Polymer-surface Interactions Research Articles

Development and evaluation of lafutidine solid dispersion via hot melt extrusion: Investigating drug-polymer miscibility with advanced characterisation

In current study, immediate release solid dispersion (SD) formulation of antiulcer drug lafutidine (LAFT) was developed using hot melt extrusion (HME) technique. Amphiphilic Soluplus® used as a primary solubilizing agent, with different concentrations of selected surfactants like PEG 400, Lutrol F127 (LF127), Lutrol F68 (LF68) were used to investigate their influence on formulations processing via HME. Prepared amorphous glassy solid dispersion was found to be thermodynamically and physicochemically stable. On the contrary, traces of crystalline LAFT not observed in the extrudates according to differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM) and Raman spectroscopy. Raman micro spectrometry had the lowest detection limit of LAFT crystals compared with XRD and DSC. Atomic Force microscopy (AFM) studies revealed drug- polymer molecular miscibility and surface interaction at micro level. 1H–COSY NMR spectroscopy confirmed miscibility and interaction between LAFT and Soluplus®, with chemical shift drifting and line broadening. MD simulation studies using computational modelling showed intermolecular interaction between molecules. Dissolution rate and solubility of LAFT was enhanced remarkably in developed SD systems. Optimized ratio of polymer and surfactants played crucial role in dissolution rate enhancement of LAFT SD. The obtained results suggested that developed LAFT has promising potential for oral delivery and might be an efficacious approach for enhancing the therapeutic potential of LAFT.

Structure of a Thermoset Polymer near an Alumina Substrate as Studied by Dissipative Particle Dynamics

We performed dissipative particle dynamics (DPD) simulations to investigate the structure and cross-link formation dynamics of a thermoset polymer while interacting with a metal-oxide surface. For characterizing the polymer–surface interactions we used the surface excess, quantifying the surface selectivity of different functional groups. Mesoscopic polymer–surface interactions are determined by matching the surface excess, as computed with atomistic molecular dynamics (MD), with those for DPD, thus realizing a coupling between the mesoscopic and atomistic scales. In the structure prior to cross-linking, we observe that some functional groups prefer to be located at the interface while others are repelled. This largely determines the final cross-linked structure near the metal-oxide interface. The initial preference for cross-links to form is in the bulk region. However, at longer times toward the equilibrium structure, the trade-off between the epoxy–alumina interactions causes migration of reacted group...

Self-Assembly of Cylinder-Forming Diblock Copolymer Thin Films

We study the self-assembly of cylinder-forming diblock copolymers confined in ultrathin films by means of dissipative particle dynamics simulations paired with an elaborate polymer model that mimics the experimental reference system (PS–PHMA) as faithfully as computationally feasible. In particular, the effects of the polymer composition, film thickness, and surface interactions are investigated systematically, where we achieve quantitative agreement with experiments. We observe that the cylindrical domains change their orientation from perpendicular to parallel with respect to the substrate twice as the film thickness is increased. Furthermore, the onset thickness and the sharpness of this transition strongly depend on the microscopic details of the copolymers and their interaction with the substrate.

Critical adsorption of a flexible polymer confined between two parallel interacting surfaces

Critical adsorption of a lattice self-avoiding bond fluctuation polymer chain confined between two parallel impenetrable surfaces is studied using the Monte Carlo method. The dependence of the mean contact number <M> on the temperature T and on the chain length N is simulated for a polymer-surface interaction E=-1. A critical adsorption of the polymer is found at T(c)=1.65 for large surface separation distance D>N(ν)b, whereas no critical adsorption is observed for small distance D<N(ν)b, where ν ≈ 0.58 is the Flory exponent and b is the mean bond length. The critical adsorption point T(c)=1.65 is the same as that of a grafted polymer. Normal diffusion is observed for the confined polymer; however, the diffusion rate is dependent on the temperature and surface separation distance.

Self-assembly of diblock copolymers under confinement

Block copolymers are a class of soft matter that self-assemble to form ordered morphologies at nanometer scales, making them ideal materials for various applications. The self-assembly of block copolymers is mainly controlled by the monomer–monomer interactions, block compositions and molecular architectures. Besides these intrinsic parameters, placing block copolymers under confinement introduces a number of extrinsic factors, including the degree of structural frustration and surface–polymer interactions, which can strongly influence the self-assembled morphologies. Therefore confinement of block copolymers provides a powerful route to manipulate their self-assembled nanostructures. In this review, we discuss the relationship between confining conditions and the resulting structures, focusing on principles governing structural formation of diblock copolymers under two-dimensional and three-dimensional confinement. In particular, the effects of commensurability condition, surface–polymer interactions, and confining geometries on the self-assembled morphologies are discussed.

