Observing Phthalate Leaching from Plasticized Polymer Films at the

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Observing Phthalate Leaching from Plasticized Polymer Films at the Molecular Level Xiaoxian Zhang and Zhan Chen* Department of Chemistry, University of Michigan, 930 North University Avenue, Ann Arbor, Michigan 48109, United States S Supporting Information *

ABSTRACT: Phthalates, the most widely used plasticizers in poly(vinyl chloride) (PVC), have been extensively studied. In this paper, a highly sensitive, easy, and effective method was developed to examine short-term phthalate leaching from PVC/phthalate films at the molecular level using sum frequency generation vibrational spectroscopy (SFG). Combining SFG and Fourier transform infrared spectroscopy (FTIR), surface and bulk molecular structures of PVC/phthalate films were also comprehensively evaluated during the phthalate leaching process under various environments. The leaching processes of two phthalates, diethyl phthalate (DEP) and dibutyl phthalate (DBP), from the PVC/phthalate films with various weight ratios were studied. Oxygen plasma was applied to treat the PVC/phthalate film surfaces to verify its efficacy on preventing/reducing phthalate leaching from PVC. Our results show that DBP is more stable than DEP in PVC/phthalate films. Even so, DBP molecules were still found to very slowly leach to the environment from PVC at 30 °C, at a rate much slower than DEP. Also, the bulk DBP content substantially influences the DBP leaching. Higher DBP bulk concentration yields less stable DBP molecules in the PVC matrix, allowing molecules to leach from the polymer film more easily. Additionally, DBP leaching is very sensitive to temperature changes; higher temperature can strongly enhance the leaching process. For most cases, the oxygen plasma treatment can effectively prevent phthalate leaching from PVC films (e.g., for samples with low bulk concentrations of DBP5 and 30 wt %). It is also capable of reducing phthalate leaching from high DBP bulk concentration PVC samples (e.g., 70 wt % DBP in PVC/DBP mixture). This research develops a highly sensitive method to detect chemicals at the molecular level as well as provides surface and bulk molecular structural changes. The method developed here is general and can be applied to detect small amounts of chemical/biological environmental contaminants.

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INTRODUCTION Because of their excellent product performance, superb process ability, relatively low cost, and high versatility, poly(vinyl chloride) (PVC) materials are widely used to make a wide range of products such as blood and urine bags, transfusion tubes, packaging materials, toys, bathroom curtains, and kitchen floors. In addition, PVC is a good insulation material for electric wires and cables because of its inherent flame-retardant nature due to its chloride content.1 During the use of these PVC products, many environmental factors such as increased oxygen content, temperature and humidity variations, application of electric fields, UV radiation exposure, and exposure to liquids (water, saliva, and blood, etc.) can alter the PVC polymer structure and properties, causing aging and breakdown of the PVC products. In order to achieve required properties such as flexibility, transparency, and durability of PVC products, additives (e.g., plasticizers) have been commonly used in PVC formulations. The amount of plasticizers in PVC varies substantially depending on the desired application of the material. In some cases, the plasticizer content in PVC can be as high as 70 wt %.1 © 2014 American Chemical Society

The most widely used plasticizers for PVC are phthalate molecules. Phthalate molecules in PVC are not chemically bound to PVC but physically interact with PVC through van der Waals forces. Therefore, they may easily diffuse to the surrounding environment.1 Recent studies indicate that human urine samples from people extensively exposed to phthalate plasticizers contain phthalate contaminants.2−4 Some phthalates, such as di(butyl) phthalate (DBP), di(ethyl) phthalate (DEP), or di(2-ethylhexyl) phthalates (DEHP), are environmental contaminants, suspected teratogens, and endocrine disruptors, etc.5−7 Therefore, leaching of these plasticizers, especially phthalates with small molecular weights such as DBP and DEP, may cause environmental and health problems. Therefore, it is of great importance to understand the leaching behavior of different phthalates in PVC. Extensive research has been performed on various aspects of migration of different plasticizers in lab or industrial setting.5,8−10 SpecifiReceived: February 5, 2014 Revised: March 21, 2014 Published: April 11, 2014 4933

dx.doi.org/10.1021/la500476u | Langmuir 2014, 30, 4933−4944

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from Fluka (St. Louis, MO). These chemicals were used as received. Figures 1a−c display the chemical structures of PVC, DEP, and DBP.

