Thermal Oxidative Aging Behaviors and Degradation Kinetics of Biobased Poly(dibutyl itaconate-co-butadiene) Elastomer

Abstract

This study investigates the aging behavior of poly(dibutyl itaconate-co-butadiene) (PDBIB), a sustainable elastomer, under thermo-oxidative, thermo-shearing, and combined conditions. The well-established commercial rubbers of styrene–butadiene rubber (SBR) and cis-1,4-polybutadiene (BR) were compared. Results indicate that the sensitivity of PDBIB to thermo-oxidative aging and thermo-shearing aging is intermediate between that of SBR and BR. Both chain degradation and cross-linking were observed during aging, with chain degradation being predominant. TGA/FTIR analysis revealed initial ester bond cleavage followed by butadiene segment scission. Thermal degradation kinetics, analyzed using the Kissinger and Flynn–Wall–Ozawa methods, showed activation energies of 196.4 and 225.7 kJ/mol for itaconate and butadiene segments, respectively. The Flynn–Wall–Ozawa method indicated a multi-step degradation process, with the activation energy ranging from 160.9 to 289.9 kJ/mol. This research provides insights into PDBIB aging and performance, aiding service life prediction and supporting its commercial potential for industrial use.

1. Introduction

With increasing environmental concerns and the depletion of petrochemical resources, the urgency for sustainable development in the rubber industry has intensified. The advancement of biobased chemicals and the use of renewable energy have garnered widespread attention and consensus.¹⁻⁴ Among biobased chemicals, itaconic acid stands out as a high-value, biomass-derived building block. Its high production yield and low cost make it ideal for synthesizing various polymers.⁵⁻⁷ Itaconic acid, with two carboxylic groups, reacts with biobased alcohols to form itaconate monomers, which are subsequently used to produce biobased elastomers.⁸⁻¹²

Wang et al.¹³ pioneered the synthesis of biobased itaconate elastomers through free-radical emulsion polymerization of biobased itaconate and diene. By varying the side chain length of itaconate and adjusting the copolymer structure, these elastomers were tailored for applications such as tire production.^14,15 Our earlier studies reported the synthesis of biobased poly(dibutyl itaconate-co-butadiene) (PDBIB) elastomers via redox-initiated emulsion copolymerization of dibutyl itaconate and butadiene. The resulting silica-reinforced nanocomposites demonstrated excellent static and dynamic mechanical properties.^16,17 However, PDBIB elastomers are susceptible to aging due to environmental factors such as oxygen, ozone, heat, light, and mechanical stress. Aging manifests as hardening, cracking, reduced elasticity, or brittleness, ultimately impairing performance and durability.^18,19

Thermal and oxidative conditions are primary contributors to rubber aging. Thermo-oxidative degradation often starts with hydrogen peroxide formation during the induction phase, initiating free-radical chain reactions with oxygen. These reactions result in macromolecular cross-linking or chain scission, accelerated by autocatalytic processes.^20,21 Mechanical shearing during processing can also induce chain scission, reducing molecular weight and altering its distribution.²² Thermo-oxidative aging mechanisms have been well-studied for many conventional rubbers. For instance, natural rubber (NR) undergoes chain breakage and molecular weight reduction when exposed to heat, oxygen, and shear forces.²³⁻²⁵ Polybutadiene rubber (BR) shows early cross-linking, while polyisoprene exhibits a delayed induction period of 10–20 h before cross-linking, despite carbonyl group formation.²⁶Trans-1,4-Poly(butadiene-co-isoprene) undergoes chain scission with carbonyl formation, reducing its elastic modulus and viscosity under thermo-shearing conditions.²⁷ Synthetic rubbers, such as 1,4- and 1,2-polybutadienes (PB), oxidize at 100 °C in air, forming polycyclohexane-like structures due to the oxidation of 1,2-PB.²⁸

The modified standard linear solid model has been used to describe the compressive stress relaxation behavior of BR, with cross-linking and oxidation being the dominant drivers of thermo-oxidative aging.²⁹ Styrene–butadiene rubber (SBR) undergoes an autocatalytic oxidative degradation process, characterized by alkyl radical formation, oxidation stages, and cross-linking.^30,31 Similarly, styrene-isoprene-butadiene rubber (SIBR) shows conjugated carbonyl formation linked to cis-1,4 and 3,4-polyisoprene units, with cross-linking as the primary mechanism.³² Nitrile butadiene rubber exhibits additive depletion, carbonyl formation, and competing cross-linking and chain scission during accelerated aging, with cross-linking dominating initially but chain scission becoming more prominent over time.³³

Despite growing interest in biobased itaconate elastomers for their favorable properties, limited research exists on their resistance to thermo-oxidative aging. This knowledge gap hampers a comprehensive evaluation of their long-term performance under high-temperature and oxidative conditions—critical for demanding applications.

