Aiming Zhang

Light-Weight Silver Plating Foam and Carbon Nanotube Hybridized Epoxy Composite Foams with Exceptional Conductivity and Electromagnetic Shielding Property

Herein, light-weight and exceptionally conductive epoxy composite foams were innovatively fabricated for electromagnetic interference (EMI) shielding applications using multiwalled carbon nanotubes (MWCNTs) and 3D silver-coated melamine foam (SF) as conductive frameworks. A novel and nontraditional polymer microsphere was used to reduce the material density. The preformed, highly porous, and electrically conductive SF provided channels for fast electron transport. The MWCNTs were used to offset the decrease in conductive pathways due to the crystal defects of the silver layer and the insulating epoxy resin. Consequently, an exceptional conductivity of 253.4 S m(-1), a remarkable EMI shielding effectiveness of above 68 dB at 0.05-18 GHz, and a thermal conductivity of 0.305 W mK(-1) were achieved in these novel foams employing only 2 wt % of MWCNTs and 3.7 wt % of silver due to the synergistic effects that originated in the MWCNT and SF. These parameters are substantially higher than that achieved for the foam containing 2 wt % MWCNTs. Also, the SF exhibited little weakening in the foamability of the epoxy blends and the compression properties of resulting foams. All the results indicated that this effort provided a novel, simple, low-cost, and easily industrialized concept for fabricating light-weight, high-strength epoxy composite foams for high-performance EMI shielding applications.

Cross-linking process of cis-polybutadiene rubber with peroxides studied by two-dimensional infrared correlation spectroscopy: a detailed tracking

cis-Polybutadiene rubber (cis-BR) is one of the typical unsaturated rubbers in mass production and is widely used. However, the detailed mechanism of its cross-linking with peroxides is still unclear so far. In this study, in situ FTIR spectra combined with the powerful PCMD2D and 2D correlation spectroscopy was used to track the detailed cross-linking process. The temperature region of cis-BR cross-linking determined by PCMW2D was within 165–195 °C. The temperature with a maximum cross-linking rate was determined at 183 °C via PCMW2D, which is identical with DSC. The generation of –˙CH– macromolecular free radicals through losing α hydrogens was observed when below 165 °C. An abnormal increase of double bonds with trans-1,4-structure during the cross-linking (165–195 °C) was observed. An obvious enhancement of –CH2– groups was also found, which indicated that a large number of the double bonds with cis-1,4- and 1,2-structure involved in cross-linking is transformed into –CH2– groups. A 5-step process for the whole cross-linking was inferred from the sequential order of group motions. The first step is DCP decomposition and free radical release. The second step is the generation of trans-1,4-structure due to the internal rotation of the cis-1,4-structure induced by free radicals at α position. The third step is the free radical addition of double bonds with a 1,2-structure, and the fourth is the free radical addition of double bonds with a cis-1,4-structure. The final step is the cross-linking via double coupling of two macromolecular free radicals. In the last step, the free radicals from the cis-1,4-structure also can be probably terminated by a chain transfer.

A Simple Way to Achieve Legible and Local Controllable Patterning for Polymers Based on a Near-Infrared Pulsed Laser

This study developed a simple way to achieve legible and local controllable patterning for polymers based on a near-infrared (NIR) pulsed laser. The polycarbonate-coated nano antimony-doped tin oxide (nano ATO) was designed as a core-shell structure that was tailored to be responsive to a 1064 nm NIR laser. The globular morphology of polycarbonate-coated nano ATO with a diameter of around 2-3 μm was observed by scanning electron microscopy and transmission electron microscopy. This core-shell structure combined the excellent photothermal conversion efficiency of nano ATO and the high char (carbon) residue of polycarbonate. The X-ray photoelectron spectroscopy results of a polymer-patterning plate after laser irradiation demonstrated that, through local controlled photochromism, the well-defined legible patterns can be fabricated on the polymer surfaces contribute to the synergistic effect consisting of polycarbonate carbonization and nano ATO photothermal conversion. Furthermore, polymers doped with a minimal content of polycarbonate-coated nano ATO can achieve a remarkable patterning effect. This novel laser-patterning approach will have wide promising applications in the field of polymer NIR pulsed-laser patterning.

New understanding on the reaction pathways of the polyacrylonitrile copolymer fiber pre-oxidation: online tracking by two-dimensional correlation FTIR spectroscopy

Polyacrylonitrile (PAN) copolymer fiber pre-oxidation has important influence on the final properties of carbon fibers. Understanding and tracking the reaction pathways of this pre-oxidation has great significance in guaranteeing the quality of the resulting carbon fibers. In this study, in situ FTIR spectroscopy in combination with scaling moving-window two-dimensional correlation spectroscopy (scaling-MW2D) and 2D correlation analysis was used to study the reaction pathways. In addition, DSC and 13C solid-state NMR were used to assist in the determination and verification of chemical structures. Scaling-MW2D revealed that pre-oxidation consists of an initial process (A, 69–223 °C) and a main process (B, 223–309 °C). From the sequential order of 2D correlation analysis, more detailed pathways were obtained. The induced reaction of comonomer units took place in process A (69–223 °C). In process B (223–309 °C), the initial cyclic structures were first generated from the induced structure formed in process A. Then, these initial cyclic structures underwent a series of oxidations and subsequent isomerization. Subsequently, a large number of AN units were immediately involved in the main cyclization reaction, and some β-amino nitriles were produced. One new understanding obtained is that the initial cyclic structures after oxidation and isomerization are the real induced “nucleus” of the main cyclization reaction, and therefore oxygen in the air plays a key role in the main cyclization of PAN. The final step is the dehydrogenation reaction on the polycyclic structures at a high temperature.

