Tuning Semi-Crystalline Polymer Thin Film Structure via Substrate Nano-Geometry Modulation

The ability to create ultra-thin functional polymer films with controlled nano-crystallinity structure for large-scale integration is at the focus of numerous experimental and theoretical works. [1-3] The main goal is to control and manipulate the assembly of building blocks to obtain structures with desired characteristics and unique properties. The geometric constraints imposed by the nanometric thickness and interfacial interactions affect the self-assembly process and, in turn, their behaviour. [4,5] In addition, non-planar substrates, such as nano-curved substrates, can further complicate the resultant structure as already observed for block copolymer thin films. [6,7] The combination of geometric effects, i.e. surface curvature, with surface free energy (SFE) may allow more finely controlled self-assemblies. To this end, we have developed a novel approach to prepare substrates with controlled nano-curvature and surface-free-energy (SFE), to be used to investigate how geometric and energetic factors affect the self-assembly of polymeric thin films. [8,9] In particular, by employing a semi-crystalline model polymer, namely poly-3-hexyltiphen (P3HT), it was observed that the crystals follow the surface curvature only in the presence of an energetic gain.[8]<br/>Here, from the structural characterization via synchrotron radiation grazing incidence X-ray diffraction (GIXRD) and the morphological characterization via atomic force microscopy (AFM), we will demonstrate a non-conformal coverage, after the polymer equilibration, showing that the crystallization strongly depends on the surface curvature. The results suggest the polymer accumulation in the interstices between the particles for the lowest curvature, leading to an in-plane crystal growth. The increase in curvature improves the nucleation probability, leading to shorter crystals on both interstices and curved portions. Moreover, we demonstrate that the crystalline fraction is strongly curvature-dependent, as the crystallization only occurs when the distance between two neighbour particles is higher than the lamellar folding period. Finally, the in-situ structural characterization during thermal cooling shows a variation in the crystallization temperature (Tc) as a function of the substrate geometry, with a decrease in the onset of the crystallization with respect to the flat substrate. These results highlight the role played by the substrate in the crystallization process, suggesting that the geometric factors affect the crystalline structure itself, with the formation of distorted crystals following the surface curvature. The reported work provides a novel and easy method to modulate the structure of polymer films by exploiting geometric distortion and interfacial interactions with possible effects on the functional properties of the polymer film. This correlation sheds more light on how these factors influence the polymer film crystallization and could allow the determination of the crystallization enthalpy and its related loss when a nanometric strain is applied. A deeper understanding of this behaviour could pave the way for the development of finely tailored crystalline structures via control of the substrate geometry and energy.<br/><br/><b>References</b><br/>[1] Gao Hanfei, et al. <i>Nature Communications</i>, <b>2019</b>, 10(1), 3912.<br/>[2] Del Valle, M. A., et al. <i>Polymers</i>, <b>2023</b>, 15(6), 1450.<br/>[3] Nguyen, et al. <i>Polymers</i>, <b>2016</b>, 8(4), 118.<br/>[4]. Verstraete, L., et al. <i>Chemical Society Reviews</i>, <b>2021</b>, <i>50</i>(10), 5884-5897.<br/>[5]. Yu, C., et al. <i>Crystals</i>, <b>2017</b>, <i>7</i>(5), 147.<br/>[6]. Yager, K. et al. <i>Soft Matter</i>, <b>2009</b>, <i>5</i>, 622-628.<br/>[7]. Kang, et al. <i>Accounts of Chemical Research</i>, <b>2022</b>, 55(16), 2224-2234.<br/>[8]. Ruffino, R., et al. <i>J. Phys. Chem. C, <b>2019</b>,</i><i> 123</i>, 8967-8974.<br/>[9]. Ruffino, R., et al. <i>Polymer</i>, <b>2021</b>, <i>230</i>, 124071.