graphene nanoplatelets composites

Accepted Manuscript Title: Improved shape memory and mechanical properties of microwave-induced thermoplastic polyurethane/Graphene nanoplatelets composites Authors: Krishan Kumar Patel, Rajesh Purohit PII: DOI: Reference:

S0924-4247(18)31608-X https://doi.org/10.1016/j.sna.2018.10.049 SNA 11095

To appear in:

Sensors and Actuators A

Received date: Revised date: Accepted date:

23 September 2018 30 October 2018 31 October 2018

Please cite this article as: Kumar Patel K, Purohit R, Improved shape memory and mechanical properties of microwave-induced thermoplastic polyurethane/Graphene nanoplatelets composites, Sensors and amp; Actuators: A. Physical (2018), https://doi.org/10.1016/j.sna.2018.10.049 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Improved shape memory and mechanical properties of microwave-induced thermoplastic polyurethane/Graphene nanoplatelets composites. Krishan Kumar Patel1*; Rajesh Purohit1 1

Maulana Azad National Institute of Technology Bhopal, 462003 India.

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Corresponding author E-mail: [email protected]

Highlights

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SMP/Graphene nanoplatelets composites containing 1 to 2 phr successfully triggered by microwave irradiation. With the addition of GNPs in polyurethane shape memory and mechanical properties were improved. Microwave induced SMP/GNPs composites have potential applications and a better choice for remote sensing and wireless applications.

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Abstract: In this research paper, the effect of microwave (MV) irradiations on shape memory and mechanical properties of SMP/GNPs composites were prepared through ex-situ polymerization. Shape memory thermoplastic polyurethane (PU) composites containing different amount of graphene nanoplatelets (GNPs) ranging from 0 to 2 phr were prepared in micro-compounder by melt mixing route. FE-SEM, stress-strain, recovery stress, DMA, DSC, shape memory test was studied. With the addition of GNPs the recovery stress, tensile stress, yield stress, constrain strain recovery, storage modulus, thermal diffusivity, shape recovery were improved. Recovery strength, constrain strain recovery and tensile strength for 2 GPU (GPU= concentration of GNPs in PU matrix) increased by 150%, 50%, 20% as compared to PU respectively. graphene nanoplatelet is a strong absorption of microwave irradiation so that SMP/GNPs composites containing 1 GPU, 1.5 GPU, and 2 GPU were successfully triggered by MV irradiation. Further shape recovery also depends on the MV frequency and concentration of GNPs. The pure specimen has no shape recovery effect in MV irradiations. So that MV-induced SMP/GNPs composites have provided the potential better choice for fast actuating remote sensing and wireless applications.

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Keywords: Shape memory polymer; Microwave; GNPs; Polyurethane; Nanocomposites Introduction: As a day by day exploring advanced research in science and technology the new smart materials [1], functionally graded [2], self-healing [1, 2], lightweight nanocomposites materials [3] takes place. In this advancement, the shape memory polymer nanocomposites play an important role over the existing composites materials. Shape memory polymers (SMP) are the class of smart self-actuating materials which are triggered by various external stimuli such as temperature, water, magnetic, electric, solution, ph, electromagnetic, microwave etc[3, 4]. Shape memory polymer can change temporary shape at 1

