Polymer brush guided templating on well-defined rod-like cellulose nanocrystals

Maria Morits ab, Ville Hynninen a, Nonappa ac, Antoine Niederberger b, Olli Ikkala ac, André H. Gröschel d and Markus Müllner *be
aDepartment of Applied Physics, Aalto University, FI-02150 Espoo, Finland
bKey Centre for Polymers and Colloids, School of Chemistry, The University of Sydney, Sydney, New South Wales 2006, Australia
cDepartment of Bioproducts and Biosystems, Aalto University, FI-02150, Espoo, Finland
dPhysical Chemistry and Centre for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, D-45127 Essen, Germany
eThe University of Sydney Nano Institute (Sydney Nano), The University of Sydney, Sydney, New South Wales 2006, Australia. E-mail: markus.muellner@sydney.edu.au

Received 30th October 2017 , Accepted 11th November 2017

First published on 24th November 2017


Cellulose is a natural biomaterial harvested from regrowing resources and it is one of the most attractive components for the construction of functional materials with low adverse ecological impact. Among various nanocelluloses, cellulose nanocrystals (CNC) are rod-like nanoparticles whose high crystallinity and stiffness make them viable candidates for templating materials. We report here on CNC-based polymer brushes used as templates for the synthesis of porous inorganic nanorods with tunable diameters and aspect ratios. The CNC were modified with initiation sites for surface-initiated polymerisation (SI-ATRP) to act as a backbone for the grafting of poly(2-(dimethyl amino)ethyl methacrylate) (PDMAEMA) brushes. Controlled polymerisation conditions allowed for adjusting the brush length and consequently the morphology of the hybrid nanomaterials. The PDMAEMA brush served as coordination and nucleation sites for the mineralisation of tetramethyl orthosilicate (TMOS) into SiO2@CNC-g-PDMAEMA hybrids. After calcination, microscopy and N2-sorption measurements revealed hollow silica nanorods with accessible micro- and meso-pores. We foresee that this strategy can be adapted to other nanocelluloses to create high-aspect ratio porous silica nanotubes, or to achieve uniform depositions in the mineralisation of other inorganic metals or metal oxide compounds.


Introduction

Cellulose is a sustainable, naturally growing and abundant biomaterial. Cellulose has experienced renewed interested among materials scientists attributed to the emergence of nanocelluloses (NC) and their exceptional mechanical properties, versatility of their modification, and diverse physical properties depending on the cellulose source.1 NC are typically anisotropic (one-dimensional, 1D) materials exhibiting nanoscale lateral dimensions including rod-like cellulose nanocrystals (CNC), higher aspect ratio cellulose nanofibers (CNF), and 3D networks of bacterial cellulose (BC). NC cover a large range of physical properties with immense application potential, which continuously motivates their incorporation into advanced functional materials.2 The network structures and biocompatibility of aero- and hydrogels made from CNF and BC have been shown to be particularly suited for bio-applications, such as drug delivery,3 wound treatment,4 tissue engineering,5 and sensing.6 NC have also been used in thermal insulation,7 as sponge material for water purification8 or components in energy storage.9

The high mechanical strength of the crystalline domains of CNC and CNF further serve as green constituents in the spinning of high tensile strength bio-fibres or reinforcements for nanocomposites.10 Through appropriate surface modification, NC have been equipped with a range of new functionalities and have been incorporated into wearable electronics,11 magnetic sponges,12 and templating methods for hybrid material synthesis.13 While pristine NC may readily find application in self-assembly,14 photonics,15 or as liquid crystals,16 new NC modification and hybridisation protocols are in demand to improve their processing, and to tune their interface energies and compatibility with else incompatible materials.17 Surface-initiated polymerisations on NC (e.g. via atom transfer radical polymerisation, ATRP, or ring-opening polymerisation, ROP) is rapidly establishing itself as a versatile tool to alter NC properties and to incorporate innovative functionality.18 Recent studies have used various polymer grafting methods to alter surface properties and render NC rods and fibres responsive to temperature19 and pH.20 Similarly, this has broadened their field of application to nanoparticulate-based drug delivery21 and enhanced their compatibility with hydrophobic polymer matrices.22

