Rheological and Dielectric Behavior of 3D-Printable Chitosan

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Rheological and dielectric behavior of 3D printable chitosan/graphene oxide hydrogels Jasim Ahmed, Mehrajfatema Mulla, and Mohammed Maniruzzaman ACS Biomater. Sci. Eng., Just Accepted Manuscript • DOI: 10.1021/acsbiomaterials.9b00201 • Publication Date (Web): 04 Apr 2019 Downloaded from http://pubs.acs.org on April 4, 2019

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ACS Biomaterials Science & Engineering

Rheological and dielectric behavior of 3D printable chitosan/graphene oxide hydrogels Jasim Ahmed1*, Mehrajfatema Mulla1, and Mohammed Maniruzzaman2

1Food

and Nutrition Program , Environment & Life Sciences Research Center, Kuwait Institute for Scientific Research, P.O. Box 24885, Safat-13109, Kuwait 2Department

of Pharmacy (Chemistry), School of Life Sciences, University of Sussex,

Falmer, Brighton, BN1 9QJ, UK. E-mail: [email protected]

*Corresponding author; E-mail: [email protected]; [email protected]

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ABSTRACT

The effect of concentration, temperature, and the addition of graphene oxide (GO) nanosheets on the rheological and dielectric behavior of chitosan (CS) solutions was studied, which finally influence the formation of the blend materials for various applications including 3D printing and packaging. Among tested acid solutions, the rheological behavior of 1% CS in acetic and lactic acid solutions was found alike, whereas the hydrochloric acid solution showed an abnormal drop in the dynamic moduli. Oscillatory rheology confirmed a distinct gel point for the CS solutions at below 10 °C. Both the G′ and G″ of the solutions increased with the loading concentrations of GO between 0.5 and 1%, and it marginally dropped at the loading concentration of 2% which is consistent with AFM observation. The steady-shear flow data fitted the Carreau model. Dielectric property measurement further confirmed that both the dielectric constant, ε′ and the loss factor, ε″ for the CS in hydrochloric acid solutions behaved differently from others. Addition of GO significantly improved both ε′ and ε″ indicating an improvement in the dielectric properties of CS/GO solutions. The dispersion of GO into the CS matrix was assessed by measuring XRD, FTIR, and microscopy of the film prepared from the solutions. Furthermore, the inclusion of GO into CS solution containing pluronic F127 (F127) base for potential 3D printing application showed positive results in terms of the printing accuracy and shape fidelity of the printed objects (films and scaffolds). The optimized composition with homogenous particle distribution indicated that up to ~50 mg/ml GO concentration (w/v of F127 base) was suitable to print both films and scaffolds for potential biomedical applications. Key words: 3D printing, bioinks, Zero shear rate viscosity, Chitosan solution, Dielectric constant, TGA.


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Introduction Among nature-derived biopolymers, chitosan (CS) poly [β-(1–4)-linked-2-amino-2-deoxy-Dglucose] – a semi-crystalline natural polymer is obtained by alkaline deacetylation of chitin. Chitosan is mostly extracted from the shells of shrimp, crabs, and krill. It has received tremendous attention because of its versatile inherent properties such as biodegradability, biocompatibility, nontoxicity, film, and hydrogel-forming capability, antibacterial activity, and oxygen impermeability.1-3 CS one of the most studied biopolymers has been investigated widely for the development of biodegradable packaging materials, coatings, micro and nanofibers, and scaffolds for tissue engineering, ink for 3D printing4, porous hydrogel beads for drug and gene delivery systems.3,5 Recently, it has been used as a solid polymer electrolyte that can be used for lithium rechargeable batteries,6 and also direct improved cell growth of electro-responsive cells under electrical stimulation.7 The industrial usage of CS is limited due to its hydrophilicity and poor mechanical properties. Various approaches have been made to improve its mechanical properties by addition of plasticizers; 8 chemical modification of the functional groups (e.g., amino and hydroxyl groups) on CS molecules, 9 and also incorporation of nanoparticles.10 Incorporation of a large variety of nanoparticles in polymers leads to significant improvement in their thermo-mechanical, rheological, and barrier properties.11 Among nanoparticles, graphene oxide (GO), a related material of graphene, appears to have a superior ability to result in substantial improvements in thermal, mechanical, crystal, conductive properties of polymers because of its excellent dispersibility in water, good biocompatibility, and facile surface functionality.12 GO and its derivative have a wide range of industrial applications including fabrication of polymer and



