Nanocomposites of waterborne polyurethane (WPU) containing graphene oxide sheets (GO) were prepared by an in-situ polymerization method at low temperature. The morphology and interface structure were characterized by atomic force microscopy (AFM), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Without undergoing complicated functionalization processes, GO can be finely embed into a WPU matrix and present high degree of orientation at high GO contents, due to the formation of chemical bonds and hydrogen bonding. From the Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD) and dynamic mechanical analysis (DMA) results, incorporation of GO exists in two ways and shows inverse effects. At a content of 2.0 wt.% GO loading, the tensile elastic modulus of the GO-WPU film increased by 193% to neat WPU. The nanocomposites also displayed 30°C higher thermal stability than WPU in thermogravimetric (TG) curves. This environment-friendly method may pave the way to design graphene-based polymer composites.
Ever since the polymer nanocomposites was developed by the Toyota Research Group (1), cost-conscious polymer filled materials have attracted wide interest. Particularly, cheap monolayer carbon material-graphene can serve as a basic building block to fabricate polymer nanocomposites with integrated performance, arising from their unique electronic, thermal and mechanical properties (2), (3), (4). But, for graphene, the most important property is to disperse into a polymeric matrix, strengthen the bonding with the polymer (5). Very recently, using graphene oxide (GO) nanosheets to reinforce polymers has generated a great deal of interest (6), (7), (8), (9). GO exhibits a special layered structure featuring numerous oxygenated functional groups: hydroxyl (-OH), epoxy (-COC), carbonyl (-C=O), and carboxyl groups (-COOH) (6), which render the GO more hydrophilic than graphene (7). Then, GO may insert water based polymers with covalently bound oxygen or non-covalently bound oxygen between the carbon layers.
Waterborne polyurethane (WPU) is a kind of environment friendly material, with applications such as elastomers, textiles, medical devices, smart actuators, etc. Because of the adjustable soft and hard segment structure, it is possible to tailor the properties of WPU through well-designed combinations of monomers (10), (11), (12), (13). Especially, it is necessary to modify WPU with the thinnest and strongest nanoparticles, such as GO, to overcome its deficiencies: low modulus and low tensile strength. But GO is a highly polar material and cannot be readily dispersed in nonpolar polymeric media, the compatibility between the GO and WPU is not good, the strength of composite material obtained by simple mechanical blending is still not high enough. Therefore, much effort has been directed toward developing the production of polymer/graphene composites: namely, melt mixing, solvent mixing, in situ polymerization and latex blending. At the same time, some researchers are trying to change GO itself to improve the exfoliation and dispersion properties by physical interaction (8) or some chemical modification (9); these methods are very complex and may decrease the activity of GO functional groups in some way. Usually, by the aide of polar solvents such as dimethylformamide (DMF) (9) and surfactant (14), the GO and polyurethane material can dissolved as a whole. But, isolating these contaminate-environment organic reagents will increase the cost of preparation.
From the results of our experiment (Figure 1), GO can be easily dispersed in the polyether polyurethane below 10°C, while aggregating above the temperature. It may be caused by the fact that GO is prone to decompose into small weak acid molecules at high temperature, which will consume salt forming agents and reduce the hydrophilic groups of WPU, resulting in aggregation. Xing et al. (15), showed that the strength of the hydrogen bonds at low temperatures water are the strongest, making effective chain extensions of WPU. So, the composites were prepared to undergo the process of chain extension and in-situ polymerization at low temperature. This way may provide a chance for GO to be a part of the chain extender and embed in the rigid segments of waterborne polyurethane.
Many recent studies on GO incorporated WPU materials have focused on block copolymers. Segmented copolymers with well-defined chemical structures are relatively unexplored. In this study, we investigated the interaction between GO and WPU segments, the influence of the GO on thermal and mechanical properties, and the morphologies of doped GO.
2.1 Raw materials
Graphite powder (99%) concentrated H2SO4 (96%), H3PO4, HCl (53%), KMnO4 were all purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Isophorone diisocyanate (IPDI), poly(tetra methylene glycol) (PTMG; molecular weight 2000 g/mol), and dimethylol propionic acid (DMPA) were all industrial grade and purchased from Hefei Anke Chemical Co. (Anhui, China). PTMG was dried at 120°C in vacuo for 1 day before use, and the other chemicals were used as received. Deionized (DI) water was laboratory self-made.
