Thermoplastic polyurethane films reinforced with carbon nanotubes: The effect of processing on the structure and mechanical properties

European Polymer Journal 49 (2013) 379–388

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Macromolecular Nanotechnology

Thermoplastic polyurethane films reinforced with carbon nanotubes: The effect of processing on the structure and mechanical properties P. Russo a,b,⇑, M. Lavorgna c, F. Piscitelli c, D. Acierno a, L. Di Maio d a

Department of Materials and Production Engineering, University of Naples Federico II, P.le Vincenzo Tecchio 80, 80125 Naples, Italy Institute of Chemistry and Technology of Polymers, National Council of Research, Via Campi Flegrei 34, 80078 Pozzuoli (Na), Italy Institute of Composite and Biomedical Materials, National Council of Research, P.le E. Fermi 1, 80055 Portici (Na), Italy d Department of Industrial Engineering, University of Salerno, Via Ponte don Melillo, 80099 Fisciano (Sa), Italy b

a r t i c l e

i n f o

Article history: Received 9 June 2012 Received in revised form 7 November 2012 Accepted 18 November 2012 Available online 1 December 2012 Keywords: Thermoplastic polyurethane Multiwalled carbon nanotubes Nanocomposites Filming process

a b s t r a c t The influence of the main film production technologies, i.e. chill-roll extrusion and film blowing on the structural characteristics and mechanical performances of films based on a commercial thermoplastic polyurethane resin reinforced with carbon nanotubes has been studied. Structural investigations by means of X-ray diffraction and FTIR spectroscopic analysis have shown how the different processing conditions determine the mutual arrangement of soft and hard domains characteristic of the polyurethane matrix as well as the orientation and the final distribution of the included nanotubes as confirmed by electron microscopy observations. The higher the carbon nanotubes content, the higher the content and the size of segregated hard domains. Furthermore the film blowing process, characterized by relatively longer cooling times with respect to the extrusion process, allows a better self-assembly of hard domains maximizing the interdomain distance and, apparently, ensuring a worse distribution of the filler. By focusing on samples with 0.5 wt.% of carbon nanotubes, an increase of the tensile modulus with respect to neat TPU ones, approximately equal to 90% and 30% has been shown for flat and blown films, respectively. Morphological and structural considerations have provided a reasonable explanation of the mechanical behavior exhibited by the investigated films. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Thermoplastic polyurethanes (TPUs) are linear block copolymers characterized by hard and soft segments. The hard segments (HS) are made from diisocyanate while the soft segments (SS) consist of long flexible polyether or polyester chains which interconnect two hard segments. In particular, the hard segments act as multifunctional tie points working as both physical crosslinks and reinforcing ⇑ Corresponding author at: Department of Materials and Production Engineering, University of Naples Federico II, P.le Vincenzo Tecchio 80, 80125 Naples, Italy. Tel.: +39 0817682268; fax: +39 0817682404. E-mail address: [email protected] (P. Russo). 0014-3057/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.eurpolymj.2012.11.008

fillers, while the soft segments primarily influence the elastic properties of TPU. Thanks to their particular chemical structure, these materials have various interesting properties such as: wide range of service temperatures and harness options, excellent tear and abrasion resistance, good resistance to nonpolar solvents, high compression and tensile strength. These properties make TPUs suitable for several products such as automotive components, medical devices, food processing equipments, and so on. Despite these appealing properties, enough space is still left to the research in order to even improve and develop the fields of application for TPUs.

