**3. Nanofibers reinforced polymers enhanced properties**

depend on the photoinitiator and monomer being used. The photoinitiators used for radical MPA vary from small molecules to large conjugated molecules. A number of groups have reported the successful application of radical MPA using a different kind of resins, homemade and commercial, and different excitation sources. Custom photoinitiators have been designed by several groups and have been shown to be effective both for low threshold powers and for the ability to use less expensive laser systems. While the benefits of custom initiators are clear, their availability is limited. Commercial resins or resins made of commercial components have the advantage of accessibility but suffer from a slightly higher power threshold for fabrication. However, for the entire laser systems used, the threshold for these resins is always well below

**Figure 2.** SEM photographs of the photolithographic patterns of (a) pure PSPI, PSPI/MMT nanofibers reinforced poly‐

MPA can achieve resolution that is considerably better than that predicted by the diffraction limit due to a combination of optical nonlinearity. The probability for MPA is proportional to In,where I is the light intensity and n is the number of absorbed photons. This effectively narrows the point-spread function (PSF) of the beam near the focal point so that it is smaller than the diffraction limit at the excitation wavelength. The real benefit of the optical nonli‐ nearity of MPA lies in the negligible absorption away from the focal point. Photoinitiator

the available power and therefore their use is completely practical.

mers with (b) 2 wt. % and (c) 3 wt. % MMT contents. [19].

170 Advances in Nanofibers

The samples that presented in this section are polymer microstructures fabricated with MPA using laser ablative synthesis generated nanofibers as reinforcement [25] [26].

#### **3.1. Mechanical properties**

NanoIndentation Tester (Figure 4) uses an already established method where an indenter tip with a known geometry is driven into a specific site of the material to be tested, by applying an increasing normal load. When reaching a pre-set maximum value, the normal load is reduced until partial or complete relaxation occurs. At each stage of the experiment, the position of the indenter relative to the sample surface is precisely monitored with a differential capacitive sensor. For each loading/unloading cycle, the applied load value is plotted with respect to the corresponding position of the indenter. The resulting load/displacement curves provide data specific to the mechanical nature of the material under examination. Established models are used to calculatequantitative hardness and elastic modulus values for such data [27]. Hardness and elastic modulus can be determined using the method developed by Oliver and Pharr [28]. The hardness represents the resistance of a material to local surface deforma‐ tion. The Indentation Testing Hardness, H, is determined from the maximum load, Fmax, divided by the projected contact area Ap at the contact depth hc [29];

**Figure 4.** NanoIndentationtester (CSM Instruments)

$$H = \frac{F\_{\text{max}}}{A\_p \text{ (hr}\_c\text{)}} \tag{1}$$

*Er* <sup>=</sup> *<sup>π</sup>*. *<sup>S</sup>*

*<sup>S</sup>* <sup>=</sup> *Fmax h max* - *h <sup>r</sup>*

1 *Er* <sup>=</sup> <sup>1</sup> - *<sup>υ</sup><sup>s</sup>*

independent since they can be related by the following equation [32];

the tensile modulus and shear modulus are calculated from [32];

*<sup>E</sup>* <sup>=</sup> <sup>3</sup>

*<sup>G</sup>* <sup>=</sup> <sup>1</sup>

subtract Eqn 6 from Eqn 7, then using transverse modulus *E*<sup>22</sup> <sup>=</sup> <sup>1</sup> <sup>+</sup> <sup>2</sup>*ηt<sup>v</sup> <sup>f</sup>*

the polymer matrix can be obtained by [31];

<sup>8</sup> *E*<sup>11</sup> +

<sup>8</sup> *E*<sup>11</sup> +

*<sup>G</sup>* <sup>=</sup> *<sup>E</sup>*

point of maximum load which can be found by [31];

is given by [29];

where S is the contact stiffness or the slope of the unloading curve shown in Figure 5 at the

The relationship between Indentation Modulus, E, and the reduced modulus, Er, of the sample

