Nanocomposites of cellulose-based adhesive and toluenesulfonic acid-doped polypyrrole prepared via colloidal dispersion

Materials

Cellulose and pyrrole (99% purity) were purchased from Acros Organic, meanwhile ammonium persulfate (APS; (NH₄)₂S₂O₈) was acquired from Friendamann Schmidt Chemicals. Dimethyl sulfoxide (DMSO) was obtained from R & M Chemicals. Dimethylformamide (DMF) was bought from Fisher Chemicals. Other chemicals were purchased from Sigma-Aldrich. Pyrrole was purified by passing through a column packed with alumina and stored at 4℃ prior to use. Other chemicals were used as received. Deionized water was used in all experiments.

Chemical modification of cellulose

Dried cellulose (3.0 g) was immersed in DMSO (90.0 mL) at 60℃ and stirred for 2 h. Sodium methylate (90.0 mL) was added to the mixture and then stirred for another hour under nitrogen atmosphere at room temperature. The product was filtered and washed with DMSO. The produced material (2.0 g) was immersed again in DMSO containing 160.0 mL of epichlorohydrin and stirred for 2 h at 50℃. The obtained product was separated and washed with distilled water and ethanol. The final product (EC) was dried in a vacuum oven for 24 h at 60℃.

Synthesis of TSA-doped PPy

TSA (0.07 mol) was dissolved in deionized water (100 mL), placed in a reaction vessel and stirred vigorously at 0℃ for 15 min. Purified pyrrole monomer (0.07 mol) was added dropwise to the reaction vessel. The mixture was stirred for 20 min. About 100 mL of APS (0.52 M) was then added dropwise to the above solution with vigorous stirring at 0℃. The mixture was stirred for another 7 h. The reaction mixture was poured into a large excess of deionized water and then filtered. The doped PPy was washed with deionized water and methanol, and dried in vacuum oven at 60℃ for 2 days.

Preparation of nanocomposite

Colloid A consisted of TSA-doped PPy nanoparticles (4.0 g), DMF (120 mL), sodium dodecyl sulfate (SDS) (2.0 g) and deionized water (12.0 mL). Meanwhile, colloid B consisted of EC (1.0 g), DMF (30.0 mL), SDS (0.5 g) and deionized water (3.0 mL). TSA-doped PPy nanoparticles were dispersed in DMF and stirred with magnetic stirrer at 400 r/min until a homogeneous dispersion was obtained. After that, SDS solution was added slowly into the mixture and stirred to form colloid A. The same procedure was followed to produce colloid B. Both colloids were mixed and stirred for 3 h to produce a uniform and homogeneous colloidal dispersion. The mixture was poured into methanol and stirred with a magnetic stirrer at 400 r/min for 30 min. The obtained product was filtered and dried in a fume hood for 24 h. The product was further dried in a vacuum oven. This procedure produced 80 : 20 composition of TSA-doped PPy and EC. Nanocomposites with compositions of 70 : 30 and 50 : 50 were also produced in the same way.

Characterization

Infrared spectra were recorded using Bruker, Fourier transform infrared (FTIR) system model Tensor 27 with OPUS 6.0 software. Samples in powder form were tested using attenuated total reflectance. The scans were carried out from 400 to 4000 cm⁻¹.

Thermogravimetric analysis (TGA) was carried out using a Mettler Toledo model TGA/SDTA 851 ^e. Samples of 5–6 mg were placed in alumina pans and heated from 30℃ to 600℃ at 10℃ min⁻¹, under a flow of nitrogen (50 mL min⁻¹). Differential scanning calorimetric analysis was carried out using a Mettler Toledo model 823 ^e. Samples of approximately 4–5 mg were placed in aluminum sealed pans and heated from 30℃ to 600℃ at 10℃ min⁻¹, under a flow of nitrogen (50 mL min⁻¹).

Morphology of the samples was examined by using a Zeiss, field emission scanning electron microscope, model Supra 46VP. Scanning electron micrograph (SEM) images were recorded using an accelerating voltage of 3–5 kV. The doping level of TSA-doped PPy was determined via elemental analysis using a CHNS/Thermo Finnigan/Eager 300 for EA 1112 elemental analyzer.

