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Terahertz Time-Domain Spectroscopy of Plasticized Poly(vinyl chloride) Stefan Sommer, Martin Koch, and Alina Adams Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b04548 • Publication Date (Web): 05 Feb 2018 Downloaded from http://pubs.acs.org on February 5, 2018

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Analytical Chemistry

Terahertz Time-Domain Spectroscopy of Plasticized Poly(vinyl chloride)

Stefan Sommera, Martin Kocha, and Alina Adamsb*

a

Faculty of Physics and Material Sciences Center, Philipps-Universität Marburg,

Renthof 5, 35032 Marburg, Germany b

Institut für Technische und Makromolekulare Chemie, RWTH Aachen University,

Templergraben 55, D-52056 Aachen, Germany

Corresponding author: E-mail: [email protected], phone: +49(0)24180-26428

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Abstract

Poly(vinyl chloride) (PVC) is today one of the most important commodity polymers. Its broad range of applications is due to the presence of plasticizers whose concentration largely impacts the microscopic and the macroscopic properties. Quantifying the concentration of plasticizer in PVC products is therefore of fundamental importance. Thus, in this paper, the applicability of terahertz (THz) timedomain spectroscopy for the characterization of plasticized PVC is for the first time evaluated in a systematic way. It could be demonstrated that the method is able to distinguish between PVC samples with different types and concentrations of plasticizers. Furthermore, a simple, fast, and efficient method is introduced to quantify the concentration of plasticizer in PVC samples of known plasticizer type but different thermal histories. The presented results are of key importance due to the need of reliable non-invasive and non-destructive analytical methods which can deliver onsite information about the remaining plasticizer concentration inside PVC products. Furthermore, it is expected that the proposed approach can be easily extended to other plasticized polymers.

Keywords: terahertz spectroscopy, plasticized PVC, plasticizer concentration, thermal aging 2 Environment ACS Paragon Plus

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Introduction Today, poly(vinyl chloride) (PVC) is the second most common produced plastic worldwide after polyolefins and has a broad range of applications in various fields of activities including medical devices, packaging, textile industry, construction, and automotive industry.1-3 PVC products consist of complex formulations including the PVC polymer and a large variety of different additives. Hereby the plasticizers make up the highest fraction of the PVC product, amounting up to 80 phr (parts per hundred rubber) of the total weight of the sample. The properties of PVC products are largely controlled by the amount of plasticizer. In particular, the plasticizers improve the flexibility of the PVC formulation by lowering its glass transition temperature Tg compared to that of the pure PVC polymer. In addition, they are expected to reduce the E-modulus, the tensile strength and the viscosity and to increase the elongation at break.3 In most PVC products, external plasticizers are chosen in order to best meet critical product requirements.4 Contrary to an internal plasticizer, the external one is not chemically bound to the polymeric chain and it can become lost from the polymer matrix with time or under the impact of various external factors.4 This, in turn, may lead to an undesired deterioration of the macroscopic properties of the material, in particular, the mechanical and electrical ones.5,6 Especially the loss of phthalate-based plasticizers, that account for most of the existing plasticizers, is of great concern due to possible environmental and human toxicity.7,8 Yet, despite of impressive efforts on introducing new approaches to minimize the plasticizer loss from PVC3,9, its long-term migration is still not known as it fails to be predicted by accelerated aging procedures and quantified onsite by nondestructive monitoring.10,11 Today various analytical techniques are widely applied to quantify the content of plasticizer in PVC. They include Fourier Transform Infrared (FTIR) spectroscopy, 3 Environment ACS Paragon Plus


Raman spectroscopy, and Nuclear Magnetic Resonance (NMR) spectroscopy11-13 with gravimetry and chromatography being the most commonly used.11 The main drawback of all these methods is that the samples have to be investigated in the laboratory and may need to undergo various preparation procedures such as plasticizer extraction. Consequently, these conventional methods cannot directly onsite quantify the concentration of the plasticizer of various PVC products. Yet, such information is the key for improving the mathematical models for predicting the plasticizer loss under various conditions and developing proper accelerated aging procedures.3,10,11 Recently, a non-destructive approach was introduced to quantify the local plasticizer concentration in a PVC product within some couple of minutes.14 It is based on proton NMR relaxometry performed with a mobile and low-cost single-sided NMR sensor.14 Single-sided NMR is a well suited analytical tool for the study of amorphous and semi-crystalline polymers.14-18 Yet, its main drawback in the case of plasticized PVC relays on the impossibility to quantify the plasticizer concentration under a threshold value when the Tg of the samples approaches the temperature where the measurements are performed (ex. room temperature). Terahertz-time domain spectroscopy (THz-TDS) is another non-invasive and non-destructive analytical method which has been successfully applied in the last years for the analysis of a large variety of materials.19-24 The THz spectra of crystalline materials consist of well-defined absorption peaks while featureless spectra are usually obtained for amorphous samples. Due to this feature, THz investigations of polymers have largely been focused on semi-crystalline polymers for quantifying their degree of crystallinity and identifying various crystalline forms and crystal transformations.25-28 Furthermore, the change in the dependence of the refractive index with the temperature at Tg has been used to quantify this 4 Environment ACS Paragon Plus

