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The investigation of exfoliation process of organic modified montmorillonite in thermoplastic polyurethane with different molecular weights
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COMPOSITES SCIENCE AND TECHNOLOGY Composites Science and Technology 68 (2008) 1815–1821 www.elsevier.com/locate/compscitech
The investigation of exfoliation process of organic modified montmorillonite in thermoplastic polyurethane with different molecular weights Xiaoyu Meng a,b, Xiaohua Du a,b, Zhe Wang a,b, Wuguo Bi a, Tao Tang a,* a
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, People’s Republic of China b Graduate School of the Chinese Academy of Sciences, Beijing 100039, China Received 30 June 2007; received in revised form 19 September 2007; accepted 29 January 2008 Available online 6 February 2008
Abstract Cloisite 30B (30B) was melt-mixed with two kinds of thermoplastic polyurethane (TPU) with different molecular weights to discern the roles of molecular diffusion and shear in the exfoliation process. The higher level of exfoliation was achieved in TPU matrix with higher molecular weight due to the appropriate viscosity. In order to have an insight into the mechanism of exfoliation, the degree of dispersion and exfoliation of 30B was characterized by wide angle X-ray diffraction and transmission electron microscopy. The layers of 30B were exfoliated via a slippage process, which was also observed in polyamide 12 nanocomposites recently. Shear played a dominating role in the process of exfoliation. The effect of morphology on rheological and mechanical properties was investigated, thus improving our understanding of TPU nanocomposites processing and optimization. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: A. Layered structures; A. Nanocomposites; A. Polymers; Exfoliation
1. Introduction Numerous works have been done on the fabrication of exfoliated organic modified montmorillonites (OMMTs)/ polymer nanocomposites via melt mixing because melt mixing technique is environmentally benign and economical favorable for industries [1–5]. Generally, the molecular diffusion and shear process are considered as the main approaches to the exfoliation of OMMTs. Molecular diffusion and shear play the different roles in different systems due to the difference of compositions in systems. On the one hand, the molecular diffusion and shear critically depend on the surface modification of montmorillonite (MMT), such as an optimal structure, size and adsorption *
Corresponding author. Tel.: +86 431 85262004; fax: +86 431 85262827. E-mail address: [email protected] (T. Tang). 0266-3538/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2008.01.012
content of surfactant. The organic modification of MMT decreases the energy of adhesion between layers, thus facilitating the molecular diffusion and shear [6,7]. The increase of the number of alkyl tails or the adsorption content of surfactants will increase the interlayer distance and weaken the attraction between adjacent platelets [8,9]. However, it also increases steric hindrance, and the molecular diffusion of polymers becomes more difficult. On the other hand, the chemical structure and melt viscosity of polymers influence the molecular diffusion and shear. Balazs proposed a structure of polymer that was beneficial to the exfoliation via molecular diffusion [10,11]. The polymer should contain fragments attracting to the MMT surface in order to promote the compatibility between the polymer and MMT; meanwhile the polymer should also contain longer fragments that are not attracted to MMT surface in order to exfoliate MMT layers. As we all known, polar polymers have stronger interaction with the surface of
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MMT than apolar polymers [12–14]. The favorable interaction makes polymers intercalate into the galleries of OMMTs easily, and shear stress can also be transferred effectively. But the resistance to diffusion of polar polymers in the interlayers also increases with the enhancement of interactions [15,16]. Therefore the exfoliation cannot be achieved via molecular diffusion in some polar matrixes [17]. In addition, the melt viscosity of polymer is also a vital factor because different melt viscosities will result in different diffusion ability and shear stress [6,18,19]. The exfoliation needs a balance between shear stress that requires high viscosity and diffusion process that needs rather low viscosity [6]. Thermoplastic polyurethane (TPU) is a kind of polar polymer having a broad application. Cloisite 30B (30B) was often used to promote the exfoliated level of MMT in TPU matrix owing to the existence of methyl bis-2-hydroxyethyl tallow as a modifier [20,21]. Generally, it was proposed that the improvement of affinity between 30B and TPU facilitated the diffusion of chains, and molecular diffusion resulted in the exfoliation of 30B [22]. However our previous work on polyamide 12 (PA12) nanocomposites [17] suggested that the stronger interaction between OMMT and PA12 blocked the further diffusion of PA12 in the interlayers although the favorable interaction facilitated the intercalation of PA12. The exfoliation of OMMT resulted from a slippage process under shear stress. TPU has similar functional groups with PA12 but different repeat units, and similar phenomenon probably occurs in TPU nanocomposites. In this work, we investigated the exfoliation mechanism via observing the intermediate states of 30B/TPU nanocomposites. The variational tendency of property with morphologies of 30B/TPU nanocomposites was also examined.
