Sorption of Benzene, 1,2-Dichloroethane, Dichloromethane, and

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Sorption of Benzene, 1,2-Dichloroethane, Dichloromethane, and Chloroform by Polyethylene Glycol, Polycaprolactone, and their Triblock Copolymers at 298.15 K Using a Quartz Crystal Microbalance Abhijeet R. Iyer, Kiranpreet Kaur, Katy York, Scott W. Campbell,* and Venkat R. Bhethanabotla*

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Department of Chemical and Biomedical Engineering, University of South Florida, Tampa, Florida 33620-5350, United States ABSTRACT: Using a quartz crystal microbalance, the sorption of several different solvents, benzene, 1,2-dichloroethane, dichloromethane, and chloroform in three triblocks of the homopolymers polycaprolactone (PCL) and polyethylene glycol (PEG) were observed at 298.15 K. The solvent−polymer interaction was modeled using a modified Flory− Huggins equation and weight-based activity coefficients at infinite dilution were obtained from it to study the trend as the PCL/PEG ratio increases in the triblock copolymer system.

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INTRODUCTION Studying solvent sorption into copolymer systems has always been difficult due to the slow equilibration caused by low diffusion rates and limited literature available. Because the QCM is mass sensitive in its operation, the time it takes for the solvent to reach equilibrium is greatly reduced due to the thin polymer film coated on its surface. The sensitivity in detecting any small increase in the weight can be detected by the change in frequency, the technique first introduced by King.1 The data reported here were obtained using a QCM apparatus in a newly designed flow system described by Iyer et al.2,3 in a previous paper from our lab. There, we reported solubilities of benzene, 1,2-dichloroethane, chloroform, and dichloromethane in the homopolymers polycaprolactone (PCL) and polyethylene glycol (PEG) and in several diblock copolymers of PCL and PEG. Here, we report solubilities of these same solvents in three triblock (PCL/PEG/PCL) copolymers at 298.15 K. The triblock copolymers studied are PCL (5000)/PEG (1000)/PCL (5000), PCL (5000)/PEG (10000)/PCL (5000), and PCL (5000)/PEG (5000)/PCL (5000), where the number in the parentheses represents the molecular weight of that segment of the polymer. The triblock copolymers used here are of the form ABA, where A and B are the individual homopolymers. Sorption literature data for benzene in PEG at 297.75 K was previously reported by Lakhanpal, Singh, and Sharma4 using the characteristic vapor pressure measurement method. Various other sorption literature data for benzene-PEG are available at higher temperatures, 303.15 K and 323.15 K.5,6 Sorption of chloroform in PEG was studied by Booth et al.7 at 298 K, a weight fraction between 0.029 and 0.811, and matched our © XXXX American Chemical Society

data set for chloroform-PEG. There are no literature data available for any of the triblock copolymers used here. The triblock of PCL/PEG finds its applications in various biomedical fields. Considering the fact that both the homopolymers PEG and PCL have been approved by the FDA for biomedical applications,8 the thermosensitive triblock copolymers are largely used as injectable hydrogels in tissue engineering and tissue regeneration due to their slower degradation rate and higher gel strength.9 The amphiphilic nature of the PEG/PCL/PEG triblock copolymer in the form of nanoparticles finds its application in the in vitro drug release. Transmission electron microscopy technique shows that the hydrophobic nanoblock of the copolymer is spherical in shape and thus finds extensive use for drug delivery.10 A recent study done by Bae and Joo11 indicates a comparison between PCL/PEG diblock copolymer and PCL/PEG/PCL triblock copolymer based on their gelation behavior where it was noticed that the diblock copolymer shows a noticeable sol phase stability at room temperature due to their potential to create more micelles.

