Self-healable castor oil based tough smart hyperbranched polyurethane nanocomposite with antimicrobial attributes

Abstract

Here, castor oil-based tough hyperbranched polyurethane/sulfur nanoparticles decorated reduced graphene oxide (HPU/SRGO) nanocomposites are fabricated with different weight% of nanohybrid. Tremendous enhancement of mechanical properties, such as tensile strength (from 7.2 to 24.3 MPa), tensile modulus (from 3.3 to 137.7 MPa), toughness (from 25.4 to 313.52 MJ m⁻³) and elongation at break (from 710% to 1456%), is observed upon incorporation of nanohybrid in HPU matrix due to strong interaction between SRGO and HPU matrix. The nanocomposite exhibited excellent repeatable self-healing (within 50–60 s at 360 W under microwave and 5–7.5 min under sunlight) and shape recovery (within 30–50 s at 360 W under microwave and 1–3 min under sunlight). The nanocomposite also demonstrated profound microbial inhibitory effect against Staphylococcus aureus, Escherichia coli and Candida albicans. Thus, the studied nanocomposite has tremendous potential for various advanced applications.

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Introduction

In recent times, immense interest is being paid to developing self-healing materials that can repair their damages using inherent nature of the materials.^1–3 The self-healing capability of these materials helps to extend their service period, facilitates maintenance and increases reliability.⁴ Recently, most self-healing materials are being developed based on encapsulation and reversible chemistry with covalent and noncovalent chemical bonds.^5–14 The encapsulation approach is achieved by the incorporation of microcapsules containing a healing agent, which readily polymerizes in the damaged area upon release from the microcapsules.^5,6 Although this approach has a great potential for internal healing, it fails to heal macroscopic damage. In addition, the irreversible healing nature is its another inadequacy; however, this limitation can be overcome using reversible chemistry.⁷ Intermolecular interactions and reversible bonds have been demonstrated to be particularly useful to introduce the ability to heal a polymer multiple times. At this juncture, it is important to mention that a new concept is developed that uses shape memory materials for improvement of the self-healing process.¹⁵ These shape-memory materials provide a mechanism to partially or fully close the crack and helps repair it. Rowan and coworkers prepared a polymer with a structurally dynamic polydisulfide network that exhibited both shape memory and self-healing properties.¹⁶ They proposed that shape-memory behavior of the polymer helps in the healing process by bringing crack surfaces into close proximity. Zhao also reported that cross-linked polyethylene/carbon black nanocomposites exhibited improved self-healing by a shape-memory effect (SME).¹⁷ Bai and his team, recently, prepared a poly(vinyl butyral)-based polymer, which also demonstrated both self-healing and SME.¹⁸ Shape-memory polymers (SMP) have the capability to recover their original shape from a temporarily deformed shape upon exposure to an external stimulus such as thermal energy, electricity, a magnetic field and light.^19–22 Structurally, SMPs are usually composed of both hard and soft segments.²³ The hard segment is a physically or chemically cross-linked structure that controls the permanent shape, and the soft segment is generally a crystalline or amorphous part of the polymer that is mainly responsible for the shape fixity.²⁴ Many polymers possess SME but polyurethanes (PU) are the most promising SMPs among them. This is due to their high recoverable strain (up to 400%), wide range of transition temperature for shape recovery, high control on the softening and retraction temperatures, inherent soft-hard segments and favorable, as well as tunable physical properties.²⁵

In this context, vegetable oil-based shape-memory PUs are in high demand due to the global scientific community search for renewable bioresources endowed with green credentials. Castor oil is one of the best industrially used vegetable oils for its unique fatty acid composition (92–95% ricinoleic acid) and easy availability.²⁶ It is pertinent to mention that monoglyceride of castor oil is a better choice as a triol compared with the oil itself in the synthesis of hyperbranched polyurethane.²⁷ The hyperbranched polymer achieved distinct interest over its linear counterpart due to unusual and desirable properties.²⁸ Further, Huang and coworkers reported that the polyurethane/graphene nanocomposite exhibited multi-stimuli responsive self-healing behavior for repeated use.²⁹ In this context, reduced graphene oxide (RGO) has some functional groups on its surface, which help to better its dispersion in the polymer matrix compared with graphene.³⁰ RGOs also have excellent thermal conductivity, and microwave (MW) and sunlight absorbing capacity that is almost equivalent to graphene.

