Melting Behavior of Bound Solvent in Isotactic Polypropylene/o-Dichlorobenzene Gel

Abstract

The melting behavior of o-dichlorobenzene in isotactic polypropylene (iPP)/o-dichlorobenzene gel was investigated by differential scanning calorimetry (DSC) and polarizing microscopy. A DSC thermogram provided two endothermic peaks for the melting of o-dichlorobenzene in the gel. The high melting temperature was almost the same as for pure o-dichlorobenzene. Therefore the solvent can be regarded as being like free solvent. However it shifted slightly to higher temperature depending on polymer concentration, indicating that the solvent was weakly bound in the gel. The low melting peak was observed at a few tens degree lower than the high melting peak. The low melting peak was very broad and decreased depending on concentration from −17.7 °C for the 5 wt % gel to −34.1 °C for the 30 wt % gel. The solvent in the gel concentrated from 10 to 60 wt % through solvent evaporation provided only one broad peak corresponding to the low melting peak around −39 °C. That no high melting peak was observed shows that the solvent with high melting temperature was weakly bound in the gel and was excluded with ease from the gel. Therefore the low melting peak corresponds to the melting of more strongly bound solvent. This indicates two bound solvent molecules in the gel, one loosely bound in the gel and the other bound more firmly by the polymer. In the 2 wt % gel, the polarinzing microscopy showed that the spherulites were isolated from others. This suggests that the solvent with high melting peak locates in the space between spherulites. The solvent in this region would be close to free solvent. The spherulites for the concentrated gel contacted adjacent ones. In this case, the molecular chains are so distorted between spherulites that the solvent molecule is weakly bound in this boundary region. This provides the high melting peak slightly shifted to higher temperature. The low melting temperature was significantly lowered depending on polymer concentration. The solvent would be strongly bound in noncrystalline phase or interphase between lamellar crystals in the spherulite.