Glycerol plasticisation of chitosan/carboxymethyl cellulose composites: Role of interactions in determining structure and properties

Biopolymers such as chitosan and cellulose continue to attract much interest as they have many appealing characteristics such as biodegradability, biocompatibility, chemical versatility and natural functionality; however, many of their properties usually require further tailoring for specific purposes. This study shows that glycerol plasticisation and the addition of graphene oxide (GO) or reduced graphene oxide (rGO) altered the properties of chitosan and a chitosan/carboxymethyl cellulose (CMC) blend. For the chitosan/CMC matrix, GO or rGO was likely to disrupt polyelectrolyte complexation (PEC) between the two biopolymers, leading to weakened mechanical properties and increased surface hydrophilicity. Conversely, glycerol assisted PEC by increasing the biopolymer chain mobility, leading to reduced surface hydrophilicity. Moreover, some synergistic effects from a combination of glycerol and GO/rGO were evident. Specifically, GO/rGO notably increased the toughness of the chitosan film on inclusion of 40 wt% glycerol. Both GO and rGO reduced the relaxation temperatures of the chitosan/CMC film with 20 wt% glycerol added, resulting in increased biopolymer chain mobility. Moreover, the bionanocomposites showed high relative permittivity (54-387). Thus, this work describes how complex interactions in multiphasic biopolymer composite systems influence structure and properties.

To fabricate biopolymer materials with enhanced properties and functionality, material hybridisation has been an interesting approach. Relying on multiple dynamic bonds such as electrostatic interaction and hydrogen bonding between different components, biopolymer materials with engineered properties may be constructed [20,21]. The concept of polyelectrolyte complexation J o u r n a l P r e -p r o o f 8 The samples were scanned over an angular range (2θ) of 6-40° with a step size of 0.0263° and a step rate of 2.16 s/step.
Fourier-transform infrared (FTIR) spectra were collected using a Bruker TENSOR 27 FTIR spectrometer with an attenuated total reflection (ATR) accessory with 32 scans for each sample over a range of 4000-500 cm −1 at room temperature (RT).
Thermo-gravimetric analysis (TGA) was undertaken using a Mettler Toledo TGA apparatus over a temperature range of 30-700 °C at 10 K/min under nitrogen.  (24 × 24 mm). At least triplicate tests were performed for each sample. The real (Z′) and imaginary (Z″) parts of impedance were acquired with a frequency (f) range of 1-10 6 Hz. The AC conductivity (admittance) (σ), the real part of relative permittivity (ε′ r ), and the imaginary part of electric modulus (M″) calculated using the following equations [50][51][52]: J o u r n a l P r e -p r o o f Here, ω is the angular frequency (= 2πf), ε 0 is the permittivity of free space (≈ 8.854 ×10 −12 F⋅m −1 ), A is the area tested of the sample (m 2 ), and t is the sample thickness (m).
The bulk resistance (R b ) was determined from the Nyquist plots of impedance (Z″ vs. Z′) from the points where the semicircle and the straight line meet. Then, the conductivity (σ dc ) was calculated using equation (4) [50,53]: Tensile tests were performed using an Instron 3367 universal testing machine with a 1kN load cell at a crosshead speed of 3 mm/min. As the specimens were in the form of thin sheets, specimen extension was measured by grip separation as recommended by ASTM Standard D882. Young's modulus (E), tensile strength (σ t ), and elongation at break (ε b ) were automatically determined using Instron Bluehill 3 software from at least seven replicates for each sample.
Contact angle data were obtained from sessile tests at RT based on Young-Laplace using an Attension Theta Lite instrument (Biolin Scientific, UK). As the contact angle kept changing after the water drop was placed on the biopolymer film surface, contact angles at 0 s and 60 s (θ c0s and θ c60s , respectively) were recorded.
J o u r n a l P r e -p r o o f was used to examine the dispersion of GO or rGO in the biopolymers, see Figure 1. All the A-samples displayed some non-dispersed particulate features of different sizes up to 100 nm, as illustrated by the bright areas in HAADF images. This could represent some chitosan structures that were not disrupted by processing or new crystals evolved (discussed in XRD results). Compared with AG2-F, AG2/GO-F and AG2/rGO-F showed very similar morphology, except that there were some large-sized flocculent substances, some of which were over several hundreds of nanometres in length. They appeared as the 'cloudy' areas with diffused bright contrast in the HAADF image, indicative of a dissolvable feature which, may be ascribed to partially exfoliated GO or rGO nanosheets. In general, this observation indicates that, due to the matching chemistry and the thermomechanical mixing, both GO or rGO nanosheets were largely exfoliated and finely dispersed in the chitosan matrix. Likely, the resultant few-layer nanosheets at a certain small size became invisible under STEM against the background of the biopolymer matrix. In particular, GO nanosheets have oxygen-containing groups (e.g. ─COOH and ─OH) and negative charges resulting from the ionisation of carboxylic acid and phenolic hydroxyl groups, which can interact adequately with the polycationic chitosan through hydrogen bonding and electrostatic attraction [47]. In agreement with this, previous studies [47,54,55] have also demonstrated the excellent dispersion of GO in chitosan materials. Although rGO is less hydrophilic and contains less negatively charges than While previous studies [45,56,57] have indicated a possible phase separation between low-and high-glycerol-content domains in plasticised biopolymer composites, no such heterogeneity was observed in both the A-and B-samples here from SEM and STEM imaging, confirming the adequate mixing of the materials.