bionanocomposites13-14, supercapacitors15, packaging and gas barrier16-17, biomedical devices18, targeted drug delivery19, tissue engineering and photothermal therapy20. CS dissolves in weak acid solutions to obtain a uniform viscous solution that can be used for various applications including membranes, films, nanocarrier for anticancer drug delivery 19, and nanofibers. Rheological properties of moderately concentrated CS solutions reflect interactions between chains of polymer dissolved in acid solutions even at acidic pH, at which the most amino groups were protonated.21-22 The protonated CS can form a composite with the negatively charged GO nanosheets, which finally produce CS/GO nanocomposite. For an efficient electrospinning process, the concentration of the polymer has to achieve a target concentration which is termed as the entanglement concentration, ce, 23 and the polymer solution concentration should be at least 2 to 2.5 times than the ce

24,

which is too high for the CS. Therefore, rheological studies of CS

solution have special importance since it can help to achieve an optimum concentration for the electrospinning, biofilm, hydrogel formation or gelation. No information is available so far on the rheological properties of CS in various aqueous acid solutions as a function of concentration or temperature at its nearest gelation temperature. Numerous studies have been carried out on CS/GO composites last few years on property improvement for various applications.25-26 Dissolving GO nanosheets into the aqueous CS solutions at a selective pH, CS/GO blend films were produced effectively by reinforcing the parallel aligned GO nanofillers.27 CS/GO matrix has been extensively used in medical applications including biosensors, medical devices to transfer the electrical signals/responses (cardiac applications), biofilm with antimicrobial properties, adsorbents for purification of trace pharmaceuticals from wastewater, and anode material in microbial fuel cells.6 Recently, there has been increased interests in 3D printing and its potential applications in biomedical engineering 4 ACS Paragon Plus Environment

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including tissue engineering and medical implants. The novel biocompatible bio-inks are equally important to transform the tissue engineering applications for the fabrication of complex geometry of personalized medical devices providing novel platform beyond the current state-of-the-art.28-30 Because of the excellent biocompatibility of GO loaded CS solution, the blend could be an ideal matrix to be seeded into 3D bioprinting ink for biomedical applications. For the purpose of study, some well-established bioinks such as pluronic F127 (mid-high viscosity range) can successfully be tailored for the personalized medical solutions. Nonetheless, limited studies carried out on rheological and dielectric behavior of CS and CS-GO in solution before feeding into 3D-printer to produce thin films or nanofibers. It is anticipated that the generated data could be helpful for developing bespoke medical devices such as scaffolds, antimicrobial hydrogels, bio-ink for 3D printing, biomedical tools or even targeted drug delivery system. The objective of the present work was to investigate the rheological and dielectric properties of chitosan in solution as influenced by aqueous solvent, concentration, temperature, and reinforcement of graphene oxide nanosheets. The film-forming solution (CS or CS/GO) is an intermediate step before getting the film intended for the packaging applications, and therefore, knowledge of the rheological and dielectric behavior of the solution could provide valuable information on the end product/s. The CS/GO solutions converted into films to elucidates the property improvement and dispersibility of GO into the CS matrix efficiently. The film properties were characterized by various analytical tools to elucidate the interaction between CS and GO in the blend. Materials and Methods Materials



