Synthesis and characterization of proton conducting membranes based on human nail keratin and sulfonated polysulfone

Synthesis and characterization of proton conducting membranes based on human nail keratin and sulfonated polysulfone has been carried out. This research was conducted to make a proton conducting membrane based on acid-base pairs. In this study, human nail was dissolved using NaOH solvent. Through a neutralization process using glacial acetic acid, then evaporation of the solution was carried out to obtain human nail keratin. Sulfonated polysulfone composite membrane-human nail keratin was prepared in mass ratio of 3.5/0.5; 3.3/0.7; 3/1. Composite membranes were characterized by functional group analysis, XRD (X-Ray Diffraction), cation exchange capacity, degree of swelling and proton conductivity. Functional group analysis of sulfonated polysulfone-human nail keratin composite membranes showed interactions at peaks of 3000-3500 cm−1. The composite membrane has the highest cation exchange capacity, which is 2.02 meq/g on the composite membrane variation of 3/1. The greatest degree of swelling in the variation of 3/1 was 33.23%. The highest proton conductivity in the mass variation of 3/1 is 4.87 x 10−5 S/cm. The results of XRD analysis showed that the composite membrane synthesized by human nail keratin and sulfonated polysulfone had semicrystalline properties.


Introduction
Energy is a human need whose development directly enhances the majority of future life.One of them is the renewal of fuel cell technology by using renewable fuels.Fuel cell is an alternative in an effort to produce electrical energy other than coal which is the main raw material in producing domestic electrical energy.Then this fuel cell consists of an anode and a cathode which are separated by a polymer membrane which functions as an electrolyte.This polymer membrane has been widely used or applied in an electronic device, including electrolytes in batteries, coating materials, and in membrane technology [1].One of the polymers that can be developed at this time is a polyaromatic one, namely polysulfone.Polysulfone is a hydrophobic polymer material that is resistant to heat (thermoplastic) up to a temperature of 190°C, stable between pH 1,5-13, has good tensile strength, is insoluble or damaged by dilute acid or base.Polysulfone has hydrophobic properties and so that to become an electrolyte it is necessary to add a sulfonate group through the sulfonation process.Sulfonation of polysulfone is carried out with a sulfonate agent, the presence of sulfonate groups will affect the thermal properties of the polymer.Many studies have been reported on sulfonated polysulfone as continuous phase and other materials as discontinuous phase such as: proton-conducting composite membranes of sulfonated polysulfone and chitosan for fuel cell application, synthesis of polymer electrolyte membrane based on

Experimental
2.1 Human Nail Keratin Preparation Sample preparation was carried out by washing the nails using detergent until clean.The washed nails were soaked with acetone solvent for 25 minutes then the nails were dried again at room temperature.The dried nails are cut into small pieces using scissors.Human nails that have been prepared as much as 30 grams are dissolved in 150 mL of 1 M NaOH solution at 70ºC until completely dissolved.Then added glacial acetic acid solution which serves to neutralize the solution (pH 7).Then the solution was heated at a temperature of 70°C on a hotplate stirrer until a gel was formed, then heated again until dry.After drying, it was mashed using a blender and characterized using FTIR.

Composite Membrane Synthesis
The sulfonated polysulfone was obtained from the synthesis of previous studies [10].The synthesis of the composite membrane was initiated by dissolving the sulfonated polysulfone dissolved in a beaker with DMF (dimethylformamide) solvent.Then human nail keratin was added in the ratio 3.5/0.5;3.3/0.7;3/1 and stirred until homogeneous, membrane casting using a petridish dish.Then heated on a hotplate with a temperature of 40°C.

