Biomass Fundamentals

Module 6: Fundamental Principles of Polymer Chemistry

A capstone course for

BioSUCCEED:

Bioproducts Sustainability: a University Cooperative Center of Excellence in EDucation

The USDA Higher Education Challenge Grants program gratefully acknowledged for support

This course would not be possible without support from:

USDA

Higher Education Challenge (HEC) Grants Program

www.csrees.usda.gov/funding/rfas/hep_challenge.html

Polymer Chemistry

• Macromolecules• Polymer Structure/Classification• Molecular Weight Definitions • Molecular Weight Distribution• Viscocity • Polymer Morphology

The Macromolecular HypothesisIn the late 1800’s it was hypotheses that large molecules - “Macromolecules” existed as a result of covalently linked smaller units, and possessed unique physical and chemical properties.

The Macromolecular Hypothesis

However the scientific community at that time was unwilling to accept such a notion, explaining high MW-molecules as being the result of inferior methodology and/or molecular association of smaller molecules.

Polymer Structure

Polymers can exist with various skeletal structures - such as linear, branched or cross-linked or network polymers.

Linear Branched Network

Polymer Structure

Variations in skeletal structure give rise to major differences in polymer properties.

– linear polyethylene has a melting point 20oC higher than that of a branched polyethylene.

– unlike most linear polymers and branched polymers, network polymers do not melt upon heating, and will not dissolve

Polymer ClassificationPolymers are commonly classified based on their

underlying molecular structure.

Polymers

Elastomers ThermosetsThermoplastics

Crystalline Amorphous

Thermoplastics

Often referred to as just “Plastics” are linear or branched polymers which soften upon heating.

They can be moulded (and remoulded) into virtually any shape

– injection moulding, extrusion

and constitute the largest portions of the polymers used in industry

Thermoplastics never achieve 100% crystallinity, but instead are semicrystalline with both crystalline and amorphous domains.

Thermoplastics

The crystalline phases of such polymers are characterized by their melting temperature (Tm).Many thermoplastics are completely amorphous and incapable of crystallization, these amorphous polymers (and amorphous phases of semicrystalline polymers) are characterized by their glass transition temperature (Tg).

– the temperature at which they transform abruptly from the glassy state (hard) to the rubbery state (soft).

Thermoplastics

Glass transition temperature (Tg)

This transition corresponds to the onset of chain motion

•below the Tg the polymer chains are unable to move and are “frozen” in position.

Both Tg and Tm increase with increasing chain stiffness and increasing forces of intermolecular attraction

Elastomers

Elastomers - crosslinked rubbery polymers - rubber networks - that can be easily stretched to high extensions (3x to 10x original dimensions) – the rubbery polymer chains become extended upon

deformation but are prevented from permanent flow by crosslinking, and driven by entropy, spring back to their original positions on removal of the stress.

Thermosets

Thermosets - normally rigid materials - network polymers in which chain motion is greatly restricted by a high degree of crosslinking

As with elastomers, they are intractable once formed and degrade rather than melt upon the application of heat.

Polysaccharides

The size of polysaccharide molecules can vary, occurring as polydispersed molecules that have a range of 100 to 100,000 monosaccharide units

– MW 16,000 - 16,000,000 daltons

There are a number of methods used to determine the molecular weight of polysaccharides

– viscosity*, light scattering, ultracentrifugation, osmometry and titration are most common

(*viscosity is routinely used, but is not an absolute method and can be used only in conjunction with one of the other methods)

Molecular Weight DistributionThe simplest, most common molecular weight is the number-average molecular weight (n)

– end-group analysis or colligative properties (b.p. elevation, osmotic pressure, etc)

others commonly used are weight-average molecular weight (w), z-average molecular

weight (z) and viscosity-average molecular

weight (u)

– light scattering (w), sedimentation equilibrium (z)

and solution viscosity (u)

Number-average molecular weight (n)– based on methods of counting the number of

molecules in a given weight of polymer• the total weight of a polymer sample, w, is the sum of the

weights of each molecular species present

11 i

iii

i MNww

1

1

1 ii

iii

ii

n

N

NM

N

wM

N = number of moleculesM = molecular weight

Weight-average molecular weight (w)

determination of molecular weight based on size rather than the number of molecules

– the greater the mass, the greater the contribution to the measurement

1

1

2

1

1

iii

iii

ii

iii

w

MN

MN

w

MwM

w = weight fractionM = molecular weight N = number of molecules

Z-average molecular weight (z)

some molecular weight determination methods (e.g. sedimentation equilibrium) yield higher molecular weight averages - z

1

1

2

1

2

1

3

iii

iii

iii

iii

z

Mw

Mw

MN

MNM

w = weight fractionM = molecular weightN = number of molecules

Example - a polymer sample consists of 9 molecules of mw 30,000 and 5 molecules of mw 50,000

000,37)59(

)000,505()000,309(

1

1

ii

iii

n

N

NMM

Number-average molecular weight (n)

Consider the previous example - 9 molecules of molecular weight 30,000 and 5 molecules of molecular weight 50,000

000,40)000,50(5)000,30(9

)000,50(5)000,30(9 22

wM

Weight-average molecular weight (w)

Consider the previous example - 9 molecules of molecular weight 30,000 and 5 molecules of molecular weight 50,000

136,42)000,50(5)000,30(9

)000,50(5)000,30(922

33

zM

Z-average molecular weight (z)

A Typical Molecular Weight Distribution Curve

200 000 400 000 600 000 800 000 1 000 000Mi (g mol-1)

104 wi

1.0

2.0

3.0

4.0 n = 100 000 g mol-1

w = 199 900 g mol-1

z = 299 850 g mol-1

Molecular Weight DeterminationIn measurements of colligative properties, each molecule contributes regardless of weight, whereas in light scattering, the larger molecules contribute more because they scatter light more effectively.

