Relationships between Crosslinking Behaviour and Morphological Development in PLA/ENR Blends Using Maleic Anhydride as a Coupling and TPV Agent

doi:10.21203/rs.3.rs-4612007/v1

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Relationships between Crosslinking Behaviour and Morphological Development in PLA/ENR Blends Using Maleic Anhydride as a Coupling and TPV Agent

https://doi.org/10.21203/rs.3.rs-4612007/v1

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Maleic anhydride (MA) can function as both a grafting agent and a thermoplastic vulcanizate (TPV) agent in thermoplastic elastomer (TPE) blends of polylactic acid (PLA) and epoxidized natural rubber (ENR), depending on the degree of crosslinking. To validate the claim, a set of formulations with MA ranges from 0 to 2.0 phr was added to the PLA/ENR blend at different matrix blend ratios and compounding sequences. To distinguish the types of networks responsible for altering the blend’s phase and melt flow behaviour, various tests including mechanical, rheological, viscoelasicity validation, visual imaging, thermal analysis and spectroscopy testing of X-ray diffraction analysis (XRD) and Fourier transform infrared (FTIR) were conducted. The results revealed that, without the presence of MA, the blends behave as typical thermoplastic elastomers (TPEs). Meanwhile, the addition of MA enhances the tensile properties and modifies the blend microstructure, indicating the presence of a coupling effect in the PLA/ENR blend. With further inclusion of more MA inside the blends, the rise in crosslinking degree transforms the blend into a thermoplastic vulcanizate (TPV) compound. Interestingly, in the molten state, the TPV blends exhibit dilatant flow behaviour, which is in contrast with the shear-thinning flow pattern of PLA.

Polylactic-acid

Epoxidize natural rubber

Thermoplastic Vulcanizate

Maleic anhydride

coupling agent

MA can function as a compatibilizer and a TPV agent in PLA/ENR blends depending on its amount.
The TPE melt initially exhibits low viscosity and shear-thinning behavior, while the TPV, with higher melt viscosity, displays dilatant flow characteristics.
The developed microstructure as a results of MA involvement control the PLA/ENR blend’s rheological behaviour, viscoelasticity, physical, thermal and mechanical properties.

The attractive decomposition features, renewable and eco-friendly nature of PLA, along with its versatile properties, processability, and ability to blend with other polymers, have made it widely used in additive manufacturing, automotive, and biomedical industries. ^{[1], [2]} Over the past two decades, blending PLA with other biopolymers and natural rubber has become a popular practice due to its simple and effective method of creating appealing new materials with significantly enhanced tensile and impact toughness. The successfulness of PLA blends is largely contribute by PLA's ability to form compatibility with other polymers, allowing it to finely tuned their morphology and enabling the blend to achieves desirable properties.^{[3], [4], [5]}

In recent years, extensive research has been devoted to analyse the properties of TPEs and TPVs based blends of natural rubber (NR) and PLA.^{[6], [7]} This interest is driven by the inherent self-healing properties of ENR, making it an attractive strategy for developing self-healing materials for applications in smart wearable electronics,^[7] supercapacitors,^[8] coatings,^[9] construction ^[10] and medical treatment,^[5] etc. Today, the focus on PLA and ENR blends has evolved leading to the development of new functional composites by incorporating mineral and natural fillers of various sizes,^[1] shapes^[6] and surface functionality.^[7] To impart self-healing properties to these blends or composites, the rubber components are chemically modified to attach active functional groups, permitting the rubber phase to be dynamically vulcanized to turn it into thermoplastic vulcanizates (TPVs) known as elastomeric alloys. Liu et al recently reported that through a tailored cross-linker with a triple dynamic covalent bond, the tensile strength of ENR/PLA TPVs were increased by 1.5 to 2.9 folds of tensile strength and increase the elongation at break of 174 % was accomplished with respect o the TPVs vulcanized by peroxide.^[11] Klinkajorn and Tanrattanakul also synthesize graft copolymer G0.60 ENR-g-MA and add it into 90PLA/10ENR blend to enhance grafting efficiency. They manage to yield a ([ENR-g-MA]-g-PLA) blend with the formation of smaller ENR rubber particles in PLA phase and the microstructure becomes smaller as the grafting amount increases.

