Anomalous phase behavior in blends of -SO 3 H terminated polystyrene with poly(n-butyl acrylate) containing a small amount of tertiary amino groups

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1 e-polymers 2008, no ISSN Anomalous phase behavior in blends of -SO 3 H terminated polystyrene with poly(n-butyl acrylate) containing a small amount of tertiary amino groups Kenji Yamamoto, Ayumi Nanakno, Hiroyasu Masunaga, Isamu Akiba * * Department of Chemistry, The University of Kitakyushu, 1-1 Hibikino, Wakamatsu, Kitakyushu , Japan; fax: ; akiba@env.kitakyuu.ac.jp (Received: 23 January, 2008; published: 2 July, 2008) Introduction Abstract: Phase behavior in the blend of -SO 3 H terminated polystyrene (PSS) with poly(n-butyl acrylate-co-n,n-dimethylaminoethyl methacrylate) containing 6.0 mol% N,N-dimethylaminoethyl methacrylate (P1) is investigated by optical microscopy and small-angle X-ray scattering (SAXS). Comparing the miscibility of polystyrene/p1 blend, it is confirmed that the miscibility of the PSS/P1 blend is drastically improved by the hydrogen bonds between -SO 3 H and tertiary amino group. In addition, two-phase region of the PSS/P1 blend is split into two regions around the stoichiometric composition, in which the molar ratio of -SO 3 H to tertiary amino group is 1:1 stoichiometry. SAXS result shows that the PSS/P1 blend at stoichiometric composition forms a block copolymer-like aggregate and it takes a disorder state. Phase behavior of an immiscible polymer blend is strongly affected by intermolecular interaction [1]. Generally, miscibility of a polymer blend is improved by introduction of intermolecular attractive interactions, such as hydrogen bonds. However, when a small amount of intermolecular hydrogen bonds are introduced to an immiscible polymer blend, the miscibility enhancement by the hydrogen bonds should compete with phase separation due to segregation force between component polymers. Such a competition of the opposite interactions should cause a characteristic feature in phase behavior of the polymer blend [2-6]. In previous studies, the author found that the blend of poly(n-vinylpyrrolidone) (PVP) and polystyrene having -SO 3 H group at the terminal position (PSS) formed nanoorganized phase structure like a block copolymer due to hydrogen bonds and segregation of PVP and PSS [6]. In this case, the polystyrene/pvp blend is a strongly segregating system. On the other hand, miscibility of a weakly segregating polymer blend should be influenced by slight change of interactions more strongly than that of a strongly segregating system. Therefore, such a hydrogen bond should cause more remarkable change in phase behavior of a weakly segregating polymer blend. Blend of polystyrene (PS) and poly(n-butyl acrylate) (PBA) has been well-known as a weakly segregated polymer blend without any specific interactions between the components [7]. In addition, -SO 3 H group can form strong hydrogen bond with tertiary amino group [4, 8]. Thus, the aim of the present study is to investigate effect 1

