- Abstract：Photodynamic therapy (PDT) has been emerged as a promising modality for cancer treatment. However, the development of drug delivery system enabling continuous release of photosensitizers (PSs) for long-term PDT treatment still remains challenges. Herein, a H2O2-responsive injectable hydrogel, covalently crosslinked by N1-(4-boronobenzyl)-N3-(4-boronophenyl)-N1,N1,N3,N3-tetramethylpropane-1,3-diaminium (TSPBA) with PVA containing polythiophene quaternary ammonium salt (PT2) polymer dots (PDots) as a photosensitizer was fabricated. Under the stimulation of H2O2, the obtained injectable hydrogel gradually degrades and releases PDots. In vitro experiments suggested that the released PDots could realize efficient tumor cells inhibition through its robust singlet oxygen generation capability upon 577 nm laser irradiation. In vivo studies demonstrated a sustained retention of PDots for at least 7 days following single-dose administration, facilitating efficient tumor inhibition with light treatments for 3 times without apparent biotoxicity. This work presents an innovative polymer dots-based composite local drug delivery system for long-term PDT in cancer treatment.Keywords：Injectable hydrogel;Polymer dots;Photodynamic therapy;Long-term;Cancer therapy
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Updated：2024-06-26 - Abstract：In prevailing p-i-n perovskite solar cells (PSCs), solution-processible fullerene molecules are widely used as electron-transporting layers (ETLs) but they typically suffer from poor uniformity and undesirable stability issues. Additionally, a separate bathocuproine (BCP) layer is needed to block hole transfer, increasing fabrication complexity and cost. Here, we address these limitations by developing a novel polymeric ETL (named PFBCP) synthesized by polymerizing C60 with BCP. This innovative material achieves both efficient electron transport and hole blocking, while its excellent uniformity minimizes interface recombination and enhances stability. Consequently, our blade-coated PSCs utilizing PFBCP achieve a high power conversion efficiency exceeding 22% and retain 91% of initial efficiency after 1200 h of light exposure. This development not only paves the way for commercially viable PSCs but also opens avenues for future ETL design to realize even more efficient and stable PSCs.Keywords：Polyfullerene;Perovskite solar cells;Electron transporter;Stability
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Updated：2024-06-26 - Abstract：Due to the mechanical stability of PP layer, the PP/HDPE double-layer microporous membrane could be prepared at a higher heat-setting temperature than that of PE monolayer membrane. In this work, the effects of heat-setting temperature on the pore structure and properties of PP/HDPE double-layer membrane were studied. With the increase of heat-setting temperature from 120 °C to 130 °C, the length of connecting bridge crystal and crystallinity in the PE layer increase due to the melting of thin lamellae and the stability of connecting bridge structure during heat-setting. The corresponding air permeability, porosity, wettability of liquid electrolyte and mechanical property of the heat-set microporous membrane increase, exhibiting better electrochemical performance. However, when the heat-setting temperature is further increased to 140 °C, higher than the melting point of PE resin, some pores are closed since the lamellae and connecting bridges melt and shrink during heat-setting, resulting in a decrease of air permeability and porosity. In contrast, there is negligible change in the PP layer within the above heat-setting temperature region. This study successfully builds the relationship between the stable pore structure and property of microporous membrane during heat-setting, which is helpful to guide the production of high-performance PP/PE/PP lithium batteries separator.Keywords：PP/HDPE bilayer microporous membranes;Heat-setting temperature;SAXS
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Updated：2024-06-26 - Abstract：Supramolecular polymers, as a type of dynamic polymers, are subordinate to the interdisciplinary field of polymer chemistry and supramolecular chemistry, whose development has greatly promoted the prosperity of new materials. Notably, molecular weight is one of the most important parameters of supramolecular polymers, which affects the physical/chemical properties and processing applications of materials. Developing new methods for characterizing the molecular weight of supramolecular polymers is crucial for advancing the development of supramolecular polymers. In this review, we elaborate and summarize three strategies for characterizing the molecular weight of supramolecular polymers that recently reported by our research group according to the characteristics of supramolecular polymers, including (1) the molecular weight distinction corresponding to variable fluorescence colors, (2) matching different molecular weights with different fluorescence lifetime, (3) transforming supramolecular polymers into mechanically interlocked polymers or covalent polymers. Besides, we also discuss the limitations of current methods for characterizing supramolecular polymers. We hope that this review can promote the development of supramolecular polymers and significantly inspire to exploit new methods to characterizing molecular weight of supramolecular polymers.Keywords：Supramolecular polymers;Molecular weight;Self-assembly;Mechanically interlocked polymers;Dynamic polymers
