Chain Growth by Propagation

R—C - C - C - C • + • C - C - C - C — R -fc- R — C - C - C - C - C - C - C - C — R

Termination by Reaction of Active End Groups

- C - C - C - C • + • R -fc- - C - C - C - C - R

Termination by Reaction with Initiator

Fig. B-16. Chain Polymerization of Polyethylene

Another characteristic of thermoplastics is that some contain totally random chain structures, known as amorphous, while others contain regions of closely folded chains and are known as semi-crystalline. A comparison of these two structures is depicted in Fig. B.17. An amorphous thermoplastic contains a massive random array of entangled molecular chains. The chains themselves are held together by strong covalent bonds, while the bonds between the chains are much weaker secondary bonds. When the material is heated to its processing temperature, it is these weak secondary bonds that breakdown and allow the chains to move and slide past one another. Amorphous thermoplastics exhibit good elongation, toughness, and impact resistance. As the chains get longer, the molecular weight increases, resulting in higher viscosities, higher melting points, and greater chain entanglement, all leading to higher mechanical properties.

Semi-crystalline thermoplastics contain areas of tightly folded chains (crystallites) that are connected together with amorphous regions. Amorphous

Amorphous Region •

Amorphous Structure

Semi-crystalline Structure

Amorphous Region •

Amorphous Structure

Semi-crystalline Structure

Fig. B-17. Amorphous and Semi-crystalline Structures thermoplastics exhibit a gradual softening on heating, while semi-crystalline thermoplastics exhibit a sharp melting point when the crystalline regions start dissolving. As the polymer approaches its melting point, the crystalline lattice breaks down and the molecules are free to rotate and translate, while non-crystalline amorphous thermoplastics exhibit a more gradual transition from a solid to a liquid. Crystallinity increases strength, stiffness, creep resistance, and temperature resistance but usually decreases toughness. By decreasing and restricting chain mobility, the tightly packed crystalline structure behaves somewhat like crosslinking in thermosets. The maximum crystallinity obtainable is about 98%, whereas metallic structures are usually 100% crystalline and exhibit much more ordered structures. In general, semi-crystalline thermoplastics used for composite matrices contain about 20-35% crystallinity.

Thermoset polymers are always amorphous. They start as low molecular weight liquids that cure by either addition or condensation reactions. A comparison of these two cure mechanisms is shown in Fig. B.18. In the addition reaction shown for an epoxy reacting with an amine curing agent, the epoxy ring opens and reacts with the amine to form a crosslink. The amine shown in this example is what is known as an aliphatic amine and would produce a crosslinked structure with only moderate temperature capability. Higher temperature capabilities can be produced by curing with what are known as aromatic amines. Aromatic amines contain the large and bulky benzene ring, which helps to restrict chain movement when the network is heated. A typical curing agent, diamino diphenyl sulfone (DDS), and a typical epoxy, tetraglycidyl methylene dianiline (TGMDA), are shown in Fig. B.19. Note that the curing agent and the epoxy both contain benzene rings. Also note that the TGMDA has four

Ring Opens

Epoxide Ring ' Ethylene


Was this article helpful?

0 0

Post a comment