Introduction

Membrane technology is one of the emerging technologies that the world has witnessed during the last few decades [1]. The reason for this emergence is certain advantages of membrane systems over their conventional counterparts. Some of these benefits are simplicity [2], compactness, low energy consumption, easy to up and down scale [3], economical and compatible with the environment regulations [4, 5]. Broadly, membranes are classified as biological and synthetic [6]. Among them synthetic membranes have been the focus of researchers due to their tunable properties. Synthetic membranes can be organic (polymeric) or inorganic (ceramic) in nature.

Polymeric membranes are less costly and offer better control of desirable properties as compared to inorganic membranes [7]. Furthermore, their film formation is easy and simple, and they can selectively transfer the chemical species. The increase in their popularity is due to the wide range of applications. They have been extensively applied in food, chemical, pharmaceutical [8], gas separations [9], aromatherapy [10], membrane distillation [11], drug delivery [12], membrane bioreactors (MBR) [13], biomedical applications ranging from artificial cells to active surfaces [14] and countless others. Various polymers for instance cellulose [15], poly ether sulfone [16], polyamide [17], polymethyl methacrylate (PMMA) [18], polyvinyl pyrrolidone (PVP) [19], poly 2-hydroxy ethyl methacrylate (HEMA) [20] etc. are exploited to use in different areas. Unfortunately, some disadvantages are associated with the polymeric membranes that limit their application like prone to fouling, low flux, poor mechanical strength, low thermal and barrier characteristics [7].

Poly vinyl alcohol (PVA) is a synthetic and water soluble polymer [21]. It is biocompatible, biodegradable, nontoxic, noncarcinogenic and inexpensive polymer [22]. Owing to its excellent film forming ability, less susceptibility to fouling, thermal confrontation, transparency, processability and capability to retain enough water [23], it has found applications in contact lenses [24], drug delivery [25], superabsorbent [26], artificial organs [27], gas separations [28], hardening of cement composites [29], food packaging [30], fuel cells [31] etc. Limited hydrophilicity [32] and dimensional instability [33] of PVA restricts its alone usage in few applications. To overcome this issue, it has been blended with natural polymers especially polysaccharides [34]. They have been observed to produce flexible, economical and effective membranes. PVA and Starch (St) have good compatibility [35]. Starch can be obtained from rice, wheat, potato and corn [36]. It is composed of 30% amylose and 70% amylopectin [37], a linear and a highly branched structure, respectively. PVA and starch are cross linked either by physical or chemical methods. Freeze-thawing (FT) cycles and lyophilization are physical crosslinking method but they are time and energy consuming [38]. Instead, chemical agents for example borax [21], carbodiimide [39], boric acid [40], epichlorohydrin [41], genipin [42] etc. are employed. Glutaraldehyde (GA) has been preferred because of its ability to support intermolecular reaction with PVA and absence of any thermal treatment required to initiative the reaction [43]. The objective of crosslinking is to enhance the mechanical stability of polymeric structure. To alter the physical properties of the membrane, plasticizers are incorporated to induce elasticity and flexibility [44]. They are nontoxic and have low molecular weight [45]. Commonly used plasticizers are poly ethylene glycol (PEG), glycerol, sorbitol etc.[46].

Inorganic compounds have upright mechanical properties, are more resistant to chemical and thermal attacks and can tolerate extreme conditions (pH, corrosive solvents, temperature etc.) [47]. Inorganic, environment friendly and non-toxic graphitic carbon nitride (g-C3N4) has been the subject of keenness. Its high thermal and chemical stability, lamellar structure, good mechanical strength and low manufacturing cost render it the most suitable filler for the polymeric membranes [48]. Other inorganic fillers such as zeolite immedazolite framework (ZIF) [49], Fe3O4 [50], silica [51], carbon nanotubes [52] etc. are reported in the literature.

