PolymerPolymer2 Categories-2 Categories-

–Naturally occurring polymers which Naturally occurring polymers which derives from plants and animals: wood, derives from plants and animals: wood, natural rubber, cotton, silknatural rubber, cotton, silk–Other natural polymer-proteins, Other natural polymer-proteins, enzymes, starches, cellulose enzymes, starches, cellulose –The other category is synthetic The other category is synthetic polymers i.e. plastics, synthetic rubbers, polymers i.e. plastics, synthetic rubbers, and fibrous materialsand fibrous materials–Properties of polymers are intricately Properties of polymers are intricately related to the structural elements of related to the structural elements of materialsmaterials

Hydrocarbon MoleculesHydrocarbon Molecules Most polymeric materials are composed of Most polymeric materials are composed of

very large molecules-mostly composed of very large molecules-mostly composed of hydrogen and carbonhydrogen and carbon

Intramolecular bonds are covalentIntramolecular bonds are covalent Saturated hydrocarbon-all bonds are singleSaturated hydrocarbon-all bonds are single These molecules have strong covalent These molecules have strong covalent

bond but weak hydrogen & van der Waals bond but weak hydrogen & van der Waals bonds between moleculesbonds between molecules

Unsaturated hydrocarbon-have double & Unsaturated hydrocarbon-have double & triple covalent bond. Possibility of other triple covalent bond. Possibility of other atom or group of atoms to attached to atom or group of atoms to attached to original moleculeoriginal molecule

Polymer MoleculesPolymer Molecules

Polymers are referred as Polymers are referred as macromolecules due to their giant sizemacromolecules due to their giant size

The molecules are in the form of The molecules are in the form of long& flexible chainslong& flexible chains

The backbone consist from string of The backbone consist from string of carbon atoms, many times each carbon atoms, many times each carbon atom singly bonds to two carbon atom singly bonds to two adjacent carbon atoms on either sideadjacent carbon atoms on either side

• Polymer = many mers

• Mer originates from meros:parts, thus, polymer means many parts or many mers

• Monomer refers to stable molecule where the polymer is synthesized

C C C C C CHHHHHH

HHHHHH

Polyethylene (PE)

mer

ClCl Cl

C C C C C CHHH

HHHHHH

Polyvinyl chloride (PVC)

mer

Polypropylene (PP)

CH3

C C C C C CHHH

HHHHHH

CH3 CH3

mer

Adapted from Fig. 14.2, Callister 6e.

The Chemistry of Polymer Molecules

Consider ethylene(C2H4) gas at ambient temperature& pressure.

How this hydrocarbon monomer transformed into a polymer?

The process happens when the initiator/catalyst react with the ethylene mer unit

(a) (b)

•The polymer chain then forms by the sequential addition of polyethylene monomer units to this active initiator-mer center.

•The active site or unpaired electron is transfered to each successive end monomer. The final result is the polyethylene molecules

Other TermsOther Terms Homopolymer-when all the repeating units Homopolymer-when all the repeating units

along a chain are of the same typealong a chain are of the same type

Copolymers-Polymers’ chains may be Copolymers-Polymers’ chains may be composed of two or more different mer unitscomposed of two or more different mer units

Bifunctional-they have 2 active bonds which Bifunctional-they have 2 active bonds which form 2 dimensional molecular structureform 2 dimensional molecular structure

Trifunctional-have 3 active bonds, which Trifunctional-have 3 active bonds, which results in 3 dimensional molecular structureresults in 3 dimensional molecular structure

MOLECULAR WEIGHTMOLECULAR WEIGHT– During polymerization, not all polymer During polymerization, not all polymer

chains have a same length which chains have a same length which results in distribution of molecular results in distribution of molecular weightsweights

– An average molecular weight is An average molecular weight is specified from various physical specified from various physical propertiesproperties

– The no. average molecular weightThe no. average molecular weight

MMnn==ΣΣxxiiMMii

– The weight average molecular weightThe weight average molecular weight

MMww==ΣΣwwiiMMii

Another alternative way:Another alternative way:– Through degree of polymerization n, which Through degree of polymerization n, which

represents the average number of mer units in represents the average number of mer units in a chain.a chain.

