4a. Polymers: A crash course - umu.se · 4a. Polymers: A crash course Brief history Natural polymer-based materials, e.g., amber and paper (manufactured from a naturally occurring

4a. Polymers: A crash course

Brief history

Natural polymer-based materials, e.g., amber and paper (manufactured from a naturally

occurring polysaccharide, cellulose), used by people for centuries, and term polymer first

used in 1833.

First entirely synthetic polymer, Bakelite, introduced in 1909.

Despite significant advances in synthesis & characterization of polymers, proper

understanding of polymer molecular structure did not come until 1920s.

Before that, scientists believed that polymers were clusters of small molecules (called

colloids), without definite molecular weights, held together by an unknown force; this

concept was known as “association theory”.

In 1922, Hermann Staudinger (previous

discovery: organic molecules with taste of

coffee) studied rubber and correctly proposed

that polymers consist of long chains of small

repeat units held together by covalent bonds.


For instance: Heinrich Wieland, 1927 Nobel laureate in chemistry, wrote to Staudinger,

“Dear colleague, drop the idea of large molecules; organic molecules with a molecular

weight higher than 5000 do not exist. Purify your products, such as rubber, then they

will crystallize and prove to be low molecular compounds!”

After presentation at conference in Dusseldorf he was highly criticized and got to hear:

The existence of a polymer is as shocking for a chemist as the observation of a 400-meter long elephant is to a zoologist!

But finally, in 1953, Staudinger received his reward for the

understanding of the concept of polymers and his

prolonged effort to establish the science of large molecules

when he was awarded the Nobel Prize in chemistry.

Contemporary response?

Very rough!


Polymer structure

Polymers made by chemical reaction (e.g. free-radical

polymerization) between monomers: long (linear or

branched) polymer chains formed

The number of repeat units in a polymer chain can

be large (n ~ 102 – 106)

The variation of polymer properties is essentially

infinite, due to huge variety of monomers.

Few examples of common polymers:

• Polystyrene (toys, electronic housings, CDs, ...)

• Polyethylene (insulation wires, plastic bags, ...)

• Polyamide or nylon (clothing)

• Polycarbonate (clear, strong; and lighter and

much higher εr than glass: excellent thin lenses)


Common synthetic polymer poly(ethylene) (PE)

comes in drastically different forms:

LDPE: branched chains → poor packing: low

density and weak material; cheap material used in

plastic bags

HDPE: linear chains → good packing: more

compact and strong material; less cheap material

used in e.g. food containers

UHMWPE: very long linear chains (large n):

extremely strong and compact material used in

bullet-proof vests & …

But polymer properties also depend

strongly on chain configuration and

packing

…even ice-hockey

rinks!



Polymer Morphology

Polymers can either be in amorphous (dis-

ordered) state or in crystalline (ordered) state

Crystalline polymers (e.g. HDPE) typically has a

complex structure, which in fact is a mixture of

crystalline (spherulites) and amorphous regions

Amorphous polymers exhibit a glass transition (Tg)

T < Tg: disordered polymer chains are essentially

static (low cp) and polymer material is hard and

brittle like a glass! (e.g. polystyrene & PMMA at RT)

T > Tg: disordered polymer chains move around

(high cp) and material is soft and flexible (e.g. LDPE

and non-vulcanized polyisoprene at RT)


Mechanical properties

PE and PS examples of thermoplastic (or

pliable) polymers: can be reshaped into new

forms at high T in viscous high-T state

Introduction of cross-links between polymer

chains non-reformable thermoset (one

gigantic molecule!)

E.g.: sulfur vulcanization of rubber

(polyisoprene) invented (thanks to fruitful

combination of accident and hard work) by

Charles Goodyear in 1839; it prevents rubber

tires from becoming too soft when hot and from

becoming too hard and brittle when cold

Number of cross-links decides whether thermoset is rubber-like (few, e.g. tires) or

stiff (many, e.g. polycarbonate for lenses, epoxy glue)


Polymer blends

Desirable to mix different polymers to attain combined properties, but very

difficult since fundamental condition for mixing (and reactions in general):

ΔGmix = ΔHmix – TΔSmix ≤ 0

almost never fulfilled

Reminder from TD: All systems spontaneously move toward state with lowest G

[& , remember electrons in SCs and metals])

Reason: (i) Low entropy gain upon mixing of two polymers, (ii) it typically costs

enthalpy to mix two dissimilar materials together

→ ΔGmix > 0

→ polymer blends often phase separate


It is possible to use

surfactants or kinetics to

minimize phase separation

Or phase separation is

desirable:

E.g. carbonated drink bottles

contain laminated sheets of

non-mixable PET (strength)

& PVA (no CO2 diffusion)

Polymer blends

The extent of phase separation often depend on very subtle differences:

+*

*

n

R2

R1

*

O*n

SO

O

O F

F

FK+ (1.33 Å)

Li+ (0.68 Å)

Rb+ (1.47 Å)+


Alternating CP = monomers in alternating fashion

Random CP = monomers in random order

Block CP = monomers joined together in blocks

Copolymers

So far homopolymers (one monomer), but combined & new types of properties attained

from design and synthesis of copolymer (CP): 2 (or more) monomers joined together

Also possible to attain new and

desired properties (e.g. solubility

& new emission color ~Eg) by

attaching pendant side groups to

the main chain:

*

*

n

R2

R1

**

n

E.g.: polymer-electrolyte-block CP with (i) soft block (RT > Tg) providing ion-transport

& (ii) hard block (RT < Tg) providing dimensional stability to prevent short circuit


Natural polymers: The key to life!

Only synthetic polymers up to now, but the world is

full of very important and inspirational natural

polymers, including ”the most advanced material”:

Deoxyribonucleic acid (DNA)

Fascinating double-helix structure first suggested by

Crick & Watson (with aid of others in 1953): 2

phosphate-sugar-based co-polymers forming helical

supermolecule via hydrogen bonding between

pendant base groups, adenine (A), cytosine (C),

guanine (G) and thymine (T)

Base groups can only “pair up” in certain order: A+T,

T+A, C+G and G+C

Which is the origin to our genetic code = who we are!


Deoxyribonucleic acid (DNA)

Cell duplication Via action of various enzymes (e.g.

polymerase): 2 polymer chains unzip and new appropriate

bases (and sugar and phosphate groups) are added to each

single chain; the result is 2 identical DNA molecules!

Protein synthesis A gene is a part of a DNA strand that codes

for one specific protein. This gene is copied and info brought

to desired place by RNA polymers (in transcription step)

The genetic code is divided into codons: series of 3 bases

(e.g. ACT). Total number of different codons: 43 = 64

One (or more) codon(s) ↔ one specific (out of 20) amino

acids

During actual synthesis of protein polymer (the translation

step), amino acids (R →) are linked together via peptide

linkages in a specific order dictated by the genetic code


• Enzymes (e.g. cellulase, polymerase) used by all living organisms to speed up

reactions

• Antibodies that bind to and neutralize specific antigens, such as bacteria and viruses

• Oxygen-carrying units in blood stream (hemoglobin)

• Structural units: skin, hair, fingernails, wool, fur, silk (keratin)

tendons, bone, teeth and again skin (collagen)

Proteins Highly versatile polypeptides (or polyamides),

containing a specific order of the 20 different amino acids

(defined by R). Many important functions; a few examples:

Proteins inspired Wallace Carothers (who actually wanted to prove that polymers were

indeed macromolecules) to invent comparatively mundane, but very useful, nylon in

1935

Flat molecule that form strong fibers used for clothing


All these cellular activities require energy; provided

during cell respiration (i.e. oxidation of glucose):

C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy

Input chemicals -- glucose and oxygen --

produced by green plants, when certain green

pigments (chlorophylls) absorb sunlight during

photosynthesis:

6CO2 + 12H2O + light energy → C6H12O6 + 6O2 + 6H2O

Significant portion of produced glucose (some is

consumed by plant in cell respiration during night) is

stored in complex carbohydrates …

…in the form of repeat units in two polymers with

nominally identical structure (isomers): cellulose and

starch…


Starch: soft polysaccharide (PS)

containing glucose repeat units,

dissolves in hot water, digestible by

humans

Cellulose: crystalline and strong PS

containing glucose repeat units, not

soluble in water, only digestible by

animals that posses the enzyme

cellulase

Small subtle difference can make all the difference in biological polymers; also

remember that mutations (mistakes in DNA replication) can cause cancer, and that

mistakes during protein folding can cause other several serious diseases, such as mad

cow disease and Alzheimer’s disease

But what’s the difference?

A. Glucose repeat units are pointing in different directions!


Synthetic polymer appeal:

Up to mid-1970s, synthetic polymers attracted interest primarily because of an appealing

set of mechanical properties:

• Easy to process into desired shapes or functions from solution and melt

• Light-weight (typically based on light elements: C, H, O, N,...)

• Strong (can be comparable to steel)

• Flexible (”plastic”)

Also important:

• “Infinitely” rich chemistry → enormous variety of materials

• Cheap production (in large quantities): simple process and common raw materials

However, no interest in electronic properties of polymers (instead opposite since lack

thereof made them useful as electrical insulation), but more on this soon-to-be change in

next lectures…

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4a. Polymers: A crash course - umu.se · 4a. Polymers: A crash course Brief history Natural polymer-based materials, e.g., amber and paper (manufactured from a naturally occurring