Friedman's differential isoconversional approach is applied to obtain effective activation energy (E-alpha) for Poly (butylene terephthalate) and PBT/15AMMT nanocomposites. The presence of 15AMMT content has largely affected E-alpha values. Further Vyazovkin and Sbirrazzuoli's approach is used to determine Hoffman-Lauritzen parameters (K-g & U-star). The crystallization regime transition (II-III) occurs for neat PBT at 189 degrees C and its nano composites is at 189-192 degrees C. K-g(III)/K-g(II) ratios are closer to theoretical value of 2. However the corresponding value of U-star is inappropriately too low with negative sign (Correlation coefficient R-2 = 0.9-0.99) and which is however highly anomalous when compared to other polymers. Setting U-star to 6300 J/mol, 4300 J/mol and 0 J/mol, resulted in poor K-g(III)/K-g(II) ratios and R-2. This imply that there is a considerable chain mobility during regime II crystallization which is explained in terms of rapid extraction of PBT chain segments by the force of crystallization, following the tube-reptation postulate.