Phase inversion, formation and stability mechanism of poly(urethane-acrylate) nanoemulsions based on block-copolymer surfmer

Accepted Manuscript Full Length Article Phase inversion, formation and stability mechanism of poly (urethane-acrylate) nanoemulsions based on block-copolymer surfmer Haihua Wang, Bei Li, Guiqiang Fei, Yiding Shen, Ke Zhu PII: DOI: Reference:

S0169-4332(18)31593-9 https://doi.org/10.1016/j.apsusc.2018.06.023 APSUSC 39531

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

10 April 2018 25 May 2018 5 June 2018

Please cite this article as: H. Wang, B. Li, G. Fei, Y. Shen, K. Zhu, Phase inversion, formation and stability mechanism of poly (urethane-acrylate) nanoemulsions based on block-copolymer surfmer, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.06.023

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Phase inversion, formation and stability mechanism of poly (urethane-acrylate) nanoemulsions based on block-copolymer surfmer Haihua Wang1,2*, Bei Li1, Guiqiang Fei1*, Yiding Shen1, Ke Zhu1 1. Shaanxi Key Laboratory of Chemical Additives for Industry, Shaanxi University of Science and Technology, Xi’an 710021, China 2. Department of Material Science and Engineering, Northwestern University, Evanston, Illinois 60208, USA Corresponding author: Haihua Wang Postal Address: Shaanxi Key Laboratory of Chemical Additives for Industry, Shaanxi University of Science and Technology, Xi’an 710021, China E-mail address: [email protected], [email protected] Corresponding author: Guiqiang Fei Postal Address: Shaanxi Key Laboratory of Chemical Additives for Industry, Shaanxi University of Science and Technology, Xi’an 710021, China E-mail address: [email protected]

Abstract: We reported herein the synthesis of block copolymers and their application in the in-situ emulsion polymerization of waterborne poly (urethane-acrylate) (WPUA) nanoemulsions based on phase inversion process. The polymerizable surfmers containing terminal unsaturated double bonds (MSA-r) were successfully synthesized via catalytic chain transfer polymerization (CCTP). Random copolymer (MSA-t) prepared through radical polymerization was utilized as control. The structure, surface tension and aggregation behavior of surfmers were characterized. The phase inversion, formation and stability of WPUA nanoemulsions were systematically investigated. Effects of MSA-r surfmer content and BMA/MMA ratio on the water resistance and thermal stability of the corresponding films were also discussed. It was found that the phase inversion mechanism of PUA prepolymer in the presence of MSA-r surfmers is

different from conventional polyurethane prepolymer. The addition of MSA-r and MSA-t surfmers alters the rheological behaviors of PUA prepolymer via PUA-surfmer interactions, as well as the phase inversion process. Compared with WPUA/MSA-t emulsion, WPUA/MSA-r nanoemulsion displayed improved stability at higher solid content. The minimum particle size of WPUA/MSA-r nanoemulsion was 88.7nm and only increased to 88.8 nm after 12-month storage, while the particle size of WPUA/MSA-t emulsion was 131.3 nm and increased to 178.9nm. Moreover, the WPUA/MSA-r film possessed improved water resistance and thermal stability.

Keywords: Catalytic chain transfer; Polymerizable sufmer; Poly (urethane-acrylate); In-situ emulsion polymerization; Phase inversion

1. Introduction Emulsion polymerization is one of the most important methods for preparing polymer materials, which has many advantages, such as environmental friendliness, high reaction rate, easy to obtain high molecular weight polymers, excellent heat dissipation and viscosity control, etc [1]. This method has gotten extensive applications in various fields including tanning, papermaking, coatings, drug delivery and so on [2-6]. Surfactants are commonly used in emulsion polymerization to control the particle size and improve the stability of the colloidal particles [7-9]. In most cases, surfactants are not covalently bound to colloidal particles, they are only physically adsorbed onto the colloidal particle surface in dynamic equilibrium with the water phase. Therefore, surfactants can be desorbed under certain external conditions; the stabilizing properties are thereby lost, resulting in particle flocculation. Additionally, some surfactants migrate to the film surface in the process of film forming, which will inevitably weaken the water resistance, gloss and adhesion of films [10,11]. To

overcome the drawbacks induced by surfactant migration, polymerizable surfactants (surfmers) have received intensive attention. They can be covalently anchored onto the particles, impeding their desorption from particles and migration to film surface [12-13]. However, migration issues still remain unless surfmers are completely polymerized [14]. An alternative solution is to employ the macrosurfactants of high molecular weight that exhibit

lower mobility. The macrosurfactants with

well-designed block or graft structure have been reported to be effective in hindering migration [15,16]. Polymers with controlled structure and molecule weight distribution (MWD) have been prepared through living radical polymerization, including radical addition-fragmentation chain transfer (RAFT) [17], and atom transfer

radical polymerization (ATRP)

