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POSS nanocomposites for superior hydrophobicity and high ductility
Composites Part B 177 (2019) 107441
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Polyurethane/POSS nanocomposites for superior hydrophobicity and high ductility Hui Zhao a, b, Wei She c, Dean Shi a, Wei Wu d, e, **, Qun-chao Zhang a, *, Robert K.Y. Li e a
Key Laboratory for the Green Preparation and Application of Functional Materials, Hubei Key Laboratory of Polymer Materials, School of Materials Science and Engineering, Hubei University, Wuhan, 430062, China b College of Light Industry and Food Engineering, Guangxi University, Nanning, 530004, China c Jiangsu Key Laboratory for Construction Materials, Southeast University, Nanjing, 211189, China d National Engineering Research Center of Novel Equipment for Polymer Processing, Key Laboratory of Polymer Processing Engineering of Ministry of Education, Guangdong Provincial Key Laboratory of Technique and Equipment for Macromolecular Advanced Manufacturing, South China University of Technology, Guangzhou, 510640, China e Department of Materials Science and Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong SAR, China
A R T I C L E I N F O
A B S T A R C T
Keywords: POSS Aqueous polyurethane Cross-linking Hydrophobicity Ductility
A novel polyhedral oligomeric silsesquioxane with di-hydroxyl and two triethoxy groups (m-POSS) was syn thesized by a facile method. Then it reacted with diisocyanate to prepare aqueous polyurethane (APU)/m-POSS nanocomposites. The di-hydroxyl groups facilitating the m-POSS dispersing into the main chain of the APU, during which two triethoxy groups served as cross-linking sites. The addition of m-POSS had little influence on the viscosity and stability of the APU/m-POSS dispersions. The APU/m-POSS nanocomposite film with 8 wt% mPOSS exhibited superior surface hydrophobicity with water contact angle up to 121� due to the migration of Si element from inner to the surface. Moreover, the Young’s modulus and tensile strength of APU/m-POSS com posite increased up to 21.8 MPa and 15.3 MPa, respectively, with the elongation at break increased up to 1000%, which was ascribed to the enhancement of the cross-linked m-POSS. The DMA results revealed that the glass transition temperature (Tg) of the APU hard segment increased significantly while a little change was observed for that of soft segments, indicating that the m-POSS preferred to locate in the APU as hard segments. This functionalized m-POSS can endow polymer matrix with superior hydrophobicity and enhanced mechanical properties.
1. Introduction Aqueous polyurethanes (APUs) are getting more and more attention for their environmental friendliness, safer and cheaper, since water can be used as a dispersion medium [1–6]. However, the poor hydropho bicity and limited mechanical properties of APUs and restrict its further applications [7,8]. To overcome these shortcomings, it is simple and effective to introduce cross-linkers to modify the chemical structures of APUs, This method can endow APUs with high modulus and fast solid ification which is benefit to the mechanical properties. Furthermore, after cured, the polyurethane will changed into thermoset and will not
dissolve in water or other solvent. However, an excessive content of cross-linkers may result in an unstable system and make the hybrid materials more brittle [9]. Polyhedral oligomeric silsesquioxane (POSS) is a special type of functional nano-filler with a hard, inorganic featured eight-corner -(SiO1.5)n-based cage that bears several organic functional side groups. The functional side groups in POSS exhibit unique feature for possibility of target chemical modification. The multiple reactive functionalities as well as inorganic nature of POSS make itself as interesting building blocks for preparing organic-inorganic hybrids with advanced func tionality. Recently, POSS have been widely used to improve the
* Corresponding author. Key Laboratory for the Green Preparation and Application of Functional Materials, Hubei Key Laboratory of Polymer Materials, School of Materials Science and Engineering, Hubei University, Wuhan, 430062, China. ** Corresponding author. National Engineering Research Center of Novel Equipment for Polymer Processing, Key Laboratory of Polymer Processing Engineering of Ministry of Education, Guangdong Provincial Key Laboratory of Technique and Equipment for Macromolecular Advanced Manufacturing, South China University of Technology, Guangzhou, 510640, China E-mail addresses: [email protected] (W. Wu), [email protected] (Q.-c. Zhang). https://doi.org/10.1016/j.compositesb.2019.107441 Received 29 July 2019; Received in revised form 2 September 2019; Accepted 11 September 2019 Available online 13 September 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.
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Composites Part B 177 (2019) 107441
Table 1 Formulations of the prepared pure APU and APU nanocomposites. Samples
APU0
APU2
APU4
APU6
APU8
IPDI (g) PTMG (g) POSS (g) DMPA (g) APTES (g) TEA (g)
60 23.5 0 26.5 20 26.5
60 22.2 1.5 26.3 20 26.3
60 20.9 3.0 26.1 20 26.1
60 19.6 4.5 25.9 20 25.9
60 18.3 6.0 25.7 20 25.7
In this study, POSS with di-hydroxyl and two triethoxy groups (mPOSS) was proposed to solve the above mentioned shortcomings. The mPOSS synthesized by ring-opening reaction between polyisobutyl-amino cage siloxane and triethoxy(3-(glycidyloxy)propyl)silane. The intro duced m-POSS can function as a chain extender and will contribute to the attachment of m-POSS chain to the APU backbone. It is expected that the viscosity of the system could be lowered during the processing and the reactions are better controlled. In addition, the attached -Si(OC2H5)3 groups in m-POSS can afford the APU chemical cross-linking functions, while not causing excessive cross-linking, and the enough Si in the POSS may make it excellent hydrophobic. The effects of m-POSS content on hydrophobicity, tensile properties, and dynamic mechanical performance for the APU/m-POSS nanocomposite films were investi gated. This new approach of m-POSS will provide new windows to synthesize other polymer nanocomposites with enhanced hydropho bicity and mechanical properties. 2. Experimental section 2.1. Materials Triethylamine (TEA), isophorone diisocyanate (IPDI), di-nbutyltindilaurate (DBTDL), 2, 2-bis (hydroxymethyl) butyric acid (DMPA), poly(tetramethylene glycol) (PTMG, Mn ¼ 2000), triethoxy(3(glycidyloxy)propyl)silane, ethanol, aminopropyltriethoxysilane (APTES), and tetrahydrofuran (THF), were obtained from Aladdin, USA. Both acetone and THF were refluxed and then distilled before use. 2.2. Synthesis of m-POSS The detail synthesis of the precursor polyisobutyl-amino cage siloxane was described in the supporting information. The mass spec trometry results in Fig. S1 confirmed the modification was successful. The obtained polyisobutyl-amino cage siloxane (4.365 g) and triethoxy (3-(glycidyloxy)propyl)silane (2.78 g) were added in dry THF � (28.58 mL) at 25 C. The mixture was stirred for 72 h, resulting in the formation of 20% of solid dihydroxyl-POSS.
