**Advances in Manufacturing and Characterization of Functional Polyesters**

Editors

**Rafael Balart Sergio Torres-Giner Octavio Fenollar Nestor Montanes Teodomiro Boronat**

MDPI • Basel • Beijing • Wuhan • Barcelona • Belgrade • Manchester • Tokyo • Cluj • Tianjin

*Editors* Rafael Balart Universitat Politecnica de ` Valencia (UPV) ` Spain

Nestor Montanes Universitat Politecnica de ` Valencia (UPV) ` Spain

Sergio Torres-Giner Universitat Politecnica de ` Valencia (UPV) ` Spain

Teodomiro Boronat Universitat Politecnica de ` Valencia (UPV) ` Spain

Octavio Fenollar Universitat Politecnica de ` Valencia (UPV) ` Spain

*Editorial Office* MDPI St. Alban-Anlage 66 4052 Basel, Switzerland

This is a reprint of articles from the Special Issue published online in the open access journal *Polymers* (ISSN 2073-4360) (available at: https://www.mdpi.com/journal/polymers/special issues/ functional polyesters).

For citation purposes, cite each article independently as indicated on the article page online and as indicated below:

LastName, A.A.; LastName, B.B.; LastName, C.C. Article Title. *Journal Name* **Year**, *Volume Number*, Page Range.

**ISBN 978-3-0365-0280-9 (Hbk) ISBN 978-3-0365-0281-6 (PDF)**

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© 2021 by the authors. Articles in this book are Open Access and distributed under the Creative Commons Attribution (CC BY) license, which allows users to download, copy and build upon published articles, as long as the author and publisher are properly credited, which ensures maximum dissemination and a wider impact of our publications.

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### **Contents**



Reprinted from: *Polymers* **2019**, *11*, 1331, doi:10.3390/polym11081331 ................ **227**

### **About the Editors**

**Rafael Balart** received his Ph.D. from the Polytechnic University of Valencia (UPV) in 2003. At present, he is Full Professor of Materials Science and Engineering at the Department of Mechanical and Materials Engineering. With more than 25 years of experience in the field of polymer and composite materials, he currently leads the Research Group on Environmentally Friendly Polymers and Composites (GiPCEco), which focuses its activity on the development and formulation of sustainable polymers and the manufacture of composite materials for industrial applications. In recent years, he has focused his research on upgrading industrial, agricultural, and agroforestry wastes with a biorefinery approach to obtain high added-value products with applications in polymers and composites.

**Sergio Torres-Giner** received a Dipl-Ing in Chemical Engineering in 2003 from the Polytechnic University of Valencia (UPV), Spain. In 2004, he achieved an MSc in Process Systems Technology at Cranfield University, England, followed by an MBA in Industrial Management in 2005 at the Catholic University of Valencia 'San Vincente Martir', Spain. He was able to complete his Ph.D. in 2010 in Food Science at the University of Valencia, Spain. He currently works as a scientist at the Research Institute of Food Engineering for Development (IIAD) in the field of macromolecular science of application interest in food packaging technology. He has more than 15 years of experience in both public research agencies and industrial R&D organizations. He has published over 80 peer-reviewed scientific papers indexed in JCR, 10 book and encyclopedia chapters, and 4 patents. His research activity has strongly contributed to advancing the knowledge on biopolymers and transferring it to applications and products for food-related applications.

**Octavio Fenollar** received his Ph.D. in Engineering from the Polytechnic University of Valencia (UPV) in 2011 and currently, he is Associate Professor of Materials Science and Engineering at the Department of Mechanical and Materials Engineering (UPV Campus d'Alcoi). He has expertise in the field of polymer processing, industrial formulations, and upgrading plastic wastes. With more than 14 years of experience at the university, he leads the research group of Environmentally Friendly Polymers and Composites (GIPCEco). He joined the Institute of Materials Technology (ITM) in 2006 and, since then, he has specialized in polymer processing and advanced characterization as well as recycling and upgrading plastic and other industrial wastes. He has wide experience in the manufacturing, formulation, optimization, and characterization of both thermoplastic and thermosetting polyesters. He is also a member of the Design Factory group at EPSA-UPV, working in additive manufacturing with polymers, mainly polyesters.

**Nestor Montanes** received his Ph.D. in Engineering from the Polytechnic University of Valencia (UPV) in 2017 and currently, he is Associate Professor of Materials Science and Engineering at the Department of Mechanical and Materials Engineering (UPV Campus d'Alcoi). With more than 10 years of experience in the university and over 10 years of experience in the packaging industry, he has actively participated in the implementation of new teaching methodologies in the classroom: reverse teaching, gamification, and so on. He joined the Technological Institute of Materials (ITM) in 2014 and specializes in polymer analysis and characterization, mainly of biopolymers and polymer-based composites. His areas of interest include polymer manufacturing, compounding, formulation, additives, polymer blends, and upgrading industrial wastes into the polymer industry.

**Teodomiro Boronat** is a Mechanical Engineer and received his Ph.D. in Engineering from the Polytechnic University of Valencia (UPV) in 2009. Currently, he is Associate Professor of Manufacturing Engineering at the Department of Mechanical and Materials Engineering (UPV Campus d'Alcoi). He is also a researcher at the Technological Institute of Materials (ITM) and specializes in polymer manufacturing processes and additive manufacturing. His main research topic is to study the relationship between processing parameters and their obtained final properties. Other areas of interest include thermoplastic manufacturing processes, thermosetting systems, rheology, and additive manufacturing.

### **Preface to "Advances in Manufacturing and Characterization of Functional Polyesters"**

In the field of polymer science, bio-based and biodegradable polymers are rapidly developing due to the rapid depletion of fossil fuels and environmental pollution deriving from traditional plastics. This trend includes aromatic polyesters derived totally or partially from natural resources as well as recycled polyesters, such as bio-based polyethylene terephthalate (bio-PET) and recycled polyethylene terephthalate (r-PET), respectively, and biodegradable aliphatic polyesters. In some cases, despite being obtained from petroleum, these polyesters can undergo biodisintegration under controlled compost soil. Examples of biodegradable petroleum derived polyesters include poly(ε-caprolactone) (PCL) and poly(butylene succinate) (PBS). Furthermore, some other biodegradable polyesters are also obtained from biomass, such as polylactide (PLA), or even microorganisms, such as polyhydroxyalkanoates (PHAs). Blends and composites based on these polyesters also represent a recurrent solution due to their good balance in terms of easy manufacturing, improved sustainability profile, and tailor-made performance. Therefore, all the above polyesters contribute to sustainable development. Furthermore, they can be functionalized by copolymerization, the use of micro- and nanoparticles or reactive compatibilizers to tailor the desired thermal and mechanical properties, barrier performance, biodegradation, and biocompatibility. Functionalization can widen their potential use in both commodity and technical areas such as packaging, textiles, automotive, building and construction, and also in specialized fields such as tissue engineering and the controlled release of drugs, electronics, or shape-memory devices.

