Micellar characteristics of diblock polyacrylate–polyethylene oxide copolymers in aqueous media

EUROPEAN POLYMER JOURNAL

European Polymer Journal 42 (2006) 593–601

www.elsevier.com/locate/europolj

Micellar characteristics of diblock polyacrylate–polyethylene oxide copolymers in aqueous media H. Desai a, D. Varade

a,*

, V.K. Aswal b, P.S. Goyal c, P. Bahadur

a

a

Department of Chemistry, South Gujarat University, Surat 395 007, India Solid State Physics Division, BARC, Trombay, Mumbai 400 085, India c IUC-DAEF, Mumbai Centre, BARC, Trombay, Mumbai 400 085, India b

Received 16 June 2005; received in revised form 20 July 2005; accepted 30 August 2005 Available online 10 October 2005

Abstract The formation and structural features of micelles from low molecular weight diblock copolymers of poly(methylmethacrylate-b-ethylene oxide) PMMA–PEO (varying in total molecular weight) and poly(butylmethacrylate-b-ethylene oxide) PBMA–PEO in water, aqueous NaCl and urea solutions were examined by surface tension, dye spectral, cloud point, viscosity and small angle neutron scattering (SANS) measurements. The increasing concentrations of NaCl reduce the onset concentration of micellization and phase separation, while urea has reverse effect. The analysis of the SANS curves revealed the presence of prolate ellipsoidal micelles in diblock copolymers at various experimental concentrations and temperatures studied. The effect of temperature, NaCl and urea on the neutron scattering profiles are more or less the same which is well supported by viscosity and surface tension measurements. The diblock copolymers form spherical micelles of aggregation number in the range of 522–664. The micelles are very temperature stable.
2005 Published by Elsevier Ltd. Keywords: Micellization; Block copolymers; Phase separation; SANS

1. Introduction Amphiphilic block copolymers are typical nonionic polymeric surfactants which have attracted considerable attention because of their outstanding solution properties and a wide range of applications. These materials are very interesting from the

*

Corresponding author. Tel.: +91 261 2258384; fax: +91 261 2256012. E-mail address: [email protected] (D. Varade). 0014-3057/$ - see front matter
2005 Published by Elsevier Ltd. doi:10.1016/j.eurpolymj.2005.08.016

point of view of fundamental research, as they exhibit self-assembling properties in the presence of a selective solvent or surface. A diblock copolymer consists of two dissimilar moieties i.e. hydrophilic moiety as polyethylene oxide (PEO) and hydrophobic moiety as polypropylene oxide (PPO) or polystyrene (PS) or polymethylmethacrylate (PMMA) or polybutylmethacrylate (PBMA), etc. attached at a common junction. These polymers when dissolved in a medium which is a good solvent for one of the block and a nonsolvent for the other, the polymer associate to form micelles. Each micelle consists of a core of the insoluble blocks surrounded by a

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H. Desai et al. / European Polymer Journal 42 (2006) 593–601

solvent swollen corona of the soluble block. In aqueous systems, block copolymers with polyethylene oxide as water soluble part have attracted renewed interest in recent years and have been extensively reviewed [1–5]. Due to increasing interest of both chemists and physicists in the field of polymeric micelles, the numbers of relevant papers have been published using several techniques [6–14]. The alteration of solution composition through the use of cosolvents and cosolutes makes accessible to the surfactant technologist a method for producing solutions with fairly precise properties. The presence of electrolytes has been found to exert a remarkable influence on the aggregation of block copolymers [15–20]. However, data on the effect of these additives influencing micellar characteristics are rather scarce although the presence of such additives have been shown to alter significantly the solvent properties as well as surface/adsorption characteristics of copolymers in solution. Micellar, rheological and clouding behavior of block copolymers in aqueous solution in the presence of added salts have been extensively studied by Bahadur et al. [17–20] and other workers in the past [21–23]. The effect of added inorganic salt on the micellization of copolymers was analogous to that of temperature. For applications where temperature cannot be altered appreciably, the addition of salt provides with an easy method for inducing micellization needed to optimize a surfactants performance. Urea, (H2N–CO–NH2), a well known protein denaturant, has been found to greatly modify the properties of aqueous solution and especially of aqueous micellar solutions [24]. Two different mechanisms have been put forward to probe the effect of urea in aqueous solutions: (i) an indirect mechanism whereby urea behaves as structure breaker and thus facilitates the hydration of nonpolar solutes [25,26] and (ii) a direct mechanism whereby urea complexes with the target molecule solute by replacing some of water molecules in the hydration shell of solute [27,28]. The indirect mechanism has gained much attention and is widely accepted [24]. The assumption that urea acts as a water structure breaker has been supported by many experimental results [29,30]. Addition of urea increases the critical micelle concentration (CMC) of nonionic [31] and ionic surfactants [32], decreases the hydrodynamic radius of nonionic [22] and ionic micelles [33]. It also raises the cloud point of aqueous nonionic surfactant solutions [22].

