Understanding Polymer Structure and Properties

POLYMER STRUCTURE D. JAGAN MOHAN New Technology Research Centre University of West Bohemia Plzen, Czech Republic

Polymers Polymers consist of long chains, which are composed of simple structural units (mers) strung together. “poly’’ = many mer mer mer mer mer mer mer mer mer mer Polymers – Natural and Synthetic chain-growth (addition) Synthetic polymers step-growth (condensation)

Molecular characteristics Size (Molecular Weight) Shape (chain twisting, entanglement etc.) Chemistry (polymer composition) Structure Linear Branched Cross linked Network Isomeric states Stereoisomers Geometrical isomers Cis Trans Isotactic Atactic Syndiotactic

Natural and Synthetic Rubber Natural rubber is too soft to be used in most applications. When natural rubber is stretched, the chains become elongated and slide past each other until the material pulls apart. In 1939, Charles Goodyear discovered that mixing hot rubber with sulfur produced a stronger more elastic material. This process is called vulcanization. disulfide bond Vulcanization results in cross-linking of the hydrocarbon chains by disulfide bonds. When the polymer is stretched, the chains no longer can slide past each other, and tearing does not occur. disulfide bond disulfide bond Vulcanized rubber is an elastomer, a polymer that stretches when stressed but then returns to its original shape when the stress is alleviated.

Chain-growth polymers (Addition) Prepared by chain reactions. Monomers are added to the growing end of a polymer chain. Ex: conversion of vinyl chloride to poly(vinyl chloride)   vinyl chloride Poly(vinyl chloride) Monomer Polymer

Step-growth polymers (Condensation) Step-growth polymers are formed when monomers containing two functional groups come together and lose a small molecule such as H2O or HCl. In this method, any two reactive molecules can combine, so that monomer is not necessarily added to the end of a growing chain. Step-growth polymerization is used to prepare polyamides, polyurethanes, polycarbonates and polyesters. Monomers  Polymer  Nylon 6,6 HCl

Molecular Structure • Physical properties of polymers depend not only on their molecular weight/shape, but also on the difference in the chain structure • Structures Network Linear Cross-linked Branched

Linear Polymers These are polymers in which monomeric units are linked together to form linear chain. These linear polymers are well packed and have high magnitude of intermolecular forces of attraction and therefore have high densities, high tensile (pulling) strength and high melting points. Some common example of linear polymers are high density polyethylene nylon, polyester, PVC, PAN etc. Polymerization by opening of Double bonds Ethylene mer units Polyethylene Chain

Branched Polymers Polymer chains can branch : • Monomers are joined to form long chains with side chains or branches of different lengths. • Irregularly packed and therefore, they have low tensile strength, low density, boiling point and melting points than linear polymers. • These branches are usually a result of side-reactions during the polymerization of the main chain • Some common examples are low density polythene, glycogen, starch etc. (Amylopectin).

Cross-linked Polymers Polymer chain Crosslink Polymer chain • A cross-link is a bond that links one polymer chain to another (Covalent or Ionic bonds). • Monomers unit are crosslinked together to form a three dimensional network polymers. • Materials often behave very differently from linear polymers • Many “rubbery” polymers are crosslinked to modify their mechanical properties; in that case it is often called vulcanization • Generally, amorphous polymers are weak and cross-linking adds strength: vulcanized rubber is polyisoprene with sulphur cross-links:

Network Polymers • Polymers that are “trifunctional” instead of bifunctional • There are three points on the mer that can react • This leads to three-dimensional connectivity of the polymer backbone • Highly crosslinked polymers can also be classified as network polymers • Examples: epoxies, phenol-formaldehyde polymers

( CH2 CH )n Polyethylene Cl Teflon ( CF2 CF2 )n ( CH2 CH2 )n PVC Homopolymer…. ….. is a polymer made up of only one type of monomer

( CH CH2CH2 CH CH CH2 )n Copolymer … …. is a polymer made up of two or more monomers Styrene-butadiene rubber

Copolymers two or more monomers polymerized together A B Why? If monomer A has interesting properties, and monomer B has (different) interesting properties, making a “mixture” of monomers should lead to a superior polymer Alternating A and B alternate in polymer chain large blocks of A units alternate with large blocks of B units Block Random A and B randomly positioned along chain Graft chains of B units grafted onto A backbone

Isomerism • compounds with same chemical formula can have quite different structures • Ex: Octane C8H18 • 2,4 Dimethyl hexane Isomerism – compounds of the same chemical composition but different atomic arrangements (i.e. bonding connectivity)

Stereoisomers of Polymers Polymers that have more than one type of side atom or group can have a variety of configurations Stereoisomerism Atactic Isotactic Syndiotactic

Isotactic • All of the R groups are on the same side of the chain Isotactic polymers are usually semicrystalline and often form a helix configuration.

