Polyelectrolyte multilayer assemblies

• Main Author: Hsieh, Meng Che
• Language: ENG
• Published: ScholarWorks@UMass Amherst 1999
• Subjects: Polymers|Plastics
• Online Access: https://scholarworks.umass.edu/dissertations/AAI9950163

The overall objective of the projects that constitute this Ph.D. thesis is an in-depth understanding of layer-by-layer assemblies. The approaches taken in this quest include: a general method of surface modification for adsorbing polyelectrolytes (Chapter 2), the mechanical integrity of layer-by-layer assemblies (Chapter 3), and the adhesion of these assemblies to a substrate (Chapter 4). The approach taken toward the development of a general method to incorporate charges on any surface to promote layer-by-layer assembly of polyelectrolytes is plasma polymerization of allylamine onto poly(tetrafluoroethylene). Plasma polymerization (10 W, 50 W) produces substrates that yield higher surface densities of amine groups at lower power. Well-defined multilayer structures, composed of poly(allylamine hydrochloride) (PAH) and poly(sodium styrenesulfonate) (PSS), are apparent in samples with greater than ∼10 layers. XPS and contact angle data indicate stratified layers. XPS analysis also indicates that the stoichiometry of the assembly process is 2 ammonium ions per sulfonate group. The average individual layer thickness for the 10 W samples and for the 50 W samples are ∼6.1 Å and 4.7 Å, respectively. The mechanical properties of layer-by-layer assemblies are determined by forming polyelectrolyte multilayer assemblies of PAR and PSS on nitrogen/hydrogen-plasma modified spandex films and yams. XPS analysis indicates that the stoichiometry of the assembly process is 1.7 ammonium ions per sulfonate group. The tensile modulus of the yams decreased to 1.24 MPa from 2.61 MPa after plasma-modification. Upon adsorption of 50 layers of polyelectrolytes, the tensile modulus increased to 7.42 MPa resulting in a calculated modulus of the 50 layers of 8.8 GPa. These layer-by-layer assemblies fail at 2.5% strain. The adhesion strength of the layer-by-layer assemblies is measured by studying the mean crack spacing of these assemblies on LPDE with varying surface charge. LDPE films, oxidized using chromic acid followed by adsorption of 50 layers, show cracks after 150% strain due to the failure of the multilayer assembly. Gold-coated LDPE films, adsorbed with an acid thiol followed by adsorption of 50 layers, show cracks after 50% strain due to the failure of the gold coating. The work of adhesion for the layers on oxidized LDPE is 370 mJ/m2, which is greater than that on plasma-modified spandex (200 mJ/m2) due to the higher surface charge density on the oxidized LDPE.