CS [(2-Amino-2-deoxy-(1→4)-β-D-glucopyranan, Poly-(1,4-β-D-glucopyranosamine)] powder from crab shells (≤1% insoluble matter; highly viscous >400 mPa.s, 1 % in acetic acid at 20 °C), and GO powder (Bulk density: 1800 kg.m-3; number of layers: 15-20 and 4-10% edge-oxidized) were procured from Sigma–Aldrich (St. Louis, Missouri, USA). According to supplier, the CS composed of about 20% β1,4-linked N-acetyl-D-glucosamine (GlcNAc) and about 80% β1,4linked D-glucosamine (GlcN) that is prepared by the partial deacetylation of chitin in hot alkali. Glacial acetic acid (AA), lactic acid (LA) and hydrochloric acid (HCA) (12 N) were purchased from VWR (France), and glycerol was obtained from SD Fine Chem (Mumbai, India). All chemicals were used as received. Chitosan solution preparation CS powders (0.25 to 2 wt%) were dissolved in 1% (v/v) aqueous solutions of selected acids (AA, LA, and HCA) at room temperature (25±2 °C). The CS solution was left stirring overnight on a magnetic stirrer (IKA, Germany) to obtain a uniform solution. The solutions were then centrifuged to remove any air bubbles, and a clear solution was obtained with no trace of residual particles. Solutions were stored at refrigerator (5±1 °C) before the rheological measurement. Chitosan/graphene oxide solution preparation Based on our earlier work31, 2% CS was taken as a base material for incorporation of GO at different loading concentrations. GO powders were dispersed in deionized water (0.5 to 2 wt% based on chitosan wt%) and sonicated (Branson 3510-DTH, CT, USA) for 30 min. GO solution was then transferred gradually into 2% CS solution under magnetic stirring and the two solutions were blended well following the recommendation of Fang M. et al.32 The final concentration adjusted to 1% (v/v) by adding required volume of 2% AA solution. The resultant solution stirred,


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centrifuged, and stored at refrigerator as mentioned earlier. All prepared solutions were made in triplicates. The CS/GO film-forming solution was cast on a rimmed silicone resin plate (5×5 cm2) and dried under room temperature for characterization of the films. pH measurement The pH of the solutions was measured by a pH Meter (Orion Star™ A111, Thermo Scientific, USA). Rheological measurement The rheological measurements of the solutions (CS and CS/GO) were performed in a Discovery Hybrid Rheometer HR-3 (TA Instruments, USA). A cone-plate geometry with a diameter of 40 mm and a cone angle 2° was employed with a gap of 57 m. The temperature selected for the measurements were 0 to 30 °C based on the gel point of the CS solutions which assessed through a few preliminary runs. Oscillatory measurements were carried out in the frequency range of 0.1 to 10 Hz within the linear viscoelastic range. The strain amplitudes were of 5% and 10% for 1% and 2% CS solutions, respectively. The steady shear flow measurements were carried out exclusively at the low shear rate of 0.1 to 10 s-1 to understand structure property of CS gel with and without the addition of nanosheets. Samples were individually loaded on the measuring geometry and allowed to stand for 2 min prior to testing. Dielectric properties measurement A microwave network analyzer (Model: N5234A PNA-L, Keysight Technologies, USA) with an open-ended coaxial cable connected to a dielectric probe (N1501A, Keysight Technologies, USA) was employed to measure the dielectric properties of CS and CS/GO solutions. The instrument was warmed up for at least 1 h and then calibrated with air, short-circuit block, and water at 20 °C. 7 ACS Paragon Plus Environment


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The sample approximately 40 ml was placed in a 50 ml glass beaker and the open co-axial probe was set into the tube. The sample was measured at 20 °C while one set of 1% CS samples also measured at 5, and 10 °C based on the gel point. The dielectric properties were measured in the frequency range of 500–3500 MHz. The dielectric spectra of the samples (dielectric constant ε′, and dielectric loss factor ε″) were automatically computed and recorded with the manufacturer supplied computer software. The penetration depth (Dp) of the sample was calculated following the equation (1).33

