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Surface Modification of Cellulose Fiber via Supramolecular Assembly of Biodegradable Polyesters by the Aid of Host-Guest Inclusion Complexation Qiang Zhao,†,‡ Shufang Wang,‡ Xinjian Cheng,† Richard C. M. Yam,§ Deling Kong,*,‡ and Robert K. Y. Li*,† Departments of Physics and Materials Science and Manufacturing Engineering and Engineering Management, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, People’s Republic of China, and Key Laboratory of Bioactive Materials, Ministry of Education, College of Life Science, Nankai University, Tianjin 300071, People’s Republic of China Received February 4, 2010; Revised Manuscript Received March 13, 2010
In this article, we report a novel surface modification method for cellulose fiber that is based on supramolecular assembly. β-Cyclodextrin (β-CD) was first covalently grafted onto the fiber surface. Then poly(ε-caprolactone) (PCL) oligomers having both ends capped with adamantane motifs (i.e., PCL-AD) were immobilized to the cellulose fiber surface through the host-guest inclusion complexation between β-CD and AD motif. FTIR-ATR and XPS analyses confirmed the successful assembly of PCL-ADs, which was further supported by the increasing trend of weight gain with the concentration of CDs on the fiber surface. Contact angle and TGA measurements reflect the enhanced hydrophobicity and thermal stability of the cellulose fiber as a consequence of this modification. The morphologies of the cellulose fiber before and after the assembly process have also been compared by SEM.
Introduction Cellulose is the most abundant raw resource in the world with a number of advantages, including renewability, biodegradability, low prices, and so on.1 In recent years, increasing attention has been paid to explore the potential of cellulose resources in a wide range of applications from biomedical devices to consumer product usage. Based on the high mechanical performance (e.g., macroscopic Young’s modulus up to 128 GPa), plant cellulose fibers have been used as reinforcement for polymer matrix to manufacture environmentally friendly composites.2-4 In the biomedical field, bacterial cellulose with a fibrous structure has been fabricated into wound dressings and tissue engineering scaffolds, taking into account its biocompatibility and high mechanical strength in the wet state.5,6 On the other hand, the surface performance of cellulose fiber (including physiochemical and biological properties) are not always satisfactory in some specific usages, thus, surface modification is often needed.2,4,6 Up to now, various chemical methods (including ring-opening polymerization (ROP),7,8 ATRP,9-11 RAFT,12 click chemistry,13 etc.) have been successfully utilized to realize the surface modification of cellulose fibers. By the aid of these approaches, different kinds of polymer chains have been covalently coupled on the cellulose surface, therefore, surface performance could be remarkably altered. However, these chemical processes are always achieved by multiple steps that are tedious and timeconsuming. Besides, a large amount of homopolymers will be * To whom correspondence should be addressed. Fax: +852-2788-7830 (R.K.Y.L.); +86-22-23498775 (D.L.K.). E-mail:
[email protected] (R.K.Y.L.);
[email protected] (D.L.K.). † Department of Physics and Materials Science, City University of Hong Kong. ‡ Nankai University. § Department of Manufacturing Engineering and Engineering Management, City University of Hong Kong.
