Controlled release from PVC matrices: Effect of four phthalate plasticizers on diffusion of THF

Eur. Polym. J. Vol. 28, No. 11, pp. 1321-1324, 1992

0014-3057/92 $5.00+0.00 Copyright © 1992 Pergamon Press Ltd

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CONTROLLED RELEASE FROM PVC MATRICES: EFFECT OF FOUR PHTHALATE PLASTICIZERS ON DIFFUSION OF THF* D. SHAILAJAand M. YASEENt Organic Coatings and Polymers Division, Indian Institute of Chemical Technology, Hyderabad 500007, India (Received 19 February 1992; in revised form 10 April 1992)

Abstract--Polymer matrices are used as dispensers for the controlled release of bioactive materials at rates effective either to kill or to attract pests in the fields. The release characteristics of the bioactive materials are directly related to the diffusion property of the matrix; these properties could be modified by incorporation of plasticizers. In order to develop a versatile monolithic controlled release device, a large number of PVC matrices have been prepared containing a di-n-alkyl phthalate (methyl, ethyl, butyl or octyl) at concentrations ranging from 10 to 30%. To assess the plasticizer efficiency, the diffusion coefficient of the tetrahydrofuran molecule in PVC matrices was studied by gravimetric desorption measurements. Fickian diffusion kinetics were observed in all cases. It was found that diffusion coefficient increases with increase in the plasticizer content but decreases with increase in the alkyl chain length.

INTRODUCTION

Controlled release (CR) as applied to agricultural chemicals is fast becoming the subject of intensive research. The principal advantage of controlled release formulations (CRF) is that they allow much less bioactive material to be used for the same period of activity. One of the most simple and promising techniques of CR is the membrane-moderated monolithic device, in which the bioactive material is dispersed in a rate controlling polymer matrix [1]. The release characteristics of the system are governed by the diffusion rates, solution levels and resultant gradients of dissolved and diffusing agent within the controlling polymer membrane. The phenomena of CR are similar to those involved in plasticizer technology. Extensive investigations in these areas can provide practical guidelines in the development of effective CR systems. Dispensers of this type are widely used in pesticide delivery and pheromone traps. Plastic laminate dispensers containing disparlure, the sex-attractant of the gypsy moth, were used by Leonhardt et al. in an emission rate study to determine the effect of temperature, dispenser thickness and size [2]. Kraxenberger et al. studied the release of Desmetryn into the gas phase from films of ethylene-(vinyl acetate) copolymer, in which herbicide was incorporated for special agricultural applications [3]. Dispensers containing the aggregation pheromone grandlure, extensively used in traps for suppressing the boll-weevil, were studied by Leonhardt et al. [4] who found that the extensive period of pheromone release is substantially influenced by the nature of the dispenser formulation and other variables. *IICT Communication No. 2989. tTo whom all correspondence should be addressed.

The transport of small molecules in polymers is strongly dependent on polymer structure and morphology. Therefore small molecules can be used as very sensitive probes to explore a polymer matrix as well as any time-dependent changes which the matrix may undergo. The polymer morphology or the stiffness of the backbone of a polymeric membrane can be altered by the addition of co-diffusants, or releasing agents commonly termed plasticizers which soften the membrane. Piasticizers serve as a transmission path for the diffusion of materials which usually have some degree of solubility in them and hence move through the polymer matrix and reach the surface of the dispenser. Diffusion of organic vapours at low concentrations in glassy PVC was examined by Berens [5] who found that the diffusivity decreases exponentially with increasing size of the penetrant molecules. The self-diffusion coefficients of plasticizers in PVC were determined by Park and Saleem [6] with n-hexadecane as probe molecule, using the thin smear radioactive method. They observed that the diffusion coefficient increases with increase in the alkyl chain length of the plasticizer and also with increase in its content. The dependence of the self-diffusion coefficient of plasticizer in plasticized PVC on the identity of the plasticizer is complicated because the plasticizing molecule is also the diffusing species. In order to isolate the effect of the plasticizing molecule on the diffusion medium, Griffiths, Krikor and Park [7] used the 1:2 double disc method to study the diffusion of n-bexadecane at low concentrations as a probe molecule in PVC plasticized with di-n-alkyl phthalates. To obtain a fundamental understanding of the influences of compounding variables and changes the matrix may undergo, the study now reported was undertaken in an effort to refine the correlations between the diffusivity, size and concentration of

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D. SHAILAJAand M. YASEEN

plasticizer molecules. The casting solvent tetrahydrofuran ( T H F ) itself is used as the probe molecule to explore PVC matrix plasticized with di-n-alkyl phthalutes. The diffusion coefficient of the probe molecule has been used as a measure of the efficiency of the plasticizer (releasing agent) in dispensing the bioactive material through the polymer matrix in actual practice. EXPERIMENTAL PROCEDURES

