Polystyrene–mesoporous diatomite composites produced by in situ activators regenerated by electron transfer atom transfer radical polymerization

Abstract

Mesoporous diatomite platelets were employed to synthesize different polystyrene/diatomite composites. Diatomite platelets were used for in situ polymerization of styrene by activators regenerated by electron transfer for atom transfer radical polymerization to synthesize tailor-made polystyrene nanocomposites. Fourier transform infrared (FTIR) spectroscopy and thermogravimetric analysis (TGA) were employed for evaluating some inherent properties of pristine diatomite platelets. Nitrogen adsorption/desorption isotherms are applied to examine surface area and structural characteristics of the diatomite platelets. Evaluation of pore size distribution and morphological studies were also performed by scanning and transmission electron microscopy. Conversion and molecular weight determinations were carried out using gas and size exclusion chromatography respectively. Addition of 3 wt% pristine mesoporous diatomite leads to increase of conversion from 85 to 94%. The molecular weight of polystyrene chains increases from 13 498 to 14 943 g mol⁻¹ by addition of 3 wt% pristine mesoporous diatomite; however, polydispersity index values increase from 1.13 to 1.37. Appropriate agreement between theoretical and experimental molecular weight in combination with low PDI values can appropriately demonstrate the living nature of the polymerization. Increasing thermal stability of the nanocomposites is demonstrated by TGA. Differential scanning calorimetry shows an increase in glass transition temperature from 92.6 to 97.8 °C by adding 3 wt% of mesoporous diatomite platelets.

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Introduction

Organic–inorganic hybrids are of great current interest since it has been found that low-volume addition of nano-objects in some cases results in significantly enhanced stiffness and strength of the polymer matrices in comparison with conventionally filled composites at the same loading.^1–3 Polymer nanocomposites are two-phase materials in which an organic polymer phase is reinforced by inorganic nanofiller(s). On the other hand, nanocomposites originate from appropriate dispersion of an ultrafine inorganic phase within a continuous organic phase.^4,5 Due to the high aspect ratio, appropriate reinforcing efficiency for nanofillers can be observed.⁶ A suitable route to distinguish between various nanocomposites is based on the number of nano dimensions of the dispersed nanofillers. In this regards, three main types of nanocomposites can be recognized: (i) nanofillers with three dimensions in the nanometer range such as silica nanoparticles. (ii) Nanofillers with two dimensions in the nanometer range such as carbon nanotubes. (iii) Nanofillers with one dimension in the nanometer range such as clay platelets.^7,8 Solution, melt intercalation, and in situ polymerization are three common methods for the preparation of nanocomposites in which in situ polymerization can be considered as one the most important and useful pathway. In this method monomer(s) is directly used in the presence of nanofillers and then polymerization can initiate via appropriate procedure (e.g. heat or radiation).^9,10

Diatomaceous earth or kieselgur is a fossil assemblage of siliceous diatom frustules. Diatoms are single-cell photosynthetic algae with an extraordinary 3D porous structure and micro to nanoscale dimensions of their frustules.^11,12 Frustules are mainly composed of amorphous hydrated silica (SiO₂·nH₂O) and therefore the main component of diatomite is also amorphous silica.¹³ According to the mineralogical classification, diatomite is categorized as non-crystalline opal-A material.¹⁴ Diatomaceous earth mineral is easily available at a low cost in large quantities since it is the most abundant form of silica on the earth.^11,12 Diatomite has many unique physical and chemical characteristics which makes it as a promising material for many applications and research fields. Some of these features are as follow: highly developed mesoporosity and/or macroporosity, high porosity, light weight, high permeability, low density, large surface area, low thermal conductivity and chemical inertness.^15–17 Diatomite has been widely used as filters, in the manufacture of explosives, as a catalyst support, in heat and sound insulation and as a kind of modifier for asphalt mixtures.^18–21

