TECHNICAL FIELD

[0001] The invention relates to transparent composite membranes, and in particular to those which comprise an electrospun component of a first polymer and an impregnated component of a second polymer.

BACKGROUND

[0002] Transparent and flexible electrospun nanofiber membranes have attracted considerable attention due to their potential use in the fields of electronic displays, solar cells, air filtration, medical, and health. Difficulties arise in designing transparent nanofibrous membranes because of significant light reflection and absorption within the fiber mesh matrix.

[0003] Optically transparent composite films, nanofibers or membrane materials have drawn considerable attention in wearable electronics, air filtration, biomedicine, flexible displays, pressure sensor, power-generating windshields for buildings and aircrafts due to their excellent mechanical flexibility and optical properties [1-4] (Reference numerals in square brackets refer to publications listed in the bibliography at the end of the description).

[0004] For many applications, transparent materials are required to exhibit not only high optical transparency, but adaptability, such as good air permeability, conductivity, flexibility, and filtration performance is also needed [5], In the past two decades, various strategies have been utilized to construct optical transparent conductive films, or nanofibers including solution casting [6], dip-coating [7], solution printing [8], and roll-to- roll technologies [9], However, these methodologies can be complex and may result in composite films which are expensive and lack uniformity and consistency [10],

[0005] Recently, electrospinning has emerged as a possible alternative for the fabrication of nanoscale fibers, mats, and membranes [11], This technique is very adaptable due to its broad processing parameters (such as polymer concentration/viscosity and variability, feed rate, spinning distance, voltage level). The technique also provides the ability to add various particles to form multifunctional mats [12],

[0006] To date, several strategies have been utilized to construct transparent nanofibers including mechanical pressure [17], hot pressing [18], polymer filling [19], and fiber/resin composites [20] or designing specific nanofiber channels for light propagation [21 , 22], as well as replacing fiber/air interfaces with fiber/composite interfaces, decreasing the thickness, and changing the structure of resulting nanofibers mats [23],

[0007] Liu et al. [24] fabricated a transparent air filter via electrospinning on a grounded collector, with a metal-coated window screen mesh. The prepared electrospun nanofibers had multimodal thickness as most of the fibers deposited on to the mesh wires, while a smaller percentage of nanofibers deposited within the mesh voids due to the different electrical field distribution. The low fiber density (small thickness) led to a higher transparency of resulting nanofibers (NFs) mats.

[0008] Zhang et al. [25] reported new fabrication technique of electronetting to develop transparent self-assembled 2D nano-architectured networks (nano-nets) from different materials. They have manipulated the dope solutions and electric field conditions around Taylor cone to achieve nano-nets with surface wettability, transparency, separation and improved air filtration properties. However, owing to complexity of process, cost, and lower mechanical reinforcement of these transparent NFs mats, they are limited only to certain applications.

[0009] Ma et al. [26] developed an electrospun NFs transparent membranes for skin care drug delivery systems. The process involved electrospinning two layers of polymer polycaprolactone (PCL) and sandwiching a layer of wax-free shellac. The fabricated opaque membrane was treated with ethanol vapor and resulting NFs become transparent due to shellac melting and filling the voids in PCL nanofibers.

SUMMARY

[0010] In accordance with the invention, there is provided a method of producing a composite sheet, the method comprising: providing a porous sheet of a matrix material; wetting the porous sheet with a solution of an impregnation material dissolved in an impregnation solvent; and directly contacting a surface of the wetted porous sheet on a solid substrate material for a period of time such that the solid substrate material draws out at least a portion of the impregnation solvent from the wetted porous sheet and allowing the impregnation solvent to dry to form the composite sheet. [0011] Each of the matrix and impregnation materials may be polymers. The solid substrate may be formed from a polymer. The solid substrate may have a consistent density throughout its volume. The solid substrate may not be a foam.

[0012] The matrix material may be polyurethane. The matrix material may be poly(methylmethacrylate).

[0013] The impregnation material may be polydimethylsiloxane.

[0014] The weight or mass ratio of impregnation to matrix materials in the composite sheet may be between 2:1 and 1 :2.

[0015] The impregnation solvent may cause the substrate material to swell. That is, the substrate material may be configured to absorb the impregnation solvent and swell as a result. The swelling ratio, S, of the substrate material in the impregnation solvent may be greater than 1.1 (or greater than 1.2). The swelling ratio, S, of the substrate material in the impregnation solvent may be less than 3. Swelling ratio, S = D/Do, where D is the length of a portion of the substrate material in the solvent and Do is the length of the dry substrate material [51], Swelling in terms of relative volume is calculated by AV = V/Vo= S ³ .

[0016] The Hildebrand Solubility Parameter, 8, of the substrate material may be within 3 cal ^1/2 cnr ^3/2 (or less than 2 cal ^1/2 cnr ^3/2 ) of the Hildebrand Solubility Parameter of the solvent. For example, the 8 _PDMS = 7.3 cal ^1/2 cnr ^3/2 and 8 _cMoroform = 9.2 cal ^1/2 cnr ^3/2 (i.e., a difference of 1.9) [51],

[0017] For a binary system, the Hildebrand-Scatchard equation relates the solubility parameters of nonpolar liquids to the enthalpy change on mixing them: where V _m is the volume of the mixture, 5, is the solubility parameter of the component i, and pi is the volume fraction of i in the mixture [51],

[0018] The substrate may have a smooth surface. A smooth support may provide a good contact with the wet membrane across its surface, and the solvent may be removed uniformly from the wet membrane via diffusion and solubilization into the polymer network of the substrate material.

[0019] The matrix and impregnation materials may be transparent in bulk.

[0020] The matrix and impregnation materials may have refractive indices with a difference of 0.15 or less. [0021] The matrix and impregnation materials may have refractive indices with a difference of 0.05 or less.

[0022] The porous sheet may be formed by electrospinning.

[0023] The matrix material may be insoluble in the impregnation solvent.

[0024] The impregnation solvent may comprise chloroform.

[0025] The matrix and impregnation materials may each comprise an electronegative element, each electronegative element being one of: oxygen, fluorine and nitrogen. At least one of the matrix and impregnation materials may comprise an electronegative element covalently bonded to a hydrogen to facilitate hydrogen bonding.

[0026] Laying the wetted porous sheet on the solid substrate material may comprise sandwiching the wetted porous sheet between two portions of the solid second material.

[0027] The wetted porous sheet may be laid on the solid substrate material the opposing side of the wetted porous sheet is not in contact with a portion of the solid second material. [0028] The substrate material may be soluble in the impregnation solvent.

[0029] According to a further aspect, there is provided a composite sheet formed by the method described herein.

[0030] The proportion of the impregnation material may vary across the thickness of the composite sheet.

[0031] One side of the composite sheet is more hydrophobic than the opposing side.

[0032] The matrix and impregnation materials may have refractive indices with a difference of 0.15 or less.

[0033] The matrix and impregnation materials may have refractive indices with a difference of 0.05 or less.

[0034] The composite sheet may be porous.

[0035] According to a further aspect, there is provided a composite sheet comprising: an electrospun matrix of a matrix material; and an impregnation material impregnated in the electrospun matrix, wherein mat and impregnation materials have refractive indices which are different by 0.15 or less.

[0036] The thickness of the composite sheet may be between 100-500 pm (e.g., around around 200 pm). The thickness may be measured using a micrometer. A nominal pressure may be applied to the composite sheet to ensure consistency in measurement (e.g., 0.1 kPa).

