Well-defined polyurethane-graft-poly(N,N-dimethylacrylamide) copolymer with a controlled graft density and grafted chain length: synthesis and its application as a Pickering emulsion

Abstract

A robust method for the synthesis of well-defined polyurethane-graft-poly(N,N-dimethylacrylamide) (PU-g-PDMA) copolymers with good control over the graft density and the grafted chain length was presented in this study. Firstly, a functional polyurethane (fPU) polymer with lateral trithiocarbonate-based chain transfer agent group was synthesized by the polyaddition reaction of 2,2-bis(hydroxymethyl)butyl 2-(ethylthiocarbonothioylthio)-2-methylpropanoate (BEMP) with hexamethylene diisocyanate (HDI), and the subsequent chain extension reaction with 1,4-butanediol (BDO). The content of the chain-transfer groups in the synthesized fPU could be well tuned by altering the mole ratio of BEMP to HDI during the synthesis of the prepolymer. The produced fPU was then applied as the macro-RAFT agent to mediate the radical polymerization reaction of the N,N-dimethylacrylamide (DMA) monomers that were initiated by azobisisobutyronitrile (AIBN), resulting in well-defined amphiphilic PU-g-PDMA graft copolymers with a known graft density and tunable grafted chain lengths. The structure of the obtained PU-g-PDMA was characterized carefully using FTIR and ¹H NMR. The average molecular weight and polydistribution of PU-g-PDMA copolymers were analyzed by GPC. The thermal properties of fPU and PU-g-PDMA copolymers were investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The amphiphilic PU-g-PDMA graft copolymers could self assemble into spherical nanoparticles with a core–shell structure in water. The core–shell structured nanoparticles could be applied as emulsifiers for the formation of stabilized toluene-in-water Pickering emulsions, and an extremely low content of emulsifiers (∼0.01%) relative to the total weight of oil and water was required.

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1. Introduction

Owing to its wide mechanical properties, from rubber to plastic, and good chemical resistance as well as good biocompatibility, polyurethane materials (PUs) are widely used in many industrial and biomedical applications.^1–3 However, some important applications are restricted by reason of their inherent limitations. For example, the application of PUs as blood-contacting materials is prohibited because of their relatively poor blood compatibility resulting from protein adsorption on the hydrophobic surfaces.⁴ Therefore, modification of PUs to satisfy their application requirements in varied fields is very significant for both academic and industrial fields.

Among the numerous developed modification techniques of materials, grafting is considered to be one of the most promising and versatile approaches to improve the surface properties of polymers for a wide variety of applications.^5,6 So far, there have been many reports on the synthesis of PU-based graft copolymers,^7–24 and the main synthetic methods can be divided into three categories, namely “graft from”,^7–16 “graft through”^17–21 and “graft to”.^22–24 For the PU-based graft copolymers synthesized by the “graft from” strategy, the grafted chains could be introduced either by radical polymerization of vinyl monomers, which were initiated by plasma treatment,^7–9 UV irradiation,^10,11 or by chemically induced radical copolymerization of vinyl monomers with the incorporated polymeric vinyl groups on the surface of PU.^12–16 It is well recognized that the grafted chain length and the graft density are very important in many application areas of graft copolymers, e.g. colloid stabilization, adhesion, lubrication, and tribology. However, synthesis of PU-based graft copolymers with controlled graft chain densities and grafted chain lengths is not easy via the above-mentioned method due to the uncontrollable polymerization process. Recently, Jin and his coworkers¹⁷ reported a protein-resistant PU-g-POEGMA copolymer that was synthesized by surface-initiated atom transfer radical polymerization (s-ATRP). A series of PU-g-POEGMA copolymers with tunable side chain lengths have been facilely obtained. However, the precise control over the graft density for the targeted graft copolymer could not be achieved because of the uncontrolled initiation process. Compared to the “graft from” strategy, PU-based graft copolymers with controlled graft chain densities and grafted chain lengths are relatively easily realized by the “graft through” technique. For this method, preformed dihydroxyl-terminated macromonomers with well-defined architectures are usually required.^18–21 The polyaddition reaction between the macromonomers and the diisocyanate is then conducted, which is followed by chain extension using low molecular weight diols, such as 1,4-butanediol, and these as-produced PUs exhibited well-defined structures with controlled graft densities and grafted chain lengths. However, the entrapment of reactive diol end groups by polymer chains and the steric hindrance effect from the nonreactive polymer chain needed to be overcome during the polyaddition reaction between diol-terminated macromonomer and diisocyanate. In addition, the precise control over the mole ratio of reactive functional groups is also difficult due to the polydispersity in the average molecular weight of polymers. The “graft to” strategy is also used for the synthesis of PU-based graft copolymers. PU polymers with reactive functional groups, such as –NCO, were firstly synthesized and then reacted with polymers containing hydroxyl or amino groups, yielding PU based graft copolymers.^22–25 Owing to the separated synthesis of the main chain and the graft chain, the structure of the final graft copolymer can be flexibly adjusted. However, the reaction efficiency and the purification of the final graft copolymer are the main challenges for this strategy. Therefore, developing a robust method for the synthesis of well defined PU-based graft copolymers with controlled graft densities and grafted chain lengths is still significant.

