It is well known that Flur is a xanthene type dye which consists of hydroxyl, carboxyl, ketone, ether, aromatic C = C like functionalities. In the present investigation, the ROP of CL and THF in the presence of PAH is analyzed. The various functional groups of R6G are ester, ether and secondary amine cation, etc. However, the secondary amine group is highly active for the polymerization of CL by ROP.
3.1 FT-IR spectroscopy
The FT-IR spectra of P1 and P2 systems are illustrated in Fig. 1a & b respectively. The stretching of symmetric and antisymmetric modes of aliphatic C-H are seen at 2868 and 2942 cm− 1 respectively. The peaks corresponding to symmetric and antisymmetric modes of aromatic C-H are noticed at 2524 and 2650 cm− 1 respectively. A peak at 1730 cm− 1 is linked to the stretching of the carbonyl group of PCL . The out of plane bending of C-H vibration and linkage of C-O-C are seen at 733 cm− 1 and 1183 cm− 1 respectively. After the diblock copolymer formation, some new peaks are assigned for THF and PAH segments. The important peaks are encircled in the spectrum. The stretching of aromatic symmetric and anti-symmetric modes are noted. The stretching of the carbonyl group is assigned as a doublet peak. The short humps at 1693 cm− 1 and 1730 cm− 1 are corresponding to the stretching of the carbonyl group of PAH and PCL . The stretching of tetrahydrofuronium ion appeared as a short peak at 1582 cm− 1. The aromatic C-H deformations are seen at 666 and 799 cm− 1 which concluded the diblock copolymer formation. The FT-IR spectrum of the P3 system exhibits the stretching modes of –OH (3440 cm− 1), C-H (2872 and 2949 cm− 1), carbonyl (1730 cm− 1) , C-N (1366 cm− 1), C-O-C (1196 cm− 1), and out of plane bending of C-H vibration (734 cm− 1) as demonstrated in Fig. 1c. The P4 system exhibits some new peaks corresponding to aromatic C-H stretching (2528 cm− 1 and 2652 cm− 1), C = O stretching (1686 cm− 1 corresponding to the C = O stretching of PAH) and aromatic C-H deformation (665 and 802 cm− 1) as illustrated in Fig. 1d. Hence, the occurrence of new peaks proved the ROP of THF while using PAH as an accelerator.
3.2 NMR analysis
The structure of homo and diblock copolymer was concluded by NMR spectroscopy. Figures 2A and B denote 1H- and 13C-NMR spectra of the P1 system. A signal for standard TMS is noticed at 0 ppm. The alkoxy proton signal of PCL is seen at 4.12 ppm. A signal at 2.3 ppm is due to the proton of -CO2CH2. A signal at 3.8 ppm is corresponding to –CH2 proton of PCL . A peak at 7.3 ppm is owing to the CDCl3 solvent. The 13C-NMR spectrum of the P1 system exhibits a signal for carbonyl carbon at 173 ppm (Figs. 2B). A signal at 64 ppm is linked to the signal of alkoxy carbon. The signals for other carbon are assigned between 20 and 40 ppm. Thus the NMR spectra concluded the structure of the P1 system. The 1H- and 13C-NMR spectra of the P2 system are demonstrated in Fig. 2C and D respectively. This system also showed the above-discussed peaks. However, there were no proton and carbon signals associating with the THF segments. It is really due to the non-solvating behavior of the PTHF units in CDCl3 solvent. Figure 3A represents the 1H-NMR spectrum of the P3 sample. The signals for the aromatic protons of R6G are identified between 6.2 and 8.2 ppm. The sharp peaks at 4.1 ppm and 2.2 ppm are attributing to the –OCH2 and –CO2CH2 protons of PCL . The other methylene protons are assigned between 1 and 1.8 ppm. A signal for CH2 protons of PCL occurs at 3.7 ppm. The occurrence of peaks associating with the aromatic and alkoxy protons proved the ROP of CL by R6G. The 13C-NMR spectrum of the P3 sample is illustrated in Fig. 3B. A signal at 172 ppm is related to the carbonyl carbon signal of PCL . A peak at 62 ppm is assigned to the signal for alkoxy carbon. The other carbon signals are related to the structure of PCL. The 1H-NMR spectrum of the P4 system is represented in Fig. 3C. This system also showed the above-discussed peaks corresponding to the structure of PCL. The proton signals for THF units did not appear owing to the poor solvation effect. The 13C-NMR spectrum of the P4 sample is represented in Fig. 3D. The signals corresponding to THF segments are absent due to poor salvation effect.