Keeping this in mind, an organic–inorganic hybrid membrane was fabricated by the solution casting technique. PVA/St entailed the polymeric matrix and g-C3N4 was incorporated as reinforcement. g-C3N4 was prepared by thermal condensation technique, using melamine as a precursor. Fourier Transform Infrared Spectroscopy (FTIR) was conducted to investigate the interaction of functional groups. Scanning Electron Microscopy (SEM) provided insight of the morphology of membranes. Phase identification was done by X-ray diffraction (XRD). Physical properties including tensile testing, moisture retention, equilibrium swelling ratio (ESR), water vapor transmission rate (WVTR), porosity and oxygen permeability were evaluated. The crosslinking efficiency was evaluated by gel fraction.

Methods and Materials

Materials

Poly (vinyl alcohol) (PVA, degree of polymerization = 1500, average molecular weight = 30,000–70,000 Da and 87–90% hydrolyzed), starch, glutaraldehyde (50% aqueous solution), ethanol (97 wt%), hydrochloric acid (31% aqueous solution) and glycerin were supplied by Daejung Korea. Sulfuric acid (95–97%) was purchased from Scharlau Spain. Sodium thiosulfate (Na2S2O3) and Sodium Hydroxide (NaOH) were obtained from Merck Germany. Potassium Iodide (KI), Sodium Chloride (NaCl) and Magnesium Chloride (MgCl2) were acquired from Ridel-de-Haen. Melamine (99%), Manganese Sulfate and Sodium Azide were bought from Sigma Aldrich USA. All the reagents were of analytical grade and used as received. Distilled water was used throughout for synthesis purpose.

Synthesis of g-C3N4

The bulk g-C3N4 was synthesized by the method reported in literature [53]. Briefly, 3 g of melamine was placed in alumina crucible and was heated to 550 °C at a heating rate of 5 °C/min in a box furnace for 4 h. Then, it was allowed to cool to ambient temperature (30 °C). After calcination, the yellow colored cluster was obtained that was ground to fine powder in pestle and mortar. The thermal exfoliation was carried out by heating bulk g-C3N4 to 500 °C at heating rate of 10 °C/min Again it was cooled to room temperature. The obtained light yellow powered was dispersed in water and ethanol solution in 3:1, and ultrasonicated for 6 h. The resulting mixture was centrifuged and dried.

Fabrication of Membranes

The membranes were prepared by the method adopted from Awais Hassan et al. with slight modifications [54]. 5 g PVA was dissolved in 35 ml of water with continuous stirring at 90 °C for 3 h. Furthermore, 3.5 g starch was dissolved in 35 ml water under continuous stirring at 110 °C for 20 min. Starch solution was cooled and then mixed with PVA. Different amount of g-C3N4 were dispersed in 10 ml water by carrying out sonication for 40 min. g-C3N4 suspension was then poured into homogenous polymer (PVA/St) solution. To prepare GA crosslinking reagent, 0.5 ml GA was mixed with 0.05 ml HCl and 10 ml ethanol. Then it was added to solution with continual stirring. Finally, 2 ml of glycerin was added and stirred for another 30 min to ensure proper mixing. The resultant solution was then casted into glass petri dishes and were left atmospheric drying for 48 h. The membranes were removed from petri dishes and stored in air tight bags.

During the cross linking of polyvinyl alcohol with glutaraldehyde, inter and intra molecular forces of hydrogen bonding takes place that would lead to acetal formation. A condensation reaction between PVA and GA takes place with the elimination reaction of small molecules i.e. (H2O) as shown in Fig. 1 [55].

Fig. 1
figure 1

Schematic depiction illustrating the synthetic route of PVA-Starch-g-C3N4 nano composites membrane

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The glutaraldehyde (GA) molecules linked the starch chains, during the cross linking the C=O double bonds are opened up and the linkages occurred between the reactive C-6 hydroxyl groups of starch and the glutaraldehyde hydrated regions as shown in Fig. 1.

The addition of PVA to the starch cross linked by glutaraldehyde, the abundant –OH groups of PVA are linked in conjugation through predominant hydrogen bonding with the –OH groups present in Starch and as a result the crosslinking occurs between PVA and starch [56].

