Synthesis of Lignin Nanofibers with Ionic-Responsive Shells: Water

Sep 18, 2012 - Aye Aye Myint , Hun Wook Lee , Bumjoon Seo , Won-Su Son , Junho Yoon , Tae Jun Yoon , Hee Jeong Park , Jihyun Yu , Jeyong Yoon ...
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Synthesis of Lignin Nanofibers with Ionic-Responsive Shells: WaterExpandable Lignin-Based Nanofibrous Mats Guangzheng Gao, James Ian Dallmeyer, and John F. Kadla* Advanced Biomaterials Chemistry Lab, Faculty of Forestry, The University of British Columbia, Vancouver, BC, Canada V6T 1Z4 ABSTRACT: A series of ionic responsive poly(N-isopropylacrylamide) (PNIPAM) surface-modified lignin nanofiber mats were prepared by aqueous surface-initiated atom transfer radical polymerization (SI-ATRP). PNIPAM brushes with various molecular weights, thickness, and grafting densities were immobilized on electrospun lignin nanofiber mats by adjusting initial monomer concentration and surface initiator density. ATR-FTIR, SEM, TGA, XPS, and water contact angle measurements confirmed successful surface modification. Analysis of the PNIPAM-modified lignin nanofiber mats (Lig-PN) found that the lower critical solution temperature (LCST) was similar to that of PNIPAM and demonstrated ionic responsive characteristics. With increasing ionic concentration, the water contact angles of the Lig-PN increased correspondingly. AFM images showed that the PNIPAM on the lignin nanofiber mat surface expanded in water and contracted in 0.5 M Na2SO4.

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surfaces for proteins41,42 and living cells,43,44 medical diagnostic devices,45,46 and functional composite surfaces,47 as well as thermoreversible separators, thermoresponsive soft actuators, automatic gel valves, and smart, reusable catalysts.48−52 Recently, we reported the synthesis of lignin-b-NIPAM copolymers by lignin macromolecular initiated atom transfer radical polymerization (ATRP).53 These modified lignins exhibited different thermosensitive characteristics depending on grafting densities and degrees of polymerization. In a continuation of this work we have immobilized PNIPAM polymer brushes onto the surface of electrospun lignin nanofiber mats by surface-initiated ATRP (SI-ATRP) under aqueous conditions. Various PNIPAM brush graft densities were obtained by adjusting the initial monomer concentrations and surface initiator cites, and the PNIPAM-modified lignin nanofiber mats (Lig-PN) exhibited environmentally sensitive characteristics, such as an ion concentration dependent LCST.

INTRODUCTION Lignin is arguably one of the most abundant natural polymers on the earth. Primarily used as a fuel in the production of wood pulp, only a small percentage is utilized for value-added products: for example, as stabilizers, dispersants, and surfactants.1 Current applications of lignin are dictated by its complex and irregular macromolecular properties.2 However, as emerging biorefinery platforms continue to evolve, lignin utilization will become significantly more important.3−8 One area of growing interest is lignin-based fibers,4,9−16 particularly as precursors to carbon fiber.5 Unfortunately, current lignin-based carbon fibers are inferior to those derived from petrochemical precursors in both strength and modulus.4 Although extensive research continues toward enhancing the mechanical properties of lignin-based fibers, other value-added nonstructural applications are needed. In fact, lignin-based fibers, particularly electrospun lignin nonwoven fibrous mats,9 may serve as unique platforms from which novel functionalized materials can be developed. Poly-N-isopropylacrylamide (PNIPAM) is an amphiphilic stimuli-responsive polymer with a lower critical solution temperature (LCST) around 32 °C. It is one of the most popular and thoroughly investigated environmentally sensitive polymers, having been studied in a variety of physical forms including linear polymer/copolymers, cross-linked gels, and surface grafted supports.17−20 PNIPAM or PNIPAM-based copolymers have been immobilized onto various substrates, including carbon nanotubes,21,22 cellulose nanocrystals,23 titanium,24 silicon or quartz,25−27 gold nanorods,28 iron oxide nanoparticles,29 alumina sheets,30 and parylene C.31 Moreover, the PNIPAM-modified surfaces have been investigated for a wide range of useful applications, such as liquid chromatography,32,33 permeation-controlled filters,34,35 chemical sensors,36−38 cell culture,39,40 attachment/detachment controllable © 2012 American Chemical Society

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EXPERIMENTAL SECTION

Materials. N-Isopropylacrylamide (97%, Aldrich) was purified by recrystallizing from n-hexane prior to use. Water was purified using a Milli-Q Plus water purification system (Millipore Corp., Bedford, MA). Methanol, dichloromethane (DCM), and N,N′-dimethylformamide (DMF) were obtained from Fisher Scientific (Ottawa, ON) and used as received. Polyethylene oxide (PEO) with a viscosity average molecular weight of 1 × 106 g/mol was purchased from Sigma-Aldrich and used as received. Softwood Kraft lignin was obtained from Westvaco Corp. (Charleston, SC). It was washed with acidified water (pH = 2) five times before drying at 105 °C for 48 h. The dried material was then washed twice with methanol, which dissolved roughly half of the lignin. The undissolved lignin was then air-dried overnight (12 h), ground Received: July 5, 2012 Revised: September 12, 2012 Published: September 18, 2012 3602

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Scheme 1

with a mortar and pestle, and washed twice with a 70/30 (v/v) mixture of CH3OH/CH2Cl2. The soluble fraction (30% of the original lignin), herein referred to as SKL, was dried on a rotary evaporator at 50 °C and further dried on a Schlenk line at 100 mTorr, 60 °C. All other reagents were purchased from Aldrich and used as received. Instrumentation. Polymer molecular weights were determined by gel permeation chromatography (GPC) on a Waters 2690 separation module fitted with a DAWN HELEOS multiangle laser light scattering (MALLS) detector from Wyatt Technology Corp (laser wavelength λ = 690 nm) and a refractive index detector (Optilab DSP) from Wyatt Technology Corp. operated at λ = 620 nm. The mobile phase was aqueous 0.1 N NaNO3 at a flow rate of 0.8 mL/min. Aliquots of 200 μL of the polymer solution were injected through two Waters Ultrahydrogel columns at 22 °C (guard column, Ultrahydrogel linear with bead size 6−13 μm, elution range 103−7 × 106 Da and Ultrahydrogel 120 with bead size 6 μm, elution range 150−5 × 103 Da) connected in series. The value of dn/dc for PNIPAM in the mobile phase at 22 °C was determined at λ = 620 nm to be 0.164 mL/g and was used for molecular weight calculation. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra were recorded using a Perkin-Elmer Spectrum One FTIR spectrophotometer, 32 scans were acquired at a resolution of 4 cm−1. Five different sites were tested for each sample, and the average value is reported. Time dependent water contact angles were determined by placing a water droplet on the sample surface, and taking a series of pictures of the droplet at 2 s intervals using a digital camera (Retiga 1300, Qimaging Co.). Water contact angles of the pure PNIPAM sample were obtained using PNIPAM films cast from acetone on a glass slide and dried at 40 °C in an oven for 1 day. X-ray photoelectron spectroscopy (XPS) was performed using a Leybold LH Max 200 surface analysis system (Leybold, Cologne, Germany) operated with a Mg Kα source, 200 W. Prior to XPS analysis, all samples were thoroughly dried under vacuum. High resolution thermogravimetric analysis (TGA) was performed on a TA Instruments Q500 using approximately 3 mg of sample under nitrogen at a heating rate of 10 °C/min. Phase transition temperatures were measured by differential scanning calorimetry (DSC, TA Instruments Q1000) using 2 mg samples with 20 μL of water in hermetically sealed aluminum pans. The lower critical solution temperature (LCST) was determined by DSC according to the literature.51,53 The samples were scanned at 5 °C/min over the temperature range of the phase transition from 5 to 50 °C, referenced against an empty pan. All temperatures were determined from the second or third heating scan. 1 H and 13C NMR were measured using a Bruker Avance 300 MHz spectrometer using D2O as the reference solvent. A total of 128 scans were acquired for 1H NMR and 20K scans for 13C NMR. SEM images were taken on Hitachi S-4700 Field Emission Scanning Electron Microscope (FESEM).

AFM observations were performed on a NanoScope III (Digital Instruments, Santa Barbara, CA) in taping mode using a commercially manufactured V-shaped silicon nitride (Si3N4) cantilever with gold on the back for laser beam reflection (Veeco, NP-S20). Lignin Nanofiber Mat Preparation. Electrospinning was performed in a vertical orientation using an operating potential of 15 kV, a solution flow rate of 0.03 mL/min, and a gap of 20 cm between the spinneret and collector.9 The SKL and PEO concentrations were 30 and 0.2 wt %, respectively. Because the as-spun fabrics are somewhat fragile, oxidative thermostabilization was carried out to induce crosslinking and improve mechanical performance.4,54 Thermostabilization was conducted using 3 × 4 inch nonwoven fabric samples mounted on PTFE-coated sample holders which held the edges of the sample. Samples were heated in a modified GC oven in air at a heating rate of 5 °C/min to 250 °C and held for 1 h. The resulting increase in mechanical properties allowed the materials to maintain their integrity during handling and heterogeneous chemical modification. Surface Initiator Modification. ATRP-initiator modified surfaces were generated by treating the lignin nanofiber mats with different molar ratios of acetyl chloride: 2-chloropropionyl chloride. A typical procedure employed a molar ratio of 1:1; specifically, 2-chloropropionyl chloride (1.25 g, 9.84 mmol), acetyl chloride (0.77 g, 9.84 mmol), and triethylamine (2.17 g, 21.40 mmol) were added dropwise to a lignin nanofiber mat (1 × 3 cm2) suspended in dichloromethane (30 mL) at 0 °C over a period of 2 h. The reaction was held at temperature for another 4 h, then allowed to warm to room temperature and left overnight. The modified surfaces were cleaned twice by ultrasonication in dichloromethane before being dried in vacuum. The dried ATRP initiator modified samples (Lig-Cl) were characterized using ATR-FTIR, TGA, SEM, AFM, XPS and water contact angle measurements. Synthesis of PNIPAM Brushes from Lig-Cl by ATRP. In a typical procedure NIPAM (0.8 g, 7.0 mmol) was dissolved in degassed water (9.2 mL) in a 25 mL Schlenk flask. Then CuCl (3.5 mg, 3.5 × 10−2 mmol) and 1,1,4,7,10,10-hexamethyltriethtlenetetramine (HMTETA; 9.6 μL, 3.5 × 10−2 mmol) were added under argon, and the system was stirred until homogeneous. The solution was then degassed by three cycles of freeze−pump−thawing, and added under argon to a second 25 mL Schlenk flask containing the Lig-Cl. After another three cycles of freeze−pump−thaw degassing, the polymerization was continued at room temperature for 24 h under argon. The polymerization was quenched by exposure air, followed by dilution with water. The resulting PNIPAM-grafted lignin fiber mats were then washed with water for 4 h, followed by methanol and dichloromethane, respectively, and then dried in vacuum. The PNIPAM-grafted LFM (Lig-PN) was characterized by water contact angle measurements, SEM, XPS, ATRFTIR, TGA, DSC, and AFM analyses. Cleavage of PNIPAM Brushes. The grafted PNIPAM brushes were cleaved from the lignin nanofiber mats by reacting the Lig-PN (1 × 2 cm2) sample with 10 mL of 2 M NaOH aqueous solution for 1 week. The resulting brown solution was removed from the reactor and 3603

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Table 1. PNIPAM Modified Lignin Nanofibersa sample

[M]0 (mol/L)

[Ac/Cl]0b

Lig-PN-1 Lig-PN-2 Lig-PN-3 Lig-PN-4 Lig-PN-5 Lig-PN-6 Lig-PN-7 Lig-PN-8 Lig-PN-9 Lig-PN-10

