Internal Reflectance Spectroscopy of Reacted Surfaces: Fluorinated Polyethylene and Polypropylene

Abstract

The measurement of the depth of reaction of F₂ into the surface of polyethylene and polypropylene is reported. The roughness of the reacted surface has required the development of a novel three-layer, coated sample and subsequent theoretical analysis and analytical techniques. The results with polypropylene can be confirmed by visible reflectance fringes; however, in polyethylene the three-layer method is, at present, the only easy way to measure the extent of fluorine modification of the polymer surface. The extent of fluorination was found to be proportional to the time of exposure to F₂ gas and to increase with elevation of the sample temperature.

Keywords

Infrared Internal reflectance spectroscopy Surface depth of reaction Polymer modification Surface fluorination

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References

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Although this presentation is concerned primarily with the discussion of the three-layer ATR depth measurement technique, we have considered the possibility of the depth of fluorination being proportional to the square root of the reaction time (a diffusion-controlled process as suggested for fluorination of other polymer materials). Our data can be fitted. to the forms y = mx + b and y = ax^b. A linear regression to y = mx + b gives r², the coefficient of goodness of fit, of 0.9997 for the 2, 6, 8, and 20 min data points for polypropylene. For y = ax^b the value of r² is 0.9823 with a = 0.3292, where b = 0.5759. A diffusion-controlled reaction should have b = 0.5000. Fitting y = mx^1/2 + b, one finds a slightly worse fit with r² = 0.9781 and m = 0.4863, b = – 0.2597. This negative y-intercept is less explainable than the positive one obtained for y = mx + b. Consideration of the 2, 4, 6, 8, and 10 min points for F-HDPE shows that the form y = ax^b fits best with b = 0.852 not 0.500, and y = mx^1/2 + b fits the data somewhat less well than the simple linear form y = mx + b. Our ATR data do not seem to support the diffusion reaction model in preference to a simple linear relation for depth of fluorination in PP and HDPE vs treatment time, at least over the range of times and treatments that were studied. A subsequent paper is planned to consider the fluorination process specifically.