Characterization
Functional group analysis by FT-IR spectra were recorded on a Bruker Alpha-P in attenuated total reflectance (ATR) in range of 4000-400 cm -1 , To determine the exchange rate of cation capacity has used the titration method.The degree of swelling has been determined by cutting the membrane with a size of 1 x 1 cm and then heated at 60 ℃ for 5 hours then weighed, dry weight was obtained.Then the membrane was immersed in distilled water for 24 hours at room temperature.The surface of the membrane was dried with a tissue and weighed then a wet weight was obtained.The proton conductivity of membrane was measured by IM 3590 Chemical Impedance Analyzer HIOKI in frequency 1 KHz, 0.05 Volt and temperature at 26 °C.The X-ray keratin diffraction (XRD) analysis were performed using X-ray diffractometer (Rigaku DMAX2200, Japan) with Cu Ka (λ = 1.5406A) radiation over therange 2θ between 0° and 70°.

Results and Discussion
In this study, human nail keratin was carried out by hydrolysis method using NaOH solvent.The manufacture of human nails begins by dissolving 30 g of human nails in 150 mL of NaOH solvent heated at a temperature of 70°C.When the nail is dissolved, the solution becomes yellow in color.This process undergoes a reaction with a NaOH base, then a hydrolysis reaction occurs between the amino acids in acid-base complex pair and its characteristics, preparation of composite membranes based on sulfonated polysulfone and activated carbon and such sulfonated polysulfone-natural zeolite-titanium dioxide [2][3][4][5].Keratin is often known from the family of scleroprotein proteins where keratin is the basic material in making up the layers of human skin.Keratin is also the basic building block of hair and nails.In addition, keratin is a large amount of the amino acid cysteine which contains a long-chain peptide fiber protein with fibers that have an elongated structure and provide structural support.Cysteine is an amino acid that contains sulphur and can form sulphur-sulphur (s-s).Keratin is a protein fiber found in many protective layers in humans and also in animals such as in the skin, hair or fur.Keratin which is abundant is the alpha part of keratin (α-keratin) which is found in hair, nails, wings and horns.While beta-keratin (β-keratin) is abundant in silk.The first two types of keratin in human nails, namely (α) hard keratin which is derived from embryological epithelial cells, is a major constituent of cornified mammalian tissues such as nails, horns, spines.And the second (β) soft keratin, is the main component of the epidermis (skin) and other epithelial tissues.Human nails have keratin consisting of about 80% hard keratin and 20% soft keratin [6][7][8].The purpose of this research report is to make composite membranes based on sulfonated polysulfone and keratin from human nails and to perform characterized by functional group analysis, XRD (X-Ray Diffraction), cation exchange capacity (CEC), degree of swelling (DS) and proton conductivity.The composite membrane was made by dissolving sulfonated polysulfone with DMF (dimethylformamide) solvent and then adding human nail keratin.The membrane was synthesized with a variation ratio of 3.5/0.5;3.3/0.7;3/1.This variation is carried out with the aim of seeing the effect of mass on the characteristics of the proton-conducting membrane that will be produced.The volume of DMF solvent used in each comparison is different, namely: 3.5/0.5 solvent 5 mL 3.3/0.7 solvent 7 mL and 3/1 solvent 8 mL.During the process of adding solvent, it is done little by little until the sulfonated polysulfone has dissolved so that human nail keratin can be added.The purpose of the synthesis of this composite membrane is to obtain better material or membrane properties compared to the properties of the constituent polymers, the obtained membrane is showed in figure 2. human nails with a strong base NaOH.Human nails are dissolved until completely dissolved.6 mL of glacial acetic acid was added (until the solution was pH 7).Glacial acetic acid aims to neutralize the pH of the solution.Checking the pH is done using pH indicator paper.The solution after being added with glacial acetic acid, the solution changed color to a dark brown color.In this process, a strong base (NaOH) reacts with glacial acetic acid to produce sodium acetate.The solution was then heated again at 70°C and stirred until a gel was formed.If the gel has formed, then the gel is heated for two days to form human nail keratin as showed in figure 1.The resulting human nail keratin is hygroscopic (easy to absorb water) and the yield of human nail keratin is 86.36% [8].