[18]

and catalytic chain transfer

polymerization (CCTP) [19]. Compared with traditional living radical polymerization, CCTP method has been certified to be a powerful method to prepare macrosurfactants with relative low molecular weight, block structure and terminal unsaturated double bonds [20]. CCTP method also possesses several advantages, i.e. low ppm levels of catalyst, mild reaction condition and abundant availability of monomers [21-23]. Macrosurfactants with block structure and terminal unsaturated double bonds prepared through CCTP method are able to combine the merits of polymerizable surfactants and macromolecular surfactants. Block-copolymer surfmers have been used in emulsion polymerization of MMA-BA-MAA polyacrylate [20], synthesis of quantum dot-polymer networks [24], nanofiber micelles [25], etc. To our best knowledge, few investigations have been attempted to prepare waterborne poly (urethane-acrylate) emulsion with the assistance of vinyl-terminated block copolymer macrosurfactants. Chen et al. has reported the possible mechanism for free radical emulsion

polymerization of polyacrylate, which is fundamentally different from the mechanism for in situ emulsion polymerization of waterborne poly (urethane-acrylate) nanoemulsions [20]. In this research, we first report the in situ emulsion polymerization of waterborne poly

(urethane-acrylate)

(WPUA)

nanoemuslions

with

the

assistance

of

block-copolymer surfme. The block copolymer PMAA-b-PBMA (MSA-r) surfmers were fabricated by the CCTP method, and the PMAA-co-PBMA random copolymer (MSA-t) was also prepared through traditional free radical polymerization as control. The structure, surface tension and aggregation behavior of surfmers are characterized. The phase inversion, formation and stability mechnisms of WPUA nanoemulsions were systematically investigated, as well as the water resistance and thermal stability of the corresponding films. The as-prepared translucent WPUA nanoemulsions can potentially be used in the field of various coatings for strengthening the cellulose fiber paper, protecting wood and metal surface. 2. Experimental 2.1 Materials Methacrylic acid (MAA, AR, Kermel, China), butyl methacrylate (BMA, AR, Kermel, China), methyl methacrylate (MMA, AR, Kermel, China), butyl acrylate (BA, AR, Kermel, China), isophorone diisocyanate (IPDI, Degussa, Germany) and 2-hydroxyethyl methacrylate (HEMA, 96%, Aladdin, China) were purified by vacuum distillation. Polycaprolactone diol (PCL, Mn=1000g/mol, DAICEL, Japan), trimethylolpropane (TMP, hydroxyl value ~37.5, Mitsubishi Chemical, Japan), and 2,2-bis(hydroxymethyl) butyric acid (DMBA, 98%, Aladdin, China) were dried at 80 °C under vacuum for 8 h before use. Triethylamine (TEA, AR, Kermel, China), butanone (MEK, AR, Kermel, China), isopropanol(IPA, AR, Kermel, China),

ammonium persulfate (APS, AR, Kermel, China), azobis(isobutryonitrile) (AIBN, AR, Yamaura, China), ammonium hydroxide solution (NH3·H2O, 25.0%, Kermel, China), and cobalt catalyst [bis-(aqua) bis-(difluoroboryl) dimethylglyoximato] cobalt(II) (CoBF, 99%, Janus-New-Materials Co., Ltd, China) were used as received. Dialysis tubing with molecular weight cut-off (MWCO) of 1000 was purchased from Union Carbide of USA. 2.2 Synthesis of polymerizable surfmer via CCTP 68 g of MEK, 0.17 g of AIBN, and 10 ppm (based on the mass of BMA) of CoBF were added to a 250 mL flask equipped with a stirrer under oxygen-free conditions, which was degassed by six freeze-pump-thaw cycles. The flask was subsequently deoxygenated by repeated vacuum/nitrogen-inletting cycles and then heated to 75 ºC with continuous stirring. Subsequently, 40 g of MAA was added dropwise over a period of 1 h. The reaction was performed at 75 ºC for another 1 h. Then, different amounts of BMA and 0.8 g AIBN were introduced. After several hours, the reaction was terminated quickly by immersing the flask in an ice-water bath. Block copolymers MSA-r were thereby obtained after removing the solvent by vacuum evaporation, which can be utilized as polymerizable surfmer due to its structural characteristic. The preparation scheme and molecular structure were shown in Figure 1A. The samples were designated as MSA-r0.5, MSA-r0.7, MSA-r1, MSA-r1.5 and MSA-r2 when the molar ratio of BMA to MAA was 0.5, 0.7, 1.0, 1.5 and 2.0, respectively. Simultaneously, PMAA-co-PMBA (MSA-t) surfmer was prepared via traditional free radical polymerization as a control sample. 68 g of MEK, 0.17 g of AIBN, 40 g of MAA and 65.95 g of BMA were added into a 250 mL flask under oxygen-free conditions. The reaction was carried out at 75 ºC for 4 h, and the final product MSA-t

