Tackiness of an electrically conducting polyurethane–nanotube nanocomposite

International Journal of Adhesion & Adhesives 30 (2010) 609–614

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Tackiness of an electrically conducting polyurethane–nanotube nanocomposite ˜ oz, Anton Santamarı´a n Mercedes Ferna´ndez, Maite Landa, M.Eugenia Mun Polymer Science and Technology Department and Polymer Institute POLYMAT, Faculty of Chemistry, University of the Basque Country UPV/EHU, P.O. Box 1072, ´n, Spain 20080 San Sebastia

a r t i c l e in fo

abstract

Article history: Accepted 29 May 2010 Available online 9 June 2010

Electrically conducting adhesive nanocomposites are obtained dispersing multiwall carbon nanotubes (MWCNT) in a thermoplastic polyurethane (TPU) by a melt mixing method. The liaison between the viscoelastic properties of the nanocomposites and their tackiness is investigated. Probe-tack results obtained at a low strain rate obey a polymer/nanotube interaction mechanism that favours tackiness, since maximum stress, strain at failure and adhesion energy increase with MWCNT concentration. However, in experiments at a high strain rate, the polymer entanglement network plays the principal role and MWCNTs only act diminishing the deformabilty of the network and reducing strain at failure and adhesion energy. & 2010 Elsevier Ltd. All rights reserved.

Keywords: Hot melt Electrical properties Composites Polyurethane

1. Introduction Multiblock polyurethanes are used as thermoplastic elastomers in many industrial applications, receiving the name of thermoplastic polyurethane elastomers, or simply thermoplastic polyurethanes (TPU). The thermodynamic incompatibility between the urethane and polyol blocks, which constitute, respectively, hard and soft segments, induces microphase separation, although this is difficult to detect [1] because of crystallization of either soft or hard segment domains [2,3]. The phase separation and subsequent crystallization lies on the basis of the use of TPUs as hot melt adhesives. At temperatures above the melting temperature of the crystallized phase the polymer is sticky or tacky, allowing immediate adhesion, whereas at room temperature the system crystallizes giving rise to a permanent weld. Advances on a theory of tack or tackiness are relatively recent [4–9]. A measure of tack is given by the energy dissipated in the debonding process in the so-called probe-tack tests. As recalled by Creton and Leibler [4], a relevant aspect is that this energy is several orders of magnitude larger than the thermodynamic work of adhesion. A few works have been published on probe-tack results of TPU hot melt adhesives [10–13]. Notwithstanding that commercial development of hot melt adhesives based on TPU has been a success in recent years [14] the possibility of dispersing carbon nanotubes (CNT) in a TPU matrix to impart a certain electrical conductivity to the adhesive has been ignored in the literature. Furthermore, in spite of the

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Corresponding author. Tel.: +34 943 018184; fax: + 34 943 212 236. E-mail address: [email protected] (A. Santamarı´a).

0143-7496/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijadhadh.2010.05.011

exponentially increasing number of papers on nanotube based nanocomposites [15], only three papers deal with tack properties of electrically conducting polymer/CNT nanocomposites [16–18]. In this paper novel features of electrically conducting TPU/CNT hot melt adhesives are presented, focusing on the liaison between viscoelasticity and probe-tack results.

2. Experimental part The investigated polyurethane is a thermoplastic polyurethane sample produced by Merquinsa (Spain) as PB121. This adhesive was specially designed for hot melt application. The hard segment is formed by the diisocyanate and the short chain diol and the soft segment is formed by the long chain diol. The diisocyanate used is an aromatic diisocyanate, diphenylmethane diisocyanate (MDI), the long chain diol is epsilon–polycaprolactone and the chain extender is a short-chain diol, 1,4-butanediol. The compositions from NMR are 10% hard segment and 90% soft segment. Nanotubes were multiwall carbon nanotubes (MWCNT) supplied by Cheap Tubes Inc. These nanotubes have specified diameters of D ¼30–50 nm and L¼10–20 mm and purity greater than 95%. Prior to melt mixing process, polymer powder was prepared from pellets using a Mill Retschs ZM 200. Then, the polymer and the MWCNT powder were stirred to obtain a homogeneous mixture. The powder mixture was blended in a Haake Mini-Lab twin-screw extruder (Thermo Electron Coorp., Hamburg, Germany). The blends were processed for 4 min at T¼ 115 1C and 100 rpm using a counter-rotating screw configuration. Nanocomposites with various CNT loading were produced. SEM

