Molecular Surface Structural Changes of Plasticized PVC Materials

Feb 27, 2013 - *E-mail: [email protected]. ... to investigate the surface and bulk structures of phthalate plasticized poly(vinyl chloride) (PVC) at the...
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Molecular Surface Structural Changes of Plasticized PVC Materials after Plasma Treatment Xiaoxian Zhang, Chi Zhang, Jeanne M. Hankett, and Zhan Chen* Department of Chemistry, University of Michigan, 930 North University Avenue, Ann Arbor, Michigan 48109, United States

ABSTRACT: In this research, a variety of analytical techniques including sum frequency generation vibrational spectroscopy (SFG), coherent anti-Stokes Raman spectroscopy (CARS), and X-ray photoelectron spectroscopy (XPS) have been employed to investigate the surface and bulk structures of phthalate plasticized poly(vinyl chloride) (PVC) at the molecular level. Two types of phthalate molecules with different chain lengths, diethyl phthalate (DEP) and dibutyl phthalate (DBP), mixed with PVC in various weight ratios were examined to verify their different surface and bulk behaviors. The effects of oxygen and argon plasma treatment on PVC/DBP and PVC/DEP hybrid films were investigated on both the surface and bulk of films using SFG and CARS to evaluate the different plasticizer migration processes. Without plasma treatment, SFG results indicated that more plasticizers segregate to the surface at higher plasticizer bulk concentrations. SFG studies also demonstrated the presence of phthalates on the surface even at very low bulk concentration (5 wt %). Additionally, the results gathered from SFG, CARS, and XPS experiments suggested that the PVC/DEP system was unstable, and DEP molecules could leach out from the PVC under low vacuum after several minutes. In contrast, the PVC/DBP system was more stable; the migration process of DBP out of PVC could be effectively suppressed after oxygen plasma treatment. XPS results indicated the increase of CO/C−O groups and decrease of C−Cl functionalities on the polymer surface after oxygen plasma treatment. The XPS results also suggested that exposure to argon plasma induced chemical bond breaking and formation of cross-linking or unsaturated groups with chain scission on the surface. Finally, our results indicate the potential risk of using DEP molecules in PVC since DEP can easily leach out from the polymeric bulk.

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INTRODUCTION Plastics play important roles in modern life. Plastics can be found everywhere, from household items to public utilities. Among all the plastics, poly(vinyl chloride) (PVC) has the second largest market share of polymeric materials due to its low production cost and versatile properties.1 Usually, PVC materials are mixed with plasticizers to obtain desired properties such as flexibility, transparency, and durability. This allows PVC to be used in a variety of products such as blood and urine bags, transfusion tubes, packaging materials, toys, bathroom curtains, and kitchen floors.2 Depending on the required properties, the amount of plasticizers in PVC can vary substantially. In some cases, the content of plasticizers can even be 70 wt %. Among all the plasticizers, phthalates dominate PVC production market due to its low cost. Di(2-ethylhexyl) phthalates (DEHP), as well as some other phthalate-based additives, including di(butyl) phthalate (DBP) and di(ethyl) phthalate (DEP), etc., have been used widely in PVC.2 More © 2013 American Chemical Society

importantly, low molecular weight phthalates, e.g. DEP and DBP, except for using as additives for polymers, are common constituents of personal care products, such as cosmetics, shampoos, and fragrances as well as printing inks and enteric coatings of pharmaceutical pills. Millions of tons of plasticizers are consumed every year globally.3 Human beings have a lot of opportunities to contact with these phthalate-added products in daily lives. However, as plasticizers in PVC, phthalate molecules do not covalently bond to the polymeric network, which permits good elasticity but also induces release of phthalates from the bulk to the environment. Consequently, as PVC materials age and break down, the release of phthalates from polymeric materials may accelerate and increase human exposure to leached phthalates. Received: January 8, 2013 Revised: February 18, 2013 Published: February 27, 2013 4008

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For the above purpose, we investigated the surface and bulk structures of a PVC/phthalate system using sum frequency generation vibrational spectroscopy (SFG). SFG is a secondorder nonlinear optical spectroscopic technique with surface sensitivity, which has been widely applied to detect chemical structural information on various polymer surfaces and buried interfaces in the past 10 years.23−41 In 2011, we developed a detection platform which is capable of providing both surface and bulk structures of materials without moving the sample via combining coherent anti-Stokes Raman scattering spectroscopy (CARS) and SFG.42 Using this platform, we reported the molecular surface and bulk structures of DEHP plasticized PVC films.43 In addition to DEHP, other phthalate additives are also widely used as plasticizers in PVC. Especially, phthalates with shorter chain lengths than DEHP, such as DEP and DBP, are much easier to mobilize and migrate from PVC matrix due to their weaker interactions with PVC than DEHP. Predictably, higher risk to human health would be induced when using these shorter chain phthalates in PVC. In this research, we continued to study the surface and bulk information on phthalates (with shorter chain lengths than DEHP) plasticized PVC films. The surface and bulk behaviors of DEP and DBP were comprehensively examined at the molecular level as they mixed with PVC to form thin films at various weight percentages. In order to verify the efficacy of surface plasma treatment on preventing or reducing phthalate migration at the molecular level, two different kinds of plasma, oxygen and argon plasma, were employed to treat the plasticized PVC. Through comparing the different surface and bulk behaviors of DEP and DBP in PVC matrix before and after plasma treatment, possible threats to the environment will be evaluated as these phthalate additives are utilized in our daily lives.

