# How much UV lamp power is needed to initiate radical polymerization?

I'm looking to purchase a UV lamp for initating some radical polymerizations. Is a hand lamp like the type for examining TLC plates (5-10W, like this) enough power to do this, or do I have to buy something a lot more expensive?

In my case, I'm looking to polymerize a fairly small sample (1mm thick, about 100 mm$^2$ of area, encased in 1mm-thick lab slides) and the chemistry will probably be MMA (initiated with AIBN). I'd like to know for my particular case, but it would be great to know if there's some rules of thumb involved (does it depend on the chemistry? does it depend on the amount? Will different power levels give different final results?)

A photopolymerization requires a photoinitiator to capture the energy. Where does AIBN absorb versus your source?

OK, that looks good. AIBN has a strong cage effect. 2,2′-Azobis(2-methylpropionitrile) extrudes nitrogen and the radicals combine without initiating polymerization. The dimer, tetramethylsuccinonitrile or 2,3-dicyano-2,3-dimethylbutane, is quite toxic. The superior initiatior is 1,1'-azobis(cyclohexanecarbonitrile), VAZO catalyst 88; J. Org. Chem. 52 2958 (1987) (e.g., Sigma-Aldrich Chemical Company).

Camphorquinone (yellow powder) can replace AIBN, shifting absorption into the visible, 380 - 510 nm. That is a tungsten-halogen bulb or a middling blue LED, GaN or InGaN maxing at 450 - 460 nm. The graph's yellow line is camphorquinone.

Molecular oxygen quenches free radical polymerizations. If things stay gooey, add a little more initiator to the starting mix to burn out the oxygen. Now, the fun part! If you treat the clean glass slides with Rain-X, (spray, wait a few minutes, wipe, lightly buff; maybe lock in at 110 C for 15 minutes) you silanize the surface and polymer will not stick. It's a miracle for treating vacuum polymerization ampoules. If you want a cleaner surface and can play chemist,

http://www.gelest.com/gelest/forms/GeneralPages/Applications/hydrophobic_materials.aspx
Gelest Aquaphobe CM, diluted by you to 8% in dichloromethane or toluene as a 5-minute soak in a closed glass container. Soak, rinse with solvent, lightly buff, oven. The active material gives off HCl, including exposure to humidity.

Sounds interesting! These hand lamps typically have two tubes with emission maxima around $\lambda$ = 254nm and 360 nm, respectively. The latter should be just right to excite AIBN and isn't cut off by Pyrex or comparable glas.

More power is obtained from a high pressure mercury lamp, such as the classical HPK 125W, but these lamps are normally used in cylindrical irradiation vessels with immersion cooling.

If you want a complete system with top- or side irradiation, replacable tubes, metal housing and ventilation have a look at the portfolio of companies like LuzChem.

It was recently pointed out to me that there's a way to do this in a principled manner. Let's assume the lamp is $4\ \mathrm W$ strong and distributes its power over 10 plates ($1000\ \mathrm{mm^2}$). The energy of a $254\ \mathrm{nm}$ photon is $7.8 \times 10^{-19}\ \mathrm J$. If the lamp is at $4\ \mathrm W$, it's putting out $4\ \mathrm{J/s}$ of energy. This corresponds to $$\frac{4\ \mathrm J}{\mathrm s}\times\frac{\text{photon}}{7.8 \times 10^{-19}\ \mathrm J} = 5.11 \times 10^{18}$$ photons per second.

Now, if the absorbance of the initiator is 0.0001, we have that the absorbed fraction of the photons is: $$\% Abs = 1 - \%T = 1 - \frac{I_1}{I_0} = 1 - 10^{-A} = 2 \times 10^{-4}$$

Since we have $5.11 \times 10^{18}$ photons per second are incoming, roughly $1 \times 10^{15}$ of them will be absorbed, which should be more than enough to initiate polymerization.

Of course, this is a very, very rough calculation, but I thought it was cool that you could, in theory, calculate that this experiment should work (which it did!)