Electrospun Nanostructured Network

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CHAPTER 3 Manufacturing, Testing and Analysis Techniques

This chapter explains the details of the manufacturing procedures used in this thesis, from materials selection and solution preparation through to formation of the nanofibres. It addresses the practical considerations of polymer processing via nanofibre spinning, collection of fibres and finally visualisation through scanning electron micrographs. Following this, the perfomance of electrspinning operations has been analysed through the measurements of fibre diameter and bead area using image processing software.

Background

The combined use of two techniques, namely electrospray and spinning, is made in a highly versatile technique called electrospinning. This technique is applicable to virtually every soluble or fusible polymer [34]. A high electric field is applied to the droplet of a fluid coming out from the tip of a die, which acts as one of the electrodes. This leads to the droplet deformation and finally to the ejection of a charged jet from the tip of the cone accelerating towards the counter electrode leading to the formation of continuous fibres. There is an ever increasing interest in this field of electrospinning, shown by both researchers in the academic institutions and industries as evident from the increased number of publications and review articles every year (refer to Section 2.5.2). The primary requirement of the process is to obtain nanofibres in continuous form with fine diameters and minimum variations. Secondly, the fibre network has to have minimum area occupied by beads to enhance the network’s porosity. Beads are considered the main demerit of electrospun fibres. These are nonporous polymeric material with different shapes. The existence of fibre beads not only reduces the fibre uniformity, but also leads to decreased fibre surface area and poor mechanical strength [83]. These two important characteristics, when achieved, render the nanofibre mats acceptable for many membrane type applications. Electrospinning is obviously a convenient method of manufacturing polymer-based multi-functional and high performance nanofibres, but to control the electrospinnability and finally to achieve the desired morphology of electrospun fibres, suitable for various uses, are challenging. It has been well established that both operating parameters and material properties affect the electrospinning process and the resulting fibre morphology [34, 77, 84]. Therefore, an understanding of the process-structure-property relationship is essential for engineering polymer nanofibres to meet the demands of applications [5]. In the present work, an attempt has been made to fabricate a nanofibrous mat by electrospinning and the results of a systematic investigation of the effects of varying manufacturing parameters on the electrospinning are reported. Poly(lactic acid) or polylactide (PLA) has been selected as the electrospinnable polymer. A particular product, PLA 3051D has been used in the entire project as the variety of products made with this grade continues to grow rapidly (refer to Secion 2.8). Polylactic acid is a renewable and biodegradable polymer that motivated this work. It is a unique polymer that in many ways behaves like polyethylene terephthalate (PET), but also performs a lot like polypropylene (PP), a polyolefin [103]. PLA is a promising polymer and electrospinning is the platform technology for generating nanofibrous mat for various purposes.

Manufacturing of Nanofibres

One attractive feature of electrospinning is the simplicity and inexpensive nature of the setup; the typical electrospinning setup consists of a syringe pump, a high voltage source, and a collector. During the electrospinning process, a polymer solution is held at a needle tip by surface tension. The application of an electric field using the high-voltage source causes a charge to be induced within the polymer, resulting in charge repulsion within the solution. This electrostatic force opposes the surface tension; eventually, the charge repulsion overcomes the surface tension, causing the initiation of a jet. As this jet travels, the solvent evaporates and an appropriate collector can be used to capture the polymer fibres. One take up roller may be fitted to collect and wind the nanofibres web. However, in the present research a stationary collector has been used to collect the nanofibres. Finally, these have been visualised under a scanning electron microscope (SEM). Initially, feed rate was controlled by gravity. After successfully achieving nanofibres in the early stage of research, a closed cabinet for electrospinning purpose has been built and a programmable syringe pump for controlled feed rate and an advanced power supply device have been applied for further research.

Electrospinning set-up

Polymer solution was delivered to the top of a needle through a glass syringe as a capillary tube. Initially, feed rate has been adjusted by giving a slope to the glass syringe, Fig. 3-1, nanofibres have been spun successfully through electrospinning of PLLA with different solvent combinations. For further research, a new set-up was built at the Centre for Advanced Composite Materials (CACM), The University of Auckland (UoA). The closed cabinet, Fig. 3-2, is made of transparent poly(methyl methacrylate) (PMMA) thick sheet, for dust protection, reduced air turbulence and extra security for the operator. The details of equipment will be discussed in the following Sub-chapters.

