Thermo-responsive shape memory polymer blends based on alpha olefin and ethylene propylene diene rubber

Polymer 78 (2015) 180e192

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Thermo-responsive shape memory polymer blends based on alpha olefin and ethylene propylene diene rubber Tuhin Chatterjee, Pranab Dey, Golok Behari Nando, Kinsuk Naskar* Rubber Technology Centre, Indian Institute of Technology, Kharagpur, 721302, West Bengal, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 May 2015 Received in revised form 30 September 2015 Accepted 1 October 2015 Available online 9 October 2015

Thermally stimulated novel shape memory polymer blend of ethylene octene copolymer (EOC) and ethylene propylene diene terpolymer (EPDM) has been developed. This novel polyolefinic blends possess superior shape memory behaviour in presence of heat. Shape memory behaviour study at 60
C of the blends shows that the blends containing higher fraction of EPDM exhibits superior shape memory behaviour in terms of shape fixity (f) and shape recovery ratio (RR). Structural similarities of the pristine components lead to higher degree of compatibility which also results better physicomechanical behaviour. Lower modulus value followed by low relaxation ratio of the blend containing higher proportion of EPDM also supports the superior shape memory behaviour of the EPDM rich blend. Thus EPDM rich blend shows better shape memory effect among the various blends. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Shape memory polymers Shape fixity Shape recovery ratio

1. Introduction In the field of modern scientific and industrial research smart materials (SMs) are gaining much more scientific and technological importance due to their ability to sense and respond to external stimuli (such as temperature, chemicals, light, pH, electric or magnetic field) that enables these materials to change their properties such as shape, colour, electrical conductivity etc. [1,2]. Shape memory polymers (SMPs), a promising class of smart materials have achieved significant importance due to their proficiency of executing unique functions in response to changes in stimuli [3e5]. On demand, SMPs can change their shape in a predefined way from a less-constrained shape/configuration to a strained temporary shape/configuration and then again reverts back to the memorized shape/configuration upon triggering by an external stimulus [6]. Unlike gradual and linear response of regular polymers to external stimulus, SMPs exhibit a significant change even in presence of small magnitude of external stimulus and that response of SMPs to external stimuli is very rapid and nonlinear in nature [7]. Depending upon the response of SMPs to external stimulus, SMPs can be of various categories, namely a) thermo-responsive (temperature dependent) [8], b) chemo-responsive (chemicals, including water, ethanol and pH dependent) [9], c) photo-

* Corresponding author. E-mail address: [email protected] (K. Naskar). http://dx.doi.org/10.1016/j.polymer.2015.10.007 0032-3861/© 2015 Elsevier Ltd. All rights reserved.

responsive (light dependent) [10], d) electro-responsive (electricity dependent) [11,12] e) magneto-responsive (magnetic field dependent) [13] etc. Among the afore mentioned different groups thermal-responsive SMPs are the most focused and concerned system because thermally actuated SMPs have found broad applications in actuators, coatings, cable applications, sporting goods and also in biomedical devices [14e16]. It is already reported that polymers like polyester [17,18], poly (ether-ether-ketone) [19], polynorbornene [20], cross-linked polyethylene [3,21] and polyurethane [22,23] exhibit shape memory phenomenon. Apart from polymer blending offers a much simpler way to fabricate SMPs in order to improve the properties or to achieve newer functions (such as improvement of mechanical properties, improvement in thermal conductivity to reduce the shape recovery induction time) of SMPs shape memory polymer blends is a matter of concern [24,25]. Existence of the separate phases which are related to the coiled structure, presence of crosslinks (covalent bonds), hydrogen or ionic bonds or physical intermolecular interactions between the polymers are the key factors for the shape memory effects (SMEs) of SMPs [26]. Formation of covalent crosslinks takes place during suitable crosslinking of the polymer whereas polymer morphology consist of segregated domains (such as hard and soft segments) generates the physical crosslinks [27]. In multiphase polymer blend systems, hard segments provide stiffness and reinforcement to the material whereas the soft segments are responsible for the thermoelastic behaviour of polymers and the shape memory behaviour is produced by the reversible phase transition of the soft segments

T. Chatterjee et al. / Polymer 78 (2015) 180e192

[26]. Although glass transition temperature (Tg) or melting temperature (Tm) of the soft segment may be the shape transition temperature (Ttrans) for this kind of blends but melting temperature are preferred because of the sharpness of the transition rather than the glass transition temperature. As a result, shape recovery temperature of the blends can be better determined [28]. Upon deformation at high temperature (T > Ttrans) the materials transform to temporary shape in which chain segments get altered and this temporary and dormant shape can be fixed by cooling below the transition temperature (T < Ttrans). Again trigerring to higher temperature (T > Ttrans) allows the material to revert back to its original shape [29]. Polymer blends based on polyethylene/polycyclooctene [5], PVDF/PMMA [30], PVDF/PVAc [30], PLA/PVAc [30], SBS/PCL [31], PCL/PLLA polyurethane [32] etc. have been reported to exhibit shape memory effects. In the present investigation a novel heterogeneous polyolefinic blend system based on ethylene octene copolymer (EOC) and ethylene propylene diene terpolymer (EPDM) rubber has been developed to study the shape memory behaviour. Both the pristine polymers contain higher percentage of ethylene. EOC acts as hard domain and therefore provides adequate stiffness and reinforcement. On the other hand, EPDM being softer is responsible for the thermoelastic behaviour of the blends. In order to study the shape memory behaviour of the EOC-EPDM blend, five different blends in uncrosslinked state having different blend ratios have been prepared and subsequently thermal and mechanical behaviour of various blend have been studied in details. Finally shape memory behaviour of EOC-EPDM blend is optimized and it has been found that shape memory character is highest at an optimum blend ratio. 2. Experimental 2.1. Materials Polyolefin elastomer, EOC (Engage 8440), with an ethylene content of 77 wt% and co-monomer content of 23 wt% was procured from Dow Chemical. EOC has the density of 0.897 gm cm3 and it has glass transition temperature (Tg) of (33
C) and melting (Tm) at 93
C. The melt flow index of EOC, measured at 190
C and 2.16 kg loads is 1.6 dg min1. Ethylene propylene diene rubber EPDM (Keltan 5508), with an ethylene content of 70 wt% and an ethylidenenorbornene content of 4.5 wt%, procured from DSM Elastomers, Netherlands was chosen as the second blend component. EPDM has the density of 0.87 gm cm3 and it has Mooney viscosity, ML(1þ4) at 125
C of 55. Glass transition temperature (Tg) of EPDM is (40
C) whereas the melting of crystal takes place at around 31
C. 2.2. Preparation of the blends All the blends of EOC and EPDM at five different blend ratios have been prepared in a batch process using a Haake Rheomix (model Rheomix 600 OS) having a mixing chamber volume of 85 cm3. The batch sizes were nearly 60 gm. Blends were prepared at a rotor (cam type) speed of 60 rpm and at a temperature of 120
C. Details of the blend compositions are given in Table 1. Table 1 Various blend composition. Blend no

