Single pulse pulsed laser polymerization

Single Pulse-Pulsed Laser Polymerization. Applying PLP in conjunction with infrared or near-infrared spectroscopic measurement of monomer conversion induced by a single laser pulse (SP-PLP) allows for the determination of the ratio of termination to propagation rate coefficients, kt/kp, in wide ranges of temperature, T, pressme, p, and monomer conversion. The SP-PLP technique was pioneered in the 1980s by Buback and co-workers (409). The monomer conversion induced by a single laser pulse, typically of 20 ns width, is measured by on-hne... 

In this chapter two novel methods are presented which enable the direct and model independent determination of chain-length dependent termination rate coefficients. Both methods are based on single-pulse pulsed-laser polymerization. After a theoretical derivation, both kinetic approaches are validated by means of computer simulations. As will be demonstrated, these simulations prove that the essential assumptions needed to gain access to the chain-length dependent coefficients are valid and do not significantly undermine the accurate and reliable determination of the termination rate coefficients. 

Time-Resolved Single-Pulse Pulsed-Laser Polymerization Obtaining from Rate Measurements ... 

To increase the accuracy of the scaling of the number MWD, there seem to be two experimental techniques (or routes) that might prove to be useful. First of all, a combination of time-resolved single-pulse pulsed-laser polymerization with subsequent analysis of the MWD of the formed polymer product can be employed. From the time-resolved trace, accurate data on the radical concentration versus time can be deduced, which can then be used in the scaling of the number MWD. An alternative method to improve the scaling accuracy might arise form the use of mass-spectrometry techniques. Polymeric material with... 

TR-SP-PLP time-resolved single-pulse pulsed-laser polymerization... 

After having validated both methods theoretically, the experimental application of one of these methods, the one based on time-resolved single-pulse pulsed-laser polymerization, turned out to fail due to the inherent noise sensitivity of this method. From the measured monomer trace versus time, only the time-resolved radical concentration could be determined accurately. Information about the chain-length dependence of termination rate coefficients, which is hidden in the latter time-resolved trace, could not be revealed with any reliability and/or accuracy. 

Summarizing the above, it can be concluded that single-pulse pulsed-laser polymerization techniques are amongst the most powerful techniques that are nowadays available to study the chain-length dependence of termination reactions. When using a dedicated kinetic analysis, as presented in this thesis, model-independent data for the chain-length dependence of kt can be obtained. These data allow for better predictions for the MWD of the final polymer product and so of the final product properties. 

PLP-SEC and (iii) single-pulse pulsed-laser polymerization coupled with online time-resolved electron-spin resonance spectroscopy (SP-PLP-EPR). The propagation rate coefficient for MCRs may be obtained via ft-PLP-SEC and SP-PLP-EPR. Termination rate coefficients kt , and kt are only accessible from SP-PLP-EPR," in which different types of radicals can simultaneously be traced as a function of time. Remaining kinetic coefficients can then be obtained via computer modeling. Table 1.5 collates kinetic coefficients for butyl acrylate polymerization as an example. 

The conversion dependenee of termination was experimentally investigated in detail via the single-pulse pulsed-laser polymerization coupled with online time-resolved near infrared spectroscopy (SP-PLP-NIR) technique. " " This method provides access to chain-length-averaged termination rate coefficients, (kt). The variation of kt) towards increasing monomer-to-polymer conversion, X, is exemplified in Figure 1.5 for bulk polymerization of MMA. 

To overcome this problem, new ways had to be explored which would give access to both kp and / or kt from a single experiment. Actually, the SIP technique already made use of this approach by combining rate data with MWD data. A major improvement in the determination of kinetic parameters, however, came with the advent of pulsed lasers into kinetic studies at the end of the 80 s. The pulsed-laser polymerization technique (PLP)... 

Figure 6.2-27 Time resolved near-infrared measurement (at 8258 5 cm ) of monomer conversion, induced by a single pulse from a KrF excimer laser (248 nm), during ethene polymerization at 190 °C and at 2550 bar (reaction mixture already containing 9.5% polyethylene before this particular pulse was applied).

Information about propagation and termination ( t) rate coefficients during a polymerization reaction are obtained from pulse sequence (PS)-PLP and single pulse (SP)-PLP experiments [21-23]. In the latter technique, monomer conversion is induced by a single excimer laser pulse typically of 20 ns width and is recorded by on-line vibrational spectroscopy with time resolution in the microsecond range. A typical monomer conversion versus time profile obtained for an ethene polymerization [24] at 190 °C, 2550 bar, and at 9.5 wt% polyethylene (from preceding polymerization) is shown in Figure 4.6-3. 

These results point to two processes, premature radical chain termination and film shrinkage, which compete in determining the ultimate polymerization conversion efficient of multifunctional acrylates. It is obvious that critical attention must be paid to the pulse repetition rate, photoinitiator concentration, and acrylate functionality in developing any photopolymerizable system for laser-initiated polymerization. Future publications on laser-initiated polymerization of multifunctional acrylates will deal with monomer extraction of partially polymerized films, mechanical properties of laser polymerized films, and the Idnetics of single-pulsed systems. 

Termination kinetics were studied using the SP-PLP technique, which combines single-pulse initiation of polymerization with in-line NIR-spectroscopic detection of monomer concentration, CmIO- Time-resolution of the measurement of monomer concentration may be as low as a few microseconds, and the time interval for analysis may be extended up to a few seconds after firing the laser pulse at t=0. 

---