The use of distinct polymer surfaces on nanorods and nanofibers to yield organic–inorganic hybrids or hybridised surfaces has not been explored in detail. Hybridising NC surfaces has been demonstrated by atomic layer deposition of inorganic oxides.23 Nanofibers have also been used in sol–gel chemistry, where a mesh of cellulose nanofibers has been coated with amorphous titania to enable the adsorption of a surfactant micelle layer for subsequent silica gel formation.24 Cellulose nanofibrils have been covered with silica via base-catalysed sol–gel chemistry in alcoholic solutions, resulting in the association of various silica spheres on the fibril surface. The rate of silica precursor hydrolysis was dependent on type of alcohol which in turn influenced the silica condensation. It was recently described that the use of an alkaline condensation of silica precursors for nanometre-precise templating on fibrous (1D) cellulosic structures is undesirable.25 The preferred formation of spherical silica morphologies during this process may be problematic in establishing uniform nanoscale layers – especially when using rod-like CNC. A more manageable approach is needed when modifying NC to retain their nanoscale shape after hybrid formation.

The success of templating inorganic materials through cylindrical polymer brushes (CPBs)26 inspired us to transfer a template-directed concept to CNC. Soft templates based on CPBs or molecular brushes have been used as anisotropic scaffolds to produce high surface area catalysts,27 mesostructured semiconductors,28 as well as metal oxide nanotubes.29 The precise control in CPB synthesis over cylinder length, diameter, and number of polymer shells30 provides a convenient handle to structure inorganic nanomaterials; particularly, 1D structures such as nanorods, nanowires and nanotubes. The use of core–shell CNCs would enable to mimic the structure of core–shell molecular brushes without the elaborate synthesis steps involved in CPB production (e.g. anionic polymerisation for higher aspect ratio backbones). Moreover, CNC may act as the backbone and the core material at the same time, and add further benefits of dimensional stability by the rigid crystal. The facile degradability of CNC would also provide straightforward access to hollow nanomaterials. The ecological advantages further support the utilisation of cellulose-based templates, which can open ways to new functional materials, depending on the type of NC used.

Herein we introduce CNC-based polymer brush nanoparticles as robust and sustainable templates for the fabrication of organic–inorganic hybrid nanomaterials. We synthesised core–shell nanorods consisting of a CNC core and polycationic poly(2-(dimethyl amino)ethyl methacrylate) PDMAEMA shell. We followed the modification of the CNC with Fourier-transformed infrared spectroscopy (FT-IR), proton nuclear magnetic resonance (1H-NMR), gel permeation chromatography (GPC), elemental analysis, and the brush polymerisation with added sacrificial initiator.31 The production of silica nanomaterials was achieved in 30 min in aqueous media by filling the amine-containing shell compartment with tetramethyl orthosilicate (TMOS) via in situ mineralisation. Hollow and highly porous silica nanorods were obtained after calcination. Given the vast access to colloidal NC and the tunability of their dimensions, our process can potentially be transferred to other cellulose-based materials and in combination with other inorganic components/precursors lead to libraries of shape-anisotropic hybrid materials.

Experimental section

Materials

Cellulose nanocrystals (CNC) were produced according to published procedures.32 The compounds 2-(dimethyl amino)ethyl methacrylate (DMAEMA, 98%), α-bromoisobutyryl bromide (BiBB, 98%), N,N′,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDETA, 99%), copper(I) chloride (CuCl) and tetramethyl orthosilicate (TMOS, 99%) were obtained from Sigma-Aldrich, and used as received. Deuterated dimethyl sulfoxide (d6-DMSO) was purchased from Cambridge Isotope Laboratories, Inc. DMAEMA was destabilised using a silica gel column prior to polymerisation.