Although the blend viscosity is expected to increase after being subjected to thermal or dynamic vulcanization methods, the correlation between the blend's rheological data and phase behavior is likely less well understood. This is because most efforts have been focused on improving the final properties of the blends and composites. Typically the flow behavior of thermoplastic melts and TPE blends largely depends on processing parameters,^[12] polymer melt strength ^[13] and the filler structural skeleton ^{[14], [15]}. In contrast, the rheological properties of TPV blends are expected to be more complex due to the emergence of gels,^[16] internal crosslinking ^[4] and formation of intermolecular bonds.^[3] According to Wu's group, the dynamic vulcanization degree of ENR is the prominent aspect in altering the blend torque in 40ENR/60PLA with dicumyl peroxide (DCP) as their vulcanizing agent.^[5] Innes et. al reported that the increase in viscosity is caused by the degradation of EPDM in their sulphur-based 50EPDM/50PP TPV. Meanwhile, Lui and coworkers indicate that the addition of Fe3O4 particles has greatly increased the viscosity and modified their magneto-responsive shape memory polymers of PLA/TPU/Fe3O4 composite crystallization properties.

Besides the other TPV agents of sulphur, peroxide and acids, the addition of MA also manages to yield a gel in PLA blends leading to the establishment of distinct microstructures and end properties. The tailored microstructure depends largely on the present functional group, interfacial interactions,^[17] and processing parameters of the polymers.^{[18], [19]} Other studies have also shown that the molecular weight and gel content of thermoplastics,^{[13], [14]} TPEs,^[19] and rubber blends ^[20] expand in proportion to the amount of MA grafting. The crosslink polymer chains were connected by the MA ring opening linkage at the active site of the surrounding neighbourhood chain. This suggests that, beyond MA’s role as a compatibilizer or grafting agent, MA has the capability to function as a TPV agent for any polymers with unsaturated sites and hydroxyl functional groups without being properly acknowledged.

Here, we conduct an experimental setup to evaluate the crosslinking behaviour of MA in the PLA/ENR blend system. To carefully determine the functions of MA, either as a grafting or TPV agent, a framework with a set of formulations varying in ingredient processing sequence, polymer matrix blend ratio, and MA content was structured. The changes that have taken place in the blend's morphology, rheological behaviour, and solid-state properties are also monitored to correlate with its mechanical properties, viscoelasticity and phase behaviour. Since MA has been widely utilized in product manufacturing and additive manufacturing, the results from this work will markedly change the perspective on and the role of MA in rich functional group bio-based polymers. This work will benefit both researchers and industry professionals, as understanding the functions of MA will provide valuable guidance for tailoring and controlling the properties of the formulated blend or composite.

Materials

Low molecular weight (M_w) L-lactic acid type polylactide PLA (Native Pultrusion Sdn. Bhd) with a density of about 1.21 g/cm³, a melting point of about 150–160°C and a degradation temperature of 240–250°C was dried for 24 hours (40°C) in an oven to remove excessive moisture. The tensile strength of neat PLA is 41.2 MPa has decreased to 38.45 MPa after undergoing thermal processing history of heated two-roll mill and hotpress. Epoxidized natural rubber (ENR) with 25 mol% epoxidation was purchased from the Rubber Research Institute of Malaysia (RRIM). Maleic anhydride (MA) was obtained from Yongchang Chemical Company in Harbin, China. Both MA and ENR were used as received. Spectroscopic-grade chloroform and acetone, acquired from HmBG Chemicals in Hamburg, Germany, were employed for gel content testing and to remove the PLA phase from the blend sheet surface for SEM sample preparation.

Sample preparation

The ingredients were mixed using an open-chamber heated two-roll mill (Guangdong Xihua Machinery Co. Ltd - RM8175A) to produce the formulated blends listed in Table 1. Formulations 1 to 3 were prepared by varying the sequence of adding materials, with each ingredient added at 6 minutes intervals, and the total compounding time was set at 20 minutes. For formulations 4 to 12, the process began with masticating the ENR for 6 minutes before gradually adding the PLA to the rubbery elastomeric material for 6 minutes, followed by the addition of MA. All specified amounts of ENR, PLA, and MA were compounded at 180°C for total accumulative time of 20 minutes until a homogeneous compound based was achieved. The back roller of the heated two-roll mill was regulated at 25 rpm, and the front roller was set to a rolling speed of 20 rpm to enhance shearing for proper blend distribution and MA dispersion. Subsequently, the compounded blends were molded using a Gotech Hot-press (GT-014-H) machine at 185°C for five minutes. The samples preparation is illustrated in Fig. 1.

