色谱  2014, Vol. 32 Issue (7): 746-752   PDF (669KB)    
扩展功能
加入收藏夹
复制引文信息
加入引用管理器
Email Alert
RSS
本文作者相关文章
Ismet KAYA
Cigdem Yigit PALA
Determination of thermodynamic properties of poly（cyclohexyl methacrylate）by inverse gas chromatography
Ismet KAYA1 , Cigdem Yigit PALA1,2    
1. Canakkale Onsekiz Mart University, Faculty of Sciences and Arts, Department of Chemistry, Polymer Synthesis and Analysis Laboratory, Canakkale 17020, Turkey;
2. Kaleseramik AR-GE Research, Kaleseramik R & D Center, Kaleseramik Canakkale Kalebodur Ceramic Industries Inc., Canakkale 17020, Turkey
Received date:2014-4-18
Corresponding author.Fax：+902862180533，E-mail：kayaismet@hotmail.com.
Abstract：In this work，some thermodynamic properties of poly（cyclohexyl methacrylate）were studied by inverse gas chromatography（IGC）. For this purpose，the polymeric substance was coated on Chromosorb W and which was filled into a glass column. The retention times（tr）of the probes were determined from the interactions of poly（cyclohexyl methacrylate）with n-pentane，n-hexane，n-heptane，n-octane，n-decane，methanol，ethanol，2-propanol，butanol，acetone，ethyl methyl ketone，benzene，toluene and o-xylene by IGC technique. Then，the specific volume（Vg0）was determined for each probe molecule. By using（1/T；lnVg0）graphics，the glass transition temperature of poly（cyclohexyl methacrylate）was found to be 373 K. The adsorption heat under the glass transition temperature（ΔHa），and partial molar heat of sorption above the glass transition（ΔH1S），partial molar free energy of sorption（ΔG1S）and partial molar entropy of sorption（ΔS1S）belonging to sorption for every probe were calculated. The partial molar heat of mixing at infinite dilution（ΔH1∞），partial molar free energy of mixing at infinite dilution（ΔG1∞），Flory-Huggins interaction parameter（χ12∞）and weight fraction activity coefficient（a1/w1）∞ values of polymer-solute systems were calculated at different column temperatures. The solubility parameters（δ2）of the polymer were obtained by IGC technique.
Key words： inverse gas chromatography（IGC）     poly（cyclohexyl methacrylate）     thermodynamic properties    

The methacrylate polymers are widely used in the manufacture of prostheses,contact lenses,adhesives,coatings,etc. [1]. The thermodynamic properties and solubility of polymers are parameters that must be known for application of polymer synthesis,economic production of polymeric materials and the process that used for this purpose. These parameters can be investigated with the inverse gas chromatography (IGC) technique. In this technique,the polymer is coated onto supported material and then filled into chromatographic columns. The solvents with known properties pass through the column by carrier gas and leave the column at different times according to the interest of the polymer. IGC technique has been used for the determination of some properties of the polymers such as the solubility parameters,melting point and glass transition temperature [2]. This method is of convenience and economics of operation. The basic tools for IGC are inexpensive,rugged,widely available,and suitable for routine laboratory applications. IGC data might be collected quite rapidly over extended temperature ranges [3]. As the molecular weights of the polymeric substances are very high and the polymeric substances are non-volatile,IGC method has been used to investigate the properties of these substances instead of normal gas chromatography. IGC was developed by Smidsrod and Guillet [4] and applied to many polymeric systems. The information in the processing steps of polymers are important parameters for higher polymer quality. The thermodynamic of polymer systems affects how these processing steps can be carried out. Therefore,the knowledge of thermodynamic data of polymer solutions is a necessity for the improvement of industrial processes.

