4a. Polymers: A crash course

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1 Brief history Natural polymer-based materials, e.g., amber and paper (manufactured from a naturally occurring polysaccharide, cellulose), used by people for centuries, and term polymer first used in First entirely synthetic polymer, Bakelite, introduced in Despite significant advances in synthesis & characterization of polymers, understanding of polymer molecular structure did not come until 1920s. Before that, scientists believed that polymers were clusters of small molecules (called colloids), without definite molecular weights, held together by an unknown force; this concept was known as association theory. In 1922, Hermann Staudinger (previous discovery: organic molecules with taste of coffee) studied rubber and correctly proposed that polymers consist of long chains of small repeat units held together by covalent bonds. proper

2 Contemporary response? Very rough! For instance: Heinrich Wieland, 1927 Nobel laureate in chemistry, wrote to Staudinger, Dear colleague, drop the idea of large molecules; organic molecules with a molecular weight higher than 5000 do not exist. Purify your products, such as rubber, then they will crystallize and prove to be low molecular compounds! After presentation at conference in Dusseldorf he was highly criticized and got to hear: The existence of a polymer is as shocking for a chemist as the observation of a 400- meter long elephant is to a zoologist! But finally, in 1953, Staudinger received his reward for the understanding of the concept of polymers and his prolonged effort to establish the science of large molecules when he was awarded the Nobel Prize in chemistry.

3 Polymer structure 4a. Polymers: A crash course Polymers made by chemical reaction (e.g. free-radical polymerization) between monomers: long (linear or branched) polymer chains formed The number of repeat units in a polymer chain can be large (n ~ ) The variation of polymer properties is essentially infinite, due to huge variety of monomers. Few examples of common polymers: Polystyrene (toys, electronic housings, CDs,...) Polyethylene (insulation wires, plastic bags,...) Polyamide or nylon (clothing) Polycarbonate (clear, strong; and lighter and much higher ε r than glass: excellent thin lenses)

4 But polymer properties also depend strongly on chain configuration and packing Common synthetic polymer poly(ethylene) (PE) comes in drastically different forms: LDPE: branched chains poor packing: low density and weak material; cheap material used in plastic bags HDPE: linear chains good packing: more compact and strong material; less cheap material used in e.g. food containers UHMWPE: very long linear chains (large n): extremely strong and compact material used in bullet-proof vests &

5 even ice-hockey rinks! 4a. Polymers: A crash course

6 Polymer Morphology Polymers can either be in amorphous (disordered) state or in crystalline (ordered) state Crystalline polymers (e.g. HDPE) typically has a complex structure, which in fact is a mixture of crystalline (spherulites) and amorphous regions Amorphous polymers exhibit a glass transition (T g ) T < T g : disordered polymer chains are essentially static (low c p ) and polymer material is hard and brittle like a glass! (e.g. polystyrene & PMMA at RT) T > T g : disordered polymer chains move around (high c p ) and material is soft and flexible (e.g. LDPE and non-vulcanized polyisoprene at RT)

7 Mechanical properties PE and PS examples of thermoplastic (or pliable) polymers: can be reshaped into new forms at high T in viscous high-t state Introduction of cross-links between polymer chains non-reformable thermoset (one gigantic molecule!) E.g.: sulfur vulcanization of rubber (polyisoprene) invented (thanks to fruitful combination of accident and hard work) by Charles Goodyear in 1839; it prevents rubber tires from becoming too soft when hot and from becoming too hard and brittle when cold Number of cross-links decides whether thermoset is rubber-like (few, e.g. tires) or stiff (many, e.g. polycarbonate for lenses, epoxy glue)

8 Polymer blends Desirable to mix different polymers to attain combined properties, but very difficult since fundamental condition for mixing (and reactions in general): ΔG mix = ΔH mix TΔS mix 0 almost never fulfilled Reminder from TD: All systems spontaneously move toward state with lowest G [&, remember electrons in SCs and metals]) Reason: (i) Low entropy gain upon mixing of two polymers, (ii) it typically costs enthalpy to mix two dissimilar materials together ΔG mix > 0 polymer blends often phase separate

9 Polymer blends The extent of phase separation often depend on very subtle differences: * R 1 R 2 * n + * O n * + Li + (0.68 Å) K + (1.33 Å) Rb + (1.47 Å) O O S O F F F It is possible to use surfactants or kinetics to minimize phase separation Or phase separation is desirable: E.g. carbonated drink bottles contain laminated sheets of non-mixable PET (strength) & PVA (no CO 2 diffusion)

