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Polyurethane composites in situ molecularly reinforced by supramolecular nanofibrillar aggregates of sorbitol derivatives
Composites Science and Technology 79 (2013) 58–63
Contents lists available at SciVerse ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Polyurethane composites in situ molecularly reinforced by supramolecular nanofibrillar aggregates of sorbitol derivatives Lei Jin a, Hong Wang b,⇑, Yajiang Yang a,⇑ a b
Institute of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China Institute of Analytical Chemistry, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
a r t i c l e
i n f o
Article history: Received 18 December 2012 Received in revised form 22 January 2013 Accepted 14 February 2013 Available online 26 February 2013 Keywords: A. Fibers A. Nanocomposites B. Interphase B. Mechanical properties
a b s t r a c t 1,3:2,4-di-O-benzylidene-D-sorbitol (DBS) can self-assemble into supramolecular nanofibers in the precursor of polyurethane followed by gelation. After UV-initiated polymerization, in situ supramolecular nanofibers reinforced polyurethane composites can be obtained. SEM studies indicate that the in situ formed supramolecular nanofibers can induce the composite interface at the molecular level, greatly improving interface compatibility of the fibers and matrices. Static and dynamic mechanical analysis indicated that the tensile strength of 12 wt% DBS nanofibers reinforced polyurethane was 2.8 times higher than that of pure polyurethane. Their breaking elongation was 2.5 times higher than that of conventional glass fibers reinforced polyurethane. In comparison with conventional fiber reinforced polymers, the utilizing of in situ formed supramolecular nanofibers not only reinforce, but also toughen the polymer matrices, and show the advantage of reducing the weight and maintaining transparency of the polymers. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction For the polymer composites, tensile strength is a key parameter of mechanical properties. As is well known, the addition of various fibers, such as conventional glass fibers, carbon fibers, aramid fibers and boron fibers into the polymer matrices is an efficient method to increase the strength of the polymer composites. However improvement of the brittleness of polymer composites still remains a challenge, which is mainly due to the poor interfacial compatibility of reinforced phases and matrices. Such interfacial compatibility involves larger diameter of conventional fibers (usually 5–20 lm), content, orientation of fibers, as well as the physicchemical properties of fiber itself. To improve this interfacial compatibility, the conventional fibers should be pretreated with modifying agents, such as silane coupling agents for glass fibers [1]. During the past decade, developing nanotechnology plays an important role in the area of polymer composites. Based on the concept of molecular composites and in situ composites [2–4], inorganic nanoparticles, like calcium carbonate nanoparticles as nanofillers were applied to reinforce polymer composites. Other typical examples are polymer/layered silicate nanocomposites [5]. The interfacial compatibility of reinforced phase and matrix was greatly improved, which can be attributed to the high surface area of nanomaterials. Furthermore, in the recent years, carbon ⇑ Corresponding authors. Tel.: +86 27 87547141; fax: +86 27 87543632. E-mail address: [email protected] (Y. Yang). 0266-3538/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compscitech.2013.02.017
nanotubes and grapheme [6–8] are also used as novel nanoscale reinforced materials, inducing the interfacial compatibility at a molecular level. Thus, reduction of the diameter of the fiber may be an effective approach. For instance, the diameter of carbon fibers, fabricated by vapor-grown process, can be in the range of 60–150 nm [9,10]. In this case, the effect of reinforce can be achieved by using small amounts of carbon fibers as a result of the improved phase interface. However, carbon fibers possess conductivity, leading to a limited application, and are usually as antistatic coating agents (discharge electronic potentials) or even shielding from radio frequency interference. Seydibeyoglu et al. reported reinforced polyurethane by using natural fibrillated cellulose [11]. Generally, cellulose is hydrophilic, but most of the polymer matrices are hydrophobic. Therefore the preparation of cellulose-reinforced polyurethane was rather complicated. Therein, a process similar to the non-woven fabrication was applied, in which, the cellulose fiber mats were firstly fabricated by filtering the water–cellulose slurry. Polyurethane composites were then prepared using compression molding, by stacking the cellulose fiber mats between polyurethane films. Recently, Stone et al. reported supramolecular nanofibers formed by the self-assembly of photopolymerizable gelator containing diacetylene for the reinforcement of ethylene oxide/epichlorohydrin copolymer. The preparation involved solution mixture, removal of solvent, photopolymerization and compression molding. The obtained nanocomposite film is, in nature, an interpenetrating polymer networks consisted of polymerized supramolecular nanofibers and the
L. Jin et al. / Composites Science and Technology 79 (2013) 58–63
