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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
super-absorbent 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.
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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.  
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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
three-dimensional 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.
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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.
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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).
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Activity. Step 1. Test some super-absorbent
polymers
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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
60-70C, 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 70C. 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.
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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.
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Exercise. Step1

• Find the right comment to the following
  statement hydrogels can turn in liquid if
• much water is added.
• 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 ?
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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!
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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.
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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? ?
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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 dont melt.
At high temperature they loose water because of
evaporation TRY AGAIN ?
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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!
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AKNOWLEDGEMENTS - ELISABETTA DELLE DONNE
(PIXEL) - FRANCESCO PINZANI AND RICCARDO ROSSI
(CONNECTIS) AND ALL THE PROJECT PARTNERS