at different temperatures.
The green bar represents the crystalline solid, which melts at the melting temperature, tee eff (Tf). Further heating leads to an equilibrated melt that is represented by the brown bar. If the melt is cooled, it normally crystallizes below the melting temperature and forms a crystalline solid.
At temperatures below the melting temperature, the melt is no longer in thermodynamic equilibrium. It is referred to as a supercooled melt.
If a melt is cooled at a sufficiently high cooling rate, the crystallization process is interrupted. A glassy solid is formed at the glass transition temperature. The glass is represented by the blue bar.
If a glass is heated, it changes to the liquid supercooled melt at the glass transition temperature. On further heating, crystallization can occur before the melting temperature is reached.
In contrast to the crystal/melt transition, the glass-to-melt or melt-to-glass transition takes place in about the same temperature range. A direct transition from the glassy solid to the crystalline solid state practically never occurs.
Slide 4: Introduction: The Glassy State
As mentioned before, a glass is formed when the melt is cooled sufficiently rapidly. This is illustrated in the diagram. The x-axis is temperature. The glass transition temperature, tee gee (Tg), and the melting temperature, tee eff (Tf), are indicated by the vertical lines. The y-axis is time and is presented logarithmically because it changes by several orders of magnitude.
The blue curve is the characteristic time of the crystallization process. This could, for example, be the time to the peak maximum in an isothermal crystallization measurement using the Flash DSC 1. At temperatures just below the melting temperature, crystallization is slow and the time is long. Crystallization becomes faster as the temperature is lowered. The characteristic time of crystallization has a minimum between the glass transition and the melting temperature. As the temperature approaches the glass transition temperature, tee gee (Tg), the crystallization rate again decreases due to the decrease in molecular mobility. Crystallization occurs in the temperature/time-region colored green.
The red curves represent two linear cooling programs that begin at the melting temperature. The curves are not straight lines because the time axis is logarithmic. The dashed curve represents a relatively slow cooling process. The cooling curve intercepts the green crystallization region and the material crystallizes. In contrast, the cooling rate of the continuous curve is so high that that the crystallization region is not reached. The sample forms a glassy solid at the glass transition temperature.
Slide 5: Introduction: The Glassy State
Since materials differ in their maximum crystallization rate, the minimum cooling rates at which a glass is formed are also different. For example, with polyethylene terephthalate, a cooling rate of less than one hundred degrees per minute (100 K/min) is sufficient to completely suppress crystallization.
As the slide shows, polypropylene behaves very differently. Even at a cooling rate of twelve hundred degrees per minute (1200 K/min), the material crystallizes in the temperature range one hundred and thirty to eighty degrees Celsius (130 to 80 °C). If the sample is cooled much faster at thirty thousand degrees per minute (30000 K/min), practically no crystallization takes place. The glass transition occurs at minus ten degrees (–10 °C). The curves were measured using the Flash DSC 1. This instrument allows the use of scanning rates of up to millions of degrees per minute (Millionen K/min).
Slide 6: Introduction: The Glassy State
During crystallization, well-ordered crystals form from the amorphous melt. The process requires a relatively large degree of molecular mobility.
The formation of a glass is however fundamentally different. With increasing supercooling, the density increases and molecular rearrangements occur more slowly. The reason for this is that, in this temperature range, cooperative movements of ensembles of particles occur. These are referred to as cooperatively rearranging regions, or CRRs. The lower the temperature, the larger the cooperatively rearranging regions become and the slower the corresponding molecular motion. In the glass transition region, the cooperative molecular rearrangements take place so slowly that they “freeze”. There is no longer any long-range molecular motion. The material becomes solid without its structure changing significantly. This process is referred to as vitrification. Like a liquid, a glass has no molecular long-range order. It is amorphous.
The properties of the crystalline phase and the glassy phase are very different. For example, a glass has higher solubility, a smaller mechanical modulus, lower brittleness, lower density, no grain boundaries and lower thermal stability compared with a single crystal.
The structure of a glass is not stable. Because molecular motion is not impossible but only very slow, so-called structural relaxation can occur. This can lead to stability problems especially during the storage of a glass just below the glass transition temperature.
Slide 7: Introduction: Solid-Liquid Transitions
Let’s now compare the two transitions from the solid state to the melt.
