Why are inorganic crystals big and beautiful, but polymer crystals are only nano$-$size

and opaque?

What kinds of vinyl polymers can crystallize?

Why are polyethylene and polypropylene rigid plastics, but ethylene$/$propylene

copolymers are rubbery?

Why is the optimum temperature for crystallization about $0\mathrm{.}9TmC?$

Why do polyethylene and nylon combine rigidity and strength with impact strength?

Why do acetal and nylon gears and bearings have such good lubricity?