The aim of this paper is to present data, obtained using surface tension, dye spectral, cloud point, viscosity and small angle neutron scattering (SANS) measurements, which describe the effect of added temperature, NaCl and urea on the onset of micellization, phase separation and structure of low molecular weight diblock copolymers varying in molecular characteristics. 2. Experimental The diblock copolymers polymethylmethacrylate (PMMA) and polyethylene oxide (PEO), PMMA– PEO 1-1, PMMA–PEO 1-2, PMMA–PEO 1-3 (having total molecular weight 2000, 3000 and 4000, where the 1-1 index denotes the PMMA and PEO block sizes of 1000 and 1000, respectively and so on) and polybutylmethacrylate (PBMA) and polyethylene oxide (PEO), PBMA–PEO 1-1 (having total molecular weight 2000), were received as gift samples from Prof. G. Riess, Mulhouse, France. The molecular characteristics of these diblock copolymers are given in Table 1. The dye, ethyl orange, from Fluka, Switzerland was purified by repeated recrystallization from ethanol and water mixture and the spectra of the aqueous solutions of the dye were recorded at different copolymer concentrations. NaCl and urea used were of AR grade purchased from Fluka, Switzerland. Triple Distilled water from an all Pyrex glass apparatus was used for the preparation of solutions. Deuterium oxide, D2O, was used for preparation of solutions for SANS measurements. 2.1. Surface tension The surface tension of diblock copolymer solutions in absence and presence of NaCl and urea was measured by drop weight method using a modified stalagmometer [9]. The assembly consists of Pyrex glass bulb of spherical shape with a capillary tube attached at filling and dropping ends. The capillary tube at the dropping end is blown into a 2-fold U shape and the tip of the end is grounded in the form of a fine cone. By this way, not only is the formation of drops of uniform shape and size ensured but also drops are allowed to break under their own weight. A thoroughly stoppered weighing bottle is attached to the dropping end through a rubber septum. The weighing bottle attached to a dropping capillary tube was placed suspended in another closed long glass tube. The stalagmometer assembly

H. Desai et al. / European Polymer Journal 42 (2006) 593–601

595

Table 1 Molecular characteristics of various block copolymers Block copolymer

PMMA–PEO 1-1 PMMA–PEO 1-2 PMMA–PEO 1-3 PBMA–PEO 1-1

Mw Acrylate block 1000 1000 1000 1000

Mw PEO block 1000 2000 3000 1000

along with the predried and preweighed weighing bottle was lowered into a thermostatic water bath maintained at the desired temperature (30 C) accurate to 0.01 C. A 30 min time of equilibrium was always allowed. Then a known number of drops (>20) of given solution and reference triple distilled water were allowed to fall into the weighing bottle in separate runs. The weight of solutions as well as triple distilled water drawn from separate runs was instantly recorded on single pan balance. The surface tension of the individual solution was then calculated from known values of surface tension of water, densities and weight of solution and water. 2.2. Dye spectral measurements Dye spectral measurements were carried out on Shimadzu (UV-160A) UV–visible spectrophotometers with matched pair of stoppered fused silica cells of 1 cm optical path length. The kmax of the dye solution in the presence of varying copolymer concentration was recorded.

