Electrospinning process control for fiber-structured poly(Bisphenol A-co-Epichlorohydrin) membrane

1.   Introduction

Thermoplastic is extensively used in many applications such as consumer products, biomedical materials, chemical sensors, filtration and separation, data storage and transmission, energy materials, and in the manufacturing process ^[1,2,3,4]. In addition to the ease of processing, mechanical performance is an important factor for thermoplastic utilization. Adding fiber materials and forming fiber composite thermoplastics to improve mechanical properties is a widely applied and well-documented practice ^{[5,6,7,8,9,10]}. Interestingly, recently it was shown that fiber-structured thermoplastics demonstrated significant improvement in mechanical properties compared with the same material having a non-fiber structure. For example, studies of Alexander and Wanasekara showed that the polypropylene PP fibers had tensile strength as high as 200 MPa ^[5,6]. Ye demonstrated that the polyethylene PE fibers had tensile strength in a range of 26 MPa–3.3 GPa, depending upon the production process, while non-fiber-structured, injection-molded, high-density polyethylene HDPE had a tensile strength of less than 50 MPa ^[6].

Electrospinning (ES) is an efficient and versatile process to fabricate highly porous membranes ^[11]. It is a straightforward technique to create continuous fibers with diameters ranging from nano- to micrometers. In this process, a high electric potential is applied to a spinneret (needle), which is connected to a syringe that holds a polymer solution. The polymer solution is fed by a syringe pump to be spun in the applied electric field. During the spinning process, the charged polymer solution is elongated to fiber-like jets and then collected on a grounded collector. The ES process has been studied for many applications, such as biomaterials, batteries, capacitors, catalysts, and filtration systems ^{[12,13,14,15,16]}. Several thermoplastic materials have been deployed to create various ES membranes. A summary of the ES-thermoplastics, solvent system, and applications are shown in Table 1.

Table 1.  A summary of electrospun thermoplastic materials and their applications.

Material	Solvent	Fiber diameter (μm)	Applications
Polyvinylidene difluoride PVdF [17,18]	N, N–dimethylacetamide DMAc	0.40–0.51	Battery separator; polymer electrolyte
	DMAc/acetone	0.38	Microfiltration membrane
	N, N–dimethylformamide DMF	0.50	Membrane distillation
Polyacrylonitrile PAN [19]	DMF	0.75	Ultrafiltration membrane
Polyethylene oxide PEO [20]	Water	0.27–0.40	Biomedical applications; solid-polymer electrolyte
Polyvinyl alcohol PVA [21]	Water	0.08–0.24	Biomedical applications; filtration membrane
Polypropylene PP [22]	Decalin	0.80	Battery separator; filtration membrane; protective clothing
PAN/PVA [18]	DMF (PAN); water (PVA)	0.15	Ultrafiltration membrane
Polyamide-6 PA-6 [23]	Formic acid	0.17	Air filtration membrane
Polyimide PI [23]	DMF	0.30	High temperature resistance, air filtration membrane

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Poly(bisphenol-A-co-epichlorohydrin) PBE, or phenoxy resin, is a copolymer of bisphenol-A with epichlorohydrin and is a widely-used thermoplastic resin. It is ductile, tough, and miscible with various polymers, due to the presence of a pendant hydroxyl group (proton donor with appropriate functional groups) ^{[24,25,26,27]}. It has been shown to increase the glass-transition temperature of functional polymer blends, such as in thermal-responsive, shape-memory applications ^{[25,26,27,28,29]}. It has been used to increase the tensile strength, elongation, elastic modulus, and flexural strength in polymer blends ^{[30,31,32,33]}. The PBE composites, and the blends with carbon nanotubes, have shown an improvement in storage modulus ^[34,35]. PBE is also used for increasing the fracture toughness as a polymer matrix and a modifier for cryogenic applications ^[36,37]. The PBE matrix exhibits significant dispersion of organic modified red mud and enhanced overall thermal stability of organic-inorganic composites ^[38]. In addition to being a vital component in polymer blends and composites, the PBE-based materials were used as coating agents and binders to improve interfacial adhesion of carbon–fiber-reinforced thermoplastic composites and have gained interest as high-performance composites for aerospace applications ^[39,40,41].