Herein, a novel method was proposed for the synthesis of well-defined amphiphilic PU-g-PDMA (polyurethane-graft-poly(N,N-dimethylacrylamide)) graft copolymers by combining the polyaddition reaction with the reversible addition fragmentation chain transfer (RAFT) polymerization. In brief, functional PU polymers containing a known content of lateral trithiocarbonate-based RAFT chain-transfer agent groups (fPU) were first synthesized by the polyaddition of a low molecular weight RAFT reagent containing diols with hexamethylene diisocyanate (HDI), followed by chain extension with 1,4-butanediol (BDO). The synthesized functional PU (fPU) was then applied as a macro-RAFT agent to mediate the radical polymerization of the N,N-dimethylacrylamide monomer initiated by azobisisobutyronitrile (AIBN), resulting in amphiphilic PU-g-PDMA graft copolymers. The structures and thermal properties of the synthesized PU-g-PDMA graft copolymers were characterized. The advantages of our procedure lie in its capability to control both the graft chain density and the graft chain length. At the same time, the type of the graft chains could be selected flexibly.

2. Experimental

2.1 Materials

2,2-Bis(hydroxymethyl)butyl 2-(ethylthiocarbonothioylthio)-2-methylpropanoate (BEMP) was synthesized according to procedures described in the literature.²⁶ N,N-Dimethylacrylamide (DMA) was purified by passing it through a dried basic alumina column and distilling over CaH₂ under reduced pressure prior to polymerization. Hexamethylene diisocyanate (HDI) and dibutyltin dilaurate (DBTDL) were purchased from Aladdin, and were used as received. Azobisisobutyronitrile (AIBN) was recrystallized twice from ethanol prior to use. Dimethylacetamide (DMAc) and 1,4-butanediol (BDO) were purified by reduced pressure distillation over CaH₂ before use.

2.2 Synthesis of functional PU (fPU)

In a typical reaction, HDI (3 g, 17.86 mmol), BEMP (3.04 g, 8.93 mmol), DBTDL (200 mg, 0.32 mmol) and 200 mL fresh DMAc were charged into a three-neck round-bottom flask under a nitrogen purge. The polyaddition reaction was carried out at 70 °C for about 40 minutes. Subsequently, BDO (804 mg, 8.93 mmol) was added and the mixture was allowed to react at 70 °C for another 10 h. After reaction, DMAc was removed under reduced pressure, and the obtained polymer was washed with anhydrous ether several times. The purified product was dried under vacuum for 24 h at room temperature.

