Molecular migration in complex industrial formulations

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Majerczak, Katarzyna ORCID: https://orcid.org/0000-0002-2909-6187 (2022). Molecular migration in complex industrial formulations. University of Birmingham. Ph.D.

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Abstract

Understanding migration of small molecules within polymer matrices (either additives already in the matrix or those infiltrating from adjacent media) is a challenge in many industrial systems. Migration can lead to undesirable changes in product properties, reduced shelf-lives and increased material wastage that is harmful for the environment. Herein, this challenge is addressed by studying the migration characteristics of model additives (surfactants of various head group chemistry, plasticiser, and fluorophore) in poly(vinyl alcohol) (PVA) films that are widely used in single unit detergent applications. The aim of this project is to investigate how lateral migration (x,y-migration) and vertical migration (z-migration) of small molecules affect film behaviour, establishing the mechanisms that influence this phenomenon, the molecular interactions associated, and their timescales.
To fully address the complex, multi-component nature of PVA-based films, complexity was introduced stepwise into the system. First, the migration mechanisms of a model additive (rhodamine B, RhB) were tracked through aqueous solutions of PVA, glycerol (a plasticiser) and surfactants of various head group chemistry (cationic, nonionic, and anionic) using fluorescence correlation spectroscopy. Specific intermolecular interactions and steric effects were revealed to be the primary factors influencing migration in formulations containing ionic surfactants and nonionic surfactants, respectively. Presence of the plasticiser was shown to decrease the diffusivity of the tracer, chiefly due to increased viscosity in the system. These solutions were then spin-coated, and RhB diffusion through the resultant films was tracked using fluorescence recovery after photobleaching. In films, an additional mechanism of migration was identified – compatibility of the components in the system, with phase separation of glycerol from the PVA matrix lowering the overall tracer diffusivity; again, intermolecular interactions and steric effects controlled tracer diffusivity in the surfactant-doped systems.
Third, the mechanism of surfactant migration was studied as films were aged under various environmental conditions (primarily relative humidity, RH). The plasticising effect of atmospheric water was demonstrated, with films restructuring to reach equilibrium molecular conformation. The nature and speed of the restructuring was dependent on surfactant concentration and its compatibility with the other components of the system. The properties of the PVA-based matrix were then measured upon exposing it to elevated temperature, with system compatibility overall increasing as the glass transition temperature was exceeded. A significant dependence on compatibility was observed while changing the properties of the matrix (increased degree of hydrolysis (DH) and molecular weight (Mw) of the polymer) as well as introducing a second surfactant to the system. However, the molecular arrangement of surfactant within the polymer matrix remained the same independent of polymer DH and Mw. Overall, observed changes could be explained by the compatibility of the four components with one another, their hydrogen bonding, and other intermolecular interactions in the system.
Finally, wetting of PVA-based film by water, dodecane, or aqueous surfactant solutions of various head group chemistry were examined. Variations in solvophilic behaviour were observed upon changing the polymer crystallinity, crystallite size, size of the free volume cavities, and polymer entanglement, regardless of liquid medium. Further, matrix hydrophilicity was identified as an important factor controlling infiltration of surfactant solutions into the polymer matrix.
Through this work, importance of environmental conditions and chemical environment on molecular migration is highlighted, providing a set of variables that need to be controlled while designing migration in industrially relevant systems as well as future directions for further investigations.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):