Solvent effect on adsorption of polymers on γ-Fe2O3 particles

Colloids and Surfaces A: Physicochemical and Engineering Aspects, 80 (1993) 85-92 0927-7757/93/$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved.

Solvent effect on adsorption particles Katsuhiko Nakamae*, Katsuya Yamaguchi Department (Received

Satoshi Tanigawa,

of Industrial Chemistry, 30 July 1992; accepted

85

of polymers on Y-Fe203

Tomoyuki

Faculty of Engineering,

Kobe

Tsujiguchi,

Shoji Okamoto,

University, Rokko,

Nada, Kobe 657, Japan

8 April 1993)

Abstract The solvent effect on the adsorption behavior of copolymers containing -OH, -COOH and -P03HZ groups onto y-Fe,O, particles in the toluene-methanol solvent system was investigated. The adsorbance and intrinsic viscosity of these polymers were measured at various methanol contents of the solvent system. The saturated adsorbance of poly(methy1 methacrylate) and copolymer containing the -OH group was made negative by addition of a small amount of methanol to the solvent system. The saturated adsorbance of copolymers containing -COOH and -PO,H, groups reached a minimum value at a certain methanol content. These saturated adsorbances were compared with the adsorbance calculated from the intrinsic viscosity. Magnetic coatings were also prepared and their magnetic properties measured. Key words: Adsorption;

y-Fe,O,

particles;

Functional

group; Intrinsic

Introduction Composite inorganic

materials

particles

consisting

find

many

of polymers applications,

and

e.g. and paints. The interaction between polymers and inorganic particles is one of the most important factors characterizing the materials and properties of these components themselves [ 1,2]. A typical example is coated magnetic recording tape. Its recording performance is greatly dependent not only on the magnetic particle itself but also on the packing density and the orientation of the magnetic particles in the coating, which is controlled by the dispersibility of the magnetic particles in the polymeric binder and the interaction of these components. We have investigated the adsorption behavior of polymers onto magnetic particles in particulate-filled

*Corresponding

rubbers,

author.

moldings

viscosity;

Poly(methy1

methacrylate)

non-aqueous solvents, and its effect on the magnetic properties of magnetic tape. Polymers having a small number of hydrophilic functional groups in a hydrophobic polymer chain are available as binders [3,4]. The interaction between the hydrophilic functional groups of the polymer and the water molecules adsorbed chemically and physically on magnetic particles affects the adsorption behavior. The degree of interaction is [3,5]

P\

-POSH2> -SOaH> -CM)H > -OH > -NC > -C-C

k

-CN

It has been verified that the conformation of polymers in the adsorbed layer is of a loop-point type, in which the hydrophilic functional groups are anchor segments and the hydrophobic segments form a loop into the solution [3,6]. These results are in good agreement with the properties of magnetic tapes using such polymers as binder. The solvent effect is very important in this

K. Nakamae et ul./Colioids Surfaces A: Physicochem. Eng. Aspects 80 (1993) 85-92

86

adsorption

system because the adsorption

of polymers among

is dominated

the polymers,

the solvents. polymers typical

effects

conditions. mation

of solvent,

about

solution, can

and which

be the under

there is no systematic

the solvent

of

particles

of the adsorption

However,

and

adsorption

on magnetic

of the polymer

factors

behavior

most certain infor-

effect on the adsorption

behavior

of copolymers,

consisting

functional inorganic

groups and hydrophobic particles in non-aqueous

of hydrophilic segments, solvents.

onto

In this study, we have investigated the solvent effect on the adsorption behavior of copolymers containing onto

-OH,

y-Fe,O,

solvent

-COOH

particles

and

-PO,H,

groups

in the toluene-methanol

system.

Experimental Materials

Magnetic y-Fe,O, particles have an average length of 0.4 urn, an average acicular ratio of l/9, and a specific 19.3 m2 gg’. Methyl

a

area measured

methacrylate

methacrylate purified

surface

(MMA),

(HEMA)

and acrylic

by distillation

nitrogen

methacrylate

under

atmosphere. (Phosmer,

by BET of

2-hydroxyethyl acid (AA) were

reduced

pressure

in

2-Acidphosphoxyethyl Yuni Chemical,

Japan) was

used after removing an inhibitor with active carbon. cr,cc’-Azobisisobutylonitoryl as an initiator was

of

reagent

grade

and

was

recrystallized.

PMMA, P(MMAlAA) and P(MMAlHEMA), (where P stands for “poly”) were synthesized using toluene

as a solvent

in a nitrogen

atmosphere

Phosmer). The toluene

and methanol

of the polymers

used as solvents

were of extra pure reagent

grade.

