Influence of Crystal Polymorphism on the Three-Phase Structure and on the Thermal Properties of Isotactic Poly(1-butene)

The influence of crystal polymorphism on the three-phase structure of isotactic poly(1-butene) (PB-1) is discussed. Crystallization of PB-1 from the melt yields the tetragonal phase, known as form II. Upon storage, it spontaneously transforms to the trigonal form I. The chain motion within the crystalline parts varies strongly with the crystal structure and affects the coupled amorphous parts. The form II−form I solid−solid transformation is accompanied by a change in the thermal properties that involves all parts of the three-phase structure. The increased density and thermal stability of the crystals cause a higher rigidity of the coupled amorphous phases. In form I PB-1, the mobile amorphous phase (MAF) has a reduced heat capacity step at the glass-transition temperature that occurs at a slightly broadened temperature. The RAF is coupled more strongly to the rigid form I crystals and relaxes at higher temperatures compared with the polymer containing modification II crystals, which are conformationally disordered. Both polymorphs display only little reversibility of the melting process. It is almost negligible in form I PB-1 and more conspicuous in the form II crystals. The different behavior arises from the varied recrystallization kinetics, which is limited in extent for the trigonal polymorph.