This study investigates the effects of thermo-oxidative and thermo-shearing aging on PDBIB, along with its thermal degradation kinetics. Rheological properties under various aging conditions were analyzed by using a rubber process analyzer (RPA), with results correlated to molecular weight changes through gel permeation chromatography (GPC) and gel content analysis. Thermogravimetric analysis coupled with infrared spectroscopy (TGA/FTIR) was employed to probe microstructural changes during thermal degradation and to elucidate degradation mechanisms. Activation energy variations during chain segment degradation were determined by using Kissinger and Flynn–Wall–Ozawa (FWO) methods. This research provides valuable insights into PDBIB aging, degradation, and performance, supporting predictions of its service life and properties. These findings enhance the commercial viability of PDBIB elastomers, ensuring that they meet industry standards for sustainable applications.

2. Experimental Section

2.1. Materials

Poly(dibutyl itaconate-co-butadiene) (PDBIB) with a Mooney viscosity (ML1 + 4 at 100 °C) of 69 and a density of 0.979 g/cm³ was an industrial-scale product provided by our research partner, Shandong Chambroad Sinopoly New Material Co., Ltd., China. Its molecular weights and compositions are summarized in Table 1. Styrene–butadiene rubber (SBR1502, ML1 + 4 at 100 °C = 52) and butadiene rubber (BR9000, ML1 + 4 at 100 °C = 48) were supplied by Qilu Petrochemical Co., Shandong, China.

Table 1. Molecular Weight and Composition of PDBIB Gum.

sample	Mn × 104 (g/mol)a	Mw × 104 (g/mol)a	Mw/Mn	1,2-Bd (mol %)b	trans-1,4-Bd (mol %)b	cis-1,4-Bd (mol %)b	DBI (mol %)b
PDBIB	21.2	81.5	3.85	8.34	46.54	18.51	26.61
SBR	14.2	45.6	3.21	/	/	/	/
BR	12.8	51.9	4.04	/	/	/	/

^aFrom GPC.

^bFrom ¹H NMR, Bd, and DBI represent the molar percentage of the butadiene and dibutyl itaconate units in the PDBIB copolymer, respectively. / means no data.

2.2. Thermo-oxidative, Thermo-shearing, and Thermo-oxidative-shearing Aging Processes for PDBIB Gum

Figure 1 illustrates the thermo-oxidative, thermo-shearing, and thermo-oxidative-shearing aging processes applied to various rubber gums (PDBIB, BR, and SBR). Thermo-oxidative aging was conducted in a hot-air oven (Gotech Testing Machines, Dongguan, China; air velocity: 1.5 m/s). The samples were aged at 100 °C for a duration of 0, 1, 3, 5, or 7 days. Thermo-shearing aging was performed using an RPA 2000 (Alpha Technologies Co., USA) under conditions of 10% strain, a frequency range of 0.02 to 15 Hz, and a temperature of 80, 100, 120, 140, 160, or 180 °C. Thermo-oxidative-shearing aging involved mastication of the PDBIB gums in an RM-200C HAPRO torque rheometer with an 85% fill factor, a rotation speed of 70 rpm, and a duration of 8 min at a temperature of 60, 80, 100, 120, 130, or 140 °C.

Figure 1.

Thermo-oxidative, thermo-shearing, and thermo-oxidative-shearing aging processes for PDBIB, BR, and SBR gums.

2.3. Preparation of the PDBIB Gum for Thermal Degradation Kinetics and Its Process

The PDBIB gum used for studying thermal degradation kinetics differed from that used in the aging study described above. This sample did not contain any antioxidants and was prepared as follows: (1) Coagulation: the PDBIB latex was added dropwise to ethanol at a volume ratio of 1:2 (latex to ethanol) with stirring at 300 rpm. Stirring continued for 10 min after the addition was complete. (2) Washing and drying: the flocculated rubber was cut into pieces and washed with ethanol at a volume five times that of the rubber for 5 min. The rubber pieces were then dried in a vacuum oven at 60 °C until a constant weight was achieved. The thermal degradation kinetics of PDBIB were studied by thermogravimetric analyzer (TGA) of type TGA2, METTLER Co., Switzerland, under nitrogen with a flow rate of 40 mL/min, a heating rate of 5, 10, 15, 20, or 30 °C/min, and a temperature range from room temperature to 900 °C.