EOC (phr)

EPDM (phr)

Designation

1 2 3 4 5

25 40 50 60 75

75 60 50 40 25

P25R75 P40R60 P50R50 P60R40 P75R25

181

2.3. Testing procedure 2.3.1. Differential scanning calorimetry (DSC) The thermal properties, such as glass transition temperature (Tg), melting temperature (Tm) and crystallinity of the soft segment, were determined using a differential scanning calorimeter (DSC 204F1, NETZSCH, Germany) coupled with an auto-sampler. Tests were carried out from 80
C to þ120
C at a heating rate of 10
C min1 under nitrogen atmosphere. In the first thermal scan the specimens were heated at a heating rate of 10
C min1 to þ120
C and then quenched to 80
C at an average cooling rate of 20
C min1 using liquid nitrogen. The second thermal scan was taken over a temperature range of 80
C to þ120
C with a heating rate of 10
C min1. Degree of crystallinity of the soft segment (EPDM) was determined from the heating curve by using the following equation

XC ¼

DHf DHc100%  Wi

 100

(1)

where Xc ¼ Percentage of crystallinity (%)

DHf ¼ Apparent melting enthalpy of crystallization (J/gm) DHc100% ¼ Extrapolated value of enthalpy of crystallization of 100% crystalline polyethylene is 290 (J/gm) Wi ¼ Weight fraction of individual polymer in the blend [33].

2.3.2. Dynamic mechanical analysis (DMA) Dynamic mechanical analyser, Metravib 50 N, France was used to perform the DMA measurements. Temperature sweep of the samples were carried out in tension mode over a temperature range of 80
C to þ100
C, at a rate of 2
C min1. The samples were scanned at a frequency of 10 Hz and a strain level of 10 mm, which was well within the linear viscoelastic region (LVR). The storage modulus (E0 ), loss modulus (E00 ), and the loss tangent (tand) were determined as a function of temperature. 2.3.3. Shape memory property (SMP) test Shape fixation of the sample to a deformed shape or temporary shape and the recovery from deformed shape to permanent shape are the main criteria for the quantification of shape memory functionality of SMPs. The so called and most widely used cyclic thermomechanical tensile test which consists of a programming module where the temporary shape is fixed and recovery module where the permanent shape is recovered was performed to characterize the shape-memory functionality of the blends. The typical test protocol of strain-controlled programming with stress free recovery (ε-s) diagram has been schematically represented in Fig. 1 [34]. Tensile testing machine (Hounsfield H10KS) with a temperature controlled chamber was used to determine the shape memory properties. Sample deformation at different stages of shape memory cycle has been schematically represented in Fig. 2. Usually cyclic thermomechanical test of a polymer is carried out at a temperature which is at least 20
C higher than that of transition temperature [35,36]. During programming module, a sample of original length (lo) was heated to a higher temperature (T  Ttransþ20) and 100% elongated to a length of (le) in the tensile tester with a constant force (Ftensile) at a crosshead speed of 50 mm min1 [37]. The new shape is then fixed by cooling the constrained specimens below the transition temperature (T  Ttrans20). Afterthat, upon release of the load instantaneous shrinkage of

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Fig. 1. Schematic representation of cyclic thermomechanical tensile tests in strain-controlled programming mode.

Fig. 2. Schematic representation of sample deformation during shape memory testing cycle.

the test specimen from length (le) to (lf) takes place as the sample is cooled. Shape fixity (f) is defined as the level of deformation that may be fixed upon rapid cooling of the deformed materials to room temperature. The value of ‘f’ is computed from the parameters defined as follows:

f ð%Þ ¼

lf  l0  100 le  l0

On the other hand, for recovery, the sample was immersed into the hot water at a temperature of (T  Ttransþ20). The recovery in hot water is instantaneous and specimen recovered its original shape as soon as it contacted hot water due to high rate of conductive heat transfer and low specimen thicknesses. The recovery in hot water represents unconstrained recovery of the shape [8]. Hence the recovery ratio (R) is determined by the equation (3) as follows:

(2)

Value of f ¼ 100% indicates no shrinkage of the test specimen upon release of the load. In the next stage, heating of the sample helps to recover the permanent shape. The recovery ratio (RR) is the level of deformation that is recovered upon heating. After fixing the shape at a lower temperature, the sample was again heated to a higher temperature (T  Ttransþ20) to recover the shape.

RR ð%Þ ¼

lf  lr  100 lf  l0

(3)

For an ideal SMP RR ¼ 100% [8,37]. 2.3.4. Morphology study To investigate the morphology of the SMP blends, intermittent contact mode atomic force microscopy, ACAFM (Agilent 5500

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183

80

Scanning Probe Microscope) was used. The resonance frequency of the tip was 146e236 kHz and the force constant was 48 N m1.

