How Big are Polymers?
It would be nice if we could assume that anyone with a high school diploma would be familiar with the molecular structure of say, water or benzene. But, to our jaundiced eye, the current staple of many high school science curriculums seems to be “Saving the Rainforest” and molecular science appears to be largely an afterthought. But that’s a different rant. The point we wish to make is that molecules like water, benzene and the like are generally called “low molecular weight” or “low molar mass” materials by polymer scientists. As a rule of thumb, molecules having molecular weights of, say, less than 500 g/mole are considered low molecular weight materials. High molecular weight polymers, on the other hand, are covalently bound, chain-like molecules that generally have molecular weights that exceed 10,000 g/mole and can be as high as 107 g/mole. Between these extremes of low and high molecular weights, there is a poorly defined region of moderately high molecular weight materials and such molecules are often referred to as oligomers.
To give you a feel for the difference between a low molecular weight material and a high molecular weight polymer, let’s next put them on a scale that we can all relate to. If we assume that a single methylene group, CH2, may be represented by a single bead in the figure above, then the gas, ethylene, is simply two beads joined together. The chain of beads shown in the figure would represent not a polymer (it’s too short), but an oligomer made up of about 100 CH2 units. Let’s further assume that the length of the ethylene molecule is 1 cm. Now, if we consider a polyethylene molecule that has a molecular weight of 700,000 g/mole, it would be made up of 700,000/14 or roughly 50,000 methylene groups. On our scale this would be equivalent to a chain roughly a quarter of a kilometer long. These are very big molecules indeed!
Many of the physical properties of polymers are simply a consequence of their large size. To also get a feel for this, let’s consider building a simple hydrocarbon chain one carbon atom at a time, filling up all the unsatisfied valences with hydrogen atoms, as shown in the figure below. If we have just one carbon atom (and hence four hydrogens), we have the gas methane, often referred to as “natural gas”, perhaps because it not only emanates from the ground, but also the rear end of cows. The next three in the series are ethane, propane and butane, which have, respectively, two, three and four carbons in their chains. These are also gases at ambient temperatures and pressures, the latter two being commonly used for heating and cooking. Liquids, commonly used as auto and jet fuels typically have carbon chain lengths of 6–12. As we increase the carbon chain lengths further, the viscosity increases and we go from liquid materials used for baby oils, to “semi-solid” materials used as soft and hard candle waxes. At even higher carbon chain lengths, typically exceeding 30,000, we encounter hard, solid polyethylenes.