In melting, well-ordered crystals are converted to a liquid; the crystalline structure is destroyed. The latent heat of fusion has to be supplied to the material. This gives rise to an endothermic peak in the DSC curve.
In the glass transition, there is mainly a change in molecular mobility. Above the glass transition, cooperative rearrangements take place. This is why the heat capacity increases at the glass transition. This causes a step in the DSC curve.
Whereas a pure substance melts at a defined temperature, the glass transition always takes place over a more or less broad temperature range. The glass transition temperature depends on the thermal history of the sample, the method used to measure it, and the measurement conditions.
Slide 8: Introduction: Behavior of the Transition
I now want to discuss the glass transition in the Enthalpy-Temperature diagram.
In the diagram on the left, the blue curve shows the effect of cooling from the melt at a relatively low cooling rate. At the glass transition temperature tee gee one (Tg1), the sample vitrifies because the cooperatively rearranging regions freeze. The red curve shows the behavior on rapid cooling. The characteristic time in this experiment is shorter. This is why the cooperatively rearranging regions freeze earlier. The glass transition temperature tee gee two (Tg2) is higher than tee gee one (Tg1). At higher cooling rates, the glass transition temperature shifts to higher temperatures. The shift depends on the material in question. As a rough guide, a change in the cooling rate of one decade causes the glass temperature to shift by two to ten degrees (2 to 10 K).
The diagram on the right illustrates the structural or enthalpy relaxation of glasses. The blue curve shows the enthalpy on cooling. If a glass is annealed below the glass transition temperature, the structure of the glass changes toward that of the corresponding liquid. This relaxation is marked by the vertical green arrow.
If the partially relaxed sample is now heated, the red curve is obtained. Up to about tee gee (Tg), the enthalpy curve has about the same slope as in cooling. The enthalpy then jumps to the curve of the supercooled melt. In the DSC heating curve, this produces a so-called enthalpy relaxation peak that overlaps the glass transition step.
The smaller the difference between the storage temperature and the glass transition temperature, the faster the enthalpy relaxation.
Slide 9: Introduction: Annealing
The diagram on the left shows a series of DSC curves of polyethylene terephthalate samples that were annealed for different times at sixty-five degrees Celsius (65 °C). The longer the sample was annealed, the larger the enthalpy relaxation peak. The enthalpy relaxation can be determined directly using temperature-modulated DSC (for example TOPEM).
Another possibility is shown in the diagram on the right. This method involves heating the sample again immediately after cooling and without annealing. This curve (Curve B) is then subtracted from the initial measurement curve (Curve A). Integration of the difference curve in the glass transition range yields the peak area of the enthalpy relaxation.
Slide 10: Different Measuring Techniques
The slide summarizes the techniques used to measure the glass transition. The blue arrow on the right shows the difference in sensitivity between the techniques with regard to measurement of the glass transition.
The first technique, TGA/DSC, makes use of a thermogravimetric analyzer equipped with a sensor that simultaneously measures the heat flow curve or temperature difference curve. The DSC curve is recorded using a linear temperature program. This technique is used for measurements above seven hundred degrees Celsius (700 °C).
Better resolution is achieved using a dedicated Differential Scanning Calorimeter, DSC. Many technical processes in industry employ high cooling rates. Reorganization can occur on heating. The Flash DSC 1 can be used to cool samples very quickly or to suppress reorganization processes.
Techniques involving temperature-modulated DSC superimpose a small periodic temperature oscillation on a linear temperature program. This allows the heat flow signal to be separated into two components, namely the reversing heat flow, which can follow the modulation, and the non-reversing heat flow, which can not.
The change in heat capacity at the glass transition appears in the reversing heat flow curve. In contrast, many overlapping events that occur simultaneously with the glass transition are observed in the non-reversing heat flow curve. This allows the glass transition to be separated from other events.
The ADSC technique uses a sinusoidal temperature modulation at a single frequency whereas the TOPEM method employs a stochastic temperature modulation. The TOPEM signal contains an entire frequency spectrum. Both ADSC and TOPEM measurements can be performed using a conventional DSC instrument.
In Thermomechanical Analysis, TMA, the length or thickness of a sample is continuously measured by a probe that rests lightly on the sample with a small force. The sample is heated using a linear temperature program. If a larger force is used, the probe penetrates the sample and measures the softening of the sample, which often begins at the glass transition.