Scheme 1. Synthetic routes for m-POSS and APU/m-POSS nanocomposites.
properties of polymer materials, such as polyolefin [10], epoxy [11], acrylics [12] and polyurethane [13]. Nanda et al. studied the influence of diamino-POSS on the dispersion and physical properties of poly urethanes [14,15]. The results revealed that the incorporation of POSS monomers increased the tensile strength, storage modulus and glass transition temperature (Tg) significantly. The surface hydrophobicity of those nanocomposites increased slightly. Moreover, this method showed no change on the nature of physical cross-linking during the curing re action, and the resultant materials had a poor storage performance, especially under a high temperature. Zhang et al. used octafunctional-POSS macromonomers as cross-linkers to synthesize POSS-polyurethane nanocomposite [16]. It was observed that the POSS particles could improve the storage modulus and Tg of the PU/POSS composites. The enhanced properties were attributed to a synergetic effect between the POSS and polyurethane, in which the network formed by the POSS cross-linker in the polyurethane composite hindered the mobility of the polyurethane chain. However, the POSS caused excessive cross-linking, which was harmful to the film formation process.
2.3. Synthesis of APU/m-POSS nanocomposites As shown in Scheme 1, firstly, both PTMG and DMPA were propor tioned in stoichiometric manner. The mixture together with acetone and DBTDL were added in a four-neck flask in argon atmosphere. The so � lution was homogenized under stirring at 60 C for 5 min. Then IPDI was added with vigorous stirring until the content of the –NCO group was maintained a prescribed value (confirmed by dibutylamine back titra tion). After 1 h, the m-POSS was added in the system and the reaction � maintained at 60 C for 3 h. Afterwards, the solution was cooled down to � 10 C, and APTES was dropped in to react with the excessive –NCO groups. Subsequently, deionized water was added, and acetone was � removed under reduced pressure at about 30 C. Finally, a series of APU/ m-POSS hybrid water dispersions with the solid content of 33 wt% were obtained. The abbreviations of APU0, APU2, APU4, APU6 and APU8 represented the content of POSS were 0, 2, 4, 6, and 8 wt% in the APU nanocomposites, as shown in Table 1. 2
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Fig. 3. Particle size distributions of all APU/m-POSS dispersions, insert is the photograph of APU/m-POSS dispersions.
Fig. 1. FT-IR spectrum of m-POSS.
the stretching speed was 300 mm/min. The gel contents Wg (%) was determined by the equation: Wg (%) ¼ m2/m1 � 100%, where m1 was the mass of the films before the extraction, and m2 was the masses of films after the extraction. The Data physics OCA20 contact angle meter was used to record the static contact angles. The surface free energies of the films were calculated according to the Owens and Wendt equation [17]: γ L(1þcosθ) ¼ 2(γdSγ dL)1/2þ2(γpSγ pL)1/2, where γdS and γ dL were the dispersion of the solid and liquid, γpS and γ pL were the polar components of the solid and liquid, γL was the surface tension of the liquid.
2.4. Characterization 1 H NMR and 13C NMR spectra were obtained by the Inova-400 sys tem at 400 MHz. FT-IR spectra were recorded on Thermo Fisher Scien tific spectrometer in the range of 4000–500 cm 1. The dispersibility of the dispersions including average particle size and polydispersity indice (PDI) were performed by dynamic light scattering (DLS) using a Malvern � Zetasizer NanoZS at 25 C. Before the experiment, all samples were diluted to about 1 wt% aqueous dispersions. To examine the storage stability, all dispersions were kept in sealed bottles at room temperature. The viscosity of the dispersions was tested by a Brookfield LVDV-II viscometer at room temperature. TEM image were taken using the Tecnai G2 F30 system with an accelerating voltage of 300 kV. Prior to the observation, the synthesized dispersions were diluted in water so that the solid content was about 0.1 wt%. Wide angle XRD were con ducted on a Bruker D8 Advance, with a diffraction angle from 5 to 80� . XPS tests were performed on a Thermo VG ESCALAB 250 spectrometer. SEM images were taken by JSM5900LV, JEOL. AFM tests were carried out using the CSPM 2003 system in the tapping mode. DMA (dynamic mechanical analyzer, Q800, TA) was used to test the dynamic me chanical properties of the product at the frequency of 1 Hz and heating � � rate of 3 C/min from 100 to 200 C. The tensile strength of the films � were carried on a universal testing machine (3367, Instron) at 25 C and
3. Results and discussion 3.1. Structural characterization of m-POSS The FT-IR spectrum of m-POSS is shown in Fig. 1. The peak located at 1116 cm 1 is corresponding to the asymmetric stretching vibration of Si–O–Si, and the peak of 3434 cm 1 is attributed to the –OH groups in the silsesquioxane cage. The peaks of 2960 and 2873 cm 1 are ascribed to the stretching vibrations of C–H while those at 1456 and 1378 cm 1 are associated with C–H bending vibrations. The 1H NMR spectrum of the functionalized m-POSS is shown in Fig. 2a. The resonance peaks at 3.78, 3.58, 334, 2.78, 2.62, 1.80, 1.72, 1.51, 1.20, 0.91 and 0.54 ppm can be assigned to its different hydrogen atoms. Firstly, the number of
Fig. 2. The NMR spectra of the m-POSS. (a) 1H NMR; (b) 3
13
C NMR.