was also purified by vacuum evaporation. 2.3 In-situ emulsion Polymerization of Waterborne Poly (urethane-acrylate) Nanoemulsions based on Polymerizable Surfmer 7 g of IPDI, 10 g of PCL, 1.8 g of DMBA, 0.3 g of TMP, 14.4 g of MMA and 3.6 g of BA were added to a 500 mL flask under oxygen-free conditions and then heated to 80 ºC with continuous stirring. The reaction was performed for 3 h. Then 1.35 g of TEA and 1.56 g HEMA were added and the reaction was allowed to continue for 0.5 h to obtain poly (urethane-acrylate) (PUA) prepolymer. Afterwards, different amounts of surfmers in 50 g aqueous solution (neutralized by NH3·H2O, 25 wt%) were added into the reaction system at 2000 rpm stirring speed for 15 min to achieve phase inversion process. Initiator solution (dissolve 0.2 g APS in 10 ml water) was then added dropwise over 1 h period to initiate the radical polymerization of vinyl monomers. The waterborne poly (urethane-acrylate) nanoemulsions (WPUA/MSA-r and WPUA/MSA-t) were thereby obtained after another 1 h reaction (Figure 1B). The WPUA/MSA-r and WPUA/MSA-t films were prepared by pouring the nanoemulsions onto clean teflon plate and dried at room temperature. 2.4 Structure Characterization FT-Raman spectroscopy was recorded on a Renishaw-invia FT-Raman spectrometer operated at 732 nm, spectra was collected with a laser power of 140 μW, and a resolution of 0.6cm-1. The nuclear magnetic resonance (NMR) spectra were observed with a Bruker ADVANCEⅢ 400 spectrometer with deuterated dimethyl sulfoxide as solvent. Gel permeation chromatograph (GPC) was performed on a gel permeation chromatography using a Waters 2695 module, a refractive index (RI), viscosity (IV), and light scattering (low angle, LALS, and right angle, RALS) triple detector setup.

HPLC tetrahydrofuran was used as eluent at a flow rate of 1ml/min. The number-average (Mn), weight-average molecular weight (Mw), and the polydispersity index (PDI = Mw/Mn) of the molecular weight distribution (MWD) are calculated using a calibration curve based on the RI detector which was constructed using narrow molecular weight poly(styrene) standards ranging from 6910 to 844000 g/mol. Mark-Houwink-Kuhn-Sakurada parameters used for the polystyrene standards logK =-3.238, a = 0.551. Surface tension was determined on a DataPhysics DCAT21 tensiometer. The particle size of emulsions was measured by using a Malvern Mastersizer 2000 particle size analyzer. The morphology observation was performed on a FEI Tecnai G2 F20 transmission electron microscope (TEM). The water absorption was calculated according to reference [26]. Contact Angle was determined by a KRÜSS DSA-100 contact angle meter. Atomic force microscopy was observed using SPI3800N/SPA400 AFM operated in the tapping mode, thin films used for AFM studies were obtained through casting the latex on a glass plate.

3. Result and discussion In this article, MSA-r block polymers with terminal unsaturated groups were synthesized via CCTP method, and subsequently utilized as polymerizable surfmer for in situ polymerization of WPUA/MSA-r nanoemulsions, as shown in Figure 1. We firstly report the application of polymerizable surfmer in in-situ emulsion polymerization and investigate the phase inversion, formation and stability of waterborne poly (urethane-acrylate) nanoemulsions. 3.1 Comparison study between MSA-r and MSA-t Figure 2A shows the FT-Raman spectra of MSA-r and MSA-t surfmers. The peak at 1640 cm-1 is assigned to the characteristic peak of C=C stretching vibration.

While this peak is not observed in the FT-Raman spectrum of MSA-t, indicating the absence of double bond. The signals ranged from 5.5 to 6.5 ppm in the NMR spectrum of MSA-r (Figure 2B) also certify the successful incorporation of C=C bonds into MSA-r chains [27]. As shown in Figure 2C, signals a1 and b1 are attributed to the protons on the terminal double bonds of MSA-r. The FT-Raman spectra, together with NMR spectra, demonstrate that we have successfully prepared MSA-r polymerizable surfmers with terminal unsaturated double bonds. The conversion as a function of reaction time for MSA-t and MSA-r is also investigated (Figure 2D). With respect to MSA-t, the conversion increases gradually with time and then levels off when the reaction time is 4 h. However, the conversion of MSA-r becomes invariable when the reaction time increases to 6 h, which is also lower than that of MSA-t. Lower molecular weight and polydispersity index (PDI) are detected for MSA-r prepared by CCTP method in comparison with MSA-t (Table 1), which is consistent with the results from previous reports [28]. With increasing the content of hydrophobic BMA monomer, the molecular weight of MSA-r increases, but still remains much lower than that of MSA-t. In addition, the average molecular weight of MSA-r increases with increasing the reaction time (Figure 3). During the process of catalytic chain transfer, CoBF catalyst also reacts with free radicals at chain ends to generate double bonds and trivalent cobalt compounds. The content of free radicals was thereby decreased, resulting in the decrease of reaction rate. This is the reason why longer reaction time is required to increase the conversion of MSA-r. On the other hand, the trivalent cobalt compounds may initiate the free monomers, leading to lower molecular weight. Moreover, iodimetry is also utilized to measure iodine value (IV) and molecular weight of MSA-r and MSA-t. The iodine value can represent the degree of

unsaturation in the organic compound. Theoretically, the number average molecular weight can be calculated by equation (1) under the assumption that each molecular chain has a terminal unsaturated double bond. Mn 