Syndiotactic R group occupies alternate side of chain

Atactic • R group occupies random side of chain • Polymers that are formed by free-radical mechanisms such as polyvinylchloride are usually atactic. Due to their random nature atactic polymers are usually amorphous

Geometrical Isomers trans trans-isoprene cis cis-isoprene H atom and CH3 groupon same side of chain H atom and CH3 group on opposite sides of chain

Cis-1,2-dibromoethane Trans 2 butene • Alkenes cannot have cis-trans isomers if a carbon atom in the double bond is attached to identical groups. identical No Cis-Trans

O O C C H N Ar' NH Ar O O 2 2 C C O O O 0~5 C 2hr, 12hr at RT DMF O O ] [ Ar' NH C C N H Ar C C H O O H O O Poly(amic acid) O 250C, 4hr -H 0 2 O O C C ] [ Ar' N Ar N C C O O Polyimide Synthesis of Polyimides Several methods are possible to prepare polyimides: Reaction between a dianhydride and a diamine Reaction between a dianhydride and a diisocyanate Applications • Membranes • Aerospace • Telecommunication • Space applications • Photolithography • House hold materials, etc.

O O O Ar' O H H N C NH C 2 C Ar' NH H O NH C 2 O O Synthesis of Polyamide-imides Ar DMF 0~5 C 2 hr, 6 hr at RT (Diamine) (Anhydride) Diamine amic acids Ar (Acid chloride) Ar Poly(amide amic acid)s

O O [ Ar' O H C HN NH C ] Ar'' C Ar' NH C C H O C NH O O O O Poly(amide amic acid) to Polyamide-imides Ar Poly(amide amic acid)s Solid 250C, 4hr –H2O Poly(amide imide)s Ar Amide group Imide group

Formation of a polyamide

Formation of a polyamide + H2O

Formation of a polyamide + H2O + H2O

Formation of a polyamide + H2O + H2O + H2O

Formation of a polyamide A polyamide “backbone” forms with R groups coming off. This protein is built with amino acids.

H H O O +H3N C C O-++H3N C C O- R2 R1 H H O O +H3N C C N C C O- + H2O H R1 R2 Proteins Amino acidsare the basic structural units of proteins. An amino acid is a compound that contains at least one amino group (-NH2) and at least one carboxyl group (-COOH) General structure of an amino acid R is the only variable group Monomers: 20 essential amino acids Peptide bond

Biodegradable polymers A biodegradable polymer is a polymer that can be degraded by microorganisms—bacteria, fungi, or algae—naturally present in the environment. Several biodegradable polyesters have now been developed [e.g., polyhydroxyalkanoates (PHAs), which are polymers of 3-hydroxybutyric acid or 3-hydroxyvaleric acid]. PHA 3-hydroxy carboxylic acid R = CH3, 3-hydroxybutyric acid Polyhydroxyalkanoate R = CH2CH3, 3-hydroxyvaleric acid PHAs can be used as films, fibers, and coatings for hot beverage cups made of paper. Bacteria in the soil readily degrade PHAs, and in the presence of oxygen, the final degradation products are CO2 and H2O

Plasticizers If a polymer is too stiff and brittle to be used in practical applications, low molecular weight compounds called plasticizers can be added to soften the polymer and give it flexibility. The plasticizer interacts with the polymer chains, replacing some of the intermolecular interactions between the polymer chains. • Since plasticizers are more volatile than the high molecular weight polymers, they slowly evaporate making the polymer brittle and easily cracked. Plasticizers like dibutyl phthalate that contain hydrolysable functional groups are also slowly degraded by chemical reactions. dibutyl phthalate

Conclusion Natural and Synthetic Polymers Homopolymers Copolymers- Alternating, block, random and graft Stereoisomers of Polymers- Isotactic, syndiotactic and atactic Geometrical Isomers – cis and trans Synthesis of Polyamide, polyimides, poly(amide imides)s. Biodegradable polymers, Plasticizers, Proteins etc

Thank You

Understanding Polymer Structure and Properties