Table 1

Formulations for PLA/ENR blend and its compounding sequence
Samples	PLA (wt %)	ENR (wt %)	MA (phr)
ENR/MA/PLA	70	30	2.0
PLA/MA/ENR	70	30	2.0
PLA/ENR/MA (2.0 MA)	70	30	2.0
80PLA/20ENR	80	20	2.0
90PLA/10ENR	90	10	2.0
0 MA	70	30	0
0.5 MA	70	30	0.5
1.0 MA	70	30	1.0
1.5 MA	70	30	1.5
^a)phr is part perhundred resin
^b)MA in phr was added according to the amount, sequential and blend ratio of the design formulation

Testing and characterization.

The tensile strength was performed by ASTM D882 using an Instron Universal Testing Machine with a crosshead movement speed of 10 mm/min. The melt flow index (MFI - GT-7100-MI) test was executed at 185°C based on the modified ASTM D 1238-90b respectively. The die diameter and length were 2.095 and 8 mm, respectively. The preheat time was regulated for 5 minutes and the calculated flow rate is by gram per 10 minutes based on the cut extrudate interval. The rheological characteristics of the blends were measured by using the single-bore capillary rheometer with LCR7001 series from Dynisco, Malaysia. The power law model was utilized to align the relationship between apparent shear stress, apparent shear rate, and flow index n (non-Newtonian index) and it is expressed in Eq. 1. The apparent viscosity (𝜂) is obtained by dividing the apparent shear stress (𝜏) over the apparent shear rate (𝛾). K is the apparent viscosity index, while n is the non-Newtonian index, which refers to the fluidity of a polymer in a viscous flow state. The applied shear rate was fixed between 50 to 2000 s^− 1 at the temperature of 190°C. Capillary die with a diameter of 1.5 mm and L/D of 6, 12 and 20 was employed for the test and Bagley correction was implemented to the collected results to obtain the blend's true viscosity value.

$$\eta \left(\gamma \right)=K{\gamma }^{n-1}$$

The sample surface morphology was analyzed using a JEOL JSM-6460 LA scanning electron microscope (SEM) from JEOL (Malaysia) Sdn. Bhd, operated at an activation voltage of 15 kV. To observe the developed microstructure, the sample was immersed in acetone for 5 minutes to dissolve the PLA on its surface. The samples were immediately dried at room temperature (27°C). Later, the samples were coated with a thin layer of silver to enhance conductivity before conducting the SEM analysis. The Fourier transform infrared (FTIR) spectra were inspected by the Perkin Elmer 2000 FTIR spectrometer using the ATR mode of 650–3650 cm^− 1 in wavelength to identify the chemical changes in the blends. Next, the phase transition temperature of the selected blends was determined by differential scanning calorimetry (DSC), performed by TA Instruments DSC Q10 (New Castle, DE, USA). Scans were taken at a rate of 10°C/min. from 30 to 180°C. Next, a Perkin Elmer D8000 dynamic mechanical analysis (DMA) test was carried out to observe the viscoelasticity of the samples following the ASTM D4440 respectively. Storage modulus (G′), loss modulus (G′′) and damping factor (tan δ) were measured as a function of temperature Eq. 2. The test was conducted in the temperature range of 20 to 120°C at a heating rate of 5°C/min. on compression-moulded rectangular bars having dimensions of 3 cm × 2 cm × 0.1 cm in size.