2.4. Characterization

The changes in storage and loss modulus of the PDBIB before and after aging were measured using an RPA 2000 (Alpha Technologies, USA) at 100 °C with a 10% strain and a frequency range of 0.02 to 15 Hz. The Mooney viscosity (ML₁₊₄^100°C) was measured using an MV Premier Mooney viscometer (Alpha Technologies, USA) at 100 °C, following ASTM D 1646-2015. The molecular weight distribution was determined by GPC (1260 Infinity II, Agilent Technology, China) with tetrahydrofuran (THF) as the solvent at 40 °C. The gel content of the PDBIB gum was determined according to ASTM D 3616-95 and calculated using the following equation

where m₀ represents the weight of the stainless-steel wire mesh, m₁ is the weight of the PDBIB gum, and m₂ is the weight of the dried PDBIB gum after dissolution in xylene at 23 ± 5 °C for 24 h.

The volatile substances produced during the thermal degradation were analyzed using a thermogravimetric analyzer-infrared spectroscope TGA/FTIR (TG209F1, Netzsch Co., Germany; Tensor27, Bruker Co., Germany). A Teflon was used to connect the TGA outlet to the FTIR gas chamber. Prior to the thermal degradation, both the Teflon tube and the FTIR gas chamber were preheated to 200 °C. Thermogravimetric conditions were as follows: nitrogen atmosphere with a flow rate of 40 mL/min and a temperature ramp of 10 °C/min from room temperature to 600 °C. The FTIR detector had a scanning range of 4000–400 cm^–1, with a resolution of 4 cm^–1 and 32 scans.

3. Results and Discussion

3.1. Thermo-oxidative Aging

Figure 2 compares the color changes of PDBIB, BR, and SBR gums during thermal oxidative aging in a hot-air oven. After aging at 100 °C for 7 days, all three gums became softer, stickier, and developed a yellowish hue. These effects became increasingly pronounced with extended aging periods. Among the three gums, SBR exhibited the most significant color change, followed by PDBIB, and then BR. This observation suggests that, based on visual appearance, the sensitivity to oxidative degradation follows the order: BR < PDBIB < SBR.

Figure 2.

Appearance changes of PDBIB, BR, and SBR gums at 100 °C under various aging times.

The aging of rubber can result in chain scission, branching, and/or cross-linking, which may occur either simultaneously or sequentially. These processes can potentially be assessed through changes in molecular weight distribution, gel content, and rheological properties.³⁴⁻³⁶ To quantify the effects of aging, we define the relative storage modulus RG′ as the ratio of the storage modulus after aging (G^′_aged) to its initial value (G^′_initial): RG′ = G^′_aged/G^′_initial. Similarly, the relative loss modulus RG″ is defined. Figure 3(a1–a3) illustrates the evolution of RG′ as a function of frequency for PDBIB, BR, and SBR gums subjected to various thermo-oxidative aging durations at 100 °C. In the frequency range of 0.02 to 15 Hz, the values of RG′ of PDBIB are always less than 1 and decrease significantly with aging time, indicating that chain scission predominates over chain branching and/or cross-linking, if the latter occurs at all. This effect becomes more pronounced with extended thermo-oxidative aging. For BR, the values of RG′ are also always less than 1. Moreover, they initially decrease with aging time up to 3 days but start to increase thereafter, particularly in the low-frequency range. This suggests that chain scission is dominant during the early stages of aging, while branching and/or cross-linking become the prevailing mechanisms after prolonged aging. In the case of SBR, the values of RG′ remain close to 1 during the first 3 days of aging but begin to increase afterward, eventually exceeding 1, especially at low frequencies. This indicates that SBR chains are not prone to thermo-oxidative aging in the first 3 days and undergo chain branching and/or cross-linking thereafter. Figure 3(b1–b3) illustrates that the relative loss modulus (RG″) of PDBIB, BR, and SBR exhibits a trend similar to that of the relative storage modulus (RG′). Thus, it can be concluded that overall, the sensitivity to thermo-oxidative aging, as reflected by changes in RG′ and RG″, follows the order: SBR < PDBIB < BR.

