Chapter 4- Structures Chapter 4- Polymer Structures

ISSUES TO ADDRESS...

What are the basic • Classification? • and chemical groups? • Nomenclature? • methods? • Molecular Weight and Degree of Polymerization? • Molecular Structures? • Crystallinity? • Microstructural features? TEM of spherulite structure in natural rubber(x30,000). • Chain-folded lamellar crystallites (white lines) ~10nm thick extend radially.

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Polymer Microstructure Polymer Microstructure

• Polymer = many mers

• Covalent chain configurations and strength: More rigid Van der Waals, H

Adapted from Fig. 14.2, Callister 6e. perspective of

Direction of increasing strength Adapted from Fig. 14.7, Callister 6e.

A zig-zag backbone structure with covalent bonds

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1 Common Examples Common Classification

- Textile fibers: , … • : that flow more easily when squeezed, pushed, stretched, etc. by a load (usually at - IC packaging materials. elevated T). – Can be reheated to change shape. - Resists for photolithography/microfabrication. • Thermosets: polymers that flow and can be molded initially but their shape becomes set upon curing. - bottles (polyethylene ). – Reheating will result in irreversible change or decomposition.

- Adhesives and . • Other ways to classify polymers. – By chemical functionality (e.g. polyacrylates, , - High-strength/light-weight fibers: polyamides, polyethers, polyeurethanes…). – Vinyl vs. non-vinyl polymers. , … – By polymerization methods (, anionic, cationic…). – Etc… - : DNA, , …

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Common Chemical Functional Groups Common Monomers

H H Methyl alcohols C C (ethene) H H

H H Propylene C C = Ethers Dimethyl Ether H C H () H H 1- Acids 2-butene trans cis Aldehydes H C C H (ethyne)

Saturated Unsaturated hydrocarbons Aromatic Phenol (loose H to add atoms) (double and triple bonds) hydrocarbons

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2 Some Common Polymers Nomenclature -based naming: Common backbone with substitutions poly______H H C C Monomer name goes here Polyacrylonitrile (PAN) H C e.g. ethylene -> polyethylene N

Vinyl polymers (one or more H’s of ethylene can be substituted) if monomer name contains more than one word:

H H H H poly(______) C C C C Monomer name in parentheses H X H X e.g. -> poly(acrylic acid)

Note: this may lead to polymers with different names but same structure. H H H H H H H H … C C C C … … C C C C … H H H H H H H H polyethylene polymethylene

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Polymerization Methods Polymerization Methods

A. Free A. Free Radical Polymerization H H 2. Propagation C C H H 1. Initiation H H H H H H H H H Radical H H H H H H H H H R transferred R C C C C R C C C C R C C C C C C C C R C C H H H H H H H H H H H H H Free radical initiator H H H H H (unpaired electron) monomer H H R C C R H H R H H H C C 2 C C Both carbon atoms will sp carbons H change from sp2 to sp3. H H H H C C H H H σ bonds 3 sp carbon H π bond

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3 Polymerization Methods Polymerization Methods

Loses water B. Stepwise polymerization (condensation) A. Free Radical Polymerization

3. Termination O O O + H N C H N C R OH O H N C 2 2 + 2 R OH R OH R N C H H H H H H H O

R C C + R R C C R H H H H

Proteins (polypeptides have similar composition) O O H O H N C + (n-1) H H H H N H C R H H H H H H C Various R groups… n

R C C + R C C R C C C C R R n H H H H H H H H C. Other methods Anionic polymerization, cationic Intentional or unintentional /impurities can also terminate. polymerization, coordination polymerization…

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Molecular Weights Number average molecular weight: N = # of polymer chains with length j ∑N jM j mo ∑N j j j Not only are there different structures (molecular arrangements) j j M = jm mass of polymer chain with length j Mn = j o …… but there can also be a distribution of molecular weights N = (m = monomer molecular weight). ∑ j ∑N j o (i.e. number of monomers per polymer molecule). j j N Note: ∑N jM j = Total weight ∑ j = Total # of polymer chains j j € Weight average€ molecular weight: 20 mers 16 mers 2 € ∑W jM j ∑N jM€j j j W N M Mw = = j = j j 10 mers ∑W j ∑N j M j j j 20 +16 +10 Average molecular weight = α€+1 M monomer =15.3M monomer ∑N jM j 3 In general: j M = If α = 0 then Mn If α = 1 then Mw € α This is what is called number average molecular weight. ∑N j M j j

