Methyl methacrylate polymerization in nanoporous confinement

Polymerization within Nanoporous Anodized Alumina Oxide Templates (AAO): A Critical Survey

In the last few years, the polymerization of monomers within the nanocavities of porous materials has been thoroughly studied and developed, allowing for the synthesis of polymers with tailored morphologies, chemical architectures and functionalities. This is thus a subject of paramount scientific and technological relevance, which, however, has not previously been analyzed from a general perspective. The present overview reports the state of the art on polymerization reactions in spatial confinement within porous materials, focusing on the use of anodized aluminum oxide (AAO) templates. It includes the description of the AAO templates used as nanoreactors. The polymerization reactions are categorized based on the polymerization mechanism. Amongst others, this includes electrochemical polymerization, free radical polymerization, step polymerization and atom transfer radical polymerization (ATRP). For each polymerization mechanism, a further subdivision is made based on the nature of the monomer used. Other aspects of "in situ" polymerization reactions in restricted AAO geometries include: conversion monitoring, kinetic studies, modeling and polymer characterization. In addition to the description of the polymerization process itself, the use of polymer materials derived from polymerization in AAO templates in nanotechnology applications, is also highlighted. Finally, the review is concluded with a general discussion outlining the challenges that remain in the field.

Manipulating Conjugated Polymer Backbone Dynamics through Controlled Thermal Cleavage of Alkyl Sidechains

The morphological stability of an organic photovoltaic (OPV) device is greatly affected by the dynamics of donors and acceptors occurring near the device's high operational temperature. These dynamics can be quantified by the glass transition temperature (T_g ) of conjugated polymers. Because flexible side chains possess much faster dynamics, the cleavage of the flexible alkyl side chains will reduce chain dynamics, leading to a higher T_g . In this work, we systematically study the T_g for conjugated polymers with controlled sidechain cleavage. Isothermal annealing of polythiophenes featuring thermally-cleavable side chains at 140 °C, a temperature that is below the melting point of polymers, was found to remove more than 95% of alkyl sidechains in 24 hours, and raise the backbone T_g from 23 to 75 °C. Coarse grain molecular dynamics simulations were used to understand the T_g dependence on side chain cleavage. X-ray scattering indicates the relative degree of crystallization remains constant over the course of isothermal annealing. The effective conjugation length is not influenced by thermal cleavage; however, the density of chromophore is doubled after the complete removal of alkyl side chains. The combined effect of enhancing T_g and conserving crystalline structures during the thermal cleavage process could provide a pathway to improving the stability of optoelectronic properties in future OPV devices. This article is protected by copyright. All rights reserved.

In Situ Synthesis of Poly(butyl methacrylate) in Anodic Aluminum Oxide Nanoreactors by Radical Polymerization: A Comparative Kinetics Analysis by Differential Scanning Calorimetry and 1H-NMR

In this work, we explore the ability to generate well-defined poly(butyl methacrylate) (PBMA) nanostructures by “in situ” polymerization of butyl methacrylate monomer (BMA). PBMA nanostructures of high and low aspect ratios have been successfully obtained through the free radical polymerization (FRP) of a BMA monomer in anodic aluminum oxide (AAO) nanoreactors of suitable size. A polymerization kinetics process has been followed by differential scanning calorimetry (DSC) and proton Nuclear Magnetic Resonance spectroscopy (¹H-NMR).The determination of the kinetics of polymerization through DSC is based on a quick and direct analysis of the exothermic polymerization process, whereas the analysis through ¹H-NMR also allows the unambiguous chemical analysis of the resulting polymer. When compared to bulk polymerization, both techniques demonstrate confinement effects. Moreover, DSC and ¹H-NMR analysis give the same kinetics results and show a gel-effect in all the cases. The number average molecular weight (Mn) of the PBMA obtained in AAO of 60–300 nm are between 30·10³–175·10³ g/mol. Even if the Mn value is lower with respect to that obtained in bulk polymerization, it is high enough to maintain the polymer properties. As determined by SEM morphological characterization, once extracted from the AAO nanoreactor, the polymer nanostructures show controlled homogeneous aspect/size all throughout the length of nanopillar over a surface area of few cm². The Young’s modulus of low aspect ratio PBMA nanopillars determined by AFM gives a value of 3.1 ± 1.1 MPa. In this work, a 100% of PBMA polymer nanostructures are obtained from a BMA monomer in AAO templates through a quick double process: 30 min of monomer immersion at room temperature and 90 min of polymerization reaction at 60 °C. While the same nanostructures are obtained by polymer infiltration of PBMA at 200 °C in about 6 h, polymerization conditions are much softer than those corresponding to the polymer infiltration process. Furthermore, the ¹H-NMR technique has been consolidated as a tool for studying the kinetics of the copolymerization reactions in confinement and the determination of monomer reactivity ratios.

