A collaborative study by researchers from three Indian Institutes of Technology (IITs) has found that adding tetrapod-shaped nanoparticles to certain synthetic plastics can significantly reduce their viscosity, making them easier and less energy-intensive to process.
Plastics owe their versatility to long molecular chains called polymers, which make them moldable and stretchable. However, many synthetic plastics with heavy, long chains become extremely thick when molten -- a property scientists call high viscosity -- making them difficult and costly to process.
A recent collaborative study titled, 'Nanotetrapods promote polymer flow through confinement induced packing frustration' by researchers from IIT Bombay, IIT Madras, and IIT Kanpur demonstrated that mixing tetrapod-shaped nanoparticles, tiny particles resembling the four-armed concrete structures used as sea wave breakers, into polymers can improve their flow. The effect was tested on polystyrene (PS), a polymer whose physical and rheological properties are well understood.
The research was led by Professor Mithun Chowdhury, who heads the Lab of Soft Interfaces at IIT Bombay, with collaborations from Professors Anindya Datta (IIT Bombay), Tarak K. Patra (IIT Madras), and Sivasurender Chandran (IIT Kanpur). The experimental work and analysis were done by Jotypriya Sarkar, Mithun Madhusudanan, Harshit Yadav, Dr. Fariyad Ali from IIT Bombay, and Dr. Sachin M. B. Gautham from IIT Madras.
"This study opens a pathway to potentially lower processing energy in the future, if we can mass synthesise precisely shaped sustainable nanoparticles," said Mr. Chowdhury, who heads the Lab of Soft Interfaces at IIT Bombay and led the research.
The idea, he said, came from a casual observation, "During a walk along Marine Drive, we saw the large concrete tetrapods used to break waves. That sparked a question: what if we used tiny versions of these shapes in thick polymer fluids?" Mr. Chowdhury thought of testing out tetrapods because of their unusual geometry. Nanoparticles of other shapes, such as spheres or rods, are known to increase viscosity rather than reduce it, he said.
Unlike spherical or rod-shaped nanoparticles, which typically increase viscosity, tetrapods reduced it. "The simulations showed that the inner curvatures of a tetrapod create regions that long polymer chains find unfavourable to enter," he explained. "This causes the lowering of the number of polymers around the nanotetrapod and thereby lets polymer chains slide past one another more easily," he added.
A visualisation of how polymer molecules behave differently around different nanoparticle shapes; the threads are polymer chains, and the red shapes are nanoparticles. Notice the lower density around tetrapods. | Photo Credit: Authors of the study
The team sourced cadmium-selenium (CdSe) tetrapods from Professor Anindya Datta's lab at IIT Bombay and incorporated them into polystyrene. Control experiments with spherical and rod-shaped CdSe nanoparticles confirmed that only tetrapods improved flow. Importantly, the addition did not compromise the polymer's mechanical or thermal integrity.
The findings also suggest that nanoparticle shape could potentially be used to tune how plastics flow. "Many applications, like coatings, adhesives, or 3D printing resins, require specific viscosity for shape retention or load bearing. There are plenty of examples of nanoparticles increasing viscosity, but our study shows it can go both ways. Compact particles like spheres or roads can thicken, Mr. Chowdhury explained.
The team is currently exploring ways to scale up the process for preparing polymer-nanoparticle composites and adapt it to different types of polymers. Key challenges remain, including large-scale nanoparticle synthesis and replacing toxic materials such as cadmium with more environmentally friendly alternatives.
"Future work will extend this to other polymers and more complex nanoparticle geometries," added Mr. Chowdhury. In the future, the group aims to develop models, using AI or machine learning techniques, to predict the behaviour and flow patterns of polymer-nanoparticle composites based on nanoparticle geometry.