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Materials Science and Engineering C 24 (2004) 23–25

Alkaline hydrolysis of modified poly(L-lactide) monolayers

Jin-Kook Leea, Chang-Sik Haa, Won-Ki Leeb,*

aDepartment of Polymer Science and Engineering, Pusan National University, Pusan 609-735, South KoreabDivision of Chemical Engineering, Pukyong National University, Yongdang-dong, Namgu, Pusan 608-739, South Korea

Abstract

In this work, hydrolysis of biodegradable poly(L-lactide) (L-PLA) and copolymers of L-lactide (L-LA) and (benzyloxycarbonyl) methyl

mophorline-2,5-dione (BMD) was investigated at the air/water interface. In order to improve the hydrophilicity of L-PLA, small amounts of

BMD were copolymerized with L-LA. NaOH was used to adjust the pH of the subphase water. Under the conditions studied here, polymer

monolayers showed much faster hydrolysis as either a subphase pH or the concentration of BMD in the copolymer is increased. This result

was explained by increasing numbers of base attack sites per unit area and increasing hydrophilicity.

D 2003 Elsevier B.V. All rights reserved.

Keywords: Poly(L-lactide); (Benzyloxycarbonyl) methyl mophorline-2,5-dione; Monolayer

1. Introduction

In recent years, there have been increasing demands for

degradable polymers, particularly to minimize polymer

waste management caused by synthetic non-degradable

polymers and for various biomedical applications. Many

hydrolyzable polyesters have been developed in the past

decade to improve specific properties, such as degradability

and biocompatibility [1–4]. A desired polymer property

often cannot be obtained from the material itself but through

chemical or physical modification such as blending and

copolymerization. Although blending is an attractive and

economical way to change properties, one limitation to

blending is the lack of miscibility between component

polymers. Much emphasis has been placed on controlling

degradability of copolymers.

Many different analytical methods have been applied to

determine the degradation rate of polyesters. One suitable

technique to study the hydrolysis behavior is to use a

Langmuir film balance to study polymers at the air/water

interface, since the hydrolysis of a polyester usually occurs

through the cleavage of ester groups and eventually produces

water-soluble oligomers and monomers. The hydrolysis of

polyester monolayers would result in a change in the

occupied area when the monolayer is maintained at a

0928-4931/$ - see front matter D 2003 Elsevier B.V. All rights reserved.

doi:10.1016/j.msec.2003.09.032

* Corresponding author. Tel.: +82-51-620-1689; fax: +82-51-625-

4055.

E-mail address: wonki071@hotmail.com (W.-K. Lee).

constant surface pressure [5,6]. Recently, Ivanova et al. [5]

reported the hydrolytic behavior of poly(DL-lactide) mono-

layers spread on acidic (pH 1.9 by HCl) and basic (pH 11.4

by Na2HPO4 and NaOH) subphases for short times.

In the present work, we copolymerized L-lactide (L-LA)

and (benzyloxycarbonyl) methyl mophorline-2,5-dione

(BMD) to improve the degradability of L-PLA. The hydro-

lytic degradation of L-PLA and copolymers of L-LA and

BMD (L-LA-co-BMD) has been investigated at the air/water

interface. The study of polyester monolayers at the air/water

interface will, therefore, give a fundamental understanding

of the hydrolytic mechanism of polyesters in molecular level

as a function of pH, time, and copolymer composition.

2. Experimental

2.1. Materials

L-LAwas obtained from Aldrich and BMD was prepared

by the method described by Wang and Feng [7]. Bulk

polymerizations of L-LA and copolymerization of L-LA

and BMD with two different BMD mol% (1 and 4 mol%)

were carried out in vacuum-sealed glass ampoules under N2

gas at a given temperature using stannous octoate as a

catalyst. The obtained copolymers (L-LA-co-BMD1 and L-

LA-co-BMD4) as shown in Scheme 1 were characterized

using 1H NMR (GEMINI 300, CDCl3: d = 1.6 (CHCH3),

d = 4.7 (NHCH), d = 7.3 (CH2COOBz)). The molecular

Fig. 1. Pressure-area isotherms of L-PLA and L-LA-co-BMD monolayer

films on subphase of pH 7.3.

Scheme 1. Structures of monomers.

J.-K. Lee et al. / Materials Science and Engineering C 24 (2004) 23–2524

characteristics of homopolymer and copolymers used in this

study are listed in Table 1.

2.2. Langmuir trough

Monolayer properties were studied by using a computer-

controlled KSV 2200 film balance held at 20 jC. A

compression rate of 30 cm2/min was used throughout. The

surface pressure could be measured with an accuracy of 0.1

mN/m. The water subphase was purified with a Mega-Pure

system, MP-6A (pH 7.3). The purified water was used as a

subphase liquid. The spreading solvent used in this study

was chloroform (Fisher, 99%+). After spreading, the solvent

was allowed to evaporate over 1 min in order to minimize

the hydrolysis during the solvent evaporation and the

compression. Reagent-grade NaOH was used to adjust the

pH of the water, unless otherwise specified. A pH meter

equipped with an electrode (Orion Research) was used to

measure the pH of solutions.