Dp 

o 2        2 2  1     1       

(1)

where λ0 values are 0.328m and 0.122m at 915 MHz and 2450 MHz, respectively. Characterization of film properties The thickness of the film was measured by a micrometer (Mituyoto Corp., Kawasaki-shi, Japan) at various random locations (at least 8 times) of each film. The morphology and roughness of the film samples were measured using the Agilent 5500 atomic force microscope (Agilent Technologies, USA). Super sharp silicon cantilevers were employed for topography analysis of GO nanosheets in chitosan matrix using the intermittent constant mode (ACAFM- tapping mode). The micrographs of the film samples were taken from a scanning electron microscope (SEM) (JEOL, JCM-6000 Plus, Tokyo, Japan) operating at 15 kV. The ATR-FTIR spectroscopy (Nicolet iS5 equipped with OMNIC software, Thermo Scientific, Madison, WI, USA) was employed to obtain the spectra of CS/GO films in the wavenumber of 4000–500 cm−1 operated at a resolution of 4 cm−1 with 32 scans. 8 ACS Paragon Plus Environment

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The wide-angle XRD of the films was performed using an X-ray diffractometer (Bruker D8 Advance, Bruker Corporation, Germany) with CuKα radiation (λ = 1.5418 Å). The recorded region of 2θ was 5–40◦, and the scanning speed was 2.0◦ min−1. Thermogravimetric analysis (TGA) of the films were performed on a Netzsch STA 449 F3 'Jupiter' simultaneous DSC/TGA instrument (AZoNetwork Ltd, Manchester, UK) under a nitrogen atmosphere (60 mL min-1). The temperature range was employed from 25 to 600 ◦C with a heating rate of 10 ◦C min−1. Bioink preparation and 3D printing of thin films In order to develop GO-based nano-solution suitable for the loading in 3D printing ink, accurately weighed GO nano-sheets were mixed with a 4% w/w polymer solution under vortex and sonication. The final concentration of the GO in the solution was optimized at 10 mg/ml (w/v). Approximately, 500 µl of GO/polymer solution was then mixed with 5 ml of already prepared pluronic F127 based bioink prior to the actual printing. The final amount of GO in the bioink was about 5 mg in 5 ml giving the final concentration of 1 mg/ml. The compositions were then further optimized to 10 mg/ml and 50 mg/ml to assess the optimum concentration for best printing outcomes. The 3D printing of the thin film (L × W × H: 2 cm × 2 cm × 1 mm and 0.5 mm) was carried out via a commercial 3D printer (SE3D, USA). Also, some scaffolds (10 mm diameter with interconnected pores) were printed for printability and shape fidelity assessment and comparison. The print resolution was set at 100 µm, and the construct was directly printed in a petri dish placed on the print bed in ambient temperature. The developed viscous printing inks were drawn into a 22 gauge-printing syringe with an internal diameter of 640 µm (~100 µm layer height) from which the inks were extruded to print the constructs. Once the fabrication process was optimized, all printed constructs were stored at room temperature for further analysis. 9 ACS Paragon Plus Environment


Data analysis Rheological and dielectric measurements were carried out in triplicate; mean values and standard deviations were calculated. Microsoft Excel software package (Microsoft Corporation, USA) was used to carry out the statistical analysis, and significant level is considered at P < 0.05. Results and discussion Rheology Effect of aqueous acid solutions Three selected aqueous acid solutions (1%) with a variable pH were examined for their contributions towards rheological behavior to 1% CS solution. The pH of the 1% AA, LA, and HCA solutions were 2.48, 2.01 and 0.65, respectively, and those values increased to 3.55, 2.92 and 0.80, respectively while CS powders were added (1-2%). It indicates the pH of the solutions was dependent on the type of acid used for the preparation. Figure 1a illustrates the mechanical spectra of 1% CS solution as influenced by the acids at 5 °C. In general, the elastic, G′ and viscous modulus, G″ increased with increasing the frequency from 0.1 to 10 Hz. A liquid-like behavior (G″>G′) was evident for both LA and AA solutions in the lower frequency range. On increasing frequency, a distinct cross-over (G′=G″) was detected at 0.63 and 0.79 Hz for LA and AA, respectively, and dynamic moduli are almost alike for both acid solutions. However, a drastic drop in the dynamic moduli was observed for the CS in HCA solution, and even no cross-over point was detected. It clearly indicated that the rheological behavior of CS solution was markedly influenced by the type of acid used. While working on a wide range of CS concentration and acids, Hamdine M et al. observed a significant difference in the rheological behavior, 34 and they argued that the difference was attributed by the ionic strength 10 ACS Paragon Plus Environment