The nanofiller Graphitic carbon nitride (g-C3N4) surface functional groups (–NH2, –NH etc.), forms H-bonding with –OH groups of starch as well as PVA –OH groups, thus interactions of starch and PVA with g-C3N4 occurs as a result of strong intermolecular forces of hydrogen bonding as shown in Fig. 2. The predominant interaction between nano filler and polymer matrix is hydrogen bonding, besides which Van der waals forces and dipole- dipole interactions are also present between these polar molecules of polyvinyl alcohol, starch and graphitic carbon nitride. These interactions lead to a good dispersion of nano filler in starch and PVA matrix during membrane fabrication [57, 58].

Fig. 2
figure 2

Cross linked membrane of PVA, Starch and g-C3N4 via GA through hydrogen bonding

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X-ray Diffraction (XRD)

The X-ray Diffraction (STOE-Germany) was used to investigate the crystalline nature of g-C3N4 and membranes. The voltage and current of X-ray source were 40 kV and 40 mA, respectively. The samples were scanned at a step size of 0.04, step time of 0.5 s/step and 2θ ranged from 10° to 45°. The wavelength of CuKα radiation was 1.540 Å. The d-spacing was evaluated by Bragg’s law [59]:

$$d = \frac{n\lambda }{{2{\text{ sin }}\theta }}$$
(1)

where n is order of reflection, \(\lambda\) is wavelength of X-rays, d is the characteristic spacing between crystal planes of a given specimen and \(\theta\) is the angle formed between incident beam and that normal to the lattice plane.

Fourier Transform Infrared Spectroscopy (FTIR)

FTIR was used to study the interfacial interactions in the membranes employing Attenuated Total Reflectance, Fourier Transform Infrared Spectroscopy (ATR-FTIR, BRUKER). The analysis was carried out in the range of 4000–500 cm−1 at resolution of 4 cm−1 and scanning frequency of 32.

Scanning Electron Microscopy (SEM)

The morphology of g-C3N4 and membranes were recognized by SEM (JSM-64900). Prior to analysis, a thin conductive layer of palladium/platinum was used to cover the samples. The cross-section of membranes was also analyzed.

Thermal Gravimetric Analysis (TGA)

Thermal Gravimetric Analysis of g-C3N4 and nanocomposite membranes was carried out in the temperature range of 20–700 °C in a nitrogen atmosphere at a gas flow of 50 ml/min.

Mechanical Properties

To analyze the mechanical properties of prepared membranes, tensile strength (TS) and elongation at break (EB) were measured. Trapezium-X Universal Testing Machine (AG-20RRKNXD Plus) manufactured by Shimadzu corporation was used for this purpose. According to ASTM D882 standard, the membranes were cut into rectangular pieces of 60 mm × 10 mm dimension. The test was conducted at a crosshead speed of 10 mm/min under 20 kN load cell. The thickness and gauge length of the samples was 0.63 mm ± 0.05 mm and 50 mm, respectively. All studies were carried out five times.

Gel Fraction

To carry out gel fraction (GF) study, the membranes were cut into 10 × 10 mm2. After weighing them initially, all samples were placed in oven until a constant weight was acquired. The weight was recorded as W1. Then, they were immersed in distilled water for 4 consecutive days. After 4 days, the samples were taken out and again placed in oven until a constant weight was attained. This weight was recorded as W2. The GF was calculated by the formula [60]:

$${\text{Gel fraction }}\left( \% \right) \, = \frac{{W_{2} }}{{W_{1} }} \times 100$$
(2)

The test was performed in triplet to minimize error.

