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Dispersion polymerization of styrene in supercritical carbon dioxide using monofunctional perfluoropolyether and silicone-containing fluoroacrylate stabilizers
European Polymer Journal 41 (2005) 1159–1167
EUROPEAN POLYMER JOURNAL www.elsevier.com/locate/europolj
Dispersion polymerization of styrene in supercritical carbon dioxide using monofunctional perfluoropolyether and silicone-containing fluoroacrylate stabilizers Nil Baran, Sennur Deniz, Mesut Akgu¨n, I. Nimet Uzun, Salih Dinc¸er
*
Chemical Engineering Department, Yildiz Technical University, Davutpasa Cad., No. 127, 34210, Esenler, Istanbul, Turkey Received 1 April 2004; received in revised form 19 November 2004; accepted 26 November 2004 Available online 22 January 2005
Abstract The free radical dispersion polymerization of styrene was carried out in supercritical carbon dioxide (scCO2) using two different stabilizers. The polymerizations are performed in the presence of poly(heptadecafluorodecyl acrylate-cotris(trimethylsilyloxy)silyllpropyl methacrylate) p(HDFDA-co-SiMA) and a commercially available carboxylic acid-terminated perfluoropolyether (KrytoxÒ 157FSL) as polymerization stabilizers. Dry, fine powdered spherical polystyrene particles were produced under optimised conditions. The resulting high yield of spherical and relatively uniform micronsize polystyrene particles were formed utilizing various amounts of p(HDFDA-co-SiMA) random copolymer. However, it was observed that KrytoxÒ 157FSL was not a good stabilizer as p(HDFDA-co-SiMA) for the dispersion polymerization of styrene. The particle diameter was shown to be dependent on the type of the stabilizer and the weight percent of the stabilizer added to the system. The effect of varying the concentrations of stabilizers and initiator, reaction time and reaction pressure upon the polymerization yield, molar mass and morphology of polystyrene have been investigated. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Dispersion polymerization; Supercritical CO2; Polystyrene particles; Stabilizer
1. Introduction Polymer industries have been using large amounts of volatile organic compounds to prepare polymer molecules, and to apply them as a reaction medium [1]. In order to minimize the impacts of chemical industry on human health and environment, there is significant interest in development of so-called Green Chemistry, the goal of which is to reduce or eliminate the use of hazard-
* Corresponding author. Tel.: +90 212 449 1925; fax: +90 212 449 1895. E-mail address: [email protected] (S. Dinc¸er).
ous substances in chemical processes and final products. One of the very promising and fast growing branches of Green Chemistry is the application of supercritical (SC) fluids as alternative solvents, reactants, or catalysts for important chemical processes. In some cases, the unique properties of SC fluids might be used as an advantage over traditional solvents. For example, the physicochemical properties (density, viscosity, diffusivity, dielectric constant, solubility parameter, etc.) of a SC fluid can be adjusted from gas-like to liquid-like values by variation of pressure and temperature. CO2 has many properties that make it an interesting solvent: it is abundant, inexpensive, nontoxic, and nonflammable. It has been proposed as a ‘‘green’’ alternative to traditional organic
0014-3057/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2004.11.032
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solvents because it is a volatile organic chemical allowed in food or pharmaceutical applications. CO2 attains the supercritical state at near-ambient temperature (Tc = 31.2 °C) and a relatively moderate pressure (Pc = 73.8 bar). Supercritical CO2 (scCO2) offers many mass transfer advantages over conventional organic solvents due to its gas-like diffusivity, low viscosity, and surface tension [2]. The use of supercritical CO2 as a polymerization medium has promising advantages with respect to conventional solvents [3,4]. However, most of vinyl polymers of practical interest are insoluble in this solvent, and dispersion polymerization in the presence of suitable steric stabilizers is the process of choice to obtain acceptable yields and molar mass [5]. The effectiveness of a surfactant is governed by two factors; first there must be sufficiently strong anchoring to the polymer particle, second the soluble segment must be chain extended into the continuous phase facilitating a negative steric interaction between particles (e.g. of sufficient solvation and chain length). These factors can be controlled by synthetic variation of both the composition and architecture of the surfactants for the polymerization reactions in CO2 medium. The successful stabilizers for dispersion polymerization in scCO2 are in the form of block copolymers which incorporate either a siloxane (e.g. polydimethylsiloxane, PDMS) or a fluorinated polymer (e.g. poly(fluorooctyl acrylate), p-FOA) as the scCO2 soluble segment: described as ‘‘CO2-philic’’ [6]. DeSimone et al. reported the first dispersion polymerization of methyl methacrylate in scCO2 using a highly soluble amorphous fluorinated polymer (poly(1,1-dihydroperfluorooctyl acrylate) (p-FOA) as a stabilizer [6]. Since then, the successful dispersion polymerization of a very wide range of vinyl monomers has been reported, including methyl methacrylate [5,7–12], 2-hydroxyethyl methacrylate [13], styrene [3,14–16], vinyl acetate [17], acrylonitrile [18], N-vinyl pyrrolidinone [19,20], glycidyl methacrylate [21,22], and copolymer of methyl methacrylate and ethyl methacrylate [23]. In this paper, we report the dispersion polymerization of styrene using p(HDFDA-co-SiMA) random copolymer synthesized in this study and commercial KrytoxÒ 157FSL as stabilizers in scCO2. The effect of varying the concentrations of stabilizer and initiator, reaction time and reaction pressure upon the polymerization yield, molar mass and morphology of the resulting polystyrene have been explored.
