# Scalable High Refractive Index polystyrene-sulfur nanocomposites via in situ inverse vulcanization

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Sep 10, 2020

Incorporation of elemental sulfur into polymers during synthesis is highly challenging due to its low solubility and compatibility with majority of organic chemicals. Further, polysulfur produced from heat treatment are highly brittle and unstable at room temperature; which also make it undesirable for use during polymer processing. Thus, in this method polysulfur chains are stabilized by reacting elemental sulfur with DIB cross-linker inside the polystyrene matrix during extrusion via in situ inverse vulcanization. A schematic representation for the preparation of transparent polystyrene-sulfur composites is shown in Fig. 1A.

Initially, pure polystyrene melted at 230 °C in an extruder, to this known amount of sulfur; DIB were added in stepwise and allowed to mix for 30 min. Under extrusion condition sulfur undergo ring opening polymerization to form a viscous polysulfur which later reacts with DIB to yield cross-linked SD copolymers. These viscous melts effectively mix with PS melts and diffuse into the matrix due to the applied shear resulting in a homogeneously dispersed transparent composites (see Fig. 1A). Colorless polystyrene changed to orange red color confirming the successful cross-linking reaction (see Fig. 1B bottom row). Similar color change was also observed when pure sulfur directly reacted with DIB (see Fig. S1). These cross-links provide structural stability to the polysulfur chains whereas DIB improves the compatibility with PS matrix. For comparison, composites were also prepared without the cross-linkers resulting in transparent yellow colored polystyrene-sulfur (PS-S) composites (see Fig. 1B top row). Optically transparent materials were obtained with up to 10 wt% of sulfur loading, excess addition resulted in agglomeration of particles and reduced transparency (see Fig. S2). Therefore, the composites with 5 and 10 wt% of sulfur loading were only prepared and used for further study.

The main advantage of these composites is their ability to scale up and processability into variety of different objects by melt and solution processing technique. Molded objects and films prepared by injection molding and hot-press methods are displayed in Fig. S3 are transparent and uniformly colored like extrudate indicate the stability and unchanged morphology of the composites. Thus, the nanocomposites are feasible to process into desire shape and size without altering the transparency. The composites also exhibits high solubility in most of organic solvent similar to polystyrene and form a clear and stable solution (see image Fig. S4); hence, enable to prepare ultra-thin films on glass slides by solution casting or spin coating method. Thus, the one step preparation and easy processability of polystyrene-sulfur composites have greater advantage over inorganic polymer composites.

In situ cross-linking reaction of SD copolymers was verified by carrying out Fourier-transform infrared spectroscopy (FT-IR) analysis (Fig. 1C). In the FT-IR spectrum of PS-SD samples the appearance of new stretching bands at 1,012 and 645 cm−1 corresponding to C–S bonding indicating successful chemical reaction between sulfur and DIB39. Intensities of these peaks are relatively weak due to the higher peak intensities of polystyrene in the same IR region. Moreover, absence of characteristic C=CH2 peak located at 900 cm−1 clearly indicates the complete consumption of DIB during cross-linking reaction. Interestingly, polystyrene peaks in the spectra’s are unchanged suggests the structural stability and non-reactivity of PS against sulfur under extrusion condition (see Fig. S6). This was also supported by solid state 1H-NMR spectra (see Fig. S7). The nature of carbon–sulfur bonding within the polymeric matrix of PS-SD was further elucidated by Raman spectroscopy analysis (see Fig. 1D). PS-SD showed S–S bonding peaks at 152, 220, and 474 cm−1 along with the ν (C–S) peak at 182 cm−1, which indicates successful chemical reaction of sulfur and DIB.