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the certain condition to permanent shape with the help of external stimuli [4]. SMPs has drawn a great attention of researchers because of their lightweight, high corrosion resistant, high water resistance, high recoverable strain (upto 800%)[4-5], bio-compatible, ease of fabrications, high electrical insulations and wide range of glass transition temperature etc [6]. the mechanism behind the SMP consists of two-phase, one is the hard phase and another is soft phase. Soft phase is an elastic phase which is responsible for switching temporary shape. And hard phase is fixing phase which is responsible for regaining original shape. The shape is changing temporarily to permanent below and above the glass transition temperature (Tg). So that glass transition temperature plays an important role in shape memory polymers [7]. Below the glass transition temperature polymer behave like a glass so it’s called as glassy state and above glass transition temperature it behaves like a rubber so it’s called rubbery state [5-6]. SMPs generally used for sensors and actuators devices for various sophisticated applications but SMPs Frequently used in aerospace, industrial, medical, automobiles, various research and developments etc. in general, shape memory polymers are triggered by the conventional method by means of heat[7], water[8], electric[9], magnetic[10], ph[5, 6], solution [6], electromagnetic[10] etc. but it’s usually impossible to trig the SMPs by conventional method during operating time especially for space and remote sensing devices[11]. So that for sophisticated wireless applications, nonconventional method comes into the picture [11-12]. Nanomaterials are most pronounced filler for SMP due to their exceptional thermal, mechanical and electrical properties. Many studies reported improvement in shape memory properties on the addition of nanofillers which includes reinforcement of SiC [10], nanoclay[13], carbon black [14-15], CNT[16], cellulose nanowhiskers, graphene [17] etc. Among these fillers, carbon-based filler is popular which include CNTs, carbon black, carbon nanopaper and graphene. In which CNTs and carbon black are extensively studied to improve mechanical and electrical properties. Research shows that the filler geometry, method of mixing and concentration are akey factor governing composite properties [14]. SMP/Graphene nanocomposites drew the researcher’s attention due to its unique mechanical and electrical properties [18-20]. Graphene is a one-atom-thick two-dimensional sheet of graphite having sp2 hybridized carbons is covalently bonded in a hexagonal manner. Its high stiffness, high aspect ratio, superior electric and thermal conductivity makes graphene a suitable candidate as reinforcement in the polymer matrix. Graphene nanoplatelets (GNPs) are recently developed a short bulk form of graphene [2125]. Many researchers [26, 27] investigated triple way photo-responsive shape memory polymer/azobenzene and graphene oxide (GO) nanocomposites film were prepared. The azobenzene and GO were acted as a heat source for heating the polymer film, which is responsible for shape recovery. Mechanical and shape memory properties were improved in UV and NIR light-responsive nanocomposites. Optical responsive shape memory polymer chain for write and erase fingerprint pattern was reported [28]. Photo-responsive shape memory polymer composites exhibit low cost, ease of available and fast shape recovery for sensors and actuators. Photo-responsive SMP graphene oxide composites were prepared by solvent casting route for photo-mechanical applications [29, 30]. In photo-mechanical polymer/GO nanocomposites graphene oxide is responsible for heat transfer in the polymer matrix. Graphene is superior thermal, mechanical and electrical properties which are used in sensors and actuators devices [31]. 2

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GNP has intermediate geometry with 6 nm thickness corresponding to 18 to 50 graphitic sheets. It has a lateral size of 25 µm and a specific surface area of 50–150 m2/g. The key factors for transferring the GNP properties to composites are: (i) homogeneous dispersion and distribution of graphene in a matrix; and (ii) type of interaction between GNPs and the polymer matrix. The electro-thermo-mechanical properties of polymer nanocomposites depend on the quality of the dispersion of nanofillers that lead to property variation over the composite structure. Researcher [11, 13, 17] also reported a few years ago MV-induced SMP/CNT nanocomposites for wireless applications which are fast actuating as compared to conventional stimuli. In this way, CNT nano particles act as a heating node of the specimen in MV conditions, where as a pure polymer (styrene-based shape memory resin) have no shape memory effect due to MV. Other researchers also investigated those MV-induced SMP/SiC nanocomposites (poly vinyl alcohol/poly acrylic acid) for remote sensing applications. In this review, SiC particles act as a heating source for the incidence microwaves which is responsible for absorbing MV and convert into heat [10-13].