Mechanical properties usually show a marked change at the glass transition. This is why Dynamic Mechanical Analysis, DMA, is the technique with the greatest sensitivity. The method measures the complex mechanical modulus consisting of the storage and loss components at selected frequencies.
Slide 11: Different Measuring Techniques
This diagram compares the three main thermal analysis techniques for measuring the glass transition.
DSC measures the heat flow exchanged between the sample and instrument. The physical quantity measured is the heat capacity, or in qualitative measurements the change in heat capacity of the sample. At the glass transition, the heat capacity changes suddenly, typically by a factor of about one point three (1.3).
In TMA, the change in length or thickness of the sample due to thermal expansion is measured as a function of temperature. The coefficient of thermal expansion or expansion coefficient of the sample changes markedly at the glass transition, by a factor of about 2. The relative change is therefore greater than with DSC. Smaller glass transitions can therefore be more easily detected by TMA than by DSC.
In DMA, the force (stress) and deformation (strain) of the sample are measured while the sample is subjected to a periodic (oscillating) stress. The frequency-dependent modulus consisting of storage and loss components are determined as a function of time, temperature and frequency. The storage modulus changes at the glass transition of polymers by about three orders of magnitude, that is by about a factor of 1000. The mechanical modulus therefore reacts very sensitively to changes in molecular mobility at the glass transition. The DMA technique has the highest sensitivity. This is particularly advantageous for highly filled systems where the intensity of the glass transition is low.
Slide 12: Different Measuring Techniques: DSC
This slide displays DSC heating curves of different polymers in their glass transition regions. The samples were measured at a heating rate of ten degrees per minute (10 K/min) except polycarbonate (PC) and polyethersulfone (PES) at twenty degrees per minute (20 K/min).
The heating curves all show clear endothermic steps. The glass transition of the polyethylene terephthalate (PET) sample looks slightly different because it is immediately followed and overlapped by an enthalpy relaxation peak. The reason for this is that the sample was annealed for a certain time below the glass transition temperature before the measurement.
Slide 13: Different Measuring Techniques: Standards
A number of different definitions are used by standards organizations and instrument companies to define the glass transition temperature, tee gee (Tg), and the step height, delta see pee (Δcp). Besides the “default” evaluation method, the STARe software also enables evaluations to be made according to the ASTM E1356 and DIN 53765 procedures as well as the so-called Richardson method. The diagram illustrates the different evaluation methods using a DSC curve with enthalpy-relaxation as an example.
All the evaluations require lines A and B. The blue line, A, is the extrapolation of the DSC curve of the glass, and the red line, B, the extrapolation of the DSC curve above the glass transition. In some cases, the positions of these two lines might have to be manually optimized. The algorithm of the glass transition evaluation also automatically determines the green tangent at the point of inflection.
The “default” midpoint glass transition temperature is the temperature at which the bisector of the angle between lines A and B intersect the measurement curve. The step height, delta cp, corresponds to distance between the intercepts of line C with lines A and B. The glass transition temperatures determined by the ASTM and DIN methods are very similar to the default value. All three glass transition temperatures have to do with the shape of the measurement curve.
The glass transition temperature according to Richardson is somewhat lower on the enthalpy-temperature curve. It is also called the fictive temperature. The temperature is evaluated in a somewhat different way and relates to the structure of the glass.
The exact definitions of the temperatures and step heights of glass transitions can be found in the Online Help of the STARe software by searching under “Midpoints” and “cp determination diagram”.
Slide 14: Different Measuring Techniques: TGA/DSC
The next slide shows a DSC curve measured at higher temperature using the DTA sensor in the TGA.
The sample was a glazing material used in the ceramic industry. The glass transition step with a midpoint at about seven hundred and fifty-nine degrees Celsius (759 °C) can be clearly seen.
Slide 15: Different Measuring Techniques: TMA
Here we see TMA measurements of the glass transitions of two different elastomers.
The upper diagram displays the TMA curves with the typical change in slope at the glass transition. The curves in the lower diagram show the corresponding linear expansion coefficients.