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c

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For instance, as widely reported in the literature [1–4], it is possible to increase and tailor some specific properties (such as mechanical or dielectric properties) of these resins through the inclusion of nanometric fillers. The use of carbon nanotubes is largely experimented as reinforcing fillers for composites thanks to their low density, excellent mechanical strength, high electrical and thermal conductivity and thermal stability. The potential benefits of carbon nanotubes as reinforcing agents in polymeric nanocomposites can be achieved only when their optimal dispersion is ensured [5–9]. However, the intrinsic nature of carbon nanotubes and their traditional methods of synthesis do not facilitate their optimal dispersion especially in viscous matrices such as polymers. Entangled and/or aggregated structures of nanotubes compacted by intermolecular van der Waals interactions are always present. At this regard, chemical modification of nanotubes is reported in the literature as a means to increase both dispersion and matrix–filler adhesion. At this regard, a key role is always played by processing conditions. In this frame, considering that the more versatile technology to get new nanocomposite materials is the melt compounding because it is compatible with already available industrial technology, special attention has to be paid to the melt mixing conditions as the screw speed and time of mixing. Usually, high shear forces and relatively long processing times have been demonstrated to be suitable to minimize nanotube aggregate formation and, thus, to enhance nanotube dispersion in various polymer matrices [10,11]. Potschke et al. [12] discussed the influence of mixing time and speed on the electrical and dielectric properties of PC/MWNT composites for concentrations below and above the percolation threshold. It is reported that increasing the screw speed leads to an improvement of the filler dispersion for MWNT contents below the percolation threshold but causes a decrease of DC conductivity for higher contents of MWNT probably because the breakage of the MWNTs is enhanced. Moreover, besides the filler content the increase of mixing time improves the state of filler dispersion. Analogously, Takase [13] has shown that the carbon nanotube agglomerate sizes are reduced by increasing the rotation speed of a twin-screw extruder. Recently, a new approach based on the dilution of commercially available masterbatch seems to be favorable as compared to the direct nanotube incorporation [14]. However, also in this case, although the MWNT agglomerates should be almost disintegrated, the dilution process has to be carried out under appropriate processing conditions to distribute the filler and avoid remaining primary masterbatch agglomerates. In light of the above considerations, this research aims to investigate the influence of different processing conditions on performances of composite film samples prepared by two traditional technologies: chill-roll extrusion and film blowing. In particular, film manufacturing was performed from nanocomposite systems previously produced by compounding with twin screw extrusion technology. Since

the compounding conditions were optimized in order to achieve the best nanotubes dispersion, in this work we will focus on the effect of secondary processing operations with particular focus to forming and cooling methods. Actually, melting and conveying of the nanocomposite polymer through the dies was performed by making use of the same extrusion apparatus (i.e., of the same barrel and screw). Thus in this work the effect of the forming technologies was assessed. In the case of cast die extrusion, a mono-axial melt drawing (draw up ratio) and a fast cooling is performed since a chill roll is used to solidify and collect the film. On the other hand, in the case of blown film extrusion, the melt undergoes a bi-axial drawing (due to the draw up and blow up ratios), and an air cooling which is quite slower with respect to the chill-roll cooling. Since the two processes strongly characterize the final products (for instance flat films are less crystalline than blown films) a sensible effect on the nano-morphology of the produced samples was expected. The analysis of products in terms of morphological and structural features of nanostructured TPU based films allowed to relate the processing aspects to their mechanical and dynamic mechanical properties.

2. Experimental Investigated systems were based on a film grade Thermoplastic PolyUrethane Elastollan 1185 A (Polyether Type) supplied by ELASTOGRAN GmbH (q = 1.12 g/cm3) (in figures and tables coded as EL 1185A), and multiwalled carbon nanotubes (MWNTs) by Shenzen Nanotechport Co. Ltd., having an average length 5–15 lm, external diameter 20–30 nm and a specific surface area equal to: 55–65 m2/g. Nanocomposite systems based on TPU resin and filler, pre-dried under vacuum at 90 °C for 8 h were prepared by melt compounding. In particular, a HAAKE Polylab PTW 24/40 corotating twin-screw extruder with a screw of 24 mm diameter and a length to diameter (L/D) ratio of 40 was used to produce the nanocomposite systems containing 0.2, 0.5 and 1 wt.% of carbon nanotubes. Processing conditions were optimized in order to achieve the best dispersion of the nanofiller. Particularly, a flat temperature profile of 200 °C along the screws and the capillary die and a screw speed of 90 r.p.m. (revolutions per minute) were employed. The extruded materials, granulated and opportunely dried, alongside the TPU pellets were used to produce respectively nanocomposite films and neat polyurethane samples by both chill-roll extrusion and film blowing techniques to obtain flat and tubular films, respectively. Although the processability of the materials was pretty good, some problems of stickiness of the films with themselves were encountered. Thus it was necessary to adopt specific procedure during the film collection. That is, in the case of chill-roll extrusion the film collection was facilitated by coupling in line a paper strip while for the blown film a trilayer polyethylene–TPU–polyethylene film was produced. In particular, 120 lm-thick single layer films were produced by a chill-roll extrusion plant on a laboratory scale. The apparatus consisted of a single screw extruder with

