The shape memory, and the mechanical and thermal properties of TPU/ABS/CNT: a ternary polymer composite

Abstract

Polymer blend nanocomposites based on thermoplastic polyurethane (TPU) elastomer, acrylonitrile butadiene styrene (ABS) and multi-walled nanotubes (MWCNTs) were prepared via a simple melt blending process. The effects of different nanotube content on the morphology, thermal, mechanical, and shape memory properties of the nanocomposites were investigated. The tensile strength and Young's modulus of the nanocomposites increased, while the elongation at break decreased with the amount of MWCNTs in the blend. The results of the impact test show that the impact strength first increased and then decreased significantly for additions of more than 2 wt% of the filler to the blend. According to the shape memory results, the shape recovery ratios and shape fixity increased significantly only with the addition of 2 wt% of MWCNTs to the blend. Differential scanning calorimetry (DSC) results showed that the addition of MWCNTs reduces the degree of crystallinity and increases the melting temperature of the blend. Furthermore, from the DSC results, we found that that the interaction of MWCNTs with TPU is stronger than with ABS. Thermogravimetric analysis (TGA) studies of the TPU/ABS/MWCNTs nanocomposites exhibit a remarkable thermal resistance much higher than that of the TPU/ABS blend. Finally, scanning electron microscopy (SEM) showed that the added MWCNTs reduced the domain size of the dispersed phase in the blends.

---

1. Introduction

Shape memory materials are some of the most applicable smart materials that are extensively used in medical devices, automobile manufacturing, aerospace, and clothing industries.^1–4 Because of their several unique properties (low cost and density, easy processing and high elastic deformation), shape memory polymers (SMPs) are considered to be leading candidates for use as shape memory materials.^5,6 These polymers are stimuli-responsive, can hold a temporary deformed shape, and can recover their original shape in the presence of an external stimulus such as light,^1,7–11 pH,^12,13 an electrical field,^5,14–20 moisture²¹ or heat.^22–25 Thermoplastic polyurethane (TPU) is one of the most commonly investigated thermo-responsive SMPs, which are known for its good elongation, excellent low-temperature properties, and high abrasion resistance.^5,26–31 The internal network of TPU generally consists of two vital segments that play special roles in the shape memory effect. The hard segment with different interactions such as hydrogen bonding, dipole–dipole interaction, and van der Waals forces functions as the physical crosslinking and memorizes the original shape.³² The additional soft segment can absorb external forces during loading and determines the switching temperature.^23,32

Despite their unique characteristics, significant weaknesses in the mechanical properties and strengths of SMPs can severely restrict their use in engineering applications.^33,34 To overcome these limitations and improve the low modulus and recovery stress of SMPs, two main strategies can be used: polymer blending and the use of nanofillers.^35,36 Polymer blending is the process of mixing two or more polymers in different compositions to improve and control some characteristic.^37,38 Since the blending process is simple compared to in situ polymerization and can be performed easily with conventional equipment and facilities, polymer blending has recently attracted much attention.³⁷ Two or more polymers are selected and mixed together in order to design a new SMP. In some cases, one of which exhibit the physical or chemical crosslinking to fix a temporary shape, and the other act as the reversible phase and somehow memorize the original shape.^6,23,39–43 But sometimes one polymer can act as the reversible and fix phase, because of its internal network which is consist of two segments. So in SMP polymer blends finding out the role of each polymer in shape memory effect is necessary for enhancing shape memory properties. As one of the most well-known SMPs, TPU has been blended with various polymers to obtain new SMPs with improved properties. In the open literature, the shape memory properties of TPU compound with phenoxy resin,⁴⁴ polybenzoxazine (PB),^45–47 polyvinyl chloride (PVC),⁴⁸ polypyrrole (PPy),⁴⁹ polystyrene–butadiene–styrene (SBS),^37,41 polylactic acid (PLA),^47,50 poly(oxyethylene) (POE),²³ polyvinylidene fluoride (PVDF),⁵¹ and polycaprolactone (PCL)⁵² has been reported. The results showed an enhancement of the mechanical properties or conductivity together with the shape memory effect. Thus, all these blends can be considered as new SMPs. The use of carbon nanotubes (CNT) is an alternative approach that not only significantly improves mechanical properties but also has a great impact on the shape memory properties is. Mohan et al.⁵ prepared a polymer blend nanocomposite based on TPU and PLA. Both thermal and electrical properties increase significantly in the presence of pristine and modified multi-walled nanotubes (MWCNTs). This new electroactive SMP shows a remarkable recoverability.⁵ In another study, Mohan et al.,¹⁶ used pristine and modified MWCNTs to enhance the mechanical and shape memory properties of the TPU/PVDF blend. Their results showed that the Young's modulus and tensile strength of the polymer blend increase remarkably with MWCNT loading.¹⁶ Sahoo et al. showed that increases of 200% and 37% in the Young's modulus and tensile strength, respectively, are observed in the PU/MWCNT (2.5%) nanocomposite.⁵³

