Two-step nucleation pathways in which disordered, amorphous, or dense liquid states precede the appearance of crystalline phases have been reported for a wide range of materials, but the dynamics of such pathways are poorly understood. Moreover, whether these pathways are general features of crystallizing systems or a consequence of system-specific structural details that select for direct versus two-step processes is unknown. Using atomic force microscopy to directly observe crystallization of sequence-defined polymers, we show that crystallization pathways are indeed sequence dependent. When a short hydrophobic region is added to a sequence that directly forms crystalline particles, crystallization instead follows a two-step pathway that begins with the creation of disordered clusters of 10–20 molecules and is characterized by highly non-linear crystallization kinetics in which clusters transform into ordered structures that then enter the growth phase. The results shed new light on non-classical crystallization mechanisms and have implications for the design of self-assembling polymer systems.
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Peptoid synthesis, MD simulations, in situ AFM, DFS and TEM characterization and high-speed AFM imaging were supported by the US Department of Energy, Office of Basic Energy Sciences, Biomolecular Materials Program at Pacific Northwest National Laboratory (PNNL) and the Lawrence Livermore National Laboratory. Development of MD potentials and peptoid designs was supported by the Materials Synthesis and Simulation Across Scales Initiative through the LDRD Program at PNNL. PNNL is a multi-program national laboratory operated for Department of Energy by Battelle under Contract No. DE-AC05-76RL01830. Work at the Lawrence Livermore National Laboratory was performed under the auspices of the US Department of Energy under Contract DE-AC52-07NA27344.
The authors declare no competing financial interests.
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Ma, X., Zhang, S., Jiao, F. et al. Tuning crystallization pathways through sequence engineering of biomimetic polymers. Nature Mater 16, 767–774 (2017) doi:10.1038/nmat4891
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