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temperature.24 Only very few studies report about the application of the THz-TDS method for amorphous polymers largely focusing on elastomers.24,29,30 For instance, it has been showed, that it can be applied for quality control during rubber extrusion and to gain information about the anisotropic optical response of optically opaque elastomers with conductive fillers.29,30 The present work investigates for the first time in a systematic way the potential of THz-TDS for characterizing plasticized un-crosslinked amorphous polymers with exemplification on plasticized PVC. Commercially available plasticized PVC is generally considered to be largely amorphous owing its extremely low degree of crystallinity and very small crystallites.31 Here it is demonstrated that all extracted optical parameters are sensitive to the presence of plasticizers in PVC over a broader range of concentrations than those accessible by single-sided NMR. Furthermore, a methodology using THz-TDS is proposed for the non-destructive quantification of the plasticizer concentration in PVC samples with known plasticizer type but different thermal histories. Calibration curves established on the fresh samples enable gaining this concentration within some couple of seconds. It is expected that the proposed methodology can be easily extended to PVC samples of other shapes than those investigated here and to other plasticized amorphous polymers.

Experimental Section Materials Industrially available PVC plates with a thickness of 1 mm and different initial amounts of DINCH (1,2-cyclohexane dicarboxylic acid diisononyl ester) and DINP (diisononyl phthalate) were studied as received. More details about the composition of the samples can be found in the Supporting Information. Pieces with defined dimensions were cut out from the PVC/DINCH plates and thermally aged by 5 Environment ACS Paragon Plus


embedding them in preheated activated charcoal in a 250 ml tin can closed with a lid. The tin can was then placed in an oven with air circulation at a temperature of 100 °C. After relevant exposure times, the samples were removed from the oven, wiped gently with a tissue to remove any residual charcoal from their surface and immediately investigated by gravimetry and terahertz spectroscopy. New pieces were used to generate samples for each exposure time.

Terahertz Time-Domain Spectrometer A standard fiber coupled THz-TDS system in transmission geometry was used to investigate the PVC samples. The photoconductive antennas are gated by a femtosecond laser at 1550 nm with a power of about 100 mW. For a detailed description of such spectrometer see for example Ref. [32]. In order to perform highly precise spectroscopy measurements, the THz beam was collimated and focused by off-axis parabolic mirrors. The whole THz path (antennas, opto-mechanics, and samples) was placed in a box purged with dry air to avoid water absorption by humidity. Thus, the recorded spectra were free of water absorptions lines. Reference measurements were carried out before and after each sample measurement. To achieve a good signal to noise a floating averaging of 1000 pulses for each measurement was performed. This procedure, which requires a total acquisition time of 44 s, helps avoiding the impact of system instabilities on the measurements and reduces experimental uncertainties.

THz Data Extraction The complex permittivity of the samples is extracted using the so called “quasi space algorithm”. This algorithm, in detail described in [33, 34] and implemented in the commercial available software TeraLyzer was used to extract the optical material


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parameters by considering the raw spectral data (transfer function) to quantify accurate values for the complex permittivity ε = ε '− iε " . The refractive index can be then determined from the real part of the complex permittivity as n = Generally, the absorption coefficient

ε' .

α increases with the frequency α ∝ f 2 for

amorphous materials.35,36 This is also the case for polar macromolecules, like PVC. For semicrystalline polymers, both the amorphous phase and the crystalline phase contribute to absorption effects. In many cases, distinct absorption peaks resulting from lattice vibrations of the polymer backbone in the crystalline regions are superimposed on the broad feature-less absorption signal from the amorphous regions.24,26 These absorption peaks are sharper in ε " versus frequency plots as shown in Ref. [27] for semi-crystalline high-density polyethylene (HDPE) samples. Therefore, the dependence of ε " instead of

α on the plasticizer concentration will be

reported.

