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Influence of Chain Length on the Self-assembly of Poly(#-caprolactone) Grafted Graphene Quantum Dots Nabasmita Maity, Priyadarshi Chakraborty, and Arun K. Nandi Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03269 • Publication Date (Web): 03 Nov 2017 Downloaded from http://pubs.acs.org on November 4, 2017

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Influence of Chain Length on the Self-assembly of Poly(ε-caprolactone) Grafted Graphene Quantum Dots Nabasmita Maity, Priyadarshi Chakraborty and Arun K. Nandi* Polymer Science Unit, Indian Association for the Cultivation of Science, Jadavpur, Kolkata700 032, INDIA

ABSTRACT: The multifarious applications of graphene quantum dots (GQDs) necessitate surface modifications to enhance their solution processability. Herein, we report the synthesis and self-assembly of GQDs grafted with poly(ε-caprolactone) (PCL) of different degrees of polymerization 3, 7, 15 and 21 produced from ring opening polymerization. Optical and morphological studies unveil the transformation of the assemblies from J-aggregates to Haggregates, accompanied by alteration in morphology from toroid to spheroid to rod like structures with increasing chain length of PCL. Functionalized GQDs with lower chain lengths of PCL at higher concentration also assembles into liquid crystalline phases as observed from birefringent textures, which are later correlated to the formation of columnar hexagonal (Colh) mesophases. However, no such behavior is observed at higher chain lengths of PCL under identical conditions. So, it is evident that variation of PCL chain length plays a crucial role in the self-assembly, which is primarily triggered by the van der Waals force between the polymer chains dictating the π-stacking of GQDs, resulting in different selfaggregated behavior.

*

For correspondence: Arun K. Nandi, Email: [email protected]

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INTRODUCTION: Graphene quantum dots (GQDs) have recently attracted immense attention because of their fascinating physical, mechanical and optoelectronic properties.1-6 They are a special kind of chromophore, whose prominent characteristics include unique excitation dependent fluorescence emission bands, size tuneable stable photoluminescence and distinct electronic properties arising from the edge effect and quantum confinement.5-8 Unlike inorganic quantum dots (QDs), they are more environmentally benign and biocompatible.1,4,8 For these alluring properties, GQDs are emerging as a desirable material for versatile applications such as bioimaging,9−11 biosensing,4,11-12 photovoltaics,1,7,13 photocatalysis14,15 and light emitting diodes.1,16 Despite such applications, structural modification of GQDs by combining with other molecules is necessary to enrich their properties for more solution processable applications.2,7,17-20 From this perspective, polymers are a better choice for the functionalization of GQDs as they can introduce additional desired properties to the unique properties of GQDs in a synergistic way. Polymers exhibit plethora of applications because of their multifarious properties which include flexibility, easy solution processability, remarkable phase behaviour, attractive self-aggregation properties etc.21-26 Recently, polymer coating of GQDs27-29 has attracted considerable interest as polymer coating acts as a barrier which results in less cytotoxic and more stable GQDs under more complex, harsh and biologically compatible conditions. However, to our knowledge, self-assembly behaviour of polymer grafted GQDs have yet not been explored and it needs to be investigated for basic understanding. Keeping all these points in mind, we envisioned the preparation of a new material capable of harnessing the properties of quantum-confined GQDs with polymers, which can yield interesting morphological features and optoelectronic properties.


