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The retardation of polybutene-1 form II to form I transition in the late stage has been investigated by means of time-resolved wide-angle X-ray diffraction. Form II samples with different lamellar thickness and constituent in amorphous phase were generated via varying crystallization temperature and molecular weight and aged at room temperature for the form II to form I transition. The II to I polymorphic transition in polybutene-1 undergoes two stages, where slow nucleation and rapid growth proceed in the first stage and extremely slow secondary nucleation and growth take place in the late stage. The degree of transition reaches a plateau value in the late stage of transition, which depends highly on crystallization temperature and molecular weight that low molecular weight and high crystallization temperature always contribute to a greater degree of transition, but is less affected by the kinetics in the first stage. According to the crystal unit cell parameters of form II and form I, there should be an extension in the normal direction of lamellae and a shrinkage in the lateral direction during phase transition, which thus would induce an additional pressure in the normal direction and the tendency to expansion in the lateral direction on the residual from II crystallites. As a result, the nucleation and further growth of the II to I transition are retarded. Low molecular weight and high crystallization temperature lead to larger content of chain ends and higher chain mobility in the amorphous phase; hence, the samples are more likely to release the unfavorable factors restraining generation of form I nuclei to some extent. The transition degree in late stage is thus an intrinsic property of PB-1 samples determined by their microstructures and would not be influenced by the transition behavior in the first stage. [ABSTRACT FROM AUTHOR]

The new crystal structure models of forms I and II of isotactic polybutene-1 have been proposed by analyzing the 2-dimensional X-ray diffraction data measured for the highly oriented samples of almost pure crystal forms. The crystal form I was found to take the hexagonal packing structure of the (3/1) helices with the space group P3̅, different from the previously reported R3̅c or R3c models. The right- and left-handed chains are packed alternately with the random directionality along the chain axis. The crystal form II was concluded to take the tetragonal unit cell of the (11/3) helical chains, the space group of which is P4̅b2. The right-handed (left-handed) chains are positioned at one site with the statistical disorder of upward and downward directionality along the chain axis. The time-dependent electron diffraction measurement showed that the crystal lattices of forms I and II are related to each other with the common 110 plane boundary, as already reported by the other researchers. By referring to the crystal structures confirmed in this study, a new phase transition mechanism has been proposed for understanding this geometrical relation between these two crystal phases. The mechanism is based on a kind of soft mode concept; the mutually opposite translational movements of the right- and left-handed chains occur along the 110 plane of the tetragonal lattice of form II. The phase angle between the neighboring unit cells along the [110] direction is π. This translational lattice vibrational mode increases the amplitude and causes the softening of the original form II unit cell into a transient structure composed of the hexagonally packed pairs of right- and left-handed chains. Then, this transient structure is stabilized to the crystal form I, during which the chain conformation changes cooperatively from (11/3) to (3/1) form by a slight change in the trans and gauche torsional angles of the skeletal chains. This newly proposed transition mechanism can explain also the formation of twin structure of form I crystals. [ABSTRACT FROM AUTHOR]

The new crystal structure models of forms I and II of isotactic polybutene-1 have been proposed by analyzing the 2-dimensional X-ray diffraction data measured for the highly oriented samples of almost pure crystal forms. The crystal form I was found to take the hexagonal packing structure of the (3/1) helices with the space group P3, different from the previously reported R3c or R3c models. The right- and left-handed chains are packed alternately with the random directionality along the chain axis. The crystal form II was concluded to take the tetragonal unit cell of the (11/3) helical chains, the space group of which is P4b2. The right-handed (left-handed) chains are positioned at one site with the statistical disorder of upward and downward directionality along the chain axis. The time-dependent electron diffraction measurement showed that the crystal lattices of forms I and II are related to each other with the common 110 plane boundary, as already reported by the other researchers. By referring to the crystal structures confirmed in this study, a new phase transition mechanism has been proposed for understanding this geometrical relation between these two crystal phases. The mechanism is based on a kind of soft mode concept; the mutually opposite translational movements of the right- and left-handed chains occur along the 110 plane of the tetragonal lattice of form II. The phase angle between the neighboring unit cells along the [110] direction is p. This translational lattice vibrational mode increases the amplitude and causes the softening of the original form II unit cell into a transient structure composed of the hexagonally packed pairs of right- and left-handed chains. Then, this transient structure is stabilized to the crystal form I, during which the chain conformation changes cooperatively from (11/3) to (3/1) form by a slight change in the trans and gauche torsional angles of the skeletal chains. This newly proposed transition mechanism can explain also the formation of twin structure of form I crystals. [ABSTRACT FROM AUTHOR]