Investigating the structural effect of electrospun nano- fibrous polymeric films on water vapor transmission

Seyed Jalil Poormohammadian ,Parviz Darvishi,*,Abdol Mohammad Ghalambor Dezfuli

1 Chemical Engineering Department,School of Engineering,Yasouj University,Yasouj,Iran

2 Physics Department,Shahid Chamran University of Ahvaz,Ahvaz,Iran

Keywords:Water vapor transmission Polymer structure Nano fiber Electrospinning Porous layer

ABSTRACT Electrospun polymers have many applications in the industry.However,the structure ofthese polymers has been less widely considered by researchers.In this work,the structural effect of electrospun and casted films of polyacrylonitrile(PAN)and polyvinylidene fluoride(PVDF)polymers on watervapor transmission were investigated.Sorption of water vapor was measured at 35,60 and 80°C and different relative humidities.The diffusion coefficients were calculated based on mass changes of the polymer sample.The water vapor transmission rate(WVTR)was also measured at 35°C and 90%relative humidity.The results indicated that electrospun nano- fibrous polymers(ESNPs)absorb much higher water vapor compared to non-porous casted polymers.The interaction of water molecules with mentioned polymers was investigated based on Flory-Huggins theory.The Flory-Huggins interaction parameter of electrospun films was less than casted films,suggesting much better interaction of water molecules with electrospun films.It was also found that electrospun films have anomalous kinetic behavior and do not obey the Fickian diffusion model.Finally,it was revealed that ESNPs show less resistance to water vapor transmission and they are good candidates for the applications of water vapor separation using membranes.

1.Introduction

Transport phenomenon of water vapor through polymers is an important and attractive subject in a variety of applications,including dehydration and dehumidification processes,food packaging,membrane gas separations,and microelectronic and display devices.Given the properties of high condensability and very small molecule size,water molecules have higher permeability compared to permanent gases such as CH4and CO2,even in hydrophobic polymers.However,the transportation of H2O molecules in the membrane polymers used as support layer is limited due to the well-known concentration polarization phenomena.The support layers are usually made by casting of polymer solutions that create layers with low porosity and flexibility.The casted films show high resistance to water vapor molecules and thus reduce the transport efficiency of water molecules through the polymers[1].Generally,selective layers account for about 30%of the total resistance to water vapor transmission,and the support layers constitute the remaining resistance[1,2].The support layers themselves are usually fabricated on a non-woven paper which brings additional resistances to water vapor transmission[1].The low porosity of polymeric films is pertinentto the dense physicalstructure and the low flexibility is related to their rigid structure.Therefore,the fabrication of polymeric layers with different morphology and structure can affect the water vapor transmission through the mats.

Similar to other small molecules,the water vapor permeation through polymers obeys the solution–diffusion mechanism[3–5].The difference is that the water molecule can interact with the polymer molecule and with itself due to its high hydrogen bonding af finity.These interactions alter certain properties of the polymer such as glass transition temperature and affectthe sorption and diffusion ofmolecules into the polymers.The sorption of water vapor by the polymer depends on the temperature and relative humidity,chemistry of the polymer and the effects of water on the properties of polymer[6–8].Another important factor on the absorption process of water vapor by polymers is the physical structure of polymer,which has been less widely considered by researchers.Changing the morphology of the polymer to obtain high contact surface area will enhance the efficiency of chemical and physical industrial processes taking place at the surface.This enhancement affects the interactions between the polymer material and the substance to which it is contacted[9].Consequently,different types of specific interactions at the polymer surface will take place that manifest themselves in quite different ways,and lead to the fact that polymers absorb the desired component avidly on their surfaces[10].By reducing the structural size of polymers to nanoscale,an enormous change in the properties ofpolymercan be achieved.Using a reliable technique such as electrospinning and shrinking the fiber diameter to sub-micron or nanometer(e.g.10×10-3-100×10-3μm),several surprising characteristics such as specific surface area to a level of 1000 m2·g-1,a million times increase in flexibility and significant increase in mechanical properties can be expected[11,12].

Diffusion of water molecules in the polymers depends on the fractional free volume(FFV),the glassy or rubbery state of the polymer,and the relative rate of water diffusion and polymer chain relaxation[13].The behavior of the polymer for transportation of molecules below and above the glass transition temperature is different due to the changes in the viscosity and molecular mobility of the polymer[14].To obtain the nature of the transport process,a relation between the polymer moisture content versus time should be determined.