The contents in the

of hydrophilic

synthesized

titration

polymers

of the polymer

functional were

in toluene

of

groups

measured solution

by with

0.1 N NaOH in methanol; both phenolphthalein and Methyl Red were used as indicators. The molecular weights and molecular weight distributions were measured by gel permeation chromatography (GPC) (Waters 6000A) and they were calibrated by monodispersed polystyrene. Table 1 shows the properties of the polymers. Sedimentation

of Y-Fe, 0,

The sedimentation volume of y-Fe, 0, in various media was measured after the glass tube (25 cm3 capacity) containing y-Fe,O, (2.0 g) and medium (20 cm”) was shaken and allowed to stand for 24 h. For the measurement of the sedimentation rate, the glass tube contained y-Fe203‘ (0.5 g) and the medium (20 cm”). The glass tube was allowed to stand for 6 h after shaking. Then it was shaken again, and the time for 5 cm of sedimentation of y-Fe, 0, was measured. Adsorption

of polymer

on y-Fe,O,

Adsorption testing was carried out as follows. Polymer solution (20 cm”) and y-Fe,O, (2.0 g) were mixed in a glass tube (25 cm3 capacity). The sample tube was shaken for 24 h and allowed to stand for an additional 48 h. The temperature was maintained at 30.0 + 0.5”C. The adsorbance of Table 1 Characterization

of the polymers

used

Polymer

Ii?,”

A,”

PMMA P(MMA-HEMA) P(MMA-AA) P(MMA-Phosmer)

67000 46000 41000 47000

113000 88000 105000 113000

Functional group contentb (mol.%)

at

70°C for 24 h. 1,4-Dioxane was used instead of toluene in the copolymerization of P(MMA-

the polymers

Characterization

interactions particles

Both the competitive

the characteristic are

the

the magnetic

and solvents

important

by

“Molecular bFunctional

5.0 2.2 0.8

weight was obtained by GPC. group content was obtained by titration.

K. Nakamae et al./Colloids Surfaces A: Physicochem. Eng. Aspects 80 (1993) 85-92

polymer on y-Fez O3 was determined by measuring the concentration of the supernatant solutions before and after the adsorption test.

87

using a solenoid coil, after the coating had been applied to poly(ethylene terephthalate) film with a doctor-blade and then dried at 80” C. The magnetic properties

were

measured

by using

a vibration

Intrinsic viscosity of polymer

sample magnetometer (VSM) (Toei Kogyo, Japan). The dispersibility and orientation of the y-Fe,O,

The viscosities of the polymer solutions were measured by Ubbelohde’s method at 30°C. The intrinsic viscosity [r] is defined by

in the magnetic coatings were evaluated from the squareness M,/M, (where M, is the residual magnetization and M, is the saturated magnetization) of the magnetic hysteresis curve.

[q] = lim !@ c-o ( C > where qsp is the specific viscosity of the and C is the polymer concentration of the The specific viscosity can be calculated time spent falling down through a fixed solution (t) and solvent (to): v],,

=

-== r - ‘lo fl0

(1) polymer solution. from the length of

t - to to

(2)

where q is the viscosity of the polymer solution and q. is that of the solvent. The intrinsic viscosity of polymers was estimated as the value of qsp/C at C = 0, which was found by extrapolation of the linear plot of u]s,/C vs. C. Mean molar volume of solvent

Results and discussion Sedimentation

of y-Fe,O,

Figure 1 shows plots of the sedimentation volume and the sedimentation rate of y-FezO, vs. the solubility parameter (SP) of the medium used. In general, a decrease in both the sedimentation volume and sedimentation rate indicates an increase in the dispersibility of the magnetic particles. In Fig. 1, the sedimentation volume and the sedimentation rate of y-Fe,O, are lowest for the media methanol, ethanol and butanol, which all contain an -OH group. In contrast, the sedimentation volume and the sedimentation rate in a hydrophobic medium such as n-hexane, toluene and benzene have larger values. The results show that v

The mean molar volume of the toluenemethanol solvent system was calculated by using the specific gravity of the mixture and the molecular weight of each component. The specific gravity was measured by using a pycnometer at 30°C.

k .-

E

E

Preparation of the magnetic coatings and their magnetic properties Magnetic paints were prepared using a ballmilling process. The vessel (50 cm3 capacity) contained the binder polymer (2.68 g), y-Fe, O3 (6.25 g), solvent (13.4 g) and stainless steel balls (8 mm in diameter). The dispersing process was carried out for 100 h. The magnetic coatings were prepared under a magnetic field (1000 Oe) for the orientation

z3 e r%

,

,,, 8

10

,,,,,I,*,,, 12 14

16 23 24

SP of medium / (Cal- crnm3)’ ‘* Fig. 1. Plots of the sedimentation volume and the sedimentation rate of y-Fe,O, particles in various media against the solubility parameter (SP) of the medium: 1, n-hexane, 2, methyl isobutyl ketone; 3, toluene; 4, benzene; 5, tetrahydrofuran; 6, dioxane; 7, butanol; 8, ethanol; 9, methanol; 10, water.

