Mechanical properties, crystallinity, and self-nucleation of carbon nanotube-polyurethane nanocomposites

Polymer Testing 79 (2019) 106011

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Material Properties

Mechanical properties, crystallinity, and self-nucleation of carbon nanotubepolyurethane nanocomposites

T

Mohammad Yazdia, Vahid Haddadi Asla,∗, Mohammadali Pourmohammadia, Hossein Roghani-Mamaqanib,c a

Department of Polymer Engineering and Color Technology, Amirkabir University of Technology, P.O. Box 15875-4413, Tehran, Iran Faculty of Polymer Engineering, Sahand University of Technology, P.O. Box 51335-1996, Tabriz, Iran c Institute of Polymeric Materials, Sahand University of Technology, P.O. Box 51335-1996, Tabriz, Iran b

A R T I C LE I N FO

A B S T R A C T

Keywords: Polyurethane Carbon nanotubes Nanoomposite Self-nucleation Crystallinity

Polyurethane nanocomposites were synthesized with different contents of oxidized multi-walled carbon nanotubes (MWCNTs). Physical and mechanical properties of the nanocomposites, such as crystallinity, dispersion of MWCNTs in the matrix, phase separation, modulus, and tensile strength were studied using differential scanning calorimetry, wide-angle X-ray diffraction, scanning electron microscopy, attenuated total reflection Fouriertransform infrared spectroscopy, and stress-strain analyses. The results showed that modulus and tensile strength were increased by incorporation of MWCNTs. In addition, phase separation of the hard and soft segments of the polyurethane matrix was increased by the addition of MWCNTs. The crystallization half-life was increased from 105 to 195 s; however, the Avrami index was reduced from 3.061 to 2.384 by the addition of MWCNTs. The width of crystals was affected by self-nucleation, where the nucleation density was varied by the addition of MWCNTs.

1. Introduction Thermoplastic polyurethanes have largely been used as multi-purpose materials [1,2]. Thermoplastic polyurethanes are multi-block copolymers consisting of hard and soft segments. The hard segment is made from a diisocyanate and a chain extender, while the soft segment is formed from polyethers or ester-based polyols. Due to incompatibility of the hard and soft segments, thermoplastic polyurethanes are microstructurally phase-separated at low temperatures [2–4]. Properties of polyurethanes depend on several parameters, such as chemical nature, thermal history, and hydrogen bonding. Therefore, the crystalline structure of the hard segment affects the rheological, mechanical, and thermal properties of polyurethanes [5–7]. Fillers can also result in polyurethane composites with improved properties [8–11]. Multiwalled carbon nanotubes (MWCNTs) have tremendously been used in the preparation of polymer nanocomposites because of their wide range of properties, such as high flexibility, low mass density, high aspect ratio, and exceptional mechanical and electrical properties [12–17]. Chemically modified MWCNTs have widely been used to reinforce polymers as a result of its high adhesion efficiency. Recent studies showed that functionalization of MWCNTs results in their high dispersion and distribution in the matrix [18–20]. Shamsi and coworkers ∗

showed that concentration of MWCNTs has a considerable effect on its dispersion in polyurethanes matrix [2]. Antolin-Ceron et al. showed that 0.5 wt% is the optimum concentration of MWCNTs in different nanocomposites [19]. Barick et al. showed that high dispersion and interaction of MWCNTs in the polyurethane matrix is achieved when the nanotubes were functionalized with acid [12]. The crystallization of polymer materials consists of initial nucleation and crystal growth. Two basic types of crystal nucleation are known as homogeneous and heterogeneous nucleation. Homogeneous nucleation is self-assembly of polymer molecules to form three-dimensional nuclei at temperatures below the melting point. In heterogeneous nucleation, nucleation sites are already incorporated in a sample and instantaneously activated with induction time. These sites can be an impurity and some crystalline components in the molten state. At insufficient temperature or time of melting, the remaining crystals can act as predefined nucleation sites after the next cooling known as self-nucleation or memory effect [21,22]. The memory from the previous crystalline structure can be erased by melting of a polymer for a long time [23]. The effects of self-nucleation in the crystallinity of polymers are in two forms. One is the fragments of crystals after insufficient melting which is an example of a molecular cluster formed in the melt [21,22]. The other relates to the effect of flow fields,

Corresponding author. E-mail address: [email protected] (V. Haddadi Asl).

https://doi.org/10.1016/j.polymertesting.2019.106011 Received 15 May 2019; Received in revised form 20 July 2019; Accepted 22 July 2019 Available online 23 July 2019 0142-9418/ © 2019 Elsevier Ltd. All rights reserved.