Identification of weak transitions using moving-window two-dimensional correlation analysis: treatment with scaling techniques

In the present study, the theory of the data treatment with scaling techniques for moving-window two-dimensional (scaling-MW2D) correlation analysis was first proposed. This new analytical method of spectroscopy can significantly enhance the resolving capacity of the moving-window two-dimensional (MW2D) correlation infrared spectroscopy in the direction of the perturbation variable. So, it strengthened the ability of MW2D to highlight the weak transitions. The in situ infrared spectra of four common polymers, including polyamide 66 (PA66), polystyrene-block-polybutadiene-block-polystyrene block copolymer (SBS), isotactic polypropylene (iPP), and polyoxymethylene (POM), were employed to illustrate the advantages of scaling-MW2D. In the applications of the present study, the conventional autocorrelation MW2D can only distinguish the melting point of PA66, the maximum crystallization temperature of POM, and the primary oxidation of SBS. However, the autocorrelation scaling-MW2D not only can more easily determine the above transitions, but also can identify PA66 brill transition, the dissociation of adsorbed water in PA66, POM secondary crystallization, the glass transition of hard blocks in SBS, and the generation of the aldehyde and hydroxyl groups during SBS oxidation. Our further study found that the selection of the scaling factor α was very important. The golden point α = 0.618 was the best value, and satisfactory application results can be achieved. The slice scaling-MW2D was also investigated. The scaling-MW2D method of spectroscopy can be used elsewhere. The application of this analytical method should not be limited to the infrared spectra, and it also should not be limited to transitions in polymers. This method can be easily extended and applied to other materials and spectra.

Rigid thermosetting epoxy/multi-walled carbon nanotube foams with enhanced conductivity originated from a flow-induced concentration effect

Herein, multi-walled carbon nanotube (MWCNT) reinforced rigid epoxy foams were innovatively prepared using expandable microspheres. A two-step technique, including prepolymerization and consecutive foaming, was employed to avoid the thermal degradation of epoxy due to the uncontrolled violent curing reaction. The effects of foaming on the MWCNT inter-connectivity, electrical percolation threshold, and through-plane electrical conductivity were investigated. Moderately foamed epoxy/MWCNT composites were confirmed to have better conductivity than their solid counterparts at the same MWCNT content. An effect we called flow-induced concentration increases the inter-connectivity by separating MWCNTs from a flowing matrix caused by thermally triggered expansion of microspheres, and changes the MWCNT distribution. For the foamed composites with 2 wt% microspheres, the electrical percolation threshold is approximately 0.3 wt% and the conductivity is obviously higher than that of the solid counterpart. Relationships between microstructures and conductivities were also discussed. The mechanical property and cellular structures of conductive foams obtained under different foaming temperatures, prepolymerization time, microsphere and MWCNT contents were respectively evaluated. All the results reveal that lightweight conductive epoxy foams with lower filler content and enhanced conductivity can be fabricated for applications in electronics, automobiles, and aerospace.

Separation of the molecular motion from different components or phases using projection moving-window 2D correlation FTIR spectroscopy for multiphase and multicomponent polymers

This study developed a new analytical method called projection moving-window 2D correlation FTIR spectroscopy (Proj-MW2D) to separate the molecular motion of groups generated from different components or phases for multiphase and multicomponent polymers. The specific implementation steps for Proj-MW2D were enumerated after the theoretical derivation and algorithm research. Two types of two-component blends, poly(L-lactide)/poly(butylene succinate) and monodisperse polystyrene/monodisperse poly(ethylene-co-1-butene), were employed to validate the concept of separating the molecular motion of groups. Results showed that the Proj-MW2D FTIR correlation technique successfully separated the molecular motion of the specific functional groups. Although MW2D and PCMW2D have the capacity to determine the multiple transitions of polymers, they cannot identify the origin of correlation intensity peaks without the help of other characterization methods. Proj-MW2D allows researchers to study the mechanism of the complex transition process for multiphase and multicomponent polymer systems. This method can be easily extended to three- or four-component polymers and to other spectra (e.g., Raman, X-ray, and UV).

