Shape Memory Polymers Synthesised For Controllable Switching Temperatures

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ScienceDirect Materials Today: Proceedings 4 (2017) 4 (2017) 11148–11153

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AMMMT 2016

Shape Memory Polymers Synthesised For Controllable Switching Temperatures Ranganatha Swamy MK1, U S Mallikarjun2, V Udayakumar3 1

Asst.prof, Department of Mechanical Engineering, School of Engineering and Technology,Jain University,Bangalore,Karnataka, India-562112, e-mail: [email protected] 2 Professor, Department of Mechanical Engineering, Siddaganga Institute of Technology, Tumkur, Karnataka, India-572103, e-mail: [email protected] 3 Professor, Department of Chemistry, Siddaganga Institute of Technology, Tumkur, Karnataka, India-572103,

Abstract Shape memory polymers (SMPs) have been synthesized by various diisocyanates and polyols for specific transition temperatures. But these types of shape memory polymers don’t have a control in varying temperatures and fail to exhibit shape memory properties. Hence it is necessary to synthesize a shape memory polymer which suits for specific applications even if there is a variation in operating temperatures which have a control on switching temperatures. This paper mainly concentrates on synthesis of shape memory polymers for controllable switching temperatures. The shape memory properties of the shape memory polymers were studied by bend test. It is found that shape memory polymers exhibits 100% shape memory effect. The shape memory properties, thermal properties and morphological characteristics are investigated by Xray difractogram (XRD), Differential scanning calorimetry (DSC), Fourier transform infrared (FT-IR) and SEM. © 2017 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of Advanced Materials, Manufacturing, Management and Thermal Science (AMMMT 2016).

Keywords: Shape memory polymer; Shape recovery; Shape memory effect; Hard

segment; Soft segment.

* Corresponding author. Tel.:9945711885; 9448166621;9242461481

E-mail address: [email protected];[email protected];[email protected]

2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of Advanced Materials, Manufacturing, Management and Thermal Science (AMMMT 2016).

