Molecular architecture

Polymer properties depend on molecular architecture (the structure of the molecules) and

the physical state of the polymer.

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HOW DO WE DEFINE MACROMOLECULAR ARCHITECTURE?

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Macromolecular architecture

• Constitution: type of atoms in the chain (backbone), type of side groups/branch groups, type of end groups, monomer sequence, molecular weight distribution

• Configuration: arrangement of neighboring atoms along the backbone or chain segments

• Conformation: the arrangement of the chain in space

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Constitution and configuration are usually set in the polymerization and/or blending processes.Conformation is a product of the polymer’s environment.

CASE STUDY: LDPE

BR-PE.aviBR-PE_2.avi1_butene.avi1_butene_2.avi

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Low density polyethylene

• The high pressure synthesis of LDPE via free radical reactions was one of the first commercial processes at supercritical conditions for the solvent (ethylene is near or above Tc).

• Constitution: polymerized from ethylene monomer in a process initiated by free radicals. Some oxygen in the monomer accelerates the process. Other free radical initiators can be used.

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Constitution

Low density polyethylene

• Statistically, local rearrangements of the chain near the free radical chain end results in short chains being formed (15-25 per 1000 monomer units; 2 – 8 carbon atoms long)

• This product has similar properties to ethylene/-olefin copolymers with 0.25 mol % -olefin.

• Some long chain branching also occurs (0.5 – 4 per 1000 monomer units long).

• Both types of branching interfere with crystallite growth and structure.

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Configuration

Low density polyethylene

• Arrangement of the backbone and side chains in 3-D. • In dilute solution, the C-C backbone will be in the zig-zag conformation,

i.e., for carbon atoms lying in the same plane.• This particular conformation represents a local state of low energy for the

chain; which can be calculated using conventional molecular dynamics methods

• Branches/side chains will disrupt the zig-zag conformation and three dimensional ‘packing’ of neighboring chains in their vicinity.

• Much of the LDPE material will form small crystallites, that can act as physical crosslinks. This means that the bulk material will be a flexible solid between Tg (Tg < -100° C; difficult to measure due to rapid crystallization) and Tm (98 < Tm < 115° C).

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Conformation

HOW DOES MOLECULAR ARCHITECTURE AFFECT PROPERTIES?

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LDPE properties

• Crystallites:– Fewer defects means higher crystallinity– Even single crystals are not 100% crystalline due to edges and corners,

and defects in the crystallite surfaces themselves.– See Table 1, Appendix B, p. 614 for different polyethylenes– Linear LDPE has ~ 80% crystallinity, Tm ~ 135 °C compared to LDPE

with 45-55% crystallinity, and a lower Tm.

• Performance:– Excellent flexibility and easy processing– Good structural strength

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Generalization

• Within one polymer family, it is possible to infer differences in performance with changes in constitution, configuration and conformation.

• It is more difficult to compare across families due to the influence of the different chemical building blocs

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SYNTHETIC POLYMERS

ConstitutionsConfigurationsHomopolymersCopolymersC, O, N are the most common elements in synthetic polymer chains Table 2.1 – elements that form long chains

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TOP 100 POLYMERS

Synthetic polymers with the highest # of literature references.This list is skewed toward thermoplastics. Thermosets and other matrix materials have greater diversity in building blocks, and therefore have fewer specific literature references.

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TOP 100/GENERAL USE

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Top 100/general use

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Top 100/general use

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TOP 100/ENGINEERING PLASTICS

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Top 100/engineering plastics

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TOP 100/THERMOSETS

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Top 100/thermosets

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TOP 100/ELASTOMERS

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Top 100/elastomers

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TOP 100/ADHESIVES

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Top 100/adhesives

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TOP 100/SPECIALTY

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Top 100/specialty

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Top 100/specialty

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Top 100/specialty

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WHERE’S THE ‘ACTION’ IN POLYMERS?