polymer matrix [12]. Stendah et al. reported supramolecular nanoribbons formed by dendron rodcoils that enable to modify the structural orientation of polystyrene. The impact strength of the resulting polystyrene can be greatly improved [13,14]. Nevertheless the preparation involved a thermal polymerization at 100 °C for 72 h and removal of residual monomer under vacuum for 1 day. In the present work, we propose a facile strategy to reinforce polyurethane by using linear supramolecular nanofibers which are formed from the self-assembly of 1,3:2,4-di-O-benzylidene-Dsorbitol (DBS). DBS is found to be capable of self-assembly into a 3D nanofibrillar network though hydrogen bonds and p–p stacking in a wide variety of organic solvents [15–17] and liquid state low molecular weight polymers (such as polyethylene glycol) [18], leading to the formation of supramolecular gels [19]. There, DBS actually served as solidifying agent. SEM and TEM studies indicated that the diameters of DBS aggregates are in the range of 10–20 nm and their length is approximately several hundred nanometers [15,20]. In general, the existence of the conventional inorganic substances (fillers or fibers) in polymer matrices results in an increase of strength [21], but a loss of toughness and transparency of the polymers. In the present study, in situ formed DBS supramolecular nanofibers not only show the effect of reinforcement, but also toughen for polyurethane composites in comparison with the conventional reinforcing methods. It could be attributed to a fact that the obtained supramolecular nanofiber reinforced polymers are not interpenetrating polymer networks. In addition, they show the advantage of reducing the weight and the transparency of polymer remains. To the best of our knowledge, little attention has been paid to the reinforcement of polyurethane by utilizing nonpolymerized in situ formed supramolecular nanofibers. 2. Experimental 2.1. Materials Glass fibers (GF, commercial name 988A, 9–13 lm in diameter and 3 mm in length) were purchased from Zhejiang Jushi Group. 1,3:2,4-di-O-benzylidene-D-sorbitol (DBS, purity 99.5%) was purchased from Wuhan Huabang Co., Polyether-based aliphatic polyurethane diacrylate (precursor of polyurethane, commercial name SM6202) was purchased from Jiangsu Sanmu Co. 1-Hydroxycyclohexyl phenyl ketone (Irgancure 184, the photoinitiator) was purchased from Ciba Geigy. All reagents were of analytical grade and used as received. 2.2. Preparation of polyurethane composites A designed amount of DBS was added into SM6202. The mixture was heated in an oil bath at ca. 100 °C until a transparent solution was obtained. Subsequently 1 wt% Irgancure 184 was added into the solution while hot. The solution was poured into a mold with 1 mm in depth, 15 mm in width and 100 mm in length, and then allowed to cool to room temperature. The formed stable gels were irradiated under an UV lamp (1000 W) for 15 min which was determined by a series of condition test. The obtained polyurethane composites containing DBS supramolecular nanofibers are denoted as DBS/PU. Depending upon the content of DBS, the samples were denoted as DBS/PU, such as 12%DBS/PU (mass fraction). As a reference, a designed amount of GF was added into SM6202. The mixture was kneaded in an internal mixer of Haake Polydrive (R600, Thermo Haake) for 30 min. The subsequent procedure was similar to the preparation of DBS/PU. The obtained polyurethane composites containing glass fibers were denoted as GF/PU, such as 12%GF/PU (mass fraction).
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2.3. Specific gravity of polyurethane composites The specific gravity of polyurethane composites was measured using a standard liquid displacement pyknometer (ISO 1183). The specific gravity of 12%DBS/PU was measured to be 1.108 using a liquid displacement pyknometer (ISO 1183). The specific gravity of 12%GF/PU was measured to be 1.182. Similarly, the pure PU was also prepared as a reference and its specific gravity was 1.095.
2.4. Measurements of mechanical properties The mechanical properties of the samples were measured by an electromechanical universal testing machine (CMT-4104, SANS) based on the test standard GB13022-91 (ISO527-3). The rate of extension was 10 mm/min.
2.5. Polarized optical microscope (POM) The hot solutions of SM6202 with DBS (or GF), as described in the Section 1.2, were dropped onto a preheated glass slide and then allowed to cool to room temperature to form stable thin layer gels. Observations were carried on a polarized optical microscopy (BX51, Olympus).
2.6. Dynamic mechanical analysis (DMA) The DMA measurements were performed on a Dynamic Mechanical Analyzer (DMS 6100, Seiko) with a tensile mode at a frequency of 1 Hz. The scanning temperature range was from 100 °C to 180 °C at a scanning rate of 3 °C/min. The sample size was 100 10 1 mm.
2.7. Field emission scanning electron microscope (FE-SEM) For the SEM image in Fig. 1a, the hot solutions of DBS and SM6202, as described in the Section 1.2, were dropped onto a preheated silicon slide and then allowed to cool to room temperature to form stable gel. The gel sample was immersed in absolute ether for 24 h to extract SM6202 and then evaporated in the air for 10 h to remove ether. The xerogels were coated by gold for the measurement by FE-SEM (Sirion 200, FEI). The accelerating voltage was 10 kV. For the SEM images in Fig. 4, the samples of three types of PU were first frozen by liquid nitrogen and then brittle ruptured. The created fractured surfaces were coated by gold for FE-SEM (Sirion 200, FEI) measurements. The accelerating voltage was 5 kV.
2.8. Transparency of polyurethane composites The transparency of polyurethane composites was characterized by measuring the absorbency of visible light. Absorbencies of the samples were recorded on a UV–Vis spectrophotometer (TU-1810, Beijing Pushi General Co.). The scanning wavelength range was from 400 nm to 700 nm.
2.9. Calculation of optimized geometries Optimized geometries of DBS were calculated by utilizing the molecular mechanics software (Spartan 06, Wavefunction Inc.) and density functional theory (DFT-B3LYP/6-31G initial geometry). The optimized structures are depicted in Fig. 1b.
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L. Jin et al. / Composites Science and Technology 79 (2013) 58–63
Fig. 1. (a) SEM image of the xerogel formed by the self-assembly of DBS in SM6202. The insert is an enlarged image of the partial area. (b) Optimized geometry of DBS as calculated by utilizing the Spartan program. (c) The driving force of DBS self-assembly may involve intra- and intermolecular hydrogen bonding between carboxyl groups and oxygen atoms in the sorbital groups, in addition to p–p stack between benzene rings.
Fig. 2. Diagram of stress/strain of fiber reinforced polyurethane composites. The insert optical photograms show the toughness of the fiber reinforced polyurethane composites.
