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Thermal, Crystallization Properties, and Micellization Behavior of HEC‑g‑PPDO Copolymer: Microstructure Parameters Effect Xue-Ting Dong, Wen-Ting Shi, Hai-Chun Dang, Wen-Yi Bao, Xiu-Li Wang,* and Yu-Zhong Wang Center for Degradable and Flame-Retardant Polymeric Materials (ERCPM-MoE), College of Chemistry, State Key Laboratory of Polymer Materials Engineering, National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), Sichuan University, 29 Wangjiang Road, Chengdu 610064, China ABSTRACT: Hydroxyethylcellulose (HEC)-graft-poly(p-dioxanone) (PPDO) copolymer (HEC-g-PPDO) was synthesized homogeneously in ionic liquid 1-butyl-3-methylimidazolium chloride by coupling PPDO onto HEC backbone in the presence of 1,6-hexamethylene diisocyanate (HDI). When the feed ratio of PPDO and HEC was adjusted, as well as the molecular weight of PPDO, a series of HEC-g-PPDO copolymers with different microstructure parameters were obtained. The chemical structure, thermal stability, and thermal transition behaviors were investigated by FT-IR, 1H NMR, TG, DSC, and nano DSC, respectively. These amphiphilic HEC-g-PPDO copolymers can self-assembly into micelles. The critical micelle concentration (CMC) and the hydrodynamic diameters were determined by fluorescence and dynamic light scattering (DLS) analyses. It was found that both the degree of substitution (DS) and polymerization (DP) of PPDO had an effect on the thermal stability, crystallization, and micellization of HEC-g-PPDO copolymers. Due to the crystallization of PPDO side chain, HEC-g-11.7PPDO0.15 whose DP and DS are 11.7 and 0.15, respectively, shows a minimum CMC value. The micelle size was in the range of 200−800 nm depending on the microstructure parameters of HEC-g-PPDO copolymers.

1. INTRODUCTION Hydroxyethylcellulose (HEC), is a nonionic and water-soluble cellulose ether and has been extensive used as a gelling and thickening agent. Due to its biocompatibility, biodegradability, and protein rejecting ability, it is often used as capsule ingredient to improve the hydrophobic drug’s dissolution in gastrointestinal fluids.1 In the past decade, polymer micelles especially based on natural macromolecules such as cellulose and chitosan, have been paid great attentions because of their stability in the blood circulation and excellent biocompatibility.2−4 Cellulose and their water-soluble derivatives such as HEC and hydroxypropylcellulose (HPC) can be tailed to have amphiphilicity after appropriate chemical modifications. Jiang et al.5 synthesized the HEC-g-PAA copolymer via free-radical graft polymerization of acrylic acid (AA) from HEC backbones. It was found that the HEC-g-PAA copolymers can self-assembly into micelles or hollow spheres by adjusting the pH value. They6 also synthesized binary graft copolymer HEC-g(PNiPAAm&PAA) (CNiPAa) by successive radical polymerization of N-isopropylacrylamide (NiPAAm) and acrylic acid from HEC and found that CNiPAa had thermal and pH sensitivity. The residual hydroxyl groups in cellulose derivatives can initiate ring-opening polymerization of lactones to prepare cellulose-grafted aliphatic polyester copolymers.7,8 Both cellulose derivatives and aliphatic polyester, such as poly(lactic acid) (PLA) and poly(ε-caprolactone) (PCL), are biodegradable polymers; therefore, the amphiphilic copolymers based on them have good biocompatibility and nontoxicity, which are necessary for the use of the copolymers as drug carriers.9 As one of the biodegradable and biocompatible aliphatic polyesters, poly(p-dioxanone) (PPDO) has the ester and ether bonds in the main chains simultaneously, showing high flexibility and good tensile strength, which has been used as surgical suture.10 Our research team synthesized a series of amphiphilic © 2012 American Chemical Society