The shorter chain phthalates like DEP and DBP leach out from polymeric matrices much easier compared to longer chain phthalates and therefore have higher potential to come into contact with humans and other animals. Recently, increasing reports have discussed toxic and hazardous influences of phthalates on human health and environments. For instance, DBP was added to a list of suspected teratogens in 2006 and was claimed to be a suspected endocrine disruptor for mammals. Several studies suggested that DEP can cause damage to the nervous system as well as to the reproductive organs in both males and females.4,5 In 2011, a food safety scandal reported in Taiwan was quickly aroused global attention. In this case, the overuse of phthalate as a clouding agent substituted for palm oil in food and drinks was found, which induced many health problems in children since phthalate may affect hormones and reproducing organs.6,7 According to thermodynamic reasons, plasticizers may tend to migrate to the PVC surface. The migration of phthalates from PVC not only leads to a progressive loss of its initial properties but, more importantly, also implies potential serious health hazards. The leached phthalates may contaminate their contacted environment and get into the human body directly. Therefore, extensive publications have been reported to investigate the possible solutions for the phthalate leaching issue.8−13 Among them, mainly two kinds of methods were included. The first is the development of new plasticizers. For example, some immobile plasticizers such as hyperbranched polymers13 or phthalate modified to be covalently bound to PVC14 have been studied to replace phthalates in flexible PVC. Some other biodegradable plasticizers known as green plasticizers have become a recent research focus.15−17 The second method reported to prevent leaching is plastic surface modification.9−11,18−21 Surface plasma treatment and other surface modification techniques have been utilized extensively in industry. For example, PVC surfaces may be coated with polymers such as acrylates or polyesters or grafted with monomers via gamma radiation to reduce or prevent plasticizer migration.10,18,19 Peroxide treatment and ultraviolet (UV) irradiation9 were also used to introduce cross-linked structures to the PVC surface.9,21 Besides these nonplasma surface modification techniques, the efficacy on preventing phthalate migration in plastics using different plasma treatments has been extensively examined.8,22 Although many research groups have previously reported migration-related issues on different kinds of phthalates in PVC, most of them focused on bulk leaching. However, understanding phthalate behaviors at plastic surfaces is important because the phthalate molecules leach out from PVC and contact the environment at the surface. The behaviors of phthalates on the PVC surface at the molecular level are still unclear. In addition, there is limited information about the surface chemical signatures of phthalates compared to those in the bulk. The lack of in situ molecular level characterization on such surfaces compared to bulk material somewhat hinders further research in this field. Therefore, we developed a comprehensive detection platform, which can give efficient information about surface chemistry of phthalate plasticized PVC matrix at the molecular level, while also being capable of providing insights into polymers’ bulk signatures with high resolution. This leads to a completed picture on how phthalate molecules behave at the surface and bulk at the same time and further verify the efficacy of various methods to reduce or stop the migration of phthalate from PVC matrix efficiently.

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EXPERIMENTAL SECTION

DBP (analytical standard) and DEP (analytical standard) were purchased from Fluka (St. Louis, MO). Tetrahydrofuran (THF; ≥99.9% purity), concentrated sulfuric acid (reagent grade), potassium dichromate, and poly(vinyl chloride) (Mw 62 000, in pellet form) were obtained from Sigma-Aldrich (St. Louis, MO) and used as received. Fused silica windows were used to deposit pure PVC, phthalate, and PVC/phthalate hybrid films for SFG and CARS spectroscopic measurements. Phthalates (DEP or DBP) and PVC were mixed with different weight percentages (seven proportions were applied: pure PVC, 5 wt % phthalate, 15 wt % phthalate, 30 wt % phthalate, 50 wt % phthalate, 70 wt % phthalate, and pure phthalate) and dissolved in THF with 1:30 weight ratio of PVC:THF. Note that all the percentages we mention in this paper are percentages by weight. Phthalates and PVC hybrid films were prepared by spin-coating (3000 rpm, 30 s) using a P-6000 spin-coater (Speedline Technologies). The films were then placed in a vacuum chamber to completely evaporate the solvent residues. Subsequently, a commercial plasma system (PE50, Plasma etch) was used to provide oxygen and argon plasma treatment on the deposited sample films. The treatment time was 10 s at 150 mbar pressure, with oxygen and argon gas flows of ∼2 sccm (standard cubic centimeters per minute) and 10 sccm, respectively. The film thicknesses were measured by a depth profilometer (Dektak 6M Stylus Surface Profilometer, Veeco), and the average thicknesses were around 200 nm. The SFG setup used in this research was a commercially available system purchased from EKSPLA. The optical setup has been reported in detail previously.24,28 The output of the Nd:YAG laser is a 1064 nm near-IR beam (20 Hz, 20 ps). The visible 532 nm input beam for SFG experiments is generated by frequency-doubling part of this 1064 nm IR beam. The IR input beam can be tuned from 1300 to 4300 cm−1. For all the SFG experiments performed here, the incident angles of the 4009