Mw Total 2000 3000 4000 2000

CP, C

CMC, wt% ST

DS

0.02 0.05 0.10 0.05

0.03 0.06 0.15 0.06

77 >100 >100 88

late the thermostated water at 30 C. The time of flow for the water was 175 s. This efflux time was kept long to minimize the need for applying kinetic corrections to the observed data. Each experiment was carried out after allowing long-time thermal stability to be reached and was repeated at least twice in order to get reproducible results. Good reproducibility can be obtained by properly cleaning the viscometer with the help of concentrated chromic acid each time before starting a set of experiments to avoid the formation of air bubbles in the viscometer. From the ratio of efflux time of the test solution, t, to that of the reference solution, t0, the relative viscosity can be calculated, gr = t/t0 by ignoring the density corrections for the dilute solutions. 2.5. Small angle neutron scattering (SANS)

2.4. Viscosity

The SANS experiments were carried out on micellar solutions of diblock copolymers prepared by dissolving known amount of copolymer in D2O. The use of D2O instead of H2O for preparing solutions provides a very good contrast between solute aggregates and solvents in a SANS experiment. The SANS measurements were carried out using SANS spectrometer at the DHRUVA Reactor, (Trombay, India) [34] in wave vector transfer Q (¼ 4p sin h=k, where 2h is the scattering angle and k is the incident neutron wavelength) range of ˚
1. The solutions were held in 5 mm 0.018–0.32 A path length UV-grade quartz sample holders with tight fitting Teflon stoppers, sealed with parafilm. The sample to detector distance was 1.8 m for all runs. The data were corrected for background, empty-cell contribution, sample transmission and normalized to absolute cross-sectional units [34].

The efflux time of dilute solutions of the mixed surfactant systems was determined with the help of an Ubbelohde-type suspended level capillary viscometer which was sealed in a glass jacket to circu-

2.5.1. Analysis of SANS data SANS is an ideal technique for determining the shape and size of the micelles. In SANS experiments, one measures the coherent differential

2.3. Cloud point (CP) Cloud point was determined at fixed concentration of the copolymer (1 wt%) in the absence and presence of varying amount of added NaCl (0– 2 M) and urea (0–2 M) by visual inspection of solutions in sealed thin-walled glass tubes with a diameter of 5 mm. The tubes were placed in a thermostat, and temperature was increased or decreased in steps of 1 C/min with 5 min equilibrium time. The first appearance of turbidity was taken as the cloud point.

H. Desai et al. / European Polymer Journal 42 (2006) 593–601

scattering cross section, dR/dX, for the sample. The expression for dR/dX per unit volume of solution for interacting micelles is given by [35,36] 2

þ hF ðQÞi ½SðQÞ
1g þ B;

ð1Þ

where nm denotes the number density of the micelles of volume Vm, qm and qs are scattering length densities of the micelle and solvent, respectively. F(Q) is the single-particle (intraparticle) form factor and S(Q) is the interparticle structure factor, and B is a constant term that represents the incoherent scattering background, which is mainly due to hydrogen in the sample. An ellipsoidal shape (a 5 b = c) of the micelles is widely used in the analysis of SANS data because it also represents the other different possible shapes of the micelles such as spherical (a = b), rod-like (a  b) and disc-like (b  a). For such an ellipsoidal micelle Z 1 2 hF ðQÞi ¼ ½F ðQ; lÞ2 dl; ð2Þ 0

hF ðQÞi2 ¼


Z

1

2 F ðQ; lÞ dl ;

ð3Þ

0

3ðsin x
x cos xÞ ; x3 1=2 x ¼ Q½a2 l2 þ b2 ð1
l2 Þ ;

F ðQ; lÞ ¼

ð4Þ ð5Þ

where a and b are, respectively, the semimajor and semiminor axes of the ellipsoidal micelle and l is the cosine of the angle between the semimajor axis and the wave reactor transfer Q. The interparticle structure factor, S(Q), is decided by the spatial arrangement of micelles in solution. Usually, S(Q) shows a peak at Qm = 2p/ D, where D is the average distance between micelles. The calculation of S(Q) depends on the spatial arrangement of micelle and on the intermicellar interactions. 3. Results and discussion The surface activity of block copolymers is a very important property in applications such as dispersion stabilization, foaming and emulsification. Surface tension measurements were performed for all the four copolymers PMMA–PEO 1-1, PMMA– PEO 1-2, PMMA–PEO 1-3, PBMA–PEO 1-1, in water at 30 C. Each copolymer showed a fairly good surface activity. The CMC data obtained from the break point in the c-log concentration plots are

