Optical Properties of Transparent Resins with Electrospun Polymer

Chapter 25

Optical Properties of Transparent Resins with Electrospun Polymer Nanofibers 1,2

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C. Krauthauser , J. M. Deitzel , D. O'Brien , and J. Hrycushko

Downloaded by AUBURN UNIV on February 20, 2016 | http://pubs.acs.org Publication Date: February 23, 2006 | doi: 10.1021/bk-2006-0918.ch025

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Center for Composite Materials, University of Delaware, Newark, DE 19716 U.S. Army Research Laboratory, Armor Mechanics Branch, Aberdeen Proving Ground, MD 21005 U.S. Army Research Laboratory, Multifunctional Materials Branch, Aberdeen Proving Ground, MD 21005 2

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Model composites have been made using electrospun Nylon 6,6 nanofibers with diameters of ~100 nm. Resins used include both transparent epoxy and vinyl ester resins. Optical measurements using a white light source have been made to evaluate the degree of light transmission, haze, and clarity for the composites. A simple model describing the transmission of light through the composite specimen has been developed. This model relates fiber volume fraction, fiber diameter, indices of refraction of both fiber and matrix, and sample thickness to the percentage of light transmitted. The model demonstrates that submicron fiber diameters are necessary in order to maintain a high degree of transparency. Predictions of the model are in good agreement with experimental data. The work shows that processing issues such as resin wet-out of the nanofiber fabric and mitigation of void formation are key factors to obtaining clear nanofiber composites.

© 2006 American Chemical Society

In Polymeric Nanofibers; Reneker, Darrell H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

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Introduction There is a need for light-weight, impact resistant transparent materials for use in a variety of applications, which could include vehicle windows and windshields, hand held protective shields, face shields, and protective eyewear. Solutions using glass and layered polymer/glass composites have the disadvantage of adding a significant weight penalty to a given application and are generally restricted to use in vehicle and building applications. Some research (1] has looked at using glass fiber to reinforce resins with a matching index of refraction. In addition to the same issues of weight associated with the laminate composites, the optical clarity of these materials is generally dependent on temperature due to difference in CTE between the resin and glass fiber as well as thermal dependence of indices of refraction. Applications requiring light weight, such as face shields and protective eyewear, usually employ a transparent polymer resin like vinyl ester, epoxy(thermoset), or polycarbonate (PC) and poly(methyl methacrylate)(PMMA)(thermoplastic). However, the impact resistance and structural properties of these materials are limited by their relatively low mechanical properties in comparison to high performance materials like Kevlar and carbon fiber reinforced composites. Reinforcement of these transparent resins with high perfomance materials is problematic due to the need to maintain optical clarity. Recent efforts by Hsieh (2] have looked at using nanoparticulate fillers and microlayering processing techniques to increase the impact performance of PC, and PC/PMMA blends, (needs a couple more examples of nano reinforcement) We propose a novel approach to increasing the impact properties of transparent polymer resins like thermosetting vinyl ester and epoxy resins, and thermoplastic resins like PC and PMMA by reinforcing these resins with high performance polymer nanofibers. These nanoscale fibers can be produced readily using the process of electrospinning [3-7]. The electrospinning process uses an electrostatically driven jet of polymer fluid (solution or melt) to form fibers with diameters rangingfrom50-500 nm. Thesefibersare most often collected in the form of a non-woven mat of randomly oriented fibers. The advantages of using nanofibers in transparent composites are many. First, the small diameter (~200nm; see Figure 2a and 2b) of thefibersare below the characteristic wavelengths of visible light (^=400-700 nm), and therefore, nanofibers dispersed in a transparent medium should not unduly scatter light in the visible range [31. Second, nanofiber textiles have orders of magnitude



355 greater specific surface area [4) than conventional fabrics, due to the small fiber diameter. The greater surface area will provide more interaction between the resin and reinforcing fiber, improving mechanical properties and potentially increasing the amount of energy dissipated during an impact event due to sliding friction associated with fiber pullout. The wide variety of polymer materials that can be electrospun [5J provide the engineer with greatflexibilityin designing a transparent composite for specific applications(ie. high performance fibers for structural applications, elastomeric fibers for toughening and impact resistance, conductivefibersfor E M shielding etc.). Finally, in addition to an improvement of mechanical properties, nanofibers can provide a continuous network connection a variety of sensors for health monitoring of the composite. In the work presented here, a simple theoretical model has been developed to predict the optical transmission properties of a transparent material reinforced with electrospun fabric. This first order model relates the variables of index of refraction, fiber volumefraction,andfiberdiameter to the optical transparency (transmission of light in the 400-700nm region) of the composite system. The transmission of light in the optical region has been measured for transparent composites made from electrospun Nylon 6,6 and are found to be in good agreement with predictions made by the model.

