which the polar and less flexible polymethacrylate backbones form disordered
layers. This structure has been confirmed through temperature-dependent wideangle X-ray scattering (WAXS) [42]. The anisotropic chain motion occurs within
the layers; conformational randomization and rotational isotropization require
extended chain units to translate from one structured unit to another. The variation
in the molecular weight of PEMA showed that a minimum chain length of five to
ten repeat units is required for this effect to occur [43]. In the vicinity of the glass
transition temperature (T g ), the time scales of the two processes for PEMA differ by
more than an order of magnitude, where the anisotropic motions follows a simple
Arrhenius law and the isotropization process follows the Williams–Landel–Ferry
(WLF) equation [7].
Recently, such peculiar chain dynamics were studied in nanoparticles onto
which PEMA was grafted [44]. Through selective
13 C labeling, different parts of
the PEMA brush were labeled: at the particle surface (brush A), in the middle
(brush B), and at the chain end (brush C). In both brush A and brush B the
isotropization is significantly slowed down, in particular at elevated temperatures (see Fig. 2a, b). The increased curvature of the data indicates a significant
increase of T g by about 20 K as well as significant changes in WLF parameters.
Remarkably, the part of the chain directly bound to the surface, brush A,
consisting of about 40 repeat units, displays virtually identical reduction in
isotropization mobility as the part in the middle of the brush, brush B, where
the labeled part is separated from the core by about 60 repeat units. This is
remarkable because the nanostructures of PEMA mentioned above involve five
to ten repeat units only.
This suggests that these structures, which are the reason for the clear separation
of the time scales of the local chain motion and the isotropization in PEMA, are
significantly affected by the presence of the nanoparticle. One can compare this
effect with the significant reduction in the chain reptation in star polymers, where
the star point does not move and chain motion can only occur via arm-retraction
[45]. In fact, from
2 H NMR on selectively deuterated four-arm star poly(butadiene),
Brereton el al. [46] found a similar behavior, namely almost uniform dynamics for
the middle part of the arm, yet significantly shorter correlation times for the chain
ends. Our work also motivated computer simulation of chain dynamics of grafted
chains. It was found that the repeat units at the end relax faster than units further
inside along the chain, as previously observed for planar brushes but at variance
with theoretical expectations [47].
This example of studying polymer chain dynamics by advanced NMR
techniques illustrates what kind of unique information this technique can provide.
Many different types of information are accessible and its site selectivity is
unmatched by other methods. In addition to the local dynamics, chain motion on
mesoscopic length scales in polymer melts have been elucidated by various NMR
techniques including DQ NMR in high and low magnetic fields [29, 48]. Last, but
not least, the translational motion of poly(ethylene) chains from the crystalline to
the noncrystalline regions and vice versa has been quantified in samples of different
morphology, unraveling the decisive role of the interface [49].
300
H.W. Spiess
layers. This structure has been confirmed through temperature-dependent wideangle X-ray scattering (WAXS) [42]. The anisotropic chain motion occurs within
the layers; conformational randomization and rotational isotropization require
extended chain units to translate from one structured unit to another. The variation
in the molecular weight of PEMA showed that a minimum chain length of five to
ten repeat units is required for this effect to occur [43]. In the vicinity of the glass
transition temperature (T g ), the time scales of the two processes for PEMA differ by
more than an order of magnitude, where the anisotropic motions follows a simple
Arrhenius law and the isotropization process follows the Williams–Landel–Ferry
(WLF) equation [7].
Recently, such peculiar chain dynamics were studied in nanoparticles onto
which PEMA was grafted [44]. Through selective
13 C labeling, different parts of
the PEMA brush were labeled: at the particle surface (brush A), in the middle
(brush B), and at the chain end (brush C). In both brush A and brush B the
isotropization is significantly slowed down, in particular at elevated temperatures (see Fig. 2a, b). The increased curvature of the data indicates a significant
increase of T g by about 20 K as well as significant changes in WLF parameters.
Remarkably, the part of the chain directly bound to the surface, brush A,
consisting of about 40 repeat units, displays virtually identical reduction in
isotropization mobility as the part in the middle of the brush, brush B, where
the labeled part is separated from the core by about 60 repeat units. This is
remarkable because the nanostructures of PEMA mentioned above involve five
to ten repeat units only.
This suggests that these structures, which are the reason for the clear separation
of the time scales of the local chain motion and the isotropization in PEMA, are
significantly affected by the presence of the nanoparticle. One can compare this
effect with the significant reduction in the chain reptation in star polymers, where
the star point does not move and chain motion can only occur via arm-retraction
[45]. In fact, from
2 H NMR on selectively deuterated four-arm star poly(butadiene),
Brereton el al. [46] found a similar behavior, namely almost uniform dynamics for
the middle part of the arm, yet significantly shorter correlation times for the chain
ends. Our work also motivated computer simulation of chain dynamics of grafted
chains. It was found that the repeat units at the end relax faster than units further
inside along the chain, as previously observed for planar brushes but at variance
with theoretical expectations [47].
This example of studying polymer chain dynamics by advanced NMR
techniques illustrates what kind of unique information this technique can provide.
Many different types of information are accessible and its site selectivity is
unmatched by other methods. In addition to the local dynamics, chain motion on
mesoscopic length scales in polymer melts have been elucidated by various NMR
techniques including DQ NMR in high and low magnetic fields [29, 48]. Last, but
not least, the translational motion of poly(ethylene) chains from the crystalline to
the noncrystalline regions and vice versa has been quantified in samples of different
morphology, unraveling the decisive role of the interface [49].
300
H.W. Spiess