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Micro Structural and Non-Isothermal Crystallization and Degradation Kinetics Studies on Manganese Thioglycolate End Capped Poly([epsilon]-Caprolactone).

INTRODUCTION

Poly(e-caprolactone) (PCL) is an aliphatic polyester widely used in bio-medical field and packaging sectors due to its high bio-compatibility, bio-degradability and thermoplasticity [1,2]. Some of its promising applications are scaffolds for tissue repairs, implants, sutures, drug delivery materials and vehicle membranes in the bio-medical field [3]. This type of polymer can be used in human because it never produces cytotoxicity byproducts during degradation [4]. However, the broad applications of PCL are restricted to some extent in terms of its low melting temperature, low abrasion, low modulus, poor barrier properties and relative high cost. PCL can be synthesized by various methods. The inherent properties of PCL can be improved by adding some inorganic or organic-inorganic hybrid nanofillers. The calcium methoxide ring opening polymerization (ROP) of CL was reported in the literature [5]. Liao and his research team [6] studied an organically modified clay initiated ROP of CL. Ti-alkoxide functionalized carbon nanotubes [7], Eosin Y [8], hydroxyl ethyl cellulose [9], salicylaldimine-Al complex [10], indolinospiropyran [11], and luminal [12] were used as an initiator in the preparation of PCL. We could not find any reports based on metal salt particularly Mn-TG initiated ROP of CL in the available literature so far. This motivated us to do the present work.

The thermal stability and thermal degradation behavior of PCL are receiving more importance besides its bio-degradability. Therefore, the perceptive of these characteristics are more useful for thermal recycling, processing and applications of PCL [13-15], Moreover, the understanding of crystallization behavior of PCL too is more important since it affects the degradation rate, bio-resorption, mechanical and physical properties of this polymer [16-18], The crystallization mechanism of PCL under nonisothermal condition was studied using Avrami, Ozawa and Liu models by Limwanich et al. [19] in 2015.Vackova et al. [20] reported about the crystallization kinetics of PCL/Ti[O.sub.2] based nanocomposites. The effect of molecular weight on the crystallization kinetics of PCL was reported by Jenkins and Harrison [21], The crystallization kinetics of PCL has also been reported by some other authors [13,21-23], Thermal degradation of PCL was investigated by many researchers [14,15]. Su et al. [24] studied thermal degradation behavior of PCL in nitrogen atmosphere using TGA technique. The effect of different nanoparticles (Si[O.sub.2], montmorillonite, multi-walled carbon nanotube) on the thermal degradation mechanism of in situ prepared PCL nanocomposites were investigated using classical kinetic models. The activation energies were determined for the prepared PCL composites [25], However, the studies related to the influence of end capping agent on the non-isothermal crystallization and degradation kinetics of PCL has not been reported much so far. In the present investigation, we report both non-isothermal crystallization and degradation behavior of metal salt end capped PCL in depth using various kinetic models. Hence, the present study is more useful to understand the crystallization and thermal degradation behavior of PCL under non-isothermal condition and this will be much more important for processing, application and thermal recycling of PCL. The Mn-TG is acting as a novel initiator for the ROP of CL. It is not only acting as an initiator but also acting as a nucleating agent. This can be evidenced by polarized optical microscopy (POM) results. But, the differential scanning calorimetry (DSC) results showed the decrease in Tc This can be explained on the basis of segmental mobility and semi-crystalline nature of PCL.

EXPERIMENTAL

Materials

All the purchased chemicals were of analytical grade and used without further purification for this work, [epsilon]-caprolactone (CL), stannous octoate (SO) and thioglycolic acid (ThGA) were purchased from Aldrich, India. Manganese carbonate (MnC03) was procured from S.D fine chemicals, India. Diethylether and chloroform were purchased from Spectrum chemicals, India.

