Introduction

In engineering applications where friction and wear are a major concern, ultra high molecular weight polyethylene (UHMWPE) is a highly preferred polymer due to its excellent wear resistance, biocompatibility, low friction and chemical inertness [1,2,3,4,5,6]. As a linear semi-crystalline polymer, it has both crystalline and amorphous phases. Chains are folded into highly oriented lamellae of thickness 10–50 nm and length 10–50 μm in the crystalline phase. In the amorphous phase, the lamellae are arranged randomly and are connected to each other through tie molecules [7]. UHMWPE is widely utilized in a wide range of applications like automotive, engineering bearings and as bio-implant materials in artificial joints etc. [8,9,10,11,12].

Despite of its unique characteristics, UHMWPE cannot be used in high temperature applications due to its low thermal stability. As per previous researches [13, 14], thermal stability can be improved by increasing the tendency of cross-linking of molecular chains in the polymer. Generally, this is achieved by irradiating UHMWPE with gamma or electron-beam (EB) radiations [15,16,17,18]. Gamma radiation dependent cross-linking is highly significant to design synthetic joints because it can also sterilize the material and thereby removing bacterial contamination [19]. C–C and C–H bonds in UHMWPE are breakdown by means of energetic gamma/EB radiations and formed alkyl radicals [20]. These radicals react with each other and also with unsaturated bonds in the polymer and as a result, cross-linking occurs. Due to irradiation, radicals are formed in both the amorphous and crystalline phase in the material. In the amorphous phase, free radicals tend to quench within 10 h or induce cross linking, while in the crystalline phase, free radicals can remain active up to eight years and migrate into amorphous phase causing embrittlement and as a result shows unexpected failure in UHMWPE [21, 22].

In order to overcome this problem, post-irradiation thermal treatment (re-melting or annealing) is utilized to remove free radicals trapped in the crystalline phase and precursor to oxidative degradation [23, 24]. However, such thermal treatment decreases the crystallinity and ductility of UHMWPE and thus sacrifices with the strength, toughness and fatigue, which results in cracking within a short period of time after its usage [22]. Another alternative treatment is the addition of antioxidants in a polymer matrix, which results in a promising alternative strategy for scavenging free radicals trapped in the crystalline phase [25].

Antioxidants, mainly hindered phenol, do not completely eliminate oxidative degradation, but they substantially inhibit the rate of auto oxidation by interfering with the radical propagation reaction [26]. Conventionally, a synthetic antioxidant such as 2,6-di-tertiary-butyl-4-methyl phenol (BHT), tertiary-butyl-4-hydroxyanisole (BHA), tertiary-butyl-hydroquinone (TBHQ), and 6-ethoxy-1,2-dihydro-2,4-trimethylquinoline (ethoxyquin, EQ) are used to enhance the thermal stability of UHMWPE[27,28,29,30,31]. However, these synthetic antioxidants are toxic in nature and have harmful effects on the human body. Therefore, natural antioxidants (Gallic acid, Dodecy gallate, Vitamin C and Vitamin E) are recommended due to their non-toxic and negligible harmful effects [32, 33]. Vitamin E (α-tocopherol) is proven to be a successful natural antioxidant applied to stabilize UHMWPE by donating a proton of phenolic HO֯ to a macro-radical and itself becoming a much less reactive phenoxy radical due to the delocalization of unpaired electron over the aromatic ring. Intensive research resulted in the application of α-tocopherol for stabilizing UHMWPE, which was used as a joint implant. Even an ASTM standard specification suggested UHMWPE blended with Vitamin E for medical and other applications [33].

Quercetin, a natural antioxidant, emerged as a potential candidate for preventing oxidative degradation in different polymeric materials [34]. This [2-(3,4-dihydroxyphenyl)-5,7-dihydroxy-4H-chromen-4one)] is a flavanol type flavonoid found in fruits, vegetables, leaves and seeds and can be easily extracted through suitable solid phase extraction [35]. Till now, studies related to thermal kinetics have not been conducted by considering Quercetin as an antioxidant in UHMWPE.

