Tuning Porosity and Functionality of Electrospun Rubber Nanofiber

Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 24544−24551

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Tuning Porosity and Functionality of Electrospun Rubber Nanofiber Mats by Photo-Crosslinking Alessandra Vitale,*,†,‡,⊥ Giulia Massaglia,†,§,⊥ Angelica Chiodoni,§ Roberta Bongiovanni,†,‡ Candido Fabrizio Pirri,†,§ and Marzia Quaglio*,∥,§ Department of Applied Science and Technology and ∥Department of Environment, Land and Infrastructure Engineering, Petroleum Engineering Group, Politecnico di Torino, 10129 Torino, Italy ‡ INSTMPolitecnico di Torino Research Unit, 50121 Firenze, Italy § Center for Sustainable Future Technologies @ PoliTo, Istituto Italiano di Tecnologia, 10129 Torino, Italy Downloaded via BUFFALO STATE on July 19, 2019 at 06:56:23 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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S Supporting Information *

ABSTRACT: The present work proposes a versatile and efficient method to fabricate rubber nanofiber membranes with a controlled morphology and tailored functionality, based on the application of photoinduced thiol-ene cross-linking reactions to electrospun mats. Besides preventing the polymer cold flow and freezing the structure obtained by electrospinning, the photocuring step finely controls the morphology of the nanofiber mats, in terms of the fiber diameter up to the nanometer range and of the membrane porosity. Nanofiber membranes are also made chemically resistant, while retaining their flexibility. Finally, the proposed approach allows imparting specific functionalities to the rubber nanofibers: the type and concentration of the functional groups can be precisely tuned by changing process parameters (i.e., thiol/ene stoichiometric ratio and irradiation dose). Active chemical groups that remain available on the surface of the nanofibers can be used for further material modifications, as here proven by two target reactions. This key result is also demonstrated with electrospun membranes embedded into a microfluidic chip, opening the way to advanced functional flexible devices. KEYWORDS: electrospinning, photo-curing, functional nanofibers, rubber nanofibers, thiol/ene chemistry

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INTRODUCTION Nanofibers prepared by electrospinning are fascinating nanomaterials with tremendous potential for application in several areas,1−4 from filtration to environmental and energy fields,5−7 chemical and biological sensing,8,9 and wound dressing.10,11 The deep impact of electrospinning and its wide application potential are surely related to the possibility to process a large set of polymers, both natural and synthetic.12,13 Interestingly, only few papers investigate the formation of rubber nanofibers by electrospinning butadiene-based polymers,14−19 and the reason is related to the difficulties in controlling the morphology of the rubbery nanofiber mat. Indeed, such polymers are characterized by high extensibility, elastic recovery, and resilience, but the fiber size and shape tend to change over time: the overall morphology of the electrospun mat is in fact easily destroyed by the cold flow of the material, which is the tendency of rubbers prior vulcanization to flow at room temperature due to their low glass transition temperature (Tg),20,21 usually well below 0 °C. Chemical cross-linking, that is, formation of covalent bonds within the polymer chains, is the way to hinder viscous flow, by increasing the temperature at which the polymer starts flowing and additionally enhancing at the same time the solvent and heat resistance of the material. Among the curing processes, © 2019 American Chemical Society

the use of light to induce cross-linking reactions has several advantages, such as short conversion times, low-energy consumption, ambient temperature operations, and selective curing with an intimate control both in time and space. In the case of rubbery polymers, the photo-crosslinking process can be made more efficient and substantially accelerated by the addition of multifunctional thiol monomers that copolymerize with carbon−carbon double bonds, such as the easily accessible vinyl groups of butadiene polymers and copolymers.22,23 Photoinitiated thiol-ene reactions are characterized by a radical-mediated step-growth mechanism, assuring high efficiency and exhibiting significant advantages compared to other types of photocuring, such as resistance to oxygen inhibition, increased network uniformity, and reduced shrinkage and shrinkage stress.24−26 Photo-crosslinking has been applied to various polymeric electrospun fibers,27−32 both in situ during the formation of the fibers and post-electrospinning, to increase their physicochemical properties (e.g., to prevent dissolution in water or other solvents) and enhance their mechanical strength. Also Received: March 14, 2019 Accepted: June 14, 2019 Published: June 14, 2019 24544