0.19 0.76 1.14 1.52 1.90 0.76 0.76 0.76 0.76 0.76

0/1 0/1 0/1 0/1 0/1 1/1 3/1 10/1 20/1 40/1

Lig-Cl diameterc D1 (nm)

Lig-PN diameterc D2 (nm)

brush thicknessd (nm)

Mnf (×10−5)

Mw/Mnf

grafting densityg, σ (chains/nm2; ×102)

± ± ± ± ± ± ± ± ± ±

790 ± 36 989 ± 83 1081 ± 81 1041 ± 51

69 ± 11 168 ± 36 214 ± 39 194 ± 21

5.05 5.93 11.58 33.97

1.45 1.51 1.44 1.22

9.92 23.59 16.25 4.91

e

e

e

e

e

1148 ± 96 1096 ± 88 1149 ± 116 935 ± 94 912 ± 120

256 ± 39 216 ± 32 157 ± 45 41 ± 20 23 ± 40

10.53 12.08 9.01 6.79 6.52

1.46 1.35 1.61 1.73 1.72

22.59 15.64 13.65 4.19 2.34

653 653 653 653 653 636 665 836 853 867

29 29 29 29 29 56 61 74 85 89

a

Condition: [M]0/[Cu(I)]0/[HMTETA]0 = 200/1/1, time = 24 h, rt. bInitial feed molar ratios of acetyl chloride/2-chloropropionyl chloride. Determined by SEM. dBrush thickness (h) was calculated by Lig-PN diameter minus Lig-Cl diameter. eNot determined. fPNIAPM cleaved from LFM surface and determined by GPC using 0.1 M NaNO3 as an eluent. gGrating density (σ) was calculated by eq 2 using density of PNIPAM (ρ) = 1.10 g/cm3. c

Figure 1. ATR-FTIR spectra of the lignin nanofiber mat (LFM) after PNIPAM surface modification. (A) ATR-FIIR spectra of unmodified, ATRP initiator modified (Lig-Cl) and PNIPAM modified (Lig-PN-2) LFM. (B) ATR-FTIR spectra of Lig-PN after reaction with various monomer initial concentrations. (C) ATR-FTIR spectra of Lig-PN after reaction of various initial initiator concentrations.

Figure 2. XPS spectra of the lignin nanofiber mat surfaces. (A) XPS spectra of unmodified, ATRP initiator modified (Lig-Cl/Ac/Cl = 0/1) and PNIPAM-modified (Lig-PN-2) lignin nanofiber mats; (B) Cl 2s spectrum of ATRP initiator modified lignin nanofiber mats (Lig-Cl/Ac/Cl = 0/1); and (C) Cl 2s spectrum of PNIPAM-modified lignin nanofiber mats (Lig-PN-2).

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the remaining insoluble PNIPAM was further washed 2 times with 2 M NaOH aqueous solution. The PNIPAM was then dissolved by ultrasonication in 0.20 M NaOH aqueous solution for 1 week, then neutralized with 0.1 M HCl and dialyzed for 2 days. The cleaved PNIPAM was obtained by freeze-drying and analyzed by NMR and GPC.

RESULTS AND DISCUSSION

ATRP Initiator Immobilization on Lignin Nanofiber Mat Surface. Lignin is a heterogeneous aromatic polyol. This implies that there are numerous hydroxyl groups on the lignin nanofiber mat surface that can be modified to create active 3604