In this study, PBE was selected as a model thermoplastic, and a PBE-porous membrane was created using the ES process. Various microstructures with different densities, elastic modulus, and tensile strength were obtained, by adjusting the concentration of the polymer solution, the solvent system, and acceleration potential. Membrane morphology and mechanical and thermal properties were studied, using scanning electron microscopy (SEM), tensile testing, and thermogravimetric analysis (TGA), respectively. This study shows that a consistent fiber structure resulted in higher Young's Modulus and tensile strength, compared with those of other morphologies. This study also identifies the process parameter ranges that reliably resulted in consistent fiber morphology in PBE material.

2.   Materials and methods

2.1.   Materials

Poly(bisphenol A-co-epichlorohydrin) (PBE, Mw = 40000 g/mol, T_g ~ 108 ℃ ^[42]), acetone (laboratory standard) and N, N–dimethylformamide (DMF, anhydrous 99.8%) were purchased from Sigma–Aldrich and used as received.

2.2.   Electrospinning experiments

The study was conducted in two stages. In stage one, the goal was to identify a suitable solvent composition. Variable acetone:DMF (%, v/v) ratios were studied, in order to identify a desirable solvent ratio for creating consistent fiber structures. Acetone:DMF (%, v/v) ratios were varied from 0:100 to 80:20, with an incremental step of a 10% increase in acetone and a 10% decrease in DMF for each data point. DMF was gradually added to the acetone and stirred, until forming a homogeneous solution. Then 0.200 g/mL of PBE was dissolved in these acetone:DMF-solvent solutions for the ES experiments. The obtained polymer solution was loaded into a 10 mL syringe, equipped with a stainless-steel gauge 18 blunt-tip needle (0.838 mm, inner diameter). The polymer solution was fed at a feeding rate of 0.3 mL/h. An acceleration potential of 25 kV was applied at the needle-tip against the collector. A stationary aluminum foil collector was set up at 15 cm (tip-to-collector distance) from the needle tip. Electrospinning experiments were conducted at 23–25 ℃ and 50–55% relative humidity.

In stage two, after a suitable acetone:DMF ratio was identified, different PBE concentrations were investigated, including 0.125, 0.150, 0.175, 0.200, 0.225, and 0.250 g/mL, at two different acceleration potentials, 20 kV and 25 kV.

2.3.   Characterizations

Surface micrographs were obtained by a field emission scanning electron microscope (FE-SEM, JEOL 7000F), operated at 2 kV. Average density of the membrane spun with each solution concentration was calculated by the weight and volume of three ES samples with an identical diameter of 7/32 inch (0.56 cm). Mechanical properties were measured by a universal testing machine (UTM, Instron 5567A) with a cross-head speed of 2 mm/min. The samples that were used to perform mechanical property measurements were prepared following the previously reported electrospinning studies ^{[43,44,45,46]}. Each ES membrane was cut into a strip of 0.25 inches (0.63 cm) width, 1 inch (2.54 cm) length, with 0.5 inches (1.27 cm) gauge-length and approximate 65 μm thickness. Each ES-sample was obtained after 40 min. of electrospinning. After drying, the thickness of the deposition was ≥65 μm in the thickest portion. A micrometer was used to measure the thickness at different locations in each sample. The portion that had a thickness of approximately 65 μm was chosen and cut into strips for testing. At least three ES samples were measured for each data point. The samples were also analyzed by thermogravimetric analysis (TGA, TA Instrument Q50) in air, in a temperature range of 35–700 ℃, at a heating rate of 10 ℃/min.

