Polyethylene-Modified Nano Silica and Its Fine Dispersion in

Article

Polyethylene-modified Nano silica and its fine dispersion in polyethylene Zhongchuan Peng, Qian Li, Huayi Li, and Youliang Hu Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 05 May 2017 Downloaded from http://pubs.acs.org on May 9, 2017

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Industrial & Engineering Chemistry Research

Polyethylene-modified Nano silica and its fine dispersion in polyethylene Zhongchuan Penga,b, Qian Lia,*, Huayi Lia,*,Youliang Hua a

Beijing National Laboratory of Molecular Sciences, CAS Key Laboratory of

Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China. b

University of Chinese Academy of Sciences, Beijing 100049, PR China.

Corresponding authors: Huayi Li ([email protected]), Qian Li([email protected]) Abstract Five types of modification reactions were used to introduce PE chains onto the surface of nano SiO2 and reactive nano SiO2 where silanol groups were chlorinated using tetrachlorosilane (SiCl4). The results show that the reaction between Si-Cl and epoxy group is more active than the others, and the highest grafting ratio is up to 41.4% deduced from the thermogravimetric analysis (TGA). The result of TEM and SEM indicate that the PE modified SiO2 particles are uniform dispersed, and the SiO2 particle size is approximately 100-300 nm. The shear rheology results reveal that the PE chains covalently attached on the surface of the nanosilica have strong interaction with PE matrix and enhance the melt strength of LDPE. The thermal properties of LDPE/SiO2 nanocomposites almost remain unchanged via TGA and differential scanning calorimetry (DSC) experiments. Key words: nanosilica; dispersion; morphology; nanocomposites; polyethylene

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1 Introduction Polymer nanocomposites have attracted great interest both in science and technology, because the addition of small amounts of nanoparticles in polymer matrix can produce a dramatic enhancement in physical, thermal and mechanical properties of polymers.1,

2

However, the strong tendency of aggregation of nanoparticles in

polymer matrix usually limits the further enhancement of the properties of nanocomposites. In all kinds of nanocomposites, the interfacial influence between the nanoparticle and the polymer matrix is a key problem. To enhance the interaction between nanoparticles and polymer matrix and achieve a highly dispersed state of nanoparticles in nanocomposites, the means of in situ polymerization3, 4 and adding functionalized nanoparticles5-8 have been widely exploited. Nanosilica, an easy available and typical zero-dimensional material, has been widely used as the filler in the preparation of nanocomposites.9-13 In general, nanosilica needed to be modified. Yangyang Sun11 used silane coupling agents to treat the surface of nanosilica, the result showed that the filler-filler interaction were reduced and achieved a mono-dispersity of nanosilica in organic solvents. In most cases, to prepare polymer/nanosilica composites, the most effective way to modify nanosilica particles is functionalized by polymer chains which are similar to the matrix. Tian-Ying Guo and coworkers14 performed the RAFT polymerization of 2-O-meth-acryloyloxyethoxyl-(2,3,4,6-tetra-O-acetyl-β-D-galactopyranosyl)-(1-4)-2,3 ,6-tri-O-acetyl-β-D-glucopyra-noside in CHCl3 solutions using cumyl dithiobenzoate as initiator agent. The living copolymer chains were subsequently grafted onto nanosilica particles modified by γ-methacryloxypropyl-trimethoxy. Salami-Kalajahi 3