12690  2 X

(1)

Where X is the iodine value. The molecular weight calculated by shows similar tendency as that measured by GPC, but larger than the results obtained from GPC measurement. The main reason is that some MSA-r chains don’t have terminal unsaturated double bond, the measured value of iodine value is lower than the practical value, thereby leading to the molecular weight enlargement. The content of MSA-r molecules with terminal unsaturated double bonds (Y) is estimated based on equation (2). Y

n' Mn  100% n M'n

(2)

Where n is the total number of molecules (mol); nˊ is the number of molecules with terminal unsaturated double bonds (mol); M n is the number average molecular weight measured by GPC, Mnˊ is the number average molecular weight measured by the iodometric method. With increasing the BMA content, the content of double bond terminated MSA-r increases and then keeps almost invariable. The terminal double bonds in PMAA macromonomers tend to react with trivalent cobalt compounds and form relatively stable covalent bonds. Therefore free radicals and divalent cobalt compounds can’t be easily formed since it is difficult to break the stable covalent bonds. Correspondingly, the amount of active catalysts in the reaction system decreases, and the probability of chain termination based on radical-radical coupling increases. This should be the main reason why the content of MSA-r with terminal double bond increases as increasing the BMA/MAA ratio.

In order to further investigate the structure of MSA-r and MSA-t, thermogravimetric (TG) and differential thermogravimetric (DTG) curves are provided, as well as the derivative weight loss curves (Figure 4). The MSA-r presents lower thermal stability than MSA-t. It can be seen from Figure 4B and Figure 4C that the thermal decomposition of MSA-r prepared by CCTP method is mainly divided into three stages. The first stage (135-229 °C) is related to the structure of the head-to-head (H-H) polymer. H-H polymers are formed when the double-bond terminated molecules are reinitiated (Figure 4D), and H-H structure may also be formed through coupling reaction. The dissociation energy of C-C bond that located at the β position of double bond in H-H polymers is about 84 KJ/mol, which is much lower than that of typical C-C bond energy (347 KJ/mol). The second stage (229-339 °C) is corresponding to the decomposition of double-bond terminated chain segment (Figure 4E). The third stage (339-450 °C) can be ascribed to the decomposition of carbon-carbon skeleton. In contrast, MSA-t only displays two degradation stages which are mainly related to degradation of H-H structure and C-C skeleton, the degradation peak of double-bond terminated chain segment isn’t detected. Derivative weight loss curves (Figure 4C) also certify that MSA-t degrades in two stages, while MSA-r degrades in three stages. A minimum weight loss of 32.7% at the second degradation stage is detected for MSA-r0.5, and the maximum rate of weight loss is -0.4535 %/°C. The weight loss at the second stage for MSA-r2 increases to 34.8%, and the maximum rate of weight loss is -0.4885 %/°C. This variation is related to the content of double bonds in MSA-r, which is consistent with the results measured by iodimetry method. 3.2 Surface tension and aggregation behavior of MSA-r polymerizable surfmers

Surface tension as a function of concentration for MSA-r and MSA-t surfmers in aqueous solution was measured, as illustrated in Figure 5A. Compared with MSA-t, MSA-r exhibits lower surface tension, which can be attributed to its more regular structure [29]. Moreover, the amount and density of MSA-t hydrophobic groups in air phase are lower than that of MSA-r (Figure 5D), which is responsible for the high surface tension of MSA-t. In addition, with the increasing of hydrophobic BMA content, the surface tension decreases from 48.88 mN/m to 42.72 mN/m. One one hand, more hydrophobic groups enter into the air phase due to the increased content of hydrophobic groups, leading to the decrease in surface tension. On the other hand, it is difficult for MSA-r molecules to adsorb stably at the air/liquid interface since the repelling force among MSA-r chain segments is strong due to its block structure. With increasing the content of hydrophobic BMA monomer, the length of hydrophobic chain increases, leading to the increase of molecular interaction, which can compensate the charge repelling force and thereby facilitate the stable adsorption of MSA-r at the air/liquid interface. The Gibbs adsorption equation (3) was applied to estimate the maximum surface excess concentration (Г∞). (3) Where R is the gas constant, T is the temperature (K), γ is surface tension (mN/m), and c is solution concentration (g/L). The minimum surface area per molecule (A) was calculated according to equation (4). (4) Where Г is surface excess concentration, Na is Avogadro’s number. The minimum surface area per molecule of MSA-r is in the range from 143 to 243 Å2, lower than