480 470

60 460 450

50

440

Wavelength, nm

dR=dX ¼ nm V 2m ðqm
qs Þ2 fhF 2 ðQÞi

70

Surface tension, mN m-1

596

40 430 30 -3

-2

-1

0

420

log [PBMA-PEO 1-1], wt%

Fig. 1. Surface tension (s) and dye spectral (d).

recorded in Table 1. A representative plot of surface tension for PBMA–PEO 1-1 is shown in Fig. 1. Aqueous solutions of dye often exhibit marked changes in their spectrum in the presence of surfactants due to the formation of dye-surfactant complex which is influenced by the charge and hydrophobicity of the interacting species [37]. The dye ethyl orange in aqueous solution shows a spectrum with a single band at its kmax = 474 nm which agrees well with the reported value [38]. The spectra of the dye for all the copolymers over the concentration range 0–1% w/v at 30 C were used to construct kmax versus copolymer concentration plots. The variation in kmax of the dye ethyl orange with copolymer concentration provides a quick method of CMC determination [39]. Such representative plot for PBMA–PEO 1-1 is also shown in Fig. 1. It can be seen that kmax remains unaltered up to a certain concentration known as ÔCMCÕ beyond which it shows a marked decrease over a range. The CMCs are in agreement with those obtained from surface tension for various copolymers. Alexandridis et al. [40] have used dye solubilization technique for determining CMCs and CMTs of various block copolymers. However, the suitability of the present method is due to the fact that dye being water soluble makes the measurement faster unlike in solubilization method which needs several hours to attain equilibrium. In the lowest concentration regime in which the kmax of the dye remains unaltered, there does not seem to be any change in the environment of the dye in the presence of copolymer molecules which remain highly extended. At higher concentration, the copolymer micellizes and dye molecules partition from water to these micelles leading to an

H. Desai et al. / European Polymer Journal 42 (2006) 593–601

ing point of water. This behavior is quite expected and is due to increased solubility of diblock copolymers with large PEO block. PMMA–PEO 1-1, block copolymer has lower CP value than PBMA–PEO 1-1, due to the less number of butylmethacrylate units (7) in the copolymer as compared to methylmethacrylate units (10) in PMMA–PEO. A representative plots of the cloud point data for aqueous solutions of copolymers PMMA–PEO 1-1 and PBMA–PEO 1-1, as a function of NaCl (0–2 M) and urea (0–2 M) are shown in Fig. 2. A linear decrease in CP is found with an increase in salt concentration as observed before for similar systems [43–46]. Detailed investigations on the effect of salts on solution and phase behavior of PEO/PPO/PEO solution have been reported by Bahadur et al. [23]. Urea is well known for its capacity of increasing solubility in water. The addition of urea shows an increase in CP with concentration. The increase in CP of nonionic surfactants on addition of urea has been observed before [22,47] and can be rationalized in terms of interactions between ethylene oxide head group and water in the presence of urea, making water a better solvent for the polymer. Viscosity measurements were performed for all the four copolymers PMMA–PEO 1-1, PMMA– PEO 1-2, PMMA–PEO 1-3, PBMA–PEO 1-1 in absence and presence of NaCl (0–2 M) and urea (0–2 M) at three different temperatures (30, 40 and 50 C). In all cases, plots of reduced viscosity against copolymer concentration are found to be linear, a behavior typical of an uncharged polymer and which yields intrinsic viscosity [g] shown in