Moisture Retention Ability

The specimen was cut into equal size and weighed initially, denoted as W1. The membranes were placed in oven at 40 °C for 6 h. After the mentioned time, the samples were taken out of the oven and were weighed again, denoted as W2. The moisture retention was calculated by the formula [61]:

$${\text{Moisture retention ability }}\left( \% \right) \, = \frac{{W_{2} }}{{W_{1} }} \times 100$$
(3)

Water Vapor Transmission Rate (WVTR)

WVTR studies were performed according to European Pharmacopeia (EP) standard [22]. The round bottles of known diameter were filled with 10 ml deionized water and samples were fixed onto them. To ensure accuracy, the mouth was sealed by means of Teflon tape. The initial weight was recorded as W1. Then, they were placed in oven at 40 °C for 24 h (t). After 24 h, the bottle was taken out and its weight was recorded as W2. The open and closed bottles served as negative and positive control, respectively. WVTR was found by following formula:

$${\text{WVTR }}\left( {{\text{g}}/{\text{m}}^{{2}} {\text{h}}} \right) \, = \frac{{(W_{1 } - W_{2 )} }}{A x t}$$
(4)

where A is the permeation area of membranes, m2.

Swelling Behavior

To estimate the swelling ratio, the membranes were cut into equal size and their initial dried weight was noted as Wd. Then, the specimen was immersed in water, blood, saline and 0.9% MgCl2. The pH of these solutions was measured. The samples were taken out after regular interval at 24 h, 48 h, 72 h and 96 h. Their weight was noted as Ws. Swelling ratio was then calculated as [62]:

$${\text{Swelling ratio }}\left( \% \right) \, = \frac{{(W_{s} - W_{d} ) }}{{W_{d} }} \times 100$$
(5)

Porosity

The density and porosity of the prepared membranes was evaluated by Archimedes Rule [63]. Ethanol was used as a displacement fluid. Briefly, a container with known volume of ethanol was weighed (W1). Then, the sample was weighed (Ws) and immersed in the fluid. After complete saturation of the sample, the container was weighed (W2) again. Afterwards, the sample was removed from container with care and container was weighed (W3). Following equations were used to perform calculations:

$$V_{s} = \frac{{W_{2} - W_{3} - W_{s} }}{{\rho_{h} }}$$
(6)
$$V_{p} = \frac{{W_{1} - W_{2 } + W_{s} }}{{\rho_{h} }}$$
(7)
$$\rho_{s} = \frac{{W_{s } \rho_{h} }}{{W_{1} - W_{2 } + W_{s} }} = \frac{{W_{s} }}{{V_{s} }}$$
(8)
$$\in = \frac{{W_{2} - W_{3} - W_{s} }}{{W_{1} - W_{3} }} = \frac{{V_{s} }}{{V_{p } + V_{s} }}$$
(9)

where Vs is the volume of the sample (ml), Vp is the volume of the pores (ml), ρs is the density of sample (g/ml), ρh is the density of ethanol (g/ml) and \(\in\) is the porosity of the sample.

Oxygen Permeability

The oxygen permeability was carried out by estimating the dissolved oxygen (DO) in the water. The sample membranes were placed on the top of the bottle containing 200 ml deionized water. The mouth of bottles was sealed with Teflon tape. Then, the bottles were placed at room temperature for 24 h. After 24 h the water was analyzed, and dissolved oxygen was determined according to Winkler’s Method [64]. The positive and negative control were an open and a closed container, respectively. The O2 permeability values were expressed as mg/l.

Results and Discussion

X-ray Diffraction (XRD) Analysis

Figure 3a shows the XRD spectrum of g-C3N4. It features two characteristic diffraction peaks. The strong peak found at 27.5° is assigned to the interlayer stacking of conjugated aromatic planes, indexed as (002) with d-spacing of 0.325 nm. In addition, the weak peak at almost 13° (d-spacing = 0.693) is the characteristic of in plane structural packing motif of tri-s-triazine units, indexed as (100) [65, 66]. This indicated the triangular nanopores with the building blocks of tri-s-triazine units. Figure 3b depicts the XRD pattern of fabricated membranes with different formulations. For PVA/St membrane without g-C3N4, only one peak is observed at 19.8° (101) plane of PVA. The intensity of this peak increased with the incorporation of g-C3N4 nanosheets. The increase in crystallinity may be attributed to the dominance of nucleating effect [67]. Furthermore, an important role is played by the strong interfacial interactions between PVA, g-C3N4 and glutaraldehyde, but this effect is less pronounced. The characteristic peak of g-C3N4 was undetectable, even at higher concentrations. This depicts the effective exfoliation and homogenous dispersion of nanosheets into the polymer matrix. The XRD analysis exposed the good compatibility of g-C3N4 with PVA, St and the added crosslinker. As the amount of g-C3N4 was increased to 0.14 g and 0.18 g, the characteristic peak of g-C3N4 appeared at 27.62 that indicates that at the said amount the agglomeration has started, and this is also clear from SEM results. The obtained results were in accordance with the ones reported in literature [66].