To the best of our knowledge, this is the first demonstration of the shape memory behavior of TPU/acrylonitrile butadiene styrene (ABS)/MWCNT composites. According to our previous study, due to its excellent mechanical properties and high impact strength, ABS can improve mechanical properties of TPU with the addition of 20 wt% ABS.⁵⁴ In the present study, the effects of MWCNT on the shape memory, mechanical and thermal properties of TPU/ABS (80/20) blend are investigated.

2. Experimental

2.1. Materials

The raw materials were obtained from commercial sources. The thermoplastic polyurethane (TPU) was supplied by Bayer Material Science (Germany) as Desmopan 385S grade. The values for the density and shore A hardness of the TPU are 1.2 g cm⁻³ and 85, respectively. ABS, which has a trade name of SD 0150, showed a density of 1.06 g cm⁻³ and an MFI of 1.80 g/10 min and was obtained from the Tabriz Petrochemical Company (Iran). Pristine multi-walled carbon nanotubes (purity ≥ 95%, outer diameter 10 to 20 nm, length 10–30 μm and density 2.1 g cm⁻³) were purchased from the Neutrino company.

2.2. Sample preparation

Commercially available TPU and ABS granules were used for the composite preparation. The TPU and ABS granules were dried in an oven for approximately 6 hours at 100 °C. The TPU/ABS (80/20) composites with different MWCNT (0.5 to 5 wt%) content were produced via a mixing process using a Brabender Plasticorder internal mixer (Germany) model W50 equipped with a Banbury type rotor design. The blending temperature was maintained at 200 °C, and the rotor speed was adjusted to 60 rpm. At the first stage, TPU and pristine MWCNTs were mixed at various weight percentages for approximately 5 min. Then, ABS granules were added to the TPU/MWCNT blend. The mixing processing time was approximately 10 min. After being discharged, the polymer blend was broken into small pieces and compression molded in the shape of square plates (1 and 3 mm thick) at 200 °C and 30 MPa for 5 min using a Toyoseiki Mini Test Hydraulic Press (Japan). Then, these sheets were cooled in a hot press. The press heaters were switched off to allow the sample to cool over a period of approximately 2 h during which 30 MPa of pressure was exerted on the sample. The nanocomposite components are listed in Table 1.

Table 1 Descriptive names and components of samples

Sample	TPU (wt%)	ABS (wt%)	MWCNT (wt%)
TPU/ABS	80	20	0
TPU/ABS/0.5CNT	79.5	20	0.5
TPU/ABS/1CNT	79	20	1
TPU/ABS/2CNT	78	20	2
TPU/ABS/3CNT	77	20	3
TPU/ABS/5CNT	75	20	5

---

2.3. Mechanical characterization

Tensile testing was performed at room temperature using a HIWA200 with standard dumbbell specimens according to ASTM D638 (Type IV). The dumbbells were punched out of the compressed polymer sheets. The tests were performed with a crosshead speed of 150 mm min⁻¹ to measure the tensile modulus, yield strength, and elongation at break of the samples at each loading level. For each sample, the averages of the results obtained from 3 individual specimens are reported.

2.4. Impact test

For the Izod impact tests, the produced composites were cut into samples with dimensions of 63.5 × 12.7 × 3.2 mm. The test was performed according to ASTM D256 on notched samples using a Zwick Impact Pendulum Tester Model 5101. For this test, the notched samples were created with a Ceast 6991. All samples were placed into the chamber for approximately 2 hours. The chamber temperature is set to −50 °C.

2.5. Shape memory properties

The shape memory test was performed using a SANTAM tensile machine model STM-50 according to the following steps: (1) record the length prior to extending (ε₀), (2) extend the sample to a strain of 100% (ε_m), (3) cool the sample to −50 °C and hold for 5 min, (4) unload the stress to zero and record the strain (ε_f), (5) heat the sample to room temperature and hold for 5 min, and (6) record the sample strain (ε_i). The shape fixing ratio (R_f) and shape recovery ratio (R_r) are determined as follows:⁵⁵where ε_m is the maximum strain reached in the cyclic tensile text, ε_f is the fixed strain of the sample that is unloaded and stabilized at −50 °C, ε_i is the final strain after the shape recovery at room temperature and N is the number of cycles.