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In the present study, nonconventional microwave (MV) induced SMP/graphene nanocomposites are introducing for wireless remote sensing applications. Experimental studies carried out different fractions of GNPs were dispersed uniformly in shape memory polyurethane using micro-compounder. Shape recovery stress and strain, thermal conductivity, viscoelastic and microwave parameters were determined and their influence on shape memory effects was studied and reported here. Because the GNPs are 2D high conductive material and it may act as an excellent heat node under microwave induced conditions. In MV-induced SMPs electromagnetic waves released with high penetration power and frequency. MV heating has a great advantage over the conventional heating because of non contacting heating, rapid heating, volumetrically heating, low cost, timesaving, easy heat controlled etc. because microwave heating is an efficient clean green source of energy for actuating such type of MV responsive SMP/GNPs composites have a great potential for various sophisticated remote sensing, nanotechnology, wireless applications etc [10-12]. 2. Experimental Details:

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2.1. Materials: The shape memory thermoplastic polyurethane (PU) granules (ether type) MM6520, in the form of pellets were obtained from SMP Technologies Inc. Japan. Graphene nanoplatelets (GNPs) size11-15 nm having specific surface area 50-80 m2/g is obtained in the form of powder from Lo-Li.Tec nanomaterials GmbH Germany.

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2.2 Sample Preparation: Shape memory polyurethane granules and GNPs were first dried in a vacuum oven at 105°C for 6 hr. A Thermo HAAKE MiniLab 3 micro-compounder with conical twin screw (Model PolyLab OS) was used for melt mixing of SMPU and GNPs at a rotor speed of 60 rpm and mixing temperature of 210°C for ten minutes. The SMP/graphene nanocomposites with different GNPs concentrations (0, 1 phr, 1.5 phr, and 2 phr), namely PU, 1GPU, 1.5 GPU and 2 GPU respectively, were prepared through melt mixing route. Different compositions of PU with 0, 1, 1.5 and 2 phr GNPs were prepared in batches of 6g each in micro-compounder. To obtain uniform composition each 6g batches of the mix was 3

extruded in the form of wire then chopped and chopped was reprocessed in micro compounder. After that melt mixed was taken in the cylinder which is already heated at 2100C and processed for Mini Injection Moulding, Thermo Scientific Germany (HAAKE Mini- Jet Pro Piston). This melt is directly injection moulded in desired shape and size ISI standard SS die at temperature 2100C, injection pressure 620bar, injection time 10 sec, post pressure 600bar for 20 sec and mould temperature 800C. Then after 5 minutes sample remove from SS die for testing and characterizations.

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2.3 Field Emission Scanning Electron Microscopy (FESEM): Morphological studies were conducted using FESEM of M/s Nova NanoSEM 430. The liquid nitrogen cryogenic fractured surfaces of SMP composites were observed after gold coating layer about 100 A. 2.4 Shape Memory Test: Shape memory (stretches and constrains recovery) test was carried out using the machine (Tinius Olsen25kt) at CSIR-AMPRI. In shape memory test sample size 50×10×1mm3 was clamped in spring-loaded grip and heated at 65°C and then force is applied at a rate of 2 mm/minute upto 50% strain. Thereafter the without releasing force the sample cool done below 30°C then releasing the load and measured the final gauge length of the sample for shape fixity. And then again clamp the stretched sample in grip and heated upto 65°C. At 65°C sample start to recover its original shape and recovery forces were continuously recorded in respect of strain rate. During shape recovery, the strain was decrease and samples try to recover its original shape. Detailed stretching stress and recovery stress were discussed in the results and discussion section.

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2.5 Tensile test: Tensile test was performed on a tensile testing machine (Tinius Olsen25kt) with wedge clamps. The tests were conducted at room temperature and a cross-head speed of 10 mm/min. the size of the samples were slandered ISI tensile SS die prepared in micro compounder through melt mixing route.

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2.6 Dynamical Mechanical Analysis: Dynamical Mechanical Analysis (DMA) is a technique for measuring thermo-mechanical properties of viscoelastic materials. It is known that shape memory properties are governed by modulus of material below and above the transition temperature. This experiment allows determination of the material’s response by the application of temperature and dynamic load. Thermo-mechanical properties of the samples have been determined using "Dynamic Mechanical Analyzer" DMs 6100 by Hitachi Instruments. The samples were injection moulded, with dimensions 40×10×1mm3. Test conditions were as follows: the measurement method was three points bending mode, loading frequency 1Hz, heating rate 20C/minute, for a temperature interval 300C to 850C. 2.7 Thermal Diffusivity: Thermal diffusivity was conducted using Thermal conductivity analyzer NETZSCH, Germany (LFA467 Hyper Flash) in nitrogen chamber temperature ranging from 25°C to 70°C. 2.8 Differential Scanning Calorimetry (DSC): DSC curves of SMP/GNPs composites were obtained using DSC 1 STARe System of M/s Mettler Toledo. The heating and cooling rates were fixed at 5°C/minute. The samples were tested at a temperature range from 10°C to 100°C in a nitrogen atmosphere. The DSC curves of as prepared test samples were obtained 4