In addition, sulfur-containing compounds and polysulfanes show significant potential as antimicrobial agents.³¹ Furthermore, both RGO and sulfur nanoparticles individually exhibited good antimicrobial activity.^31,32 Microbial contamination or infections are severe threats to humans. Various environmental factors lead to the fouling of polymeric materials. Microbial fouling is the vital force that causes degradation of polymeric materials in the course of time. Interestingly, vegetable oil-based polyurethane exhibited biodegradability on exposure to different microbes.³³ Keeping this in mind, it is desirable to fabricate polymers with repeated healing capability and excellent shape recovery along with antibacterial attributes for different demanding applications. Therefore, a sulfur nanoparticle-decorated RGO (SRGO) nanohybrid may be the right choice to prepare antimicrobial polymer nanocomposites.

Herein, we describe the fabrication of hyperbranched PU/SRGO (HPU/SRGO) nanocomposites with different weight% of SRGO. The mechanical properties, self-healing ability, shape-memory behavior and antimicrobial properties of the nanocomposites were also explored.

Experimental

Materials

Graphite flakes (60 meshes, purity 99%) and castor oil were obtained from Sigma-Aldrich, India. 1,4-Butanediol (BD) and sodium thiosulphate were purchased from Merck, India, and used as received. Poly(ε-caprolactone)diol (PCL, Solvay Co., Mn = 3000 g mol⁻¹) was also used as received. Lemon (Citrus limon) was collected from the local area. Monoglyceride of the castor oil was prepared as previously reported²⁷ and GO was prepared according to a modified Hummers method.³⁴

Preparation of SRGO nanohybrid

SRGO nanohybrid was prepared as reported in our earlier work.³⁵ Briefly, a thiosulfate solution was prepared by dissolving 64 mg of Na₂S₂O₃ in 50 mL millipore water. Then, 35 mL GO (1 mg mL⁻¹) was added into the solution and sonicated it for 30 min to obtain a GO-dispersed thiosulfate solution. Then, 10 mL of lemon juice was added into the solution and the mixture was stirred for 60 min under ambient conditions to form the nanohybrid. This prepared nanohybrid, SRGO, was sonicated for 10 min. The resulting suspension was washed by repeated centrifugation with millipore water and acetone, and then dried in an oven at 60 °C.

Preparation of HPU/SRGO nanocomposite

The nanocomposite was prepared by in situ polymerization technique in a three-necked round bottomed flask, equipped with a nitrogen gas inlet, a mechanical stirrer and a Teflon septum. PCL (0.002 mol, 6 g), BD (0.004 mol, 0.36 g) and dispersion of SRGO in DMAc (different weight%: 0.5, 1 and 2 with respect to total weight of nanocomposite) were taken into the flask with the desired amount of xylene (maintaining solid content at 40%). After dissolving PCL, TDI (0.007 mol, 1.22 g) was added dropwise using a syringe into the reaction mixture at room temperature. Then, the reaction was continued for 3 h at a temperature of (70 ± 2) °C to obtain the desired viscous mass, which was treated as the pre-polymer.

Then, this pre-polymer was cooled to room temperature and monoglyceride of castor oil (0.002 mol, 0.74 g) as a triol was added into it with the required amount of TDI (0.002 mol, 0.35 g). The temperature was then raised to 110 ± 2 °C and continuously stirred for 2.5 h to complete the reaction, as indicated by the absence of isocyanate band at 2270 cm⁻¹ in the FTIR spectrum. HPU was also prepared without using SRGO. HPU with 0.5, 1 and 2 weight% of SRGO were encoded as HPU/SRGO0.5, HPU/SRGO1 and HPU/SRGO2, respectively.

