Electrospun Fiber Bonding

SEM images of PA6 electrospun membranes thermally bonded onto viscose non-woven [M. Faccini, C. Vaquero, and D. Amantia, Journal of Nanomaterials, vol. 2012, Article ID 892894, 9 pages, 2012. doi:10.1155/2012/892894. This work is licensed under a Creative Commons Attribution 3.0 Unported License.]

Electrospun nanofibers are often coated onto a substrate such that the substrate provides the mechanical robustness while the coated nanofibers provide the functional performance. Application that uses this layered configuration includes air filter media and clothing. To ensure durability of the layered composite, the adhesion between the nanofibers and the underlying substrate needs to be good. Unfortunately, this can be challenging as the actual contact points between the two layers may be very low which leads to poor bonding strength. When there is debonding between the electrospun membrane and the supporting substrate, it is important to determine the source of the failure. It may be poor adhesion between the fiber and the substrate or separation of fibers from the substrate bonded fibers. Guo et al (2021) showed that with electrospun polyethylene terephthalate (PET) on a PET microfiber supporting substrate, the electrospun PET nanofiber was completely debonded from the substrate in a peel test when the membrane is thin. However, for thicker PET membrane, the breaking interface is within the electrospun membrane and adhesion strength reduces with increasing membrane thickness. Hence for thicker membrane, the weak link is at the interfiber bond instead of membrane substrate adhesion.

For in situ electrospun fibers on a substrate, there are several factors affecting its adhesion strength. Three possible factors that affect fiber to substrate adhesion are surface contact area, substrate surface asperities and electrostatic residual charges [Zhang et al 2020]. With greater surface contact area between the fibers and substrate, there is a greater amount of van der Waal attraction between them. Thinner electrospun fibers generally give greater density of fibers per unit area. Therefore, factors that give rise to thinner fibers will yield stronger adhesion of the electrospun layer to the substrate. Substrate surface asperities work as a mechanical lock onto the electrospun fiber layer. Zhang et al (2020) showed that electrospun polyvinylpyrrolidone (PVP) layer adhered more strongly to rough wood pulp paper to smooth silicon paper. In electrospinning, there will be charges in the electrospun fiber. Depending on the substrate material, the speed of electrostatic decay may differ. When the speed of decay is slow as electrospinning to a non-conducting substrate, the residual charges on the fibers will repel incoming fibers. Therefore, the deposited fiber layer will be loose on the substrate and there is less contact between the fibers and the substrate surface. This will result in less adhesion to the substrate surface.

The relatively low number of actual contact points between the nanofiber and the underlying substrate is due to random deposition of continuous fibers. As the continuous fibers are laid down on the bottom substrate, the part of the fiber that comes into contact with the bottom substrate will be supporting the later part of the fiber. Even with applied pressure, the "second" layer of nanofiber may not come into contact with the substrate. Therefore, the bonding strength between the nanofiber layers and the substrate relies on just a single "layer" of nanofiber that is touching the substrate. The strength of such small contact areas is obviously low.

To improve the bonding strength, it is necessary to increase the number of nanofiber "layers" that participate in the adhesion with the bottom substrate. One method is to fuse the layers of nanofibers with the underlying substrate at the points of contact. Between nylon nanofibers layer and a cellulose-based air filter media substrate, Lei L et al (2006) used formic acid vapor to encourage fusion of the nanofibers at the contact points. This has been shown to improve the abrasion property of the resultant coated filter media. Using heat and pressure such as ironing to improve bonding between fibers and base substrate has also been attempted. However, the adhesion is not satisfactory for electrospun polyamide-6 nanofibers on cellulose-based nonwoven material [Hakala and Heikkila 2011].