Control over the molecular weight, polydispersity index (PDI), composition, architecture, and functionality of polymers have been long-standing goal for polymer chemists.^22,23 Living polymerization such as anionic and cationic polymerization, ring opening metathesis, and group transfer polymerization are powerful pathways to synthesize tailor-made polymers with desired compositions and architectures.^24,25 These methods however, face severe limitations in which can restrict their applications (e.g. the absence of water or air, and highly sophisticated catalysts).^25,26 Despite these methods, free radical polymerization (FRP) presents more tolerant to moisture and other impurities and therefore can facilitate polymerization process. In addition, application wide variety of monomers, large reaction temperature, and various polymerization systems and media are other benefits of FRP. However, FRP also suffers other deficiencies such as disability to produce pure block copolymers and control over the molecular weight and PDI.^27,28 To overcome the limitations of FRP and living polymerization methods and combining the advantages of both of them, controlled/living radical polymerization (CRP) systems have been developed.^22,23 Three main procedures of CRP are namely nitroxide-mediated polymerization (NMP),²⁹ atom transfer radical polymerization (ATRP),³⁰ and reversible addition fragmentation chain transfer (RAFT).³¹ Commercial availability of all necessary reagents, mild polymerization conditions, great industrialization prospects in comparison with other living polymerization processes, ability to polymerization of a wide variety of monomers, performance in homo and heterogeneous polymerization media, and ability to adjust for a given system by modifying the complexing ligand are some unique characteristics of ATRP systems.^28,32,33

A review of literatures indicates that there is some research on the application of diatomite as filler to synthesize polymer/diatomite composites. Karaman et al. have prepared polyethylene glycol (PEG)/diatomite composite as a novel form-stable composite phase change material (PCM) in which the PCM was prepared by incorporating PEG in the pores of diatomite.³⁴ Li et al. have synthesized conducting diatomite by polyaniline on the surface of diatomite. Linkage of polyaniline on the surface of diatomite is attributed to the hydrogen bond between the surface of diatomite and polyaniline macromolecules.³⁵ Li et al. have also prepared fibrillar polyaniline/diatomite composite by one-step in situ polymerization. According to their results, the polyaniline/diatomite composite can be applied as fillers for electromagnetic shielding materials and conductive coatings.³⁶ In addition, other studies such as investigating the effects of extrusion conditions on die-swell behavior of polypropylene/diatomite composite melts and crystallization behaviors and foaming properties of diatomite-filled polypropylene composites have been performed.^37,38

In this study, abundant advantages of ATRP were employed to synthesize well-defined polymeric nanocomposites. Well-defined polymer matrices were synthesized by a newly developed initiation technique, namely activators regenerated by electron transfer for atom transfer radical polymerization (ARGET ATRP). Effect of the pristine diatomite platelets on the ARGET ATRP and thermal properties of the produced nanocomposites are discussed in detail.

Experimental

Materials

Diatomite earth sample was obtained from Kamel Abad-Azerbaijan, Iran. It was dispersed in 100 mL distilled water by magnetic stirring and then it was kept constant until some solid impurities were dispersed. The particles were separated with filter paper and dried at 100 °C for 8 h. Styrene (St, Aldrich, 99%) was passed through an alumina-filled column, dried over calcium hydride, and distilled under reduced pressure (60 °C, 40 mm Hg). Copper(II) bromide (CuBr₂, Fluka, 99%), N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDETA, Aldrich, 99%), ethyl alpha-bromoisobutyrate (EBiB, Aldrich, 97%), tin(II) 2-ethylhexanoate (Sn(EH)₂, Sigma, 95%), anisole (Aldrich, 99%), tetrahydrofuran (THF, Merck, 99%), neutral aluminum oxide (Aldrich, 99%) were used as received.

Synthesis of polystyrene via ARGET ATRP

ARGET ATRP of styrene was performed in a 150 mL lab reactor which was placed in an oil bath thermostated at desired temperature. Typical batch of polymerization was run at 110 °C with the molar ratio of 150 : 1 : 0.1 : 0.15 : 0.1 for [styrene] : [EBiB] : [CuBr₂] : [PMDETA] : [Sn(EH)₂], giving a theoretical molecular weight of 15 663 g mol⁻¹ at 100% conversion. At first, styrene (20 mL), anisole, CuBr₂ (0.02 g, 0.11 mmol), PMDETA (0.03 mL, 0.17 mmol) were transferred into the reactor. The resultant solution was degassed and back-filled with nitrogen three times, and then left under N₂ with stirring for 30 minutes at room temperature. Then a predeoxygenated solution of Sn(EH)₂ (0.03 mL, 0.11 mmol) in anisole was injected into the reactor. When the majority of the metal complex has formed, EBiB (0.17 mL, 1.16 mmol) was also added to initiate the polymerization. Subsequently, the reactor temperature was increased to 110 °C. At the beginning of the reaction, polymerization media was light green and it was gradually changed to light brown which is an appropriate evidence for ATRP equilibrium establishment.

