Iranian Journal of Polymer Science and Technology1016325534220210622Stabilization of Polymer Solar Cells and Their Importance in Photovoltaic SystemsStabilization of Polymer Solar Cells and Their Importance in Photovoltaic Systems99129180710.22063/jipst.2021.1807FASamira AgbolaghiChemical Engineering Department, Faculty of Engineering, Azarbaijan Shahid Madani University, P.O. Box 5375171379, Tabriz, IranOmid Mohammadi-VanyarChemical Engineering Department, Faculty of Engineering, Azarbaijan Shahid Madani University, P.O. Box 5375171379, Tabriz, IranSaleheh AbbaspoorMechanical Engineering Department, School of Engineering, Damghan University, P.O. Box 36716-41167, Damghan, IranJournal Article20210714Nowadays, the use of renewable energy resources has been considered as one of the imminent issues in human life. By converting solar energy into electricity, solar cells can meet most of the needs of communities for domestic and industrial use. Meanwhile, polymer solar cells have received much attention in recent years for their acceptable performance and easy manufacturing method. Nevertheless, researchers are trying to simultaneously decrease the cost of preparation and increase efficiency. In addition to the high efficacy of polymer solar cells, their stability in the manufacturing process is a challenge. For decades, rapid advances in photovoltaic cells have strongly linked the world of polymers and photovoltaic energies. Stable solar cells can be used in a variety of applications, such as space station equipment, solar vehicles, sensors, traffic lights, clocks and watches and more. Due to their high sensitivity to environmental factors, degradability and susceptibility to oxidation, polymer solar cells are very important in performance and stability. Polymer solar cells with high sensitivity to environmental factors and configuration containing of degradable and oxidizing materials have attached much attention in the discussion of stability and performance. In the present study, effective properties in reducing the stability of polymer solar cells, including semi-stable morphology, oxygen, heat, stress, etc., will be discussed. Moreover, outstanding methods for increasing stability such as architectural manipulation and morphology of the active layer, reverse configuration, optimization of buffer layers, stable electrodes, molecular restructuring of polymers and other components, etc. are reviewed and in each section, the principal researches are briefly discussed. Nowadays, the use of renewable energy resources has been considered as one of the imminent issues in human life. By converting solar energy into electricity, solar cells can meet most of the needs of communities for domestic and industrial use. Meanwhile, polymer solar cells have received much attention in recent years for their acceptable performance and easy manufacturing method. Nevertheless, researchers are trying to simultaneously decrease the cost of preparation and increase efficiency. In addition to the high efficacy of polymer solar cells, their stability in the manufacturing process is a challenge. For decades, rapid advances in photovoltaic cells have strongly linked the world of polymers and photovoltaic energies. Stable solar cells can be used in a variety of applications, such as space station equipment, solar vehicles, sensors, traffic lights, clocks and watches and more. Due to their high sensitivity to environmental factors, degradability and susceptibility to oxidation, polymer solar cells are very important in performance and stability. Polymer solar cells with high sensitivity to environmental factors and configuration containing of degradable and oxidizing materials have attached much attention in the discussion of stability and performance. In the present study, effective properties in reducing the stability of polymer solar cells, including semi-stable morphology, oxygen, heat, stress, etc., will be discussed. Moreover, outstanding methods for increasing stability such as architectural manipulation and morphology of the active layer, reverse configuration, optimization of buffer layers, stable electrodes, molecular restructuring of polymers and other components, etc. are reviewed and in each section, the principal researches are briefly discussed. http://jips.ippi.ac.ir/article_1807_d098f0da97a924014db739912279bbb9.pdfIranian Journal of Polymer Science and Technology1016325534220210622Dispersion of Glycidyl POSS-modified Silica Nanoparticles in EpoxyDispersion of Glycidyl POSS-modified Silica Nanoparticles in Epoxy131142180910.22063/jipst.2021.1809FAMohammad Saleh