and his coworker15 prepared PMMA/silica nanoparticles using graft through method. The silanol groups of the nanosilica were functionalized with methyl methacrylate groups. The modified nanosilica with vinyl groups was mixed with methyl methacrylate monomers and polymerized using RAFT agent. The nanosilica particles were covalently grafted to the side chains of the PMMA. The nanocomposites containing 7 wt% of silica nanoparticles achieved the best improvement of mechanical and thermophysical properties. It has been extensively proved that the modification of nanosilica by covalent polymer chain is a best way to prepare polymer/nanocomposite. Up until now, few literatures reported the modification of nanosilica by polyolefins. A much greater challenge for polymer/nanoparticles is the preparation of polyolefin nanocomposites. Most polyolefins are nonpolar and composed of crystalline region and amorphous region, which likely promotes inhomogeneous nanoparticle distribution and aggregation.16 In order to prepare nanoparticles that were miscible with the matrix and evenly dispersed in the polyolefin matrix, the nanoparticles surface has to be modified with matrix-miscible polyolefin. Vincent Monteil and coworker17 obtained functional nanosilica via condensation reaction between trimethoxy(7-octen-1-yl)silane and surface hydroxyl of the silica, and the functional nanosilica were copolymerized with ethylene using nickel catalyst, so this way enhanced the dispersion of nanosilica in polyethylene. Another way to modify nanosilica by polyolefin is via the reaction between nanosilica and a functional polyolefin. In our previous work, many chain end functionalized 4


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polyethylenes were prepared with high efficiency via click chemistry.18,

19

In the

present work, the chain end functionalized polyethylene (Cef-PE) is used to modify the nanosilica via the approach of “grafting to”. LDPE/nanosilica nanocomposites were directly prepared by melt mixing, and the dispersion of silica in LDPE matrix were studied by TEM and rheology. 2 Experimental Section 2.1 Materials Vinyl-terminated polyethylene (v-PE, Mn=500g/mol) and chain end functionalized polyethylene (Cef-PE), which denoted as Cef-Si(OCH3)3, Cef-OH, Cef-COONa and e-PE were synthesized according to the literature.19 Scheme 1 shows the structure of the Cef-PEs. Low density polyethylene (LDPE, MFR = 1.22g/10min) was supplied by Yanshan Petrochemical Co. LTD., Beijing, China. Tetrachlorosilane(SiCl4) was purchased from J&K Scientific. Nanosilica/water suspension was purchased from Bo Yu(Beijing) Materials Technology Co., Ltd. The nanosilica suspension was treated by centrifuge and dried at 50 oC under vacuum to get silica powder. The particle sizes of the SiO2 particles are approximately 100-300 nm and the surface area is 200 m2/g determined by BET. Triethylamine(Et3N) and toluene were purchased from Sinopharm Chemical Reagent Beijing Co., Ltd. The toluene was deoxygenated by nitrogen purge before refluxing for 48 h, and then distilled over sodium. Dimethylin dichloride (Me2SnCl2) and hexamethylphosphoramide(HMPA) were purchased from Aladdin. 2.2 Analytical techniques 5



A stress controlled rheometer (AR 2000ex) equipped with parallel-plate geometry with a diameter of 25 mm was used to study the dynamic rheological properties in a frequency range from 0.01 to 100 Hz. The gap was fixed at 1200 mm. To prevent the thermal degradation of the matrix, all the rheological experiments were conducted in a nitrogen atmosphere at 180 oC To prevent the thermal degradation of the matrix, all the rheological experiments were conducted in a nitrogen atmosphere at 180 oC). The frequency sweep was conducted in a frequency range from 0.01 to 100 Hz). Differential scanning calorimetry (DSC) was conducted using a TA Q2000 thermal analyzer under a nitrogen atmosphere with a heating rate of 10 oC/min in a temperature range of 50-200 oC for dynamic scanning, and the Tm was determined in the second scan. Thermogravimetric analysis (TGA) was performed at a heating rate of 20 oC/min in nitrogen atmosphere from 50 to 700 °C on Perkin-Elmer TGA-1 instrument. Transmission electron microscopy (TEM) was carried out on a Jeol JEM2200 transmission electron microscope using an acceleration voltage of 100 kV. Samples for TEM were prepared by ultramicrotomy. Scanning electron microscope (SEM) was carried on SU80200 (Hitachi High-Tech corporation, Japan). 2.3 Modification of nanosilica via “grafting to” method 2.3.1 Synthesis of SiO2-PE1 0.5 g of nanosilica and 2 g of Cef-Si(OCH3)3 were dispersed in 150 ml of toluene. 2 ml of HCl (30 wt%) and 2 ml of H2O were added into the suspension. The mixture was heated at 80 oC under stirring for 24 h. The product was poured into an amount of methanol and filtered. Then the crude product was purified by successive Soxhlet 6