2.6. Thermal analysis

Differential scanning calorimetry (DSC) under a nitrogen atmosphere was performed using a Mettler-Toledo DSC-1. Approximately 5 mg samples were collected from the molded sheets and encapsulated in closed-aluminum pans. The tests were performed under non-isothermal conditions by heating the sample from −100 °C to 240 °C at the rate of 10 °C min⁻¹. The samples were maintained at 240 °C for 5 min to eliminate any previous thermal history and then cooled to −100 °C at the cooling rate of 10 °C min⁻¹, and this temperature was held for 5 min. Finally, the second heating to 240 °C was performed at the same heating rate of 10 °C min⁻¹.

The thermal stability of the samples was studied using a Mettler-Toledo thermogravimetric analyzer (TGA). All specimens were heated at the heating rate of 10 °C min⁻¹ from room temperature to 600 °C under a nitrogen atmosphere.

2.7. Morphological characterization

Scanning electron microscopy (SEM) was performed using a VEGA\TESCAN to examine the fracture surface morphology of the TPU/ABS blends. The acceleration voltage for each sample was 20 kV. The fracture surface was prepared by fracturing the samples in liquid nitrogen. The fractured surfaces were etched by dipping the samples in butyl acetate (a good solvent of ABS) for 4 h at room temperature. The etched samples were subsequently dried in a vacuum oven for 6 h at 80 °C. Finally, prior to the SEM observations, all specimens were coated with a thin gold layer.

3. Result and discussions

3.1. Mechanical properties

Fig. 1 exhibits the representative stress–strain curves of TPU/ABS and the TPU/ABS/CNT nanocomposites. All samples show the typical tensile elastomer behavior without any yield, necking or strain hardening, namely, the stress increases gradually with increasing strain. The increases of the tensile strength and elongation at the break lead to the increase in the area under the stress–strain curve; this area is important in shape memory polymers because it represents the strain energy stored during stretching and drives the strain recovery upon the release of the stress in the rubbery state of the elastomer. As shown in Table 2, the integral areas were calculated for the TPU/ABS and its nanocomposites. According to the results listed in Table 2, the TPU/ABS/3CNT nanocomposite shows much higher integral area compared to TPU/ABS and the other nanocomposites, indicating that TPU/ABS/3CNT shows a much higher driving force for shape recovery in its rubbery state. However, we found that the addition of CNTs into the TPU/ABS blends induces the apparent changes of the tensile behaviors (tensile strength, Young's modulus, and elongation at break) of the nanocomposites. The results obtained from the tensile experiments for TPU/ABS and its nanocomposites are presented in Table 2. As observed, with increasing CNT content, the Young's moduli of nanocomposites are largely improved from 42.6 MPa for TPU/ABS to 108.4 MPa for TPU/ABS with 5 wt% CNTs. It was also found that the maximum tensile strength was achieved when 2 wt% CNT were added to the blend. This improvement in the modulus and tensile strength of the nanocomposites is related to the inherent stiffness of MWCNTs. Beyond this value (above 2 wt%), the tensile strength started to decrease, possibly due to the agglomeration of MWCNTs in the TPU/ABS blend. This gives rise to the defects in the blend and decreases the adhesion of the nanofiller to the blend. However, it is apparent that MWCNT decreased the toughness of the TPU/ABS blend by reducing the strain at the break dramatically from approximately 500% to 350% for the samples containing 5 wt% of MWCNT. This may be due to the poor interfacial adhesion between the MWCNTs and blend and also the defects that may be created by the addition of MWCNTs.

Fig. 1 Stress–strain curves of the TPU/ABS blend and its nanocomposites.