Electrospun nano- fibrous materials have been investigated by many researchers in a broad range of industrial applications[12],including clothing[15],air filtration[16,17],ultra filtration[18,19],osmosis process[20,21],and in energy recovery ventilators[22].Other applications of these specific materials can be cited in drug delivery[23],tissue engineering,regenerative medicine[24]and active materials for photonics and electronics[25].The permeation performance of ESNP was characterized and increased by over a full order of magnitude,especially with low fiber diameters.However,the sorption and diffusion of water vapor through these materials is not reported in the previous works.In contrast,sorption and diffusion of water vapors in the casted polymers has been studied by many researchers[7,26–33].Prausnitz and co-workers[34,35]investigated the sorption and diffusion of water in several glassy polymers,including PAN,poly(methyl methacrylate)(PMMA),poly(acrylic acid),and poly(n-vinyl-2-pyrrolidone)by the Flory–Huggins model and the Zimm and Lundberg clustering function[36].They found that poly(n-vinyl-2-pyrrolidone)absorbs large amounts of water so that its glass transition temperature is reduced below experimental temperature.However,other polymers such as PMMA and PANabsorb a smallamountofwater,which has little effect on their glass transition temperature.

In nearly all of the previous works,the transport of water vapor was investigated in dense polymeric films fabricated by casting method.To the best of our knowledge,no report has been published to investigate in detail the process of water vapor sorption and diffusion through ESNPs and enhance its transmission rate.Therefore,this work aimed to synthesize polymeric films with high performance for sorption,diffusion and transport of water vapors.For this purpose,polymeric nanofibrous films of PAN and PVDF were fabricated by electrospinning method.The sorption isotherms of water vapor were determined and the Flory–Huggins interaction parameter was calculated.The water uptake was specified from the experimental data and the diffusion coefficient was calculated based on the plots of water uptake at initial times of the experiment.The WVTR through the fabricated films was measured and the resistances to water vapor were calculated.In addition,the casting layers ofpolymers were also fabricated and their performance in water vapor transmission was compared with electrospun nano- fibrous layers.

2.Theory of the Work

To investigate the physical and chemical interactions of water molecule with the polymer,an appropriate model should be used for determining the related parameters from experimental data.In the sorption behavior,the Flory–Huggins theory is frequently used to fit the sorption isotherms of vapor penetrants in polymers[37,38].

To investigate the diffusion behavior of polymeric films,Fick's second law[39]is used.In non-porous casted films,the equations of diffusion in the plane sheet are applied,while in ESNPs,the cylindrical form of Fick's law is necessary to be considered.There is no global model to cover all types of water diffusion through polymers[40].The Fickian,case II and anomalous diffusion are the three mechanisms that may occur when water molecules diffuse through the polymers[41].The common method for characterizing the diffusion behavior is the mass gain of polymers during the absorption process[42].

A quantitative measure of gas transport through the polymer is the flux or permeation rate of the gas.The permeability of water molecules within a polymeric film is defined as follows[1,43]:

For comparing the performance of thin films with different thicknesses,permeance is used instead ofpermeability which is defined as the permeability divided by the film thickness(P/l)[44].

2.1.Sorption isotherms

The moisture content at any time in the polymer is calculated from the following equation:

The equilibrium concentration of the absorbed water vapor in the polymer sample is calculated as follows[45]:

where the number 22414 is the volume(cm3)of 1 mol of penetrant at standard temperature and pressure.The interaction parameter of water vapor with the polymer molecules can be obtained from the Flory–Huggins theory[37],which is based on the Lattice model[46].According to this model,the activity of component 1 can be calculated as follows:

where χ12is the dimensionless Flory–Huggins interaction parameter determined by intermolecular forces.This parameter can be used as a fitting parameter or calculated independently from the solubility parameters[47,48]:

In Eq.(4),if1/b is setto zero,the slope ofstraightlineversus ?2gives the Flory–Huggins parameter.The change in glass transition temperature of the polymer during the water sorption can be calculated from the Flory–Fox equation[49]:

