Chapter 4- Polymer Structures. Chapter 4- Polymer Structures. Polymer Microstructure. Polymer Microstructure ISSUES TO ADDRESS...

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1 hapter 4- Polymer Structures hapter 4- Polymer Structures ISSUES T ADDESS... What are the basic lassification? Monomers and chemical groups? Nomenclature? Polymerization methods? Molecular Weight and Degree of Polymerization? Molecular Structures? rystallinity? Microstructural features? TEM of spherulite structure in natural rubber(x30,000). hain-folded lamellar crystallites (white lines) ~10nm thick extend radially. Polymer Microstructure Polymer Microstructure Polymer many mers ovalent chain configurations and strength: More rigid Van der Waals, Adapted from Fig. 14.2, allister 6e. Polyethylene perspective of molecule Direction of increasing strength Adapted from Fig. 14.7, allister 6e. A zig-zag backbone structure with covalent bonds 1

2 ommon Examples - Textile fibers: polyester, nylon - I packaging materials. - esists for photolithography/microfabrication. - Plastic bottles (polyethylene plastics). - Adhesives and epoxy. - igh-strength/light-weight fibers: polyamides, polyurethanes, Kevlar - Biopolymers: DNA, proteins, cellulose ommon lassification Thermoplastics: polymers that flow more easily when squeezed, pushed, stretched, etc. by a load (usually at elevated T). an be reheated to change shape. Thermosets: polymers that flow and can be molded initially but their shape becomes set upon curing. eheating will result in irreversible change or decomposition. ther ways to classify polymers. By chemical functionality (e.g. polyacrylates, polyamides, polyethers, polyeurethanes ). Vinyl vs. non-vinyl polymers. By polymerization methods (radical, anionic, cationic ). Etc ommon hemical Functional Groups ommon ydrocarbon Monomers Ethylene (ethene) Alcohols Methyl alcohols Propylene (propene) Ethers Dimethyl Ether 1-butene Acids Acetic acid 2-butene Acetylene (ethyne) trans cis Aldehydes Formaldehyde Saturated hydrocarbons (loose to add atoms) Unsaturated hydrocarbons (double and triple bonds) Aromatic hydrocarbons Phenol 2

3 Some ommon Polymers Polyacrylonitrile (PAN) ommon backbone with substitutions N Vinyl polymers (one or more s of ethylene can be substituted) X X Monomer-based naming: poly Nomenclature Monomer name goes here e.g. ethylene -> polyethylene if monomer name contains more than one word: poly( ) Monomer name in parentheses e.g. acrylic acid -> poly(acrylic acid) Note: this may lead to polymers with different names but same structure. polyethylene polymethylene Polymerization Methods Polymerization Methods A. Free adical Polymerization 1. Initiation Free radical initiator (unpaired electron) sp 2 carbons monomer σ bonds π bond adical transferred sp 3 carbon A. Free adical Polymerization 2. Propagation Both carbon atoms will change from sp 2 to sp 3. 3

4 Polymerization Methods Polymerization Methods A. Free adical Polymerization 3. Termination Intentional or unintentional molecules/impurities can also terminate. B. Stepwise polymerization N 2 N 2. ther methods Anionic polymerization, cationic polymerization, coordination polymerization N 2 Proteins (polypeptides have similar composition) N n Various groups N N n Loses water (condensation) (n-1) Molecular Weights Not only are there different structures (molecular arrangements) but there can also be a distribution of molecular weights (i.e. number of monomers per polymer molecule). Average molecular weight 20 mers 16 mers 10 mers M monomer M monomer This is what is called number average molecular weight. Number average molecular weight: N M N M n N N Note: N M Total weight Weight average molecular weight: W M N M 2 M w W N M In general: N M α1 M N M α N Total # of polymer chains W N M If α 0 then N # of polymer chains with length M mass of polymer chain with length ( monomer molecular weight). M n If α 1 then M w 4