3. Results and discussion Taking advantage of DBS self-assembly into supramolecular nanofibers in organic solvents [22,23], DBS was firstly dissolved in the liquid state precursor of polyurethane (SM6202) and then self-assembled into a 3D nanofibrillar network, and then resulting in the gelation of SM6202. Fig. 1a shows an SEM image of the morphology of DBS supramolecular nanofibers formed in SM6202. A number of nanofiber bundles with diameters of tens of nanometers were observed. The formation of nanofiber bundles may involve the preparation of the SEM sample by solvent extraction although the reported diameter of single DBS supramolecular nanofiber was in the range of 10–20 nm [15,20]. Fig. 1b and c shows the optimized geometry of DBS simulated by molecular mechanics software (Spartan 06, Wavefunction Inc) and the DBS aggregates are formed by hydrogen bonds and p–p stacking. Under the synergistic effect of multiple hydrogen bonds and p–p stacking, the DBS supramolecular nanofibers are supposed to be relatively strong although their strength could not be determined in this work. SM6202 gels with DBS supramolecular nanofibers were then polymerized under UV irradiation, leading to the formation of
in situ nanofiber reinforced polyurethane composites. Fig. 2 shows the stress–strain diagram of non-reinforced polyurethane (pure PU), 12 wt% DBS supramolecular nanofibers reinforced polyurethane (12%DBS/PU) and 12 wt% conventional glass fiber reinforced polyurethane (12%GF/PU). We note that the content (12%) of both types of fibers was determined by considering their whole properties, both tensile strength and breaking elongation according to the experimental results (the stress–strain diagrams of the reinforced polyurethane by using varied contents of both fibers were not shown here). As shown in Fig. 2, pure PU without any fibers has a rather low tensile strength and breaking elongation. The tensile strength of 12%DBS/PU increased from 1.5 MPa to 4.3 MPa, 2.8 times higher than that of pure PU. Its breaking elongation was also increased from 31% to 45%. In the case of 12%GF/PU, the breaking elongation was only 60% of the pure PU although its tensile strength was higher. While the breaking elongation of 12%DBS/ PU was 2.5 times higher than that of 12%GF/PU. From the inserted optical photograms of testing samples in Fig. 2, we see that 12%DBS/PU exhibits excellent toughness. In contrast, 12%GF/PU shows obvious brittleness. As is well known, reinforcement and toughening is usually mutual restricted in the modification of polymer composites, as in the case of 12%GF/PU in this work. From the results of Figs. 1 and 2, we conclude that DBS supramolecular nanofibers served both as reinforcing and toughening agents for the polyurethane matrix. This can most likely be attributed to the excellent compatibility between in situ formed supramolecular nanofibers and the polymer matrix. It is worth noting that a key advantage of in situ formed molecular composites is their light weight in comparison with that of conventional fillers and fibers. Here, the specific gravity of 12%DBS/PU is 1.108, closed to that of pure PU (1.095) (see Section 2), while the specific gravity of 12%GF/PU is 1.182, an increase by 6.6%. Therefore, the in situ formed supramolecular nanofibers reinforced polymer composites show a clear potential for applications requiring lightweight materials, such as aerobats, vehicles and vessels. Mechanical properties are generally macroscopic in nature. To well understand the effect of either reinforcement and toughening of supramolecular nanofibers for the polyurethane, we measured the microscopic phase structures of both nanofibers and glass fibers reinforced polyurethane by using polarized optical microscope (Fig. 3) and field emission scanning electron microscope (Fig. 4). Under the polarizing optical microscope, pure PU as a blank sample does not show any birefringence behavior (Fig. 3a), indicating that the polymer matrix is amorphous. Beyond that,
L. Jin et al. / Composites Science and Technology 79 (2013) 58–63
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Fig. 3. POM images of SM6202 gels formed by (a) 0%, (b) 3%, (c) 5%, (d) 8%, (e) 10%, (f) 12%, (g) 15% and (h) 18% of DBS (Magnification 100).
all of the samples (Fig. 3b–h) exhibit distinct Maltese cross extinction, a typical birefringence of spherulites formed by 3D nanofibrillar network [19,24]. As shown in Fig. 3, the size and the distribution of spherulites in the matrix depend upon the content of DBS. The size and distribution of the spherulites formed by 12%DBS seems to be more uniform in comparison with other samples containing varied amounts of DBS (Fig. 3f). This is also a reason that the sample of 12%DBS/PU was selected for further study. Thus, the reinforcing mechanism may involve 3D nanofibrillar networks supporting molecular chains of the matrix. Furthermore, spherulites also play a role similar to that of the fillers. And flexible nanofibers may play a significant role in maintaining sufficient toughness of the matrix. Fig. 4 shows the SEM images of brittle fracture surfaces of, respectively, pure PU, 12%GF/PU and 12%DBS/PU. Pure PU showed a smooth fracture surface (Fig. 4a), in which the white lines may be attributed to the inherent craze of polymer in the brittle rupture. In the fracture surface of 12%GF/PU, protruded fibers and the small holes left when the fibers are withdrawn can be clearly observed (Fig. 4b). It is a typical brittle rupture, indicating that the compatibility between glass fibers and matrix is poor. In the case of 12%DBS/PU (Fig. 4c), a rough and uneven fracture surface can be observed, which can be attributed to partial adhesion when brittle rupture has occurred. In addition, no phase interface was observed, implying good compatibility between the in situ formed nanofibers and the matrix. Conventional inorganic glass fibers cannot induce such compositing effects at a molecular level. The above discussion is based on static mechanical properties. Due to the phase transition and relaxation of polymer composites are sensitive to temperature variation and the frequency of external forces. Therefore, we measured dynamic mechanical properties of pure PU, 12%GF/PU and 12%DBS/PU. Fig. 5 shows their stretching
mode dynamic mechanical spectra. In the range of temperature from –100 to 250 °C, their storage modulus (E0 ) and loss modulus (E00 ) exhibited a decreasing tendency with an increase of temperature (Fig. 5a and b). In the case of 12%DBS/PU, it was found that a secondary transition occurred at ca. 170 °C. From the view point of supramolecular chemistry, DBS can self-assemble into fiber-like aggregates in the precursor of PU (SM6202) as mentioned above. These aggregates can also disassemble at a certain temperature depending upon the concentration of DBS [19]. Thus, we speculate that the secondary transition at ca. 170 °C can be attributed to the disassembly of DBS fiber-like aggregates. The dynamic storage modulus (E0 ) is the most important property to assess the load-bearing capability of composite materials [25], characterizing the stiffness of materials. The loss modulus (E00 ) characterizes the damping property of materials. The ratio of E0 and E00 is the so-called mechanical loss factor (tand). This quantity is a measure of balance between the elastic phase and the viscous phase in a polymeric structure, or, in other words, the balance of stiffness and toughness. The results shown in Fig. 5a and b indicate that either glass fibers or DBS supramolecular fibers can significantly improve the stiffness of pure PU in the temperature of routine use. In Fig. 5, the maximum value of tand of pure PU is much higher than that of both 12%GF/PU and 12%DBS/PU because of its amorphous structure and relatively free motion of molecular chains [26]. In the presence of reinforced fibers, the motion of molecular chains is restricted, leading to the low tand [27,28]. According to the definition of tand, this implies that the material has certain damping capabilities. From Fig. 5, the maximum tand of 12%DBS/ PU is higher than that of 12%GF/PU, and the corresponding temperature (13.8 °C) is lower than that of the latter (25.4 °C), even lower than that of pure PU (20.5 °C). This implies that 12%DBS/PU has the
Fig. 4. SEM images of brittle fracture surfaces of pure PU (a), 12%GF/PU (b) and 12%DBS/PU (c).