Fig. 1. WAXD profiles and TEM images of TPU nanocomposites mixed at 185 °C for 10 min: (a) 5 wt.% 30B/TPU-1; (b) 5 wt.% 30B/TPU-2.
2. Experimental part 2.1. Materials A commercial organically modified montmorillonite, Cloisite 30B (30B) with methyl bis-2-hydroxyethyl tallow as a modifier was purchased from Southern Clay Co. Two kinds of polyether-based thermoplastic polyurethane (TPU) supplied by Urethane Company at Yantai were employed in this work, and signed with TPU-1 (Mw = 177000) and TPU-2 (Mw = 98000) for TPU with higher and lower molecular weight, respectively. Both TPUs consist of a 1000 g/mol poly(tetramethylene oxide) (PTMO) soft segment with a 4,40 -methylene diphenyl diisocyanate (MDI) and 1,4-butanediol (BDO) hard segment. The hard segment concentrations of the TPU-1 and TPU-2 are 55 and 35 wt.%, and the Shore Hardness of TPU-1 and TPU2 are 85A and 75A, respectively. The materials were dried in a vacuum oven at 80 °C for 12 h before using.
Fig. 2. The complex viscosities of two kinds of TPU at 185 °C.
2.2. Sample preparation 30B was melt-mixed with the TPUs at 185 °C in a Brabender with a rotating speed of 100 rpm. The anneal experi-
Fig. 3. WAXD of 5 wt.% 30B/TPU-1 nanocomposites mixed at 185 °C for different times.
X. Meng et al. / Composites Science and Technology 68 (2008) 1815–1821
ment was performed in a quartz tube at 185 °C under N2. TPU-1 for anneal was smashed to powder and mixed with 30B in a stirring vessel at room temperature, and the mixture was pressed to a disk. All systems contained 5 wt.% 30B. 2.3. Characterization Wide angle X-ray diffraction (WAXD) was carried out with a Rigaku model Dmax 2500 with a Cu Ka radiation. The morphologies of the composites were observed by transmission electron microscope (JEM, JEOL1011) on microtome sections of the composites. Ultrathin sections were cryogenically cut at a temperature of 80 °C using a Leica Ultracut. Rheological measurements were performed on a PHYSICA MCR 300 at 185 °C under a nitrogen atmosphere. Parallel plates of 25 mm diameter were
Fig. 4. WAXD curves of 5 wt.% 30B and TPU-1 mixture annealed at 185 °C for different times.
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used and the sample disk was 1 mm thick and 25 mm in diameter. The complex viscosity and storage modulus were measured as a function of angular frequency (ranging from 0.1 to 100 rad/s). The static mechanical properties were measured with Instron 1121 tensile testing machine, and the crosshead rate was set at 20 mm/min. For each data point, five samples were tested and the average value was taken. The molecular weights of the TPUs were determined by a Waters 410 GPC. All the samples were dried in a vacuum oven at 80 °C for 12 h before characterization. 3. Results and discussion 3.1. The exfoliated process of Cloisite 30B in TPU nanocomposites In order to investigate the roles of molecular diffusion and shear in the exfoliation process, 30B was melt-mixed with the TPUs with different molecular weights, i.e. TPU1 and TPU-2. In Fig. 1, the (0 0 1) diffraction peak of 30B disappears after mixing with the TPUs. However, TEM images show that the dispersion of 30B in TPU-1 is obviously better than that in TPU-2 matrix. The platelets of 30B are homogeneously exfoliated in TPU-1 matrix, but there are still many smaller aggregates in TPU-2 matrix. The exfoliation of OMMTs requires a balance between shear stress requiring high viscosity level and diffusion process requiring low viscosity [6]. Both the viscosities of TPUs are not very high and they can quickly diffuse into the interlayers of 30B, but the melt viscosity of TPU-2 is too low to provide adequate shear stress (Fig. 2). Correspondingly 30B can be fully exfoliated in TPU-1 matrix owing to the appropriate melt viscosity of TPU-1 which
Fig. 5. TEM images of 5 wt.% 30B/TPU-1 nanocomposites mixed at 185 °C for different times: (a) 0.5 min; (b) 1 min; (c) 2 min; (d) 3 min; (e) 5 min and (f) 7.5 min.
