聚合物在超临界二氧化碳中的溶胀行为 叶树集 陈鸣才** 黄玉惠 丛广民 (中国科学院广州化学研究所 广州 510650) Abstract A systematic study was undertaken in supercritical carbon dioxide to investigate the swelling behavior of six polymers, namely, LDPE, PP, PA6, EVA, PS, and PU. Changes in appearance and weight had been observed with the changes of the critical factors such as swelling temperature, swelling pressure, swelling time, decompression time, and dimension of samples. The weight change of polymers after treatment in mass with time showed a dependence on the logarithm of time. Key words supercritical carbon dioxide；swelling temperature；swelling pressure；swelling time；decompression time；desorption curve 摘要 研究了LDPE、PP、PA6、EVA、PS、PU等六种聚合物在超临界CO2中的溶胀情况，观测了溶胀温度、溶胀压力、溶胀时间和降压时间对聚合物溶胀前后重量变化和聚合物形态的影响以及溶胀后聚合物中的CO2的解吸曲线。 关键词 超临界CO2 溶胀温度 溶胀压力 溶胀时间 降压时间 解吸曲线 Swelling Behavior of Polymers in Supercritical Carbon Dioxide Ye Shuji,Chen Mingcai**, Huang Yuhui, Cong Guangmin (Guangzhou Institute of Chemistry, CAS, Guangzhou 510650) INTRODUCTION Supercritical carbon dioxide (SC-CO2) represents an environmental benign alternative to organic solvents for it offers a number of advantages including low human toxicity, non-combustibility, chemical inertness, leaving no residues, natural occurrence, low cost, environmental acceptability and so on. SC-CO2 technology has been widely used in supercritical fluid extraction, recrystallization, chromatography, chemical reaction, dyeing and drying. In the field of polymer science, SC-CO2 can swell many polymers, it plays a useful role as a solvent or medium in a wide range of polymerization processes and modification of polymers. The study of SC-CO2 swelling polymers may offer some basic data for polymerization and modification of polymers in SC-CO2. The interaction of SC-CO2 with polymers is complex, so different polymers will be expected to show a wide range of interactions with SC-CO2[1]. The interaction of SC-CO2 with polymers has been widely studied[1-3], however systematic study about the effect on the swelling behavior of polymers in SC-CO2 by critical factors, such as swelling temperature, swelling pressure, swelling time and decompression time, has been seldom addressed thoroughly. In this article we will address the effect on polymers due to these factors. EXPERIMENTAL Materials Six different polymers are commercially available: Low-density polyethylene(LDPE 1F7B) is the product of Yanshan Petrochemical Corp. Ltd.; Polypropylene(PP) is the product of Formosa Plastics Corp. of USA; Polyamide-6(PA6) is the product of Guangzhou Tianyu Petrochemical Corp. Ltd.; Ethylene-vinyl acetate copolymer(EV101) is the product of Taiwan; Polystyrene (PS666D) is the product of Yanshan Petrochemical Corp. Ltd.; Polyurethane (Bagflex 50T) is the product of Bayer Chemical Corp. of German. The carbon dioxide gas(99.9% purity) used in all experiment was purified by molecular sieve. Preparation of Polymer Samples Polymer samples were prepared into sheets by compression molding with a planet vulcanizing machine at different temperature (listed in table1).The sheets were cut out into 1×4cm sizes to have uniform surface area for all the samples.
Table1 the Temperature of Compression Molding
polymer | LDPE | PP | PA6 | EVA | PS | PU
Temperature(℃) | 125 | 170 | 220 | 80 | 100 | 170
Swelling Experiments Samples were purified with ethyl alcohol (chemically pure) before the swelling experiment. The swelling experiments were carried out in a system comprised of a 0.1 L, high-pressure equipment which has been described previously[4], and were performed by the following steps: (a) inject carbon dioxide gas to the autoclave through a high-pressure pump until the pressure reaches 6.0Mpa; (b) increase temperature to the swelling temperature for 30 minutes; (c) inject carbon dioxide gas to the autoclave again until the pressure reaches swelling pressure; (d) swell the samples; (e) depressurize; then (f) remove the samples from the autoclave. After the swelling experiment, the weight of polymers was monitored gravimetrically(by taking the weights up to 0.1milligram) at once, and then did it again at different time intervals. The experiments were repeated twice at the same condition, and the reported data was an average of two values, with a standard deviation of 0.5%. The polymers after treatment were stored in the room whose temperature varies from 23 to 26℃ and relative humidity varies from 65% to 70%. RESULTS AND DISCUSSION Changes in Appearance Among the investigated polymers, the transmission of 0.3mm-thick polyurethane decreased, and some bubbles appeared in the 1.0mm-thick polyurethane under the treatment conditions that temperature is 