Experimental

Electrospun fiber mats Nylon 6,6 sub-micron fiber textiles have been fabricated using the electrospinning process. Nylon 6,6 was spunfroma solution of forming acid at a concentration of 20% by wt. The electrospinning voltage was 8 kV and the distance between syringe needle and collection plate was - 15 cm. Estane*" fibers were spunfroma binary solvent of Tetrahydrofuran (THF,75%)/ Dimethyl Formamide(DMF, 25%) at a concentration of 5% by weight. The spinning voltage for the Estane " solution was 7 kV, and the distance from syringe needle to collection plate was - 12 cm. Each type of electrospun mat was examined using field emission scanning electron microscopy and the average fiber diameter was measuredfromthe micrographs. 1

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Electrospun fiber composites Thin film Nylon-6,6 electrospun fiber composites were made using both Epon 828 epoxy and Derkane Vinyl ester resin. In order to eliminate air bubbles the thermoset resin was held under vacuum to -10 psi for 30 minutes, prior to infusion. After the resin was degassed, the electrospun fabric was slowly lowered into the resin in order to get complete infiltration. The resin infused fabric was then placed between two glass slides that had been treated with a thin layer of mold release, and compressed under a pressure of -25 psi. The composite was allowed to gel at room temperature. The samples were then postcured for one hour at 130 ° C. Optical properties of the electrospun fiber composite were characterized with respect to total transmission, haze and optical clarity using a B Y K Haze-Gard Plus apparatus (ASTM D 1003). The illumination source used in this technique was white light. A major question to be confronted is what are the factors controlling transmission, and thus, controlling optical transmission. There are many factors controlling optical transmission of light through a media, such as orientation of inclusions, characteristic sizes of the inclusions, volume fraction of the inclusions, and so forth. These issues will be discussed in further detail in the next section.

Theory: A look at fiber size and transparency There are many parameters, such as transmittance, haze, clarity, that can be used as a measure of the level of transparency that characterizes a particular specimen. Transmittance is traditionally defined as the ratio of the intensity of the transmitted light through a specimen to the intensity of the incident light on the specimen. The ASTM D 1003 defines haze as that percentage of transmitted light which in passing through the specimen deviatesfromthe incident beam by more than 2.5° on average, whereas clarity is evaluated in an angle smaller than 2.5°. In this section, in an attempt to present a simple theory for understanding the effects on transparency of infusing nano-scale fibers into transparent resins, our focus will be exclusively on determining transmittance; the issues of haze and clarity will be treated elsewhere. There are many factors that can affect optical transmittance, which include fiber diameter and volumefractionof thefibersin the resin. Optical transmittance, as given above, is expressed as [1]:

(i)


357 As is known, the scattering of electromagnetic signals by any material is related to the optical heterogeneity of that material. In addition to scattering, the material could also absorb some of the energy of the electromagnetic signal. These two pieces combined affect the optical clarity of the incident light, and make up the attenuation that diminishes the optical clarity, thus Attenuation = Scattering + Absorption When incident light of intensity I traverses a slab of heterogeneous material over an optical path d , the transmitted intensity can be given by (Beer's Law) 0


0

I,=I exp(-Ad ) 0

(2)

o

assuming that effects from multiple scattering can be ignored, and that phase and wavelength of the light after the scattering is unchanged (coherent scattering). Here, A is the attenuation coefficient, and is given by

where n is the particle density for the i-th material, G t

a i

and C7 are the si

absorption and scattering cross-sections, respectively, for the i-th material. In this paper, we will assume that the inclusions are, to good approximation, nonabsorbing. For non-absorbing particles, only the scattering term is important, and thus A = ^jT n G . Furthermore, it is assumed for this paper that the i non-woven mat can be approximated as a collection of non-interacting, randomly oriented, cylindricalfiberswith high aspect ratio. By non-interacting, it is meant that there is no weave associated with the collection, and furthermore, the individual fibers are, on average, sufficiently farfromneighboringfibers(greater than 4-5 fiber diameters). {