Preparation of polystyrene/diatomite platelets composites via in situ ARGET ATRP

For preparation of polystyrene/diatomite platelets composites, a desired amount of pristine diatomite platelets were dispersed in 10 mL of styrene and the mixture was stirred for 22 hours. Then, the remaining 10 mL of styrene was added to the mixture. Subsequently, polymerization procedure was applied accordingly. Designation of the samples with the percentage of pristine diatomite platelets is given in Table 1. In this designation, PS–ARG refers to neat polystyrene and PS–ARG–DT “X” implies different nanocomposites of polystyrene with various percentages of pristine diatomite platelets loading (PS: polystyrene, ARG: ARGET ATRP, and DT: diatomite platelets).

Table 1 Designation of the samples

Sample	Method of preparation	Proportion of pristine diatomite platelets (wt%)	Dispersion time prior to the polymerization (h)
PS–ARG	ARGET ATRP	0	—
PS–ARG–DT 1	In situ ARGET ATRP	1	22
PS–ARG–DT 2	In situ ARGET ATRP	2	22
PS–ARG–DT 3	In situ ARGET ATRP	3	22

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Separation of polystyrene chains from diatomite platelets and catalyst removal

For separating of polymer chains from pristine diatomite platelets, nanocomposites were dissolved in THF. By high-speed ultracentrifugation (10 000 rpm) and then passing the solution through a 0.2 micrometer filter, polymer chains were separated from diatomite platelets. Subsequently, polymer solutions passed through an alumina column to remove catalyst species.

Characterization

FTIR spectrum of the pristine diatomite was achieved using FTIR spectroscopy on a Bruker FTIR spectrophotometer, within a range of 400–4400 cm⁻¹. Materials porosity was characterized by N₂ adsorption/desorption curves obtained with a Quntasurb QS18 (Quntachrom) apparatus. The surface area and pore size distribution values were obtained with the corrected BET equation (Brunauer–Emmett–Teller). In addition, specific surface area measurements were also performed with NS-93 apparatus (Towse-e-Hesgarsazan Asia, Iran). Surface morphology of the pristine diatomite was examined by SEM (Philips XL30) with acceleration voltage of 20 kV. The transmission electron microscope, Philips EM 208 (The Netherlands), with an accelerating voltage of 120 kV was employed to study the morphology of the pristine diatomite sample. The specimens were prepared by coating a thin layer on a mica surface using a spin coater. Gas chromatography (GC) is a simple and highly sensitive characterization method and does not require removal of the metal catalyst particles. GC was performed on an Agilent-6890N with a split/splitless injector and flame ionization detector, using a 60 m HP-INNOWAX capillary column for the separation. The GC temperature profile included an initial steady heating at 60 °C for 10 min and a 10 °C min⁻¹ ramp from 60 to 160 °C. The samples were also diluted with acetone. The ratio of monomer to anisole was measured by GC to calculate monomer conversion throughout the reaction. Size exclusion chromatography (SEC) was used to measure the molecular weight and molecular weight distribution. A Waters 2000 ALLIANCE with a set of three columns of pore sizes of 10 000, 1000, and 500 Å was utilized to determine polymer average molecular weight and polydispersity index (PDI). THF was used as the eluent at a flow rate of 1.0 mL min⁻¹, and calibration was carried out using low polydispersity polystyrene standards. Proton nuclear magnetic resonance spectroscopy (¹H NMR) spectra were recorded on a Bruker 300 MHz ¹H NMR instrument with CDCl₃ as the solvent and tetramethylsilane as the internal standard. Thermal gravimetric analysis (TGA) was carried out with a PL thermogravimetric analyzer (Polymer Laboratories, TGA 1000, UK). Thermograms were obtained from ambient temperature to 700 °C at a heating rate of 10 °C min⁻¹. Thermal analysis were carried out using a differential scanning calorimetry (DSC) instrument (NETZSCH DSC 200 F3, Netzsch Co, Selb/Bavaria, Germany). Nitrogen at a rate of 50 mL min⁻¹ was used as purging gas. Aluminum pans containing 2–3 mg of the samples were sealed using DSC sample press. The samples were heated from ambient temperature to 220 °C at a heating rate of 10 °C min⁻¹.

Results and discussion

Since inherent characteristics of the nano-filler can influence on the kinetics of in situ polymerization and consequently nanocomposite properties, inherent features of the pristine diatomite platelets are evaluated.

