KE-100.3410Polymer Properties

Steve Spoljaric, PhD

Polymer Properties (5 credits)

Learning outcomes:

After completing the course, the student understands:

1) The basics of polymer physics and structure hierarchy of polymers

2) The basics of the most important methods of polymer analysis

3) Theories of polymer rheology

4) Stability and degradation of polymers

5) The ability to calculate the above mentioned phenomena

Polymer Properties

Content: • Structure hierarchy in polymers• Theoretical aspects:

• Polymer analysis• Physics• Rheology

• Stability and degradation of polymers• Calculation of polymer properties

Lectures and corresponding book chapters (Fried)

• Lecture I (Structure and Molecular Weight) – Chapters 1 and 3.3

• Lecture II (Crystallinity) – Chapter 4

• Lecture III (Spectroscopic analysis techniques) – Chapters 2.6 and 4

• Lecture IV (Rheology) – Chapters 5 and 11

• Lecture V (Solution fractionation and permeation) – Chapters 3.2 and 12.1

• Lecture VI (Stability, degradation and microscopy) – Chapter 6

Schedule for the course

• 6 Lectures on Mondays from 10.15-12.00– Lecture notes available on MyCourses– 5 Problem solving exercises on

Thursdays from 8.15-10.00 • Chemistry building, Luentosali 4, Room C301

• Textbook for the course: Polymer Science and Technology, Fried JR.

• Exam: 8.12. at Luentosali1, Luentosali2– (Chemical Technology, Kemistintie 1)

Other recommended references

• Spectroscopy of Polymers, Jack L. Koenig– Good for further information about spectrsocopy

and charactersiaton (Lecture III)

• Polymer Physics, Ulf Gedde– A good all-round textbook, includes some

information about microscopy (Lecture VI)

Classification of polymers• Monomer and thus polymer

• Oil (or coal, gas) based • Bio-based

• Polymers• Synthetic• Natural

• Polymerization mechanism• Step polymerization• Chain polymerization

• Structure• Thermoplastic• Thermoset

• Product• Biodegradable or stable

Structure-property correlations

Structure considerations:• Linear / branched / cross-

linked• Thermoplastic / thermoset • Molecular weight /

molecular weight distribution

• Homopolymer – copolymer– Block, alternating,

random, graft• Copolymer / blend

Properties:• Thermal properties

– Glass transition temperature, melting temperature

– Amorphous or partly crystalline polymer

– Operating temperature range for continuous use

• Mechanical properties– Tensile stress and strain,

modulus, hardness, impact strength

http://www.pcn.org/Technical%20Notes%20-%20Periodic%20Table%20of%20Polymers.htm

Questions?

1. How much new material was in the first few slides? Was it mostly reviewing what you have learned before?

2. Have you completed the course ‘Polymeerien valmistus’ (Polymer synthesis)?

3. What would you like to know more about regarding polymers? What do you think will be the topics in this course?

4. Any comments or wishes at this moment?

Polymerisation overview

• Step polymerisation• Chain polymerisation

– Radical polymerisation• Emulsion polymerisation• Ionic polymerisation (anionic and cationic)• Co-ordination polymerisation

– Living radical polymerisation• Atom Transfer Radical Polymerization (ATRP)• Stable Free Radical Polymerization (SFRP)• Reversible Addition-Fragmentation chain Transfer (RAFT)

• Ring-opening polymerisation• Co-polymerisation

To begin with:

• What affects the molecular weight in different polymerization mechanisms? – Step polymerization

• (stoichiometric equivalence of functional groups)– Radical polymerization

• (initiator concentration, chain transfer)– Living polymerization

• (monomer and initiator concentration)

• How is crosslinking achieved in thermoset materials?– What are the crosslinks?

