"Chemical Modification" means that we'll change the actual covalent bonds that make cellulose what it is. After we modify it chemically, it's no longer cellulose, but something else altogether ! Depending on how we change the cellulose, we can engineer the physical properties to meet specific needs. (Sometimes, as in the case of old movie films, meeting one need can create more problems, but more about that later !)
In cellulose, glucose molecules are attached to each other such that the polymer is the most extended that it can be. The structure below shows a section of cellulose made from four glucose molecules. (A typical cellulose molecule varies in length, from about 2000 to more than 25000 glucose residues*!)
See three -OH groups that are attached to each ring? They can hydrogen bond quite well, which they do with adjacent cellulose chains. The result is a very stiff and strong material that can hold up the tallest trees, which, as a matter of fact, it does! The major structural component of wood is cellulose.
What would happen if we took away the ability of cellulose to hydrogen bond? The polymer chains would be much more free to move around and become all tangled up. The advantage to doing that is to be able to do more with this material, like casting films and spinning fibers.
So, how do we take away the hydrogen bonding? We have to block the OH group with something that won't hydrogen bond. A relatively cheap and easy way to do this is to replace them with either acetate groups, to make cellulose acetate, or nitro groups, to make cellulose nitrate.
Make sense? Here's a comparison of the molecular structures of cellulose acetate, cellulose nitrate, and plain old cellulose. See the acetate and nitro groups that replaced the OH groups?
Neither one of these structures can hydrogen bond anymore! Think about the differences that hydrogen bonding (or the lack of hydrogen bonding) can make in the physical properties.
| ||Picture a bowl full of cooked spaghetti and you'd be pretty close to what a polymer is like without hydrogen bonding (or any other kind of intermolecular interactions that are relatively strong).|
Remember that the cellulose acetate and cellulose nitrate can't hydrogen bond. So, as the cellulose -OH groups are replaced, the chains spread apart and get more tangled up. Now THIS is something that we can work with! Unlike those stiff hydrogen-bonded cellulose chains, that tangled-up mess of polymer chains can be cast into films. Just imagine taking a big handful of that cooked spaghetti and smooshing it around on a plate! You can't do that with the raw stuff.
Now, why on earth would anyone want to do this? Let's take a look at the applications of cellulose acetate and cellulose nitrate.
* "Physical structure of cellulose microfibrils: Implications for biogenesis", 1991. S. Kuga , R.M. Brown, Jr. , in "Biosynthesis and Biodegradation of Cellulose", C. H. Haigler and P. Weimar, eds., 125-142, Marcel Dekker, New York