2. Experimental 2.1. Materials Styrene (Acros) was freed from inhibitor by washing in a 5% aqueous NaOH solution and distilled water, and
drying over anhydrous Na2SO4. 3-[Tris(trimethylsilyloxy)silyl]-propyl methacrylate (SiMA, Aldrich) and Heptadecafluorodecyl acrylate (HDFDA, Aldrich) were used as received. The stabilizer, KrytoxÒ 157FSL (nominal M n ¼ 2500 g/mol) was kindly supplied by DuPont (Turkey), and used as received. 2,2 0 -Azobis-isobutyronitrile (AIBN, Acros) was recrystallized in methanol before use. Methanol (LabScan) and methylene chloride (LabScan) were used as received. Carbon dioxide was obtained from HABAS (Turkey) with analytical grade (purity 99.99%). 2.2. Preparation of p(HDFDA-co-SiMA) copolymer (HDFDA-co-SiMA) random copolymer via radical polymerization in bulk was synthesized from the reaction of HDFDA and SiMA with 50:50 w% monomer ratio. Polymerization was carried out as follows: the 10 g of HDFDA and SiMA were placed into a glass pyrex tube with 0.05 g of AIBN (0.5 w% of monomers) as an initiator. After the tube was purged with nitrogen for 10 min, the reacting monomers were degassed by freeze-thawing cycles. Then, the tube containing the monomers was immersed in a constant heating bath at 65 ± 1 °C for 24 h in nitrogen atmosphere. At the end of the desired time, the tube contents were dissolved in methylene chloride, and then reprecipitated in methanol. The copolymer was filtered, and dried under vacuum. p(HDFDA-co-SiMA) synthesized is white tacky solid. The reaction yield was determined gravimetrically as 98 w%. The chemical structures of p(HDFDA-co-SiMA) random copolymer and KrytoxÒ 157FSL are shown in Fig. 1. 2.3. Dispersion polymerization of styrene using p(HDFDA-co-SiMA) and KrytoxÒ 157FSL The polymerization reactions were carried out in a 40 ml stainless steel reactor. The reactor was charged with styrene (20 w/v% of reactor volume), desired amounts of AIBN (w% with respect to monomer), and p(HDFDA-co-SiMA) (w% with respect to monomer), and then the system was purged with CO2 flow to remove oxygen for 15 min. An ISCO (Model No. 260D) automatic syringe pump was used to pressurize the reactor with CO2 to approximately 90 ± 5 bar, and the reaction mixture was heated to desired temperature in a heating bath. As the reaction vessel was heated, the remaining CO2 was added to the system, until the desired pressure was reached. After the reaction conditions were obtained, polymerization was started with stirring by a magnetic stirrer for specified reaction times. At the end of the reactions, the reactor was cooled in an ice bath, and CO2 was vented slowly from the reactor, and bubbled through methanol in order to collect any polymer particles, which sprayed out during the venting
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Fig. 1. Chemical structures of the stabilizers used for styrene polymerization in scCO2.
process. The polymer product was then removed from the reactor. To quantify the reaction yield, the reactor was rinsed with methylene chloride to collect any residual polymer product. The collected product was dried under vacuum, and the polymer conversion was determined gravimetrically.
PN 4 di d w ¼ Pi¼1 N 3 i¼1 d i
ð2Þ
where di is the diameter of particle i, and N is the total number of particles measured in the SEM images. As an illustration, a particle size distribution histogram is shown in Fig. 2.
2.4. Polymer characterization The molar mass of the resulting polystyrene were determined by gel permeation chromatography (GPC) using a Waters Chromatograph equipped with Agilent 1100 RI detector and Waters HR 5E, 4E, 3, 2 narrowbore column set. Tetrahydrofuran (THF) was used as an eluent, and the elution rate was 0.3 ml/min. The morphology of the polymer particles was determined with scanning electron microscope (SEM) (JEOL-5410LV). The number-average particle size and particle size distributions were determined by measuring the diameters of 50–150 particles in the SEM images. Number (dn) and weight (dw) average particle diameters were calculated from the particle size distribution histograms using the following equations [6]: PN di d n ¼ i¼1 ð1Þ N
3. Results and discussion The polymerization of styrene was carried out using p(HDFDA-co-SiMA) and KrytoxÒ 157FSL as stabilizers in supercritical CO2 in a 40 ml stainless steel reactor. The concentration of styrene was based on 20% (w/v) of the reactor volume, and the amounts of stabilizer and AIBN were determined on the basis of the weight percentage of styrene used in the reaction system. The effect of the concentrations of stabilizer and initiator, reaction time and reaction pressure on the polymerization yield, molar mass and morphology of the polystyrene were investigated for each stabilizer used. The products obtained were generally white solid or oily solid at low yields. The polymerization yield explicitly depends on the reaction time and stabilizer concentration. 3.1. Solubility of p(HDFDA-co-SiMA) in scCO2
Fig. 2. Illustrative particle size distribution histogram.
To determine the solubility of 1 w/v% of p(HDFDAco-SiMA) in CO2, 1.05 g of the stabilizer was added to a 105 ml high pressure view cell followed by the addition of CO2 to 60 bar. The cell was then heated to 65 °C and more CO2 was charged, while the contents were mixed using a magnetic stirrer. The pressure was then slowly increased up to 400 bar by the addition of CO2 until the stabilizer was dissolved completely, giving an optically clear, one-phase, homogeneous solution. For these experiments, the cloud point was defined as the lowest CO2 pressure at which the stabilizer became completely dissolved. The cloud point pressure of the stabilizer in CO2 (1 w/v%, 65 °C) was observed as 275 bar through the high pressure view cell. In addition, the cloud point pressure of p(HDFDA) was observed as 249 bar under the same conditions. However, the cloud point pressure of p(SiMA) could not be observed below 520 bar at 65 °C.
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3.2. Dispersion polymerization of styrene in scCO2 using p(HDFDA-co-SiMA) Data for the polymerization of styrene in scCO2 with and without addition of stabilizer are summarized in Table 1. The polymerization conducted in the absence of the stabilizer in scCO2 resulted in precipitation of the resulting polymer, which fouled the reactor, and proceeded to undesirably low conversion forming a low molar mass product and unstable colloidal dispersion of polystyrene. In contrast, the polymerizations that were carried out in the presence of the stabilizer resulted in both higher yields and higher molar mass, and formed a remarkably stable colloidal dispersion of polystyrene. At the end of the reactions, a dry white free-flowing powder remained in the reaction vessel, after CO2 was vented. Polymeric stabilizers for steric stabilization are effective in the dispersion polymerization. A role of the stabilizing molecule is to attach to the surface of the growing polymeric particle by physical adsorption and prevent the particles from aggregating by steric stabilization. When p(HDFDA-co-SiMA) random copolymer is used as stabilizer, micron-size polystyrene particles were produced with good latex stability (entries 2–6 in Table 1 and Fig. 3). The particles were spherical with a relatively narrow particle size distribution. Since the HDFDA segment having higher solubility in CO2 is present in copolymer structure, the random copolymer with the high CO2-philicity allows the successful stabilization of styrene polymerization in CO2. However, when used as stabilizer in the dispersion polymerization of styrene, the p(HDFDA-co-SiMA) random copolymer synthesized in this work yielded micron-sized polystyrene particles similar to when fluorinated acrylic homopolymers used [15], in contrast to submicron particles obtained when poly(1,1-dihydroperfluorooctyl acrylate-b-styrene) and poly(styrene-b-dimethylsiloxane) diblock copolymers [14,3] are used as stabilizers.
