Thermochimica Acta 518 (2011) 101–106

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Thermochimica Acta

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Thermoporometry study of coagulation bath temperature effect on polyacrylonitrile ﬁbers morphology

Payman Sobhanipour a,∗, Reza Cheraghi b, Alex A. Volinsky c a Textile Engineering Department, Amirkabir University of Technology, Tehran 15914, Iran b Textile Engineering Department, Isfahan University of Technology, Isfahan 84154-83111, Iran c University of South Florida, Department of Mechanical Engineering, Tampa, FL 33620 USA article info abstract

Article history: The effect of coagulation bath temperature on the morphology of wet-spun polyacrylonitrile fibers was Received 18 November 2010 investigated by scanning electron microscopy and thermoporometry, a calorimetric technique based on Received in revised form 4 February 2011 lowering liquid triple point temperature inside the pores. Fibers were fabricated at two coagulation bath Accepted 8 February 2011 temperatures of 5 ◦C and 60 ◦C. The shape of nascent fibers cross-section transforms from bean to circular Available online 21 February 2011 and pore size increases with coagulation temperature. Porosity parameters including average pore size and pore size distribution, pore volume and internal surface area were determined by thermoporometry. Keywords: The average pore size and pore volume of the fibers increase with coagulation temperature. Low heating Polyacrylonitrile ◦ Wet spinning rate of 0.1 C/min during thermoporometry measurements is the key parameter to ensure that test condi- Coagulation bath temperature tions are close to equilibrium. This study shows that thermoporometry can be employed to characterize Meso-porosity closed meso-porosity of wet-spun fibers inaccessible by other standard porosimetry methods. Thermoporometry © 2011 Elsevier B.V. All rights reserved.

1. Introduction polymer rich phases. Polymer rich phase forms the fiber matrix which can solidify through different mechanisms, including gela- Light weight carbon fibers (CFs) have outstanding mechani- tion and/or crystallization. Polymer lean phase, which is full of cal properties (high tensile strength and elastic modulus), which solvent and non-solvent, forms the fiber pores. This kind of phase makes them excellent engineering materials for aerospace, aviation separation is called liquid–liquid de-mixing and is responsible for and marine industries, and for use in high quality sporting goods the appearance of porous structures [11]. Shape, size and distribu- [1–3]. Different precursors are employed to produce CFs, includ- tion of the pores strongly depend on the exchange rate between the ing polyacrylonitrile (PAN) fibers, viscose rayon, mesophase pitch. solvent and the non-solvent controlled by the process conditions, PAN fibers are most popular among carbon precursors because such as coagulation bath composition, temperature, and polymer of their high carbon yield, economic processing and competitive solution additives [12]. pricing, high crystallinity, and continuous carbon backbone which Coagulation bath temperature has significant effects on the facilitates the cyclization reaction [4–6]. It is clear that final CFs’ coagulation process by controlling the coagulation rate, i.e. properties strongly depend on the properties of PAN precursor exchange rate between solvent/non-solvent, and significantly fibers, i.e. orientation, crystallinity, linear density, shape of cross- affects physical properties of PAN fibers [13–15]. At low bath tem- section, porosity and co-monomer types [7–10]. peratures the rate of solvent out-diffusion is higher than the rate PAN fibers are fabricated by different spinning methods like wet of non-solvent diffusing in. Consequently, fiber structure collapses spinning, dry spinning and dry-jet-wet spinning. Meanwhile, wet and non-circular shapes including bean or dog bone cross-sections spinning is the main fabrication method for PAN fibers employed by form. Also, slower coagulation results in less skin formation, leading many researchers [6–10]. In this method polymer solution enters to smaller and fewer voids. the coagulation bath after passing through the spinneret orifices. According to the International Union of Pure and Applied Chem- Here, non-solvent-induced phase separation (NIPS) is responsible istry (IUPAC) recommendations, there are three pore size ranges, for the fibers coagulation. Thermodynamic instability takes place including micro-pores (R < 2 nm), meso-pores (2 nm < R < 50 nm) in the polymer solution and separates it into polymer lean and and macro-pores (R > 150 nm) [16]. For PAN fibers to be processed into CFs, the presence of meso-pores, macro-pores and especially closed pores significantly affects processing [17]. Therefore, suffi- cient knowledge of PAN precursor porosity is essential. There are ∗ Corresponding author. Tel.: +98 21 6454 2600; fax: +98 21 6640 0245. several methods to determine pore size and pore size distribution of E-mail address: [email protected] (P. Sobhanipour).