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2. Experimental

orientation, and deformation of polymer molecules in the molten state [24]. Self-nucleation is a function of many experimental conditions such as heating rate, melting temperature, storage time, etc. Melting temperature plays an important role in determination of the overall crystallinity. The overall crystallinity rate increases with increasing of the remaining nuclei or the initial degree of crystallinity [25]. The reported experimental conditions generate widely distributed self-nucleation data. Re-usable results are obtained in well-controlled test conditions [26]. The total crystallinity content is a function of linear growth rate of crystal and also initial nucleation rate. To clarify the details of the nucleation mechanism, direct observation of nucleation is necessary [23]. Fillon et al. [27] divided the temperature range of self-nucleation (TS) into three domains. The first domain is the temperature range giving full melting, and the samples do not retain the previous memory of the crystal. This phenomenon occurs when TS is several degrees higher than the polymer's melting temperature (Tm). The second domain is observed at lower self-nucleation temperatures. This region is characterized by a significant increase in the nucleation density and displacement of the peak temperature of the crystallinity (Tc) to higher values by cooling from the TS. The third domain is shown at lower than the self-nucleation temperature in minor melting and heat treatments. Fillon and coworkers studied the effect of experimental variables such as temperature and time on self-nucleation. They observed that modification of the self-nucleation temperature in the second domain significantly affects the concentration of active nuclei [28]. Fernandez et al. [29] examined the isothermal and non-isothermal crystallinity of the linear polyurethanes family. According to the crystallinity half-life, the crystallinity of polyurethane increases by increasing the number of methylene units in the polymer structure. A study on the nucleation and crystallinity of poly(ethylene oxide), polycaprolactam, and polyethylene in a wide range of copolymer types of AB and ABC by Müller et al. [30] showed that lack of a large number of heterogeneities leads to homogeneous nucleation in crystalline parts in the copolymers. The number of crystals originating from homogeneous nuclei cannot be produced by the self-nucleation in the second domain; therefore, the second domain disappears. Begenir et al. [31] prepared melting networks of elastomeric copolymers. The crystallinity kinetics of these networks in the molten state was analyzed by two different methods by differential scanning calorimetry (DSC). The Avrami kinetic parameters of crystallinity (n) and crystallization rates driven from both the models showed similar dependency on crystallization temperature, polymer type, and hardness. The value of n (2.59–3.41) shows that all the networks produced from this type of polymers have similar crystal nucleation and growth mechanisms even at different conditions. Lorenzo et al. [30] showed that the crystal fragments in the second domain are not detectable by DSC, polarized optical microscopy (PLOM), and rheology. They also showed that the second domain of self-nucleation is a transient phenomenon that may disappear by giving sufficient time at self-nucleation temperature. One of the most commonly used methods for the preparation of polyurethane nanocomposites is extruder melt compounding at high temperatures [32]. It is important to obtain an optimal temperature for the preparation of polyurethane nanocomposite. In addition, effect of MWCNTs on the self-nucleation of polyurethane has not comprehensively been studied. The main aim of this study is synthesis of polyurethane nanocomposites, studying effect of MWCNTs on the self-nucleation, and finding an optimal temperature for the preparation of nanocomposites. For this purpose, the effect of MWCNTs and its content on the crystallinity at different temperatures was investigated by the Avrami equation and also DSC analysis.

