  We present the results of analytic calculations and numerical simulations of
the behaviour of a new class of chain molecules which we call thick polymers.
The concept of the thickness of such a polymer, viewed as a tube, is
encapsulated by a special three body interaction and impacts on the behaviour
both locally and non-locally. When thick polymers undergo compaction due to an
attractive self-interaction, we find a new type of phase transition between a
compact phase and a swollen phase at zero temperature on increasing the
thickness. In the vicinity of this transition, short tubes form space filling
helices and sheets as observed in protein native state structures. Upon
increasing the chain length, or the number of chains, we numerically find a
crossover from secondary structure motifs to a quite distinct class of
structures akin to the semi-crystalline phase of polymers or amyloid fibers in
polypeptides.
