up to 125 rad/s. This means that the samples exhibited a viscoelastic liquid character below 125 rad/s and that the matrix average molecular weight and molecular weight distribution did not change significantly during processing. These results demonstrated the restriction of chain mobility of the PLA matrix by HAp particles and no significant degradation of the matrix. Furthermore, Liu et al believed that the decrease in complex viscosity of PLA/HAp composites with incorporated inorganic SiO2-MgO-CaO whiskers (SMCWs) was caused by the increase in molecular chain distances of PLA due to the addition of rigid particles. A similar implication was also found in the work of Barczewski et al. where they reported the plasticising effect in the PLA composites filled with linseed cake (LC). Our PLA/HAp composites with 20% HAp showed significant differences in the viscoelastic behaviour in the melt (Figure ). PLA/20nHAp showed G" > G' in the whole frequency range, indicating the behaviour of viscoelastic liquid, whereas PLA/20µHap showed scattered values of G' and G" in the low frequency range with the crossover point (G'=G") at ~ 4 kPa and at a frequency around 40 rad/s suggesting some instabilities. The results can clearly indicate strong changes in the behaviour of the PLA matrix. Based on the literature cited above, we can assume that they can be caused by the degradation of PLA chains, the structural changes of the PLA matrix resulting from the incorporation of HAp particles or a combination of both. The stability of dispersions and the structural strength of PLAbased composites with 20% HAp in the molten state varied with the particle size of the filler . Furthermore, the composites with 40% HAp significantly increased the complex viscosity in the whole frequency range compared to the PLA matrix (Figure ). Also, both PLA/40nHAp and PLA/40µHAp composites showed G'>G" in the whole range of frequencies and the curves occurred almost as parallel straight lines (Figure ). This represents typical behaviour of stable dispersion with gel-like structure due to intermolecular interactions and formation of three-dimensional filler networks , clearly showing that the percolation threshold was exceeded. These results proved that the addition of 40 % of HAp to PLA caused a strong interaction between the PLA matrix and HAp particles, and such an appearance has already been described in the literature . The aforementioned interaction network for composites with 40% HAp addition is also shown in Figure and described in Section 3.1.