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Macromolecules 2009, 42, 1400-1403
Notes Hydration and Dynamic Behavior of a Cyclic Poly(N-isopropylacrylamide) in Aqueous Solution: Effects of the Polymer Chain Topology Yuichi Satokawa,† Toshiyuki Shikata,*,† Fumihiko Tanaka,‡ Xing-ping Qiu,§ and Franc¸oise M. Winnik§ Department of Macromolecular Science, Osaka UniVersity, Toyonaka, Osaka 560-0043, Japan; Department of Polymer Chemistry, Graduate School of Engineering, Kyoto UniVersity, Katsura, Kyoto 615-8510, Japan; and Faculty of Pharmacy and Department of Chemistry, UniVersite de Montreal, CP 6128 Succursale Centre Ville Montreal QC H3C 3J7, Canada ReceiVed October 22, 2008 ReVised Manuscript ReceiVed December 26, 2008 The coil-to-globule collapse of a macromolecule, which takes place as its solution is heated through its lower critical solution temperature (LCST), has been scrutinized by polymer scientists for many years.1-4 In the case of aqueous polymer solutions, the heat-induced phase transition is accompanied by the release into bulk of water molecules bound to the polymer chain in its extended form. Thus, thermosensitive waterborne materials tend to undergo a large change in volume and/or hydrophilicity when subjected to a small change in temperature. Thermoresponsive materials are major components of many microfluidics devices,5 sensors,6 supports for cell culture and tissue engineering,7 chromatography columns,8 and drug delivery systems.9 Poly(Nisopropylacrylamide) (PNIPAM) is the best known thermoresponsive water-soluble polymer. The LCST of aqueous PNIPAM solutions is ∼32 °C. For polymers of sufficiently high molecular weight (Mn > 15.0 × 103 g mol-1), the LCST has a very weak dependence on solution concentration (c) and on the sample molecular weight. The LCSTs of PNIPAM oligomers, however, depend significantly on the number of N-isopropylacrylamide (NIPAM) repeating units and on the chemical structure of the chain ends.10 Recent studies of aqueous solutions of cyclic PNIPAMs have revealed that the cyclic topology also significantly affects the temperature and the enthalpy of the phase transition as well as the size, stability, and density of the mesoglobules formed above the LCST.11-13 These observations suggest that the cyclic topology affects the hydration and/or dehydration characteristics of PNIPAM. However, there is no direct evidence that the hydration state of cyclic PNIPAM differs from that of its linear counterpart. Dielectric relaxation (DR) measurements, which yield with high precision the relaxation times of electric dipoles in solution, are a powerful means to probe the dynamics of any system containing molecules and/or groups bearing electric dipoles.4 * Corresponding author. E-mail:
[email protected]. † Osaka University. ‡ Kyoto University. § Universite de Montreal.
This technique was applied recently to study aqueous solutions of linear PNIPAMs of Mw ) 7.5 × 103 and 300 × 103 g mol-1. The experiments provided information on the temperaturedependent relaxation behavior of both the PNIPAM chains and the solvent, water, molecules. It was estimated that the hydration number per NIPAM unit (m) was ∼11 for PNIPAM chains in their extended conformation in cold water. It was demonstrated also that, of the 11 molecules of water, 5-6 are bound to the amide functional group of each NIPAM unit via hydrogen bonds.4 The additional 5-6 water molecules form hydrogen bonds with the water molecules closest to the amide groups. The total hydration number decreases sharply as the solution approaches the phase transition temperature. The exact hydration level of the NIPAM unit in phase-separated PNIPAM solutions cannot be determined by DR measurements due to difficulties in the analysis of data obtained from inhomogeneous solutions. We report here the results of a DR study of aqueous solutions of a cyclic PNIPAM (c-PNIPAM, Mn ) 12.8 × 103 g mol-1 and Mw/Mn ) 1.1) and a linear PNIPAM sample of