Adhesion of a viscoelastic polymer to wet or compliant substrates

Pressure sensitive adhesives (PSAs) are viscoelastic polymers designed to attach to a variety of substrates. PSAs need light pressure to adhere to surfaces and do not undergo any chemical reaction to bond. Pressure sensitive adhesives (PSAs) are industrially produced viscoelastic polymers that are ubiquitous in a broad range of industries like electronics, automotive, biomedical and office supplies. PSAs are soft adhesives that can typically bear large strains. May studies have investigated and continue to investigate the adhesion and debonding of PSAs to rigid substrates in air. However, behavior and performance of PSA in wet conditions or to soft substrates is relatively less well known. The objectives of this thesis are to understand 1) the role of water in contact formation and debonding between PSA and rigid glass substrate 2) the role of acrylic acid functional groups in adhesion of PSA at different pH and 3) the effect of a soft substrate in adhesion and debonding of PSA. In this thesis, the role of water in contact formation and debonding between PSA and rigid glass substrate has been investigated using a model acrylic pressure sensitive adhesive (PSA) composed of 2-ethylhexylacrylate-co-acrylic acid. Probe-tack adhesion measurements along with bottom view imaging are used to characterize adhesion of the PSA with hydrophilic and hydrophobic rigid probes in air and in DI water. First contact is made between thin film of PSA (~25 μm) and spherical glass probe at constant force using a force feedback loop through LabView. The polymer is let to relax for a dwell time of 100 s. During this duration, contact is first made homogeneously in air and then grows with time until it reaches a plateau. However, heterogeneous contact formation is observed in water. As the probe is retracted after 100 s, debonding forces are measured. A comparison of maximum force of debonding revealed lower adhesion of PSA in water irrespective of surface functionality of probe. However, a higher decrease in adhesion is observed for hydrophilic probe. Additionally, fingering instabilities are observed during debonding of PSA from all glass probes in air and hydrophobic probe in water. Fingering wavelength analysis reveals an increase in surface energy of PSA in water. Overall, hydration of PSA and trapped water defects between PSA and probe lead to decrease in adhesive strength of the PSA. One factor in adhesion in aqueous environment is the effect of pH. PSAs can be exposed to a variety of pH conditions. A better understanding of effect of pH on acrylic PSAs are important for optimized designed of adhesive polymer. In this thesis the bonding and debonding of 2 PSAs were investigated in air and aqueous solution with hydrophobic glass probes. PSAs chosen were poly(2-ethylhexyl acrylate), or poly(2-EHA) with no acrylic acid content, and 2-ethylhexylacrylate-co-acrylic acid, or poly(2-EHA-co-AA), with 5wt% acrylic acid content. Bonding and debonding of both PSAs were characterized in air, pH 3, pH 6, pH 8 and pH 11 with hydrophobic spherical glass probe along with in situ imaging. Presence of acrylic acid comonomer led to strong dependence of adhesion on pH. Receding contact angle measurements can measure the properties of PSA exposed to pH solution. Deprotonation of acrylic acid comonomer was characterized through contact angle titration curve revealing a pKa of poly(2-EHA-co-AA) as 8.7. Changes in surface properties of PSA with pH was found to be the leading cause of pH dependent adhesion. Additionally, addition of PSAs to soft materials is of great interest and has practical applications like adhesion to skin. Choosing polydimethylsiloxane (PDMS) as a model substrate, role of substrate compliance was investigated on adhesion and debonding of PSA with soft substrates. Elastic modulus of PDMS was varied by changing the crosslinking ratio of PDMS and surface energy of PDMS was increased by performing oxygen plasma treatment. Probe-tack measurements were performed in sphere-plane geometry and revealed insignificant effect of compliance on adhesive strength. A key observation was change in debonding mechanism from fingering instabilities to edge crack propagation. Influence of surface energy of soft substrate, compliance of soft substrate and debonding or retraction velocity was noted. Elastohydrodynamics was found to be the leading cause of transition of instabilities from fingering to edge crack propagation