Synthesis of ThGA decorated Mn Nanoparticle

To prepare ThGA decorataed Mn NP, the required quantity of MnC[O.sub.3] was suspended in 100 mL of deionized water under stirring condition. Then 150 mL of deionized water containing ThGA was added slowly to the above solution. The reaction was carried out in the presence of nitrogen for 30 min. The reaction mixture was turned into milky white slurry at the end of the reaction. The obtained white product was collected separately by filtration. Further, it was dried at 110[degrees]C in hot air oven for 6 hrs. Thus, the prepared white powder was ThGA decorated Mn NP and it is represented as Mn-TG. It was further stored under nitrogen atmosphere to avoid aerial oxidation.

Synthesis of ThGA decorated Mn NP end Capped PCL

An entail quantity of CL was taken in a 50 mL RB flask. The prepared ThGA decorated Mn NP was added to the above solution. They were mixed together for 10 min at room temperature under an inert atmosphere. This mixture was kept in an oil bath at 160[degrees]C for 2 h to initiate ROP of CL [8]. Then, it was removed from the oil bath and cooled to room temperature. The obtained solid mass was dissolved in 10 mL of chloroform for further purification purpose. Then it was re-precipitated by the addition of 200 mL of diethyl ether. The precipitate was dried under fume hood. Thus obtained white precipitate was ThGA decorated Mn NP end capped PCL. It was stored for further analytical studies.

Characterization of the Sample

Various analytical techniques were employed to analyze the synthesized ThGA decorated Mn NP end capped PCL. The Fourier transform infrared (FTIR) spectra was recorded using Shimadzu 8400S spectrometer, Japan make in the range of 400-4,000 [cm.sup.-1] in order to confirm the structure of functional groups present in PCL. The testing sample was prepared like a thin pellet by mixing 200 mg of spectral grade KBr by applying the pressure of 7 tons to record FTIR spectrum. The structure of the PCL was further analyzed by taking [sup.1]H and [sup.13]C nuclear magnetic resonance (NMR) spectra in CDC13 solvent using a Bruker Biospin high resolution digital 300 MHz NMR spectrometer, USA. The surface morphology of the PCL was investigated using Park XE7 atomic force microscopy (AFM). The particle size and morphology of the prepared PCL was executed using a JEOL 2100 tunnelling electron microscope (TEM). The sample was evacuated at 60[degrees]C before the high-resolution transmission electron microscopy (HRTEM) measurement. The microstructure of the prepared sample was analyzed to confirm the spherulitic growth of PCL using Olympus BX51 (POM). The thermal behavior such as melting and crystallization temperatures of the PCL was examined by using a Toledo DSC 822e DSC. 3-5 mg of samples were heated and cooled at five different heating and cooling rates from 10 to 30 [degrees]C/min in the temperature range of -50[degrees]C to 100[degrees]C under nitrogen atmosphere. The second heating and cooling scan was considered in order to erase the previous thermal history. The thermal stability and thermal degradation mechanism of PCL was assessed by using a TG/DTA 6200 thermal analyzer in the temperature range of 30[degrees]C-600[degrees]C at five different heating rates, 10-30[degrees]C/min under air atmosphere.

Non-Isothermal Crystallization Kinetics

The non-isothermal crystallization kinetics of TG decorated Mn NP end capped PCL was studied to comprehend the nucleation and actual dimensional crystal growth. The process of crystallization of polymer in molten state is always coupled with considerable amount of heat release, which can be measured accurately with the help of DSC [24]. Hence, the evolution of heat released during the crystallization process is linearly proportional to the evolution of crystallinity. The relative crystallinity ([X.sub.t]) is set as a function of crystallization temperature according to the following expression:

[mathematical expression not reproducible] (1)

where d[H.sub.c] is the enthalpy of crystallization at an infinitesimal time interval dt. [T.sub.0] and [T.sub.[infinity]] are the onset and end of the crystallization temperatures respectively. The Avrami equation-based kinetic models have been used to describe crystallization kinetics effectively by many researchers as it describes how nucleation propagates under non-isothermal condition. The non-isothermal crystallization process of TG decorated Mn NP end capped PCL can be studied by using the following Avrami equation:

