Evidence of Cooperativity among van der Waals Interactions in Segmented Polysiloxane Alok Sarkar, † Meenal Mehra, † Debarshi Dasgupta, † Lalit Negi, ‡ and Anubhav Saxena* ,† † Corporate R&D Center, Momentive Performance Materials Inc., Survey No. 09, Hosur Road, Electronic City (West), Bangalore 560100, India ‡ Momentive Performance Materials Inc., B-3, Sipcot, Oragadam, Kancheepuram Dist., Sriperumpudur 602105, India * S Supporting Information ABSTRACT: The cooperativity of van der Waals interactions in biomolecules and supramolecular assemblies has drawn significant scientific attention due to its offering of seamless opportunities to generate new materials. However, the potential of such cooperativity in synthetic polymers has not yet been explored. In this study, we demonstrate a strategy to achieve cooperativity via enhancement of weak interactions in a designed siloxane copolymer. Segmentation of siloxane copolymer with specific alkyl chain length (C18) induces cooperativity in van der Waals interaction among the alkyl chains, resulting in interdigitated as well as end-to-end packing. The cooperativity of van der Waals interactions is also evident by an excess melting enthalpy of ΔH = 1.86 kJ/mol in the designed block siloxane copolymer as compared to its control counterpart, i.e., a compositionally similar random siloxane copolymer. The dynamic relationship of cooperativity and physical properties is established by various analytical tools and further substantiated by disturbing the regularity of the block segments using a trace amount (1 per 99 octadecyl units) of a dissimilar alkyl chain as “dopant”. 1. INTRODUCTION The cooperativity of van der Waals forces has been well exemplified by Mother Nature through the creation of unique macromolecular assemblies of biomolecules. 1-3 Theoretical and experimental studies have demonstrated the cooperative effect of van der Waals forces in impelling the structure, stability, dynamics, and function of organic as well as coordination compounds. 4 In synthetic polymers, harnessing the cooperativity among weak interactions can control and tune their properties for a wide range of relevant applications, but there are not many such reports. 5-10 Polysiloxane is a high-demand synthetic polymer used across different applications with ∼$15 billion market size. 11-13 Guiding supramolecular architects in flexible siloxane polymer chains with noncovalent interactions is limited mainly due to their low T g (146 K) and extremely low rotational energy barrier about the Si-O-Si bonds in comparison to C-C bonds in organic polymers. 14 There are few studies on hierarchical assemblies of siloxanes (prepared using complex synthetic routes) in the presence of strong ionic bond, quadrupole H-bonding or π-π interactions, 15-17 but the critical role of weak van der Waals forces in regulating structure and properties is least understood. Alkyl-modified siloxanes are now well-known in the silicone industry, 18-23 and identifying the role of cooperativity in amplifying van der Waals interactions may reveal new phenomena. Therefore, a quantitative understanding of the role of weak van der Waals forces in polysiloxane may lead to better control of its material properties. Herein, we envisaged a model segmented and random polysiloxane system with varying alkyl chain length (C8, C12, and C18) (Scheme 1) to demonstrate the quantitative impact of weak van der Waals forces on its thermal and morphological attributes. We believe that weak interactions like van der Waals, which occur only within the short distances, can be realized more effectively in the segmented siloxane wherein the alkyl side chains are placed in relatively more closely and orderly fashion (Scheme 1). Thus, despite composition remains the same, segmentation in the backbone is expected to give differentiated performance with respect to thermal, mechanical, and morphological properties. The cooperativity among the alkyl chains and its effect on thermal properties were established by variable temperature small-angle X-ray diffraction (SAXD), Fourier transform infrared spectroscopy (FTIR), and calorimetric studies. The effect of cooperativity in the segmented siloxane polymer 1c was further demonstrated by disturbing the regularity of the octadecyl chain in block segments using a trace amount of a dissimilar alkyl chain as “dopant”. Received: July 31, 2018 Revised: October 27, 2018 Article pubs.acs.org/Macromolecules Cite This: Macromolecules XXXX, XXX, XXX-XXX © XXXX American Chemical Society A DOI: 10.1021/acs.macromol.8b01636 Macromolecules XXXX, XXX, XXX-XXX Downloaded via KAOHSIUNG MEDICAL UNIV on November 15, 2018 at 01:13:30 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.