BordbarDepartment of Adhesive and Resin, Faculty of Petrochemical,, Iran Polymer and Petrochemical Institute, P.O. Box: 14975-112, Tehran, IranAli SalimiDepartment of Adhesive and Resin, Faculty of Petrochemical,, Iran Polymer and Petrochemical Institute, P.O. Box: 14975-112, Tehran, IranMajid KarimiDepartment of Polymerization Engineering, Faculty of Engineering; Iran Polymer and Petrochemical Institute, P.O. Box: 14975-112, Tehran, IranJournal Article20210714<strong>Hypothesis</strong>: Proper dispersion of silica nanoparticles in epoxy resin leads to promising improvements in mechanical and thermal properties of nanocomposite. A comparative study of dispersion between GPOSS-modified silica nanoparticles and neat silica nanoparticles shows a measure of GPOSS efficiency in dispersion of silica nanoparticles.<br /><strong>Methods</strong>: Nanoparticle dispersion was investigated through polymer cumulative behavior such as rheological parameters and viscosity measurement. Using the ultrasonic technique, a pre-dispersed compound containing 20 %wt silica nanoparticles and GPOSS was first prepared. The pre-dispersed compound was then diluted to a final percentage of silica nanoparticles of 2, 5 and 10 %wt. An atomic force microscope (AFM) was also used to further investigate the dispersion of silica nanoparticles in the pre-dispersed compound.<br /><strong>Findings</strong>: According to the rheometry test results, all samples with pre-dispersed compound showed lower viscosity than their corresponding counterparts. It seems that the GPOSS is able to lower the composition viscosity through minimizing interfacial interactions between silica nanoparticles as well as possible interactions between epoxy chains and silica nanoparticles. In comparison with the sample prepared without pre-dispersed compound, the viscosity of the composition containing 10% (wt) silica nanoparticles was drastically reduced, i.e. from 246000 cP to 39000 cP. AFM surface images of the pre-dispersed compound represent the presence of particles with a statistical accuracy of 95% in the range of 12 nm to 20 nm, proving that the silica nanoparticles are well dispersed in GPOSS. The interesting finding of this study was that a pre-dispersed compound of GPOSS and silica nanoparticles not only improves the dispersion of silica nanoparticles and hence the final mechanical properties, but also improves the easy use through reducing the viscosity of the epoxy-based composition.<strong>Hypothesis</strong>: Proper dispersion of silica nanoparticles in epoxy resin leads to promising improvements in mechanical and thermal properties of nanocomposite. A comparative study of dispersion between GPOSS-modified silica nanoparticles and neat silica nanoparticles shows a measure of GPOSS efficiency in dispersion of silica nanoparticles.<br /><strong>Methods</strong>: Nanoparticle dispersion was investigated through polymer cumulative behavior such as rheological parameters and viscosity measurement. Using the ultrasonic technique, a pre-dispersed compound containing 20 %wt silica nanoparticles and GPOSS was first prepared. The pre-dispersed compound was then diluted to a final percentage of silica nanoparticles of 2, 5 and 10 %wt. An atomic force microscope (AFM) was also used to further investigate the dispersion of silica nanoparticles in the pre-dispersed compound.<br /><strong>Findings</strong>: According to the rheometry test results, all samples with pre-dispersed compound showed lower viscosity than their corresponding counterparts. It seems that the GPOSS is able to lower the composition viscosity through minimizing interfacial interactions between silica nanoparticles as well as possible interactions between epoxy chains and silica nanoparticles. In comparison with the sample prepared without pre-dispersed compound, the viscosity of the composition containing 10% (wt) silica nanoparticles was drastically reduced, i.e. from 246000 cP to 39000 cP. AFM surface images of the pre-dispersed compound represent the presence of particles with a statistical accuracy of 95% in the range of 12 nm to 20 nm, proving that the silica nanoparticles are well dispersed in GPOSS. The interesting finding of this study was that a pre-dispersed compound of GPOSS and silica nanoparticles not only improves the dispersion of silica nanoparticles and hence the final mechanical properties, but also improves the easy use through reducing the viscosity of the epoxy-based composition.http://jips.ippi.ac.ir/article_1809_82cdc3d41ee110f96e0534051ad53e4f.pdfIranian Journal of Polymer Science and Technology1016325534220210622Barium