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Preparation and properties of non-ionic polyurethane surfactants
Colloids and Surfaces A: Physicochem. Eng. Aspects 363 (2010) 16–21
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Preparation and properties of non-ionic polyurethane surfactants Jie Zheng a,b , Jianxin Luo a,b , Dewen Zhou a,b , Tengfei Shen a,b , Huan Li a,b , Liyan Liang a , Mangeng Lu a,∗ a b
Guangzhou Institute of Chemistry, Chinese Academy of Science, Guangzhou 510650, PR China Graduate School of the Chinese Academy of Science, Beijing 100049, PR China
a r t i c l e
i n f o
Article history: Received 17 December 2009 Received in revised form 25 March 2010 Accepted 5 April 2010 Available online 10 April 2010 Keywords: Polyurethane Non-ionic surfactants Micelle Emulsification Solubilization
a b s t r a c t A series of non-ionic polyurethane surfactants were synthesized by the reaction of isophorone diisocyanate with glycerol monostearate and mono methoxy polyethylene glycol. The chemical structure of the polyurethane surfactant was confirmed by Fourier transform infrared spectroscopy and 1 H NMR. These tailor-made surfactants exhibit excellent surface activity, but do not have a definite critical micelle concentration. The effects of pH and salt resulted in a decrease in surface tension. It was found that cloud points of the surfactants raise with increasing ethoxylated content. The emulsifying capability is also affected by the ethoxylated content and attenuated as the hydrophilic nature of monomer declines. The micelle can form from the surfactant solution and the size is increased with the solution concentration. The solubilization capability has been proved by UV spectra. © 2010 Published by Elsevier B.V.
1. Introduction Surfactant is such a kind of substance that when dissolved in a solvent at low concentration has the ability to decrease interfacial force significantly. Due to the special physical properties, it has been applied in industrial production especially in emulsion polymerization region. The remains of traditional low molecular surfactants always affect the quality of stripping film, furthermore the remains will pollute environment because they are hard to degrade or recycle [1–5]. Hence new types of surfactants have been devised for increasing performance of emulsion and reducing pollution [6–10]. Polyurethane with amphiphilic molecular structure which has an appropriate balance of hydrophilic and hydrophobic groups could be applied similarly like traditional low molecular surfactants [11,12]. It is considered that it is easy to obtain the amphiphilic polyurethane because the structure of polyurethane could be modified in a wide range by varying the composition including the isocyanates, polyols, chain extenders, blocking agents. In recent years, some correlative researches had been carried out and proved that polyurethane surfactants have well surface activity properties [13–16]. The surfactants commonly used in process of emulsion polymerization could be divided into ionic type and non-ionic type. Compare with ionic surfactants, the non-ionic surfactants are not easily affected by pH values [17], but ‘cloud point’ may limit their direct employment [18]. The non-ionic polyurethane surfactants reported [19,20] had shown excellence in surface activity, but the drawback is the cloud point
∗ Corresponding author. Tel.: +86 020 85232978. E-mail address: [email protected] (M. Lu). 0927-7757/$ – see front matter © 2010 Published by Elsevier B.V. doi:10.1016/j.colsurfa.2010.04.001
of low temperature which make direct use impossible in emulsion polymerization. In this paper, the novel non-ionic polyurethane surfactants which contain a mono-tail and bi-head groups were synthesized by introducing a long alkyl chain as hydrophobic moiety and mono methoxy polyethylene glycol as hydrophilic moiety. These tailor-made non-ionic surfactants could be used in a broad temperature range to conquer the current drawbacks and may endow latex a lot of advantages. 2. Experiment 2.1. Materials Glycerol monostearate (GM, Union chemistry, China), mono methoxy polyethylene glycol 500 (MPEG-500, Hannong, South Korea), mono methoxy polyethylene glycol 1000 (MPEG-1000) and mono methoxy polyethylene glycol 2000 (MPEG-2000) were purchased from commercial company, reagents above mentioned were dealt with vacuum drying at 65 ◦ C before use. 3-Isocyanatomethyl-3,5,5-trimethylcyclohexylisocyanate (isophorone diisocyanate, IPDI) with a purity > 99% was purchased from Degussa, dibutyl tin dilaurate solution (DBTL) with a content >90% was used without further purification. Distilled acetone was free from moisture using 4A molecular sieve before use. Water used in this experiment was de-ionized and distilled. 2.2. Synthesis of non-ionic polyurethane surfactants The basic recipes for non-ionic polyurethane surfactants are presented in Table 1. The polyaddition reaction was conducted in a 500 mL four-necked round bottom flask equipped with a mechan-
J. Zheng et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 363 (2010) 16–21
17
Table 1 The basic recipes for non-ionic polyurethane surfactants. Tag
IPDI
GM
MPEG-500
MPEG-1000
MPEG-2000
Pus IGM500 Pus IGM1000 Pus IGM2000
2 2 2
1 1 1
2 0 0
0 2 0
0 0 2
ical stirrer and a condenser under the protection of nitrogen. IPDI and GM were dissolved in acetone, and then the obtained solution was charged into the reactor with a drop of DBTL. The reaction was taken at a constant temperature at 80 ± 2 ◦ C and lasted several hours until the NCO group content reached a designed value determined by titration with dibutylamine. The prepolymer contained NCO groups was then capped with mono methoxy polyethylene glycol for 2–3 h. Afterwards drying production was obtained while acetone was removed by distillation. The structure of non-ionic polyurethane surfactant is shown in Fig. 1.
