Investigation of Water Diffusion in Low-Density Polyethylene by

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Investigation of Water Diffusion in Low-Density Polyethylene by Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy and Two-Dimensional Correlation Analysis Mengyin Wang,† Peiyi Wu,*,† Saurav S. Sengupta,‡ Bharat Indu. Chadhary,‡ Jeffrey M. Cogen,‡ and Bin Li§ †

The Key Laboratory of Molecular Engineering of Polymers(Ministry of Education) and Department of Macromolecular Science and Laboratory of Advanced Materials, Fudan University, Shanghai 200433, People’s Republic of China ‡ The Dow Chemical Company, 171 River Road, Piscataway, New Jersey 08854, United States § Dow Chem (China) Co. Ltd., 3D217, Shanghai Dow Center, 936 Zhang Heng Road, Shanghai, 201203, P.R. China ABSTRACT: Water diffusion through low density polyethylene (LDPE) film was investigated by attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and two-dimensional correlation spectroscopy (2DCOS) over a temperature range of 2580 C. The 2DCOS spectra of water diffusion in LDPE at different temperatures were very similar. The broad OH stretching vibration (ν(OH)) was split into four bands assigned to three different types of water molecules, namely, bulk water, cluster water, and free water, which will be defined. It appeared that as water molecules disperse in LDPE, cluster water with moderate hydrogen bonds diffused faster than bulk water with strong hydrogen bonds. Additionally, the hydrogen bonds between water molecules may have been further weakened or even broken to form free water, possibly due to limited free volume. On the basis of a Fickian model, the diffusion coefficients could be calculated from overall, fast and slow diffusion process, respectively. The diffusion coefficients were observed to increase with temperature, possibly due to increased swelling of molecular chain, more free volume, and reduced size of the penetrants.

1. INTRODUCTION The diffusion of small molecules through a polymer matrix is an important process, which is closely related with their applications in different fields, such as packaging, drug delivery, and film filtration.13 During the past several decades, extensive research interest has been focused on diffusion phenomena. Various experimental techniques have been utilized for these investigations, such as gravimetric method,4 high performance liquid chromatography (HPLC),5 Fourier transform infrared spectroscopy (FTIR),6 nuclear magnetic resonance (NMR),7 and Raman spectroscopy.8 Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy is a particularly appropriate technique for penetrants that are responsive to infrared. In such studies, one side of the polymer film is exposed to the penetrants and the other side is in contact with the ATR crystal. The spectra can be collected in real time as the penetrants diffuse toward the interface between polymer and ATR crystal. This in situ measurement can overcome many problems associated with “blot and weigh” immersion techniques.9 Additionally, saturation artifacts, a major problem involved with IR spectroscopy,10,11 are avoided in the ATR-FTIR spectra. For these reasons, ATRFTIR is a powerful method for investigation of diffusion through polymer materials and has been widely used in this area.1219 From the resultant spectra, it is possible to measure the mass transfer process until equilibrium, calculate the diffusion coefficients, and monitor the interactions between penetrants and polymers. Water molecules can assume several configurations depending on their interaction with surrounding molecules. This makes the observation of water molecules difficult, even by IR spectroscopy r 2011 American Chemical Society

which is a very sensitive technique for studying hydrogen bonds. It has been found that in a polymer matrix, there are four different states of OH stretching vibration of water molecules and there are some differences in these states between different polymer materials.14 These have been attributed to the fact that water molecules can form four different types of hydrogen bonds in different polymer matrixes, including strong hydrogen bonds, moderately strong hydrogen bonds, moderately weak hydrogen bonds, and weak hydrogen bonds, each of which gives rise to a pair of symmetrical and asymmetrical stretching vibrations.2026 Band broadening in this region may also complicate the assignment of OH stretching bands. Therefore, it is difficult to give a clear assignment of different types of water in diffusion processes. Two dimensional correlation spectroscopy (2DCOS), first proposed by Noda,27 is a technique where the spectral intensity is defined as a function of two independent spectral variables. It has two attractive features: spectral resolution enhancement and sequential order determination. By spreading the original data over the second dimension, features not readily observable in conventional spectra can be emphasized. Additionally, 2DCOS can be used to probe the specific sequence of the spectral intensity changes under a certain external perturbation, such as temperature, time, and pressure. Owing to these advantages, 2DCOS is an excellent method to investigate complex chemical or physical processes with highly overlapped bands and very Received: November 3, 2010 Accepted: April 6, 2011 Revised: March 18, 2011 Published: April 06, 2011 6447