1 - *υ<sup>i</sup>* 2

*Ei*

2 *<sup>E</sup>* +

Where ν<sup>i</sup> is the Poisson's ratio of the indenter, ν<sup>s</sup> is the Poisson's ratio of the sample and Ei

the modulus of the indenter. The elastic stress–strain characteristics of a randomly nanofibers dispersed polymer are expressed by three elastic constants, namely, Young's modulus E, Poisson's ratio n νs, and shear modulus G. Only two of these three elastic constants are

By substituting the values of E, and νs, the shear modulus can be calculated. A thin layer containing randomly oriented discontinuous nanofibers exhibits planar isotropic behavior. The properties are ideally the same in all directions in the plane of the layer. For such a layer,

5

1

where E11 and E22 are the longitudinal and transverse tensile moduli for a unidirectional discontinuous nanofiber lamina of the same nanofiber aspect ratio, and same nanofiber volume fraction as the randomly oriented discontinuous nanofiber composite. Rearrange Eqn 7 and

Poisson's ration *ν<sup>s</sup>* =*v <sup>f</sup> ν <sup>f</sup>* + *vmνm* and *v <sup>f</sup>* + *vm* =1, the volume fraction ratio of the nanofibers in

<sup>2</sup> *Ap* (*<sup>h</sup> <sup>c</sup>*) (2)

Nanofibers Reinforced Polymer Composite Microstructures

http://dx.doi.org/10.5772/57101

<sup>2</sup> (1 <sup>+</sup> *<sup>ν</sup>s*) (5)

<sup>8</sup> *E*<sup>22</sup> (6)

<sup>4</sup> *E*<sup>22</sup> (7)

1 - *ηtv <sup>f</sup>*

*Em*, *η<sup>t</sup>* <sup>=</sup> (*<sup>E</sup> <sup>f</sup>* / *Em*) - <sup>1</sup>

(*<sup>E</sup> <sup>f</sup>* / *Em*) <sup>+</sup> <sup>2</sup> ,

(3)

173

(4)

is

Where Ap (hc) is the projected area of indenter contact at distance, hc from the tip and hc is the depth of the contact of the indenter with the test piece at Fmax. Which can be expressed by *hc* =*h max* - *ε* (*h max* - *hr*) , where hmax is the maximum indentation depth at Fmax, hr is the point of intersection of the tangent to the unloading curve with the depth axis, and ε is a constant depending on the nonlinearity of the unloading curve (Figure 5). For modified Berkovich indenter, *Ap* (*hc* ) =24. 5 *hc* <sup>2</sup> [30]. The elastic modulus represents the overall stiffness of the reinforced polymer network and the reduced modulus of the indentation contact, Er, is given by [29];

Nanofibers Reinforced Polymer Composite Microstructures http://dx.doi.org/10.5772/57101 173

$$E\_r = \frac{\sqrt{\pi} \cdot S}{2\sqrt{A\_p \left(\hbar\_c\right)}}\tag{2}$$

where S is the contact stiffness or the slope of the unloading curve shown in Figure 5 at the point of maximum load which can be found by [31];

**3.1. Mechanical properties**

172 Advances in Nanofibers

NanoIndentation Tester (Figure 4) uses an already established method where an indenter tip with a known geometry is driven into a specific site of the material to be tested, by applying an increasing normal load. When reaching a pre-set maximum value, the normal load is reduced until partial or complete relaxation occurs. At each stage of the experiment, the position of the indenter relative to the sample surface is precisely monitored with a differential capacitive sensor. For each loading/unloading cycle, the applied load value is plotted with respect to the corresponding position of the indenter. The resulting load/displacement curves provide data specific to the mechanical nature of the material under examination. Established models are used to calculatequantitative hardness and elastic modulus values for such data [27]. Hardness and elastic modulus can be determined using the method developed by Oliver and Pharr [28]. The hardness represents the resistance of a material to local surface deforma‐ tion. The Indentation Testing Hardness, H, is determined from the maximum load, Fmax,

divided by the projected contact area Ap at the contact depth hc [29];