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Fig. 5. Dynamic mechanical analysis of pure PU (black curve), 12%GF/PU (blue curve) and 12%DBS/PU (red curve). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. Absorbency of 12%GF/PU, 12%DBS/PU and pure PU in the range of visible light.
highest toughness. Generally, the corresponding temperature of the maximum tand can be defined as the transition temperature for the transition between the glass state and the viscoelastic state, i.e., the glass transition temperature (Tg). Considering that the routine use of polyurethane is in the range of the high elastic state, and therefore, the lower Tg leads to a broader range of the high elastic state. In other words, polyurethane composites can be applied in a larger range of temperatures. The results shown in Fig. 5 reveal the fact that the in situ formed DBS supramolecular nanofibers significantly improve the whole properties of polyurethane composites. It is worth noting that the transparency is a commendable inherent feature of polymer materials in comparison with metal and other inorganic materials. However, most of the polymer composites are non-transparent due to the presence of reinforced fibers or fillers. Fig. 6 shows the absorbency of 12%GF/PU, 12%DBS/ PU and pure PU. In the range of 400–700 nm (visible light), 12%GF/PU shows larger absorbency than that of 12%DBS/PU and pure PU, implying the poor transparency of 12%GF/PU. To quantitatively characterize their transparency, we measured absorbencies of three samples at the wavelength of 550 nm (median value of the visible light). The absorbencies of pure PU, 12%DBS/PU and
Fig. 7. Schematic illustration of in situ formed DBS supramolecular nanofibers reinforced polyurethane in comparison with conventional glass fibers reinforced polyurethane.
L. Jin et al. / Composites Science and Technology 79 (2013) 58–63
12%GF/PU were found to be 0.062, 0.161 and 1.005, respectively. The transparency of 12%DBS/PU was 6.2 times higher than that of 12%GF/PU. From the inserted optical photograms of three samples, 12%DBS/PU still maintains an excellent transparency which could be attributed to the good compatibility between nanoscaled fibers and PU matrix. Therefore, this work also provides a new strategy to fabricate optical polymer composites requiring transparency, such as photoelectric material and photonic crystal films [29,30].
4. Conclusions Fig. 7 schematically summaries the preparation protocols of polyurethane composites reinforced by in situ formed DBS supramolecular nanofibers and by conventional glass fibers as reference. SEM images of brittle fracture surfaces of both reinforced PU indicated that no phase interface was observed in the case of DBS supramolecular nanofibers reinforced PU. The study of mechanical property indicated that DBS supramolecular nanofibers served both as reinforcing and toughening agents for the polyurethane matrix. This can most likely be attributed to the excellent compatibility between in situ formed supramolecular nanofibers and the PU matrix. By contrast, the conventional glass fibers only served as reinforcing agents. In addition, DBS supramolecular nanofibers reinforced PU shows the advantage of reducing the weight and maintaining transparency of the polymers. Acknowledgements We gratefully acknowledge funding for this work provided by the National Basic Research Program of China (2012CB812500) and the National Natural Science Foundation of China (51073062). We also thank HUST Analytical and Testing Center for the SEM measurements. References [1] Xie Y, Hill CAS, Xiao Z, Militz H, Mai C. Silane coupling agents used for natural fiber/polymercomposites. Compos A-Appl Sci Manuf 2010;41:806–19. [2] Takayanagi M, Ogata T, Morikawa M, Kai T. Polymer composites of rigid and flexible molecules: System of wholly aromatic and aliphatic polyamides. J Macromol Sci B 1980;17:591–615. [3] Hwang WF, Wiff DR, Benner CL, Helminiak TE. Composites on a molecular level: phase relationships, processing, and properties. J Macromol Sci B 1983;22:231–57. [4] Dencheva N, Denchev Z, Oliveira MJ, Funari SS. Microstructure studies of in situ composites based on polyethylene/polyamide 12 blends. Macromolecules 2010;43:4715–26. [5] Pavlidou S, Papaspyrides CD. A review on polymer-layered silicate nanocomposites. Prog Polym Sci 2008;33:1119–98. [6] Coleman JN, Khan U, Blau WJ, Gun’ko YK. Small but strong: a review of the mechanical properties of carbon nanotube-polymercomposites. Carbon 2006;44:1624–52.