Figure 3.

Evolution of the relative storage modulus (RG′) and that of the relative loss modulus (RG″) as a function of frequency for PDBIB (a1,b1), BR (a2,b2), and SBR (a3,b3) subjected to various aging times at temperature 100 °C.

3.2. Oxygen Free Thermo-shearing Aging

The oxygen-free thermo-shearing aging of PDBIB, BR, and SBR was conducted in an RPA of type RPA 2000 at a temperature range of 80 to 180 °C with a strain of 10%, and the frequency range was from 0.02 to 15 Hz. Figure 4 shows RG′ and RG″ as a function of frequency for various temperatures. The values of RG′ are always smaller than 1 for all three rubber gums, suggesting that under the specified aging conditions, chain scission dominates over chain branching and/or cross-linking if the latter occurs at all. However, while they decrease significantly with increasing aging temperature from 80 to 180 °C for PDBIB and BR, indicating that chain scission becomes more serious when they are subjected to higher aging temperature under the frequency of 0.02 to 15 Hz, for SBR in the low frequency range (<1 Hz), they decrease with increasing aging temperature from 80 to 140 °C and then start to increase with a further increase in aging temperature from 160 to 180 °C, suggesting that chain branching/cross-linking occurs and tends to partly compensate for chain scission. The values of RG″ for PDBIB, BR, and SBR follow almost the same trends as those of RG′. Overall, the sensitivity to thermo-shearing aging, as reflected by changes in RG′ and RG″, follows the order: SBR < PDBIB < BR.

Figure 4.

Evolution of the RG′ and RG″ as a function of frequency for PDBIB (a1,b1), BR (a2,b2), and SBR (a3,b3) subjected to various shearing temperatures.

3.3. Thermo-oxidative-shearing Aging

Thermo-oxidative-shearing aging involved mastication of the PDBIB gum in a RM-200C HAPRO torque rheometer with an 85% fill factor, a rotation speed of 70 rpm, and a duration of 8 min at a temperature of 60, 80, 100, 120, 130, or 140 °C. The processed samples were dissolved in THF solvent, and the fully dissolved samples, which have been filtered, some gels or macromolecules, were then subjected to GPC analysis. Table 2 presents the molecular weights, molecular weight distributions (M_w/M_n), gel contents, and Mooney viscosities of PDBIB gums subjected to the aforementioned thermo-oxidative-shearing aging conditions. Overall, as the temperature increases from 60 to 140 °C, the molecular weights of the soluble fraction of the PDBIB gum decrease, while there was no regular variation of M_w/M_n due to complicated aging behaviors, the gel content increases, and the Mooney viscosity decreases. These findings clearly indicate that under the specified thermo-oxidative-shearing conditions, both chain scission and cross-linking occur simultaneously, with both processes becoming more prominent at higher temperatures. The observed decrease in Mooney viscosity with increasing temperature suggests that the increase in gel content cannot offset the effects of chain scission on Mooney viscosity.

Table 2. Molecular Weight and Gel Content of PDBIB Gums after Shearing with Different Temperatures^a.

sample	initial	60 °C	80 °C	100 °C	120 °C	130 °C	140 °C
Mw (×104 g/mol)	81.5	93.7	90.8	84.2	73.9	/	/
Mn (×104 g/mol)	21.2	25.8	18.6	22.8	15.3	/	/
Mw/Mn	3.85	3.63	4.88	3.69	4.83	/	/
gel content (%)	3.5 ± 0.3	5.6 ± 0.3	6.9 ± 0.4	11.2 ± 0.6	14.7 ± 0.8	20.9 ± 1.3	18.3 ± 1.2
ML1 + 4 at 100 °C	69	65	64	60	54	51	49

^a/ means there is no data from GPC because of too much gel generate.

Figure 5 shows the G′ and G″ of the PDBIB at 100 °C after thermo-oxidative shearing aging at various temperatures as a function of frequency. When the aging temperature is 80 °C or below, the values of G′ and G″ are almost the same as those of the initial PDBIB. They become significantly smaller than those of the initial PDBIB when the aging temperature is 100, 120, or 140 °C, indicating that the increase in gel content cannot offset the effects of chain scission on G′ and G″. These results align with those of Mooney viscosity.

Figure 5.

Shear modulus G′ (a) and loss modulus G″ (b) of PDBIB after thermo-oxidative-shearing aging at various temperatures.