formed during some grafting processes, which also makes the strategy not industrial-acceptable. Cyclodextrins (CDs) are a kind of cyclic oligosaccharides consisting of 6, 7, or 8 glucose units linked by R-(1f4) glucosidic bonds, named R-, β-, or γ-cyclodextrin, respectively.14 The structure of CDs is just like a hollow truncated cone with a hydrophobic cavity and a hydrophilic exterior. The cavity structure of CDs allows the ability to include different types of guest compounds, including organic small molecules15 as well as high molecular weight polymers (such as polyethylene glycol, (PEG))16-21 to form inclusion complexes (ICs). One of the well-known examples is β-CD/adamantyl complex. Due to the near-perfect (precise) size match between β-CD and AD, the interaction between them is extremely high with association constant of about 3 × 104 M-1.15 In this context, this interaction has been employed as driving force to construct supramolecular assemblies with various architectures, including supramolecular polymers22 and copolymers,23 polymer networks,24-26 and micelles.27 Over the years, CDs have been covalently attached onto surface of cellulose fibers with the aim to stabilize and slowly release different functional molecules, such as dye, fragrance,28 and antimicrobial agent.29 On the other hand, relevant studies on CD grafted cellulose are mainly confined to the binding and controlled release of small molecular compounds. Very few studies can be found to focus on the immobilization of long chain polymers which could further modify and (or) functionalize the surface performance of the cellulose fibers more substantially. In this study, we will exploit the host-guest inclusion complexation between β-CD and AD as driving force to immobilize the polymer chains onto cellulose fiber surfaces and, therefore, realize meaningful fiber surface modification. β-CDs were first covalently grafted onto the cellulose surface and guest polymers poly(ε-caprolactone) (PCL) with both ends capped with adamantane groups (abbreviated as PCL-AD) were syn-
10.1021/bm100140n 2010 American Chemical Society Published on Web 04/01/2010
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Scheme 1. Synthesis Pathway for the Cellulose-CD and PCL-AD and the Conceptual Illustration for the Assembly Process of Cellulose-CD with Guest Polymer PCL-AD
thesized as well. Then guest polymers were supramolecularly assembled onto the fiber surface by fitting AD motifs inside the cavities of grafted β-CDs. Ideally, the surface properties of the cellulose fibers could be fine-tuned by the appropriate choice of guest polymers with certain intrinsic properties and chain length. The surface compositions, structures, morphologies, and properties of the modified cellulose fibers will be characterized.
Experimental Section Materials. Whatman No.1 filter paper (98% R-cellulose) with density of 88 g · m-2 and thickness of 0.18 mm was used as cellulose substrate without preliminary treatment. β-Cyclodextrin (β-CD; CAVAMAX W7, 98%) was purchased from Wacker Fine Chemicals (Germany). Poly(εcaprolactone) diol (Mn ) 2000), epichlorohydrin (EPI, Acros, 99+%), dibutyltin dilaurate (Aldrich), 1-adamantyl isocyanate (Aldrich, 97%), sodium hydroxide (Aldrich), and other chemicals were used as received. Synthesis of β-CD Grafted Cellulose (Cellulose-CD). The grafting reaction of β-cyclodextrin onto the cellulose matrix was accomplished by using EPI as coupling agent through a one-step procedure (Scheme 1).30 First pure cellulose sheet with dimension of 15 × 15 mm was pretreated by swelling in aqueous NaOH solution (8 wt %) for 24 h at room temperature with the aim to break down the strong hydrogen bonds between cellulose molecular chains and, thus, obtaining more free hydroxyl groups for the subsequent reactions.12 At the same time, β-CD was dissolved in the aqueous NaOH solution at given concentration, and the pretreated cellulose was then loaded into this alkaline β-CD solution and kept still for 30 min. The desired amount of EPI was thus added into the reaction system, and the reaction was allowed to proceed at 50 °C for 6 h (Table 1). The product (cellulose-CD) was collected and washed with deionized water repeatedly to remove ungrafted CD and residual NaOH. At last, the product was dried at room temperature for 1 day and then at 80 °C for 2 days. Synthesis of Adamantyl End-Capped Polyester (PCL-AD). A total of 1.5 g of hydroxyl end-capped PCL (PCL-diol) was dissolved in 20 mL of chloroform (dried over molecular sieves). To this solution, 2-fold excess (mole ratio) of 1-adamantyl isocyanate (AD-NCO) and a trace amount of catalyst dibutyltin dilaurate (about 10 µL) was added. The reaction was proceeded at 60 °C with stirring for 16 h. Subsequently, the reaction system was precipitated into 150 mL of diethyl ether, which was cooled with an external ice water bath and kept still overnight.
