polymers Article
Surface Modification of Wood Flour via ARGET ATRP and Its Application as Filler in Thermoplastics Martin Kaßel, Julia Gerke, Adrian Ley and Philipp Vana * Institute of Physical Chemistry, University of Göttingen, Tammannstr. 6, D-37077 Göttingen, Germany;
[email protected] (M.K.);
[email protected] (J.G.);
[email protected] (A.L.) * Correspondence:
[email protected]; Tel.: +49-(0)551-39-12753 Received: 21 February 2018; Accepted: 20 March 2018; Published: 22 March 2018
Abstract: Wood flour is particularly suitable as a filler in thermoplastics because it is environmentally friendly, readily available, and offers a high strength-to-density ratio. To overcome the insufficient interfacial adhesion between hydrophilic wood and a hydrophobic matrix, a thermoplastic polymer was grafted from wood flour via surface-initiated activators regenerated by electron transfer-atom transfer radical polymerization (SI-ARGET ATRP). Wood particles were modified with an ATRP initiator and subsequently grafted with methyl acrylate for different polymerization times in the absence of a sacrificial initiator. The successful grafting of poly(methyl acrylate) (PMA) was demonstrated using attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and water contact angle (WCA) measurements. To confirm the control over the polymerization, a cleavable ATRP initiator was immobilized on the particles, allowing the detachment of the grafted polymer under mild conditions. The grafted particles were incorporated into a PMA matrix using solvent casting and their influence on the mechanical properties (Young’s modulus, yield strength, and toughness) of the composite was investigated. Tensile testing showed that the mechanical properties improved with increasing polymerization time and increasing ratio of incorporated grafted particles. Keywords: ARGET ATRP; wood flour; composite; surface modification; graft polymerization; mechanical properties; tensile testing; solvent casting; poly(methyl acrylate) (PMA)
1. Introduction Composites are made by combining two or more components to give a new material with enhanced properties. One early example of composites is the utilization of mud bricks reinforced with twigs or straw as a construction material [1]. The idea of composites was not invented by mankind; nature already provided high-performance composites in the form of wood. Wood has a highly sophisticated cellular structure which consists of rigid cellulose fibers embedded in a soft matrix of lignin and hemicellulose [2,3]. Wood is a particularly suitable filler for composites because it is cheap, renewable, very abundant on earth, and offers a high strength-to-density ratio. Over the last decades there has been a rapid growth in interest in wood-reinforced thermoplastics due to their outstanding properties such as high durability, light weight, and low maintenance [4]. Additionally, wood and polymer wastes can be used in these products, which will help to close the loop for conserving natural resources (cradle-to-cradle concept) [5]. However, wood has several disadvantages such as poor wettability, dimensional stability (shrinking and swelling behavior), and susceptibility to microorganisms [1,6]. Furthermore, the hydrophilic nature of wood, which originates from the high amount of surface hydroxyl groups, leads to poor compatibility in a hydrophobic thermoplastic matrix and, therefore, to low tensile strength and durability [2,7]. Graft polymerization is a powerful tool Polymers 2018, 10, 354; doi:10.3390/polym10040354
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for surface modification to achieve better adhesion to the matrix. Graft polymerization can be done through treatment of the wood surface with aqueous solutions of hydrogen peroxide with ferrous iron or ceric ammonium nitrate, resulting in the formation of radicals on the surface. Afterwards, the resulting wood surface is immersed in a suitable monomer solution (compatible with the intended polymer matrix) for a certain period of time [8]. However, the radicals are formed under harsh conditions which result in a partial destruction of the surface. A more elegant way is the utilization of surface-initiated reversible-deactivation radical polymerization (RDRP) methods, since they can be conducted under mild conditions and the properties of the polymer can be tailored [9,10]. Atom transfer radical polymerization (ATRP) is one of the most commonly applied reversible-deactivation radical polymerization techniques. Particularly, the broad range of commercially available monomers, simple experimental setup, and readily available initiators and catalysts have contributed to its popularity in both industrial and academic science [11,12]. Surface-initiated ATRP (SI-ATRP) is a well-suited technique for the modification of surfaces with polymers, due to the fact that a vast amount of surface-tethered functional groups can be converted into ATRP initiators in a straightforward fashion with commercially available substances [12–14]. In particular, natural substances offer numerous superficial hydroxyl groups, which can be used for graft polymerizations. SI-ATRP has been successfully conducted on silk [15]; chitosan [16]; rice husk [17]; lignin [18,19]; cellulose [20–27]; and lignocellulosic material, such as jute fibers [28], wood pulp [29,30], and wood in form of flour and fibers [31–34]. Furthermore, SI-ATRP can be conducted exclusively on the surface without the presence of an initiator in solution. Therefore, the formation of an untethered polymer, which otherwise has to be removed after the polymerization, is prevented [35]. Besides these advantages, ATRP has two decisive drawbacks. Firstly, the polymerization must be conducted under a completely inert atmosphere because oxygen acts as a radical trap. Secondly, the required amount of transition metal, most commonly Cu(I), can be problematic with regards to toxicology and economics [13]. In 2006, a novel, more robust, and environmentally friendly ATRP technique called activators regenerated by electron transfer (ARGET) ATRP was introduced [36,37]. ARGET ATRP relies on slow and steady in situ regeneration of the Cu(I) from Cu(II) by a reducing agent throughout the polymerization. Due to the constant Cu(I) generation and large excess of reducing agent, ARGET ATRP can be performed with a significant reduction of copper and under limited amounts of oxygen. The aim of this study was to modify the surface of wood flour used as a reinforcing filler material in a hydrophobic polymer matrix. To the best of our knowledge, there are no studies on the modification of wood flour via surface-initiated ARGET ATRP with respect to further use as a filler material in thermoplastics. Utilization of SI-ARGET ATRP allowed us to graft a hydrophobic polymer on the wood surface to improve the compatibility between wood and the polymer matrix. Both the amount of surface-tethered polymer as well as the amount of grafted particles incorporated into the matrix were investigated with regard to their influence on the mechanical properties of the composite. 2. Materials and Methods 2.1. Materials Acetone (ACS reagent, VWR Chemicals, Radnor, PA, USA); anisole (99%, Acros Organics, Geel, Belgium); ascorbic acid (AsAc, reagent grade, Sigma Aldrich, Saint Louis, MO, USA); azobisisobutyronitrile (AIBN, ≥98%, Fluka, Buchs, Switzerland), which was recrystallized from methanol; 2-bromoisobutyryl bromide (BIBB, 98%, Sigma Aldrich); Copper(II) bromide (CuBr2 , 99%, Sigma Aldrich); 2-cyano-2-propyl dodecyl trithiocarbonate (98%, Sigma Aldrich); dichloromethane (DCM, HPLC Grade, Fluka); 4-dimethylaminopyridine (DMAP, 99%, Sigma Aldrich); dithiothreitol (DTT, Cleland’s reagent, ≥99%, Sigma Aldrich); 2-hydroxyethyl disulfide (technical grade, Sigma Aldrich); methanol (ACS reagent, VWR Chemicals); methyl acrylate (MA, 99%, Acros Organics), which was passed through a column containing aluminum oxide (basic, Brockmann I, 150 mesh, Sigma Aldrich) before use and stored at 4 ◦ C until use; oxalyl chloride (98%, Alfa Aesar, Heverville, MA,
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USA); propylene glycol methyl ether acetate (PGMEA, ≥99.5%, ReagentPlus® , Sigma Aldrich); N, N 0 , N 0 , N”, N”-pentamethyldiethylenetriamine (PMDETA, 99%, Sigma Aldrich); 2-propanol (IPA, ≥99.5%, Sigma Aldrich); pyridine (Py, SeccoSolv, Merck KGaA, Darmstadt, Germany); succinic anhydride (≥99%, Sigma Aldrich); tetrahydrofuran (THF, ACS reagent, VWR Chemicals); toluene (Tol, ACS reagent, VWR Chemicals); triethylamine (TEA, ≥99%, Sigma Aldrich); wood flour (Arbocel® , natural raw cellulose, Grade C 100, J. Rettenmaier & Söhne GmbH & Co. KG, Rosenberg, Germany, particle size was 70–150 µm with spruce as dominant component). 2.2. Pretreatment of Wood Flour Wood flour was extracted using a Soxhlet apparatus using dichloromethane for 16 h, followed by drying at 60 ◦ C under reduced pressure. Finally, the flour was ground with a mortar and pestle and kept in a desiccator until use. 2.3. Immobilization of the ATRP Initiators The immobilization of the initiator was inspired by Schwellenbach et al. [38]. One gram of dry wood flour was suspended in a solution of 100 mL DCM (90 wt %) containing triethylamine (TEA, 5 wt %). The resulting suspension was cooled to 0 ◦ C and 2-bromoisobutyryl bromide (BIBB, 5 wt %) was added dropwise over 15 min and stirred for additional 30 min. The reaction mixture was stirred at room temperature for 23 h. Afterwards, the flour was filtered off and washed with copious amounts of tetrahydrofuran, acetone, 2-propanol, and dichloromethane; dried at 60 ◦ C under reduced pressure; and stored in a desiccator until use. The immobilization of the disulfide initiator was conducted in a similar fashion, but in a solution of DCM (93 wt %) containing pyridine (2 wt %) and initiator (5 wt %). 2.4. Synthesis of Poly(Methyl Acrylate) (PMA) as Polymer Matrix Methyl acrylate (MA, 5.3 mL, 58.1 mmol, 480 eq.), 2-cyano-2-propyl trithiocarbonate (40 mg, 0.166 mg, 1.0 eq.), and azobisisobutyronitrile (AIBN, 2.0 mg, 12.0 µmol, 0.1 eq.) were dissolved in toluene (10.5 mL) and degassed with argon for 10 min at 0 ◦ C. The mixture was polymerized at 60 ◦ C for 23 h, cooled down to room temperature, precipitated twice in methanol, and dried at 80 ◦ C under reduced pressure. The molecular weight of poly(methyl acrylate) (PMA) was 3.6 × 104 g·mol−1 , the dispersity was 1.1, and it had a glass transition temperature of 19 ◦ C (measured via DSC). 2.5. Polymerization on Wood Surfaces via ARGET ATRP The initiator-modified wood particles (200 mg) were suspended in a solution of methyl acrylate (10 g, 116 mmol, 400 eq.), anisole (10 g, 93 mmol), CuBr2 (6.5 mg, 29 µmol, 0.1 eq.), and PMDETA (50 mg, 290 µmol, 1.0 eq.) before ascorbic acid (AsAc, 51 mg, 290 µmol, 1.0 eq.) was added. The flask was sealed and degassed with argon for 5 min. Afterwards, the reaction mixture was heated to 60 ◦ C for the specific period (0.75 to 9.5 h). When the polymerization time was reached, the particles were filtered off, washed with tetrahydrofuran and acetone, dried at 60 ◦ C under reduced pressure, and stored in a desiccator. 2.6. Synthesis and Cleavage of the Disulfide Initiator The synthesis of the ATRP initiator containing a disulfide bond was synthesized according to literature, except that the acid chloride was freshly prepared before immobilization. The cleavage of the disulfide-modified grafted wood (100 mg) was conducted using DTT (160 mg) and TEA (0.3 mL) in THF (10 mL). Afterwards, the wood particles were washed with an excess of THF and dried [39]. 2.7. Preparation of Composite Materials Poly(methyl acrylate) (PMA, 4 g, see Section 2.4) was dissolved in propylene glycol methyl ether acetate (PGMEA, 8 mL) to obtain a highly viscous solution. Wood particles (0–10 wt %, see Section 2.5)
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were added to this solution and the mixture was stirred until the particles were well dispersed (4 to 6 h). Dogbone-shaped specimens were prepared via solvent casting of the mixture by slowly drying over several days at 100 ◦ C under reduced pressure. 2.8. Methods 2.8.1. Size-Exclusion Chromatography (SEC) Molar mass distributions were determined with an Agilent 1260 infinity system (Agilent, Santa Clara, CA, USA) consisting of an autosampler (Agilent 1260 ALS G1329B), an isocratic HPLC pump (Agilent 1260 Infinity ISO), a PSS-SDV (Polymer Standard Services) precolumn, three PSS-SDV separation columns (8 × 300 mm, particle size 10 µm, pore sizes of 106 , 105 , and 103 Å) and an RI detector (Agilent 1260 G1362A). The setup was maintained at 35 ◦ C and tetrahydrofuran (THF) was used as the eluent (flow rate of 1 mL·min−1 ). The system was calibrated via a conventional calibration method using narrowly PMMA standards (PSS, MP = 800 to 1,600,000 g·mol−1 ) and toluene as internal standard. The molar masses of PMA were obtained using the Mark–Houwink parameters (K = 1.95 × 10−2 mL·g−1 , a = 0.660) according to the principle of universal calibration [40,41]. All polymer samples (4 mg·mL−1 ) were filtered prior to injection (VWR, PTFE filter, 0.45 µm). 2.8.2. Attenuated Total Reflectance Fourier-Transform Infrared Spectroscopy (ATR-FTIR) ATR-FTIR spectra were recorded using a Bruker IFS 88 (Bruker Optik GmbH, Ettlingen, Germany), equipped with a Harrick MVP 2 StarTM ATR unit, a KBr beam splitter, a MCT detector, a globar, and a tungsten halogen lamp. The spectra were recorded with 32 scans per sample, in a range from 750 to 4000 cm−1 , and at a resolution of 2 cm−1 . 2.8.3. Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray Spectroscopy (EDX) Scanning electron microscopy was recorded on an FEI Quanta FEG 250 (FEI Company, Hillsboro, OR, USA) equipped with an Everhart-Thornley detector. Prior to investigation, the samples were coated with gold. The images were taken using an acceleration voltage of 30 kV in high vacuum with a magnification of 2000. Energy-dispersive X-ray spectroscopy measurements were conducted three times per sample using the same acceleration voltage but recorded with a SSD (Silicon Drift Detector, EDAX Inc., Mahwah, NJ, USA). Bromine content was given as weight percent (wt %). Errors were given as the error of the respective integral. 2.8.4. Differential Scanning Calorimetry (DSC) DSC measurements were performed using a NETZSCH DSC 214 Polyma (Netzsch, Selb, Germany) with an automatically controlled liquid nitrogen cooling device in a temperature range from −50 to 150 ◦ C. Measurements were performed with a heating rate of 10 ◦ C·min−1 and a constant nitrogen flow of 40 mL·min−1 . 2.8.5. Tensile Testing Tensile testing was conducted on a ZWICK & ROELL Z2.5 instrument (Zwick ROELL, Ulm, Germany) equipped with an air conditioner at 24.9 ± 0.3 ◦ C. All measurements were performed with a strain rate of 0.25 mm·s−1 and a loading speed of 25 mm·min−1 . The cross section of the specimen was taken at three different places and averaged. The contact pressure was 0.8 bar. Tensile data (Young’s modulus E, toughness UT , and yield strength σy ) reported herein were averages taken from at least three specimens per sample. The Young’s modulus (E) was calculated in the linear region of 0.05% to 0.20% elongation. Errors were given as maximum error. The data were collected and analyzed with the computer program testXpert II (Zwick ROELL) or ORIGINPRO 8.5G (Originlab, Northampton, MA, USA).
