Immobilization of Water-Soluble 6 ... - ACS Publications

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Immobilization of Water-Soluble 6‑Carboxylcellulose on Poly(ethylene terephthalate) Films Monitored by a Quartz Crystal Microbalance with Dissipation Sergiu Coseri,*,† Aleš Doliška,‡ and Karin Stana Kleinschek‡ †

“Petru Poni” Institute of Macromolecular Chemistry of Romanian Academy, 41A Grigore Ghica Voda Alley, 700487 Iasi, Romania Laboratory for Characterization and Processing of Polymers, Faculty of Mechanical Engineering, University of Maribor, Smetanova 17, SI-2000 Maribor, Slovenia

‡

ABSTRACT: Microcrystalline cellulose has been oxidized employing the newly reported N-hydroxyphthalimide−cerium ammonium nitrate system, at alkaline pH using sodium hypochlorite and sodium bromide in water, after an ultrasonication treatment. The water-soluble oxidized cellulose fraction (OCws) thus obtained has been analyzed by Fourier transform infrared and 13 C NMR and used subsequently for adsorption onto aminolyzed poly(ethylene terephthalate) films deposited onto sensors of a quartz crystal microbalance with dissipation detection (QCM-D). The successful immobilization was monitored using QCM-D, and the morphology of the films was studied by means of atomic force microscopy.

1. INTRODUCTION Cellulose-based materials have limited applications in various studies of cellulose interactions, mainly because of their inhomogeneity and microscopic roughness, which exclude ab initio powerful surface analysis techniques, available for the other kinds of water-soluble biopolymers. However, different approaches to overcome these impediments were envisaged. One of the easiest ways to achieve these goals is to perform chemical reactions on cellulose.1,2 Surface derivatization of cellulose becomes thus one essential technique for further applications such as packaging due to high gas-barrier properties, display, filters for fine chemical preparation, composites, and medicine and health care.1 Generally, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) is widely used for surface carboxylation of different sorts of cellulose nanofibrils.2−4 The reaction is carried out in water, and primary hydroxyl groups in cellulose are selectively converted to sodium carboxylates by using sodium hypochlorite as the primary oxidant and sodium bromide as the cocatalyst. The main issue of this protocol is to obtain a reasonable amount of water-soluble oxidized cellulose without a dramatic decrease of the length of the macromolecular chain, maintaining thus the mechanical properties of the original (starting) material. Recently, the introduction of N-hydroxyphthalimide5,6 as a mediator for the selective oxidation of cellulose fibers, type viscose, has been reported.7,8 This method implies the use of one cocatalyst to generate active radical species, i.e., a phthalimide N-oxyl (PINO) radical, and also the presence of sodium hypochlorite and sodium bromide. When this oxidizing protocol was applied, a lower depolymerization of the macromolecular chain was observed.9 On the other hand, poly(ethylene terephthalate) (PET), a semicrystalline and semiaromatic thermoplastic polyester, has emerged as an excellent material with a broad area of applications, from heavy industry to biomedical engineering as an unfailing component of artificial blood vessels, tissue prostheses, scaffold to support the regeneration of tissue-engineered organs, and many others.10−13 Unfortunately, PET, like other synthetic polymers, © XXXX American Chemical Society

is inert and hydrophobic, and its surface is not entirely compatible with blood, causing phenomena such as red blood cell destruction and coagulation, which eventually lead to blood clots.10 Because the main interaction between living cells and exogenous materials takes place essentially on the interfacial layer, many surface modifications of the PET surface, consisting of chemical breaking of ester bonds,11−14 grafting polymerization,15−18 and plasma treatments,19,20 have been proposed. In this work, aminolysis using triethylenetetramine was used as a simple and efficient method for PET surface modification. The quartz crystal microbalance with dissipation detection (QCM-D), which is based on the piezoelectric properties of quartz, has lately been extensively used as a very sensitive tool (up to the nanogram scale) to study the adsorption of various polymers21 or proteins22 on different surfaces. The main goal of this study was to achieve the preparation of the watersoluble C6 oxidized cellulose (OCws) employing NHPI as the mediator and to investigate its adsorption onto PET activated films deposited on quartz crystals using QCM-D. The newly formed structures could represent a valuable material with large applications in the biomedical field because it is well-known that oxidized cellulose itself has unique bioabsorbable properties, being a biocompatible material used in medicine especially as an effective hemostatic agent.23−25

2. EXPERIMENTAL SECTION 2.1. Materials. Microcrystalline cellulose (Avicel, PH-101) and 1,1,2,2-tetrachloroethane (≥98%) were purchased from Fluka. N-Hydroxyphthalimide (NHPI), ceric ammonium nitrate (CAN), sodium bromide, and 10% sodium hypochlorite were of laboratory grades (Sigma Aldrich) and were used without further Received: February 28, 2013 Revised: April 17, 2013 Accepted: May 11, 2013