[0037] The mesh or fabric preferably has a filament or yarn diameter of 0.2 to 5 pm. The nanofiber diameter was measured from the images collected by using scanning electronic microscope. The images were taken over several locations on a composite membrane, to provide sufficient data for statistical analysis. The diameter may be a mean diameter of the strands visible in multiple SEM locations.

[0038] The matrix material and/or impregnation material may comprise one or more of polyvinyliden chloride (PVDC), polyvinylidene fluoride (PVDF), polyhexamethylen adipamide (PA6.6), polydodecanamide (PA12), polypropylene (PP), polycaproamide (PA6), polyethylene terephthalate (PET), ethylene monochlor trifluor ethylene (E-CTFE), ethylene tetrafluor ethylene (ETFE), polyethylene (PE), polyoxymethylen (POM), polycaprolactone (PCL), polysulfone (PS), polyvinylbutyral (PVB), polyacrylate, high density PE, fluorized ethylenepropylene (FEP), perfluoralkoxy (PFA), polyacrylonitrile (acrylic fibers) (PAN), polyundecanamide (PA11), polyphenylensulfide (PPS), polyethylene terephthalate (PET), polyhexamethylen sebacinamid (PA6.10), polyethylene naphtalate (PEN), aliphatic polyamide, aromatic polyamide, polyurethane (Pll), polyvinyl alcohol (PVA), polylactide (PLA), polybenzimidazole (PBI), polyethylenoxide (PEO), poly(butylene terephthalate), polyvinylchloride (PVC), cellulose, cellulose acetate (CA), polypropylene (PP), polyetherimide (PEI), polydimethylsiloxane (PDMS), polyaniline, poly(ethylene naphthalate), styrenebutadiene, polystyrene, poly(vinyl butylene), polymethylmethacrylate (PMMA), poly(N-isopropylacrylamide (PolyNIPAM), polysaccharides and their derivatives, biomolecules and their derivatives.

[0039] The solvent may comprise one or more of: ether, tetrahydrofuran (THF) and xylene, toluene, dichloromethane, chloroform, and acetone.

[0040] The impregnation material may be different from the matrix material. The material of the impregnation material may be same as the material of the solid substrate.

[0041] The solid substrate may be rigid. The solid substrate may be flat.

[0042] A porous material may be a solid containing void spaces. A porous material may be permeable to a fluid (e.g., gas and/or liquid).

[0043] Allowing the solvent to dry may comprise one or more of: absorbing solvent into the substrate away from the composite sheet; and evaporating the solvent into the atmosphere. Absorbing solvent into the substrate may include absorbing solvent into the body or bulk of the substrate (e.g., into pores, cavities or gaps away from the contacted surface).

[0044] An advancing contact angle is measured when the liquid wets the previously dry surface. This is typically done with the so-called needle method, where water is gradually applied on the surface. When the contact line starts to move, the advancing contact angle can be measured. A receding angle is the contact angle between a liquid and a solid which has already been wetted with the liquid and which is in the course of being de-wetted. Like the advancing angle, the receding angle is a dynamic contact angle. Dynamic contact angles may be measured in accordance with ISO 19403-6:2017. The values of contact angle given in this disclosure are mean averages.

[0045] The water vapor transmission rate (WVTR) is a measure of the passage of water vapor through a substance. It is a measure of the permeability for vapor barriers. ASTM F 1249 is a standard test method for measuring the water vapor transmission rate through flexible barrier materials used in packaging or industrial applications.

[0046] The solution may comprise between 5 and 25 wt% of the impregnation material.

[0047] The porous sheet may be wetted in the solvent for a period of between 10 and 120 minutes.

BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Various objects, features and advantages of the invention will be apparent from the following description of particular embodiments of the invention, as illustrated in the accompanying drawings. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of various embodiments of the invention. Similar reference numerals indicate similar components.

Figure 1 is a flow chart of the method of production of a composite sheet.

Figure 2a is a schematic diagram of electrospinning and post-treatment process. Figure 2b are perspective views of different ways of providing a supporting substrate: (a) the substrate is a glass petri dish, (b) the substrate is a plastic petri dish, (c) the substrate is tissue paper, (d) the substrate is a PDMS substrate.

Figure 3a is a graph of the transmittance of Pll and various PU/PMMA composite sheets as a function of wavelength. Figure 3b is a graph of the transmittance at 700nm for two wetting periods as a function of PMMA concentration.

Figure 3c are photograph of a Pll porous sheet and PU/PMMA composite sheets overlying text.

Figure 4 are SEM images of a Pll porous sheet and composite nanofibrous membranes at different concentration and immersion time in PMMA/chloroform solutions.

Figure 5a is a graph of the transmittance various PU/PDMS composite sheets as a function of wavelength.

Figure 5b is a graph of the transmittance at 700nm for two wetting periods as a function of PDMS concentration.

Figure 5c are photograph of various PU/PDMS composite sheets overlying text.

Figure 6 are SEM images of composite nanofibrous membranes at different concentration and immersion time in PDMS/chloroform solution.

Figure 7 is a graph of typical stress-strain curves of pure PU nanofiber mat and composite nanofibrous membranes with different contents of PDMS and immersion time.

Figure 8a-b are graphs of advancing and receding water contact angle of pure PU and composite nanofibrous membranes: figure 8a corresponds to the front side (away from the PDMS) and figure 8b corresponds to the back side (next to the PDMS).

Figure 9a-b are graphs of FTIR spectra and TGA respectively of pristine polyurethane and composite nanofibrous membranes.

Figure 10a-b are graphs of DSC thermograms of pristine PU and composite nanofibers membranes and amplified peaks.

DETAILED DESCRIPTION

Introduction

[0049] As described in the background section above, there has been significant work done to design and manufacture optically transparent composite films, including using electrospinning. [0050] Factors that influence the transmittance of electrospun nanofiber mats include the refractive indices of the component materials, the thickness of the nanofiber mats, and the transparency of base materials [14], Electrospun nanofiber mats are composed of fibers and air with high porosity. Due to large difference in refractive index at the fiber/air interface that contributes to light reflection and absorption losses inside the nanofiber mat [1], Similarly, high areal density (i.e. , related to how much material there is per unit area, typically measured in terms of mass per unit area) result in decrease in light transmittance and increase absorption capacity [15, 16], However, high areal density is required for most membrane applications. Consequently, the ordinary electrospun nanofiber mats are nontransparent and to prepare nanofiber-based membrane with high transparency is still a challenge. So, a change in design approach is required to find the optimal conditions.

[0051] To generate transparent composite nanofibrous membranes, certain embodiments of the method comprise electrospun NFs and a post-treatment. The NFs mats were impregnated into polymers solution and then dried on a soft substrate. Thermoplastic polyurethane (Pll) was used to prepare nanofibers mat followed by reinforcing fibers in a polymer solution. To evaluate the effect of selected polymers, the concentration and the immersion time on the transparency of NFs are investigated. In addition, the morphology, mechanical properties, wettability, and light transmittance of post-treated composite nanofibrous membranes are characterized. Our process improves the transparency of Pll NFs while maintains its mechanical strength with controlled solvent evaporation after impregnation. This way, current work provides a possibility to develop an optically flexible transparent NFs for potential applications such as filtration, displays, pressure sensors, wound dressings, and window glass material for aircraft.

