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Thermo-responsive nanofibrillated cellulose by polyelectrolyte adsorption
European Polymer Journal 49 (2013) 2689–2696
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Thermo-responsive nanofibrillated cellulose by polyelectrolyte adsorption Emma Larsson a,b, Carmen Cobo Sanchez a, Christian Porsch a, Erdem Karabulut a, Lars Wågberg a,b, Anna Carlmark a,b,⇑ a KTH Royal Institute of Technology, School of Chemical Science and Engineering, Department of Fibre and Polymer Technology, Teknikringen 56, SE-100 44 Stockholm, Sweden b KTH Royal Institute of Technology, Wallenberg Wood Science Centre, Teknikringen 56, SE-100 44 Stockholm, Sweden
a r t i c l e
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Article history: Received 13 March 2013 Accepted 26 May 2013 Available online 5 June 2013 Keywords: Nanofibrillated cellulose (NFC) Polyelectrolyte Adsorption Thermoresponsive Atom transfer radical polymerization (ATRP) Block copolymer
a b s t r a c t In this study, thermo-responsive nanofibrillated cellulose (NFC) has been produced by the adsorption of thermo-responsive polyelectrolytes to the NFC. Three block copolymers were synthesized in which the polyelectrolyte block was composed of quaternized poly(2-(dimethylamino)ethyl methacrylate) (qPDMAEMA) and the thermo-responsive block was composed of poly(di(ethylene glycol) methyl ether methacrylate) (PDEGMA). The block copolymers were synthesized employing atom transfer radical polymerization (ATRP) and the PDMAEMA block was utilized as a macroinitiator for the polymerizations of PDEGMA. The length and charge of the PDMAEMA block were kept constant in all three block copolymers, while three different molecular weights of the PDEGMA block was synthesized. The PDMAEMA block was quaternized to introduce positive charges and the block copolymers were subsequently adsorbed onto the negatively charged NFC that was dispersed in water. The lower critical solution temperatures (LCSTs) of the free block copolymers in solution were analyzed by dynamic light scattering (DLS). The composites were analyzed by QCM-D, FT-IR and TGA, which clearly showed an adsorption of the block copolymer onto the NFC. The grafted NFC showed a thermo-responsive behavior in solution upon heating and cooling, thus supporting that the properties of the polyelectrolyte can be transferred to the cellulose. By this methodology, thermo-responsive NFC materials can be produced in a straight-forward manner in water dispersions, without performing any chemical reactions on the NFC. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Cellulose, being one of the most abundant naturally occurring polymers on earth, attracts much attention due to its durability, good mechanical properties and considerably low price. These properties, in combination with its biodegradability, make cellulose a highly interesting material for development of new biobased products. Recently, the nanostructural components of cellulose, such as ⇑ Corresponding author at: KTH Royal Institute of Technology, School of Chemical Science and Engineering, Department of Fibre and Polymer Technology, Teknikringen 56, SE-100 44 Stockholm, Sweden. Tel.: +46 8 790 8027; fax: +46 8 790 8283. E-mail address: [email protected] (A. Carlmark). 0014-3057/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.eurpolymj.2013.05.023
nanofibrillated cellulose (NFC) (also earlier known as micro fibrillated cellulose (MFC)) first presented in the early 1980s, have attracted increasing attention [1–6]. NFC is the resulting product when cellulose fibers are separated into individual microfibrils or small microfibril aggregates by, for example, high pressure homogenization. Due to its non-toxicity, application areas for the material, such as food, pharmaceuticals, cosmetics, and other functional materials are envisioned [3–6]. By modifying the surface of NFC, a new functionality can be incorporated into the material, for example by the grafting of functional polymers from the surface [7,8]. A functionality of special interest is responsiveness to different stimuli. Responsive polymers have lately received much attention due to their
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ability to show conformational changes triggered by an external stimuli. This ability has made them especially attractive for potential use in the biomedical field, but they can also find use as stimuli responsive surfaces and sensors [9–12]. Introduction of a stimuli-responsive functionality to a surface can be used both to control the interactions with other compounds, as well as to control the release of chemicals from the surface [9]. The stimuli-responsive properties of polymers are influenced by the chemical environment of the responsive groups [13–18]. The grafting of a cellulose substrate with thermo-responsive polymers enables cellulose to be utilized in high technology material applications, such as sensors and smart filters. There are few reports in literature, to the knowledge of the authors, where cellulose has been modified with thermo-responsive polymers [19–27]. Malmström et al. grafted poly(N-isopropylacrylamide) (PNIPAAm) from the surface of filter paper by atom transfer radical polymerization (ATRP) and obtained a thermo-responsive cellulose surface [19]. The same group grafted copolymers of di(ethylene glycol) methyl ether methacrylate (DEGMA) and oligo(ethylene glycol) methyl ether methacrylate (OEGMA) from the surface of hydroxypropyl cellulose, and the grafted cellulose showed preserved, but altered, thermo-responsive behavior, in comparison to the linear PDEGMA and POEGMA analogues [20]. In a recent study by Estevez et al., a cellulose based temperature responsive fabric that can switch between a superhydrophobic and a superhydrophilic state has been synthesized by grafting the fabric with PNIPAAm brushes. The fabric is envisioned to be used for water collection from the humidity in air [21]. The grafting-from methodology, however, is less viable in the case of grafting from NFC, as NFC typically is dispersed in water and this would require tedious solvent exchange in order to incorporate the initiator functionality on the surface of the NFC. A more straight-forward approach is to graft polymers to the surface, using electrostatic interactions and, hence, avoid performing chemical reactions on the NFC. In a number of cases, block copolymers have been synthesized in which one block is made up of a polyelectrolyte and the other of a thermo-responsive polymer [12,14–18,28–35]. Since the polyelectrolyte block can adsorb to oppositely charged surfaces, the responsive properties of the thermo-responsive block is bound to the surface by physical adsorption [12,17,30,32–34,36]. In a study by Carlmark and Wågberg et al., PNIPAAm based permanent polyelectrolytes were synthesized and adsorbed to negatively charged NFC in a QCM-D study. The adsorption measurements were performed both in single layers and multilayers and the copolymers showed thermo-responsiveness, in the same region as PNIPAAm homopolymer, after adsorption [34]. In this study, three block copolymers, all containing a thermo-responsive block based on PDEGMA, having a lower critical solution temperature (LCST) in the region 26–28 °C [37–39], and a quaternized poly(2-(dimethylamino)ethyl methacrylate) (qPDMAEMA) polyelectrolyte block, have been synthesized by ATRP and adsorbed to NFC in water suspension. The length and charge of the qPDMAEMA blocks were kept constant in all three block copolymers, while the molecular weight of the PDEGMA block was varied.
The block copolymers were adsorbed to NFC dispersed in water solutions, which rendered a thermo-responsive NFCmaterial, without any chemical reactions being performed on the surface of the NFC during grafting. 2. Experimental 2.1. Materials Ethyl a-bromoisobutyrate (EBiB) (98%), 1,1,4,7,10, 10-hexamethyltriethylenetetramine (HMTETA) (97%), copper(I)chloride (99%), copper(II)chloride (97%) were purchased from Sigma–Aldrich. Iodomethane (P99%) was purchased from Riedel-de Haën. TrizmaÒ base (tris base) (P99.9%) was purchased from Sigma–Aldrich. 2-(Dimethylamino)ethyl methacrylate (DMAEMA) (98%, Aldrich) was passed through a column of activated, basic aluminum oxide prior to use to remove the inhibitor. Di(ethylene glycol) methyl ether methacrylate (DEGMA) (95%, Aldrich) was passed through a column of activated, neutral aluminum oxide prior to use to remove the inhibitor. Diethyl ether (P99.8%), acetone (100%), dichloromethane (DCM) (P99.8%). n-heptane (P99.8%) was purchased from VWR. Tetrahydrofuran (THF) (P99.7%) was purchased from Chemtronica. Milli-Q water was used. Nanofibrillated cellulose (NFC), utilized for the composite production, was prepared according to the procedure described by Isogai et al. [40]. The cellulose fibres used were a mixture of Norwegian spruce (60%) and Scots pine with a hemicellulose content lower than 5% by weight. It was delivered form Domsjö Fabriker, Örnsköldsvik, Sweden. Before homogenization the fibres were oxidised according to Isogai et al. [40] and then homogenized in a high pressure homogenizer [41]. Before use the NFC was pretreated according to Wågberg et al. [42] in order to remove all aggregated fibrils and to make sure that all the carboxyl group were in the Na+ form. The charge density of the fibres before homogenization was determined by conductometric titration [43] to be 720 leq/g. The NFC dispersion used for QCM-D measurements was prepared from Generation 2 (Gen 2) NFC by the supplier (Innventia AB, Stockholm, Sweden), according to a procedure described by Wågberg et al. [41]. It had a charge density of 550 leq/g [35]. 2.2. Methods Fourier transform infrared spectroscopy (FT-IR) was collected 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 32 scans were recorded. Dynamic light scattering (DLS) was performed using a Malvern Zetasizer NanoZS. The analyses were performed at a polymer concentration of 3 g L1, in phosphate buffered saline (PBS) buffer and tris base, resulting in a pH = 8.3. Thermogravimetric analysis (TGA) was performed on a TA Instruments Hi-Res TGA 2950 analyzer under a N2 flow of 50 ml min1, at a heating rate of 10 °C min1. The samples were heated from 40 to 700 °C. Field-emission scanning electron microscopy (FE-SEM) images were recorded on a Hitachi S-4800 FE-SEM. The
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samples were mounted on a split mount substrate with carbon tape and coated for 5 s with a carbon coater (Cressington 108carbon/A) followed by 30 s of a platinum/palladium sputter coater (Cressington 208HR). 1 H NMR spectra were recorded at room temperature with the aid of a Bruker Avance 400 MHz spectrometer, using CDCl3 and D2O as solvents. Tetramethylsilane (TMS) (CDCl3) and the solvent residual peak (D2O) were used as internal standards. Size exclusion chromatography (SEC) was performed, using dimethylformamide (DMF) (0.2 ml min1) as the mobile phase at 50 °C, using a TOSOH EcoSEC HLC8320GPC system equipped with an EcoSEC RI detector and three columns (PSS PFG 5 lm; Microguard, 100 Å, and 300 Å) (MW resolving range: 300–100,000 Da) from PSS GmbH. A conventional calibration method was created using narrow linear poly(methyl methacrylate) (PMMA) standards. 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. Polyelectrolyte titration (PET) was performed with a 716 DMS Titrino from Metrohm (Switzerland). Potassium polyvinyl sulfate (KPVS) was used as the titrant, and orthotoluidine blue (OTB) was used as the indicator. The color change was recorded with a fotoelektrischer Messkopf 2000 from BASF and the amount of KPVS needed to reach the equilibrium point was calculated according to the method developed by Horn [44]. The adsorption of polyelectrolyte copolymers to negatively charged NFC was measured using a quartz crystal microbalance with dissipation monitoring (QCM-D from Q-sense AB Sweden). The QCM crystals used were AT-cut quartz crystals with silica sputtered active surfaces prepared as follows. The crystals were washed with Milli-Q water, ethanol and Milli-Q water and then blown dry with N2 before being placed in an air plasma cleaner (Model PDC 002, Harrick Scientific Corporation (USA)) at reduced pressure for 120 s at an effect of 30 W. The QCM crystals were first treated with polyethylenimine (PEI) as anchoring polymer for the NFC, and then the NFC was adsorbed to the modified quartz crystal from Milli-Q water. The polyelectrolytes were adsorbed to the NFC at a concentration of 100 mg L1 in tris base pH = 8.3. The rinsing steps were performed using tris base. The adsorbed mass of polymer is related to the change in frequency according to the Sauerbrey model [45] (Eq. 1), which assumes that the adsorbed polymer forms rigid layers, and that the layers contain both the adsorbed polymer and the water immobilized by the adsorbed polymer layer.
m¼C
Df n
ð1Þ
where m = adsorbed mass per unit area (mg m2), C = sensitivity constant, 0.177 [mg (m2 Hz)1), Df = change in resonant frequency (Hz), n = overtone number. The thermo-responsive behavior of modified NFC in solution was investigated by heating a well dispersed water solution of modified NFC to a temperature above the LCST of the PDEGMA block.
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2.2.1. ATRP of DMAEMA Acetone (45 g, 50 wt%), HMTETA (2.08 ml, 7.63 mmol), and EBiB (560.1 ll, 3.82 mmol) were added to a 100 ml round bottom flask equipped with a magnetic stirrer. The reaction vessel was placed in an ice/water-bath and DMAEMA (45 g, 286.24 mmol) was added. The reaction flask was sealed with a rubber septum, evacuated (5 min) and backfilled with argon (5 min). Cu(I)Cl (37.8 mg, 3.81 mmol) was added to the reaction mixture and the reaction vessel was degassed with two additional cycles of evacuation/backfilling. The flask was placed in a pre-heated oil bath (50 °C), and left to react for 1 h (~30% monomer conversion). Aliquots of 0.1 ml were taken out every 15 min for analysis by 1H NMR and SEC to ensure the termination of the reaction at around 30% monomer conversion. The reaction was terminated by addition of 150 mg of Cu(II)Cl2 followed by one cycle of degassing with vacuum and argon, as previously described, to ensure the preservation of living end-groups. The acetone was removed by rota-evaporation and the reaction mixture was re-dissolved in THF before being passed through a short column of Al2O3 (activated, neutral) to remove copper complexes from the product. The reaction mixture was concentrated by rota-evaporation and precipitated twice in cold heptane. The polymer was dried under vacuum overnight. 2.2.2. ATRP of DEGMA from PDMAEMA macroinitiator The general procedure used for the polymerization of DEGMA from the PDMAEMA macroinitiator is as described below. The amounts of monomer and acetone were altered in the different reactions while the monomer conversion was kept constant at approximately 30%. PDMAEMA (840 mg, 0.12 mmol) was added to a 50 ml round bottom flask, dissolved in acetone (50 wt% in relation to monomer) and immersed in an ice bath. HMTETA (65.28 ll, 0.24 mmol) and DEGMA was added and the reaction vessel was sealed and degassed with one cycle of evacuation/ backfilling. Cu(I)Cl (11.9 mg, 0.12 mmol) was added and the reaction mixture was degassed with two additional cycles of evacuation/backfilling. The flask was placed in an oil bath pre-heated to 50 °C and left to react until the desired monomer conversion (30%) had been reached. To monitor the reaction aliquots of 0.1 ml were taken out every 30 min for analysis by 1H NMR and SEC. The reaction was terminated by exposure to air. The acetone was removed by rota-evaporation and the reaction mixture was re-dissolved in THF before being passed through a column of Al2O3 (activated, neutral) to remove copper complexes from the product. The THF was removed by rota-evaporation and the product was dissolved in DCM and precipitated twice in cold diethyl ether. The final polymer was dried under reduced pressure overnight. 2.2.3. Quaternization of PDMAEMA block The general procedure for quaternization of the PDMAEMA block in the block copolymer is as described below. Block copolymer (2 g) was dissolved in THF (25 ml) in a 50 ml round bottom flask equipped with a magnetic stirrer. Iodomethane (3 times excess to the stoichiometric amount of DMAEMA repeating units) dissolved in 15 ml
