Polymer Synthesis & Characterization

Professors Kinam Park & Luis Solorio

Purdue UniversityBiomedical Engineering

Polymers

Crosslinked Gel (1940s)

Synthetic Polymers

Branched Polymers (1960s)

Dendrimers (1980s)

Linear Polymers (1930s)

Homopolymer

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N N N N NN

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Block copolymer

Natural Polymers

O

OH

OH

CH 2OH

O OO

OH

OH

CH 2OH

Cellulose

DNA

N C

R

C

H

H O

Protein

Polymer Structures and Classifications

Linear Branched Network

A linear skeletal structure consists of a chain polymer with two ends

Branched polymers have side chains of significant length covalently bound to the main chain at junction points

Network polymers are 3D dimensional structures with interconnected covalently bound chains-These are cross-linked polymers-The density of cross-links will dictate the properties of the material

The skeletal structure will dictate the overall physical properties of the polymer-Branches tend to reduce the melting point-Network polymers do not melt or dissolve (but may swell)

Modified from “Biomaterials Science” Second Edition

Polymer Structures and Classifications

-A—A—A—A—A—A—A—A-

-A—B—A—B—A—B—A—B-

-A—B—B—B—A—B—B—A-

-B—B—B—B—A—A—A—A-

-A—A—A—A—A—A—A—A-— —

—

B—B—B—B-

B—B—B—B

-B—B—B

Consists of one species of monomer. The term can be expanded to consider an single type of repeat unit.

Statistical Copolymer-Consists of two units with the repeat units obeying Markovian statistical laws (Statistics for finite chains). Random copolymers are a subclass in which the distribution is truly random.

Homopolymers

Copolymers

Alternating Copolymer-Consists of two units two units that alternate along the polymer chain. Properties of statistical and alternating copolymers tend to be intermediate of the homopolymers

Block Copolymer-repeat units exist in long units (blocks)

Graft Copolymer-Branched copolymers, with the branches consisting of a different chemical structure than the main chain. Block and graft copolymers have unique properties relative to the constituent homopolymers.

Modified from “Fundamentals of Materials Engineering” Second Edition

Molecular Configuration and Stereochemistry

Atoms linked in the same order, but differ in the spatial arrangement of the the atoms

Stereoisomer:

Steric order, isotactic, syndiotactic, atacticTacticity:

Side Groups can alter the polymer properties

Head-to-tail configuration:Typically the dominant configuration

Head-to-Head:Less common typically due to steric hindrance

Polymer CrystallinityPolymer Crystallinity refers to the idea that the polymer chains can be packed in a way that creates an ordered array.

With atoms, the structure is either completely crystalline (salts) or completely amorphous (water)

Polymers have mixed structures due to their size and complexityChain disorders and misalignment lead to amorphous regions

Effects of Crystallinity:Crystallinity increases the polymer densityAlters release kinetics of drugsChanges the mechanical properties of the material Changes the polymer solubility and meltingRandom arrangements of block co-polymers reduces crystallinity

Measuring Tg, Tm, Tc, and Crystallinity: Differential Scanning Calorimetry

Measures the amount of heat absorbed or evolved from a sample under isothermal conditions.

An empty reference pan is used and heating of the sample occurs with respect to the reference pan, and are heated at the same rate

Measuring the difference in the amount of heat it takes to maintain the same rate of heating

Plot is the difference in the heat output between the two heaters at a given temperature

http://pslc.ws/macrog/dsc.htm

Heat Flow qt

Rate of Heating Tt

Cp

qtTt

qT

Determining the Tg

The glass transition temperature (Tg)Occurs in amorphous and semicrystalline polymers.

Results from a reduction in segmental motion (several repeat units) and elimination of conformational changes.

Corresponds to the transition from a rubbery to rigid solid.

Is not a first order thermodynamic transition (occurs across a small range of temps).

Below the Tg the polymer is solid and above it is rubbery.

Polystyrene: Tissue culture plastic Tg is ~90°CPolyisoprene: Natural rubber Tg ~ -90°C

Backbone flexibility and side chains can be used to modify the polymer Tg.

Determining the Tc and Tm

The crystallization temperature (Tc)Occurs after the Tg, and is marked by a reduction in the heat flow

A temperature where the polymer chains have enough energy to form ordered structures and is used to help determine the polymer crystallinity

The melting temperature (Tm)The polymer chains can move around freely and requires an increase in the heat flow. The increase in energy is used to melt the crystalline regions

Do all polymers have a Tg and Tm?