Crystalline structure
The crystalline structures of the different biopolymer and composite films were revealed by XRD, see diffractograms in Figure 2. As shown in Figure 2 (a), all the A-samples displayed similar XRD patterns. The reflections are all attributed to the crystal lattice of chitosan [58]. As discussed earlier [27], the crystalline patterns shown here are largely different from those of unprocessed chitosan and, therefore, are in the main due to processing-induced re-crystallisation. Clearly, AG4-F displayed weaker peak intensities than the A-samples with 20 wt% glycerol, indicating a greater J o u r n a l P r e -p r o o f 12 and hydrogen bonding may facilitate the arrangement of the attached chitosan chains to form an ordered structure along the rigid template offered by GO [47]. The content of glycerol or the addition of GO or rGO did not vary the amorphous nature of this dual-biopolymer system. Irrespective of matrix type, the addition of either GO or rGO is not observed to cause apparent changes in the FTIR spectrum. Likely, the interactions involving GO and rGO are not visible in the FTIR spectra due to their low content in the matrices, but they are also not very IR-active materials. Figure 4 shows the TGA data for the different films presented as the derivative of weight loss as a function of temperature. For AG2-F, the major thermal decomposition of chitosan occurred between 215 °C and 385 °C, with its peak temperature (T d , when the weight loss occurs at the maximum rate) being 286 °C. Immediately before this major weight loss, there was a small peak between 195 °C and 215 °C, associated with the initial de-polymerisation of the biopolymer.

Thermal stability
Compared with AG2-F, AG2/GO-F and AG2/rGO-F exhibited slightly lower T d (284 °C and 281 °C, respectively). The GO used in this study is relatively thermally unstable with the major mass loss occurring between about 150 and 240 °C peaked at 207 °C [49], presumably due to pyrolysis of the labile oxygen-containing functional groups, whereas rGO is more thermally stable [66]. The thermal decomposition of glycerol starts as early as 155 °C and concludes at 237 °C [67], which is also more

Molecular relaxations
DMTA was used to study the molecular relaxation of the different films, with tan δ plots as a function of temperature shown in Figure 5. For all the samples, two transitions could be identified.
A weak one occurring at sub-zero temperatures is associated with a β-relaxation of chitosan attributed to the side chain or lateral group motions of chitosan interacting with small molecules such as water or glycerol by hydrogen bonding. At higher temperatures, there was a much more prominent transition attributed to the α-transition (glass transition) of chitosan [68,69]. For AG2-F, the peak temperature of the β-transition (T β ) was −40 °C and the peak temperature of the α-transition (T α ) was at 54 °C. AG/rGO-F showed unchanged relaxation peak temperatures whereas AG/GO-F displayed    Table S1. BG2/GO-F and BG2/rGO-F had the lowest ε′ r values at 1 kHz, 56±3 and 54±14, respectively. AG4-F had the highest ε′ r at 1 kHz, 387±110, followed by BG4/GO-F whose ε′ r at 1 kHz was 307±72. Thus, these biopolymer materials have potential for use in some electronic applications such as energy storage. to a higher f, suggesting a decreased relaxation time. Obviously, more plasticiser in the system could increase polymer chain mobility and thus, make ions and associated dipoles more mobile. The influence of GO or rGO inclusion on the relaxation process was minor. counteract the plasticisation effect of glycerol and assist re-crystallisation (see XRD results), but also facilitate uniform stress distribution and minimise stress concentration, leading to increased mechanical properties [47,55]. However, this mechanical reinforcement effect was not apparent for the B-samples with 40 wt% glycerol, due to the less effective interactions between GO or rGO and biopolymer chains in the polyelectrolyte-complexed system.

Mechanical properties
The Shore D hardness of the different samples was measured as shown in Figure S2, which correspond well with the trends obtained for E and σ t data. This means glycerol plasticisation and the addition of GO or rGO affected the surface rigidity in the same way as for the tensile properties.

Surface wettability
The surface wettability of the different films is reflected by θ c0s and θ c60s as shown in Figure 8.