Fig. 1 presents FTIR spectrum of the pristine diatomite platelets. The peaks at 3434 cm⁻¹ and 1634 cm⁻¹ correspond to the stretching vibrations of physically adsorbed water and zeolitic water, respectively.³⁹ Although diatomite platelets are rehydrated, during the preparation process and obtaining the spectrum some water molecules may be re-adsorbed.⁴⁰ The strong peak at 1098 cm⁻¹ is attributed to the stretching mode of siloxane (Si–O–Si). In addition, the peak at 471 cm⁻¹ is associated with the asymmetric stretching mode of siloxane bonds. The peak at 796 cm⁻¹ is also attributed to the vibration of O–H.^39,41,42

Fig. 1 FTIR spectrum of the pristine diatomite nanoparticles.

Fig. 2 represents nitrogen adsorption/desorption isotherms of the pristine diatomite platelets. The shape of isotherm is similar to the IV type isotherms according to the IUPAC classification and confirms that diatomite has mesoporous structure.^43,44 The hysteresis is associated with the filling and emptying of the mesopores by capillary condensation.⁴⁵ A sharp increase in the nitrogen adsorbed quantity near the relative pressure of 1 demonstrates the existence of macropores in the pure diatomite and therefore non-uniform pore size distribution can be concluded.⁴⁶

Fig. 2 Nitrogen adsorption/desorption isotherm of the pristine diatomite platelets.

Extracted data from the nitrogen adsorption/desorption isotherm is summarized in Table 2.

Table 2 Extracted data from nitrogen adsorption/desorption isotherm of the pristine diatomite platelets

BET surface area	Langmuir surface area	Average pore diameter (4 V A^−1 by BET)	BJH plot dp,peak (area)
			Adsorption branch	Desorption branch
11.92 m^2 g^−1	19.82 m^2 g^−1	36.91 nm	4.76 nm	3.71 nm

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Fig. 3 illustrates TGA result of the pristine diatomite platelets. As it can be seen two main mass losses are resolved in the TGA curve. The first mass loss is mainly ascribed to the dehydration of diatomite (the weight loss below 160 °C is due to the removal of physisorbed water whilst the weight loss at 160–300 °C can be attributed to the expulsion of chemisorbed water molecules). The second weight loss in the temperature range of 400–700 °C is a gradual and slow loss and may be associated with the dehydroxylation of the silanols of diatomite. Pristine diatomite platelets leave 92.31% char at 700 °C.

Fig. 3 TGA graph of the pristine diatomite platelets.

Fig. 4 displays SEM images of the pristine diatomite platelets in two different magnifications. According to these images, diatomite platelets are composed of plaque plate particles with spherical-shaped pores. These plates have regular pores and sometimes are aggregated. This structure of the diatomite is suitable for the synthesis of various composites and makes it as an appropriate catalyst support.

Fig. 4 SEM images of the pristine diatomite platelets in two different magnifications.

TEM images are employed to evaluate the morphology and pore condition of the pristine diatomite platelets. TEM image of the pristine diatomite platelets is shown in Fig. 5. As it can be seen, pristine diatomite belongs numerous regularly spaced rows of pores in its structure that this observation is confirmed with SEM images. In addition, average pore diameter from TEM images is estimated between 35 and 40 nm that these results are in coincidence with the extracted data from the nitrogen adsorption/desorption isotherm.

Fig. 5 TEM image of the pristine diatomite platelets.

Although ATRP can provide a robust route to synthesize a wide variety of desired materials, existence of relatively high concentration of metal catalyst in the products is a critical point in which must be considered. Among different pathway to remove of metal catalyst from the products, decrement of its concentration during polymerization is more preferred. Activators regenerated by electron transfer (ARGET) for ATRP is an appropriate procedure to reduce concentration of transition metal complex in the final products.⁴⁷ In addition, usage of ARGET ATRP can facilitate synthesis of high molecular weight polymers. General mechanism of ARGET ATRP is shown in Fig. 6.

Fig. 6 General mechanism for ARGET ATRP.

Food and Drug Administration (FDA) approved tin(II) 2-ethylhexanoate [Sn(EH)₂], ascorbic acid, and glucose are some examples for environmentally accepted reducing agents which are employed in ARGET ATRP. Sn(EH)₂ reductant can reduce deactivators (Mtⁿ⁺¹) species that accumulate when radicals irreversibly terminate to reproduce original Mtⁿ species which are needed for general ATRP.^48,49 In overall, ARGET ATRP can open a suitable practical synthetic pathway to prepare well-defined polymers with various compositions and architectures. Fig. 7 illustrates general procedure for synthesis of tailor-made polystyrene chains via ARGET ATRP in the presence of pristine diatomite platelets.