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To fulfil our aim, we have synthesized four poly(ε-caprolactone) (PCL) functionalized GQDs (S1-S4) having different chain lengths of PCL. Owing to the availability of native -OH functional groups of GQDs, PCL is anchored from it via ring opening polymerization (ROP) of ε-caprolactone (CL).30-33 Moreover, biodegradability and biocompatibility of PCL make it a better choice among different aliphatic polyesters.34-36 The as synthesized products undergo self-assembly process producing a green emitting soft gel in CHCl3. Accordingly, we investigate their morphological features to get an insight into the self-aggregation behaviour which demonstrates that as the length of tethered PCL chain increases, GQDs aggregate via different modes of π-π stacking, resulting in the construction of nanostructures of diverse shapes and dimensions. Although different modes of stacking (J and H aggregates) are well known in molecular gels,37-39 it would be interesting to observe this phenomenon in this GQD grafted polymers of varying chain lengths. In addition, we observe that the self-assembly of as synthesized disk-like materials (S1 and S2) is extended further forming lyotropic liquid crystal (LC) phases in their gel phase in CHCl3. Such highly concentrated carbon-based liquid crystalline phases could be promising from the perception of their attractive properties and applications as they comprise with the intrinsic properties of graphene.40,41 Herein, we report a striking impact of the chain length alteration on the self-assembly of PCL functionalized GQDs (S1-S4) to yield nanorings (toroids), nanospheres and nanorods with favourable offset or face-to-face stacking arrangements in CHCl3 medium. We also report here the self-assembly of S1 and S2 into columnar mesophases at higher concentrations (15 wt%).


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EXPERIMENTAL Materials: Graphite powder, stannous 2-ethylhexanoate [Sn(Oct)2], (Aldrich, USA), hexane (Merck, Mumbai, synthetic grade), FeCl3 and H2O2 (Loba Chemie, Mumbai, India) were used as received. Tetrahydrofuran, chloroform (Rankem, New Delhi, analytical grade) and toluene (Merck, Mumbai, 99%) were distilled before use. The monomer ε-caprolactone (CL) (Aldrich, USA) was dried over calcium hydride (CaH2) at room temperature for 48 h and was purified by distillation under reduced pressure. Graphene quantum dots (GQDs) preparation: Graphene quantum dots (GQDs) were prepared from graphene oxide (GO) (Scheme S1) using simple sono-Fenton method.42 First, GO was produced from graphite powder using Hummer's method43 and was dispersed in deionised water by stirring and ultrasonication for 6 h. To this dispersion H2O2 (30%) was added under stirring followed by a drop wise addition of aqueous FeCl3 solution (2 mg/mL). After sonication for about 6 hours, the reaction mixture was dialyzed in deionised water for 3-4 days to ensure the complete removal of iron. The resulting solution after dialysis was lyophilised to obtain solid GQDs. Synthesis of poly(ε-caprolactone) grafted from GQDs (S1, S2, S3 and S4): (a) Synthesis of S1. Poly(ε-caprolactone) (PCL) functionalized GQDs was synthesized using ROP of CL catalyzed by Sn(Oct)2.30-33 In a typical procedure (Figure 1a), 15 mg of GODs was taken in a dried and nitrogen purged glass ampule and was dispersed in 3 ml of dry toluene by ultrasonication for 1 h. To this dispersion, 0.3 mL of CL (0.002 mol) was added under nitrogen atmosphere followed by drop wise addition of 100 µl (0.031 × 10-2 mol) Sn(Oct)2. The mixture was degassed through three freeze-pump-thaw cycles and was placed in an oil 4 ACS Paragon Plus Environment

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Figure 1. (a) Synthetic Scheme and (b) 1H NMR Spectra of S(1-4) in CDCl3.


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bath thermostated at 90 oC under constant stirring. After 3 h, the polymerization was terminated by cooling to room temperature (30 oC). The product was purified by precipitating into large excess of cold hexane three times and was dried under vacuum. To remove any PCL homopolymer, the collected solid mass was further dissolved in THF and was centrifuged. The process was repeated twice and the residue was dried under vacuum. The degree of polymerization (DP) was determined by 1H-NMR spectrum from the signal integral ratio of d and d´ peaks32,33 (Figure 1b). Yield (gravimetric) = 93%. (b) Synthesis of S2, S3 and S4. S2, S3 and S4 were synthesized in a similar way as in S1 by varying the monomer amounts and polymerization times (Table S1). The DP for grafted polymer chains of each set was calculated using 1H NMR spectra. Yields (gravimetric) of S2, S3 and S4 were 94%, 91% and 90% respectively. 1

