In the last nine years, a sustained effort has been made to synthesize composites based on polystyrene (PS) and various carbon nanoparticles, such as multi-walled carbon nanotubes [1
], single-walled carbon nanotubes [3
], fullerene [3
], reduced graphene oxide [4
], graphene oxide (GO) [6
], GO functionalized with phosphor-based organic compounds [7
], and sulfonated GO [8
]. The interest in these composite materials was reported to be for use as flame retardants [9
] and non-volatile memory devices [10
]. In the case of the PS/GO composites, the following six synthesis methods were used until now: (i) suspension polymerization [11
], (ii) microemulsion polymerization [12
], (iii) electrostatic self-assembly [13
], (iv) reversible addition fragmentation chain transfer [14
], (v) in situ polymerization followed by melt process [15
], and Pickering emulsion polymerization [8
]. Using X-ray photoelectron spectroscopy (XPS) and IR spectroscopy, the mechanisms proposed for the microemulsion and suspension polymerizations of styrene in the presence of GO were demonstrated to induce: (i) the opening of ether cycles on the GO surface [11
] and (ii) the appearance of ester groups [12
]. Using scanning electron microscopy (SEM) [11
], transmission electron microscopy (TEM) [11
] and X-ray diffraction (XRD) [11
], the morphological and structural properties of the PS/GO composites were reported.
In order to obtain composites based on expandable PS and GO, in this work the attention was focalized on the radical polymerization of styrene in the presence of the GO sheets and pentane. The influence of the GO sheets’ weight on the PS spheres’ size was analyzed using SEM and dynamic light scattering (DLS). Using Raman scattering and IR spectroscopy, a polymerization mechanism of styrene in the presence of GO and pentane was reported. For the first time, the photoluminescence (PL) of the PS/GO composite was evidenced and the role of the GO sheets in the PS spheres’ PL was also reported. A photo-degradation process of the PS/GO composites with a GO sheets concentration of 5 wt.% is also highlighted in this work by PL.
2. Materials and Methods
Styrene, benzoyl peroxide (BPO), dimethylformamide (DMF), ethanol and benzene were purchased from the Sigma-Aldrich Company Ltd, Poole, UK and used without further purification. Other compounds were purchased from: (i) S.C. Nordic Invest S.R.L., such as H2SO4, H3PO4, and HCl; (ii) S.C. “Hipocrate 2000” S.R.L., such as H2O2, (iii) Fluka, the graphite powder, and (iv) Merck, such as KMnO4.
The GO membranes were prepared by oxidative-exfoliation method [16
]. In detail, 742 mL H2
(95–97%, SC Nordic Invest SRL, Cluj-Napoca, Romania) was mixed by stirring with 83 mL H3
(85%, SC Nordic Invest SRL, Cluj-Napoca, Romania) and then with 7.5 g graphite (powder, <0.1 mm, Fluka, (Leicestershire, England). Then, 33 g KMnO4
(99%, Merck, Darmstadt, Germany) was added slowly over the obtained suspension placed in an ice bath. After 2 days of ambient conditions, the reaction suspension was placed again by stirring in an ice bath and 550 mL of H2
(3%, SC “Hipocrate 2000” SRL, Bucharest, Romania) was slowly added. There followed steps of centrifugation (5000 rpm/10 min)-decantation-washing-sonication (15 min) in 550 mL H2
O, 275 mL HCl (37%, SC Nordic Invest SRL, Cluj-Napoca, Romania) and 275 mL ethanol (absolute, SC Nordic Invest SRL, Cluj-Napoca, Romania). The last two processes were repeated two times. The obtained product was dispersed in 550 mL H2
O and kept for 4 days in a sealed jar. After that, about 90% of the suspension volume from the upper part was harvested and spread onto a large glass support and placed for drying in open air under ambient conditions. GO as membranes (i.e., unsupported film) was obtained by mechanical exfoliation of the obtained GO film.
The radical polymerization of styrene (2 g) was started in the presence of benzene (40 mL) and BPO (0.04 g) at a temperature of 90 °C. The reaction was performed using a 1-L flat-bottom polymerization vessel equipped with a reflux condenser, mechanical stirrer, thermometer and two access inlets for nitrogen and pentane, respectively. A nitrogen bubbling was performed prior to the start of the polymerization reaction. In order to obtain expandable PS, four hours after the start of the polymerization reaction pentane was added to the reaction mixture (4 mL) as a foaming agent and the reaction was conducted at 120 °C for a further two hours. The polymerization reaction was stopped by adding about 500 mL ethanol when the temperature of the reaction mixture reached room temperature. Afterwards, filtration of the crude product and drying at a temperature of 60 °C were performed until a constant weight was achieved. Finally, a white powder of expandable PS was obtained.
With the intention of obtaining PS/GO composites, different GO amounts, i.e., 11, 22, 43, 66, 90 and 112 mg, were suspended in 10 mL DMF and added to the styrene mixture. Following all the above-mentioned steps in the case of expandable PS synthesis, PS/GO composites of gray color having a GO concentration equal to 0.5, 1, 2, 3, 4 and 5 wt.%. were obtained.
SEM pictures of GO membranes and PS/GO composites were recorded with a Zeiss Gemini 500 scanning electron microscope (Zeiss, Oberkochen, Germany).
The distribution of the PS spheres’ size in the case of compounds synthetized in this work was determined by a static light scattering method with Fritsch Analysette 22 NanoTec equipment from Fritsch GmbH, Idar-Oberstein, Germany, which has a measuring range from 10 to 2 mm. The light is obtained from two linear polarized lasers (one green, i.e., λ = 532 nm and another in IR, λ = 850 nm) and both forward and back-scattered signals are recorded by an array of detectors. For each sample, a background signal from the dispersion fluid (bi-distilled water) was recorded and then dispersions of the materials in water with an opacity of 10% were cycled five times through the particle size analyzer in order to obtain the best homogeneity. The last three measurement cycles provided virtually identical distributions. The values recorded from the fifth cycle are presented in this work.
Raman spectra of the GO membranes, the expandable PS and the PS/GO composites were recorded with a Raman spectrophotometer, T64000 model, from Horiba Jobin Yvon (Palaiseau, France) equipped with an AR laser (the excitation wavelength used was equal to 514 nm).
IR spectra of the GO membranes, the expandable PS and the PS/GO composites were recorded with a FTIR spectrophotometer, Vertex 80 model, from Bruker (Billerica, MA, USA).
PL and photoluminescence excitation (PLE) spectra of the expandable PS and the PS/GO composites were recorded with a Flurolog-3 spectrometer, FL3-22 model from Horiba Jobin Yvon (Palaiseau, France), under an excitation wavelength equal to 327 nm.