Search

In this paper we examine the relative knotting probabilities in a lattice model of ring polymers confined in a cavity. The model is of a lattice knot of size $n$ in the cubic lattice, confined to a cube of side-length $L$ and with volume $V=(L{+}1)^3$ sites. We use Monte Carlo algorithms to approximately enumerate the number of conformations of lattice knots in the confining cube. If $p_{n,L}(K)$ is the number of conformations of a lattice polygon of length $n$ and knot type $K$ in a cube of volume $L^3$, then the relative knotting probability of a lattice polygon to have knot type $K$, relative to the probability that the polygon is the unknot (the trivial knot, denoted by $0_1$), is $\rho_{n,L}(K/0_1) = p_{n,L}(K)/p_{n,L}(0_1)$. We determine $\rho_{n,L}(K/0_1)$ for various knot types $K$ up to six crossing knots. Our data show that these relative knotting probabilities are small so that the model is dominated by lattice polygons of knot type the unknot. Moreover, if the concentration of the monomers of the lattice knot is $\varphi = n/V$, then the relative knot probability increases with $\varphi$ along a curve that flattens off as the Hamiltonian state is approached.

We investigate a lattice model of comb polymers and derive bounds on the exponential growth rate of the number of embeddings of the comb. A comb is composed of a backbone that is a self-avoiding walk and a set of $t$ teeth, also modelled as mutually and self-avoiding walks, attached to the backbone at vertices or nodes of degree 3. Each tooth of the comb has $m_a$ edges and there are $m_b$ edges in the backbone between adjacent degree 3 vertices and between the first and last nodes of degree 3 and the end vertices of degree 1 of the backbone. We are interested in the exponential growth rate as $t \to \infty$ with $m_a$ and $m_b$ fixed. We prove upper bounds on this growth rate and show that for small values of $m_a$ the growth rate is strictly less than that of self-avoiding walks.

Polymers in confined spaces are compressed and have reduced conformational entropy, and will partially or fully escape from confinement if conditions are suitable. This is in particular the case for a polymer grafted in a pore. The escape of the polymer from the pore may be considered a partial translocation from the pore into bulk solution, and the resulting conformational readjustment of the polymer has characteristics of a thermodynamic phase transition. In this paper a lattice self-avoiding walk model of a star polymer grafted in a pore is examined numerically using the PERM algorithm. We show that the arms of the grafted lattice star escape one at a time as the length of the pore is reduced, consistent with earlier results in the literature. Critical points for the escape transitions are estimated for square and cubic lattice models and we also examine various properties of the model as it undergoes escape transitions.

We investigate a lattice model of comb copolymers that can adsorb at a surface and that are subject to a force causing desorption. The teeth and the backbone of the comb are chemically distinct and can interact differently with the surface. That is, the strength of the surface interaction can be different for the monomers in the teeth and in the backbone. We consider several cases including (i) the uniform case where the number of teeth is fixed and the lengths of the branches in the backbone and the lengths of the teeth are all identical, (ii) the case where the teeth are short compared to the branches in the backbone, (iii) the situation where the teeth are long compared to the backbone, and (iv) the case where the number of teeth approaches infinity. We determine the free energies in the thermodynamic limit and discuss the nature of the phase diagrams of the model.

The distribution of monomers in a coating of grafted and adsorbing polymers is modelled using a grafted staircase polygon in the square lattice. The adsorbing staircase polygon consists of a bottom and a top lattice path (branches) and the asymptotic probability density that vertices in the lattice are occupied by these paths is determined. This is a model consisting of two linear polymers grafted to a hard wall in a coating. The probability that either the bottom, or the top, path of a staircase polygon passes through a lattice site with coordinates $(\lfloor \epsilon n \rfloor,\lfloor \delta \sqrt{n}\rfloor)$, for $0 < \epsilon < 1$ and $\delta\geq 0$, is determined asymptotically as $n\to\infty$. This gives the probability density of vertices in the staircase polygon in the scaling limit (as $n\to\infty$): $\displaystyle {\mathbb P}_r^{f,(both)} (\epsilon,\delta) = \frac{\delta^2(15\,\epsilon^2(1 - \epsilon)^2 - 12\, \delta^2 \epsilon(1 - \epsilon) + 4\, \delta^4 ) }{3\sqrt{\pi\,\epsilon^7(1 - \epsilon)^7}} \, e^{-\delta^2/\epsilon(1 - \epsilon)} .$ The densities of other cases, grafted and adsorbed, and at the adsorption critical point (or special point), are also determined. In these cases the most likely and mean trajectories of the paths are determined. The results show that low density regions in the distribution induces entropic forces on test particles which confine them next to the hard wall, or between branches of the staircase polygon. These results give qualitative mechanisms for the stabilisation of a drug particle confined to a polymer coating in a drug delivery system such as a drug-eluding stent covered by a grafted polymer.