A

dx.doi.org/10.1021/ie400645j | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

Article

Figure 1. Simplified oxidation scheme of C6 in cellulose mediated by NHPI−CAN.

frequency in air with the QCM-D before and after spin coating. The adsorbed mass of the PET film was calculated according to the Sauerbrey equation (1).26

purification. Poly(ethylene terephthalate) (PET; Mylar A, DuPont, Taijing Films, Luxembourg) and triethylenetetramine (TETA) 60% (v/v) aqueous solutions (Sigma Aldrich) were used as received. The water used for sample solution preparation and rinsing was of Milli-Q ultrapure grade with a resistivity of 18.2 MΩ cm. 2.2. Cellulose Oxidation. Oxidation of microcrystalline cellulose was carried out as follows: 1 g of cellulose was dispersed in distilled water (30 mL) and sonicated for 30 min [ultrasonication frequency 24 kHz, amplitude 35%, pulse method (30 s ultrasonication, 15 s pause)]. After ultrasonication, the volume of the solution was doubled by adding distilled water. NHPI (0.15 mmol), sodium bromide (1 mmol), and CAN (0.15 mmol) were added. After the pH was adjusted to 10.5 with a few drops of 0.5 M hydrochloric acid, a volume of sodium hypochlorite corresponding to 10 mmol g−1 cellulose was added. The pH was carefully checked with a pH meter instrument and maintained at 10.5 by adding a 0.5 M sodium hydroxide solution. After 4 h, the reaction was stopped by quenching of the unreacted NaOCl with 5 mL of methanol and then acidified to a pH of 6.8. The slurry was then centrifugated (13.200 rpm for 20 min), and the supernatant was collected, precipitated with ethanol, filtered, and washed several times with an excess of ethanol. The water-soluble oxidized cellulose was redissolved in water, dialyzed against water, and finally freeze-dried. 2.3. Fourier Transform Infrared (FTIR) Measurements. IR absorption spectra of microcrystalline cellulose and fully oxidized cellulose samples were recorded using a Bruker Vertex 70 spectrometer at a scan range from 4000 to 650 cm−1, at a resolution of 2 cm−1, and 32 scans. Samples were measured as KBr pellets. 2.4. NMR Determinations. The NMR spectra were obtained on a Bruker Avance DRX 400 MHz spectrometer, equipped with a 5 mm QNP direct detection probe and z gradients. 2.5. PET Film Preparation by Spin Coating. The QCMD analyses were performed on model PET surfaces, which were prepared using the spin-coating technique. The quartz crystals (supplied by Q-Sense AB) were AT-cut quartz with gold-plated electrodes and with gold on the active surface. The fundamental frequency of quartz crystals is f 0 ≈ 4.95 MHz, and sensitivity constant C = 0.177 mg m−2 Hz. Prior to spin coating, all crystals were cleaned in a 5:1:1 mixture of Milli-Q water, H2O2 (30%), and NH4OH (25%) for 5 min at 70 °C. The spincoated PET films were prepared by dissolving 1 wt % PET films in 1,1,2,2-tetrachloroethane and refluxing the mixture until a clear solution was obtained (generally less than 15 min). After the solution has cooled to room temperature, it was filtered through a 0.2 μm Acrodisc GHP filter. An aliquot of 30 μL was spread on a 14 mm quartz crystal and spin-coated at a maximum of 2000 rpm at an acceleration of 2500 rpm s−1 for 60 s. Thereafter, the crystals were dried in a vacuum oven at 100 mbar and 30 °C overnight. PET film thicknesses (founded to be 48 ± 5 nm) were estimated by measuring the quartz crystal resonance

Δm = C

Δf n

(1)

where Δm is the mass change, C is a proportionality constant (known as the “Sauerbrey constant”) equal to 17.7 ng Hz−1 cm−2, Δf is the frequency change, and n is the overtone number of oscillations. The deposited PET thickness was then calculated from the calculated Sauerbrey mass and density of the PET film. In our case, we took a value for bulk PET, i.e., 1300 kg m−3. The thickness was calculated according to eq 2. hf =

107 (nm cm−1) × Δm (ng cm−2) 109 (ng g −1) × ρef (g cm−3)

(2)

where hfis the layer thickness and ρef the density of the adsorbed layer. Atomic force microscopy (AFM) imaging of model PET films (the area of studied images was 1 × 1 μm2) showed that films were smooth and uniform with an average roughness Sa = 0.25 nm. 2.6. QCM-D Measurements. A QCM-D type Q-Sense (Goteborg, Sweden) was used in this study. The experiments were performed at 30 °C. The QCM-D technique provides valuable information in real time on the adsorption of various components onto a quartz crystal. The method principle is based on the change in the resonance frequency of a quartz crystal because of an increase of mass as a consequence of the deposition of different materials. The Sauerbrey equation is applicable only to rigid films. Therefore, to check the validity of the Sauerbrey equation, another parameter, namely, the energy dissipation, should be considered. The QCM-D technique offers the advantage of the simultaneous measurement of both the resonance frequency and energy dissipation.27 During the voltage discontinuity, the energy from the oscillating crystal dissipates outside of the system. Dissipation is defined as the ratio between the lost energy during one oscillation cycle and the stored energy in the system, eq 3.