As seen in Table 1, both the yield and the numberaverage molar mass of polystyrene increase with increase in the concentration of p(HDFDA-co-SiMA). The number-average particle diameter is affected by the concentration of the stabilizer. Although the particle size increases with p(HDFDA-co-SiMA) concentration from 1 to 2.5 w%, the diameter of the particles decreases from 6.64 to 2.37 lm when the concentration of p(HDFDAco-SiMA) increases from 2.5 to 10 w%. In the presence of a large amount of the stabilizer, it is believed that the oligomer polystyrene particles can rapidly adsorb the stabilizer prior to aggregation with other particles. Therefore, there is an increase in the number of stable nuclei with higher stabilizer content and correspondingly smaller particles are produced [6]. As the concentration of stabilizer increases, the molar mass of the resulting PS increases and the PDI becomes narrower while the particle size of PS decreases. The inverse correlation between the particle size and the molar mass observed here is explained by examining the locus of polymerization [24]. It is reported that monomer swollen polymer particles represent the dominant proportion of polymerization locus in the case of small particles due to efficient capture of oligomeric radicals generated in continuous phase (here CO2 phase). The molar mass of polymer chains, which propagate in the polymeric particles are usually higher than those which propagate in the continuous phase due to the gel effect inside the particles. The smaller particles tend to have a fewer number of radicals per particle and less bimolecular termination is expected in the smaller particles than in the larger particles in CO2 systems [24]. The effect of reaction time on the polymerization of styrene is shown in Table 2. Both the yield and the molar mass increase with reaction time, although there is a slight decrease in molar mass when going for 36–48 h. This can be explained by a gel effect. In many organic solvents, gel effect occurs between 20% and 80% conversion for dispersion polymerizations [6]. Once the parti-
Table 1 The effect of p(HDFDA-co-SiMA) concentration on the polymerization of styrenea Entry
p(HDFDA-co-SiMA) (w/w%)
Yield (w%)
M n 103 (g/mol)b
PDIb,c
dn(lm)d
PSDe
Appearancef
1 2 3 4 5 6
0 1 2.5 5 7.5 10
51.73 68.88 80.33 90.57 92.20 98.08
21.2 26.7 38.1 42.8 44.4 44.1
6.581 3.027 2.553 3.131 2.132 2.611
na 4.23 6.64 3.94 3.85 2.37
na 1.267 1.547 1.264 1.321 1.237
Oil and solid White powder White powder White powder White powder White powder
a
Reaction conditions: 1% AIBN, 350 ± 5 bar, 65 °C and 36 h. As determined by GPC analysis. c PDI, polydispersity index of the molar mass distribution, M w =M n . d dn, mean particle diameter. e PSD, dispersity index of the particle size distribution, dw/dn. f Appearance of polymer in cell directly after venting. b
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Fig. 3. SEM images of polystyrene for (a) 0%, (b) 1%, (c) 7.5%, (d) 10% of p(HDFDA-co-SiMA) (1% AIBN; at 350 ± 5 bar and 65 °C for 36 h).
cles are formed, it is believed that the polymerization takes place primarily in the monomer swollen particles. Polymerization in highly viscous particles results in a gel effect which leads to an increase in the rate of polymerization and an increase in the molar mass of the polymer [6]. The gel effect arises relatively from retardation of termination rate, and monomer diffusion is hindered at high polymer conversion. However, polymerizations carried out in scCO2 offer the advantage of increasing the diffusivity of monomers into the growing polymer particles to maintain a sufficient rate of propagation, which in turn effectively facilitates the gel effect. Data in Table 2 show that the reaction rate is slow, and spherical polystyrene particles could not be isolated because of the large amount of unreacted monomer in the 12 h reaction (Fig. 4a). On the other hand,
when the conversion was low, difficulties arose in the isolation of the PS particles due to the high solubility of PS in the unreacted styrene monomer. Thus, no particles could be isolated from this experiment (entry 1 in Table 2). SEM images of the particles obtained from high conversion reactions at different reaction times studied are shown in Fig. 4b–d. The polystyrene particle diameter increased from 3.68 lm to 7.11 lm when the reaction time increased from 24 h to 48 h at constant stabilizer concentration. As expected, increasing the reaction time resulted in the particle size increase due to the decrease in the unreacted styrene monomer. A primary advantage of employing scCO2 as a reaction medium lies in the ability to tune the solvent density and dielectric constant by simply changing either
Table 2 The effect of reaction time on the polymerization of styrenea Entry
t (h)
Yield (w%)
M n 103 (g/mol)b
PDIb,c
dn (lm)d
PSDe
Appearancef
1 2 3 4
12 24 36 48
32.80 76.35 90.57 95.81
10.4 25.3 42.8 38.9
3.169 3.254 3.131 3.080
na 3.68 3.94 7.11
na 1.399 1.264 1.280
Oil and solid White powder White powder White powder
a
Reaction conditions: 1% AIBN, 5% (w/w) p(HDFDA-co-SiMA), 350 ± 5 bar and 65 °C. As determined by GPC. c PDI, polydispersity index of the molar mass distribution, M w =M n . d dn, mean particle diameter. e PSD, dispersity index of the particle size distribution, dw/dn. f Appearance of polymer in cell directly after venting. b
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Fig. 4. SEM images of polystyrene at (a) 12 h, (b) 24 h, (c) 36 h, (d) 48 h. (1% AIBN; 5% (w/w) p(HDFDA-co-SiMA); at 350 ± 5 bar and 65 °C).
temperature or pressure. The effect of pressure on the polymerization can be clearly seen by comparing Fig. 5a and b: changing the density of the continuous phase by manipulating the pressure affects morphology. The polymer formed at 150 bar does not form particles, and after its isolation at 36 h, a conversion of 46.25%
is obtained (entry 1 in Table 3). Although the critical pressure of styrene-CO2 system at 65 °C is 120 bar [25], a stabilized polystyrene and high yield could not be obtained at 150 bar, because p(HDFDA-co-SiMA) is insoluble in CO2 at this pressure, and canÕt stabilize the polystyrene particle. At polymerization pressures
Fig. 5. SEM images of polystyrene at (a) 150 bar, (b) 250 bar, (c) 350 bar. (1% AIBN; 5% (w/w) p(HDFDA-co-SiMA); at 65 °C for 36 h).