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extractions to remove unreacted Cef-Si(OCH3)3 using boiling toluene and dried at 70oC under vacuum to give SiO2-PE1. 2.3.2 General procedure for synthesis of SiO2-PE2, SiO2-PE3, SiO2-PE4, SiO2-PE5 A surface modification method to get the reactive nanosilica according to the literature.20 0.5 g of nanosilica was dispersed in 150 ml of SiCl4. The mixture was heated at 60 oC under stirring for 20 h. 80 ml of toluene was added in the mixture, and then the unreacted SiCl4 was removed by distillation at atmospheric pressure. The reactive nanosilica which containing the bond of Si-Cl were stored in toluene and denoted as SiCl3-SiO2. SiCl3-SiO2 was used to synthesis SiO2-PE2, SiO2-PE3, SiO2-PE4, SiO2-PE5 via ‘graft to’ method. 2 g of Cef-COONa, Cef-OH, e-PE, Cef-Si(OCH3)3 and appropriate catalyzers were added into the 0.5 g of SiCl3-SiO2 in toluene, respectively. The mixture was heated at 80 oC under stirring for 20 h. The products ware poured into an amount of methanol and filtered. Then the crude product was purified by successive Soxhlet extractions to remove unreacted chain-end functional polyethylenes using boiling toluene and dried at 70oC under vacuum to give SiO2-PE2 to SiO2-PE6. 3 Results and discussion 3.1 Modification of nanosilica The modifications of nanosilica by chain-end functional polytheylene via the approach of “grafting to” is shown in Scheme 1. In general, nanosilica is modified by the reaction between the Si-OH bond of nanosilica and the -Si(OCH3)3 groups of 7



small organic molecule21.

SiO2-PE1 was synthesized using this method.

Cef-Si(OCH3)3 was covalently linked to the silica by the bond of Si-O-Si via the reaction between the -OH of nanosilica and the -Si(OCH3)3 of Cef-Si(OCH3)3. However, the grafting ratios were low because of the low reactivity of Si-OH, as deduced from the TGA experiments reported in the following part. In order to improve the grafting ratios, surface modification by SiCl4 was taken to get the reactive nanosilica containing the reactive bond of Si-Cl. The SiO2-PE2, SiO2-PE3, SiO2-PE4 and SiO2-PE5 were synthesized via the reactions between the bond of Si-Cl and -COONa,22 -OH,22-24 epoxy,25-27 -Si(OCH3)3,28, 29 respectively. Scheme 2 shows the reaction mechanism between Si-OCH3 and the bond of Si-Cl.

Scheme 1 The functional route of nanosilica 8


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Scheme 2 The mechanism of reaction between Si-OCH3 and Si-Cl catalyzed by FeCl3. After the “graft to” reactions, the modified nanosilica particles were purified by extraction with toluene to ensure that the unreacted Cef-PEs were removed entirely. The FTIR spectra of pure nanosilica, SiO2-PE1, SiO2-PE2, SiO2-PE3, SiO2-PE4 and SiO2-PE5 are shown in Figure 1. The absorption bands at 1026 cm-1 and 798 cm-1 are assigned to the asymmetric stretching vibration and symmetric stretching vibration of Si-O-Si. The stretching vibration peak and bending vibration peak of Si-OH are located at 3373 cm-1 and 1640 cm-1. The peaks at 2918 cm-1 and 2849 cm-1 correspond to the asymmetric stretching vibration of C-H. From the analysis by FTIR, it indicates that Cef-PEs have been covalently linked to the surface of nanosilica.

