The glass transition is defined as the reversible transition that can affect the amorphous portions of any material in general. This transition takes such amorphous structures from a state of being hard and relatively brittle (in other words, a "glassy" state), into a viscous or rubbery state, as a function of gradual temperature increase. Any such amorphous solid that can undergo a glass transition is called a glass. The inverse transition, which occurs by cooling a viscous liquid into the glass state, is consequently referred to as vitrification.
The glass-transition temperature Tg of a material itself denotes the temperature range over which this glass transition takes place. As a result of its definition, in relation to the fact that the rubbery state above the glass transition is by itself also still in the solid state, this Tg quantity is always lower than the melting temperature, Tm, of the material under consideration.
Applications to Polymer Science
The concept of glass transition temperature is particularly relevant in polymer science. In this context, IUPAC defines the glass transition as the process through which a polymer melt changes on cooling into its corresponding glass state, or vice versa upon heating.
Polymers (plastics, elastomers or rubbers) are in general made up of long chains of molecules, and may be amorphous or crystalline in nature. The structure of a polymer is defined in terms of a physical property known as its crystallinity, according to which the following distinction can be drawn:
- Amorphous polymers, defined in general by their relatively random molecular structure, which is characterized by an ill-defined melting temperature. As a result of this, such amorphous materials soften only very gradually as the temperature rises, via the glass transition temperature itself. Some examples of this class of materials include the polymers PC, GPPS, PMMA, PVC, ABS.
- Crystalline or Semi-crystalline polymers, characterized by a highly-ordered molecular structure. This type of polymers does not soften as the temperature rises, but instead it is defined by a sharply-occurring melting point. Some examples include the Polyolefins, PEEK, PET and POM polymers.
Thus, in summary, crystalline polymers have a well-defined melting point, while amorphous polymers are characterized by a glass transition temperature, and semi-crystalline polymers can have both melting point and a glass transition temperature. The concept of glass transition temperature (Tg), marking the transition from glassy state to rubbery state, therefore only applies to amorphous and semi-crystalline polymers. The glassy state is hard and brittle, while the rubbery one is soft and flexible.
According to the final application, some of these polymers are used above Tg, and some are used below. Hence, an accurate understanding of Tg is essential in the selection of materials for such uses (for example, applications requiring rigid and hard polymers at ambient temperature would require corresponding high values of Tg). The magnitude of Tg itself depends on the overall mobility of the main polymer’s chain, and for most synthetic polymers the value of Tg falls between 170 K and 500 K.
The following diagram summarizes the behaviour of semi-crystalline polymers (amorphous with some percentage of crystalline structure, which is the case for the vast majority of polymers): when we slowly heat this kind of polymer and increase the temperature, the polymer changes from the glassy state to rubbery state first, via the glass transition temperature, and finally to the viscous liquid state via the corresponding melting temperature:
Ref.: CHEM3020: POLYMER CHEMISTRY lecture slides (Unit-3: Crystallinity and Glass Transition Temperature) by Prof. Rafique Ul Islam, Department of Chemistry, MGCU, Motihari (http://www.mgcub.ac.in/pdf/material/20200503035034f039fc3842.pdf)
The behaviour described so far can be understood in terms of the structure of glassy materials, which are formed typically by structures consisting in long chains, networks of linked atoms, or those that possess a complex underlying molecular structure. Normally, such materials have in fact a high viscosity in the liquid state. When fast cooling occurs to a temperature at which the crystalline state is expected to be the most stable one, the resulting molecular movement can in fact become very slow and clumsy, such that the material cannot possibly adopt a truly crystalline conformation. Therefore, the random arrangement of the molecules characteristic of the liquid state continues to dominate until the viscosity corresponds to that of the solid state of the material. The term glassy therefore indicates the persistence of such a constant non-equilibrium state. In fact, a path to the state of lowest energy (the fully crystalline state) might not exist at all.
When lowering the temperature of an amorphous material from its initially liquid state, there is in fact no sudden change in volume at the solidification point (Tf), such as it would typically occur in the case of a purely crystalline material (via a classical first-order phase transition, with its characteristic associated volume discontinuity). Instead, at the glass transition temperature, Tg, of an amorphous material, there is a gradual change in the slope of the volume of the material as a function of temperature. The comparison between the behaviours of crystalline materials (1) and amorphous materials (2) is portrayed in the figure below. The intersection point of the two straight line segments of curve (2) in this way consequently defines the quantity Tg.
Ref.: “Thermal Properties of Polymers”; course by Haldia Institute of Technology
The specific volume measurements explained previously, as performed on an amorphous polymer, can be carried out in a dilatometer instrument at a slow heating rate. In this experimental set-up, a material sample is inserted within a glass bulb, and a surrounding confining liquid (typically mercury) is poured into the bulb, and is also made to fill partially a thin glass capillary tube. This tube allows relatively small changes in polymer volume to be measured via the corresponding change in the height of the mercury in the capillary.
While the dilatometer method is the most precise fashion of determining the glass transition temperature, it is rather cumbersome and delicate. Hence, measurements of Tg are often made through an alternative method called differential scanning calorimeter (DSC). In this other type of instrument, the heat flow into or out of a small (10 - 20 mg) material sample is measured during the course of a linear temperature increase.
Of course, there exists several other methods as well to determine Tg, such as:
- Specific heat measurements
- Thermo mechanical analysis
- Thermal expansion measurement
- Micro-heat-transfer measurement
- Isothermal compressibility
- Heat capacity
Tg and Mechanical Properties
The transition from the glass to the rubbery state is an essential characteristic of the polymer’s mechanical behaviour, denoting a region of significant alterations in the physical properties of the polymer, such as its hardness and elasticity.
