Citation: Hosseinnezhad, R.; Elumalai, D.; Vozniak, I. Approaches to Control Crazing Deformation of PHA-Based Biopolymeric Blends. Polymers 2023, 15, 4234. https:// doi.org/10.3390/polym15214234 Academic Editors: Chen Xiao, Peichuang Li, Wanhao Cai and Yuancong Zhao Received: 31 August 2023 Revised: 12 October 2023 Accepted: 23 October 2023 Published: 26 October 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). polymers Article Approaches to Control Crazing Deformation of PHA-Based Biopolymeric Blends Ramin Hosseinnezhad * , Dhanumalayan Elumalai and Iurii Vozniak * Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, 90-363 Lodz, Poland; dhanumalayan.elumalai@cbmm.lodz.pl * Correspondence: ramin.hosseinnezhad@cbmm.lodz.pl (R.H.); iurii.vozniak@cbmm.lodz.pl (I.V.) Abstract: The mechanical behavior of polymer materials is heavily influenced by a phenomenon known as crazing. Crazing is a precursor to damage and leads to the formation of cracks as it grows in both thickness and tip size. The current research employs an in situ SEM method to investigate the initiation and progression of crazing in all-biopolymeric blends based on Polyhydroxyalkanoates (PHAs). To this end, two chemically different grades of PHA, namely poly(hydroxybutyrate) (PHB) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBHV), were melt-blended with polybutyrate adipate terephthalate (PBAT). The obtained morphologies of blends, the droplet/fibrillar matrix, were highly influenced by the plasticity of the matrices as well as the content of the minor phase. Increasing the concentration of PBAT from 15 to 30 wt.% resulted in the brittle to ductile transition. It changed the mechanism of plastic deformation from single craze-cracking to homogeneous and heterogeneous intensified crazing for PHB and PHBHV matrices, respectively. Homogeneous tensile crazes formed perpendicularly to the draw direction at the initial stages of deformation, transformed into shear crazes characterized by oblique edge propagation for the PHBHV/PBAT blend. Such angled crazes suggested that the displacement might be caused by shear localized deformation. The crazes’ strength and the time to failure increased with the minor phase fibers. These fibers, aligned with the tensile direction and spanning the width of the crazes, were in the order of a few micrometers in diameter depending on the concentration. The network of fibrillar PBAT provided additional integrity for larger plastic deformation values. This study elucidates the mechanism of crazing in PHA blends and provides strategies for controlling it. Keywords: crazing; biopolymer; deformation; PHA; PBAT 1. Introduction Polyhydroxyalkanoates (PHAs) are a family of biodegradable and biocompatible poly- mers with diverse applications in various fields. Among them, polyhydroxybutyrate (PHB), polyhydroxybutyrate-co-hydroxyvalerate (PHBV), and poly(4-hydroxybutyrate) (P4HB) are the most commonly studied biopolymers [1,2]. Despite the significant advantages of PHAs, the widespread adoption of them in various industries has been hindered by certain drawbacks, e.g., low thermal and mechanical properties. To address these limitations while maintaining biodegradability, they are often blended with other biodegradable polymers, such as polylactide (PLA) and polycaprolactone (PCL). However, the inadequate compati- bility and poor interfacial adhesion of the pairs result in phase separation, heterogeneous structures, and lower-than-expected mechanical performance [3–5]. In particular, PHAs can be mixed together with co-polyesters like poly butylene-co-succinate-co-adipate (PBSA) and polybutyrate adipate terephthalate (PBAT) at high temperatures and then cooled to create a solid blend that combines the desirable properties of both. The compatibility of these pairs is influenced by the ratio of compounds, the chemical structure of the polymers, and the processing conditions used to create the blend [6,7]. Zytner et al. conducted a successful analysis of PHBV/PBAT blends, reporting that the transition in morphology Polymers 2023, 15, 4234. https://doi.org/10.3390/polym15214234 https://www.mdpi.com/journal/polymers