Temperature-induced polymorphic phase transitions in gels and aerogels of nylon 11

Unprecedented Thermally Induced Structural Changes in Aerogels of Poly(3-hydroxybutyrate) during Heating

“…SEM image shows highly interconnected macroporous structures consisting of fibrillar morphology with a fiber diameter ∼100–400 nm. A similar kind of fibrillar morphology was observed in aerogels of semicrystalline polymers. , The WAXS pattern of the 5 wt % PHB aerogel (Figure b) shows reflections at 2θ = 13.5° (020), 17.0° (110), 20° (021/110), 21.5° (101), 22.5° (111), 25.5° (121/031), 27.0° (040), and 30.0° (002) corresponding to the α form similar to that of PHB gel. , This indicates that the crystal structure of PHB remains the same even after the solvent-exchange and freeze-drying processes. Figure c shows the DSC thermograms of PHB aerogels (5 wt %) and melt-crystallized PHB in the heating process at 10 °C/min.…”

“…A similar kind of fibrillar morphology was observed in aerogels of semicrystalline polymers. 5,16 The WAXS pattern of the 5 wt % PHB aerogel (Figure 2b) shows 27,28 This indicates that the crystal structure of PHB remains the same even after the solvent-exchange and freezedrying processes. Figure 2c shows the DSC thermograms of PHB aerogels (5 wt %) and melt-crystallized PHB in the heating process at 10 °C/min.…”

“…In bulk samples, usually, the δ form of sPS transforms to the γ form before transforming into the α form at a higher temperature. , Rastogi et al claimed that the enhanced chain mobility in the amorphous phase of gel triggered the unusual δ to β transition . Ashitha and Gowd reported the reverse Brill transition in nylon-11 gels due to the presence of excess solvents in the amorphous region . Recently, Pan and co-workers reported that the chain entanglement in the amorphous region alters the crystal-to-crystal transitions in polymers like PLA (stereocomplex) and polybutene-1. ,, This group used the freeze-drying process to control the degree of chain entanglements in polybutene-1 and demonstrated that the chain disentanglements favored the crystal transition from form II to form I due to the reduced nucleation barrier and enhanced chain mobility in the disentangled polybutene-1 .…”

“…2,7,13,15 Recently, our group investigated the phase transition behavior of semicrystalline polymers in gels and aerogels and observed unusual phase transitions in syndiotactic polystyrene (sPS), nylon-11 and poly(vinylidene fluoride). 6,11,16,17 Biobased polyesters are known to exhibit complicated polymorphism due to the strong inter/intramolecular interactions during processing and polymer−solvent interactions. 18−21 Thus, the formation of thermoreversible gels of biodegradable polyesters in solvents is expected to influence the formation of the crystal structure of these polymers in the gel state as well as in aerogels.…”

“…Aerogels are open porous materials that possess low density, high porosity, large sound absorption, low sound velocity, and low thermal conductivity. , Among the aerogels, semicrystalline polymer-based aerogels attracted immense interest due to their ease of production and environment-friendliness. − These polymers readily undergo thermoreversible gelation in various solvents by forming gel networks made up of polymer crystalline phases, which are thermally labile. − ,, The inclusion of crystalline networks within the gels and aerogels provides the required mechanical robustness, flexibility, and adequate porosity. The solvent extraction by suitable procedures from thermoreversible gels leads to the corresponding aerogels. − , Among these, biodegradable polymer-based thermoreversible gels and aerogels gained attention due to their unique applications in the biomedical field to biodegradable packaging. ,,,, Polymer aerogels are generally produced by freeze-drying or supercritical drying methods and they exhibit less entangled amorphous chains compared to their bulk counterparts. ,,, Recently, our group investigated the phase transition behavior of semicrystalline polymers in gels and aerogels and observed unusual phase transitions in syndiotactic polystyrene (sPS), nylon-11 and poly­(vinylidene fluoride). ,,, …”