Material characterization

FTIR spectra of HPU and its nanocomposites were taken by a Nicolet (Madison, USA) FTIR impact 410 spectrophotometer over the wavenumber range 4000–400 cm⁻¹ using KBr pellets. XRD was carried out at room temperature (ca. 25 °C) by a Rigaku X-ray diffractometer (Miniflex, UK) over the range 2θ = 2–70° at a scanning rate of 2° min⁻¹. TGA was performed by a thermal analyzer, TGA4000, Perkin Elmer, USA, with a nitrogen flow rate of 30 mL min⁻¹ at a heating rate of 10 °C min⁻¹. Differential scanning calorimetry (DSC) was performed by DSC 6000, Perkin Elmer, USA, at 2 °C min⁻¹ heating rate under the nitrogen flow rate of 30 mL min⁻¹ from −40 to 120 °C. The tensile strength and elongation at break were measured on a rectangular strip with dimensions 80 × 10 × 0.50 mm³ by the help of the Universal Testing Machine (UTM), Jinan WDW 10, China, with a 500 N load cell at crosshead speed of 20 mm min⁻¹. Tensile modulus of the sample are obtained from the slopes of the linear areas in the stress–strain curves and toughness of the sample are calculated by integrating the stress–strain curves. Tensile strength is measured as the maximum stress withstood by the sample before damage.

Shape memory test

To study the shape-memory behaviour under MW and direct sunlight, the bending test was performed. The sample was folded into a ring form at 60 °C followed by quenching into an ice-salt bath for 5 min at −10 °C. Then, the shape recovery of the nanocomposite films was achieved by exposure to MW irradiation of 360 W for 30–60 s and direct sunlight (11 am–2 pm, at the Tezpur University campus, altitude: 26.63 °N 92.8 °E in the month of March on sunny days with average temperature: 32 ± 1 °C and humidity: 70% ± 1%). The shape recovery was calculated using the following equation

Shape recovery (%) = {(90 − θ)/90} × 100 (1)

where θ in degrees denotes the angle between the tangential line at the midpoint of the sample and the line connecting the midpoint and the end of the curved samples.

Self-healing test

To evaluate healing performance, the above rectangular strips of nanocomposite were cut (10 × 0.2 × 0.015 mm³ in dimension) in the transverse direction by a razor blade, and the crack was separately healed by sunlight and MW. Healing efficiency was calculated as the ratio of tensile strength values of the nanocomposites before to after healing. The tensile strengths of the pristine and the healed samples were measured using the same UTM. Tensile strengths of pristine HPU and nanocomposites with different loadings of nanohybrid were measured for at least 5 samples in each case, before and after the healing process. The optimal healing time for each case is defined as the shortest time required to achieve the best healing efficiency under the given conditions. For MW healing, a domestic MW oven (800 W) at an operating frequency of 2.45 GHz and a MW power of 360 W was used. Samples were placed in the middle position of the rotating disc on a glass plate inside the MW oven with a chamber having interior dimension of 20 × 19 × 14 cm³. Sunlight healing was performed under direct sunlight (11 am–2 pm) at the Tezpur University campus (altitude: 26.63 °N 92.8 °E) in the month of March on sunny days [average temperature (34 ± 1 °C) and humidity (74% ± 1%), light intensity: 90 000–100 000 lux].

Antimicrobial activity

Microbial strains used for the study were Staphylococcus aureus (ATCC 11632), Escherichia coli (ATCC 10536) and Candida albicans (ATCC 10231).

Minimum inhibitory concentrations (MIC) were calculated for the nanomaterials and HPU, as well as for the nanocomposites. A micro-dilution technique was adopted for the assay.³⁶ S. aureus and E. coli were cultured in Nutrient Broth (NB, HiMedia, India) for 24 h at 37 °C. C. albicans was grown in Potato Dextrose Broth (PDB, HiMedia, India) for 48 h at 28 °C inside an incubator. Serial dilution was carried out for the samples (from stock solutions with concentration 30 mg mL⁻¹) using 1% dimethylsulfoxide (DMSO, HiMedia, India). Samples (100 μL) were incubated with the microbial cultures (100 μL) in 96 well plates at their specific concentrations. Streptomycin and nystatin (HiMedia, India) were taken as the positive controls. After incubation of 24 and 48 h, respectively, for bacteria and fungus, 40 μL of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) was poured into each well. Change in color of the media to dark blue indicated the viable microbial cells, whereas no change in color indicated the dead cells.

Further, microbial growth patterns were studied in the presence of HPU and the nanocomposite films. Microbial cultures were taken in 15 mL test tubes in the presence of the films and incubated for a specific time period. Growth was determined by recording their UV absorbance at 600 nm. Test tubes without film were considered as controls. Further, test tubes containing HPU were taken for comparative study. An SEM image was taken for E. coli as a representative species to observe the fate of the bacteria adhered to the films.