40℃, pressure is 10.0MPa or 14.0MPa, swelling time is 2h and decompression time is 1h; Heavy deformation was observed in polystyrene, and microcell was noticed under the treatment conditions that temperature is 60℃. The deformation of PS was increasing with the treatment temperature, pressure and swelling time, which was interpreted that SC-CO2 has a strong swelling effect on PS and can reduce its grass transition temperature from 100℃ to as low as the experimental temperature by increasing its free volume[5-6]. No significant change in appearance and color was observed in other polymers. Weight Changes of Polymers The effect of treatment condition on the weight change of polymers can be ascertained from the data noted in Table2～4. The fundamental mechanisms that contribute to the weight changes are: (a) CO2 and other small molecules are absorbed by or dissolved in the polymers, which leads to an increase in the weights of the samples; and (b) either the polymers or some agents, such as monomers, oligomers, additive, or placticizers in the polymers are dissolved or extracted from polymer material, which leads to a decrease in the weight of the samples[7]. In general, the weight change shows a similar trend for all the different treated polymers. The amount of polymers or some agents such as monomers, oligomers, additive, or placticizers in the polymers dissolved or extracted from polymers are increasing with temperature while that of CO2 and other small molecules absorbed by or dissolved in the polymers are decreasing. So the weight change of polymers is decreasing with temperature. Moreover, the amount of polymers or some agents such as monomers, oligomers, additive, or placticizers in the polymers dissolved or extracted from polymers are increasing with pressure while that of CO2 and other small molecules absorbed by or dissolved in the polymers are also increasing. So the weight change of polymers will have maximum or minimum value with pressure. On the same reason, the weight change of polymers will have maximum or minimum value with the swelling time. The process of decompression is actually the swelling process in which swelling temperature and pressure are changed, so the effect of decompression time is similar as the one of swelling time. The weight change of PS is increasing with the swelling pressure or swelling time because its volume changes in the swelling process. In addition, we found the weight change is increasing with the thickness of sample, with the exception of PA6. These effects often occur simultaneously, but there are many other effects, such as crystallization of polymers, which will be discussed in our other articles.
Table2 Observed Weight Changes of Polymers with Temperatures and Pressures*.
polymer | thickness (mm) | Weight Change(%)
40℃ | 60℃
8.0 MPa | 10.0 MPa | 12.0 MPa | 14.0 MPa |  | 8.0 MPa | 10.0 MPa | 12.0 MPa | 14.0 MPa
LDPE | 0.4 | 0.2 | 0 | -0.2 | -0.3 | 0.3 | 0 | 0.3 | 0.3
| 1.1 | 1.1 | 2.0 | 0.9 | 1.6 | 0.9 | 0.7 | 1.2 | 1.0
PP | 0.3 | 0.2 | 0.5 | 0.1 | 0.4 | 0.6 | 0.1 | 0.2 | 0.1
| 1.1 | 1.0 | 1.5 | 0.7 | 1.4 | 0.7 | 0.5 | 0.9 | 0.9
PA6 | 0.25 | 2.3 | 6.0 | 2.1 | 2.9 | 0.1 | 1.2 | 0 | 0.6
| 1.0 | 0.9 | 1.9 | 1.0 | 0.7 | 2.7 | 1.0 | 1.0 | 0.9
EVA | 0.6 | 0.7 | 1.2 | 0.3 | 1.8 | 0.1 | 0.3 | 0.2 | 0.4
| 1.15 | 1.5 | 2.1 | 2.8 | 4.8 | 0.4 | 1.1 | 1.2 | 2.2
PS | 0.5 | 4.1 | 4.8 | 5.5 | 6.2 | 2.0 | 3.5 | 6.1 | 5.2
PU | 0.3 | 0.6 | 1.7 | 1.6 | 1.4 | 0.4 | 0.1 | -0.1 | 0.2
| 1.0 | 3.4 | 7.6 | 4.4 | 5.6 | 1.4 | 3.0 | 2.7 | 3.4
*Treated with CO2 for 2h and decompression time was 1h.
Table3. Observed Weight Changes for Polymers with Treatment Time* |  | Table4. Observed Weight Changes for Polymers with Decompression Time*
polymer | thickness (mm) | Weight Change(%)
1h | 2h | 4h | 6h
LDPE | 0.4 | 0.7 | 0 | 0.1 | 0
| 1.1 | 2.1 | 2.0 | 1.8 | 1.4
PP | 0.3 | 0.2 | 0.5 | 0.2 | 0.1
| 1.1 | 1.2 | 1.5 | 1.2 | 1.1
PA6 | 0.25 | 3.4 | 6.0 | 4.3 | 3.2
| 1.0 | 1.3 | 1.9 | 1.4 | 1.2
EVA | 0.6 | 2.6 | 1.2 | 1.8 | 1.9
| 1.15 | 5.2 | 2.1 | 4.5 | 3.5
PS | 0.5 | 4.4 | 4.8 | 5.5 | 6.1
PU | 0.3 | 0.5 | 1.7 | 1.6 | 1.0
| 1.0 | 4.4 | 7.6 | 6.2 | 5.1
polymer | thickness (mm) | Weight Change(%)
0.5h | 1h | 2h | 3h
LDPE | 0.4 | -0.1 | 0 | 0.1 | 0.2
| 1.1 | 1.6 | 2.0 | 2.0 | 1.1
PP | 0.3 | -0.2 | 0.5 | 0.2 | 0
| 1.1 | 1.0 | 1.5 | 1.2 | 0.9
PA6 | 0.25 | 3.1 | 6.0 | 4.6 | 4.5
| 1.0 | 0.9 | 1.9 | 1.7 | 1.4
EVA | 0.6 | 1.6 | 1.2 | 3.0 | 2.1
| 1.15 | 4.0 | 2.1 | 5.4 | 3.5
PS | 0.5 | 6.5 | 4.8 | 5.7 | 5.7
PU | 0.3 | 1.4 | 1.7 | 1.6 | 2.3
| 1.0 | 5.6 | 7.6 | 6.8 | 5.2
*Treated with CO2 at 40℃, 10.0MPa and decompression time was 1h. |  | *Treated with CO2 at 40℃, 10.0MPa and treatment time was 2h.