2.2.Diffusion behavior

Assuming a constant diffusion coefficient,the integration of the second law of Fick[39]at a given temperature and pressure leads to the following equation for a dense flat sheet of the polymer[42,50]:

where Mtand Meqare the mass of diffusing componentabsorbed by the polymer sample at time t and at equilibrium,respectively.Under short immersion time conditions,Eq.(7)transforms into the following equation[51–53]:

As a result,the diffusivity D can be determined from the initial slope of moisture absorption versus the square root of time:

The cylindrical form of Eq.(7)is presented by Crank[54]as follows:

where bns are the roots of J0(bn)=0.J0is the Bessel function of the first kind of order zero.Also,for a small time duration,Eq.(10)takes the following form:

In general,the adsorption process of water vapor through polymers can be written in the following form:

3.Material and Methods

3.1.Chemicals

Polyacrylonitrile(PAN,average molecular weight=100000)was provided by Esfahan Polyacryl Company,Iran.Polyvinylidene fluoride(PVDF,average molecular weight=444000)was purchased from Arkema,France.N,N-dimethylformamide(DMF,purity=99 wt%)and ethanol were supplied from Samchun,Korea.N-methyl-2-pyrrolidone(NMP)was purchased from Merck Millipore Company.All chemicals were used as received without further purification.Table 1 gives the properties of materials used in this study and Fig.1 shows the chemical structure of PAN and PVDF.

Table 1 The properties of materials

The chemical structure of PVDF consists of carbon backbones with hydrogen and fluorine atoms on both sides symmetrically.It has a high thermal stability and good electrical properties due to the polarity of alternating groups in the polymer chain.PAN is a superabsorbent polymer[56]and has a very polar group,which polarizes it more than PVDF and is expected to be more attractive for water molecules[57].

3.2.Film preparation

3.2.1.Fabrication of electrospun films

Fig.2 shows a schematic diagram of the electrospinning set-up.The electrospinning process involves the use ofan electric field to form fiber.As a result of electric field,the pendant droplet of the polymer solution at the syringe tip first takes the shape of the hemisphere and then the conicalshape.When the intensity ofelectric field overcomes the surface tension ofpolymer solution,the solution is ejected towards the metallic collector.Severalparameters can affectthe formation of nano fibers.The most effective parameters include the distance between the syringe tip and the collector,solution concentration,solution viscosity,molecular weightofpolymer,dielectric constantofthe solvent,feeding rate,orifice diameter,the applied voltage,humidity and the surface tension of polymer solution[11].

PAN solution was prepared by mixing 10 wt%of PAN powders with DMF and then stirring at 60°C for 6 h until a well-dissolved solution with a light yellow color was achieved.Stirring was continued at room temperature overnight.When the bubbles formed in the solution were removed,the solution was placed in a 5 ml syringe with a spinneret at the head.The spinneret was installed at a distance of 15 cm from the 10 cm×20 cm flat-grounded copper collector.A 25 kV voltage was applied from a high voltage DC source and allows the polymer solution to jet from the syringe to the collector.The feed rate of the solution was kept constant at 1.2 ml·h-1using a controlled syringe pump.The electrospinning of PAN solution lasted for 7 h to obtain an appropriate thickness of the support layer.

To collect the nano fibers conveniently,a non-woven polyester support with high porosity(＞60%)was used on the copper plate.This support may create some resistance to water vapor transport,but it helps to easily prepare smooth electrospun films.

PVDF solution was prepared by dissolving 12 wt%of PVDF polymer into the NMP/ethanol(70:30 v/v)solvent.To prepare the electrospinning solution,the polymer was dissolved in NMP and stirred at 60°C for 6 h.Then,stirring was continued overnight at room temperature.After that,ethanol was added to the solution and stirred for 2 h before electrospinning.The feed rate of solution was 1.0 ml·h-1,the distance from the needle to copper plate was 10 cm and the voltage was 16 kV.The electrospinning of PAN solution was continued for 6 h to obtain the desired thickness for the support layer.

3.2.2.Preparation of casted films

The casted layers were prepared by solvent casting using the polymer solutions as dense flat layers on glass plates.Then,the casted films were dried atroom temperature for 24 h.This procedure produces non-porous polymeric films(NPPFs)with flat sheet geometry.The supportlayers are semi-porous polymeric films(SPPFs)fabricated by phase inversion method.In order to compare the WVTR of these layers with electrospun films,the polymeric support layers of PAN and PVDF were synthesized by casting the polymer solutions via phase inversion method.These layers have microporous structure but their geometry is unknown.Therefore,it is not possible to investigate the sorption isotherm ofwater vapor in the mats and only WVTR tests were considered.The thickness of dried films was measured using a micrometer(Mitutoyo MDC 0–1'PF,Japan)at ten different locations,where the average of them is reported in Table 2.