that of MSA-t (373 Å2), indicating MSA-r tends to adsorb at the air/liquid interface in tight-packed vertical arrays (Figure 5D). With increasing of BMA content, Г∞ decreases, while A increases from 143 to 243 Å2. Micelle aggregation behavior of MSA-r and MSA-t surfmers was studied by fluorescence emission spectrum using pyrene as the probe. The intensity ratio of the first to the third vibronic peak of pyrene (I1/I3) was measured as a function of surfmer concentration (Figure 5B). The curves show a typical sigmoidal decrease around CMC. At concentrations below CMC, pyrene molecules locate in a polar environment. When the concentration is larger than CMC, the pyrene migrates into a more hydrophobic environment, resulting in an abrupt decrease in I1/I3 ratio. The CMC values obtained from the first inflection point agrees well with the values obtained surface tension tests. The CMC value decreases with increasing the hydrophobic BMA content. With further increasing the surfmer concentration, the second inflection point is observed, after which the I1/I3 value keeps constant, suggesting that all pyrene molecules enter into the micelles. Noticeably, MSA-t is the first to reach the CMC point, since the MSA-t with random structure can quickly adsorb at the air/liquid interface in tight-packed horizontal arrays and reach the CMC point antecedently. MSA-t is also the last to reach the second inflection point, suggesting higher surfmer concentration is demanded to encapsulate all pyrene in micelles. 3.3 Phase inversion and formation of poly (urethane-acrylate) nanoemulsions The as-prepared MSA-r and MSA-t surfmers are utilized as surfactants in in-situ emulsion

polymerization

of

waterborne

poly

(urethane-acrylate)

(WPUA)

nanoemulsions via phase inversion process (PIC). Previously, PMAA-b-PBA has been adopted as surfactant in the emulsion polymerization of polyacrylate latexes, and the possible surfmer-stabilized emulsion polymerization mechanism was investigated

[20]. However, WPUA was prepared through step-growth polymerization and phase inversion, which use completely different polymerization mechanism from the radical polymerization of polyacrylate, Figure 6 shows the variation of viscosity and conductivity with water content (based on the weight of PUA prepolymer) for PUA prepolymers prepared with different ratios of BMA/MMA. The mixture of MMA and BA are utilized as continuous phase during the preparation of PUA prepolymer. Water is added into PUA prepolymer together with MSA-t or MSA-r surfmers to initiate the phase inversion process. As increasing the water content, the viscosity begins with a slight decrease followed by a dramatic increase, and then decreases abruptly again. The conductivity firstly increases and then descends. The point corresponding to the maximum viscosity or conductivity is defined as phase inversion point (PIP). Although the critical water content at the PIP determined by viscosity test and conductivity test is different owing to disparate measuring mechanisms, they can provide a point of reference. Significantly, a nonconventional behavior was detected for the phase inversion of PUA prepolymer. For conventional polyurethane prepolymer in nonaqueous media, a sort of microionic lattices is formed through the association of unsolvated “salt segment” based on Coulomb forces. This association can be quickly taken apart or eliminated with the addition of small quantities of water, resulting in sharp decrease in viscosity [31]. In contrast, as increases the water content to 20%, the viscosity of PUA/MSA-r prepolymers is almost invariable, and the viscosity of PUA/MSA-t prepolymer starts to increase when the water content is 10%. It indicates that the addition of MSA-r and MSA-t surfmers alters the rheological behaviors of PUA prepolymer via PUA-surfmer interactions. With the incorporation of surfmers, the

association of the ion centers in PUA is weakened due to the repulsive electrostatic force between PUA backbone and anionic surfmer, resulting in the slight decrease of PUA prepolymer viscosity at stage 1. A further increase in the water content leads to a conspicuous increase in viscosity (Stage 2). At stage 2, the hydrophobic segments start to lose their solvation sheath, and aggregate together to form hydrophobic associates, the viscosity increases correspondingly. However, with continuous increase of the water content, the interpolymeric networks are broken down and assemble into O/W colloidal micelles. Emulsions of lower viscosity are thereby obtained (Stage 3).

The viscosity keeps decreasing until the completion of phase

inversion. Finally, the viscosity almost keeps constant (Stage 4). The schematic model of phase inversion is shown in Figure 7. Minimal water content is required for PUA/MSA-r1 to reach the PIP and complete the phase inversion, owing to the low CMC and surface tension. It is also favorable to increase the maximum solid content of WPUA nanoemulsion (Table 2). It is also worthy to note that the viscosity of PUA/MSA-t is much higher than that of MSA-r, as well as the increasing rate of viscosity. This pehnomenon can be ascribed to enhanced interactions and chain entanglements because of its random structure and higher molecular weight (Figure 7B). The water content at PIP determined by conductivity test is much lower than that determined by viscosity test, since the mobility of free charges is greatly hindered due to the abrupt increase of MSA-t viscosity, expediting the decrease of conductivity. Conductivity-water content curves display that the conductivity increases and then decreases with increasing water content. The salt groups are ionized when the water molecules enter into the microionic lattice, resulting in the increase of conductivity. Afterwards, the conductivity decreases during the formation of colloidal

