Exploration on structural morphology and properties of novel poly(urethane-imide)/TiO₂ bionanocomposites derived from L-tyrosine based diol

Abstract

In this study, an optically active biodegradable poly(urethane-imide) (PUI) derived from L-tyrosine was synthesized. Since biopolyurethanes cannot be fully competitive with conventional thermoplastics and are sometimes too weak for practical use, these biopolymers have been formulated and associated with nano-sized fillers. For the preparation of PUI/titanium dioxide (TiO₂) bionanocomposites (BNC)s, the high-intensity ultrasound was applied. The PUI/TiO₂ BNCs were characterized by Fourier transform infrared spectroscopy, powder X-ray diffraction, transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and UV–vis spectroscopy. The TEM and SEM results specified that the nanoparticles were dispersed homogeneously in PUI matrix on nanoscale. TGA/DSC confirmed that the heat stability of the novel BNCs was improved. UV–vis studies indicated that the resulting BNCs had high absorbance value than those values of pure PUI.

Keywords:

• TiO₂ nanoparticle
• bionanocomposite polymers
• transmission electron microscopy
• ultrasonic irradiation
• X-ray diffraction

1. Introduction

Recently, many studies have been devoted to the biopolymers and biodegradable polymers, because of increasing environmental concern and decreasing fossil resources. Biologically functional polymers have attracted large interest both in academic and industrial fields for pharmaceutical, biomedical, agricultural, and environmental applications [1–3].

Bionanocomposites (BNC)s are organic/inorganic hybrid biomaterials, which are produced by incorporating nano-sized fillers into a biopolymer matrix. Depending on the nanofiller chosen, the nanocomposite materials could show drastic modifications in their properties, like improved mechanical properties, barrier properties, or a change in their thermal and electrical conductivity [4]. They exhibit improved properties with preservation of the material biodegradability, without ecotoxicity. Such materials are mostly intended to biomedical applications and different short-term applications, e.g. packaging, hygiene, or agriculture devices [5–7]. Polymer nanocomposite materials could be fabricated by incorporating different inorganic nanoparticles (NPs) or fillers such as ZnO, SiO₂, carbon nanotubes, and other metal NPs into the polymer matrix [8–11]. Titanium dioxide (TiO₂) is a nontoxic, cheap, and photostable material, which has been applied in food packaging material, cosmetics, and in environmental treatments such as water and air purification, water disinfection and sterilization because of its unique properties such as good stability, high refractive index, UV resistance, strong photocatalytic activity, and chemical stability [12–14]. Polymer optical materials have been extensively utilized in optical fibers, optical waveguides, lenses, optical coatings, and optoelectronic devices as substitutes for inorganic optical materials. Since the refractive index of the polymers is usually in the range of 1.3–1.7, the application of polymers as high refractive index materials is limited [15–19]. Lately, the preparation of inorganic/organic hybrid materials with high refractive index has attracted substantial interest [20–28]. A technique for producing high refractive index nanocomposites and transparency in the fields of clear coatings, optical waveguides, and anti-reflection coatings for optical components was developed by incorporating premade inorganic colloidal particles into a polymer matrix. This method provides full synthetic control over the particle size, size distribution, and the surface properties of NPs [29–31]. Due to the high surface-area/particle-size ratio, there is a strong tendency for NPs to aggregate, therefore decreasing the resultant mechanical properties of the nanocomposite materials [32]. Some studies have shown that fully and uniformly dispersing NPs in the polymer matrix can noticeably improve the properties of materials, such as mechanical and thermal properties, barrier performance, and flame retardancy [33–35]. Consequently, it is necessary to disperse the NPs without aggregation in organic binders. One approach is breaking down the agglomerated NPs applying mechanical shear forces such as ultrasonic irradiation [36–38]. Ultrasonic has been broadly used in medicine, jointing, cleaning, machining, chemistry, and preparing nano-sized materials [39–41]. Lately, ultrasonic irradiation has been used to synthesize the nanocomposites [38]. Nevertheless, this approach will be restricted because of the limited interaction between the inorganic materials and the polymeric matrix, compared with the very strong interaction between individual NPs. In order to improve homogeneous dispersion of NPs and better compatibility between the NPs and polymer matrix, many researches were focused upon the surface modification of NPs and the use of different coupling agents such as trialkoxy silanes was suggested. [29,30,37,42,43] The objective of this study is the efficient preparation of PUI/TiO₂ BNC using TiO₂ NPs modified by KH550 (with high refractive index, 2.5, in the crystal form of anatase and some small amount of rutile form), via ultrasonic irradiation process. The influence of interface structure on the thermal and the UV–vis absorber properties of the BNC are also discussed with the aid of thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and UV–vis spectrometric instrument.