With Tc and Tm we can calculate the crystallinity…but how?

Are there other ways to measure crystallinity?

Tc

Tm

Determining the Tc and Tm

To determine the crystallinity, we need to determine the heat of crystallization, heat of melting, and the heat of melting for 1 g of polymer

Heat of Crystalization (HC ) AreaTC

T

xMass

Heat of Melting (HM ) AreaTM

T

xMass

H ' HM HC

H *M Heat of melting 1 gram of polymer

Mc H 'HM

* Crystalline MassAreaTC

JKsg

AreaTc

T

JKsg

Ks

Jg

HC Jg

g J H '

HM* J

Jg

g

http://pslc.ws/macrog/dsc.htm

Polymer Molecular Weight

What is different about a polymer molecular weight and the molecular weight of a molecule?

Defining the Polymer Molecular Weight:

Mn- Number average molecular weightMw- Weight-average molecular weightMz- Z average molecular weightMv- Viscosity average molecular weight

They all describe the molecular weight of the polymer, but why are they different and what do those differences mean

Mn and Mw are the most commonly used molecular weights, but they all determine a different aspect of the polymer structure

Mn < Mv ≤ Mw < MzTensile Strength Flow properties

Flex Life Moldability

Mw Mn

Mz Mv

Polymer Molecular Weight: Number Average

City PopulationWest Lafayette, IN 40,103St. Louis, MO 998,954Greenville, IL 7,000Weeki Wachee, FL 12

Mn: Most often used by chemists…but why?

What are colligative properties and why would Mn be the most appropriate to use when measuring?

How is it calculated?

Total: 1,046,069Mn: 261,517

Average person doesn’t live in a town that is 261,517 people large…

Polymer Characteristics: Weight Average

Mn: Most often used when mechanical properties are important

More predicative of polymer properties than Mn

How is it calculated?

City PopulationWest Lafayette, IN 40,103St. Louis, MO 998,954Greenville, IL 7,000Weeki Wachee, FL 12

City Population FractionWest Lafayette, IN .04St. Louis, MO .95Greenville, IL .01Weeki Wachee, FL .000001

Mw: 955, 545Much closer to the most probable city that people on the list come from

What are the implications of this with respect to the polymer for predicting properties

Polydispersity Index: Width of the molecular weight distribution of the polymer

PDI= Mw/Mn

Always greater than 1

Gel Permeation Chromatography

Gel permeation chromatography uses organic solvents, so the polymer must be hydrophobic

Size exclusion chromatography is used for hydrophilic substances (sometimes called gel filtration chromatography)

Fast and simple way to determine the Mw, Mn, and PDI of the polymer

What effect does the polymer size have on the elution time?

What are some other ways that the Mn can be determined?

Polymer Structures and Classifications

Polymers

Thermoplastic Elastomers Thermosets

Crystalline Amorphous Hydrogels

Thermosets

Highly crosslinked network polymers

Tend to be amorphous, rigid, and brittle

Have a high modulus, but minimal elongation

Do not undergo melt upon heating, so they can not be reprocessed, can only be broken down/degraded

Examples: Cross-linked Epoxy and phenolic resins

These polymers have restricted chain movement due to the cross-linking

The high degree of cross-linking leads to a more rigid polymer

Tg Tm Density (g/mL) Crystal (%)Epoxy N/A N/A 1.11-1.4 0Phenol-Formaldehyde N/A N/A 1.25-1.3 0

The crystallinity is low due to interference of crystal structures by the cross-links

Thermoset Examples

Elastomers

Elastomers are cross-linked rubbery polymers

Usually amorphous, with a low to moderate modulus

These polymers can be stretched to high extensions and recover rapidly

The elastic nature is a result of the low cross-link density and the molecular structure

Entropy drives the return to the original structure/spring back

Can these polymers be melted and reprocessed?

Tg Tm Density (g/mL)

Crystal (%)

Polyurethanes -53 -67 120-150 1.05 ~10-15%Silicone rubber (Silastic) -120 -123 N/A .99-1.5 0

Polyfunctional silanols facilitate networking

Silicones consist of repeat units of siloxane.Used in tubing, catheter occluders, breast prostheses, vascular ties, wound dressings, oxygenator membranesDoesn’t degrade in the bodyHigh gas transmission

PolyurethaneContains both crystalline and amorphous domains in the backbone

Cross-links are caused by the intermolecular forces in the crystalline domains (physical cross-link)

Crystalline and amorphous regions phase separate

Amorphous region provides elasticity

An elastomer that can be melt processed!