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provides enough shear stress to exfoliate the platelets. Therefore shear probably results in the ultimate exfoliation of 30B in TPU matrix. The process of exfoliation is crucial to the research of exfoliation mechanism, thus the intermediate states of the morphology in 30B/TPU-1 nanocomposite were characterized. When mixed for 0.5 min, the position of (0 0 1) diffraction peak for 30B shifts to lower angle comparing with the peak of original 30B (Fig. 3). It suggests that TPU-1 chains diffuse into the interlayers of 30B quickly in the initial period of melt mixing due to the appropriate viscosity and favorable interaction. With the increase of mixing time, the peak only shifts slightly and then remains unchanged, whereas the intensity of peak decreases gradually. When mixed for 10 min, the diffraction peak disappears. It suggests that the further diffusion of the TPU chains in the interlayers is difficult, and the stronger interaction between 30B and TPU chains resists the further diffusion [15,16]. Therefore the exfoliation of 30B does not result from the molecular diffusion of TPU chains. In order to prove that the further molecular diffusion in 30B/TPU-1 system is difficult, the mixture of 30B and TPU-1 powder was annealed at 185 °C. The WAXD results suggest that the chains of TPU-1 intercalate into the interlayers of 30B when the mixture was annealed for 10 min (Fig. 4). However, the interlayer distance does not change even if annealing time was extended to 2 h. This means that the molecular diffusion mainly occurs in the initial period. Although the stronger interaction between 30B and TPU makes the intercalation of TPU easier, it also hinders the further diffusion of molecules in the galleries. Therefore it is difficult to achieve the exfoliation of 30B only via molecular diffusion in TPU nanocomposites. The similar phenomenon was found in polyamide 12 (PA12) nanocomposites [17], in which the exfoliation of OMMT was achieved via a slippage process of platelets. TEM images provide the evidence for the morphology evolution of 30B/TPU-1 nanocomposites (Fig. 5). When mixed for 0.5 min, many agglomerates of 30B exist in the matrix. When mixed for 1 min, the sizes of agglomerates become smaller. When mixed for 2 min, many longer particles appear in the matrix, and the aspect ratio of particles increases obviously. The results of WAXD (Fig. 3) and TEM suggest that the platelets of 30B slip under the shear field, and the longer particles are formed in the matrix (Fig. 5c). With further increase of mixing time, the platelets keep on slipping and even separate each other, thus the lengths of particles decrease again. Finally, the layers are exfoliated homogenously in the TPU matrix. The statistical results provide the detail information of slippage and help to analyze the TEM images better. Fig. 6 compares the average length and distributions of particle length in 30B/TPU nanocomposites mixed for 1, 2 and 3 min. The lengths of particles in the images were measured manually with a vernier caliper and transformed to actual sizes basing on the scale. The number average length is defined as
Fig. 6. Distribution histograms of particle length in 30B/TPU-1 nanocomposites mixed for different times at 100 rpm (a) 1 min; (b) 2 min and (c) 3 min.
P l¼
li N i N total
ð1Þ
where li is the length of a particle, N i is the number of particles with the length of li , N total is the total number of particles. When counting the length distribution, the lengths of particles were approximations, such as 84 nm and 87 nm
X. Meng et al. / Composites Science and Technology 68 (2008) 1815–1821
were thought as 80 nm and 90 nm, and the percentages of these approximations were calculated. The similar approximate method was taken in previous paper [23]. Although the statistical average length will be smaller than the true diameter [24], the change trend of distribution of particle length still provides detail information about the exfoliation process. In the statistical histograms (Fig. 6), the number average length is about 100 nm when mixed for 1 min. and the main length distribution locates at the region between about 50 and 130 nm. When mixed for 2 min, the average length of particles increases to about 135 nm, and the main distribution shifts to the region of larger length. The content of longer particle increases obviously, implying the lateral slippage of platelets under shear field occurs in the matrix. When mixed for 3 min, the average length decreases to about 116 nm, and the content of longer particle also decreases. It proves that the platelets keep on slipping and separate. Therefore the average length of particles increases firstly and then decreases. The above results show that the molecular diffusion of TPU chains mainly occurs in the initial period of mixing, and the exfoliation of 30B is achieved by a slippage process, which is
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dominated by shear stress. The process is similar to that of PA12 nanocomposites [17]. The stronger interaction between 30B and TPU improves the transferred efficiency of shear stress provided by TPU matrix. 3.2. The correlation of properties and morphology of TPU nanocomposites The rheological property is not only an important property of polymer nanocomposites, but also a powerful tool for inspecting the morphology of nanocomposites. The low frequency region of rheological curves is especially sensitive to the structure of OMMTs in nanocomposites [25,26]. When 30B is introduced into TPU-1 matrix, both the complex viscosity and storage modulus enhance greatly comparing with neat TPU at the terminal of curves, and the storage modulus of nanocomposite exhibits solid-like behavior (Fig. 7). With the increase of mixing time, the storage modulus of 30B/TPU-1 nanocomposites enhance gradually. It should be attributed to the formation of net-work via the slippage process and the exfoliated degree of 30B is improved gradually.
Fig. 7. Rheological properties of 5 wt.% 30B/TPU-1 nanocomposites mixed at 185 °C for different times: (a) complex viscosity and (b) storage modulus.
Fig. 8. The variational tendency of mechanical properties of 30B/TPU-1 nanocomposites with the increase of mixing time (a) Young’s modulus and (b) stress–strain curve.