– Number average degrees of polymerization Number average degrees of polymerization (n(nnn),),

nnnn =M =Mnn/m/m

– Weight average degrees of polymerization (nWeight average degrees of polymerization (nww),),

nnww =M =Mww/m /m

where m is the mer molecular weightwhere m is the mer molecular weight– For a copolymer, m is determined from For a copolymer, m is determined from

m= m= Σ Σ ffjjmmjj

fj=chain fraction

mj=molecular weight of mer

Computation of Average Molecular Computation of Average Molecular Weights & Average Degrees of Weights & Average Degrees of PolymerizationPolymerization– Assume that the molecular weight Assume that the molecular weight

distributions are for poly (vinyl chloride):distributions are for poly (vinyl chloride):

Compute: a) the no. average molecular weight

b) the no. average degree of polymerization

c) the weight average molecular weight

(a) (b)

a) The data taken is from (a) and presented in the above table, the summation of all xiMi products yields the no. average molecular weight, which is 21,150 g/mol

b) No average degree of polymerization is computed from mer molecular weight. For PVC each mer has-2 carbon atoms, 3 hydrogen atoms, 1 chlorine atoms.

– Atomic weights: C= 12, H= 1, Cl=35.5, thus Atomic weights: C= 12, H= 1, Cl=35.5, thus

m= 2(12 g/mol)+3(1 g/mol)+35.5 g/molm= 2(12 g/mol)+3(1 g/mol)+35.5 g/mol

= 62.5 g/mol= 62.5 g/mol

&&

nnnn =M =Mnn/m = (21,150 g/mol)/(62.5 g/mol)/m = (21,150 g/mol)/(62.5 g/mol)

= 338= 338

(c)(c)

The data taken is from (b), the summation of all wiMi products yields the weight average molecular weight, which is 23,2000 g/mol

MOLECULAR SHAPEMOLECULAR SHAPE– Polymer chain molecules are illustrated to be Polymer chain molecules are illustrated to be

straight for easy understandingstraight for easy understanding– A more accurate model is shown with 109A more accurate model is shown with 109oo

between carbon atoms that forms a zigzag between carbon atoms that forms a zigzag patternpattern

Figure (b) shows a straight chain segment resulted when the chain atoms are well positioned

– Chain bending & twisting are possible when Chain bending & twisting are possible when there is chain atoms rotation into other there is chain atoms rotation into other positions as depicted in (c) positions as depicted in (c)

– This could result as in picture below:This could result as in picture below:

– Polymers consist of large no. of molecular Polymers consist of large no. of molecular chains, each of which may bend, coil & kink chains, each of which may bend, coil & kink as in the pictureas in the picture

– The shape of the polymer chain molecule are The shape of the polymer chain molecule are responsible for a number of important responsible for a number of important characteristics characteristics

– Some mechanical & thermal characteristics of Some mechanical & thermal characteristics of polymer are a function of the ability of the polymer are a function of the ability of the chain segments to experience rotation in chain segments to experience rotation in response to applied stress or thermal vibrationsresponse to applied stress or thermal vibrations

– Rotational flexibility is dependent on mer Rotational flexibility is dependent on mer structure& chemistrystructure& chemistry

– For example: The region of a chain segment For example: The region of a chain segment that has a double bond (C=C) is rotationally that has a double bond (C=C) is rotationally rigidrigid

– Also, the introduction of a bulky or large side Also, the introduction of a bulky or large side group of atoms restricts rotational movementgroup of atoms restricts rotational movement

MOLECULAR STRUCTUREMOLECULAR STRUCTURE

•Four general molecular structures:

•Linear polymers

•Branched polymers

•Cross-linked polymers

•Network polymers

Linear PolymersLinear Polymers

– The mer units are joined together end to end The mer units are joined together end to end in single chains represents as a mass of in single chains represents as a mass of spaghettispaghetti

– May have extensive van der Waals & May have extensive van der Waals & hydrogen bonding between chainshydrogen bonding between chains

– Examples: PE, PVC, polystyrene, PMMAExamples: PE, PVC, polystyrene, PMMA

Branched PolymersBranched Polymers

– Side branch chains are connected to the Side branch chains are connected to the backbonebackbone

– The branches resulted from side reactions The branches resulted from side reactions that occur during the polymer synthesisthat occur during the polymer synthesis

– The chain packing efficiency is reduced The chain packing efficiency is reduced by the formation of side branches which by the formation of side branches which lowers the polymer densitylowers the polymer density

Cross-linked PolymersCross-linked Polymers

– Adjacent linear chains are joined one to another Adjacent linear chains are joined one to another at various positions by covalent bondsat various positions by covalent bonds

– Crosslinking process is achieved either by Crosslinking process is achieved either by synthesis or nonreversible chemical reaction synthesis or nonreversible chemical reaction carried out at elevated temperaturecarried out at elevated temperature

– Is accomplished by additive atoms or molecules Is accomplished by additive atoms or molecules covalently bonded to the chainscovalently bonded to the chains