Polymer selection

The present study uses PLA synthesized from L optical isomer [denoted poly(L-lactic acid) (PLLA)] as the electrospinnable polymer. It is a biodegradable, thermoplastic and aliphatic polyester derived from renewable resources. PLA properties are also comparable to those properties of PET. PLA can be formulated to be either rigid or flexible and can be copolymerised with other materials. Typical properties of PLA made by NatureWorks®LLC (the 3051D grade used in this project) are discussed in Table 2-3 (Section 2.8.3). The variety of products made with PLA 3051D continues to grow rapidly [106]. This common product of PLLA has been dissolved in the mixture of dichloromethane (DCM) dimethyl formamide (DMF) and successfully electrospun resulting in fibre diameters ranging from 50 nm to about 400 nm depending on the proportion of the solvents and polymer concentration.

Materials

Electrospinning of PLLA was accomplished using solutions of PLLA (using NatureWorks®LLC 3051D) with a weight average molar mass about 1.044×105 g mol-1 and polydispersity 1.352 [as measured using gel permeation chromatography (GPC)]. Some solvents, such as dichloromethane (DCM), methanol, tetrahydrofuran (THF) hexafluoroisopropanol (HFIP), Trifluoroacetic acid (TFAA) and dimethyl formamide (DMF), were used to find out the suitable combination of solvents for successful electrospinning of PLLA. Finally, the combination of DCM and DMF was used for the entire study as it produced fine and uniform nanofibres consistently. For manufacturing PLLA-based conducting nanofibres, dodecylbenzene sulphonic acid (DBSA) doped polyaniline (PANi) mixed with PLLA has been dissolved in a common solvent (mixtures of chloroform and DMF), and the mixture was successfully electrospun to engineer bead-free superfine PANi/PLLA nanofibres. A tetra butyl ammonium bromide (TBAB) was used as an additive to increase the conductivity of the solution. Aniline (99%), procured from Acros Organics, ammonium persulphate (98%), purchased from Ajax Chemicals and DBSA obtained from Acros Organics were used to synthesise DBSA doped PANi (PANi-DBSA). Chloroform (98%), DCM (98%) and DMF (98%) and TBAB were purchased from Sigma-Aldrich and used without further purification. Three saturated salt solutions have been made from lithium chloride (Sigma-Aldrich), sodium chloride (Ajax Finechem Pty Ltd) and potassium sulphate (Merck) to maintain the required relative humidity (RH) inside the electrospinning cabinet.
All experiments were performed in the set-up, Fig. 3-2, which was contained in a cabinet made of transparent PMMA sheets. The set-up included a power supply capable of generating high voltage, a syringe as a capillary tube and a stationary collector as a target. Polymer solution was delivered to the top of a needle through a hypodermic glass syringe (Popper & Sons, Inc) with capacity of 5 ml as a capillary tube and the flow of the liquid spinnable polymer was controlled using a programmable syringe pump, shown in Fig. 3-3 (a) (Cole-Parmer Hz 50/60, cat# 789100C). The same type of hypodermic needle (20G1TW, 0.9 × 25 mm from BD PrecisionGlideTM Needle) was used throughout the experimental work. The power supply, Fig. 3-3 (b), was a Spellman DEL HVPS INST 230-30R unit that could generate up to 30 kV. A metal screen and a heavy metal stand (pictures are shown in following Sub-chapter 3.2.5) have been made for collecting nanofibres.