H NMR (CDCl3, 400 MHz) [Figure 1b]: δ (ppm) = 1.2-1.5 (m, 2Hc); 1.5-1.7 (m, 4Hb); 2.2-

2.5 (m, 2Ha); 3.66 (t, 2Hd′); 4.07 (t, 2Hd). Preparation of the gels: A solution (3 wt %) of S1 was prepared in CHCl3 in a glass vial. It was sonicated for 10 minutes and was heated under sealed condition to make it homogeneous and then was allowed to cool at room temperature (30 oC). It became viscous and gradually transformed into gel after 72 h. To determine its critical gelation concentration (CGC) in CHCl3, solutions of S1 were prepared with varying concentrations (10, 6, 4, 3, 2, 1 and 0.5 wt %) and were monitored after complete aging (10 days). It was noticed that gels were formed until 3 wt % which is taken as CGC in CHCl3. CGC for other three samples (S2-S4) was found to be similar to that of S1 gelation.


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Sample preparation for the self-assembly studies in solution state: All UV-Vis and PL spectral studies were carried out with CHCL3 solutions (0.3 wt %) of samples (S1-S4) after 3h of equilibration time. To compare the spectral features in their deassembled state, spectra were recorded in THF. Characterization: All 1H NMR spectra were recorded in CDCl3 using a Bruker 400 MHz NMR spectrometer. Fourier transform infrared (FTIR) spectral studies were performed using a Shimadzu FT-IR instrument (model 8400S). X-ray photoelectron spectroscopy (XPS) was performed in an Omicron Nano Technology 0571 XPS instrument using a focused mono-chromatized MgKα X-ray radiation source (1253.6 eV). The ultraviolet-visible (UV-Vis) absorption spectra were examined using a UV-Vis spectrophotometer (Hewlett-Packard, Model 8453) in a quartz cell with a thickness of 0.1 cm in the wavelength range of 190-1100 nm. The photoluminescence (PL) spectra were obtained using a quartz cell of 1 cm path length with a Fluoromax-3 fluorimeter (Horiva Jovin Yvon) for excitation at a wavelength of 270 nm. Raman spectra of the samples were recorded using a Lab Spec Raman spectroscope (JY T6400) with 514 nm argon laser for a scanning duration of 40 s. Rheological experiment of gel sample was performed with an advance rheometer (AR 2000, TA Instrument, USA) using cone plate geometry on a peltier plate. The diameter of the plate was 40 mm and the cone angle was 4° with a plate gap of 121µm. The morphology of the samples were monitored using a transmission electron microscope (TEM JEOL, 2010EX) operated at an acceleration voltage of 200 kV. For the surface morphological investigation, atomic force microscopy (AFM, Veeco, model AP 0100) was conducted in the non-contact mode at a resonance frequency of the tip end ~250 KHz. Field-emission scanning electron microscopy (FESEM) (Jeol GSM5800) was conducted on the sample drop casted on glass cover slip and coated with platinum


