Chapter 4 Structures of Polymers

Structures of Solid Materials general Structures of Solid Materials Crystalline materials Materials which have crystal structure. dense, ordered packing. atoms pack in periodic, 3D arrays @ long range atomic order. tend to have higher (max.) bond energies. Occurs for: normal solidification conditions. Typical of: metals, many ceramics & certain polymers. Energy r typical neighbor bond length typical neighbor bond energy typical neighbor bond length bond energy crystal structure amorphous structure crystalline SiO2 noncrystalline SiO2 metals ceramics polymers SC, BCC, FCC, HCP crystal structures Others: silicate, glass, carbon ceramics. AX, AX2 & ABX3 type crystal structures, Linear, branched, cross-linked, network structures (as molecular chains) (refer to atomic arrangement in crystalline materials) Noncrystalline materials Materials which have amorphous structure. non dense, random packing. atoms have no periodic arrays @ short range order. tend to have lower (min.) bond energies. Occurs for: complex structures @ rapid cooling. Typical of: polymers, some ceramics & metals. (refer to atomic arrangement in noncrystalline materials)

Structures of Solid Materials Characteristics of polymer molecules metals ceramics polymers Chemistry (repeat unit composition) Size (molecular weight) Shape (chain, twisting, entanglement, etc) Structure (as polymer molecular chains) In general All polymer are made from organic materials. natural polymers – plants & animals (organic materials), such as: wood, rubber, cotton, wool, leather, silk etc. synthetic polymers – R&D from organic materials, such as: polyethylene, polypropylene, polystyrene, Kevlar, nylon etc. Most polymeric materials are… hydrocarbons (made up of H & C) – organic materials. composed of very large molecular chains (in 2D array) with organics group (methyl, ethyl, phenyl) @ side group of various atoms (O, Cl, etc). 2 classifications: Thermoplastic polymers. Thermosetting polymers. SC, BCC, FCC, HCP crystal structures Others: silicate, glass, carbon ceramics. AX, AX2 & ABX3 type crystal structures, Linear, branched, cross-linked, network structures (as molecular chains) Linear, branched, cross-linked, network Linear Cross-linked Branched Network Isomeric states Stereo isomers Geometrical isomers cis trans Isotactic Syndiotactic Atactic

Structures of Solid Materials repeat unit repeat unit Structures of Solid Materials C H Polyethylene (PE) H Polypropylene (PP) C CH3 Polymers ceramics Metals repeat unit Repeat units: Smaller structural entities that are repeated along the molecular chain. Cl C H Poly(vinyl chloride) (PVC) Chemistry of polymer molecules Polymer molecules (composition & structure) polymer is defined as many repeat unit (in terms of molecular chains). these macromolecules are composed of repeat units. 2 types of bond exist: intramolecular bonds: covalent (between C – C). intermolecular bonds: hydrogen & van der Waals (between C – H @ C – other atoms (O, Cl etc)). ex: ethane, C2H6 Adapted from Fig. 4.2, Callister & Rethwisch 3e. General terms C2H6 composition (chemical formula) Intramolecules Covalent bond (strong) structure Intermolecules Hydrogen & van der Waals bonds (weak)

Structures of Solid Materials Other hydrocarbons @ organic groups: Methyl, CH3 Ethyl, C2H5 Phenyl, C6H5 Alcohols (methyl alcohol), CH3OH @ R – OH Ethers (dimethyl ether), C2H6OH @ R – OH – R Acids (acetic acid), CH3O2H Aldehydes (formaldehyde) Aromatic hydrocarbons (phenol) Structure of phenyl group free radical, R from organic groups Polymers ceramics Metals Chemistry of polymer molecules Polymer molecules (composition & structure) composed of hydrocarbons molecules. saturated & unsaturated hydrocarbons. Unsaturated hydrocarbons double & triple bonds between 2 carbon atoms. - unstable & can form new bonds. ex: ethylene or ethene, C2H4 Saturated hydrocarbons also known as paraffin molecules/compounds. each carbon singly bonded to 4 other atoms. ex: ethane, C2H6 double bond ex: acetylene or ethyne, C2H2 others: methane, CH4, propane, C3H8, butane, C4H10, pentane, C5H10 & hexane, C6H14 triple bond

Structures of Solid Materials Isomerism hydrocarbon molecules (compounds) with same composition (chemical formula) can have quite different structures (atomic arrangement). ex: C8H18 Polymers ceramics Metals Octane (normal)  2,4-dimethylhexane Chemistry of polymer molecules Polymer molecules Polymerization 2 steps of polymerization: reaction between R (catalyst species/free radical) & monomer (from unsaturated hydrocarbon) to form solid polymer materials. Monomer: Small molecule from which polymer is synthesized.

