Covalently γ-aminobutyric acid-functionalized carbon nanotubes: improved compatibility with PHBV matrix
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The improvement of compatibility between carbon nanotubes (CNTs) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) was achieved using CNT functionalized with γ-aminobutyric acid (GABA). The efficiency of the CNT functionalization with GABA was evaluated by X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (FT-IR), Raman spectroscopy, and transmission electron microscopy (TEM). The PHBV/CNT nanocomposites were produced in the molten state with the addition of 0.5 wt% of CNT (pristine, oxidized and functionalized with GABA) and characterized concerning the Izod impact strength tests. The impact fracture morphologies were analyzed using scanning electron microscopy. The results showed that GABA was covalently attached to CNT, resulting in the detection of nitrogen in the XPS survey, the shift of carbonyl peak wavelength on FT-IR, and a higher degree of structural disorder, detected by Raman and observed in TEM images. The impact strength was not significantly affected by the introduction of CNT; however, the impact fracture mechanism was changed from fragile to ductile when CNT was functionalized with GABA. These results are promising for the production of environmentally friendly nanocomposites with superior properties.
KeywordsNanocomposite PHBV Carbon nanotube Compatibility Functionalization
The increasing accumulation of waste in landfills linked to environmental pollution has been making researchers to seek alternatives to conventional plastics . In this context, biodegradable matrices, such as poly(3-hydroxybutyrate-co-3-hydroxyvalerate), PHBV, have attracted the attention of the scientific community.
PHBV is biodegradable and biocompatible natural polyester from polyhydroxyalkanoates family (PHAs), which have properties that make them promising candidates to replace conventional thermoplastics . However, PHBV has some shortcomings, such as brittleness, low mechanical properties and thermal stability, and a narrow processing window [3, 4]. To overcome the limitations of PHBV properties, the incorporation of nanoparticles has been a widely studied alternative [5, 6, 7, 8, 9, 10, 11, 12]. Among nanoparticles, carbon nanotubes (CNTs) are specially investigated to improve properties of matrices, due to their outstanding electrical, optical, and mechanical properties .
The incorporation of CNT into PHBV has shown to change the mechanical properties, wettability, thermal stability, and crystallinity degree [10, 14, 15]. However, CNT has a great tendency to form bundles, making its dispersion into matrices difficult. Therefore, the process of CNT functionalization is extensively employed to enhance the dispersion and compatibility, producing nanocomposites with improvements in their mechanical properties . Usually, functionalization made with the same polymer used as the matrix results in better compatibility due to improved interfacial adhesion between the filler and the matrix [4, 8]. However, functionalizing CNT with polymers requires several steps and is much time-consuming process.
Some studies have shown that when CNT is functionalized with a molecule which interacts with the polymer repeating unit, the wettability of CNT by the polymer is improved, which provides enhanced interfacial compatibility between the polymer and the filler [17, 18]. In view of these assertions, in this work, CNT was functionalized with γ-aminobutyric acid (GABA), which has a molecular structure similar to that of hydroxybutyrate, the main monomer on PHBV chain. GABA is a four-carbon nonprotein amino acid which is involved in the transmissions in the mammalian central nervous system [19, 20].
In this study, γ-aminobutyric acid-functionalized multi-walled carbon nanotubes were prepared and characterized. To our knowledge, covalently GABA-functionalized CNTs were not reported yet and could be used to reinforce not only PHBV but also polyesters such as poly(ε-caprolactone), poly(lactic acid), among others. PHBV/CNT nanocomposites with pristine and oxidized CNTs were also prepared to compare the effectiveness of the functionalization process. The results demonstrated that when GABA is attached to the CNT surface, the impact fracture of PHBV nanocomposite is modified, ranging from brittle to ductile. The findings suggest that increasing CNT-GABA concentration into PHBV matrix could lead to great improvements in the mechanical properties of this nanocomposite.