102 P. Sobhanipour et al. / Thermochimica Acta 518 (2011) 101–106 the porous beds including sorption (1–100 nm), mercury porosimetry (1 nm to 10 ␮m), electron microscopy (10 nm to 1 ␮m), optical microscopy (1 ␮m), X-ray methods (0.1–100 nm) [18] and novel standard porosimetry method (0.3 nm to 300 ␮m) of Volfkovich et al. [19]. The numbers in parentheses show the pore size range detectable by each method. Each of these methods has advantages and limitations, but probing closed pores is still problematic. Thermoporometry (TPM) is an alternative calorimetric method of meso-pores characterization based on lowering liquid triple point temperature inside the pores. When liquid is confined in meso-pores, its surface is no longer planar and has curvature. This configuration causes liquid to melt/freeze at temperatures lower than its normal transition temperature [20–25]. Brun et al. [26] derived a relationship between the temperature drop (T) and pore radius (rp) for melting of ice inside the pores from Gibbs–Duhem fundamental equation by considering the pores as cylinders with circular cross-section: 32.33 r =− + 0.68 for 0 > T > −40 ◦C (1) p T where the value of 0.68 nm is the thickness of water layer that does not participate in any transition, named non-freezable water. It can be seen that T inversely depends on rp. This means that water transition temperature inside the pores gradually approaches its bulk temperature with increasing rp. One can calculate other porosity parameters, including pore volume (Vp) and internal surface area (Sp) from the pore size distribution (PSD) curve, i.e. differential pore volume verses pore radius. In order to transform a thermo- Fig. 1. Schematic of the laboratory wet spinning apparatus. Here, 1 is the dope graph of a differential scanning calorimeter (DSC) to PSD curve, Eq. vessel, 2 is the filter pack (10 ␮m filter mesh), 3 is nitrogen gas tank, 4 is fine gear (2) is employed [20]: pump that feeds the dope into the coagulation bath 5 through spinneret 6. (200 orifices, L/D = 1.2, 50 ␮m diameter). The nascent fibers are collected by the take-up dV dQ dt d(T) 1 roller 7 and washed with deionized water. p = (2) drp dt d(T) drp mHf (T)(T) where dQ/dt is the heat flow, dt/d(T) is the reverse of the heating [29]. DMSO was purchased from Merck (Germany) and employed to dissolve PAN. Deionized water was used wherever necessary. rate, and m is the mass of a dry porous sample. Also, Hf(T) and (T) are temperature-dependent heat of fusion and ice density, respectively. Parameter d(T)/drp is derived from Eq. (1). Hf(T) and (T) 2.2. Wet spinning functions are described by Eqs. (3) [26] and (4) [20]: Schematic of laboratory wet spinning apparatus is shown in =− − 2 − − Hf (T) 0.155(T T0) 11.39(T T0) Fig. 1. The PAN powder and DMSO are mixed in dope vessel (1) and − 332 for 0 > T > −25 ◦C (3) stirred until a clear and homogenous polymer solution is prepared. Then the dope is driven to the filter pack (2) (10 ␮m filter mesh) by nitrogen gas (3). A fine gear pump (4) feeds the dope into the coagulation bath (5) through spinneret (6) (200 orifices, L/D = 1.2, 50 ␮m (T) = 0.917(1.032 − 0.000117T) (4) diameter). Finally, the nascent fibers are collected by take-up roller There are a few studies dealing with applications of TPM method (7) and washed with enough deionized water for complete solvent to determine PAN films and fibers porosity [27,28] and this paper extraction. Water inside the pores acts as the probe liquid respon- investigated how TPM can be employed to characterize meso- sible for the samples porosimetry according to the TPM method, porosity in the wet-spun PAN fibers. thus the samples were kept in fresh deionized water to avoid water In this paper, PAN fibers were spun in the system of water and evaporation. dimethyl sulfoxide (DMSO) at two coagulation bath temperatures Two-step spinning process was performed. The following spin- of 5 ◦C (PAN1) and 60 ◦C (PAN2) through wet spinning and their ning parameters were used: 20% dope concentration (PAN %), morphology was investigated by scanning electron microscopy 2.07 cm2/min gear pump throughput, 65:35 DMSO:water bath con- (SEM) and TPM. centration, 2.1 m/min take-up roller velocity, and 0.39 jet draw ratio. The main difference between the two processes is the coag- ◦ 2. Experimental results ulation bath temperature. PAN1 and PAN2 are the products of 5 C and 60 ◦C bath temperatures, respectively. The apparent jet draw 2.1. Materials ratio was taken as the ratio of linear velocity of the take-up roller (V2) to the linear velocity of the jet (V1), i.e. V2/V1. PAN copolymer (acrylonitrile:methyl acrylate:sodium 2- methyl-2 acrylamidopropane sulfonate = 94:5.6:0.4) in the form 2.3. Thermoporometry of powder was supplied by Polyacryl Co. (Isfahan, Iran) with the viscosity average molecular weight of 104,000 g/mol, which was Differential scanning calorimeter (Mettler Toledo DSC823, obtained using an Ubbelohde viscometer and Mark-Houwink Switzerland) was employed for thermal analysis, calibrated with equation. The Mark–Houwink K and ˛ parameters for N,N- deionized water. Nascent fibers were picked up from deionized dimethylformamide at 25 ◦C are 0.0243 ml/g and 0.75, respectively water and cut carefully to 2 mm approximate length. TPM in melt- Author's personal copy

