Abstract
Biodegradable thermoplastic starch (TPS) composites with isometric titanium dioxide nanoparticles (TiO2; diameter 100 nm) and elongated titanate nanotubes (TiNT; diameter 20 nm and aspect ratio >50) were prepared from wheat and tapioca starch. The preparation was based on our recently developed two-step procedure consisting of the solution casting (SC) followed by the melt mixing (MM), which had been shown to yield highly homogeneous TPS in our previous study. In this work we demonstrated that the type of the TPS matrix and the type of the filler had significant impact on the morphology and the properties of the final composites. Multiple microscopic techniques (LM, SEM and TEM) evidenced that the TPS/TiO2 composites exhibited a very homogeneous dispersion of the filler, while the TPS/TiNT composites contained micrometer-size agglomerates of TiNT. Moreover, all composites with the wheat starch matrix (TPS(w)) showed a higher filler agglomeration than the corresponding composites with the tapioca starch matrix (TPS(t)). Rheological experiments showed that the TiO2 and TiNT fillers had quite small impact on the viscosity of the TPS(w) matrix, probably due to slightly higher agglomeration, poorer dispersion and weaker matrix-particle interactions. On the other hand, the TPS(t) matrix was influenced by both fillers significantly: the TiO2 nanoparticles with almost ideal dispersion formed a physical network in the TPS(t) matrix, which significantly increased the viscosity of the composite, whereas the TiNT nanotubes seemed to destruct the TPS(t) matrix partially, resulting in decreased viscosity of the composite. DMTA results confirmed the rheological measurements: Storage moduli (G') showed that TPS(t) and its composites with TiO2 were stiffer than the corresponding TPS(w) samples, while the TPS(t)/TiNT composites were less stiff than TPS(w)/TiNT. Also loss moduli (G) confirmed the difference between tapioca starch and wheat starch composites, which differed by their glass transition temperatures (Tg of TPS(w) < Tg of TPS(t)). The rheological and DMTA results were supplemented and supported by IR, XRD and TGA measurements.
Highlights
Starch is one of the cheapest and the most abundant natural polymers
We focused our attention on the comparison of the systems prepared by our two-step procedure (SC + melt mixing (MM)), using two types of starches and two types of TiO2-based particles
Higher magnification scanning electron microscope (SEM) (Figure 2) micrographs showed that the TiO2 particles tended to envelope plasticized, but notfully-merged starch granules after solution casting (SC) (Figures 2A,C), while the following MM step resulted in complete merging of starch granules and very homogeneous distribution of TiO2 nanoparticles (Figures 2B,D)
Summary
Starch is one of the cheapest and the most abundant natural polymers. On this account, starch-based materials are employed in many applications, for example, in household, agriculture, textile, pharmacy, or medicine (Bertolini, 2010; Sarka et al, 2011, 2012; Saiah et al, 2012; Xie et al, 2013; Ghavimi et al, 2015; Campos-Requena et al, 2017; Javanbakht and Namazi, 2017; Kuswandi, 2017; Liu et al, 2017a,b). The semicrystalline granular starch itself decomposes before melting when processed in classic devices (Biliaderis, 2009). The starch plasticization is influenced by many factors, mostly: the starch source (wheat, tapioca, corn etc.; Ao and Jane, 2007; Biliaderis, 2009; Bertolini, 2010; Liu et al, 2010), the plasticizer type and amount (Dai et al, 2008; Pushpadass et al, 2008), and the plasticization method and conditions (Altskar et al, 2008; Liu et al, 2013; Xie et al, 2013)
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