Mechanical properties. CS was blended with glycerol, CMC, and LNR through solution mixing at 80°C for 1 h. The mixed solutions were cast onto films, followed by drying at 60°C for 24 h. Figure 1 shows the stress–strain curves of the CSG/LNR blends with 0–20 phr of CMC. The Young’s modulus was calculated from the slope at the early stage of the stress–strain curve. The CSG/LNR blend showed a low Young’s modulus of 0.3 MPa, a maximum tensile strength of 0.5 MPa, and an elongation at break of 30% (Table 1). The addition of CMC resulted in an increase in the values of Young’s modulus and maximum tensile strength. The CSG/LNR/CMC10 blend exhibited a Young’s modulus of 6.7 MPa, a maximum tensile strength of 8 MPa, and an elongation at break of 80%, all the values higher than those for the blends where CMC was added at 0, 2, and 5 phr. The CSG/LNR/CMC20 blend exhibited the highest Young’s modulus (18.2 MPa) and maximum tensile strength (18 MPa) and the lowest elongation at break because of the high tensile strength and brittle properties of CMC. The toughness and tensile properties of the CSG/LNR blend were improved by adding CMC, particularly for the CSG/LNR/CMC10 sample. The toughness of the sample is related to the area under the stress–strain curve [21]. The Young’s modulus of starch increased with the CMC content [22], and a high interfacial reaction improved the mechanical properties of the polymer blends, which has been reported previously [23]. The improvement in the tensile properties was attributed to the compatibility of CSG/CMC, occurrence of the interfacial reaction of CMC/LNR, and crosslinking inside the LNR phase.
Table 1
Young's modulus, maximum tensile strength, and elongation at break of CSG/LNR blends with 0–20 phr CMC.
Sample | Young's modulus (MPa) | Maximum tensile strength (MPa) | Elongation at break (%) |
CSG/LNR | 0.3 ± 0.08a | 0.5 ± 0.08a | 30.1 ± 2.40b |
CSG/LNR/CMC2 | 2.4 ± 0.07b | 1.7 ± 0.12b | 35.4 ± 2.89c |
CSG/LNR/CMC5 | 2.8 ± 0.08c | 2.9 ± 0.21c | 33.3 ± 3.21c |
CSG/LNR/CMC10 | 6.7 ± 0.08d | 8.0 ± 0.43d | 79.9 ± 4.35d |
CSG/LNR/CMC20 | 18.2 ± 0.08e | 18.0 ± 2.1e | 7.80 ± 0.51a |
Means with different lowercase superscript letters in the same column are significantly different (P < 0.05). |
Morphology. The morphologies of the samples were observed using SEM. The samples were disintegrated in liquid nitrogen, and then the LNR phase on the fracture surface was extracted using an immersion technique in toluene at 60°C for 1 h. Figure 2 shows the fracture surface images of the CSG/LNR and CSG/LNR/CMC blends with 2, 5, 10, and 20 phr of CMC. The CSG/LNR blend exhibited a black circle representing the LNR particles extracted using toluene because NR was dissolved in toluene [24]. The LNR particle sizes of the CSG/LNR blend were 1–3 µm. The addition of CMC at 2, 5, 10, and 20 phr resulted in the dispersion of the LNR rubber particles (1–3 µm) in the CSG matrix. The LNR formed small rubber particles in the CSG matrix, while the addition of CMC did not reduce the particle size of the LNR. The improvement in the tensile properties was probably due to the high tensile properties of CMC, interfacial crosslinking of CSG/LNR, and crosslinking inside the LNR phase.
Solubility and swelling. The solubility and swelling of the samples were measured by dissolving the samples in distilled water at 25°C for 24 h. The CSG film was prepared through a controlled mixing of starch with glycerol (70/30 %w/w). The solubility and swelling degree of the CSG/LNR film were 41 and 65%, respectively (Fig. 3). The solubility of CSG/LNR/CMC2 decreased to 22% owing to the formation of interfacial crosslinking between the CMC and LNR. The elevated CMC content increased the solubility of the CSG/CMC/LNR blends because of the large amount of hydrophilic materials. The degree of swelling increased with the CMC content. CS is a hydrophilic material [25], whereas CMC forms a gel in water [18, 26]. The increase in the swelling degree was an evidence of the hydrophilic properties of CS and CMC [25, 27], the swelling ability of CMC [28], and the interfacial crosslinking of CSG/LNR through CMC.