In this paper,we have examined the interactions of poly(cyclohexyl methacrylate) with alcohols (polar) and alkanes (nonpolar),acetone,ethyl methyl ketone,benzene,toluene and o-xylene solute probes by using IGC in the temperature range of 353-453 K. Flory-Huggins interaction parameter ( x ∞12) and weight fraction activity coefficient (a1/w1)∞ of poly(cyclohexyl methacrylate)-solute systems selected were determined by IGC technique. Also,we have determined the solubility parameter ( δ 2) of the poly(cyclohexyl methacrylate) by IGC technique. The glass transition temperature (Tg) of poly(cyclohexyl methacrylate) was found to be 373 and 371 K from IGC and differential scanning calorimetry (DSC) measurements,respectively.

1 Theoretical
The probe specific retention volume (V0g),corrected to 0℃ was calculated from the standard chromatographic relation:

where △t (=tp-tg) is the difference between the retention times of the probe (tp) and the methane (tg); F is the flow rate of the carrier gas measured at room temperature (Tr); w is the mass of the polymeric stationary phase; Pi and P0 are the inlet and outlet pressures,respectively.

The partial molar heat of sorption ( △HS1) and the partial molar free energy of sorption (△GS1) of the probe adsorbed by the polymer,is given by Equation (2) and (3) [5, 6, 7, 8]:

By incorporating Equation (2) and (3) we calculated the entropy of sorption of solutes as follows:

The adsorption heat of probes adsorbed by the poly(cyclohexyl methacrylate) is given by the following equation where △Ha is the adsorption enthalpy and R is the ideal gas constant [9]:

The weight fraction activity coefficient,(a1/w1)∞,the partial molar free energy ( △ G∞1) and the average partial molar heat of mixing ( △ H∞1) at infinite dilution were calculated according to the following equations. Heats of vaporization ( △ Hv) for the probes were obtained from the heats of solution and heats of mixing by using the following relation [9]:

where B11 is the second viral coefficient of the organic solute in the gaseous state; P01 is the vapor pressure of the probes at the column temperature (T,K); M1 is the molecular weight of the probe; V1 is the molar volume of the solute. The values of P01 and B11 have been calculated in literatures [10, 11, 12].

The molar volume of the solute (V1) was calculated using the following equation [11]:

where Vc is the critical molar volume and qr is the reduced density of the solute given by the following equation.

where zc is the critical compressibility factor and Tc is critical temperature [13].

The Flory-Huggins parameter ( x ∞12) characterizing the interactions of a vapor-phase probe with a polymer is determined by Equation (12):

where R is the gas constant; v2 is the specific volume of the polymer.

Solubility parameter of the probe is calculated as follows [14]:

where δ1 is solubility parameter of probes; △ Hv is the molar enthalpy of vaporization for the probe at temperature T (K).

The solubility parameter of the polymer ( δ2) can be calculated by using the following equation:

If the left hand side of this equation is plotted against δ1,a straight line with a slope of 2δ2/RT and an intercept of (-δ22/RT) is obtained. The solubility parameter of polymer ( δ2) can be determined from both the slope and intercept of the straight line [15]. The studies indicated that the inverse gas chromatography method gives good information on polymeric systems after careful analysis [9, 10, 11, 12, 14, 16, 17, 18]. The Flory-Huggins interaction parameter ( x∞12) and solubility parameter ( δ2) of naphthenic and paraffinic base oils had been determined from Emam M. N. et al. by IGC technique [19].

2 Experimental
2.1 Materials

Fourteen polar and non-polar probes were used in this study. They were selected to provide different chemical natures and polarities. n-Pentane,n-hexane,n-heptane,n-octane,n-decane,methanol,ethanol,2-propanol,butanol,acetone,ethyl methyl ketone,benzene,toluene and o-xylene,were from Aldrich Chemical Co. Poly(cyclohexyl methacrylate) was supplied by Across Organics in powder form of Registry No. 1849402. Poly (cyclohexyl methacrylate) was in white powder form. Refractive index (n20/D) and density of poly (cyclohexyl methacrylate) were 1.506 5 and 1.1 g/mL at 25℃,respectively. Chromosorb W (45-60 mesh) was supplied from Sigma Chemical Co.