Table 1. Synthesis of Cellulose-CDs by Using EPI as Coupling Agent in Aqueous Alkaline Solution reaction grafting CD sample temperature H2O β-CD NaOH EPI/CD ratio fraction code (°C) (mL) (g) (g) (mole ratio) (wt%) (wt%) CeCD1 CeCD2 CeCD3 CeCD4 CeCD5 CeCD6
50 50 50 35 35 35
30 20 20 20 20 10
7.5 10 7.5 10 10 5
6 5 5 4 4 2
1:8 1:8 1:11.6 1:8 1:3 1:2
4.7 6.27 8.57 13.65 5.73 3.81
2.83 3.79 4.40 8.27 4.58 3.28
The precipitates thus formed were vacuum filtered and precipitated from chloroform solution in cold methanol again. The final products were obtained by vacuum filtration and dried at 45 °C.31 FTIR (ATR, cm-1): 3441, 3376 (w, ν(NH)), 2943 (m, νas(CH2)), 2864 (m, νs(CH2)), 1721 (s, ν(CdO)), 1519 (m, δ(NH)) (Supporting Information, Figure S1). 1H NMR (300 MHz, CDCl3, δ): 4.55 (s, 2H, O-(CdO)-NHadamantyl), 4.24 (t, 4H, CH2-(CdO)-OCH2CH2O), 4.06 (t, 2nH, CH2-O(CdO)-CH2CH2CH2CH2CH2, PCL), 3.70 (t, 4H, CH2-(CdO)OCH2CH2O), 2.31 (m, 2nH, CH2-O-(CdO)-CH2CH2CH2CH2CH2, PCL), 2.07 (s, 6H, CH, adamantyl), 1.93 (s, 12H, NH-C-CH2-, adamantyl), 1.69 (m, 4nH, CH2-O-(CdO)-CH2CH2CH2CH2CH2, PCL), 1.38 (m, 2nH, CH2O-(CdO)-CH2CH2CH2CH2CH2, PCL).22,31,32 The conversion of terminal groups was close to almost 100%, as evidenced by the ratio of characteristic signals (δ ) 1.93 and 3.70; Supporting Information, Figure S2).33 Supramolecular Assembly between Cellulose-CD and Guest Polyester. First DMF solution of guest polymer with desired concentration was prepared. Then cellulose-CDs with the same dimension as mentioned earlier (15 × 15 mm) were put into the as-prepared solution and incubated in a reciprocal shaker (160 rpm) at room temperature overnight. Then the solution was decanted, and the treated celluloseCDs were put into deionized (DI) water and ultrasonically rinsed repeatedly to precipitate and remove the free (uncomplexed) guest polymers. Eventually, the assembled cellulose-CDs were dried at room temperature for 1 day and followed by 45 °C for 2 days. At the same time, pure cellulose without CD grafting has been treated by the same procedure with the aim of comparison. Characterization. Fourier transform infrared (FT-IR) spectra were recorded on a Perkin-Elmer 300 spectrometer at single attenuated total
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reflectance (ATR) mode. The ATR-crystal is ZnSe. The spectral resolution is 4 cm-1 and 20 scans were coadded for each spectrum. X-ray photoelectron spectroscopy (XPS) analysis was performed on a PHI 5082 spectrometer (Physical Electronics) using an Al KR X-ray source (1486.6 eV) operating at 15 KV under a current of 24 mA. Samples were placed in an ultrahigh vacuum chamber (10-10 Torr) with electron collection by a hemispherical analyzer at takeoff angle of 45°. Data were curve analyzed by using XPSPEAK41 software. The surface morphology of cellulose-CDs and assembled ones was observed by a scanning electron microscope (JEOL JSM-820) with an accelerating voltage of 10 kV. The surfaces were gold-coated before analysis. The surface contact angle of pedant drops of DI water on the cellulose surface was measured with a SL 200B contact angle meter (Solon Tech Co., Ltd., Shanghai, China), which is software-controlled at room temperature. The volume of each water drop is about 2 µL, and the images were recorded continuously at a speed of 3 frames/ second. Thermogravimetric analysis (TGA) was carried out on a TGA Q50 (TA Instruments) system. The samples were heated at the rate of 10 °C/min from 30 to 800 °C, under a helium flow rate of 40 mL/min.