Solution preparation

Supplied PLLA pellets were granulated in a grinder for easy and quick dissolution in different solvents. The weight average molar mass of PLLA has been found at about 1.044×105 g mol-1 by GPC and polymer was found to be amorphous from XRD, as discussed in Chapter 5. Concentrations of 4% and 7% were made by dissolving 1.0 and 1.75 g pellets respectively in the mixture of 25 ml of DCM/DMF at a proportion of 90/10, 80/20, 70/30 and 60/40 (by volume). At a later stage, polymer concentrations of 4, 7 and 10% (w/v) [x% w/v means x g of solute in 100 ml of solvent] were made by adding 1, 1.75 and 2.5 g PLLA powders, respectively in a mixture solution of 25 ml, prepared by mixing DCM and DMF (60:40 by volume) with 15 ml and 10 ml, respectively. Solutions were prepared with the aid of ultrasonication for 2 hours at room temperature. Ultrasonication offers great potential in the processing of liquids by improving the mixing and chemical reactions through alternating low-pressure and high-pressure waves in the liquid. A single solvent or different combinations of solvents (methanol, DCM, DMF, THF, HFIP, TFAA, chloroform and acetone) mixed in different proportions, were made to investigate the ability of solvent to make fine and uniform fibres. For producing PLLA-based conducting nanofibres, PLLA was dissolved in CHCl3/DMF mixtures with proportions (v/v, CHCl3:DMF) 50:50, and 70:30, to make a PLLA concentration of 10 % (w/v). The PLLA solution (2 parts by volume) was then mixed with a DBSA-doped PANi solution (1 part by volume) to give solutions with 6.7 % (w/v) PLLA and 1.7% (w/v) DBSA-doped PANi in 67:33 and 80:20 CHCl3:DMF. 1% (w/v) TBAB was added to half of the solution for the testing. The details will be discussed in the respective Sub-chapters. All solutions were prepared and stored at the same room temperature (200C) before the electrospinning process started.

Electrospinning operation

Polymer solution was delivered to the top of a needle through a programmable syringe pump at a rate of 0.5 – 2 ml hr -1. The power supply used had a capacity of 30 kilovolts (kV). However, 6-15 kV was sufficient for this study. The positive electrode from the high voltage supply was connected to the needle. A heavy metal stand, Fig. 3-4 (b), as collector was attached to the other end of the electrode, which serves as the negative terminal. In the process, polymer solution is charged to a very high electrical potential. Because of the electric field, a charge is induced on the liquid surface. The mutual charge repulsion causes a force opposite to the surface tension of the polymer solution. As the electric field increases, the hemispherical surface of the solution at the tip of the capillary tube extends to form a cone-like structure, a “Taylor cone”, [65] as discussed earlier in the theory of electrospinning. A metal screen, Fig. 3-4 (a), to collect nanomat/nanotextiles for other testing purposes (tensile strength, thermal behaviour, etc.) has been used. A heavy metal stand, Fig. 3-4 (b), was used for holding the aluminium (Al) stubs to collect the nanofibres for visualisation in SEM. The distance between the collector and the needle tip was varied in the range of 60 – 120 mm. During electrospinning of the PLLA the spin-ning started at a voltage of 7 kV as the cone started forming. After increasing to 8 kV, a clear jet of fibres was established. Electrospun fibres were projected directly on to the SEM stubs which were polished and cleaned beforehand. Some electrospinning conditions used in this study are given in Table 3-1.
After projecting the nanofibres for 15 – 20 seconds on the SEM stub, the process was stopped. The stubs were dried for 8 hours in the atmosphere and then collected in a closed plastic container for visualisation in the SEM. The resultant nanofibres were found to be in the range of 50 – 400 nm depending on the parametric conditions, which was satisfactory indeed.

Nanotextiles

The textile industry, considered as a traditional industry, is an important and necessary part of the global manufacturing industry and provides employment to several million (or even more) people. With increasing international competition, it has been generally recognised that manufacturing traditional textile products may no longer be sufficient to sustain a viable business, and the textile industry may have to move towards more innovative and higher-quality products. Conventional mechanical fibre-spinning technologies cannot produce robust fibres with a diameter smaller than about 2 μm [108]. Nanotextile, a more generalised term, is a flexible material consisting of a network of natural or artificial fibres (average diameter of 50-1000 nm) that are woven or non-woven. The use of textiles is pervasive in both product design and architectural applications. In the present study, PLLA nanofibres have been collected on the metal screen, as nonwoven mesh that consists of thousands of individual fibrils with diameters in the nanorange. From a textile viewpoint, these nanofibrous mats, Figs. 3-5 (a) and (b), have been termed as nanotextile/nanomat in the manuscript; however, the emphasis here is focused on electrospinning.