cally, diffusion parameters such as activation energy and diffusion coefficient, etc., have been systematically determined under different conditions (various temperatures, voltages, etc.) for specific plasticizers.5 However, most of such studies focused on the bulk materials using conventional methods, which take days or even months. For instance, traditionally migration behaviors of different plasticizers can be quantitatively evaluated by tracing the mass loss from a bulk sample5 using an analytical balance or monitoring the change of the plasticizer content in a solution in contact with a PVC sample using a chromatograph.11 Similar to bulk properties, the surface behaviors of PVC also play an important role for migration because phthalates leach out from PVC through the surface, and the surface directly contacts the surrounding media. However, compared to bulk studies, there are very few investigations on the surface molecular structures of PVC films during the plasticizer leaching process. Moreover, detection of small amounts of chemicals like phthalate can be quite challenging. In the past several decades, quartz crystal microbalances (QCM) were widely applied to detect small amounts of adsorbed materials by measuring the change in frequency of a quartz crystal resonator.12,13 However, it is vital to elucidate the surface and bulk molecular structures of the phthalate/PVC system during the phthalate leaching process in a highly sensitive detection system to understand the underlined mechanism for phthalate leaching/diffusion. Especially when a surface treatment method is developed to reduce/prevent phthalate leaching from plasticized PVC, it is important to understand how the surface structure is influenced/altered by the surface treatment to optimize the method to prevent phthalate leaching. In this study, a highly surface-sensitive technique, sum frequency generation vibrational spectroscopy (SFG), was combined with a bulk measurement method, Fourier transform infrared spectroscopy (FTIR), to investigate phthalate leaching from plasticized PVC films into air for two PVC/phthalate systems at the molecular level. In addition to SFG and FTIR data obtained before and after the phthalate leaching process, a precleaned substrate was introduced here as a sensor to “deposit” the phthalate molecules leached out from the PVC matrix. Subsequently, SFG was used to detect/trace the small amounts of molecules adsorbed on the substrate. Two phthalate molecules with different chain lengths, DEP and DBP, were studied in this paper. Since the phthalate content in PVC product can vary greatly, and the bulk content of plasticizers can affect the leaching process significantly,10 here we varied the phthalate contents in the sample by mixing PVC and phthalate with different weight ratios. We studied four phthalate/PVC samples with 5, 15, 30, and 70 wt % phthalate in the mixture bulk. In addition, we studied pure PVC and pure phthalate films as control. Two other factors, temperature and heating time, were also studied in this paper to elucidate their influences on the phthalate leaching process for various samples. Finally, the efficacy of a very widely used surface treatment, oxygen plasma treatment, on preventing/suppressing phthalate leaching from PVC into air was evaluated at the molecular level.

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Figure 1. Molecular structures of (a) PVC, (b) DEP, and DBP (c). (d) Schematic illustration showing the sample arrangement to study phthalate leaching from PVC matrix into air under different temperatures. Fused silica and calcium fluoride (CaF2) windows were utilized to deposit pure PVC, two kinds of phthalates, and PVC/phthalate hybrid films for SFG and FTIR spectroscopic studies. Phthalates (DEP or DBP) and PVC were mixed with various weight ratios and were dissolved in THF with 1:30 weight ratio of PVC:THF. Spin-coating (3000 rpm, 30 s) was used to fabricate pure and hybrid films with a P6000 spin-coater (Speedline Technologies). Half of the as-deposited film samples were exposed to oxygen plasma for 10 s at 150 mbar by using a commercial plasma system (PE-50, Plasma etch). The film thicknesses were measured by a depth profilometer (Dektak 6 M Stylus Surface Profilometer, Veeco), and the average thicknesses were around 200 nm. SFG has been widely applied to study surfaces and interfaces of many materials.14−36 The theories and experimental details of SFG have been extensively reported in the previous publications.14,37,38 Since the same SFG spectrometer was reported previously, the experimental setup and operation details of SFG and FTIR are presented in the Supporting Information. No quantitative analysis will be included in this paper from FTIR results because such information has been reported extensively before. Here we only want to use FTIR results to follow the changes qualitatively before and after the phthalate leaching to support highly sensitive SFG detection. Figure 1d shows the sample arrangement designed for probing phthalate molecules that migrated from the PVC/phthalate samples to precleaned substrates. One (or more) spin-coated PVC/DBP film(s) was put into a clean glass dish (Pyrex), and a precleaned CaF2 window was placed on top of it to “adsorb” the leached phthalate molecules. This setup was sealed with aluminum foil to prevent possible contamination from the environment during the heating process and placed in an oven. To further confirm that the samples are not contaminated during heating, a blank window was placed in a second clean glass dish and sealed with aluminum foil and then placed in the oven together with the dish containing the PVC/phthalate films. No SFG signal was observed from these blank windows after heating, indicating that there was no contamination in the experiment. After heating, the original samples (assigned as sample A in Figure 1d) were studied using FTIR and SFG, while the clean window placed above (assigned as sample B in Figure 1d) was examined by SFG and

EXPERIMENTAL SECTION

PVC (Mw 62 000, in pellet form) and tetrahydrofuran (THF; ≥99.9% purity) were purchased from Sigma-Aldrich (St. Louis, MO). DEP (analytical standard) and DBP (analytical standard) were purchased 4934


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Figure 2. FTIR spectra of DEP/PVC samples (a) before and (b) after oxygen plasma treatment.