Ferrite-containing Polymer Composite Magnets: Magnetic and Mechanical PropertiesBarium Ferrite-containing Polymer Composite Magnets: Magnetic and Mechanical Properties143154181010.22063/jipst.2021.1810FAAlireza RedaeiDepartment of Polymer and Chemical Engineering, Faculty of Engineering, Yazd University, Postal Code 8915818411, Yazd, IranMohsen HakimiDepartment of Physics, Faculty of Science; Yazd University, Postal Code 8915818411, Yazd, IranElham Hashemi Sarvestani1. Department of Polymer and Chemical Engineering, Faculty of Engineering, Yazd University, Postal Code 8915818411, Yazd, IranJournal Article20210714<strong>Hypothesis</strong>: In recent years, much attention has been paid to the use of BaFe<sub>12</sub>O<sub>19</sub> barium ferrite magnetic material in the manufacture of polymer composite magnets. Hence, nitrile butadiene rubber-polyvinyl chloride (NBR-PVC) polymer composite magnets containing barium ferrite alloy powder BaFe<sub>12</sub>O<sub>19</sub> with particles less than 5 microns were made with optimal properties<br /><strong>Methods</strong>: The effect of different amounts of barium ferrite magnetic powder in the polymer substrate on the magnetic, morphological and mechanical properties of barium ferrite polymer magnets was investigated using SEM images and VSM diagrams and tensile test.<br /><strong>Findings</strong>: NBR-PVC/ BaFe<sub>12</sub>O<sub>19</sub> composite magnets containing 70 %wt NBR and 30 %wt PVC with 100 units of base rubber (phr) including 2 units of sulfur as baking agent, 4 units of zinc oxide as activator, 3 units of stearic acid as lubricant, 2 units of tetramethyl thioram disulfide (TMTD) and 2 units of mercapto-benzythiazyl disulfide (MBTS) as accelerator and barium ferrite powder BaFe<sub>12</sub>O<sub>19</sub> were fabricated with a particle size of less than 5 microns and a combination of different percentages as a magnetic agent. Among the samples made, N-P/F80 composite magnets with a ratio of 20:80 (powder:polymer) had the highest saturation magnetism of Ms and the residual magnetization of Mr and the lowest intrinsic coercivity of Hc. As expected, the best sample in terms of tensile strength was N-P/F20 with a powder:polymer ratio of 80:20, and finally, according to all the results related to the magnetic, morphological and mechanical properties of the samples, the most suitable one in terms of good particle dispersion in the polymer substrate and suitable tensile strength and appropriate magnetic properties of N-P/F50 with 50:50 (powder:polymer) ratio was selected.<strong>Hypothesis</strong>: In recent years, much attention has been paid to the use of BaFe<sub>12</sub>O<sub>19</sub> barium ferrite magnetic material in the manufacture of polymer composite magnets. Hence, nitrile butadiene rubber-polyvinyl chloride (NBR-PVC) polymer composite magnets containing barium ferrite alloy powder BaFe<sub>12</sub>O<sub>19</sub> with particles less than 5 microns were made with optimal properties<br /><strong>Methods</strong>: The effect of different amounts of barium ferrite magnetic powder in the polymer substrate on the magnetic, morphological and mechanical properties of barium ferrite polymer magnets was investigated using SEM images and VSM diagrams and tensile test.<br /><strong>Findings</strong>: NBR-PVC/ BaFe<sub>12</sub>O<sub>19</sub> composite magnets containing 70 %wt NBR and 30 %wt PVC with 100 units of base rubber (phr) including 2 units of sulfur as baking agent, 4 units of zinc oxide as activator, 3 units of stearic acid as lubricant, 2 units of tetramethyl thioram disulfide (TMTD) and 2 units of mercapto-benzythiazyl disulfide (MBTS) as accelerator and barium ferrite powder BaFe<sub>12</sub>O<sub>19</sub> were fabricated with a particle size of less than 5 microns and a combination of different percentages as a magnetic agent. Among the samples made, N-P/F80 composite magnets with a ratio of 20:80 (powder:polymer) had the highest saturation magnetism of Ms and the residual magnetization of Mr and the lowest intrinsic coercivity of Hc. As expected, the best sample in terms of tensile strength was N-P/F20 with a powder:polymer ratio of 80:20, and finally, according to all the results related to the magnetic, morphological and mechanical properties of the samples, the most suitable one in terms of good particle dispersion in the polymer substrate and suitable tensile strength and appropriate magnetic properties of N-P/F50 with 50:50 (powder:polymer) ratio was selected.http://jips.ippi.ac.ir/article_1810_18240150eb962372e998b81fa9fcb4d6.pdfIranian Journal of Polymer Science and