Table 2 Mechanical properties of the pristine TPU/ABS and its nanocomposites

Samples	Tensile strength (MPa)	Young's modulus (MPa)	Strain at break (%)	Integral area (MPa)
TPU/ABS	27.4(±1.54)	42.6(±2.25)	497.4(±9.67)	7470.4
TPU/ABS/0.5CNT	37.6(±0.11)	52.7(±1.16)	485.8(±1.58)	9808.6
TPU/ABS/1CNT	37.5(±1.23)	70.66(±3.5)	473.5(±9.95)	9937.1
TPU/ABS/2CNT	38.2(±0.56)	95.4(±2.5)	437.0(±7.13)	10 293.7
TPU/ABS/3CNT	35.7(±1.18)	102.1(±1.74)	381.7(±8.56)	8167.3
TPU/ABS/5CNT	34.7(±0.92)	108.4(±2.12)	349.6(±6.17)	7144.3

---

In this section, we investigate the effect of MWCNTs on the impact resistance of the TPU/ABS blend. As we know, a crack is generated due to an impact as the impact propagates toward a region with a low interfacial interaction. On the other hand, if the adhesion between the filler and the polymer is very strong, fillers restrict the mobility of the matrix molecules. After performing impact test, we found that with the incorporation of the MWCNTs in the TPU/ABS blend, first, impact resistance increased from 4.25 kJ m⁻² for TPU/ABS to 7.23 kJ m⁻² for TPU/ABS/2CNT, and then with the further increase in the concentration of MWCNTs above 2 wt% led to a decrease of the impact resistance to the value of 6.01 kJ m⁻² for the TPU/ABS/5CNT nanocomposite. Therefore, with the addition of more than 2 wt% of the filler to the blend, the impact strength decreases significantly. This may be due to the agglomeration of MWCNTs that leads to a poor interfacial adhesion and increases the amount of defects. The results of the impact resistance for TPU/ABS blend and its nanocomposites are shown in Fig. 2.

Fig. 2 Impact resistance for the TPU/ABS blend and its nanocomposites.

3.2. Shape memory

Then, the shape memory properties of the TPU/ABS blend and its nanocomposites were investigated. The shape memory properties can be characterized by the shape fixing (R_f) and shape recovery (R_r) ratios. In the shape memory polyurethane, the hard segment phase is responsible for the shape recovery, while the soft segment phase plays the major role in the shape fixity.³⁰ Shape-memory polymers exhibited great shape fixity and shape recovery caused by the high rubbery modulus and slow chain relaxation. Shape recovery is one of the most important parameters used to assess the quality of shape-memory polymers. It can be calculated from the results of the thermomechanical cycles according to eqn (2). The results of the shape recovery for all samples with different amounts of MWCNTs are summarized in Fig. 3. The shape recoveries of all shape memory polymers are in the range of 70 to 98%.⁵⁷ According to our obtained results, for the MWCNT addition of less than 2 wt%, the shape recovery ratios increase with increasing MWCNTs content. However, when the MWCNTs content reaches 3 wt%, the shape recovery ratios decreased faintly. This significant improvement in R_r can be due to the homogenous dispersion of MWNT (Fig. 9) in the polyurethane molecules and in particular with the hard segment regions. The MWCNTs in the TPU/ABS blend act as physical constraints that could diminish the mobility of the hard segment chains and lead to a longer relaxation time and an improvement in the shape recovery of the blend. However, with an excess loading of 2 wt%, the shape recovery is diminished because of the poor dispersion of the MWCNTs and the formation of agglomerates. As discussed in the previous section, TPU/ABS/2CNT has a much higher integral area under stress–strain curve compared with TPU/ABS nanocomposites, indicating that more energy is stored in the temporary shape. The improvement of the storage energy can increase the shape recovery ratio of the samples.

Fig. 3 Shape recovery of all samples with different amounts of MWCNT.

The shape fixity is another key parameter for assessing the ability of the shape memory polymers to maintain a temporary shape after the applied load is removed. The shape fixity can be calculated using eqn (1) from the thermomechanical cycle data. Fig. 4 is the summary of the shape fixity for all TPU/ABS nanocomposites. The shape fixity for all shape memory polymers is in the range of 65–90%.⁵⁶ According to the obtained results, due to the larger rubbery modulus, the shape fixity increased with the MWCNTs loading.⁵⁷ By the addition of MWCNT in TPU, covalent crosslinks successfully formed between CNT and soft segment chains. So the density of sub-chains in the polymer make greater and as a result the rubbery modulus increase.⁵⁸ Previous studies showed that the rubbery modulus can increase by a factor of 2 to 5 by loading only small amount of CNTs (1 to 5 wt%).³³ It means that incorporation of nano-sized fillers improve rubbery modulus and therefore by increasing strain-induce crystallization, shape fixity enhance.³³

Fig. 4 Shape recovery of all samples with different amounts of MWCNT.