Fig. 7 General procedure for preparation of polystyrene/diatomite platelets composites via in situ ARGET ATRP.

Number and weight average molecular weights, polydispersity index, and effect of the pristine diatomite platelets on these parameters are evaluated by SEC analysis. SEC traces of the neat polystyrene and extracted polystyrene chains from different composites are shown in Fig. 8. According to this figure, SEC traces of the neat polystyrene and all of the composites display monomodal peaks. Pure polystyrene reveals narrow distribution and low PDI value which can demonstrate successful ARGET ATRP is established.

Fig. 8 SEC traces of the neat polystyrene and its various composites.

According to the results, ARGET ATRP of styrene without diatomite platelets results in well-defined polystyrene with low PDI value. In addition, an increase in final conversion and molecular weight were occurred by adding pristine diatomite platelets. By addition of only 3 wt% of the pristine diatomite platelets, final conversion increases from 85 to 94%. Positive effect of diatomite platelets on the molecular weight of the samples can be interpreted by abundant pendant hydroxyl groups of the pristine diatomite. It is demonstrated that polar solvents (especially hydroxyl containing ones like water, phenol, and carboxylic acids) exert a rate acceleration effect on the polymerization systems for increasing radical activation rate and also reducing radical recombination rate. Numerous pendant hydroxyl groups on the surface of the pristine diatomite can possibly cause a polarity change into the reaction medium. In addition, negatively charged surface (pendant hydroxyl groups) could absorb and gather positively charged catalyst (Cu ions) and consequently enhances the chain growth rate. The accelerating effect of other nano-fillers such as nanoclay and MCM-41 nanoparticles on polymerization rate was also reported elsewhere.^50–52 Moreover, PDI values of the polystyrene chains increases by the addition of pristine diatomite platelets. Pristine diatomite platelets acts as an impurity in the polymerization medium and therefore causes the molecular weight distribution of the resultant polymers to be increased; PDI increases from 1.13 to 1.37 by 3 wt% loading of diatomite platelets.^53,54 Table 3 summarized the extracted data from SEC traces of the neat polystyrene and its different nanocomposites.

Table 3 Molecular weights and PDI values of the extracted polystyrenes resulted from SEC traces

Sample	Reaction time (h)	Conversion (%)	Mn (g mol^−1)		Mw (g mol^−1)	PDI
			Exp.	Theo.
PS–ARG	25	85	13 498	13 314	15 253	1.13
PS–ARG–DT 1	25	88	13 965	13 783	16 898	1.21
PS–ARG–DT 2	25	92	14 115	14 410	18 067	1.28
PS–ARG–DT 3	25	94	14 943	14 723	20 472	1.37

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Theoretical molecular weight is calculated by using eqn (1):

where, [St]₀ and [EBiB]₀ are initial concentration of the monomer and initiator, p is final conversion, and M_monomer represents molecular weight of styrene which is equals to 104.15 g mol⁻¹.

An appropriate agreement between the theoretical and experimental molecular weights in combination with low PDI values (PDI < 1.4) can be considered as an appropriate evidences for controlled nature of the polymerization. Also, color change of the reaction media during polymerization is an evidence of successful ARGET ATRP equilibrium establishment.

¹H NMR spectra of the neat polystyrene and its different composites reveal a signal at 4.15 ppm, which originates from the hydrogen atom at the end of polystyrene chains (–CHBr), demonstrates the living feature of the polymerization. Moreover, characteristic peaks of polystyrene chains like benzene rings, methylene (–CH₂–), and tertiary carbon hydrogen [–CH(C₆H₅)–] appears in the spectra in the range of around 7.40–6.25, 1.71–1.15, and 2.31–1.71 ppm, respectively.⁵⁵

Fig. 9 represents TGA thermograms of weight loss as a function of temperature in the temperature window of 30–700 °C in addition to their corresponding differential thermogravimetric (DTG) for the neat polystyrene and its different composites.

Fig. 9 (a) TGA and (b) DTG thermograms of the neat polystyrene and its different composites.

