Stephen Mazur

Viscoelastic effects in the coalescence of polymer particles

Abstract A study was made of the coalescence of linear acrylic resins over a much greater dynamic range than previously investigated. The rate of neck growth between a sphere and a flat slab of the same material was measured in situ by optical microscopy and found to exhibit a number of features typical of polymer viscoelastic response. Results are compared with independent measurements of the melt rheology (torsional creep compliance). An analytic approximation is proposed which allows the neck-growth kinetics to be predicted from the recoverable creep compliance and viscosity of the melt. It predicts qualitative trends consistent with the limited data on molecular weight, structure, and particle size variations. This analysis helps resolve some apparent contradictions among earlier studies and implies a qualitative difference in the coalescence mechanisms for large particles (5 μm) and colloids (

GROWTH OF ADHESIVE CONTACTS FOR MAXWELL VISCOELASTIC SPHERES

Coalescence of a Maxwell viscoelastic sphere to a frictionless and flat rigid plane is analyzed to study the transition from initial elastic adhesion to viscous sintering. Deformation is driven by surface tractions due to the surface energy. The formulation for surface forces consistently combines direct van der Waals attraction across the gap ahead of the contact edge with curvature-based tractions normal to the sphere surface. These two contributions to the surface traction result in two different modes of contact growth. The initial elastic contact and the early stage of time-dependent contact growth are in a zipping mode of contact closure dominated by direct attractive forces. The later stage of sintering is by stretching of the contact and is dominated by curvature-based tractions. The transition from the initial elastic contact to the zipping mode of contact growth is viscoelastic. For a given sphere radius, kinetics of the zipping mode of contact growth scale with a characteristic viscous sinterin...

Neighborship partition of the radial distribution function for simple liquids

In a disordered system of N particles, the probability of finding any neighbor at a given distance from a reference particle is described by the radial distribution function G(r). In many instances it would also be useful to know the distribution functions P(1,r),P(2,r),...,P(n,r) corresponding to each subset of ‘‘neighborship;’’ namely, the nearest, next nearest, ...nth nearest neighbor. While G(r) is a two‐particle correlation function, each P(n,r) is formally dependent upon all levels of multiparticle correlations. By means of suitable approximations relating higher‐order correlations to two‐particle correlations, it becomes possible to calculate P(n,r) from G(r). This approach takes advantage of the fact that G(r) is often accurately known from either experiment or theory. For the case of hard spheres, the superposition approximation and a first‐order correction to it are found to give P(1,r) in good agreement with simulations and earlier analytic results, even at high densities. P(n,r) for neighborsh...

Sintering of silver interlayers in polyimide films

Annealing at 220–420 °C and exposure to laser light were used to induce sintering of silver metal interlayers in polyimide films. The sintering process was followed by transmission electron microscopy, electrical conductivity, optical reflectivity measurements, and x‐ray scattering. We find that upon sintering at temperatures below Tg≂380 °C of the polymide film, sintering of the silver interlayer is induced. A dramatic increase in both the optical reflectivity (from 32% to 95%) and a hundredfold increase in electrical conductivity are observed upon sintering. Both the average size and the connectivity of the silver particles in the interlayer are increased in the process; however, there is no apparent change in the polymer. High‐speed (∼100 ns) high spatial resolution laser writing (∼1 μm) is feasible by a selective sintering process. It is, therefore, of potential interest as a basis for write‐once, high‐density optical information storage. It is suggested that a surface diffusion mechanism is responsib...

