Effects of citric acid-functionalized ZnO nanoparticles on the structural, mechanical, thermal and optical properties of polycaprolactone nanocomposite films

doi:10.1016/j.matchemphys.2017.05.023

Materials Chemistry and Physics

Volume 197, 15 August 2017, Pages 129-137

https://doi.org/10.1016/j.matchemphys.2017.05.023 Get rights and content

Highlights

•
PCL/ZnO-CA NCs were fabricated via ultrasonic irradiation, as a green method.
•
Modified ZnO NPs have good compatibility and proper dispersion in the PCL matrix.
•
UV shielding and wettability were increased by the addition of modified ZnO.
•
E-modulus of the NCs showed about 2-fold increase compared to that of blank polymer.
•
Homogeneous distribution of ZnO-CA NPs in PCL led to transparent NC films.

Abstract

In this paper, nanocomposite (NC) films of citric acid (CA)-modified zinc oxide (ZnO) and polycaprolactone (PCL) were fabricated by solvent casting and evaporation method. The prepared films were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, field emission scanning electron microscopy (FE-SEM), transmission electron microscopy and thermogravimetric analysis. Then, the effect of the CA-ZnO content on physiochemical properties of the PCL/ZnO-CA NCs was studied. The results of mechanical tests showed enhanced tensile strength and Elastic modulus for the prepared NCs. The FE-SEM images showed relative changes in the morphology of the NCs by increasing the nanoparticles (NP)s content. In vitro bioactivity test of the obtained NCs was performed in simulated body fluid which showed the formation of hydroxyapatite and indicated that the new NCs are bioactive. It was further found that the NPs concentration in the NC films had affected their optical absorption, thermal properties and surface wettability.

Graphical abstract

Introduction

Nanocomposites (NC)s are commonly materials containing nanofillers with a mean diameter of 1–100 nm dispersed throughout an organic polymer matrix [1]. NCs have aroused much interest over the past two decades [2] because they show unexpected properties, such as large increases in thermal stability, electronic and optical properties mechanical strength and electrical conductivity [3], [4], [5], [6], [7]. Wide ranges of nanoparticle (NP)s as nanofillers, compatibilising agents, and polymers as a matrix have been used in fabrication of the NCs [8], [9]. Amongst many of the nanofillers used in these materials, inorganic NPs are preferred as fillers due to their superior mechanical strength, thermal stability, corrosion resistance and antimicrobial properties [10]. Zinc oxide (ZnO) is one of the five compounds of zinc that are generally recognized as safe (GRAS) by the Food and Drug Administration (FDA) [11]. It also comes under the group of II–VI semiconductors with wide direct band gap energy (3.37 eV) and large excitonic binding energy of (60 meV) that exhibited high chemical stability, low toxicity [12], [13], powerful oxidation strength [14] and antibacterial properties [15].

Several strategies have been reported for the preparation of polymer-ZnO NCs including melt-compounding process [16], hydrothermal method [17], sol–gel deposition [18], melt-blending [19], solvent casting and evaporation method [20], in which chloroform, dichloromethane, toluene or a mixture of solvents were used to dissolve a polymer matrix [21]. Since unmodified NPs experience significant van der Waals interactions, they tend to aggregate in suspension [22]. But modification of NP surfaces with polymers or surface-active molecules suppress the NP aggregations [23] and made them more compatible with the polymer matrices. Modified NPs also exhibit good dispersion characteristics in the polymer matrix, because of reduced inter-particle forces and exhibit higher stability [24].

Recently, NCs based on biodegradable polymers like starch or polycaprolactone (PCL) have been in the focus of attention, due to their application as biodegradable, impermeable packaging materials [25]. PCL is a biodegradable [26], [27] and semi-crystalline polymer composed of hexanoate repeat units [28]. The presence of nonpolar methylene groups in the repeating units lead to unusual properties that are similar to polyolefin, but hydrolytically unstable aliphatic-ester linkage made it degradable. The chemical structure of PCL has also caused the polymer to be compatible with numerous polymers [29]. Therefore, it has been widely used in development of controlled drug delivery systems [30], surgical sutures [31] or for tissue engineering (such as guided bone or tissue regeneration) [32]. However, some drawbacks such as low thermal stability, poor mechanical properties, and barrier properties to gases limited its applications [33]. Several studies have revealed improved properties of PCL-based NCs, by the use of multi-walled carbon nanotubes [34], [35], silica [36], bioactive glass nanofiber [37] and clay [38] as reinforced nanofillers.

Citric acid (CA) is a natural and organic acid with multi-carboxylic structure found in various citrus fruits and it can be used as a crosslinking agent [39], [40]. The three carboxyl groups of citric acid can treat with hydroxyl groups of the ZnO NPs to produce ester linkage.

The main objective of the present study is to fabricate PCL/ZnO-CA NC films using a solvent casting method with different weight percent of ZnO-CA NPs as the nanofiller. Then, the effect of NP percentages on the improvement of property of PCL films will be investigated by evaluating the film properties such as morphology, optical, mechanical, and thermal properties.

Section snippets

Material

PCL with Mw = 70000–90000 g mol⁻¹ was purchased from Sigma-Aldrich. It was used without any further purification. ZnO powder with average particle size of <25 nm and surface areas of >80 m² g⁻¹ was obtained from Neutrino company (Tehran, Iran). CA, dichloromethane (DCM) were provided from Merck chemical company (Germany). The NPs were dried at 400 °C for 4 h.

NC films preparation

First, ZnO NPs were modified with biocompatible CA (Fig. 1). In this regard, 0.1 g of ZnO NPs were dispersed in 10 mL of deionized water

Results and discussion

NC films were fabricated by incorporating the ZnO-CA NPs within the PCL matrix that CA has the main role of providing a bio-safe modifier for the NPs. Fig. 1, Fig. 2 shows a schematic illustration of the overall process to obtain PCL-based NCs. Within the NCs, carbonyl groups of the matrix can form hydrogen bond interactions with CA and ZnO. The organic CA modifiers facilitate ZnO to disperse in organic DCM solution and interact with hydrophobic PCL chains [43], [44]. The modified NPs and the

Conclusions

The purpose in this work was to investigate the effect of low contents of modified ZnO NPs on the thermal degradation behavior, water contact angle and their optical, morphological and mechanical properties. In this regard, surface modification of ZnO NPs with CA was performed through simple ultrasonic-assisted technique as a green and fast method. Then, the ZnO-CA NPs were incorporated into the PCL matrix to prepare PCL/ZnO-CA NC films. FT-IR analysis verified the modification of NPs. The XRD

Acknowledgments

The authors acknowledge the partial financial support provided by the Research Affairs Division Isfahan University of Technology (IUT), Isfahan, Iran. The authors are also grateful to National Elite Foundation (NEF), Iran Nanotechnology Initiative Council (INIC) and Center of Excellence in Sensors and Green Chemistry Research (IUT), for financial support.