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3.2. Dispersions of APU/POSS hybrids
Table 2 The dispersion properties of APUs nanocomposite dispersions. Samples
Appearance
Numberaverage particle size (nm)
PDI
Viscosity (mPa�s)
Storage stability
APU0
Translucent, blue light Translucent, blue light Translucent, blue light Translucent, blue light Translucent, blue light
36
0.07
47
>30 days
42
0.20
52
>30 days
49
0.28
50
>30 days
55
0.34
55
>30 days
68
0.44
53
>30 days
APU2 APU4 APU6 APU8
The physical properties of all APU-based dispersions are presented in Fig. 3 and Table 2. It is observed that all the products have similar translucent appearance. The average particle size and PDI increased with the increasing m-POSS content, which is due to an increase in crosslinking density [18]. In addition, the higher m-POSS content results in more urethane and less urea residuals. It is speculated that the size of the urea group is slightly smaller than that of the urethane one, which will contribute to the formation of smaller particle [19]. The value of PDI increases from 0.07 to 0.44 when the content of m-POSS increases up to 8 wt%. The viscosity of APUs nanocomposite dispersions shows a slight increase with increase in m-POSS content. Moreover, no obvious change is observed for all the dispersions after 30 days storage. These results indicated that the addition of m-POSS had only a slight influence on the viscosity and storage performance of APU nanocomposites [18,20]. TEM images of APU0 and APU8 are shown in Fig. 4. The diameter of most particles in Fig. 4a is below 40 nm, while the particles in Fig. 4b have a broader size distribution and there seem to be some particle agglomeration. The heterogeneity in particle size and shape would be caused by the incorporation of silicon element and strengthened by the increase of m-POSS content. These results indicated that the incorpo ration of m-POSS have little influence on the APU stability.
hydrogen atoms at position k was the most, so the peak k was the highest, and the hydrogen atoms at position a could be identified by the same method, the hydrogen atoms at position b, d, e and f were next to oxygen atom, so they were in the lowest filed, the hydrogen atoms at position g and h were next to nitrogen atom, so they were in the higher filed than hydrogen atoms at position b, d, e and f. The hydrogen atoms at position c, m, and j were next to Si, so they were in the highest field. The 13C NMR spectrum shows resonance peaks at 73.82, 71.43, 68.03, 58.43, 50.95, 44.42, 25.76, 23.91, 23.01, 22.50 and 18.36 ppm as shown in Fig. 2b, the position can. The above FT-IR spectrum and NMR results confirmed that the m-POSS was synthesized successfully.
3.3. Structural analysis of the APU/POSS nanocomposite films As shown in Fig. 5a, the FT-IR spectra of all the films have similar
Fig. 4. TEM image of (a) APU0 and (b) APU8.
Fig. 5. (a) FT-IR spectra of the APUs; (b) Si–O–C spectra in the region around 994 cm 4
1
.
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Fig. 6. (a) XRD patterns of the hybrid films and m-POSS; (b) XPS survey spectrum of APU8 film; (c) Si2p, (d) C1S, and (e) O1S high-resolutions of APU8 film; (f) XPS results of Si concentration.
characteristic peaks. The peaks at 2936 cm 1 and 2855 cm 1 are cor responding to the anti-symmetry and symmetry stretching vibration of CH2, respectively. The peak of 3336 cm 1 is ascribed to N–H stretching vibration, and the peak at 1529 cm 1 is due to bending vibration of N–H. Meanwhile, the band at 1714 cm 1 is related to the stretching vibration of C¼O. These results indicated that the isocyanate groups transformed to urethane moieties completely. Fig. 5b shows the spectrum from 900 to 1100 cm 1. It can be seen that the peak of 994 cm 1 in APU0 dis appeared in the APU/POSS hybrid materials, indicating an increase of
the degree of cross-linking. The XRD patterns of m-POSS and APU nanocomposite films are shown in Fig. 6a. The peak centered at 8.1� corresponds to the (101) plan of the m-POSS [21]. The diffraction peak around 19.8� is ascribed to the amorphous phase in the polyurethane, which displays a decreasing trend as the m-POSS content increases. With the increase of m-POSS, the APU6 and APU8 showed an additional peak at about 8.1� , and the intensity of the peak increased with the increase of POSS con tent, also indicating that the POSS moieties were chemically linked to
Fig. 7. SEM images of the film surfaces of (a) APU0, (b) APU2, (c) APU4, (d) APU6, and (e) APU8. 5
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of the APU films can contribute to the migration of Si element from the inner to the surface of APU films [22,23], which indicates that the feasibility of the strategy for simultaneously designing the silane groups to the end of molecular chain and its main chain backbone. Fig. 7 shows the SEM images of all the films’ surfaces. It is observed that APU0 and APU2 exhibit rough surfaces. With the increasing content of m-POSS, the surfaces of the samples become smoother. The AFM images of the surfaces are presented in Fig. S2. It is noted that the APU0 film has a root-mean-square (RMS) surface roughness value (Rq) of 17.17 nm, whereas the Rq of APU4 is 9.04 nm and that of APU8 is 4.40 nm. This basically has two reasons: firstly, the mobility of chain segments is restricted and the silane including m-POSS moved to the surface, secondly, the silane including m-POSS moved to the surface.
Table 3 Values of the contact angle and surface free energy (SFE) for the APU/m-POSS nanocomposite films. Samples APU0 APU2 APU4 APU6 APU8
Contact angle (o)
SFE (mJ/m2)
Hexadecane Water
Water
γps
γds
γts
13.5 12.7 11.2 9.8 9.5
84.0 93.8 104.7 115.8 121.0
5.41 2.10 0.23 0.04 0.01
25.29 25.37 25.51 24.62 22.53
30.70 27.47 25.74 24.66 22.54
the polymer chain. Fig. 6b shows the XPS survey spectrum of APU8. The peaks at 285, 400 and 533 eV are corresponding to C1s, N1s and O1s, respectively. Moreover, the presence peak of Si2p (102 eV) and Si2s (153 eV) confirm that the m-POSS was incorporated into APU backbone successfully. The high-resolution spectrum of Si2p on the surface of the APU8 film and the corresponding deconvolution results are presented in Fig. 6c. The Si–O–C at 102.6 eV bond is derived from the –Si(OC2H5)3 moieties, which creates during the film-forming stage, and the Si–O–Si at about 102 eV indicated the occurrence of cross-link networks. In addition, it is noted that the bonds of Si–O–C and Si–O–Si can be observed in the corresponding deconvolution results of C1S and O1S in Fig. 6d, e, which were consistent with the results of Si2p spectrum. Fig. 6f shows the Si concentrations in the bulk and on the surface of the APU films. As expected, the concentration of Si element on the film surface increases with the increasing m-POSS content. In addition, it is noted that the experimental value of Si concentration on the surface of APU8 is 4.09%, which is much higher than the theoretical value (2.08%) for APU8. This phenomenon is speculated that the lower surface tension
3.4. Surface hydrophobicity and mechanical properties of the hybrids The incorporation of siloxane networks in the APU was important to its surface energy and surface hydrophobicity [24,25]. Table 3 sum marizes the values of the static contact angle and surface free energy of the films. It is observed in Fig. 8 that the water contact angle increases from 84.0� to 121� with the increasing of m-POSS content. The corre sponding total surface energy in Table 3 reduces from 30.7 to 22.5 mJ/m2, which is in consistence with the XPS results. This is because the presence of Si–O–Si network structure in the APU matrix provided a thermodynamic driving force for its migration from inner to the surface, and the smoother surface for the m-POSS, resulting in reducing the surface energy and rendering the surface hydrophobicity [24,25]. The stress-strain curves of the APU/m-POSS nanocomposite films are shown in Fig. 9a. A typical rubber-like elasticity behavior is observed for
Fig. 8. Water drops on the hybrid films and schematic representation of the effect of m-POSS on the water wettability of the hybrid film surfaces.