DOI: 10.1021/acsami.9b04599 ACS Appl. Mater. Interfaces 2019, 11, 24544−24551

Research Article

ACS Applied Materials & Interfaces photo-crosslinking of butadiene-based fibers was carried out in few works, with the aim of hindering the polymer cold flow and obtaining stable fibers.33−35 However, in these cases, the control on the fiber size was very limited (i.e., fiber diameters in the micrometer range were obtained), and no control of the mat morphology was attempted. In this work, we propose a versatile method, based on the coupling of electrospinning and thiol-ene photo-crosslinking, that allows fine-tuning the morphology of butadiene-based nanofiber mats, in terms of the fiber diameter up to the nanometer range and of the membrane porosity. Even more interestingly, such an approach is also demonstrated to easily and efficiently turn rubber nanofibers into active and functional nanomaterials suitable to be embedded in complex devices. The ability to add different functionalities to polymeric nanofibers in a controlled way, at the same time keeping simple the overall process, is currently one of the hot topics in electrospinning technology. By carefully selecting the composition of the electrospinnable solution (e.g., the type of rubber, its vinyl content, the amount of thiol cross-linker, and the thiol/ene ratio) and photocuring process parameters (in particular the time between electrospinning and irradiation and the light dose), it is possible to differently and independently control the mat surface area, its porosity, and the amount of active functional groups available both in the bulk and on the surface of the cross-linked nanofibers. Several “click” reactions24,36,37 can then be applied both for surface modification by grafting and for further tailoring mechanical and thermal properties38,39 of the rubber nanofibers. Two target reactions (i.e., iodination of the double bonds and Ellman’s test of the thiols) are selected herein to quantitatively analyze and prove the availability of the thiol/ene groups on the fiber surface at the end of the process. The presence of active functionalities on electrospun membranes is also demonstrated when they are embedded into a flexible microfluidic device. While the present work describes polybutadiene fibers photocured with thiols, evidently the proposed method can be applied to other electrospinnable polymers and other functionalities: for instance, other chemistries such as azide− alkyne40−42 and Diels−Alder cycloaddition43 can be employed.

Figure 1. Scheme of the process to obtain functional nanofiber membranes with a well-defined morphology, which couples a first electrospinning step and a subsequent photocuring process. Rubbery nanofibers are formed by solution-based electrospinning of a butadiene-based polymer and then curing by a photoinduced thiolene cross-linking reaction.

ratio between vinyl unsaturations and thiol groups Rvinyl/thiol equal to 8.2 and a constant amount of photoinitiator (being the viscosity of the resulting solutions adequate for electrospinning). After the electrospinning step, the fiber mats are photo-crosslinked with a fixed exposure dose d, which is the product of light intensity and irradiation time (the manner in which the dose is selected will be discussed in the next section). The obtained electrospun membranes are uniform (inset of Figure 2a), the diameter of the fibers is around 220 nm, and the morphology of the rubber mats is maintained over time, as a chemical network is formed. The effect of the electrospinning flow rate on the morphology of the nanofiber mats was investigated, obtaining the results reported in Figure 2a and Figure S3 of the Supporting Information. As expected, the electrospinning flow rate and the average fiber diameter are proportional,1,44,45 and thus the mat porosity decreases by increasing the flow rate (Figure 2a). Porosity data reported in Figure 2a were evaluated by analyzing field emission scanning electron microscopy (FESEM) images, and the reliability of the method used was confirmed by measuring the mat porosity from density values, as reported in the Experimental Section. Results obtained with the latter technique were always included in the error limits of the data resulting from the former method. Moreover, the final morphology of the membrane can also be finely controlled by introducing a time interval between the electrospinning step and light exposure (i.e., waiting time, tw), as shown in Figure 2. In this way, the spontaneous change of the electrospun mat morphology due to the polymer cold flow is exploited. Exposing the nanofiber membrane to UV light immediately after the electrospinning step (tw = 0 min), it is possible to retain its morphology as obtained on the collector, thus using the electrospinning as the controlling process step. However, as clearly shown in Figures 2b,c, during the waiting time tw, the viscous cold flow of the uncured polymer not only increases the fiber diameter (which goes from an average value of 220 nm for tw = 0 min to 410 nm for tw = 30 min) but also

