Metallic –Electropositive: give up electrons Ionic –Electronegative/Electropositive Colavent –Electronegative: want electrons –Shared electrons along bond direction Types of Bonds  Types of Materials Isotropic, filled outer shells e- Close-packed structures

H “The Graduate” 1967 Mr. McGuireMr. McGuire: I want to say one word to you. Just one word. Benjamin: Yes, sir. Mr. McGuire: Are you listening? Benjamin: Yes, I am. Mr. McGuire: Plastics. Benjamin: Just how do you mean that, sir? Benjamin Mr. McGuire Benjamin Mr. McGuire Benjamin Long chain molecules with repeated units Molecules formed by covalent bonds Secondary bonds link molecules into solids C C H H H methane C H many units

Polymer Synthesis Traditional synthesis –Initiation, using a catalyst that creates a free radical –Propagation –Termination R  + C=C R…… C – C  + C=C R…… C – C  +  C – C……R unpaired electron C=C H H H H  R – C – C   R……C – C – C – C   R –(C-C) n – R

Polydispersity Traditional synthesis  large variation in chain length number average # of polymer chains molecular weight # of polymer chains of M i total number of chains molecular weight weight average weight of polymer chains of M i total weight of all chains width is a measure of polydispersity = weight fraction Degree of polymerization –Average # of mer units/chain Average chain molecular weight by number by weight mer molecular weight

New modes of synthesis “Living polymerization” –Initiation occurs instantaneously –Chemically eliminate possibility of random termination –Polymer chains grow until monomer is consumed –Each grows for a fixed (identical) period

Polymers Homopolymer –Only one type of ‘mer’ Copolymer –Two or more types of ‘mers’ Block copolymer –Long regions of each type of ‘mer’ Bifunctional mer –Can make two bonds, e.g. ethylene  linear polymer Trifunctional mer –Can make three bonds  branched polymer

Polymers Linear Branched Cross-linked C CCC C C CC C C = C H H H H

Polymers C CCC C C CC C C = C H H H H 109.5° H out H in Placement of side groups is fixed once polymer is formed Example side group: styrene R = R

C CCC C C CC C R RR R C = C H H Cl H Isotactic C CCC C C CC C R RRR Syndiotactic CCCC C C CC C R RR R Atactic

Thermal Properties –Thermoplastics Melt (on heating) and resolidify (on cooling) Linear polymers –Thermosets Soften, decompose irreversibly on heating Crosslinked Crystallinity Linear: more crystalline than branched or crosslinked Crystalline has higher density than amorphous

How do we know about structure? This part of lecture done by hand on the white board Introduction to x-ray powder diffraction Up to Bragg’s law, schematic XRD pattern