Fig. 2. FTIR spectrogram of non-ionic polyurethane surfactant (Pus IGM1000).
determining the solubilization of styrene in surfactant aqueous solution.
2.3. Characterization Analect RFX-65A Fourier transform infrared spectroscopy was taken for acquiring infrared spectrum using pressed disc method. The 1 H NMR spectrum was recorded with a Bruker DRX-400 (300 MHz) spectrometer using CDCl3 as a solvent. A JEM 100CX II transmission electron microscope (TEM) was taken to observe the conformation of micelles. The samples for TEM were prepared by negative staining method using a 2% phosphotungstic acid (PTA) as a staining agent. The surface tension measurements were carried out with a Solon A210 surface tensiometer at 28 ◦ C. The non-ionic polyurethane surfactant aqueous solutions in a series of concentrations had been prepared firstly at room temperature and then the obtained solutions were tested by surface tensimeter. In addition, the environmental factors have been considered including pH values and electrolytes. The hydrochloric acid and sodium hydroxide solutions were added to the polyurethane surfactant aqueous solutions for adjusting the pH value of the system in a broad range from 1.8 to 13.4. The obtained solutions with different pH value were then tested by surface tensiometer. The electrolyte here used was sodium chloride. A series concentration of sodium chloride solutions were prepared by the polyurethane surfactant aqueous solutions. The obtained solutions were then measured by surface tensiometer too. The cloud point was obtained by determination of the temperature while 1% diluted surfactant aqueous solution turned pellucid to visibly turbid in an heating water bath, and the mean value of 3 tests was finally confirmed as the cloud point. To investigate emulsifying capability, 3 types of monomers butyl acrylate (BA), methacrylate ester (MMA) and styrene (St) were taken to blend with 2% diluted surfactant aqueous solution respectively at equal volumes of 5 mL each for 10 min using an magnetic stirring apparatus, the volume of separate water was observed to scale emulsifying capability at different times. The Shimadzu UV-2550 ultraviolet–vis spectrometer was used for
3. Results and discussion 3.1. Characterization of non-ionic polyurethane surfactant The FTIR spectrum of non-ionic polyurethane surfactant is shown in Fig. 2. The broad peak found at 3342 cm−1 is N–H stretching vibration, and the peak at 1540 cm−1 is due to N–H flexural vibration and C–N stretching vibration. A strong C–H stretching vibration occurs at 2869 cm−1 . Another strong C O band stretching vibration at 1724 cm−1 belongs to ester. The peaks at 1245 and 1043 cm−1 respectively belong to C–O–C symmetric and dissymmetrical stretching vibration of ester. The C–O–C group of polyethylene glycol gives a peak at about 1112 cm−1 . With the characteristic absorption peaks between 2200 and 2280 cm−1 vanished, it can be approved that –NCO in the product has totally reacted. The 1 H NMR spectrum of non-ionic polyurethane surfactants was shown in Fig. 3. The proton respondence at 7.24 ppm is related to NH of urethane. The peaks displayed at 0.83–1.22 and 2.03–2.28 ppm belong to CH3 and CH2 in acyclic chain. The proton CH2 of ethoxylated and CH mainly locate at 3.35–3.70 ppm. The proton CH2 joined with carboxyl are found at 4.02–4.17 ppm. Both FTIR and 1 H NMR results can provide evidence of synthesis of non-ionic polyurethane surfactants is successful. 3.2. Surface tension measurements The surface activity of non-ionic polyurethane surfactant can be determined from the measurement of the interfacial tension as a function of solute concentration. The results are shown in Fig. 4. The surface tension curves show an obvious descending trend with the increasing concentration of surfactant aqueous solution. The
Fig. 1. The structure of non-ionic polyurethane surfactant.
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Fig. 3.
J. Zheng et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 363 (2010) 16–21
1
H NMR spectrogram of non-ionic polyurethane surfactant (Pus IGM1000).
surface excess i is defined as surface excess concentration of component i and given by i =
ni A
,
(1)
where A is the interfacial area and ni is the total number of moles of component i in the surface phase . According to Gibbs adsorption isotherm: i =
−1 d , RT d ln c
(2)
where is the surface tension, c is the concentration of surfactant, R is the gas constant and T is the absolute temperature. The minimum interfacial area per molecular am is calculated by amin =
1 Nav i
(3)
where Nav is Avogadro’s number. From Eqs. (1) and (2), rises with the increase of ni while A and c are invariant. The relative surface properties were calculated by the equations above and the results were shown in Table 2. One of the advantages of non-ionic surfactant is the effects from pH values are less compared to ionic surfactants. The effect of pH value to these tailor-made polyurethane surfactant aqueous solutions was given in Fig. 5. Both acidic and alkaline circumstances which are regulated by adding HCl or NaOH solution can induce the reduction of surface tension, but pH value has less effect on
Fig. 4. Surface tension curves of polyurethane surfactants.