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Fig. 2. SEM images of wet-spun PAN1 (5 ◦C bath) fiber cross-section. Fig. 3. SEM images of wet-spun PAN2 (60 ◦C bath) fiber cross-section. ing mode was executed, i.e. the melting thermograph was recorded. coagulated fibers deviates from circular. This happens in the 5 ◦C About 2–5 mg of water-saturated wet-spun fibers were placed into coagulation bath. In this bath, J2 > J1, and the coagulating fibers are DSC pan and immediately cooled to −60 ◦C, and held for 15 min consequently collapsed and non-circular cross-section fibers are at this temperature to ensure that water inside all pores froze. formed. The exact shape of the fibers cross-section also depends on Heating was performed from −60 ◦C to 5◦ C with 0.1 ◦C/min heat- the coagulation bath concentration. In this work the coagulation ing rate. These heating tests were repeated three times to evaluate bath concentration was 65% DMSO. In the work of Ismail et al. [30] reproducibility. both cold and harsh water bath was employed to make dry-jet- wet-spun PAN fibers, which resulted in the dog bone cross-section 2.4. Scanning electron microscopy shape. Since circular cross-section is the best for heat treatment in order to produce CFs, and PAN fibers have better properties at low The morphology of nascent fibers was investigated by SEM coagulation temperatures, there are still many attempts to produce (Philips XL30). Samples were picked up from deionized water and circular shape PAN fibers in cold coagulation baths. ◦ dried at 70 C in an oven for several hours. Then, a string of fibers The dominance of J2 flow over J1 in cold bath is better explained was smeared with Epoxy resin mixed with hardener and left to by the non-solvent molecules mobility. As a general idea, at lower be solidified in 50 ◦C oven for several hours. Finally, prepared temperatures the mobility of non-solvent molecules decreases, composite was fractured in liquid nitrogen to produce smooth thus J1 vector weakens and this condition consequently causes J2 to cross-sections. take over J1. Thus tension polarization on the surface is responsible for the fiber geometry. However, in warm bath, J1 and J2 vectors 3. Results and discussion are in balance and there is no tension polarization. This explana- tion is convincing because of a good interaction between water and 3.1. Morphology of nascent PAN fibers DMSO. These arguments are schematically demonstrated in Fig. 4. By examining SEM images more carefully, it was found that SEM cross-sections of wet-spun PAN1 and PAN2 fibers are the morphology of the wet-spun PAN fibers is free of macrovoids shown in Figs. 2 and 3, respectively. Clearly, cross-section shape (MVs) for both PAN1 and PAN2. MVs are large cavities about changed from bean to circular when coagulation bath tempera- 10–50 ␮m in diameter and their formation during coagulation ture increased, which is in good agreement with literature results process, especially in ternary systems, including non-solvent, sol- [1,4,8,10]. This behavior is explained by the solvent and non-solvent vent and polymer, is an undesirable phenomenon that weakens counter-diffusion. During coagulation process there are two oppo- mechanical properties of membranes and fibers [12,31]. Rapid site flows, one is the diffusion of the non-solvent into the polymer exchange of the non-solvent/solvent, i.e. fast precipitation facil- solution (J1) and the other is the diffusion of the solvent (J2) itates the formation of MVs proposed by Kim et al. [32]. It is out of the solution. When J1 =/ J2 the cross-section shape of the interesting to note that although coagulation rate increases with Author's personal copy

