最新刊期
卷 ， 期 4 ， 2017
- 摘要：As a sort of intelligent material, shape memory polymer material is outstanding in various aspects, such as lower density, high recoverable strain, and in response to a vast array of external stimuli. According to whether the shape memory process is reversible or not, the shape memory effect could be classified into two categories: one-way shape memory effect and two-way shape memory effect. The one-way shape memory process is actually irreversible, in which the recovered permanent shape cannot revert back to the temporary shape by merely altering the external stimulus and a re-programming process is necessarily needed. On the contrary, the two-way one is fully reversible, meaning that the sample would be able to vary between distinguished shapes reversibly without external re-shaping process applied by user. As a consequence, two-way shape memory polymer materials have high practical values and broad application prospects, which have aroused wide attention and become one of the hot spots of current researches. This work presents a brief review of two-way shape memory crystalline polymer and its composites that have been investigated and developed in recent years, including quasi two-way shape memory crystalline polymer under constant external stress and two-way shape memory crystalline polymer plus its composites without external stress. More specifically, the former consists of chemically and physically cross-linked crystalline polymer with persistent external stress, while the latter is comprised of polymer laminates, polymer/elastomer core/shell composites, chemically cross-linked crystalline polymer possessing dual-cure network architectures, chemically cross-linked two-component crystalline polymer, chemically cross-linked crystalline polymer with a broad melting transition, and physically cross-linked crystalline polymer. More importantly, preparation methods, influencing factors and the corresponding two-way shape memory mechanism of the aforementioned materials are introduced and discussed in detail. In the end, the future developments and research directions of these polymer materials are proposed as well.关键词：Two-way shape memory effect;Crystalline polymer;Composites;Mechanism;External stress
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发布时间：2021-05-18 - 摘要：Isotactic polypropylene (iPP) is a semi-crystalline polymer which exhibits polymorphisms, designated respectively as monoclinic α-, trigonal β-, orthorhombic γ-and tetragonal e-form. Due to the industrial and economic value, α-and β-iPP have been widely studied, and the transformation behavior between these two phases has been extensively investigated. Therefore, studies in recent years with the focus on such topic are generally reviewed in this article. Originally, the growth rate ofβ-iPP is over that of α-iPP between high critical temperature (141 ℃) and low critical temperature (100-105 ℃). But when the temperature is higher than 141 ℃ or lower than 100 ℃, β → α-iPP phase transformation could occur, for α-iPP is kinetically favorable. This growth transformation can be realized by two-step crystallization of iPP. Besides, such phase transformation would take place during melting and recrystallization because α-iPP crystals are thermodynamically more stable. Moreover, the β → α-iPP phase transformation can also be observed during deformation at room temperature or high temperature, and microporous membranes can be prepared via this process due to the higher density of α-iPP. Such products could be used as gas exchange membrane, filter membrane or separator of lithium-ion batteries. Since the transformation mechanism during deformation is still controversial, the article also reviews two proposed ideas about it. One is that the transformation during stretching should experience partial melting and recrystallization, and the other is just a solid-solid phase transformation. In addition, there are problems remaining that should be further explored about the β → α-iPP phase transformation. For example, it is still unknown whether there is crystallographic relationship between new formed α-iPP and original β-iPP during two-step crystallization, or it is just realized by a nucleation process. So, this will be an appealing direction for researchers in the field of polymer physics.关键词：Isotactic polypropylene;α-Form isotactic polypropylene;β-Form isotactic polypropylene;Phase transformation
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发布时间：2021-05-18
Reviews