*<sup>H</sup>* <sup>=</sup> *Fmax*

Where Ap (hc) is the projected area of indenter contact at distance, hc from the tip and hc is the depth of the contact of the indenter with the test piece at Fmax. Which can be expressed by *hc* =*h max* - *ε* (*h max* - *hr*) , where hmax is the maximum indentation depth at Fmax, hr is the point of intersection of the tangent to the unloading curve with the depth axis, and ε is a constant depending on the nonlinearity of the unloading curve (Figure 5). For modified Berkovich

reinforced polymer network and the reduced modulus of the indentation contact, Er, is given

*Ap* (*<sup>h</sup> <sup>c</sup>*) (1)

<sup>2</sup> [30]. The elastic modulus represents the overall stiffness of the

**Figure 4.** NanoIndentationtester (CSM Instruments)

) =24. 5 *hc*

indenter, *Ap* (*hc*

by [29];

$$\mathbf{S} = \frac{F\_{\text{max}}}{h\_{\text{max}} \cdot h\_r} \tag{3}$$

The relationship between Indentation Modulus, E, and the reduced modulus, Er, of the sample is given by [29];

$$\frac{1}{E\_r} = \frac{1 \cdot \upsilon\_s^2}{E} + \frac{1 \cdot \upsilon\_i^2}{E\_i} \tag{4}$$

Where ν<sup>i</sup> is the Poisson's ratio of the indenter, ν<sup>s</sup> is the Poisson's ratio of the sample and Ei is the modulus of the indenter. The elastic stress–strain characteristics of a randomly nanofibers dispersed polymer are expressed by three elastic constants, namely, Young's modulus E, Poisson's ratio n νs, and shear modulus G. Only two of these three elastic constants are independent since they can be related by the following equation [32];

$$G = \frac{E}{2\left(1 + \nu\_s\right)}\tag{5}$$

By substituting the values of E, and νs, the shear modulus can be calculated. A thin layer containing randomly oriented discontinuous nanofibers exhibits planar isotropic behavior. The properties are ideally the same in all directions in the plane of the layer. For such a layer, the tensile modulus and shear modulus are calculated from [32];

$$E = \frac{3}{8}E\_{11} + \frac{5}{8}E\_{22} \tag{6}$$

$$G = \frac{1}{8}E\_{11} + \frac{1}{4}E\_{22} \tag{7}$$

where E11 and E22 are the longitudinal and transverse tensile moduli for a unidirectional discontinuous nanofiber lamina of the same nanofiber aspect ratio, and same nanofiber volume fraction as the randomly oriented discontinuous nanofiber composite. Rearrange Eqn 7 and subtract Eqn 6 from Eqn 7, then using transverse modulus *E*<sup>22</sup> <sup>=</sup> <sup>1</sup> <sup>+</sup> <sup>2</sup>*ηt<sup>v</sup> <sup>f</sup>* 1 - *ηtv <sup>f</sup> Em*, *η<sup>t</sup>* <sup>=</sup> (*<sup>E</sup> <sup>f</sup>* / *Em*) - <sup>1</sup> (*<sup>E</sup> <sup>f</sup>* / *Em*) <sup>+</sup> <sup>2</sup> , Poisson's ration *ν<sup>s</sup>* =*v <sup>f</sup> ν <sup>f</sup>* + *vmνm* and *v <sup>f</sup>* + *vm* =1, the volume fraction ratio of the nanofibers in the polymer matrix can be obtained by [31];

**Figure 5.** Typical indentation load–displacement curves for the reinforced polymer

$$\varpi\_f = \frac{\frac{8\left(3G \cdot E\right)}{E\_m} \cdot 1}{\left(\frac{\left(E \cdot \left(\frac{\cdot}{\cdot} \right) \cdot E\_m\right) \cdot 1}{\left(E \cdot \left(\frac{\cdot}{\cdot} \right) \cdot E\_m\right) + 2}\right) \left(2 + \frac{8\left(3G \cdot E\right)}{E\_m}\right)}\tag{8}$$