particles, since some ionic groups are wrapped inside the colloidal particles and the concentration of ionic groups reduces with increasing water content. It is also observed that the maximum conductivity at PIP decreases owing to the decline of hydrophilic group content, as well as the amount of free charges. In order to further understand the phase inversion process of PUA prepolymer, the variation of viscosity and conductivity with water content for PUA prepolymer prepared with different MSA-r1 surfmer content is also investigated. The viscosity at stage 2 decreases with increasing surfmer content, while at stage 3 the viscosity increases with surfmer content. At stage 2, the viscosity decreases due to the enhanced repulsion electrostatic force between PUA backbone and anionic surfmer with increasing the amount of MSA-r1 surfmer. At stage 3 of O/W colloidal particles formation, the surfmers isolate their hydrophobic groups from the aqueous phase by associating them with the hydrophobic segments on polymer chains. Enhanced hydrophobic association can be achieved with increasing the surfmer content, resulting in increased viscosity. Moreover, the maximum conductivity increases with increasing the MSA-r1 surfmer content owing to the increase in the content of hydrophilic groups. 3.4 Stability and water resistance of poly (urethane-acrylate) nanoemulsions Table 2 summarizes the maximum solid content, the water content at PIP, and the coagulum of WPUA/MSA nanoemulsions. In comparison with WPUA/MSA-t, the coagulum of WPUA/MSA-r1 decreases significantly from 7.42% to 1.07%, the maximum solid content increases from 20% to 40%, indicating WPUA/MSA-r1 nanoemulsion was endowed with enhanced stability at higher solid content. It can be attributed to the improved compatibility between WPUA and MSA-r1 surfmer [32]. With increasing the surfmer content from 1.3% to 3.8%, the maximum solid

content increases from 35% to 40%, and then decreases when the surfmer content is 5%. The coagulum decreases from 5.06% to 1.07%, suggesting increased stability with addition of appropriate amount of MSA-r surfmer. The BMA/MAA ratio also has a significant influence on the stability of WPUA/MSA-r nanoemulsions. Increasing BMA content is beneficial to improve the nanoemulsion stability. WPUA-MSAr1 shows the lowest coagulum, i.e., the best colloidal stability. The phenomenon is different from the results obtained by Chen et al [20]. It has been reported that the stability of polyacrylate emulsion increases with increasing PMAA content when PMAA-b-PBA surfmers were utilized as polymerizable macrosurfactants in emulsion polymerization of MMA-BA-MAA. In contrast, the stability of WPUA/MSA-r nanoemulsion displays different tendency which be due to the different emulsion polymerization methods and stability mechanisms for WPUA and polyacrylate. The variation of particle size and distribution with time is also an indicative of emulsion stability (Figure 9). It has been reported by Chen et al. [20] that stable polyacrylate latex can’t be obtained when only the block macrosurfactant is utilized as the surfactant; the particle size also increases with the addition of macrosurfactant, since micelle nucleation and droplet nucleation simultaneously predominate the polymerization process. However, the particle size decreases from 105.0 nm to 88.7 nm with the incorporation of MSA-r surfmer, and the stability of WPUA/MSA-r nanoemulsion gets improved in the absence of conventional surfactant, owing to the different emulsion polymerization mechanisms for polyacrylate and PUA. In-situ emulsion polymerization based on phase inversion process is utilized in this research. The interchain repulsion force is increased with the incorporation of surfmer before phase inversion. Therefore, it is easier for polymer chains to rearrange themselves due to increased mobility, resulting in the effective allocation of hydrophilic segments on

the colloidal particle surface and the association of hydrophobic groups inside the colloidal particles. Under such circumstances, micelles with core-shell structure are formed (Figure 10 A), and the particle size is thereby decreased. Nevertheless, the particle size increases when the MSA-r surfmer content is greater than 3.8%. Free surfmer micelles are easily formed at higher surfmer content, correspondingly the amount of surfmer adsorbed on the surface of colloidal particles decreases. Consequently, larger exposed hydrophobic regions are thereby produced on the particle surface, which can increase the frequency of particle-particle collisions and thereby raise the particle size [33]. The variation tendency of particle size with storage time further proves the above-mentioned phenomenon. After storage for 12 months, the increasing rate of particle size decreases from 5.3% to 0.1% and then increases greatly to 14.3% (Figure 9B), indicating the stability gets improved and then decreases. The results are consistent with the stability results evaluated by coagulum. The ratio of BMA to MMA in surfmer also plays an important role in the particle size and emulsion stability. The particle size decreases from 149.5 nm to 88.7 nm and then increases to 118.5 nm with increasing the ratio of BMA to MMA. This phenomenon can be explained based on the surface tension and aggregation behavior of MSA-r surfmer. With respect to MSA-r, the ability to reduce the surface tension is restricted at lower PBMA content, resulting in higher cmc value and the reduction in the number of micelles. Additionally, it is difficult for surfmer with shorter hydrophobic chain segment to anchor on the colloidal particles, which is also responsible for the higher particle size at lower PBMA content. However, larger micelles are also easily formed when the length of hydrophobic segment increases to a certain value, leading to the increased particle size. That’s the reason why the particle size shows a decreasing then increasing trend. With increasing the PBMA