Figure 3. FTIR spectra of PVC/DBP samples with various DBP weight percentages before (a) and after (b) oxygen plasma treatment for 10 s show similar spectral features. Therefore, it is believed that the short-term plasma treatment only affects the surface but not the bulk.

chain ends). The SFG spectra of pure DEP and DBP films are very similar; two main peaks at ∼2880 cm−1 (methyl symmetric stretching) and 2945 cm−1 (Fermi resonance of CH3) were observed but differed in the ratios of peak intensities. As the bulk concentration of phthalate in the mixture increases, the phthalate spectral features gradually increase and finally dominate the SFG spectra as the bulk concentration of phthalate exceeds a certain value (30 wt % for both phthalates). In the previous publication,39 we also applied CARS to study bulk changes of DEP/PVC and DBP/PVC samples before and after oxygen plasma treatment. In this study we will report the FTIR results on the studies on the bulk structures of phthalate plasticized polymers, which are well correlated to our previous CARS results. The FTIR spectra we detected here cover a much broader frequency range compared to CARS. Figures 2 and 3 display the typical FTIR spectra of pure PVC, DEP, and PVC/DEP mixture films as well as pure PVC, DBP, and PVC/DBP mixture films before and after exposing to oxygen plasma for 10 s. These FTIR spectra are complicated, with many signatures. Here we will focus on some typical peaks to distinguish the spectral features and identify some differences

FTIR to examine the adsorbed phthalates which were leached out from the PVC/phthalate sample. To further confirm that the detected SFG signals were contributed by the migrated phthalates, attenuated total reflectance FTIR (ATR-FTIR), a technique with higher sensitivity than conventional FTIR, was also utilized here to examine if the leaching molecules were phthalate or not. That is, a precleaned ZnSe crystal was placed above the PVC/DBP films during the heating process, and subsequent ATR-FTIR measurement on this ZnSe crystal would indicate whether phthalate signals can be detected or not.

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RESULTS AND DISCUSSION Although both PVC and two phthalate molecules consist of methyl and/or methylene groups (shown in Figures 1a−c), our previous research demonstrated that it is feasible to distinguish these functional groups in the SFG spectra.39,40 The SFG spectra of PVC/DEP and PVC/DBP mixture samples before and after oxygen plasma treatment have been reported previously.39 Our previous research shows that the SFG spectrum of the pure PVC film is dominated by a 2915 cm−1 peak (symmetric stretching of CH2 groups) and has a weak 2880 cm−1 peak (symmetric stretching of methyl groups at the 4935


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Figure 4. Comparisons of FTIR spectra collected from the PVC/DBP samples with 0 wt % DBP or pure PVC (a), 5 wt % DBP (b), 15 wt % DBP (c), 30 wt % DBP (d), 70 wt % DBP (e), and 100 wt % DBP (f) before and after plasma treatment after annealing the samples at 70 °C overnight (14 h).

(1000−1800 cm−1). Likewise, for pure DBP and DEP, two peaks around 1725 cm−1 (CO stretch) and 1280 cm−1 (conjugated aromatic ester COO groups in phthalate) were chosen to represent both phthalates in the fingerprint region. A peak around 2963 cm−1 was utilized to study DBP, and a peak around 2984 cm−1 was used to study DEP in the CH stretching frequency region.

in various spectra. For pure PVC, two pronounced peaks around 2906 cm−1 (assigned to the asymmetric stretching mode of CH2) and 2954 cm−1 (assigned to the stretching mode of CH) were observed in the CH stretching frequency region (2800−3000 cm−1). The two strongest peaks, one around 1428 cm−1 assigned to the CH bending mode and the other at 1256 cm−1 contributed from CH2 bending modes,41−44 were chosen to represent PVC in the FTIR spectra in the fingerprint region 4936


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Figure 5. SFG spectra of the PVC/DBP films (sample A) without (a) and with a short time of plasma treatment (b) after annealing at 70 °C overnight.