In the present work, the thermal stability of UHMWPE samples blended with different concentrations (0.1–1.0 wt%) of Quercetin have been investigated (heating rate: 5 °C/min.) through the thermogravimetric (TGA/DTA) technique. The activation energies of these samples are also determined for different reaction mechanisms by adopting model fitting Coats and Redfern kinetic method. Further, thermograms of a maximum thermally stable sample are obtained at four different heating rates (5, 10, 15, 20 °C/min) and complexities involved during thermal degradation are resolved with deconvolution methods. Afterwards activation energies of deconvoluted peaks are determined by using both integral (Straink model) and differential (Friedman model) kinetic models. The reaction mechanism is identified using integral master plots and the pre-exponential factor is calculated with the help of activation energy and identified reaction mechanism. Finally, thermodynamic parameters, i.e., change in entropy (∆S), change in enthalpy (∆H) and change in Gibbs free energy (∆G) are determined.

Materials and methodology

Material

UHMWPE (Ultra high molecular weight polyethylene) and Quercetin, in powder form, are procured from Sigma Aldrich, USA. The quoted molecular weight of UHMWPE is 3 × 106–6 × 106 g/mol and density is 0.94 g/ml. The purity of Quercetin powder is ≥ 95% analyzed through High Performance Liquid Chromatography (HPLC).

Sample preparation

Quercetin (Q) with ten different weights ranging from 0.05 to 0.5 g, with interval of 0.05 g, is added in ethanol to prepare 1 wt% solution [36]. The solution (pale yellow colour) is poured into UHMWPE powder to prepare ten different samples (UHMWPE-Q) with different concentrations (0.1–1.0 wt%). For proper homogenization of ethanolic-quercetin solution in UHMWPE matrix, stirring was done with the help of magnetic stirrer for 30 min. Then, mixture was dried in an oven at 50 °C to evaporate ethanol. Afterwards, dried UHMWPE-Q powder was kept in a desiccator loaded with anhydrous blue silica gel to further dry ethanol and moisture content, if any [37].

Moulding of sample

The dried UHMWPE-Q powder is compacted, in a cylindrical designed mould, at room temperature (25 °C) and 5 MPa pressure. Then, these samples are melted at an elevated temperature (140 °C) and compressed at 15 MPa pressure. Finally, UHMWPE-Q samples are gradually cooled with a dwelling time of 10 min and cylindrical shaped (diameter: 12 mm; height 40 mm) samples are prepared.

These UHMWPE-Q samples, with different quercetin concentrations, are cut into small pieces (~ 0.5 mm thickness; ~ 5 mg weight) for thermogravimetric analysis.

Thermogravimetric analysis (TGA) techniques

These samples are placed one by one in an alumina crucible and subjected to thermogravimetric analysis (Hitachi STA 7200 TGA analyzer). Thermograms (mass loss as a function of temperature) are recorded at 5 °C/min heating rate in temperature range 50–600 °C, in nitrogen environment at 100 ml/min gas flow rate. These thermograms are analyzed, through ORIGIN software, to identify the maximum thermally stable sample. This identified sample is further subjected to the thermogravimetric analysis at four different heating rates (5, 10, 15 and 20 °C/min) and mass loss as a function of temperature is recorded.

Theoretical kinetic approach

Kinetic models

Thermograms obtained through TGA/DTA technique are analyzed for kinetic studies [38]. For thermal degradation, the rate of reaction is described as:

$$\frac{d\alpha }{dt}=k\left(T\right)f\left(\alpha \right)$$
(1)

where \(k\left(T\right)\) is the reaction rate constant, \(f\left(\alpha \right)\) is the differential form of an ideal reaction model, \(T\) is the absolute temperature in Kelvin, \(t\) is the time duration of the reaction (in min) and \(\alpha \) is the degree of conversion, which can be written as:

$$\alpha =\frac{{m}_{o}-{m}_{t}}{{m}_{o}-{m}_{\infty }}$$
(2)

where \({m}_{0}\) is the initial mass, \({m}_{t}\) is the mass at time \(t\), \({m}_{\infty }\) is the final mass of the sample.

The rate constant \(k\left(T\right)\) can be expressed by Arrhenius equation:

$$k\left(T\right)=A{e}^{-{E}_{a}/RT}$$
(3)

\(A\left({min}^{-1}\right)\), \({E}_{a}\left(kJ/mol\right)\) and \(R\left(8.314 J/Kmol\right)\) are the pre-exponential factor, activation energy and gas constant, respectively.