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The result consists with those of WAXD (Fig. 3) and TEM (Fig. 5). The mechanical properties are also influenced by the morphology of nanocomposites. In Fig. 8a, the variational tendency of Young’s modulus of 30B/TPU-1 nanocomposite with mixing time coincides with the rheological properties. As the mixing time increases, the degree of exfoliation of 30B enhances (Figs. 3 and 5). Therefore the modulus of nanocomposite is improved gradually. The stress–strain curves of 30B/TPU-1 nanocomposites mixed for different times are shown in Fig. 8b. When mixed for 1 min, the tensile strength improves obviously, and the elongation decreases. Compared with the results for 1 min, the tensile strength and elongation for 3 min increase slightly, indicating the better dispersion of 30B in the matrix. However, the tensile strength and elongation decrease when the mixing time is prolonged continuously. It should be attributed to the degradation of TPU matrix. It was known that the organic modifier can accelerate the degradation of TPU [22], so the degree of degradation will increase with prolonging the mixing time. Dan et al. reported the similar phenomenon in TPU composites [20]. In their system, the tensile properties improved when the 30B/TPU was extruded firstly. However, the tensile properties decrease when extruded again because of the degradation of TPU. Therefore the long processing time is not beneficial to the tensile properties. In this work, the mixing time was prolonged to 10 min in order to discern the process of exfoliation. In the future work, the shear stress can be strengthened and thus the processing time can be shortened to decrease the degradation of polymers. 4. Conclusions A slippage process of MMT layers was observed in the exfoliation evolution of 30B/TPU nanocomposites, and the process was found in OMMT/PA12 nanocomposites before. Shear stress dominates the slippage process of MMT layers and results in the exfoliation. The molecular diffusion in the initial period of mixing decreases the attraction between OMMT layers, and increases the contacting sites between platelets and TPU, thus making OMMT platelets slip easier under shear field. The interaction between 30B and polymers efficiently transfers the shear stress provided by TPU matrix onto the OMMT layers. The appropriate viscosity of polymers ensures the intercalation of molecules and provides adequate shear stress. Therefore adequate shear stress, the appropriate molecular diffusion and interaction between OMMT and polymer are required to optimize the formation of nanocomposites. The control of morphology is crucial to the improvement of properties of nanocomposites. Acknowledgement We thank the financial supports from the National Natural Science Foundation of China for the Outstanding
Youth Fund (No. 50525311), general Project (No. 50473029) and the Fund for Creative Research Groups (No. 50621302). Also Jilin Bureau of Science and Technology supports the research (No. 20060319). References [1] Kojima Y, Usuki A, Kawasumi M, Okada A, Fukushima Y, Kurauchi T, Kamigaito O. Mechanical-properties of nylon 6-clay hybrid. J Mater Res 1993;8(5):1185–9. [2] Kawasumi M, Hasegawa N, Kato M, Usuki A, Okada A. Preparation and mechanical properties of polypropylene-clay hybrids. Macromolecules 1997;30(20):6333–8. [3] Alexandre M, Dubois P. Polymer-layered silicate nanocomposites: preparation, properties and uses of a new class of materials. Mater Sci Eng R 2000;28(1–2):1–63. [4] Ray SS, Okamoto M. Polymer/layered silicate nanocomposites: a review from preparation to processing. Prog Polym Sci 2003;28(11):1539–641. [5] Zhang YQ, Lee JH, Rhee JM, Rhee KY. Polypropylene-clay nanocomposites prepared by in situ grafting-intercalating in melt. Comp Sci Tech 2004;64(9):1383–9. [6] Bousmina M. Study of Intercalation and exfoliation processes in polymer nanocomposites. Macromolecules 2006;39(12):4259–63. [7] Kadar F, Szazdi L, Fekete E, Pukanszky B. Surface Characteristics of layered silicates: influence on the properties of clay/polymer nanocomposites. Langmuir 2006;22(18):7848–54. [8] Fornes TD, Hunter DL, Paul DR. Nylon-6 nanocomposites from alkylammonium-modified clay: the role of alkyl tails on exfoliation. Macromolecules 2004;37(5):1793–8. [9] Zhao ZF, Tang T, Qin YX, Huang BT. Effects of surfactant loadings on the dispersion of clays in maleated polypropylene. Langmuir 2003;19(18):7157–9. [10] Balazs AC, Singh C, Zhulina E. Modeling the interactions between polymers and clay surfaces through self-consistent field theory. Macromolecules 1998;31(23):8370–81. [11] Balazs AC, Singh C, Zhulina E, Lyatskaya Y. Modeling the phase behavior of polymer/clay nanocomposites. Acc Chem Res 1999;32(8):651–7. [12] Fermeglia M, Ferrone M, Pricl S. Computer