Polymers for special uses
1. Basics
What a polymer is, main characteristics of polymers, different class of polymers ...
2. Biocompatible polymers
3. Expanded polymers
4. Bioplastics
5. Super-absorbent polymers
Absorption occurs when a liquid, such as water, is taken up (or absorbed by) a solid. When this happens, the
solid often grows in size (swells). Many polymers can also absorb large quantities of liquid, and by doing so,
they find usefulness in many industries and aspects of every day life: they are called super-absorbent polymers
and, when the absorbed liquid is the water, the swollen material is called hydrogel. The water absorbed in
hydrogels gives them the typical transparency and softness. Few well known applications of synthetic superabsorbent polymers are disposable nappies, hair gel, water crystals, contact lenses and some prosthesis. An
example of natural super-absorbent polymer is collagen, a protein that is found exclusively in animals: it is
the main component of connective tissue, and is the most abundant protein in mammals. Collagen
is the polymer responsible for the formation of the classic gelatin that is its hydrogel. Did you
know that your muscles and internal organs are hydrogels? Sounds weird? Just think you have
lots of proteins in your body and these proteins are surrounded by an enormous quantity of water.
The property of certain polymers of absorbing large quantities of water is due to their microscopic
structure and to their chemical affinity with water. Polymer chains of a super-absorbent polymer
are not free: they are arranged in a sort of three-dimensional network thanks to some links among
chain segments. This particular arrangement of polymer chains is called crosslinked structure.
When water meets this structure is incorporated and entrapped in it and the result is a swollen soft material:
a gel . A funny application of super-absorbent polymers is a well known toy: the ‘slime’..Such material
is a hydrogel, mainly composed by polyvinylalcohol polymer that is firstly dissolved in water and then
crosslinked by a molecule called borax.
Homopolymers
Homopolymers are made by one monomer type:
Chemical affinity
All molecules, both small ones and polymers, interact with each other, attracting
each other through electrostatics. Some molecules are drawn to each other more
than others. Polar molecules stick together better than non polar molecules. For
example, water and methane have similar molecular weights. Methane's weight is
sixteen and water's is eighteen. Methane is a gas at room temperature, and water is a
liquid. This is because water is very polar, polar enough to stick together as a liquid,
while methane is very non polar, so it doesn't stick together very well at all.
Then, polymers rich with polar chain segments have good affinity with water, while
non polar polymers have no affinity.
Solutions and gels
The left figure is the representation of uncrosslinked polymer chains. If
they have a good affinity with water their meeting produces a solution.
It means a liquid composed of polymer chains, each one completely
surrounded by water molecules. Macroscopically, a solution is uniform
in each point and appears transparent, colored or colorless. Moreover, it
can be more or less viscous as a function of the amount of polymer that
is dissolved.
If polymers are crosslinked some links between chains or between
segments of the same chain are established. The links, pictured as knots in
the right figure, can be more ore less strong (i.e covalent bonds or
electrostatic interactions as hydrogen bonds). When water meets this
structure the chains are not able to separate and leave but the affinity with
the liquid let it enter more and more, causing a swelling of the threedimensional network. Characteristics of gels depend on polymer and on
linkage density: the more crosslinked the material, the more elastic and
stronger it is (low swelling), while low crosslinking produces very soft
and weak gels (high swelling). Just think about difference between contact
lenses and gelatin.
Activity. Step 1. Test some super-absorbent polymers
Description of the activity
The experience aims to show the behaviour of four super-absorbent materials that belong to our
everyday life. Gummy bears and sheet gelatin contain collagen, a super-absorbent natural polymer
(extracted from leaf), while disposable nappies and water crystals are based on super-absorbent
synthetic polymers. Finally, a simple analysis of the results is proposed.
Activity. Step 1. Test some super-absorbent polymers
Materials
3 transparent plastic cups half-filled with tap water , a graduated cylinder or graduated transparent cup,a kitchen
balance (division: 1 g), gummy bears, disposable nappy, sheet gelatin, water polymer crystals (available at plant
stores).
Procedure
Gummy bears
1. Obtain two different colored gummy bears. Weight one, then place it in the plastic cup with tap water.
2. Wait for at least 1h, then place the wet bear close to the dry bear. What do you observe? What about softness,
transparency, shape and size?
3. Weight the swollen bear to evaluate the quantity of water that has been absorbed.
4. Repeat step 2 and 3 since the equilibrium has been reached, meaning that no more change is detected.
5. Finally, try to calculate the percent of swelling (in terms of percentage increase of weight).
Sheet gelatin
…………….
Water Crystals
………………
Disposable nappy
1. Take a piece of the inside material of a disposable nappy (about 1g). Place it in a transparent graduated cup (better
if a graduated cylinder is available) and shape it on the bottom of the cup.
2. Register the volume of the dry material
3. Then add one tablespoon of water and stir gently. Repeat this action since all the material has became transparent.
4. Register the final volume of the swollen plastic and try to calculate the percent of swelling (in terms of percentage
increase of volume).
Activity. Step 1. Test some super-absorbent polymers
Conclusions and teaching notes
Gummy bears contain collagen together with other ingredients such as sugars, fruit juices, waxes.
Sheet gelatin is made of pure collagen.
Inside material of disposable nappies is a synthetic polymer called sodium polyacrylate.