3.   Results and discussion

3.1.   Solvent composition

The use of the acetone and DMF solvent system in this study was informed by the established understanding of the vapor pressure effect. The boiling point and vapor pressures are 56 ℃ for acetone vs. 153 ℃ for DMF, and 30.79 kPa for acetone vs. 0.49 kPa for DMF, at 25 ℃, respectively ^[47,48]. Acetone has a relatively high vapor pressure, at 25 ℃. Hence, the acetone evaporated quickly, which was expected to assist in obtaining thin fibers. The DMF, on the other hand, evaporated slowly, which was anticipated to provide flexibility and allow the spun jets to elongate ^[49,50,51]. The rheology of the acetone and DMF binary mixture was thoroughly studied ^[52,53]. Previous research has investigated the effects of vapor pressure, boiling point, and viscosity of the various binary solvents on morphology ^{[49,51,54,55,56,57]}. As the solution jets were spun in the electric field, the dielectric constant was suspected to be a factor in producing a membrane with the desired morphology. Hence, dielectric constants of the acetone and DMF solvents were provided in this report.

Different acetone:DMF (%, v/v) ratios were studied to identify a suitable solvent ratio for consistent fiber structures. It was found that a minimum of 20% DMF was needed to fully dissolve 0.200 g/mL of PBE and form a homogeneous solution. Vapor pressure of the mixed solvent at each acetone:DMF ratio was the summation of the partial vapor pressure of acetone and DMF, and calculated by Eq 1 ^[58]. The dielectric constant of the solvent changes as a function of the composition. O. Kolling experimentally determined and calculated the dielectric constants of the acetone:DMF system at 25 ℃ by Eq 2, where X_Acetone is the mole fraction of acetone ^[59]:

Using Eq 2, the dielectric constants of the various solvents tested in this study were calculated and are summarized in Table 2. It was observed that a high dielectric constant resulted in a nonporous membrane. Figure 1 shows the typical morphology of ES-PBE membranes with different solvent ratios. Porous structures of the ES-PBE membrane were obtained, with amounts of DMF up to 60% (Figure 1a–c). Fiber structures were obtained up to a dielectric constant ($\epsilon$ of approximately 28 (Table 2). Although a homogeneous solution can be obtained with pure DMF, acetone is required to control the morphology, because acetone evaporates faster than DMF, and the rapid evaporation is needed to reduce the diameter of the spinning jets prior to being collected on the grounded collector. When the amount of acetone was 40–50% (Figure 1d, e), a mixed fiber-bead structure was obtained. When the amount of acetone was less than 40% (Figure 1f–h), there were no fibers observed, and the membrane became nonporous. A consistently fine fiber structure (Figure 1c) was reliably obtained with an acetone:DMF ratio of 60:40 (%, v/v). Thus, this solvent ratio was selected to be used in future experiments, in order to rule out any variation in solvent composition.

Table 2.  Dielectric constant of the mixed solvents at different ratios of acetone:DMF.

Acetone:DMF % (v/v)	Moles of solvent (mol)		Mole fraction of acetone	Vapor pressure (kPa)	Dielectric constant	Membrane structure
	Acetone	DMF
80:20	0.103	0.031	0.769	23.779	24.400	Fiber with large diameter distribution
70:30	0.088	0.045	0.660	20.476	26.143
60:40	0.073	0.059	0.555	17.298	27.820	Fiber with small diameter distribution
50:50	0.060	0.072	0.454	14.237	29.435	Fiber-bead
40:60	0.047	0.084	0.356	11.288	30.992
30:70	0.034	0.096	0.263	8.444	32.493	Nonporous membrane; process turned to electrospray
20:80	0.022	0.108	0.172	5.699	33.941
10:90	0.011	0.119	0.084	3.050	35.339
0:100	0.000	0.129	0.000	0.490	36.690

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Figure 1.  Effect of solvent composition of acetone:DMF (%, v/v) to morphology of the ES-PBE membrane: (a) 80:20; (b) 70:30; (c) 60:40; (d) 50:50; (e) 40:60; (f) 30:70; (g) 20:80; and (h) 10:90 (700× magnification, scale bar corresponds to 10 μm).