Fig. 9. (a) Strain-stress curves; (b) The gel content of the APU nanocomposite films. 6
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Fig. 10. Fracture surface SEM images of (a) APU0, (b) APU2, (c) APU4, (d) APU6 and (e) APU8.
incorporation of the m-POSS, which indicated an improvement of the toughness. The SEM images of the fracture surfaces of the APU/m-POSS nanocomposite films are shown in the Fig. 10a–e. It can be seen that the fracture surface became rough when m-POSS was added into the APU, which exhibited a ductile fracture and indicated strong resistance to the break [27–30]. The higher elongation at break means higher motion ability in the plastic zone when more m-POSS were introduced. The m-POSS particles may be incorporated in the hard segments and the small m-POSS crystals act as physical cross-linking points in the struc ture, which would absorb large amount of energy and improved the toughness under the external strain, as shown in the Fig. 11. The DMA curves for the APU nanocomposite films are shown in Fig. 12. It is observed that the two peaks in Fig. 12a are corresponding to the Tg of the soft segments (Tg1) and the hard segments (Tg2), respec tively. The corresponding temperatures of Tg1, Tg2 and ΔTg ¼ (Tg2 -Tg1) are listed in Table S1. It is noted that the Tg2 increases significantly from 60.1 to 159.2 � C, while Tg1 shows a slight decrease from 68.6 to 75.4 � C. These results indicate that the m-POSS prefers to disperse in the APU as hard segments [27–30]. In addition, the value of ΔTg
Fig. 11. Schematic showing the breakage of polymer chains with the increase of tensile stress.
all the sample. With the addition of m-POSS, the mechanical properties of the APUs/m-POSS nancomposites are significantly enhanced. The Young’s modulus increased from 2.1 to 21.3 MPa and the tensile strength increased from 2.2 to 15.8 MPa, which could be attributed to nano-reinforcement of the m-POSS and enhanced cross-linking density from 56% to 87%, as shown in the Fig. 9b [26]. It is noted that the elongation at break of the films was also greatly improved by the
Fig. 12. The DMA curves of APU nanocomposite films. (a) tanδ; (b) storage modulus. 7
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increases significantly with the increasing m-POSS content, which im plies the enhanced micro-phase separation between the hard and soft segments [27–30], thus resulting in enhanced mechanical properties. The storage modulus of the APU nanocomposites in Fig. 12b shows a rapid decrease around 70 � C, which was related to the chain relaxation of the soft segments. Moreover, the storage modulus significantly in creases with the increasing m-POSS content throughout the whole temperature range.
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4. Conclusions A novel technique to functionalize POSS with dihydroxyl and two triethoxy groups (m-POSS) was successfully demonstrated. APU/mPOSS nanocomposites were fabricated through the reaction between isocyanate and hydroxyl groups. The incorporation of m-POSS showed little effect on the viscosity and storage performance of the APU dis persions. The DMA tests revealed that the m-POSS was preferentially located in the hard segment domains, which resulted in a significant increase in the hard segments. The APU/m-POSS nanocomposite films had a high ductility (elongation at break over 1000%) with significantly enhanced Young’s modulus and tensile strength simultaneously. The improvements were attributed to the m-POSS acting both as nano-fillers and cross-linkers in the APU system. Acknowledgements This work was supported by the grants from the National Key Research and Development Program of China (No. 2016YFB0302000), the Science and Technology Major Projects of Hubei Province (No. 2017AAA115), the Fundamental Research Funds for the Central Uni versities (No. 2019MS062), and City University of Hong Kong (No. SRG 7004918). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.compositesb.2019.107441. References [1] Ahmadi Y, Yadav M, Ahmad S. Oleo-polyurethane-carbon nanocomposites: effects of in-situ polymerization and sustainable precursor on structure, mechanical, thermal, and antimicrobial surface-activity. Compos B: Eng 2019;164:683–92. [2] Wu W, He H, Liu T, Wei R, Cao X, Sun Q, et al. Synergetic enhancement on flame retardancy by melamine phosphate modified lignin in rice husk ash filled P34HB biocomposites. Compos Sci Technol 2018;168:246–54. [3] Bahrami S, Solouk A, Mirzadeh H, Seifalian AM. Electroconductive polyurethane/ graphene nanocomposite for biomedical applications. Compos B: Eng 2019;168: 421–31. [4] Zhao D, Hamada H, Yang Y. Influence of polyurethane dispersion as surface treatment on mechanical, thermal and dynamic mechanical properties of laminated woven carbon-fiber-reinforced polyamide 6 composites. Compos B: Eng 2019;160:535–45. [5] Jing QF, Liu Q, Li L, Dong ZL, Silberschmidt V. Effect of graphene-oxide enhancement on large-deflection bending performance of thermoplastic polyurethane elastomer. Compos B: Eng 2016;89:1–8. [6] Shi Y, Liu C, Liu L, Fu L, Yu B, Lv Y, et al. Strengthening, toughing and thermally stable ultra-thin MXene nanosheets/polypropylene nanocomposites via nanoconfinement. Chem Eng J 2019;378:122267.
