  By means of sophisticated Monte Carlo methods, we investigate the
conformational phase diagram of a simple model for flexible polymers with
explicit thickness. The thickness constraint, which is introduced geometrically
via the global radius of curvature of a polymer conformation, accounts for the
excluded volume of the polymer and induces cooperative effects supporting the
formation of secondary structures. In our detailed analysis of the temperature
and thickness dependence of the conformational behavior for classes of short
tubelike polymers, we find that known secondary-structure segments like helices
and turns, but also ringlike conformations and stiff rods are dominant
intrinsic topologies governing the phase behavior of such cooperative tubelike
objects. This shows that the thickness constraint is indeed a fundamental
physical parameter that allows for a classification of generic polymer
structures.
