j o u r n a l o f MEMBRANE

SCIENCE E L S E V I E R Journal of Membrane Science 125 (1997) 213-218

Release rates from semi-crystalline polymer microcapsules formed by interfacial polycondensation

S.K. Yadav, Kartic C. Khilar, A.K. Suresh * Department of Chemical Engineering Indian Institute of Technology-Bombay Powai, Bombay 400 076, India

Received 8 December 1995; revised 21 May 1996; accepted 27 May 1996

Abstract

Microencapsulation of active agents, for their controlled release, can be brought about by the method of interfacial polycondensation [ 1]. In this paper, the permeabilities of the polyurea microcapsules for encapsulated cyclohexane have been determined. It is shown that the product of the permeability and membrane thickness can be changed over at least an order of magnitude by changing the degree of crystallinity of the polymer forming the membranes.

Keywords: Polyurea; Microencapsulation; Controlled release; Crystallinity; Permeability

1. Introduction

Polymer microcapsules have found numerous ap- plications as carriers of various active agents such as agrochemicals and pharmaceuticals [2,3]. Microcap- sules can be used to either protect the encapsulated materials from the environmental conditions or to release the active agent in a sustained and controlled manner into the surrounding medium. Such micro- capsules may be produced by carrying out an interfa- cial polycondensation reaction at the surface of drops containing the material to be encapsulated. Rates of release from such microcapsules depend largely on the structure of the polymer formed, which in turn is influenced by the conditions employed in the prepa- ration. It would therefore be useful to study the mechanism of release and how it is influenced by the structure of the polymer. Since similar procedures

* Corresponding author.

are often used to form the thin selective barrier in reverse osmosis and gas separation membranes [4], any understanding of the correlation between release phenomena and polymer structure will be of impor- tance in these areas as well. Many studies of struc- ture-permeability correlations in polymeric mem- brane materials [4,5] concern classes of polymers in which differences in physical structure are brought about by systematically changing the chemical struc- ture of the monomers used. Such studies show the degree of crystallinity influences the permeability significantly, primarily through diffusivity. Our pre- vious studies on poly(hexamethylene)urea microcap- sules [1] indicate that it is possible to vary the degree of crystallinity of the membrane wall, for the same polymer, through the choice of preparation condi- tions. Microcapsules produced under high rates were shown to have more amorphous walls. In the present paper we extend the earlier studies to the behaviour in release of such polyurea microcapsules made un-

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214 S.K. Yadav et al. / Journal of Membrane Science 125 (1997) 213-218

der different conditions, and examine the dependence of release rates on the degree of crystallinity of the wall material.

2. Microcapsule preparation

Poly(hexamethylene)urea microcapsules were formed in this work by carrying out an interfacial polycondensation reaction in an emulsion between hexamethylene-1,6-diisocyanate (HMDI) dissolved in cyclohexane, and hexamethylene-l ,6-diamine (HMDA) dissolved in water. The organic phase formed the dispersed phase. The progress of the reaction was monitored by measuring the pH of the continuous phase [6]. The reaction was assumed to be completed when the pH became constant, and the microcapsules were filtered out. All reactions were carried out at the ambient temperature of 27-28°C. Further details of the procedure are reported else- where [1,7].

The parameters varied in the preparation were: monomer mole ratio R (ratio of the number of moles of HMDI to the number of moles of HMDA taken), the number of moles of the limiting monomer n L, and the ratio of the dispersed phase (cyclohexane) to the continuous phase in the final reaction mixture ( V d / V c ) . Table 1 shows the conditions employed for the samples used in these studies.

3. Characterization

The capsules produced had a size range of 0.5 to 10 p~m as determined by Galai CIS-1 particle sizer. The number average size was 1.6 ixm. The polymer wall thickness could not be directly measured. How- ever, since the reaction went to completion in every

case (as determined by pH measurement), an average wall thickness could be calculated from stoichiom- etry and the average size of the capsules. The poly- mer density, pp in these calculations was taken as 1000 k g / m 3.

