AA_AMPS水凝胶的交联聚合反应动力学及其机理_英文_

1、CATALYSIS, KINETICS AND REACTORSChinese Journal of Chemical Engineering, 19(2 285291 (2011Crosslink Polymerization Kinetics and Mechanism of HydrogelsComposed of Acrylic Acid and 2-Acrylamido-2-methylpropaneSulfonic Acid*LIAO Liewen (廖列文1,*, YUE Hangbo (岳航勃2,* and CUI Yingde (崔英德1,21 Institute of Gr

2、een Chemical Engineering, Zhongkai University of Agriculture and Engineering, Guangzhou 510225,China2 School of Materials Science and Engineering, Northwestern Polytechnical University, Xian 710072, ChinaAbstract Crosslink polymerization kinetics of poly(acrylic acid-co-2-acrylamido-2-methylpropane

3、sulfonic acid, AA/AMPS hydrogels, was investigated by using dilatometry in the presence of sodium persulfate as initiator and N,N-methylene bis(acrylamide as crosslinker. It was found that the reaction for the crosslink polymerization of AA/AMPS hydrogels had orders of 0.58, 1.14, and 0.86 with resp

4、ect to the initiator, AMPS, and AA, respectively.From the Arrhenius plots, the activation energy of the crosslink polymerization was found to be about 140 and 89 kJ·mol1 in the presence and absence of the crosslinker, respectively, in the temperature range from 45 to 65 °C. It was noted th

5、at the crosslinker had effects on the reaction order of the initiator and the activation energy due to the formation of cross-linked networks, which was verified by Fourier transfer infrared (FTIR spectrum. To further confirm the influences of the cross-linked network structure on kinetic parameters

6、 of the crosslink polymerization, a mechanism was proposed, which highlights the different termination routes between free radical polymerization and crosslink polymerization. These results suggest that dilatometry provides a convenient tool for crosslink polymeri-zation study, and confirm that the

7、cross-linked networks are formed in the crosslink polymerization.Keywords hydrogel, polymerization, cross-linked networks, kinetics, dilatometry1 INTRODUCTIONPolymerization kinetics, which focuses on the relationships among many factors such as polymeriza-tion rate, molecular mass, initiator concent

8、ration, mono-mer concentration and temperature, is a great concern 1, 2. The research methods for the polymerization kinetics reported in literature mainly include nuclear magnetic resonance (NMR 3, 4, UV-vis 5, Fou-rier-transform near infrared 6, photo-DSC (differen-tial scanning calorimetry/DSC 7-

9、9, viscometry 10, 11, pulsed laser techniques 12, 13, and dilatometry 11, 14.Hydrogel, three-dimensional cross-linked poly-meric networks, can swell or collapse reversibly in response to environmental variables such as pH, tem-perature, ionic strength and electric field. It is exten-sively used in a

10、 variety of applications from the area of artificial sensors to the biomedical fields 15-18. Solu-tion free radical polymerization is the most commonly used technique to prepare hydrogels 19, 20. However, little consideration is given to crosslink polymeriza-tion due to the nature of networks format

11、ion 21.The dilatometry provides a simple and effective method to determine the volumetric polymerization shrinkage kinetics 22, 23. The volume of the polym-erization mixture decreases with time during the po-lymerization, since monomer has lower density while polymer has higher density. Moreover, th

12、ere is a close relationship between the volume change of the po-lymerization mixture and the monomer conversion rate 24, 25. The aim of this study is to obtain the kinetic equation for crosslink polymerization of poly(acrylic acid-co-2-acrylamido-2-methylpropane sulfonic acid (AA/AMPS hydrogel by di

13、latometry and to investi-gate the influences of cross-linked networks on the kinetic parameters. The mechanism of crosslink po-lymerization is also proposed in comparison with free radical polymerization.2 EXPERIMENTAL2.1 Materials and apparatus2-Acrylamido-2-methylpropane sulfonic acid (AMPS and ac

14、rylic acid (AA from Guangzhou Shuang-Jian Trading Ltd., China, were used as received. Initiator sodium persulfate and crosslinker N,N-methylene bis(acrylamide supplied by Tianjin Kermel Chemical Reagent Ltd., China, were used without further treatment.Dilatometer: consists of a glass cylinder and a

15、capillary (volume: 10 ml; diameter: 1 mm; Fourier transfer infrared (FTIR 2000 spectrometer (Pekin- Elmer, USA.2.2 Polymerization kinetics measurementsThe crosslinker and the initiator were added to the aqueous solution containing monomers AA and AMPS, and the volume of the mixture was adjusted to 2

