CHAPTER 14 Polymer Structures

ME 260: Introduction to Engineering Materials

Chapter 15. Polymer Structures 14.1

CHAPTER 14CHAPTER 14

Polymer Structures

ME 260: Introduction to Engineering Materials

Chapter 15. Polymer Structures 14.2

INTRODUCTION

Natural Polymers Leather, wood, rubber, cellulose, cotton, wool.Synthetic polymers Synthesized from small organic molecules.

CHEMISTRY OF POLYMER MOLECULES

Hydrocarbons (many organic materials)= composed of hydrogen and carbon

Covalent Bonding

H-C-H

H

H

Methane Ethylene

Similarly

ME 260: Introduction to Engineering Materials

Chapter 15. Polymer Structures 14.3

POLYMERISATIONMers = small structural entities making up the ‘poly’mer.

Ethylene(C2H4) gas

CAN BE CONVERTED TO POLYETHYLENE(PE) Solid

Catalyst

bonding satisfied for C atom

bonding unsatisfied for this C atom

ME 260: Introduction to Engineering Materials

Chapter 15. Polymer Structures 14.4

Angle =109o

RESULT

ME 260: Introduction to Engineering Materials

Chapter 15. Polymer Structures 14.5

Similarly

Methyl group

Unpaired electron

ME 260: Introduction to Engineering Materials

Chapter 15. Polymer Structures 14.6

ME 260: Introduction to Engineering Materials

Chapter 15. Polymer Structures 14.7

MOLECULAR WEIGHT Number average

Weight-average

Not all chains same length

ME 260: Introduction to Engineering Materials

Chapter 15. Polymer Structures 14.8

Molecular weight of mer j

Degree of polymerizationFor copolymers

ME 260: Introduction to Engineering Materials

Chapter 15. Polymer Structures 14.9

Linear Branched

Crosslinked Networke.g. Epoxy e.g. Rubber

e.g. Polyethylene, Nylon

Lower density

ME 260: Introduction to Engineering Materials

Chapter 15. Polymer Structures 14.10

COPOLYMERSPolymers composed of two or more different ‘mer’ units

HOMOPOLYMERS

Bifunctional, trifunctional mers

ME 260: Introduction to Engineering Materials

Chapter 15. Polymer Structures 14.11

Styrene-butadiene rubber (SBR) random copolymerUsed in automobile tires.

ME 260: Introduction to Engineering Materials

Chapter 15. Polymer Structures 14.12

POLYMER CRYSTALLINITY

= Packing of molecular chains so as to produce an ordered atomic array.

Small molecules (Methane, H2O) either either totally crystalline (as solids) or amorphous (as liquids)

However POLYMERS only partially crystalline (semi-crystalline)Max. crystallinity ~ 95%.

ME 260: Introduction to Engineering Materials

Chapter 15. Polymer Structures 14.13

Crystalline polymer = higher density

Properties greatly affected by degree of crystallinity (crystalline = stronger and more resistant to softening by heat. important to quantify degree crystallinity

ME 260: Introduction to Engineering Materials

Chapter 15. Polymer Structures 14.14

Slower cooling rates from the melt favor higher crytallinity (giving time for ordered configurations)

Linear Polymers Crystallization easily accomplished, since no restriction to chain alignment

Branched polymers NEVER highly crystalline Side branches interfere with crystallization Network Polymers AMORPHOUS

COPOLYMERS

More Irregular and random mer less crystallinity

Alternating and block copolymers likelihood of crystallization.

Random and Graft copolymers Normally Amorphous