content, the increasing rate of particle size also decreases from 8.9% to 0.1% and then increases to 9.4% after 12-month storage. Therefore, it is concluded that WPUA/MSA nanoemulsions with smaller particle size, improved homogeneity and stability can be obtained through controlling the ratio of BMA to MMA and surfmer content. It is also worthy to note that the particle size of WPUA/MSA-t emulsion is 131.3 nm, and the increasing rate of WPUA/MSA-t particle size reaches 36.3%, suggesting that the stability of WPUA/MSA-t is lower than that of WPUA/MSA-r. It is difficult for hydrophilic groups to migrate to the particle surface due to the high viscosity caused by enhanced chain entanglements and interchain interaction among PUA and MSA-t surfmer chains. As a consequence, the charge density on the particle surfaces decreases and the particle size increases. It can also explain the reason why WPUA/MSA-r1 nanoemulsion displays apparent core-shell morphology and no core-shell morphology is detected for WPUA/MSA-t (Figure 9). Conspicuous particle agglomeration is also observed for WPUA/MSA-t after 12-month storage, leading to the increase of particle size, which is in good agreement with particle size results. The water absorption and contact angle of WPUA/MSA films were illustrated in Figure 13. The water resistance and contact angle are also affected by the surfmer content and PBMA content. The water absorption and contact angle of WPUA/MSA-t film are 14.71% and 59.4°, respectively, while the water absorption and contact angle of WPUA/MSA-r1 film are 4.33% and 84.5°, demonstrating enhanced water resistance. With increasing the surfmer content, the water resistance decreases. The water resistance reached the maximum when the ratio of BMA to MMA is 1.5 (WPUA/MSA-r1.5). The water absorption and contact angle are 3.87% and 92.3%, respectively. Atomic force microscope (AFM) was utilized to monitor the morphology of

film-air interface (Figure 11). Little change takes place in the morphology of WPUA/MSA-r surface after water penetration for 24 h. While the morphology of WPUA/MSA-t surface undergoes a significant change due to dissolution of MSA-t in water caused by the migration of MSA-t onto the film surface. In contrast, MSA-r surfmer can react with vinyl-terminated polyurethane chains and vinyl monomers to build covalent bonds between surfmer and polymers, which can effectively suppress the surface migration. FTIR spectra for the film-substrate and film-air interface are also studied. The adsorption peaks in the FTIR spectra of WPUA/MSA-r interfaces are almost identical. However, apparent changes are observed for the adsorption peaks in the FTIR spectra of WPUA/MSA-t interfaces due to the surfmer migration. The thermal behavior analysis displays that the WPUA/MSA-r film with the addition of 2.5% MSA-r surfmer was endowed with the best thermal stability (Figure 12), but the thermal stability decreases when the surfmer content increases to 5%. The improved thermal stability also suggests improved interaction and compatibility, and vice versa. WPUA/MSA-r film also possesses enhanced thermal stability than WPUA/MSA-t, although the thermal stability of MSA-r is much lower than that of MSA-t (Figure 4). It can be concluded that the thermal stability can be improved through building covalent bonds among surfmer and PUA chains.

4. Conclusion A series of waterborne poly (urethane-acrylate) nanoemulions have been fabricated through in situ emulsion polymerization based on phase inversion process, using double-bond terminated block copolymers (MSA-r) as polymerizable surfmer. Random copolymer (MSA-t) was also prepared via free radical polymerization as control. GPC, together with iodimetry method, demonstrates the concurrent presence of terminal-unsaturated and saturated MSA-r molecules. With increasing the

BMA/MAA ratio from 0.5 to 1.0, the content of terminal-unsaturated MSA-r increased from 72.3% to 82.3%. Different from the phase inversion of conventional polyurethane prepolymer, the phase inversion process of PUA/MSA-r prepolymer mainly displays four stages: the viscosity keeps almost invariable at stage 1 due to the repulsive electrostatic force between PUA backbone and anionic surfmer, and then increases via hydrophobic association at stage 2. Afterwards, the viscosity starts to decrease when the interpolymeric networks are broken down and O/W colloidal micelles are formed at stage 3. Finally, the viscosity levels off at stage 4. Moreover, PUA/MSA-t system presents much higher viscosity during the phase inversion process owing to the enhanced interactions and chain entanglement. The function and mechanisms of polymerizable surfmer in emulsion polymerization of polyacrylate [34] and in situ emulsion polymerization of WPUA nanoemuslions are different. The particle size decreases with increasing the surfmer content and the BMA/MAA ratio. The BMA/MAA ratio and surfmer content play significant roles in the stability, particle size of WPUA/MSA-r nanoemulsions, as well as the water resistance and thermal stability of corresponding films. The foregoing results can provide fundamental theory on the preparation of stable WPUA nanoemulsions based on polymerizable surfmer.

ACKNOWLEDGMENTS This work is supported by SRF for ROCS, SEM (Grant Number 2012[1707]); the Key Research and Development Projects of Shannxi Province (Grant Number 2017GY-154); and the Key Laboratory Project of Shannxi Province project (Grant Number 16JS011).