Results and Discussion The results depicted in Figure 1 for PVC/DINCH and Figure S1 for PVC/DINP show that the extracted optical parameters, the imaginary part of the permittivity (Figure 1a and S1a) and the refractive index (Figure 1b and S1b), are sensitive to the concentration of the plasticizer inside the investigated samples but the extent strongly depends on the THz frequency for the imaginary part of the permittivity. Contrary to this, the sensitivity of the refractive index seems to be largely frequency-independent. The lower the concentration of plasticizer, the higher is the value of the refractive index. The imaginary part of the permittivity shows for both types of plasticizers a spectral feature at around 1.81 THz. The intensity of this peak becomes more pronounced at lower concentrations of plasticizer.



Figure 1. THz frequency-dependent (a) imaginary part of the permittivity and (b) refractive index of PVC/DINCH samples with different plasticizer concentrations.

Plasticized PVC has a heterogeneous morphology consisting of non-plasticized and plasticized regions.13,37,38 The non-plasticized regions are generally considered to be composed of a very low amount of small microcrystals or/and rigid amorphous polymer chains whose mobility is largely independent of the concentration of plasticizer.37,38 The plasticized regions are composed of regions consisting of plasticized polymer chains whose mobility is strongly dependent on the concentration of plasticizer and regions composed of additives, largely represented by the plasticizer molecules.14 An increase in the plasticizer concentration leads to a decrease in the fraction of non-plasticized regions and to a decrease in the glass transition temperature Tg.38,39 The observed decrease for n with increasing plasticizer concentration (Figure 1b and S1b) can be thus related to higher free volume at the measurement temperature. This higher free volume is due to the higher temperature difference to the corresponding Tg, as reported in literature.40 This decrease happens at lower extent for the same concentration of DINP as compared to DINCH due to its lower plasticization effect as observed by NMR.14 Mathematical seen, as described later on, the observed dependencies relates also to the lower values of the refractive index of the pure plasticizers compared with the non-plasticized PVC. 8 Environment ACS Paragon Plus

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It is generally well accepted that commercial available PVC has a lower degree of crystallinity of only a few percent.3,41 Yet, less conclusive data are reported about the effect of plasticizer content on the degree of crystallinity.41 One study showed that the degree of crystallinity as quantified by Differential Scanning Calorimetry (DSC) decreases with increased plasticizer concentration.42 DSC measurements (Figure S2) on selected samples investigated in our study seems to indicate a similar behavior. Therefore, the detected peak at around 1.81 THz (Figure 1b and S1b) could be related to the existence of these crystalline regions. This assignment is along with the earlier assignment of this spectral feature to a lattice mode using farinfrared measurements on highly crystalline PVC samples.43 As reported for other semicrystalline polymers, further contribution to the broad spectral feature should comes from the vibrational density of states originating from the non-crystalline regions. In the case of the PVC samples they would arise from low-frequency correlated libration motions in the non-plasticized amorphous regions and lowfrequency non-correlated libration motions in the plasticized amorphous regions. The dependence of the extracted THz parameters for the two types of PVC/plasticizer samples with the plasticizer content is depicted in Figure 2. One can clearly observe that the optical parameters are sensitive not only to the content of the plasticizer but also to the type of plasticizer. They have higher values for the samples containing DINP than for those containing DINCH in the whole range of investigated concentrations. Mathematically seen, this is also due to the lower values of the optical parameters of pure DINCH compared to the pure DINP. The observed dependencies of the optical parameters on the plasticizer concentration c is then further exploited to establish correlation curves which can be in turn used to quantify the plasticizer concentrations for samples containing the same type of plasticizer but unknown thermal histories. The correlation of the various 9 Environment ACS Paragon Plus


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optical parameters with the plasticizer concentration can best be expressed with the help of linear fits as described by the Equations (1)-(4):

nPVC/DINCH = 1.63 − 0.00135× cDINCH,

(1)

ε "PVC/DINCH = 0.20 − 0.00147× cDINCH,

(2)

n PVC/DINP = 1 .63 − 0 . 0009 × c DINP ,

(3)

ε " PVC/DINP = 0 . 19 − 0 . 00116 × c DINP .

(4)

Figure 2. Correlation curves of the optical parameters with the concentration of plasticizer established for fresh PVC samples with two different types of plasticizers. The reported ε " in (a) corresponds to the values at 1.81 THz. The refractive index values displayed in (b) are the average of the values between 0.3 THz and 1 THz. The lines depict the fit results using the Equations (1)-(4). The correlation coefficients for all fits are higher than 0.99.