Initiator-dependent kinetics of lyotropic liquid crystal-templated thermal polymerization

In this study, we show that how the locus of initiation can change kinetics and mechanical properties of polymerized lyotropic liquid crystals.

Poly(diethylene glycol methylether methacrylate) Brush-Functionalized Anodic Alumina Nanopores: Curvature-Dependent Polymerization Kinetics and Nanopore Filling

We report on the synthesis and characterization of poly(diethylene glycol methylether methacrylate) (PDEGMA) brushes by surface-initiated atom transfer radical polymerization inside ordered cylindrical nanopores of anodic aluminum oxide with different pore radii between 20 and 185 nm. In particular, the dependence of polymerization kinetics and the degree of pore filling on the interfacial curvature were analyzed. On the basis of field emission scanning electron microscopy data and thermal gravimetric analysis (TGA), it was concluded that the polymerization rate was faster at the pore orifice compared to the pore interior and also as compared to the analogous reaction carried out on flat aluminum oxide substrates. The apparent steady-state polymerization rate near the orifice increased with decreasing pore size. Likewise, the overall apparent polymerization rate estimated from TGA data indicated stronger confinement for pores with increased curvature as well as increased mass transport limitations due to the blockage of the pore orifice. Only for pores with a diameter to length ratio of ∼1, PDEGMA brushes were concluded to grow uniformly with constant thickness. However, because of mass transport limitations in longer pores, incomplete pore filling was observed, which leads presumably to a PDEGMA gradient brush. This study contributes to a better understanding of polymer brush-functionalized nanopores and the impact of confinement, in which the control of polymer brush thickness together with grafting density along the nanopores is key for applications of PDEGMA brushes confined inside nanopores.

Thermal Degradation of Polystyrene under Extreme Nanoconfinement

Extreme nanoconfinement has been shown to significantly affect the properties of materials. Here we demonstrate that extreme nanoconfinement can significantly improve the thermal stability of polystyrene (PS) and reduce its flammability. Capillary rise infiltration (CaRI) is used to infiltrate PS into films of randomly packed silica nanoparticles (NPs) to produce highly confined states. We demonstrate that as the NP size is decreased, increasing the degree of confinement, the isothermal degradation time is dramatically increased, by up to a factor of 30 at 543 K for PS confined in ∼3 nm pores. The activation energy of PS degradation is also increased, by 50 kJ/mol in the most confined state (∼3 nm pores). We demonstrate that the degradation proceeds through the film surface and from the center of large holes toward NP surfaces, indirect evidence that the process is diffusion limited. The surface-driven process dramatically reduces char formation even in large NP packings that show no significant changes in their dynamics and glass transition temperature (T_g) compared to the bulk.

Polymerization in soft nanoconfinement of lamellar and reverse hexagonal mesophases

This work describes the kinetics of thermal polymerization in nanoconfined domains of lyotropic liquid crystal (LLC) templates by using chemorheological studies at different temperatures. We investigate lamellar and reverse hexagonal LLC phases with the same concentration of the monomeric phase. Results show that the mesophase structures remain intact during thermal polymerization with very slight changes in the domain size. The polymerization rate decreases in the nanoconfined structure compared to the bulk state due to the segregation effect, which increases the local monomer concentration and enhances the termination rate. Additionally, the polymerization rate is faster in the studied reverse hexagonal systems compared to the lamellar ones due to their lower degree of confinement. A higher degree of confinement also induces a lower monomer conversion. Differential scanning calorimetry confirms the obtained results from chemorheology.