2.2 Instrumentation and procedure of thermodynamic studies

A Shimadzu GC-2010 model gas chromatograph equipped with a dual flame ionization detector was used. Dried nitrogen gas (research grade) was used as carrier gas. Methane was used as a non-interacting marker to correct the dead volume in the column. Pressures at inlet of the column read from GC were used to compute corrected retention volumes by the usual procedure. Flow rates were measured with a soap bubble flow meter at the end of the column. A flow rate of about 15 cm3/min was used throughout our experiment. The glass tube (2.1 m × 3.2 mm i. d.) was washed with acetone and was annealed prior to use. A column packing material was prepared by coating 45-60 mesh size Chromosorb W treated with polymer. An amount of 0.5 g poly(cyclohexyl methacrylate) was dissolved in 100 mL of tetrahydrofuran (THF). An amount of 5 g of the solid supporting material was added to this solution and kept stirring afterwards. The solvent was removed by continuous stirring and slow evaporation under partial vacuum in a rotary evaporator. The prepared material was packed into the glass tube [14, 16]. The column was conditioned with fast carrier gas (N2) flow rate for 48 h prior to use. The probes were injected into the column with an auto sampler. Three consecutive injections were made for each probe at each set of measurement and three values of retention time with inverse gas chromatography method were averaged. An injection volume was selected as 0.1 μL. Methane was synthesized in the laboratory by the reaction of sodium acetate with sodium hydroxide [10]. DSC analyses were carried out between 20-250 ℃ (in N2,10 ℃/min) using Perkin Elmer Pyris Sapphire DSC.

3 Results and Discussion

The specific retention volumes (V0g) of 14 probes were obtained by loading poly(cyclohexyl methacrylate) at a series of temperatures. Probes of different chemical natures and polarities (n-alkanes,alcohols,ketones,aromatics) were selected for this study. The V0g values of these probes were calculated according to Equation (1) and are given in Table 1 and Fig. 1,respectively. The averages of three values of retention time measured with inverse gas chromatography method were used in calculation of V0g of each probe at different temperatures. V0g values changed with the molecular weight of each group of solvents. Also,V0g values of probes on poly(cyclohexyl methacrylate) were decreased with increased temperature. According to IGC and DSC analyses,Tg of poly (cyclohexyl methacrylate) was found as 100 and 98 ℃,respectively.

Table 1 Variation of V0g of selected organic solvent systems at different column temperatures using poly(cyclohexyl methacrylate) as stationary phase

Fig.1 V0g of selected probes at different temperatures

The (a1/w1)∞ and x ∞12 values obtained using Equation (6) and (12) respectively are shown in Table 2. The values of x ∞12 greater than 0.5 represent unfavorable polymer-solvent interactions while values lower than 0.5 indicate favorable interactions in dilute polymer solutions [17]. The following rules have been formulated by Guillet et al [15]: (a1/w1)∞ <5: good solvents; 5<(a1/w1)∞<10: moderate solvents; (a1/w1)∞ >10: bad solvents.

Table 2 Poly(cyclohexyl methacrylate)-solute interaction coefficient ( x ∞12) and weight fraction activity coefficients (a1/w1)∞of selected organic solvents at various temperatures

The (a1/w1)∞ data in Table 2 indicate that n-pentane,acetone and benzene are good solvents; n-hexane,n-heptane,methanol,ethanol,2-propanol,butanol,ethyl methyl ketone,toluene and o-xylene are moderate solvents; n-octane,n-decane are bad solvents for poly(cyclohexyl methacrylate). Similar results were obtained according to the interaction parameters. x ∞12 and △G∞1 were found to be related to the number of carbons in the series and temperature.

x ∞12 ,△G∞1, (a1/w1)∞ and △H∞1at infinite dilution of the solutes showed dependence on the number of carbons in the series (except for alcohols). These values increased with increasing number of carbons in the series. But in all series,the values of x ∞12 , (a1/w1)∞ and △G∞1 decreased with increase in the column temperature. △G∞1 and △ GS1 calculated from Equation (7) and (3) respectively,are shown in Table 3.

