Transparent Composite

Electrospun fibers have been successfully demonstrated as fillers for improvement in the mechanical property of composites. When the diameter of the nanofibers are sufficiently small, the reduction in light scattering by the nanofibrous fillers are able to give rise to transparent composite. For this property, the diameter of the fibers should preferably be less than 100 nm. Bergshoef et al [1999] has successfully constructed transparent composite using electrospun nylon fibers with diameter of about 100 nm as fillers and epoxy as the matrix material. However, fibers with larger diameter has also been tested and has shown good transparency. Antoine (2013) used polyacrylonitrile (PAN) hollow fibers constructed using coaxial nozzle with diameter of 1.18 µm as fillers within a poly (methyl methacrylate) (PMMA) matrix. Transmittance for pure PMMA was 92% and transmittance with 5% PAN reinforcements and 10% PAN reinforcements were 85% and 80% respectively. With 5% PAN, the ultimate tensiles stress and Young's modulus of the composite were increased by 58.3% and 50.4% respectively. An important criterion to achieve optical transparency is the absence of voids within the composite. Parameters such as viscosity of the matrix material and porosity of nanofibrous membrane need to be optimized such that complete impregnation of the nanofibers within the matrix can be achieved. Using Polyamide 6 nanofibrous membrane as fillers, Stachewicz et al [2012] optimized the concentration of polyvinyl alcohol solution to completely fill the pores within the nanofibrous membrane to achieve optical transparency with improved mechanical strength and toughness.

Surface topography of the filler fibers may also affect the interfacial compatibility with the matrix material. Xuan et al (2021) tested the effect of electrospun poly (L-polylactic acid) (PLLA) fibers with smooth and porous surface topography when embedded within a poly(methyl methacrylate) (PMMA) matrix. Electrospun fibers with two different pore sizes were produced by dissolving PLLA in different organic solvents to induce phase separation in a high humidity environment. Comparing the light transmittance of the composite, pure PMMA has the highest transparency of 89% at wavelength of 589 nm. For PMMA composite containing smooth surface PLLA fibers, the transmittance drops to 80.16%. With porous fibers, the drop in transmittance was less and the composite containing the fibers with larger pores has a transparency of 88.72%. This is despite smooth surface fibers having the smallest diameter. The better light transmittance may be due to greater interfacial compatibility between the porous PLLA fibers and the PMMA matrix. The presence of PLLA fibers regardless of surface topography in the composite also significantly improves the mechanical properties of PMMA. However, if the difference in the refractive index between the filler and the matrix material is high, the resultant image behind the composite may appear hazy if it is not viewed directly behind the composite. Thus similarity in the refractive index between the two components is preferred [Yu et al 2010]. Stafiej et al (2018) demonstrated the optical properties of electrospun polycaprolactone (PCL) fibers, both randomly oriented and cross aligned, embedded in alginate hydrogel. The fiber diameter for random and aligned ranged from 42 to 255 nm and 83 to 305 nm respectively. Analysis using UV-Vis-spectrophotometer for visual light area ranging from 400 to 800 nm showed that the composite exhibited a low transparency at a small wavelength and an increase in transparency with an increase in wavelength. The optical transparency reduces with increasing fiber thickness with similar curve profile. Between composite reinforcement using cross-aligned fibers and randomly oriented fibers, the former showed better transparency and this can be attributed to greater inter-fiber spacing.

Perhaps the most straight forward way to ensure similarity in refractive index is to use the same material for both the fillers and matrix. Chen et al (2011) constructed polyimide (PI) composite films with highly aligned polyimide (PI) fibers as filler materials. Electrospinning was carried out for polyimide precursor (poly(amic acid)) to form aligned nanofibers. Imidization process was then carried out through heat treatment of the electrospun precursor fibers. These fibers were soaked in dilute solution of poly(amic acid) before imidization through the same thermal treatment. While the as-spun membrane is opaque, the self-reinforced polyimide (PI) composite film showed considerable transparency although it is not quantitatively measured. Mechanical strength and elongation at break with the PI nanofibers reinforcement showed an improvement of about 97% and 46% respectively over neat PI films. Interestingly, there are cases where the addition of nanofibers increases the transparency of the otherwise neat material. Gavande et al (2023) demonstrated this using nylon 6 nanofibers as fillers for epoxy resin. The electrospun nylon 6 (N6) nanofibers have a diameter of less than 100 nm and its refractive index is close to that of the epoxy resins. The optical transmittance of cured epoxy films of YD epoxy, YDJR epoxy mixture, and JR epoxy were 56%, 60%, and 62% respectively. With the addition of nylon 6 (N6) into its matrix, the optical transmittances became 66%, 72%, and 80% for the N6/YD, N6/YDJR, and N6/JR epoxy composites, respectively. The improved transparency with the addition of nanofibers may be due to good interfacial bonding and wettability between the N6 nanofibers and the epoxy resins. As the resin penetrates the N6 nanofiber mat, the interconnecting fibers guide the resin into the interior and reduce the generation of microbubbles which would have reduced the clarity of the cured resin.