si

Let VJJ be the volumefractionof the i-th material, and v be the t

volume of a single particle of material i, then it is clear that fl = VJ-J IV-, and i

thus


358 Letting G

to be the so-called geometrical cross section, we define the

g i

scattering efficiency, Q, by

(5)

a = —


and thus

v

'

i

Here it becomes necessary to specify the types and extension of the materials embedded in the transparent resin. To a good approximation, there are two basic types of inclusions in the transparent resin: infinitely long cylinders and spheres. Each has to be taken into account when determining the attenuation factor, thus there will be 2 basic terms:

A = A +A C

(7)

S

For cylindrical strands, it is clear that (T = djL , whererf/isthe fiber strand g

2

diameter and L is the strand length, and v = fld L/4. Thus, f

For normally unpolarized incident light, Q can be expressed as: c

£

Q (n n d A)=-t i\B f c

fy

Mt

ft

+\K()

nl

where

_

J.(mxyr.(x)-mJ:(mx)j (x) a

J (mx)H'Xx)-rnJ' (mx)H (x) n

tt

n


(9)

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mJ (mx)H' (x)-J'Xmx)H (x) n

n

n

Here, £ = 2 when n > 0 and £ = 1, x = w

0

in the matrix material, m = n^ I n

/ A and A is the wavelength

is the relative refractive index of the

M

polymer fiber material («/) and the matrix material (n ), J is a Bessel function of the first kind, H is a Hankel function of the second kind, and the primes denote differentiation of the functions with respect to their arguments. It is useful to have some bounds on what can be expected in terms of the transmission ratio for the case of cylindrical fibers perfectly infused (no gaps, voids, bubbles, etc.) by a transparent resin. The electrospun fibers that will be considered experimentally have a characteristic diameter of M

n


n

d f ~ 100 — 300 nm, the volume fraction of the fibers in the matrix material goes as Vj- ~ 0.10, the thicknesses of the resin infused samples would go as d

0

~ 0.05 — 0.10 nm, and the wavelength of light going through the matrix

material goes as X ~ 400 - 700 nm. In terms of indices of refraction, the fiber material was chosen to have an index of refraction reasonably close to that of the resin material, thus H

nylon6 6

=

%

n

polyurethane = 1 -5 — 1.6

n

n

1-53

=1.55-1.60

epoxy

*^

polyester

=1.52-1.54

Within this range of values, X, mx ~ 0.45 — 2.36. For our purposes, it is practical to choose n

vinyester

= 1.52, thus m = 1.00658. If we consider the

above samples, allowing only the fibers to be the scatterers, and taking 200 nm as the average fiber diameter, and 550 nm as the average wavelength of light, the predicted transmissions are 99.31%, 99.34%, and 99.48% for the samples in Figures 6A, B, C respectively. When compared to the 95%-98% transmission as measured, there is clearly a not insignificant role being played by the inclusions and voids that have resultedfromprocessing. One can go through a similar analysis of considering the voids and inclusions. We consider these voids and inclusions as essentially spherical balls made up mostly of air, with a characteristic diameter of approximately 5


360 microns, and volume fraction of approximately 0.1%; the index of refraction, n would be 1.0. For these spherical inclusions, we derive through a fashion similar i9

to that done above the attenuation factor. Clearly, (T = TVd] 14 where d is g

s

the diameter of the sphere and V = 7fd\ 16, thus 5

A

J I l & l

(

1

0

)


The scattering efficiency for non-absorbing spheres is given by

where

¥\

jx) -mif/ {mxy„ (x) n

¥' (mxK (x) -my/ {mx)C' {x) n

tt

tt

n

m y/' (mx)£ (x) - y/ (mx^l (x) n

n

tt

and

again, J„ is a Bessel function of the first kind, H„ is a Hankel function of the second kind, with x = nd IX and A is the wavelength of light in the matrix $

media. If we assume the inclusions have an average diameter of approximately 5 microns, a volume fraction of approximately 0.1%, and the average wavelength