Network PolymersNetwork Polymers

– Trifunctional mer units, having 3 active covalent Trifunctional mer units, having 3 active covalent bonds to form 3-D networksbonds to form 3-D networks

– A polymer that is highly crosslinked maybe A polymer that is highly crosslinked maybe classified as a network polymerclassified as a network polymer

– These materials have distinctive mechanical & These materials have distinctive mechanical & thermal propertiesthermal properties

– Polymers can have more than one distinctive Polymers can have more than one distinctive structural typestructural type

MOLECULAR CONFIGURATIONSMOLECULAR CONFIGURATIONS– Consider the mer unit where R represents an atom or side group other than H (Cl, CH3)

– A possible arrangement when the R side groups of successive mer units bond to alternate carbon atoms. This is designated as a head -to-tail configuration

– Its complement the head-to-head configuration occurs when R groups bond to adjacent chain atoms

ISOMERISMISOMERISM--polymer molecules with same polymer molecules with same composition but different atomic arrangementscomposition but different atomic arrangements

STEREOISOMERISMESTEREOISOMERISME--denotes the denotes the

situation insituation in which atoms are linked together in which atoms are linked together in the same order (head-to-tail) but different in their the same order (head-to-tail) but different in their spatial arrangementspatial arrangement

– ISOTACTIC CONFIGURATIONISOTACTIC CONFIGURATION

All the R groups are situated on the same side All the R groups are situated on the same side of the chainof the chain

– Syndiotactic ConfigurationSyndiotactic Configuration

The R groups alternate sides of the chainThe R groups alternate sides of the chain

– Atactic ConfigurationAtactic Configuration

Random positioning of R groupsRandom positioning of R groups Conversion of one stereoisomer to another will Conversion of one stereoisomer to another will

involve severing of bonds reformation after involve severing of bonds reformation after appropriate rotationappropriate rotation

GEOMETRICAL ISOMERISMGEOMETRICAL ISOMERISM– Also possible within mer units having double bond Also possible within mer units having double bond

between chain carbon atomsbetween chain carbon atoms– Bonded to the carbon atoms participating with Bonded to the carbon atoms participating with

double bond is a single sided bonded atom or radicaldouble bond is a single sided bonded atom or radical

– Let’s say the CH3 group & the H atom on the same Let’s say the CH3 group & the H atom on the same side. This is a CIS structureside. This is a CIS structure

– For the trans structure, the CH3 & H reside on For the trans structure, the CH3 & H reside on opposite chain sides. This isomerism also can’t be opposite chain sides. This isomerism also can’t be convert by simple rotation because the chain double convert by simple rotation because the chain double is too rigidis too rigid

COPOLYMERSCOPOLYMERS

a)Random copolymer-two different units are randomly dispersed along the chain

b)Alternating copolymer-the 2 mer units alternate chain positions

c) Block copolymer-the identical mers are clustered in blocks along the chain

d) Graft copolymer-homopolymer side branches of one type maybe grafted to homopolymer main chains that are composed of a different mer

THERMOPLASTIC & THERMOSETTING THERMOPLASTIC & THERMOSETTING POLYMERSPOLYMERS– Classification scheme based on the materials behavior Classification scheme based on the materials behavior

with rising temperaturewith rising temperature– Thermoplastic polymer have linear & branched structures; Thermoplastic polymer have linear & branched structures;

they soften when heated & harden when cooled.they soften when heated & harden when cooled.– The process reversible & can be repeatedThe process reversible & can be repeated– It is not reversible if the temperature is raised to a point It is not reversible if the temperature is raised to a point

where molecular vibrations are violent enough to break where molecular vibrations are violent enough to break the primary covalent bondsthe primary covalent bonds

– Thermosets polymer, once having hardened, will not Thermosets polymer, once having hardened, will not soften upon heating, their structures are cross-linked & soften upon heating, their structures are cross-linked & network. network.

– At initial heat treatment, covalent cross-links are formed At initial heat treatment, covalent cross-links are formed between adjacent molecular chains to anchor the chains between adjacent molecular chains to anchor the chains together which resist vibrations & rotations at high together which resist vibrations & rotations at high temperaturetemperature

– Crosslinking is usually extensiveCrosslinking is usually extensive– Thermoset polymers are harder & stronger than Thermoset polymers are harder & stronger than

thermoplasticsthermoplastics

POLYMER CRYSTALLINITYPOLYMER CRYSTALLINITY Atomic arrangement in polymer crystals is

more complex than in metals or ceramics (unit cells are typically large and complex).

More crystallinity: higher density, more strength, higher resistance to dissolution and softening by heating!

