Design and processing of star-shaped PLA architectures: a promising tool to tailor conventional linear PLLA performances

In this Thesis, the manipulation of poly(lactide) (PLA) macromolecular architecture, through introduction of controlled star-shaped branching, was exploited to enhance ductility, toughness, degradability and reduce viscosity -for better melt flow during manufacturing- of industrially employed PLA, which is typically a linear semicrystalline PLA. These features are instrumental to the development of flexible and highly biodegradable all-PLA-based products with improved processability, without altering the chemical nature of the material, thus making poly(lactide) materials more competitive on the market with respect to petroleum-based commodities. Firstly, the plasticization performance of short-branched, star-shaped PDLLA as green and compatible additive of conventional linear PLLA was assessed. The miscibility of these two components and the positive effect of increasing branching contents on final mechanical, thermal, and biodegradation properties were evaluated, along with a comparative analysis of the resulting ductile and highly biodegradable star/linear PLA blends with respect to other existing polymer products. Next, the scalability of the star/linear PLA blends production was investigated, by employing typical large-scale manufacturing techniques for thermoplastic polymers, i.e. extrusion -coupled to compression molding- and injection molding. In addition to reduce typical linear PLLA brittleness, progressively higher star-shaped contents were found to gradually decrease the melt viscosity of the final material, while increasing the shear-thinning behaviour, thus facilitating the melt flow during manufacturing and improving overall processability. Moreover, the high compatibility between branched and linear PLA allowed their efficient blending through a single-step injection molding process, avoiding prior mixing by melt extrusion, thus reducing the typically rapid PLA thermo-oxidative degradation, and resulting in materials with enhanced properties. The possibility to tailor final PLA material properties by varying also the branching degree and tacticity of the selected branched modifier was finally assessed. To this aim, a novel and versatile lactide ROP protocol was developed, giving access to a library of multi-armed poly(lactide) architectures with variable topology (i.e. linear, star, comb PLAs with increasing number of arms) and stereoconfiguration (i.e. PLLA and PDLLA homopolymers; PLLA-b-PDLLA and PDLLA-b-PLLA copolymers). The combined effect of these parameters on final PLA materials properties was systematically investigated, in terms of macromolecular conformation, crystallizability, rheological behaviour and degradability and confirmed the efficient modulation of polymer flexibility (few chain entanglements) and rheology (reduced viscosity), compared to the conventionally used linear semicrystalline PLA. The observed physico-chemical and mechanical properties, as well as the faster hydrolytic degradation kinetics, make the synthesized branched poly(lactide)s extremely interesting for future industrial development of flexible and easily processable PLA-based materials with enhanced biodegradability.