Tan 𝛿 = 𝐺″/𝐺′ (2)

The thermal stabilities of PLA/ENR TPE and TPV blends were investigated using a thermogravimetric analysis (TGA) instrument (PL-150, Mettler Toledo, Switzerland) at a heating rate of 10°C/min. in the temperature range of 25 to 700°C under nitrogen atmosphere. The onset temperature of degradation and the degradation temperature were measured from the 5% weight loss and maximum weight loss rate temperature, respectively. Eventually, the crystallinity of the projected blend was analysed by using the Rigaku X-ray diffractometer (XRD), Model Smartlab 2018 (Tokyo, Japan). The measurements were recorded with Cu Kα radiation at a scan rate of 0.1 s per step in the range of 2θ = 2° to 90°. The crystallinity percentage (X_c) was calculated from the relation in Eq. 3 where A_a represents the area of the amorphous peak while A_c represents the area of the crystalline peak.

$${x}_{c }\left(\%\right)= \frac{{A}_{c}}{{A}_{c}+ {A}_{a}} \times 100\%$$

Meanwhile, the gel content percentage (%) was gathered by dissolving the samples (~ 10 g) in a chloroform at room temperature (27 ⁰C) under constant mechanical stirring (20 rpm) for 30 minutes. The remaining gel residuals were dried in an oven and it was calculated as in Eq. 4 where W_g and W_s were the weight of the dried gel and sample respectively.

$$Gel \left(\%\right)= \frac{{W}_{g}}{{W}_{s}} \times 100\%$$

Although most reports suggest that blend compatibility is achieved through successful interfacial interaction between PLA and ENR, deeper questions on the density of the grafting, specific location of the grafting and types of grafting or crosslinking that appear in the blend are still undetermined. To address these questions, a set of design formulations with 2.0 phr MA was added in the PLA/ENR blend at different compounding ingredient sequent and polymer ratios. It can be seen that significant differences in tensile properties were observed for each sample shown in Figs. 2a and Fig. 2b, with the values detailed in Table 2. The ENR/MA/PLA blend exhibited the lowest tensile strength at 2.5 MPa, followed by ENR/PLA/MA at 4.6 MPa, and PLA/MA/ENR at 9.7 MPa. Additionally, the gel content percentage shown in Fig. 2c indicated that the ENR/MA/PLA blend had the highest gel content with respect to the other formulations.

According to Zhang and colleagues, the addition of sebacic acid depending on the acid amount in 80PLA/20ENR reactive blend allows the crosslinking degree to be regulated on the mENR side. This degree of crosslinking controls the diameter, size, and distribution of rubber particles.^[19] In the case of the ENR/MA/PLA compound, the incorporation of MA into ENR during compounding, before adding PLA, caused the ENR to form crosslinks among its neighbouring chains through MA grafting linkages. This resulted in the formation of larger ENR rubber particles dispersed within the PLA phase, leading to the progressive development of more gels and a reduction in the blend's melt flow index (MFI) value, as shown in Fig. 2d. It was also noticed that the error bars for the ENR/MA/PLA MFI results are larger than the other compounds. These could be contributed by various factors such as imbalanced flow properties or known as the unlaminar flow, a wide range of polymer molecular weight distribution, foreign substances and also the agglomeration of fillers.^{[21], [22], [23], [24]} By observing the condition of ENR/MA/PLA blend, the eyewitness development of larger yellowish ENR dispersed phase is obviously display in Fig. 1. The apparent phase separation developed between the PLA and ENR created weak stress concentration point which resulted in an intense reduction in the blend tensile properties. The reason for the development of ENR dispersed phase in PLA will be further discussed in other section.

Table 2

Tensile properties of PLA/ENR blend at different ingredient compounding sequent and matrix blend ratios.
Samples	Tensile strength (MPa)	Tensile modulus (MPa)	Elongation at Break (%)
ENR/MA/PLA	2.5 ± 0.6	150.7 ± 10.2	7.2 ± 2.8
PLA/MA/ENR	9.7 ± 1.1	578.2 ± 23.9	6.4 ± 1.7
ENR/PLA/MA	4.6 ± 0.8	358.8 ± 11.3	14.5 ± 3.4
20ENR/80PLA	14.3 ± 2.5	547.7 ± 18.0	11.3 ± 1.4
10ENR/90PLA	25.5 ± 3.4	713.7 ± 32.5	8.5 ± 1.2