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Table 3 The effect of pressure on the polymerization of styrenea Entry
P (±5 bar)
Yield (w%)
M n 103 (g/mol)b
PDIb,c
dn (lm)d
PSDe
Appearancef
1 2 3 4
150 250 300 350
46.25 90.30 94.18 90.57
10.8 38.4 34.0 42.8
3.346 2.516 2.724 3.131
na 4.63 4.84 3.94
na 1.123 1.617 1.264
Oil and solid White powder White powder White powder
a
Reaction conditions: 1% AIBN, 5% (w/w) p(HDFDA-co-SiMA), 65 °C and 36 h. As determined by GPC. c PDI, polydispersity index of the molar mass distribution, M w =M n . d dn, mean particle diameter. e PSD, dispersity index of the particle size distribution, dw/dn. f Appearance of polymer in cell directly after venting. b
of 250–350 bar, the polymerization yield, the molar mass and the particle diameter are not affected significantly.
Table 4 The effect of KrytoxÒ 157FSL as stabilizer on the polymerization of styrenea
3.3. Dispersion polymerization of styrene in scCO2 using KrytoxÒ 157FSL
Entry
Krytox (w/w%)
Yield (w%)
Mn (g/mol)b
PDIb,c
Appearanced
1 2 3 4 5 6
0 1 5 10 15 20
34.53 49.17 49.20 51.06 71.66 72.86
3290 3290 2700 2750 3200 3290
1.332 1.581 1.633 1.614 1.636 1.584
Oil and solid Oil and solid Oil and solid Oil and solid White solid White solid
Data for the polymerization of styrene in scCO2 with and without addition of KrytoxÒ 157FSL as stabilizer are summarized in Table 4. The polymerization conducted in the absence of the stabilizer KrytoxÒ 157FSL in scCO2 resulted in precipitation of the resulting polymer, which fouled the reactor. However, the reactions that were carried out in the presence of the stabilizer resulted in both low yields and low molar mass, and formed generally unstabilized polystyrene products. It was observed that the yield and the molar mass of polystyrene increase with the concentration of KrytoxÒ 157FSL. At the end of the reactions, an oily solid material remained for 0–10 w/w% KrytoxÒ 157FSL concen-
a
Reaction conditions: 5% AIBN, 350 ± 5 bar, 65 °C and 36 h. As determined by GPC. c PDI, polydispersity index of the molar mass distribution, M w =M n . d Appearance of polymer in cell directly after venting. b
tration, after CO2 was vented out. The SEM images of polystyrene particles are shown in Fig. 6. Although higher yields as white solid were obtained in entries 5
Fig. 6. SEM images of polystyrene for (a) 0%, (b) 1%, (c) 15% of Krytox 157FSL. (5% AIBN; at 350 ± 5 bar and 65 °C for 36 h).
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Table 5 The effect of initiator concentration on the polymerization of styrenea Entry
AIBN (w/w%)
Yield (w%)
Mn (g/mol)b
PDIb,c
Appearanced
1 2 3 4 5
1 5 10 15 20
40.54 51.06 69.41 71.41 80.88
11940 2750 2440 1630 1350
3.446 1.614 2.137 1.551 1.664
Oil and solid Oil and solid Oil and solid White solid White solid
Reaction conditions: 10% (w/w) KrytoxÒ 157FSL, 350 ± 5 bar, 65 °C and 36 h. b As determined by GPC. c PDI, polydispersity index of the molar mass distribution, M w =M n . d Appearance of polymer in cell directly after venting. a
and 6 using KrytoxÒ 157FSL, an expected stabilization of polystyrene was not observed as seen in SEM image of Fig. 6c. Thus, SEM images of Fig. 6 in general show aggregated particles with nonuniform distribution. Howdle and coworkers [26–29] have reported that KrytoxÒ 157FSL successfully stabilized the polymerization of acrylate monomers with high yield and molar mass at very low loading. They have mentioned that the stabilizer forms stable dispersion by the formation of a hydrogen bonding between the carboxylic acid group in KrytoxÒ 157FSL and ester group in acrylate monomers. However, there is no literature data regarding the polymerization of styrene using KrytoxÒ 157FSL. Since styrene has no ester group, the type of stabilization mentioned in literature is not effective. The effect of initiator concentration on the polymerization of styrene is shown in Table 5. Although the polymerization yield increases with the initiator concentration, use of KrytoxÒ 157FSL as stabilizer results in undesirably low conversion forming a low molar mass product and an unstable colloidal dispersion of polystyrene.
4. Conclusion The polymerization of styrene was carried out using p(HDFDA-co-SiMA) and KrytoxÒ 157FSL as stabilizers in supercritical CO2. The cloud point pressure of p(HDFDA-co-SiMA) in CO2 was observed as 275 bar through the high pressure view cell. The cloud point pressures of p(HDFDA) and p(SiMA) were also measured separately in CO2 at the same conditions, and were observed as 249 bar for p(HDFDA). However, the cloud point pressure of p(SiMA) could not be observed below 520 bar at 65 °C. The effect of the concentrations of stabilizers, reaction time and pressure on the polymerization yield, molar mass and morphology of polystyrene product were investigated for each stabilizer
used. p(HDFDA-co-SiMA) is an effective stabilizer for the polymerization of styrene in scCO2. Dry, white, free-flowing, micron-size polystyrene particles were obtained in high yields and high molar mass. It was observed that the yield and the molar mass of polystyrene increase both with the concentration of p(HDFDA-co-SiMA) and the reaction time. However, KrytoxÒ 157FSL was not as good stabilizer as p(HDFDA-co-SiMA) in the polymerization of styrene in supercritical CO2. As a result, lower molar mass polymers and lower yields were obtained using KrytoxÒ 157FSL. In general, KrytoxÒ 157FSL forms stable dispersion with a monomer by forming H-bonds, but the relatively inferior stabilization here might be due to its lack of formation of hydrogen bonds with styrene.
Acknowledgment This work was supported by DPT (Turkish State Planning Organization, project no. 22-DPT-07-01-01) and YTU-BAPK (Yildiz Technical University, Organization of Scientific Research Project, project no. 21-0701-02 and 22-07-01-04). The contributions of P. Topuz of YTU for SEM images and Prof. G. Hizal of ITU for molar mass measurements is acknowledged.
