1. Introduction With the recent developments in synthesis strategies of novel macromolecules, it is now possible to tune the macromolecular size and shape by covalent and non-covalent interactions. The covalent interactions can be effectively tapped to form block copolymers (BCPs) which has numerous applications such as, an etch mask for nano/meso scale patterning, photo- voltaic devices, drug delivery, etc [1–6]. Apart from the cited applications, BCPs are also being explored for their applicability as photonic materials [7–12]. Among the numerous strategies of controlled/living free-radical polymerization for the synthesis of BCPs, reversible addition–fragmentation chain transfer (RAFT) polymerization process stays in the forefront. The advantage of RAFT polymerization is that, it re- sults in polymers with controlled molecular weight and with narrow polydispersities and offers a wide 589 Optical and rheological studies on weak gel-sol transition in aqueous solutions of poly(N-isopropylacrylamide)-block-polystyrene S. Sanjeevi Prasath 1 , J. Brijitta 1* , B. V. R. Tata 2 , R. G. Joshi 3 , K. Chennakesavulu 4 , Deepak K. Gupta 3 1 Centre for Nanoscience and Nanotechnology, Sathyabama University, Chennai-600119, India 2 Department of Physics, Indian Institute of Technology Palakkad, Palakkad-678557, India 3 Condensed Matter Physics Division, Materials Science Group, Indira Gandhi Centre for Atomic Research, Kalpakkam-603102, India 4 Department of Chemistry, Sathyabama University, Chennai-600119, India Received 23 November 2016; accepted in revised form 21 February 2017 Abstract. The optical and rheological properties of aqueous solutions of block copolymer composed of low molecular weight poly(N-isopropylacrylamide)-b-polystyrene are studied as a function of temperature. From light scattering measurements the block copolymer solution is found to form micelles at very low concentrations and the critical micellar concentration is identified as 0.005 wt%. Apart from the concentration dependence, a unique temperature dependent micelle formation is noted at 34 °C. Further, temperature dependent refractive index measurements shows that the refractive index increases with temperature (beyond the lower critical solution temperature, 31.6 °C of the polymer), and is attributed to the stable rearrange- ment of the proximal hydrophobic isopropyl-polystyrene chains in the collapsed polymer so as to overcome the steric hin- drance effects offered by the hydrophobic chains. In the polymer concentrations investigated for rheological studies, the so- lution flows, yet manifested solid like behavior with G′> G″ with the modulus being frequency dependent and the magnitude of G′ two-fold higher than G″ implying a weak gel state. Weak gel states are in general noted at high temperatures in most of the polymer systems, contrary to this, in our studies weak gel state is observed at lower temperature. Further, a transition from weak gel to sol state is observed at slightly elevated temperatures. The reason for the existence of weak gel state below the lower critical solution temperature is due to the micellar entanglements of poly(N-isopropylacrylamide)-b-polystyrene with one another and whereas above the lower critical solution temperature disentanglement of the micelles makes the system behave like a viscoelastic liquid. Keywords: rheology, block copolymer, PNIPAM, light scattering, micelles eXPRESS Polymer Letters Vol.11, No.7 (2017) 589–599 Available online at www.expresspolymlett.com https://doi.org/10.3144/expresspolymlett.2017.56 * Corresponding author, e-mail: brijitta@live.com © BME-PT