Technology1016325534220210622Thin Film Polyamide Membranes Containing Modified Manganese Dioxide Nanotubes for Removal of Sodium and Copper IonsThin Film Polyamide Membranes Containing Modified Manganese Dioxide Nanotubes for Removal of Sodium and Copper Ions155172181110.22063/jipst.2021.1811FAZeynab FallahnejadChemical Engineering Faculty, Babol Noshirvani University of Technology, P.O. Box 484, Babol Iran,Gholamreza Bakeri1. Chemical Engineering Faculty, 2. Advanced Membrane Research Lab; Babol Noshirvani University of Technology, Babol Iran,A.F. IsmailAdvanced Membrane Technology Research Centre (AMTEC), Universiti Teknologi, Malaysia, 81310 Skudai, Johor Darul Takzim, MalaysiaJournal Article20210714<strong>Hypothesis</strong>: Today, with the development of different indus tries and the disposal of untreated was tewaters, environmental pollution and pollution of water resources are increasing very rapidly. Membrane technology is an advanced and hopeful way to treat water and was tewater. Nanofiltration technology is widely used in water treatment and desalination of seawater.<br /><strong>Methods</strong>: The performance of thin film polyamide membranes containing unmodified and modified manganese dioxide nanotubes was inves tigated. After hydrothermal synthesis of manganese dioxide nanotubes, the inner surface of the nanotubes was modified with polydopamine, and then, their performance in thin film polyamide membranes (in terms of monovalent/divalent ions rejection and permeation flux) was inves tigated.<br /><strong>Findings</strong>: Unmodified and modified nanotubes were characterized by Fourier transform infrared spectroscopy (FTIR), Brunauer-Emmett-Teller (BET) and X-ray diffraction analysis (XRD). In addition, the morphology and s tructure of the thin film membranes were inves tigated by FESEM tes t and the performance of the membranes was s tudied in terms of permeation flux, contact angle and rejection of sodium and copper ions. The maximum pure water flux, 18.6 L/m<sup>2</sup>h, was obtained for the membrane containing 0.10 %wt modified nanotube; an increase of 21.88% compared to the neat membrane. Creation of tiny pores on the surface of the membranes through hydrophilic nanotubes resulted in higher flux while there are extra routes through the nanotubes for water permeation. The maximum rejection of sodium ion (97.02%) for the membrane containing 0.2 %wt modified nanotubes could be related to the s tacking of the nanotubes and more spatial hindrance, reduction in the diameter of the nanotube due to the coating and permeation of water through the nanotubes.<strong>Hypothesis</strong>: Today, with the development of different indus tries and the disposal of untreated was tewaters, environmental pollution and pollution of water resources are increasing very rapidly. Membrane technology is an advanced and hopeful way to treat water and was tewater. Nanofiltration technology is widely used in water treatment and desalination of seawater.<br /><strong>Methods</strong>: The performance of thin film polyamide membranes containing unmodified and modified manganese dioxide nanotubes was inves tigated. After hydrothermal synthesis of manganese dioxide nanotubes, the inner surface of the nanotubes was modified with polydopamine, and then, their performance in thin film polyamide membranes (in terms of monovalent/divalent ions rejection and permeation flux) was inves tigated.<br /><strong>Findings</strong>: Unmodified and modified nanotubes were characterized by Fourier transform infrared spectroscopy (FTIR), Brunauer-Emmett-Teller (BET) and X-ray diffraction analysis (XRD). In addition, the morphology and s tructure of the thin film membranes were inves tigated by FESEM tes t and the performance of the membranes was s tudied in terms of permeation flux, contact angle and rejection of sodium and copper ions. The maximum pure water flux, 18.6 L/m<sup>2</sup>h, was obtained for the membrane containing 0.10 %wt modified nanotube; an increase of 21.88% compared to the neat membrane. Creation of tiny pores on the surface of the membranes through hydrophilic nanotubes resulted in higher flux while there are extra routes through the nanotubes for water permeation. The maximum rejection of sodium ion (97.02%) for the membrane containing 0.2 %wt modified nanotubes could be related to the s tacking of the nanotubes and more spatial hindrance, reduction in the diameter of the nanotube due to the coating and permeation of water through the nanotubes.http://jips.ippi.ac.ir/article_1811_2b530f630ddf0823b47815a083ec1303.pdfIranian Journal of Polymer Science and Technology1016325534220210622Fibrous Structures Fabricated from