Models for relaxation in glass-forming liquids based on sequentially correlated displacements

Abstract Among the simplest models for correlated molecular motion in dense liquids are vacancy-mediated displacements in which only those molecules immediately adjacent to vacancies (packing defects of suitable topography) can be displaced at any instant. Any other molecule must await the arrival of a vacancy which requires an appropriate sequence of prior displacements. Examples of this kind of model for hard spheres and idealized polymer chains (chains of hard spheres) are reviewed. Configurational relaxation of the liquid may be represented by the auto-correlation for spheres adjacent to vacancies, a function which is derived and solved for various concentrations of vacancies. Relaxation is found to include a time domain which is non-exponential and resembles the empirical relaxation laws observed for dielectric and mechanical properties of real supercooled liquids. In particular, for independent hard spheres relaxation always converges to a simple exponential at long times, similar to the empirical Davidson-Cole expression which gives a good account of relaxation in many small molecule liquids. On the other hand, for chains of hard spheres, where adjacent pairs are required to move simultaneously, non-exponential decay can persist indefinitely for vacancy concentrations below a critical value which is consistent with the form of decay observed for polymeric liquids and glasses (Kohlrausch-Williams-Watts and Havriliak-Negami equations).

Configurational relaxation in dense liquids based on sequentially correlated displacements. Origin and limitations of nonexponential decay

A model for configurational relaxation in a hard sphere liquid is described in which it is assumed that individual atomic displacements are sequentially dependent upon the displacement of other atoms, these sequences originating with atoms adjacent to a small number of vacancies (packing defects) scattered randomly throughout the sample. This results in an inhomogeneous distribution of waiting times for displacement of atoms located at different distances from the vacancies. The inhomogeneity is formalized as a normalized distribution in s, the state of interference. Solution of the master equations yields a description of the redistribution of atoms among states at equilibrium. Configurational relaxation is represented by the autocorrelation for atoms neighboring vacancies. Relaxation exhibits a time regime characterized by nonexponential decay, followed by transition to a simple exponential decay at longer times. Persistence of the nonexponential regime increases with decreasing vacancy concentration. T...

Primitive subunits: Models for cooperative motion in glass‐forming liquids. Application to an idealized polymer melt

A family of mechanistic models are proposed to describe cooperative molecular displacements in glass‐forming liquids. These models assume that within each cooperative domain motion occurs by sequences of discrete, localized events, but that each of these events involves synchronous displacement of a smaller cluster of neighboring components. The size and properties of this cluster, described as a ‘‘primitive subunit,’’ are assumed to reflect intrinsic details of local structure, not sensitive to external degrees of freedom. However, the length of the sequence of events by which each subunit moves is assumed to be a statistical function of some communal degree of freedom such as the free volume. Two examples are explored for the case of a polymer melt in which idealized conformational rearrangements are constrained by steric interferences. The distribution of lengths for cooperative sequences of events required to remove these interferences is derived as a function of a parameter β related to free volume. ...

Metal and semi-metal precipitates in Nuclepore membranes

Au, Pt, Ag, Sb, Te, In and Ge were precipitated by a salt-reducing agent reaction into the pores of Nuclepore membranes 10 to 600 nm pore size range. The precipitates form filament-like structures inside the pores and a dendritic growth on the surface. The permeation of water as a function of pore size in the Nuclepore membranes was measured by tracing the diffusion through the membrane with tritium. The results are modelled and theoretical predictions are compared with the experimentally observed kinetics for metal deposition in the membranes. Also the d.c. electrical properties of the metal precipitates were measured. The anisotropic electrical properties of these systems are dictated by the morphology of the precipitated metal on the membrane.

Metal Interlayers in Polymer Films: A Survey of Deposition Processes, Morphology, Patterning Methods and Physical Properties

Metal interlayer deposition comprises a new family of wet-chemical processes by which a dense, electrically continuous metal film can be grown within the bulk of a pre-existing polymer film. The present article reviews three generic forms of the process, namely: electroless, polymer-mediated electrochemical, and carrier-mediated electrochemical deposition. Experimental examples are given with particular emphasis on polyimides as the matrix material. Mechanistic principles which govern the kinetics and morphology of the deposition are described.

Transport Regulated Electrochemical Reactions in Polyimide Films

A film of ODA/PMDA polyimide can accept electrons from a cathode in a conventional electrochemical cell. When metal ions, such as Ag+, are included in the electrolyte solution, these can permeate into the film where they are reduced via the polymer to submicron metal particles. Under suitable conditions the deposition process can be controlled to produce a continuous metal interlayer embedded within the polymer. The thickness and position of this interlayer may be controlled with great precision and high resolution patterns can be made.