11MSE 2090: Introduction to Materials Science Chapter 14, Polymer StructuresChapter Outline: Polymer Structures¾ Hydrocarbon and polymer molecules¾ Chemistry of polymer molecules¾ Molecular weight and shape¾ Molecular structure and configurations¾ Thermoplastic and thermosetting polymers¾ Copolymers¾ Polymer crystals and degree of crystallinity¾ Defects and diffusion in polymersOptional reading: noneChaperone/structural protein by Choudhury et al. 2MSE 2090: Introduction to Materials Science Chapter 14, Polymer Structures¾ Polymers – materials consisting of polymer moleculesthat consist of repeated chemical units (`mers') joined together, like beads on a string. Some polymer molecules contain hundreds or thousands of monomers and are often called macromolecules.¾ Polymers may be natural, such as leather, rubber, cellulose or DNA, or synthetic, such as nylon or polyethylene.Polymers: IntroductionMany of important current research problems and technological applications involve polymers. Living organisms are mainly composed of polymerized amino acids (proteins) nucleic acids (RNA and DNA), and other biopolymers. The most powerful computers - our brains - are mostly just a complex polymer material soaking in salty water. We are just making first smallsteps towards understanding of biological systems. Silk fiber is produced by silk worms in a cocoon, to protect the silkworm while it metamorphoses in a moth. 3MSE 2090: Introduction to Materials Science Chapter 14, Polymer Structures¾ Most polymers are organic in their origin and are formed from hydrocarbon molecules¾ Each C atom has four e-that participate in bonds, each H atom has one bonding e-Hydrocarbon molecules (I)Methane, CH4Propane, C3H8Ethane, C2H6Examples of saturated (all bonds are single ones) hydrocarbon molecules:4MSE 2090: Introduction to Materials Science Chapter 14, Polymer StructuresDouble and triple bonds can exist between C atoms (sharing of two or three electron pairs). Molecules with double and triple bonds are called unsaturated. Unsaturated molecules are more reactiveHydrocarbon molecules (II)Ethylene, C2H4Acetylene, C2H2H-C≡C-HIsomers are molecules that have the same composition (contain the same atoms) but have different atomic arrangement. An example is butane and isobutane:Butane → C4H10← Isobutanephysical properties (e.g. boiling temperature) depend on the isomeric state25MSE 2090: Introduction to Materials Science Chapter 14, Polymer StructuresMany other organic groups can be involved in polymer molecules. In table below R represent a radical, an organic group of atoms that remains as a unit and maintains their identity during chemical reactions (e.g. CH3, C2H5, C6H5)Hydrocarbon molecules (III)6MSE 2090: Introduction to Materials Science Chapter 14, Polymer StructuresPolymer molecules¾ Polymer molecules can be very large (macromolecules)¾ Most polymers consist of long and flexible chains with a string of C atoms as a backbone¾ Side-bonding of C atoms to H atoms or radicals¾ Double bonds are possible in both chain and side bonds ¾ Repeat unit in a polymer chain (“unit cell”) is a mer¾ Small molecules from which polymer is synthesized is monomer. A single mer is sometimes also called a monomer.polyethylene (e.g. paraffin wax for candles)(mer)7MSE 2090: Introduction to Materials Science Chapter 14, Polymer StructuresChemistry of polymer molecules (I)¾ Ethylene (C2H4) is a gas at room temp and pressure¾ Ethylene transform to polyethylene (solid) by forming active mer through reaction with initiator or catalytic radical (R.)¾ (.) denotes unpaired electron (active site)3. Termination when two active chain ends meet each other or active chain end meet with initiator or other species with single active bond:2. Rapid propagation ~1000 mer units in 1-10 ms:Polymerization: 1. Initiation reaction:8MSE 2090: Introduction to Materials Science Chapter 14, Polymer StructuresChemistry of polymer molecules (II)hydrogen atoms in polyethylene are replaced by fluorine: polytetraflouroethylenePTFE – Teflonevery fourth hydrogen atom in polyethylene is replaced with chlorine: poly(vinyl chloride) PVCevery fourth hydrogen atom in polyethylene is replaced with methyl group (CH3): polyproplylene PPMore examples on pp. 539-540 of the textbook39MSE 2090: Introduction to Materials Science Chapter 14, Polymer StructuresChemistry of polymer molecules (III)¾ When all mers are the same, the molecule is called a homopolymer¾ When there is more than one type of mer present, the molecule is a copolymer¾ Mer units that have 2 active bonds to connect with other mers are called bifunctional¾ Mer units that have 3 active bonds to connect with other mers are called trifunctional. They form three-dimensional molecular network structuresPhenol-formaldehyde (trifunctional)Polyethilene(bifunctional)10MSE 2090: Introduction to Materials Science Chapter 14, Polymer Structures¾ The molecular weight (chain length) is controlled by the synthesis process: Relative rates of initiation, propagation, termination steps of polymerization¾ Formation of macromolecules during polymerization results in a distribution of chain lengths and molecular weights¾ The average molecular weight can be obtained by averaging the masses with the fraction of times they appear (number-average molecular weight) or with the mass fraction of the molecules (weight-average molecular weight).∑=iinMxM∑=iiwMwMnumber-average:weight-average:wiis weight fraction of chains of length ixiis number fraction of chains of length iMolecular weight11MSE 2090: Introduction to Materials Science Chapter 14, Polymer StructuresMolecular weight: Example illustrating the difference between number-average and weight-averagestudent weight mass (lb)1 1042 1163 1404 1435 1806 1827 1918 2209 22510 380What is the average weight of 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?Solution: The first step is to sort the students into weight ranges (let’s use 40 lb ranges).weight range # of students mean weight number fraction weight fractionNiMixiwi81-120 2 110 0.2 0.117121-160 2 142 0.2 0.150161-200 3 184 0.3 0.294201-240 2 223 0.2 0.237241-280 0 - 0 0.000281-320 0 - 0 0.000321-360 0 - 0 0.000361-400 1 380 0.1 0.2021881=∑iiMN10=∑iN∑=iiiNNx∑=iiiiiMNMNw12MSE 2090: Introduction to Materials Science Chapter 14, Polymer StructuresMolecular weight: