Emma
Larsson
a,
Samuel A.
Pendergraph
a,
Tahani
Kaldéus
b,
Eva
Malmström
a and
Anna
Carlmark
*a
aKTH Royal Institute of Technology, School of Chemical Science and Engineering, Department of Fibre and Polymer Technology, Teknikringen 56, SE-100 44 Stockholm, Sweden. E-mail: annac@kth.se; Fax: +468 790 8283; Tel: +468 790 8027
bKTH Royal Institute of Technology, School of Chemical Science and Engineering, Wallenberg Wood Science Center, Teknikringen 56, SE-100 44 Stockholm, Sweden
First published on 24th December 2014
The photoinduced controlled radical polymerisation (CRP) technique has been utilised to graft methyl acrylate (MA) and di(ethylene glycol) ethyl ether acrylate (DEGA) from filter paper. Grafting of MA was performed from α-bromoisobutyryl bromide functionalised papers. The amount of polymer grafted on the surface could be regulated by modifying the target DP of the reaction. SEC of cleaved linear polymer grafts showed that the grafting from filter papers proceeded with different kinetics compared to polymerisation from a free initiator added to the reaction mixture, resulting in higher dispersity. Furthermore, filter papers were polymerised with α-chloro-ε-caprolactone by surface-initiated ring opening polymerisation, yielding linear grafts containing initiating functions through-out the main chain. This functionality was subsequently utilised for the photoinduced CRP grafting of DEGA, yielding a graft-on-graft structure, which resulted in a thermoresponsive cellulose surface.
Atom transfer radical polymerisation (ATRP) is one of the most utilised controlled polymerisation methods as it can be employed to polymerise a wide variety of monomers. End group fidelity is maintained throughout polymerisation, making it an attractive method for the synthesis of block copolymers and polymers with advanced architectures.1,6–9 Guan and Smart discovered that visible light had an effect on the ATRP of methyl methacrylate (MMA).15 They observed that the rate of polymerisation was increased by light compared to when the reaction was performed in the dark. They also discovered that there was an increase in the control of the polymerisation when it was exposed to light during the reaction. More recently, several studies using ultra-violet (UV) and visible light have been demonstrated to trigger metal complex mediated radical polymerisations. Examples of this include work by Hawker and co-workers, in which a bipyridine/iridium complex was used to absorb visible light and control the reaction.16,17 Matyjaszewski and co-workers utilised pyridine-based ligands with copper bromide to polymerise methacrylates with sunlight/visible light.18,19 While these methods were effective, their synthetic procedures required unconventional ligands. Yagci and co-workers reported the first photoinduced controlled radical polymerisation (CRP) utilising common ATRP reactants, polymerising MMA using copper(II) bromide (Cu(II)Br2) and N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDETA) to form the catalyst complex.20,21 Haddleton and co-workers have recently reported the use of a system based on Cu(II)Br2 and tris(2-(dimethylamino)ethyl)amine (Me6TREN) to form a catalyst complex, used for the CRP of methyl acrylate (MA) in DMSO by exposure to UV light.22 The system is similar to a SET–LRP system previously utilised by the Haddleton group.23–25 To further investigate potential applications of this technique, MA polymerisation was also performed in a continuous flow reactor.26 To broaden the scope of this polymerisation technique, Haddleton and co-workers investigated additional solvents and monomers that can be utilised.27 While metal complex-mediated photoinduced CRP has produced well-defined polymers in solution, there are no reports on the effect of grafting polymers on cellulose surfaces utilising this technique.
ATRP and ARGET-ATRP have previously been employed for the grafting of cellulose surfaces in several studies.7–9,28–31 Barner-Kowollik and co-workers have reported on the utilisation of light to graft polymeric chains onto cellulosic surfaces but, as has been shown, “grafting-from” cellulose typically results in higher grafting densities than “grafting-to” cellulose.11,12 The method developed by Haddleton and co-workers, utilising UV light to induce polymerisation, is an attractive alternative for the “grafting-from” of a surface, as it facilitates the possibility to control the chain growth by turning the light source off and on. Furthermore, it requires smaller amounts of copper than traditional ATRP, while still resulting in high conversions and low dispersities (Đ) of the final polymer. In this work, we present the first use of this technique from a cellulose substrate. In addition, we characterise and compare free polymers formed from the sacrificial initiator in solution with the grafted polymer by initiating polymerisation on the surface from a cleavable initiator. The grafted surfaces were characterised by field emission scanning electron microscopy (FE-SEM), Fourier transform infrared spectroscopy (FT-IR), contact angle measurements (CAM) and thermogravimetric analysis (TGA).
In order to increase the functionality on the cellulose substrate, surface initiated ring-opening polymerisation (SI-ROP) was employed to graft ε-caprolactone (ε-CL) and α-chloro-ε-caprolactone (αClεCL) from the surface of filter paper. The naturally inherent OH-groups found in cellulose were utilised as initiators for this polymerisation. There are several publications where a cyclic monomer/lactone, containing a halogen functionality in the α-position, has been polymerised by ROP, which allows for further derivatisation by ATRP.32–36 Pan and co-workers copolymerised αClεCL and ε-caprolactone (ε-CL) in different proportions followed by ARGET-ATRP of N-isopropylacrylamide (NIPAAm), producing graft copolymers with well-defined structures.32 A similar approach was utilised herein to create a polymer brush from the cellulose surface, containing numerous ATRP-initiating sites. The sites were subsequently utilised for the photoinduced CRP of di(ethylene glycol) ethyl ether acrylate (DEGA), in a graft-on-graft approach, resulting in a thermoresponsive cellulose paper.
1H NMR spectra were recorded at room temperature with a Bruker Avance 400 MHz spectrometer, using CDCl3 solvent. Tetramethylsilane (TMS) and the solvent residual peak were used as internal standards.
Fourier transform infrared spectroscopy (FT-IR) was performed using a Perkin-Elmer Spectrum 2000 FT-IR equipped with a MKII Golden Gate, single reflection ATR System from Specac Ltd, (London, UK). The ATR-crystal used was a MKII heated Diamond 45° ATR Top Plate. For each spectrum 16 scans were recorded.
Size exclusion chromatography (SEC) was performed using two separate systems. System 1: A TOSOH EcoSEC HLC-8320GPC system equipped with an EcoSEC RI detector and three columns (PSS PFG 5 μm; Microguard, 100 Å, and 300 Å) (MW resolving range: 300–100000 Da) from PSS GmbH was used for the analysis. Dimethylformamide (DMF) (0.2 mL min−1, 50 °C) was used as the mobile phase. A conventional calibration method was employed using narrow linear poly(methyl methacrylate) (PMMA) standards (800–1600000 Da). Corrections for flow rate fluctuations were made using toluene as an internal standard. PSS WinGPC Unity software version 7.2 was used to process data. System 2: A Verotech PL-GPC 50 Plus system equipped with a PL-RI detector and two PLgel 5 μm MIXED-D (300 × 7.5 mm) columns from Varian was used for the analysis. Chloroform (CHCl3) (1 mL min−1, 30 °C) was used as the mobile phase. A conventional calibration method was employed using narrow polystyrene standards (PS) (162–371100 Da). Corrections for flow rate fluctuations were made using toluene as an internal standard. Cirrus GPC Software was used to process data.
Field emission scanning electron microscopy (FE-SEM) images were recorded on a Hitachi S-4800 FE-SEM. The samples were mounted on a substrate with carbon tape and coated with 5 nm of palladium.
Contact angles were measured at 50% RH and 23 °C on a KSV instrument CAM 200 equipped with a Basler A602f camera, using 5 μL droplets of Milli-Q water.
Sample namea | DPtarget | Conversionb (%) | M n, theo (g mol−1) | M n, SEC (g mol−1) | Đ |
---|---|---|---|---|---|
a Samples have been denoted as follows: BiB-PMAx(y) where x represents the reactions target DP and y = 1 or 2 denotes duplicates. S–S before the sample name indicates that the free polymer has been formed in the reaction with a cleavable initiator attached to the filter paper. Cleaved S–S before the sample name represents cleaved polymer for the respective reactions. b Monomer conversion calculated from 1H NMR. c M n, theo calculated from the conversion according to 1H NMR assuming 100% initiator efficiency. d Results obtained from DMF-SEC. | |||||
BiB-PMA300(1) | 300 | 88 | 22700 | 24100 | 1.1 |
BiB-PMA300(2) | 300 | 84 | 21700 | 29000 | 1.1 |
BiB-PMA600(1) | 600 | 91 | 47000 | 61300 | 1.1 |
BiB-PMA600(2) | 600 | 88 | 45500 | 59600 | 1.1 |
S–S BiB-PMA300(1) | 300 | 89 | 23000 | 28800 | 1.1 |
Cleaved S–S BiB-PMA300(1) | 300 | — | — | 64000 | 1.3 |
S–S BiB-PMA300(2) | 300 | 78 | 20100 | 27200 | 1.1 |
Cleaved S–S BiB-PMA300(2) | 300 | — | — | 38400 | 1.4 |
S–S BiB-PMA600(1) | 600 | 73 | 37700 | 41300 | 1.1 |
Cleaved S–S BiB-PMA600(1) | 600 | — | — | 47900 | 1.5 |
S–S BiB-PMA600(2) | 600 | 81 | 41800 | 49700 | 1.1 |
Cleaved S–S BiB-PMA600(2) | 600 | — | — | 66600 | 1.4 |
The appearance of the carbonyl peak (1730 cm−1) in the FT-IR spectrum (Fig. 1) of the grafted filter papers clearly showed that filter papers had been grafted with the polymer. The carbonyl peak intensity in the spectrum revealed that all the papers had been grafted with relatively large amounts of the polymer. The agreement in the intensity of the peaks from duplicate samples demonstrated that there was more polymer grafted on the filter papers grafted in the reactions with target DP 600, showing that it was possible to regulate the amount of grafted polymer by altering the target DP using a sacrificial initiator. The intensity of the carbonyl peak in the spectrum was significantly higher than the typical intensities reported for polymer grafted filter paper,28,29,40–42 showing the efficiency of the photoinduced CRP method. CAM of the grafted papers showed that they were hydrophobic with contact angles above 120°. However, it should be noted that the analysed filter paper is a rough inhomogeneous material and the contact angles should only be considered as indications of the change in hydrophobicity and not as absolute values. The structures of the grafted filter papers were also compared to BiB modified filter papers treated in the same manner as the grafted filter papers (Fig. 2). This treatment caused no change in the FT-IR spectra or CAM and hence behaved similar to untreated filter paper. Similarly, unmodified filter paper present during polymerisation from a sacrificial initiator showed no carbonyl peak after being subjected to the same washing procedure as the grafted filter papers. Grafted and unmodified filter papers were analysed by FE-SEM (Fig. 2) and it appeared that the grafting of PMA from the filter papers created an open fibre structure. However, this was believed to be caused by the DMSO solvent.
DMSO is known to cause swelling of cellulose fibres and the theory is that the swelling facilitates the possibility for larger amounts of polymer to be grafted due to a larger available surface area, and as the polymerisation proceeded it started to disintegrate the fibre network. This idea was supported by investigating traditional ATRP for the grafting of MA in DMSO from filter paper, which also resulted in a large amount of grafted polymer compared to previously reported results for MA grafting in other organic solvents.28,29
Scheme 1 UV induced controlled radical polymerisation of MA from S–S BiB modified filter paper and subsequent cleavage of the grafted chains with DTT. |
The chains grafted from the S–S BiB modified filter papers were cleaved by the utilisation of DTT, as previously reported.38 FT-IR spectra (Fig. S1†) of the filter papers before and after cleavage revealed that the intensity of the carbonyl peak was strongly reduced after cleavage indicating that a majority of the polymer had been cleaved off the surface. However, a small signal from the carbonyl remained which suggested that some polymers remained on the substrates. Additional cleavage attempts were performed with TCEP and also DTT in DMSO, but it was not possible to remove any additional polymer. A plausible explanation to this could be that some of the grafting occurred inside of the swollen fibre wall, and that the total removal of this polymer was not achieved under the chosen experimental conditions, even when the polymer had been cleaved from the surface. FE-SEM images of the S–S BiB modified filter papers (Fig. S2†), before and after cleavage, support the result from the FT-IR that most of the polymer was removed from the filter paper during the cleaving reactions.