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of THF was added to the flask under vigorous stirring. The reaction was left to react at room temperature overnight. The quaternized polymers were dissolved in deionized water, loaded to a pre-soaked dialysis membrane (MWCO 6000-8000) and dialyzed against water. The product was isolated by freeze-drying. 2.2.4. Adsorption of quaternized block copolymers to NFC in solution The general procedure for adsorption of block copolymers to NFC was performed according to the following procedure. NFC/water dispersion (10 g, 0.87 wt% of NFC) was diluted with Milli-Q water (80 ml, tris base, pH 8.3) and dispersed by ultrasonication, utilizing an ultraturrax. Two cycles, of each 3 min, with a pulse of 3 s on and 5 s off were performed. The amplitude of the instrument was set to 30% and the NFC solution was cooled on an ice bath during the procedure to avoid heating of the sample. Quaternized block copolymers (stoichiometric amounts to NFC charges) were dissolved in tris base (10 ml) and added drop wise to the NFC solution, and the solution was left to stir for 1 h. The modified fibers were washed by centrifugation (4500 rpm) twice, being re-dispersed with one ultrasonication cycle as described above. The water phase was analyzed by PET to ensure that the polymer had adsorbed to the NFC, and that the adsorbed polymer did not detach during ultrasonication. 2.2.5. Preparation of dry composites of NFC and quaternized block copolymers Composites of NFC fibers and block copolymers were made by first preparing dispersions of modified NFC fibers (75 mg) in aqueous solutions (250 ml) by ultrasonication for 3 min, as previously described. Secondly, the composites were formed by vacuum filtration through a 0.65 micron filter. The composites were left to dry at room temperature, in air, for 2 days. 3. Results and discussion Controlled radical polymerization techniques enable the production of block copolymers that are well-defined in terms of molecular weight and dispersity. Due to this, ATRP was chosen as polymerization technique in this work and block copolymers of PDMAEMA and PDEGMA were successfully synthesized, with varying length of the PDEGMA block. The reaction schemes for the syntheses
performed can be seen in Scheme 1 and characteristics of the polymers are reported in Table 1. In the ATRP of the PDMAEMA macroinitiator the reaction was terminated by addition of Cu(II)Cl2 to ensure that the active end groups were preserved. The structure of the synthesized polymer was analyzed with 1H NMR, but unfortunately the end-group functionality could not be determined. However, chain extension with PDEGMA, utilizing the PDMAEMA as a macroinitiator, indicated that almost all end groups had preserved initiating activity, as shown by the monomodal peak in the SEC crude samples from the block copolymer synthesis (See ESI, Fig. 9). Three different block copolymers, with three different degrees of polymerization of the PDEGMA blocks, were successfully synthesized, as shown by 1H NMR and SEC (See ESI). SEC results show that the PDMAEMA homopolymer had a low dispersity, which was also seen for the two block copolymers with the shortest PDEGMA blocks (PDMAEMA-block-PDEGMA234 and PDMAEMA-blockPDEGMA409). The dispersity of the block copolymer with the longest PDEGMA block (PDMAEMA-block-PDEGMA489) was slightly higher (1.4), but the relatively low dispersities for all copolymers indicate that the polymerization systems used provided good control over the final polymers. The PDMAEMA was charged by the quaternization of the nitrogen in the PDMAEMA repeating unit using an earlier described procedure [46]. The quaternization was qualitatively detected by 1H NMR and the degree of quaternization was quantitatively determined by PET and found to be around or above 90 % for all three block copolymers, as shown in Table 1. The quaternized polymers could, due to their strong interactions with the column material, not be analyzed by SEC. 1H NMR results, however, indicate that the PDEGMA blocks remain unaffected by the quaternizations, as the peaks in the NMR originating from the DEGMA repeating units have their chemical shift in similar positions as prior to quaternization and do not change their relative integral values. The thermo-responsive properties of the synthesized block copolymers in aqueous solution were analyzed by DLS. All three block copolymers showed good water solubility at room temperature, while they formed aggregates at elevated temperatures. Initially, the lower critical solution temperatures (LCSTs) of the polymers were analyzed in tris base solution, since tris base was selected for the adsorption to NFC. However, the LCST was difficult to determine in this solvent as the block copolymers
Scheme 1. Schematic image of the block copolymers synthesized in this study.
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a b c d e f
Polymera
Mn SECb (g mol1) Dispersityb LCSTc (°C) Charged density (leq/g) Degree of quaternizatione (%) Charge ratiof
PDMAEMA PDMAEMA-block-PDEGMA234 PDMAEMA-block-PDEGMA409 PDMAEMA-block-PDEGMA489 qPDMAEMA-block-PDEGMA234 qPDMAEMA-block-PDEGMA409 qPDMAEMA-block-PDEGMA489
6,100 50,400 83,200 97,800 – – –
1.19 1.11 1.22 1.43 – – –
– – – – 27 23 23
598 380 356
– – – 88 88 97
0.92 1.12 1.17
(PDMAEMA-block-PDEGMAn): n denotes the number average degree of polymerization for the PDEGMA blocks determined by SEC. Determined by DMF SEC (MMA calibration). Measured by DLS, assessed at a concentration of 3 g l1 in PBS. Determined by PET. Determined by PET as a percentage out of the total number of quaternizable groups available in each block copolymer. Charge ratio of Polyelectrolyte: NFC, determined by adsorption of polyelectrolytes to NFC surfaces in a QCM-D.
displayed wide transitions, especially qPDMAEMAblock-PDEGMA234 which contains the highest ratio of qPDMAEMA to PDEGMA (see ESI). The LCST study was instead performed in PBS in accordance with Saunders et al. whom also utilized quaternized PDMAEMA as polyelectrolyte [35]. In PBS all three block copolymers showed a clear phase transition upon heating and their LCSTs are reported in Table 1. DLS results, i.e. hydrodynamic diameter, for the three polymers in PBS can be seen in Fig. 1. From the results it can be seen that the polymer with the shortest thermoresponsive block has a higher LCST than the two block copolymers with longer thermo-responsive blocks. The two polymers with the longer thermo-responsive blocks show similar LCSTs, close to that expected of PDEGMA homopolymer in PBS buffer (which is around 25 °C) [39]. The higher LCST of the block copolymer with the shortest thermo-responsive block is strongly believed to be a consequence of more repulsive charges between the higher charged polymers in solution. The quaternized block copolymers were adsorbed to NFC in accordance with a procedure from Ikkala et al. [47]. The adsorption was performed in a tris base solution to ensure that the carboxyl groups of the anionic NFC were dissociated. The dissociation should ensure a strong ionic binding
Fig. 1. DLS results for the three block-copolymers in PBS showing how the hydrodynamic diameter of the polymers changes with temperature.
between the NFC and the block copolymers [47]. After the adsorption, the materials were thoroughly washed, in order to remove excess unbound block copolymer. To further quantify how the block copolymers adsorb to negatively charged NFC, an adsorption of qPDMAEMAblock-PDEGMA234 to a NFC modified QCM quartz crystal was performed. The adsorption was performed by first adsorbing an anchoring layer of PEI to the QCM crystal. This was followed by the adsorption of NFC from Milli-Q water and a subsequent adsorption of block copolymer from a tris base solution. Between each adsorption step the surface was rinsed with tris base solution. The results from the QCM-D study can be seen in Fig. 2. From the adsorption measurement it can clearly be seen that the polymers adsorb to NFC and that they do not desorb when the surface is rinsed with a tris base solution. It is also clearly visible that the change in frequency increases when the adsorbed NFC is rinsed with tris base solution, which is probably due to a higher swelling of the NFC in this medium. From the change in frequency the amount of polymer adsorbed can be calculated using the Sauerbrey model [45]. Unfortunately, it is problematic to give a quantitative estimate of the absolute charge matching between the polyelectrolyte and the NFC since the adsorbed values from the QCM also contain the immobilized water. However, by assuming that the immobilized water, after rinsing with tris base, is the same for the two components, the charge ratios could be calculated. For all three polymers, the amount of polymer adsorbed gave a charge ratio to NFC of around 1, reported in Table 1, showing that the adsorption of the polyelectrolyte is mainly driven by electrostatic interactions. The polyelectrolyte modified NFC’s, i.e. the composite materials, were analyzed with TGA, FT-IR and SEM. Prior to these analyses, the materials were dried by filtration and evaporation for 2 days. The FT-IR curves attained from the dry composites can be seen in Fig. 3, and are compared to those of neat NFC. In the FT-IR curve a peak corresponding to the carbonyl stretching, attributed to the ester group in the block copolymer, appears at 1723 cm1, clearly showing that the block copolymers have adsorbed to the NFC. As can be seen, the intensity of this peak is slightly higher for the composites produced from the block copolymer
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Fig. 2. QCM-D show adsorption of PEI around t=400 s. Washing with tris base at around t=750 s. Adsorption of NFC around t=1100 s. Washing with tris base around t=1800 s. Adsorption of quaternized block copolymers at around 2250s. Washing with tris base at around t=2800 s. The left y-axis shows the fundamental frequency change as calculated from the 3rd overtone. The right y-axis displays the change in energy dissipation following the adsorption.