10 Copolymers So far homopolymers (one monomer), but combined & new types of properties attained from design and synthesis of copolymer (CP): 2 (or more) monomers joined together Alternating CP = monomers in alternating fashion Random CP = monomers in random order Block CP = monomers joined together in blocks E.g.: polymer-electrolyte-block CP with (i) soft block (RT > T g ) providing ion-transport & (ii) hard block (RT < T g ) providing dimensional stability to prevent short circuit Also possible to attain new and desired properties (e.g. solubility & new emission color ~E g ) by attaching pendant side groups to the main chain: * * n * R 1 R 2 * n

11 Natural polymers: The key to life! Only synthetic polymers up to now, but the world is full of very important and inspirational natural polymers, including the most advanced material : Deoxyribonucleic acid (DNA) Fascinating double-helix structure first suggested by Crick & Watson (with aid of others in 1953): 2 phosphate-sugar-based co-polymers forming helical supermolecule via hydrogen bonding between pendant base groups, adenine (A), cytosine (C), guanine (G) and thymine (T) Base groups can only pair up in certain order: A+T, T+A, C+G and G+C Which is the origin to our genetic code = who we are!

12 Deoxyribonucleic acid (DNA) Cell duplication Via action of various enzymes (e.g. polymerase): 2 polymer chains unzip and new appropriate bases (and sugar and phosphate groups) are added to each single chain; the result is 2 identical DNA molecules! Protein synthesis A gene is a part of a DNA strand that codes for one specific protein. This gene is copied and info brought to desired place by RNA polymers (in transcription step) The genetic code is divided into codons: series of 3 bases (e.g. ACT). Total number of different codons: 4 3 = 64 One (or more) codon(s) one specific (out of 20) amino acids During actual synthesis of protein polymer (the translation step), amino acids (R ) are linked together via peptide linkages in a specific order dictated by the genetic code

13 Proteins Highly versatile polypeptides (or polyamides), containing a specific order of the 20 different amino acids (defined by R). Many important functions; a few examples: Enzymes (e.g. cellulase, polymerase) used by all living organisms to speed up reactions Antibodies that bind to and neutralize specific antigens, such as bacteria and viruses Oxygen-carrying units in blood stream (hemoglobin) Structural units: skin, hair, fingernails, wool, fur, silk (keratin) tendons, bone, teeth and again skin (collagen) Proteins inspired Wallace Carothers (who actually wanted to prove that polymers were indeed macromolecules) to invent comparatively mundane, but very useful, nylon in 1935 Flat molecule that form strong fibers used for clothing

14 All these cellular activities require energy; provided during cell respiration (i.e. oxidation of glucose): C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O + Energy Input chemicals -- glucose and oxygen -- produced by green plants, when certain green pigments (chlorophylls) absorb sunlight during photosynthesis: 6CO H 2 O + light energy C 6 H 12 O 6 + 6O 2 + 6H 2 O Significant portion of produced glucose (some is consumed by plant in cell respiration during night) is stored in complex carbohydrates in the form of repeat units in two polymers with nominally identical structure (isomers): cellulose and starch

15 Starch: soft polysaccharide (PS) containing glucose repeat units, dissolves in hot water, digestible by humans Cellulose: crystalline and strong PS containing glucose repeat units, not soluble in water, only digestible by animals that posses the enzyme cellulase But what s the difference? A. Glucose repeat units are pointing in different directions! Small subtle difference can make all the difference in biological polymers; also remember that mutations (mistakes in DNA replication) can cause cancer, and that mistakes during protein folding can cause other several serious diseases, such as mad cow disease and Alzheimer s disease

16 Synthetic polymer appeal: Up to mid-1970s, synthetic polymers attracted interest primarily because of an appealing set of mechanical properties: Easy to process into desired shapes or functions from solution and melt Light-weight (typically based on light elements: C, H, O, N,...) Strong (can be comparable to steel) Flexible ( plastic ) Also important: Infinitely rich chemistry enormous variety of materials Cheap production (in large quantities): simple process and common raw materials However, no interest in electronic properties of polymers (instead opposite since lack thereof made them useful as electrical insulation), but more on this soon-to-be change in next lectures