- 摘要：The core-shell nanoparticles of poly (methyl methacrylate)/poly (isoprene-co-styrene) (PMMA/PIS) were prepared using a two-stage semi-batch microemulsion polymerization, i.e., PMMA spherical seeds were synthesized in the first stage and the PIS shells around the seeds were formed in the second stage. The polymer particles with their diameter of 19-200 nm with a narrow distribution were achieved. It was found that there existed an optimal sodium dodecyl sulfate (SDS) amount, potassium persulfate (KPS) amount and an agitate speed, which were, for example, 1.0 wt% of SDS amount, 0.10 wt% of KPS amount and 200 r/min of agitation speed, in order to control the particle sizes of PMMA/PIS core-shell nanoparticles. After the second polymerization stage, a direct hydrogenation process was applied to saturate the residual carbon-carbon double bonds of PIS in the PMMA/PIS nanoparticles synthesized and the hydrogenation was realized by diimide reduction in presence of hydrazine and hydrogen peroxide using boric acid as a promoter without any organic solvent and precious metal catalysts. The effects of particle size, ratios of hydrazine hydrate, and hydrogen peroxide on the hydrogenation degree were investigated in detail. The hydrogenation degree of PMMA/PIS was determined using FTIR, 1H-NMR and titration of Na2S2O3. Hydrogenation degree of more than 95% was achieved and no cross-linking was detected, when the particle size of the latex was less than 200 nm, the molar ratio of hydrogen peroxide to hydrazine was 1.8:1, the molar ratio of hydrazine to C＝C double bonds was 2:1, the reaction temperature was 50 ℃, and the reaction time was 6 h. Detailed characterizations of the microstructure, the molecular weight and heat resistance of PMMA/PIS and hydrogenated PMMA/PIS were conducted by TEM, GPC and TGA. The TEM results indicated that PMMA/PIS nanoparticles were well structured core-shell nanoparticles, in which PMMA was the core and PIS was the shell as designed. GPC results confirmed the hydrogenation of PMMA/PIS overwhelmed the crosslinking process during hydrogenation operation. The decomposition temperature of the hydrogenated PMMA/PIS was 382 ℃ while that of the parent PMMA/PIS was 341 ℃, which indicated that the heat resistance of the hydrogenated PMMA/PIS was significantly improved.关键词：Poly (isoprene-co-styrene);Microemulsion polymerization;Core-shell nanoparticles;Diimide hydrogenation
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发布时间：2021-05-18 - 摘要：Started from phenol and 4, 4-dichloro-methyl-biphenyl, biphenyl-type novolac (BN) was synthesized by Friedel-Crafts alkylation reaction. Phthalonitrile biphenyl novolac (PBN) was then synthesized through nucleophilic substitution between 4-nitro-phthalonitrile and BN. Hollow PBN microsphere was prepared by high-temperature foaming for the first time. Structure and thermal properties of PBN and its cured products were examined by Fourier transform infrared spectroscopy (FTIR), 1H nuclear magnetic resonance spectroscopy (1H-NMR), gel permeation chromatography (GPC), thermal gravimetric analysis (TG) and laser particle measurement, respectively. Structure of the PBN microspheres was characterized by optical microscopy and scanning electron microscope (SEM). FTIR showed that nucleophilic substitution occurred for phenolic hydroxyl group and phthalonitrile was successfully grafted to BN backbone. Grafting rate of phthalonitrile was 53.63% calculated from1H-NMR analysis. GPC analysis indicated that PBN and BN had similar dispersion index, indicating that nucleophilic substitution did not alter the molecular structure of BN main chain. FTIR analysis of the cured PBN indicated that the cure mechanism was dependent on the grafting rate of phthalonitrile. PBN underwent crosslinking through phenol-mediated reaction of its nitrile groups. By self-curing without curing agents, cured PBN formed rigid structures with isoindoline and phthalocyanine rings. With curing agents, triazine ring was formed in cured PBN, in addition to isoindoline and phthalocyanine rings. TG analysis showed that char yield of cured PBN was 67% by self-curing and it was 70% with curing agents used when curing was done in argon at 1000 ℃, while those figures became 70% and 47% at 600 ℃ in air. Hollow PBN microsphere was highly spherical as seen under SEM. The number average particle diameter was 562 μm and the volume average particle diameter was 583 μm, respectively, with a polydispersity index of 1.04.关键词：Phthalonitrile;Biphenyl novolac;Hollow microspheres;Thermal performance
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发布时间：2021-05-18 - 摘要：The formation mechanism of flower-like nanostructured polyaniline prepared in the presence of L-valine was explored by a series of in situ 1H-NMR experiments. Depending on the signal evolution of the major species, the flower-like nanostructured polyaniline was observed to be formed through three continuous stages. At first, aniline and L-valine were packed loosely into micelle-like aggregates before the polymerization, and the phenazine-like oligomers were abundantly formed within this kind of micelles; subsequent steady aggregation through p-p stacking interactions rendered the oligomers to arrange into structurally regular polyaniline nanosheets due to their highly conjugated fused rings; further intermolecular packing of these nanosheets under guidance of L-valine through hydrogen bonds generated the desired flower-like architectures. Moreover, a careful comparison between the samples prepared under varied concentration of L-valine and aniline revealed that the formation of the flower-like nanostructure was closely related to the following characters