Materials technology

Research projects

Adhesion of polymer-based soft materials: from liquid to solid (2018-2020)

Tribology of soft materials plays a crucial role in natural, biological and technological systems. In the present project we focus on the technological systems involving rubber (elastomer) adhesion. In general, we distinguish three major contributions to rubber adhesion acting at different length scales: bulk viscoelasticity, roughness and molecular mobility. In many practical applications, e.g. lubricated devices, contact resides in liquid environment. There adhesion force can strongly depend on the state of the liquid phase, which can depend on the time of stationary contact ("contact ageing"). In order to tackle this complexity we propose to use polydimethylsiloxane (PDMS) and PDMS-based composite materials as a model material.

Within the framework of the project we will pursue the following objectives.

First, it is of particularly important objective of the project to reveal dependence of adhesion on the PDMS cross-linking density, when the material is transformed from the liquid state to the solid state. The chemical kinetics of PDMS allows it to undergo this transformation within approximately one day at room temperature after mixing the source ingredients. We will study PDMS interaction force with counter-body upon repeated contacts right after the PDMS mixture is prepared. The PDMS sample is first in the liquid state during the first few hours, and in this case the PDMS/counter-body interaction is purely adhesive, i.e., no repulsive interaction. In the fluid state the attraction could be explained as formation of capillary bridges, with negligible influence of the fluid viscosity until close to the gel-point where PDMS forms a percolating network. During this transition the pull-off contact mechanics is characterized by formation of "strings" similar to pressure sensitive adhesives, see the figure below and the paper "Contact mechanics for polydimethylsiloxane: from liquid to solid" in our library. These transient effects will be investigated both on the macroscopic and microscopic levels in atomic force microscopy (AFM), and scanning electron microscopy (SEM).

Second, we will elaborate the effect of surface topography on the adhesion mechanics for differently cross-linked materials with randomly rough and patterned surfaces. There the focus will lie in the macroscopic adhesion tests in the regimes of non-percolating and percolating contact that will be analysed with the contact mechanics models for rough surfaces, and compared with the nano-adhesion tests that we will carry out in parallel.

Third, we will reveal the adhesion of PDMS-based composite materials filled with nanoobjects of various shapes and sizes, including nanoparticles (Ag, Ni, Cu, Au, ZnO, Cu2O, TiO2, AlN), nanowires (Ag, ZnO), Ag nanochains, ZnO nanosheets. Those fillers with a wide variety of geometry are expected to have different mechanical behaviour in cross-linked networks and change adhesion properties that will be observed macroscopically. In the complimentary nanotribological tests we will find the friction and adhesion forces for individual nano-objects.

Fourth, we will conduct macroscopic adhesion tests in liquid environment for solid and porous PDMS, and PDMS with defined surface roughness as a series of contact ageing experiments. Those tests will be complemented by contact angle measurements and will help to understand the dewetting and squeezed-out mechanisms in those model systems in the regimes of non-percolating and percolating contact.

In general, this project is distinguished among the others and benefit from the following progressive-multiscale-transient approach: a) step-by-step way from the simple and known adhesion systems towards more complex and application relevant ones [progressive], b) linking macroscopic and microscopic scales of the same phenomenon [multiscale], c) mechanical pull-off coupled with liquid-to-solid transformation due to the convenient cross-linking ability of the chosen base material [transient].

The project is registered in ITMO University (St.-Petersburg, Russia) and has granted support from the Russian Science Foundation in 2018-2020.