1 - [X.sub.t] = exp (-[Z.sub.t][t.sup.n]) (2)

where [X.sub.t] stands for relative crystallinity at crystallization time, n is the Avrami exponent and [Z.sub.t] describes the crystallization rate constant. The kinetic parameters n and [Z.sub.t] will depend on nucleation and growth mechanism of crystals. The Eq. 2 can also be written in the following form:

ln[-ln(1-[X.sub.t])] = ln[Z.sub.t] + 1 1nt (3)

The non-isothermal crystallization process of PCL was attempted using the combined Avrami and Ozawa equation which is shown in Eq. 4:

log([PHI]]) = logF(T)-blog(t) (4)

where [PHI] represents the cooling rate, F(T) is crystallization rate constant and t refers to crystallization time and b is Ozawa exponent. The kinetic parameters F(T) and b will depend on the nucleation and growth mechanism of crystals respectively.

Kissinger suggested the following mathematical expression in order to find out energy of activation ([E.sub.a]) during the crystallization process. The crystallization peak temperature ([T.sub.c]) is measured from DSC curves at different cooling rates to make Kissinger plot.

d[log([psi]/[T.sup.2.sub.c)]/d(1/[T.sub.c]) = [E.sub.a]/R (5)

The following equation is obtained by integrating Eq. 5,

Log (psi]/[T.sup.2.sub.R] 1/[T.sub.c] (6)

where [E.sub.a] is the energy of activation, R denotes the universal gas constant, [T.sub.c] is the crystallization peak temperature at which reaction rate reaches maximum and [psi] is the cooling rate.

Non-Isothermal Degradation Kinetics

The kinetic parameters for the thermal degradation of prepared PCL were computed by non-isothermal approach with the help of three well known models such as Flynn-Wall-Ozawa (FWO), Kissinger and Auggis-Bennet kinetic analysis methods. The reaction conversion (a) can be determined by using the following mathematical expression:

[alpha] = [W.sub.o] - [W.sub.t]/w-[w.sub.f] (7)

where [W.sub.o] is the initial sample weight, [W.sub.t] is the weight at any temperature and [W.sub.f] is the sample weight at the end of the process.

FWO method

FWO method [24] is one of the integral methods used to analyze the degradation behavior of PCL under non-isothermal condition. The [E.sub.a] can be determined without knowing the reaction order and differential data with the help of this method. The proposed mathematical expression for this model is given as follows:

[E.sub.a] = -R/1.052 x [DELTA][lambda]v[beta]/1.052 [DELTA](1/T) (8)

where [E.sub.a] describes the activation energy, [beta] is the heating rate, R is the gas constant and T is the temperature. The plots of [beta] versus 1/T will give a straight line for each a value in this method. The activation energy can be determined from the slope value.

Kissinger Model

The thermal degradation process of PCL can also be studied by Kissinger model [26]. The reaction rate reaches its maximum at the thermal degradation temperature ([T.sub.d]) according to this model. The degree of conversion ([alpha]) is constant at [T.sub.d]. Nevertheless, the degree of conversion ([alpha]) at [T.sub.d] varies with the heating rate. The Kissinger equation is shown as follows:

ln ([beta]/[T.sup.2.d] = 1n (AR/E) = E/R[T.sub.d] (9)

The plot of 1n([beta]/[T.sup.2]d) versus l/[T.sub.d] is used to determine the activation energy ([E.sub.a]). This method also provides the value of A apart from the [E.sub.a] value.

Auggis-Bennet Model

It is also one of the important classical kinetic models. This model was proposed by Auggis and Bennet [27] to evaluate activation energy by using the following mathematical expression:

ln ([beta]/[T.sub.d] = E/R[T.sub.d] + lnA (10)

where [T.sub.d] is the degradation temperature. It can be determined from the differential thermal analysis curve. The [E.sub.a] value can be estimated from the slope of the straight line plot of ln([beta]/[T.sub.d]) versus 1/[T.sub.d].