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EUROPEAN POLYMER JOURNAL www.elsevier.com/locate/europolj
Dispersion polymerization of styrene in supercritical carbon dioxide using monofunctional perfluoropolyether and silicone-containing fluoroacrylate stabilizers Nil Baran, Sennur Deniz, Mesut Akgu¨n, I. Nimet Uzun, Salih Dinc¸er
*
Chemical Engineering Department, Yildiz Technical University, Davutpasa Cad., No. 127, 34210, Esenler, Istanbul, Turkey Received 1 April 2004; received in revised form 19 November 2004; accepted 26 November 2004 Available online 22 January 2005
Abstract The free radical dispersion polymerization of styrene was carried out in supercritical carbon dioxide (scCO2) using two different stabilizers. The polymerizations are performed in the presence of poly(heptadecafluorodecyl acrylate-cotris(trimethylsilyloxy)silyllpropyl methacrylate) p(HDFDA-co-SiMA) and a commercially available carboxylic acid-terminated perfluoropolyether (KrytoxÒ 157FSL) as polymerization stabilizers. Dry, fine powdered spherical polystyrene particles were produced under optimised conditions. The resulting high yield of spherical and relatively uniform micronsize polystyrene particles were formed utilizing various amounts of p(HDFDA-co-SiMA) random copolymer. However, it was observed that KrytoxÒ 157FSL was not a good stabilizer as p(HDFDA-co-SiMA) for the dispersion polymerization of styrene. The particle diameter was shown to be dependent on the type of the stabilizer and the weight percent of the stabilizer added to the system. The effect of varying the concentrations of stabilizers and initiator, reaction time and reaction pressure upon the polymerization yield, molar mass and morphology of polystyrene have been investigated. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Dispersion polymerization; Supercritical CO2; Polystyrene particles; Stabilizer
1. Introduction Polymer industries have been using large amounts of volatile organic compounds to prepare polymer molecules, and to apply them as a reaction medium [1]. In order to minimize the impacts of chemical industry on human health and environment, there is significant interest in development of so-called Green Chemistry, the goal of which is to reduce or eliminate the use of hazard-
* Corresponding author. Tel.: +90 212 449 1925; fax: +90 212 449 1895. E-mail address: [email protected] (S. Dinc¸er).
ous substances in chemical processes and final products. One of the very promising and fast growing branches of Green Chemistry is the application of supercritical (SC) fluids as alternative solvents, reactants, or catalysts for important chemical processes. In some cases, the unique properties of SC fluids might be used as an advantage over traditional solvents. For example, the physicochemical properties (density, viscosity, diffusivity, dielectric constant, solubility parameter, etc.) of a SC fluid can be adjusted from gas-like to liquid-like values by variation of pressure and temperature. CO2 has many properties that make it an interesting solvent: it is abundant, inexpensive, nontoxic, and nonflammable. It has been proposed as a ‘‘green’’ alternative to traditional organic
0014-3057/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2004.11.032
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solvents because it is a volatile organic chemical allowed in food or pharmaceutical applications. CO2 attains the supercritical state at near-ambient temperature (Tc = 31.2 °C) and a relatively moderate pressure (Pc = 73.8 bar). Supercritical CO2 (scCO2) offers many mass transfer advantages over conventional organic solvents due to its gas-like diffusivity, low viscosity, and surface tension [2]. The use of supercritical CO2 as a polymerization medium has promising advantages with respect to conventional solvents [3,4]. However, most of vinyl polymers of practical interest are insoluble in this solvent, and dispersion polymerization in the presence of suitable steric stabilizers is the process of choice to obtain acceptable yields and molar mass [5]. The effectiveness of a surfactant is governed by two factors; first there must be sufficiently strong anchoring to the polymer particle, second the soluble segment must be chain extended into the continuous phase facilitating a negative steric interaction between particles (e.g. of sufficient solvation and chain length). These factors can be controlled by synthetic variation of both the composition and architecture of the surfactants for the polymerization reactions in CO2 medium. The successful stabilizers for dispersion polymerization in scCO2 are in the form of block copolymers which incorporate either a siloxane (e.g. polydimethylsiloxane, PDMS) or a fluorinated polymer (e.g. poly(fluorooctyl acrylate), p-FOA) as the scCO2 soluble segment: described as ‘‘CO2-philic’’ [6]. DeSimone et al. reported the first dispersion polymerization of methyl methacrylate in scCO2 using a highly soluble amorphous fluorinated polymer (poly(1,1-dihydroperfluorooctyl acrylate) (p-FOA) as a stabilizer [6]. Since then, the successful dispersion polymerization of a very wide range of vinyl monomers has been reported, including methyl methacrylate [5,7–12], 2-hydroxyethyl methacrylate [13], styrene [3,14–16], vinyl acetate [17], acrylonitrile [18], N-vinyl pyrrolidinone [19,20], glycidyl methacrylate [21,22], and copolymer of methyl methacrylate and ethyl methacrylate [23]. In this paper, we report the dispersion polymerization of styrene using p(HDFDA-co-SiMA) random copolymer synthesized in this study and commercial KrytoxÒ 157FSL as stabilizers in scCO2. The effect of varying the concentrations of stabilizer and initiator, reaction time and reaction pressure upon the polymerization yield, molar mass and morphology of the resulting polystyrene have been explored.