8
Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Polyurethane/POSS nanocomposites for superior hydrophobicity and high ductility Hui Zhao a, b, Wei She c, Dean Shi a, Wei Wu d, e, **, Qun-chao Zhang a, *, Robert K.Y. Li e a
Key Laboratory for the Green Preparation and Application of Functional Materials, Hubei Key Laboratory of Polymer Materials, School of Materials Science and Engineering, Hubei University, Wuhan, 430062, China b College of Light Industry and Food Engineering, Guangxi University, Nanning, 530004, China c Jiangsu Key Laboratory for Construction Materials, Southeast University, Nanjing, 211189, China d National Engineering Research Center of Novel Equipment for Polymer Processing, Key Laboratory of Polymer Processing Engineering of Ministry of Education, Guangdong Provincial Key Laboratory of Technique and Equipment for Macromolecular Advanced Manufacturing, South China University of Technology, Guangzhou, 510640, China e Department of Materials Science and Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong SAR, China
A R T I C L E I N F O
A B S T A R C T
Keywords: POSS Aqueous polyurethane Cross-linking Hydrophobicity Ductility
A novel polyhedral oligomeric silsesquioxane with di-hydroxyl and two triethoxy groups (m-POSS) was syn thesized by a facile method. Then it reacted with diisocyanate to prepare aqueous polyurethane (APU)/m-POSS nanocomposites. The di-hydroxyl groups facilitating the m-POSS dispersing into the main chain of the APU, during which two triethoxy groups served as cross-linking sites. The addition of m-POSS had little influence on the viscosity and stability of the APU/m-POSS dispersions. The APU/m-POSS nanocomposite film with 8 wt% mPOSS exhibited superior surface hydrophobicity with water contact angle up to 121� due to the migration of Si element from inner to the surface. Moreover, the Young’s modulus and tensile strength of APU/m-POSS com posite increased up to 21.8 MPa and 15.3 MPa, respectively, with the elongation at break increased up to 1000%, which was ascribed to the enhancement of the cross-linked m-POSS. The DMA results revealed that the glass transition temperature (Tg) of the APU hard segment increased significantly while a little change was observed for that of soft segments, indicating that the m-POSS preferred to locate in the APU as hard segments. This functionalized m-POSS can endow polymer matrix with superior hydrophobicity and enhanced mechanical properties.
1. Introduction Aqueous polyurethanes (APUs) are getting more and more attention for their environmental friendliness, safer and cheaper, since water can be used as a dispersion medium [1–6]. However, the poor hydropho bicity and limited mechanical properties of APUs and restrict its further applications [7,8]. To overcome these shortcomings, it is simple and effective to introduce cross-linkers to modify the chemical structures of APUs, This method can endow APUs with high modulus and fast solid ification which is benefit to the mechanical properties. Furthermore, after cured, the polyurethane will changed into thermoset and will not
dissolve in water or other solvent. However, an excessive content of cross-linkers may result in an unstable system and make the hybrid materials more brittle [9]. Polyhedral oligomeric silsesquioxane (POSS) is a special type of functional nano-filler with a hard, inorganic featured eight-corner -(SiO1.5)n-based cage that bears several organic functional side groups. The functional side groups in POSS exhibit unique feature for possibility of target chemical modification. The multiple reactive functionalities as well as inorganic nature of POSS make itself as interesting building blocks for preparing organic-inorganic hybrids with advanced func tionality. Recently, POSS have been widely used to improve the
* Corresponding author. Key Laboratory for the Green Preparation and Application of Functional Materials, Hubei Key Laboratory of Polymer Materials, School of Materials Science and Engineering, Hubei University, Wuhan, 430062, China. ** Corresponding author. National Engineering Research Center of Novel Equipment for Polymer Processing, Key Laboratory of Polymer Processing Engineering of Ministry of Education, Guangdong Provincial Key Laboratory of Technique and Equipment for Macromolecular Advanced Manufacturing, South China University of Technology, Guangzhou, 510640, China E-mail addresses: [email protected] (W. Wu), [email protected] (Q.-c. Zhang). https://doi.org/10.1016/j.compositesb.2019.107441 Received 29 July 2019; Received in revised form 2 September 2019; Accepted 11 September 2019 Available online 13 September 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.
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Table 1 Formulations of the prepared pure APU and APU nanocomposites. Samples
APU0
APU2
APU4
APU6
APU8
IPDI (g) PTMG (g) POSS (g) DMPA (g) APTES (g) TEA (g)
60 23.5 0 26.5 20 26.5
60 22.2 1.5 26.3 20 26.3
60 20.9 3.0 26.1 20 26.1
60 19.6 4.5 25.9 20 25.9
60 18.3 6.0 25.7 20 25.7
In this study, POSS with di-hydroxyl and two triethoxy groups (mPOSS) was proposed to solve the above mentioned shortcomings. The mPOSS synthesized by ring-opening reaction between polyisobutyl-amino cage siloxane and triethoxy(3-(glycidyloxy)propyl)silane. The intro duced m-POSS can function as a chain extender and will contribute to the attachment of m-POSS chain to the APU backbone. It is expected that the viscosity of the system could be lowered during the processing and the reactions are better controlled. In addition, the attached -Si(OC2H5)3 groups in m-POSS can afford the APU chemical cross-linking functions, while not causing excessive cross-linking, and the enough Si in the POSS may make it excellent hydrophobic. The effects of m-POSS content on hydrophobicity, tensile properties, and dynamic mechanical performance for the APU/m-POSS nanocomposite films were investi gated. This new approach of m-POSS will provide new windows to synthesize other polymer nanocomposites with enhanced hydropho bicity and mechanical properties. 2. Experimental section 2.1. Materials Triethylamine (TEA), isophorone diisocyanate (IPDI), di-nbutyltindilaurate (DBTDL), 2, 2-bis (hydroxymethyl) butyric acid (DMPA), poly(tetramethylene glycol) (PTMG, Mn ¼ 2000), triethoxy(3(glycidyloxy)propyl)silane, ethanol, aminopropyltriethoxysilane (APTES), and tetrahydrofuran (THF), were obtained from Aladdin, USA. Both acetone and THF were refluxed and then distilled before use. 2.2. Synthesis of m-POSS The detail synthesis of the precursor polyisobutyl-amino cage siloxane was described in the supporting information. The mass spec trometry results in Fig. S1 confirmed the modification was successful. The obtained polyisobutyl-amino cage siloxane (4.365 g) and triethoxy (3-(glycidyloxy)propyl)silane (2.78 g) were added in dry THF � (28.58 mL) at 25 C. The mixture was stirred for 72 h, resulting in the formation of 20% of solid dihydroxyl-POSS.