100

80

CP, oC

alteration in its microenvironment which manifests as a decrease in kmax, at CMC, the copolymer molecules exert enough hydrophobic interactions leading to micellization. The CMCs determined from both the methods provides almost identical values. The surface tension measurements for all the copolymers were also made in presence of NaCl and urea solution (figure not shown). The CMC of copolymers is not significantly changed in the presence of NaCl. SANS and viscosity data (to be discussed later) also revealed that presence of NaCl have almost negligible influence on the micellization and micelle structures of this block copolymers in aqueous solution. Usually ionic surfactants showed a marked decrease in CMC in the presence of added salt, though highly hydrophilic PEO/PPO/PEO block copolymers have shown drastic decrease in the CMC in the presence of salt possibly at high temperatures due to the enhanced hydrophobicity in PPO in presence of salt [18,19]. The surface tension of block copolymer solutions increased (and the surface activity decreased) in the presence of urea. Addition of urea shifts the surface tension curve to higher copolymer concentrations, compared to the ones at zero urea concentration without affecting the features of the curve. The CMC values of the copolymers in the presence of urea were higher than those for copolymers in water, intimating that the micelle formation becomes more difficult upon increasing urea concentration. The increase of CMC in nonionic surfactants caused by urea was attributed by Schick [41] to increase hydration of the PEO segments caused by a reduction of the cooperative structure of water. According to Briganti et al. [42], enhanced solubility of the hydrophobic tail in urea–water solutions was the dominant factor in determining the CMC in nonionic surfactants, which is also supported by the findings of DasGupta and Moulik [32]. The clouding is ascribed to the increase in the size of polymer aggregates, leading eventually to phase separation into a polymer rich and water rich phases. Therefore, its occurrence reflects the balance between polymer–polymer and polymer–water interactions. The clouding behavior of nonionic surfactants in water is an interesting feature involving their practical usefulness. The cloud point (CP) of all four copolymers in water is shown in Table 1. Copolymers 1-1, with 50% PEO had CP less than 100 C while those having larger hydrophilic PEO block lengths i.e. 1-2 or 1-3, had CP above the boil-

597

60

40

0.0

0.5

1.0

1.5

2.0

2.5

[Additive], M

Fig. 2. Cloud points of PBMA–PEO 1-1 (circle) and PMMA– PEO 1-1 (triangle). Filled symbols for urea and open symbols for NaCl.

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H. Desai et al. / European Polymer Journal 42 (2006) 593–601

Table 2 Intrinsic viscosities of diblock copolymer micelles in aqueous NaCl and urea at different temperatures Additive (M)

Intrinsic viscosities, dl g
1 PBMA–PEO, 1000:1000

PMMA–PEO, 1000:1000

PMMA–PEO, 1000:2000

PMMA–PEO, 1000:3000

30 C

40 C

50 C

30 C

40 C

50 C

30 C

40 C

50 C

30 C

40 C

50 C

NaCl 0 1.0 2.0

0.068 0.052 0.042

0.064 0.050 0.042

0.055 0.047 0.040

0.080 0.063 0.057

0.072 0.060 0.057

0.064 0.054 0.049

0.097 0.090 0.085

0.093 0.087 0.082

0.086 0.083 0.079

0.125 0.106 0.093

0.120 0.103 0.090

0.108 0.080 0.079

Urea 1.0 2.0

0.070 0.076

0.066 0.074

0.061 0.068

0.084 0.088

0.066 0.071

0.065 0.070

0.099 0.105

0.095 0.098

0.091 0.098

0.130 0.133

0.124 0.130

0.112 0.116

Table 2. Representative plots of reduced viscosity versus copolymer concentrations in water at 30 C are shown in Fig. 3. The copolymer PMMA–PEO 1-3 in water at 30 C remains molecularly dissolved in the concentration range studied as its CMC is observed to be 0.10 wt% thus high intrinsic viscosity, [g] = 0.125 dl g
1 can be anticipated. There is a gradual increase in intrinsic viscosity of copolymers in water at particular temperature with increase in number of repeat unit in PEO content and such a behavior in quite usual. Among the PMMA–PEO 1-1 and PBMA–PEO 1-1, [g] is lowest (i.e. 0.068 dl g
1) for PBMA–PEO 1-1 because there are only 7 repeat units in PBMA block while 10 in PMMA block. As reflected from Table 2, intrinsic viscosities showed a slight linear decrease with increase in temperature and NaCl concentration. The decrease in intrinsic viscosities observed could be considered due to the progressive dehydration of the hydrated PEO shell leading to more compact

micelles which may account for the observed trend. Such a behavior in viscosities was observed by several workers in the past [48]. In the presence of urea all the copolymer shows a slight increase in intrinsic viscosities with increase in urea concentration, which is attributed to increase hydration of the PEO segments caused by a reduction of the cooperative structure of water [41]. The characteristic neutron scattering curves for PMMA–PEO 1-2 aqueous solution at different concentrations of 1, 2 and 5 wt% (w/v) at 30 C are shown in Fig. 4. The curves have common characteristics at all the three concentrations. The coherent differential scattering cross section (dR/dX) shows relatively strong Q dependence. The measured SANS distribution is typical of that obtained from micellar solutions [49]. In view of this and the fact that scattering cross section of individual polymer molecules are much smaller than those seen above, indicates that surfactant molecules are associating to form micelles.