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visible and the IR input beams were 60° and 55° with respect to the surface normal, respectively. The diameters of the two input beams at the surface were around 500 μm. The SFG spectra were collected in the ssp (s-polarized signal output, s-polarized visible input, and ppolarized IR input) polarization combination. As we reported previously, our SFG system can be used to collect CARS spectra as well.42 For CARS, the 532 nm visible beam was overlapped with a frequency tunable visible beam generated from the OPG/OPA system rather than the frequency tunable IR beam. In this research, CARS spectra were detected using the ssss polarization combination (spolarized signal output, pump, probe, and Stokes input beams). Both SFG and CARS spectra were measured by using the reflection geometry. For each sample, spectra were collected at least at five different spots; for each spot, five spectra were detected to examine the film homogeneity and reduce the influence of the detection noise. X-ray photon electron spectroscopy (XPS) was also used to study plasticized PVC samples. XPS characterization (Kratos AXIS Ultra DLD XPS) was performed at the Electron Microbeam Analysis Laboratory (EMAL) of the University of Michigan. A monochromatic Al source was utilized to provide an energy resolution better than 0.5 eV (measured from a standard silver sample). Charge neutralization was applied to eliminate electron charges on the surfaces of samples during all measurements. Software CASA XPS was implemented for spectra fitting.

centers in the SFG and CARS spectra, permitting the possibility to distinguish similar functional groups from different molecules. For instance, the CH2 groups in DBP and DEP are next to each other in aliphatic chains, and this CH2 symmetric stretching signal usually appears at about 2850 cm−1 according to previous literature.44 For PVC, however, CH2 groups are located in every other position along the PVC backbone. Isotope-labeled experiments on PVC as well as relevant IR studies carried out on poly(vinyl alcohol) (PVA), whose methylene groups possess similar surrounding environments as PVC, provided sufficient evidence that the symmetric stretching peaks of CH2 groups in PVC are centered at 2910− 2915 cm−1.45−48 Figure 2a exhibits the SFG spectra taken from the surfaces of the pure PVC film, the PVC/DBP hybrid films with different DBP bulk concentrations (5% DBP, 15% DBP, 30% DBP, 50% DBP and 70% DBP) as well as the pure DBP film. We can see that the SFG spectrum of the pure PVC film is dominated by a peak around 2915 cm−1, which can be assigned to the symmetric stretching signal of CH2 groups in the PVC backbone. There is also a weak spectral peak around 2880 cm−1 which may be due to the symmetric stretching modes of methyl groups at the end of the PVC long chain, as reported previously.44 Even though the overall concentration of methyl end groups is much lower than PVC backbone groups, the end groups are usually more flexible so that they can segregate to the surface to generate SFG signal. For the pure DBP film, two main peaks around 2880 and 2945 cm−1 are present in the SFG spectrum, indicating unique signatures of DBP compared to PVC. These two peaks are generated from the methyl symmetric stretching and Fermi resonance, respectively.43 Different from the weak and broadened 2880 cm−1 peak shown in the PVC spectrum, the methyl symmetric stretching peak here is much sharper and stronger. Two small peaks around 2855 and 2920 cm−1 were observed in the SFG spectrum of DBP as well. These two weaker peaks can be assigned to methylene symmetric and asymmetric stretching in DBP, respectively. The SFG spectra of PVC/DBP hybrid films with different DBP bulk concentrations are displayed in Figure 2a as well. First, we found that the DBP spectral signature (two speaks

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RESULTS AND DISCUSSION The chemical structures of PVC, DBP, and DEP are shown in Figure 1. We can see in Figure 1a that PVC is a linear polymer

Figure 1. Molecular structures of PVC (a), DEP (b), and DBP (c).

with chlorides on alternating carbon centers. DEP (Figure 1b) and DBP (Figure 1c) are esters of phthalic acids with ethyl and butyl groups in both of the ester chains, respectively. Although both methyl and methylene groups exist in PVC and phthalate molecules at the same time, different local chemical environments surrounding the functional groups yield different peak

Figure 2. SFG ssp spectra of DBP/PVC (a) and DEP/PVC (b) samples with different phthalate bulk concentrations. 4010