Medical application exampleCitation data – one way to look for future trendsSciFinder Scholar1985 - 2005

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POLYMER SCIENCE DIRECTIONS

Medical applications are a rich applications area for polymers.Local variations in surface roughness at the nanoscale can induce strains in cell membranes, leading to the growth of F-actin stress fibers that span the length of the cell.W.E. Thomas, D. E. Discher, V. P. Shastri, Mechanical regulation of cells by materials and tissues, MRS Bulletin, 35 (2010), 578-583.

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Cells feel their environment

• Tissues are hydrated natural polymers with controlled elasticity

• Most animals cells require adhesion to a solid to be viable

• Tissue elasticity (~ kPa’s) is important for regulating cell growth, maturation and differentiation. Brain – 0.2 < E < 1 kPa; fat – 2 < E < 4 kPa; muscle – 9 < E < 15 kPa; cartilage – 20 < E < 25; bone – 30 < E < 40 kPa

• Nanoroughness seems to affect a number of cell processes

• 3D scaffolding is important• Mechanotransduction: cells adhere to surfaces via

adhesive proteins attached to adaptor proteins, to the actomyosin cytoskeleton.

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TOPICS 1-5

There has been a major decline in publications on the physical properties of polymers, dropping from ~23% in 1985-9 to ~13% over the last two years. Plastics manufacturing, processing and fabrication has been relatively steady, but may have declined recently. Information on fundamental polymer chemistry seems to be nearly constant at 10% of the total. Information about pharmaceutical applications is increasing significantly, from 6% to ~12% recently.

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TOPICS 6-10

The synthetic high polymers category seems to be discontinued. Coatings and inks seem to be fairly steady at 2 %. There is significant growth in the areas of electric phenomena, optical, and radiation technology. These three areas together constitute ~11% of the current publication volume.

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TOPICS 11-15

There may be some renewed interest in the near term in photochemistry applications. Synthetic elastomers show some fluctuations, as do textiles and fibers. However, both surface chemistry + colloids, and biochemical methods show significant growth.

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TOPICS 16-20

Ceramics applications seem to be decline, despite that fact the ceramers are creating new interest. Cellulosics seems to be relatively steady, while general biochemistry is growing. The textile topic is now combined with fibers, and fossil fuel applications are dropping off dramatically.

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POLYMERS FOR SPECIFIC APPLICATION AREAS

Semiconductor industryBiomedical devices

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Semiconductor industry• PolyBenzImidazole• Polyimide• PolyAmide-Imide• PolyEtherImide• PolyEtherEtherKetone• Polyphenylene Sulfide• Polyvinylidene Fluoride• Ethylene-ChloroTriFluoro-

Ethylene• Ethylene-TetraFluoro-Ethylene• Polyethylene Terephthalate

• Polyfluorene derivatives• Polymer transport layers:

– Polyanaline (PANI)– Poly(3,4-

ethylenedioxythiophene)/Poly(styrenesulfonate) [PEDOT-PSS]

• Polypyrrole• Polythiophene• Polydiacetylene• Polyaryl ethers containing

Perfluorocyclobutyl (PFCB) linkages; replacement for PMMA

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Biomedical applications• Polyanhydrides• Polyphosphazenes• Polyurethanes• Polyvinylchloride (PVC)• Polyethylene (PE)• Polypropylene (PP)• Polymethylmethacrylate

(PMMA)• Polystyrene (PS)• Polyester

• Polyamide• Polyacetal• Polysulfone• Polycarbonate• Polysiloxane• Polylactide (PLA)• Polyglycolide (PGA)• Poly(glycolide-co-lactide); PLGA• Poly(dioxanone)• Poly(carbonate)

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CARBON-CARBON BASED CHAINS

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POLYACRYLICS

Poly(acrylic acid). PAA_2.aviPolyacroleinPolyacrylamidePolyacrylonitrilePoly(methyl methacrylate)Poly(2-hydroxyethyl methacrylate)

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POLY(OLEFINS)