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analyzed with the computer program testXpert II (Zwick ROELL) or ORIGINPRO 8.5G (Originlab, Northampton, MA, USA). Polymers 2018, 10, 354
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2.8.6. Thermogravimetric Analysis (TGA) performed with a NETZSCH TG 209 F3 Tarsus instrument 2.8.6.Thermogravimetric Thermogravimetricanalysis Analysiswas (TGA) (Netzsch, Selb, Germany). The sample was placed in an aluminum oxide crucible and heated from Thermogravimetric analysis was performed with a NETZSCH TG 209 F3 Tarsus instrument room temperature to 1000 °C with a heating rate of 10 °C·min−1 and under a constant nitrogen flow (Netzsch, Selb, Germany). The sample was placed in an aluminum oxide crucible and heated from of 20 mL·min−1. The data were collected and analyzed with the computer program NETZSCH Proteus room temperature to 1000 ◦ C with a heating rate of 10 ◦ C·min−1 and under a constant nitrogen flow of Thermal Analysis 6.1.0 (Netzsch). 20 mL·min−1 . The data were collected and analyzed with the computer program NETZSCH Proteus Thermal Analysis 6.1.0 (Netzsch). 2.8.7. Water Contact Angle (WCA) Measurements 2.8.7.Water Watercontact Contactangle Angle (WCA) Measurements measurements were conducted using the static sessile drop technique on a Dataphysics OCA 15EC (Dataphysics Instruments GmbH, Filderstadt, Germany) with a measuring Water contact angle measurements were conducted using the static sessile drop technique on a range from 0 to 180°, equipped with an LED backlight and a VGA camera with 752 × 480 pixels. Dataphysics OCA 15EC (Dataphysics Instruments GmbH, Filderstadt, Germany) with a measuring Measurements were conducted at 21 °C and with droplets of 5 μL demineralized water. Prior to WCA range from 0 to 180◦ , equipped with an LED backlight and a VGA camera with 752 × 480 pixels. measurements, the wood flour was dried at 60 °C under reduced pressure and pressed into a pellet Measurements were conducted at 21 ◦ C and with droplets of 5 µL demineralized water. Prior to WCA with a diameter of 1 cm. measurements, the wood flour was dried at 60 ◦ C under reduced pressure and pressed into a pellet with a diameter of 1 cm. 2.8.8. Fluorescence Microscopy 2.8.8.Dried Fluorescence Microscopy wood flour containing thiol groups was immersed in a solution of 32 μg·mL−1 fluorescence dye (Abberior LIVE 510) in PBS (phosphate-buffered saline, pH and shaken in the dark. Dried wood flour containing thiol groups was immersed in a7)solution of 32 overnight µg·mL−1 fluorescence Afterwards, the flour was washed four times with PBS, centrifuged, and put on a microscope dye (Abberior LIVE 510) in PBS (phosphate-buffered saline, pH 7) and shaken overnight in the glass dark. slide (Thermo Fisher, MA, USA). Fluorescence images wereand taken with microscope Leica Afterwards, the flour Waltham, was washed four times with PBS, centrifuged, put on aa microscope glass DMi8 (Leica Microsystems GmbH, Germany) equipped with taken a 63× with oil immersion objective. slide (Thermo Fisher, Waltham, MA,Wetzlar, USA). Fluorescence images were a microscope Leica The excitation was performed with an argon krypton laser, which was set to a gain of 500 mV and an DMi8 (Leica Microsystems GmbH, Wetzlar, Germany) equipped with a 63× oil immersion objective. intensity of 2%.was Theperformed excitationwith wavelength set tolaser, 488 nm andwas the detection range set mV to 500–570 The excitation an argonwas krypton which set to a gain of 500 and an nm. intensity of 2%. The excitation wavelength was set to 488 nm and the detection range set to 500–570 nm.
3. 3. Results Results and and Discussion Discussion 3.1. Polymerization of Wood Surfaces via ARGET ATRP ATRP Prior to to modification, modification,the thewood wood flour was extracted for h16 h with a Soxhlet apparatus flour was extracted for 16 with a Soxhlet apparatus using ◦ using dichloromethane as solvent and afterwards C under reduced pressure. dichloromethane (DCM)(DCM) as solvent and afterwards dried dried at 60 at °C60under reduced pressure. The The modification poly(methyl acrylate) (PMA) performed a two-step process, outlined modification withwith poly(methyl acrylate) (PMA) waswas performed in aintwo-step process, outlined in in Scheme 1. First, the ATRP initiator was immobilized on the surface; second, polymer chains were Scheme 1. First, the ATRP initiator was immobilized on the surface; second, polymer chains grafted via via surface-initiated surface-initiated ARGET ARGET ATRP. ATRP. The conducted through through esterification esterification grafted The immobilization immobilization was was conducted of the surface hydroxyl groups with 2-bromoisobutyryl bromide (BIBB) in the presence of DCM of the surface hydroxyl groups with 2-bromoisobutyryl bromide (BIBB) in the presence of DCM as as solvent and solvent and triethylamine triethylamine (TEA) (TEA) as as auxiliary auxiliary base. base. O Br
O
OH DCM, 0 °C
rt, TEA
O
O
Br
Br
O
O CuBr2, PMEDTA, AsAc anisole, 60 °C
O
O
O Br n
Scheme of methyl methyl acrylate acrylate from from wood wood surfaces. surfaces. Scheme 1. 1. Initiator Initiator immobilization immobilization and and ARGET ARGET ATRP ATRP of
The grafting of of poly(methyl poly(methyl acrylate) acrylate) was was performed performed by by placing placing the the initiator-functionalized initiator-functionalized The grafting particles in a one-pot mixture of methyl acrylate (MA), anisole, Copper(II) bromide (CuBr (CuBr2),), particles in a one-pot mixture of methyl acrylate (MA), anisole, Copper(II) bromide 2 pentamethyldiethylenetriamine (PMDETA), and ascorbic acid (AsAc). The suspension was purged pentamethyldiethylenetriamine (PMDETA), and ascorbic acid (AsAc). The suspension was purged with argon argon for for 55 min. min. The was conducted conducted at at 60 60 °C ◦ C with with The polymerization polymerization was with polymerization polymerization times times ranging ranging between 45 min and 9.5 h. The choice of anisole as solvent is crucial because on the one a between 45 min and 9.5 h. The choice of anisole as solvent is crucial because on the onehand, hand,it itis is good solvent for the catalyst system and on the other hand, it is a poor solvent for AsAc. Since AsAc a good solvent for the catalyst system and on the other hand, it is a poor solvent for AsAc. Since AsAc is a very strong reducing agent, poor solubility limits its reduction potential, preventing high radical concentrations and, therefore, loss of control over the polymerization [42]. Additionally,
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is a very strong reducing agent, poor solubility limits its reduction potential, preventing high radical is a very strong and, reducing agent,loss poor limits reduction potential, preventing high concentrations therefore, ofsolubility control over theitspolymerization [42]. Additionally, the radical ligand the ligand PMDETA was used in a tenfold excess in order to ensure the formation of the intended concentrations and, in therefore, of control over the polymerization Additionally, the ligand PMDETA was used a tenfoldloss excess in order to ensure the formation[42]. of the intended metal metal ligand complex thein polymerization, the washed with anexcess excess of PMDETA was usedAfter in [43,44]. a tenfold excess order tothe ensure thesamples formation of the intended ligand complex [43,44]. the After polymerization, samples were were washed with anmetal of tetrahydrofuran. The grafted samples were characterized by FTIR spectroscopy, scanning electron complex [43,44]. The After the samples polymerization, the samples werespectroscopy, washed with an excess of tetrahydrofuran. grafted were characterized by FTIR scanning electron microscopy thermogravimetric (TGA), (TGA), and water contact angle (WCA) measurements. tetrahydrofuran. The grafted samplesanalysis wereanalysis characterized by FTIR spectroscopy, scanning electron microscopy (SEM), (SEM), thermogravimetric and water contact angle (WCA) Figure 1 shows the recorded FTIRthe spectra of untreated woodand (A), (B), microscopy (SEM), thermogravimetric analysis water contact (WCA) measurements. Figure 1 shows recorded FTIR(TGA), spectra of initiator-immobilized untreated wood angle (A), wood initiatorand PMA-grafted wood polymerization of 2spectra h (C) and h (D).ofFor of5simplicity, measurements. Figure 1with shows the recorded FTIR of 5untreated (A), immobilized wood (B), and PMA-grafted woodtimes with polymerization times 2 wood hreason (C) and hinitiator(D). For spectra of other polymerization times are exclusively shown in the supporting materials S1). immobilized wood (B), and PMA-grafted wood with polymerization times of 2 h (C) h (D). For reason of simplicity, spectra of other polymerization times are exclusively shown inand the 5(Figure supporting It can beofclearly seen polymerization results in polymerization thetimes emergence ofresults four distinct 1725, reason simplicity, spectra of polymerization are exclusively shown in theatsupporting materials (Figure S1).that It can beother clearly seen that in the peaks emergence of 1435, four −1 (shown in red) attributed to PMA [45]. The intensity of the PMA peaks increases 1153, and 826 cm −1 materials (Figure S1). 1435, It can1153, be clearly seen polymerization results to inPMA the emergence of four distinct peaks at 1725, and 826 cmthat (shown in red) attributed [45]. The intensity −1 (shown in blue) decreases with progressing whereas the peaks intensity of1435, the wood at 1100–900 −1 (shown distinct peaks at 1725, 1153, peaks and 826 cm−1 (shown in red)peaks attributed to PMA The intensity of the PMA increases whereas the intensity of cm the wood at 1100–900 cm[45]. in blue) polymerization time [46]. −1 of the PMAwith peaks increases whereas the intensity of the wood peaks at 1100–900 cm (shown in blue) decreases progressing polymerization time [46]. decreases with progressing polymerization time [46].
Figure 1. FTIR spectra of untreated wood (A); initiator-immobilized wood (B); and PMA-grafted Figure 1. FTIR spectra of untreated wood (A); initiator-immobilized wood (B); and PMA-grafted wood wood with polymerization times of 2 wood h (C) and h (D). Figure 1. FTIR spectra of untreated (A);5 initiator-immobilized wood (B); and PMA-grafted with polymerization times of 2 h (C) and 5 h (D). wood with polymerization times of 2 h (C) and 5 h (D).
SEM studies were employed to capture the surface topography of unmodified wood (Figure 2a), SEM studies studies were were employed employed to capture capture the surface surface topography ofparticles unmodified wood (Figure 2a), 2a), SEM to the topography unmodified initiator-functionalized wood (Figure 2b), and PMA-grafted woodof afterwood 5 h (Figure 2c). initiator-functionalized wood (Figure 2b), and and PMA-grafted wood particles particles after 55 hh (Figure (Figure 2c). initiator-functionalized wood (Figure 2b), PMA-grafted wood after Before modification, wood exhibits a fibrous structure with a smooth surface. Immobilization of 2c). the Before modification, wood exhibits a fibrous structure with a smooth surface. Immobilization of the Before modification, wood exhibits a fibrous structure with a smooth surface. Immobilization of the ATRP initiator resulted in no significant topographical change. After graft polymerization, however, ATRP initiator in significant change. Afterlayer graft [34]. polymerization, however, however, ATRP initiator resulted in no no significant topographical change. After graft polymerization, the surface wasresulted rough and irregular duetopographical to a clearly visible polymer the surface the surface was was rough rough and and irregular irregular due due to to aa clearly clearly visible visible polymer polymer layer layer [34]. [34].
Figure 2. SEM images of untreated wood flour (a); initiator-functionalized wood flour (b); and PMAgrafted2.wood with a polymerization time(a); of 5initiator-functionalized h (c). Figure SEM flour images of untreated wood flour wood flour (b); and PMAFigure 2. SEM images of untreated wood flour (a); initiator-functionalized wood flour (b); grafted wood flour with a polymerization time of 5 h (c). and PMA-grafted wood flour with a polymerization time of 5 h (c).
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Thermogravimetric analysis to study the thermal behavior of the grafted Thermogravimetric analysiswas wasconducted conducted to study the thermal behavior of the samples. grafted The thermogram (TG) and the respective derivation (DTG) of unmodified and modified wood as well samples. The thermogram (TG) and the respective derivation (DTG) of unmodified and modified as of pure PMA in Figure 3, and the corresponding decomposition temperatures (Ton : wood as well asare of depicted pure PMA are depicted in Figure 3, and the corresponding decomposition onset degradation temperature, T : temperature at the maximal degradation state) and mass losses max temperatures (Ton: onset degradation temperature, Tmax: temperature at the maximal degradation ◦ C with a maximum (ML) are Table Untreated wood starts degrade wood sharplystarts at 200 state) andsummarized mass losses in (ML) are1.summarized in Table 1. to Untreated to degrade sharply at ◦ ◦ at 360 C and a total mass loss of 69% in this stage. Afterwards, at 450 C, wood decomposes at 200 °C with a maximum at 360 °C and a total mass loss of 69% in this stage. Afterwards, atfurther 450 °C, a much slower steady rate. This result is in good agreement with decomposition measurements wood decomposes further at a much slower steady rate. This result is in good agreement with of the three main wood components—cellulose, hemicellulose, and lignin. The degradation of decomposition measurements of the three main wood components—cellulose, hemicellulose, and ◦ C and 250–380 ◦ C, whereas lignin cellulose and hemicellulose takes place at a high rate at 200–380 lignin. The degradation of cellulose and hemicellulose takes place at a high rate at 200–380 °C and ◦ [47]. Initiator immobilization (Wood ) results in a decrease of slowly decomposes at 180–900 250–380 °C, whereas lignin slowlyCdecomposes at 180–900 °C [47]. InitiatorBrimmobilization (WoodBr) ◦ C and a similar T the onset temperature to 160 . Accordingly, it canTmax be .concluded thatitinitiator results in a decrease of the onset temperature tomax 160 °C and a similar Accordingly, can be immobilization on the wood surface decreases thermal stability, due to two reasons: first, concluded that initiator immobilization on the wood surface decreases thermal stability,esterification due to two of cellulose its crystallinity; second,itshydrogen bromide may be hydrogen formed, which catalyzes reasons: first,damages esterification of celluloseand damages crystallinity; and second, bromide may ◦ the decomposition of wood [31,33]. Pure PMA starts to decompose at 220 C with a maximum be formed, which catalyzes the decomposition of wood [31,33]. Pure PMA starts to decompose at 220 decomposition rate atdecomposition 400 ◦ C and a total 98%. Grafted samples ) show a °C with a maximum rate mass at 400loss °C of and a total mass loss of (Wood-PMA 98%. Graftedxhsamples two-step degradation profile; the steps fluently merge into each other and can be assigned to a wood (Wood-PMA xh) show a two-step degradation profile; the steps fluently merge into each other and can ◦ C) and a PMA decomposition stage (T ◦ C). decomposition stage (T max approx. 360 max approx. 400 be assigned to a wood decomposition stage (Tmax approx. 360 °C) and a PMA decomposition stage Inmax general, graft polymerization of PMA results in of anPMA increased stability (T approx.surface-initiated 400 °C). In general, surface-initiated graft polymerization resultsthermal in an increased ◦ indicated by a decreased loss belowmass 200 loss C. Abelow longer200 polymerization time leads to antime increased thermal stability indicatedmass by a decreased °C. A longer polymerization leads T in the wood stage and, therefore, a shift to pure PMA. A closer look at the mass losses both toon an increased Ton in the wood stage and, therefore, a shift to pure PMA. A closer look at theatmass stagesatshows that the ML decreases in the wood stage progressing time (Table 1, losses both stages shows that the ML decreases in thewith wood stage withpolymerization progressing polymerization blue arrow), but increases in the PMA stage (Table 1, red arrow). time (Table 1, blue arrow), but increases in the PMA stage (Table 1, red arrow).
Figure Figure 3. 3. Thermograms Thermogramsand andderivations derivationsof of untreated untreated wood, wood, Wood WoodBrBr, ,PMA-grafted PMA-graftedwood woodsamples, samples, and pure PMA. and pure PMA.
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TablePolymers 1. Thermal degradation temperatures and mass losses (MLs) of untreated wood, Wood , Polymers 2018, 10, x FOR PEER REVIEW 8 of Br16 2018, 10, x FOR PEER REVIEW 8 of 16 PMA-grafted samples, and pure PMA. Table 1. Thermal degradation temperatures mass losses of untreated wood, WoodBr, PMATable 1. Thermal degradation temperatures and massand losses (MLs) of (MLs) untreated wood, Wood Br, PMAgrafted samples, pure PMA. grafted samples, and pureand PMA.
Wood Stage
PMA Stage
Wood Stage PMA Stage Wood/◦Stage PMA ◦ CStage T on T ML/% max C Tmax/°CML/% ML/% T max /Tmax T on/°C /°C Ton/°C Tmax/°C ML/% Tmax/°C ML/% ML/% 200 360 360 69 Wood Wood Wood 200 200 360 69 - Br 160 360 360 71 WoodBr WoodBr Wood160 160 360 71 - 0.75h 160 360 360 40 410410 410 20 20 Wood-PMA 0.75h Wood-PMA Wood-PMA 0.75h 160 160 360 40 Wood-PMA 170 365 365 37 410410 410 39 39 Wood-PMA 170 2h 170 2h Wood-PMA 2h 365 37 Wood-PMA 5h 190 370 28 410 58 Wood-PMA 190 370 410 Wood-PMA 5h 190 370 28 410 58 5h Wood-PMA 9.5h 190 - 95 Wood-PMA Wood-PMA 9.5h 190 190 400400 400 95 9.5h 220 - 98 PMA 220 220 -PMA PMA 400400 400 98
/◦ C
In addition to the determination of the thermal of the composites, can be In addition to the determination of the thermal behaviorbehavior of the composites, TGA canTGA be used to used to
ratio of polymer grafted on the particles. closer look at the decomposition ofcan the be pureused In addition to determination the of the composites, estimateestimate the the ratiothe of polymer graftedof on the thermal particles. Abehavior closerAlook at the decomposition ofTGA the pure polymer reveals that it is almost completely decomposed at 450 °C. Under the assumption that wood polymer reveals it is almost completely decomposed at 450 Under the assumption that wood to estimate the ratio ofthat polymer grafted on the particles. A °C. closer look at the decomposition of the and in the composites exhibit same thermal as the pureand wood and purethe PMA, the ◦wood and PMA in PMA thethat composites exhibitcompletely the samethe thermal behaviorbehavior as the pure pure PMA, pure polymer reveals it is almost decomposed at 450 C. Under the assumption that mass exclusively should exclusively belong to the undecomposed part ofatwood at this temperature. The leftover leftover mass should belong to the undecomposed part of wood this temperature. The wood and PMA polymer in the composites exhibit the same thermal behavior as the pure wood and pure PMA, can be calculated the following Equations (1)–(3),awhere w represents polymer content content can be calculated with thewith following Equations (1)–(3), where given awgiven represents mass fractions of the respective the leftover should belong to the undecomposed part of wood at this temperature. themass massthe fractions ofexclusively the respective species. species. The polymer content can be calculated withwthe following Equations wpolymer 1 wwood (1)–(3), where a given w represents (1) 1 wwood (1) polymer the mass fractions of the respective species. (2) w 0.24 w 450°Cw wood (2) 0.24 wood w ° w 1° (3) (1) wpolymer =polymer 11 − wwood wpolymer w (3) 0.24 0.24 In general, mass fraction of polymer grafted polymer ) in a composite corresponds to 1 minus In general, the massthe fraction of grafted (wpolymer)(w inpolymer a composite corresponds to 1 minus ◦C = w450 wwood (wwood), as can be seen in0.24 Equation (1). Untreated wood decomposes to a remaining (2) the massthe of mass woodof (wwood wood), as can be seen in Equation (1). Untreated wood decomposes to a remaining of the 24%temperature at the temperature of (as 450can °C be (as seen can be seen in 3), Figure 3),means whichthat means w450 mass of mass 24% at of 450 °C in ◦Figure which the that massthe mass C w = 1 − fraction of the measured grafted wood (w 450°C ) can be expressed by Equation (2). Combination of (3) polymer fraction of the measured grafted wood (w450°C) can be expressed by Equation (2). Combination of 0.24 Equations (1) and (2) results in the final relation of w polymer and w450°C. The determined residual masses Equations (1) and (2) results in the final relation of wpolymer and w450°C. The determined residual masses In general, the mass of grafted polymer (wwpolymer ) in a composite corresponds to 1 minus 450°C 450fraction °C and calculated polymer are shown As can from be seen w450°C at w 450 °Catand calculated polymerpolymer content content wpolymer are shown in Tablein 2. Table As can2.be seen thefrom the data, the residual mass of the grafted particles at 450 °C decreases with progressing polymerization mass of wood (w ), as can be seen in Equation (1). Untreated wood decomposes to a data, the residual woodmass of the grafted particles at 450 °C decreases with progressing polymerization remaining time,leads which toamounts higher amounts of polymer grafted polymer according to Equation (3). ◦ grafted time, which to leads higher of according to Equation (3).