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Figure 3. CARS ssss spectra of DBP/PVC (a) and DEP/PVC (b) samples with different phthalate concentrations.

around 2880 and 2945 cm−1) can be detected by SFG even at a very low DBP bulk concentration (5%), demonstrating that DBP molecules present on the surface even at this low concentration. Second, as the concentration of DBP increases from 5% to 30%, CH3 signals (∼2880 and ∼2945 cm−1) become stronger while CH2 signals (2920 cm−1) dramatically decrease, demonstrating that the DBP signatures gradually dominate the SFG spectra as its bulk concentration increases. Third, as the bulk concentration of DBP exceeds 30%, the SFG spectra become very similar, dominated by the DBP contribution. This suggests that at a DBP bulk concentration of 30% or greater the surfaces of these hybrid films are mainly covered by DBP molecules. Because the peaks at 2880 cm−1 detected from the plasticized PVC samples are much sharper and stronger than the ∼2880 cm−1 peak in the pure PVC spectrum, we believe that these peaks are generated from the methyl groups in DBP molecules. There are small peaks around 2915−2920 cm−1 in the SFG spectra of 30%−70% DBP hybrid films, which become almost undetectable in the spectrum from the film with 70% DBP. Very likely this peak is contributed by the methylene asymmetric stretching mode in DBP here. Figure 2b displays the SFG spectra of PVC/DEP system. Similar to DBP, the SFG spectrum of pure DEP is mainly contributed from CH3 vibration modes. Two dominant speaks, 2880 and 2945 cm−1, can also be assigned to CH3 symmetric stretching and Fermi resonance, respectively. Additionally, the relative intensity of these two peaks is different from the DBP spectrum. For DBP, the intensities of these two peaks are comparable, and the 2880 cm−1 peak is slightly stronger. In the case of DEP, however, the CH3 symmetric stretching peak is much weaker than Fermi resonance. Also, a small peak around 2915 cm−1 is generated from the surface of the pure DEP film. This peak can be assigned to the methylene asymmetric stretching mode, similar to that of DBP. In addition to pure DEP and PVC spectra, the SFG spectra generated from the PVC/DEP hybrid films are also displayed in Figure 2b. Generally, as the bulk concentration of DEP increases from 5% to 70%, the trend in SFG spectral variation is similar to the case of DBP. At the low concentration region (5%−15%), DEP signals (2880 and 2945 cm−1) can be clearly detected from the hybrid film/air surfaces and gradually increase in intensity as

the bulk concentration of DEP increases. When the bulk concentration of DEP is greater than or equal to 30%, the SFG spectra are dominated by DEP signatures: only two strong CH3 peaks and a very weak 2915 cm−1 peak are observed in the spectra, indicating that the surfaces are mainly covered by DEP molecules. In order to compare the surface signatures of phthalate plasticized PVC with its bulk signatures, CARS measurements were subsequently carried out on the corresponding samples studied above, shown in Figures 3a and 3b, respectively. Clearly, both PVC/DBP and PVC/DEP spectra are dominated by PVC bulk signatures when the percentages of phthalates are lower than 30% because PVC is the major component in these samples. Contributions from the PVC CH2 symmetric stretching modes (2915 cm−1) and the relatively weak peak (∼2975 cm−1) can be observed in these CARS spectra. The ∼2975 cm−1 signal in PVC is from the stretching of C−H in CHCl.45,47,49,50 With more phthalate in the sample, this peak is shifted to 2970 cm−1 and can also be contributed by the methyl groups in the plasticizer. Symmetric stretching peaks are much stronger than those from the asymmetric modes in the ssss CARS spectrum, which provides further evidence that the 2915 cm−1 signal is from the CH2 symmetric stretch in PVC. As the percentage of DBP reaches 30%, except for the PVC characteristic peaks, three DBP peaks (∼2860, ∼2945, and ∼3075 cm−1) appear gradually, which are generated from the phthalate CH2 asymmetric stretch, Fermi resonance of CH3 symmetric stretch, and the vibration of phenyl groups (υ20b/ B1), respectively.51 This is reasonable because the samples contain more phthalate molecules. Additionally, it is noticeable that CARS spectra of pure DBP and DEP films are remarkably different. For DBP, the spectrum signature is very similar to the signals generated from 70% DBP films; the only difference is the relative intensity ratio of peaks. The peak at 2910 cm−1 is assigned to the CH2 asymmetric stretching mode of DBP here. However, this peak becomes almost undetectable in the spectrum of DEP, which may due to the chain length differences of two phthalates and also the influence of different local environments on the molecules vibrations. In order to further understand the role that plasma treatment plays in the phthalate migration, we studied the changes of 4011

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Figure 4. SFG ssp spectra of DBP/PVC (a) and DEP/PVC (b) samples with different phthalate concentrations after oxygen plasma treatment.