According to the Fig. 9, thermal stability of the neat polystyrene is lower than all of the nanocomposites. In addition, thermal stability of the neat polystyrene is improved by adding pristine diatomite platelets which by increasing diatomite content, an increase in degradation temperatures was observed. In the high loading of diatomite platelets, a considerable improvement will be obtained. In general, by increasing temperature in the TGA graphs three separated steps can be identified: (i) evaporation of the water molecules results in the weight loss between 100 and 150 °C. (ii) Degradation of volatile materials such as residual monomer and low molecular weight oligomers (at the temperature window around 180–350 °C). (iii) Degradation of the synthesized polymer and nanocomposites (the main degradation step) is also presented at the temperatures above 380 °C. Extracted data from TGA are graphically illustrated in Fig. 10. Degradation temperature of the samples versus amount of degradation is employed to show that addition of diatomite platelets in the polystyrene matrix, results in an improvement of thermal stabilities of the composites (T_X: temperature threshold at which X% of neat polystyrene and its composites is degraded). Char values at 650 °C are also reported in Table 4.

Fig. 10 Graphical illustration of temperature and degradation relationship.

Table 4 Extracted data from TGA and DTG thermograms for the neat polystyrene and its composites

Sample	TGA	DTG (°C)
	Char (%) at 650 °C	Start point	Peak point	End point
PS–ARG	2.04	322	417	461
PS–ARG–DT 1	3.79	327	421	472
PS–ARG–DT 2	5.29	338	428	473
PS–ARG–DT 3	6.82	341	431	475

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Increasing of degradation temperature of the composites by adding pristine diatomite platelets content is attributed to the high thermal stability of diatomite platelets and also interaction between diatomite platelets and polymer matrix.⁵⁶ Physical interaction between polystyrene chains and surface of the diatomite platelets is an important factor for increasing thermal stability of the nanocomposites.⁵⁷ Additionally, hindrance effect of diatomite platelets on the polystyrene chains movement and restriction of oxygen permeation by these sheets are the other reasons for higher thermal stability of the composites. Similar conclusions are also reported in the case of polymer/clay nanocomposites.^52,58 Table 4 summarized the extracted data from TGA thermograms and DTG curves. Pristine diatomite platelets leave 92.31% char after complete degradation at 700 °C.

Determination of glass transition temperature (T_g) and also evaluation the effect of diatomite platelets on the chain confinement of the samples are performed by DSC. Fig. 11 shows DSC thermograms of the neat polystyrene and its different composites. Temperature range of 25–175 °C is used to describe DSC results in heating path. Since diatomite platelets do not bear any transitions in this range of temperature, therefore only thermal transition of polymers is observed. In these experiments, samples are heated from room temperature to 220 °C to remove their thermal history. Then, they cooled to room temperature to distinguish the phase conversion and other irreversible thermal behaviors. Finally, samples are heated from room temperature to 220 °C to obtain T_g values.

Fig. 11 DSC thermograms of the neat polystyrene and its different composites (heating path).

According to the Fig. 11, an obvious inflection in the heating path is occurred in which shows glass transition temperature of the samples. Corresponding inflection in the cooling path is also appeared. Since there is not another peak in the cooling path which indicates that the structure of synthesized polystyrene and its composites are mainly amorphous and they have not gone through crystallization phenomenon. Table 5 summarized the extracted T_g values of the samples.

Table 5 Extracted T_g of the neat polystyrene and its various composites

Sample	Mn (g mol^−1)	PDI	Tg (°C)
PS–ARG	13 498	1.13	92.6
PS–ARG–DT 1	13 965	1.21	94.2
PS–ARG–DT 2	14 115	1.28	96.7
PS–ARG–DT 3	14 943	1.37	97.8

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According to the Table 5, T_g value of the neat polystyrene is lower than all of the composites and an increase in T_g values is occurred by increasing of diatomite platelets content. Increasing T_g values by adding pristine diatomite platelets content in the polymer matrix can be attributed to the confinement effect of the diatomite platelets. The rigid two-dimensional diatomite platelets can restrict the steric mobility of the polystyrene chains and causes the inflection in the DSC graphs starts at higher temperatures. Similar conclusions are also reported in the case of polymer/clay nanocomposites.^52,58

Conclusions

In situ ARGET ATRP of styrene was employed to synthesize well-defined polystyrene and its nanocomposites in the presence of pristine mesoporous diatomite platelets. Mesoporous structure, existence of plaque plate particles with spherical-shaped pores, silica as the main constituent, and existence of numerous regularly spaced rows in its structure are some inherent features of the pristine diatomite platelets. In situ ARGET ATRP of styrene in the presence of pristine mesoporous diatomite leads to increment of conversion from 85 to 94%. Moreover, molecular weight and PDI values increase from 13 498 to 14 943 g mol⁻¹ and from 1.13 to 1.37 respectively. Improvement in thermal stability of the nanocomposites and increasing T_g values from 92.6 to 97.8 °C was also observed by incorporation of 3 wt% of pristine mesoporous diatomite platelets.