Furthermore, in order to check the repeatability and durability of shape memory properties, 8 cyclic tensile test has been performed on TPU/ABS and TPU/ABS/2CNT samples and the long term shape memory property of these samples has been investigated. The shape recovery and fixity ratio of TPU/ABS and TPU/ABS/2CNT is depicted in Fig. 5. The results showed when the cycle number increased the recovery ratio decreased in both samples. When the cycles repeat, the frozen crystals is formed in the direction of external force and this fact reduce the recovery ratio.^16,59 Also it is generally observed for thermoplastic elastomers that the shape recovery behavior of first two cycles is somehow different from the other. This fact is attributed to the distribution of crystalline phase and thermomechanical history of chain formation.^33,60

Fig. 5 Variation of shape recovery and fixity ratio of TPU/ABS and TPU/ABS/2CNT nanocomposites as a function of cycle number.

3.3. DSC analysis

In shape memory polymers, the soft domains are the reversible phase, and their crystallization and melting behaviors control the reversibility. For the nanocomposites, the crystallization and melting behaviors of the soft segments are critically influenced by the phase morphology and location of the fillers. The DSC cooling thermograms in the glass transition (T_g) region and heating thermograms of TPU/ABS and its nanocomposites are shown in Fig. 6. It may be observed that all samples show two T_g that change smoothly with MWCNTs loading. The T_g1 and T_g2 values correspond to pure TPU and pure ABS, respectively. As shown in Table 3, the T_g1 increased from −39.3 °C for TPU/ABS to −38.2 °C for TPU/ABS with 3 wt% MWCNTs because of the hindrance of the segmental motion of the TPU chains by the intercalation of MWCNTs.⁵⁶ Furthermore, the T_g2 decreased due to the poor surface interaction of the MWCNTs with the ABS molecules. These results are due to the strong interaction between TPU and MWCNTs and show that the MWCNTs prefer to be found in the TPU phase. We found that the addition of MWCNTs reduced the degree of TPU crystallinity in the blend. However, the T_m shift about 20 °C to higher temperatures for the nanocomposites containing 5 wt% of MWCNTs. These results corresponds to the formation of smaller and less ordered crystals of TPU in the presence of SWCNTs.⁶¹ Furthermore, the reduction in the value of crystallinity with the increase of SWNT content could be attributed to the reduction of the amount of TPU in the nanocomposites.⁶²

Fig. 6 DSC curves of the TPU/ABS blend and its nanocomposites (a) cooling scan and (b) second heating scan.

Table 3 Thermal properties of TPU/ABS blend and its nanocomposites obtained from DSC results

Samples	Tg1 (°C)	Tg2 (°C)	Tm (°C)	ΔHf (J g^−1)
TPU/ABS	−39.3	127.1	166.5	22.3
TPU/ABS/1CNT	−38.2	125.5	184.7	18.7
TPU/ABS/3CNT	−39.2	125.5	185.7	16.3
TPU/ABS/5CNT	−38.2	125.5	186.1	14.7

---

3.4. TGA analysis

The thermal stability data for the pure TPU/ABS and its nanocomposites in a nitrogen atmosphere are presented in Fig. 7. Fig. 7a shows the weight change as the function of the increasing temperature (TGA curves), while Fig. 7b shows the derivative of the weight change curves (DTG curves). The onset temperature of degradation (T_onset) was defined by the intersection point of the two tangent lines and the temperature at which the investigated process ends was defined as T_end. The key thermal degradation parameters were determined from the TGA diagram. Additionally, based on the DTG curves the maximum rate of weight loss can be determined that signifies the peak temperature (T_p). These thermal stability parameters determined from Fig. 7 are listed in Table 4. As observed in this table, the addition of MWCNTs increased the T_onset, T_p and T_end of TPU/ABS maximum to 4.45, 5.8 and 58.3 °C, respectively. The increases of these thermal stability parameters may be due to the high thermal stability of the MWCNTs, the thermal conductivity of the MWCNTs and the interaction between the blend and the MWCNTs.

Fig. 7 (a) Thermogravimetric analysis and (b) derivative thermogravimetric curves of the TPU/ABS blend and its nanocomposites.

Table 4 Characteristic degradation temperatures of TPU/ABS blend and its nanocomposites obtained from TGA results

Sample	Tonset (°C)	Tp (°C)	Tend (°C)	Residual weight (%)
TPU/ABS	294.26	406.67	515	4.49
TPU/ABS/1CNT	298.71	411.67	544.17	6.49
TPU/ABS/3CNT	297.55	412.5	555.1	7.45
TPU/ABS/5CNT	296.42	410.17	573.3	9.99

---

3.5. SEM and TEM observation

The content of MWCNTs has affected the microstructure of the TPU/ABS blend. For a detailed morphological study and in order to show the dispersion of CNTs in the blend transmission electron microscopy (TEM) is used.^63,64 Fig. 9 for the microtome-cut TPU/ABS/2CNT sample show the perfect dispersion of CNTs in TPU matrix. Also as it is obvious CNTs are mostly localized in TPU matrix which shows stronger interfacial interaction between CNTs and TPU.