Figure 5. CARS ssss spectra of DBP/PVC (a) and DEP/PVC (b) samples with different phthalate concentrations after the oxygen plasma treatment.

reported before.43 Compared to the spectrum of PVC shown in Figure 2a, the main peak undergoes broadening and shifts from 2920 to 2930 cm−1. The appearance of the 2930 cm−1 peak may result from the overlap of two peaks (2920 and 2940 cm−1) and/or peak shift induced by the loss of chlorine in the PVC long chains due to the possible PVC chain scission. This will be further interpreted by XPS results shown below. The SFG spectrum of 5% DBP maintains this characteristic peak but displays a stronger CH3 symmetric (2880 cm−1) peak, which may be due to the increase of DBP on the surface. It is also interesting to observe that at and above a DBP bulk concentration of 15% all SFG spectra show very similar DBP signatures, mainly two CH3 peaks (2880 and 2945 cm−1) in the spectra. This spectral trend found after oxygen plasma treatment may be explained by DBP segregation to the surface under the vacuum condition during the plasma treatment, which led to a higher concentration of DBP near the surface. More experimental evidence is needed to further support this conclusion.

surface and bulk molecular signatures of PVC and phthalates after plasma treatment. Oxygen and argon, two of the most commonly used kinds of plasma for polymer surface treatment, were utilized in this research to treat the surfaces of PVC/DBP and PVC/DEP films. The corresponding SFG and CARS spectra detected after the plasma treatment are displayed in Figure 4 (SFG results of samples after oxygen plasma treatment), Figure 6 (SFG results of samples after argon plasma treatment), Figure 5 (CARS results of samples after oxygen plasma treatment), and Figure 7 (CARS results of samples after argon plasma treatment). First, oxygen plasma treatment was performed on both PVC/ DBP and PVC/DEP samples. Figure 4a shows the SFG spectra collected from the PVC/DBP hybrid films in air. Clearly, for the pure PVC film, the main change in the spectrum signature is that a broad peak around 2930 cm−1 now dominates the spectrum. The CH3 symmetric stretching peak around 2880 cm−1 can also be detected. This spectrum is very similar to that of a PVC film after atmosphere plasma treatment for 5 s 4012

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Figure 6. SFG ssp spectra of DBP/PVC (a) and DEP/PVC (b) samples with different phthalate concentrations after the argon plasma treatment.

of DEP from PVC during the pumping process and also indicates SFG possesses better surface sensitivity than CARS. Besides oxygen plasma, argon plasma was applied to treat the surfaces of the plasticizers, PVC, and plasticized PVC films. According to literature, argon plasma treatment exhibits better effects on the inhibition of phthalate leaching from PVC to the environment than other plasma treatments.8 Figures 6 and 7 display the corresponding SFG and CARS results on the two types of samples after the argon plasma treatment, respectively. The SFG spectra detected from the PVC/DBP films after the argon plasma treatment are shown in Figure 6a. The SFG signal intensity detected from the pure PVC film after the argon plasma treatment becomes much weaker compared to the signal before plasma treatment. The PVC methylene peak around 2920 cm−1 in 5% DBP sample is also weaker than that in the spectrum collected before the argon plasma treatment. The SFG spectra detected from other DBP plasticized PVC samples show similar DBP signatures, indicating that the surfaces are covered by DBP molecules after the argon plasma treatment. We can also see from Figure 6 that the 2880 cm−1 peak becomes much weaker in the spectrum collected from the PVC film after the argon plasma treatment. This may be interpreted as an increase in disordered methyl end groups on the surface, induced by exposure to the argon plasma. The substantial decrease of the PVC methylene peak in the SFG spectra detected from the samples containing low-concentration DBP is not clear here. It may be due to the disordering of surface groups and the loss of certain surface elements such as chlorine or hydrogen, cross-linking, or the formation of unsaturated groups with chain scission caused by the argon plasma treatment.52,53 Further details will be examined by CARS and XPS results below. Figure 6b presents the SFG results of PVC/DEP samples after the argon plasma treatment. Similarly, for samples containing low concentrations of DEP, the PVC methylene symmetric stretching signal becomes much weaker and the DEP characteristic peaks become stronger compared to those detected before the plasma treatment. At DEP concentrations of 15% and higher, all the SFG spectra display very similar features to that detected from the pure DEP spectrum. This can also be explained by the migration of DEP