3.3.Water vapor sorption and permeation tests

Fig.1.Chemical structure of(a)PAN and(b)PVDF.

Fig.2.A schematic diagram of electrospinning set-up.

Table 2 The characteristics of the fabricated films

To measure the water vapor sorption,squared specimens(40 mm×50 mm)of each film were cut and dried for 24 h over their glass transition temperature to obtain the initial mass(W0)of each sample.The water uptake of the films was measured gravimetrically using the sorption chamber test.The films of fibers were sealed and clamped in a controllable temperature–humidity chamber.The system was evacuated overnight to degas the polymeric films.Then,the water vapor was introduced into the chamber and the vapor exchange with ambient kept the humidity inside the chamber constant.The relative humidity was monitored by a dew point sensor(S212,CSi-tec,Germany).The wet mass(Wt)was measured by taking out the sample from the chamber and weighing immediately with an electronic balance(ae ADAM,PW 254,USA)with an accuracy of 0.0001 g.The mass of sample was measured during 30 s to minimize the amount of water lost by evaporation.Simultaneously,the mass of sample was recorded using a real-time data logger[58,59].Each test was performed four times to reduce the uncertainty of experiments.

The WVTR was measured according to ASTM E 96/E96M-10[60]using a self-made lab approach.Fifty percent of the test cell volume was filled with water and covered with a test module fabricated from polyvinylchloride(PVC).The testcellwas placed in an arid environment and a circular uncoated film(d=40 mm)ofeach sample was fixed into the module.The driving force for water vapor transport is a constant relative humidity(RH)differential.The RH in the cell was monitored and measured using the dew point sensor.The WVTR(g·m-2·d-1)was calculated according to the following equation:

4.Results and Discussion

4.1.Film characterization

Fig.3.The SEM images of(a)PAN and(b)PVDF nano fibers.

Fig.3 shows the SEM images of ESNP films at 5000×magnification.The electrospun nano fibers of PAN(Fig.3a)and PVDF(Fig.3b)exhibit uniform and smooth surfaces with random orientation.The fibers have a good morphology with very small amounts of bead.The diameter of PVDF nano fibers is higher than PAN nano fibers.This difference is mainly due to the higher concentration of PVDF solution,which increases its viscosity and molecular weight,and reduces the tip-to-collector distance.The higher viscosity of the solution increases the diameter of fiber[61–63].Deitzel et al.[64]demonstrated that the fiber diameter increases with raising the polymer concentration according to a power law relationship.Demir et al.[65]showed that fiber diameter is proportional to the cube ofpolymer concentration.Another factor for the higherdiameterofPVDF layeris related to itsmolecularweight.Given thatthe molecular weight(MW)is proportionalto the polymerchain length,a high MW implies a high degree of polymer chain entanglement.The Berry number,an indication ofthe degree ofpolymer chain entanglement,is the product of intrinsic viscosity and polymer concentration.A high Berry number indicates a high degree of polymer chain entanglement.A Berry number thatcorrelates positively with the fiber morphology and diameter,implies a higher MWwould resultin a larger fiberdiameter and fewer beads[11].Since the fibers have more time and distance to elongate themselves in the instable zone,the further the distance between the spinneret and the metal screen,the finer the fiber diameter will be.Doshi and Reneker[62]reported that the jet diameter decreases with increasing syringe tip distance to collectorand suggested thatthe fiberdiameterwould decrease with increasing distance from the spinneret.

The nano fiber diameters were measured using the KLONK image software after the calibration of images according to their scale bars.For each polymer,the diameter of 100 different nano fibers was determined and the average value is given in Table 2.To determine the porosity of layers,the specimens with known area and dry mass were immersed in deionized water for 24 h.The porosity of the films was calculated using the following equation[66]:

Table 2 represents the characteristics of the fabricated films.