361


is 550 nm, the transmission from the combination of the non-woven mat and "air" inclusions for the samples given in Figures 6A, 6B, and 6C are 93.51%, 94.39%, and 95.95%, respectively. Since it was assumed that the volume fraction of the "air" inclusions was constant, the theoretical predictions seem to lag behind the experimental measurements. It is worthwhile to note briefly and in a general way the effects of the different parameters of the model (volume fraction, the ratio of the fiber and matrix indices of refraction, fiber diameters, etc.). In general, for toughening and durability issues of reasonably thickfilmsof transparent resins, to maintain a relatively high level of transmittance, it is important to keep the fiber diameters at the submicron level, as the following graph illustrates (Figure 1). Of singular importance is the ratio between thefiberand matrix indices of refraction. The scattering efficiency is very sensitive to this ratio, and if the ratio significantly departs from 1 (>1.5), there are profound effects on the scattering efficiency, and thus the transmission, without a concomitant offset in either the volume fraction orfiberdiameter or both. To maintain good transmission, it is, in some sense, a balancing act between these three parameters. This is very important when one wishes to consider afibermaterial with an index of refraction that departs significantly from the index of refraction of the resin material.

Results and Discussion

Electrospunfibermats as a suitable textile for making durable transparent composites Fiber mats were electrospun from a 20%( by wt.) solution of Nylon 6,6 in formic acid. Figure 2a shows an SEM micrograph of a typicalfibermat, while 2b shows a histogram of the distribution of fiber diameters. From this micrograph we see that the electrospun fibers are uniform in shape and do not exhibit any of the radical deviations in fiber morphology that have been reported elsewhere [4]. Additionally, it is clear that the electrospun fiber mats are continuous and that the fiber aspect ratio is essentially infinite to a good approximation. Figure 2b shows the distribution of fiber diameters measured from several SEM micrographs taken from different areas of a piece of electrospun nylon fabric. The distribution is log normal, and the majority fiber diameters are below 150nm (Figure 2a) and all are below 300 nm, which is well below the wavelength of visible light (400-700 nm). These results show that the properties of the Nylon 6,6 electrospun fiber mat are consistent with the assumptions made in the theoretical model discussed above.


In Polymeric Nanofibers; Reneker, Darrell H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006. th

Figure I. Predicted transmission as a function offiber diameter. Indices of refraction for nylon 6,6 (nj) and polyester resin (n^) were obtainedfrom the Polymer Handbook, 4 edition.



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Figure 2 a - SEM picture of electrospun Nylon 6,6 non-woven mat

140

T

100

127

153

180

207

233

260

Diameters(nm)

Figure 2b - Histogram of diameters for Nylon Fibers


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Nanofiber composite fabrication Initial attempts in making a nanofiber composite focused on the infusion of non-woven electro-spun mats of Nylon 6,6 with a commercially available amine based epoxy resin, Epon 828. An example of the electrospun nylon fabric is shown in Figure 3. As can be seen, the fabric is opaque in air, and has a definite texture that is the resultfromthe collection of the electrospun fibers on a metal screen. Pieces of fabric were then immersed in a small amount of resin and allowed to gel at room temperature (Figure 4). A qualitative visual inspection of the sample in Figure 4 illustrates the relatively high degree of visual clarity that is maintained. However, when the samples were backlit, two observations were made that were not obvious when the samples were viewed in reflected light. First, a faint grid pattern, corresponding to the texture of the fabric was clearly evident. This is not surprising since it has been observed [4] that electrospunfiberswill fall preferentially on the conductive portions of a patterned collection target, resulting in a regular variation infibervolume fraction. Secondly, a noticeable degree of haze was detected in the region of the fiber mat in the composite. Inspection of the samples using optical microscopy (Figure 5), revealed the presence of voids rangingfrom1 to a few tens of microns in diameter throughout the sample. These observations illustrate two key challenges in the manufacture of transparent composites with electrospun fibers, the need for a uniform distribution of fibers throughout the matrix and the importance of complete wetting of the electrospun fabric with the matrix resin. To address these concerns, several modifications were made. Our subsequent efforts were devoted to the infusion of vinyl ester into electrospun non-woven mats of Nylon 6,6. The migration to vinyl ester was due primarily to the greatly reduced viscosity over the epoxy resin. Initially, the volume fraction of the non-woven mat in the resin was very small (

Optical Properties of Transparent Resins with Electrospun Polymer

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