dx.doi.org/10.1021/ie102221a | Ind. Eng. Chem. Res. 2011, 50, 6447–6454

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suitable for the study of water diffusion processes in polymer materials. Since low density polyethylene (LDPE) is widely used in packaging applications, it is desirable to study water diffusion processes in LDPE, calculate the diffusion coefficients, and monitor different forms of water involved in the process. McCall, et al.28 have studied solubility and diffusion of water in LDPE by means of desorption experiments and measured the dependence of the solubility and diffusion on the state of oxidation. The oxygen groups on the polymer could interact with the water molecules, but were not strong enough to fully prevent them from diffusing in the polymer. Metayer, et al.29 have used a new high performance method based on permeation measurements to study diffusion of water in various polymer films. In this paper, water diffusion in low density polyethylene was investigated by ATR-FTIR, and 2DCOS was utilized to analyze the resultant spectra. Diffusion of water through LDPE was studied at 25, 40, 60, and 80 C. Diffusion coefficients at different temperatures were determined by continuously monitoring the water absorbance bands. The results obtained from 2DCOS provide an insight into the different states of water molecules in the LDPE matrix and provide a clear understanding of the dynamic diffusion process.

2. MATERIALS AND EXPERIMENTAL 2.1. Materials. LDPE pellets provided by Dow Chemical were prepared into films for diffusion investigation. The polymer density was 0.920 g/cm3 and its melt index was 2 dg/min (190 C, 2.16 kg); 2.0 wt % LDPE was dissolved in xylene at 130 C and stirred continuously at 70 C to obtain a homogeneous solution. A small amount of the transparent solution was dropped in a glass-surface vessel in fume hood at room temperature and a film was recovered after solvent evaporation. To remove residual xylene, the sample was aged in a vacuum oven for 5 days at 40 C. The resulting film had a thickness of approximately 30 μm. 2.2. Thermal Transitions and Crystallinity of LDPE Film. Thermal analysis of the LDPE film was performed by differential scanning calorimetry (DSC) under a nitrogen atmosphere. About 4 mg of sample was heated from room temperature to 200 C, held under isothermal conditions for 5 min, cooled to 25 C, and then heated up to 200 C again (all with heating and cooling rates were 10 C/min). The melting temperature (Tm), the crystallization temperature (Tc), and fusion enthalpy (ΔHm) were determined from the DSC traces. The degree of crystallinity (Xc) was calculated based on the following method:

X c ¼ ΔH m =ΔH m 0 100

between two adjacent spectra was about 1 min and the time range was 1 h. The measured spectral range was 4000650 cm1. The equation proposed by Fieldson and Barbari,12 Eq1, is very useful to calculate the diffusion coefficient of a single penetrant through polymer materials from the ATR-FTIR data sets based on a Fickian diffusion model: At A0 8γ ¼ 1 π½1 expð 2γLÞ A¥ A0 " # ¥ expðgÞ½f expð 2γLÞ þ ð 1Þn ð2γÞ ð2Þ ð2n þ 1Þð4γ2 þ f 2 Þ n¼0

