1 Introduction

Poly(ε-caprolactone) (PCL) is a well known bio-medical candidate due to its bio-compatibility, bio-degradability, low cytotoxicity and low melting temperature (Tm). Generally, as we have described previously [1,2,3] PCL is synthesized through the ring opening polymerization (ROP) of CL in the presence of an initiator and a catalyst. In order to initiate the ROP of CL we used various functionalities, such as –OH, –NH2, –CO2H, –SO3H, –SH, –CONH2; among them the –OH group exhibited the highest yield. For instance, Oshimura and Takasu [4] reported the controlled ROP of CL in the presence of sulphonimide as a chemical initiator. Functionalized PCL was synthesized in the presence of stannous octoate (SO) as a catalyst [5]. In 2015, Hemmati et al. [6] synthesized PCL for bio-medical applications using propargyl alcohol as a chemical initiator. PCL was also synthesized in the presence of a cross linking agent and its enzymatic degradation studied [7]. Aminoacids initiated ROP of CL has also been reported in the literature [8]. Octadecanoic acid was used as an initiator for the ROP of CL [9]. Tributyltin alkoxides initiated ROP of CL was carried out and its non-isothermal degradation kinetics studied [10]. Microwave assisted methacrylic acid initiated ROP of CL was reported in the literature by Koopmans et al. [11]. The graphene oxide nanosheet initiated ROP of CL was reported by Wang and his research team [12]. Samarium acetate as a chemical initiator for the ROP of CL has been reported in the literature [13]. Benzyl alcohol [14] and luminol [15] were also used for the ROP of CL. Meenarathi et al. [16] used near infrared dye grafted MWCNT system, Fe3O4/acidfuchsin dye [17], Ag/acidfuchsin dye [18], bromophenol blue dye [19] and Fe3O4/alizarinred dye [20] initiators for the ROP of CL. However, despite a thorough literature survey, we could not find any report describing a non-isothermal degradation kinetics study of magnetic and fluorescent PCL. Because of their potential usage for various bio-medical purposes this leads the authors to do the present investigation.

Since PCL is used in the bio-medical engineering field it is necessary to find out its thermal properties, particularly its thermal degradation behavior. The non-isothermal degradation kinetics of PCL based triblock copolymer was reported within the last year [21]. In addition the thermal degradation kinetics of blends of poly(trimethylene carbonate) with PCL was studied by Marquez et al. [22]. Aoyagi et al. [23] reported the degradation kinetics of PCL and its blend with PLLA and poly(hydroxybutyrate). Su et al. [24] published their results on the degradation kinetics of PCL with the Ea value of 70 kJ/mol. Sivalingam et al. [25] also studied the thermal degradation kinetics of PCL. Our literature survey indicated that there have been no research publications are available on the degradation kinetics of PCL/Fe3O4–CR nanocomposite system. The novelty of the present investigation is the study of the influence of the chemical structure of two dyes methyl orange (MO) and CR on the thermal degradation behavior of PCL. The chemical structure of dye plays a vital role on the thermal degradation behavior of PCL. The MO contains one tertiary amino group, one azo group and one –SO3Na group whereas the CR contains two azo groups, two amino groups and two –SO3Na groups. Hence, the chemical structure of dye can influence the thermal degradation behavior of PCL. Apart from their fluorescent nature, they can initiate the ROP of CL. For these reasons it is necessary to study the influence of chemical structure of dyes on the thermal and spectral behavior of PCL. The main aim of the present investigation is to study the influence of chemical structure of dyes on the thermal degradation behavior of PCL.

2 Experimental

2.1 Materials

Ferric chloride (FeCl3), ferrous sulphate (FeSO4), sodium hydroxide (NaOH), methyl orange (MO) and congo red (CR) dye were purchased from Nice Chemicals private limited, India. Double distilled water (DDW) was used for making solutions. ε-caprolactone (CL, monomer) and stannous octoate (SO, catalyst) were purchased from Sigma Aldrich company limited, USA and used as-received. Chloroform and diethyl ether were purchased from Spectrum Chemicals private limited, India.

