Non-Isothermal Degradation Mechanism of Micro/Nano Titanium Dioxide-Enhanced Polycaprolactone Biocomposite

Abstract

Understanding the degradation behavior of polymer composites is crucial for their practical application, especially in areas such as biomedicine and environmental engineering. In this study, we investigated the influence of titanium dioxide (TiO₂) particle size and content, containing 0.5, 1, 2, 5, and 10 wt% m/nTiO₂, on the degradation mechanism of biodegradable polycaprolactone (PCL) biocomposites. The degradation kinetics of the prepared biocomposites were evaluated using the Friedman method in conjunction with multivariate nonlinear regression facilitated by the Netzsch Thermokinetics software. The results indicate different degradation mechanisms for PCL biocomposites containing TiO₂ microparticles compared to biocomposites containing TiO₂ nanoparticles. However, the PCL biocomposites with TiO₂ microparticles showed a three-step degradation process, and the PCL biocomposites with TiO₂ nanoparticles exhibited a four-step degradation process. This difference can be attributed to the observed agglomeration of TiO₂ nanoparticles within the PCL matrix, which leads to an additional diffusion step in the degradation process. Interestingly, the addition of TiO₂ particles did not change the basic degradation mechanism of PCL but prolonged the degradation process to a higher conversion range. These findings shed light on the complicated interplay between the properties of the filler particles and the behavior of the polymer matrix and provide valuable clues for the design and optimization of biodegradable biocomposites.

1. Introduction

Polycaprolactone (PCL) is a promising biopolymer from renewable sources synthesized by the ring-opening polymerization of ε-caprolactone (ε-CL) initiated by a suitable catalyst [1,2]. PCL is known for its exceptional biodegradability and flexibility and has attracted considerable attention in various fields such as food packaging, tissue engineering, wound dressings and drug delivery applications [2,3]. In addition, PCL is a prime candidate for producing nanocomposite materials due to its commendable stability profile [4]. Despite its numerous advantages, PCL encounters limitations, such as high production costs, relatively low melting temperature and modest mechanical properties, which hinders its wide industrial use [2]. Integrating inorganic particles or utilizing the synergistic effects of inorganic nanoparticles integrated into the polymer matrix or the hybrid (organic/inorganic) nature of the designed composites represents a promising way to solve these problems [5,6]. The resulting polymer nanocomposites usually exhibit improved properties compared to their pure polymer counterparts, which include mechanical robustness, thermal and dimensional stability, chemical resistance, and optical and electrical properties [7,8]. Titanium dioxide (TiO₂) is an important material in various scientific and industrial fields due to its inertness, non-toxicity and cost-effectiveness. As an inert substance, TiO₂ remains chemically stable under a wide range of environmental conditions, ensuring its longevity and reliability in various applications. Its non-toxicity not only highlights its safety for human health and environmental welfare but also enhances its suitability for numerous biomedical and consumer-oriented applications. In addition, the cost-effectiveness of TiO₂ makes it a particularly attractive option for large-scale production and use in various industries, enabling advances in areas ranging from photocatalysis to pharmaceuticals. The distinctive combination of inertness, non-toxicity, and affordability makes titanium dioxide a cornerstone of today’s scientific and industrial landscape with multiple uses and significance [2]. Incorporating TiO₂ nanoparticles into different types of polymer matrices can produce synergistic effects, mainly improving the mechanical performance of polymer nanobiocomposites for applications in various fields [7]. Therefore, polymer/TiO₂ nanobiocomposite materials have found applications in catalysis, bioengineering, food packaging, biotechnology, and biomedicine [9,10,11]. However, the size of the inorganic particles incorporated into the polymer matrix has a great influence on the properties of the resulting composites. Xu et al. [12] have reported that smaller TiO₂ particles (<30 nm) exhibit more active photocatalytic properties of TiO₂ compared to larger (micro) particles. The PCL/TiO₂ nanobiocomposites are not “new” and have already been prepared and characterized. Indeed, the morphology and thermal properties [2,13] have been studied. In our previous work [2], we characterized PCL/n,mTiO₂ biocomposites by differential scanning calorimetry (DSC), dynamic mechanic analysis (DMA), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA). DSC measurements showed that the slight shift of the glass transition to lower temperatures in the PCL/TiO₂ micro- and nanobiocomposites could be due to the higher mobility of the amorphous phase of PCL. The crystallinity of PCL in the PCL/TiO₂ biocomposites was higher than that of the pure PCL. According to the TGA results, the thermal stability of PCL was improved by the addition of TiO₂ microparticles compared to TiO₂ nanoparticles. Finally, it was found by SEM analysis that the TiO₂ microparticles were better dispersed in the PCL matrix than the TiO₂ nanoparticles, which formed more aggregates (agglomeration). The influence of micro and nano TiO₂ particles on the properties of PCL-based biocomposites before and after UV radiation has been discussed in detail [14]. The impact of TiO₂ morphology on the structure, crystallization kinetics, and properties of PCL biocomposites have also been studied [15]. Monteiro et al. [16] characterized the interaction between the PCL chains and TiO₂ nanoparticles using X-ray diffraction (XRD), infrared spectroscopy (FTIR), low-field nuclear magnetic resonance (NMR), TGA, and DSC analysis. Samples of PCL/TiO₂ hybrid with different TiO₂ ratios, ranging from 0.05% w/w to 0.35% w/w concerning PCL concentration, were produced using a solvent casting technique. Later authors confirmed an interaction between the PCL chains and the TiO₂ nanoparticles. They also showed, by XRD and DSC analyses, that the incorporation of TiO₂ transforms the semi-crystalline structure of PCL into a less ordered structure. Finally, Monteiro et al. observed through TGA analysis that the introduction of TiO₂ nanoparticles decreased the thermal resistance of PCL. There is limited information on the kinetic analysis of the thermal degradation of PCL/TiO₂ nanobiocomposites. Mofekeng and Luyt [13] studied the effects of blending and the low content of titanium dioxide nanoparticles on the thermal degradation behavior of PLA and PCL. Although the latter authors used the Flynn–Ozawa–Wall method to calculate the apparent activation energies, they did not calculate the other two kinetic parameters of the so-called “kinetic triplet”: the pre-exponential factor (A) and a kinetic model (f(α)).

The main purpose of this work was to investigate how different particle sizes affect the mechanism of the thermal degradation of PCL/TiO₂ nanobiocomposites. This was conducted by calculating the true kinetic triplets of their non-isothermal degradation using a kinetic model combining the isoconvertional Friedman method and multivariate non-linear regression integrated into the Netzsch Thermokinetic Professional software, version 3.1. [17].

2. Materials and Methods

The PCL (polycaprolactone 440744-500G, average Mn 70,000–90,000 g mol⁻¹ by GPC, Mw/Mn < 2, density 1.145 g mL⁻¹ at 298 K) was supplied by Sigma-Aldrich, Darmstadt, Germany. Titanium dioxide (TiO₂) technical micropowder (denoted as mTiO₂, particles average size 0.1 μm) and TiO₂ nanopowder (denoted as nTiO₂, particles 21 nm, commercial grade Aeroxide P25) also supplied by Sigma-Aldrich, Darmstadt, Germany, were used as the inorganic filler. Neat PCL and its biocomposites containing 0.5, 1, 2, 5, and 10 wt% m/nTiO₂ were prepared using a laboratory Brabender mixer, Duisburg, Germany, at a temperature of 393 K and a screw speed of 60 min⁻¹, with a total blending time of 7 min. Final specimens for the analysis were prepared by compression molding (hydraulic press Dake Model 44-226, Grand Haven, MI, USA) at 413 K. Thermogravimetric (TGA) measurements of the above samples were performed using a TA Instruments Q500 system analyzer, New Castle, DE, USA. Non-isothermal thermogravimetry was performed at heating rates of 5, 10, and 20 K min⁻¹ in a temperature range of 396–873 K in nitrogen atmosphere (60 cm³ min⁻¹). Further details on the thermal stability and characteristic degradation temperatures, such as the temperature at which 5% weight loss occurs (T_5%), the temperature of the maximum degradation rate (T_max), and the final degradation temperature (T_f), of the investigated biocomposites can be found in our previous work [2]. Since thermogravimetry is recognized as a suitable and reliable method for monitoring the thermal degradation of polymeric materials [18,19,20], the results of non-isothermal thermogravimetry from our previous work [2] are used in this work for the kinetic analysis of PCL/TiO₂ biocomposites.

