Solvent gradient fractionation of Polybutene-1 resin and its molecular weight dependency of Form II to I transformation

Abstract

The influence of the molecular weight on the crystallization and crystalline phase transformation in polymorphic polybutene-1 (PB-1) is a controversial issue in literature. We suppose the main reason comes from the used commercial samples with broad molecular weight distribution (MWD). To overcome this, one homo-polymer PB-1 resin is effectively separated by solvent gradient fractionation (SGF) according to molecular weight to obtain fractions with narrow MWD for the first time. The appropriate solvent system is selected, i.e. the good solvent and poor solvent are 1,2,4-trimethylbenzene (TMB) and n-butyl cellosolve (BCS), respectively. Most of fractions have narrow polydispersity index less than 1.4. The molecular weights of SGF fractions gradually increase from 3.19 × 104 g/mol to 1.21 × 106 g/mol when the content of good solvent increases in the mixed solvent. In addition, the influence of the molecular weight on the transformation from Form II to Form I at 25 °C is investigated by differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) methods. It is found that the higher molecular weight is beneficial to the transformation in the initial stage (0–8h) and the lower molecular weight is beneficial to the transformation in the late stage (over 72h), i.e. the molecular weight has a double influence on the transformation rate from Form II to Form I. And the reasons are also discussed. Our results clarify the influence of molecular weight on transformation from Form II to Form I for PB-1 resin at 25 °C.