One question per week: Basics of polymers – What are the factors that affect the glass transition temperature?
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The glass transition temperature is an important indicator of the performance of polymer materials. So, what are the factors that affect the glass transition temperature?
The temperature at which a polymer transitions from the elastomeric state to the glassy state is the temperature at which amorphous polymers (including the amorphous portions in crystalline polymers) change from the glassy state to the elastomeric state, or vice versa. It represents the lowest temperature at which the macromolecular chains of amorphous polymers can move freely, and is usually denoted by Tg. There is no fixed value; it often changes depending on the measurement method and conditions. An important process parameter for polymers. Above this temperature, the polymer exhibits elasticity ； Below this temperature, polymers exhibit brittleness, which must be taken into account when using them as plastics, rubbers, synthetic fibers, etc. For example, the glass transition temperature of polyvinyl chloride is 80°C. However, he is not the upper limit for the working temperature of the product. For example, the operating temperature of rubber must be above its glass transition temperature; otherwise, it loses its high elasticity. Amorphous materials can be distinguished into three states based on their mechanical properties: the glassy state, the hyperelastic state, and the viscous flow state. Polymer materials in the elastomeric state undergo a transition from the elastomeric state to the glassy state as the temperature decreases; this transition is known as the glass transition. Its transition temperature is called the glass transition temperature Tg. If the temperature of a high-elasticity material increases, the polymers will undergo a transition from the high-elasticity state to the viscous flow state, and the temperature at which this transition occurs is known as the viscous flow temperature Tf. When a glassy polymer undergoes a phase transition at the Tg temperature, its modulus drops by three orders of magnitude, causing the material to change suddenly from a hard solid to a soft elastomer, thereby completely altering its functional properties. Many other physical properties of polymers, such as volume (specific volume), thermodynamic properties (specific heat capacity, enthalpy), and electromagnetic properties (dielectric constant and dielectric loss, width of nuclear magnetic resonance absorption lines, etc.), also undergo significant changes. Polymers used as plastics lose their plastic properties and turn into rubber when the temperature rises above their glass transition temperature. What we usually refer to as plastics and rubbers is determined by whether their Tg is above or below room temperature. Materials with a Tg below room temperature are rubbers, while those with a Tg above room temperature are plastics. Therefore, from a processing perspective, Tg is the upper limit temperature for the use of amorphous thermoplastics and the lower limit temperature for the use of rubbers. Tg is one of the characteristic temperatures of polymers and can be used as an indicator to characterize them. There are many factors that affect the glass transition temperature. Since the glass transition temperature is the transition temperature at which the chain segments of a polymer go from being frozen to becoming mobile, and chain segment movement is achieved through intramolecular rotation of the single bonds in the main chain, any factor that affects the flexibility of the polymer chains will have an impact on Tg. Factors such as the introduction of rigid or polar groups, cross-linking, and crystallization, which reduce the flexibility of polymer chains or increase intermolecular forces, all lead to an increase in Tg ； Factors that increase the flexibility of polymer chains, such as the addition of plasticizers or solvents and the introduction of flexible groups, all result in a decrease in Tg.
Since the glass transition is a phenomenon related to molecular motion, and molecular motion is closely connected to molecular structure, factors such as the flexibility of molecular chains, intermolecular forces, as well as copolymerization, blending, and plasticization are all important internal factors that affect the Tg of polymers. In addition, external conditions such as applied force, the rate of force application, and the rate of temperature increase are also significant influencing factors.
1. Chemical Structure
(1) Chain Flexibility
The flexibility of molecular chains is the most important factor determining a polymer’s Tg. The greater the flexibility of the main chain, the lower the glass transition temperature. Polymers with saturated single bonds in their main chains have low Tg, as the molecular chains can rotate around these bonds internally; this is especially true when there are no polar side groups present, resulting in an even lower Tg. Among different types of single bonds, those with lower barriers to internal rotation have lower Tg. For example, polymers with isolated double bonds in their main chains have relatively low Tg, although the double bonds themselves cannot rotate internally, the α-single bonds adjacent to them can rotate more easily, thus leading to a lower Tg. For instance, styrene-based rubbers have low glass transition temperatures.
