Stretchable, Sensitive Strain Sensors with a Wide Workable Range and Low Detection Limit for Wearable Electronic Skins.

With the rapid development of wearable electronics, a multifunctional and flexible strain sensor is urgently required. Even though enormous progress has been achieved in designing high-performance strain sensors, the conflict between high sensitivity and a large workable range still restricts their further advance. Herein, a "point to point" conductive network is proposed to design and fabricate a carbon black/polyaniline nanoparticles/thermoplastic polyurethane film (CPUF). The designed structure renders CPUF composites with a wide sensitive range (up to 680% strain), highly sensitive response with a low detection limit of 0.03% strain, and high gauge factor (GF) of 3030.8, together with good sensing stability, fast response/recovery time (80 ms/95 ms), and good durability even after 10000 stretching/releasing cycles. CPUF composites are assembled as wearable strain sensors with the ability of precisely detecting full-range human motions and organic solvents, showing a potential application in human-machine interaction and environmental monitoring.

Synergistic Effect of Pressurization Rate and ß-Form Nucleating Agent on the Multi-Phase Crystallization of iPP.

Using a homemade pressure device, we explored the synergistic effect of pressurization rate and ß-form nucleating agent (ß-NA) on the crystallization of an isotactic polypropylene (iPP) melt. The obtained samples were characterized by combining small angle X-ray scattering and synchrotron wide angle X-ray diffraction. It was found that the synergistic application of pressurization and ß-NA enables the preparation of a unique multi-phase crystallization of iPP, including ß-, γ- and/or mesomorphic phases. Pressurization rate plays a crucial role on the formation of different crystal phases. As the pressurization rate increases in a narrow range between 0.6-1.9 MPa/s, a significant competitive formation between ß- and γ-iPP was detected, and their relative crystallinity are likely to be determined by the growth of the crystal. When the pressurization rate increases further, both ß- and γ-iPP contents gradually decrease, and the mesophase begins to emerge once it exceeds 15.0 MPa/s, then mesomorphic, ß- and γ- iPP coexist with each other. Moreover, with different ß-NA contents, the best pressurization rate for ß-iPP growth is the same as 1.9 MPa/s, while more ß-NA just promotes the content of ß-iPP under the rates lower than 1.9 MPa/s. In addition to inducing the formation of ß-iPP, it shows that ß-NA can also significantly promote the formation of γ-iPP in a wide pressurization rate range between 3.8 to 75 MPa/s. These results were elucidated by combining classical nucleation theory and the growth theory of different crystalline phases, and a theoretical model of the pressurization-induced crystallization is established, providing insight into understanding the multi-phase structure development of iPP.

Polymorph selection during melt crystallization of the isotactic polybutene-1 homopolymer depending on the melt state and crystallization pressure.

This work investigated the crystalline forms obtained from melt crystallization in the isotactic polybutene-1 (iPB-1) homopolymer via manipulation of the temperature at which samples were melted (Tmelt) and crystallization pressure (Pcry). Unlike the results under atmospheric conditions where the molten sample crystallized into the pure form II and the crystallization temperature and kinetics were affected obviously by Tmelt, the melted sample crystallized into forms II or I' under high pressure, depending on Tmelt and Pcry. The content of form I' decreases with increasing Tmelt or decreasing Pcry. Meanwhile, the critical pressure for the formation of pure form I' increases with increasing Tmelt. The formation of form I' is attributed to the memory effect of the melt which preserved some ordered sequence of crystal and the high pressure (Pcry) which suppressed the nucleation and growth of the kinetically favored form II, which results in the formation of form I'. In addition, the melt crystallized form II transforms to form I under high pressure conditions; thus forms I, I' and II are observed. The relative contents of the three crystalline forms on samples for different Tmelt and Pcry are obtained in this work. The result shows that the crystalline forms in melt crystallization of iPB-1 can be customized by regulating the melt state and crystallization conditions.

Structure and Mechanical Properties of Multi-Walled Carbon Nanotubes-Filled Isotactic Polypropylene Composites Treated by Pressurization at Different Rates.

