poly (vinyl methyl ether) blend revealed by two-dimensional correlation resonance light scattering spectroscopy

Polymer Testing 28 (2009) 456–460

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Phase separation in polystyrene/poly (vinyl methyl ether) blend revealed by two-dimensional correlation resonance light scattering spectroscopy Jin Yang b, Xudong Chen a, c, *, Ruowen Fu b, Yunbo Li a, Wei-ang Luo b, Mingqiu Zhang b, c, ** a

Institute of Polymer Science, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, PR China Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, PR China c OFCM Institute, PR China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 January 2009 Accepted 5 March 2009

Two-dimensional (2D) correlation resonance light scattering (RLS) spectroscopy has been successfully applied to investigate phase separation of polystyrene (PS)/poly (vinyl methyl ether) (PVME) film by using a conventional spectrofluorimeter. 2D synchronous correlation RLS spectrum indicates that the RLS peak intensity drastically increases with a rise in temperature due to aggregation of chromophores (i.e. phenyl rings) in PS particles in the course of phase separation. In addition, as concluded by 2D asynchronous correlation RLS spectrum, RLS has higher sensitivity than conventional light scattering. For RLS, the closer to the absorption band, the more sensitive it is to the aggregation during phase separation. By means of moving-window two-dimensional (MW2D) correlation spectrum based on autocorrelation calculations, the cloud point (370 K) was determined, which is in good agreement with the literature. On the other hand, time evolution of RLS intensity at various temperatures distinctly shows that phase separation of PS/PVME film involves two mechanisms, i.e. spinodal decomposition (SD) and nucleation and growth (NG). Accordingly, 2D correlation RLS proves to be a very simple and sensitive method to monitor phase separation in polymer blends and might supplement the existing characterization tools. Ó 2009 Elsevier Ltd. All rights reserved.

Keywords: Phase separation PS/PVME Absorption band 2D correlation Resonance light scattering

1. Introduction Much attention has been paid for years to the behavior of phase separation in polymer blends, from both theoretical and experimental viewpoints. Theories were propounded to predict the decay of one-phase state in the special case of binary mixture of polymers. They are

* Correspondence to: Institute of Polymer Science, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, PR China. Tel./fax: þ86 20 84113498. ** Correspondence to: Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, PR China. Tel./fax: þ86 20 84113498. E-mail addresses: [email protected] (X. Chen), ceszmq@mail. sysu.edu.cn (M. Zhang). 0142-9418/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2009.03.005

referred to as spinodal decomposition (SD) in an initially unstable system [1,2] and nucleation and growth (NG) in a metastable system [3]. A variety of experimental methods were employed to study the related problems, such as differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA), atomic force microscopy (AFM), small-angle laser light scattering (SALLS), smallangle neutron scattering (SANS), etc. [4–8] Each of these techniques has its advantages and drawbacks, depending on the systems and problems of interest. For instance, thermal analysis is very convenient to observe miscibility in polymer blends, but sometimes it is hard to clearly identify Tgs of the blends. Compared with AFM, light scattering methods are superior because one can analyze the relaxation rate as a function of scattering vector and one can follow, in situ, a rapid growth of the fluctuations without introducing an additional physical process such as freezing

J. Yang et al. / Polymer Testing 28 (2009) 456–460

a given phase-separated state for the measurements by quenching the specimen down to temperatures below Tg. However, conventional light scattering technique (e.g. laser light scattering) inevitably relies on expensive equipment and is a sophisticated process. What is more, since a He-Ne laser (with fixed wavelength at 633 nm) acts as the light source in most measurements [4], polymer blends with an absorption band in this region cannot be studied. This might also be the reason why light scattering at or near the absorption band region during phase separation was rarely studied in the past. Recently, resonance light scattering (RLS) has attracted researchers’ interests because of its simplicity, rapidity and sensitivity. It has been used to detect biological macromolecules, amounts of inorganic ions and complex formation by focusing light scattering near the absorption wavelength of the aggregated molecular species [9–17]. In particular, in many cases RLS experiments can be easily conducted using a conventional spectrofluorometer by synchronously scanning both the lamp and detector monochromators through the wavelength range of interests. To the authors’ knowledge, however, RLS is mainly employed in the field of analytical chemistry [16–19], and has not yet been applied to the investigation of solid polymer materials. Phase separation is a phenomenon resulting from particles aggregation. Since RLS is good at tracing particle aggregation in solution so long as the particles contain chromophores [16,17], it might also be applicable for studying phase separation in polymer blends. In this work, phase separation behavior of a representative binary blend of polystyrene (PS) and poly (vinyl methyl ether) (PVME) is investigated in terms of RLS by focusing the light scattering near the absorption band. The blend has been extensively investigated and exhibits an accessible lower critical solution temperature (LCST) [20,21]. Here, its composition is set at PS/PVME ¼ 3/7, so that the PS plays the role of dispersed phase [22]. To separate the overlapped bands in RLS spectra and to emphasize the variations in intensities of important peaks, two-dimensional (2D) correlation spectroscopy is introduced. 2D correlation RLS analysis shows that RLS has higher sensitivity than conventional light scattering.