Synthesis of CNC-g-PDMAEMA brushes

CNC were freeze-dried and pre-modified with BiBB through chemical vapour deposition (CVD) as suggested by earlier works.33 In a second step, the pre-modified CNC were re-dispersed in dimethylformamide (DMF) and further esterified in solution using BiBB to yield CNC-Br.33,34 PDMAEMA brushes were grafted from the CNC-Br following a procedure modified from a previously reported protocol.35 Briefly, CNC-Br (50 mg, n(Br) = 0.039 mmol) were dispersed in 20.58 mL DMF. Subsequently, DMAEMA (1.3 mL, 7.73 mmol) and PMDETA (10.1 mg, 0.06 mmol) were added and the mixture was degassed by three freeze–pump–thaw cycles (using N2) in a Schlenk flask. Then, CuCl (3.83 mg, 0.039 mmol) was added under a gentle N2-stream before the frozen mixture was again evacuated to high vacuum. The polymerisation was stopped after 60 min, cooled to room temperature and exposed to air. The product was dialysed to methanol and then to water.

Preparation of SiO2@CNC-g-PDMAEMA hybrids

A total of 42.5 mL of TMOS was added dropwise to 850 mL of an aqueous CNC-g-PDMAEMA dispersion (c = 0.25 g L−1) at 15 °C under vigorous stirring. The mixture was stirred for 30 min before it was diluted with 1 L of ethanol. The mixture was then centrifuged at 14[thin space (1/6-em)]000 rcf for 10 min and washed with ethanol and water, aided by ultrasound. Note, that the AFM studies were performed on small batches of silica hybrids whereas the TEM and surface area studies were performed on two different but larger batches.

Characterisation of polymer brushes and silica hybrids

Elemental analysis was performed by the microanalysis service at Mikroanalytisches Labor Pascher, Germany. Thermogravimetric analysis (TGA) was performed on a TA Instruments Q500. Samples were heated from 25 °C to 700 °C at a heating rate of 10 K min−1 under N2 flow, followed by purging with oxygen at 700 °C for 10 min. FT-IR measurement was performed on a Nicolet 380 instrument equipped with an ATR cell. 1H-NMR spectra were recorded in deuterated solvents using a 300 MHz Bruker Avance system at 25 °C. GPC measurements were performed on an UFLC Shimadzu Prominence system at 50 °C using DMAc/LiBr as eluent and a flow rate of 1.0 mL min−1. GPC samples were dissolved and pressed through a 200 nm Nylon filter prior to injection. The system was calibrated using PMMA standards. N2-Sorption was measured on a Tristar II (Micrometrics) and specific surface area and porosity were analysed with TriStar II 3020 software package V1.04.

Atomic force microscopy (AFM)

AFM height images were recorded in air using a Bruker Multimode-8 with tapping-mode cantilevers (48 N m−1, Tap190Al-G, Budget Sensors, Bulgaria). Prior to AFM measurement, freshly cleaved mica was dropcast with a solution of CNC-Br or CNC hybrids in methanol and immediately blown dry under a N2 stream. NanoScope analysis software was used to investigate the AFM images.

Transmission electron microscopy (TEM)

TEM images were taken with an Ultra-Scan 1000 CCD camera (Gatan) on a FEI Tecnai 12 Bio-Twin instrument in bright field mode, operating at an accelerating voltage of 120 kV. Images were processed with GMS1.3.3 (Gatan) software. Hybrids dispersed in slightly acidic water (c = 0.25 g L−1) were dropcast onto carbon-coated copper grids and blotted after 30 s with filter paper.