Similarly, oxygen plasma was utilized to treat the surface of PVC/DEP samples as well. The corresponding SFG spectra are exhibited in Figure 4b. Different from the PVC/DBP results, except for the pure DEP, all spectra are very similar to each other and are also similar to the pure PVC spectrum detected after the oxygen plasma treatment. The only difference is that the main peak position shifts from 2930 to 2938 cm−1 as the concentration of DEP molecules increased from 0 to 15%. This peak then stays unchanged in the spectra of samples with higher bulk phthalate concentrations. This may be due to more contributions of the CH3 Fermi resonance peak at ∼2945 cm−1 in the SFG spectra of DEP. Considering that DEP is a shortchained molecule and has weak interactions with PVC, one hypothesis is that some DEP molecules migrated to the film surface and then leached out from the sample during the vacuum pumping process. This leads to a PVC surface similar to the pure PVC film at a molecular level. Therefore, all the PVC/DEP sample surfaces became very similar after the oxygen plasma treatment. This hypothesis will be further confirmed by CARS and XPS results. In order to further understand the effects of oxygen plasma treatment on our samples, CARS measurements were carried out on the samples to examine the bulk structures (Figure 5). Not surprisingly, all the CARS results of PVC/DBP are very similar after the oxygen plasma treatment, indicating that any plasma-induced reactions only occur on the sample surfaces. Clearly, no direct evidence was found to indicate that DBP molecules leached from the PVC/DBP hybrid films. However, in the case of PVC/DEP films, the CARS spectra display remarkable changes after the oxygen plasma treatment (shown in Figure 5b). Except for the 70% DEP sample, all CARS spectra become very similar just as that collected from the pure PVC film. Characteristic peaks assigned to DEP (2880 and 2945 cm−1 CH3 peaks and 3075 cm−1 phenyl peak) decrease substantially in the 70% DEP sample. Furthermore, no CARS signal can be detected from the pure DEP films after the plasma treatment (due to the evaporation of DEP in the vacuum). The disappearance or pronounced reduction of DEP signatures in the CARS spectra confirms our hypothesis on the evaporation 4013

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Figure 7. CARS ssss spectra of DBP/PVC (a) and DEP/PVC (b) samples with different phthalate concentrations after the argon plasma treatment.

Table 1. XPS Analysis on the Chemical Components of DEP/PVC Samples with Different Phthalate Concentration before and after Plasma Treatment

a

atomic cons (%)

PVC(Ba)

PVC(AOb)

PVC(AAc)

30% (B)

30% (AO)

30% (AA)

70% (B)

70% (AO)

70% (AA)

C 1s O 1s Cl 2p

70.2 0.8 29.0

73.8 13.9 7.3

69.5 10.0 20.5

71.5 1.0 27.5

74.0 16.2 9.8

70.7 8.6 20.6

71.2 1.2 27.6

72.4 17.0 10.7

69.2 9.8 20.9

B: before plasma treatment. bAO: after oxygen plasma treatment. cAA: after argon plasma treatment.

Table 2. XPS Analysis on the Chemical Components of DBP/PVC Samples with Different Phthalate Concentration before and after Plasma Treatment

a

atomic cons (%)

PVC(Ba)

PVC(AOb)

PVC(AAc)

30% (B)

30% (AO)

30% (AA)

70% (B)

70% (AO)

70% (AA)

C 1s O 1s Cl 2p

70.2 0.8 29.0

73.8 13.9 7.3

69.5 10.0 20.5

71.2 0.9 27.9

66.7 19.1 3.6

71.1 16.5 12.4

71.2 1.0 27.8

73.8 25.3 0.9

75.8 16.8 7.4

B: before plasma treatment. bAO: after oxygen plasma treatment. cAA: after argon plasma treatment.

regardless of the DEP bulk concentration, the chemical compositions of the PVC/DEP hybrid films before or after oxygen/argon treatment detected using XPS are not very different, close to that of the pure PVC film surface. This further indicates that migration and leaching of DEP indeed happen during the pumping process. This conclusion is consistent with our SFG and CARS results. Table 2 reveals the changes of surface chemical components for PVC/DBP samples before and after oxygen/argon plasma treatments. Table 2 shows very similar XPS fitting results for all PVC/DBP samples before treatment compared to the XPS spectra of PVC/DEP samples before treatment. For example, the surface chemical compositions of plasticized PVC containing 30% DBP and 70% DBP resemble the pure PVC film surface, indicating that similar migration process of phthalates occurred during the high-vacuum XPS pumping process. However, for all films after oxygen plasma treatment, oxygen signals are significantly higher while chlorine signals become lower than the samples with the plasma treatment, implying that the addition of oxygen and loss of chlorine are caused by oxygen treatment on the surfaces of DBP hybrid films. A similar trend can also be observed from the argon plasma treated samples, but only differ in the addition/

in the plasticized PVC to the surface and possible partial leaching out during the pumping process. Consequently, this leads to a higher concentration of DEP on the surface, enhancing the DEP signals in the SFG spectra. Figure 7 displays the CARS spectra of the two types of samples after argon plasma treatment, indicating very similar bulk structures to those observed after the oxygen plasma treatment in Figure 5. These results also support the hypothesis that phthalate molecules undergo a segregation process to the sample surface. Especially, low molecular weight DEP molecules are more volatile in vacuum, resulting in a large loss of DEP molecules in the bulk after the exposure to argon plasma. All the results shown above demonstrate that DEP molecules much more easily segregate to the surface and leach out from the polymeric matrix than DBP. In order to further clarify the details on surface changes during the plasma treatment process, XPS measurements were performed on various films before and after the plasma treatment. The fitting results of XPS survey spectra in a large region for two types of plasticized PVC samples are illustrated in Table 1 (PVC/DEP) and Table 2 (PVC/DBP). First we will discuss the changes of chemical components on the surfaces of all PVC/DEP hybrid films as listed in Table 1. As expected, 4014