This book is divided into thirteen chapters that gather a series of research articles focused on the manufacturing and characterization of functional polyesters. The book starts with two chapters devoted to the synthesis of functional copolyesters with adjustable properties depending on the polymerization conditions, their monomer contents, or thermal and mechanical post-treatments. Thereafter, the book follows with four chapters covering the development of functional polyesters as resorbable materials in bone tissue engineering. This part of the book is based on research articles reporting the use of blends and nanocomposites based on PCL, PLA, and PHA that can satisfy most of the current requirements for bone fixation of load-bearing devices in terms of both mechanical performance and degradation rates. The next two chapters describe the potential application of functional polyesters in the development of sound absorbing materials and sustainable advanced composites. It then continues with two more chapters focused on the packaging field, which show that PLA and its green composites with food waste derived fillers can be used for compostable food containers and disposable articles with high performance. Finally, the book ends with three chapters that discuss strategies to valorize the waste of polyester-based materials such as those found in bottles for water and beverages, food trays, or fibers for textiles. All these research articles describe the use of nanostructured functional fillers and/or reactive additives to increase the physical–mechanical properties of r-PET and their blends with virgin polyester such as bio-PET by reinforcement or a process of the chain-extension mechanism of the polyester chains. All the studies gathered in this section indicate that secondary recycling of bio-based but non-biodegradable polyesters can be feasible from environmental and economic points of view.

The present book can be of interest to a diverse audience of researchers and professionals working in any area of polymer science and engineering and in academia, industry, or government, including but not limited to scientists, engineers, consultants, etc. This book also intends to make original and cutting-edge research works in the field of synthesis, characterization, manufacturing, and applications of functional polyesters accessible to a broad readership.

### **Rafael Balart, Sergio Torres-Giner, Octavio Fenollar, Nestor Montanes, Teodomiro Boronat**

*Editors*

### *Editorial* **Advances in Manufacturing and Characterization of Functional Polyesters**

**Rafael Balart 1,\*, Nestor Montanes 1, Octavio Fenollar 1, Teodomiro Boronat <sup>1</sup> and Sergio Torres-Giner 2,\***


Received: 13 November 2020; Accepted: 26 November 2020; Published: 29 November 2020

In the last few years, a remarkable growth in the use of functional polyesters has been observed. This trend comprises the development of aromatic polyesters derived either from renewable resources or recycling processes on account of the rapid depletion of fossil fuels and the cost of extracting polymers from petroleum. Furthermore, biodegradable aliphatic polyesters are also being rapidly developed due to pollution deriving from traditional plastics. The latter group includes both bio-based and petroleum derived polyesters that are biodegradable, that is, they can undergo biodisintegration under controlled compost soil or natural conditions. Moreover, blends and composites based on these novel polyesters represent a recurrent and cost-effective solution due to their good balance in terms of easy manufacturing, improved sustainability profile, and tailor-made performance. All these polyesters do not only contribute to sustainable development but they can also be functionalized to tailor their desired properties in terms of thermal and mechanical properties, barrier performance, biodegradation, and biocompatibility. For instance, copolymerization and the use of micro- and nanoparticles or reactive compatibilizers can widen the potential use of polyesters in both commodity and technical areas such as packaging, textiles, automotive, building and construction, and also in specialized fields such as tissue engineering and controlled release of drugs, electronics or shape-memory devices. The present Special Issue gathers a series of thirteen articles focused on the manufacturing and characterization of functional polyesters.

In terms of technical applications, the synthesis of functional materials is increasingly important for optical devices and electronics. In this regard, Jeong et al. [1] developed a novel copolyester for next-generation flexible devices in optics. To this end, authors synthesized by melt polymerization and subsequent solid-state polycondensation (SSP) a copolyester, named PCITN, based on 2,6-naphthalene dicarboxylic acid (NDA), terephthalic acid (TPA), 1,4-cyclohexanedimethanol (CHDM), and isosorbide (ISB). Polymerization carried out in two steps led to a PCITN copolyester having a high molecular weight (Mw = 68,900 g/mol), which was successfully shaped thereafter into films by extrusion at 290 ◦C. The resultant randomly-oriented PCITN films were preheated for 25 min and then tensile stretched and uniaxially cold drawn at 150 ◦C in machine direction (MD) at different draw ratios (λ = 1~4) to increase molecular chain orientation. The uniaxially oriented films showed high glass transition temperature (Tg of up to 140 ◦C) and Young's modulus (E = 2.6 GPa), which was ascribed to the use of the low segmental mobility and high thermal stability of the building block ISB. The PCITN films also presented lower water absorption (0.54 wt.%) and birefringence (n = 0.09) when compared with other conventional substrate polymers used for flexible electronics such as polyimide (PI), poly(ethylene 2,6-naphthalate) (PEN), and polyethylene terephthalate (PET). Copolymerization in two stages was also explored by Safari et al. [2] to develop random copolyesters with adjustable properties

depending on their crystallinity. Authors synthesized poly(butylene succinate)-*ran*-poly(ε-caprolactone) (PBS-*ran*-PCL) copolyesters by transesterification/ring opening polymerization (ROP) reaction of 1,4-butanediol (BD), dimethyl succinate (DMS), and ε-caprolactone (CL) followed by polycondensation at reduced pressure. The PBS-*ran*-PCL copolyesters showed isodimorphic behavior with a controllable balance between comonomer inclusion and exclusion, where at some intermediate compositions the crystal lattice of each one of the components partially tolerate the presence of the other. In particular, the pseudo-eutectic point was found at 55 mol.% CL, where both PBS- and PCL-rich phases crystallized, whereas single PBS- and PCL-rich crystals were respectively formed at compositions based on lower and higher CL-unit molar contents. Furthermore, the phase crystallization of PBS-*ran*-PCL with pseudo-eutectic composition was successfully controlled by varying the isothermal crystallization temperature.

Only specifically developed functional polyesters can fulfill the specific challenges of the constantly developing biomedical field. At present, biodegradable aliphatic polyesters are being intensively studied as resorbable materials in bone tissue engineering, particularly for low-stress parts such as small orthopedic plates, rods or bone screws. Accordingly, these polyesters can develop scaffolds and biomaterial devices with desired geometries and special functionalities to attain osteoconductive properties. However, polyesters cannot satisfy most of the current mechanical requirements for bone fixation of load-bearing devices. In this regard, the use of micro- and nano-scale mineral particles with high hardness and different bioactives is very promising. For instance, nanoparticles of hydroxyapatite (nHAs) were used by Ivorra-Martinez et al. [3] to improve the mechanical performance of the poly(3-hydroxybutyrate-*co*-3-hydroxyhexanoate) [P(3HB-*co*-3HHx)] copolyester. It was observed that the incorporation of the osteoconductive nanofiller yielded balanced properties to the microbial copolyester in terms of strength and ductility, showing values of tensile modulus (Et) and elongation at break (εb) ranging from approximately 1 up to 1.7 GPa and 6.5 to 19.4%, respectively. As a result, the P(3HB-*co*-3HHx)/nHA nanocomposites presented a closer mechanical performance to that found in the natural bone when compared to titanium (Ti) and alloys of metals such as stainless steel and cobalt-chrome (Co–Cr) alloys.