Friedman Method

Friedman method [24] is known to be an iso-conversional method. It proposes the following mathematical expression to analyze the thermal degradation mechanism based on Arrhenius equation:

ln([d.sub.[alpha]]/dt)=ln [Qaf([alpha])] - [E.sub.a]/RT (11)

where R is the gas constant, Q is the total heat released by the reaction, [alpha] is the conversion rate at time t and T is the temperature. The slope value of [E.sub.a]/R can be determined from the straight line plot of In(da/dt) versus 1/T. It also provides the [E.sub.a] values at different stages of degradation reaction.

RESULTS AND DISCUSSION

The main aim of this investigation is to find out the amount of energy consumed by the Mn-TG end capped PCL for its nonisothermal crystallization process and non-isothermal degradation process through various kinetic models. Meanwhile, the synthesized Mn-TG end capped PCL was characterized by FTIR, NMR, HRTEM and AFM like analytical tools. Structural, thermal and surface morphology data of Mn-TG end capped PCL is required for the bio-medical field particularly in drug delivery application.

FTIR Study

Figure 1 a displays the FTIR spectrum of ThGA decorated Mn NP. The -OH stretching vibration is observed at 3,471 [cm.sup.-1]. The characteristic peaks assigned at 2,858 and 2,943 [cm.sup.-1] are observed to be C-H stretching and anti-symmetric stretching vibrations respectively. The carbonyl stretching (C=O) is seen at 1,741 [cm.sup.-1]. The C-H bending vibration is appeared at 1,556 [cm.sup.-1]. The other peaks at 1,410 and 1,173 [cm.sup.-1] in the FTIR spectrum are found to be C-S and C-O-C stretching vibrations of TG. The C-H out of plane bending vibration (OPBV) of TGA is noticed at 730 [cm.sup.-1]. The M-O stretching is seen at 628 [cm.sup.-1]. FTIR spectrum of Mn-TG initiated ROP of CL is shown in Fig. 1b. Peaks observed at 2,864 and 2,944 [cm.sup.-1] are due to the C-H stretching and anti-symmetric stretching vibrations of PCL. The carbonyl group (C=0) of PCL is appeared at 1,722 [cm.sup.-1] [28], A sharp peak at 1,180 [cm.sup.-1] is found to be the C-O-C ester linkage of PCL. A peak at 729 [cm.sup.-1] is attributed to the C-H OPBV of PCL. The presence of carbonyl and C-H vibration are used to confirm the PCL formation.

NMR study

The [sup.1]H-NMR and [sup.13]C-NMR spectra were used to confirm the chemical structure of TG decorated Mn NP end capped PCL. Figure 2a illustrates the [sup.1]H-NMR spectrum of TG decorated Mn NP end capped PCL. The solvent peak of CD[Cl.sub.3] is appeared at 7.2 ppm. A peak assigned at 4.1 ppm is due to the presence of alkoxy proton [28]. The methylene proton adjacent to carbonyl group is noticed at 2.3 ppm [28]. The other protons of PCL are seen between 1.38 and 1.68 ppm in the spectrum. All the existing peaks in the NMR spectrum notably confirmed the actual structure of PCL. Figure 2b depicts the [sup.13]-NMR spectrum of TG decorated Mn NP end capped PCL. The triplet peak lying between 76.5 and 77.4 ppm in the spectrum is noticed to be solvent signal. The peak at 173.3 ppm is linked with the carbon signal of carbonyl group of PCL. The peak at 64.4 ppm is seen to be alkoxy carbon signal. The methylene carbon lying nearer to the carbonyl group is perceived at 34.02 ppm [28]. All the carbon signals in the spectrum authentically confirmed the actual structure of PCL.