[0052] Various aspects of the invention will now be described with reference to the figures. For the purposes of illustration, components depicted in the figures are not necessarily drawn to scale. Instead, emphasis is placed on highlighting the various contributions of the components to the functionality of various aspects of the invention. A number of possible alternative features are introduced during the course of this description. It is to be understood that, according to the knowledge and judgment of persons skilled in the art, such alternative features may be substituted in various combinations to arrive at different embodiments of the present invention.

Method [0053] As shown in figure 1 , the method of producing a composite sheet comprises first providing a porous sheet of a matrix material. In this embodiment, the matrix material is a polymer which is transparent in bulk. In this embodiment, the porous sheet is formed from an electrospun polymer such as polyurethane. In other embodiments other materials may be used.

[0054] Electrospinning is a fiber production method that uses electric force to draw charged threads of polymer solutions or polymer melts up to fiber diameters in the order of a few hundred nanometers to several microns.

[0055] In this case, the fibers are directed onto a target to form a porous mesh sheet. Using a mesh sheet as a base provides a base matrix structure to define the shape of the ultimate composite sheet, as well as to provide the composite sheet with mechanical properties from the electrospun matrix.

[0056] After the porous sheet has been created, it is wetted with a solution of an impregnation material dissolved in a solvent. In this case, the wetting involves immersing the porous sheet in the solution for a period of time. It will be appreciated that the material of the porous sheet may be insoluble in the solvent, or at least relatively insoluble compared to the impregnation material. For example, the impregnation material may be PM MA or PDMS and the solvent may be chloroform. PMMA and PDMS are readily soluble in chloroform, whereas PU is not.

[0057] This step allows the impregnation material to be positioned within the pores (or gaps) in the porous material.

[0058] The last step relates to removing the solvent from the wetted sheet. The inventors have identified that simply allowing the solvent to evaporate (e.g., into the atmosphere) causes the wetted sheet to shrink and deform and/or become white which is undesirable. To address this, the method comprises contacting a surface of the wetted porous sheet on a solid substrate material for a period of time to allow the solid substrate material to draw out at least a portion of the solvent from the wetted porous sheet and allowing the solvent to dry to form the composite sheet.

[0059] The solid substrate may swell in the presence of the solvent to allow solvent to be extracted into the solid substrate. The substrate material may also be soluble in the solvent. The substrate may be a polymer (e.g., a swelling polymer with the solvent). For example, the substrate may comprise PDMS. [0060] PDMS is a polymer chain network. The solvent can diffuse, penetrate and dissolve into PDMS. See the attached paper for complete mechanism. Very importantly, the PDMS support keeps the membrane uniform and flat as the solvent evaporates into air. PDMS has a swelling ratio, S, of 1 .39 for the solvent chloroform where S = D/D _o , where D is the length of PDMS in the solvent and D _o is the length of the dry PDMS [51],

[0061] The inventors tried polystyrene film, glass, tissue, paper towels, cotton towel. None of them gave a transparent membrane after drying for chloroform solvent. We also tried to clip the membrane in air to dry and holding the edges of the membrane. This did not work either.

[0062] The support materials should preferably have the properties to draw the excessive solvent rapidly and uniformly. A smooth PDMS support can be in good contact with the wet membrane, and the solvent is removed uniformly from the wet membrane via diffusion and solubilization into the polymer network of PDMS.

[0063] The use of the solid substrate may help prevent shrinkage and deformation of the porous sheet as it dries. It may also create a non-uniform profile of impregnation material across the thickness of the composite sheet. Typically, there is a greater concentration of impregnation material towards a side near a solid substrate, as migration of the solvent draws the impregnation material towards that side.

[0064] Where the material of the solid substrate is soluble in the solvent (e.g., PDMS), some migration of the solid substrate material into the wetted mesh may also occur.

[0065] In addition, a side dried next to the solid substrate is typically smoother than a side which has been dried through evaporation.

[0066] The result is a dry composite sheet, with properties arising from the combination of the porous sheet and the impregnation material. The dry composite sheet may comprise a combination of the porous sheet and the impregnation material.

[0067] It will be appreciated that as the solvent is removed via drying, gaps may appear, and the impregnation material will solidify and attach to the porous sheet. The gaps may allow the composite sheet itself to be porous, which may allow the composite sheet to allow gas (e.g., water vapour) to pass through it.

[0068] Regarding the physical properties of the composite sheet, the mechanical properties will be dictated by the mechanical properties of the matrix material, the impregnation material, the relative amounts of matrix and impregnation materials the structure of the porous sheet and the interaction between the matrix and impregnation materials. The ratio of impregnation to matrix materials is adjustable by varying the immersion time and the concentration of the immersion polymer solution. For example, by using the 3% PMMA solution with immersion time of 1 hour, we can obtain the mass ratio (impregnation material/matrix) around 1 :1. By using 5% PMMA, we can obtain the mass ratio up to 2:1. By reducing the immersion time to 5 -10 mins we can lower the ratio.

[0069] The inventors have found that the mechanical properties of the composite sheet are strongly dependent on the impregnation material. For example, a Pll porous sheet is inherently flexible. When a Pll porous sheet is combined with a brittle impregnation material (e.g., PMMA), the composite sheet may also be brittle (particularly at high levels of impregnation material), whereas when a Pll porous sheet is combined with a flexible impregnation material (e.g., PDMS), the composite sheet is also flexible.

[0070] Regarding the optical properties, the interaction between the impregnation material and the matrix material is important. A number of factors may affect the overall transparency of the composite sheet.

[0071] The first is the relative refractive indices of the matrix and impregnation materials. If there is a significant difference in the refractive indices, the boundary between the matrix and impregnation materials will form an interface which will scatter light. In the context of an electrospun mesh, the overall effect of this scattering is to give the composite sheet a white opaque appearance. The inventors have found that a refractive index difference of less than 0.15 allows a composite sheet to allow some light to pass through without being scattered. A refractive index difference of less than 0.05 permits a relatively transparent composite sheet.

[0072] Another factor is how the matrix and impregnation materials bond. For example, if the impregnation material did not bond to the surface of the matrix material, and instead formed separate domains of material within the gaps of the mesh matrix, even more interfaces would be formed between each of the matrix and impregnation materials and the air. This would give rise to additional scattering and cause a higher degree of opacity. Bonding between the matrix material and the impregnation material may arise from Van der Waals interactions, dipole-dipole forces, hydrogen bonding and/or other bonds. Stronger bonds may help ensure that the impregnation material is positioned in direct contact with the matrix material as the wetted porous material is dried, thereby reducing the number of scattering interfaces. To form a strong interaction between the matrix and impregnation materials, each of the matrix and impregnation materials may comprise dipole bonds. Dipole bonds are interactions between components of polymers which have a dipole moment created by the presence of an electronegative element, (e.g., oxygen, fluorine and/or nitrogen) which can draw electrons away from other elements forming a permanent dipole. These electronegative elements may form part of the monomer within a polymer to help ensure a strong connection at any point along the length of the polymer. It will also be appreciated that the interaction layer between the matrix and impregnation materials may have an intermediate refractive index between that of the bulk of the two materials. This may also help prevent scattering from the interface.

[0073] Another factor is the surface roughness of the composite sheet. By drying the composite sheet, one or both of the surfaces of the composite sheet may be smoother. This may facilitate the coherent transmission of light through the composite sheet.

Experimental

[0074] The present disclosure includes the experimental verification of the method of fabrication of transparent composite nanofibrous membranes by combining electrospinning and post-treatment of polyurethane (Pll) nanofibers with poly(methylmethacrylate) (PMMA) and polydimethylsiloxane (PDMS).