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EXPERIMENTAL SECTION

Materials. The triblock copolymers PCL/PEG/PCl with varying molecular weights were obtained from Polysciences. Inc. The three triblock copolymers chosen based on their average molecular weights are PCL (5000)/PEG (1000)/PCL (5000), PCL (5000)/PEG (5000)/PCL (5000), and PCL Received: April 25, 2018 Accepted: August 7, 2018

A

DOI: 10.1021/acs.jced.8b00338 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

(5000)/PEG (10000)/PCL (5000) where the numbers in the parentheses represents the molecular weight of that segment of the polymer. The four solvents, benzene, 1,2-dichloroethane, chloroform, and dichloromethane were obtained from SigmaAldrich with 99.99% purity. The 5 MHz crystal tested for this experimentation was obtained from Phillip Technologies. The SRS QCM-200 5 MHz crystals were 1 in. in diameter, 0.013 in. thick, have goldcoated electrode wrapped upon on both sides, and have a smooth, polished surface finishing. Thin films (∼0.5 μm) of triblock copolymers were spin coated on the 5 MHz crystal using a Laurell WS-400 B spin coater. Apparatus and Procedure. The experimental apparatus and procedure have been explained in an earlier publication by Iyer et al.2 The basic principle underlying the quartz crystal microbalance (QCM) is that a frequency change results when some mass is deposited on the quartz crystal as a result of sorption. The Sauerbrey12 equation quantifies this effect by expressing the weight fraction w1 of sorbed solvents w1 =

Δf Δf + Δf0

d ln Ps(T ′) d ln Ps(T ) σa1 σ = σT′ + σT + v a1 dT ′ dT V

(4)

where Ps is the solvent vapor pressure at cell temperature T and bubbler temperature T′, σv is the uncertainty in volumetric flow rate, and σT and σT′ are uncertainties in cell and bubbler temperatures. The uncertainty in both temperatures was 0.01 K and that in volumetric flow rate obtained was 1%. Overall, the uncertainty in activity was less than 1.5%. The uncertainty σw1 in weight fraction was σw1 =

σ Δf Δf0

+

( )ijjjk ( ) w1 1 − w1

1 + w1 1 − w1

σ Δf0 y Δf0

zz z {

(5)

Δf and Δf 0 were defined earlier and σΔf and σΔf0 are uncertainties in these frequency shifts. The uncertainty in weight fraction was found to be 0.0004 or less.

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RESULTS Experimental Data. The frequency versus time shown in Figure 1 was plotted to study the change in frequency as the

(1)

where Δf refers to the frequency shift from pure polymer to polymer plus sorbed solvent and Δf 0 refers to the frequency change between bare crystal and crystal coated with a polymer layer. Solvent activities were determined from2 ÄÅ ÅÅ P − P s(T ) y1P Å 1 a1 = s expÅÅÅ B11 ÅÅ P1 (T ) RT ÅÅÇ ÑÉ P(1 − y1)2 (2B13 − B11 − B33) ÑÑÑ ÑÑ + ÑÑ RT ÑÑ (2) ÑÖ where y1 is the mole fraction of solvent in the combined gas stream passing over the crystal, P is total pressure, P1s(T) is the solvent vapor pressure at cell temperature T, B11 is the second virial coefficient of pure solvent, B33 is the second virial coefficient of nitrogen, and B13 is the second virial cross coefficient. The virial coefficients were calculated from the Tsonopolous equation.13 Vapor pressures P1S at both the bubbler and cell temperatures were obtained using the Antoine equation log10 P1S (bars) = A −

B T (K) + C

Figure 1. Frequency versus time curve for benzene-PCL (5000)/PEG (1000)/PCL (5000).

sorption process takes place. It was noticed that the frequency decreases as the solvent vapors are deposited on the copolymer film surface. The resulting frequency change corresponds to the weight fraction of the solvent in the thin film. The experimental data depicted in Figure 1 are considered to be well equilibrated as observed from the leveling off of the frequency shifts. Experimental data for the QCM apparatus is of the form of a graph between solvent weight fraction and solvent activity for the four solvents in PEG, PCL, PCL (5000)/PEG (5000)/PCL (5000), PCL (5000) PEG (10000)/PCL (5000), and PCL (5000)/PEG (1000)/PCL (5000) at 298.15 K which are shown in Figures 2−5. The LabView software which automates the experiment is capable of producing 10 data points for each solvent but based on our previous study the lowest 8, 5, and 4 data points are reported for 1 2-dichloroethane, chloroform, and dichloromethane, respectively, as at higher solvent activities two phenomenon can occur, a second phase may be formed as explained by Iyer et al.2 or the polymer may become rubbery, resulting in higher resistances. The values in Tables 2−5 are for the data points wherein a vapor phase of the solvent and nitrogen are in equilibrium with the polymer phase.