HRTEM Study

The TEM image of ThGA decorated Mn NP end capped PCL is shown in Fig. 3a. It confirms the presence of TG decorated metal salt in PCL matrix. The size of the Mn salt is varied from 10 to 25 nm which confirms that the metal NP is in the nanosize. The shape of the metal salt is more or less spherical in shape. The particles of Mn salt are agglomerated due to their high surface energy. Also, the grains in the micrograph seem to have good interconnectivity between them. Figure 3b depicts the selected area of electron diffraction (SAED) pattern of Mn-TG end capped PCL. The appearance of the concentric circles in TEM image is due to the crystalline nature of PCL.

AFM Study

The surface topography of the sample was further analyzed with the help of AFM technique. The main aim of this study is to understand the nature of material surface. The functional performance of the material mainly depends on the surface texture. The 2D and 3D micrographs are analyzed in order to assess the roughness and grain orientation of PCL. Figure 4a and b depicts the 2D and 3D AFM images of Mn-TG end capped PCL system. The yellow region in the micrograph is the PCL matrix and the presence of sharp white peaks is due to the incorporated ThGA decorated metal salt. The AFM images revealed that nano-sized metal salts are dispersed uniformly throughout the polymer matrix. Similar to TEM result, the particles seem to be agglomerated due to high surface energy. Moreover, grains possess different irregular shape, spacing and size. The particle size of the metal is found to be <25 nm (Fig. 4c). The root mean square and average surface roughness were measured as 8.9 and 6.8 nm, respectively from the 2D micrograph. It can be seen that the surface has both peaks and valleys. However, the valleys are more predominant than peaks in the micrograph of PCL which is further confirmed by the obtained negative Skewness moment (-1.276). It can be understood well while comparing 3D image of AFM with 2D. Moreover, the distribution of peak's height is somehow symmetrical in the 3D micrograph.

DSC Profile

The DSC measurement was performed to analyze the melting temperature ([T.sub.m]) and crystallization temperature ([T.sub.c]) of PCL. The polymer sample was subjected to thermal treatment both heating and cooling in the temperature range of -30[degrees]C to 100[degrees]C at different heating rate of 10, 15, 20, 25, and 30[degrees]C/min in nitrogen atmosphere during the analysis. The main purpose of this measurement is to know the crystallization behavior of the prepared ThGA decorated Mn NP end capped PCL under non-isothermal condition. Figure 5a-e describes the DSC heating scan of PCL system at different heating rates. The[T.sub.m]of PCL notably rises from 66.8[degrees]C to 73.9[degrees]C with increasing heating rates. This is due to the fast scanning rate of the sample. This is in good agreement with the literature report [29], This is further confirmed by plotting the heating rate against [T.sub.m] of Mn-TG end capped PCL as shown in Fig. 5f. The reason for this is the amount of energy needed to melt the substance is decreased with increasing heating rates. The polymer chains may possibly get relaxed due to the faster scanning process. Figure 5g-k shows the crystallization exotherms of PCL at different cooling rates of 10, 15, 20, 25, and 30[degrees]C/min respectively. It was revealed that the [T.sub.c] value shifted towards the lower temperature from 37.5[degrees]C to 31.6[degrees]C with the increasing cooling rates. It is further confirmed by plotting [T.sub.c] against cooling rate as shown in Fig. 51. All the acquired data related to the DSC measurement is mentioned in Table 1.

In this investigation, non-isothermal crystallization and nonisothermal degradation kinetic studies were done by following the universally accepted procedure [30-32], The amount of energy required for the crystallization and degradation of Mn-TG end capped PCL was measured and compared with the literature value.

non-Isothermal Crystallization Kinetics

The non-isothermal crystallization kinetics of PCL was mainly carried out to predict crystallization rates and crystal shapes as they are used to define the structure and properties of the materials. PCL is one of the most important and widely used biodegradable thermoplastic polymers. Hence, it is really essential to study about the non-isothermal process of PCL under nonisothermal conditions which are closer to industrial processing conditions. The degree of crystallinity ([X.sub.c]), crystallization peak temperature ([T.sub.c]) and crystallization enthalpy ([DELTA][H.sub.c]) were measured from the crystallization exotherms of PCL at various cooling rates (Fig. 5g-k). It was observed that [T.sub.c] value notably shifts towards lower temperature with the increasing cooling rates.