Another challenge that polyesters is facing in the biomedical area is related to their inadequate degradation rates, which would increase the risk of requiring additional chirurgical interventions, eventually causing adverse tissue reactions or even infections. In this context, Torres et al. [4] also employed nHAs and halloysite nanotubes (HNTs) to increase the surface wettability of hydrophobic and hydrophilic sets of poly(ε-caprolactone) (PCL), polylactide (PLA), and their blends as well as poly(2-hydroxyethyl methacrylate) (PHEMA) and its copolymer with ethyl methacrylate (EMA), that is, poly(2-hydroxyethyl methacrylate-*co*-ethyl methacrylate) P(HEMA-*co*-EMA). Authors demonstrated that both the blending of PCL with PLA and the incorporation of nHAs and HNTs provided hydrophilic units and decreased the crystallinity of PCL, favoring the accessibility of water molecules to the ester linkages. Therefore, in comparison with the unfilled PCL/PLA blend, mass loss increased up to 48% after incubation for 12 weeks in phosphate buffered saline (PBS) during the evaluation of their degradation rate. Consequently, the mechanical properties of the polyesters decreased above 60% after the incubation time due to the higher hydrolytic cleavage achieved. Similarly, Zhao et al. [5] developed nanocomposites of PLA with 1 wt.% of nanoparticles and whiskers of magnesium oxide (MgO) that were chemically modified with stearic acid. The in vitro degradation of the nanocomposites was analyzed by PBS soaking for up to 12 months, showing that the addition of the stearic acid-modified MgO accelerated the PLA's water uptake rate, especially for the whiskers. Furthermore, the dissolution of MgO through the neutralization of the acidic product of the PLA degradation contributed to regulate the pH value of PBS. The in vivo results of the histological morphologies attained with the nanocomposites further suggested that the presence of MgO can improve bone repair since, when dissolving, it releases magnesium ions (Mg2+) that can activate a variety of enzymes that promote the synthesis of proteins. In addition, PLA-based materials can also serve as a drug carrier for controlled release applications. Micro- and nanoencapsulation using HNTs is promising to deliver different active and bioactive compounds such as antioxidants, antimicrobials, antibiotics, or growth factors

(GFs). In this regard, Montava-Jorda et al. [6] developed PLA composites with HNTs loadings of 3, 6, and 9 wt.%. Authors found that water uptake of PLA increased due to the hydrophilic nature of the nanotubes, which offer a high surface area with hydroxyl (–OH) groups, and thus can favor the degradation rate of the resultant wound dressings.

One of the most exciting areas of functional polyesters is the development of sound absorbing materials. It has been demonstrated that, for instance, nonwoven structures of polyester fibers are potential candidates in noise reducing panels. The research article of Yang et al. [7] estimated the non-acoustic parameters of high-loft nonwoven panels made of a commercial polyester blend composed of 45 wt.% staple polyester, 30 wt.% hollow polyester, and 25 wt.% bicomponent polyester provided by the Technical University of Liberec (Liberec, Czech Republic). The panels, with densities ranging from 16.93 to 45.56 kg/m3, were manufactured by perpendicular laying technology and were tested by means of the Bayesian reconstruction procedure, implementing the Johnson-Champoux-Allard-Lafarge model and Markov chain Monte Carlo optimization technique. The inversed method showed that the polyester-based panels were homogeneous along with the panel thickness, presenting the same inferred tortuosity. Mean relative differences of airflow resistivity and porosity of 0.019 and 0.004 were respectively attained, which mostly affected the thermal characteristic length. In the textile area, linen (*Linum usitatissimum*) is among the most usable and profitable plants. One of its processing by-products is flax fiber (FF), which can also be applied to replace glass fiber (GF) as a suitable reinforcement in advanced composites of polyesters for several engineering applications such as panel boards and insulation panels. However, lignocellulosic fibers habitually show a poor interfacial adhesion with hydrophobic polyester matrices such as PLA. In this context, Agüero et al. [8] evaluated the influence of different compatibilization strategies on the performance of PLA/FF composites. In particular, the compatibilization routes consisted of silanization with (3-glycidyloxypropyl) trimethoxysilane (GPTMS) and reactive extrusion (REX) with a commercial random copolymer of poly(styrene-*co*-glycidyl methacrylate) (PS-*co*-GMA, Xibond™ 920, Polyscope, Geleen, The Netherlands), a multi-functional epoxy-based styrene-acrylic oligomer (ESAO, Joncryl ADR 4368®, BASF S.A., Barcelona, Spain), and maleinized linseed oil (MLO). Among the tested routes, the petroleum derived ESAO yielded the highest mechanical resistance and toughness improvement and also the highest thermal stability due to the chain-extension or cross-linking effect on PLA, whereas the most ductile green composites were attained with MLO by plasticization.

Focusing on the packaging field, the PLA biopolyester is widely used for food containers (e.g., food films and trays) or disposable articles (e.g., straws and cutlery). However, it habitually results in extremely brittle materials with low ductility and toughness. To improve the impact properties of PLA, Lascano et al. [9] incorporated an oligomer of lactic acid (OLA) in the contents of 5–20 wt.%. At a loading of 15 wt.% OLA, it was observed a percentage increase of nearly 171% in the impact strength of PLA. Furthermore, for low deformation angles, the OLA-containing PLA samples successfully recovered over 95% of their original shape, while for the highest angles they still reached a recovery of approximately 70%. In packaging applications, the development of eco-friendly or green composites by the valorization of agrofood waste also shows several benefits including the enhancement of the environmental profile, improved biodegradability, lightweight, or cost reduction. Among the wide variety of agricultural wastes and food processing by-products, almond (*Prunus amygdalus* L.) shell powder (ASP) was used by Ramos et al. [10] as the filler for a commercial biopolyester blend (INZEA F2®, Nurel, Zaragoza, Spain) at 10 and 25 wt.%. In this study, the lignocellulosic filler was first subjected to two grinding levels, yielding sizes of 125–250 and 500–1000 μm, and MLO and low-functionality ESAO (Joncryl ADR 4400®, BASF S.A.) were added as compatibilizers. Authors reported that ASP successfully improved the biodisintegration rate of PLA under composting conditions, while full disintegration was obtained after 90 and 28 days for the green composites containing 10 and 25 wt.% ASP, respectively. Authors concluded that the presence of ASP at high contents produced a significant discontinuity in the polyester matrix, facilitating water penetration and favoring the growth of microorganisms responsible for biodisintegration.