Polyethylene

Degree of crystallinity is determined by:

Rate of cooling during solidification: time is necessary for chains to move and align into a crystal structure

Mer complexity: crystallization less likely in complex structures, simple polymers, such as polyethylene,crystallize relatively easily

Chain configuration: linear polymers crystallize relatively easily, branches inhibit crystallization, network polymers almost completely amorphous, crosslinked polymers can be both crystalline and amorphous

Isomerism: isotactic, syndiotactic polymers crystallize relatively easily - geometrical regularity allows chains to fit together, atactic difficult to crystallize

Copolymerism: easier to crystallize if mer arrangements are more regular - alternating, block can crystallize more easily as compared to random and graft

How to calculate crystallinity?

POLYMER CRYSTALSPOLYMER CRYSTALS

Polymer molecules are often partially crystalline (semicrystalline), with crystalline regions dispersed within amorphous material.

Fringed-micelle model

Crystalline

Amorphous

Recently : Studies focus on Polymer single crystals grown from dilute solution

Each platelet will consist of a number of molecule; however the average chain length is much greater than the thickness of the crystallite

Spherulites: Aggregates of lamellar crystallites ~ 10 nmthick, separated by amorphous material. Aggregatesapproximately spherical in shape.

Photomicrograph ofspherulite structureof polyethylene

Schematic representation of the detailed

structure of a sperulite.

Polymers Application in Biomedical Polymers Application in Biomedical EngineeringEngineering

The structures of polymers determine their utilization The structures of polymers determine their utilization in various medical domain i.e.: surgery, dermatology, in various medical domain i.e.: surgery, dermatology, ophthalmology pharmacyophthalmology pharmacy

Popular thermoplastic polymers for biomaterials: Popular thermoplastic polymers for biomaterials: polyolefin, Teflon® (fluorinated hydrocarbons), poly polyolefin, Teflon® (fluorinated hydrocarbons), poly (methyl methacrylate) (PMMA), poly (hydroxyethyl (methyl methacrylate) (PMMA), poly (hydroxyethyl methyacrylate) (PHEMA, Hydron ®), polyvinyl chloride methyacrylate) (PHEMA, Hydron ®), polyvinyl chloride (PVC), polycarbonate, nylon, polyester (Dacron ®)(PVC), polycarbonate, nylon, polyester (Dacron ®)

Popular thermosettings polymers for biomaterials: Popular thermosettings polymers for biomaterials: butyl rubber, chlorosulfonated polyethylene (Hypalon butyl rubber, chlorosulfonated polyethylene (Hypalon ®), epichlorohydrin rubber (Hydrin ®), polyurethane ®), epichlorohydrin rubber (Hydrin ®), polyurethane (Biomer ® etc.,) natural rubber, & silicon rubber (Biomer ® etc.,) natural rubber, & silicon rubber (Silastic ®).(Silastic ®).

Polymer Specific Properties Biomedical UsesPolyethylene Polyethylene

PolypropylenePolypropylene

TetrafluoroethyTetrafluoroethylenelene

Low cost, easy to process, Low cost, easy to process, excellent electrical excellent electrical insulator, excellent insulator, excellent chemical resistance, tough chemical resistance, tough & flexible even at low & flexible even at low temperaturetemperature

Excellent chemical Excellent chemical resistance, weak resistance, weak permeability to water permeability to water vapors, good vapors, good transparency & surface transparency & surface reflectionreflection

Chemical inertness, Chemical inertness, exceptional weathering & exceptional weathering & heat resistance, heat resistance, nonadhesive, very low nonadhesive, very low coefficient of frictioncoefficient of friction

Tubes for various Tubes for various catheters, hip joint, catheters, hip joint, knee joint prosthesesknee joint prostheses

Yarn for sutures, Yarn for sutures, surgerysurgery

Vascular & auditory Vascular & auditory prostheses, prostheses, catheters, tubescatheters, tubes

PVCPVC Excellent resistance Excellent resistance to abrasion, good to abrasion, good dimensional stability, dimensional stability, high chemical high chemical resistanceresistance

Flexible or semiflexible Flexible or semiflexible medical tubes, catheter, medical tubes, catheter, inner tubes, components of inner tubes, components of dialysis installation & dialysis installation & temporary blood storage temporary blood storage devicedevice

PolyacetalPolyacetalss

PMMAPMMA

Stiffness, fatigue Stiffness, fatigue endurance, endurance, resistance to creep, resistance to creep, excellent resistance excellent resistance to humidity, gas & to humidity, gas & solvent actionsolvent action

Optical properties, Optical properties, exceptional exceptional transparency, easy transparency, easy thermoformation & thermoformation & weldingwelding

