One might not think that this little methyl group would make a whole lot of difference in the behavior and properties of the polymer, but it does. As it turns out, how soft or hard a polymer is at a given temperature is determined by what we call chain mobility , that is how well the polymer chains wiggle past and around each other. The more they can move, the softer the polymer is. It helps here to think the scene in the movie Raiders of the Lost Ark , in which our hero Indiana Jones finds himself in an Egyptian temple and ankle deep in slithery poisonous snakes. Polymer chains are like those snakes.
Smooth snakes can move past each other very easily.
But if you could imagine those snakes having giant barbed spikes all up and down their backs like some of the dinosaurs in another Steven Spielberg film, you can see that they might not be moving around quite as much on the floor of that Egyptian temple. Their spikes would catch on each other, and slithering would become quite difficult. Without that extra methyl group getting in the way, they can slither all they want. If the polymer chains can slither and wiggle past and around each other easily, the whole mass of them will be able to flow more easily. So a polymer which can move around easily will be soft, and one which can't will be hard, to put it simply.