Fig. 3. FT-IR spectrum of a dried neat NFC film and dried composites with varying PDEGMA block length, prepared by vacuum filtration.
containing the longer PDEGMA blocks, which could indicate that a larger mass of polymer is adsorbed when longer PDEGMA blocks are utilized. Since the qPDMAEMA block was the same in all three block copolymers, this difference is attributed to the molecular weight of the PDEGMA block. Differences between the spectra of NFC and the composites can also be seen in the cellulose fingerprint region, were peaks appear around 1250 cm1, 1100 cm1, and 850 cm1 in the spectra of the composites. In Fig. 4 the thermograms from the TGA-analyses are shown for composites of all three block copolymers and of neat NFC. The results clearly show a different degradation behavior of the pure NFC, in comparison with those of the composite materials, when heated under nitrogen atmosphere. NFC degrades in a two-step process, in which the first step (250–380 °C) is known as thermal cracking, and the second step starting at 380 °C is where NFC undergoes
Fig. 4. TGA results for a dried NFC film and the three nanocomposites prepared with block-copolymers of different PDEGMA block lengths.
carbonization [48]. As can be seen in Fig. 4, the composite materials have two different slopes in their curves, indicating two different types of degradation in each sample. The first degradation (270–330 °C) most probably corresponds to the block copolymers, and the second degradation (330–460 °C) to the degradation of NFC. The results from the TGA indicate that the composite formed have a relatively high temperature stability, as little degradation occurs at temperatures below 250 °C under nitrogen atmosphere. FE-SEM was used to compare neat NFC films to composites made from the polymer containing the shortest PDEGMA block. Images were captured at both 5000 and 25,000 magnification as shown in Fig. 5. In the images with a 5000 magnification, a clear difference in appearance between the materials can be seen, in which the layers in the neat NFC film are more closely packed than in the composite film.
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Fig. 5. FE-SEM images of (a and c) neat NFC films and (b and d) composite films made from NFC and qPDMAEMA-block-PDEGMA234.
Fig. 6. (a) Image of modified NFC in aqueous solutions, with increasing molecular weight of the PDEGMA block from left to right, below LCST. (b) Image of modified NFC in aqueous solutions, with increasing molecular weight of the PDEGMA block from left to right, above LCST.
In the images with 25,000 magnification this difference can be seen even more evidently. There may naturally be several explanations to this phenomena but one obvious explanation is that the adsorbed polyelectrolyte forms a steric barrier on the surface of NFC. When the water is removed, and the fibrils forced towards each other, this steric barrier will prevent the close-packing of the NFC that is detected in the neat NFC film. From the adsorption measurements it could be calculated that 8.9 mg/m2 NFC adsorbed 7.5 mg/m2 polyelectrolyte and assuming that the immobilized water is about the same for the NFC and the polyelectrolyte it is obvious that the surfaces of the fibrils are saturated with polymer which can explain why the fibrils do not pack in the same way as in the neat NFC. To investigate the thermo-responsive properties of NFC modified with block copolymers, vials with grafted NFC in aqueous tris base solutions (0.083% dry weight of NFC) were heated above the LCST of PDEGMA. Images can be seen in Fig. 6, in which three samples of NFC, physically grafted with the three quaternized block copolymers, is
displayed prior to, and just after, heating. As can be seen in the figure, the grafted NFC forms aggregates when the solution is heated, clearly indicating that the PDEGMA block is above its LCST and that it has provided the grafted NFC in solution with thermo-responsive properties. When the solution was left to cool down, the grafted NFC redispersed in the aqueous solutions, showing that the behavior is reversible. Aqueous solutions of ungrafted NFC did not display a thermo-responsive behavior when heated hence; the responsiveness of the grafted NFC originates from the block copolymer adsorption. 4. Conclusions In this paper, block copolymers were synthesized using standard ATRP conditions, in which one block was composed of PDMAEMA and the other block was composed of the thermo-responsive polymer PDEGMA. Three block copolymers were produced with varying length of the PDEGMA block, while the PDMAEMA block length was
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kept constant. All three block copolymers had narrow polydispersities (1.1 > Ð < 1.4). Quaternization of the PDMAEMA block, in all three block copolymers, introduced cationic charges and the formed polyelectrolytes had thermo-responsive properties with LCSTs ranging from 23 to 27 °C, measured by DLS in PBS buffer. The block copolymers were adsorbed to negatively charged NFC in water dispersion by electrostatic interactions. FT-IR and TGA of the dried composite material showed that the polyelectrolytes had been adsorbed onto the NFC, also indicating that the longer the block of the PDEGMA, the more polyelectrolyte, calculated by mass, was adsorbed. This was further supported by adsorption measurements performed in the QCM-D. From TGA analyses it was also clear that the composite materials exhibited a different degradation behavior compared to that of neat cellulose. Thermo-responsive behavior could finally be showed by ocular inspection of solutions of grafted NFC while heating and cooling. Hence, by this methodology, thermo-responsive NFC materials can be produced in a straight-forward manner in water dispersion, without performing any chemical reactions on the NFC. Acknowledgements The authors would like to thank Wallenberg Wood Science Center (WWSC), the Swedish Research Council (VR) and the Swedish Research Council Formas for financial support. Kasinee Prakobna and Anna Svensson are thanked for assistance with FE-SEM and polyelectrolyte titration measurements, respectively. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.eurpolymj.2013.05.023. References [1] Turbak AF, Snyder FW, Sandberg KR. J Appl Polym Sci: Appl Polym Symp 1983;37:815–27. [2] Henriksson M, Berglund LA, Isaksson P, Lindström T, Nishino T. Biomacromolecules 2008;9:1579–85. [3] Eichhorn SJ, Dufresne A, Aranguren M, Marcovich NE, Capadona JR, Rowan SJ, et al. J Mater Sci 2010;45:1–33. [4] Siqueira G, Bras J, Dufresne A. Polymers. vol. 2. Basel, Switzerland; 2010. p. 728–765. [5] Siro I, Plackett D. Cellulose. vol. 17. Dordrecht, Netherlands; 2010. p. 459–494. [6] Klemm D, Kramer F, Moritz S, Lindström T, Ankerfors M, Gray D, et al. Angew Chem Int Edit 2011;50:5438–66. [7] Lönnberg H, Fogelström L, Berglund L, Malmström E, Hult A. Eur Polym J 2008;44:2991–7. [8] Roy D, Semsarilar M, Guthrie JT, Perrier S. Chem Soc Rev 2009;38:2046–64.