Course content / overview• Structure and average molecular weights• Solid-state properties:

– Crystallinity– Thermal transitions

• Characterization methods:– Thermoanalytical methods (DSC, DMTA)– Structure analysis (FTIR and Raman, solution and solid NMR,

fractionation SEC)• Rheological properties and their measurement• Solubility and gas permeability• Microscopy methods, X-ray analysis• Stability and degradation

Polymer Structure

Stereochemistry of polymerization

• Conformation:– Different orientations of atoms and substituents in a molecule– Result from rotations around single bonds– Can be changed by bond rotations– Extended chain / Helix

• Configuration:– Isomers differ in the spatial arrangement of their atoms and

substituents; that can only be changed by breaking and reforming primary chemical bonds

Tacticity

• Regularity in the configurations of successive stereocenters determines the overall order of the polymer chain:– If the R groups are randomly distributed on the two sides of the

planar zigzag polymer, the polymer does not have order and is atactic

– Ordered structures are isotactic and syndiotactic – When the sterocenter of each repeating unit has the same

configuration the structure is termed isotactic– When the stereocenter alternate from one repeating unit to the

next the structure is syndiotactic• Even more complicated structures can be seen in some

1,2-disubstituted polymers

Tacticity: structures

Atactic

Syndiotactic

Isotactic

Tacticity: influences

• In general, isotactic and syndiotactic polymers are partly crystalline and atactic are amorphous

• In addition to crystallinity, tacticity affects also other properties such as thermal and mechanical properties

• The tacticity of a polymer is determined by several factors:– Polymerization temperature– Pressure– Catalyst– Solvent

Tacticity in commercial polymers

• Polypropylene (PP) used and sold is mostly isotactic but it is also possible to produce syndiotactic or atactic polymer

• Polyvinylchloride (PVC) is usually atactic but contains some syndiotactic structures which results in some crystallinity in the material. It is also possible to produce highly syndiotactic and isotactic PVC

Isotactic

Atactic

Syndiotactic

Isomerism

• In chemistry, isomers are molecules with the same molecular formula but different chemical structures.

• In other words, isomers contain the same number of atoms of each element, but have different arrangements of their atoms in space

• Unsaturated carbons (double or triple bonds) result in different isomers:– 1,3-butadiene (dienes in general)– Cis and trans isomers possible– Results in geometric isomerism (isotactic, syndiotactic or atactic)

CH2 CH CH CH2

1,3-butadieeni

CH2 CH

CH CH2

n

1,2-poly(1,3-butadieeni)

cisC CCH2

H

CH2

Hn

C CCH2

H

H

CH2n

trans

1,4-poly(1,3-butadieeni)

Geometric isomerism

• Geometric isomers have the same structural formulas but differ in the arrangement of groups at a single atom, at double bonds, or in rings

• Polybutadiene isomers:

Isotactic Isotactic

Syndiotactic

Optical isomerism (enantiomerism)

• Optical isomers (also called enantiomers) are mirror images that are not superimposable (that is, they can’t be put on top of one another)

• They differ in only one characteristic; their interaction with plane polarized light

• More general in bio-based polymers– An example is synthetic polymer is poly(propylene oxide)

Lactic acid-based materials

• Commercial optically-active polymers are lactic acid based

• Lactic acid is available both in optically-active enantiomers as well as an optically-inactive racemic mixture

• Synthetic lactic acid is always a mixture of the two isomers, whereas in bio-technically produced lactic acid the L/D ratio can be tailored with the fermentation process

• Lactic acid based polymers are either optically-active or not depending on the monomer used

C COOH

OH

H

H3C*

C COOH

OHH3C

H*

D-MAITOHAPPO

L-MAITOHAPPO

DL-MAITOHAPPO (50/50 seos)

Lactic acid monomers

D,L-lactic acid is a mixture of the twoL-lactic acid

D-lactic acid

Lactide monomers

C

C

O

C

C

O

O

O

H

H3C

H

CH3

C

C

O

C

C

O

O

O

H

H

H3C

CH3

C

C

O

C

C

O

O

O

H

H3CH

CH3

[D]

[D]

[D]

[L][L]

[L]

*

*

*

*

*

*

D,L-LAKTIDI(50/50 seos)

MESO-LAKTIDID-LAKTIDIL-LAKTIDI

Polylactide is polymerized with Ring Opening Polymerization (ROP)