EOC P25R75

2.3.5. Mechanical properties Ultimate tensile strength of the blends was determined according to ASTM D412 using dumb-bell shaped specimens in a universal tensile testing machine (Hounsfield H10KS) at a constant cross-head speed of 200 mm min1at room temperature. Tensile modulus at 100% elongation, of the blends at room temperature and at 60
C was also recorded respectively at a cross-head speed of 50 mm min1. To evaluate the tension set values of the blends, all the blends were stretched upto 100% in the tensile direction at a cross-head speed of 200 mm min1 and in the 100% stretched condition the specimens were kept for 10 min. After 10 min the specimens were relaxed back from the stressed condition to the unstressed condition and the percentage change in length was measured after 15 min and the percentage set was calculated by the following equation no (4):

Tension set ð%Þ ¼

Change in length  100 Original length

(4)

To assess the stress relaxation study of the blends, decay of stress was measured during tension set test. Values of stress were noted after each 30 s interval and it was continued upto 10 min. As per ASTM D2240 standard (through parallel contact of the specimen to the Durometer pressure foot, specimens thickness of minimum 6 mm, were subjected to the direct indentation without shock and with just sufficient force to overcome the spring force), both Shore A and Shore D hardness of the specimens were measured using Shore A Durometer hardness tester (Rex Durometers) and Shore D Durometer hardness tester (Bowers Mertrology, UK) respectively. 3. Results and discussion 3.1. Mixing energy curve Rise of torque during mixing of polymers inside the internal mixer is a complex combination shear and elongational flow. Mixing curves (torque-time plots) of the blends and the pristine polymers are shown in Fig. 3. From the plots it can be clearly noticed that all the plots consists of two peaks. The first peak is due to an increment in mixing torque (rise of viscosity) due to resistance exerted on the rotor by the unmolten polymer (EOC) followed by a viscosity decrease due to the complete melting of EOC. Afterwards, addition of EPDM onto the molten EOC phase again implies the rise in the torque value (rise of second peak) because of rise of viscosity. After a certain specified time of mixing, uniform torque value indicates the proper level of mixing and also suggests the attainment of the steady state value [38]. P25R75 blend was attributed to highest equilibrium mixing torque which may be attributed to higher melt viscosity of the EPDM phase as compared to the EOC phase. 3.2. Melting and crystallization behaviour Glass transition, melting and crystallization behaviour of the above blend systems are demonstrated in Fig. 4. From the literature it can be observed that both the polymers have Tg well below the room temperature and the difference between these two polymers in terms of glass transition temperature is not so wide which results a single Tg value in DSC thermogram. The single Tg as obtained from DSC also indicate higher degree of

Torque (Nm)

70

P40R60

60

P50R50

50

P60R40 P75R25

40

EPDM

30 20 10 0

0

1

2

3

4

5

6

7

8

Time (min) Fig. 3. Torque vs. time plot of pristine polymers and blends at 120
C and at 60 rpm rotor speed.

compatibility of the two phase which has been discussed later (section 3.7). However, the small shift in Tg values (more or less 1
C) is due to difference in blend composition. From the literature as well as our experimentation it has been found that the Tg value of EPDM is 40.23
C and Tg value of EOC is 33.67
C. Therefore, depending on the blend composition, Tg value shifts towards higher temperature. As a result, Tg of P25R75 blend is 39.39
C, whereas Tg value of P75R25 blend is 35.11
C. On the other hand above the room temperature there are two major transitions; one is above 31
C and other one is above 93
C. Former corresponds to the melting of the crystalline domain of the EPDM phase and the second one is of the melting of crystalline domain EOC phase. Melting temperature (Tm) and crystallization temperature (Tc) of the two base polymers and that of various blends are given in Table 2. From the table it can be clearly observed that the percentage crystallinity of the soft segment gradually reduces with decrease of EPDM content. Therefore P25R75 blend exhibits the highest degree of soft segment crystallinity whereas the degree of soft segment crystallinity is minimum for P75R25 blend system.

3.3. DMA analysis Tand can be defined as the ratio of loss modulus to the storage modulus and it can be written as follows:

Tand ¼

00
Loss modulus E Storage modulusðE0 Þ

(5)

The maximum tand peak from DMA curve is often considered as a measure of the glass transition temperature (Tg). From Fig. 5, it can be clearly seen that with increase of EPDM content, tand values of the blends at the glass transition region increases. In case of virgin rubber (EPDM), the entire rubber chain segments are exposed to dynamic transition which leads to the higher tand values (higher dissipation of energy) at the transition region. Now with gradually addition of thermoplastic EOC phase (from 25 parts to 75 parts) into the blends, the availability of the chain segments to the dynamic transition becomes lower. This is due to the immobilization of the chain segments which comes closure to the plastic phase and as a result those chain segments cannot contribute to the dynamic transition which leads to lower dissipation of energy

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Fig. 4. DSC plot of the blends (a) Second heating and (b) first cooling curves.

