Study of thin films of kraft lignin and two DHPs by means of single-molecule force spectroscopy (SMFS)

The adhesion of single and associated lignin chains to a substrate has been studied by means of single-molecule force spectroscopy (SMFS). Softwood kraft lignin (KL) and two lignin polymer models (dehydrogenation polymers, DHPs) based on coniferyl alcohol (DHPc.alc.)and coniferaldehyde (DHPc.ald.) were in focus. The desorption force from the "silicon nitride SMFS tip" for the KL was significantly greater than that of the DHPs. The higher desorption force was interpreted as being due to the interaction of carboxyl groups through hydrogen bonding with the tip as well as to the less compact polymeric layer at the interface. The distribution of the extended chain lengths was determined, and self-association of lignin chains was observed. For both KL and the DHPc.ald., chains were extended significantly beyond the limit that would be expected for polymers with the corresponding degree of polymerization. The alpha-carbon on the DHPc.alc. has a strong intramolecular hydrogen bonding interaction with the adjacent aryl ether, which inhibits the possibility of the ether to participate in intermolecular hydrogen bonding with nearby lignin chains. Thus, the self-association for KL and DHPc.ald. was found to be dominated by intermolecular hydrogen bonding with carboxylic groups and aryl ether functionalities.