The linear fits predict very close values for n and ε " for both types of samples at zero plasticizer concentration confirming thus the reliability of the used fit approach. In addition, under the assumption that the observed correlations are linear in the whole range of possible plasticizer concentrations, they enable the quantification of the optical parameters of non-plasticized PVC and of the pure 10 Environment ACS Paragon Plus

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plasticizers. For non-plasticized PVC, the estimated average values for n and ε " were 1.63 and 0.2, respectively. At 100% concentration the optical parameters of the plasticizers themselves can be obtained. They are n=1.5, ε " = 0 . 05 for DINCH and n=1.54, ε " = 0 . 08 for DINP. As expected, the extracted values for n from the THz measurements are higher than the reported values in literature at visible frequencies (see values in SI). Moreover, a relatively linear correlation between the values is observed (Figure S4), in agreement with earlier results for silicate glasses.35

Figure 3. Correlation of DINCH concentrations in PVC samples with different aging times as predicted by THz and gravimetry. The results of all aged samples are included. The continuous line is the fit result with a correlation factor higher than 0.99 using the equation c THz = − 0 . 56 + 1 . 02 × c gravimetry .

To test the applicability of the proposed method for quantifying the concentration of plasticizer on samples with known type of plasticizer, both optical parameters were measured also for aged DINCH/PVC samples. During the used aging procedure, the plasticizer migrates on the surface of the plates and from there it is adsorbed by the activated charcoal. One observes a reduction in the size of the plates with increased aging time as the polymer chains fill the empty place left by the plasticizer molecules. The extracted THz parameters (Figure S5) were then used in



conjunction with the Eq. (1)-(2) to predict the remaining plasticizer concentration in the aged samples. The determined concentrations by THz show excellent agreement with the plasticizer concentration quantified by gravimetry (Figure 3) in the whole range of the investigated concentrations, demonstrating thus the reliability of the method. In conclusion, the present work demonstrates for the first time that timedomain THz spectroscopy is a well suited analytical tool for characterizing plasticized PVC. Both the refractive index and the imaginary part of the permittivity are sensitive to the concentration and type of plasticizer. In addition, the spectral feature of ε " at 1.81 THz seems to be characteristic solely for PVC samples24 highlighting thus the possibility to use it as a fingerprint for chemical identification of this type of polymer from other polymer materials. Furthermore, the proposed methodology using correlation curves shows great potential for quantifying the remaining plasticizer concentration in PVC samples with known type of plasticizer and thermal history. Important advantages of the proposed methodology are its rapidity and simplicity and thus it is expected that in combination with a mobile THz spectrometer this will enable in the future the onsite characterization of plasticized PVC samples.

Supporting Information. Description of the used samples, calculation of the remaining plasticizer concentration in an aged PVC sample, THz frequencydependent ε " and n for the PVC/DINP samples (Figure S1), typical DSC traces of two PVC samples with different concentrations of plasticizer (Figure S2), lost mass with the aging time for DINCH/PVC samples (Figure S3), correlation of the estimated n from THz results for the un-plasticized PVC and pure plasticizers with the corresponding values reported at visible frequencies (Figure S4), and changes of ε "


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and n for

DINCH/PVC samples exposed to accelerated aging for various times

(Figure S5).

Acknowledgments. The authors are grateful to the referees for their useful comments. AA thanks Professor B. Blümich from RWTH Aachen University for his constant support.

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(13) Garnaik, B.; Sivaram, S. Macromolecules 1996, 29, 185-190. (14) Adams, A.; Kwamen, R.; Woldt, B.; Graß, M. Macromol. Rapid Comm. 2015, 36, 2171-2175. (15) Adams A. TrAc 2016, 83, 107-119. (16) Adams, A.; Piezeck, A.; Schmitt, G.; Siegmund, G. Anal. Chim. Acta 2015, 887, 163-171. (17) Zhang, J.; Adams, A. Polym. Degrad. Stab. 2016, 134, 169-178. (18) Kwamen, R.; Blümich, B.; Adams, A. Macromol. Rapid Comm. 2012, 33, 943-947. (19) Jepsen, P. U.; Cooke, D. G.; Koch, M. Laser Photon. Rev. 2011, 5, 124-166. (20) Baxter, J. B.; Guglietta, G.W. Anal. Chem. 2011, 83, 4342–4368. (21) McIntosh, A.I.; Yang, B.; Goldup, S.M.; Watkinson, M.; Donnan, R.S. Chem. Soc. Rev. 2012, 41, 2072–2082. (22) Sibik, J.; Zeitler, J.A. Adv. Drug Deliv. Rev., 2016, 100, 147-157 (23) Tonouchi, M. Nat. Photonics 2007, 1, 97 – 105. (24) Wietzke, S.; Jansen, C.; Reuter, M.; Jung, T.; Kraft, D.; Chatterjee, S.; Fischer, B.M.; Koch M. J. Mol. Struct. 2011, 1006, 41–51. (25) Vieira, F.S.; Pasquini, C. Anal. Chem. 2014, 86, 3780−3786. (26) Suzuki, H.; Ishii, S.; Otani, C.; Hoshina, H. Eur. Polym. J. 2015, 67, 284–291. (27) Sommer, S.; Raidt, T.; Fischer, B. M.; Katzenberg, F.; Tiller, J.; Koch M. J. Infrared Millim. Terahertz Waves 2016, 37, 189-197. (28) Li, H.; Ye, H.-M.; Yang, Y. Polymer Testing 2017, 57, 52-57. (29) Peters, O.; Schwerdtfeger, M.; Wietzke, S.; Sostmann, S.; Scheunemann, R.; Wilk, R.; Holzwarth, R.; Koch, M.; Fischer, B. M. Polymer Testing 2013, 32, 932–936. (30) Okano, M.; Watanabe, S. Sci. Rep. 2016, 6, 39079. 14 Environment ACS Paragon Plus

