A study of water dehydration in nylon 6 as a function of temperature using two-dimensional (2D) correlation near-infrared (NIR) analysis

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1 Journal of Molecular Structure 833 (2007) A study of water dehydration in nylon 6 as a function of temperature using two-dimensional (2D) correlation near-infrared (NIR) analysis Yan Xu, Peiyi Wu Department of Macromolecular Science, The Key Laboratory of Molecular Engineering of Polymers, Ministry of Education, Fudan University, Shanghai , PR China Received 7 August 2006; received in revised form 12 September 2006; accepted 12 September 2006 Available online 30 October 2006 Abstract The dehydration progress of nylon 6 was investigated at two diverent temperatures, using near-infrared (NIR) spectroscopy. The twodimensional (2D) correlation technique was applied to analyse the experimental data. In the region of the O H combination band ( cm 1 ), the asynchronous spectra of the two temperatures resolve the complex band into three component bands centered at 5280, 5230, and 5140 cm 1, respectively, indicating the existence of three species of water molecules in diverent states of hydrogen bonding. From the study of the 2D correlation analysis at two temperatures, we have suggested diverent dehydration mechanisms of the polyamide 6 soaked with water for 50 and 80 C Elsevier B.V. All rights reserved. Keywords: Dehydration; Hydrogen bonding; Near-infrared spectroscopy; Nylon; Two-dimensional (2D) correlation spectroscopy 1. Introduction The role played by moisture in polymers is of paramount importance. Through forming hydrogen bonds, moisture has a signiwcant impact on physical properties of many polymers. Nylon is one of the most commonly used engineering plastics. Since Wrst synthesized in 1935, this material has gained wide acceptance owing to their good thermal stability and mechanical properties. Besides nylon is readily processable by extrusion molding. However, most of the nylons are not recommended for extremely wet applications. They generally absorb more moisture than other bearing materials, swelling as much as 5% in fully immersed applications, and therefore, may be dimensionally unstable in wet applications requiring close tolerances. On the other hand, the glass transition * Corresponding author. Tel.: ; fax: address: peiyiwu@fudan.edu.cn (P. Wu). temperature (T g ) for dry nylon generally drops for wet nylon. Hence, the investigation of moisture absorption and dehydration is necessary for a better understanding of the material transport mechanism. Near-infrared (NIR) spectroscopy was discovered in the 18th century and was not developed until the 1960s [1 3]. In this region absorption bands corresponding to overtones and combinations of OH, NH, and CH fundamental vibrations are dominating. They generally have the largest anharmonicity constants due to the involvement of the light hydrogen atom. Therefore, NIR analysis is suitable for studying polymers. Other spectroscopic analysis techniques, such as midinfrared spectroscopy (IR) [4,5] and Raman [6 8] have already been applied to study the sorption and desorption of small molecules in polymer. However, these methods have certain shortages. In the N H stretching region of the infrared area, the intensity of the bands due to the hydrogen-associated species is so strong that the information of the bands due to the free N H group is obscured /$ - see front matter 2006 Elsevier B.V. All rights reserved. doi: /j.molstruc

2 146 Y. Xu, P. Wu / Journal of Molecular Structure 833 (2007) Secondly, some of the band positions are very close to each other in the infrared area, making it diycult to distinguish between them. For example, the N H stretching bands are located around cm 1, while the O H stretching bands are ranging from 3500 to 3600 cm 1. And the position of the O H bending (»1637 cm 1 ) is neighboring with those of amide I (»1640 cm 1 ) and amide II (»1545 cm 1 ). On the contrary, overtones and combination bands of both the amides and the O H stretching vibrations are well resolved and do not overlap. In the NIR region, the O H group has speciwc absorption bands at 5200 cm 1 (ν 2 + ν 3 ; ν 2 OH bending mode, ν 3 antisymmetric OH stretching mode) and 6940 cm 1 (ν 1 + ν 3 ; ν 1 symmetric OH mode), [9] compared with the absorption bands at about 4900 cm 1 (combination) and 6500 cm 1 (Wrst overtone) of the N H group [9]. Eventually, IR and Raman analysis are strict to the testing conditions, especially, the thickness of the test Wlm and the materials used as transmission cell windows. As a result, NIR analysis has been widely used for the characterization