simulation of nylon-6/ organoclay nanocomposites: prediction of the binding energy. Fluid Phase Equilib 2003;212(1–2):315–29. [13] Tanaka G, Goettler LA. Predicting the binding energy for nylon 6,6/ clay nanocomposites by molecular modeling. Polymer 2002;43(2):541–53. [14] Toth R, Coslanich A, Ferrone M, Fermeglia M, Pricl S, Miertus S, Chiellini E. Computer simulation of polypropylene/organoclay nanocomposites: characterization of atomic scale structure and prediction of binding energy. Polymer 2004;45(23):8075–83. [15] Manias E, Chen H, Krishnamoorti R, Genzer J, Kramer EJ, Giannelis EP. Intercalation kinetics of long polymers in 2nm confinements. Macromolecules 2000;33(21):7955–66. [16] Hu XS, Zhang WH, Si MY, Gelfer M, Hsiao B, Rafailovich M, Sokolov J, Zaitsev V, Schwarz S. Dynamics of polymers in organosilicate nanocomposites. Macromolecules 2003;36(3):823–9. [17] Meng XY, Wang Z, Zhao ZF, Du XH, Bi WG, Tang T. Morphology evolutions of organically modified montmorillonite/polyamide 12 nanocomposites. Polymer 2007;48(9):2508–19. [18] Fornes TD, Yoon PJ, Keskkula H, Paul DR. Nylon 6 nanocomposites: the effect of matrix molecular weight. Polymer 2001;42(25):9929–40. [19] Kim SW, Jo WH, Lee MS, Ko MB, Jho JY. Effects of shear on melt exfoliation of clay in preparation of nylon 6/organoclay nanocompoistes. Polym J 2002;34(3):103–11. [20] Dan CH, Lee MH, Kim YD, Min BH, Kim JH. Effect of clay modifiers on the morphology and physical properties of thermoplastic polyurethane/clay nanocomposites. Polymer 2006;47(19):6718–30.
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[24] Fornes TD, Paul DR. Modeling properties of nylon 6/clay nanocomposites using composite theories. Polymer 2003;44(17): 4993–5013. [25] Solomon MJ, Almusallam AS, Seefeldt KF, Somwangthanaroj A, Varadan P. Rheology of polypropylene/clay hybrid materials. Macromolecules 2001;34(6):1864–72. [26] Lee KM, Han CD. Rheology of organoclay nanocomposites: effects of polymer matrix/organoclay compatibility and the gallery distance of organoclay. Macromolecules 2003;36(19):7165–78.
COMPOSITES SCIENCE AND TECHNOLOGY Composites Science and Technology 68 (2008) 1815–1821 www.elsevier.com/locate/compscitech
The investigation of exfoliation process of organic modified montmorillonite in thermoplastic polyurethane with different molecular weights Xiaoyu Meng a,b, Xiaohua Du a,b, Zhe Wang a,b, Wuguo Bi a, Tao Tang a,* a
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, People’s Republic of China b Graduate School of the Chinese Academy of Sciences, Beijing 100039, China Received 30 June 2007; received in revised form 19 September 2007; accepted 29 January 2008 Available online 6 February 2008
Abstract Cloisite 30B (30B) was melt-mixed with two kinds of thermoplastic polyurethane (TPU) with different molecular weights to discern the roles of molecular diffusion and shear in the exfoliation process. The higher level of exfoliation was achieved in TPU matrix with higher molecular weight due to the appropriate viscosity. In order to have an insight into the mechanism of exfoliation, the degree of dispersion and exfoliation of 30B was characterized by wide angle X-ray diffraction and transmission electron microscopy. The layers of 30B were exfoliated via a slippage process, which was also observed in polyamide 12 nanocomposites recently. Shear played a dominating role in the process of exfoliation. The effect of morphology on rheological and mechanical properties was investigated, thus improving our understanding of TPU nanocomposites processing and optimization. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: A. Layered structures; A. Nanocomposites; A. Polymers; Exfoliation
1. Introduction Numerous works have been done on the fabrication of exfoliated organic modified montmorillonites (OMMTs)/ polymer nanocomposites via melt mixing because melt mixing technique is environmentally benign and economical favorable for industries [1–5]. Generally, the molecular diffusion and shear process are considered as the main approaches to the exfoliation of OMMTs. Molecular diffusion and shear play the different roles in different systems due to the difference of compositions in systems. On the one hand, the molecular diffusion and shear critically depend on the surface modification of montmorillonite (MMT), such as an optimal structure, size and adsorption *
Corresponding author. Tel.: +86 431 85262004; fax: +86 431 85262827. E-mail address: [email protected] (T. Tang). 0266-3538/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2008.01.012