Table 2 Glass transition temperature (Tg), Melting temperature (Tm), crystallization temperature (Tc) and percentage crystallinity of soft segment of the virgin polymers and the various blends. Sample name

Tg (
C) of the blend

Tm (
C) of EPDM

Tm (
C) of EOC

Tc (
C) of EPDM

Tc (
C) of EOC

Percentage (%) crystallinity of soft segment

EPDM P25R75 P40R60 P50R50 P60R40 P75R25 EOC

40.23 39.39 38.11 37.17 36.12 35.11 33.67

32.30 31.75 31.64 31.59 31.25 31.28 e

e 94.12 93.71 94.22 94.55 94.53 96.44

19.73 14.91 15.68 16.10 16.72 17.14 e

e 68.22 72.64 75.00 76.58 79.80 81.82

6.79 4.86 3.88 3.56 3.23 2.43 e

(lower tand values) [39]. Another peak in the tand versus temperature plot was also observed in the temperature range of (45e50)
C. This peak corresponds to a0 transition of EOC phase which arise due to molecular motion in the crystalline zone [40,41]. This a0 transition is the characteristic peak of EOC phase only. From the figure it can be

Fig. 5. Tand versus temperature plot of the various blends and base components.

clearly viewed that a0 transition in the blends increases with increase proportion of EOC. In case of neat EPDM, no such kind of transition occurs whereas a0 transition peak height is maximum in case of neat EOC. In case of EOC, a0 transition peak arises much below its melting temperature. With higher the EOC content in the blend system peak height increases gradually and with increase of EOC content peak also shifts slightly towards higher temperature. Storage moduli of the blends and the virgin components have been shown in Fig. 6. At low temperature region (80
C) also the storage modulus of EPDM is 919 MPa whereas the same for EOC is 1858 MPa. With increase of temperature there is a sharp decrease in the storage modulus value which implies glass to rubber transition phenomenon. Virgin EPDM shows a clear rubbery plateau region. With increase of EPDM content the rubbery plateau region gradually increases in the blend system whereas it is vice-versa for the higher EOC content blend. But from the figure one distinguished phenomenon is clear that throughout the temperature range the storage modulus value of the blends containing higher EOC content is always higher than the blends containing lower proportion of EOC. This may be due to the higher modulus of pristine EOC than that of pristine EPDM. Storage modulus values of the blends at three different temperatures (50
C, 0
C, þ50
C) have been summarized in Table 3. P75R25 (higher EOC content blends) shows the highest storage modulus and P25R75 (higher EPDM content blends) shows the lowest value among the blends. Intermediate blends show storage modulus value in between the storage modulus of that two terminal blends.

T. Chatterjee et al. / Polymer 78 (2015) 180e192

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Fig. 7. Typical SME cycle of the blends. Fig. 6. Storage modulus versus temperature plot of the different blends and the pristine polymers.

Table 3 Storage modulus values of the blends at different temperatures. Sample name

E0 (MPa) at (50
C)

E0 (MPa) at (0
C)

E0 (MPa) at (þ50
C)

P25R75 P40R60 P50R50 P60R40 P75R25

997.49 1083.83 1223.91 1277.02 1322.56

28.77 43.37 65.20 82.41 125.34

7.12 11.19 14.70 17.60 25.00

3.4. Thermomechanical properties It is well accepted by other investigators that the ratio of the elastic modulus in glassy state and rubbery state for smart materials, expressed as [E0 (Tg20)/E0 (Tgþ20)] is an indicator of the shape memory behaviour of an SMPs. The ratio of elastic modulus in glassy state and rubbery state must be at least 20 for efficient shape recovery of SMPs from its deformed shape to permanent shape [42]. Storage modulus as a function of temperature has already been plotted in Fig. 6. The ratio of storage modulus (E0 ) at (Tg20) and at (Tgþ20) obtained from the plot, has been given in Table 4. From the table it can be clearly observed that with increasing EOC content, the value of the ratio of E0 (Tg20)/E0 (Tgþ20) is gradually becoming lower. This clearly implies that the blend with higher EPDM content will show better shape memory behaviour, i.e., means P25R75 blend should exhibit the better ability to retain its temporary shape and also it will show efficient shape recovery from its secondary or deformed shape to permanent shape upon actuation than the other blends. 3.5. Shape memory properties The scenario of the shape memory properties, namely shape

fixity (SF), shape recovery ratio (RR) of the blends are discussed in this section. Considering the condition of cyclic thermomechanical test condition (mentioned in section 2.3.3) and also followed by DSC melting endotherm of EPDM crystal (Tm z 32
C) and a0 transition of EOC phase (z50
C) shape memory behaviour of the different blends was tested at 60
C only. During stretching of the blends at 60
C, the chains of the soft and hard phases get oriented along the direction of stretching. Upon cooling to room temperature in the stretched condition EPDM crystallizes and the crystallinity of the EPDM phase restricts the relaxation of the blends upon unloading and hence the sample length is not anticipated to change much more. Higher the crystallite formation in the EPDM phase better is the shape fixity value of the blend. However immediate shrinkage of the deformed sample takes place after the release of the tensile load due to entropic elasticity which reveals the partial crystallization of the EPDM phase [23,37]. As a result the sample could not maintain its length in the stretched condition (le). Typical SME cycle of the different blends in strain-controlled mode is shown in Fig. 7. From the graph itself, it can be seen that P25R75 blend shows higher shape fixity (f) whereas P75R25 blend shows the lowest value of shape fixity. Since the shape memory property test has been carried out at 60
C (above the melting of EPDM crystal), EPDM phase in the blend system mainly contributes to the shape fixity and shape recovery properties of the various blends. Therefore higher the EPDM content better will be the shape fixity and shape recovery (RR) of the blend system. Shape fixity ratio of the different blends after cooling is depicted in Fig. 8. Therefore P25R75 blend which contains 75% EPDM and 25% EOC shows higher shape fixity value whereas P75R25 blend containing 75% EOC and 25% EPDM shows poor shape fixity which can be clearly observed from the figure. Intermediate blends show the behaviour according to the ratio of the two components. Unconstrained shape recovery from the temporary shape to permanent shape of the specimens has been attributed to the

Table 4 Values of storage modulus (E0 ) at (Tg20
C), (Tgþ20
C) and E0 (Tg20)/E0 (Tgþ20).