Again, HPU and the nanocomposite films (1.5 cm × 1.5 cm × 0.3 mm) were laid on solidified agar plates, on which inoculums of bacteria and fungus were spread with the help of a spreader. Plates were incubated at 24 and 48 h at 37 and 28 °C in an incubator, respectively. Microbial growth inhibition over the films was witnessed by capturing the photographs of the plates in a Nikon Coolpix camera and analyzed visually.³⁷

Results and discussion

Preparation

The HPU/SRGO nanocomposite was prepared by an in situ technique using a monoglyceride of castor oil as a branching moiety (triol) and SRGO as reinforcing nanomaterial. The key factors for successful preparation of the HPU nanocomposite are concentration of the reactants (especially the branching moiety), addition rate of the triol moiety, reaction time and temperature.³⁸ In first step of the polymerization process, a SRGO dispersion in DMAc was incorporated to provide a chance to react the nanohybrid with a few isocyanate terminated prepolymer chains. This provides strong interfacial interactions between HPU chains and the nanohybrid.³⁴ In the second step of the reaction, the multifunctional moiety (monoglyceride) was slowly added in a very dilute solution (15% in xylene) to avoid gel formation. Moreover, reaction temperature was gradually elevated from room temperature to 110 °C.³⁸

Characterization

FTIR spectra of HPU and its nanocomposites are shown in Fig. 1. The formation of urethane linkage in HPU and its nanocomposite was confirmed by the presence of characteristic bands for N–H deformation vibration (1070–1090 cm⁻¹), C–O stretching vibration (1150–1170 cm⁻¹), amide II (N–H bending vibration and C–N stretching vibration, 1560–1580 cm⁻¹), amide I (C=O stretching vibration 1675–1685 cm⁻¹), and 3430 cm⁻¹ (O–H free and N–H stretching vibrations).^25,27 The increase in broadening of the –OH band and shifting of the C=O band to 1675 from 1685 cm⁻¹ were observed with the increase in the amount of SRGO in the nanocomposite.³⁰ This indicated the presence of interactions among polymer chains and SRGO, which are increased with the SRGO content.

Fig. 1 FTIR spectra of (a) HPU, (b) HPU/SRGO0.5, (c) HPU/SRGO1 and (d) HPU/SRGO2.

In the XRD patterns of HPU and its nanocomposites, two distinct peaks at 2θ = 21.1° (corresponding to d-spacing of 0.419 nm) and 23.4° (corresponding to d-spacing of 0.381 nm) are observed for the crystals of PCL moiety of HPU (Fig. 2).³⁹

Fig. 2 XRD patterns of (a) HPU, (b) HPU/SRGO0.5, (c) HPU/SRGO1 and (d) HPU/SRGO2.

In the nanocomposites, minor shifting of PCL peaks towards a higher angle was noticed, due to the formation of a dense structure compared to pristine HPU.²⁶ A slight increase in the peak intensity of the PCL moiety was also observed with increase in the amount of SRGO due to the nucleating effect of SRGO. Importantly, distinct peaks for SRGO are not observed in the XRD patterns of nanocomposites because a small amount of SRGO was used to fabricate the nanocomposite.³⁰

Mechanical properties

The stress–strain profiles of neat HPU and its nanocomposites are shown in Fig. 3. The nanocomposite demonstrated extensive enhancement in mechanical properties after the incorporation of a small amount of SRGO. Mechanical properties, such as tensile strength, tensile modulus, elongation at break and toughness, are summarized in Table 1. The presence of strong interfacial interactions provided efficient load transfer ability between the nanohybrid and the polymer matrix.²⁶ This resulted in the outstanding mechanical properties of the nanocomposite. Moreover, due to the formation of the covalent bonds between prepolymer chains and functional groups of nanohybrid, hard segments of the HPU chains become stiff, resulting in high modulus and strength.³⁰ All the nanocomposites exhibited dose-dependent mechanical properties. HPU and HPU/SRGO0.5 demonstrated typical elastomeric stress–strain profile, whereas HPU/SRGO1 and HPU/SRGO2 demonstrated typical flexible plastic stress–strain profile. From Fig. 3, it is also clear that the tensile modulus and strength of nanocomposite are enormously enhanced in HPU/SRGO1 and HPU/SRGO2, compared with HPU/SRGO0.5. This indicated that a small amount of nanohybrid (0.5 weight%) does not establish appropriate interfaces, and hence there might be a lack of load transfer from the polymer matrix to the nanohybrid. This is directly reflected in low improvement in the mechanical properties of HPU/SRGO0.5.