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initiating sites for controlled polymerization.2 The synthetic pathway employed to modify the lignin nanofiber mats is shown in Scheme 1. The lignin nanofiber mat surface was modified with varying ratios of 2-chloropropionyl chloride/acetyl chloride (Ac/Cl) to produce surfaces with different initiator densities (Table 1). N-Isopropylacrylamide (NIPAM) was then polymerized from the macroinitiator surface (Lig-Cl) through surface initiated atom transfer radical polymerization (SIATRP). ATR-FTIR (Figure 1) and X-ray photoelectron spectroscopy (XPS; Figure 2) analyses of the macroinitiator-immobilized surfaces clearly shows the incorporation of chloropropionyl and acetyl ester moieties. Compared with the original lignin nanofiber mat (LFM), the ATR-FTIR spectrum of the Lig-Cl (Figure 1A) shows the CO stretching vibrations at 1765 cm−1 along with a decrease in the O−H stretching band envelope (3600−3200 cm−1), consistent with chloropropionyl modification. Likewise, in addition to the C 1s (284.60 eV) and O 1s (533.30 eV) peaks associated with lignin, the surface-modified ATRP initiator sample, Lig-Cl also shows a small peak at 198.63 eV, corresponding to the Cl 2s peak of the chloropropionyl groups (Figure 2A). Polymerization of NIPAM Brushes on the Lignin Nanofiber Mat Surface by ATRP. SI-ATRP is a useful technique to modify solid substrate surfaces with polymer brushes. In the present study PNIPAM chains were grafted from the Lig-Cl surface under aqueous conditions. A series of PNIPAM grafted lignin nanofiber mats, Lig-PNs were prepared by varying the initial monomer (NIPAM) concentration ([M]0) as well as the initiator site density ([Ac/Cl]0; Table 1). SI-ATRP resulted in the appearance of several new peaks in the FTIR spectrum (Figure 1A) of the Lig-PN sample corresponding to the amide stretching (νCO ∼ 1645 cm−1; νN−H ∼ 3300 cm−1) and bending (δN−H ∼ 1539 cm−1) bands of the surface grafted PNIPAM. As expected, increasing the initial monomer concentration ([M]0) increased the height ratio between the grafted PNIPAM amide stretching band (νCO ∼ 1645 cm−1) and the Lig-Cl initiator ester stretching band (νCO ∼ 1765 cm−1; Figure 1B). Successful PNIPAM grafting from the lignin nanofiber mat surface was further shown by thermal gravimetric analysis (TGA; Figure 3); for example, the residual weight at 600 °C of the Lig-PN-2 was about 22 wt % as compared to almost 60 wt % for the lignin nanofiber mat (LFM). Moreover, the Lig-PN thermogram shows two distinct decomposition profiles. The initial onset of decomposition ∼250 °C for the Lig-PN is in good agreement with that of our previously reported lignin-gNIPAM copolymers (∼252 °C).53 SEM analysis of the lignin nanofibers mats before (Lig-Cl) and after (Lig-PN-2) surface-initiated ATRP are shown in Figure 4. An increase in lignin nanofiber mat diameter is clearly evident after SI-ATRP. Table 1 summarizes the increase in fiber diameter after SI-ATRP for the various reaction conditions. The surface PNIPAM brush thickness showed a direct relationship with initial monomer concentration, [M]0, and surface initiator concentration ([Ac/Cl]0); increasing with increasing [M]0, and surface initiator sites. For example, the nanofiber mats prepared with an initial surface initiator concentration [Ac/Cl]0 ratio of 0/1, increased in grafted brush thickness from 69 ± 11 nm to 194 ± 21 nm as the [M]0 was increased from 0.19 to 1.52 mol/ L. At 1.90 mol/L (sample Lig-PN-5 in Table 1), the PNIPAM surface grafting became uncontrollable and the lignin nanofiber

Figure 3. TGA of unmodified lignin nanofiber mat (LFM), initiator modified lignin nanofiber mat (Lig-Cl), PNIPAM immobilized lignin nanofiber mat (Lig-PN-2), and pure PNIPAM.

Figure 4. SEM images of lignin nanofiber mat surface before (A and C) and after (B and D) PNIPAM surface modification. The Lig-PN SEM images (B and D) are of Lig-PN-2.

