New process strengthens thermoplastic and polylactic acid blends

In nano-IR imaging, the layer structure of the pure PVDF/PLLA mixture (left) and the layer with SAD additive (right) can be clearly distinguished. Light and dark colors correspond to the PLLA and PVDF phases, respectively. Adding SAD reduces the domain size in two phases. Credit: Eindhoven University of Technology/HZB
Bio-based thermoplastics are manufactured from renewable organic materials and can be recycled after use. Blending bio-based thermoplastics with other thermoplastics can improve their elasticity. However, the interfaces between materials within these blends may need to be strengthened to achieve optimal properties.
A team from Eindhoven University of Technology in the Netherlands is currently researching at BESSY II how a new process can produce thermoplastic blends with high interfacial strength from two base materials. Images taken at the new nanostation at the IRIS beamline showed that: During the process, a nanocrystalline layer is formed, which improves the performance of the material.
Bio-based thermoplastics are considered environmentally friendly because they are sourced from non-petroleum-based raw materials and can be recycled like standard thermoplastics. The thermoplastic substrate is polylactic acid (PLA), which can be made from sugar cane or corn. Researchers around the world are working to optimize the properties of PLA-based plastics, including by blending them with other thermoplastic substrates. But this is a really difficult challenge.
Now, a team led by Professor Ruth Cardinaels at the Eindhoven University of Technology has shown how PLA can be successfully mixed with another thermoplastic. They developed a process in which certain PLA-based copolymers (such as SAD) are formed during manufacturing. This results in the formation of a particularly stable (steric) crystalline layer at the interface between the different polymer phases, which facilitates the mixing of the two raw materials (ICIC strategy). The research is published in the journal ACS Applied Materials & Interfaces.
With BESSY II, we have discovered a process that ensures that the mechanical properties of blended thermoplastics are significantly improved. To that end, we examined samples containing pure 50% blends of thermoplastic PLA and polyvinylidene fluoride (PVDF), as well as PLA-based copolymers, at the IRIS beamline of BESSY II.
Stereo complex crystal at interface
Using infrared spectroscopy at the IRIS beamline, PhD student Hamid Ahmadi was able to demonstrate the formation of a PLA-based copolymer SAD. Further X-ray measurements showed how the formation of SAD affects the crystallization behavior.
The new nanoimaging and spectroscopy capabilities of the IRIS beamline enable advanced chemical visualization and identification from sample areas as small as 30 nm. This precision was critical in determining that stereocomplex crystals were present only at the interface. Infrared nanoscopy images showed a 200–300 nm thick layer of stereocomplex crystals at the interface.
Reason for increased stability
The formation of stereocomplex crystals at the interface increases the stability and crystallization temperature. Nucleation at the interface accelerates the overall crystallization process within the PLLA/PVDF blend. Moreover, the interfacial crystalline layer improves the mechanical stress transfer between the phases and thus improves the tensile properties. Elongation at break increases by up to 250%.
“By elucidating the location and distribution of crystalline layers within the sample, we can now better understand the mixing steps,” Ahmadi says.
“By developing a new strategy, we have paved the way for the development of high-performance polymer blends,” Cardinaels added.
Further information: Hamid Ahmadi et al, “Toughening Imiscible Polymer Blends: The Role of Interface-Crystallization-Induced Compatibility Explored Through Nanoscale Visualization,” ACS Applied Materials & Interfaces (2024). DOI: 10.1021/acsami.4c10829
Provided by Helmholtz German Research Center Association
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