Results and Discussion Preparation of the Celluloses that are Surface-Grafted with β-CDs (Cellulose-CDs). β-CDs were first covalently tethered on the cellulose surface by using EPI as coupling agent, which is a typical strategy for the synthesis of β-CD derivatives and polymers. The grafting reaction was monitored by FTIR at ATR mode. Pure cellulose is characterized by the intense peaks at 1161 cm-1 (antisymmetric bridge oxygen stretching), 1110 cm-1 (antisymmetric in-phase ring stretching), and 1055 and 1031 cm-1 (C-O stretching; Supporting Information, Figure S3).1 After the grafting reaction, the intensity of the peaks at 1161, 1110, and 1055 cm-1 has been lowered, and the most intensive peak at 1031 cm-1 became broad. These results are similar to those obtained from cyclodextrin-EPI polymers.34 It should be noted that during the grafting reaction, CDs can also be homopolymerized by the aid of EPI and some CDoligomers or CD-polymers were synthesized at the same time. Besides, EPI can cross-link the neighboring cellulose fibers, which will be discussed in the later section of this paper. Successful grafting of CDs onto the cellulose substrate has been supported by the significant weight gain (i.e., grafting ratio) after the grafting reaction since unbound CDs (i.e., CD-oligomer or CD-polymer) have already been removed by repeated washing with water. The grafting ratios of the cellulose-CD could be tuned within the range from 4.7 to 13.7 wt % by changing the concentration of CDs and EPI/CD mole ratio (Table 1). In general, grafting ratios increase as the increase of these two parameters. The EPI/ CD mole ratio should be lower than 10:1 to guarantee the crosslinking would not take place. In addition, an effective NaOH concentration (i.e., 16 wt %) is crucial for the reaction. Assembly of Guest Polymers onto the Cellulose-CD Surface. The immobilization of guest polymer PCL onto the surface of cellulose-CD has been realized through the host-guest inclusion complexation between the AD moieties in guest polymers and grafted CDs. First, cellulose-CDs were incubated in the DMF solution of PCL-AD to capture the guest polymers via molecular recognition. It has been reported that the cavity of CDs can include and bind the AD motif in polar solvent DMF, however, the binding strength is lower than that in aqueous medium.27 Therefore, after attaining the adsorption equilibrium, the cellulose-CDs were put into the aqueous
Zhao et al. Table 2. Summary of the Weight Variation for Pure Cellulose and Cellulose-CDs after the Assembly Process with Guest Polymersd cellulose substrate pure cellulose CeCD1 CeCD2 CeCD3 CeCD4 CeCD5 CeCD6
CD grafting weight gain weight gain weight gain ratio (wt%) (wt%)a (wt%)b (wt%)c 0 2.83 3.79 4.40 8.27 4.58 3.28
1.45 4.29 6.80 6.91 7.34
-0.96 2.52 4.46 4.12 4.01 1.93 1.79
5.29 8.55 15.3 5.51
Assembly parameters: CPCL-AD ) 20 mg/mL, without ultrasonication. b Assembly parameters: CPCL-AD ) 20 mg/mL, ultrasonication. c Assembly parameters: CPCL-AD ) 40 mg/mL, ultrasonication. d Sample code: for example, the CeCD4 treated by the procedure b was denoted as CeCD4-b. a
medium to precipitate the uncomplexed PCL-ADs. At the same time, with the removal of DMF molecules, the interaction between grafted CDs and AD motifs will be enhanced. The weight gain due to the assembly process demonstrates an increasing trend with the CD content for the cellulose-CDs (Table 2). In contrast, pure cellulose only has a relatively low weight gain, which is caused by the nonspecific absorption of guest polymers on cellulose (i.e., filter paper). Furthermore, the complexed cellulose-CDs were post-treated by applying ultrasonication in the rinsing process. This process can remove the absorbed PCL-AD more completely, whereas it also destroyed the structural integrity of the cellulose mat to some extent, thus, leading to the weight loss in pure cellulose. CD grafted celluloses remained intact and no detectable flaw has been identified. A reasonable explanation for this phenomenon is that during the surface grafting reaction the coupling agent EPI can also cross-link the neighboring cellulose fibers, therefore, the structural properties of the cellulose matrix has been strengthened.8 The concentration of PCL-AD solution is also a key factor to determine the assembly efficiency. When the concentration was increased from 20 to 40 mg/mL, the weight gain has been increased accordingly (Table 2). This is because, at higher concentrations, the CDs grafted on the cellulose matrix are more likely to capture the guest polymer in PCL-AD solution. Because the