Materials with negative Poisson's ratios (Le. those that expand in the transverse when tensile load is applied in the longitudinal) are also known as 'auxetic' materials. While there are examples of naturally occurring auxetic materials, the increasing interest in synthesising this property in various materials and at various levels of physical magnitude from nano-scale to macro-scale has led to increasing research over the past two decades. As well as elasticity theory, much theoretiCal modelling has shown the potential benefits of these materials in different situations, r' such as self-cleaning filters and molecular sieves, push-fasteners, medical bandages and sutures, smart textiles and energy-absorbing protective materials. One branch of auxetic research concentrates on the production of auxetic polymers at the microstructural level. The production of au;