2.3 Synthesis of PU-g-PDMA graft copolymer

In a typical reaction, 150 mg of fPU (1.38 mmol RAFT groups per gram of polymer), 2.05 g of DMA (20.7 mmol), 4.85 mg of AIBN (0.03 mmol) and 20 mL of fresh DMAc were charged into a dried round bottomed flask. The flask was sealed after degassing O₂ and then immersed in an oil bath at 65 °C. After 7 h of polymerization, the reaction was stopped by diluting with DMAc. The polymer was then precipitated in diethyl ether. After multiple precipitations in anhydrous ether, filtration and washing with methanol, the purified product was dried under vacuum at room temperature. The conversion of monomer (C%) and the relative mass fraction (R%) of PDMA in the PU-g-PDMA graft copolymer were determined gravimetrically and calculated using the following equations:

C% = (W₂ − W₁)/W_M

R% = (W₂ − W₁)/W₂

where W_M, W₁, and W₂ are the masses of the DMA monomer, fPU, and the final PU-g-PDMA copolymer, respectively.

2.4 Self assembly of the PU-g-PDMA copolymer in water

In a typical procedure, 2 mg of PU-g-PDMA copolymer (R% = 91.9%) was dissolved in 3 mL of DMF. And then, 12 mL of pure water was pumped into the solution at a rate of 0.5 mL h⁻¹ with the assistance of gentle stirring. After addition of water, the mixture was transferred into a bag and dialyzed against pure water. The final concentration of micelles was 0.12 mg mL⁻¹.

2.5 Characterization and test

FTIR spectra were recorded on a Perkin-Elmer Spectrum One FTIR spectrometer by the KBr pellet method. ¹H NMR spectra were recorded with a Bruker AV-400 NMR spectrometer. Molecular weight and polydispersity index (PDI) of polymers were determined by GPC, which was performed on a Waters 1515 GPC equipped with a Waters 2414 differential refractive index detector in DMF at 80 °C with a row rate of 1.0 mL min⁻¹. A narrow-polydispersity polystyrene was used as a calibration standard. Thermogravimetric analysis (TGA) was performed on a TA SDT Q600 instrument under a nitrogen atmosphere at a heating rate of 10 °C min⁻¹ in the range from 25 to 700 °C. Differential scanning calorimetry (DSC) was performed on a TA instruments Q10 differential scanning calorimeter under a nitrogen atmosphere. Samples were quickly heated to 200 °C and kept for 5 min to remove thermal history, then cooled to −20 °C at a rate of 10 °C min⁻¹, and finally reheated to 200 °C at a rate of 10 °C min⁻¹. All DSC traces were from the second heating to minimize effects of thermal history. Glass transition temperatures (T_g) were taken as the midpoint of the transition. Elemental analysis (EA) was performed with a Perkin-Elmer CHN 2400 analyzer. The morphologies of aggregates formed by the self assembly of the PU-g-PDMA copolymer in water were observed by transmission electron microscopy (TEM) measurements. The TEM measurement was carried out with a microscope (JEOL2010) operated at an acceleration voltage of 200 kV. Optical micrographs (OM) were collected with an optical microscope (Leica, DM 4500P).

3. Results and discussion

3.1 Synthesis of fPU

In this study, fPU containing lateral trithiocarbonate-based RAFT chain-transfer agent groups was synthesized by two successive processes, and the overall synthesis route was shown in Scheme 1.

Scheme 1 Synthesis route of the PU-g-PDMA copolymer.

Firstly, a PU prepolymer with lateral trithiocarbonate-based RAFT chain-transfer agent groups and end –NCO functions was synthesized by the polyaddition reaction of BEMP with HDI (Scheme 1a). And the produced PU prepolymer was then subjected to chain extension with BDO, affording the fPU (Scheme 1b). The mole ratio of HDI to BEMP was strictly fixed to be 2 during the preparation of the PU prepolymer, and the mole number of [–NCO] was slightly less than that of [–OH] provided by both BEMP and BDO.