Sample name

Tg ( C)

E0 (Tg20) (MPa)

E0 (Tgþ20) (MPa)

E0 (Tg20)/E0 (Tgþ20)

P25R75 P40R60 P50R50 P60R40 P75R25

29 29 29 28.5 28

980 1060 1167 1250 1300

40.7 60.5 86.9 116 173

24.07 17.52 13.42 10.77 7.51

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T. Chatterjee et al. / Polymer 78 (2015) 180e192

140

P25R75 P40R60

Shape fixity (f) (%)

120

P50R50 P60R40

100

P75R25

86.71

81.07

78.67

40:60

50:50

80

75.15

74.41

60:40

75:25

60 40 20 0

25:75

Blend ratio Fig. 8. Shape fixity values of the different blends.

influence of heat only. Reheating the specimens above the melting range of the soft segments (above the shape transition temperature) helps to recover the permanent shape of the specimens. It is evident that, reheating of the specimens allows the melting of the crystallites, formed during cooling of the specimens in the stretched condition and shape recovery begins. Once the melting starts the sample gets relaxed from the stretched condition to the original non-deformed state. In the present study to assess the shape recovery behaviour of the blends, recovery study has been carried out both in the heating chamber and also in hot water [37]. The shape recovery ratio of the blends is shown in Fig. 9. Shape recovery ratio (RR) of the blends also follows the same trend like the shape fixity (f) of the blends. P25R75 blend (containing 75% EPDM and 25% EOC) shows the highest percentage of shape recovery whereas shape recovery ratio for the P75R25 blend (containing 25% EPDM and 75% EOC) is the lowest. Higher proportion of EPDM results higher shape fixity (f) due to formation of higher level of EPDM crystallites and the melting of all the crystallites in presence of heat also implies highest percentage of shape recovery. One interesting fact to note that the shape recovery of the specimens increase drastically in presence of hot water than that of shape recovery in hot chamber which may be due to instantaneous heat

100 Recovery at hot chamber Recovery at hot water

Recovery (R) (%)

95

95

90

90

85

85

80

80

75

75

70

70

65

65

60

25:75

40:60

50:50

60:40

75:25

Blend ratio Fig. 9. Shape recovery ratio (%) of the various blends.

60

3.6. Morphology study To get an insight about the developed morphology after mixing atomic force microscopy (AFM) has been carried out. Although in our work five different blend ratios have been chosen but AFM study has been carried out only for the two terminals (P25R75 and P75R25) and one intermediate (P50R50) blend system. Fig. 12(aec) represent the respective phase images of the P25R75, P50R50 and P75R25 blends. In the corresponding phase images the yellow regions indicate the presence of hard domains (EOC phase) and the brown region represents the soft domains (EPDM phase) [43]. The corresponding phase image depicts uniform distribution of the hard and soft domains and also indicates microphase separation of the hard and soft segments. From the phase images it can be clearly noticed that in P25R75 blend, EOC phase become dispersed in the soft continuous EPDM matrix whereas in P75R25 blend reverse phenomenon has been observed. In the intermediate blend P50R50, both the phases are continuous which leads to the formation of co-continuous phase morphology. On the other hand from all the phase images, it can be noticed that both EOC and EPDM phases form highly physical entanglements and this physical entanglement helps to exhibit shape memory behaviour for the above blend systems. 3.7. Mechanical properties

Recovery (R) (%)

100

transfer to the specimens from the hot water to the specimens whereas rate of heat transfer in the heating chamber to the specimens is much lower. Therefore, shape recovery ratio for the P25R75 blend in the hot chamber is 81.41%, whereas shape recovery ratio is 92.10% for P25R75 blend in contact with hot water. Typical shape memory effect of the polymer blend in hot water is shown in Fig. 10. From Fig. 10 it can be observed that in hot water more or less complete recovery is of the sample taking place. Sample recovery in hot water is higher than recovery in heating chamber and also instantaneous because of fast heat transfer from the hot water to the sample. Therefore, to complete the recovery of the sample from e) to j) only 10e15 s time is sufficient that has been observed by the authors during the measurement. However, shape recovery of different blend in heating chamber varies with respect to time. The shape recovery of different blend as a function of time has been shown in Fig. 11. For all the blends, recovery time was not more than 10 min, i.e., within 10 min recovery of various blends are complete. Only there were the differences in percentage of recovery. The figure depicts that maximum percentage of recovery of all the blends has taken place within 6 min. It can be observed that the fastest shape recovery is shown by P25R75 blend, whereas shape recovery is slowest in case of P75R25 blend. As a result, percentage recovery of P25R75 blend after 2 min is 62% whereas it is 37% in case of P75R25 blend.

Fig. 13 depicts the stress-strain curves for the virgin polymers as well as their blends. Inset is the mechanical properties of the pristine polymers. Neat EPDM exhibits tensile strength of 8.0 MPa and elongation at break of 1337% whereas pristine EOC shows a tensile strength of 30.3 MPa and elongation at break of 1230%. From the graph it can be clearly viewed that inspite of having higher ethylene content EPDM still shows rubbery behaviour whereas EOC shows behaviour like thermoplastics. Neat EOC shows necking phenomenon which is absent in case of EPDM which can be clearly seen from Fig. 13 [44]. Necking phenomenon is also observed in case of blends. With increase of EOC content, tendency to show necking phenomenon is also higher. For this reason P75R25 blend (contains 75% of EOC phase) shows maximum necking behaviour whereas P25R75

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Fig. 10. Photo series demonstrating the shape memory behaviour of the blend system: a) Initial sample; b) softening of the sample in hot water; c) fixation of the temporary shape by emerging in cold water; d) fixed temporary shape; e) immersion of the temporary sample of the sample in hot water for recovery to its initial length; (fei) recovery of the sample in different stages; j) sample after final recovery.