The large surface area of nanofibers provides better interaction between the polymer chains and nanofibers. The nanofibrous structures act as a preferential nucleation site for crystalline phases which cause an increase in the modulus and hardness. Hardness, which is directly related to the flow strength of a material, depends on the effective load transfer between the matrix and the reinforcement phase in the nanofibers reinforced polymer. Strong interfacial bonding between the matrix and the nanofibers is essential for efficient load transfer to obtain high strength. Plastic deformation in polymers occurs by nucleation and propagation of shear bands which in the unreinforced polymer matrix propagate unhindered as there are no barriers for their movement. Thus, the presence of nanofibers in the nanofibers reinforced polymers could offer resistance for the propagation of shear bands. A good mechanical interlocking and the presence of obstacles to the motion of shear bands are the reasons for the enhancement of

Nanofibers Reinforced Polymer Composite Microstructures

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175

Many techniques for the synthesis of conductive polymers have been developed. Most conductive polymers are prepared by oxidative coupling of monocyclic precursors. One challenge is usually the low solubility of the polymer. An important property of the electrically conductive reinforced polymer is the positive temperature coefficient (PTC) effect. PTC reflects the increase in electrical resistivity of the composites during the heating process, and hence decreasing the electrical conductivity of the reinforced polymer. PTC materials have so many potential applications, including sensors, self-regulating heaters, and switching materials. The PTC behavioroccurs as a result of the difference in the coefficients of thermal expansion

Electrical properties of compositions made of nanofibers, dispersed in polymers, are mainly characterized by the formation of a conduction network by contact conditions between neighboring nanofibers in the network. Carbon nanofibers (CFN's) dispersed in the developed resin was used as a conductive composition materials in several applications. The conductivity of the nanofibers reinforced polymer films can be measured using two points probe testing device from SVSLabsInc as shown in Figure 6. Four probe systems can also be used. The voltage passing through the nanofibers reinforced polymer film can be recorded, as well as the current and the value of the resistance (and hence the conductivity) can be obtained using Ohm's law.

hardness and elastic modulus in polymer dispersed nanofibers.

*3.1.1. Electrical conductivity*

between the fillers and the matrix.

**Figure 6.** Two points probe testing device (SVSLabsInc).

Where vm is the volume fraction of the polymer matrix,ν<sup>f</sup> and ν<sup>m</sup> are Poisson's rations of nanofibers and polymer matrix respectively, Ef and Em are Young's moduli for nanofibers and polymer matrix respectively. By substituting the appropriate values in Eqn 8, the volume fraction of the nanofibers in the polymer matrix can be calculated. An important function of the matrix in a nanofiber-reinforced composite material is to provide lateral support and stability for nanofibers under longitudinal compressive loading like the one applied by the indenter head. In polymer matrix composites with which the matrix modulus is relatively low compared to the nanofiber modulus, failure in longitudinal compression is usually initiated by localized buckling of nanofibers. In general, the shear mode of failure is more important than the extensional mode of failure and the shear mode is usually controlled by the matrix shear modulus as well as nanofiber volume fraction.

A viscoelastic polymer has both elastic component and a viscous component. Applying stress to a polymer causes molecular rearrangement due to changing positions by parts of the long polymer chain (creep). Polymers remain solid even when these parts of their chains are rearranging in order to accompany the stress which creates a back stress in the material. The material no longer creeps when the back stress is of the same magnitude as the applied stress. If the original stress is taken away, the accumulated back stresses will cause the polymer to get back to its original form with the help of its elastic component. The polymer loses energy when a load is applied then removed with the area between loading and unloading curves (Figure 5) being equal to the energy lost during the loading cycle. Thus permanent plastic deformation of the polymer occurs even when the applied load is removed.