mat was cover with a very thick PNIPAM layer; as a result, we were not able to measure the brush thickness. When the initial monomer concentration was held constant at [M]0 = 0.76 mol/L, a maximum in brush thickness (∼256 ± 39 nm) was obtained when [Ac/Cl]0 = 1/1. Increasing the initiator site density, [Ac/Cl]0 = 0/1 or decreasing the initiator site density, [Ac/Cl]0 = >3/1 both resulted in a reduction in brush thickness (Table 1). In fact, the brush thickness decreased more than 200 nm when the [Ac/Cl]0 changed from 1/1 to 40/1. Cleavage of PNIPAM Brushes from the Lig-PN Surfaces. PNIPAM is an environmentally sensitive polymer that undergoes temperature-dependent conformational changes in aqueous solutions. Its LCST is affected by ionic concentration, wherein at high aqueous NaOH or HCl concentrations PNIPAM becomes insoluble. By contrast lignin is soluble in aqueous NaOH solution because of its large number of phenolic and aliphatic hydroxyl groups. Therefore, we utilized this fact to design a procedure to cleave and separate 3605

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Figure 5. Analysis of the PNIPAM cleaved from the surface of the modified lignin nanofiber mats; (A, B) 1H and 13C NMR spectrum of the cleaved PNIPAM in D2O, respectively (Lig-PN-2); (C, D) GPC traces of the cleaved PNIPAM using 0.1 Na2NO3 as an eluent (C, Lig-PN-2; D, Lig-PN-6).

where ρ is the density of surface brush and NA is Avogadro’s number. If it was on the smooth fiber surface, eq 1 could be written as

(dissolve) the lignin from the PNIPAM brushes. Basically, after a one-week of reaction in 2.0 M NaOH aqueous solution, the Lig-PN suspension went from a brown solid in a clear liquid to a white solid in a brown solution, indicating lignin cleavage. The brown lignin-containing solution was removed and the remaining white membrane was ultrasonically dissolved in 0.2 M NaOH/water to continue digestion for another week. The PNIPAM brushes were then obtained after neutralization, dialysis, and freeze-drying. 1H and 13C NMR confirmed the cleaved PNIPAM structure (Figure 5A,B). GPC analyses of the cleaved PNIPAM brushes (also shown in Figure 5) showed single narrow monomodal peaks with polydispersities in the range of 1.22−1.73 (Table 1). The number-average molecular weight, Mn of the PNIPAM brushes increased from ∼5 × 105 Da to almost 34 × 105 Da as initial monomer concentration was increased from 0.19 to 1.52 mol/L. The behavior with respect to initiator site concentration followed a similar trend as was observed with the calculated brush thickness, wherein there was an increase in Mn to a maximum of ∼12 × 105 Da followed by a decrease. The brush height (h) on a flat surface is related to the molecular weight Mn and the graft density (σ) of the chains, as defined by the following equation55 h=

σ × Mn ρ × NA

σ=

ρ × NA × (D2 2 − D12) 4 × M n × D1

(2)

where σ is the graft density, Mn is the number average molecular weight of the grafted PNIPAM chains, ρ is the density of PNIPAM (1.10 g/mL), NA is Avogadro’s number, D2 is the diameter of the PNIPAM modified lignin nanofiber (Lig-PN-#), and D1 is the diameter of the initiator modified lignin nanofibers (Lig-Cl). The calculated graft densities are listed in Table 1. Graft density increased from 9.92 × 10−2 to 23.59 × 10−2 chains/nm2 when [M]0 increased from 0.19 to 0.76 mol/L, and decreased with when [M]0 was increased to more than 1.14 mol/L. This phenomenon maybe due to the exothermic character of the NIPAM polymerization. In reactions where the [M]0 was higher than 1.14 mol/L, a clear white layer, PNIPAM aggregation, appeared to form on the surface of the lignin nanofiber mats as soon as the ATRP was initiated. This exothermic polymerization made the lignin nanofiber mat surface temperature higher than the LCST of PNIPAM, which then aggregated and resulted in the decrease in grafting density. By contrast, when the [M]0 was relatively low, 0.76 mol/L, the grafting density decreased continually from 23.59 × 10−2 to 2.34