Fig. 5. The effect of pH to surface tension.
surface tension compared with general ionic surfactant aqueous solution. In the meantime the effect from additives as electrolyte is investigated (Fig. 6). Salt effect acts on hydrophobic and hydrophilic groups simultaneously, but it presents the main action taken place at the hydrophobic portion as salting out or salting in. Salting out makes surface tension depressed, but salting in can cause solubility and surface tension incremental. In 0–1.0 mol L−1 NaCl solution, the surface tension decreases a little as the electrolyte concentration grows. As the salt was added, the repulsion between hydrophobic and hydrophilic groups on the surface was reduced. The oriented arrangement is closer, hereby the surface tension decline. However it is considered that the amount of electrolyte has little effect on the surface tension of the non-ionic polyurethane surfactant aqueous solutions and mainly presents as salting out effect. 3.3. Cloud point Cloud point is an important parameter for non-ionic surfactant and depends on the chemical structure of surfactant. For polyoxyethylene (PEO) non-ionic surfactants, the cloud point increases with increasing ethoxylated content for a given hydrophobic group. For emulsion polymerization, it is requested the cloud point should approach the polymerized reactive temperature or even higher while using non-ionic surfactants independently. If its cloud point
Fig. 6. The effect of electrolyte to surface tension.
J. Zheng et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 363 (2010) 16–21
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Table 2 Surface properties of non-ionic polyurethane surfactant aqueous solutions. Mn (g mol−1 )
Surfactant Pus IGM500 Pus IGM1000 Pus IGM2000
CMC (×10−5 mol L−1 )
1803 2803 4803
3.82 3.32 1.09
cmc (mN m−1 ) 43.93 45.81 48.83
is lower than the reactive temperature, it cannot be used independently and should compound with ionic surfactants. General reactive temperature of emulsion polymerization is at about 80 ◦ C with potassium persulfate as initiator. High temperature breaks the hydrogen bonds and effects solution stability. The results were shown in Table 3. It illustrates that the cloud point rises with the increase of ethoxylated chain, thereby Pus IGM1000 and Pus IGM2000 should be more suited with emulsion polymerization, but Pus IGM500 is not suited for use in emulsion polymerization, as its cloud point is well below the threshold of 80 ◦ C. It accounts for that more hydrophilic ethoxylated chains can strengthen hydrogen bonds with water and improve solution capability of non-ionic polyurethane surfactants in aqueous phase.
i cmc (mol m−2 ) −6
1.12 × 10 1.04 × 10−6 1.96 × 10−6
am (nm2 ) 1.48 1.60 8.48
ated with head group hydration. While a0 is large enough to make Pc ≤ 1/3, the shape of micelles would be spherical. The non-ionic polyurethane surfactants contain a single hydrophobic alkyl chain and double hydrophilic ethoxylated chains. Double head groups have sufficient large area to meet the condition Pc ≤ 1/3, hereby,
3.4. Emulsifying capability For scaling emulsifying capability, the separation rate-time curve was set up (Fig. 7). Separation rate is calculated as the following equation: Rs = Vs × 100/V0 , where Rs is the separation rate in %, Vs is the separation volume of water in mL, and V0 is the initial volume of emulsion in mL. The testing time is from 0 to 24 h. For the same kind of surfactant, the stability of emulsion became worse which can be scaled by the increase of the separation rate as the hydrophilic property of monomer weakened. For the same kind of monomer, the separation rate and the time to achieve equilibrium declined as the chain length grew. The Pus IGM2000 presented well performance on emulsifying capability, the separation rates for emulsions obtained by different monomers were below 5%, especially for MMA the separation is not happened. The difference among these three surfactants is the hydrophilic chain length. The increasing ethoxylated chains provide more hydrogen bond can make the emulsion more stable, and offer the steric hindrance to particles. The results aforementioned indicated that the phase separation depends not only on the hydrophilic chain length of the surfactants, but also on the substrate. 3.5. Structure of micelle TEM is one of effective tools for investigating the structures of micelles. The images in Fig. 8 illustrate the shape and size of micelles in aqueous solution at different concentration. It proves that the surfactants will aggregate spontaneously to form spherical micelles even at a low concentration 0.1 g L−1 . The shape is considered to be governed by molecular geometric consideration which must play an important role to cause the spontaneous action in micellisation process. According to the critical packing parameter (Pc ) equation: Pc =
V la0
(4)
where V is the volume of the hydrophobic tail, l is the maximum extended chain length of the tail and a0 is the interfacial area of head group. If V and l are invariant, Pc is governed by a0 which is associTable 3 Cloud points of non-ionic polyurethane surfactants. Tag
Pus IGM500
Pus IGM1000
Pus IGM2000
Cloud point/◦ C
54 ± 0.5
88 ± 0.5
97 ± 0.5
Fig. 7. Separation rate versus time curves with different monomers: (a) Pus IGM500, (b) Pus IGM1000 and (c) Pus IGM2000.
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J. Zheng et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 363 (2010) 16–21
Table 4 The absorbance of styrene solution at 277 nm. Medium
H2 O
Absorbance
2.001
Pus IGM500 solution (g L−1 )
Pus IGM1000 solution (g L−1 )
Pus IGM2000 solution (g L−1 )
0.01
0.1
1
0.01
0.1
1
0.01
0.1
1
2.250
>5
>5
2.058
2.599
>5
2.016
2.256
>5
Fig. 8. The micelle structure of Pus IGM1000 solution at different concentration: (a) 0.1 g L−1 and (b) 10 g L−1 .
the micelles represent globular particles. The size increases several times at 10 g L−1 associated with increment of aggregation number. The size of these micelles is governed by energetic consideration. According to Gibbs–Helmholtz equation (G0 = H0 − T S0 ), where G0 is the net free energy change upon micellisation, H0 is the enthalpy change, S0 is the entropy change and T is the absolute temperature. At room temperature H0 is small and positive, and S0 is large and positive which is considered as the main contribution to the negative G0 , and account for micellisation is an entropy-driven process. The driving force from the increment of S0 relies on the formation of micelles to expel the highly structured water around the hydrophobic chain back to ordinary bulk water there accounting for the increment of aggregation number. Therefore, the size of micelles rises while the concentration of surfactant increases. 3.6. Solubilization In this experiment, UV spectrophotometry has been used to clarify the capacity of non-ionic polyurethane surfactants to solubilize hydrophobic low molecular organic compound. Both lyophobic inner core of micelles where the hydrophobic groups aggregate together and interface between ethoxylated chain and alkyl chain can be the positions for solubilization, and the model had been conjectured by Dong et al. [20]. Styrene has been chosen as the solubilized substance for its hydrophobicity and the apparent characteristic absorbance is at 277 nm. Via the survey of enhanced absorbance at 277 nm, whether the surfactant solution solubilizes styrene can be proved. Table 4 shows the UV absorbance at 277 nm of different solution. The absorbance in surfactant solution aqueous is higher than in water. Solubilization results from micelles in solution. Due to the molecule number of surfactant decrease as the ethoxylated content increases at the same mass concentration, the number of micelle varies correspondingly. Hereby, the absorbance declines with the increasing of ethoxylated content at the same mass concentration. 4. Conclusion Non-ionic polyurethane surfactants have been synthesized successfully by the reaction of isophorone diisocyanate with glycerol monostearate and mono methoxy polyethylene glycol. Due to the
amphipathic structure of surfactant which contains a mono-tail and double head groups, these non-ionic polyurethane do have obvious surface activity represented by reduction of surface tension, however they do not have CMC which is different from traditional surfactants and some special block copolymers with surface active properties. The variation of pH and salt extra added slightly induced the reduction of surface tension. TEM results showed the existence of micellisation process in surfactant solution. The structure of micelle is globular and the size will change in times while the concentration increases. UV spectrophotometry had been used to prove the micellar solubilization of the surfactant solution. In addition, these surfactants show good emulsifying capabilities compared with the conventional low molecular weight surfactants. The results illustrate that the increasing ethoxylated content and hydrophilic monomer facilitate the stability of the emulsion. The cloud points of Pus IGM1000 and Pus IGM2000 are over 80 ◦ C, and provide the possibility of using these novel non-ionic polyurethane surfactants directly as emulsifier in emulsion polymerization. Acknowledgement The authors appreciate the support from the Natural Science Foundation of Guangdong Province of China (Project No: 8251065004000001). References [1] B.W. Greene, D.P. Sheetz, In situ polymerization of surface-active agents on latex particles. II. The mechanical stability of styrene/butadiene latexes, J. Colloid Interface Sci. 32 (1970) 96–100. [2] B. Kronberg, J. Kuortti, P. Stenius, Competitive and cooperative adsorption of polymers and surfactants on kaolinite surfaces, Colloids Surf. 18 (1986) 411–425. [3] A. Guyot, K. Tauer, Reactive surfactants in emulsion polymerization, Adv. Polym. Sci. 111 (1994) 43–65. [4] K. Holmberg, Polymerizable surfactants, Prog. Org. Coat. 20 (1992) 325–337. [5] C.S. Chern, Emulsion polymerization mechanisms and kinetics, Prog. Polym. Sci. 31 (2006) 443–486. ˜ D.C. Sherrington, K. [6] M.J. Unzue, H.A.S. Schoonbrood, J.M. Asua, A.M. Goni, Stähler, K.H. Goebel, K. Tauer, M. Sjöberg, K. Holmberg, Reactive surfactants in heterophase polymerization. VI. Synthesis and screening of polymerizable surfactants (surfmers) with varying reactivity in high solids styrene–butyl acrylate–acrylic acid emulsion polymerization, J. Appl. Polym. Sci. 66 (1997) 1803–1820. [7] A. Dworak, I. Panchev, B. Trzebicka, W. Walach, Poly(␣-t-butoxy--styryloglycidol): a new reactive surfactant, Polym. Bull. 40 (1998) 461–468.
J. Zheng et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 363 (2010) 16–21 [8] D.Y. Zhao, Q.S. Huo, J.L. Feng, B.F. Chmelka, G.D. Stucky, Nonionic triblock and star diblock copolymer and oligomeric surfactant syntheses of highly ordered, hydrothermally stable, mesoporous silica structures, J. Am. Chem. Soc. 120 (1998) 6024–6036. [9] N. Mureau, H. Trabelsi, F. Guittard, S. Geribaldi, Preparation and evaluation of monodisperse nonionic surfactants based on fluorine-containing dicarbamates, J. Colloid Interface Sci. 229 (2000) 440–444. [10] X.S. Li, J. Turánek, P. Knötigová, H. Kudláˇcková, J. Maˇsek, S. Parkin, S.E. Rankin, B.L. Knutson, H.J. Lehmler, Hydrophobic tail length, degree of fluorination and headgroup stereochemistry are determinants of the biocompatibility of (fluorinated) carbohydrate surfactants, Colloids Surf. B: Biointerfaces 73 (2009) 65–74. [11] E.A. ISMAIL, Preparation of water-soluble polyurethane surfactants, J. Appl. Polym. Sci. 68 (1998) 1531–1536. [12] H.J. Adler, K. Jahny, B.V. Birnbrich, Polyurethane macromers—new building blocks for acrylic hybrid emulsions with outstanding performance, Prog. Org. Coat. 43 (2001) 251–257. [13] H. Lim, Y. Lee, I.J. Park, S.B. Lee, Synthesis and surface property of aqueous fluorine-containing polyurethane, J. Colloid Interface Sci. 241 (2001) 269–274.
