Investigation of nano-size montmorillonite on enhancing polyvinyl alcohol–starch blends prepared via solution cast approach

Abstract

It was found that the addition of montmorillonite (MMT) at low amount (<5 phr) exhibited reinforcing effect in neat PVOH, whereas higher amount can cause agglomeration leading to decrease of mechanical strength. Nevertheless, such inferior effect is not observed when MMT added in 50% PVOH–starch blends with the tensile strength increased in relation to the amount of MMT added. Besides, higher enthalpy of melting was detected in MMT added PVOH–starch blends via differential scanning calorimetry. This indicates that addition of MMT in PVOH–starch blend showed favorable molecular bonding interactions. From the morphology analysis, PVOH and starch seemed to interact in homogenous condition through the exfoliation of MMT as observed. It was also found that the infrared wavenumber of the hydroxyl group for both neat PVOH and 50% PVOH–starch samples reduced gradually with the increasing amount of MMT. The addition of MMT has shown the “red shift” effect indicating the formation of hydrogen bonding induced by MMT. In conclusion, the blending of MMT in both PVOH and PVOH–starch are favorable to produce superior properties of the resulting blends.

Introduction

Starch-based polymer blend have gained the attention of the researchers since decades ago [1]. Starch has the potential to develop as the biodegradable plastic materials because it is available abundantly in nature. Researchers have introduced starch in biodegradable polymer production because of its renewability, biodegradability, wide availability and low cost [1], [2], [3]. However, starch itself is weak and lack of mechanical strength to withstand external loads. Starch also does not exhibit viscoelastic characteristic likes thermoplastics. Thus, it is crucial to transform starch into thermoplastic starch with reversible processability characteristics. Typically, thermoplastic starch is a form of starch compound added compatible plasticizer and undergoes gelatinization to form continuity interaction within the macromolecules [4]. Most of the time, a compatible polymer is added to further provides stronger structural support so that the blends possess better mechanical strength compared to starch merely [5]. As a result, some researchers employed polyvinyl alcohol (PVOH), polylactic acid, polycaprolactone and etc. to blend with starch [5], [6], [7]. These polymers are inherently polar with the ability to interact well with starch via hydroxyl functional group [5].

According to the previous study of Sin et al. [8], the blending of PVOH with starch exhibited synergistic interaction as shown by differential scanning calorimetry analysis. This is due to the presence of hydrogen bonding interaction found in between the PVOH and starch [9]. Nevertheless, the mechanical properties of the polymer blends still depend on the structural arrangement of polymer blends. In other words, the irregularity of the molecular sizes of PVOH and starch can affect the structural packing thus weakening mechanical strength. Previous study have shown that polymer blending with layered silicate nanocomposite films can improve the mechanical, barrier, thermal and moisture adsorption properties of packaging material [9]. Montmorillonite (MMT) is one of the most widely used types of smectite clay in polymer nanocomposites nanofillers. Special attention has been paid to MMT minerals because of their small, extremely large surface areas and exfoliation and intercalation properties. Dimonie et al. [5] reported that the shifting of the X-ray diffraction peaks of the melt processed MMT added PVOH–starch bio-hybrids towards small angle reveal formation of certain interactions between the silicate and the original polymers. Moreover, such interaction can be further enhanced by adding treated MMT which exhibited a basal spacing enlarged due to the interaction degree between the hydroxylic oxygen from the PVOH–starch and the treated functional group from the silicate galleries [10], [11]. This interaction was also exhibited by the increment of glass transition for 4–11 °C indicating the favorable blends [12]. In the nutshell, the scope of current study is broader to utilize nano-size montmorillonite (MMT) as the reinforcing component on the PVOH–starch blends.