Absorption Of Gas In Semicrystalline Polymer Solubility And Modeling

In gas-phase polymerization, an accurate understanding of absorption of small molecule compound in semicrystalline polymers is of crucial importance. It can be seen that the rate of the polymerization reaction and the design of devolatilization equipment all depend directly on the solubility of gases such as olefins and alkanes in semicrystalline polymer. In this paper we focus on the absorption of three kinds of small molecule compound and their mixtures in semicrystalline polyethylene. Five sections are summarized as follows:1. Experimental apparatus and schemeSolubility measurements were performed using a pressure-decay method. The apparatus was designed and applicable to absorption of gaseous small molecule pure compound or their mixtures in solid state semicrystalline polymers. Experimental scheme was consisted of examination of accuracy, pretreatment of particle, properties of matter and dimension determination, and absorption experiment. And experimentation steps were described in detail.2. Solubility data analysis in binary systemThe solubility of ethylene, n-hexane and iso-pentane in three types of polyethylene samples were examined with the crystallinities range of 20-72%, at temperatures of 60-90℃ and pressures up to 2MPa of ethylene, 19-100kPa of n-hexane, and 80-300kPa of iso-pentane respectively. It was shown that the solubilities of gases in semicrystalline polymer are mainly related to temperature, polymer crystallinity, and property of gas. The solubilities increased with increasing pressure and gas molecular size, decreasing temperature and crystallinity. The solubility of ethylene could be described by Henry's law and that of n-hexane or iso-pentane were non-linear. The absorption occurred only within the amorphous regions of the polymer, while the solubility per unit mass of amorphous polymer decreased with increasing crystallinity. Further, the influence of the change of temperature and crystallinity differed with the species of gases. Henry's law coefficients for ethylene decreased with increasing crystallinity, but the velocity change was the same with increasing temperatureand had nothing to do with crystallinity. The effect of solubilities of n-hexane and iso-pentane on temperature decreased with increasing crystallinity.3. Solubility data analysis in ternary systemSolubility data of ethylene/iso-pentane/PE and ethylene/n-hexane/PE were presented for PE crystallinity 48.6% at temperatures of 70-90°C and total pressures up to 2MPa with the partial pressures of iso-pentane 80-190kPa and those of hexane 20-90kPa. Compared with the solubilities of the corresponding binary system, the factors in the ternary system were discussed about the partial pressure of the second component and temperature. It was found that the presence of iso-pentane or n-hexane gives rise to an increase in the solubility of ethylene whereas the presence of ethylene decreases the solubility of iso-pentane or n-hexane. Because of coexistence of two kinds of gases, there is a special phenomenon in ternary systems unlike binary systems, which is that the change of solubility of one gas is due to the existence of the other one.4. Multiplicity solubility model of gas in semicrystalline polymerA multiplicity solubility model of the binary system was presented on temperature and crystallinity. The model was based on activity coefficient model combined with rubber elasticity theory, and taken into account the relationship between crystallinity and temperature. The only adjustable parameter in this model was 5 , which meant the amount of elastically ineffective chains of the total polymer and was characteristic of the polymer. Experimental data were used for the single-parameter fitting and fitting error was within about 12%. It was found that the single-parameter is merely dependent on the properties of PE used. Contrast with Flory-Huggins model and UNIFAC-M-H method, it was shown that the correlation between the crystallinities of semicrystalline polymers and temperatures must be taken into account, in order to fit well the solubility data of alkane, olefin and aromatic hydrocarbons in polyethylene, especially at the temperature range near the melting point of polymer. The four free energy contributions to the total gas activity were experimentally determined to be the combinatorial contribution about 47-60%, the free-volume contribution about 12-25%, the elastic effect about 22-35%, but the interactional contribution zero.The contributions changed with the gas molecular size.5. Co-absorption model of gas mixture in semicrystalline polymerThe phenomenon of absorption of two gases mixtures in semicrystalline polymer was contrasted with corresponding binary systems and summarized. The results presented showed that the co-absorption phenomenon in ternary systems is due to temporary plasticization of semicrystalline polymer by gases sorbed, which leads to the change of crystallinity. Thus we modeled the solubility of ternary systems, and placed stress on the effect due to absorption of gas on crystallinity of polymer. The co-absorption experimental data could be fitted by the use of the model in two cases, and the relative root mean square errors for fitting the solubility data of n-hexane and iso-pentane were 4.64% and 5.13% respectively.