Reduced graphene oxide composites with water soluble copolymers having tailored lower critical solution temperatures and unique tube-like structure

Nanohybrids of graphene with water soluble polymer were synthesized using ‘grafting from’ method. GO, prepared by modified Hummers’ method, was first reacted with sodium azide. Alkyne-terminated RAFT-CTA was synthesized by reaction of propargyl alcohol and S-1-dodecyl-S’-(α,α‘-dimethyl-α”-acetic acid) trithiocarbonate. RAFT-CTA was grafted onto the GO sheets by facile click-reaction and subsequently, N-isopropylacrylamide (NIPAM) and N-ethyleacrylamide (NEAM) were polymerized on graphene sheets via RAFT polymerization method. The respective copolymers with different ratios were also prepared. The nanohybrids were characterized by FTIR, XRD, TGA, Raman, SEM, and AFM. Both SEM and AFM clearly showed rod-like structures for rGO-PNEAM. XRD showed a small peak at 2θ = 19.21°, corresponding to d-spacing ≈ 4.6 Å. In addition, the nanohybrids showed a very broad temperature range for the LCST in water between ca. 30 and 70 °C.


Results and discussion
FT-IR spectroscopy provides information about the structures of the materials, as shown in Fig.  SI1. The FT-IR spectra of GO clearly showed the characteristic absorption peaks at 1091, 1392, 1631, 1735, and 3332 cm -1 , corresponding to the stretching modes of C-O-C, C-OH, C=C, C=O and O-H, respectively. For GO-N3, peak at 2054 cm -1 was observed and could be assigned to the absorption of the azide groups while alkyne-RAFT shows a weak peak at 2120 cm -1 attributed to alkyne group, and trithioester group present a peak at 1090 cm -1 . After clicking GO-N3 and alkyne-RAFT together, the peaks related to azide and alkyne groups disappeared and stretching bands of alkyl group on the RAFT chain were observed in the spectrum. In addition, the intensity of C-O and O-H peaks were significantly reduced, indicating that during click reaction, the GO sheets have been reduced as well. 1 This is an added advantage for click-coupling reactions to obtain graphene sheets without the need for a further reduction step. In the polymer nanohybrid spectra, the typical absorption peaks of polyacrylamides include an absorption around 1550 cm -1 due to N-H bending, 1600 cm -1 C=O and 3500 cm -1 N-H stretching. 2   SI2 shows Raman data for GO, GO-N3, rGO-RAFT, rGO-PNIPAM, and rGO-PNEAM. Graphite shows only one peak (G band), which is related to sp 2 hybridization in a graphitic two dimensional hexagonal lattice, while GO showed another peak named D band at 1360 cm -1 ascribed to sp 3 hybridization (C atoms of defects and disorder) besides the G band that was observed at 1586 cm -1 . In graphitic materials functionalization leads to a blue shift in G band and a red shift in D band. Shift in G band to higher wavenumbers could be due to the influence of defects and isolated double bonds produced by the reduction process of GO. However, D band still existed after functionalization since it was difficult to restore GO completely into graphene even after reduction. 3,4 The D bands displayed by homopolymer nanohybrids were detected at 1351 and 1354 cm -1 and for copolymer ones were observed at 1341, 1345, and 1342 cm -1 a relatively lower value when compared to that displayed by GO (1360 cm -1 ), indicating the chemical modification of GO surface or electron transfer between the polymers and GO. ID/IG is used to get information about the disordered and ordered domains of graphene. Grafting has led to an increase in ID/IG in the following order: rGO-PNIPAM > rGOcopolymer 1:1 > rGO-PNEAM = rGO-copolymer 3:1 > rGO-copolymer 1:3 > GO-N3 > rGO-RAFT > GO (Table SI1). The increased ID/IG ratio reflects decreased average size of the sp 2 domains in the related nanocomposites. 5 The zeta potential, average size and conductivity of the polymer nanocomposites were measured at 25°C (Fig. SI3). The Z-average for GO, rGO-PNIPAM, rGO-PNEAM, and rGOcopolymer 1:1 were 437, 1300, 1004, 1339 nm, while PDI was 0.477, 0.653, 0.761, and 0.741, respectively. The increase in size of the nanomaterials were due to the introduction of polymers on the graphene sheets. Even though the size of the graphene sheets reduced in each step of functionalization, the increase in the average size of the nanocomposites could be attributed to this fact that water-soluble polymers stretch out in water.
This is also the reason why the polydispersities are relatively high. Depending on the angle under which laser checks the nanohybrids, their size varies, which causes a broadening of the size distribution.  An estimation of the grafting density of the can be done when combining several geometric considerations.
Considering the molar mass of 448 g/mol of the RAFT-agent including the coupling groups, we can conclude that the composite contains ca. 89.3 wt. % RGO and 10.7 wt. % RAFT-agents.
2. Bonaccorso et al. 6 reported the theoretical specific surface area (SSA) to be 2630 m²/g. Considering the graphene content, this translates into ca. 2347 m²/g for the RGO-RAFT composite. This assumes that the influence of the oxygen and of the sheet edges is negligible.
3. Based on these numbers and the Avogadro-number, the grafting density can be calculated to be 6.37×10 16 RAFT-molecules per m², which translates into 15.7 nm² per grafted RAFT-molecule. This number includes both sides of the RGO-sheet. 4. When assuming that the 15.7 nm² encompass a circular area, the radius would be 2.23 nm. The length of the RAFT-agent including the coupling group is 2.7 nm (assuming a stretched molecule) and, thus, on approximately the same length scale. It is clear that this assumption is not 100% correct, as -when assuming an ideal hexagonal packing of the grafted groups -the area should be hexagonal, but the grafting does not follow an ordered but rather a statistical packing and consequently, we can only calculate average areas per RAFT-agent. For the purpose of this estimation, the assumption of circular areas of influence is sufficient for getting an idea of the grafting density.
Considering that based on these assumptions 2 RAFT-molecules are ca. 2 radii (=4.47 nm) separated, while the RAFT-agents' length is somewhat less than that, it becomes clear that this grafting density automatically leads to a higher surface roughness.