Effect of Graphene oxide or Functionalized Graphene Oxide on the Copolymerization Kinetics of Styrene/n-butyl Methacrylate

Nanocomposite materials based on copolymers of styrene and n-butyl methacrylate with either graphene oxide (GO) or functionalized graphene oxide (F-GO) were synthesized using the in-situ bulk radical copolymerization technique. Reaction kinetics was studied both experimentally and theoretically using a detailed kinetic model also taking into account the effect of diffusion-controlled phenomena on the reaction kinetic rate constants. It was found that the presence of GO results in lower polymerization rates accompanied by the synthesis of copolymers having higher average molecular weights. In contrast, the presence of F-GO did not seem to significantly alter the conversion vs time curves, whereas it results in slightly lower average molecular weights. The first observation was attributed to side reactions of the initiator primary radicals with the hydroxyl groups on the surface of GO, resulting in lower initiator efficiency, whereas the second to grafted structures formed from copolymer macromolecules on the F-GO surface. The copolymerization model predictions including MWD data were found to be in satisfactory agreement with the experimental data. At least four adjustable parameters were employed and their best-fit values were provided.

The application of spatially restricted geometries as a unique route to produce well-defined poly(vinyl pyrrolidones) via free radical polymerisation

We report, for the first time, the metal-free green synthesis of linear poly(vinyl pyrrolidone) (PVP) homopolymers of molecular weight higher than 100 kg mol-1 and narrow dispersities via thermal and photo-induced free radical polymerisation carried out within alumina nanoporous membranes acting as "nanoreactors".

Determination of homopolymerization kinetics and copolymerization with methyl methacrylate of diethyl 9-(methacryloyloxy)-2-oxo-nonylphosphonate, 9-(methacryloyloxy)-2-oxo-nonylphosphonic acid and diethyl 9-(methacryloyloxy)-nonylphosphonate

The three polymerizable monomers diethyl 9-(methacryloyloxy)-2-oxo-nonylphosphonate M1, 9-(methacryloyloxy)-2-oxo-nonylphosphonic acid M2 and diethyl 9-(methacryloyloxy)-nonylphosphonate M3 are examined for their free radical polymerization and copolymerization activity in methanol between 40 and 65°C. Polymerization proceeds readily through a thermal free radical initiation. The intensity exponents for the monomer and initiator are only slightly over 1 and approximately 0.5, respectively. This is in accordance with the results typically observed for an ideal free radical polymerization with bimolecular termination. The kinetics of copolymerization with methyl methacrylate (MMA) are monitored by online 1H nuclear magnetic resonance (NMR) spectroscopy. Two copolymerization reactions for each pair of co-monomers are sufficient to evaluate the kinetic data using the Jaacks method, the Fineman-Ross method and the nonlinear least squares method. All three methods give similar results for particular monomer/MMA couples.

Studies on the hard confinement effect on the RAFT polymerization of a monomeric ionic liquid. Unexpected triggering of RAFT polymerization at 30 °C

For the first time, the RAFT polymerization of a monomeric ionic liquid under hard confinement was successfully carried out.

Polymerization of Monomeric Ionic Liquid Confined within Uniaxial Alumina Pores as a New Way of Obtaining Materials with Enhanced Conductivity

Broadband dielectric spectroscopy (BDS) and differential scanning calorimetry (DSC) have been employed to probe dynamics and charge transport of 1-butyl-3-vinylimidazolium bis(trifluoromethanesulfonyl)imide ([bvim][NTf₂]) confined in native uniaxial AAO pores as well as to study kinetics of radical polymerization of the examined compound as a function of the degree of confinement. Subsequently, the electronic conductivity of the produced polymers was investigated. As observed, polymerization carried out at T = 363 K proceeds faster under confinement with some saturation effect observed for the sample in pores of smaller diameter. Obtained results were discussed in the context of the very recent reports showing that the free volume of the confined material is higher with respect to the bulk one. It was also noted that conductivity of poly[bvim][NTf₂] is significantly higher with respect to the macromolecules obtained upon bulk polymerization. Moreover, charge transport of the confined macromolecules is even higher when compared to the bulk monomeric ionic liquid at some thermodynamic conditions. Additionally, the molecular weight, M_w, of the confined-synthesized polymers is significantly higher with respect to the bulk-synthesized material. Interestingly, both parameters, (i) the enhancement of σ_dc and (ii) the increase in M_w, can be tuned and controlled by the application of the appropriate confinement. Consequently, those results are quite promising in the context of development of the fabrication of polymerized ionic liquids (PILs) nanomaterials with unique properties and morphologies, which can be further easily applied in the field of nanotechnology.