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Updated：2024-06-26 - Abstract：Toughening the petroleum-based epoxy resin blends with bio-based modifiers without compromising their modulus, mechanical strength, and other properties is still a big challenge in view of the sustainability. In this study, a bio-based liquid crystal epoxy resin (THMT-EP) with an s-triazine ring structure was utilized to modify a petroleum-based bisphenol A epoxy resin (E51) with 4,4'-diaminodiphenylsulfone (DDS) as a curing agent, and the blended systems were evaluated for their thermal stability, mechanical properties, and flame retardancy. The results showed that the impact strength of the blended system initially increased and then decreased with the increase in THMT-EP content, and it reached the a maximum value of 26.5 kJ/m2 when the THMT-EP content was 5%, which was 31.2% higher than that of E51/DDS. Notably, the flexural strength, modulus, and glass transition temperature of the blended system were all simultaneously improved with the addition of THMT-EP. At the same time, the addition of THMT-EP enhanced the flame retardancy of the system by increasing the char yield at 700 °C and decreasing the peak heat release rate and total heat release rate. This work paves the way for a more sustainable improvement in the comprehensive performance of epoxy resin.Keywords：Epoxy resin;Toughening;Bio-based;Thermotropic liquid crystal
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Updated：2024-06-26 - Abstract：Owing to its high production volume and wide range of applications, polyethylene has gained a great deal of attention, but its low surface energy and non-polar nature have limited its application in some important fields. In this study, ethylene/11-iodo-1-undecene copolymers were prepared and used as the intermediates to afford a series of imidazolium-based ionomers bearing methanesulfonate (CH3SO3−), trifluoromethanesulfonate (CF3SO3−), or bis(trifluoromethane)sulfonimide (Tf2N−) counteranions. The tensile test results showed that the stress-at-break (7.8−25.6 MPa) and the elongation-at-break (445%−847%) of the ionomers could be adjusted by changing the counterion species and the ionic group contents. Most importantly, the ionomers exhibited marvelous antibacterial activities against Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli). The ionomers bearing Tf2N− exhibited antibacterial activities >99% against both S. aureus and E. coli when ionic content reached 9.1%. The imidazolium-based ionomers prepared in this work demonstrated excellent comprehensive properties, especially high-efficient and broad-spectrum antibacterial ability, exhibiting the potential for the application as the antibacterial materials in packaging, medical, and other fields.Keywords：Polyethylene;Ionomer;Polymerization catalysis;Imidazolium;Antibacterial material
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Updated：2024-06-26 - Abstract：Shear-thinning hydrogels have emerged for endoscopic submucosal dissection, while wound intervention after surgery has rarely been mentioned. Herein, a catechol-modified chitosan hydrogel with shear-thinning property was developed for simultaneously facilitating endoscopic submucosal dissection and postoperative wound healing. Benefiting from the shear-thinning and self-healing characteristics, the as-prepared hydrogel showed easily endoscopic injectability. It also performed very well as submucosal cushion, which could remain above 70% after injection for 120 min in ex vivo porcine large intestine model. In fact, the cushion height of normal saline dramatically decreased to 46% of the initial height at 30 min. Ag nanoparticles encapsulated into the network endowed the hydrogel with almost reached 100% antibacterial effect against E. coli and S. aureus. The hemolysis ratio of the hydrogel was calculated to be as low as 0.8%. Combined with good hemocompatibility and cytocompatibility, the as-prepared hydrogel displayed much higher in vivo wound closure and healing efficacy than normal saline. These results demonstrated the superiority of the shear-thinning chitosan hydrogel in facilitating clinical endoscopic submucosal dissection surgery.Keywords：Shear-thinning hydrogel;Endoscopic mucosal dissection;Wound healing;Catechol-modified chitosan;Silver nanoparticles
