Plasma treatment

Plasma modification of polymeric materials is an extremely useful way of tailing a polymer into a desired material by utilizing the selective chemistry and molecular structure on the surface (Kiaei et al. 1995; Lee et al. 1991; Oehr et al. 1999). It is an efficient way to produce functional groups such as hydroxyl groups, amino groups, and carboxylic groups. Plasma can also initiate polymerization reactions of monomers on a surface. Graft polymerization can be carried out by plasma treatment either directly or indirectly. For the former, monomers are exposed to the plasma environment on the objective surface. At the same time, indirect polymerization can also be initiated by functional groups produced by the plasma.

Kou et al. (2003) grafted a glycopolymer to polypropylene microporous membrane (PPMM) surface by direct plasma polymerization (Fig. 1a). A monomer containing sugar moieties, a-allyl glucoside (AG), was synthesized and grafted to PPMMs (Fig. 1b). In that case the PPMM was immersed in an AG solution of N,N-dimethylformamide for a predetermined time, and the solvent was evaporated in a vacuum oven. The coated AG monomer was then grafted chemically onto the membrane surface by N2 plasma. Bovine serum albumin (BSA) was used as a model protein to evaluate the anti-protein-fouling characteristic of this modified PPMM. As shown in Fig. 2, increases in the AG grafting degree leads to a reduction in the amount of protein deposited to the membrane surface. This is due to the hydrophilicity and biocompatibility of the grafted AG.

Liu et al. (2003) tethered a polypeptide, poly(y-stearyl-L-glutamate) (PSLG), to the PPMM surface by plasma treatment (Fig. 3). First, the PPMM was treated by plasma under an atmosphere of ammonia to introduce amino groups to the surface. Then, a monomer, N-carboxyanhydride (NCA), was subjected to ring-opening polymerization initiated by the amino groups on

Fig. 1. Molecular structure of a-allyl glucose (AG; a) and poly (a-allyl glucoside) (PAG)-modified polypropylene microporous membrane (PPMM; b)

Fig. 1. Molecular structure of a-allyl glucose (AG; a) and poly (a-allyl glucoside) (PAG)-modified polypropylene microporous membrane (PPMM; b)

i i—i—|—i—|—i—|—i—i—i—|—i—|—i—|—i—i—i—|—i— 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5

Fig. 2. a-d Effect of AG grafting degree on bovine serum albumin (BSA) adsorption (a-d: 0, 0.82,1.86, 3.46 wt.%, respectively)

i i—i—|—i—|—i—|—i—i—i—|—i—|—i—|—i—i—i—|—i— 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5

BSA concentration (mg/ml)

Fig. 2. a-d Effect of AG grafting degree on bovine serum albumin (BSA) adsorption (a-d: 0, 0.82,1.86, 3.46 wt.%, respectively)

the membrane surface, and PSLG was grafted onto the membrane surface. There are two mechanisms of ring-opening polymerization of NCA (Fig. 3b) and the active monomer mechanism, which is initiated by hydroxyl groups, must be depressed to improve the grafting degree. Thus y-(aminopropyl) triethanoxysilane (y-APS) was used to eliminate the effect of hydroxyl groups by reacting with them. The results of BSA adsorption experiments are shown in Fig. 4.

Fig. 3. Poly(y-stearyl-L-glutamate) (PSLG)-modified PPMM (a) and mechanisms for the synthesis of polypeptide from N-carboxyanhydride (NCA) monomers (b)

Fig. 4. BSA adsorption onto different PPMMs. ■ original membrane; ▼ NH3 -plasma-treated membrane; O PSLG-grafted membrane without y-(aminopropyl) triethanoxysilane (y-APS) treatment; A PSLG-grafted membrane with y-APS treatment

BSA concentration (mg/ml )

Fig. 4. BSA adsorption onto different PPMMs. ■ original membrane; ▼ NH3 -plasma-treated membrane; O PSLG-grafted membrane without y-(aminopropyl) triethanoxysilane (y-APS) treatment; A PSLG-grafted membrane with y-APS treatment

For the ammonia-plasma-treated membranes, BSA adsorption was reduced slowly as a result of the hydrophilic groups generated on the membrane surface. However, what surprised us was that the PSLG-y-APS-PPMM exhibited greatly increased protein adsorption, even larger than the nascent PPMM. This could be interpreted by the conformation of PSLG on the surface. It is known that polypeptide exhibits a-helix and coil conformations under different conditions. For the poly(y-stearyl-L-glutamic acid) with long stearyl groups, Poche et al. (1995) speculated that the grafted PSLG in solutions formed an a-helix conformation that was supported by the intramolecular hydrogen bonds, with the peptide main chains in the cores and y-stearyl long side chains stretched outside. This molecular model of PSLG could be used to explain the BSA adsorption results. For the PSLG-PPMM, the amount of adsorbed BSA increased a little; this could be ascribed to the existence of -OH groups, which led to low polymerization degree. For the PSLG-y-APS-PPMM, the polymerization degree of grafted chains increased, the stearyl long side chains stretched outside, thus greatly increasing the surface hydrophobicity, resulting in an increase in BSA adsorption. This characteristic was used to immobilize lipase in our further work.

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