preventing/reducing DBP migration from the PVC/DBP system below. Subsequently, the leaching behaviors of DBP molecules in PVC/DBP systems were investigated under different temperatures. Since leaching at higher temperatures should be more prominent compared to that at low temperatures, we first study the phthalate leaching at 70 °C. Because the glass transition temperature of PVC is ∼80 °C and the boiling point of DBP is ∼340 °C,1 we believe that 70 °C should be appropriate to study the migration behavior of DBP from PVC. Figure 4 compares the FTIR spectra collected from PVC/DBP films without and with plasma treatment (sample A in Figure 1d) with various DBP bulk contents after heating at 70 °C overnight (14 h). By tracing the PVC and DBP feature peaks discussed above (peaks at 2954, 2906, 1428, and 1256 cm−1 for PVC and peaks at 2963, 1725, and 1280 cm−1 for DBP), the spectral changes for each PVC/DBP film were clearly observed in Figures 4a−f. Since the spectral features changed dramatically after the longterm 70 °C heating, the corresponding FTIR spectra before and after heating were not overlapped to show the differences. Instead, the spectra are displayed offset. For pure PVC, there is almost no change after 70 °C heating treatment for both films without and with plasma treatment, showing that the heating effect on the PVC network (70 °C for 14 h) is not substantial. Thus, the spectral changes observed from PVC/DBP films which will be discussed below must be induced by other reasons (e.g., changes from DBP) rather than the thermoinduced changes in PVC. For PVC/DBP films with DBP bulk concentrations of 30 wt % and lower, Figures 4b−d show very similar spectral signatures as pure PVC, indicating that almost all DBP molecules in these samples migrate out from PVC/DBP films into air for both types of films without and with plasma treatment. The above results suggest that short-term oxygen plasma treatment did not affect the migration process substantially after heating the samples at 70 °C overnight. However, it is interesting to observe from Figures 4e−f that almost no DBP signal can be detected by FTIR after heating for films without plasma treatment. Differently, FTIR spectra still contain some DBP spectral features after heating for plasma-

After identifying the typical FTIR peaks for pure PVC and phthalates, we then studied the FTIR spectra without plasma treatment (shown in Figures 2a and 3a). As the DEP bulk concentration increases, the PVC signal intensities (e.g., 2954, 2909, 1428, and 1256 cm−1) gradually decrease, while the DEP signature peaks (e.g., 2984, 1725, and 1280 cm−1, Figure 2a) gradually dominate the FTIR spectra of the DEP plasticized PVC. The same trend was observed from the DBP plasticized PVC samples (Figure 3a). We then exposed the PVC/phthalate samples to oxygen plasma for 10 s. For DBP plasticized PVC samples (Figure 3b), almost nothing changed in the FTIR spectra for the whole series of PVC/DBP samples. Only the signal intensity of the CO stretching at 1725 cm−1 increases slightly, which is likely caused by oxygen radicals from oxygen plasma reacting with carbon atoms in the PVC chains. We therefore believe that oxygen plasma treatment only affects the surface of PVC/DBP samples (from our previous study39) and does not substantially impact the bulk structure. Differently, in the case of DEP plasticized PVC, the FTIR spectral signatures for PVC/DEP hybrid films and pure DEP film changed dramatically after the oxygen plasma treatment. Comparing to the corresponding spectra before plasma treatment, the DEP peaks became less pronounced, while the PVC signals became relatively stronger. In particular, one extreme example is that the FTIR spectrum from the pure DEP film shows no detectable DEP signals after the plasma treatment. In order to clarify the reason for these dramatic changes, a similar vacuum pumping process was performed on a pure DEP sample to simulate the pumping process used during the plasma treatment. FTIR spectrum was collected from the pure DEP film subsequently (with pumping but not plasma treatment, Figure S1 in the Supporting Information) shows that almost no FTIR DEP signals were detected, suggesting that the disappearance of the FTIR signals was caused by the evaporation of DEP molecules induced by pumping, not the plasma treatment. The possible mechanism for the phthalate loss is discussed in the Supporting Information. From this data, we believe any treatment involves long-term vacuum pumping should be avoided for PVC/DEP products. Therefore, we will focus on the discussion of the efficiency of plasma treatment on 4937


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Figure 6. SFG spectra of the samples B after annealing at 70 °C overnight. SFG spectra collected from samples B facing 5 wt % DBP (a), 15 wt % DBP (b), 30 wt % DBP (c), and 70 wt % DBP (d).

