Adsorption of diblock copolymers on stripe-patterned surfaces

Two-dimensional Monte Carlo simulations of coarse-grained poly(3-hexylthiophene) (P3HT) adsorbed on striped substrates

We investigate the structural phases of single poly(3-hexylthiophene) (P3HT) polymers that are adsorbed on a two-dimensional substrate with a striped pattern. We use a coarse-grained representation of the polymer and sophisticated Monte Carlo techniques such as a parallelized replica exchange scheme and local as well as non-local updates to the polymer's configuration. From peaks in the canonically derived observables, it is possible to obtain structural phase diagrams for varying substrate parameters. We find that the shape of the stripe pattern has a substantial effect on the obtained configurations of the polymer and can be tailored to promote either more stretched out or more compact configurations. In the compact phases, we observe different structural motifs, such as hairpins, double-hairpins, and interlocking "zipper" states.

Interactions of complex polymers with nanoporous substrate

With the advance of polymer synthesis, polymers that possess unique architectures such as stars or cyclic chains, and unique chemical composition distributions such as block copolymers or statistical copolymers have become frequently encountered. Characterization of these complex polymer systems drives the development of interactive chromatography where the adsorption of polymers on the porous substrate in chromatography columns is finely tuned. Liquid Chromatography at the Critical Condition (LCCC) in particular makes use of the existence of the Critical Adsorption Point (CAP) of polymers on solid surfaces and has been successfully applied to characterization of complex polymer systems. Interpretation and understanding of chromatography behaviour of complex polymers in interactive chromatography motivates theoretical/computational studies on the CAP of polymers and partitioning of these complex polymers near the CAP. This review article covers the theoretical questions encountered in chromatographic studies of complex polymers.

Polymers undergoing inhomogeneous adsorption: Order parameters for a partially directed walk model

We consider partially directed walk models of polymers undergoing inhomogeneous adsorption. The inhomogeneity can be in the polymer, in the surface, or in both. For the cases where the polymer is either a homopolymer or a strictly alternating copolymer and where the surface is either homogeneous or has stripes of width 1, we calculate detailed order parameters and show that these provide important information about the ways in which the polymer adsorbs.

Critical adsorption of a flexible polymer on a stripe-patterned surface

The adsorption and dynamics of a polymer chain on a stripe-patterned surface composed of periodical attractive and neutral stripes are studied by using Monte Carlo simulation. The critical adsorption temperature Tc and pattern-recognition temperature Tr are estimated from the desorption probability, surface contact number, and bridge number. A phase diagram presenting three polymer states, including a desorbed state above Tc, a multi-stripe adsorbed state at an intermediate temperature Tr < T < Tc, and a single-stripe adsorbed state below Tr, is provided for infinitely long chains. Normal diffusion is always observed for a polymer in the direction parallel to the stripe even at low temperature. But in the direction perpendicular to the stripe, the polymer can freely diffuse above Tc, whereas the polymer is confined to one attractive stripe below Tr. However, the adsorbed polymer can hop from one attractive stripe to another at the intermediate temperature Tr < T < Tc.

Interplay of Coil-Globule Transition and Surface Adsorption of a Lattice HP Protein Model

An end-grafted hydrophobic-polar (HP) model protein chain with alternating H and P monomers is studied to examine interactions between the critical adsorption transition due to surface attraction and the collapse transition due to pairwise attractive H–H interactions. We find that the critical adsorption phenomenon can always be observed; however, the critical adsorption temperature T_CAP is influenced by the attractive H–H interactions in some cases. When the collapse temperature T_c is lower than T_CAP, the critical adsorption of the HP chain is similar to that of a homopolymer without intrachain attractions and T_CAP remains unchanged, whereas the collapse transition is suppressed by the adsorption. In contrast, for cases where T_c is close to or higher than T_CAP, T_CAP of the HP chain is increased, indicating that a collapsed chain is more easily adsorbed on the surface. The strength of the H–H attraction also influences the statistical size and shape of the polymer, with strong H–H attractions resulting in adsorbed and collapsed chains adopting two-dimensional, circular conformations.