K. Nakamae et al./Colloids Surfaces A: Physicochem. Eng. Aspects 80 11993) 85-92

88 the surface of y-Fe,O, interacts

strongly

groups

of the medium

particles with

is hydrophilic

hydrophilic

and

functional

methanol decreases fraction

molecules.

solvent system. The mean molar volume linearly with the increase in the molar of methanol.

The linear

relationship

indi-

In this study, we used the toluene-methanol solvent system. Toluene is a hydrophobic solvent and hardly interacts with y-Fe, 0,) whereas methanol is hydrophilic and interacts with y-Fez03 strongly. Competitive adsorption of polymer and

cates that each component - toluene and methanol - has the same molar volume in any formation of the solvent system. Thus the interaction between toluene and methanol is not important in this solvent system. This result allows us to handle the

methanol onto the y-Fe203 surface is important in this system. The adsorbance of polymer will decrease if methanol adsorbs onto y-Fe,O, preferentially. Another effect of the methanol is the change in solubility of the polymer in the solvent system. Toluene is a good solvent and methanol is a poor solvent for the polymers used. Addition of methanol makes the solubility of the polymer decrease, and the decrease in solubility of the polymer may increase the adsorbance of the polymer onto y-FezO,.

adsorption

Mean molar volume of the solvent system Figure 2 shows the relationship between the mean molar volume of the solvent molecule and the molar fraction of methanol in the toluene-

-0

0.2

0.4

0.6

0.8

mixture

behavior

Adsorption

Fig. 2. Relationship between the mean molar volume solvent molecule and the molar fraction of methanol toluene-methanol solvent system.

Figure 3 shows adsorption isotherms of PMMA, P(MMA-HEMA), P(MMA-AA) and P(MMA-Phosmer) on y-Fe,O, particles in toluene at 30°C. The adsorbance of each polymer increases with the equilibrium concentration of the polymer but shows a plateau above about 1.5 g/l00 cm3. The P(MMA-Phosmer) shows the highest saturated adsorbance and PMMA the lowest. The shape of the adsorption isotherm does not change on addition of methanol. Hence, noting the saturated adsorbance at various methanol contents of the solvent system, we can discuss the

Equilibrium of the in the

in the solvent systems.

of polymers

1.0

Molar fraction of methanol / -

of the polymer

system as in monosolvent

1

2

concentration

/g - 100cms3

Fig. 3. Adsorption isotherms of the polymers on y-Fe20, in toluene at 30°C: 0, PMMA; a, P(MMA-HEMA); 0, P(MMA-AA); 0, P(MMA-Phosmer).

K. Nakamae et al./Colloids Surfaces A: Physicochem. Eng. Aspects 80 (1993) 85-92

89

relationship between the methanol content and the saturated adsorbance. Figure 4 shows the relationship between the

groups, which show a stronger interaction with y-Fe,O, than the -OH group of methanol. Thus these polymers can adsorb on y-Fe,O, even in the

saturated adsorbance of PMMA, P(MMAHEMA), P(MMA-AA) and P(MMA-Phosmer) on y-FezO, particles and the methanol content of the

toluene-methanol system. However, the complicated relationship between the adsorbance of these polymers and the methanol content cannot be

solvent system. The saturated polymer decreases suddenly

explained clearly from the viewpoint of the interaction between the polymers and y-Fe,O,. The

adsorbance of each with addition of a

small amount of methanol. The saturated bance of PMMA and P(MMA-HEMA)

adsorwas

negative in the toluene-methanol system. PMMA no hydrophilic functional groups. contains P(MMA-HEMA) contains the same hydrophilic -OH groups as methanol. Therefore methanol shows a stronger interaction with y-Fe,O, than these polymers. It is believed that methanol adsorbs onto y-FezO, preferentially to these polymers. However, saturated adsorbance of the P(MMA-AA) and P(MMA-Phosmer) was positive for all methanol contents. It reached a minimum value at a methanol content of about 30 vol.% and increased with methanol content over 20-30 vol.%. These polymers have -COOH groups or -PO,H,

adsorption behavior was then compared with the solution behavior of the polymers in the solvent system. Intrinsic

viscosity

Figure 5 shows the relationship between the intrinsic viscosity of the polymer solution and the methanol content of the solvent system. The intrinsic viscosity of each polymer solution reached a maximum value at a methanol content of lo-30 vol.%. The intrinsic viscosity of the polymer solution is related to the gyration of the polymer molecular chain in the solution. Flory’s equivalent sphere model [7] gives IIr,