from L-valine: (1) being amphoteric, which could have served as the buffering agent to prevent abrupt protonation of aniline and to induce several aniline molecules to share the charges together so that the electron density at each reactive site on aniline was nearly identical before the polymerization, the formation of phenazine-like oligomers was therefore favored with a steady proton induced aggregation of these oligomers; (2) locating at the outer-most layer of the aggregates upon the formation of L-valine stabilized monomer micelle due to its amphiphilic feature, which could not only stabilize the monomers and the nascent produced oligomers but also would allow the aggregation of the oligomers to form nanosheets and the subsequent formation of the nanoflowers in the micelle or through fusion of the micelles respectively. This way, any irregular structural arrangement induced by the disturbance from the reaction system or the surroundings might be eliminated, leading to the highly thermodynamic stable product to be produced eventually.关键词：Phenazine-like oligomers;Nanosheets;π-π Stacking interactions;Hydrogen-bonds
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发布时间：2021-05-18 - 摘要：A new kind of accordion-type chain-folded poly (ester urea) was designed and synthesized by polyesterfication of the monomer with dicarboxylic groups, 4, 4'-(carbonyldiimino) dibenzoic acid and the monomer with dibromo groups, 3, 5-bis (bromoalkoxyl) benzoate in polar solvents at ambient temperature. The kinetics of the polyesterification reaction was monitored by proton nuclear magnetic resonance (1H-NMR) tracking. It was found that the rate of the polyesterification was highly dependent on the length of the side alkyl chains in the monomer bearing dibromine groups. A shorter alkyl chain was more favorable for the polyestification owing to the different solubility of the monomers, the monomer bearing a long alkyl chain was more soluble. The number average molecular weight and the structure of the obtained poly (ester urea) were characterized by 1H-NMR and Fourier transform infrared spectroscopy (FTIR). The number average molecular weight of the poly (ester urea) was measured to be about 2 × 104. Differential scanning calorimetry (DSC) result showed that the melting point of the poly (ester urea) was 57 ℃, suggesting that the presence of crystallinity in the poly (ester urea). While the result from thermogravimetric analysis (TGA) displayed that the poly (ester urea) was highly thermal-stable with an obvious thermal degradation around 407 ℃. Because of the presence of the hydrogen-bond between the urea groups and π-π interaction between the phenyl units, self-assembly behavior of the poly (ester urea) in a mixed organic solvent of chloroform and methanol was characterized by transmission electron microscope (TEM). The result revealed that lamellar aggregates formed in the poly (ester urea) after self-assemble for 4 h. However, the lamellar aggregates ultimately transformed into thermodynamically stable vesicles after standing for 3 days. The thickness of the vesicle wall was estimated to be about 7 nm, close to the calculated size of the folded chain. Moreover, the result of X-ray diffraction (XRD) confirmed the presence of ordered structure in the polymer, demonstrating further that the poly (ester urea) adopts a chain-folded conformation.关键词：Chain-folded;Poly (ester urea);Polyesterification;Self-assembly
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发布时间：2021-05-18 - 摘要：To make up for the limited mechanical strength of polyurethane (TPU) material, carbon nanotube (CNT) and graphene (G) were added as reinforced phase into TPU matrix. The mechanical property of this CNT-G/TPU nanocomposite as well as its self-healing property under microwave processing were studied. Firstly, the solution mixing method was adopted to prepare the polyurethane nano-composite filled with carbon nanotube and graphene. Then the mechanical property of this G-CNT/TPU nanocomposite was investigated by tensile testing. Mechanics principle analysis and scanning electron microscopy (SEM) were used to study the mechanisms of mechanical enhancement of this material. Secondly, self-healing property of these nanocomposites under microwave electromagnetic fields was also studied through the crack-repair processing, tensile testing and SEM characterization. Moreover, the mechanisms of both mechanical enhancement and self-healing under microwave radiation were studied. The results indicated that, in terms of mechanical properties, there was synergetic reinforcement effect between the graphene and CNT, which could further improve the mechanical performance of G-CNT/TPU nanocomposites when compared with the samples that were only reinforced with CNT or grapheme. With the ratio of graphene to CNT at 3:1, the mechanical properties of G-CNT/TPU composites reached the optimum value of 70.6 MPa. The tensile strength of G-CNT/TPU composites increased by 67% when compared with that of pure TPU materials, by 18% when compared with that of G/TPU composites, and increased by 25% when compared with that of CNT/TPU composites. In term of the self-healing of cracks in the material under microwave radiation, the results showed that these G-CNT/TPU composites could be healed effectively and rapidly by microwave electromagnetic radiation. Besides, it was shown that the self-healing efficiency could be further improved by the synergy effect between graphene and CNT. With the ratio of graphene to CNT at 3:1, the self-healing of G-CNT/TPU composite reached the best result, which was about 117% of the virgin composite.关键词：Carbon nanotubus;Graphene;Polyurethane;Self-healing;Mechanical properties