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3. Results and discussion The dependence of the AC conductivity on frequency is presented in Fig. 1. The general results resemble those presented in recent years in the literature [22–31] for dispersions of MWCNT in thermoplastic matrixes such as polycarbonate, polyamide/ABS blends and polypropylene. A DC conductivity, sDC, can be determined from the frequency independent plateau and a power law behaviour is observed for AC conductivity above a critical frequency, oc. The absence of a DC plateau is only revealed for TPU sample and 1 wt% MWCNT concentration nanocomposite, which leads us to conclude that the electrical percolation lies between 1 and 2 wt%. Certainly this is a relatively high percolation concentration, as compared with the thresholds given in the review of Bauhofer and Kovacs [32] for a large series of polymer/CNT nanocomposites, notwithstanding we have to consider that our dispersions are prepared by melt mixing, under conditions which are closer to industrial procedures [33]. The percolation threshold value reported for an

-3

10

10

σ (S/cm)

microphotographs (not shown) allow us to measure at the level of microscale, revealing the presence of dispersed bundles or agglomerates of approximately 6–10 nanotubes. Very few papers deal with TPU/CNT nanocomposites prepared using a melt blending process [19–21], but they refer similar blending conditions when compared to our system; major differences are found in terms of temperature which is dependent on the studied polyurethane type. The viscoelastic behaviour of compression moulded (T¼115 1C) samples was investigated using an AR G2 (TA Instruments) with parallel-plate fixture (12 mm diameter). Dynamic frequency sweep experiments in the linear regime were conducted in a wide range of temperature (70–150 1C). Probe-tack tests were carried out in a Rheometric Expansion System (ARES, Rheometric Scientific) with aluminium parallelplate fixture (8 mm diameter). The adhesive was deposited as a 300 mm thick layer and the upper plate is brought into contact with it and subsequently removed at constant rate. During the compression phase the plate was descended at 0.1 mm/s constant rate. Once the gap between plates is 50 mm, the displacement is held constant for 60 s. Experiments were conducted at T¼100 1C using two debonding rates: 0.00314 and 3.14 mm/s. The required force for debonding was measured as a function of time. The evaluation of strain e during debonding is based on the elongational strain rate obtained by the equation de/dt ¼Vdeb/h0, where Vdeb is the debonding velocity and h0 is the thickness at the beginning of the experiment. Extensional flow experiments were performed in the ARES rheometer using the extensional viscosity fixture or EVF. The EVF design is based on the original Meissner concept to elongate the sample within a confined space by expelling the sample with rotary clamps. Instead of the rotary clamps, two cylinders are used to wind up the sample: one cylinder is rotating and the other is measuring the force. The sample is mounted vertically and the sample length L0 is reduced to 12.7 mm. Compression moulded samples with 18 mm length, 10 mm width and 0.7 mm thickness were used. Hencky strains in the range 0.01–3 s  1 were used. Test temperature was set to T¼70 1C in order to avoid sagging of the sample during the measurement. AC–DC conductivity measurements were performed on the compression moulded samples (thickness 1 mm) in the frequency range between 10  2 and 107 Hz using the Dielectric Analysis option (DETA) included in ARES Rheometer coupled to a Novocontrol interface (broad band dielectric converter). Electrodes are 25 mm diameter stainless steel plates.

10

10

-5

-7

-9 TPU 1% MWCNT 2% MWCNT 4% MWCNT 6% MWCNT 8% MWCNT

-11

10

-13

10

10-2

10 0

10

2

10

4

10

6

ω (Hz) Fig. 1. Dependence of the AC conductivity on frequency for TPU sample and TPU/ MWCNT nanocomposites of the indicated concentrations (T¼ 23 1C).

106 10

5

10 4 G', G'' (Pa)

610

G' TPU G'' TPU G' 1% G'' 1% G' 2% G'' 2% G' 4% G'' 4% G' 6% G'' 6%

10 3 10 2 10 1 10 0 10 -1 10 -3

10 -2

10 -1

10 0

10 1

10 2

ω (Hz) 0

Fig. 2. Storage modulus, G , and loss modulus, G00 , as a function of frequency for TPU sample and TPU/MWCNT nanocomposites of the indicated concentrations. The frequencies marked by the arrows are equivalent, in terms of experimental time, to the strain rates applied during debonding (see Section 3).