3.3 UV-visible spectral analysis
Figure 4a demonstrates the UV-visible spectrum of pristine Flur. It exhibited one absorption peak at 490 nm relating to the monomeric structure of Flur . The absorbance peak was highly suppressed for the P1 (Fig. 4b) and P2 (Fig. 4c) systems. This is due to the degradation of Flur dye at 160 oC. The –OH group of Flur is active towards the ROP of CL. Moreover, the carboxyl group of Flur is active only for the polymerization of THF in the presence of PAH as an accelerator. But it is proved that Flur is degraded at a lower temperature since it is not having any halogen or nitro group. Figure 4d illustrates the UV-visible spectrum of pure R6G. The absorption peaks at 534 nm (monomeric) and 502 nm (dimeric) are assigned to the structure of R6G . The P3 system (Fig. 4e) shows an absorption peak at 532 nm and a small hump at 499 nm associating with the structure of the R6G. The UV-visible absorption spectrum of the P4 system shows a peak at 528 nm due to the monomeric form of R6G (Fig. 4f). The absorption peak for R6G was blue-shifted during the ROP of THF and CL. This explains the decrease in the conjugation length of R6G. This can also be explained by the decrease in the size of R6G during the ROP of THF and CL. In comparison, the xanthene type dyes (Flur) are not suitable for the ROP of CL at 160 oC, whereas the R6G dye is stable at the same experimental conditions. In our previous publications , a similar effect was observed while using xanthene type dyes.
3.4 Fluorescence emission spectrum (FES) study
Flur is a well-known dye molecule, and it can exhibit a fluorescence property. The FES spectra of the P1 (Fig. 5a) and P2 systems (Fig. 5b) are given here for the sake of comparison. Both the spectra showed an emission peak at 521 nm . Unfortunately, the peak’s intensity was found to be very low. This may be due to (i) the presence of a low quantity of dye molecules, (ii) degradation of Flur dye during the ROP of CL at 160 oC for 2 hours. Generally, both homopolymer and diblock copolymer exhibited a very low intensity than the pristine Flur dye. This suggests that the degradation of Flur dye may occur during the polymerization of CL. The FES of the P3 sample exhibits an intensity of 80 cps at 563 nm  as illustrated in Fig. 5c. The peak is red-shifted to 568 nm for the P4 sample after the formation of the diblock copolymer (Fig. 5d). This concluded the ROP of THF and CL in the presence of R6G dye initiator.
3.5 DSC analysis
PCL is a semi-crystalline polymer, and hence it exhibits a melt transition peak (Tm). The Tm was found to be 68.1 oC (Table 1) for the P1 system as demonstrated in Fig. 6a . The Tm value of the P2 system was decreased to 64.5 oC (Fig. 6b). This indicates that the Tm value of the PTHF is almost equal to the Tm of PCL. It absorbs moisture due to hydrophilic nature. Hence, there is a reduction in Tm value. The endothermic peak was observed at 63.8 oC for the P3 system  attributing to the Tm of PCL (Fig. 6c). The DSC thermogram of the P4 system exhibited the Tm at 61.2 oC (Fig. 6d). In comparison, the diblock copolymer exhibited lower Tm value than the homo PCL. The appearance of a single Tm confirmed the homogeneity of the diblock copolymer. In the overall comparison, the homopolymer exhibited higher Tm values, and particularly the Flur dye yielded a higher Tm value. The P1 sample exhibited the highest Tm value of 68.1oC owing to the hydrophobic nature of PCL.