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发布时间：2021-05-18 - 摘要：A series of polyimide based covalent organic framework materials for lithium batteries anode were designed and synthesized. In a first step, dianhydride, namely pyromellitic dianhydride (PMDA), and naphthalene-1, 4, 5, 8-tetracarboxylic dianhydride (NTCDA) with different covalent organic frameworks were chosen to build blocks, which were utilized to polymerize with melamine to fabricate dianhydridebased polyimides (PIs). The obtained materials were heat-treated to improve their crosslinking degree and the stability. The materials showed a stable electrochemical performance when it was used in lithium ion battery cathode. The electrochemical test revealed that the material exhibited good durability, large reversible capacity and satisfied rate capacity. The result of rate capacity test showed that a reversible capacity of PI-1 of 471 mAh·g-1 was reached after 150 cycles at 150 mA·g-1, and the reversible capacity of PI-1 reached to 2 A·g-1 at 122.1 mAh·g-1, which could be maintained even at current density of 2 A·g-1, indicating that the material was of good rate capacity. Compared with the low molecular materials, the polymerization of the material avoided the decomposition during the redox process to improve the stability. Meanwhile, the high conjugated structural and polar groups on polymer was believed to have improve the electrical conductivity and the accommodation capability with Li ion. At the same time, the stability of the materials was increased by polymer crosslinking, resulting in the capacity loss in the process of charging and discharging of the lithium ion battery cathode, which was caused by the redox reaction. High degree of conjugate structure unit improved the carrier in the process of charging and discharging and the speed of the ion moving in and out. These materials would potentially afford a way to develop novel organic anode materials with excellent durability and rate capacity for LIBs.关键词：Polyimide;Covalent organic frameworks;Copolymer;Li-ion battery
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发布时间：2021-05-18 - 摘要：Attention has been always paid to long-term antibacterial property of biomaterial surface. In this study, the antibacterial hydroxyapatite (HA) and silver (Ag) composite coatings are prepared in situ using pulse electrochemical driven by regulation of chitosan. The influence of both Ag+ and the salt on the composition and the morphology of the composite coatings was investigated. The underlying mechanism regulated to HA and Ag nanoparticles formation by chitosan (CS) was studied. The study showed that the optimized experimental conditions were the following: potential of-1.3 V, Ag+ concentration of 0.06 g×L-1, Ca2+ concentration of 5 mmol·L-1. Under these conditions, the biological activity of the composite coatings, their antibacterial properties and physiological stability were examined. The results showed that the composite coatings were composed of nano-spherical with three-phases of HA, Ag and CS, where Ag and HA nanoparticles were uniformly distributed in the composite coatings, and covered by a layer of CS. The composite coatings were immersed in supersaturated calcium phosphate solution (SCPS) for mineralization for 10 days at 37 ℃. The biomimetic mineralization process of the surface of the composite coatings produced fine aticular neat rows of HA with preferential growth of the (002) crystal plane at 25.8°. This indicated that the composite coatings could form apatite in the fast mineralized solution. Also, it was found that bioactivities of the composite coatings were pretty good. In order to examine the physiological stability of the composite coating, the composite coating was immersed in phosphate buffer solution (PBS) at 37 ℃. In composite coatings, the presence of CS reduced the double ion release rates of Ca2+ and Ag+, leading to good physical stability. The antibacterial testing of the composite coatings on bacterial strains, including Escherichia coli and Staphylococcus aureus, illustrated antibacterial properties with rates over 99%.关键词：Electrochemical deposition;Hydroxyapatite;Composite coatings;Bioactivity;Antimicrobial properties
