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Experimental and Theoretical Investigations of PAN Molecular Weight Increase in Precipitation Polymerization as a Function of H<sub>2</sub>O/DMSO Ratio
Experimental and Theoretical Investigations of PAN Molecular Weight Increase in Precipitation Polymerization as a Function of H2O/DMSO Ratio
Carbon letters. 2010. Mar, 11(1): 22-27
Copyright ©2010, Korean Carbon Society
  • Received : February 02, 2010
  • Accepted : March 03, 2010
  • Published : March 30, 2010
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About the Authors
Jing Zhang
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing, 100029, China
Fengjing Bu
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing, 100029, China
Yongqiang Dai
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing, 100029, China
Liwei Xue
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing, 100029, China
Zhixian Xu
Dept. of Chemistry and Chemical Engineering, Anyang Normal University,Henan, 455000, China
Seung Kon Ryu
Dept. of Chemical Engineering, Chungnam National University, Daejeon 305-764, Korea
Riguang Jin
State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing, 100029, China
jin.riguang@163.com, skryu@cnu.ac.kr
Abstract
The precipitation polymerization of acrylonitrile (AN) was carried out in a mixture solution of dimethyl sulfoxide(DMSO) and water at 50~65 o C using α,α’-azobisisobutyronitrile (AIBN) as an initiator. The increased molecular weight polyacrylonitrile (PAN) was prepared with increasing the H 2 O/DMSO ratio from 10/90 to 80/20. The viscosity average molecular weight of H 2 O/DMSO solvent was 4.4 times larger than that of H 2 O/DMF solvent, and precipitation polymerization was accelerlated due to the far decreased chain transfer effect of DMSO. Based on the experimental results,the increased PAN molecular weight was regarded as the summation of two mechanisms: i) particle-particle aggregation and ii) particle-radical attachment. The theoretical equation derived from the mechanisms was well coincided with the experimental results showing the linear relationship between the viscosity average molecular weight and the H 2 O/DMSO ratio.
Keywords
1. Introduction
As a type of high-performance inorganic fiber, carbon fiber has lots of good superiorities such as specific strength, high modulus, lightfastness, corrosion resistance, fatigue, creep resistance, electrical conductivity, excellent heat transfer performance, and etc. [1 , 2] . When temperature is as high as 2000 o C, carbon fiber is the only material that the strength does not fall in an inert environment. It is well known that the properties of high performance polyacrylonitrile (PAN) based carbon fiber are determined strongly by the nature of precursor, and preparing high molecular weight polymer is one of the effective ways to obtain high strength fiber [3 - 6] .In general, a second monomer such as acrylic ester and a third monomer such as itaconic acid were added to monomer in order to increase the molecular weight of PAN in solution polymerization.
Recently, precipitation polymerization is increasing the interests in the preparation of high molecular weight PAN due to the advantage of small chain transfer reaction than solution polymerization in using a second monomer [7 - 10] .Bahrami [11] reported the effect of H 2 O/dimethyl formamide(DMF) on the conversion rate and intrinsic viscosity during precipitation polymerization using acrylonitrile and acrylic acid as monomers and H 2 O and DMF as mixed solvent system. Wang [12] studied the effect of monomer concentration,initiator concentration, ratio of H 2 O/dimethyl sulfoxide (DMSO) on the precipitation polymerization, and suggested that precipitation polymerization was more effective than solution polymerization in preparation of high performance carbon fibers. Zhang [13] also studied the effect of precipitation polymerization conditions on the conversion rate using H 2 O/DMSO mixture solution.Wang [14] reported the relationship between apparent activation energy and the ratio of H 2 O/DMSO. However,there are few studies on the relationship between molecular weight increase of PAN and H 2 O/DMSO ratio, especially relating to the theoretical mechanism of PAN molecular weight increase.
Therefore, the purpose of this study is experimental and theoretical investigations of PAN molecular weight increase in precipitation polymerization as functions of H 2 O/DMSO ratio and intrinsic viscosity. To obtain the purpose, i)precipitation polymerization of acrylonitrile was carried out by changing the solvent ratio of H 2 O/DMSO at different temperature, ii) molecular weight and intrinsic viscosity of the obtained polymer were measured in DMF solution, and iii) model mechanism of precipitation polymerization was suggested in relation between molecular weight increase of PAN and H 2 O/DMSO ratio.
2. Experimental
- 2.1. Materials
Acrylonitrile (AN, Beijing Chemical Reagents Co.) was freed from inhibitors by distillation at 76~78 o C before polymerization. α,α’-azobisisobutyronitrile (AIBN) was purified by recrystallization. Dimethyl sulfoxide (DMSO)and deionized water (H 2 O) were used as polymerization solvents. Dimethyl formamide (DMF) solution was used to compare the intrinsic viscosity of the polymer with that of DMSO solution.