Materials Commercial PVC (PR 124) supplied by Chemplast, Madras, India, was used. It has a weight-average molecular weight of 190,375 and a number-average molecular weight of 97,600 as determined by the Gel Permeation Chromatograpy. Dimethyl phthalate (DMP), dibutyl phthalate (DBP) and dioctyl phthalate (DOP) of laboratory grade were obtained from Quality Products, Bombay, and diethyl phthalate (DEP) from Fluka. THF of analytical grade, free from peroxides, was obtained from Ranbaxy, India. PVC was purified by precipitating its solution in THF with methanol followed by prolonged drying at 60° under vacuum.

Procedure Membranes with uniform thickness of 75 + 2 #m containing a certain amount of the plasticizer were prepared with four di-n-alkyl phthalates (methyl, ethyl, butyl and octyl) with concentrations ranging from 10 to 30% by weight using solution casting [8]. The non-adhering property of a mercury surface has been used for preparing the films. A 6% (by weight) solution in THF containing appropriate amounts of PVC and plasticizer was poured over a mercury surface in a tray of 20 x 15 cm 2 area. The assembly was left in a chamber for 20 hr for evaporation of the solvent. The resultant film was of uniform thickness except for the small peripheral portions. A specimen of 15 x 10cm 2 was prepared by discarding these portions. The time-dependent weight loss of the polymer membranes was determined by the gravimetric desorption method [9]. The amount of THF molecules in the membrane was taken as the initial weight. The film was hung in a chamber of regulated ventilation maintained at 25° and ambient pressure. The weight loss in the film was recorded periodically using a balance (accuracy of 0.0001 g) housed inside the chamber. Experimental data were plotted as M / M , vs t ~/2, where M, is the weight desorbed at time t and M~ is the weight desorbed after equilibrium is attained. RESULTS AND DISCUSSION

Diffusion kinetics Typical plots, shown in the figures, were obtained for the desorption of T H F from the film at time t following the Fick equation:

(g,/go~) = (4/•) (Dt /n ) '/2. Therefore

O = x (10/4) 2. Diffusion coefficient, D, of the penetrant/desorbing molecule in or from a polymer membrane was estimated from the slope 0 of the linear portion of the graph o f fraction loss (Mt/Mo~) vs t 1/2, where M r a n d Moo are cumulative masses desorbed from the polymer film of thickness l at times t and t~o respectively.

The applicability of the Fickian diffusion kinetics was confirmed in all the cases by the initial linearity of Mt/Mo~ vs t 1/2 plots followed by a plateau of slow weight loss continuing long beyond the anticipated Fickian equilibrium time which is not shown in the figures.

Plasticizer concentration The overall mechanism of diffusion depends on many factors relating to such aspects as the size, shape, composition and concentration of the penetrant and the polymer composition, structure and morphology. Generally, the process of diffusion is visualized as a sequence of unit diffusion jumps under the influence of a chemical gradient. Table 1 shows the values obtained for the diffusion coefficient of T H F in PVC plasticized with various di-n-alkyl phthalates over a range of concentrations. The diffusion coefficients decrease with decrease in the phthalate content at r o o m temperature. This observation can be explained by the free volume theory, proposed by Fujita and Kishimoto [10], where the diffusion coefficient is directly related to the probability of hole formation or the probability of the free volume domain available per unit time and the unit volume for diffusive jumps as determined by the polymer chain segmental mobility. The local chain segmental mobility of a polymer is enhanced by the presence of a plasticizer, resulting in a lowering of the glass transition temperature. Thus, as the plasticizer concentration decreases with time, the free volume within the polymer becomes exponentially harder to generate and therefore the diffusion coefficient decreases. Quantitatively, this concentration dependence [11] can be expressed as D c = e ac where D c is the diffusivity at concentration c, and a is a positive real constant. The unplasticized or rigid PVC has the lowest diffusion coefficient for T H F , where the local chain segmental mobility is not found because of the absence of the free volume domains caused by the plasticizer molecules which help in diffusion jumps.