A molecular dynamics study of poly(N-isopropylacrylamide) endgrafted on a model cylindrical pore surface

Structure and dynamic behavior of the thermo-responsive polymer poly(N-isopropylacrylamide) (PNIPAM) endgrafted onto the inner surface of a simple cylindrical pore model that resembles a carbon nanotube (CNT) with a diameter of 8.4 nm is studied as a function of temperature, of surface-polymer interaction strength, and of pore water content. A free PNIPAM chain in water shows thermo-responsive behavior with a lower critical solution temperature (LCST) of about 305 K. We have investigated two different strengths of PNIPAM-pore interactions. In the strong interaction case, which corresponds to force-field parameters taken without change from the AMBER force field, the endgrafted PNIPAM chain collapses onto the surface at all temperatures studied and hence does not adopt a brush structure. In the weak interaction case the PNIPAM-pore interaction strengths were scaled by a factor of 10, and the temperature-responsive behavior of the PNIPAM chain re-emerges. End-to-end distances, radii of gyration, density profiles, number of hydrogen bonds, and radial distribution functions demonstrate the temperature-dependent structural changes of endgrafted PNIPAM in the pore. Analysis of the translational motion of water molecules in the pore shows that the ratio of the water self diffusion coefficient in a pore with a free pore surface relative to the self diffusion coefficient in a pore containing an end-grafted PNIPAM molecule is less strongly reduced above the LCST than below the LCST, where the chain is in a more extended state.

Confined chiral polymer nematics: Ordering and spontaneous condensation

We investigate condensation of a long confined chiral nematic polymer inside a spherical enclosure, mimicking condensation of DNA inside a viral capsid. The Landau-de Gennes nematic free-energy Ansatz appropriate for nematic polymers allows us to study the condensation process in detail with different boundary conditions at the enclosing wall that simulate repulsive and attractive polymer-surface interactions. By increasing the chirality, we observe a transformation of the toroidal condensate into a closed surface with an increasing genus, in some respects akin to the ordered domain formation observed in cryo-microscopy of bacteriophages.

Molecular dynamics simulation of rupture in glassy polymer bridges within filler aggregates

We present a series of nonequilibrium molecular dynamics simulations, investigating the rupture mechanisms in glassy polymer films confined between two solid surfaces. Such systems provide a useful model for the strong nonlinear reinforcement of rubber by colloidal filler particles. Depending on the degree of confinement three qualitatively different rupture modes have been found, which originate from the interplay of internal (polymer-polymer) and external (polymer-wall) interactions. In very thin films we observe the formation and stretching of many single-chain bridges between the confining surfaces. Progressing to thicker samples we observe fewer bridges, consisting of bundled polymer chains, and eventually just one large bridge in thick specimens. The yield stress and the elongational modulus of the polymer films have been calculated from the stress-strain curves at various temperatures and confinements and their behavior has been analyzed in terms of polymer-polymer and polymer-surface interaction energies. The thinnest films (5 monomer diameters) are always glassy in our simulations, while the others display a glass transition temperature around 0.50-0.55 (in units ε(0)/k(B) of the Lennard-Jones interaction energy), depending on their thickness. This range of values, which has been determined using both the nonequilibrium tensile simulations and equilibrium diffusion data, agrees with the transition temperature previously found by shear simulations [Baljon and Robbins, Science 271, 482 (1996)].

Band renormalization of a polymer physisorbed on graphene investigated by many-body perturbation theory

Many-body perturbation theory at the $G_0W_0$ level is employed to study the electronic properties of poly(\emph{para}-phenylene) (PPP) on graphene. Analysis of the charge density and the electrostatic potential shows that the polymer-surface interaction gives rise to the formation of only weak surface dipoles with no charge transfer between the polymer and the surface. In the local-density approximation (LDA) of density-functional theory, the band structure of the combined system appears as a superposition of the eigenstates of its constituents. Consequently, the LDA band gap of PPP remains unchanged upon adsorption onto graphene. $G_0W_0$ calculations, however, renormalize the electronic levels of the weakly physisorbed polymer. Thereby, its band gap is considerably reduced compared to that of the isolated PPP chain. This effect can be understood in terms of image charges induced in the graphene layer, which allows us to explain the quasi-particle gap of PPP versus polymer-graphene distance by applying a classical image-potential model. For distances below 4.5 {\AA}, however, deviations from this simple classical model arise which we qualitatively explain by taking into account the polarizablity of the adsorbate. For a quantitative description with predictive power, however, we emphasize the need for an accurate ab-initio description of the electronic structure for weakly coupled systems at equilibrium bonding distances.