Hard tissue replacementHard tissue replacement

Bone cement, intraocular Bone cement, intraocular lenses, contact lenses, lenses, contact lenses, fixation of articular fixation of articular prostheses, denturesprostheses, dentures

PolycarbonPolycarbonateate

Rigidity & toughness Rigidity & toughness up to 140up to 140oo, , transparency, good transparency, good electrical insulator, electrical insulator, physiological physiological inertnessinertness

Syringes, arterial tubules, Syringes, arterial tubules, hard tissue replacementhard tissue replacement

PolyethylenPolyethylene e terephtalatterephtalatee

PolyamidePolyamide

PolyurethaPolyurethane ne

Silicone Silicone rubberrubber

Transparency, good Transparency, good resistance to traction resistance to traction & tearing, resistance & tearing, resistance to oils, fats, organic to oils, fats, organic solventsolvent

Very good Very good mechanical mechanical properties, good properties, good thermal properties, thermal properties, good chemical good chemical resistance, resistance, permeable to gasespermeable to gases

Exceptional resistance Exceptional resistance to abrasion, resistance to abrasion, resistance to breaking, very high to breaking, very high elasticity modulus at elasticity modulus at compression traction & compression traction & sheering remarkablesheering remarkable

Good thermal Good thermal stability, resistance to stability, resistance to atmospheric & atmospheric & oxidative agents, oxidative agents, physiological physiological inertnessinertness

Vascular, laryngeal, Vascular, laryngeal, esophageal prostheses, esophageal prostheses, surgical sutures, knitted surgical sutures, knitted vascular prosthesesvascular prostheses

Tubes for intracardiac Tubes for intracardiac catheters,surgical sutures, catheters,surgical sutures, dialysis devices dialysis devices components,heart mitral components,heart mitral valves, suturesvalves, sutures

Adhesives, dental Adhesives, dental materials, blood pumps, materials, blood pumps, artificial heart & skinartificial heart & skin

Burn treatment, shunt, Burn treatment, shunt, mammary prostheses, mammary prostheses, maxillofacial implantsmaxillofacial implants

Polyethylene & Polypropylene Polyethylene & Polypropylene – Ultra High Molecular Weight Poly (Ethylene) is used to Ultra High Molecular Weight Poly (Ethylene) is used to

fabricate acetabular cups in artificial hips, bearing fabricate acetabular cups in artificial hips, bearing surface of some knee prostheses, blood contacting tubesurface of some knee prostheses, blood contacting tube

– Polypropylene (Prolene ®) sutures are widely used Polypropylene (Prolene ®) sutures are widely used clinicallyclinically

UHMWPE acetabular cup

(From -http://economy.mse.uiuc.edu/figure/hyq2.gif)

Prolene suture (blue color)

(From-http://www.constipated.com/Chapters/13_how_surg.html)

Perfluorinated PolymersPerfluorinated Polymers– PTFEPTFE

For a heart valve, it serves as a sewing For a heart valve, it serves as a sewing ring/ receptor for suturesring/ receptor for sutures

Other application- shunts to carry Other application- shunts to carry cerebral spinal fluid from hydrocephalic cerebral spinal fluid from hydrocephalic patientpatient

Middle ear drain tubes, suturesMiddle ear drain tubes, sutures

Heart valve

(From-http://www.ctsnet.org/stjude/product/26)

Acrylic PolymersAcrylic Polymers– PMMAPMMA– Used for contact lenses, bone cement, Used for contact lenses, bone cement,

dentures, maxillofacial prosthesesdentures, maxillofacial prostheses

PMMA contact lenses

(From-

http://www.sterlingint.com/images/lens.jpg)

 AP fluoroscopy of completed vertebroplasty with polymerized polymethylmethacralate (PMMA) bone cement in place. -Bassewitz al, 2000

PolyurethanesPolyurethanes– Vascular tubes, artificial heart assist Vascular tubes, artificial heart assist

devicesdevices

Trileaflet of artificial heart(Frommembers.evansville.net/ict/angioflex-200.jpg )

Polyurethane Intra-Vascular Catheter Tubing (Fromwww.harvardapparatus.com)

PolyamidesPolyamides– Applications in intracardiac catheters, Applications in intracardiac catheters,

components in dialysis device, suturescomponents in dialysis device, sutures

Silicone RubberSilicone Rubber– The most widely used is polydimethyl-The most widely used is polydimethyl-

siloxanesiloxane– Catheters, silicone prostheses: finger, Catheters, silicone prostheses: finger,

toe, mammary, maxillofacial surgerytoe, mammary, maxillofacial surgery