Optimisation of Surface-Initiated Photoiniferter-Mediated Polymerisation under Confinement, and the Formation of Block Copolymers in Mesoporous Films

Abstract

Nature as the ultimate inspiration can direct, gate, and selectively transport species across channels to fulfil a specific targeted function. Harnessing such precision over local structure and functionality at the nanoscale is expected to lead to indispensable developments in synthetic channels for application in catalysis, filtration and sensing, and in drug delivery. By combining mesoporous materials with localised charge-switchable poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) brushes, precisely controlling pore filling and exploring the possibility of incorporating two different responsive polymers, we hope to approach the precision control of natural systems in the absence of an external force. Here, we report a simple one-step approach to prepare a mesoporous silica thin film with~8 nm pores functionalised with a photoiniferter by combining sol-gel chemistry and evaporation-induced self-assembly (EISA). We show that surface-initiated photoiniferter-mediated polymerisation (SI-PIMP) allows the incorporation of a high polymer content up to geometrical pore blocking by the simple application of UV light in the presence of a monomer and solvent, proceeding in a controlled manner in pore sizes below 10 nm, with the potential to tune the material properties through the formation of surface-grafted block copolymers.

Influence of Nanoconfinement on the pKa of Polyelectrolyte Functionalized Silica Mesopores

“…Although a change in the polymer amount did lead to distinguishable pK a values it has to be noted that the variation is not very pronounced and difficult to separate from the error of pKa determination. Based on previous findings this only minor influence of polymer amount variation has to be expected since the chain length variation of oligomers in SP mesoporous systems between a median filled pore (≈50%) and highly filled pores (>90%) is not larger than 2–3 repetition units as determined in a previous study 10b,11. Additionally, since the mesopore necks are only ≈3 nm wide and very probably partially blocked by polymer regardless of the polymer amount and pore filling degree ions that enter the pore are in the range of the Debye screening length and thus subjected to the strong influence of the electric double layer 15.…”

“…DMAEMA was chosen as positively charged monomer, while the negatively chargeable MEP was chosen since theoretical data on pKa shifts are available from Gilles et al,9b and because it can carry two charges per repetition unit allowing to investigate complex charge situation within the spatial confinement of mesopores. (N,N(diethylamino)dithiocarbamoylbenzyl(trimethoxy)silane (SBDC) as photoiniferter was covalently grafted to the silica mesopore wall following a previously reported and well‐established procedure allowing a controlled polymerization in nanoconfined pores 10a,b,11. This grafting process resulted into a photoiniferter density of 0.7 molecules*nm −2 as already reported in a previous study 11.…”

“…To investigate the effect of positive charge generation within the spatial confinement of silica mesopores, PDMAEMA was selected. PDMAEMA represents an ideal second candidate for this model since it is a widely used polymer in the context of responsive surface modifications 10d,23. The pH‐dependent charging and the corresponding gating behavior of PDMAEMA modified mesoporous silica is shown in Figure a.…”

“…Controlled polymerization and thus precise control on pore filling and charge density in nanopores has been demonstrated by atom transfer radical polymerization, reversible addition‐fragmentation chain transfer, or surface initiated photoiniferter polymerization 10. Only very recently it has been shown that not only the amount of polymer but also the architecture of the polymer chains can be controlled and block‐cooligomers in pores can be generated 10a,11. Thereby, pH responsive polymers are most extensively investigated 5c,12.…”

“…A wide range of pH responsive polymers, such as poly(vinylpyridine) (PVP), poly(2‐(dimethylamino)ethyl methacrylate) (PDMAEMA), poly(acrylic acid) (PAA) are reported. Additionally multicharged polymers, such as (2‐(methacryloyloxy)ethyl phosphate) (MEP), carboxybetaine methyl acrylate (CBMA) or block copolymers of PDMAEMA‐ b ‐PMEP have been polymerized in nanoporous materials and their gating properties have been studied in detail 5d,e,9c,d,10a,11–13. They have also been proven to be particularly suitable for drug‐release applications, as differences in the physiological pH value can be used and no additional external stimulus is required 14.…”