2.1. Materials Poly(tetramethylene glycol) (PTMG: Mn = 1000 g mol−1, SigmaAldrich) and 1,6-hexamethylene diisocyanate (HDI, Merck) were used as the polyurethane components. 1,4-butanediol (BD, Sigma-Aldrich) was used as the chain extender. Oxidized multi-walled carbon nanotubes (MWCNT-COOH, Sigma-Aldrich) were used as the nanofiller. According to the specifications provided by the company, the carboxyl content of MWCNT-COOH is 8 wt %, the average outer diameter is 9.5 nm, and the length is 1.5 μm. Dimethylformamide (DMF, Merck) was used as the solvent. 2.2. Synthesis methods PTMG (2.5 g) and HDI were charged into a round bottom flask (in OH to NCO molar ratio of 1:1.05) under nitrogen atmosphere and stirred at 70 °C for 4 h. The prepolymer and chain extender were mixed and poured into an aluminum mold, and the polyurethanes were postcured at 60 °C for 24 h. For this purpose, MWCNTs (0, 0.3, 0.6, and 1 wt % of polyurethane) was ultrasonically agitated in DMF and stirred in the solution of polyurethane and DMF. After evaporation of the solvent, the nanocomposites were cured in an oven. The MWCNTs content is reported in Table 1. 2.3. Measurement Hydrogen bonding among the hard segments was studied by attenuated total reflection Fourier-transform infrared spectroscopy (ATRFTIR). ATR-FTIR spectra were obtained with Equinox 55 (Bruker, Germany). The DSC thermograms were obtained with Shimadzu DSC60 (Japan) from −70 to 200 °C with a heating rate of 10 °C/min in nitrogen. Structure of the polyurethanes was evaluated using wideangle X-ray diffraction (WAXD). WAXD profiles were obtained by Siemens-D5000. Scanning electron microscopy (SEM) images were recorded by using a Vega SEM instrument (Tescan, Czech Republic). Samples were prepared by drying a drop of the sample dispersion on a mica surface using a spin coater. Tensile testing was performed with a SANTAM tensile analyzer (SANTAM SMT-50, Iran) at ambient temperature. The dimension of samples is 40 mm × 10 mm × 0.2 mm. An elongation rate was set at 40 mm/min. Self-nucleation studies were performed with Shimadzu DSC-60 in N2 atmosphere. To find the SN temperature, the following thermal protocols were applied to each sample. Thermal history was removed by heating the sample to 200 °C and keeping the sample for 1 min at this temperature. The semi-crystalline morphology was created by cooling from the melt to 25 °C at a rate of 20 °C/min. The samples were heated from 25 °C to a chosen selfseeding temperature (TS) and kept at this temperature for 1 min. The DSC cooling scan was recorded by cooling to 25 °C at 20 °C/min. After stabilization period of 1 min at 25 °C, the samples were heated from 25 to 200 °C at the rate of 20 °C/min for recording the DSC heating scan. The domain borders from Domain I to Domain II can be easily determined by examining the cooling runs from the selected TS. When the samples are in Domain I, the crystallization peak's temperature (upon cooling from TS) does not vary by varying TS. If TS values are Table 1 Designation of the samples by varying MWCNTs content.

2

Sample

MWCNTs content (wt %)

HF HC1 HC3 HC6

0 0.1 0.3 0.6

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Table 2 HBI and DSP of samples.