3.4. Thermal Oxidation Mechanism

PDBIB contains both double bonds and ester moieties. The double bonds are particularly vulnerable to thermo-oxidative degradation through free radical reactions, which lead to simultaneous chain scission and cross-linking, as illustrated in Figure 6.^30,37 The aging behavior of PDBIB indicates that the temperature plays a pivotal role in controlling its degradation. Higher aging temperatures result in more pronounced chain scission and increased cross-linking. Consequently, the molecular weights of the soluble fraction decrease, while the gel content (representing the percentage of the insoluble fraction) increases significantly.

Figure 6.

Schematic diagram of the probable thermal aging mechanism for PDBIB.

The probable thermal aging mechanism of PDBIB can be described as follows: first, there is a radical generation and initial degradation stage. In the initial stage of thermo-oxidation, the high temperature leads to the high reactivity of α-hydrogen in itaconate rubber, making it prone to forming allylic radicals and other alkyl radicals. The double bonds, being highly reactive, are easily attacked, weakening the rubber structure. This stage is dominated by degradation, causing chain scission and the formation of small molecular fragments and radicals, which serve as active centers for subsequent reactions. Then, as the generation of radicals increases, some radicals begin to undergo coupling reactions, forming cross-linked structures that enhance structural stability. Many allylic radicals and double bonds participate in cross-linking, slowing further degradation. However, due to continuous oxygen exposure, new radicals keep forming, balancing cross-linking and degradation. As aging progresses, higher temperatures promote hydrogen abstraction, generating alkyl- and hydroperoxide radicals. This leads to the breakdown of the cross-linked network. While some cross-linking reactions still occur, degradation becomes the dominant process, resulting in a significant decline in the overall elasticity modulus of the rubber.

3.5. Thermal Degradation Behaviors

There has been limited research on the degradation behaviors of PDBIB. To better understand this process, thermogravimetric analysis combined with infrared spectroscopy has been employed. The TGA/FTIR technique enables the study of the thermal decomposition and gas-phase products generated during degradation, offering a more comprehensive analysis of the degradation mechanisms.^38,39

The thermal decomposition process of PDBIB and its degradation gas information during the whole process are shown in Figure 7. Figure 7a illustrates that the thermal decomposition of PDBIB occurs in two distinct stages: the first stage results in a weight loss of approximately 57.49%, followed by a second stage with a weight loss of 39.61%. Figure 7b shows the degradation gas information during the thermal decomposition process with time or temperature. There are clear peaks in the absorbance at certain wavenumbers, indicating the formation of specific gaseous products as the polymer undergoes thermal degradation. As time progresses, some peaks grow in intensity, suggesting that these decomposition products become more abundant. This 3D-FTIR plot allows for tracking the generation and transformation of chemical species in real time during thermal degradation of PDBIB.

Figure 7.

(a) TGA and derivative thermogravimetry (DTG) curves of PDBIB, (b) 3D-FTIR stereogram of PDBIB thermal degradation gas, (c) 3D-FTIR curves of the PDBIB thermal degradation gas under different typical temperatures, and (d) detailed 2D-FTIR curves of PDBIB thermal degradation gas under different typical temperatures.