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RESULTS AND DISCUSSION Control of the Nanofiber Mat Morphology. To control the butadiene-based membrane morphology, the proposed process is based on two independent subsequent steps, as sketched in Figure 1: (i) electrospinning of a polymer solution and (ii) irradiation of the nanofiber mat containing a selected amount of cross-linker and photoinitiator. These two steps can be optimized independently the one to the other, guaranteeing a high versatility to the process. In fact, the electrospinning step can be specifically defined to control the starting morphology of the nanofiber mats, while the photocuring reaction can be implemented to set their final morphology and functionality. In this work, an anionic styrene-butadiene copolymer, with a low vinyl content and a Tg ≈ −70 °C (Figure S1 of the Supporting Information), is selected as a model system, and a trifunctional thiol monomer is used as a cross-linker. The chemistries employed are detailed in Figure S2 of the Supporting Information. For optimizing the electrospinning process, the composition of the butadiene-based solutions is kept constant, with a molar 24545


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ACS Applied Materials & Interfaces

Figure 2. Control of the rubber fiber mat morphology. (a) Effect of the electrospinning flow rate on the mat porosity, measured by analyzing FESEM images. Samples were irradiated immediately after the electrospinning step (waiting time tw = 0 min). The inset represents a FESEM image of a sample obtained by electrospinning with a flow rate of 0.1 mL h−1. (b) Effect of the waiting time between the electrospinning step and the light exposure (tw) on the mat porosity. The inset represents a zoom of the graph for short waiting times (up to 100 min). The error bars on data points of graphs in (a,b) represent the standard deviation of at least five measurements of the same conditions; when the error bar is not visible, it means that it is within the symbol size. (c) FESEM analysis of the effect of tw. Samples in (b,c) were obtained by electrospinning with a flow rate of 0.1 mL h−1. All samples, in (a−c), have Rvinyl/thiol = 8.2 and were irradiated with an exposure dose d = 9 J cm−2.

Figure 3. Control of the photocuring reaction for rubber electrospun nanofibers. (a) Insoluble fraction as a function of exposure dose d for fibers prepared by a system with Rvinyl/thiol = 8.2 and by a thiol-free system (no cross-linker was added to the electrospinning solution). The inset demonstrates the flexibility of a photo-crosslinked nanofiber membrane (irradiated with an exposure dose d = 9 J cm−2). (b) Conversion of vinyl and thiol groups with exposure dose d for fibers with Rvinyl/thiol = 8.2. Data were obtained by FT-IR spectroscopy analysis. (c,d) Evolution of unreacted vinyl and thiol groups concentration, respectively, with d for fibers with different ratios between vinyl unsaturations and thiol groups Rvinyl/thiol. Data were measured by FT-IR spectroscopy. Processing conditions used: electrospinning flow rate = 0.1 mL h−1, tw = 0 min.