Phenolic polymer–surface interactions from ab initio computations

Test calculations show that the diamond surface binding energy of C13H11O2, the simplest model for phenolic, is virtually the same as that of C6H5. Using the C6H5 model, we compare the binding to a diamond surface, a graphene sheet, a (10, 0) nanotube, and a silica surface. The binding energy is more than 5 eV for the silica and 2.85 eV for the diamond surface. As expected, the binding energy of a second molecule at a site adjacent to the first molecule is larger than the first binding energy for the graphene sheet and the carbon nanotube, since the first C6H5 bond breaks a π bond and the second molecule bonds to the unpaired π electron created by adding the first molecule. For all of the systems, adding a C2 unit between the surface and the C6H5 group increases the binding by at least 0.51 eV and up to 2.3 eV. Part of this increase is due to the intrinsically stronger bonding for the sp hybridization and part due to a decrease in the surface–C6H5 repulsion.

General approach to polymer chains confined by interacting boundaries. II. Flow through a cylindrical nano-tube

The Laplace-Green's function methods of Paper I are extended to describe polymers confined in interacting, impenetrable cylindrical geometries, whose treatment is far more challenging than the slit and box geometries considered in Paper I. The general methods are illustrated with calculations (as a function of the polymer-surface interaction) of the free energy of confinement, the radial density profile, and the average of the drag force in the free draining limit, quantities that will be used elsewhere to analyze experiments of Wu and co-workers involving the flow of polymers through nanopores. All these properties are evaluated by numerical inverse Laplace transforms of closed form analytical expressions, a significant savings over the traditional eigenfunction approaches. The example of the confinement free energy for a 3-arm star polymer illustrates the treatment when a closed form expression for the Laplace transform is unavailable.

Electric Glue: Electrically Controlled Polymer-Surface Adhesion

Polymer-surface interactions provide a basis for nanoscale design and for understanding the fundamental chemistry and physics at these length scales. Controlling these interactions will provide the foundation for further manipulation, control, and measurement of single molecule processes. It is this direction of control over nanoscale polymer-surface interactions that we explore with electric glue. The adhesion between surfaces and single molecules is manipulated based on an externally controlled potential in electric glue.

Subnanometer Replica Molding of Molecular Steps on Ionic Crystals

Replica molding with elastomeric polymers has been used routinely to replicate features less than 10 nm in size. Because the theoretical limit of this technique is set by polymer-surface interactions, atomic radii, and accessible volumes, replication at subnanometer length scales should be possible. Using polydimethylsiloxane to create a mold and polyurethane to form the replica, we demonstrate replication of elementary steps 3-5 Å in height that define the minimum separation between molecular layers in the lattices of the ionic crystals potassium dihydrogen phosphate and calcite. This work establishes the operation of replica molding at the molecular scale.

How silanization of silica particles affects the adsorption of PDMS chains on its surface

AbstractA series of six fumed silica types, with different surface areas in the 50–400 m2/g range, were modified by grafting with trimethylchlorosilane. The grafting reaction was controlled by elemental analyses, surface hydroxyl titration, and combustion techniques. The silica surface energy was determined as a function of silanization degree by inverse gas chromatography (IGC). Adsorption of a series of poly(dimethylsiloxane) elastomers with molecular weights ranging between 4 and 420 kg/mol on silica was followed using flow microcalorimeter (FMC). IGC results show that free adsorption energies of two series of alkanes and siloxanes as well as the dispersive component of the surface energy were found to decrease monotonously with surface silanization and so does the polymer molar heat of adsorption. FMC results indicate, however, that the conformation of the macromolecules on silica depends on the silica surface area but remains unaffected by the surface treatments. A given polymer chain was found to remain adsorbed on the surface preserving its same conformation until its molar heat of adsorption falls bellow a critical value. These findings offer a better monitoring of surface–polymer interactions as it defined a comprehensive relationship between the degree of modifications of the filler surface and polymer adsorption conditions. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010

Conformational properties of a polymer tethered to an interacting flat surface

The mean square end-to-end distance 〈R2〉 is dependent on the polymer-surface interaction ɛ. It reaches minimum near critical adsorption point (CAP) ɛc≈−0.29. The result provides an alternate method to determine ɛc.

Filamentous Biopolymers on Surfaces: Atomic Force Microscopy Images Compared with Brownian Dynamics Simulation of Filament Deposition

Nanomechanical properties of filamentous biopolymers, such as the persistence length, may be determined from two-dimensional images of molecules immobilized on surfaces. For a single filament in solution, two principal adsorption scenarios are possible. Both scenarios depend primarly on the interaction strength between the filament and the support: i) For interactions in the range of the thermal energy, the filament can freely equilibrate on the surface during adsorption; ii) For interactions much stronger than the thermal energy, the filament will be captured by the surface without having equilibrated. Such a ‘trapping’ mechanism leads to more condensed filament images and hence to a smaller value for the apparent persistence length. To understand the capture mechanism in more detail we have performed Brownian dynamics simulations of relatively short filaments by taking the two extreme scenarios into account. We then compared these ‘ideal’ adsorption scenarios with observed images of immobilized vimentin intermediate filaments on different surfaces. We found a good agreement between the contours of the deposited vimentin filaments on mica (‘ideal’ trapping) and on glass (‘ideal’ equilibrated) with our simulations. Based on these data, we have developed a strategy to reliably extract the persistence length of short worm-like chain fragments or network forming filaments with unknown polymer-surface interactions.