copolymers based on PPDO and found that they can selfassembly into aggregates with different morphologies11,12 due to the crystallization of PPDO segment. Besides this, hydrophobic drugs such as ibuprofen can be controlled release from these amphiphilic copolymers-based carriers.13 The solubility of HEC is poor in most common solvents, which means its modification has to be processed in a heterogeneous solution.14 Ionic liquids (ILs) and low-melting-point salts only consist of cations and anions, which have been considered as the green solvent to replace volatile organic compounds (VOCs) due to their unique physicochemical properties.15,16 Many publications demonstrated that ILs, especially for hydrophilic ionic liquids such as 1-butyl-3-methylimidazolium chloride ([BMIM]Cl), and 1-allyl-3-methylimidazolium chloride ([AMIM]Cl), have excellent dissolution ability and are suitable for cellulose and its derivatives modification.17−19 The high chloride concentration and activity in [BMIM]Cl, which is assumed highly effective in breaking the extensive hydrogenbonding network present in cellulose and its derivatives.20,21 Although there is the growing body of literature concerning hydrophobic modification of water-soluble cellulose derivatives, relatively little is known about the effect of microstructure parameters of cellulose based amphiphilic copolymers on their thermal stability, crystallization, and micellization. Therefore, in this study, the PPDO chain with different degrees of polymerization (DP) was homogenously grafted onto HEC in 1-butyl-3-methylimidazolium chloride ([Bmim]Cl). When the ratio of PPDO and HEC was adjusted, a series of HEC-g-PPDO Received: Revised: Accepted: Published: 14037

April 4, 2012 September 28, 2012 October 3, 2012 October 3, 2012 dx.doi.org/10.1021/ie300873a | Ind. Eng. Chem. Res. 2012, 51, 14037−14046

Industrial & Engineering Chemistry Research

Article

Scheme 1. Synthetic Route of HEC-g-PPDO Copolymers

Figure 1. FT-IR spectra of neat HEC and PPDO and HECg-11.7PPDO0.27.

copolymers with different degrees of substitution (DS) were synthesized. The influence of DP and DS values of HEC-g-PPDO copolymers on their thermal and crystallization properties were discussed in detail. Besides this, the critical micelle concentration and the size of the micelles were investigated.

Figure 2. Typical (A) and enlarged (B) 1H NMR spectra of HECg-11.7PPDO0.27.

2. EXPERIMENTAL SECTION 2.1. Materials. PDO (99.9%) was provided by the Pilot Plant of the Center for Degradable and Flame-Retardant Polymeric Materials (Chengdu, China). It was dried over CaH2 and then distilled before use. HEC (Mv = 90 000 g/mol, DS = 1.5, MS = 2.5) was purchased from J&K Chemical Co. (Shanghai, China) and was used after being dried under vacuum to remove the absorption water. 1-Butyl-3-methylimidazolium chloride ([Bmim]Cl) (99%) purchased from Chengjie Chemical Co. Ltd. (Shanghai, China) with a purity of 99% and was dried in vacuum at 100 °C about 4−5 h before use. Stannous octoate (SnOct2) (95%) and 1,6-hexamethylene diisocyanate (HDI, AR grade) were purchased from Sigma-Aldrich and used without purification. The other reagents were used as received. A dialysis bag (MD34, cut off molecular weight 3500 Da) was purchased from Beijing Solarbio Science & Technology Co. Ltd. (Beijing, China).

2.2. Preparation of Single Hydroxyl Terminated PPDO (PPDO−OH). PPDO with a single terminal hydroxyl group was prepared through the ring-opening polymerization (ROP) of PDO using butyl alcohol as an initiator and SnOct2 as a catalyst. When the polymerization was completed, the crude product of PPDO was dissolved in CHCl3 and precipitated in methyl alcohol. The final product was dried under vacuum until the weight was constant. The synthetic route of PPDO−OH prepolymer was shown in Scheme 1. By adjusting the ratio of butyl alcohol and PDO, we can obtain PPDO with different DP values. The DP values were calculated on the basis of their H NMR results. 2.3. Preparation of HEC-g-PPDO. A certain amount of ionic liquid (1-butyl-3-methylimidazolium chloride) was charged into a dried flask and heated. After the ionic liquid melted, the flask was degassed in a vacuum line and filled with N2 until there 14038

dx.doi.org/10.1021/ie300873a | Ind. Eng. Chem. Res. 2012, 51, 14037−14046


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10 mg of HEC-g-PPDO copolymer was dissolved in 2 mL of DMF (C = 5 mg/mL), and then the solution was dripped into 3 mL of double distilled water. After being evenly mixed, all of the solution was transferred into a dialysis bag (3500 Da molecular weight cut off) and dialyzed in 2 L double distilled water for 2 days to remove DMF. During the dialysis process, double distilled water was replaced every 12 h. Finally, the solution was filtrated through 0.45 μm microfilter to remove the aggregates in the solution, and the micelle solution was obtained. 2.5. Characterizations. Fourier transform infrared (FTIR) spectra of the samples were recorded on a Nicolet FT-IR 170SX spectrometer in a range from 500 to 4000 cm−1. The samples were milled into powders and then mixed and laminated with KBr pellets. The resolution and scanning time were 4 cm−1 and 32 times, respectively. The microstructure parameters of the obtained copolymers were characterized by a Bruker AV II-400 spectrometer at ambient temperature, using CDCl3 or DMSO-d6 as the corresponding solvent and tetramethylsilane (TMS) as the internal chemical shift standard.