The crystal structure of the samples was analysed by wide-angle XRD. The polymer sample were pre- pared from the microcapsules, and their powder X-ray diffraction (XRD) patterns were recorded on a Rigaku goniometer (RIGAKU-DMAX/1C), as described in [1]. The samples showed prominent crystalline peaks in the region 10 ° < 2 0 < 30 °. Degrees of crystallinity (X c) of the samples were therefore calculated in this range, by drawing an amorphous halo and comparing the area under the crystalline peaks to the total area under the diffractogram [1,8].

4. Experiments to determine membrane perme- ability

The permeabilities of the polyurea membrane forming the wall of the microcapsules for cyclohex- ane, were determined by monitoring the rate of release of cyclohexane into a n-heptane environ- ment. For these studies, the microcapsules produced as described above were filtered and washed 3-4 times with distilled water. Filtered microcapsules were lightly blotted to remove surface moisture and taken in a release cell containing 100 ml n-heptane. The ratio of the dispersed phase (i.e. microcapsules) to the continuous phase was maintained at 0.5% (wt/wt). Samples of the continuous phase were taken at intervals with a syringe, fitted with filter paper to exclude the microcapsules, and analyzed for cyclo- hexane by gas chromatography (supplied by CIC, India) using FID and N 2 gas as a carrier. Ethyl acetate was used as internal standard. The column

Table 1 Properties of the polyurea microcapsules produced under different conditions. Vo/V c stands for the ratio of volumes of the dispersed to the continuous phase, used in the preparation of the microcapsules Sample no. Vd/V c n L X 104 (mo1) R 6f (nm) X c (%) P X 10 s (m/s) Pt~f X 1016 (m2 / s )

s8 0.25 240.0 1.25 36 26 0.65 2.34 s13 0.25 60.3 5.0 9.2 21 3.75 3.45 s14 0.045 3.0 5.0 4.6 33 1.20 0.55 s43 0.043 24.0 1.25 36 30 0.23 0.83

S.K. Yadav et a l . / Journal of Membrane Science 125 (1997) 213-218 215

used was a 1/8" SS column, 4 m in length and was packed with 20% SE-30 on Chromosorb WHP, (80/100 mesh). Oven and Injection port tempera- tures were kept fixed at 70 and 150°C, respectively. The pressures of N 2 and H 2 gas at the column inlet were kept at 2 and 1.6 k g / c m 2, respectively. A calibration plot was developed for cyclohexane in n-heptane solutions using ethyl acetate as internal standard and was found to be linear in the concentra- tion range of interest.

5. Results and discussion

Figs. 1 and 2 show the XRD patterns of two of the polyurea samples used in these studies. A consid- erable difference in the degree of crystallinity is obvious. Table 1 shows the degrees of crystallinity (X c) of the polyurea shell material, calculated from such diffractograms, together with other pertinent data for the samples used in these studies. The sample s13 is the most amorphous (X c = 21%) and the sample s14, the most crystalline (X c = 33%).

Fig. 3 shows the data from the release experi- ments for samples s13 and s14 whose XRD's have been given in Figs. 1 and 2. Here the fraction of encapsulated cyclohexane released is plotted as a function of time. In these plots Mo~ (mass of cyclo- hexane released at large time) is calculated from the final concentrations which the concentration-time curves asymptotically approach at large times. The release rates are observed to be significantly differ- ent for the two samples. In both cases however, the data follow the trend expected of a diffusional re- lease process. The data can therefore be interpreted in terms of a permeation model. In the following, we shall process the release data to estimate the perme- ability for the different cases considered.

A quasi-steady state material balance for cyclo- hexane in the releasing environment yields, per unit volume of the continuous phase, gives:

dC - / ~ = ~ P ( C m - C) (1)

dt

where C m and C are, respectively, the concentrations of cyclohexane inside the capsules and in n-heptane

S.ggM

2 . 5

I , I , I I I z 5.911 !g.09 ~ .OO 59. Bg 4g.gO 59.00

2 0 - ~

Fig. 1. XRD pattern of the polyurea shell material (sample s13) formed under the fol lowing conditions: Vd/V c = 0.25, R = 5.0,

n , = 60.3 × 10 -4 mol.