Fig. 3 Stress–strain profiles of (a) HPU, (b) HPU/SRGO0.5, (c) HPU/SRGO1 and (d) HPU/SRGO2.

Table 1 Mechanical properties of HPU and its nanocomposites

Sample	Tensile strength (MPa)	Tensile modulus (MPa)	Elongation at break (%)	Toughness (MJ m^−3)
HPU	7.2 ± 0.5	3.3 ± 0.2	710 ± 20	25.4 ± 1.2
HPU/SRGO0.5	16.5 ± 0.7	13.6 ± 0.5	1186 ± 35	138.2 ± 2.2
HPU/SRGO1	21.6 ± 1.1	130.64 ± 1.2	1372 ± 30	268.9 ± 3.4
HPU/SRGO2	24.3 ± 1.3	137.74 ± 2.2	1456 ± 45	313.5 ± 4.3

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Toughness of the nanocomposite was also enhanced by the higher amounts of nanohybrid, and all the nanocomposite demonstrated excellent toughness compared with pristine HPU. Similar to our earlier report on the graphene-based nanocomposite, the HPU/SRGO nanocomposite also exhibited higher elongation at break compared to pristine HPU, and it was found to increase with increasing nanohybrid content.³⁰

The enhancement of elongation at break is due to the alignment of polymer chains along the loading direction during the initial stress and sliding of layers of RGO at high stress. The elasto-plastic behavior of graphene sheets may also be another reason for this observation.⁴⁰

Thermal properties

Thermal stability of the prepared nanocomposites were evaluated by thermogravimetric analysis; the thermograms are shown in Fig. 4. HPU and its nanocomposite show almost similar thermograms but the nanocomposites demonstrate higher thermal stability compared with pristine HPU. According to the thermograms, initial degradation temperature (T_initial) and midpoint degradation temperature (50% weight loss) are summarized in Table 2. The improvement in thermal stability of the nanocomposite may be attributed to the so-called “tortuous path” effect of RGO, which delays the escape of volatile degradation products and restricts the movement of polymeric chains due to the presence of strong interfacial interactions between HPU and the nanohybrid.⁴¹

Fig. 4 TG thermograms of HPU and its nanocomposites.

Table 2 Thermal properties of HPU and its nanocomposites

Sample	Tinitial (°C)	Midpoint degradation temperature (°C)	Melting point of soft segment (°C)	Degree of crystallization (%)
HPU	277.23	398.41	48.4	27.62
HPU/SRGO0.5	305.48	417.85	49.06	33.15
HPU/SRGO1	306.71	420.39	51.12	35.14
HPU/SRGO2	308.75	421.23	53.29	37.25

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The degree of crystallinity and melting temperature (T_m) of the prepared nanocomposite were evaluated by DSC analysis (Fig. 5). The crystallinity found in the nanocomposite is due to the presence of a crystalline PCL moiety in the soft segment of HPU. The dose-dependent degree of crystallinity and T_m values were observed for the nanocomposite with the content of nanohybrid (Table 2). These results are due to the fact that the nanohybrid may help to orient the HPU chains in a particular direction and thereby restrict the mobility of the polymeric chains.

Fig. 5 DSC curves of HPU and its nanocomposites.

Shape memory behavior

Shape memory behaviours of HPU/SRGO nanocomposites under MW and sunlight are shown in Fig. 6. All the nanocomposites demonstrated excellent shape fixity and recovery under exposure to the mentioned stimuli. Shape recovery of the nanocomposite was faster and more efficient upon exposure to MW compared with sunlight. This may be due to excellent MW absorbing capacity of RGO as compared with that of sunlight. Shape recovery time and ratio under different stimuli are tabulated in Table 3. The nanocomposite exhibited better shape recovery compared to HPU. Due to the homogenous distribution of SRGO in the HPU matrix, high stored elastic strain energy is generated in the nanocomposite, which helps the nanocomposites to achieve high recovery stress by releasing the stored elastic strain energy.^25,38 The shape recovery also found to be increased with increase in loading of nanohybrid.

Fig. 6 Shape-memory behaviors of HPU and its nanocomposites under direct sunlight.