was no bubble, and the process was repeated three times. Under a nitrogen atmosphere, HEC and PPDO with various feed ratios were charged into the flask, stirred, and heated. After the mixture was melted, the flask was degassed in a vacuum line and filled with N2 until there was no bubble. This process was repeated three times. Then a certain amount HDI, in which the molar ratio of PPDO−OH to HDI was 1:1, was injected into the flask, the coupling reaction was processed for 1 h, then the flask was cooled in the ice water. The crude product was precipitated in a mixed solvent of anhydrous methyl alcohol and ethyl ether. To eliminate the cross-linked HEC, the precipitated products were dissolved in DMF and then the insoluble substances were filtered. After that, the product was dried in vacuum oven until constant weight. The synthetic route of HEC-g-PPDO was also shown in Scheme 1. 2.4. Preparation of HEC-g-PPDO Micelles. The detailed preparation process of HEC-g-PPDO micelle was as follows: Table 1. Structure Parameters and CMC Values of HEC-gPPDO Copolymers PPDO samplea

HEC:PPDO (mol:mol)

Mn

DP

DS

HEC-g-11.7PPDO0.35 HEC-g-11.7PPDO0.27 HEC-g-11.7PPDO0.15 HEC-g-11.7PPDO0.10 HEC-g-11.7PPDO0.06 HEC-g-2.8PPDO0.13 HEC-g-6.6PPDO0.17 HEC-g-16.5PPDO0.19 HEC-g-23.1PPDO0.14

1:0.2 1:0.1 1:0.05 1:0.03 1:0.02 1:0.05 1:0.05 1:0.05 1:0.05

1269 1269 1269 1269 1269 349 748 1750 2487

11.7 11.7 11.7 11.7 11.7 2.8 6.6 16.5 23.1

0.35 0.27 0.15 0.10 0.06 0.13 0.17 0.19 0.14

CMC (mg/mL)

Table 2. TG Data of HEC, PPDO, and HEC-g-PPDO Copolymers

20.4 × 10−2 10.0 × 10−2 5.62 × 10−2 3.16 × 10−2 1.00 × 10−2 56.2 × 10−2 24.5 × 10−2 17.0 × 10−2

a

HEC-g-nPPDOm: n is the DP value of PPDO chain; m is the DS of copolymers.

sample

T5% (°C)

T1max (°C)

HEC PPDO HEC-g-11.7PPDO0.35 HEC-g-11.7PPDO0.27 HEC-g-11.7PPDO0.10 HEC-g-2.8PPDO0.13 HEC-g-6.6PPDO0.17 HEC-g-23.1PPDO0.14

268 179 210 210 207 246 236 216

333 230 242 244

246

T2max (°C)

276 276 276 284 286 280

Figure 3. TG and DTG curves of HEC-g-PPDO with different microstructure parameters. 14039



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Figure 4. DSC cooling run of HEC-g-PPDO copolymers with different DS values (A) or with different DP values (B).

Figure 5. DSC heating scans of HEC-g-PPDO copolymers with different DS values (A) or with different DP values (B).

The thermal stability of the obtained copolymers was measured by a NETZSCH 209F1 TA Instruments from room temperature to 500 °C at a heating rate of 10 °C/min under a dynamic nitrogen flow of 50 mL/min. Conventional DSC measurement was performed with Q200DSC (TA Co., USA) in aluminum pan under nitrogen atmosphere. The samples (5−10 mg) were preheated up to 150 °C and kept for 3 min to eliminate the thermal history, then cooled to −50 °C at a rate of 10 °C/min. After that, the samples were heated up to 120 °C at a rate of 10 °C/min. The thermal transition behaviors of the obtained micelles without water were also investigated by the conventional DSC. The detailed procedures were as follows: a certain volume of the solution taken from HEC-g-PPDO micelles was quenched in the liquid nitrogen, which made the morphology of aggregates fixed. Then the frozen samples were freeze-dried at −50 °C under vacuum. The dried aggregates were collected and studied by DSC without erasing the thermal history. Microcalorimetric measurements were performed on NANO DSC (TA Instruments-Waters LLC, New Castle, DE) at an