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Updated：2024-06-26 - Abstract：Janus films with asymmetric physical/chemical properties have attracted considerable attention due to their promising applications in personal thermal management, electronic skins, sensors, actuators, etc. However, traditional methods for fabricating Janus films conventionally need the assistance of an interface or auxiliary equipment, which are usually complex and time-consuming. Herein, flexible poly(vinyl alcohol) (PVA)/graphene oxide (GO)/h-BN (recorded as PVA/GO/h-BN) Janus films with thermally, optically, and electrically anisotropic properties are fabricated by a simple density deposition self-assembly method, which just utilizes the density difference between GO and h-BN during water evaporation. Experimental results show that the two sides of the acquired Janus films have obvious asymmetric characteristics. In the original state of the PVA/GO/h-BN Janus films, the thermal conductivity of the GO side (10.06 W·m–1·K–1) is generally lower than that of the h-BN side (10.48 W·m–1·K–1). But after GO is reduced, the thermal conductivity of the rGO side reaches 12.17 W·m–1·K–1, surpassing that of the h-BN side. In addition, the relative reflectance of the h-BN side of Janus film is also significantly higher than that of the rGO side, and the surface resistance difference between the two sides is about 4 orders of magnitude. The prepared PVA/GO/h-BN Janus films show great application potential in human thermal management, light conversion switches, and electronic skins. This study provides a simple and versatile strategy for fabricating Janus films with multifunctional (such as thermal, optical, and electrical) anisotropies.Keywords：Janus films;PVA/GO/h-BN;Density deposition self-assembly method
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Updated：2024-06-07 - Abstract：Polyelectrolyte (PE) gels, distinguished by their unique stimuli-responsive swelling behavior, serve as the basis of broad applications, such as artificial muscles and drug delivery. In this work, we present a theoretical model to analyze the electrostatics and its contribution to the swelling behavior of PE gels in salt solutions. By minimizing the free energy of PE gels, we obtain two distinct scaling regimes for the swelling ratio at equilibrium with respect to the salt concentration. We compare our predictions for the swelling ratio with experimental measurements, which show excellent agreement. In addition, we employ a finite element method to assess the applicability range of our theoretical model and assumptions. We anticipate that our model will also provide valuable insights into drug adsorption and release, deformation of red blood cells, 4D printing and soft robotics, where the underlying mechanism of swelling remains enigmatic.Keywords：Gels;Polyelectrolytes;Swelling;Stimuli-responsive;Electrostatic interaction;Cell model
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Updated：2024-06-06 - Abstract：Salt-doped block copolymers have widespread applications in batteries, fuel cells, semiconductors, and various industries, where their properties crucially depend on phase separation behavior. Traditionally, investigations into salt-doped diblock copolymers have predominantly focused on microphase separation, overlooking the segregation between ionic and polymeric species. This study employs weak segregation theory to explore the interplay between phase separation dominated by the polymer-modulated mode and the salt-out-modulated mode, corresponding to microscopic and macroscopic phase separations, respectively. By comparing diblock copolymers doped with salts to those doped with neutral solvents, we elucidate the significant role of charged species in modulating phase behavior. The phase separation mode exhibits a transition between the polymer-modulated and salt-out-modulated modes at different wavenumbers. In systems doped with neutral solvents, this transition is stepwise, while in salt-ion-doped systems, it is continuous. With a sufficiently large Flory-Huggins parameter between ions and polymers, the salt-out-modulated mode becomes dominant, promoting macrophase separation. Due to the solvation effect of salt ions, salt-doped systems are more inclined to undergo microphase separation. Furthermore, we explore factors influencing the critical wavenumber of phase separation, including doping level and the Flory-Huggins parameters between two blocks and between ions and polymeric species. Our findings reveal that in a neutral solvent environment, these factors alter only the boundary between micro- and macro-phase separations, leaving the critical wavenumber unchanged in microphase separation cases. However, in a salt-doped environment, the critical wavenumber of microphase separation varies with these parameters. This provides valuable insights into the pivotal role of electrostatics in the phase separation of salt-doped block copolymers.Keywords：Block copolymer;Phase separation;Field theory;Salt-doped
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Updated：2024-06-06 - Abstract：Polyelectrolyte solutions are more variable than uncharged macromolecule due to electrical interaction between charged molecules and surrounding counterions. Therefore, the subject of polyelectrolyte solutions has attracted a wide range of interests in both basic and applied research, and has also been extensively explored. However, the understanding of the molecular dynamics and conformation of polyelectrolytes in solution remains to be deepened, and universal consensus on some key issues have not been reached. Many methods have contributed to solving the above problems in different ways, including dielectric relaxation spectroscopy (DRS). In this perspective, we briefly reviewed the history of dielectric spectroscopic research on polyelectrolyte solution, with emphasis on summarizing our efforts. In particular, we expound the characteristics of DRS and its ability to obtain the internal information of the system of interest. Finally, we evaluate the advantages and limitations of the dielectric method and discussed future prospects of this field.Keywords：Polyelectrolyte;Dielectric relaxation spectroscopy;Chain conformation;Thermosensitive microgel;PNIPAM microgel;Spherical polyelectrolyte brushes