The red curves correspond to the measurement of a solvent-polymerized styrene-butadiene-rubber sample. The glass transition temperature is at about minus ten degrees Celsius (–10 °C) and can be determined as the onset of the change of slope of the measurement curve or as the point of inflection of the step in the expansion coefficient curve.
The black curves represent the measurement of a blend of emulsion-polymerized styrene-butadiene-rubber and butadiene rubber. The glass transitions of these two components lie relatively close together. They are much more clearly separated in the expansion coefficient curve. The separation is more difficult to measure by DSC because only a relatively broad step is measured.
Slide 16: Different Measuring Techniques: DMA
This slide shows typical DMA curves of a powder coating measured in the shear mode at a frequency of one hertz (1 Hz).
The black curve is the storage modulus, gee prime (G′), the red curve the loss modulus, gee double prime (G″), and the blue curve the loss factor, tan delta. The loss factor is the ratio of the loss modulus and the storage modulus. The y-axis in the upper diagram is plotted logarithmically because the change in the modulus is large at the glass transition.
During the glass transition, the storage component of the shear modulus changes by more than two orders of magnitude. The loss modulus exhibits a peak with a maximum at about one hundred and thirteen degrees Celsius (113 °C). This temperature agrees well with the onset temperature in the semi-logarithmic presentation of the storage modulus curve. The tan delta curve also exhibits a peak whose maximum temperature at one hundred and twenty degrees (120 °C) is somewhat higher than that of the peak of the loss modulus.
Frequently, one of these three temperatures is defined as the glass transition temperature. In this case, it is always important to state the frequency at which the measurement was performed. Furthermore, we recommend that the loss modulus is used to determine the glass transition temperature because the peak in the tan delta curve depends on the value of the modulus in the rubbery plateau.
Slide 17: Different Measuring Techniques: DMA
The slide shows a measurement of the glass transition of a powder coating by DMA in the shear mode at several different frequencies. The frequencies range from zero point one hertz (0.1 Hz) to one thousand hertz (1000 Hz).
The curves show that the glass transition is dependent on the measurement frequency. At higher frequency, the glass transition shifts to higher temperatures. The glass transition temperature determined from the maximum of the loss modulus curve, gee double prime(G″), is about one hundred and eight degrees Celsius (108 °C) at zero point one hertz (0.1 Hz), and one hundred and twenty-seven degrees (127 °C) at one thousand hertz (1000 Hz).
The position of the measurement curve depends on the measurement frequency and the actual characteristic frequency of molecular movement. At higher frequencies, rapid molecular rearrangements are excited that take place at higher temperatures. This is why the measured glass transition temperature is higher at higher frequencies. Measurements of the frequency dependence of glass transitions allow the temperature dependence of molecular rearrangement processes to be studied.
The measurement curves show that the storage modulus is about five mega-pascals (5 MPa) at one hundred and twenty five degrees (125 °C) and a frequency of one hertz (1 Hz). At one thousand hertz (1000 Hz), however, the modulus is about 100 times larger at this temperature. This clearly demonstrates that the mechanical behavior of a material should be measured at frequencies at which it is intended to be used. This is particularly important because conventional extrapolation procedures often fail for modern complex materials.
Slide 18: Industries and Applications
The evaluation of glass transitions plays an important role in practically all application fields of thermal analysis in which amorphous or semicrystalline materials and substances are used. The glass transition is frequently used as a thermal event to identify or compare materials and to characterize them with regard to their structure or use. The table lists a number of industries and examples of applications.
Slide 19: Application 1: Influence of structure and composition
In filled materials, the filler influences the glass transition. If the material is semicrystalline, the crystallites act like a filler and have a similar influence on the glass transition of the remaining amorphous constituents.
The slide shows measurement curves of samples of polyethylene terephthalate with degrees of crystallinity ranging from 0.4% to 20.5%. In the inserted graph, the black points show the glass transition temperature and the blue points the step height, delta cp, as a function of the degree of crystallinity.
The step height decreases linearly with increasing degree of crystallinity because the amorphous content in the sample becomes smaller and smaller. The step height is a measure of the content of the mobile amorphous phase in the sample.
The glass transition temperature remains unchanged up until a degree of crystallinity of about 15% after which it increases. The molecular mobility in the amorphous phase is reduced due to interaction with the crystalline phase.