The non-isothermal crystallization kinetics of PCL was studied using Avrami equation as shown in Eq. 3. The plot of ln[-ln(1-[X.sub.t])] versus ln(r) was made to determine the Avrami exponent (n) and crystallization rate constant ([Z.sub.t]) as given in Fig. 6a-e. The plot showed a straight line for all the five cooling rates. Hence, the Avrami equation is valid for the PCL. The Avrami exponent (n) and the crystallization rate constant ([Z.sub.t]) were measured from the slope and intercept of the plots. The kinetic parameters, n and Zt are temperature dependent. The heating rate will be changed continuously during non-isothermal crystallization process. Hence, this will affect the rate of nuclei formation and spherulitic growth. The n values were estimated to be in the range of 2.81-3.08 which clearly indicates a three dimensional spherulitic crystal growth. The spherulitic growth of PCL was further confirmed by polarized optical micrograph as shown in Fig. 7. The nucleated crystals were found to be in the spherulitic shape. The [Z.sub.t] values were calculated as 0.25-0.91 for various cooling rates.

The combined equation of Avrami and Ozawa is also used to study the non-isothermal crystallization kinetics of PCL as given in Eq. 4. The crystallization rate constant (F[T\) and Ozawa exponent (b) were estimated from the plot of log([PHI]) versus log(t) as mentioned in Fig. 6f. The F(T) and b values were determined as 1.5 and 3.5, respectively. The obtained b value is above 3 suggesting the 3D spherulitic crystal growth [33], The captured POM images during the crystallization process also showed the same spherulitic growth of PCL (Fig. 7). Both Avrami and combined Avrami-Ozawa models revealed the same 3D type of crystal growth of PCL.

The activation energy (Ea) of crystallization was derived under various cooling rates using Kissinger model as given in Eq. 1. The energy required for the transportation of molecules from a molten state to an interface of growing crystal can be determined by using this model. Figure 8a displays the Kissinger plot of ln([PHI]/[T.sup.2.sub.c]) versus [T.sub.c] for Mn-TG NP end capped PCL. The calculated [E.sub.a] value was found to be 149.09 kJ [mol.sup.-1]. The obtained [E.sub.a] value is higher when compared with pure PCL (142 kJ [mol.sup.-1]) [34], This is due to the incorporation of nano-sized ThGA decorated Mn into PCL chain. It restricts the transportation ability of polymer chains during crystallization process resulting with higher [E.sub.a] value. The crystallization occurs at a higher temperature since there is sufficient time to activate nucleation when the cooling rate is slower. The crystallization rate coefficient (CRC) is determined by plotting crystallization temperature ([T.sub.c]) against cooling rate ([PHI]). This CRC value was determined from the slope of the plot of [PHI] versus [T.sub.c] as shown in Fig. 8b. The slope value was determined to be -0.3. The rate of crystallization is faster for the polymer with the shorter repeating units than a polymer with longer or branched repeating units. The plot of [T.sub.m] against [T.sub.c] shows a linear relation between them as shown in Fig. 8c.

TGA Profile

The thermal stability of ThGA decorated Mn NP end capped PCL was inspected using TGA technique. The PCL was heated at different heating rates such as 10, 15, 20, 25, and 30[degrees]C/min in air atmosphere to assess its thermal degradation behavior. The obtained TGA curves for all the five heating rates are shown in Fig. 9a-e. It was observed that all the obtained TGA curves exhibited single step degradation process which took place between 270[degrees]C and 382[degrees]C. The decomposition temperature (Td) of PCL was found to be increased while increasing the heating rate from 10 to 30[degrees]C/min. The similar trend was observed for initial and final decomposition temperatures of PCL with the rising heating rate [35], The major weight loss accompanied with the breaking of PCL backbone started around 270[degrees]C for the heating rate of 10[degrees]C/min. The % weight residue remains above 470[degrees]C was increased from 2.32% to 10.4% with the increasing heat rate. Figure 9f-j displays the derivative thermogram. Here, the degradation temperature ([T.sub.d]) is shifted to higher temperature while increasing the heating rate.

