Materials of Engineering ENGR 151 POLYMER STRUCTURES

LEARNING OBJECTIVES Understand different molecular and crystal structures of polymers What are the general structural and chemical characteristics of polymer molecules? What are some of the common polymeric materials, and how do they differ chemically? ow is the crystalline state in polymers different from that in metals and ceramics?

POLYMER A solid, nonmetallic (typically organic) compound of high molecular weight Consisting of small repeat units (mer)

WAT IS A POLYMER? Poly many mer repeat unit C C C C C C repeat unit Polyethylene (PE) C C C C C C Cl repeat unit Cl Cl Poly(vinyl chloride) (PVC) C 3 Polypropylene (PP) C C C C C C C 3 repeat unit C 3 Adapted from Fig. 14.2, Callister & Rethwisch 9e.

POLYMERS Naturally occurring wood, rubber, cotton, wool, leather, silk Proteins, enzymes, starches, cellulose Production polymers are synthetic Inexpensive Superior properties

YDROCARBON MOLECULES Organic molecules Many are hydrocarbons (contain and C) ydrocarbons Covalent bonds (sharing electrons) Carbon (4 valence e - ) ydrogen (1 valence e - )

MOLECULES Unsaturated: molecules with double/triple bonds Each C atom is not bonded to max four other atoms More atoms or group of atoms can attach to C atom Saturated: molecules with single bonds only No new atoms can join without removal of other atoms

POLYMER COMPOSITION Most polymers are hydrocarbons i.e., made up of and C Saturated hydrocarbons Each carbon singly bonded to four other atoms Example: Ethane, C 2 6 C C

YDROCARBONS Double bond between carbon atoms Sharing of two pairs of electrons Triple bond between carbon atoms Sharing of three pairs of electrons

UNSATURATED YDROCARBONS Double & triple bonds somewhat unstable can form new bonds Double bond found in ethylene or ethene - C 2 4 C C Triple bond found in acetylene or ethyne - C 2 2 C C

SIMPLE YDROCARBONS Paraffin family (chain-like structure): Methane C 4 Ethane C 2 6 Propane C 3 8 Butane C 4 10 Pentane C 5 12 exane C 6 14 Strong bonds within molecule, weak with other molecules (low melting point) Boiling temperatures increase with molecular weight

ISOMERISM Molecules having different atomic arrangements Example: Butane Normal butane boils at -0.5 C Isobutane boils at -12.3 C Butane Isobutane

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

POLYMER MOLECULES Polymer molecules are relatively large in size (macromolecules) compared to hydrocarbon molecules Backbone of carbon chain polymer molecules are carbon strings (long and flexible) Side bonding allowed Radical positioning

OTER ORGANIC GROUPS Contain radicals (groups of atoms that remain intact during chemical reactions) Alcohols (radical connected to O) Ethers (2 radical connected to O) Acids (radical, O, C/O double bond) Aldehyde (radical,, C/O double bond)

MER UNITS Mer: successively repeated units along chain Polymer: many mers Monomer: stable molecule small molecule from which polymer is synthesized

TE CEMISTRY OF POLYMER MOLECULES Ethylene (C 2 4 ): polyethylene formed from temperature and pressure (catalytic reaction) PE is solid polymer Active mer (PE unit) formed from reaction Catalyst and ethylene Causes chain-like formation with unpaired electron

POLYMERIZATION AND POLYMER CEMISTRY Free radical polymerization R + f ree radical C C monomer (ethy lene) R C C initiation R C C + C C R C C C C propagation dimer Initiator: example - benzoyl peroxide C O O C 2 C O = 2 R

CEMISTRY AND STRUCTURE OF POLYETYLENE Adapted from Fig. 14.1, Callister & Rethwisch 9e. Note: polyethylene is a long-chain hydrocarbon - paraffin wax for candles is short polyethylene

POLYETYLENE Angle between single bonded carbon atoms ~ 109 (zig-zag pattern) Bond length ~ 0.154 nm Replace with Fl Polytetraflouroethylene, PTFE Teflon Flourocarbon

