Apparently. It seems that they flow r e a l l y s l o o w l y. The reason they can still hold your shopping is that the
polymer molecules (
polyethylene - or rather
LDPE) are a sort of tree shape that forms stronger
network interactions than unbranched chains. Like many substances formed from chains (
jelly/
jello,
dough,
hair)
alignment of the chains makes the material stretchy while tangling makes it stiff. Heating the plastic bag a little (too much will burn it - you KNOW that's bad!) will melt it and it will flow. Think cooked
spaghetti (mmmm..spaghetti!).
LDPE, on the other hand, is so entangled with inter-chain contacts that flow is impossible. However, once the force on a bag exceeds a certain limit, it deforms plastically - that is it goes all stretchy and dumps your eggs and milk on the floor. Hungry now :-(