Glycidol Copolymeric Green

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Highly Branched Polycaprolactone/Glycidol Copolymeric Green Plasticizer by One-Pot Solvent-Free Polymerization Kyu Won Lee, Jae Woo Chung, and Seung-Yeop Kwak ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01356 • Publication Date (Web): 29 May 2018 Downloaded from http://pubs.acs.org on May 30, 2018

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ACS Sustainable Chemistry & Engineering

Highly Branched Polycaprolactone/Glycidol Copolymeric Green Plasticizer by One-Pot Solvent-Free Polymerization Kyu Won Leea, Jae Woo Chungb,*, Seung-Yeop Kwaka,*

a

Department of Materials Science and Engineering, and Research Institute of Advanced

Materials (RIAM), Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Korea b

Department of Organic Materials and Fiber Engineering, Soongsil University, 369 Sangdo-ro, Dongjak-gu, Seoul 06978, Korea

*

Corresponding Author: Seung-Yeop Kwak (E-mail: [email protected]) Tel.: +82-2-880-8365, Fax: +82-2-885-1748 Jae Woo Chung (E-mail: [email protected]) Tel: +82-2-828-7047, Fax: +82-2-817-8346;

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Abstract This study aims to develop a simple, low-cost method for synthesis of highly branched polycaprolactone (hbPCL) for use as effective “green” plasticizers for poly(vinyl chloride) (PVC). We demonstrate the facile synthesis of hbPCL with tunable molecular architecture using glycidol as a branching monomer. A series of hbPCL is prepared via one-pot, solventfree copolymerization of ε-caprolactone and glycidol, wherein the molecular architecture is readily controlled by varying the molar ratio of glycidol to ε-caprolactone. Further, studying the kinetics of copolymerization reveals the preferential reaction of glycidol over ε-caprolactone, resulting in a multi-arm star-like copolymer after the ring-opening of the two monomers. The crystallization ability of hbPCL is found to gradually weaken with the introduction of the branching structure, and its molecular mobility is improved substantially by esterification with butyric anhydride, following which, a maximum mobility is realized at an intermediate level of branching. The butyl-esterified hbPCL (hbPCL-C4) is miscible with PVC, and their mixtures have excellent flexibility comparable to that of PVC/bis(2ethylhexyl) phthalate (DEHP). In particular, the stretchability of PVC/hbPCL-C4 is superior to that of PVC/DEHP, owing to its better structural homogeneity. Furthermore, PVC/hbPCL-C4 shows outstanding migration stability with the weight loss after extraction being >85% lower than that of PVC/DEHP.

Keywords: highly branched polymer, polycaprolactone, glycidol, copolymerization, green plasticizer


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Introduction Over the past 20 years, polycaprolactone (PCL) has attracted considerable attention owing to its excellent biocompatibility and biodegradability, low melting/glass transition temperatures, high miscibility with a variety of commercial polymers, and potential to be synthesized from a monomer derived from sustainable resources.1–5 Owing to these unique properties, PCL has been widely investigated in both academic and industrial fields for various applications, e.g., as a drug delivery system,6,7 scaffold in tissue engineering,8,9 food packaging material,10 and thermoplastic elastomer.11 One of the most attractive and valuable applications of PCL is an eco-friendly plasticizer replacing phthalate, a lowmolecular-weight organic compound primarily used as a plasticizer in poly(vinyl chloride) (PVC).12–14 Phthalate easily migrates out of the matrix during processing or use, thereby causing severe deterioration of the physical properties of PVC products and potentially acts as an environmental hormone for human health hazard.15,16 Several studies have demonstrated that flexible PCL segments show plasticizing effects in PVC, and PCL plasticizers have better migration stability than phthalate owing to their high molecular weights. However, the plasticizing efficiency of PCL is quite lower than that of phthalate, which is a challenge to be overcome for the practical application of PCL plasticizers. Highly branched polymers have the characteristics of low melting point, low crystallinity, low melt viscosity, low chain entanglement, high solubility, and high end-group concentration compared to linear polymers.17–20 In this regard, we anticipated that introducing a branching structure into the PCL backbone could enhance its molecular mobility, and hence provide sufficient flexibility to PVC. Indeed, our group previously


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reported that hyperbranched poly(ε-caprolactone) (HPCL) exhibits a high plasticizing efficiency concurrent with its excellent migration stability.21 However, despite the distinguishing performance of HPCL, its complicated manufacturing processes, including the synthesis of macromonomers via protection-polymerization-deprotection steps and polycondensation of the macromonomers, impede their large-scale production at a low cost. Therefore, a simple synthetic method should be developed for the highly branched PCL to be commercially available. To date, several synthetic strategies have been reported for the preparation of highly branched PCLs. These strategies can be mainly classified into two approaches: (i) Step growth and (ii) chain growth polymerization. The former approach is based on selfcondensing polymerization of intrinsically branched AB2−type macromonomers, as described

above.22–25

Meanwhile,

the

latter

approach

involves

multibranching

copolymerization of ε-caprolactone (CL) and branching AB2 monomer. For example, Frey et al. reported the synthesis of hyperbranched copolyesters through combined ring-opening polymerization/AB2 polycondensation of CL and 2,2-bis(hydroxymethyl)butyric acid.26 Although this method is conveniently accessible in one-pot, it still requires the use of a toxic solvent, expensive catalysts, and a continuous water removal system during the polymerization. Recently, Irvine et al. successfully prepared highly branched PCLs using 4,4’-bioxepanyl-7,7’-dione (BOD) as a branching agent.27 In this work, PCLs with a welldefined branching structure were achieved via one-pot bulk copolymerization of CL and BOD in the presence of commercially relevant tin 2-ethylhexanoate (Sn(Oct)2) catalyst. However, this strategy required a separate process to synthesize BOD, and the



thermal/physical properties of highly branched PCLs were very close to those of linear PCL as a result of the extremely low content of the branching BOD unit in the final product. Glycidol, one of the most versatile derivatives of renewable glycerol, has been widely used as a raw material in the chemical synthesis of polyglycerols, glycidyl ethers, and polyurethanes, as well as in perfumes, cosmetics, surface coatings, detergents, and paints.28–30 In particular, since glycidol is converted to the glycerol branching unit after anionic/cationic ring-opening polymerization, a number of studies have demonstrated the availability of glycidol to produce highly branched polymers via copolymerization with other monomers.31–33 From this perspective, we hypothesized that glycidol would be a suitable branching monomer for PCL, if it can be ring-opened under the typical conditions used for synthesizing PCL, i.e., a one-pot, solvent-free, and Sn(Oct)2-catalyzed polymerization. However, to date, there have been only a few reports describing the ringopening of glycidol in the presence of Sn(Oct)2.34,35 Furthermore, to the best of our knowledge, there are no reports on the ring-opening multibranching copolymerization of CL with glycidol. In this work, we present the facile synthesis of PCL with tunable molecular architecture using glycidol as a novel branching agent. The highly branched PCLs (hbPCLs) were synthesized via Sn(Oct)2-catalyzed bulk copolymerization of CL and glycidol in a single pot. We achieved hbPCLs with distinctly different branching structures by varying the feed molar ratio of glycidol with respect to the core material, and systematically investigated the structure-properties relationship of the obtained hbPCLs to gain an in-depth understanding


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of their plasticization behaviors. According to our study, the hbPCLs could prove to be promising candidates for the development of sustainable, safe, and feasible plasticizers.



