Confined Glassy Dynamics in a Star-Shaped Polymer …
281
the IR absorption bands at ν = 840, 906, 1155, and 980 cm
−1 (Fig. 7) whereby the
former three are assigned to the amorphous state while the latter is present in the
crystalline state [32, 33]. To quantitatively examine the oscillator strength of these
bands Voigt functions are fitted to the spectra and the integrated absorbance (area
under the curve) as a measure of the oscillator strength is extracted (Fig. 7).
The oscillator strengths of all analyzed bands exhibit a lag time t 0 of about 2
× 10
3 s (approx. 30 min) between the temperature decrease from 210 °C to T c
initializing the crystallization and the appearance of first effects of crystallization on
molecular vibrations. This delayed crystallization is known for iPS from previous
studies [32]. Its duration depends on the crystallization temperature; for T c = 190 °C
a value of 12 min was reported which is in the range of the ~30 min determined
here. This lag time was not observed in the BDS measurements reported here since
such short crystallization times were not investigated. As expected, the oscillator
strength of the three amorphous bands (at 840, 906, and 1155 cm
−1 ) drops upon
crystallization after the lag time indicating a reduction of the number density of
amorphous segments. For the two bands at 840 and 1155 cm
−1 a reduction by about
10% is observed which is in accord with the degree of crystallinity deduced from the
DSC measurements. However, while the reduction of these amorphous IR absorption
bands has a characteristic time of 2–3 × 10
3 s, the crystallization traced via DSC
happens much slower (Figs. 1 and 8).
The absorption band at 906 cm
−1 exhibits a much larger drop in oscillator strength
of about 37% which is comparable to the reduction of ε in the BDS measurements. When fitted to a single exponential, the extracted time constant is 3 ×
10
3 s which corresponds to those found for the other two analyzed amorphous IR
bands, but is almost 10 times shorter than the time constant extracted from the BDS
results. However, in contrast to the two bands at 840 and 1155 cm
−1 , the integrated
absorbance of the band at 906 cm
−1 can be determined with better accuracy which
reveals a mismatch of the single exponential fit with the data (Fig. 7f). Using a double
exponential, the evolution of the oscillator strength and thus reduction of amorphous
moieties due to crystallization can be described much better, the respective time
constants are 2.2 × 10
3 s and 2.6 × 10
4 s. While the former is similar to that one
from the single exponential fit (and those of the other two amorphous bands), the
latter corresponds well with the time constant of the reduction in ε. This suggests
a multi-step process in the course of crystallization whereby only the slower step is
resolved in the case of BDS measurements.
In addition, the absorption band at 980 cm
−1 is assigned to an out-of-plane ring
deformation sensitive to the crystalline moiety [32, 34]. Thus, this band grows significantly upon crystallization (at times longer than the lag time t 0 ). The single exponential fit yields a characteristic time of 2.8 × 10
3 s which is very similar to those
of the amorphous bands. Like in the case of the band at 906 cm
−1 a deviation from
the single exponential fit is evident, consequently, a double exponential is fitted to
the spectra, as well. The resulting time constants are 2.3 × 10
3 and 2.3 × 10
4 s
which agrees well with the values obtained from the band at 906 cm
−1 . Thus, on the
basis of a second band distinct from that at 906 cm
−1 , another strong indication for
a multi-step process in the course of crystallization has been derived independently.
281
the IR absorption bands at ν = 840, 906, 1155, and 980 cm
−1 (Fig. 7) whereby the
former three are assigned to the amorphous state while the latter is present in the
crystalline state [32, 33]. To quantitatively examine the oscillator strength of these
bands Voigt functions are fitted to the spectra and the integrated absorbance (area
under the curve) as a measure of the oscillator strength is extracted (Fig. 7).
The oscillator strengths of all analyzed bands exhibit a lag time t 0 of about 2
× 10
3 s (approx. 30 min) between the temperature decrease from 210 °C to T c
initializing the crystallization and the appearance of first effects of crystallization on
molecular vibrations. This delayed crystallization is known for iPS from previous
studies [32]. Its duration depends on the crystallization temperature; for T c = 190 °C
a value of 12 min was reported which is in the range of the ~30 min determined
here. This lag time was not observed in the BDS measurements reported here since
such short crystallization times were not investigated. As expected, the oscillator
strength of the three amorphous bands (at 840, 906, and 1155 cm
−1 ) drops upon
crystallization after the lag time indicating a reduction of the number density of
amorphous segments. For the two bands at 840 and 1155 cm
−1 a reduction by about
10% is observed which is in accord with the degree of crystallinity deduced from the
DSC measurements. However, while the reduction of these amorphous IR absorption
bands has a characteristic time of 2–3 × 10
3 s, the crystallization traced via DSC
happens much slower (Figs. 1 and 8).
The absorption band at 906 cm
−1 exhibits a much larger drop in oscillator strength
of about 37% which is comparable to the reduction of ε in the BDS measurements. When fitted to a single exponential, the extracted time constant is 3 ×
10
3 s which corresponds to those found for the other two analyzed amorphous IR
bands, but is almost 10 times shorter than the time constant extracted from the BDS
results. However, in contrast to the two bands at 840 and 1155 cm
−1 , the integrated
absorbance of the band at 906 cm
−1 can be determined with better accuracy which
reveals a mismatch of the single exponential fit with the data (Fig. 7f). Using a double
exponential, the evolution of the oscillator strength and thus reduction of amorphous
moieties due to crystallization can be described much better, the respective time
constants are 2.2 × 10
3 s and 2.6 × 10
4 s. While the former is similar to that one
from the single exponential fit (and those of the other two amorphous bands), the
latter corresponds well with the time constant of the reduction in ε. This suggests
a multi-step process in the course of crystallization whereby only the slower step is
resolved in the case of BDS measurements.
In addition, the absorption band at 980 cm
−1 is assigned to an out-of-plane ring
deformation sensitive to the crystalline moiety [32, 34]. Thus, this band grows significantly upon crystallization (at times longer than the lag time t 0 ). The single exponential fit yields a characteristic time of 2.8 × 10
3 s which is very similar to those
of the amorphous bands. Like in the case of the band at 906 cm
−1 a deviation from
the single exponential fit is evident, consequently, a double exponential is fitted to
the spectra, as well. The resulting time constants are 2.3 × 10
3 and 2.3 × 10
4 s
which agrees well with the values obtained from the band at 906 cm
−1 . Thus, on the
basis of a second band distinct from that at 906 cm
−1 , another strong indication for
a multi-step process in the course of crystallization has been derived independently.