Positron annihilation lifetime spectroscopy (PALS) measurements, in the temperature range 110-480 K, are given for three linear, amorphous polymers. Based on these measurements, a method is proposed for evaluating the hole volume in amorphous thermoplastics. Our studies show that hole volume is composed of static and dynamic components. We demonstrate that the dynamic component, which is a result of the thermal vibrations of the molecular chains, is strongly correlated to thermodynamic volume/density fluctuations, The static hole volume is interpreted as "frozen-in" fluctuations, which are manifested as nanometer-sized flaws in the packing of molecular chains. The results from PALS measurements reported in our work are in very good agreement with results from small-angle X-ray scattering measurements.