[9] Tripathy SK, Kumar J, Nalwa HS, Editors. Handbook of polyelectrolytes and their applications. vol. 1. Polyelectrolytebased multilayers, self-assemblies and nanostructures; 2002. [10] Richter A, Bund A, Keller M, Arndt K-F. Sensor Actuator B: Chem 2004;B99:579–85. [11] Liu R, Fraylich M, Saunders BR. Colloid Polym Sci 2009;287:627–43. [12] Liu R, Saunders BR. J Colloid Interf Sci 2009;338:40–7. [13] Hahn M, Goernitz E, Dautzenberg H. Macromolecules 1998;31:5616–23. [14] Bokias G, Hourdet D, Iliopoulos I. Macromolecules 2000;33:2929–35. [15] Liu R, De Leonardis P, Cellesi F, Tirelli N, Saunders BR. Langmuir 2008;24:7099–106. [16] Patrizi ML, Diociaiuti M, Capitani D, Masci G. Polymer 2009;50:467–74. [17] Masci G, Ladogana RD, Cametti C. J Phys Chem B 2012;116:2121–30. [18] Bokias G, Mylonas Y. Macromolecules 2001;34:885–9. [19] Lindqvist J, Nyström D, Östmark E, Antoni P, Carlmark A, Johansson M, et al. Biomacromolecules 2008;9:2139–45. [20] Porsch C, Hansson S, Nordgren N, Malmström E. Polym Chem 2011;2:1114–23. [21] Yang H, Zhu H, Hendrix MMRM, Lousberg NJHGM, de With G, Esteves ACC, et al. Adv Mater 2013;25:1149. Weinheim, Germany. [22] Bokias G, Mylonas Y, Staikos G, Bumbu GG, Vasile C. Macromolecules 2001;34:4958–64. [23] Pan K, Zhang X, Cao B. Polymer International 2010;59:733–7. [24] Zhang Z, Chen L, Zhao C, Bai Y, Deng M, Shan H, et al. Polymer 2011;52:676–82. [25] Mohd A, Mohd CI, Ahmad N, Halib N, Ahmad I. Carbohydrate Polymers 2012;88:465–73. [26] Zoppe JO, Venditti RA, Rojas OJ. Journal of Colloid and Interface Science 2012;369:202–9. [27] Cui G, Li Y, Shi T, Gao Z, Qiu N, Satoh T, et al. Carbohydrate Polymers 2013;94:77–81. [28] Dautzenberg H, Gao Y, Hahn M. Langmuir 2000;16:9070–81. [29] Masci G, Diociaiuti M, Crescenzi V. J Polym Sci Part A: Polym Chem 2008;46:4830–42. [30] Zhang J, Zhou Y, Zhu Z, Ge Z, Liu S. Macromolecules 2008;41:1444–54. Washington, DC, United States. [31] Liu R, Cellesi F, Tirelli N, Saunders BR. Polymer 2009;50:1456–62. [32] Liu R, Tirelli N, Cellesi F, Saunders BR. Langmuir 2009;25:490–6. [33] Fraylich Michael R, Liu R, Richardson Stephen M, Baird P, Hoyland J, Freemont Anthony J, et al. J Colloid Interf Sci 2010;344:61–9. [34] Utsel S, Malmström EE, Carlmark A, Wågberg L. Soft Matter 2010;6:342–52. [35] Shahidan NN, Liu R, Cellesi F, Alexander C, Shakesheff KM, Saunders BR. Langmuir 2011;27:13868–78. [36] Masci G, De Santis S, Cametti C. J Phys Chem B 2011;115:2196–204. [37] Han S, Hagiwara M, Ishizone T. Macromolecules 2003;36:8312–9. [38] Lutz J-F, Hoth A. Macromolecules 2006;39:893–6. [39] Lutz J-F, Hoth A, Schade K. Des Monomers Polym 2009;12:343–53. [40] Saito T, Hirota M, Tamura N, Kimura S, Fukuzumi H, Heux L, et al. Biomacromolecules 2009;10:1992–6. [41] Wågberg L, Decher G, Norgren M, Lindström T, Ankerfors M, Axnas K. Langmuir 2008;24:784–95. [42] Fall AB, Lindström SB, Sundman O, Ödberg L, Wågberg L. Langmuir 2011;27:11332–8. [43] Katz S, Beatson RP, Scallan AM. Svensk Papperstidning 1984;87:R48–53. [44] Horn D. Progr Colloid Polym Sci 1978;65:251–64. [45] Sauerbrey G. Zeitschrift fuer Physik 1959;155:206–22. [46] Paneva D, Mespouille L, Manolova N, Degee P, Rashkov I, Dubois P. J Polym Sci Part A: Polym Chem 2006;44:5468–79. [47] Wang M, Olszewska A, Walther A, Malho J-M, Schacher FH, Ruokolainen J, et al. Biomacromolecules 2011;12:2074–81. [48] Liu A, Walther A, Ikkala O, Belova L, Berglund LA. Biomacromolecules 2011;12:633–41.
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Thermo-responsive nanofibrillated cellulose by polyelectrolyte adsorption Emma Larsson a,b, Carmen Cobo Sanchez a, Christian Porsch a, Erdem Karabulut a, Lars Wågberg a,b, Anna Carlmark a,b,⇑ a KTH Royal Institute of Technology, School of Chemical Science and Engineering, Department of Fibre and Polymer Technology, Teknikringen 56, SE-100 44 Stockholm, Sweden b KTH Royal Institute of Technology, Wallenberg Wood Science Centre, Teknikringen 56, SE-100 44 Stockholm, Sweden
a r t i c l e
i n f o
Article history: Received 13 March 2013 Accepted 26 May 2013 Available online 5 June 2013 Keywords: Nanofibrillated cellulose (NFC) Polyelectrolyte Adsorption Thermoresponsive Atom transfer radical polymerization (ATRP) Block copolymer
a b s t r a c t In this study, thermo-responsive nanofibrillated cellulose (NFC) has been produced by the adsorption of thermo-responsive polyelectrolytes to the NFC. Three block copolymers were synthesized in which the polyelectrolyte block was composed of quaternized poly(2-(dimethylamino)ethyl methacrylate) (qPDMAEMA) and the thermo-responsive block was composed of poly(di(ethylene glycol) methyl ether methacrylate) (PDEGMA). The block copolymers were synthesized employing atom transfer radical polymerization (ATRP) and the PDMAEMA block was utilized as a macroinitiator for the polymerizations of PDEGMA. The length and charge of the PDMAEMA block were kept constant in all three block copolymers, while three different molecular weights of the PDEGMA block was synthesized. The PDMAEMA block was quaternized to introduce positive charges and the block copolymers were subsequently adsorbed onto the negatively charged NFC that was dispersed in water. The lower critical solution temperatures (LCSTs) of the free block copolymers in solution were analyzed by dynamic light scattering (DLS). The composites were analyzed by QCM-D, FT-IR and TGA, which clearly showed an adsorption of the block copolymer onto the NFC. The grafted NFC showed a thermo-responsive behavior in solution upon heating and cooling, thus supporting that the properties of the polyelectrolyte can be transferred to the cellulose. By this methodology, thermo-responsive NFC materials can be produced in a straight-forward manner in water dispersions, without performing any chemical reactions on the NFC. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Cellulose, being one of the most abundant naturally occurring polymers on earth, attracts much attention due to its durability, good mechanical properties and considerably low price. These properties, in combination with its biodegradability, make cellulose a highly interesting material for development of new biobased products. Recently, the nanostructural components of cellulose, such as ⇑ Corresponding author at: KTH Royal Institute of Technology, School of Chemical Science and Engineering, Department of Fibre and Polymer Technology, Teknikringen 56, SE-100 44 Stockholm, Sweden. Tel.: +46 8 790 8027; fax: +46 8 790 8283. E-mail address: [email protected] (A. Carlmark). 0014-3057/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.eurpolymj.2013.05.023
nanofibrillated cellulose (NFC) (also earlier known as micro fibrillated cellulose (MFC)) first presented in the early 1980s, have attracted increasing attention [1–6]. NFC is the resulting product when cellulose fibers are separated into individual microfibrils or small microfibril aggregates by, for example, high pressure homogenization. Due to its non-toxicity, application areas for the material, such as food, pharmaceuticals, cosmetics, and other functional materials are envisioned [3–6]. By modifying the surface of NFC, a new functionality can be incorporated into the material, for example by the grafting of functional polymers from the surface [7,8]. A functionality of special interest is responsiveness to different stimuli. Responsive polymers have lately received much attention due to their
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ability to show conformational changes triggered by an external stimuli. This ability has made them especially attractive for potential use in the biomedical field, but they can also find use as stimuli responsive surfaces and sensors [9–12]. Introduction of a stimuli-responsive functionality to a surface can be used both to control the interactions with other compounds, as well as to control the release of chemicals from the surface [9]. The stimuli-responsive properties of polymers are influenced by the chemical environment of the responsive groups [13–18]. The grafting of a cellulose substrate with thermo-responsive polymers enables cellulose to be utilized in high technology material applications, such as sensors and smart filters. There are few reports in literature, to the knowledge of the authors, where cellulose has been modified with thermo-responsive polymers [19–27]. Malmström et al. grafted poly(N-isopropylacrylamide) (PNIPAAm) from the surface of filter paper by atom transfer radical polymerization (ATRP) and obtained a thermo-responsive cellulose surface [19]. The same group grafted copolymers of di(ethylene glycol) methyl ether methacrylate (DEGMA) and oligo(ethylene glycol) methyl ether methacrylate (OEGMA) from the surface of hydroxypropyl cellulose, and the grafted cellulose showed preserved, but altered, thermo-responsive behavior, in comparison to the linear PDEGMA and POEGMA analogues [20]. In a recent study by Estevez et al., a cellulose based temperature responsive fabric that can switch between a superhydrophobic and a superhydrophilic state has been synthesized by grafting the fabric with PNIPAAm brushes. The fabric is envisioned to be used for water collection from the humidity in air [21]. The grafting-from methodology, however, is less viable in the case of grafting from NFC, as NFC typically is dispersed in water and this would require tedious solvent exchange in order to incorporate the initiator functionality on the surface of the NFC. A more straight-forward approach is to graft polymers to the surface, using electrostatic interactions and, hence, avoid performing chemical reactions on the NFC. In a number of cases, block copolymers have been synthesized in which one block is made up of a polyelectrolyte and the other of a thermo-responsive polymer [12,14–18,28–35]. Since the polyelectrolyte block can adsorb to oppositely charged surfaces, the responsive properties of the thermo-responsive block is bound to the surface by physical adsorption [12,17,30,32–34,36]. In a study by Carlmark and Wågberg et al., PNIPAAm based permanent polyelectrolytes were synthesized and adsorbed to negatively charged NFC in a QCM-D study. The adsorption measurements were performed both in single layers and multilayers and the copolymers showed thermo-responsiveness, in the same region as PNIPAAm homopolymer, after adsorption [34]. In this study, three block copolymers, all containing a thermo-responsive block based on PDEGMA, having a lower critical solution temperature (LCST) in the region 26–28 °C [37–39], and a quaternized poly(2-(dimethylamino)ethyl methacrylate) (qPDMAEMA) polyelectrolyte block, have been synthesized by ATRP and adsorbed to NFC in water suspension. The length and charge of the qPDMAEMA blocks were kept constant in all three block copolymers, while the molecular weight of the PDEGMA block was varied.