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Contents lists available at SciVerse ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Polyurethane composites in situ molecularly reinforced by supramolecular nanofibrillar aggregates of sorbitol derivatives Lei Jin a, Hong Wang b,⇑, Yajiang Yang a,⇑ a b
Institute of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China Institute of Analytical Chemistry, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
a r t i c l e
i n f o
Article history: Received 18 December 2012 Received in revised form 22 January 2013 Accepted 14 February 2013 Available online 26 February 2013 Keywords: A. Fibers A. Nanocomposites B. Interphase B. Mechanical properties
a b s t r a c t 1,3:2,4-di-O-benzylidene-D-sorbitol (DBS) can self-assemble into supramolecular nanofibers in the precursor of polyurethane followed by gelation. After UV-initiated polymerization, in situ supramolecular nanofibers reinforced polyurethane composites can be obtained. SEM studies indicate that the in situ formed supramolecular nanofibers can induce the composite interface at the molecular level, greatly improving interface compatibility of the fibers and matrices. Static and dynamic mechanical analysis indicated that the tensile strength of 12 wt% DBS nanofibers reinforced polyurethane was 2.8 times higher than that of pure polyurethane. Their breaking elongation was 2.5 times higher than that of conventional glass fibers reinforced polyurethane. In comparison with conventional fiber reinforced polymers, the utilizing of in situ formed supramolecular nanofibers not only reinforce, but also toughen the polymer matrices, and show the advantage of reducing the weight and maintaining transparency of the polymers. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction For the polymer composites, tensile strength is a key parameter of mechanical properties. As is well known, the addition of various fibers, such as conventional glass fibers, carbon fibers, aramid fibers and boron fibers into the polymer matrices is an efficient method to increase the strength of the polymer composites. However improvement of the brittleness of polymer composites still remains a challenge, which is mainly due to the poor interfacial compatibility of reinforced phases and matrices. Such interfacial compatibility involves larger diameter of conventional fibers (usually 5–20 lm), content, orientation of fibers, as well as the physicchemical properties of fiber itself. To improve this interfacial compatibility, the conventional fibers should be pretreated with modifying agents, such as silane coupling agents for glass fibers [1]. During the past decade, developing nanotechnology plays an important role in the area of polymer composites. Based on the concept of molecular composites and in situ composites [2–4], inorganic nanoparticles, like calcium carbonate nanoparticles as nanofillers were applied to reinforce polymer composites. Other typical examples are polymer/layered silicate nanocomposites [5]. The interfacial compatibility of reinforced phase and matrix was greatly improved, which can be attributed to the high surface area of nanomaterials. Furthermore, in the recent years, carbon ⇑ Corresponding authors. Tel.: +86 27 87547141; fax: +86 27 87543632. E-mail address: [email protected] (Y. Yang). 0266-3538/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compscitech.2013.02.017
nanotubes and grapheme [6–8] are also used as novel nanoscale reinforced materials, inducing the interfacial compatibility at a molecular level. Thus, reduction of the diameter of the fiber may be an effective approach. For instance, the diameter of carbon fibers, fabricated by vapor-grown process, can be in the range of 60–150 nm [9,10]. In this case, the effect of reinforce can be achieved by using small amounts of carbon fibers as a result of the improved phase interface. However, carbon fibers possess conductivity, leading to a limited application, and are usually as antistatic coating agents (discharge electronic potentials) or even shielding from radio frequency interference. Seydibeyoglu et al. reported reinforced polyurethane by using natural fibrillated cellulose [11]. Generally, cellulose is hydrophilic, but most of the polymer matrices are hydrophobic. Therefore the preparation of cellulose-reinforced polyurethane was rather complicated. Therein, a process similar to the non-woven fabrication was applied, in which, the cellulose fiber mats were firstly fabricated by filtering the water–cellulose slurry. Polyurethane composites were then prepared using compression molding, by stacking the cellulose fiber mats between polyurethane films. Recently, Stone et al. reported supramolecular nanofibers formed by the self-assembly of photopolymerizable gelator containing diacetylene for the reinforcement of ethylene oxide/epichlorohydrin copolymer. The preparation involved solution mixture, removal of solvent, photopolymerization and compression molding. The obtained nanocomposite film is, in nature, an interpenetrating polymer networks consisted of polymerized supramolecular nanofibers and the