w450°C
the mass of 24% at the temperature of 450 C (as can be seen in Figure 3), which means that the mass ◦ C ) can be expressed by Equation (2). Combination of fraction of the measured Table grafted wood of(wthe 450 2. Estimation amount grafted for different polymerization Table 2. Estimation of the amount of graftedofPMA for PMA different polymerization times. times. ◦ C . The determined residual Equations (1) and (2) results in the final relation of wpolymer and w 450 /%polymer/% wpolymer/% w450°C/%w450°Cw masses w450◦ C at 450 ◦ C and calculated polymer0.75hcontent 24 wpolymer are0 shown in Table 2. As can be Wood-PMA Wood-PMA 0.75h 24 0 ◦ seen from the data, the residual mass of the grafted at Wood-PMA 2h 16 Wood-PMA 2h 16particles 33 450 33C decreases with progressing Wood-PMA 5h 13 46 according to Equation (3). polymerization time, which leadsWood-PMA to higher amounts polymer 5h 13of grafted 46 Wood-PMA 9.5h Wood-PMA 9.5h 4
4
83
83
Table 2. Estimation of the amount of grafted PMA for different polymerization times.
DSC measurements were conducted to complete the thermal characterization of the PMADSC measurements were conducted to complete the thermal characterization of the PMAgrafted(the wood (the are spectra are depicted in the supporting materials, Figure All samples grafted wood spectra depicted in the supporting materials, Figure S2). All S2). samples showedshowed w to the /% w /%which is similar a glass transition temperature (T±g)1of°C, 19450 ± 1◦ C°C, g of the matrix polymer and a glass transition temperature (Tg) of 19 which is similar to the polymer Tg of theTmatrix polymer and became more pronounced with increasing polymerization time. As a result, utilization became more pronounced with increasing polymerization time. As a result, utilization of theseof these Wood-PMA 24 0 0.75h as in a filler in thermoplastic does not the change the thermal behavior of the resulting grafted grafted particlesparticles as a filler thermoplastic does not change thermal behavior of the resulting Wood-PMA2h 16 33 composite and, therefore, itsofscope of application. composite and, therefore, its scope application. Wood-PMA 13 46 5h Water contact angle measurements were performed to evaluate the hydrophobic character of Water contact angle measurements were performed to evaluate the83 hydrophobic character of Wood-PMA 4 9.5h the grafted particles. Prior to measurements, the flour was compressed to obtain a flat surface. Figures the grafted particles. Prior to measurements, the flour was compressed to obtain a flat surface. Figures 4a–cthe show the contact angles for unmodified wood and PMA-grafted wood with polymerization 4a–c show contact angles for unmodified wood and PMA-grafted wood with polymerization times of 2 and 5 h. The unmodified wood surface is very hydrophilic due to numerous hydroxyl DSC times measurements conductedwood to complete characterization ofhydroxyl the PMA-grafted of 2 and 5 h.were The unmodified surface is the verythermal hydrophilic due to numerous groups which results in an immediate absorption of the water and swelling of the sample. In groups which results in an immediate absorption of the water and swelling of the sample. In contrast,contrast,
wood (the spectra are depicted in the supporting materials, Figure S2). All samples showed a glass transition temperature (Tg ) of 19 ± 1 ◦ C, which is similar to the Tg of the matrix polymer and became more pronounced with increasing polymerization time. As a result, utilization of these grafted particles as a filler in thermoplastic does not change the thermal behavior of the resulting composite and, therefore, its scope of application. Water contact angle measurements were performed to evaluate the hydrophobic character of the grafted particles. Prior to measurements, the flour was compressed to obtain a flat surface. Figure 4a–c show the contact angles for unmodified wood and PMA-grafted wood with polymerization times
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of 2 and2018, 5 h.10,The unmodified wood surface is very hydrophilic due to numerous hydroxyl groups Polymers x FOR PEER REVIEW 9 of 16 Polymers 2018, 10, x FOR PEER REVIEW 9 of 16 which results in an immediate absorption of the water and swelling of the sample. In contrast, grafting ◦ and ◦ and 123°, respectively, of PMA on the surface 2 or over 5 h leads angles of 112 which are grafting of on surface 22 or 55 h leads contact angles of 112° grafting of PMA PMA on the theover surface over or to hcontact leads to to contact angles of123 112°, respectively, and 123°, respectively, maintained for several minutes and demonstrate the successful grafting of PMA. which are maintained for several minutes and demonstrate the successful grafting of PMA. which are maintained for several minutes and demonstrate the successful grafting of PMA.
Figure Figure 4. 4. Water Water contact contact angle angle measurements measurements of of untreated untreated wood wood (a) (a) and and PMA-grafted PMA-grafted wood wood with with Figure 4. Water contact angle measurements of untreated wood (a) and PMA-grafted wood with polymerization times of 2 h (b) and 5 h (c). polymerization times of 2 h (b) and 5 h (c). polymerization times of 2 h (b) and 5 h (c).