Bonding between fibers and with substrate is also dependent on the fiber material. While some materials are naturally "sticky", others are more inert. One way of improving adhesion of more inert fiber materials is by blending a "sticky" material into it. In order to improve the adhesion properties of polycaprolactone (PCL), Yalcinkaya et al (2016) blended polyvinyl butyral (PVB) into PCL solution for electrospinning into fibers. A disadvantage of blending a sticky material with the desired material is that the mechanical strength of the desired material is weakened. Nabzdyk et al (2015) showed that the mechanical strength of fiber mixture is superior to blended fibers based on nondegradable poly(ethylene terephthalate) (PET) and biodegradable poly(glycolic acid) (PGA ) nanofibers. For a mechanically strong material, its strength derives from its crystallinity or strong intermolecular bonds. In a blended system, such molecular arrangement and bonds are disrupted. This will significantly reduce the strength of the polymer compared to its homogeneous form. Kozior et al (2019b) showed that if the electrospun solution is a common solvent to the base substrate, it is possible to induce good bonding between the two. To demonstrate this effect, Kozior et al (2019b) used two different base substrate material, poly(lactic acid) (PLA) and the thermoplastic polyurethane (TPU) and polyacrylonitrile (PAN) dissolved in dimethyl sulfoxide (DMSO) as the solution to be electrospun. With PLA as the substrate, the electrospun PAN fibers were unable to bond to it. However, with TPU as the substrate, the electrospun PAN fibers showed very good adhesion. This is because DMSO is a solvent for TPU but not for PLA. During electrospinning, residual solvent on the PAN fibers were able to partially dissolve the base TPU leading to good bonding between the two as demonstrated by Martindale abrasion tests.

Heat with compression may also be used to bind electrospun fibers to the bottom substrate. An additive may also be used to improve the affinity between the fibers and the substrate. Shen et al (2021) added polyhedral oligomeric silsesquioxane (POSS) to the electrospinning solution to improve binding between graphene and the electrospun fibers. Three POSS molecules were tested, octaepoxycyclohexyl-dimethylsilyl (EP)-, trisnorbornenylisobutyl (NB)-, and octaphenyl (MS)-POSS. The functional groups of EP-POSS are flexible while the functional groups of the other two are rigid. With electrospun poly(acrylonitrile-co-vinyl acetate) copolymer (PAN-co-VA), the addition of EP-POSS increases the coverage by graphene on the electrospun membrane from 69% to 98% while the other two POSS molecules does not improve graphene coverage. Therefore, for binding with graphene, hydrophobic molecules with flexible chains are preferred.

Application of a slow or heat curing adhesive on the bottom substrate prior to coating with nanofibers may also be used to bond the two materials. However, the adhesive solution must be able to penetrate through the layers of nanofibers at contact points prior to curing to achieve good bonding. Faccini M et al (2012) first selectively deposit adhesive powder on a substrate. Electrospun fibers are subsequently deposited over the adhesive powder and substrate before hot pressing to fuse the fibers to the substrate by melting the adhesive powder. Mohammadian and Haghi (2014) demonstrated the feasibility of establishing good bonding between electrospun fibers and an underlying substrate using a polypropylene spun-bond nonwoven (PPSN) layer as the melt adhesive layer for hot-press lamination. By adjusting the lamination temperature, they were able to completely melt the adhesive layer for penetration into the nanofiber web. However, as the adhesive layer fills in the pores between the nanofibers, the resultant air permeability of the multi-layered fabric suffered.

Inducing chemical bonding between electrospun fibers and the base substrate will certainly improve bonding strength. Plasma treatment is a common method of introducing surface free radicals for further reactions. Vitchuli et al (2012) used plasma pre-treatment on a woven fabric substrate before electrospun deposition of nylon nanofibers on it. The resultant adhesion strength and durability of the adhesion was significantly improved. With plasma pre-treatment to generate free radicals, strong cross-linking bonds can be formed between the electrospun fibers and the fabric. Janu et al (2023) treated the polypropylene (PP) supporting fabrics using low-pressure plasma oxygen treatment before polycaprolactone (PCL) nanofibers were electrospun onto it. The adhesion between the fibers and the fabric as measured by its work per area (W/A) was almost 5x higher than untreated fabric. Improved adhesion from the plasma treated fabric may be attributed to greater oxygen containing reactive groups. High surface area of the electrospun nanofibers may also provide more bonding sites and greater proximity to the fabric surface where any improvement in the molecular bonding strength can be magnified. Plasma modification with amine reactive groups on the supporting fabric gave a lower bonding strength compared to oxygen containing reactive groups. With plasma treatment, the type of reactive groups need to be investigated for optimum bonding strength between the electrospun fibers and the supporting fabric.