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prior to observation. The optical images were studied using a polarized optical microscope (Leitz Biomed) equipped with a digital camera. The wide-angle X-ray scattering (WAXS) experiments were performed using a Bruker AXS diffractometer (model D8 Advance) fitted with a Lynx Eye detector operated at a voltage of 40 kV and at a current of 40 mA. Samples were scanned in the range of 2θ = 5-40o at a scan rate of 0.3 s per step with a step width of 0.02o. Small-angle X-ray scattering (SAXS) experiments were performed in the 2θ range of 0.4-10° at a scan rate of 1 s per step. The thermal study was made by differential scanning calorimetry (PerkinElmer, Diamond DSC) calibrated with indium before each experiment. The heating rate was 10o/ min and the DSC scans for second heating is presented here. RESULTS AND DISCUSSION Synthesis and Characterization: Graphene quantum dots (GQDs), produced from graphene oxide as per our previously reported sono-Fenton reaction procedure,42 is used to synthesize four poly(ε-caprolactone) (PCL) functionalized GQDs (S1-S4) of different length of pendent polymer chains via the ring opening polymerization (ROP) of ε-caprolactone (CL) (Figure 1a, Table S1). Briefly, GO sheets are disintegrated into large number of tiny particles called GQDs by using 30% H2O2 and FeCl3 (Fenton’s reagent) followed by vigorous sonication (Scheme S1). In Figure 2a the TEM images of GQDs (black spots) are shown and the sizes of GQDs, presented in the distribution cure (Figure 2a, inset), lie in the range 1.8 to 4.3 nm with an average diameter of 2.5 ± 0.5 nm. At the inset of Figure 2a also the HRTEM image of GQDs is presented showing the fringe pattern with a lattice parameter of 0.24 nm. They are characterized by Raman spectra, X-ray diffraction (XRD), FT-IR and XPS spectra. The Raman spectrum (Figure 2b) shows D, G and 2D bands. The D and G bands correspond to the disorderinduced and Raman allowed phonon mode of vibrations at 1353 and 1607 cm-1, respectively and the 2D band appears at ~2800 cm-1 8 ACS Paragon Plus Environment

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as a broad band. Figure 2c demonstrates the XRD pattern indicating that GQDs have a broad peak at 2θ = 27.1o, corresponding to the interlayer spacing of 0.33 nm (002 plane).44

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Figure 2. (a) TEM micrographs (inset: HRTEM image of single GQD showing the lattice spacing of 0.24 nm and size distribution), (b) Raman spectrum (inset: aqueous dispersions of GQDs under UV-light), (c) XRD pattern and (d) FT-IR spectrum of GQDs. Now, depending on the defects and oxygenated functional groups present in GQDs, this peak can shift a little bit. Additionally, a peak corresponding to the interlayer spacing of 0.24 nm is also observed at 2θ = 36.7o.44 To verify the particle size of GQDs further from XRD data, we have used Scherrer equation:

߬=

Kλ βcosθ


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where, ࣎ = average crystallite size, K is a dimensionless shape factor. It has a typical value of about 0.9, β = full width (in radians) at half the maximum intensity (FWHM), θ = Bragg angle in degree, λ = X-ray wavelength. Calculations give the values of 4.07 and 3.80 nm for the peaks at 2θ = 27.1o (FWHM = 2.1o) and 36.7o (FWHM = 2.3o), respectively and it indicate that although the result from XRD data is not an excellent agreement with the average size determined by TEM data, but it resides within the size distribution range determined using TEM data. In the FT-IR spectrum (Figure 2d) GQDs have characteristic peaks at 3373, 1403, 1270 and 1050 cm-1 for the stretching vibrations of -OH, C-O bonds, CO stretching of C-OH and the epoxide groups, respectively. It exhibits another peak in the range of 1600-1750 cm-1, which can be ascribed to the merging of two peaks at 1638 and 1710 cm-1 for C=C and >C=O stretching vibrations, respectively.42,45 The XPS spectrum of GQDs is shown in Figure S1 and it exhibits two strong peaks for C1s and O1s. In Figure 3a, the C1s XPS spectrum is deconvoluted into four peaks centered at 284.5 eV (C=C), 285.2 eV (C-C), 286.2 eV (C–O) and 288.6 eV(C=O). Figure 3b shows that the O1s peak can also be decomposed into two peaks centered at 531.4 eV and 532.6 eV for C=O and C-OH/C-O-C groups, respectively. The presence of C-O and C=O peaks in Figure 3a and 3b confirms that the as-synthesized GQDs contain hydroxyl, epoxy and carboxyl groups as oxygenated functional groups. To functionalize PCL from GQDs surface, ROP of CL initiated from the hydroxy groups30–33 of GQDs is used by heating CL with a dispersion of GQDs in toluene at 90 oC in presence of Sn(Oct)2 catalyst (Figure 1a). Figure 1b shows the 1H-NMR spectra of all the synthesized products along with the assignments for different protons. The signal integral ratio of d′ and d protons corresponding to the terminal CL unit and all other CL units, respectively46 is used for the determination of the degree of polymerization (DP) of grafted PCL chains. The DP is calculated to be 3, 7, 15 and 21 for S1, S2, S3 and S4, respectively. 10 ACS Paragon Plus Environment