butadiene vinyl content, and a similar viscosity in solution, an important difference of the effect of tw can be detected. This is due to the different Tg and in particular to the different microstructure (see the Supporting Information) of the rubber polymers. Therefore, the effective window of waiting time and quality of the electrospun nanofiber mat morphology should be tuned depending on intrinsic properties of the material. Control of the Fiber Functionality. In the previous section, we have demonstrated that the proposed method, which combines electrospinning with photo-crosslinking, allows controlling the morphology of nanofiber mats. Actually, it also offers the unique possibility to tune the chemical functionalities of the nanofibers, which gives great opportunities in terms of designing smart materials with precisely selected functional properties, as both thiol groups and

enhances connections among nanofibers. This results in the progressive collapse of the nanofibrous morphology and in the reduction of both the mat surface area and its porosity (from 62% for tw = 0 min to 26% for tw = 15 days). Comparing Figures 2a,b, it is clear that the waiting time allows the mat porosity control over a much larger range, with respect to the electrospinning flow rate. The behavior of another styrene-butadiene copolymer was also explored: results showing the effect of the waiting time tw on the mat morphology are reported in the Supporting Information (Figure S4). A more rapid change of the electrospun mat morphology is evident and the control of the cold flow is less effective. Thus, although the two investigated butadiene-based copolymers have a similar composition in terms of the styrene/butadiene ratio and 24546


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ACS Applied Materials & Interfaces carbon−carbon double bonds can be used for further modification reactions. Interestingly, it has been demonstrated that for a successful fabrication of nanofibers starting from thiol-ene monomers (i.e., low molecular weight ene monomers, instead of unsaturated polymers), only a stoichiometric ratio can be used.46 On the contrary, we explored offstoichiometry systems, varying the vinyl-to-thiol ratio Rvinyl/thiol, and used the exposure dose d as an additional process parameter. Keeping fixed the composition of the starting polymeric solution, electrospinning parameters, and value of tw, we analyzed the effect of d on the photocuring reaction and on the presence of reactive groups on the final fibers. To this aim, the photo-crosslinking process was monitored by measuring the insoluble fraction of the mats and the disappearance of the vinyl and thiol groups by Fourier transform infrared (FT-IR) spectroscopy (Figure 3). It was assumed that the thiol-ene cross-linking reaction involves the pendent vinyl double bonds, which are demonstrated to be much more reactive toward thiyl radicals than the 2-butene double bonds of the butadiene chain.22,23 Figure 3a shows the insoluble fraction of electrospun mats as a function of the irradiation dose d, when Rvinyl/thiol = 8.2: essentially, the polymer becomes completely insoluble after a dose of 9 J cm−2. In fact, butadiene-based polymers with a low vinyl content undergo efficient photo-crosslinking processes because intramolecular reactions (i.e., when two thiol groups of the same cross-linker molecule react with two neighboring vinyl double bonds located on the same polymer chain) rarely take place.47 Accordingly, in the absence of thiol, insolubilization hardly occurs upon UV exposure (Figure 3a), as crosslinking is not efficient, resulting only from the homopolymerization of the few pendent vinyl double bonds. The effect of the photo-crosslinking reaction on the mechanical properties of the rubber nanofiber mats was evaluated by tensile stress−strain analyses (Figure S5 in the Supporting Information). For instance, an irradiation with an exposure dose d = 9 J cm−2 increases the membrane elastic modulus from 0.9 MPa (for uncross-linked mats) to 3.3 MPa (for cross-linked mats). The stretchability is instead slightly reduced: the average elongation at break is in fact >200% and ≈100% for uncross-linked and cross-linked fiber membranes, respectively. Nonetheless, while fully cross-linked, the nanofiber membranes remain very flexible, as shown in the inset of Figure 3a, and their rubbery behavior is maintained (i.e., Tg = −62 °C after photo-curing). The conversion curves of the thiol and vinyl groups upon UV irradiation are reported in Figure 3b, as a function of the exposure dose (i.e., varying the irradiation time and keeping fixed the light intensity). The thiol-vinyl copolymerization initially proceeds very rapidly, until the thiol monomer is fully consumed: this corresponds to a light dose of 2.5 J cm−2. Upon further irradiation (d > 2.5 J cm−2), the conversion of the vinyl double bonds continues very slowly, even in the absence of thiols, because of homopolymerization. This is confirmed by the rate of decrease of the unreacted vinyl groups, which is found to be the same as for a thiol-free system (Figure S6 in the Supporting Information). In fact, the copolymerization of the vinyl groups of polybutadiene with the trithiol cross-linker is favored over the homopolymerization, even though the vinyl concentration is much higher than the thiol concentration, as the propagating alkyl radicals are more reactive toward the thiol group than toward the vinyl double bond.22

