Strain-induced crystallization (SIC) is a phenomenon in which an initially amorphous solid material undergoes a phase transformation due to the application of strain. Polymers are subjected to external flow fields during their manufacture where polymer chains undergo a stretching process that causes the development of strain-induced crystallites, different crystal morphologies, and the orientation of polymer chains within the amorphous phase. A similar phenomenon is exhibited in natural rubber whereby the underlying physical mechanism was identified by Flory (1974 Nobel Prize for chemistry). Due to the high applicative importance of natural rubber, this phenomenon has been widely investigated. In particular, in situ, real-time X-ray diffraction experiments have been developed, while other theories that have been proposed still essentially rely on the basic Flory’s idea that chain stretching promotes crystallization. However, in most practical situations, SIC is almost entirely driven by kinetic phenomena, which were not addressed explicitly by Flory and therefore call for further research.
On this account, CNRS scientists: Dr Paul Sotta and Dr Pierre-Antoine Albouy, from the Advanced Polymers and Materials Laboratory (Laboratoire Polymères et Matériaux Avancés), the Solid State Physics Laboratory (Laboratoire de Physique des Solids) and the Laboratory Engienering of Polymer Materials (Ingénierie des Matériaux Polymères) in France explored the theory of SIC as developed by Flory within a somehow innovative perspective, in analogy to the liquid−gas phase transformation. Their work is currently published in the research journal, Macromolecules.
Equilibrium properties were considered first by the researchers. They proposed a simple lever rule to relate the crystallinity index to the local and global draw ratios. then proposed and discussed simple experiments within the developed framework so as to enlighten on some fundamental aspects of SIC. By acknowledging that the order parameter for SIC is the draw ratio of the amorphous phase, the importance of directly measuring this parameter was emphasized.
The authors then measured and analyzed the time dependence of the crystalline content in relation to strain relaxation of the remaining amorphous fraction in a quite simple manner using the lever rule mentioned above. They derived an explicit equation which compared well to SIC kinetics measured in various situations. Additionally, the crystallization kinetics was seen to be also fundamental to explaining the hysteretic behavior observed in dynamical conditions as well as the extraordinary resistance to failure of natural rubber.
In summary, the study painted a unified picture of strain-induced crystallization in natural rubber in which the basic physical ingredient of the seminal theory by Flory, that is, the entropic relaxation of amorphous chains associated to SIC, was implemented in the form of a simple phase equilibrium, similar to, e.g., liquid−gas equilibrium of a pure substance. The key importance of measuring the local draw ratio concomitantly to the crystallinity index, as proposed recently in various publications by the authors, was emphasized. In a statement to Advances in Engineering, Dr Paul Sotta said that the importance of understanding crystallization kinetics of polymers submitted to large dynamical strain goes well beyond the particular case of SIC in natural rubber, as this is one basic concept in polymer physics and is relevant in a vast majority of polymer manufacturing processes.
Paul Sotta, Pierre-Antoine Albouy. Strain-Induced Crystallization in Natural Rubber: Flory’s Theory Revisited. Macromolecules 2020, volume 53, page 3097−3109.