Polymer surface oxidation

The surface of any material is different from the interior or bulk of the material. Differences can be chemical or physical, or both. An example is rust on iron. In polymers, surface oxidation can also exist. Special effects on polymer surfaces include contamination, plasticization, grafting, and a host of other chemical changes of modifications, some wanted, and some adventitious see Figure 12.1. 

A particularly important challenge for biomaterials is the possibility to sterilize them without affecting their functional properties. XPS is a valuable tool to study the effect of sterilization on the surface properties of the materials. In the case of metals, the fate of the oxide layer, which is related to the biocompatibility, is of interest. ° For polymers, surface oxidation possibly brought by the sterilization must be examined. °... 

The durability in use and reuse of the hydrocarbon polymers and their performance during recycling can be readily controlled. Since the catabolic enzymes cannot biodegrade polymers in the inhospitable hydrophobic environment at the polymer surface, oxidatively resistant hydrocarbon polymers are essentially non-degradable. As was seen in Chapter 9, extensive modification of the polymer surface is a prerequisite to biological attack. In particular, both thermal and photooxidation can be employed to provided a suitable surface environment for microbial colonization and the rate at which these processes occur can be controlled by the use of the appropriate antioxidants and stabilizers [7]. Photooxidation of the hydrocarbon polymers is particularly relevant to the control of litter, whereas thermal oxidation is a prerequisite for aerobic composting. 

Polymer Surface Oxidation Induced by Irradiation with Electron Beams Svorcik et al. [96] have irradiated PE foils, 15 pm thick (Mjj = 180,000, p = 0.945 g/cm ) in air and at room temperature, with a 14.89 MeV electron beam. The electron flux was 247 Gy/min and the samples were irradiated to fluences ranging from 57.6 to 576 kGy. 

Other Methods. In the three previous methods described for treating polymers, surface oxidation took place. Other direct methods of oxidation have also been used to modify polymer adherends effectively. With flame treatment in air, an oxidizing flame briefly ( 0.01-0.1 sec) impinges on the surface. XPS analysis" has shown that amide surface groups are generated as well as typical oxidation functionality. Treatment by a blast of hot air has also been used to create surface macro-radicals that then combine with oxygen. 

The polymer surfaces oxidized gradually along the sample length with increasing plasma exposure time and thus the wettability gradient was created. 

Surface Fluorination of Polymers. Fluorocarbon-coated objects have many practical appHcations because the chemically adherent surface provides increased thermal stabiHty, resistance to oxidation and corrosive chemicals and solvents, decreased coefficient of friction and thus decreased wear, and decreased permeabiHty to gas flow. Unusual surface effects can be obtained by fluorinating the polymer surfaces only partially (74). 

Poljraer surfaces can be easily modified with microwave or radio-frequency-energized glow discharge techniques. The polymer surface cross-links or oxidizes, depending on the nature of the plasma atmosphere. Oxidizing (oxygen) and nonoxidizing (helium) plasmas can have a wide variety of effects on polymer surface wettability characteristics (92). 

Other SSIMS studies of polymer surfaces have included perfluorinated polyether [3.21], low-density polyethylene [3.22], poly(ethylene terephthalate) [3.23], and the oxidation of polyetheretherketone [3.24]. 

Standard two-compartment H cell. The catholyte consisted of 3.25 M nitric acid and was separated by a medium-porosity sintered glass frit. Experiments were also carried out to determine if surface oxidation of hydrocarbon polymers could be obtained in an electrolyte consisting only of nitric acid. 

UV irradiation on a polymer surface produces chemical modification as well as wettability and bondability improvement. It causes chain scission and oxidation on polymer surfaces. -iven in the presence of an inert gas [45]. Carbonyls are found to be introduced onto polyethylenes on UV irradiation. Sivram et al. [46] have used photochemical treatments for surface modification of polymers. They have generated surfaces of vaying surface energies by simple organic reactions. 

When a polymer is soaked in a heavily oxidative chemical liquid, such as chromic anhydride-tetrachloroeth-ane, chromic acid-acetic acid, and chromic acid-sulfuric acid, and treated under suitable conditions, polar groups are introduced on the polymer surface and the surface characteristics are improved [49,50]. The sur-... 

In addition, there are many surface modification processes that use triplet sensitizers to permit oxidation reactions. In a typical process, polyisocyanate is applied on a polyolefin together with a sensitizer such as benzo-phenone and then irradiated with UV light. As shown in Eq. (15) the sensitizer has an oxidizing effect to produce hydroxyl groups over the polymer surface. These hydroxyl groups finally react with isocyanate to provide a functional polymer [56,57]. 

Oxidative Reactions Caused by Ozone and Atomic Oxygen on Polymer Surfaces... 

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