2 Experimental

2.1 Materials

Toluene 2,4-diisocyanate (TDI), dimethylformamide (DMF), coupling agent (γ-amidopropyltriethoxyl silicane) (APS), and NPs (TiO₂) with average particle sizes of 35–50 nm were purchased from Merck chemical Co. (Darmstadt, Germany) and Nanosabz. Co. (Tehran, Iran) and were used without further purification.

2.2 Characterization

X-ray diffraction (XRD) measurements of the BNCs and its components were recorded using a Philips X’PERT MPD with a copper target at 40 kV and 30 mA. The powder samples were spread on a sample holder and the diffractograms were recorded in the range 5–80° at the speed of 0.05°/min. Fourier transform infrared spectroscopy (FTIR) were recorded on (Jasco-680, Japan) spectrophotometer. First, the samples were dried, and then mulled with KBr pellets. UV–vis spectra were recorded on a UV/vis/NIR spectrophotometer, JASCO, V-570 with solid pellets sample of BNCs. The surface morphology of the samples was monitored with transmission electron microscopy (TEM), scanning electron microscopy (SEM), and atomic force microscopy (AFM). TEM was performed on a Philips, CM120 (Netherland) microscope operating at an accelerating voltage of 100 kV. SEM was obtained using a Cambridge Oxford (UK) 7060 scanning electron microscope connected to a four-quadrant backscattered electron detector with resolution 1.38 eV. The samples were dusted on a double-sided carbon tape placed on a metal stub and coated with a layer of gold to minimize charging effects. AFM were obtained using a Multimode Nanoscope III instrument (Digital Instrument Inc., New York, USA) with noncontact tapping mode with a silica probe (NSC 11) and a frequency of 463 kHz. TGA and DSC data for the BNCs were recorded, on Perkin Elmer instrument at a heating rate of 10 and 20 °C/min, under nitrogen atmosphere by IPPI.

2.3 Apparatus

The reaction was performed on an Auburn Hills, MI, USA ultrasonic cell crusher. Ultrasonic irradiation was carried out with the probe of the ultrasonic horn immersed directly in the mixture solution system with frequency 2 × 10⁸ Hz and high power level (100%).

2.4 Polymer synthesis

The PUI was synthesized from N,N′-(pyromellitoyl)-bis-L-tyrosine dimethyl ester as nontoxic aromatic diol (1) which was reported in our previous work [44] with TDI under gradual heating [45]. Here is a brief procedure. A three-necked flask containing diol 1 (0.10 g, 1.74 × 10⁻⁴ mol) and dibutyltin dilaurate (DBTDL) and 2 mL of DMF was magnetically stirred under nitrogen atmosphere at room temperature for 30 h. A solution of (0.04 g, 1.74 × 10⁻⁴mol) of TDI in 1 mL of DMF was added dropwise through a dropping funnel over 15 min. The mixture was stirred at room temperature for 1 h and then it was heated at 80 °C for 8 h – then was heated gradually from 100 to 110 °C for 4 h. The mixture was cooled down to room temperature, poured into 15 mL of methanol to precipitate yellow flocculate. The product was filtered, washed with methanol several times, and dried at 80 °C (Scheme 1).

Scheme 1 Synthesis of the PUI.