SEC analysis of the cleaved polymer showed that the polymerisation from the surface did not result in well-defined polymers as the bulk polymerisations. The polymer cleaved from the surface had a higher dispersity and also a higher molecular weight than the polymer formed from the sacrificial initiator. This result was not in agreement with the earlier study performed using ARGET-ATRP where there was a good correlation between the grafted polymer and the polymer formed from a sacrificial initiator.38 A difference between the previous study and this study was that the polymer in this study was not fully cleaved from the surface. However, as the grafted polymer showed a higher dispersity, with a higher molecular weight than the polymer formed from the sacrificial initiator (Fig. S4 and S5†), it seems unlikely that the removal of all polymers would decrease the dispersity. A possible explanation for the difference between the studies could be that the Cu(I) species, which in this study was created due to UV-light exposure, reacted quickly after formation. Effectively, this would have caused the species to react more readily on the areas of the filter papers most exposed to the UV-light. Another possible explanation for the higher dispersity of the grafted polymer chains could be that the swelling, followed by disintegration of the fibres, continuously caused more initiating sites to be exposed on the filter paper surface. This in turn would have resulted in a continuous initiation of new polymer chains from the surface.
CAM of the grafted and cleaved filter papers showed that the grafted papers had a hydrophobic surface with stable contact angles above 120°. The cleaved filter papers were highly hydrophilic and adsorbed water drops instantly (Table S1†) which further corroborates the cleavage of the PMA chains from the surface.
The reaction scheme for the copolymerisation grafting of αClεCL and εCL, and the subsequent grafting of DEGA, from filter paper can be seen in Scheme 2. Conversions, molecular weights and dispersities of the free polymers were determined from 1H NMR and SEC (Table 2, Fig. S6–S8†). The target DP for all three monomer compositions for the copolymerisation of αClεCL and εCL was 100. The final molecular weights of the polymers formed from the sacrificial initiator were significantly lower for all three polymerisations than the theoretical molecular weight calculated from 1H NMR. This indicated that water may have been present and initiated polymerisation or caused hydrolysis, despite careful drying of all reactants prior to the polymerisations. The rather broad dispersities of the final polymers from the ROP indicated that the polymerisations were not well controlled. However, a good control of the ROP was not the scope of this study, and was therefore not of major concern. The monomer composition of the polymers, determined by 1H NMR, was in good agreement with the ratio of the co-monomer feed, showing that it was possible to regulate the ratio of Cl units incorporated in the final polymers. This result was important, as the incorporated Cl units were further used as initiators for the polymerisation of DEGA. The grafted filter papers were analysed by FT-IR (Fig. 3). As shown from the peak of the carbonyl in the spectra, polymers have been grafted from all three filter papers. Despite the relatively low intensity of the carbonyl peak, the grafted papers were hydrophobic when analysed with CAM. In FE-SEM images of the P(αClεCL-co-εCL) grafted filter papers (Fig. 4) the fibrillar structure of the fibres is still visible after grafting, also indicative of only small amounts of grafted polymer. Unfortunately, it was not possible to determine the monomer ratio in the polymer grafted from the filter papers, as the grafted polymer could not be separated from the cellulose. It was assumed that the monomer ratio of the grafted polymer was the same as for the polymer formed in bulk.
