Plasma Induced Graft CoPolymerization

Plasma is the fourth state of material and is composed of electrons, ions, free radicals, atoms, and molecules. There are two subdivisions for plasma: thermal equilibrium and nonthermal equilibrium plasma. For current use on polymers, the subdivision of nonthermal equilibrium is frequently used, which constitutes different plasma species having different temperatures. More precisely, the electrons usually have much higher temperature than the heavier particles (ions, atoms, molecules). In a plasma, important parameters include average electron temperature, ranging from 1 to 10 eV, electron density, varying from 109 to 1012 cm-3, and degree of ionization, lying between 10-6 to 0.3 [27]. It is notable that, in plasma, compared to the high temperature of the electron, heavy particles have temperature around ambient condition (300 K or 0.025 eV) [28]. This heavy particle temperature is obviously suitable for treating many temperature-sensitive polymers, which will otherwise undergo degradation if exposed to a hot gas. Although the hot electrons, which are light and easily accelerated by the applied electromagnetic fields, can provide gas molecule energy through inelastic collision with them, to produce a chemically rich environment. Due to the advantages of plasma surface modifications, this technique is widely used in the polymer field. Many papers regarding this technique and its application are available [28,44,55]. The advantages of plasma surface include the following [27].

1. Modification can be limited to the surface layer, leaving the properties of the bulk material unaltered. Typically, the modification depth is several hundred angstroms.

Figure 4.3. Schematic of surface activation.

Substrate

Activated substrate

2. Nearly all polymer surfaces can be modified by plasma, no matter how the surface is composed.

3. It is possible to choose the type of chemical modification for the polymer through the choice of gas used in gas plasma.

4. Gas plasma treatment can avoid the problems encountered in wet chemical techniques such as residual solvent on the surface.

5. Plasma surface treatment is fairly uniform over the whole surface.

4.5.2.1. Mechanism

In plasma, as mentioned, many species exist:electrons, ions, and photons. The reaction of these species with the polymer can result in free radicals in the polymer main chain. The generation of free radicals can happen in two ways: the bombardment of the ions and photons can provide energy higher than the covalent C-C or C-H bonding energy and can break the C-C, or C-H bond to form C free radicals on the midpoint of the polymer main chain [56]; or through elastic or inelastic collision of the electron in the plasma with the polymer, the H can be abstracted, resulting in C free radicals.

When these activated polymers are exposed to monomers with unsaturated bonds, the radicals on the polymer play the role of initiator as in conventional polymerization and starts polymerization . This is called plasma-induced graft co-polymerization. One typical example is shown as follows.

1. Radicals are formed when the polymer are exposed to an inert gas (such as Argon) plasma (Figure 4.3).

2. Then the activated surface is exposed to the monomer (such as acrylic acid) with an unsaturated bond in the form of gas or liquid; a polymer layer is grafted on the polymer surface (Figure 4.4).

Another approach for this would be to expose the plasma-treated surface in air to form peroxides. This peroxide, after exposure to ultraviolet, decomposes and

Acrylic acid

Grafted polyacrylic acid side

COOH

Figure 4.4. Grafting onto activated surface.

can later act as initiator to start a polymerization when the surface is exposed to monomers with unsaturated bond [32].

4.5.2.2. Applications

Via plasma surface modification, the surface properties, such as printability, wettability, hardness, and biocompatibility can be improved to a certain degree, thus this technique is widely used in fields such as print, plastic, and especially, these days, bioengineering. In this field, it can be used to make the surface more compatible, sterilizing the surfaces, to produce certain surface morphology to regulate cell functions, or to create a barrier to reduce undesirable diffusion of small molecules into or out of the substrate. Table 4.3 lists briefly some materials that have been plasma-surface modified in the bioengineering field.

In one instance, nonwoven mesh composed of polysulphone (PSU) ultrafine fibers (diameter, 1~2 ^m) were fabricated via the electrospinning technique and

Table 4.3 Plasma surface modified materials employed in bioengineering field

Modified

material

Modification method

Modification effect

Application field

Reference

PTFE and

Use argon plasma to

Reduced fibrinogen

Vascular graft

57

PET

activate surface , then graft collagen IV and laminin

adsorption and platelet adhesion

PU

Si+ and N+ ion implantation

Improved wettability, anticoagulability and anticalcific behavior

Medical field

33,58

PLLA

Ammonia plasma

Improved growth in human vascular endothelial cell and rabbit microvascular endothelial cell

Cell transplantation and in vivo regeneration of vascular tissue

59

PMMA

Plasma activation

Reduced cell adhesion

Artificial intraocular

60

followed by PEG

and decreased

lens

grafting

incidents of retroprosthetic membrane formation after keratoprosthesis surgery

SR

Argon plasma glow discharge followed by grafting PHEMA

Improved cell attachment and growth

Artificial cornea

61

SR

Ion implantation

Reduced biofouling and building up of thrombosis

Catheter

31,62

Titanium

Ion(nitrogen,

Improved wear

Orthopedic and

32,45,63

calcium)

resistance, improved

dental

implantation

new bone regeneration

Cobalt-

Ion implantation

Improved wear

Artificial hip and

24

chromium

resistance

joint

then surface modified using plasma-induced grafting towards development of a novel affinity membrane [64]. After heat treatment of the nanofiber mesh, car-boxyl groups were introduced onto the PSU fiber surfaces through grafting co-polymerization of methacrylic acid (MAA) initiated by Ce(IV) after an air plasma treatment of the PSU fiber mesh. Toluidine Blue O (TBO), a dye which can form a stable complex with carboxyl groups, was used as a model target molecule to be captured by the PMAA-grafted PSU fiber mesh. Furthermore, the carboxyl groups on the PMAA-grafted PSU fiber mesh can be used as coupling sites for immobilization of other protein ligands. Bovine serum albumin (BSA) was chosen as a model protein ligand to be immobilized into the PSU fiber mesh with an encouraging capacity of 17 Mg/mg, which implies other protein ligands can also be attached using the same method.

In addition to physical blending, to covalently graft ECM protein such as collagen and gelatin on nanofiber surfaces is the ultimate goal in development of biomimetic tissue engineering scaffolds. One of the most popular methods to co-valently attach protein molecules on a polymer surface is grafting poly(methacrylic acid) on the biomaterial surface to introduce carboxyl groups at first, followed by the grafting of the protein molecules using water-soluble carbodiimide as the coupling reagent [65,66]. Nonwoven PET nanofiber meshes were prepared by electro-spinning technology and were surface grafted with gelatin. The PET NF was first treated in formaldehyde to yield hydroxyl groups on the surface, followed by the grafting polymerization of methacrylic acid (MAA) initiated by Ce (IV). Finally the PMAA-grafted PET nanofiber was grafted with gelatin using water-soluble carbodiimide as coupling agent. For gelatin grafting on PCL nanofiber surfaces, the PCL nanofiber was first treated with air plasma to introduce -COOH groups on the surface followed by covalent grafting of gelatin molecules with the water-soluble carbodiimide as the coupling agent. Both the gelatin-modified PET and PCL nanofibers showed improved endothelial cell compatibility to those of the unmodified nanofibers [8]. Moreover, the gelatin-modified aligned PCL nanofiber also showed strong abilities to affect endothelial cell orientation.

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