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. 2018 Nov 16;3(11):15666–15678. doi: 10.1021/acsomega.8b02029

Supramolecular Route for Enhancing Polymer Electrospinnability

Deepika Malpani 1, Asha Majumder 1, Pratick Samanta 1, Rajiv K Srivastava 1, Bhanu Nandan 1,*
PMCID: PMC6643600  PMID: 31458222

Abstract

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Electrospinning of polymers typically requires high solution concentrations necessitated by the requirement of sufficient chain overlaps to achieve the required viscoelastic properties. Here, we report on a novel supramolecular approach, involving polymer/surfactant complexes, which allows for a significant reduction in the solution concentration of polymer for electrospinning. The approach involved supramolecular complexation of poly(4-vinylpyridine) (P4VP) with a surfactant, dodecylbenzenesulfonic acid (DBSA), via ionic interactions. The supramolecular complexation of P4VP with DBSA led to a significant increase in the solution viscosity even at a DBSA/4VP molar ratio as low as 0.05. Furthermore, the solution viscosity of the P4VP/DBSA complex increased significantly with the DBSA/4VP molar ratio. The increase in the viscosity for the P4VP/DBSA complexes was plausibly due to the formation of physical cross-links between P4VP chains driven by hydrophobic interactions between the surfactant tails. The formation of such physical cross-links led to a significant decrease in the solution concentration needed for the onset of semidilute entangled regime. Thus, the P4VP/DBSA complexes could be electrospun at a much lower concentration. The critical solution concentration to obtain bead-free uniform nanofibers of P4VP/DBSA complexes in dimethylformamide was reduced to 12% (w/v), which was not possible for neat P4VP solution even up to approximately 35% (w/v). Furthermore, small-angle X-ray scattering and polarized optical microscopy results revealed that the electrospun nanofibers of P4VP/DBSA complexes self-assembled in lamellar mesomorphic structures similar to that observed in bulk. However, the electrospun nanofibers exhibited significantly improved lamellar order, which was plausibly facilitated by the preferred orientation of P4VP chains along the fiber axis.

Introduction

Electrospinning is a relatively simple and versatile method to produce fibrous materials in the nano- and micrometer dimensions.1−3 In a typical electrospinning process, a polymer solution or melt is ejected from a small nozzle under the influence of a strong electric field. The buildup of electrostatic charges on the surface of a liquid droplet induces the formation of a jet, which is subsequently stretched to form a continuous ultrathin fiber.4 The electrospun nanofibers possess remarkable features, such as desirable fiber length, well-controlled diameter, and large surface area-to-volume ratio. As a result, the electrospun nanofibers have potential applications in many fields, including reinforced composites, tissue engineering, drug delivery, filtration industry, supports for enzymes and catalysts, sensing materials, optoelectronic devices, etc.5−8 Solution properties (viz. viscosity, conductivity, surface tension) and process parameters (viz. applied voltage, tip-to-collector distance, and flow rate) are the two major groups of factors that govern the morphology and mechanical properties of electrospun fibers.8,9 The past studies have shown that one of the most important factors influencing the formation of continuous electrospun fibers is the formation of effective topological entanglements between polymer chains in the solution. The entanglements between polymer chains act as physical cross-links, which resist the deformation imposed by the electrostatic stretching forces during electrospinning and, hence, maintain the chain connectivity crucial for the formation of continuous fibers. Hence, the molecular weight of the polymer and solution concentration play an important role in governing the elctrospinnability of the polymers. Increasing either or both of them results in an effective increase in the entanglement density of the chains in solution, which has been used as one of the most effective tools for making the polymer electrospinnable. The fundamental studies done on the electrospinning process have shown that for a polymer solution to be electrospinnable, the polymer molecular weight and solution concentration need to be well above the critical molecular weight for entanglement (Me) and critical solution concentration for onset of semidilute entangled regime (Ce), respectively, to facilitate sufficient topological constraints in solution during electrospinning.10−15

The requirement of either high molecular weight or high solution concentration, however, limits the applicability of electrospinning as a general tool for the spinning of polymer nanofibers especially for the case of polymers such as polyelectrolytes, biopolymers, or rigid polymers, which do not entangle easily. Hence, several strategies have been used in the past to improve the electrospinnability of such polymers. The most common approach has been to blend the nonelectrospinnable polymer with a secondary electrospinnable polymer for improving adequate entanglements.16−18 The entanglement density could also be increased by tuning the chain conformation in solution.19−21 Khan et al. showed that alginate, having a rigid conformation in water, could be made electrospinnable by the use of glycerol as a polar co-solvent. The use of glycerol as a co-solvent was shown to increase the chain flexibility and, hence, effective chain entanglements between the polymer chains.19 Similarly, Pellerin et al. showed that poly(4-vinylpyridine) (P4VP) chain conformation, in dimethylformamide (DMF), could be tuned by adding a poor nonsolvent such as nitromethane. In this case, the presence of nonsolvent induces chain coiling, which was responsible for increase in chain entanglements and hence improved electrospinnability.20 Georgiadou et al. also found that electrospinnability of polylactides could be improved by taking a combination of good solvent (acetone) and poor solvent (DMF). The reduction of surface tension of polymer solutions has also been found to improve electrospinnability. In this case, either a combination of solvents of low and high surface tensions could be used or a surface active agent could be added in the polymer solution to reduce the droplet formation during electrospinning.21