The large surface area of nanofibers provides better interaction between the polymer chains and nanofibers. The nanofibrous structures act as a preferential nucleation site for crystalline phases which cause an increase in the modulus and hardness. Hardness, which is directly related to the flow strength of a material, depends on the effective load transfer between the matrix and the reinforcement phase in the nanofibers reinforced polymer. Strong interfacial bonding between the matrix and the nanofibers is essential for efficient load transfer to obtain high strength. Plastic deformation in polymers occurs by nucleation and propagation of shear bands which in the unreinforced polymer matrix propagate unhindered as there are no barriers for their movement. Thus, the presence of nanofibers in the nanofibers reinforced polymers could offer resistance for the propagation of shear bands. A good mechanical interlocking and the presence of obstacles to the motion of shear bands are the reasons for the enhancement of hardness and elastic modulus in polymer dispersed nanofibers.

#### *3.1.1. Electrical conductivity*

*v <sup>f</sup>* =

**Figure 5.** Typical indentation load–displacement curves for the reinforced polymer

Where vm is the volume fraction of the polymer matrix,ν<sup>f</sup>

nanofibers and polymer matrix respectively, Ef

174 Advances in Nanofibers

shear modulus as well as nanofiber volume fraction.

8 (3*G* - *E* ) *Em* - 1

(*<sup>E</sup> <sup>f</sup>* / *Em*) <sup>+</sup> <sup>2</sup> )(2 <sup>+</sup> <sup>8</sup> (3*<sup>G</sup>* - *<sup>E</sup>* )

polymer matrix respectively. By substituting the appropriate values in Eqn 8, the volume fraction of the nanofibers in the polymer matrix can be calculated. An important function of the matrix in a nanofiber-reinforced composite material is to provide lateral support and stability for nanofibers under longitudinal compressive loading like the one applied by the indenter head. In polymer matrix composites with which the matrix modulus is relatively low compared to the nanofiber modulus, failure in longitudinal compression is usually initiated by localized buckling of nanofibers. In general, the shear mode of failure is more important than the extensional mode of failure and the shear mode is usually controlled by the matrix

A viscoelastic polymer has both elastic component and a viscous component. Applying stress to a polymer causes molecular rearrangement due to changing positions by parts of the long polymer chain (creep). Polymers remain solid even when these parts of their chains are rearranging in order to accompany the stress which creates a back stress in the material. The material no longer creeps when the back stress is of the same magnitude as the applied stress. If the original stress is taken away, the accumulated back stresses will cause the polymer to get back to its original form with the help of its elastic component. The polymer loses energy when a load is applied then removed with the area between loading and unloading curves (Figure 5) being equal to the energy lost during the loading cycle. Thus permanent plastic

deformation of the polymer occurs even when the applied load is removed.

*Em*

) (8)

and ν<sup>m</sup> are Poisson's rations of

and Em are Young's moduli for nanofibers and

( (*<sup>E</sup> <sup>f</sup>* / *Em*) - <sup>1</sup>

Many techniques for the synthesis of conductive polymers have been developed. Most conductive polymers are prepared by oxidative coupling of monocyclic precursors. One challenge is usually the low solubility of the polymer. An important property of the electrically conductive reinforced polymer is the positive temperature coefficient (PTC) effect. PTC reflects the increase in electrical resistivity of the composites during the heating process, and hence decreasing the electrical conductivity of the reinforced polymer. PTC materials have so many potential applications, including sensors, self-regulating heaters, and switching materials. The PTC behavioroccurs as a result of the difference in the coefficients of thermal expansion between the fillers and the matrix.

Electrical properties of compositions made of nanofibers, dispersed in polymers, are mainly characterized by the formation of a conduction network by contact conditions between neighboring nanofibers in the network. Carbon nanofibers (CFN's) dispersed in the developed resin was used as a conductive composition materials in several applications. The conductivity of the nanofibers reinforced polymer films can be measured using two points probe testing device from SVSLabsInc as shown in Figure 6. Four probe systems can also be used. The voltage passing through the nanofibers reinforced polymer film can be recorded, as well as the current and the value of the resistance (and hence the conductivity) can be obtained using Ohm's law.