2.2 Fabrication GO/WPU nanocomposites membrane
GO was synthesized based on the modified Hummers method (16). GO particles were diluted using DI water (~0.001 mg/ml–0.1 mg/ml) and mildly sonicated in a bath sonicator for 20 min. The GO dispersion was mixed with anionic aliphatic prepolymer (17) below 10°C. Afterwards, the homogeneous aqueous dispersion underwent the process of emulsification and chain extending by ethanediamine, which resulted in a brownish yellow emulsion with a solid content of around 20% and with up to 2 wt.% graphene content. The mixture was poured into a smooth glass and dried at an elevated temperature from 50°C to −120°C by 10°C/h to remove the solvent completely. Later, it was air-dried at room temperature for 2 days and stored in a desiccator to remove the moisture. The obtained product was named GO-WPU membrane. For example, GO-WPU-2% is a WPU that contains two parts of GO per 100 parts of polymer solid. When the amount of GO is above 2 wt.%, GO is prone to aggregate in the WPU emulsion.
The particle size and distribution of WPU dispersions were measured on a Zetasizer Nano analyzer ZS90 (Zata Nano-ZS90, Malvern, UK). Attenuated total reflection-Fourier transform infared (ATR-FTIR) spectra of WPU membranes were recorded on an FTIR (FT-IR, Thermo Nicolet, USA), Thermo Nicolet-67. X-ray diffractometer (XRD, Rigaku Corporation D/MAX2500V, Japan) was used to characterize the microstructure of WPU membranes. The GO-WPU emulsion were dropped on a copper grid for direct transmission electron microscopy (TEM, Hitachi-H800, Japan) examination. GO-WPU membrane profile preparation by ultramicrotome (Cambridge Instruments ULTRACUTE 970114), and detected by a transmission electron microscopy (TEM) instrument (JEOL, JEM-100SX, Japan). Freeze-fractured surfaces of the samples in liquid nitrogen were examined using a field emission scanning electron microscopy (SEM) (Hitachi-SU8020, Japan). Five nanometer of Au was sputtered (Denton Desk V Sputter system) on the film surface before imaging. Atomic force micrographs (AFM) (Fastscan, Bruker, Germany) were acquired using a silicon tip on a silicon nitride cantilever in the tapping mode. Dynamic mechanical analysis (DMA) measurements were performed on a DMA Instrument Q800 dynamic mechanical analyzer (DMA242E, Netzsch, Germany) in the film tension mode at a frequency of 1 Hz and a temperature ramp of 3°C min−1. Glass transition temperature of soft segment (Tgs) were determined according to the peak values in tanδ curves. The Young’s modulus at 100% (E100) was reported. Samples with rectangle shape test pieces were used. The thermal property of WPU membranes was measured on a Shimadzu DTG-60H (TG209 F3, NETZSCH, Germany) under a nitrogen flow of 50 ml/min with a heating rate of 10°C/min. Tensile properties of WPU membranes were tested according GB/T 1040.3 standard using an universal testing machine with a test speed of 10 mm/min and a clamp distance of 30 mm.
3 Results and discussion
3.1 Synthesis and characterization of GO-WPU emulsion
AFM observation provides direct evidence for the GO sheet thickness. The thickness of the GO sheet is about 2.5–5 nm (Figure 2), which is thicker than that of a monolayered GO sheet. The possible reason may be the folding characteristic of thin GO sheets as shown in Figure 3A. TEM images presents the morphology of emulsion samples (Figure 3A–C). A thin GO sheet is generally flat but rolled on the edge (Figure 3A), which arises from the existence of abundant functional groups, such as epoxy, carboxyl and hydroxyl groups, bonded to both sides of the graphene sheets which result in folds and distortions on the sheets (18). Prepolymerized polyurethane emulsions have a much more irregular bubble pattern (Figure 3B), while the GO-WPU dispersions display some solid spheres on the edge of GO sheets (Figure 3C). The morphology change was attributable to the chain extending and the in-situ polymerization process. In some way, it indicates that the polyurethane chain can only join with the edge of the GO, where these carboxyl and hydroxyl groups exist. The possible polymerization processes of GO-WPU are illustrated in cartoon patterns (Figure 4). Similar to other recent studies (19), by the successful chain extending, long chains of polyurethane easily assemble and are apt to be a sphere. Owing to the hydrophilic oxygenated groups of the GO sheet, these negatively charged and hydrophilic spheres are mostly located at the edge of the GO sheet surface. This interaction may prevent the aggregation of GO sheets.
In order to explore the formation mechanism of GO-WPU, the particle size distribution of different WPU emulsion were measured. As shown in Figure 5, the double-peak distribution of neat WPU showed the propagation of the chain segments and formed urea (15). The emulsion of GO-WPU emerges as three particle-distribution peaks, and eventually two peaks at the high content doping sample. This phenomenon shows the influence of doped GO content on the WPU structure, which is specifically described in Section 3.2.
