High Entropy Materials
Study on the growth and arrangement of functional crystals guided by coalescence and stacking behaviors of P(VDF-TrFE) ferroelectric lamellae
Chia-Hung Pan * Wei-Ting Li Jrjeng Ruan
Department of Materials Science and Engineering, National Cheng-Kung University
The low temperature phase of the poly(vinylidenefluoride-co-trifluoroethylene) (P(VDF-TrFE)) have polar and piezoelectric properties and is a rare characteristic in the polymer crystal phase. However, it requires oriented stacking of crystalline domains to truly harvest this rare merit of polymer crystals as useful properties of thin film. In this work, the growth habits of the crystal phases in the film were investigated, as being limited by the blending of the amorphous poly(methyl methacrylate) (PMMA). This research aims to explore the phase interactions between PVDF-TrFE crystalline phase and PMMA amorphous phase, and corresponding impacts on the stacking of crystalline lamellae.
Introduction
After the mixing with poly(methyl methacrylate) (PMMA), the oriented coalescence and assembly of PVDF-TrFE lamellar crystals via selected thermal treatment is observed as shown on Figure 1.
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Figure 1. (a) In the film without PMMA, annealing at 125oC for 60 minutes. (b) The mixing ratios PVDF-TrFE/PMMA of studied mixture are all equal to 90/10. After the stay at 100 oC for 60 minutes, and(c)After slowly raising the temperature to 125 oC (0.5 oC/min) and staying for 60 minutes .
Two-dimensional oriented stacking of ferroelectric lamellar crystals
The continuous growth of crystalline lamellae is able to cause the shrinkage and coalescence of PMMA domains, and therefore make ZnO crystalline nanorods to stand up as shown in Figure 3. Hence, a guiding mechanism for the alignment of the functional inorganic nanorods is disclosed. Consequently, the characteristic polarities of ZnO nanorods and polymer crystalline lamellae are superimposed. This should help to harvest and maximize piezoelectric properties of the organic/inorganic hybrid films.
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Figure 2. (a)The selective distribution of ZnO nanorods within PMMA domains was found to gradually stand up as being compressed by surrounded amorphous PMMA domain. (b)The diagram is a schematic.
Guided assembly behavior and evolved standing arrays of ZnO nanorods
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Figure 3. (a) The proposed model of PVDF-TrFE lamellar crystal coalescence via secondary crystallization. The green color represent the molecular segments within amorphous region. Since the regular molecular conformation are less required for HT-form crystals, the secondary crystallization of linking amorphous segments is able to assemble neighboring lamellar crystals together. Hence, the thickening and decrease number of lamellar crystals are observed. (b)The interplanar spacing of the P(VDF-TrFE)/PMMA (90/10) blending system at different temperatures was analyzed by In-situ SAXS at a rate of 1oC/minute. (c)Analysis of powder samples of P(VDF-TrFE)/PMMA by in-situ X-ray diffraction experiment.
Crystals coalescence upon enhanced secondary crystallization
After the solution casting of PCBM on P(VDF-TrFE)/PMMA thin films, PCBM tends to selectively distribute into PMMA globular domains, as shown in Figure 2.
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Figure 4. (a)After the deposition of PCBM on P(VDF-TrFE)/PMMA blend thin film, PCBM tends to assembly into PMMA globular domains. (b)Through the heating to 125 oC and further annealing for 120 minutes, the shrinkage and deformation of PMMA domains is observed, which is conceived due to oriented continuous lamellae growth above the Curie temperature. (c)The PCBM crystals were observed to grow along the extension direction of PMMA domains. The extension of the PMMA domains also causes the bending of the surrounding lamellar crystals. (d) Further alignment of crystal growth and stacking upon extended annealing at 125 °C for 12 hours.
Guided assembly and crystallization of organic semiconductor PCBM
After the primary stage of crystallization of poly (vinylidenefluoride-co-trifluoroethylene), enhanced secondary crystallization of residue amorphous molecular segments during annealing beyond Curie temperature has been studied as an essential background for observed assembly and coalescence behaviors of lamellar crystals. Upon collateral molecular dragging during secondary crystallization, streak-like entangled regions evolve and serve as interaction mediums for mutual drawing among lamellar crystals to gradually adjust the growth and stacking orientation, leading to several-micrometer spread of oriented stacking of lamellar crystals. For dispersed poly (methyl methacrylate)(PMMA) amorphous domains, anisotropic deformation is generally observed upon continuous expansion of arrayed lamellar crystals, and thus compression effect of crystallization advancement on amorphous domains of disparate component is experimentally identified. Inside deformed PMMA domains, the inclusive crystallization of semiconductive methanofullerene is observed to advance favorably along less compressed regions. The anisotropic densification of surrounding polymeric matrixes is thus acknowledged able to guide the advancement of inclusive crystallization.