Polylactides

Partly crystallinePLA

AmorphousPLA

Average molecular weights

Average molecular weights

http://www.ias.ac.in/initiat/sci_ed/resources/chemistry/MolWeight.pdf

Average molecular weights

• Number average molecular weight:

• Weight average molecular weight

• Viscocity average molecular weight

MM

ni i

i

n

n

MM M

Mw

i i

i

i i

i i

w

w

n

n

2

a

ii

aii

vn

n11

M

MM

Average molecular weights

• Z-average molecular weight

• Polydispersity

ni = Number of molecules with molecular weight Mi

wi = weight fraction with molecular weight Mi

a = constant, depends on polymer/solvent combination

2

3

M

MM

ii

iiz

n

n

M

M

w

n

PDI

vM

Molar mass distribution for a narrow and broad polystyrene (molar mass averages are also given)

http://www.polymer.de/solutions/molar-mass-determination.html

Molar mass vs. molecular weight

• Molar mass is the mass of one mole of a substance

• Molecular weight is the mass of one molecule of a substance

Determination of average molecular weights

• Primary (absolute values) methods:– Osmometry (Mn)

– Scattering (Mw)

– Sedimentation (Mz) Z-average molecular weight is obtained from centrifugation data

• Secondary (relevant to reference or calibration) methods:– Gel permeation chromatography (GPC), also called size

exclusion chromatography (SEC) to obtain molecular weight distribution

– Intrinsic viscosity for determining viscosity average molecular weight

Method Relative/ absolute

Average molecular

weight

Range of molecular

weight

Comments

Ebulliometry A < 104 Insensitive

Cryoscopy A < 104 Small sample size

Membrane osmometry

A 5x103 - 106 Difficulties with the membrane, slow measurement

Vapor pressure osmometry

A < 3x104 Only few suitable solvents

End-group analysis A 102 - 3x104 Restricted to low molecular weights, assumption of 2 end-group/molecule

Light scattering A 104 - 107 Insensitive for small molecules, slow measurement

MALLS, LALLS A 104 - 106  

Ultracentrifugation A 2x103 - 107 Slow measurement, expensive instrument

Viscometry S% 102 - 107 Simple measurement, inexpensive instrument

SEC (GPC) S% all 102 - 107 Gives molecular weight distribution. Requires calibration.

Mn

Mn

Mn

Mn

Mn

Mw

Mw

Mw

vM

Determination of average molecular weights

Number average molecular weight

Number average molecular weight

• Determination methods based on various phenomena or factors:– End groups– Property changes

• Osmotic pressure• Increase in boiling point• Decrease in freezing point or vapour pressure

Osmometric measurement

• Pure solvent and a dilute solution of polymer in the same solvent are placed on opposite sides of a semi-permeable membrane

• Membrane will allow the solvent to pass through but will retain the polymer molecules in solution

• In equilibrium the difference in the heights of the solvent and solution in capillaries can be used to calculate the osmotic pressure

Osmometric measurement

• Van’t Hoff equation for the osmotic pressure of an ideal, dilute solution:

p = osmotic pressure

c = concentration

R = gas constant

T = temperature (K)

= number average molecular weight (g/mol)

g = gravitational constant 9.80665 m/s2

ρ = solvent density

RT

cnM

c

RT

n

M

Mn

gh

Osmometric measurement

• Van’t Hoff equation is for ideal, dilute solutions. In real solutions the equation will be following:

• For the determination of molecular weight, 4-6 pressure measurements with different concentrations are required. When solutions are dilute enough, p/c can be obtained by extrapolation of c to 0. Average molecular weight can be calculated from:

• Polymer concentration is g/dm3 and p/c in J/kg

c

RTBc Cc

n

M2 .... B, C are virial coefficients

limc nc

RT

0

M

Osmometric measurement: determining π/c

PS in toluene

PS in acetone

c (g/dm3)

Osmometric measurement: challenges

• Simple experimental procedure, but can be very time consuming

• Performance of the membrane can be a problem

• Membrane can let some smaller polymer molecules through and this will result in an artificially-higher Mn value

• Thus, the method is considered accurate for molecular weights above 20,000 g/mol

• The upper limit for molecular weight is 500,000 g/mol due to inaccuracy in measuring small osmotic pressures

Vapor-pressure osmometry

• When a polymer is added to a solvent, the vapor pressure of the solvent is lowered due to the decrease in solvent activity

• Osmometry uses two matched thermistors that are in a closed chamber containing saturated solvent vapor

• A drop of solvent and a drop of dilute polymer solution is placed on thermistors

• Condensation of solvent vapor onto the solution causes the temperature of the solution thermistor increase until vapor pressure of solution equals to that of a solvent

Vapor-pressure osmometry cont.