(3)

where T represents the cell temperature. To calculate vapor pressures at the bubbler temperature, T is replaced with bubbler temperature T′ in eq 3. Values of constants14 A, B, and C are given in Table 1. Uncertainties in calculated activities were calculated using Table 1. Values of Coefficients Used in Equation 3 for Calculating Solvent Vapor Pressures Antoine parameter

benzene

A B C

4.018 1203.835 −53.226

dichloromethane chloroform 4.585 1521.789 −24.67

4.208 1233.129 −40.953

dichloromethane 4.527 1327.016 −20.474 B



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Figure 2. Comparison of activity versus weight fraction for benzene in PCL2, PCL (5000)/PEG (10000)/PCL (5000), PCL (5000)/PEG (1000)/PCL (5000), PCL (5000)/PEG (5000)/PCL (5000), and PEG2 at 298.15 K. Solid curves refer to fits to eq 7.

Figure 5. Comparison of activity versus weight fraction for dichloromethane in PCL2, PCL (5000)/PEG (10000)/PCL (5000), PCL (5000)/PEG (1000)/PCL (5000), PCL (5000)/PEG (5000)/ PCL (5000), and PEG2 at 298.15 K. Solid curves refer to fits to eq 7.

could be due to the linear stability of the molecule imparted because of similar weight fraction of individual homopolymer in it. Data Correlation. The experimental data were put to a Flory−Huggins two parameter model where the excess Gibbs energy, GE is given by the equation ⌀ ⌀ NGE = N1 ln 1 + N2 ln 2 + ⌀1⌀2(N1 + rN2) RT X1 X2 (A ⌀1 + B⌀2)

(6)

where Xi is mole fraction (1 for solvent and 2 for the polymer), ⌀i is volume fraction, r is the ratio of molar volumes of the solvent and polymer, Ni is number of moles, and, A and B are the two parameters. Table 6 provides the values for molecular weights and molar volumes of the solvents and the polymers used in this system. From eq 2, the expression for solvent activity a1 can be derived:

Figure 3. Comparison of activity versus weight fraction for 1,2dichloroethane in PCL2, PCL (5000)/PEG (10000)/PCL (5000), PCL (5000)/PEG (1000)/PCL (5000), PCL (5000)/PEG (5000)/ PCL (5000), and PEG2 at 298.15 K. Solid curves refer to fits to eq 7.

1y i ln a1 = ln ⌀1 + jjj1 − zzz⌀2 + (2 × (A − B) × ⌀1 + B) r{ k × ⌀2 2

(7)

The A and B interaction parameters serve as an important tool to study miscibility. They are obtained by minimizing the sum of the squares of the differences between experimental and calculated activities. The values of A and B and the resulting deviation in weight fraction are given for each solvent− polymer system in Table 7 and include results of fits to data reported earlier for systems containing the homopolymers. The model represents the experimental weight fractions to within an average between 0.001 and 0.01. The two parameter Flory−Huggins modeling was used to calculate the infinite dilution activity coefficient as follows

Figure 4. Comparison of activity versus weight fraction for chloroform in PCL2, PCL (5000)/PEG (10000)/PCL (5000), PCL (5000)/PEG (1000)/PCL (5000), PCL (5000)/PEG (5000)/PCL (5000), and PEG2 at 298.15 K. Solid curves refer to fits to eq 7.

ij M yz 1 − lnjjj 1 r zzz jM z r (8) k 2 { ∞ Here, Ω is the infinite dilution activity coefficient, B is a parameter obtained from Flory−Huggins modeling, r is the ratio of molar volumes of the polymer and the solvent, and M1 and M2 are the molecular weights of the solvent polymer, respectively.