PHBV was provided by NaturePlast, with 2 mol% of hydroxyvalerate units, molecular weight (Mw) from 400,000 to 500,000 g mol−1, and was used as received. Multi-walled carbon nanotubes (MWCNTs) were supplied by Nanostructured & Amorphous Materials, Inc. (Houston, TX, USA) with specification #1229Y, with a minimum purity of 95%. They have a diameter in the range of 20–30 nm and length in the range of 10–30 μm and were used as supplied. Gamma-aminobutyric acid (GABA) was obtained from Sigma-Aldrich with a minimum purity of 99%. Nitric acid (HNO3) 65% and dimethyl sulfoxide (DMSO) P.A. ACS from Neon Comercial (Brazil) were used as received.
2.2 Functionalization of MWCNT
The first step was the oxidation of MWCNT, based on our previously reported methodology . In this step, pristine MWCNT (CNT) was oxidized in nitric acid 6 M at 120 °C for 5 h. After the oxidation period, the sample (CNT-Ox) was centrifuged (ROTINA 420R, Hettich) and washed thoroughly with distilled water until the removal of all remaining acid.
2.3 Production of PHBV/CNT nanocomposites
PHBV/CNT nanocomposites with 0.5 wt% of CNT were produced by melt mixing, followed by hot compression molding. Neat PHBV sample and PHBV/CNT, PHBV/CNT-Ox, and PHBV/CNT-GB samples were processed in a high-speed mixer (DRAIS mixer produced by MH Equipamentos Ltda, model MH50-H) rotating at 3000 rpm and mixing chamber with a capacity of 65 g of material. The mixing, melting, and homogenization of the composites occur due to the high friction generated between the rotor and the material. The polymer mass was previously oven-dried for 4 h at 40 °C. After 1 min of mixing, the homogenized composites were collected and pressed into 3.2-mm-thick plates with dimensions according to ASTM D256-06 for the mechanical impact tests in a hydropneumatic press (MH Equipamentos Ltda, model PR8HP) at 200 °C with a pressure of 5 bar for 3 min and then cooled for 2 min.
2.4 Characterization of CNT
X-ray photoelectron spectrometry (XPS) measurements were taken in an X-ray photoelectron spectrometer K-Alpha (Thermo Scientific) using the Al Kα X-ray beam with a spot size of 400 µm, pass energy of 200 eV, and energy step size of 1 eV. Fourier transform infrared (FT-IR) analysis was conducted in a Spectrum One PerkinElmer spectrometer in the 4000–400 cm−1 range, using KBr pellet technique (0.5:300 mg). Raman spectra of the materials were collected with a LabRAM HR Evolution model Raman Spectrophotometer (Horiba) equipped with a camera and using an Ar laser (532 nm). The spectra were taken between 1100 and 1800 cm−1 with an acquisition time of 30 s, 3 cycles. The transmission electron microscopy (TEM) was performed using a JEOL JEM-2100 instrument (JEOL Ltd., Tokyo, Japan) with an accelerating voltage of 200 kV. Energy-dispersive spectrometer (EDS) from Oxford Instruments, model X-MaxN 80T was used.
2.5 Characterization of PHBV/CNT nanocomposites
Notched Izod impact strength test was performed according to ASTM D256-06 on PHBV/CNT, PHBV/CNT-Ox, and PHBV/CNT-GB samples. A CEAST/Instron Impactor (model 905) was used with standard Izod-type hammers to perform the test following the Izod model. Impact loading was done with a 0.5 J hammer. Samples were notched using CEAST/Instron equipment, V-notch impact test specimens with depth of 2.54 ± 0.1 mm. The test was carried out with a temperature of 25 °C and 50% humidity, and the samples were maintained under these conditions for 48 h before testing. Ten specimens were tested for each condition. Results were analyzed using one-way analysis of variance (ANOVA) and Tukey’s multiple comparisons test on GraphPad Prism 6 (GraphPad Prism 6 Software Inc. USA). The results were presented as mean ± standard deviation (n = 10). Mean values at P < 0.05 were considered significant.