457

a FLS920 Combined Fluorescence Lifetime and Steady State Spectrometer (Edinburgh, England) using a heating rate of 3 K/min. Both excitation and emission monochromator wavelengths were coupled (Dl ¼ 0) and adjusted to simultaneous scan. To minimize the influence of reflected light, 45 -angle sample geometry was employed (see Fig. 1). All the RLS spectra have been calibrated by division of the signal intensity at each wavelength by the corresponding source intensity. 3. Results and discussion Fig. 2 shows light scattering spectra of PS/PVME (3/7) film collected by a spectrofluorimeter from 300 K to 387 K where the phase separation usually occurs [23,24]. It can be observed that shoulder peaks appear in the range 270–340 nm, which is near the maximum absorbance wavelength (265 nm) of the film (see the inset of Fig. 2). In addition, the light scattering intensities in the whole range measured drastically increase with a rise in temperature. Pasternack et al. [25] declared that the scattering cross section of a system, Csca, the ratio of the rate of energy scattering out of the incident beam (in all directions) to the intensity of the incident beam, can be expressed as

  2   m 1 Cabs ¼ pg2 4cIm m2 þ 2

(1)

2. Experimental 2.1. Materials Film samples were prepared by casting from 10 wt% toluene solutions of the polymer blend containing 30 wt% PS (Mw ¼ 354,000 g/mol, PDI ¼ 1.05) and 70 wt% PVME (Mw ¼ 90,600 g/mol, PDI ¼ 1.95) onto quartz plates at room temperature. Then, the films were further heat treated in a vacuum oven at 65  C for 24 h. We found this procedure resulted in homogeneous films that were about 3 mm thick. 2.2. Methods UV-visible absorption spectra were recorded on a UV3150 spectrometer (Shimadzu, Japan). The film thickness was determined by a Thin Film Measurement System (Filmetric, U.S.A.). The RLS spectra were collected with

Fig. 1. Optical scheme for RLS measurements (with 45 -angle sample geometry and a quartz substrate).

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J. Yang et al. / Polymer Testing 28 (2009) 456–460

where c is the molar concentration of the scattering material and 3(l0) is the molar absorption coefficient atl0. Therefore, the increment of n and k with the fluctuation of c can be specified by 2

vn 2:3cl0 ¼ vc 2p2

Z

N

0

3ðlÞ dl l  l2

(8)

2 0

vk 2:33ðl0 Þl0 ¼ 4p vc

Fig. 2. RLS spectra of PS/PVME (3/7) film collected at various temperatures (from 300 K to 387 K at approximately 3 K increment). The inset shows the absorption spectrum of this film at 300 K.

2  2   m 1 Csca ¼ pg2 ð8=3Þc4 m2 þ 2

(2)

where m stands for the complex refractive index, g the scatter radius, Im the imaginary part within the parenthesis, c ¼ ð2pgnmed Þ=l0 , l0 the wavelength of incident light, and nmed the refractive index of the medium. Accordingly, the contribution of aggregation of chromophores to the great light-scattering enhancement influences the amounts of absorption, A(l0), and scattering, Isca(l0), as follows [16,26].