Scheme 1. Synthetic routes for m-POSS and APU/m-POSS nanocomposites.
properties of polymer materials, such as polyolefin [10], epoxy [11], acrylics [12] and polyurethane [13]. Nanda et al. studied the influence of diamino-POSS on the dispersion and physical properties of poly urethanes [14,15]. The results revealed that the incorporation of POSS monomers increased the tensile strength, storage modulus and glass transition temperature (Tg) significantly. The surface hydrophobicity of those nanocomposites increased slightly. Moreover, this method showed no change on the nature of physical cross-linking during the curing re action, and the resultant materials had a poor storage performance, especially under a high temperature. Zhang et al. used octafunctional-POSS macromonomers as cross-linkers to synthesize POSS-polyurethane nanocomposite [16]. It was observed that the POSS particles could improve the storage modulus and Tg of the PU/POSS composites. The enhanced properties were attributed to a synergetic effect between the POSS and polyurethane, in which the network formed by the POSS cross-linker in the polyurethane composite hindered the mobility of the polyurethane chain. However, the POSS caused excessive cross-linking, which was harmful to the film formation process.
2.3. Synthesis of APU/m-POSS nanocomposites As shown in Scheme 1, firstly, both PTMG and DMPA were propor tioned in stoichiometric manner. The mixture together with acetone and DBTDL were added in a four-neck flask in argon atmosphere. The so � lution was homogenized under stirring at 60 C for 5 min. Then IPDI was added with vigorous stirring until the content of the –NCO group was maintained a prescribed value (confirmed by dibutylamine back titra tion). After 1 h, the m-POSS was added in the system and the reaction � maintained at 60 C for 3 h. Afterwards, the solution was cooled down to � 10 C, and APTES was dropped in to react with the excessive –NCO groups. Subsequently, deionized water was added, and acetone was � removed under reduced pressure at about 30 C. Finally, a series of APU/ m-POSS hybrid water dispersions with the solid content of 33 wt% were obtained. The abbreviations of APU0, APU2, APU4, APU6 and APU8 represented the content of POSS were 0, 2, 4, 6, and 8 wt% in the APU nanocomposites, as shown in Table 1. 2
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Fig. 3. Particle size distributions of all APU/m-POSS dispersions, insert is the photograph of APU/m-POSS dispersions.
Fig. 1. FT-IR spectrum of m-POSS.
the stretching speed was 300 mm/min. The gel contents Wg (%) was determined by the equation: Wg (%) ¼ m2/m1 � 100%, where m1 was the mass of the films before the extraction, and m2 was the masses of films after the extraction. The Data physics OCA20 contact angle meter was used to record the static contact angles. The surface free energies of the films were calculated according to the Owens and Wendt equation [17]: γ L(1þcosθ) ¼ 2(γdSγ dL)1/2þ2(γpSγ pL)1/2, where γdS and γ dL were the dispersion of the solid and liquid, γpS and γ pL were the polar components of the solid and liquid, γL was the surface tension of the liquid.
2.4. Characterization 1 H NMR and 13C NMR spectra were obtained by the Inova-400 sys tem at 400 MHz. FT-IR spectra were recorded on Thermo Fisher Scien tific spectrometer in the range of 4000–500 cm 1. The dispersibility of the dispersions including average particle size and polydispersity indice (PDI) were performed by dynamic light scattering (DLS) using a Malvern � Zetasizer NanoZS at 25 C. Before the experiment, all samples were diluted to about 1 wt% aqueous dispersions. To examine the storage stability, all dispersions were kept in sealed bottles at room temperature. The viscosity of the dispersions was tested by a Brookfield LVDV-II viscometer at room temperature. TEM image were taken using the Tecnai G2 F30 system with an accelerating voltage of 300 kV. Prior to the observation, the synthesized dispersions were diluted in water so that the solid content was about 0.1 wt%. Wide angle XRD were con ducted on a Bruker D8 Advance, with a diffraction angle from 5 to 80� . XPS tests were performed on a Thermo VG ESCALAB 250 spectrometer. SEM images were taken by JSM5900LV, JEOL. AFM tests were carried out using the CSPM 2003 system in the tapping mode. DMA (dynamic mechanical analyzer, Q800, TA) was used to test the dynamic me chanical properties of the product at the frequency of 1 Hz and heating � � rate of 3 C/min from 100 to 200 C. The tensile strength of the films � were carried on a universal testing machine (3367, Instron) at 25 C and
3. Results and discussion 3.1. Structural characterization of m-POSS The FT-IR spectrum of m-POSS is shown in Fig. 1. The peak located at 1116 cm 1 is corresponding to the asymmetric stretching vibration of Si–O–Si, and the peak of 3434 cm 1 is attributed to the –OH groups in the silsesquioxane cage. The peaks of 2960 and 2873 cm 1 are ascribed to the stretching vibrations of C–H while those at 1456 and 1378 cm 1 are associated with C–H bending vibrations. The 1H NMR spectrum of the functionalized m-POSS is shown in Fig. 2a. The resonance peaks at 3.78, 3.58, 334, 2.78, 2.62, 1.80, 1.72, 1.51, 1.20, 0.91 and 0.54 ppm can be assigned to its different hydrogen atoms. Firstly, the number of
Fig. 2. The NMR spectra of the m-POSS. (a) 1H NMR; (b) 3
13
C NMR.