aromatic polyester based polyurethane filled with MWCNTs [34] is 0.5% and, on the other hand, large values, such as 1.5% and 4%, have been reported for polycaprolactone, PCL [35]. A comparison with PCL is pertinent because our TPU sample contains 90% of this polymer. The effect of MWCNTs on both dynamic viscoelastic functions, elastic modulus G0 and loss modulus G00 , is presented in Fig. 2. The elastic modulus shows a plateau at low frequencies for the highest concentrations, 4 and 6 wt%, reflecting a significant alteration with respect to the terminal viscoelatic response of the TPU polyurethane. In the case of polymer/CNT nanocomposites, the existence of a practically frequency independent elastic modulus is associated with a combined carbon nanotube/ polymer network [22,23,26–40]. This has been used as a way to evaluate the mechanical or rheological percolation of a number of polymer matrixes [41]. In view of our results, a mechanical percolation threshold between 2% and 4% can be deduced. Therefore, the percolation threshold achieved by electrical measurements (see above) is lower than the value achieved by rheological measurements. This is not surprising considering that

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611

90

1.2 10 5 TPU 2% MWCNT 4% MWCNT 6% MWCNT

8 10 4 Stress (Pa)

δ (degree)

1 10 5

45

TPU 1% MWCNT 2% MWCNT 4% MWCNT 6% MWCNT

0

101

102

6 10 4 4 10 4 2 10 4

103

104

105

0

106

0

1

2

3

G* (Pa)

electrical conductivity in polymer/CNT nanocomposites results from an electron hopping/tunnelling mechanism, which implies a certain tube–tube distance [42]. The contact between polymer chains and carbon nanotubes gives rise to a chain mobility restriction, which may be investigated using a tan d relaxation observed at low frequencies [43,44]. In this paper we use the so-called Mavridis–Shroff [45] or van Gurp–Palmen [46] plots, phase angle d versus complex modulus Gn, which also reflect specific relaxations at large times (low frequencies) of heterogeneous polymer systems [47–50]. Instead of d tending to 901 as Gn decreases (as frequency does), as observed in homogeneous polymer melts with no overall restrictions to chains motion, the phase angle should decrease as viscous dissipation is reduced by the presence of carbon nanotubes. This assertion is confirmed by the decrease in phase angle observed for 4% and 6% MWCNT concentrations and hinted in the case of 2% (Fig. 3). We remark that similar phase angle values are observed for the nanocomposites and the pure TPU sample at high frequencies or high Gn values, as the entanglement plateau is approached. This implies that the maximum observed for 4% and 6% MWCNT concentrations actually separates to modes of polymer motion restrictions: chain entanglements and chain/ MWCNT interactions. Obviously, only entanglements are present in the case of the neat TPU. Polymer chain motions involving short times, like entanglement slippage, are permitted for TPU/MWCNT nanocomposites, and therefore, phase angle values are similar to those of TPU matrix at high frequencies (high Gn values in Fig. 3). But chain mobility which requires large times, typically the motion of the whole chain, is hindered by carbon nanotubes producing the decrease in d observed in Fig. 3 at low frequencies. This drastic alteration of the viscoelastic response, provoked by carbon nanotubes, may affect some properties of the nanocomposites linked to practical purposes. This concerns, for instance, the potential use of TPU/MWCNT nanocomposites as semiconducting adhesives. Actually the TPU matrix here considered is commercially employed as a hot melt adhesive, because it is tacky or sticky in the molten state and gives rise to a permanent adhesion as the polymer crystallizes on cooling at room temperature. The question posed is how the presence of nanotubes, which raises the electrical conductivity to the level of semiconductors, can change the tackiness of the matrix,

Stress (Pa)

Fig. 3. Phase angle d as a function of complex modulus Gn for TPU sample and TPU/MWCNT nanocomposites of the indicated concentrations. The oscillatory frequency increases from left to right. The arrows denote the frequencies equivalent, in terms of experimental time, to the strain rates applied during debonding (see Section 3).

4 10

5

3.5 10

5

3 10

5

2.5 10

5

4 strain

5

6

7

8

TPU 2% MWCNT 4% MWCNT 6% MWCNT

2 10 5 1.5 10

5

1 10

5

5 10

4

0

0

5

10

15

20

25

strain Fig. 4. (a) Stress–strain curves obtained in low velocity probe-tack experiments. Debonding velocity 0.00314 mm/s, which corresponds to a strain rate of 0.0628 s  1, as explained in Section 2. (b) Stress–strain curves obtained in high velocity probe-tack experiments. Debonding velocity 3.14 mm/s, which corresponds to a strain rate of 62.8 s  1, as explained in Section 2. A summary of the results is presented in Table 1.