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Figure 3. (a) C1s and (b) O1s XPS spectra of GQDs, (c) C1s and (d) O1s XPS spectra of PCL functionalized GQDs (S2). The FT-IR spectra of PCL functionalized GQDs (S1-S4) (Figure S2) exhibit bands at 1725, 3443 and 1047 for >C=O, O-H and C-O stretching, respectively and in every case, the spectra indicate the presence of GQDs signature peak at 1635 cm-1 (C=C stretching). In addition, each spectrum exhibits the bands at 1296, 1243 and 1188 cm-1 for C-O stretching of ester and at 2946 and 2865 cm-1 for aliphatic C-H stretching corresponding to the characteristic peaks of pendant PCL chains.33 Again the XPS spectrum of PCL functionalized GQDs (S2) reveals the presence of two peaks for C1s and O1s (Figure S1). The C1s XPS spectra (Figure 3c) can be deconvoluted into four peaks, arising from C=C (284.5 eV), C-C (285.2 eV), C-O (286.2 eV) and C=O (288.6 eV) groups as observed for GQDs. In the O1s XPS spectra 11 ACS Paragon Plus Environment

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(Figure 3d) two peaks are observed at 531.4 and 532.6 eV, corresponding to the C=O and COH/C-O-C groups, respectively. While comparing the XPS spectra of GQDs before and after PCL grafting, it is noticed that the relative intensity of C=O peak from that of C=C for C1s (Figure 3a, c) increases after grafting and this is more prominent in C-OH/C-O-C peak for O1s spectra (Figure 3b, d), which is resulted from the carbonyl groups of pendent PCL chains. The photoluminescence (PL) spectra (Figure S3a, S3b) of S1 and S4 i.e. the GQDs with shortest and longest PCL chains, exhibit the characteristic excitation dependent PL emission spectra of GQDs, indicating that polymer attachment does not disturb the inherent properties of GQDs. A careful look on the spectra indicate that although the peak positions are almost independent of chain length, but PL intensity significantly increases with the chain length, probably due to formation of more stable excitons of GQDs at highly wrapped condition by the longer polymer chains, prohibiting quenching with the medium. Self-assembly: The self-assembling behaviour of S1 is tested by cooling a hot solution of it (CGC~3 wt%) in chloroform (CHCl3), resulting in the formation of almost transparent, green emitting gel (Figure 4a) after 72 h. The rheological experiment (Figure S4) indicates frequency independent storage modulus (G′) and it is also greater than the loss modulus (G″), confirming the gel nature of the sample. The UV-Vis spectroscopic studies show that the absorption maximum of S1 is red-shifted with enhanced fluorescence intensity (Figure 4b) in CHCl3 compared with solutions in THF, which does not promote any self-assembly. These spectral results suggest that the aromatic GQDs overlap in J-type stacking mode,47-49 which influences the self-assembly of S1 to arrange themselves with slipped packing and finally forming toroidal nanostructure (Figure 4c).