=

46 (S2>)3’2 M

where

[YI] is the intrinsic

viscosity,

0

40

(S2)

is the

0.6

0

IO

20

30

40

50

60

Methanol content 1~01% Fig. 4. Relationship between the saturated adsorbance of the polymers on y-Fe,O, and the methanol content of the solvent system: 0, PMMA, @, P(MMA-HEMA); 0, P(MMA-AA); 0, P(MMA-Phosmer).

10

20

30

50

60

Methanol content / vol% Fig. 5. Relationship between the intrinsic viscosity of the polymer solution and the methanol content of the solvent system: 0, PMMA; @, P(MMA-HEMA); 0, P(MMA-AA); c), P(MMA-Phosmer).

K. Nakamae et al./Colloids Surfaces A: Physicochem. Eng. Aspects 80 (1993) 85-92

90

mean square radius of gyration, molecular weight of the polymer

M is the (mean) and 4 is a con-

stant given by 4 =0.01588

; 3’2N

(4)

0 where N is Avogadro’s because M is a constant,

where A, is a saturated gives the molecule constant adsorbed

and an(S’)

adsorbance

area occupied by an adsorbed polymer on the surface of y-Fe,O,, with a a related to the effects of neighboring molecules (overlapping, rejection, etc.).

Combining

Eqs. (3), (4) and (6) gives

constant. In this study, Eq. (3) can be simplified

to where p is a constant

ct11 @zG2 )3’2

(5)

The gyration of the polymers is widest at a methanol content of lo-30 vol.% because the intrinsic viscosity is a maximum (Fig. 3). This fact shows that a solvent mixture containing lo-30 vol.% methanol is a better solvent for the polymers than pure toluene, even though methanol is a precipitant for the polymers. The solubility parameter (SP) of PMMA with a poor-hydrogen-bonding solvent is 8.9912.7 calli cm312, but with a moderatehydrogen-bonding solvent it is 8.5-l 3.3 calli cm3j2 [S]. The difference in these ranges shows that moderate-hydrogen-bonding solvents are generally better solvents for PMMA than poorhydrogen-bonding solvents. In addition, the SP of PMMA is approximately located between that of toluene (SP = 8.9) and that of methanol (SP = 14.5). Therefore it seems that PMMA dissolves better in a solvent mixture containing a certain amount of methanol than it does in pure toluene, which is a typical poor-hydrogen-bonding solvent. This behavior is not necessarily unusual. For example, a mixture of benzene and methanol is a better solvent for ethylene-vinyl alcohol copolymer pure benzene or pure methanol [9].

than

P=

0.06318 a

given by

M 1’3

(8)

(-1N

Here p depends hardly at all on the solvent. In this study, the value of p was determined by substituting both the observed saturated adsorbance and the value of [++I in pure toluene in Eq. (7); the calculated saturated adsorbance in the solvent mixture was given by Eq. (7) from this value of p. Figure 6 shows the relationship between the saturated adsorbance of PMMA, P(MMAHEMA), P(MMA-AA) and P(MMA-Phosmer) on y-Fe,O, particles and the methanol content of the

4PMMApiiKz-

either

P(MMA-Phosmer)

Calculated adsorbance If the polymer adsorbs onto y-Fe,O,, keeping the same gyration structure as in the solution, and the adsorption of the polymer on y-Fe,O, is in a monomolecular layer, we have

0

20

40

60

0

20

40

60

Methanol content / ~01%

(:)(&)

Am=

(6)

Fig. 6. Plots of saturated adsorbances of polymers at various methanol contents: 0, observed values; 0, calculated values.

K. Nakamae

et al./Colloids

Surfaces

A: Physicochem.