external pressure of 3.0 atm using deionized water as an external reference. The cell volume was 0.33 mL. The heating rate was 1 °C/min. The testing sample with a concentration of 4.0 mg.mL−1 in deionized water was degassed at 25 °C for 10 min and was treated as followed: First, it was kept at approximate 95 °C for 5 min and then reduced to 25 °C immediately. Finally, a certain volume of sample incubated at various times was placed in the NANO−DSC to implement a first heating scan. The particles were filtered through a Millipore 0.45 μm filter and then their hydrodynamic diameters and distribution were measured by dynamic light scattering (DLS) (Brookhaven model BI-200SM) and 9000AT correlator using a Innova304 He−Ne laser (1 W, λ = 532 nm) at a fixed scattering angle (θ) of 90 °C and a correlation measurement time of 2 min with cumulate analysis and CONTIN software. To investigate the assembly behavior of HEC-g-PPDO copolymer, the critical micelle concentration (CMC) was determined by fluorescence analysis using pyrene as a fluorescence probe. The concentrations of sample solution were varied from 3.16 × 10−4 to 1 mg/mL containing 6 × 10−5 mol/L pyrene. 14040



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Table 3. Thermal Transition Behaviors of HEC-g-PPDO Copolymers cooling scan samples HEC PPDO HEC-g-11.7PPDO0.35 HEC-g-11.7PPDO0.27 HEC-g-11.7PPDO0.10 HEC-g-11.7PPDO0.06 HEC-g-2.8PPDO0.13 HEC-g-6.6PPDO0.17 HEC-g-11.7PPDO0.15 HEC-g-16.5PPDO0.19 HEC-g-23.1PPDO0.14 a

second heating scan

Tc (°C)

ΔHc (J/g)

Tg (°C)

Tc (°C)

ΔHc (J/g)

Tm (°C)

ΔHm (J/g)

χca (%)

34.5 36.1 20.3

38.3 8.2 3.4

−22.9 −22.2 −25.1 −23.7 −22.6 −23.1 −20.0 −24.9 −19.7 −20.6

20.6 26.0 24.6 22.8

30.7 24.0 12.2 5.53

97.7 100.5 98.7 98.7 100.1

86.4 41.6 23.9 8.0 2.3

61.2 47.6 30.4 17.8 7.5

26.0 23.0 29.8 27.4

0.4 1.0 3.5 12.5

101.5 98.2 104.0 102.9

2.8 12.0 17.3 28.6

6.2 20.6 22.3 42.6

20.3 47.9

3.4 14.4

The crystallinity (χc) of the PPDO chains was calculated by using following equation:

χc =

ΔHm × 100% ΔHm 0 × PPDO (wt %) ÷ 100

where ΔHm0 is the melting enthalpy per gram of PPDO in its completely crystalline state (141.2 J/g) and PPDO (wt %) is the content of PPDO block in weight fraction.

The solutions were oscillated by ultrasonic for half an hour before fluorescence measurements. Fluorescence spectra were recorded on CARY Eclipse fluorescence spectrophotometer (Varian, U.S). The slit openings were set at 10 mm (excitation) and 1 mm (emission), respectively. The excitation and emission wavelength was 335 and 390 nm, respectively. The scanning rate was 60 nm/min and the response time was 2 s.