Contact angle. The water droplet contact angle is related to the hydrophilicity and surface tension of materials. Figure 4 shows the contact angle of CSG and the CSG/LNR blends with 0–20 phr of CMC at 3 min. The CSG exhibited a low contact angle of 61°. The contact angle of the CSG/LNR blends increased with increasing CMC content, especially for 20 phr. CS is a polar material, whereas amphiphilic CMC combines polar and non-polar structures [27]. The increase in the contact angle of the CSG/LNR blend was probably caused by small hydrophobic rubber particles that were finely dispersed in the CSG matrix. The increase in the contact angle of CSG/LNR/CMC2 was possibly due to the interfacial crosslinking between LNR and CMC and the non-polar portion of CMC. The addition of 5–20 phr of CMC increased the contact angle to 85–90°, respectively, owing to the enhanced non-polar portion of CMC.
Reaction mechanism. The reaction mechanisms of CSG, CMC, and LNR were investigated using FTIR. Figure 5 shows the FTIR spectra of CMC, CSG, and the CSG/LNR blends with 0–20 phr of CMC. The FTIR spectra of LNR (cis-1,4 polyisoprene) exhibited C‒H stretching (2960, 2927, and 2852 cm− 1), C = C stretching (1661 cm− 1), C‒H deformation of stretching ‒CH2‒ (1448 cm− 1), C‒H deformation of ‒CH3 (1376 cm− 1), and C = C‒H (835 cm− 1) [29]. The CSG spectra exhibited peaks at 1643 (‒OH bending), 1016, and 929 cm− 1 (‒CO stretching) [30]. The CMC spectra exhibited peaks at 3040 (‒OH stretching), 2897 (‒CH stretching), 1602 (COO−), and 1427 cm− 1 (COO−Na+) [31]. The spectra of the CSG/LNR blend exhibited a combination of the individual CSG and LNR spectra. The CSG/LNR/CMC blend presented an increase in peak intensities at 1602 (COO−) and 1427 cm− 1 (COO−Na+) of CMC. To study the reaction mechanism of the blend, the LNR phase was extracted from the CSG/LNR and CSG/LNR/CMC blends. The spectra of CSG, CMC, LNR, and the extracted LNR are shown in Fig. 6. The extracted LNR from the CSG/LNR (Fig. 6d) exhibited spectra similar to those of pure LNR (Fig. 6c) with peaks at 1661 (C = C stretching) and 835 cm− 1 (C = C‒H). Furthermore, in the LNR extracted from the CSG/LNR/CMC blend, the peak at 1661 cm− 1 (C = C) shifted to 1657 cm− 1 and increased in intensity (Fig. 6e). This indicated a new ‒C‒O peak due to the reaction between CMC and LNR. The intensity of the peak at 835 cm− 1 (C = C‒H) decreased (Fig. 6e) compared to that of pure LNR (Fig. 6c) owing to the reduction of the C = C‒H structure in the LNR chain. The Na+ ion in CMC is considered a Lewis acid catalyst [32]. The crosslinking at the C = C structure of NR is accelerated by a Lewis acid catalyst, as reported previously [33]. It was confirmed that the C = C of the LNR structure reacted with the COO− of CMC as the Na+ ion in CMC acted as a catalyst. The suggested reaction is shown in Fig. 7. CSG showed high compatibility with CMC owing to their structural similarity and interaction between the ‒OH groups (Fig. 7a); whereas, a reaction occurred between the C = C of LNR and COO− of CMC (Fig. 7b). These interactions and reactions improved the interfacial crosslinking, LNR crosslinking, mechanical properties, and water resistance of the CSG/LNR/CMC blends.