Some polymers are used below their Tg (in glassy state), like polystyrene, poly(methyl methacrylate) etc., which are hard and brittle. Their Tgs are in fact higher than room temperature. Some other polymers on the other hand are used above their Tg (in the rubbery state), for example rubber elastomers like polyisoprene, polyisobutylene etc. They are thus soft and flexible in nature, and their Tgs are consequently less than room temperature.
Identifying the Tg of polymers is often used for better conceiving the potential applications of such polymers, and thus it represents a key step in commercial polymer R&D. Altering the Tg of polymers can in fact profoundly affect the physical and mechanical properties of solids, and consequently its fine-tuning can be exploited for improving such materials properties as handling characteristics, solubility, etc.
An additional important property of polymers, also highly sensitive to their temperatures, is constituted by their mechanical response properties, as indicated primarily by the onset of either elastic or plastic behaviour when the material is subjected to external stress loading. Glass is a good example of a completely elastic material for instance, as long as the temperature is kept below its Tg. It will then remain elastic until it reaches its fracture point, otherwise known as the glass elastic limit. The Tg of glass occurs between 510 and 560 degrees C, and therefore at room temperature glass will always behave as a brittle solid, in accordance with its well-known behaviour of shattering when subject to major stress.
Another polymer known as polyvinyl chloride (PVC) has a Tg of 83 degrees C, making it suitable for cold water pipes for instance, but dysfunctional for hot water, since at such high temperatures it would then get severely weakened. According to its Tg, PVC thus also behaves in a brittle fashion at room temperature. Moreover, adding a small amount of plasticizer to PVC can decrease its Tg to minus 40 degrees C. This insertion can consequently help transform the PVC into a soft and flexible material at room temperature, thus making it ideal for applications such as garden watering hoses. A plasticized PVC hose can, however, still become brittle and hence easily breakable in winter, when temperatures approach its transformed value of Tg.
In summary, considering the above-mentioned examples, we deduce that the value of the polymer’s Tg in comparison to ambient temperature is what determines the choice of that particular material in the context of a particular application, with regards to the set of its mechanical response properties.
Materials properties impacting the glass transition temperature
The state of a polymer (solid, rubbery or molten) depends on the presence and absence of segmental and molecular movements, and in turn these movements depend on size and geometry, and can be impacted by the following factors:
1. Chain flexibility: chain flexibility is one of the important factors which affects Tg. Inherent chain flexibility is determined by the nature of the polymer backbone, and by the size and shape of the groups directly attached to it. For example, aliphatic C-C and C-O bonds show quite a high flexibility, while the introduction of ringed structures causes stiffening of the chain. This stiffening of the chain in turn causes an increase in the glass transition temperature. Bulky groups attached to the polymer backbone can also reduce the overall chain’s flexibility.
2. Intermolecular interactions: Segmental rotations are also affected by intermolecular interactions, such as dipole-dipole interactions, induction forces, van der Waals forces, hydrogen bonding etc. These types of interactions increase the rigidity of the polymeric structure, and hence increase its glass transition temperature.
3. Molecular weight: Polymer having low molecular weight have a higher number of chain ends compared to polymers having high molecular weights. Chain ends have less strain and become more active than the chain backbone, and this causes greater molecular mobility. Therefore, an increase in molecular weight decreases the glass transition temperature of the polymer. On the other hand, it is reasonable to assume that with an increase of chain-end concentration, the glass transition temperature increases correspondingly.
4. Co-polymerization: Random co-polymerization results in additional structural disorder, and thus decreases the resulting molecular packing. Therefore, the glass transition temperature is often lowered under such circumstances. The glass transition temperature of random copolymers typically depends on the composition of the various constituent monomeric units, as well as on their individual values of Tg.
5. Cross-linking: Cross-linking introduces restrictions and stiffness within the polymer’s main structure. Therefore, this has the effect of increasing its corresponding glass transition temperature.
6. Plasticizer: Plasticizers are generally low-molecular weight substances, which are then added within the polymer. This process causes a separation of the various polymer chains, resulting in a reduction of the cohesive forces and in an generalized increase in molecular mobility. Plasticizers can therefore help to reduce the brittleness of polymers, hence resulting in lower values for the corresponding glass transition temperature.
Summarized below are Tg values characteristic of certain classes of materials:
- Wikipedia (https://en.wikipedia.org/wiki/Glass_transition)
- Omnexus website (https://omnexus.specialchem.com/polymer-properties/properties/glass-transition-temperature)
- CHEM3020: POLYMER CHEMISTRY lecture slides (Unit-3: Crystallinity and Glass Transition Temperature) by Prof. Rafique Ul Islam, Department of Chemistry, MGCU, Motihari (http://www.mgcub.ac.in/pdf/material/20200503035034f039fc3842.pdf)
- “Glass Transition Temperature of Polymer”; course by Atma Ram Sanatan Dharma College, University of Delhi (https://www.arsdcollege.ac.in/wp-content/uploads/2020/04/Glass-transition-Temperature-and-factors-affecting-1.pdf)
- “Glass transition temperature: Basics and application in pharmaceutical sector” by N.R. Jadhav et al.: Asian Journal of Pharmaceutics - April-June 2009 (https://www.asiapharmaceutics.info/index.php/ajp/article/download/246/111)
- “Thermal Properties of Polymers”; course by Haldia Institute of Technology (https://hithaldia.in/faculty/sas_faculty/Dr_P_K_Khatua/Lecture%20note%204%20Polymer.pdf)
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