- 2.2. Precipitation polymerization
Precipitation polymerization of AN was carried out in a 1.0 L three-necked glass reactor by adding 0.3 L different H 2 O/DMSO ratio from 10/90 to 80/20 (wt/wt) as solvent to 0.1 L AN for 2 h at 50, 55, 60 and 65 o C under pure nitrogen atmosphere. After polymerization, enough distilled water was added into the reactor to precipitate the produced polymer. The precipitated polymer was washed with distilled water and ethanol successively, filtered with Buchner funnel,and dried under vacuum at 40 o C till a constant weight was obtained.
- 2.3. Intrinsic viscosity measurement
Intrinsic viscosity [η] of the obtained polymers was measured in DMF solution using an Ubbelohde viscometer in a water bath at constant temperature of 35 ± 0.1 o C following the Kashyap’s method [15] . The viscosity average molecular weight M η was calculated from the following equation [16] :
Lager Image
3. Results and Discussion
- 3.1. Experimental study
Fig. 1 shows the SEM images of PAN obtained from H 2 O/DMSO ratio of 10/90 and 80/20 in precipitation polymerization at 55 o C for 2 h. The morphology of PAN obtained from small ratio of H 2 O (10/90) shows loosely aggregated large particles. While that of obtained from 80/20 shows big spheres composed of tightly aggregated small particles. These images suggested that there might be immense number of micro-droplet monomers in 80/20 ratio solvent which was divided by H 2 O at the beginning of precipitation polymerization. And these immense micro-droplet monomers proceeded the precipitation polymerization, then generated plenty of precipitation particles, finally resulted in the
Lager Image
SEM images of PAN obtained from H2O/DMSO ratio of(a) 10/90 and (b) 80/20 in precipitation polymerization at 55oCfor 2 h.
Lager Image
Relationship between H2O/DMSO ratio and viscosityaverage molecular weights of PAN.
increase of intrinsic viscosity and viscosity average molecular weight. The size and shapes of aggregation were depended on the H 2 O/DMSO ratio, and higher molecular weight PAN precursor can be obtained from increasing the H 2 O/DMSO ratio.
Fig. 2 shows the viscosity average molecular weights of PAN as a function of H 2 O/DMSO ratio obtained at 50, 55,60 and 65 o C. The viscosity molecular weight linearly increased as the increase of H 2 O/DMSO ratio from 10/90 to 80/20. This was because the number of micro-droplet monomer increased as the increase of H 2 O/DMSO ratio,and the increased micro-droplet were aggregated until they became big precipitation particles, resulted in the molecular weight increase, which made the solution more turbid and viscous. Wang [12] and Zhang [13] have reported the similar results on the effect of H 2 O/DMSO ratio. On the other hand, the viscosity average molecular weight decreased as the increase of reaction temperature at the fixed ratio of H 2 O/DMSO. This was believed high
Lager Image
Viscosity average molecular weights of PAN from H2O/DMSO and H2O/DMF solvent in precipitation polymerization at60oC.
reaction temperature promoted the suspension of microdroplet rather than particles aggregation, resulted in the decrease of viscosity.
Fig. 3 shows the viscosity average molecular weights of the PAN as a function of H 2 O/DMSO and H 2 O/DMF ratio obtained from precipitation polymerization at 60 o C. The viscosity average molecular weight of H2O/DMSO solvent was 4.4 times larger than that of H 2 O/DMF solvent. Bahrami [11] reported very similar data to those of our H 2 O/DMF shown in Fig. 3 . The higher value of H 2 O/DMSO system was due to the decreased chain transfer effect of the DMSO.The chain transfer constant of water is almost zero and that of DMSO is much less than DMF [10] . Therefore, H 2 O/DMSO solvent system is far better than H 2 O/DMF system for the precipitation polymerization to obtain increased molecular weight of PAN.
Lager Image
Aggregation of nascent PAN particles in precipitation polymerization.
- 3.2. Theoretical study
Based on the experimental results, summation of two mechanisms, particle-particle aggregation and particleradical attachment can be suggested in the formation of the increased molecular weight PAN.
Mechanism I was regarded as a particle-particle aggregation.In the precipitation polymerization of acrylonitrile, the obtained polymer was not dissolved in the H 2 O/DMSO mixture solution. Once it was generated, it became nascent particle and suspended in the solution. As the core, the nascent particle aggregated with other particles until it became a larger particle in the precipitation polymerization.This process can be regarded as n-dimensional aggregation as shown in Fig. 4 .
The number of micro-droplet monomer increased as the increase of H 2 O/DMSO ratio due to the division of droplet by H 2 O, which made the solution more turbid and viscous. From this consideration, the following equation can be proposed.
Lager Image
Where, [P] is the concentration of the nascent precipitation particle, and following hypothesis can be suggested. F (P) is the number of aggregated monomers in multi-dimensional direction in precipitation polymerization. When the nascent particle aggregated with other one particle, F (P) will be proportional to [P] showing the following equation.
Lager Image
Where, F (P) is the number of aggregated monomers in multi-dimensional direction in precipitation polymerization,
k ij : aggregation rate constant,
i : dimension,
j : serial number of particles.
When the nascent precipitation particle aggregated with other two particles, F (P) will be proportional to the square of [P], and the aggregation rate constant of the second particle will be half of the first particle in the contribution of aggregation. Therefore, the F (P) 2 can be expressed as the following equation.
