Styrene – acrylate / carbon nanotube nanocomposites : mechanical , thermal , and electrical properties

Styrene–acrylonitrile copolymer (SAN) and multi-walled carbon nanotube (CNT) nanocomposites were prepared by solution casting from aqueous solution. The composites were prepared with different CNT concentrations. CNTs significantly affected mechanical properties of the obtained materials. The elastic modulus and ultimate strength of the composite films were improved though ultimate elongation reduced by increasing the CNT content. Apart from studying stress–strain characteristics, some calorimetric investigations were carried out to explain the specific deformation behaviour from the structure viewpoint. Dielectric measurements indicated an increase in AC conductivity and dielectric permittivity already at a small CNT addition.


INTRODUCTION
* Nowadays carbon nanotube/polymer nanocomposites have attracted much attention due to their extraordinary properties that make them ideal candidates for various multifunctional material systems.Intense research has focused on various carbon nanotube filler containing composite materials with enhanced mechanical, electrical, and thermal properties.Most significantly, they are considered to be ideal candidates for mechanical reinforcement of polymer materials.Carbon nanotubes (CNTs) are known to have an extremely high Young's modulus of up to 1 TPa and tensile strength approaching 10-200 Gpa [1].All these properties open up broad possibilities for the use of CNTs as lightweight, highly elastic, and very strong composite fillers.However, the development of CNT/polymer nanocomposites has been largely hindered by the lack of good dispersion of CNTs in the polymer matrix.It has been reported that the enhancement of mechanical properties of the polymer/CNT composites is limited * Corresponding author, jurisbitenieks@inbox.lv due to the phase separation of the polymer matrix and CNT.Therefore CNTs are generally considered to be nonuniformly dispersed in the polymer matrix because they have very large surface areas and strong van der Waals interactions, which can lead to the formation of very strongly bound CNT aggregates [2][3][4][5][6][7].Multiple approaches are used to improve dispersion of CNTs in a polymer matrix: in situ polymerization [8], ultrasound assisted solvent casting [7,9], latex concept [10,11], compatibilizer containing melt blending [12] a.o.By using these technologies CNTs have been successfully introduced in a broad range of thermoplastic and thermosetting polymer matrices (mainly in polyolefins, polyamides, thermoplastic polyesters, unsaturated polyesters, and epoxies).
Much less research has been done on polymers such as polyvinylacetate, styrene-acrylonitrile copolymer (SAN), and polyvinyl alcohol [13][14][15].Generally an increase in mechanical and electrical properties of CNT modified nanocomposites is reported [14,15].Taking into account these aspects investigations SAN/CNT nanocomposites seem particularly interesting, especially considering broad potential application of these com-posites in coating applications like antistatic coatings and films.
In this paper, we report on a simple method for preparing SAN/CNT composite films.In order to achieve good CNT dispersion in the SAN matrix during composite preparation intensive ultrasound mixing was used.Considerably improved mechanical, electrical, and thermophysical properties of these nanocomposite films were observed as compared to the pure SAN material.