of nylon for decades [10 14]. The basic concept of 2D correlation spectroscopy was originally proposed and generalized by Noda [15 18]. By spreading spectral peaks over the second dimension, the visualization of complex spectra consisting of many overlapped bands is simpliwed and spectral resolution is enhanced. That is, it is possible to identify the bands not readily accessible from one-dimensional spectra. Moreover, we can obtain a set of dynamic spectral signals by monitoring any physical variables, not only time but also temperature, concentration, and so on. In this way, 2D correlation spectroscopy can provide information about the relative rate or the sequential order of the spectral changes of molecular segments taking place based on the external perturbation. In the present study, the dehydration of nylon 6 was investigated by NIR spectroscopy at 50 and 80 C, respectively. The 2D correlation was also performed in the NIR regions. Furthermore, the interactions between H 2 O molecules and nylon 6 are discussed on the basis of the hydrogen bonds. 2. Experimental 2.1. Materials Melt nylon 6 Wlm (thickness»1 mil) was used for the experiments. For comparative dehydration experiments, the original Wlm was cut into four specimens and two of which were prepared by suspending the Wlms at room temperature for over 12 h in water bath. The specimens were heated at 50 and 80 C, respectively, and NIR spectra were recorded at diverent time intervals. The information about the sample pretreatment in this investigation is listed in Table 1. Specimens were put in separate water baths so that surface contact was prevented. All the specimens were wiped prior to the analysis. Table 1 Sample information Sample Preparation Treatment A Immersed in a water bath Heated at 80 C for about 1.5 h over 12 h B Immersed in a water bath Heated at 50 C for about 2 h over 16 h C Kept in a desiccator Heated at 80 C for about 1.5 h D Kept in a desiccator Heated at 50 C for about 2.5 h 2.2. Analytical methods Near-infrared spectroscopy The near-infrared spectra were recorded at a resolution of 4 cm 1 with a Nicolet Nexsus 470 FT-IR/NIR spectrometer equipped with a liquid-nitrogen-cooled MCT detector. Glass disks were used for the transmission cell windows, which give no absorbance along the near-infrared region. All the spectra were measured from 3000 to 11,000 cm Two-dimensional correlation spectroscopy The data treatment was performed in Pocha software which was composed by D. Adachi (Kwansei-Gakuin University). The one-dimension average spectrum was shown as a reference at the side and the top of the 2D correlation maps. In these maps unshaded regions indicate positive correlation intensities, while shaded regions indicate negative correlation intensities. 3. Results and discussion The NIR absorption spectra of the original and pretreated nylon 6 are shown in Fig. 1. It can be clearly seen that the spectrum of the dry sample is diverent from the wet sample in several regions. The intensity of the peaks around 7000 and 5200 cm 1 was strongly enhanced after the sample was immersed in water for 12 h. The assignment of the Absorbance A... C wavenumber(cm -1 ) Fig. 1. Near-infrared spectra recorded from the original Wlms of sample A ( ) and sample C (ƒ) at room temperature

3 Y. Xu, P. Wu / Journal of Molecular Structure 833 (2007) Table 2 Assignment of the absorption bands in the NIR spectrum (see Fig. 1) of nylon 6 Wavenumber (cm 1 ) Tentative assignment *ν(CH 2 ) *ν as (CH 2 )+δ(ch 2 ) ν 1 (OH) + ν 3 (OH) 6750 ν 1 (OH) + ν 3 (OH)and 2*ν(NH) f *ν(NH) b *ν as (CH 2 ) *ν s (CH 2 ) ν 2 (OH) + ν 3 (OH) (s 0 ) 5230 ν 2 (OH) + ν 3 (OH) (s 1 ) 5140 ν 2 (OH) + ν 3 (OH) (s 2 ) ν(nh) b +amide I ν(nh) b +amide II 4718 Amide I + 2*amide II ν(nh) b + amide III ν s (CH 2 )+δ(ch 2 ) ν s (CH 2 )+δ(ch 2 ) 4230 ν s (CH 2 )+γ w (CH 2 ) 4193 ν s (CH 2 )+γ(ch 2 ) Absorbance min wavenumber (cm -1 ) Fig. 2. Near-infrared spectra of sample A in the range cm 1 recorded every 10 min during isothermal treatment at 80 C. former is well-known to be the combination of symmetric O H stretching and antisymmetric O H stretching (ν 1 + ν 3 ), and the latter is attributed to the combination of O H bending and antisymmetric O H stretching (ν 2 + ν 3 ) [9]. The bands of interest, including hydroxy, amide, and methylene groups are identiwed in the spectra and their assignments are listed in Table 2. The assignments of the bands generally agree with those reported in the published literature [4,19]. In Fig. 2, the NIR spectra of sample A are shown as a function of time during isothermal heat treatment at 80 C. From this Wgure it is clear that the intensities of several