Also, to observe the microstructure of the TPU/ABS nanocomposites by SEM micrograph, the ABS domains were preferentially etched by dipping the samples in butyl acetate for 4 h. The SEM micrographs of the freeze-fractured surfaces after etching are shown in Fig. 8, where the black domains indicate the positions of the extracted ABS phase. As the MWCNT content increased, more and smaller black domains corresponding to the ABS phase were observed in the micrographs. The ABS domains size, which depends on the quantity of MWCNTs, was determined using SEM with ImageJ software. The calculated size distributions for all of the samples are shown in Fig. 10. The SEM micrographs and size distribution data showed that the mean diameter of the ABS holes decreased from 0.91 μm for TPU/ABS/1CNT to 0.48 μm for TPU/ABS/5CNT. This occurs because, at first, MWCNTs were added to the TPU, which has a lower viscosity than the ABS,^65–67 by adding MWCNTs to the matrix, the rheological properties of the TPU change.^68,69 The MWCNT in TPU increases the viscosity tremendously during processing, and much higher stress is required to blending the ternary blends. As a result, ABS phase is broke into much smaller sizes. The covalent interaction between the MWCNTs and the polymer matrix significantly improve the interfacial interaction; therefore, they considerably influence the mechanical properties. These data confirm that the interaction of CNT with TPU is stronger than with ABS. Therefore, CNTs tend to be located in the TPU phase.

Fig. 8 SEM micrographs of TPU/ABS nanocomposites with different amounts of MWCNT.

Fig. 9 TEM images of TPU/ABS/2CNT nanocomposite.

Fig. 10 Normal size distribution of ABS particle diameters in the TPU matrix.

4. Conclusion

In this paper, the melt mixing method is used to prepare TPU/ABS nanocomposites with different MWCNT content. The thermal and mechanical properties as well as the morphology of the TPU/ABS/CNT nanocomposite were studied. The studies of the mechanical properties showed that the maximum value of tensile strength was obtained at 2 wt% while the Young's modulus is enhanced with increasing amount of CNTs in the matrix. Furthermore, the strain at the break decreased from approximately 497% for TPU/ABS to 350% for TPU/ABS/5CNT. By performing the impact test, we found that the maximum growth of impact resistance can be achieved only by the addition of 2 wt% of MWCNTs to the TPU/ABS matrix. According to the shape memory results, the shape recovery ratio of the nanocomposites increases with additional MWCNT content until 2 wt% and then decreases. We also found that the shape fixity increased with increasing amount of MWCNTs in the TPU/ABS blend due to the larger rubbery modulus. The results obtained from non-isothermal crystallization of TPU/ABS and the nanocomposites performed using the differential scanning calorimetry technique showed two distinct glass transition temperatures that change smoothly with MWCNT loading. Using the thermogravimetric analyzer, it was found that the thermal stability increases with the SWNTs content in the TPU/ABS matrix. Finally, the SEM observations and size distributions data indicate that the mean diameter of the ABS domains size decreased with increasing MWCNT content.