Excellent blood compatibility

Properties are controlled by the composition (hard/soft segment ratios)

Elastomer Example

Thermoplastics

Most typical type of polymer, and typically called “plastic”

Consist of linear or branched polymer, and is not cross-linked

Can have both amorphous and crystalline regions

Moderate to high tensile properties (5-100 Mpa), moderate elongation (1-100%), and undergo plastic deformation at high strains

They can be molded into any shape, and remolded into new shapes

Tg Tm

Low density polyethylene -20 95-115High density polyethylene -25 135-138Polypropylene (isotactic) 0 165Polytetrafluoroethylene(Gortex)

-10 327

Polyethylene Terephthalate(Dacron)

69-82 265-270

Polystyrene 116 137

Thermoplastic Example

Polymer Structures and Classifications

Polymer Type Cross-linkDensity

MolecularWeight

Crystallinity(%)

Tg (C) Tm (C) Characteristics

Thermosets High ∞ 0 n/a n/a High modulus, brittle, insoluble, and no melting

Elastomers

-Rubbers low ∞ 0-Low < -50°C n/a Low modulus and high extension. No melting. Insoluble, but may swell

-Silicones low ∞ 0-Low < -50°C n/a Low modulus and high extension. No melting. Insoluble, but may swell

-Linear 0 105-106 Low <-50°C n/a Low modulus and high extension will melt and can be dissolved

Thermoplastics

-Tough 0, branched, or linear

105-106 Low-Mod <RT 100-200Intermediate modulus, may undergo plastic deformation on high extension

-Rigid 0 or branched

105-106 0-Low >RT 100-250 High Modulus and Brittle

-Fibers linear 105-106 high >RT >200 High Modulus

Polymer Synthesis

Requirements for Polymerization:Monomer must be capable of chemically reacting to at least two other monomer molecules

What does this mean for the functionalization of a monomer?

What does this mean with respect to the number of potential reaction mechanisms?

Polymer Reactions

Condensation Addition

Condensation Polymerization: Results in a loss of atoms during the polymerization

Addition Polymerization: Will yield polymers with repeat units without losing atoms

Less common now due to overlap in reaction features

Wallace Carothers (1896-1937)

Polymer Synthesis Classification

Step-growth Polymerization (Step Polymerization):Polymer chain grows step-wise by reactions occurring between any two molecular species

A—A—AA—A + A

A—AA + A

A—A—A—AA—A—A + AA—A + A—A

A—A—A—A—A—AA—A—A—A—A + AA—A—A—A + A—A

A—A—A + A—A—A

Dimer

Trimer

Tetramer

Hexamer

Chain-growth Polymerization (Chain Polymerization):Polymer chains grow by reaction of the monomer with a reactive end-group on the polymer chain.

Polymer Synthesis Classification

AI—A—AAI—A + A

A—AAI + A

A—A—A—AAI—A—A + AAI—A + A—A

AI—A—A—A—A—AAI—A—A—A—A + AAI—A—A—A + A—A

AI—A—A + A—A—A

Dimer

Trimer

Tetramer

Hexamer

Step Polymerization Example

‐C00HH00C‐

CH3COOH + CH3CH2OH CH3COOCH2CH3 + H2O

What are the functional groups?Will this polymerize?

Ester Linkage of acetic acid and ethanol:

Ester Linkage of terephthalic acid and ethylene glycol:

+ HOCH2CH2OH   ‐C00CH2CH2OH + H2OH00C‐

Will this reaction scheme propagate?

What would happen with a tri-functional monomer?

Is this an example of a polycondensation or polyaddition?

What if the molecule reacts with itself?

How can this be avoided?

