A numerical analysis of the loop tack test is presented to study the behavior of the strip and the influence of several factors, and the results are compared with experimental ones. The numerical results can be applied to model the performance of a pressure sensitive adhesive (PSA). Since the simulation of the loop tack test includes geometrical and material nonlinearities, it is solved numerically by the finite element method. The finite element program ABAQUS is used throughout the research. As the teardrop shaped loop is pushed down onto the adhesive and then pulled up, the variation of the loop behavior is investigated using two-dimensional (2D) and three-dimensional (3D) models. A bilinear elastic-plastic constitutive law is used for the strip. The deformation of the pressure sensitive adhesive is approximated as uniaxial extension of independent adhesive strands. A Winkler-type nonlinear elastic foundation and a viscoelastic foundation are used to model the PSA. A nonlinear elastic spring function is used, which is composed of a compression region for the bonding phase and a tension region for the debonding phase. A debonding failure criterion is assumed, in which an adhesive strand will debond when it reaches a certain length. During the bonding phase, it is assumed that the loop is perfectly bonded, and the contact time is not included. Curves of the pulling force versus the top displacement (i.e., tack curves) are obtained throughout the simulation. A parametric study is made with respect to the nonlinear spring function parameters, experimental uncertainties, and strip thickness. Anticlastic bending behavior is shown in the 3D analysis, and the contact patterns are presented. The effects of the elasticity modulus of the PSA for the elastic foundation and the displacement rate for the viscoelastic model are investigated. / Ph. D.

The loop tack test measures the tack (instant grip) of an adhesive. An analytical model of this test seems to be lacking and is the subject of this research. The strip is investigated using several mathematical formulations, and the solutions are obtained numerically. The loop is created from a flexible elastic strip that is bent into a teardrop shape, with its ends clamped together. The strip is tested in a cycle, in which the loop is first pushed onto the surface, compressing the adhesive. Then the loop is pulled up, and gradually debonds from the substrate. The loop is assumed to be nonlinearly elastic and inextensible. The mechanics of the loop tack test are studied in order to determine the impact of various factors on adhesive performance. These factors include the stiffness of the backing, the stiffness and thickness of the adhesive, the elongation of the adhesive before debonding, and the contact time. The relationship between the applied force and the vertical deflection of the loop's ends is determined, as well as that between the applied force and the contact length. Also, the maximum "pull - off" force needed to remove the substrate from the loop is obtained from the results. Shapes of the loop during the cycle are found. This research will increase understanding of the behavior of the adhesive and backing during the loop tack test. With the computer model that has been developed, any set of parameters and conditions can be analyzed, and improvements can be made in the test procedure. / Master of Science

Adhesion between different objects is happening everywhere. Without it, simple procedures like walking or holding something in a hand or attaching a postage stamp would be impossible. The life itself depends on adhesion on all levels, starting from the interactions between the living cells. Adhesion between two substrates is a complex phenomenon, which at present is still not well understood. There are several factors determining the strength of adhesion: (i) molecular interactions at interface, (ii) mechanical properties of adhesive, and (iii) area of contact between adhesive and probing surface. Two surfaces are tacky when they possess the right balance between these factors. Controlling the adhesion of materials is important in many fields ranging from industrial purposes to biomedical applications and everyday usage. There is a demand for “smart” materials with integrated functionalities that make them responsive, switchable, biocompatible, anti-bacterial, more energy efficient, or autonomous. In particular, materials for such cutting-edge applications like cell culture, drug delivery, tissue engineering, biosensors, anti/biofouling, microfluidics, climbing robots, sport equipment and many others require adjustable/tuneable adhesive properties. Many efforts were directed towards fabrication of materials with either weak or strong adhesion depending on the field of application. However, design of “smart” surfaces with reversibly switchable/controllable adhesion is still a highly challenging task. Therefore, the thesis aims on design of smart polymeric materials with responsive / adaptive adhesion properties. For this, fabrication and investigation of two types of switchable polymer layers based on stimuli-responsive polymer brushes will be performed. The first group is dealing with thermoresponsive polymer brushes: poly-(N-isopropylacrylamide) and two types of biocompatible polyethylene glycol-based systems. These polymer layers undergo phase transition below and above LCST between hydrophilic and hydrophobic states. The second part of the work is related to solvent-responsive comb-like and block copolymer brushes consisted of hydrophilic PEG and hydrophobic PDMS biocompatible and biodegradable polymers.

Adhesion between different objects is happening everywhere. Without it, simple procedures like walking or holding something in a hand or attaching a postage stamp would be impossible. The life itself depends on adhesion on all levels, starting from the interactions between the living cells. Adhesion between two substrates is a complex phenomenon, which at present is still not well understood. There are several factors determining the strength of adhesion: (i) molecular interactions at interface, (ii) mechanical properties of adhesive, and (iii) area of contact between adhesive and probing surface. Two surfaces are tacky when they possess the right balance between these factors. Controlling the adhesion of materials is important in many fields ranging from industrial purposes to biomedical applications and everyday usage. There is a demand for “smart” materials with integrated functionalities that make them responsive, switchable, biocompatible, anti-bacterial, more energy efficient, or autonomous. In particular, materials for such cutting-edge applications like cell culture, drug delivery, tissue engineering, biosensors, anti/biofouling, microfluidics, climbing robots, sport equipment and many others require adjustable/tuneable adhesive properties. Many efforts were directed towards fabrication of materials with either weak or strong adhesion depending on the field of application. However, design of “smart” surfaces with reversibly switchable/controllable adhesion is still a highly challenging task. Therefore, the thesis aims on design of smart polymeric materials with responsive / adaptive adhesion properties. For this, fabrication and investigation of two types of switchable polymer layers based on stimuli-responsive polymer brushes will be performed. The first group is dealing with thermoresponsive polymer brushes: poly-(N-isopropylacrylamide) and two types of biocompatible polyethylene glycol-based systems. These polymer layers undergo phase transition below and above LCST between hydrophilic and hydrophobic states. The second part of the work is related to solvent-responsive comb-like and block copolymer brushes consisted of hydrophilic PEG and hydrophobic PDMS biocompatible and biodegradable polymers.