• The difference in temperatures is recorded in terms of difference in resistance DR, which is calibrated by use of a standard low molecular weight sample

• Several polymer concentrations are measured and extrapolation of DR/c to zero concentration yield Mn (requires calibration constant obtained with the standard sample hence VPO is secondary method)

Vapor-pressure osmometry: measurement principles

• For the characterization of low molecular weight oligomers and polymers, vapor-pressure osmometry is recommended

Closed chamber

Pure solvent

Ebullioscopic method

• The determination of molecular weight, in which the differential thermometer is used to measure the rise in boiling point

• Boiling point of the polymer solution is compared to the boiling point of the solvent. An increase in boiling point is comparable to polymer properties:

• Top limit of the measurement is 50 000 g/mol

nS

Abb W

WKT

M

1000

Tb = increase in Tb

Kb = ebullioscopy constant

WA = mass of the polymer

WS = mass of the solvent

Cryoscopic method

• Resembles ebullioscopy method

• Compares the freezing point of a polymer solution and pure solvent

• For dilute solutions:

• Top limit of the measurement is 50 000 g/mol

Kf = cryoscopic constant

nS

Aff W

WKT

M

1000

End group method

• The structure of the polymer chain must be known!!!

• End groups to be either quantitatively determined with appropriate reagents (titration) or determined by IR or NMR

• IR: Number average molecular weight can be calculated assuming the chain is linear with no branch points– Number of end groups of the polymer is thus one or two per

molecule– End groups are not spectroscopically coupled to the remainder of

the chain, low molecular weight analogs can be used for calibration– Particularly useful for insoluble systems (eg. fluorocarbons)

Viscosity average molecular weight

Viscosity average molecular weight

• Based on the determination of intrinsic viscosity of a polymer solution through measurements of solution viscosity

• Widely used for routine molecular weight determination– Cheap equipment and simple measurement procedure

• In this method, the viscosities of pure solvent and dilute polymer solutions are compared

• To determine the molecular weight, the empirical constants K and a, that are specific for a given polymer, solvent and temperature, need to be known. Extensive libraries of the values for constants exist for different polymer solvent combinations

Staudinger equation

• Staudinger equation is valid only for a relatively narrow molecular weight distribution

sp mK c M

hsp = specific viscosity

c = solution concentrationKm = constant for specific polymer-solvent combination

Mark-Houwink equation

• Mark-Houwink equation is valid for broad molecular weight distribution:

Km vaM

[h] = intrinsic viscosity = viscosity average molecular weight

K and a = constants for particular polymer-solvent combination

For flexible polymer chains a is 0.5 - 0.8, for stiff and rod like chains a is 2.0.

Mv

Mark-Houwink equation: K and a constants

• Values for the constants can be found in:– ‘Polymer Handbook’, Bandrup, J., Immergut, EH. (ed.)

• Some examples of polymer/solvent combinationsPolymeeri Liuotin T oC Kx104 a

lineaarinen PE tetraliini 130 4,60 0,72

tetraliini 105 1,65 0,83

p-ksyleeni 105 1,62 0,83

polystyreeni tolueeni 30 1,05 0,72

metyylietyyliketoni 30 3,14 0,61

polyvinyylikloridi tetrahydrofuraani 25 1,63 0,77

Viscosity parameters

Capillary viscometers are used to carry out the measurement:– Ostwald-Fenske or Ubbelodde type capillary– Constant temperature, typically 25 or 30°C– The time required for a dilute solution and pure solvent to pass

through the capillary is measured– 4-6 polymer solutions are measured– Efflux times need to be more than 100 s, which can then be

used to calculate the relative viscosity:

r t t / /0 0t = efflux time for dilute polymer solutionT0 = efflux time for pure solvent

h = dynamic viscosity of the solutionh0 = dynamic viscosity of the solvent

Capillary viscometers (‘U’ tubes)