2. Experimental 2.1. Materials Styrene (Acros) was freed from inhibitor by washing in a 5% aqueous NaOH solution and distilled water, and
drying over anhydrous Na2SO4. 3-[Tris(trimethylsilyloxy)silyl]-propyl methacrylate (SiMA, Aldrich) and Heptadecafluorodecyl acrylate (HDFDA, Aldrich) were used as received. The stabilizer, KrytoxÒ 157FSL (nominal M n ¼ 2500 g/mol) was kindly supplied by DuPont (Turkey), and used as received. 2,2 0 -Azobis-isobutyronitrile (AIBN, Acros) was recrystallized in methanol before use. Methanol (LabScan) and methylene chloride (LabScan) were used as received. Carbon dioxide was obtained from HABAS (Turkey) with analytical grade (purity 99.99%). 2.2. Preparation of p(HDFDA-co-SiMA) copolymer (HDFDA-co-SiMA) random copolymer via radical polymerization in bulk was synthesized from the reaction of HDFDA and SiMA with 50:50 w% monomer ratio. Polymerization was carried out as follows: the 10 g of HDFDA and SiMA were placed into a glass pyrex tube with 0.05 g of AIBN (0.5 w% of monomers) as an initiator. After the tube was purged with nitrogen for 10 min, the reacting monomers were degassed by freeze-thawing cycles. Then, the tube containing the monomers was immersed in a constant heating bath at 65 ± 1 °C for 24 h in nitrogen atmosphere. At the end of the desired time, the tube contents were dissolved in methylene chloride, and then reprecipitated in methanol. The copolymer was filtered, and dried under vacuum. p(HDFDA-co-SiMA) synthesized is white tacky solid. The reaction yield was determined gravimetrically as 98 w%. The chemical structures of p(HDFDA-co-SiMA) random copolymer and KrytoxÒ 157FSL are shown in Fig. 1. 2.3. Dispersion polymerization of styrene using p(HDFDA-co-SiMA) and KrytoxÒ 157FSL The polymerization reactions were carried out in a 40 ml stainless steel reactor. The reactor was charged with styrene (20 w/v% of reactor volume), desired amounts of AIBN (w% with respect to monomer), and p(HDFDA-co-SiMA) (w% with respect to monomer), and then the system was purged with CO2 flow to remove oxygen for 15 min. An ISCO (Model No. 260D) automatic syringe pump was used to pressurize the reactor with CO2 to approximately 90 ± 5 bar, and the reaction mixture was heated to desired temperature in a heating bath. As the reaction vessel was heated, the remaining CO2 was added to the system, until the desired pressure was reached. After the reaction conditions were obtained, polymerization was started with stirring by a magnetic stirrer for specified reaction times. At the end of the reactions, the reactor was cooled in an ice bath, and CO2 was vented slowly from the reactor, and bubbled through methanol in order to collect any polymer particles, which sprayed out during the venting
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Fig. 1. Chemical structures of the stabilizers used for styrene polymerization in scCO2.
process. The polymer product was then removed from the reactor. To quantify the reaction yield, the reactor was rinsed with methylene chloride to collect any residual polymer product. The collected product was dried under vacuum, and the polymer conversion was determined gravimetrically.
PN 4 di d w ¼ Pi¼1 N 3 i¼1 d i
ð2Þ
where di is the diameter of particle i, and N is the total number of particles measured in the SEM images. As an illustration, a particle size distribution histogram is shown in Fig. 2.
2.4. Polymer characterization The molar mass of the resulting polystyrene were determined by gel permeation chromatography (GPC) using a Waters Chromatograph equipped with Agilent 1100 RI detector and Waters HR 5E, 4E, 3, 2 narrowbore column set. Tetrahydrofuran (THF) was used as an eluent, and the elution rate was 0.3 ml/min. The morphology of the polymer particles was determined with scanning electron microscope (SEM) (JEOL-5410LV). The number-average particle size and particle size distributions were determined by measuring the diameters of 50–150 particles in the SEM images. Number (dn) and weight (dw) average particle diameters were calculated from the particle size distribution histograms using the following equations [6]: PN di d n ¼ i¼1 ð1Þ N
3. Results and discussion The polymerization of styrene was carried out using p(HDFDA-co-SiMA) and KrytoxÒ 157FSL as stabilizers in supercritical CO2 in a 40 ml stainless steel reactor. The concentration of styrene was based on 20% (w/v) of the reactor volume, and the amounts of stabilizer and AIBN were determined on the basis of the weight percentage of styrene used in the reaction system. The effect of the concentrations of stabilizer and initiator, reaction time and reaction pressure on the polymerization yield, molar mass and morphology of the polystyrene were investigated for each stabilizer used. The products obtained were generally white solid or oily solid at low yields. The polymerization yield explicitly depends on the reaction time and stabilizer concentration. 3.1. Solubility of p(HDFDA-co-SiMA) in scCO2
Fig. 2. Illustrative particle size distribution histogram.
To determine the solubility of 1 w/v% of p(HDFDAco-SiMA) in CO2, 1.05 g of the stabilizer was added to a 105 ml high pressure view cell followed by the addition of CO2 to 60 bar. The cell was then heated to 65 °C and more CO2 was charged, while the contents were mixed using a magnetic stirrer. The pressure was then slowly increased up to 400 bar by the addition of CO2 until the stabilizer was dissolved completely, giving an optically clear, one-phase, homogeneous solution. For these experiments, the cloud point was defined as the lowest CO2 pressure at which the stabilizer became completely dissolved. The cloud point pressure of the stabilizer in CO2 (1 w/v%, 65 °C) was observed as 275 bar through the high pressure view cell. In addition, the cloud point pressure of p(HDFDA) was observed as 249 bar under the same conditions. However, the cloud point pressure of p(SiMA) could not be observed below 520 bar at 65 °C.
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3.2. Dispersion polymerization of styrene in scCO2 using p(HDFDA-co-SiMA) Data for the polymerization of styrene in scCO2 with and without addition of stabilizer are summarized in Table 1. The polymerization conducted in the absence of the stabilizer in scCO2 resulted in precipitation of the resulting polymer, which fouled the reactor, and proceeded to undesirably low conversion forming a low molar mass product and unstable colloidal dispersion of polystyrene. In contrast, the polymerizations that were carried out in the presence of the stabilizer resulted in both higher yields and higher molar mass, and formed a remarkably stable colloidal dispersion of polystyrene. At the end of the reactions, a dry white free-flowing powder remained in the reaction vessel, after CO2 was vented. Polymeric stabilizers for steric stabilization are effective in the dispersion polymerization. A role of the stabilizing molecule is to attach to the surface of the growing polymeric particle by physical adsorption and prevent the particles from aggregating by steric stabilization. When p(HDFDA-co-SiMA) random copolymer is used as stabilizer, micron-size polystyrene particles were produced with good latex stability (entries 2–6 in Table 1 and Fig. 3). The particles were spherical with a relatively narrow particle size distribution. Since the HDFDA segment having higher solubility in CO2 is present in copolymer structure, the random copolymer with the high CO2-philicity allows the successful stabilization of styrene polymerization in CO2. However, when used as stabilizer in the dispersion polymerization of styrene, the p(HDFDA-co-SiMA) random copolymer synthesized in this work yielded micron-sized polystyrene particles similar to when fluorinated acrylic homopolymers used [15], in contrast to submicron particles obtained when poly(1,1-dihydroperfluorooctyl acrylate-b-styrene) and poly(styrene-b-dimethylsiloxane) diblock copolymers [14,3] are used as stabilizers.