16、0 ml by adding deionized water. Then, the mixture was well stirred to dissolve and transferred into a glassReceived 2010-08-16, accepted 2011-01-10.* Supported by the National Natural Science Foundation of China (20176007, 20376087.Chin. J. Chem. Eng., Vol. 19, No. 2, April 2011 286cylinder, which w

17、as connected with a capillary. En-trapping air in the capillary was excluded before thedilatometer was immersed in a constant temperaturebath. Liquid level in the capillary increased because ofthe thermal expansion. Then the liquid level stoppedrising and kept at an equilibrium state. When the liq-u

18、id level began to fall, volume changes in the capillarywere immediately monitored at each 5 min until theend of the experiments, i.e., the liquids level reachedthe bottom of the capillary. Experimental conditionsfor the crosslink polymerization kinetic study ofAA/AMPS hydrogels are summarized in Tab

19、le 1.Table 1 Summary of experimental conditions for thecrosslink polymerization kinetic study ofAA/AMPS hydrogelsCrosslinker/%AA/mol·L1AMPS/mol·L1Initiator/%T/°C0 5.60.60.5450.8 6.90.31.0501.21.5552.06065The amount of crosslinker or initiator is mass percent (% oftotal monomers mass.2

20、.3 Fourier transfer infrared (FTIR spectroscopy measurementsTo prepare dried gel samples for FTIR measure-ments, monomers AA and AMPS were dissolved in aqueous solution (20 ml, followed by addition of ini-tiator sodium persulfate and crosslinker N,N-methylenebis(acrylamide. The mixture was well stir

21、red and transferred into a plastic pipe (diameter: 6 mm; length:20 cm and put in a constant temperature water bath of50 °C for 2 h to produce AA/AMPS hydrogel. Afterthe hydrogel was synthesised, it was immersed into deionized water for three days, with changing the wa-ter twice a day to elimina

22、te unreacted monomers andlinear polymer chains. Finally, the swollen hydrogelwas put into a vacuum oven for 12 h and the dried gelwas ready for use in the spectrum test. FTIR spectrumwas obtained by using FTIR 2000 with KBr pellets containing dried AA/AMPS hydrogel. The scanningrange was 500-4000 cm

23、1 and the resolving capabilitywas 4 cm1.3 RESULTS AND DISCUSSION3.1 Kinetic equation of the crosslink polymerizationFor binary radical copolymerization, the kinetic equation is given byp dM/dIMx nR t k=(1 where R p is the polymerization rate, k is the polymeri-zation rate constant, I is the initiato

24、r concentration, M is the monomer concentration, x and n denote the reaction order of initiator and total reaction order, re-spectively. Here, the reaction order considered is the total reaction order, the reaction order of monomers, and the reaction order of initiator.Single factor analysis was app

25、lied to determine the reaction order of monomers AA and AMPS. When the initiator keeps at a constant concentration, Eq. (1 is simplified aspdM/dMnR t K=(2 where K is the apparent polymerization rate constant. The integral equation of Eq. (2 is(1(1lnM/M(1MM(1n nKt nKt n+=(3 where 1n=refers to the fir

26、st-order reaction while 2n= refers to the second-order reaction. The residual monomer concentration after reaction time t is calcu-lated by0aMM(1/V V=(4 where M0 is the initial monomer concentration, V is the volume change of the system after polymerization time t, V a is the volume change of the sy

27、stem during the whole polymerization. Replacing the monomer concentration M in Eq. (3 with Eq. (4, we obtain(a(1a(1ln1/(1(11/1MnnV V Kt nKt nV V+=+(5There is a relationship between the volume change of the polymerization mixture and the mono-mer conversion rate. This volume change can be ob-tained b

28、y using dilatometry and V value is closely related to the liquid level change in the capillary(0tV A h h A h=(6 where h0 and h t denote the liquid level height in the capillary at the beginning and at time t of the polym-erization, respectively, h is the height change of liq-uid level, and A is the

29、cross-sectional area of the cap-illary. Replacing V in Eq. (5 with Eq. (6, we haveaaln1(11111(2Mh AKt nVVnA h K t=+=(7V a value is often viewed as a constant as it is calcu-lated by(a0M P1/V V d d=(8 where d M and d P are the monomer and polymer densi-ties, respectively, at certain temperature, and

