This is the first time I've seen a color version of a picture of a chain-folded polymer crystal. I like it a lot. It sure beats the black and white drawings that I put in my dissertation. Source
Except in exceptional circumstances, polymers do not crystallize in an extended-chain conformation (spaghetti in a box) but instead are repeatedly folded along the chain and re-enter the crystal. You can see this in the blue and green chains above - the red ones appear to be too short to fold over and re-enter. The length of the chain folds depends on the temperature and can be changed by annealing - another topic for another day.
This is from a simulation effort, but it still is better than the idealized hand drawings like this: