Thermal Properties, Isothermal Decomposition by Direct Analysis in Real-Time-of-Flight Mass Spectrometry and Non Isothermal Crystallization Kinetics of Poly(Ethylene-co-Vinyl Alcohol)/Poly(ε-Caprolactone) Blend

Abstract

A series of poly(ethylene-co-vinyl alcohol)/poly(ε-caprolactone) blends with different compositions were prepared using solvent casting. The miscibility of this pair of polymers was investigated using differential scanning calorimetry (DSC), and proved by a negative Flory interaction parameter value calculated from the Nishi–Wang equation. The miscibility of this blend was also confirmed by scanning electronic microscopy (SEM). The thermal behaviors of the obtained materials were investigated by DSC, thermogravimetric analysis, and direct analysis in real-time–time-of-flight mass spectrometry and the results obtained were very relevant. Furthermore, the crystalline properties of the obtained materials were studied by DSC and X-ray diffraction where the Ozawa approach was adopted to investigate the non-isothermal crystallization kinetics. The results obtained revealed that this approach described the crystallization process well.

1. Introduction

Many systems, which were based on the chemical combination of different monomers by random, block, and graft copolymerization, have been developed to improve the properties of polymers over the years [1,2,3,4]. The development and commercialization of new polymers are extremely expensive and generally require many years. However, employing a process that is based on polymer blends, which are inexpensive and easy to process, often reduces the production time and cost. This approach is also associated with shortcomings and has not been developed as rapidly as expected, partly because most physical blends of different high molecular weight polymers are quite immiscible [5,6,7,8]. Put differently, when mixed, the blend components will likely be separated into phases predominantly containing their respective kinds. This characteristic, combined with the generally low physical forces of attraction across the phase boundaries, usually imparts immiscible blend systems with poor mechanical properties. Despite this shortcoming, different physical blend systems have been commercialized, as will be discussed subsequently.

Poly(ε-caprolactone) (PCL), which is also called a Shapelock (US) or polymorph (UK), is a non-hazardous semicrystalline polymer that can be readily shaped by hand because of its low melting point (60 °C). Furthermore, this polymer can be decomposed by the hydrolysis of the ester linkages in its backbone under physiological conditions [8]. One of the principal routes for preparing this polymer is the ring-opening polymerization of ε-caprolactone in the presence of stannous octoate as the catalyst [9]. Furthermore, the competitive viscoelastic and rheological properties of PCL enormously facilitate the manufacturing and handling of many polyesters into a wide range of implants and other devices [10,11,12,13,14]. This biodegradable and biocompatible polymer, which is approved by the US Food and Drug Administration and prepared by relatively inexpensive production routes, offers a promising basis for its utilization as long-term degradable implants. This polymer can be physicochemically manipulated according to specific anatomy sites to control the biodegradation process [15,16].

Furthermore, poly(ethylene-co-vinyl alcohol) (E-VAL) is a semicrystalline, biodegradable [17], biocompatible [18,19] copolymer that possesses excellent mechanical and gas barrier properties. Because of its double hydrophilic/hydrophobic character, this copolymer can be controlled through its composition of hydrophobic/hydrophilic comonomers, thereby favoring its application, such as tissue engineering, in the biomedical field through the hydrophilic character of the cells [20]. This copolymer is synthesized by the polymerization of ethylene with vinylacetate to yield poly(ethylene-co-vinylacetate), followed by the hydrolysis of the acetate substituent. Moreover, only a few investigations have been reported regarding the application of this copolymer in the biomedical domain. It is well known that certain properties of semicrystalline polymers are directly related to the dynamic behavior of the crystallization of a polymer [21,22]. Generally, a polymer can crystallize under isothermal or non-isothermal conditions upon heat treatment. The relevant kinetic behaviors are generally studied to elucidate the mechanism by which crystallization evolves in polymer materials. The kinetics of isothermal crystallization is widely applied to describe the evolution of a crystalline system in the polymer matrix [23,24,25]. However, some studies have focused on the non-isothermal crystallization of polymers through their processes, thereby reflecting the industrial conditions [26,27,28]. According to the literature, regarding the kinetics of isothermal crystallization, many models have been developed, although only some of them (Jeziorny [29], Ziabocki [30]), and Ozawa [31] are suitable for describing non-isothermal kinetics. To properly control the rate of crystallization and obtain the desired physical and mechanical properties of a material, kinetic studies via suitable mathematical models must be conducted [32]. The competitive viscoelastic and rheological properties of PCL and E-VAL greatly facilitate their manufacture and handling in a wide range of implants and other devices. However, these accessories have been developed on an industrial scale primarily via thermal molding under certain heating temperature and pressure conditions. Therefore, a study of the thermal stability of these materials is required.

In this work, a series of polymer blends involving PCL and E-VAL with different compositions were prepared via the solvent casting route, after which the crystalline and thermal properties of the resulting materials were studied. To achieve this, differential scanning calorimetry (DSC) analysis was employed to characterize the miscibility of this pair of polymers and study the non-isothermal crystallization kinetics of the obtained material. Therein, the Ozawa model was applied to determine the different crystallization parameters. Furthermore, thermogravimetric analysis/differential thermal analyses (TGA/TDA) was conducted to study the thermal stability of the obtained material and the nature of the resulting fragments from the thermal decomposition of the blends.

2. Materials and Methods

2.1. Materials

E-VAL (27% ethylene) and PCL (${\overline{M}}_w$ = 45,000 g mol⁻¹) was supplied by Sigma-Aldrich (Schnelldorf, Germany). Dimethylformamide (DMF) (purity, 98%) was purchased from Panreac (Chicago, IL, USA). These chemicals were utilized without prior purification.

2.2. Blend Preparation

The E-VAL/PCL blend was prepared by the solution casting method. The polymer and copolymer were dissolved separately in DMF in a 50 mL Erlenmeyer flask at 80 °C, with stirring until complete dissolution was achieved, as observed by the formations of viscous solutions. Thereafter, the two solutions were mixed with continuous stirring to form an E-VAL/PCL/DMF ternary solution. To obtain a film sample with perfectly homogeneous thickness, the solution was poured into a Teflon petri dish, after which they were all carefully deposited horizontally on a square Styrofoam plate, floating on the surface of a crystallizer that was filled with water and kept to dry for 48 h at ambient temperature. To extract the residual solvent that was crusted on the obtained film, the Teflon plate containing the polymeric material was vacuum-dried for 24 h at 60 °C. Furthermore, a series of E-VAL/PCL blends containing 10, 25, 50, 75, and 90 wt% E-VAL was prepared by the same method, and

Table 1. Preparation conditions of the E-VAL/PCL blends in DMF.

Sample	E-VAL:PCL (wt%)	E-VAL (g)	PCL (g)
E-VAL	100:0	10.0	0
E-VAL/PCL90	90:10	9.0	1.0
E-VAL/PCL75	75:25	7.5	2.5
E-VAL/PCL50	50:50	5.0	5.0
E-VAL/PCL25	25:75	2.5	7.5
E-VAL/PCL10	10:90	1.0	9.0
PCL	0:100	0	10

presents the experimental conditions.

2.3. Characterization

The polymer, copolymer, and their blends were characterized by different techniques. The X-ray diffraction (XRD) patterns of the E-VAL copolymer, PCL homopolymer, and their blends were recorded on a Rigaku D_max 2000 X-ray diffractometer that was equipped with a Cu anode tube at a voltage and generator current of 40 kV and 100 mA, respectively. Furthermore, the diffraction angle was in the range of 0–80 two theta. The samples were utilized as thin films, although the pure naphthalene sample was analyzed as powder microparticles. Surface morphology analyses of dried samples were performed on a Jeol JSM-6360LV scanning electron microscope (SEM) (Japan) at an accelerating voltage of 20 kV. The surface of samples was first sputter-coated with a thin layer of gold and then observed at a magnification range of 5000×. The DSC thermograms were obtained by a Shimadzu DSC-60 system, which was previously calibrated with indium. Additionally, 8–10 mg of the polymer sample was packed in aluminum DSC pans before they were placed in the DSC cell. The samples were heated from 30 °C to 200 °C at heating rates of 5, 7, 10, and 20 °C min⁻¹ in nitrogen gas and kept at 200 °C for 10 min to destroy any nucleus that might act as a crystal seed. In the cooling mode, the samples were cooled to 50 °C at cooling rates of 5, 7, 10, 15, and 20 °C min⁻¹. For a uniform thermal history of all the samples, the data were obtained from the second scan runs of all the samples. No degradation phenomena were observed in all the DSC thermograms of E-VAL, PCL, and their blends. Noting that the T_g value was taken as the midpoint of the heat capacity change with temperature and the T_m and T_c at the top of the melting and crystallization changes with temperature, respectively. The TGA/DTA measurements were conducted on a Shimadzu TGA-60 system in dynamic nitrogen gas. Afterward, 4–10 mg of the polymer or copolymers was carefully loaded into the TGA aluminum pan and heated from 25 °C to 600 °C at a heating rate of 20 °C min⁻¹. The isothermal decomposition was detected by an Accu-time-of-flight (ToF) LC-plus JMS-T100 LP mass spectrometer (JEOL, Tokyo, Japan) that was equipped with direct analysis in real-time (DART) ion source (IonSense, Saugus, MA, USA) without any prior preparation of the sample. The volatile components of the extract were evaporated from a stream of helium that was heated at 350 °C, after which they were ionized by the excited metastable helium atoms before entering the ion source of the ToF mass spectrometer. In the positive ionization mode, each molecule was transformed into a protonated ion [M + H]⁺ or a non-protonated radical molecular ion [M⁺]. Since the ionization process was considered “soft,” only a little or no fragmentation occurred, such that each peak in the spectrum corresponded to a given compound. The high-resolution mass spectra were recorded using the following experimental conditions: needle voltage: 3000 V, helium flow-rate: 0.5 L/min., detector voltage: 2200 V. The mass calibration was achieved using a mixture of standard polyethylene glycols PEG 200 and PEG 600. The mass spectra were recorded in the range 50 to 1000 Da, with a mass resolution between 4800 and 5450. The spectral data were stored and processed using Mass Center software. The surface morphology of the PE-co-VAL, PCL, and PE-co-VAC dried films, which were coated with a gold grid, was analyzed by field emission scanning electron microscopy (JEOL-JSM-2100F scanning electron microscope at accelerating voltage of 15 kV). The samples were first sputter-coated with a thin layer of gold before they were observed at a magnification range of 300–3000×.

3. Results

3.1. Miscibility

The miscibility of the E-VAL/PCL mixture with different compositions was recently proven by DSC and confirmed by Fourier-transform infrared (FTIR) analysis in our previous study [33]. The DSC analysis revealed the presence of a single T_g in the thermograms of the blends and its negative deviation from that of the ideal, and these indicated the interaction between the chains of different natures. The comparison of the FTIR spectra of the blends with their components revealed the presence of hydrogen bonds, thereby confirming the interaction of the two different polymers.

3.2. SEM Analysis

Figure 1 shows the SEM micrographs of the surface morphology of E-VAL, PCL, and E-VAL/PCL blends containing 25, 50, 75, and 90 wt% of E-VAL content. A homogeneous and smooth surface was observed in the micrographs of the E-VAL and PCL components. The SEM micrographs of the blends also showed smooth and uniform monophasic surface morphologies comparable to those of pure E-VAL and PCL devoid of any stress could be reflecting the repulsion forces causing the immiscibility. Noting that, an immiscible blend shows dull and heterogenic surfaces, proving the existence of stress resulting from the contraction of each constituent due to the electrostatic repulsion forces indicating the non-miscibility. The ripples or the wrinkles seen in photos of some samples probably resulted from bloating due to the evaporation of residual DMF during the vacuum drying period at 60 °C. Under these conditions, the vapor released lifts the film and then deposits it on the Teflon plate after the total evaporation of the solvent.

3.3. DSC Analysis

3.3.1. Glass Transition and Melting Temperature

The DSC thermograms of the E-VAL/PCL blends and their pure components revealed T_g and the apparent T_m ($T_m^{ap})$, as presented in

Table 2. Comparative data of the E-VAL/PCL blend with different compositions, obtained by DSC.

Blend System		PCL		E-VAL
	Tg (oC)	Tmap (oC)	∆Hm (J∙g−1)	Tmap (oC)	∆Hm (J∙g−1)
E-VAL	60	-	-	183	73.70
E-VAL/PCL90	39	52	8.34	180	47.83
E-VAL/PCL75	17	56	14.18	178	31.32
E-VAL/PCL50	−12	67	45.46	177	27.02
E-VAL/PCL25	−33	65	69.19	175	24.57
E-VAL/PCL10	−53	64	73.12	167	19.65
PCL	−59	62	75.1	-	-

ΔH_m = 72.5 J/g for E-VAL32% [35].

. According to the T_g values of the blend, regarding all the investigated compositions, the presence of only one transition temperature, which was localized between those of their pure components, indicated the miscible behavior of this pair of polymers in their amorphous forms. The T_g value of PCL shifted toward higher temperatures when the E-VAL content of the blend increased. According to Qui et al. [34], the appearance of a single T_g for the blend indicated the full miscibility on a 20–40 nm scale.

Two melting temperatures were observed in each thermogram. They were attributed to each constituent in the blend in which $T_m^{ap}$ of E-VAL shifted moderately toward the low temperatures (from 183 °C to 167 °C), whereas that of PCL passed by a maximum of 67 °C when the composition was 50 wt%. The decrease in T_m of a semicrystalline polymer in the blend was caused by the thermodynamic interactions between the two different polymeric chains. According to the Flory–Huggins theory [36,37], a miscible blend is characterized by the magnitude of the reduction in T_m, which allows the assumption of the value of the interaction energy involved. T_m of a polymer is generally affected by thermodynamic factors and morphological parameters, especially the thickness of the crystal. It was also revealed from the data in

Table 3. Parameters of the Hoffman–Weeks plot for E-VAL in the E-VAL/PCL blends.

System	${\mathit{\phi }}_{\mathit{E}-\mathit{V}\mathit{A}\mathit{L}}$	$({\mathit{\phi }}_{\mathit{E}-\mathit{V}\mathit{A}\mathit{L}}^2)\times {10}^2$	${\mathit{T}}_{\mathit{m}}^{\mathit{o}} {(}^{\mathbf{o}}\mathbf{C})$	Tc (oC)	η
E-VAL	1.0	100	186	157	0.24
E-VAL/PCL90	0.895	80.10	183	154	0.17
E-VAL/PCL75	0.740	54.76	181	152	0.14
E-VAL/PCL50	0.487	23.72	176	150	0.14
E-VAL/PCL25	0.210	4.41	175	148	0.11
E-VAL/PCL10	0.095	0.90	173	145	0.11

Regarding the E-VAL/PCL blends, $T_m^o$

that the heat of fusion (∆H_m), which was attributed to PCL in the blend, increased significantly with the E-VAL content, whereas that of E-VAL similarly decreased. This appeared to be directly related to the degree of depression in the crystallinity of each component as the concentration of the other increased.

To dissociate the thermodynamic effects on the decrease in T_m from the morphological parameters, we applied the equilibrium melting point (T^o_m) data. According to the Hoffman–Weeks approach [38], T^o_m is given by Equation (1):

$$T_m^{ap}=\eta T_c+\left(1-\eta \right)T_m^o$$

(1)

This method is based on the isothermal crystallization of the polymer at different T_c, as displayed in Figure 2 for the pure components and blends containing equal amounts of each constituent, and the plot of the variations in $T_m^{ap}$ with T_c. Moreover, $T_m^o$ and $\eta$ were obtained from the intercept of the straight line, $T_m^{ap}=T_c,$ and the slope of the Hoffman–Weeks plot (Figure 3), respectively. According to the same authors, the value of η varied between zero and one and could be considered as a parameter for evaluating the stability of crystals undergoing melting. Indeed, η = 0 indicates that the crystals are perfectly stable, so that $T_m^{ap}=T_m^o$ at all the crystallization temperatures. Conversely, η = 1 represents intrinsically unstable crystals. Table 3 lists the η and $T_m^o$ values of the E-VAL and E-VAL/PCL blends with different compositions. As shown in these data, $T_m^o$ of E-VAL was observed at 186 °C, which agreed with the result in the literature (187 °C) [39].

Regarding the E-VAL/PCL blends $T_m^o$ of the E-VAL phase decreased with the PCL content to reach a minimum of 173 °C when the PCL content of the blend was 10 wt%. In other terms, the maximum extent of this T_m depression in the E-VAL/PCL10 blend was 13 °C. Generally, the η values of E-VAL in its blend (0.24–0.11) were relatively small, thereby revealing the stability of the E-VAL crystals in the blend. This parameter decreased slowly as the PCL content of the blend increased, practically losing half when the concentration of PCL in the blend was 10%. This suggests that the stability of the E-VAL crystals increased relatively, i.e., their lamellar thicknesses increased, when blended with PCL. This fact is probably attributed to the morphological effect.

3.3.2. Flory Interaction Parameter

The Flory interaction parameter (${\chi }_{1,2}$) and the interaction energy density (B), which characterized the interactional dynamics between the two different macromolecules in the blend, were determined by the Nishi–Wang equation (Equation (2)) [40] based on the Flory–Huggins theory. According to Qui et al. [34], regarding the crystalline polymer (1)/crystalline polymer blend (2), as in our case, T_c applied in the equation of Hoffman-Weeks to determine $T_m^o$ of 1 must be higher than that applied to determine $T_m^o$ of 2. Under this condition, 1 acts as a crystalline polymer, whereas 2 is an amorphous polymer. For polymers with high molecular masses, the Nishi–Wang equation can be written as follows:

$$\frac{1}{T_m^o\left(blend\right)}=\frac{R{V}_2{\chi }_{1,2}}{\Delta H^o{V}_1}{\phi }_1^2+\frac{1}{T_m^o\left(pure\right)}$$

(2)

where, $T_{m\left(pure\right)}^o$ and $T_{m\left(blend\right)}^o$ are the equilibrium melting temperatures of the pure crystallizable component possessing a high melting point and the blend, ∆H° is the enthalpy of melting 100% of the 1 repeated units, and V_i and φ₁ are the molar volume of the repeated units and the volume fraction of polymer (i) in the blend. Applying the data that were obtained from the literature, ΔH_uE-VAL = 4.22 kJ mol⁻¹ [41], V_uE-VAL = 37.80 cm³ mol⁻¹ [41], and V_uPCL = 99.65 cm³ mol⁻¹ [42]. The ${\chi }_{1,2}$ and B values were deduced from the slope and intercept of the linear curve, indicating the inverse of $T_{m\left(blend\right)}^o$, which was obtained from the data in Table 3, vs. the square of ${\phi }_{E-VAL}$, as displayed in Figure 4.

The χ_1,2 value, which was estimated from the slope of the linear fit, was −0.012 ± 0.002, and the estimated B value, which was equal to χ_1,2 RTV₁⁻¹ [43], was −1.21 ± 0.20 J/cm³ of PCL obtained at 186 °C. According to the Flory–Huggins theory, a negative χ_1,2 value indicated that the miscibility of this pair of polymers was caused by specific interactions between the different chains in the blend. These interactions were relatively weak because of the hydrogen bonds in the blend, as previously highlighted by FTIR analysis. The amorphous fraction of the E-VAL/PCL blend is thermodynamically miscible and can form a compatible blend in the melt state.

3.3.3. Crystallization Temperature

In the polymer blend domain, it is well known that the crystallization behavior of each component in a blend depends on its miscibility with the other components, its physicochemical properties, and crystallization conditions. According to Hu et al. [43], the crystallization of one component in a blend affects the morphology, crystallization, and mechanical properties of the second component. Figure 5 shows the DSC cooling thermograms of pure semicrystalline E-VAL, PCL, and their blends, obtained in the non-isothermal crystallization process at a cooling rate of 20 °C min⁻¹. Expectedly, these thermal curves exhibited two distinct exothermic peaks for each blend. The peaks were attributed to the crystallization temperature of each polymer in the blend, and

Table 4. Temperatures and heats of crystallization of E-VAL/PCL blends and their pure components.

System E-VAL/PCL	PE-VAL			PCL
	Tc (oC)	∆Hc (J/g)	Xc (%)	Tc (oC)	∆Hc (J/g)	Xc (%)
E-VAL	157	−54.18	48.20	-	-	-
E-VAL/PCL90	148	−52.11	42.56	18	−5.76	5.22
E-VAL/PCL75	152	−26.32	34.22	23	−10.08	9.65
E-VAL/PCL50	152	−23.09	28.34	22	−17.27	15.02
E-VAL/PCL25	154	−12.50	23.08	24	−25.91	22.10
E-VAL/PCL10	154	−10.21	15.21	25	−39.59	33.50
PCL	-	-	-	26	−65.54	52.30

illustrates their values.

The percentage crystallization of each component in the blend was calculated from Equation (3) [44], and Table 4 lists the results.

$$X\left(\%\right)=\frac{\Delta H_m-\Delta H_c}{w\times \Delta H_m^o}\times 100,$$

(3)

where ΔH_m and $\Delta H_m^o$ are the heat of fusion of the semicrystalline polymer in the blend and that of the pure polymer with 100% crystallinity for E-VAL = 157.83 J g⁻¹ [44] and PCL = 135.5 J g⁻¹ [45], respectively. ΔH_c and w are the heat of crystallization of the semicrystalline polymer and its weight fraction, respectively.

The normalized crystallinity of PCL in the blend decreased dramatically from 52.30% to 5.22% and was accompanied by an increase in the crystallinity of E-VAL from 15.21% to 48.20% when the E-VAL content passed from 0 to 100 wt%. The presence of E-VAL with PCL in the blend severely reduced their respective crystallinities.

The DSC cooling thermograms of the blends and their pure components were obtained at cooling rates of 5, 10, 15, and 20 °C min⁻¹. Figure 6 shows the comparison of the thermal curves of E-VAL/PCL50 and E-VAL/PCL75, employed as examples, with those of the pure components. As shown in these curve profiles, the crystallization temperature shifted to the right as the cooling rate decreased. A significant depression was also observed in the crystallization heat with decreasing cooling rate. This was also observed by different researchers utilizing different pairs of polymers [46,47].

3.4. XRD Analysis

Figure 7 shows the XRD patterns of the E-VAL/PCL blends and their components. The spectra of the blend with different compositions revealed the combination of the diffraction peaks of each localized pure polymer of E-VAL at 2θ = 20 and 29.22°, which were assigned to the (110) and (200) reflection planes, respectively, and to the orthorhombic forms of polyethylene [48] and PCL at 16°, 22°, and 24°, which are assigned to the reflection planes of (110), (111), and (200), respectively [49,50]. The absence of new diffraction peaks in the spectra of the blend indicated the non-formation of a new crystalline structure. Put differently, E-VAL and PCL preserved their distinct initial crystalline structures in the blend, and the miscibility was realized only in their amorphous parts.

3.5. Thermal Stability

Very few reports are available in the literature on the thermal decomposition of E-VAL compared with the abundant reports on their poly(ethylene) (PE) and poly(vinyl alcohol) (PVA) components. Among these reports, only a very limited number of articles discussed the results of the thermal decomposition of this copolymer in detail. Alvarez et al. [51] suggested that the products, which were obtained from the first step between 387 °C and 407 °C, involved the decomposition of a greater part of the vinyl alcohol units, whereas the second step involved the decomposition of the ethylene units at higher temperatures. According to different authors [52], the thermal decomposition of PVA principally leads to the production of water and carbon dioxide because of the elimination reactions. Conversely, Kumar and Sing [53] investigated the thermal degradation of PE in which the main products were different volatile saturated and unsaturated hydrocarbons. Thus far, there are no published articles on the fragments, which are obtained from combining ethylenic alcohol and vinyl alcohol units in a copolymer. Regarding the thermal decomposition of PCL, Persenaire et al. [54] and Vogel et al. [55] concluded that the decomposition process proceeded in two steps: the first caused a statistical rupture of the PCL chains to produce H₂O, CO₂, and hexanoic acid, and the second is characterized by an unzipping depolymerization process, which causes the regeneration of a monomer (ε-caprolactone). Although very few authors have discussed this subject, our contribution in this section is to present a more precise idea of the fragments that are obtained from the thermal decompositions of the two polymers and their blends. To achieve this goal, two complementary techniques were applied to elucidate the decomposition process of the blend and its components. The first, which was non-isothermal, was performed by TGA to elucidate the thermal stability of these materials and their different stages of decomposition, and the second, which was isothermal, was conducted by DART-ToF-MS to determine the main fragments that were obtained from the decomposition, which could not be analyzed by TGA.

3.5.1. TGA Analysis

The thermal decompositions of the E-VAL/PCL blends and their pure components were conducted by TGA/derivative-TGA (DTGA) analysis, and Figure 8 lists the obtained thermal curves. The thermograms of pure E-VAL revealed the ultimate decomposition step, which started at 360 °C, whereas that of pure PCL revealed two decomposition steps in which the first and second were at 310 °C (weight loss 9.09 wt%) and 435 °C (weight loss 90.91 wt%), respectively. As shown in the profiles of the E-VAL/PLC curves, similar trends to those of pure E-VAL were observed. These thermograms exhibited only one decomposition temperature, except that containing 90 wt% E-VAL (E-VAL/PCL90) exhibited three distinct decomposition temperatures at 310^oC (weight loss 9.09 wt%), 435^oC (weight loss 27 wt%), and 445 °C (weight loss 63.91 wt%). Enhanced thermal stability was generally observed in PCL when E-VAL was incorporated in the copolymer matrix. Indeed, the decomposition temperature of the blend shifted positively from 320 °C to 350 °C according to the E-VAL content of the blend. For clarification, just observe the effect of adding 10 wt% E-VAL, which contributed to an increase in the stability of PCL by 20 °C. By contrast, it was observed that the addition of only 10 wt% PCL to the blend dramatically reduced the decomposition temperature of E-VAL from 360 °C to 250 °C. This finding also revealed the molecular interactions between E-VAL and PCL in the blend. A similar conclusion was also reported on the thermal decomposition of the acetylated starch/poly(ethylene-co-vinyl alcohol) by Jiang et al. [56] These authors also revealed the second decomposition of all the samples at 425 °C. Therein, more than 80 wt% of the polymers or blends were volatilized in form of small molecules, such as carbon dioxide, ethylene, aldehyde, and ketone.

3.5.2. DART-ToF-MS Analysis

Figure 9 lists the spectra of the isothermal decompositions of the blends and their pure components, as obtained by DART-ToF-MS at 350 °C. The E-VAL spectrum revealed a series of signals, which accrued from the fragmentation of this copolymer, and

Table 5. Principal fragments obtained by the isothermic decomposition of pure E-VAL.

No	Experimental Mass	Calculated Mass	Formula
1	195.12201	195.12183	C8H18O5
2	223.09488	223.09703	C12H15O4
3	235.20339	235.20619	C16H27O
4	256.17643	256.17869	C13H24N2O3
5	344.23557	344.23112	C17H32N2O5
6	388.25476	388.25734	C19H36N2O6

presents their main formulas. These compounds were produced by a radical mechanism, which involved chain scission, followed by the random transposition of the different groups that were detached and recombined afterward. As shown in these data, the absence of monomers indicated the absence of depolymerization reaction. However, the intense signal, which was observed at 223.09 m/z, was probably attributed to the isomers of the ferulic acid ethyl ester (C₁₂H₁₅O₄) containing 6° of unsaturation. The less-intense signals at 195.12, 235.20, and 344.23 m/z were probably assigned to the tetramer of the vinyl alcohol (C₈H₁₈O₅), the isomers of (2E,4S,6E,11R)-4,11-dihydroxy-2,6-dodecadienoic acid (C₁₂H₂₀O₄), and the isomers of C₂₀H₃₄O₇ containing four degrees of unsaturation, respectively. Conversely, the isothermal decomposition of pure PCL generated five main compounds and a small amount of the monomer, as presented in

Table 6. Principal fragments obtained by the isothermic decomposition of PCL.

No	Experimental Mass	Calculated Mass	Formula
1	129.08866	129.09155	C7H12O2
2	143.10588	143.10720	C8H14O2
3	198.16233	198.16198	C12H21O2
4	223.09488	223.09703	C12H15O4
5	235.20927	235.20619	C16H27O
6	285.20753	285.20658	C16H29O4
7	341.23820	341.23280	C19H33O5

. However, the spectrum of the E-VAL/PCL50 blend was revealed by its data in

Table 7. Principal fragments obtained by the isothermal decomposition of E-VAL/PCL50.

No	Experimental Mass	Calculated Mass	Formula
1	115.07872	115.07590	C6H11O2 (monomer)
2	151.09929	151.09971	C9H13NO
3	195.12345	195.12325	C8H19O5
4	212.15016	212.14979	C7H16N8
5	239.14970	239.14946	C10H23O6
6	256.17762	256.17735	C11H22N5O2
7	283.17585	283.17568	C12H27O7
8	299.18481	299.18585	C16H27O5
9	327.20244	327.20189	C14H31O8
10	343.21010	343.21206	C18H31O6

. In addition to the signals that were attributed to the two pure components, other signals indicating the presence of new fragments were obtained from a possible transposition and combination of the radical groups from one chain to a neighboring chain of a different nature. This could also be considered as proof of the miscibility of this system since a radical transposition reaction can only proceed when the two components are close, as in a miscible blend.

3.6. Non-Isothermal Crystallization Kinetics

3.6.1. The Relative Degree of Crystallization

The relative degree of crystallinity (X_T), as a function of T_c, is expressed by Equation (4) [57]:

$$X_T=\frac{{{\displaystyle \int }}_{T_0}^T\left(\frac{\mathrm{dH}}{\mathrm{dt}}\right)\mathrm{dt}}{{{\displaystyle \int }}_{T_0}^{T\infty }\left(\frac{\mathrm{dH}}{\mathrm{dt}}\right)\mathrm{dt}}$$

(4)

where T₀ and $T_{\infty }$ are the starting and ending T_c that were obtained at the starting and ending inflections of the crystallization peak, respectively, and H is the instantaneous enthalpy of the crystallization process. After substituting the areas of the DSC thermograms, Equation (5) would be expressed as follows:

$$X_T=\frac{A_T}{A_{\infty }}$$

(5)

where A_T and $A_{\infty }$ are the areas under the DSC curves from T = T₀ to T = T and the total area under the crystallization curve, respectively. X_T could be determined by employing this equation at a specific temperature. During non-isothermal crystallization, the variation in the crystallization time with the crystallization temperature follows Equation (6):

$$t=\frac{\left(T_0-T\right)}{\beta }$$

(6)

where T and β are the temperature at a crystallization time, t, and the cooling rate, respectively. The integration of the exothermic peaks during the non-isothermal scans afforded X_T, as a function of the temperature, as shown in Figure 10 for E-VAL, PCL, and the E-VAL/PCL50 blend.

Because of the effect of retardation on crystallization, all the curves exhibited an approximately sigmoid pattern. The typical plots of X_t vs. time for E-VAL, PCL, and their blends with equal composition were traced by combining Equations (4)–(6), as shown in Figure 11.

Similar to the plots of X_T vs. temperature, all the curves exhibited largely sigmoid patterns, and the slopes of these curves at each point corresponded to the rate of crystallization. As observed from these sigmoid profiles, the rate of crystallization was almost constant between 20% and 80% of X_T. At the later stage, the curves appeared flat because of spherulite impingement [58].

3.6.2. Half-Time of the Crystallization

The half times (t_1/2) for completing the crystallization for the pure polymer, pure copolymer, and their blends, as deduced from the curves, indicate the variation in X_t vs. time at 50% X_T, and

Table 8. Temperatures and t_1/2 of the crystallization of E-VAL and the E-VAL/PCL blends obtained by the non-isothermal crystallization kinetics.

Samples	β (oC.min−1)	Tc (oC)		t1/2 (min)
		PCL	E-VAL	PCL	E-VAL
PCL	5	21	-	1.8	-
	10	19	-	0.8	-
	15	16	-	0.7	-
	20	10	-	0.6	-
E-VAL/PCL10	5	30	157	1.8	2.4
	10	27	156	0.8	1.2
	15	26	153	0.5	0.8
	20	25	152	0.5	0.5
E-VAL/PCL25	5	30	156	2.0	2.2
	10	28	154	1.0	1.1.
	15	26	152	0.7	1.0
	20	25	151	0.6	0.8
E-VAL/PCL50	5	30	154	2.2	1.7
	10	28	155	1.2	0.9
	15	27	153	0.8	0.7
	20	25	152	0.5	0.6
E-VAL/PCL75	5	19	157	1.2	2.6
	10	16	154	0.8	1.9
	15	13	152	0.6	1.4
	20	11	150	0.5	1.2
E-VAL/PCL90	5		161	1.0	1.6
	10		159	0.7	1.3
	15		158	0.6	1.1
	20		157	0.5	0.8
E-VAL	5	-	155	-	1.0
	10	-	152	-	0.9
	15	-	150	-	1.0
	20	-	148	-	0.5

