284
M. Tress et al.
crystalline fraction and the CAF corresponds to the ratio of crystallinities extracted
from DSC and BDS (1:4). Apparently, different peaks can reflect distinct moieties
with rather subtle differences. E.g. the peak at 840 is assigned to iPS segments in an
amorphous structure, i.e. its integrated absorbance drops only, when the segments
are incorporated into the crystalline phase (the CAF still contributes to this peak).
In contrast, the peak at 906 cm
−1 , is sensitive to mobility of the segments, i.e. its
integrated absorbance drops already when the iPS segments experience the constrains
in the CAF (only the MAF contributes).
4 Conclusion
BDS measurements of a star-shaped polymer based on a polyhedral oligomeric
silesquioxane (POSS) molecule as center and isotactic polystyrene arms reveal a
faster mean relaxation time of about 1 decade in the amorphous fraction in the semicrystalline state compared to the purely amorphous state. Such a signature is typical
for spatial confinement and suggests tremendous constraints within the material. A
decomposition of the respective relaxation time distributions yields three fractions of
different dynamics: a rigid amorphous fraction (RAF) around the crystallites which
is well known from the literature, a mobile amorphous fraction (MAF) which was
considered identical to the purely amorphous state, and a confined amorphous fraction (CAF) of enhanced dynamics presumably located around the POSS centers. The
latter is assigned to chain stretching of the arms due to the volume reduction upon
crystallization. This is promoted by the POSS centers which suppress crystallization
in their surrounding as well as the star-shape which connects arms from all directions
and, thus, prevents a relaxation by contraction. Investigation of the kinetics reveals
an exponential decay of both relaxation strength and mean relaxation time with time
constants of about 1–2 × 10
4 s.
Complementary FTIR measurements of crystalline and amorphous moieties with
a better time resolution reveal a multi-step process with a lag time of about 30 min
before actual changes can be observed. Then, a fast mechanism initializes the crystallization with time constants of about 2–3 × 10
3 s before a second transition follows
with a 10 times slower rate corresponding to the BDS results. This suggests an intricate interplay between the crystallization of iPS arms giving rise to conformational
changes and structural constraints in the sample’s complex chain architecture.
Acknowledgements MT is grateful to the Alexander von Humboldt foundation for granting
his Feodor-Lynen postdoc fellowship. AMA appreciates financial support from the Deutsche
Forschungsgemeinschaft (German Research Foundation, DFG) grant number AN 1523/1-1. AMA
and FK grateful thank the DFG for granting project B08 within the Sonderforschungsbereich
(Collaborative Research Center) SFB/TRR 102.
M. Tress et al.
crystalline fraction and the CAF corresponds to the ratio of crystallinities extracted
from DSC and BDS (1:4). Apparently, different peaks can reflect distinct moieties
with rather subtle differences. E.g. the peak at 840 is assigned to iPS segments in an
amorphous structure, i.e. its integrated absorbance drops only, when the segments
are incorporated into the crystalline phase (the CAF still contributes to this peak).
In contrast, the peak at 906 cm
−1 , is sensitive to mobility of the segments, i.e. its
integrated absorbance drops already when the iPS segments experience the constrains
in the CAF (only the MAF contributes).
4 Conclusion
BDS measurements of a star-shaped polymer based on a polyhedral oligomeric
silesquioxane (POSS) molecule as center and isotactic polystyrene arms reveal a
faster mean relaxation time of about 1 decade in the amorphous fraction in the semicrystalline state compared to the purely amorphous state. Such a signature is typical
for spatial confinement and suggests tremendous constraints within the material. A
decomposition of the respective relaxation time distributions yields three fractions of
different dynamics: a rigid amorphous fraction (RAF) around the crystallites which
is well known from the literature, a mobile amorphous fraction (MAF) which was
considered identical to the purely amorphous state, and a confined amorphous fraction (CAF) of enhanced dynamics presumably located around the POSS centers. The
latter is assigned to chain stretching of the arms due to the volume reduction upon
crystallization. This is promoted by the POSS centers which suppress crystallization
in their surrounding as well as the star-shape which connects arms from all directions
and, thus, prevents a relaxation by contraction. Investigation of the kinetics reveals
an exponential decay of both relaxation strength and mean relaxation time with time
constants of about 1–2 × 10
4 s.
Complementary FTIR measurements of crystalline and amorphous moieties with
a better time resolution reveal a multi-step process with a lag time of about 30 min
before actual changes can be observed. Then, a fast mechanism initializes the crystallization with time constants of about 2–3 × 10
3 s before a second transition follows
with a 10 times slower rate corresponding to the BDS results. This suggests an intricate interplay between the crystallization of iPS arms giving rise to conformational
changes and structural constraints in the sample’s complex chain architecture.
Acknowledgements MT is grateful to the Alexander von Humboldt foundation for granting
his Feodor-Lynen postdoc fellowship. AMA appreciates financial support from the Deutsche
Forschungsgemeinschaft (German Research Foundation, DFG) grant number AN 1523/1-1. AMA
and FK grateful thank the DFG for granting project B08 within the Sonderforschungsbereich
(Collaborative Research Center) SFB/TRR 102.