Structures of Solid Materials A listing of 10 common polymeric materials Polymeric materials Structure repeat unit Polymers ceramics Metals Chemistry of polymer molecules Polymer molecules

Structures of Solid Materials A listing of 10 common polymeric materials Polymeric materials Structure repeat unit Polymers ceramics Metals Chemistry of polymer molecules Polymer molecules

Structures of Solid Materials Molecular size - not all chains in a polymer are of the same length. i.e., there is a distribution of molecular weights. short molecule chains Polymers ceramics Metals Low M long molecule chains high M - molecular weight, M: Mass of a mole of chains. Size of polymer molecules Molecular weight distribution Mi = mean (middle) molecular weight of size range i xi = number fraction of chains in size range i wi = weight fraction of chains in size range i

Structures of Solid Materials Molecular size Student Weight mass (lb) 1 104 2 116 3 140 4 143 5 180 6 182 7 191 8 220 9 225 10 380 Molecule weight calculation ex: average mass of a class Polymers ceramics Metals What is the average weight of the students in this class: Based on the number fraction of students in each mass range? Based on the weight fraction of students in each mass range? answer: The first step is to sort the students into weight ranges. Using 40 lb ranges gives the following table: Size of polymer molecules Calculate the number and weight fraction of students in each weight range as follows: total number total weight

Structures of Solid Materials chain Polymers ceramics Metals Shape of polymer molecules Molecular shape Twisting also known as 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. Entanglement

Structures of Solid Materials (1) Linear structures (2) Branched structures polymers are synthesized in which… repeat units are joined together end to end in single chains. flexible & spaghetti like. common polymers: Polyethylene Poly(vinyl chloride) Polystyrene Poly(methyl methacrylate) Nylon Fluorocarbons polymers are synthesized in which… side-branch chains are connected to the main chains. the branches may… result from side reactions that occur during synthesis. reduced the packing efficiency & lowered the polymer density. - common polymers: high-density polyethylene (HDPE) low-density polyethylene (LDPE) Polymers ceramics Metals Structure of polymer molecules Molecular chains structures (3) Cross-linked structures (4) Network structures polymers are synthesized in which… adjacent linear chains are joined one to another at various positions by covalent bonds. accomplished by additive atoms that are covalently bonded to the chains. common polymers: rubber elastic materials. polymers are synthesized in which… multifunctional monomers forming 3 @ more active covalent bonds make 3D networks. polymer that is highly cross-linked may also be classified as a network polymer. these polymers have distinctive mechanical & thermal properties. common polymers: epoxies polyurethanes phenol-formaldehyde shows the structure of the molecular chains. all structures contain van der Waals & hydrogen bonding between the chains. B ranched Cross-Linked Network Linear secondary bonding

Structures of Solid Materials Stereo isomers Stereoisomers are mirror images – can’t superimpose without breaking a bond. ex: E B A D C mirror plane Polymers ceramics Metals - Tacticity – stereoregularity or spatial arrangement of R units along chain. - It can be isotactic, syndiotactic & atactic. Isotactic configuration Structure of polymer molecules - all R groups on same side of chain. Molecular chains structures – isomeric states shows the molecular configurations for polymers. to change configurations, it must break bonds. Syndiotactic configuration - R groups alternate sides. Geometrical isomers - either cis @ trans isomerism. cis trans H atom and CH3 group on same side of chain. - H atom and CH3 group on opposite sides of chain. Atactic configuration - R groups randomly positioned. ex: cis-isoprene ex: trans-isoprene

Structures of Solid Materials Thermoplastic polymers soften when heated (eventually liquefy) & harden when cooled. processes that are reversible & repeated. structures: most linear polymers & some branched polymers with flexible chains. properties: soft & less dimensional stability. common polymers: polyethylene. poly(vinyl chloride). polystyrene. poly(ethylene terephthalate). Polymers ceramics Metals Classification of polymeric materials Polymers classification Copolymers random block graft alternating - 2 @ more monomers polymerized together. - 4 types: 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 – Thermosetting polymers permanently hard during their formation & do not soften when heated. processes that are not reversible & repeated. structures: most network polymers & cross-linked polymers. properties: hard, strong & high dimensional stability. common polymers: epoxies. phenolics. polyester resins.

Structures of Solid Materials Polymer crystallinity ordered atomic arrangements involving molecular chains. crystal structures in terms of unit cells. ex: thin platelets with chain folds at faces. polyethylene unit cell. Polymers ceramics Metals 10 nm Adapted from Fig. 4.12, Callister & Rethwisch 3e. Polymer crystallinity chain folded structure polyethylene unit cell crystalline region amorphous - polymers rarely 100% crystalline. difficult for all regions of all chains to become aligned. degree of crystallinity expressed as % crystallinity. some physical properties depend on % crystallinity. heat treating causes crystalline regions to grow & % crystallinity to increase. Adapted from Fig. 14.11, Callister 6e. (Fig. 14.11 is from H.W. Hayden, W.G. Moffatt, and J. Wulff, The Structure and Properties of Materials, Vol. III, Mechanical Behavior, John Wiley and Sons, Inc., 1965.)

Structures of Solid Materials Polymer single crystal single crystals – only for slow and carefully controlled growth rates. electron micrograph – multilayered single crystals (chain-folded layers) of polyethylene. Polymers ceramics Metals Polymer crystallinity Polymer semi crystal some semicrystalline polymers form spherulite structures. alternating chain-folder crystallites and amorphous regions. spherulite structure for relatively rapid growth rates. Cross-polarized light used - a maltese cross appears in each spherulite Photomicrograph – Spherulites in polyethylene

End of Chapter 4