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发布时间：2021-05-18 - 摘要：Nano-structured fibers of the synthesized poly (p-dioxanone-co-trimethylene carbonate) (PDT) were prepared by electrospinning technique. Their physicochemical and biological properties were evaluated with the standards for biomedical materials, and it was found that the content of trimethylene carbonate (TMC) units in PDT copolymer could change these properties. As observed by the scanning electron microscopy (SEM), more TMC units in the copolymer would lead to severer fiber bonding, which probably is due to the ductility of TMC segments. The results of static water contact angle testing for the electrospun membranes demonstrated that the hydrophilicity of PDT was between that of PPDO and PTMC. The growth status of L929 mouse fibroblasts on the as-spun membranes of PDT and PPDO was evaluated by the fluorescent micrographs and MTT assay. It was shown that the cells grew well, which suggested good biocompatibility of these copolymers and their suitability in biomedical fields. The in vitro biodegradability of the electrospun membranes in phosphate buffered saline (PBS) and PBS with lipase was separately investigatedvia measurements of mass loss, Mw retention, as well as variations in crystallinity, thermal properties, pH value, and morphology. After 35 days of degradation, mass and Mw of the polymers significantly reduced while slight differences appeared in pH values before and after. Among the samples, the pH value exhibited the most descent for PBS with PPDO and the least for PBS with PTMC; pH decrement of PBS with PDT was in the middle. These results pointed out that the existence of TMC units in the polymers reduced acidic products in the degradation process, which was good for implanted materials. The changes of SEM images of PTMC at different degradation time indicated that PTMC degraded by surface erosion. And the changes of thermal properties of PDT indicated that PTMC was susceptible to enzymatic degradation by lipase. The lipase enzymatic degradation rate of the polymers containing TMC segments was faster than the hydrolytic degradation rate, which illustrated this point.关键词：p-Dioxanone;Trimethylene carbonate;Electrospun membranes;Degradation
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发布时间：2021-05-18 - 摘要：On the surface of self-made PEDOT-PSS/PVA conductive textiles, compact and uniform PEDOT-PSS conductive coating is formed by in situ polymerization, and the resulted composite conductive textile is called in situ PEDOT-PSS/PVA. The synthesis conditions of the in situ polymerization are optimized. The morphology, structure and properties of the in situ PEDOT-PSS/PVA are measured. It is shown that the optimum molar ration of the reactant is [PSS]:[FeCl3]:[EDOT] = 2.1:1.4:1 with a reaction time of 10 h. The resultedin situ PEDOT-PSS/PVA has excellent conductive property with a low surface resistance of 2 Ω/□ and electron conductivity of 360 S/cm under the optimized synthesis condition, which may be resulted from the synergistic action between PEDOT-PSS inside the fiber and the surface coating, forming the connected and effective conductive path to facilitate the electron transport. FTIR analysis indicates that there exists strong bonding force between base textile and PEDOT-PSS coating. Besides, the in situ PEDOT-PSS/PVA has strong near infrared shielding capability. The XPS measurements show that the sulfur content of the in situ PEDOT-PSS/PVA is much higher than that of the PEDOT-PSS/PVA, implying a higher PEDOT-PSS conductive component, which is also responsible for its low surface resistance. Moreover, the electromagnetic shielding measurements demonstrate that the electromagnetic shielding effectiveness of thein situ PEDOT-PSS/PVA can reach to 12 dB with a shielding efficiency of 75% compared with the 0 dB of PEDOT-PSS/PVA. In the range of 4 ~ 18 GHz, the most reflectivity of electromagnetic wave is between-5 and-10 dB, which can absorb 75% ~ 90% electromagnetic wave. Furthermore, the in situ PEDOT-PSS/PVA composite conductive textile presents a higher stability, good fastness to washing and acid resistance. The results indicate that the in situ polymerization is an effective method to improve the conductive property of PEDOT-PSS/PVA conductive textile, and the resulted in situ PEDOT-PSS/PVA composite conductive textile has an important application prospect as a novel kind of military electromagnetic wave absorption material.关键词：PEDOT-PSS;Conductive textile;In situ polymerization;Electromagnetic wave absorption properties
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发布时间：2021-05-18 - 摘要：Novel anti-dripping polyethylene terephthalate (PET) fabrics were successfully prepared by two-step gamma radiation-induced grafting polymerization of glycidyl methacrylate (GMA)/divinylbenzene (DVB) and dimethyl vinylphosphonate (DMVP) onto the surface of PET fabrics. The grafting yield increased with the increase of absorbed dose; under the absorbed dose of 21 kGy and 200 kGy, it reached 105.9% and 10.2% for GMA/DVB and DMVP, respectively. Similarly, it was noted that the grafting yield increased with monomer concentration while decreased with the dose rate. Moreover, it was confirmed by FTIR, XPS and TGA analyses that PET fabrics had been successfully modified. FTIR result showed the characteristic vibration absorption