As seen in Table 1, both the yield and the numberaverage molar mass of polystyrene increase with increase in the concentration of p(HDFDA-co-SiMA). The number-average particle diameter is affected by the concentration of the stabilizer. Although the particle size increases with p(HDFDA-co-SiMA) concentration from 1 to 2.5 w%, the diameter of the particles decreases from 6.64 to 2.37 lm when the concentration of p(HDFDAco-SiMA) increases from 2.5 to 10 w%. In the presence of a large amount of the stabilizer, it is believed that the oligomer polystyrene particles can rapidly adsorb the stabilizer prior to aggregation with other particles. Therefore, there is an increase in the number of stable nuclei with higher stabilizer content and correspondingly smaller particles are produced [6]. As the concentration of stabilizer increases, the molar mass of the resulting PS increases and the PDI becomes narrower while the particle size of PS decreases. The inverse correlation between the particle size and the molar mass observed here is explained by examining the locus of polymerization [24]. It is reported that monomer swollen polymer particles represent the dominant proportion of polymerization locus in the case of small particles due to efficient capture of oligomeric radicals generated in continuous phase (here CO2 phase). The molar mass of polymer chains, which propagate in the polymeric particles are usually higher than those which propagate in the continuous phase due to the gel effect inside the particles. The smaller particles tend to have a fewer number of radicals per particle and less bimolecular termination is expected in the smaller particles than in the larger particles in CO2 systems [24]. The effect of reaction time on the polymerization of styrene is shown in Table 2. Both the yield and the molar mass increase with reaction time, although there is a slight decrease in molar mass when going for 36–48 h. This can be explained by a gel effect. In many organic solvents, gel effect occurs between 20% and 80% conversion for dispersion polymerizations [6]. Once the parti-
Table 1 The effect of p(HDFDA-co-SiMA) concentration on the polymerization of styrenea Entry
p(HDFDA-co-SiMA) (w/w%)
Yield (w%)
M n 103 (g/mol)b
PDIb,c
dn(lm)d
PSDe
Appearancef
1 2 3 4 5 6
0 1 2.5 5 7.5 10
51.73 68.88 80.33 90.57 92.20 98.08
21.2 26.7 38.1 42.8 44.4 44.1
6.581 3.027 2.553 3.131 2.132 2.611
na 4.23 6.64 3.94 3.85 2.37
na 1.267 1.547 1.264 1.321 1.237
Oil and solid White powder White powder White powder White powder White powder
a
Reaction conditions: 1% AIBN, 350 ± 5 bar, 65 °C and 36 h. As determined by GPC analysis. c PDI, polydispersity index of the molar mass distribution, M w =M n . d dn, mean particle diameter. e PSD, dispersity index of the particle size distribution, dw/dn. f Appearance of polymer in cell directly after venting. b
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Fig. 3. SEM images of polystyrene for (a) 0%, (b) 1%, (c) 7.5%, (d) 10% of p(HDFDA-co-SiMA) (1% AIBN; at 350 ± 5 bar and 65 °C for 36 h).
cles are formed, it is believed that the polymerization takes place primarily in the monomer swollen particles. Polymerization in highly viscous particles results in a gel effect which leads to an increase in the rate of polymerization and an increase in the molar mass of the polymer [6]. The gel effect arises relatively from retardation of termination rate, and monomer diffusion is hindered at high polymer conversion. However, polymerizations carried out in scCO2 offer the advantage of increasing the diffusivity of monomers into the growing polymer particles to maintain a sufficient rate of propagation, which in turn effectively facilitates the gel effect. Data in Table 2 show that the reaction rate is slow, and spherical polystyrene particles could not be isolated because of the large amount of unreacted monomer in the 12 h reaction (Fig. 4a). On the other hand,
when the conversion was low, difficulties arose in the isolation of the PS particles due to the high solubility of PS in the unreacted styrene monomer. Thus, no particles could be isolated from this experiment (entry 1 in Table 2). SEM images of the particles obtained from high conversion reactions at different reaction times studied are shown in Fig. 4b–d. The polystyrene particle diameter increased from 3.68 lm to 7.11 lm when the reaction time increased from 24 h to 48 h at constant stabilizer concentration. As expected, increasing the reaction time resulted in the particle size increase due to the decrease in the unreacted styrene monomer. A primary advantage of employing scCO2 as a reaction medium lies in the ability to tune the solvent density and dielectric constant by simply changing either
Table 2 The effect of reaction time on the polymerization of styrenea Entry
t (h)
Yield (w%)
M n 103 (g/mol)b
PDIb,c
dn (lm)d
PSDe
Appearancef
1 2 3 4
12 24 36 48
32.80 76.35 90.57 95.81
10.4 25.3 42.8 38.9
3.169 3.254 3.131 3.080
na 3.68 3.94 7.11
na 1.399 1.264 1.280
Oil and solid White powder White powder White powder
a
Reaction conditions: 1% AIBN, 5% (w/w) p(HDFDA-co-SiMA), 350 ± 5 bar and 65 °C. As determined by GPC. c PDI, polydispersity index of the molar mass distribution, M w =M n . d dn, mean particle diameter. e PSD, dispersity index of the particle size distribution, dw/dn. f Appearance of polymer in cell directly after venting. b
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Fig. 4. SEM images of polystyrene at (a) 12 h, (b) 24 h, (c) 36 h, (d) 48 h. (1% AIBN; 5% (w/w) p(HDFDA-co-SiMA); at 350 ± 5 bar and 65 °C).
temperature or pressure. The effect of pressure on the polymerization can be clearly seen by comparing Fig. 5a and b: changing the density of the continuous phase by manipulating the pressure affects morphology. The polymer formed at 150 bar does not form particles, and after its isolation at 36 h, a conversion of 46.25%
is obtained (entry 1 in Table 3). Although the critical pressure of styrene-CO2 system at 65 °C is 120 bar [25], a stabilized polystyrene and high yield could not be obtained at 150 bar, because p(HDFDA-co-SiMA) is insoluble in CO2 at this pressure, and canÕt stabilize the polystyrene particle. At polymerization pressures
Fig. 5. SEM images of polystyrene at (a) 150 bar, (b) 250 bar, (c) 350 bar. (1% AIBN; 5% (w/w) p(HDFDA-co-SiMA); at 65 °C for 36 h).