Table 3 Shape memory properties of the nanocomposites

Stimulus	HPU		HPU/SRGO0.5		HPU/SRGO1		HPU/SRGO2
	Shape recovery time	Shape recovery (%)	Shape recovery time	Shape recovery (%)	Shape recovery time	Shape recovery (%)	Shape recovery time	Shape recovery (%)
MW	80 ± 2 s	95.4 ± 0.2	70 ± 2 s	96.8 ± 0.1	60 ± 2 s	97.6 ± 0.2	45 ± 2 s	98.6 ± 0.1
Sunlight	7 ± 0.5 min	95.7 ± 0.1	2.5 ± 0.2 min	96.2 ± 0.1	2 ± 0.3 min	97.4 ± 0.2	1 ± 0.1 min	98.5 ± 0.1

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More unlocked oriented chains are generated owing to increase in crystallinity of the nanocomposite on incorporation of nanohybrid (as obtained from DSC and XRD results). These unlocked chains can produce an instantaneous retractive force upon elimination of the load because of the elastic entropy, and hence it improved the shape recovery.

Self-healing properties

Tensile tests and optical images were used to examine the self-healing behavior of the nanocomposite. Fracture of the nanocomposite is effectively healed by exposure of direct sunlight and MW, as shown in Fig. 7. The healing efficiency of nanocomposite under sunlight and MW are also shown in Fig. 7. The healing efficiency of the nanocomposite depends on loading of SRGO, power input of MW and exposure time (Fig. 8). In this context, it is pertinent to mention that pristine HPU does not heal upon exposure of different stimuli even after a long time. All the nanocomposites were effectively healed within 30–50 s under low MW power (360 W) and within 5–7.5 min under direct sunlight. During the healing process, SRGO absorbed energy from the stimulus, and then transferred this energy to the HPU matrix.

Fig. 7 (a) Digital and optical microscopic photographs of cracked and healed nanocomposite films; healing efficiency of the nanocomposites under (b) sunlight and (c) MW (360 W); and (d) repeatable healing efficiency of the nanocomposite under sunlight and MW.

Fig. 8 The healing efficiency of the nanocomposite at different MW power input of (a) 180 W, (b) 360 W and (c) 540 W; and (d) representative stress–strain curves of HPU/SRGO 2 before crack and after healing the crack.

The soft segment of the HPU is melted by gaining the energy as its T_m is around 50 °C. Thus, the crack could repair with a higher mobility of the soft segment of HPU. At the same time, the hard segment of HPU helps to retain its original shape. The shape memory properties of nanocomposite also played a vital role in the healing process.¹⁷ When energy is transferred to HPU chains by SRGO, it was activated, and then the internal stress was released. Therefore, the notched surface of the crack was first healed with the help of the recovery force from the bottom before complete repairing of the crack.¹⁸ Here, it is important to notice that HPU shows a SME but not self-healing ability, although both the processes depend on the rearrangement of the polymeric chains. The energy requirement to activate the polymeric chain for rearranging its orientation and gaining its original shape is less compared with diffusing the polymeric chains in the crack site. A huge amount of energy is needed to melt a particular segment of the polymer and allow subsequent diffusion. Pristine HPU has only one polar group, which is only capable to absorb some energy from the stimulus for rearranging them and return from a deformed state (temporary shape) to their original (permanent) shape. However, this amount of energy may not be sufficient for the polymeric chains' diffusion, and as a result pristine HPU only demonstrates SME. In contrast, the SRGO nanohybrid has good capability to absorb energy from MW and sunlight, and hence SRGO absorbs and transfers more amount of the energy to the HPU matrix of the nanocomposite. Therefore, the soft segment of HPU is melted in the nanocomposite and diffuses in the cracked places to heal it.

Repeated healing is a daunting challenge to the material scientist and important for the self-healing materials, which could greatly increase the service lifetime. In our work, self-healing was achieved by the rearrangement of soft segments of HPU with the assistance of the SME, thus the healing of the prepared nanocomposite could be repeated again and again. Thus, even after the fifth cycle of the experiment, the healing ability of the nanocomposite remains almost same under both sunlight and MW (Fig. 7).