Fig. 3
figure 3

XRD Pattern of a g-C3N4 and b PVA/St/GCN nanocomposite membranes

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Fourier Transform Infrared Spectroscopy (FTIR)

The FTIR spectrum of g-C3N4 nanosheets is depicted in Fig. 4a. The peak at 810 cm−1 is related to s-triazine ring mode [68]. The distinctive peaks appearing at 1250–1640 cm−1 was attributed to the C–NH–C and N–C3 stretching vibration modes [69]. A broad peak at 3180 cm−1 is assigned to stretching modes of primary and secondary amines and intermolecular hydrogen bonds.

Fig. 4
figure 4

FTIR spectrum of a g-C3N4 and b PVA/St/GCN nanocomposite membranes

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Figure 4b indicates the FTIR spectrum of nanocomposite membranes. As the formulations were made from the polymers sharing of similar functional groups, the spectra obtained were quite alike. In case of pure PVA/St membrane (without g-C3N4), the peak at 3200 cm−1 confirmed the stretching of hydroxyl groups and peak near 2900 cm−1 specified sp3 hybridized hydrocarbon chains (stretching of –CH groups) [70]. When g-C3N4 was introduced into the polymeric matrix the characteristic peaks of g-C3N4 appeared at 810 cm−1 and 1200–1600 cm−1. The peak appearing at 3270–3280 cm−1 is due to the presence of hydroxyl groups. The addition of g-C3N4 caused a little variation in intensity of stretching of hydroxyl groups. The peak shifting and decrease in relative intensity suggested the formation of strong interfacial interactions between residual –NH or –NH2 of g-C3N4 and –OH groups of PVA and Starch that arises due to the hydrogen bonding. The strong band near 1150 cm−1 appeared as a result of intra and intermolecular interactions of hydroxyl groups. A band at 1095 cm−1 is due to the crosslinking reaction between PVA and glutaraldehyde (C–O–C) group. The absorption band at 1740 cm−1 is attributed to stretching of unreacted aldehyde group present in glutaraldehyde [43].

Scanning Electron Micrographs (SEM)

SEM is a promising technique for topographical analysis of the specimens. It provides valuable information about size and shape of the material used [71]. Figure 5a displays the scanning electron micrograph of g-C3N4 nanosheets. The result was quite similar to graphene like crenelated two-dimensional morphology of g-C3N4 nanosheets. Its ultrafine structure was also confirmed [48].

Fig. 5
figure 5

SEM images of a g-C3N4, surface morphology of PVA/St, b 0.00 g GCN, c 0.02 g GCN, d 0.06 g GCN, e 0.1 g GCN, f 0.14 g GCN, g 0.18 g GCN and cross-section of PVA/St, h 0.00 g GCN, i 0.02 g GCN, j 0.06 g GCN, k 0.1 g GCN, l 0.14 g GCN, m 0.18 g GCN

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The morphology of formulated membranes is depicted in Fig. 5b–g. The surface of the membranes was dense, and no visible pores were found, even at higher magnifications. As anticipated, a smooth and homogenous surface for pure PVA and starch was obtained. This indicated the miscibility of components with each other, as presented in Fig. 3b. Similar results have been reported in literature [72]. The smooth surface of PVA/St/g-C3N4 composite membrane designated that the addition of crosslinker, GA, not only favored the network formation with polymers but also permitted an effective dispersion of g-C3N4. Furthermore, the proper dispersion might be the result of ultrasonication [48]. As the g-C3N4 content was increased, partial agglomeration of nanosheets was seen on the surface of PVA/St/0.14GCN hybrid (Fig. 5f) and it became more noticeable in PV0A/St/0.18GCN (Fig. 5g).