EXPERIMENTAL
FINNDISP A 10 SAN dispersion was selected as the matrix material.According to the manufacturer's specifications, it is an unplasticized SAN dispersion stabilized with an anionic emulsifying system.
Bayer multi-walled carbon nanotubes Baytubes C 150 P were added to the polymer matrix in the amounts from 0.01 to 2 wt%.The nanocomposites were manufactured applying the latex concept.Sodium dodecylsulphate (SDS) was used to stabilize the corresponding SAN/CNT dispersions.
To disperse CNTs in the SAN matrix Hielscher UIS250V ultrasound sonication equipment was used.First CNTs were dispensed in 50 mL of distilled water by sonication for about 30 min.To stabilize the solution SDS was added (~ 1% of the solution).After that the obtained aqueous dispersion of CNTs was combined with the polymer suspension and mixed for 30 min with Heidolph RZR 2051 mechanical agitator, and then repeatedly sonicated for 30 min.SAN dispersions containing different concentrations of CNTs were cast in Teflon forms and dried at ambient temperature for 3-4 days.
Tensile stress-strain characteristics were determined by using a Zwick/Roell BDO020 universal testing machine (speed of the upper moving clamp 1 mm/min for Young's modulus E determination and 50 mm/min for measuring stress-strain characteristics at yield and at break (σ Y , ε Y , σ B , and ε B respectively)).
Differential scanning calorimetry (DSC) measurements were carried out with Mettler Toledo DSC 1/200W equipment.Glass transition temperature (T g ) was determined within temperature scans from -10 °C to + 150 °C at heating rate 10 °C/min.
Measurements for thermogravimetric analysis (TGA) were made using Mettler Toledo TGA1/SF equipment.Changes in weight in relation to change in temperature from + 25 °C to + 800 °C were determined at heating rate 20 °C/min.
Thermal conductivity was determined with a NETZSCH model LFA 447 NanoFlash TM diffusivity apparatus.The measurements were conducted between room temperature and 80 °C.The front and back sides of the sample were coated with graphite.
The measurements of dielectric properties of the films were carried out with a Novocontrol Concept 40 broadband dielectric spectrometer.Dielectric measurements of the selected polymer nanocomposites were performed in a wide frequency region (typically from 10 -2 to 10 6 Hz).Permittivity (ε ′) and the real part of AC conductivity (σ ′) were recorded.