By analyzing the specific wavenumber positions and the timing of the peaks, the sequence of bond breakages and chemical changes occurring in PDBIB could be determined as it degrades under heat. Therefore, the infrared spectra at typical temperatures in Figure 7b were refined to produce Figure 7c,d, with the aim of further pinpointing key information. As can be seen from the initial sample infrared spectrum, the characteristic peak at 2964 cm^–1 was typically associated with C–H stretching vibrations, possibly originating from the C–H bonds in the alkyl side chains (−CH₃ or –CH₂−) of the itaconate rubber molecules, the characteristic peak at 1727 cm^–1 was C=O stretching vibration peak, the characteristic peak at 1170 cm^–1 was C–O–C stretching vibration peak in di-n-butyl itaconate, and the characteristic peak at 968 cm^–1 was the deformation vibration of the C–H structure of trans-1,4 butadiene chain segment. The peak position at 1727 cm^–1 of the C=O group moved to the lower wavenumber, which compared with the general C=O peak position. This was because the side ester group of PDBIB related to the main chain methylene group, which was the electron-pushing group leading the peak position of the carbonyl group, moved to the lower wavenumber. From the infrared spectra of decomposed gas, the characteristic peak at 2964 cm^–1 has always existed in the decomposition gas, the characteristic peak at 2360 cm^–1 appeared from 308 °C of decomposed gas, and the intensities of these two peaks reach their maximum at 386 and 397 °C decomposed gas. The characteristic peak at 2360 cm^–1 was the absorption of CO₂, which may be generated through decarboxylation reactions triggered by the cleavage of C=O and C–O–C bonds in the ester groups. The variation in the characteristic peak at 2964 cm^–1 reflects the degradation or rearrangement of the C–H bonds. From the infrared spectra at 349 °C of decomposed gas, and the characteristic absorption peaks of C–O stretching vibration and the ester carbonyl C=O at 1043 and 1741 cm^–1 appeared, respectively, and the characteristic peaks at 1043 cm^–1 no longer appeared after 397 °C. While the characteristic peak of carbonyl C=O at 1741 cm^–1 has always existed, reflecting the relative stability of the carbonyl groups, and the stretching vibration peak of C–O–C in di-n-butyl itaconate at 1170 cm^–1 has been significantly weakened compared with the initial sample, but has always existed in the decomposition gas, indicating the stability of oxygen bridge structures at high temperatures. It was presumed that PDBIB undergoes a two-step thermal degradation; the first stage involves the decomposition of side chains of the itaconate segment, particularly the C–O bond in ester groups, and part of the C–O–C bonds preferentially breaks, generating small gaseous molecular products like CO and CO₂. The second stage focuses on the main chain breakdown, and the 968 cm^–1 peak disappears entirely, confirming the decomposition of the butadiene segments. Therefore, the weight loss peak at 383.5 and 447.1 °C of DTG was attributed to the weight loss peak of the itaconate segment and the butadiene chain segment, respectively.

The objective of this work is to explore the mechanisms of thermal degradation and carbonization in PDBIB, with the purpose of understanding how it behaves under high-temperature conditions. The thermal decomposition behavior of PDBIB was systematically investigated to understand its thermal stability. Figure 8 showed the thermal decomposition process of PDBIB from room temperature to 600 °C with nitrogen atmosphere and then changed to an oxygen atmosphere from 600 to 900 °C. There was a weight loss peak occurring after changing to an oxygen atmosphere. This weight loss was 3.14%, which proved the existence of a residual carbon oxidation process. This observation suggested that during thermal decomposition, PDBIB underwent molecular chain carbonization due to cross-linking.

Figure 8.

TGA and DTG curves of PDBIB under nitrogen and oxygen atmosphere.

3.6. Thermal Degradation Kinetics

In thermal analysis, particularly when using TGA under nonisothermal conditions, the mass change of a sample at various heating rates can be recorded to analyze the kinetics of thermal decomposition. This approach aims to determine the activation energy (E_a) of the material, providing insights into its decomposition behavior, and helps to assess the stability and breakdown mechanisms of the polymer by quantifying the energy. The Kissinger^40,41 and Flynn–Wall–Ozawa (FWO)^42,43 methods are two commonly used models for this purpose. The TGA and DTG curves of PDBIB with different heating rates under a nitrogen atmosphere are shown in Figure 9, and the relevant data are listed in Table 3.

Figure 9.

TGA and DTG curves of PDBIB with different heating rates under nitrogen atmosphere.

Table 3. Data of TG and DTG of PDBIB in Nitrogen Atmosphere^a.

heating rate (°C/min)	T5% (°C)	TPK1 (°C)	TPK2 (°C)	W1 (%)	W2 (%)
5	329.3	372.7	431.3	46.37	48.84
10	332.0	383.2	445.2	55.10	39.33
15	340.8	392.0	450.8	57.34	37.62
20	347.0	398.7	456.3	58.23	37.08
30	367.5	408.5	458.5	60.53	35.38

^aT_5%: the onset temperature for 5% weight loss; T_PK1 and T_PK2 correspond to the peak temperatures of the first and second weight loss range, respectively; W₁ and W₂ correspond to the weight loss values of the first and second ranges, respectively.

Based on the data in Table 3, as the heating rate increases from 5 to 30 °C/min, T_5% rises from 329.3 to 367.5 °C, with a temperature difference of 38.2 °C, indicating that the temperature at which the sample begins to degrade increases with the heating rate. Similarly, T_PK1 and T_PK2 show a similar trend, increasing by 35.8 and 27.2 °C, respectively. These results indicate that the higher the heating rate, the higher the temperature corresponding to the maximum degradation rate, which is characteristic of the thermal lag effect. As the heating rate increases, the weight loss W₁ slightly increases from 46.37% to 60.53%, and W₂ as the percentage of weight loss in the second stage decreases, dropping from 48.84% to 35.38%. This indicates that the heating rate has different effects on degradation behavior in different stages.

