Bio-based thermoplastics are derived from renewable organic materials and are recyclable after their usage. Their strength can be enhanced by combining them with other thermoplastics. Nonetheless, sometimes the junction where these materials meet needs to be improved to achieve the best properties. A research group from Eindhoven University of Technology in the Netherlands has explored at BESSY II how a new method allows for the creation of thermoplastic blends with strong interfaces using two base materials. Observations made at the latest nano station of the IRIS beamline revealed that during this process, nanocrystalline layers develop, thereby boosting the performance of the resulting materials.
Bio-based thermoplastics are seen as eco-friendly options because they originate from non-petroleum-based resources and can be recycled like conventional thermoplastics. An example of such a thermoplastic base material is Polylactic acid (PLA), which can be made from sugarcane or corn. Researchers globally are striving to enhance the characteristics of PLA-based plastics, including blending them with other thermoplastic materials. However, this presents a significant challenge.
A new method for improved blends
A team led by Prof. Ruth Cardinaels at TU Eindhoven has demonstrated effective mixing of PLA with another thermoplastic. They invented a method during production that generates certain PLA-based copolymers (e.g., SAD), which promote the blending of the two materials by forming particularly stable (stereo)-crystalline layers at the boundaries of the different polymer phases, a technique referred to as the ICIC strategy.
Discoveries at the IRIS-Beamline
At BESSY II, they uncovered the processes that result in significantly improved mechanical properties of blended thermoplastics. They examined pure 50% blends of PLA and polyvinylidene fluoride (PVDF), including samples with PLA-based copolymers, at the IRIS beamline of BESSY II.
Stereocomplex crystals at the boundaries
Using infrared spectroscopy at the IRIS beamline, PhD student Hamid Ahmadi was able to show the creation of the PLA-based copolymer SAD. Subsequent X-ray analysis revealed how SAD influences the crystallization process. The advanced nano imaging and spectroscopy capabilities at this beamline allows for precise chemical analysis and identification of sample areas as small as 30 nm. This level of detail was essential in determining that the stereocomplex crystals were exclusively found at the interface, with infrared nanoscopy images revealing a layer of stereocomplex crystals measuring 200-300 nm thick at these boundaries.
Factors contributing to enhanced stability
The presence of stereocomplex crystals at the interfaces boosts stability and increases the crystallization temperature. The nucleation that occurs at the interface speeds up the overall crystallization process of the PLLA/PVDF blend. Furthermore, the crystalline layer at the interface enhances the transfer of mechanical stresses among the phases, resulting in improved tensile properties; the elongation at break can increase by as much as 250%.
“By identifying the location and distribution of the crystalline layer in our samples, we gained a better understanding of the mixing process,” says Hamid Ahmadi. “With our new strategy, we have paved the way for the development of high-performance polymer blends,” adds Ruth Cardinaels.