Experimental Section Materials Glycidol, 1,4-benzenedimethanol (BDM), Sn(Oct)2 and bis(2-ethylhexyl) phthalate (DEHP) were purchased from Sigma-Aldrich Ltd., Korea. ε-caprolactone and butyric anhydride was purchased from Tokyo Chemical Industry Co., Ltd., Japan. Dichloromethane (DCM), tetrahydrofuran (THF), n-hexane, sodium bicarbonate (NaHCO3), and magnesium sulfate (MgSO4) were sourced from Daejung Chem., Korea. PVC resin LS100 was provided by LG Chem. Ltd., Korea. The thermal stabilizer SONGSTAB CZ-400 (Ca/Zn complex) was supplied by Songwon Co., Ltd., Korea. All chemicals were used as received without purification. Synthesis of highly branched polycaprolactone (hbPCL) via the copolymerization of CL and glycidol The typical procedure used for the synthesis of hbPCL is as follows: 0.415 g of BDM (3 mmol) and 13.697 g of CL (0.12 mol, 40 equiv.) were added to a 250 mL three-necked, round-bottom flask, and different quantities of glycidol (0.444 g, 1.111 g, 2.222 g, and 3.334 g which are 2, 5, 10, and 15 equivalents with respect to BDM) were added to the reaction mixture to control the branched architecture of hbPCLs. The mixture was stirred vigorously under heating for 30 min to obtain a homogeneous mixture. Then, the mixture was heated in an oil bath at 130 °C, and a catalytic amount (0.2 equiv.) of Sn(Oct)2 was added. The reaction mixture was allowed to react at the same temperature for 16 h under


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argon atmosphere, and then quenched with 100 mL of n-hexane. This sample is referred to as hbPCLx, where x indicates the average length of branched segments of the hbPCLs. The same procedure was used for the synthesis of the linear PCL (LPCL) without the addition of glycidol into the reaction mixture. To investigate the kinetics of copolymerization, aliquots of the reaction mixture were retrieved at different time intervals and dissolved in DMSO-d6 for NMR analysis. In the kinetic experiments, DMSO-d6 was used as a solvent instead of chloroform-d since the intermediates are polar and did not dissolve in chloroform-d. Esterification of the free hydroxyl end groups of the hbPCL A known quantity of hbPCL (for e.g., 10 g) and an excess of butyric anhydride (1.5 equiv. corresponding to the hydroxyl groups in hbPCL) were allowed react at 120 °C for 3 h under argon atmosphere. The reaction mixture was dissolved in DCM, washed several times with 1 M NaHCO3, and then dried with MgSO4. The solution was subsequently filtered and concentrated under reduced pressure. The polymer product was dried for 24 h at 60 °C under vacuum to obtain butyl-esterified hbPCL (hbPCLx-C4). Preparation of PVC films PVC resin (3.0 g, 100 parts per hundred resin (phr)), plasticizer (1.8 g, 60 phr), and thermal stabilizer (0.06 g, 2 phr) were added to 50 mL of THF and vigorously stirred for 2 h at room temperature. The mixture was subsequently poured into a glass Petri dish and dried in an oven at room temperature for 24 h and then at 40 °C for 24 h to completely remove the residual solvent. After drying, flexible PVC films with a thickness of approximately 0.20



mm were obtained. Characterization 1

H NMR and

13

C NMR spectra were recorded in chloroform-d at 600 MHz on a Bruker

Avance 600 spectrometer. Size exclusion chromatography (SEC) was carried out at 35 °C in THF (flow rate = 1.0 mL min−1) on a Waters Alliance e2695 equipped with a refractive index detector and three Waters Styragel columns. Number-average molecular weight (Mn) and molecular weight distribution (Mw/Mn) were calculated by calibrating with polystyrene standards. Differential scanning calorimetry (DSC; Netzsch DSC 200 F3) was performed under a nitrogen atmosphere over the temperature range of −100 to 140 °C at a heating rate of 10 °C min−1. X-ray diffraction (XRD; New D8 Advance, Bruker) patterns were collected at room temperature using Cu Kα radiation (λ = 1.541 Å), at a voltage of 40 kV and current of 40 mA, over the Bragg angle (2θ) range of 15−30° with scan rate of 2° min−1. Steady state viscosity measurements were carried out using a stress-controlled rheometer (RS-1, Thermo Fisher Scientific, Germany) in the temperature range of 60 to 100 °C over the shear rate range of 1–500 s−1. Thermogravimetric analysis (TGA) was carried out under flowing nitrogen up to 700 °C and a heating rate of 10 °C min−1. Fourier transform infrared (FTIR) spectra were acquired using a Thermo Scientific Nicolet 6700. Dynamic mechanical analysis (DMA; TA Instruments DMA 2980) was conducted in tension mode at a frequency of 1 Hz at −60 to 110 °C with the heating rate of 3 °C min−1. The oscillatory amplitude and static force were 15 µm and 0.01 N, respectively. The mechanical properties of the film samples were evaluated using an Instron-5543 universal testing machine (UTM) at a strain


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rate of 20 mm min−1 with a 50 N static load cell. UV-Vis spectra were recorded at ambient temperature using a Perkin Elmer Lambda25 spectrophotometer over the wavelength range of 400–900 nm. The migration stability was determined by estimating the amount of plasticizer thath migrated out of samples placed under harsh conditions. The specimens for test were prepared with the dimensions of 20 × 20 × 0.20 mm3. The extraction test was carried out based on ASTM D5227; the specimens were immersed in 1 L of n-hexane and stirred at 50 °C for 2 h. The degree of migration was evaluated by gravimetric analysis.