progressively decreased, the crystallization peak upon cooling forms specific TS; then, the sample has crossed from Domain I into Domain II. The Domain border between Domain II and Domain III can be detected by the appearance of an annealing peak on the subsequent heating scan after SN [33]. 3. Results and discussion Attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FT-IR) was used to evaluate chemical structure of polyurethane and its nanocomposites, as shown in Fig. 1. Absence of the cyanate band at about 2260 cm−1 shows that the polyurethane reaction was completed. The vibration band at 3322 cm−1 is assigned to the stretching mode of the hydroxyl groups. The characteristic bands for the amine groups hydrogen-bonded with carbonyl, the amine groups hydrogenbonded with ether, and the free amine groups appear at 2856, 2798, and 2938 cm−1, respectively [34]. In addition, the peak at 1476 cm−1 is attributed to the N–H deformation [35,36]. The free and hydrogenbonded carbonyl groups with amine groups show bands at 1684 and 1636 cm−1, respectively [37]. The band for hydrogen-bonded carbonyl groups appears like a shoulder beside the free carbonyl's band. The hydrogen-bonded carbonyl peak of the HF sample is not observed because of lower sensitivity of the ATR method. The peaks at 1105 and 1065 cm−1 are assigned to the free ether groups and the ether groups hydrogen-bonded with amine groups [38]. The peak at 1536 cm−1 is ascribed to the ethylene groups. The hydrogen bonding index (HBI) or R index (Equation (1)) was used to determine the amount of hydrogen bonding between the hard phases [19].

DSP

HC1 HC3 HC6

0.0641 0.0839 0.0843

0.0602 0.0774 0.0778

1 − X (t ) = exp[−Zt t n]

(3)

log{ −ln[1 − X (t )]} = n log t + log Zt

(4)

where, X (t), n, t, and Zt are the relative degree of crystallinity, index of Avrami, crystalization time, and constant growth rate of the crystals, respectively. From the logarithmic plot of Equation (4) (Fig. S1 in Supporting Information), n and Zt can be extracted as the slope and yintercept, respectively. As shown in Fig. S1, all the curves are divided into the initial and second stages of crystallinity. As presented in Table 3, the Avrami index in the initial and second stages of crystallinity is in the range of 2.38–3.66 and 1.32–1.77, respectively. In the second stage, the spherulites are converted from two-dimensional to the mixture of one-dimensional and two-dimensional shapes because of their penetration and compression. On the other hand, Zt is increased in the second stage, which indicates a higher crystallinity rate. With the addition of MWCNTs in the initial stage of crystallinity, decrease of n indicates tendency for the formation of spherulites. In addition, the crystallinity rate is reduced by increasing MWCNTs, which is the

(1)

where, AC]O, bonded is the height of hydrogen-bonded carbonyl peaks and AC]O, free is the height of free carbonyl group bands in the absorbance mode. The degree of phase separation (DPS) (degree of hard segments linked to hard segments) can be obtained using Equation (2) [19].

DPS = R R + 1

R or HBI

urethane moieties in the hard phase increases, which finally results in increase of DPS [19]. The difference between DPS of HC3 and HC6 is lower compared with the difference between HC1 and HC3, which can be attributed to the formation of a network between nanotubes at contents of higher than 0.3%. Differential scanning calorimetry (DSC) was used to investigate phase separation, crystallization, and melting behavior of the polyurethane and its nanocomposites. Local restructuring of the hard segments, association and mixing of the hard and soft segments, and melting of the nano- or microdomains of the hard segments are important [5]. As shown in Fig. 2, the pure polyurethane shows a peak at around −50 °C as a result of the crystallinity of polyol. The endothermic peak at about −17 °C is related to melting of the soft segment crystals. By the addition of nanotubes, this peak is slightly removed to higher temperatures. The melting enthalpy of the soft segment crystals decreases with increasing MWCNTs content, indicating that the soft segment regularity is disturbed by the nanotubes. The endothermic peak is observed at 47 °C, which is related to interaction of the hard and soft segments. This peak was not significantly changed by the addition of MWCNTs to the polyurethane matrix. In the second heating, this peak is shifted to higher temperatures and combined with the peak at higher temperatures. This suggests that phase separation is more affected by temperature than the addition of nanotubes. The exothermic peak observed at about 113 °C is related to the hard segment crystals. With the addition of MWCNTs and increasing its content, the peak is shifted to higher temperatures and crystallization enthalpy is significantly increased. This indicates that the nanotubes affected the hard phase crystallinity as a result of interaction between the functional groups of the nanotubes surface and hard phase of polyurethane. The wide endothermic peak ascribed to the melting of the hard segment is observed in 130–170 °C, which is reduced by the addition of MWCNTs content. Physical and mechanical properties of semi-crystalline polymers depend on morphology, crystallinity, and degree of crystallinity. Some of mineral reinforcing materials in polymer nanocomposites (clay, carbon nanotubes, calcium carbonate, etc) affect crystalization nucleation [39–42]. To investigate the effects of nanotubes on the crystallization kinetics, the modified Avrami equation (Equations (3) and (4)) was used.