Lager Image
When the nascent precipitation particle aggregated with other three particles, F (P) will be proportional to the cube of [P], and the aggregation rate constant of the third particle will be one-third of the first particle in contribution of aggregation.
Lager Image
When the nascent precipitation particle aggregated with other n particles, F (P) will be proportional to [P] n , and the aggregation rate constant of the n th particle will be one-n th of the first particle in contribution of aggregation.
Lager Image
If there was no particle to aggregate, F(P) will be proportional to [P] 0 , and expressed as the following equation.
Lager Image
Therefore, all the aggregated nascent particles will be the summation of F (P) 1 + F (P) 2 ----- + F (P) n , and the relationship between F(P) and [P] can be obtained as follows.
Lager Image
If each nascent precipitation particle has same ability for precipitation and kij is the function of temperature, it can be regarded to k ij =k ji =k 00 =k 11 = k nn , and the equation (8)can be simplified as follows.
Lager Image
The equation is exactly part of the Taylor series expansion,so the equation (9) can be changed as the following form:
Lager Image
Therefore, the total aggregated number of nascent precipitation particles is
Lager Image
Where Q is the quantity of precipitates and K 1 is the
Lager Image
Attachment of particles to PAN free radicals in precipitationpolymerization.
overall aggregation rate constant.
Mechanism II was regarded as particle-radical attachment. When the weight ratio of H 2 O/DMSO was small, the number of nascent precipitation particle will be limited. The propagation radical ( M •) can mainly move in a large range in DMSO solvent, resulted in the reaction easily terminate and viscosity average molecular weight(M η ) small. When the ratio of H 2 O/DMSO increased, the number of nascent precipitation particles also rapidly increased. In this process, each propagation radical will be sited on everywhere of nascent precipitation particles.These propagation radicals only react in DMSO, and the propagation radicals may not easily move with the decrease of DMSO. Therefore, the termination opportunity of adjacent two propagation radicals will be reduced. In this case, propagation radical will have much more opportunity to react and form lager molecule. Finally,molecular weight will be increased constantly as the increase of H 2 O/DMSO ratio as shown in Fig. 5 .
Based on the physical concept above, it can be put forward a model similar to the previous. When the nascent particle attached to one other particle, F ( M •) will be proportional to [M•] and expressed as the following equation.
Lager Image
Where, F (M• ): number of attachment monomer in multidimensional direction in precipitation polymerization,
k 11 : attachment rate constant,
i : dimension,
j : serials number of particles.
When the nascent particle reacted with two other particles, F ( M •) will be proportional to the square of [M•], and the attachment rate constant of the second particle will be half of the first particle in the contribution of attachment.
Lager Image
When the nascent particle attached to three other particles, F(M •) will be proportional to the cube of [M•] , and the attachment rate constant of the third particle will be onethird of the first particle in contribution of attachment.
Lager Image
When the nascent monomer particle attached to n other particles, F(M •) will be proportional to [M&•] n , and the attachment rate constant of the n th particle will be one-n th of the first particle in contribution of attachment.
Lager Image
If there is no monomer particle for attachment, F(M •) will be proportional to [M•] 0 .
Lager Image
Therefore, all the attached nascent monomer particles will be the summation of F(M•) 1 + F(M •) 2 +…+ F(M •) n and the relationship between F(M •) and [M•] can be obtained as follows.
Lager Image
If each first born particle has same ability for precipitation and k ij is the function of temperature, it can be regarded as k ij = k ji = k 00 = k 11 =… k nn , and the equation (17) can be simplified as follow as :
Lager Image
The equation (18) was exactly part of the Taylor series expansion, so it can be changed as the following form:
Lager Image
The total attached monomer of nascent precipitation particles are
Lager Image
Where Q is the quantity of precipitates and K 1 is the overall attachment rate constant.Therefore, from (11) and (20):
Lager Image
Taking logarithm on both sides of the equation:
Lager Image
The viscosity average molecular weight can be calculated by the summation of (2) and (22), and the final equation can be suggested as follows.
Lager Image
Where K 3 is inversely proportional to K 2 and K 4 = K 1 /K 2
From (23), it can be found that there is a linear relationship between the viscosity average molecular weight and the H 2 O/DMSO ratio. Therefore, the theoretical equation derived from the mechanism was well coincided with the experimental results showing the linear relationship between the viscosity average molecular weight and the H 2 O/DMSO ratio.
4. Conclusions
In the precipitation polymerization PAN molecular weight increase was depended on the increase of H 2 O/DMSO ratio due to the increased micro-droplets, which made the solution more turbid and viscous, resulted in the molecular weight increase. Using H 2 O/DMSO solvent system was far better than using H 2 O/DMF system in precipitation polymerization and obtained increased molecular weight PAN precursor because of decreased chain transfer effect of DMSO. Based on the experimental results, the increased molecular weight of PAN was regarded as the summation of two mechanisms: particle-particle aggregation and particleradical attachment. The theoretical equation derived from the mechanisms can be used to estimate the viscosity average molecular weight of the newly polymerized polyacrylonitrile (PAN) precursor.
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