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Updated：2024-06-05 - Abstract：We present the results of molecular dynamics simulations of steady shear between a pair of neutral polymer brushes, as well as a pair of charged polymer brushes in the strongly compressed regime. The results of the molecular dynamic simulations of neutral and polyelectrolyte brushes in implicit solvent including normal forces, shear forces, viscosities and friction coefficients as a function of separation between brushes, are presented in the study. The comparison of the simulation results of neutral and charged brushes shows that the charged brushes is in the quasi-neutral regime, and the dependence of viscosity on the separation distance show the similar power law of neutral brushes. Our simulation results confirm that the implicit solvent simulations of polyelectrolyte brushes that ignore hydrodynamics interaction are in agreement with the scaling predictions qualitatively because of screening of hydrodynamic interaction and long-range electrostatic interactions on the correlation length scale. Both of neutral and charged brushes show the lubrication properties that the friction coefficient decreases with the separation decreases at enough large loads. However, a maximum of friction coefficients is observed for polyelectrolyte brushes, which is in contrast to the neutral brushes with monotonical dependence.Keywords：Polymer brush;Frictions coefficient;Polyelectrolyte;Lubrication
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Updated：2024-06-05 - Abstract：The evolution of high-frequency communication has accentuated the significance of controlling dielectric properties in polymer media. Traditionally, it has been theorized that rigid molecular chains lead to lower dielectric loss. However, the validity of this proposition at high frequencies remains uncertain. To scrutinize the correlation between chain flexibility and dielectric properties, we synthesized six poly(ester imide)s (PEIs) with systematically varied molecular chain flexibilities by modifying the ester's substitution on the aromatic ring. The introduction of ester bonds bestowed all PEI films with a low dielectric dissipation factor (Df), ranging from 0.0021 to 0.0038 at 10 GHz in dry conditions. The dry Df displayed a pattern consistent with volume polarizability (P/V). Unexpectedly, PI-mmm-T, featuring the most flexible molecular chain, exhibited the lowest dielectric loss under both dry (0.0021 @ 10 GHz) and hygroscopic (0.0029 @ 10 GHz) conditions. Furthermore, the observed increase in Df after humidity absorption suggests that the high dielectric loss of PEI in applications may be attributed to its hygroscopic nature. Molecular simulations and characterization of the aggregation structure revealed that the smaller cavities within flexible molecular chains, after close stacking, impede the entry of water molecules. Despite sacrificing high-temperature resistance, the precursor exhibited enhanced solubility properties and could be processed into high-quality films. Our research unveils new insights into the relationship between flexibility and high-frequency dielectric loss, offering innovative perspectives on synthesizing aromatic polymers with exceptional dielectric properties.Keywords：5G communications;Poly(ester imide)s;Molecular chain flexibility;Dielectric properties;Humidity absorption
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Updated：2024-06-05 - Abstract：Olefin polymerization is one of the most important chemical reactions in industry. This work presents a strategy that emphasizes the synergistic meta/para-steric hindrance of N-aryl groups and electronic effects in newly synthesized neutral salicylaldiminato nickel catalysts. These nickel(II) catalysts exhibit exceptional thermostability, ranging from 30 °C to 130 °C, demonstrating enhanced catalytic activities and broadly regulated polyethylene molecular weights (3−341 kg·mol−1) and controlled polymer branch density (2−102 brs/1000C). The preferred catalyst Ni3 with concerted steric and electronic effects enables the production of solid-state semi-crystalline polyethylene materials at temperatures below 90 °C. Notably, Ni3 exhibits an impressive tolerance of 110 °C and can withstand even the challenging polymerization temperature of 130 °C, leading to the production of polyethylene wax and oil. Also, functionalized polyethylene is produced.Keywords：Polyolefin;Salicylaldiminato ligand;Nickel catalyst;Oil;Wax