The block copolymers were adsorbed to NFC dispersed in water solutions, which rendered a thermo-responsive NFCmaterial, without any chemical reactions being performed on the surface of the NFC during grafting. 2. Experimental 2.1. Materials Ethyl a-bromoisobutyrate (EBiB) (98%), 1,1,4,7,10, 10-hexamethyltriethylenetetramine (HMTETA) (97%), copper(I)chloride (99%), copper(II)chloride (97%) were purchased from Sigma–Aldrich. Iodomethane (P99%) was purchased from Riedel-de Haën. TrizmaÒ base (tris base) (P99.9%) was purchased from Sigma–Aldrich. 2-(Dimethylamino)ethyl methacrylate (DMAEMA) (98%, Aldrich) was passed through a column of activated, basic aluminum oxide prior to use to remove the inhibitor. Di(ethylene glycol) methyl ether methacrylate (DEGMA) (95%, Aldrich) was passed through a column of activated, neutral aluminum oxide prior to use to remove the inhibitor. Diethyl ether (P99.8%), acetone (100%), dichloromethane (DCM) (P99.8%). n-heptane (P99.8%) was purchased from VWR. Tetrahydrofuran (THF) (P99.7%) was purchased from Chemtronica. Milli-Q water was used. Nanofibrillated cellulose (NFC), utilized for the composite production, was prepared according to the procedure described by Isogai et al. [40]. The cellulose fibres used were a mixture of Norwegian spruce (60%) and Scots pine with a hemicellulose content lower than 5% by weight. It was delivered form Domsjö Fabriker, Örnsköldsvik, Sweden. Before homogenization the fibres were oxidised according to Isogai et al. [40] and then homogenized in a high pressure homogenizer [41]. Before use the NFC was pretreated according to Wågberg et al. [42] in order to remove all aggregated fibrils and to make sure that all the carboxyl group were in the Na+ form. The charge density of the fibres before homogenization was determined by conductometric titration [43] to be 720 leq/g. The NFC dispersion used for QCM-D measurements was prepared from Generation 2 (Gen 2) NFC by the supplier (Innventia AB, Stockholm, Sweden), according to a procedure described by Wågberg et al. [41]. It had a charge density of 550 leq/g [35]. 2.2. Methods Fourier transform infrared spectroscopy (FT-IR) was collected 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 32 scans were recorded. Dynamic light scattering (DLS) was performed using a Malvern Zetasizer NanoZS. The analyses were performed at a polymer concentration of 3 g L1, in phosphate buffered saline (PBS) buffer and tris base, resulting in a pH = 8.3. Thermogravimetric analysis (TGA) was performed on a TA Instruments Hi-Res TGA 2950 analyzer under a N2 flow of 50 ml min1, at a heating rate of 10 °C min1. The samples were heated from 40 to 700 °C. Field-emission scanning electron microscopy (FE-SEM) images were recorded on a Hitachi S-4800 FE-SEM. The
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samples were mounted on a split mount substrate with carbon tape and coated for 5 s with a carbon coater (Cressington 108carbon/A) followed by 30 s of a platinum/palladium sputter coater (Cressington 208HR). 1 H NMR spectra were recorded at room temperature with the aid of a Bruker Avance 400 MHz spectrometer, using CDCl3 and D2O as solvents. Tetramethylsilane (TMS) (CDCl3) and the solvent residual peak (D2O) were used as internal standards. Size exclusion chromatography (SEC) was performed, using dimethylformamide (DMF) (0.2 ml min1) as the mobile phase at 50 °C, using a TOSOH EcoSEC HLC8320GPC system equipped with an EcoSEC RI detector and three columns (PSS PFG 5 lm; Microguard, 100 Å, and 300 Å) (MW resolving range: 300–100,000 Da) from PSS GmbH. A conventional calibration method was created using narrow linear poly(methyl methacrylate) (PMMA) standards. 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. Polyelectrolyte titration (PET) was performed with a 716 DMS Titrino from Metrohm (Switzerland). Potassium polyvinyl sulfate (KPVS) was used as the titrant, and orthotoluidine blue (OTB) was used as the indicator. The color change was recorded with a fotoelektrischer Messkopf 2000 from BASF and the amount of KPVS needed to reach the equilibrium point was calculated according to the method developed by Horn [44]. The adsorption of polyelectrolyte copolymers to negatively charged NFC was measured using a quartz crystal microbalance with dissipation monitoring (QCM-D from Q-sense AB Sweden). The QCM crystals used were AT-cut quartz crystals with silica sputtered active surfaces prepared as follows. The crystals were washed with Milli-Q water, ethanol and Milli-Q water and then blown dry with N2 before being placed in an air plasma cleaner (Model PDC 002, Harrick Scientific Corporation (USA)) at reduced pressure for 120 s at an effect of 30 W. The QCM crystals were first treated with polyethylenimine (PEI) as anchoring polymer for the NFC, and then the NFC was adsorbed to the modified quartz crystal from Milli-Q water. The polyelectrolytes were adsorbed to the NFC at a concentration of 100 mg L1 in tris base pH = 8.3. The rinsing steps were performed using tris base. The adsorbed mass of polymer is related to the change in frequency according to the Sauerbrey model [45] (Eq. 1), which assumes that the adsorbed polymer forms rigid layers, and that the layers contain both the adsorbed polymer and the water immobilized by the adsorbed polymer layer.
m¼C
Df n
ð1Þ
where m = adsorbed mass per unit area (mg m2), C = sensitivity constant, 0.177 [mg (m2 Hz)1), Df = change in resonant frequency (Hz), n = overtone number. The thermo-responsive behavior of modified NFC in solution was investigated by heating a well dispersed water solution of modified NFC to a temperature above the LCST of the PDEGMA block.