To gain further insight of the membranes, cross-section micrographs are shown in Fig. 5h–m. No pores were found for pure PVA/St blend. Furthermore, smooth morphology is observed. However, PVA/St/ g-C3N4 formulations have shown pores. g-C3N4 was dispersed into the polymetric matrix. The preferential orientation of 2D fillers is parallel to the membrane surface [73] and was observed here too. Moreover, no cracks or defects were observed. The alignment of g-C3N4 was disturbed at higher content. This can be attributed to the surface activation energy. It compelled adjacent nanosheets to restack together into agglomerates [74, 75].

Thermal Gravimetric Analysis (TGA)

Figure 6a illustrates the TGA curve for g-C3N4. The result demonstrated its unique behavior. g-C3N4 is thermally stable up to 620 °C. Therefore, it is suitable to be applied at normal and even high temperatures. According to the studies reported to date, g-C3N4 is the most stable allotrope of carbon nitride [48].

Fig. 6
figure 6

TGA Curve of a g-C3N4 and b PVA/St/GCN nanocomposite membranes

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The TGA for PVA/St and different formulations of PVA/St/GCN is also analyzed and shown in Fig. 6b. In these curves, three different regions can be observed. The initial weight loss at 70–200 °C is because of the solvent release absorbed by the polymer and loss of volatile matter (e.g. water and glycerin). In the subsequent stage, the degradation of starch and PVA occurs. This is chiefly on account of dehydration of hydroxyl groups and followed by the formation of other carbon species. At this stage, oxygen functional groups are removed, and polyene intermediates are formed. Commonly, scission and decomposition of PVA chains occurs at a temperature above 400 °C. A monotonic decrease in weight is observed until the complete degradation of PVA (almost 240 °C). Starch is more thermally stable as compared to PVA, owing to the presence of cyclic hemiacetal structures in it [72]. Carbonization occurs at last stage, from 500 to 700 °C. It is corresponded to the backbone degradation of crosslinked PVA and starch. Studies have been reported indicating the shift of degradation temperature from 250 to 350 °C [38]. It evidences that crosslinking plays a vital role in enhancing the thermal stability. The results thus obtained confirms the thermal stability of nanocomposite membranes up to 280 °C.

Mechanical Properties

One of the key advantages of incorporating inorganic fillers with polymeric membranes are to improve their mechanical properties and this was fully utilized here. The tensile testing further confirmed the interfacial interactions between polymers and g-C3N4. With the increase in g-C3N4 amount from 0.02 to 0.14 g, the tensile strength also enhanced, compared with pure PVA/St membranes. The increase in strength was credited to the homogenous dispersion of g-C3N4 and strong interfacial interactions between g-C3N4 and GA [74, 76]. The improvement in tensile strength was also result of hydrogen bonding. The tensile strength decreased when g-C3N4 was further increased to 0.18 g. This was attributed to the deterioration of lamellar structure due to the accumulation of g-C3N4 nanosheets [76]. It should be worth noticing that the mechanical strength was still much higher than the pure PVA/St membrane. The results are shown in Fig. 7a.

Fig. 7
figure 7

Mechanical properties a tensile strength and b elongation at break of PVA/St/GCN nanocomposite membranes

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The results of elongation at break are shown in Fig. 7b and were accordance to literature [60, 77]. The elongation at break decreased gradually with increasing g-C3N4 content, the results can be recognized as the addition of g-C3N4 into polymeric matrix might accelerate and undermine the break elongation of the membrane. Besides, high g-C3N4 might upsurge the hydrophilic susceptibility and decrease the entanglement degree which resulted in reduction of flexibility. The high crosslinking density was also responsible for this behavior. Furthermore, the hardness and brittleness of the membranes enhanced with the increase in g-C3N4 content. This also leads to the reduction in elongation.

