Improving mechanical and thermal properties of polymer physical poly (vinylidene fluoride)/propylene carbonate gels

Significance 

Crystalline gels are formed by cooling solutions consisting of semi-crystalline polymers and solvents. The resulting gels are non-covalently bonded and generally exhibit weak mechanical properties. As such, various methods such as mixing with nanoparticles or fibers and blending with other polymers have been employed to enhance their mechanical properties. Consequently, the crystalline structures and properties of polymeric materials are also of great significance and thus must be considered. They can be controlled in preparation or in post-process. For instance, post-process treatments such as freeze-thawing and high-temperature annealing have been employed to control the structures and properties of various polymeric materials. Even though previous studies have reported an increase in the break stress and Young’s modulus and crystallinity, little has been reported about the simultaneous increase in the break stress, break strain and Young’s modulus.

Noburu Osaka and Keiji Hamamoto at the Okayama University of Science in Japan recently investigated the effects of thermal annealing near peak melting temperature (Tm) on the thermal and mechanical properties of polymer physical gels. The gels comprised of polyvinylidene fluoride (PVDF) and propylene carbonate (PC) with 3D networks of interconnected spherulites while the post-thermal treatment involved annealing near peak melting temperature. The authors purposed to simultaneous determine the strengthening, stiffening and toughening of polymer physical gels. Their work is published in the journal, Polymer.

The authors observed a simultaneous stiffening, toughening and strengthening in the gels annealed near Tm in contrast to that prepared at high gelation temperatures which exhibited brittleness due to the increase in Young’s moduli, decrease in the break strain and fewer and larger spherulites with increased crystallinity. Consequently, an increase in peak melting temperatures which could be attributed to the polymorphic transition of the PVDF crystals from one dominant phase to the other dominant phase was noted. For instance, polymer physical gels annealed near melting temperature below 95°C shifted by more than 200°C towards a higher temperature. This also led to thickening of the PVDF crystal.

The study is the first to successfully apply self-nucleation method to enhance thermal and mechanical properties of polymer physical gels with interconnected spherulites. Morphological changes of the spherulites in the crystalline gels were observed especially during the deformation process. For instance, from the scattering measurements and microscopic observations, the duo reported enhanced recrystallization during the cooling process while during the heating process, small PVDF crystals melted at annealing temperature near melting temperature. However, the recrystallization process continued over the thermal treatment phase. This was also the reason for the growth of the spherulites during heating and annealing processes.

According to the researchers, annealing at a temperature far below the Tm insignificantly affected the spherulite morphologies thus retaining a fracture surface almost similar to that obtained before stretching. This was ascribed to the fact that the gels exhibited weak bonds at the interspherulitic boundaries enabling they ease in fracturing. However, gels annealed near melting temperature exhibited larger spherulites with enhanced spherulitic boundaries bonds thereby enabling stretching. Generally, high crystallinity and strong interspherulitic boundaries were responsible for enhanced mechanical and thermal properties of polymeric gels annealed at near melting temperature.

Reference

Osaka, N., & Hamamoto, K. (2018). Simultaneous stiffening, strengthening and toughening of poly (vinylidene fluoride)/propylene carbonate gـels by thermal annealing near peak melting temperature. . Polymer, 141, 132-142. .

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