Haifeng Shi

Fabrication, characterization, and supercooling suppression of nanoencapsulated n-octadecane with methyl methacrylate–octadecyl methacrylate copolymer shell

Supercooling of micro- and nanoencapsulated phase change material is widely observed as their diameters depress to a limitation upon cooling. The aim of this study is to suppress the supercooling of nanoencapsulated n-octadecane (NanoC18) using a novel copolymer consisting of long n-alkyl side chains as shell. Nanoencapsulations of n-octadecane with various compositions of poly(methyl methacrylate-co-octadecyl methacrylate) copolymer as shells were carried out by means of miniemulsion polymerization. Fabrication, morphology, diameter distributions, phase change behaviours, and thermal stabilities of nanocapsules were investigated using Fourier transformed infrared spectroscopy, a field-emission scanning electron microscope, a transmission electron microscope, particle size distribution analysis, differential scanning calorimetry, and thermogravimetric analysis. The results indicate that a series of nanocapsules with core/shell structure and spherical shapes are fabricated with average diameters ranging from 373 to 398xa0nm. The average thickness of the shells is about 60xa0nm. All the NanoC18 crystallize into a stable triclinic phase via a metastable rotator phase (RI) from the liquid phase. The crystallization temperature of n-octadecane within poly(methyl methacrylate) nanocapsules is considerably lower than that in bulk phase. Supercooling is effectively suppressed using the comb-like copolymer with crystallizable n-octadecyl side chains as shell. Octadecyl methacrylate is not only employed as a reactive costabilizer to suppress the influence of Ostwald ripening during the formation of nanocapsules but also as a functional monomer in the composition of the copolymer shell in order to suppress the supercooling of NanoC18.

Thermo-regulated sheath/core submicron fiber with poly(diethylene glycol hexadecyl ether acrylate) as a core

Side-chain crystallizable comb-like polymers can be used as polymeric thermal energy-storage materials. Diethylene glycol hexadecyl ether acrylate (C16E2AA) was synthesized with sodium alkoxide using diethylene glycol hexadecyl ether (C16E2) and acryloyl chloride as reactants. C16E2AA was prepared by free radical polymerization to form a comb-like polymeric phase change material, poly(diethylene glycol hexadecyl ether acrylate) (PC16E2AA). A series of sheath/core composite submicron fibers were coaxially electrospun using PC16E2AA as the core and poly(acrylonitrile-co-vinylidene chloride) as the sheath. Results indicate that C16E2AA and PC16E2AA were synthesized in high yield. The average molecular weight and polydispersity index of PC16E2AA were 29,800u2009g/mol and 3.17, respectively. PC16E2AA melted at 33.8℃ and crystallized at 25.8℃. The melting and crystallization enthalpy for PC16E2AA were 90 and 85u2009J/g, respectively. PC16E2AA was thermally stable below 326℃. Coaxial submicron fibers with smooth surface and average diameter ranging from 341 to 371u2009nm were electrospun. The optimum feed rate of the sheath and core components for fabricating thermo-regulated submicron fibers with high enthalpies were 0.125 and 0.375u2009mL/h, respectively. The fibers can absorb 50u2009J/g at approximately 37℃ and release 48u2009J/g of heat at approximately 32℃.

Crystalline structure and phase behavior of N-alkylated polypyrrole comb-like polymers

We report the crystalline structure and phase behavior of a series of comb-like polymers (PPy-Cn), which were prepared via N-alkylation reactions between a pyrrole monomer and n-alkyl bromides (n = 18–26). Analysis by temperature-dependent X-ray diffraction and Fourier transform infrared spectroscopy (FTIR), as well as differential scanning calorimetry reveals that the crystal structure, thermal events and packing mode are affected by PPy backbones, which exhibit stronger rigidity than those of previously reported comb-like polymers. Remarkably, variable-temperature (VT)-FTIR proves the formation of orthorhombic phase at very low temperature, while VT-wide-angle X-ray diffraction only presents the hexagonal phase, illustrating that FTIR is a very sensitive tool for analyzing the micro-packing mode of molecular chains. The enhanced side-chain crystallizable carbon atoms indicate that PPy polymeric backbones play a negative role in the regular packing of nano-crystallites formed by alkyl groups.