0.15 PMMA-PEO 1-2 5 wt%

40

0.10

dΣ/dΩ , cm-1

Reduced viscosity, dlg-1

50

0.05

30 2 wt%

20 1 wt%

10 0.00 0.0

0.5

1.0

1.5

2.0

2.5

[Copolymer], wt%

Fig. 3. Reduced viscosity vs copolymer concentration at 30 C. PBMA–PEO 1-1 (s), PMMA–PEO 1-1(n), PMMA–PEO1-2 ($) PMMA–PEO 1-3 (%).

0

0.01

Q, A-1

0.1

Fig. 4. SANS profiles for different concentrations of PMMA– PEO 1-2 at 30 C.

H. Desai et al. / European Polymer Journal 42 (2006) 593–601

It is seen that SANS distribution at high Q is almost independent of surfactant concentration. This suggests that at least one of the dimensions of the micelle is independent of surfactant concentration. The measured SANS profiles are monotonically decreasing with Q, and there is no indication of the correlation peak. Thus, we assumed S(Q) = 1 in the analysis. F(Q) was calculated assuming prolate (a > b) ellipsoidal shapes for the micelles. qs for the solvent has a value of 6.38 · 1010 cm
2 and the value of qm was calculated assuming that scattering is from the hydrophobic core of the micelle. The SANS data were fitted to Eq. (1) using nonlinear least square fitting program. It was found that our experimental SANS intensities were best matched with the values obtained by using a prolate ellipsoidal model. In the analysis, the semiminor axis (b) and the semimajor axis (a) were obtained as the fitted parameters. The aggregation number (Nagg) of the micelles is then calculated as Nagg = Vm/Vh, where Vm is the micellar volume that equals (4/3)pa2b and Vh is the volume of hydrophobic part of block copolymer. From the estimated value of Nagg, the number density of micelles, nm, i.e., the number of polymer molecules per unit volume, is calculated by the following relation: nm =cm
3 ¼

CN A  10
3 ; N agg

ð6Þ

where C is the concentration of copolymer in mol dm
3 and NA is the AvogadroÕs number. All the parameters that characterize the micellar associates diblock copolymers as extracted from the above procedure are listed in Table 3. Fig. 5 shows the effect of (A) temperature, (B) added NaCl and (C) urea on SANS distributions of PMMA–PEO 1-2 (1 wt%). The values of micellar parameters obtained from SANS analysis are shown in Table 3. There does not appear to be any significant influence of temperature on the micellar characteristics of copolymer as reflected from the observed neutron scattering profile. Mortensen et al. [50] also observed the similar effect of temperature on diblock copolymer of polystyrene–polyethylene oxide, PS–PEO 1-3 and concluded that copolymer forms spherical micelles which are thermally stable up to phase separation. Here, it appears that increase in temperature neither affects hydrophobic core nor dehydrating the hydrated PEO shell. However, it may be assumed that temperature variation might be distorting water structure, lead-

599

Table 3 Values of semimajor axis (a), semiminor axis (b), axial ratio (a/b), aggregation number (Nagg) and number density (nm) of the prolate ellipsoidal micelles of PMMA–PEO 1-2 (1 wt %) ˚) ˚) nm (cm
3) a (A b (A a/b Nagg Concentration, wt% 1 97 28 2 93 28 5 86 28