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3.2. Dispersions of APU/POSS hybrids
Table 2 The dispersion properties of APUs nanocomposite dispersions. Samples
Appearance
Numberaverage particle size (nm)
PDI
Viscosity (mPa�s)
Storage stability
APU0
Translucent, blue light Translucent, blue light Translucent, blue light Translucent, blue light Translucent, blue light
36
0.07
47
>30 days
42
0.20
52
>30 days
49
0.28
50
>30 days
55
0.34
55
>30 days
68
0.44
53
>30 days
APU2 APU4 APU6 APU8
The physical properties of all APU-based dispersions are presented in Fig. 3 and Table 2. It is observed that all the products have similar translucent appearance. The average particle size and PDI increased with the increasing m-POSS content, which is due to an increase in crosslinking density [18]. In addition, the higher m-POSS content results in more urethane and less urea residuals. It is speculated that the size of the urea group is slightly smaller than that of the urethane one, which will contribute to the formation of smaller particle [19]. The value of PDI increases from 0.07 to 0.44 when the content of m-POSS increases up to 8 wt%. The viscosity of APUs nanocomposite dispersions shows a slight increase with increase in m-POSS content. Moreover, no obvious change is observed for all the dispersions after 30 days storage. These results indicated that the addition of m-POSS had only a slight influence on the viscosity and storage performance of APU nanocomposites [18,20]. TEM images of APU0 and APU8 are shown in Fig. 4. The diameter of most particles in Fig. 4a is below 40 nm, while the particles in Fig. 4b have a broader size distribution and there seem to be some particle agglomeration. The heterogeneity in particle size and shape would be caused by the incorporation of silicon element and strengthened by the increase of m-POSS content. These results indicated that the incorpo ration of m-POSS have little influence on the APU stability.
hydrogen atoms at position k was the most, so the peak k was the highest, and the hydrogen atoms at position a could be identified by the same method, the hydrogen atoms at position b, d, e and f were next to oxygen atom, so they were in the lowest filed, the hydrogen atoms at position g and h were next to nitrogen atom, so they were in the higher filed than hydrogen atoms at position b, d, e and f. The hydrogen atoms at position c, m, and j were next to Si, so they were in the highest field. The 13C NMR spectrum shows resonance peaks at 73.82, 71.43, 68.03, 58.43, 50.95, 44.42, 25.76, 23.91, 23.01, 22.50 and 18.36 ppm as shown in Fig. 2b, the position can. The above FT-IR spectrum and NMR results confirmed that the m-POSS was synthesized successfully.
3.3. Structural analysis of the APU/POSS nanocomposite films As shown in Fig. 5a, the FT-IR spectra of all the films have similar
Fig. 4. TEM image of (a) APU0 and (b) APU8.
Fig. 5. (a) FT-IR spectra of the APUs; (b) Si–O–C spectra in the region around 994 cm 4
1
.
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Fig. 6. (a) XRD patterns of the hybrid films and m-POSS; (b) XPS survey spectrum of APU8 film; (c) Si2p, (d) C1S, and (e) O1S high-resolutions of APU8 film; (f) XPS results of Si concentration.
characteristic peaks. The peaks at 2936 cm 1 and 2855 cm 1 are cor responding to the anti-symmetry and symmetry stretching vibration of CH2, respectively. The peak of 3336 cm 1 is ascribed to N–H stretching vibration, and the peak at 1529 cm 1 is due to bending vibration of N–H. Meanwhile, the band at 1714 cm 1 is related to the stretching vibration of C¼O. These results indicated that the isocyanate groups transformed to urethane moieties completely. Fig. 5b shows the spectrum from 900 to 1100 cm 1. It can be seen that the peak of 994 cm 1 in APU0 dis appeared in the APU/POSS hybrid materials, indicating an increase of
the degree of cross-linking. The XRD patterns of m-POSS and APU nanocomposite films are shown in Fig. 6a. The peak centered at 8.1� corresponds to the (101) plan of the m-POSS [21]. The diffraction peak around 19.8� is ascribed to the amorphous phase in the polyurethane, which displays a decreasing trend as the m-POSS content increases. With the increase of m-POSS, the APU6 and APU8 showed an additional peak at about 8.1� , and the intensity of the peak increased with the increase of POSS con tent, also indicating that the POSS moieties were chemically linked to
Fig. 7. SEM images of the film surfaces of (a) APU0, (b) APU2, (c) APU4, (d) APU6, and (e) APU8. 5
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of the APU films can contribute to the migration of Si element from the inner to the surface of APU films [22,23], which indicates that the feasibility of the strategy for simultaneously designing the silane groups to the end of molecular chain and its main chain backbone. Fig. 7 shows the SEM images of all the films’ surfaces. It is observed that APU0 and APU2 exhibit rough surfaces. With the increasing content of m-POSS, the surfaces of the samples become smoother. The AFM images of the surfaces are presented in Fig. S2. It is noted that the APU0 film has a root-mean-square (RMS) surface roughness value (Rq) of 17.17 nm, whereas the Rq of APU4 is 9.04 nm and that of APU8 is 4.40 nm. This basically has two reasons: firstly, the mobility of chain segments is restricted and the silane including m-POSS moved to the surface, secondly, the silane including m-POSS moved to the surface.
Table 3 Values of the contact angle and surface free energy (SFE) for the APU/m-POSS nanocomposite films. Samples APU0 APU2 APU4 APU6 APU8
Contact angle (o)
SFE (mJ/m2)
Hexadecane Water
Water
γps
γds
γts
13.5 12.7 11.2 9.8 9.5
84.0 93.8 104.7 115.8 121.0
5.41 2.10 0.23 0.04 0.01
25.29 25.37 25.51 24.62 22.53
30.70 27.47 25.74 24.66 22.54
the polymer chain. Fig. 6b shows the XPS survey spectrum of APU8. The peaks at 285, 400 and 533 eV are corresponding to C1s, N1s and O1s, respectively. Moreover, the presence peak of Si2p (102 eV) and Si2s (153 eV) confirm that the m-POSS was incorporated into APU backbone successfully. The high-resolution spectrum of Si2p on the surface of the APU8 film and the corresponding deconvolution results are presented in Fig. 6c. The Si–O–C at 102.6 eV bond is derived from the –Si(OC2H5)3 moieties, which creates during the film-forming stage, and the Si–O–Si at about 102 eV indicated the occurrence of cross-link networks. In addition, it is noted that the bonds of Si–O–C and Si–O–Si can be observed in the corresponding deconvolution results of C1S and O1S in Fig. 6d, e, which were consistent with the results of Si2p spectrum. Fig. 6f shows the Si concentrations in the bulk and on the surface of the APU films. As expected, the concentration of Si element on the film surface increases with the increasing m-POSS content. In addition, it is noted that the experimental value of Si concentration on the surface of APU8 is 4.09%, which is much higher than the theoretical value (2.08%) for APU8. This phenomenon is speculated that the lower surface tension
3.4. Surface hydrophobicity and mechanical properties of the hybrids The incorporation of siloxane networks in the APU was important to its surface energy and surface hydrophobicity [24,25]. Table 3 sum marizes the values of the static contact angle and surface free energy of the films. It is observed in Fig. 8 that the water contact angle increases from 84.0� to 121� with the increasing of m-POSS content. The corre sponding total surface energy in Table 3 reduces from 30.7 to 22.5 mJ/m2, which is in consistence with the XPS results. This is because the presence of Si–O–Si network structure in the APU matrix provided a thermodynamic driving force for its migration from inner to the surface, and the smoother surface for the m-POSS, resulting in reducing the surface energy and rendering the surface hydrophobicity [24,25]. The stress-strain curves of the APU/m-POSS nanocomposite films are shown in Fig. 9a. A typical rubber-like elasticity behavior is observed for
Fig. 8. Water drops on the hybrid films and schematic representation of the effect of m-POSS on the water wettability of the hybrid film surfaces.