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polymer matrix [12]. Stendah et al. reported supramolecular nanoribbons formed by dendron rodcoils that enable to modify the structural orientation of polystyrene. The impact strength of the resulting polystyrene can be greatly improved [13,14]. Nevertheless the preparation involved a thermal polymerization at 100 °C for 72 h and removal of residual monomer under vacuum for 1 day. In the present work, we propose a facile strategy to reinforce polyurethane by using linear supramolecular nanofibers which are formed from the self-assembly of 1,3:2,4-di-O-benzylidene-Dsorbitol (DBS). DBS is found to be capable of self-assembly into a 3D nanofibrillar network though hydrogen bonds and p–p stacking in a wide variety of organic solvents [15–17] and liquid state low molecular weight polymers (such as polyethylene glycol) [18], leading to the formation of supramolecular gels [19]. There, DBS actually served as solidifying agent. SEM and TEM studies indicated that the diameters of DBS aggregates are in the range of 10–20 nm and their length is approximately several hundred nanometers [15,20]. In general, the existence of the conventional inorganic substances (fillers or fibers) in polymer matrices results in an increase of strength [21], but a loss of toughness and transparency of the polymers. In the present study, in situ formed DBS supramolecular nanofibers not only show the effect of reinforcement, but also toughen for polyurethane composites in comparison with the conventional reinforcing methods. It could be attributed to a fact that the obtained supramolecular nanofiber reinforced polymers are not interpenetrating polymer networks. In addition, they show the advantage of reducing the weight and the transparency of polymer remains. To the best of our knowledge, little attention has been paid to the reinforcement of polyurethane by utilizing nonpolymerized in situ formed supramolecular nanofibers. 2. Experimental 2.1. Materials Glass fibers (GF, commercial name 988A, 9–13 lm in diameter and 3 mm in length) were purchased from Zhejiang Jushi Group. 1,3:2,4-di-O-benzylidene-D-sorbitol (DBS, purity 99.5%) was purchased from Wuhan Huabang Co., Polyether-based aliphatic polyurethane diacrylate (precursor of polyurethane, commercial name SM6202) was purchased from Jiangsu Sanmu Co. 1-Hydroxycyclohexyl phenyl ketone (Irgancure 184, the photoinitiator) was purchased from Ciba Geigy. All reagents were of analytical grade and used as received. 2.2. Preparation of polyurethane composites A designed amount of DBS was added into SM6202. The mixture was heated in an oil bath at ca. 100 °C until a transparent solution was obtained. Subsequently 1 wt% Irgancure 184 was added into the solution while hot. The solution was poured into a mold with 1 mm in depth, 15 mm in width and 100 mm in length, and then allowed to cool to room temperature. The formed stable gels were irradiated under an UV lamp (1000 W) for 15 min which was determined by a series of condition test. The obtained polyurethane composites containing DBS supramolecular nanofibers are denoted as DBS/PU. Depending upon the content of DBS, the samples were denoted as DBS/PU, such as 12%DBS/PU (mass fraction). As a reference, a designed amount of GF was added into SM6202. The mixture was kneaded in an internal mixer of Haake Polydrive (R600, Thermo Haake) for 30 min. The subsequent procedure was similar to the preparation of DBS/PU. The obtained polyurethane composites containing glass fibers were denoted as GF/PU, such as 12%GF/PU (mass fraction).
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2.3. Specific gravity of polyurethane composites The specific gravity of polyurethane composites was measured using a standard liquid displacement pyknometer (ISO 1183). The specific gravity of 12%DBS/PU was measured to be 1.108 using a liquid displacement pyknometer (ISO 1183). The specific gravity of 12%GF/PU was measured to be 1.182. Similarly, the pure PU was also prepared as a reference and its specific gravity was 1.095.
2.4. Measurements of mechanical properties The mechanical properties of the samples were measured by an electromechanical universal testing machine (CMT-4104, SANS) based on the test standard GB13022-91 (ISO527-3). The rate of extension was 10 mm/min.
2.5. Polarized optical microscope (POM) The hot solutions of SM6202 with DBS (or GF), as described in the Section 1.2, were dropped onto a preheated glass slide and then allowed to cool to room temperature to form stable thin layer gels. Observations were carried on a polarized optical microscopy (BX51, Olympus).
2.6. Dynamic mechanical analysis (DMA) The DMA measurements were performed on a Dynamic Mechanical Analyzer (DMS 6100, Seiko) with a tensile mode at a frequency of 1 Hz. The scanning temperature range was from 100 °C to 180 °C at a scanning rate of 3 °C/min. The sample size was 100 10 1 mm.
2.7. Field emission scanning electron microscope (FE-SEM) For the SEM image in Fig. 1a, the hot solutions of DBS and SM6202, as described in the Section 1.2, were dropped onto a preheated silicon slide and then allowed to cool to room temperature to form stable gel. The gel sample was immersed in absolute ether for 24 h to extract SM6202 and then evaporated in the air for 10 h to remove ether. The xerogels were coated by gold for the measurement by FE-SEM (Sirion 200, FEI). The accelerating voltage was 10 kV. For the SEM images in Fig. 4, the samples of three types of PU were first frozen by liquid nitrogen and then brittle ruptured. The created fractured surfaces were coated by gold for FE-SEM (Sirion 200, FEI) measurements. The accelerating voltage was 5 kV.
2.8. Transparency of polyurethane composites The transparency of polyurethane composites was characterized by measuring the absorbency of visible light. Absorbencies of the samples were recorded on a UV–Vis spectrophotometer (TU-1810, Beijing Pushi General Co.). The scanning wavelength range was from 400 nm to 700 nm.
2.9. Calculation of optimized geometries Optimized geometries of DBS were calculated by utilizing the molecular mechanics software (Spartan 06, Wavefunction Inc.) and density functional theory (DFT-B3LYP/6-31G initial geometry). The optimized structures are depicted in Fig. 1b.
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Fig. 1. (a) SEM image of the xerogel formed by the self-assembly of DBS in SM6202. The insert is an enlarged image of the partial area. (b) Optimized geometry of DBS as calculated by utilizing the Spartan program. (c) The driving force of DBS self-assembly may involve intra- and intermolecular hydrogen bonding between carboxyl groups and oxygen atoms in the sorbital groups, in addition to p–p stack between benzene rings.
Fig. 2. Diagram of stress/strain of fiber reinforced polyurethane composites. The insert optical photograms show the toughness of the fiber reinforced polyurethane composites.