**Figure 6.** Two points probe testing device (SVSLabsInc).

Electrical conductivity in CNF's reinforced polymer is generated as a result of the direct contact between CNF's and tunneling resistance determined by the width of the insulating resin around the CNF's. Thermal expansion caused by heat gradient increases gap width between contiguous CNF's and reduces the number of conductive pathways which results in a decrease in electrical conductivity. The temperature dependence of the electrical conductivity can be explained by the general theory of the thermal fluctuations. The resistivity of the junction is given by;

$$
\rho = \rho\_o e^{-T\_y/\left\{T \ast T\_o\right\}} \tag{9}
$$

fabricated with lower repetition rate. This is a direct result of the matrix volumetric expan‐ sionwhich cause more contiguous CFN's to get disconnected and reduce number of electrical

0 2 4 6 8 10

**Figure 7.** Pulse repetition frequency effect on the relative conductivity of the nanofibers reinforced polymer micro‐

Magnetic neodymium-iron-boron (NdFeB) nanofibers and nanoparticles, have become one of the most important spot in the research field of magnetic nanomaterials to meet the demand for miniaturization of electronic components recently. They have been successfully prepared by various techniques like the sol-gel auto-combustion method [33], co-precipitation [34], hydrothermal method [35], reverse micelles [36], microemulsion method [37], alternate sputter‐

The total magnetization of a nanofiber is given by the vectorial sum of all single magnetic moments of the atoms. As for the atomic magnetic moments in nanofibers, the average magnetization will be zero in the absence of magnetic field since all magnetic moments are randomly directed in space. When a magnetic field is applied by the substrate, the magnetic moments orient in the direction of the field and give rise to a net magnetization of the

In ferromagnetic materials (Figure 8 (a) ), the saturation magnetization MS of the magnetic nanofibers reinforced polymer microstructures at room temperature is higher than that of the pure polymer. The larger coercivitiesHC that we see could be as a result of the oxide Fe. Figure 8 (b) shows the room temperature M-H curves for the polymer (Ormocer). It has a weak diamagnetic response to the applied field, but appears to have a small amount of unknown impurity which contributes to a very weak non- linear deviation near zero fields that could be a paramagnetic effect. Small traces of paramagnetic impurities can be found in almost all

**Relative pulse repetition frequency**

Mean nanofiber size (nm) Relative conductivity




**Relative**

**conductivity**

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177





Nanofibers Reinforced Polymer Composite Microstructures

pathways.

structures.

nanofibers [40].

ing [38], pulsed laser deposition [39], and many others.

**Mean**

*3.1.2. Magnetic enhancement*

**nanofiber size (nm)** 

Where the constants ρ0, T1, and T0 depend essentially on the characteristics of the tunnel junctions, which are supposed to be functions of various parameters such as filling factor, filler size and shape, sample processing. The relative resistivity (ρr) can be used to characterize the intensity of the PTC effect:

$$\rho\_r = \log\left(\frac{\rho\_{140}}{\rho\_{20}}\right) \tag{10}$$

Where ρ140 and ρ20 are the resistances of the composites at two different temperatures, for example, 140 and 20o C. Thus the relative conductivity can be expressed as;

$$S\_r = \log\left(\frac{\rho\_{20}}{\rho\_{140}}\right) \tag{11}$$

This represents how sensitive and to what extent the reinforced polymer resistance responds after being stimulated by the temperature change. The electrical sensitivity of a reinforced polymer fabricated by femtosecond laser material processing can be expressed by;

$$K\_s = \frac{\Delta S\_r}{\Delta \omega\_r} \tag{12}$$

Where Ksis the electrical sensitivity, ΔS<sup>r</sup> is the change in relative conductivity and Δωr is the change in relative repetition rate of the femtosecond laser that can be expressed by;

$$
\omega\_r = \frac{\omega}{\omega\_{\rm min}} \tag{13}
$$

Where ωmin is the minimum repetition rate used for the generation of nanofibers. For example, The equation of the straight line obtained from Figure 7 is Sr=0. 17 ω<sup>r</sup> - 10. 61 17 which indicate that the electrical sensitivity Ks is about 0. 17. The higher the value of the electrical sensitivity, the more sensitive is the reinforced polymer to change in temperature and thus less conduc‐ tivity at higher temperatures. Also, the PTC effect of nanofibers reinforced polymers fabricated with higher laser repetition rate is weaker than that of nanofibers reinforced polymer s fabricated with lower repetition rate. This is a direct result of the matrix volumetric expan‐ sionwhich cause more contiguous CFN's to get disconnected and reduce number of electrical pathways.