Effect of the fillers on the structural behavior of polystyrene was analyzed by XRD. Figure 2 shows the XRD patterns comparison between PS with PS-S and PS-SD composites at different sulfur loading. PS shows a broad XRD peak at 2θ = 20° confirming its amorphous nature, similarly PS-S and PS-SD composites are also displayed amorphous peak in the same range, however; the peak intensities are reduced according to the sulfur content. It is also interesting to note that the sharp crystalline sulfur peaks (see insight image) are completely absent even in 10 wt% loaded composites samples, indicating the change in sulfur morphology after the extrusion. This can be explained as follows; during melt mixing process, both PS and polysulfur turns to viscous melts and hence the polysulfur chains can easily diffuse into the PS matrix under applied shear to form a homogeneous polymer blends. When these polymers are extruded they are allowed to cool naturally to the room temperature, during cooling process diffused polysulfur chains solidifies much faster than PS due to the difference in solidification rate. Thus, the mobility of polysulfur chains are arrested inside the PS matrix and results in random dispersion. Consequently, when these polysulfur chains revert back to the monomeric octasulfur form, their crystalline structure is destroyed which leads to the non-crystalline amorphous sulfur. In case of PS-SD the polysulfur chains reacts with the DIB to form highly cross-linked copolymers which further restricts the chain mobility and increases the amorphous nature. Thus, the sulfur crystalline peaks are completely absent in the composites. Furthermore, faster solidification of sulfur also interfere with the crystalline arrangement of PS chains and reduces the lamellar thickness, hence, increasing in sulfur loading resulted in decreased peak intensity. Similar behaviors was also observed in our previous work when sulfur is blended with high density polyethylene40.

Thermal stability of the PS-S and PS-SD nanocomposites was examined through thermogravimetric analyses (TGA). In Fig. S8, pure sulfur shows one step decomposition initiated around 200 °C and completely decomposes around 300 °C. However, nanocomposites showed two-step degradation, first step degradation started at around 205 °C was estimated to the decomposition of sulfur and is comparable to the amount of initial sulfur feed and the second decomposition step was by the polystyrene matrix. The amount of sulfur loading is comparable with the initial degradation of sulfur suggest complete incorporation of sulfur during extrusion processing.

In Fig. 3, DSC thermogram of pure sulfur shows three endothermic transitions at 106, 124, and at 184 °C, which are assigned for the solid–solid transition of sulfur from orthorhombic to monoclinic form, melting of monoclinic sulfur and for the polymerization of sulfur, respectively. However, these melt transition peaks are completely absent in both PS-S and PS-SD composites suggests that the crystalline sulfur changed to the amorphous form41. These results further supports the formation of amorphous sulfur and the absence of crystalline peaks in XRD.

SEM and TEM images were used to study the surface morphology in the PS composites. Figure 4a shows the SEM image of elemental sulfur particles in random size, whereas neat PS in Fig. 4b display the uniform smooth surface, similar smooth surface was also observed in PS-S and PS-SD composites (se Fig. 4c,d) indicate high level of dispersion and compatibility of sulfur with PS matrix. EDX and elemental mapping of the composites confirms the presence of sulfur, however the particles are not observed in the SEM image due to their smaller size. Uniform distribution of sulfur and carbon elemental images further supports the formation of homogeneous composites. Sulfur dispersion and their particle size was studied by TEM using thin films prepared by ultramicrotome (films thickness around 100 nm). In Fig. 4e, PS shows plane surface, whereas, the samples PS-S-5 and PS-SD-10 displayed the uniformly dispersed dark particles on the smooth surface without any agglomeration (see Fig. 4f,g), these particles are completely absent in the pure polystyrene. Enlarged image of PS-S-10 and PS-SD-10 in Fig. 4h,i respectively, suggest the dark particles are around 5 nm in size and are arises due to the higher electron density of sulfur. EDX analyses further confirms the presence of sulfur in the dark spots. High transparence of the composites can be related to the nano size and non-agglomerated sulfur and SD particles.