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3.2.   PBE concentration and acceleration potential

The solvent ratio of 60:40 (%, v/v) acetone:DMF was used in experiments with different concentrations and acceleration potentials. Concentrations ranging from 0.125 to 0.250 g/mL were tested under two acceleration potentials, 20 kV and 25 kV. At low concentrations, ranging from 0.125 to 0.175 g/mL (Figure 2a–c and 2a'–c'), a mixed fiber-bead structure was observed with beads dominant at both acceleration potentials. When the concentrations were increased, this structure gradually transitioned to a fiber-dominant structure. The fiber structure was obtained at concentrations of 0.200–0.250 g/mL (Figure 2d–f and 2d'–f') at both acceleration potentials. At the concentration of 0.200 g/mL, fine and consistent fiber structure was reliably obtained (Figure 2d, d'). At the higher concentration of 0.250 g/mL (Figure 2f, f'), thicker fibers were obtained. At the concentration of 0.125 g/mL, a powder-like deposition was formed on the collector.

Figure 2.  Effects of the concentration and the acceleration potential at 20 kV (a–f) and 25 kV (a'–f') on the morphology: (a) and (a') 0.125 g/mL; (b) and (b') 0.150 g/mL; (c) and (c') 0.175 g/mL; (d) and (d') 0.200 g/mL; (e) and (e') 0.225 g/mL; (f) and (f') 0.250 g/mL (3000× magnification, scale bar corresponds to 1 μm).

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It is known that the morphology of the ES membrane may be affected by the acceleration potential. The high acceleration potentials (20 kV and 25 kV) in this study were chosen based on previous studies ^[21,60,61]. A high-acceleration potential results in a stronger electric field. Hence, the charged solution jets, at high acceleration potential, move faster and produce fibers with a smaller diameter than at the low acceleration potential ^[61]. The distribution of the fiber diameter (Figure 2d–f and 2d'–f') was measured from 50 different fibers in each SEM figure. The membranes created from the solution concentrations of 0.225 g/mL and 0.250 g/mL had either a wide distribution or showed a bimodal distribution of the fiber diameter. In addition, average fiber diameter expanded with an increase in solution concentration. Similar observations were reported in other studies of electrospun thermoplastics ^[19,21,61]. The average fiber diameters created from the solution concentration of 0.200 g/mL were 0.49 ± 0.14 μm (spun at 20 kV, Figure 2d) and 0.29 ± 0.02 μm (spun at 25 kV, Figure 2d'). An acceleration potential of 20 kV enables better control over the total deposition thickness, due to a slower deposition rate, as compared with that of 25 kV. In this study, 20 kV was used in subsequent experiments.

3.3.   TGA study of ES-PBE membrane

Figure 3 shows the TGA thermogram of the ES-PBE membranes and pellets. The ES-PBE membranes did not show significant weight change until 370 ℃ (initial decomposition temperature). Two decomposition steps occurred: the first one at 370–420 ℃ and the second one between 480–650 ℃. This observation is similar to Corres' study on thermal decomposition of the PBE powder under ambient atmosphere ^[27]. Corres et al.'s study shows that the presence of oxygen in an ambient atmosphere affects the degradation behavior and results in two decomposition steps in air, as opposed to a single decomposition step in a nitrogen environment ^[27].

Figure 3.  TGA thermogram illustrating thermal decomposition temperatures (~370 ℃) and thermal stability of the ES-PBE membranes.

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The ES-PBE membrane was further examined at temperatures slightly higher than the material's glass-transition temperature T_g ~ 108 ℃ ^[42] (Figure 4). Three ES samples, obtained by the same processing conditions (solution concentration = 0.200 g/mL, acceleration potential = 20 kV), were cut into squares of 2 cm (0.78 inch) by 2 cm. The samples were heated in a box furnace, from room temperature to 110 ℃ (slightly higher than the T_g), 115 ℃, and 118 ℃, then held for 1 h. At 110 ℃, a change in the fiber morphology was already noticeable (compared to Figure 2d). At 118 ℃, some fibers were fused together. Even though ES-PBE membranes do not start to decompose until 370 ℃, it is probably necessary to set the service temperature lower than the T_g for applications that demand integrity of the membrane morphology and structure.