(containing 25% EOC phase) shows least necking behaviour. In between this two terminal blends; necking phenomenon gradually reduces with decrease of EOC content from P60R40 to P40R60. In case of blends, with increase addition of EOC phase tensile

100

Recovery (R) (%)

90 80 70 60 50 P25R75

40 30 20 0

P40R60

3.8. Mechanical modelling

P50R50

Depending upon the relative proportions of the components mechanical properties of the blends differs from each other. Various theoretical models like series model (Voight model), parallel model (Reuss model) have been studied to demonstrate the change in modulus and tensile strength with composition of the blends [41]. Parallel coupling signifies that the strains of all the polymer components are equal and each component contributes to the final mechanical properties as per the mixing rule and hence the resulting mechanical properties of the blends become

P60R40

10

P75R25

0

1

2

3

4

5

6

7

8

strength steadily increases. Tensile strength (T.S.) for P25R75 blend is 15.7 MPa whereas for P50R50 blend 22.5 MPa followed by 24.3 MPa for P75R25 blends. On the other hand, elongation at break stepwise decreases with increasing amount of EOC. Like tensile strength, modulus values of the blends show the same trend. Tensile strength (T.S.), elongation at break (%EB), modulus at different percentages (100, 200, 300%) of strain, and hardness of the pristine polymers and blends are enlisted in Table 5. At 300% of strain, modulus of P25R75 (lowest EOC content) blend is only 2.7 MPa whereas the modulus of P75R25 (highest EOC content) is 5.1 MPa. Hardness (both Shore A and Shore D) of the blends also increases with the increase of EOC content. Shore A hardness for P25R75 blend is of 72.2 whereas it is 85.3 in case of P75R25 blend system.

9 10 11

Time (min) Fig. 11. Recovery with respect to time for various blends.

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T. Chatterjee et al. / Polymer 78 (2015) 180e192

Fig. 12. AFM phase images of (a) P25R75, (b) P50R50, (c) P75R25 blend.

independent of interfacial adhesion. Mathematically parallel model can be written as follows:

M ¼ ∅1 M1 þ ∅2 M2

(6)

where M is the mechanical property of the blend under consideration and M1 and M2 are the particular properties of the components 1 and 2 respectively. f1 and f2 are the volume fraction of the components respectively. On the other hand, as per the lowest lower bound series model, it is assumed that the components are arranged in series with the applied stress. Therefore contributions of the components to the blend system are expressed by the inverted rule of mixing. Interfacial adhesion plays the key role in determining the ultimate properties as the stress is transmitted through the present interface. Series model is expressed as:

1 ∅ ∅ ¼ 1þ 2 M M1 M2

Fig. 13. Stress-strain curves of the pristine polymers and their blends.

(7)

Several researchers have studied this kind of models to characterize the compatibility or miscibility study of the various blend systems [33]. Fig. 14 shows the comparison between the

T. Chatterjee et al. / Polymer 78 (2015) 180e192

189

Table 5 Mechanical properties of the virgin polymers and their blends. Sample code

T.S. (MPa)

EPDM P25R75 P40R60 P50R50 P60R40 P75R25 EOC

8.0 15.7 19.6 22.5 23.3 24.3 30.3

± ± ± ± ± ± ±

EB (%)

0.2 0.4 0.3 0.1 0.3 0.5 0.3

1337 1240 1207 1145 1107 1097 1230

± ± ± ± ± ± ±

M100 (MPa) 13 25 21 7 15 19 23

1.0 1.7 2.5 2.8 3.0 3.8 5.4

± ± ± ± ± ± ±

0.02 0.05 0.05 0.03 0.02 0.05 0.04

M200 (MPa) 1.4 2.2 2.8 3.3 3.6 4.4 5.6

± ± ± ± ± ± ±

M300 (MPa)

0.02 0.05 0.05 0.03 0.02 0.05 0.04

1.7 2.7 3.3 3.9 4.2 5.1 6.4

± ± ± ± ± ± ±

0.02 0.05 0.05 0.03 0.02 0.05 0.04

Hardness (Shore A)

Hardness (Shore D)

54.1 72.2 75.6 81.2 83.1 85.3 91.4

10.2 17.5 18.2 23.3 28.5 33.1 36.8

Note: M100, M200 and M300 represent modulus at 100%, 200% and 300% of strain.

Tensile strength (MPa)

35

Experimental Parallel Series

30 25 20 15 10 5 0

0

20

40

60

80

100

EOC (%) Fig. 14. Experimental and theoretical values of tensile strength for various blends as a function of EOC content.

experimental and theoretical data of the tensile strength of EOC/ EPDM blends as a function of EOC content. From the figure it is evident that experimental values show significant positive deviations which suggest better properties for the blend systems over the theoretical predictions and it also implies the occurrence of synergism which may be due to the formation of sufficient physical entanglements between the two phases and indicates very good compatibility between the components in this blend system [45]. Higher degree of compatibility of the blend components is also supported by DSC thermogram where single glass transition was obtained for each and every blend.

depicts the relaxation phenomenon of the various blends at room temperature. From Fig. 16(a) it can be noticed that the decay in stress (stress relaxation) is relatively lower for the blend containing lower fraction of EOC. With gradual increase of EOC fraction, decay of stress becomes higher. It means P75R25 blend relaxed at the fastest rate whereas stress relaxation for P25R75 blend is the slowest one. This result also supports the tension set behaviour of the blends and it also indicates that highest EOC containing blend possess highest viscous nature which results faster decay of stress value. Table 6 demonstrates the decay of stress value of the various blends at room temperature. Therefore decay of stress for P25R75 blend is 0.54 MPa only whereas decay of stress is of 1.58 MPa for P75R25 blend. Another interesting phenomenon, relaxation ratio of the blends with respect to time at 100% strain at room temperature has been plotted in Fig. 16(b). From the figure it can be observed that relaxation ratio is highest for the P75R25 blend and minimum for the P25R75 blend. It is a well-known fact that lower relaxation ratio results low hysteresis loss ratio and also indicates the higher recovery of a sample from the temporary shape to permanent shape [20]. Considering this factor, it can be stated that SMP blend which shows low relaxation ratio will exhibit better shape recovery behaviour. This also suggests that the P25R75 blend (low EOC containing blend) will show the better shape recovery behaviour. From the shape memory study, it can be seen also that P25R75 blend possess highest shape recovery among all the blends which supports the relaxation ratio results of the different blend system. On the other hand P75R25 blend shows highest relaxation ratio and it also shows poor shape recovery behaviour.