Recently, supramolecular interactions such as hydrogen-bonding, ionic interactions, and hydrophobic interactions have also been used to decrease the overlap concentration for polymer solutions to improve the polymer elctrospinnability.18,22−25 The supramolecular interactions basically produce physical cross-links between polymer chains such that the entanglement density increases leading to electrospinnability at lower solution concentrations. Pellerin and co-workers used bifunctional small organic molecules and divalent metal ions to produce physical cross-links between polymer chains via hydrogen-bonding or coordination interactions. They demonstrated that the solution concentration for spinning poly(4-vinylpyridine) (P4VP) in DMF could be reduced by a factor of two using their approach.20,23 Similarly, Khan and co-workers used a polymeric additive having long pendant hydrophobic groups to reduce the electrospunnable concentration of poly(ethylene oxide).18,22

In addition to the above, few works have been reported in the past showing that even small molecules could also be electrospun without the addition of polymer, proving that supramolecular interactions could be strong enough to allow electrospinning without breaking of the electrospun jet. For example, electrospinning of nonpolymeric species like phospholipids, gemini surfactant, cyclodextrins, small peptide, and recently tannic acid has been demonstrated, which involved supramolecular interactions.26−31

The use of supramolecular interactions, apart from improving polymer electrospinnability, potentially could also be used to develop nanostructured and functional nanofibers. Such an approach has been used extensively in the past to prepare bulk materials or thin films having ordered and hierarchical nanostructures.32,33 In this case, amphiphilic molecules with long alkyl tails such as dodecylbenzenesulfonic acid (DBSA) or pentadecylphenol were used to form supramolecular complexes with polymers such as P4VP, resulting in the formation of comb polymers. These so-called comb polymers exhibit interesting properties and could be used to impart functionalities to the otherwise nonfunctional polymers. One of the interesting aspects of such comblike supramolecular polymers is that they can undergo microphase separation to form well-ordered nanostructures at room temperature. The self-organization of the supramolecular complexes in this case is basically driven by a balance between the attractive and repulsive interactions between the polymer and surfactant.34−36 Such supramolecular comblike polymers have several potentially interesting applications in optoelectronics, molecular electronics, photonic band gap materials, porous materials for filtration, and controlled release and absorption.37−40

In the present work, we demonstrate that the supramolecular binding of an amphiphilic molecule, i.e., DBSA could be used as a novel method to enhance interchain interactions in stiff polymers such as P4VP via hydrophobic interactions. It will be shown that the supramolecular approach adopted in the present study enabled the reduction in solution concentration, for effective electrospinning of P4VP, by a factor of more than 2. The improved electrospinnability could be achieved even at a DBSA/4VP molar ratio as low as 0.05. The improved electrospinnability was further corroborated using rheological measurements, which demonstrated a significant increase in solution viscosity as well as the elasticity after supramolecular complexation. Furthermore, the electrospun nanofibers prepared from P4VP/DBSA comb polymers self-assembled in lamellar mesomorphic structures with improved order compared to that in bulk.

Results and Discussion

Fourier Transform Infrared (FTIR) Study of P4VP(DBSA)x Complexes

The interaction between P4VP and DBSA during complex formation involves the protonation of pyridinic nitrogen of P4VP by the acidic sulfonic groups (−SO3H) and the subsequent association of the positive pyridinium ions and the negative sulfonate ions (−SO3–). This ionic association between the two is shown in Figure 1a.

Figure 1.

Figure 1

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(a) General molecular structure of the ionic complexation between P4VP and DBSA. Infrared spectra for DBSA, P4VP(DBSA)x (x = 0.05, 0.10, 0.25, 0.50, 0.75, 1.00), and neat P4VP in regions (b) 900–1100 cm–1 and (c) 1500–1800 cm–1.

The chemical association of DBSA molecules to P4VP was ascertained by FTIR, as shown in Figure 1b,c. As per the past studies on similar complexes, the most affected bands of P4VP are the stretching bands at 1597, 1570, and 993 cm–1.32,41,42Figure 1b,c shows the spectrum of P4VP(DBSA)x complexes with x = 0.05, 0.10, 0.25, 0.50, 0.75, and 1.00 as well as neat P4VP. Figure 1b,c represents the detailed spectrum insight of P4VP(DBSA) complexes and neat P4VP, in ranges of 900–1100 and 1500–1800 cm–1, respectively. Figure 1b depicts that the pure P4VP absorption band at 993 cm–1 vanished on complete protonation and gets replaced by two new bands of almost similar strength at 1008 and 1033 cm–1. Figure 1c demonstrates the characteristic band of pure P4VP at 1570 and 1597 cm–1 due to the pyridine ring stretching. On protonation, the bands are replaced by an extra band at 1637 cm–1, which can be attributed to the hydrogen-bonded pyridinium ring. On complete protonation, the band at 1570 cm–1 fully vanished and the band at 1637 cm–1 gains maximum intensity. Thus, the FTIR spectrum attested the successful complexation of DBSA with P4VP.