Figure 4.  SEM micrographs demonstrate macroscopic morphology deformation of the ES-PBE membrane (solution concentration = 0.200 g/mL, and acceleration potential = 20 kV) at tested temperatures: (a) 110 ℃; (b) 115 ℃; and (c) 118 ℃ (3000× magnification, scale bar corresponds to 1 μm).

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3.4.   Mechanical property

Elastic modulus and tensile strength of the ES-PBE membranes were measured and listed in Table 3. In comparison to the reported nonporous, cast PBE, the electrospun samples had a similar or higher elastic modulus but exhibited lower tensile strength ^[63]. The relatively large standard deviation of tensile strength and elastic modulus was typical for the ES fibers deposited on the stationary collector, due to the random depositions. This observation agrees with a previous study by Al-Attabi et al. ^[64]. Differences in the tensile strength between the electrospun membrane and the raw materials have also been seen in previous electrospinning studies on other thermoplastics, for example, PVdF (1.2–7.5 vs 42–43 MPa) and polyvinyl chloride PVC (0.90 vs 56.6 MPa) ^{[65,66,67,68]}. As presented in Figure 5, membranes with the mixed fiber-bead structure (solution concentrations < 0.200 g/mL) had much lower tensile strength than that of the fiber structure (solution concentration ≥0.200 g/mL). Although the tensile strength of the fiber-structured membranes was similar, the membranes with a more consistent fiber structure had a higher elastic modulus (Figure 6). It was observed that the membranes produced from a concentration of 0.200 g/mL had the highest elastic modulus and tensile strength among all the samples tested.

Table 3.  Physical and mechanical properties of ES samples, which correspond to different polymer solution concentrations at an acceleration potential of 20 kV.

Concentration of polymer solution (g/mL)	Membrane structure	Average density (g/cm3)	E (GPa)	TS (MPa)
0.150	Fiber-bead	0.345 ± 0.029	1.129 ± 0.746	0.135 ± 0.065
0.175	Fiber-bead	0.263 ± 0.019	3.322 ± 0.736	0.294 ± 0.022
0.200	Consistent fiber	0.420 ± 0.056	9.125 ± 2.573	1.260 ± 0.195
0.225	Inconsistent fiber	0.368 ± 0.008	2.229 ± 0.994	1.055 ± 0.353
0.250	Inconsistent fiber	0.302 ± 0.005	5.019 ± 2.309	1.271 ± 0.112
PBE pellet	-	1.180 [62]	-	-
Cast PBE (thickness = 120 μm) [63]	Nonporous	-	1.2 [63]	41 [63]

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Figure 5.  Tensile strength (MPa) correspondence with the polymer solution concentration (g/mL).

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Figure 6.  Elastic modulus (GPa) correspondence with the polymer solution concentration (g/mL).

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4.   Conclusion

In this report, porous PBE membranes were created by an electrospinning process. The thermal stability of the electrospun samples was confirmed to be similar to that of PBE pellets. In this study, the density, tensile strength, and elastic modulus varied, depending on the membrane morphology. Consistent fiber morphology resulted in high tensile strength and elastic modulus. The concentration of polymer solution, solvent composition, and acceleration potential were important factors influencing membrane morphology. Solvent composition determines the dielectric constant, which affects the spinning rate of the solution jets, and, hence, the membrane morphology. In addition, solvent composition is an important parameter for controlling morphology, due to the boiling point and the vapor pressure of each solvent component. PBE is used as an adhesive, coating agent, additive, and matrix to other functional materials. The porous, non-woven membranes of PBEs, with a high thermal stability similar to that of PBE pellets, may be desirable for applications at elevated temperatures that other ES membranes cannot withstand.

Conflict of interests

Authors have declared that no competing interests exist.