The impact fracture surface morphology of the nanocomposites was observed by scanning electron microscopy (SEM) using an Inspect S50-FEI Company® (Oregon, USA) microscope, operating at 7.5 keV with detectors of secondary electrons. The samples were fixed on aluminum stubs and covered with gold. The roughness of the impact fracture surface was quantified (n = 3) through profilometry using a mechanical profilometer KLA Tencor P-7.
3.1 X-ray photoelectron spectroscopy
Elemental surface composition of CNT, CNT-Ox, and CNT-GB
Atomic concentration (%)
The surfaces of CNT and CNT-Ox consist of C and O, with C1s and O1s peaks, near 533 eV and 285 eV, respectively, however, with different ratios. For CNT, a small amount of 2.0% of oxygen was detected, which is attributed to functional groups on the tube’s edges . After oxidation, the oxygen content increased from 2.0 to 4.7%, indicating that oxygen-containing functional groups were introduced onto the CNT surface. In the spectrum of CNT-GB, an additional peak near 400 eV is observed, corresponding to N1s, indicating the presence of nitrogenated functional group on the CNT-GB surface , confirming the CNT functionalization with GABA.
3.2 Infrared spectroscopy
Pristine CNT shows a band at 1575 cm−1, which is attributed to the vibration of carbon skeleton . After acid oxidation, CNT-Ox presented a new peak at 1720 cm−1, related to the (O=C–OH) stretching vibrations of the carboxylic acid group [18, 24, 25]. After GABA-functionalization, the peak at 1720 cm−1 was shifted to lower wavenumber (1706 cm−1) and had its shape changed. This is due to the presence of amide groups on the CNT-GABA surface, resulting from the covalent attachment of GABA. This covalent bond is changing the behavior of carbonyl groups, once amide carbonyl groups appear at lower wavenumbers compared to the carbonyl from the carboxylic acid group [18, 23, 25]. Another evidence of GABA grafting to CNT-GABA surface is the disappearance of the band at 1261 cm−1, which is attributed to C–O stretching of aromatic groups of the nanotube structure . The intensity of this band was reduced for CNT-Ox and completely disappeared for CNT-GABA, reflecting the consumption of these C–O groups for the GABA attachment. This result confirms that GABA is not adsorbed, but covalently attached to the CNT surface. Another evidence is the increase in the intensity of the peak at 1575 cm−1, which is characteristic and very intense peak of GABA, as can be seen in Fig. 3.
3.3 Raman spectroscopy
The ratio between the integrated areas of the D and G bands (ID/IG), and D′ and G bands (ID′/IG) is useful to provide an approach regarding the extent of functional groups introduced on the CNT surface, by measuring the sp3 atoms in the structure . Increased values of ID/IG and ID′/IG were obtained for oxidized and GABA-functionalized CNT. CNT-P exhibited an ID/IG ratio of 0.89 and ID′/IG ratio of 0.07, while for CNT-Ox and CNT-GB, the ratio ID/IG was 1.01 and 1.25, respectively; and the ID′/IG ratio was 0.08 and 0.11, respectively. These values indicate that the surface structure of the nanotubes was modified, with the introduction of defects and an increased number of sp3-hybridized carbons . In this particular case, the increase in the ID/IG and ID′/IG ratio values is due to covalent binding of carboxylic and hydroxyl groups emerging from oxidation for CNT-Ox, and covalent binding of GABA groups on CNT-GB.
Raman position and FWHM of CNT, CNT-Ox, and CNT-GB
The broadening of the FWHM on G band also confirms that the structural order of CNT was diminished after both functionalizations , providing another evidence that the attachment of functional groups could be successfully performed.
3.4 Transmission electron microscopy
3.5 Notched Izod impact strength
The impact strength of neat PHBV, which was 38.6 J/m, was significantly reduced to 34.0 J/m after the addition of CNT. According to the Tukey’s test, both samples are statistically different, displaying a reduction of about 12% in the impact resistance value. This reduction was expected since the addition of CNTs increases the stiffness of the polymer matrix and consequently reduces the impact strength, and similar results were found in the literature . Moreover, PHBV/CNT nanocomposite suffered a reduction in the impact strength because the dispersion of CNTs into PHBV matrix was not good enough, changing the tenacity mechanism and consequently reducing the energy absorption [38, 39]. After functionalization, the impact resistance was slightly increased if compared to PHBV/CNT. Samples produced with CNT-Ox and CNT-GB presented average values of impact resistance lower than that observed for neat PHBV; however, statistical analysis showed that the three samples PHBV, PHBV/CNT-Ox, and PHBV/CNT-GB have similar impact strength.