(1) 3606

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× 10−2 chains/nm2 with decreasing initiator site concentration ([Ac/Cl]0 from 0/1 to 40/1). The TGA weight loss profile for the Lig-Cl (Figure 3) is approximately 10 wt % lower than that of the lignin nanofiber mats (LFM), and reflects the amount of initiating sites on the Lig-Cl surface.56 However, the surface grafting density of the corresponding PNIPAM grafted nanofibers (Lig-PN-2) was determined to be less than 24 × 10−2 chains/nm2 (Table 1). This means that in the Lig-PN-2 sample not all of the available surface-grafted initiators participated in the SI-ATRP of NIPAM. XPS analyses of the modified lignin nanofibers mat surfaces are summarized in Table 2. Initially the Cl/C ratio of the lignin Table 2. Experimental O/C, N/C, and Cl/C Molar Ratios Obtained from XPS sample

O/C

N/C

Cl/C

lignin Lig-Cl PNIPAM (theoretical) Lig-PN Lig-PN-1 Lig-PN-2 Lig-PN-3

0.28 0.29 0.17 0.14 0.13 0.16

0 0 0.17 0.15 0.13 0.15

0 0.05 0 ∼0 ∼0 ∼0

Figure 6. Effect of salt concentration on LCST of PNIPAM and PNIPAM-modified lignin nanofiber mats with different grafting densities.

the LCST of the Lig-PNs, despite a 20-fold increase in grafting density (Figure 6). To further investigate and correlate the surface properties of PNIPAM immobilized on the lignin nanofiber mats, time related water contact angle measurements were made as a function of Na2SO4 aqueous solution concentration (Figure 7). Both Lig-Cl and the lignin nanofiber mat surfaces exhibited hydrophobic characteristics where their water contact angles were at about 130° with little change throughout the concentration series (Figure 7A,B). By contrast, the pure PNIPAM and PNIPAMmodified lignin nanofiber mats (Lig-PN) exhibited quite different characteristics (Figure 7C,D). At low salt concentration, 0.1 M Na2SO4, the time dependent water contact angles of PNIPAM and Lig-PN-2 initially decreased rapidly but leveled off after a period of time, perhaps due to water evaporation. In the case of PNIPAM, the water contact angle dropped rapidly to ∼45° in about 40 s then remained rather stable. Increasing the Na2SO4 salt concentration from 0.1 − 0.3 M decreased the time over which the water contact angle changed to less than 10 s. At 0.4 and 0.5 M Na2SO4 the contact angles increased to about 90° and remained stable over the experimental measurement time. These results demonstrate the ion induce hydrophobic character of PNIPAM. In the case of the Lig-PN-2, a much more significant but slower decrease in water contact angles with time was observed for low salt concentrations. The water contact angles dropped from an initial value of ∼120° to ∼45° and ∼90° after 120 s for 0.1 and 0.2 M Na2SO4, respectively. The longer initial water contact angle decay times likely result from nanofiber surface roughness. At 0 and 0.1 M salt concentration a logarithmic decrease is observed and the final contact angles for PNIPAM and the Lig-PN-2 sample reach the same level, about 40 and 45°, respectively. Under these conditions, the surface PNIPAM brushes would assume an extended-chain form. However, at 0.2 M Na2SO4, the Lig-PN-2 water contact angles decrease linearly with time. At this salt concentration, the LCST of Lig-PN-2 (∼23.5 °C) is approximately room temperature, and the LigPN-2 surface PNIPAM brushes change between extended hydrophilic and shrunken hydrophilic states. Further increasing the salt concentration beyond 0.3 M drops the LCST below room temperature (21.1 °C at 0.3 M Na2SO4) and the Lig-PN-2 becomes a completely hydrophobic material and the water