The measurement curves also show that the glass transition becomes broader at higher degrees of crystallinity. Again, the reason for this is the influence of the crystallites on molecular mobility in the amorphous phase.
Slide 20: Application 2: Compatible mixtures: plasticizer
If liquids can be homogeneously mixed and sufficiently strongly supercooled, a homogeneous glass with a glass transition temperature dependent on the composition forms from the mixture.
The example displays DSC measurement curves of a system consisting of polyvinylacetate and a plasticizer. The inserted graph shows that the glass transition temperature decreases with increasing plasticizer content. The curve can be used to describe the activity of the plasticizer in the polymer or to determine the plasticizer content in a sample.
Slide 21: Application 3: Compatible mixtures: PVP and CO2
Carbon dioxide also acts as a plasticizer for some materials. A good example is poly-vinyl-pyrrolidone or PVP, which is employed as an excipient in the pharmaceutical industry.
The diagram on the left displays high-pressure DSC measurement curves of PVP at different pressures of carbon dioxide in the glass transition region. The curves show that the glass transition temperature decreases with increasing CO2 content. The graph on the right shows values of the glass transition temperature plotted against increasing pressure.
Slide 22: Application 4: Overlapped glass transition
As we have already heard, different thermal events often occur simultaneously during a DSC measurement. This slide shows a good example of this, namely the analysis of a powdered pharmaceutical formulation consisting of an amorphous and a crystalline component using TOPEM. The powder was produced by spray drying and contained residual moisture. The residual water acts as a plasticizer and influences the production process.
In a conventional DSC curve of the substance, the broad peak due to the evaporation of water would overlap all the other thermal events. This would mask the glass transition and make evaluation difficult.
However, as shown in the slide, the use of temperature-modulated DSC, in this case TOPEM, enables the different events to be separated from one another. The upper diagram displays the specific heat capacity curve. This is proportional to the reversing heat flow. The lower diagram shows the non-reversing heat flow curve.
In the heat capacity curve, the glass transition of the amorphous component appears at about sixty degrees Celsius (60 °C). The heat capacity then decreases significantly due to the reduction in mass through loss of water. The small peak just above one hundred and twenty degrees (120 °C) is due to the melting of the crystalline component.
In the non-reversing heat flow curve, the broad peak shaded green is caused by the evaporation process of the water. The evaluation of the peak area yields a water content of 6.4%. This corresponds to the change in the specific heat capacity. Besides the broad evaporation peak, the curve also exhibits two sharp peaks at about sixty degrees (60 °C) and one hundred and twenty-five degrees (125 °C). The first peak lies in the region of the glass transition and is due to enthalpy relaxation. The second peak is due to melting of the crystalline component.
Slide 23: Application 5: Curing and glass transition
Systems that cure such as adhesives, coating materials, prepregs and other thermosetting plastics have a glass transition that depends on the composition of the system and the degree of cure. Glass transition measurements provide very important information about such systems.
The postcuring curves were obtained from an epoxy-amine system. Samples were cured for different times at one hundred degrees Celsius (100 °C), then rapidly cooled and heated again. The heating curves show the glass transition followed by the exothermic postcuring peak. The higher the degree of cure before the heating measurement, the smaller the postcuring peak. The degree of cure can be determined by comparing the area of a peak with that of the uncured sample. The curves show that the glass transition temperature is shifted to higher temperature with increasing degree of cure.
If the glass transition temperature exceeds the curing temperature, in this case one hundred degrees Celsius (100 °C), enthalpy relaxation takes place in the partially cured glass during the reaction without the degree of cure increasing significantly. The glass transition temperature of the cured material can be obtained by heating the sample again.
In the inserted graph, the glass transition temperatures are plotted as a function of the reaction conversion determined from the peak areas. Diagrams like this can be used to estimate the degree of cure of a sample from the glass transition temperature and obtain valuable information about the curing system.
Slide 24: Application 6: Glass transition overlapped by postcuring
It is often difficult to determine the glass transition by means of DSC measurements if the glass transition is overlapped by a postcuring peak. This problem arises for example with systems that are almost completely cured. Temperature-modulated DSC is then very useful.
The TOPEM measurement shown here was performed at the rather high underlying heating rate of ten degrees per minute (10 K/min) in order to prevent significant postcuring from occurring on heating. This would cause the glass transition to shift. The total heat flow curve is red, the non-reversing heat flow curve blue and the reversing heat flow curve green.