P. Russo et al. / European Polymer Journal 49 (2013) 379–388

Tensile tests were performed by using an Alpha Technologies Tensometer Mod. 2020 on rectangular specimens stretched along the longitudinal direction. Experiments were carried out at room temperature. Tests were performed with a 5 KN load cell setting an initial speed of the traverse of 10 mm/min for the evaluation of the Young modulus and then moving to 50 mm/min until failure of the specimen. Dynamic–mechanical measurements were done by a DMA Tritec 2000 in tensile mode at a constant frequency of 1 Hz, over the temperature range –40–140 °C. The viscoelastic behavior of TPU systems was investigated in terms of dynamic moduli and damping ability by heating each film sample at a rate of 2°/min. 3. Results and discussion 3.1. Morphological analysis It is well known that the dispersion of nanofillers is often affected by aggregation phenomena which determine the reduction of their effectiveness. Since the probability of any aggregation of the dispersed phase increases with its content, particular attention has been devoted to the level of dispersion achieved in samples containing the highest concentration of carbon nanotubes taken into account (1% by weight). Fig. 1 compares cryo-fractured surface images of samples prepared by (a) chill-roll extrusion and (b) film blowing techniques, respectively, at the same magnification. In general, isolated carbon nanotubes embedded in the polymer matrix have been observed over the full range of nanocomposite concentrations studied. However, in terms of distribution of the filler, it seems that chill-roll extrusion technology can ensure better dispersion of the included nanotubes than blowing conditions. 3.2. Structural aspects WAXS spectra (data not shown) have indicated that all samples are completely amorphous, regardless the method

Fig. 1. Comparison of TEM micrographs of TPU based film samples containing 1% by weight of MWNT and obtained by (a) flat film extrusion and (b) film blowing, respectively.

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screw diameter (D) of 12 mm and L/D ratio of 24, a flat die of 200  0.3 mm  mm (width  thickness) and a chill roll. Extrusion temperature was set at 200 °C and chill-roll temperature was set at 15 °C. Three layers coextruded blown films were produced by making use of a laboratory apparatus which consisted of three single screw extruders similar to the above described one, equipped with a spiral mandrel coextrusion die and a take up apparatus (Collin teach line). The use of the co-extrusion process with polyethylene layers allowed to separate the polyurethane film, having a thickness approximately equal to 80 lm, after the collection. For both the processes, the temperature profile was set at 200 °C along the whole extruders and dies. Morphological features were studied by using a transmission electron microscope (FEI Tecnai G2 Spirit TWIN) equipped with an emission source LaB6. Film samples, previously incorporated into a cyanoacrylate resin, were cut with a diamond blade. The nominal thickness of the samples was 200 nm. Structural investigations were carried out by simultaneous small- and wide-angle X-ray measurements using an Anton Paar SAXSess (Austria) diffractometer (40 kV and 50 mA) equipped with a CuKa radiation (wavelength equal to 0.1542 nm) source and an image plate detector. The spectra were collected at room temperature in transmission mode. The scattering intensity as function of q vector (q = (4p/k)sinh, where k is the wavelength and 2h is the scattering angle) was corrected for the dark current, background and Porod constant. The spectra were smeared and normalized for the primary beam intensity and the sample thickness. FT-IR analysis in attenuated total reflectance (ATR) mode was performed to quantify the extent of TPU phase separation and study the effect of the multiwalled carbon nanotubes (MWNT) on the process of nucleation and growth of TPU hard domains. FTIR-ATR spectra were collected using a Nicolet 560 infrared spectrophotometer equipped with a ZnSe crystal. The spectra were collected as the average of 168 scans with a resolution of 4 cm1 over the spectral range 400–4000 cm1.

