Crystallization of Adsorbed Polymer Monolayers

Abstract

We present atomic force microscopy results and corresponding computer simulations on crystallization of quasi-2D monolayers of polyetheleneoxid adsorbed onto bare silicon wafers. Fingerlike branched patterns with a characteristic width w resulted from crystallization at temperatures T c below the melting point T m . w decreased exponentially as ͑T m 2 T c ͒ increased. The patterns are explained by a simple model considering the interplay of transport on the surface and the probability of attachment to the crystal. [S0031-9007(98)05902-X] PACS numbers: 61.41. + e, 81.10.Aj Crystallization can be considered as a process of molecular self-assembly, resulting in fascinating patterns such as snowflakes or spherulites [1][2][3][4][5]. The growth of polymeric crystals is complicated by the need of cooperative movements of the large number of connected monomers [6][7][8][9][10]. Problems such as chain folding [11,12], lamellar thickening [13,14], fractionation effects [6,8], or chain diffusion [15][16][17] affect molecular organization and consequently the resulting patterns. Specific molecular architectures [18,19] or restricted geometries [20 -22] impose further constraints. The influence of a solid surface may modify or even prevent polymer order [20][21][22]. Here, we focus on crystallization close to an interface in quasi-2D geometry [23] for strongly adsorbed polymers. Although there is an extensive literature [2,6-19] on the crystallization of polymers, we are still lacking comprehensive insight, especially for crystallization in thin films and close to interfaces. Recent work indicates that polymers may be unable to crystallize in ultrathin films [20]. Competition between adsorption and crystallization, difficulties to nucleate a crystal, and other surface induced effects may be anticipated.For our studies, we used low molecular weight polyethyleneoxid (PEO), a well investigated polymer [6,8,10,12,13,24], adsorbed onto the oxide surface of UV-ozone cleaned silicon wafers. Samples were prepared from dilute toluene solutions of dried [25] polymer by spin coating a thin film (about 50-100 nm) onto the wafers. Annealing these films in the molten state led to pseudodewetting [26] thereby forming holes and, eventually, tiny droplets on top of an adsorbed monolayer. Pseudodewetting could be arrested by crystallization which always started in the thick parts of the sample, e.g., at the rims, and not within the monolayer. This allowed the initial nucleation and the subsequent crystal growth process to separate in space. Consequently, our resulting patterns were due exclusively to crystal growth and were not affected or modified by nucleation processes within the monolayer [27].Atomic force microscopy (AFM) allowed visualization of the resulting patterns (see Fig. 1). The molecules, predominantly planar within the adsorbed layer [24], could crystallize by attaching to the already crystalline rim. They thereby attained a mostly vertical and highly stretched conformation (schematically indicated in the inset of Fig. 1). ...

Crystal Growth of Isotactic Polystyrene in Ultrathin Films: Film Thickness Dependence

“…1 f in very thin films, the situation looks similar to the DLA growth: the crystal structure hence should become open fractal aggregates. In fact, this has been demonstrated in the Monte Carlo simulation [3,5] on crystallization of adsorbed polymer monolayers, where the thicker the lamellar thickness becomes, the wider of the width of branches and more open the structure becomes. It is worth noting that the dendritic tip can grow steadily even in the diffusion controlled growth as in Fig.…”

“…For the film thinner than the entanglement distance, i.e. the diameter of the tube in the reptation model (∼ 7 nm [17]), entanglement effect on diffusion constant is small; the diffusion constant D should remain almost constant with film thickness [3]. Still the G(d) decreases with decreasing film thickness.…”

“…Crystal growth of polymers in thin films has recently studied on morphology and growth rate [1,2,3,4,5,6]. The crystal growth rate is reduced in thin films [7,8,1,2,4,6].…”

Crystalline and Spherulitic Morphology of Polymers Crystallized in Confined Systems

“…Different crystalline morphologies are generally observed with the reduction of film thickness, as depicted in Figure 1b-e. The formation of such different crystalline patterns in polymer ultrathin films is attributed to the crystal growth in ultrathin films being generally a diffusion-controlled process [59,60].…”

How far from stability can we go using gammasphere and the FMA?

“…The recoils are identified following the GSI method of correlated alphas, but the RDT method can be then used to identify prompt gamma rays emitted shortly after formation of the nuclei while it is still rotating. A fist experiment, producing the Z = 102 element 254 No in the 208 Pb( 48 Ca,2n) 254 No reaction has been completed successfully [15] with the identification of gamma-rays in the groundstate rotational band. The deformation was found to be ␤ 2 = 0.25, close to some theoretical predictions, and the intensities of the band indicate the shell-stabilized fission barrier is robust against rotation.…”