peaks of epoxy group and P―O for modified PET at 905 and 1041 cm-1, respectively. Through XPS, the characteristic peaks of phosphorus were recognized, including P2s and P2p. TGA experiment indicated that after PDMVP grafting, thermal stability of the fabric was enhanced by the introduced phosphorus and also the formed protecting carbon layer. Compared with original PET fabrics, the tensile strength of modified PET fabrics was increased by 0.79 kN/m and 0.82 kN/m for grafting of PGMA/DVB and PDMVP, and their elongation at break increased by 11.68% and 9.69%, respectively. The results indicated that the mechanical properties of modified PET fabrics were improved at a certain extent. Additionally, the performance of anti-dripping was improved remarkably after radiation modification due to the introduction of crosslinking network and phosphorus-containing PDMVP chain. The time of per droplet generation during the combustion of PET was 3.0 s/n, and that of PET-g-PGMA/DVB was 17.0 s/n; there was no droplet after further grafting of PDMVP. The morphologies of PET fabrics before and after radiation modification as well as after combustion were observed by SEM and it was found that the modified PET fabrics by grafting of PGMA/DVB and PDMVP retained the fabric structure after combustion.关键词：Anti-dripping;PET fabrics;GMA;DMVP;Radiation-induced grafting
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发布时间：2021-05-18 - 摘要：Based on the dissolution of cellulose in aqueous LiOH/urea solvent system at low temperature, a series of cellulose/full-vulcanized carboxylic styrene-butadiene elastomeric nanoparticles (CSB ENP) composite films were successfully prepared for the first time. A certain amount of CSB ENP was introduced by blending the latex with the aqueous LiOH/urea solvent prior to dissolving of native cellulose. Cellulose/CSB ENP composite films with CSB ENP content of 2 wt% ~ 20 wt% were prepared from the resulting cellulose/CSB ENP complex solutions by coagulating with 5 wt% H2SO4/10 wt% Na2SO4 aqueous solution. Their structure and properties were investigated by TEM, SEM, X-ray diffraction, solid-state NMR, thermogravimetry and tensile testing. The TEM and SEM results indicated that the CSB ENPs with particle size of 50 ~ 200 nm were embedded uniformly in the matrix of regenerated cellulose film, which possessed homogeneous micro/nanoporous structure. The results from WAXD and solid-state NMR demonstrated that the crystallinity of regenerated cellulose was barely affected by the addition of CSB ENP and there was no chemical reaction between cellulose and CSB ENP. The cellulose/CSB ENP composite films exhibited a good optical transmittance, suggesting certain miscibility of the two components. The thermal stability of the composite films was improved to some extent compared with that of the pure regenerated cellulose film. The tensile testing results showed that low content of CSB ENP could toughen the regenerated cellulose films probably due to the fact that the highly cross-linking structure of CSB ENP prevented entanglement between the rubber particles. And thus the CSB ENP with a characteristic of small particle size and large specific surface area could disperse in the cellulose matrix in nano scale. Moreover, there existed intermolecular hydrogen-bond interaction between the carboxyl groups of CSB ENP and the hydroxyl groups of cellulose. The composite film with 5 wt% of CSB ENP showed the best tensile strength and elongation at break. When the content of CSB ENP increased, the mechanical property of the composite films decreased gradually. This was possibly caused by the aggregation of excessive elastomeric nano-particles.关键词：Cellulose;Elastomeric nano-particles;Composite films;Mechanical property
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发布时间：2021-05-18 - 摘要：The commercial bleached bamboo was oxidized to TEMPO-oxidized cellulose nanofibrils (TOCNs) with hundreds of nanometers in length, less than 5 nm in width and the thickness less than 1 nm, which was done through TEMPO (2, 2, 6, 6-tetramethylpiperidine-1-oxyl radical)-mediated oxidation and successive mild disintegration in water solution combined with high frequency ultrasonic treatment. The shape and dimensions of these sheets (termed "nanoribbons" hereafter), composed of many mono-and multi-layered molecular sheets, were analyzed by scattering and spectroscopy techniques to understand the structural changes. The morphology of the raw material and two types of TOCNs nanoribbons were characterized by field emission scanning electron microscope (FESEM). And the three-dimensional morphologies of the same two TOCNs samples were measured through transmission electron microscope (TEM) and atomic force microscope (AFM). The results revealed that the TEMPO-mediated oxidization systems processed in cellulose Iβ intra-layer hydrogen bonds and the long axis direction of elementary fibrils (composed by 36 cellulose chains), which was explained based on the three dimensional size difference and the change of the crystalline structure in TOCNs nanoribbons. The structures of