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4 Molecular Weight: Different Notations Molecular Weights Why do we care about weight average MW? -some properties are dependent on MW (larger MW polymer chains can In Lecture Notes In Callister Textbook contribute to overall properties more than smaller ones). ∑N jM j Mn = ∑xiMi j i Mn = N Distribution of ∑ j polymer weights Ni NiMi j xi = wi = ∑N j ∑N j M j j j 2 € ∑N jM j € j Examples – Mw = € Mw = ∑wiMi ∑N M € Light scattering: larger molecules scatter more light than smaller ones. j j i j 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.

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Polydispersity and Degree of Polymerization Example 1

Mw Polydispersity: ≥ 1 Compute the number-average degree of polymerization for , M n given that the number-average molecular weight is 1,000,000 g/mol.

When polydispersity = 1, system is monodisperse. What is “mer” of PP? C3H6 € Degree of Polymerization: Mer molecular weight of PP is mo=3AC+6AH =3(12.01 g/mol)+6(1.008 g/mol) M n = 42.08 g/mol Number avg degree of polymerization nn = mo Number avg degree of polymerization Mw Weight avg degree of polymerization n 6 w = M n 10 g /mol mo nn = = = 23,700 € mo 42.08g /mol

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5 Example 2 (a, b, and c) Example 2 (cont.) A. Calculate the number and weight average degrees of polymerization and polydispersity for a polymer sample with the following distribution. B. If the polymer is PMMA, calculate number and weight average molecular weights. Avg # of monomers/chain Relative abundance 10 5 CH3 100 25 M if monomer is methylmethacrylate (5C, 2O, and 8H) | 500 50 w -CH2-C- 1000 30 So m = 5(12)+2(16)+8(1)= 100 g/mol 0 | 5000 10 50,000 5 CO2CH3 jN jN M m ∑ j j ∑ j j n = n = 0 = n m m N N M n = nnmo = 2860.4(100g /mol) = 286,040g /mol o 0 ∑ j j ∑ j j 5 *10 + 25 *100 + 50 * 500 + 30 *1000 + 10 * 5000 + 5 * 50000 M w = nwmo = 35,800(100g /mol) = 3,580,000g /mol = = 2860.4 5 + 25 + 50 + 30 + 10 + 5 M 3,580,000 Polydispersity: w = ~ 12.52 2 2 M n 286,040 ( jmo ) N j j N j M w 1 ∑ j ∑ j n = = = Note: m0 cancels in all these! w m m N ( jm ) jN € o o ∑ j j o ∑ j j 5 *102 + 25 *1002 + 50 * 5002 + 30 *10002 + 10 * 50002 + 5 * 500002 = = 35,800 5 *10 + 25 *100 + 50 * 500 + 30 *1000 + 10 * 5000 + 5 * 50000 €

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Example 2 (cont.) Sequence isomerism

C. 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 For an asymmetric monomer and weight average degrees of polymerization and polydispersity? T H T H

jN T H + T H T H H T M ∑ j j n = n = Add 5 more monomers of length 10 …. n m N o ∑ j j H T T H 10 *10 + 25 *100 + 50 * 500 + 30 *1000 + 10 * 5000 + 5 * 50000 = = 2750 10 + 25 + 50 + 30 + 10 + 5 e.g. poly(vinyl fluoride): e.g. PMMA j2N Mw ∑ j j H C H C H C nw = = = 35,800 H C 3 3 3 m ∑ jN H F H H F H H H 3 O O O O O O O o j j O C C C C C C C C H C H C H C H C Note: significant change in number average (3.8 %) H H H F H H H F but no change in weight average! C C C C C C C C € H to T T to T H CH3 H CH3H CH3 H CH3 H to H H to T Mw 3,580,000 H to T Polydispersity: = ~ 13 Random arrangement H to T Mn 275000 Exclusive H to T arrangement (Why?)

