265029 Controlling Crystal Phase Transition From Form II to I in Isotactic Poly-1-Butene Using CO2

Monday, October 29, 2012
Hall B (Convention Center )
Yang Xu1, Tao Liu1, Lei Li1 and Ling Zhao2, (1)East China University of Science and Technology, Shanghai, China, (2)East China University of Science and Technology, Shanghai, China

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 (SCO2) 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 (Ct,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.

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