Gel Fraction

Gel fraction (GF) test is performed to quantitatively assess the effectiveness of the crosslinker used. The GF value ranges in 80–86%. There was a slight decrease with the addition of g-C3N4. Figure 8a shows that the gel fraction of pure PVA/St to be almost 86%. This was the maximum value attained. This behavior suggests the entanglement of PVA chains [60]. The decrease in gel fraction values with the addition of g-C3N4 may be due to the reduction in entanglement reactions [78]. The obtained results demonstrated the effectiveness of glutaraldehyde as a crosslinker for PVA and St membranes. Similarly, the crosslinked membranes showed better mechanical and thermal stability [38].

Fig. 8
figure 8

a Gel fraction, b moisture retention ability and c water vapor transmission rate of PVA/St/GCN nanocomposite membranes

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Moisture Retention Ability

The moisture retention ability demonstrates the water loss from the membranes. Moisture retention ability with respect to g-C3N4 content is exhibited in Fig. 8b. No substantial difference in the values was observed. The moisture retention of all the membranes was < 90%. This is credited to the fact that PVA, Starch and g-C3N4 are hydrophilic in nature, thus they can easily combine with the water molecules to form hydrogen bonds [79]. The water gets locked into the robust structure and barely get out of the network. The hydroxyl group (OH) of PVA and St have the ability to absorb and retain considerable amount of moisture [80]. The neutral groups assisted in water retention by contributing in hydrogen bonding [81]. But the higher water retention is not always preferable, all depends on the application. The suggested application of PVA/St/GCN nanocomposite membranes is wound dressings. In that case, high moisture retention values are favored. Because moist bed accelerates the tissue regeneration process [32]. Moreover, in a wet environment the healing of skin takes places without inflammation or eschar formation [82].

Water Vapor Transmission Rate (WVTR)

The WVTR behavior of PVA/St/g-C3N4 films is illustrated in Fig. 8c. It was observed that with the rise in amount of g-C3N4, the WVTR also increases. The negative and positive controls were a closed and an open bottle, respectively. For the open bottle, the WVTR value was 372.47 g/m2 h. But the values reduced to a large extent when the bottle was covered with the membranes. The WVTR of pure PVA/St film is 65.23 g/m2 h. The WVTRs for PVA/St/0.02GCN, PVA/St/0.06GCN, PVA/St/0.1GCN, PVA/St/0.14GCN and PVA/St/0.18GCN were 72.54, 76.811, 80.47, 85.35 and 91.44 g/m2h, respectively. The results suggested that the formulated nanocomposite membranes have ability to act as a barrier in preventing the water loss. This again encourage its usage as wound dressings. The increase in WVTR is ascribed to the introduction of hydrophilic g-C3N4 [83], having more ability to attract moisture. Moreover, the hydrophilic nature of glycerin also enhanced water permeability through the membranes [84] (Table 1).

Table 1 The detailed description for fabrication of PVA/Starch g-C3N4 nanocomposite membranes
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Swelling Behavior

Swelling behavior of the formulated membranes was investigated against distilled water, 0.9% NaCl solution, 0.9% MgCl2 solution and blood. A higher degree of swelling was observed for crosslinked PVA films. The factors effecting the swelling are pH, temperature, swelling environment, nature of polymers and degree of crosslinking [71]. The hydrophilic functional groups of g-C3N4, PVA and Starch were involved in the swelling of the films. The swelling is affected by the pattern of ionization in the buffer solutions and hydrogen bonding between water molecules [85]. As the crosslinking density decreases, swelling percentage increases [86] and this may be due to reduction in entanglement of polymeric chains. The swelling reached its maximum value in water, as shown in Fig. 9a. In NaCl and MgCl2 solutions, the electrostatic repulsion of ionic charges induced the swelling, that blocks the accumulation of polymer chains and tend to expand the network [87]. With the increase in ionic strength, the osmotic pressure between the polymer network and solution decreased, that delayed the penetration of water molecules into the network of polymer chains [86]. This was the reason for reduction in swelling for NaCl and MgCl2, as shown in Fig. 9b and c. In case of blood (Fig. 9d), the swelling reached its maximum value. This may be ascribed to the fact that crosslinking intensify the molecular spacing between the chains and deteriorate the hydrogen bonding [88]. Moreover, an additional osmotic pressure is created due to the ionic nature of blood that enhances the electro-neutrality effect and causes swelling [89].