heating for 14 h, and PVC/DBP films with various DBP bulk ratios behave differently (detected in the sample bulk) during the migration process of DBP molecules, especially for the samples with high concentrations of DBP after the plasma treatment. Consequently, in order to clarify more details about the DBP leaching to understand the possible mechanisms underlined for these above observations, SFG measurements were performed to study the surfaces of PVC/DBP films with/ without plasma treatment after heating at 70 °C overnight (Figure 5). We first collected SFG spectra from PVC/DBP samples (samples A) without the plasma treatment after heating at 70 °C overnight (Figure 5a). For the pure PVC sample, the SFG spectrum shows a decrease for both the 2880 and 2920 cm−1 peaks, indicating that either the PVC surface became more disordered or these CH functional groups on the surface changed orientation after heating at 70 °C.40 For the 5 and 15 wt % DBP plasticized PVC samples, SFG spectral signatures became completely different from those collected before heating.39 After heating, the DBP spectral features (strong 2880 and 2940 cm−1 peaks) became more pronounced while the PVC spectral signatures were relatively weak (2920 cm−1 peak), implying that heating could enhance the segregation of DBP molecules to the PVC surface even for mixtures with low bulk DBP concentration (e.g., 5 wt % DBP),

treated samples. Especially, exposure to oxygen plasma for 10 s was found to be capable of partially suppressing the leaching of DBP from plasticized PVC for the sample with 70 wt % DBP after 70 °C heating overnight. Even for the pure DBP film treated with oxygen plasma, some DBP FTIR signal can still be detected after heating the sample at 70 °C overnight. The above results suggest that although short-term oxygen plasma treatment cannot totally prevent DBP from leaching out from PVC matrix after long-term heating at 70 °C, the formation of surface “protection layer” by oxygen plasma treatment would reduce the migration of DBP in high concentration films including the pure DBP film. In order to more clearly compare the trend in the spectra from the samples without and with the plasma treatment, we redraw the FTIR results shown in Figure 4 in the form of Figure S2 (Supporting Information). It is interesting that although all FTIR spectra collected from films without plasma treatment (Figure S2a, Supporting Information) exhibit very similar PVC-like spectra, the intensities of all peaks gradually decrease as the original concentration of DBP in bulk is reduced because of the gradually lowered weight proportions of PVC in bulk. Therefore, we believe these results are reliable. The FTIR results in Figure 4 indicate that DBP molecules leach out from the DBP/PVC mixtures extensively after 70 °C 4938


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Figure 7. FTIR spectra collected from PVC/DBP samples without and with plasma treatment with 0 wt % DBP, i.e., PVC (a), 5 wt % DBP (b), 15 wt % DBP (c), 30 wt % DBP (d), 70 wt % DBP (e), and 100 wt % DBP (f) before and after annealing at 30 °C overnight.

and the surface was covered mainly by DBP molecules after long-term 70 °C heating. In addition, Figure 5a shows that for the PVC/DBP films with relative higher bulk DBP ratios, e.g., 30 and 70 wt %, although the DBP spectral features (2880 and 2945 cm−1) still remained after heating, the overall intensities were much lower

than those before heating. The reduced spectral intensities may be induced by more randomly orientated surface functional groups or less coverage of the methyl groups from DBP molecules on the PVC surface after heating. Since at higher temperatures the DBP molecules segregate to the surface more, it is more likely that after heating at 70 °C, 4939


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Figure 8. SFG spectra collected from the PVC/DBP films (sample A) without (a) and with (b) plasma treatment after heating at 30 °C overnight.

the surfaces became more disordered;, i.e., the methyl groups on the surfaces have a broadened orientation distribution.40 The SFG results shown in Figure 5a are consistent with the FTIR results exhibited in Figure 4a. Figure 5b displays the SFG spectra collected from the plasma-treated PVC/DBP films after heating at 70 °C overnight. For plasma-treated pure PVC, SFG spectral features are very similar to those from the sample before heating,39 but the intensity of the 2880 cm−1 peak decreased, suggesting that after heating the methyl end groups adopt a broader orientation distribution or there was a smaller amount of surface methyl groups. Most likely the end methyl groups on the surface are more disordered. Except for the pure PVC, the SFG spectral features for other films shown in Figure 5b are very similar, exhibiting very typical DBP spectral signatures. Compared to the corresponding SFG spectra before heating,39 the broad PVC peak around 2936 cm−1 (induced by oxygen plasma on PVC) disappeared for the 5 wt % DBP film. A new peak appeared at 2945 cm−1, contributed by methyl groups. We hypothesize that the heating process at 70 °C provides energy for DBP molecules to segregate to the near surface. Longer heating time enables DBP molecules to more likely penetrate the possible “protection layer” (made by the plasma treatment) on top of the sample formed by the oxygen plasma through surface cross-linking, etc.,39 and reach the top surface then finally leach out of the sample film. If this hypothesis is correct, a shorter heating time at the same temperature (70 °C) could have different impacts. For example, in this case fewer DBP molecules could penetrate the surface “protection layer” and segregate to the top layer. Thus, the SFG spectra detected should be very similar to those before heating. We will report the results obtained from shorter heating time below. The results obtained from the studies on PVC/DBP films annealed at 70 °C for 14 h show that a substantial amount of DBP molecules segregated to the sample surface and left the sample. This is true for samples both without and with plasma treatment. Therefore, we should be able to detect SFG signals from DBP (leached from the PVC matrix) deposited on the sample B. SFG spectra were collected from the sample B placed on top of the 5 wt % DBP (a), 15 wt % DBP (b), 30 wt % DBP (c), and 70 wt % DBP (d) samples after heating at 70 °C