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1. Introduction Shape memory polymers (SMPs), are polymeric materials which have the ability of recovering from a ‘fixed’ temporary shape to a ‘memorized’ permanent shape in a controlled manner upon exposure to an external stimulus. Although heat remains the ‘intrinsic’ stimulus, shape memory effect can be triggered by using various stimuli such as light, electric current, magnetic field, and moisture [1]. SMPs exhibit superior properties of light weight, large shape recovery rate, low cost and easy processing as compared to shape memory alloys have promoted their rapid development and commercialization [2-4]. SMPs usually consist of two phases: “fixed phase” and “reversible phase” which can also be called as hard and soft segments, respectively [5,6]. The hard segments usually contain long sequences of hydrogen bonding sites and serve as the physical cross-links [7]. Cross-link points prevent the neighboring chains slipping from each other when it is subjected to deformation and consequential stress build-up [8]. The ‘switching’ or transformation temperature (Ttrans), enabling the material to return to its permanent shape, is either linked with the glass transition (Tg) or with the melting temperature (Tm) [9]. Thermally actuated SMA shape recovery is accompanied by a moderate change in elastic modulus (10 to 80 GPa), large recovery stresses (~ 1000 MPa), and low recovery strains (< 8 %), and high thermal hysteresis. In contrast, in thermally actuated SMPs deformation is stored elastically as macromolecular chain orientation; hence, there is an entropic potential for shape recovery. Upon actuation shape recovery is accompanied by a large change in elastic modulus (from 0.002 GPa to 2 GPa), low recovery stresses (~ 1 to 10 MPa), large recovery strains (>300%), and thermal hysteresis [10]. However, SMPs also have some drawbacks, such as low deformation stiffness and low recovery stress. To overcome these deficiencies, shape memory polymer have been synthesized by addition of various filler elements like graphite, grapheme, carbon nanotubes and carbon black etc, have been developed in practical applications to satisfy demand. The results of studies on addition of fillers indicate that SMPs have higher strength, higher stiffness and certain special characteristics determined by what fillers are added, which can offer further advantages over SMPs. Potential applications for SMP exist in almost every area of daily life: from self repairing auto bodies to kitchen utensils, from switches to sensor, from intelligent packing to tools. Some of the specific applications are SMP hinges, SMP booms, SMP reflector antenna, morphing structures [11]. In this paper research work is carried out by varying the composition of diisocyanate concentration and the shape memory properties were evaluated. 2. Materials and Methods 2.1 Materials The materials used in this work were procured from sigma-aldrich. The polyols used here is Polyethylene glycol (Mw~10,000), poly(ethylene terephthalate) (Mw~10,000) which is also called as soft segment, Diisocyanates used are 4,4′-diphenylmethane diisocyanate (MDI), Isophorone diisocyanate (IPDI) which are also called as hard segments, were refrigerated before use, dimethylformamide (DMF) as a solvent which was stored under molecular sieves, Polyethylene glycol (Mw~200), as a crosslinking agent, 1, 4-butanediol (BDO) as a chain extender, dibutyltin dilaurate as a catalyst. Remaining chemical were used as received. 2.2 Method of preparation A 500 ml round-bottom, three-necked separable flask equipped with a mechanical stirrer, nitrogen inlet, thermometer. SMP was synthesized by solution polymerization under dry nitrojen by a prepolymer method. Initially PEG (Mw~10,000) and poly(ethylene terephthalate) (Mw~10,000) was reacted by stirring with IPDI in the presence of catalyst for 2h at 90ºC, to prepare a prepolymer with terminal C=O group. This step is followed by addition of MDI and PEG (~200) as a crosslinking agent at 90ºC and reacted for 1hr to get a polymer with NCO group. Then prepolymer was subsequently chain extended with adding BDO for 1 h at 60ºC. The SMP films were cast by pouring the polymer resin on preheated glass mould coated with a releasing agent. Then the films were baked at 60°C for 12 h, 80°C for 24h and 100°C for 8h, respectively, in a oven. The chemical composition of Soft segment, Hard segment is listed in table 1.

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Table 1: showing chemical compositions of SMPs prepared.(SS-Soft segment,HS-Hard segment) Sample ID

SS (wt%)

HS (wt%)

SMP-1

45

55

SMP-2

50

50

SMP-3

55

45

SMP-4

58

42

3. Results and Discussions 3.1 Evaluation of Shape Memory properties

Fig.1. Schematic representation of shape memory effect Fig.1 shows the detailed view of shape memory effect, fig.1(a) shows the actual sample which has been synthesized in the aforementioned method, it is showing the specimen before deformation that is application of load, fig.1(b) shows that specimen is deformed into coil or spiral shape and the deformation is fixed by external load and left for few minutes.fig.1(c) shows the specimen after recovery that is after application of external stimulus that is heat. A straight strip (size: 70×3×1mm) of the specimen was kept in an oven for few min at Tg +20°C; then the sample was bent to a storage angle θo in a ‘U’ shape with the radius of 20 mm in the soft rubbery state, after that the sample was placed in cold water with the external con-straint to freeze the elastic deformation energy for few min. The sample after deforming elastically placed in an oven at elevated temperature, and it recovered to an angle θN. The value of the shape recovery ratio (RN) can be calculated by Equation (1). RN =

Equation…

(1)

Experiment was conducted for all the prepared samples and the values were tabulated in the table.2 Table 2: Experimental data for measurement of shape memory effect by bend test. Sample

D (mm)

T (mm)

θo

θN

RN (SME%)

SMP-1

20

1.0

1.0

97

50

SMP-2

20

1.0

1.0

120

40

SMP-3

20

1.0

1.0

154

20

SMP-4

20

1.0

1.0

100

170

As per the values stated in the above table the composition of soft segment 58% and hard segment 42% exhibits the 100% shape memory effect.