References

1. D. Ratna and J. Karger-Kocsis, J. Mater. Sci., 2008, 43, 254–269.
2. R. Vaia, Nat. Mater., 2005, 4, 429–430.
3. S. M. Lai and Y. C. Lan, J. Polym. Res., 2013, 20, 140.
4. Y. Liu, H. Du, L. Liu and J. Leng, Smart Mater. Struct., 2014, 23, 023001.
5. M. Raja, S. H. Ryu and A. M. Shanmugharaj, Eur. Polym. J., 2013, 49, 3492–3500.
6. G. M. Lin, G. X. Sui and R. Yang, J. Appl. Polym. Sci., 2012, 126, 350–357.
7. A. Lendlein and S. Kelch, Angew. Chem., Int. Ed., 2002, 41, 2034–2057.
8. C. Liu, H. Qin and P. Mather, J. Mater. Chem., 2007, 17, 1543–1558.
9. P. T. Mather, X. Luo and I. A. Rousseau, Annu. Rev. Mater. Res., 2009, 39, 445–471.
10. J. Hu, Advances in shape memory polymers, Elsevier, 2013.
11. H. Lu, Y. Yao, W. M. Huang, J. Leng and D. Hui, Composites, Part B, 2014, 62, 256–261.
12. H. Lv, J. Leng, Y. Liu and S. Du, Adv. Eng. Mater., 2008, 10, 592–595.
13. H. Lu, Y. Liu, J. Leng and S. Du, Smart Mater. Struct., 2009, 18, 085003.
14. Y. Liu, H. Lv, X. Lan, J. Leng and S. Du, Compos. Sci. Technol., 2009, 69, 2064–2068.
15. J. Leng, H. Lv, Y. Liu and S. Du, Appl. Phys. Lett., 2007, 91, 144105.
16. M. Raja, S. H. Ryu and A. Shanmugharaj, Colloids Surf., A, 2014, 450, 59–66.
17. J. W. Cho, J. W. Kim, Y. C. Jung and N. S. Goo, Macromol. Rapid Commun., 2005, 26, 412–416.
18. F.-P. Du, E.-Z. Ye, W. Yang, T.-H. Shen, C.-Y. Tang, X.-L. Xie, X.-P. Zhou and W.-C. Law, Composites, Part B, 2015, 68, 170–175.
19. J. Leng, X. Lan, Y. Liu, S. Du, W. Huang, N. Liu, S. Phee and Q. Yuan, Appl. Phys. Lett., 2008, 92, 014104.
20. S. Chen, S. Yang, Z. Li, S. Xu, H. Yuan, S. Chen and Z. Ge, Polym. Compos., 2015, 36, 439–444.
21. W. Huang, B. Yang, L. An, C. Li and Y. Chan, Appl. Phys. Lett., 2005, 86, 114105.
22. Y. Kagami, J. P. Gong and Y. Osada, Macromol. Rapid Commun., 1996, 17, 539–543.
23. E. Kurahashi, H. Sugimoto, E. Nakanishi, K. Nagata and K. Inomata, Soft Matter, 2012, 8, 496–503.
24. H. Meng and G. Li, Polymer, 2013, 54, 2199–2221.
25. J. Hu, Shape Memory Polymers: Fundamentals, Advances and Applications, Smithers Rapra, 2014.
26. M. Raja, A. Shanmugharaj and S. H. Ryu, Soft Mater., 2008, 6, 65–74.
27. M. Raja, A. Shanmugharaj, S. H. Ryu and J. Subha, Mater. Chem. Phys., 2011, 129, 925–931.
28. S. M. Oh, K. M. Oh, T. D. Dao, H. i. Lee, H. M. Jeong and B. K. Kim, Polym. Int., 2013, 62, 54–63.
29. J. T. Choi, T. D. Dao, K. M. Oh, H.-i. Lee, H. M. Jeong and B. K. Kim, Smart Mater. Struct., 2012, 21, 075017.
30. W. M. Huang, B. Yang and Y. Q. Fu, Polyurethane shape memory polymers, CRC Press, 2011.
31. G. Yu, H. Chen, W. Wang, Y. Zhou, J. Zhang and Y. Li, Polym. Compos., 2016 DOI:10.1002/pc.24115.
32. M. Fonseca, B. Abreu, F. Gonçalves, A. Ferreira, R. Moreira and M. Oliveira, Compos. Struct., 2013, 99, 105–111.
33. H. Koerner, G. Price, N. A. Pearce, M. Alexander and R. A. Vaia, Nat. Mater., 2004, 3, 115–120.
34. S. Hashmi, R. Abishera, H. C. Prasad, H. Bhargaw and A. Naik, Int. J. ChemTech Res., 2014, 6, 1873–1876.
35. M. Y. Razzaq and L. Frormann, polymer composites, 2007, 28, 287–293.
36. M. Haghayegh and G. Mir Mohamad Sadeghi, Polym. Compos., 2012, 33, 843–849.