Solution Properties

Conductivity

Solution conductivity is a crucial parameter in the process of electrospinning as the viscoelastic solution undergoes deformation due to the repulsion of charges present on spinning jet surface to produce fibers with reduced diameters. Hence, the conductivities of neat P4VP and P4VP(DBSA)x complex solutions were measured to evaluate the influence of surfactant added to the solution. The conductivity of the DMF, used in this work, was also measured for a systematic comparison of all of the prepared solutions. Figure 2 shows the conductivities of neat P4VP and P4VP(DBSA)x complex solutions at different molar ratios. The conductivity measurements were done at 1% (w/v) of P4VP in DMF. The conductivity showed significant increase after the addition of DBSA and was found to increase with increase in the molar ratio, x. The conductivity increased from 3.1 μS/cm, for P4VP solution without DBSA, to 336 μS/cm, when the DBSA/4VP molar ratio was 1. This significant increase in the conductivity mostly was due to the ionic interaction between the pyridinic nitrogen of P4VP and sulfonate head of DBSA, as shown in Figure 1a. The increase in the solution conductivities were expected to favor the electrospinning process. In this case, the increase in the accumulated charge density on the spinning jet would facilitate the stretching of the viscous jet and should yield fibers with reduced diameter. However, as will be shown later, at increased content of DBSA in the solution, the fiber diameters were found to increase, which suggested that conductivity was not the primary reason for the improvement in electrospinnability as reported in this work and that the viscoelastic properties of the solution had more significant impact.

Figure 2.

Figure 2

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Conductivity as a function of DBSA/4VP molar ratio at 1 wt % P4VP.

Surface Tension

It is well known that fiber formation in electrospinning process is facilitated by lower surface tension of the solution. In the present study, the solvent used, i.e., DMF, had a surface tension of 37 mN/m. As shown in Figure 3, the addition of P4VP in DMF had a negligible influence on the surface tension. The addition of DBSA, which is a surfactant, was expected to lower the surface tension especially at a very high content of DBSA. As shown in the figure, the complexation of P4VP with DBSA does result in some decrease in the surface tension. However, the decrease was very small up to the DBSA/4VP molar ratio of 0.25. A further increase in the molar ratio resulted in a slight decrease in the surface tension of the solution. The surface tension data showed that the change in surface tension of the solution, on addition of DBSA, was not significant enough to be considered as the primary factor influencing the electrospinnability of the solution.

Figure 3.

Figure 3

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Surface tension as a function of DBSA/4VP molar ratio.

Rheological Results

The rheological studies were carried out in both steady shear and dynamic mode to ascertain the detailed viscoelastic properties of the P4VP(DBSA) complexes at different solution concentrations.

Steady Shear Rheological Measurements

Steady-state rheological measurements of the P4VP(DBSA)x complexes, dissolved in DMF, were done in the solution concentration range of 4–30% (w/v). The detailed viscosity versus shear rate plots for complexes, at each molar ratio and different solution concentrations, are provided in the Supporting Information (Figure 1S). The results revealed that the neat P4VP solution exhibited Newtonian behavior in the shear rates used in the experiments (0.1–1000 s–1) at all investigated solution concentrations. The solutions exhibited almost Newtonian behavior even for the P4VP(DBSA) supramolecular complexes at molar ratios between 0.05 and 0.25. However, as will be discussed later, the shear viscosities were found to increase significantly as the DBSA content in the solution was increased. As the molar ratio was further increased to 0.5, the solution started to exhibit shear thinning behavior at high shear rates when the concentration was more than 16%. Furthermore, as the molar ratio was further increased up to 1.0, the shear thinning behavior was observed to occur at comparatively much lower shear rates. The zero shear viscosities of the P4VP(DBSA) complex solutions at four different concentrations are shown in Figure 4. The figure shows that the zero shear viscosity of the P4VP(DBSA) complexes increased as the molar ratio of the complexation increased. Furthermore, at high solution concentrations, where the increase in the zero shear viscosity up to a molar ratio of 0.1 was modest, the increase was more substantial at molar ratio of 0.25 and higher. Hence, a higher density of the DBSA bound to the P4VP chains led to a significant increase in the solution viscosity.

Figure 4.

Figure 4

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Zero shear viscosity at 4, 10, 16, and 30% (w/v) as a function of different molar ratios of DBSA/4VP.

The significant increase in the viscosity of P4VP(DBSA) complexes as well as the non-Newtonian behavior observed in solutions, at high molar ratios, strongly indicated the presence of increased entanglement and/or physical interactions between polymer chains in the solvent. The shear thinning behavior is generally attributed to the structural transitions of temporary three-dimensional (3D) networks taking place at high shear rates. Increased entanglement density and/or physical interactions are known to result in the formation of such 3D network structures of polymer chains. In Figure 5, the increase of the zero shear viscosity with x is more significant for the highest P4VP concentration. At higher solution concentrations, the hydrophobic alkyl tails were expected to be in close proximity with each other resulting in increased hydrophobic interactions, which were likely to result in much higher P4VP chain entanglements leading to additional contribution toward rise in solution viscosity. As shown in the figure, the hydrophobic associations of the long alkyl tails work as alternate entanglement regions, which then are responsible for increase in viscosity as well as viscoelastic properties. The model also highlights the importance of the increased DBSA density bound to the P4VP chains on the rise in viscosity. The higher binding fraction is necessary for effective hydrophobic associations of the alkyl tails.

Figure 5.

Figure 5

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Schematic explaining two interactions at low and high concentrations: the supramolecular interaction between P4VP and clustering of P4VP chains due to the supramolecular interaction between hydrophobic tails of DBSA.