Results and discussion Synthesis of highly branched polycaprolactone A series of hbPCL containing glycerol branching units was prepared through Sn(Oct)2catalyzed bulk copolymerization of CL and glycidol, as depicted in Scheme 1. To accurately characterize the chemical structure of the obtained copolymers, 1,4benzenedimethanol (BDM) bearing an aromatic ring was selected as the core material. To achieve hbPCLs with different molecular architectures, the feed molar ratios of glycidol to the core BDM were adjusted to 2, 5, 10, and 15. LPCL was prepared as the linear counterpart serving as a control. Figure 1a presents the representative 1H NMR spectrum of hbPCL with the assignment of characteristic proton peaks of PCL. The broad peak observed in the range of 4.69 to 3.27 ppm is attributed to the protons in the glycerol branching units produced after the ring-opening of glycidol.36 As shown in Figure 1b, the relative peak intensities of terminal (f) to inner (e) units increased with increasing molar ratio of glycidol, indicating a high end- group concentration, which is characteristic of highly branched polymers. The degree of polymerization of CL (DPCL) and glycidol (DPglycidol) were calculated from the ratios of the protons corresponding to the CL segment (2.30 ppm) and the integral of all the protons in the glycerol unit to the protons in the core (7.33 ppm). As displayed in Table 1, DPCL and DPglycidol are in good agreement with the target values, implying that the ring-opening reaction of glycidol was highly quantitative under Sn(Oct)2 catalyst. The molar fraction of glycidol (fglycidol) was calculated from the composition, as determined using 1H NMR spectroscopy. The average length of branched segments ()


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of hbPCLs was estimated using the equation, = DPCL/(fcore + DPglycidol),

(1)

where, fcore is the functionality of the core. The values decreased gradually from 19.8 to 2.3 with increasing DPglycidol, indicating a more compact and globular structure of the obtained PCLs (Table 1). Figure 1c shows the SEC traces of all the PCL samples, exhibiting unimodal peaks with moderate molecular weight distributions. The number average molecular weight (Mn) of the PCL samples decreased steadily from 5362 to 1400 g mol−1 as the DPglycidol increased (Table 1), which is consistent with the descending trend of . The obtained PCLs can be regarded as oligomers considering their Mn. However, note that the actual molecular weight of PCLs may be larger than the values estimated by SEC analysis, because highly branched polymers are generally smaller than linear polymers with the same molecular weight. Therefore, highly branched PCLs with controlled molecular structures were successfully obtained by multibranching copolymerization of CL and glycidol. To elucidate the macromolecular architecture of the synthesized hbPCLs, we studied the kinetics of copolymerization on the basis of time-dependent NMR measurements. Figure 2a shows the 1H NMR spectra of samples retrieved at different time intervals during the copolymerization of CL and glycidol. The NMR spectra exhibit individual signals corresponding to the protons of methine (a), methylene (b and c) of glycidol, and methylene (d) of CL, with a decrease in the intensities of the monomer peaks and simultaneous increase in the characteristic peak (d’) of the PCL segments over the course of the



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copolymerization. For quantitative analysis, the conversion of each monomer is plotted as a function of the reaction time (Figure 2b). The conversion rate of glycidol is observed to be significantly faster than that of CL. This higher reactivity of glycidol may be ascribed to the strong ring-strain in the epoxide moiety of glycidol. Thus, the branch points formed mainly during the early stage of polymerization, resulting in multi-arm star-like PCL, as illustrated in Figure 3c.39 The copolymerization with glycidol induced variations in the molecular architecture of PCL, which would in turn affect various properties of PCL, including its ability to crystallize, molecular mobility, and plasticizing performance when incorporated into the PVC matrix. 13

C NMR spectroscopy was used to investigate the chemical structure of hbPCLs in

detail. The glycidol monomer produces a glycerol branching unit with one primary and one secondary hydroxyl group after the ring-opening reaction. As the secondary hydroxyl group is known to have lower reactivity than the primary hydroxyl group to the initiation of CL monomer, the reaction of the secondary hydroxyl group is necessary to confirm whether glycidol indeed acts as a branching agent.37,38 Figure 3a shows the

13

C NMR spectra of

LPCL and hbPCLs corresponding to the carbons of the ester groups, in which, LPCL shows only one peak at 173.5 ppm, whereas the hbPCL samples show an additional peak at 173.7 ppm due to the esters generated from the secondary hydroxyl group. This could be rationalized by the fact that the reaction temperature in our experiments was higher than the temperature typically used for polymerizing PCL, which might promote the reactivity of secondary hydroxyl groups in the glycerol branching unit to initiate CL. On one hand, the hbPCLs exhibit individual signals arising from carbon atoms in the core, glycerol unit, and


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CL repeating unit, as shown in Figure 3b and c. It is clear that the peak intensities corresponding to the terminal PCL segment (1’, 2’, and 3’) increased in proportion to the content of the branching unit in the hbPCL. This further confirms that highly branched structures were introduced into the PCL main chain owing to copolymerization with glycidol. The DEPT 135 and

13

C NMR spectra were recorded to accurately assign peaks of the

glycerol units (Figure S2). The peaks corresponding to the dendritic (D), linear (L13, L14) and terminal (T) units of glycerol were observed at 75−60 ppm, and the peak corresponding to the dendritic units (DCL1 and DCL2) connected to the caprolactone unit were also observed at 67.6, 65.2 ppm and 66.5, 64.9 ppm, respectively. These results indicate that not only the homopolyglycerol moiety but also the glycerol unit exists as a branching point between linear oligocaprolactone segments in the molecular structure of hbPCL. We measured inverse-gated (IG) 13C NMR for quantitative analysis of hbPCLs. Unfortunately, the content of glycerol could not be obtained due to the very low intensity of the peaks of the glycerol unit as well as the partial overlap of signals and the noise level inherent to IG NMR (Figure S3). However, a notable finding is that as the content of glycerol increases, the proportion of DCL2 relative to DCL1 increases, indicating that the dendritic glycerol units present in the linear oligocaprolactone segments are relatively dominant. In other words, as the ratio of glycerol in hbPCL increases, it becomes closer to an ideal highly branched polymer in which branching points are randomly distributed throughout the molecular structure.



Crystallization behavior and molecular dynamics of hbPCL DSC was used to investigate the effect of branched architecture on the crystallization behavior of hbPCLs. Figure 4a shows the 2nd DSC heating traces of hbPCLs and LPCL. LPCL shows a sharp and intense endothermic peak at 52.9 °C similar to that of a typical PCL, whereas hbPCLs show broader peaks with relatively lower intensities. Additionally, as plotted in Figure 4b, the maximum endothermic (melting) temperature of the PCL samples decreased inversely with an increase in the end-group concentration (i.e., as the branching level increased). Similar to the observation for the melting temperatures (Tms), the crystallization temperatures (Tcs) of hbPCLs shift towards lower temperature, and concurrently, the degree of crystallinity (Xc) decreased with increasing branching level (Figure 4c). These results indicate that the crystallization ability of hbPCLs weakened by the introduction of the branching structure. The branch points in hbPCLs would act like structural defects, and hence disturb the molecular packing and long-range ordering of the hbPCLs. In particular, hbPCL3.3 and hbPCL2.3 showed much lower Xc values of 26.0 and 16.1%, suggesting that the length of the linear backbone segments decreased to an effective level to hinder the crystallization of hbPCL. Apart from DSC, XRD was used to determine the crystal structure of hbPCLs at room temperature. As shown in Figure 4d, the XRD patterns of LPCL19.8, hbPCL9.6, and hbPCL5.8 exhibit the characteristic diffraction peaks of the PCL crystal, corresponding to its (110), (111), and (220) planes.40 When the branched segments are smaller, the characteristic peaks became less intense and shifted to higher angles, suggesting the decrease in the degree of crystallization and the interplanar distance of the polymers, respectively. In contrast, the XRD patterns of hbPCL3.3 and hbPCL2.3