Polylactic Acid and Nanofibrillated Chitosan/
Zinc Oxide NanoparticlesFibrous Structures Fabricated from
Polylactic Acid and Nanofibrillated Chitosan/
Zinc Oxide Nanoparticles173190181210.22063/jipst.2021.1812FAMarziyeh Ranjbar-MohammadiTextile Group, Faculty of Engineering, University of Bonab, Postal Code 5551761167, Bonab, IranParinaz ShakooriTextile Group, Faculty of Engineering, University of Bonab, Postal Code 5551761167, Bonab, IranZahra Arab-BafraniDepartment of Biochemistry and Biophysics, Faculty of Medicine, Golestan University of Medical Sciences, Postal Code 4934174515, Gorgan, IranErfan ZabihiDepartment of Polymer Engineering, Faculty of Engineering, Golestan University, Postal Code 49361-79142, Gorgan, IranJournal Article20210714<strong>Hypothesis</strong>: Nowadays, the use of mixtures of natural and synthetic polymers in the production of biological scaffolds has been considered by researchers because of their ability to achieve the desired properties. <br /><strong>Methods</strong>: Nanofibers from polylactic acid (PLA) and nanofibrillated chitosan/zinc oxide nanoparticles (CS/ZnO) with three different blend ratios of 1:1, 2:1 and 1:0 were fabricated by electrospinning method. In order to reduce the number of experiments and thus reduce the cost of materials and time, nine different experiments were performed using Taguchi test design method with three factors: PLA concentration (PLA 7, 9 and 11% by wt), CS/ZnO concentration (5, 10 and 20% by wt) and three different CS/ZnO ratios of 1:1, 2:1 and 1:0. The contact angle and morphology of the produced scaffolds were evaluated using scanning electron microscopy (SEM). <br /><strong>Findings</strong>: The results of scanning electron microscopy showed that with increasing PLA concentration, the beads and spindle-like morphologies are lost and the fibers are almost smooth and uniform. The results showed that by increasing the CS/ZnO concentration from 5% to 20%, the diameter of nanofibers first decreased and then slightly increased. The contact angle of fabricated samples decreased with increasing CS/ZnO concentration from 5% to 10%. Also from the samples obtained by Taguchi method, nanofiber sample containing PLA (7%, CS/ZnO 2: 1) with CS/ZnO concentration of 10%, due to having a smaller diameter (345±30 nm), very thin structure and lower contact angle (101°) was reported as the optimal sample. The contact angle, morphology and surface roughness for the optimum sample were examined and the surface roughness for the optimal sample was about 178 nm. Cell culture studies on the optimal sample was successfully performed.<strong>Hypothesis</strong>: Nowadays, the use of mixtures of natural and synthetic polymers in the production of biological scaffolds has been considered by researchers because of their ability to achieve the desired properties. <br /><strong>Methods</strong>: Nanofibers from polylactic acid (PLA) and nanofibrillated chitosan/zinc oxide nanoparticles (CS/ZnO) with three different blend ratios of 1:1, 2:1 and 1:0 were fabricated by electrospinning method. In order to reduce the number of experiments and thus reduce the cost of materials and time, nine different experiments were performed using Taguchi test design method with three factors: PLA concentration (PLA 7, 9 and 11% by wt), CS/ZnO concentration (5, 10 and 20% by wt) and three different CS/ZnO ratios of 1:1, 2:1 and 1:0. The contact angle and morphology of the produced scaffolds were evaluated using scanning electron microscopy (SEM). <br /><strong>Findings</strong>: The results of scanning electron microscopy showed that with increasing PLA concentration, the beads and spindle-like morphologies are lost and the fibers are almost smooth and uniform. The results showed that by increasing the CS/ZnO concentration from 5% to 20%, the diameter of nanofibers first decreased and then slightly increased. The contact angle of fabricated samples decreased with increasing CS/ZnO concentration from 5% to 10%. Also from the samples obtained by Taguchi method, nanofiber sample containing PLA (7%, CS/ZnO 2: 1) with CS/ZnO concentration of 10%, due to having a smaller diameter (345±30 nm), very thin structure and lower contact angle (101°) was reported as the optimal sample. The contact angle, morphology and surface roughness for the optimum sample were examined and the surface roughness for the optimal sample was about 178 nm. Cell culture studies on the optimal sample was successfully performed.http://jips.ippi.ac.ir/article_1812_001ebd1fd39c8a1b4e834f148819286b.pdfIranian Journal of Polymer Science and Technology1016325534220210622Effect of Nucleating Agent Concentration on Non-Isothermal Melt Crystallization Kinetics of b-Nucleated Impact Polypropylene CopolymerEffect of Nucleating Agent Concentration on Non-Isothermal Melt Crystallization Kinetics of b-Nucleated Impact Polypropylene Copolymer191202181310.22063/jipst.2021.1813FAMina FarahaniDepartment of Plastics, Faculty of Processing, Iran Polymer and Petrochemical Institute,