summarizes the results obtained. As shown in these curve profiles, t_1/2 of pure PCL and E-VAL generally decreased slightly as the cooling rate increased, passing from 1.8 to 0.6 min for the first polymer and 1.0 to 0.5 min for the second as the cooling rate passed from 5 to 20 °C min⁻¹. Similar values and trends were also observed by Yung et al. [59] and Zhu et al. [60]. The variation of t_1/2 vs. the E-VAL content of the E-VAL/PCL system attained a minimum of 1.6 min at 5 °C min⁻¹ for the blend containing 90 wt% E-VAL. In the same direction contrast to the pure E-VAL, the t_1/2 values decreased with increasing cooling rate. This is because, at a relatively high PCL content, the molecules of the E-VAL clusters can restrict the motion of the PCL molecular chains, although they cannot act as a heterogeneous nucleating agent in the non-isothermal crystallization process to accelerate crystallization. However, with an increased PCL content, the molecular chains of these polymer clusters could act as a barrier to restrict the thermal motion of the molecular chains of E-VAL, thereby retarding the formation of crystals. Therefore, the addition of a large amount of PCL can delay the overall crystallization process.

3.6.3. Ozawa Parameters

The Ozawa relationship (Equation (7)) [31], which is an extension of the Avrami equation (Equation (8)) [61], was adopted to investigate the non-isothermal crystallization of the pure polymer and its blend at various cooling rates.

$$1-{\mathrm{X}}_{\mathrm{T}}=\exp (-\frac{{\mathrm{k}}_{\mathrm{T}}}{{\mathsf{\beta }}^{\mathrm{m}}})$$

(7)

$$1-{\mathrm{X}}_{\mathrm{t}}=\exp (-{\mathrm{kt}}^{\mathrm{n}})$$

(8)

The Avrami equation, which is originally applied to isothermal crystallization, could also be applied to non-isothermal crystallization by assuming that the specimen was cooled at a constant cooling rate. Accordingly, X_t and X_T represent the relative degrees of crystallinity at a time (t) and temperature (T), respectively; k and k_T are the rate constant of the crystallization kinetics and cooling function of the non-isothermal crystallization at temperature (T), respectively; β is the cooling rate; and n and m are the isothermal Avrami and Ozawa exponents based on the dimensions of the crystal growth and nucleation mechanism, respectively. Thus, Equation (9) can be linearized as follows:

$$\ln [-\ln (1-{\mathrm{X}}_{\mathrm{T}}\left)\right]={\mathrm{lnk}}_{\mathrm{T}}-\mathrm{mln}\mathsf{\beta }$$

(9)

As shown in the plots of ln[−ln(1 − X_t)] vs. ln(β) for E-VAL and the E-VAL/PCL50 system, which were adopted as examples (Figure 12), all the specimens containing pure E-VAL and E-VAL/PCL with different PCL contents exhibited straight lines. This revealed that the Ozawa equation (Equation (7)) perfectly described the primary non-isothermal crystallization process.

Table 9. Ozawa parameters representing the non-isothermal crystallization kinetics of pure E-VAL, PCL, PCL in E-VAL/PCL, and E-VAL in E-VAL/PCL.

Sample	Tc (oC)		m		kT
	PCL	E-VAL	PCL	E-VAL	PCL	E-VAL
PCL	14	-	1.73	-	3.7 × 10−3	-
	15	-	1.60	-	4.4 × 10−3	-
	17	-	1.87	-	7.4 × 10−3	-
	19	-	2.67	-	1.9 × 10−3	-
E-VAL/PCL10	16	145	1.45	0.72	1.5 × 10−3	18.91
	18	150	1.46	0.74	1.4 × 10−3	13.46
	20	155	1.72	0.75	1.1 × 10−3	10.17
	22	160	2.39	0.82	1.0 × 10−3	5.99
E-VAL/PCL25	19	146	1.17	0.96	7.1 × 10−2	9.47
	21	151	1.32	1.21	1.1 × 10−2	6.56
	23	155	1.53	1.26	1.9 × 10−3	4.64
	25	158	2.10	1.31	6.6 × 10−4	3.05
E-VAL/PCL50	25	147	0.60	1.21	1.4 × 10−1	0.17
	27	153	1.0	1.23	2.0 × 10−2	0.14
	28	155	1.2	1.62	2.7 × 10−3	0.04
	31	157	1.5	1.80	2,8 × 10−4	0.06
E-VAL/PCL75	23	155	0.54	0.87	4.6 × 10−2	0.37
	25	157	0.83	1.10	7.1 × 10−3	0.19
	27	160	0.94	1.20	3.4 × 10−3	0.10
	30	162	1.21	1.52	3.2 × 10−3	0.04
E-VAL	-	148	-	1.97	-	1.8 × 10−2
	-	150	-	2.12	-	94.63
	-	152	-	2.21	-	86.48
	-	155	-	2.33	-	84.77

summarizes k_T and m, which were deduced from the intercepts and slopes of these curves, respectively. According to these obtained data, the m value of pure E-VAL slightly increased from 1.97 to 2.33, whereas that of pure PCL passed from 1.73 to 2.67 with the crystallization temperature.