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used for the production of the films i.e., chill-roll extrusion and film blowing, and irrespective of multiwalled carbon nanotubes (MWNTs) presence. The morphology of TPU matrix, consisting of hard and soft domains, was examined by SAXS analysis in order to evaluate any effect related to film processing and MWNTs presence. The SAXS profiles for the neat TPU and the nanocomposites films prepared by chill-roll extrusion and film blowing are reported in Fig. 2(a) and (b), respectively. In all curves the broad scattering feature observed in the region 0.5–1 nm1 is associated to the interdomain distance between neighboring hard domains [7], whereas the intensity at low q values (i.e., q lower than 0.3 nm1) is ascribed to the scattering of MWNTs objects as individual tubes or small ropes [15]. In details, for the nanocomposite samples prepared by chill-roll extrusion, the higher the filler content, the higher the scattered intensity at low q values. This suggests an increase of the number of scattering objects by increasing the MWNT content. Conversely, for the nanocomposite films prepared by blowing method the intensity profiles at low q values are close each other, indicating a negligible effect of the MWNTs content at least in the range so far considered. This confirms that in the film samples produced by blowing technology the MWNTs are not completely dispersed. The carbon nanotubes likely present the initial agglomerate structure which, diffracting at q values lower than 0.1 nm1 (the lower limit of the equipment

Intensity (A.U.)

(a)

100

10

1

0.1

EL 1185A 0.2wt % MWNTs 0.5wt % MWNTs 1wt % MWNTs 1 -1

q (nm )

(b)

100

Intensity (A.U.)

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382

10

used in this work), contributes to level off the scattered intensity in the 0.1–0.3 nm1 q values region. However the intensity level off at low q value may be also ascribed to the presence of a constant number of scattering objects which does not depend on the MWNT content [16]. The interdomain spacing, L as the separation distance between TPU hard domains, has been determined from the Lorentz corrected plot, i.e. I(q)q2–q curves by using Bragg’s equation [17,18]:

L¼

2p qmax

where qmax is the scattering vector corresponding to the maximum IðqÞ  q2 value. The Lorentz corrected plots are shown in Fig. 3, whereas the corresponding values of the interdomain spacing are listed in Table 1. For either preparation methods it is observed that the higher the MWNT content the higher the interdomain spacing. Additionally, for constant MWNT content the interdomain spacing values of nanocomposite film samples prepared by blowing method are higher than those of films prepared by chill-roll extrusion. The FT-IR spectroscopy results allow to quantify the segregation of TPU hard domains through the estimation of the extent of hydrogen bonding between N–H and C=O groups belonging to hard segments. This estimation may be obtained by the absorbance of carbonyl peaks in the region from 1680 to 1750 cm1. As an example, Fig. 4 reports the carbonyl region for the nanocomposite flat films containing 0.5 wt.% of MWNT with the identification of carbonyl peaks by deconvolution method (by using Origin software). Three different peaks are observed at 1730, 1712 and 1700 cm1 which are attributed to free carbonyl (i.e., non-hydrogen bonded), hydrogen bonded carbonyl groups involved in disordered hard domains and strongly hydrogen bonded carbonyl groups involved in ordered hard domains, respectively [19]. From the absorbance values it is possible to calculate the hydrogen bonding index R as absorbance ratio, i.e. ðA1700 þ A1712 Þ=A1730 . Moreover, assuming reasonably that in the investigated spectral region the extinction values do not change with the wavenumbers [20,21], it is possible to estimate the degree of phase separation (DPS) as [20,22,23]:

DPS ¼ 1

0.1

EL 1185A 0.2wt% MWNTs 0.5wt% MWNTs 1wt% MWNTs 1 -1

q (nm ) Fig. 2. SAXS profiles for TPU and nanocomposites prepared by (a) chillroll extrusion and (b) film blowing methods.