By substituting the value of \(k\left(T\right)\) in Eq. (1), one can write:

$$\frac{d\alpha }{dt}=A{e}^{-{E}_{a}/RT}f\left(\alpha \right)$$
(4)

For non-isothermal TGA experiment, at linear heating rate \(\beta =dT/dt\), the final equation for the rate of reaction can be obtained by modifying Eq. (4) as:

$$\frac{d\alpha }{dT}=\frac{A}{\beta }{e}^{-{E}_{a}/RT}f\left(\alpha \right)$$
(5)

By considering the above expression (5) as a base, different kinetic models are formulated [39,40,41,42,43,44,45]. These models follow different temperature integral approximations for the evaluation of kinetic parameters.

In the present study, Coats and Redfern (CR) [39] kinetic model is adopted to determine the activation energy of the considered samples. They utilized asymptotic series expansion [46, 47] and developed the following relation:

$$ln\left(\frac{g(\alpha )}{{T}^{2}}\right)=ln\left(\frac{AR}{\beta {E}_{a}}\right)+ln\left(1-\frac{2RT}{{E}_{a}}\right)-\frac{{E}_{a}}{RT}$$
(6)

where \(g\left(\alpha \right)\) is the integral form of an ideal reaction model. With the help of this model, the reaction mechanism is identified by plotting the different values of \(g(\alpha )\) at different degree of conversion \(\left(\alpha \right)\)(Table 1).

Table 1 Reaction mechanisms involved in thermal decomposition processes in solids
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Deconvolution kinetic method

The second stage of thermal decomposition of UHMWPE (Figs. 2 and 3), with different quercetin concentrations, is very complex. To resolve this complexity, deconvolution kinetic method is adopted. In the present study, the deconvolution is performed by using the Bi-Gaussian asymmetric function:

$$y=\left(\begin{array}{c}{y}_{0}+ H{e}^{-0.5{\left(\frac{x-{x}_{c}}{{w}_{1}}\right)}^{2}}\, for\, x<{x}_{c}\\ {y}_{0}+ H{e}^{-0.5{\left(\frac{x-{x}_{c}}{{w}_{2}}\right)}^{2}}\, for\, x\ge {x}_{c}\end{array}\right)$$
(7)

where \({y}_{0}\) is baseline, \(H\) is maximum height, \(x\) is the independent variable, \({x}_{c}\) is the central value; \({w}_{1}\) and \({w}_{2}\) are the width of left and right side of the Gaussian peak.

The activation energy corresponding to each deconvoluted peak is determined for the second stage using ICTAC recommended iso-conversional [48] kinetic models. These are Starink (SR) integral kinetic model [44] and Friedman (FR) differential kinetic model [45]. Starink model is written as:

$$ln\left(\frac{\beta }{{T}^{1.92}}\right)=\mathrm{ln}\left(\frac{AR}{{E}_{a}}\right)+\mathrm{ln}\left(\frac{df\left(\alpha \right)}{d\alpha }\right)-1.0008\frac{{E}_{a}}{RT}$$
(8)

and Friedman kinetic model is written as:

$$ln\left(\beta \frac{d\alpha }{dT}\right)=ln A+lnf\left(\alpha \right)-\frac{{E}_{a}}{RT}$$
(9)

By using these models, activation energies are determined from the slopes obtained through the plots \(ln\left(\beta /{T}^{1.92}\right)\) vs. \(1000/T\) (Eq. 8) and \(ln(\beta d\alpha /dt)\) vs. \(1000/T\) (Eq. 9), respectively.

