Polyethylene glycol Tethering

Surface graft polymerization is one of the most commonly used methods of grafting, including radical, radiation, plasma, and ultraviolet initiated graft polymerization. In general, grafting modification changes the surface properties of a polymer membrane in a defined selective way while preserving its bulk/macroporous structure. The physicochemical properties of the membrane surfaces change to a great extent after modifications. Therefore, interactions with solutes such as proteins with a membrane can be changed accordingly, which may cause protein resistance.

Many researchers have developed various technologies to realize surface grafting. Meanwhile, a great variety of monomers were chosen to improve the protein resistance of PAN membranes. Ulbricht and Belfort (1996) studied the surface and permeation properties of PAN and polysulfone membranes after treatment with low-temperature plasma and subsequent grafting with HEMA. They indicated that PAN ultrafiltration membranes hydrophilized by simple water/He plasma treatment exhibited a strongly reduced impact of protein adsorption on membrane fouling, and hence enabled higher filtrate fluxes with the same protein retention. Based on the results of BSA ultrafiltration, they also proposed that the reduction of protein surface adsorption by increased membrane surface hydrophilicity could diminish the tendency of protein fouling, thus compensating for a loss in hydraulic permeability due to the grafting modification.

Being different from some monomers such as acrylamide and acrylic acid, poly(ethylene glycol) (PEG) is well-known for its extraordinary ability to resist protein adsorption because of its hydrophilicity, large excluded volume, and unique coordination with surrounding water molecules in an aqueous medium (Nie et al. 2003,2004b, d; Xu et al. 2005). Surface-grafted PEG has also rendered ultrafiltration membranes resistance to protein fouling. In order to improve the protein resistance and blood compatibility of the PANCMA membrane, the carboxyl groups on the membrane surfaces were converted into anhydride groups by refluxing the membranes in acetic anhydride, and then these anhydride groups are subjected to an esterifica-

Fig. 6. Illustration of the preparation of acrylonitrile/maleic acid copolymer membrane and its grafting with poly(ethylene glycol) (PEG)

tion reaction with PEG containing hydroxyl end groups, as illustrated in Fig. 6.

For surface grafting modification, the grafting degree or the amount of grafted polymer is important to the surface properties of membrane. The grafting degree can be tuned by changing the conditions of modification during radiation-, plasma-, or ultraviolet-initiated graft polymerization. In order to investigate the effect of the grafting degree of PEG on the protein resistance of the membrane, PANCMA membranes with different contents of maleic acid were fabricated and then PEG was immobilized onto these membrane surfaces according to the procedure described above. The typical characteristics of the PANCMA and PEG-grafted membranes are summarized in Table 1. It can be seen from Table 1 that the grafting degree of PEG increased with the content of maleic acid, and the hydrophilicity (depicted as water contact angle) increased in the same order.

To investigate the filtration performances of this series of membranes, dynamic protein adsorption experiments were carried out. Typical results for the permeation fluxes of water and BSA solution through all membranes are shown in Figs. 7 and 8. The symbols, Jw0 and Jp are defined as the flux

Table 1. Typical characteristics of the acrylonitrile/maleic acid copolymer (PANCMA) and poly(ethylene glycol) (PEG)-grafted membranes. CA Water contact angle, GD grafting degree

Polymer3

Contentb

CA (°Cc)

Polymerd

GDe

CA (°C)

PANCMA04

3.69

48.2

PANCMA04-;

-PEG400

7.19

31.4

PANCMA07

7.48

42.4

PANCMA07-

-PEG400

13.17

24.2

PANCMA11

11.45

37.1

PANCMA11-;

-PEG400

16.84

21.1

a,bThe number following "PANCMA" refers to the mol content of maleic acid in the copoly-

mer cMeasured by the sessile drop method dPEG400-grafted PANCMA membrane with different maleic acid contents e Calculated from the weights of a membrane before and after grafting reaction (wt%)

a,bThe number following "PANCMA" refers to the mol content of maleic acid in the copoly-

mer cMeasured by the sessile drop method dPEG400-grafted PANCMA membrane with different maleic acid contents e Calculated from the weights of a membrane before and after grafting reaction (wt%)

Fig.7. Permeation fluxes of deionized water (/w0, hatched column) and BSA solution (Jp, shaded column) through: PANCMA04 (1), PANCMA07 (2), PANCMA11 (3), PANCMA04-g-PEG400 (4), PANCMA07-g-PEG400 (5), and PANCMA11-g-PEG400 membranes (6). The number following "PANCMA" and "PEG" refers to the mole content of maleic acid and PEG, respectively, in or grafted onto the copolymer, respectively. PANCMA acrylonitrile/maleic acid copolymer