RESULTS AND DISCUSSION
Stress-strain characteristics of SAN/CNT nanocomposites as functions of CNT wt% are presented in Fig. 1. Results indicate that addition of CNTs shifts the stress-strain curves of the nanocomposites towards the higher stress and lower strain regions.Most intensive change of stress-strain characteristics of the nanocomposites is observed at the lowest CNT concentration (below 0.5 wt% of CNT), where the modulus of elasticity E and yield strength σ Y increase approximately 1.3 and 2.3 times for SAN/CNT nanocomposites.At CNT concentration above 0.5 wt% the increase in E and σ Y starts to slow down.The breaking stress σ B varies nonmonotonically; however, these changes are not very significant.The effect of CNT content on the ultimate elongation ε B is more pronounced.Thus ε B of the nanocomposite with 2 wt% loading of CNTs is 3 times lower than that of unfilled SAN.
It should be noted that a markedly nonlinear relationship of E as a function of CNT weight content, characterized by a fast increase of E at low nanofiller contents (0.5-1%), and followed by a slower increase or even decrease of E upon introduction of nanotubes at higher weight contents was observed in other studies as well [16,17].The main reason for the decreased efficiency of reinforcement of the material is the worsening of nanotube dispersion.There is ample evidence from other studies [18,19] that at higher nanofiller contents (usually above 3 to 6 and higher wt%) the composite contains clusters (even entanglements) of nanotubes randomly crimped and aggregated in bound bunches.From the composite theory it is also well known that elastic modulus of a nanocomposite is a function of the theoretical elastic modulus of the filler, aspect ratios of the fillers, and the degree of fibre alignment.
Some calorimetric investigations were carried out to explain the specific deformation behaviour from the structure viewpoint.DSC measurements revealed that glass transition temperature (T g ) of the investigated nanocomposites increased from ~ 15 °C to ~ 19 °C showing significant dependence on the CNT content (Fig. 2).Increase in T g indicates that CNTs greatly limit macromolecular movements of SAN, thus increasing the stiffness and reducing the deformability of the nanocomposites.This is mentioned also in the literature [20]  where higher T g of the nanocomposites is attributed to the thorough dispersion of graphene in the polymethylmetacrylate and the strong chemical bonding between the nanofiller and the polymer matrix.The filler is stated to prevent the motion of polymethylmetacrylate chain segments in the nanocomposites.TGA measurements (Fig. 3) of the investigated SAN/CNT nanocomposites showed that the temperature at which significant weight losses started was shifted to higher values when the CNT content was increased.
These results are most probably due to CNTs' thermal properties: CNTs improve thermal stability of nanocomposites by dissipating the applied heat through the whole sample volume making it more resistant to thermal degradation.These results were explained with thermal conductivity measurements.For example, the increase of the thermal conductivity λ for the SAN nanocomposite with CNT content of 0.5 wt%, as seen in Fig. 4, was about 34% compared to the matrix thermal conductivity.
Dielectric spectroscopy measurements in the frequency range from 0.1 Hz to 1 MHz showed a sharp increase in the real part of AC conductivity σ ′ (see Fig. 5) already at a small CNT addition (e.g.0.5% of CNT concentration).Moreover, above 0.5 wt% of CNT loading (σ ′ = 10 -11 S/cm) the AC conductivity levelled off (σ ′ = 10 -6 S/cm), indicating that the conductivity percolation threshold of the composite was reached and its conductivity was now controlled by the conducting CNTs which had formed a network of CNTs within the   SAN matrix.In other words, below this concentration the nanocomposites were resistant to electrical flow, whereas above this value they were conductive.Similar results are reported with copolyester CNT nanocomposites obtained with melt blending where percolation threshold was observed at much higher CNT concentrations (between 5% and 7% of CNT weight content) by raising σ ′ from 10 -10 S/cm up to 10 -6 S/cm [21].At the same time a decrease in dielectric permittivity ε ′ for SAN/CNT nanocomposites with CNT content increased from 1 to 2 wt% can be observed due to the high conductivity of these samples (see Fig. 6).This decrease in ε ′ is due to the mobility decrease in the nanocomposite above the percolation composition as the filler network acts as a hindrance to the dipole orientation.Thus, charge separation is reduced as a result of the decreased polymer segmental mobility, which translates into a decrease in the dielectric constant [21].
However, at the smaller CNT concentration ε ′ basically remained linear in the whole frequency range.This indicates that CNT has superior properties compared to other carbon based fillers, as mentioned also elsewhere [22], where SAN samples filled with nanoscale graphite sheets showed a high permittivity ε ′ increase from ~ 10 2 to ~ 10 5 at filler contents from 5 to 6 wt%.However, in the case of SAN/CNT samples the same permittivity ε ′ increase from ~ 10 2 to ~ 10 5 was observed already at the CNT content of 1 and 2 wt%.
Besides permittivity (Fig. 6) also conductivity measured in AC mode showed a percolation threshold in the CNT range from 0.5 to 1 wt% (Fig. 7), thus testifying on the development of an effective CNT conductive network in the polymer matrix at such small concentrations and an effect on nanocomposite electrical properties.

CONCLUSIONS
A simple solution casting method was applied to prepare SAN/CNT nanocomposites with enhanced mechanical and electrical properties.Mechanical tests showed that, compared with pure SAN, the tensile modulus of the composite was improved by a factor of 1.3 at a significantly small CNT loading amount of 0.5 wt%.TGA measurements showed T g increase from ~ 15 °C to ~ 19 °C by adding CNTs from 0.01 to 2 wt%.Measurements of thermal properties testified on a noticeable increase in the thermal conductivity (34% increase at 0.5 wt% CNT).This increase in the thermal properties leads to a higher thermal decomposition point.
Similarly to the improvement of the mechanical properties of the nanocomposite, a significant improvement was observed in their electrical properties at CNT concentration of 0.5 wt%.This indicates that a good CNT dispersion was achieved by using a common CNT dispersion technique.
The results on SAN/CNT nanocomposites presented in this paper may lead to some practical applications, for example in the manufacture of antistatic adhesives and coatings.

Fig. 2 .
Fig. 2. SAN/CNT nanocomposite glass transition temperature as a function of CNT weight fraction.