The obtained fPU was characterized by FTIR spectroscopy. Fig. 1B showed the FTIR spectrum of fPU. As shown in Fig. 1B, the typical absorption peaks at 2970–2860, 1721 and 1256 cm⁻¹, were assigned to the stretching vibration of C–H, –C=O and C–N of the fPU backbone, respectively.^27–29 The N–H bending vibration absorption peak of fPU appeared at 1576 cm⁻¹, while the N–H stretching vibration absorption at about 3336 cm⁻¹ was overlapped by the vibration peak of the absorbed H₂O.³⁰ The characteristic peak originating from the –C=S group appeared at 1074 cm^−131,32 in the FTIR spectrum of fPU, indicating the successful incorporation of RAFT groups onto the PU backbone. Fig. 2 showed the ¹H NMR spectra of BEMP, fPU and PU-g-PDMA. The ¹H NMR spectrum of BEMP, shown in Fig. 2A, agreed with our earlier report.²⁶ The ¹H NMR spectrum of fPU was shown in Fig. 2B, in which the respective resonances were clearly assigned, and these agreed with the earlier reports.^33–35 Both FTIR and ¹H NMR analyses revealed that a PU with lateral trithiocarbonate-based RAFT chain-transfer agent groups had been successfully synthesized. The content of the incorporated RAFT groups onto the fPU backbone was measured by EA. Based on the EA, the S content in the present fPU product was 13.25%, which was nearly consistent with the theoretical value of 12.53%. Therefore, BEMP could be considered to be consumed completely and converted to fPU, and the content of RAFT groups was thus calculated to be about 1.38 mmol per gram of fPU. In fact, the degree of polymerization could be adjusted by altering the stoichiometrical mole ratio of [HDI]/[BEMP] during the synthesis stage of the PU prepolymer. Decreasing the mole ratio of [HDI]/[BEMP] could improve the degree of polymerization of the PU prepolymer, and more RAFT groups would be introduced onto the final fPU backbone.^36,37 Our control experiment also proved this conclusion. The mole ratio of HDI to BEMP was changed to be 4/3 during the synthesis of the PU prepolymer, and the other reaction parameters were the same. After chain extension with BDO, the content of the S in the final fPU was 15.75%, which was almost consistent with the theoretical value of 16.16%. Correspondingly, the content of RAFT groups was about 1.64 mmol per gram of fPU. In other words, the content of RAFT groups on the fPU backbone could be adjusted, and the adjustable RAFT group implied that the graft density of PU-based copolymers was adjustable.

Fig. 1 FTIR spectra of (A) BEMP, (B) fPU, (C) PDMA and (D) PU-g-PDMA copolymer (R% = 79.7 wt%).

Fig. 2 ¹H NMR spectra of (A) BEMP, (B) fPU with 1.38 mmol RAFT group per gram of polymer and (C) PU-g-PDMA copolymer (R% = 79.7 wt%).

3.2 Synthesis of the PU-g-PDMA copolymer

It is well known that trithiocarbonate-based RAFT chain-transfer agents demonstrate good capabilities to mediate the controlled radical polymerization of many vinyl monomers.^38–42 Therefore, the introduced lateral trithiocarbonate-based RAFT chain-transfer agent groups on the PU backbone provided a tool for the synthesis of PU based graft copolymers with controlled grafted chain length. In the present study, a hydrophilic DMA monomer was selected as a graft monomer to synthesize amphiphilic graft copolymers. Besides its hydrophilicity, another reason for selecting DMA was the result of its known controlled radical polymerization in the presence of 2-(ethylthiocarbonothioylthio)-2-methylpropanoate (EMP).^43,44 The graft polymerization of DMA monomers was conducted in DMAc at 65 °C using AIBN as the initiator, and fPU containing 1.38 mmol of lateral trithiocarbonate-based RAFT groups per gram of polymer was used as the macro-RAFT agent (Scheme 1c). A series of parallel polymerization reactions were carried out with the same reaction parameters other than reaction time. The monomer conversion and ln([M]₀/[M]) versus polymerization time were plotted and presented in Fig. 3A. As shown in Fig. 3A, the monomer conversion increased with the prolongation of the reaction time. The monomer conversion reached 24.4% after 1 h of polymerization. After 7 h of polymerization, 85.5% of the monomer had been consumed. The semilogarithmic ln([M]₀/[M]) versus reaction time was also plotted in Fig. 3A. As shown in Fig. 3A, the nearly linear semilogarithmic plot of ln([M]₀/[M]) versus reaction time suggested that the polymerization was pseudo first-order with respect to the monomer. It means that the concentration of active centers is almost constant throughout the polymerization of DMA monomer.⁴⁵ Consequently, the polymerization process of DMA was controlled, and the grafted chain length could be adjusted by altering the polymerization reaction time. The graft density of the obtained graft copolymer was considered to be equal to the amount of RAFT groups on the fPU backbone (vide infra). Here, the relative mass fraction of the PDMA component, R%, in the dual graft copolymer of PU-g-PDMA was employed to represent the variation of the grafted chain length with time. For example, R% was 79.7 wt% for 1 h of polymerization, and reached 91.9 wt% after 7 h of polymerization.