A method to construct perfect 3D polymer/graphene oxide core–shell microspheres via electrostatic self-assembly

In this study, a method to construct perfect three-dimensional (3D) polymer/graphene oxide (GO) core–shell microspheres was proposed via electrostatic self-assembly. 2D GO nanosheets were successfully wrapped onto polymer microspheres to form a perfect 3D core–shell structure with uniform shell thickness under the action of an electrostatic attraction force. The GO nanosheets, with a thickness of 1.5–2 nm and an area over 2 × 1 μm2, were firstly prepared from graphite, and then cationic polystyrene (PS) microspheres with 0.246% and 0.715% surface concentrations of –N(CH3)3+ were successfully synthesized. After that, PS/GO core–shell microspheres were constructed from the GO nanosheets and the cationic PS microspheres. It was found that different cationic PS microspheres led to different assembly speeds. SEM and TEM images of rippled silk waves on the surface of the PS/GO core–shell microspheres not only indicated the perfect polymer/GO core–shell structure, but also demonstrated the strong binding between the two materials. It was also revealed that the thicknesses of the shells of the PS/GO core–shell microspheres were under good control, and the thicknesses of shells from different cationic PS microspheres were 9–13 nm and 80–100 nm. The method proposed here has proved to be a valuable tool for the assembly of 3D microstructures from polymers and graphene oxide (or graphene).

Two-dimensional correlation infrared spectroscopy reveals the detailed molecular movements during the crystallization of poly(ethylene-co-vinyl alcohol)

In this paper, quantitative spectroscopic evidence for the important role of hydrogen bonds during the cooling crystallization of poly(ethylene-co-vinyl alcohol) (EVOH) was successfully obtained. Furthermore, the detailed molecular movements during the crystallization of EVOH were revealed via two-dimensional correlation infrared spectroscopy. Two cooling crystallization processes were detected at 159 °C and around 101–105 °C via the combination of DSC and the newly proposed scaling-MW2D correlation FTIR spectroscopy. These two crystallization processes were defined as the primary crystallization (region I) and the secondary perfection process (region II) of EVOH. The methods for calculating the enthalpies of the pair-bonded hydrogen bonding (ΔHh), vinyl alcohol (VA) repeating unit crystallization (ΔHC-VA), VA repeating unit diffusion into the crystal lattice (ΔHl-VA), and ethylene (ET) repeating unit crystallization (ΔHC-ET) were established via van ’t Hoff plots. For regions I and II, the contributions of the pair-bonded hydrogen bonding of the VA repeating units to the entire EVOH crystallization were 42.8% (ΔHh = −90.4 ± 5.2 kJ mol−1) and 64.6% (ΔHh = −75.8 ± 3.1 kJ mol−1), respectively. However, the contributions of the ET crystallization to the EVOH crystallization were 19.4% (ΔHC-ET = −41.0 ± 2.0 kJ mol−1) and 32.7% (ΔHC-ET = −38.3 ± 0.8 kJ mol−1), which were only half of the contributions of the pair-bonded hydrogen bonding. 2D correlation analysis was used to investigate the sequential order of the groups’ movement in the crystallization. It was found that region I had 3 steps. The first step is the formation of the hydrogen bonds in the VA repeating unit, and the second step is the diffusion of the VA repeating unit into the crystal lattice, resulting in the primary crystallization. The third step is the ET repeating unit crystallization accompanied by the movement of the VA repeating unit without hydrogen bonding in the amorphous region. Region II had 4 steps. The first step is also the generation of the hydrogen bonds in the VA repeating unit. The second step is a local rearrangement of the lattice in the imperfect crystalline of the VA repeating unit, called the secondary perfection process. The third step is the movement of the VA repeating unit without hydrogen bonding in the amorphous region. The fourth step is the weak crystallization of the ET repeating unit at a low temperature.

Polypropylene elastomer composite for the all-vanadium redox flow battery: current collector materials

In this study, a carbon-based polypropylene thermoplastic elastomer (PP-elastomer) composite for current collectors of an all-vanadium redox flow battery (VRB) was successfully prepared. The volume resistivity of the PP-elastomer composite was 0.47 Ω cm. Its tensile strength and elongation at break were 6.6 MPa and 250%, respectively. In addition, the good flow property in processing means this composite has potential for the mass industrial production of current collectors. The single cell and the cell stack of a VRB equipped with the composite current collectors were assembled for battery tests, including cyclic voltammetry, long-term performance, long-term stability, and oxidation corrosion. To evaluate the stability and the performance of the cell stack under a long-term operating condition, tests with more than 2300 charge–discharge cycles were carried out. The coulombic efficiency (CE) and voltage efficiency (VE) of the cell stack were maintained at around 93% and 80% during 2300 charge–discharge cycles, and energy efficiency (EE) held at around 75%. The results proved that a VRB equipped with composite current collectors has good stability and performance. Furthermore, long-term corrosion tests indicated that the PP-elastomer composite could endure the strong corrosion of pentavalent vanadium and concentrated sulfuric acid. The composite materials prepared in this study are more suitable than other materials for producing the current collectors. The corrosion resistance of composite materials is much better than that of the graphite, and the mechanism is also discussed.