It is important to notice that the amount of exposure dose does not affect the electrospun mat morphology, as demonstrated by Figure S7 in the Supporting Information. In fact, the average fiber diameter varies in the range 205−260 nm by using a dose of 3−15 J cm−2. However, the UV irradiation allows preventing the polymer cold flow, and consequently the loss of the nanofiber mat morphology with time, only if sufficient cross-links are formed in the material. Figure S8 of the Supporting Information shows that a UV dose d of 9 J cm−2 completely stabilizes the nanofibers: after 1 month in ambient air, neither the fiber diameter nor the mat porosity has changed. Therefore, the UV exposure dose, while not influencing the nanofiber mat morphology, can be used to precisely control their functionality. Modulating the thiol/ene stoichiometry over a wide range allows fine control of the concentration of the chemical groups remaining in the nanofibers and thus the functionality of the electrospun mat. Figure 3c,d describes the effect of Rvinyl/thiol, which is modified by changing the thiol cross-linker content in the electrospinning solution. The concentration of the active vinyl groups after the photo-crosslinking reaction diminishes by decreasing the ratio Rvinyl/thiol, while the concentration of active thiol groups increases, as expected. The minimum value of Rvinyl/thiol that could be used is 0.2; below this value, the polymer solution cannot be processed because of its rheological properties. Indeed, the thiol/ene stoichiometric ratio can be adjusted not only by modifying the concentration of the thiol but also by choosing a cross-linker with a different functionality degree (results obtained with a tetrafunctional thiol cross-linker are reported in Figure S9 of the Supporting Information). The control of the rubber nanofiber functionality is undoubtedly a relevant result; interestingly, the proposed method allows achieving it by simply controlling two parameters, namely, the exposure dose d and vinyl-to-thiol ratio Rvinyl/thiol. To obtain functional nanofibrous materials, it is important that unreacted functional groups are present on the nanofibers and can be still available for further functionalization; this will make the electrospun mats useful in a wide range of applications, such as catalysis, selective filtration, and biosensing.48−51 To demonstrate the accessibility of the functional groups on the rubber nanofibers, two different target reactions are selected: the iodination reaction, which tests the reactivity of the CC bonds, and the Ellman’s reaction, which shows the availability and activity of the thiol groups. An electrospun mat prepared from a solution with Rvinyl/thiol = 8.2 and irradiated with an exposure dose d = 9 J cm−2, therefore presenting a high content of unreacted vinyl groups (specifically 1.5 mol kg−1), was immersed in a iodine solution. A fiber mat with a content of unreacted thiol groups equal to 1.9 mol kg−1 (Rvinyl/thiol = 0.2, d = 9 J cm−2) was instead immersed in the Ellman’s test solution. After a defined time (t1), the iodine solution became colorless, while the Ellman’s solution turned to yellow, confirming the occurrence of both test reactions (Figure 4a,b). UV−vis absorbance of the solutions before the immersion of the electrospun mat (t0) and after it, at the completion of the reaction (t1), is compared in Figure 4a and in 4b. These data indicate and prove the reactivity of the nanofibers, also demonstrating that the unreacted functional groups are effectively covalently linked to the cross-linked polymeric fibers (see also Figure S10 in the Supporting Information). Moreover, negative control experi24547