overnight (Figure 6). SFG signals similar to those collected from DBP with two dominant peaks around 2880 and 2945 cm−1, respectively, were detected from corresponding sample B’s, indicating that the DBP molecules leached from the PVC/ DBP films were effectively adsorbed to the surface of the clean windows. This also shows that SFG is sensitive enough to detect small amount of DBP deposited on the surface. In addition, the SFG intensities detected from the samples without the plasma treatment are stronger than those collected from the plasma-treated films for all four types of samples (5 wt % DBP, 15 wt % DBP, 30 wt % DBP, and 70 wt % DBP). As the bulk DBP concentration increases, the detected SFG signals increase, likely due to that fact that more DBP are deposited. ATR-FTIR was also used to detect the DBP molecules leached from the samples, using a similar sample preparation method for sample B shown in Figure 1d. Here, clean ZnSe crystals were placed on top of the PVC/DBP films to adsorb DBP molecules leached from PVC. Since ATR-FTIR is not as sensitive as SFG, only ZnSe crystals on top of the 70 wt % DBP film after heating at 70 °C overnight exhibit weak DBP signals (shown in Figure S3, Supporting Information). Nevertheless, this agrees with the SFG data that the leached DBP molecules were adsorbed on the sample B. Compared to the ATR-FTIR method, SFG is much more sensitive to detect DBP leached from the PVC/DBP films. Distinct SFG signals can be observed from small amount of leached DBP molecules when such amounts are well below the detection limit of ATR-FTIR. Until now, we developed a simple, nondestructive, and efficient method which is capable of providing ultrahigh sensitive detection of phthalate molecules that have migrated out of PVC/DBP films directly. Next, we will verify the sensitivity and feasibility of this method. Since many plasticized PVC products are used around room temperature, we examined samples annealed at 30 °C for 14 h. We also studied samples under shorter time annealing (at 30 and 70 °C for 2 h), the results of which are exhibited in the Supporting Information. Figure 7 displays the FTIR spectra collected from PVC/DBP films with various DBP bulk contents before and after annealing at 30 °C overnight (14 h). To better compare the spectral changes after 30 °C heating here, the FTIR spectra from 4940


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Figure 9. SFG spectra of the samples B after annealing at 30 °C overnight. SFG spectra collected from samples B facing 5 wt % DBP (a), 15 wt % DBP (b), 30 wt % DBP (c), and 70 wt % DBP (d).

for preventing DBP from leaching from the PVC/DBP film. Almost no change can be detected from the FTIR spectra contributed from the plasma-treated PVC/DBP hybrid films (>15 wt %) before and after heating (Figures 7d−f), showing that no DBP leaching occurred or the DBP leaching amount is below the detection limit of FTIR. We then used the newly developed SFG detection method to study the DBP leaching phenomena. SFG spectra were first collected from the PVC/ DBP films (sample A) without (Figure 8a) and with (Figure 8b) plasma treatment after heating at 30 °C overnight. Without plasma treatment, the PVC surface change after 30 °C heating for overnight (Figure 8a) is similar to that after 70 °C heating for overnight (Figure 5a). The signal is dominated by the CH2 symmetric stretching peak around 2920 cm−1. For all the PVC/DBP hybrid films, SFG spectra exhibit similar spectral features, dominated by two DBP signature peaks at 2880 and 2945 cm−1 (Figure 8a), indicating that all the surfaces of PVC/DBP hybrid films are covered by ordered CH3 groups from DBP molecules. Different from those shown in Figure 5a for samples after 70 °C heating, here the intensities of SFG peaks gradually increased as the concentration increases,

samples before heating (already shown in Figure 3) were presented here together with those from the samples after heating (Figure 7). Almost no spectral changes can be seen from the samples before and after heating shown in Figures 7a−c. This shows that the long-term 30 °C heating did not change the bulk of PVC/DBP films with DBP of 15 wt % or lower (including pure PVC). Therefore, for 5 wt % DBP and 15 wt % DBP, FTIR results show that DBP molecules do not leach out from PVC/DBP films after heating at 30 °C for 14 h (or the leachable amount is below the detection limit), regardless of the oxygen plasma treatment on the sample. However, for the PVC/DBP hybrid films with DBP higher than 15 wt % without the plasma treatment, the FTIR spectral features changed after heating at 30 °C overnight (Figures 7d− f). Specifically, the signals generated from PVC (for example, CH2 bending peak at 1256 cm−1) become more pronounced, while the signals contributed from DBP (for instance, the peak around 1280 cm−1) decreased. These results suggest that the overnight heating even at 30 °C induces migration of DBP molecules from the PVC/DBP films to the environment. It is exciting to observe that the oxygen plasma treatment is effective 4941