Finally, disposal of waste polyester-based materials has become an urgent environmental problem in the last decades. For instance, PET, which is widely used worldwide in bottles for water and beverages, food trays or fibers for textiles, produces a plastic waste that is neither biodegradable nor compostable, and recycling currently represents the only solution. Although the post-consumer uses of recycled polyethylene terephthalate (r-PET) streams has increased, it is currently limited to low contents in mixed formulations with virgin PET due to the weakening of its physical-mechanical properties as a result of the MW reduction during processing derived from the cleavage of the polyester chains. Thus, the use of nanostructured additives and/or reactive additives has emerged as a novel route to deal with this technical issue. In the study of Dominici et al. [11], anhydrous calcium terephthalate anhydrous salts (CATAS), a nanometric metal organic framework consisting of calcium ions (Ca2<sup>+</sup>) as metal clusters coordinated to TPA as organic ligand, were developed and incorporated by melt processing into rPET at different two levels of loadings, that is, 0.1–1 wt.% and 2–30 wt.%. Authors found that 0.4 wt.% of CATAS led to the formation of a rigid amorphous fraction due to the aromatic interactions (π−π conjugation) between the rPET matrix and Ca-based salts, which was located at the rPET/CATAS interface. In contrast, tangible changes were observed below this threshold, whereas a restriction of rPET/CATAS molecular chains mobility was detected above 0.4 wt.% CATAS due to the formation of percolation networks with hybrid mechanical characteristics. In another study, Montava-Jorda et al. [12] melt-mixed at 15–45 wt.% partially bio-based polyethylene terephthalate (bio-PET) with r-PET flakes that were obtained from pre-consumer waste streams of the food-use bottle industry and PS-*co*-GMA at 1–5 parts per hundred resin (phr) of polyester blend. For the polyester blend containing 45 wt.% of r-PET, the addition of 5 phr of the reactive compatibilizer led to an enhancement in the elongation-at-break value from 10.8 to 378.8%, whereas impact strength also increased from 1.84 to 2.52 kJ/m2. It was concluded that, due to a chain-extension mechanism based on the reaction of the–OH and carboxyl (–COOH) terminal groups of both bio-PET and r-PET chains with the multiple groups of glycidyl methacrylate (GMA) present in PS-*co*-GMA, branched and larger interconnected macromolecules were formed and contributed to improve the ductile performance of the polyester blends. Additionally, in terms of improving the mechanical or secondary recycling of polyesters with other polymers, Jorda et al. [13] reported binary blends of bio-PET with polyamide 1010 (PA1010), a fully bio-based polyamide (bio-PA), compatibilized with PS-*co*-GMA. Authors reported that the addition of 30 wt.% of PA1010, which provides a final renewable content of nearly 50 wt.% in the polymer blend, yielded an immiscible droplet-like structure in which PA1010 droplets are embedded in the bio-PET matrix. However, the intrinsic stiffness of the biopolyester was improved by the bio-PA also the addition of PS-*co*-GMA at 3 phr, which induced a remarkable reduction of the droplet size from approximately 4 to 1 mm. According to these studies, secondary recycling of bio-based but non-biodegradable polyesters can be feasible from environmental and economic points of view.

From the above, the Special Issue *Advances in Manufacturing and Characterization of Functional Polyesters* published in *Polymers,* brings together a broad range of research works dealing with functional polyesters to update the "state-of-the-art" knowledge in this field. Functionalization of polyesters was successfully achieved by means of copolymerization, the use of additives at micro- and nano-scale or reactive compatibilizers. The resultant materials can find potential uses in technical applications, such as optical devices and electronics, in the biomedical field for tissue engineering and controlled release of drugs, or as sound absorbing panels, textiles and apparel, advanced composites, and sustainable food packaging.

**Author Contributions:** All the guest editors wrote and reviewed this editorial letter. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research work was funded by the Spanish Ministry of Science and Innovation (MICI) project number MAT2017-84909-C2-2-R.

**Acknowledgments:** S.T.-G. acknowledges MICI for his Ramón y Cajal contract (RYC2019-027784-I). The guest editors thank all the authors for submitting their work to this Special Issue and for its successful completion. We also acknowledge all the reviewers participating in the peer-review process of the submitted manuscripts for

enhancing their quality and impact. We are also grateful to Chris Chen and the editorial assistants of Polymers who made the entire Special Issue creation a smooth and efficient process.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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*Article*

### **High Thermal Stability, High Tensile Strength, and Good Water Barrier Property of Terpolyester Containing Biobased Monomer for Next-Generation Smart Film Application: Synthesis and Characterization**

**Jaemin Jeong 1,**†**, Fiaz Hussain 1,2,**†**, Sangwon Park 1, Soo-Jung Kang <sup>1</sup> and Jinhwan Kim 1,\***


Received: 24 September 2020; Accepted: 19 October 2020; Published: 23 October 2020

**Abstract:** This research synthesizes novel copolyester (PCITN) containing biobased isosorbide, 1,4-cyclohexandimethanol, terephthalic acid, and 2,6-naphthalene dicarboxylic acid and characterize its properties. The PCITN copolyester was extruded into film, and its performance properties including: tensile strength, Young's modulus, thermal, dimensional stability, barrier (water barrier), and optical (birefringence and transmittance) were analyzed after uniaxial stretching. The films have higher *T*g, *T*m, dimensional stability, and mechanical properties than other polyester-type polymers, and these performance properties are significantly increased with increasing stretching. This is due to the increased orientation of molecular chains inside the films, which was confirmed by differential scanning calorimetry (DSC), X-ray diffraction (XRD), and birefringence results. Good water barrier (0.54%) and lower birefringence (n: 0.09) of PCITN film compared to poly(ethylene terephthalate) (PET), poly(ethylene 2,6-naphthalate) (PEN), and polyimide (PI) films, used as conventional substrate materials for optical devices, make it an ideal candidate as performance material for next-generation flexible devices.

**Keywords:** bio-based; copolyester; dimensional stability; flexible optical devices; uniaxial stretching; birefringence; and barrier properties

#### **1. Introduction**

Biobased polymers have gained the attention of scientists due to the rapid depletion of fossil fuels and increasing environmental pollution [1–4]. With the advancement and innovation of biotechnology and chemical industry, a wide range of biobased monomers such as isosorbide (ISB) [5], lactic acid [6], tannic acid [7], 2,5-furandicarboxylic acid [8], succinic acid [9], etc. have been developed and are being used widely for the development of biobased polymers. However, their poor mechanical and thermal properties limit their industrial applications for engineering polymers.

