고분자에서 다성분 물질의 확산

홍성욱

한국과학기술연구원 고분자하이브리드연구센터

Multicomponent Diffusion in Polymers

Seong-Uk Hong

Polymer Hybrid Center, KIST

Introduction

Diffusion of small molecules in polymeric materials is important in many areas where polymers are acting as barriers, such as food packaging and protective clothing, or as membranes for separation. For many years, polymer/penetrant mutual diffusion coefficients were measured extensively using techniques such as gravimetric sorption[1,2] FTIR-ATR spectroscopy[3,4] and capillary column inverse gas chromatography (CCIGC)[5,6]. However, the diffusivity data measured using one technique have rarely been compared with the experimental results obtained from other measurement techniques. Moreover, in these studies, most of the data was for polymer/penetrant binary systems. However, many diffusion processes in actual applications consist of more than one penetrant. Therefore, experimental results for multicomponent diffusion processes in polymers are needed to provide a framework for structure-property guide lines.

In this study, vapor sorption FTIR-ATR (Fourier Transform Infrared Attenuated Total Reflectance) spectroscopy was introduced and combined with a conventional sorption balance[7]. Mutual diffusion coefficeients of methyl ethyl ketone (MEK) in polyisobutylene (PIB) were measured at various temperatures and penetrant activities using the gravimetric sorption balance, FTIR-ATR spectroscopy, and CCIGC techniques. The diffusion coefficients determined from the three methods are then compared with each other. In addition, the diffusion behavior of MEK/toluene mixtures in PIB was investigated using vapor sorption FTIR-ATR spectroscopy[8]. FTIR-ATR Fickian diffusion models for both ternary and binary systems were used to determine diffusion coefficients. Diffusion coefficients for mixtures in PIB are then compared with those determined from pure component diffusion experiments.

Experimental

Materials and Sample Preparation

PIB was purchased from Scientific Polymer Products, Inc, and characterized from the manufacturer as follows: Mv = 4,700,000 [GPC], density = 0.92 g/cm3, refractive index = 1.51, and Tg = -64oC. The toluene (J.T. Baker 9460-03) and MEK (Aldrich 27069-5) used in this study were of reagent and HPLC grades, respectively, and used without further purification. To obtain samples for the FTIR-ATR spectroscopy and gravimetric sorption experiments, the ATR crystal was placed in the slot of an aluminum block with the ATR surface flush with that of the block. A 3 wt. % solution of PIB in toluene was poured over the ATR crystal and dried for 24 h in a clean hood. After drying, the films were placed in a vacuum oven to remove residual solvent. The films were kept in the oven for at least 48 h at 50oC. No residual toluene was detected spectroscopically in the film attached to the ATR crystal. After removal of solvent, the ATR crystal was removed from the block for the FTIR experiments, and the remaining film in the aluminum block was used for the gravimetric sorption experiments. The capillary column used in this study was prepared by Supelco, Inc. (Bellefonte, PA), from PIB supplied by the author. The column had an inside diameter of 530 □m and was 15 m long. The PIB coating was applied by a static coating technique, and the thickness of coating was 3 □m.

Diffusion Experiments

To conduct the gravimetric sorption experiments, the PIB film was hanged directly on the quartz spring. Each experiments consist of determining the weight gain of the polymer sample from the spring extension using the cathetometer as a function of time. If the data are plotted as fractional approach to equilibrium vs. the square root of time, the diffusion coefficient can be obtained from the initial slope of the plot. Measurements of the infrared spectra were obtained using a Mattson Research Series 1 FTIR spectroscopy, with a horizontal ATR cell purchased from Graseby Specac, Inc. The ATR crystal used in the experiment was a silicon trapezoid, also obtained from Graseby Specac. The presence of toluene in PIB film was measured through the aromatic ring C-C streching band, while the presence of MEK was measured through the carbonyl (C=O) streching band as a function of time. Chromatograms were obtained using a Hewlett Packard 6890 gas chromatograph equipped with a flame ionization detector (FID), a splitless capillary injector, and a circulating oven. Dry-grade air and high-purity hydrogen were used for the FID detector. Small amounts of penetrant were injected in the column as liquids. Partition and diffusion coefficients were determined using the time domain fitting analysis, starting with the initial estimates from the moment analysis. Diffusion experiments for the PIB/MEK system were conducted using the three techniques at various penetrant activities and temperatures in the range 40-60oC, while the diffusion behavior of MEK/toluene mixtures of varying composition in PIB was studied at 50oC using vapor sorption FTIR-ATR spectroscopy.