• The time for the fluid to fall or rise between marks A and B determines the flow rate (Q)

• The pressure drop (DP) is calculated from the head of fluid

• The viscosity can be determined from these values and the geometry of the viscometer

Ostwald Uppelohde Reverse flow

http://www.adhesivestoolkit.com/Docs/test/Physical%20Analysis%20-%20Viscometry.xtp

Viscosity parameters

• Specific viscosity can be calculated from viscosities or the efflux times:

• The viscosity number or reduced viscosity is obtained by dividing specific viscosity by the solution concentration:

sp

t t

t

0

0

0

0

0

0

cc

spred

Molecular weight determination, Rheological measurements in Polymers: Polymer Characterization and Analysis, Ed. J.I. Krowchwitz, John Wiley & Sons Inc, 1990

Viscosity parameters

• Logarithmic viscosity number, inherent viscosity:

• Limiting viscosity number, intrinsic viscosity is obtained from the parameters of logarithmic viscosities

inhr

c c

lnln

0

Viscosity measurement

• Unit used in concentration is g/cm3

cc c

sp

c0

0

00limlim

limln

limln /

c

r

cc c0 0

0

Viscosity measurement

• Measuring viscosity for several polymer solutions with different concentrations, the intrinsic viscosity can be determined graphically:

sp /C

inh

red

CHuggins/Kraemer Plot

Weight average molecular weight

Light scattering in nature

• Why is the sky blue?• How can we see clouds?• Lord Rayleigh 1871

• When light passes through matter, most of the light continues in its original direction but a small fraction is scattered in other directions

incident beam

scattered light

Light scattering in the lab

• In the lab we can control polarization, wavelength (l) and intensity (Ii) of the coming light

• We can detect the scattered light (Is) as a function of angle (q)

• Scattered light is proportional to the concentration and molecular weight of the polymer

Light scattering

• The weight average molecular weight can be obtained directly only by scattering experiments

• Most commonly used is light scattering from dilute polymer solution

• Polymer molecules in solution cause stronger scattering than solvent molecules– The intensity of the light is proportionate to the absolute

molecular weight of the particle causing the scattering

Light scattering procedure

• Polymer is dissolved in appropriate solvent

• The particles have interactions in solution and self organize– Local changes in concentration and density in solution

• Light scattering is observed in different angles

• Measurements with several concentrations and angles

Light scattering

Intensity distribution of LS at various angles for a small particle (dashed line) and a large polymer molecule (solid line)

Light scattering instrumentation

Conventional LS instrumentation showing incident and scattered light, sample cell, and photomul’plier (Fried et al.)

Light scattering

In order to determine molecular weight, the following information must be known/determined:

• Refractive index for the solvent• Difference between refractive indices for the solution and solvent• Polymer concentration• Absolute light intensity for certain scattering angles

Light scattering

• Rayleigh formula:

• Problem is determining R(q) value, which should be known for all values

1 2 202

40

2

00, 0Mw b c

n

N R

dn

dcI

c

R

n0 = refractive index for solvent

l = wavelength of the light

N0= Avogadro number

Rb= Rayleigh constant

I0= light intensity at 90o angle that scattered from solvent

c = polymer concentration in solution

= measuring angle

R = measured intensity, contains the angle and correction for concentration

Low-Angle Laser Light scattering (LALLS)

• Helium-Neon lasers have replaced conventional light sources in scattering instruments

• The high intensity of the light sources permits measurements at much smaller angles (2–10 o). Then, the scattering by small spherical particles can be expressed:

• Only at one single angle chain dimensions cannot be obtained

Kc

RA c

w

12 2M K

n

N

dn

dc

2 2 2

04

2

Multi-Angle Laser Light-Scattering (MALLS)

• Several detectors (18 angles, 20–150 o )

