307 research outputs found
Sharp Fronts Due to Diffusion and Viscoelastic Relaxation in Polymers
A model for sharp fronts in glassy polymers is derived and analyzed. The major effect of a diffusing penetrant on the polymer entanglement network is taken to be the inducement of a differential viscoelastic stress. This couples diffusive and mechanical processes through a viscoelastic response where the strain depends upon the amount of penetrant present. Analytically, the major effect is to produce explicit delay terms via a relaxation parameter. This accounts for the fundamental difference between a polymer in its rubbery state and the polymer in its glassy state, namely the finite relaxation time in the glassy state due to slow response to changing conditions. Both numerical and analytical perturbation studies of a boundary
value problem for a dry glass polymer exposed to a penetrant solvent are completed. Concentration profiles
in good agreement with observations are obtained
Free boundary problems in controlled release pharmaceuticals. I: diffusion in glassy polymers
This paper formulates and studies two different problems occurring in the formation and use of pharmaceuticals via controlled release methods. These problems involve a glassy polymer and a penetrant, and the central problem is to predict and control the diffusive behavior of the penetrant through the polymer. The mathematical theory yields free boundary problems which are studied in various asymptotic regimes
Drug diffusion from polymeric delivery devices: a problem with two moving boundaries
An existing model for solvent penetration and drug release from a spherically-shaped polymeric drug delivery device is revisited. The model has two moving boundaries, one that describes the interface between the glassy and rubbery states of polymer, and another that defines the interface between the polymer ball and the pool of solvent. The model is extended so that the nonlinear diffusion coefficient of drug explicitly depends on the concentration of solvent, and the resulting equations are solved numerically using a front-fixing transformation together with a finite difference spatial discretisation and the method of lines. We present evidence that our scheme is much more accurate than a previous scheme. Asymptotic results in the small-time limit are presented, which show how the use of a kinetic law as a boundary condition on the innermost moving boundary dictates qualitative behaviour, the scalings being very different to the similar moving boundary problem that arises from modelling the melting of an ice ball. The implication is that the model considered here exhibits what is referred to as ``non-Fickian'' or Case II diffusion which, together with the initially constant rate of drug release, has certain appeal from a pharmaceutical perspective
Swelling and drug release in polymers through the theory of Poisson–Kac stochastic processes
Experiments on swelling and solute transport in polymeric systems clearly indicate that the classical parabolic models fail to predict typical non-Fickian features of sorption kinetics. The formulation of moving-boundary transport models for solvent penetration and drug release in swelling polymeric systems is addressed hereby employing the theory of Poisson–Kac stochastic processes possessing finite propagation velocity. The hyperbolic continuous equations deriving from Poisson–Kac processes are extended to include the description of the temporal evolution of both the Glass–Gel and the Gel–Solvent interfaces. The influence of polymer relaxation time on sorption curves and drug release kinetics is addressed in detail
Neutral and Charged Polymers at Interfaces
Chain-like macromolecules (polymers) show characteristic adsorption
properties due to their flexibility and internal degrees of freedom, when
attracted to surfaces and interfaces. In this review we discuss concepts and
features that are relevant to the adsorption of neutral and charged polymers at
equilibrium, including the type of polymer/surface interaction, the solvent
quality, the characteristics of the surface, and the polymer structure. We pay
special attention to the case of charged polymers (polyelectrolytes) that have
a special importance due to their water solubility. We present a summary of
recent progress in this rapidly evolving field. Because many experimental
studies are performed with rather stiff biopolymers, we discuss in detail the
case of semi-flexible polymers in addition to flexible ones. We first review
the behavior of neutral and charged chains in solution. Then, the adsorption of
a single polymer chain is considered. Next, the adsorption and depletion
processes in the many-chain case are reviewed. Profiles, changes in the surface
tension and polymer surface excess are presented. Mean-field and corrections
due to fluctuations and lateral correlations are discussed. The force of
interaction between two adsorbed layers, which is important in understanding
colloidal stability, is characterized. The behavior of grafted polymers is also
reviewed, both for neutral and charged polymer brushes.Comment: a review: 130 pages, 30 ps figures; final form, added reference
A New Phenomenon: Sub-Tg, Solid-State, Plasticity-Induced Bonding in Polymers
Polymer self-adhesion due to the interdiffusion of macromolecules has been an
active area of research for several decades [70, 43, 62, 42, 72, 73, 41]. Here,
we report a new phenomenon of sub-Tg, solid-state, plasticity-induced bonding;
where amorphous polymeric films were bonded together in a period of time on the
order of a second in the solid-state at ambient temperatures nearly 60 K below
their glass transition temperature (Tg) by subjecting them to active plastic
deformation. Despite the glassy regime, the bulk plastic deformation triggered
the requisite molecular mobility of the polymer chains, causing
interpenetration across the interfaces held in contact. Quantitative levels of
adhesion and the morphologies of the fractured interfaces validated the sub-Tg,
plasticity-induced, molecular mobilization causing bonding. No-bonding outcomes
(i) during the compression of films in a near hydrostatic setting (which
inhibited plastic flow) and (ii) between an 'elastic' and a 'plastic' film
further established the explicit role of plastic deformation in this newly
reported sub-Tg solid-state bonding
Transport of hydrocarbons in silicone rubber-polystyrene copolymers
Imperial Users onl
- …