Evidence of Cooperativity among van der Waals Interactions in Segmented Polysiloxane

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”.

Microstructure Induced Rigidity of Polysiloxane Yielding Hierarchical Self‐Assembly of Alkyl Side Chains

“…The present study reveals that random alkyl siloxane 1 exhibits a distinct OD transition, whereas the compositionally identical block 2 or full alkylated alkyl siloxane 3 do not allow such reorganization to happen. The additional evidence of enhanced rigidity in block polymer was garnered from the detailed FTIR analysis of polymers 1 and 2 reported earlier . It is well‐known that the CH peaks for n‐alkane with “all‐trans” conformational orientation appears at 2850 and 2920 cm −1 for symmetric and asymmetric stretching, respectively.…”

“…The first set of reflections, therefore, indicates the presence of highly ordered interdigitated siloxane backbone with a primary interlamellar spacing distance of 2.71 nm whereas, the second one corresponds to a small portion of siloxane backbone packed in end‐to‐end fashion with an interlamellar spacing distance of 5.1 nm. The fact that two different kinds of packing coexist in polymer 2 at 298 K has already been established in our earlier publication by using variable temperature SAXS, DSC, and FTIR techniques ( Scheme ) …”

“…At 303 K, the CH peaks of the octadecyl side chains for polymer 1 (2854 cm −1 and 2923 cm −1 ) appear at considerably shorter wavelength compared to polymer 2 (2852 cm −1 and 2921 cm −1 ) indicating an increased conformational disorder in polymer 1 arising due to higher flexibility of the backbone ( Figure ). However, it is to be noted that at 333 K where these polymers are in the amorphous state, both polymers 1 and 2 exhibited identical CH stretching vibration peaks (2855 cm −1 and 2925 cm −1 ) corresponding to their fully conformational disorder state with “trans–gauche” orientation …”

“…However, it is to be noted that at 333 K where these polymers are in the amorphous state, both polymers 1 and 2 exhibited identical CH stretching vibration peaks (2855 cm −1 and 2925 cm −1 ) corresponding to their fully conformational disorder state with "trans-gauche" orientation. [24] Owing to the higher flexibility, polymer 1 forms anisotropic fibrillar morphology while crystallizing from solution in decamethyl cyclopentasiloxane (D 5 ) (Figure 7a), whereas polymer 2 continues to grow platelet-like bigger lamellar crystals (Figure 7b). A strongly birefringent high aspect ratio objects account for the anisotropic growth of fibrillar crystallites with poor lamellar definition in polymer 1.…”

“…In our previous work, we have established that the weak interactions like van der Waals, which occur only within the short distances, can be realized more effectively in the block siloxane like polymer 2 wherein the alkyl side chains are placed in relatively more closely and orderly fashion leading to a relatively more stable interdigitated packing of the side chains as compared to end-to-end packing in a random polysiloxane 1. [24,25] In this paper, we further unveiled that at low temperature, where the siloxane backbone tends to lose their flexibility and mobility, the interdigitated crystals can retain their original hexagonal form whereas the end-to-end crystals undergo a reversible order-disorder (OD) transition from a less-ordered hexagonal to an well-ordered orthorhombic structure. The OD transitions in the later case were also found to be responsible for a major restructuring of the backbone conformation leading to a fully disordered state of the siloxane chain at low temperature.…”