4.2.Sorption isotherm

Itwas found that ESNPs absorb moisture much more effectively than corresponding NPPFs.The absorption tests have been carried out at different relative humidities.Fig.4 indicates the moisture absorption versus time at 90%,55%and 23%RHs and temperature of 35°C.The saturated moisture content of electrospun PVDF was more than 40 times higher than the NPPF of PVDF and this value was about 32 times for PAN polymer at 90%RH.The important property to reach equilibrium in the system is the chemical potential.The chemical potential is the sum of internal(intrinsic)and external(surface)chemical potentials[67]:

The high porosity,geometry and nanoscale size of ESNPs make the surface interaction of polymer with water molecules much higher than NPPFs.Since the external potential contribution in NPPFs is negligible,the polymer film swells.At the surface,additional contributions to chemical potential from van der Waals,electrostatic and hydration interactions are important.These result in a positive external contribution of chemical potential,which leads to manifest itself as an increase in water concentration.

Comparison of the data at differentrelative humidities revealed that absorption of water vapor by PAN nano fibers at 90%RH was more than the 55%and 23%RHs by about 34%and 60%,respectively.The increases in water vapor absorptions for PVDF nano fibers at similar conditions were about 37%and 50%,respectively.At higher relative humidities,there are more water molecules that are able to contact with polymer molecules,and therefore,ESNPs have a greater capacity to absorb water molecules.In NPPFs,there is not much difference between the water vapor absorption at different relative humidities that shows poor interactions between the water vapor molecules and the casted layers.Since there is not much surface area for the interaction of water molecules with polymer molecules,the casted layers reach very soon to equilibrium than ESNPs.

As itis illustrated in Fig.4,PANabsorbs more water than PVDF in both electrospinning and casting processes.PAN consists of a semi-crystalline polymer with high polar nitrile groups and dipole moment.There are many other polar polymers that have good interactions with water molecules and can be grouped based on theirchemicalstructure.Acrylics include acrylic acid,acrylamide,maleic anhydride polymers and copolymers.Amine-functionalpolymers such as allylamine,ethyleneimine,and oxazoline contain amine groups in their main or side-chains.Poly vinyl alcohol(PVA)and poly vinyl acetate(PVAc)are also good examples of polar polymers that contain a vinyl group in their structure.However,most of these polymers have high solubility in water and dissolve through hydrogen bonds,making themnotsuitable fordurable waterrelated processes.For example,poly(N-vinyl-2-pyrrolidone)(PVP)and poly(2-hydroxyethylmethacrylate)(PHEMA)are polar polymers due to vinyl and large oxygen-containing acrylic groups in their structures,respectively.They can absorb more water vapor in their casted forms[34]than PAN polymer but both of them are soluble in water.On the other hand,polysulfone(PSF)is an example of a polar polymer which absorbs lower water vapor than casted PAN[28].The presence of moisture does not chemically degrade polysulfone but it can cause foam formation and makes the polymer a little clumsy[68].In this manner,superabsorbent polymers such as PAN are good candidates for the mechanisms relevant to water operations,as they are capable of absorbing considerable amounts of water without dissolving via hydrogen bonds[56,69].

Fig.4.Moisture content versus time of absorption for the fabricated films at 35°C and a relative humidity of(a)90%(b)55%and(c)23%.electrospun PAN;electrospun PVDF;nonporous casted PAN;and nonporous casted PVDF.

The non-porous casted PAN and PVDF polymers absorb only small amounts of water(2.22%for PAN and 1.1%for PVDF under saturated conditions).Fig.5 compares our results with the data obtained by Prausnitz and co-workers[34]for the casted PAN polymer.The water mass fraction of the polymer is plotted against water vapor activity.Our data represents a slightly higher water vapor absorption than the work of Prausnitz and co-workers[34].

Fig.5.The equilibrium water mass fraction of casted PAN at 35°C and different water activities.

Fig.6 shows the water weightfraction ofallfabricated films atdifferent activities.Electrospun PAN gained the highest water mass fraction,while the casted PVDF gains the lowest value.

Fig.6.Equilibrium water mass fraction at 35°C and different water vapor activities:electrospun PAN;electrospun PVDF;non-porous casted PAN;and nonporous casted PVDF.

The change in the glass transition temperature of the polymers during water vapor absorption is also important because it has a profound in fluence on transport properties.Fig.7 illustrates the glass transition changes of the fabricated films versus water vapor activity based on Eq.(6),where the glass transition temperature is equal to-135°C[28,70].