D=

E lost 2ππstored

(3)

For data analysis, the final frequency shift of the third overtone after all rinsing steps was compared. All measurements were conducted three times on independently prepared surfaces. An average frequency and a dissipation shift were calculated from these values. 2.7. AFM. The Agilent 5500 AFM multimode scanning probe microscope (Digital Instruments, Santa Barbara, CA) has been used to determine the surface morphology of the PET films, in tapping mode. The images were taken at a size of 1 × 1 μm2 under an ambient atmosphere at room temperature, with a resonance frequency of 210−490 kHz and a force constant of 12−110 N m−1 using silicon cantilevers of type ATEC-NC-20 B



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amounts of negatively charged groups were formed.28 In the case of microcrystalline cellulose oxidation, mediated by NHPI, the main fraction (70%) consists of incompletely oxidized cellulose, with an amount of negatively charged groups of a maximum of 150 mmol kg−1 (as determined by potentiometric titration) that remained insoluble. The difference (30%) is constituted by OCws, which is fully oxidized at the primary OH groups (Figure 1). The lower amount of the soluble oxidized fraction is usually attributed to the poor accessibility of the primary hydroxyl groups, engaged in both intra- and intermolecular hydrogenbonding networks.29 The FTIR spectra of the original material and the oxidized OCws are presented in Figure 2. The spectrum of the original material shows characteristic absorption of cellulose chains: a broad band in the 3100− 3600 cm−1 region due to the OH stretching vibration, a band at 2900 cm−1 corresponding to the CH stretching vibration, the 1638 cm−1 adsorption band of bound water, 1431 cm−1 assigned to symmetric the CH2 bending vibration, and 896 cm−1 assigned to the COC stretching at β-(1→4)-glycosidic linkages, a marker of an “amorphous” region.30 After oxidation, a new and broad absorption band around 1736 cm−1 appears as a result of the presence of carboxylate groups. The absorption band at 1614 cm−1 can be also attributed to the CO stretching of free carboxylate groups because the absorption band at 1424 cm−1 represents the CO symmetric stretching of dissociated carboxyl groups.4 On the other hand, slight reductions of the bands related to the hydrogen-bond stretching vibration of OH groups at 3420 cm−1 and to stretching vibrations of CH at 2904 cm−1 are observed. Figure 3 shows the 13C NMR spectrum of C6 fully oxidized water-soluble cellulose (OCws). The spectrum can be regarded as a pure typical spectrum of cellouronic acid because the signal of the C6 primary hydroxyl groups of cellulose around 61 ppm is completely absent, but the other six main signals corresponding to the C2, C3, and C5 atoms at 72.79, 74.18, and 75.03 ppm, respectively, the C4 atom at 80.68 ppm, the C1 atom at 102.34 ppm, and newly formed COOH groups at 174.38 ppm are present. No resonance in the region of 190−210 ppm was observed; therefore, neither aldehydes nor keto groups were formed after the reaction.31 The introduction of amino functional groups on the PET surface was achieved by aminolysis reaction with multifunctional amines, i.e., TETA. This reaction yields new amide bonds on the surface and also provides free −OH

produced by Nanosensors, Neuchatel, Switzerland. The software SPIP 6.0.6, Image Metrology A/S, Hørsholm, Denmark, was used to evaluate the AFM data. PET films, which were TETA activated using both protocols before and after adsorption with OCws, were subjected to AFM measurements.

3. RESULTS AND DISCUSSION The NHPI−CAN system proved its oxidizing efficiency toward cellulose fibers like viscose, modal, or lyocell when certain

Figure 2. FTIR spectra of the original and oxidized (OCws) cellulose samples.

Figure 3. 13C NMR spectrum of C6 fully oxidized (OCws) cellulose samples.

