5.54.1 Morphology of the Dispersed Filler
(NR)-TEOS shows well-assembled aggregates of spherical particles and a homogeneous filler network; (NR)-TESPT and (NR)-TESPD have inhomogeneous shape
with both spherical and anisotropic particles, but the aggregates are less compact
than in (NR)-TEOS, probably due to a stronger filler–rubber interaction. As for
(NR)-TMSPM, large anisotropic particles are visible, while responsible of strong
aggregation, this being due to interaction between thiol groups (Fig. 5.45). On the
basis of morphological properties, a different rigidity of the rubber composite, due
to a different cross-linking extent, is expected. The investigation was performed by
the spin probe method.
5.54.2 Spin Probe Procedure
ESR measurements were performed on composites (NR)-TEOS, (NR)-TESPT,
(NR)-TESPD, and (NR)-TMSPM and on the reference sample (NR) SiO 2 after
introduction of the TEMPOL spin probe in the rubber matrix. TEMPOL (in
Fig. 5.46) is a nitroxide radical largely used in the procedures assisted by the ESR
spectroscopy. The radical embedded into the matrix allows to detect the motion of
the polymer molecules. In fact, the room-temperature ESR spectrum of TEMPOL is
sensitive to the dynamics of the nitroxide molecules, which is determined by the
rotational correlation time s R (time required for a complete radical rotation around
its axis). Fast motion occurs when nitroxide has no or very few mobility restrictions
by the matrix. In this case, the resulting isotropic ESR spectrum shows three sharp
well-resolved resonances with small hyperfine coupling due to interaction with the
nitrogen nuclear spin (I = 1) (Fig. 5.47).
If the nitroxide mobility is limited by physical or chemical interaction with the
surrounding, the line width and the hyperfine coupling increase. If the motion is
fully hindered, the ESR anisotropic lines resemble those of a powder.
The spectra of NR composites here discussed show the ESR spectrum reported
in Fig. 5.48.
A very strong broad anisotropic signal (g 1 = 2.023, g 2 = 2.005, g 3 = 1.981)
associable to spin probe slow motion in a high-density matrix is observed. The
resonances overlap to the much less intense narrow isotropic lines due to fast
motion spin probe in the free volumes of the matrix. Figure 5.48 reports the
hyperfine coupling values of the outer resonances for the two species (iso or fast
and aniso or slow motion). From the highness of the iso- and aniso-resonances, it
has calculated the ratio I ANISO /I ISO to which the probe motion in the rubber matrix
can be associated. Higher value of this ratio means prevalence of slow motion. All
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5 The Symmetry Properties Describe the Electronic Structure …
(NR)-TEOS shows well-assembled aggregates of spherical particles and a homogeneous filler network; (NR)-TESPT and (NR)-TESPD have inhomogeneous shape
with both spherical and anisotropic particles, but the aggregates are less compact
than in (NR)-TEOS, probably due to a stronger filler–rubber interaction. As for
(NR)-TMSPM, large anisotropic particles are visible, while responsible of strong
aggregation, this being due to interaction between thiol groups (Fig. 5.45). On the
basis of morphological properties, a different rigidity of the rubber composite, due
to a different cross-linking extent, is expected. The investigation was performed by
the spin probe method.
5.54.2 Spin Probe Procedure
ESR measurements were performed on composites (NR)-TEOS, (NR)-TESPT,
(NR)-TESPD, and (NR)-TMSPM and on the reference sample (NR) SiO 2 after
introduction of the TEMPOL spin probe in the rubber matrix. TEMPOL (in
Fig. 5.46) is a nitroxide radical largely used in the procedures assisted by the ESR
spectroscopy. The radical embedded into the matrix allows to detect the motion of
the polymer molecules. In fact, the room-temperature ESR spectrum of TEMPOL is
sensitive to the dynamics of the nitroxide molecules, which is determined by the
rotational correlation time s R (time required for a complete radical rotation around
its axis). Fast motion occurs when nitroxide has no or very few mobility restrictions
by the matrix. In this case, the resulting isotropic ESR spectrum shows three sharp
well-resolved resonances with small hyperfine coupling due to interaction with the
nitrogen nuclear spin (I = 1) (Fig. 5.47).
If the nitroxide mobility is limited by physical or chemical interaction with the
surrounding, the line width and the hyperfine coupling increase. If the motion is
fully hindered, the ESR anisotropic lines resemble those of a powder.
The spectra of NR composites here discussed show the ESR spectrum reported
in Fig. 5.48.
A very strong broad anisotropic signal (g 1 = 2.023, g 2 = 2.005, g 3 = 1.981)
associable to spin probe slow motion in a high-density matrix is observed. The
resonances overlap to the much less intense narrow isotropic lines due to fast
motion spin probe in the free volumes of the matrix. Figure 5.48 reports the
hyperfine coupling values of the outer resonances for the two species (iso or fast
and aniso or slow motion). From the highness of the iso- and aniso-resonances, it
has calculated the ratio I ANISO /I ISO to which the probe motion in the rubber matrix
can be associated. Higher value of this ratio means prevalence of slow motion. All
140
5 The Symmetry Properties Describe the Electronic Structure …