Confined Glassy Dynamics in a Star-Shaped Polymer …
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Additionally, several chains can become connected if parts of them are joined in the
same crystallite. This connection acts as crosslink which further inhibits mechanical
stress relaxation.
A more subtle effect is the impact on the polymer segments of the chains protruding
from the crystallites. Due to the immobilization of chain segments in the crystallite,
the adjacent segments which are not part of the crystalline order are still limited
in their mobility. While structural aspects of crystallization are typically addressed
with scattering techniques [6–8] and thermodynamic methods [9, 10], such mobility
implications require dynamical methods that directly probe molecular quantities
[11, 12]. The development of the so-called rigid amorphous fraction (RAF), i.e. a
transition zone from the normal amorphous domain to the crystallite, with a reduced
mobility has been demonstrated by dielectric spectroscopy investigations in several
polymers [2, 11]. To some extent, the RAF corresponds to the interfacial layer in
polymer-nanocomposites where attractive interactions between nanoparticles and
polymer segments generate a layer of segments with reduced mobility at the interface
of the particles [13, 14]. This effect is fundamental to the property enhancement of
nanocomposites.
Here we present a case where the dynamics in the amorphous fraction of the
polymer becomes faster after partial crystallization of a star-shaped polymer. The
reason for this unexpected and counterintuitive response is the complex architecture of the chains and its interplay with constraints arising from the crystallization
process. Further we will demonstrate how dielectric spectroscopy can be used to
resolve regions of different dynamics; in fact, the signature of a RAF is found as well
as normal amorphous behavior and a considerable fraction which exhibits characteristics of dynamics in confinement [15, 16]. Finally, complimentary measurements
by means of infrared spectroscopy reveal a lag time of about 30 min in-between
the temperature jump initializing the crystallization process and first impacts on
structure-specific molecular vibrations become detectable. After this lag time, a fast
mechanism with a time constant around 2–3 × 10
3 s primes the crystallization and
a second process follows which is about 10 times slower and matches to the BDS
results. This suggests a multi-step process which is probably a result of the interplay
of constraints imposed by the crystallization and the complex architecture of the
molecules.
2 Sample Preparation and Measurements
2.1 Synthesis
Vinyl-terminated isotactic polystyrene (iPS) was synthesized by polymerization
of styrene on post-metallocene titanium catalysts using 1,9-decadiene as chain
transfer agent in toluene under dry argon atmosphere. Octakis (dimethylsilyloxy)
silesquioxane was obtained from Hybrid Plastics and used as received. It was
269
Additionally, several chains can become connected if parts of them are joined in the
same crystallite. This connection acts as crosslink which further inhibits mechanical
stress relaxation.
A more subtle effect is the impact on the polymer segments of the chains protruding
from the crystallites. Due to the immobilization of chain segments in the crystallite,
the adjacent segments which are not part of the crystalline order are still limited
in their mobility. While structural aspects of crystallization are typically addressed
with scattering techniques [6–8] and thermodynamic methods [9, 10], such mobility
implications require dynamical methods that directly probe molecular quantities
[11, 12]. The development of the so-called rigid amorphous fraction (RAF), i.e. a
transition zone from the normal amorphous domain to the crystallite, with a reduced
mobility has been demonstrated by dielectric spectroscopy investigations in several
polymers [2, 11]. To some extent, the RAF corresponds to the interfacial layer in
polymer-nanocomposites where attractive interactions between nanoparticles and
polymer segments generate a layer of segments with reduced mobility at the interface
of the particles [13, 14]. This effect is fundamental to the property enhancement of
nanocomposites.
Here we present a case where the dynamics in the amorphous fraction of the
polymer becomes faster after partial crystallization of a star-shaped polymer. The
reason for this unexpected and counterintuitive response is the complex architecture of the chains and its interplay with constraints arising from the crystallization
process. Further we will demonstrate how dielectric spectroscopy can be used to
resolve regions of different dynamics; in fact, the signature of a RAF is found as well
as normal amorphous behavior and a considerable fraction which exhibits characteristics of dynamics in confinement [15, 16]. Finally, complimentary measurements
by means of infrared spectroscopy reveal a lag time of about 30 min in-between
the temperature jump initializing the crystallization process and first impacts on
structure-specific molecular vibrations become detectable. After this lag time, a fast
mechanism with a time constant around 2–3 × 10
3 s primes the crystallization and
a second process follows which is about 10 times slower and matches to the BDS
results. This suggests a multi-step process which is probably a result of the interplay
of constraints imposed by the crystallization and the complex architecture of the
molecules.
2 Sample Preparation and Measurements
2.1 Synthesis
Vinyl-terminated isotactic polystyrene (iPS) was synthesized by polymerization
of styrene on post-metallocene titanium catalysts using 1,9-decadiene as chain
transfer agent in toluene under dry argon atmosphere. Octakis (dimethylsilyloxy)
silesquioxane was obtained from Hybrid Plastics and used as received. It was