{'fraction_non_alphanumeric': 0.02479253112033195, 'fraction_numerical': 0.013485477178423237, 'mean_word_length': 4.893031784841075, 'pattern_counts': {'":': 0, '<': 1, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Conformational rearrangements in thin films of polydimethylsiloxane melt

Synchrotron X-ray reflectivity (XRR) confirms the formation of a quasi-immobilized layer in thin films of polydimethylsiloxane (PDMS) melts near silica surfaces. This layer (40-60A) has a lower density than the bulk value, and its thickness varies slightly with PDMS molecular weight. Formation of this layer is very rapid for PDMS melts with low molecular weights (below entanglement limit for these molecules) but takes 5-10 hours for higher molecular weights (close to and above their entanglement value).


Introduction
Restricted geometries diminish the number of possible molecular configurations in liquids and give rise to ordered structures. 1 For thin films of oligomers and polymers, the region adjacent to the substrate is very sensitive to the shapes and chemical structures of the liquid molecules. Further, depending upon the surface specific interactions and inherent flexibility of the molecular backbones, the molecules may be positionally aligned (layered) at the interface and/or change their shapes to form extended, flat conformations at the interface. 2 Although for short unentangled chains of a polymer liquid (melt) embedded between two parallel impenetrable walls, a surface orientation of the chains parallel to the substrates is predicted 3 , recent small-angle neutron scattering experiments show either unperturbed gaussian 4 or distorted conformation 5 of polymeric melt molecules close to an impenetrable interface compared to the bulk. For complex molecules of non-polar van der Waals liquids, the region adjacent to the silicon substrate has a more complicated organization of molecules with a lower density than in bulk. 6 In the case of complex molecules such as elastomers, formation of a gel-like layer of lower density has been confirmed by neutron reflectivity. 7 In this paper, we describe our structural studies of molecular arrangements in thin films of polydimethylsiloxane (PDMS) melts near solid substrate surfaces. PDMS is a homopolymer with repeat units consisting of -(O-Si (CH 3 ) 2 )-. A fundamental property of polysiloxane family is the highly pronounced inherent conformational flexibility of the thin main-chain backbones, which results in the high mobility of thin segments and entire molecules. In our previous studies of thin films of low molecular weight PDMS (M w < 2000 g/mol) at silica interfaces, we had confirmed that molecular layering was induced. 8 In this case, the interaction between the Si-O groups of PDMS molecules and silica surface is believed to be very weakly adhesive via van der Waals' bonds. 9 Electronic structure studies have previously determined that the dominant adhesive interaction is between the H atoms on PDMS methyl groups and the O atoms in the surface hydroxyl groups. 10 These interactions cause more stable physical bonding between the macromolecules and the hydroxylated silicon surface and lead to flat conformational structures for higher PDMS molecules perpendicular to the substrate. Some evidence of such ordering has been obtained for PDMS molecules with a molecular weight of 2000g/mole. 8 It should be noted that arrangements at the interface are governed by an interplay between the energetics of surface-specific interactions and the entropic factors associated with molecular conformations. In fact, recent NMR studies on higher molecular weight (M w = 1.6×10 4 g/mol) PDMS molecules show uniaxial chain segment ordering in polymer films, even in the absence of surface specific interactions. 11 Also, using only relatively simple physical considerations based on a self-consistent-field lattice model, 12 it was shown for a model polymer melt that (1) there are oscillations of segment potential of about 4 layers for chain molecules in the interfacial region near the wall, and (2) the fraction of molecules with flat conformation increases towards the wall.
The time scales for structural ordering at the interface are governed by the surface diffusion properties in thin liquid films. The diffusion in thin liquid films near an interface is considerably reduced in a two-dimensional process compared to a threedimensional process. 13 Friction measurements on PDMS melts show a variety of shearrelated property changes as a function of film thickness. 14,15 These effects were attributed to different sliding mechanisms of the physiobsorbed and mobile molecular layers. Also, in thin films the presence of an interfacial "solid-like" layer for PDMS films on silica surface has been observed using quasielastic neutron scattering, 16 nuclear magnetic resonance 17 and electrostriction measurements. 18 Atomic force microscopy measurements show that the time scales for structural changes in PDMS melts at a solid surface are much longer than in the bulk, and that the formation of an immobilized layer at mica and silica surface typically takes 10 hrs for M w = 18 000 g/mol PDMS molecules. [19][20][21] However, the authors could not give a value for the layer thickness, because in atomic force microscopy the zero distance is derived from the straight part of the force curve itself and not from an independent quantity.
In the present work, we have sought to examine geometrical confinement effects, and the time evolution of conformational arrangements in thin films of long-chain polydimethylsiloxane melts at hydroxylated surface of silicon substrate.
Prior to preparing the films, the wafers were removed from the water and blown dry under a stream of nitrogen. We made thin films from dilute solutions of PDMS in hexane (0.5-1.0 wt%). The solution was poured over horizontal silicon substrates, and then the hexane was allowed to evaporate in a closed chamber (to slow the evaporation rate). The liquid films we studied were ~ 1-3 μm thick.
XRR studies were performed at the beam lines X23B of the National Synchrotron Light Source and MRCAT of the Advanced Photon Source using a Huber four-circle diffractometer in the specular reflection mode (i.e., incident angle equal to exit angle). Xrays of energy E = 10 keV (λ = 1.24 Å) were used for these measurements. The beam size was 0.20-0.30 mm vertically and 1.0 mm horizontally. The samples were placed under helium during the measurements to reduce background scattering and radiation damage. The experiments were performed at room temperature. The off-specular background was measured and subtracted from the specular counts.