Analyzing the activity-weight fraction curves from Figures 2−5, which include homopolymer results from the earlier study,2 it can be seen that the individual homopolymers tend to have lower solvent weight fractions than the copolymers. The sorption of all the four solvents is least in PEG homopolymer and the maximum in the PCL (5000)/PEG (5000)/PCL (5000) triblock copolymer. The high sorption of solvents in the PCL (5000)/PEG (5000)/PCL (5000) triblock

ln Ω∞ = B + 1 −

C



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Table 2. Experimental Data for Weight Fraction w1 of Benzene in PCL (5000)/PEG (10000)/PCL(5000), PCL (5000)/PEG (1000)/PCL (5000), and PCL (5000)/PEG (5000)/PCL (5000) at 298.15 K as a Function of Benzene Activity a1 w1

activity a1

PCL (5000)/PEG (10000)/PCL (5000)

PCL (5000)/PEG (1000)/PCL (5000)

PCL (5000)/PEG (5000)/PCL (5000)

0.070 0.136 0.205 0.267 0.330 0.392 0.448 0.509 0.565 0.620

0.009 0.017 0.024 0.032 0.040 0.048 0.057 0.067 0.078 0.091

0.010 0.019 0.029 0.039 0.049 0.061 0.073 0.085 0.100 0.114

0.012 0.023 0.035 0.047 0.059 0.072 0.087 0.102 0.122 0.142

Table 3. Experimental Data for Weight Fraction w1 of 1,2-Dichloroethane (DCE) in PCL (5000)/PEG (10000)/PCL(5000), PCL (5000)/PEG (1000)/PCL (5000), and PCL (5000)/PEG (5000)/PCL (5000) at 298.15 K as a Function of DCE Activity a1 w1

activity a1

PCL (5000)/PEG (10000)/PCL (5000)

PCL (5000)/PEG (1000)/PCL (5000)

PCL (5000)/PEG (5000)/PCL (5000)

0.069 0.133 0.202 0.263 0.326 0.388 0.444 0.505

0.019 0.037 0.054 0.071 0.090 0.118 0.169

0.019 0.038 0.055 0.073 0.090 0.109 0.129 0.152

0.030 0.059 0.085 0.110 0.134 0.157 0.181 0.212

Table 4. Experimental Data for Weight Fraction w1 of Chloroform in PCL (5000)/PEG (10000)/PCL(5000), PCL (5000)/ PEG (1000)/PCL (5000), and PCL (5000)/PEG (5000)/PCL (5000) at 298.15 K as a Function of Chloroform Activity a1 w1

activity a1

PCL (5000)/PEG (10000)/PCL (5000)

PCL (5000)/PEG (1000)/PCL (5000)

PCL (5000)/PEG (5000)/PCL (5000)

0.079 0.153 0.228 0.293 0.358

0.036 0.071 0.112 0.183

0.042 0.079 0.113 0.144 0.217

0.069 0.125 0.176 0.222 0.281

Table 5. Experimental Data for Weight Fraction w1 of Dichloromethane (DCM) in PCL (5000)/PEG (10000)/PCL(5000), PCL (5000)/PEG (1000)/PCL (5000), and PCL (5000)/PEG (5000)/PCL (5000) at 298.15 K as a Function of DCM Activity w1

activity a1

PCL (5000)/PEG (10000)/PCL (5000)

PCL (5000)/PEG (1000)/PCL (5000)

PCL (5000)/PEG (5000)/PCL (5000)