Specific Viscosity versus Concentration Plots

The steady shear rheological data were further analyzed by plotting the specific viscosity of the P4VP(DBSA) complexes as a function of solution concentration from where the different concentration regimes were determined. Figure 6 shows the double logarithmic plots of specific viscosity versus solution concentration at each DBSA/4VP molar ratio investigated in this work. According to the past studies, the different regimes are basically separated by three critical concentrations (C): C* (onset of semidilute unentangled regime), Ce (onset of semidilute entangled regime), and CD (onset of concentrated regime).43−45 The viscosity exhibits an exponential rise with concentration, and each regime follows power-law relationship

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where n is the scaling exponent. For polymer solution in a good solvent, it has been found that ηsp ∼ C1.0 in the dilute regime and ηsp ∼ C1.25–2 in the semidilute unentangled regime, whereas ηsp ∼ C3.7–4.8 in the semidilute entangled regime. The actual value of the power-law exponent was found to depend on the solvent nature, i.e., theta or good solvent.44,46 The semidilute entanglement regime is known to play a crucial role in predicting electrospinnabilty as well as directs the microstructure of nanofibers electrospun from a polymer solution. It has been reported that Ce is the critical concentration at which beaded fiber formation starts and, in general, uniform bead-free nanofibers are produced at solution concentrations which are 2–2.5 times that of Ce.22,47,48 In the concentrated regime, the dimensions of chains do not have any dependence on concentration and also do not follow the polymer solution thermodynamics since the effect of polymer–solvent interactions on chain dimensions is negligible in highly concentrated solutions.43

Figure 6.

Figure 6

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Specific viscosity as a function of P4VP concentration at different DBSA/4VP molar ratios.

Specific viscosity scaling factors for the experimental solutions in the semidilute unentangled, semidilute entangled, and concentrated regimes were determined from the linear slopes in the log–log plots of specific viscosity versus concentration as shown in Figure 6. The values of slope along with the critical concentrations of each regime are reported in Table 1. Since the rheology was performed on solutions with concentration greater than 4 wt %, the dilute regime for most of the complexes was not accessible. From Figure 6, it could be ascertained that for complexes with molar ratio from 0 to 0.5, the semidilute unentangled and entangled regimes were accessible within the concentration range in which solutions were investigated. The neat P4VP solution in DMF showed a scaling relationship of ηsp ∼ C1.99 for the semidilute unentangled regime and ηsp ∼ C3.54 for semidilute entangled regime. The exponents were in close agreement with those reported in the past for different concentration regimes.23,44,46 The two discrete regimes were found to meet at approximately 12 wt %, which corresponded to the transition from semidilute unentangled to semidilute entangled regime. Hence, in this case, 12 wt % was the critical entanglement concentration (Ce) from which the electrospinnable solution concentration could be ascertained.

Table 1. Summary of Exponents and Critical Regime Predicted from Rheological Behavior.

sample semidilute unentangled regime slope semidilute entanglement regime slope onset of semidilute entanglement regime (Ce) (w/v) onset of concentrated regime (CD) (w/v)
neat P4VP 1.99 3.54 12%  
1:0.05 2.00 3.14 12%  
1:0.10 1.80 2.88 10%  
1:0.25 1.78 3.06 9.4%  
1:0.50 2.20 3.47 8%  
1:0.75   3.50   25%
1:1.00   3.54   25%
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On addition of DBSA in different stoichiometric ratios in P4VP(DBSA)x complexes (0.05 ≤ x ≤ 0.50), the scaling relationships for the two distinct concentration regimes were found to be ηsp ∼ C1.78–2.20 and ηsp ∼ C3.06–3.54, respectively. The scaling exponents, n, were again in agreement with the theoretical predictions. More significantly, a gradual decrease of Ce was observed from 12 to 8 wt % as the molar ratio was increased up to 0.50. This was attributed to the increased junction points created in the solution, due to the hydrophobic interactions between the DBSA tails anchored to the P4VP, which induced more entanglement between the polymer chains, as shown schematically in Figure 5. The decrease in Ce was expected to favor electrospinning at lower solution concentrations.

The supramolecular complexes at higher molar ratios, i.e., P4VP(DBSA)0.75 and P4VP(DBSA)1.00, also revealed two distinct concentration regimes in the specific viscosity vs concentration plots. However, in this case, the value of the scaling exponent, n, revealed that the semidilute unentangled concentrated regime was plausibly lower than the lowest concentration used in the present study. The value of the scaling exponent, n, suggested that the two concentration regimes observed corresponded to semidilute entangled and concentrated regimes. The concentrated regime was not observed for the complexes until molar ratio of 0.5 even at the highest solution concentration studied. The concentrated regime started emerging only from P4VP(DBSA)0.75 molar ratio as evidenced by the drastic change in viscosity magnitude. This transition from semidilute entangled to concentrated regime in both P4VP(DBSA)0.75 and P4VP(DBSA)1.00 took place at 25 wt % solution concentration, and the intersection point was denoted by CD (onset of concentrated regime). There are two major driving factors for the appearance of this regime: first one is the electrostatic interactions of the P4VP chains with the sulfonate head group of the surfactant and hydrophobic interactions between the alkyl tails of the DBSA molecules anchored to different P4VP chains at multiple locations and the second one is the P4VP chain entanglements, which bring about the formation of dynamic 3D network in solution.