For some polymers, post electrospinning curing or binding of the fibers may help adhesion with the bottom substrate. While the electrospun polymer may not be chemically bonded to the bottom substrate, bonding between the fibers may interlock with the supporting substrate especially if the bottom substrate is porous. Al Kayal (2020) constructed a bilayered fibrin/poly(ether)urethane (PEU) scaffold with PEU phase-inversed membrane as the base supporting the electrospun layer of fibrin fibers. After electrospinning of fibrin, polymerization was obtained by thrombin solution spraying onto it. Adhesion of the fibrin fibers on the PEU base was found to be strong through qualitative peeling.

For thicker electrospun membrane layer, a low viscosity light curable resin/adhesive may also be used for bonding to the substrate. Such resin may be introduced from the top and allowed to penetrate through the thickness and to the base substrate before curing. Wang et al (2013) used AZ 5214 positive photoresist to form micro-chambers on the electrospun membrane and for adhesion to the glass substrate below. An obvious benefit of this method is that the full thickness of the fiber layer is in contact with the resin thus providing maximum bonding through its depth and to the substrate.

(A) Schematic diagram of AZ 5214 penetrating through the porous structure, and bonding with the glass substrate and the surrounding fibers (cross section). (B) and (C) Digital camera pictures of the top view of real products. (D) Micro-chambers filled with different color inks. [Wang et al. PLoS ONE 2013; 8(12): e82888. doi:10.1371/journal.pone.0082888.g003. This work is licensed under a Creative Commons Attribution 4.0 International.]

Where electrospun fibers are combined with 3D fuse deposition to form an assembly, creating a good fusion between the electrospun fibers and the 3D printed mesh may just be a matter of the order of the process. Pensa et al (2019) selected a blend of electrospun poly-epsilon;-caprolactone (PCL) and gelatin for electrospinning and poly(lactic acid) (PLA) for 3D fuse deposition. Instead of using fused deposition to form the supporting substrate first, electrospinning was carried out to form PCL/gelatin membrane. Fuse deposition was then carried out to lay down the supporting substrate on the electrospun membrane. The PCL component of the electrospun PCL/gelatin membrane has a relative low melting point. During fuse deposition where the higher melting point PLA was extruded as a melt, the higher temperature of the extruded PLA filament would result in partial melting of the electrospun membrane hence causing the supporting PLA substrate to bond with the electrospun membrane.

In most cases, bonding of the electrospun fibers are with a substrate at the bottom. Kozior et al (2019) suggested an alternative method where the bonding and main supporting structure is on top of the electrospun fibers with the help of 3D printing. For their method, the electrospun membrane is first formed and 3D printing is subsequently used to deposit strips of poly(lactic acid) (PLA) fiber on it. However, there are several important points to take note. The distance between the printing nozzle and the nanofiber membrane needs to be precise due to the high temperature of the printing nozzle. However, too great a distance will result in poor contact between the printed fiber and the electrospun membrane, giving a very uneven surface. At optimal distance, the printed fibers will adhere to the top layer of the electrospun membrane and is unable to penetrate through the depth of thick membrane.

The requirement for good bonding between nanofiber and substrate is more of an industrial product development issue rather than basic academic research thus there are limited published studies on this area. However, for eventual translation of academic research results on nanofiber performance to commercial product, the reliability and durability of the nanofiber-substrate composite needs to be addressed.

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