Temperature-responsive inclusion complex of cationic PNIPAAM diblock copolymer and g-cyclodextrin† Giuseppe Lazzara,‡ a Gerd Olofsson, a Viveka Alfredsson, a Kaizheng Zhu, b Bo Nystr€ om b and Karin Schill  en * a Received 25th November 2011, Accepted 27th February 2012 DOI: 10.1039/c2sm07252a Aqueous mixtures of g-cyclodextrin (g-CD) and the thermosensitive cationic diblock copolymer poly(N-isopropylacrylamide)-b-poly(3-acrylamidopropyl)trimethylammonium chloride (PNIPAAM 24 -b-PAMPTAM(+) 9 ) or the PNIPAAM homopolymer PNIPAAM 47 have been investigated using various experimental methods. Solid g-CD–polymer inclusion complexes (pseudopolyrotaxanes) form at ambient temperatures in fairly concentrated CD solutions. The NMR measurements showed that the stoichiometry of the inclusion complexes is close to two NIPAAM units per CD molecule. The cationic block of the copolymer is not incorporated into the CD cavity. Synchrotron radiation X-ray diffraction spectra of the solid inclusion complexes indicate a compact columnar structure of CD molecules threaded onto the PNIPAAM chains. In water, square-shaped cyclodextrin aggregates were found to co-exist with single cyclodextrin molecules. In mixed solutions of g-CD and PNIPAAM 24 -b-PAMPTAM(+) 9 these aggregates disintegrate with time as inclusion complexes are formed and the kinetics was studied using time-resolved static and dynamic light scattering and cryo-TEM. Steady-state fluorescence spectroscopy measurements reveal that the CD molecules dethread from the PNIPAAM chains upon increasing the temperature to 40  C, which is above the lower critical solution temperature of PNIPAAM. Introduction The word polyrotaxane is generally used to identify supramo- lecular structures that are formed by a linear axis (the polymer chain) with several ring-shaped molecules like cyclodextrin (CD) threaded onto it. 1 The structures have bulky moieties bound to each end of the polymer chain so that the ring molecules cannot dethread due to steric constraint. If there are no stoppers at the end of the polymer chains, the word pseudopolyrotaxane is used. The net attractive intermolecular interactions behind this type of inclusion complex formation are hydrophobic interaction and van der Waals forces. 2–4 The pseudopolyrotaxanes and poly- rotaxanes formed by block copolymers and CD molecules are extensively investigated because they have tunable structures and they can be used in several application fields based on molecular recognition and molecular switches. 1,5 Thorough control of such supramolecular structures by using external stimuli and the understanding of the involved mechanisms is a challenge for the design of smart nanomaterials. 3 The CDs are cyclic oligosaccharides formed by glucopyranose units. They have a truncated cone shape with a hollow cavity, which may incorporate hydrophobic solutes or polymer chains. It has been observed that several CD rings can thread a polymer chain assuming either a close compact 1,6 or a loose 6 structure depending on the nature of the CD. If copolymers are consid- ered, the size of the CD cavity determines which block will be enclosed. Moreover, Fujita et al. 7 reported that the CD can be moved from one block to another by varying the temperature. It is interesting to note that CD molecules in water have a tendency to form clusters, 8–10 which have been considered strategic in the formation of pseudopolyrotaxanes. The forma- tion of an inclusion complex between Nylon-6 and a-CD has been used to manipulate the polymorphic crystal structure, crystallinity and subsequent orientation of the polymer chain. 11 Furthermore, it has been shown that supramolecular inclusion complexes with CD can control the unwinding and rewinding of a double helix of oligoresorcinol, 12 the stretching of polymer brushes 13 and the formation of functional pseudopolyrotaxanes for drug and gene delivery. 14,15 Promising results in the field of nanoelectronics have been presented by CD-threaded polymers. 16 a Division of Physical Chemistry, Department of Chemistry, Center for Chemistry and Chemical Engineering, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden. E-mail: Karin.Schillen@fkem1.lu.se b Department of Chemistry, University of Oslo, P.O. Box 1033, Blindern, N-0315 Oslo, Norway † Electronic supplementary information (ESI) available: Chemical shifts for g-CD protons in the presence of polymers. Time-resolved SLS data for an aqueous mixture of 10 wt% g-CD and 1 wt% PNIPAAM 72 . Time-resolved SLS and DLS data for an aqueous mixture of 10 wt% g-CD and 0.5 wt% PNIPAAM 24 -b-PAMPTMA(+) 9 . See DOI: 10.1039/c2sm07252a ‡ Present address: Department of Chemistry, University of Palermo, Viale delle Scienze, IT 90128 Palermo, Italy. This journal is ª The Royal Society of Chemistry 2012 Soft Matter , 2012, 8, 5043–5054 | 5043 Dynamic Article Links C < Soft Matter Cite this: Soft Matter , 2012, 8, 5043 www.rsc.org/softmatter PAPER Published on 23 March 2012. Downloaded by Lund University on 02/10/2013 23:12:17. View Article Online / Journal Homepage / Table of Contents for this issue