  This work introduces a methodology for the statistical mechanical analysis of
polymeric chains under tension controlled by optical or magnetic tweezers at
thermal equilibrium with an embedding fluid medium. The response of single
bonds between monomers or of entire groups of monomers to tension is governed
by the activation of statistically interacting particles representing quanta of
extension or contraction. This method of analysis is capable of describing
thermal unbending of the freely jointed or wormlike chain kind, linear or
nonlinear contour elasticity, and structural transformations including effects
of cooperativity. The versatility of this approach is demonstrated in an
application to double-stranded DNA undergoing torsionally unconstrained
stretching across three regimes of mechanical response including an
overstretching transition. The three-regime force-extension characteristic,
derived from a single free-energy expression, accurately matches empirical
evidence.