3. Results and discussion Taking advantage of DBS self-assembly into supramolecular nanofibers in organic solvents [22,23], DBS was firstly dissolved in the liquid state precursor of polyurethane (SM6202) and then self-assembled into a 3D nanofibrillar network, and then resulting in the gelation of SM6202. Fig. 1a shows an SEM image of the morphology of DBS supramolecular nanofibers formed in SM6202. A number of nanofiber bundles with diameters of tens of nanometers were observed. The formation of nanofiber bundles may involve the preparation of the SEM sample by solvent extraction although the reported diameter of single DBS supramolecular nanofiber was in the range of 10–20 nm [15,20]. Fig. 1b and c shows the optimized geometry of DBS simulated by molecular mechanics software (Spartan 06, Wavefunction Inc) and the DBS aggregates are formed by hydrogen bonds and p–p stacking. Under the synergistic effect of multiple hydrogen bonds and p–p stacking, the DBS supramolecular nanofibers are supposed to be relatively strong although their strength could not be determined in this work. SM6202 gels with DBS supramolecular nanofibers were then polymerized under UV irradiation, leading to the formation of
in situ nanofiber reinforced polyurethane composites. Fig. 2 shows the stress–strain diagram of non-reinforced polyurethane (pure PU), 12 wt% DBS supramolecular nanofibers reinforced polyurethane (12%DBS/PU) and 12 wt% conventional glass fiber reinforced polyurethane (12%GF/PU). We note that the content (12%) of both types of fibers was determined by considering their whole properties, both tensile strength and breaking elongation according to the experimental results (the stress–strain diagrams of the reinforced polyurethane by using varied contents of both fibers were not shown here). As shown in Fig. 2, pure PU without any fibers has a rather low tensile strength and breaking elongation. The tensile strength of 12%DBS/PU increased from 1.5 MPa to 4.3 MPa, 2.8 times higher than that of pure PU. Its breaking elongation was also increased from 31% to 45%. In the case of 12%GF/PU, the breaking elongation was only 60% of the pure PU although its tensile strength was higher. While the breaking elongation of 12%DBS/ PU was 2.5 times higher than that of 12%GF/PU. From the inserted optical photograms of testing samples in Fig. 2, we see that 12%DBS/PU exhibits excellent toughness. In contrast, 12%GF/PU shows obvious brittleness. As is well known, reinforcement and toughening is usually mutual restricted in the modification of polymer composites, as in the case of 12%GF/PU in this work. From the results of Figs. 1 and 2, we conclude that DBS supramolecular nanofibers served both as reinforcing and toughening agents for the polyurethane matrix. This can most likely be attributed to the excellent compatibility between in situ formed supramolecular nanofibers and the polymer matrix. It is worth noting that a key advantage of in situ formed molecular composites is their light weight in comparison with that of conventional fillers and fibers. Here, the specific gravity of 12%DBS/PU is 1.108, closed to that of pure PU (1.095) (see Section 2), while the specific gravity of 12%GF/PU is 1.182, an increase by 6.6%. Therefore, the in situ formed supramolecular nanofibers reinforced polymer composites show a clear potential for applications requiring lightweight materials, such as aerobats, vehicles and vessels. Mechanical properties are generally macroscopic in nature. To well understand the effect of either reinforcement and toughening of supramolecular nanofibers for the polyurethane, we measured the microscopic phase structures of both nanofibers and glass fibers reinforced polyurethane by using polarized optical microscope (Fig. 3) and field emission scanning electron microscope (Fig. 4). Under the polarizing optical microscope, pure PU as a blank sample does not show any birefringence behavior (Fig. 3a), indicating that the polymer matrix is amorphous. Beyond that,
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Fig. 3. POM images of SM6202 gels formed by (a) 0%, (b) 3%, (c) 5%, (d) 8%, (e) 10%, (f) 12%, (g) 15% and (h) 18% of DBS (Magnification 100).
all of the samples (Fig. 3b–h) exhibit distinct Maltese cross extinction, a typical birefringence of spherulites formed by 3D nanofibrillar network [19,24]. As shown in Fig. 3, the size and the distribution of spherulites in the matrix depend upon the content of DBS. The size and distribution of the spherulites formed by 12%DBS seems to be more uniform in comparison with other samples containing varied amounts of DBS (Fig. 3f). This is also a reason that the sample of 12%DBS/PU was selected for further study. Thus, the reinforcing mechanism may involve 3D nanofibrillar networks supporting molecular chains of the matrix. Furthermore, spherulites also play a role similar to that of the fillers. And flexible nanofibers may play a significant role in maintaining sufficient toughness of the matrix. Fig. 4 shows the SEM images of brittle fracture surfaces of, respectively, pure PU, 12%GF/PU and 12%DBS/PU. Pure PU showed a smooth fracture surface (Fig. 4a), in which the white lines may be attributed to the inherent craze of polymer in the brittle rupture. In the fracture surface of 12%GF/PU, protruded fibers and the small holes left when the fibers are withdrawn can be clearly observed (Fig. 4b). It is a typical brittle rupture, indicating that the compatibility between glass fibers and matrix is poor. In the case of 12%DBS/PU (Fig. 4c), a rough and uneven fracture surface can be observed, which can be attributed to partial adhesion when brittle rupture has occurred. In addition, no phase interface was observed, implying good compatibility between the in situ formed nanofibers and the matrix. Conventional inorganic glass fibers cannot induce such compositing effects at a molecular level. The above discussion is based on static mechanical properties. Due to the phase transition and relaxation of polymer composites are sensitive to temperature variation and the frequency of external forces. Therefore, we measured dynamic mechanical properties of pure PU, 12%GF/PU and 12%DBS/PU. Fig. 5 shows their stretching
mode dynamic mechanical spectra. In the range of temperature from –100 to 250 °C, their storage modulus (E0 ) and loss modulus (E00 ) exhibited a decreasing tendency with an increase of temperature (Fig. 5a and b). In the case of 12%DBS/PU, it was found that a secondary transition occurred at ca. 170 °C. From the view point of supramolecular chemistry, DBS can self-assemble into fiber-like aggregates in the precursor of PU (SM6202) as mentioned above. These aggregates can also disassemble at a certain temperature depending upon the concentration of DBS [19]. Thus, we speculate that the secondary transition at ca. 170 °C can be attributed to the disassembly of DBS fiber-like aggregates. The dynamic storage modulus (E0 ) is the most important property to assess the load-bearing capability of composite materials [25], characterizing the stiffness of materials. The loss modulus (E00 ) characterizes the damping property of materials. The ratio of E0 and E00 is the so-called mechanical loss factor (tand). This quantity is a measure of balance between the elastic phase and the viscous phase in a polymeric structure, or, in other words, the balance of stiffness and toughness. The results shown in Fig. 5a and b indicate that either glass fibers or DBS supramolecular fibers can significantly improve the stiffness of pure PU in the temperature of routine use. In Fig. 5, the maximum value of tand of pure PU is much higher than that of both 12%GF/PU and 12%DBS/PU because of its amorphous structure and relatively free motion of molecular chains [26]. In the presence of reinforced fibers, the motion of molecular chains is restricted, leading to the low tand [27,28]. According to the definition of tand, this implies that the material has certain damping capabilities. From Fig. 5, the maximum tand of 12%DBS/ PU is higher than that of 12%GF/PU, and the corresponding temperature (13.8 °C) is lower than that of the latter (25.4 °C), even lower than that of pure PU (20.5 °C). This implies that 12%DBS/PU has the
Fig. 4. SEM images of brittle fracture surfaces of pure PU (a), 12%GF/PU (b) and 12%DBS/PU (c).