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2.2.1. ATRP of DMAEMA Acetone (45 g, 50 wt%), HMTETA (2.08 ml, 7.63 mmol), and EBiB (560.1 ll, 3.82 mmol) were added to a 100 ml round bottom flask equipped with a magnetic stirrer. The reaction vessel was placed in an ice/water-bath and DMAEMA (45 g, 286.24 mmol) was added. The reaction flask was sealed with a rubber septum, evacuated (5 min) and backfilled with argon (5 min). Cu(I)Cl (37.8 mg, 3.81 mmol) was added to the reaction mixture and the reaction vessel was degassed with two additional cycles of evacuation/backfilling. The flask was placed in a pre-heated oil bath (50 °C), and left to react for 1 h (~30% monomer conversion). Aliquots of 0.1 ml were taken out every 15 min for analysis by 1H NMR and SEC to ensure the termination of the reaction at around 30% monomer conversion. The reaction was terminated by addition of 150 mg of Cu(II)Cl2 followed by one cycle of degassing with vacuum and argon, as previously described, to ensure the preservation of living end-groups. The acetone was removed by rota-evaporation and the reaction mixture was re-dissolved in THF before being passed through a short column of Al2O3 (activated, neutral) to remove copper complexes from the product. The reaction mixture was concentrated by rota-evaporation and precipitated twice in cold heptane. The polymer was dried under vacuum overnight. 2.2.2. ATRP of DEGMA from PDMAEMA macroinitiator The general procedure used for the polymerization of DEGMA from the PDMAEMA macroinitiator is as described below. The amounts of monomer and acetone were altered in the different reactions while the monomer conversion was kept constant at approximately 30%. PDMAEMA (840 mg, 0.12 mmol) was added to a 50 ml round bottom flask, dissolved in acetone (50 wt% in relation to monomer) and immersed in an ice bath. HMTETA (65.28 ll, 0.24 mmol) and DEGMA was added and the reaction vessel was sealed and degassed with one cycle of evacuation/ backfilling. Cu(I)Cl (11.9 mg, 0.12 mmol) was added and the reaction mixture was degassed with two additional cycles of evacuation/backfilling. The flask was placed in an oil bath pre-heated to 50 °C and left to react until the desired monomer conversion (30%) had been reached. To monitor the reaction aliquots of 0.1 ml were taken out every 30 min for analysis by 1H NMR and SEC. The reaction was terminated by exposure to air. The acetone was removed by rota-evaporation and the reaction mixture was re-dissolved in THF before being passed through a column of Al2O3 (activated, neutral) to remove copper complexes from the product. The THF was removed by rota-evaporation and the product was dissolved in DCM and precipitated twice in cold diethyl ether. The final polymer was dried under reduced pressure overnight. 2.2.3. Quaternization of PDMAEMA block The general procedure for quaternization of the PDMAEMA block in the block copolymer is as described below. Block copolymer (2 g) was dissolved in THF (25 ml) in a 50 ml round bottom flask equipped with a magnetic stirrer. Iodomethane (3 times excess to the stoichiometric amount of DMAEMA repeating units) dissolved in 15 ml
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of THF was added to the flask under vigorous stirring. The reaction was left to react at room temperature overnight. The quaternized polymers were dissolved in deionized water, loaded to a pre-soaked dialysis membrane (MWCO 6000-8000) and dialyzed against water. The product was isolated by freeze-drying. 2.2.4. Adsorption of quaternized block copolymers to NFC in solution The general procedure for adsorption of block copolymers to NFC was performed according to the following procedure. NFC/water dispersion (10 g, 0.87 wt% of NFC) was diluted with Milli-Q water (80 ml, tris base, pH 8.3) and dispersed by ultrasonication, utilizing an ultraturrax. Two cycles, of each 3 min, with a pulse of 3 s on and 5 s off were performed. The amplitude of the instrument was set to 30% and the NFC solution was cooled on an ice bath during the procedure to avoid heating of the sample. Quaternized block copolymers (stoichiometric amounts to NFC charges) were dissolved in tris base (10 ml) and added drop wise to the NFC solution, and the solution was left to stir for 1 h. The modified fibers were washed by centrifugation (4500 rpm) twice, being re-dispersed with one ultrasonication cycle as described above. The water phase was analyzed by PET to ensure that the polymer had adsorbed to the NFC, and that the adsorbed polymer did not detach during ultrasonication. 2.2.5. Preparation of dry composites of NFC and quaternized block copolymers Composites of NFC fibers and block copolymers were made by first preparing dispersions of modified NFC fibers (75 mg) in aqueous solutions (250 ml) by ultrasonication for 3 min, as previously described. Secondly, the composites were formed by vacuum filtration through a 0.65 micron filter. The composites were left to dry at room temperature, in air, for 2 days. 3. Results and discussion Controlled radical polymerization techniques enable the production of block copolymers that are well-defined in terms of molecular weight and dispersity. Due to this, ATRP was chosen as polymerization technique in this work and block copolymers of PDMAEMA and PDEGMA were successfully synthesized, with varying length of the PDEGMA block. The reaction schemes for the syntheses
performed can be seen in Scheme 1 and characteristics of the polymers are reported in Table 1. In the ATRP of the PDMAEMA macroinitiator the reaction was terminated by addition of Cu(II)Cl2 to ensure that the active end groups were preserved. The structure of the synthesized polymer was analyzed with 1H NMR, but unfortunately the end-group functionality could not be determined. However, chain extension with PDEGMA, utilizing the PDMAEMA as a macroinitiator, indicated that almost all end groups had preserved initiating activity, as shown by the monomodal peak in the SEC crude samples from the block copolymer synthesis (See ESI, Fig. 9). Three different block copolymers, with three different degrees of polymerization of the PDEGMA blocks, were successfully synthesized, as shown by 1H NMR and SEC (See ESI). SEC results show that the PDMAEMA homopolymer had a low dispersity, which was also seen for the two block copolymers with the shortest PDEGMA blocks (PDMAEMA-block-PDEGMA234 and PDMAEMA-blockPDEGMA409). The dispersity of the block copolymer with the longest PDEGMA block (PDMAEMA-block-PDEGMA489) was slightly higher (1.4), but the relatively low dispersities for all copolymers indicate that the polymerization systems used provided good control over the final polymers. The PDMAEMA was charged by the quaternization of the nitrogen in the PDMAEMA repeating unit using an earlier described procedure [46]. The quaternization was qualitatively detected by 1H NMR and the degree of quaternization was quantitatively determined by PET and found to be around or above 90 % for all three block copolymers, as shown in Table 1. The quaternized polymers could, due to their strong interactions with the column material, not be analyzed by SEC. 1H NMR results, however, indicate that the PDEGMA blocks remain unaffected by the quaternizations, as the peaks in the NMR originating from the DEGMA repeating units have their chemical shift in similar positions as prior to quaternization and do not change their relative integral values. The thermo-responsive properties of the synthesized block copolymers in aqueous solution were analyzed by DLS. All three block copolymers showed good water solubility at room temperature, while they formed aggregates at elevated temperatures. Initially, the lower critical solution temperatures (LCSTs) of the polymers were analyzed in tris base solution, since tris base was selected for the adsorption to NFC. However, the LCST was difficult to determine in this solvent as the block copolymers
Scheme 1. Schematic image of the block copolymers synthesized in this study.
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a b c d e f
Polymera
Mn SECb (g mol1) Dispersityb LCSTc (°C) Charged density (leq/g) Degree of quaternizatione (%) Charge ratiof
PDMAEMA PDMAEMA-block-PDEGMA234 PDMAEMA-block-PDEGMA409 PDMAEMA-block-PDEGMA489 qPDMAEMA-block-PDEGMA234 qPDMAEMA-block-PDEGMA409 qPDMAEMA-block-PDEGMA489
6,100 50,400 83,200 97,800 – – –
1.19 1.11 1.22 1.43 – – –
– – – – 27 23 23
598 380 356
– – – 88 88 97
0.92 1.12 1.17
(PDMAEMA-block-PDEGMAn): n denotes the number average degree of polymerization for the PDEGMA blocks determined by SEC. Determined by DMF SEC (MMA calibration). Measured by DLS, assessed at a concentration of 3 g l1 in PBS. Determined by PET. Determined by PET as a percentage out of the total number of quaternizable groups available in each block copolymer. Charge ratio of Polyelectrolyte: NFC, determined by adsorption of polyelectrolytes to NFC surfaces in a QCM-D.
displayed wide transitions, especially qPDMAEMAblock-PDEGMA234 which contains the highest ratio of qPDMAEMA to PDEGMA (see ESI). The LCST study was instead performed in PBS in accordance with Saunders et al. whom also utilized quaternized PDMAEMA as polyelectrolyte [35]. In PBS all three block copolymers showed a clear phase transition upon heating and their LCSTs are reported in Table 1. DLS results, i.e. hydrodynamic diameter, for the three polymers in PBS can be seen in Fig. 1. From the results it can be seen that the polymer with the shortest thermoresponsive block has a higher LCST than the two block copolymers with longer thermo-responsive blocks. The two polymers with the longer thermo-responsive blocks show similar LCSTs, close to that expected of PDEGMA homopolymer in PBS buffer (which is around 25 °C) [39]. The higher LCST of the block copolymer with the shortest thermo-responsive block is strongly believed to be a consequence of more repulsive charges between the higher charged polymers in solution. The quaternized block copolymers were adsorbed to NFC in accordance with a procedure from Ikkala et al. [47]. The adsorption was performed in a tris base solution to ensure that the carboxyl groups of the anionic NFC were dissociated. The dissociation should ensure a strong ionic binding
Fig. 1. DLS results for the three block-copolymers in PBS showing how the hydrodynamic diameter of the polymers changes with temperature.