Scheme 2 Surface initiated ring opening polymerisation of αClεCL and εCL followed by UV induced controlled radical polymerisation of DEGA from Whatman 1 filter paper. |
Fig. 3 FT-IR spectra of poly(αClεCL24-co-εCL76) grafted filter paper; before and after grafting with DEGA. |
Sample namea | DPtarget | Conversionb (%) | M n, theo (g mol−1) | M n, SEC (g mol−1) | Đ |
---|---|---|---|---|---|
a Samples have been named accordingly poly(αClεCLx-co-εCLy), where x and y equal the molar composition in the polymer. The addition of -g-PDEGA(z) indicates that the poly(αClεCLx-co-εCLy) has been used as a macro initiator for the polymerisation of DEGA, and z = 1 or 2 has been given to differentiate duplicate samples. b Monomer conversion calculated from 1H NMR. c M n, theo. calculated from the conversion according to 1H NMR assuming 100% initiator efficiency. d Results obtained from CHCl3-SEC. | |||||
Poly(αClεCL24-co-εCL76) | 300 | >99 | 36700 | 13700 | 1.6 |
Poly(αClεCL40-co-εCL60) | 300 | >99 | 38400 | 23100 | 1.5 |
Poly(αClεCL70-co-εCL30) | 300 | >99 | 41500 | 9200 | 1.6 |
Poly(αClεCL24-co-εCL76)-g-PDEGA(1) | 100 | 71 | 999000 | 174000 | 1.6 |
Poly(αClεCL24-co-εCL76)-g-PDEGA(2) | 100 | 71 | 999000 | 13000 | 1.6 |
Poly(αClεCL40-co-εCL60)-g-PDEGA(1) | 100 | 75 | 1732000 | 192000 | 1.5 |
poly(αClεCL40-co-εCL60)-g-PDEGA(2) | 100 | 74 | 1709000 | 208000 | 1.5 |
Poly(αClεCL70-co-εCL30)-g-PDEGA(1) | 100 | 59 | 2374000 | 301000 | 1.4 |
Poly(αClεCL70-co-εCL30)-g-PDEGA(2) | 100 | 60 | 241300 | 304000 | 1.4 |
Conversions, molecular weights and dispersities of PDEGA-grafted poly(αClεCLx-co-εCLy), used as a sacrificial macroinitiator, were determined by 1H NMR and SEC (Table 2, Fig. S6–S8†). The target DP, calculated from the ratio of CL containing repeating units in the polymers, was 100 for all reactions. The Mn of the polymers, as determined by SEC, was significantly lower than the theoretical Mn. This difference in Mn was to a large extent caused by the comb structure of the polymers, which has a smaller hydrodynamic volume than the linear polymers used for calibration. Since macroinitiators with several initiating groups were used for the polymerisation, it was difficult to draw any conclusion regarding the dispersity of the PDEGA grafts. However, it is clear that the dispersity does not increase, which indicates the formation of relatively well-defined PDEGA grafts during the reactions.
FT-IR spectra of the grafted filter papers (Fig. 3) showed a large increase in the intensity of the carbonyl peak, showing that the grafting of DEGA from the poly(αClεCLx-co-εCLy) grafted filter papers was successful. FE-SEM images of the graft-on-graft filter papers (Fig. 4) showed a smoothening of the fibre surface also indicative of a successful polymer grafting. The synthesis of graft-on-grafts from cellulose in a two-step reaction, compared to the previous study involving a total of five steps, enabled a higher grafting density than when a BiB modified cellulose was utilised. We believe this modification to be an attractive method for creating high grafting densities as well as complex architectures on cellulosic and other bio-fibre based surfaces containing hydroxyl groups.
PDEGA is a thermoresponsive polymer with an LCST in the range 9.0–16.5 °C.44–46 Previous work has shown that cellulose fibres grafted with types of polymers produced thermoresponsive materials.47,48 The filter paper modified in the graft-on-graft approach with PDEGA grafted from the poly(αClεCL-co-εCL) was tested for thermo-responsive behaviour in the form of switchable hydrophobicity. CAM showed that the PDEGA grafted filter papers adsorb a water drop instantly at low temperature (approx. 4 °C). When the temperature was increased above the LCST of PDEGA, to approx. 50 °C, the water contact angle increased to around 90°. The adsorption of water at the lower temperature shows that the properties of the grafted PDEGMA had a larger influence on the hydrophilic/hydrophobic properties of the papers than the grafted poly(αClεCL-co-εCL) and that the thermoresponsive properties of PDEGMA were transferred to the cellulose surface. This was in agreement with earlier results reported.29
Footnote |
† Electronic supplementary information (ESI) available: Additional FT-IR, FT-SEM, CAM and SEC results. See DOI: 10.1039/c4py01618a |
This journal is © The Royal Society of Chemistry 2015 |