The total heat flow curve corresponds to the conventional DSC curve. Only the postcuring peak can be recognized. The reaction enthalpy appears to be about ten joules per gram (10 J/g). In the reversing heat flow curve, the glass transition is observed at about one hundred degrees Celsius (100 °C). The non-reversing heat flow curve shows the reaction peak without the change due to heat capacity. The actual reaction enthalpy is almost sixteen joules per gram (16 J/g), that is, more than 50% greater than that determined from the conventional DSC curve.
Slide 25: Application 7: Influence of structure and composition
This slide compares DSC and DMA measurements of a polyurethane elastomer.
The DSC curve in the upper diagram shows a relatively narrow glass transition and a small melting peak.
The lower diagram shows DMA loss factor or tan delta curves. The red curve was measured at one hertz (1 Hz) and the blue curve at ten hertz (10 Hz). Melting is observed in both curves as a small peak at about fifty degrees Celsius (50 °C). In contrast to DSC, in the DMA curves the glass transition appears as a relatively broad peak in the temperature range minus forty degrees to plus forty degrees (–40 °C to 40 °C). Besides the maximum at about minus twenty degrees (–20 °C), the tan delta peaks also exhibit a shoulder at about zero degrees (0 °C). The DMA curves show that this sample of polyurethane has soft and hard segments whose transition temperatures significantly differ. This level of resolution cannot be achieved by DSC.
A further point of interest is that the blue curve is shifted slightly to higher temperature in the glass transition region because time-dependent effects are frequency dependent. In contrast, the melting peak is observed at the same temperature at both measurement frequencies.
Slide 26: Summary
Let me now summarize what we have discussed during the webinar.
We have seen that, at sufficiently high cooling rates, theoretically any material can form a glass. A glass is not in a state of thermodynamic equilibrium. When it is stored or annealed, enthalpy relaxation occurs. This can be measured by DSC. The transition from a supercooled melt to a glass and vice versa is the glass transition. In this process, there is a significant change in molecular mobility.
In a DSC measurement, the glass transition is observed as a step in the heat capacity curve. The glass transition temperature, tee gee (Tg), and the step height, delta cp, can be determined from the heat flow curve. In the high temperature range, glass transitions are measured using a TGA with a heat flow or a DTA sensor. Glass processes can be investigated at high cooling and heating rates using the Flash DSC 1.
The use of temperature-modulated DSC increases the sensitivity of the measurement and separates the glass transition from other possibly overlapping effects. Since the frequency can also be varied in these measurements, information is also obtained about the frequency dependence of the glass transition.
TMA is often used to measure the change in thermal expansion at the glass transition. The glass transition can also be detected due to the softening of the sample on heating by measuring the penetration of a probe into the sample.
DMA is the most sensitive technique for measuring glass transitions because the modulus can change by several orders of magnitude. The technique is therefore often used when only very weak glass transitions occur in a DSC measurement. The relatively large frequency range of DMA allows the frequency dependence of the glass transition to be studied.
Slide 27: Summary
Frequently, glass transition measurements are used to compare materials or to detect changes. The characteristic quantities that can be determined from the DSC curves change.
The table summarizes some of the changes that can occur. For example, when the degree of crystallinity increases, it is mainly the step height that decreases. A small increase in the glass transition temperature and a broadening of the step might also possibly occur.
Slide 28: For More Information on the Glass Transition
Finally, I would like to draw your attention to further information on the glass transition that you can download from METTLER TOLEDO Internet pages.
Articles on thermal analysis and applications from different fields are published twice a year in our well-known, METTLER TOLEDO thermal analysis UserCom customer magazine. The slide lists different UserCom articles that specifically relate to glass transition studies.
Our Collected Applications handbooks and the “Thermal Analysis in Practice” reference handbook also contain many examples of the measurement of glass transitions and are recommended for further study.
Slide 29 For More Information on the Glass Transition
In addition, you can download information about webinars, application handbooks or information of a more general nature from the Internet addresses given on this slide and so ensure that you are always fully up-to-date in thermal analysis.
Slide 30 Thank You
This concludes my presentation on the glass transition. Thank you very much for your interest and attention.