for heating as well as for cooling cycles. The heating results were analyzed further in the results and discussion section.

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2.9 Microwave induced shape recovery test: Effect of MV was conducted in a household microwave oven with proper modifications (IFB Model: 30SC3) and infrared thermal imager (FLIR-E6 with MSX) both were supplied by Technical System Pvt. Ltd. The microwave power 120 W and frequencies were adjustable from 0.25 GHz to 2.45 GHz having distance between the permatron was 30cm. before carrying test the samples were deformed into temporary shape. 3. Results and Discussion:

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FESEM is used to observe the cryogenic fractured surface and morphology of SMP/GNPs composites. Fig. 1(a) showed that the lateral size of GNP was found to be 1-6 µm and present in the form of aggregates. In Fig 1 (b) fractured surface morphology of PU, which shows that smooth, layered and uniform orientation. Fig 1(c-e) the fracture surface roughness was increased with the additions of GNPs in the PU matrix.

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Fig. 1 Cryogenic fractured surface morphology FE-SEM image of (a) GNPs Cross section (b) PU (c) 1 GPU (d) 1.5 GPU and (e) 2 GPU SMP/GNPs Surface roughness was indicated that more uniform and random distribution of GNPs and also observed that strong interfacial interaction between GNPs and polymer matrix. Random and homogeneous distribution of nanoplatelets helps to superior properties. In Fig 1 (e) microcracks and agglomeration also observed in some places which are responsible for the inferior mechanical as well as shape memory properties.

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Shape memory test was carried out in the tensile testing machine (Tinius Olsen25kt) with spring-loaded clamps under the controlled hot chamber at CSIR-AMPRI. Details procedure was already discussed in the experimental section. Shape memory tests (stretch and constrain recovery) were shown in Fig 2. Recovery strength was improved with the addition of GNPs. Maximum recovery strength was observed near about 3MPa for 2 GPU which is 3 times more than the pure polyurethane (PU). During stretching time the internal energy stored by GNPs in the form of elastic strain energy which helps the improved recovery strength. With the addition of GNPs property such as Stretch and recovery strength, both were enhanced. A similar observation was also reported by various researches reinforcement with CNT/SiC in the PU matrix [16, 11]. The recovery strength of PU, 1 GPU, 1.5 GPU, and 2 GPU were observed as 1.2 MPa, 2.2 MPa, 2 MPa, and 3 MPa respectively. From Fig. 2 observed that the constrained strain recoveries of SMP/GNPs composites were also improved with the addition of GNPs. The maximum strain recovery under constrains loading was 60 % for 1.5 GPU which is higher as compared to PU only 45 % strain recovery. The recovery strength and recovery strain were not in increasing trend as GNPs addition it may due to agglomeration of GNPs within the matrix which was observed in SEM image. Fig. 3 shows that the constrained and free shape/strain recovery of SMP/GNPs composites. With the addition of nanoplatelets as reinforcement, the strain recovery was enhanced. Free recovery was much higher than the constrained recovery.