(d) 3/1.Functional group analysis using of FTIR spectroscopy.From figure 3 have shown that the presence of sulfonated sulfonate groups for sulfonated polysulfone which is typical of the stretching vibrations of the OH and O=S=O groups from the sulfonate groups which form broad peaks in the wave number region of 3431 cm -1 and absorption band of 1141 cm -1.Where these wavelengths do not exist in polysulfone compounds [5].Based on the spectrum FTIR from figure 3, keratin has a typical absorption band, namely at 3268 cm -1 which is the absorption band of the hydroxyl group and at the peak of the wave number is 2956 cm -1  which is the stretching vibration of CH.In region at wave number of 1642 cm -1 is the absorption band of the C=O stretching vibration of the amide I group, and at 1552 cm -1 is the absorption band of the NH amide II bending vibration which is a characteristic of keratin.Then in the area of wavelength of 1396 cm -1 is the bending CH vibration, and at wavelength of 1248 cm -1 is the stretching CN vibration (Amide III).From the spectrum FTIR results obtained are the same, it is concluded that the results obtained are most likely keratin [10].
The spectrum results obtained for the three mass ratios are almost the same as the spectrum for sulfonated polysulfones, only a slight change in width occurs due to the presence of functional groups present in human nail keratin in sulfonated polysulfones.According to previous research [3][4][5], the wave number shift occurs due to the interaction between the polymers making up the composite.This indicates that the sulfonated polysulfone and keratin have physical interactions so that the resulting spectrum is not much different.According to the Nafion membrane, the degree of swelling of 30% can be compared to the degree of swelling in keratin which is higher so that it retains more water.However, if too much water is absorbed, the membrane becomes less stable due to the high hydrophilic nature of the membrane.Optimal water absorption greatly affects the performance of a membrane because sufficient water absorption can facilitate proton transfer so that it has more hydrophilic properties.The hydrophilic membrane will contain more water which is absorbed by the membrane so that the proton transfer will be better.At the degree of swelling the water absorbed in large quantities can cause a decrease in the mechanical properties and selectivity of a membrane so that it cannot be applied to fuel cell.In the mass ratio 3/1 from composite membranes have a fairly large the cation exchange capacity compared to other composite membranes by obtaining CEC of 2.02 meq/g.From figure 4 shows that the mass ratio of 3.5/0.5;3.3/0.7;3/1 composite membrane has an increase in each of its cation exchange capacity.This is because there is a sulfonate group SO3H which has the ability to exchange its H + ions with Na + ions from NaOH solution.
From table 1, showed that the degree of swelling is directly proportional to the cation exchange capacity because the more water is absorbed, the better the proton transport will be.In this study using keratin where this keratin has very hygroscopic properties.Previous studies [11], the degree of swelling of human nail keratin is 36% so that the highwater content in the membrane will facilitate the passage of proton transport.
From figure 5, showed that the sulfonated polysulfone has a proton conductivity that reaches the highest at a temperature of 25°C with of 4.87 x 10 -5 S/cm.According to previous research [12], the proton conductivity of the Nafion membrane is 10 -1 S/cm with a temperature below 100°C.From the table, it can be compared that the proton conductivity of the Nafion membrane is higher than that of sulfonated polysulfone.However, sulfonated polysulfones can still be used because their proton conductivities meet the standard for fuel cell applications.Nafion membrane has been proven to have a high proton conductivity and good chemical stability, but this membrane still has drawbacks besides being expensive and this Nafion membrane does not provide good performance in direct methanol fuel cell [13].
The proton conductivity of the composite membrane 3.5/0.5 has a high at 25°C at 2.35 7 x 10 -5 S/cm, for composite membrane 3.3/0.7,high at 25°C is 3.69 x 10 -5 S/cm and composite 3/1 high at 25°C is 4.87 x 10 -5 S/cm.This shows the effect of temperature where the higher the temperature, the lower of the proton conductivity in a membrane, and vice versa, the lower the temperature, the higher the conductivity [14].The highest proton conductivity among the three comparisons above is composite membrane 3/1 with a value of 4.87 x 10 -5 S/cm, due to the influence of many additions of human nail keratin which can increase protons.
