Polymer Transition
Polymer Transition is an intermediate state at the time of heating and cooling of polymeric materials. Solid thermosetting polymeric substance changes to rubbery and rubbery thermoplastic polymer turns into liquid after applying specific temperatures.
Polymer Transition Regions
Polymer transition regions occur within very short temperature intervals over which the properties of a polymer change a lot. Glass transition and the crystal melting transition is the example of polymer transition.
Glass transition temperature (Tg)
The glass transition temperature is an important characteristic of polymeric substances which is represented by Tg. The three units of glass transition temperature are Degrees Fahrenheit (°F), Kelvin (K), and Degrees Celsius (°C). The glass transition temperature of most synthetic polymers lies between 170 K to 500 K. Glass transition temperature is the temperature of an amorphous polymer at which it changes from a rigid “glassy/hard” state to a flexible “soft/rubbery” state which is a polymer transition.
This temperature relates to a polymeric substance’s strength and capabilities for various applications. This transition temperature has a strong connection with some polymer’s mechanical properties like tensile strength, modulus of elasticity, impact resistance, and operational temperature range.
Glass transition temperature can be shown by the following graph:
Below Tg: Due to lack of mobility, the polymers are hard and brittle like glass.
Above Tg: Due to some mobility, the polymers are soft and flexible like rubber.
Effect of Glass transition temperature on Polymer property:
An amorphous portion of a semi-crystalline solid shows the property of glass transition temperature. The glass transition is a property of only the amorphous portion of a semi-crystalline solid. The crystalline portion of the polymer remains unchanged during the glass transition.
Amorphous polymers have a random or disordered molecular structure shape. They have glassy-state properties like brittleness, stiffness, and rigidity below the glass transition temperature. They do not have a sharp melting point. Their polymeric chains are attached in such a way that they have free spaces between them. As a result, free volumes are responsible to move at lower temperatures and create changes a lot during heating. When heat is applied, they gradually turn into softening to the point they become a leathery or rubbery shape. Examples of some amorphous polymers are polystyrene (PS) and polymethyl methacrylate (PMMA), Polyvinyl Chloride (PVC), etc.
Factors influencing Tg for Polymer transition :
Chain Stiffness: The polymer chain flexibility and strength depend on the groups present in the polymer backbone. Those groups which decrease the flexibility and strength of polymeric chains are called stiffening groups. Some stiffening groups are given below:
As an example, Polyethylene terephthalate (PET) is a stiffer molecule than Polyethylene adipate (PEA) because a benzene ring is not as flexible as a chain of CH2 groups. The Tg of Polyethylene terephthalate is 690C and the Tg of polyethylene adipate is -50 to-70oC.
Intermolecular Forces: Intermolecular forces have a close connection with the Tg because strong intermolecular forces lead to a higher glass transition temperature. Intermolecular forces of polymer combination of the dipole-dipole forces and hydrogen bonding.
Dipole-dipole forces:
Dipole-dipole forces arise from the strong attraction between polar groups. As an example, Polyvinyl chloride (PVC) has higher intermolecular forces than polypropylene (PP) because of the dipole-dipole forces from the C-Cl bond in PVC. Polyvinyl chloride (PVC) has Tg of 810C, whereas Polypropylene Tg is −20oC.
Hydrogen Bonding: Hydrogen bonding creates an effect on the polymer molecule during heating in which the polymer molecule contains -OH or -NH groups. It creates strong intermolecular forces among polymer molecules. As an example, Polyvinyl alcohol (PVA) has hydrogen bonding and its Tg is 85oC. Polypropylene (PP) has no hydrogen bonding and its Tg is −20oC.
C. Pendant Groups: There are some effects of glass transition temperature on polymeric molecules for having the presence of different types of pendant groups.
1. Bulky pendant groups: There are many bulky pendant groups such as a benzene ring connecting with its neighboring chains and restricting the freedom rotation and creating a higher Tg value. As an example, Polypropylene has a Tg value of -20oC whereas polystyrene has a Tg value of 100oC.
2. Flexible pendant groups: There are many flexible pendant groups such as the aliphatic chain which is responsible to pack the polymeric chain closely and increasing the rotational movement and creating lower Tg value. As an example, poly methyl methacrylate (PMMA) has a Tg value of 1050C whereas poly Polypropylene has a Tg value of -20oC.
D. Cross-linking: The cross-linked between the polymeric chains restricts rotational motion and increases Tg. As an example, Melamine resin Tg is 1500C whereas Polypropylene has a Tg value -20oC.
e. Plasticizers: Plasticizers are additives that can be added to plastics for increasing their flexibility and workability. They reduce the intermolecular forces between the polymer chains. As a result, the value of Tg is decreased in the presence of plasticizers.
Mechanical Properties of Polymers
Polymer mechanical properties relate to behavior under stress.
Some mechanical properties of the polymer are discussed below:
Stress: Stress is defined as the force per unit area of polymeric materials. The stress units are N/m2 or Pa.
Mathematically, stress can be noted as the following:
σ = F/A
Where, σ= stress, F= force, and A= Cross-sectional area of the sample.
Strain: Strain is defined as extension per unit length area. It is a kind of ratio of lengths, so it has no units. Mathematically, strain can be represented by the following formula:
ε = ΔL/L0; ΔL = L-L0
Where,
ε =strain,
L0 = Original length of a polymeric material,
L =Length after it has been changed, and
ΔL =Expansion of the polymeric material which is the difference between these two lengths.
Tensile strength: The tensile strength of a polymer can be defined as the maximum stress that a polymer can bear before fracture when it is allowed to be stretched or pulled. The units of tensile strength are Pascals or psi (pounds per square inch).
1 MPa = 145 psi
% Elongation to break: Elongation at break is the amount of deformation capability a material can withstand before crack formation. % Elongation uses at the time of polymeric materials testing and assessment for engineering and various manufacturing applications.
Young’s Modulus: Young’s modulus or tensile modulus is the important mechanical property of polymer which is the ratio of stress to strain. The slope of the stress-strain curve also expresses Young’s modulus. The stress-strain curves are not straight-line plots because of changing the modulus with strain. Metals and fibrous materials have a higher Young’s modulus than elastomers.
The mechanical properties of some materials are given below:
References:
1.Ebewele, R. O. (2000). Polymer science and technology. CRC press.
2.Chanda, M. (2006). Introduction to polymer science and chemistry: a problem-solving approach. CRC press.
3.Gowariker, V. R., Viswanathan, N. V., & Sreedhar, J. (1986). Polymer science. New Age International.
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