[Phalcone42]

Hello,

I was wondering if anyone knows of any methods of attaching a silicone (such as PDMS) to solid silica surface chemically.
Alternatively does anyone know if silicones can be grown off of a silica substrate, and if so, what are the methods for doing so?

Thanks

[Enthalpy]

No-one answers? Then here are my thoughts, hoping to stimulate better answers from other members. Silica was a common substrate when I worked in microelectronics a billion years ago, but silicon rubber wasn't, so I don't know present-day usual methods.

Silicone rubber adheres naturally well to silica and glass, and silicone rubber itself is mechanically weak, so if you haven't already done it, just try pouring the silicone resin with mixed cross-linker on silica. A few random attempts at the silica surface, like washing with a strong acid, or a strong base, or solvents, may improve the adhesion. Hydrofluoric acid is a seducing option but please be very careful with it - or rather, let only try by an experienced chemist.

Grow silicone: one has to distinguish the polycondensation, which grows macromolecules that make an oil or a thick liquid, from subsequent cross-linking that creates a few chemical bonds between these macromolecules to get an elastomer. Cross-linking is often called "polymerization" improperly. Though, some trichloro reactant mixed in the dichloro can cross-link the polycondensate at the same step.

PDMS resulting often from Cl-Si(CH₃)₂-Cl and water
https://en.wikipedia.org/wiki/Silicone#Synthesis
you might imagine to modify silica's surface so it ends with Si-Cl. These would then starting points for the silicone macromolecules made there by water.

This would need subsequent cross-linking. The polycondensation produces lots of HCl or other acids, unacceptable if you plan semiconductor circuits on you silica, or if you want a silica-fibre reinforced silicone coating to protect a metal from corrosion. That many chemical bonds with silica are also overkill, because the rubber's resistance results from the few cross-links.

Or you might try to put functional groups at silica's surface that react with the cross-linking agent. Then you'd mix the silicone resin with the cross-linker and pour on the silica. There are many cross-linking methods so the answer isn't simple
http://www.essentialchemicalindustry.org/polymers/silicones.html

[Phalcone42]

Thank you for the input.

A follow up question would be how difficult do you think it is to adhere PDMS to silica at both ends? My understanding of polymers is that due to their nature, there is going to be a great deal of steric hindrance preventing the ends from reacting at the silica surface.
Any ideas for how to increase the chance of this silicone-silica "bridge" occuring? I am thinking running the reaction with a dilute solution of the PDMS in a theta solvent might help.


Btw project is making microcomposite materials.

[Enthalpy]

I hope you'll get better answers from more knowledgeable people here.

Macromolecules use to bend easily. Preventing it is difficult; this is one trick that makes para-aramide stiff and strong, that the bond pairs prevent rotations all the way and that the repeated pattern is straight.

Already alkanes of moderate length bend statistically at several C-C bonds at room temperature. My (correct?) understanding is that polysiloxanes do it much more, because the bigger silicon atom and the naked oxygen atom ease the rotations, and this explains why silicone oils have a wide temperature range. Representations of Pdms often put all methyls at the same side, but twisting much every second Si-O suppresses the hindrance by methyls and makes the chain bendable. I attach an estimation of Pdms by Pm3, for whatever credibility software has. You can see that the methyls are nicely separated.

So my prophecy would be that attaching both ends of a polysiloxane is no more difficult than a single one, because the chain twists at random permanently. A longer chain eases this.

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Will you make exotic materials for me, please? When I'm a mechanical engineer, I need unusual properties more than extreme stiffness and resistance. For instance big volume compressibility, slow sound propagation, very high or very low elastic rebound, and more.

So if your material doesn't have the property you sought, don't give it up! Search instead for other exotic attributes. It may let some gasses pass selectively, or get hugely birefringent under uniaxial pressure, cool a lot when elongated, reduce its length or volume at higher temperature, bloat in solvents, have shape memory erased by heat... Any such property would be precious in some use.

[Phalcone42]

You are correct, the Si-O bond has great freedom in bond rotation, making the RMS end-to-end distance for PDMS generally closer to a random walk then other, stiffer polymers. I guess my question was more along the lines of "does this improve or hinder the chances of a chain reacting at both ends?" I would prefer to keep a flexible chain.


In response to your exotic materials statement, I'm expecting this material to have a high fracture toughness along a wide temperature range, as well as have interesting optical properties.

[wildfyr]

Higher chain flexibility will increase the reacticity of the end group. However the storm gets I vidal fat or for this is probably the glass transition temperature. Above this, reacticity will be greatly increased.

[Phalcone42]

@wildfyr, Thanks for the input, but that is quite the typo.

However the storm gets I vidal fat or for this is probably the glass transition temperature.

I didn't catch what you were saying about the glass transition temperature, but PDMS has one of the lowest Tg's as far as polymers go (~-120°C), so I will likely be working above it.