C bonded R ¼ C bonded þ C free R þ 1

where Cbonded is the overall concentration of hydrogen bonded carbonyl groups and Cfree is the concentration of free carbonyl groups. The degree of phase separation values, reported in Table 1, slightly increases with the MWNT content for both flat and blown nanocomposite films, in agreement with the findings of Xia et al. [24]. Fig. 5 shows the interdomain spacing values as function of the degree of phase separation for TPU and the related nanocomposite films. The interdomain spacing values increase linearly with the degree of phase separation (DPS) for both types of

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Flat films: EL 1185A 0.5wt% MWNTs 1wt% MWNTs

20

I(q)q

2

Blown films: EL 1185A 0.5wt% MWNTs 1wt% MWNTs

10

0 0.5

1.0

1.5

2.0

-1

Fig. 3. Lorentz corrected plot for TPU and nanocomposites.

Table 1 Interdomain spacing and phase separation degree values of TPU and nanocomposites. Samples (MWNT content)

Interdomain spacing (nm)

Phase separation degree

Chill-roll extrusion

EL 1185A 0.2 wt.% 0.5 wt.% 1 wt.%

7.08 7.40 7.50 7.60

0.710 0.740 0.750 0.770

Film blowing

EL 1185A 0.2 wt.% 0.5 wt.% 1 wt.%

7.50 8.32 8.66 8.88

0.740 0.758 0.760 0.770

Absorbance, AU

Preparation method

1650

1675

1700

1725

1750

1775

-1

Wavenumbers, cm

Fig. 4. FTIR carbonyl region of the nanocomposite flat film sample containing 0.5 wt.% MWNT.

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q (nm )

384

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Interdomain spacing, nm

9.0

8.5 Film blowing Chill-roll extrusion

8.0

7.5

7.0

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0.71

0.72

0.73

0.74

0.75

0.76

0.77

DPS Fig. 5. Interdomain spacing values as function of degree of phase separation for neat TPU and nanocomposites prepared by chill-roll extrusion and film blowing.

nanocomposite films. In particular, the higher the DPS the higher the interdomain distance between hard and soft domains. Furthermore, the blown film samples always display DPS and interdomain spacing values slightly higher than those of the samples prepared by chill-roll extrusion. From these findings it can be inferred that irrespective of the processing technology the increase of MWNT content leads to an increment of hard domains size. Furthermore by comparing the neat samples as well as samples with identical MWNT contents, the films obtained by chill-roll procedure present a morphology mainly constituted by small sized hard domains, homogeneously distributed within the soft amorphous phase; whereas the blown films show larger hard domains. In fact the interdomain spacing values exhibited by samples prepared by film blowing are higher than those exhibited by flat films, despite the overall amounts are somewhat comparable as confirmed by DPS values. The different morphologies observed for nanocomposites film samples can be ascribed to the effect of the different cooling rates on the mechanism of nucleation, growth and self-assembly of hard domains. At this regard, it is worth to remind that in the case of film blowing, the sample cooling rate performed in air is slower with respect to the one applied during the chill-roll extrusion in which case the film sample is cooled by direct contact with a temperature controlled chill roll (Troll = 15 °C), usually located relatively near to the slit die of the extruder head. These conditions allowed to freeze the TPU morphology consisting of small nucleated hard domains dispersed throughout the matrix. At relatively low cooling rates, as for the film blowing technology, the hard segments maintain sufficient mobility for a longer time. Thus the hard segments as well

as the nucleated hard domains may further self-assembly to form larger hard domains. In order to confirm the suggested mechanism, both neat TPU and nanocomposites flat film samples with 0.5 wt.% MWNT were annealed at 85 °C for 15 min. The sample annealing was performed just to simulate the sample thermal treatment ascribed to the longer cooling step which takes place during blowing process. After the annealing treatment the samples were rapidly cooled to ambient temperature. The Lorentz corrected plots, shown in Fig. 6 confirm that, due to the annealing treatment, the distribution of the interdomain spacing values become more homogeneous as shown by the reduction of the width at half-height as well as the average interdomain spacing value increases. For instance it increases from 7.3 to 9.3 nm and 7.6 to 9.4 nm, respectively, for TPU and nanocomposite films. This confirms that low cooling rates allow the growth or aggregation of small hard domains into larger domains likely through a self-assembling mechanism [18]. This structural scenario is also confirmed by the dynamic mechanical results obtained for the annealed nanocomposite containing 0.5 wt.% of carbon nanotubes (data not shown for sake of brevity). It is shown that the storage modulus of the annealed material is lower than that of the untreated material all over the investigated temperature range. Both at –80 °C and at 20 °C this reduction is approximately equal to 32.2% and 19.5%, respectively. This softening of the material is ascribed to the TPU morphology modification. In details the growth or aggregation of hard domains brings about to a reduction of macromolecular constraints sites and the amorphous phase results less hindered to viscoelastic relaxations. This structure modification takes place both during the annealing treatment of

