Chapters-12&14-2023

Chapter 12 - 3 Examples of Ceramics

Chapter 12 - 6 Ceramic Crystal Structures Mainly ionic bonding with some covalent bonding Cations – Positively charged metallic ions Anions – Negatively charged nonmetallic ions Electrical neutrality – net charge = 0 Ex: Na 1+ Cl 1- , Ca 2+ F 2 1- Size effect – determine the maximize # of nearest oppositely charged neighbors - - - - + unstable - - - - + stable - - - - + stable (Smallest stable size) (not always true but we will not discuss other situations) ß

Chapter 12 - 9 AX Crystal Structures -- NaCl Example: NaCl (rock salt) structure MgO and FeO r Na = 0.102 nm r Na / r Cl = 0.564 \ cations prefer O H sites r Cl = 0.181 nm Can be treated as Na fcc + Cl fcc with the displacement of <100>/2 between the two lattices Coordination # = 6

Chapter 12 - 12 Most common elements on earth are Si & O Silicate Ceramics Si 4+ O 2- The basic unit of silicate, with -4 charge associated with it More covalent character than ionic Quartz: crystalline Glass fibre: amorphous

Chapter 12 - 15 Polymer Composition Most polymers are hydrocarbons – i.e., made up of H and C Methane 14

Chapter 12 - 18 Unsaturated Hydrocarbons • Double & triple bonds somewhat unstable – can form new bonds – Double bond found in ethylene or ethene - C 2 H 4 – Triple bond found in acetylene or ethyne - C 2 H 2 C C H H H H C C H H 14

Chapter 12 - 21 Chemistry and Structure of Polyethylene Note: polyethylene is a long-chain hydrocarbon - paraffin wax for candles is short polyethylene bond angle = 109.5 ° 14

Chapter 12 - 22 MOLECULAR WEIGHT • 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 (Sometimes called molecular mass, molar mass) 14

Chapter 12 - 23 x i = number fraction of chains in size range i M n = total wt of polymer total #of molecules MOLECULAR WEIGHT DISTRIBUTION M n = Σ x i M i M W = Σ w i M i w i = weight fraction of chains in size range i M i = mean (middle) molecular weight of size range i Number-average molecular weight: ࠵? ࠵? Weight-average molecular weigh: ࠵? ࠵? 14 Important

Chapter 12 - 24 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? 14

Chapter 12 - 25 Molecular Weight Calculation (cont.) Solution: The first step is to sort the students into weight ranges. Using 40 lb ranges gives the following table: weight number of mean range students weight N i W i mass (lb) mass (lb) 81-120 2 110 121-160 2 142 161-200 3 184 201-240 2 223 241-280 0 - 281-320 0 - 321-360 0 - 361-400 1 380 Σ N i Σ N i W i 10 1881 total number total weight Calculate the number and weight fraction of students in each weight range as follows: x i = N i N i ∑ w i = N i W i N i W i ∑ For example: for the 81-120 lb range x 81 − 120 = 2 10 = 0.2 w 81 − 120 = 2 x 110 1881 = 0.117 14

Chapter 12 - M w = w i M i ∑ = (0.117 x 110 + 0.150 x 142 + 0.294 x 184 + 0.237 x 223 + 0.202 x 380) = 218 lb M n = x i M i ∑ = (0.2 x 110 + 0.2 x 142 + 0.3 x 184 + 0.2 x 223 + 0.1 x 380) = 188 lb 26 Molecular Weight Calculation (cont.) weight mean number weight range weight fraction fraction W i x i w i mass (lb) mass (lb) 81-120 110 0.2 0.117 121-160 142 0.2 0.150 161-200 184 0.3 0.294 201-240 223 0.2 0.237 241-280 - 0 0.000 281-320 - 0 0.000 321-360 - 0 0.000 361-400 380 0.1 0.202 M w = w i M i ∑ = 218 lb 14

Chapter 12 - 27 Degree of Polymerization, DP DP = average number of repeat units per chain where m = average molecular weight of repeat unit for copolymers this is calculated as follows: m = Σ f i m i C C C C C C C C H H H H H H H H H H H H H H H H H C C C C H H H H H H H H H ( ) DP = 6 mol. wt of repeat unit i Chain fraction DP = M n m An alternative way of expressing the average chain size of a polymer is DP 14

Chapter 12 - 28 Effect of Molecular Weight on Mechanical Properties 14

Chapter 12 - 29 Molecular Structures for Polymers B ranched Cross-Linked Network Linear secondary bonding 14

Chapter 12 - 30 Thermoplastic and Thermosetting Thermoplastic - soften when heated and harden when cooled (secondary bonding diminished through molecular motion) - the hardening and softening are reversible processes - relatively soft - most linear and branched polymers - common ones are polyethylene, polystyrene and polyvinylchloride Thermosetting - become hard during formation and do not soften on heating - network polymers – extensive covalent cross-links between chains - 10-50% of chain repeat units are cross-linked - Generally harder and stronger with better dimensional stability than thermoplastics - Vulcanised rubbers, epoxies, and some polyester resins (understanding the meaning is enough) 14

31 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 block graft alternating 14

Chapter 12 - 32 Crystallinity in Polymers • Ordered atomic arrangements involving molecular chains • Crystal structures in terms of unit cells • Example shown – polyethylene unit cell 14

Chapter 12 - 33 Polymer Crystallinity (cont.) 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 and % crystallinity to increase. crystalline region amorphous region % crystallinity 100 ) ( ) ( ´ = - - a a r r r r r r c s s c ࠵? ! is the density of fully crystalline structure ࠵? " is the density of fully amorphous structure ࠵? # is the density of the specimen concerned 14

Chapter 12 - 34 • Ceramic materials have covalent & ionic bonding. • Structures are based on: -- charge neutrality -- maximizing # of nearest oppositely charged neighbors. • Structures may be predicted based on the ratio of the cation and anion radii. • Polymer molecules – very large molecular chains that are composed of repeat units along the chain. • Polymer molecules can be characterized by their size (molecular weight), shape (chain twisting, coiling, bending) , and structure (linear, branched, crosslinked, network). • Polymers can be partially crystallized. Percent crystallinity depends on processing condition, chemistry and structure of the polymers. Summary