In PET bottles, polymers are entwined in a disorderly manner. Accordingly, the longer a polymer chain is, the stronger the bond is, and the shorter a polymer chain is, the weaker the bond is. Although this principle generally holds in solid-state physics, it is clear that a different theory is applicable for the surface of the material. These findings were revealed by an international collaborative research team composed of Lead Professor Keiji Tanaka and Associate Professor Daisuke Kawaguchi of Kyushu University, Faculty of Engineering. The findings of this research were published in Nature.
Since polymer materials such as plastics exist in a glass state rather than as crystals, the time-temperature conversion law, whereby the elastic modulus changes constantly with time and temperature above the transition temperature, is applicable to these materials. This means that the polymer strength is dependent on the length of the polymer chain. However, a nanocreep test conducted by the research group on polystyrene revealed that the time-temperature conversion law did not hold true at the surface layers, and even if the polymer chain was short, it was firmly bonded. This is because when only the surface temperature is high, the ends of the polymer chains are strongly bonded to the low-temperature internal structure, resulting in pseudo entanglement.
Professor Tanaka commented, "Researchers in the industry expressed that they had always suspected this pseudo-entanglement phenomenon, although they did not understand the reason. Using this system, we would like to develop a mechanism that allows polymer materials to be linked together and peeled off as needed."
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