2.5 In vitro toxicity and soil biodegradability test

In vitro toxicity of PUI derived from N,N′-(pyromellitoyl)-bis-L-tyrosine dimethyl ester was evaluated based on the growth of air-born spores on culture media [44,45]. Also, the soil burial degradation test of PUI was conducted in aerobic soil condition in the presence of natural micro-organisms [44,45].

2.6 Preparation of PUI/TiO₂ BNCs

Preparation of PUI/TiO₂ BNCs was carried out according to literature procedure in an ultrasonic wave irradiation process [29,37]. Here is the brief procedures. NPs were firstly treated with 10 wt.% of silane coupling agent (APS). Different amounts of TiO₂ (5, 10, 15, and 20 wt.%) modified NPs were mixed with the PUI and the mixture was dispersed in 20 mL of absolute ethanol, followed by irradiation with high-intensity ultrasonic wave for 4 h. After irradiation, the resulting suspension was cooled to room temperature and then centrifuged, and the precipitate was washed twice with absolute ethanol and distilled water, respectively. The solid was dried in vacuum at room temperature for 8 h. The obtained product was kept for further characterization.

PUI/TiO₂ (10 wt.%) BNC spectroscopic data: ν _max (KBr)/cm⁻¹: 3380, 2928, 2870, 1722, 1631, 1535, 1123, 1060, 756.6, 655, and 542.

3 Results and discussion

3.1 In vitro toxicity and soil biodegradability test of polymer

According to former works [44,45], it could be concluded that, PUI derived from N,N′-(pyromellitoyl)-bis-L-tyrosine dimethyl ester as nontoxic diphenolic monomer, is biologically active and biodegradable under microbial growth and soil burial test due to microbial invasion. Therefore, this polymer may be classified as biologically active and biodegradable material.

3.2 FT-IR spectroscopy

From the FT-IR spectra of the pure TiO₂, TiO₂ modified with APS, pure PUI, PUI/TiO₂ BNCs, and TiO₂ NP after removal of PUI by dissolving BNC in solvent (Figure 1), it can be seen that in pure TiO₂ (Figure 1a), ν(OH) stretching band and bending band are viewed at 3423 and 1637 cm⁻¹, respectively. Based on Bezrodna et al. reported paper [46], the observed broad bands at 3426–3500 cm⁻¹ were ascribed to hydroxyl groups and some varying interactions between hydroxyl groups on TiO₂, respectively. The bands at 643 and 540 cm⁻¹ are assigned to the anatase nano-TiO₂ [47]. In the spectrum of TiO₂ modified with APS (Figure 1b), the bands at 2928 and 2870 cm⁻¹ were assigned to alkyl groups [–(CH₂) _n –], however, their intensity are weak. The N–H bending vibration of primary amine is observed at around 1605 cm⁻¹. The appearance of these bands indicates that functional groups in modifier (aminopropyl group) are existed on the surface of modified TiO₂. Furthermore, the peak at around 925 cm⁻¹ appeared in both spectra which indicate that the condensation reaction between silanol groups and surface hydroxyl group took place. Li et al. [48] showed the FT-IR spectrum of titania–silica composite and assigned the peak at around 960–910 cm⁻¹ to the stretch vibration band of Ti–O–Si. In addition, the broad band at around 1050 cm⁻¹ corresponded to Si–O–Si bond is observed which indicates the condensation reaction happened between silanol groups. The carbonyl group of urethane is shown in the region of 1725 cm⁻¹ (Figure 1(c)). In the spectrum of 10 wt.% PUI/TiO₂ BNC (Figure 1(d)), the characteristic peaks of pure PUI and TiO₂ are still maintained, but it could be concluded that the structure of PUI may be affected by the presence of the modified TiO₂. In order to understand the possible reactions between functional groups of modified TiO₂ and the PUI matrix, TiO₂ NP was isolated from the final BNC by dissolving BNC polymer in N,N-DMF and filtering it, following by precipitation and washing with DMF two times. The solid was dried in vacuum at 100 °C for 8 h and then was characterized by FT-IR. From this data (Figure 1(e)), it is implied that the NH₂ groups from modified TiO₂ may be reacted with the methyl ester pendant group of PUI.