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The tensile stress-strain test was conducted in the tensile testing machine (Tinius Olsen25kt) at room temperature for ISI standard tensile specimens of SMP/GNPs composites. Test specimen’s results shown in Fig 4. The maximum tensile strength was enhanced with the addition of GNPs in the matrix. The increased strength may be strong interfacial bonds between the polymer and GNPs. The maximum tensile strength of 2 GPU was 58 MPa which is 20 % higher as compared to PU only 46 MPa. From stress-strain Fig. 4 also observed that the yield point shift toward higher values as addition of more GNPs. The yield strength of PU, 1 GPU, 1.5 GPU, and 2 GPU was 40MPa, 45MPa, 50MPa, and 54MPa respectively. Increased elastic zone help to promote the large strain energy stored by GNPs which definitely promote the higher recovery strength. 7

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Storage modulus with respect to the temperature of different compositions of SMP/GNPs composites was shown in Fig 5. The storage modulus plays an important role in viscoelastic materials. As the storage modulus increases the mechanical properties such as elastic modulus and strength were increased . With the addition of GNPs, the storage modulus was increased remarkably at 25°C as compared to pure polyurethane. The maximum storage modulus for 2 GPU is 3200MPa which is higher than PU 2600 only MPa . With the addition of higher GNPs the yield stress, stored energy and shape recovery stress were increased. In another hand Fig. 5 also clearly shows that storage modulus for composites was drastically decreased about 48°C as compared to the pure polyurethane which may help the narrow glass transition zone. Many researches were also reported that with the addition of nanoparticles such as CNT, SiC the storage modulus were increased [16, 11].

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Fig. 7 Energy dissipation factor curve SMP/GNPs as a function of temperature

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The loss modulus variation with temperature was plotted in Fig 6. It measure the stored energy transfer in the form of heat. Fig. 6 clearly shows that with the addition of GNPs loss modulus peaks increases which was indicated for fast heating. The embedded GNP acts as heat conducting source so that as much as a higher percentage of GNP added fast shape recovery may happen. The maximum peak values were decreased which govern the more loss modulus associated with increasing as higher loading of GNPs shown in Fig 7. The positive peaks shift of the SMP/GNPs indicates proper interfacial bonding and physical interactions between the polyurethane and GNPs.

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increases because the specific heat capacity (Cp) increased. Thermal diffusivity is directly proportional to the thermal conductivity of materials which is shown by equation (1) below. α = K / (ρ Cp).............Eq. (1) Where α = thermal diffusivity, K= thermal conductivity, ρ = density, and Cp specific heat capacity

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Thermal diffusivity curve clearly shows that thermal conductivity was increased with the addition of more nanoplatelets within the PU matrix. Increasing the thermal conductivity reduced the shape recovery time of SMP/GNPs nanocomposites. The thermal conductivity of 2 GPU composites was 20 % higher than the PU specimen. Similar observations were also reported by the various researches [12, 15], with the addition of SiC and CNT nanoparticles in the polymer matrix the thermal conductivity was increased [11, 16]. In another hand, the increased thermal conductivity of composites helps to promote fast specimen heat and response under the microwave induced shape memory as well as conventional thermoresponsive shape memory polymer.

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Fig. 9 DSC thermal cooling curve of SMP/GNPs

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State (rubbery to glassy state vies versa) transition temperature analysis was calculated from the Differential scanning calorimetric (DSC) curve by using the half height. In DSC cooling curves transition start from rubbery state to glassy state shown in Fig 9. Whenever in the DSC heating curve, the transition starts from glassy state to rubbery state which is not shown in this paper. In a thermal cooling curve, the transition starts from a higher temperature and end with lower temperature. Fig. 9 clearly shows that with increasing higher percentage of GNPs in the PU matrix transition temperature was shifted towards lower temperature which helps the fact recovery of shape memory. Decrease Tg helps shape recovery at low temperature. When deformed specimen heated at that glass transition temperature (Tg) or above Tg the specimen recover its original shape. Because when specimen heated above Tg it’s become glassy state to rubbery state. In DSC cooling curve clearly shown that above Tg it’s become a rubbery state which is responsible for shape recovery of SMP. Low transition temperature was also supported in MV-induced fast shape recovery of composites specimen. For 2 GPU the transition temperature was observed as very close to PU it may happenbecause of agglomeration of GNPs in the PU matrix. Detailed transitions temperature and Tg for cooling curve were shown in Table 1. The GNPs in the PU matrix were acted as a heating source for SMP composites which may help to promote the fast heating of the specimen. The almost same observation was also reported in various research papers, glass transition temperature was suppressed with the addition of SiC/CNT nanoparticles within the polymer matrix [11, 16].