Result and discussion

Thermal decomposition behavior of UHMWPE

Fig. 1 presents the thermogravimetric (TG), its derivative (DTG) and second derivative (SDTG) curves of UHMWPE, which is compressed at optimized compression parameters (Temperature: 140 °C and Pressure: 15 MPa). Thermogram is recorded in temperature region 50–600 °C at 5 °C/min heating rate, in nitrogen environment. As per Fig. 1, UHMWPE shows three thermal decomposition stages. No noticeable mass loss is observed in Stage-I, which lies in the temperature region 50–208 °C. In the second stage (~ 208–539 °C), the mass gain is observed at ~ 233 °C temperature. Here, alkyl radicals (R•) combine with oxygen, present in the amorphous phase and form peroxy radicals (ROO•) [49,50,51].These peroxy radicals abstract hydrogen from adjacent hydrocarbon chains and convert into hydro-peroxide (ROOH) by forming alkyl radicals. These alkyl radicals further continue oxidative degradation. The O–O bond of peroxy-radical and hydro-peroxide is relatively thermally unstable and breakdown into RO• and OH, resulting in mass loss first slowly and then abruptly by forming low molecular weight carboxylic acid, alcohols, ketones, esters, water, olefins, paraffins etc. [52,53,54]. Around 468 °C, Diels–Alder Reaction favours the formation of cyclic products and as a result, unexpected mass gain is observed up to 539 °C. Second stage is highly active and variety of complex reactions are formed during thermal degradation, as clearly observed in SDTG curve (Fig. 1). In Stage-III, after 539 °C, sample is complete volatized and no loss in mass is observed.

Fig. 1
figure 1

Thermogravimetric (TG), its derivative (DTG) and Second Derivative (SDTG) curves of pure UHMWPE, at 5 °C/min heating rate

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Optimization of quercetin concentration in UHMWPE through thermal stability

To observe the impact of quercetin concentrations on thermal stability of UHMWPE, recorded thermograms (at 5 °C/min heating rate) are presented in Fig. 2 and Fig. S1a, b. These thermograms show similar thermal decompositions stages as observed in pure UHMWPE (Fig. 2). However, the maximum decomposition temperature (Tmax) is different at different quercetin concentrations (Table 2). Firstly, thermal stability increases with an increase in quercetin concentration up to 0.4 wt% (Fig. 2); then it decreases with an increase of concentration from 0.5 to 0.7 wt% (Fig. S1a) and finally, it decreases even below the pure UHMWPE (Fig. S1b). The enhancement in thermal stability up to 0.4 wt% may be due to scavenging of formed free radicals of UHMWPE by abstracting hydrogen from the hydroxyl group of quercetin, leave phenoxy radicals of quercetin that can be further stabilized by delocalization of their electrons and forms various stable aromatic compounds of low molecular weight. However, an increase in quercetin concentration (0.5–0.7 wt%) lowers its efficiency as radical scavenger. This might be due to the reaction of quercetin directly with the oxygen present in UHMWPE. Here, in concentration 0.1–0.7 wt%, quercetin acts as antioxidant in UHMWPE. At higher concentration (0.8–1.0 wt%), quercetin transformed into pro-oxidant by forming various oxidative products [55], resulting in even lower thermal stability of pure UHMWPE. The above discussion reveals that quercetin act as antioxidant at lower concentration only and observed maximum thermal stability of UHMWPE at 0.4 wt%.

Fig. 2
figure 2

TG/mass-loss curves of UHMWPE with different Quercetin (Q) concentration (0.0, 0.1, 0.2, 0.3, 0.4 wt%)

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Table 2 Maximum decomposition temperature \(\left({T}_{max}\right)\) of Stage-II at different Quercetin concentrations
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To ascertain that maximum thermal stability of UHWMPE at 0.4 wt% quercetin only, activation energy at different quercetin concentration for different reaction mechanisms are obtained (Table 3) by utilizing Coats and Redfern Kinetic model. The table also revealed that values of activation energy are maximum at 0.4 wt% as compared to other concentrations.

Table 3 Activation energies (kJ/mol) at different quercetin concentrations, for different reaction mechanisms, obtained through Coats and Redfern model
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Thermal kinetic parameters of UHMWPE at optimized quercetin concentration

In order to explore different thermal kinetic parameters, thermograms of UHMWPE with quercetin (0.4 wt% concentration) are obtained at four different heating rates (5, 10, 15, 20 °C/min.) and are presented in Fig. 3. It is observed that these TG curves are of similar trend and shows three decomposition stages in which Stage-II is highly active and complex. In addition, TG curves also shifted towards higher temperature side with an increase in heating rate. Similar trends are also observed in plots (Fig. S2a, b) of the degree of conversion \((\alpha )\) and change of degree of conversion \(\left(d\alpha /dt\right)\) as a function of temperature. This may be due to the delay participation of radicals, in thermal decomposition, resulting in thermal lag, which leads to a change in thermal kinetic parameters. Further, peaks (Fig. S2b) also reveal the complexities involved in thermal reactions in Stage-II.