Safety measures

During this project, primary safety measures were taken against two risks: from the chemicals and from the high electrical voltage. Safety glasses, protective gloves and clothes, and good ventilation were compulsory. The flammability of solvents properties has been considered with the use of high voltages and as a result only small quantities of solution have been used. The electric risk added obvious problems of insulation. The power supply has been protected by a very sensitive fuse, as the risk of electrocution and electric arc were very important if the air contained a high level of moisture. A person standing within a distance of 50 cm can receive projections of both fibres and the chemicals. All the chemicals were handled with gloves and within a fume cupboard. DCM and DMF are harmful if swallowed or inhaled and may be harmful by skin contact [109]. During electrospinning of the PLLA-PANi mat, special care was taken in regard to chloroform. A mask was worn at all times and solution preparation was done strictly within the fume cupboard. The electrospinning operation was performed in the closed cabinet, Fig. 3-3, for dust protection, reduced air turbulence and for the extra security of the operator. However, some polymers might emit unpleasant or even harmful smells, so the processes should be conducted within chambers having a ventilation system. Furthermore, as a DC voltage in the range of 10 – 30 kV was necessary to generate the electrospinning, the researchers must be careful to avoid touching any part of the charged jet during manipulation.

Sample Preparation

Nanofibres were collected on a stationary collector in the electrospinning process. Aluminium (Al) stubs were kept on the stationary collector through the holes made on it, and nanofibres were projected directly on to the metal stub. The stubs were designed for the purpose of visualisation under a scanning electron microscope (SEM). After projecting the nanofibres for 15 – 20 seconds on the Al stub, the electrospinning process was stopped for a while. The stubs were taken out, dried for 8 hours in the atmosphere and then collected in an air-tight plastic container. Silica gel was kept inside the container to keep the nanofibre samples completely dry. A definite process of cleaning and polishing the Al stubs was established, Figs. 3-6 (a) – (d), by the trial and error method. Old and previously used stubs were put in acetone and ultrasonicated for 30 minutes. Then these were dried and carefully rubbed on emery paper to make a clear surface. During rubbing, Al stubs, Fig. 3-6 (c), were finger-rotated and polishing was continued until a smooth white surface was created. This process was followed by another acetone wash. Finally, the stub was dried and polished with a metal-polish paste. During polishing, changing the position of the stubs was very important to ensure a uniformly clean and glossy surface throughout the stub face. The surface started shining as soon as the stubs were polished with a soft fleecy cloth. Finally, the stubs were immersed in acetone and ultrasonicated for 30 minutes and were dried in soft tissue paper. The used stubs could be cleaned with less effort after reusing for the same purpose. A previous coating can be removed in an ultrasonic bath with acetone. A clear and bright surface could be achieved by simply using a metal polish, avoiding the use of emery paper. Cleaning time and effort depended on how dirty the stubs were and for what purpose these have previously been used. By trial and error, cleaning and polishing process could be established and degree of cleaning required could be estimated visually. The surfaces of the stubs could be checked under a microscope in the middle of the cleaning and polishing process.

Techniques of Analysis

The electrospun nanofibres were studied under a Philips XL30S scanning electron microscope (SEM) after the nanomat had been coated with platinum in the Department of Chemical and Materials Engineering. Clear pictures of beaded or nonbeaded nanofibres were taken, and afterwards, by using image processing software (UTHSCSATM ImageTool), measurements of nanofibre diameter and bead area were taken and analysed.

Scanning electron microscopy (SEM)

The SEM is a high resolution microscopic tool to produce largely magnified images which are formed by using electrons rather than light. It utilises an electron beam to excite the electrons near the surface of a specimen, most commonly using an Everhart-Thornley detector to detect the low energy ‘secondary’ electrons that are emitted as a result of this excitation. There are two main types of electron microscope, the transmission electron microscope (TEM) and the scanning electron microscope (SEM). Light which is used as a source of illumination for the sample in light microscopy has a wavelength of approximately 500 nm and therefore the resolution of the light microscope is limited. In practice, this means that the maximum magnification obtainable with a light microscope is X1000. Electrons used to view the sample in an electron microscope have an effective wavelength of about 0.1 nm and so allow much greater resolution and therefore greater magnification of the sample [110]. In the SEM, Fig. 3-7, the image is formed and presented by a very fine electron beam, which is focused on the surface of the specimen. The beam is scanned over the specimen in a series of lines and frames called a raster, just like the (much weaker) electron beam in an ordinary television.
The sputter coater, Polaron SC 7640, was used for coating standard SEM samples. It is designed to give a very thin, minimal metal coating suitable for SEM viewing. The normal target used is platinum (Pt) or gold (Au). In the case of visualisation of PLA nanofibres, the samples were coated with Pt for 300 seconds.