Agglomerate particles act reducing the impact strength of polymer matrices . In the case of PHBV/CNT-Ox and PHBV/CNT-GB, the functionalization acted positively, leading the impact strength value similar to that of neat PHBV. In this case, the improvement in impact strength may be related to good dispersion of CNT and increased compatibility between the filler and the polymer matrix.
Improved interfacial compatibility is essential for obtaining nanocomposites with superior mechanical properties due to a continuous interface formed between the polymer and the matrix [4, 8] and better dispersion of the nanotubes [41, 42]. Improvements in mechanical and thermal properties are observed when good compatibility is obtained between CNTs and the matrix .
3.6 Scanning electron microscopy (impact surface)
Figure 7e shows a greater magnification from Fig. 7d and allows to identify some ears on the fracture surface of PHBV/CNT-GB, which are an indication of local plastic deformation . Such localized plastic deformation shown in Fig. 7d consists of the formation of fibrils and microvoids, since in thermoplastic polymers, the primary deformation mechanism is called crazing, which is a localized plastic deformation. This region of plastic deformation fracture consists of a highly misaligned and stretched region, involving tearing of materials .
Although the impact resistance values were not significantly changed with the addition of CNT-GB, the fracture surface morphology shows that the functionalization with GABA changes the fracture mode from relatively fragile to ductile, evidenced by the regions with plastic deformation. This behavior may be justified because GABA has a similar structure to the hydroxybutyrate monomer from PHBV. Better compatibility with improved interfacial adhesion will be achieved when a nanoparticle is functionalized with the same polymer used as matrix [4, 8]. Moreover, the mechanical properties of nanocomposites are directly affected by the dispersion of CNT in the matrix, the presence of effective functional groups, and strong interfacial adhesion between nanoparticle and polymer matrix .
The nanocomposites reported in this work were produced with 0.5 wt% of CNT; however, the results obtained are promising for producing nanocomposites with higher concentrations of CNT-GB, which could result in much improvement in the mechanical properties of PHBV.
3.7 Impact surface roughness
The compatibility of CNT-GB with PHBV matrix was significantly improved, once GABA has a similar structure to PHBV, leading to better polymer–filler interactions , which was responsible for changing the fracture mode of this sample.
The unprecedented functionalization of multi-walled carbon nanotubes with GABA was successfully reported. The presence of nitrogen from GABA was detected by XPS, and FT-IR confirmed its covalent attachment on the CNT surface. Raman and TEM contributed to evidence the functionalizations, showing higher structural disorders for functionalized samples, as expected. Nanocomposites with 0.5 wt% of CNT, CNT-Ox and CNT-GB and PHBV were produced to evaluate the effectiveness of CNT functionalization in the compatibility between the filler and the matrix. The addition of CNT-GB showed changes in the impact fracture mode of PHBV from fragile to ductile, evaluated through SEM images and average roughness of the impact fracture surface, which was increased by about 300% for PHBV/CNT-GB compared to neat PHBV. This result suggests that the compatibility between CNT-GB and PHBV was greatly improved, being a promising result for the production of biodegradable thermoplastic nanocomposites.
We would also thank the Nanostructured Soft Materials Laboratory and Brazilian Nanotechnology National Laboratory, LNNano, for the use of XPS facility.
This study was funded by the Brazilian Funding institutions: FAPESP (Fundação de Amparo à Pesquisa do Estado de São Paulo) [2017/24873-4; 2018/12035-7, 2016/19978-9, 2017/27079-7], FINEP (Financiadora de Estudos e Projetos) [01.13.0328.03], and CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior).
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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