Dynamic Rheological Measurements

The rheological behavior of P4VP(DBSA) complex solutions was further analyzed by carrying out the frequency sweep measurements. The temporary entanglements formed by ionic and hydrophobic interactions of P4VP and DBSA in DMF behave similarly to the chemical cross-links. However, unlike the case for chemical cross-links, here, under deformation, the chains can move past each other influencing the viscoelastic properties of the formulations. These properties can be evaluated by oscillatory mode of the rheometer. The detailed viscoelastic behavior of neat P4VP and its complexes with DBSA in DMF solutions, at different concentrations, under dynamic mode of rheometer by frequency sweep analysis is shown in the Supporting Information (Figure 2S). The magnitude of the elastic moduli (G′) and viscous moduli (G″) was found to increase as the solution concentration increased. The cross-over between the two dynamic moduli was not observed within the studied frequency range; however, it appeared to shift toward lower frequency with increasing solution concentration. The addition of DBSA resulted in a progressive increase of both the dynamic moduli at each solution concentration. This behavior was more explicit from Figure 7, which compares the dynamic moduli of the P4VP(DBSA) complexes at three different concentrations. The figure clearly shows the increase in the elastic and loss moduli as the molar ratio was increased. Furthermore, cross-over frequencies were also observed to shift to lower frequency with increase in the DBSA molar ratio, suggesting an increase in the characteristic relaxation time of the system.22 Most interestingly, a sudden increase in the elastic modulus was observed at much lower solution concentrations as the molar ratio of the DBSA was increased in the complexes as shown in Figure 8, where storage moduli of different solution concentrations are given at 10 rad/s as a function of different molar ratios of DBSA/4VP. For x = 1.0, the critical concentration after which the enhanced elasticity was observed was found to be 16 wt %. This clearly revealed an enhanced elastic behavior of the solutions at higher molar ratios of DBSA. These results further provided strong evidence for the formation of higher degree of chain entanglements driven by hydrophobic interactions between the DBSA alkyl chains bounded to the P4VP chains. The enhanced elasticity of the polymer solutions was expected to favor the electrospinning process at lower concentrations.

Figure 7.

Figure 7

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Dynamic frequency sweep rheograms of elastic (G′, closed symbols) and loss (G″, open symbols) depicted for P4VP at (a) 10%, (b) 16%, and (c) 30% (w/v) with different P4VP/DBSA molar ratios.

Figure 8.

Figure 8

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Storage moduli of different solution concentrations at 10 rad/s as a function of different molar ratios of DBSA/4VP.

Electrospinnability and Its Correlation to Solution Properties

The electrospinnability of the neat P4VP and its complexes with DBSA was ascertained by carrying out the electrospinning experiments under similar conditions. The morphologies of the electrospun fibers showed transition from droplets, beaded fibers to almost bead-free fibers. The scanning electron microscopy (SEM) images of the electrospun fibers of neat P4VP at different solution concentrations are shown in Figure 9. The figure shows that transition from droplets to beaded fibers (Cbf) occurred at 12 wt %. This corresponded well with the rheological results, which revealed the start of semidilute entangled regime (Ce) at the same solution concentration for neat P4VP. Interestingly, even though the density of beads decreased as the solution concentrations increased, completely bead-free fibers were not obtained even up to 35 wt % concentration (∼3Ce) under the used spinning conditions. It is well known that the P4VP chains are semiflexible in nature and has a very high critical entanglement molecular weight (Me ∼ 30–40 kg/mol).23 Hence, high solution concentrations are necessary for sufficient entanglements to buildup for attaining the required viscoelastic behavior necessary to obtain bead-free nanofibers.

Figure 9.

Figure 9

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SEM images of electrospun fibers of neat P4VP solutions in DMF at different concentrations, 8–30% (w/v).

The addition of DBSA, however, resulted in a remarkable improvement in electrospinnability of the P4VP solutions even at very low molar ratios. Figures 10, 11, 12, and 13 show the SEM images of the electrospun fibers obtained at different solution concentrations as the molar ratio of DBSA was varied from 0.05 to 0.5. The critical concentrations corresponding to the onset of beaded and bead-free fibers are summarized in Table 2. At the lowest molar ratio of 0.05, the critical concentration at which beaded fibers formed was found to be 12%, whereas the onset of bead-free fibers was observed at a solution concentration of 25 wt %, which was significantly lesser than that for neat P4VP. It was remarkable that even though the rise in viscosity and elasticity of polymer solutions composed of P4VP(DBSA)0.05 complexes were not significant, the decrease in the effective solution concentration needed to obtain bead-free fibers was substantial. Hence, it is plausible that at such low molar ratios, the improvement in the electrospinnability result from a combined favorable effect of increase in viscoelastic properties as well as conductivity of the solution. However, as the molar ratio was further increased, the effect of change in the viscoelastic properties became more pronounced. In the case of P4VP(DBSA)0.1 complex, where the Ce was observed at 10% solution concentration, the transition from droplets to beaded fibers (Cbf) was also observed at 10%. Moreover, the solution concentration corresponding to the onset of bead-free fibers (Cf) further reduced to 20%. The hydrophobic interactions now were sufficiently high to increase the interchain contact points, which facilitated electrospinning of bead-free fibers. Further increase in the molar ratio of DBSA to 0.25 and 0.5 led to the reduction in the Cf to 16 and 12%, respectively. This was 1.7 and 1.5 times of Ce observed for respective complex solution and corresponded reasonably well with the electrospinnability correlation with the Ce as observed in some other systems.47 Hence, at a molar ratio of 0.5, bead-free electrospun nanofibers could be obtained at solution concentrations almost more than 3 times lower compared to that needed for neat P4VP. This could be attributed to the formation of a well-developed network of topological entanglements, driven by hydrophobic interactions of surfactant tails, which leads to a highly viscous system with an increased contribution of the elastic forces. The significant increase in viscosity strongly resists the electrical stretching as well as surface tension forces that favor bead or droplet formation during the electrospinning process.