[0075] In summary, randomly organized Pll nanofibers were prepared via electrospinning. These Pll nanofibers mats were then impregnated with different concentrations (i.e., 5, 10, and 15 wt%) of PMMA and PDMS solutions for different impregnation times (i.e., 30 and 60 min) to prepare transparent composite nanofibrous membranes. The resulting composite nanofibrous membranes exhibit excellent performance including optical transparency (i.e., up to 82% for PMMA and ~ 6% for PDMS), high hydrophobicity, mechanically robust, good water vapor transmission rate (WVTR, ~ 1 kg/m ² day), and good thermal stability.

[0076] As a result of the composite nanofibers membranes’ optical transparency, suitable mechanical strength, and high hydrophobicity, they have a wide variety of potential uses including, for example, windshields for rapidly moving objects, displays, masks, wound dressings and other medical equipment.

Materials [0077] The polymer thermoplastic polyurethane (Pll, Tecoflex EG-80A) was bought from Noveon™, Inc. (USA). Acetone (ACS grade, 99.7%) and N, /V-dimethylformamide (Anhydrous, 99.8%, DMF) were purchased from Sigma Aldrich™ and used as solvent to prepare electrospinning polymer solution. The polymers to prepare post-treatment solutions; poly(dimethylsiloxane) (PDMS) and poly(methyl methacrylate) (PMMA) were purchased from Sigma-Aldrich, USA. Chloroform (ACS grade, 99.9%) of analytical grade were purchased from Fisher Scientific™ USA, and was used as received.

Fabrication of Polyurethane (PU) Nanofibers

[0078] Figure 2a is a schematic of the method of manufacturing the composite sheets.

[0079] A series of polyurethane (PU) concentrations were trialed to manufacture uniform PU nanofibers. One successful precursor mixture was PU dissolved in DMF: acetone (at a 1 :1 weight ratio) to form an electrospinning solution with a concentration of 18 wt% PU (Table 1). To get a homogenous solution; the mixture was stirred for 24 h at 50 °C and then sonicated to avoid aggregation.

[0080] The lab-scale electrospinning process is explained in previous study by the inventors [27], A specific volume of PU solution 201 (6 ml) was filled into a syringe 202 with a blunt end stainless needle of gauge 25 (inner diameter of 0.260 mm). The grounded steel rotating cylinder 203 (diameter: 20 cm), which was covered by aluminum foil as the backing layer. The electrospinning was conducted at optimized conditions. The voltage 204 applied to generate the uniform nanofiber 205 was 11 kV (0.73 kV/cm), flow rate of 1.5 mL/hr, and at a working distance of 15 cm between the needle tip to a grounded drum collector (rotating speed: 300 rpm). The environmental electrospinning temperature and relative humidity (RH) were 25 ± 1 °C and 25-30%, respectively.

[0081] The electrospun nanofiber mat was detached from aluminum foil and dried in a vacuum oven at 70 °C for 12 h to remove any residual electrospinning solvent to produce the porous sheet.

[0082] Table 1 : Conditions for electrospinning PU nanofibers

Preparation of PU Composite Nanofibers Reinforced PDMS and PMMA [0083] The electrospun nanofiber composite sheets were prepared using a solution impregnation method. Square shaped nanofibrous porous samples were cut into size of 30 mm x 30 mm and placed in a glass petri dish 210. The image 211 is an SEM image of the porous sheet 213 just before immersion.

[0084] Immersion solutions were prepared using different concentration (i.e., 5, 10, and 15 wt%) of PDMS and PMMA in chloroform solvent (Table 2). Then, 30 ml of each solution was measured and poured into the petri dish containing a nanofiber mat. The petri dish was sealed with polyethylene film for different period of time (i.e., 30 and 60 min) to investigate the effect of time on nanofibers transparency.

[0085] After infiltration for 30 and 60 min at 25 ± 1 °C, the prepared electrospun nanofiber composite wetted sheet 220 is transferred to the clean petri dish 221 containing a PDMS substrate porous substrate 222 for drying in sealed environment for 12 h. The image 223 is a photo showing text overlaid with samples of the dried composite sheet to provide a visual indication of transparency. The PDMS substrate is smooth.

[0086] Table 2b shows post-treatment solutions composition and conditions

[0087] The resulting composite nanofibrous membranes were sent to characterization to investigate the influence of PDMS and PMMA solution impregnation. It was observed that the effect of PDMS and PMMA solutions on resulting composite transparency and morphology was different and was investigated through various characterizations.

Composite Sheet Characterization

[0088] Light transmittance (Transparency) of composite nanofibrous membranes was observed on a UV-vis spectrometer (Lambda™ 850, PerkinElmer™) in visible light wavelength range of 350-800 nm at 25 °C. The surface morphology of the pristine PU electrospun nanofiber and composite nanofibrous membranes were examined using a field-emission scanning electron microscope (FE-SEM, Mira 3, Tescan™, Czech Republic).

[0089] The successful impregnation of PDMS and PMMA was confirmed by the identification of functional groups on the surface of the electrospun composite nanofibers using an attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR, Alpha-P™ FTIR spectrometer, Bruker, USA). Differential scanning calorimetry (DSC, TA Instruments™ Q1000 V9.9 Build 303) was conducted to determining polymer crystallinity based on the heat required to melt the polymer. The surface water contact angle (WCA) of the PU composite nanofibrous membranes with and without post-treatment were measured using the sessile drop method by a drop shape analyzer KRUSS™ GmbH 22453 Hamburg, Germany.

[0090] The crystallinity of composite nanofibrous membranes with/without post-treatment can be determined with DSC by quantifying the heat associated with melting (fusion) of the polymer. In this work, samples are analyzed over the temperature range from -50 °C to 180 °C. A heating rate of 10 °C/min was used with a nitrogen atmosphere around the sample.

[0091] Thermal properties of composite nanofibrous membranes were studied by thermogravimetric analysis (TGA, TA-Q500™, USA) using ~ 10 mg in the temperature range -40 to 600°C, at heating rate of 10 °C/min under nitrogen atmosphere. The mechanical properties of all the composite nanofibrous membranes with/without posttreatment were examined using an Instron Corporation Automated Materials Testing system at room temperature.

Characterization of PU/PMMA Composite NFs

[0092] As discussed above, to prepare transparent composite nanofiber membranes, PU white nanofiber mats (or porous sheets) were soaked in the PMMA/chloroform medium with different concentrations of PMMA (i.e., 5, 10, and 15 wt%) for different impregnation times (i.e., 30 and 60 min).

[0093] After impregnation, treated composite nanofibers were taken out and transferred to different substrates for drying as shown schematically in Figure 2.

[0094] Figure 2a shows one experiment where immersed composite NFs (or wetted porous sheet) 220a was transferred to clean glass petri dish 221a and left it out for 2 hr at room temperature in fume hood. The treated composite NFs membrane stuck to the glass, and it was hard to remove.

[0095] Figure 2b shows a second experiment where the glass petri dish of figure 2a was replaced with a plastic petri dish 221b, but it was found that the plastic petri dish dissolves in the impregnation solution when the wetted porous sheet 221 b was put into the plastic petri dish.

[0096] Figure 2d shows a third experiment in which tissue paper 231 was used to separate the drying wetted porous sheet 220d from the plastic petri dish 221 d. The wetted porous sheet was sandwich between two sheets of tissue paper as shown in figure 2d. Although the wetted porous sheet was relatively soft/flexible after drying, most of PMMA was absorbed into the tissues rather than in the composite sheet.