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P. Russo et al. / European Polymer Journal 49 (2013) 379–388 (9.37nm)

(9.345nm)

6

6

(7.63nm)

2

I(q)q

I(q)q

2

(7.319nm)

4

4

2

2 EL 1185A Annealed EL 1185A

0 0.5

1.0

0.5wt% MWNTs Annealed 0.5wt% MWNTs

0

1.5

2.0

0.5

1.0

1.5

2.0

-1

q (nm )

-1

q (nm )

flat samples and the longer cooling step of the blowing process. 3.3. Tensile test results Regarding the tensile behavior, essentially studied along the longitudinal direction, the comparison between performances of pure TPU films with respect to those containing carbon nanotubes (see Table 2) has shown significant effects related to both the included filler and the production technology. In particular, flat film samples show an increase in the stiffness and tensile strength with increasing content of carbon nanotubes at the expense of elongation at break that is reduced by more than 40%, with respect to the pure matrix, apparently especially in presence of 0.5 wt.% filler loading. For blown films, an analogous increase of the tensile stiffness is shown and the tensile strength seems to be halved with less pronounced effects about the elongation at break which slightly increases from sample with 0.2 wt.% to 1 wt.%. These facts may be explained invoking again the relationships between processing and structure features of the investigated film samples. In fact, the flat film production is usually characterized by monoaxial drawing whereas the film blowing technique is characterized by a bi-axial orientation with deformation of the polymeric stream achieved in the molten state for both technologies. Thus, under the same content and level

Table 2 Tensile parameters of all investigated film samples. Samples (MWNT content) Flat film EL 1185A 0.2 wt.% 0.5 wt.% 1 wt.% Blown film EL 1185A 0.2 wt.% 0.5 wt.% 1 wt.%

E (MPa)

ebr (%)

rmax (MPa)

7.2 ± 1.1 11.6 ± 0.6 13.7 ± 1.4 14.6 ± 0.4

791 ± 22 502 ± 43 491 ± 27 461 ± 31

24.4 ± 2.2 32.3 ± 1.9 34.7 ± 1.2 26.6 ± 1.5

10.1 ± 1.0 12.6 ± 2.8 13 .2 ± 4.1 11.1 ± 1.7

730 ± 23 574 ± 57 627 ± 56 608 ± 21

24.3 ± 3.8 9.7 ± 2.1 10.4 ± 1.7 11.9 ± 1.1

of filler distribution, under the same kind of interface and so on, the chill-roll extrusion technology may offer a greater ability to frozen the unstable structure of the matrix as well as the filler orientation along the flow direction with respect to the film blowing process. Indeed, given that generally the presence of carbon nanotubes affects the TPU morphology, some results, such as the improvement of stiffness of composite samples with respect of neat TPU based ones, can be primarily explained just in terms of a balance between the stiffening due to the filler as well as the orientation and/or organization of the dispersed phase in the amorphous regions and the TPU softening for the mutual organization of the hard matrix segments and domains. 3.4. Dynamic-mechanical properties Dynamic mechanical evaluations in terms of storage modulus are reported in Fig. 7(a) and (b) for flat and blown films, respectively. In all cases, the samples filled with carbon nanotubes exhibit a stiffening effect all over the considered temperature range. This effect, mainly shown for flat film samples, in case of blown ones appears to be relevant especially in the rubbery region. In particular, in case of flat films, benefits obtained by 0.2 wt.% of MWNTs are slightly enhanced by increasing the filler content. About blown films, instead, a content of 0.5 wt.% of MWNTs is not enough to increase the storage modulus of the film in the glassy region with a positive influence only in the rubbery zone. The doubling of the filler content leads to an up-word shift of the E0 curve. To better emphasize these considerations, the storage modulus values, estimated in the glassy and in rubbery region, respectively, for the two kind of film samples are reported in Table 3. The analysis of the viscoelastic behavior confirms the reorganization of the different phases dictated by the rate of cooling of the film and by the presence of carbon nanotubes. For blown films, the slow cooling allows the aggregation of the hard domains that, in the final structure, will take dimensions more and more big with increasing the content of nanotubes. For this, in agreement with the mor-