A linear polymer grafted to a hard wall and underneath an AFM tip can be modelled in a lattice as a grafted lattice polymer (or self-avoiding walk) compressed underneath a piston approaching the wall. As the piston approaches the wall the increasingly confined polymer escapes from the confined region to explore conformations beside the piston. This conformational change is believed to be a phase transition in the thermodynamic limit, and has been argued to be first order, based on numerical results in reference [12]. In this paper a lattice self-avoiding walk model of the escape transition is constructed. It is proven that this model has a critical point in the thermodynamic limit corresponding to the escape transition of compressed grafted linear polymers. This result relies on the analysis of ballistic self-avoiding walks in slits and slabs in the square and cubic lattices. Additionally, numerical estimates of the location of the escape transition critical point is reported based on Monte Carlo simulations of self-avoiding walks in slits and in slabs.

The distribution of monomers along a linear polymer grafted on a hard wall is modelled by determining the probability distribution of occupied vertices of Dyck and ballot path models of adsorbing linear polymers. For example, the probability that a Dyck path passes through the lattice site with coordinates $(\lfloor \epsilon n \rfloor,\lfloor \delta \sqrt{n}\rfloor)$ in the square lattice, for $0 < \epsilon < 1$ and $\delta\geq 0$, is determined asymptotically as $n\to\infty$ and this uncovers the probability density of vertices along Dyck paths in the limit as the length of the path $n$ approaches infinity: $$\hbox{P}_r (\epsilon,\delta) = \frac{4\delta^2}{\sqrt{\pi\,\epsilon^3(1-\epsilon)^3}} \, e^{-\delta^2/\epsilon(1-\epsilon)}\ .$$ The properties of a polymer coating of a hard wall and the density or distribution of monomers in the coating is relevant in applications such as the stabilisation of a colloid dispersion by a polymer or in a drug delivery system such as a drug-eluding stent covered by a grafted polymer.

Two ring polymers close to each other in space may be either in a segregated phase if there is a strong repulsion between monomers in the polymers, or intermingle in a mixed phase if there is a strong attractive force between the monomers. These phases are separated by a critical point which has a $\theta$-point character. The metric and topological properties of the ring polymers depend on the phase, and may change abruptly at the critical point. In this paper we examine the thermodynamics and linking of two ring polymers close in space in both the segregated and mixed phases using a cubic lattice model of two polygons interacting with each other. Our results show that the probability of linking is low in the segregated phase, but that it increases through the critical point as the model is taken into the mixed phase. We also examine the metric and thermodynamic properties of the model, with focus on how the averaged measures of topological complexity are related to these properties., Comment: Key words: Links, lattice polygons, polymer collapse, Monte Carlo methods

The surface entropic exponents of half-space lattice stars grafted at their central nodes in a hard wall are estimated numerically using the PERM algorithm. In the square half-lattice the exact values of the exponents are verified, including Barber's scaling relation and a generalisation for $2$-stars with one and two surface loops respectively. This is the relation \[ \gamma_{211}=2\,\gamma_{21}-\gamma_{20},\] where $\gamma_{21}$ and $\gamma_{211}$ are the surface entropic exponents of a grafted $2$-star with one and two surface loops respectively, and $\gamma_{20}$ is the surface entropic exponent with no surface loops. This relation is also tested in the cubic half-lattice where surface entropic exponents are estimated up to $5$-stars, including many with one or more surface loops. Barber's scaling relation and the relation \[ \gamma_{3111}=\gamma_{30}-3\,\gamma_{31}+3\,\gamma_{311} \] are also tested, where the exponents $\{\gamma_{31},\gamma_{311},\gamma_{3111}\}$ are of grafted $3$-stars with one, two or three surface loops respectively, and $\gamma_{30}$ is the surface exponent of grafted $3$-stars.

We consider a model of star copolymers, based on self-avoiding walks, where the arms of the star can be chemically distinct. The copolymeric star is attached to an impenetrable surface at a vertex of unit degree and the different monomers constituting the star have different interaction strengths with the surface. When the star is adsorbed at the surface it can be desorbed by applying a force, either at a vertex of degree 1 or at the central vertex of the star. We give some rigorous results about the free energy of the system and use these to establish the general form of the phase diagrams, and the orders of certain phase transitions in the system. We also consider the special case of spiders, ie stars constrained to have all degree 1 vertices in the surface.