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reactive than the pendent vinyl double bonds and thus do not contribute significantly to the photo-crosslinking process. Use of the Functional Nanofiber Mats in Microfluidic Devices. A rubber electrospun membrane was embedded into a polydimethylsiloxane (PDMS) microfluidic system to fabricate a functional device, as shown in Figure 5a (details

Figure 5. PDMS microfluidic device with an embedded electrospun functional membrane showing unreacted thiol groups: before, t0 = 0 min (a), and after, t1 = 8 min (b), the introduction of the Ellman’s reagent solution. The FESEM image in the inset of (a) illustrates the morphology of the mat before the interaction with the Ellman’s reagent solution. The nanofiber mat was obtained by electrospinning a solution with Rvinyl/thiol = 0.2 and flow rate of 0.1 mL h−1, and subsequent UV irradiation with exposure dose d = 9 J cm−2. In (c), the flexibility of the PDMS microfluidic device with the embedded nanofiber membrane is shown.

Figure 4. Availability of functional groups on the electrospun rubber nanofibers. (a) UV−vis spectra of a iodine solution before (t0 = 0 min) and after (t1 = 30 min) the immersion of a fiber mat containing unreacted vinyl groups. For the electrospinning solution, Rvinyl/thiol = 8.2. (b) UV−vis spectra of the Ellman’s reagent solution before (t0 = 0 min) and after (t1 = 5 min) the immersion of a fiber mat containing unreacted thiol groups. For the electrospinning solution Rvinyl/thiol = 0.2. (c) Concentration of available CC double bonds and thiol groups on the nanofibers, as a function of the molar ratio between vinyl unsaturations and thiol groups Rvinyl/thiol. Data are obtained by UV−vis spectroscopy analyses as in (a,b). For all samples, in (a−c), the electrospinning flow rate was 0.1 mL h−1, the waiting time tw = 0 min, and the exposure dose d = 9 J cm−2.

on the chip preparation are reported in the Supporting Information). The photo-cured mat chosen has a high surface area (porosity = 62%) and high content of unreacted thiol groups (i.e., 1.9 mol kg−1) and is completely insoluble in organic solvents. To prove the functionality of the device, the Ellman’s reagent solution is injected into the microfluidic chip and, because of the presence of active thiol groups in the membrane, its color turns to yellow after 8 min (Figure 5b). This result demonstrates that, as expected, thiol groups are present on the nanofibers and are still reactive even after the assembling of the device. Interestingly, the elastomeric behavior of the electrospun membrane makes it perfectly suitable for applications in flexible and stretchable devices: for instance, as shown in Figure 5c, the fabricated microfluidic chip can be easily bended without delamination nor separation of the nanofiber mat from the PDMS layers. Thus, it is demonstrated that advanced functional and flexible devices can be produced by integrating rubber electrospun mats and exploiting the presence on their nanofibers of a well-controlled concentration of active chemical groups. Thanks to the easy modulation of the nanofiber mat porosity and functionality, the use of these functional devices in (bio)sensing can be envisaged.

ments were performed to verify the accuracy of the results and are reported in Figure S11 of the Supporting Information: in the absence of reactive groups, the UV−vis absorbance of the tested solutions does not change. The same experiments were repeated with other electrospun membranes prepared with a different molar ratio between vinyl unsaturations and thiol groups Rvinyl/thiol. The amount of functional groups available on the nanofibers after the photocuring step was quantitatively measured by UV−vis analyses. Data in Figure 4c show that by increasing Rvinyl/thiol, the amount of available CC double bonds is enhanced, while the amount of available thiol groups is reduced. It is interesting to notice that the iodination reaction can detect all of the unsaturations that are present in the polymeric chain, whereas data reported in Figure 3 consider only the vinyl unsaturations. Therefore, data of Figure 4c count also the backbone 2-butene double bonds of the polybutadiene chain, which are less 24548