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have no peaks, indicating a fully amorphous phase because of their lower Tms than room temperature. In fact, LPCL19.8 and hbPCL9.6 are powders and hbPCL5.8 exists in a waxy state, whereas hbPCL3.3 and hbPCL2.3 are gel-like materials, as shown in Figure 4e. Considering that the molecular mobility of a plasticizer is closely related to its plasticization efficiency, it is necessary to investigate the molecular dynamics of hbPCLs in order to understand their plasticizing behaviors in PVC. The glass transition temperature (Tg) refers to the temperature at which the amorphous polymer chains change from a glassy to a rubbery state. At temperatures above Tg, the polymer chains become flexible with a free volume; therefore, Tg is generally considered to be an indicator of the cooperative motion of polymer segments. We therefore measured and compared Tg of hbPCLs to evaluate their molecular mobility with respect to the branching structure. As the branching level in hbPCL increases, however, the number of hydroxyl end groups increases and as a consequence, the segmental motion of hbPCL is hindered by inter- and intramolecular hydrogen bonds.41 Accordingly, we modified the hydroxyl end groups of the polymers to butyl ester groups to eliminate the hydrogen-bonding effect (Figure S4).42 As shown in Figure 5a, unmodified hbPCLs has a single Tg at approximately −60 °C similar to that of LPCL regardless of their . Meanwhile, the butyl-esterified hbPCLs (hbPCL-C4s) have significantly lower Tgs than hbPCLs, reflecting their enhanced molecular mobility owing to the removal of hydrogen bonds. The Tg of hbPCL-C4s decreased gradually as decreased until became 3.3 and slightly increased thereafter (Figure 5a). In other words, the Tg showed a minimum at the intermediate branching level. This result suggests that the branching point in hbPCL contributed to the enhancement in the molecular



mobility at the initial stage of introduction, however it acted as an obstacle to the segmental motions along the polymer beyond a certain level. However, it should be noted that the molecular mobility of semi-crystalline polymers could be decreased due to the crystalline confinement.43,44 LPCL19.8-C4, hbPCL9.6-C4, and hbPCL5.8-C4 still exhibited sharp melting peaks in their DSC curves (Figure S5), indicating that the polymer chains in the amorphous region would be constrained by the crystalline region. Furthermore, given the fact that the plasticizer is uniformly distributed among the PVC chains to form a homogeneous mixture in the plastic material, it is necessary to study the molecular dynamics of hbPCL-C4s in the absence of external factors such as crystallization, aggregation, and chain entanglements. According to the lubricity theory of plasticization mechanism, a plasticizer serves as a lubricant for PVC, thus providing softness and ductility to the rigid PVC backbone.45,46 Therefore, the rheological property of a plasticizer is considered to be the most direct and reliable predictor of its plasticizing performance. In this study, the steady shear viscosity was evaluated to investigate the molecular dynamics of hbPCL-C4s and LPCL-C4. Figure 5b shows the shear rate dependence of the steady shear viscosity (η) for hbPCL-C4s and LPCL-C4 in their melt state. For all samples, the measured viscosity was found to be independent of the shear rate, indicating a Newtonian fluid behavior.47 This suggests that none of the samples have any aggregation or entanglement of chains in the network. The η values at 60 °C were found to decrease in the order of LPCL-C4 (2.29 Pa·s) > hbPCL9.6C4 (0.55 Pa·s) > hbPCL2.3-C4 (0.45 Pa·s) > hbPCL3.3-C4 (0.29 Pa·s) > hbPCL5.8-C4 (0.24 Pa·s). The η values for hbPCL-C4s and LPCL-C4 showed the same order at other


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temperatures (70−100 °C) (Figure S6). This descending trend of η is similar to that of Tg, with a difference that the lowest value was observed for hbPCL5.8-C4 (Figure 5c). In other words, hbPCL5.8-C4 had the most enhanced molecular mobility among the PCL samples. This can be attributed to the result of excluding the restricted molecular motion due to the crystallization of the linear segments. In our copolymer, increasing the molar ratio of glycidol resulted in a large number of end groups as well as a more compact branched structure of the hbPCL-C4 (Figure 5d). The former provides more free volume to the polymer chains, thus enhancing the molecular mobility.48 In contrast, the latter reduces the length of the linear backbone segments, thereby decreasing the flexibility of the chain.49 We speculate that the molecular mobility of hbPCL-C4 is significantly affected by the increase in the end-group concentration at the initial stage of the introduction of the branching structure; however, the decrease in the flexible linear PCL segments begin to dominate after the critical branching point is passed. As a result, hbPCL-C4 with a moderately branched architecture had the highest molecular mobility among the PCLs of various architectures. Therefore, through detailed analysis of the correlation between the branching structure of hbPCL-C4s and their dynamics, hbPCL5.8-C4 is expected to exhibit the best plasticizing performance in the PVC. We performed TGA and compared the thermal stability of hbPCL-C4s with that of DEHP. As shown in Figure S7, hbPCL-C4s degraded at a higher temperature than DEHP, indicating that hbPCL-C4s have better thermal stability than DEHP. Note that the 5% weight-loss temperature (Td5) of hbPCL-C4s was between 310 and 375 °C, which is above



the extrusion temperature range of PVC (180–200 °C). Meanwhile, the Td5 of DEHP was 185 °C, suggesting that some of DEHP may be lost during processing. Plasticizing effects of hbPCL on the PVC Plasticized PVC films containing 60 phr (~38 wt%) of hbPCL-C4s and LPCL-C4 were fabricated individually via solution blending, and a PVC film with 60 phr of DEHP was also prepared in the same manner. As shown in Figure 6a, all the FTIR spectra of plasticized PVCs have characteristic bands at 2940, 1730, 1430 and 625 cm−1 corresponding to the C–H stretching, C=O stretching, C–H bending (methylene), and C–Cl stretching, respectively. PVC/hbPCL-C4s and PVC/LPCL-C4 show an additional IR band at 1165 cm−1 corresponding to C–O stretch of the ester group, whereas the C–O stretching band of PVC/DEHP shifted to 1273 cm−1 due to the aromatic group in DEHP. Figure 6b–g show the FTIR spectra of the plasticizers and the corresponding PVC mixtures in the carbonyl region. Apparently, all the plasticizers showed a significantly shifted C=O band after mixing with PVC, implying that the C=O groups of the plasticizer strongly interact with the polar moieties (C–Cl bonds) of PVC, and thus, these components are compatible with each other.50 In addition to the shift of the carbonyl bands, the ratio of the amorphous and crystalline bands of plasticized PVC indicates the compatibility between the plasticizer and PVC. As the plasticizer can only solvate the amorphous regions of PVC, the relative intensities of the amorphous bands decrease when the plasticizer is more compatible with PVC.51 Figure 6h and i show the FTIR spectra of plasticized PVCs in the wavenumber ranges of 1440–1420 cm−1 and 660–580 cm−1, corresponding to the methylene deformation


