Atomic oxygen polymers

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

The low concentrations of ozone normally present in the atmosphere are sufficient to cause severe oxidation and cracking in polyolefins (2-13) and many other polymers such as polystyrene (6,11,12,14.15). poly(vinyl chloride) ( y, J 2,J 6) and rubbers (11,12,17-20). Where the ozone concentration is increased by air pollution, higher altitudes or the present of electrical machinery, the rate of degradation considerably increases.The prevention of such degradation represents a matter of considerable economic interest since it can greatly improve service life of polymers and plastics. An additional application of this research is to apprise the suitability of polymers for upper atmosphere application (Space Shutle flights) where a plentitude of ozone and atomic oxygen prevails. 

The most obvious change in an oxidation of the PP surface results in an increase in the wettability of the polymer. The PP shows a great decrease in contact angle with water or water/alcohol (3 1) after treatment with ozone or ozone-UV irradiation or atomic oxygen (Table 1). 

A PP sample after ozonization in the presence of UV-irradiation becomes brittle after 8 hrs of exposure, whereas the same effect in ozone is noticeable after 50-60 hours.Degradation of polymer chain occurs as a result of decomposition of peroxy radicals. The oxidation rapidly reaches saturation, suggesting the surface nature of ozone and atomic oxygen against of PP as a consequence of limited diffusion of both oxygen species into the polymer. Ozone reacts with PP mainly on the surface since the reaction rate and the concentration of intermediate peroxy radicals are proportional to the surface area and not the weight of the polymer. It has been found that polyethylene is attacked only to a depth of 5-7 microns (45). 

Atomic oxygen oxidation of polymers has been reported by a few authors (46,50). Experiments were limited to the measurements of weight-loss data and changing of the wetteability (46-48), and only two papers were devoted to the study mechanism of atomic oxygen oxidation of polydienes (49,50). 

The PP samples exposed to atomic oxygen show for the formation of polymer peroxy radicals (P00 ), which give almost identical ESR spectrum as in the case of ozone reaction (Fig.l). The ESCA spectra (Fig.5) indicate that atomic oxygen oxidation is more effective than ozonization. These results support our assumption that ozonization... 

Mechanism of the reactions which are belived to occur when a polymer such as PP is exposed to atomic oxygen are following ... 

Reaction of Atomic Oxygen [0( P)] with Polybutadienes and Related Polymers... 

Reneker, D. H., and L. H. Bolz. Effect of atomic oxygen on the surface morphology of polyethylene. Paper Presented at Symposium on Plasma Chemistry of Polymers, American Chemical Society Meeting, Philadelphia, April 6-11, 1975. 

Bawn, Bell, Fitzsimmons, and Ledwith (20) have suggested that transfer reaction in the bulk polymerization of THF must involve either hydride ion abstraction from the alpha methylene of THF or of tetra-methyleneoxy units in the polymer, or degradative oxonium ion formation with the ethereal oxygen atoms of polymers of the type discussed in Section IIID4. 

The analytical depth profiling for these systems (e.g. the polystyrene data is shown in Figure 5) revealed that the reaction is essentially confined to the topmost monolayer of material ( ). This is entirely reasonable in terms of the plasma chemistry since the most prominent reactive species is atomic oxygen f ich is expected to have an extremely short mean free path in hydrocarbon polymers. This serves as a very good example of the powerful nature of XPS when applied to the study of the surface modification of polymers. 

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