High Quality SiO2‐like Layers by Large Area Atmospheric Pressure Plasma Enhanced CVD: Deposition Process Studies by Surface Analysis

AbstractThis work reports on the main competing processes and their contribution to the properties of SiO2 layers on polymers in large area AP‐PE‐CVD from ArN2O2‐HMDSO mixtures. The detailed space resolved surface analysis on the statically deposited films showed smooth SiOx films in the vicinity of the gas injection, as deposited by HMDSO radicals. At the gas effluent, due to HMDSO depletion, non‐depositing species interact with the polymer and induce rough deposits with high carbon content. The competition of plasma–polymer surface interaction with HMDSO radicals deposition, is further confirmed from the analysis of films grown on polymers with a “protecting” layer and with reverse gas flow direction. Under web roll conditions, HMDSO radicals deposition is dominant, resulting in high quality SiOx layers along the whole electrode length.magnified image

The influence of adsorbate-surface interaction energy on adsorption and recognition of diblock copolymers on patterned surfaces

The adsorption and pinning of a single diblock chain on patterned surfaces with varying block-surface interactions are investigated using Monte Carlo simulations. Previous investigations [K. Sumithra and E. Straube, J. Chem. Phys. 125, 154701 (2006)] on diblock copolymers on stripe-patterned surfaces have shown that the adsorption takes place in two stages for certain specific adsorbate-surface interactions. We have found that interesting changes from the usual adsorption behavior occurs for a series of interaction parameters for the copolymer on the surface patterns. It is also seen that the junction point of the different blocks gets pinned to the interface between the different surface sites. It is possible to get the polymer blocks completely localized on respective sites by suitably adjusting the polymer-surface interactions. We find that unlike in the random heteropolymer adsorption on random surface where the second transition is the result of the rearrangement of the already adsorbed polymers, in this case, it is seen that the process not only involves rearrangement but also optimizes the energetics of the chain by optimizing the extent of adsorption. The static and conformational properties of the copolymer are also discussed.

Quantification of chemical–polymer surface interactions in microfluidic cell culture devices

Microfluidic cell culture devices have been used for drug development, chemical analysis, and environmental pollutant detection. Because of the decreased fluid volume and increased surface area to volume ratio, interactions between device surfaces and the fluid is a key element that affects the performance and detection accuracy of microfluidic devices, particularly if fluid is recirculated by a peristaltic pump. However, this issue has not been studied in detail in a microfluidic cell culture environment. In this study, chemical loss and contaminant leakage from various polymer surfaces in a microfluidic setup were characterized. The effects of hydrophilic coating with Poly (vinyl alcohol), Pluronic(R) F-68, and multi-layer ionic coating were measured. We observed significant surface adsorption of estradiol, doxorubicin, and verapamil with PharMed BPT tubing, whereas PTFE/BPT and stainless steel/BPT hybrid tubing caused less chemical loss in proportion to the fraction of BPT tubing in the hybrid system. Contaminants leaching out of the BPT tubing were found to be estrogen receptor agonists as determined by estrogen-induced green fluorescence expression in an estrogen responsive Ishikawa cell line and also caused interference with an estradiol enzyme-linked immunosorbent assay (ELISA) assay. Stainless steel/BPT hybrid tubing caused the least interference with ELISA. In summary, polymer surface and chemical interactions inside microfluidic systems should not be neglected and require careful investigations when results from a microfluidic system are compared with results from a macroscale cell culture setup.

Adsorption and self-assembly of linear polymers on surfaces: a computer simulation study

The adsorption and self-assembly of linear polymers on smooth surfaces are studied using coarse-grained, bead-spring molecular models and Langevin dynamics computer simulations. The aim is to gain insight on atomic-force microscopy images of polymer films on mica surfaces, adsorbed from dilute solution following a good-solvent to bad-solvent quenching procedure. Under certain experimental conditions, a bimodal cluster distribution is observed. It is demonstrated that this type of distribution can be reproduced in the simulations, and rationalized on the basis of the polymer structures prior to the quench, i.e., while in good-solvent conditions. Other types of cluster distribution are described and explained. Measurements of the fraction of monomers bound to the surface, the film height, and the radius of gyration of an adsorbed polymer chain are also presented, and the trends in these properties are rationalized. In addition to providing insight into experimental observations, the simulation results support a number of predicted scaling laws such as the decay of the monomer density as a function of distance from the surface, and scaling of the film height with the strength of the polymer-surface interactions.