Among various biomass monomers, isosorbide (1,4:3,6-dianhydrohexitol; ISB) is well known due to its superior performance properties. It is derived from glucose, and its bifunctional hydroxyl groups facilitate the condensation or addition polymerization. Its unique rigid structure and chirality significantly improve the glass transition temperature (*T*g) and transparency of the resulting polymers. Many efforts have been done to develop the ISB-based polyesters [10], epoxies [11], polyurethane [12],

and polycarbonates [13,14], which are not only environmentally friendly but also have superior thermal and optical properties. However, it is very difficult to get a high molecular weight product using ISB alone due to its low reactivity and degradability during the polymerization process.

Poly(ethylene terephthalate) (PET) and poly(ethylene naphthalate) (PEN) are well known conventional homopolyesters. High crystallizability, low *T*g, and inferior barrier properties of PET act as obstacles in its commercial applications, especially at high temperatures (>100 ◦C). PEN has good thermal stability, however, expensive raw materials used for its synthesis, high birefringence, and necking of the film during stretching limit its commercial applications. To replace the conventional polymeric substrate for flexible electronics, materials need to offer ease of processing, high *T*g, good dimensional and thermal stability, good optical, and moisture barrier properties. The properties of the polymers can be significantly improved by blending [15]. The performance properties of the polyester-based composites are significantly higher than copolyesters [16]. However, polyester-based composites are not suitable for the application where transparency and flexibility are required. So, it would be of great interest to develop a biobased transparent and flexible copolyester for versatile industrial applications.

This research focuses on the synthesis of terephthalic acid (TPA), 2,6-naphthalene dicarboxylic acid (NDA), 1,4-cyclohexanedimethanol (CHDM), and isosorbide (ISB)-based copolyester that has high *T*g and good mechanical, thermal, and barrier properties, even with low ISB content compared to conventional polyesters. It is important to note that the low reactivity of ISB prevents the obtaining of high molecular weight polymer products, which limits the commercial application of polymers as engineering plastics [17–19]. To overcome this problem, CHDM was incorporated to facilitate between TPA, NDA, and ISB. Finally, PCTIN copolyesters containing 25 mole% of ISB (feed ratio) with high weight average molecular weight (Mw: 68,900 g/mol) and *T*<sup>g</sup> (121 ◦C) were successfully synthesized by two-step melt polymerization. Novel PCITN copolyester film was fabricated using a melt extrusion process to extend their commercial applications and analyze their performance properties. We significantly improved the thermal, mechanical, dimensional stability, and barrier properties, which is the weak point of bio-based polymers, by utilizing additional monomers: CHDM, TPA, and NDA.

In this study, the fabrication and characterization of biobased novel PCITN film are being reported to explore its versatile industrial applications such as a smart film. The fabricated semi-crystalline film was uniaxially stretched (λ1–λ4) in machining direction (MD) at 150 ◦C. The influence of stretching on thermal, mechanical, dimensional stability, optical, and barrier properties was studied. Analyzed performances were found to increase linearly with increasing stretching, which is due to increased molecular orientation in the stretched films. It is obvious from the results that based on the unique performance properties of PCITN compared to PET, PEN, and Polyimide (PI) [20,21], this novel material has the potentional to be used as a performance material in next-generation flexible electronic devices.

#### **2. Materials and Methods**

#### *2.1. Materials*

TPA (99.9%), ISB (99.8%), and CHDM (99.8%) with 70 mole% trans-isomers were purchased from SK Chemicals (SKC; Suwon, Korea). NDA (99.8%) was purchased from BASF (Ludwigshafen, Germany). The ultra-pure (99%) catalyst (titanium-N-butoxide (TNBT) and thermal stabilizer (phosphorous acid) were used as received from Sigma Aldrich (Soul, South Korea). The high-purity monomers were used as received. High purity (99.9%) solvents, like deuterated chloroform (CHCl3-d) (Sigma Aldrich, Soul, South Korea), deuterated trifluoroacetic acid (TFA-d) (Sigma Aldrich, Soul, South Korea), and o-chlorophenol (Sigma Aldrich, Soul, South Korea) used for the characterization of specimens, were used as received without any further purification.

#### *2.2. Synthesis of Copolyester Followed by Solid-State Polycondensation (SSP)*

#### 2.2.1. Copolyester Synthesis

PCITN copolyester was melt polymerized using two different 5 L batch reactors of the pilot-scale melt polymerization reactor [22]. For esterification, the residual oxygen was removed from the reactor by purging pure nitrogen (N2). All the chemicals: TPA (249 g, 1.5 moles), NDA (973 g, 4.5 moles), CHDM (1089 g, 6.8 moles), ISB (331 g, 2.3 moles) titanium butoxide catalyst (3.906 g; 300 ppm relative to the total expected weight of the product), and phosphorous acid stabilizer (0.4850 g; 100 ppm relative to the total expected weight of the product) were charged into the esterification reactor at a molar ratio of diacid:diol = 1.0:1.5 for PCITN. The temperature of the reaction mixture was increased slowly up to 270 ◦C and held at this temperature until the completion of the esterification reaction. At this stage, water (by-product) generated during esterification reaction has been distilled out. After, this product was transferred into the polycondensation reactor. For transesterification, the temperature was gradually increased to 300 ◦C, the vacuum was reduced to 0.5 Torr, and these conditions were maintained until the completion of the reaction. Torque meter value (Nm) was used to determine the end-point of the reaction. The stirring speed was reduced gradually as the degree of polymerization increased. The polycondensation reaction continued isothermally at 90 RPM, 60 RPM, and 30 RPM until the torque value reached 1.6 Nm at each RPM. Finally, the reactor pressure was returned to atmospheric pressure by purging N2 gas to avoid the oxidative degradation of the polymer. The resulting PCITN copolyester was extruded from the reactor, quenched, and pelletized. The schematic diagram for the melt polymerization of PCITN is shown in Scheme 1.

**Scheme 1.** The schematic diagram for the melt polymerization of PCITN copolyester.

Solid-state polycondensation (SSP) is an important step of the polymer industry as it significantly increases the molecular weight of synthesized polymers making them suitable for a wide range of commercial applications as an engineering plastic [23–25]. SSP of synthesized PCITN copolyester was also carried out at optimized conditions (Temperature and time) under the nitrogen atmosphere. The

synthesized PCITN copolyester was thoroughly characterized for its chemical structure, thermal, and degradation behavior after SSP at 250 ◦C for 20 h.

#### 2.2.2. Film Fabrication and Uniaxial Stretching

The extrusion grade PCITN copolyester having a high molecular weight (*M*w: 68,900) and intrinsic viscosity (IV: 0.77) was successfully fabricated into a film using the melt extrusion process at 290 ◦C. The randomly-oriented film was successfully fabricated (thickness; 550 μm and width; 18 cm) and wound up on a cold stainless-steel winder roll (25 ◦C).