(2) Side Groups
The polarity of side groups has a significant impact on both the internal rotation of molecular chains and intermolecular interactions. The stronger the polarity of the side groups, the higher the Tg. The relationship between the Tg of certain olefinic polymers and the polarity of their side groups is shown in Table 2.
Table 2: Relationship between the polarity of side groups and Tg in olefinic polymers
Additionally, increasing the number of polar groups on the molecular chain can also raise the Tg of the polymer. However, once the number of polar groups exceeds a certain value, the electrostatic repulsion between them becomes stronger than the attractive forces, resulting in increased distances between molecular chains and a decrease in Tg. An increase in the steric hindrance caused by side groups also hinders internal rotation of molecular chains, thereby increasing the Tg. It should be noted that the presence of side groups does not always lead to an increase in Tg. The symmetry of side groups on the main chain also has a significant impact on Tg. In polyvinylidene chloride, the symmetric substitution of polar side groups results in partial cancellation of dipoles, reducing the overall molecular polarity and lowering the barrier to internal rotation, thereby increasing flexibility and resulting in a lower Tg compared to polyvinyl chloride. In polyisobutylene, each chain segment has two symmetrical methyl side groups, which increases the distance between main chains, weakens interchain interactions, lowers the barrier to internal rotation, and increases flexibility, resulting in a lower Tg compared to polypropylene. Furthermore, when polymers contain flexible side groups, as these side groups grow larger, they increase the distance between molecules and weaken intermolecular interactions, leading to what is known as “internal plasticization,” and thus the Tg decreases.
(3) Geometric Isomerism
The glass transition temperatures of monosubstituted olefinic polymers such as polyacrylates and polystyrene are hardly affected by their stereochemistry, whereas the Tg of disubstituted olefinic polymers depends on their stereochemical structure. Generally, isotactic polymers have lower Tg, while syndiotactic polymers have higher Tg. In cis-trans isomers, trans configurations usually result in harder molecular chains and higher Tg.
(4) Introduction of Ionic Bonds
The presence of ionic bonds between molecular chains can significantly increase the Tg. For example, adding metal ions to polyacrylic acid raises its Tg, with the effect depending on the valence of the ions. Using Na+ can raise the Tg from 106°C to 280°C, while replacing Na+ with Cu2+ can raise the Tg to 500°C.
2. Influence of Other Structural Factors
(1) Copolymerization
The Tg of random copolymers lies between the Tg of the monomers that make up the copolymer, and it changes linearly or non-linearly as the composition of the copolymer changes. In non-random copolymers, the simplest case is alternating copolymers, which can be considered as homopolymers composed of two monomers in a repeating unit, so they have only one Tg. Block or graft copolymers are more complex in terms of their Tg.
(2) Crosslinking
As the number of crosslinks increases, the free volume of the polymer decreases, and the movement of molecular chains becomes more restricted. The average length of chains between adjacent crosslinks decreases, resulting in an increase in Tg.
(3) Molecular Weight
An increase in molecular weight leads to an increase in Tg, especially at low molecular weights. This effect becomes less pronounced once the molecular weight exceeds a certain level.
(4) Plasticizers and Diluents
Plasticizers also have a significant impact on Tg. Polymers with high glass transition temperatures can have their Tg significantly reduced upon the addition of plasticizers. For example, pure polyvinyl chloride has a Tg of 78°C and behaves like a hard plastic at room temperature. However, when 45% plasticizer is added, the Tg drops to -30°C, allowing it to be used as a substitute for rubber. The glass transition temperature of starch also changes significantly after being mixed with water.
1. Chemical Structure
(1) Chain Flexibility
The flexibility of molecular chains is the most important factor determining a polymer’s Tg. The greater the flexibility of the main chain, the lower the glass transition temperature. Polymers with saturated single bonds in their main chains have low Tg, as the molecular chains can rotate around these bonds internally; this is especially true when there are no polar side groups present, resulting in an even lower Tg. Among different types of single bonds, those with lower barriers to internal rotation have lower Tg. For example, polymers with isolated double bonds in their main chains have relatively low Tg, although the double bonds themselves cannot rotate internally, the α-single bonds adjacent to them can rotate more easily, thus leading to a lower Tg. For instance, styrene-based rubbers have low glass transition temperatures.