PolyethyleneChlorinated polyethylenePolypropylenePoly(1-butene)Poly(isobutylene)PolystyrenePoly(2-vinyl pyridine)

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POLYDIENES

Poly(1,4-butadiene) BR (butadiene rubber)Polyisoprene NR (natural rubber)Polychloroprene CR, NeoprenePolynorbornenePoly(pentenamer) Ring-opening monomer

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Configurations for dienes

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POLYHALOCARBONS

Lower costs than ethylene-based polymersBond energies: C-F: 461 kJ/mol; C-H: 377 kJ/mol; C-Cl: 293 kJ/mol; C-Br: 251 kJ/mol; C-I: 188 kJ/molPTFE + copolymers with hexafluoropropylene, perfluoropropylvinyletherPTFCEPVCPVDFPVDC

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OTHER VINYL POLYMERS

Poly(vinyl acetate)- inexpensive emulsion systems (paints)Poly(vinyl alcohol) - adhesivesPoly(vinyl formal) -adhesivesPoly(vinyl butyral) - adhesivesPoly(vinyl methyl ether) – adhesives, plasticizersPoly(2-vinyl pyrrolidone) -

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CARBON-NITROGEN CHAINS

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POLYAMIDES

Perlons –[NH-CO-R]- ring-opening polymerizationsNylons – [NH-R-NH-CO-R1-CO-]; Nylon x,y; Nomex, Kevlar

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POLYIMIDES

Poly(isocyanic acid), Nylon 1- [NH-CO-]; [CO-NR-CO-]Kapton

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POLYIMINES/POLYURETHANES

Polyimines – [NH-CHR-]; polymerization of nitriles, hydrogenation; poly(ethylene imine) is a flocculating aidPoly(carbodiimides) – from isocyanates to give open cell rigid foams for reaction injection moldingPolyurethane foams: diisocyanate + dialcohol to [-R-NH-CO-OR-]. Fibers, films, paints, adhesives, foams, elastomers, image reproduction.Polyureas: [-R-NH-CO-NH-]

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CARBON-OXYGEN CHAINS

Polyacetals: [-CHR-O]; engineering thermomplastics, low water absorption, good wear resistance.Polyethers [-R-O]. Poly(ethylene oxide) – water soluble packaging films, surfactants. Poly(propylene oxide) – polyurethane intermediate, lubricants, surfactants. Poly(tetrahydrofuran) – thermoplastic elastomers and artifical leather.Epoxies: based on epichlorihydrin. coatings, circuit boards, adhesives, composites, road coatings.Bisphenol A is a major component.PEEK: poly(ether ether ketone)

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CARBON-OXYGEN CHAINS

Phenolic resinsPolyesters:

aliphatic – lactones and self-condensed and - hydroxy acids, diols + dicarboxylic acids or chlorides, aromatic – polycarbonates, poly(ethylene terephthalate), PBT, crosslinked- phthalic anhydride + glycerol; alkyd paints (now mostly replaced with latex paints)

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CARBON-SULFUR CHAINS

Polysulfides: poly(phenylene sulfide) – Ryton, can be made to be conductive by addiitives; poly(alkylene sulfide)s – Thiol RubberPolysulfones: oxidation of polysulfides, or polymerization via nucleophilic substitution. Aromatic polysulfones have high use temperatures.

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INORGANIC POLYMERS

Boron nitride – ‘parquet’ polymers, 2000 C.Poly(siloxanes)-oils, elastomers, resins, inert. Poly(dichlorophosphazene) – no uses; replace chlorine with alkoxy, aryloxy or amino groups

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GENERAL TYPES OF CONSTITUTIONS AND CONFIGURATIONS

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LINEAR HOMOPOLYMERS

Both chain and step polymerizations yield linear homopolymers.Vinyl polymers: head-to-head addition and head-to-tail additionTacticity: stereoregular arrangement of C-C backbone polymers atactic – no regular repeating structure; amorphous, no Tm

syndiotactic – functional group is on alternate sides of the C-C chain; Tg and Tm

isotactic – functional group is on the same side of the C-C chain; Tg and Tm

Elastomers: cis and trans configurations. Tg and Tm

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NONLINEAR HOMOPOLYMERS

Comb, ladder, semiladder, star, dendrimerIUPAC Compendium of Chemical Terminology, 2nd Ed., 1997.