Cryo-TEM and electron tomography (ET)

Cryo-TEM measurements were performed on a JEM 3200FSC microscope (JEOL) at 300 kV in bright field mode. Images were acquired with a CCD camera (Gatan) using an Omega-type Zero loss energy filter. The specimen temperature was maintained at −187 °C. Vitrified samples were prepared using a FEI Vitrobot Mark IV (in 100% humidity). 3 μL SiO2@CNC-g-PDMAEMA suspension was pipetted onto a 200-mesh copper grid with holey carbon support film, blotted with filter paper and plunged into a −170 °C ethane/propane mixture. Vitrified samples were cryo-transferred to the microscope. SerialEM software package (version 3.2.2) was used to acquire cryo-ET tilt series (±69° angles with 2° increment steps) under low dose mode.36 Image pre-alignment and fine-alignment of the tilt image series, were performed using IMOD.37 Images were binned twice to minimise noise and computation time. The maximum entropy method (MEM) reconstruction scheme was performed using a custom-made script with regularisation parameter value of λ = 1.0 e−3.38 UCSF chimera package was used for the volumetric graphics and analyses.

Results and discussion

The synthetic approach towards hollow silica nanorods required three key steps which are schematically summarised in Fig. 1. Rod-shaped CNC are firstly modified with ATRP initiation sites (Fig. 1A), to graft a polymer brush from the CNC surface (Fig. 1B). The resulting core–shell polymer brush can then be used as a nanoreactor template to produce organic–inorganic hybrid nanomaterials, where the polymer shell facilitates spatial control of hybrid formation (Fig. 1C).
image file: c7py01814b-f1.tif
Fig. 1 (A) Surface-modified CNC-Br are grafted with PDMAEMA brushes via SI-ATRP to yield (B) CNC-g-PDMAEMA core–shell templates that are used as (C) nanoreactors for the selective mineralisation (hybrid formation) of silica within the PDMAEMA brush shell.

Following procedures reported by Edgar et al.,32 we produced hydrophilic CNC with an average length of L = 212 ± 53 nm, diameter of ∼7 nm (Fig. 2A) and negative ζ-potential (Fig. S1 and S2 in ESI). The surface of the pristine CNC is covered with hydroxyl groups (and few sulfate ester groups), and was modified using α-bromisobutyryl bromide (BiBB) to incorporate initiation sites for SI-ATRP. To ensure high surface coverage of ATRP initiators, the pristine CNC were modified in a two-step procedure – using BiBB in a chemical vapour deposition (CVD) and a subsequent solution procedure.33,34 The surface modification was monitored by FT-IR spectroscopy. The subsequent appearance of a new peak at 1728 cm−1 was attributed to the formation of ester bonds leading to the increase of a carbonyl peak (Fig. 2B-II). Elemental analysis revealed a bromine content on the modified CNCs (CNC-Br) of fBr = 6.18 wt%, equalling a degree of substitution (DS) of CNC surface groups of ∼34%.33,34


image file: c7py01814b-f2.tif
Fig. 2 (A) TEM of pristine CNC. The scale bar is 500 nm. (B) FT-IR analysis of pristine (0) CNC (black line), (I) and (II) CNC-Br (blue line, after CVD; green line, after solution modification) and (III) polymer-grafted CNC (red line). The developing signal at 1728 cm−1 underlines the introduction of carbonyl bonds. In addition, signals at 1450–70 cm−1 correspond to methyl groups of PDMAEMA. See Fig. S4A for a larger figure.

The CNC-Br crystals were then used as shape-anisotropic backbones and grafted with a polymer brush to yield a core–shell polymer brush template. SI-ATRP was used to graft PDMAEMA side chains. Kinetic studies revealed good control over the surface grafting process (Fig. 3A). It was previously shown that comparable molecular weights and molecular weight distribution of free and grafted polymer chains can be achieved during SI-ATRP in the sacrificial initiator approach.22a Therefore, GPC and the sacrificial initiator approach were used to qualitatively assess the polydispersity of PDMAEMA (Đ = 1.29, see Fig. S3 in ESI). Fig. 3B shows the assigned 1H-NMR spectrum of the PDMAEMA-grafted CNC (CNC-g-PDMAEMA).


image file: c7py01814b-f3.tif
Fig. 3 SI-ATRP of DMAEMA from CNC-Br. (A) Polymerisation kinetics and (B) assigned 1H-NMR spectrum of CNC-g-PDMAEMA in d6-DMSO.