Isca ðl0 Þ ¼ Csca ðN=VÞI0 ðl0 Þ ¼

2  24p3 vn4med MI0 ðl0 Þ m2  1 m2 þ 2 rl4 0

(3) Aðl0 Þ ¼ 2:31 ðN=VÞCabs L   0  2 m 6pnmed m 1 Im ¼ 2:31 2 rV l0 m þ2

(4)

where v is the volume of the scatterer, N/V is the number of scatterers per unit volume, m0 is the mass of scattering material, I0(l0) is the incident intensity, and M and r are molar mass and density of scattering material, respectively. Since the expression of m can be given as [27]

According to equations (8) and (9), with l0 moving towards absorption bands, vn/vc and vk/vc both increase. Specifically, in the high frequency section of the absorption band, vn/vc counteracts vk/vc because vn/vc is negative but vk/vc is positive, whereas in the low frequency section of the absorption band, vn/vc accords with vk/vc because both vn/ vc and vk/vc are positive. In general, when l0 is slightly higher than lmax, m reaches a maximum, explaining why the maximum enhanced RLS often occurs at the red end of the absorption band [26,29]. Thus, the drastic increase in light scattering intensity near the absorption band, shown in Fig. 2, should be interpreted as the enhanced RLS due to aggregation of the chromophores (i.e. phenyl rings) in PS particles during phase separation. However, the subtle spectral variations cannot be distinguished because the peaks in the RLS spectra are overlapped. As a powerful spectrum analysis method, 2D correlation spectroscopy first proposed by Noda [30] can directly observe spectral correlation variation along both spectral variables (e.g., wavelength) and perturbation variables (e.g., temperature) axes. [31,32] This novel technique developed in recent years, has been used to analysis all kinds of spectra, such as such as IR, Raman, fluorescence, UV-visible, chromatography, NMR, X-ray, and dielectric spectroscopy [32]. The generalized 2D correlation function can be broken down to a synchronous correlation function F(l1,l2):

Fðl1 ; l2 Þ ¼

(5)

where n and k portray effects of refractive and absorption of pffiffiffiffiffiffi ffi a scattering system, respectively, and i ¼ 1. The relationship between n and molar absorption coefficient 3(l) at any wavelength within the absorption band can be described with the Kronig–Kramers equation [26,28]: 2

2:3cl0 n ¼ nmed þ 2p2

k ¼

2:33ðl0 Þcl0 4p

Z 0

N

3ðlÞ dl l20  l2

(6)

(7)

k 1 X yi ðl1 Þyi ðl2 Þ k  1 i¼1

(10)

and an asynchronous correlation function J(l1,l2):

Jðl1 ; l2 Þ ¼ m ¼ n  ik

(9)

k k X 1 X yi ðl1 Þ Nij yj ðl2 Þ k  1 i¼1 j¼1

(11)

where y(l1) and y(l2) are the dynamic spectra at l1 and l2; k is the total number of data points in the dynamic spectra; and Nij is defined by :

Nij ¼ 0 if i ¼ j or Nij ¼

1

pðj  iÞ

if isj

(12)

The 2D maps of the synchronous and asynchronous functions are called synchronous and asynchronous spectra, respectively. Fig. 3 presents the 2D correlation RLS spectra, which are based on all the spectra shown in Fig. 2. Fig. 3(a) and (b) show the 2D synchronous and asynchronous correlation RLS spectra, respectively. Obviously, an auto-

J. Yang et al. / Polymer Testing 28 (2009) 456–460

Fig. 3. 2D correlation RLS spectra based on all the spectra in Fig. 2. The contour levels represent the projection of correlation intensity. Key: (a) 2D synchronous correlation RLS spectrum; (b) 2D asynchronous correlation RLS spectrum.

peak is perceived at 296 nm (Fig. 3(a)), suggesting that the drastic change in RLS intensities appears at around 296 nm. On the other hand, since the appearance of an asynchronous cross-peak J(l1,l2) indicates that the bands l1 and l2 vary out of phase with each other, the two cross-peaks (i.e. J (285, ~320) and J (285, ~360)) in the upper left triangle in Fig. 3(b) imply that the peak in Fig. 2 should be split into three separate peaks at 285, ~320 and ~360 nm. It is worth noting that the light scattering peak at ~360 nm should be attributed to the conventional light scattering rather than RLS because this light scattering is very far away from the absorption band [26,29]. Furthermore, according to the rules of Noda [30–32], the two positive cross-peaks reveal that changes in the intensity of RLS at 285 nm occur prior to the changes at ~320 and ~360 nm. A careful observation of Fig. 3(b) shows that J (~360, ~320) is negative, which manifests that the RLS intensity changes at ~320 nm occur earlier than the changes at ~360 nm. Accordingly, the sequence of phase separation induced changes in light scattering intensity is as follows: 285 nm > 320 nm > 360 nm (the symbol ‘‘>’’ refers to ‘‘prior to’’). Therefore, it can be concluded that (1)

