Types of Bonds



Types of Materials

Isotropic, filled outer shells • Metallic – Electropositive: give up electrons • Ionic – Electronegative/Electropositive • Colavent – Electronegative: want electrons – Shared electrons along bond direction + + + + e e + + + + e + + + + + + Close-packed structures

“The Graduate” 1967

Mr. McGuire

: I want to say one word to you. Just one word.

Benjamin

: Yes, sir.

Mr. McGuire

: Are you listening? units

Benjamin

: Yes, I am.

Mr. McGuire

: Plastics . many

Benjamin

: Just how do you mean that, sir?

methane

H

Long chain molecules with repeated units Molecules formed by covalent bonds Secondary bonds link molecules into solids H C H C H C H C H C H C H C H C H H H H H H H H

H C

http://en.wikipedia.org/wiki/File:Polyethylene-repeat-2D.png

Polymer Synthesis

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

Polydispersity

• Traditional synthesis  large variation in chain length

M Average chain molecular weight

number average

M n

  molecular weight

i

of polydispersity

M w

# of polymer chains of

M i

total number of chains weight average

M w

 

w M i

weight of polymer chains of

M i

total weight of all chains = weight fraction molecular weight • Degree of polymerization – Average # of mer units/chain

n n



M n m n w



M w m

by number mer molecular weight by 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  • Trifunctional mer – Can make three bonds  linear polymer branched polymer

• Linear

C C

• Branched

C C

Polymers

C C C C C H H C = C H H

• Cross-linked

Polymers

H out H in C C C C 109.5

° C C C C C H H C = C H H R

Placement of side groups is fixed once polymer is formed Example side group: styrene

R =

Cl H C = C H H Isotactic C R C C R C C R C C R C C C R C C C R R C C C C R C Syndiotactic Atactic C R C C C R R C C C R C C

• 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