Confinement effects in ionomers: a high-field pulsed electron spin resonance spectroscopy study
Introduction
The formation of nanoscopic structures by self-assembly of molecules is a topic of current interest in both biology [1] and materials science [2], [3], [4] since they may control the properties on a macroscopic scale without requiring significant changes in the overall molecular design. We have studied the self-organization of ionomers [3], [4] by continuous wave (CW) and pulsed high-field electron spin resonance (ESR) by dissolving suitably-designed ionic radicals (spin probes). These probes confine themselves at the interface between the polymer and the clusters where the ionic parts of the polymer chains segregate [5], [6]. Due to the high orientational resolution of high-field ESR we can conclude that the anisotropy of the dynamics of the spin probes is virtually uniaxial. The highly constrained dynamics of the ionic clusters in a diblock copolymer is evidenced.
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Sample preparation and characterization
End-capped poly(isoprene) homopolymer (PI-S10) and poly(styrene)–poly(isoprene) diblock copolymers (PS–PI-S24) were prepared by anionic polymerization and subsequent introduction of the ionic end groups as described elsewhere [7], [8]. Their schematic representation is shown in Fig. 1 and the molecular characteristics are listed in Table 1. Table 1 also lists the glass-transition temperatures of the homopolymer and the poly(styrene) and poly(isoprene) fractions of the diblock copolymer. Fig. 2
Results
The main broadening mechanism of the ESR lineshape is the coupling between the reorientation of the spin probe and the relaxation of the electron magnetization M via the anisotropy of the Zeeman and the hyperfine magnetic interactions. When the molecule rotates, the coupling gives rise to fluctuating magnetic fields acting on the spin system. The resulting phase shifts and transitions relax the magnetisation and broaden the resonance [14].
The first characterization of the reorientation process
Discussion
As noted above, the reorientation is much slower for K-TEMPO than for TEMPO. Due to the similar molecular shape and size of the two spin probes, the immobilization of K-TEMPO is ascribed to the effective attachment to the ionic clusters of the ionomers.This is in agreement with an earlier CW ESR study at X-band frequencies (9.4 GHz) [10].
However, CW ESR spectroscopy both at X- and W-band frequencies is unable to characterize the slow reorientation of K-TEMPO since the lineshape does not exhibit
Conclusions
Confinement effects in ionomeric materials have been studied by pulsed high-field ESR spectroscopy. Both homopolymers and diblock copolymers have been investigated. The reorientation of spin-probes which have been confined at the interface between the ionic cluster and the polymer exhibits clear uniaxial features. Due to the good orientation selectivity of high-field ESR, this conclusion can already be drawn from the raw data.
The probe dynamics, and thus the dynamics of chain segments adjacent
Acknowledgments
Financial support by the DFG Schwerpunkt `Polyelektrolyte' and the DFG Schwerpunkt `Hochfeld-EPR in Biologie, Chemie und Physik' is gratefully acknowledged.
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