Results and Discussions
The polymer melt structure can be described as "a heap of spaghetti". Depending  Additional structural information can be obtained by fitting the XRR curves to a model. In order to find the specific electron density profile, we have fitted the reflectivity curves using the Gaussian-step model. We have assumed that the liquid film consists of a region of uniform bulk density and a immobilized layer close to substrate with variable electron density, thickness and interfacial widths. We have also accounted for silicon and silicon oxide substrate layers. The normalized reflectivity can be written as where q z = (4π/λ) sin θ ; λ is the wavelength of the X-ray beam; θ is the incident angle;  Figure 3. A ~40 Å thick reduced density region is formed at the interface between the hydroxylated surface of the silica substrate and PDMS bulk. The electron density of this layer is 5-7% less than the bulk density. A number of fresh samples were prepared and measured by XRR. Analysis of these data allows us to conclude that thickness of immobilized layer depends on molecular weight of PDMS (length of polymer chain) and varies from 40-45 Å (for the shortest chains, DMS-T15 sample), 50-60 Å (DMS-T23) and 60-70 Å (for the longest molecules studied, DMS-T31). If we compare these values with the radius of gyration, we can see that thickness of this layer is approximately 3R g for DMS-T15 and decreases to 1.5R g for DMS-T31. Unfortunately, the fitting process is not precise enough to allow quantitative conclusions about how the electron density of the immobilized layer varies as a function of PDMS molecular weight.
We only can claim the presence of a trend: with increasing molecular weight the electron density of this layer approaches the density of bulk PDMS.
Why is the region with reduced density formed in deposited PDMS films and why do the time scales for formation vary with molecular weight? As already mentioned, PDMS molecules have attractive interactions with the hydroxylated surface of the silicon substrate. This should keep the chains at the substrate surface and lead to increased (at least for size of PDMS strand 8-10 Å) or oscillated (with the same scale) density.
Presumably due to chain entanglement, we do not see any manifestations of layering or other ordering at the film-substrate interface (see, for example, refs. 8 and 15 for a detailed discussion of ordering in thin films of short-chain PDMS as a function of molecular weight or applied shear). We speculate that such a picture is valid only for very short times after film deposition. This state is kinetically metastable. Since in pure bulk polymers the average shape of chains in contact with the surface is determined only by entropic factors, the reduced configurational entropy of confined polymer chains can only increase by (partially) leaving the surface and assuming a random coil configuration. 29 The loss of configurational freedom (relative to the bulk) by polymers in proximity to the surface produces an "entropic repulsion" between the polymer and the wall 30 and as a consequence, the appearance of a depletion layer at the interface. 31