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Fig. 5. Dynamic mechanical analysis of pure PU (black curve), 12%GF/PU (blue curve) and 12%DBS/PU (red curve). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. Absorbency of 12%GF/PU, 12%DBS/PU and pure PU in the range of visible light.
highest toughness. Generally, the corresponding temperature of the maximum tand can be defined as the transition temperature for the transition between the glass state and the viscoelastic state, i.e., the glass transition temperature (Tg). Considering that the routine use of polyurethane is in the range of the high elastic state, and therefore, the lower Tg leads to a broader range of the high elastic state. In other words, polyurethane composites can be applied in a larger range of temperatures. The results shown in Fig. 5 reveal the fact that the in situ formed DBS supramolecular nanofibers significantly improve the whole properties of polyurethane composites. It is worth noting that the transparency is a commendable inherent feature of polymer materials in comparison with metal and other inorganic materials. However, most of the polymer composites are non-transparent due to the presence of reinforced fibers or fillers. Fig. 6 shows the absorbency of 12%GF/PU, 12%DBS/ PU and pure PU. In the range of 400–700 nm (visible light), 12%GF/PU shows larger absorbency than that of 12%DBS/PU and pure PU, implying the poor transparency of 12%GF/PU. To quantitatively characterize their transparency, we measured absorbencies of three samples at the wavelength of 550 nm (median value of the visible light). The absorbencies of pure PU, 12%DBS/PU and
Fig. 7. Schematic illustration of in situ formed DBS supramolecular nanofibers reinforced polyurethane in comparison with conventional glass fibers reinforced polyurethane.
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12%GF/PU were found to be 0.062, 0.161 and 1.005, respectively. The transparency of 12%DBS/PU was 6.2 times higher than that of 12%GF/PU. From the inserted optical photograms of three samples, 12%DBS/PU still maintains an excellent transparency which could be attributed to the good compatibility between nanoscaled fibers and PU matrix. Therefore, this work also provides a new strategy to fabricate optical polymer composites requiring transparency, such as photoelectric material and photonic crystal films [29,30].
4. Conclusions Fig. 7 schematically summaries the preparation protocols of polyurethane composites reinforced by in situ formed DBS supramolecular nanofibers and by conventional glass fibers as reference. SEM images of brittle fracture surfaces of both reinforced PU indicated that no phase interface was observed in the case of DBS supramolecular nanofibers reinforced PU. The study of mechanical property indicated that DBS supramolecular nanofibers served both as reinforcing and toughening agents for the polyurethane matrix. This can most likely be attributed to the excellent compatibility between in situ formed supramolecular nanofibers and the PU matrix. By contrast, the conventional glass fibers only served as reinforcing agents. In addition, DBS supramolecular nanofibers reinforced PU shows the advantage of reducing the weight and maintaining transparency of the polymers. Acknowledgements We gratefully acknowledge funding for this work provided by the National Basic Research Program of China (2012CB812500) and the National Natural Science Foundation of China (51073062). We also thank HUST Analytical and Testing Center for the SEM measurements. References [1] Xie Y, Hill CAS, Xiao Z, Militz H, Mai C. Silane coupling agents used for natural fiber/polymercomposites. Compos A-Appl Sci Manuf 2010;41:806–19. [2] Takayanagi M, Ogata T, Morikawa M, Kai T. Polymer composites of rigid and flexible molecules: System of wholly aromatic and aliphatic polyamides. J Macromol Sci B 1980;17:591–615. [3] Hwang WF, Wiff DR, Benner CL, Helminiak TE. Composites on a molecular level: phase relationships, processing, and properties. J Macromol Sci B 1983;22:231–57. [4] Dencheva N, Denchev Z, Oliveira MJ, Funari SS. Microstructure studies of in situ composites based on polyethylene/polyamide 12 blends. Macromolecules 2010;43:4715–26. [5] Pavlidou S, Papaspyrides CD. A review on polymer-layered silicate nanocomposites. Prog Polym Sci 2008;33:1119–98. [6] Coleman JN, Khan U, Blau WJ, Gun’ko YK. Small but strong: a review of the mechanical properties of carbon nanotube-polymercomposites. Carbon 2006;44:1624–52.
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