Grafting polystyrene on Cellulose (CNC) by surface initiated Abstract Atom Transfer Radical Polymerization (SI ATRP) Zhen Zhang, Gilles Sebe, Xiaosong Wang Grafting polymers on the surface of nanoparticles is a very promising way to prepare hybrid nanomaterials or nanocomposites. if the polymer grafting is in a controlled way, including grafting density, chain length and dispersity, well designed nanocomposites can be obtained. In this paper, polystyrene was grafted on the surface of Cellulose NanoCrystal (CNC) nano-initiator by surface initiated Atom Transfer Radical Polymerization (SI ATRP), which was confirmed by FTIR, TGA, DTG, DSC and DLS. To initiate SI ATRP on CNC, it is necessary to introduce initiating sites on the surface of CNC to prepare CNC nano-initiators first. 1 Introduction Grafting soft polymer onto the surface of rigid nanoparticles by covalent bonds is a very promising bottom up approach to prepare nanocomposites[1, 2], which can not only take fully advantage of the synergistic effect of the soft and rigid parts to obtain optimal balance between durability, mechanical properties and thermal properties, but also introduce novel functionalities [3, 4]. The diversity of nanoparticles and polymers offers a big resource pool of new ideas. Fabrication of nanomaterials with designed structures is essential for nanotechnology. However, grafting polymers on the surface of nanoparticles in a controlled way to prepare well designed nanocomposites is still a challenge, including controlled grafting density, chain length and dispersity of grafted polymer. Cellulose nanocrystal (CNC) is extracted from cellulose by acid hydrolysis with diameter from 5 to 20 nm and length from 50 to 500 nm which depends on different resources of cellulose and extraction method. Due to the excellent properties of CNC [5], for example, renewable, biocompatible, excellent mechanical properties with low density, potential to modification due to abundant hydroxyl group on the surface, self-assembly to chiral nematic crystal and low thermal expansion coefficient, CNC have drew a lot of attention in recent years in the applications of nano-metal particles stabilizer, transparent substrate for electronic, nano-template, reinforcing agent for nanocomposites, emulsifier for Pickering emulsion, and so on. With the scalable production in the demonstration plant, the cost of CNC will decrease dramatically in the near future, and the applications of CNC will be exploited much more in industry field. However, the hydrophilic nature of CNC surface limits its further application due to the poor miscibility with hydrophobic materials[6]. Sometimes it is necessary to modify the surface for specific applications. The modification of the surface also can introduce some other novel functionality. Grafting polymer on CNC is a very
effective and promising method to by modify the surface and introduce novel functionalities[7]. Since the development of the Atom Transfer Radical Polymerization (ATRP) in 1995[8], it has become a very important method and widely use living polymerization technique to prepare well-controlled polymeric chains. A variety of monomer can be polymerized by ATRP with well-controlled molecular weight and low dispersity. ATRP has also been initiated efficiently and extensively on the surface of different kinds of substrate (defined as surface initiated (SI) ATRP) to yield hybrid materials by grafting polymers precisely. SI ATRP is a grafting from method, which can obtain high graft density of polymer on the surface when compared with grafting to method [9]. 2 Results and discussion 2.1 Preparation of CNC nano-initiator: CNC-Br Before grafting polymer on CNC by SI ATRP, initiating sites Br were introduced on the surface of CNC. BIBB was used to modify the surface of CNC with TEA and DAMP as the catalyst. In the modification process, the dispersibility of CNC was crucial for the modification. So it is necessary to disperse CNC in DMF by sonication. As shown in Figure 1, compared with the FTIR of CNC, the new strong absorption peak at 1739 cm -1 was attributed to the C=O of the modified moieties, which indicated the successfully modification of CNC by BIBB. Figure 1. FTIR of CNC and CNC-Br The thermal stability of CNC and CNC-Br were also characterized by TGA and DTG, as shown in Figure 2. As CNC was more hydrophilic than CNC-Br, CNC showed a little more weight loss than CNC-Br below 100 C due to absorbed moisture. Normally the main decomposition temperature range of cellulose is between 320 to 400 C with the maximum degradation temperature (Tmax) at 390 C [10]. The sulfuric ester groups on the surface of CNC catalyzed the thermal degradation at low temperature [11]. The main decomposition temperature range of CNC was between
250 to 320 C with the onset decomposition temperature (Tonset) at 280 C and Tmax at 297 C respectively. The obviously decrease of thermal stability of CNC-Br was due to the introduction of Br, which may eliminate HBr upon heating and the HBr formed catalyzed decomposition of CNC at lower temperature [10]. Figure 2. (A) TGA of CNC and CNC-Br; (B) DTG of CNC and CNC-Br 2.2 Grafting PS on CNC nano-initiators by SI ATRP As the initiator sites on the surface of CNC are negligible to some extent when compared with sacrificial initiator, it is easy to tailor the length of grafted polymer by changing the initial molar ratio of monomer to sacrificial initiator and the conversion. Moreover, the addition of sacrificial initiators will increase the concentration of Cu 2+ in the system, which limits the termination to obtain a better controlled ATRP. Although the molecular weight of free polymer initiated by sacrificial initiator was not exactly the same with the grafted polymer, the free polymer was still a tool to study the grafted polymer. Now sacrificial initiator has been extensively used in SI ATRP to have a better controlled ATRP and provide some information about the grafted polymer. In this paper, EBiB was used as the sacrificial initiator in SI ATRP system. From the comparison of FTIR spectra of CNC-Br, CNC-g-PS 2h (reaction time was 2 h) and the free PS, as shown in Figure 3, it was easy to conclude that PS was successfully grafted on CNC-Br by SI ATRP. After grafting PS, the peaks in CNC-g-PS FTIR spectra, for example, 3025 cm -1 (C-H stretching of PS), 1494 cm -1 (C=C stretching of PS), and 700 cm -1 (C-H bending of PS), were the characteristic peaks of PS.