Morphological study of nanofibres using Imagetool

SEM images were used in conjunction with image analysis software, UTHSCSATM ImageTool [111] to determine the average diameter of the PLLA nanofibre and the area of beads. In order to ensure the accuracy of the results, experiments were replicated five times. From each experimental SEM-micrograph, eight individual fibre strands were considered and therefore, from five repetitions, a total of 40 fibres were considered from the same set of experimental conditions to estimate the range of diameters and their variations. For an individual nanofibril, measurements of diameter were taken in five different places and finally average diameter has been calculated. Similarly for an individual bead, area was calculated using the software, and the total area occupied by beads was calculated by adding all of them. On the basis of measurements of five repetitions, derived from imagetool software, the average diameter, standard deviation and finally S/N ratios [112] were calculated for analysing the parametric effect. S/N ratios take into account both the degree of variability in the response data and the closeness of the average response to the desired value that is explained in Sub- chapter 4.2.2.1.

Manufacturing of Nanotextile from Poly(L-lactic acid) (PLLA)

Electrospinning of PLLA was accomplished using solutions of PLLA in different concentrations of various solvents. In the early stage of research, the solvents – namely DCM, methanol, DMF, HFIP, THF and TFAA- were used to make a nanofibrous mat for finding the performances of the solvents in individual or in combinations – those are shown in scanning micrographs in the next Sub-chapter. Finally, the combination of DCM and DMF performed well and different proportions have been checked to find out the ability to make uniform nanofibres consistently.
PLLA was dissolved in a mixture of DCM and DMF (60:40 by volume) to prepare different concentrations. As recommended in reported studies [1, 42, 71], a concentration of 4 – 10% (w/v) (x% w/v means x g of solute in 100 ml of solvent) and a feed rate of 0.5 – 2 ml hour -1 were chosen for this study. It is to be noted that although a feed rate of 0.5 ml hour -1 performed well at times, it had a tendency to clog the needle tip. So precautions had been taken by cleaning the tip frequently. As the electric field increased beyond 7 kV, the hemispherical surface of the solution at the tip of the capillary tube extended to form a cone-like structure. A constant volume flow rate was maintained using a syringe pump, set to keep the solution at the tip of the tube without dripping. The electric potential, the flow rate, and the distance between the capillary tip and the collection screen were adjusted so that a stable jet was obtained. Once a stable jet was produced, electrospinning was continued for a considerable time.
Commercial aluminium foil used for collecting the nanofibres, was kept at a distance so that the solvent could evaporate and nanofibres could get enough flight time to dry. Aluminium stubs that had been well cleaned to create a smooth and glossy surface, were used to collect the sample for visualisation under SEM. The distance between the collector and the needle tip was maintained at 80 – 120 mm; white nanofibres adhered smoothly on the aluminium foil with a voltage in the range of 7 – 12 kV. The power supplied should be adequate to overcome the viscosity and surface tension of the polymer solution to form and sustain the jet from the pipette. An increase in electric potential tends to stretch the polymer more, resulting in decreased fibre diameter up to a certain level. A maximum voltage of 12 kV has been found suitable for effective stretching; further increase in voltage generates too much acceleration on the polymer solution resulting in nanofibres with irregular sizes due to insufficient flying time for solvent evaporation. Three saturated salt solutions were made from lithium chloride, sodium chloride and potassium sulphate to maintain the required relative humidity (RH) inside the cabinet. Temperature was controlled inside the electrospinning cabinet using ice and / or hot air. At the beginning of the research, the ambient temperature (200C) and humidity (40%) were kept unchanged, where the effects of other parameters were examined.