Reversible activation/deactivation polymerisations, like ATRP, allow for straightforward control of polymer side chain length during the grafting process. This, in turn, enabled a modular tuning of the PDMAEMA polyelectrolyte shell. FT-IR allowed to qualitatively confirm the presence of PDMAEMA on the grafted CNC (Fig. 2B-III). Further increase of the carbonyl peak and a small shift relative to the carbonyl signal of CNC-Br confirmed the incorporation of PDMAEMA (see also Fig. S4A in ESI). Moreover, a pronounced band – corresponding to the methyl groups on the amine of PDMAEMA – appeared around 1450–1470 cm−1.

To evaluate the effect of PDMAEMA brush length on the morphology of silica shells, three core–shell polymer brushes (CNC-g-PDMAEMA) with varying degrees of polymerisation (DP) have been synthesised. The brush lengths were adjusted to DP of 10, 25, and 50, respectively. The DP was calculated using the monomer conversion and assuming that all initiation sites have been initiated during the polymerisation.

AFM was used to investigate the morphological changes of the CNC-Br after polymer grafting and hybrid formation. The observed lengths were comparable to the TEM analysis; however, AFM overestimates the size of the nanocrystals due to the tip convolution phenomena. AFM on the CNC-Br on mica confirmed the presence of rigid CNC-Br nanorods (Fig. 4A) with smooth surface topographies (see cross-sectional analysis in Fig. S5 in ESI). The AFM cross-section of polymer-grafted CNC, however, exhibited rougher surface pattern (see Fig. S6 in ESI) when compared to CNC-Br. Further differences were observed in the phase images after polymer grafting, where a core–shell structure could be observed for CNC-g-PDMAEMA50 (Fig. S6D in ESI). Shorter side chains made it difficult to differentiate the CNC core, however, the morphology changed significantly for all brush templates after the hybrid formation (Fig. 4B–D).


image file: c7py01814b-f4.tif
Fig. 4 AFM height images of (A) pristine CNC-Br and their corresponding silica hybrids, namely (B) SiO2@CNC-g-PDMAEMA10, (C) SiO2@CNC-g-PDMAEMA25, and (D) SiO2@CNC-g-PDMAEMA50. Numbered dashed lines indicate areas of cross-sectional analysis, which can be found in the ESI Fig. S5 and S7A–D. All scale bars are 200 nm.

Van Opdenbosch et al. found the typical basic condensation of TEOS to produce a silica coating on fibrous cellulosic structures as undesirable due to the preferred formation of spherical morphologies during this process.25 A recent study by Liu et al. highlights the formation of such silica spheres on top of bacterial cellulose nanofibers.39 Contrary to state-of-the-art silica shell formation in alcohols, we produced silica coated CNC nanohybrids (SiO2@CNC-g-PDMAEMA) in water at pH 7 (15 °C). By mixing an aqueous template brush suspension with a labile silica precursor TMOS for 30 min, we achieved a uniform silica coating. The tertiary amine groups of PDMAEMA can catalyse the hydrolysis of TMOS and at the same time act as a localised nanoreactor that facilitates silica condensation within the PDMAEMA shell. Weakly negatively-charged transitional complexes are formed during TMOS hydrolysis and condensation (such as silicic acids),25,40 which then interact with the protonated tertiary amines of PDMAEMA. Given the timeframe and conditions of this coating method, SiO2 formation occurs favourably in the polymer shell – which in turn avoids condensation reactions between single template particles or the formation of single SiO2 nanoparticles.