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and C–Cl stretching, respectively. In all cases, the amorphous (1434 and 610 cm−1) and crystalline (1426 and 636 cm−1) bands are observed in these regions. Further, the ratios of absorbance intensities of the amorphous and crystalline bands (A1434/1426 and A610/636) were estimated. Despite a slight difference in the order of the two ratios, it is obvious that the ratios of PVC/hbPCL3.3-C4 and PVC/hbPCL2.3-C4 are significantly higher than those of PVC/LPCL19.8-C4, PVC/hbPCL9.6-C4, and PVC/hbPCL5.8-C4 (Figure 6j). This indicates that hbPCL3.3-C4 and hbPCL2.3-C4 are less compatible with PVC. This might be due to the high concentration of the glycerol branching unit, which renders the molecules more polar and restricted. The ratios of the absorbance intensities of PVC/DEHP were found to be as high as those of PVC/hbPCL3.3-C4 and PVC/hbPCL2.3-C4. Consequently, all the plasticized PVCs are considered to be miscible mixtures; however, hbPCL3.3-C4, hbPCL2.3-C4, and DEHP are relatively less compatible with PVC than LPCL19.8-C4, hbPCL9.6-C4, and hbPCL5.8-C4. The addition of a plasticizer imparts free volume to PVC, thereby lowering its Tg. Therefore, the plasticizing ability of the plasticizer can be evaluated by comparing the reduction in Tg. As shown in the DSC thermograms, all the PVC mixtures have a single Tg at much lower temperature than that of neat PVC, indicating the homogeneity of all the mixtures and adequate plasticization of PVC (Figure 7a). Remarkably, the Tg values of PVC/hbPCL- C4s were found to be comparable to that of PVC/DEHP, despite the much bulkier molecular structures of hbPCL-C4s relative to that of DEHP (Figure 7c). This can be rationalized by the synergistic effect of the flexible linear PCL segments and a large number of the highly mobile end groups of hbPCL-C4. From the perspective of the free



volume theory of the plasticizing mechanism, free-moving end chains in addition to the flexible linear segments effectively create free volume in the PVC, resulting in a further reduction in the softening temperature. Although the plasticization efficiency of hbPCLC4s was slightly lower than that of low-molecular-weight DEHP, it was significantly higher than that of LPCL-C4, confirming that hbPCL-C4s are sufficient as alternative plasticizers. In particular, hbPCL5.8-C4 showed superior plasticizing ability among the hbPCL-C4s, which correlates well with the results of the molecular dynamics of hbPCL-C4s. Therefore, the proper branching structure could promote the molecular mobility of hbPCL-C4s, thus increasing the degree of plasticization of the PVC mixtures. The distribution in the glass transition for neat PVC and PVC mixtures was determined based on the difference between the onset temperature (Tonset) and the end temperature (Tend). As shown in Figure 7c, PVC/hbPCL3.3-C4, PVC/hbPCL2.3-C4, and PVC/DEHP exhibited broad glass transition temperatures compared to other mixtures, which is relevant to high fluctuation in the local concentration of hbPCL-C4 in miscible blends.52,53 This implies that these mixtures had poorer structural homogeneity than others, which is related to lower compatibility between the plasticizer and PVC, as determined by FTIR analysis. The viscoelastic properties of PVC/hbPCL-C4 were also characterized to evaluate the plasticizing ability of hbPCL-C4. Figure 7b shows the storage modulus (E’) and loss tangent (tan δ) of neat PVC and plasticized PVC as a function of temperature. The E’ decreases steadily as the temperature increases, indicating the dissipation of energy. The E’ curves for plasticized PVC are located below and to the left of that of neat PVC, implying that the stiffness and rigidity of PVC decreased with the addition of the plasticizer.


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Similarly, the tan δ peaks of plasticized PVCs shift to a lower temperature than that of neat PVC (Figure 7b). The α relaxation temperature (Tα relaxation) of plasticized PVCs increased in the order, PVC/DEHP (25.0 °C) < PVC/hbPCL5.8-C4 (28.5 °C) < PVC/hbPCL9.6-C4 (30.0 °C) < PVC/hbPCL3.3-C4 (32.1 °C) < PVC/LPCL19.8-C4 (35.3 °C) < PVC/hbPCL2.3-C4 (36.3 °C). This result indicates that the moderately branched architecture of hbPCL-C4 effectively improved its plasticizing ability, which is in good agreement with DSC results. The full width at half maximum (FWHM) of the tan δ peak was estimated to assess the structural homogeneity of the plasticized PVC. As shown in Figure 7c, the obtained FWHM values are consistent with the distribution in the glass transition temperatures, confirming the relatively lower uniformity of PVC/hbPCL3.3-C4, PVC/hbPCL2.3-C4, and PVC/DEHP. Next, we carried out tensile tests to investigate the impact of the plasticization efficiency and structural homogeneity of PVC/hbPCL-C4 on their mechanical properties. All the plasticized PVCs display typical stress-strain characteristics of flexible PVC (Figure 8a). The plasticized PVCs simultaneously exhibit much smaller stress and much larger strain during their deformation compared to neat PVC, because the plasticizer weakened the intermolecular friction forces between the PVC chains. The tensile strength and elongation at break of each plasticized PVC sample were determined to compare their mechanical properties in greater detail (Figure 8b). As expected, PVC/hbPCL5.8-C4 has the lowest tensile strength among the PVC/hbPCL-C4s with the strength being slightly higher than that of PVC/DEHP. Notably, in case of the elongation at break, PVC/hbPCL5.8-C4



exhibited the largest value of 397%, which is even greater than that of PVC/DEHP by 21%. Such exceptional stretchability of PVC/hbPCL5.8-C4 can be attributed to its high degree of plasticization concurrent with the good structural homogeneity. For the plasticized PVC with poor homogeneity, a high concentration of the plasticizer would induce structural defects in the system, resulting in rapid crack propagation and subsequent fracture under the tensile load.54,55 As shown in Figure 8b, the elongation at break decreased substantially for PVC/hbPCL3.3-C4 (338%) and PVC/hbPCL2.3-C4 (312%), which supports the above assumption. Consequently, it can be concluded that the structural integrity as well as the high degree of plasticization played crucial roles in enhancing the stretchability of flexible PVC. The transparency of plasticized PVC is an essential property for its practical applications. As displayed in Figure 9a, PVC/hbPCL5.8-C4 and PVC/DEHP are highly transparent, whereas PVC/LPCL19.8-C4 and PVC/hbPCL9.6-C4 contain randomly distributed impurities on their surfaces. This is likely due to the formation of PCL crystals as a result of the high tendency of LPCL19.8-C4 and hbPCL9.6-C4 to crystallize in PVC.56 PVC/LPCL19.8-C4 had more pronounced impurities than PVC/hbPCL9.6-C4 due to the much higher crystallinity of LPCL19.8-C4. In contrast, PVC/hbPCL3.3-C4 and PVC/hbPCL2.3-C4 were observed to be foggy throughout the surface, which is attributed to the lower uniformity of these mixtures.57 As observed in the UV-Vis spectra (Figure 9b), the transmittances of PVC/hbPCL5.8-C4 and PVC/DEHP are as high as 90% over the visible wavelength range, whereas those of the other PVC films are considerably lower than those of the former mixtures. Therefore, both crystallinity and dispersity of the plasticizer