Selection of solvents

The choice of solvent is critical as the properties of the polymer solution have the most significant influence on the electrospinning process [1]. The dielectric constant of a solvent has a major contribution in making a spinnable polymer solution. Generally, a solution with greater dielectric constant reduces the bead formation and the diameter of the resultant electrospun fibre [1]. Solutions with high conductivity will have a greater charge-carrying capacity than solutions with low conductivity. Thus the fibre jet of highly conductive solutions will be subjected to a greater tensile force in the presence of an electric field than will a fibre jet from a solution with a low conductivity. In addition, the volatility of solvent plays a major role in the formation of nanostructures by influencing the phase separation process [16, 113]. As the fibre jet travels through the atmosphere towards the collector, a phase separation occurs before the solid polymer fibres are deposited. This process is greatly influenced by the volatility of the solvent. To find a suitable combination of solvents, various solvents [DCM, methanol, DMF, HFIP, THF, TFAA] were examined in individual or in combination of two. Zong et al. used a mixed solvent of DCM and DMF with a ratio of 60:40 to dissolve the semi-crystalline PLLA [42]. Lee et al. had investigated chloroform as a good solvent [108]. However, there are concerns about safety with chloroform whereas HFIP and THF have been reported safe, easy and convenient solvents by many researchers. These solvents have been used as well, but spinnability (Figs. are shown in Sub-chapter 3.5.3), was not acceptable.
DMF with a high dielectric constant is an effective solvent component. Together with a dielectric constant, DMF has low vapour pressure and high boiling point that can make the solution more effectively spinnable. More electric charges carried by the electrospinning jet impose higher elongation forces, resulting in finer fibres [42, 54]. DCM is a unique solvent to dissolve aliphatic polyesters like PLLA. However, due to its low boiling point (39.80C) and high volatility, the tip of the spinneret/needle is easily clogged while the polymer solutions are electrospun in this solution. Thus, DMF (boiling point 1530C) can be added in a significant proportion to increase the boiling point of the resultant solution and finally to achieve dry and fine fibres. Different proportions of DCM and DMF have been made for making polymer solutions to extract suitable performances, (Figs. are shown in Sub-chapter 3.5.3) of electrospinning. However, a mixture of DCM and DMF (60:40 by volume) consistently produced fine, uniform and beaded/non-beaded nanofibres. It was supported by Zong et al., they used a mixed solvent of DCM and DMF with the same proportion to dissolve the semi-crystalline PLLA [42]. The details of the performances are discussed in Chapter 4, parametric analysis.

TABLE OF CONTENTS
Abstract
Dedication
Acknowledgements
Table of Contents
List of Tables
List of Figures
Nomenclature
Glossary of Terms & Abbreviations
1 Introduction
2 Literature Review
2.1 Nanomaterials and Composites
2.2 Nanocomposites
2.3 Nanomechanics and Nanosciences
2.4 Nanofibres
2.5 Electrospinning
2.6 Electrospun Nanostructured Network
2.7 Different Factors of Electrospinning
2.8 Poly(lactic acid)(PLA) as Electrospinnable Polymer
3 Manufacturing, Testing and Analysis Techniques
3.1 Background
3.2 Manufacturing of Nanofibres
3.3 Sample Preparation
3.4 Techniques of Analysis
3.5 Manufacturing of Nanotextile from Poly(L-lactic acid) (PLLA)
3.6 Manufacturing of PLLA-based Conducting Nanofibrous Mat
3.7 Concluding Remarks
4 Parametric (Factorial) Study of Electrospinning
4.1 Quality of Nanofibres
4.2 A Parametric Analysis of Electrospinning
4.3 What does Taguchi Analysis tell us about Best Parameter Combinations?
4.4 Full Statistical Analysis and Bead-free Network
4.5 Concluding Remarks
5 Characterisation of Nanofibrous Mat
5.1 Introduction to Various Properties of Nanofibrous Mat
5.2 Molecular Structure
5.3 Mechanical Properties of Nanofibres
5.4 Characterisation of Polymer Solution
6 Regression Analysis of Electrospinning Process
6.1 Introduction and Requirement of a Model
6.2 Inclusion of Parameters in the Model
6.3 Multiple Regression Analysis (MRA)
6.4 Results and Discussion
6.5 Validations of MRA and Suitability of the Model
6.6 Concluding Remarks
7 Nanofibrous Mat using Multiple Syringes
7.1 Introduction
7.2 Manufacturing of Nanotextiles using Multiple Syringes
7.3 Sample Preparation and Study of Nanofibrous Structure
7.4 Comparative Studies of Morphologies using Two Techniques
7.6 Discussion with Merits and Demerits
8 Conclusions
8.1 Conclusions
8.2 Achievements and Recommendations for Further Work
9 References
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