Results and Discussion

Mutual Diffusion coefficients for the PIB/MEK system using the three techniques are shown in Figure 1. In this figure, open and solid symbols represent data measured from FTIR-ATR and CCIGC experiments, respectively, while values from gravimetric sorption experiments are shown as crosses. The diffusion coefficients at finite solvent concentrations determined gravimetrically and spectroscopically agree very well with each other. For example, the diffusion coefficient at 60oC and a penetrant vapor pressure of 145.1 mmHg determined from the FTIR-ATR experiment is 3.85×10-8 (cm2/s), while the value measured using the sorption balance is 3.72×10-8 (cm2/s). In addition, the diffusion coefficients obtained from the two methods are correlated as a function of temperature and concentration using the Vrentas-Duda free-volume theory[9] and are shown as solid lines in the figure. The correlation results show that the diffusion coeffficients measured using the CCIGC, considered to be at infinite dilution, agree very well with those determined from gravimetric and spectroscopic techniques when extrapolated to the limit of zero MEK concentration. For example, the diffusion coefficient at 50oC using the CCIGC was 1.25×10-8 (cm2/s), while the value from free-volume correlation was 1.18×10-8 (cm2/s).

Fig. 1. Mutual diffusion coefficients and theoretical correlations for PIB/MEK system.         Open and solid symbols represent data measured from FTIR-ATR and               CCIGC experiments, respectively, while the values from gravimetric sorption         experiments are shown as crosses.

For the PIB/MEK/toluene system, the diffusion coefficients were measured at 50oC using the FTIR-ATR spectroscopy. The values of the diffusion coefficients from the diffusion models for both binary and ternary systems are given in Table 1 for three experiments. Because the aromatic C-C bands of toluene were very small, diffusion coefficients for toluene could not be measured reliably for mixtures corresponding to toluene mole fraction, x1, in the liquid source lower than 0.5. The results show that "crossterm" diffusion coefficients for MEK, D21, were consistently two or more orders of magnitude less than the "mainterm" diffusion coefficients, D22. For toluene, however, the "crossterm" diffusion coefficients were significant and appear to increase as the MEK concentration of the liquid mixture, and hence, the MEK concentration in PIB, is increased. Because the diffusion coefficient of toluene is lower than that of MEK and FTIR-ATR experiments measure concentrations near the crystal-polymer interface, the results suggest that the diffusion coefficient of toluene is affected by concentration gradients of both MEK and toluene whereas that of MEK may be only affected by the concentration gradient of MEK itself for the three mixture compositions studied here. Because the "crossterm" diffusion coefficient for MEK, D21, was found to be very small for three mixture compositions with x1 > 0.5, the diffusion coefficient of MEK was determined for the remaining mixtures, using only the MEK diffusion data, with the binary diffusion model. This assumes that the effect of toluene concentration gradients will be even less for x1 < 0.5. The results compared well with those determined from pure MEK transport data. The results suggest that if a second penetrant does not interact with the first one and has the lower diffusion coefficient, the addition of the second penetrant simply reduces the activity of the system and may not affect the diffusion behavior of the first penetrant under certain experimental conditions for sorption kinetics. Further studies are needed with other systems to elucidate the conditions over which this result can be applied generally.

Table 1. Diffusion Coefficients for PIB/MEK/Toluene System at 50oC

	Experiment #1	Experiment #2	Experiment #3
Liq. source temp.	40oC	40oC	36oC
x1	0.771	0.558	0.558
y1	0.474	0.279	0.276
Toluene activity	0.484	0.369	0.309
MEK activity	0.183	0.325	0.252
Toluene conc. (g/ g PIB)a	0.071	0.061	0.040
MEK conc. (g/ g PIB)a	0.019	0.031	0.022
D (toluene) (cm2/s)b	6.18×10-9	6.80×10-9	6.52×10-9
D11 (cm2/s)	6.09×10-9	5.88×10-9	5.56×10-9
D12 (cm2/s)	8.21×10-10	3.96×10-9	3.95×10-9
D (MEK) (cm2/s)b	1.91×10-8	2.22×10-8	1.95×10-8
D21 (cm2/s)	6.56×10-15	4.98×10-10	3.41×10-10
D22 (cm2/s)	1.91×10-8	2.05×10-8	1.81×10-8

a Concentrations at equilibrioum.

b Regressed from binary model.

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