8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

P4.18Fri 911:10-14:00The Influence of light absorption and shadowing amongsegments of a chain on the kinetics of ultravioletdepolymerizationAhmet Giz, 1 Nazmi Postacioglu, 1 and Ezgi Erdogan 11 Istanbul Technical University, Fen-Edebiyat Fakultesi, Maslak 34469, Istanbul,TurkeyIn UV degradation a high energy photon deposits its energy on a single bond and breaks it. Since insolution the polymer is considered to be a random coil it is generally assumed that the probabilityof a photon impacting a given bond is independent of the position of the bond on the chain andthe length of that chain. This assumption is valid for UV treatment in bulk or in concentratedsolutions, however in dilute solutions individual bonds are not statistically equivalent. In dilutesolutions, segments of the chain are ”shadowed” by other segments within the same chain whichare closer to the light source at a given time. When a photon hits a bond and breaks it, this obviouslyprotects the other bonds which lie on the path the photon would have followed. The probability ofbond ”j” being shadowed by bond ” i ” is proportional to the solid angle subtended and inverselyproportional to the square of the distance between the bonds, 1/r 2 ij. The average r ij , is proportionalto |i − j| a where a is equal to 1/3 for poor, 1/2 for theta and 3/5 for good solvents. The shadowingprobability is then proportional to 1/|i − j| 2 . The total probability that the unit j is shadowed isproportional to the sum of these terms for all units in the chain. It depends on the chain length,chain conformation and whether the segment lies near the middle or near an end of the chain. HereUV degradation of polymer solutions is simulated and resulting molecular weight evolutions areinvestigated taking the effect of shadowing among chain segments into account. Simulations showthat with shadowing effect, ultraviolet chain scission is not completely random. The effect is subtlebut it is expected to be observable in the (1/M n ) versus time, d(1/M n ) /dt versus time and HIversus time graphics if the polydispersity of the polymer is low.18