Non-Isothermal Degradation Kinetics

The thermal degradation of ThGA decorated Mn NP end capped PCL was analyzed using TGA measurements at five different heating rates such as 10, 15, 20, 25, and 30[degrees]C [min.sup.-1] under air atmosphere. The [E.sub.a] for the degradation of PCL was determined with the help of model free iso-conversional kinetic methods. The TGA thermograms of PCL as well as their derivative plots with five heating rates are displayed in Fig. 9a-j. It was observed that the temperature at which maximum weight loss rate occurred on the TGA curve was considerably shifted to higher temperature with the increasing heating rates from 10 to 30[degrees]C/min. The peak area of the derivative plots was increased in accordance with increasing heating rates.

The apparent rise in degradation temperature ([T.sub.d]) with respect to the heating rates is shown in Fig. 10a. Hence, it was concluded that [T.sub.d] is increased proportionally with the increasing heating rates. The [E.sub.a] for the thermal degradation of PCL was calculated by the well-known non-isothermal kinetics models [35]. In FWO model, the [E.sub.a] value was estimated as 135.8 kJ [mol.sup.-1] from the plot of ln[beta] versus 1,000/[T.sub.d] as shown in Fig. 10b. In Auggis-Bennet method, the [E.sub.a] value was determined as 130.7 kJ [mol.sup.-1] from the plot of ln([beta]/[T.sub.d]) versus 1,000/[T.sub.d] as shown in Fig. 10c. The plot of ln([beta]/[T.sup.2.sub.d]) versus 1,000/[T.sub.d] was made to estimate [E.sub.a] value associated with degradation process which was found to be 125.77 kJ [mol.sup.-1] by Kissinger method as shown in Fig. lOd. It was noticed that the E.d values were found to be within the error limit only. Kissinger method yielded a low [E.sub.a] value among the three models. This is because of the denominator on its Y-axis caption. In comparison, the system consumed 1.18 time higher [E.sub.a] value for crystallization rather than the degradation.

The [E.sub.a] was further calculated for the degradation of the MnTG end capped PCL system by Friedman method. The [E.sub.a] was determined from the plot of ln(d[alpha]/dt) versus 1/T for Friedman method as shown in Fig. 1 la-e. It was found to be in the range of 76.6-111.2 kJ [mol.sup.-1] for different heating rates. The decreasing trend was noticed from the plots for all heating rates. Figure 11 f indicates the plot of [E.sub.a] against reaction extent which confirms the linear plot.

CONCLUSION

The Mn-TG end capped PCL was prepared by bulk polymerization method. The presence of carbonyl group at 1,722 [cm.sup.-1] and C-O-C ester linkage in the FTIR spectrum was used to confirm the functionality of PCL. The assigned peaks at 4.1 ppm for alkoxy proton and at 2.3 ppm for methylene proton in [sup.1]H-NMR spectrum as well as the assigned peaks at 173.3 ppm for carbon signal of carbonyl group and at 64.4 ppm of alkoxy carbon signal in [sup.13]C-NMR spectrum were also used to confirm the chemical structure of PCL. The particle size of metal salt was found to be in the nano range (10-25 nm) by TEM. This is further confirmed with the help of AFM. The gradual decrease in crystallization temperature of PCL in accordance with rising cooling rates was observed. The POM images confirmed the spherulitic growth of PCL. The TGA thermogram exhibited single step degradation process. The degradation temperature ([T.sub.d]) of PCL was shifted to higher temperature while increasing the heating rate. The [E.sub.a] for the degradation of polymer under non-isothermal condition was calculated by using different classical kinetics models.
SYMBOLS AND ABBREVIATIONS

