Biodegradable poly(lactic acid)/chitosan-modified montmorillonite nanocomposites: Preparation and characterization

Abstract

In this study, the biodegradable poly(lactic acid) (PLA)/montmorillonite (MMT) nanocomposites were successfully prepared by the solution mixing process of PLA polymer with organically-modified montmorillonite (m-MMT), which was first treated by n-hexadecyl trimethyl-ammonium bromide (CTAB) cations and then modified by biocompatible/biodegradable chitosan to improve the chemical similarity between the PLA and m-MMT. Both X-ray diffraction data and transmission electron microscopy images of PLA/m-MMT nanocomposites indicate that most of the swellable silicate layers were disorderedly intercalated into the PLA matrix. Mechanical properties and thermal stability of the PLA/m-MMT nanocomposites performed by dynamic mechanical analysis and thermogravimetric analysis have significant improvements in the storage modulus and 50% loss in temperature when compared to that of neat PLA matrix. The degradation rates of PLA/m-MMT nanocomposites are also discussed in this study.

Introduction

The biodegradable and biocompatible polymers have caused significant attention from both ecological and biomedical perspectives in the past decade [1]. In general, synthetic polymers produced from petrochemical products have low recovery/reproduction rates and are not easily degraded in the environment. Rapid growth of municipal waste is driving the efforts toward biodegradable/biocompatible polymers that can be used as renewable resources for polymer manufacturing and reduce the waste volume of synthetic polymers. The most popular and important biodegradable polymers are aliphatic polyesters [2], [3], [4], [5], [6], [7], [8], [9], [10], such as poly(lactic acid) (PLA), poly(glycolic acid) (PGA), poly(e-caprolactone) (PCL) and poly(3-hydroxybutyrate) (PHB), among which PLA has received the most attention due to its renewable resources [2], biocompatibility, biodegradation, excellent thermal/mechanical properties, and superior transparency of the processed materials [3]. Therefore PLA is widely used in medical application such as surgical implants [4], tissue culture [5], resorbable sutures [6], wound closure, and controlled release systems [7], [8], [9], [10].

PLA produced from renewable resources is a linear aliphatic thermoplastic polyester and is readily biodegradable through hydrolytic and enzymatic pathways [2], [11], [12], [13]. PLA can be also synthesized by condensation polymerization of the lactic acid monomers or by ring-opening polymerization of lactide monomers, and those monomers are obtained from the fermentation of corn, potato, sugar beat, and sugar cane [14]. In general, grades of commercial PLA are copolymers of poly(l-lactide) and poly(dl-lactide). The amount of d-enantiomers is known to affect the physical properties of PLA, such as the melting temperature and the degree of crystallinity. Many investigations have been also performed to enhance the impact resistance of PLA and compete with low cost commodity polymers. Considerable progress has been made to enhance the mechanical properties by blending PLA with other biodegradable and nonbiodegradable polymers [15]. From a biomedical point of view, the mechanical properties of neat PLA might not be adequate for high-load-bearing application [16] which is necessary with additional incorporation of reinforced filler, such as clay [17], [18], [19], [20], [21] or calcium phosphate in the crystalline form of hydroxyapatite (HA) [22].

Polymer/clay nanocomposites have received significant attentions from both academic and industrial area in recent years due to the excellent enhancement in physical and/or chemical properties relative to the neat polymer matrix [23], [24], [25]. The polymer/clay nanocomposites is prepared by the intercalation of monomers or polymers into swellable layered silicate hosts. In most cases, the synthesis involves either melt-direct polymer intercalation by using a conventional polymer extrusion process or intercalation of a suitable monomer and then exfoliating the layered host into their nanoscale elements by subsequent polymerization [26], [27], [28]. The high aspect ratio layered silicate affects the mechanical, physical and thermal properties of the synthesizing polymer/clay nanocomposites.

Recently, the enhancement of physical properties of PLA by addition of clay has been extensively reported in the literature [17], [18], [19], [20], [21]. The fabrications of PLA/clay nanocomposites were mixed with the PLA matrix and organically-modified clay using melt blending [17], [18], [19] or solution intercalation [20], [21]. All results indicate that the fabricated nanocomposites are mostly intercalated even though with the various surface modifications of clay.

In this report, we suggest a new method to prepare the PLA/m-MMT nanocomposites by the solution mixing process of PLA polymer with organically-modified montmorillonite (m-MMT), which was first treated by n-hexadecyl trimethyl-ammonium bromide (CTAB) cations and then modified by biocompatible/biodegradable chitosan in an aqueous solution containing 1 wt% of lactic acid to improve the chemical similarity between the PLA and m-MMT. A conceptual illustration of the preparation of m-MMT is shown in Fig. 1. The crystallization behavior and thermal/mechanical properties of prepared nanocomposites were measured by X-ray diffraction (XRD), differential scanning calorimeter (DSC) and dynamic mechanical analysis (DMA). In vitro degradation behaviors of prepared nanocomposites have been systematically investigated for up to 7 weeks in phosphate buffer saline (PBS) solution at 37.5 °C. The change of molecular weight as a function of degradation time has been also discussed.