216 S.K. Yadav et al. / Journal of Membrane Science 125 (1997) 213-218

at any instant. /~ denotes the rate of release of cyclohexane from the microcapsules in moles per second per unit volume of the continuous phase, and is written in terms of a permeability P and the interfacial area ~ [9]. The permeability P would vary directly as diffusivity D, and inversely as the mem- brane thickness 6f and the partition coefficient K for cyclohexane between the organic phase (continu- ous or dispersed) and the polymer.

C m and C can be related by a material balance for cyclohexane at any time t, as

Vc. C m -: C m i - C (2 ) v~p

where Cmi is the initial concentration of cyclohexane inside the microcapsule, m o l / m 3, and Vch and Vop are, respectively, the volumes of continuous and dispersed phases.

Combining Eqs. (1) to (2) and integrating be- tween the limits (t = 0, C = 0), and (t = t, C = C) and rearranging, we obtain,

C ~- ~vfmi [ 1 -- exp( - f i t ) ] (3)

where fl = [(aP)/~b~] and qb~ = (Vdp)/(Vch + Vop) = volume fraction of dispersed phase.

08

~ 0,4 Xc(~) kl-,/ oo 33

0.2 ~ ~ 21

0.0 ~ ~ - 0 50 100 150 200 2%0

"Time (s)

Fig. 3. Fractional release of cyclohexane as a function of time for samples s13 and sl4.

We observe from Eq. (3) that a semi-log plot of [1 - (1/th~XC/Cmi)] with time should give a straight line if the assumed diffusional mechanism can de- scribe the release process. The slope of this straight line plot can be used to estimate the permeability.

2 . 5

I z

~ . N l | . N I i I i I I

21.gll ~ l . U 4n .m 5e.W

2 0 ~

Fig. 2. XRD pattern of the polyurea shell material (sample s14) formed under the following conditions: VJV~ = 0.045, R = 5.0, nm = 3 × 10 - 4 mol .

S.K. Yadav et al. / Journal of Membrane Science 125 (1997) 213-218 217

Fig. 4 shows such plots. Only three are shown for the sake of clarity. The data between 20 and 80% of the response fall on a straight line. Normally in such transient experiments the very small time and the very large time data are prone to errors [10]. The permeabilities estimated from such plots are tabu- lated in Table 1. Among the samples studied, a variation in P from 6.5 X 10 -9 to 3.75 X 10 -8 m / s is seen.

In Fig. 5 we plot the product of permeability P and estimated film thickness, ~f, as a function of X c. The product Prf depends only on the diffusivity and partition coefficient, and hence is a function only of the polymer structure in the present set of experi- ments. Fig. 5 shows the strong dependence of the p r o d u c t (et~f) on the degree of crystallinity. The work of Gao and Lu [4] shows that, while the partition coefficient is likely to be only a weak function of X c, the diffusivity is likely to be a strong function. If it is assumed that diffusion occurs pri- marily through amorphous domains, with the crys- talline domains acting as barriers, P~f may be as- sumed to vary as 1 - X c. Such a trend is seen, although more data is needed, as well as independent determinations of partition coefficients, before a definitive correlation can be established. In any case, an order of magnitude variation in this product is

20

1 ' 5 f

o r e > - ' 1 0

I

o

( D "~0.5

0.0 0

D A

o o A

[3 E)D S14

f / I t I I ~ I I I I L I _ . _ x ~

~o 20 so ,~o so 60 70 Time (s)

Fig. 4. ln[C=//(Cmi -(1//f~v)C)] as a function of time. Prepara- tion conditions of the samples are shown in the Table 1.

5.6 \

3 . 2

28 to

O24

×2.0

.. 1.6

1.2

0.8

0.4

15 20

\

\, '\,

\ \ \

\ \ ' \

I i_

25 x~ (~ )

I I q 1 ~ J

30 35 40

Fig. 5. Correlation between product of permeability and the estimated thickness and the degree of crystallinity.

seen, which is attributable solely to differences in structure.