5 Molecular Weight: Different Notations In Lecture Notes M n M w N M N N M 2 N M M n x i N i N In allister Textbook M w x i M i i w i N im i N M w i M i i Molecular Weights Why do we care about weight average MW? -some properties are dependent on MW (larger MW polymer chains can contribute to overall properties more than smaller ones). Distribution of polymer weights Examples Light scattering: larger molecules scatter more light than smaller ones. Infrared absorption properties: larger molecules have more side groups and light absorption (due to vibrational modes of side groups) varies linearly with number of side groups. Polydispersity and Degree of Polymerization Example 1 Polydispersity: M w M n 1 ompute the number-average degree of polymerization for polypropylene, given that the number-average molecular weight is 1,000,000 g/mol. When polydispersity 1, system is monodisperse. Degree of Polymerization: Number avg degree of polymerization Weight avg degree of polymerization n n M n n w M w What is mer of PP? Mer molecular weight of PP is Number avg degree of polymerization 3 6 3A 6A 3(12.01 g/mol)6(1.008 g/mol) g/mol n n M n 106 g /mol 42.08g /mol 23,700 5

6 Example 2 (a, b, and c) A. alculate the number and weight average degrees of polymerization and polydispersity for a polymer sample with the following distribution. n n M n m 0 m 0 Avg # of monomers/chain elative abundance ,000 5 N N N N 5 *10 25 * * * * * n w M w 1 ( ) 2 N N ( ) 2 N N Note: m 0 cancels in all these! 5 * * * * * * *10 25 * * * * * ,800 Example 2 (cont.) B. If the polymer is PMMA, calculate number and weight average molecular weights. M w if monomer is methylmethacrylate (5, 2, and 8) So m 0 5(12)2(16)8(1) 100 g/mol M n n n (100g /mol) 286,040g /mol M w n w 35,800(100g /mol) 3,580,000g /mol Polydispersity: M w 3,580,000 M n 286,040 ~ Example 2 (cont.) Sequence isomerism. If we add polymer chains with avg # of monomers 10 such that their relative abundance changes from 5 to 10, what are the new number and weight average degrees of polymerization and polydispersity? For an asymmetric monomer T T n n M n N N Add 5 more monomers of length * * * * * * T T T T T T n w M w 2 N N Polydispersity: 35,800 Note: significant change in number average (3.8 %) but no change in weight average! M w 3,580,000 M n ~ 13 e.g. poly(vinyl fluoride): F to T F F to T to T andom arrangement F e.g. PMMA to T to T to T Exclusive to T arrangement (Why?) 6

7 Polymer Molecular onfigurations egularity and symmetry of side groups affect properties Polymer Geometrical Isomerism egularity and symmetry of side groups affect properties Polymerize an it crystallize? Melting T? Stereoisomerism: (can add geometric isomerism too) Syndiotactic Alternating sides cis-structure trans-structure with 3 to form rubber is-polyisoprene trans-polyisoprene Isotactic n one side Atactic andomly placed -onversion frone isomerism to another is not possible by simple rotation about chain bond because double-bond is too rigid! - onversion frone stereoisomerism to another is not possible by simple rotation about single chain bond; bonds must be severed first, then reformed! -See Figure 4.8 for taxonomy of polymer structures Polymer Structural Isomerism Some polymers contain monomers with more than 1 reactive site e.g. isoprene trans-isoprene 4 3 Polymer Microstructure ovalent chain configurations and strength: More rigid Van der Waals, trans-1,4-polyisoprene trans-1,2-polyisoprene 3,4-polyisoprene Direction of increasing strength Adapted from Fig. 14.7, allister 6e n 2 n 3 2 Note: there are also cis-1,4- and cis-1,2-polyisoprene 2 n 2 3 Long branching Short branching Star branching Dendrimers 7

8 opolymers andom, Alternating, Blocked, and Grafted Synthetic rubbers are often copolymers. e.g., automobile tires (SB) Molecular Structure ow do crosslinking and branching occur in polymerization? 1. Start with or add in monomers that have more than 2 sites that bond with other monomers, e.g. crosslinking polystyrene with divinyl benzene Styrene-Butadiene ubber random polymer stryene ontrol degree of crosslinking by styrene-divinyl benzene ratio polystyrene styrene divinyl benzene crosslinked polystyrene Monomers with trifunctional groups lead to network polymers. Branching in polyethylene (back-biting) Same as Molecular Structure adical moves to a different carbon ( transfer) Polymerization continues from this carbon Process is difficult to avoid and leads to (highly branched) low-density PE. When there is small degree of branching you get high-density PE. Example 3 Nitrile rubber copolymer, co-poly(acrylonitrile-butadiene), has M n 106,740g /mol alculate the ratio of (# of acrylonitrile) to (# of butadiene). 3 3 x g/mol 3 3 x g/mol 1 N 1 x g/mol m g/mol We need to use an avg. monomer MW: f 1 m 0 m m 1 m n n ,4-addition product M n 106, g /mol n n x g/mol 6 6 x g/mol f 1 m 1 f 2 m 2 f 1 (m 1 m 2 ) m 2 f 2 1 f m g/mol f 2 f : 3 8