The mechanical properties of the PS and their composites were studied by DMTA and UTM. Figure 5a–c shows the loss tangent (tan δ), storage modulus (G′) and loss modulus (G″) versus temperature graphs of PS, PS-S-5, PS-S-10, PS-SD-5 and PS-SD-10, respectively. Single glass transition temperature (Tg) was observed in tanδ curve for PS, PS-S and PS-SD hybrid samples, shown in Fig. 5a–c and reported in Table 1. All the tanδ curves show uniformity with homogeneous nature of the hybrid materials. The tan δ curves of the PS, PS-S-5, PS-S-10, PS-SD-5 and PS-SD-10 hybrid materials show the glass transition temperature and tanδ max at 109.7 °C, 1.51; 95.4 °C, 2.01; 93.9 °C, 2.16; 99.3 °C, 1.84; and 102.2 °C, 1.76 , respectively. PS has a Tg of 109.7 °C, which was shifted to 95.4 °C and 93.9 °C after incorporation of sulfur (5% and 10%) into polymer matrix, suggest a unhindered segmental motion of PS chains. This could be due to the phase change of elemental sulfur, which act as a plasticizer in the hybrid materials. A comparison of glass transition temperature between PS-S and P-SD hybrid materials show that hybrid materials prepared from sulfur copolymer cross-linker (SD) were harder than the sulfur (S) based hybrid materials. This could be due to the strong cross-linking between the sulfur chains and formation of interpenetrating (IPN) network in PS matrix, which increased the cross-linked density in PS-SD-5 and PS-SD-10 hybrid materials. The observed Tg and storage modulus values for the series PS-SD-5 and PS-SD-10 show that, the Tg and crosslinking density increase with increasing SD content in the hybrid materials. Figure 5b,c shows the G′ and G′′ plots of PS, PS-S-5, PS-S-10, PS-SD-5 and PS-SD-10 hybrid materials. The storage modulus measures the stiffness and crosslinking density of the hybrid materials and loss modulus measures the glass transition temperature, reported in Table 1. Decrease in storage modulus was observed for the PS-S-5 and PS-S-10 hybrid materials compare to pure PS polymer and increase in storage modulus was observed for PS-SD-5 and PS-SD-10 compare to PS-S-5 and PS-S-10 hybrid materials at 40 °C and Tg + 5 °C region. However, storage modulus and crosslinking density both were decreased after incorporating sulfur and sulfur copolymer cross linker. The crosslink density (νe) of PS, PS-S-5, PS-S-10, PS-SD-5 and PS-SD-10 hybrid materials was calculated by using Eq. (1).

$$upsilon {text{e }} = {text{E}}^{prime}left( {{text{G}}}^{prime} right)/{text{3RT}}$$

(1)

where R is the universal gas constant, and T the temperature in K. G′ values in the rubbery region at T > Tg were taken to calculate υe by using Eq. (1).

The G′ values of PS-SD-5 and PS-SD-10 hybrid materials at temperatures 40 °C suggest the presence of good cross-linking and hardness. The glass transition temperature of the hybrid materials from tan δ curves and loss modulus (G′) curves shows very similar data, this could be due to the homogeneous arrangement of the molecular structure in the hybrid materials.

Effect of elemental sulfur and SD copolymer nanoparticles on tensile strength, elastic modulus and elongation at break of the PS-S and PS-SD hybrid materials were investigated. The stress vs strain curve and data are reported in Fig. 5d and Table 2. The UTS, Young’s Modulus and the elongation at break determined from Fig. 5d were presented in Fig. 6 show that UTS and Modulus decrease with increasing the elemental sulfur content. The ultimate tensile strength (UTS) of the PS-S and PS-SD hybrid materials slightly decreased for sulfur and SD loaded samples, i.e., 66.46, 62.03, 58.19, 50.27 and 54.31 MPa for PS, PS-S-5, PS-S-10, PS-SD-5 and PS-SD-10 hybrid samples respectively. However, with increasing SD concentration to 10 wt% resulted in increased UTS to 54.31 and modulus to 2283 MPa. This could be due to the formation of interpenetrating network (IPN) in the hybrid material. Interesting results were observed in elongation at break (%), where all the data close to same with slightly increase in elongation after addition of sulfur in PS polymer matrix. However, the mechanical properties of these PS-sulfur composites are much more superior to the inverse vulcanized copolymers (see Table 2)42. Inverse vulcanized materials are highly brittle to handle, whereas, in this case the PS-S and PS-SD composites exhibited mechanical strength similar to neat PS. The high mechanical strength and one step preparation method can be advantage to prepare in bulk by scale up process and can also be explored in wider application areas.