∑

g ¼

ð3Þ

ð2n þ 1Þπ 2L

ð4Þ

f ¼

where At is the integrated area of a characteristic IR band closely associated with the penetrant diffusion at time t, and A¥ is an equilibrium value; γ is the penetration depth of the evanescent wave, L is the thickness of the polymer film, and D is the diffusion coefficient. The diffusion process can be further divided into two parts: fast diffusion (D1); and slow diffusion (D2). The former may be related to monomeric and dimeric water, and the latter may possibly be due to the cluster and bulk water.13,16 The two diffusion coffecients associated with these two processes can be described as follows:15 At x1 A0 8γ ¼ 1 π½1 expð 2γLÞ x1 ðA¥ A0 Þ " # ¥ expðg1 Þ½f expð 2γLÞ þ ð 1Þn ð2γÞ ð2n þ 1Þð4γ2 þ f 2 Þ n¼0

∑

ð5Þ At x2 A0 8γ ¼ 1 π½1 expð 2γLÞ x2 ðA¥ A0 Þ " # ¥ expðg2 Þ½f expð 2γLÞ þ ð 1Þn ð2γÞ ð2n þ 1Þð4γ2 þ f 2 Þ n¼0

∑

ð6Þ

ð1Þ

where ΔH0m is the melting enthalpy of 100% crystalline LDPE, about 288 J/g.30 2.3. ATR-FTIR Diffusion Experimental. The time-resolved ATR-FTIR measurements were carried out at 25, 40, 60, and 80 C, respectively, using a Nicolet Nexus Smart ARK FTIR spectrometer equipped with a DTGS detector and a ZnSe IRE ATR crystal. The LDPE film was sandwiched between a ZnSe IRE ATR crystal and a piece of filter paper mounted on the ATR cell for 5 min to equilibrate with the cell temperature. Distilled water was injected onto the filter paper and data collection was initiated by a macro program. The spectra were collected at a resolution of 4 cm1 by accumulating 32 scans. The time interval

Dð2n þ 1Þ2 π2 t 4L2

g1 ¼

D1 ð2n þ 1Þ2 π2 t 4L2

ð7Þ

g2 ¼

D2 ð2n þ 1Þ2 π2 t 4L2

ð8Þ

The values of x1 and x2 are related to the proportion of the fast and slow diffusion process (x1 = A1¥/A¥, x2 = A2¥/A¥ and x1 þ x2 = 1), and the diffusion coefficients (D, D1, and D2) can be calculated through the nonlinear curve fitting according to eq 1, eq 4, and eq 5, respectively, from the absorbance intensity variations of the water ν(OH) stretching bands versus time. 6448

dx.doi.org/10.1021/ie102221a |Ind. Eng. Chem. Res. 2011, 50, 6447–6454


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Figure 1. Thermal transitions of LDPE film from second heat curve (A) and cooling rate curve (B).

Table 1. Band Assignments for the ATR-FTIR Spectra of LDPE Film33,34 wavenumber (cm1)

Figure 2. ATR-FTIR spectra of LDPE film at 25 C in the range of 4000650 cm1.

2.4. 2D Correlation Analysis. The generalized 2D correlation analysis was performed by the 2D software named 2D Shige (developed by Shigeaki Morita, Kwansei Gakuin University, Nishinomiya, Japan). The synchronous correlation intensity Φ(ν1, ν2) and the asynchronous one Ψ(ν1, ν2) is calculated as following:31

Φðν1 , ν2 Þ ¼ Ψðν1 , ν2 Þ ¼

1 ~y ðν1 ÞT ~y ðν2 Þ m1

ð9Þ

1 ~yðν1 ÞT N~y ðν2 Þ m1

ð10Þ

where ~(v,t)is y the dynamic spectrum and N is the HilbertNoda transformation matrix. According to the rules of Noda:31 (1) a positive Ψ(ν1, ν2) (ν1 > ν2) means spectral intensities at ν1, ν2 change in the same direction, while a negative one represents the opposite direction; (2) if Ψ(ν1, ν2) > 0, a positive Φ(ν1, ν2) means ν1 changes before ν2 and a negative one means ν1 changes after ν2. This rule is reversed if Ψ(ν1, ν2) < 0.