content of surfactant. The organic modification of MMT decreases the energy of adhesion between layers, thus facilitating the molecular diffusion and shear [6,7]. The increase of the number of alkyl tails or the adsorption content of surfactants will increase the interlayer distance and weaken the attraction between adjacent platelets [8,9]. However, it also increases steric hindrance, and the molecular diffusion of polymers becomes more difficult. On the other hand, the chemical structure and melt viscosity of polymers influence the molecular diffusion and shear. Balazs proposed a structure of polymer that was beneficial to the exfoliation via molecular diffusion [10,11]. The polymer should contain fragments attracting to the MMT surface in order to promote the compatibility between the polymer and MMT; meanwhile the polymer should also contain longer fragments that are not attracted to MMT surface in order to exfoliate MMT layers. As we all known, polar polymers have stronger interaction with the surface of
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MMT than apolar polymers [12–14]. The favorable interaction makes polymers intercalate into the galleries of OMMTs easily, and shear stress can also be transferred effectively. But the resistance to diffusion of polar polymers in the interlayers also increases with the enhancement of interactions [15,16]. Therefore the exfoliation cannot be achieved via molecular diffusion in some polar matrixes [17]. In addition, the melt viscosity of polymer is also a vital factor because different melt viscosities will result in different diffusion ability and shear stress [6,18,19]. The exfoliation needs a balance between shear stress that requires high viscosity and diffusion process that needs rather low viscosity [6]. Thermoplastic polyurethane (TPU) is a kind of polar polymer having a broad application. Cloisite 30B (30B) was often used to promote the exfoliated level of MMT in TPU matrix owing to the existence of methyl bis-2-hydroxyethyl tallow as a modifier [20,21]. Generally, it was proposed that the improvement of affinity between 30B and TPU facilitated the diffusion of chains, and molecular diffusion resulted in the exfoliation of 30B [22]. However our previous work on polyamide 12 (PA12) nanocomposites [17] suggested that the stronger interaction between OMMT and PA12 blocked the further diffusion of PA12 in the interlayers although the favorable interaction facilitated the intercalation of PA12. The exfoliation of OMMT resulted from a slippage process under shear stress. TPU has similar functional groups with PA12 but different repeat units, and similar phenomenon probably occurs in TPU nanocomposites. In this work, we investigated the exfoliation mechanism via observing the intermediate states of 30B/TPU nanocomposites. The variational tendency of property with morphologies of 30B/TPU nanocomposites was also examined.
Fig. 1. WAXD profiles and TEM images of TPU nanocomposites mixed at 185 °C for 10 min: (a) 5 wt.% 30B/TPU-1; (b) 5 wt.% 30B/TPU-2.
2. Experimental part 2.1. Materials A commercial organically modified montmorillonite, Cloisite 30B (30B) with methyl bis-2-hydroxyethyl tallow as a modifier was purchased from Southern Clay Co. Two kinds of polyether-based thermoplastic polyurethane (TPU) supplied by Urethane Company at Yantai were employed in this work, and signed with TPU-1 (Mw = 177000) and TPU-2 (Mw = 98000) for TPU with higher and lower molecular weight, respectively. Both TPUs consist of a 1000 g/mol poly(tetramethylene oxide) (PTMO) soft segment with a 4,40 -methylene diphenyl diisocyanate (MDI) and 1,4-butanediol (BDO) hard segment. The hard segment concentrations of the TPU-1 and TPU-2 are 55 and 35 wt.%, and the Shore Hardness of TPU-1 and TPU2 are 85A and 75A, respectively. The materials were dried in a vacuum oven at 80 °C for 12 h before using.
Fig. 2. The complex viscosities of two kinds of TPU at 185 °C.
2.2. Sample preparation 30B was melt-mixed with the TPUs at 185 °C in a Brabender with a rotating speed of 100 rpm. The anneal experi-
Fig. 3. WAXD of 5 wt.% 30B/TPU-1 nanocomposites mixed at 185 °C for different times.
X. Meng et al. / Composites Science and Technology 68 (2008) 1815–1821
ment was performed in a quartz tube at 185 °C under N2. TPU-1 for anneal was smashed to powder and mixed with 30B in a stirring vessel at room temperature, and the mixture was pressed to a disk. All systems contained 5 wt.% 30B. 2.3. Characterization Wide angle X-ray diffraction (WAXD) was carried out with a Rigaku model Dmax 2500 with a Cu Ka radiation. The morphologies of the composites were observed by transmission electron microscope (JEM, JEOL1011) on microtome sections of the composites. Ultrathin sections were cryogenically cut at a temperature of 80 °C using a Leica Ultracut. Rheological measurements were performed on a PHYSICA MCR 300 at 185 °C under a nitrogen atmosphere. Parallel plates of 25 mm diameter were
Fig. 4. WAXD curves of 5 wt.% 30B and TPU-1 mixture annealed at 185 °C for different times.