ThGA        thioglycolicacid
PCL         Poly(e-caprolactone)
FTIR        Fourier Transform Infrared
KBr         Potassium bromide
SO          stannous octoate
NMR         nuclear magnetic resonance
[E.sub.a]   energy of activation
HRTEM       high resolution transmission electron microscopy
DSC         differential scanning calorimetry
TGA         thermogravimetry analysis
ROP         ring opening polymerization
CL          [epsilon]-caprolactone
POM         polarized optical microscopy
NP          nanoparticle
[T.sub.m]   melt transition temperature
[T.sub.c]   crystallization temperature
CRC         crystallization rate coefficient
[T.sub.d]   degradation temperature
Mn-TG       thioglycolicacid functionalized Manganese


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Caption: FIG. 1. (a) FTIR spectrum of ThGA decorated Mn nanoparticles. (b) FTIR spectrum of Mn-TG end capped PCL. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 2. (a) [sup.1]H-NMR spectrum (b) 13C-NMR spectrum of Mn-TG end capped PCL. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 3. (a) HRTEM image (b) SAED pattern of Mn-TG end capped PCL system.

Caption: FIG. 4. (a) 2D AFM image (b) 3D AFM image (10 pm) (c) 3D AFM image (2.5 pm) of Mn-TG end capped PCL system. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 5. (a-e) DSC heating scan (f) plot of heating rate against [T.sub.m] (g-k) cooling scan (1) plot of cooling rate against [T.sub.c] of Mn-TG end capped PCL system. [Color figure can be viewed at wileyonlinelibrary.coml

Caption: FIG. 6. (a-e) Avrami plot for Mn-TG end capped PCL system at the cooling rates of 10[degrees]C, 15[degrees]C, 20[degrees]C, 25[degrees]C, and 30[degrees]C [min.sup.-1] (f) Avrami-Ozawa plot. [Color figure can be viewed at wileyonlinelibrary.coml

Caption: FIG. 7. Polarized optical microscope image of Mn-TG end capped PCL system [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 8. (a) Kissinger [E.sub.a] plot (b) CRC plot (c) the plot of [T.sub.m] against [T.sub.c] for Mn-TG end capped PCL system. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 9. (a-e) TGA (f-j) derivative thermograms of Mn-TG end capped PCL system at five different heating rates. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 10. (a) Effect of heating rate on the [T.sub.d] of Mn-TG end capped PCL system, (b) FWO plot, (c) Auggis-Bennet plot, (d) Kissinger plot for Mn-TG end capped PCL system. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 11. (a-e) Friedman plot for Mn-TG end capped PCL system at the heating rates of 10, 15, 20, 25, and 30[degrees]C/min (f) plot of [E.sub.a] against reaction extent. [Color figure can be viewed at wileyonlinelibrary.com]
TABLE 1. DSC data of Mn-TG end capped PCL system.

Heating rate     [T.sub.c]     [DELTA][H.sub.c]    [X.sub.c]
([degrees]C)    ([degrees]C)     ([Jg.sup.-1])        (%)

10                  37.5             72.1             53
15                  35.7             73.8            54.2
20                  34.6             72.4            53.2
25                  32.7             74.2            54.5
30                  31.6             77.5            56.9

Heating rate    [E.sub.a]     [T.sub.m]     [DELTA][H.sub.m]
([degrees]C)      kJ/mol     ([degrees]C)     ([Jg.sup.-1])

10                 651           66.8             115.1
15                               69.4             110.9
20                               69.5             111.4
25                               72.7             114.5
30                               73.9             117.5

Heating rate           [DELTA]T
([degrees]C)    ([T.sub.m]-[T.sub.c])
                     ([degrees]C)

10                       29.3
15                       33.7
20                       34.9
25                        40
30                       42.3

[X.sub.c], ([DELTA][H.sub.f]/[DELTA][H.sup.*.sub.f]) X 100,
([DELTA][H.sup.*.sub.f] = 136 J/g).
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Author:Mahalakshmi, S.; Alagesan, T.; Parthasarathy, V.; Anbarasan, R.
Publication:Polymer Engineering and Science
Article Type:Report
Date:Mar 1, 2019
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