Samples in pellet form were analyzed using a frequency response analyzer (HIOKI 3532-50 LCR HiTester) over a frequency range of 10²–10⁶Hz at room temperature to measure the electrical conductivity. Meanwhile, thermal conductivity was measured by using a C-Therm Tci thermal analyzer.

FTIR spectroscopy

Figure 1 shows FTIR spectra of cellulose and EC. The two spectra are similar to each other. The peak observed at 898 cm⁻¹ is attributed to glucose ring stretching in cellulose.^10,11 Other peaks that attributed to cellulose are observed at 2890 cm⁻¹ (C–H stretching), 1373 cm⁻¹ (O–H in-plane deformation) and 1053 cm⁻¹ (C–O stretching). However, there are additional three peaks observed for EC. The bands at 1588 cm⁻¹ (C–O stretching), 952 cm⁻¹ (C–H bending) and 1263 cm⁻¹ are due to epoxide groups. ¹² This indicated that the epoxy group has been incorporated successfully onto the cellulosic chain.

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Figure 1.

FTIR spectra for cellulose (a) and epoxypropyl cellulose (b).

FTIR spectra for PPy and TSA-doped PPy nanoparticles, as shown in Figure 2, are also similar to each other, indicating the main polymer chains are alike. The bands at 1562 cm ^{− 1} (C = C stretching), 1479 cm ^{− 1} (C–N stretching), 1195 cm ^{− 1} (C–H in-plane bending) and 1044 cm ^{− 1} (C–H in-plane deformation vibration) are the characteristics for PPy. All observed peaks are in accordance with those reported previously.^13,14 The broad peak above 2000 cm ^{− 1} is more obvious in the spectrum for TSA-doped PPy. The peak at 1004 cm ^{− 1} of TSA-doped PPy FTIR spectra is attributed to the mode of in-plane deformation vibration of N⁺H₂ on protonated nitrogen which is an important characteristic for doped polypyrrole. ¹⁵

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Figure 2.

FTIR spectra for PPy (a) and TSA-doped PPy (b).

Figure 3(a) shows FTIR spectra of EC, 3(b) TSA-doped PPy (pH 1) and 3(c) shows an example of TSA-doped PPy:EC composite. FTIR spectra for nanocomposites were more similar to TSA-doped PPy due to its higher content.

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Figure 3.

FTIR spectra of epoxypropyl cellulose (a), TSA-doped PPy (b) and TSA-doped PPy:EC composite (80:20) (c).

Elemental analysis

Charge carriers for doped PPy are usually thought to be responsible for electronic mobility along the polymeric chain. ¹⁶ It means that the doping level will affect some properties of the produced nanocomposite including electrical and thermal conductivities. Calculation of the doping level was made according to the following equation:

Doping level ( % ) = ( ( S N ) 2 . 3 ) × 100

(1)

where S and N are the sulfur and nitrogen content calculated from elemental analysis, respectively. Results of elemental analysis and doping levels are shown in Table 1. The sample prepared at pH 1 gave the highest doping level.

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Table 1.

Doping level of doped PPy as calculated from the elemental analysis.

Samples	N (%)	C (%)	S (%)	H (%)	Doping level (%)
TSA-doped PPy, pH 1	12.60	53.79	7.20	4.40	24.80
TSA-doped PPy, pH 3	13.19	54.32	6.50	3.95	21.44
TSA-doped PPy, pH 4	13.73	54.35	5.46	4.21	17.29

Morphology

SEMs of TSA-doped PPy are shown in Figure 4. TSA-doped PPy particles prepared at pH 3 and pH 4 are spherical in shape but the one prepared at pH 1 is rod-shaped particle. The diameter of rod-shaped particles quite varies, the smallest one was around 80–100 nm. TSA-doped PPy particles prepared at pH3 are smaller (76–100 nm) compared to the one prepared at pH 4 (97–254 nm). In aqueous solution, it is easy to form TSA micelles due to its hydrophilic nature. Pyrrole/TSA supramolecular clusters might be formed in the reaction system due to the hydrophobic pyrrole and hydrophilic dopant. Therefore, it is reasonable to propose that TSA micelles and pyrrole/TSA clusters might act as templates in the formation of TSA-doped PPy particles. As a result, their content including free pyrrole in the reaction system affect the morphology and size variation of the doped PPy. ¹⁷ Synthesis condition might be the factor that causes either TSA micelles or pyrrole/TSA clusters react as template.