Eng. Aspects 80 (1993

solvent system. The plots of the observed saturated adsorbance are the same as in Fig. 3. The shape of the curve of calculated values is similar to that of the observed pation of the

values. This is evidence for the partici-

of the solution polymer.

in the adsorption

According

to

the

I 85-92

91

0.8 c

behavior comparison

between the observed and calculated saturated adsorbances, it is demonstrated that an increase in the observed saturated adsorbance with a methanol content over the range 20-30 vol.% is caused by a decrease in the gyration of the polymers. The calculated value is larger than the observed value in the cases of PMMA, P(MMA-HEMA) and P(MMA-AA). The reason for this seems to be the preferential adsorption of methanol. The calculated saturated adsorbance of P(MMA-Phosmer) agrees with the observed value. Thus the saturated adsorbance of P(MMA-Phosmer) is mainly governed by the interaction between the polymer and y-Fe,O, particles. Methanol can hardly prevent the adsorption of P(MMA-Phosmer) on y-Fe,O, particles because the -OH group of methanol shows a weaker interaction with y-Fe,OJ particles than does the -P03H, group of P(MMAPhosmer).

Magnetic property Figure 7 shows the relationship between the squareness of the magnetic coatings and the methanol content of the solvent system. The squareness is strongly

dependent

on the dispersion

0.4;

I 10

I 20

I 30

I 40

I 50

I 60

Methanol content / vol% Fig. 7. Relationship between the squareness M,/M, of the magnetic coatings and the methanol content of the solvent system: 0, PMMA; 0, P(MMA-AA).

tion is derived from the interaction between methanol and the particles. However, the squareness of the magnetic coating when using P(MMA-AA) also decreases rapidly with increase in methanol content up to 5 vol.%, but the behavior of the squareness above 10 vol.% is very complicated. The polymer could adsorb on y-FezOS in the presence of methanol, but the observed value is lower than the calculated value. This behavior suggests the occurrence of competitive adsorption of the polymer and methanol, which contributes to the complicated behavior of the dispersion and orientation.

and the

orientation of y-Fe,03 in the magnetic coatings and approaches 1.0 when the dispersion and the orientation are highly improved. The squareness of the magnetic coating using PMMA as a binder decreases rapidly with the increase in methanol content up to 5 vol.%. The decrease in the dispersion and the orientation arises from the decrease in the saturated adsorbance of PMMA on y-Fe, O3 (see Fig. 3). However, the squareness increases with methanol content above 10 vol.%. The saturated adsorbance of the polymer is negative in this range. Thus the increase in the dispersion and the orienta-

Conclusion The solvent effect on the adsorption behavior of PMMA, P(MMA-HEMA), P(MMA-AA) and P(MMA-Phosmer) onto y-Fe,O, in the toluenemethanol solvent system has been investigated. The adsorption behavior is mainly controlled by two factors: the competitive adsorption of methanol, and the solution behavior of the polymer. Addition of methanol influences both these factors. In the cases of PMMA and P(MMA-HEMA), which have either no or only weak hydrophilic

K. Nakamae et al.lColloids Surfaces A: Physicochem. Eng. Aspects 80 (1993) 85-92

92

functional groups, the former becomes nant factor, whereas the latter becomes nant

factor

in

the

which has a strongly

case

the domithe domi-

2 T.F. Tadros, 3

of P(MMA-Phosmer),

hydrophilic

functional

group.

4 5

Acknowledgment

6

The authors thank Nissha Research for financial support.

Aid for Academic

7 8

References 1 D.T. Wu, A. Yokoyama SPSJ Int. Polym.

and R.L. Setterquist, Conf., Japan, 1990, p. 136.

Prepr.

3rd

9

Prepr. 3rd SPSJ Int. Polym. Conf., Japan, 1990, p. 126. K. Sumiya, S. Watatani, F. Hayama, K. Nakamae and T. Matsumoto, Kobunshi Ronbunshu, 35 (1978) 565. K. Nakamae, S. Tanigawa, N. Hirayama, K. Yamaguchi and T. Matsumoto, J. Adhes., 21 (1987) 229. K. Nakamae, S. Tanigawa, K. Sumiya and T. Matsumoto, Colloid Polym. Sci., 266 (1988) 1014. K. Sumiya and T. Matsumoto, J. Jpn. Sot. Color. Mater., 58 (1984) 211. P.J. Flory, Principles of Polymer Chemistry, Cornell University Press, Ithaca, NY, 1953. E.A. Grulke, in J. Brandrup and E.H. Immergut (Eds.), Polymer Handbook, 3rd edn., Wiley, New York, 1989, pp. VII-5 19. T. Matsumoto, K. Nakamae and T. Ochiumi, Sen’i Gakkaishi, 30 (1974) 398.