Kinetic Analysis

The kinetic analysis of the solid-state reactions that are ruled by a single process is based on Equation (1):

$$\frac{d\alpha }{dt}\cong \beta \frac{d\alpha }{dT}=A\cdot \exp \left(-\frac{E_a}{RT}\right)\cdot f(\alpha )$$

(1)

where E_a is the activation energy, A is the pre-exponential factor, f(α) is the reaction model, α is the degree of conversion, β is the linear heating rate (°C min⁻¹), T is the absolute temperature (K), R is the general gas constant (J mol⁻¹ K⁻¹), and t is the time (min). It is suggested that before kinetic analysis, the complexity of the process should be investigated by determining the dependence of E_a on α by isoconversional methods. Before delving into kinetic analysis, it is advisable to explore the intricacies of the process by examining the relationship between activation energy (E_a) and conversion (α) through isoconversional methods. Based on the recommendations of the Kinetics Committee of the International Confederation for Thermal Analysis and Calorimetry (ICTAC) [21,22,23], the principle, the basic equations of fundamental principles, the equations governing solid-state reactions, and the application of the isoconversional Friedman method (FR) are described. The experimental approach to kinetic analysis entails elucidating the dependence of E_a on α through FR plots, followed by analysis using Netzsch Thermokinetic Professional software, version 3.1. Comprehensive details regarding this methodology, software utilization, and an overview of the kinetic models can be found in the referenced literature [17,24,25,26].

3. Results

Kinetic Analysis

The collected data from the non-isothermal thermogravimetric analysis of PCL + m/n TiO₂ biocomposites were used to evaluate the effect of the addition of TiO₂ micro- and nanoparticles on the degradation mechanism of PCL. The first step is to determine the dependence of E_a on α using the Friedman method. From Figure 1, it can be concluded that E_a depends on α for all the samples studied. From a kinetic point of view, this undoubtedly indicates a complex degradation mechanism, and consequently, the process cannot be described only by a single reaction model and a single pair of Arrhenius parameters. The solution to this problem is to use the model fitting multivariate nonlinear regression method [21,22]. The presence of two or more inflection points or maxima for all samples studied (Figure 1) indicates that degradation occurs in at least two main steps, which is consistent with the results of TGA and DTG [2].

Looking more closely at the apparent activation energy of PCL degradation calculated by the Friedman method, it can be seen that there are three areas with different E_a values. Neat PCL has already been the subject of our study. Also, in our previous study [26], we concluded that the thermal degradation of PCL can be adequately described by three individual kinetic triplets. The PCL/TiO₂ biocomposites also showed three different areas of apparent activation energy. However, the exceptions to this rule are the samples of the PCL/nTiO₂ biocomposites with a higher proportion of TiO₂ nanoparticles, 95/5 and 90/10, respectively. The values of the apparent activation energy for each degradation phase and conversion range are summarized in

Table 1. The range of apparent activation energy values of thermal degradation of PCL/m(n)TiO₂ blends obtained by Friedman method in kJ mol⁻¹.