3.2 Chemical and morphology analyses of the GO-WPU membrane
The chemical structure of the membranes was analyzed by ATR-FTIR. The corresponding function groups are shown in Figure 6. The FTIR bands of GO-WPU are similar to that of WPU and different from GO, arising from the low doping content of GO. There are four characteristic peaks both in the GO-WPU and WPU samples: N-H stretching vibrations (3321 cm−1), N-H bending vibrations (1535 cm−1), C=O stretching vibrations (1710 cm−1) and the C-N stretching vibrations (1349 cm−1) group, which confirm the formation of polyurethane chains (20). But with careful observation, there is a significant change between GO-WPU and WPU. In contrast, the peak’s shoulder of N-H (3300–3600 cm−1), C=O (1710–1750 cm−1) weakened and -C-O-C (1040–1100 cm−1) strengthened when GO was added to the polymer. Furthermore, comparing Figure 6B, the FTIR spectra of varying percentage of GO doped WPU membranes’ relative intensity of C=O decreases with the increase of the GO doping amount, while the relative intensity of C-O-C increases accordingly. Considering the main intermolecular interactions between -OH and -N=C=O groups, the urethane group -NHCO in GO-WPU are different from that in neat WPU. All these may be expected owing to the in-situ polymerization process at low temperature. At the same time, GO and polyurethane segments may be linked by hydrogen bonding too, because many oxygenated functional groups exist on the edge, as illustrated in Figure 4.
The peak at 1710 cm−1 (C=O) is ascribed to the ordered hydrogen bonding carbonyl (21), which reflects the formation of N-H…O=C bonding between soft segments and hard segments, also in the hard segments itself of WPU. The decreasing intensity of the peak at 1710 cm−1 shows the weakened bonding. It may be arising from the fact that the connection of GO-WPU reduces the hydrogen bonding between the center part of WPU itself. On the basis of the ATR-FTIR results, it can be deduced that the incorporation of GO exists in two ways: being blended with WPU by hydrogen bonding, or being embedded into the hard segment of WPU by in-situ polymerization and acts as a “barrier”.
Figure 7 shows the XRD profiles for GO, WPU and GO-WPU. It is clear that neat WPU shows two crystalline peaks at 2θ=7.11° and 19.56°. It might be the small crystalline structure of diffraction from the large crystals in PTMG (22). GO exhibits a crystalline diffraction peak at 10.8° (corresponding to interlayer spacing of about 0.834 nm) (14), which disappears in all the GO-WPU samples. Furthermore, with the increase of GO content, the diffraction peak of 7.11 decreases gradually until it disappears. From this experimental observation, we may infer that GO is embedded in the matrix of WPU to a certain degree. The higher the content of GO in the polyurethane segment, the more limitations there are on the migration of the segment, which disturbs the chain regularity of polyurethane. So, the absence of a diffraction peak in the XRD pattern does not necessarily suggest the lack of stacked GO platelets (23), as can be confirmed by TEM and SEM analysis.
3.2.3 The morphological interaction of GO in WPU membrane
The morphology of the GO-WPU membrane was detailed using the TEM images. The pristine WPU membrane presents a smooth surface with some stripes, due to the cutting action (Figure 8A). Figure 8B–D shows that the low dopant GO nanoparticles always float on the surface, indicating that interfacial bonding between GO and anionic polyurethane is not enough to resist the electrostatic attraction. It is visible that the random floating GO distribution is not uniform, in accordance with the multi peak distribution of particle size at low content GO samples depicted in Figure 5.
More importantly, the GO-WPU nanocomposite materials of more than 0.9% content reveal a highly aligned direction (Figure 8E–H). GO-WPU-2% show the characteristic protruding ridges of GO sheets running parallel to each other (Figure 8F). Judging from this fact, strong interfacial bonding may exist between GO and WPU after the in-situ polymerization processes. The conclusions are in agreement with the study of Yousefi et al. (24). This phenomenon has been observed in some nanoplatelet reinforcements, including clay (25), (26), (27) and CNT (28). Recently, it is also reported that GO and reduced GO could form a self-aligned structure during the evaporation process through layer by-layer stacking in polymers (29), (30) or water (31), (32). It was thought that the origin of self-orientation can be ascribed to different mechanisms: gravitational forces (30), shear flow (33), evaporation of solvent (34) and the formation of hydrogen bonds between the polymer and nanofillers (35).
Field emission scanning electron microscopy (FESEM) of the freeze-fractured surface was investigated to explore the interfacial interaction of GO in the WPU membrane. Figure 9 showed the typical polyester profile, some light coiled sheet features can be observed after the incorporation of GO sheets into the polymer (Figure 9B). This coiled sheet increases begining with the GO-WPU-0.09% membrane (Figure 9C–F). The GO curled sheet exerted from the fracture surface (Figure 9B–C), is attributed to a weak interfacial bond. With the increasing of GO content, some interspace dispersed in the samples and interestingly appeared to be randomly oriented, extended and embedded in the polymer (Figure 9D–F). Comparing all the fractured surfaces, higher doping content samples (GO-WPU-0.3%, GO-WPU-0.9%, GO-WPU-2%) disclose an extraordinary divergence in the interfacial interaction between the polyurethane matrix. The main reason for this phenomenon is the strong interaction of the oxygenated group on the GO surface and the urea group in the WPU hard segment (8). The interaction destroys the original segment mixing and forms a new segment morphology.