[0097] In a fourth experiment shown in figure 2c, PDMS film 222 was prepared in glass petri dish 221c and utilized as substrate. Of the four experiments, the PDMS substrate worked the best because the composite nanofiber 220c did not stick onto the substrate and the PDMS with its hydrophobic surface did not absorb PMMA.

[0098] The resulting PMMA composite sheets were optically transparent within the visible light range (e.g., between 380 to 700 nanometers), even though the Pll porous sheet precursor is white in color as shown in Figure 3a-c.

[0099] Figure 3a shows the transparencies of the Pll porous sheet and of various PU/PMMA composite sheets, including:

• PU base (347);

• PU with 5% PMMA immersed for 30 min (346);

• PU with PMMA 5% immersed for 60 min (345);

• PU with PMMA 10% immersed for 30 min (344);

• PU with PMMA 10% immersed for 60 min (343);

• PU with PMMA 15% immersed for 30 min (342); and

• PU with PMMA 15% immersed for 60 min (341).

[0100] Figure 3b is a graph of the transmittance at different concentration and at specific visible wavelength 700 nm.

[0101] For example, the PU/PMMA composite nanofiber with PMMA 15 wt% for both 30- and 60-min immersion time displayed exceptional light transmittance of -84% (Figure 3a) which is approximately 82% higher than that of the pristine PU nanofibers. Figure 3b presents the transmittance vs PMMA concentration at different immersion time and at specific visible wavelength of 700 nm for better understanding and comparison.

[0102] It was observed that immersion time did not influence the transmittance of resulting composite nanofibers as much as the PMMA concentration. This may be because the neat Pll nanofiber mat absorbed PMMA up to its maximum capacity in short period of time (i.e. , 30 min) so a longer impregnation time did not influence absorption capacity much as it was already saturated. On the other hand, with an increase of PMMA concentration the composite nanofibers showed considerable improvement in the light transmittance. However, the increased thickness of the PMMA film formed at higher concentration more significantly influences the mechanical properties of resulting composite sheet. For example, those with a higher concentration of PMMA were more brittle. This may allow producers to optimize the process for particular applications depending on whether optical or mechanical properties of the composite sheet are more important.

[0103] Figure 3c are photographs of a pristine Pll nanofiber mat and PU/PMMA composite sheets. The text behind the pure Pll porous sheet is relatively hard to read. Consistent with the graphs shown in figure 3a and 3b, composite sheets with a higher ratio of PMMA are easier to read.

[0104] Regarding the optical properties of the composite sheets, when incident light encounters an interface, it may be reflected, transmitted, refracted, scattered and/or absorbed. The light reflection generally attributes to the maximum loss of the light, which is associated with the refractive index (Rl) of the interface materials [28], The greater the Rl difference between two materials forming the interface, the more the light is reflected and the less the light is transmitted [29],

[0105] For a Pll porous sheet with a large number of fiber/air interfaces and a highly porous structure, there is a significant loss of the light on to the interface due to the large difference in their RIs. Polyurethane and air refractive indices (RIs) are 1.54 [30] and 1.0 respectively, and this causes significant light reflection and absorption at the Pll-air interface with a small amount of light being transmitted [31],

[0106] After post-treatment air/fiber interface is replaced with PMMA (Rl = 1.49)/ fiber interface and small Rl difference (0.05). The PMMA/PU interface contributes to less light reflection and more light transmission [32], In addition, coating the porous sheet with an impregnation material with a refractive index intermediate between that of the matrix material and the air may also reduce reflection and increase transmission.

[0107] However, although the resulting PU/PMMA composite nanofibers are highly transparent, the resulting composite sheet is more brittle due to brittleness of PMMA, that negatively influence on to the integrity of Pll nanofiber. The PMMA brittle layer dominants the flexibility of the nanofibers of the porous sheet.

[0108] Figure 4 presents FE-SEM surface morphologies of pristine Pll and composite nanofibrous membranes prepared with the post-treatment of PMMA and PDMS matrices. It can be seen that, because of the porous structure of Pll nanofibrous mat, the PMMA/chloroform and PDMS/chloroform solutions were easily penetrated and fill into the pores during the solution impregnation process.

[0109] The resulting composites nanofibers portrayed in Figure 4 revealed significantly different morphologies based on the concentration and immersion time in respective impregnation solutions applied. Pll nanofiber mat impregnation in low concentrations (5 wt %) of PMMA/chloroform solution led to accumulation of PMMA on both fibers and voids between them in the form of film. However, still some of fiber structure visible on the surface at lower concertation and immersion time.

[0110] For samples with a higher PMMA concentration and immersion time, the nanofibers can no longer be seen, and only smooth film can be seen at PMMA 10 and 15 wt% with 60 min of immersion time. A complete PMMA film formed on to the nanofiber dominates the material properties, and so the resulting composite sheet has less mechanical integrity and become brittle.

Characterization of PU/PDMS Composite Sheets

Transparency Evaluation

[0111] In another set of experiments, composite sheets were produced similar to those described above, except that instead of PMMA, the impregnation material was PDMS (polydimethylsiloxane). The same post-treatment process to prepare PU/PDMS composite sheet as was used for the PU/PMMA composite sheet.

[0112] The transmittance results and digital photos of prepared composite nanofibers at different concentration of PDMS and impregnation time are presented in Figure 5a-c. Figure 5a is a graph of the transparencies of PU/PDMS composite nanofibrous membranes:

• Pll with PDMS 5% immersed for 30 min (546);

• Pll with PDMS 5% immersed for 60 min (545);

• Pll with PDMS 10% immersed for 30 min (544);

• Pll with PDMS 10% immersed for 60 min (543);

• Pll with PDMS 15% immersed for 30 min (542); and

• Pll with PDMS 15% immersed for 60 min (541).

[0113] Figure 5b is a graph of transmittance at different concentration and at specific visible wavelength 700 nm.

[0114] Figure 5c are photographs of a pristine Pll nanofiber mat and PU/PMMA composite nanofiber mats. These photos confirm that the text is more readable through a composite PU/PDMS sheet than the initial PU porous sheet as shown in figure 3c.

[0115] Transmittance results indicate enhancement of transparency up to a certain limit at all the concentration and immersion time of the PDMS matrix. The transparencies of PU/PDMS composite nanofiber membranes are much lower than PU/PMMA composite due to higher RIs difference between interfaces of the matrix and impregnation materials. PDMS Rl is 1.43 [33], however, PDMS/PU interface had higher Rl difference (0.11) which attributes to higher light reflection at the PDMS/PU interfaces and less light transmission than is the case for a PMMA/fiber interface. Nevertheless, the transmittance of the composite sheet is higher than that of the initial porous material. There may be less reflection on the air/PDMS interfaces.

[0116] Another reason for the lower transmittance for PDMS/fiber interfaces relates to the molecular weight (MW) of the base polymers. PDMS has much lower MW than PMMA and, as a result, PDMS post-treatment solution has lower viscosity at all concentrations utilized in this work. This low viscous PDMS solution has small molecular chains than PMMA. Less viscous PDMS may penetrate the pores readily and form a thin, soft and uniform coating layer around the fiber. A uniform coating layer may reduce light scattering, which is desirable for improving transmittance.