Figure 4. Proposed surface functionalization of PET with TETA and subsequent reaction with OCws. C



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and −NH2 groups, which serves for further anchoring of OCws, as proposed in Figure 4. It is well-known that, with increasing reaction temperature, the degree of functionalization increases as well.14 However, a higher temperature easily favors undesired phenomena such as chain scission, formation of oligomers, and low molecular mass fragments, which are eventually released during aminolysis and rinsing processes.29 It has been reported that, at 30 °C, the aminolysis reaction proceeds without any weight loss for at least 80 min, whereas at 60 °C, even at early stages (up to 10 min) degradation and weight loss occur.32 To ensure a higher content of NH2 groups, we performed the aminolysis reaction using two strategies: (i) ex situ, by immersing the PET films deposited on QCM crystals into a 60% (v/v) aqueous TETA solution for 5 min at 60 °C; (ii) in situ, in the QCM-D chamber, by introducing a continuous flow of a 0.1 mL min−1 60% (v/v) aqueous TETA solution at 30 °C for 60 min. Under these conditions, no degradation or weight loss occur.14 Figure 5

Figure 6. Changes in the frequency (Δf 3) and dissipation (ΔD3) over time after in situ deposition of OCws on TETA-treated PET films.

Figure 5. Changes in the frequency (Δf 3) and dissipation (ΔD3) of the PET films reacted in situ with a 60% (v/v) aqueous TETA solution for 60 min at 30 °C.

Figure 7. ΔD−Δf plot of the QCM-D data for adsorption of OCws on TETA-treated PET films.

presents the QCM frequency (Δf 3) and dissipation (ΔD3) shifts for the PET film activated in situ. It can be observed that, once the TETA solution is introduced into the QCM cell, the frequency decreases up to −1150 Hz, as a consequence of the difference in density of the TETA solution compared with pure water. Also, the ΔD3 value becomes higher, reaching 420 (10−6). However, after

rinsing with water, the change in the frequency reduced its value by −190 Hz because of removal of excess TETA from the quartz crystal surface, which confirms the cross-linking reaction of PET with TETA.14 This cross-linking reaction increases the molecular weight of the sample, causing a significant reduction of the frequency. After the successful aminolysis of the PET films, the water solution of OCws (0.2 mg mL−1) was allowed D



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Figure 8. AFM topography images (1 × 1 μm2) of a pure PET film (left) and OCws deposited onto TETA-treated PET films, ex situ (middle) and in situ (right).

further successfully immobilized onto activated PET films. The process was in situ monitorized using a QCM-D. The activation of the PET films was achieved through chemical reaction with a multifunctional amine, i.e., TETA. The deposition of OCws was supplementarily confirmed by AFM, which revealed a homogeneously distributed immobilized layer on the modified PET surfaces.

to react with a TETA-treated PET film, at a flow rate of 0.1 mL min−1. Figure 6 shows the changes in the frequency and dissipation of the third overtone for OCws immobilized on TETA-treated PET films. At the first stage of the reaction, there is a considerable shift of the frequency of −46 Hz. After 4 min, the frequency shift slightly decreases, reaching a constant value of −35 Hz, which is maintained even after rinsing with water. This behavior can be due to chemical linkage between the COOH groups of carboxyl cellulose and the NH2 groups of PET treated films, as proposed in Figure 4. The dissipation value reaches a maximum of 8 (10−6) after 4 min of OCw adsorption, then remaingin constant at 5 (10−6) for the whole adsorption process, confirming the irreversibility of OCw adsorption on PET-modified films. The changes in the QCM-D energy dissipation were plotted against the shift in the frequency, in so-called ΔD−Δf profiles (Figure 7). This profile is useful and provides valuable information regarding the adsorption mechanism. From Figure 7, we can conclude that OCw adsorption is a highly irreversible reaction because at the end of the experiment the ΔD−Δf profile does not change. Generally, when the process is reversible and desorption occurs, the ΔD−Δf profile shows a small loop at the end of the process.33 For the ex situ aminolyzed PET samples, QCM-D adsorption of OCws was to a very low extent (the shift of the frequency below −4 Hz), whereas the unmodified PET films did not adsorb any OCws. AFM images of OCws deposited on PET-modified surfaces are presented in Figure 8. The adsorbed OCws drastically changed the topography of the pure PET film. As Figure 8 shows, the morphology of the OCws deposited onto PET-modified films depends essentially on how the activation process of PET films is performed. When PET films were ex situ reacted with TETA and then OCws adsorbed on them, the film surfaces become fully covered by cellulose fibrils with different shapes and sizes. There are visible clusters with higher sizes. In contrast, when the reaction of PET films with TETA was monitored by QCM-D, in the cell chamber (in situ), the OCws adsorbed much more uniformly with a highly narrow distribution of cellulose fibrils. The root-mean-square roughness was found to be 7.41 nm for the ex situ activated films and 2.84 for those in situ activated.

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AUTHOR INFORMATION

Corresponding Author

*Tel./fax: +40 211299. E-mail: [email protected]. Notes

The authors declare no competing financial interest. Members of the European Polysaccharide Network of Excellence (EPNOE).

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ACKNOWLEDGMENTS The research leading to these results has received funding from the European Union’s Seventh Framework Programme (FP7/ 2007-2013) under Grant 264115-STREAM.

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REFERENCES

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Immobilization of Water-Soluble 6 ... - ACS Publications

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