Introduction to Engineering Materials ENGR2000 Chapter 14: Polymer Structures Dr. Coates
14.1 Introduction Naturally occurring polymers Wood, rubber, cotton, wool, leather, silk Synthetic polymers Plastics, rubbers, fibers
14.2 ydrocarbon Molecules Many organic materials are hydrocarbons Composed of hydrogen & carbon Covalent bonding
Saturated & Unsaturated hydrocarbons Saturated Molecules with all single bonds No new atoms may be joined without the removal of others that are already bonded Unsaturated Molecules with double or triple covalent bonds
Paraffin Family 5
14.3 Polymer Molecules Polymers are often called macromolecules Covalent bonds Molecules in the form of long & flexible chains
Mers, polymers & monomers Mer units - structural entities which are repeated along the chain Polymer many mers Monomer a stable molecule from which a polymer is synthesized
Polymerization and Polymer Chemistry Free radical polymerization R + free radical R C C C C monomer (ethylene) R C C + C C R C C Initiator: example - benzoyl peroxide dimer C C initiation propagation Free radical C O O C 2 C O = 2 R 8
Chemistry and Structure of Polyethylene Adapted from Fig. 14.1, Callister & Rethwisch 9e. Note: polyethylene is a long-chain hydrocarbon - paraffin wax for candles is short polyethylene 9
Bulk or Commodity Polymers 10
Bulk or Commodity Polymers (cont) 11
Isomerism Isomerism two compounds with same chemical formula can have quite different structures for example: C 8 18 normal-octane C C C C C C C C = 3 C C 2 C 2 C 2 C 2 C 2 C 2 C 3 3 C ( C 2 ) C 6 3 2,4-dimethylhexane 3 C C 3 C C 2 C C 3 C 2 C 3
Molecular weight, M: Mass of a mole of chains. MOLECULAR WEIGT Low M high M Not all chains in a polymer are of the same length i.e., there is a distribution of molecular weights 13
MOLECULAR WEIGT DISTRIBUTION Fig. 14.4, Callister & Rethwisch 9e. M i x i w i = mean molecular weight of size range i = number fraction of chains in size range i = weight fraction of chains in size range i 14
Molecular Weight Calculation Example: average mass of a class Student Weight mass (lb) 1 104 2 116 3 140 4 143 5 180 6 182 7 191 8 220 9 225 10 380 What is the average weight of the students in this class: a) Based on the number fraction of students in each mass range? b) Based on the weight fraction of students in each mass range? 15
Solution: The first step is to sort the students into weight ranges. Using 40 lb ranges gives the following table: Molecular Weight Calculation (cont.) weight number of mean number weight Calculate the number and weight range students weight fraction fraction of fraction students in each weight N i W i xrange i as follows: w i mass (lb) mass (lb) 81-120 2 110 0.2 0.117 121-160 2 142 0.2 0.150 161-200 3 184 0.3 For example: 0.294 for the 81-120 lb range 201-240 2 223 0.2 0.237 241-280 0-0 0.000 281-320 0-0 0.000 321-360 0-0 0.000 361-400 1 380 0.1 0.202 total number SN i SN i W i 10 1881 total weight 16
Degree of Polymerization, DP DP = average number of repeat units per chain ( ) DP = 6 C C C C C C C C C C C C Chain fraction mol. wt of repeat unit i 17
14.7 Molecular Structure Characteristics of a polymer depend on Its molecular weight Molecular shape Structure of molecular chains
Linear Polymers Mer units are joined together end to end in single chains Flexible long chains (like a mass of spaghetti) Van der Waals & hydrogen bonds between the chains Polyethylene, polyvinyl chloride, nylon, etc.
Branched Polymers Side branch chains are connected to the main ones Linear polymers form branched polymers when side reactions that occur during synthesis form the branches Chain packing efficiency reduced->lower density Ex. low density polyethylene
Crosslinked Polymers Adjacent linear chains are joined one to another at various positions by covalent bonds Crosslinking may be accomplished by additives Vulcanized rubber
Network Polymers Multifunctional mer units, with three or more active covalent bonds, form 3-D networks Epoxies, phenol-formaldehydes
Questions ow might the length of the chain affect the likely phase (hard solid, waxy solid, liquid) at room temperature? Melting temperature? Why? ow might molecular weight affect modulus and strength? Why? 23
14.9 Thermoplastic and Thermosetting Polymers Thermoplastic polymers Soften when heated Eventually liquefy arden when cooled Processes are totally reversible & may be repeated Linear & branched polymers
Thermoplastic polymers The structure of polyethylene (a) the basic monomer (b) the double bond in the monomer is opened (c) monomers are linked together (d) secondary bonds between the polymer chains
14.9 Thermoplastic and Thermosetting Polymers Thermosetting polymers Become permanently hard when heated Do not soften or liquefy arder & stronger than thermoplastic polymers Cross-linked & network polymers
Thermoset polymers The structure of crosslinked rubber (a) double bonds along the length of the polymer chains (b) formation of crosslinks (primary bonds) between the chains
14.11 Polymer Crystallinity Packing of molecular chains so as to produce an ordered atomic array Linear polymers crystallization is easily obtained No restrictions to prevent chain alignment Crosslinked and network polymers amorphous
Practice Problems: 14.3, 14.5, 14.6, 14.13, 14.16