Further AFM studies of the above-mentioned core–shell CNC-g-PDMAEMAn (n = 10, 25, 50) brushes were used to investigate the effect of side chain length on the hybrid formation. Height images (Fig. 4) and their corresponding cross-sectional analyses (see Fig. S7 in ESI, and dashed lines in Fig. 4) revealed increasingly rough surfaces on the hybrid nanorods; in contrast to the height profile of pristine CNC-Br (see Fig. S7A). In addition, the overall height and thickness of the hybrids increased with growing polymer shells, suggesting that the thickness of the silica shell can be adjusted via the overall length of the polymer brush. A similar phenomena was observed in the use of molecular brush templates, where very long polyelectrolyte chains were able to induce topographical bundle formation to yield SiO2 structures with a spiked surface.27 While the AFM analysis suggest increasing surface roughness, TEM images underlined the formation of uniform silica coatings of individual CNC (Fig. 5 and Fig. S8 in ESI). Similarly, FT-IR measurements of the silica nanohybrids showed strong signals for SiO2 (Fig. S4B in ESI) suggesting that the polymer shell is mostly embedded within the SiO2 network structure.


image file: c7py01814b-f5.tif
Fig. 5 (A) TEM, (B) cryo-TEM, and (C/D) SEM images of SiO2@CNC-g-PDMAEMA15. Scale bars are (A/B) 100 nm, (C) 500 nm and (D) 200 nm. The inset in (B) shows the diameter across one SiO2@CNC hybrid using a grey-scale analysis. Additional analysis can be found in ESI S8.

CNC are known for its extraordinary stiffness, providing a very robust and stiff core within the template brush. The CNC core was easily observed in TEM, due to the comparatively lower contrast of the organic cellulose in comparison to the inorganic silica shell. Due to the relatively uniform CNC core, all hybrid nanorods showed a comparable diameter of the core compartment. The inset in Fig. 5A highlights the core–shell structure of as-synthesised SiO2@CNC-g-PDMAEMA15 hybrids. Note, these nanohybrids are not hollow until further heat treatment.

The SiO2@CNC-g-PDMAEMA15 hybrids were further investigated using cryo-TEM, grey-scale analysis and scanning electron microscopy (SEM). Cross-sectional analysis on the cryo-TEM image in Fig. 5B indicated that the diameter of the silica hybrids was in range of 29–36 nm, which is significantly larger than the diameter of the CNC-Br before the modification steps. Core diameters derived from grey-scale analysis on dried SiO2 hybrids matched the average diameter of pristine CNC (∼7 nm, Fig. 2A; compared to 6–9 nm in Fig. S8 in ESI), and revealed a continuous silica shell thickness of 12–16 nm. SEM images highlighted the general porosity of the dried SiO2@CNC-g-PDMAEMA15 hybrid materials (Fig. 5C and D). Qualitatively, the loosely packed aerogel-like networks of hybrid nanorods indicated various cavities and pore sizes at the largest scale. Quantitatively, N2-physisorption experiments were performed to quantify the specific surface areas and pore sizes of the hybrid materials. The measurements revealed relatively low accessible surface area for the as-synthesised hybrid material (Fig. 6A, black plots). After calcination, i.e. the removal of all organic matter within the hybrids, the surface area increased drastically (Fig. 6A, red plots). N2-Physisorption measurements indicated the appearance of micro-pores at lower relative pressures. This highlights the removal of polymer brush side chains, leading to microporous channels <2 nm and an increase in specific surface area. Similarly, meso-pores have developed after calcination, as shown by the prominent hysteresis in the physisorption plots (Fig. 6A, red plots). The appearance of meso-pores is largely attributed to the removal of the CNC core, providing a meso-cavity that is accessible through the formed micro-pores.


image file: c7py01814b-f6.tif
Fig. 6 (A) N2-Physisorption measurements of as-synthesised (black) and calcined (red) SiO2@CNC-g-PDMAEMA15 (1) and SiO2@CNC-g-PDMAEMA25 (2). (B) TGA measurements of SiO2@CNC-g-PDMAEMA15 (green, dashed line) and SiO2@CNC-g-PDMAEMA25 (blue, solid line). Insets show respective TEM images of as-synthesised hybrids. Scale bars are 100 nm.