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should be carefully considered to achieve transparent PVC. The migration stability of plasticized PVC was estimated by leaching tests conducted under harsh experimental conditions (heated in n-hexane at 50 °C for 2 h). When flexible PVC is applied as a packaging material and comes into contact with oily foods, the plasticizer tends to migrate out from the PVC matrix. The use of n-hexane as the extraction medium for plasticizer is intended to accelerate the migration of plasticizer in harsher environments, as n-hexane readily swells PVC at high temperature and has a solubility profile similar to cooking oil. The weight loss of PVC products caused by the migration of the plasticizer must not exceed 5.5% for their practical usage.58 As shown in Figure 10, the weight losses of PVC/hbPCL-C4s ranged from 2.4 to 4.6%, which are below the limit of 5.5%, whereas that of PVC/DEHP was found to be extremely high at 36%. Such remarkable migration stability of PVC/hbPCL-C4 can be attributed to the high molecular weight (i.e., bulky molecular structure) of hbPCL-C4 along with numerous ester groups in the molecular structure, which can strongly interact with PVC. Moreover, even if a small amount of hbPCL-C4 migrated out during processing or in use, the PCL-based plasticizer is believed to be harmless to humans and environment owing to its well-known biocompatibility and biodegradability. Thus, our hbPCL-C4s will be applicable in various PVC products requiring long-term stability and non-toxicity, such as medical devices, interior materials, infant toys, and food packaging.



Conclusions The synthesis of hbPCL was accomplished in a straightforward manner via the Sn(Oct)2catalyzed, one-pot, and solvent-free copolymerization of ε-caprolactone and glycidol. A series of hbPCL with different lengths of linear backbone segments was obtained by simply varying the molar ratio of glycidol. The study of the copolymerization kinetics revealed that the resulting copolymers had multi-arm star-like architectures. With the introduction of the branching structure, the crystallization ability and molecular mobility of hbPCLs changed drastically. The molecular mobility of hbPCL was considerably enhanced by butyl esterification, and the mobility of the butyl-esterified hbPCL reached a maximum at an intermediate branching level (for hbPCL5.8-C4). Indeed, hbPCL5.8-C4 was miscible with PVC, and showed excellent plasticizing effect, which is comparable to that of DEHP. In particular, the stretchability of PVC/hbPCL5.8-C4 was better than that of PVC/DEHP, owing to its better structural homogeneity. Furthermore, PVC/hbPCL-C4s showed outstanding migration stability compared to that of PVC/DEHP. Consequently, our hbPCLs are extremely attractive as sustainable, safe, and feasible plasticizers for versatile applications of flexible PVC.


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Supporting Information 13

C NMR spectra of hbPCLs and LPCL, 1H NMR spectra of hbPCL and hbPCL-C4, DSC

thermograms of hbPCL-C4s and LPCL-C4, temperature dependences of the viscosity for PCL samples, and TGA curves for PCL samples and DEHP.

Corresponding Author E-mail: [email protected] E-mail: [email protected] Notes The authors declare no competing financial interest.

Acknowledgements This work was supported in part by the Korean Ministry of Science and Technology under the National Research Laboratory (NRL) Program (RIAM).



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Table 1 Molecular characterization of highly branched polycaprolactone (hbPCL) and linear polycaprolactone (LPCL) Samplea

DPCL

DPglycidol

fglycidol

Mn (g mol−1)

Mw/Mn

LPCL19.8

39.5

0

0

19.8

5362

1.65

hbPCL9.6

39.3

2.1

0.05

9.6

2986

1.47

hbPCL5.8

40.1

5.0

0.11

5.8

1961

1.38

hbPCL3.3

39.5

10.1

0.20

3.3

1610

1.28

hbPCL2.3

39.4

15.1

0.28

2.3

1400

1.17

a

In the sample notation, hbPCLx and LPCLx, x indicates the average length of branched segments ().



Page 36 of 59

Scheme 1. Synthetic scheme for the Sn(Oct)2-catalyzed bulk copolymerization of εcaprolactone and glycidol.

n

ε-caprolactone

glycidol n

130 °C, 16 h

HO OH

1,4-benzenedimethanol

Sn(Oct)2-catalyzed bulk copolymerization n n

n


Page 37 of 59 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a)

(b)

b, d

(c)

e f

e LPCL19.8

c

f

solvent

eluent

LPCL19.8

a

hbPCL9.6 hbPCL9.6

1 2 hbPCL5.8 hbPCL5.8

g, h, i

hbPCL3.3

hbPCL3.3

8

7

6

5

4

3

2

1

0

Chemical shift (ppm)

1

1

b

2 a 1

2

b

1 a

g

d c

hbPCL2.3

i

d c

hbPCL2.3

h

e 8

f

7

6

5

4

3

2

1


0

18

19

20

21

22

Elution time (min)

Figure 1. (a) Representative 1H NMR spectrum of hbPCL with the proton assignments, (b) 1 H NMR spectra of LPCL and hbPCLs with different branch structures, and (c) SEC traces of the synthesized hbPCLs and LPCL.



(a)

(b)

100 80

Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 59

a b, c

Kglycidol > KCL

60 40

glycidol CL

20

DMSO-d6

d

0 0

12 10 8

2

4

6

8

10

12

Time (h)

(c)

6 5 4 3 2 1.5 1 0.5

a 3.0

b 2.8

d 2.6

c

0

d’ (ring-opened) 2.4

2.2

one-pot copolymerization

2.0


multi-arm star copolymer

Figure 2. (a) 1H NMR spectra of samples retrieved after different time intervals during the copolymerization of CL and glycidol with a feed molar ratio of [CL]:[glycidol]:[BDM] = 40:5:1, (b) plots of the monomer conversion versus time, and (c) schematic illustration of the molecular architecture of the resulting copolymer on the basis of the reactivity ratios of the monomers.