Stage of Reaction	PCL/mTiO2
	100/0	99.5/0.5	99/1	98/2	95/5	90/10
Stage I	0.0–158.8	137.3–210.1	140.1–158.6	101.3–159.4	32.0–160.7	13.4–171.3
Conversion range	0.00–0.05	0.01–0.05	0.01–0.20	0.01–0.05	0.01–0.10	0.01–0.10
Stage II	177.5–192.8	170.3–190.9	160.0–183.2	182.2–190.1	167.2–177.7	176.6–186.1
Conversion range	0.05–0.70	0.05–0.70	0.20–0.80	0.05–0.70	0.10–0.80	0.10–0.60
Stage III	0.0–283.2	0.0–285.2	0.0–230.5	4.8–215.0	118.8–286.3	0.0–426.9
Conversion range	0.70–1.00	0.70–1.00	0.80–1.00	0.70–1.00	0.80–1.00	0.60–1.00
Stage of Reaction	PCL/nTiO2
	100/0	99.5/0.5	99/1	98/2	95/5	90/10
Stage I	0.0–158.8	127.9–163.2	0.0–170.1	77.5–156.5	110.6–145.9	96.2–135.7
Conversion range	0.00–0.05	0.00–0.05	0.00–0.05	0.00–0.10	0.00–0.05	0.00–0.20
Stage II	177.5–192.8	168.8–182.9	177.5–189.8	160.1–190.8	166.9–190.8	149.1–172.9
Conversion range	0.05–0.70	0.05–0.55	0.05–0.55	0.10–0.75	0.05–0.20	0.20–0.40
Stage III	0.0–283.2	0.0–302.1	167.7–303.7	0.0–278.2	187.6–192.5	178.8–188.3
Conversion range	0.70–1.00	0.55–1.00	0.55–1.00	0.75–1.00	0.20–0.70	0.40–0.75
Stage IV	-	-	-	-	16.8–276.5	71.7–284.9
Conversion range	-	-	-	-	0.70–1.00	0.75–1.00

. In addition to the apparent activation energy values, the kinetic analysis heavily relied on discerning the shape and trajectory of both experimental data points and isoconversional lines extracted from Friedman plots (Figure 2). Particularly in cases of multistep reactions, deriving precise insights into individual reaction types remains challenging. However, Friedman’s analytical framework provides valuable cues regarding the initial reaction phase. Notably, if the experimental points exhibit a shallower slope compared to the isoconversional lines during the early stages of the reaction (α = 0.02–0.10, as depicted in Figure 2c), it strongly suggests the presence of a diffusion-controlled reaction process [26].

Utilizing the F-test, correlation coefficient, and congruence of activation energy (E_a) values derived from the Friedman method, we discerned the most suitable fit of the f(α) function, effectively characterizing the kinetic model of degradation. A comprehensive summary of these calculations is presented in

Table 2. Probable kinetic models for thermal degradation of PCL/mTiO₂ microbiocomposites: F-test analysis and correlation coefficients from multivariate non-linear regression.

Stage of Reaction	Parameter	PCL/mTiO2
		100/0	99.5/0.5	99/1	98/2	95/5	90/10
Stage I	Ea1/ kJ mol−1	32.4 ± 0.1	141.8 ± 3.5	153.3 ± 1.4	152.5 ± 5.4	56.7 ± 0.0	42.1 ± 0.3
	log A1	−0.7 ± 0.0	13.5 ± 0.5	9.3 ± 0.1	14.3 ± 0.5	1.6 ± 0.0	0.4 ± 0.0
	n	-	1.9 ± 0.4	0.7 ± 0.1	2.7 ± 0.9	-	-
	Model	D4	Cn	Cn	Fn	D4	D4
Stage II	Ea2/kJ mol−1	177.2 ± 0.8	170.2 ± 0.5	160.5 ± 4.1	189.8 ± 2.0	176.9 ± 1.1	176.4 ± 1.1
	log A2	11.2 ± 0.1	10.9 ± 0.0	10.4 ± 0.3	12.5 ± 0.1	11.8 ± 0.1	11.1 ± 0.0
	n	1.4 ± 0.0	1.4 ± 0.0	1.1 ± 0.2	1.0 ± 0.1	1.6 ± 0.0	1.4 ± 0.0
	Model	Cn	An	An	Cn	Cn	Cn
Stage III	Ea3/kJ mol−1	2.0 ± 0.4	264.0 ± 7.1	217.9 ± 13.1	212.0 ± 9.5	165.6 ± 0.7	10.1 ± 6.0
	log A3	−3.2 ± 0.0	17.8 ± 0.5	14.9 ± 1.1	14.3 ± 0.8	10.7 ± 0.0	−2.6 ± 0.4
	n	0.1 ± 0.0	2.0 ± 0.0	1.9 ± 0.1	1.6 ± 0.1	0.8 ± 0.0	0.4 ± 0.1
	Model	Fn	Fn	Fn	Fn	Fn	Fn
Correlation coefficient, r2		0.99993	0.99991	0.99988	0.99986	0.99999	0.99992

and

Table 3. Predicted kinetic models for thermal degradation of PCL/nTiO₂ nanobiocomposites: F-test analysis and correlation coefficients from multivariate non-linear regression.