For this reason, w are greater than n , except when all molecules are of the same weight and w = n

Molecular Weight Distribution The narrower the molecular weight range, the closer are the values of w and n , and the ratio w / n may thus be used as an indication of the breadth of the molecular weight range in a polymer sample.

The ratio is called the polydispersity index, and any system having a range of molecular weights is said to be polydispersed

A Typical Molecular Weight Distribution Curve

200 000 400 000 600 000 800 000 1 000 000Mi (g mol-1)

104 wi

1.0

2.0

3.0

4.0 n = 100 000 g mol-1

w = 199 900 g mol-1

z = 299 850 g mol-1

Polymer Solution Viscosity

When a polymer is dissolved in a solvent and then subjected to flow through a narrow capillary it exerts a resistance to that flow. This resistance is very informative. • It provides information on the size of the polymer• Its Flexibility and shape in solution • Its interactions with the solvent it is disolved in.

Polymer Solution Viscosity

For dilute solutions the ratio between flow time of a polymer solution (t) to that of the pure solvent (to) is effectively equal to the ratio of their viscosity ( h / ho)

oorel t

t

o

orelsp t

tt 1

As this has a limiting value of unity, a more useful quantity is specific viscosity (hsp)

Intrinsic Viscosity [η]

To eliminate concentration effects, the specific viscosity (hsp ) is divided by concentration and extrapolated to zero concentration to give intrinsic viscosity [h]

cKcc H

o

osp 2

Thus plotting hsp/c vs c, the intercept is the intrinsic viscosity [h] and from the slope, KH (Huggins constant, typically between 0.3 - 0.9) can be determined

Intrinsic Viscosity Determination

0.2 0.4 0.6 0.8 1.0

C (g dl-1)

2.0

2.5

3.0

3.5

co

o

[h]

KH[η2]

Intrinsic viscosity [h] can be related to molecular weight by the Mark-Houwink-Sakurada Equation Applicable for a given polymer-solvent system at a given temperature

aυMK

Viscosity-Molecular Weight Relations

Log [h] vs log M (w or n) for a series of fractionated

polymers produces log K (intercept) and a (slope)

A wide range of values have been published– a ~ 0.5 (randomly coiled polymers)

~ 0.8 (rod-like, extended chain polymers)– K between 10-3 and 0.5

Solvent Temp oC

K (x10-3) ml g-1

a MW (x10-3)

Method

Cellulose Cadoxen Cuprammonium

25 25

33.8 8.5

0.77 0.81

20-100 10-100

SD OS

Amylose DMSO Water

25 20

1.25 13.2

0.87 0.68

20-300 30-220

LS LS

Dextran Linear Branched

Water Water

25 34

97.8 10.3

0.50 0.25

2-10 80

LS LS

Typical Mark-Houwink-Sakurada Equation Constants for Several Polysaccharides

Solvent Temp oC

[ dl g-1

a K (x10-3) ml g-1

MW

Kraft Lignin Dioxane 25 0.06 0.12 1638 50,000

Celluose CED 25 1.81 0.75 54.0 50,000

xylan CED 25 2.16 1.15 0.85 50,000

Typical Intrinsic Viscosities, a and K values for Several Naturally Occurring Polymeric Materials

The degree of expansion or shape of the molecular coils of a polymer can be ascertained from its a values (Table 2) • lignin (Newtonian sphere), cellulose (non-freedraining

coil) and xylan (freedraining coil)

Viscosity-average molecular weight (u)– viscosity, like light scattering, is greater for the larger-

sized polymer molecules than the smaller ones, and is much closer to Mw than Mn a

iii

i

aiia

i

aii

MN

MNMwM

1

1

1

11

1

w = weight fraction N = number of molesM = molecular weighta = A constant

– When a = 1, u= w , usually a ~ 0.5-0.9

– a is a measure of the the hydrodynamic volume of the polymer

– varies with polymer, solvent and temperature

Polymer Morphology

The ultimate properties of any polymer (plastic, fiber, or rubber) result from a combination of molecular weight and chemical structure.

Polymers require a particular MW, which depends largely on the chemical structure, to have desirable mechanical properties.

Molecular Weight

Mechanical Property

Polymer MorphologyThe mechanical properties result from attractive forces between molecules– dipole-dipole interactions, H-bonding, induction forces,

London forces or ionic bonding, ion-dipole interactions

CO

O

R

CO

O

R+

+

-

-

C

HN

O

R

C

HN

O

R

+

+

-

-

A lower MW polyamide will produce good fiber properties as compared to the polyester H-bonding

H-bondingdipole-dipole

• Hydrogen Bonding– A dipole-dipole interaction for hydrogens bonded to

electronegative elements• Electrostatic Interaction

Polymer Morphology

HO

R

HO

R

HO

R

HO

R

Weak bond ~ 5 kcal mol-1 (c-c ~ 81 kcal mol-1 )Require short bond distance ~ 2.5Å (c-c ~ 1.46Å)

very importantin cellulose

Polymer Morphology

Intermolecular forces drop off very rapidly with distance important polymer molecules be able to pack together closely to achieve maximum cohesive strength.

ex. Natural Rubber

unstretched state - molecules are randomly distributed

low modulus

stretched state - molecules become aligned, at 600% elongation high modulus

(2000 times higher than unstretched)

unstretched - amorphous / stretched - crystalline