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Yayın Molecular alignment during gel formation from methyl methacrylate: An excimer fluorescence study(Taylor & Francis Ltd, 2005-05) Kaya Aktaş, Demet; Erdoğan, Matem; Pekcan, Mehmet ÖnderGlass transition during bulk polymerization was studied in free-radical cross-linking copolymerization (FCC) of methyl methacrylate (MMA) using the steady-state fluorescence (SSF) technique. Naphthalene (N) was used as a monomer and excimer forming probe. Changes in the viscosity of the pregel solutions due to gel formation dramatically enhance both monomer and excimer fluorescent yield of N molecules. The reaction time at which the monomer and excimer intensities exhibit a sudden increase corresponds to the reaction time at which the rate of polymerization becomes maximum resulting from the gel effect. This effect was used to study the gelation of MMA, as a function of time, in various N concentrations. The results were interpreted in the view of percolation theory. The gel fraction, beta, and weight average degree of polymerization, gamma, exponents beta=0.40 +/- 0.02 and gamma=1.70 +/- 0.07 were found in agreement with percolation results for both monomer and excimer measurements, respectively.Yayın Study on critical behaviour in N-isopropyl acrylamide gels by using fluorescence technique(Taylor & Francis Group, 2006-11) Kaya Aktaş, Demet; Pekcan, Mehmet ÖnderThe steady state fluorescence (SSF) technique was used to study the sol-gel transition for the solution free radical crosslinking copolymerization of N-isopropyl acrylamide (NIPA), with N,N'-methylenebis (acrylamide) (BIS) as crosslinker in the presence of ammonium persulfate (APS) as an initiator. Pyranine (8-hydroxypyrene-1,3,6-trisulfonic acid, trisodium salt, HPTS) was used as a floroprobe for monitoring the polymerization. Pyranine molecules start to bind to NIPA polymer chains upon the initiation of the polymerization, thus the spectra of the bonded pyranines shift to the shorter wavelengths. The get fraction exponent beta and the weight average degree of polymerization exponent gamma' agree best with the mean-field (Flory-Stockmayer) results near the gel point for various crosslinker contents.
