Architected Sustainability: Enhancing Biobased Copolyesters for Film Applications Through Pentanediol and Branching Agents

Linear low-density polyethylene (LLDPE), in particular, has become indispensable in the flexible packaging industry because of its durability, chemical resistance, and low cost. Yet, its end-of-life options remain alarmingly limited: it neither biodegrades under natural conditions nor recycles efficiently within current waste systems. As concerns about microplastic accumulation intensify, particularly in soils and aquatic ecosystems, the demand for truly sustainable alternatives is no longer just an academic interest—it’s a societal imperative. This study emerges against that backdrop, at a time when materials scientists are being asked not just to improve performance, but to rethink what polymers can be. In recent years, biodegradable aliphatic–aromatic copolyesters have attracted attention for their potential to bridge the gap between environmental compatibility and mechanical robustness. Among them, poly(butylene adipate-co-terephthalate) (PBAT) has gained commercial ground due to its flexibility and composition. However, PBAT still relies on petrochemical monomers and falls short in stiffness, a key limitation in certain applications. To overcome these limitations, in new research paper published in Polymer Engineering & Science, the team led by Professor Brian Grady at the University of Oklahoma—with contributions from researchers Hesham Aboukeila, Onkar Singh, John Klier, and Professor George Huber at the University of Wisconsin–Madison—designed a biobased polyester that could deliver comparable or improved performance. Their approach centered on replacing 1,4-butanediol with 1,5-pentanediol (PeD), a five-carbon diol that can be sourced renewably from biomass. Because of its chemical structure, PeD introduces variations in flexibility and packing efficiency, with the potential to enhance mechanical stiffness while maintaining elasticity. But monomer substitution was only part of the strategy. To fine-tune processability, especially for film-blowing applications, the researchers introduced controlled branching through the incorporation of glycerol and hexane-1,2,5,6-tetrol which can emulate the melt strength and viscoelastic response of LLDPE—characteristics critical to industrial extrusion processes. However, the team was also aware that such modifications might interfere with crystallization kinetics. Addressing this delicate balance required a comprehensive, multi-technique investigation to assess how branching, composition, and morphology intersect to define the final polymer’s utility.

The authors synthesized a high molecular weight poly(pentylene adipate-co-terephthalate), or PPeAT60, dialing in the monomer feed at a 40:60 molar ratio of adipic to terephthalic acid. That balance wasn’t chosen arbitrarily—it was aimed at pushing the thermal transitions closer to LLDPE’s, giving the authors a better shot at compatibility with film-blowing applications. But thermal tuning alone wasn’t going to get us there. They also incorporated branching agents—glycerol and HTeO—during polycondensation, hoping they’d quietly adjust the melt behavior without throwing crystallization entirely off balance. After synthesis, the research team turned to characterization. GPC results were reassuring: all formulations, branched and unbranched, came in well above 140,000 g/mol, hitting the threshold they needed for melt processing. TGA showed they could tolerate heat up to ~350°C which suggested no red flags for thermal degradation. But when authors ran DSC, they observed that the branched variants showed slightly lower melting points and enthalpies which indicated they introduced some disruption in the crystal structure, even with modest branching. Then came the crystallization data, which was hard to ignore. Unlike PBAT or LLDPE, PPeAT60 just didn’t crystallize readily under standard cooling. In the second DSC heat, the melting peak had all but vanished. Isothermal runs which confirmed that the crystallization was slow, and branching made it slower. And yet, despite the soft crystallinity, mechanical performance held steady. Linear PPeAT60 actually doubled the stiffness of PBAT while keeping similar tensile strength and elongation. Rheology gave the researchers more to think about. Glycerol-branching boosted zero-shear viscosity, likely from more entanglements. And in extension, they saw clear strain hardening—something LLDPE didn’t show at all. Still, the branched samples paid a price in lower elongational viscosity. As they suspected, tuning melt strength through branching came with trade-offs. But the direction felt promising.

What makes the new study truly impactful is the way it challenges a long-standing assumption in polymer science—that when it comes to sustainability and performance, you can’t have both. Here, Professor Brian Grady and colleagues demonstrated that it’s actually possible to move toward environmentally responsible materials without forfeiting the mechanical functionality that industries depend on. By replacing conventional butanediol with 1,5-pentanediol—a monomer derived from renewable biomass—they were able to maintain, and in some respects improve, critical physical properties. Notably, stiffness increased in the pentanediol-based copolyesters, even as flexibility was largely preserved. Equally important are the authors’ findings around melt behavior. The introduction of low levels of branching through glycerol and HTeO didn’t just slightly alter the structure—it meaningfully shifted how the material responded under shear and elongational flow. Melt viscosity increased modestly, and more notably, they observed clear strain hardening in extensional tests. This is precisely the kind of rheological behavior that improves film blowing performance, especially in preventing premature bubble breakage. While these modifications were subtle, their impact on processing stability is promising. They highlight an approach where structural fine-tuning at the molecular level can be used to engineer macroscale processability—a rare but powerful convergence.

Additionally, while the study initially highlighted the slow crystallization kinetics of PPeAT60—particularly in branched variants—as a potential barrier to scale-up, it’s encouraging to hear that this issue has now been successfully addressed. According to Professor Grady, the team has since identified a solution that’s already been filed as a patent. This advancement suggests that the material’s scalability for industrial film processing is indeed feasible. That development not only strengthens the case for PPeAT60 as a viable LLDPE alternative, but also speaks to the momentum behind this line of research. Rather than just checking a box for sustainability, the work reflects a growing mindset in polymer science—one that embraces the complexity of balancing mechanical performance with environmental responsibility