Explaining the Thermal Behavior of Polymers

Thermal behavior of polymers

When explaining the thermal behavior of polymers, at sufficiently low temperatures all polymers are hard and rigid. As temperature rises, each polymer eventually obtains sufficient energy to enable its chains to move. This movement makes the polymer behave like a viscous liquid.

Change of specific volume of a polymer with respect to the temperature

Similar masses of the three different morphologies (amorphous, semi-crystalline, and crystalline) of the same polymer are taken and the variation of its specific volume is observed with respect to the temperature.

Volume change in amorphous polymers 

The A-D curve represents the thermal behavior of the amorphous polymer. Initially, it has the highest volume of semi-crystalline and crystalline polymers. From point “A” to point “B” the polymer is in a glassy state. As the temperature increases, intermolecular forces like hydrogen bonds and dipole-dipole bonds are being broken down.

So, the polymer chains become more flexible. They start to slip each other. So, at point “B” the polymer starts to obtain a rubbery texture. The temperature at which the polymer became a rubbery texture is the Glass transition temperature (T_g) 

If the temperature continuously increases, the polymer texture becomes a rubbery state of viscous liquid. (B-C curve). At point “C” the polymer has become a completely viscous liquid. At this temperature, polymer molecules are free to move through the polymer matrix. However, amorphous polymers do not show sharp melting points. Their melting temperature is somewhere in between points “B” and “C”. If the temperature is increased beyond the melting temperature, the viscosity of the polymer is reduced. 

Volume change in perfectly crystalline polymers 

The perfectly crystalline polymer has the lowest volume compared to the amorphous and semi-crystalline polymers. The H-D curve represents the thermal behavior of a perfectly crystalline polymer. They do not show any glass transition. Crystalline polymers directly become melt states from a glassy state as the temperature increases. At this point, “I” polymer crystals are being melted.

This temperature is the crystalline temperature (T_c). When the temperature increases beyond the crystalline temperature, the polymer becomes a viscous liquid. This particular temperature is its melting temperature. Generally, perfectly crystalline polymers have the highest melting temperatures. Because due to the crystalline structure, they have strong intermolecular forces. So, it needs high energy to break down the intermolecular forces. 

Volume change in semi-crystalline polymers 

Semi-crystalline polymers have intermediate initial volume. They show both glass transition temperatures and sharp melting temperatures. E-A curve shows the thermal behavior of a semi-crystalline polymer. Glass transition temperature of semi-crystalline polymers is higher than the T_g of amorphous polymers. And the T_m is lower than the T_m of crystalline polymer. 

Factors affecting the Glass transition temperature

When the polymers at below their glass transition temperature the motions of the polymer chains are frozen. When sufficient thermal energy is supplied, chain segments move cooperatively and a transition from a glassy state to a rubbery state begins to take place. This transition is the “Glass transition” of the polymers. Glass transition is a reversible process. Several factors affect the glass transition temperature of polymers. 

i. Chain flexibility 

The most important factor that affects the glass transition temperature is the flexibility of the polymer chains. It is a measure of the ability of polymer chains to rotate. Polymers with flexible chains have low T_g and rigid chains have high T_g. Normally polymer chains with [CH₂-CH₂], [CH₂-O-CH₂], and [Si-O-Si] links are very flexible. So, they have low T_g values. Polymers with benzene rings are rigid, so they have high T_g values. 

ii. Molecular structure

Polymers with bulky pendant groups prevent the rotation of the backbone. Therefore with the increase of the bulkiness of the pendant group, the T_g will also be increased. This is the steric effect. 

The configurational effect such as cis-trans isomerism and tacticity also affect the T_g of polymers. Different stereo structures of the same polymer have different T_g values. For example, isotactic, syndiotactic, and atactic morphologies of polymethyl methacrylate have three different T_g values. 

As well, the cis and trans isomerism of polybutadiene has two different T_g values. (cis-polybutadiene -165K and trans-polybutadiene – 255K)

iii. Molecular mass 

The T_g is increased with the increase of the molar mass of the polymer. But at very high molar masses T_g is constant. 

iv. Branching and cross-linking 

When crosslinks or branches are present in the polymer, molecular motions are restricted. So, T_g is increased. 

Factors affecting the melting temperature

Melting temperature (T_m) is the temperature at which the crystalline phase of a polymer becomes a viscous liquid. Several factors affect the melting temperature of polymers. Those are,

• Symmetry
• Intermolecular forces
• Tacticity
• Branching
• Molecular mass 

a. Symmetry

The symmetry of the chain affects both T_m and the ability to form crystals. Polymers like polyethylene and Teflon are highly symmetrical. So, their T_m is also high. Chains containing irregular units reduce the ability of a polymer to be crystallizing. Therefore, Tm would be 

lower. Polymers with para substitutions have axis symmetry. So, they can crystallize easily. The incorporation of trans double bonds maintains the chain symmetry and can crystallize.

b. Intermolecular forces

Any interaction between chains in the crystals will help to hold the structure together. This will increase the melting temperature. Polymers containing polar groups such as -Cl, -CN, -OH can have strong dipole-dipole interactions. Polymers with hydrogen bonds such as polyamides have high melting points. 

c. Tacticity

If a polymer chain contains a large number of pendant groups, it will increase rigidity. But it also increases the difficulty of close packing to form crystals. If the pendant groups are arranged in a regular pattern along the chain, it will help to form crystals. Isotactic and syndiotactic polymers have a regular pattern, so, they will tend to be crystallized. Atactic polymers have no regular pattern so, they will be amorphous. 

I. Atactic

Repeating units and pendent groups have no regular stereochemical arrangement through the polymer backbone. 

II. Isotactic

All the repeating units and pendent groups have the same regular stereochemical configuration. 

III. Syndiotactic

Repeating units and pendent groups have alternative stereochemical configurations. 

d. Branching

If the chains are sufficiently branched, the polymer cannot peak into crystals and T_m will be lower. As an example, linear polyethylene has high density and high T_m than branched polyethylene. 

e. Molecular mass

The chain ends of the polymer molecules are always free to move. Let’s consider an equal mass of the same polymer with high molar mass and low molar mass. The polymer with high molar mass has a smaller number of chain ends. So, it has a high T_m. Thus, the polymer with low molar mass has a higher number of chain ends. So, it has a low T_m.