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used and the sample disk was 1 mm thick and 25 mm in diameter. The complex viscosity and storage modulus were measured as a function of angular frequency (ranging from 0.1 to 100 rad/s). The static mechanical properties were measured with Instron 1121 tensile testing machine, and the crosshead rate was set at 20 mm/min. For each data point, five samples were tested and the average value was taken. The molecular weights of the TPUs were determined by a Waters 410 GPC. All the samples were dried in a vacuum oven at 80 °C for 12 h before characterization. 3. Results and discussion 3.1. The exfoliated process of Cloisite 30B in TPU nanocomposites In order to investigate the roles of molecular diffusion and shear in the exfoliation process, 30B was melt-mixed with the TPUs with different molecular weights, i.e. TPU1 and TPU-2. In Fig. 1, the (0 0 1) diffraction peak of 30B disappears after mixing with the TPUs. However, TEM images show that the dispersion of 30B in TPU-1 is obviously better than that in TPU-2 matrix. The platelets of 30B are homogeneously exfoliated in TPU-1 matrix, but there are still many smaller aggregates in TPU-2 matrix. The exfoliation of OMMTs requires a balance between shear stress requiring high viscosity level and diffusion process requiring low viscosity [6]. Both the viscosities of TPUs are not very high and they can quickly diffuse into the interlayers of 30B, but the melt viscosity of TPU-2 is too low to provide adequate shear stress (Fig. 2). Correspondingly 30B can be fully exfoliated in TPU-1 matrix owing to the appropriate melt viscosity of TPU-1 which
Fig. 5. TEM images of 5 wt.% 30B/TPU-1 nanocomposites mixed at 185 °C for different times: (a) 0.5 min; (b) 1 min; (c) 2 min; (d) 3 min; (e) 5 min and (f) 7.5 min.
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provides enough shear stress to exfoliate the platelets. Therefore shear probably results in the ultimate exfoliation of 30B in TPU matrix. The process of exfoliation is crucial to the research of exfoliation mechanism, thus the intermediate states of the morphology in 30B/TPU-1 nanocomposite were characterized. When mixed for 0.5 min, the position of (0 0 1) diffraction peak for 30B shifts to lower angle comparing with the peak of original 30B (Fig. 3). It suggests that TPU-1 chains diffuse into the interlayers of 30B quickly in the initial period of melt mixing due to the appropriate viscosity and favorable interaction. With the increase of mixing time, the peak only shifts slightly and then remains unchanged, whereas the intensity of peak decreases gradually. When mixed for 10 min, the diffraction peak disappears. It suggests that the further diffusion of the TPU chains in the interlayers is difficult, and the stronger interaction between 30B and TPU chains resists the further diffusion [15,16]. Therefore the exfoliation of 30B does not result from the molecular diffusion of TPU chains. In order to prove that the further molecular diffusion in 30B/TPU-1 system is difficult, the mixture of 30B and TPU-1 powder was annealed at 185 °C. The WAXD results suggest that the chains of TPU-1 intercalate into the interlayers of 30B when the mixture was annealed for 10 min (Fig. 4). However, the interlayer distance does not change even if annealing time was extended to 2 h. This means that the molecular diffusion mainly occurs in the initial period. Although the stronger interaction between 30B and TPU makes the intercalation of TPU easier, it also hinders the further diffusion of molecules in the galleries. Therefore it is difficult to achieve the exfoliation of 30B only via molecular diffusion in TPU nanocomposites. The similar phenomenon was found in polyamide 12 (PA12) nanocomposites [17], in which the exfoliation of OMMT was achieved via a slippage process of platelets. TEM images provide the evidence for the morphology evolution of 30B/TPU-1 nanocomposites (Fig. 5). When mixed for 0.5 min, many agglomerates of 30B exist in the matrix. When mixed for 1 min, the sizes of agglomerates become smaller. When mixed for 2 min, many longer particles appear in the matrix, and the aspect ratio of particles increases obviously. The results of WAXD (Fig. 3) and TEM suggest that the platelets of 30B slip under the shear field, and the longer particles are formed in the matrix (Fig. 5c). With further increase of mixing time, the platelets keep on slipping and even separate each other, thus the lengths of particles decrease again. Finally, the layers are exfoliated homogenously in the TPU matrix. The statistical results provide the detail information of slippage and help to analyze the TEM images better. Fig. 6 compares the average length and distributions of particle length in 30B/TPU nanocomposites mixed for 1, 2 and 3 min. The lengths of particles in the images were measured manually with a vernier caliper and transformed to actual sizes basing on the scale. The number average length is defined as
Fig. 6. Distribution histograms of particle length in 30B/TPU-1 nanocomposites mixed for different times at 100 rpm (a) 1 min; (b) 2 min and (c) 3 min.
