The isothermal crystallizations of three kinds of commercial isotactic polypropylene have been studied by DSC and rheometric experiments

in a range of temperatures where the rate of crystallization is moderate. As the crystallization proceeds

volume contraction induces tensile force upon the parallel plates. The tensile force relaxed quickly in liquid states

but after a critical amount of the relative crystallinity

it starts to accumulate in the static test

that is

with the motionless parallel plates. A new method to determine the liquid-solid transition by the static tensile force is proposed and compared with two dynamic methods of detecting liquid-solid transition

that is

the power-law modulus theory and the yield modulus model. The tensile force method predicted considerably earlier transition than the dynamic methods

and the corresponding DSC data indicate relative crystallinity of larger than 0.2 at the transition times. The limitation of dynamic methods and other possible errors have been analyzed. While the dynamic methods are suitable for slow crystallization

the tensile force method is more appropriate for the crystallization of moderate rates. Moreover

it has the advantage of almost not disturbing the crystallizing materials before the transition.