Water crystals are made of a synthetic polymer called polyacrylamide.
Collagen, sodium polyacrylate and polyacrylamide are polymers different in terms of chain structure, number and
strength of links in the crosslinked structure. They all are characterized by capacity of absorbing a great quantity of
water, but, depending of their structure, the rate of absorption and the percent of absorbed water is different. Accordingly,
also the appearance of the gels is different: i.e. swollen pure collagen (gelatin) is very soft while swollen water crystals
are soft but less than gelatin.
Transparency is a common characteristic of the four kinds of hydrogels, indeed it depends on inside water.
The percent of swelling can be calculated by the following formula (if weights are registered):
% of swelling =
weight of swollen material – weight of material before swelling
weight of material before swelling
x 100
or by the following (if volumes are registered):
% of swelling =
volume of swollen material – volume of material before swelling
volume of material before swelling
x 100
Activity. Step 2. Test some super-absorbent polymers
Description of the activity
The experience aims to test four kinds of super-absorbent materials that belong to our everyday life. Gummy
bears and sheet gelatin contain collagen, a super-absorbent natural polymer (extracted from leaf), while
disposable nappies and water crystals are based on super-absorbent synthetic polymers. The behavior of the
materials in presence of water as a function of temperature and addition of table salt is studied and justified.
Materials
Procedure
Conclusions and teaching notes
Gummy bears contain collagen together with other ingredients such as sugars, fruit juices, waxes.
Sheet gelatin is made of pure collagen.
Inside material of disposable nappies is a synthetic polymer called sodium polyacrylate.
Water crystals are made of a synthetic polymer called polyacrylamide.
They all are characterized by capacity of absorbing a great quantity of water, but, depending of their structure, the
rate of absorption and the percent of absorbed water is different. Accordingly, also the appearance of the gels is
different: i.e. swollen pure collagen (gelatin) is very soft while swollen water crystals are soft but less than
gelatin.
The behavior is different also when the polymers are tested with hot water. Indeed materials containing collagen
likely dissolve in water at 60-70°C, because weak links, responsible of crosslinked structure, are broken by
giving moderate energy, thereby preventing gel formation. Crosslinked polyacrylamide and sodium polyacrylate
claim stronger links that cannot be broken at 70°C.
Table salt has an interesting role in water absorption when disposable nappies are tested: in this case the addition
of sodium chloride to swollen material allows the water to escape, thus causing the collapsing of gel. The
phenomenon can be justified by a sort of competition between sodium chloride and water: there is higher affinity
between sodium chloride and polymer than between water and polymer.
Exercise. Step1
Find the right comment to the following statement: ‘hydrogels can turn in liquid if
much water is added’.
a) Correct: hydrogels are materials soluble in water, as suggested by the prefix hydro-.
b) No, hydrogels are hydrophobic materials, as suggested by the prefix hydro-.
c) Correct. If much water is added, hydrogel swells more and more till the crosslinked
structure breaks.
d) Not correct: hydrogels can turn in liquid only when heated.
e) Not correct: hydrogels can turn in liquid when links that form crosslinked structure
break: if much water is added, simply it is not completely absorbed.
Exercise. Step1
Find the right comment to the following statement: ‘hydrogels can turn in liquid if
much water is added’.
a) Correct: hydrogels are materials soluble in water, as suggested by the prefix hydro-.
the prefix hydro- suggests a relationship between gel and water,
not necessarily the solubility
TRY AGAIN 
Exercise. Step1
Find the right comment to the following statement: ‘hydrogels can turn in liquid if
much water is added’.
e) Not correct: hydrogels can turn in liquid when links that form crosslinked structure
break: if much water is added, simply it is not completely absorbed.
Correct choice
COMPLIMENTS!
Exercise. Step2
At room temperature gelatin is a hydrogel. By moderate heating it becomes a
solution. Why?
a) Because collagen is able to dissolve in water when heated.
b) Because collagen, if heated, becomes a liquid, thus the system seems a solution.
c) Because gelatin, if heated, melts.
d) Because links among collagen chains are weak and can be broken by giving
moderate energy, thereby collapsing the gel.
e) Because chemical affinity between water and collagen raises by heating, thereby
changing the gel in solution.
Exercise. Step2
At room temperature gelatin is a hydrogel. By moderate heating it becomes a
solution. Why?
a) Because collagen is able to dissolve in water when heated.
Correct, but not complete. Why is it able to dissolve in water
when heated? 
Exercise. Step2
At room temperature gelatin is a hydrogel. By moderate heating it becomes a
solution. Why?
c) Because gelatin, if heated, melts.
gels are complex systems composed by a liquid entrapped in a cage of
polymer, they don’t melt. At high temperature they loose water because of
evaporation
TRY AGAIN 
Exercise. Step2
At room temperature gelatin is a hydrogel. By moderate heating it becomes a
solution. Why?
d) Because links among collagen chains are weak and can be broken by giving
moderate energy, thereby collapsing the gel. .
Correct choice
COMPLIMENTS!
AKNOWLEDGEMENTS
- ELISABETTA DELLE DONNE (PIXEL)
- FRANCESCO PINZANI AND RICCARDO ROSSI
(CONNECTIS)
AND ALL THE PROJECT PARTNERS