Fig. 1. ATR-FTIR results for the HF and its nanocomposites with different MWCNTs contents.

HBI = R= A C= O,bonded/A C= O,free

Samples

(2)

The values of HBI and DPS for all the samples are shown in Table 2. Accordingly, DPS increases by increasing the amount of MWCNTs. This phenomenon is attributed to the presence of hydroxyl groups on the nanotubes. By increasing the content of MWCNTs, hydrogen bonding between the hydroxyl groups of nanotubes and carbonyl groups of the 3

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Fig. 2. DSC curves for the HF and its nanocomposites with different MWCNTs contents: a) First heating, b) second heating, and c) cooling processes.

4

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well-dispersion of MWCNTs in the polyurethane matrix. In addition, interaction of the hard phase with the nanotubes can be other reason for disappearance of the diffraction peak of MWCNTs in nanocomposites. Peak intensity of the plane (001) is also reduced in the nanocomposites; however, its intensity increases with increasing the amount of MWCNTs. The full width at half maximum (FWHM) of the peaks as an indicator of crystallinity, is greatly reduced for nanotubes by its incorporation to the polyurethane matrix. SEM images for the MWCNTs and HC1, HC3, and HC6 nanocomposites are shown in Fig. 4. Accordingly, MWCNTs with tubular structures are clearly shown in Fig. 4 (a). Uniform dispersion and distribution of the nanotubes in the matrix of HC1, HC3, and HC6 nanocomposites are shown in Fig. 4(b–d), respectively. This shows that oxidation of nanotubes results in its well dispersion in the polar polyurethane matrix because of the polar-polar interactions of the polymer chains with the carboxylic acid groups of MWCNTs and also the hydrogen bonding. Stress-strain analysis was used to study the effect of MWCNTs and its content on the mechanical properties of polyurethane nanocomposites, and the results are shown in Fig. 5. All the samples exhibit elastic behavior at low stresses, which make it possible for estimation of the Young's modulus. In addition, plastic behavior was observed at high stresses. The HF sample shows a higher modulus in comparison with the MDI-based polyurethane. The higher modulus of the HDI-based sample is due to the crystallization ability of HDI-based polyurethane [44]. Mechanical properties of the nanocomposites are dependent on the dispersion, shape, bonding ratio, and concentration of the filler [44]. As shown in Table 4, increasing the content of MWCNTs resulted in 172% increase of modulus in the HC6 sample. The high modulus of MWCNTs, which is due to its high aspect ratio as well as its structure results in higher modulus values of the nanocomposites. In addition, increasing the interaction between polyurethane chains and MWCNTs and hydrogen bonding reduce the mobility of polyurethane chains, leading to increase of the nanocomposites modulus. Because of the continuation of the soft segment elongation and its crystallinity during elongation and also rotation and orientation of the hard segment, a plateau is observed at high stresses. The slope of the plateau is decreased with the addition of MWCNTs, which is because of the reduction of crystallinity of the soft segment during elongation. Tensile strength, elongation at break, and toughness are lower in nanocomposites, because of restriction of movement of polyurethane chains by MWCNTs. However, increasing amount of MWCNTs, interaction between the nanotubes and the polyurethane, and hydrogen bonding result in higher elongation at break, tensile strength, and toughness. In heterogeneous nucleation, nuclei are already present in a dispersed manner in the sample and activated instantaneously or with an induction time. These sites can be attributed to an impurity in a bulk sample or some crystal components in the melt. Even at higher than the melting point, in insufficient temperature or time of melting, the remaining crystals can act as the predefined nucleation sites after the next cooling [21,22]. Such a phenomenon is referred to as self-nucleation or memory effect. DSC analysis is used to identify the domains of selfnucleation and evaluate the kinetics of crystallization at different temperatures. The cooling curves at various temperatures are shown in Figs. S2–S5. In addition, the results extracted from DSC curves are presented in Table 5. The results show no significant change for crystallization temperature (Tc) of polyurethane in 172–200 °C, which is related to the first domain of SN. By decreasing temperature from 171 to 165 °C, Tc is increased. Such an increment depends on the initial concentration of polymer crystal nuclei. In polymers with high concentrations of crystalline nuclei, Tc is lower (about 2–4 °C) than polymers with several times higher initial concentration of crystalline nuclei (about 30 °C) [45]. Fernandez-d'Arlas observed an increase of 30 °C in Tc in the research on self-nucleation in polyurethanes by using MDI and BD as the hard component and PTMG as the soft section [33]. Due to higher crystallinity of MDI compared to HDI and higher concentrations