• Absolute molar mass (Mn, Mw, - Mz) and size (Rz , rh)

• No standard references, more reliable than calibration techniques– Information about molecules and particles (conformation,

branching, partly cross-linked fractions)– Co-polymer, protein, aggregate detection– Easy and fast to use

DAWN 8+

• 8 angles (23-155 °)

• Molar mass range <10³-107 g/mol

• Molecular size range 10-500 nm

• Laser wavelength 658 nm – (50mW GaAs linear polarized laser)

• All solvents compatible (aqueous and organic)

incident beam

photodiode detectors

Gel permeation chromatography (GPC )

aka: Size-exclusion chromatography (SEC)

Gel permeation chromatography (GPC)

www.waters.com

Most widely used method for routine determination of molecular weight and molecular weight distribution is GPC

Separates samples of polydisperse polymers into fractions of narrower molecular weight distributions

Principle of GPC:

Polystyreenigeeli

Polymeerimolekyyli

Polystyrene gel

Polymer molecules

GPC measurement

• Columns are packed with small, highly porous beads or gel– Pore diameters of the beads range from 10 to 107 Å– Approximately the dimensions of polymer molecules in solution

• During GPC operation, pure pre-filtered solvent is continuously pumped through the columns at a constant flow rate. Then a small amount of dilute polymer solution is injected into the solvent stream and carried to through the columns

• Polymer molecules diffuse from this mobile phase into the stationary phase in the pores. The smallest polymer molecules penetrate deeply into the pores whereas the largest molecules pass through the columns first

GPC measurement

Comprehensive Analytical Chemistry vol 53: Molecular Characterization and Analysis of Polymers, editors John M.Chalmers, Robert J. Meier, 2008

GPC measurement

• The concentration of polymer molecules in each eluting fraction is monitored by a detector, such as IR

• The elution volumes are calibrated with know Mn standards (PS and PMMA standards are most common)

• Only apparent molecular weights are determined unless:– Molecular weight standards exist of the same composition and

topology as the samples– LALLS or viscometry is used– Mark–Houwink parameters of the standards and sample are

known

GPC

• GPC is the most widely used technique for the analysis of polymers

• Can be used for samples soluble in organic and aqueous eluents and molecular weights from approximately 100 to several million

• Yields all molecular weight averages and the molecular weight distribution

• The analysis of the molar mass distribution of polyethylene and polypropylene samples by GPC requires elevated temperature operation for dissolution

Room temperature GPC and GPC operating at higher temperature (for polyolefins 120ᵒC)

Effect of molecular weight on polymer properties

Properties dependent on Molecular Weights and Molecular Weight Distributions (MWD)

• Melt Viscosity• Tensile Strength• Toughness or Impact Strength• Resistance to heat• Corrosive properties

Polymer

Monodisperse (uniform)

Proteins, nucleic acids

Polydisperse (non-uniform)

All synthetic polymers

Effect of the molecular weight on polymer properties

• Great effect on properties

• Some properties have a limit, like Tg and strength at break, whereas some can change without any limit as the molecular weight increases. Dependence of Tg on average molecular weight:

- A and B are experimental constants

- Tg, is glass transition temperature of a extremely high molecular weight polymer

T TA

Bg gn

, M

Effect of the molecular weight on polymer properties

• Many mechanical properties exhibit similar dependence on molecular weight:

P = property valueK’ = constantP P K n '/M

Effect of the molecular weight on polymer properties

• Tensile strength of PS in proportion to molecular weight

Tensile strength (Mpa)

Tens

ile s

tren

gth

(Mpa

)

Inverse of molecular weight (106/M)

Effect of molecular weight and PDI on properties

• Examples of how average molecular weight and molecular weight distribution affect different properties:

Property:+ = improves- = worseo = little effect

Property Molecular weight increases

Molecular weight distribution narrows

Tensile strength + +Tensile strain + -Strength at yield + -Ductility + +Brittleness + -Hardness + -Softening point + +Melt viscosity + +Adhesion - -Chemical endurance

+ +

Solubility - o

Next week:

CrystallinityThermal transitions