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We report the influence of side chain hydrolysis on the evolution of nanoscale structure in thin films fabricated by the reactive layer-by-layer (LbL) assembly of branched poly(ethylenimine) (PEI) and poly(2-vinyl-4,4-dimethylazlactone) (PVDMA). LbL assembly of PEI and PVDMA generally leads to the linear growth of thin, smooth films. However, assembly using PVDMA containing controlled degrees of side chain hydrolysis leads to the growth of thicker films that exhibit substantial nanoscale roughness, porosity, and have resulting physicochemical behaviors (e.g., superhydrophobicity) that are similar to those of some thicker PEI/PVDMA coatings reported in past studies. Our results reveal that the degree of PVDMA partial hydrolysis (or carboxylic acid group content) influences the extent to which complex film features develop, suggesting that ion-pairing interactions between hydrolyzed side chains and amines in PEI promote the evolution of bulk and surface morphology. Additional experiments demonstrate that these features likely arise from polymer/polymer interactions at the surfaces of the films during assembly, and not from the formation and deposition of solution-phase polymer aggregates. When combined, our results suggest that nanoporous structures and rough features observed in past studies likely arise, at least in part, from some degree of adventitious side chain hydrolysis in the PVDMA used for film fabrication. Our results provide useful insight into molecular-level features that govern the growth and structures of these reactive materials, and provide a framework to promote nanoscale morphology reliably and reproducibly. The principles and tools reported here should prove useful for further tuning the porosities and tailoring the physicochemical behaviors of these reactive coatings in ways that are important in applied contexts.
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Interfacial wetting-induced nanorheology of thin polymer films
The rheological response and chain dynamics of thin polymer films underpin nanoscale polymer processing, yet molecular confinement alters such behavior. Using the interfacial wetting force of an immiscible liquid droplet to deform the films and linear elastic theory to describe the time evolution of the deformation profile, we demonstrated that the linear viscoelastic spectra, i.e., the frequency-dependent storage and loss moduli, of nanoscale polymer films are experimentally accessible over a wide frequency range. Our measurements on polystyrene nanofilms evidence an acceleration of polymer diffusion at a large confining length scale, i.e., at film thicknesses of hundreds of nanometers. This long-range perturbation in chain dynamics was interpreted as the fast relaxation of surface chains with reduced entanglements provoking loosening of entanglement constraints of the underlying chains, allowing the accelerated reptation mobility at the surface to extend deeply into the film interior. This suggests a surface-induced constraint release effect dominating the dynamics and rheology of polymers confined at a large length scale.
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- Award ID(s):
- 2011750
- PAR ID:
- 10680516
- Publisher / Repository:
- Physical Review Research
- Date Published:
- Journal Name:
- Physical Review Research
- Volume:
- 7
- Issue:
- 2
- ISSN:
- 2643-1564
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1103/PhysRevResearch.7.023226