3. RESULTS AND DISCUSSION 3.1. Preparation of HEC-g-PPDO. FT-IR was used to investigate the chemical structure of HEC-g-PPDO. Figure 1 shows the FTIR spectra of HEC, PPDO and the HEC-g-PPDO. For pure HEC, the band at 3485−3450 cm−1 is due to the associating vibration hydroxyl group. The band at 2924 cm−1 is ascribed to C−H stretching of −CH2 groups. The stretching vibration of glucose ring appears at 1611 cm−1.22 Compared with the FTIR spectrum of pure HEC, some characteristic peaks of PPDO were observed in HEC-g-PPDO spectrum. For example, the peak located at 1741 cm−1 was ascribed to the carbonyl stretching vibration of PPDO side chain; the peaks found at 1421 and 1201 cm−1 belonged to the methylene flexural vibration and C−O−C vibration of PPDO, respectively.23 For HEC-g-PPDO, the vibration absorption of hydroxyl group found at 3462 cm−1 can be still seen, which illustrated that there existed unreacted hydroxyl groups of HEC. The fine microstructure parameters of HEC-g-PPDO will be analyzed by NMR. Figure 2 shows the H NMR spectra PPDO-g-HEC. The methyl and methylene protons (Hd, He, Hf) of n-butyl alcohol appeared at 0.90, 1.31, and 1.57 ppm, respectively. The peaks at 4.16, 3.70, and 4.22 ppm were assigned to the a-, b-, and c-methylene of PPDO graft chain.24 From the enlarged H NMR spectrum of PPDO-g-HEC (Figure 2B), the detailed proton’s chemical shifts of anhydroglucose unit can be determined. The multipeaks at 3.41−3.45 ppm were ascribed to H2, H3, H4 of anhydroglucose unit.25 The singlet at 3.84 ppm was belonged to H5 of HEC. The chemical shifts of methylene protons (H6, H7, H8) of HEC were found at 3.50− 3.55 ppm.26

Figure 6. Plots of I335/I333 versus log C for HEC-g-PPDO copolymers with different DS values (A) or with different DP value (B).

On the basis of the above results, the degree of substitution (DS) of PPDO in the copolymer was determined from the 14041



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Figure 7. DSC heating scan curves of the frozen dried HEC-g-PPDO micelles. All of these measurements were obtained without erasing the thermal history.

Table 4. Thermal Characteristic of Frozen-Dried HEC-gPPDO Micelles sample

Tm1

Tm2

ΔHm1+2 (J/g)

χc (%)

HEC-g-11.7PPDO0.35 HEC-g-11.7PPDO0.27 HEC-g-11.7PPDO0.10 HEC-g-11.7PPDO0.06 HEC-g-6.6PPDO0.17 HEC-g-11.7PPDO0.15 HEC-g-16.5PPDO0.19

90.8 83.7 86.6 84.1 89.3 89.1 89.9

102.8 95.8 97.7 102 100.6 100.8 100.3

39.3 26.4 9.6 4.2 3.9 14.4 19.5

45.0 33.6 21.4 13.6 8.7 24.8 25.1

signal intensities of PDO and HEC units. The signals of H-5 proton of HEC and Hd of PPDO can be clearly differentiated from the other protons of HEC and PPDO. Therefore, DS values of PPDO branches were calculated as follows: DS = Id /3I 5

On the basis of the HNMR spectra, a series of HEC-g-PPDO copolymers were prepared by adjusting the different molecular ratio of HEC and PPDO, and the structural parameters of HEC-g-PPDO are summarized in Table 1. In the following text, HEC-g-PPDO copolymers were named as HEC-g-nPPDOm, in which n was the DP value of PPDO chain and m was the DS of copolymers. 3.2. Thermal Stability of HEC-g-PPDO. Figure 3 shows the TG and DTG curves of HEC-g-PPDO with different DS and DP values. The data determined from TG curves are listed in Table 2. It can be clearly found that the thermal stability of HEC-g-PPDO copolymers was better than that of pure PPDO due to the introduction of HEC. The 5% weight loss temperature (T5%) of HEC-g-PPDO was enhanced from 179 °C (pure PPDO) to 216−246 °C. The maximum decomposition temperature of HEC and PPDO was 333 and 230 °C, respectively. For the concerned HEC-g-PPDO copolymers, only those with high PPDO length or density showed the decomposition range of PPDO side chain. For example, when the DP value of PPDO was fixed at 11.7, two decomposition ranges, which were ascribed to the decomposition of PPDO and HEC, can only be found for those copolymers with DS values higher than 0.15. When the DS value was fixed at about 0.15, for PPDO with low DP values, such as HEC-g-2.8PPDO0.13 and HEC-g-6.6PPDO0.17, only HEC decomposition was found. When the DP value of the PPDO side chain was increased to 11.7 or 23.6, both decomposition ranges were found. Compared with the

Figure 8. Nano DSC first heating scan curves of HEC-g-PPDO micelles in aqueous solution.

decomposition temperature of HEC, the T2max of HEC-g-PPDO were low. This can be due to the fact the residual hydrogen bond was further broken when the PPDO side chain was grafted onto cellulose backbone.27 3.3. Thermal Transition Behaviors of HEC-g-PPDO. The DSC cooling and heating scans of HEC-g-PPDO with different structure parameters are shown in Figures 4 and 5. And the thermal transition temperatures, the corresponded enthalpies as well as relative crystallinity are listed 14042