30、V0 is theChin. J. Chem. Eng., Vol. 19, No. 2, April 2011 287initial volume of the system. Equation (7 shows that the relationship between h and t gives the total reaction order of the polym-erization. For example, the linearity of 1/h versus 1/t indicates that the total reaction order is two (n =2.I

31、n our experiments the mixture containing dif-ferent concentrations of monomers AA and AMPS was applied to determine the total reaction order of the crosslink polymerization of AA/AMPS hydrogels and the results are illustrated in Fig. 1. Plots of 1/h ver-sus 1/t at different monomer concentration are

32、 linear with correlation coefficients higher than 0.99. Accord-ing to Eq. (7, the total reaction order of the crosslink polymerization of AA/AMPS hydrogel is two. Figure 1 Plots of 1/h vs . 1/t at T =55 °C (ini =1%, cross =0.8%, by mass C AA =5.6 mol·L 1, C AMPS =0.6 mol·L 1; C AA =6.

33、9mol·L 1, C AMPS =0.3 mol·L 1; C AA =5.6 mol·L 1, C AMPS = 1.2 mol·L 1The apparent polymerization rate constant K can be calculated from the line of best fit with Eq. (70M bK a = (9 where a and b are the slope and the intercept of the line of best fit for 1/h versus 1/t , respect

34、ively. Eq. (9 shows that the slope of K /K 0 versus M/M 0 repre-sents the reaction order of the monomer. Here single factor analysis is applied to determine the reaction order of monomers AA and AMPS, i .e ., the kinetic experiments were conducted by varying AMPS con-centration while fixing concentr

35、ations of other materi-als used in the polymerization.Figure 2 shows the relationships of the experi-mental and linear fitting results at different AMPS concentrations. The equations in Fig. 2 are from the line of best fit, from which the reaction order of AMPS is 1.14, the average slope of the line

36、 of best fit, (1.15 1.13/2y =+. In addition, the reaction order of AA is readily obtained by subtracting the reaction or-der of AMPS from the total reaction order, 21.14= 0.86. It suggests that AA has less effect on the crosslink polymerization rate than AMPS due to the higher re-activity of sulfoni

37、c group in AMPS than carboxylgroup in AA.(a C AA =5.6 mol·L 1(b C AA =6.9 mol·L 1Figure 2 Plots of K /K 0 vs . M/M 0 at T =55 °C(C AMPS =0.3, 0.6, 1.2 mol·L 1, ini =1%, cross =0.8%, by mass experimental result; linear fitting resultSingle factor analysis was also applied to deter

38、-mine the reaction order of initiator sodium persulfate. The kinetic experiments were conducted with different initiator concentrations in the presence and absence ofthe crosslinker. The results are shown in Fig. 3.Figure 3 Plots of lg(K /K 0 vs . lg(I/I 0 at T =55 °C(C AA =5.6 mol·L 1, C

39、AMPS =0.6 mol·L 1, ini =0.5%, 1%, 1.5%, 2%, by masscross /% (by mass: 0.8; 0 The slope of the line of best fit for lg(K /K 0 ver-sus lg (I/I 0 is the reaction order of the initiator. Ac-cordingly, the reaction order of initiator sodium per-sulfate are 0.58 and 0.5 in the presence and absence of

40、crosslinker N ,N -methylene bis(acrylamide, respectively.Chin. J. Chem. Eng., Vol. 19, No. 2, April 2011 288Obviously, the crosslinker concentration produces an effect on the reaction order of the initiator. This effect may be caused by the formation of cross-linked net-works within the hydogel, whi

41、ch will be verified by the FTIR spectrum of AA/AMPS hydrogel. FTIR spectroscopy was used for the characteriza-tion of AA/AMPS hydrogel. The hydrogel sample exhibits important absorption bands from FTIR spec-troscopy measurements as shown in Fig. 4. It is asso-ciated with stretching vibration of two

42、S O bonds in sulfonic group 26, at =1150 cm 1. Strong band from carbonyl group associated with carboxyl group is verified (at =1708 cm 1. Amide group contribution is observed with absorption ranging from =1550- 1650 cm 1. Characteristic alkyl (R CH 2 stretching modes from =2850-3000 cm 1 are observe

43、d 27. In summary, the characteristic bands of monomers AA and AMPS clearly indicate the occurrence of copoly-merization of AA and AMPS. An import absorption peak from NH stretching vibration is verified at a frequency of =3296 cm 1. This vibrational band is mostly attributed to crosslinker N ,N -met