P.O. Box 14975-112, Tehran, IranYousef JahaniDepartment of Plastics, Faculty of Processing, Iran Polymer and Petrochemical Institute,
P.O. Box 14975-112, Tehran, IranJournal Article20210719<strong>Hypothesis</strong>: b-Nucleation in synergy to rubbery phase of impact polypropylene copolymer (b-IPC) leads to enhanced impact strength at low temperature. The extent of crystallinity is a major factor affecting the mechanical performance and impact strength. An important step to develop the application of this polymer on industrial scale is to study its crystallization kinetics especially in non-isothermal mode which is more closely related to industrial processes. For this purpose, the effect of beta nucleating agent concentration on the non-isothermal crystallization kinetics of µ-IPC has been investigated in this article by theoretical models. <br /><strong>Methods</strong>: Non-isothermal melt crystallization kinetics of b-IPC samples with two different amounts of calcium pimelate as the beta nucleating agent, prepared in solution blending method, was investigated at various heating rates of 1, 10 and <br />25°C/min using differential scanning calorimetry.<br /><strong>Findings</strong>: The results showed that the total crystallinity improved by increasing the content of b-nucleating agent (b-NA). Also, increasing the cooling rate and increasing the concentration of the nucleating agent were in favor of beta crystal formation. On the other hand, the results of calculating the half-time for crystallization, changes in conversion rate with relative crystallization, Mo's analysis and the evaluated activation energy based on Kissinger method showed that the higher the share of beta crystal and the lower the share of alpha, the crystallization kinetics of b-IPC slowed down. Therefore, increasing the concentration of beta nucleating agent reduces the rate of crystallization of b-IPC. The Ozawa model was not accurate enough due to the presence of secondary crystallization, while the Mo's analysis was well able to elucidate the effect of the concentration of the nucleating agent on the crystallization kinetics.<strong>Hypothesis</strong>: b-Nucleation in synergy to rubbery phase of impact polypropylene copolymer (b-IPC) leads to enhanced impact strength at low temperature. The extent of crystallinity is a major factor affecting the mechanical performance and impact strength. An important step to develop the application of this polymer on industrial scale is to study its crystallization kinetics especially in non-isothermal mode which is more closely related to industrial processes. For this purpose, the effect of beta nucleating agent concentration on the non-isothermal crystallization kinetics of µ-IPC has been investigated in this article by theoretical models. <br /><strong>Methods</strong>: Non-isothermal melt crystallization kinetics of b-IPC samples with two different amounts of calcium pimelate as the beta nucleating agent, prepared in solution blending method, was investigated at various heating rates of 1, 10 and <br />25°C/min using differential scanning calorimetry.<br /><strong>Findings</strong>: The results showed that the total crystallinity improved by increasing the content of b-nucleating agent (b-NA). Also, increasing the cooling rate and increasing the concentration of the nucleating agent were in favor of beta crystal formation. On the other hand, the results of calculating the half-time for crystallization, changes in conversion rate with relative crystallization, Mo's analysis and the evaluated activation energy based on Kissinger method showed that the higher the share of beta crystal and the lower the share of alpha, the crystallization kinetics of b-IPC slowed down. Therefore, increasing the concentration of beta nucleating agent reduces the rate of crystallization of b-IPC. The Ozawa model was not accurate enough due to the presence of secondary crystallization, while the Mo's analysis was well able to elucidate the effect of the concentration of the nucleating agent on the crystallization kinetics.http://jips.ippi.ac.ir/article_1813_d669a34eac068176171829f31d694298.pdf