Page 39 of 59 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a)

(b)

(c) 2

A a

1

f

1’ 3’

A

3 2’ 4

b

a

4

e

b f

solvent

e

A

4

1 from glycerol unit

174.5

174.0

173.5

173.0


172.5 80

78

76

74

72

70

68

66

2

2’ 3’

3

1’

64

62

36


34

32

30

28

26

24


Figure 3. 13C NMR spectra of LPCL19.8 (black line), hbPCL5.8 (blue line), and hbPCL2.3 (red line) corresponding to the carbon atoms of (a) ester groups, (b) BDM, glycerol unit, and terminal CL, and (c) inner CL.



Exo

(b) LPCL19.8

Heat flow

hbPCL9.6

52.9 °C

hbPCL5.8

41.2 °C 26.0 °C

hbPCL3.3

Endo

46.2 °C 34.7 °C 20.7 °C

hbPCL2.3

4.7 °C

-90

-60

-30

0

30

60

90

120

Maximum endotherm temperature (oC)

(a)

60

LPCL19.8

50

hbPCL9.6 40

hbPCL5.8 30

hbPCL3.3

20 10

hbPCL2.3 0 5

o

15

20

25

30

10 X End-group concentration

(c)

(d) 27.3 °C (Xc =52.3%)

Exo

(110)

Intensity (a.u.)

13.6 °C (49.3%)

hbPCL9.6 1.1 °C (44.1%)

hbPCL5.8 −3.7 °C (26.0%)

hbPCL3.3

-60

(200)

22.0°

23.7°

LPCL19.8 hbPCL9.6 hbPCL5.8 hbPCL3.3

−15.7 °C (16.1%)

hbPCL2.3 -90

(111) 21.4°

LPCL19.8

Endo

10 4

Temperature ( C)

Heat flow

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

hbPCL2.3 -30

0

30

60

Temperature (oC)

90

120

21

22

23

24

2theta (°)

Figure 4. Crystallization characteristics of hbPCLs and LPCL. (a) 2nd DSC heating curves, (b) the maximum endothermic temperatures as a function of the end group concentration, (c) DSC cooling curves, (d) XRD patterns, and (e) phases of hbPCLs and LPCL.


Page 40 of 59

(c)

10

LPCL19.8-C4 hbPCL9.6-C4 hbPCL5.8-C4

LPCL19.8 −60.0 −60.6

hbPCL5.8

Exo

Viscosity (Pas)

hbPCL9.6

−58.2

hbPCL3.3

hbPCL3.3-C4 hbPCL2.3-C4

1

−62.0

hbPCL2.3

0.1

-70 -75

-85

10

100

1000

Shear rate (1/s)

(d)

−66.8

−70.2

hbPCL9.6-C4 −76.1

Endo

hbPCL5.8-C4

−80.6

hbPCL3.3-C4 hbPCL2.3-C4

−79.1

Length of linear segments

LPCL19.8-C4

1

-80

-90

1

−60.9

-65

19.8

9.6

5.8

3.3

2.3

Length of linear segments

30

50

25

40

20 30 15 20 10 10

5 0

-90

-80

-70

Temperature (oC)

-60

-50

Steady shear viscosity (Pas)

(b)

4 10 X End-group concentration

(a)

Heat flow

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Glass transition temperature (°C)

Page 41 of 59

0 LPCL19.8-C4

hbPCL9.6-C4

hbPCL5.8-C4

hbPCL3.3-C4

hbPCL2.3-C4

Samples

Figure 5. (a) DSC thermograms of hbPCLs and LPCL (upper) and those of hbPCL-C4s and LPCL-C4 (lower) and (b) steady shear viscosity as a function of shear rate for hbPCL-C4s and LPCL-C4. (c) Dependence of the glass transition temperature and steady shear viscosity on the linear backbone length. (d) Variation in the length of the linear segments and end-group concentration of hbPCL-C4s and LPCL-C4.



PVC mixture 1723.6 1727.5

(d)

PVC/hbPCL3.3-C4

Transmittance (a.u.)

C–O stretch (ester)

PVC/hbPCL2.3-C4

1734.8 1729.3

4000

3200

2000

1600

1200

Absorbance (normalized)

Absorbance (normalized)

Amorphous 1434 cm–1

Crystalline 1426 cm–1

1734.4 1730.1

1760

1720

1680

1680 −1

hbPCL3.3-C4

PVC mixture

1735.9 1730.2

1760

1720

1680 −1

Wavenumber (cm ) DEHP PVC mixture 1728.2 1722.3

1800

1760

−1

1720

Wavenumber (cm )

(j) 0.90

1.10 0.971

0.85

0.80

0.946

0.906

0.941

Amorphous 610 cm–1

0.900 0.869

0.75

Average

640

620

600

580

−1

Wavenumber (cm )

0.95 PVC/ DEHP

660

PVC/ hbPCL2.3-C4

Wavenumber (cm )

1420

PVC/ hbPCL3.3-C4

1425 −1

PVC/ hbPCL5.8-C4

1430

PVC/ hbPCL9.6-C4

1435

1.05

1.00

Crystalline 636 cm–1

0.70 1440

1680 −1

Wavenumber (cm )

A1434/A1426

(i)

PVC mixture

1720

Wavenumber (cm )

(g)

hbPCL2.3-C4

1800

1760

1800

1680 −1

Wavenumber (cm )

800

−1

Wavenumber (cm )

(h)

1720

1723.5 1727.7

1800

PVC mixture

1760

PVC mixture

(e)

hbPCL5.8-C4


C–H C–Cl bend stretch C=O (methylene) C–O stretch stretch (aromatic ester)

1680 −1

1800

C–H stretch

1720

Wavenumber (cm )

(f) PVC/DEHP

1760

hbPCL9.6-C4

A610/A636


1800

PVC/hbPCL5.8-C4


PVC/hbPCL9.6-C4

LPCL19.8-C4


PVC/LPCL19.8-C4

(c) Transmittance (a.u.)

(b) Transmittance (a.u.)

(a)

PVC/ LPCL19.8-C4

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 59

Figure 6. (a) FTIR spectra of plasticized PVCs. FTIR spectra showing the carbonyl region of (b) LPCL19.8-C4, (c) hbPCL9.6-C4, (d) hbPCL5.8-C4, (e) hbPCL3.3-C4, (f) hbPCL2.3C4, (g) DEHP and the corresponding PVC mixtures. FTIR spectra of plasticized PVC for (h) methylene deformation and (i) C–Cl stretching regions. (j) Ratios of the absorbance intensities of amorphous and crystalline bands of plasticized PVCs.