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could crystallize. And the crystallization peak was more obvious with the increase of DS value. In the heating scans of HEC-g-PPDO (Figure 5), when the DS values were fixed, all the samples showed the melting temperatures except HEC-g-2.8PPDO0.13. The samples containing longer PPDO chains such as HEC-g-11.7PPDO0.15, HECg-16.5PPDO0.19, and HEC-g-23.1PPDO0.14 even showed cold crystallization peaks during the heating scans. When the DP values were fixed at 11.7, all the samples showed cold crystallization peaks in their DSC curves except HEC-g-11.7PPDO0.06. On the basis of the results, we can draw a conclusion that the crystallization behavior of HEC-g-PPDO copolymer was mainly attributed to the crystallization of PPDO side chains. Due the poor ordering and regularity of HEC, it cannot crystallize. Crystalline properties of PPDO blocks in HEC-g-PPDO copolymer were enhanced with the increase of DP or DS values. Besides this, the relative crystalline degree of HEC-g-PPDO was lower than that of PPDO, which can be ascribed to the fact that the crystallization PPDO side chains were hindered by the HEC backbones. 3.4. Micellization Behaviors of HEC-g-PPDO Aqueous Solution. Polymer micelle was formed only when the concentration of polymer was higher than the critical micelle concentration (CMC). HEC-g-PPDO copolymer can form a micelle due to its amphiphilic nature in which the hydrophobic PPDO segments formed a core and the hydrophilic HEC backbone extended into the aqueous solution to being a shell. The CMC values of HEC-g-PPDO were determined by fluorescence spectrometry using pyrene as a probe. Usually, the intensity ratio of I1/I3 (I333/I335) of pyrene changed greatly when the micelle was formed. This was a reflection that pyrene molecules transferred from a water media to the hydrophobic core of micelles. By comparing the intensity ratios of I1/I3

Table 5. Thermal Characteristic of HEC-g-PPDO Micelles in Aqueous Solution from Nano DSC Heating Scan sample

Tm (°C)

ΔHm (kJ/mol)

HEC-g-11.7PPDO0.35 HEC-g-11.7PPDO0.27 HEC-g-11.7PPDO0.10 HEC-g-11.7PPDO0.06 HEC-g-6.6PPDO0.17 HEC-g-11.7PPDO0.15 HEC-g-16.5PPDO0.19

88.4 88.2 88.7 89.1 88.4 89.0 88.2

17093.8 14995.9 7198.6 4869.2 6264.7 8818.8 14370.2

in Table 3. The relative crystallinity is calculated by the following equation: χc =

ΔHm ΔHm0

× PPDO(wt %)

(1)

ΔHm and ΔHm are the melting enthalpy of the samples and the completely crystalline PPDO,28 respectively. From the DSC results we can see that pure HEC had no glass transition temperature, cold crystallization, and melting peaks during DSC heating scans. As for HEC-g-PPDO copolymers, their thermal transition behaviors had great relationships with their DP and DS values. When the DS values were fixed at around 0.15, a crystallization peak can only be observed in the cooling scan of HEC-g-23.1PPDO0.14 (Figure 4). As far as the others samples were concerned, no obvious crystallization peaks were found. This illustrated that when the DS values were low, only the copolymers containing long PPDO chain can crystallized at the cooling rate of 10 °C/min. When the length of PPDO chain was fixed at 11.7, it was found that only samples whose DS values were 0.27 and 0.34 (HEC-g-11.7PPDO0.27, HEC-g-11.7PPDO0.35) 0

Figure 9. Hydrodynamic diameters and distribution of HEC-g-PPDO with different DS values. 14043



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Figure 10. Hydrodynamic diameters and distribution of HEC-g-PPDO with different DP values.

