4a. Polymers: A crash course - umu.se · 4a. Polymers: A crash course Brief history Natural...
Transcript of 4a. Polymers: A crash course - umu.se · 4a. Polymers: A crash course Brief history Natural...
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.
4a. Polymers: A crash course
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!
4a. Polymers: A crash course
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)
4a. Polymers: A crash course
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!
4a. Polymers: A crash course
4a. Polymers: A crash course
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)
4a. Polymers: A crash course
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)
4a. Polymers: A crash course
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
4a. Polymers: A crash course
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)+
4a. Polymers: A crash course
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
4a. Polymers: A crash course
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!
4a. Polymers: A crash course
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
4a. Polymers: A crash course
• 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
4a. Polymers: A crash course
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…
4a. Polymers: A crash course
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!
4a. Polymers: A crash course
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…