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					Benjamin Dach                                                                      03/27/09
Second-year report


        Crosslinked Nanocomposite Networks from Polymer-Silica Hybrid
                                Nanoparticles

Background

     Organic–inorganic hybrid materials represent an important field of research in material

science as they have applications in optics, electronics, ionics, mechanics, energy,

environment, biology and medicine.1,2 This versatility arises from the benefits and desirable

properties of both constituents: the inorganic component is mechanically and thermally stable

and the organic component is flexible and functional. Silica nanoparticles dispersed within

polymer matrices by mechanical, ultrasonic, or solvent-aided mixing have been reported to

display increased mechanical properties3,4 like modulus, flexural, tensile, and impact strength

up to a certain silica content (2.5%).5 Thereafter, mechanical properties are decreased, while

fracture toughness6 and thermal stability7,8 are improved. Sulfonated polyimide-silica

nanocomposite membranes were shown to function as proton conducting solid electrolytes for

fuel cells.9 Additionally, fumed silica has been used to control the nanoparticle dispersion

within a polymer matrix to create ultrapermeable, reverse-selective nanocomposite

membranes that allow heavier vapors to permeate through more easily than light gases.10

While mechanically mixed nanocomposites have attracted much attention, covalently

crosslinked polymer-silica nanocomposite networks are a related class of new materials which

may exhibit completely novel properties. This work investigates the formation of polymer-

silica hybrid nanocomposite networks that are covalently crosslinked by the 1,3-dipolar

cycloaddition of alkyne and azide end groups, a ‘click’ reaction. (Scheme 1)

Research Objectives

   1) Covalently graft polystyrene and poly(tertbutyl acrylate) to silica nanoparticles.
   2) Characterize the polymer grafted nanoparticles by thermal and spectroscopic analyses.
   3) Create polymer-silica hybrid nanocomposite networks.
   4) Investigate the properties of the networks and the mechanisms of formation.

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Benjamin Dach                                                                            03/27/09
Second-year report




           Scheme 1. Nanocomposite network formation via Hüisgen 1,3-dipolar cycloaddition.



Synthesis of Polymer-Silica Hybrid Nanocomposite Networks

     Scheme 2 illustrates the synthetic route from bare silica nanoparticles to polymer-silica

nanocomposite networks. All reactions are kept under inert atmosphere at 80°C for 24 h. 1 g

of silica nanoparticles (10 – 20 nm), 1, and 10 mL of anhydrous toluene are mixed in a flask.

1 mL of 3-bromoundecyltrichlorosilane in 3 mL of toluene is added dropwise and stirred. 2a,

and all modified nanoparticles, are recovered by centrifugation at 3000 rpm for 30 min, then

redispersed in toluene and centrifuged. This cycle is repeated six times. 1 g of 2a and a

solution of 0.4 g of NaN3 in 20 mL DMF are then added to a flask and stirred. The particles

are washed with water to remove excess NaN3 and centrifuged to recover 2. Meanwhile, 1 g

of 1 and 2.5 g of o-propargyloxy-N-triexoxysilylpropyl urethane are stirred into 100 mL of

anhydrous toluene. This yields 3 after a cycle of centrifugation. Heterobifunctional α-alkyne-

ω-bromopoly(styrene) and α-alkyne-trimethylsilyl-ω-azidopoly(tert-butyl acrylate) (Mw = 10

kD) are then covalently bound to the surfaces of 2 and 3, respectively. 50 mg of polymer and

50 mg of 2 or 3 are added to these ‘click’ reagents: 12 mg of Cu(I)Br, 5 mg of Na-(L)-

ascorbate, 5 mg of TBTA, and 50 μL of PMDETA mixed in 10 mL of DMF. 4a and 5a are

recovered after centrifugation. 4a is then modified by a NaN3 reaction to yield 4 after washing

with water and centrifuging. 5a is modified by a TMS deprotection reaction involving K2CO3

and a 10:1 solution of dichloromethane to methanol to yield a free alkyne on the surface of 5.

The polymer-silica hybrid nanoparticles, 4 and 5, are then ‘clicked’ together to form a

crosslinked nanocomposite network, 6. This is accomplished by adding 50 mg of 4 and 50

mg of 5 to a flask with the aforementioned ‘click’ reagents and stirring.

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Benjamin Dach                                                                                        03/27/09
Second-year report




                 Scheme 2. Synthetic route to PS-PtBA silica nanocomposite network formation




Figure 1. IR spectra of (a) 1 [black] (b) 2 [blue] (c) 3 [green] (d) SiO2-‘click’-SiO2 [red] (e) 4 [grey]


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Benjamin Dach                                                                                      03/27/09
Second-year report
Infrared Spectroscopy

      Figure 1 shows diffuse reflectance Fourier transform infrared spectra (DRIFTS) of bare

and modified silica. All samples have strong peaks at 1100 cm-1 (Si-O) due to silica. 2 and 4

show peaks at 2100 cm-1 (azide stretch). The red curve (d) is of 2 and 3 ‘clicked’ together and

shows a decrease in the azide peak intensity. 2, 3, (d), and 4 all possess a stretch at 2900 cm-1

for C-H bonds. 4 has a doublet peak at 1485 cm-1 due to the C=C bonds in polystyrene.