Table 3 The Avrami parameters for the HF and its nanocomposites with different MWCNTs contents. Sample

n1

Zt1

n2

Zt2

t1/2 (s)

HF HC1 HC3 HC6

3.061 2.384 2.285 2.405

4.487E-6 8.545E-6 5,33E-6 2.32E-6

1.634 1.318 1.794 1.747

0.0005687 0.0007755 3,83E-5 5.41E-5

105 114 173 189

intrinsic effect of the nanotubes on the crystallinity of polyurethane. In the second stage, the crystallinity index is slightly increased by the addition of MWCNTs, which is accompanied by a significant reduction in the growth rate of the crystals. Therefore, presence of MWCNTs reduces the penetration and compression of spherulites. The half-life of crystallinity can be calculated from Equation (5). 1/ n

ln 2 ⎞ t1/2 = ⎛ ⎝ Zt ⎠ ⎜

⎟

(5)

Increase of t1/2 by the addition of MWCNTs to the polyurethane matrix indicates the spatial inhibition of the nanotubes for crystal formation. X-ray diffraction (XRD) is used to study the crystallinity of polyurethane and also distribution of MWCNTs in the nanocomposites. XRD patterns of the pure polyurethane and MWCNTs, HC1, HC3, and HC6 are shown in Fig. 3. Polyurethane shows a diffraction peak at about 2θ = 20° for the reflective plane (011) with an inner spacing of 4.44 Å, which is due to the regular structure of a short domain of hard and soft segments within the structure of the polyurethane amorphous phase [12]. The peak becomes slightly wider and its intensity decreases with the addition of MWCNTs, which means that MWCNTs affect the regular microstructure of the soft and hard phases. The XRD pattern of the MWCNTs shows a sharp diffraction peak at about 2θ = 26° and a wide peak at 2θ = 44°, which are related to the reflective planes (200) and (001), respectively with an inner spacing of 3.42 and 2.04 Å. The peaks at 2θ = 54.3 and 77.5° are respectively ascribed to the reflective planes (004) and (110) [43]. XRD patterns of the nanocomposites show all the peaks assigned to the pure MWCNTs and polyurethane. The nanotubes main diffraction peak is due to the plane (200), which is completely disappeared in XRD patterns of the nanocomposites. This can be due to the homogeneous dispersion of MWCNTs in the polyurethane matrix. Using ultrasonic mixing in the preparation of the solutions resulted in

Fig. 3. XRD pattern for the HF and its nanocomposites with different MWCNTs contents. 5

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Fig. 4. SEM images for the (a) MWCNTs, (b) HC1, (c) HC3, and (d) HC6 nanocomposites. Table 5 The effect of self-nucleation on thermal parameters. HF

Fig. 5. Stress-strain curves for the HF and its nanocomposites with different MWCNTs contents.