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[14] I.W. Cheong, M. Nomura, J.H. Kim, Water-soluble polyurethane resins as emulsifiers in emulsion polymerization of styrene: nucleation and particle growth, Macromol. Chem. Phys. 202 (2001) 2454–2460. [15] B.S. Kim, H.Y. Jeong, B.K. Kim, Surface characterizations of polyurethanes having different types of soft segment, Colloids Surf. A: Physicochem. Eng. Aspects 268 (2005) 60–67. [16] H.T. Lee, S.Y. Wu, R.J. Jeng, Effects of sulfonated polyol on the properties of the resultant aqueous polyurethane dispersions, Colloids Surf. A: Physicochem. Eng. Aspects 276 (2006) 176–185. [17] D. Myers, Surfaces, interface, and colloids: principles and applications, 2nd ed., John Wiley, New York, 1999. [18] K. shinoda, The formation of micelles, in colloidal surfactants: some physic chemical properties, Academic Press, New York, 1963. [19] Y. Dong, Y. Jin, D.Q. Wei, Synthesis and solution properties of novel nonionic functional polyurethane surfactants, J. Appl. Polym. Sci. 100 (2006) 2956– 2962. [20] Y. Dong, Y.Y. Jin, D.Q. Wei, Surface activity and solubilization of a novel series of functional polyurethane surfactants, Polym. Int. 56 (2007) 14– 21.
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Preparation and properties of non-ionic polyurethane surfactants Jie Zheng a,b , Jianxin Luo a,b , Dewen Zhou a,b , Tengfei Shen a,b , Huan Li a,b , Liyan Liang a , Mangeng Lu a,∗ a b
Guangzhou Institute of Chemistry, Chinese Academy of Science, Guangzhou 510650, PR China Graduate School of the Chinese Academy of Science, Beijing 100049, PR China
a r t i c l e
i n f o
Article history: Received 17 December 2009 Received in revised form 25 March 2010 Accepted 5 April 2010 Available online 10 April 2010 Keywords: Polyurethane Non-ionic surfactants Micelle Emulsification Solubilization
a b s t r a c t A series of non-ionic polyurethane surfactants were synthesized by the reaction of isophorone diisocyanate with glycerol monostearate and mono methoxy polyethylene glycol. The chemical structure of the polyurethane surfactant was confirmed by Fourier transform infrared spectroscopy and 1 H NMR. These tailor-made surfactants exhibit excellent surface activity, but do not have a definite critical micelle concentration. The effects of pH and salt resulted in a decrease in surface tension. It was found that cloud points of the surfactants raise with increasing ethoxylated content. The emulsifying capability is also affected by the ethoxylated content and attenuated as the hydrophilic nature of monomer declines. The micelle can form from the surfactant solution and the size is increased with the solution concentration. The solubilization capability has been proved by UV spectra. © 2010 Published by Elsevier B.V.
1. Introduction Surfactant is such a kind of substance that when dissolved in a solvent at low concentration has the ability to decrease interfacial force significantly. Due to the special physical properties, it has been applied in industrial production especially in emulsion polymerization region. The remains of traditional low molecular surfactants always affect the quality of stripping film, furthermore the remains will pollute environment because they are hard to degrade or recycle [1–5]. Hence new types of surfactants have been devised for increasing performance of emulsion and reducing pollution [6–10]. Polyurethane with amphiphilic molecular structure which has an appropriate balance of hydrophilic and hydrophobic groups could be applied similarly like traditional low molecular surfactants [11,12]. It is considered that it is easy to obtain the amphiphilic polyurethane because the structure of polyurethane could be modified in a wide range by varying the composition including the isocyanates, polyols, chain extenders, blocking agents. In recent years, some correlative researches had been carried out and proved that polyurethane surfactants have well surface activity properties [13–16]. The surfactants commonly used in process of emulsion polymerization could be divided into ionic type and non-ionic type. Compare with ionic surfactants, the non-ionic surfactants are not easily affected by pH values [17], but ‘cloud point’ may limit their direct employment [18]. The non-ionic polyurethane surfactants reported [19,20] had shown excellence in surface activity, but the drawback is the cloud point
∗ Corresponding author. Tel.: +86 020 85232978. E-mail address: [email protected] (M. Lu). 0927-7757/$ – see front matter © 2010 Published by Elsevier B.V. doi:10.1016/j.colsurfa.2010.04.001
of low temperature which make direct use impossible in emulsion polymerization. In this paper, the novel non-ionic polyurethane surfactants which contain a mono-tail and bi-head groups were synthesized by introducing a long alkyl chain as hydrophobic moiety and mono methoxy polyethylene glycol as hydrophilic moiety. These tailor-made non-ionic surfactants could be used in a broad temperature range to conquer the current drawbacks and may endow latex a lot of advantages. 2. Experiment 2.1. Materials Glycerol monostearate (GM, Union chemistry, China), mono methoxy polyethylene glycol 500 (MPEG-500, Hannong, South Korea), mono methoxy polyethylene glycol 1000 (MPEG-1000) and mono methoxy polyethylene glycol 2000 (MPEG-2000) were purchased from commercial company, reagents above mentioned were dealt with vacuum drying at 65 ◦ C before use. 3-Isocyanatomethyl-3,5,5-trimethylcyclohexylisocyanate (isophorone diisocyanate, IPDI) with a purity > 99% was purchased from Degussa, dibutyl tin dilaurate solution (DBTL) with a content >90% was used without further purification. Distilled acetone was free from moisture using 4A molecular sieve before use. Water used in this experiment was de-ionized and distilled. 2.2. Synthesis of non-ionic polyurethane surfactants The basic recipes for non-ionic polyurethane surfactants are presented in Table 1. The polyaddition reaction was conducted in a 500 mL four-necked round bottom flask equipped with a mechan-
J. Zheng et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 363 (2010) 16–21
17
Table 1 The basic recipes for non-ionic polyurethane surfactants. Tag
IPDI
GM
MPEG-500
MPEG-1000
MPEG-2000
Pus IGM500 Pus IGM1000 Pus IGM2000
2 2 2
1 1 1
2 0 0
0 2 0
0 0 2
ical stirrer and a condenser under the protection of nitrogen. IPDI and GM were dissolved in acetone, and then the obtained solution was charged into the reactor with a drop of DBTL. The reaction was taken at a constant temperature at 80 ± 2 ◦ C and lasted several hours until the NCO group content reached a designed value determined by titration with dibutylamine. The prepolymer contained NCO groups was then capped with mono methoxy polyethylene glycol for 2–3 h. Afterwards drying production was obtained while acetone was removed by distillation. The structure of non-ionic polyurethane surfactant is shown in Fig. 1.