Figure 3. The FTIR spectra of CNC-Br, CNC-g-PS 2 h and free PS The kinetic research of the free PS was studied first. Figure 4A showed the monomer conversion and ln (M0/Mt) vs reaction time curve, in which the monomer conversion and ln (M0/Mt) was calculated by 1 H NMR. The liner increase of ln (M0/Mt) vs time indicated a first-order kinetic with a constant concentration of the radical. Figure 4B showed the Mn and dispersity of free PS vs monomer conversion curve, and the Mn and dispersity was determined by SEC. The Mn increased linearly with the monomer conversion, and all the dispersity values (6-1.13) are below 1.15. The kinetic research results demonstrated the well-controlled living polymerization of free PS by ATRP. A 0.30 Monomer conversion 0.25 0.20 0.15 0.10 5 monomer conversion ln ([M 0 ]/[M t ]) Fit line for ln ([M 0 ]/[M t ]) 0.3 0.2 0.1 ln([m 0 ]/[M t ]) Mn B 15000 10000 5000 Mn Dispersity Linear fit for Mn 1.4 1.2 Dispersity 0 0 1 2 3 4 5 6 7 8 Time (h) 0 0 5 0.10 0.15 0.20 0.25 0.30 Monomer conversion Figure 4. The kinetic research of free PS by ATRP (A) Monomer conversion and ln(m0/mt) vs reaction time; (B) Mn and dispersity of free PS vs monomer conversion
A 0.8 0.6 Free PS 2h Free PS 4h Free PS 6h Free PS 7.3h PS Mn=250000 B 2.5 2.0 CNC-Br free PS 2H free PS 4H free PS 6H free PS 7.3H Weight 0.2 Deriv. Weight 1.5 0.5 100 200 300 400 500 100 200 300 400 500 600 Temperature ( o C) Temperature ( o C) Figure 5. The TGA (A) and DTG (B) vs reaction time of free PS CNC-Br CNC-g-PS 2H by ATRP free PS 7.3H by ATRP 2.8 2.4 CNC-Br CNC-g-PS 2H by ATRP free PS 7.3H by ATRP 0.8 2.0 Weight 0.6 Deriv. Weight 1.6 1.2 0.8 0.2 100 200 300 400 500 600 Temperature ( o C) 100 200 300 400 500 600 Temperature (h) Figure 6. The TGA and DTG of CNC-g-PS The thermal stability of the free PS and CNC-g-PS was characterized by TGA and DTG. As shown in Figure 5A, the Tonset of the free PS increased a little bit with the increase of the Mn of the PS. Then Mn of PS used here as a comparison was 250, 000 g/mol. The main decomposition temperature range of PS was from 350 to 450 C (Figure 5B). Figure 6 showed the TGA and DTG of CNC-g-PS. After grafted with PS, the thermal stability of CNC-g-PS increased a lot. 3 Conclusion BIBB was employed to modify CNC to prepare CNC-Br nano-initiator for SI ATRP. And then PS was grafted on CNC-Br by SI ATRP with presence of sacrificial initiator. The successful grafting was confirmed by FTIR, TGA, DTG and so on. 4 References 1. Harrisson, S., et al., Hybrid Rigid/Soft and Biologic/Synthetic Materials: Polymers Grafted onto Cellulose Microcrystals. Biomacromolecules, 2011. 12(4): p. 1214 1223. 2. Tsai, Y. and W. C. Wang, Polybenzyl methacrylate brush used in the top down/bottom up approach for nanopatterning technology. Journal of Applied Polymer Science, 2006. 101(3): p. 1953 1957.
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