Although the PLA/MA/ENR blend exhibits better tensile strength at 9.7 MPa compared to other blend sequences, it reveal the lowest elongation at break (EB) value of 6.4%, indicating the brittleness of the sample. This brittleness of the sample is due to weak interfacial adhesion between PLA and ENR, as the MA initiates networking through the hydroxyl terminals in the PLA chains, leaving fewer hydroxyl terminals available to form proper grafting with ENR chains.^{[25], [26]} A gel content test was conducted to determine the type of grafting holding the PLA chains, and the results in Fig. 2c showed that the blend stipulate the lowest gel content value. This mean that the MA-PLA grafting linkage is primarily hold by non-chemical-force of intermolecular bonding. The enhancement of intermolecular bonding induced by MA among the PLA chains have significantly improves PLA's mechanical properties. This is supported by the MFI test result in Fig. 2d, where the melted blend exhibits a higher MFI value of 79.4 g/10min in correspond to other blend ingredient sequences. Meanwhile, in compounds with different PLA/ENR blend ratios, the tensile strength and modulus drop significantly when higher ENR weight percentages are deployed. This corresponds to the inherent low strength and modulus of ENR. ^[15]

Meanwhile, the tensile properties of a 70PLA/30ENR blend without and with MA addition ranging from 0 to 2.0 phr are shown in Fig. 3a. The results indicate that the tensile strength of the TPE blend is 10.4 MPa which is lower without the MA grafting effect. This is attributed to the generated sea-island structure as a result of weak interfacial adhesion and poor blend miscibility. ^{[10], [27], [28]} According to Bernardez's group, the compatibilization issues stem from the polarity difference between PLA and ENR. Weak interfacial interactions cause phase separation, which occasionally lowers the sample's tensile properties. This observation aligns with the findings for the TPE blend. Notably, adding MA mitigated this issue and improved the tensile strength and elastic modulus (Fig. 3b) of the blend, suggesting that MA effectively acts as a compatibilizer in the system. However, at 1.5 phr MA and above, the blend's tensile properties rapidly decrease due to the brittleness of the samples.

Table 3

The tensile properties value of 70PLA/30ENR blend with MA loading ranges from 0 to 2.0 phr.
Samples	Tensile strength (MPa)	Tensile modulus (MPa)	Elongation at Break (%)
0.0 MA	10.4 ± 1.7	361.7 ± 24.2	7.2 ± 2.8
0.5 MA	13.1 ± 1.0	421.2 ± 29.9	10.5 ± 2.7
1.0 MA	18.5 ± 0.8	478.8 ± 34.3	14.5 ± 3.4
1.5 MA	7.1 ± 0.6	407.7 ± 34.0	11.3 ± 1.4
2.0 MA	4.6 ± 0.8	351.8 ± 30.3	8.5 ± 3.4

To further understand the established microstructure, the MFI test was performed and the results are plotted in Fig. 3c. The blend with 1.0 phr MA showed the highest MFI value compared to other PLA/ENR blends. Interestingly, at MA contents above 1.0 phr, the MFI value starts to decrease. Since MFI provides an indirect measurement of molecular weight, these findings highlight the contradictory effects of MA on inducing chemical grafting to the PLA and ENR which will be discussed later. Further investigation of the MFI extrudates exhibited in Fig. 3d reveals that the TPE blend had the highest die swell reading of 96.7%. The value narrowed to 58.5% when 1.0 phr MA was added to the blend. At 1.5 and 2.0 phr MA, a peculiar surface texture with numerous small bulges appeared randomly in the extrudates. To address the effects of MA on the blend's microstructure and morphology, further investigations were carry out on the modified blend surface samples with acetone using the SEM.

Figure 4 illustrates the microstructure of PLA/ENR blends and the image of the TPE blend reveals a typical co-continuous and sea-island structure characteristic of immiscible PLA/ENR blends. After the addition of 0.5 phr MA loading, the size of the sea-island domain decreased and the contact area between PLA and ENR expanded. Previously, Esmizadeh et al. reported that the diameter of NR in a 50PLA/50NR blend with graphene nanosheets (GNS) became smaller after successful grafting and compatibilization with ENR50. This finding aligns with the present study, where tensile properties improved after the grafting effect by the 0.5 phr MA. At 1.0 phr MA, the formation of homogeneous crosslinking between the ENR was detected after the removal of PLA. The crosslinking structure is not visible to the naked eye, but the SEM images clearly reveal the features of the insoluble ENR crosslinking network, alongside with a micro-scale blank region presumably filled by the PLA matrix. At 0.5 phr MA, the formation of the ENR crosslinking build up is not detected due to the insufficient amount of MA to attract the hydroxyl groups in the dispersed ENR.^{[2], [29]} This indicates that MA acts only as a compatibilizer or grafting agent at this particular concentration. Conversely, at 1.0 phr MA, MA has acted as a TPV agent by predominantly targeting the ENR chains, leading to phase separation between PLA and ENR which results in isolated ENR crosslinking skeleton. The established crosslinking restricts the existing PLA and ENR chain mobility, thereby minimizing the extrudate die swell. ^{[27], [28], [30]}