[0117] During post-treatment very thin and soft layer of PDMS formed due to softness of PDMS. Unlike the PU/PMMA composite sheets described above which are brittle due to hardness of PMMA, the PU/PDMS composite sheets appeared to be soft. This is consistent with PDMS films having a smaller young modulus (i.e. 1 ~ 3 MPa) [34] than PMMA films (i.e., 2 GPa) [35], The resulting PU/PDMS composite sheets are therefore flexible, transparent, and nanofibers sustain their integrity that would be useful for various applications.

SEM Morphology Evaluation

[0118] Figure 6 are SEM images of composite nanofibrous membranes at different concentration and immersion times in PDMS/chloroform solution. Pll porous sheet impregnation in PDMS/chloroform solution demonstrated different behavior than was observed with PMMA/chloroform solution (see figure 4).

[0119] As shown in Figure 6 at lower concentration (5 wt%) the PDMS is deposited onto the fibers while still leaving empty voids (i.e., pores). At higher concentration impregnated composite sheet has a reduced porosity which depends on solution concentration and immersion time.

[0120] Even with 15 wt% PDMS and a 60 min immersion time, a smooth film is formed but a fiber matrix structure can still be clearly seen. Because of this, PDMS/chloroform- based composite nanofibers retain the flexibility, elasticity, and mechanical properties of the underlying porous material matrix, at least up to certain limit.

[0121] This different behaviour of PDMS may be attributed to its low molecular weight and the hardness of PMMA. The molecular weight of PMMA is approximately 120,000 g/mol and of PDMS is around 27,000 g/mol. However, at the same concertation PDMS exhibits lower viscosity than PMMA solution. Hence, PDMS forms relatively soft and flexible composite sheets with improved mechanical properties relative to PMMA composite nanofibrous membranes. Meanwhile, both composite nanofibrous membranes showed lower mechanical characteristics than the underlying porous material, because the film formed during the impregnation process causes the fibers to lose their mechanical integrity. However, these composite nanofibers considered for further characterization and have shown promise for various practical applications.

Mechanical Properties Evaluation

[0122] The mechanical properties of PU/PDMS composite nanofiber reinforced with PDMS different concentration and immersion time were measured. Figure 7 is a graph of stress-strain curves of pure Pll nanofiber mat and composite nanofibrous membranes with different contents of PDMS and immersion time:

• PU base (747);

• PU with PDMS 5% immersed for 30 min (746);

• PU with PDMS 5% immersed for 60 min (745);

• PU with PDMS 10% immersed for 30 min (744);

• PU with PDMS 10% immersed for 60 min (743);

• PU with PDMS 15% immersed for 30 min (742); and

• PU with PDMS 15% immersed for 60 min (741).

[0123] The electrospun PU porous sheet alone (without impregnation with PDMS) was tested for mechanical properties. The neat PU nanofiber mat displayed a tensile strength of - 16.69 MPa (e.g. corresponding to the ultimate tensile strength), while the composite nanofiber at low concentrated (5 wt%) solution presented tensile strength of -4.13 MPa.

[0124] Therefore, electrospun nanofibers post-treatment by the impregnation may be less useful for applications requiring strong tensile strength of membranes, as also described by other researchers [36, 37], Moreover, with an increase the concentration of PDMS, the tensile strength of resulting composite nanofiber membrane increased due to relatively thick film and high fiber entanglement.

[0125] At the same time, these composite nanofibrous membranes exhibited decline in their tensile strength compared to pristine PU nanofiber because PDMS film cracked on stretching. Despite lower tensile strength, the composite nanofiber at 60 min immersion time displayed higher strain at the yielding point than the base PU porous sheet. This increase in stretching of composite nanofibrous membranes associated with the PDMS intrinsic characteristics (i.e., adhesive). During tensile testing of these composite, the increased stretching force was related with the friction between fibers and impregnated PDMS film that led to delaying composite failure. Consequently, more PDMS absorbed in nanofibers resulted in a higher strain, as shown in the tensile testing results of the composites in the Figure 7.

Contact Angle Analysis

[0126] Hydrophobicity is an important parameter for evaluating the wettability of a fabricated membrane surface. Results of contact angles (i.e., advancing, and receding) were obtained for pristine PU nanofibers and composite nanofibrous membranes with different concentration of PDMS and immersion time, as shown in Figure 8a-b. These graphs show that there is a difference in hydrophobicity between a side which is dried or partially dried in contact with the solid substrate and the opposite side which is dried in the atmosphere not in contact with the solid substrate. In this case, figure 8a corresponds to the front side (away from the solid PDMS substrate) and figure 8b corresponds to the back side (next to the solid PDMS substrate). Variations in hydrophobicity through the composite sheet may be useful where water transport through the sheet is important (e.g., in a wound dressing to control how the wound dries).

[0127] The contact angles were measured on both sides of the as-prepared membranes, since the solvent evaporation process different on either side of composite nanofiber membranes. The bottom side of the composite membranes was in contact with the PDMS substrate for all the prepared samples, which influenced the surface morphology of the membranes and may lead to smoother surfaces, as compared to the top side exposed to the air. This change in surface morphology of resulting composite nanofibers membrane have a significant effect on the surface wettability (i.e. , contact angle), as shown in Figure 8a-b. In general, electrospun nanofibers have significant surface roughness, which can be attributed to their highly porous structure [38],

[0128] Pll is a hydrophobic polymer, and its electrospinning produces rough nanofibers surface as can be seen in the Figure 8 graphs. Polyurethane films usually have contact angle around 70°, while Pll nanofiber exhibited super-hydrophobicity with static contact angle of 128° [39], These results are well described by the Cassie and Baxter model, which correlates the superhydrophobic behavior to the surface roughness and the porosity of the fabricated membrane. This model deliberates the wetting phenomenon in heterogeneous surfaces, whereby the air is entrapped in the porous structure, as shown in equation 1 [40, 41]: cos 9 ^CB = f _s l + cos 0 _e ) — 1 (1)

[0129] where 6 ^CB is the contact angle heterogeneous surface, f _s is solid/liquid contact area fraction and 6 _e is the Young’s contact angle of the flat surface [42],

[0130] This model also appropriately correlates the wettability behavior of the heterogeneous, PDMS- reinforced composite nanofibers membranes.

[0131] According to the Cassie and Baxter model, the increase in the contact angle observed is due to larger fraction of air entrapped, leading to higher surface roughness and making the surface more hydrophobic [43], In general, an increase in surface roughness makes the hydrophobic surface, leading to an increase in the contact angle [44], The as-prepared composite nanofiber membranes exhibited an increase in hysteresis (e.g., difference between the advancing and receding contact angle) and advancing contact angles with a decrease in receding contact angle, as the concentration and immersion time of PDMS increased. Although the PDMS-reinforced membranes are relatively smoother in comparison to pure Pll nanofibers surface, it seems that an increase in PDMS reinforcement renders the surface slightly rougher, as visible by the slowly increasing static contact angle. To understand this phenomenon better, it is important to account for the compositional changes in the membrane as well.

[0132] It is interesting to note that PDMS has a static contact angle of 107°, which is higher than that of polyurethane [45], Thus, the formation of PDMS layer on the nanofiber surface has a stronger influence on the wettability of the membrane, as the concentration and immersion time of PDMS increases. The decrease in the dominance of the polyurethane nanofiber along with the changing surface morphology of the membranes work in conjunction, resulting in increasing hydrophobicity of the PDMS-reinforced, membranes.

Water Vapor Transmission Rate (WVTR) Analysis

[0133] WVTR is another parameter to determine wettability of different surfaces and their permeability capacity. Choosing a WVTR for a film depends on its application. For instance, WVTR and oxygen transmission rate should be balanced to select an appropriate film for packaging because the packaging should provide a suitable environment for the contained product’s long-term stability [46], Similarly, wound dressings may require a high WVTR to help absorb exudate from wound while providing a moist environment wound to accelerate the healing process.

