Interfacial Interactions, Crystallization, and Molecular Dynamics of Renewable Poly(Propylene Furanoate) <i>In Situ</i> Filled with Initial and Surface Functionalized Carbon Nanotubes and Graphene Oxide
Panagiotis Α. Klonos, Lazaros Papadopoulos, George Z. Papageorgiou, Apostolos Kyritsis, P. Pissis, Dimitrios Ν. Bikiaris
Abstract
The relatively new biobased polyester poly(propylene furanoate), PPF, belongs in a class of polymers prepared from renewable resources and expected to replace its fossil-based homologues. Due to its potential in future applications and for the optimization of the material properties such as the mechanical performance, manipulation of crystallinity is crucial, which is rather slow for PPF. By following previous work in semicrystalline polymers, crystallization rate enhancement is achieved here by in situ introduction to the PPF matrix of low amounts, 1 wt %, of carbon nanotubes (CNT) and graphene oxide (GO) platelets. A combination of complementary techniques are employed for this work, which include nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectroscopy for the nanocomposites structure, X-ray diffraction (XRD) for the crystalline structure, differential scanning calorimetry (DSC) for thermal transitions, and broad-band dielectric spectroscopy (BDS) for molecular dynamics. Crystallization is accelerated in the nanocomposites due to the fillers acting as additional crystallization nuclei, whereas the degree of crystallinity is barely enhanced. The latter is correlated with alternations in the semicrystalline morphology (XRD, DSC). Furthermore, we investigate the additional role of surface functionalization of these fillers by proper groups in order to promote the polymer–filler interaction degree. The enhanced interfacial interactions were confirmed by FTIR and the suppressed glass transition strength. Interestingly, this enhancement was found to actually result in a deceleration of crystallization, as the formed interfacial bound polymer fraction does not seem to contribute to crystallization. In general, the effects imposed by the CNTs were stronger as compared to GO, most probably due to the larger aspect ratio of the nanotubes. Regarding molecular mobility, the glass transition was studied in the amorphous and the semicrystalline state. The segmental polymer mobility, calorimetric Tg and dynamic α relaxation, is governed in the amorphous state by the low molecular weight (MW ≈ 10k–20k) and, further, by crystallinity. On the other hand, the strength of glass transition is suppressed by the formation of interfacial bound polymer (interactions). Finally, we were able to record the local polymer dynamics below Tg (i.e., β relaxation), which seems to be sensitive to structural changes of the matrix, and the global chain dynamics above Tg (Normal Mode, NM, relaxation), dependent on the low MW, as expected.