Optical transparency of the PS composites was investigated using UV–Vis-near-IR transmition spectroscopy over a broad optical window (200–3,000 nm) to determine the effect of sulfur and samples thickness on optical properties. Samples with thickness around 15 μm and 2 mm were prepared by drop casting and injection molding method respectively. The thin films of PS, PS-S-5 and PS-S-10 appeared transparent, whereas, PS-SD-5 and PS-S-10 are slightly yellow in color. However, the molded samples of PS-S composites appear faint yellow and the inverse vulcanized PS-SD samples exhibits deep orange color (see insight image in Fig. 7a,b respectively). The thin films PS-S-5 and PS-SD-5 with 5 wt% sulfur loading are 85% transparent near IR region similar to pure polystyrene, however, increasing sulfur content to 10 wt% reduced the transparency to around 70%. Whereas, the optical transparency for thicker molded samples (2 mm disc) diminished to 60% in comparison to thin films (see Fig. 7a,b). This could be due to the increase in sample thickness as well as the absorbance of C-H vibrations from polystyrene and DIB co-monomer in the near-IR 1,500–3,000 nm range. Expended zone of the UV–Vis spectra between 200–800 nm for both thin films and molded samples are presented in Fig. 7c,d respectively. The spectra clearly indicate that the optical transparency of the PS-SD-10 thin films is higher than the PS-S-10. It has been observed that the larger nanoparticles aggregates in a polymer matrix steeply increase the intensity of scattered light, which would induce a dramatic decrease in the transparency44. Sulfur shows partial solubility in toluene, thus, the thin films prepared in toluene at lower sulfur concentration (5 wt%) showed higher transparency, however, when the loading increased the undissolved sulfur particles partially aggregates which resulted in reduced transparency. Whereas, in case of PS-SD samples the SD copolymers are completely soluble in toluene and hence form homogeneous solution of SD particles, thus displays higher transparency. In molded samples all samples displayed similar transparency around 60%. It is worth to note that the 15 µm‐thin films exhibit cutoff wavelengths (λcutoff) of 270–310 nm and the molded samples exhibited low transparency below 600 nm in the visible spectrum, as readily evidenced by the yellow to orange color of the composites material. However, the retention of high transparency in the visible (above 600 nm) was observed.

Sulfur containing polymer displayed HIR due to the high atomic refraction property of sulfur atom. Thus, the amount of sulfur and their degree of molecular packing in the polymers play an important role in controlling RI properties1. Hence, in recent studies, highly cross-linked sulfur rich copolymers prepared via inverse vulcanization using elemental sulfur displayed High Refractive Index (n > 1.7)29. In these sulfur rich copolymers the S–S bonds are largely IR inactive due to low organic content make them to use in IR imaging in near and mid-infrared region also and thus imparted high n to the sulfur copolymers and demonstrated the ability to use in IR imaging. The properties of these sulfur copolymers can be easily tunable by varying the cross-linkers amount.

Similarly, in current study the cross-linked sulfur nanoparticles with S–S bonds were generated inside the polystyrene matrix during extrusion process, these polymer composites can be easily molded in to different shape by molding or hot-press method. To measure RI, the composites were molded in to 2 mm thick circular molds and the variation in RI with respect to the wavelength are measured and are presented in Fig. 8. Pristine PS shows RI n = 1.601 at 402.9 nm, whereas, addition of elemental sulfur in the matrix increased the RI n = 1.656, and RI increased with increase in the sulfur loading confirms the role of S–S bonds in improving RI. Cross-linking these sulfur with DIB further improved the RI to 1.652 to 1.673 according to the SD copolymers loading (5 and 10 wt%) respectively. The higher RI of PS-SD compared to the PS-S are due to the higher dipolar arising from the cross-linked structure and the presence of aromatic DIB in PS matrix.

Abbe’s number (νD), a key parameter to measure refractive index dispersion, is also an important for optical materials used in the visible region. νD can be calculated from bellow Eq. (2)

$$nu_{{{text{D}} }} = n_{D} – { 1}/n_{F} – n_{C}$$

(2)

where: nD, nF, and nC are the refractive indices of the material at the wavelengths of the sodium D (589.3 nm), hydrogen F (486.1 nm), and hydrogen C (656.3 nm) lines, respectively.

The Abbe’s numbers calculated from Eq. (2) for pristine PS, PS-S and PS-SD at 486, 589, and 656 nm wavelength using RI in Fig. 8 are calculated from Table S1. The obtained high Abbe’s number for the PS-S and PS-SD molds are in the range ~ 30 ± 1, indicate the high level dispersion of sulfur and SD copolymer in PS matrix, this also supports the increased RI in the composites.