Interfacial behavior of pH responsive ampholytic heteroarm star block terpolymers Rajesh Kodiyath a , Ikjun Choi a , Brendan Patterson a , Constantinos Tsitsilianis b , Vladimir V. Tsukruk a, * a School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245, USA b Department of Chemical Engineering, University of Patras, 26504 Patras and Institute of Chemical Engineering and High Temperature Processes (FORTH/ICE-HT), Greece article info Article history: Received 8 October 2012 Received in revised form 5 December 2012 Accepted 10 December 2012 Available online 26 December 2012 Keywords: Star block terpolymers LangmuireBlodgett films pH-responsive films abstract We report the pH-controlled surface behavior of amphiphilic PS n (P2VP-b-PAA) n heteroarm star block terpolymers, (n ¼ number of arms, 9 and 22), bearing amphoteric diblock arms with varying polyvalent ionic charges (i.e. negative, positive, and zwitterionic) at the airewater and airesolid interfaces. We investigated the assembly of these pH-sensitive star terpolymers in Langmuir and LangmuireBlodgett monolayers under different pH conditions of the subphase. The p-A isotherms acquired at variable pH conditions revealed a distinct aggregation behavior of surface micelles which is dependent on the ionization of the polyelectrolyte blocks and the number of arms. The star block copolymer with a small number of arms (n ¼ 9) was found to exhibit a strong pH-dependent phase transformation under compression. The pH responsive (zwitterionic) behavior results in changes in surface morphologies from circular micelles to complex labyrinth structures. In contrast, star polymers with a larger number of arms (22) and a crowded branched architecture show stable circular domain morphology without the internal reorganization under variable conditions. The observed variety of surface behaviors is attributed to the fine balance of intra molecular interactions caused by the highly branched chain architecture composed of both acidic and basic blocks. This study proposes a novel strategy for pH induced tunable surface assembly of responsive ultrathin films with multicompartments. We indeed demonstrated that the enhanced responsiveness of star polymers to wide pH range and their diverse morphology at the interface is enabled by coupling amphoteric concept and tailored molecular architecture. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Macromolecular engineering in combination with self-assembly techniques allows for the development of a number of strategies to achieve tunable nanomaterials for different applications such as drug delivery, catalysis, and microelectronics [1e5]. Highly branched macromolecular architectures offer a further possibility for the design of novel nanostructures [6e8]. Multi-armed star copolymers, hyperbranched polymers, dendritic polymers, dendrimer-like polymers, and star polymers with hyperbranched or dentritic fragments attached to the end of arms are of great interest because of their peculiar behavior in solutions, the bulk state, and at interfaces [9e18]. This peculiar morphology and physical behavior arises from their unique and diverse chain architectures relative to traditional linear block copolymers [19e 21]. For instance, micrometer-long uniform nanofibers were obtained from amphiphilic functionalized hyperbranched mole- cules possessing irregular flexible cores capable of forming multiple hydrogen bonds [22e25]. Recently, star-shaped polymers with different blocks confined to a single core, named heteroarm or miktoarm star copolymers, have attracted significant attention because of their well defined macromolecular architectures and can be considered as model branched systems with colloidal and polymeric properties [26e33]. As a result of their star-shaped chain architecture, they can show peculiar micelle aggregation in bulk, solution, and at interfaces. In fact, star copolymers show a critical micellar concentration which is a few orders of magnitude higher that of linear counter parts while the association number is significantly lower than that observed for linear counter parts [34e38]. Star copolymers bearing chemically different arms possess a variety of morphologies such as spherical micelles, stripes, and disk-like micelles depending upon the copolymer composition and solvent selectivity [36,39e42]. For instance, the assembly of amphiphilic heteroarm star polymers composed of hydrophobic and hydrophilic arms polystyrene/poly(2-vinylpyridine) (PS n P2VP n ) in * Corresponding author. E-mail address: vladimir@mse.gatech.edu (V.V. Tsukruk). Contents lists available at SciVerse ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer 0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2012.12.031 Polymer 54 (2013) 1150e1159