Substrate‐Induced and Thin‐Film Phases: Polymorphism of Organic Materials on Surfaces

Abstract

2233wileyonlinelibrary.com to that of the bulk liquid; [ 7 ] order typically exists over a few molecular layers before rapidly decaying to bulk behavior as the distance from the wall increases. [ 8 ] Similar ordering effects can also be observed in the quasiliquids which form on the surface of many solids due to surface melting (also known as premelting): [ 9,10 ] the existence of a liquid-like layer a few molecules thick on the surface of a solid below the melting point of the bulk material. [ 11 ] The structure within this layer is also found to be different to that of the bulk isotropic liquid, [ 12 ] highlighting the ordering occurring at the interface. A converse effect, surface freezing, is also observed in some liquids (primarily n -alkanes with 16 ≤ n ≤ 50), where a solid monolayer may exist at an interface up to ≈30 °C above the bulk melting point while the rest of the compound is molten. [13][14][15] Such phase behavior is strongly dependent on molecular shape and has only been observed for chain molecules, which then form layers similar to self-assembled monolayers (SAMs) at the interface with the liquid bulk. [ 16 ] Further examples of molecular ordering at interfaces can be seen in materials which display liquid crystal (LC) phases, a phase behavior also dependent on molecular shape and generally observed for molecules with a highly anisotropic shape (e.g., rod-like, disc-like or bowl-like molecules). [ 17 ] LCs have properties of both solids and liquids and display long range order which may be orientational and/or positional; various types of LC phases (mesophases) are possible and normally exist only over a certain temperature range (i.e., they are thermotropic); a further property of LCs is that they generally reach thermodynamic equilibrium in the bulk and at interfaces as a result of their short relaxation time. Nematic (N) phases of calamitic LCs (rod-like molecules with cylindrical symmetry) have no positional order but an orientational order which can be described by a unit vector, n , known as the director, which represents the preferred molecular orientation ( Figure 1 , left). A scalar order parameter can also be defi ned which is related to the average angle the long molecular axes make to the director, n .Increased order is seen in smetic phases (Sm) which have both orientational and positional order; for example in a smetic A phase (SmA) molecules form layers (positional ordering of the molecular mass centers) which are oriented normal to the plane of the layers (orientational order), while smetic C phases (SmC) form layers where molecules are oriented with the long molecular axes tilted at an angle to the molecular layer normal However, the factors that drive such a process are not clearly understood or studied in depth. In this feature article, we review the current state of understanding concerning SIPs, giving examples of systems where SIPs have been observed, discussing their origins, and which questions remain to be answered. The role of the substrate in controlling the growth a...

Structural stability of naphthyl end-capped oligothiophenes in organic field-effect transistors measured by grazing-incidence X-ray diffraction in operando

“…Future studies should expand these experiments to Na2T nanofibers [23] and other naphtyl substituted oligothiophenes and surface effects as shown elsewhere for α, ω-dihexylsexithiophenes and other materials [16,38]. They may also benefit from reflectivity experiments as demonstrated for polythiophenes elsewhere [39].…”

Is zeroth order crystal structure prediction (CSP_0) coming to maturity? What should we aim for in an ideal crystal structure prediction code?

“…The organic electronic community are obviously interested in substrate-induced polymorphs. 57 Are these cases where full thermodynamic treatment including the interfacial effects would predict these structures? Concomitant polymorphs raise further questions 37,58,59 as to whether we could ever devise a computational method that would allow the condent prediction of the conditions needed to crystallise phase pure samples.…”

Surface-Induced Frustration in Solid State Polymorphic Transition of Native Cellulose Nanocrystals

“…3,5,9 The extent (degree) of crystallinity and the quality of the polymorphs can be tuned in various ways, e.g., by helical inducers for polymers. [10][11][12] The presence of an interface can significantly affect the crystalline structure of materials, small molecules 13 and polymers 14,15 alike. The impact of geometrical confinement in forms of droplets, 16,17 thin films, 15,[18][19][20][21][22][23] adsorbed structures, 24,25 composite phases, [26][27][28] and blend patterns 14,29,30 on polymer crystals specifically have been comprehensively investigated.…”

“…33 From the fundamental perspective of interfacial polymer polymorphs, the most common approach is to study crystallization from solution or melt onto a solid surface or perhaps observe the effect of temperature as post treatment that leads to crystallization. [13][14][15] Biopolymers, particularly proteins, have also received fair attention in this respect. 34,35 Furthermore, efforts to elucidate crystallization and melting of monomolecular polymer layers on atomically smooth surfaces have recently undergone an upsurge because of the popular appeal of 2D substrates like graphene.…”

“…The conversion from cellulose I to III was performed by first preparing the EDA-cellulose complex from freeze dried CNCs and rinsing the EDA away with methanol -altogether an established procedure for cellulosic fibers 43 but not previously reported for CNCs. This change to cellulose III in bulk dispersion was determined with 13 C MAS NMR both using Cross Polarization (CP) ( Figure S3) and single pulse (SP) (Figure 4 and S2), the latter being used as a complement for CP experiments, more sensitive but known to be not quantitative. Cellulose I and cellulose III can be identified by the difference in chemical shift of the C6 peak, resonating at about 65 ppm and 63 ppm (Table S1), respectively, for cellulose I and cellulose III.…”