3. RESULTS AND DISCUSSION 3.1. Thermal Properties of LDPE Film. The DSC curve of the LDPE film is shown in Figure 1. The Tm (109 C) and Tc (94 C) were determined directly from the thermograms as the temperatures of the main endotherms and exotherms, respectively. Based

assignment

2912, 2848

stretching of CH2

1740

stretching of CdO

1473, 1462 1378

bending of CH2 bending of CH3

1365, 1353, 1303

wagging of CH

729, 719

rocking of CH

on eq 1, the crystallinity of LDPE prepared by solution casting is about 42.91%. 3.2. Time-Resolved ATR-FTIR Spectra. The ATR-FTIR spectrum of LDPE film prepared by solution casting is shown in Figure 2. The characteristic bands and their assignments are summarized in Table 1. The band at 1740 cm1 may be attributed to the stretching vibration of the carbonyl group caused by the trace oxidation of LDPE at high temperature.32 During the diffusion process at different temperatures, the intensities of bands which were closely related to water molecules (OH stretching vibration in the spectral region of 3700 3100 cm 1 and the OH bending vibration in 1700 1570 cm 1 ) changed with the diffusion time, as shown in Figure 3. Except for the spectra collected at 80 C, their intensities all increased in the first 1 h. In the first 30 min of diffusion process at 80 C, the intensities of OH stretching vibration (ν(OH)) and OH bending vibration (δ(OH)) both increased, but subsequently decreased in the last 30 min (perhaps due to moisture evaporation). This study focuses on changes in intensity of ν(OH), since it is stronger and varies more significantly than δ(OH). 3.3. Two-Dimensional Correlation Analysis of Water Changes. Thirty spectra at equal time intervals of 1 min in the range of 37003100 cm1 were collected for 2D correlation analysis to study the diffusion process at 25, 40, 60, 80 C. For the four different temperatures, the synchronous correlation spectra were very similar, and the intensities of correlation peaks were all positive. The corresponding asynchronous correlation spectra, shown in Figure 4, will be discussed in detail, since more useful information can be derived from them. In the resulting contour maps, the 2D correlation spectra were shown in the center, and 6449



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Figure 3. Time-resolved ATR-FTIR spectra measured during water diffusion in LDPE in the range of 37003100 and 17001570 cm1 collected at (A) 25 C from 0 to 60 min, (B) 40 C from 0 to 60 min, (C) 60 C from 0 to 60 min, (D) 80 C from 0 to 30 min, and (E) 80 C from 31 to 60 min.

the time-averaged 1D spectrum was shown at right and on top. The red and green patterns denote positive and negative correlation peaks, respectively. In the contour maps, there are three cross peaks above and to the left side of the main diagonal line, the locations of which are listed in Table 2. The data indicate that all the broad OH stretching bands of water molecules during diffusion at 25, 40, 60, and 80 C are split into four separate bands, which overlap in the 1D ATR-FTIR spectra. From low to high wavenumber, the four bands can be assigned to symmetric OH stretching (νs(OH)) band of tetrahedrally coordinated water, antisymmetric OH stretching (νas(OH)) band of cluster water not fully coordinated, and νs(OH) and νas(OH) of free water, respectively.14,18,19 The slow diffusion may be dominated by the first two states of water, while the fast diffusion is controlled by the other two. The characteristic bands assigned to different

water states display some temperature dependence and shift to high wavenumber with temperature rising, because the hydrogen bonds will be weakened by high temperature.35,36 Accordingly, during diffusion of water in LDPE, there were four different states of water molecules, namely, bulk water with strong hydrogen bonds, cluster water with moderate hydrogen bonds, and free water with very weak hydrogen bonds. In addition to enhancing spectral resolution, 2DCOS also provides useful information on the different water types. The asynchronous contour maps at different temperatures were very similar. In the upper left side of the main diagonal line, the asynchronous cross peak between bulk water and cluster water was positive, while the ones between free water and cluster water were negative. For diffusion process at 25 C, in the asynchronous contour maps presented in Figure 4A, there was one positive cross peak at (3471, 3282 cm1) and two negative cross 6450