9 Vulcanization Molecular Weight and rystallinity See also sect. in hpt. 8 rosslinking in elastomers is called vulcanization, and is achieved by irreversible chemical reaction, usually requiring high temperatures. Molecular weight, Mw: Mass of a mole of chains. Sulfur compounds are added to form chains that bond adacent polymer backbone chains and crosslinks them. Unvulcnaized rubber is soft and tacky an poorly resistant to wear. Tensile strength (TS): e.g., cis-isoprene Single bonds --often increases with M w. --Why? Longer chains are entangled (anchored) better. Stress-strain curves % rystallinity: % of material that is crystalline. Double bonds (mn) S --TS and E often increase with % crystallinity. --Annealing causes crystalline regions to grow. % crystallinity increases. (S)m (S)n crystalline region amorphous region Adapted from Fig , allister 6e. Polymer rystallinity polyethylene Volume fraction of crystalline component. Some are amorphous. Some are partially crystalline (semi-crystalline). Why is it difficult to have a 100% crystalline polymer? %crystallinity ρc ( ρ s ρa ) 100% ρ s ( ρc ρa ) ρs density of specimen in question ρa density of totally amorphous polymer ρc density of totally crystalline polymer %crystallinity Mcrystalline Mtotal ρ V ρ 100% c c 100% c fc 100% ρsvs ρs Mtotal Mcrystalline Mamophous Using definition of volume fractions: Ms Mc Ma ρsvs ρcvc ρava V V ρs ρc c ρa a Vs Vs V fc c Vs V fa a Vs ρc fc ρa fa ρc fc ρa (1 fc ) fc ( ρc ρa ) ρa ρ ρa fc s ρc ρa Substituting in f c into the original definition: ρ ( ρ ρa ) %crystallinity c s 100% ρ s ( ρc ρa ) 9

10 Polymer rystallinity Degree of crystallinity depends on processing conditions (e.g. cooling rate) and chain configuration. Semi-rystalline Polymers Fringed micelle model: crystalline region embedded in amorphous region. A single chain of polymer may pass through several crystalline regions as well as intervening amorphous regions. ooling rate: during crystallization upon cooling through MP, polymers become highly viscous. equires sufficient time for random & entangled chains to become ordered in viscous liquid. hemical groups and chain configuration: More rystalline Less rystalline Smaller/simper side groups Larger/complex side groups Linear ρ ρa fc s ρc ρa ighly branched rystalline volume fractions Important rosslinked, network Isotactic or syndiotactic andom Semi-rystalline Polymers hain-folded model: regularly shaped platelets (~10 20 nm thick) sometimes forming multilayers. Average chain length Semi-rystalline Polymers Spherulites: Spherical shape composed of aggregates of chain-folded crystallites. >> platelet thickness. Natural rubber ross-polarized light through spherulite structure of PE. 10

