Lyn Bently, of Dunedin, asks :-

What determines whether or not a plastic bag rustles, something that is embarrasing at the theatre?

Neil Edmonds, a chemist with Auckland Industrial Technologists Ltd, responded.

The noise factor is due to a combination of factors such as the (plastic) film thickness, the area under compression, the average molecular weight of the polymer chains and more importantly, the type of plastic.

Plastics materials are polymers or long-chain molecules, and display a property known as the glass transition temperature. This is the temperature at which the material becomes flexible on heating, due to sufficient energy becoming available to allow for movement of chemical groups in the polymer molecules. A classic example of the importance of the glass transition temperture is shown by a demonstration often carried out in secondary schools using rubber tubing and liquid nitrogen. After immersion for a few minutes in the liquid nitrogen the rubber becomes brittle and, when dropped or hit, shatters like glass.

Different plastics have different glass transition temperatures. Polyethylene has a very low glass transition temperture, and hence polyethylene bags make little noise when crushed at room temperature, as they are in their 'flexible state'. Plastics used for confectionery bags (such as cellulose acetate) have glass transition tempertures over 100 Celsius and so are brittle materials at room temperature. Hence they make more noise when compressed.

The story becomes even more complex when one also considers crystallinity in plastics. But the main answer is to take your lollies in a polythene bag.