Figure 1 FT-IR spectra of (a) pure nano-TiO₂, (b) Modified TiO₂ with APS, (c) Pure PUI, (d) 10% modified nano-TiO₂/PUI, and (e) nano-TiO₂ after removed PUI from BNC.

3.3 XRD data

Figure 2 shows the powder XRD patterns of the PUI (a), NP TiO₂ (b), and (10 wt.%) PUI/TiO₂ BNC (c). From the spectra, it can be observed that the pure PUI is the amorphous polymer and exhibit no anisotropic behavior, which may be due to the presence of the aromatic structures in the main chain, thus limiting the molecular mobility of the polymer. From Figure 2(c), it was observed that the most of the characteristic peaks at 2θ values of anatase TiO₂ are kept intact (compared with Figure 2(b)). The average crystalline size of nano-TiO₂, which is determined from the half-width of the diffraction using the Debye-Scherrer equation, is approximately 30–50 nm. Meanwhile, it is interesting to note that all the diffraction peaks become broad in these cases comparing with the XRD spectrum of TiO₂, which should be attributed to the presence of the small-sized NPs [49]. The findings indicate that ultrasonic irradiation reduces the crystallite size of anatase due to the generation of many localized hot spots in the solution by the ultrasound irradiation, which further gives rise to the homogeneous formation of a large number of seed nuclei, leading to a smaller particle size [39–41].

Figure 2 XRD patterns of (a) pure PUI, (b) pure nano-TiO₂, and (c) 10 wt.% PUI/TiO₂ BNC.

3.4 SEM and TEM observations

The direct verifications of the formation of BNCs are provided by TEM investigation. Figure 3(a) and (b) show the morphology of 5% unmodified TiO₂/PUI and 5% modified TiO₂/PUI BNCs. The micrographs confirmed that the titania particles were well dispersed in the PUI matrix (Figure 3(b)). As can be seen, the average size of PUI/TiO₂ BNCs is 35–50 nm. Moreover, this result was consistent with the XRD result mentioned previous. According to the SEM micrograph, as shown in Figure 4, the morphology of pure PUI does not show any nano-sized distribution of polymer (Figure 4(a)). The dimension of agglomerates for the unmodified-TiO₂ filler composite is about 3 μm in average (Figure 4(b)), and that of the modified counterpart is about 50–100 nm (Figure 4(c)–(h)). The surface pretreatment with coupling agent on the nano-sized TiO₂ fillers could really decrease the tendency of aggregation of the NP TiO₂ fillers in the PUI matrix which indicates the formation of a BNC.

Figure 3 TEM micrographs of (a) 5 wt.% unmodified nano-TiO₂ /PUI BC and (b) 5 wt.% modified nano-TiO₂/PUI BNC.

Figure 4 SEM micrographs of (a) pure PUI, (b) 5 wt.% unmodified nano-TiO₂/PUI, (c,d) 5 wt.% modified nano-TiO₂/PUI BNC, (e,f) 10 wt.% TiO₂/PUI BNC, and (g,h) 15 wt.% TiO₂/PUI BNC.

3.5 AFM observation

The topographic morphology of PUI surfaces was scanned by AFM (Figure 5). Compared the surface morphology of pure PUI with BNC, a number of small bulges are observed on the interfaces. The bulges should be TiO₂ particles because the size of the bulge varies with the particle size of TiO₂ and so is close to the particle size of the used TiO₂, indicating that some nano titania particles exist at the surface of PUI. AFM also reveals the size of the filler particles is about 50–150 nm, which is in the range of small aggregates of primary particles of 50 nm.

Figure 5 Typical AFM topographic images of (a,b) pure PUI and (c,d) 5 wt.% TiO₂/PUI BNC.

