Synthesis and unique characteristics of biobased high Tg copolyesters with improved performance properties for flexible electronics and packaging applications
Graphical abstract
Introduction
Biobased polymers have gained significant attention in the polymer industry due to their environmentally friendly properties [1], [2], [3]. They have many advantageous properties such as energy conservation, biodegradation, reduction in carbon dioxide, and environmental pollution compared to petroleum-based conventional polymers [4], [5]. They have offered material scientists a unique opportunity to replace conventional hazardous polymers and are being used in various industrial applications such as film, sheet, wound dressing, cosmetics, etc. [6], [7]. However, the poor thermal, barrier and mechanical behaviors restrict their versatile commercial applications as an engineering plastic.
Among various biobased materials, isosorbide (1,4:3,6-dianhydrohexitol; ISB) is well known renewable monomer which has the potential to be used for addressing environmental issues. Due to the unique rigid structure and bifunctional hydroxyl groups, it has been used for the development of biobased polymers, pharmaceuticals, and cosmetics [8], [9]. The ISB based polymers and copolymers are transparent in nature and exhibit high glass transition temperature (Tg) due to the unique molecular structure and chirality of ISB [10], [11]. A lot of literature is available for the development of environment-friendly polyesters, epoxides, and polyurethane containing biobased ISB moiety [12], [13], [14]. Superior performance properties of biobased polymers containing ISB such as Tg, thermal, mechanical, and optical properties make them ideal candidates to address the environmental issues and demanding requirements of the commercial market. However, when ISB is used alone, due to the low reactivity and thermal degradation it becomes very difficult to synthesize high molecular weight polymeric product during the melt synthesis process.
Poly(ethylene terephthalate) (PET) with acceptable performance properties has a strong position among various types of commercially available conventional aromatic semicrystalline polymers. However, high crystallinity and low Tg limits its commercial application as an engineering plastic especially at elevated temperature (higher than 100 °C). When linear diol; ethylene glycol of PET is replaced with thermally stable 1,4-cyclohexandimethanol (CHDM), the resulting PCT homopolyester has a much higher Tg (88 vs 79 °C) and melting temperature (Tm; 300 vs 260 °C) than conventional PET. However, due to a higher Tm and limited processing window of PCT, it undergoes thermal and thermo-oxidative degradation during the melt polymerization, which results in low molecular weight and brittle product [15]. The thermal properties of PET can also be significantly enhanced by replacing terephthalic acid (TPA) with rigid and thermally stable naphthalene dicarboxylic acid (NDA), the resulting homopolyester poly(ethylene naphthalate) (PEN) homopolyester has a much higher Tg (around 120 vs 79 °C) and Tm (270 vs 260 °C) compared to PET. Soon after the discovery of PEN in 1969 [16], this polymer finds a strong position as a performance material for versatile industrial applications due to its superior thermal, barrier, electrical, and mechanical properties. However, high raw material cost, inferior optical behavior, and necking of PEN film act as barriers for its industrial applications.
Novel copolyesters containing thermally stable diacid moieties (TPA, and NDA), and cyclic diol moieties (CHDM and biobased ISB) (PCITN, shown in Scheme 1) with high Tg, good mechanical, optical, and barrier properties can be a potential environmentally friendly material which can contribute to controlling of the environmental pollution, depletion of fossil foils and energy sources. The low reactivity of ISB acts as a hindrance to getting high molecular weight polymeric products [10], [17]. In order to get a higher molecular polymeric product during the melt synthesis, a second diol; CHDM was used to facilitate between diacid moieties (TPA, NDA) and ISB. Conclusively, in the first step of this research work, a series of PCITN with different content of NDA, ranging from 0 to 100 mol% with a fixed amount of ISB content, 25 mol% (feed ratio) were synthesized. In the second step, the best composition (PCITN70) of synthesized copolyesters was selected and the effect of ISB content on the thermal properties was analyzed. Based on the exceptional thermal properties, PCI6TN70 was successfully fabricated into film and its thermal, dimensional stability, optical, and barrier properties were compared with conventional polymeric substrate materials (PET & PEN) used in the flexible optical and electronic devices. Compared to conventional polyesters, synthesized PCITN copolyesters have a wide processing window, high Tg, good thermal, optical, and barrier properties which is a weak point of general biobased polymers. It is important to note that synthesized PCI6TN70 can be used in the form of smart film, nanofibers, or any desired shape and its exceptional performance properties make it suitable for versatile unique applications such as; medical, optical devices, flexible electronics, and packaging.
Section snippets
Materials
Purified monomers: naphthalene dicarboxylic acid (NDA) (99.9%), terephthalic acid (TPA) (99.9%), Isosorbide (ISB) (99.8%), and 1,4-cyclohexandimehtanol (CHDM, Cis: Trans isomers, 30:70) (99.8%) were kindly supplied by SK Chemicals (Korea). The titanium-N-butoxide (TNBT, 99%) and phosphorous acid (99%), deuterated chloroform (CHCl3-d, 99.9%), deuterated trifluoroacetic acid (TFA-d, 99.9%), and o-chlorophenol (OCP, 99.9%) were purchased from Sigma-Aldrich, Korea. All the monomers and chemicals
Solid-state polymerization of the synthesized resins
There are versatile industrial applications that require a high molecular weight of polymers. SSP at optimized condition (Temperature and time) significantly improve the physical, chemical, thermal, and mechanical properties of the resultant product since it improves the regular arrangement of the molecular chains and amorphous regions are reduced [21], [22], [23]. All the synthesized PCITN copolyesters were thoroughly characterized for their structure, degradation behavior, and thermal
Conclusion
This research work has shown that novel copolyesters (PCITN) of ISB, CHDM, TPA, and NDA monomers, with high Tg, wide processing window, and high thermal stability can be synthesized by pilot-scale melt polymerization reactor. SSP at optimized temperature and time significantly improves the thermal properties of the synthesized copolyesters. DSC and TGA results showed that all the copolyesters were semi-crystalline and random copolyesters. It was also found that ISB noticeably improves the Tg of
Declaration of interests
Authors have no interests to declare.
Acknowledgments
The authors would like to express appreciation to BASF and HEC Pakistan (Ref. No. 463/IPFP-II(Batch-I)/SRGP/NAHE/HEC/2020/110) for financial support.
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