As synthesized, randomly-oriented film specimens were preheated for 25 min before tensile stretching and then uniaxially cold drawn (at 150 ◦C) in MD using a tensile stretching machine with a speed of 1.3 mm/sec. To freeze the orientation of aligned polymeric chains obtained during drawing, the stretched films were cooled down to room temperature using cold air without removing the stress. The films with different draw ratios (λ) (1~4) were thus obtained and they were characterized for their thermal, mechanical and thermal degradation behavior; dimensional stability; and optical and barrier properties.

#### *2.3. Characterization*

The IV (dL/g) of PCITN copolyester was measured in the o-chloroform (OCP) solvent (15 g/15 mL) using an automated Ubbelohde viscometer (no. 1C) at 30 ◦C. The number average molecular weight (*M*n), weight average molecular weight (*M*w), and polymer dispersity index (PDI) were measured by gel permeation chromatography (GPC, Agilent, Santa Clara, California, United States) using m-cresol as a mobile phase at a velocity of 0.7 mL/min. The separation was performed with two Shodex LF804 columns at 100 ◦C equipped with a Malvern TDA 305 refractive index detector. The sample solutions were filtered using a polytetrafluoroethylene microporous membrane (Merck Millipore, 0.45 μm pore size) before injection. *M*n, *M*w, and PDI (*M*w/*M*n) were determined from universal calibration with polystyrene standards.

For the actual chemical composition of synthesized copolyester, proton nuclear magnetic resonance ( 1H-NMR, Bruker Cooperation, Billerica, Massachusetts, United States) spectra were obtained at 25 ◦C by a Unity Inova 500NB High Resolution 500 MHz NMR Console. Deuterated chloroform (CHCl3-d) and deuterated trifluoroacetic acid (TFA-d) (1:1) were used as solvent and tetramethylsilane (TMS) as an internal standard and as reference for chemical shifts (parts per million, ppm). Chemical shifts were expressed in ppm (parts per million) relative to an internal standard (TMS).

Thermal properties of polymer pallets were analyzed by differential scanning calorimeter (DSC Q20, TA instruments, New Castle, Delaware, USA) thermograms. About 8–10 mg of the sample was melted at 300 ◦C and then quenched to 40 ◦C at a cooling rate of 200 ◦C/min. Then, the temperature was raised from 40 to 300 ◦C at a heating rate of 10 ◦C/min under N2 atmosphere. Thermal decomposition behavior was analyzed by thermo-gravimetric analyzer (TGA Q50, TA instruments, New Castle, Delaware, USA) thermograms. For this, the sample placed in the alumina pan of TGA was heated from 40–600 ◦C at a scan rate of 10 ◦C/min under a constant N2 flow of 50 mL/min. The temperature at which 5% weight loss is observed (*T*id5%), the onset temperature of main degradation (*T*d50%), and residue at 600 ◦C (%) were recorded from TGA analysis.

#### *2.4. Characterization of Film Properties*

Fabricated film specimens were thoroughly characterized using DSC for their *T*g, *T*m, cold crystallization temperature (*T*cc), cold crystallization enthalpy (ΔHc in J/g), and the melting enthalpy (ΔHm in J/g). Only, the first heating thermogram (40 to 300 ◦C) of DSC was recorded to study the effect of stretching on the thermal properties of the films. The degree of crystallinity (*X*c) of all the film specimens was also calculated from XRD analysis by the integration of corresponding amorphous and crystalline peaks. For accurate results, fabricated films were analyzed four times. *T*g was estimated as the onset point in heat capacity associated with a transition.

The dimensional stability of the film specimens was determined using a thermo-mechanical analyzer (TMA6100, Seiko Exstar 6000, Seiko Instrument Inc., Chiba, Japan). For this, the values for the coefficient of linear thermal expansion (CTE) of the film specimens (60 <sup>×</sup> <sup>5</sup> <sup>×</sup> 0.1 mm3) were recorded from 30 to 120 ◦C with a heating rate of 5 ◦C/min and analyzed.

The influence of the stretching on the crystalline structure of the films was analyzed by high-power wide-angle X-ray diffraction (XRD, D8 Advance, Bruker Cooperation, Billerica, Massachusetts, United States) spectroscopy. For this, spectrograms of film specimens were obtained using CuKα radiations (λ = 0.154 nm) in the 2θ range of 20 to 50◦ with a scanning speed of 2θ/min. Tensile testing of the developed samples was carried out using a universal testing machine (UTM, E3000, E3000, Instron, Norwood, Massachusetts, USA), with a crosshead speed of 50 mm/min. Samples were prepared (50 mm × 5 mm) according to the standard test method, ASTM D882 [26], before the testing. For the reliability and accuracy of the mechanical data, the film specimens with different draw ratios (λ) were analyzed four times. The tensile strength was measured as the ultimate tensile strength at the fracture point while Young's modulus was measured from the initial slope of the S-S curve (strain < 0.1%). The water barrier (%) property was determined by the cup method by following the standard test method ASTM D570-98 [27]. Before the testing, the film samples were conditioned for 1 week at 23 ± 2 ◦C and 65% RH. The film specimens were placed in the in DI water at 23 ± 2 ◦C and continued to record the values for the wet weight until they reached a steady-state, usually after 11–12 h.

The water absorption (%) of samples was determined using the wet weight (g) and dry weight (g) data as follows:

Water absorption (%) = [Wet weight (g) − Dry weight (g)] ÷ Dry weight (g) × 100%

Optical properties: transmittance and birefringence (Δn) of fabricated films were determined using a UV–Vis spectrophotometer (S-400, Scinco, Seoul, Korea) and a wide range 2-D birefringence analyzer system (WPA-100-L, Photonic Lattice Inc., Sendai, Japan), respectively. The percentage transmittance (%) for wavelengths from 300–800 nm was recorded and analyzed. For Δn measurement, retardation of white light in the absence of any sample between the light source and detector was used as a reference. The retardation of the film sample was measured against the reference and analyzed. The Δn values of film specimens were determined from their retardation (R) and thickness (d) values using the formula as follow:

Δn = R/d

#### **3. Results and Discussion**

#### *3.1. Actual Composition of PCITN Copolyester*

The composition of monomers: ISB, CHDM, TPA, and NDA was estimated from the integration of peak intensities of each proton of each monomer. The actual chemical composition of synthesized PCITN copolyester was determined by 1H-NMR spectroscopy. 1H-NMR and assignments of characteristic chemical shifts (δ) (ppm) are shown in Figure 1. Characteristics δ (ppm) peaks at 4.60, 2.12, 1.95, and 1.35 are assigned to trans-CHDM. While δ (ppm) peaks at 4.70, 2.25, 1.83, and 1.70 ppm are assigned to cis-CHDM isomers. δ (ppm) peaks observed at 4.52, 5.84, 5.54, 5.10, 5.80, and 4.36 ppm are assigned to hydrogen atoms 1, 2, 3, 4, 5, and 6 of the ISB linked with terephthalic units. The peaks for 1 and 6 overlap with the hydrogens (11) of the CHDM moiety. δ (ppm) peaks observed at 4.50, 5.73, 5.50, 5.06, 5.78, and 4.34 ppm are assigned to hydrogen atoms 1 , 2 , 3 , 4 , 5 , and 6 of ISB between the naphthalic unit and the terephthalic unit. The peaks for 1 and 6 overlap with the hydrogens (11) of the CHDM moiety.

