ORIGINAL PAPER Conformation mediated preferential swelling of amphiphilic block copolymer ultrathin films Jayanta Kumar Bal 1 & Manabendra Mukherjee 2,3 & Lucky Dildar 1 & Alain Gibaud 4 Received: 8 March 2017 /Accepted: 5 November 2017 # Springer Science+Business Media B.V., part of Springer Nature\ 2017 Abstract Steady-state and dynamic swelling characteristics of PEO-PPO-PEO copolymer ultrathin films comprising 2- layered structure, where the bottom layer (L1) is made of one PEO (hydrophilic)-PPO (hydrophobic) bilayer and the top layer (L2) is made of two alternating PEO-PPO bilayers, have been studied at room temperature in precisely controlled environment of 10–93% relative humidity (RH). Swelling, which is monitored by measuring the thickness of the films using X-ray reflectivity (XRR), is found to be largely dissim- ilar in differently thick films prepared by varying the concen- tration of mother solutions from 1 g/L to 4 g/L. This is well explained considering a preferential-swelling of amphiphilic block copolymer, where only the hydrophilic PEO blocks contribute in swelling and PPO acts a barrier in the diffusion of water molecules. The swelling kinetics is governed by the thickness/width and coverage of outer PPO block layer. Keywords Copolymer . Structure . Conformation . Swelling . Diffusion . Hydrophobic/hyhdrophilic . Ultrathin film . X-ray reflectivity Introduction Copolymers have received eminent interest owing to their combination of unique physical, chemical properties and their potential applications in the fields of sensors, coating, mem- brane, drug delivery, medical treatments and other biological applications [1–10]. Particularly in medical stream, hydrophil- ic poloxamers i.e., linear PEO-PPO-PEO triblock copolymers are appropriate in treating severe heart failure and necrosis of muscle cells by refurbishing the structural integrity of injured cell membranes whereas hydrophobic poloxamers can facili- tate the passage of molecules across lipid membrane [11]. A characteristic feature of block copolymer systems is the repul- sion between unlike blocks, which leads to microphase sepa- ration at mesoscopic length scales. The reason behind these structural and swelling studies of such triblock copolymer lies in the fact that the self-assembly of block copolymers into well-defined structure has opened up numerous possibilities for the applications in drug delivery, biomembrane activities and in material science (adhesive properties, lubricants, mem- branes and coatings), lithography and electronic devices (light emitting diode, photodiodes and transistors etc.). The structural properties of polymer thin films deviate con- siderably from that of the bulk due to the effect of confine- ment. In contrast to homopolymers, amphiphilic copolymers face more complicated situation due to inclusion of additional parameters, such as the polarity and length of the constituent blocks. With decreasing film thickness these parameters be- come increasingly important in defining the morphology. In our previous study [12] we looked upon these matters. We observed that the surface and interface free energies as well as the conformation of copolymer in mother solution of selective-solvent were influencing the structure of this kind of polymer. The significant difference in surface energy of the two blocks and the selective interaction of substrate * Jayanta Kumar Bal jayanta.bal@gmail.com 1 Department of Physics, Abhedananda Mahavidyalaya, University of Burdwan, College Road, Sainthia 731234, India 2 Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700064, India 3 Adjunct Prof at Department of Physics, B.R.Ambedkar Bihar University, Muzaffarpur 842001, Bihar, India 4 LUNAM Université, IMMM, Faculté de Sciences, Université du Maine, UMR 6283 CNRS, 72000 Le Mans Cedex 9, France J Polym Res (2017) 24:222 https://doi.org/10.1007/s10965-017-1384-3