1. Polymer Structure Polyolefins with side chains have stereocenters on every other carbon With so many stereocenters, the stereochemistry can be complex. There are three main stereochemical classifications for polymers.

2. Molecular configurations Head-to-tail configuration Bonded to alternate carbons on the same side Where, R: Alkyl radical

3. Molecular configurations continue…. Head-to-head configuration Bonded to adjacent carbon atoms

4. Isotactic configuration Stereoisomerism Molecular configurations continue…. R groups are situated on the same side of the chain

5. Syndiotactic On alternate sides Molecular configurations continue….

6. Atactic At random position Source: William Callister 7 th edition, chapter 14, page 504 Conversion from to another is only by severing branches and through new reaction

7. Geometric Isomerism CIS-Isoprene eg., Natural rubber Attacked by acids/alkalis Molecular configurations continue…. TRANS-Isoprene eg., Gutta Percha Highly resistant to acid/alkalis

8. Molecular configurations continue…. Geometric Isomerism continue… TRANS- isoprene –Highly resistant to acids/alkalis

9. Why is this important? Tacticity affects the physical properties Atactic polymers will generally be amorphous, soft, flexible materials Isotactic and syndiotactic polymers will be more crystalline, harder and less flexible Polypropylene (PP) is a good example Atactic P is a low melting, viscous and sticky. Isoatactic P is high melting (176º), crystalline, tough material. Syndiotactic P has similar properties, but is very clear.

10. Polymer Morphology All properties of any polymer (plastic, fiber, or rubber) result from a combination of molecular weight and chemical structure. Molecular Weight Mechanical Property

11. Polymer Morphology The mechanical properties result from attractive forces between molecules dipole-dipole interactions, H-bonding, London forces, ion-dipole interactions. H-bondingdipole-dipole

12. Nylon 66 hydrogen-bonded structure for crystallites of an amide-type polymer of hexanedioic acid and 1,6-hexanediamine.

13. Polymer Crystallinity Crystallinity: Packing of chains to produce ordered atomic array. As crystallinity is increased in a polymer: 1. Density increases 2. Stiffness, strength, and toughness increases 3. Heat resistance increases

14. 1.An amorphous polymer is one with no crystallites. If the attractive forces between the chains are weak the motions of the chain are not in some way severely restricted as by cross-linking or large rotational barriers, such a polymer low tensile strength and when stressed to undergo plastic flow in which the chains slip by one another 2. Crystallline polymer : Consider a polymer such as nylon, which has strong intermolecular forces When the material is subjected to strong stress in one direction, usually above Tg, so that some plastic flow can occur, the material elongates and the crystallites are drawn together and oriented along the direction of the applied stress 3. Elastomers usually are amorphous polymers.

15. Factors for Crystallization Slower cooling promotes crystal formation and growth Mechanical deformation, as in the stretching of a heated thermoplastic, tends to align the structure and increase crystallization Plasticizers (chemicals added to a polymer to soften it) reduce the degree of crystallinity.

16. Crystallinity characteristics Degree of crystallinity depends on rate of cooling; need sufficient time to result in ordered configuration. Crystalline if chemically simple polymer. e.g., polyethylene, PTFE, even if rapidly cooled. Polymer Crystallinity

17. Amorphous if network polymer. Crystalline if linear polymer (no restrictions to prevent chain alignment) Amorphous: Atactic stereoisomer. Crystalline: Isotactic or Syndiotactic stereoisomer Amorphous: If bulky/large side-bonded group. Crystalline: Simple straight chain Polymer Crystallinity

18. Polymer Crystallinity continue… Amorphous: Most copolymers (and more irregular/ random mers) Crystalline: Alternating or block polymers Amorphous: Random or graft polymers Crystalline: Strong, more resistant to dissolution by softening by heat

19. Polymer crystals Fringed micelle model Aligned small crystalline regions (crystallites or micelles) Amorphous regions in-between platelets of crystals (10- 20 nm thick) (  10  m long)

20. ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning ™ is a trademark used herein under license. The effect of temperature on the structure and behavior of thermoplastics.

21. Crystal Structures Fe 3 C – iron carbide – orthorhombic crystal structure