3.6 UV–vis spectroscopy

UV–vis spectroscopy of BNCs coatings containing various loadings of modified TiO₂ NPs is shown in Figure 6. For comparison between different BNCs coatings, the UV–vis absorbance spectrum of the pure PUI has been used as a reference (i.e. made to have zero absorbance). It is clear from Figure 6 that the increase in TiO₂ NP loadings leads to notably increased absorption in UV-wavelength region.

Figure 6 UV–vis absorption of PUI BNC coatings (5, 10, 15, and 25) wt.%.

3.7 TGA/DSC thermal data

Figure 7 showed the thermogravimetric curves of the pure PUI and PUI/TiO₂ BNCs with the different NP contents. It was evident that the pure PUI had a low decomposition temperature of about 120 °C which was further improved when TiO₂ was introduced. However, the initial temperature of the BNCs weight loss was increased with increasing TiO₂ content until at a weight loss of 40%, this increase in the thermal stability may result from the high thermal stability of TiO₂ network and the physical cross-link points of the TiO₂ particles, which limited the movement of the molecular chain of PUI. After addition of TiO₂ NPs with high-melting point to the polymer matrix, the TiO₂ particles can serve as a good thermal cover layer, avoiding the direct thermal decomposition of polymer matrix by heat. In addition, the TiO₂ is a nanoscale particle, which offers a larger surface area and improves the effect of thermal cover.

Figure 7 TGA curves of PUI BNC coatings (5, 10, 15, and 25) wt.% and pure TiO₂.

In order to estimate the effect of surface modification of NP TiO₂ on the thermal stability of the PUI BNCs, thermal scans were carried out from 50 to 300 °C by the DSC method. As shown in Figure 8, the thermal stability of the PUI polymer can be significantly improved by the inclusion of TiO₂ nanofillers (T _g increased from 137.30 °C for pure PUI to 141.80, 154.92, and 164.47 °C for 5, 10, and 25%, respectively). As a result, mobility of the matrix molecules was restricted to certain extent, and creep resistance of the BNCs became much higher than the pure PUI.

Figure 8 DSC curves of PUI BNC coatings (5, 10, and 25) wt.% and pure TiO₂.

3.8 Mechanism illustration for the preparation of BNCs

The surface modification of TiO₂ NP and interaction with the PUI matrix are schematically illustrated in Figure 9. Under ultrasonic, silanol groups of APS generated by hydrolysis can interact with hydroxyl groups on the TiO₂ surface and form the modification layer –NH₂ groups. Generally, the main effects of sonication are due to cavitation or the growth and explosive collapse of microscopic bubbles on a microsecond time scale. At the same time, ultrasonic cavitation can generate a rigorous environment of local temperature up to 5000 K and local pressure up to 500 atm [37]. Under such conditions, the modified NPs might be dispersed absolutely and will combine with PUI via the amino of coupling agent by physical and may be chemical absorption. The amine groups on APS surface may be reacted with methyl ester pendant groups of the PUI chains to form the interface structure based on covalent bonding. As a result, the dispersity of NPs increases with the decrease in surface energy of NPs, and the polymer on the NP surface has a steric dispersing and stabilizing effect, which can be observed from the photographs of TEM and SEM.

Figure 9 Mechanism illustration of the ultrasonic-induced reaction in the PUI formed on the NPs.

4 Conclusions

In this investigation, novel PUI/TiO₂ BNCs were prepared via ultrasonic process. The results showed that surface treatment of TiO₂ NPs with APS improves NPs dispersion, thermal properties, and UV protection of BNCs coating. A possible mechanism of ultrasonic-induced composite reaction was proposed based on the experimental results. DSC studies indicated that the thermal stability of the BNCs was improved with increasing NPs contents. The UV–vis studies showed that the BNCs possessed high absorbance value than those of pure polymer.

Acknowledgments

We wish to express our gratefulness to the Research Affairs Division, Isfahan University of Technology (IUT), for partial financial support. Further partial financial support from National Elite Foundation (NEF), Centre of Excellency in Sensors and Green Chemistry (IUT), and Iran nanotechnology Initiative Council (INIC) is gratefully acknowledged.