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0.45 THF CHCl3

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Figure 4. (a) Digital images of S1 gel (6 wt %) in CHCl3 under day (left) and UV light (right) (b) Absorption and emission spectra of S1 in THF (black and solid line) and in CHCl3 (red and dashed line), (c,d) AFM images (e) HRTEM image (inset: FESEM image; scale bar, 100 nm) of S1 gel in CHCl3 (f) Magnified image of the area in red box of (e) showing the lattice spacing of 0.24 nm. To unveil the nanostructure of the aggregates we have used the atomic force microscopy (AFM) which clearly indicates formation of toroidal structures (Figure 4c, 4d and S5a) with an outer diameter of around 220 nm and an inner cavity diameter of nearly 65 nm (height~ 2.5 ± 0.2 nm), which is in good agreement with both the transmission electron microscopy (TEM) (Figure 4e) and scanning electron microscopy (SEM) images (inset of 13 ACS Paragon Plus Environment

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Figure 4e, Figure S6a). The presence of GQDs is confirmed by taking a closer look to the TEM image (Figure 4f), which clearly shows distinguishable GQDs domain. To explore the significant role of polymer chain length on self-assembly behaviour, if any, we have studied the self-assembly behaviour of other three samples, S2, S3 and S4. When their hot CHCl3 solutions are cooled, gelation is observed in each case similar to that of S1. However, the UV/Vis spectra of CHCl3 solutions of S2 give rise to a different outcome. On changing the solvent from THF to CHCl3, S2 displays a blue shifted absorption maximum with reduced fluorescence intensity (Figure 5a), suggesting H-type stacking47-49 of GQDs. Inspection of AFM image (Figure 5c, S5b) of S2 reveals the presence of spherical aggregates (width ~190-230 nm; height ~8-9 nm), which is quite different from the nano structures observed for S1. Here, the alteration in the self-assembly pathway can serve to cause structural difference as a result of increasing chain length48 of PCL segments. All the observations together with the SEM and TEM image analysis (Figure 5d, inset) demonstrate that S2 self-assembles via face-to-face packing i.e. stack on each other which further aggregates [Figure S5b(i, ii)] giving core-shell type spherical nanostructure where the shell is produced from the PCL chains extending outwards. Moreover, S3 and S4 showed the similar optical spectral behaviour as that of S2, therefore, only the result for S4 is shown in Figure 5b. Briefly, S4 consists of H-aggregates, as evidenced by the blue-shifted absorption maximum and fluorescence quenching in CHCl3 with respect to THF solutions. Characterization using AFM imaging demonstrates that self-assembly of S4 gives rise to formation of rodlike nanostructure (Figure 5e, S5c). To corroborate the result, we have further studied the SEM and TEM images (Figure 5f, inset), which clearly reveals the nanorod formation. In this context, the morphological investigation of S3 using TEM and SEM (Figure S6b, inset) studies also unveiled formation of similar type of rodlike nanostructure as that of S4, indicating that both S3 and S4 follows similar type 14 ACS Paragon Plus Environment

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self-assembly pathway to give H-type aggregations. To gain an insight into the rodlike nanostructure formation, we have performed AFM study of freshly prepared solution of S4 (0.3 wt %) in CHCl3 and AFM images (Figure 6a, S7) clearly indicate that distinct spherical


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objects (width ~180-210 nm; height ~ 7.3 ± 1 nm) started to adhere together forming extended nanorod structure (width ~ 210 nm; height ~ 2.3 ± 0.2 nm) upon aging after 72h (Figure 5e). The heights of spherical objects somewhat decrease due to flattening which is more likely to happen during extended nanorod formation. The FTIR spectral measurements (Figure 6b) reveal that the band due to O-H stretching vibration in the region 3430-3460 cm-1, appears as a broad band at a lower wave number region in xerogel phase compared to the S1-S4 powder (Figure S2). In addition, the spectra suggest that the shifts to lower frequency gradually decrease with decreasing chain length. This suggests that the strength of H-bonding is comparatively lower in S1 from that of other samples after gelation, which is in accordance with the respective mode of π-π stacking of GQDs in the grafted samples.48 Moreover, the FTIR peak of >C=O band (1725 cm-1) becomes broader showing a new peak at 1717 cm-1 in the xerogels (Figure S8), supporting formation of H-bonds between –OH and >C=O groups of tethered chains.

a % Transmittance

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1.0 μm

b (i) (ii) (iv) (iii) 3150

3300 3450 3600 -1 Wave number (cm )