2.2 Synthesis of CR functionalized Fe3O4

10 g FeCl3 and 5 g FeSO4 each were dissolved in 100 mL DDW separately; then the two solutions were mixed at room temperature under vigorous stirring conditions. After the vigorous stirring for 5 min, a NaOH solution was added drop by drop (10 g NaOH dissolved in 100 mL DDW) till the attainment of a strongly alkaline pH (ca 12.5). The addition process was continued after the alkaline pH for the next 2 h. The Fe3O4 formation was confirmed by the appearance of a dark colored precipitate (Scheme 1) [26, 27]. At the end of reaction the reactants were transferred to a 500 mL beaker and put on a magnetic bar and kept for 5 min. The black colored precipitate settled down and the remaining supernatant solutions were pipetted out. 50 mL of fresh DDW was added to the Fe3O4 nanocrystals, stirred well and kept on the magnetic bar for 5 min. In the same way the process of purification was continued 3 times. The excess NaOH, FeSO4 and FeCl3 were removed by the repeated washing and the precipitate dried at 110 °C for 8 h. The thus obtained fine black powder is known as Ferrite (Fe3O4). The thus obtained Fe3O4 nanocrystals were ground and stored in a zipper lock polyethylene bag. In the case of CR or MO dye functionalized Fe3O4 synthesis, the above said procedure was followed in the presence of 5 g of CR or MO dye powder in 100 mL DDW without adding NaOH. The end product then was a dark brown powder the CR functionalized Fe3O4 nanohybrid system [18]. A similar procedure was repeated for the preparation of Fe3O4/MO nanohybrid system.

Scheme 1
scheme 1

Synthesis of Fe3O4–CR end capped PCL

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2.3 Synthesis of poly(ɛ-caprolactone)/Fe3O4–CR nanocomposite

1 g CL monomer was accurately weighed in a 25 mL capacity round bottomed flask (RBF). 0.001 g of SO catalyst was added into the RBF. 0.015 g of Fe3O4–CR or Fe3O4–MO nanohybrid initiator was accurately weighed and mixed with the RBF. The ROP of the CL was carried out at five different mass loadings [M] of CL 0.5, 1.0, 1.5, 2.0 and 2.5 g. Here the ratio between the monomer and catalyst, [M/C] was maintained at 1000. After the physical mixing, the RBF was kept in an oil bath at 160 °C and heated under N2 atmosphere for 2 h (Scheme 1). During the course of the reaction the color of the Fe3O4–CR changed and it became a highly viscous mass [18]. Even though, the ROP of the CL was carried out by a bulk polymerization method, it was further purified by a re-precipitation method. The highly viscous mass was dissolved in 25 mL chloroform and re-precipitated by the addition of 250 mL of diethyl ether. The total content was transferred into a 500 mL beaker and evaporated to dryness under fume hood for overnight. After the evaporation and the aerial drying process, a dark brown color product was obtained. This was again weighed and stored in a zipper lock bag and named as Fe3O4/CR end capped PCL. The mechanism behind the initiation of ROP of CL can be explained on the basis of co-ordination insertion mechanism. In the present investigation, SO was used as a catalyst for the ROP of CL and this co-ordinated with the –NH2 group of Fe3O4/CR nanohybrid system in the first step. The ROP of CL was carried out at 160 °C under N2 atmosphere for 2 h. Now this nanohybrid is acting as a heterogeneous initiation system because the added initiator is not completely soluble in CL monomer due to difference in melting and boiling points. In the second step, the co-ordinated nanohybrid is interacted with the CL through –C–O–C linkage. In such a way the ring structure of CL was opened and one hydrogen atom from the –NH2 group of nanohybrid was transferred to the PCL chain end. Finally, it produced the PCL/Fe3O4–CR nanocomposite system. A similar mechanism was followed by the PCL/Fe3O4–MO system.

2.4 Characterization techniques

The UV–visible spectrum was measured by dissolving the sample in CHCl3 solvent. The instrument used here was a 3600 NIR spectrophotometer, (Shimadzu Corporation, Japan) from 200 to 800 nm. The same sample solution was subjected to fluorescence emission measurement from 350 to 700 nm by using a SL174 (Elico Analytical Instruments, India) instrument. The FTIR spectrum was recorded with the help of a 8400 S (Shimadzu Corporation, Japan) model instrument by a KBr pelletization method from 400 to 4000 cm−1. 3 mg polymer was ground with 200 mg of spectral grade KBr and made into a disc under the pressure of 7 tons. Field emission scanning electron microscopy (FESEM) (better resolution at higher magnification in the whole range of accelerating voltages even for beam sensitive specimens are the advantages of FESEM over SEM) was used to examine the morphological behaviour of the polymer with the help of S4800 (FESEM-Hitachi Japan) instrument. The % compositions of the elements present in the PCL were determined by electron dispersive energy (EDX, thermal source). The surface morphology of the samples were characterized by JSM 6300, JEOL product, SEM, USA instrument. The PCL semi-crystalline powder sample was subjected to gold coating under vacuum for 15 min followed by SEM imaging. The melting temperatures (Tm) of the polymer samples were determined by using a Thermal Analyst 2000 Differential Scanning Calorimeter 910S, (Dupont, USA) model instrument. All the measurements were done under N2 atmosphere in a temperature range of RT to 100 °C with 10 °C/min heating rate. Thermal stabilities of the polymer samples were measured by 951 thermogravimetric analyzer (Dupont, USA). Thermograms were recorded under an air atmosphere in a temperature range of 30–800 °C at five different heating rates 5, 10, 15, 20 and 25 °C/min. X-ray diffraction was carried out using an advanced scanning instrument from 2θ value of 2°–60° at the scanning rate of 2°/min (XRD, BrukerXS08 model instrument). Molecular weight of the polymer samples were carried out using gel permeation chromatography (GPC), (Perkin Elmer Series 200) (USA) in THF solvent and poly(styrene) as an internal standard.

From the FTIR spectrum the relative intensities were calculated, from which the order of reaction was determined using Eq. (1),

$${\text{FTIR-RI}}\,{\text{of}}\,[{\text{C}}{=}{\text{O/C}}{-}{\text{H}}] = {\text{A}}_{1726} / {\text{A}}_{730}$$
(1)

where A1726 is the corrected peak area of a peak at 1726 cm−1 and A730 is the corrected peak area at 730 cm−1.

The rate of polymerization (Rp) was calculated from the absorption value of UV–visible spectra and fluorescence emission intensity value of fluorescence emission spectra, as given in Eq. (2).

$${\text{Rate}}\,{\text{of}}\,{\text{polymerization}}\,\left( {\text{Rp}} \right) \, = \frac{{{\text{Abs}} .\,\left( {\text{or}} \right)\,{\text{FEI}}}}{\text{VtM.W}} \times 1000$$
(2)

where V, is total volume of the reactants; t, reaction time in seconds; M.W, molecular weight of CL, monomer.

3 Results and discussion

The results and discussion part of the present investigation is subdivided into four parts namely, (1) characterization of Fe3O4–CR nanohybrids, (2) characterization of PCL/Fe3O4–CR nanocomposites, (3) non-isothermal degradation kinetics of the PCL/Fe3O4–CR nanocomposite system, (4) non-isothermal degradation kinetics of the PCL/Fe3O4–MO nanocomposite system.

3.1 Characterization of the Fe3O4–CR nanohybrid system

3.1.1 FTIR study

Characterization results for the above synthesized Fe3O4–CR nanohybrid system are shown in Fig. 1 as characterized by various analytical techniques; FTIR, UV–visible, FES, VSM and FESEM. The FTIR spectrum of the pristine Fe3O4 is shown in Fig. 1a. The broad –OH stretching appearing at 3400 cm−1 is due to the hydroxyl stretching vibrations of the intercalated water molecules. A twin peak appeared at 576 and 619 cm−1 corresponded to the metal oxide (M–O) stretching vibration of Fe3O4. This is in accordance with an earlier report from our research team [26]. Figure 1b is ascribed to the FTIR spectrum of Fe3O4–CR nanohybrid system. Here also one can see the –OH and M–O stretching bands corresponding to the Fe3O4–CR nanohybrid system. A small peak appeared at 1727 cm−1 is due to the C=O stretching of a carbonate. The C–N stretching of the CR dye was observed at 1359 cm−1 and the aromatic bending vibrations of CR dye appeared at 699, 753 and 839 cm−1 [1, 28]. The –SO2 and –SO3−Na+ stretchings [18] of CR appeared at 1070, 1166 and 1231 cm−1 respectively. A sharp peak at 1450 cm−1 can be explained on the basis of N=N structure of CR dye. The p-disubstituted benzenoid structure of CR dye was observed at 1590 cm−1. Thus, the appearance of new peaks confirmed the existence of strong interactions between the CR dye and Fe3O4 through the amino groups of the CR. Hence, the FTIR spectrum confirmed the functional groups present in the Fe3O4–CR nanohybrid system.