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3.2 PXRD analysis PXRD was used to determine crystalline structures of the polymers. D8 Discover X-ray diffractometer equipped with Cu Kα radiation of a wavelength of 1.542 Å was used to investigate the crystalline structures of the SMPUs, with a scanning angle 2θ between 5 to 40° At 35 KeV and 35 mA. The major peaks in the fig.2, 20.05º,24.27º,37.00º explains there is more crystallization which has been taken place. The crystallographic planes are found to be 010,011 and 110 respectively. The results indicate that the SMP-3 is having more strength and rigidity. The curves also states that there is presence of both the crystalline and amorphous phases which are most required for fast recovery process. P3

2000 1900 1800

2th=24.271 °,d=3.66418

1700 1600 1500 1400

1200

2th=20.053 °,d=4.42447

Lin (Counts)

1300

1100 1000 900 800 700

2th=37.004 °,d=2.42741

600 500 400 300 200 100 0 3

10

20

30

40

50

60

70

80

2-Theta - Scale File: SAIFXR160430C-03 (P3).raw - Step: 0.020 ° - Step time: 27.5 s - WL1: 1.5406 - kA2 Ratio: 0.5 - Generator kV: 35 kV - Generator mA: 35 mA - Type: 2Th/Th locked 1) Obs. Max: 24.272 ° - FWHM: 0.801 ° - Raw Area: 37.50 Cps x deg. 2) Obs. Max: 20.057 ° - FWHM: 0.431 ° - Raw Area: 12.50 Cps x deg. Operations: Smooth 0.284 | Background 4.571,1.000 | Import

Fig.2. PXRD curve of sample SMP-3. 3.3 FT-IR The FTIR spectra were recorded on a Bruker instrument FTIR Spectra shows absorption bands that enable to determine if certain functional groups are present in a molecule. The FT-IR spectra of the SMPs presented in Fig.3 shows the growth in the transmittance peaks of the crosslinked SMPs at 1961.43-2236.90cm−1, which are responsible for the (C=0) carbonyl groups and the N–H (Hydroxyl) bonds, respectively. Here the intensity of peak is medium and broad depicting less strength in the bond. Here the intensity of peak is strong and sharp depicting more strength in the bond.

Fig.3. FT-IR spectra of sample SMP-4.

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3.4 Thermal analysis DSC was performed in Perkin Elmer DSC-7 with a heating rate of 10 °C/min and a temperature range of 20°C to 250°C. Samples of about 10.5~15.0 mg were put into the platinum sample holder, and the tests were conducted under a nitrogen gas flow. The thermal properties including transition temperature and melting temperature was measured. As shown in fig.4 the sample SMP-4 has sharp transition temperature that is at 47ºC and melting temperature at 100ºC. The decrease in transition and melting temperatures shows that as we increase the content of graphite it is getting more rigidity and the strength has been increased.

Fig.4. DSC curves of sample SMP-P4 Table 3: Experimental data of glass transition temperature, melting temperature and heat of fusion. Sample

Tg (ºC)

Tm (ºC)

ΔHf (Jg-1)

SMP-1

39

65

52

SMP-2 SMP-3 SMP-4

44 49 57

69 75 84

56 64 72

Table 3. shows the values of glass transition temperature and melting temperature with respect to heat flow.Here we can see as the glass transition temperature increases the melting temperature range also increases. This shows that there is a great change in crosslinking phenomenon and also the switching temperatures are also changing with respect to changing environment. 3.5 Morphological study 3.5.1 SEM Fig.5 shows SEM images of the SMP filled with graphite. Morphologically it was absorved that crystalline phases are more in percentage compared to amorphous phase. SEM images were taken in the magnification resolution of X1500 and X2,000. It was observed that the region corresponding to fast fractures is almost mirror-like in appearance (it has a smooth, shiny surface). SMP-4(a) depicts the presence of less crystalline phase compared to smp-4 (b) hence the specimen is having more strength and rigidity.