By looking at the samples and the captured SEM images for 2.0 phr MA, the sample surface morphology shows signs of vast phase separation with clear visible spots filled by PLA. This suggests an increase in crosslinking density within the ENR as MA continues to accumulate the nearby ENR chains in the blend leading to the formation of larger ENR particles and clusters within the PLA domain. This observation is further supported by the appearance of small bulges on the 1.5 phr MA MFI extrudate surface and the phenomenon becomes more pronounced at 2.0 phr MA as mentioned earlier. Furthermore, it is also detected that the error bars for the MFI become more pronounced as the MA content exceeds 1.5 phr. This is attributed to the flow instabilities of the blended TPV melts. The actual size of the accumulated ENR gel particles, however, is challenging to determine using SEM or TEM due to the association of weak intermolecular forces presence in the co-continuous PLA phase.^{[26], [31]} During the hot press process, distinct voids can be clearly seen on the blend's sample surface. The voids were filled by the low-viscosity PLA melts, explaining the previously contradictory MFI results. This phenomenon is due to the pronounced phase separation between PLA and ENR, with the low-viscosity PLA melt dominating over the high-viscosity ENR. The formation of TPVs with excessive ENR crosslink particles and clusters causes them to behave like fillers in a composite system. To address the emerging TPV particles in the ENR/PLA blend comprehensively, a capillary rheometer test was conducted to observe the flow behavior of the formulated blend at different shear rates (γ) and MA content.

Interestingly, the rheological data gathered in Fig. 5 behaves differently at low and high γ. Although the results are synchronized with the MFI at low γ, the melt experiences a sudden increase in shear stress (𝜏) when high γ is applied. The melted TPE blend 𝜏 remains moderate at high γ with respect to the TPV blend. Without proper grafting or phase interactions, the immiscible blends easily slip against each other under shear forces and essentially the case is similar for other component PLA and ENR blend studies.^{[16], [32]} The addition of 0.5 phr MA has noteworthy induced proper grafting and compatibilization to the blend causing the sudden increase in 𝜏 as the γ escalates. Nonetheless, at 1.0 phr MA, the 𝜏 depreciates although the gel amount increases. The reversal results and outcomes are explained by the formation of ENR self-crosslinking sites which manifest themself in the form of small clusters or particles. These particles isolate themselves from the PLA matrix causing the low viscosity PLA melt to govern the system thereby lowering the overall blend viscosity. This explains the marked increase in MFI value from 7.28 g/10 min to 60.7 g/10 min.

Despite the evidence from the gel content and SEM images, another indication of particle emergence is demonstrated by the obvious dilatant flow pattern and shear thickening effect in the 1.0 phr MA TPV blend. Previously, Hsiao et al affirmed the effect of surface roughness and topography on the shear-thickening behaviour of PMMA colloidal particles. While. Hu and coworkers extended the investigation on the effect of crosslink density by changing the amount of cross-linker during synthesis.^{[25], [26]} The tailored colloidal particle showed that the crosslinker induces heterogeneity during oligomer precipitation, resulting in size-monodisperse rough particles that contribute to the dilatancy and strong thickening when subjected to tangential force. This severe thickening effect is described as discontinuous shear thickening which suggests that the enhancement in surface asperities moves the onset of both shear thickening and dilatancy to the critical limit. These types of compounds have the ability to solidify during flow or in other words termed as shear jamming which can cause machine breakdown during processing. Therefore, it should be aware that the addition of MA can possibly lead to shear jamming as a result of the TPV melt’s shear thickening effects.