3.3 Thermal and mechanical properties of the GO-WPU membrane
DMA experiments were used to investigate the effect of GO content on the dynamic modulus and mechanical loss of different samples. All samples exhibited clear thermal transitions near −60°C, which corresponded to the Tgs of the WPU soft segment (36). As shown in Figure 10, the storage modulus values decrease noticeably after the application of GO in the period of glass transition. According to the ATR-FTIR results, the doped GO may reduce the H-bonding between the polymer segment, making the soft segment more flexible, this is called plasticizing effects (37), and results in the lower storage modulus. On the contrary, in the range of −50 to 100°C, the storage modulus values of GO-WPU increases compared with neat WPU, this can be explained by the restraint of the GO “barrier” mentioned in Section 3.2. According to the ideal rubber theory (38), (39), (40), the relationship between the peak value of storage modulus, crosslinking density, molecular weight and the crosslinks (Mc) can be interpreted by equation 
where ρ, density; T, the absolute temperature; R, gas constant; N, moles of sub-chains per volume.
In the case of GO-WPU, the increased plateau modulus with GO content is due to the decreased Mc or increased N. These are two aspects affecting the doping graphene oxide: GO linked by hydrogen bonding weakened the force between chain segments, making soft segments more flexible; GO embedded in polyurethane hard segment by an in-situ polymerization reaction, constrains the mobility of the soft segment. All the above hypothesis will be further proved by tensile testing.
Figure 11 shows the results of thermogravimetry with the decomposition temperature at 250°C for the hard segment and 350°C for the soft segment, respectively. Except for GO-WPU-0.03%, the thermal stability of other GO doped samples were improved. The bad thermal properties of GO-WPU-0.03% is due to the bad dispersion of the graphene nanoplatelets in the matrix polymer (41), (42). Highly doped GO (over 0.9 wt.%) revealed notable orientation of graphene sheets in the WPU membrane (Figure 8E,F), resulting in increasing thermal stability. Unprecedentedly, thermal stability of GO-WPU-2% increased by approximately 30°C with a 37% weight loss. This is obviously due to the thermal insulation barrier of highly aligned GO. Moreover, the composite residual weight percentages shows less weight loss up to 800°C with the feed content of GO. The enhanced thermal conduction in the presence of a high aspect ratio GO sheet may hinder the diffusion of decomposition products.
3.3.3 Tensile tests
Tensile tests were performed to evaluate the mechanical properties of the GO-WPU membrane. The distinctive stress-strain curves are presented in Figure 12, and compared in Table 1. It is clear that the addition of 2.0 wt.% GO to WPU effectively improve the tensile modulus but deteriorate the elongation at break. The tensile elastic modulus of the film increased by 193% to neat WPU. These results indicate the effect of the load transfer between GO nanoparticles and the WPU matrix. On the contrary, the elongation at break of GO-WPU-2% samples decreases by 51% to that of WPU, meaning there is less elastic deformation in doped membrane. Indeed, it often adversely affected the tensile strengths of the membrane with increasing weight fraction. These results show that these molecular rearrangements constrained the motion of the segment by a stronger interaction between GO and WPU. It was consistence with ATR-FTIR or XRD analysis.
|Sample||E: elastic modulus (MPa)||ε: elongation at break (%)||σ: tensile strength (MPa)|
With the change of the graphene doping, the elongation at breaks, elastic modulus and the tensile strength change irregularly, which may be ascribed to the different forms of existence and inverse effect of GO doping.
The high performance of GO-WPU membranes were prepared by in-situ polymerization at low temperature, without undergoing a complicated functionalization process. Distribution of GO in waterborne-polyurethane membrane exhibited high aligned characteristics depending on the doping content. Incorporation of GO exists in two ways and have inverse effects: blending with WPU by H-bonding, or embedded into a hard segment of WPU by in-situ polymerization. The GO-WPU nanocomposite membrane exhibited a significant improvement in terms of their mechanical and thermal properties due to the strong interactions between GO and the segmented WPU. The modulus and thermal stability of the GO-WPU-2% are 193% greater and 30°C higher than those of the neat WPU membrane, respectively. All these confirm the fact that the assembly of the structural blocks affects the properties of the “tailored” GO-WPU membrane. However, the polymerization mechanism of the GO-WPU and the character of composite membrane are to be investigated further.
This project was financially support provided by the Natural Science Foundation of China (no. 21476055).
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