Characterization of St, ES, CSP and CSP-GMA
The modified starch particles exhibit significant changes in their microscopic morphology, which can be visually observed through SEM. Figure 4 presents the scanning electron microscopy images of St and ES. From Fig. 4, it can be observed that the surface of St displays a uniform and smooth morphology. In contrast, the surface of ES becomes noticeably rougher, and there are interactions between the starch particles[25, 26]. This is due to the formation of a shell structure on the starch particle surface by stearoyl chloride and acryloyl chloride as a result of the esterification reaction. These observations indicate that the esterification modification of the starch particles was successfully performed, resulting in the synthesis of esterified modified starch with the expected blend structure.
(a) St (x1000); (a1) St (x3000); (b) ES (x1000); (b1) ES (x3000)
FTIR is a crucial technique for identifying functional groups and determining the chemical structure of substances. By analyzing the FTIR spectra of St, ES, CSP, and CSP-GMA, it can be observed from Fig. 5 that St exhibits an -OH stretching vibration peak at 3306 cm− 1, C-O stretching vibration peaks at 998 cm− 1 and 1078 cm− 1, and a C-O-C absorption peak at 1149 cm− 1. In comparison to St, the presence of stearoyl chloride in ES introduces a new C = O stretching vibration peak at 1740 cm− 1. These results confirm the successful preparation of ES. The FTIR spectra also show prominent C-H bending vibration peaks at 1446 cm− 1 and 1376 cm− 1. Compared to the ES spectrum, the absorption peak at 1727 cm− 1 in the CSP spectrum significantly increases[27], further indicating the successful grafting of EA onto ES.
From Fig. 5, it is evident that CSP-GMA exhibits an absorption peak at 933 cm− 1, characteristic of the epoxy group from GMA, confirming the successful modification of CSP by GMA.
The particle size distribution of the emulsion obtains by the soap-free emulsion polymerization method is tested, as shown in Fig. 6. The particle size of CSP-GMA ranges from 160 nm to 420 nm, with an average particle size of approximately 280 nm. Additionally, the solid content of the emulsion reached 31.4%. After purification and precipitation of the product, the monomer conversion rate is measured to be 94.6%.
Mechanical Properties Analysis
Impact strength reflects the material's resistance to impact and helps in assessing its brittleness and toughness. Figure 7 shows the impact strength of PBS/CSP-GMA blends. It can be observed that as the CSP-GMA content increases, the impact strength of the material first increases and then decreases. With an increase in St content, the impact strength of the material consistently decreases. The pure PBS sample has an impact strength of 70 J/m. When the CSP-GMA content is increased to 5%, the impact strength slightly improves, reaching 109 J/m at 20% content, which is the optimal impact strength. Beyond this content, the impact strength gradually decreases. Compared to PBS/St blends, PBS/CSP-GMA blends show a significant improvement in impact strength, increasing by 179% at a 20% filler content. This difference is attributed to the better compatibility of CSP-GMA within the PBS matrix compared to St, allowing for effective stress transfer. CSP-GMA, as a hard-core soft-shell particle, provides strength through its core and absorbs impact energy through its outer shell, preventing fractures blend and absorbing impact energy. When the content exceeds 20%, the impact strength decreases due to the non-uniform distribution of core-shell particles causing agglomeration, resulting in decreased overall performance[28]. Starch, as a filler, increases the blend viscosity and rigidity, leading to higher energy consumption during impact. Increased starch content and poor compatibility with the PBS matrix result in uneven distribution, causing a continuous decrease in the impact strength of PBS.
Elastic modulus describes the degree of deformation of a material under stress and is a key indicator of its elastic properties. Elongation at break represents the material's ability to stretch before breaking, usually expressed as a percentage. Higher elongation at break indicates greater ductility and toughness.
Figure 8 and 9 show the elongation at break and elastic modulus of PBS/CSP-GMA and PBS/St blends, respectively. From Figs. 8 and 9, it is evident that as the CSP-GMA content increases, the elastic modulus of the blends exhibits a negative growth trend, while the elongation at break first increases and then decreases, reaching its maximum at a CSP-GMA content of 15%. Conversely, as the St content increases, the elongation at break shows a negative growth trend, while the elastic modulus increases.
When the CSP-GMA content reaches 15%, the blend exhibits the best toughness, with an elongation at break of 392%. Compared to PBS/St, using CSP-GMA as a filler significantly enhances the elongation at break. This is because untreated starch forms chemical bonds with the material's molecules, increasing the overall viscosity and restricting molecular mobility, making the material harder to break and stretch. GMA-modified CSP, as a core-shell particle, enhances the adhesion between particles and the matrix through the GMA on its surface, improving interfacial strength, resisting crack propagation, and maintaining high tensile performance. The decrease in elongation at break beyond a 15% content is due to the agglomeration of CSP-GMA, hindering molecular movement and reducing toughness[26].
As shown in Fig. 9, increasing the CSP-GMA content gradually decreases the elastic modulus, while PBS/St exhibits an increasing trend. Starch, as a high molecular weight compound, enhances the viscosity and strength of the material through molecular interactions, leading to increased elastic modulus[28]. CSP-GMA, with its soft outer shell layer, effectively improves the flexibility of the blend, resulting in a gradual decrease in elastic modulus.
Observation of Microstructure of blend
Scanning electron microscopy (SEM) is commonly used to study the microstructure of materials. Figure 10 (a1, a2) shows SEM images of the impact fracture surfaces of PBS/St blends, and Fig. 10 (b-c5) displays SEM images of the impact fracture surfaces of PBS and PBS/CSP-GMA blends with varying CSP-GMA content.
