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.

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.

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%.

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.