considering the accepted liaison between viscoelasticity and tack results [7,8,51–56]. We, therefore, investigate how the modification of the viscoelasticity affects the tack results of our composites. In Fig. 4 stress–strain diagrams obtained from probe-tack experiments at two different debonding velocities are presented. The corresponding strain rates, deduced as explained in Section 2, are 0.0628 and 62.8 s  1 for Fig. 4a and b, respectively. Since the debonding strain rate is the ratio of the debonding velocity over the sample thickness it comes to be the inverse of the time involved in each probe-tack experiment. The correspondence between the debonding strain rate and the corresponding frequency (inverse of time) at which d and Gn are obtained is marked by the arrows in Fig. 3. The corresponding frequencies are also marked in G0 (o) and G00 (o) plots of Fig. 2 to facilitate the explanation of the results. The probe-take experiment carried out at the low strain rate with neat TPU gives rise to a stress–strain diagram typical of a socalled ‘‘viscous adhesion’’ [7,53] characterized by a force which goes rapidly to a maximum and then decreases slowly to zero, giving a low adhesion energy. The rest of the results, which include the nanocomposites at low and high strain rates and neat

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Table 1 Summary of tack parameters: maximum stress, ss, strain at failure, ef, and energy of adhesion Ead, determined from stress–strain results of Fig. 4a and b. vd ¼0.003 mm/s

TPU TPU + 2% CNT TPU + 4% CNT TPU + 6% CNT

vd ¼3 mm/s

Ead(J/m2)

rs(Pa)

ef

Ead(J/m2)

rs(Pa)

ef

3.3 7.84 10.8 11.3

5.12E+ 04 7.77E+ 04 1.18E+ 04 1.18E+ 04

6.5 8.0 8.83 8.94

116 116 106 111

3.63E +05 3.66E +05 3.61E +05 3.67E +05

24.0 20.7 17.8 17.2

TPU at high strain rate, follows the ‘‘viscoelastic adhesion’’ pattern [7,53] characterized by a decrease in force to a plateau value, associated with a fibrillation process. The fibrillation mechanism involved in viscoelastic-like adhesion contributes to a higher adhesion energy, which is measured as the area under the stress– strain debonding curve. The values of the adhesion energy, Ead, maximum stress, ss, and strain at failure, ef, of neat TPU and nanocomposites are presented in Table 1. The results are interpreted according to d versus complex modulus Gn results, as well as by G0 (o) and G00 (o) results. The viscous-like adhesion, observed only in the case of TPU at the low strain rate (0.0628 s  1 in Fig. 4a), is analysed in the first term. The correspondence between the low strain rate and the frequency (marked with the arrow at low Gn values) indicates that in the case of neat TPU, d phase is close to 901, denoting a clearly viscous dominant behaviour. Times involved in the debonding process correspond to G00 4G0 , as can be seen in Fig. 2. The fact that the time involved at the low strain rate experiment is longer than the relaxation time for entanglement slippage motion (disentanglement) is significant, because the effectiveness of the entanglement network to impart elasticity to the melt is practically lost. Therefore, for pure TPU the inexistence of an effective entanglement network at the low strain rate experiment reduces considerably the strength of the polymer, leading to an absence of fibrillation and to very low adhesion energy. However, when the probe-tack experiment is performed at a high strain rate (62.8 s  1 in Fig. 4b) a plateau zone is developed and viscoelastic-like adhesion is observed. The viscoelastic state that corresponds to this rapid debonding is now predominantly elastic, d o451 or G0 4G00 , because the experimental times are shorter than the times for entanglement slippage and, consequently, the effect of entanglement network prevails. It can be assumed that entanglements impart enough mechanical strength to the system to favour fibrillation and, therefore, viscoelastic-like behaviour. In the case of nanocomposites of 4% and 6% MWCNT concentration the elastic predominant state prevails (Figs. 2 and 3) at times or frequencies involved in debonding, at both, low and high strain rate. Meanwhile, a viscoelastic-like adhesion behaviour is observed, whatever be the applied debonding velocity (Fig. 4a and b). This corresponds to the existence of a combined carbon nanotube/polymer network for the nanocomposites, besides the entanglement network of the TPU matrix. The nanocomposite containing 2% MWCNT concentration constitutes an interesting limit case. Certainly the existence of a carbon nanotube/polymer network cannot be claimed, according to the results of Figs. 2 and 3 (d 4451 or G00 4G0 ), but the results of Fig. 4a indicate that a certain fibrillation (diffuse plateau in the stress curve) occurs during debonding. This is an indicator of a certain strength, which in this case should be given by isolated carbon nanotube/polymer interactions. Within this framework, we call the attention to the fact that viscoelastic criteria have been used to establish the frontier between elastic-like adhesives (with