Fig. 3 (A) The monomer conversion and ln([M]₀/[M]) versus the polymerization time; (B) GPC traces of PU-g-PDMA copolymers at different polymerization times; (C) GPC traces of the PU-g-PDMA copolymers at different [fPU] : [DMA] ratios. Polymerization conditions: the content of RAFT group was 1.38 mmol per gram of fPU, [fPU] : [AIBN] = 7 : 1, and the monomer concentration was 10 wt%.

The average molecular weight and the polydistribution of fPU and the obtained graft copolymers were analyzed by GPC. Fig. 3B showed GPC profiles of PU-g-PDMA copolymers with different R% values. As shown in Fig. 3B, the M_n and PDI of fPU were 7000 g mol⁻¹ and 1.21, respectively. After grafting with PDMA chains, all the GPC traces of PU-g-PDMA graft copolymers showed a clear shift towards the high molecular weight region by comparison with fPU. GPC traces of PU-g-PDMA copolymers presented a symmetrical monomodal peak with a narrower distribution. The M_n of PU-g-PDMA graft copolymers increased with the prolongation of the polymerization reaction time, while PDI showed a minor change. The detailed GPC results were displayed in Table 1. All these suggested that the graft copolymerization of DMA initiated by AIBN in the presence of fPU was controllable throughout the polymerization reaction. Fig. 3C showed GPC traces of PU-g-PDMA graft copolymers obtained at various [DMA] : [fPU] ratios and the same reaction time. The [DMA] : [fPU] ratio ranged from 50 to 200. As shown in Fig. 3C, GPC traces of PU-g-PDMA graft copolymers showed a symmetrical monomodal peak. M_n presented an obvious shift towards the high molecular region with the increase of [DMA] : [fPU], whereas PDI showed a minor alternation (Table 1). These implied that the controlled radical polymerization of DMA mediated by fPU could be applied to a broader range of monomer concentrations.

Table 1 RAFT polymerization of DMA mediated by the fPU macro-RAFT agent at different reaction conditions

Entry	[DMA] : [fPU]^a	Reaction time (h)	Mn,GPC (g mol^−1)	Mw,GPC (g mol^−1)	PDI
a The content of RAFT groups was 1.38 mmol per gram of fPU; the molar ratio of [fPU] : [AIBN] was 7 : 1; the concentration of the DMA monomer was 10 wt%; the polymerization temperature was 65 °C.
1	0	—	7000	8500	1.21
2	100 : 1	1	17 600	22 000	1.25
3	100 : 1	2	20 600	24 800	1.21
4	100 : 1	4	21 200	29 400	1.39
5	100 : 1	5	22 100	29 300	1.33
6	100 : 1	7	25 900	32 900	1.27
7	50 : 1	4	17 000	21 800	1.29
8	200 : 1	4	34 900	45 000	1.29