Fig. 3
figure 3

TG/mass-loss curves of UHMWPE + Quercetin (0.4 wt%) at four different heating rates (5, 10, 15 and 20 °C/min)

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Deconvolution kinetic method

The complexities involved in thermal reactions (Stage-II) are resolved through deconvolution method. Adopting this, peaks are deconvoluted by applying the Bi-Gaussian asymmetric function (Eq. 7). At 5 °C/min heating rate, two peaks at two different temperatures (432 and 444 °C) are obtained (Fig. 4). Similar trends are also observed (two peaks at two different temperatures) for other heating rates after deconvolution (Fig. S3a–c). These peaks are presented separately in Fig. 5a, b. Further, degree of conversion \((\alpha )\) values are generated and presented as a function of temperature at different heating rates (Fig. S4a, b). Figs. 5 and S4) reveals that peaks are shifted towards higher temperatures, which substantiates the thermal lagging.

Fig. 4
figure 4

Deconvoluted peaks in Stage-II of UHMWPE + Quercetin (0.4 wt%) at 5 °C/min heating rate

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Fig. 5
figure 5

\(d\alpha /dt\) as a function of Temperature in Stage-II of UHMWPE + Quercetin (0.4 wt%) for a Peak 1 and b Peak 2, at different heating rates

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Estimation of activation energy

To estimate activation energies for Peak 1 and Peak 2, linear fitted plots are generated at different conversion values (Fig. S4a, b) by adopting Starink (Fig. 6a, b) and Friedman (Fig. S5a, b) models. The obtained linear fitted parameters (slope, standard error (S.E.), R2 value) and activation energy are given in Tables 4 and 5. It is observed that the values of activation energies based on Starink model are less as compared to Friedman model. Further, the activation energy value of Peak 1 decrease with increase in degree of conversion \(\left(\alpha \right)\) value while for Peak 2 it increases for both integral and differential models.

Fig. 6
figure 6

Linear fitted curves at different conversion values obtained through Starink model for a Peak 1 and b Peak 2

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Table 4 Linear fitting parameters obtained through Starink model for Peak 1 and Peak 2
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Table 5 Linear fitting parameters obtained from Friedman model for Peak 1 and Peak 2
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Determination of specific reaction mechanism

The reaction mechanism involved in thermal decomposition of the considered sample (UHMWPE + quercetin (0.4 wt%)) is determined through integral master plots method. As per this method, thermal decomposition reaction can be expressed [56] as:

$$g\left(\alpha \right)=\frac{{AE}_{a}}{\beta R}P(u)$$
(10)

where \(g\left(\alpha \right)\) is the integral function at different degree of conversion \(\left(\alpha \right)\),\(u={E}_{a}/RT\) and \(P\left(u\right)\) is the temperate integral that can be expressed as:

$$P\left(u\right)\approx \frac{{e}^{\left(-1.0008u-0.312\right)}}{{u}^{0.92}}$$
(11)

To determine reaction mechanism, the above expression can be expressed as

$$\frac{g(\alpha )}{g(0.5)}=\frac{P(u)}{P(0.5)}$$
(12)

By utilizing Starink model-based activation energy, experimental values are generated and presented as a function of the degree of conversion \(\left(\alpha \right)\) at different heating rates (Figs. 7, 8, S6 and S7). Further, master plots are generated for different reaction mechanisms (Table 1) and introduced in these figures. The comparison between experimental values and master plots reveals that Peak 1 and Peak 2 favours A2 and A3 (random nucleation) reaction mechanisms, respectively. These reaction mechanisms may be due to semi-crystalline nature of UHMWPE, which restricts the mobility of free radicals in amorphous phases and facilitate random nucleation [57].