Isotactic polypropylene filled with 1 wt.% multi-walled carbon nanotubes (iPP/MWCNTs) were prepared, and their crystallization behavior induced by pressurizing to 2.0 GPa with adjustable rates from 2.5 to 1.3 × 104 MPa/s was studied. The obtained samples were characterized by combining wide angle X-ray diffraction, small angle X-ray scattering, differential scanning calorimetry, transmission electron microscopy and atomic force microscopy techniques. It was found that pressurization is a simple way to prepare iPP/MWCNTs composites in mesophase, γ-phase, or their blends. Two threshold pressurization rates marked as R1 and R2 were identified, while R1 corresponds to the onset of mesomorphic iPP formation. When the pressurization rate is lower than R1 only γ-phase generates, with its increasing mesophase begins to generate and coexist with γ-phase, and if it exceeds R2 only mesophase can generate. When iPP/MWCNTs crystallized in γ-phase, compared with the neat iPP, the existence of MWCNTs can promote the nucleation of γ-phase, leading to the formation of γ-crystal with thicker lamellae. If iPP/MWCNTs solidified in mesophase, MWCNTs can decrease the growth rate of the nodular structure, leading to the formation of mesophase with smaller nodular domains (about 9.4 nm). Mechanical tests reveal that, γ-iPP/MWCNTs composites prepared by slow pressurization display high Young's modulus, high yield strength and high elongation at break, and meso-iPP/MWCNTs samples have excellent deformability because of the existence of nodular morphology. In this sense, the pressurization method is proved to be an efficient approach to regulate the crystalline structure and the properties of iPP/MWCNTs composites.

Pressure-jump induced rapid solidification of melt: a method of preparing amorphous materials.

By using a self-designed pressure-jump apparatus, we investigated the melt solidification behavior in rapid compression process for several kinds of materials, such as elementary sulfur, polymer polyether-ether-ketone (PEEK) and poly-ethylene-terephthalate, alloy La68Al10Cu20Co2 and Nd60Cu20Ni10Al10. Experimental results clearly show that their melts could be solidified to be amorphous states through the rapid compression process. Bulk amorphous PEEK with 24 mm in diameter and 12 mm in height was prepared, which exceeds the size obtained by melt quenching method. The bulk amorphous sulfur thus obtained exhibited extraordinarily high thermal stability, and an abnormal exothermic transition to liquid sulfur was observed at around 396 K for the first time. Furthermore, it is suggested that the glass transition pressure and critical compression rate exist to form the amorphous phase. This approach of rapid compression is very attractive not only because it is a new technique of make bulk amorphous materials, but also because novel properties are expected in the amorphous materials solidified by the pressure-jump within milliseconds or microseconds.

A Study of the Pressure-Induced Solidification of Polymers.

By using a self-designed pressure-jump apparatus, we investigated the melt solidification behavior in the rapid compression process for poly-ethylene-terephthalate (PET), polyether-ether-ketone (PEEK), isotactic polypropylene (iPP), high-density polyethylene (HDPE), and the living polymer sulfur. The experimental results clearly show that crystallization could be inhibited, and some melts were solidified to the full amorphous state for PET, PEEK, and sulfur. Full amorphous PEEK that was 24 mm in diameter and 12 mm in height was prepared, which exceeded the size obtained by the melt quenching method. The bulk amorphous sulfur thus obtained exhibited extraordinarily high thermal stability, and an abnormal exothermic transition to liquid sulfur was observed at around 396 K. Since the solidification of melt is realized by changing pressure instead of temperature and is not essentially limited by thermal conductivity, it is a promising way to prepare fully amorphous polymers. In addition, novel properties are also expected in these polymers solidified by the pressure-jump within milliseconds.

Simulation of Jetting in Injection Molding Using a Finite Volume Method.