Fig. 2. FTIR spectrogram of non-ionic polyurethane surfactant (Pus IGM1000).
determining the solubilization of styrene in surfactant aqueous solution.
2.3. Characterization Analect RFX-65A Fourier transform infrared spectroscopy was taken for acquiring infrared spectrum using pressed disc method. The 1 H NMR spectrum was recorded with a Bruker DRX-400 (300 MHz) spectrometer using CDCl3 as a solvent. A JEM 100CX II transmission electron microscope (TEM) was taken to observe the conformation of micelles. The samples for TEM were prepared by negative staining method using a 2% phosphotungstic acid (PTA) as a staining agent. The surface tension measurements were carried out with a Solon A210 surface tensiometer at 28 ◦ C. The non-ionic polyurethane surfactant aqueous solutions in a series of concentrations had been prepared firstly at room temperature and then the obtained solutions were tested by surface tensimeter. In addition, the environmental factors have been considered including pH values and electrolytes. The hydrochloric acid and sodium hydroxide solutions were added to the polyurethane surfactant aqueous solutions for adjusting the pH value of the system in a broad range from 1.8 to 13.4. The obtained solutions with different pH value were then tested by surface tensiometer. The electrolyte here used was sodium chloride. A series concentration of sodium chloride solutions were prepared by the polyurethane surfactant aqueous solutions. The obtained solutions were then measured by surface tensiometer too. The cloud point was obtained by determination of the temperature while 1% diluted surfactant aqueous solution turned pellucid to visibly turbid in an heating water bath, and the mean value of 3 tests was finally confirmed as the cloud point. To investigate emulsifying capability, 3 types of monomers butyl acrylate (BA), methacrylate ester (MMA) and styrene (St) were taken to blend with 2% diluted surfactant aqueous solution respectively at equal volumes of 5 mL each for 10 min using an magnetic stirring apparatus, the volume of separate water was observed to scale emulsifying capability at different times. The Shimadzu UV-2550 ultraviolet–vis spectrometer was used for
3. Results and discussion 3.1. Characterization of non-ionic polyurethane surfactant The FTIR spectrum of non-ionic polyurethane surfactant is shown in Fig. 2. The broad peak found at 3342 cm−1 is N–H stretching vibration, and the peak at 1540 cm−1 is due to N–H flexural vibration and C–N stretching vibration. A strong C–H stretching vibration occurs at 2869 cm−1 . Another strong C O band stretching vibration at 1724 cm−1 belongs to ester. The peaks at 1245 and 1043 cm−1 respectively belong to C–O–C symmetric and dissymmetrical stretching vibration of ester. The C–O–C group of polyethylene glycol gives a peak at about 1112 cm−1 . With the characteristic absorption peaks between 2200 and 2280 cm−1 vanished, it can be approved that –NCO in the product has totally reacted. The 1 H NMR spectrum of non-ionic polyurethane surfactants was shown in Fig. 3. The proton respondence at 7.24 ppm is related to NH of urethane. The peaks displayed at 0.83–1.22 and 2.03–2.28 ppm belong to CH3 and CH2 in acyclic chain. The proton CH2 of ethoxylated and CH mainly locate at 3.35–3.70 ppm. The proton CH2 joined with carboxyl are found at 4.02–4.17 ppm. Both FTIR and 1 H NMR results can provide evidence of synthesis of non-ionic polyurethane surfactants is successful. 3.2. Surface tension measurements The surface activity of non-ionic polyurethane surfactant can be determined from the measurement of the interfacial tension as a function of solute concentration. The results are shown in Fig. 4. The surface tension curves show an obvious descending trend with the increasing concentration of surfactant aqueous solution. The
Fig. 1. The structure of non-ionic polyurethane surfactant.
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Fig. 3.