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Figure 4. Asynchronous correlation spectra in the range of 37003100 cm1 calculated from time-resolved ATR-FTIR spectra of water diffusion in LDPE films collected at (A) 25 C from 0 to 30 min, (B) 40 C from 0 to 30 min, (C) 60 C from 0 to 30 min, and (D) 80 C from 0 to 30 min.

Table 2. Separate ν(OH) Bands in 37003100 cm1 Differentiated by 2DOCS wavenumber (cm1) 25 C

3685

3617

3471

3282

40 C

3681

3619

3482

3286

60 C

3669

3619

3482

3315

80 C

3675

3625

3505

3324

peaks at (3685, 3471 cm1) and (3617, 3471 cm1) in the upper left side of the main diagonal, while the corresponding synchronous correlation peaks were all positive. On the basis of the rules by Noda,31 the band at 3471 cm1 changed prior to the other three bands, and the total sequential order is 3471 f 3282 f 3617, 3685 cm1. The four asynchronous contour maps shown in Figure 4 were close to each other, namely, changes of water states during diffusion at different temperature all followed the same sequence: cluster water f bulk water f free water. In this system, aggregated water molecules first diffuse into LDPE gradually, and bulk water diffuses more slowly than cluster water because it has stronger hydrogen bonds and its size is larger. With increasing time, more and more water molecules are confined in limited space and their motion is more restricted than before, which may lead to the breaking of strong hydrogen bonds between water clusters to form some smaller, less accessible associations and even the free water. For the study of water evaporation, the last 30 spectra collected at 80 C were examined using 2D correlation analysis. The synchronous and asynchronous contour maps are shown in Figure 5. There is one synchronous autopeak at 3320 cm1, versus

one positive asynchronous cross peak at (3623, 3503 cm1) and one negative asynchronous cross peak at (3503, 3284 cm1). The broad ν(OH) band is composed of three bands: 3623, 3503, and 3284 cm1 and the sequential order is 3284f3623f3503 cm1, which is different than the former results. The intensity change of νas(OH) assigned to the free water may be very weak, so it could not been found in 2DCOS contour maps. During the desorption process, the strong hydrogen bonds in bulk water may be weakened and even broken with the formation of cluster or free water. Comparing to bulk water, the cluster and free one may evaporate at the same time with generation, so their decreasing rates were smaller than bulk water and the apparent intensity of bulk water decreased prior to them. The intensity of free water changed faster than cluster water, perhaps because its weaker hydrogen bond resulted in evaporation at 80 C. 3.4. Evaluation of Water Diffusion Coefficient in LDPE. The diffusion curves of water in LDPE illustrated in Figure 6 are in line with that reported in the literature for water in other polymer systems.16 The diffusion curves obtained using the methods proposed in section 2.3 are shown in Figure 6, which indicate that there is a distinct variation in diffusion coefficients as a function of temperature. Compared to fast diffusion, the water bulk reaches the ATR crystal after some certain lag because of their larger size. The detailed data are tabulated in Table 3, including the diffusion coefficients (D, D1, and D2) and the proportion of the fast and slow diffusion processes (x1 and x2). It is clear from Table 3 that the diffusion coefficient D1 (fast diffusion) is much larger than D2 (slow diffusion). On going from 25 to 80 C, the diffusion coefficients increase with temperature. This is consistent with the fact that at higher 6451



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Figure 5. (A) Synchronous and (B) asynchronous 2DIR correlation spectra in the 37003100 cm1 region calculated from the time-resolved ATRFTIR spectra of water evaporation through LDPE film at 80 C.