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Table 3 The effect of pressure on the polymerization of styrenea Entry
P (±5 bar)
Yield (w%)
M n 103 (g/mol)b
PDIb,c
dn (lm)d
PSDe
Appearancef
1 2 3 4
150 250 300 350
46.25 90.30 94.18 90.57
10.8 38.4 34.0 42.8
3.346 2.516 2.724 3.131
na 4.63 4.84 3.94
na 1.123 1.617 1.264
Oil and solid White powder White powder White powder
a
Reaction conditions: 1% AIBN, 5% (w/w) p(HDFDA-co-SiMA), 65 °C and 36 h. As determined by GPC. c PDI, polydispersity index of the molar mass distribution, M w =M n . d dn, mean particle diameter. e PSD, dispersity index of the particle size distribution, dw/dn. f Appearance of polymer in cell directly after venting. b
of 250–350 bar, the polymerization yield, the molar mass and the particle diameter are not affected significantly.
Table 4 The effect of KrytoxÒ 157FSL as stabilizer on the polymerization of styrenea
3.3. Dispersion polymerization of styrene in scCO2 using KrytoxÒ 157FSL
Entry
Krytox (w/w%)
Yield (w%)
Mn (g/mol)b
PDIb,c
Appearanced
1 2 3 4 5 6
0 1 5 10 15 20
34.53 49.17 49.20 51.06 71.66 72.86
3290 3290 2700 2750 3200 3290
1.332 1.581 1.633 1.614 1.636 1.584
Oil and solid Oil and solid Oil and solid Oil and solid White solid White solid
Data for the polymerization of styrene in scCO2 with and without addition of KrytoxÒ 157FSL as stabilizer are summarized in Table 4. The polymerization conducted in the absence of the stabilizer KrytoxÒ 157FSL in scCO2 resulted in precipitation of the resulting polymer, which fouled the reactor. However, the reactions that were carried out in the presence of the stabilizer resulted in both low yields and low molar mass, and formed generally unstabilized polystyrene products. It was observed that the yield and the molar mass of polystyrene increase with the concentration of KrytoxÒ 157FSL. At the end of the reactions, an oily solid material remained for 0–10 w/w% KrytoxÒ 157FSL concen-
a
Reaction conditions: 5% AIBN, 350 ± 5 bar, 65 °C and 36 h. As determined by GPC. c PDI, polydispersity index of the molar mass distribution, M w =M n . d Appearance of polymer in cell directly after venting. b
tration, after CO2 was vented out. The SEM images of polystyrene particles are shown in Fig. 6. Although higher yields as white solid were obtained in entries 5
Fig. 6. SEM images of polystyrene for (a) 0%, (b) 1%, (c) 15% of Krytox 157FSL. (5% AIBN; at 350 ± 5 bar and 65 °C for 36 h).
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Table 5 The effect of initiator concentration on the polymerization of styrenea Entry
AIBN (w/w%)
Yield (w%)
Mn (g/mol)b
PDIb,c
Appearanced
1 2 3 4 5
1 5 10 15 20
40.54 51.06 69.41 71.41 80.88
11940 2750 2440 1630 1350
3.446 1.614 2.137 1.551 1.664
Oil and solid Oil and solid Oil and solid White solid White solid
Reaction conditions: 10% (w/w) KrytoxÒ 157FSL, 350 ± 5 bar, 65 °C and 36 h. b As determined by GPC. c PDI, polydispersity index of the molar mass distribution, M w =M n . d Appearance of polymer in cell directly after venting. a
and 6 using KrytoxÒ 157FSL, an expected stabilization of polystyrene was not observed as seen in SEM image of Fig. 6c. Thus, SEM images of Fig. 6 in general show aggregated particles with nonuniform distribution. Howdle and coworkers [26–29] have reported that KrytoxÒ 157FSL successfully stabilized the polymerization of acrylate monomers with high yield and molar mass at very low loading. They have mentioned that the stabilizer forms stable dispersion by the formation of a hydrogen bonding between the carboxylic acid group in KrytoxÒ 157FSL and ester group in acrylate monomers. However, there is no literature data regarding the polymerization of styrene using KrytoxÒ 157FSL. Since styrene has no ester group, the type of stabilization mentioned in literature is not effective. The effect of initiator concentration on the polymerization of styrene is shown in Table 5. Although the polymerization yield increases with the initiator concentration, use of KrytoxÒ 157FSL as stabilizer results in undesirably low conversion forming a low molar mass product and an unstable colloidal dispersion of polystyrene.
4. Conclusion The polymerization of styrene was carried out using p(HDFDA-co-SiMA) and KrytoxÒ 157FSL as stabilizers in supercritical CO2. The cloud point pressure of p(HDFDA-co-SiMA) in CO2 was observed as 275 bar through the high pressure view cell. The cloud point pressures of p(HDFDA) and p(SiMA) were also measured separately in CO2 at the same conditions, and were observed as 249 bar for p(HDFDA). However, the cloud point pressure of p(SiMA) could not be observed below 520 bar at 65 °C. The effect of the concentrations of stabilizers, reaction time and pressure on the polymerization yield, molar mass and morphology of polystyrene product were investigated for each stabilizer
used. p(HDFDA-co-SiMA) is an effective stabilizer for the polymerization of styrene in scCO2. Dry, white, free-flowing, micron-size polystyrene particles were obtained in high yields and high molar mass. It was observed that the yield and the molar mass of polystyrene increase both with the concentration of p(HDFDA-co-SiMA) and the reaction time. However, KrytoxÒ 157FSL was not as good stabilizer as p(HDFDA-co-SiMA) in the polymerization of styrene in supercritical CO2. As a result, lower molar mass polymers and lower yields were obtained using KrytoxÒ 157FSL. In general, KrytoxÒ 157FSL forms stable dispersion with a monomer by forming H-bonds, but the relatively inferior stabilization here might be due to its lack of formation of hydrogen bonds with styrene.
Acknowledgment This work was supported by DPT (Turkish State Planning Organization, project no. 22-DPT-07-01-01) and YTU-BAPK (Yildiz Technical University, Organization of Scientific Research Project, project no. 21-0701-02 and 22-07-01-04). The contributions of P. Topuz of YTU for SEM images and Prof. G. Hizal of ITU for molar mass measurements is acknowledged.
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