Le Chatelier’s Principle

Henry Le Chatelier (1850-1936)

“A system at equilibrium, when subjected to a disturbance, responds in a way that tends to minimize the effect of the disturbance”

‐C00HH00C‐ + HOCH2CH2OH   ‐C00CH2CH2OH + H2OH00C‐

‐C00CH2CH2OH + H2OH00C‐

Removal of the water can be used to drive a reaction forward and increase the polymer molecular weight

Chain Polymerization

Chain polymerization typically requires 3 steps:

1. Initiation2. Propagation3. Termination

Free radical polymerization is the most widely practiced method of chain

InitiationFree radical active center is made, typically in 2 steps

1. An initiator creates the first free radical2. The free radical is added to a monomer

Free radicals can be formed through homolytic scission of a single bond or single electron transfer to or from a molecule

Homolysis can be created with heat (Thermolysis), light (photolysis), or a Redox reaction-Peroxides, Benzoin ethers, Fe2+

‐C0‐0‐C‐

0 0

‐C0

0.Δ

2

Chain Polymerization

Chain polymerization typically requires 3 steps:

1. Initiation2. Propagation3. Termination

Free radical polymerization is the most widely practiced method of chain

InitiationFree radical active center is made, typically in 2 steps

1. An initiator creates the first free radical2. The free radical is added to a monomer

Free radicals can be formed through homolytic scission of a single bond or single electron transfer to or from a molecule

Homolysis can be created with heat (Thermolysis), light (photolysis), or a Redox reaction-Peroxides, Benzoin ethers, Fe2+

-C0-0-C-0 0

-C00.Δ

2

Chain Polymerization

PropagationThe growth of the polymer chain by sequential adding of monomer to the reactive centerAddition occurs on the time scale of millisecondsThe reaction tends to go head-to-tail

R—CH2—CH + CH2—CH —— —

X X

.R—CH2—CH—CH2—CH — —

X X

.

TerminationThe growth of the polymer chain is terminated.

-Combination, coupling of two growing chains-Disproportionation, hydrogen atom is abstracted from one growing chain to another

R—CH2—CH + CH2—CH — —X X

.R—CH2—CH—CH—CH2— —

X X

.

Bulk Reactions

The reaction mixture is only monomer and initiator

As the reaction proceeds, heat is generated

This makes controlling the reaction difficult

The polydispersity index can be quite high

Solution PolymerizationEasier to control the system viscosity

Easier temperature control

Solvent selection is important

Monomers and the solvent must be soluble in the selected solvent

Poor polymer solvent results in polymer precipitation

Professor Paula Hammond, Synthesis of Polymers, Massachusetts Institute of Technology

Interfacial Reactions

Professor Paula Hammond, Synthesis of Polymers, Massachusetts Institute of Technology

Interfacial Polymerization Reactions

Reactants diffuse to the interface and react

Forms a high molecular weight polymer

To refresh monomer, you remove the film

Not kinetically controlled, but diffusion controlled

Nylon is produced this way

Graft Polymers/Surface PolymerizationAtom Transfer Radical Polymerization (ATRP)Controlled reversible-deactivation radical polymerization in which the deactivation of the radicals involves reversible atom transfer or reversible group transfer catalyzed usually, though not exclusively, by transition-metal complexes. Tight control of the radical concentration is the key feature.

Well defined co-polymers with narrow molecular weight distribution, high degree of functionality, and controllable molecular weights

ATRP monomers: styrenes, acrylates, acrylamides, and acrylonitriles

Graft Polymers/Surface Polymerization

"Synthetic polymer coatings for long-term growth of human embryonic stem cells", Nature Biotechnology (2010)

Emulsion Reactions

II

II

I

I = Initiator

Amphiphilic monomers collect at the water surface until it saturates

The monomers then arrange into stable structures called micelles in the water

The initiator diffuses into the micelle to initiate the reaction

Polymerization drives an increase in particle size

Dendrimers

Conformational changesLow generation dendrimers (0, 1 and 2) possess asymmetric shape and open structure and become globular as higher generations are achieved. As the dendrimer grows, it becomes densely packed and extends out to the periphery, and due to steric hindrance further elongation or branching stops (Fischer, Vogtle, 1999). With increase in each generation, the branch density increases leading to the formation of internal cavities and a large number of terminal end groups.

Physicochemical propertiesSome of the important physicochemical properties of dendrimers originate from their overall conformation, and their cascade. Dendrimers in solution possess rheological properties as they form closely packed ball like structures. The terminal end groups of the dendrimers render them high solubility, miscibility and high reactivity (Frechet, 1994).

Dendri-means tree likeDivergent synthesis starts at the core and grows the branches outConvergent synthesis starts from the formation of branches and reacts the branches with a coreDendrimer generation refers to the branching

Chemical Vapor Deposition

[2.2] paracyclophane monomers are added to the system

The system is put under vacuum

Argon is used as a carrier gas

The monomer sublimates and enters the furnace

Pyrolysis leads to polymerization into poly(p-xylenes)

Chilling chamber draws the reacting oligomers toward a surface where they continue to polymerize