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Figure 8. C 1s high-resolution spectra taken from (a) PVC film, (b) PVC film after the oxygen treatment, and (c) PVC film after the argon plasma treatment; (d) 30% DBP film, (e) 30% DBP film after the oxygen treatment, and (f) 30% DBP film after the argon plasma treatment; (g) 70% DBP film, (h) 70% DBP film after the oxygen treatment, and (i) 70% DBP film after the argon plasma treatment.

reduction amounts of different atoms. For instance, after argon treatment, the oxygen signals become higher than the samples without the plasma treatment but not as much as the ones after the oxygen plasma treatment. These results suggest that some oxygen remains in the vacuum chamber during the argon plasma treatment, inducing some oxidation reactions on the polymer surface in addition to the physical ablation caused by the pure argon plasma treatment.52,53 Different from the unchanged surfaces of samples without the plasma treatment, in the case of oxygen plasma treated samples, chlorine signals decrease while oxygen signals increase as the bulk DBP concentration increases. This trend is reasonable since the amounts of DBP molecules on the film’s surface likely increase as the concentration of DBP increases. These results also imply that, for the PVC/DBP samples, oxygen treatment may be an effective method to prevent/ reduce DBP leaching out from PVC matrix. In the case of argon treatment, however, the situation becomes more complicated. As the concentration of phthalate increases from 0 to 30%, oxygen signals increase while chlorine signals decrease. However, the oxygen signals almost stay unchanged although chlorine signals still decrease as phthalates concentration increases from 30% to 70%. Note that the carbon signals increase as the total amount of oxygen and chlorine becomes lower as the concentration of DBP increases from 0 to 70%. Extensive reports in the literature claimed that physical ablation is the main effect of argon plasma treatment. Under the argon plasma exposure, hydrogen would be abstracted to form free radicals at or near the surface which then interact to form crosslinks and unsaturated groups with chain scission.53 Therefore, as the concentration of DBP increases from 0 to 70%, the increase of carbon with the reduction of oxygen and chlorine

implies that more unsaturated groups or cross-links may be formed on the surface under argon plasma exposure. To further explore details about surface chemistry after the plasma treatment, e.g., to probe the formation of new bonds or breakage of some original chemical bonds, etc., C 1s highresolution spectra were taken from PVC/DBP and PVC/DEP samples before and after oxygen or argon plasma treatment. A typical C 1s envelope includes structure that offers precise chemical information about a sample; however, spectral fitting is necessary to provide more quantitative information about the sample. Here, all the C 1s high-resolution spectra were fitted, and the corresponding fitting results as well as the experimental results are displayed in Figures 8 and 9. All the fitting parameters are listed in Table 3. Figures 8 and 9 show the C 1s high-resolution spectra of PVC/DBP and PVC/DEP series of films separately. Comparing all samples before and after plasma treatment, both C−C/ C−H and C−O/CO signals become higher while C−Cl become lower, indicating breaking of the C−Cl bond as well as the formation of new bonds such as C−C/C−H or C−O/C O bonds after the plasma treatment. Clearly, for all kinds of DEP samples (before plasma treatment, after oxygen/argon plasma treatment), similar spectra are observed for pure PVC, plasticized PVC containing 30% DEP, and 70% DEP films, further confirming that DEP molecules have leached out from PVC matrix during the plasma treatment process, and the surfaces are dominated by the PVC component (shown in Figure 9). Similar results were also observed for PVC/DBP samples before the plasma treatment (shown in Figure 8). That is to say, without the plasma treatment, DBP molecules would migrate to the surface and leach from the PVC to the vacuum in the XPS chamber. These results are consistent with previous 4015

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Figure 9. C 1s high-resolution spectrum taken from (a) 30% DEP film, (b) 30%DEP film after oxygen treatment and (c) 30%DEP film after argon plasma treatment, (d) 70% DEP film, (e) 70% DEP film after oxygen treatment, and (f) 70% DEP film after argon plasma treatment.