**Figure 7.** Pulse repetition frequency effect on the relative conductivity of the nanofibers reinforced polymer micro‐ structures.

#### *3.1.2. Magnetic enhancement*

Electrical conductivity in CNF's reinforced polymer is generated as a result of the direct contact between CNF's and tunneling resistance determined by the width of the insulating resin around the CNF's. Thermal expansion caused by heat gradient increases gap width between contiguous CNF's and reduces the number of conductive pathways which results in a decrease in electrical conductivity. The temperature dependence of the electrical conductivity can be explained by the general theory of the thermal fluctuations. The resistivity of the junction is

Where the constants ρ0, T1, and T0 depend essentially on the characteristics of the tunnel junctions, which are supposed to be functions of various parameters such as filling factor, filler size and shape, sample processing. The relative resistivity (ρr) can be used to characterize the

Where ρ140 and ρ20 are the resistances of the composites at two different temperatures, for

This represents how sensitive and to what extent the reinforced polymer resistance responds after being stimulated by the temperature change. The electrical sensitivity of a reinforced

Where Ksis the electrical sensitivity, ΔS<sup>r</sup> is the change in relative conductivity and Δωr is the

Where ωmin is the minimum repetition rate used for the generation of nanofibers. For example, The equation of the straight line obtained from Figure 7 is Sr=0. 17 ω<sup>r</sup> - 10. 61 17 which indicate that the electrical sensitivity Ks is about 0. 17. The higher the value of the electrical sensitivity, the more sensitive is the reinforced polymer to change in temperature and thus less conduc‐ tivity at higher temperatures. Also, the PTC effect of nanofibers reinforced polymers fabricated with higher laser repetition rate is weaker than that of nanofibers reinforced polymer s

C. Thus the relative conductivity can be expressed as;

*<sup>ρ</sup><sup>r</sup>* =log ( *<sup>ρ</sup>*<sup>140</sup> *ρ*<sup>20</sup>

*Sr* =log ( *<sup>ρ</sup>*<sup>20</sup> *ρ*<sup>140</sup>

polymer fabricated by femtosecond laser material processing can be expressed by;

*Ks* <sup>=</sup> <sup>∆</sup> *<sup>S</sup> <sup>r</sup>* ∆ *ω<sup>r</sup>*

change in relative repetition rate of the femtosecond laser that can be expressed by;

*<sup>ω</sup><sup>r</sup>* <sup>=</sup> *<sup>ω</sup> ωmin*


) (10)

) (11)

(12)

(13)

*ρ* =*ρoe*

given by;

176 Advances in Nanofibers

intensity of the PTC effect:

example, 140 and 20o

Magnetic neodymium-iron-boron (NdFeB) nanofibers and nanoparticles, have become one of the most important spot in the research field of magnetic nanomaterials to meet the demand for miniaturization of electronic components recently. They have been successfully prepared by various techniques like the sol-gel auto-combustion method [33], co-precipitation [34], hydrothermal method [35], reverse micelles [36], microemulsion method [37], alternate sputter‐ ing [38], pulsed laser deposition [39], and many others.

The total magnetization of a nanofiber is given by the vectorial sum of all single magnetic moments of the atoms. As for the atomic magnetic moments in nanofibers, the average magnetization will be zero in the absence of magnetic field since all magnetic moments are randomly directed in space. When a magnetic field is applied by the substrate, the magnetic moments orient in the direction of the field and give rise to a net magnetization of the nanofibers [40].

