Mark Egorov

Tuning of cross-linking and mechanical properties of laser-deposited poly (methyl methacrylate) films

The chemical composition, amount of cross-linking and its influence on the mechanical properties of poly(methyl methacrylate) (PMMA) thin films produced by pulsed laser deposition (PLD) at a wavelength of 248nm under ultrahigh vacuum were investigated by infrared spectroscopy, scanning electron microscopy, size-exclusion chromatography, thermogravimetric analysis, and nanoindentation experiments. The films consist of two components, one fraction with a molecular weight well below that of the target material and a second fraction, which is cross-linked. Compared to bulk material, the Young’s modulus of the film is increased. The amount of cross-linking in the film can be tuned by the applied laser fluence leading to changes of the mechanical properties.

Modeling Termination Kinetics of Non-Stationary Free-Radical Polymerizations

Pulsed-laser induced polymerization is modeled via an approach presented in a previous paper. An equation for the time dependence of free-radical concentration is derived. It is shown that the termination rate coefficient may vary significantly as a function of time after applying the laser pulse despite of the fact that the change in monomer concentration during one experiment is negligible. For the limiting case of T >> c -1 (k p M) -1 , where c is a dimensionless chain-transfer constant, k p the propagation rate coefficient and M the monomer concentration, an analytical expression for k 1 is derived. It is also shown that time-resolved single pulse-laser polymerization (SP-PLP) experiments can yield the parameters that allow the modeling of k t in quasi-stationary polymerization. The influence of inhibitors is also considered. The conditions are analyzed under which M(t) curves recorded at different extents of laser-induced photo-initiator decomposition intersect. It is shown that such type of behavior is associated with a chain-length dependence of k t .

Modeling the effects of primary radicals in free-radical polymerization

The effect of non-ideal initiator decomposi tion, i.e., decomposition into two primary radicals of different reactivity toward the monomer, and of primary radical termination, on the kinetics of steady-state free-radical polymerization are considered. Analytical expressions for the exponent n in the power-law dependence of polymerization rate on initiation rate are derived for these two situations. Theory predicts, that n should be below the classical value of 1/2. In the case of non-ideal initiator decomposition, n decreases with the size of the dimensionless parameter a ≡(k t2 /k d2 )t m /k t , where k a is the termination rate coefficient for the reaction of a non-propagating primary radical with a macroradical, k dz is the first-order decomposition rate coefficient of non-propagating (passive) radicals, r in is initiation rate, and k t is the termination rate coefficient of the active radicals. In the case of primary radical termination, n decreases with the size of the dimensionless parameter β ≡ k 1,s r 1/2 in /k ps Mk 1/2 t.1 , where k t,s is the termination rate coefficients for the reaction of a primary (short) radical with a macroradical, k t,1 is the termination rate coefficients of two large radicals, k r,s is the propagation rate coefficient of primary radicals and M is monomer concentration. As k t is deduced from coupled parameters such as k t /k p , the dependence of k p on chain length is also briefly discussed. This dependence is particularly pronounced at small chain lengths. Moreover, effects of chain transfer to monomer on n are discussed.

Analysis of the Phenomenon of Autoacceleration in Free Radical Polymerization

Free radical polymerization provides a good example of a process for which nonlinear kinetics of a complex system is governed to a great extent by the physical properties of the medium. The kinetic scheme of bulk and solution polymerizations includes the following reactions: Initiation:

Critically evaluated termination rate coefficients for free-radical polymerization : Experimental methods

I\xrightarrow{{{{k}_{J}}}}2{{R}^{*}}

Modeling termination kinetics in free-radical polymerization using a reduced number of parameters

{{R}^{*}} + M\xrightarrow{{{{k}_{{pi}}}}}{{R}_{1}}