Preparation and Characterization of Cross-Linked Hydroxypropyl

Chapter 29

Downloaded by UNIV MASSACHUSETTS AMHERST on October 11, 2012 | http://pubs.acs.org Publication Date: October 15, 1996 | doi: 10.1021/bk-1996-0648.ch029

Preparation and Characterization of Cross-Linked Hydroxypropyl Cellulose Hydrophilic Films 1

Cheng Qian Song , Morton H. Litt, and Ica Manas-Zloczower Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, OH 44106

Hydroxypropyl cellulose(HPC) films with isotropic or liquid crystalline structures have been prepared by photoinitiated crosslinking of HPC in dimethylacetamide(DMAc) solutions. The equilibrium swelling degree for both kinds of films depended on the crosslinking conditions. p-Nitrophenol, indophenol sodium, and Chrome Black Τ were used to evaluate the permeation behavior of HPC films. The influence of penetrant size, shape and interaction with film molecules on the diffusivity was also investigated.

Fundamental studies of the transport of small organic molecules through polymeric films or membranes have been actively pursued for many years (1-6). The investigations generally included a wide variety of polymers and penetrants over a broad range of experimental conditions. Moreover, hydrophilic films largely used in biomedical applications, have also been studied extensively (7-13). However, there are no reported data on penetration studies through liquid crystalline hydrogel films. A comparison of the permeation properties of isotropic and liquid crystalline films in this study correlates the structural regularities with the permselectivity. The permselectivity of a film depends on its structure and properties. Characteristics of the structure include the average pore size and size distribution. The molecular interaction between penetrant and film molecules is another major factor which determines the film separation efficiency. Hydroxypropyl cellulose(HPC) is a hydrophilic polymer with a semi-rigid rod backbone. HPC films can not be used in biomedical applications because the polymer is water soluble. However, HPC can be crosslinked in both the isotropic and liquid 1

Current address: Research and Development Center, Montell Polyolefins, 912 Appleton Road, Elkton, MD 21921

0097-6156/96/0648-0490S15.00/0 © 1996 American Chemical Society In Film Formation in Waterborne Coatings; Provder, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.


29. SONG ET AL.

Cross-Linked HPC Hydrophilic Films

491

crystalline states by using photoinitiated crosslinking (14). While keeping their hydrophilicity, such crosslinked HPC films can be used as hydrogel films and are considered the first example of hydrogel films which can be obtained in either isotropic or liquid crystalline states. The HPC films we have prepared could have solute sieving properties as do dialysis or ultrafiltration membranes. Methods often used to determine permeation properties and to estimate pore size of such films are bubble pressure/solvent permeability (15,16), gas adsoφtion/desorption (17,18), thermoporometry (17), high resolution electron microscopy (18) and selective solute sieving (19-21). Of the techniques mentioned above, the solute sieving method using well-characterized pure solutes of known size and shape provided information most relevant for characterizing the films. The methodology is quite general and has been widely applied to various biological transport barriers and films (22). Experimental Materials. Food Grade HPC-LF (weight-average molecular weight 95,000) from Aqualon Company was used after drying in vacuo at 60 °C for 24 hours. Anhydrous Ν,Ν-dimethylacetamide (DMAc) (Aldrich) was used as received. Hexamemoxymemylmelamine (HMMM) (Pfaltz & Bauer Inc.), the crosslinking agent, and triphenylpyrylium trifluoromethanesulfonate (TPTS)(Lancaster Synthesis Ltd.), the cationic photocatalyst, were used without further purification. l,4-Diazabicyclo[2.2.2]octane (DABCO) (Aldrich, used as received) was added to prevent premature crosslinking of the samples. The penetrants, p-nitrophenol (Matheson Coleman & Bell Manufacturing Chemists), indophenol sodium (Aldrich Chemical Co., Inc.), Chrome Black Τ (Crompton & Knowles Corp.) and methylene blue chloride (Merck & Co., Inc.) were used without further purification. Concentration Measurement. The U V absorption spectra of penetrants and their concentrations were determined using a Perkin-Elmer Lambda 9 UV/VIS/NIR spectrophotometer. Preparation of Films. Isotropic and liquid crystalline HPC films were prepared from 30 wt% and 50 wt% HPC/DMAc solutions respectively. The solution compositions are listed in Table I. The films were fabricated as follows: 1. A solution of DMAc containing crosslinker (HMMM), photocationic catalyst (TPTS) and base (DABCO) was made up as needed before use. The solution was then mixed with HPC powder which had been dried in vacuo at 60 °C for 24 hours. 2. The suspension was mixed by hand for several minutes to dissolve the polymer and then was allowed to stand for one hour in order to achieve uniformity and to eliminate most of the air bubbles. 3. The solution was then poured onto a Pyrex glass plate with a spacer (0.3 mm) and an upper glass plate was clamped over the lower plate containing the polymer solution.

In Film Formation in Waterborne Coatings; Provder, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

In Film Formation in Waterborne Coatings; Provder, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

b

0

0

Table I Formula of HPC solutions for the preparation of films TPTFS HMMAf HPC-LF DMAc (g) (g) (g) (g) 0.13 0.065 4.67 2.00

0.039 0.13 2.00 2.00 50 Crosslinker: Hexamethoxymethylmelamine (HMMM). Photoinitiator: 2,4,6-Triphenylpyrylium trifluoromethanesulphonate (TPTFS). Base: l,4-Diazabicyclo[2.2.2]octane (DABCO).

Cone. (wt%) 30


1.59

DABCCf (mg) 2.65

29.

SONG E T AL.

493

Cross-Linked HPC Hydrophilic Films


4. After standing for 30 minutes, the samples were U V irradiated (GE U V lamp, 275W; sample to lamp distance was kept at about 25 cm) for a preset time at room temperature. 5. The samples were postcured at 65 °C for various times as specified. 6. The films were demolded from the glass plates and immersed in D M A c containing 1 wt% DABCO for two hours to neutralize and remove the acid. 7. The films were then rinsed in distilled water several times and kept in distilled water until they were used. Permeability Measurement. If a film of thickness L and area A separates two chambers containing a penetrant at different concentrations, the permeation of the penetrant from the high concentration chamber to the low concentration chamber can be calculated according to Fick's first law (23): F=D(C C )/L r

(1)

2

where F is the flux of penetrant passing through the film, C and C are the concentrations of penetrant on either side of the film and D is the diffusion coefficient. The surface concentrations C, and C can be correlated with the bulk concentrations in the feed cell, C and the receiving cell, C respectively: x

2

2

f

p

C^CfS;

C =C S 2

(2)

r

where S is the partition coefficient. Substituting eqn.(2) into eqn.(l) gives: F=DS(C C )/L

(3)

F=P(C C )/L

(4)

r

r

or, r

r

where P=DS is called the permeability coefficient. The above derivation is based on the assumption of infinite cells so that the concentrations on each side of the film can be assumed constant. For finite cells, the derivation must be modified to consider the change in concentrations during the course of diffusion. By definition the flux, F, can be expressed as: F=dQ/(Adt)=VdC/(Adt)

(5)

where dQ is the amount of penetrant transported during the time interval dt, A is the effective area of the film, V is the volume of the cells, and dC is the concentration change during the time interval dt. Combining equations (4) and (5) and integrating the result with the boundary condition C =0 at t

Preparation and Characterization of Cross-Linked Hydroxypropyl

Recommend Documents