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Fiber cross section 0 Warm J2 Bath J 1 -0.4

J Cold 2 -0.8 Bath

J W/g Flow, Heat 1 5 oC Bath Fig. 4. Counter-diffusion of non-solvent/solvent in cold and warm baths. -1.2 60 oC Bath bath temperature, there are no MVs in the wet-spun PAN2 fibers. -2 0 2 Here high DMSO content of coagulation bath lowers the diffusion Temperature, ºC of non-solvent molecules into the coagulating fiber and balances J1 and J vectors. 2 Fig. 6. DSC curves of water in warm and cold baths nascent fibers cooled to −60 ◦C Cross-section SEM images of wet-spun PAN1 and PAN2 fibers and heated by 0.1 ◦C/min. taken at higher magnification (Figs. 2b and 3b) show that the size and distribution of the pores are completely different for 5 ◦C and 60 ◦C baths. It is found that pore size increases with coagulation By induction of phase separation through the non-solvent addi- bath temperature, but pore distribution decreases. For PAN2 fibers tion (NIPS), thermodynamic instability occurs in the system and there are sparse pores with the size of about 150–300 nm. How- separates it into polymer rich phase (fiber matrix) and polymer ever, for PAN1 fibers there is a widespread distribution of the pores lean phase (fiber pores). Polymer concentrations in initial solution with sizes between a few nanometers and 150 nm. This difference (Cp) and at the critical point (Cc) determine the path through which is explained by the coagulation mechanism, which is a compli- coagulation occurs. When Cp =/ Cc nucleation and growth (NG) of cated process, especially for fibers in wet spinning. Although not polymer rich phase (Cp < Cc) or NG of polymer lean phase (Cp > Cc) considered in this paper, rheological phenomena contribute to the happens [33]. The NG mechanism is responsible for the formation morphology of wet-spun fibers in addition to kinetic and thermo- of spherical pores (spongy structure). According to Dong et al. [2] dynamic effects. studies of water/DMSO/PAN system phase behavior, the Cc value is A tool to study phase behavior and coagulation mechanism of a about 5%. Since the Cp value in our system is 20%, NG of polymer ternary system is a phase diagram which shows non-solvent, sol- lean phase undoubtedly happens. Spherical pore shape of the wet vent and polymer compositions at given pressure and temperature spun fibers (Figs. 2b and 3b) confirms the NG mechanism. How- (Fig. 5). The phase diagram has three areas, including one phase ever, the difference between size and distribution of the pores in area (I), meta-stable area (II) and two phase area (III). One phase the wet-spun fibers is still questionable. and meta-stable areas are separated by binodal or liquid–liquid When NG is dominant, the phase separation path goes through phase boundary. Also spinodal curve severs meta-stable and two the meta-stable area demonstrated in Fig. 5 by vectors 1 and 2. In phase areas from each other. The intercept of binodal and spinodal cold bath coagulation is slow or delayed, i.e. coagulation traverses a curves is called the critical point from which the phase separation longer distance in the meta-stable area. This condition increases the starts. One can obtain binodal curve through titration of polymer nucleation density. Vector 1 in the phase diagram pertains to PAN1 solution by adding non-solvent (cloud point experiment) or calcu- phase separation path. Also, as discussed earlier, when J2 > J1, poly- lating it through thermodynamic models of polymer solutions, e.g. mer concentration in the polymer rich phase gradually increases. Flory–Huggins model. Spinodal curve can only be calculated from This situation fixes the coagulating matrix and terminates pores thermodynamic models [12]. growth. Vice versa in warm bath, phase separation path crosses the meta-stable area rapidly (instantaneous de-mixing). Then nuclei of the polymer lean phase have little time for growth and nucleation Polymer density is smaller than in the cold bath. However, due to the non- solvent penetration into the coagulating fibers, polymer lean phase can grow and the pore size increases. Vector 2 in Fig. 5 shows the Binodal coagulation path for PAN2. II Spinodal 3.2. Meso-porosity in nascent PAN fibers