Fig. 9. (a) Strain-stress curves; (b) The gel content of the APU nanocomposite films. 6
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Fig. 10. Fracture surface SEM images of (a) APU0, (b) APU2, (c) APU4, (d) APU6 and (e) APU8.
incorporation of the m-POSS, which indicated an improvement of the toughness. The SEM images of the fracture surfaces of the APU/m-POSS nanocomposite films are shown in the Fig. 10a–e. It can be seen that the fracture surface became rough when m-POSS was added into the APU, which exhibited a ductile fracture and indicated strong resistance to the break [27–30]. The higher elongation at break means higher motion ability in the plastic zone when more m-POSS were introduced. The m-POSS particles may be incorporated in the hard segments and the small m-POSS crystals act as physical cross-linking points in the struc ture, which would absorb large amount of energy and improved the toughness under the external strain, as shown in the Fig. 11. The DMA curves for the APU nanocomposite films are shown in Fig. 12. It is observed that the two peaks in Fig. 12a are corresponding to the Tg of the soft segments (Tg1) and the hard segments (Tg2), respec tively. The corresponding temperatures of Tg1, Tg2 and ΔTg ¼ (Tg2 -Tg1) are listed in Table S1. It is noted that the Tg2 increases significantly from 60.1 to 159.2 � C, while Tg1 shows a slight decrease from 68.6 to 75.4 � C. These results indicate that the m-POSS prefers to disperse in the APU as hard segments [27–30]. In addition, the value of ΔTg
Fig. 11. Schematic showing the breakage of polymer chains with the increase of tensile stress.
all the sample. With the addition of m-POSS, the mechanical properties of the APUs/m-POSS nancomposites are significantly enhanced. The Young’s modulus increased from 2.1 to 21.3 MPa and the tensile strength increased from 2.2 to 15.8 MPa, which could be attributed to nano-reinforcement of the m-POSS and enhanced cross-linking density from 56% to 87%, as shown in the Fig. 9b [26]. It is noted that the elongation at break of the films was also greatly improved by the
Fig. 12. The DMA curves of APU nanocomposite films. (a) tanδ; (b) storage modulus. 7
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Composites Part B 177 (2019) 107441
increases significantly with the increasing m-POSS content, which im plies the enhanced micro-phase separation between the hard and soft segments [27–30], thus resulting in enhanced mechanical properties. The storage modulus of the APU nanocomposites in Fig. 12b shows a rapid decrease around 70 � C, which was related to the chain relaxation of the soft segments. Moreover, the storage modulus significantly in creases with the increasing m-POSS content throughout the whole temperature range.
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4. Conclusions A novel technique to functionalize POSS with dihydroxyl and two triethoxy groups (m-POSS) was successfully demonstrated. APU/mPOSS nanocomposites were fabricated through the reaction between isocyanate and hydroxyl groups. The incorporation of m-POSS showed little effect on the viscosity and storage performance of the APU dis persions. The DMA tests revealed that the m-POSS was preferentially located in the hard segment domains, which resulted in a significant increase in the hard segments. The APU/m-POSS nanocomposite films had a high ductility (elongation at break over 1000%) with significantly enhanced Young’s modulus and tensile strength simultaneously. The improvements were attributed to the m-POSS acting both as nano-fillers and cross-linkers in the APU system. Acknowledgements This work was supported by the grants from the National Key Research and Development Program of China (No. 2016YFB0302000), the Science and Technology Major Projects of Hubei Province (No. 2017AAA115), the Fundamental Research Funds for the Central Uni versities (No. 2019MS062), and City University of Hong Kong (No. SRG 7004918). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.compositesb.2019.107441. References [1] Ahmadi Y, Yadav M, Ahmad S. Oleo-polyurethane-carbon nanocomposites: effects of in-situ polymerization and sustainable precursor on structure, mechanical, thermal, and antimicrobial surface-activity. Compos B: Eng 2019;164:683–92. [2] Wu W, He H, Liu T, Wei R, Cao X, Sun Q, et al. Synergetic enhancement on flame retardancy by melamine phosphate modified lignin in rice husk ash filled P34HB biocomposites. Compos Sci Technol 2018;168:246–54. [3] Bahrami S, Solouk A, Mirzadeh H, Seifalian AM. Electroconductive polyurethane/ graphene nanocomposite for biomedical applications. Compos B: Eng 2019;168: 421–31. [4] Zhao D, Hamada H, Yang Y. Influence of polyurethane dispersion as surface treatment on mechanical, thermal and dynamic mechanical properties of laminated woven carbon-fiber-reinforced polyamide 6 composites. Compos B: Eng 2019;160:535–45. [5] Jing QF, Liu Q, Li L, Dong ZL, Silberschmidt V. Effect of graphene-oxide enhancement on large-deflection bending performance of thermoplastic polyurethane elastomer. Compos B: Eng 2016;89:1–8. [6] Shi Y, Liu C, Liu L, Fu L, Yu B, Lv Y, et al. Strengthening, toughing and thermally stable ultra-thin MXene nanosheets/polypropylene nanocomposites via nanoconfinement. Chem Eng J 2019;378:122267.
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