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The structure of the graft copolymer was characterized by FTIR and ¹H NMR. Fig. 1D showed the FTIR spectrum of the PU-g-PDMA copolymer. By comparison with the FTIR spectrum of PDMA, shown in Fig. 1C, the vibration absorption peaks at 3347 cm⁻¹ and 1637 cm⁻¹ in Fig. 1D were assigned to N–H and –C=O of PDMA.^46,47 Furthermore, the intensities of peaks at 2860, 1721 and 1256 cm⁻¹, corresponding to C–H, –C=O and C–N, were obviously strengthened. All these suggested that the PDMA chains have been grafted on the PU backbone.^48,49 The ¹H NMR spectrum of the PU-g-PDMA copolymer was shown in Fig. 2C. Besides the resonance peaks originating from the protons of fPU, clear resonance peaks at δ = 3.09, 2.64 and 1.60 ppm, corresponding to –N(CH₃)₂, –CCH₂CH and –CCH₂ protons of the grafted PDMA chains, were observed,^50,51 revealing the successful grafting of PDMA chains on the PU backbone. Graft efficiency is an important parameter for graft copolymerization, which can be evaluated by ¹H NMR.^51,52 For the present system, the resonance peak at 1.75 ppm in the ¹H NMR spectrum of fPU, corresponding to the protons of the –C(CH₃)₂ group attached to SC(S)SC₂H₅, almost completely disappeared after the incorporation of PDMA chains, as shown in Fig. 2C. The proton resonance peak of the –C(CH₃)₂ group of PU-g-PDMA appeared at 0.85 ppm in Fig. 2C.^52,53 The complete disappearance of the resonance signal at 1.75 ppm in the ¹H NMR spectrum of PU-g-PDMA implies that all lateral trithiocarbonate-based RAFT chain-transfer agent groups on fPU have taken part in the graft copolymerization reaction. Therefore, the graft density of the obtained graft copolymer was equal to the amount of RAFT groups on the fPU backbone.

3.3 TGA and DSC analyses

The thermal stability of fPU and PU-g-PDMA copolymers were investigated by TGA. Fig. 4A displayed the TGA curves of the PDMA homopolymer, fPU and PU-g-PDMA copolymers with different values of R%. The PDMA homopolymer (M_n = 6000 g mol⁻¹, PDI = 1.08) was synthesized by RAFT polymerization mediated by BEMP. As shown in Fig. 4A, pure PDMA showed a two-step decomposition process. The first mass loss was about 2 wt% between 200 °C and 270 °C, which was resulted from the evaporation of the absorbed water.⁵⁴ The second stage with about 88 wt% mass loss started at 350 °C and ended at 514 °C. The TGA trace of fPU presented two degradation stages. The first degradation with about 11 wt% weight loss occurred between 180 °C and 250 °C, which may originate from the evaporation of the absorbed water and the pyrolysis of the lateral groups of fPU. The second degradation with about 86 wt% loss that occurred between 250 °C and 470 °C should be caused by the pyrolysis of the fPU backbone.⁵⁵ TGA traces of PU-g-PDMA copolymers with different R% were also shown in Fig. 4A. We can see that the PU-g-PDMA (R% = 79.7 wt%) presented a three-stage degradation process. The first weight loss at about 7 wt% occurred between 190 °C and 270 °C, and the second decomposition process, about 30 wt% of weight loss, occurred between 270 °C and 370 °C. These two segments of decomposition arose from the evaporation of absorbed water and the partial pyrolysis of PU. The third degradation stage occurred between 370 °C and 520 °C, and corresponded to almost 61 wt% weight loss, was attributed to the pyrolysis of grafted PDMA. Compared with the PU-g-PDMA copolymer (R% = 79.7 wt%), PU-g-PDMA (R% = 91.9 wt%) presented a reduced weight loss (about 13 wt%) between 190 °C and 370 °C and an increased weight loss (about 80%) between 370 °C and 520 °C. Clearly, the weight loss tendency was approximately consistent with the respective relative mass fraction in the different graft copolymers.

Fig. 4 (A) TGA curves of the PDMA homopolymer, fPU with 1.38 mmol RAFT group per gram of polymer and PU-g-PDMA copolymers with different values of R%; (B) DSC heating curves of the PDMA homopolymer, fPU with 1.38 mmol RAFT group per gram of polymer and PU-g-PDMA copolymers with different values of R% (c, 79.7 wt%; d, 91.9 wt%).