nanofiber mat was zero, which after initiator modification (LigCl) increased to 0.05. Following surface brush grafting, the Cl 2p peak (198.63 eV) became weak (Figure 2c) and the Cl/C ratio was near zero, and a new peak component with a binding energy (BE) of about 402.10 eV corresponding to N 1s (Figure 2A) appeared, indicative of the PNIPAM brush growth on the Lig-Cl surface. The experimental molar ratios of C/O, C/N, and C/Cl are about 0.14, 0.15, and 0, where the theoretical values of PNIPAM are 0.17, 0.17, and 0, respectively. The positive correlation of the experimental molar ratios C/O, C/N, and C/ Cl, with theoretical values for the PNIPAM-modified lignin nanofiber mat surface, further confirms the desired surface modification. Ionic Effect on Thermal Characterization of PNIPAM Grafted LFM (Lig-PN). PNIPAM is a thermal-sensitive polymer, which exhibits a phase transition at a lower critical solution temperature (LCST) of ∼32 °C in aqueous solution, which can be controlled by specific ionic effects.57,58 Figure 6 illustrates the effect of sodium sulfate concentration on the LCST for samples of Lig-PN with different grafting densities (Lig-PN-2 and Lig-PN-10) along with pure PNIPAM. As previously reported for PNIPAM,57,58 a change in slope of the LCST dependence on salt concentration is observed at ∼0.13 M Na2SO4. This can be attributed to a single LCST at low salt content, which separates into two distinct transitions at higher salt content.57 At low salt concentrations below 0.1 M, the LigPNs have similar LCST behavior as that of pure PNIPAM, decreasing from 32 to 26.5 °C. However, as the concentration of Na2SO4 increased beyond 0.2 M, the LCST of Lig-PNs decreased continually to 16 °C at 0.5 M, with a relatively larger slope than that of pure PNIPAM. Literature reports indicate that there are temperature related morphology changes associated with PNIPAM brush densities. For example, a PNIPAM modified silicon sheet exhibited temperature related morphology changes between various brush density samples due to PNIPAM brush lateral interaction.27 However, PNIPAM grafting densities did not show any effect on 3607

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Figure 7. Effect of salt concentration on water contact angles of lignin nanofiber mats. (A) Unmodified lignin nanofiber mats. (B) Initiator-modified lignin nanofiber mats (Lig-Cl [Ac/Cl = 1/1]). (C) PNIPAM film. (D) PNIPAM-modified lignin nanofiber mats (Lig-PN-2).

Figure 8. AFM images of PNIPAM modified lignin nanofibers (Lig-PN-2). (A) Lig-PN-2 in air. (B) Lig-PN-2 in water. (C) Lig-PN-2 in 0.5 M Na2SO4 aqueous solution.

modified lignin nanofibers were clearly observed, while in water, the nanofibers were difficult to discern, suggesting that the extended PNIPAM brushes covered the profile of lignin nanofiber and obscure the images. The same fibers when exposed to a 0.5 M Na2SO4 aqueous solution once again became resolvable by AFM. Such behavior indicates that the ionic

contact angles stabilize at about 120°. Under these conditions, the surface PNIPAM brushes assume a globular contractedchain form. The effect of ion concentration on the PNIPAM surfacemodified lignin nanofiber mats was further investigated through AFM (Figure 8). AFM studies were conducted in air, water, and 0.5 M Na2SO4 aqueous solutions. In air, the dry PNIPAM3608

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concentration induces expansion and contraction responses in the lignin nanofiber surface grafted PNIPAM brushes.

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CONCLUSION Lignin nanofiber mats were modified with PNIPAM brushes through SI-ATRP under aqueous conditions. ATR-FTIR, TGA, XPS, water contact angles, SEM, and AFM demonstrated successful surface modification. Confirmation of the modification was obtained by cleaving the PNIPAM brushes from the lignin nanofiber mats with aqueous NaOH and subsequent analysis through both NMR and GPC. Different brush molecular weights and grafting densities were obtained by controlling the initial monomer concentration and initial initiator molar feed ratios. The LCST of the PNIPAM-modified lignin nanofiber mats was found to be dependent on ionic concentration; hydrophilic in water and at low Na2SO 4 concentrations (0−0.3 M) and hydrophobic at Na 2 SO 4 concentration beyond 0.3 M. AFM images illustrated that the PNIPAM brushes exhibited ionic responsive characteristics, expanding in water and contracting in a 0.5 M Na2SO4 aqueous solution. These nanofibrous functionalized materials may serve as a platform for the development of thermoresponsive separation and purification devices.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (604) 827-5254. Fax: (604) 822-9104. E-mail: john. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank LIGNOWORKS, the NSERC Biomaterials and Chemicals Strategic Research Network, and its supporting member companies for supporting this research.

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REFERENCES

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