increase of temperature, the perfect crystal melted (96−103 °C). The crystallinity based on ΔHm1 and ΔHm2 are listed in Table 4. From the data, we also can see that copolymer with longer PPDO or denser PPDO blocks have higher crystalline ability. Furthermore, the thermal transition behaviors of HEC-gPPDO micelles in water are investigated by NANO−DSC, which are shown in Figure 8. In agreement with the conventional DSC, the results showed that heating scan curves of HEC-g-PPDO micelles in water exhibited the melting transition of crystalline PPDO, and ΔHm was increased with the increase of DP or DS values (shown in Table 5). Both conventional and nano DSC results show that PPDO with a DP value of 11.7 was easy to crystallize whether with or without water. This strong crystalline ability made it have not only low CMC value but also different micelle morphologies, which will be discussed in the next paper. 3.5. Size of HEC-g-PPDO Micelles. Figures 9 and 10 showed the size and size distribution of HEC-g-PPDO with different DS and DP values examined by DLS. The detailed results of micelles size as a function of DS or DP values are shown in Figure 11. From Figure 11, it can be seen clearly that the size of HEC-g-PPDO micelles was increased with the increase of DS value. The same results can also be found for HEC-g-PPDO with different DP values. This illustrated that the increase of DS or DP values made the content of hydrophobic chain increase, which facilitated the growth of the hydrophobic core of polymeric micelles. Compared with the results reported by Jiang,5,6 it was found that the size of HEC-g-PPDO micelles was larger. This can be due to the strong hydrophobicity and crystallization of PPDO branches.32 The surface tension of micelle was enhanced when the hydrophobic segment of copolymer became long and dense, which would result in the increase of micelles’ diameters.33,34

(I333/I3335) of pyrene, we can determine the CMC value of HEC-g-PPDO copolymer by the interception of two straight lines.29 The plots of I335/I333 ratio versus log C for HEC-g-PPDO copolymers with different DS values or with different DP value are shown in Figure 6. The detailed CMC values of HEC-g-PPDO copolymer determined from Figure 6 are listed in Table 1. From Table 1 it can be seen that the CMC values of HEC-gPPDO copolymer have great relationships with their microstructure parameters. When the length of PPDO chain was fixed at 11.7, it was found that the CMC value was increased with the increase of DS values. For example, the CMC value of HEC-g-11.7PPDO0.06 was 1.00 × 10−2 mg/mL and then it was increased to 20.4 × 10−2 mg/mL for HEC-g-11.7PPDO0.35. When the effect of PPDO length on CMC values was investigated, a strange phenomenon was found. A minimum CMC value was obtained for HEC-g-11.7PPDO0.15. When the DP of PPDO was enhanced to 16.5 or 23.1, however, CMC values were increased. The lower CMC value means that this micelle could remain stable in solution even after extreme dilution and preserve stability without dissociation after intravenous injected into much larger volume of blood for systemic circulation.30 This result illustrated that micelle formed by PPDO with DP value of 11.7 was more stable than the others. Because PPDO block is semicrystalline, its crystalline behavior plays an important role in the micelle forming. The crystalline property of frozen-dried HEC-g-PPDO micelles is examined by DSC, which is shown in Figure 7 and the relative data are shown in Table 4. Compared with the results shown in Figure 5, the DSC heating curves of frozen-dried HEC-g-PPDO micelles are complicated. Double endothermic peaks assigned to the melting of the crystalline PPDO chains in the micelles appeared. This illustrated that the crystallization of PPDO blocks in the micelles was imperfect making the imperfect crystalline melted at lower temperature (84−91 °C).31 With the 14044



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ACKNOWLEDGMENTS This work was supported financially by the National Natural Science Foundation of China (20974066, 51121001), the Excellent Youth Foundation of Sichuan (2011JQ0007), Program for Changjiang Scholars and Innovative Research Team in University of Ministry of Education of China (IRT1026), and the Program of International S & T Cooperation (2011DFA51420).

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Figure 11. Effect of DP and DS values on the size of HEC-g-PPDO micelles.

4. CONCLUSIONS HEC-g-PPDO copolymer was synthesized successfully by coupling PPDO onto HEC backbone in ionic liquid. The thermal stability of the copolymer was enhanced compared with pure PPDO. Only samples with high DP and DS values showed two decomposition range. Due to the poor ordering and regularity of HEC, only the PPDO side chain can crystallize. And the crystallization of PPDO was enhanced with the increase of DP or DS values. When the length of the PPDO chain was fixed at 11.7, it was found that the CMC value was increased with the increase of DS values. A minimum CMC value was obtained for HEC-g-11.7PPDO0.15 when the DS values were fixed. With the further increase of the DP value to 16.5 or 23.1, however, CMC values were increased. This can be ascribed to the crystallization and hydrophobicity of PPDO side chain during the micelle preparation. The micelle size was in the range of 200−800 nm depending on the microstructure parameters of HEC-g-PPDO copolymers.

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