Figure 10.

Figure 10

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SEM images of electrospun fibers of P4VP(DBSA)0.05 complex solutions in DMF at different concentrations, 4–0% (w/v).

Figure 11.

Figure 11

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SEM images of electrospun fibers of P4VP(DBSA)0.10 complex solutions in DMF at different concentrations, 4–30% (w/v).

Figure 12.

Figure 12

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SEM images of electrospun fibers of P4VP(DBSA)0.25 complex solutions in DMF at different concentrations, 4–30% (w/v).

Figure 13.

Figure 13

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SEM images of electrospun fibers of P4VP(DBSA)0.50 complex solutions in DMF at different concentrations, 4–30% (w/v).

Table 2. Morphological/Rheological Property Relationshipa.

sample Cbf (w/v) Cea (w/v) Cf (w/v) Ceb (w/v)
neat P4VP 12% 12% not up to 35% not even up to approx. 3Ce
P4VP(DBSA)0.05 12% 12% 25% 2.08Ce
P4VP(DBSA)0.10 10% 10% 20% 2.08Ce
P4VP(DBSA)0.25 10% 9.4% 16% 1.7Ce
P4VP(DBSA)0.50 8% 8% 12% 1.5Ce
P4VP(DBSA)0.75        
P4VP(DBSA)1.00        
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a

Where: Cbf = onset of beaded fiber concentration, Cf = onset of bead-free fiber concentration, Cea = onset of semidilute entangled concentration, Ceb = minimum Ce for uniform fiber formation.

Interestingly, even though further increase in DBSA molar ratio to 0.75 and 1.0 led to a dramatic increase in the solution viscosities and the electrospinnability of these solutions could be visually observed at comparatively lower concentrations, the SEM analysis did not show the presence of fibrous structures on the surface. As already discussed, Ce values at these molar ratios were lower than the studied minimum solution concentration (4%) studied, whereas the concentrated regime, which was not observed at lower molar ratios, could be observed at 25% signifying a highly entangled polymer solution. Hence, the absence of fibers, as ascertained from microscopy tools, was surprising. However, it must be noted that DBSA is highly hygroscopic and it is plausible that, at higher molar ratios, the P4VP(DBSA) complex takes up a lot of moisture such that the fiber morphology is destroyed. This was indeed found to be the case since a quick morphology observation, immediately after electrospinning, under an optical microscope indeed revealed the fibrous morphology. Hence, at higher molar ratios, the present approach has a limitation of fiber instability due to high moisture absorption. However, in future, we plan to carry out more studies on such complexes at higher molar ratios to understand the effect of moisture on fiber stability. We would also like to note that DBSA could be removed by treatment with an aqueous alkali solution such that neat P4VP fibers could be obtained. Our future work will focus on this aspect as it also provides a pathway to produce porous fibers.

Effect of Complexation on Fiber Diameter

The relative contribution of different factors on the electrospinnability could also be understood by observing the variation of fiber diameters with respect to different parameters such as solution concentration, molar ratio, and viscosity. Figure 14 shows the variation of fiber diameters as a function of molar ratio at two different solution concentrations, i.e., 20 and 30%. The figure shows an increase in fiber diameter as the DBSA/4VP molar ratio increases at the same solution concentration. These results are significant as they provide good information about the dominating factor influencing electrospinnability as the DBSA concentration was increased in the complex. The three major factors that could be considered influencing the fiber formation here are the conductivity, surface tension, and viscoelastic forces. As shown earlier, the surface tension was not found to change much in the complexes relative to that of neat P4VP and, hence, its effect could be neglected. The conductivity increased significantly, as well as viscosity and elasticity of the solution as the DBSA content was increased. The increase in conductivity was expected to decrease the fiber diameter due to enhanced electrical stretching of the spinning jet. The fact that the fiber diameter actually increased suggested that the increased viscosity as well as elasticity of the solution had a dominating influence on the formation of fibers. The fiber diameter was found to increase almost linearly with the increase in the DBSA/4VP molar ratio. This was indeed due to the formation of well-developed network of physical entanglements, as molar ratio increased, leading to a significant rise in the viscosity and elasticity of the polymer solution. The influence of conductivity on electrospinnability, at higher molar ratios, was expected to be negligible.

Figure 14.

Figure 14

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Diameter of electrospun fibers of 20 and 30% (w/v) neat P4VP and P4VP(DBSA) complexes as a function of various molar ratios of DBSA/4VP.

The fiber diameter was further correlated with the solution concentration as well as zero shear viscosity of the P4VP(DBSA) complexes. Figures 15 and 16 show the variation in fiber diameter with respect to solution concentration and zero shear viscosity, respectively. As expected, the fiber diameter increased with solution concentration. The increase in chain entanglements at higher concentration resists the jet stretching induced by the electric field forces and, hence, thicker fibers are formed. In general, the correlation between fiber diameter (D) and solution concentration (C) followed a power-law relation, which could be expressed as D α Cn. The scaling exponent, n, was found to be between 1.4 and 1.8 for neat P4VP as well as for most of the complexes. The exponent values were found to be nearly similar to those reported elsewhere.20 It must be mentioned that the scaling exponent for P4VP(DBSA)0.1 complexes were found to be slightly lesser than other samples. This could plausibly be due to the combined contribution of rheological and conductivity effects, which had opposite effect on the fiber diameter.