Modulus (MPa)

Tensile Strength (MPa)

Elongation at break (%)

Ductility (J/ m3)

HF HC1 HC3 HC6

19.71 34.87 46.45 53.69

16.37 9.90 11.74 12.81

314.38 50.53 55.36 61.89

3881.96 351.24 370.90 525.31

HC3

HC6

TS

TC

Tm

TS

TC

Tm

TS

TC

Tm

TS

TC

Tm

200 184 180 178 177 176 175 174 173 172 171 170 169 168 167 166 165 164

103 101 98 97 96 95 95 114 99 103 107 118 111 113 116 115 114 101

– 157 156 155 155 155 155 156 155 154 154 154 149 149 153 151 151 152

200 174 171 170 169 168 167 166 165 164 – – – – – – – –

133 133 134 134 135 136 134 133 127 133 – – – – – – – –

– 152 153 153 154 154 154 154 153 153 – – – – – – – –

200 184 180 178 177 176 175 174 173 172 171 170 169 168 167 166 165 164

131 133 133 133 132 132 132 130 131 131 131 129 131 131 128 126 119 126

– 154 154 155 155 156 156 154 156 156 156 153 156 156 153 155 155 152

200 174 171 170 169 168 167 166 165 164 – – – – – – – –

134 134 133 133 133 133 133 132 128 128 – – – – – – – –

– 153 153 153 154 154 154 154 154 153 – – – – – – – –

All the temperatures are in oC.

Table 4 Mechanical properties data extracted from the stress-strain analysis for the HF and its nanocomposites with different MWCNTs contents. Sample

HC1

As shown in Figs. S3–S5, increase of Tc was not observed by the addition of MWCNTs to the polyurethane matrix. The initial temperature of the third domain can be identified from the heating thermograms of DSC at various temperatures, as shown in Figs. S6–S9. In the third domain, the melting peak shows a small shoulder, which is located at 164 °C in the pure polyurethane. In the nanocomposites, the selfnucleation and nucleation agent have no significant effect on the increase of Tm. The typical example for this behavior is polypropylene with a crystallinity temperature of up to 30 °C, which its Tm increases about 1 °C or remains constant. The incomplete stability of the polymer crystals is due to the increase of the difference in Tc and Tm. This is confirmed by a slope of fewer than 45° of the experimental data of crystallinity of the Hoffman-Wicks equation in the isothermal condition. In terms of equilibrium condition, Tm=Tc and the slope of the line is 45°. In non-equilibrium conditions, the slope of the graph is less than

of crystalline nuclei, an increase of 15 °C in Tc is acceptable. The second domain of SN happened in 171 to 165 °C. In the presence of nanotubes, the second domain of self-nucleation is eliminated, which indicates that MWCNTs have a significant effect on polymer crystallinity as nucleating agents. 6

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Table 6 Effect of self-nucleation on crystallization kinetics parameters for HF.

Table 9 Effect of self-nucleation on crystallization kinetics parameters for HC6.

HF o

TS ( C)

n1

Zt1

n2

Zt2

t1/2 (s)

200 184 180 178 177 176 175 174 173 172 171 170 169 168 167 166 165 164

4.103 3.831 3.705 3.613 3.524 3.527 3.472 3.950 3.318 2.879 2.861 2.526 3.168 2.840 2.960 2.880 3.331 1.907

3.72E-9 3E-8 4.64E-8 6.55E-8 8.58E-8 5.67E-8 1.19E-7 6.43E-9 3.50E-7 3.26E-6 3.72E-6 9.36E-6 7.12E-7 3.38E-6 7.02E-7 2.35E-6 1.23E-7 4.27E-5

1.803 1.656 1.706 1.744 1.868 1.878 1.757 1.847 1.646 1.422 1.385 1.709 1.561 1.578 2.087 1.829 2.161 2.955

0.00022 0.00074 0.00052 0.00040 0.00020 0.00020 0.00035 0.00018 0.00068 0.00180 0.00202 0.00031 0.00072 0.00065 3.42E-5 0.00018 2.45E-5 2.47E-7

104 84 86 88 91 90 89 104 79 71 69 85 77 75 102 81 107 161

n1

Zt1

n2

Zt2

t1/2 (s)