different nanoribbons were analyzed by Fourier infrared absorption spectrum (FTIR), X-ray diffraction (XRD) and cross polarization and magic angle spinning solid-state 13C nuclear magnetic resonance (CP/MAS 13C-NMR). FTIR analysis indicated that structure of the cellulose nanofibrils was changed during oxidization, significant amount of C6 carboxylate groups were selectively formed on each cellulose nanofibril surface by TEMPO-mediated oxidation without any changes to the original crystalline structure. XRD analysis indicated that the cellulose elementary fibrils were broken mainly along its (101) planes in TEMPO/NaClO/NaClO2 and TEMPO/NaClO/NaBr oxidized systems under high frequency ultrosonic treatment, while the Iβ crystalline structure was retained. CP/MAS 13C-NMR results were in accordance with the analyses from XRD and Raman spectra. Moreover, the oxidization scheme was important to explain the structures of the final products. This research has important significance for design, development and application of ultrafine biomass cellulose nanoribbons.关键词：TEMPO-oxidization;High frequency ultrasonication;Cellulose nanofibrils;Three dimensional size;Oxidation mechanism
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发布时间：2021-05-18 - 摘要：A novel and efficient flame retardant additive poly (biphenyl phenoxy phosphate) (PBPP) was prepared by reaction of biphenol with dichloro phenoxyphosphate. The structure of PBPP was confirmed by Fourier transform infrared spectroscopy (FTIR). Thermogravimetric analysis (TGA) result demonstrated that the initial decomposition temperature of PBPP was 253 ℃, with a char residue of 51.41 wt% at 700 ℃. PBPP blended with ammonium polyphosphate (APP) at the ratio of 1:2 was incorporated into epoxy resins and cured by m-phenylenediamine (m-PDA) to form a flame retardant thermoset (FR-EP). The effect of intumescent flame retardant (IFR) composed of PBPP and APP on epoxy resin was investigated by limiting oxygen index (LOI), vertical burning (UL-94), TGA, cone calorimetry (CONE) and scanning electron microscope (SEM). Comparing epoxy resin retardant common used with 30 wt% of APP, this PBPP based flame retardant with PBPP amount of 10 wt% showed similar retardant effect as the classical one. At this low addition amount of PBPP, this PBPP/APP modified FR-EP past UL-94 V0 grade and gave 29.6% as the LOI value at the same time. TGA test results revealed that the incorporation of PBPP/APP into epoxy resins stimulated the degradation of thermosets at initial degradation stage and charring formation. The char yield of the FR-EP thermosets increased from 15.02 wt% to 22.17 wt% at 700 ℃. According to the cone test, heat release rate (HRR), total heat release (THR) and total smoke release (TSR) were all decreased dramatically in the FR-EP. The cone test showed that the av-HRR, pk-HRR, and av-EHC of the FR-EP were lowered by 45.7%, 51.0% and 21.1%, respectively, in comparison with those of the pure EP. The char residue of the new FR-EP was about 80 times of that observed in the pure EP. Moreover, PBPP/APP promoted the epoxy resins thermosets forming more homogeneous and compact char layer during combustion as seen by SEM. Char layer prevented the inner matrix from further decomposition and combustion, which enhanced by consequence the flame retardancy of the FR-EP thermosets. The tensile strength of EP was also affect by loading of the flame retardant with a decrease at lower loading and an increased at higher loading.关键词：Poly (biphenyl phenoxy phosphate);Ammonium polyphosphate;Epoxy resin;Intumescent flame retardant;Synergistic
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发布时间：2021-05-18 - 摘要：The polyhedral oligomeric silsesquioxane (POSS) based fluorinated acrylate copolymer was successfully synthesized via free radical solution polymerization, which was confirmed by nuclear magnetic resonance (NMR) and gel permeation chromatography (GPC). POSS and fluorinated monomers were separately introduced into the reaction. The copolymer solution was firstly prepared by dissolving the copolymer in a solvent mixture of 1, 1, 2-trichlorotrifluoroethane (F113) and ethyl acetate (EA). The copolymer film was then fabricated by simply drop-casting one layer of the solution on glass substrate. The influence of the volume ratio of EA to F113 on the microstructure morphology, element component, roughness and hydrophobicity of the film surface were investigated by scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), atomic force microscope (AFM) and contact angle instrument. The experimental results revealed that POSS aggregated on the top of the film and formed nanoscale particles, which led to rough film surface withgreat hydrophobicity. The complex and rough structure was simply fabricated due to the effect of both POSS aggregation and organic microphase separation between the fluorinated and non-fluorinated segments. Although fluorinated segments and POSS were competed to migrate onto the film surface, the content of POSS aggregation on the surface would decrease alone with the increase of F113 in the solvent mixture, while fluorine content was contrary to that. Consequently, both the surface roughness and surface energy decreased. The phenomenon suggested that F113 was helpful for fluorinated segments to migrate onto the film surface, which ensured much more room for fluorinated parts on the surface and reasonably restrained the capability of POSS aggregation on the surface. The hydrophobicity of the film increased with the content of F113. With pure F113 as the solvent, the contact angle of the film formed increased to the maximum of 135.0°, demonstrating well-performed hydrophobicity. At the same time, the fluorine content and average roughness of the film surface were 45.25% and 93.4 nm, respectively.