Desorption of Carbon Dioxide

Figure1 Weight change of LDPE after being treated for 2h at 40℃ vs. time (dots are observed data and curve is fitting curve).

The absorption/dissolution of carbon dioxide in polymers was indirectly indicated by the weight change of polymers after treatment. In accordance with Fickian diffusion kinetics, this changes vs. the square root of desorption time(t1/2) was initially linear[8-9]. But in our experiments, this change in mass with time showed a dependence on the logarithm of time, which is consistent with the result of Yeong-Tarng Shieh[3]. It is worth mentioning that the logarithm rule is often thought as non-Fickian behavior, in our option, it is a special Fickian behavior and the reason will be given in our other paper.Weight change of PS after treatment was showed in Fig1 and other polymers had the similar changes.we have plotted the weight changes of these six polymers and fitted the changes with the function: W=W0+ΣAiEXP{-(t-t0)/τi}(i=1,2). As a result, the fitting curve showed satisfactory agreement with the observed data. A conclusion was given that the crystalline polymers, such as LDPE, PP and PA6, showed a less amount of CO2 absorption, while the amorphous polymers, such as EVA, PS and PU, showed a greater amount of CO2 absorption.

Solubility of Carbon Dioxide in Polymers

In our experiments, the weight change of polymers after treatment in mass with time showed a dependence on the logarithm of time. We fitted the changes with the function: W=W0+ΣAiEXP{-(t-t0)/τi}(i=1,2). Theoretically, W0 equals the weight of polymers which CO2 was desorbed completely. The solubility of CO2 in polymers can be calculated by the function: solubility of CO2 (wt%)=(Wa-W0)/Wb×100, here Wa is the weight of polymer after removal from the treatment autoclave, Wb is the weight of polymer before experiment. The solubility of CO2 was listed in table5. The results of table5, except PS, showed that there was not the trend that solubility of CO2 in polymers was increasing with the swelling pressure, which was interpreted that the swelling behavior of polymers in SC-CO2 was affected by many factors. The contrast between table5 and table2 indicated that the second mechanism that contributes to the weight changes can be obviously observed in these polymers, namely, LDPE, PP, EVA, PS and PU, under some experimental conditions. After 5 months, the weight of polymers was monitored again and the results showed that the weight of polymers after 5 months deviated the weight of W0 less than 0.2%, with the exception of PA6.

Table5 Solubility of Carbon Dioxide in Polymers*
polymer | thickness(mm) | Solubility of CO2 (wt%)
40℃ | 60℃
8.0 MPa | 10.0 MPa | 12.0 MPa | 14.0 MPa |  | 8.0 MPa | 10.0 MPa | 12.0 MPa | 14.0 MPa
LDPE | 1.1 | 1.6 | 3.5 | 1.8 | 1.8 |  | 0.8 | — | 1.4 | 1.2
PP | 1.1 | 1.1 | 1.4 | 0.7 | 1.3 |  | 0.8 | — | 0.9 | 1.0
PA6 | 1.0 | 0.3 | 0.8 | 0.8 | 0.5 |  | — | — | — | —
EVA | 1.15 | 1.6 | 3.4 | 2.9 | 5.3 |  | — | 1.0 | 1.3 | 2.3
PS | 0.5 | 3.5 | 4.2 | 4.6 | 5.9 |  | — | 3.4 | 4.8 | 7.0
PU | 1.0 | 3.3 | 8.0 | 5.1 | 6.2 |  | — | 3.0 | 2.9 | 6.8
*Treatment time was 2h and decompression time was 1h.

CONCLUSIONS

While SC-CO2 is a relatively poor solvent for most polymers, it can swell most polymers including those that are generally considered solvent-resistant. A conclusion was given that crystalline polymers such as LDPE, PP and PA6, have a weak interaction with SC-CO2, while amorphous polymers, such as EVA, PS and PU, have a strong interaction with SC-CO2. These data may offer a guide to supercritical application, such as supercritical precision cleaning, supercritical foaming of polymers, fractionation of polymers, modification of polymers and synthesis of polymers in SC-CO2.

The project was supported by the National Natural Science Foundation of China and Guangdong province Natural Science Foundation.

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