**Figure 1.** 1H-NMR spectra of PCITN copolyester.

The δ peaks at 3.40~3.81 ppm are assigned to protons of CH2OH at the chain end of the polymer chain. The δ peaks of diacid moieties: TPA (7) and NDA (8 and 9) appear at 8.11~8.21 ppm. However, the δ peak at 8.73 ppm represented by 10 in the figure is assigned to the hydrogen atom of NDA, which enables us to calculate the actual chemical composition of diacid moieties present in PCITN copolyester. The amount of each monomer in PCITN copolyester was determined by the integration of their corresponding δ peaks. Analyzed results (Table 1) indicate synthesized PCITN has a higher amount of CHDM and TPA than feeding monomers, which indicates that CHDM and TPA have higher reactivity than isosorbide and NDA, respectively. Our findings are also supported by the previous research works [28–30].

**Table 1.** Characteristics of synthesized PCITN copolyester pallets


<sup>a</sup> Glass transition temperature (*T*g), cold crystallization temperature (*T*cc), melting temperature (*T*m), and melting enthalpy ΔHm (J/g) were determined by differential scanning calorimeter (DSC). <sup>b</sup> Number average molecular weight (*M*n, g/mol), weight average molecular weight (*M*w, g/mol), and polymer dispersity index (PDI) were determined by gel permeation chromatography (GPC). <sup>c</sup> The intrinsic viscosity (IV) was determined in o-chloroform (OCP) solvent by using automated Ubbelohde viscometer (no. 1C) at 30 ◦C. <sup>d</sup> Actual chemical composition of each monomer in PCITN was determined by proton nuclear magnetic resonance (1H-NMR) spectroscopy.

#### *3.2. Mechanical Behavior of PCITN Films*

Tensile testing of the film specimens with different draw ratios (λ) was conducted and the representative stress-strain (S-S) curves are shown in Figure 2. PCITN films showed high tensile strength, as can be seen in Figure 2a. The mechanical properties of as-synthesized PCITN film (λ1) show the ultimate tensile strength as 50.5 MPa and strain as 35.8%. In the case of films having different stretching (λ) such as λ2, λ3, λ3.5, and λ4, the values for the tensile strength are 71.2, 101.5, 132.8, and 154.5 MPa, respectively. It is obvious from the results that as the λ is increased, both tensile strength (MPa) and Young's modulus (GPa) are also increased (Figure 2b). Initially, tensile strength increased linearly with increasing stretching (λ1–λ3) due to an increment in the molecular orientation of the polymeric chain along the stretching direction. Then, a rapid increment in tensile was observed at higher stretching (λ3–λ4) due to the stress-induced crystallization (SIC). Additionally, the film with the highest stretching (λ4) exhibited the lowest strain at break. It can also be seen from the S-S curve that a decrement in the strain at break is observed with increasing λ. These findings can be attributed to the higher degree of molecular orientation during the stretching of the films. Lee et al. and Hoik et al. also found that mechanical properties of the polymeric materials are directly influenced by the molecular orientation [31,32]. Moreover, the unique V-shaped structure of ISB with a 120◦ angle between the rings causes the hindrance of polymerization of polymer chains [33]. So, mechanical performance can be affected by the orientation of polymeric chains.

**Figure 2.** Mechanical properties of cold-drawn PCITN films: (**a**) Tensile strength and (**b**) Young's modulus as a function of draw ratio (λ).

#### *3.3. Direct Evidence of SIC by High Power X-ray Di*ff*raction (XRD) Analysis*

The XRD spectrum of the developed PCITN film specimens at the ambient room temperature from 5 to 50 is shown in Figure 3. Three diffraction peaks for the randomly oriented PCTIN film with λ1 that appeared at 7.84◦, 18.72◦ and 42.90◦ are corresponding to the Miller indices of (001), (112) and (105), respectively. The peaks observed at 18.70◦ and 42.92◦ are the characteristic peaks of amorphous regions, while the peaks observed at 7.84◦ are corresponding to the crystalline regions of undrawn (DR1) film. This indicates that as-synthesized PCITN film is semi-crystalline, and it has relatively low crystalline regions compared to stretched films. However, based on the XRD analysis, the reduction in amorphousness of PCITN film can be seen as the intensity of the amorphous peaks decreases with increasing λ. The new peaks are observed at 15.76◦, 17.68◦, 20.12◦, and 23.08◦ at higher stretching (λ > 3), indicating that at the microscopic level the regular arrangement of molecular chains of the films increases with increasing stretching. These peaks are attributed to the new crystallites that appeared due to stretching and they are attributed to the Miller indices of (010), (111), (110), and (100), respectively [34,35]. It indicates the appearance of the new crystalline regions at the expense of amorphous regions due to the SIC at higher stretching (λ > 3).

**Figure 3.** X-ray diffraction pattern of PCITN films stretched at different draw ratios (λ).

#### *3.4. Thermal Properties and SIC Analysis of PCITN Film*

The unique chemical structure of ISB with high thermal stability and low segmental mobility makes it an interesting building block for a polymer having high thermal properties, especially the *T*g. Thermal properties of the developed uniaxially stretched PCITN films with different λ were analyzed (Figure 4), and the results are summarized in Table 2. It is obvious from DSC analysis that the thermal properties and *X*<sup>c</sup> are directly influenced by the λ of the film. *T*<sup>g</sup> was improved linearly while both *T*cc and ΔHcc were reduced with increasing λ. The gradual reduction in ΔHcc indicates that thermally induced crystallites are reduced, and stress-induced crystallites are increased with increasing λ. ΔHm (J/g) and *X*<sup>c</sup> (%) also confirm the appearance of SIC at higher stretching (λ >3). Initially, the thermal properties and crystallinity of the stretched films increase linearly with λ due to the gradual growth of molecular chains along the stretching direction. Then, a rapid increment is observed at higher stretching, which can be attributed to new crystallites due to SIC. The effect of the stretching (λ) on thermal and *X*<sup>c</sup> (%) behavior observed in this study can be due to increased symmetry of polymeric chains, which was improved significantly by stretching. The beauty of this analysis is that its findings are compatible with the previously reported research [36–38].