In the PLA and ENR blend, a similar effect of particle development was observed with an increase in ENR crosslinking. To confirm the presence of chemical crosslinking, a gel content test was conducted. Figure 6a displays that crosslinking is evident in all blends containing MA, with the amount of gel increasing as more MA is added. To support the claim, the TGA results in Fig. 6b confirm an excess residual of 6.9% after the burning test. Since the crosslink particles consist of both covalent bonds and intermolecular bonding interactions, the final residual shows less than the actual collected gel, which is 19.7%. Additionally, the thermal stability of the TPV with crosslinking outcomes shifts to a higher temperature in comparison to the TPE blend. This significant temperature shift is due to the higher energy required to break the large 3D crosslinking network of covalent bonds.^{[16], [33]}

Figure 7a depicts the DSC thermograms of PLA/ENR blends with MA content ranges from 0 to 2.0 phr. The thermal properties results predominantly originate from the thermal transition curves of PLA, with the melting temperature (T_m) of the TPE and TPV remaining consistent with the PLA setting, around 157°C. However, the glass transition temperature (T_g) of the PLA component shifts to a higher temperature as the gel content increases. This shift can be attributed to two possible reasons. First, the increase in chemical crosslinking in ENR restricts the PLA chain mobility, thus enhancing the T_g of the PLA in the blend [22]. Second, the improvement in miscibility between PLA and ENR due to the growing presence of weak forces such as hydrogen bonding, Van der Waals forces, and other interactions within the system.^[13] These intermolecular forces contribute to the stiffness of the PLA molecular chains, stabilizing the PLA T_g as more MA is added to the PLA/ENR blend. Conversely, the cold crystallization temperature (T_c) disclosed a contradictory pattern to the T_g value where it decreases as more MA is added. The influence of ENR viscosity significantly alters the blend T_c in which the value drops from 113.9°C to 101.7°C for 2.0 phr MA.

The addition of more MA in the blend has triggered higher ENR crosslinking causing the viscosity of the ENR phase to increase. The increase in ENR viscosity prevented PLA crystallite growth by hindering chain folding, which induced a lower T_c value. Besides, it is also noticed in Fig. 7b that the XRD diffraction peak intensity positions at 29.8 ⁰ and 33.7 ⁰ for 1.0 phr MA became slightly visible with respect to the TPE blend. The sharpness of the 1.0 phr MA blend intensity could be contributed by the developed “grafted” or “crosslink junction”. However, the crystallization percentage (X_c) calculated from the XRD shows not much difference except for 2.0 phr MA which suggests that the addition of a more amorphous ENR phase to PLA has resulted in chain mobility restriction to the PLA.^[33] On top of that, as the amount of crosslinking increases, the effect of chain restriction becomes more profound, and for that reason, the X_c value reduces.

Table 4

DSC and XRD data for 70PLA/30ENR blend with MA loading ranges from 0 to 2.0 phr.
Samples	T_g (°C)	T_c (°C)	T_m (°C)	X_c (%)
0.0 MA	59.8	113.9	157.8	57.4
0.5 MA	63.5	113.2	157.6	57.6
1.0 MA	63.8	104.0	157.2	57.2
2.0 MA	65.1	101.7	157.5	55.4

To further correlate the effect of grafting and the evolution of TPV with the viscoelastic properties, the DMA test was put up for the compounded blend. Figure 8a represents the storage modulus and Fig. 8b displays the tan δ versus temperature for the PLA/ENR blend with 0 to 1.0 phr MA. The TPE blend obtained the highest storage modulus and tan δ value with respect to other MA filled blend. A sharp drop in 0.5 phr MA blend storage modulus and tan δ was recorded in comparison to the TPE blend and the value tremendously shows a decreasing trend with the increase in 1.0 phr MA content. The DMA results for 1.5 and 2.0 phr MA are not obtainable due to the brittleness, stiffness and failure of the sample during testing. Another reason is the dominance of the PLA domain as a result of phase separation after the formation of the ENR crosslink particle. On the other hand, the TPVs blend tan δ peak at 74.3 ⁰ for PLA and 103.5 ⁰ for ENR is noticeably shifted towards a higher temperature corresponding to 0.5 phr MA with 77.1 ⁰ for PLA and 109.8 ⁰ for ENR. As for the 1.0 phr MA content, the tan δ peak slightly shifts backward to 75.3 ⁰ for PLA and 104.9 ⁰ for ENR in comparison to the 0.5 phr MA blend. This happened when the developed ENR particles started to appear and showcased signs of phase separation between the PLA and ENR depicted in previous SEM morphology. For that reason, the peak relocates to a lower temperature region.^{[9], [34]}