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(31) Chanda, M.; Roy, S.K. Plastics Technology Handbook, 4th ed.; CRC Press:Boca Raton, 2007. (32) Vieweg, N.; Rettich, F.; Denninger, A.; Roehle, H.; Dietz, R.; Göbel, T.; Schell, M. J. Infrared Millim. Terahertz Waves 2014, 35, 823-832. (33) Scheller, M.; Jansen, C.; Koch, M. Opt. Commun. 2009, 282, 1304-1306. (34) Scheller, M. J. Infrared Millim. Terahertz Waves, 2014, 35, 638–648. (35) Naftaly, M.; Miles, R. E. Proc.IEEE, 2007 95, 1658-1665. (36) Chantry, G. W.; Fleming, J. W.; Smith, P. M. Chem. Phys. Lett. 1971, 10, 473-477. (37) Soni, P. L.; Geil, P. H.; Collins, E. A. J. Macromol. Sci. Phys. B 1981, 20, 479-503. (38) Liu, Y.; Zhang, R.; Wang, X.; Sun, Chen, P.W.; Shen, J.; Xue, G. Polymer 2014 , 55 , 2831-2840. (39) Gomez Ribelles, J.L.; Diaz-Calleja, R. ; Ferguson, R.; Cowie, J.M.G. Polymer 1987, 28, 2262–2266. (40) Krause, S.; Lu, Z.-H. J. Polym. Sci.: Polym. Phys. 1981, 19, 1925-1928. (41) Wypych, G. PVC Degradation and Stabilization, 3rd ed.; ChemTec Publishing: Toronto, 2015. (42) Zou, J.; Su, L.; You, F.; Chen, G.; Guo, S. J. Appl. Polym. Sci. 2011, 121, 1725–1733. (43) Goldstein, M.; Stephenson, D.; Maddams, W. F. Polymer 1983, 24, 823-826.



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Figure 1. THz frequency-dependent (a) imaginary part of the permittivity and (b) refractive index of PVC/DINCH samples with different plasticizer concentrations. 79x55mm (300 x 300 DPI)

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Figure 1. THz frequency-dependent (a) imaginary part of the permittivity and (b) refractive index of PVC/DINCH samples with different plasticizer concentrations. 79x55mm (300 x 300 DPI)


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Figure 2. Correlation curves of the optical parameters with the concentration of plasticizer established for fresh PVC samples with two different types of plasticizers. The reported in (a) corresponds to the values at 1.81 THz. The refractive index values displayed in (b) are the average of the values between 0.3 THz and 1 THz. The lines depict the fit results using the Equations (1)-(4). The correlation coefficients for all fits are higher than 0.99. 79x56mm (300 x 300 DPI)



Figure 2. Correlation curves of the optical parameters with the concentration of plasticizer established for fresh PVC samples with two different types of plasticizers. The reported in (a) corresponds to the values at 1.81 THz. The refractive index values displayed in (b) are the average of the values between 0.3 THz and 1 THz. The lines depict the fit results using the Equations (1)-(4). The correlation coefficients for all fits are higher than 0.99. 79x56mm (300 x 300 DPI)


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Figure 3. Correlation of DINCH concentrations in PVC samples with different aging times as predicted by THz and gravimetry. The results of all aged samples are included. The continuous line is the fit result with a correlation factor higher than 0.99 using the equation . 79x56mm (300 x 300 DPI)



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