2 of a small amount of hydrogen bonds on phase behavior of the weakly segregating blend consisted of PS having a terminal -SO 3 H group (PSS) and PBA containing small amount of tertiary amino groups. Results and discussion Polymers used in this study are PSS, PS and poly(n-butyl acrylate-co-n,ndimethylaminoethyl methacrylate) containing 6 mol% N,N-dimethylaminoethyl methacrylate (P1) (Scheme 1). The PSS and PS have same degree of polymerization (43-mer) because the PS was produced during the living anionic synthesis of the PSS. In the PSS/P1 blend, hydrogen bond between terminal -SO 3 H group of PSS and tertiary amino group of P1 is formed. The PS/P1 blend is used as a control. The PSS/P1 and PS/P1 blends are represented as PSS/P1(x) and PS/P1(x), respectively. Here, x in the parenthesis denotes weight fraction of P1. SO 3 H 43 Hydrogen Bonding SO 3 H N PSS O O O O 43 H Weak Segregation N PS P1 Scheme 1. Molecular structure and characteristics of PSS, PS, and P1. Fig. 1 shows phase diagram of the PSS/P1 blend. The closed circle and solid line indicate transition temperature and the shaded area shows two-phase region of the PSS/P1 blend. As a control, the transition temperature of the PS/P1 blend is indicated by open circle and dotted line in Fig. 1. The transition temperatures were determined by phase contrast optical microscopy for the PSS/P1 and the PS/P1 blends at various temperatures. The blends take homogeneous state at higher temperature than the transition temperatures and the transitions are thermally reversible. Homogeneous region of the PSS/P1 blend is much larger than that of the PS/P1 blend. This result means that the miscibility is extremely improved by introduction of a terminal -SO 3 H group to PS. Therefore, such hydrogen bond has significant effects on miscibility enhancement of the blend, even if its amount is just a little. In addition, the two-phase region splits into two species in PSS/P1 blend. A polymer blend usually shows a simple dome-type phase diagram. In addition, the PS/P1 blend as a control shows usual phase behavior. Hence, the unusual phase behavior in the PSS/P1 blend should be related to the hydrogen bond between 2

3 terminal -SO 3 H group and tertiary amino group. The composition shown as vertical dashed line in Fig. 1 corresponds to stoichiometric composition (φ st ), where molar ratio of terminal -SO 3 H group of PSS to tertiary amino group of P1 is 1:1 stoichiometry. The φ st is located at the cleft of the two-phase regions. This means that the miscibility of the PSS/P1 blend is particularly improved around the φ st. Tanaka et. al. theoretically predicted the phase diagrams of polymer-polymer or polymeroligomer mixtures containing an end-functional polymer as one component at least [9, 10]. According to their predictions, miscibility of the mixture is sufficiently improved and unstable region splits into two regions around the φ st due to the formation of block copolymer (BCP)-like aggregate. In addition, the miscibility enhancement around the φ st becomes remarkable with an increase of binding energy between different components. In the PSS/P1 blend, the BCP-like aggregate can be formed as shown in Scheme 1. In addition, the hydrogen bond between -SO 3 H group as a terminal group of the PSS and tertiary amino group is sufficiently strong. Accordingly, the two-phase region should be split into two species in the PSS/P1 blend. In this case the BCP-like or graft copolymer-like aggregates are formed through the hydrogen bond between a terminal -SO 3 H group and a tertiary amino group. Therefore, the other features of a block or graft copolymers should be observed. Fig. 1. Phase diagram of PSS/P1 and PS/P1 blends. Closed circle and solid line show PSS/P1 blend. The shaded area is two-phase region of PSS/P1 blend. Open circle and dotted line show cloud point of PS/P1 blend. PS/P1 blend forms homogeneous mixture at higher temperature than the cloud point. At the concentration indicated by dashed line, molar ratio of -SO 3 H of PSS to tertiary amino group of P1 is equal to 1 (stoichiometric composition). Fig. 2 shows small-angle X-ray scattering (SAXS) profile from the PSS/P1 (0.7) at ambient temperature. The vertical axis is intensity (I(q)) and horizontal one the 3