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6 Polymer Molecular Configurations Polymer Geometrical Isomerism • Regularity and symmetry of side groups affect properties • Regularity and symmetry of side groups affect properties

Can it crystallize? Polymerize Melting T?

H H cis-structure trans-structure • Stereoisomerism: (can add geometric isomerism too) Syndiotactic with R= CH to form rubber Alternating sides 3 Cis-polyisoprene trans-polyisoprene

Isotactic Atactic -Conversion from one isomerism to another is not possible by simple On one side Randomly placed rotation about chain bond because double-bond is too rigid!

- Conversion from one stereoisomerism to another is not possible by simple -See Figure 4.8 for taxonomy of polymer structures rotation about single chain bond; bonds must be severed first, then reformed!

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Polymer Structural Isomerism Polymer Microstructure Some polymers contain monomers with more than 1 reactive site • Covalent chain configurations and strength: e.g. isoprene 2 4 CH More rigid 3 Van der Waals, H 1 C CH2 H2C C H 3 trans-isoprene

Direction of increasing strength trans-1,4-polyisoprene trans-1,2-polyisoprene 3,4-polyisoprene Adapted from Fig. 14.7, Callister 6e.

CH H 3 H 2 H 2 C 2 C C C Short branching C C C n H H H C n 2 H C n 3 CH C H C CH3 2 H2C Long branching Note: there are also cis-1,4- and cis-1,2-polyisoprene Star branching Dendrimers

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7 Molecular Structure

• Random, Alternating, Blocked, and Grafted How do crosslinking and branching occur in polymerization?

• Synthetic rubbers are often copolymers. 1. Start with or add in monomers that have more than 2 sites that bond with other monomers, e.g. crosslinking with divinyl benzene e.g., automobile tires (SBR) … …

stryene polystyrene

Styrene- Rubber random polymer … Control degree of + … crosslinking by -divinyl styrene benzene ratio divinyl benzene crosslinked polystyrene Monomers with trifunctional groups lead to network polymers.

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Molecular Structure Example 3 Nitrile rubber , co-poly(-butadiene), has Branching in polyethylene (back-biting)

H H H n 2000 2 2 2 Mn = 106,740g /mol n = C C C H C CH H 2 2 R C C C H2 H2 H Calculate the ratio of (# of acrylonitrile) to (# of butadiene). Same as 3 C = 3 x 12.01 g/mol H 4 C = 4 x 12.01 g/mol H 3 H = 3 x 1.008 g/mol € H H € 6 H = 6 x 1.008 g/mol C C H 1 N = 1 x 14.007 g/mol CH H CH Radical moves to a different carbon 2 2 m = 54.09 g/mol m = 53.06 g/mol 0 C CH 0 C CH 2 2 (H transfer) C 1,4-addition product R C R H H2 H H2 We need to use an Mn 106,740 mo = = = 53.57g /mol Polymerization continues from this carbon avg. monomer MW: nn 2000 m o = f1m1 + f2m2 = f1(m1 −m2) + m2 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. m −m 53.37 − 54.09 f2 0.7 f = 0 2 = = 0.7 f = 1−f = 0.3 = → 7 : 3 1 € 2 1 f 0.3 m1 −m2 53.06 − 54.09 1

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8 Vulcanization See also sect. in Chpt. 8 Molecular Weight and Crystallinity • Crosslinking in is called vulcanization, and is achieved by irreversible , usually requiring high temperatures. • Molecular weight, Mw: Mass of a mole of chains.

• Sulfur compounds are added to form chains that bond adjacent polymer backbone chains and crosslinks them. • Unvulcnaized rubber is soft and tacky an poorly resistant to wear. • Tensile strength (TS): --often increases with Mw. e.g., cis-isoprene Single bonds Stress-strain curves --Why? Longer chains are entangled (anchored) better. • % Crystallinity: % of material that is crystalline. --TS and E often increase with % crystallinity. crystalline (S)m --Annealing causes Double bonds region + (m+n) S crystalline regions to grow. (S)n % crystallinity increases. amorphous region

Adapted from Fig. 14.11, Callister 6e.