37. J.-H. Wu, C.-H. Li, Y.-T. Wu, M.-T. Leu and Y. Tsai, Compos. Sci. Technol., 2010, 70, 1258–1264.
38. P. Poomalai and Siddaramaiah, J. Macromol. Sci., Part A: Pure Appl. Chem., 2005, 42, 1399–1407.
39. K. Inomata, K. Nakagawa, C. Fukuda, Y. Nakada, H. Sugimoto and E. Nakanishi, Polymer, 2010, 51, 793–798.
40. I. S. Kolesov, K. Kratz, A. Lendlein and H.-J. Radusch, Polymer, 2009, 50, 5490–5498.
41. L.-S. Wang, H.-C. Chen, Z.-C. Xiong, X.-B. Pang and C.-D. Xiong, Mater. Lett., 2010, 64, 284–286.
42. S. C. Li, L. N. Lu and W. Zeng, J. Appl. Polym. Sci., 2009, 112, 3341–3346.
43. H. Zhang, H. Wang, W. Zhong and Q. Du, Polymer, 2009, 50, 1596–1601.
44. H. M. Jeong, B. K. Ahn and B. K. Kim, Eur. Polym. J., 2001, 37, 2245–2252.
45. N. Erden and S. C. Jana, Macromol. Chem. Phys., 2013, 214, 1225–1237.
46. S. Jana, Polymers, 2014, 6, 1008–1025.
47. X. Jing, H.-Y. Mi, M. R. Salick, T. Cordie, W. C. Crone, X.-F. Peng and L.-S. Turng, J. Cell. Plast., 2014, 50, 361–379.
48. H. M. Jeong, J. H. Song, S. Y. Lee and B. K. Kim, J. Mater. Sci., 2001, 36, 5457–5463.
49. N. G. Sahoo, Y. C. Jung, N. S. Goo and J. W. Cho, Macromol. Mater. Eng., 2005, 290, 1049–1055.
50. X. Jing, H. Y. Mi, X. F. Peng and L. S. Turng, Polym. Eng. Sci., 2015, 55, 70–80.
51. M. Behl, U. Ridder, Y. Feng, S. Kelch and A. Lendlein, Soft Matter, 2009, 5, 676–684.
52. S. H. Ajili, N. G. Ebrahimi and M. Soleimani, Acta Biomater., 2009, 5, 1519–1530.
53. N. G. Sahoo, Y. C. Jung, H. J. Yoo and J. W. Cho, Compos. Sci. Technol., 2007, 67, 1920–1929.
54. G. Demma, E. Martuscelli, A. Zanetti and M. Zorzetto, J. Mater. Sci., 1983, 18, 89–102.
55. A. Saralegi, S. C. Fernandes, A. Alonso-Varona, T. Palomares, E. J. Foster, C. Weder, A. Eceiza and M. A. Corcuera, Biomacromolecules, 2013, 14, 4475–4482.
56. Q. Meng, J. Hu and S. Mondal, J. Membr. Sci., 2008, 319, 102–110.
57. M. Ahmad, J. Luo, B. Xu, H. Purnawali, P. J. King, P. R. Chalker, Y. Fu, W. Huang and M. Miraftab, Macromol. Chem. Phys., 2011, 212, 592–602.
58. S. Han and B. C. Chun, Composites, Part A, 2014, 58, 65–72.
59. M. Raja, S. H. Ryu and A. Shanmugharaj, Eur. Polym. J., 2013, 49, 3492–3500.
60. M. C. Boyce, S. Socrate, K. Kear, O. Yeh and K. Shaw, J. Mech. Phys. Solids, 2001, 49, 1323–1342.
61. Y. Mosleh, N. G. Ebrahimi, A. Mahdavian and M. Ashjari, Iran. Polym. J., 2014, 23, 137–145.
62. A. Fereidoon, M. G. Ahangari and S. Saedodin, J. Macromol. Sci., 2009, 48, 196–211.
63. S. K. Yadav and J. W. Cho, Appl. Surf. Sci., 2013, 266, 360–367.
64. G. Sun, G. Chen, J. Liu, J. Yang, J. Xie, Z. Liu, R. Li and X. Li, Polymer, 2009, 50, 5787–5793.
65. A. Barick and D. Tripathy, Appl. Clay Sci., 2011, 52, 312–321.
66. S. Jafari, P. Pötschke, M. Stephan, H. Warth and H. Alberts, Polymer, 2002, 43, 6985–6992.
67. X.-Q. Liu, Y. Wang, W. Yang, Z.-Y. Liu, Y. Luo, B.-H. Xie and M.-B. Yang, J. Mater. Sci., 2012, 47, 4620–4631.
68. X.-Q. Liu, W. Yang, B.-H. Xie and M.-B. Yang, Mater. Des., 2012, 34, 355–362.
69. J.-P. Cao, J. Zhao, X. Zhao, F. You, H. Yu, G.-H. Hu and Z.-M. Dang, Compos. Sci. Technol., 2013, 89, 142–148.

---