(2) Side Groups
The polarity of side groups has a significant impact on both the internal rotation of molecular chains and intermolecular interactions. The stronger the polarity of the side groups, the higher the Tg. The relationship between the Tg of certain olefinic polymers and the polarity of their side groups is shown in Table 2.
Table 2: Relationship between the polarity of side groups and Tg in olefinic polymers
Additionally, increasing the number of polar groups on the molecular chain can also raise the Tg of the polymer. However, once the number of polar groups exceeds a certain value, the electrostatic repulsion between them becomes stronger than the attractive forces, resulting in increased distances between molecular chains and a decrease in Tg. An increase in the steric hindrance caused by side groups also hinders internal rotation of molecular chains, thereby increasing the Tg. It should be noted that the presence of side groups does not always lead to an increase in Tg. The symmetry of side groups on the main chain also has a significant impact on Tg. In polyvinylidene chloride, the symmetric substitution of polar side groups results in partial cancellation of dipoles, reducing the overall molecular polarity and lowering the barrier to internal rotation, thereby increasing flexibility and resulting in a lower Tg compared to polyvinyl chloride. In polyisobutylene, each chain segment has two symmetrical methyl side groups, which increases the distance between main chains, weakens interchain interactions, lowers the barrier to internal rotation, and increases flexibility, resulting in a lower Tg compared to polypropylene. Furthermore, when polymers contain flexible side groups, as these side groups grow larger, they increase the distance between molecules and weaken intermolecular interactions, leading to what is known as “internal plasticization,” and thus the Tg decreases.
(3) Geometric Isomerism
The glass transition temperatures of monosubstituted olefinic polymers such as polyacrylates and polystyrene are hardly affected by their stereochemistry, whereas the Tg of disubstituted olefinic polymers depends on their stereochemical structure. Generally, isotactic polymers have lower Tg, while syndiotactic polymers have higher Tg. In cis-trans isomers, trans configurations usually result in harder molecular chains and higher Tg.
(4) Introduction of Ionic Bonds
The presence of ionic bonds between molecular chains can significantly increase the Tg. For example, adding metal ions to polyacrylic acid raises its Tg, with the effect depending on the valence of the ions. Using Na+ can raise the Tg from 106°C to 280°C, while replacing Na+ with Cu2+ can raise the Tg to 500°C.
2. Influence of Other Structural Factors
(1) Copolymerization
The Tg of random copolymers lies between the Tg of the monomers that make up the copolymer, and it changes linearly or non-linearly as the composition of the copolymer changes. In non-random copolymers, the simplest case is alternating copolymers, which can be considered as homopolymers composed of two monomers in a repeating unit, so they have only one Tg. Block or graft copolymers are more complex in terms of their Tg.
(2) Crosslinking
As the number of crosslinks increases, the free volume of the polymer decreases, and the movement of molecular chains becomes more restricted. The average length of chains between adjacent crosslinks decreases, resulting in an increase in Tg.
(3) Molecular Weight
An increase in molecular weight leads to an increase in Tg, especially at low molecular weights. This effect becomes less pronounced once the molecular weight exceeds a certain level.
(4) Plasticizers and Diluents
Plasticizers also have a significant impact on Tg. Polymers with high glass transition temperatures can have their Tg significantly reduced upon the addition of plasticizers. For example, pure polyvinyl chloride has a Tg of 78°C and behaves like a hard plastic at room temperature. However, when 45% plasticizer is added, the Tg drops to -30°C, allowing it to be used as a substitute for rubber. The glass transition temperature of starch also changes significantly after being mixed with water.
The flexibility of molecular weight, side chain structure, chain length, and so on
The third floor explained in great detail that there are many factors that affect the glass transition temperature. Any factor that affects the flexibility of polymer chains will have an impact on Tg. Factors such as the introduction of rigid or polar groups, cross-linking, and crystallization, which reduce the flexibility of polymer chains or increase intermolecular forces, all lead to an increase in Tg ； Factors that increase the flexibility of polymer chains, such as the addition of plasticizers or solvents and the introduction of flexible groups, all result in a decrease in Tg.
Generally speaking, the factors that affect the glass transition temperature of polymers are as follows: 1. The influence of the main chain structure; 2. The influence of substituents; 3. The influence of cross-linking, branching, and molecular weight; 4. The influence of copolymerization