3.2. 3.2. Selective Selective Cleavage Cleavage of of the the Grafted Grafted Polymer Polymer under under Mild Mild Conditions Conditions 3.2. Selective Cleavage of the Grafted Polymer under Mild Conditions Usually, Usually, aa sacrificial sacrificial initiator initiator is is deployed deployed to to spectroscopically spectroscopically follow follow the the progress progress of of the the Usually, a and sacrificial initiator is deployed tothe spectroscopically follow the progress ofFree the polymerization to obtain information about control over the polymerization [12]. polymerization and to obtain information about the control over the polymerization [12]. Free polymerization and to obtain information about can the control over the polymerization [12]. Free polymer polymer polymer formed formed from from the the sacrificial sacrificial initiator initiator can be be analyzed analyzed by by size-exclusion size-exclusion chromatography chromatography formed from the sacrificial initiator can be analyzed by size-exclusion chromatography (SEC), which (SEC), (SEC), which which provides provides estimations estimations of of the the molecular molecular weight weight and and dispersity dispersity of of the the polymer polymer grafts grafts provides estimations of the molecular weight and dispersity of the polymer grafts [48,49]. Alternatively, [48,49]. [48,49]. Alternatively, Alternatively, polymer polymer grafts grafts can can be be cleaved cleaved off off and and analyzed analyzed directly, directly, but but this this often often requires requires polymer grafts can beleads cleaveddegradation off and analyzed directly, but this often requires harsh conditions harsh harsh conditions conditions and and leads to to degradation of of the the surface surface [50,51]. [50,51]. In In this this study, study, no no sacrificial sacrificial initiator initiator and leads to degradation of the surface [50,51]. In this study, no sacrificial initiator was utilized, was was utilized, utilized, because because the the formation formation of of free free polymer polymer was was undesired. undesired. Instead, Instead, an an ATRP ATRP initiator initiator because the formation of free polymer was undesired. Instead, an ATRP initiator bearing a disulfide bearing bearing aa disulfide disulfide group group was was immobilized immobilized on on the the surface, surface, allowing allowing the the detachment detachment of of the the polymer polymer group was immobilized on the surface, allowing the detachment of the polymer grafts under mild grafts under mild conditions [52,53]. Subsequently, the detached polymer was analyzed by grafts under mild conditions [52,53]. Subsequently, the detached polymer was analyzed by SEC SEC to to conditions [52,53]. Subsequently, the detached polymer was analyzed by SEC to receive information receive receive information information about about the the molar molar mass mass and and dispersity. dispersity. The The synthesis synthesis of of the the disulfide disulfide initiator initiator is is about theinmolar mass and dispersity. The synthesis of the disulfide initiator is outlined in Scheme 2. outlined Scheme 2. outlined in Scheme 2. O O HO HO
S S S S
Br Br OH OH
Br Br HO HO
S S S S
O O O O
O O 1. O O O 1. O Br Br 2. 2 2. (COCl) (COCl)2
O O Cl Cl
O O O O
S S S S
O O O O
Br Br
Scheme Scheme 2. Synthesis of of the the disulfide-containing disulfide-containing ATRP ATRP initiator initiator for subsequent detachment detachment of polymer Scheme 2. 2. Synthesis Synthesis of the disulfide-containing ATRP initiator for for subsequent subsequent detachment of of polymer polymer grafts under mild conditions (according to literature [39]). grafts grafts under under mild mild conditions conditions (according (according to to literature literature [39]). [39]).
The immobilization this initiator was conducted comparably to the immobilization of BIBB The immobilization immobilization of of this this initiator initiator was was conducted conducted comparably comparably to to the the immobilization immobilization of of BIBB BIBB The of except that pyridine (Py) was used as an auxiliary base. It is evident from the energy-dispersive Xexcept that that pyridine pyridine(Py) (Py)was wasused usedasasananauxiliary auxiliary base. evident from energy-dispersive Xexcept base. It It is is evident from thethe energy-dispersive X-ray ray spectroscopy (EDX) data given in Table 3 that the immobilization was successful as indicated by ray spectroscopy (EDX) data given in Table 3 that the immobilization was successful as indicated by spectroscopy (EDX) data given in Table 3 that the immobilization was successful as indicated by the the bromine content of which is similar to the bromine content of the resulting resulting bromine content of 2.0%, 2.0%, which is very very similar the obtained obtained bromine content of 1.7% 1.7% resulting bromine content of 2.0%, which is very similar to thetoobtained bromine content of 1.7% with with BIBB as initiator. It is known in the literature that the extent of polymerization (length of polymer with BIBB as initiator. It is known in the literature that the extent of polymerization (length of polymer BIBB as initiator. It is known in the literature that the extent of polymerization (length of polymer chains chains and is influenced by the total of sites on the chains and dispersity) dispersity) is not not greatly greatly influenced thenumber total number number of initiator initiator sites onsubstrate the substrate substrate and dispersity) is not greatly influenced by theby total of initiator sites on the [50], [50], which means that the grafting densities of BIBB-modified and disulfide-initiator-modified wood [50], which means that the grafting densities of BIBB-modified and disulfide-initiator-modified wood which means that the grafting densities of BIBB-modified and disulfide-initiator-modified wood should should be Afterwards, the particles were grafted with methyl should be comparable. comparable. Afterwards, the disulfide-modified disulfide-modified particles were methyl be comparable. Afterwards, the disulfide-modified particles were grafted withgrafted methylwith acrylate via acrylate via ARGET ATRP for 2 or 5 h. acrylate via ARGET ATRP for 2 or 5 h. ARGET ATRP for 2 or 5 h. Table 3. both immobilized ATRP initiators. Table 3. Comparison Comparison of of both both immobilized immobilized ATRP ATRP initiators. initiators. Table 3. Comparison of
ATRP ATRP initiator initiator BIBB BIBB BIBB Disulfide Disulfide
ATRP initiator Disulfide
Base Base Base TEA TEA TEA Py Py Py
Br Br wt wt % % (EDX) (EDX) Br wt % (EDX) 1.7 1.7 ±± 0.7 0.7 1.7 ± 0.7 2.0 ±± 0.6 2.02.0 0.60.6 ±
The The resulting resulting disulfide-containing disulfide-containing particles particles were were treated treated with with aa solution solution of of 1,4-Dithiothreitol 1,4-Dithiothreitol (DTT) and TEA in tetrahydrofuran (THF) at room temperature for several days (DTT) and TEA in tetrahydrofuran (THF) at room temperature for several days to to cleave cleave off off the the grafted polymer. DTT is a reducing agent bearing two thiol groups which react selectively with grafted polymer. DTT is a reducing agent bearing two thiol groups which react selectively with the the disulfide disulfide bond bond of of the the surface-tethered surface-tethered initiator initiator [39,54]. [39,54]. In In the the reaction, reaction, the the disulfide disulfide group group of of the the
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The resulting disulfide-containing particles were treated with a solution of 1,4-Dithiothreitol (DTT) and TEA in tetrahydrofuran (THF) at room temperature for several days to cleave off the grafted polymer. DTT is a reducing agent bearing two thiol groups which react selectively with the disulfide Polymers 2018, 10, x FOR PEER REVIEW 10 of 16 bond of the surface-tethered initiator [39,54]. In the reaction, the disulfide group of the substrate is reduced, to a leading surface-tethered thiol moiety thiol and the release therelease polymer intopolymer the solution substrateleading is reduced, to a surface-tethered moiety andofthe of the into with a thiol end group, as can be seen in Scheme 3. the solution with a thiol end group, as can be seen in Scheme 3. Polymers 2018, 10, x FOR PEER REVIEW
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SH
S S SH + + + Br Br HS S S SH substrate is reduced, leading to a surface-tethered thiol moiety and the release of the polymer into the solution with a thiol end HO group,OH as can be seen in Scheme 3. HO OH SHreaction of PMA-grafted wood containing a disulfide linker. Scheme 3. Schematic cleavage S S Scheme 3. Schematic cleavage reaction SH of PMA-grafted wood containing a disulfide linker. S S
Br
+
SH
+ HS
Br
+
OH and the obtained peak maximum of the number The polymer was analyzed HO by SEC average HO OH The polymer was analyzed by SEC and the obtained peak maximum of the number average molar mass (Mn,max) and the dispersity (Đ) are displayed in Table 4. The data clearly show a sufficient Scheme Schematic cleavage reaction ofdisplayed PMA-grafted containing a disulfide molar ) 3.and the dispersity (Đ)an are in wood Table 4. The clearlylinker. show a sufficient n,max controlmass over(M the polymerization given by increasing molar mass withdata progressing polymerization control over the polymerization given by an increasing molar mass with progressing polymerization time and The similar dispersities. polymer was analyzed by SEC and the obtained peak maximum of the number average time and similar dispersities.
molar mass (Mn,max) and the dispersity (Đ) are displayed in Table 4. The data clearly show a sufficient Tablegiven 4. Properties of cleaved polymer control over the polymerization by an increasing molar massgrafts. with progressing polymerization Table 4. Properties of cleaved polymer grafts. time and similar dispersities. 3 −1
t/h
Mn,max/10 g·mol
Đ
3 g·mol−1 t/hTable M n,max /10 2 4. Properties 290 1.84 of cleaved polymer grafts.Đ
410 1.88 1.84 290 t/h Mn,max/103 g·mol−1 Đ 410 1.88 2 290 1.84 ATR-FTIR spectroscopy was utilized to confirm the complete removal of the surface-tethered 5 410 1.88 2 5
5
polymer with DTT. Figure 5 shows the spectra of the disulfide initiator-immobilized wood (A), PMAATR-FTIR spectroscopy was utilized to confirm the complete removal of the surface-tethered grafted wood containing disulfide linker (B), and DTT-cleaved wood substrate (C). Afterwood cleavage, ATR-FTIR was utilized to confirm the disulfide complete removal of the surface-tethered polymer with DTT.spectroscopy Figure 5 shows the spectra of the initiator-immobilized (A), −1 (red region) is substantially decreased and the resulting spectrum the polymer signal at 1725 cm polymer with Figure 5disulfide shows thelinker spectra(B), of the initiator-immobilized (A), PMAPMA-grafted woodDTT. containing anddisulfide DTT-cleaved wood substratewood (C). After cleavage, grafted wood containing disulfideof linker (B), andwood DTT-cleaved wood1a); substrate (C). Afterthe cleavage, looks very similar the spectrum untreated (see Figure this successful the polymer signal to at 1725 cm−1 (red region) is substantially decreased and confirms the resulting spectrum the polymer signal at 1725 cm−1 (red region) is substantially decreased and the resulting spectrum cleavage of the polymer. looks very similar to the spectrum of untreated wood (see Figure 1a); this confirms the successful looks very similar to the spectrum of untreated wood (see Figure 1a); this confirms the successful
cleavage of the cleavage ofpolymer. the polymer.
