The crystalline polymer preferably has a crystallinity of at least 20%, particularly at least 30%, especially at least 40%, as measured by DSC. Suitable polymers include polyolefins, in particular polyethylene; copolymers of olefins with copolymerisable monomers, e.g. copolymers of ethylene and one or more fluorinated monomers e.g. tetrafluoroethylene, or one or more carboxyl- or ester-containing