Cl signals become lower. The CO signal especially becomes much higher as the concentration of DBP increases from 0 to 70%. These results may also suggest that the increased amounts of phthalate molecules in O2 plasma-treated hybrid films. As a consequence, combining SFG and especially CARS results, this proves again the effectiveness of O2 plasma for reducing phthalate migration. However, for the Ar plasma-treated samples, although CO signals increase a little bit as the concentration of DBP increases from 30% to 70%, the C−O signals decrease at the same time, which makes the total C−O/ CO coverage lower. Meanwhile, the C−Cl signals are almost unchanged. Consequently, it is difficult to judge if argon treatment is effective for preventing phthalate molecules inside the PVC matrix from leaching under vacuum conditions or not. However, the increasing amounts of C−C/C−H as DBP bulk content increases imply that as exposed to Ar plasma, polymeric surface may undergo chain scissions and formation of unsaturated bonds. These data are consistent with SFG data shown in Figure 6. We can get a clear picture of how these two kinds of phthalates behave in PVC based on the previous analysis. DEP was proved to be a very unstable additive, which can easily leach out from the polymeric matrix. CARS results showed very little DEP signal from the DEP/PVC mixtures under a low vacuum condition for a very short time, indicating only small amount of DEP remained on the PVC surface. On the other hand, our studies indicate that DBP is much more stable than

Table 3. Analysis Results from C 1s Peaks in the XPS Spectra atomic cont (%)

C−C/C−H

C−Cl

PVCa PVC(AOb) PVC(AAc) 30% DBP 30% DBP(AO) 30% DBP(AA) 70% DBP 70% DBP(AO) 70% DBP(AA) 30% DEP 30% DEP(AO) 30% DEP(AA) 70% DEP 70% DEP(AO) 70% DEP(AA)

50.8 56.5 57.0 50.3 57.4 59.6 44.6 59.1 62.7 50.9 56.2 57.9 44.3 56.3 57.7

49.2 26.6 33.1 49.7 19.7 26.9 45.5 15.3 26.7 49.1 26.7 32.8 46.2 26.2 30.6

C−O

CO

12.1 9.9

4.9

16.3 7.7 9.9 9.0 2.0

6.6 5.8

12.2 6.0 9.9 12.3 7.0

16.6 8.6 5.0 3.4 4.8 4.7

a Nontreated samples. bAO: after oxygen plasma treatment. cAA: after argon plasma treatment.

SFG and CARS results. Notice that weak C−O signals were detected on the 70% DEP/DBP surface before plasma treatment, which may suggest that a small amount of phthalate molecules stay on the surface. These results are consistent with the SFG results shown in Figures 4 and 6. For the PVC/DBP samples after oxygen treatment, both C− C/C−H and C−O/CO signals become higher while the C− 4016

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Langmuir DEP. DBP can remain in PVC films under a low-vacuum condition but will also gradually segregate to the surface and leach out from PVC if placed under high vacuum for a long period of time. O2 plasma treatment was shown to be an effective method to prevent/reduce DBP leaching out from PVC through a combination of CARS and XPS studies. However, the effect of argon plasma treatment is not clear in our studies. Future work will focus on the dynamic studies on the PVC/phthalate systems after plasma treatment and in-situ observation of the migration process of different phthalate molecules under different conditions (e.g., temperature or time) at the molecular level, further elucidating the efficiency of plasma treatment on solving phthalate migration/leaching issues.

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CONCLUSION

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AUTHOR INFORMATION

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ACKNOWLEDGMENTS

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REFERENCES

Article

This research is supported by the NSF (CHE 1111000). XPS spectra were obtained at the University of Michigan’s Electron Microbeam Analysis Laboratory.

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In this paper, two kinds of phthalate molecules have been studied to elucidate their different surface and bulk behaviors in PVC/phthalate hybrid systems. From SFG results, we found that phthalate signals increased gradually as the phthalate bulk concentration increased from 0 to 70% in both DEP/PVC and DBP/PVC systems, indicating phthalates were present on the surface even at very low bulk concentrations. However, after the oxygen plasma treatment, for the DBP/PVC system, the SFG signal was dominated by DBP contributions especially as the bulk concentration of DBP exceeded 15%. In comparison, the DEP/PVC surfaces became very similar, dominated by the PVC characteristic peaks except for the high DEP bulk concentration sample (70%). SFG results showed similar phthalate signatures for the surfaces of both DBP and DEP series of samples after the argon plasma treatment. Additionally, CARS measurements provided a clear picture of bulk information for these two systems, showing that bulk structure was not significantly changed after two kinds of surface treatments for the DBP/PVC system. For the DEP/PVC system, however, all spectra became very similar to that of PVC after two kinds of plasma treatment, showing the small molecules DEP almost completely leached out from PVC during the plasma pumping process. XPS results confirmed the different behaviors of DBP and DEP as plasticizers in PVC, indicating DEP molecules more easily migrated and leached from PVC while DBP molecules were more stable. The differences of two kinds of plasma were investigated in detail by analyzing high-resolution C 1s spectra, which gave further evidence on local environment changes, i.e., formation of new bonds and breaking of C−Cl bonds after plasma treatment. In summary, we demonstrated different behaviors of two phthalate molecules with different chain lengths on the surface and bulk of PVC and the resulting surface and bulk changes induced by two different types of plasma treatment. Our results illustrated embedded danger in using DEP as a plasticizer as it can easily leach out from PVC.

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 4017

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