On the effective application of star-shaped polycaprolactones with different end functionalities to improve the properties of polylactic acid blend films

The aim of this work was to improve and disclose novel properties of polylactic acid (PLA)-based films, using adhoc synthesized star-shaped polymers characterized by short polycaprolactone (PCL) arms having the samelength but different end functionalities, namely: hydroxyl (coded as PCL-OH), carboxyl (coded as PCL-COOH) and pyrenyl (PCL-Pyr) groups. The properties of the films prepared by adding the synthesized PCLs to the polymer matrix were compared with those of films based on PLA and a commercial linear PCL (coded as PCL-L), which is commonly used in the development of PLA/PCL blends. The comparison of film morphology, evaluated by Field Emission Scanning Electron Microscopy (FE-SEM) measurements, showed that the dimensions of the PCL domains in the systems containing the star-shaped PCLs were smaller and adhered better to the polymer matrix than those of the films prepared from the commercial linear PCL. Moreover, it was found that the specific characteristics of the PCL domains, as well as the thermal properties of the films, depended on the nature of the end functionalities of the synthesized PCL. The peculiar dispersion of PCL domains in the films containing starshaped PCLs resulted in a better elongation at break (ca. 40%) than the PLA/PCL-L films (8%), while maintaining an acceptable stiffness. Contact angle measurements performed on the developed films showed an influence of the PCL functionalities also on their wettability, with the PLA/PCL-COOH systems exhibiting the highest hydrophilicity (contact angle of 71.9). To assess the effect of the star-shaped PCLs on the adsorption capacity of amino-functionalized compounds, the films were contacted with a solution containing pararosaniline, an aminodye. Interestingly, the films based on PCL-COOH showed the highest tendency to retain the cationic dye. This indicates that the functionalities conferred to the polymer system by the functionalized PCL allowed interactions with positively charged molecules. The enzymatic hydrolysis behaviour was studied by applying a cutinase, a well-known enzyme hydrolysing various polyesters. It is of outmost relevance to underline that films containing star-shaped PCLs exhibited slower and more controlled hydrolysis kinetics when compared to PLA/PCL systems and that this property could be tuned by changing the PCL end functionality.