Stage of Reaction	Parameter	PCL/nTiO2
		100/0	99.5/0.5	99/1	98/2	95/5	90/10
Stage I	Ea1/kJ mol−1	32.4 ± 0.1	143.6 ± 0.6	130.4 ± 0.7	128.5 ± 0.6	104.7 ± 0.0	98.4 ± 0.3
	log A1	−0.7 ± 0.0	8.0 ± 0.0	7.0 ± 0.1	6.9 ± 0.0	7.0 ± 0.0	6.8 ± 0.0
	n	-	-	-	-	-	-
	Model	D4	D4	D4	D4	D4	D4
Stage II	Ea2/kJ mol−1	177.2 ± 0.8	168.2 ± 0.9	178.2 ± 1.4	160.4 ± 1.1	207.2 ± 0.3	210.0 ± 0.3
	log A2	11.2 ± 0.1	10.9 ± 0.1	11.7 ± 0.1	10.2 ± 0.1	13.9 ± 0.0	13.9 ± 0.0
	n	1.4 ± 0.0	1.0 ± 0.0	1.2 ± 0.0	1.0 ± 0.0	-	-
	Model	Cn	Cn	Cn	Cn	D4	D4
Stage III	Ea3/kJ mol−1	2.0 ± 0.4	294.1 ± 4.0	295.4 ± 3.7	252.5 ± 5.9	211.9 ± 1.9	198.5 ± 4.0
	log A3	−3.2 ± 0.0	20.6 ± 0.3	20.7 ± 0.3	17.4 ± 0.5	14.2 ± 0.1	12.5 ± 0.2
	n	0.1 ± 0.0	2.1 ± 0.0	1.8 ± 0.0	1.8 ± 0.0	1.4 ± 0.0	2.6 ± 2.5
	Model	Fn	Fn	Fn	Fn	Cn	Cn
Stage IV	Ea4/kJ mol−1	-	-	-	-	211.9 ± 1.9	198.5 ± 4.0
	log A4	-	-	-	-	14.2 ± 0.1	12.5 ± 0.2
	n	-	-	-	-	1.4 ± 0.0	2.6 ± 2.5
	Model	-	-	-	-	Fn	Fn
Correlation coefficient, r2		0.99993	0.99996	0.99994	0.99995	0.99998	0.99993

. Additionally, Figure 3 provides a visual depiction, contrasting the experimental data points with the curves computed through multivariate non-linear regression, across both pristine PCL and the chosen compositions of PCL/TiO₂ biocomposites.

The optimal alignment between the experimental data and presumed kinetic models revealed a three-stage degradation mechanism for the PCL sample, characterized by consecutive reactions: A→B→C→D. In the initial stage of thermal degradation, the apparent activation energy (E_a) for neat PCL stands at 32.4 kJ mol⁻¹, indicative of a three-dimensional diffusion reaction (Ginstling–Brounstein type, D4). Transitioning to the second stage, the mechanism is described by an nth-order reaction with autocatalysis (Cn), demonstrating a notably higher E_a value of 177.2 kJ mol⁻¹. Lastly, the third stage manifests as a nth-order reaction (Fn), featuring a significantly lower apparent activation energy (2.0 kJ mol⁻¹) compared to the preceding stages. In our previous work [24], the thermal degradation of neat PCL was described by three kinetic models as follows, Cn→An→Fn, with quite different values of the apparent activation energies. This difference can be attributed to the different conditions of sample preparation in our previous work [26], in particular the higher temperature and pressure used in the sample preparation.