Table 1 DSC, TGA and GPC data
3.6 TGA study
The thermal properties of the P1 system were analyzed by TGA. The decomposition of the P1 system occurs in two-steps (Fig. 7a). The major mass loss at 410 oC (Table 1) is attributing to the decomposition of PCL . The minor mass loss at 500 oC is responsible for the decomposition of Flur dye attached to the chain end of the PCL. The decomposition of the P2 sample occurs in two-steps (Fig. 7b). The major mass loss at 200 oC is attributing to the decomposition of PTHF units . The minor mass loss at 385 oC is responsible for the decomposition of PCL. The thermal stability of hydrophobic PCL was higher than the hydrophilic PTHF segments. The decomposition of the P3 system occurs in two-steps (Fig. 7c). The major mass loss at 422 oC is attributing to the decomposition of PCL. The minor mass loss at 525 oC is related to the decomposition of the R6G dye. The decomposition of the P4 system occurs in three-steps (Fig. 7d). The minor mass loss at 214 oC is attributing to the decomposition of PTHF units. The major mass loss at 393 oC is because of the decomposition of PCL. The minor mass loss at 475 oC is ascribed to the decomposition of the R6G dye. The homo PCL showed higher thermal stability than the diblock copolymer. This is because of the attached hydrophilic PTHF units to the hydrophobic PCL backbone. In the overall comparison, the P3 sample revealed the highest Td value of 422 oC for the R6G end-capped PCL. It means that the R6G dye yielded the highest thermal stability for the PCL system
3.7 GPC study
The initiating efficiency of the Flur and R6G dyes on the ROP of THF and CL was confirmed by GPC measurements. The P1 system exhibited the Mw, Mn, PD values of 12763 g/mole, 7295 and 1.7 respectively (Table 1). For the P2 system, the Mw was increased to 14388 g/mole with the simultaneous increase of Mn (8688). This is indicated in Fig. 8a&b for the prepared P1 and P2 systems. The increase in Mw of the P2 system proved the polymerization of THF by the ROP method while using PAH as an accelerator and Flur as a chemical initiator. The GPC profile concluded the polymerization of CL with R6G initiator. The Mw, Mn, PD values of the P3 system are 5516 g/mole, 3064 and 1.8 respectively (Fig. 8c). Similarly, the Mw, Mn, PD values of the P4 system were determined as 6348 g/mole, 3341, and 1.9 respectively (Fig. 8d). The increase in Mw proved the ROP of THF while using PAH and R6G as an initiator. As usual, the Mw of the diblock copolymer was higher than the homopolymer. The R6G initiator system yielded a low molecular weight than the Flur initiator system. This is due to the poor ionizing nature of -NH group. This proved a higher ring-opening capability of –OH group than the N-H group. Sivabalan et al.  proved the ROP ability of different functional groups toward CL and also concluded the same.
3.8 SEM and FE-SEM study
The surface morphology of the P1 sample is illustrated in Fig. 9a. It seems to be like a dry-sky with some microvoids . These voids can be accommodated by drug molecules while loading drugs. Hence, it proved that PCL is an ideal candidate for drug release applications. The FE-SEM micrograph of the P2 system is demonstrated in Fig. 9b. The morphology looks like a dry-sky with some NPs, and this can be explained as follows. The formation of polymer NPs at the interface of the polymers is due to the combination of hydrophilic PTHF and hydrophobic PCL. The polymer NPs are the key elements for the active drug release process. The size of the particles was estimated as 50–80 nm. Figure 9c illustrates the SEM micrograph of the P3 system. The surface texture is looking like a broken stone  with the spherical particles of size about 800 nm. Here also, one can see the various structures like microvoids and micro rods. The presence of microvoids is very much useful for the drug-carrying purpose. The surface texture of the P4 sample is analyzed by using the FE-SEM image (Fig. 9d). It exhibited a broken stone-like texture with the homogeneously dispersed spherical NP of size approximately 90 nm. The formation of polymer nanoparticle at the interface is due to the interaction of hydrophobic and hydrophilic segments.