The magnitude of the permeability values found may be compared with what is expected on the basis of the diffusivity values for these membranes re- ported in the kinetic studies of Yadav et al. [7]. Diffusivity values for HMDA in those experiments were estimated to be in the range 10 -13 tO 10 -14

m2/s, again depending on X c. Since the polyurea microcapsules used in the present experiments were formed in contact with water, the membrane would be water-swollen. Assumption of a partition coeffi- cient K of the order of 1 /100 between such water- swollen membranes and an organic phase gives us permeabilities of the same order of magnitude as observed.

6. Conc lus ions

The release of cyclohexane from polyurea micro- capsules is determined by diffusion across the poly- meric membrane. The data on release show that the release rate is strongly influenced by the degree of crystallinity of the polymer shell. For cyclohexane diffusion through the water-swollen polyurea mem- branes use in this work, the permeability was in a range from 6.5 x 10 -9 to 3.75 X 10 -8 m / s . An

218 S.K. Yadav et al. / Journal of Membrane Science 125 (1997) 213-218

order of magnitude variation in the product P~f (a quantity proportional to the diffusivity) has been shown to result from a variation in the degree of crystallinity of 21 to 33%.

7. List of symbols

area per unit volume of the continuous phase ( l / m )

C concentration of cyclohexane in n-heptane (mol /m 3)

C m concentration of cyclohexane at any time in- side the microcapsule (mol /m 3)

Cmi initial concentration of cyclohexane inside the microcapsule (mol /m 3)

D effective diffusivity of cyclohexane through polyurea shell (mZ/s)

K effective partition coefficient (Ratio of concen- tration in organic phase to that in polymer at equilibrium)

M t mass of cyclohexane release at any time t M~ mass of cyclohexane release at large time n L number of moles of the limiting monomer P permeability of cyclohexane through polymer

film (m/s) R mole ratio of monomers (HMDI to HMDA) /~ rate of release of cyclohexane per unit volume

of the continuous phase from all microcapsules present in the releasing environment (mol /m 3 s)

Vch volume of continuous phase (n-heptane) (m 3) Vdp volume of disperse phase (microcapsules) (m 3) X c degree of crystallinity (%)

7.1. Greek le t ters

/3 defined in Eq. (3) ~f final estimated thickness of the polyurea shell

(nm) 0 angle of diffraction of X-ray pp density of polyurea formed (kg/m 3) ~b,, defined in Eq. (3)

References

[1] S.K. Yadav, N. Ron, D. Chandrasekharam, K.C. Khilar, A.K. Suresh and V.M. Nadkarni, Polyureas by interracial polycon- densation: preparation and properties, J. Macromol. Sci., Phys., (1995) in press.

[2] T. Kondo, in E. Matijevic (Ed.), Microcapsules: Their Prepa- ration and Properties in Surface and Colloid Science, Vol. 10, Plenum Press, New York, 1978, p. 1.

[3] T. Kondo, Preparation and permeability characteristics of microcapsules membranes, J. Controlled Release, 11 (1990) 215.

[4] T. Gao and F. Lu, Structure/permeability relationships of polyamide: imides, J. Appl. Polym. Sci., 54 (1994) 1965.

[5] S.A. Stem, R. Vaidyanathan and J.R. Pratt, Structure/perma- bility relationships of silicon containing polyimides, J. Mem- brane Sci., 49 (1990) 1.

[6] S.K. Yadav, A.K. Suresh and K.C. Khilar, Microencapsula- tion in polyurea shell by interracial polycondensation, AIChE J., 36(3) (1990) 431.

[7] S.K. Yadav, K.C. Chilar and A.K. Suresh, Microencapsula- tion in polyurea shell: kinetics and film structure, AIChE J., (1995) in press. D. Campbell and J.R. White, Polymer Characterization: Physical Techniques, Chapman and Hall, London, 1989. T. Kondo, in Y. Osada and T. Nakagawa (Ed.), Membrane Science and Technology, Marcel Dekker, New York, 1992. A.K. Suresh, T. Sridhar and O.E. Potter, Mass transfer and solubility in autocatalytic oxidation of cyclohexane, AIChE J., 34(1) (1988) 55.

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