Fig. 1
figure 1

FTIR spectrum of a Fe3O4, b Fe3O4–CR nanohybrid, UV–visible spectrum of c CR, d Fe3O4–CR nanohybrid, Fluorescence emission spectrum of e CR, f Fe3O4–CR nanohybrid, VSM loops of g Fe3O4, h Fe3O4–CR nanohybrid, i PCL/Fe3O4–CR nanocomposite system

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3.1.2 UV–visible study

Figure 1c, d represent the UV–visible spectrum of CR before and after the nanohybrid formation, respectively. The UV–visible spectrum of CR (Fig. 1c) exhibited two peaks, at 446 nm (corresponding to the monomeric form of CR dye) and 292.8 nm (corresponding to the dimeric form of CR dye). Recently, we reported [29] the UV–visible spectrum of CR dye before and after chemical grafting reaction. The nanohybrid (Fig. 1d) again exhibited two peaks (499.2 and 343.2 nm), corresponding to the monomeric and dimeric structures of CR with red shifting. The red shift by 53 nm is attributed to the existence of chemical interaction between Fe3O4 and the amino groups of the CR dye. If the –SO3Na group of the CR dye interacts with the Fe3O4 surface automatically the structure of Fe3O4 will be degraded due to the very high acidic pH and more electronegativity nature.

3.1.3 Fluorescence spectroscopy study

The advantage of using dye moieties (as a fluorescence probe in bio-medical engineering field) is its fluorescence activity. The CR dye has fluorescence property even after the nanohybrid formation with Fe3O4 and nanocomposite formation with PCL. Hence it is necessary to study the fluorescence property with the help of fluorescence emission spectrum measured under quantitative conditions. Figure 1e indicates the fluorescence emission spectrum of pure CR dye at the wavelength of 523 nm. After the nanohybrid formation the emission peak was blue shifted to 520.7 nm (Fig. 1f) due to the decrease in crystal size [29]. This can be explained as follows: the size of the Fe3O4 crystals was reduced due to the surface functionalization by CR dye. As a result of an increase in Surface Plasmon Resonance (SPR) effect, the electronic clouds on ferrite were extended to the CR dye. Such an electronic cloud will reduce the crystal size of the Fe3O4. The reduction in size of the Fe3O4 crystal is further supported by FESEM analysis.

3.1.4 VSM study

The first and foremost characterization technique for magnetic material is the VSM technique. Figure 1g, h indicate the VSM loops of pristine Fe3O4 and its nanohybrid, respectively. Before the surface functionalization reaction, the Fe3O4 exhibited a magnetic moment value of 62.0 emu/g. For the same Fe3O4 after the structural modification with CR dye, the VSM value was suppressed to 55.1 emu/g. The decrease in VSM value confirmed the existence of chemical interactions between Fe3O4 surface and CR dye molecules. We noted a similar observation [26] due to the surface functionalization and encapsulation effects offered by a drug molecule towards ferrite. Figure 1i shows the VSM loop of the PCL/Fe3O4–CR nanocomposite system; it will be discussed in the following sections.

3.1.5 FESEM study

Figure 2a shows the FESEM image of the nano Fe3O4, with spherical and rod shapes. The sizes of the sphere were calculated as approximately 100 nm. The important point noted here is the synthesis of ferrite under strongly alkaline condition leads to the formation of different size and shape (non homogeneity) particles. Figure 2b indicates the FESEM image of the Fe3O4–CR nanohybrid system. The preparation procedure is mentioned in the experimental part. It was found that the size of the Fe3O4 nanoparticles were reduced or compressed. The average size of the Fe3O4 nanoparticles was determined as 22 nm. The decrease in particle size with homogeneity confirmed the existence of chemical interaction between Fe3O4 and CR dye. Figure 2b also shows a nanosquare and nanocube. This must have been formed due to the agglomerisation of nanorods under the influence of the CR dye. From the FESEM analysis we concluded that after the nanohybrid formation the size of the Fe3O4 could be reduced as we also showed previously [18, 26].

Fig. 2
figure 2

FESEM image of a Fe3O4 and b Fe3O4–CR nanohybrid, XPS of c Fe3O4 and d Fe3O4/CR nanohybrid system

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3.1.6 XPS study

The interaction between Fe3O4 and CR dye were confirmed by XPS. Figure 2c indicates the XPS of pristine Fe3O4. The spectrum showed peaks corresponding to O1s (532.2 eV), Fe2p3/2(712.7 eV), Fe2p1/2 (716.5 eV) and Fe3p (54.2 eV). Figure 2d represents the XPS of Fe3O4/CR nanohybrid system. Here also one can observe the above mentioned peaks with some new peaks such as S2p (164.1 eV), C1s (286.7 eV), N1s (397.3 eV) and Na1s (1067.4 eV) due to the added CR dye. Thus the XPS confirmed the nanohybrid formation between Fe3O4 and CR dye.

