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We're currently studying "the chemistry of contact lenses", which is mostly polymers and gels. I just looked at the package of two pure hydrogel lenses made out of the same monomers, but they have different water content? Why?

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There are a few things that influence the properties of polymers:

  • Molecular weight of the chains, i.e. how many monomers are joined in each chain, can drastically alter the polymer's properties. For example, for polyethylene, UHMWPE has extremely long chains and produces a very strong material suitable for bullet-resistant vests, whereas LLDPE has much shorter chains that produce a weaker material that is much more ductile (think about how easy it is to stretch a plastic shopping bag or the like).

  • Another factor that affects a polymer's properties is branching. Branching is when a polymer is not made up of nice straight chains, but chains that occasionally fork off into several different chains. The difference between LLDPE and LDPE is that LDPE has a lot more branching. This keeps the chains from packing as nicely so LDPE isn't as strong.

  • The other big factor (other than adding another monomer to make a copolymer) is the introduction of crosslinking. Crosslinking is where different chains are connected to one another, often by addition of a molecule that can bond to a chain at both ends, e.g. rubber vulcanization crosslinks the different chains with linear sulfur chains. Natural rubber is very different than what most people think; it's a soft sticky material that doesn't hold its shape. The polyisoprene chains it's composed of aren't crosslinked and can easily slide past one another, but once crosslinked, the chains are connected and aren't free to move as much. Depending on the degree of crosslinking, you can go from something somewhat soft, like a bouncy ball, to something very rigid, like a car tire.

Hydrogels are not simple linear polymers. While they're mostly made from a single monomer, small additions of related molecules are used to control how they polymerize. N.B. These apply generally to silicones, other polymers may have other branching/termination processes. If the monomer only has two sites at which it can connect to other monomers, naturally the only structure that can be formed is a linear chain. One can control the length of the chains by adding a small amount of a terminator—a monomer that can only make one connection. If a terminator reacts with a growing chain, it stops the growth as it can't connect to anything else. The more terminator added, the shorter chains are, on average. Similarly, if you add a monomer that has more than two reactive groups, branching and crosslinking will occur. Often, a non-crosslinked polymer is supplied as a liquid, then a crosslinking agent is added to harden the polymer. Dow Corning has a great primer on the actual chemistry involved.

What makes a hydrogel special is that it's a polymer network that encapsulates water, i.e. it's an extensively crosslinked polymer that is hydrophilic. If the polymer was just a series of individual chains, it would just dissolve in water since nothing is holding the different chains together. When crosslinked, water can get in, but the mesh of polymer is linked together enough to hold its shape, though it may swell to accommodate the water. Depending on the degree of crosslinking, more or less water can fit within the structure. A less linked polymer can fit more water since there are fewer crosslinks resisting, but is less rigid when hydrated for the same reason (think Jell-O). A more crosslinked polymer won't absorb as much water, but will be much more rigid (like a contact lens). Contact lenses have to balance holding their shape, oxygen permeability, surface hydrophilicity, etc. so there are many reasons why, despite being based on the same primary monomer, two different brands may have a different blend of the other stuff that lets one control the hydrogel's properties.

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When a polymer is produced, the final result does not only rely on what the monomers were. For example, there is "extended" polystyrene and "extruded" polystyrene. Or there are polystyrene sulfonate chemical polymers and etc.

Since nowadays, we need polymers to do many things for us (sheathing, sheltering, and sometimes even heat conductivity), we usually tend to modify their structure to make them more temperature-resistant, harder, stronger and ...; and that is because researches on new polymers are not always cheap or successful.

Let's explain with an example. Imagine you have $10000$ ethylene monomers that you will need to make a polymer from. Each of the PE polymers require $100$ ethylene molecules to form. Since you need this thing to be a little bit stronger, you would pressurize it, heat it, liquify it etc. So the result is a set of well-arranged polymer molecules [You can imagine lines of pupils standing in a school's yard, waiting to go to their classes] They are resistant to minor temperature rises and can be used in places where hard polymers are needed. (actually, not yet, but you are currently imagining :))

On the other hand, Imagine the raw "plastic" that is industrially produced with $10000$ monomers and the polymers are made of 100 ethylene molecules. This time it seems as if the pupils are dispersed through the school yard, though in tact with each other. You can easily conclude that this polymer is potentially weaker and is more vulnerable to temperature differences.

The first example is called "extruded polyethylene" and the 2nd one is called "expanded polyethylene". Notice that the monomers are the same, but the result is different in as big of a range that includes usage to specific heat capacity.

This is not the only case. You remember that I mentioned PS sulfonate. Sulfur, in here, is added to polystyrene to make chain-like "links" to polymers. For example, imagine the lines of pupils again. These sulfonates act as some older students that hold the pupils still and do not let them escape. These chains make polystyrene sufonate ideal for being an agent for dying clothes (since it becomes resistant to dissolution).

In your case, Hydrogel monomers are the same, but as I described above, either the how of their production or additional materials that modify the behavior of the Hydrogel or both are different.

Sorry, I didn't find any appropriate applicable images without license and I hope the description is not that vague. Hope I've helped.

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I don't konw much about contact lenses. But for hydrogel, the reason may be as followed:

Conventional synthesized hydrogel is chemical crosslinked polymer network. Except the monomer, another reagent named crosslinker with multi-functionality such as the MBAA(N,N'-Methylenebisacrylamide)is needed. Then the swelling ratio or the water content of a hydrogel is strongly affected by the relative amount of crosslinker. Higher the crosslinking density, lower the water content.

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  • $\begingroup$ It's better if you use capital letter after a full stop. $\endgroup$ – Aditya Dev Apr 8 '16 at 8:42

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