J. Zheng et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 363 (2010) 16–21
1
H NMR spectrogram of non-ionic polyurethane surfactant (Pus IGM1000).
surface excess i is defined as surface excess concentration of component i and given by i =
ni A
,
(1)
where A is the interfacial area and ni is the total number of moles of component i in the surface phase . According to Gibbs adsorption isotherm: i =
−1 d , RT d ln c
(2)
where is the surface tension, c is the concentration of surfactant, R is the gas constant and T is the absolute temperature. The minimum interfacial area per molecular am is calculated by amin =
1 Nav i
(3)
where Nav is Avogadro’s number. From Eqs. (1) and (2), rises with the increase of ni while A and c are invariant. The relative surface properties were calculated by the equations above and the results were shown in Table 2. One of the advantages of non-ionic surfactant is the effects from pH values are less compared to ionic surfactants. The effect of pH value to these tailor-made polyurethane surfactant aqueous solutions was given in Fig. 5. Both acidic and alkaline circumstances which are regulated by adding HCl or NaOH solution can induce the reduction of surface tension, but pH value has less effect on
Fig. 4. Surface tension curves of polyurethane surfactants.
Fig. 5. The effect of pH to surface tension.
surface tension compared with general ionic surfactant aqueous solution. In the meantime the effect from additives as electrolyte is investigated (Fig. 6). Salt effect acts on hydrophobic and hydrophilic groups simultaneously, but it presents the main action taken place at the hydrophobic portion as salting out or salting in. Salting out makes surface tension depressed, but salting in can cause solubility and surface tension incremental. In 0–1.0 mol L−1 NaCl solution, the surface tension decreases a little as the electrolyte concentration grows. As the salt was added, the repulsion between hydrophobic and hydrophilic groups on the surface was reduced. The oriented arrangement is closer, hereby the surface tension decline. However it is considered that the amount of electrolyte has little effect on the surface tension of the non-ionic polyurethane surfactant aqueous solutions and mainly presents as salting out effect. 3.3. Cloud point Cloud point is an important parameter for non-ionic surfactant and depends on the chemical structure of surfactant. For polyoxyethylene (PEO) non-ionic surfactants, the cloud point increases with increasing ethoxylated content for a given hydrophobic group. For emulsion polymerization, it is requested the cloud point should approach the polymerized reactive temperature or even higher while using non-ionic surfactants independently. If its cloud point
Fig. 6. The effect of electrolyte to surface tension.
J. Zheng et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 363 (2010) 16–21
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Table 2 Surface properties of non-ionic polyurethane surfactant aqueous solutions. Mn (g mol−1 )
Surfactant Pus IGM500 Pus IGM1000 Pus IGM2000
CMC (×10−5 mol L−1 )
1803 2803 4803
3.82 3.32 1.09
cmc (mN m−1 ) 43.93 45.81 48.83
is lower than the reactive temperature, it cannot be used independently and should compound with ionic surfactants. General reactive temperature of emulsion polymerization is at about 80 ◦ C with potassium persulfate as initiator. High temperature breaks the hydrogen bonds and effects solution stability. The results were shown in Table 3. It illustrates that the cloud point rises with the increase of ethoxylated chain, thereby Pus IGM1000 and Pus IGM2000 should be more suited with emulsion polymerization, but Pus IGM500 is not suited for use in emulsion polymerization, as its cloud point is well below the threshold of 80 ◦ C. It accounts for that more hydrophilic ethoxylated chains can strengthen hydrogen bonds with water and improve solution capability of non-ionic polyurethane surfactants in aqueous phase.
i cmc (mol m−2 ) −6
1.12 × 10 1.04 × 10−6 1.96 × 10−6
am (nm2 ) 1.48 1.60 8.48
ated with head group hydration. While a0 is large enough to make Pc ≤ 1/3, the shape of micelles would be spherical. The non-ionic polyurethane surfactants contain a single hydrophobic alkyl chain and double hydrophilic ethoxylated chains. Double head groups have sufficient large area to meet the condition Pc ≤ 1/3, hereby,
3.4. Emulsifying capability For scaling emulsifying capability, the separation rate-time curve was set up (Fig. 7). Separation rate is calculated as the following equation: Rs = Vs × 100/V0 , where Rs is the separation rate in %, Vs is the separation volume of water in mL, and V0 is the initial volume of emulsion in mL. The testing time is from 0 to 24 h. For the same kind of surfactant, the stability of emulsion became worse which can be scaled by the increase of the separation rate as the hydrophilic property of monomer weakened. For the same kind of monomer, the separation rate and the time to achieve equilibrium declined as the chain length grew. The Pus IGM2000 presented well performance on emulsifying capability, the separation rates for emulsions obtained by different monomers were below 5%, especially for MMA the separation is not happened. The difference among these three surfactants is the hydrophilic chain length. The increasing ethoxylated chains provide more hydrogen bond can make the emulsion more stable, and offer the steric hindrance to particles. The results aforementioned indicated that the phase separation depends not only on the hydrophilic chain length of the surfactants, but also on the substrate. 3.5. Structure of micelle TEM is one of effective tools for investigating the structures of micelles. The images in Fig. 8 illustrate the shape and size of micelles in aqueous solution at different concentration. It proves that the surfactants will aggregate spontaneously to form spherical micelles even at a low concentration 0.1 g L−1 . The shape is considered to be governed by molecular geometric consideration which must play an important role to cause the spontaneous action in micellisation process. According to the critical packing parameter (Pc ) equation: Pc =
V la0
(4)
where V is the volume of the hydrophobic tail, l is the maximum extended chain length of the tail and a0 is the interfacial area of head group. If V and l are invariant, Pc is governed by a0 which is associTable 3 Cloud points of non-ionic polyurethane surfactants. Tag
Pus IGM500
Pus IGM1000
Pus IGM2000
Cloud point/◦ C
54 ± 0.5
88 ± 0.5
97 ± 0.5
Fig. 7. Separation rate versus time curves with different monomers: (a) Pus IGM500, (b) Pus IGM1000 and (c) Pus IGM2000.