200 174 171 170 169 168 167 166 165 164

3.666 2.845 2.754 2.797 2.664 2.576 2.319 2.180 2.169 2.002

6.06E-8 5.82E-6 9.36E-6 7.48E-6 1.62E-5 2.60E-5 6.46E-5 0.00010 3.37E-5 7.54E-5

1.805 1.478 1.411 1.440 1.415 1.455 1.494 1.569 2.197 2.210

0.00013 0.00117 0.00155 0.00138 0.00165 0.00144 0.00120 0.00089 3.07E-5 2.64E-5

84 61 59 60 55 52 57 58 98 89

n1

Zt1

n2

Zt2

t1/2 (s)

200 190 187 184 180 178 177 176 175 174 173 172 171 170 169 168 167 166 165 164

3.174 2.891 2.843 3.230 3.332 3.328 3.345 3.241 3.255 3.307 3.116 3.089 3.001 2.942 2.682 2.555 2.797 2.104 2.132 2.352

7.11E-7 3.68E-6 4.70E-6 5.88E-7 3.33E-7 3.57E-7 3.25E-7 5.33E-7 5.40E-7 2.66E-7 1.01E-7 1.20E-6 1.95E-6 1.65E-6 8.17E-6 1.39E-5 3.33E-6 6.70E-5 2.61E-5 1.97E-5

1.654 1.529 1.635 1.641 1.631 1.610 1.613 1.577 1.579 1.673 1.526 1.578 1.521 1.583 1.531 1.537 1.563 1.670 2.128 1.645

0.00033 0.00065 0.00056 0.00034 0.00036 0.00041 0.00042 0.00051 0.00053 0.00024 0.00070 0.00057 0.00076 0.00041 0.00076 0.00074 0.00047 0.00040 2.68E-5 0.00033

77 67 66 79 79 78 78 77 75 87 74 73 71 81 69 69 80 81 118 86

Zt1

n2

Zt2

t1/2 (s)

200 174 171 170 169 168 167 166 165 164

2.863 2.735 2.880 2.894 2.860 3.056 3.054 2.614 2.255 2.160

1.56E-6 5.38E-6 2.39E-6 2.89E-6 3.15E-6 1.10E-6 1.20E-6 6.19E-6 2.64E-5 3.70E-5

1.746 1.639 1.747 1.658 1.732 1.832 1.757 1.961 2.101 2.021

0.00016 0.00046 0.00025 0.00043 0.00031 0.00018 0.00025 9.89E-5 5.08E-5 6.75E-5

94 74 80 73 76 79 77 84 91 99

4. Conclusion Crystallinity, self-nucleation, and mechanical properties of the polyurethane nanocomposites containing different contents of MWCNTs were comprehensively studied. Modulus and tensile strength of the nanocomposites were increased by the increase of MWCNTs content. MWCNTs affected the spatial shape of the crystals and reduced the crystals dimensions and Avrami index from 3.061 to 2.384. The crystallization rate is decreased by the addition of MWCNTs, and crystallization half-life was increased from 105 to 195 s. Comparison of the Avrami equation and crystallinity temperature results shows that the presence of MWCNTs affected the nucleation density, and self-nucleation influenced the width of crystals.

Table 8 Effect of self-nucleation on crystallization kinetics parameters for HC3. TS (oC)

n1

index n. In the first domain, n decreases with decreasing temperature as a result of reduction of energy and movement of the chains. In the second domain, a significant increase in n is observed, and then the index remains constant by decreasing the temperature. In the third domain, n is decreased by the reduction of temperature similar to the first domain. The constant crystal growth rate has an inverse relationship with the Avrami index. In the first and third domains, the constant growth rate of the crystal is increased when the index reduces. In the second domain, Zt also decreases. Temperature memory has no significant effect on the crystallization half-life. Also, the second stage of crystallinity shows independent behavior relative to the temperature memory. The comparison of Avrami equation results and crystallinity temperature shows that the presence of MWCNTs affects the nucleation density, and self-nucleation influences the width of crystals.

Table 7 Effect of self-nucleation on crystallization kinetics parameters for HC1. TS (oC)

TS (oC)

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