关键词：Polyhedral oligomeric silsesquioxane;Fluorinated acrylate polymer;Hydrophobicity;Phase separation;Roughness
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发布时间：2021-05-18 - 摘要：For tire tread rubbers A1 and A2 which were the same kind of rubber but with different cross-linking densities, dynamic tensile experiment was conducted to obtain the curves of storage modulus and loss modulus versus loading frequency. In order to quantitatively analyze the energy loss of rubber under dynamic load with different frequencies and temperatures, a viscoelastic constitutive equation based on generalized Maxwell model was developed. An algorithm of non-linear regression was proposed to fit the curves of dynamic modulus versus loading frequency respectively in low frequency phase (10-25 Hz) and high frequency phase (25-60 Hz). And then the parameter values of storage modulus and loss modulus in the form of generalized Maxwell model were obtained. The dynamic tensile process of the tread rubber was simulated by finite element code Abaqus, and the loss tangent was calculated; hence the changing characteristic of loss tangent under different frequencies was revealed. Through the comparison of experimental results and simulated results of the loss tangent, it was proved that the viscoelastic constitutive model and its parameter determination method could be used to accurately analyze the tread rubber's dynamic tensile performance. Quantitative predictions of the tread rubber stress-strain hysteresis loop and the energy dissipation in a full deformation cycle under different temperatures and frequencies were performed, and then the change rule of the energy dissipation under such circumstance was explained. The results reveal that the energy dissipation of tread rubbers is gradually increased as the loading frequency increases, whereas its dependence on frequency is gradually reduced as the temperature increases. Meanwhile, the structure-function relationship of tread rubber was explored through the comparative testing of cross-linking density. Although the cross-linking density of tread rubber A2 was slightly higher than that of tread rubber A1, the energy loss of tread rubber A2 was larger than that of tread rubber A1 under the same conditions due to the higher proportion of the dangling chain end in the molecular chain.关键词：Rubber;Energy loss;Dynamic modulus;Viscoelastic model;Finite element method
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发布时间：2021-05-18 - 摘要：Molecular dynamics simulations have been used to study the micro structures and properties of graphene/polyethylene composite, and tensile properties of the composite have also been analyzed using a uniaxial tension simulation. The results show that, different from the pure polyethylene model, multi adsorption layers are formed by polyethylene molecules on the graphene surface due to equilibrium structure of the composite model. These adsorption layers are dynamic stable, through which the polyethylene molecules can migrate during the simulation progress. "Adsorption curing" occurs in the adsorption layers, where the polyethylene molecules become more extended and ordered, and their movements are inhibited in the direction perpendicular to the graphene surface. Tension simulation suggests improvement in tensile properties of the polyethylene matrix brought by graphene. In elastic region and yield region, the graphene can inhibit compression deformation in directions perpendicular to the strain, which keeps the stability of "adsorption curing" structures and causes sharp increase of stress in the composite model before yield strain. In softening region, the graphene bends greatly in one of the perpendicular directions and the "adsorption curing" structures are damaged, which results in stress decrease after the yield strain. The strain increase leads to not only change of non-bond interactions in the first two regions but change of intramolecular bonding energy in the last region for the composite system. The yield stress of composite increases with the increase of strain rate, which, however, has no effects on the general trend of the stress-strain curves for the model.关键词：Molecular dynamics simulation;Graphene;Polyethylene;Uniaxial tension;Composite
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发布时间：2021-05-18
Research Article
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