Figure 5. FTIR spectra of disulfideinitiator-immobilized initiator-immobilized wood PMA-grafted wood containing a Figure 5. FTIR spectra of disulfide wood(A); (A); PMA-grafted wood containing a Figure 5. FTIRlinker spectra ofa disulfide initiator-immobilized wood (A); PMA-grafted disulfide with polymerization timeof of 55 h h (B); (B); and wood (C).(C).wood containing a disulfide linker with a polymerization time andDTT-cleaved DTT-cleaved wood disulfide linker with a polymerization time of 5 h (B); and DTT-cleaved wood (C).
The thiol functionalities thesurface, surface, which which result of the disulfide bonds, The thiol functionalities onon the resultfrom fromthe thereduction reduction of the disulfide bonds, werethiol madefunctionalities visible by staining with a fluorescence dye and were subsequently observed with confocal The on the surface, which result from the reduction of the disulfide bonds, were made visible by staining with a fluorescence dye and were subsequently observed with confocal microscopy. The dye bears a maleimide moiety which readily reacts with thiol groups via a thiol-ene were made visible by staining with a fluorescence dye and werereacts subsequently with confocal microscopy. The dye bears a maleimide moiety which readily with thiolobserved groups via a thiol-ene reaction. After immobilization of the dye, the sample was excited at 488 nm and detected at 500–570
reaction. immobilization of the dye, the samplethiol wasgroups excitedand at 488 and detected at 500–570 nm. After A green color shows the presence of former the nm intensity of the color is nm. A green color shows the presence of former thiol groups and the intensity of the color is correlated with the initiator density. Figure 6 shows a dye-treated wood particle after cleavage of the correlated with theimage initiator density. Figuresurface 6 shows a dye-treated wood particle after cleavage polymer. The reveals a satisfying coverage of the wood particle with a few regionsof the with The significantly higher athiol concentration. wood, which particle was used in a few control polymer. image reveals satisfying surface Untreated coverage of the wood with regions showed no fluorescence after treatment with the dye (image not shown). with experiment, significantly higher thiol concentration. Untreated wood, which was used in a control
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microscopy. The dye bears a maleimide moiety which readily reacts with thiol groups via a thiol-ene reaction. After immobilization of the dye, the sample was excited at 488 nm and detected at 500–570 nm. A green color shows the presence of former thiol groups and the intensity of the color is correlated with the initiator density. Figure 6 shows a dye-treated wood particle after cleavage of the polymer. The image reveals a satisfying surface coverage of the wood particle with a few regions with significantly higher thiol concentration. Untreated wood, which was used in a control experiment, showed no fluorescence after treatment with the dye (image not shown). Polymers 2018, 10, x FOR PEER REVIEW 11 of 16
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Figure 6. Fluorescence image of stained wood flour. Figure 6. Fluorescence image of stained wood flour.
3.3. Preparation and Properties of Wood-Reinforced Thermoplastics Figure 6. Fluorescence Thermoplastics image of stained wood flour. 3.3. Preparation and Properties of Wood-Reinforced PMA-grafted wood flour was incorporated in a PMA matrix and its influence on the mechanical PMA-grafted wood flourwas was incorporated in auniaxial PMA matrix its influence onpolymer the mechanical 3.3. Preparation and Properties of Wood-Reinforced Thermoplastics properties of the composites investigated using tensileand testing. The matrix PMA properties of the composites was investigated using uniaxial tensile testing. The matrix polymer PMA was synthesized via wood reversible (RAFT) Dogbone specimens PMA-grafted flour addition-fragmentation was incorporated in a PMA matrixpolymerization. and its influence on the mechanical was synthesized reversible addition-fragmentation (RAFT) polymerization. Dogbone specimens were prepared using solvent was casting. Tensileusing testing was performed atThe room temperature with a properties ofvia the composites investigated uniaxial tensile testing. matrix polymer PMA were prepared solvent Tensile testing wasofperformed room temperature with was synthesized via reversible (RAFT) polymerization. Dogbone specimens constant strainusing of 0.25 mm·s−1casting. . Aaddition-fragmentation constant weight percent wood flouratwith a varying amount of a werestrain prepared using solvent Tensile testing was performed at flour room temperature withE, a the constant of 0.25 mm ·s−1 . casting. A constant weight of wood with a modulus varying amount grafted polymer was incorporated in the matrix andpercent the influence on the Young’s −1. A constant weight percent of wood flour with a varying amount of constant strain 0.25the mm·s strength σy, ofwas and toughness UT the wasmatrix recorded. of pureonPMA and composites of E, of yield grafted polymer incorporated in andSamples the influence the Young’s modulus grafted polymer was incorporated theUmatrix and the influence on the the Young’s modulus E, the flour PMAinserved as references to evaluate reinforcing character of the theunfunctionalized yield strength σwood the in toughness was recorded. Samples of pure PMA and composites y , and T yield wood strength σy, and the toughness UT was stress-strain recorded. Samples ofchosen pure PMA and composites of flour. Figure 7 shows typical plots, from a data set ofcharacter three of grafted unfunctionalized wood flour in PMA served as references to evaluate the reinforcing unfunctionalized wood flour in PMA served as references to evaluate the reinforcing character of the measurements per sample. The corresponding mechanical characteristics are summarized in Table 5. of the grafted wood flour. Figure 7 shows typical stress-strain plots, chosen from a data set of three grafted wood flour. Figure 7 shows typical stress-strain plots, chosen from a data set of three
measurements per sample. The corresponding characteristics summarized in 5. Table 5. measurements per sample. The corresponding mechanical mechanical characteristics are are summarized in Table
Figure 7. Representative pure PMA PMAand andwood-reinforced wood-reinforced thermoplastics. Figure 7. Representativestress-strain stress-strain curves curves of of pure thermoplastics. Figure 7. flour Representative stress-strain curves of pure PMA and wood-reinforced thermoplastics. Wood (5 wt %) with varying amounts of grafted polymer was incorporated into matrix. Wood Wood flour (5 wt %) with varying amounts of grafted polymer was incorporated into thethe matrix. flour (5 wt %) with varying amounts of grafted polymer was incorporated into the matrix. Table 5. Mechanical characteristics and wood-reinforced wood-reinforced thermoplastics. Wood flour Table 5. Mechanical characteristicsof ofpure pure PMA PMA and thermoplastics. Wood flour (5 (5 wt with %) with varying amountsofofgrafted graftedpolymer polymer was the matrix. wt %) varying amounts was incorporated incorporatedinto into the matrix.