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using sensitive SFG detection, we found that DBP molecules leach from the PVC/DBP films even after heating at 30 °C for 2 h. After treating with oxygen plasma for only 10 s, no DBP molecules were observed to leach from the PVC/DBP samples (from 5 to 70 wt % DBP) after heating at 30 °C for 2 h. More interestingly, no DBP molecules were found to leach from the plasma-treated PVC/DBP films with DBP bulk concentration lower than 70 wt % after heating at 70 °C within 2 h. For the 70 wt % DBP sample, some leached DBP molecules were detected from the sample B after heating at 70 °C for 2 h, but the leaching occurred at a much less degree. This research further demonstrated the sensitivity of SFG detection on phthalate leaching and showed that plasma treatment can prevent or substantially reduce phthalate leaching. Since the sensitivity of SFG is submonolayer of the leached molecules adsorbed on a surface, assuming one molecule has an area of 1 nm2, and if SFG can detect 20% of the monolayer, for 1 cm2 sample B surface, the detection limit will be ∼30 ng. Table 1 lists all the phthalate leaching results we obtained in this study.

indicating that CH3 groups at the surface become more ordered after 30 °C heating than 70 °C heating. Compared with the SFG results of PVC/DBP hybrid films reported previously,39 DBP molecules may segregate to the surface of the PVC films and leach from the films after heating at 30 °C. To test this possibility, SFG spectra were directly collected from the sample B after 30 °C heating. As expected, two pronounced DBP peaks can be detected in the SFG spectra of sample B’s for all the PVC/DBP hybrid films without plasma treatment after 30 °C heating (Figure 9), indicating that the DBP molecules can easily leach out from the PVC/DBP films even at room temperature. This process is quite slow for low-concentration samples (5 and 15 wt %); thus, the signal change cannot be easily detected by FTIR. Herein, we again show that SFG has an excellent sensitivity in detecting plasticizer leaching. SFG spectra were also collected from the plasma-treated PVC/DBP samples after 30 °C heating overnight (Figure 8b). Compared with the corresponding SFG spectra detected from the samples before heating, almost no change was detected for all the PVC/DBP samples. In detail, for pure PVC and 5 wt % DBP films, two broad peaks around 2880 and 2936 cm−1 dominate the SFG spectra, similar to those detected from the samples before heating,.39 For the PVC/DBP hybrid films with DBP bulk concentration of 15 wt % and more, all SFG spectra exhibit two sharp DBP signature peaks around 2880 and 2945 cm−1, which are also similar to those detected from the samples before heating. SFG results show similar surface structures of plasma-treated PVC/DBP samples before and after 30 °C overnight heating. This indicates that no DBP molecules segregate to the surface for the 5 wt % DBP film (plasmatreated) after overnight heating at 30 °C. For the samples with DBP bulk concentrations higher than 5 wt %, it is difficult to conclude whether phthalate molecules leach from PVC matrix or not because the surface is dominated by DBP before and after heating. We again used clean windows as sample B to study the leached DBP molecules from plasma-treated PVC/DBP samples. SFG spectra were collected from these samples B (Figure 9). Interestingly, almost no signal was detected from sample B for the 5 and 15 wt % DBP films after overnight 30 °C heating, indicating that plasma treatment effectively prevents the migration of DBP from 5 and 15 wt % DBP films. However, for other PVC/DBP films with higher DBP bulk concentrations, DBP signals with different peak intensities and ratios were detected by SFG, indicating that DBP molecules leach from the PVC/DBP hybrid films. Generally, the intensities of the DBP peaks detected here from sample B’s are smaller than those collected from the samples before plasma treatment, proving that oxygen plasma treatment can effectively reduce the migration of DBP from PVC matrix at 30 °C. Additionally, we can also conclude that the DBP bulk concentration plays very important roles in the phthalate migration. The lower the bulk phthalate concentration, the less leaching was detected. The above results are consistent with the previous observations.1,11 Furthermore, to better demonstrate the sensitivity of the SFG technique to detect phthalate leaching, we applied FTIR and SFG to study PVC/DBP samples without and with plasma treatment after heating the samples for 2 h at 30 and 70 °C, respectively. The corresponding FTIR results and SFG spectra collected from both sample A and sample B are presented in the Supporting Information (Figures S5−S7: heating at 30 °C for 2 h; Figures S8−S10: heating at 70 °C for 2 h). In summary,