Fig.7.Glass transition temperature changes of the films:electrospun PAN;electrospun PVDF;non-porous casted PAN;and non-porous casted PVDF.

High absorption of water vapor causes the electrospun PAN to have the highest change of glass transition temperature,where its state changes from glassy to rubbery during the experiment.Non-porous casted PANabsorbs smallamounts ofwater,and its glass transition temperature changes from 95 °C to 76.2 °C at high water vapor activities.The change in glass transition temperature of electrospun PVDF is more than the casted one.

Fig.8 represents the Flory–Huggins interaction parameter at different water vapor activities of polymeric films.The lower interaction parameter(in some cases negative)of electrospun films implies that the films have higher molecular interactions with water molecules than non-porous casted polymers.This high interaction is related to the geometry and nano-structure of ESNPs.Since they have a cylindrical nanoscale shape and the porosity of their films is very high,extensive surfaces are exposed to water molecules.

Fig.8.The Flory–Huggins interaction parameter of fabricated films at 35 °C and different water vapor activities.electrospun PAN;electrospun PVDF;nonporous casted PAN;andnon-porous casted PVDF.

Based on Scatchard–Hildebrand theory[46],Eq.(5)calculates the interaction parameters from the solubility parameters of polymer and water.This equation is not able to calculate the interaction parameters in differentactivities and needs the solubility parameters.The solubility parameters of water[71],PAN[71]and PVDF[72]are 47.9 MPa-5,26.09 MPa-5and 23.2 MPa-5,respectively.Based on these values,the interaction parameter for PAN–H2O is 3.34 and for PVDF–H2O is 2.28.Itis clearthatthese valuesare notin accordance with the Flory–Huggins equation,especially for ESNPs.Therefore,Eq.(5)cannot be used for an accurate estimation of polymer–water interactions and only provides a good guide for qualitative considerations.

4.3.The kinetics behavior

Based on the high changes of glass transition temperatures in Fig.7,anomalous diffusion is expected in the films during the experiment.Fig.9 shows the line ln(Mt/Meq)versus ln(t)for NPPF and ESNP.The slope of these lines gives the exponent parameter α in Eq.(12).This parameter is very close to 0.5 for NPPFs of PAN and PVDF,suggesting Fickian diffusion for these films.It is more than 0.5 for ESNPs which shows anomalous diffusion behavior.Table 3 presents the polymer diffusion coefficients that are calculated by Eq.(8)for NPPFs and Eq.(10)for ESNPs.

Fig.9.Water vapor sorption atearly time of(a)casted films non-porous PANand nonporous PVDF and(b)electrospun filmsPAN and PVDF(T=35°C and RH=90%).

Table 3 also compares the diffusion coefficient of non-porous casted PAN film with previous works.Fieldson and Barbari[73]reported the values of 0.606P × 10-10and 1.236 × 10-10cm2·s-1for diffusion coefficient of water in PAN at 57 and 102°C,respectively.In the present study,the diffusion coefficient of casted PAN film was higher than the results of Fieldson and Barbari[73]and Stannett et al.[74]but it was lower than the resultofPrausnitz and co-workers[34].To the bestofourknowledge,there are no reported work for diffusion coefficients of water vapor in casted PVDF and nano- fibrous layers.It was found that the diffusion coefficient of ESNPs is much higher than NPPFs.This is due to the greater flexibility of polymer chains and the higher contact area of water molecules[11]with ESNPs than NPPFs.

Table 3 The kinetic properties of fabricated films

Fig.10 indicates the plot of Mt/Meqversus the square root of time over film thickness( fiber radius for ESNPs)and compared with the results calculated from Fickian diffusion model(Eq.(7)for NPPFs and Eq.(10)for ESNP).This figure indicates that the experimental data of NPPFs are in good agreement with the results calculated from the Fickian diffusion model.However,the experimental data of ESNPs have much deviation from the Fickian diffusion model and obey the anomalous diffusion model.

Fig.10.Water uptake at 35°C and 90%relative humidity of(a)non-porous casted PAN;(b)non-porous casted PVDF;(c)electrospun PAN;and(d)electrospun PVDF.