Figure 15.

Figure 15

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Diameter of electrospun fibers of neat P4VP and P4VP(DBSA) complexes as a function of P4VP concentrations. The numbers in parentheses depict the scaling factor (n) according to the correlation D α Cn.

Figure 16.

Figure 16

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Diameter of electrospun fibers of neat P4VP and various P4VP(DBSA)x complexes as a function of zero shear viscosity. The numbers in parentheses depict the scaling factor (n) according to the correlation D α (ηo)n.

The fiber diameter was also found to correlate with the zero shear viscosity (ηo) through a similar scaling law, which could be expressed as D α (ηo)n. It is well known that viscosity increase has a direct influence on the increase of electrospun fiber diameter.13,14,49,50 To corroborate the influence of viscoelastic properties, the dependence of the electrospun fiber diameter on zero shear viscosities was ascertained as shown in Figure 16. Most of the earlier studies have shown the scaling exponent to be around 0.8.13,48 However, recently, Hashimoto et al. have shown that the scaling exponent may have a value in the range of 0.41–0.80.46 In the current study too, the scaling exponents were found to be in the range of 0.44–0.51, which corroborates well with the findings of Hashimoto et al. Here also, the lower value of scaling exponent in the case of P4VP(DBSA)0.10 complexes may be attributed to the combined contribution of rheological and conductivity effects.

Superstructure of P4VP/DBSA Electrospun Nanofibers

It has been shown in the past that P4VP(DBSA) and similar other comb-shaped complexes display propensity to self-organize due to repulsion between the polymer–polymer backbone and nonpolar alkyl tail.32−36,41 The self-organization results in the formation of dominantly lamellar structure composed of a layer composed of polymer backbone together with DBSA head group and another layer consisting of alkyl tails. It was expected that similar self-organized structure may also form in the electrospun nanofibers of P4VP(DBSA) complex. This was ascertained using small angle X-ray scattering (SAXS) analysis shown in Figure 17. The SAXS intensity profile shows an intense scattering peak at q ∼ 2 nm–1 and a higher order peak at 4 nm–1. The 1:2 ratio of the two peaks suggested that the scattering peaks could be attributed to the diffraction from the self-organized lamellar structures formed by the P4VP(DBSA) complexes. The interlamellar distance (d) corresponding to this scattering peak was found to be ∼3 nm (d = 2π/qmax) and is similar to that reported for P4VP(DBSA) self-assembled structure in bulk. Interestingly, the higher order peak was much more intense for the nanofiber samples, suggesting plausibly a higher degree of lamellar order. This could be attributed to the chain orientation, which results during elongational flow facilitating the formation of lamellar structures, as shown schematically in Figure 17 for complexes in bulk and electrospun fibers. The formation of self-organized lamellar structure was further corroborated by the observed birefringence in the electrospun nanofibers using a polarized optical microscope, as shown in Figure 4S in the Supporting Information.

Figure 17.

Figure 17

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SAXS data for P4VP(DBSA)0.5 complex in film and electrospun fibers along with the insets of expected lamellar organization of P4VP(DBSA) complexes in film and electrospun fiber.

Conclusions

The rheology and electrospinnability of poly(4-vinylpyridine) (P4VP) complexed with dodecylbenzenesulfonic acid (DBSA), at different DBSA/4VP molar ratios, were investigated. The complexation resulted in a comblike polymer with P4VP forming the hydrophilic main chain, whereas the alkyl chains of DBSA constituted the hydrophobic brushes tethered to the main chain. The solution viscosities of these comblike polymers were found to increase significantly as the DBSA/4VP molar ratio of the complex was increased from 0 to 1. The increase in viscosity was attributed to the formation of higher degree of topological entanglements driven by the hydrophobic interactions between the alkyl tails of DBSA. The increase in topological entanglements was also corroborated by the fact that the critical concentration, for transition from semidilute unentangled regime to semidilute entangled regime, decreased from 12 wt % for neat P4VP to less than 4 wt % for P4VP(DBSA) complexes with stoichiometric DBSA/4VP molar ratios. Furthermore, the dynamic rheological experiments showed that the storage modulus and, hence, elasticity of the solutions also increased significantly as the molar ratio of complexation increased. The increase in the elasticity of the P4VP(DBSA) complexes further provided solid evidence for the increase in entanglements with increased DBSA fraction in the complexes. The resultant increase in the viscosity and elasticity of the solution led to a dramatic improvement in the electrospinnability of P4VP. The critical solution concentration to obtain bead-free uniform nanofibers of P4VP(DBSA) complexes was reduced to 12 wt %, which was not possible for neat P4VP solution even up to approximately 35 wt %. Hence, the present study provides a novel and simple approach for improving the electrospinnability of stiff polymers, which generally requires very high solution concentrations for bead-free nanofibers formation. Furthermore, small-angle X-ray scattering and polarized optical microscopy results revealed that the electrospun nanofibers of P4VP(DBSA) complexes self-assembled in lamellar mesomorphic structures similar to those observed in bulk. However, the electrospun nanofibers exhibited significantly improved lamellar order which, might have been facilitated by the preferred orientation of P4VP chains along the fiber axis.