no fibrillation and very low Ead) and adhesives able to fibrillate in probe-tack experiments [54,55]. But, to our knowledge, the viscoelastic conditions for the transition from a viscous-like adhesion to the formation of fibrils have not been investigated. Deepening on this aspect of tack is out of the scope of this work, but it is worth pointing out that, as observed in our laboratory (non-published results), for the neat TPU viscous-like adhesion occurs below d ¼881 and Gn ¼ 110 Pa (plots like those of Fig. 3), whereas above d values a plateau is observed in stress–strain curves. A quantitative analysis of the debonding experiments, in terms of the parameters presented in Table 1, leads to some apparently contradictory results. The energy of adhesion, Ead, the maximum stress, ss, and the strain at failure, ef, determined at low debonding velocity increase with nanotube concentration. However, the opposite trend is observed in Ead and ef, when experiments are carried out at high strain rate. To explain these results the role played by each interaction (polymer/polymer chain entanglement and nanotube/polymer interaction) must be analysed. When only nanotube/polymer interactions are effective, which happens at low debonding velocities, increase in the nanotube concentration produces a tackiness enhancement. A dissipation mechanism, associated to polymer/nanotube interface, which provides a high adhesion energy increase in toughness without sacrificing strength, is explained for waterborne carbon nanotube nanocomposites in a recent paper of Wang et al. [17]. Our results obtained at the low strain rate (debonding velocity 0.00314 mm/s and strain rate e ¼0.0628 s  1) respond to this polymer/nanotube interaction mechanism, since both, maximum stress and strain at failure, as well as adhesion energy, increase with MWCNT concentration. Obviously, the absence of any nanotube/matrix interface in neat TPU would justify the found low Ead, ss and ef values, as well as the lack of the stress plateau. The trend changes when both, entanglement network and carbon nanotube/polymer network, are effective, as happens at high debonding velocities and frequencies. Considerably higher Ead, ss and ef values than for low debonding velocities are obtained (compare Fig. 4a and b) and these parameters decrease as MWCNT concentration increases (Table 1). These results suggest that at the high strain rate (debonding velocity 3.14 mm/s and strain rate e ¼62.8 s  1) the entanglement network plays the most important role. The presence of carbon nanotube/polymer interactions reduces the deformability of the entanglement network, stiffening the matrix. This gives rise to a

1.2 10 5

elongational stress (Pa)

612

9 10 4

6 10 4

3 10 4

0 0

1.2

2.3

3.5

elongational strain Fig. 5. Extensional stress as a function of Hencky strain measured at T¼70 1C and extensional rate¼ 3 mm/s.

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larger resistance to elongational strain (higher values of the elastic modulus E) as the concentration of MWCNTs increases (Fig. 5). The behaviour observed at the high strain rate satisfies the viscoelastic criterion which establishes that higher values of tan d/E favour the development of fibrils [57,58]. Similar values of tan d are deduced for pure TPU and nanocomposites at the frequency corresponding to the high strain rate tack experiment (arrows in right part of Fig. 3), but the elastic modulus increases with MWCNT concentration as seen in Fig. 5. Consequently, lower tan d/E values are obtained as nanotube concentration is increased, a result which is compatible with the reduction of stress plateau extension observed in Fig. 4b.

4. Conclusion Our approach for an interpretation of the probe-tack results of TPU/MWCNT nanocomposites includes the following hypothesis: (a) at long experimental times (low debonding velocity and oscillatory frequency) the entanglement network is not effective (because entanglement slippage at large times): the strength of the matrix is not enough to facilitate fibrillation and only a viscous-like adhesion is observed. (b) At long experimental times no entanglement network effect is observed for nanocomposites, but the effect of the carbon nanotube/polymer network is noted, giving rise to a fibrillation mechanism during debonding (plateau in stress–strain curves) and viscoelastic-likes adhesion. (c) At short experimental times (high debonding velocity and oscillatory frequency) the entanglement network is manifested in neat TPU (d o451; G0 4G00 ), giving enough strength to produce a fibrillation process and viscoelastic-like adhesion. (d) At short experimental times the effect of both, carbon nanotube/polymer network and entanglement network, is noticed for nanocomposites and a viscoelastic-like adhesion is observed. The results obtained at the low strain rate obey a polymer/ nanotube interaction mechanism that favours tackiness, since maximum stress, strain at failure and, consequently, adhesion energy increase with MWCNT concentration. However, for high strain rate probe-tack experiments the entanglement network plays the principal role and MWCNTs only act diminishing the deformability of the network. This leads to a reduction of the strain at failure and adhesion energy as carbon nanotube concentration is increased.

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