comparable molecular weight (l-PNIPAM, Mn ) 10.0 × 103 g mol-1 and Mw/Mn ) 1.1) bearing an alkyne residue on one end and an azide on the other end (see Figure 1). The two functional end groups are able to couple via “click chemistry”, leading to cyclic polymers.11 This R-azido-ω-propargyl-PNIPAM was prepared by reversible addition-fragmentation transfer (RAFT) polymerization of NIPAM, following a reaction sequence similar to a procedure described previously11 (see Supporting Information). As a consequence of this reaction sequence, each PNIPAM cycle contains a triazole group. In order to minimize the effect of this moiety on the hydration and dynamics of c-PNIPAM in water, we synthesized a cyclic polymer having as high a molecular weight as possible given the experimental constraints of end-to-end polymer cyclization reactions. The changes with angular frequency (1.0 × 107 s-1 < ω < 1.3 × 1011 s-1) of the real and imaginary parts (ε′ and ε′′) of the complex permittivity were determined for aqueous solutions of c-PNIPAM and l-PNIPAM kept at various temperatures (T) between 15 °C and the solutions’ LCST. The total spectra of the solutions, ε′ and ε′′, were decomposed into a number of Debye-type relaxation modes and a constant permittivity (ε∞) in order to determine the exact dielectric contribution of PNIPAMs in water (∆ε′ and ∆ε′′) according to the following standard procedure. The real and imaginary parts, ε′ and ε′′, were decomposed into the summation of the relaxation functions of the mode i ε′ )
εi
∑1 + ω τ i)1
2 2 i
+ ε∞,
ε′′ )
εiωτi
∑1 + ω τ i)1
2 2 i
(1)
where τi and εi represent respectively the relaxation time and strength for the mode i.4 The fastest relaxation mode was assigned to bulk water molecules in the solutions (ε′w and ε′′w). It was the strongest signal in all spectra and had a relaxation time close to that of
10.1021/ma802375w CCC: $40.75 2009 American Chemical Society Published on Web 02/02/2009
Macromolecules, Vol. 42, No. 4, 2009
Notes 1401
Figure 1. Chemical structures of c-PNIPAM and l-PNIPAM used in this study.
Figure 2. Dependence of ε′ and ε′′ on ω for an aqueous solution of the c-PNIPAM at c ) 1.2 M and 24.6 °C. This figure also contains the contribution of the polymer, ∆ε′ and ∆ε′′, and that of pure water, ε′wp and ε′′wp; thin broken lines represent the calculated curves due to the literature.14 ∆ε′ and ∆ε′′ for an aqueous solution of the l-PNIPAM at the same conditions are also shown with thick solid lines.
pure water (ε′wp and ε′′wp).14 In Figure 2, we present the dielectric spectra, ε′ and ε′′, ∆ε′ ) ε′- ε′w - ε∞, ∆ε′′ ) ε′′ ε′′w, ε′wp, and ε′′wp as functions of the frequency, ω, recorded for an aqueous c-PNIPAM solution (c ) 1.2 M in monomer units or 121 g L-1) at 24.6 °C as a typical result; we also carried out dielectric relaxation experiments for solutions of c ) 0.47 and 0.84 M. The ∆ε′ and ∆ε′′ spectra involved two major relaxation processes, a sharp one and a broader one, at frequencies of 4 × 1010 and ∼108 s-1, respectively. Similar dielectric spectra were obtained for all the samples examined below their LCSTs. The data of ∆ε′ and ∆ε′′ for an aqueous solution of the l-PNIPAM in the same condition are shown in Figure 2 with thick solid lines. The ω dependences of ∆ε′ and ∆ε′′ for both the c-PNIPAM and l-PNIPAM are similar to each other within the 3 × 109-1011 s-1 ω range. However, ∆ε′ and ∆ε′′ for the l-PNIPAM solution are significantly smaller than those for the c-PNIPAM solution for ω < 109 s-1. The ratio of the relaxation strength of bulk water to that of pure water (Φ ) εw/εwp ) 0.59 for the solution shown in Figure 2) is related to the hydration value, m, by the equation Φ ) (1 j wc, where φ is the volume fraction of - φ)/(1 - φ/2) - 10-3mV j pc) and V j p and V j w are the partial molar PNIPAMs (φ ) 10-3V volumes of the NIPAM repeating unit and of water, respectively. Under these conditions, we obtain hydration numbers of ∼12 for c-PNIPAM in water below 30 °C (
