Properties Of Crystalline Polymers

Views: 0 Author: Site Editor Publish Time: 2020-08-25 Origin: Site

The general properties of crystalline polymers are: distinct and sharp melting point, higher specific gravity, higher tensile strength and tensile modulus, lower ductility and impact strength, creep resistance, tend to be translucent or opaque, higher fatigure resistance, difficult to bond using adhesives and solvents, higher chemical resistance and resistance to stress cracking, and good for bearing and wear, as well as for structural application. Tab. 1. 4 lists the properties of amorphous and crystalline polymers.

The properties of amorphous and crystalline polymers

Polymer

Properties

Amorphous polymers

1. Soften over a wide range of temperatures

2. Lower specific gravity

3. Lower tensile strength and tensile modulus

4. Higher ductility and impact strength

5. Lower creep resistance

6. Tend to be transparent

7. Higher dimensional stability

8. Lower fatigue resistance

9. Bond well using adhesive and solvents

10. Lower chemical resistance and resistance to stress cracking

crystalline polymer

1. Distinct and sharp melting point

2. Higher specific gravity

3. Higher tensile strength and tensile modulus

4. Lower ductility and impact strength

5. Higher creep resistance

6. Tend to be translucent or opaque

7. Higher fatigue resistance

8. Difficult to bond using adhesives and solvents

9. Higher chemical resistance and resistance to stress cracking

10. Good for bearing and wear, as well as for structural application

The properties of semicrystalline polymers are determined not only by the degree of crystallinity, but also by the size and orientation of the molecular chains. Plastics crystallinity can be controlled by modifying processing techniques. It is possible to anneal polymers after processing to ensure that proper crystallization has taken place. Annealing involves holding the polymer above Ts but below Tm for a specific time to both encourage and control the growth of the crystalline structure. Equally, It is possible to quench or rapidly cool plastic workpieces to reduce the formation of crystals

High nucleation and growth rates can also be achieved if heterogeneous nucleation is used. In this case, nucleation is initiated by seeding with a foreign particle, which is typically a polymer similar to the base polymer but with a higher melting point. Some commercial products incorporate special nucleating agents to produce a high degree of crystallization.

It is also possible to induce directional crystallization by stretching polymers below Tm to stretching. create crystalline filaments, fibers or sheets with a crystalline structure oriented in the direction of stretching. This cold-drawing technique is used extensively in fiber and film production to produce

high strength fibers and films and in the production of PET bottles with injection blow molding.