[0134] Table 3 below presents the WVTR of pure Pll nanofiber and PU/PDMS composite nanofibrous membranes prepared in this study. These results indicate that, with an increase in concentration of PDMS, the WVTR values decrease compared with pristine Pll nanofiber mat (1.26 kg/m ² day).

[0135] This is because the higher concentration of PDMS partially occludes the porous structure of the nanofiber mat. Similarly, the higher the impregnation time, the lower the WVTR, due to dense PDMS film formed which increase barrier resistance for permeation. However, the WVTR of the composite sheet is important depending on its final application and it can be manipulated accordingly via different techniques.

[0136] Table 3. WVTR of neat Pll nanofibers and PU/PDMS composite nanofibrous membranes

FTIR Analysis of Composite Nanofibers

[0137] Figure 9a is a graph of FTIR spectra of pristine polyurethane and composite nanofibrous membranes:

• PU base (947);

• PU with PDMS 5% immersed for 30 min (946);

• PU with PDMS 5% immersed for 60 min (945);

• PU with PDMS 10% immersed for 30 min (944);

• PU with PDMS 10% immersed for 60 min (943);

• PU with PDMS 15% immersed for 30 min (942); and

• PU with PDMS 15% immersed for 60 min (941).

[0138] Figure 9b is a graph of TGA of pristine polyurethane and composite nanofibrous membranes:

• PU base (947b);

• PU with PDMS 5% immersed for 30 min (946b);

• PU with PDMS 5% immersed for 60 min (945b);

• PU with PDMS 10% immersed for 30 min (944b);

• PU with PDMS 10% immersed for 60 min (943b);

• PU with PDMS 15% immersed for 30 min (942b); and

• PU with PDMS 15% immersed for 60 min (941b).

[0139] Polyurethane structure confirmed with the typical absorption bands at 1727 cm-1 and 1702 cm ^-1 which are attributed to the stretching frequencies of N-H and C=O bonds present in urethane linkage [47], [0140] In the FTIR spectra of composite nanofibers membranes, the continuous decrease of the characteristic peak at 1727.9 cm ^-1 shows success of impregnation process. During impregnation the urethane linkage (N-H) makes a hydrogen bond with PDMS silane group oxygen. In this example, the presence of the electronegative element nitrogen and the electronegative element oxygen forms permanent dipoles within each polymer which then allow the two polymers to form a strong attraction to each other. With the increase of concentration of PDMS and immersion time a stronger reaction occurred, which explains the decline in peak intensity. With a stronger reaction intensity, the transparency of resulting composite nanofibrous membranes increased and these results can be confirmed from transparency data in Figure 5a. This indicates that a strong interaction between the matrix and impregnation materials can improve the transparency of the composite sheet.

DSC and TGA Analysis of Composite Nanofibers Membranes

[0141] Thermogravimetric analysis (TGA) was conducted to investigate the thermal stability of neat polyurethane and composite nanofibrous membranes as shown in Figure 9b. Results indicate that TGA of pristine polyurethane at which ~5% weight loss observed was 302 °C, whereas the composites formed at different PDMS concentrations and immersion time also exhibited similar TGA values [48], All the prepared membranes decompose at same time with similar thermal stability. The composite curves initial weight is higher than pristine Pll that indicates of success of post-treatment process. Pure Pll nanofiber membrane weight loss of 50% at a temperature of 383 °C, weight loss of 90% at a temperature of 438 °C, whereas the composite nanofibrous membranes weight loss of 50% at a temperature range 430 to 475 °C depending on the PDMS concentration and immersion time.

[0142] Moreover, at highest temperature of -600 °C, the Pll curve approaches nearly to zero which confirms the complete decomposition of Pll; while the composites curves indicate that some PDMS remains at that temperature. PDMS contains silicon which does not decompose at 600 °C to a gaseous form, which is why composite curves shows 1.5 - 3 % weight residual in the end of TGA analysis.

[0143] Overall, the weight loss at 0-200 °C was same for all the fabricated membranes that make them viable/stable for various applications with low operating temperatures. [0144] DSC analysis was carried out to examine thermal properties of pristine Pll and composite nanofibers membranes at different concentration and immersion time as illustrated in Figure 10a-b. Figure 10a shows the DSC thermograms of pristine Pll and composite nanofibers membranes, and figure 10b shows the amplified peaks:

• PU base (1047);

• PU with PDMS 5% immersed for 30 min (1046);

• PU with PDMS 5% immersed for 60 min (1045);

• PU with PDMS 10% immersed for 30 min (1044);

• PU with PDMS 10% immersed for 60 min (1043);

• PU with PDMS 15% immersed for 30 min (1042); and

• PU with PDMS 15% immersed for 60 min (1041).

[0145] The glass transition (T _g ) and melting temperatures (T _m ) of the neat PU, the and composite nanofibers membranes are shown in Table 4 below. The glass transition temperature of pristine PDMS ranged between -124 and -126 °C, as reported in literature values [49], The incorporation of PDMS into PU was also found to result in a decrease in T _g values of the composite than T _g of the neat PU (12.86 °C). It can be seen from Figure 10b the slight shift in curves and change in intensity confirms the effect of PDMS. In addition, with an increase of PDMS content T _g of the composite nanofiber membrane decreased to lower values. In the case of T _m , only small deflections in curves are observed. The melting characteristics peaks almost showing similar intensities with slight shift, with increasing PDMS content. These results also propose that the crystallization of the resulting composite membranes decreases due to good phase mixing of the chemically bonding between urethane and silane linkage of PU and PDMS respectively [50],

[0146] Table 4: DSC results (i.e. , T _g and T _m ) of the pristine PU and composite nanofibers membranes

Conclusion [0147] In conclusion, the present disclosure describes a method for preparing a transparent, hydrophobic, and mechanically robust composite nanofibrous membranes using a solution impregnation technique.

[0148] The obtained PDMS composites have good mechanical properties, thermal stability, and flexibility. The results indicate small but significant increase in transparency of PDMS composite nanofiber membranes with an increase of polymer concertation and immersion time. TGA results proved the thermal stability of composite in between 0-200 °C. DSC analysis showed that T _g of as prepared PDMS composites decreased which confirms the successful formation of composites. These membranes were characterized in terms of their optical transparency, mechanical properties, wettability, and WVTR. The technique to prepare optically transparent nanofibrous membranes has been proposed in this work. However, this work provides opportunities to develop of transparent, breathable and high-performance nanofiber-based membranes.

[0149] The obtained PMMA composites with a significant increase in transparency, but with some loss of mechanical flexibility and strength.

[0150] Although the present invention has been described and illustrated with respect to preferred embodiments and preferred uses thereof, it is not to be so limited since modifications and changes can be made therein which are within the full, intended scope of the invention as understood by those skilled in the art.