Table 3 ( △ G)∞1 and △ GS1 of sorption by using poly(cyclohexyl methacrylate) as the stationary phase and selected organic solvents as mobile phase

△ H∞1 values of probes at infinite dilution were calculated using Eqation (8). ln (a1/w1)∞ were plotted against T-1/K-1 (Fig. 2). △Ha and △ HS1 of poly(cyclohexyl methacrylate)-probe systems were calculated by plotting lnV0g against T-1/K-1 using Equation (5) and (2),respectively. △Hv values of probes were found according to Equation (9). Table 4 shows the experimentally obtained sorption heats ( △ HS1),molar heats of mixing ( △ H∞1) and adsorption heats ( △ Ha) in temperature ranges of 353-373 K,423-453 K and 353-373 K,respectively.

Fig.2 Variation of (a1/w1)∞ with T-1/K-1 for (a) n-pentane,n-hexane,n-heptane,n-octane,n-decane; (b) methanol, ethanol,2-propanol,butanol; (c) acetone,ethyl methyl ketone,benzene,toluene and o-xylene

The number of carbon atoms of each probe is different from each other. △ Ha becomes more exothermic with increasing CH2 groups in each group of probes due to the increasing surface areas of probes. △ HS1 becomes more exothermic with more CH2 groups in each group probe. The attraction forces between poly(cyclohexyl methacrylate) and ketones are actually a combination of two types: dispersive forces between the CH2 groups of the ketones and the methyl group of poly(cyclohexyl methacrylate) and the interaction of the C=O groups of the ketones with the C=O groups of poly(cyclohexyl methacrylate) via dipole-dipole interactions. Also,alcohols have higher exothermic △ HS1 values than hydrocarbons,ketones and aromatics since there are hydrogen bond interactions between poly(cyclohexyl methacrylate) and alcohols. △ GS1 of these probes on poly(cyclohexyl methacrylate) are of positive values. According to thermodynamic rules,△ G is of negative value for spontaneous events. For this reason,interactions between polymer and probes are weak.

△ H∞1 values of n-hydrocarbons changed from 3.84 to 6.27 kcal/mol as seen from Table 4. △ H∞1 values of alcohols changed from 3.88 to 5.39 kcal/mol,while the values of ketones changed from 3.40 to 3.88 kcal/mol and the values of aromatics changed from 1.36 to 1.98 kcal/mol. Based upon these results the probes with low △ H∞1values were accepted as solvent-polymer systems and the others were taken as non-solvent-polymer systems.

Table 4 △ HS1 (383-403 K),△ H∞1 (423-453 K),△ Ha (353-373 K),and △ Hv of selected organic solvents on poly(cyclohexyl methacrylate)

DiPaola-Baranyi et al [7] determined that △ H∞1 values for aromatic solvents changed from -0.01 kcal/mol to 0.3 kcal/mol in polystyrene (PS),and from 0.3 kcal/mol to 1.1 kcal/mol in polymethyl acrylate (PMA). These values for the same polymers changed from 0.6 kcal/mol to 2.5 kcal/mol and from 2.5 kcal/mol to 4.1 kcal/mol in n-hydrocarbons. According to these results the probes with small △ H∞1 values were suitable for solvent-polymer systems and those with large △ H∞1 values were suitable for nonsolvent-polymer systems [7].

The solubility parameter ( δ 2) of a polymer can be determined by Equation (14) [18]. δ 2 was determined from either slope or intercept of a straight line obtained by plotting the left-hand-side of Equation (14) versus δ1 (Fig. 3). δ2 of poly(cyclohexyl methacrylate) was found as 5.17,4.60,4.18,3.74 (cal/cm3)0.5 and 5.17,4.38,3.87,3.22 (cal/cm3)0.5 at 423,433,443 and 453 K,respectively (Table 5).