between the NFC and the block copolymers [47]. After the adsorption, the materials were thoroughly washed, in order to remove excess unbound block copolymer. To further quantify how the block copolymers adsorb to negatively charged NFC, an adsorption of qPDMAEMAblock-PDEGMA234 to a NFC modified QCM quartz crystal was performed. The adsorption was performed by first adsorbing an anchoring layer of PEI to the QCM crystal. This was followed by the adsorption of NFC from Milli-Q water and a subsequent adsorption of block copolymer from a tris base solution. Between each adsorption step the surface was rinsed with tris base solution. The results from the QCM-D study can be seen in Fig. 2. From the adsorption measurement it can clearly be seen that the polymers adsorb to NFC and that they do not desorb when the surface is rinsed with a tris base solution. It is also clearly visible that the change in frequency increases when the adsorbed NFC is rinsed with tris base solution, which is probably due to a higher swelling of the NFC in this medium. From the change in frequency the amount of polymer adsorbed can be calculated using the Sauerbrey model [45]. Unfortunately, it is problematic to give a quantitative estimate of the absolute charge matching between the polyelectrolyte and the NFC since the adsorbed values from the QCM also contain the immobilized water. However, by assuming that the immobilized water, after rinsing with tris base, is the same for the two components, the charge ratios could be calculated. For all three polymers, the amount of polymer adsorbed gave a charge ratio to NFC of around 1, reported in Table 1, showing that the adsorption of the polyelectrolyte is mainly driven by electrostatic interactions. The polyelectrolyte modified NFC’s, i.e. the composite materials, were analyzed with TGA, FT-IR and SEM. Prior to these analyses, the materials were dried by filtration and evaporation for 2 days. The FT-IR curves attained from the dry composites can be seen in Fig. 3, and are compared to those of neat NFC. In the FT-IR curve a peak corresponding to the carbonyl stretching, attributed to the ester group in the block copolymer, appears at 1723 cm1, clearly showing that the block copolymers have adsorbed to the NFC. As can be seen, the intensity of this peak is slightly higher for the composites produced from the block copolymer
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Fig. 2. QCM-D show adsorption of PEI around t=400 s. Washing with tris base at around t=750 s. Adsorption of NFC around t=1100 s. Washing with tris base around t=1800 s. Adsorption of quaternized block copolymers at around 2250s. Washing with tris base at around t=2800 s. The left y-axis shows the fundamental frequency change as calculated from the 3rd overtone. The right y-axis displays the change in energy dissipation following the adsorption.
Fig. 3. FT-IR spectrum of a dried neat NFC film and dried composites with varying PDEGMA block length, prepared by vacuum filtration.
containing the longer PDEGMA blocks, which could indicate that a larger mass of polymer is adsorbed when longer PDEGMA blocks are utilized. Since the qPDMAEMA block was the same in all three block copolymers, this difference is attributed to the molecular weight of the PDEGMA block. Differences between the spectra of NFC and the composites can also be seen in the cellulose fingerprint region, were peaks appear around 1250 cm1, 1100 cm1, and 850 cm1 in the spectra of the composites. In Fig. 4 the thermograms from the TGA-analyses are shown for composites of all three block copolymers and of neat NFC. The results clearly show a different degradation behavior of the pure NFC, in comparison with those of the composite materials, when heated under nitrogen atmosphere. NFC degrades in a two-step process, in which the first step (250–380 °C) is known as thermal cracking, and the second step starting at 380 °C is where NFC undergoes
Fig. 4. TGA results for a dried NFC film and the three nanocomposites prepared with block-copolymers of different PDEGMA block lengths.
carbonization [48]. As can be seen in Fig. 4, the composite materials have two different slopes in their curves, indicating two different types of degradation in each sample. The first degradation (270–330 °C) most probably corresponds to the block copolymers, and the second degradation (330–460 °C) to the degradation of NFC. The results from the TGA indicate that the composite formed have a relatively high temperature stability, as little degradation occurs at temperatures below 250 °C under nitrogen atmosphere. FE-SEM was used to compare neat NFC films to composites made from the polymer containing the shortest PDEGMA block. Images were captured at both 5000 and 25,000 magnification as shown in Fig. 5. In the images with a 5000 magnification, a clear difference in appearance between the materials can be seen, in which the layers in the neat NFC film are more closely packed than in the composite film.
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Fig. 5. FE-SEM images of (a and c) neat NFC films and (b and d) composite films made from NFC and qPDMAEMA-block-PDEGMA234.
Fig. 6. (a) Image of modified NFC in aqueous solutions, with increasing molecular weight of the PDEGMA block from left to right, below LCST. (b) Image of modified NFC in aqueous solutions, with increasing molecular weight of the PDEGMA block from left to right, above LCST.
In the images with 25,000 magnification this difference can be seen even more evidently. There may naturally be several explanations to this phenomena but one obvious explanation is that the adsorbed polyelectrolyte forms a steric barrier on the surface of NFC. When the water is removed, and the fibrils forced towards each other, this steric barrier will prevent the close-packing of the NFC that is detected in the neat NFC film. From the adsorption measurements it could be calculated that 8.9 mg/m2 NFC adsorbed 7.5 mg/m2 polyelectrolyte and assuming that the immobilized water is about the same for the NFC and the polyelectrolyte it is obvious that the surfaces of the fibrils are saturated with polymer which can explain why the fibrils do not pack in the same way as in the neat NFC. To investigate the thermo-responsive properties of NFC modified with block copolymers, vials with grafted NFC in aqueous tris base solutions (0.083% dry weight of NFC) were heated above the LCST of PDEGMA. Images can be seen in Fig. 6, in which three samples of NFC, physically grafted with the three quaternized block copolymers, is
displayed prior to, and just after, heating. As can be seen in the figure, the grafted NFC forms aggregates when the solution is heated, clearly indicating that the PDEGMA block is above its LCST and that it has provided the grafted NFC in solution with thermo-responsive properties. When the solution was left to cool down, the grafted NFC redispersed in the aqueous solutions, showing that the behavior is reversible. Aqueous solutions of ungrafted NFC did not display a thermo-responsive behavior when heated hence; the responsiveness of the grafted NFC originates from the block copolymer adsorption. 4. Conclusions In this paper, block copolymers were synthesized using standard ATRP conditions, in which one block was composed of PDMAEMA and the other block was composed of the thermo-responsive polymer PDEGMA. Three block copolymers were produced with varying length of the PDEGMA block, while the PDMAEMA block length was
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kept constant. All three block copolymers had narrow polydispersities (1.1 > Ð < 1.4). Quaternization of the PDMAEMA block, in all three block copolymers, introduced cationic charges and the formed polyelectrolytes had thermo-responsive properties with LCSTs ranging from 23 to 27 °C, measured by DLS in PBS buffer. The block copolymers were adsorbed to negatively charged NFC in water dispersion by electrostatic interactions. FT-IR and TGA of the dried composite material showed that the polyelectrolytes had been adsorbed onto the NFC, also indicating that the longer the block of the PDEGMA, the more polyelectrolyte, calculated by mass, was adsorbed. This was further supported by adsorption measurements performed in the QCM-D. From TGA analyses it was also clear that the composite materials exhibited a different degradation behavior compared to that of neat cellulose. Thermo-responsive behavior could finally be showed by ocular inspection of solutions of grafted NFC while heating and cooling. Hence, by this methodology, thermo-responsive NFC materials can be produced in a straight-forward manner in water dispersion, without performing any chemical reactions on the NFC. Acknowledgements The authors would like to thank Wallenberg Wood Science Center (WWSC), the Swedish Research Council (VR) and the Swedish Research Council Formas for financial support. Kasinee Prakobna and Anna Svensson are thanked for assistance with FE-SEM and polyelectrolyte titration measurements, respectively. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.eurpolymj.2013.05.023. References [1] Turbak AF, Snyder FW, Sandberg KR. J Appl Polym Sci: Appl Polym Symp 1983;37:815–27. [2] Henriksson M, Berglund LA, Isaksson P, Lindström T, Nishino T. Biomacromolecules 2008;9:1579–85. [3] Eichhorn SJ, Dufresne A, Aranguren M, Marcovich NE, Capadona JR, Rowan SJ, et al. J Mater Sci 2010;45:1–33. [4] Siqueira G, Bras J, Dufresne A. Polymers. vol. 2. Basel, Switzerland; 2010. p. 728–765. [5] Siro I, Plackett D. Cellulose. vol. 17. Dordrecht, Netherlands; 2010. p. 459–494. [6] Klemm D, Kramer F, Moritz S, Lindström T, Ankerfors M, Gray D, et al. Angew Chem Int Edit 2011;50:5438–66. [7] Lönnberg H, Fogelström L, Berglund L, Malmström E, Hult A. Eur Polym J 2008;44:2991–7. [8] Roy D, Semsarilar M, Guthrie JT, Perrier S. Chem Soc Rev 2009;38:2046–64.
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