POLYVINYL CLORIDE (PVC) Variant from PE Every fourth hydrogen replaced with Cl atom Replace each Cl with C 3 (polypropylene, PP) Table 14.3 Polystyrene (PS) nylon, polyester (complex polymeric structure)

BULK OR COMMODITY POLYMERS 23

BULK OR COMMODITY POLYMERS (CONT) 24

Bulk or Commodity Polymers (cont) 25

CEMISTRY OF POLYMERS omopolymer: repeating units of the same type along a chain Copolymer: chains composed of two or more mer units Monomer: Bifunctional: may bond with two other units to form 2-D chain-like molecular structure Trifunctional: may bond with three other units to form 3-D network structure

MOLECULAR WEIGT Molecular weight, M: Mass of a mole of chains. Low M high M Not all chains in a polymer are of the same length i.e., there is a distribution of molecular weights

MOLECULAR WEIGT Large chains = large molecular weight Average molecular weight: Number-average molecular weight ( ): Divide chains into different size ranges M n Determine number of fraction chains within each size range M xm n i i M i = mean molecular weight of size range i x i = fraction of number of chains within size range

MOLECULAR WEIGT Weight-average molecular weight ( ): M wm w i i M w M i = mean molecular weight of size range i w i = weight fraction of molecules within same size interval

DEGREE OF POLYMERIZATION n = average number of mer units in a chain Number-average degree of polymerization (n n ): Weight-average degree of polymerization (n w ): m = mer molecular weight n n n w M m M m n w

EXAMPLE PROBLEM 14.1 (PG 542)

MOLECULAR WEIGT Melting temperature increases with increased molecular weight (up to 100,000 g/mol) Polymers with short chains (100 g/mol) are liquids/gases at room temp 1000 g/mol (paraffin wax, soft resin) 10,000-several million g/mol (solid polymers)

POLYMERS MOLECULAR SAPE Molecular Shape (or Conformation) chain bending and twisting are possible by rotation of carbon atoms around their chain bonds note: not necessary to break chain bonds to alter molecular shape Adapted from Fig. 14.5, Callister & Rethwisch 9e.

CAIN END-TO-END DISTANCE, R Fig. 14.6, Callister & Rethwisch 9e.

MOLECULAR SAPE Single chain molecule can have a zigzag structure due to range of 109 Responsible for large elastic extension of rubber, also mechanical and thermal characteristics Bending, coiling and kinking properties Double bonds are rotationally rigid

LEARNING OBJECTIVES Understand different molecular structures of polymers Analyze different molecular configurations of polymers Discuss thermoplastic & thermosetting polymers Understand different copolymers and review polymer crystals

MOLECULAR STRUCTURE Polymer Characteristics Defined by: Molecular weight Shape Structural differences in chains Linear polymers Branched polymers Crosslinked polymers Network polymers

MOLECULAR STRUCTURE Each circle represents a repeat unit

MOLECULAR STRUCTURES FOR POLYMERS secondary bonding Linear B ranched Cross-Linked Network Adapted from Fig. 14.7, Callister & Rethwisch 9e.

LINEAR POLYMERS Mer units are joined end to end in single chain Flexible, spaghetti-like Van der Waals & hydrogen bonding between chains Examples: polyethylene, polyvinyl chloride, polystyrene, polymethyl methacrylate, nylon, flourocarbons

BRANCED POLYMERS Branches connected to main chain Caused by side reaction during polymer synthesis More branches = lower density Linear polymers can be synthesized to branch

CROSSLINKED POLYMERS Linear chains joined together Covalent bonds maintain joints Synthesis or non-reversible chemical reactions at high temperature Additive molecules/atoms (covalently bonded to chains) Rubber materials (elastic) Formation of crosslinks (vulcanization)

NETWORK POLYMERS Three dimensional network Formed from mers with three covalent bonds ighly crosslinked polymer = network polymer Epoxies (unique bonding characteristic)

14.8 MOLECULAR CONFIGURATIONS Properties influenced by: Regularity & symmetry of side group arrangement (two or more side atoms/groups) ead-to-tail configuration Alternating More likely to occur ead-to-head configuration (R) groups adjacent to each other Polar repulsion occurs