20 18

Tension set values of the blends are shown in Fig. 15 which clearly demonstrates that with increase of EOC content in the blend, percent tension set gradually increases. It indicates that with increase of EOC content, elastic nature of the blend gradually reduces while the viscous behaviour increases on the other hand. Therefore, P25R75 blend (containing 25% EOC and 75% EPDM) shows a set value of 8.2% whereas P75R25 blend shows 18.3% set value due to containing highest amount of EOC. This also indicates that, P25R75 blend exhibits highest elastic behaviour than that of other blends. It is well known that, the force required to maintain a constant strain that is applied on a viscoelastic material, gradually decreases with time and this phenomenon is called “stress relaxation” [46]. Therefore during tension set test change of stress value after each 30 s interval was noted down as the decay of stress value. Fig. 16

Tension set (%)

3.9. Tension set and stress relaxation

16 14 12 10 8

18.3

P25R75 P40R60 P50R50

14.7

P60R40

12.3

P75R25

10.2 8.2

6 4 2 0 Fig. 15. Tension set values of the blends.

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T. Chatterjee et al. / Polymer 78 (2015) 180e192

4.5

Stress (MPa)

P50R50

3.5

P60R40 P75R25

3.0 2.5 2.0 1.5

Relaxation ratio (%)

P40R60

4.0

1.0

50

P25R75

P25R75 P40R60

40

P50R50 P60R40 P75R25

30 20 10 0

0

100

200

300

400

500

600

700

0

2

4

6

Time (sec)

Time (min)

(a)

(b)

8

10

Fig. 16. (a) Stress relaxation versus time curves (b) Relaxation ratio versus time of the different blends in tension mode at room temperature.

Table 6 Initial stress values, final stress values and decay of stress values of the various blends. Sample code

Initial stress (MPa)

Final stress (MPa)

Decay of stress (MPa)

P25R75 P40R60 P50R50 P60R40 P75R25

1.92 2.57 2.86 3.34 4.10

1.38 1.86 2.06 2.14 2.52

0.54 0.71 0.80 1.20 1.58

3.10. Tensile modulus of the blends at 60
C and at room temperature Typical tensile stress-strain plots of the blends at room temperature as well as at 60
C and are shown in Fig. 17(a and b). It clearly demonstrates that modulus at 100% strain of the various blends at room temperature is higher than that of the modulus at 60
C. This phenomenon is simply due to the melting of the crystal of the EPDM phase at 60
C [37]. Table 7 shows the tensile modulus value of the various blends at room temperature and also at 60
C. Percentage reduction in tensile modulus of the blends is also given. Percentage reduction in tensile modulus is maximum (66.67%) for P25R75 blend and minimum (44.70%) for P75R25 blend. Higher the EPDM content in the blend higher is the percentage reduction in tensile modulus due to complete melting of the EPDM crystals at 60
C. Low modulus at room temperature and at 60
C in case of P25R75 blend also supports the better shape recovery of the blend [37]. 3.11. Stress relaxation and relaxation ratio at 60
C Stress relaxation study has also been performed during shape memory testing. It means that decay of stress also takes place during cooling of the specimen from 60
C to a temperature which is below the transition temperature holding at 100% of strain. Stress relaxation phenomenon and the relaxation ratio study are also

carried out and it has been depicted in Fig. 18. Here also the same phenomenon of stress relaxation was observed like the stress relaxation phenomenon observed at room temperature. Decay of stress is higher for the EOC rich blend. With increase in EPDM content, stress relaxation phenomenon gradually decreases as depicted in Fig. 18(a). On the other hand Fig. 18(b) reports the relaxation ratios of the blends with respect to time. In this case, the relaxation ratio is minimum for the lowest EOC containing blend means for P25R75 blend and maximum for P75R25 blend system that contains 75% EOC. Thus based on the postulation of relaxation ratios upon the shape recovery behaviour mentioned in (Section 3.9) it can be stated that P25R75 blend gives better shape recovery behaviour while the P75R25 blend exhibits poor shape recovery and all the results of relaxation ratio behaviour supports the shape memory behaviour of the EOC/EPDM blend systems [23]. 4. Conclusions A detailed investigation of the shape memory behaviour of thermally stimulated polyolefinic blends based on EOC and EPDM has been pursued. It is evident from this study that a strong correlation exists between the crystallinity of soft segment and the shape memory behaviour of the blends. From DSC and DMA analyses it has been found that there are two sharp distinguished transitions at 32
C and 93
C corresponding to melting of crystallite of EPDM and EOC phase respectively. SMP test was carried out at 60
C (shape transition temperature) which was about 25
C higher than the melting temperature of the EPDM crystallites. EOC-EPDM blend having higher EPDM-rich phase shows better shape memory behaviour in terms of shape fixity (f) and shape recovery ratio (R) value. Cooling below the transition temperature results higher degree of crystallite formation that provides better shape fixity for P25R75 blend and again reheating of the stretched specimen above the shape transition temperature also provides better shape recovery due to the melting of the crystals that helps the material to relax back from its strained shape to unconstrained shape. Thermomechanical analysis from DMA study also correlates with the better shape memory behaviour of the P25R75 blend

T. Chatterjee et al. / Polymer 78 (2015) 180e192

6

6

P25R75 P40R60

5

P25R75 P40R60

5

P50R50

P60R40

Stress (MPa)

Stress (MPa)

P50R50

4

P75R25

3 2 1 0

191

P60R40

4

P75R25

3 2 1

0

20

40

60

80

100

0

120

0

20

40

60

Strain (%)

Strain (%)

(a)

(b)

80

100

120

Fig. 17. Typical stress-strain plot of the SMP blends at (a) room temperature and (b) at 60
C stretching upto 100% of strain.