4 scattering vector, q, where q is defined as q=(4π/λ)sinθ (λ: wave length of incident beam, θ: scattering angle). The SAXS profile indicates broad peak. The SAXS profile from usual miscible polymer blend does not show such a peak. On the contrary, the SAXS profile from block copolymer in disordered state shows such a broad peak due to the connection between different polymers [11]. Therefore, the BCP-like aggregate is formed in the PSS/P1 (0.7) blend by the hydrogen bond between a terminal -SO 3 H group of PSS and a tertiary amino group of P1 and it takes disordered state. Accordingly, it is concluded that the formation of the BCP-like aggregate by the hydrogen bond between terminal -SO 3 H of PSS and tertiary amino group of P1 causes such a unique phase behavior of the PSS/P1 blend. In addition, this experimental result shows an agreement with theoretical prediction of the phase behavior in an associating polymer blend containing an end-functional polymer. Fig. 2. SAXS profile of PSS/P1(0.7) blend at room temperature. Experimental part Sample Preparation PSS and PS were synthesized by living anionic polymerization [6, 12-14]. P1 was synthesized by free radical copolymerization of n-butyl acrylate and N,Ndimethylaminoethyl methacrylate using AIBN as an initiator. M n and polydispersity index (PDI) of the PSS were 4.5x10 3 and 1.1, respectively, determined by GPC and 1 H-NMR. It was confirmed by the elemental analysis and 1 H-NMR that the PSS has one -SO 3 H group in a PSS chain. The PS had same degree of polymerization and polydispersity as PSS. M n and PDI of the P1 were 3.4x10 3 and 1.6, respectively, determined by GPC and 1 H-NMR. Content of N,N-dimethylaminoethyl methacrylate in the P1 was 6.0 mol% determined by 1 H-NMR. 4

5 Blend samples were prepared by solution mixing using CHCl 3. Component polymers weighed to desired composition were dissolved in CHCl 3 and stirred until the solution became clear. Subsequently, the solvent was evaporated on glass plate at room temperature. The resultant blend samples were further dried under reduced pressure. Measurements Molecular weights of these polymers were determined by combination of 1 H-NMR spectroscopy and gel permeation chromatography calibrated by standard polystyrene. Contents of functional groups in the polymers were determined by 1 H- NMR spectroscopy and elemental analyses. 1 H-NMR measurements were carried out using a JEOL JNM-ECP500 NMR spectrometer. Elemental analyses were performed by Yanaco MT-5 CHN and S corder. Cloud point measurements were performed using a Nikon E-600 optical microscope equipped with a Linkam heating stage to precisely control temperature. SAXS measurement was performed at the BL-40B2 station of SPring-8 in Japan. Two-dimensional SAXS pattern was obtained by a Rigaku R-AXIS IV++ imaging plate. The one-dimensional SAXS profile (I(q) vs. q) was obtained by converting from the two-dimensional SAXS pattern by circular averaging. Acknowledgements Elemental analysis and 1 H-NMR measurements were performed using the apparatus at the Instrumentation Centre of The University of Kitakyushu. SAXS measurement was performed under approval of the SPring-8 Advisory Committee (the approved number 2007A1243). This study was financially supported by Grant-in-Aid for Young Scientists from Japanese Ministry of ECSST. References [1] Coleman, M. M.; Graf, J. F.; Painter, P. C. Specific Interaction and the Miscibility of Polymer Blends; Technomic : Lancaster, [2] Tanaka, F. Adv. Colloid Interface Sci. 1996, 63, 23. [3] Hobbie, E. K.; Han, C. C. J. Chem. Phys. 1996, 105, 738. [4] Akiba, I.; Murata, S.; Masunaga, H.; Sasaki, K. Composite Interfaces 2006, 13, 415. [5] Akiba, I.; Ohba, Y.; Akiyama, S. Macromolecules 1999, 32, [6] Akiba, I.; Masunaga, H.; Sasaki, K.; Shikasho, K.; Sakurai, K. Polymer 2004, 45, [7] Miwa, Y.; Usami, K.; Yamamoto, K.; Sakaguchi, M.; Sakai, M.; Shimada, S. Macromolecules 2005, 38, [8] Akiba, I.; Masunaga, H.; Murata, S.; Sasaki, K. e-polymers 2006, 036. [9] Tanaka, F.; Ishida, M.; Matsuyama, A. Macromolecules 1991, 24, [10] Tanaka, F.; Ishida, M. Macromolecules 1997, 30, [11] Mori, K.; Tanaka, H.; Hashimoto, T. Macromolecules 1987, 20, 381. [12] Akiba, I.; Jeong, Y.; Sakurai, K. Macromolecules 2003, 36, [13] Quirk, R. D.; Kim, J. Macromolecules 1991, 24, [14] Masunaga, H.; Sasaki, K.; Akiba, I. e-polymers 2006,