By comparing Fig. 10 (a) with Fig. 10 (c), it is evident that the dispersion and compatibility of CSP-GMA with PBS resin are significantly better than those of Starch. This improvement is attributed to the epoxy groups in GMA, which enhance the interfacial interaction between CSP and PBS chains[29]. From Fig. 10 (c1) to Fig. 10 (c5), as the CSP-GMA content increases, the blend surface transitions from smooth to rough. This roughness results from plastic deformation of the material, which absorbs a significant amount of energy during the process. At this stage, numerous crazes appear on the fracture surface. A small number of crazes can impart some strength to the material, allowing it to bear a certain load.
Additionally, some voids appear on the fracture surface. These voids are formed by CSP-GMA particles absorbing a substantial amount of external force. These particles act as stress concentrators within the blend, absorbing excessive energy and causing the soft shell layer of the particles to deform and debone from the PBS matrix, leading to void formation. CSP-GMA particles effectively transfer and disperse the stress during impact, reducing local stress concentration in the material. This promotes crazing and shear yielding in the matrix near the particles, thereby enhancing the material's toughness. However, with a further increase in CSP-GMA content, particle agglomeration occurs, resulting in internal stress imbalances in the matrix. This leads to an increase in the number of crazes and a subsequent decrease in impact performance.
DSC Analysis
Differential Scanning Calorimetry (DSC) tests are conducted on PBS and PBS/CSP-GMA blends. Figure 11 presents the DSC analysis curves of PBS and PBS/CSP-GMA blends. From the figure, it can be deduced that the addition of CSP-GMA slightly reduces the crystallization temperature (Tc) and melting temperature (Tm) of PBS. As the CSP-GMA content increases, both Tc and Tm exhibit a decreasing trend[30], with Tc and Tm dropping by approximately 6.0°C and about 3.0°C compared with PBS (the Tc of PBS is 86.8°C and the Tm of PBS is 116.1°C), respectively. This reduction in Tc and Tm for the blends indicates that the addition of CSP-GMA makes the blends more easily crystallizable, thereby improving their processability. The crystallinity of the blends also decreases compared to PBS, weakening the intermolecular interactions within PBS molecules and making the material more flexible[31]. As the addition of CSP-GMA disrupts the integrity of the PBS molecular chain, it hinders the movement of the molecular chain, reducing the area of regular arrangement, which results in a decrease in the crystallinity of the blend[30].
(a)Melting curve; (b) Crystallization curve
Table 2 presents the detailed DSC analysis data of PBS and PBS/CSP-GMA. The crystallinity of PBS is calculated according to Eq. (3):
\(\:{X}_{C}=\frac{\varDelta\:{H}_{m}}{\varDelta\:H}\times\:\frac{1}{w}\times\:100\%\) (3)
where ΔHm is the melting enthalpy of the blend sample, ΔH is the melting enthalpy of the fully crystallized polymer (the complete melting enthalpy of PBS is about 110.5 J·g− 1), and \(\:w\) is the mass fraction of PBS in the blend.
Table 2
DSC data of PBS and PBS/CSP-GMA blends
| Sample | Tm/°C | Tc/°C | △Hm/J·g− 1 | △Hc/J·g− 1 | Xc/% |
| PBS | 116.1 | 86.8 | 78.61 | 43.63 | 71.14 |
| PBS/CSP-GMA5% | 113.3 | 83.4 | 53.88 | 57.69 | 51.33 |
| PBS/CSP-GMA10% | 112.7 | 83.2 | 53.76 | 57.49 | 54.06 |
| PBS/CSP-GMA15% | 112.5 | 81.8 | 53.69 | 56.23 | 57.16 |
| PBS/CSP-GMA20% | 112.3 | 81.3 | 53.52 | 53.70 | 60.54 |
| PBS/CSP-GMA25% | 112.5 | 80.5 | 52.11 | 51.87 | 62.88 |
Thermogravimetric Analysis (TGA)
Figure 12 shows the TGA curves of the blends, and Fig. 13 presents the DTG curves of the blends. From Fig. 12, it can be observed that the addition of CSP-GMA15% significantly improves the thermal stability of PBS blends. It can be seen that with increasing CSP-GMA content, the T5% temperature of PBS/CSP-GMA blends first increases and then decreases. When the CSP-GMA content is 15%, the T5% is highest, reaching 361.3°C, which is 17.8°C higher than that of PBS. This phenomenon indicates that the energy required for the decomposition of the blend is highest at this point, and the thermal stability of the material is best. This is because the added CSP-GMA is well dispersed in the PBS matrix at this ratio, with no agglomeration, resulting in a very stable material structure. From the DTG curves in Fig. 13, it can also be observed that CSP-GMA has little impact on Tmax of the material. When the CSP-GMA content is 15%, the weight loss rate is minimized, and the material exhibits the highest stability at high temperatures, making it less prone to decomposition[32].
X-ray Diffraction Analysis (XRD)
XRD is a commonly used method for characterizing the crystalline structure of materials. XRD tests are performed to analyze the crystalline structure of PBS/St and PBS/CSP-GMA blends, and the results are compared with PBS. Figures 14 and 15 show the XRD analysis curves of PBS/St and PBS/CSP-GMA, respectively. It can be clearly observed that PBS exhibits two characteristic crystalline peaks at 20.4° and 23.2° Corresponding to the (020) and (110) crystal planes, respectively. The PBS/St curve in Fig. 14 and the PBS/CSP-GMA curve in Fig. 15 also display these characteristic peaks, indicating that adding pure St, the variation in filler content or the addition of modified starch-based CSP-GMA does not affect the position of PBS characteristic peaks. The nucleation mechanism, crystal structure, and crystal morphology of the blend remain unchanged[33].