3.2 Characterization of poly(ε-caprolactone)/Fe3O4–CR nanocomposite

3.2.1 FTIR study

The second part of the present investigation was the characterization of the PCL/Fe3O4–CR nanocomposite system. Here the [M] was varied 0.5, 1.0, 1.5, 2.0 and 2.5 g and followed the bulk polymerization technique. The FTIR spectra are shown in Fig. 3a–e. The metal oxide stretching vibration was observed at 446 cm−1. The C–H out of plane bending vibration was observed at 738 cm−1. The ester C–O–C linkage was observed at 1179 cm−1. The C–N stretching of CR dye was observed at 1373 cm−1. The ester carbonyl group of PCL is noted at 1723 cm−1 [30, 31]. The aliphatic C–H stretchings are noted at 2859 and 2949 cm−1 corresponding to the C–H symmetric and anti-symmetric stretching of PCL, respectively. The –OH stretching can be seen at 3440 cm−1. Peak at 1039 and 1113 cm−1 are associated with the –SO2 stretching of CR dye. Thus the FTIR spectrum confirmed the functional groups expected to be present in both PCL and the Fe3O4–CR nanohybrid system. The important point noted here is that while increasing the [CL] from 0.5 to 2.5 g, the corrected peak areas of the carbonyl stretching (1723 cm−1) and C–H out of plane (738 cm−1) bending vibrations were increased in a linear manner. In order to find out the order of the reaction, the plot of log [M] versus log(RI[C=O/C–H]) was drawn (Fig. 3f). The plot showed an increasing trend with the slope value of 0.18. This indicated that 0.18 mol of CL was required to form one mole of PCL. The kinetic study indicated that Fe3O4–CR nanohybrid system was an effective initiator towards the ROP of CL. Our result is in accordance with our earlier report [20]. During the polymerization process, the cyclic structure of CL underwent ring opening process and attached one by one through co-ordination insertion mechanism. While increasing the concentration of CL, the number of repeating units added with the polymer chain is increased. In order to find out the role of CL monomer during the course of the reaction, the above said kinetic plot was made. The rate at which the ring structure of CL was opened in the presence of fixed concentration of initiator, catalyst, time and temperature and added with each other, lead to the polymer formation. The present investigation followed the 0.18 order of reaction with respect to CL concentration.

Fig. 3
figure 3

FTIR spectrum of PCL/Fe3O4–CR nanocomposite synthesized at [M] of a 0.50 g, b 1.5 g, c 2.0 g, d 2.5 g, e 3.0 g and f plot of log[M] versus log(RI[C=O/CH])

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3.2.2 UV–visible spectroscopy study

Figure 4 indicates the UV–visible spectra of the PCL/Fe3O4–CR nanocomposite system. The monomer concentration was varied from 0.5 to 2.5 g. It was found that while increasing the [M], the absorbance values were correspondingly decreased (Fig. 4a–e). This is due to the decrease in number of initiator species [18]. The number of initiator species per unit area is reduced while increasing the concentration of CL monomer. In order to find out the binding constant, the plot of (1/weight of CL) versus A0/A − A0 (Fig. 4f) was drawn and the plot was found to be nearly a linear line. The binding constant value was determined from the ratio of the intercept and slope value of the plot as 5.13 × 105 [M]−1. When compared with our prior report [18] the present system yielded a good binding constant value. This can be explained on the basis of chemical structure of an initiator species.

Fig. 4
figure 4

UV-visible spectrum of PCL/Fe3O4–CR nanocomposite synthesized with [M] of a 0.50 g, b 1.0 g, c 1.5 g, d 2.0 g, e 2.5 g and f plot of Ao/[A − Ao] versus 1/[M]

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3.2.3 FES of PCL/Fe3O4–CR nanocomposite

The binding constant and number of binding sites were determined from the fluorescence emission spectrum of PCL/Fe3O4–CR nanocomposite synthesized at various monomer concentrations. While increasing the monomer concentration from 0.5 to 2.5 g the fluorescence emission intensity (FEI) was found to be decreased (Fig. 5a–e). This is due to the decrease of initiator concentration for the ROP of CL. From the emission intensity value one can find out the order of ROP reaction by plotting log (weight of CL) versus log Rp (Fig. 5f). The plot was found to be nearly a straight line with decreasing trend. It means while increasing the CL concentration, the fluorescence emission intensity was decreased. The order of reaction was determined as 0.31. This indicated that 0.31 g of monomer was required to form one mole of PCL (based on FEI value). From the fluorescence emission spectrum it was also possible to find out the number of binding sites and binding constant for CR dye with PCL. The binding constant and the number of binding sites were determined from the plot of log(weight of CL) versus log(I0 − I/I) (Fig. 5g). The plot showed a straight line with the slope value of 1.21 and the intercept value of 3.01 (determined by linear fit method). The slope value gives the binding constant whereas the intercept value gives the number of binding sites. In case of CR, we have two amino groups and two –SO3Na groups. During the course of the reaction the –SO3Na group may be converted to a –SO3H group. One amino group is bonded with the Fe3O4. The remaining three functional groups (1-amino group and 2-SO3H groups) might be involved in the ROP of CL [18]. Again this confirmed the existence of chemical interaction between CR dye and PCL chains. The number of binding sites exactly matches with the structure.