To investigate the thermal decomposition characteristics of PDBIB, the Kissinger method provides a useful approach to determine the activation energy associated with its degradation process, which analyzes the peak temperatures of decomposition at different heating rates, offering a direct way to calculate the activation energy. This method is widely used for polymers and other materials to evaluate thermal stability by observing how the decomposition temperature shifts with heating rate. By applying the Kissinger method, we aim to obtain precise kinetic parameters that will help us understand the stability and thermal behavior of PDBIB under varying conditions. The Kissinger model is expressed as follows

where T_max is absolute temperature, K; β is the heating rate (K/min); A is a pre-exponential factor; ΔE_a is activation energy, kJ/mol; and R is the gas constant, which is 8.314 J/(K·mol). The Kissinger method was according to the different heating rate β and corresponding peak temperature T_max on the DTG diagram; a straight line with slope −ΔE_a/R can be obtained by plotting ln(β/T²_max) versus 1/T_max to calculate the activation energy of E_a.

The Kissinger method was used to process the DTG curves of PDBIB with heating rates of 5, 10, 15, and 20 °C/min. Based on the above equation, a plot of ln(β/T²_max) versus 1/T_max was created, as shown in Figure 10. The E_a for thermal decomposition of PDBIB gums can be calculated from the slope of the plot. The activation energies of the itaconate chain segment and butadiene chain segment are 196.4 and 225.7 kJ/mol, respectively. This suggested that butadiene chain segments require a higher temperature for degradation than itaconate segments, indicating different thermal stability characteristics within the polymer.

Figure 10.

ln(β/T²_max) versus 1/T_max curves of PDBIB.

To better understand the kinetics of thermal decomposition, the FWO method is also an effective approach for determining the activation energy without assuming a specific reaction model. This technique is widely used to analyze the thermal stability of materials, especially in the study of polymer aging and degradation. For example, research on NR composites has shown that the FWO method can provide valuable insights into the multistage degradation process, indicating higher activation energies when antioxidants are used, which enhances thermal stability and aging resistance.^25,44 Applying the FWO method here will allow us to systematically evaluate the activation energy of PDBIB, thereby gaining a deeper understanding of its decomposition behavior. The FWO model is expressed as follows

where T is absolute temperature at a given conversion (α), K; β is the heating rate (K/min); A is a pre-exponential factor; ΔE_a is activation energy, kJ/mol; and R is the gas constant, which is 8.314 J/(K·mol). f(α) is a dynamic mechanism function in the differential form. The conversion (α) is defined based on the change in the sample’s mass during the reaction process. It represents the extent of the reaction and is calculated using the following formula

where W₀ is the initial mass of the sample (mass at the beginning of the reaction), W_t is the sample mass at a given temperature T, and W_f is the final mass of the sample (mass at the end of the reaction, representing the residual mass). The conversion (α) was fixed at specific values (e.g., 0.1, 0.2, ..., 0.9). For each fixed conversion, the temperatures (T) correspond to different heating rates. The FWO method was based on TG data by plotting ln β versus 1/T, and a straight line with a slope of −1.052ΔE_a/R was obtained.

The FWO method was used to process the TG curves of PDBIB with heating rates of 10, 15, 20, and 30 °C/min. The relationship between ln β and 1/T was plotted according to the formula, which is illustrated in Figure 11a. The Fitting formulas R² and E_a of PDBIB were determined with a conversion rates range of 20% to 90%, as shown in Table 4. Most R² values are close to 0.99, indicating an excellent fit of the data to the linear model. For higher conversion levels (e.g., 60% and 70%), the R² values drop significantly (0.89 and 0.83), suggesting a reduced reliability of the linear approximation in these ranges. This may indicate a change in reaction mechanisms or increased complexity at higher conversions. The thermal degradation activation energies of PDBIB range from 160.9 to 289.9 kJ/mol. As shown in Figure 11b, the activation energy–conversion curves exhibit complexity throughout the entire thermal degradation process, indicating that the thermal degradation reaction of PDBIB involves multiple steps and is highly intricate. As the conversion rate increases, the activation energies of thermal degradation initially increase and then decline. The maximum value for the degradation activation energy occurs at a conversion rate of approximately 60%. When a conversion rate is below 60%, it mainly corresponds to the thermal degradation of itaconate chain segments; when a conversion rate above is 60%, it mainly relates to the degradation of butadiene main chains. The degradation activation energy of the butadiene chain segment is greater than that of the itaconate chain segments. However, the activation energy of PDBIB increased significantly within the conversion rate range of 40% to 60%, reaching its maximum value at 60%. This is probably due to the carbonization reaction of PDBIB, and the removal of small molecules on the molecular chain requires more energy. As the temperature continues to rise, carbonization reactions form a carbon layer, leading to an increase in thermal degradation activation energy. At higher conversion rates, the structure of the carbon layer is destroyed, and the degradation activation energy begins to decrease.