Fig.3 Variation of the term [δ 21/(RT)- x ∞12 /V1] with δ 1 at the column temperatures of (a) 423 K, (b) 433 K,(c) 443 K and (d) 453 K for poly(cyclohexyl methacrylate)

Table 5 Variation of δ 2 with poly(cyclohexly methacrylate) temperature

These values were in compliance with the values found previously [20]. The solubility parameters obtained from the slopes and intercepts of the plots were in good agreement with each other. Comparing the δ2 values of poly(cyclohexyl methacrylate) at different temperatures,it showed that the solubility parameters decreased with increasing temperature.

4 Conclusions

Inverse gas chromatography technique was successfully applied to determine the glass transition temperature of poly(cyclohexyl methacrylate). Some thermodynamic properties were obtained for poly(cyclohexyl methacrylate)-solute systems such as adsorption heat,partial molar heat of sorption,partial molar free energy of sorption,partial molar heat of mixing at infinite dilution,partial molar free energy of mixing at infinite dilution,Flory-Huggins interaction parameter and weight fraction activity coefficient values. The solubility parameter of the polymer was determined with inverse gas chromatography technique. The results obtained are in good agreement with those of polymer-solvents and polymer-non-solvents systems. The technique is relatively uncomplicated and the data reduction is carried out by a computer.

参考文献
[1]Jang Y S, Kang J W, Byun H S. J Ind Eng Chem, 2010, 16(4): 598  
[2]Aydin S. [MS Dissertation]. Istanbul, Turkey: Yildiz Technique University, 2005
[3]Shyamala M, Pragati Ranjan S, Sharma J V C. International Journal of Pharma Sciences, 2013, 3(2): 201
[4]Guillet J E, Purnel J H. New Developments in Gas Chromatography: 322 Progress in Gas Chromatography. New York, USA: Wiley-Interscience, 1973: 323, 187
[5]Smidsrod O, Guillet J E. Macromolecules, 1969, 2(3): 272  
[6]Braun J M, Guillet J E. Macromolecules, 1977, 10(1): 101  
[7]DiPaola-Baranyi G, Guillet J E. Macromolecules, 1978, 11(1): 228  
[8]Galin M, Maslinco L. Macromolecules, 1985, 18(11): 2192  
[9]Kaya I, Ozdemir E. Polymer, 1999, 40 (9): 2405  
[10]Kaya I. [PhD Dissertation]. Elazig, Turkey: Firat University, 1995
[11]Kaya I, Demirelli K. Polymer, 2000, 41(8): 2855  
[12]Papadopoulou S K, Panayiotou C. J Chromatogr A, 2012, 1229: 230  
[13]Reid C R, Prausnitz J M, Sherwood T K. The Properties of Gases and Liquids. 2nd ed. New York, USA: McGraw-Hill, 1977
[14]Kaya I, Ozdemir E, Coskun M. J Macromol Sci A, 1996, A33(1): 37  
[15]Guillet J E, Purnel J H. Advances in Analytical Chemistry and Instrumentation Gas Chromatography. New York, USA: John Wiley & Sons, 1973
[16]Pala C Y. [MS Dissertation]. Canakkale, Turkey: Canakkale Onsekiz Mart University, 2013
[17]Klein J, Jeberien H E. Macromol Chem Phys, 1980, 181(6): 1237  
[18]Kaya I, Ilter Z, Senol D. Polymer, 2002, 43(24): 6455  
[19]Emam M N, Ahmed E E. Chinese Journal of Chromatography, 2007, 25(6): 871  
[20]Bicerano J. Prediction of Polymer Properties. 3rd ed. NewYork, USA: Marcel Dekker, Inc., 2002