ISOMERISM Recall: different atomic configuration for same composition (butane/isobutane) Stereoisomerism Geometrical isomerism

STEREOISOMERISM Isotactic ( groups on same side) Syndiotactic ( alternates sides) Atactic (random) ROTATION NOT ALLOWED! (must sever first) Polymer can exhibit more than one configuration 3-D image

STEREOISOMERISM ISOTACTIC Isotactic R groups on same side of the polymer chain

STEREOISOMERISM SYNDIOTACTIC Syndiotactic R groups on alternate sides of the polymer chain

STEREOISOMERISM ATACTIC Atactic R groups randomly positioned along the polymer chain

ISOMERISM Geometrical Isomerism Double bonds between C atoms C atoms bonded to other atom or radical (side group) Cis structure (same side, cis-polyisoprene) Natural rubber Trans structure (opposite sides, trans-polyisoprene) Conversion from cis to trans not possible as bond rotation is not allowed double bond is rigid Cis Trans

POLYMER STRUCTURE CLASSIFICATION Chemistry Molecular size Molecular weight Molecular shape Degree of twisting, coiling, bending Molecular structure The way units (mers) are joined together

POLYMER STRUCTURE CLASSIFICATION

TERMOPLASTIC & TERMOSETTING POLYMERS Further classify polymers Based on response to rising temperature Thermoplasts: Soften when heated, eventual liquefaction arden when cooled Reversible, repeatable Typically linear polymer, branched polymer Fabricated using heat and pressure

TERMOSETTING POLYMERS Thermosets: Permanently hard when heat is applied No softening with heat During heating, covalent crosslinks are formed Crosslinks serve as anchors 10%-50% crosslinking arder & stronger that thermoplastics Examples: Vulcanized rubbers & epoxies,

COPOLYMERS Consists of two mer units Different sequencing possible: Random copolymer Alternating copolymer Block copolymer Graft copolymer

COPOLYMERS Two or more monomers polymerized together random A and B randomly positioned along chain alternating A and B alternate in polymer chain block large blocks of A units alternate with large blocks of B units graft chains of B units grafted onto A backbone A B random alternating block graft

POLYMER CRYSTALLINITY Packing of chains to form ordered atomic array Polyethylene Small molecule polymers: Completely crystalline (solid) Completely amorphous (liquid) Typical polymers: Semi-crystalline

CRYSTALLINITY IN POLYMERS Ordered atomic arrangements involving molecular chains Crystal structures in terms of unit cells Example shown polyethylene unit cell

DEGREE OF CRYSTALLINITY Range of complete amorphous to 95% crystalline c ( s a ) % crystallinity 100 ( ) ρ s = density of specimen s c a ρ a = density of totally amorphous polymer ρ c = density of perfectly crystalline polymer

DEGREE OF CRYSTALLINITY - EXAMPLE

DEGREE OF CRYSTALLINITY Depends on rate of cooling during solidification Chains must assume ordered configuration for crystallinity Not favored for complex mers Not easily prevented for simple mers Linear polymers (crystalline) Branch polymers (mostly amorphous) Network/crosslink polymers (amorphous)

CRYSTALLINITY Stereoisomerism Atactic (amorphous) Isotactic/syndiotactic (crystallize easier due to regularity) Bulky side-bonding groups = less crystallization Copolymers Alternating and block (more likely to crystallize) Random and graft (not as much likelihood)

CRYSTALLINITY Crystalline polymers resistant to diffusion and softening by heat

POLYMER CRYSTALS Fringed-Micelle model Chain molecule can pass through crystallite and amorphous regions Chain-Folded model Chains fold back on each other to form platelets Spherulites (like metal grains) Crystalline lamellae separated by amorphous material

POLYMER CRYSTALLINITY Crystalline regions thin platelets with chain folds at faces Chain folded structure Fig. 14.12, Callister & Rethwisch 9e. 10 nm

OMEWORK W (Due Monday, May 15 th ) 14.3, 14.5, 14.7, 14.25, 14.26