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Fig.5. SEM images of the sample SMP-4(a) and SMP- 4 (b) 4. Conclusions From the present work it can be concluded that shape memory polymers exhibit a controllable switching temperatures with the addition of poly(ethylene terephthalate). Shape memory properties is found to be 100% and PXRD analysis determines the crystalline structures and ensures that the crystallinity got enhanced with the increasing MDI content. The transition temperature found to be 47ºC and melting temperature found to be 100ºC. FT-IR depicts the presence of both carbonyl and hydroxyl bonds which are most required for having the shape memory effect. SEM images shows the morphologies of SMP in which it shows the presence of crystallinity. References [1] Xiaofan Luo* and Patrick T Mather “Design strategies for shape memory polymers” Science direct, Current Opinion in Chemical Engineering 2013, 2:103-111, 2211-3398/$-2012 Elsevier Ltd. http://dx.doi.org/10.1016/j.coche.2012.10.006. [2] Tobushi, H.; Haray, H.; Yamaday, E.; Hayashiz, S. Thermomechanical properties in a thin film of shape memory polymer of polyurethane series. Smart Mater. Str. 1996, 5, 483-491. [3] Liang, C.; Rogers, C.A.; Malafeew, E. Preliminary investigation of shape memory polymers and their hybrid composites. Smart Struct. Mater. 1991, 24, 97-103. [4] Hayashi, S.; Ishikawa, N.; Jiordano, C. High moisture per-meability polyurethane for textile application. J. Coated Fabrics 1993, 23, 74-83. [5] Lee, H.S.; Wang, Y.K.; Macknight, W.J.; Hsu, S.L. Spectroscopic analysis of phase-separation kinetics in model polyurethanes. Macromolecules 1988, 21, 270-273. [6] Xia, H.S.; Song, M.; Zhang, Z.Y.; Richardson, M. Microphase separation, stress relaxation, and creep behavior of polyurethane nanocomposites. J. Appl. Polym. Sci. 2007, 103, 2992-3002. [7] Coleman, M.M.; Lee, K.H.; Skrovanek, D.J.; Painter, P.C. Hydrogen-bonding in polymers. 4. Infrared temperature studies of a simple polyurethane. Macromolecules 1986, 19, 2149-2157. [8] Seymour, R.W.; Allegrezza, A.E.; Cooper, S.L. Segmental orientation studies of block polymers.i. hydrogen-bonded polyurethanes. Macromolecules 1973, 6, 896-908. [9] Hu J., Zhu Y., Huang H., Lu J.: Recent advances in shape–memory polymers: Structure, mechanism, functionality, modeling and applications. Progress in Polymer Science, 37, 1720-1763 (2012). DOI: 10.1016/j.progpolymsci.2012.06.001. [10] Kornbluh, R., Pelrine, R., Shastri, S. V., Full, R. J., Meijer, K., SRI Report Number 433-PA-00-013, Menlo Park, CA. [11] Yanju Liu1, Haiyang Du1, Liwu Liu1 and Jinsong Leng2 “Shape memory polymers and their composites in aerospace applications: a review” Smart Materials and Structures,IOP publishing, Smart Mater. Struct. 23 (2014) 023001 (22pp) doi:10.1088/09641726/ 23/2/023001. [12] Lendlein, A.; Langer, R. Biodegradable, Elastic Shape-Memory Polymers for Potential Biomedical Applications. Science. 2002, 296, 16731676. [13] Lendlein, A.; Kelch, S. Shape-Memory Polymers. Angew. Chem. Int. Ed. 2002, 41, 2034-2057. [14] Lendlein, A.; Schmidt, A. M.; Langer, R. AB-Polymer Networks Based on Oligo(ɛ-caprolactone) Segments Shwoing Shape-Memory Properties. Proc. Natl. Acad. Sci. 2001, 98, 842-847. [15] U.S. Mallik, V. Sampath, “Effect of composition and ageing on damping characteristics of Cu-Al-Mn shape memory alloys” Materials Science and Engineering A 478 (2008) 48-55. [16] V. Sampath and U.S. Mallik, “Influence of minor additions of boron and zirconium on shape memory properties and grain refinement of a Cu-Al-Mn shape memory alloy” ESOMAT 2009, 05028 (2009) DOI:10.1051/esomat/200905028.