Sample Sample purePMA PMA pure Compositeunf unf Composite Composite1h Composite1h Composite2h
E/MPa E/MPa 88 ±± 33 77 ±± 33 11 ± 1 11 ± 1 20 ± 2
σσyy/MPa /MPa 0.27 0.27±±0.06 0.06 0.30 0.30±±0.17 0.17 0.40 ± 0.03 0.40 ± 0.03 0.58 ± 0.02
UU T/MPa T/MPa 126 ± 31 126 ± 31 5050 ± 17 ± 17 106 ± 18 106 ± 18 235 ± 16
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Table 5. Mechanical characteristics of pure PMA and wood-reinforced thermoplastics. Wood flour (5 wt %) with varying amounts of grafted polymer was incorporated into the matrix. Sample
E/MPa
σy /MPa
U T /MPa
pure PMA Compositeunf Composite1h Composite2h Composite5h Composite7h Composite9.5h
8±3 7±3 11 ± 1 20 ± 2 26 ± 1 29 ± 2 36 ± 2
0.27 ± 0.06 0.30 ± 0.17 0.40 ± 0.03 0.58 ± 0.02 0.73 ± 0.02 0.74 ± 0.01 0.79 ± 0.02
126 ± 31 50 ± 17 106 ± 18 235 ± 16 297 ± 5 398 ± 15 299 ± 68
Pure PMA exhibits the typical behavior of a thermoplastic above its glass transition temperature. After a small increase of stress in the initial elastic region, the sample starts to flow due to a lack of internal cohesion until an elongation of about 1100% [55]. Incorporation of unfunctionalized wood flour (Compositeunf ) resulted in a similar Young’s modulus and a slightly increased yield strength, but the more noticeable change was a halving of the toughness. An increase in yield strength can be explained by the inherently stiff character of wood flour which leads to a higher force required. The dramatic drop in toughness is due to two reasons: first, incorporated stiff particles may disrupt the ability of unbranched polymer chains to flow past each other; second, unfunctionalized wood particles possess a highly hydrophilic surface, which results in poor interfacial adhesion with the hydrophobic polymer matrix and, therefore, an insufficient stress transfer between both phases [2,7]. In contrast, all composites made of grafted wood flour exhibit overall enhanced mechanical properties over pure PMA and the composite made with unfunctionalized wood flour. In general, the longer the polymerization time, the higher the reinforcing effect. The Young’s modulus increases almost linearly with the polymerization time, whereas the yield strength seems to start off linear as well but reaches saturation. However, the toughness—for which the area under the curve is a measure—seems to increase rapidly to a maximum at a polymerization time of between 5 and 7 h and decrease afterwards. It is important to note that the reinforcing character cannot purely be explained by a better interfacial adhesion mediated by longer polymer chains on the particle surface. Additionally, the intrinsic composition of the particles plays an important role. From a practical point of view, addition of the same weight fraction of grafted particles regardless of the polymerization time is a more natural approach compared with addition of a constant mass fraction of wood. Nevertheless, longer polymerization times result in lower wood-to-polymer ratios of the particles, as can be seen in Table 2. Based on the assumption that the discovered reinforcing effect exclusively results from the inherent mechanical properties of wood and the grafted polymer has no significant effect on the composite, a lower wood-to-polymer ratio should lead to weakened mechanical properties. Therefore, this effect counteracts the increased interfacial adhesion of grafted particles. However, tensile tests clearly showed that longer polymerization times are beneficial for the mechanical properties, probably due to an increased ratio of surface-bound polymer molar mass to matrix polymer molar mass. Consequently, this effect, in combination with the improved compatibility between wood and the polymer matrix, plays the major role in the reinforcing character. Furthermore, wood particles with a constant amount of grafted polymer were incorporated in the matrix in different mass percentages. For this purpose, wood particles with a polymerization time of 2 h were added in 3, 5, 7, and 10 wt % into a PMA matrix and the mechanical properties of the composites were determined. Based on the preparation method of the composite, addition of 10 wt % grafted particles was the maximum amount which could be added, because otherwise the dogbone specimen exhibited a very heterogenous structure. The resulting stress–strain curves are depicted in Figure 8 and the corresponding mechanical characteristics are summarized in Table 6. Generally, the higher the added amount of grafted wood particles, the higher the Young’s modulus and tensile strength, whereas the toughness of the composite reaches a maximum at around 5 to 7 wt %. The Young’s
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modulus and yield strength are not significantly affected after addition of 3 wt % grafted particles, but start to rapidly increase with higher addition rates and seem to reach a saturation afterwards. As a result, addition 7 PEER wt %REVIEW seems to be the optimum regarding the examined mechanical characteristics. Polymers 2018, 10, x of FOR 13 of 16
Figure curves of of pure pure PMA PMAand andwood-reinforced wood-reinforcedthermoplastics thermoplasticswith witha Figure 8. 8. Representative Representative stress-strain stress-strain curves aconstant constantamount amountofofgrafted graftedpolymer polymerand andvarying varyingweight weightpercentages. percentages. Table 6. Mechanical characteristics of pure PMA and wood-reinforced thermoplastics with a constant Table 6. Mechanical characteristics of pure PMA and wood-reinforced thermoplastics with a constant amount of grafted polymer and varying weight percentages. amount of grafted polymer and varying weight percentages.
Sample pure PMA E/MPa pure PMA ±3 Composite2h (3 wt 8%) Composite2h (3 wt %) 8±1 Composite2h (5 wt %) Composite2h (5 wt %) 20 ± 2 %)± 3 Composite2hComposite (7 wt %) 2h (7 wt22 Composite2hComposite (10 wt %) 2h (10 wt22%) ±8 Sample
E/MPa 8±3 8±1 20 ± 2 22 ± 3 22 ± 8
σy/MPa σy ± /MPa 0.27 0.06 0.27 ±±0.01 0.06 0.32 0.32 ± 0.01 0.58 ± 0.02 0.58 ± 0.02 0.65 0.65 ±±0.01 0.01 0.74 0.74 ±±0.14 0.14
UT/MPa 126 ± 31 U T /MPa 97 ± 13 126 ± 31 235 ± 16 97 ± 13 235 ± 16 228 ± 34 228 ± 34 166 ± 49 166 ± 49
4. 4. Conclusions Conclusions The The modification modification of of wood wood flour flour was was accomplished accomplished via via surface-initiated surface-initiated ARGET ARGET ATRP ATRP of of methyl methyl acrylate with an environmentally friendly reducing agent (AsAc) in the absence of a sacrificial acrylate with an environmentally friendly reducing agent (AsAc) in the absence of a sacrificial initiator. initiator. Theofamount graftedon polymer on could the wood could bevia estimated via TGA measurements The amount graftedof polymer the wood be estimated TGA measurements and control and control over the polymerization (polymer length and dispersity) was verified via detachment of over the polymerization (polymer length and dispersity) was verified via detachment of the polymer the polymer under mild conditions. Additionally, wood-flour-reinforced thermoplastics with PMA under mild conditions. Additionally, wood-flour-reinforced thermoplastics with PMA as matrix were as matrix were produced and by means of tensile testing. Addition of unfunctionalized produced and investigated byinvestigated means of tensile testing. Addition of unfunctionalized wood flour in wood flour in PMA results in worse mechanical properties than pure PMA, whereas of PMA results in worse mechanical properties than pure PMA, whereas addition of graftedaddition wood flour grafted wood flour leads to increased Young’s modulus, yield strength, and toughness. The findings leads to increased Young’s modulus, yield strength, and toughness. The findings herein presented herein presented may also be extended to other polymer-wood composites. may also be extended to other polymer-wood composites. Supplementary Materials: The Thefollowing followingareare available online at http://www.mdpi.com/s1. Figure S1: FTIR Supplementary Materials: available online at http://www.mdpi.com/2073-4360/10/4/354/s1. Figure S1: FTIR spectra of untreated wood (A), initiator-immobilized wood (B), and PMA-grafted wood with spectra of untreated wood (A), initiator-immobilized wood (B), and PMA-grafted wood with polymerization polymerization times 45 min (C), and 2 h (D), h (E), and 9.5 (F). curves Figure of S2:PMA-grafted DSC curves of PMA-grafted wood times of 45 min (C), 2 hof(D), 5 h (E), 9.5 h5(F). Figure S2: h DSC wood and pure PMA. ◦ C. and pure PMA. All measured samples showed a glass transition temperature of 19 ± 1 All measured samples showed a glass transition temperature of 19 ± 1 °C. Acknowledgments: We acknowledge the MaFo Holz (Materialforschung Holz) PhD program of the state of Acknowledgments: We acknowledge the Holz (Materialforschung Holz) Sartorius PhD program of the state for of Lower Saxony (Lichtenberg fellowship toMaFo Martin Kaßel) for financial support, AG (Göttingen) Lower Saxony (Lichtenberg fellowship to Martin Kaßel) for financial support, Sartorius AG (Göttingen) for financial support (Adrian Ley) and contributing the fluorescence dye, and the working group of Stefan Hell (MPI Göttingen) givingLey) us the opportunity to use fluorescence microscope. financial supportfor (Adrian and contributing the the fluorescence dye, and the working group of Stefan Hell (MPI Göttingen) for giving us the opportunity to use the fluorescence microscope. Author Contributions: Martin Kaßel conceived and designed the experiments. Martin Kaßel and Julia Gerke performed the experiments and analyzed the data. Adrian Ley conducted the SEM, EDX, and fluorescence measurements. Martin Kaßel wrote the manuscript. Philipp Vana monitored the process and improved the manuscript. Conflicts of Interest: The authors declare no conflict of interest.
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Author Contributions: Martin Kaßel conceived and designed the experiments. Martin Kaßel and Julia Gerke performed the experiments and analyzed the data. Adrian Ley conducted the SEM, EDX, and fluorescence measurements. Martin Kaßel wrote the manuscript. Philipp Vana monitored the process and improved the manuscript. Conflicts of Interest: The authors declare no conflict of interest.
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