guest polymers are both end-capped with AD groups, there are two possibilities for their assembly: (1) singleend immobilization and (2) double-end immobilization (Scheme 1). A rough approximation on the basis of the results obtained indicates that both assembly manners coexist. Surface Compositions, Structures, Morphologies, and Properties of Assembled Cellulose-CDs. The surface chemical structures were characterized by FTIR analysis at ATR mode (Figure 1). After the assembly process, FTIR spectra of cellulose-CDs demonstrate an intense peak at about 1723 cm-1, which is arising from the stretching of carbonyl group in PCLAD. The relative intensity of the characteristic peak increases with the CD concentration on the cellulose fiber surface, which is in accord with results concerning weight measurement. Additional peaks have been indentified at 2943 and 2866 cm-1, which belong to the asymmetric and symmetric stretching of CH2 group of the PCL-AD, respectively. From the spectrum of neat cellulose treated by the same process, only a weak CdO peak was observed, which is due to the nonspecific absorption of the guest polymer, as mentioned earlier. X-ray photoelectron spectroscopy (XPS) provides a suitable tool to analyze the variation in chemical composition of the cellulose surface which is due to the surface modification (Figure 2). Carbon and oxygen atoms are the main constituents for the
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Table 3. Surface Chemical Compositions of Cellulose-CD (CeCD4) before and after the Assembly Process, Determined from the Deconvolution of C1s Peaks in XPS Spectra C1 (C-C, C-H) binding energya (eV) CeCD4 CeCD4-c a
C2 (C-O)
C3 (O-C-O)
C4 (O-CdO)
285.0
286.6
288.2
289.0
9.19% 30.92%
76.02% 45.03%
14.79% 6.61%
17.45%
Theoretical value.
Figure 1. ATR-FTIR spectra of neat cellulose and cellulose-CDs after the assembly process with guest polymer: (a) cellulose-b; (b) CeCD5b; (c) CeCD4-b; (d) CeCD4-c.
Figure 3. Scanning electron micrographs (SEMs) of the celluloseCD prior to and after the assembly process at low and high magnification: A1-A2, CeCD4; B1-B2, CeCD4-c.
estimated according to eq 1 to be about 67 wt %. It should be noted that after the assembly process there is still a fraction of cellulose fiber surfaces that have not been covered by PCL, which is due to the relatively low molecular weight of PCL utilized in this study.
%C(cellulose-CD)x + %C(PCL-AD) · (1 - x) ) %C(assembly)
Figure 2. XPS spectra and high-resolution C1s peaks for the cellulose-CD (CeCD4) before and after the assembly process.
surface of cellulose-CD before and after the assembly process, and the corresponding atomic content can be quantitatively determined from the integration of C1s and O1s signals. After the assembly process, the C/O ratio has been evidently increased because the assembled polymer PCL has a higher carbon atomic fraction. On the basis of these data, the mole faction of the PCL guest polymer assembled on the surface of CeCD4 has been
(1)
where x is the mole percentage of the cellulose-CD, and %C is the carbon atomic content of the corresponding component. More detailed analysis has been performed through the deconvolution of the high-resolution C1s signals (Figure 2). The decomposed multiple peaks have been assigned to the corresponding functional groups with the correlative data summarized in Table 3.35 Results demonstrate that C1s spectrum of celluloseCD exhibits three main peaks with the relative ratio very similar to the reported data for pristine cellulose filter paper.36 Because cellulose and CD both consist of anhydrous glucose units (AGUs), and no additional chemical bonds were introduced after the grafting reaction of β-CD. After the immobilization of PCLADs, the C-C/C-H contribution (C1) significantly increases, while a new peak belonging to O-CdO (C4) appears. All these results provide the additional evidence to confirm the presence of the PCL on the cellulose surface. The surface morphology of the cellulose-CDs prior to and after the assembly of PLC-ADs was compared by using scanning electron microscopy (SEM; Figure 3). The cellulose-CD has a somewhat smooth surface and exhibits visible undulations along the axial direction, which does not show evident contrast with the pure cellulose without grafting reaction (Supporting Information, Figure S4). After the assembly procedure, the surface
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Figure 4. Evolution of water droplets on the surfaces of pure cellulose (A1∼A4), CeCD4 (B1∼B4), and CeCD4-b (C1∼C4) recorded at a speed of 3 frames/second.