In ferromagnetic materials (Figure 8 (a) ), the saturation magnetization MS of the magnetic nanofibers reinforced polymer microstructures at room temperature is higher than that of the pure polymer. The larger coercivitiesHC that we see could be as a result of the oxide Fe. Figure 8 (b) shows the room temperature M-H curves for the polymer (Ormocer). It has a weak diamagnetic response to the applied field, but appears to have a small amount of unknown impurity which contributes to a very weak non- linear deviation near zero fields that could be a paramagnetic effect. Small traces of paramagnetic impurities can be found in almost all materials and hence, it is not surprising in these polymers. However, these trace impurities do not affect the quality of our ferromagnetic composites. Figure 8 (c) shows the magnetic measurements of the nanofibers reinforced polymers which has a coercive field HC of approximately 260 Oe at room temperature. This large coercivity suggests that oxide nano‐ fibrers are present on the microstructure.

modified for immobilization of probe DNA via covalent linking or electrostatic interactions. The conductivity of the conducting polymer electrochemically deposited on the electrode surface can be modulated by changing the pH of the medium, the electrochemical potential, and/or the electrolyte. Because of these characteristics and other advantages, conducting polymers could be utilized extensively for the construction of biosensors including DNA

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179

Noble metals are known to create strong chemical bonds with compounds terminating with a thiol (-SH) group. In order to achieve immobilization of nucleic acids on solid substrates, researchers have developed techniques by which DNA molecules can be linked to a thiol group. In this case, the thiolated terminal of molecules is chemisorbed on the substrate, while the DNA portion of the molecules are standing parallel to each other and away from the substrate. Since they are formed quickly, resulting in a well-defined and reproducible surface, and are stable under normal laboratory conditions, gold electrodes are fabricated using

Reinfoced polymer microstructuring can be utilized in neomerous application. Researchers prepared dye-sensitized solar cells using micro/nanofibers reinforced polymer Ti02 porous films [42]. This result in cells with enhanced light collection. They applied a technique which opens an alternative way for manufacturing solar cells on an industrial scale. Ti02 micro/nanocomposite structured electrodes for quasi-solid-state dye-sensitized solar cells [43]. These revolutionary nano-structured ultra thin film solar PV products will provide affordable clean renewable energy for everyone. Another unique technology has been developed that absorbs and converts more sunlight throughout the day by utilizing special kind of nanofibers reinforced polymers. This result in a dramatic increase in total power output. Each nanofiber increases the total PV surface area by an incredible 6-12 times over current other thin film products on the market today. Figure 9 shows luminescent solar concentrators (LSCs)

comprising CdSe core/multishell quantum dots (QDs) developed by Bomm et al [44].

**Figure 9.** (a) Photograph of a P (LMA-co-EGDM) plate containing CdSe core/multishell QDs (illuminated by a UV-lamp) illustrating the concentrator effect and (b) TEM image of a QD-LSC/P (LMA-co-EGDM) nanofibers reinforced polymer

showing single QDs and a few small QD aggregates [44].

sensors which, to the best of our knowledge, is reported for the first time.

femtosecond laser material processing or other techniques [41].

**4.2. Energy applications**

It can be seen that the magnetization of the generated nanofibers reinforced polymer increases compared to pure polymer. This increase in magnetization is expected below the superpara‐ magnetic- ferromagnetic transition temperature, which is above 300K for the magnetic nanofibers of 20 nm average size due to reduced thermal activation energy. Nanoparticle interactions, which depend on the iron concentration in the polymer matrix, strongly influence the remnant magnetization. Since agglomeration of nanoparticles into nanofibers is observed in all our polymer nanofibers reinforced polymer samples, interactions are expected to play a significant role in the magnetic response. These interactions lead to a non- linear increase in MR as the concentration of iron is increased. The magnetic interactions are generally expected to be dipolar in nature, although in strongly coupled clusters, exchange interactions are also possible.

**Figure 8.** Room temperature M-H curves (a) Magnetic nanofiberous structures, (b) Ormocer, and (c) magnetic nano‐ fibers reinforced polymer.
