I'm talking about the fancy ones you get when you're in the hospital. These are made of a polyester film made by DuPont called Mylar. The balloons are made of a sandwich, composed of Mylar and aluminum foil. Materials like this, made of two kinds of material, are called composites . A special family of polyesters are polycarbonates .
Polyesters have hydrocarbon backbones which contain ester linkages, hence the name.
I realize that terephthalate is not the kind of word most English-speaking mouths are used to saying, but with practice you should be able to say it with only a slight feeling of awkwardness when it rolls off your tongue. The ester groups in the polyester chain are polar, with the carbonyl oxygen atom having a somewhat negative charge and the carbonyl carbon atom having a somewhat positive charge. The positive and negative charges of different ester groups are attracted to each other. This allows the ester groups of nearby chains to line up with each other in crystal form, which is why they can form strong fibers . The inventor who first discovered how to make bottles from PET was Nathaniel Wyeth.