Fig. 7
figure 7

Comparison between experimental values (represented with symbols) and theoretical master curves obtained for different nucleation mechanisms, for Peak 1

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Fig. 8
figure 8

Comparison between experimental values (represented with symbols) and theoretical master curves obtained for different nucleation mechanisms, for Peak 2

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Estimation of pre-exponential factor and thermodynamic parameters

For determination of pre-exponential factors (A), Eq. (10) is rewritten by replacing \(g\left(\alpha \right)\) with A2 reaction mechanism for Peak 1 (Eq. 13) and A3 reaction mechanism for Peak 2 (Eq. 14) as

$${\left[-ln\left(1-\alpha \right)\right]}^{1/2}=\frac{{AE}_{a}}{\beta R}P\left(u\right)$$
(13)

and

$${\left[-ln\left(1-\alpha \right)\right]}^{1/3}=\frac{{AE}_{a}}{\beta R}P(u)$$
(14)

By plotting linear fitted curves between \({\left[-ln\left(1-\alpha \right)\right]}^{1/2}\) and \(\left({E}_{a}/\beta R\right)P\left(u\right)\) for Peak 1 and \({\left[-ln\left(1-\alpha \right)\right]}^{1/3}\) and \(\left({E}_{a}/\beta R\right)P\left(u\right)\) for Peak 2, pre-exponential factors at different heating rate are determined and given in Tables 6 and 7.

Table 6 Pre-exponential factor and thermodynamic parameters of Peak 1 at different heating rates
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Table 7 Pre− exponential factor and thermodynamic parameters of Peak 2 at different heating rates
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For thermodynamic parameters [58, 59] viz. entropy change \((\mathrm{\Delta S})\), enthalpy change \((\mathrm{\Delta H})\), and Gibbs free energy change \((\mathrm{\Delta G})\), the following equations are utilized:

$$\mathrm{\Delta S}=Rln\left(\frac{Ah}{e\mathrm{\varkappa }{k}_{b}{T}_{max}}\right)$$
(15)
$$\mathrm{\Delta H}={{E}_{a}-RT}_{max}$$
(16)

and

$$\mathrm{\Delta G}=\mathrm{ \Delta H}-{T}_{max}\mathrm{\Delta S}$$
(17)

where \(e\), \(\mathrm{\varkappa }\), \({k}_{b}\) and \(h\) are Neper number (2.7183), transition number (Equal to unity for monomolecular reaction), Boltzmann’s constant (1.38 \(\times \) 10–23 Js−1) and Planck’s constant (6.626 \(\times \) 10–34 Js−1), respectively. By utilizing maximum decomposition temperature \(\left({T}_{max}\right)\) in these equations, values of considered thermodynamic parameters are determined (Tables 6 and 7). It is observed from these tables that \(\Delta S\) values are negative and values of \(\Delta G\) and \(\Delta H\) are positive for both the peaks. This reveals that the thermal decomposition of UHMWPE + Quercetin (0.4 wt%) is endergonic, non-spontaneous and unfavourable.

Conclusions

The thermal decomposition of UHMWPE shows three decomposition stages and Stage-II (~ 208–539 °C) is highly complex and active. Mixing of quercetin in UHMWPE matrix doesn’t alter the decomposition stages. However, the temperature regions of decomposition stages are varied. At low (up to 0.7 wt%) concentration, quercetin acts as an antioxidant in UHMWPE matrix whereas it behaves as a pro-oxidant at higher concentration (0.8–1.0 wt%). Further, at 0.4 wt% quercetin concentration thermal stability of UHMWPE is maximum. Complexities involved in Stage-II are resolved through deconvolution by splitting into two peaks. Obtained activation energies, through integral (Starink) and differential (Friedman) kinetic models, at different heating rates are higher for Peak 1 than Peak 2. Here, random nucleation reaction mechanism favours the thermal decomposition. Additionally, negative value of change in entropy \((\Delta S)\) and positive value of change in enthalpy \((\Delta H)\) and change in Gibbs free \((\Delta G)\) energy indicates that thermal decomposition of UHMWPE is non-spontaneous and endergonic. The study brings new prospects for optimizing minimal quercetin concentration to provide maximum thermal stability to UHMWPE for higher temperature applications.