Figure 6. Integrated band areas of the OH stretching bands during water diffusion through LDPE at (A) 25, (B) 40, (C) 60, and (D) 80 C. Experimentally measured band areas (0) and mathematically simulated band areas for fast (O) and slow diffusion (Δ) of water through the films. The black solid line indicates the sum of simulated fast and slow water.

temperatures swelling of molecular chains increases, thus creating more free volume. Additionally, the proportion of slower diffusion was reduced with increasing temperature, so the proportion of free water with very weak hydrogen bonds increased. Diffusion process may also be associated with the degree of clustering of water. Temperature increase can reduce the cluster size, which makes the water diffusion through LDPE easier. The diffusion coefficient was plotted against the inverse temperature in Figure 7. From an Arrhenius-type equation for

Table 3. Diffusion Coefficients and Proportions of Different Diffusion Processes for Water in LDPE Film at Different Temperature

6452

D1 (cm2 3 s1)

x1(%)

D2 (cm2 3 s1)

x2(%)

D (cm2 3 s1)

25 C 40 C

4.78 109 5.92 109

51.7 54.5

7.67 1010 7.54 1010

48.3 45.5

8.38 1010 1.63 109

60 C

4.24 109

78.6

9.58 1010

21.4

2.79 109

80 C

8

31.1

5.61 109

1.69 10

68.9

9

3.19 10



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diffusion coefficients and the proportion of fast diffusion all increased with higher temperature. This is consistent with the fact that the higher the temperature is, the higher the swelling is and thus the free volume is considerably more. In addition, the diffusion process may be closely related with the degree of clustering of penetrants, and high temperature can reduce the cluster size of water molecules, which makes the diffusion process faster than at low temperature.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

Figure 7. Arrhenius plot of the overall water diffusion coefficient for the LDPE film. The solid line represents the best fit with the Arrhenius equation.

the diffusion coefficient, D ¼ D0 expð Ed =RTÞ

ð11Þ

the activation energy for diffusion (Ed) is calculated to be about 29.6 kJ/mol. This value is on the order of the average hydrogen bond energy for water (15.5 kJ/mol),17 consistent with the breaking of hydrogen bonds between water molecules during diffusion in LDPE.

4. CONCLUSION Water diffusion in LDPE film at different temperature is investigated by ATR-FTIR spectroscopy. In this paper, different types of water can be distinguished: free (weakly hydrogen bonded), clustered (moderately strong hydrogen bonded), and bulk water (strongly hydrogen bonded). Generalized 2D correlation spectroscopy is utilized to probe the change of different states of water during diffusion. From the 2DCOS spectra, it is found that there were at least three states of water in the LDPE film during the water diffusion process at different temperature: bulk water, cluster water, and free water. According to the sequential order rules, the detailed water diffusion process appears to be as follows: Water first disperses into LDPE film in the form of cluster water and bulk water. Since the hydrogen bond of cluster water is weaker and its degree of water clustering is smaller, the cluster water diffuses faster than bulk water. With increasing time, more and more water molecules become confined in the limited free volume and their motion is restricted. The hydrogen bonds between cluster water and bulk water may be weakened or even broken, which causes the formation of free water. In the later period collected at 80 C, the density of ν(OH) vibration began to decrease because water molecules evaporated at high temperature. On the basis of the 2DCOS spectra calculated from the last 30 spectra, the broad ν(OH) band could be split into three separate bands, assigned to bulk water, cluster water, and free water, respectively. And the corresponding sequence of different states of water molecules was that the bulk water first decreased and then the free water changed prior to the cluster water. The hydrogen bond of free water is weaker and its size is smaller, so it evaporates faster than cluster water. According to a Fickian model, the diffusion coefficients at different temperatures could be evaluated. The three different

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Investigation of Water Diffusion in Low-Density Polyethylene by

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