became rough and some patches can be clearly observed, which are attributed to the assembled PCL-AD guest polymers.11 It is worth noting that there are some small particles present on the surface of the cellulose-CD after the assembly process, although the quantity of them is relatively low. As we have mentioned earlier, these particles are caused by the nonspecific absorption of PCL-ADs that have not been completely removed by posttreatment. The surface hydrophilic/hydrophobic performance was characterized by static contact angle analysis. Because of the high hydrophilicity of pure cellulose and mesh structure, the water droplet cannot stay on the surface of the neat (i.e., untreated) filter paper and was absorbed immediately. Surface grafting reaction does not alter the hydrophilicity of the cellulose matrix because the grafted CD ring has the same composition as the cellulose chain, as discussed before. In contrast, the surface performance of assembled cellulose-CD behaved differently, that is, the water droplet stayed on it for a much longer time, with the contact angle decreasing progressively (Figure 4). This is an indication of the enhanced hydrophobic nature as a result of the immobilization of PCL-ADs. On the other hand, because the molecular weight of assembled PCLs is relatively low (Mn ) 2000), such short polymer chains cannot form a continuous layer to fully cover the fiber surface, which has already been evidenced by FTIR, XPS and SEM analyses. Therefore, the droplet was finally absorbed by the cellulose substrate. In this study, we only aim to explore the viability of this modification methodology. The surface performance could be further optimized by selection of polymers with suitable chain length and intrinsic properties. The thermal stability of the cellulose-CD prior to and after assembly of the guest polymer (i.e., PCL-AD) has been compared (Figure 5). The thermal degradation of cellulose-CD occurred mainly in the range of 250 to 400 °C, with a residual weight of about 16 wt %, which is comparable to that of neat cellulose under similar conditions. The main residues are attributed to the formation of char, because the decomposed products levoglucosans are easily volatilized and have been blown out during the heating process.12 The decomposition behavior of cellulose-CD is similar to that of neat cellulose because the primary compositions (i.e., glucose units) in both materials are the same. PCL-AD decomposed from 200 to 460 °C by almost 100%, only leaving a negligible residue. In
Figure 5. Thermogravimetric analysis (TGA) curves of cellulose-CD (CeCD4), PCL-AD, and cellulose-CD after the assembly process (CeCD4-c) at a heating rate of 10 °C/min.
contrast, the degradation of assembled cellulose-CD proceeded through two stages.37 The first one ranges from 250 to about 390 °C, which is very similar to that of the uncomplexed counterpart. With further increasing the temperature, the weight continued to decrease until stabilizing at temperatures above 650 °C (second stage), resulting in a weight retention between those of cellulose-CD and PCL-AD. The broadened decomposition temperature range reflects the improved thermal stability of assembled cellulose-CD. This is because at an early stage the cellulose matrix decomposed leaving the char as the residue, but guest polymers (PCL-AD) on the cellulose surface were less decomposed and remained in the melt state. The polymer melt act as cement to bind the char into a protection layer which can delay the thermal degradation of cellulose matrix below it, thus, leading to the formation of second stage decomposition.
Conclusions In this research, β-CDs were covalently grafted onto the cellulose fiber surface (cellulose-CD), and oligomer PCLs that are both end-capped with adamantane groups (PCL-AD) were successfully synthesized as well. Through host-guest inclusion complexation between β-CDs and AD motifs, guest polymer
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PCL-ADs were immobilized onto the fiber surface. The successful assembly of guest polymers was supported by weight measurement, ATR-FTIR, and XPS analyses. Surface morphology, hydrophilic/hydrophobic performance, and thermal stability have thus been altered. Compared with the conventional grafting polymerization, this novel method holds the advantage of modular nature, that is, the chain length of the assembled polymers and their concentration on the fiber surface can be precisely controlled. Furthermore, this method allows the attachment of multiple components at one time; when assembling biomolecules, the bioactivity of them could be largely maintained. Acknowledgment. This work was supported by the Project of City University of Hong Kong (No. ARG 9667015), National Program on Key Basic Research Project (973 Program; No. 2005CB623904), and NSFC Key Project (No. 50830104). We thank Dr. Yeung in BCH Department of City University of Hong Kong for NMR analysis and Dr. Boris Tong at Hong Kong University for contact angle analysis. Supporting Information Available. FTIR spectra of cellulose-CDs and PCL-AD, 1H NMR of PCL-AD, as well as the SEM of pure cellulose. This material is available free of charge via the Internet at http://pubs.acs.org.
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BM100140N