These results are similar to those reported in the literature: 1.93–2.27 for E-VAL [47] and 2.1–2.5 for PCL [62]. Conversely, the values of m for E-VAL and PCL in the E-VAL/PCL blend increased with the temperature, indicating that the incorporation of E-VAL into the PCL matrix greatly influenced the growth of the crystals. A slight increase in m, which was observed as the temperature increased, revealed a dramatic change in the crystallization mechanism. This was probably because of an increase in the viscosity of the blend, which was caused by the weak rupture of the hydroxyl–hydroxyl groups of E-VAL to favor the relatively high interactions between the hydroxyl and carbonyl groups of E-VAL and PCL, respectively, thereby supporting the miscibility behavior of this pair of polymers. From these data, it was generally revealed that the m value of E-VAL was dramatically reduced by the addition of PCL. For example, at 155 °C, m passed from 2.33 to 0.75 when the PCL content of the blend varied from 0 to 90 wt%. Similar results were also obtained for the m values of PCL, although they were less-noticeable with increasing E-co-VAL contents. Moreover, an increase in the m value is generally attributed to the change from sporadic to instantaneous nucleation [63]. According to typical polymer crystallization reports, an m value of ~2 indicated that the crystal growth occurred in the nuclei and was sporadic and spherical. Heterogeneous nucleation has been attributed to the fractional m values that were obtained during polymer crystallization [64]. The k_T value, which is related to the overall crystallization rate, decreased as the temperature decreased, indicating that for E-VAL and the E-VAL/PCL blend with these compositions, the crystallization rate increased as the temperature decreased, notably in the case of pure E-VAL in 3.2.2.

3.6.4. Crystallization Activation Energy (E_c)

E_c, which is associated with the overall crystallization process, was evaluated from the rates of crystallization and temperature that were recorded at the top of the crystallization peak of the polymer employing the Kissinger equation (Equation (10)) as follows [65]:

$$E_c=\frac{d[\ln \left(\beta {/\mathrm{T}}_c^2\right)}{d\left({1/\mathrm{T}}_c\right)}R$$

(10)

where R and T_c are the gas constant and peak crystallization temperature, respectively. E_c was obtained from the slope of $RLn\left(\frac{\beta }{T_c^2}\right)$ vs. 1/T, which was plotted, as shown in Figure 13. As shown by the curve profiles of the pure polymer, copolymer, and blends, straight lines were obtained, and

Table 10. Activation energies of the crystallization of pure E-VAL, PCL, PCL in E-VAL/PCL, and E-VAL in E-VAL/PCL.

Sample	Ea (kJ.mol−1)
	PCL	E-VAL
PCL	−120.0	-
E-VAL/PCL10	−241.1	−295.2
E-VAL/PCL25	−262.7	−275.2
E-VAL/PCL50	−291.0	−266.2
E-VAL/PCL75	−144.7	−206.4
E-VAL/PCL90	−132.3	−194.2
E-VAL	-	−180.8

summarizes the obtained E_c values. These data revealed negative values for E_c, indicating that the crystallization process was accompanied by the release of heat. These data revealed that the $E_c$ values of pure PCL and E-VAL were −120 and −145 kJ mol⁻¹, respectively, which are slightly inferior to those obtained by Wei et al. (−145 kJ mol⁻¹) [66] and Zhua et al. (−239 kJ mol⁻¹) [60] for PCL and E-VAL, respectively. These values were severely affected by the incorporation of one of the components into the other. Indeed, $E_c$ of PCL in the blend passed by a maximum of 291 kJ mol⁻¹ with an equal ratio of E-VAL/PCL, whereas that of E-VAL decreased continuously. For all the cases and at any composition, the $E_c$ values of the blends were superior to those of their pure constituents. According to Yang et al. [67], a more negative value of the activation energy means that more heat would be released for the crystallization. In this case, this indicates that it is more challenging to perform the crystallization process of each component in the blend than in their respective pure states.

4. Conclusions

Conclusively, the objective of this investigation, i.e., the miscibility of the E-VAL/PCL blend in the investigated composition range, was achieved by obtaining a negative value (−1.21) for the Flory interaction parameter between its components through the Nishi–Wang equation. Generally, the thermal stability of the obtained material, which was analyzed by TGA, revealed a drastic increase in the thermal stability of PCL with increasing E-VAL contents of the blend. For example, the incorporation of only 10 wt% E-VAL increased the thermal stability of PCL by 20 °C. Conversely, the addition of only 10 wt% PCL to the blend dramatically decreased the decomposition temperature of E-VAL. The isothermal decompositions of the pure components and their blends, which were analyzed by DART-ToF-MS at 350 °C, revealed the non-generation of monomers for E-VAL. However, a series of the isomers of ferulic acid ethyl esters, the tetramers of vinyl alcohol (C₈H₁₈O₅), the isomers of (2E,4S,6E,11R)-4,11-dihydroxy-2,6-dodecadienoic acid (C₁₂H₂₀O₄), and the isomers of C₂₀H₃₄O₇ were observed. Conversely, pure PCL generated five main compounds, as well as a few monomers. In addition to the fragments of the two components, the thermal fragmentation of E-VAL/PCL at the same temperature generated new compounds probably through the transposition and recombination of the radical groups from one chain to a neighboring chain of different natures. Furthermore, this finding can be employed as supplementary proof of the miscibility of this polymer system. The non-isothermal crystallization kinetics, which was obtained for the E-VAL/PCL blends and their components, revealed that the Ozawa model well-described their crystallization behaviors. It was observed from this investigation that the Ozawa exponent decreased as one of the two components in the blend increased. The Kissinger equation was adapted to determine the activation energies for the investigated blends and their pure components. Indeed, the Kissinger plots were obtained, and their slopes exhibited negative values of E_c, which indicated that the crystallization process was exothermic and that the activation energies were severely affected by the incorporation of one of the components into the blend.