References

1. M. S. Ozmusul, C. R. Picu, S. S. Sternstein and S. K. Kumar, Macromolecules, 2005, 38, 4495–4500.
2. C. Gao and G. Chen, Compos. Sci. Technol., 2016, 124, 52–70.
3. Z. Hu and G. Chen, J. Mater. Chem. A, 2014, 2, 13593–13601.
4. M. Zanetti, S. Lomakin and G. Camino, Macromol. Mater. Eng., 2000, 279, 1–9.
5. Z. Hu and G. Chen, Adv. Mater., 2014, 26, 5950–5956.
6. F. Hussain, M. Hojjati, M. Okamoto and R. E. Gorga, J. Compos. Mater., 2006, 40, 1511–1575.
7. K. Xu, G. Chen and D. Qiu, J. Mater. Chem. A, 2013, 1, 12395–12399.
8. D. W. Schaefer and R. S. Justice, Macromolecules, 2007, 40, 8501–8517.
9. S. Varghese and J. Karger-Kocsis, Polymer, 2003, 44, 4921–4927.
10. K. Khezri, H. Roghani-Mamaqani, M. Sarsabili, M. Sobani and S. Mirshafiei-Langari, Polym. Sci., Ser. B, 2014, 56, 909–918.
11. Y. Jia, W. Han, G. Xiong and W. Yang, Sci. Technol. Adv. Mater., 2007, 8, 106–109.
12. W.-T. Tsai, C.-W. Lai and K.-J. Hsien, J. Colloid Interface Sci., 2006, 297, 749–754.
13. S. Nenadovic, M. Nenadovic, R. Kovacevic, L. Matovic, B. Matovic, Z. Jovanovic and J. Grbovic Novakovic, Sci. Sintering, 2009, 41, 309–317.
14. D. Liu, W. Yuan, P. Yuan, W. Yu, D. Tan, H. Liu and H. He, Appl. Surf. Sci., 2013, 282, 838–843.
15. S. Akin, J. M. Schembre, S. K. Bhat and A. R. Kovscek, J. Pet. Sci. Eng., 2000, 25, 149–165.
16. B. Gao, P. Jiang, F. An, S. Zhao and Z. Ge, Appl. Surf. Sci., 2005, 250, 273–279.
17. Y. S. Al-Degs, M. F. Tutunju and R. A. Shawabkeh, Sep. Sci. Technol., 2000, 35, 2299–2310.
18. T. Yi-qiu, Z. Lei and Z. Xing-you, Construct. Build. Mater., 2012, 36, 787–795.
19. Y. Jia, W. Han, G. Xiong and W. Yang, J. Colloid Interface Sci., 2008, 323, 326–331.
20. N. Caliskan, A. R. Kul, S. Alkan, E. G. Sogut and I. Alacabey, J. Hazard. Mater., 2011, 193, 27–36.
21. M. Stankovic, M. Gabrovska, J. Krstic, P. Tzvetkov, M. Shopska, T. Tsacheva, P. Bankovic, R. Edreva-Kardjieva and D. Jovanovic, J. Mol. Catal. A: Chem., 2009, 297, 54–62.
22. P. B. Zetterlund, Y. Kagawa and M. Okubo, Chem. Rev., 2008, 108, 3747–3794.
23. K. Khezri and H. Roghani-Mamaqani, Mater. Res. Bull., 2014, 59, 241–248.
24. K. Matyjaszewski, J. Qiu, D. A. Shipp and S. G. Gaynor, Macromol. Symp., 2000, 155, 15–29.
25. Y. Zhen, S. Wan, Y. Liu, H. Yan, R. Shi and C. Wang, Macromol. Chem. Phys., 2005, 206, 607–612.
26. M. F. Cunningham, Prog. Polym. Sci., 2008, 33, 365–398.
27. J. Qiu, B. Charleux and K. Matyjaszewski, Prog. Polym. Sci., 2001, 26, 2083–2134.
28. W. Braunecker and K. Matyjaszewski, Prog. Polym. Sci., 2007, 32, 93–146.
29. C. J. Hawker, A. W. Bosman and E. Harth, Chem. Rev., 2001, 101, 3661–3688.
30. Y. Kagawa, P. B. Zetterlund, H. Minami and M. Okubo, Macromol. Theory Simul., 2006, 15, 608–613.
31. G. Moad, E. Rizzardo and S. H. Thang, Aust. J. Chem., 2006, 59, 669–692.