In order to predict the jetting and the subsequent buckling flow more accurately, a three dimensional melt flow model was established on a viscous, incompressible, and non-isothermal fluid, and a control volume-based finite volume method was employed to discretize the governing equations. A two-fold iterative method was proposed to decouple the dependence among pressure, velocity, and temperature so as to reduce the computation and improve the numerical stability. Based on the proposed theoretical model and numerical method, a program code was developed to simulate melt front progress and flow fields. The numerical simulations for different injection speeds, melt temperatures, and gate locations were carried out to explore the jetting mechanism. The results indicate the filling pattern depends on the competition between inertial and viscous forces. When inertial force exceeds the viscous force jetting occurs, then it changes to a buckling flow as the viscous force competes over the inertial force. Once the melt contacts with the mold wall, the melt filling switches to conventional sequential filling mode. Numerical results also indicate jetting length increases with injection speed but changes little with melt temperature. The reasonable agreements between simulated and experimental jetting length and buckling frequency imply the proposed method is valid for jetting simulation.

Unusual structural evolution of poly(lactic acid) upon annealing in the presence of an initially oriented mesophase.

Uniaxial deformations of amorphous poly(lactic acid) (PLA) films were performed at two different temperatures, 70 and 80 °C, at various draw strains. The samples deformed at 70 °C showed a strain-induced mesophase, and the structural ordering and thermal stability increased as the draw strain increased. Further annealing was performed in situ at constant length at the drawing temperature of 70 °C for the films drawn to strains of 100% and 230%. Unusually, we found that after annealing, the crystal structure of the film at lower strain was more ordered than the one at higher strain. Further investigations revealed that upon annealing the structural evolution followed a distinct molecular mechanism for the samples stretched to the two draw strains. For the sample drawn to 100%, the mesophase melted very quickly upon annealing, resulting in chain randomization and the release of the constraints on the thermodynamic relaxation of the oriented amorphous chains. The chain relaxation motions had a beneficial effect on the occurrence of the conformational rearrangements that are necessary for crystalline ordering. By contrast, for the 230% sample, the melting of the mesophase was slow and most of the chain orientations were preserved upon annealing. As a result, a less ordered crystal structure was formed since the local relaxation motions that are necessary for promoting crystalline order via conformational rearrangements were hindered.

Identifying the phase behavior of biodegradable poly(hexamethylene succinate-co-hexamethylene adipate) copolymers with FTIR.

Whether a phase separation or a cocrystallization occurs in poly(hexamethylene succinate-co-hexamethylene adipate) (P(HS-co-HA)) copolymers was studied with a combination of wide-angle X-ray diffraction (WAXD) and Fourier transform infrared (FTIR) spectroscopy. With HA as the majority, the presence of HS comonomers leads to weakening and broadening of (10l) peaks in the X-ray fiber diffraction patterns, while a crystal structure similar to PHS is formed in the copolymer with HS as the majority. The X-ray diffraction patterns imply possible cocrystallization between HS and HA comonomers, but cannot lead to an unambiguous conclusion, which was clarified with the compensative tool of FTIR. Following the characteristic absorption bands of crystals, cocrystallization of HS and HA comonomers was observed in copolymers with HA comonomer as the majority during which HA initiated the nucleation at high temperatures. With HA as minority, cocrystallization of HS and HA can still be achieved with a fast quenching to below 0 degrees C, while a phase separation occurs and only HS comonomer crystallizes at high temperatures. This demonstrates that P(HS-co-HA) has an asymmetric phase diagram. Because of the sensitivity to local conformations, FTIR spectroscopic method is demonstrated to be a powerful tool on study phase behaviors of polymers with similar crystal structure.

Phase transition of [C(n)-mim][PF6] under high pressure up to 1.0 GPa.

Behavior of the phase transition of an ionic liquid, [Cn-mim][PF(6)], has been investigated under pressures up to 1.0 GPa by using a high-pressure differential thermal analysis (DTA) apparatus. The T versus P phase diagrams of [BMIM][PF(6)] and [EMIM][PF(6)] are constructed. The DTA curve of [BMIM][PF(6)] shows one endothermal valley in heating course at each given pressure, which indicates that a simple phase transition from solid to liquid has taken place under high pressure and that the melting point is an increase function of pressure. However, the DTA curve of [EMIM] x [PF(6)] shows two endothermal valleys in the heating course within the tested pressure range, implying that there may exist another phase. After treatment of [EMIM][PF(6)] at different temperatures under high pressure, the structures of the recovered samples are also investigated by wide-angle x-ray scattering. By considering the results above, it indicates that another crystalline phase exists between the solid and liquid of [EMIM][PF(6)].