Fig. 5
figure 5

Fluorescence emission spectra of PCL/Fe3O4–CR nanocomposite synthesized at [M] of a 0.50 g, b 1.5 g, c 2.0 g, d 2.5 g, e 3.0 g, f plot of log(weight of monomer) versus log Rp and g plot of log(weight of monomer) versus log[Io − I/I]

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3.2.4 DSC study

The melting temperature (Tm) of PCL synthesized at five different heating rates was determined by DSC. The DSC thermogram exhibits one endothermic peak at 65 °C due to the melting of PCL (Fig. 6a–e). It was very interesting to note that while increasing the heating rate, the Tm value of PCL was increased linearly. While increasing the heating rate between 5 and 25 °C/min, the Tm value was varied between 64.4 and 68.4 °C. This can be explained on the basis of fast scanning of PCL chains. We observed similar results [18]. Figure 6f indicates the plot of Tm versus heating rate of PCL. The plot showed an increasing trend. This can be explained on the basis of fast scanning speed.

Fig. 6
figure 6

DSC thermogram of PCL/Fe3O4–CR nanocomposite synthesized at [M] = 1.50 g and at various heating rates of a 5 °C/min, b 10 °C/min, c 15 °C/min, d 20 °C/min, e 25 °C/min and f plot of Tm versus heating rate

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Hereafter, for further analysis, the PCL synthesized at [M] of 1.5 g was considered for the sake of convenience.

3.2.5 VSM study

The VSM study reveals that after the nanocomposite formation (in solid disc form), the magnetization value was decreased to 21.5 emu/g (Fig. 1i). The decrease in VSM value of Fe3O4–CR end PCL was due to the encapsulation effect (i.e.) the Fe3O4 nanoparticles occupied the free space available between the polymer chains. The VSM study confirmed our prior results that both surface functionalization reaction and nanocomposite formation suddenly reduced the VSM value of Fe3O4 [32].

3.2.6 GPC, FESEM analysis and XRD analysis

In order to confirm the PCL formation GPC measurements were made. The GPC of PCL synthesized at the [M] = 1.50 g is shown in Fig. 7a. The Mw, Mn and polydispersity (PD) values were noted as 8417 g/mol, 6119 and 1.37 respectively. The PD value confirmed the absence of crosslinking reactions during the ROP reaction. Figure 7b is the FESEM image of PCL/Fe3O4–CR nanocomposite system synthesized at [M] = 1.50 g. Here one can see the broken stone like morphology. This is the peculiar surface morphology of the PCL. Some micro voids can also be seen on the surface of PCL. This will help us to load the drug molecules in the bio-medical applications. The Fe3O4 nanoparticles are uniformly distributed on the PCL backbone. Spherical and triangle shaped (marked by a circle) Fe3O4 nanoparticles were present on the surface of the PCL. The size was calculated as 15–20 nm. When compared to the pristine Fe3O4 and its nanohybrid systems, the nanocomposite system exhibited the smallest ferrite size. This can be explained on the basis of the applied thermal energy and the mechanical (agitation) force might help to reduce the size of Fe3O4 [32]. The semi-crystalline nature of the PCL was confirmed by XRD analysis as shown in Fig. 7c. The diffractogram showed crystalline peaks at 21.3°, 22.0° and 23.7° corresponding to d110, d111 and d200 crystalline planes of PCL [33]. The crystalline peaks of ferrite were appeared at 15.8°, 29.9°, 36.3°, 38.4°, 40.3°, 43.7°.

Fig. 7
figure 7

a GPC trace, b FESEM image and c XRD of the PCL/Fe3O4–CR nanocomposite system synthesized at [M] = 1.50 g

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3.3 Non-isothermal degradation kinetics of PCL/Fe3O4–CR nanocomposite system

3.3.1 TGA study

The TGA thermogram of pristine –OH end capped PCL is given in Fig. 8a with a single step degradation process. A major weight loss was noticed at 392.3 °C due to the degradation of PCL backbone [17]. For the sake of comparison, the simple Fe3O4 (0.0085 g) loaded –OH end capped PCL is given in Fig. 8b with a single step degradation process. The major weight loss corresponding to the degradation of PCL backbone occurred at 388.4 °C. Above 450 °C the system exhibited 7.4% weight residue remained. This is corresponding to the loaded Fe3O4 nanoparticles. When compared with the degradation temperature (Td) of pristine PCL with PCL/Fe3O4 nanocomposite system, the later exhibited the lower Td due to increase in viscosity of the reaction medium during its synthesis in the presence of Fe3O4. The PCLs were synthesized in the presence of water as an initiator and SO as a catalyst. This leads to the decrease in Mw of the PCL. Here the main aim of the present research work is to study the non-isothermal degradation kinetics of PCL synthesized in the presence of Fe3O4/CR nanohybrid system.