References

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Figure Captions Figure 1. Schematic preparation of MSA-r polymerizable surfmer and waterborne poly(urethane-polyacrylate) nanoemulsions based on polymerizable surfmer. Figure 2. (A) FT-Raman spectra of MSA-r surfmer prepared via CCTP method and MSA-t via conventional free radical polymerization. (B) 1H NMR spectrum of MSA-r polymerizable surfmer prepared via CCTP method. (C) Magnification of 1H NMR spectrum for MSA-r polymerizable surfmer. (D) Curves of Conversion vs reaction time for MSA-t and MSA-r. Figure 3. (A) Gel permeation chromatography (GPC) curves for MSA-r1 at different reaction times. (B) Molecular weight and polydispersity index (PDI) as a function of reaction time for MSA-r1. Figure 4. (A) TGA curves of MSA-t and MSA-r prepared at different MAA/BMA molar ratios. (B) Differential thermogravimetric (DTG) curves of MSA-t and MSA-r. (C) The derivative weight loss curves of MSA-t and MSA-r. (D) Formation of H-H linkages during CCTP radical polymerization. (E) Process of the decomposition caused by terminal double bond. Figure 5. (A) Variation of surface tensions with concentration of MSA-Co and MSA aqueous solution. (B) Variation of I1/I3 with concentration of MSA-Co and MSA aqueous solution. (C) Adsorption parameters of MSA-t and MSA-r at the air-liquid interface. (D) Aggregation behaviors of MSA-t and MSA-r at the air-liquid interface. Figure 6. (A) Variation of viscosity with water content for PUA prepolymer solutions prepared with addition of surfmers of different BMA/MAA molar ratios during the phase inversion process. (B) Variation of conductivity with water content for PUA prepolymer solutions during the phase inversion process. Figure 7. Schematic models for the phase inversion process of PUA prepolymer at

different stages in the presence of (A) MSA-r surfmer and (B) MSA-t surfmer Figure 8. (A) Variation of viscosity with water content for PUA prepolymer solutions prepared with different MSA-r1 surfmer content during the phase inversion process. (B) Variation of conductivity with water content for PUA prepolymer solutions prepared with different MSA-r1 surfmer content during the phase inversion process. Figure 9. (A) Particle size and distributions of WPUA/MSA nanoemuslions prepared with different polymerizable surfmer content. (B) Particle size and distributions of WPUA/MSA nanoemuslions after 12-month storage prepared with different polymerizable surfmer content. (C) Particle size and distributions of WPUA/MSA nanoemuslions prepared with different polymerizable surfmer content. (D) Particle size and distributions of WPUA/MSA nanoemuslions after 12-month storage prepared with different MAA/BMA molar ratios. Figure 10. (A) TEM images of WPUA/MSA-r1.5 nanoemuslion (a-1) before and (a-2) after 12-month storage. (B) TEM images of WPUA/MSA-t nanoemuslion (b-1) before and (b-2) after 12 month storage. (C) Water resistance of WPUA/MSA-r films prepared with different polymerizable surfmer content (c-1) before and (c-2) after 12 month storage. Figure 11. (A) AFM topographic images of WPUA/MSA-r1.5 film (a-1) before and (a-2) after immersion in water for 24 h. (B) AFM topographic images of WPUA/MSA-t film (b-1) before and (b-2) after immersion in water for 24 h. (C) FTIR spectra for WPUA/MSA-r film surface and film-substrate interface. (D) FTIR spectra for WPUA/MSA-t film surface and film-substrate interface. Figure 12. (A) Thermal stability of WPUA/MSA films prepared with different polymerizable surfmer content. (B) Thermal stability of WPUA/MSA nanoemuslions prepared with different MAA/BMA molar ratios.

Table 1. The molecular weight, PDI and double bond content of MSA-t and MSA-r prepared at different monomer ratios. Sample MSA-t MSA-r0.5 MSA-r0.7 MSA-r1 MSA-r1.5 MSA-r2

Mn(×103)

Mw×103

PDI

Mnˊ (×103)

15.4 3.3 4.0 4.5 5.2 5.9

31.1 5.7 6.6 7.8 9.1 9.8

2.02 1.72 1.65 1.73 1.75 1.66

/ 4.6 5.0 5.5 6.5 7.1

Iodine value/g / 5.47 5.12 4.61 3.93 3.57

Y/% / 72.3 80.8 82.3 81.0 83.0

Table 2. Maximum solid content and water content at PIP point for WPUA/MSA nanoemulsions Emulsion

Maximum solid content

Water content at PIP

Coagulum

WPUA/MSA-t

20%

55%

7.42%

WPUA/MSA-1.3%

35%

40%

5.06%

WPUA/MSA-2.5%

40%

45%

1.07%

WPUA/MSA-3.8%

40%

53%

1.28%

WPUA/MSA-5.0%

35%

57%

6.72%

WPUA/MSA-r0.5

25%

55%

3.15%

WPUA/MSA-r0.7

35%

50%

4.21%

WPUA/MSA-r1

40%

45%

1.07%

WPUA/MSA-r1.5

35%

47%

1.54%

WPUA/MSA-r2

35%

50%

3.56%

Highlights Block copolymer surfmer with terminal double bond is synthesized with CCTP method. The surfmer is utilized for in situ emulsion polymerization of WPUA nanoemulsion. Comparison study was made between block surfmer and surfmer with random structure. The phase inversion, formation mechanism and stability of nanoemulsion were studied. The variation of morphology, water resistance and thermal stability were investigated.