Table 7 Tensile modulus of the blend at room temperature and at 60
C. Sample name

Modulus at 100% strain (MPa) at room temp.

Modulus at 100% strain (MPa) at 60
C

Reduction in modulus (%)

P25R75 P40R60 P50R50 P60R40 P75R25

1.89 2.48 2.64 3.37 3.87

0.63 1.11 1.36 1.60 2.14

66.67 55.24 48.48 46.74 44.70

P40R60 P50R50

2.0

P60R40 P75R25

1.6 1.2 0.8 0.4

Relaxation ratio (%)

Stress (MPa)

60

P25R75

2.4

P25R75

50

P40R60

40

P60R40

P50R50 P75R25

30 20 10 0

0

100

200

300

400

Time (sec)

(a)

500

600

0

100

200

300

400

500

Time (sec)

(b)

Fig. 18. (a) Stress relaxation versus time curves (b) Relaxation ratio versus time of the different blends in tension mode at 60
C.

600

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T. Chatterjee et al. / Polymer 78 (2015) 180e192

rather than the other blends. Better shape memory behaviour of the highest EPDM containing blends was also supported by the lower degree of stress relaxation and lowest relaxation ratio of the blends as compared to the other blends. Thus P25R75 blend (EPDMrich) shows better shape memory behaviour amongst all the blends. Shape memory behaviour of the EOC-EPDM blends even without any crosslinking is primarily due to very good compatibility between the two pristine polymers owing to the structural similarities. The high degree of compatibility between the virgin components basically helps in formation of sufficient physical entanglements between the two phases. Structural similarity and high degree of compatibility of the pristine polymers enhances the physicomechanical properties of the blends. The authors would like to study in near future the influence of covalent crosslinks formation (either through chemical crosslinks or through electron beam treatment) in the blends to study further shape memory behaviour. Acknowledgement The authors would like to acknowledge Board of Research in Nuclear Sciences (Sanction No: 2013/35/34/BRNS/1165), Department of Atomic Energy (DAE), Mumbai, India, for providing financial support to carry out the work. Authors also would like to thank Mr Debdipta Basu (IPF Dresden, Germany) for his kind help in thermal characterization. Also thanks to Mr. Pradip Das, Mr. Syed Mushtaq, Mr. Parijat Ray, Mr. Padmanabhan R., Mr. Syed Mohammad Reffai, Mr. Partheban M. References [1] Z.L. Wang, Z.C. Kang, Functional and Smart Materials, Plenum Publishing Corp, New York, 1998, p. 514. [2] T. Xie, Recent advances in polymer shape memory, Polymer 52 (2011) 4985e5000. [3] I.S. Kolesov, H.J. Radusch, Multiple shape-memory behavior and thermalmechanical properties of peroxide cross-linked blends of linear and shortchain branched polyethylenes, eXPRESS Polym. Lett. 2 (2008) 461e473. [4] R. Biju, C. Gouri, C.P.R. Nair, Shape memory polymers based on cyanate esterepoxy-poly (tetramethyleneoxide) co-reacted system, Eur. Polym. J. 48 (2012) 499e511. [5] H.J. Radusch, I.S. Kolesov, U. Gohs, G. Heinrich, Multiple shape-memory behavior of polyethylene/polycyclooctene blends cross-linked by electron irradiation, Macromol. Mater. Eng. 297 (2012) 1225e1234. [6] A. Lendlein, S. Kelch, Shape-memory polymers, Angew. Chem. Int. Ed. 41 (2002) 2034e2057. [7] M. Behl, J. Zotzmann, A. Lendlein, Shape-memory polymers and shapechanging polymers, Adv. Polym. Sci. 226 (2010) 1e40. [8] I.S. Gunes, F. Cao, S.C. Jana, Effect of thermal expansion on shape memory behavior of polyurethane and its nanocomposites, J. Polym. Sci. Part B Polym. Phys. 46 (2008) 1437e1449. [9] C.C. Wang, W.M. Huang, Z. Ding, Y. Zhao, H. Purnawali, Cooling-/waterresponsive shape memory hybrids, Compos. Sci. Technol. 72 (2012) 1178e1182. [10] A. Lendlein, H.Y. Jiang, O. Jünger, R. Langer, Light-induced shape-memory polymers, Nature 434 (2005) 879e882. [11] N.G. Sahoo, Y.C. Jung, J.W. Cho, Electroactive shape memory effect of polyurethane composites filled with carbon nanotubes and conducting polymer, Mater. Manuf. Process 22 (2007) 419e423. [12] R. Vaia, Nanocompositesdremote-controlled actuators, Nat. Mater. 4 (2005) 429e430. [13] M.Y. Razzaq, M. Anhaltb, L. Frormanna, B. Weidenfellerb, Mechanical spectroscopy of magnetite filled polyurethane shape memory polymers, Mater. Sci. Eng. A 471 (2007) 57e62. [14] B.K. Kim, New frontiers of shape memory polymers, eXPRESS Polym. Lett. 4 (2010) 589. [15] S. D'hollander, G. Van Assche, B. Van Mele, F. Du Prez, Novel synthetic strategy toward shape memory polyurethanes with a well-defined switching temperature, Polymer 50 (2009) 4447e4454. [16] S.M. Kang, S.J. Lee, B.K. Kim, Shape memory polyurethane foams, eXPRESS Polym. Lett. 6 (2012) 63e69. [17] X. Zheng, S. Zhou, X. Li, J. Weng, Shape memory properties of poly(D,Llactide)/hydroxyapatite composites, Biomaterials 27 (2006) 4288e4295.

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