Moving beyond mechanical reinforcement, electrospun nanofibers have been tested as carrier of dye and embedded within a matrix for the purpose of optical tagging. In the work by Yu et al [2010], poly(vinyl butyral) nanofiber was used as the carrier for the dye and poly(methyl methacrylate) was used as the matrix. The similarity in their refractive index allows the construction of transparent composite although the diameter of the poly(vinyl butyral) nanofiber was about 500 nm. The UV visible dye was found to be stable and was visible under UV after electrospinning and embedding in the matrix.

Transparent conducting polymer film has also been fabricated combining inorganic nanofiber film with a polymer matrix. Techniques such as hot pressing and spin coating have been successfully used to fabricate conductive composite film containing nanofibers. Ostermann et al [2011] electrospin poly(vinyl pyrrolidone) with dispersed antimony doped tin oxide (ATO) nanoparticles. Sintering was then carried out to remove the polymer, leaving behind free standing ATO nanofiber mat. Using poly(styrene) or poly(vinyl pyrrolidone) as the matrix for the ATO nanofiber mat, a conductive and transparent composite film with resistivity of 20 kΩ/cm was fabricated by hot pressing. Hwang et al (2014) constructed a conductive transparent composite film using silver nanofibers, single-walled carbon nanotube (SWCNT) and poly-(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS). Silver nanofibers were formed by calcination of electrospun silver nitrate/polyvinyl pyrrolidone solution. A solution containing SWCNT and PEDOT:PSS was layered over the silver fiber coated quartz glass using spin coating. At ten deposition cycles, the transmittance drops to 65% but the surface resistance was reduced from more than 3 x 10⁶Ω/sq at zero coating to 2 Ω/sq [Hwang et al 2014]. Given that SWCNT itself is highly conductive and silver nanofiber without SWCNT coating exhibits high resistance, it is unclear whether silver nanofiber has contributed to the conductivity of the composite membrane or the improved conductivity is due to SWCNT alone.

Table 1. Transparent composite with nanofiber filler

Nanofiber Filler	Filler Description	Matrix	Reference
Nylon-4,6	Randomly oriented nanofibers. Fiber diameter, 30-200 nm	Epoxy	Bergshoef et al 1999
Poly(vinyl butyral)	Randomly oriented nanofibers. Fiber diameter 500 nm	Poly(methyl methacrylate)	Yu et al 2010
Polyamide 6	Randomly oriented nanofibers. Fiber diameter, 134 nm	Polyvinyl alcohol	Stachewicz et al 2012
Polyacrylonitrile	Randomly and aligned nanofibers membrane. Fiber diameter, 500 to 600 nm	Epoxy	Ren 2013
Polyacrylonitrile	Randomly oriented hollow nanofibers membrane. Fiber diameter, 1.18 µm	poly (methyl methacrylate)	Antoine 2013

Spaces may be created on the electrospun membrane to increase its optical transparency in a hybrid structure. To enhance the transparency of electrospun aligned nanofibers, Kong et al (2017) used laser to create perforations of diameters in the range of 100 to 200 µm at intervals of 50 to 100 µm on the membrane. This perforated electrospun poly (lactic-co-glycolide) (PLGA) membrane was sandwiched between collagen gels and compressed to give a hybrid construct. With the perforation, the optical transmittance was 15 fold higher than hybrid construct with non-perforated membranes. This hybrid construct was found to exhibit an optical transmittance of 63% and this was increased to 72% after 7 days of immersion in PBS solution.

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