All samples of the PCL/mTiO₂ biocomposites showed the best fit of the experimental data with the assumed kinetic models for the three-step degradation mechanism with consecutive reactions, which was confirmed by the high correlation coefficient above 0.9999. To simplify the discussion of the obtained kinetic results, it can be summarized that the first stage of degradation of the PCL/mTiO₂ biocomposites can be described by either the Cn or Fn kinetic model (Table 2). However, at a higher mTiO₂ content (5–10 wt.%), the Ginstling–Brounstein-type kinetic model (D4) became the rate-controlling factor. At the beginning of thermal degradation, a solid or highly viscous melt system was formed, and the mass transfer processes became rate-controlling. Hence, the decomposition products must diffuse to the surface to be vaporized, and the diffusion of volatile products toward the surface is the rate-controlling process in the first stage [27]. This ultimately proves that in the first stage, a higher content of mTiO₂ (inorganic substance) interferes with the thermal degradation of PCL. However, the second degradation stage of all PCL/mTiO₂ biocomposites is described by either the Cn or An kinetic model. These calculations undoubtedly indicate that the addition of TiO₂ microparticles has a negligible effect on the degradation kinetics of PCL. Since the main degradation process occurs in the conversion range of 0.10–0.70 and the Cn and An models belong to the same accelerating reaction type [26], it can be summarized that, as in the case of neat PCL, the above kinetic models correctly describe the degradation process of all PCL/mTiO₂ biocomposites.

The values of the apparent activation energy of the degradation of all PCL/mTiO₂ biocomposites, calculated by the Friedman method, are practically in the same range of values (170–190 kJ mol⁻¹) (Table 2). Finally, the third stage of degradation of the PCL/mTiO₂ biocomposites is characterized by the kinetic model of Fn (Table 2). This is the final confirmation that the addition of TiO₂ microparticles did not affect the mechanism of the thermal degradation of PCL. In the case of PCL/nTiO₂ nanobiocomposites, the results are undoubtedly clear. All PCL/nTiO₂ samples showed the best fit of the experimental data with the assumed kinetic models for the three-step degradation mechanism with consecutive reactions (r² > 0.9999). However, samples 95/5 and 90/10, whose degradation occurred through four stages with four individual kinetic triplets, are an exception to this pattern. In the first stage of degradation, the Ginstling–Brounstein-type kinetic model (D4) is undoubtedly the rate-controlling process for all the samples studied. The second step of the degradation of PCL/nTiO₂ nanobiocomposites is described exclusively by the nth-order reaction with auto-catalysis (Cn), with higher E_a values (Table 3). The situation is quite different for samples 95/5 and 90/10. The thermal degradation of these samples is described by the kinetic model D4, which can be seen in Figure 2c. Moreover, at least for the majority of PCL/nTiO₂ samples, the third and the last steps of the degradation are characterized exclusively by the nth-order-type reaction (Fn). In contrast to this, the third and fourth steps of the degradation of the PCL/nTiO₂ samples with 5 and 10 wt.% TiO₂ can be described by the Cn and Fn models, respectively. The dominance of diffusion as the rate-controlling process is apparent across all samples up to α = 0.05, with one exception noted in samples featuring the highest addition of TiO₂ nanoparticles. In these cases, (specifically, compositions with ratios of 95/5 and 90/10), diffusion governs the rate up to α = 0.20 and α = 0.40, respectively. The TiO₂ nanoparticles prolong the random cleavage by cis-elimination and the specific cleavage of the chain end by cleavage from the hydroxyl end of the polymer chain of PCL, according to the mechanism of the thermal degradation of neat PCL investigated by Persenaire et al. [28], which is described by the Cn kinetic model in this work (Table 3). The NETZSCH Thermokinetics software, version 3.1, [17] recognized this situation as two diffusion stages with different kinetic parameters (E_a and ln A). Theoretically, if these stages are considered as one stage, then the thermal degradation of these samples could also be described with a three-stage process and all PCL/nTiO₂ nanobiocomposites would have shown the same kinetic scheme and the same mechanism of thermal degradation. Comparing the values in Table 2 and Table 3, the difference between the effect of TiO₂ micro- and nanoparticles on the thermal degradation of PCL is mainly limited to the starting point of degradation (lower conversion range) due to the size of the inorganic particles incorporated into the polymer matrix. In our previous study [2], the phase structure of the samples was observed using an SEM microscope. It was found that the TiO₂ microparticles were uniformly dispersed in the PCL matrix, while the TiO₂ nanoparticles formed aggregates. The same work also shows that the addition of TiO₂ microparticles improves the thermal stability of PCL compared to TiO₂ nanoparticles (TGA). Further evidence can be found in the work of Mofekeng and Luyt [13], who studied the effects of the addition of TiO₂ nanoparticles on the surface properties, thermal stability, and apparent activation energy of the thermal degradation of PLA/PCL blends. The latter authors found that the nanoparticles in the blend were preferentially located in the PLA phase, which they explained by the high interfacial tension between PCL and the TiO₂ nanoparticles, which they attributed to the difference between the polar components of their surface free energies. Moreover, Mofokeng and Luyt [13] found that the nanoparticles improved the thermal stability of PLA more significantly, especially at lower contents where they observed less agglomeration. Ultimately, through a comparative analysis of the apparent activation energies, notably elevated in the presence of the nanoparticles, the researchers deduced that the nanoparticles’ interaction with volatile degradation products hampered their diffusion from the molten polymer sample. This phenomenon dominates any catalytic effect of the nanoparticles on the degradation kinetics of PCL [13].