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for RLS (i.e. 285 and 320 nm), the closer to the absorption band the more sensitive it is to aggregation during phase separation; and (2) RLS is more sensitive than conventional light scattering (i.e. 360 nm) in the process of phase separation in PS/PVME. To further study spectral correlation variation along perturbation variables (i.e. temperature in this work) during phase separation, Fig. 4 shows the moving-window two-dimensional (MW2D) correlation RLS spectrum based on autocorrelation calculations from Fig. 2. When MW2D correlation analysis based on autocorrelation spectrum is performed, each row of the MW2D correlation spectrum matrix is directly extracted from a diagonal line of F(l1,l2) matrix, namely, the autocorrelation intensity of synchronous 2D correlation spectrum at the same two spectral variables. The moving procedure has been discussed in detail elsewhere [33]. Clearly, it is seen from Fig. 4 that a positive correlation intensity peak appears at 370 K, which means the PS particles start to aggregate and initially lead to the drastic increase in RLS intensity at this temperature. It is worthwhile to point out this temperature is in good agreement with cloud point (Tc) in the literature (97  C) [34]. By applying the methods commonly used in light scattering experiments for the analysis of dynamics [35], as shown in Fig. 5, we plot ln IRLS as a function of time at various temperatures. For simplicity, IRLS here is the RLS intensity at 296 nm according to the result obtained in Fig. 3(a). Interestingly, it can be seen that for higher temperatures (i.e. T ¼ 373 K, 376 K and 379 K), ln IRLS increases linearly with time, while for lower temperature (i.e. T ¼ 370 K), ln IRLS increases nonlinearly with time. This different behavior strongly implies that the phase separation might occur in different manners. Hashimoto et al. [36] reported that spinodal temperature (Ts) for PS/PVME (3/7) blend was 99.2  C (z372 K). Hence, it is reasonable to conclude that, in the case of T ¼ 373 K, 376 K and 379 K that are higher than Ts, phase separation occurs due to SD mechanism, which allows a rapid concentration fluctuation and domain growth. On the other hand, in the case of T ¼ 370 K that are lower than Ts, phase separation occurs

Fig. 4. MW2D correlation RLS spectrum based on autocorrelation calculations from Fig. 2.

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Acknowledgements One of the authors (X.D. Chen) acknowledges the financial support from the program of National Natural Science Foundation of China (Grant no. 50673104 and 50673101) and Natural Science Foundation of Guangdong province (Grant no. 7003702).

References [1] [2] [3] [4] [5]

Fig. 5. Time evolution of ln IRLS of PS/PVME (3/7) film at various temperatures.

[6] [7] [8] [9]

due to NG mechanism, the kinetics of which is rather slow compared with that of SD mechanism [37]. Cahn predicted exponential growth of scattering intensity during SD, which has been confirmed to be true in some metal alloys, glasses, liquids and also in many polymer blends [38–41]. The results in Fig. 5 also obey his consideration. This means that one can easily estimate spinodal temperature from the data of RLS measurements and Cahn’s criterion without the necessity of other complicated calculations.

[10] [11] [12] [13] [14] [15] [16] [17] [18] [19]

4. Conclusions In summary, 2D correlation RLS spectroscopy has been successfully applied to the investigation of light scattering near the absorption band during phase separation of PS/ PVME film by using a conventional spectrofluorimeter. Three main outcomes are obtained as follows. (1) For RLS at 285 and 320 nm, the closer to the absorption band the more sensitive it is to the aggregation during phase separation; (2) RLS is more sensitive than conventional light scattering (i.e. 360 nm) in the process of phase separation in PS/PVME; (3) The cloud point determined by MW2D correlation spectrum based on autocorrelation calculations is in good agreement with that in the literature. In addition, the dynamic analysis of RLS intensity obviously shows that the phase separation involves two different mechanisms, i.e. SD and NG. The results suggest that 2D correlation RLS is a very simple and sensitive means to monitor phase separation and might supplement the existing characterization tools. More quantitative light scattering information can be obtained by multiple-angle measurements at various directions. This method would be extended to the study of interaction in solid polymer blends containing absorption band for the possible change of electronic coupling.