Plasticizer size In general the diffusion coefficient of the small molecule should increase with increase in the size of the plasticizer molecule. In contrast, it appears from the table that the diffusion coefficients of T H F decrease noticeably with increase in the size of the alkyl groups i.e. from methyl to octyl, for a particular

Table 1. Diffusion coefficientsof THF from plasticized PVC membranes Diffusion coefficientD × 108cm2.min-' Percent concentration of di-n-alkyl phthalate plasticizers Plasticizer 10.0 15.0 17.5 20.0 22.5 25.0 27.5 30.0 DMP 0.51 0.58 0.64 0.71 0.72 0.79 0.83 0.86 DEP 0.42 0.49 0.58 0.64 0.66 0.72 0.79 0.88 DBP 0.36 0.42 0.44 0.50 0.53 0.51 0.65 0.73 DOP 0.34 0.37 0.40 0.46 0.49 0.51 0.58 0.65 Note: Diffusion coefficient of unplasticized PVC membrane (blank) is 0.25 × 10-Scm2.rain -I.

Controlled release from PVC matrices

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0.40 --

O

0.35

• ~

0.30 0.25 8

"-. 0.20 o

0.15 0.10 0.05

0

I

I

I

L

5

10

15

20

t 1/2 (min1/2) Fig. 1. Desorption of T H F from PVC films plasticized with 10% di-n-alkyl phthalates as a function of t 1/2

concentration. This effect is probably a result of the diffusion process which is dominated by the immobilization of the penetrant T H F molecules resulting from the polar group interactions and the Van der Waal interactions between the T H F and phthalate molecules. Thus we found an increase in the degree of interaction between the two molecules resulting in a decrease of chain segmental mobility and therefore reduction in the diffusion coefficients. The diffusion coefficients give an indirect method of measuring the plasticizer efficiency. On this

at 25°. basis, it appears from Table 1 and Figs 1-3 that the low molecular weight phthalates are more efficient than the high molecular weight ones. This investigation also showed that, at medium plasticizer concentration (17.5-20%), the membranes were easy to handle and had appropriate diffusion coefficients. Thus an understanding of the solution-diffusion model of transport helps in the prediction of controlled release rates and also in the design and development of specific polymeric formulations for particular agents and applications.

0.40 0.35 0.30 8

./

0.25 0.20 0.15 0.10 •

0.05 I 5

I 10

I 15

DOP

I

20

t 1/2 (min1/2)

Fig. 2. Desorption of THF from PVC films plasticized with 17.5% di-n-alkyl phthalates as a function of t ~/2 at 25°.

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D. SHAILAJAand M. YAS~r~

0.35 -

o

~

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0.25 8

0.20 0.15 -

/-'~"

* Blank

"*J

0.05

0

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I 10

J 15

J 20

t 1/2 (min1/2) Fig. 3. Desorption of THF from PVC film plasticized with 27.5% di-n-alkyl phthalates as a function of t ~/2 at 25°. CONCLUSIONS 1. The diffusion coefficient of T H F increases with increasing concentration of the plasticizer in plasticized PVC. The effect is greater for D M P and DEP and could be due to the greater internal flexibility of the plasticizer molecules. 2. The diffusion coefficient decreases slightly with increase in the alkyl chain length of the phthalates plasticizing PVC. This effect could be due to increase in polar group and Van der Waal interactions resulting in the immobilization of the T H F molecules. 3. Concentrations (17.5-20%) of the plasticizers gave appropriate diffusion coefficients which could be suitable for use in the controlled release monolithic device; membranes with higher concentrations are soft to handle because of the increased flexibility imparted by the plasticizer.

2. 3. 4. 5. 6. 7. 8. 9. 10.

REFERENCES

1. T. J. Roseman and N. F. Cardarelli. Controlled Release Technologies: Methods, Theory and Applications (edited

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by A. F. Kydonieus), Vol. I, Chap. 2. CRC Press, Florida (1980). B. A. Leonhardt, E. D. Devilbiss and J. R. Plimmer. J. Econ. Entomol. 72, 319 (1979). M. Kraxenberger, G. Pfister and M. Bahadir. J. appL Polym. Sci. 34, 2069 (1987). B. A. Leonhardt, W. A. Dickerson, R. L. Ridgway and E. D. Devilbiss. J. Econ. Entomol. 81, 937 0988). A. R. Berens and H. B. Hopfenberg. J. Membr. Sci. 10, 283 (1982). G. S. Park and M. Saleem. J. Membr. Sci. 18, 177 (1984). P. J. F. Gritiiths, K. G. Krikor and G. S. Park. Polym. Additives (edited by J. E. Kresta), Chap. 5. Plenum Press, New York (1984). M. Yaseen and K. V. S. N. Raju. Prog. Org. Coat. 10, 125 (1982). M. Saleem, Abdul-Fathah, A. Asfour and D. Dekee. J. appl. Polym. Sci. 37, 617 (1989). C. A. Kumins and T. K. Kwei. Diffusion in Polymers (edited by J. Crank and G. S. Park), Chap. 4. Academic Press, London (1968). A. Aitken and R. M. Barrer. Trans. Faraday Soc. 51, 116 (1955).