1 DSC melting curves for water in nascent PAN fibers are displayed Critical 2 in Fig. 6. There are two endothermic peaks for each curve. The left III peak pertains to melting of ice inside the meso-pores while the Point remaining ice outside of meso-pores melts at normal transition I temperature of water recognized by the right peak. The peaks are completely apart, which confirms that heating was done close to Solvent Non Solvent equilibrium conditions. For PAN1 fibers, pore water had a weaker melting peak than for PAN2 fibers. Since the area of pore water peak Fig. 5. Delayed de-mixing in the cold bath (vector 1) and instantaneous de-mixing in the warm bath (vector 2) during PAN fibers formation; non-solvent: water, solvent: pertains to Vp, it is expected that coagulated fibers in the warm bath dimethyl sulfoxide and polymer: polyacrylonitrile. have higher Vp than coagulated fibers in the cold bath. Author's personal copy

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Table 1 0.12 Calorimetric data of pore water peaks and related pore sizes.

PAN1, 5 ◦C bath PAN2, 60 ◦C bath ◦ − − Onset temp. ( C), pore size (nm) 1.36 (19.05 nm) 1.78 (22.52 nm) /g/nm o 3 5 C Bath Peak temp. (◦C), pore size (nm) −0.6 (33.01 nm) −0.79 (66.66 nm) ◦ Final temp. ( C), pore size (nm) −0.43 (39.63 nm) −0.63 (98.64 nm) 0.08