Table 1. Migration of DBP Molecules from PVC/DBP Samples with Various DBP Bulk Concentrations with and without Plasma Treatment under Different Heating Conditionsa experimental conditions 30 °C, 2 h

30 °C, 14 h

70 °C, 2 h

70 °C, 14 h

without plasma plasmatreated without plasma plasmatreated without plasma plasmatreated without plasma plasmatreated

5 wt % DBP

15 wt % DBP

30 wt % DBP

70 wt % DBP

yes

yes

yes

yes

no

no

no

no

yes

yes

yes

yes

no

no

yes

yes

yes

yes

yes

yes

no

no

no

yes

yes

yes

yes

yes

yes

yes

yes

yes

“Yes” means leaching was detected while “no” indicates that no leaching occurred.

a

The results listed in Table 1 show the following: First, DBP molecules leach from all the PVC/DBP films (with 5−70 wt % DBP) into environment at 30 °C, which is close to room temperature. However, this process is very slow and is difficult to detect from the bulk structure changes using FTIR. Second, heating temperature plays an important role in the leaching behavior of phthalates. Higher temperature facilitates the leaching process of DBP from the PVC/DBP film. Third, the bulk concentration of DBP influences the phthalate leaching. Finally, we found that short-term plasma treatment can prevent or greatly suppress the phthalate leaching from the PVC/DBP films. As we discussed above and in a previous publication,39 oxygen plasma could induce surface oxidation, possible chain scission, and cross-linking. We believe that a “protection” layer could be formed after plasma treatment on the surface; thus, phthalate leaching could be reduced. Note that the leaching processes studied here are based on the PVC/phthalate model systems. For real PVC products containing thermostabilizers, etc., the leaching process should be much slower. However, the 4942


Langmuir fundamentals for the leaching processes, such as temperature dependences, bulk concentration effects, and plasma treatment influences, should be similar. For the DEP/PVC samples, oxygen plasma treatment was not suitable to prevent/reduce phthalate leaching. However, the discussed SFG detection method can be used to study the leaching process of DEP. FTIR and SFG experiments were carried out to study the PVC/DEP systems without plasma treatment after heating. The results showed that even just after heating at 30 °C for 2 h, pronounced DEP signals can be detected from the sample B placed above the 5 wt % DEP film, and significant changes can also be observed from FTIR spectra (data not shown here), indicating that DEP leaches from the PVC matrix easily. These results are consistent with our previous report.39

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REFERENCES

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CONCLUSION In this paper, on the basis of the previous understanding on the surface and bulk behaviors of the two phthalate/PVC systems,39,40 we applied a simple but efficient method to directly observe molecular leaching behaviors of the two different phthalate molecules from plasticized PVC samples to the environment. We used SFG and FTIR to detect molecular level information about the surface and bulk of plasticized PVC without and with oxygen plasma treatment after heating under different temperatures for different time durations. DEP leaches from the plasticized PVC films easily, and plasma treatment is not useful to prevent DEP from leaching. For DBP/PVC films, without plasma treatment, DBP can leach after heating at 30 or 70 °C for both short and long times. Surface treatment by oxygen plasma will efficiently prevent/reduce the leaching process of DBP. We demonstrated that SFG is very sensitive to detect phthalate leaching, and the detection limit of this method will be as low as ∼30 ng. Also, we believe the method using SFG and FTIR to study the leaching process is general and should be widely applicable to detect small amounts of chemical and even biological environment contaminants with extreme sensitivity and at the same time provide molecular surface structural information. In our recent study, we also applied secondary ion mass spectrometry (SIMS) as a supplemental tool to SFG and FTIR to study surface and bulk changes of phthalate plasticized PVC after UV irradiation.45 We believe that the incorporation of SIMS study in the future will provide further understanding on phthalate leaching mechanisms. ASSOCIATED CONTENT

S Supporting Information *

Details about SFG experimental setup, FTIR spectrum collected from the pure DEP film with and without pumping, the possible mechanism for the phthalate loss, and a serial of SFG and FTIR results showing various migration processes of DBP from PVC under short-time treatment. This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

This research is supported by the NSF (CHE 1111000). The authors thank Ms. Jeanne M. Hankett for insightful discussions.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Z.C.). Notes

The authors declare no competing financial interest. 4943


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Observing Phthalate Leaching from Plasticized Polymer Films at the

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