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Fig. 6. Lorentz corrected plots for (a) TPU and (b) 0.5 wt.% MWNT nanocomposite sample obtained by chill-roll extrusion both before and after the annealing treatment.

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386

Fig. 7. Tensile storage modulus (E’) thermograms of neat EL 1185A and its nanocomposite film samples obtained by (a) flat film extrusion and (b) film blowing, respectively.

phological data, the increase in carbon nanotubes concentration, results in an increase in the storage modulus over the entire range of temperatures only for the sample containing 1 wt.%. For lower filler contents, the same parameter shows an improvement only in the rubbery region where this effect can be interpreted as a greater consistency of the amorphous phase due to the inclusion of rigid nanoparticles. Viceversa for flat films, with morphology apparently constituted by small hard domains dispersed in the soft

phase of the matrix, the inclusion of nanotubes induces an increase in the storage modulus only in the glassy region with an effect dependent on content of the same. This result is obtained despite the simultaneous slight decrement of the stiffness of the surrounding soft phase due to the increase of hard domains size and the reduction of macromolecular constraints site. For temperatures above the glass transition, the response of the amorphous regions already well supported by the particular morphology and, therefore, with module higher than that of the blown film

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Samples(MWNT content)

E’80 °C (MPa)

EL 1185A 0.2 wt.% 0.5 wt.% 1.0 wt.%

1050 1787 70 1887 80 2156 105 Flat film

% variation respect to plain samples

E’40 °C (MPa)

% variation respect to plain samples

E’80 °C (MPa)

15.30 21.30 22.40 20.60

39 51 39

1041 1170 12 1172 13 1722 65 Blown film

Table 4 Loss factor geometric parameters of all investigated film samples. Samples

Hmax

WMH

Hmax

WMH

EL 1185A 0.2 wt.% MWNTs 0.5 wt.% MWNTs 1.0 wt.% MWNTs

0.37 0.33 0.34 0.34 Flat film

46 43 43 39

0.40 0.32 0.29 0.30 Blown film

43 41 40 40

in the same temperature region, appears to be only slightly influenced by the further addition of the reinforcing elements. For similar reasons, a lowering of the mechanical damping ability (reduction in the height of the tan d signal) has been detected for both nanocomposite blown film and flat film samples with respect to neat TPU based ones. More precisely, considering some geometric parameters of the signal damping, such as the intensity (Hmax) and breadth at half-height (WMH), collected in Table 4, both are affected and, specifically, reduced by the inclusion of nanotubes. In particular, the inclusion of the filler mainly affects the peak intensity in case of flat films and the amplitude of the signal for blown films. This behavior can be again attributed to the mutual dispersion of hard and soft segments of the matrix as well as to the level of distribution of carbon nanotubes that likely constrain the dissipative macromolecular movements of the host matrix chains. In particular the effect of carbon nanotubes constraints seem to be more relevant since the morphology of TPU matrix exhibits a reduction of the macromolecular constraint sites due to the aggregation of hard domains promoted from carbon nanotubes presence. In particular about reinforced flat films, the inclusion of the nanotubes has resulted in a lowering of the damping signal tan d with respect to the reference neat TPU sample, with an effect especially less marked than in the case of blown films as well as independent of the content of reinforcement. 4. Conclusion TPU based films, containing multi-walled carbon nanotubes were prepared by two typical production technologies: chill-roll extrusion and film blowing. The results have shown that processing technology, in addition to influencing the dispersion of included nanotubes, also affects the mutual arrangement of hard and soft domains characterizing the polyurethane matrix and, therefore, the potential applications of produced films. In details