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of the acid solutions and their pKas. Furthermore, the viscosity decreases with pH due to the presence of large amounts of small ions e.g. Cl− which exhibited screening/shielding effect of the electrostatic force of the anionic group in solution and increased the chain flexibility.35 The solubility of CS in acid solutions was entirely related to the pH and the ionic strength, and therefore, lower ionic strength of weak acid (e.g., LA) provides a higher mechanical rigidity over a stronger one (HCA).36 The steady flow behavior of 1% CS in LA and AA solutions (1% CS in HCA was too dilute for the steady shear measurement, and thus, data related to HCA are excluded) indicating that the shear stress, τ increased with increasing the shear rate, while the corresponding apparent viscosity, η decreased confirming a shear-thinning behavior of those solutions. Both flow curves showed a similar trend with the shear rate although the magnitudes of τ and η of AA had the edge over the LA. It indicates that both acid anions (acetate and lactate) followed similar electrostatic effects on the CS solutions within the studied pH range (3.55 and 2.92). Rinaudo M. et al. reported the relative viscosity of chitosan was constant between pH 3.5 and 5.4.37 Based on these observations, further studies were carried out using only AA as a solvent for preparing the CS solution.



Figure 1. Effect of selected aqueous acid solutions on rheology of 1% chitosan at 5 °C (a) oscillatory and (b) steady shear measurement.


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Effect of temperature ramp CS (1 and 2 wt %) in 1% acetic acid solutions were non-isothermally heated from 0 to 30 °C at a heating rate of 5 °C/min to examine the thermal gelation (Figure 2a). At lower temperature, both solutions exhibited solid-like behavior, and a distinct gel point (Tgel; G′=G″) was detected at 9 and 11.6 °C, for 1 and 2% CS solutions, respectively. Furthermore, it can be seen that CS chains entangled with each other at 2% concentration, and produced a stiffer gel almost 3 times higher than 1% CS solution. The gel point temperature provides information about the suitability of the solution for the practical applications (e.g., hydrogel or liquid-form for drug delivery). Furthermore, the CS could form self-assembled hydrogel below the gel temperature without any cross-linking agent. It is quite interesting to study the gel structure at the vicinity of the gel point. Figures 2b and 2c illustrate how gel point shifts during the isothermal heating of 1% and 2% CS in 1% AA solutions. At the studied temperature range (0-30 °C), CS solution exhibited a distinct cross-over point at a particular frequency (critical frequency), which shifted gradually from 0.5 to 4 Hz with increasing the temperature. It indicates a shorter relaxation time for the CS when the temperature is increased, and furthermore, the chain entanglement loosens at higher temperature.



(a)

(b)


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Figure 2. (a) Effect of temperature ramp on gelation behavior of 1 and 2% chitosan in 1% acetic acid solution, (b) and (c) effect of isothermal heating on gelation behavior of 1% and 2% chitosan in 1% acetic acid solution at selected temperatures. Effect of chitosan concentration At 10 °C, the CS solutions exhibited a distinct gel point (G′=G″) in the studied concentration range (0.25 to 2%), and a representative rheogram is illustrated in Figure 3 at a loading concentration of 2%. The oscillatory strain values were ranged between 2 and 10% for the selected CS concentrations (0.25 to 2%). It indicates that the gel formation was not only depending upon the concentration of CS but on the frequency of the measurement also. The cross-over frequency (CO) signals the relaxation time for the polymer chain. At a lower concentration (0.25%), the gel point detected at a higher frequency (7.94 Hz) with lower relaxation time. On the contrary, the maximum relaxation time (0.25 Hz) was obvious for the highest concentration level (2%), and equaled to the time taken by the polymer molecules in the solution to disentangle. The cross-over frequency, CO