ν(ch 2 ) overtone and combination bands remain almost constant, while the intensities of the 5200 and 7020 cm 1 bands are reduced signiwcantly with time. After being heated for 60 min, the intensity of the strong band at 5200 cm 1 diminishes to almost zero. The spectrum of the wet nylon 6 sample at room temperature (see Fig. 1) is characterized by a broad absorption band ranging from 7200 to 6500 cm 1. On being heated, the intensity of this region decreases, but the reduction for the 6750 cm 1 band is much less signiwcant than that for the 7020 cm 1 band. The reason is that the 6750 cm 1 combination band is superimposed by the new 2 ν(nh) f. Considering the complexity and the comparatively low absorbance of this region, we use the isolated band of the 5200 cm 1 band for the evaluation of the dehydration progress as a function of time. Fig. 3 shows the synchronous and asynchronous maps of sample A in the spectral region of cm 1. The synchronous map develops a strong autopeak at 5181 cm 1, indicating that the change of the O H group is prominent. No cross peaks were found in the synchronous map. The corresponding asynchronous map shown in Fig. 3B depicts four major cross peaks. The appearance of the negative peak at (5280, 5230cm 1 ) and the positive peak at (5230, 5140cm 1 ) reveals that out-of-phase spectral changes occur at these wavenumbers and that the change at 5230cm 1 occurs before or at a higher rate than those at 5280 and Fig. 3. Synchronous (A) and asynchronous (B) 2D NIR correlation spectra of sample A in the cm 1 region. (The unshaded and shaded areas in the contour maps represent positive and negative peaks, respectively.)

4 148 Y. Xu, P. Wu / Journal of Molecular Structure 833 (2007) Fig. 4. Synchronous (A) and asynchronous (B) 2D NIR correlation spectra of sample B in the cm 1 region. (The unshaded and shaded areas in the contour maps represent positive and negative peaks, respectively.) 5140cm 1. This result indicates that the 5181cm 1 band (ν 2 (OH) + ν 3 (OH)) has three origins. The speciwc assignment of these three bands has already been carefully discussed [20,21]. Thus the three component bands at 5280, 5230, and 5140cm 1 are assigned, respectively, to free H 2 O molecules (s 0 ), H 2 O molecules with one active hydrogen bond (s 1 ) and H 2 O molecules with two active hydrogen bonds (s 2 ). Though the positions of the bands in this investigation do not agree perfectly with the reference, it is possible that the shift of the band position arises from diverent interactive forces between water and the polyamide. From these considerations, the H 2 O molecules with one active hydrogen bond (s 1 ) decrease earlier than those with two active hydrogen bonds (s 2 ) and free H 2 O molecules (s 0 ). An additional 2D correlation analysis for the Wrst 50 min (not shown here) reveals that the change s 2! s 1 hardly takes place at the beginning. Perhaps the energy at 80 C is large enough for s 2 to change to s 0 directly. However, in view of the lower energy needed for the change s 1! s 0 than that for s 2!s 0, the intensity of s 1 is expected to reduce faster than s 0. Fig. 4 shows the synchronous and asynchronous maps of sample B in the spectral region of cm 1. The synchronous map is almost the same as Fig. 3A, while the corresponding asynchronous spectrum develops only a pair of cross peaks. However, a careful examination reveals that there are two centers in the cross peak, with the location at (5280, 5140cm 1 ) and (5240, 5140cm 1 ), respectively. The sign of this cross peak indicates that the intensity change at 5140cm 1 band proceeds earlier than those at 5240 and 5280cm 1. The result suggests that the intensity of H 2 O molecules with two active hydrogen bonds (s 2 ) decrease earlier than those with one active hydrogen bond (s 1 ) and free H 2 O molecules (s 0 ), which is somewhat diverent from the 80 C case. A rational explanation is that the energy at 50 C is not suycient for the change s 1! s 0 to arise. Thus the change s 2! s 1 is dominant. The peak area of 5200 cm 1 versus time is plotted to further prove the behavior of water at diverent temperatures. Fig. 5A and B show signiwcant diverences in the isothermal dehydration process at 50 and 80 C. The decrease of intensity versus time is less pronounced at 50 C than at 80 C. Furthermore, for nylon 6 heated at 80 C, the intensity of 5200 cm 1 decreases exponentially, and it tends to level ov to zero after 1 h. In contrast, for the sample heated at 50 C, A B Intensity Intensity Time(s) Time/s Fig. 5. Mass decrease of water as a function of time at (A) 80 C; (B) 50 C. The data point dimensions take into account the experimental errors.

