Table of Contents
Why does polyethylene crystallize?
Polymers can crystallize upon cooling from melting, mechanical stretching or solvent evaporation. Crystallization affects optical, mechanical, thermal and chemical properties of the polymer.
What is the crystal structure of polyethylene?
The crystal structure of polyethylene was first determined by Bunn (2). The orthorhombic unit cell, identical with the crystal structure of straight-chain alkanes, is the most stable crystal structure. The unit cell with the space group notation Pnam-D2h contains 4 CH2 groups and the all-trans chains are parallel (Fig.
Why does the elongation of the polymer increase crystallinity?
Crystallinity: The crystallinity of the polymer increases strength, because in the crystalline phase, the intermolecular bonding is more significant.
How will the crystallinity of a polymer be affected by the addition of a plasticizer?
When a plasticizer (such as DOP) is added, its molecules will be placed in various sites between the chains. So PVC will become more flexible & the Tg will go down as if the polymer has adopted the NR character, nearly.
How does crystallinity affect density?
Crystallinity defines the degree of long-range order in a material, and strongly affects its properties. The more crystalline a polymer, the more regularly aligned its chains. Increasing the degree of crystallinity increases hardness and density.
How does branching affect crystallinity?
Short branches interfere with the formation of crystals, that is, they reduce the amount of crystallinity whereas long branches undergo side chain crystallization because they are able to form lamellar crystals of their own.
What is the geometric structure of the unit cell of polyethylene?
Polyethylene Unit Cell Each polyethylene chain is in an extended planar zigzag conformation. The c axis is always along the chain direction.
How physical properties and chemical structure affect solubility of polymers?
Long chains can be densely entangled. This makes it difficult for solvent molecules to penetrate. So longer chains dissolve more slowly than short ones. As crosslinking increases, solubility decreases.