Fig.7. Permeation fluxes of deionized water (/w0, hatched column) and BSA solution (Jp, shaded column) through: PANCMA04 (1), PANCMA07 (2), PANCMA11 (3), PANCMA04-g-PEG400 (4), PANCMA07-g-PEG400 (5), and PANCMA11-g-PEG400 membranes (6). The number following "PANCMA" and "PEG" refers to the mole content of maleic acid and PEG, respectively, in or grafted onto the copolymer, respectively. PANCMA acrylonitrile/maleic acid copolymer of deionized water and BSA solution, respectively. As can be seen from Fig. 7, for all PANCMA membranes, both /w0 and Jp membranes increased with the content of carboxyl groups, and the PEG-grafted membranes had higher fluxes than all nongrafted membranes. However, it was interesting that the Jw0 and Jp of the PANCMA11-g-PEG400 (i.e., the membrane with the largest grafting degree) of PEG, were smaller than that of PANCMA07-g-PEG400 (i. e., a membrane with a moderate grafting degree). This may be due to the pore plugging of the membrane with the high graft degree of PEG. Figure 8 shows the percent of total fouling [(1 - Jp/Jw0) x 100], the flux recovery following water cleaning [(Jw1 - Jp)/(Jw0 - Jp) x 100, where Jw1 denotes water permeation flux after water cleaning], and the flux recovery as a result of chemical cleaning [(Jw2 - Jwi)/(Jw0 - Jp) x 100, where Jw2 denotes the water permeation flux after chemical cleaning]. It can be clearly seen that the PEG-grafted membranes possessed lower total fouling and higher flux recovery.

Figure 9 shows the results of static protein adsorption for membranes with different contents of carboxyl groups or grafting degree of PEG. It

Membranes

Fig.8. Flux changes of: PANCMA04 (1), PANCMA07 (2), PANCMA11 (3), PANCMA04-g-PEG400 (4), PANCMA07-g-PEG400 (5), and PANCMA11-g-PEG400 (6) membranes during filtration (hatched columns) and after water cleaning (black columns) and chemical cleaning (crosshatched columns)

Membranes

Fig.8. Flux changes of: PANCMA04 (1), PANCMA07 (2), PANCMA11 (3), PANCMA04-g-PEG400 (4), PANCMA07-g-PEG400 (5), and PANCMA11-g-PEG400 (6) membranes during filtration (hatched columns) and after water cleaning (black columns) and chemical cleaning (crosshatched columns)

can be seen that, the higher the content of carboxyl groups or PEG, the smaller the amount of static BSA adsorption. In addition, the PEG-grafted membranes had a greater degree of protein resistance than the PANCMA membranes. This result implied that the grafting modification with PEG rendered the membrane somewhat protein resistant.

To investigate the protein resistance of the PEG-grafted membrane, PEGs with various molecular weights were grafted onto the PANCMA membrane. Water contact angle and water absorption measurements were used to characterize the relative hydrophilicity of the membrane surface. It can be seen from Fig. 10 that the PEG400-grafted membrane showed the greatest degree of water absorption and the smallest water contact angle. This is interesting and can be simply explained as follows. Although the PANCMA04-g-PEG200 membrane has the largest number of PEG chains per unit area, namely grafting density, the chain is short and of relatively low hydrophilicity. On the other hand, it is more difficult to graft PEG600 and PEG1000 onto the membrane surface; therefore, the hydrophilicity of the corresponding membranes also decreases. It seems that to obtain an optimal protein-resistant membrane, PEG of a moderate molecular weight should

Fig.9. Relationship between static BSA adsorption and maleic acid (MA) content in PANCMA for the original (squares) and PEG-grafted (triangles) membranes

Membrane

Fig. 10. Effects of the molecular weight of PEG on the water contact angle (open circles, left y axis) and water absorption (filled triangles, right y axis) of: polyacrylonitrile (PAN; 1), PANCMA3 (2), PANCMA04-g-PEG200 (3), PANCMA04-g-PEG400 (4), PANCMA04-g-PEG600 (5), and PANCMA04-g-PEG1000 (6) membranes

Membrane

Fig. 11. Relationship between the molecular weight of PEG and static BSA adsorption on: PAN (1), PANCMA04 (2), PANCMA04-g-PEG200 (3), PANCMA04-g-PEG400 (4), PANCMA04-g-PEG600 (5), and PANCMA04-g-PEG1000 (6) membranes

Membrane

Fig. 11. Relationship between the molecular weight of PEG and static BSA adsorption on: PAN (1), PANCMA04 (2), PANCMA04-g-PEG200 (3), PANCMA04-g-PEG400 (4), PANCMA04-g-PEG600 (5), and PANCMA04-g-PEG1000 (6) membranes be suitable because of the balance between the grafting density and the length of the hydrophilic chain segments. Figure 11 shows the results of static protein adsorption experiments on membranes grafted with different molecular weights of PEG. It can be seen that little protein is adsorbed on PEG-grafted membranes.

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