Copolymerization Kinetics of Dopamine Methacrylamide during PNIPAM Microgel Synthesis for Increased Adhesive Properties

Abstract

Combining stimuli-responsive properties of gels with adhesive properties of mussels is highly interesting for a large field of applications as, e.g., in life science. Therefore, the present paper focuses on the copolymerization of poly(N-isopropylacrylamide) (PNIPAM) microgels with dopamine methacrylamide (DMA). A detailed understanding of reaction kinetics is crucial to figure out an optimized synthesis strategy for tailoring microgels with adhesive properties. The present study addresses the influence of relevant synthesis parameters as the injection time of DMA during the microgel synthesis and the overall reaction time of the microgel. Reaction kinetics were studied by mass spectrometry of time samples taken during the microgel synthesis. This allowed us to determine the monomer consumption of NIPAM, the cross-linker N,N′-methylenebisacrylamide (BIS), and DMA. A second-order reaction kinetics was found for DMA incorporation. The amount of DMA incorporated in the resulting microgel was successfully determined by a combination of UV−vis and NMR spectroscopy to level off limitations of both methods. The dependence of the hydrodynamic radius on temperature was determined by DLS measurements for the microgels. While an early injection of DMA stops the PNIPAM polymerization due to scavenging, it greatly enhances the reaction speed of DMA. The faster reaction of DMA and the incomplete NIPAM and BIS conversion also compensate for shorter reaction times with respect to the incorporated amount of DMA. On the contrary, a later injection of DMA leads to a full NIPAM monomer and BIS cross-linker consumption. An overall reaction time of 60 min ensures the DMA incorporation. Longer reaction times lead to clumping. First adhesion tests show an increased adhesion of P(NIPAM-co-DMA) microgels compared to pure PNIPAM microgels, when mechanical stress is applied.

Influence of Dopamine Methacrylamide on Swelling Behavior and Nanomechanical Properties of PNIPAM Microgels

“…15,30 The reaction parameters were optimized in a previous publication. 15 While an early injection of DMA prohibits the PNIPAM polymerization due to the scavenging ability of DMA, a later injection ensures the PNIPAM polymerization, but slows down the reaction kinetics of DMA. An injection time of DMA of t DMA = 10−15 min and an overall reaction time of t reac = 60 min ensure the full NIPAM monomer conversion while still leading to large DMA incorporations.…”

“…The derivative d R H /d T was calculated. While the temperature at the minimum of the derivative function equals the VPTT, the temperature where d R H /d T evens out describes the VPT completion temperature (VPT CT). ,, Evening out was defined as a maximum R H difference of 2.5 nm for a difference in temperature of 5 °C.…”

“…We assume that the observed sharp VPT in our study is a consequence of our synthesis protocol, differing from other literature procedures. 19,25 In the present study, DMA is added later during the reaction, 15 when BIS is already fully consumed. This might lead to a different distribution of the more hydrophobic DMA within the microgel network.…”

Aqueous microgels with engineered hydrophobic nano-domains

“…The incorporation of hydrophobic compartments in colloidal stable aqueous microgels can give rise to the demand for a specific adjustment of the synthesis procedure, such as comonomer injection time, 40 that will influence the resulting microgel structure. A pre-defined functional group distribution within the microgel can be controlled, e.g.…”

Preface to the Françoise M. Winnik Special Issue

“…This special issue celebrates her career with contributions from colleagues across the world. The included papers represent many topical areas, including microgels, elastomers, microbubbles, microemulsions, supramolecular assemblies, and thin films, with critical physical properties enabling applications as broad-ranging as carbon capture, medicine, and electronics. − Nonetheless, many of the papers are fundamental in nature, reflecting Professor Winnik’s enduring commitment to and interest in basic colloid and interfacial science.…”