3. Results and discussion

The Langmuir technique has been used to measure the

hydrolytic degradation of polyester monolayers on a mo-

lecular scale, since most polyesters are capable of forming

monolayers due to their hydrophilic/hydrophobic balance

and the low molecular weight oligomers and monomers

generated by hydrolysis dissolve into water. Despite THE

extensive investigation of the hydrolytic behavior of thick

polyester films [4,8], few studies have been made on the

hydrolysis of monolayers at the air/water interface [5,6].

The surface pressure-area isotherms for L-PLA and its

copolymer monolayers were measured on the subphase of

pH 7.3 (Fig. 1). The plateau region at ca. 8.5 mN/m in the

isotherm of the L-PLA monolayer was interpreted as a phase

transition and a formation of three-dimensional structure by

Ivanova et al. [5]. However, this plateau region disappeared

on increasing BMD content in the copolymers (L-LA-co-

Table 1

Characteristics of synthesized polymers

Mw (Mw/Mn) BMD (mol%)

L-PLA 20,000 (2.9) –

L-LA-co-BMD1 121,000 (1.6) 1

L-LA-co-BMD4 35,000 (1.7) 4

BMD) because of the decrease of regularity and the change

of hydrophilicity.

Fig. 2 shows a plot of the area ratio, A/A0, vs. time of

L-LA-co-BMD4 monolayer at a constant surface pressure of

7 mN/m on subphases of different pHs, where A0 and A

represent the areas occupied by the film at time 0 and t,

respectively. The initial time, t= 0, was considered when

the surface pressure reaches a desired surface pressure,

meaning that the effect of dissolving low molecular

oligomers due to hydrolysis was neglected during the

compression. This effect is considered to be marginal, if

any, since there is little difference in A/A0 ratios with

hydrolysis time. The measured kinetic curves follow a

typical sigmoid shape, that is, the faster the reduction of

A/A0 ratio, the smaller the real initial area. The extent of

area reduction (Fig. 2) appears to increase upon higher

subphase pH. This trend would result from increasing

number of the base attack sites per ester bond unit. The

data (Fig. 2) indicate that the reduction fraction of the

original area was approximately 0.07, 0.16, and 0.3 (ex-

perimental error of F 2%) after a hydrolysis time of 60 min

Fig. 2. Area ratio vs. time for L-LA-co-BMD4 monolayer films at a constant

surface pressure of 7 mN/m on subphases of different pHs.

Fig. 3. Fraction of dissolved molecules into subphase of pH 10.5 for L-PLA

and its copolymer monolayer films maintained at a constant surface

pressure of 7 mN/m.

J.-K. Lee et al. / Materials Science and Engineering C 24 (2004) 23–25 25

exposed to pH rates of 10.3, 10.4, and 10.5, respectively.

Our previous work showed that the hydrolysis of polyesters

is strongly affected by the concentration of the active

sodium ion [6]. The concentration of Na+ ions in the

subphase of pH 10.5 is nearly 3 times of that of pH 10.3.

However, the dissolved molecules at pH 10.5 are nearly 4.5

times. This result suggests that the higher concentration of

degradation medium, the faster hydrolysis (acceleration

effect).

The fraction of dissolved molecule (1�A/A0) with time

of various polyester monolayer films on the subphase of pH

10.5 at a constant surface pressure of 7 mN/m was calcu-

lated from A/A0 data (Fig. 3). The extent of hydrolysis,

under the condition studied here, follows the order: L-LA-

co-BMD4>L-LA-co-BMD1>L-PLA. The data (Fig. 3) indi-

cate that the reduction fraction of the original area was

approximately 0.37, 0.3, and 0.26 (experimental error of

F 1%) for L-LA-co-BMD4, L-LA-co-BMD1, and L-PLA,

respectively, after a hydrolysis time of 90 min exposed to

pH 10.5. This result indicates that the hydrolytic degrad-

ability of L-PLA was significantly increased when small

amounts of BMD were copolymerized to L-PLA. When 4

mol% of BMD was copolymerized to L-LA, the rate of

alkaline hydrolysis was much accelerated over 40% (accel-

erating effect). Also, this copolymer can be modified to a

functional biodegradable polymer with reactive side-chain

group by catalytic hydrogenation [8].

4. Conclusions

To control the hydrolysis of L-PLA monolayers, a small

amount of hydrophilic BMD were introduced to L-PLA.

Since the hydrolysis of polyester monolayers maintained at

a constant surface pressure leads to the reduction in the area

occupied by films, the rate of hydrolysis of Langmuir

monolayers of L-PLA and L-LA-co-BMD was investigated

at the air/water interface. When they are exposed to a basic

subphase, the extent of hydrolysis was increased with

increasing either pH of the subphase or BMD in the

copolymer. The rate of alkaline hydrolysis of L-PLA was

much accelerated by copolymerization with small amounts

of BMD.

Acknowledgements

This work was supported by grant No. R01-2002-000-

00034-0 (2003) from the Basic Research Program of the

Korea Science and Engineering Foundation.

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