Reexamination of Slow Dynamics in Semidilute Solutions: From Correlated Concentration Fluctuation to Collective Diffusion Guangcui Yuan, † Xiaohong Wang, † Charles C. Han,* ,† and Chi Wu* ,‡ State Key Laboratory of Polymer Physics & Chemistry, Joint Laboratory of Polymer Science & Materials, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China, and Department of Chemistry, The Chinese UniVersity of Hong Kong, Shatin, N.T., Hong Kong ReceiVed January 10, 2006; ReVised Manuscript ReceiVed March 26, 2006 ABSTRACT: Two different polymer systems, poly(N-isopropylacrylamide) (PNIPAM)/H 2 O and poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO)/H 2 O, were examined by laser light scattering(LLS). In both cases, a single relaxation mode was observed in dilute solution which is related to the mutual diffusion of separated polymer chains. As the polymer concentration increases and enters the semidilute regime, one fast and one slow relaxation modes were observed. The fast mode corresponds to cooperative diffusion of chain segments inside each “blob”. Utilizing the thermal-sensitive properties of these two systems, we followed the disentanglement of transient network in the semidilute solutions through temperature-induced chain shrinkage, but without changing the overall concentrations. Meanwhile, we can follow the slow mode of the semidilute solution changes from long-range correlated concentration fluctuation of transient network to collective diffusion of aggregates or micelles. The present results clearly reveal that the slow modes in these two different systems have the same nature. Introduction Semidilute solutions of polymers have been the focus of attention for more than two decades. Because of the existence of overlapping and entanglement of polymer chains, new dynamical processes involving interchain interactions and disentanglement come into the problem. Many theoretical and experimental works have been invested to clarify the properties of these systems. Among these, dynamics and relaxation processes investigated by dynamic LLS are of particular interest. A number of dynamic LLS experiments on semidilute solutions have detected the deviation of the intensity-intensity time correlation function from a single-exponential decay, which is simultaneously with a fast and so-called slow mode. 1-20 It is a widely held view that the fast mode is related to the cooperative diffusion of chain segments in each “blob”, 21 while the slow mode has been assigned to a variety of origins, and the interpretation is far from a comprehensive resolution. For example, a general consensus in the angular, concentration, and chain length dependence and a clear understanding regarding the origins of the slow mode are still lacking. For homopolymer in semidilute solutions, the origins of the slow mode have been mainly postulated to be related to reptation of a clustering of polymer chains through the entangled coils, 1-4 or to the center-of-mass motion of the entire polymer chain in its surroundings, 5-7 or to the viscoelastic properties of the network. 14-17 As for block copolymer in a selective solvent, the chain dynamics in semidilute solutions is more complicated and system dependent, i.e., a good solvent for one block and a poor or nonsolvent for another block; segregation effects are responsible for a wide range of structures depending on the number of blocks, their degree of polymerization, their incom- patibility, and the solvent quality. 22,23 A common feature of these systems is that multiple dynamic modes could be observed by dynamic LLS. The slow mode is tentatively attributed to cooperative rearrangements of microdomains, 24 or to cooperative diffusion of nodes, 25 or to viscoelastic relaxation of textures embedded in the solution, 26 or to “long-range density fluctua- tions” or “cluster” relaxation. 27,28 The above interpretations about the origins of the slow mode are all speculative. In general, the slow modes observed in block copolymer systems are thought to be different in nature with that observed in homopolymer systems. 20 Actually, for polymers with high molecular weight, the overlap concentration (C*) is low, and the semidilute regime extends to a low concentration (in terms of g L -1 ), 29 where the segment density is so low that the extent of interpenetration or overlapping between polymer chains is rather limited. The precursor stage of a uniform transient network formation apparently extends over an unexpectedly broad concentration range where most of the reported experiments were carried out. 30 This precursor stage inevitably exists in all polymer/solvent systems, not depending on polymer chain structures. One possible explanation about the deviation of the intensity- intensity time correlation function measured in dynamic LLS from a single-exponential decay is that, in the experimental concentration range, the correlation of segments at different “blob” is not sufficiently “screened” by chain overlapping. 21 In other words, the slow mode originates from the concentration fluctuation of segments belonging to different “blobs” in the solution which fluctuations take place dependently. The extent of correlation is directly related to the incorporation of various interactions, such as polymer-polymer, polymer-solvent, and solvent-solvent interaction, which can be attractive or repulsive. In the present study, the system of PNIPAM/H 2 O was selected because of its thermal-responsive properties which exhibits a lower critical solution temperature (LCST) around 32 °C, 32 and the system of PEO-PPO-PEO/H 2 O was selected because of the well-known fact that it can exhibit temperature-induced micellization phenomenon at a certain concentration range. 33 Although the homopolymer and the triblock copolymer have * To whom correspondence should be addressed: Ph +86 010 82618089; Fax +86 010 62521519; e-mail c.c.han@iccas.ac.cn; e-mail chiwu@ cuhk.edu.hk. † The Institute of Chemistry, CAS. ‡ The Chinese University of Hong Kong. 3642 Macromolecules 2006, 39, 3642-3647 10.1021/ma060060a CCC: $33.50 © 2006 American Chemical Society Published on Web 04/15/2006