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Fig. 10 MW – induced shape recovery test of SMP/GNPs (2 GPU at 2.45 GHz), Group A infrared thermal imager and group B digital image.

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Infrared thermal image analyzer was used to study the thermal response of specimen under microwave during the shape recovery. In this process Fig. 10 (group A, IR thermal camera and group B digital image by IR imager) shows that shape recovery of 2 GPU SMP/GNPs nanocomposite under 2.45 GHz. Firstly the sample was deformed in a straight strip (temporary shape) and shape recovery test was started under the microwave oven. Whenever time started the sample got heated due to graphene nanoplatelets act as a heat node which is converting microwave energy into heat. Within 30 seconds the sample recovers its original “S” shape almost 90% shape recovery which is as fast as conventional heating. Whenever 2 GPU SMP composite with conventional heating was taken 90 min to recover it’s 90% original shape recovery. Details comparative analysis of MV-induced and conventional as well as MV+ convection shows in Fig 11. As compared to the conventional heating microwave heating exhibits the very fast recovery because specimen heated volumetrically. Fig. 11 clearly shows that when SMP/GNPs composite 2 GPU tested in a microwave oven with 50%MV+convection mode the recovery results give the superior then convection heating.

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Fig. 12 shows Comparative analysis of SMP/GNPs composites with different frequencies of MV. In this analysis, we found that with the increase of both GNPs concentration and incidence frequency shape recovery was increased. Further analysis shows that the pure polyurethane (PU) specimen has not exhibited any shape recovery effect, which concluded that the PU has not heated under microwave because there are no GNPs participate for heating the specimen. On another hand the materials which absorbed MV irradiation are called as dielectric materials. In dielectric materials, storage modulus and loss modulus plays the important role to convert the MV irradiation into heat [10, 11]. Loss modulus is directly proportional to the absorbing heat in dielectric materials for microwave induced shape memory polymer composites. The shape recovery effect of microwave irradiation on pure polymers (styrene-based resin and poly vinyl alcohol/ poly acrylic acid) have almost zero previous reported by [10]. 100

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Conclusions: Shape recovery of SMP/GNPs composites enhances under the microwave irradiation as compared to conventional heating. In MV irradiation 2 GPU specimens fully recover its original shape within 30 seconds but the PU specimen has no shape recovery effect under MV. With increasing the frequency of MV and GNPs concentration the shape recovery was improved.

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Tensile strength and yield strength both were increased with increase GNPs concentration. The maximum strength and yield strength were increased by 20 % and 35 % for 2 GPU as compared to PU respectively. Stretch and Recovery strength both were improved by the addition of GNPs, recovery strength for 2 GPU increased by 150% as compared to PU. And constrained strain recovery for 1.5 GPU was increased by 50 % as compared to PU. Further free strain recovery enhanced by the addition of GNPs.

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With the addition of GNPs, thermal diffusivity increased which is responsible for fast heating and glass transition temperature was decreased. Further, the glass transition zone becomes narrow as adding a more GNPs concentration. The elastic modulus of SMP/GNPs was increased with the addition of GNPs. Maximum storage modulus for 2 GPU is 3200MPa and for PU only 2600 MPa at 25°C.Loss modulus increases with increase of GNPs concentration which indicates the more stored energy converted into heat which is essential for thermoresponsive shape memory polymer. Tan d peak shift towards the higher temperature which indicates the proper interfacial bonding and physical interaction between the Polyurethane and GNPs.

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Acknowledgement: Author Krishan Kumar Patel would like to thank the National Institute of Technology Bhopal India, for granting R&D fund for this work. And I would like thank to CSIR-AMPRI Bhopal India, for providing characterization facility. Authors also declare that there is no conflict of interest.

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Biography

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Krishan Kumar Patel: Ph.D. research scholar, Mechanical Engineering Department, MANIT Bhopal MP India 462003.

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Rajesh Purohit: Associate professor, Mechanical Engineering Department, MANIT Bhopal MP India 462003.

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