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Attractive bridging interactions in dense polymer brushes in good solvent measured by atomic force microscopy
Diane Goodman1, Jayachandran N Kizhakkedathu, Donald E Brooks
1Department of Chemistry, University of British Columbia, Vancouver, British Columbia, V6T 2B5, Canada.
Langmuir : the ACS Journal of Surfaces and Colloids
|April 20, 2005
Summary
Polymer brushes on latex particles exhibit repulsive forces due to excluded volume interactions. At low grafting densities, attractive bridging forces were observed, influenced by monomer type and density, particularly in poly(N-isopropylacrylamide) brushes.
Area of Science:
- Surface Science
- Polymer Chemistry
- Nanotechnology
Background:
- Polymer brushes grafted onto surfaces are crucial in controlling interfacial properties.
- Understanding interaction forces, such as repulsion and attraction, is key for designing advanced materials.
- Atomic Force Microscopy (AFM) is a powerful tool for probing nanoscale forces.
Purpose of the Study:
- To investigate interaction forces of various polymer brushes (PDMA, PMEA, PNIPAM, PMEA-b-PNIPAM) in aqueous media using AFM.
- To determine the influence of grafting density, molecular weight, and monomer type on brush interactions.
- To elucidate the mechanism of attractive forces, specifically bridging interactions.
Main Methods:
- Synthesis of polymer brushes via surface-initiated atom transfer radical polymerization on latex particles.
- Characterization of polymer brushes (hydrodynamic thickness, molecular weight, grafting density, PDI) using dynamic light scattering, gel permeation chromatography, and multiangle laser light scattering.
- Force measurements using Atomic Force Microscopy (AFM) to probe inter-particle and tip-surface interactions.
Main Results:
- Poly(N,N-dimethylacrylamide) (PDMA) and poly(methoxyethylacrylamide) (PMEA) brushes showed purely repulsive forces, increasing with molecular weight and grafting density due to excluded volume effects.
- At low grafting densities, PDMA exhibited an attractive bridging force, attributed to interactions between free chain ends and the AFM tip.
- Poly(N-isopropylacrylamide) (PNIPAM) brushes displayed bridging forces at lower grafting densities than PDMA, indicating monomer-dependent bridging. This bridging was also observed in PMEA-b-PNIPAM block copolymer brushes, confirming PNIPAM's role.
Conclusions:
- Bridging interactions in polymer brushes are dependent on both grafting density and the specific monomer chemistry.
- The presence of PNIPAM in block copolymers can induce attractive bridging forces, which are tunable with block length.
- AFM force measurements provide valuable insights into the complex interplay of forces governing polymer brush behavior at interfaces.