Pulsed-laser-initiated polymerization

The pulsed laser-initiated polymerization of multifunctional acrylates has been investigated using a photocalorimeter. In the case where a limited munber of pulses are delivered to the sample, the extent of conversion decreases as the laser repetition rate increases. However, for a large number of pulses, higher repetition rates do not lead to decreases in the overall degree of conversion. 

Propagation rate coefficients for styrene, vinyl acetate, and various acrylates and methacrylates were determined by the so-called PLP-SEC technique, which combines pulsed-laser-initiated polymerization (PLP) with subsequent polymer analysis via size-exclusion chromatography (SEC). The PLP-SEC experiment measures the product of propagation rate coefficient, kp, and monomer concentration, Cm ... 

To illustrate more clearly the nature of free radical polymerization, it is instructive to examine the values of the individual rate constants for the propagation and termination steps. A number of these rate constants have been deduced, generally using nonstationary-state measurements such as rotating sector techniques and emulsion polymerization [26]. Recently, the lUPAC Working Party on Modeling of kinetics and processes of polymerization has recommended the analysis of molecular weight distributions of polymers produced in pulsed-laser-initiated polymerization (PLP) to determine values of... 

Data taken from Refs. [30-32] data in parentheses determined by pulsed-laser-initiated polymerization. Ref [29], Ref [33], Ref [34], Ref [35]. 

Free-radical polymerization in aqueous solution is of significant industrial importance. To model polymerization processes and product properties, reliable rate coefficients for the individual reaction steps are required. The propagation rate coefficient, kp, may be precisely obtained by the PLP-SEC method, which combines pulsed-laser initiated polymerization with subsequent... 

A number of nonsteady polymerization rate techniques can be used to measure ftp [11]. The most widely used method involves pulsed-laser-induced polymerization in the low monomer conversion regime. Briefly, a mixture of monomer and photoinitiator (Section 6.5.3) is illuminated by short laser pulses of about 10 ns (10 sec) duration. The radicals that are created by this burst of ligh propagate for about 1 sec before a second laser pulse produces another crop of radicals. Many of the initially formed radicals will be terminated by the short, mobile radicals created in the second illumination. Analysis of the number molecular weight distribution of the polymer produced permits the estimation of ftp from the relation... 

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. 

Keywords aqueous-phase polymerization free-radical polymerization methacrylic acid PLP-SEC propagation rate coefficients pulsed-laser initiation water-soluble monomers... 

Summaiy In this short review, selected experimental approaches for probing the mechanism and kinetics of RAFT polymerization are highlighted. Methods for studying RAFT polymerization via varying reaction conditions, such as pressure, temperature, and solution properties, are reviewed. A technique for the measurement of the RAFT specific addition and fragmentation reaction rates via combination of pulsed-laser-initiated RAFT polymerization and j,s-time-resolved electron spin resonance (ESR) spectroscopy is detailed. Mechanistic investigations using mass spectrometry are exemplified on dithiobenzoic-acid-mediated methyl methacrylate polymerization. 

From this we can see that knowledge of k f and Rf in a conventional polymerization process readily yields a value of the ratio kp fkt. In order to obtain a value for kf wc require further information on kv. Analysis of / , data obtained under non-steady state conditions (when there is no continuous source of initiator radicals) yields the ratio kvlkx. Various non-stcady state methods have been developed including the rotating sector method, spatially intermittent polymerization and pulsed laser polymerization (PLP). The classical approach for deriving the individual values of kp and kt by combining values for kp kx. with kp/k, obtained in separate experiments can, however, be problematical because the values of kx are strongly dependent on the polymerization conditions (Section... 

From the cure profile recorded after a short and intense UV or pulsed laser exposure 18, one can evaluate the actual rate at which the polymer chains are growing. By calculating the ratio Rp/[R ], where [R ] is the number of initiating radicals generated by the UV exposure, we found that 5.104 acrylate double bonds have polymerized per second, for each initiating radical. From this value, the average time for the addition of one monomer unit was calculated to be 20 jlls. 

As a result of this study, it should be apparent that both C.W. and pulsed laser beams are capable of very efficiently initiating the cross-linking polymerization of multifunctional acrylate photoresists, provided adequate initiators are used to absorb the laser photons and generate the reactive species. The main advantages of this technology arise from the specific properties of the laser emission ... 

The time to measure spectra of this quality under high-pressure conditions has been about I min. The absolute time scale of the experiment depends on the method of initiation. In thermally initiated (spontaneous) polymerizations reaction time can be several hours or even days. In contrast, in excimer laser-initiated free radical polymerizations application of a few laser pulses each of about 20 ns duration can induce changes between subsequent spectra as on this figure. 

Figure 3.72. Dependence of lateral resolution and depth resolution on average power of a mode-locked Ti sapphire laser (wavelength 763 nm, 82MHz repetition, 130 fs pulse width) during TP initiated polymerization of a urethane acrylate resin. (From Ref. [575] with permission of SPIE—The International Society for Optical Engineering.)...

Figure 3.73. Volume size of voxels assuming ellipsoid structure as a function of the inverse of the scan speed. The voxels were obtained by TP initiated crosslinking radical polymerization of acrylates in the presence of poly (styrene-co-acrylonitrile) as binder and an amino-substituted distyrylbenzene as TP active initiator using a pulsed laser (150-fs pulses at a 76-MHz repetition rate or 85-fs pulses at a repetition rate of 82 MHz). (From Ref. [133] with permission of the Technical Association of Photopolymers, Japan.)...

A number of anomeric D-hex-2-ulopyranosyl azides have been synthesized and their photochemistry examined. For both a- and P-azides, the major photoproducts arise from cleavage of the C-2C-3 bond and migration of the C-3 carbon to the nitrene centre. Decomposition reactions of a glycidyl azide polymer have been induced by pulsed laser infrared pyrolysis and UV photolysis of thin films at 17-77 K and monitored by IR spectroscopy. The initial step is elimination of N2 and formation of imines, which decompose on warming, possibly with secondary polymerization. 

The use of lasers to initiate the polymerization of both monofunctional and multifunctional monomers has been reported in a number of papers during the past decade. Decker (1) was the first to demonstrate that pulsed lasers could be effectively used to obtain relatively high degrees of polymerization for trimethylolpropane triacrylate. He showed that even for pulsed lasers which deliver up to gigawatts of peak power, polymerization could be effectively carried out over a wide range of conditions (1). 

Surface radicals produced in the exposed areas may also serve directly as polymerization initiators. However, their concentration will not be very high because they may be rapidly deactivated at the relatively high temperatures reached locally and because of possible reaction with volatile reaction products. Still, it was reported that poly(hydroxyethyl methacrylate) (PHEMA) could be grafted from PDMS surfaces treated with a C02-pulsed laser via the peroxide route [10]. However, the capability of the method to obtain surface structures was not explored in this case. 

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