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Polymer Crystallinity Volume fraction of crystalline component.

polyethylene • Some are amorphous. Mcrystalline ρ V ρ %crystallinity 100% c c 100% c f 100% • Some are partially crystalline (semi-crystalline). = × = × = c × Mtotal ρsVs ρs • Why is it difficult to have a 100% crystalline polymer?

ρ (ρ − ρ ) Mtotal= Mcrystalline + Mamophous Using definition of volume fractions: %crystallinity = c s a × 100% € ρ (ρ − ρ ) Ms = Mc + Ma Vc Va s c a fc = fa = ρsVs = ρcVc + ρaVa Vs Vs Vc Va ρs = ρc + ρa = ρcfc + ρafa = ρcfc + ρa(1−fc ) = fc (ρc − ρa ) + ρa Vs Vs ρ = density of specimen in question € s € ρs − ρa ρa = density of totally amorphous polymer fc = ρ − ρ ρc = density of totally crystalline polymer c a €

ρc (ρs − ρa ) Substituting in fc into the original definition: %crystallinity = × 100% ρ (ρ − ρ ) € s c a

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9 Polymer Crystallinity Semi-Crystalline Polymers

Degree of crystallinity depends on processing conditions (e.g. Fringed micelle model: crystalline region embedded in amorphous region. cooling rate) and chain configuration. A single chain of polymer may pass through several crystalline regions as well as intervening amorphous regions. Cooling rate: during crystallization upon cooling through MP, polymers become highly viscous. Requires sufficient time for random & entangled chains to become ordered in viscous liquid.

Chemical groups and chain configuration:

ρs − ρa More Crystalline Less Crystalline fc = ρc − ρa Smaller/simper side groups Larger/complex side groups Linear Highly branched Crystalline volume fractions Important

Crosslinked, network € Isotactic or syndiotactic Random

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Semi-Crystalline Polymers Semi-Crystalline Polymers

Spherulites: Spherical shape composed of aggregates of chain-folded crystallites. Chain-folded model: regularly shaped platelets (~10 – 20 nm thick) sometimes forming multilayers. Average chain length >> platelet thickness.

Natural rubber

Cross-polarized light through spherulite structure of PE.

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10 Diblock copolymers Thermoplastics vs Thermosets T • Thermoplastics: Callister,rubber --little cross linking viscous Fig. 16.9 --ductile mobile liquid Tm liquid tough --soften w/heating plastic --polyethylene (#2) Tg polypropylene (#5) partially crystalline crystalline polystyrene (#6) solid solid Representative polymer-polymer phase behavior with different • Thermosets: Molecular weight architectures: Adapted from Fig. 15.18, Callister 6e. A) Phase separation with mixed --large cross linking (10 to 50% of mers) LINEAR homopolymers. --hard and brittle B) Mixed LINEAR homopolymers and --do NOT soften w/heating Tm: melting over wide range of T DIBLOCK copolymer gives depends upon history of sample surfactant-like stabilized state. --vulcanized rubber, , consequence of lamellar structure C) Covalent bond between blocks in polyester resin, phenolic resin thicker lamellae, higher Tm. DIBLOCK copolymer give T : from rubbery to rigid as T lowers F. Bates, Science 1991. microphase segregation. g

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Packing of Polymers What Are Expected Properties?