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Updated：2024-06-05 - Abstract：The self-consistent field theory (SCFT) was employed to numerically study the interaction and interpenetration between two opposing weak polyelectrolyte (PE) brushes formed by grafting weak PE chains onto the surfaces of two long and parallel columns with rectangular-shaped cross-section immersed in a salty aqueous solution. The dependences of the brush heights and the average degree of ionization on various system parameters were also investigated. When the brush separation is relatively large compared with the unperturbed brush height, the degree of interpenetration between the two opposing PE brushes was found to increase with increasing grafting density and bulk degree of ionization. The degree of interpenetration also increases with the bulk salt concentration in the osmotic brush regime. Numerical results further revealed that, at a brush separation comparable to the unperturbed brush height, the degree of interpenetration does not increase further with increasing bulk degree of ionization, bulk salt concentration in the osmotic regime and grafting density. The saturation of the degree of interpenetration with these system parameters indicates that the grafted PE chains in the gap between the two columns retract and tilt in order to reduce the unfavorable electrostatic and steric repulsions between the two opposing PE brushes. Based on salt ion concentrations at the midpoint between the two opposing brushes, a quantitative criterion in terms of the unperturbed brush height and Debye screening length was established to determine the threshold value of the brush separation beyond which they are truly independent from each other.Keywords：Polyelectrolyte;Polymer brush;Self-consistent field theory
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Updated：2024-06-05 - Abstract：Plasma treatment is necessary to optimize the performance of biomaterial surfaces. It enhances and regulates the performance of biomaterial surfaces, creating an effective interface with the human body. Plasma treatments have the ability to modify the chemical composition and physical structure of a surface while leaving its properties unaffected. They possess the ability to modify material surfaces, eliminate contaminants, conduct investigations on cancer therapy, and facilitate wound healing. The subject of study in question involves the integration of plasma science and technology with biology and medicine. Using a helium plasma jet source, applying up to 18 kV, with an average power of 10 W, polymer foils were treated for 60 s. Plasma treatment has the ability to alter the chemical composition and physical structure of a surface while maintaining its quality. This investigation involved the application of helium plasma at atmospheric pressure to polyamide 6 and polyethylene terephthalate sheets. The inquiry involves monitoring and assessing the plasma source and polymer materials, as well as analyzing the impacts of plasma therapy. Calculating the mean power of the discharge aids in assessing the economic efficacy of the plasma source. Electric discharge in helium at atmospheric pressure has beneficial effects in technology, where it increases the surface free energy of polymer materials. In biomedicine, it is used to investigate cytotoxicity and cell survival, particularly in direct blood exposure situations that can expedite coagulation. Comprehending the specific parameters that influence the plasma source in the desired manner for the intended application is of utmost importance.Keywords：Atmopheric pressure plasmas;Plasma-polymer interaction;Plasma Medicine;Plasma diagnosis;Polymer characterization
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Updated：2024-05-23 - Abstract：The thermochromic mechanism and the structure-property regulation principle of reversible thermochromic polydiacetylene (PDA) materials have always been a challenging issue. In this work, a series of diacetylene monomers (m-PCDA) containing phenyl and amide or carboxyl groups were synthesized from 10,12-pentacosadiynoic acid (PCDA) through the esterification or amidation reactions. The effects of the number and the distribution of the functional groups in m-PCDA molecules on their solid-state polymerization capability, and the thermochromic mechanism of their corresponding polymers (m-PDA) were investigated and discussed in detail. The results show that the m-PCDA monomers containing both benzene ring and groups that can form hydrogen bonding interactions have strong intermolecular interaction, and are easy to carry out the solid phase polymerization under 254-nm UV irradiation to obtain the corresponding new thermochromic m-PDA materials. The thermochromic behavior of m-PDA depends on its melting process. The initial color-change temperature (blue to red) is determined by the onset melting temperature, and the temperature range in which reversible color recovery can be achieved by repeat heating-cooling treatment is determined by its melting range. According to the proposed thermochromic mechanism of PDA, various new PDA materials with precise thermochromic temperatures and reversible thermochromic temperature ranges can be designed and synthesized through the appropriate introduction of benzene ring and groups that can form hydrogen bonding interactions into the molecular structure of linear diacetylene monomer. This work provides a perspective to the precise molecular structure design and the property regulation of the reversible thermochromic PDA materials.Keywords：Polydiacetylene;Thermochromism;Hydrogen bonding;Chemical modification