Antimicrobial activity

SRGO exhibited the lowest MIC against both bacteria and fungus (Table 4). As sulfur nanoparticles and RGO showed a strong antimicrobial effect, the nanohybrid demonstrate a synergistic effect against the tested microbes. However, indirect contact of SRGO with the microbial strains suppressed the inhibitory action in case of the nanocomposite. Thus, a high dose of the nanocomposite was required to show the inhibitory effect. MIC values suggest that the nanocomposite can inhibit both gram positive and gram negative bacteria, though the effect is more pronounced for the former. Contrarily, HPU exhibited an inhibitory effect at a very high dose, which implies that SRGO is mainly responsible for conferring antimicrobial efficacy to the nanocomposites. Antifungal activity of the nanohybrid, as well as nanocomposite, was also found to be significantly effective. However, in each case, the reduction of growth was observed when incubated with HPU/SRGO2 films. Statistical measurements (Two way ANOVA) revealed that the MIC values were significantly different from each other, with LSD 0.63 and p < 0.05.

Table 4 MIC against bacteria and fungus

Microbe	HPU (μg mL^−1)	RGO (μg mL^−1)	Sulfur nanoparticles (μg mL^−1)	SRGO (μg mL^−1)	HPUSRGO (μg mL^−1)
S. aureus	180 ± 2.6	22.7 ± 1.57	18.3 ± 0.57	13.7 ± 1.50	33.7 ± 1.52
E. coli	245 ± 3.2	31.0 ± 1.00	21.7 ± 1.52	19.3 ± 0.57	43.0 ± 2.00
C. albicans	272 ± 3.6	47.0 ± 1.00	33.7 ± 1.50	29.0 ± 1.00	63.3 ± 1.52

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It is clearly visible from Fig. 9(a–c) that microbial growth exponentially increased with time in the controls. On the other hand, the presence of HPU/SRGO2 inhibited the growth rate of each microbe, considered under the test. Another, interesting conclusion derived from the assay is that the growth rate of the strains are not considerably hampered by the presence of HPU. This again validates the efficient antimicrobial activity of SRGO. Microorganisms adhered to the antimicrobial surfaces lost their morphological integrity.⁴² Cell membrane lysis is the vital factor for the decrement in growth rate. To ascertain this, E. coli was considered as a representative microbe for SEM analysis. From Fig. 9(d–f) it is observed that membrane disruption occurred in the bacterial cells that attached to the HPU/SRGO2 surface. Red marks indicate the irregularity in the cellular structures and agglomeration of the dead cells. However, bacteria attached to HPU surface did not lose their cellular structure. This confirmed the microbial growth-resisting potential of HPU/SRGO2. This antimicrobial property is very useful for the fabrication of various advanced materials that could prevent microbial contamination and infections.

Fig. 9 MIC of nanocomposite against (a) Candida ablicans, (b) Escherichia coli, (c) Staphylococcus aureus; and SEM image of E. coli cells adhered to (d) HPU, (e) HPU/RGO and (f) HPU/SRGO.

Another assay was performed to verify the possibility of microbial growth, in the proximity of HPU and HPU/SRGO2. Thus, images were taken and analyzed for the films, laid in Petri plates with each of the test microorganisms (Fig. 10). Images confirmed that none of the microbes could grow over the HPU/SRGO 2 surface within an area of 1.5 cm², whereas significant growth was witnessed over the surface of HPU. This indicated that the nanocomposite did not allow microbial fouling over its surface, even in close contact with the microbes. The overall study endorses the material as an advance antimicrobial nanocomposite with high mechanical attributes.

Fig. 10 Growth of (a) Staphylococcus aureus, (b) Escherichia coli and (c) Candida albicans in close proximity of HPU/SRGO.

Conclusions

In summary, we developed a tough HPU/SRGO nanocomposite that demonstrated excellent self-healing and shape recovery along with significant antimicrobial activity. The nanocomposite exhibited improved mechanical and thermal properties after the incorporation of a small amount of nanohybrid owing to good interaction between nanohybrid and HPU. The nanocomposite also showed excellent rapid and repeatable self-healing and shape recovery by MW and sunlight. The presence of SRGO nanohybrid in nanocomposite provided good antimicrobial activity against gram positive and gram negative bacteria, along with fungus. Thus, the multifunctional smart nanocomposite has great potential in the domain of advanced materials.

Acknowledgements

S.T. sincerely acknowledges the receipt of his Senior Research Fellowship from the Council of Scientific and Industrial Research (CSIR), India. The authors express their gratitude to SAP (UGC), India through grant no. F.3-30/2009(SAP-II) and FIST program-2009 (DST), India through the grant no. SR/FST/CSI-203/209/1 dated 06.05.2010.

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