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DENDRIMERS

‘dendron’ = tree (arborol; cascade molecule)Repeatedly branched molecules; nearly sphericalUsually symmetric around a core

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www.wikipedia.com/dendrimer

SYNTHESIS

Divergent: 1978- Vogtle, 1981 Denkewalter (Allied Chemical), 1983, Tomalia (Dow Chemical); 1985, NewkomeConvergent: 1990, Frechet

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TYPES

Low molecular weight: dendrimers, dendronsHigh molecular weight: dendronized polymers, hyperbranched polymers,Polymer brushDendrimer functionalization: mimics the site of biological moleculesAdd water solubility by using hydrophilic groups for the outermost layerControl of: toxicity, chirality, crystallinity,

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GENERATIONS

Convergent – three addition cycles; 3rd generation dendrimerEach generation increases the size by 2

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EXAMPLE-POLY(AMIDOAMINE) PAMAM

0 generation: ethylene diamine + methyl acrylate + ethylene diamineLow generations: flexible molecules with no inner regionsG3 to G4: internal space separate from outer shellG7: solid-like particles with dense surfacesMultiple steps mean that synthesis is difficult

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DIVERGENT SYNTHESIS

Multifunctional coreEach step must be driven to completion to ensure symmetry

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CONVERGENT SYNTHESISSmall molecules end up on the sphere surface

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DRUG DELIVERY

Encapsulation of hydrophobic compounds and anticancer drugsMonodisperse, hydrophilicity, variable functionalityCovalent links; ionic bonds, encapsulation

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COPOLYMERS

Long sequences: block, graft, star, blend,…Networks: crosslinked, interpentrating, co-terminous

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Network: phenol-formaldehyde resin

IPN: two crosslinked polymers not bonded to each other

Semi-interpenetrating polymer network: crosslinked epoxy with vinyl polymer

CONFORMATIONS

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RANDOM COIL CONFORMATION

Coil shapes are dynamicThe most probable shape is bean-like: oblate ellipsoid (3 unequal axes; 1.36/0.78/0.50)Least probable shapes are spherical, rod-like

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€

ρ0 =k ⋅Mw

R02

( )3 / 2 =

k1 ⋅Mw

Mw( )3 / 2 ∝

1

Mw

R0 = end-to-end distance; proportional to the square root of molecular weight

THETA SOLVENT

Chain assumes a minimum conformation in solution; beyond this T, the polymer precipitatesGood solvent: polymer expands beyond the random coil conformationGlobular proteins have conformation similar to random coils.

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Human serum albumin conformation. www.wikipedia.orgShape changes with pH, T, salts, ionic strength, ion types.

CRYSTALLINE, LINEAR POLYMERS

Extended chain – repulsive groupsRandom coil – interactions between chain/chain and chain/solvent units are similarFolded chain – zig-zag conformation for C-C backbone systemsHelix – nylons, PP

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WHICH POLYMERS CRYSTALLIZE?

Isotactic, syndiotactic C-C backbone chains: polypropyleneUnsubstituted linear addition polymers: polyoxymethyleneAddition polymers with di-substituted vinyl groups: poly(vinylidene dichloride) Straight chain condensation polymers: PETSymmetric ring-containing condensation polymers: poly(phenylene terephthalamide)Some nonstereoregular asymmetric polymers: poly(vinyl alcohol)Helix: bulky side groups, low energy conformation

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TERPOLYMER SYSTEM

Polyacrylonitrile; polystyrene; polybutadieneCrystalline; amorphous; elastomer

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MIXTURES, COPOLYMERS

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