Experimental Section

Materials

Poly(4-vinylpyridine) (P4VP) with a weight-average molecular weight (Mw) of 160 000 g/mol and dodecylbenzenesulfonic acid (DBSA) with 95% purity were purchased from Sigma-Aldrich. Both the polymer and surfactant were used without further purification. The solvent used for electrospinning, i.e., dimethylformamide (DMF), was supplied by Fisher Scientific and used as provided. The pKa values of P4VP, DBSA, and DMF are 5, −1.8, and −0.3, respectively.51,52

Sample Preparation

P4VP(DBSA)x complexes were prepared in DMF solutions, where the molar ratio, x, indicates the number of surfactant molecules per 4-vinylpyridine (4VP) repeat unit in P4VP and was varied from x = 0.05 to x = 1.00. The polymer concentration was varied from 4 to 30% (w/v) at each of the aforesaid x values of DBSA/4VP, to investigate the concentration and stoichiometric effects on the solution properties as well as the morphology of the electrospun fibers. In the first step, P4VP and DBSA solutions were prepared separately in DMF and then magnetically stirred at room temperature (25–27 °C) until both the solutions give a clear appearance. In the second step, DBSA solution is transferred dropwise into the P4VP solution. The solutions then were allowed for further magnetic stirring at ambient temperature (25–27 °C) for 2–4 days or more until a clear homogenous solution is observed.

Electrospinning

To prepare electrospun fibers, each of the solutions of P4VP/DMF and P4VP–DBSA/DMF complexes were taken into a syringe and mounted on a syringe pump of an electrospinning machine (E-Spin Nano). A high-voltage DC power supply of 20 kV was applied to the needle. The working distance between syringe tip and the collector plate was kept approximately at 20 cm and a flow rate of 0.5 mL/h was used.

Characterization

FTIR Study

Chemical and compositional characteristics of the pure and prepared complexes were evaluated by infrared spectra by using an FTIR spectrophotometer (Nicolet IS 50 FTIR, Thermo Fischer Scientific) with an attenuated total reflection unit attached. Samples were placed on a diamond crystal surface and measured in the range of 4000–400 cm–1. An average of 64 scans was taken for each measurement.

Conductivity Measurements

The electrical conductivities of the samples were measured using a digital conductivity meter (Labtronics Microprocessor) with a dip-type conductivity electrode having double-walled glass jacket and a cell constant of 1 cm–1. The temperatures of the solutions were maintained using a water bath. The conductivity electrode was first calibrated with a freshly prepared solution of 0.01 KCl with known conductivity at 25 °C. The reproducibility of the conductivity measurement was within ±0.5 μS/cm.

Surface Tension Measurements

Surface tension measurements of solutions were conducted using Wilhelmy plate method using a tensiometer (Data Physics) at room temperature. Before each experiment, the plate was cleaned using burner flame and ethanol to avoid any probable contamination of solutions due to plate. Instrument was calibrated with double-distilled water, which has a known surface tension of 72 mN/m. The precision of the measurements of surface tension with the tensiometer was within ±0.03 mN/m.

Rheological Measurements

The rheological behavior of prepared P4VP/DBSA complex solutions were evaluated with an Anton Paar MCR702 (single drive) stress-controlled rheometer using cone-plate measuring systems (cone radii, 50 and 75 mm; cone angle, 1°) with 0.150 and 0.05 mm truncations, respectively. Rheology of each of the solutions was measured in both steady shear as well as dynamic modes of the rheometer at 25 °C. The steady shear mode was used to determine solution viscosities in zero shear and various shear regimes as a function of shear rate ranging from 0.1 to 1000 s–1. The specific viscosity (ηsp) was determined by the following formula

graphic file with name ao-2018-02029n_m002.jpg 2

where ηs is the solvent viscosity and ηo represents the zero shear viscosity. The viscosity of DMF was calculated as 0.771 mPa s at 25 °C. The dynamic mode of the rheometer was used to determine viscoelastic regime of each solution. All of the dynamic studies were carried out at a constant strain falling in linear viscoelastic regime with varying angular frequencies ranging from 0.1 to 100 rad/s.

Scanning Electron Microscopy

The morphology of the obtained electrospun fibers was observed by using a ZEISS EVO 18 scanning electron microscope. Samples were mounted on a holder and gold-sputtered to enable the conduction of electrons on the sample surface. The fiber diameter was determined using ImageJ.

Small-Angle X-ray Scattering

Small-angle X-ray scattering experiment was performed on SAXSess mc2 (Anton Paar Gmbh, Graz, Austria). The X-ray source used was a sealed copper tube (40 kV/50 mA; wavelength, 0.1542 nm), and the detector was an imaging plate. Exposure time allotted was 30 min per scan.

Polarized Optical Microscopy

A Leica DM 2500P optical microscope was used to detect the presence of mesomorphic order in the nanofibers of complexes under crossed polarizers.

Acknowledgments

This work was supported by the Department of Science and Technology (DST). The authors also acknowledge Anton Paar India Pvt. Ltd. for rheological measurements at their facility.

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b02029.

  • Viscosity vs shear rate plots for all samples; dynamic rheological data; polarized optical microscope image of P4VP(DBSA) complex nanofibers (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao8b02029_si_001.pdf (706.7KB, pdf)

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Supplementary Materials

ao8b02029_si_001.pdf (706.7KB, pdf)

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