0.106 0.196 0.282 0.352

0.038 0.075 0.128

0.038 0.072 0.103 0.133

0.059 0.109 0.153 0.189

The infinite dilution activity coefficient (weight based) values are plotted as a function of PCL weight fraction in the copolymers in Figure 6. The molecular weight of each segment of the polymer has been mentioned in Materials. In case of triblock copolymers, it can be seen that for all the four solvents, the activity coefficient value decreases for PCL/PEG ratio from 0 to 0.67, after which it starts to increase until it reaches pure PCL homopolymer. For the case of diblock copolymers, the infection point can be seen for PCL/PEG ratio of 0.8, after which the activity coefficients increase. For both the diblock and triblock copolymers, the values for infinite dilution activity

coefficients at a constant PCL/PEG ratio was maximum for benzene, followed by DCE, DCM, and chloroform at the bottom. The traditional way of finding these infinite dilution activity coefficients is by gas chromatographic technique or gas stripping technique.15 It is important to understand the infinite dilution activity coefficient values as it acts as an important parameter for synthesis, design of thermal separation process16

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CONCLUSION Isothermal solubilities of benzene, 1, 2-dichloroethane, chloroform, and dichloromethane in (PCL/PEG/PCL) triblock D



Article

copolymers from previous work.2 It was observed that there were similar trends in infinite dilution activity coefficient for all the four solvents with PCL/PEG ratio.

Table 6. Molar Mass (M) and Molar Volume (V) of the Solvents and Polymers M/g·mol‑1

species

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V/cm3·mol‑1

Solvents benzene 1, 2-dichloroethane chloroform dichloromethane

78.11 98.95 119.37 84.93

92.41 78.97 80.17 64.02

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Polymers polycaprolactone (PCL) PCL (5000)/PEG (10000)/PCL (5000) PCL (5000)/PEG (1000)/PCL (5000) PCL (5000)/PEG (5000)/PCL (5000) polyethylene glycol (PEG)

14000 20000 11000 15000 2000

Venkat R. Bhethanabotla: 0000-0002-8279-0100

12216.40 17575.56 9610.96 13150.80 1666.67

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Science Foundation, grant number EEC- 1301054 is gratefully acknowledged.

Table 7. Parameters Used in the Two Parameter Flory− Huggins Model and Deviation in Weight Fractions system

A

B

del w

Benzene-PEG2 benzene/PCL(5k)-PEG (10k)-PCL(5k) benzene/PCL(5k)-PEG (5k)-PCL(5k) benzene/PCL(5k)-PEG (1k)-PCL(5k) benzene-PCL2 DCE-PEG2 DCE/PCL(5k)-PEG (10k)-PCL(5k) DCE/PCL(5k)-PEG (5k)-PCL(5k) DCE/PCL(5k)-PEG (1k)-PCL(5k) DCE-PCL2 chloroform-PEG2 chloroform/PCL(5k)-PEG (10k)-PCL(5k) chloroform/PCL(5k)-PEG (5k)-PCL(5k) chloroform/PCL(5k)-PEG (1k)-PCL(5k) chloroform-PCL2 DCM-PEG2 DCM/PCL(5k)-PEG (10k)-PCL(5k) DCM/PCL(5k)-PEG (5k)-PCL(5k) DCM/PCL(5k)-PEG (1k)-PCL(5k) DCM-PCL2

−4.3 1.28 0.67 0.95 0.5 −1.7 −0.74 0.74 0.33 0.61 −2.8 −1.96 −0.48 −1.12 −0.24 −1.63 −1.62 0.26 −0.11 0.1

2.87 0.94 0.6 0.75 0.98 2.31 0.71 −0.11 −0.2 0.74 1.7 0.43 −0.54 −0.42 0.13 1.8 0.47 −0.31 −0.42 0.44

0 0.001 0.001 0 0.002 0.001 0.008 0.002 0.001 0.001 0.002 0.012 0.004 0.01 0.003 0.001 0.006 0.001 0.001 0.001

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Corresponding Authors

REFERENCES

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copolymers at 298.15 K. are reported. The experimental results were interpreted using a modified two-parameter Flory− Huggins equation. Infinite dilution activity coefficient values for solvents in the triblock copolymers were determined and compared to results for the same solvents in diblock

Figure 6. Comparison of infinite dilution weight-based activity coefficient versus PCL/PEG ratio for triblock and diblock copolymers. E



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Sorption of Benzene, 1,2-Dichloroethane, Dichloromethane, and

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