11 F. Bates, Science Diblock copolymers epresentative polymer-polymer phase behavior with different architectures: A) Phase separation with mixed LINEA homopolymers. B) Mixed LINEA homopolymers and DIBLK copolymer gives surfactant-like stabilized state. ) ovalent bond between blocks in DIBLK copolymer give microphase segregation. Thermoplastics: --little cross linking --ductile --soften w/heating --polyethylene (#2) polypropylene (#5) polycarbonate polystyrene (#6) Thermoplastics vs Thermosets Thermosets: --large cross linking (10 to 50% of mers) --hard and brittle --do NT soften w/heating --vulcanized rubber, epoxies, polyester resin, phenolic resin T mobile liquid crystalline solid viscous liquid allister, rubber Fig tough Tm plastic Tg partially crystalline solid Molecular weight Adapted from Fig , allister 6e. T m : melting over wide range of T depends upon history of sample consequence of lamellar structure thicker lamellae, higher T m. T g : from rubbery to rigid as T lowers Packing of Polymers Packing of spherical atoms as in ionic and metallic crystals led to crystalline structures. ow polymers pack depend on many factors: long or short, e.g. long (- 2 -) n. stiff or flexible, e.g. bendy - sp 3. smooth or lumpy, e.g., DPE. regular or random single or branched slippery or sticky, e.g. - covalent (nonpolar) oined via vdw. Analogy: onsider dried (uncooked) spaghetti (crystalline) vs. cooked and buttered spaghetti (amorphous). pile of long stiff spaghetti forms a random arrangement. cut into short pieces and they align easily. andle wax more crystalline than PE, even though same chemical nature. What Are Expected Properties? Would you expect melting of nylon 6,6 to be lower than PE? nylon 6,6 N N N 6 4 ydrogen bonds N N N 6 4 a) What is the source of intermolecular cohesion in Nylon vs PE? b) ow does the source of linking affect temperature? Van der Waals bonds polyethylene With -bonds vs vdw bonds, nylon is expected to have (and does) higher melting T. 11

12 What Are Expected Properties? Which polymer more likely to crystallize? an it be decided? Linear syndiotactic polyvinyl chloride Linear isotactic polystyrene What Are Expected Properties? Which polymer more likely to crystallize? an it be decided? Networked Phenol-Formaldehyde (Bakelite) Linear and highly crosslink cis-isoprene 2 0 For linear polymers, crystallization is more easily accomplished as chain alignment is not prevented. rystallization is not favored for polymers that are composed of chemically complex mer structures, e.g. polyisoprene. Networked and highly crosslinked structures are near impossible to reorient to favorable alignment. Linear and syndiotactic polyvinyl chloride is more likely to crystallize. The phenyl side-group for PS is bulkier than the l side-group for PV. Generally, syndiotactic and isotactic isomers are equally likely to crystallize. Not possible to decide which might crystallize. Both not likely to do so. What Are Expected Properties? Which polymer more likely to crystallize? an it be decided? alternating Poly(Polystyrene-Ethylene) opolymer random poly(vinyl chloride-tetra-fluoroethylne) copolymer Detergents Soap is a detergent based on animal or vegetable product, some contain petrochemicals water detergent grease What properties of soap molecules do you need to remove grease? green end must be hydrophilic. Why? pposite end must be hydrocarbon. Why? Alternating co-polymer more likely to crystallize than randones, as they are always more easily crystallized as the chains can align more easily. Water must be like oxygen (hoard electrons and promote -bonding) e.g., oxy-clean grease 12

13 Simple polymer: Elmers glue Borax SLIME! Simple polymer: Elmer s glue Borax SLIME! hemistry Elmer s glue is similar to poly (vinyl alcohol) with formula: ydrolyzed molecule acts in a condensation reaction with PVA, crosslinking it. this is a ST, n15 chain of poly(vinyl alcohol) Borax is sodium tetraborate decahydrate (B 4 Na ). The borax actually dissolves to form boric acid, B() 3. This boric acid-borate solution is a buffer with a p of about 9 (basic). Boric acid will accept a hydroxide - from water. B() B() 4-3 p9.2 rosslinked B() B() 4-3 p9.2 ydrolyzed molecule acts in a condensation reaction with PVA, crosslinking it. rosslinking ties chains via weak non-covalent (hydrogen) bonds, so it flows slowly. ange of Bonding and Elastic Properties Is slime a thermoset or thermoplastic, or neither? Thermoset bonding ovalent bonds form crosslinks Slime? -bonds form crosslinks Thermoplastic bonding Induced dipolar bonds form crosslinks Summary Polymers are part crystalline and part amorphous. The more lumpy and branched the polymer, the less dense and less crystalline. The more crosslinking the stiffer the polymer. And, networked polymers are like heavily crosslinked ones. Many long-chained polymers crystallize with a Spherulite microstructure - radial crystallites separated by amorphous regions. Where is nylon? Stiffness increases ptical properties: crystalline -> scatter light (Bragg) amorphous -> transparent. Most covalent molecules absorb light outside visible spectrum, e.g. PMMA (lucite) is a high clarity tranparent materials. 13