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Figure 4.

SEM images of TSA-doped PPy prepared at pH 1 (a), pH 3 (b) and pH 4 (c).

Figure 5 shows that doped PPy particles are well dispersed in the matrix. This shows that colloidal dispersion method is a suitable method to produce cellulose-based adhesive and TSA-doped PPy nanocomposites.

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Figure 5.

SEM images of nanocomposites (TSA-doped PPy: Epoxypropyl cellulose); 70:30 (a) and 50:50 (b).

Thermal stability

Based on TGA results (Figure 6), samples synthesized as lower pH showed a better thermal stability. This is probably due to the increase in doping level that increases the presence of aromatic group in doped PPy and subsequently improves the thermal stability. ¹⁶

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Figure 6.

TGA (a) and DTG (b) curves for PPy and TSA-doped PPy.

It can be seen in Figure 7(a) that cellulose exhibited two stages of mass loss. The mass loss below 100℃ is attributed to the release of absorbed moisture. The mass loss attributable to the thermal decomposition of cellulose started around 300℃. For the EC, there are three steps of weight loss. The first one is due to evaporation of water. The second step observed at 230℃ indicated the decomposition of epoxy group that has been incorporated into the cellulose backbone. Meanwhile, the third mass loss started around 300–330℃ is attributed to cellulose decomposition.

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Figure 7.

TGA (a) and DTG (b) curves for cellulose, epoxypropyl cellulose, PPy, TSA-doped PPy and epoxypropyl cellulosae: TSA-doped PPy nanocomposite.

As shown in Figure 8, the first mass loss of TSA-doped PPy composite is below 100℃, due to the release of adsorbed moisture in the composite. The second mass loss, ranging from 130℃ to 230℃, can be attributed to degradation of the epoxy group that has been incorporated into cellulose chains. The third mass loss started around 280–330℃ corresponded to TSA-doped PPy. This step can also be attributed to the decomposition of cellulose because its degradation occurs at similar temperature.

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Figure 8.

TGA (a) and DTG (b) curves for nanocomposites.

Electrical and thermal conductivities

Based on results shown in Table 2, PPy and TSA-doped PPy gave the reading of electrical conductivity in semiconductive range. The delocalization of π electrons along the polymeric backbone, co-existing with low ionization potentials and high electron affinities leads to unique electrical properties of conjugated polymer including PPy and doped PPy. ¹³ The higher doping level of TSA-doped PPy synthesized at pH 1 (25%) compared to others resulted in higher electrical and thermal conductivities. The rod-shaped nanoparticles of TSA-doped PPy are closely packed and thus may lead to the inter-particle electron hopping and high electrical conductivities relatively. ¹⁴

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Table 2.

Results of thermal conductivity and electrical conductivity.

Samples	Electrical conductivity (S cm−1)	Thermal conductivity K(W m−1 K−1)
PPy	5.422 × 10−6	0.343
TSA-doped PPy, pH 4	9.955 × 10−4	0.406
TSA-doped PPy, pH 1	8.422 × 10−3	0.431
TSA-doped PPy, pH 3	1.407 × 10−3	0.422
TSA-doped PPy : EC (80 : 20)	1.076 × 10−3	0.399
TSA-doped PPy : EC (70 : 30)	3.751 × 10−5	0.371
TSA-doped PPy: EC (50 : 50)	3.350 × 10−8	0.237

However, as expected the nanocomposites showed a decrease in electrical conductivity since cellulose-based adhesive is an insulator. This non-metal material causes interface resistance which is believed to be the blockage for the thermal conductivity. ¹⁸ The results presented in Table 2 show that the obtained nanocomposites gave the reading of thermal conductivity in the range of conventional polymer. ¹⁹