• molecular weight
• layered silicates nanocomposites
• functionalization
• polypropylene

In this work, functionalized polypropylenes with controlled architecture were prepared in the melt under mechanical mixing and successively assessed as compatibilizers for PP based nanocomposites.
In the first part of the work, PP radical functionalization with maleic anhydride (MAH) was investigated in the presence of selected molecules (furan derivatives) limiting the degradation process of PP macroradicals. First, the traditional PP radical functionalization with MAH and peroxide carried out in an internal mixer at 200°C was accurately described and pointed out the significant occurrence of the polymer chain degradation (by beta-scission reaction), which was also favored by increasing the MAH content in the feed. Indeed, the MAH graft reaction terminated by H-transfer thus contributing to keep alive PP macroradicals which represented active sites for MAH grafting as well as for polymer chains scission. The process, carried out with an excess of MAH compared with peroxide, allowed to obtain a grafting efficiency higher than 1. On the contrary, the PP radical functionalization with the butyl 3-(2-furanyl) propenoate (BFA) in the molten state led to functionalized polymers characterized by molecular weight (MW) conservation and an efficiency of the grafting process very close to 1, thus suggesting that most likely the BFA graft reaction did not terminate by H-transfer reaction. Moreover, by optimizing the feed conditions (stoichiometric conditions between primary radicals and BFA and high amount of peroxide), functionalized polymers having good functionalization degrees (FD) and controlled MW could be obtained, thus highlighting that the BFA-assisted radical functionalization of PP with MAH was a process able to generate a wide range of functionalized PPs characterized by tunable FD and MW.
A combination of different characterization techniques was adopted to have further information about the reactions involved in such complex system. In particular, UV spectroscopy highlighted different BFA species grafted onto the PP backbone and rheological measurements pointed out the presence of long chain branches (LCB) in some samples. In addition, successive extraction procedure on selected samples emphasized in few cases the presence of local cross-linked fractions and a homogeneous grafting along all the macromolecular chains of both the molecules (BFA and MAH) within the different fractions. Investigations about the BFA reactivity towards primary radicals (performed without PP) in presence or not of MAH highlighted that BFA was able to oligomerize only in very specific conditions and that the addition (even low) of another monomer (MAH or styrene) led to the homopolymerization of this second monomer. It was also pointed out that reaction products formed between BFA, MAH (when added to the system) and primary radicals terminated by coupling or disproportionation reactions. Further investigations involving the use of 2,3-13C2 MAH evidenced the grafting of MAH oligomers to the PP particularly in the presence of BFA.
In the second part of this work, functionalized PP samples having different type and amount of grafted functional groups as well as variable MW were selected and tested as compatibilizers for the preparation of PP layered silicates nanocomposites. The attention was focused on the influence of FD and MW on the morphology, thermal and mechanical properties of the nanocomposites. Initially the functionalized PPs were used as matrix and to well define the final properties of the composites, a preliminary investigation about their tensile, dynamic thermo-mechanical, thermal properties and crystallization behavior was performed. A relevant exfoliation degree of low amount of organophilic layered silicate (1-5 wt%) was obtained by using PP samples having both high MW (larger than 230,000D) and FD (around 0.9 % mol). Finally, the functionalized PPs were used at lower concentration (5 wt%) as compatibilizers of the system pure PP/organophilic-phyllosilicate and in this case one exfoliated and two semi-intercalated semi-exfoliated morphologies were evidenced depending on the number of PP units between two functional groups. The nanocomposites were characterized by enhanced thermal stabilities and Young’s modulus with respect to the pure PP and by a good elongation at break in some cases. Moreover, the dynamic thermo-mechanical properties were slightly improved (with respect to the PP) even if the general morphology of prepared nanocomposites was not completely homogeneous. Indeed, an optimization of the process and feed conditions could certainly improve the main properties of PP nanocomposites prepared here.