o 60 C Bath Water behavior in tiny nano-pores differs from the bulk. In fact, equilibrium temperature for a solid/liquid transition is determined 0.04 by the radius of curvature of the interface between the solid and the liquid phases. In the bulk state water surface is planar and it melts/freezes at around 273.15 K. However, when water is confined in a nanometer pore, its surface is no longer planar and has curvature with high surface-to-volume ratio. Thus water phase Differential Pore Volume, cm 0 transition temperature shifts below 273.15 K. It is important to note that the heating rate in a TPM experiment is the key parameter 10 40 70 100 and should be set correctly. It means that the low heating rate is Pore Radius, nm essential to keep the test conditions close to equilibrium. At high Fig. 7. PSD curves of warm and cold baths wet-spun PAN fibers determined by TPM. heating rates pore water and bulk water peaks approach each other and overlap, i.e. melting of ice in very small pores is not captured. 3 This situation makes TPM calculations inaccurate and sometimes By integrating the PSD curves, Vp is determined to be 1.06 cm /g impossible. Arbab et al. [28] used the heating rate of 1 ◦C/min in for PAN1 fibers and 1.46 cm3/g for PAN2 fibers. By comparing the their TPM porosimetry studies. The overlapped pore and bulk water Vp values of PAN1 and PAN2 fibers, it is recognized that the pore endothermic peaks showed that this heating rate was chosen to volume values are close to each other as explained earlier. If it some extent high. Also in the work of Price and Bashir [27], the is assumed that the pores are true cylinders with circular cross- ◦ heating rate of 4 C/min seems to had the same problem. The effect section, then internal surface area is calculated through Sp =2Vp/rp. 2 2 of heating rate on TPM results was also discussed by Zeman et al. Therefore, Sp for PAN1 is calculated at 64.24 m /g verses 43.77 m /g [34] and Landry [20]. for PAN2 fibers. Since both fibers have approximately similar values Table 1 shows the onset, peak and final temperatures of pore of the pore volume and rp for PAN2 fibers is about twice of PAN1, water endothermic peaks for PAN1 and PAN2 fibers. Numbers in PAN1 have more internal surface area than PAN2. parentheses demonstrate the correlated pore size calculated using Eq. (1). To obtain T, the peak temperature of the right bulk peak 4. Conclusions was considered to be 0.4 ◦C for PAN1 and −0.3 ◦C for PAN2. There- fore, the rp for PAN1 is about 33 nm with 19–40 nm range. For PAN2 In this work polyacrylonitrile (PAN) fibers were spun at two ◦ ◦ fibers, rp is about 67 nm with 23–99 nm range. It is clear that rp coagulation bath temperatures of 5 C and 60 C and the mor- increases with coagulation bath temperature. Also PSD becomes phology of nascent fibers was investigated by scanning electron better distributed with increasing coagulation bath temperature. microscopy and calorimetric porosimetry method of thermo- The difference between the melting temperatures of bulk water porometry. SEM images showed that by increasing the coagulation arises from the fact that the weight ratio of water to dry porous bath temperature, the shape of the fiber cross-section changes from sample affects the area and location of bulk water melting peak bean to circular with larger pore size. These observations were demonstrated by Landry [20]. In fact bulk water melting peak area explained by the counter-diffusion of non-solvent/solvent and by increases and the peak shifts slightly to a higher temperatures as the phase behavior of water/PAN/DMSO system. Meso-porosity of water to dry sample weight ratio increases. In the same way, pore PAN fibers was investigated through TPM and porosity parame- water melting peak is moved to higher temperatures, therefore, ters including average pore size (rp), pore volume (Vp) and internal temperature depression (T) remains constant. surface area (Sp) were calculated. Results showed that rp and Vp PSD curves for wet-spun PAN fibers are displayed in Fig. 7, calcu- increase with coagulation bath temperature. Wet-spun fibers at lated through equations (1)–(4). It is observed that although PAN1 low coagulation temperature have higher surface area Sp than wet- have smaller pores than PAN2, their volume is higher, signified by a spun fibers at high coagulation temperature. large peak area. This finding is in good agreement with SEM obser- This study also showed that TPM method can be employed to vations. From Figs. 2b and 3b, PAN1 fibers have smaller pores than characterize meso-porosity of wet-spun PAN fibers which affects PAN2 but the pores are distributed around the fiber cross-section. the properties of final carbon fibers. The main feature of the method Therefore, it is expected for PAN1 fibers to have more internal sur- is its ability to detect closed pores, inaccessible by other standard face area than for PAN2. porosimetry methods. By comparing the results of TPM and SEM, it is found that pore size distribution of PAN1 is between 19 and 40 nm according to TPM Acknowledgements method and changes from a few nanometers to 150 nm from SEM. Also, PAN2 showed a distribution of 23–99 nm via TPM technique The authors are thankful to Mr. Rezaei from Tarbiat Modares and 150–300 nm according to SEM images. This comparison shows University for SEM work. Alex A. Volinsky would like to acknowl- that the pore size distribution from two methods for each sample edge support from the National Science Foundation. is a bit different; however, the trend, i.e. the increase in pore size with coagulation temperature is in agreement. This difference was References expected because TPM only detects meso-pores (2–50 nm) while SEM visualizes both meso-pores and especially macro-pores. In [1] R. Dong, M. Keuser, X. Zeng, J. Zhao, Y. Zhang, C. Wu, D. Pan, Viscometric other words, each method works in its respective size range. measurement of the thermodynamics of PAN terpolymer/DMSO/water system Author's personal copy

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