Molecular weight averages with antioxidant

Figure 6. Molecular weight averages vs. residence time at 135 °C MDPE with antioxidants.

Increased age and smoking are associated with an increased oxidative stress and depletion of low-molecular weight antioxidants in many tissues including the retina (Beatty et al., 2000). Epidemiological studies indicate that on average, smokers develop late stage AMD 10 years earlier than nonsmokers (Kelly et al., 2004 Klein et al., 1998 Mitchell et al., 2002 Thornton et al., 2005 Tomany et al., 2004). 

A reliable procedure for determination of molecular parameters number, weight and z-averages of the molecular weight (Mj, i = n, w and z respectively) for polyethylenes, PE, by means of Size Exclusion Chromatography, SEC, has been developed. The Waters Sci. Ltd. GPC/LC Model 150C was used at 135 C with trichlorobenzene, TCB, as a solvent. The standard samples as well as commercial stabilized and not stabilized PE-resins were evaluated. The effects of sampling, method of solution preparation, addition of antioxidant(s), thermal and shear degradation were studied. The adopted procedure allows reproducible determination of and M , with a random error of 4% and M2, with 9%, within 2 to 72 hrs from the initial moment of preparation of solutions. 

Sample Preparation. The UHMWPE used in this study was a HiFax 1900 (Himont, Inc.) with an average molecular weight of 2-8 x 10. The UHMWPE was added to paraffin oil to concentrations from 2 to 5% w/w. To avoid polymer degradation at high temperatures, the polymer was stabilized with approximately 0.5% of BHT antioxidant. The mixture was stirred at 150 C. A solution was obtained that was clear until it was cooled to about 120 C when it became opaque and transformed into a gel-like or pseudo-gel. 

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