3750

Figure 6. (a) AFM image of freshly prepared (30 mins) S4 solution (0.3 wt %) in CHCl3 (b) FT-IR spectra of the O-H stretching vibrations for (i) S1, (ii) S2, (iii) S4 xerogel and (iv) S1 powder. Thus, variation in chain length plays vital role in the self-assembly which is driven mainly by the van der Waals force among the polymer chains, determining the final outcome. Increased length of grafted PCL chains helps in solubility in CHCl3. Considering the chain length dependence, if we enumerate all the data, then it can be proposed that the self- assembly occurs via H-bonding and van der Waals interactions of PCL chains together 16 ACS Paragon Plus Environment

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with π-π interaction among GQDs. As the chain length decreases, van der Waals and Hbonding force between the tethered polymer chains become relatively weak, which induces the neighbouring molecules to slide into a loosely packed arrangement favouring offset stacking of GQDs. On the other hand, increasing tethered chain length causes stronger interchain interaction facilitating the GQDS to stack face-to-face i.e. H-aggregate formation. Corroborating all experimental evidences, a model has been proposed in Scheme 1 illustrating the different modes of self-assembly of PCL functionalized GQDs. Scheme 1. Proposed model for the self-assembly of PCL functionalized GQDs.


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To examine the impact of π-π interactions on the PL behaviour of these J- and H-type aggregates in their constrained self-assembled state i.e. in gel state, we have compared the emission spectra of S1 and S4 gels with their respective solution states in CHCl3. Compared to the emission spectrum of solution state (Figure 4b), S1 gel (Figure S9a) exhibits a broad red-shifted band in the range of 390-630 nm showing a maximum at 467 nm, justifying the green emissive nature of the gel. The emission spectrum of S4 gel (Figure S9b) is also red-shifted to 422 nm relative to that of its solution state (Figure 5b). These results are indicative of effective π-π interactions in both the gels produced by stacking of GQDs. Now, compare to the S1 gel, the emission maximum of S4 gel shows a lower shift by 45 nm. As S1 and S4 gels are produced from different aggregated states, their fluorescence properties are originated from different environment and as a consequence they exhibit different emission maxima. These results clearly indicate that the molecules of S1 and S4 interact differently with each other in their respective gel states both in the ground and excited states. Lyotropic liquid crystal (LC) behaviour: An interesting observation is noticed in S1 and S2 gels at a concentration of 15 wt%. Disk-like S1 and S2 in their gel state self-assemble into LC phases,50 but no mesophase behaviour is noticed for S3 and S4 under identical condition. Below 15 wt% concentration, all the samples are isotropic in nature. Birefringent textures, observed under a polarised optical microscope (POM) for 15 wt% of S1 and S2 gel, are shown in Figure 7a and 7b, respectively. To identify the LC phases, both the samples are investigated using X-ray diffraction (XRD) technique. In the small-angle X-ray scattering (SAXS) of both the samples (Figure 7c, 7d) four reflections are visible in the ratio of 1: 1/√3: 1/2: 1/√7, which is in agreement with columnar hexagonal (Colh) lattice (Table 1). The wide-angle X-ray scattering (WAXS) profile displays a broad peak at d = 4.34 Å for S1 (inset of Figure 7c) and at 4.41 Å


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a

b

d

1.25

1.50

o

Intensity (a.u)

c Intensity (a.u)

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1.75

q (A-1 )

0.10

0.15

0.20

0.25

1.5 o 2.0 q (A-1)

0.10

0.15

0.20 o

o -1

0.25

0.30

-1

q (A )

q (A )