32. K. Matyjaszewski and J. Xia, Chem. Rev., 2001, 101, 2921–2990.
33. Z. Cheng, X. Zhu, L. Zhang, N. Zhou and J. Chen, J. Macromol. Sci., Part A: Pure Appl.Chem., 2003, 40, 371–385.
34. S. Karaman, A. Karaipekli, A. Sarı and A. Bicer, Sol. Energy Mater. Sol. Cells, 2011, 95, 1647–1653.
35. X. Li, C. Bian, W. Chen, J. He, Z. Wang, N. Xu and G. Xue, Appl. Surf. Sci., 2003, 207, 378–383.
36. X. Li, X. Li and G. Wang, Appl. Surf. Sci., 2005, 249, 266–270.
37. S. Hu, X. Zhu, W. Hu, L. Yan and C. Cai, Polym. Bull., 2013, 70, 517–533.
38. J. Z. Liang, Polym. Test., 2008, 27, 936–940.
39. G. Sheng, H. Dong and Y. Li, J. Environ. Radioact., 2012, 113, 108–115.
40. P. Yuan, D. Liu, D. Tan, K. Liu, H. Yu, Y. Zhong, A. Yuan, W. Yu and H. He, Microporous Mesoporous Mater., 2013, 170, 9–19.
41. Y. Yu, J. Addai-Mensah and D. Losic, Sci. Technol. Adv. Mater., 2012, 13, 015008.
42. I. Rea, N. M. Martucci, L. De Stefano, I. Ruggiero, M. Terracciano, P. Dardano, N. Migliaccio, P. Arcari, R. Taté, I. Rendina and A. Lamberti, Biochim. Biophys. Acta, 2014, 1840, 3393–3403.
43. N. Garderen, F. J. Clemens, M. Mezzomo, C. P. Bergmann and T. Graule, Appl. Clay Sci., 2011, 52, 115–121.
44. Y. Du, J. Yan, Q. Meng, J. Wang and H. Dai, Mater. Chem. Phys., 2012, 133, 907–912.
45. Z. Sun, X. Yang, G. Zhang, S. Zheng and R. L. Frost, Int. J. Miner. Process., 2013, 125, 18–26.
46. D. Liu, P. Yuan, D. Tan, H. Liu, T. Wang, M. Fan, J. Zhu and H. He, J. Colloid Interface Sci., 2012, 388, 176–184.
47. W. Jakubowski, K. Min and K. Matyjaszewski, Macromolecules, 2006, 39, 39–45.
48. H. Chen, L. Yang, Y. Liang, Z. Hao and Z. Lu, J. Polym. Sci., Part A: Polym. Chem., 2009, 47, 3202–3207.
49. N. Chan, M. F. Cunningham and R. A. Hutchinson, Macromol. Chem. Phys., 2008, 209, 1797–1805.
50. H. Roghani-Mamaqani, V. Haddadi-Asl, M. Najafi and M. Salami-Kalajahi, Polym. Compos., 2010, 31, 1829–1837.
51. A. Asfadeh, V. Haddadi-Asl, M. Salami-Kalajahi, M. Sarsabili and H. Roghani-Mamaqani, NANO, 2013, 8, 1350018.
52. H. Roghani-Mamaqani, V. Haddadi-Asl, M. Najafi and M. Salami-Kalajahi, J. Appl. Polym. Sci., 2011, 120, 1431–1438.
53. H. Roghani-Mamaqani and K. Khezri, J. Appl. Polym. Sci., 2016, 23, 190–204.
54. M. Salami-Kalajahi, V. Haddadi-Asl, S. Rahimi-Razin, F. Behboodi-Sadabad, M. Najafi and H. Roghani-Mamaqani, J. Polym. Res., 2012, 19, 9793–9804.
55. K. Khezri, V. Haddadi-Asl, H. Roghani-Mamaqani and M. Salami-Kalajahi, J. Polym. Eng., 2012, 32, 235–243.
56. M. Sarsabili, K. Kalantari and K. Khezri, J. Therm. Anal. Calorim., 2016, 126, 1261–1272.
57. M. A. Ver Meer, B. Narasimhan, B. H. Shanks and S. K. Mallapragada, ACS Appl. Mater. Interfaces, 2010, 2, 41–47.
58. H. Roghani-Mamaqani, V. Haddadi-Asl, M. Najafi and M. Salami-Kalajahi, AIChE J., 2011, 57, 1873–1881.

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