Due to the restriction on the molecular mobility of chain segments adjacent to crystals, an interfacial layer is formed at the boundary of crystalline and amorphous phases. The interfacial layer commonly referred to as rigid amorphous fraction (RAF) exhibits distinct properties, which include gas solubility in the region. The present thesis investigates the effect of rigid amorphous fraction on the solubility of physical blowing agents in semi-crystalline polymers using PLA and CO2 as an example. The phase compositions of PLA samples are characterized through differential scanning calorimetry and are incorporated into the analysis of CO2 solubility determined using a magnetic suspension balance. A mathematical approach based on the three phase model of semi-crystalline polymers is applied to differentiate the overall solubility into the contributions of RAF and MAF (mobile amorphous fraction). The results confirm that the solubility of CO2 in RAF (~4%) is significantly lower than in MAF (~7%).