Thermogravimetric Analysis

      The presence and the amount of grafted polymers on silica surfaces can be estimated by

thermogravimetric analysis (TGA).11 TGA is performed on a TA Q50 instrument at a scan

rate of 10°C/min under nitrogen atmosphere. Figure 2 shows the weight loss as the silica

nanoparticles are heated to 800°C. For bare silica, 1, the observed weight loss of 5.33% is

due to silanol group dehydroxylation. The alkyne(3), azide(2), PS(4), and PtBA(5) modified

                                                                                nanoparticles lost 8.95%, 12.48%,

                                                                                22.58%, and 23.23% of their

                                                                                weights, respectively. These

                                                                                results indicate that the bare silica

                                                                                particles were successfully

                                                                                modified, because the successive

                                                                                increase in weight loss is related

                                                                                to the pyrolysis of organic

                                                                                compounds.

Figure 2. TGA degradation curve of (a) 1 [black] (b) 3 [grey] (c) 2 [blue] (d) 4 [green] (e) 5 [red]
             (a)                          (b)                           (c)                            (d)




Figure 3. TEM images of (a) bare silica nanoparticles and (b), (c), (d) crosslinked nanoparticle aggregates


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Benjamin Dach                                                                            03/27/09
Second-year report
Dynamic Rheology

      Dynamic rheology measures the elastic (G′) and viscous (G″) moduli of a sample. For

weakly agglomerated systems, the presence of particle structures introduces both viscous and

elastic effects into the rheological response. If the particles flocculate into a volume-filling

network structure the material is classified as a gel as it shows predominantly elastic

properties12 with G′ > G″. The dynamic rheology of crosslinking hybrid nanoparticles is

measured using a TA Instruments AR2000 rheometer. A 60 mm diameter steel parallel-plate

geometry with a 25 μm gap is used and a 5% oscillatory strain is applied at the rate of 1 Hz. 5

mg of 3 and 5 mg of 4 are added to 2 mg of Cu(I)Br, 1 mg of Na-(L)-ascorbate, 0.1 mg of

TBTA, and 5 μL of PMDETA mixed in 1 mL of DMF and stirred briefly in a vial. This

‘click’ solution is deposited under the rheological steel plate geometry and heated to 60°C. A

DMF solvent trap is used to prevent sample evaporation. Figure 4 indicates that after 20 min

of oscillatory strain the value of G′ shoots up to nearly 100 times that of G″. According to

theory, when G′ > G″, it is due to a network formation. Thus it has been shown that polymer-

silica hybrid nanoparticles have been crosslinked to form nanocomposite networks.

                                                                         Future Work

                                                                         We will begin to investigate the

                                                                         self-organization properties and

                                                                         phase behavior of a new class of

                                                                         matrix-free nanocomposites:

                                                                         nanoparticles decorated with block

                                                                         copolymer brushes. In the coming

                                                                         months, we will ‘click’ another

                                                                         polymer to the silica, forming a
Figure 4. Rheology measurements of a crosslinked nanocomposite network
                                                                         diblock copolymer on the surface.
Then we will use HF to etch away the SiO2 and study the diblock copolymer self-assembly.


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Benjamin Dach                                                                    03/27/09
Second-year report

References
1. C. Sanchez, B. Juliàn, Ph. Belleville, M. Popall, J. Mater. Chem. 15, 3559 (2005).

2. P. Judeinstein, C. Sanchez, J. Mater. Chem. 6, 511 (1996).

3. Y. P. Zheng, R. C. Ning, Y. Zheng, J. Reinforced Plast. Compos. 24, 223 (2005).

4. K. W. Liang, G. Z. Li, H. Toghiani, J. H. Koo, C. U. Pittman, Chem. Mater. 18, 301
(2006).

5. P. Singh, A. Kaushik, and A. Kirandeep, J. Reinforced Plast. Compos. 25, 119 (2006).

6. G. Ragosta, M. Abbate, P. Musto, G. Scarinzi, L. Mascia, Polymer 46, 10506 (2005).

7. Q. Chen, R. W. Xu, J. Zhang, D. S. Yu, Macromol. Rapid Commun. 26, 1878 (2005).

8. T. M. Lee, C. C. M. Ma, J. Polym. Sci. Part A: Polym. Chem. 44, 757 (2006).

9. C. H. Lee, S. Y. Hwang, J. Y. Sohn, H. B. Park, J. Y. Kim, Y. M. Lee, Journal of Power
Sources 163, 339 – 348 (2006).

10. T. C. Merkel, B. D. Freeman, R. J. Spontak, Z. He, I. Pinnau, P. Meakin, A. J. Hill,
Science 296, 519 (2002).

11. J. Che, B. Luan, X. Yang, L. Lu, X. Wang, Mater. Lett. 59, 603 (2005).

12. C.W. Macosko, Rheology: Principles, Measurements and Applications, VCH, New York,
NY, 1994.




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