Why IPNs?
So what's so important about IPNs anyway? Well that is what this page
is going to tell you. Before this question can be answered, there are
a few more things you should understand about IPNs.
IPNs as a Class of Heterogeneous Materials
IPNs are one of the three general classes of heterogeneous materials, along with
composites and
blends. IPNs differ from composites
because there is no load-bearing reinforcing agent like fibers put into the material. Blends
and IPNs are more closely related but there are distinct differences. Blends generally consist
of two or more polymers that are mixed together and none of the components are
crosslinked to
any appreciable extent. On the other hand, IPNs are comprised of two polymeric components that
are both crosslinked. The crosslinking is what really separates IPNs from blends because it
affects what kind of morphology is produced. More on this in a minute!
Blends vs. IPNs
Before going any further, there is a little quiz available
to test your knowledge of polymer blends. Go ahead and take it. It should take you all of two
minutes to finish it. If you're a little rusty on polymer blends, there are a few links at the
end of the quiz that will give you a refresher on this subject. The material is not too
complicated but there are a few concepts you should be familiar with before going much farther.
So what kind of difference does crosslinking make in a mixture of polymers or putting the
question a bit more bluntly, what is the difference between IPNs and blends? Well, crosslinking
a multi-component polymer system still buys you the things associated with
thermoset material:
a broad glass transition temperature, chemical insolubility (but capable of being swelled), and
some increased mechanical properties (especially creep and flow). While these properties are important in IPNs, it doesn't
quite hit the mark on why we would be interested in multi-component systems. After all, you can
get these types of properties from crosslinking a homopolymer such as
poly(methyl methacrylate) or
natural rubber.
By combining two different polymeric materials, one usually hopes to obtain something that at
the very least has properties that are somewhere in between those of the two components.
Sometimes if you are really lucky, you might be able to make a new material that shows a
synergistic increase in one or more of the properties of the material. This is the motivation
behind blending two polymers together. So where does IPNs fit into this? Well, you already know
that mixing two polymers together often results in a phase-separated system. This isn't always a
bad thing but in most cases you end up with a new material with lousy properties. There are
a few tricks people have learned to try get around this though, such as using compatibilizers.
Click here to learn more about these
tricks. Using IPN materials circumvents the problem of gross phase separation seen in many blends
by using the idea that "All because they want to phase-separate doesn't mean that they are
able to phase separate.
The Grade School Experiment
Isn't it weird how little boys and girls can't stand to be around one another and then, one day,
you see a complete change in their attitudes toward each other? Well the little boys and girls
that can't stand each other can be used to model the differences between a blend and an IPN.
Imagine packing a room with boys and girls and we'll assume that the boys and girls are mixed
while filling the room. Almost immediately, you'll see boys going to talk to other boys and
if at all possible, avoiding girls. The girls will pretty much do the same thing. You might
eventually see the room's population completely divided, with boys on one side of the room and
girls on the other. This behavior is shown
below. What we have here is a simulation of the behavior of an immiscible polymer blend,
where the boys represent one polymer component and the girls represent the other.
Now imagine that all the children enter the room and before the boys and girls
start separating, the teacher tells the boys to grab on to the boys closest to them (hold
hands, grab a shirt, hook an arm, etc.) and
tells the girls to do the same thing with the closest girls. Now you have a situation where
the boys and girls can't separate even though they still want to. Its like a big game
of Twister. The boys and girls are each forming a network and the networks are interpenetrating
each other. The boys and girls don't like the situation but they are entangled with each
other to such an extent that they can't separate. This concept is shown below and is the working
idea behind IPN materials.
Putting It all Together
The point of the little experiment described above it that while the two components of an IPN
still want to phase separate, they can't. This brings up the important consideration of kinetics
versus thermodynamics. Thermodynamics dictates that the components should phase separate. However
if there is no pathway for this to occur, such as in this case by creating interpenetrating networks,
then the components can't phase separate.
To sum it all up, IPNs control the phase separation in a multi-component blend by the formation
of networks. The supression of phase-separation isn't complete, but that is discussed in the
Anatomy of an IPN. For now it is sufficient to list the major benefits
of IPNs compared to blends:
- IPNs provide a way to control the phase-separation that occurs in multi-component systems.
The control of phase-separation allows multi-phase materials to be made which have useful
mechanical, permeation, and optical properties.
- By using a multi-component system, one has the potential of obtaining materials with a
range of properties and perhaps generating a synergistic effect on one or more of the
properties.
- The use of networks allows materials to be made that are insoluble in solvents.
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