44、hylene bis(acrylamide. FTIR spectrum in Fig. 4 confirms the formation of AA/AMPS hydrogel with cross-linked network structure. The crosslinker acts as reactive sites among polymer chains of AA and AMPS to induce the crosslink polymerization.3.2 Activation energy of the crosslink polymeri-zationThe a

45、ctivation energy of polymerization can becalculated by Arrhenius equation2a d ln /d /K t E RT = (10(21a 21lg /2.303(1/1/K K E R T T = (11 where E a is the activation energy, R is the molar gas constant, and T is the thermodynamic temperature.Before calculating the activation energy of crosslink poly

46、merization of AA/AMPS hydrogels, we evaluated the effects of temperature on the total reac-tion order of the crosslink polymerization. The kinetic experiments were conducted in the temperature range from 45 to 65 °C by dilatometry and the results areshown in Fig. 5. The linear relationship of 1

47、/h versus 1/t reveals that the total reaction order of AA/AMPS hydrogels is two as discussed previously.Figure 5 Plots of 1/h vs . 1/t at different temperatures(C AA =5.6 mol·L 1, C AMPS =0.6 mol·L 1, ini =1%, cross =0.8%, by mass T /°C: 45; 50; 55; 60; 65According to Eq. (11, the act

48、ivation energy can be calculated from the slope of the line of lgKversus 1/T . Arrhenius plots are drawn in Fig. 6, which show a good linear relationship. The activation energy of crosslink polymerization of AA/AMPS hydrogels isFigure 4 FTIR spectrum of AA/AMPS hydrogel(C AA =5.6 mol·L 1, C AMP

49、S =0.6 mol·L 1; ini =1%, cross =0.8%, by massChin. J. Chem. Eng., Vol. 19, No. 2, April 2011 289 calculated to be about 140 and 89 kJ·mol 1 in the presence and absence of crosslinker N ,N -methylene bis(acrylamide, respectively. It should be pointed out that the crosslinker has effects on

50、the activation energy. Since the molecular motion of monomer or initiator is impaired by the cross-linked networks (Fig. 7, the crosslink polymerization system needs more energy (higher activation energy to overcome these extra ob-stacles compared with the free radical polymerization. 3.3 Mechanism

51、of the crosslink polymerizationA mechanism is proposed for the crosslink po-lymerization in comparison with the free radical po-lymerization as illustrated in Fig. 7, to further analyze the effects of the crosslinker on the reaction order of the initiator and the activation energy (Figs. 3 and 6.At

52、termination step of the free radical polymeri-zation in Fig. 7 (a, the propagating radicals at the end of the polymer chain tend to free themselves from the restraint of polymer chains and collide each other to terminate. For the crosslink polymerization, however, the following two issues related to

53、 the termination step should be considered. On one hand, the chains of macromolecules are entangled to form the cross-linked networks, which will bring more obstacles to the ter-mination of propagating radicals Fig. 7 (b. On the other hand, the cross-linked networks will produce negative effects on

54、the initiation and propagation step Fig. 7 (b. In other words, the propagation with growth of chains to higher molecular mass polymer takes place very rapidly without the influence of the cross-linked networks. The free radicals and the propa-gating radicals may stop growing and terminating due to t

55、his negative effect. An effect found in many po-lymerizations is called the gel effect or Trommsdorff effect, which may cause an auto-acceleration of the reaction 28. The cross-linked networks increases the viscosity, which will decrease the termination rate since the bulky growing polymer radicals

56、cannot dif-fuse easily through the medium as seen in Fig. 7. Thus, the possibility for two polymer radicals to approach each other and participate in a termination process becomes less.In the classical free radical polymerization sys-tem, one may expect the polymerization rate, R p , to scale with t

57、he initiator concentration, I, to the 1/2 power 21, i .e ., x =1/2 in Eq. (12p IMx m R K = (12where x =0.5-1.0, m =1-1.5. For the polymerization system of AA/AMPS hydrogel, the reaction order ofinitiator sodium persulfate is 0.5 in the absence of theFigure 6 Arrhenius plots of lg K vs . 1/T(C AA =5.

58、6 mol·L 1, C AMPS =0.6 mol·L 1, ini =1%, by mass cross =0.8% (by mass; cross =0% (by mass Figure 7 Schematic illustration of the free radical polymerization and the crosslink polymerization290 Chin. J. Chem. Eng., Vol. 19, No. 2, April 2011 crosslinker, indicating the free radical polymerization route. The reaction order of 0.58 in the presence of the crosslinker indicates the crosslink polym