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Chapter 4 Structures of Polymers. Structures of Solid Materials general Crystalline materials -Materials which have crystal structure. -dense, ordered.

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Presentation on theme: "Chapter 4 Structures of Polymers. Structures of Solid Materials general Crystalline materials -Materials which have crystal structure. -dense, ordered."— Presentation transcript:

1 Chapter 4 Structures of Polymers

2 Structures of Solid Materials general Crystalline materials -Materials which have crystal structure. -dense, ordered packing. -atoms pack in periodic, 3D long range atomic order. -tend to have higher (max.) bond energies. -Occurs for: normal solidification conditions. -Typical of: metals, many ceramics & certain polymers. metalsceramicspolymers Noncrystalline materials -Materials which have amorphous structure. -non dense, random packing. -atoms have no periodic short range order. -tend to have lower (min.) bond energies. -Occurs for: complex rapid cooling. -Typical of: polymers, some ceramics & metals. SC, BCC, FCC, HCP crystal structuresAX, AX 2 & ABX 3 type crystal structures, Others: silicate, glass, carbon ceramics.Linear, branched, cross-linked, networkstructures ( as molecular chains ) Energy r typical neighbor bond length typical neighbor bond energy Energy r typical neighbor bond length typical neighbor bond energy crystal structure amorphous structure crystalline SiO 2 noncrystalline SiO 2 (refer to atomic arrangement in crystalline materials) (refer to atomic arrangement in noncrystalline materials)

3 Structures of Solid Materials metalsceramicspolymers SC, BCC, FCC, HCP crystal structuresLinear, branched, cross-linked, network Characteristics of polymer molecules Chemistry (repeat unit composition) Size (molecular weight) Shape (chain, twisting, entanglement, etc) Structure (as polymer molecular chains) Cross-linkedLinearBranchedNetwork Isomeric states Stereo isomers Geometrical isomers IsotacticSyndiotacticAtactic cistrans 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, side group of various atoms (O, Cl, etc). - 2 classifications: Thermoplastic polymers. Thermosetting polymers. AX, AX 2 & ABX 3 type crystal structures, Others: silicate, glass, carbon ceramics.Linear, branched, cross-linked, networkstructures ( as molecular chains )

4 Structures of Solid Materials PolymersceramicsMetals Chemistry of polymer molecules - 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 – C – other atoms (O, Cl etc)). - ex: ethane, C 2 H 6 CCCCCC HHHHHH HHHHHH Polyethylene (PE) repeat unit Cl CCCCCC HHH HHHHHH Poly(vinyl chloride) (PVC) repeat unit HH HHHH Polypropylene (PP) CCCCCC CH 3 HH H repeat unit Adapted from Fig. 4.2, Callister & Rethwisch 3e. Repeat units: Smaller structural entities that are repeated along the molecular chain. Intramolecules Covalent bond (strong) Intermolecules Hydrogen & van der Waals bonds (weak) C2H6C2H6 composition (chemical formula) structure General terms Polymer molecules (composition & structure)

5 Structures of Solid Materials PolymersceramicsMetals Chemistry of polymer molecules Polymer molecules (composition & structure) -composed of hydrocarbons molecules. saturated & unsaturated hydrocarbons. Saturated hydrocarbons - also known as paraffin molecules/compounds. - each carbon singly bonded to 4 other atoms. ex: ethane, C 2 H 6 - others: methane, CH 4, propane, C 3 H 8, butane, C 4 H 10, pentane, C 5 H 10 & hexane, C 6 H 14 Unsaturated hydrocarbons - double & triple bonds between 2 carbon atoms. - unstable & can form new bonds. ex: ethylene or ethene, C 2 H 4 ex: acetylene or ethyne, C 2 H 2 double bond triple bond Other organic groups: Methyl, CH 3 Ethyl, C 2 H 5 Phenyl, C 6 H 5 Alcohols (methyl alcohol), CH 3 R – OH Ethers (dimethyl ether), C 2 H 6 R – OH – R Acids (acetic acid), CH 3 O 2 H Aldehydes (formaldehyde) Aromatic hydrocarbons (phenol) free radical, R from organic groups Structure of phenyl group

6 Structures of Solid Materials PolymersceramicsMetals Chemistry of polymer molecules Polymer molecules - hydrocarbon molecules (compounds) with same composition (chemical formula) can have quite different structures (atomic arrangement). ex: C 8 H 18 Octane (normal) 2,4-dimethylhexane  Polymerization Isomerism - reaction between R (catalyst species/free radical) & monomer (from unsaturated hydrocarbon) to form solid polymer materials. 2 steps of polymerization: Monomer: Small molecule from which polymer is synthesized.

7 Structures of Solid Materials PolymersceramicsMetals Chemistry of polymer molecules Polymer molecules A listing of 10 common polymeric materials Polymeric materials Structure repeat unit

8 Structures of Solid Materials PolymersceramicsMetals Chemistry of polymer molecules Polymer molecules A listing of 10 common polymeric materials Polymeric materials Structure repeat unit

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

10 Structures of Solid Materials PolymersceramicsMetals Size of polymer molecules Molecular size Molecule weight calculation ex: average mass of a class 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? StudentWeight mass (lb) answer: The first step is to sort the students into weight ranges. Using 40 lb ranges gives the following table: total number total weight Calculate the number and weight fraction of students in each weight range as follows:

11 Structures of Solid Materials PolymersceramicsMetals Shape of polymer molecules Molecular shape - 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. chain Twisting Entanglement

12 Structures of Solid Materials PolymersceramicsMetals Structure of polymer molecules Molecular chains structures - shows the structure of the molecular chains. - all structures contain van der Waals & hydrogen bonding between the chains. (1) Linear structures (2) Branched structures (3) Cross-linked structures (4) Network 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 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 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 B ranched Cross-Linked Network Linear secondary bonding

13 Structures of Solid Materials PolymersceramicsMetals Structure of polymer molecules Molecular chains structures – isomeric states Stereo isomers Geometrical isomers - shows the molecular configurations for polymers. - Stereoisomers are mirror images – can’t superimpose without breaking a bond. - ex: Isotactic configuration Syndiotactic configuration Atactic configuration E B A D CC D A B E mirror plane - Tacticity – stereoregularity or spatial arrangement of R units along chain. - It can be isotactic, syndiotactic & atactic. - to change configurations, it must break bonds. - all R groups on same side of chain. - R groups alternate sides. - R groups randomly positioned. cistrans -H atom and CH 3 group on same side of chain. - H atom and CH 3 group on opposite sides of chain. ex: cis-isopreneex: trans-isoprene - either trans isomerism.

14 Structures of Solid Materials PolymersceramicsMetals Classification of polymeric materials Polymers classification 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). - 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. Thermosetting polymers - more monomers polymerized together. - 4 types: 1.random – A and B randomly positioned along chain. 2.alternating – A and B alternate in polymer chain. 3.block – large blocks of A units alternate with large blocks of B units. 4.graft – chains of B units grafted onto A backbone. A – B – random block graft alternating Copolymers

15 Structures of Solid Materials PolymersceramicsMetals 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. 10 nm Adapted from Fig. 4.12, Callister & Rethwisch 3e. chain folded structure polyethylene unit cell - 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. crystalline region amorphous region Adapted from Fig , Callister 6e. (Fig 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.)

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

17 End of Chapter 4


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