Figure 7. Polarized optical microscopy (POM) images of (a) S1 and (b) S2 gels (15 wt %), small angle XRD pattern of (c) S1 and (d) S2 gels (inset: respective wide-angle XRD pattern) in their Colh phase. for S2 (inset of Figure 7d), corresponding to the average interchain separation between the aliphatic polymer chains. In addition, a peak arising from the π-π interactions of the aromatic cores is also visible at d = 3.33 Å and 3.36 Å for S1 and S2 respectively. To investigate the thermal behaviour, differential scanning calorimetry (DSC) was performed under N2 flow at a heating rate of 10o/min. An endothermic peak (Figure S10) is observed in the temperature range of 24-56 oC for S1 and 21-48 oC for S2 and is attributed to the melting temperature (Tm) of gels. Observed d-Spacings in Colh phases determined from the locations of Bragg reflections and thermal analysis of lyotropic LC gels are summarised in Table 1. We have left the S2 gel in air and monitored the changes of WAXS pattern with time due to 19 ACS Paragon Plus Environment

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evaporation of chloroform (Figure S11). It is evident from the figure that gel at 3 hrs has liquid crystalline broad diffraction peaks and sharp crystalline peaks starts to appear at 6 hrs and at 48 hrs the gel gets dried completely producing PCL crystals. Thus it may be concluded from the results that S2 forms lyotropic liquid crystal in the gel state but at the solid state it is purely crystalline. Table 1. Observed d-spacings in Colh phases and thermal analysis of lyotropic LC gels.

d (Å) Compound in CHCl3 (15 wt%) S1 S2

2nd Heating

Colh

Tm

61.9, 35.56, 29.51, 22.69, 4.34, 3.33

( C) 39.5

∆H (J/g) 17.3

57.3, 33.17, 27.71, 21.22, 4.41, 3.36

33.2

6.1

o

CONCLUSION: So variation of PCL chain length grafted from GQDs plays a vital role in the selfassembly which is driven mainly by the van der Waals force among the polymer chains guiding the π-stacking of GQDs in chloroform, determining the selfaggregated behaviour. Amongst the four PCL grafted GQDs (S1-S4) of different degree of polymerization, 3, 7, 15 and 21, only S1 gel shows toroidal morphology produced from J-aggregate formation; however S2 shows spherical morphology arising from H-aggregate formation. On increasing the chain length of PCL grafted on GQDs, S3 and S4 also exhibit H-aggregate formation but producing rodlike morphology. Liquid crystalline mesophases are produced in S1 and S2 gel at 15 wt% concentration but no mesophase behaviour is noticed for S3 and S4 gels under identical condition. Small and wide angle x-ray scattering indicate formation of columnar hexagonal (Colh) mesophase with the presence of π-π stacking in S1 and S2 gel, but with increasing chain length in S3 and S4 no such mesophases are noticed. The outcome of the current study is indeed encouraging to explore exciting possibilities of self-assembled 20 ACS Paragon Plus Environment

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polymer functionalized GQDs with different polymers which will help in growing the scope of GQDs for wide range of applications. ACKNOWLEDGEMENTS: We gratefully acknowledge CSIR, New Delhi (Grant No. 02(0241)/15/EMR-II) for financial support. N. M. acknowledges CSIR, New Delhi for the fellowship. We acknowledge Prof. Sandeep Kumar of Raman Research Institute, Bangalore for illuminating discussion on the lyotropic liquid crystal formation. SUPPORTING INFORMATIONS: Additional Table (Table S1), Scheme (Scheme S1) and Figures (Figure S1-S11) are presented in supporting information. The Supporting Information is available free of charge on http://pubs.acs.org/. REFERENCES: 1. Gupta, V.; Chaudhary, N.; Srivastava, R.; Sharma, G. D.; Bhardwaj, R.; Chand, S. Luminscent Graphene Quantum Dots for Organic Photovoltaic Devices. J. Am. Chem.

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Table of Content Influence of Chain Length on the Self-assembly of Poly(ε-caprolactone) Grafted Graphene Quantum Dots Nabasmita Maity, Priyadarshi Chakraborty and Arun K. Nandi* Polymer Science Unit, Indian Association for the Cultivation of Science, Jadavpur, Kolkata700 032, INDIA


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Grafted Graphene Quantum Dots - ACS Publications - American

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