The results of the XRD are in the form of a diffractogram of the relationship between the diffraction peaks at the angle and the intensity.This analysis to find the crystallinity of the membrane can be done using XRD (X-Ray Diffraction) with a range of 3-70°, scan rate of 5°/min and using Cu-Kα radiation.Figure 6 show that the sulfonated polysulfone has an amorphous peak and is located at the peak of the 2θ angle area of 19.3911° with an intensity of 261.8392.Therefore, the amorphous phase causes there to be more free space which allows a lot of movement between the ions to increase.in previous research.21regarding sulfonated polysulfone as membranes used for polymer electrolyte fuel cells, with XRD analysis results showing amorphous structures.In the XRD results, the crystallinity of the PSF membrane can be shown by the high and low peak intensity obtained, the higher the peak intensity, the greater the crystallinity of the membrane.From diffractogram the isolation of human nail keratin which has a peak in the 2θ angle area of 20.2491° with an intensity of 461.3566.
According to previous research [15], the structure of extracted keratin is semi-crystalline to amorphous.
Diffractogram from the isolation of human nail keratin did not show any distinct peaks in each plane, it was seen that the keratin from human nails had broad peaks indicating that the keratin structure was semicrystalline to amorphous.This amorphous phase causes more empty space, which allows more ion movement to increase.The structure results from the isolation of human nail keratin which has mechanical properties that are in the middle of amorphous and crystalline properties.According to [16] the diffractogram of human nail keratin has the highest peak in the 2θ area of 20.2°.So the results in this study have the highest peak in the 2θ area of 20.2491°.The diffractogram of the composite membrane 3.5/0.5 form a semi-crystalline phase, which shows a sharp peak that increases at an angle of 2θ with an intensity of 31.9491° and has an intensity of 433.3573.This is caused by the addition of sulfonated polysulfone which is more than keratin so that it affects the crystallinity of the membrane.Therefore, the diffractogram produced by the membrane provides amorphous and semicrystalline to amorphous phases formed on the composite membrane by hydrogen bonds between polymer chains with one another.The composite membrane diffractogram with a ratio of 3.3/0.7 shown in the picture above shows that the maximum intensity is at 2θ with an angle of 31.9751° with an intensity of 420.3981 which forms a semicrystalline phase.It has a slightly wider peak than 3.5/0.5 composite membrane.This is because the addition of keratin in the ratio of the composite membranes 3.3/0.7 is more than the ratio of composite membranes 3.5/0.5 so that it affects the crystallinity formed.Diffractogram of the composite membrane with a ratio of 3/1 form a semicrystalline phase.This is because, the more human nail keratin is added, the greater the effect of crystallinity on sulfonated polysulfone.Therefore, the formation of a wide peak at an angle of 2θ obtained 32.0011° with an intensity of 344.907.The proton-conducting membrane of the semicrystalline phase greatly affects the proton transport process [17].The larger the semi-crystalline phase formed, the better the proton-conducting process that will occur.

Conclusion
Human nail keratin has been successfully carried out and a composite membrane has been synthesized from sulfonated polysulfone and human nail keratin with mass variation of 3.5/0.5;3.3/ 0.7; 3/1.Proton conducting composite membranes have been successfully characterized by group analysis showing interactions at peaks of 3000-3500 cm -1 .The highest of cation exchange capacity was 2.02 meq/g in the variation of composite membrane 3/1.The highest degree of swelling is 33.23 % in the variation of PSF composite membrane 3/1.The highest conductivity of the composite membrane in the mass variation of 3/1 was 4.87 x 10 -5 S/cm.The results of the XRD analysis showed that the sulfonated polysulfone composite membrane and human nail keratin had semicrystalline properties at an angle of 2θ which was obtained at 32.0011° with an intensity of 344.907 which was owned by the best peak at variation of 3/1.

Figure 1 .
(a) Dissolving human nails with NaOH solvent (b) the results of human nail keratin.

Figure 3 .
Figure 3. Spectrum FTIR of proton conducting membrane sulfonated polysulfone and composite membranes.

Figure 4 .
Figure 4.The cation exchange capacity of proton conducting membranes.

Table 1 .
Degree of swelling (DS) of proton conducting membranes.