Decoding Thermoplastic Starch (TPS) Microstructure

Unveiling the microscopic organization of thermoplastic starch (TPS) involves a nuanced exploration of constituent molecules and particles. Numerous factors, encompassing processing conditions, raw materials, and additives, intricately shape the TPS microstructure. This intricate arrangement significantly influences mechanical, thermal, and rheological properties, thereby dictating TPS applications across diverse industries, including packaging and biodegradable materials.

Morphological Insights into TPS Films

Evaluation of amylose, amylopectin, and starch films’ morphology unfolds through advanced microscopy techniques such as light microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM).

Networked Surface Structure

Starch films exhibit a network structure on the surface, believed to emerge from crystalline or co-crystallized amylose and amylopectin strands. This network formation necessitates high moisture content or a slow cooling rate during processing, ensuring ample molecular mobility. Distinct film-forming conditions can lead to phase-separated structures, with amylose-rich and amylopectin-rich phases under specific circumstances.

Impact of Phase Separation

Under certain conditions, phase separation in starch films occurs, influencing their properties. Amylose’s ability to form a continuous network faster than phase separation leads to observable heterogeneity in structure. This phenomenon limits co-crystallization possibilities, resulting in lower crystallinity compared to non-phase-separated starch films.

Surface Features and Protrusions

Distinct features, such as rounded protein aggregate protrusions and identified granule remnants (ghosts), add complexity to film surfaces. Ghosts, remnants of starch granule envelopes, impact film properties due to their elastic/plastic properties.

Influence of Plasticizers on Morphology

The addition of glycerol as a plasticizer affects granular disruption levels, contributing to varied morphology in corn starch films. The use of glycerol retards granule gelatinization, influencing crystalline domains’ formation in films.

Unraveling Crystallinity in TPS

The crystallinity in TPS is a dynamic interplay influenced by diverse factors, including composition, processing conditions, and storage. Three distinct types of crystallinity are observed: residual crystallinity, crystallinity induced by processing, and crystallinity induced by post-processing aging.

Role of Plasticizers in Crystallinity

Plasticizers play a pivotal role in TPS crystallinity. The complexity arises from their dual impact: increasing plasticizing effect enhances chain mobility, favoring crystalline domain formation, while decreasing molecular weight hinders crystallization.

Impact of Amylose Concentration

Amylose concentration is a key determinant, affecting crystallinity in potato starch materials. Films with higher amylose content exhibit increased crystallinity, showcasing the linear dependence between VH-type crystallinity and amylose content.

Storage-Induced Changes

Storage conditions contribute to post-processing changes in crystallinity. Retrogradation or recrystallization of starch molecules into a B-type double helix structure occurs when TPS materials are stored above the glass transition temperature (Tg).

In summary, TPS microstructure intricacies, morphological nuances, and crystallinity dynamics underscore the complex interplay of factors shaping its properties. Understanding these facets is imperative for optimizing TPS for diverse applications and advancing sustainable material development.

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