Abstract
Syndiotactic polystyrene (sPS) is a crystalline polymer which has a unique property; it is able to form cocrystals with a wide range of chemical compounds, in which the guest molecules are confined in the vacancies of the host sPS crystalline region. Recently, it has been found that even polyethylene glycol oligomers with a molecular weight of more than several hundreds can be introduced into the sPS crystalline region. It is quite important to know how such a long-chain molecule is stored in the host sPS lattice. To tackle this issue, a new simultaneous measurement method combing small-angle neutron scattering and Fourier transform infrared spectroscopy (SANS/FTIR), which has been recently developed by the authors, was applied to an sPS cocrystal with polyethylene glycol dimethyl ether with a molecular weight of 500 (PEGDME500). The temperature-dependent changes of the SANS profile and FTIR spectrum were followed from room temperature up to 413 K for a one-dimensionally oriented SANS/PEGDME500 cocrystal sample. The intensity of the reflections due to the stacking of crystalline lamellae showed a significant temperature dependence. The two-dimensional pattern in the high Q region of SANS also changed depending on temperature. The combined information obtained by SANS and FTIR suggested that PEGDME500 molecules are distributed in both the crystalline and amorphous regions in the low-temperature region close to room temperature, but they are predominantly included in the amorphous region in the high-temperature region. It was also suggested by the two-dimensional SANS profile that PEGDME500 molecules in the crystalline region have an elongated structure along the thickness direction of the crystalline lamellae.
Highlights
Syndiotactic polystyrene is a crystalline polymer that possesses some intriguing properties
The combined information obtained by small-angle neutron scattering (SANS) and Fourier-transform infrared (FTIR) suggested that PEGDME500 molecules are distributed in both the crystalline and amorphous regions in the low-temperature region close to room temperature, but they are predominantly included in the amorphous region in the high-temperature region
The intensity of the lamellar reflection increases remarkably with temperature, in dramatic contrast to the sPS/triethyleneglycol dimethyl ether (TEGDME) cocrystal whose lamellar reflections wane as the temperature increases (Kaneko et al, 2015). It follows that there is an evident difference in the temperature dependence of the guest distribution between the deuterated syndiotactic polystyrene (d-sPS)/TEGDME and d-sPS/PEGDME500 cocrystals, since the distribution of protonated guest molecules is the main factor to determine the scattering length density (SLD) in the host d-sPS matrix
Summary
Syndiotactic polystyrene (sPS) is a crystalline polymer that possesses some intriguing properties. In the previous simultaneous SANS/FTIR study using a short polyethylene glycol dimethyl ether (PEGDME), triethyleneglycol dimethyl ether (TEGDME) with a molecular weight of 178, hardly any of the guest molecules remained in the amorphous region at elevated temperatures because of the high volatility (Kaneko et al, 2015). In response to this point, we employed a longer PEGDME with a molecular weight of about 500 (PEGDME500). On the basis of the SANS and FTIR results, it is demonstrated that PEG chains take a characteristic molecular shape and orientation in the crystalline region, which is very different from that of those residing in the amorphous region
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