• Packing of “spherical” atoms as in ionic and metallic crystals led to • Would you expect melting of ,6 to be lower than PE? crystalline structures. O O  H  H H H nylon 6,6   ||   || polyethylene  |   |  C C − C − C − • How polymers pack depend on many factors: − N −  C − N − −  C − N − −  |   |  • long or short, e.g. long (-CH2-)n. |   |   | H H 3     • stiff or flexible, e.g. bendy C-C sp . H  H 6 H  H 4 H + • smooth or lumpy, e.g., HDPE. + + + Van der+ Waals+ bonds • regular or random + Hydrogen bonds+ € + + • single or branched € + + H H • slippery or sticky, e.g. C-H covalent (nonpolar) joined via vdW.  H O  H O   ||   || − C − C −  |   |  C C Analogy: Consider dried (uncooked) spaghetti (crystalline) vs. − N −  C − N − −  C − N − − H H  |   |  cooked and buttered spaghetti (amorphous). |   |   |     • pile of long “stiff” spaghetti forms a random arrangement. H  H 6 H  H 4 H • cut into short pieces and they align easily. a) What is the source of intermolecular€ cohesion in Nylon vs PE? Candle wax more crystalline than PE, even though same € b) How does the source of linking affect temperature? chemical nature. With H-bonds vs vdW bonds, nylon is expected to have (and does) higher melting T.

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11 What Are Expected Properties? What Are Expected Properties?

Which polymer more likely to crystallize? Can it be decided? Which polymer more likely to crystallize? Can it be decided? Networked Linear and highly crosslink Linear syndiotactic Linear isotactic polystyrene Phenol-Formaldehyde cis-isoprene (Bakelite)

+ H

+ H20

• For linear polymers, crystallization is more easily accomplished as • Networked and highly crosslinked structures are near impossible chain alignment is not prevented. to reorient to favorable alignment. • Crystallization is not favored for polymers that are composed of chemically complex mer structures, e.g. polyisoprene.

• Linear and syndiotactic polyvinyl chloride is more likely to crystallize. • The phenyl side-group for PS is bulkier than the Cl side-group for PVC. • Not possible to decide which might crystallize. Both not likely to do so. • Generally, syndiotactic and isotactic isomers are equally likely to crystallize.

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What Are Expected Properties? Detergents

Which polymer more likely to crystallize? Can it be decided? • Soap is a detergent based on animal or vegetable product, some contain alternating random water Poly(Polystyrene-Ethylene) poly(vinyl chloride-tetra-fluoroethylne) Copolymer copolymer detergent

grease

• What properties of soap molecules do you need to remove grease? • “green” end must be “hydrophilic”. Why? • Opposite end must be hydrocarbon. Why?

Water must be like oxygen (hoard • Alternating co-polymer more likely to crystallize than random ones, as they electrons and promote H-bonding) are always more easily crystallized as the chains can align more easily. e.g., oxy-clean® grease

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12 Simple polymer: Elmers glue + Borax  SLIME! Simple polymer: Elmer’s glue + Borax  SLIME!

Chemistry Elmer’s glue is similar to “poly (vinyl )” with formula: Hydrolyzed molecule acts in a condensation reaction with PVA, OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH crosslinking it.

this is a SHORT, n=15 chain of poly(vinyl alcohol) - +  B(OH)3 + 2H2O  B(OH)4 + H3O pH=9.2

Borax is sodium tetraborate decahydrate (B4Na2O7 • 10 H2O).

The borax actually dissolves to form boric acid, B(OH)3. Crosslinked This boric acid-borate solution is a buffer with a pH of about 9 (basic). Boric acid will accept a hydroxide OH- from water.

- +  B(OH)3 + 2H2O  B(OH)4 + H3O pH=9.2

Hydrolyzed molecule acts in a condensation reaction Crosslinking ties chains via weak non-covalent with PVA, crosslinking it. (hydrogen) bonds, so it flows slowly.

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Range of Bonding and Elastic Properties Summary • Polymers are part crystalline and part amorphous. Is “slime” a thermoset or , or neither? • The more “lumpy” and branched the polymer, the less dense and less crystalline. Thermoset Slime? Thermoplastic • The more crosslinking the stiffer the polymer. And, bonding bonding networked polymers are like heavily crosslinked ones. • Covalent bonds • Induced dipolar bonds • Many long-chained polymers crystallize with a Spherulite form crosslinks • H-bonds form form crosslinks crosslinks microstructure - radial crystallites separated by amorphous regions.

• Optical properties: crystalline -> scatter light (Bragg) Stiffness increases amorphous -> transparent. Most covalent molecules absorb light outside visible spectrum, Where is nylon? e.g. PMMA (lucite) is a high clarity tranparent materials.

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