Fig. 8
figure 8

TGA thermogram of a pristine PCL and b PCL/Fe3O4 nanocomposite system

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The third part of the characterization of PCL/Fe3O4–CR nanocomposite system is the non-isothermal degradation kinetic study. The degradation kinetics of PCL was characterized by TGA. PCL synthesized at [M] = 1.50 g was taken. The PCL was subjected to degradation at various heating rates such as 5, 10, 15, 20 and 25 °C/min. The system exhibited a single step degradation process due to the degradation of the PCL backbone with the liberation of CO2 gas (Fig. 9a–e). It was quite interesting to note that, while increasing the heating rate, the degradation temperature of PCL proportionally increased (Fig. 9f). Above the Tm, the chains can move away from each other. While increasing the heating rate, the secondary forces of attraction between the polymer chains were broken and set free to move. In order to find out the energy of activation (Ea), for the degradation of the PCL backbone, three models were tried. First the Ea value was determined by the FWO method [34, 35]. In this method, the plot of ln(β) versus 1/Td (Fig. 9g) was made, from which the Ea and A values were calculated as 158 kJ/mol and 15.8 respectively. β-is the heating rate and Td is the degradation temperature. The second method was the Auggis and Bennet method. Here the plot of ln(β/Td) versus 1/Td (Fig. 9h) was drawn with the decreasing trend. The Ea and A values were calculated as 146 kJ/mol and 8.4 respectively. Finally, the Kissinger method was adopted by drawing 1/Td versus ln(β/T 2d ) (Fig. 9i). The plot showed a decreasing trend and from the slope value the Ea value was calculated as 134 kJ/mol. The pre-exponential factor A was calculated from the intercept of the plot as 0.958. This is in accordance with our earlier results [34]. This degradation kinetics indicated that ~ 150 kJ/mol was required for the degradation of the PCL chains.

Fig. 9
figure 9

TGA thermogram of PCL/Fe3O4–CR nanocomposite system synthesized at [M] = 1.50 g recorded at the different heating rates of a 5 °C/min, b 10 °C/min, c 15 °C/min, d 20 °C/min, e 25 °C/min, f plot of heating rate versus Td, g FWO plot, h Auggis–Bennet plot and i Kissinger plot

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3.3.2 Friedman method and KAS method

The ultimate aim of the present investigation is determination of Ea by different methods that include the iso-conversional method too. The Ea for the degradation of PCL was calculated by Friedman method by drawing a plot between ln(dα/dt) versus 1/T (Fig. 10a–e). The α indicates the reaction extent. The plot showed a linear decreasing trend for all the five heating rates. From the slope value of all five heating rates, the Ea value was calculated. While increasing the heating rates, the Ea value was slowly increased. It varied between 143 and 152 kJ/mol. The Ea value for the degradation of PCL was also determined by integral isoconversional method (KAS). The Ea for each heating rate was determined and plotted against the extent of reaction (α) (Fig. 10f–j). It was found that the Ea value linearly increased with the α value. The Ea value varied between 144 and 270 kJ/mol at each heating rate. The linear increase in Ea values were attributed to the rate-limiting step of a random chain scission reaction during the process of heating. This is in accordance with a report by Goswami et al. [36]. The Ea values determined by this method is very nearer to the value determined by the other methods.

Fig. 10
figure 10

a–e Friedman plot, f–j KAS plot for PCL-Fe3O4–CR nanocomposite system synthesized at [M] = 1.50 g recorded at the different heating rates of a 5 °C/min, b 10 °C/min, c 15 °C/min, d 20 °C/min and e 25 °C/min

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3.3.3 DSC study of PCL/Fe3O4–MO nanocomposite system

The DSC thermograms of PCL at the different heating rates are given in Fig. 11a–e. The Tm of the PCL was found to be varying from 62 to 73 °C with increasing heating rate [18]. This is due to the fast scanning of PCL chains. For clear understanding, the plot of heating rate versus Tm is given in Fig. 11f.