In summary, this work has built upon previous work [2,14] to provide further insight into the effects of TiO₂ particle size on thermal degradation and the mechanisms of such effects in neat PCL and its micro and nano PCL/TiO₂ biocomposites. This approach involves using kinetic analysis and modelling and linking these to empirical data to confirm that the addition of TiO₂ does not intrinsically alter those mechanisms involved in the thermal degradation of PCL. However, these studies show that when agglomeration occurs in composite systems at higher loadings (5, 10 wt%) of TiO₂ nanoparticles, there is a delay in the onset of degradation explained by a slowing of internal diffusional processes involved in degradation. This additional phenomenon is not observed with neat PCL at the same loadings of micro TiO₂ or lower loadings of nano TiO₂. The dispersion and binding characteristics of TiO₂ particles within the PCL matrix are pivotal factors governing the material’s properties and behavior, particularly concerning degradation as well as mechanical performance. At lower loadings of nano TiO₂, typically below 5 wt%, effective dispersion within the PCL matrix is observed. Well-dispersed nanoparticles furnish a substantial interfacial area for interaction with the PCL matrix, enhancing material properties. Conversely, as nanoparticle loadings increase, agglomeration tendencies escalate due to the heightened surface energy of the nanoparticles, fostering their propensity to coalesce rather than remaining uniformly dispersed. These agglomerates introduce defects within the matrix, which significantly influence material properties and impede internal diffusion processes crucial for degradation. Moreover, agglomerates impose a convoluted path for water molecules, retarding their penetration rate into the matrix and subsequently decelerating hydrolytic degradation. A comprehensive comprehension of TiO₂ nanoparticle dispersion and binding within the PCL matrix is paramount for tailoring the properties of PCL-based biocomposites. The implementation of suitable dispersion techniques and nanoparticle surface modifications can effectively enhance the mechanical and degradation characteristics of these biocomposites. While not a fundamental material science study, it is hoped that the insights provided here can assist material scientists and engineers in better understanding, designing, and further developing PCL biocomposites fit for a given purpose.

4. Conclusions

By using the isoconversional Friedman method in combination with multivariate nonlinear regression, supported by Netzsch Thermokinetic Professional software, version 3.1, the true kinetic triplets determining the non-isothermal degradation of PCL/TiO₂ biocomposites were calculated. The degradation of PCL/TiO₂ biocomposites proceeded via a three-step mechanism characterized by successive reactions. The exception was the PCL/nTiO₂ nanobiocomposites with 5 and 10 wt% nTiO₂ (ratios 95/5 and 90/10), where the degradation proceeded in four different stages due to the agglomeration of TiO₂ nanoparticles in the PCL matrix, which was also observed in the SEM analysis in our previous research. The resulting agglomeration led to an additional diffusion stage that retarded the diffusion of volatile PCL degradation products from the fused PCL/nTiO₂ nanobiocomposite. Importantly, our previous findings and the results presented in this work together suggest that the addition of TiO₂ particles did not fundamentally alter the mechanism of PCL thermal degradation.