3.5 3.3 3.0

664 610 522

2.993 · 1015 6.516 · 1015 19.153 · 1015

Temperature, C 30 97 45 97 60 97

28 29 30

3.5 3.3 3.2

664 688 711

2.993 · 1015 2.888 · 1015 2.795 · 1015

NaCl, M 1.0 2.0

97 97

29 31

3.3 3.1

688 735

2.888 · 1015 2.704 · 1015

Urea, M 1.0 101 2.0 105

25 21

4.0 5.0

643 584

3.091 · 1015 3.403 · 1015

ing to an increase in entropy of the system, the situation favorable to the stacking of normal spherical micelles which eventually leads to phase separation at its cloud point. Such an observation for highly hydrophilic block copolymer F88 is reported in past [17]. Fig. 5(B) shows almost identical neutron scattering profile with increasing salt concentration, indicating the weak influence of NaCl on aggregation and micellar characteristics of block copolymer micelles in aqueous medium. More or less identical values of micellar parameters also provide an evidence for the same. Surface tension and viscosity measurements exhibiting no/negligible changes in CMCs and intrinsic viscosities (discussed earlier) also support this observation. This behavior shown by diblock copolymers is entirely different in comparison to that of salt induced micellization and sphere to rod transition in of PEO–PPO–PEO triblock copolymers documented by Jain et al. [17] and Jorgensen and coworkers [51]. Salt induced micellization is probably due to induced hydrophobicity of PPO block which is otherwise hydrophilic at/below ambient temperature. Scattering functions show that the intensity decreases slightly as the urea concentration increases (Fig. 5C). The presence of urea in aqueous solution of copolymers leads to its complexation with spherical aggregation of ethylene oxide units in PEO shell of micelles and this result into increased hydration of PEO shell consequently steric repulsion originates and becomes significant with increasing urea concentration.

600

H. Desai et al. / European Polymer Journal 42 (2006) 593–601 20

PBMA-PEO 1-1

15

60 45 30

dΣ/dΩ , cm-1

dΣ/dΩ , cm-1

15

20

A

Temperature oC

10

PMMA-PEO 1-2

10 PMMA-PEO 1-3

5

5

0

0 0.01

0.1

Q, A-1

20

B [NaCl], M

Fig. 6. SANS profiles for various copolymers at 30 C.

dΣ/dΩ , cm-1

15 2 1 0

Table 4 SANS data showing the effect of different size of hydrophobic and hydrophilic block of diblock copolymer (1 wt%) ˚ ) b (A ˚ ) a/b Nagg nm (cm
3) Copolymer a (A

10

5

PBMA–PEO 1-1 PMMA–PEO 1-2 PMMA–PEO 1-3

0 20

C [Urea], M

dΣ/dΩ , cm-1

15 0 1 2

10

5

0 0.01

0.1

Q, A-1

Fig. 5. SANS profiles for PMMA–PEO 1-2 at different temperature, in presence of NaCl and urea concentrations.

However, here the urea concentration appears to be quite less to generate very strong steric repulsion not causing any appreciable change in aggregation number for PMMA–PEO micelles. Effect of variation in molecular weight of PEO hydrophilic block (1000, 2000 and 3000) and hydrophobic block PMMA and PBMA of fixed molecular weight 1000, on SANS distributions were made and are shown in Fig. 6. Scattering curves show that the contribution from interparticle structure factor S(Q)

75 97 86

36 28 24

2.1 3.5 3.4

510 664 447

3.897 · 1015 2.993 · 1015 4.446 · 1015

is not significant and can be neglected. The values of micellar parameters obtained from SANS analysis are shown in Table 4. The scattered intensity of these copolymers is in the order PBMA–PEO 11 > PMMA–PEO 1-2 > PMMA–PEO 1-3. The smaller values of scattered intensity for PMMA– PEO 1-3 as compared to PMMA–PEO 1-2 may be due to former being more hydrophilic than latter. PMMA–PEO 1-3 has higher CMC and less aggregation number which may cause the molecules in its hydrophobic core to be in the coiled. Surface tension data (Table 1) reveals that among all acrylates, PBMA–PEO 1-1 is more hydrophobic and surface active having lowest CMC. Therefore higher values of its aggregation number determined here are quite justified. 4. Conclusions The micellization and structure of PMMA–PEO and PBMA–PEO varying in molecular characteristics with and without NaCl and urea has been studied by using different methods. The addition of NaCl decreases while urea increases the onset concentration of micellization and phase separation. The effect of increase in temperature, NaCl and urea concentration on the neutron scattering profiles for

H. Desai et al. / European Polymer Journal 42 (2006) 593–601

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