(b)

(c) Glass transition temperature ( C)

Neat PVC

Exo 83.6

−3.9

PVC/LPCL19.8-C4

−11.7

1

PVC/hbPCL9.6-C4

−14.3

o

10

0.1

PVC/hbPCL5.8-C4

Neat PVC PVC/LPCL19.8-C4 PVC/hbPCL9.6-C4 PVC/hbPCL5.8-C4 PVC/hbPCL3.3-C4 PVC/hbPCL2.3-C4 PVC/DEHP

35 -10 30 -15

25

-20

50

50

45

45

40

40

35

35

PVC/hbPCL2.3-C4

o

o

tanδ

0.4

−20.7

FWHM ( C)

PVC/hbPCL3.3-C4 −6.3

Endo

-5

0.6

−13.6

Tend − Tonset ( C)

Heat flow

Tend

Storage Modulus (MPa)

100

Tonset

40

0

o

(a)

0.2

PVC/DEHP

0.0

40 o

60

80

100

-60

-40

-20

0

20

40

60

80

100

Temperature (oC)

o

PVC/ DEHP

20

PVC/ hbPCL2.3-C4

0

Temperature ( C)

PVC/ hbPCL3.3-C4

-20

PVC/ hbPCL5.8-C4

-40

PVC/ hbPCL9.6-C4

-60

PVC/ LPCL19.8-C4

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


α relaxation temperature ( C)

Page 43 of 59

Figure 7. (a) DSC thermograms of neat PVC and plasticized PVCs. (b) Temperature dependence of the storage modulus and loss tangent of neat PVC and plasticized PVCs. (c) Comparison of the plasticization efficiencies and structural homogeneities of plasticized PVC samples.



(a)

40

Neat PVC PVC/LPCL19.8-C4 PVC/hbPCL9.6-C4 PVC/hbPCL5.8-C4 PVC/hbPCL3.3-C4 PVC/hbPCL2.3-C4 PVC/DEHP

Stress (MPa)

30

20

10

0 0

100

200

300

400

Tensile strength (MPa)

(b)

20

420

18

390

16

360

14

330

12

300

10

270

Elongation at break (%)

Strain (%)

PVC/ DEHP

PVC/ hbPCL2.3-C4

PVC/ hbPCL3.3-C4

PVC/ hbPCL5.8-C4

PVC/ hbPCL9.6-C4

PVC/ LPCL19.8-C4

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 44 of 59

Figure 8. (a) Representative stress-strain curves of neat PVC and plasticized PVCs. (b) Variation in the tensile strength and elongation at break of neat PVC and plasticized PVCs.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(b) 100 80

Transmittance (%)

Page 45 of 59

60 40 20 0 400

PVC/LPCL19.8-C4

PVC/hbPCL3.3-C4

PVC/hbPCL9.6-C4 PVC/hbPCL5.8-C4

PVC/hbPCL2.3-C4 PVC/DEHP

500

600

700

800

900

Wavelength (nm)

Figure 9. (a) Digital images and (b) UV-Vis spectra of plasticized PVCs.



35

36.0

30 25 20 15 10

Regulation (5.5%)

5 4.5

3.7

PVC/ DEHP

4.6

PVC/ hbPCL2.3-C4

2.4

PVC/ hbPCL3.3-C4

0

PVC/ hbPCL9.6-C4

0

PVC/ hbPCL5.8-C4

Weight loss after migration (%)

40

PVC/ LPCL19.8-C4

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 10. Weight loss of plasticized PVCs heated in n-hexane at 50 °C for 2 h.


Page 46 of 59

Page 47 of 59 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC

Highly branched polycaprolactone is synthesized using glycidol as a novel branching monomer to be applied as a green plasticizer.



Scheme 1. Synthetic scheme for the Sn(Oct)2-catalyzed bulk copolymerization of ε-caprolactone and glycidol. 87x56mm (600 x 600 DPI)


Page 48 of 59

Page 49 of 59 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. (a) Representative 1H NMR spectrum of hbPCL with the proton assignments, (b) 1H NMR spectra of LPCL and hbPCLs with different branch structures, and (c) SEC traces of the synthesized hbPCLs and LPCL. 67x32mm (600 x 600 DPI)



Figure 2. (a) 1H NMR spectra of samples retrieved after different time intervals during the copolymerization of CL and glycidol with a feed molar ratio of [CL]:[glycidol]:[BDM] = 40:5:1, (b) plots of the monomer conversion versus time, and (c) schematic illustration of the molecular architecture of the resulting copolymer on the basis of the reactivity ratios of the monomers. 62x29mm (600 x 600 DPI)


Page 50 of 59

Page 51 of 59 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. 13C NMR spectra of LPCL19.8 (black line), hbPCL5.8 (blue line), and hbPCL2.3 (red line) corresponding to the carbon atoms of (a) ester groups, (b) BDM, glycerol unit, and terminal CL, and (c) inner CL. 57x24mm (600 x 600 DPI)



Figure 4. Crystallization characteristics of hbPCLs and LPCL. (a) 2nd DSC heating curves, (b) the maximum endothermic temperatures as a function of the end group concentration, (c) DSC cooling curves, (d) XRD patterns, and (e) phases of hbPCLs and LPCL. 75x39mm (600 x 600 DPI)


Page 52 of 59

Page 53 of 59 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. (a) DSC thermograms of hbPCLs and LPCL (upper) and those of hbPCL-C4s and LPCL-C4 (lower) and (b) steady shear viscosity as a function of shear rate for hbPCL-C4s and LPCL-C4. (c) Dependence of the glass transition temperature and steady shear viscosity on the linear backbone length. (d) Variation in the length of the linear segments and end-group concentration of hbPCL-C4s and LPCL-C4. 76x40mm (600 x 600 DPI)



Figure 6. (a) FTIR spectra of plasticized PVCs. FTIR spectra showing the carbonyl region of (b) LPCL19.8-C4, (c) hbPCL9.6-C4, (d) hbPCL5.8-C4, (e) hbPCL3.3-C4, (f) hbPCL2.3-C4, (g) DEHP and the corresponding PVC mixtures. FTIR spectra of plasticized PVC for (h) methylene deformation and (i) C–Cl stretching regions. (j) Ratios of the absorbance intensities of amorphous and crystalline bands of plasticized PVCs. 120x102mm (600 x 600 DPI)


Page 54 of 59

Page 55 of 59 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. (a) DSC thermograms of neat PVC and plasticized PVCs. (b) Temperature dependence of the storage modulus and loss tangent of neat PVC and plasticized PVCs. (c) Comparison of the plasticization efficiencies and structural homogeneities of plasticized PVC samples. 82x39mm (600 x 600 DPI)



Figure 8. (a) Representative stress-strain curves of neat PVC and plasticized PVCs. (b) Variation in the tensile strength and elongation at break of neat PVC and plasticized PVCs. 81x123mm (600 x 600 DPI)


Page 56 of 59

Page 57 of 59 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 9. (a) Digital images and (b) UV-Vis spectra of plasticized PVCs. 74x113mm (600 x 600 DPI)



Figure 10. Weight loss of plasticized PVCs heated in n-hexane at 50 °C for 2 h. 41x36mm (600 x 600 DPI)


Page 58 of 59

Page 59 of 59 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Highly branched polycaprolactone is synthesized using glycidol as a novel branching monomer to be applied as a green plasticizer. 43x22mm (600 x 600 DPI)


Glycidol Copolymeric Green

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