% variation respect to plain samples

E’40 °C (MPa)

% variation respect to plain samples

10.6 19.8 22.1 24.3

92 108 129

from morphological analyses it seems that the chill-roll extrusion conditions are able to provide better dispersion of nanotubes with respect to film blowing ones, at the same filler loading and extrusion temperatures. Furthermore the film obtained by chill-roll extrusion show smaller hard domains dispersed throughout the amorphous soft phase which acting as macromolecular constraints site contribute to stiffen the structure. On the other side the films obtained by blowing process show comparable amount of hard phase exhibiting bigger domain sizes. In both case the size of hard domains increases with the carbon nanotube content. For this reason the blowing films exhibit lower elastic modulus and higher elongation at break compared to the flat films. References [1] H. Koerner, J. Kelley, J. George, L. Drummy, P. Mirau, N.S. Bell, J.W.P. Hsu, R.A. Vaia, ZnO nanorod – Thermoplastic polyurethane nanocomposites: morphology and shape memory performance, Macromolecules 42 (2009) 8933–2942. [2] Chavarria F, Paul DR. Morphology and properties of thermoplastic polyurethane nanocomposites: effect of organoclay structure. Polymer 2006;47:7760–73. [3] Mishra K. Ananta, P.R. Rajamohanan, Golok B. Nando, S. Chattopadhyay, Structure–property of thermoplastic– polyurethane–clay nanocomposite based on covalent and dualmodified laponite, Adv. Sci. Lett. 4 (2011) 65–73. [4] Barick AK, Tripathy DK. Effect of organically modified layered silicate nanoclay on the dynamic viscoelastic properties of thermoplastic polyurethane nanocomposites. App. Clay Sci. 2011;52:312–21. [5] Cadek M, Coleman JN, Ryan KP, Nicolosi V, Bister G, Fonseca A, et al. Reinforcement of polymers with carbon nanotubes: the role of nanotube surface area. Nano Lett. 2004;4:353–6. [6] Hunley MT, Potschke P, Long TE. Melt dispersion and electrospinning of non-functionalized multiwalled carbon nanotubes in thermoplastic polyurethane. Macromol. Rapid Communication 2009;30:2102–6. [7] Zhan YH, Patel R, Lavorgna M, Piscitelli F, Khan A, Xia HS, et al. Processing of polyurethane/carbon nanotubes composites using novel minimixer. Plast. Rubb. & Comp. 2010;39:400–10. [8] Smart S, Fania D, Milev A, Kamali Kannangara GS, Lu M, Martin D. The effect of carbon nanotube hydrophobicity on the mechanical properties of carbon nanotube-reinforced thermoplastic polyurethane nanocomposites. J. App. Polym. Sci. 2010;117:24–32. [9] Chen W, Tao X, Liu Y. Carbon nanotube-reinforced polyurethane composite fiber. Comp. Sci. Tech. 2006;66:3029–34. [10] Lin B, Sundararaj U, Potschke P. Melt mixed of polycarbonate with multi-walled carbon nanotubes in miniature mixers. Macromol. Mater. Eng. 2006;29(3):227–38. [11] Li Y, Shimizua H. High-shear processing induced homogeneous dispersion of pristine multiwalled carbon nanotubes in a thermoplastic elastomer. Polymer 2007;48(8):2203–7. [12] Potschke P, Dudkin SM, Alig I. Dielectric spectroscopy of melt processed polycarbonate-multiwalled carbon nanotube composites. Polymer 2003;44(17):5023–30. [13] H. Takase, High conductive PC/CNT composites with ideal dispersibility. In: Schulte K. Ed. Carbon nanotube (CNT) – Polymer Composites International Conference. Hamburg, Germany: TUHH, ISBN 3-930400-73-1, 2005.

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Table 3 Dynamic–mechanical parameters for all investigated film samples.

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