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Updated：2024-05-23 - Abstract：Elastomers with high mechanical toughness can guarantee their durability during service life. Self-healing ability, as well as recyclability, can also extend the life of materials and save the consuming cost of the materials. Many efforts have been dedicated to promoting the mechanical toughness as well as the self-healing capability of elastomers at the same time, while it remains a challenge to balance the trade-off between the above properties in one system. Herein we proposed a molecular design driven by dual interactions of acylsemicarbazide hydrogen bonding and Cu2+-neocuproine coordination simultaneously. By introducing the reversible multiple hydrogen bonds and strong coordination bonds, we successfully fabricated an extremely tough and self-healing elastomer. The elastomer can achieve an impressive top-notch toughness of 491 MJ/m3. Furthermore, it boasted rapid elastic restorability within 10 min and outstanding crack tolerance with high fracture energy (152.6 kJ/m2). Benefiting from the combination of dynamic interactions, the material was able to self-repair under 80 °C conveniently and could be reprocessed to restore the exceptional mechanical properties.Keywords：Metal-Ligand coordination;Hydrogen bond;Ultra-tough elastomer;Self-healing
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Updated：2024-05-17 - Abstract：Shish crystals are crucial to achieving high performance low-dimensional ultra-high molecular weight polyethylene (UHMWPE) products. Typically, high stretch and shear flow fields are necessary for the formation of shish crystals. In this study, UHMWPE gel films with reserved shish crystals were prepared by gel molding, the structural evolution and properties of UHMWPE films stretched at temperatures of 100, 110, 120 and 130 °C were investigated by in situ small-angle X-ray scattering (SAXS)/ultra-small-angle X-ray scattering (USAXS)/wide-angle X-ray diffraction (WAXD) measurements as well as scanning electron microscopy (SEM) and differential scanning calorimetry (DSC) measurements. Our findings showed that the reserved shish crystals can facilitate the formation and structural evolution of shish-kebab crystals during the hot stretching. Additionally, the reserved shish crystals promote the structural evolution of UHMWPE films to a greater extent when stretched at 120 and 130 °C, compared to 100 and 110 °C, resulting in higher crystallinity, orientation, thermal properties, breaking strength and Young's modulus. Compared to UHMWPE high-entangled films with reserved shish crystals prepared by compression molding, UHMWPE low-entangled films with reserved shish crystals prepared by gel molding are more effective in inducing the formation and evolution of shish-kebab crystals during the hot stretching, resulting in increased breaking strength and Young's modulus.Keywords：UHMWPE low-entangled films;Reserved shish crystals;Structural evolution;Hot stretching
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Updated：2024-05-17 - Abstract：The traditional high-temperature preparation process of polyimide can cause many problems and limits the wider application in extreme conditions. An important challenge to be solved urgently is the reduction of imidization temperature. In this work, twelve kinds of polyimide films with different chain rigidity were prepared at low temperature of 200 °C, in the absence or presence of imidazole used as the catalyst. The molecular rigidity and free volume were theoretically calculated, and relationship between structure and properties were systematically studied. The results show that imidization reaction under low temperatures is significantly affected by the rigidity of molecular chains. The rigid structure of polyimide is not conducive to the low-temperature imidization, but this adverse effect can be eliminated by adding catalyst, resulting the notably increased imidization degree. The optical and thermal properties can be improved to a certain extent for the chemically catalyzed system, resulting in relatively higher heat resistance and thermal stability. While the mechanical performance could be determined by complicating factors, greatly different from polyimide films prepared by high temperature method. To investigate aggregation structures of films, the effect of chain rigidity and catalyst on the stacking or orientation of molecular chains was further elaborated. This work can contribute to the understanding of chemically catalyzed imidization that is rarely reported in the existing research, and will provide guidance for the low-temperature preparation of high-performance polyimides.Keywords：Polyimide;Imidization;Catalyst;Structure and properties;Aggregation structures
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Updated：2024-05-17
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