(180j) Controlling Crystal Phase Transition From Form II to I in Isotactic Poly-1-Butene Using CO2

Controlling Crystal Phase Transition
from Form II to I in Isotactic Poly-1-butene Using CO₂

Yang
Xu, Tao Liu^*, Lei Li and Ling Zhao^*

State Key Laboratory of Chemical Engineering, East China
University of Science and Technology, Shanghai 200237, People's Republic of
China

*To
whom correspondence should be addressed. E-mail: liutao@ecust.edu.cn, zhaoling@ecust.edu.cn.

  Isotactic
poly-1-butene (iPB-1) is a polymorphous semicrystalline
polyolefin.
Crystallized from
melt under atmospheric pressure, form II can be obtained
and gradually transform into form I in more than 3 months. The phase transition
is beneficial to the mechanical, thermal and physical properties of iPB-1
products. Li et al. [1] discovered that pressurized CO₂ can
effectively promote the phase transition of form II to I, during which CO₂
diffusion and CO₂-induced the phase transition take place
simultaneously. In this work, a model combining CO₂ diffusion in and
CO₂-induced phase transition was proposed to calculate the CO₂
concentration as well as the phase transition in iPB-1 sheets.

The
intrinsic kinetics of CO₂-induced phase transition from form II to I
at 40 ^oC was investigated using in-situ
high-pressure Fourier transform infrared spectroscopy and correlated
by Avrami equation. Thin iPB-1 films (20-30 µm) were used to eliminate the effect of CO₂
diffusion. As shown in Fig. 1, the CO₂-induced phase transition can
be divided into three stages. The duration of the first stage decreased with
increasing CO₂ pressure and disappeared at pressures upper than 3 MPa. The rate constant and apparent Avrami
exponent of the second stage both increased with CO₂ pressure.

Fig. 1. Avrami
curves of the phase transition from II to I in thin iPB-1 films at 40 ^oC and different CO₂ pressures.

Fig. 2. Flow chart of the coupling model.

Fig. 3.
Calculation from the proposed model (solid lines) and FTIR results (points) in
iPB-1 sheets with thicknesses of 0.2 mm (blue) and 0.4 mm (red) at 40 ^oC and CO₂ pressures of 6 MPa (upper) and 4 MPa (lower).

Since
the phase transformation only occurs in the crystal regions, the diffusion of
CO₂ in iPB-1 is considered to be independent from the phase transition.
The flow chart of the coupling model is depicted in Fig. 2. The iPB-1 sheet was
divided into 2n+1 slices in the thickness direction. The solubility (S_CO2) and diffusivity (D) of CO₂ at 40 ^oC and a desired pressure were experimentally
measured using magnetic suspension balance, and the distribution of CO₂
concentration (C_t,y)
was calculated from the Fick's second law. The intrinsic
kinetic data were used to determine the relative content of form I in each
slice at the next time point. In order to verify the proposed model, 0.2 and
0.4 mm thick iPB-1 sheets were treated with 4 and 6 MPa
CO₂. The model values (lines) agreed well with the experiment
results (points), as illustrated in Fig. 3.

[1]
Li L, Liu T, Zhao L,
Yuan W-k. Macromolecules 2009;42:2286-90.