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J. Zheng et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 363 (2010) 16–21
Table 4 The absorbance of styrene solution at 277 nm. Medium
H2 O
Absorbance
2.001
Pus IGM500 solution (g L−1 )
Pus IGM1000 solution (g L−1 )
Pus IGM2000 solution (g L−1 )
0.01
0.1
1
0.01
0.1
1
0.01
0.1
1
2.250
>5
>5
2.058
2.599
>5
2.016
2.256
>5
Fig. 8. The micelle structure of Pus IGM1000 solution at different concentration: (a) 0.1 g L−1 and (b) 10 g L−1 .
the micelles represent globular particles. The size increases several times at 10 g L−1 associated with increment of aggregation number. The size of these micelles is governed by energetic consideration. According to Gibbs–Helmholtz equation (G0 = H0 − T S0 ), where G0 is the net free energy change upon micellisation, H0 is the enthalpy change, S0 is the entropy change and T is the absolute temperature. At room temperature H0 is small and positive, and S0 is large and positive which is considered as the main contribution to the negative G0 , and account for micellisation is an entropy-driven process. The driving force from the increment of S0 relies on the formation of micelles to expel the highly structured water around the hydrophobic chain back to ordinary bulk water there accounting for the increment of aggregation number. Therefore, the size of micelles rises while the concentration of surfactant increases. 3.6. Solubilization In this experiment, UV spectrophotometry has been used to clarify the capacity of non-ionic polyurethane surfactants to solubilize hydrophobic low molecular organic compound. Both lyophobic inner core of micelles where the hydrophobic groups aggregate together and interface between ethoxylated chain and alkyl chain can be the positions for solubilization, and the model had been conjectured by Dong et al. [20]. Styrene has been chosen as the solubilized substance for its hydrophobicity and the apparent characteristic absorbance is at 277 nm. Via the survey of enhanced absorbance at 277 nm, whether the surfactant solution solubilizes styrene can be proved. Table 4 shows the UV absorbance at 277 nm of different solution. The absorbance in surfactant solution aqueous is higher than in water. Solubilization results from micelles in solution. Due to the molecule number of surfactant decrease as the ethoxylated content increases at the same mass concentration, the number of micelle varies correspondingly. Hereby, the absorbance declines with the increasing of ethoxylated content at the same mass concentration. 4. Conclusion Non-ionic polyurethane surfactants have been synthesized successfully by the reaction of isophorone diisocyanate with glycerol monostearate and mono methoxy polyethylene glycol. Due to the
amphipathic structure of surfactant which contains a mono-tail and double head groups, these non-ionic polyurethane do have obvious surface activity represented by reduction of surface tension, however they do not have CMC which is different from traditional surfactants and some special block copolymers with surface active properties. The variation of pH and salt extra added slightly induced the reduction of surface tension. TEM results showed the existence of micellisation process in surfactant solution. The structure of micelle is globular and the size will change in times while the concentration increases. UV spectrophotometry had been used to prove the micellar solubilization of the surfactant solution. In addition, these surfactants show good emulsifying capabilities compared with the conventional low molecular weight surfactants. The results illustrate that the increasing ethoxylated content and hydrophilic monomer facilitate the stability of the emulsion. The cloud points of Pus IGM1000 and Pus IGM2000 are over 80 ◦ C, and provide the possibility of using these novel non-ionic polyurethane surfactants directly as emulsifier in emulsion polymerization. Acknowledgement The authors appreciate the support from the Natural Science Foundation of Guangdong Province of China (Project No: 8251065004000001). References [1] B.W. Greene, D.P. Sheetz, In situ polymerization of surface-active agents on latex particles. II. The mechanical stability of styrene/butadiene latexes, J. Colloid Interface Sci. 32 (1970) 96–100. [2] B. Kronberg, J. Kuortti, P. Stenius, Competitive and cooperative adsorption of polymers and surfactants on kaolinite surfaces, Colloids Surf. 18 (1986) 411–425. [3] A. Guyot, K. Tauer, Reactive surfactants in emulsion polymerization, Adv. Polym. Sci. 111 (1994) 43–65. [4] K. Holmberg, Polymerizable surfactants, Prog. Org. Coat. 20 (1992) 325–337. [5] C.S. Chern, Emulsion polymerization mechanisms and kinetics, Prog. Polym. Sci. 31 (2006) 443–486. ˜ D.C. Sherrington, K. [6] M.J. Unzue, H.A.S. Schoonbrood, J.M. Asua, A.M. Goni, Stähler, K.H. Goebel, K. Tauer, M. Sjöberg, K. Holmberg, Reactive surfactants in heterophase polymerization. VI. Synthesis and screening of polymerizable surfactants (surfmers) with varying reactivity in high solids styrene–butyl acrylate–acrylic acid emulsion polymerization, J. Appl. Polym. Sci. 66 (1997) 1803–1820. [7] A. Dworak, I. Panchev, B. Trzebicka, W. Walach, Poly(␣-t-butoxy--styryloglycidol): a new reactive surfactant, Polym. Bull. 40 (1998) 461–468.
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