**Figure 4.** DSC thermograms of the fabricated PCITN films.

**Table 2.** Influence of stretching on the thermal properties and *X*c of PCITN film.


#### *3.5. Thermal Stability of the Fabricated PCITN Films*

Thermal degradation behavior of the fabricated PCITN films stretched at different λ were determined using TGA and the corresponding thermograms, indicating the high thermal stability, are shown in Figure 5. The temperature corresponding to 5% wt loss of the initial weight and residue (%) at 600 ◦C were recorded from TGA thermograms, and the results are summarized in Table 2. It is evident from the TGA thermograms that all the analyzed samples have a one-stage decomposition, indicating that they have a random copolymer structure. The residue (%) at 600 ◦C is increased with increasing λ due to increased crystallinity (%) of the resultant stretched polymeric films. The same finding was reported by Um et al. [39]. The high thermal stability of the fabricated film makes it suitable for applications where high thermal stability is required.

**Figure 5.** TGA thermograms of the fabricated PCITN films.

#### *3.6. Improving the Dimensional Stability of Uniaxially Oriented Films*

Dimensional stability is one of the most important parameters of polymer films for their various industrial applications, especially in the field of flexible electronic devices. The dimensional stability of the fabricated films was determined as their CTE behavior, and the results are shown in Figure 6. It is found that the thermal stability of the PCITN film improves with increasing λ. The least thermal expansion of PCITN film with λ4 indicates that it has the highest dimensional stability, which is due to the higher degree of molecular orientation in this film than other films. Just like conventional plastic films, novel PCITN films also show an undesirable dimensional change around *T*g. This phenomenon can be attributed to the facts: (a) the molecular relaxation due to the increased mobility of polymer chains and (b) a rapid shrinkage or expansion due to the residual frozen stress within the stretched film structure. Compared to stretched films, the unstretched film expands abruptly around its *T*<sup>g</sup> due to increased mobility of molecular chains. CTE behavior of the PCITN films was improved after uniaxial stretching. It can be attributed to the fact that the orientation of molecular chains is significantly improved with increasing λ. It is also reported by the previous researches that that increased molecular orientation improves the dimensional stability of the resultant films [40,41].

**Figure 6.** The coefficient of linear thermal expansion (CLTE) plots between 30–120 ◦C for the uniaxially stretched PCITN films.

#### *3.7. Water Barrier Property*

It is well known that good barrier properties of smart polymeric films are critical for their industrial applications especially for next-generation flexible electronic applications. The results for the water barrier of fabricated PCITN in comparison with commercial PET and PEN films are shown in Figure 7. It shows that PCITN film has a better water barrier compared to conventional homopolyester films. This superior water barrier is due to the presence of rigid ISB and cycloaliphatic CHDM units present in the synthesized PCITN films compared to PET and PEN films. This superior water barrier is due to the presence of rigid ISB and cycloaliphatic CHDM units present in the synthesized PCITN films compared to PET and PEN films. It is also obvious from Figure 7 that water barrier property is also improved with increasing λ. The increased water barrier of PCITN film with increasing λ is due to the well-known facts of the improved orientation of polymeric chains and the appearance of new crystallites due to SIC at higher stretching. It is important to note that our findings of water barrier properties are also supported by XRD and DSC results (Sections 3.3 and 3.4). It is important to note that our findings of water barrier properties are also supported by XRD and DSC results (Sections 3.3 and 3.4).

**Figure 7.** Comparison of the water absorption (%) of developed PCITN films and commercial PET and PEN homopolyester films.

#### *3.8. Birefringence of As-Synthesized and Uniaxially Stretched PCITN Films*

The birefringence can be used to evaluate the molecular orientation and relaxation phenomena of the polymer films. To analyze the orientation development during uniaxial cold-drawing of PCITN films, birefringence has been measured for drawn PCITN films, and the results in comparison with uniaxially stretched PET film [37] are shown in Figure 8. It can be seen from the results (Figure 8) that like PET film, PCITN film also exhibits three regimes during stretching. In the first regime, for λ values up to 1.0~2.5, birefringence increases linearly with increasing λ. In regime II, for λ values up to 3.0~4.0, a rapid increment in the birefringence is observed. In regime III (λ > 4), birefringence reaches its saturation where further stretching cannot cause a significant variation in birefringence. The birefringence results indicate how the molecular orientations are developed during the uniaxial stretching of PCITN. In regime I, the birefringence of isotropic and randomly-oriented films increases linearly with increasing λ due to the steady growth of molecular chains along the stretching direction. An abrupt increment of birefringence in regime II is attributed to the development of SIC phases and the reduction of amorphousness of the stretched films. In regime III, birefringence reaches its saturation due to the achievement of maximum alignment polymer chains. Our results for the birefringence are also supported by the previous research works [37,42]. Our research indicates that novel PCITN film with lower birefringence can be used as an alternative performance material to conventional PET and PEN films for optical applications (PEN > PET > PCITN) [43].

**Figure 8.** Influence of uniaxial stretching on the birefringence of PCITN films in comparison with uniaxially stretched PET film.

It is worthy to note that the analyzed properties of the developed PCITN film are superior or comparable to the conventional polymeric substrates for flexible electronic devices (Table 3) [20,37,44–46]. Uniaxially stretched PCTIN has better optical (Yellow color vs transparent) and water barrier properties (water absorption; 1.8% vs 0.20%) than PI film [47]. Compared to conventional polymer (PET, PEN, and PI), fabricated semi-crystalline PCITN film has unique performance characteristics such as high *T*g, wide processing window, good thermal degradation behavior, low CTE, good water barrier, and low birefringence, which make this novel smart film an ideal substrate for next-generation flexible electronics.


**Table 3.** Characteristics of uniaxially stretched PCITN film in comparison with PET, PEN, and PI films; conventional polymeric substrate for flexible electronics

The analyzed performance properties of PCITN can be further improved by heat setting and biaxial stretching. The availability of raw materials, ease of synthesis, thermal stability, and wide processing window of PCITN make it suitable for scale-up at the industrial scale. So, it is very easy to develop PCITN film on a commercial scale by adopting the reported methods.

#### **4. Conclusions**

A novel PCITN copolyester containing a biobased monomer, isosorbide, with a high *T*g and wide processing window was synthesized by melt polymerization. The unique structure of ISB increased the rigidity and *T*g of synthesized copolyester. PCTIN films were successfully fabricated and thoroughly characterized for their thermal, mechanical, optical, and barrier properties after the uniaxial cold drawing (λ = 1~4) at optimized conditions. The performance properties of PCITN were significantly improved due to the increase in molecular chain orientation with increasing λ. The good thermal behavior, high strength, good dimensional stability, good barrier, and optical properties are very useful for polymers as a smart film for industrial applications. The unique characteristics of PCITN compared to conventional polymers make it at futuristic performance material for flexible devices.

**Author Contributions:** Conceptualization, J.J. and F.H.; methodology, J.J. and F.H.; validation, S.P.; formal analysis, J.J. and F.H.; investigation, J.J. and F.H.; writing—original draft preparation, J.J. and F.H.; writing—review and editing, J.K.; supervision, S.-J.K. and J.K.; All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** The author would like to express appreciation to BASF for financial support.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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