DSC traces of the PDMA homopolymer, fPU and PU-g-PDMA copolymers with different R% from the second heating under nitrogen atmosphere were recorded and were shown in Fig. 4B. It was found that all polymers exhibited a characteristic glass transition. The values of glass transition temperature (T_g) of the PDMA homopolymer and fPU were 107 °C, and 31.6 °C, respectively. For PU-g-PDMA copolymers, as expected, only one T_g was observed. The DSC measurements indicated that the PU-g-PDMA copolymer with more PDMA content showed a higher T_g. For example, the values of T_g of PU-g-PDMA with 79.7 wt% and 91.9 wt% of PDMA were 85.9 °C and 103.3 °C, respectively. For the graft copolymer, T_g can also be estimated using the Fox equation:⁵⁶

1/T_g = W_A/T_gA + W_B/T_gB

where W_A and W_B represent the mass fraction of component A and component B in the dual graft copolymer, respectively; T_gA, T_gB and T_g represent the glass transition temperature of component A, component B and the dual graft copolymer, respectively.

Using above formula, T_g values of PU-g-PDMA copolymers with 79.7 wt%, 91.9 wt% of PDMA content were estimated to be 72.1 °C and 89.3 °C, which showed the same tendency as that obtained by DSC measurement. Both TGA and DSC results suggested that the relative content of PDMA estimated by the gravimetrical method was reliable.

3.4 Formation of toluene-in-water Pickering emulsions

The PU-g-PDMA copolymer possesses a hydrophobic PU backbone and many hydrophilic PDMA grafted chains. Such amphiphilic PU-g-PDMA copolymers can self assemble into spherical particles with PU as the core and PDMA as the shell in pure water. Fig. 5A showed a DLS trace of the obtained dispersion. The average hydrodynamic diameter (〈D_h〉) of the aggregates formed by the self assembly of the PU-g-PDMA copolymer was almost 100 nm with a PDI of 0.45. Typical TEM images of nanoparticles were displayed in Fig. 5B and C and the nanoparticles showed spherical core–shell structures with an average size of about 80 nm. The size estimated by TEM was a bit smaller than that from the DLS measurement, this is the result of shrinkage of particles during drying.

Fig. 5 (A) DLS trace of aggregates formed by self assembly of the PU-g-PDMA copolymer (R% = 91.9 wt%) in water; (B, C) typical TEM images of nanoaggregates.

The star-like polymer nanoparticles with a core–shell structure have been proved to possess excellent interfacial performance and could be applied as emulsifiers to generate ultrastabilized Pickering emulsions.^57–60 In this study, a highly stabilized toluene-in-water Pickering emulsion (V_toluene/V_water = 1) was formed using the obtained spherical nanoparticles as emulsifiers, and the weight percent of the emulsifiers was about 0.01% versus the total weight of the oil and water for emulsion formation. As shown in Fig. 6A, the emulsion layer thickness was about 50% with respect to the total liquid layer. According to optical microscopy, the average droplet size was about 48 μm (Fig. 6B). The formed Pickering emulsion showed a high stability, and no obvious change in appearance or droplet size after storage for one month at room temperature (Fig. 6C).

Fig. 6 (A) Photograph of the toluene-in-water Pickering emulsion (V_toluene/V_water = 1) stabilized by 0.01% of polymer nanoparticles; typical optical microscopy images of droplets of a toluene-in-water Pickering emulsion after homogenization (B) and one month storage at room temperature (C).

4. Conclusion

PU-g-PDMA copolymers with controlled graft densities and grafted chain lengths have been successfully synthesized by combining polyaddition reaction with RAFT polymerization. The content of RAFT groups on the PU backbone could be adjusted by altering the mole ratio of [–NCO]/[–OH] during the preparation of the prepolymer, which could be used as a potential tool to control the graft density. The graft copolymerization of DMA initiated by AIBN presented a controlled style in the presence of fPU. The M_n of PU-g-PDMA copolymers increased with the prolongation of the polymerization time, while the values of PDI showed minor changes. Amphiphilic PU-g-PDMA copolymers could self assemble into stabilized spherical nanoparticles in pure water. The formed spherical nanoparticles demonstrated excellent emulsifying performances, and an ultrastabilized toluene-in-water Pickering emulsion could be generated at an extremely low particle content (∼0.01 wt%) versus the total mass of the oil and water for the emulsion formation.

Acknowledgements

The authors thank the financial supports from the National Natural Science Foundation of China (21574112) and Open Project of Hunan Provincial University Innovation Platform (15K123).

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