A detailed analysis of the three-phase structure of isotactic poly(butene) was conducted by conventional and temperature-modulated calorimetry. The development of the crystalline, mobile amorphous, and rigid amorphous fractions was analyzed as a function of thermal history, upon isothermal and non-isothermal crystallization. It was found that, under the chosen experimental conditions, the amount of rigid amorphous phase (w(RA)) in PB-1 ranges from w(RA) = 0.14 to 0.23, with higher values formed when the polymer is crystallized at low temperatures or at high cooling rates from the melt. Comparison of total and frequency-dependent reversing heat capacity curves suggested that the rigid amorphous phase of isotactic poly(I-butene) vitrifies after completion of the crystallization process and that its full mobilization takes place at around 50 degrees C. The exact temperature of complete devitrification is slightly affected by the thermal history of the material. An effort to link the mechanical properties of PB-1 to the three-phase structure was attempted, and a correlation of Young's modulus with the solid fraction at the temperature of analysis, composed of crystalline and rigid amorphous phases, was proposed. (c) 2008 Elsevier Ltd. All rights reserved.