Figure 1 FTIR spectrum of silica nanoparticles and functional silica 9



3.2 Grafting density of PE-grafted silica nanoparticles The grafting ratios (Gr) and the grafting density (Gd) were determined by thermal-gravimetric analysis (TGA). The polyethylene chains on the surface of SiO2 were selectively eliminated by heating at 20 oC/min under air from 50 oC to 750 oC. The Gr and Gd were calculated via the follow equation,14, 30 respectively:

Gr

1

Gd

In the equation, was the difference between the weight loss of the grafted samples (PE) and the reference samples (the weight loss of hydroxyl of silica), Sspe was the specific surface (nm2/g) of the silica, MPE was the molecular weight of the grafted polyethylene, NA was the Avogadro number. The Gd and Gr were calculated via the weight loss which was obtained by TGA (Figure 2), and the Gr, Gd and the weight loss of the samples were summarized in Table 1. The Gd ranges from 0.18 to 1.04 chains/nm2. The biggest Gr (41.4%) is achieved by SiO2-PE4, which indicates that the reactivity of Si-Cl with epoxy group is more active than the others.

Figure 2 The TGA curves of pure nano SiO2, SiO2-PE1, SiO2-PE2, SiO2-PE3, SiO2-PE4 and SiO2-PE5 10


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Table 1 The summary of Gr, Gd and the weight loss of sample. Sample

Weight loss (%)

Gr (%)

Gd (chains/nm2)

SiO2-PE1

13.0

10.6

0.64

SiO2-PE2

9.9

7.0

0.42

SiO2-PE3

14.3

12.2

0.73

SiO2-PE4

32.7

41.4

2.49

SiO2-PE5

14.6

14.0

0.84

3.3 Investigation of the properties of LDPE/nanosilica nanocomposites The nanocomposites of LDPE/SiO2-PE4 and LDPE/SiO2 with different contents of nano SiO2 were prepared by melt-mixing method using a HAAKE Minilab-II internal mixer (Mess-Technic GmbH, Germany) at 190 oC with a screw speed of 50 rpm for 5 min. Table 2 shows the compositions of the nanocomposites. Sample LDPE

Table 2 Compositions of the PE/SiO2 nanocomposites LDPE SiO2-PE4 SiO2 SiO2 content g g wt% g 5.00 0

N-2

4.69

0.31

-

2

N-4

4.39

0.61

-

4

N-6

4.08

0.92

-

6

N-8

3.78

1.22

-

8

P-6

4.70

-

0.30

6

11



3.3.1 Morphology and dispersion of nanosilica in the composites

Figure 3 SEM micrographs of pure silica nanoparticles

Figure 4 TEM micrographs of the modified and the unmodified silica nanoparticles dispersed in LDPE matrix: N-6 (A) and P-6 (B) SEM (Figure 3) shows the micrographs of pure silica nanoparticles. The particle size is approximately 100-300 nm. TEM technique was used to evaluate the dispersion state and the morphology of silica nanoparticles in the polymer matrix. Figure 4 shows the micrographs of the modified and the unmodified silica nanoparticles dispersed in LDPE matrix. It is obvious that the modified nanosilica 12


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(N-6) shows finer dispersion and distribution in LDPE matrix from Figure. 4A, and has uniform particle size which compared to the pure silica nanoparticles. The unmodified nanosilica aggregated all and displayed uneven distribution state in the LDPE matrix (Figure 4B). Meanwhile, the particle size of the modified nanosilica by PE chains dispersed in LDPE matrix was approximately 100-300 nm. 3.3.2 Dynamic rheological properties of LDPE/nanosilica nanocomposites

Figure 5 Frequency response of (A) storage modulus G’, (B) loss modulus G’ and (C) viscosity of the LDPE/SiO2 nanocomposites Figure 5 displays the variation of storage and loss moduli versus angular frequency for the nanocomposites with different loading of the modified nano SiO2. It is obvious that nanosilica has a dramatic effect on the rheology property of LDPE. As we can see, both storage and loss moduli exhibit the same behavior that their values have little reduction at high frequency with the increase of nanosilica content. The result of the storage modulus G’ dramatically increased at low frequencies with the filler loading increase indicates that both low frequency range (0.01~1) and high frequency (100~500) motion of LDPE chains have been changed and the interaction between matrix

and

functional

nanosilica

was

dramatically

enhanced.