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FESEM images of a sample, obtained with the same conditions (in terms of magnification, i.e., 500k magnification); standard deviation was calculated and reported. Images on both sides of the mats were analyzed. The mat porosity percentage was also measured considering the density of the electrospun mat ρmat and density of a continuous film of the same material ρbulk, as [1 − (ρmat/ρbulk)] × 100. The insoluble fraction of the membranes was determined by measuring the weight loss after 24 h of toluene extraction at room temperature. The kinetics of the light-induced cross-linking of electrospun mats was studied quantitatively by FT-IR spectroscopy in transmission mode, by following the decrease upon UV exposure of the absorption band characteristic of the vinyl double bond at 910 cm−1 and of the SH group at 2565 cm−1. During the measurement, the UV exposure dose d was adjusted by varying the irradiation time. The percent degree of conversion was determined from the integration of the absorption band of the reactive functionality before (A0) and after UV exposure for a defined time (At), as [1 − (At/A0)] × 100. Percentage of unreacted groups was calculated from the corresponding FT-IR absorption peak area, considering the absorbance of the sample before UV irradiation as 100% (i.e., all groups are unreacted). As FT-IR measurements were recorded in real time (simultaneous irradiation of the sample and FT-IR spectra acquisition) with a horizontal setup, the analysis was performed on the same sample at exactly the same spot. FT-IR analyses were performed on at least three samples under the same conditions to confirm the reproducibility of the results: a variation lower than 4% was obtained. However, data reported correspond to measurement series conducted on a single sample. For determining the presence of active CC bonds, an iodination reaction was performed. A fiber membrane (Rvinyl/thiol = 8.2) was obtained by electrospinning (flow rate = 0.1 mL h−1), irradiated with UV light (exposure dose d = 9 J cm−2), and then washed with a THF/ DMF 4:1 solution. A small piece of the membrane (1 × 5 cm2) was cut and inserted together with the Al substrate in a I2 2.5 mM aqueous solution. Few drops of HCl 1 M solution were added and the reaction was followed by UV−vis spectroscopy for 30 min. For determining the presence of active thiol groups, Ellman’s reagent DTNB was used. A fiber membrane (Rvinyl/thiol = 0.2) was obtained by electrospinning (flow rate = 0.1 mL h−1), irradiated with UV light (exposure dose d = 9 J cm−2), and then washed with a THF/DMF 4:1 solution. A small piece of the membrane (1 × 5 cm2) was cut and inserted together with the Al substrate in a DTNB 20 mM aqueous solution, prepared with a 0.1 M buffer solution (pH = 7.4). The reaction was followed by UV−vis spectroscopy for 5 min. For both reactions, UV−vis spectra were acquired on diluted solutions (1:20 vol/vol) with a Jenway 6850 UV−vis spectrophotometer. The same UV−vis analyses were repeated in the presence of different electrospun mats and used to quantitatively determine the amount of CC double bonds and thiol groups available on the fibers. Stress−strain measurements were performed at ambient temperature on an Instron model 5566 dynamometer system, using a 10 kN load cell at a crosshead speed of 5 mm min−1. Young’s modulus was determined from the stress−strain curves. At least three replicates were performed for each sample.

CONCLUSIONS This work proposes a two-step method based on electrospinning and photo-crosslinking for the efficient preparation of rubber nanofiber membranes with a well-controlled morphology and functionality. The proposed process allows preventing the butadiene-based polymer cold flow, which causes a change of the fiber size and a loss of mat porosity and surface area, and to increase the chemical resistance of the material, while its flexibility is maintained. More interestingly, the morphology and functionality of the nanofiber mats can be finely controlled in an easy way. In fact, the desired morphology (in terms of fibers diameter and membrane porosity) can be set by tuning the electrospinning parameters and waiting time before irradiation, at the end of the electrospinning step. However, the density of unreacted groups present on the surface of the rubber mats can be tailored by changing the thiol/ene stoichiometric ratio and light dose. Such reactive chemical groups are available for further reactions: it is demonstrated that even when the nanofiber membrane is embedded into a microfluidic device, its functionalities are accessible and active. Hence, the proposed method is indeed ideally suited for preparing functional membranes for a wide range of applications, such as chemical and biological sensing, energy devices, stretchable electronics, and (bio)filtration.

