Concentration Mediated Structural Transition of Triblock Copolymer Ultrathin Films Jayanta K. Bal,* ,† Manabendra Mukherjee, ⊥ Nicolas Delorme, † Milan K. Sanyal, ⊥ and Alain Gibaud † † LUNAM Universite ́ , IMMM, Faculte ́ de Sciences, Universite ́ du Maine, UMR 6283 CNRS, Le Mans Cedex 9, 72000, France ⊥ Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700064, India * S Supporting Information ABSTRACT: X-ray reflectivity, atomic force microscopy, X-ray photoelectron spectroscopy, and contact angle measure- ment techniques are used to study the structural changeover as a function of concentration of poly(ethylene oxide)-poly- (propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymer diluted in toluene spin-coated as ultrathin films on hydrophilic Si substrate. A lamellar structure made of three alternating incomplete bilayers is observed until the concentration of copolymer solution attains a threshold value of about 3.6-4 g/L. Around this concentration and beyond, the entanglement of polymer chains takes place during drying and the growth of a homogeneous film made of complete bilayers on Si substrate is observed. The strong hydrophilic nature of the Si substrate dictates the growth of this amphiphilic copolymer. We evidence that the lower part of the films is made of hydrophilic PEO blocks attached to the substrate while the hydrophobic PPO blocks are directed toward air. ■ INTRODUCTION Amphiphilic block copolymers have emerged as smart materials for combining antagonist properties of different molecules by building a macromolecule in which the antagonist polymer blocks are attached by a covalent bond. The ability of these block copolymers to undergo microphase separation has generated significant interest in their application for forming ordered morphologies. 1-10 The conformation of any macromolecule in solution is governed by the equilibrium between the interaction strengths of the polymer segments among themselves and with the molecules of the solvent. 11,12 This equilibrium is broadly referred to as the “solvent quality” and is described by a variety of parameters such as the Hildebrand solubility 13 and the Flory parameters. 14 According to the Hildebrand solubility theory, solubility parameter (describes as the square root of the cohesive energy density of a material) of a good solvent is closer to that of the macromolecule, whereas Flory theory suggests that the smaller (or more negative) the Flory parameter, the more favorable the monomer-solvent interaction. For a block copolymer like PEO-PPO-PEO, this balance becomes more complex due to the contributions from the interactions of two different building blocks, namely, PEO block, which is hydrophilic, and PPO, hydrophobic. Therefore, an unusual circumstance appears when these interactions involve a solvent which is good for one of the units but bad for the other(s). This is called a selective solvent, being selective to the block whose solvation is stronger. Thus, it leads to an amphiphilic behavior of the polymer, and as a consequence, it has a tendency to self- assemble according to the nature of surfactant solutions. The self-assembly of block copolymers into well-defined structure has opened up numerous possibilities for applications in drug delivery, 15,16 biomembrane activities, 17 and in material science (adhesive properties, lubricants, membranes, and coatings), 18,19 lithography 20 and electronic devices (light emitting diode, photodiodes, and transistors, etc.). 21 Further- more, such copolymers are well-known as reducing agents for different types of nanoparticle formation. 22-30 A detailed study of this polymer solution has been made with or without nanoparticles. Thus, at first the growth of pure copolymer is essential prior to further understanding. The adsorption and desorption behaviors of pure symmetric triblock copolymers from aqueous solutions onto solid surfaces were studied via ellipsometry, 31 quartz crystal microgravimetry (QCM), 32 total internal fluorescence spectroscopy (TIRF), 33 and surface plasmon resonance spectroscopy (SPR). 34 In this regard, nondestructive X-ray reflectivity (XRR) technique 35,36 would be more advantageous as it simultaneously provides the information about thickness (resolution ∼ angstrom) and electron density, which is proportional to the mass density or coverage of the grown film on solid surfaces. To the best of our knowledge, no such studies have been made so far revealing the structure of ultrathin films of asymmetric triblock copolymer prepared by spin coating. In this work, we demonstrate that the structure of ultrathin films made of highly asymmetric PEO-PPO-PEO copolymer on Si surface can be tuned by varying the concentration of polymer solutions. Vertical stacking of three alternating bilayers for all the films with increasing density of the upper two bilayers with Received: January 21, 2014 Revised: April 16, 2014 Published: April 30, 2014 Article pubs.acs.org/Langmuir © 2014 American Chemical Society 5808 dx.doi.org/10.1021/la5002607 | Langmuir 2014, 30, 5808-5816