- 1 - DOI: 10.1002/marc.((please add manuscript number)) Article type: Communication Controlling Thermochromism in a Photonic Block Copolymer Joseph J. Walish, Yin Fan, Andrea Centrone and Edwin L. Thomas* ––––––––– Dr. J. J. Walish Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA Y. Fan Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA Dr. A. Centrone Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA and Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, MD 20899-6203, USA Prof. E. L. Thomas Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA and Brown School of Engineering, Rice University, Houston, TX 77030, USA E-mail: (elt@rice.edu) ––––––––– The tunable properties of stimulus-responsive materials attract great interest in a variety of technological applications. Photonic gels are a new class of these materials which can be tuned to reflect different wavelengths of light. Controlling this reflected color via temperature-induced changes of self-assembled photonic materials is important for their application in sensors and displays. In this work, the thermochromic behavior of a PS-P2VP photonic gel was found to originate from a temperature-induced change in the pKa of the P2VP blocks. Control was obtained through the manipulation of the solution pH. The findings of this work provide the basis for understanding and controlling the properties of thermochromic block copolymers fostering their use in technologically relevant applications.