Fig. 9
figure 9

Swelling behavior of PVA/St/GCN nanocomposite membranes against a water, b 0.9% NaCl, c 0.9% MgCl2, d blood

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Porosity

Porosity more depends on the fabrication process of membrane, rather than the compositions [90]. Archimedes method was selected to determine the density and porosity of membranes, owing to the extensive errors and unsuitable results of other methods [91]. Porosity is the crucial factor as it is responsible for enhancing adsorption and surface area [92]. The rate of solvent diffusion is largely dependent on it. Solvent diffusion is facilitated and aids in interaction with the functional groups present in the network [93]. Furthermore, permeability is also affected by density and porosity. Table 2 enlists the porosity and density of the membranes. For porosity, a decreasing trend is observed with the increase in g-C3N4 content. This may be due to the filling of pores, thus making the network more compact but on the cost of consumption of some hydrogen bonds [93, 94]. The porosity was found in the range of 66–75% and highest percentage was found for PVA/St/0.02GCN.

Table 2 Porosity and oxygen permeability of PVA/St/GCN nanocomposite membranes
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Oxygen Permeability

The obtained results of oxygen permeability are shown in Table 2. The analysis was carried out adopting Winkler’s Method and the amount of dissolved oxygen (DO) is measured in water. A divalent manganese solution was added into the flasks followed by the addition of Sodium Hydroxide (NaOH), a strong base. By doing this, DO rapidly oxidized into equal amount of divalent manganese ions (Mn2+) to a higher valency, manganese dioxide, MnO2 (Mn4+). MnO2 remained in the form of precipices in the solution. Subsequently, solution was acidified by adding Potassium Iodide. As a result, free iodine ions were produced, that were equal to the actual concentration of oxygen present in receipt water. The water was titrated with sodium thiosulfate solution (0.025 N). Eventually, starch solution was used as an indicator. The endpoint was achieved when the solution turned blue [95]. The dissolved oxygen for open and closed flask was found to be 12.60 ± 3.4 mg/l and 6.00 ± 2.8 mg/l, respectively. Usually, DO in water lies in the range 7 to 14.5 mg/l at 35 °C [96]. For the bottles covered with membranes, the oxygen permeability ranged from 6.91 ± 2.5 to 7.92 ± 3.1 mg/l. The obtained results indicated that adequate amount of oxygen could penetrate into the network and provide sufficient supply of oxygen. Thus, these membranes can be suitable for energy production, tissue engineering and cell repair [97].

Conclusions

In this study, a novel PVA/Starch/g-C3N4 nanocomposite membrane was prepared by the solution casting technique. g-C3N4 was synthesized by “thermal oxidation etching process”. For this purpose, melamine was utilized as a precursor. The polymeric membranes with excellent swelling ability and superior mechanical strength were fabricated by incorporating varying amounts of g-C3N4 into the polymeric matrix of PVA and Starch. g-C3N4 improved the mechanical properties of the membranes. Incorporation of g-C3N4 resulted into its parallel alignment that significantly improved its water barrier properties. The hybrid membranes showed remarkable swelling abilities up to 96 h. FTIR analysis confirmed the hydrogen bonding interactions between NH groups of g-C3N4 and OH groups of PVA and starch. The hydrogen bonding resulted into the increase in thermal and mechanical properties. The tensile strength was increased from 11 MPa of pure membrane to 30 MPa for PVA/St/0.14 GCN. The membranes were enough porous to facilitate the adsorption process. The high moisture retention up to 95% was credited to hydrophilicity of PVA, starch and g-C3N4. The maximum gel fraction value achieved was 86% that validate the efficiency of crosslinker. Also, it confirms the strong interfacial interactions among PVA, glutaraldehyde and g-C3N4. Additionally, the membranes were also permeable to oxygen. The obtained results suggested that g-C3N4 has the potential to be utilized as a versatile filler into polymeric membranes. Moreover, these membranes could also be used as wound dressings [22].