By observing the FTIR results for the TPE and TPV blends in Fig. 9, it can be seen that the absorption peak at 1750 cm⁻¹ becomes more intense at 1.0 phr MA. The increase in intensity indicates an enhancement of the ester group and a reduction of the epoxy group, which are responsible for the chemical crosslinking in the TPV blend. The epoxide groups attached to the ENR chains were bridged through the MA ring-opening linkage, forming a larger ENR network. This network led to the evolution of the blend microstructures observed in the SEM images.^{[35], [36]} Additionally, it was also noticed that the absorption peak at 1133 cm⁻¹, corresponding to the C-O-C polysaccharide of PLA, was also increased. This suggests that the addition of more MA elevates the physical crosslinking within the PLA chains. To properly describe the relationship, Scheme 1 graphically illustrates the suggest structures for TPE and TPV. Without MA, the TPE blend with PLA and ENR chains are held together by weak intermolecular forces such as Van der Waals interactions, hydrogen bonding, CH-CH interactions, H-H bonds, etc. These forces, which highly depend on the distance between polymer chains, result in the development of the blend's inferior properties. Conversely, the addition of MA linkage sites in the ENR chains has resulted in the expansion of the blend's crosslinking. Depending on the tailored amount of crosslinking, this may allow MA to function either as a coupling agent or TPV agent.

Based on the findings and results, it can be concluded that the incorporation of MA into the PLA/ENR blend, across various sequential stages and blending ratios, profoundly impacts the blend's homogeneity, melt flow behaviour and solid state properties. In the absence of MA, the blend exhibits typical TPE behaviour with surface interaction issues stemming from the immiscibility of ENR and PLA. The addition of 0.5 phr MA effectively induces a grafting effect within the PLA/ENR blend, resulting in the reduction of sea-island domain sizes and the expansion of the blend contact area. Conversely, at 1.0 phr MA, a notable transition to a TPV blend was observed as a more pronounced crosslinking effect is developed in the system. The added MA linkages are more likely to target the ENR chains, causing the ENR chains to be attracted away from the PLA sites, which is visibly observed through SEM images. With 2.0 phr MA, the blend crosslinking intensifies further, exacerbating phase segregation issues of macrosize voids on the blend surface. In short, the amount of MA chain linkage determines its role as either a grafting or TPV agent in which significantly influencing the overall properties of the blend.

The authors declare no conflict of interest.

Funding

The authors would like to express their appreciation for the funding provided by the Ministry of Higher Education (MOHE) Malaysia’s Fundamental Research Grant Scheme (FRGS) under grant number FRGS/1/2023/TK05/UNIMAP/02/17.

Author Contributions:

Mr. Muhamad Al-Haqqem Abdul Hadi, a PhD student, conducted the experimental work and drafted the main manuscript under the supervision of Dr. Nor Azura Abdul Rahim. Assoc. Prof. Dr. Yeoh Cheow Keat served as co-supervisor, providing valuable advice and suggestions for the research. Dr. Roshasnorlyza Hazan assisted with the DMA data and its analysis. All authors contributed to editing the manuscript to minimize grammatical and typographical errors.

Acknowledgments

We would like to thank Mr. Nasir Ibrahim, Mr. Norzaidi Zainol and Mr Mohd Faisal Rusli for their technical assistance while conducting the research work.

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Scheme 1 is available in the Supplementary Files section.

Supportinginformation.docx
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Scheme 1. PLA/ENR blend with TPE physical crosslinking and TPVs mechanism with combined intermolecular forces and chemical crosslinking link by MA ring opening linkages

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Relationships between Crosslinking Behaviour and Morphological Development in PLA/ENR Blends Using Maleic Anhydride as a Coupling and TPV Agent

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