Fig. 11
figure 11

DSC thermogram of PCL/Fe3O4–MO nanocomposite synthesized at [M] = 1.50 g at various heating rates of a 5 °C/min, b 10 °C/min, c 15 °C/min, d 20 °C/min, e 25 °C/min and f plot of heating rate versus Tm

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3.4 Non-isothermal degradation kinetics of PCL/Fe3O4–MO nanocomposite system

3.4.1 TGA profile

The fourth part of the present investigation was the non-isothermal degradation kinetic study of PCL/Fe3O4–MO nanocomposite system synthesized at [M] = 1.50 g at different heating rates of 5, 10, 15, 20 and 25 °C/min. The influence of MO structure on the thermal degradation behavior of PCL was investigated with the aid of TGA. The PCL sample synthesized at the [M] = 1.50 g was considered. As for the DSC studies, the sample was heated under air atmosphere at five different heating rates, namely, 5, 10, 15, 20 and 25 °C/min. The TGA thermograms of PCL/Fe3O4–MO nanocomposite system recorded at various heating rates are given in Fig. 12a–e. The system showed a two step degradation process of PCL. The first slow degradation step up to 250 °C was ascribed to the removal of moisture and breaking of linkage between initiator (Fe3O4–MO) and PCL chains. The second major weight loss around 400 °C was attributed to the degradation of PCL backbone. While increasing the heating rates, the Td value increased proportionally (Fig. 12f). Various kinetic models are used in order to find out the Ea value for the degradation of PCL chains. For this purpose, FWO plot (Fig. 12g), Auggis–Bennet plot (Fig. 12h) and Kissinger plot (Fig. 12i) were drawn and the Ea values were calculated as 218.09, 196.45 and 174.72 kJ/mol, respectively. The Ba thioglycolate end capped PCL consumed nearly 120 kJ/mol for its own degradation under air atmosphere [37]. The Fe3O4-dye end capped PCL consumed lower amount of thermal energy for its degradation. This indicated that the Ea for the Td of PCL depended on the nature of end capping agent as well as the molecular weight of PCL. When compared with the CR based system, the present MO based system consumed higher amounts of thermal energy for the degradation of the PCL chains. This proved that the structure of dye (nature of initiating functional group and other functional groups available) played a vital role on the thermal property of PCL.

Fig. 12
figure 12

TGA thermogram of PCL/Fe3O4–MO nanocomposite system synthesized at [M] = 1.5 g recorded at the different heating rates of a 5 °C/min, b 10 °C/min, c 15 °C/min, d 20 °C/min, e 25 °C/min, f plot of heating rate versus Td, g FWO plot, h Auggis–Bennet plot and i Kissinger plot

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3.4.2 Friedman method and KAS method

For the present system the Ea value were also determined by the Friedman method by plotting 1/T versus ln(dα/dt) (Fig. 13a–e). As usual, the plot showed a decreasing trend with increasing temperature. The Ea value varied between 117 and 146 kJ/mol. The Ea values for the degradation of PCL were also determined by the KAS method. Figure 13f–j indicates the plot of reaction extent (α) versus activation energy. The activation energy varied between 54 and 117 kJ/mol. With increasing the reaction extent, the Ea value very slowly increased. While increasing the heating rate, the Ea value increased considerably. In the case of the PCL-Fe3O4–CR system, while increasing the reaction extent, the Ea value was increased in a linear manner for any given heating rate. In comparison we conclude that the degradation kinetics of PCL can be altered by the end group attached to the PCL chain end.

Fig. 13
figure 13

a–e Friedman plot, f-j KAS plot for PCL-Fe3O4–MO nanocomposite system synthesized at [M] = 1.50 g recorded at the different heating rates of a 5 °C/min, b 10 °C/min, c 15 °C/min, d 20 °C/min and e 25 °C/min

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4 Summary

From the above study the niche points are presented here as summary. The red shift in the UV–visible spectrum of CR dye confirmed the nanohybrid formation with Fe3O4. The reduction in VSM value of Fe3O4 nanoparticle also confirmed the nanohybrid formation. The FTIR relative intensity of [C=O/C–H] was found to be increased while increasing the [M]. This is due to the decrease in initiating species concentration. While increasing the weight of CL, the absorbance value was found to be reduced. The same results were observed in the case of FES while increasing the weight of CL. The Tm value of PCL increased while increasing the [M] due to the availability of more number of monomeric units. The degradation temperature of PCL was increased while increasing the heating rates. The Ea value was determined from the Kissinger plot as 134.25 kJ/mol for the degradation of PCL backbone in PCL/Fe3O4–CR system.