showed an exponential relationship with the concentration (C), at 10 °C (eqn. 2). At low concentration, the flow (e.g., deformation, orientation, etc.) of the polymer behaves like separate molecules, 38 and it changes with increasing the polymer concentration where the chain starts to intertwine and entangles by hindering the motion and increase the viscosity.39 Calero N. et al., observed that the G′ was not traced in a rheometer below a concentration level of 1.5 wt%, whereas, a concentration of 0.25% in this work was found to be sufficient for tracing the gel point.40 Furthermore, it has been reported that the CS solutions (1.5 to 8% w/v) remained in the sol state with G″>G′ at room temperature to 95 °C41. Such differences could be attributed to the difference in chitosan molecular weights, solvent-polymer interaction, degree of deacetylation, and also equipment and its set up (geometry, temperature, and frequency range employed).

Figure 3. Effect of concentration on viscoelastcity of chitosan solution in 1% acetic acid at 10 °C.


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The complex viscosity, * and apparent viscosity,  of CS solutions increased with increasing concentration, C, and followed a power-type relationship (eqn. 3-4) at a constant frequency of 1 Hz/ 1 s-1 at 10 °C (gelation temperature). 𝜔𝐶𝑂 = 11.97𝑒 ―2.06𝐶

(R2=0.97)

(2)

𝜂 ∗ = 7.71 𝐶2.20

(R2=0.99)

(3)

𝜂 = 18.62 𝐶2.49

(R2=0.99)

(4)

Effect of graphene oxide on oscillatory rheology Effect of GO loading concentration on 2% CS solution during isothermal heating at 5 and 20 °C is illustrated in Figure 4. Both dynamic moduli increased with the increase of frequency for the CS and the CS/GO samples. A distinct cross-over point was noticed in the lower frequency range (0.98), and the calculated activation energy was found to be 1.25 kJ/mol within the studied temperature range.

―𝐸𝑎

𝜂0 = 𝑘𝐴𝑒

𝑅𝑇

(5)

The Carreau model was also fitted well (R2 =0.99) for the control 2% CS solutions and CS/GO (Table 1b). It can be seen, the loading concentration of GO and the temperature both have significant effects on the Carreau model parameters. Below the cross-over point (8.5 °C), the η0 values increased with GO loading with an exception at 0 °C for 2%. At 10 and 20 °C, the η0 values dropped while 1% GO was added to CS solution, however, increased, thereafter. At 30 °C, the η0 values dropped with the loading concentration. These obtained η0 data revealed that the reinforcement of GO into the CS matrix is dependent on the selection of temperature whether it is below or above the gelation temperature. The change of time constant, λ follows a similar pattern of η0 with the temperature as well as the flow index, n. The 2% CS solutions exhibited non21 ACS Paragon Plus Environment


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Newtonian behavior with the n values ranged between 0.41 and 0.68 which is reflecting that the solutions become less shear thinning fluid with increasing temperature. Table 1b. Carraeu model parameters for 2% chitosan in 1% aqueous acetic acid solutions at selected graphene oxide concentrations and temperatures considering η∞ = 0. Temperature (°C)

GO (%)

η0 (Pa.s)

λ (s)

n (-)