Adsorption and pattern recognition of polymers at complex surfaces with attractive stripelike motifs

We construct the complete structural phase diagram of polymer adsorption at substrates with attractive stripelike patterns in the parameter space spanned by the adsorption affinity of the stripes and temperature. Results were obtained by extensive generalized-ensemble Monte Carlo simulations of a generic model for the hybrid organic-inorganic system. By comparing with adhesion properties at homogeneous substrates, we find substantial differences in the formation of adsorbed polymer structures if translational invariance at the surface is broken by a regular pattern. Beside a more specific understanding of polymer adsorption processes, our results are potentially relevant for the design of macromolecular pattern recognition devices such as sensors.

Pulling alternating copolymers adsorbed on a striped surface

We consider a partially directed walk model of a strictly alternating copolymer adsorbing on a striped surface where the energy is associated with the numbers of the two types of monomers adsorbed on the two types of surface sites. A force is applied to the last monomer and the polymer responds to this force, sometimes by desorbing. The force can be applied at various angles, with the surface component parallel or perpendicular (or at some other angle) to the stripe direction. The desorption behavior is strongly dependent on the force direction and the response gives information about the shape and direction of the polymer adsorbed on the surface, especially at low temperatures. In some cases the ground state is degenerate and this also has an important effect on the temperature dependence of the critical force needed for desorption. We give a complete solution of the problem using generating function techniques and an approximate treatment that is especially useful at low temperatures and helps in our physical understanding of the situation.

The differences in surfactant adsorption on carbon nanotubes and their bundles

Dissipative particle dynamics simulations of a mesoscale model are performed to investigate the concentration dependence of surfactant adsorption on small-diameter carbon nanotubes and their bundles. Adsorption is found to follow fundamentally different mechanisms in the two cases because of the heterogeneity of the bundle surface and the difference in diameter of bundles compared to that of individual tubes. Whereas aggregation dominates adsorption on individual tubes, on bundles it is largely a Langmuir-type process. High adsorption energy sites on the outer surface of bundles, where surfactant molecules can interact with two tubes simultaneously, dominate at low coverage. They also cause adsorption on bundles to become significant well before adsorption on individual tubes starts. The difference in the adsorption mechanisms leads to a crossover point at higher concentrations, where the adsorbed amount per surface area on individual tubes becomes larger than that for the bundles.

Adsorption and freezing of diblock copolymers on stripe-patterned surfaces: A scaling analysis

We present the results of scaling analysis of diblock copolymers adsorbed on stripe-patterned surfaces of various widths. Our previous studies [K. Sumithra and E. Straube, J. Chem. Phys. 125, 154701 (2006)] show that the adsorption of diblock copolymer on patterned surfaces yields two peaks in the specific heat capacity, thereby indicating two transition. In the current study, we characterize these two transitions. The scaling of the adsorption energy data proves that the first peak in the heat capacity curve is, in fact, associated with the adsorption transition. We found that for this transition the classical scaling laws are obeyed and that the critical crossover exponent is unaltered with respect to the case of homogeneous polymers. However, we found a change in the scaling exponent in the case of parallel component of the radius of gyration. It is evident from the scaling analysis of the parallel component of the radius of gyration that the chain is stretched along the direction of the stripes. The scaling plot shows, for (square root <Rg parallel2>)/Nnu, an exponent of approximately 0.55 which is much different from that expected of a self-avoiding chain (nud=2-nu)/phi which is 0.25. The observed value is closer to an exponent of (nud=1-nu)/phi=0.69, for a completely stretched chain in one dimension. The perpendicular component of the radius of gyration <Rg perpendicular2> shows deviation from the power law and the slope is steeper than the expected value of -2. We have also defined an order parameter to characterize the second transition and have found that it corresponds to a freezing transition where there are only a few dominant conformations. The perpendicular component of the radius of gyration also supports this information.