At

low

frequency(Figuer 5C), the viscosity of the nanocomposites has been greatly enhanced with the SiO2 loading increased, which ascribed to the contribution of the interaction 13



between matrix and functional nanosilica. At lower frequency, both the storage modulus and viscosity significantly increased with the increase of SiO2 concentrations, which ascribes to the stronger interactions between the PE chains of functional SiO2 and LDPE matrix in nanocomposites. From Figure 5C, the nanocomposites shows obviously shear thinning performance with the particle loading increased at high frequency. The rheological behaviors of the composite are essentially important for processing. Also, the formation of a percolated system can be detected by characterizing the complex viscosity (η*), storage modulus (G’), and loss modulus (G’’) as a function of frequency.31-33 Minoru Terano1, 34 prepared PP/PP-g-SiO2 nanocomposites and studied the percolation threshold which was 4.0-10.0 wt% via rheological behavior. In our research, both G’ and η* significantly increased by the addition of the silica at low frequency. A plateau appeared (Figure 5A) instead of the terminal flow at the content over 4.0 wt%. The transition in G’ at low frequency indicates that the nanocomposites have reached a rheological percolation, at which the nanocomposites form a network structure and greatly impede the motion of the polymer chains.

14


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Figure 6 Schematic illustration of the interaction between silica and matrix polymers at different particle/polymer number ratio The composites exhibit lower Newtonian plateau and higher values of complex viscosity and moduli at low frequency as the filler content increases. The rheological response was affected not only by silica content but also the silica treatment.35, 36 The effect of particle/polymer number ratio on percolation threshold has been attributed to the formation of ‘‘polymer/particle’’ network. A likely structural model is illustrated in Figure 6. As shown in Figure 6 (when N < Nmax), we believed that one particle was bridged by one or some PE chains to form intrachain polymer/particle complex. When N > Nmax, the large aggregates of silica particles would be presented, and the particle/particle interactions due to the polyethylene chains which attached on the silica has a dramatic effect on the rheology behavior.37-39

Figure 7 Frequency response of (A) storage modulus G’, (B) loss modulus G’’ and (C) viscosity of N-6 and P-6 15



The effect of modification of nanosilica by long chain PE on the rheology behaviour of LDPE (the samples N-6) is compared with that without modification (P-6) and the results are shown in Figure 7. The samples of N-6 and P-6 are comprised of the same loading of nano SiO2. The dependences of the storage modulus G’, loss modulus G’’ and viscosity of the sample P-6 is almost the same with those of pure LDPE, which indicates that the nanosilica without modification has little interaction with LDPE chains. However, there are greatly increase of the storage modulus and the viscosity in low frequency, ascribe to the stronger interaction between the PE chains of functional SiO2 particles and LDPE matrix in N-6 nanocomposites. The stronger interaction of filler-polymer results in the higher modulus and viscosity. The increasing of storage modulus and viscosity in low frequency indicated that the grafted PE chains of functional SiO2 made an important contribution on the melt-strengthen of nanocomposites. From Figure 7C, the N-6 nanocomposites shows notably shear thinning performance at high frequency, which should be caused by the disentanglement between the grafted PE chains and the LDPE chains. 3.3.3 Thermal properties of the LDPE/SiO2-PE4 nanocomposites

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Figure 8 TGA curves of LDPE/SiO2-PE4 nanocomposites and pure LDPE in nitrogen atmosphere

Figure 9 DSC curves of LDPE/SiO2-PE4 nanocomposites for various SiO2 contents:(A) Cooling, (B) Heating