R2

0

0

670.10

15.18

0.41

0.99

1

762.24

21.46

0.45

0.99

2

703.84

13.53

0.41

0.99

0

421.22

13.38

0.51

0.99

1

465.22

16.84

0.54

0.99

2

544.98

19.59

0.47

0.99

0

361.21

15.46

0.53

0.99

1

325.71

13.09

0.56

0.99

2

409.61

18.53

0.49

0.99

0

305.35

25.90

0.56

0.99

1

212.12

13.08

0.61

0.99

2

257.28

19.64

0.53

0.99

0

151.28

15.64

0.64

0.99

1

138.64

12.63

0.68

0.99

2

132.52

17.68

0.61

0.99

5

10

20

30

Dielectric properties


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Effect of aqueous acid solutions The dielectric properties of 1% (v/v) acid solutions with and without 1 wt% CS were measured as a function of frequency at 20 °C (Figures 5a and 5b). Both AA and LA solutions with/without CS showed a similar pattern for the dielectric constant, ′ although they superimposed with each other as the frequency was increased from 500 to 3500 MHz, and those values narrowly ranged between 75 and 78. The ′ values (74-83) for HCA solutions differed from other acid solutions, and those values dropped with addition of CS (72 to 76). The higher ′ values for acid solutions are expected since acid solutions contain 99% of free water in the system, which generated more polarizable dipole moments per unit volume of the sample. A relatively higher ′ of HCA indicates the charge carrier accumulates at the interface as the relaxation time is different for the components. The dielectric loss factor, ε″ - the most important parameter for the microwave heating and it measures the heating efficiency (the higher the loss factor the faster was the heating) in a microwave oven. The ′′ values for AA and LA solutions increased as function of frequency (Figure 5b). The ′′ values for AA solution were significantly lower than those measured for the LA solution throughout the frequency range. Furthermore, the ′′ values for CS dissolved in AA were higher than the LA in the frequency range of 500 to 1500 MHz, and superimposed, thereafter. It indicates in spite of different solution properties, the CS solutions showed a similar ε″ values due to same degree of dissociations in the solutions. For HCA, the ′′ decreased with increasing the frequency. However, incorporation of CS in HCA solutions reduced the ′′ significantly with the frequency range. A distinct difference between HCA with other solutions was attributed by the electrolytic effect, ionic dissociations and solvent-CS interaction. Effect of temperature



The dielectric properties of 1% CS in AA solutions were measured at selected temperatures (5, 10 and 20 °C) (Fig. 5c). The ε′ was almost frequency independent from 500 to 2500 MHz, and decreased thereafter. The ε″ remained constant within 1000 MHz, and increased exponentially at higher frequencies. No significant difference was observed for both ε′ and ε″ values in the temperature range of 5 to 10 °C. It is believed that the mobility of ions/dipole orientation were restricted with a longer relaxation time below the gel point temperature. The dielectric parameters decreased when the temperature increased to 20 °C. The decrease in both ε′ and ε″ values for CS solutions at higher temperature indicated the absence of dielectric relaxation in the higher frequency range. This observation is in consistent with the earlier report.44 Effect of concentration The ′ values decreased with increasing the concentration of CS in solutions from 0.5 to 2% although the drop was not significant between 0.5 and 1% solutions. The drop in the ′ values was obvious for 2% CS solutions because of the less availability of water in the solutions. For many starch/water systems, the decrease of the ′ was not significant with the concentration.45 The ′′ values increased with the concentration in the solutions; the values remained insignificant with frequency between 0.5 and 1% concentrations. The ′′ values at 2% solutions dropped significantly at above 2000 MHz. It has been opined that the increase in the concentration of solute in solutions decreased the dielectric loss factor.46-47 Effect of graphene oxide concentration The dielectric properties of the CS solution matrix (2%) and GO loaded CS solutions measured at 20 °C are shown in Figure 5d. Both ′ and ′′ were highly frequency dependent (500-3500 MHz) for the CS/GO solutions; this could be beneficial for their wide applications. The ′ of CS solutions 24 ACS Paragon Plus Environment

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was increased significantly by the addition of GO from 0.5 to 2 wt% in the whole frequency range. The GO possesses high aspect ratio and have a functional group to improve dielectric constant effectiveness. The higher ′ values of the low-frequency range were resulted by the dipoles and charge carriers, which had sufficient time to align themselves in the direction of the electric field leading to enhanced polarization.48 The high ε′ values of the CS/GO solutions indicated their ability to store charge under the influence of electric field. At the low-frequency range (

Rheological and Dielectric Behavior of 3D-Printable Chitosan

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