[20] [21] [22] [23] [24] [25] [26]

[27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41]

P.G. De Gennes, J. Chem. Phys. 72 (1980) 4756–4763. P. Pincus, J. Chem. Phys. 75 (1981) 1996–2000. K. Binder, J. Chem. Phys. 79 (1983) 6387–6409. K. El-Mabrouk, M. Belaiche, M. Bousmina, J. Colloid. Interf. Sci. 306 (2007) 354–367. G.D. Wignall, R.G. Alamo, J.D. Londono, L. Mandelkern, F.C. Stehling, Macromolecules 29 (1996) 5332–5335. A. Jha, A.K. Bhowmick, J. Appl. Polym. Sci. 78 (2000) 1001–1008. S. Swier, D.K. Van, M.B. Van, J. Polym. Sci. Polym. Phys. 41 (2003) 1824–1836. J.T. Cabral, J.S. Higgins, T.C.B. McLeish, S. Strausser, S.N. Magonov, Macromolecules 34 (2001) 3748–3756. H.Q. Luo, S.P. Liu, Z.F. Liu, Q. Liu, N.B. Li, Anal. Chim. Acta 449 (2001) 261–270. S. Liu, Y. Chen, Z. Liu, X. Hu, F. Wang, Microchimica Acta 154 (2006) 87–93. Y. Li, X. Chen, M. Zhang, W. Luo, J. Yang, F. Zhu, Macromolecules 41 (2008) 4873–4880. L. Qi, Z.Q. Han, Y. Chen, J. Chromatogr. A. 1110 (2006) 235–239. S.P. Liu, G.M. Zhou, Z.F. Liu, Fresenius. J. Anal. Chem. 363 (1999) 651–654. N. Li, H. Luo, S. Liu, Spectrochim. Acta. A. 60 (2004) 1811–1815. X.X. Dai, Y.F. Li, W. He, Y.F. Long, C. Huang, Talanta 70 (2006) 578–583. R.F. Pasternack, P.J. Collings, Science 269 (1995) 935–939. S. Liu, H. Luo, N. Li, Z. Liu, W. Zheng, Anal. Chem. 73 (2001) 3907–3914. J. Anglister, I.Z. Steinberg, J. Chem. Phys. 78 (1983) 5358–5368. G. Arena, L.M. Scolaro, R.F. Pasternack, R. Romeo, Inorg. Chem. 34 (1995) 2994–3002. T. Nishi, T.T. Wang, T.K. Kwei, Macromolecules 8 (1975) 227–234. L.P. McMaster, Macromolecules 6 (1973) 760–773. H. Yang, M. Shibayama, R.S. Stein, N. Shimizu, T. Hashimoto, Macromolecules 19 (1986) 1667–1674. G. Beaucage, R.S. Stein, T. Hashimoto, H. Hasegawa, Macromolecules 24 (1991) 3443–3448. J.K. Kim, H.H. Lee, H.W. Son, C.D. Han, Macromolecules 31 (1998) 8566–8578. R.F. Pasternack, C. Bustamante, P.J. Collings, A. Giannetto, E. Gibbs, J. Am. Chem. Soc. 115 (1993) 5393–5399. W. Lu, B.S. Fernandez Band, Y. Yu, Q.G. Li, J.C. Shang, C. Wang, Y. Fang, R. Tian, L.P. Zhou, L.L. Sun, Y. Tang, S.H. Jing, W. Huang, J.P. Zhang, Microchimica Acta 158 (2007) 29–58. G.A. Miller, J. Chem. Phys. 82 (1978) 616–618. J. Anglister, I.Z. Steinberg, J. Chem. Phys. 74 (1981) 786–791. Z.H. Cheng, Analyst 123 (1998) 1401–1406. I. Noda, J. Am. Chem. Soc. 111 (1989) 8116–8118. I. Noda, Appl. Spectrosc. 54 (2000) 994–999. I. Noda, J. Mol. Struct. 799 (2006) 2–15. S. Morita, H. Shinzawa, R. Tsenkova, I. Noda, Y. Ozaki, J. Mol. Struc 799 (2006) 111–120. T. Hashimoto, J. Kumaki, H. Kawai, Macromolecules 16 (1983) 641–648. M. Zuo, M. Peng, Q. Zheng, Polymer 46 (2005) 11085–11092. M. Shibayama, R.S. Stein, C.C. Han, Macromolecules 18 (1985) 2179–2187. E.D. Siggia, Phys. Rev. A. 20 (1979) 595–605. J.W. Cahn, J. Chem. Phys. 42 (1965) 93–99. H.E. Cook, Acta Metal 18 (1970) 297–306. H.L. Snyder, P. Meakin, J. Chem. Phys. 79 (1983) 5588–5594. J.S. Langer, Acta Metal 21 (1973) 1649–1659.