5 Y. Xu, P. Wu / Journal of Molecular Structure 833 (2007) a fairly linear trend is observed for intensity versus time, and the dehydration does not seem to come to an end even after 2-h heating. The continuous line in Fig. 5A is an interpolation of the experimental data with a second-order exponential decay, whereas in Fig. 5B, it is a Wt of the experimental data with a second-order polynomial. 4. Conclusion FTNIR spectroscopy has been successfully utilized to determine the dehydration progress of nylon 6 at diverent temperatures. The results show that the H 2 O molecules absorbed in nylon 6 have three diverent structures: free H 2 O molecules (s 0 ), H 2 O molecules with one active hydrogen bond (s 1 ) and H 2 O molecules with two active hydrogen bonds (s 2 ). From detailed investigations on the combination band ν 2 +ν 3 (ν 2 O H bending mode; ν 3 antisymmetric O H stretching mode) distinct mechanisms of dehydration at 50 and 80 C are proposed. The energy at 80 C is large enough for the change s 2!s 0 to occur, therefore, s 2 and s 1 transform into s 0 synchronously at the earlier step of heating. On the other hand, the energy is comparatively low at 50 C and the changes s 1!s 0 and s 2! s 0 are diycult to take place. In this case, the change s 2! s 1 is predominant and only a small part of s 1 and s 2 transforms into s 0. Acknowledgements The authors gratefully acknowledge the Wnancial support by the National Science of Foundation of China (NSFC) (No , Nos , , No ), the Qimingxing Project (No. 01QE14011), the Shuguang Project (No. 01SG05), the National Basic Research Program of China (2005CB623800) and PHD Program of MOE ( ). References [1] H. Dannenberg, W.R. Harp, Anal. Chem. 28 (1956) 86. [2] R.F. Goddu, D.A. Delker, Anal. Chem. 30 (1958) [3] K. Whetzel, W.E. Roberson, M.W. Krell, Anal. Chem. 30 (1958) [4] M.A. Czarnecki, P.Y. Wu, H.W. Siesler, Chem. Phys. Lett. 283 (1998) 326. [5] I. Linossier, F. Gaillard, M. Romand, J.F. Feller, J. Appl. Polym. Sci. 66 (1997) [6] Y. Maeda, N. Tsukida, H. Kitano, T. Terada, J. Yamanaka, J. Phys. Chem. 97 (1993) [7] Y. Maeda, H. Kitano, Spectrochim. Acta Part A 51 (1995) [8] N.E. Schlotter, J. Phys. Chem. 94 (1990) [9] H.W. Siesler, Y. Ozaki, S. Kawata, H.M. Heise (Eds.), Near-Infrared Spectroscopy: Principles, Instruments, Applications, Wiley-VCH, Weinheim, 2002, pp [10] Y. Ozaki, Y.L. Liu, I. Noda, Macromolecules 30 (1997) [11] J.B. Reeves, T.H. Blosser, A.T. Balde, B.P. Glenn, J. Vandersall, J. Dairy Sci. 74 (1991) [12] J.E. Rodgers, S. Lee, Textile Res. J. 61 (1991) 531. [13] P.Y. Wu, H.W. Siesler, Chem. Mat. 15 (2003) [14] J.E. Rodgers, S. Lee, Abstr. Pap. Am. Chem. Soc. 198 (1989) 33, cell. [15] I. Noda, Bull. Am. Phys. Soc. 31 (1986) 520. [16] I. Noda, Appl. Spectrosc. 47 (1993) [17] I. Noda, J. Am. Chem. Soc. 111 (1989) [18] I. Noda, Appl. Spectrosc. 44 (1990) 550. [19] P.Y. Wu, Y.L. Yang, H.W. Siesler, Polymer 42 (2001) [20] V. Fornes, J. Chaussidon, J. Chem. Phys. 68 (1978) [21] M. Takeuchi, G. Martra, S. Coluccia, M. Anpo, J. Phys. Chem. B 109 (2005) 7387.