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EXPERIMENTAL SECTION

Materials. Commercial anionic styrene-butadiene linear block copolymer Kraton D1102, with 28 wt % bound styrene, 10 wt % vinyl, and low cis-structure content, was kindly provided by Kraton. A trifunctional thiol was selected as a cross-linking agent: trimethylolpropane tris(3-mercaptopropionate), from Sigma-Aldrich. 2,4,6Trimethylbenzoyl(diphenyl)phosphine oxide (Lucirin TPO from BASF) was chosen as a photoinitiator. Tetrahydrofuran (THF), dimethylformamide (DMF), toluene, and Ellman’s reagent 5,5′dithio-bis(2-nitrobenzoic acid) (DTNB) were purchased from SigmaAldrich and used as received. Iodine solution 0.05 M was purchased from Sigma-Aldrich and diluted with deionized water. All other chemicals were obtained from Sigma-Aldrich. Electrospinning and Photocuring Process. A NANON 01A electrospinner setup (MECC CO., LTD.) was used in this work. It is equipped with a high voltage power supply (HVU-30P100) and syringe pump operating with a flow rate ranging from 0.1 mL h−1 up to 99.9 mL h−1. Electrospinning was performed with 18 wt % polymer solutions in THF/DMF 4:1, with 1 wt % (with respect to the polymer) of photoinitiator. Solutions containing different amounts of the thiol cross-linker were used: 0, 3, 10, and 50 wt % (with respect to the polymer). Electrospinning was conducted with a flow rate of 0.1−1.2 mL h−1, applied voltage of 24 kV, and a tip-to-collector working distance of 13 cm. Aluminum foils were used as the substrate during electrospinning. Each sample was prepared by electrospinning 0.06 mL of polymer solution; fiber mats with a thickness of around 25 μm were obtained. The membranes were irradiated by means of a high-pressure mercury−xenon lamp equipped with an optical fiber (Hamamatsu LC8), with a UV light intensity of 15 mW cm−2. The UV exposure dose d was adjusted by varying the irradiation time: the time window used was 0 s to 15 min. Characterizations. Morphology of all samples was examined by FESEM (ZEISS, Auriga and Supra 40). Prior to FESEM examination, samples were coated with a 10 nm thick Pt film via sputtering, carried out by QUORUM Q150T ES applying a current of 50 mA for 30 s. Additionally, the FESEM images were processed with an imaging software (ImageJ) that allows better characterization of the morphological properties of the mats. In particular, the porosity distribution of the nanofiber mats was measured by analyzing five

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b04599. Dynamic mechanical thermal analysis; details of the chemistries used; effect of the electrospinning flow rate; effect of the waiting time on another styrene/butadiene polymer; tensile stress−strain behavior of the electrospun mats; concentration of unreacted vinyl groups for a thiol-free system and for a system containing a tetrafunctional thiol cross-linker; effect of the exposure dose; proof of the nanofiber mat morphology stability; 24549


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effect of washing procedure; UV−vis spectroscopy control experiments; and microfluidic chip fabrication (PDF)

AUTHOR INFORMATION

Corresponding Authors

* E-mail: [email protected] (A.V.). * E-mail: [email protected] (M.Q.). ORCID

Alessandra Vitale: 0000-0002-8682-3125 Giulia Massaglia: 0000-0003-2363-0307 Marzia Quaglio: 0000-0003-3707-8760 Author Contributions ⊥

A.V. and G.M. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge Kraton and Synthos for kindly providing us the styrene-butadiene copolymers. REFERENCES

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Tuning Porosity and Functionality of Electrospun Rubber Nanofiber

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