Volume 38, Number 4
February 22, 2005
© Copyright 2005 by the American Chemical Society
Communications to the Editor Polymer Blend Compatibilization by Gradient Copolymer Addition during Melt Processing: Stabilization of Dispersed Phase to Static Coarsening Jungki Kim,† Maisha K. Gray,‡ Hongying Zhou,§ SonBinh T. Nguyen,§ and John M. Torkelson*,†,‡ Department of Chemical and Biological Engineering, Department of Materials Science and Engineering, and Department of Chemistry, Northwestern University, Evanston, Illinois 60208 Received November 27, 2004 Revised Manuscript Received January 10, 2005
Introduction. It has long been appreciated that blending of immiscible polymers can allow for synergistic tunability of material properties.1-5 However, compatibilization of immiscible blends remains an academic and technological challenge. As optimal properties often rely on an average dispersed-phase diameter less than several microns, the stabilization of the dispersed phase domain size against coarsening, taken as the definition6 of compatibilization, is key to processing immiscible blends. Many compatibilization strategies have been tested,1 most involving methods that theoretically lead to a reduction in interfacial tension7 and/or to steric hindrance against coalescence.6 These strategies include the addition of premade block,8-12 tapered block,13 graft,14 and random15,16 copolymers during melt processing. The use of added block/graft copolymers has led to compatibilization in selected small-scale studies but has not been commercialized, due in part to the very low critical micelle concentration5,17,18 (cmc) that prevents sufficient copolymer from reaching blend interfacial regions during melt processing. Random copolymer addition typically leads to †
Department of Chemical and Biological Engineering. Department of Materials Science and Engineering. § Department of Chemistry. * To whom correspondence should be addressed. E-mail:
[email protected]. ‡
encapsulation of the dispersed phase and thus does not lead to compatibilization even in small-scale studies.15 A strategy that has seen some commercial success is reactive compatibilization, resulting in the in situ production of block or graft copolymer at the blend interfaces during melt processing.1,19,20 Reactive processing by solid-state shear pulverization (SSSP) has also yielded compatibilization.21-23 The advent of nitroxide-mediated controlled radical polymerization24 and atom-transfer radical polymerization25 has led to the production of a new class of copolymer called gradient copolymers26-33 that possess a gradual change in composition along the chain length. Gradient copolymers may also be made by ring-opening metathesis polymerization.34 Theoretical studies35,36 indicate that gradient copolymers dispersed in homopolymer have much higher cmcs and exhibit broader interfacial coverage than block copolymers of the same composition, suggesting that gradient copolymers may be effective blend compatibilizers. The simplicity of controlled radical polymerization also makes gradient copolymers attractive for academic studies and technological application. Here we demonstrate for the first time that compatibilization of an immiscible blend can be achieved via the addition of gradient copolymer during melt processing. By comparing several blend systems in terms of the evolution of the number-average dispersed phase diameter, Dn, of melt mixed samples after static annealing, we show that coarsening can be eliminated or suppressed upon melt mixing by addition of gradient copolymer to the blend. Experimental Section. Two polystyrene (PS) samples were used in this study: PSa (Mn ) 26 600, Mw/Mn ) 1.09, Pressure Chemical) and PSb (bimodal molecular weight distribution: Mn1 ) 36 900, Mw1/Mn1 ) 1.88, Mn2 < 2000, Scientific Polymer Products). Poly(methyl methacrylate) (PMMA) (Mn ) 16 200, Mw/Mn ) 1.84, Scientific Polymer Products) and poly(n-butyl methacrylate) (PnBMA) (Mn ) 97 700, Mw/Mn ) 2.27, Aldrich) were the dispersed phases. Styrene (S), methyl methacrylate
10.1021/ma047549t CCC: $30.25 © 2005 American Chemical Society Published on Web 01/25/2005
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Communications to the Editor
(MMA), and n-butyl methacrylate (nBMA) (Aldrich) were deinhibited with inhibitor remover (Aldrich) and dried over CaH2. Gradient copolymers of S/MMA (SgradMMA) and S/nBMA (SgradnBMA) were made using alkoxyamine 29, 2,2,5-trimethyl-3-(1-phenylethoxy)4-phenyl-3-azahexane,37 as unimolecular initiator. Semibatch polymerization involved addition of MMA or nBMA to an initially all S reaction mixture, which was purged with N2 for 30 min prior to polymerization. SgradMMA (Mn ) 37 700, Mw/Mn ) 1.6338) and SgradnBMA (Mn ) 94 300, Mw/Mn ) 1.4738) were obtained after 8 h at 93 °C and 5 h at 105 °C, respectively. Low conversion (