Bibliography

1. C Wang, N Meng, AA Babar, et al. (2022) ACS Nano 16: 119. Doi: 10.1021 /acsnano.1 C09055

2. R Xia, CJ Brabec, H-L Yip, Y Cao (2019) Joule 3: 2241. Doi: https://doi.Org/10.1016/j.joule.2019.06.016

3. W-R Huang, Z He, J-L Wang, J-W Liu, S-H Yu (2019) iScience 12: 333. Doi: https://doi.Org/10.1016/j.isci.2019.01.014

4. H Ren, L Zheng, G Wang, et al. (2019) ACS Nano 13: 5541. Doi:

10.1021/acsnano.9b00395

5. L Liu, HY Li, YJ Fan, et al. (2019) 15: 1900755. Doi: https://d0i.0rg/l 0.1002/smll.201900755

6. K Shikinaka, K Aizawa, N Fujii, et al. (2010) Langmuir 26: 12493. Doi: 10.1021/la101496b F Mirri, AWK Ma, TT Hsu, et al. (2012) ACS Nano 6: 9737. Doi: 10.1021/nn303201g Z Liu, Y Xie, J Zhao, et al. (2018) Thin Solid Films 657: 24. Doi: https://doi.Org/10.1016/j . tsf .2018.05.005 S Bae, H Kim, Y Lee, et al. (2010) Nature nanotechnology 5: 574. Doi: 10.1038/nnano.2010.132 Z Ding, Y Zhu, C Branford-White, et al. (2014) Materials Letters 128: 310. Doi: https://d0i.0rg/l 0.1016/j.matlet.2O14.04.165 A Greiner, JH Wendorff (2007) 46: 5670. Doi: https://d0i.0rg/l 0.1002/anie.200604646 TA Arica, M Guzelgulgen, AA Yildiz, MM Demir (2021) Materials Science and Engineering: C 120: 111720. Doi: https://doi.Org/10.1016/j.msec.2020.111720 H Yano, J Sugiyama, AN Nakagaito, et al. (2005) 17: 153. Doi: https://d0i.0rg/l 0.1002/adma.200400597 Y Xiao, H Luo, R Tang, J Hou (2021) Polymers (Basel) 13: 506. Doi: 10.3390/polym13040506 L Du, Y Zhang, X Li, et al. (2020) 137: 48657. Doi: https://d0i.0rg/l 0.1002/app.48657 F Li, Q Li, H Kim (2013) Applied Surface Science 276: 390. Doi: https://d0i.0rg/l 0.1016/j.apsusc.2O13.03.103 C Wang, J Zhao, L Liu, et al. (2021) Engineering. Doi: 10.1016/j.eng.2021.02.018 N Kurokawa, A Hotta (2018) Polymer 153: 214. Doi: https://d0i.0rg/l 0.1016/j.polymer.2018.08.018 Z Xu, M Wu, W Gao, H Bai (2020) 32: 2002695. Doi: https://d0i.0rg/l 0.1002/adma.202002695 Y Yao, J Tao, J Zou, et al. (2016) Energy & Environmental Science 9: 2278. Doi: 10.1039/C6EE01011C J Xiong, W Shao, L Wang, et al. (2022) Journal of Colloid and Interface Science 607: 711. Doi: https://doi.Org/10.1016/j.jcis.2021.09.040 F Zuo, S Zhang, H Liu, et al. (2017) 13: 1702139. Doi: https://d0i.0rg/l 0.1002/smll.201702139 N Pan, J Qin, P Feng, B Song (2019) Nanoscale 11 : 13521. Doi: 10.1039/C9NR02810B C Liu, P-C Hsu, H-W Lee, et al. (2015) Nature Communications 6: 6205. Doi: 10.1038/ncomms7205 S Zhang, H Liu, N Tang, J Ge, J Yu, B Ding (2019) Nature Communications 10: 1458. Doi: 10.1038/s41467-019-09444-y K Ma, Y Qiu, Y Fu, Q-Q Ni (2018) Journal of Materials Science 53: 10617. Doi: 10.1007/sl 0853-018-2241-4 AA Shah, YH Cho, S-E Nam, A Park, Y-l Park, H Park (2020) Journal of Industrial and Engineering Chemistry 86: 90. Doi: https://doi.Org/10.1016/j.jiec.2020.02.016 C Tang, HJCPAAS Liu, Composites: Part A 39 (2008) 1638-1643 M Wu, Y Wu, Z Liu, H Liu (2012) Journal of Composite Materials 46 (21) 2731. Doi: 10.1177/0021998311431995 H-W Chiu, RA Mayr, L Yean (2012) US Patent 8,270,089 B2. B Li, S Pan, H Yuan, Y Zhang (2016) Composites Part A: Applied Science and Manufacturing 90: 380. Doi: https://doi.Org/10.1016/j.compositesa.2016.07.024 1 Minin, O Minin, G Shuvalov (2021) Dielectric Wavelength-Scaled Metalenses Based on an Anomalous Apodization Effect for Photoconductive Optical-to- Terahertz Switches. K Raman, TRS Murthy, GM Hegde (2011) Physics Procedia 19: 146. Doi: https://doi.Org/10.1016/j.phpro.2011 .06.139 THN Dinh, E Martincic, E Dufour-Gergam, PY Joubert (2017) Journal of Sensors 2017: 8235729. Doi: 10.1155/2017/8235729 B Briscoe, L Fiori, E Pelillo (1998) Journal of Physics D-applied Physics - J PHYS- D-APPL PHYS 31 : 2395. Doi: 10.1088/0022-3727/31/19/006 C Tang, M Wu, Y Wu, H Liu (2011) Composites Part A: Applied Science and Manufacturing 42: 1100. Doi: https://doi.Org/10.1016/j.compositesa.2011.04.015 M Tian, Y Gao, Y Liu, et al. (2007) Polymer 48: 2720. Doi: 10.1016/j.polymer.2007.03.032 N Nuraje, WS Khan, Y Lei, M Ceylan, R Asmatulu (2013) Journal of Materials Chemistry A 1 : 1929. Doi: 10.1039/C2TA00189F M Aliofkhazraei (Ed.). (2015) Wetting and Wettability, BoD-Books on Demand. 1 Sas, RE Gorga, JA Joines, KA Thoney (2012) Journal of Polymer Science Part B: Polymer Physics 50: 824. Doi: https://doi.org/10.1002/polb.23070 A B D Cassie, S Baxter (1944) Transactions of the Faraday Society 40: 546. II Cengiz, C Elif Cansoy (2015) Applied Surface Science 335: 99. Doi: https://doi.Org/10.1016/j.apsusc.2O15.02.033 Z Pan, F Cheng, B Zhao (2017) Polymers (Basel) 9. Doi: 10.3390/polym9120725 PA Tran, TJ Webster (2013) Int J Nanomedicine 8: 2001. Doi: 10.2147/ijn.S42970 R Bartali, E Morganti, L Lorenzelli, et al. (2017) Micro & Nano Letters 12. Doi: 10.1049/mnl.2017.0230 Z Song, H Xiao, Y Zhao (2014) Carbohydrate Polymers 111 : 442. Doi: https://d0i.0rg/l 0.1016/j.carbpol.2014.04.049 PK Behera, P Mondal, NK Singha (2018) Polymer Chemistry 9: 4205. Doi: 10.1039/C8PY00549D M Marlina, S Jabar, R Rahmi, S Saleha, S Nurman (2017) Oriental Journal of Chemistry 33: 199. Doi: 10.13005/ojc/330122 T Dollase, HW Spiess, M Gottlieb, R Yerushalmi-Rozen (2002) Europhysics Letters (EPL) 60: 390. Doi: 10.1209/epl/i2002-00276-4 F Joki-Korpela, T Pakkanen (2011) European Polymer Journal 47: 1694. Doi: 10.1016/j.eurpolymj.2011.06.006 J N Lee, C Park and G M Whitesides (2003) Anal. Chem., 75, 6544-6554