Thermogravimetric analysis (TGA) was used to determine the weight content of the inorganic material, revealing that high amounts of silica were incorporated within brush templates; 68% for SiO2@CNC-g-PDMAEMA15 and 69% for SiO2@CNC-g-PDMAEMA25 (Fig. 6B). Although the calcined nanomaterials revealed no major overall change in morphology in TEM (Fig. 7), the development of highly porous materials was evident in the surface area measurements of the calcined materials. Table 1 summarises the specific surface areas and the pore sizes of the as-synthesised and calcined nanomaterials. Fig. 7B shows a 3D electron tomographic reconstruction (ET) of a calcined (hollow) nanorod, where the large internal meso-pore becomes visible. The pore diameters of the hollow SiO2 nanorods obtained via the N2-physisorption measurement mirrored the previously established CNC diameters of ∼7 nm (see Fig. S9 in ESI).


image file: c7py01814b-f7.tif
Fig. 7 (A) TEM image of calcined SiO2@CNC-g-PDMAEMA15. (B) 3D ET reconstruction of one hollow SiO2@CNC-g-PDMAEMA15 nanorod. The scale bars are (A) 100 nm and (B) 10 nm.
Table 1 Overview of surface areas and pore size distributions of silica hybrids before and after calcination
Hybrid composition Surface areaa (as-synthesised) [m2 g−1] Surface areaa (calcined) [m2 g−1] Micropore volumeb (as-synthesised) [cc g−1] Micropore volumeb (calcined) [cc g−1] Pore volumec (as-synthesised) [cc g−1] Pore volumec (calcined) [cc g−1]
a Single point surface area. b BET. c BJH adsorption cumulative surface area of pores.
SiO2@CNC-g-PDMAEMA15 74 494 0.0674 0.2820 0.6272
SiO2@CNC-g-PDMAEMA25 106 551 0.00003 0.0678 0.3715 0.6867


Conclusions

We demonstrated the use of a sustainable colloidal rod template material in the production of tailor-made hybrid and inorganic nanomaterials. By proof-of-concept, we developed an effective template-directed synthesis of micro- and meso-porous silica nanorods by grafting CNC with PDMAEMA brushes followed by the formation of SiO2 networks within the polymer brush layer. Subsequent calcination yielded highly porous and hollow nanorods which may find potential application as insulating aero-gels, catalyst supports or storage and delivery systems. The versatility of the polymer surface grafting allows the process to be extended to graft other functional polymers and to produce customised template brushes. Inspired by the templating of various functional hybrids through molecular brushes, we foresee possibilities for CNC, tunicates and cellulose nanofibers to become versatile templating materials with well-defined dimensions and improved template stability. The incorporation of CNC into polymer nanoparticle-based structuring of inorganic matter enables straightforward production procedures, reduces synthesis steps and effort, while incorporating scalable materials of the future.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

We thank Leena Nolvi for the help with N2-sorption measurements. The authors made use of instrumentation at the Key Centre for Polymers and Colloids (KCPC, University of Sydney), the Aalto University Nanomicroscopy Center (Aalto-NMC) premises and the Aalto University Bioeconomy Facilities. M. Morits is grateful for financial support from the Tiina and Antti Herlin Foundation. O. Ikkala acknowledges the Academy of Finland's Centre of Excellence Programme (2014–2019) and support from the ERC-2011-AdG (291364-MIMEFUN). A. H. Gröschel acknowledges Evonik Industries and the German Research Foundation (DFG) for support through an Emmy Noether Independent Junior Research Group (2017–2022, #376920678). M. Müllner acknowledges support from the Selby Research Foundation and the Australian Nanotechnology Network Overseas Travel Fellowship.

Notes and references

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Footnote

Electronic supplementary information (ESI) available: CNC ξ-potential and length distribution, GPC of sacrificial PDMAEMA, FT-IR analysis, additional AFM, TEM and cross-section analyses, pore distributions, and cryo-electron tomography tilt series of silica nanostructures. See DOI: 10.1039/c7py01814b

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