Figure 11.

(a) ln β–1/T curves of PDBIB and (b) relationship between E_a and conversion rates of PDBIB in the thermal degradation process.

Table 4. Activation Energies (E_a) of PDBIB Gums under Different Conversions.

conversion (%)	fitting formulas	R2	Ea (kJ/mol)
20	y1 = 14.75 – 8.84·x1	0.99	160.88
30	y2 = 14.74 – 8.97·x2	0.99	163.30
40	y3 = 16.53 – 10.32·x3	0.99	187.86
50	y4 = 17.23 – 9.38·x4	0.96	199.77
60	y5 = 23.90 – 15.92·x5	0.89	289.86
70	y6 = 21.77 – 14.77·x6	0.83	268.85
80	y7 = 19.19 – 13.15·x7	0.99	239.39
90	y8 = 17.72 – 12.33·x8	0.99	224.49

In the analysis of PDBIB thermal degradation, both Kissinger and FWO methods were employed to calculate the activation energies of the process. The Kissinger method is simple and convenient, allowing for a quick estimation of the activation energy for polymer degradation. Compared with the FWO method, it provides less information and is unable to describe the changes in activation energy throughout the entire degradation process. In a word, these two methods can provide valuable insights into the thermal degradation behaviors of PDBIB, highlighting the distinct contributions of its chemical segments and the multistep nature of the degradation process.

4. Conclusions

Poly(dibutyl itaconate-co-butadiene) (PDBIB), a novel biobased itaconate elastomer, exhibited intermediate sensitivity to thermo-oxidative aging and thermo-shearing aging when compared with BR and SBR, the well-established commercial rubbers. Results indicate that both chain degradation and cross-linking were observed during aging, with chain degradation being predominant. The TGA/FTIR results showed that the bond between the ester group and the main chain was broken first, the weight loss ratio of the main chain of the itaconate segment reached the maximum at about 386 °C, and the weight loss peak at 447 °C was attributed to the weight loss peak of the butadiene chain segment according to the weight loss ratio. The thermal degradation activation energies of the itaconate chain segment and the butadiene chain segment are 196.4 and 225.7 kJ/mol, respectively, from the Kissinger method. The FWO method showed that the thermal degradation reaction of PDBIB involves multiple steps, and the E_a of PDBIB ranges from 160.9 to 289.9 kJ/mol. As the conversion rate increases, the activation energies of thermal degradation initially rise and then decline. The maximum value for degradation activation energy occurs at a conversion rate of approximately 60%. TGA results proved that there was a molecular chain carbonization process, which was caused by cross-linking during the thermal decomposition process. This work demonstrates that PDBIB can undergo thermo-oxidative processing like conventional rubbers, making it suitable for rubber products. Its renewable, biobased origin, reduced carbon footprint, and alignment with industry sustainability goals enhance both environmental benefits and commercial viability, making it a promising alternative for sustainable rubber applications.

Data Availability Statement

Data will be made available on request.

Author Contributions

Fulan Hao: investigation, methodology, data curation, validation, writing-original draft preparation, writing-reviewing, and editing. Hui Yang, Yaping Ma, Qingzhi Du, and Ju Han: investigation, methodology, data curation, and validation. Haijun Ji: writing-reviewing. Runguo Wang: conceptualization, project administration, funding acquisition, writing-reviewing, and supervision. Guo-Hua Hu: data curation and writing-reviewing and editing. Liqun Zhang, Laurent Falk: conceptualization, project administration, data curation, data curation, and supervision.

The authors declare no competing financial interest.