Table 3 Thermal properties of LDPE/SiO2-PE4 nanocomposites for various SiO2 contents Sample ∆Tm(oC) Hm(J/g) Tc(oC) ∆Hc(J/g) LDPE

106.3

85.5

92.5

68.5

N-2

107.2

79.1

93.8

62.0

N-4

106.7

76.8

94.2

60.5

N-6

106.0

75.6

94.3

58.8

N-8

106.9

75.4

93.7

49.9

The TGA curves of the LDPE/SiO2-PE4 nanocomposites and the pure LDPE are 17



shown in Figure 8. Compared to the pure LDPE, there are about 5% weight loss of LDPE/SiO2-PE4 in the temperature range from 200 oC to 450 oC, mainly due to the loss of a small amount of PE which graft onto the nanosilica in the LDPE/SiO2-PE4 samples as Figure 2 shows. The DSC curves of LDPE/SiO2-PE4 nanocomposites with different silica contents are shown in Figure 9. The thermal properties of the nanocomposites are listed in Table 3. The melting temperature almost remain unchanged. The crystallization temperature has a little increase with the addition of SiO2-PE4. The contribution ascribed to the nanosilica particles which play the role of crystal nucleus. 4. Conclusion Four kinds of chain end functional polyethylene were used to modify nanosilica and SiCl4 activated nanosilica. The results showed that the reaction between Si-Cl and epoxy group is more active than the others, which achieved the highest grafting ratio to 41.4% deduced from the TGA. Comparing to the unmodified nanosilica, the modified nanosilica shows finer dispersion in LDPE matrix from TEM, and revealed that the particle size is 100-300 nm with narrow size distribution and uniform dispersity. The rheology results revealed that the addition of modified SiO2 significantly enhanced the melt strength of LDPE, which indicated that a strong interaction between LDPE matrix and PE-SiO2 happened. In addition, it is found that the thermal properties almost remain unchanged via TGA and DSC experiments. In summary, it is proved that the modification of nanosilica by functionalized polyethylene is a versatile and efficient route to improve the dispersion of nanosilica 18


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in LDPE matrix. Acknowledgements The authors express thanks for the financial support from the National Science Foundation of China (Grant No. 51403216) and PetroChina Innovation Foundation. References (1) Umemori, M.; Taniike, T.; Terano, M., Influences of polypropylene grafted to SiO2 nanoparticles on the crystallization behavior and mechanical properties of polypropylene/SiO2 nanocomposites. Polym. Bull. 2012, 68, 1093. (2) Meer, S.; Kausar, A.; Iqbal, T., Attributes of Polymer and Silica Nanoparticle Composites: A Review. Polym. Plast. Technol. Eng. 2015, 55, 826. (3) Ishitake, K.; Satoh, K.; Kamigaito, M.; Okamoto, Y., Stereospecific Free Radical and RAFT Polymerization of Bulky Silyl Methacrylates for Tacticity and Molecular Weight Controlled Poly(methacrylic acid). Macromolecules 2011, 44, 9108. (4) Qu, M.; Meth, J. S.; Blackman, G. S.; Cohen, G. M.; Sharp, K. G.; Van, K. J., Tailoring and probing particle-polymer interactions in PMMA/silica nanocomposites. Soft Matter 2011, 7, 8401. (5) Kawashima, S.; Hou, P.; Corr, D. J.; Shah, S. P., Modification of cement-based materials with nanoparticles. Cem. Concr. Compos. 2013, 36, 8. (6) Gao, X.; Hu, G.; Qian, Z.; Ding, Y.; Zhang, S.; Wang, D.; Yang, M., Immobilization of antioxidant on nanosilica and the antioxidative behavior in low density polyethylene. Polymer 2007, 48, 7309. (7) Chen, J. H.; Rong, M. Z.; Ruan, W. H.; Mai, Y. L.; Zhang, M. Q., A Comparative 19



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