4.4.Effectoftemperature on watervapor sorption and diffusion ofESNP films

There are many industrial applications of water vapor transmission at temperatures higher than 35°C.One example is dehydration of natural gas,which usually occurs at temperatures of 60°C or higher.This motivated us to investigate the sorption and diffusion behavior of ESNPs at this temperature range.The moisture content of ESNPs of PAN and PVDF versus the square root of time over fiber radius is shown in Fig.11 at 90%RH and temperatures of 35,60 and 80°C.

The equilibrium water vapor sorption of ESNPs at 35°C is more than 60 and 80°C.It is related to the solubility of water vapor in the polymers,which often decreases by increasing the temperature[75].However,the required time to reach the equilibrium state of water vapor sorption is lower at higher temperatures.It means that at higher temperatures,the water molecules have higher diffusion coefficients and diffuse faster into the ESNPs(Table 4).The higher diffusion coefficient of water molecules in ESNPs is due to more flexibility of polymer chains at higher temperatures.

4.5.Film resistances in water vapor permeation

As mentioned earlier,support layers are semi-porous polymeric films fabricated by casting the polymer solutions via phase inversion method.They exhibit high resistances to WVTR and cause an internal concentration polarization in the membrane.Fig.12 compares the WVTR performance of ESNPs with SPPFs in the presence and without the polyester support layer.It was found that ESNPs transfer water vapor much higher than the SPPFs.The WVTR of electrospun layers is about an order of magnitude higher than the casted layers.This high rate of water vapor is because of the high porosity and flexibility of electrospun films.In ESNPs,the high flexibility ofpolymer chains causes the water molecules to move easier through the mats.The polyester support layer decreases the WVTR in the films.This layer can be removed butitenhances the mechanicalproperties ofthe films.Ofcourse,the high WVTR due to the high porosity of ESNPs is not surprising but the main goal is to estimate the resistance contribution of each support layer.

The transport resistance of each layer is the inverse of its permeance and the overall resistance is the sum of layer resistances in series[76,77].Table 5 summarizes the resistance contribution of each layer.The resistances of SPPFs are much higher than ESNPs.These highresistances are due to the very low porosity and less flexibility of the polymer chains of casted layer.It was found that about 20%to 25%of the resistances are related to the polyester support layers.

Fig.11.Moisture contentofESNP films for(a)PANand(b)PVDF at90%RHand 35,60 and 80°C.

Table 4 Diffusion coefficient of water vapor in ESNP films at 60 and 80°C

Fig.12.Water vapor transmission rate of the films at 35°C and RH=90%with and without polyester support layer.

Table 5 Permeance and resistance ofthe films to water vapor transmission and the relative contribution of each layer to the overall resistance of the support layer.PSS stands for polyester support.1 GPU=10-6 cm3(STP)·cm-2·s-1·(cm Hg)-1.1 cm Hg=1333.22 Pa

5.Conclusions

Nano- fibrous polymers have in fluenced many aspects of science in the industry due to their unique properties.Therefore,the present work was devoted to investigate the performance of electrospun polymeric films in water vapor transport,and comparing its proficiency with non-porous and semi-porous casted layers.The ESNPs showed much better performance in both water vapor sorption(in comparison to NPPF)and WVTR(in comparison to SPPF)because of its specific properties such as higher porosity, flexibility and specific surface area.It was found that the sorption of water vapor in ESNPs is about 40 times of NPPFs.Besides,the diffusion behavior of ESNPs is non-Fickian,while it is Fickian in NPPFs.The fabricated layers are a good candidate to use as a supportlayer ofthe polymeric membranes in water vapor separation technology.In the future work,the potential of the produced layers for water vapor separation motivated us to consider them as support layers in the polymeric membranes for dehydration of natural gas.

Nomenclature

A surface area,cm2

a1water activity

B molar volume ratio of polymer to penetrant

C concentration,cm3·cm-3

D diffusion coefficient,cm2·s-1

l film thickness,cm

M moisture content

MWspenetrant molecular weight,g·mol-1

N flux,cm3·s-1

P permeability,Barrer

P1downstream water vapor partial pressure,cmHg

P2upstream water vapor partial pressure,cmHg

R universal gas constant,J·mol-1·K-1

r nano fiber radius,cm

T system temperature,K

Tgglass transition temperature,K

t time,s

W sample mass,g

δ solubility parameter,MPa0.5

? film porosity

?2polymer volume fraction

ν1water molar volume,cm3·mol-1

ρppolymer density,g·cm-3

χ12Flory–Huggins interaction parameter