P l¼
li N i N total
ð1Þ
where li is the length of a particle, N i is the number of particles with the length of li , N total is the total number of particles. When counting the length distribution, the lengths of particles were approximations, such as 84 nm and 87 nm
X. Meng et al. / Composites Science and Technology 68 (2008) 1815–1821
were thought as 80 nm and 90 nm, and the percentages of these approximations were calculated. The similar approximate method was taken in previous paper [23]. Although the statistical average length will be smaller than the true diameter [24], the change trend of distribution of particle length still provides detail information about the exfoliation process. In the statistical histograms (Fig. 6), the number average length is about 100 nm when mixed for 1 min. and the main length distribution locates at the region between about 50 and 130 nm. When mixed for 2 min, the average length of particles increases to about 135 nm, and the main distribution shifts to the region of larger length. The content of longer particle increases obviously, implying the lateral slippage of platelets under shear field occurs in the matrix. When mixed for 3 min, the average length decreases to about 116 nm, and the content of longer particle also decreases. It proves that the platelets keep on slipping and separate. Therefore the average length of particles increases firstly and then decreases. The above results show that the molecular diffusion of TPU chains mainly occurs in the initial period of mixing, and the exfoliation of 30B is achieved by a slippage process, which is
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dominated by shear stress. The process is similar to that of PA12 nanocomposites [17]. The stronger interaction between 30B and TPU improves the transferred efficiency of shear stress provided by TPU matrix. 3.2. The correlation of properties and morphology of TPU nanocomposites The rheological property is not only an important property of polymer nanocomposites, but also a powerful tool for inspecting the morphology of nanocomposites. The low frequency region of rheological curves is especially sensitive to the structure of OMMTs in nanocomposites [25,26]. When 30B is introduced into TPU-1 matrix, both the complex viscosity and storage modulus enhance greatly comparing with neat TPU at the terminal of curves, and the storage modulus of nanocomposite exhibits solid-like behavior (Fig. 7). With the increase of mixing time, the storage modulus of 30B/TPU-1 nanocomposites enhance gradually. It should be attributed to the formation of net-work via the slippage process and the exfoliated degree of 30B is improved gradually.
Fig. 7. Rheological properties of 5 wt.% 30B/TPU-1 nanocomposites mixed at 185 °C for different times: (a) complex viscosity and (b) storage modulus.
Fig. 8. The variational tendency of mechanical properties of 30B/TPU-1 nanocomposites with the increase of mixing time (a) Young’s modulus and (b) stress–strain curve.
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The result consists with those of WAXD (Fig. 3) and TEM (Fig. 5). The mechanical properties are also influenced by the morphology of nanocomposites. In Fig. 8a, the variational tendency of Young’s modulus of 30B/TPU-1 nanocomposite with mixing time coincides with the rheological properties. As the mixing time increases, the degree of exfoliation of 30B enhances (Figs. 3 and 5). Therefore the modulus of nanocomposite is improved gradually. The stress–strain curves of 30B/TPU-1 nanocomposites mixed for different times are shown in Fig. 8b. When mixed for 1 min, the tensile strength improves obviously, and the elongation decreases. Compared with the results for 1 min, the tensile strength and elongation for 3 min increase slightly, indicating the better dispersion of 30B in the matrix. However, the tensile strength and elongation decrease when the mixing time is prolonged continuously. It should be attributed to the degradation of TPU matrix. It was known that the organic modifier can accelerate the degradation of TPU [22], so the degree of degradation will increase with prolonging the mixing time. Dan et al. reported the similar phenomenon in TPU composites [20]. In their system, the tensile properties improved when the 30B/TPU was extruded firstly. However, the tensile properties decrease when extruded again because of the degradation of TPU. Therefore the long processing time is not beneficial to the tensile properties. In this work, the mixing time was prolonged to 10 min in order to discern the process of exfoliation. In the future work, the shear stress can be strengthened and thus the processing time can be shortened to decrease the degradation of polymers. 4. Conclusions A slippage process of MMT layers was observed in the exfoliation evolution of 30B/TPU nanocomposites, and the process was found in OMMT/PA12 nanocomposites before. Shear stress dominates the slippage process of MMT layers and results in the exfoliation. The molecular diffusion in the initial period of mixing decreases the attraction between OMMT layers, and increases the contacting sites between platelets and TPU, thus making OMMT platelets slip easier under shear field. The interaction between 30B and polymers efficiently transfers the shear stress provided by TPU matrix onto the OMMT layers. The appropriate viscosity of polymers ensures the intercalation of molecules and provides adequate shear stress. Therefore adequate shear stress, the appropriate molecular diffusion and interaction between OMMT and polymer are required to optimize the formation of nanocomposites. The control of morphology is crucial to the improvement of properties of nanocomposites. Acknowledgement We thank the financial supports from the National Natural Science Foundation of China for the Outstanding
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