Thermodynamic Aspects of the Disappearance of Antiplasticization in Slightly Plasticized Polymer Films at High Temperature To the Editor: In a previous paper1 we reported the effects of plasticizers on the permeation of water through and mechanical properties of cellulose acetak, that is, antiplasticization in slightly plasticized polymer film. The effects of triacetin and three M eren t molecular weights of polyethylene glycol on the permeation of water through and mechanical properties of celluloee acetate were investigated. At 37 "C, the permeability of cellulose acetate to water decreased with increasing plasticizer to a minimum and then increased with higher concentrations of plasticizer. Low plasticizer concentrations caused a decrease in permeability to water by antiplasticization. The term "antiplasticization" has been ascribed to the mechanical behavior obeerved on adding certain lowmolecular-weight materials to polymers in which there occurs an increase in modulus, or a decrease in compliance, and the appearance of brittleness.- The phenomenon of antiplasticization at low plasticizer concentration has been explained in many papers.'-11 However, the real reasons for antiplasticization at low plasticizer concentration are not clear yet. Basically, it ia supposed that antiplasticization ariaea from an interaction between the polymer and the plasticizer molecules and decreaees the molecular mobility of the polymer. This antiplasticization effect was confirmed by mechanical measurementa of polymer-free films all at the same experimental temperature in our previous paper; the creep compliance decreased with increasing plasticizer at low plasticizer concentration and then increased with higher plasticizer concentration. We also found that when the experimental temperature is higher than the g l a ~transition ~ temperature (T,) of the polymer films, the creep compliance of the more highly plasticized cellulose acetate films will be higher than that of the less highly plasticized cellulose acetate films.The order of the creep compliance of these fdms was as follows: [cellulose acetate10 w t % polyethylene glycol (PEG 60011 > (cellulose acetate4 wt 8 PEG 600) > (cellulose acetate). When the temperature is raised above the TBof the polymer films,the polymer films contain enough energy to overcome the interaction between the polymer and plasticizer molecules and the antiplasticization effect disappears. Williams, Landel, and Ferry12 related the time and temperature interdependence of the viacoelastic properties of polymers by an equation of the following form:

In eq 1,a,is the time shift factor, C, and C2 are constants, T is the experimental temperature, and Ts is the reference temperature. Equation 1is normally called the WLF equation and it gives the relative increase in time for the same viecoelastic behavior to occur at T that occurs at the higher T,. When a,values are obtained for these relaxations, they do obey the WLF equation (Figure 1). Experimentally, the relationship between the a, values and T was correlated by an equation of the Arrhenius form13: lw22-3549~~12001229$02.50/0 Q 1992,American Pharmaceutical Association

0 -

-1

-

-2

-

-3

-

-4 "

t

430

440

450

460

470

480

490

500

Temperature (K) Flgure 1-The WLF relationship between temperature and shM factor. Key: (m) cellulose acetate; (0)cellulose-PEG600 (5%); (0)cellulose acetate-PEG600 (10%).

H RT

lnaT= --

In eq 2, R is the gas constant, and H is the relaxation activation energy. Thus, plots of In a, versus 1/T for a relaxation yield straight lines,not curves as in the WLF case. The plots of In a,of cellulose acetate versus 1/T (Figure 2) are almost straight lines for each of the cellulose acetate films studied. The relaxation activation energies of them films were

-1

'

2.0

I

2.1

l/T

I

2.2 (1/K) (10--3)

I

2.3

Arrhenius relationship between temperature and shM factor. Key: (W) cellulose acetate; (0)dlulose-PEG 600 (5%); (0) cellulose acetate-PEG 600 (10%). Flgure 2-The

Journal of Pharmaceutical Sciences I 1229 Vol. 82, No. 12, December 1992

References and Notes 1. Guo, J-H.; Roberteon, R. E.; Amidon, G. L., eubmitted for publi-

cation in J. Controlled Releme.

2. Brow, 9. L.; Semon, W. L. Znd. Eng. Chem. 1935,27,667-672. 3. Walter, A. T.J. Polym. Sci. 1954,13,207-228. Plasticizer 4. Jackson, W. J., Jr.; Caldwell, J. R. Plasticizcrtion 6. 6. 7. 8.

0

Figure +The

5

10

PEG400 level (%, w h ) effect of PEG 600 on the relaxation ecthration energy of

cellulose acetatefree films.

obtained with eq 2 (Figure 3). Thus, the relaxation activation energyof celluloseacetatedecreaeee with increasingPEG 600 levels when the experimentaltemperature is higher than the T, of the polymer films. Free volume playa an important role in the relaxation time of polymers.When diluent (i.e.,solvent or plasticizer)L added to an undiluM polymer, the relaxation tima deuease rapidly.14 Be ~ ~ t h e ~ i n ~ ~ ~ ~ ~ l y a or collapee of free volume, the relaxation time kcmasea as the freevolume decseasee.Fox and Flory16 indicated that an incmue in free volume dimhished the relaxation activation energy. Became the exktence of plasticizer immasea the freevolume of a polymer,a decreaaeinthe relaxationactivationenergywith the plmti&erlevelLexpected.

1230 I Journal of Phermaceuticel Sciences Vd. 82, No. 12, December 1992

Proce88e8; Gould, R. F., Ed.; American Chemical SOQety:Washington. D.C.; Chapter 10. U o . N.: Nakagawa, T. PoZym. J. 1973,4, 143-163. 6, k.U.; HOG 9:M.; &, B. C. J. Korean SOC.Text. Eng. Chem. 1987,24, f 3 4 . Jackson, W. J., Jr.; Caldwell, J. R. Adv. Chem. Ser. 1965, 48, 1AK-196. - -- - - -. Jackson, W. J., Jr.; Caldwell, J. R. J. Appl. Polym. Sci. 1967,11,

221-226. 9. Jackeon, W. J., Jr.; Caldwell, J. R. J. Appl. Polym. Sci. 1967,11, 227-241. 10. Roberteon, R. E.; Joynson, C. W. J. Appl. Polym. Sci. 1972. 16, 733-738. 11., Seare J. K.; Darby, J. R. The Technology of Plasticizers; Wiley: New frork, 1982; pp 203-216. 12., Williams, M. L.; Landel, R. F.; Ferry, J. D. J. Am. Chem. SOC. 1956,77,3701-3707. 13. M o n k , J. J.; MacKnight, W. J. Introduction to Polymer Vimcoekretic~;Wiley: New York, 1983; pp 100-109. Wiley: NewYork, 14. Ferry, J. D. ViecoelasticPtvpertie~ofPolymere; 1980; pp 292-361. 16. Fox, T. G.; Flory, P. J. J. Appl. Phy8. 19S0,21,681.

JIAN-HWAGuo*' RICHARDE. ROBERTS ON^ GORDON L. AMIDO$

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*3M Pharmaceuticals 3M Center, 270-4S-02 St. Paul, MN 55144-100. t'Depertment o t hof Materials e ~ Science o n and Engineering The University of Mlchigfm Ann h r , MI 481082136. SCollege of Pharmacy The University of Michigan, Ann Arbor, MI 461081065.

Received October 3,1991. Accepted for publication June 29. 1992.