Soy-based Polymers for Surface Modification and Interactions with Lignocellulosic Materials

Recent environmental concerns about the use of synthetic materials that are often used to maintain our quality of life has triggered a significant amount of research to develop new technologies and to adopt sustainable, bio-based materials. Cellulose, lignin and other plant-derived macromolecules including proteins from soybeans have witnessed recent, renewed interest by the industrial and scientific communities. For example, soybean proteins have been proposed for a variety of applications, including wood adhesives, bio-plastics, composites and functional materials that may include synthetic polymers. Despite its importance in such systems or materials, very little is known about the fundamental nature of the interactions between soy proteins and other polymers. Therefore, this work addresses this issue by a systematic investigation of the interactions between soy proteins with the two most abundant macromolecules in the biosphere, namely, cellulose and lignin and with the most widely used synthetic polymer, polypropylene (PP).;The adsorption of the main soy protein globulins, glycinin (11S) and beta-conglycinin (7S), was studied by using ultrathin films of cellulose, lignin and PP (as well as reference silica and organic self-assembled monolayers (SAMs) surfaces) that were used as substrates. The extent and dynamics of adsorption was monitored by using quartz crystal microgravimetry with dissipation (QCM-D), surface plasmon resonance (SPR) as well as complementary techniques including circular dichroism (CD) and atomic force microscopy (AFM).;QCM-D experiments indicated that soy protein adsorption was strongly affected by changes in the physicochemical environment. An increased adsorption of glycinin on silica (by 13%) and cellulose (by 89%) was observed with the increased ionic strength of the aqueous solution, from 0 to 0.1 M NaCl. This highlights the relevance of electrostatic interactions in the adsorption process. In contrast, the adsorption of beta-conglycinin was reduced (by 25 and 57 % on silica and cellulose, respectively). Similarly, the addition of 10 mM of 2-mercaptoethanol (a denaturing agent) reduced the mass adsorbed for both proteins.;The amounts of 11S and 7S adsorbed on lignin and self-assembled 1-dodecanethiol monolayers were higher when the protein was in the native state if compared to that after chemical denaturation (by using urea and 2-mercaptoethanol). Urea-denatured proteins adsorbed more extensively onto the hydrophobic SAM monolayes. The reduction in water contact angle after protein adsorption (≈40° and 35° for native 11S and 7S, respectively) suggests strong nonspecific interactions between the protein and the substrates, favoring conformational changes at the interface that contribute to exposure and rearrangement of hydrophobic and hydrophilic amino acid residues.;The adsorption on polypropylene thin films and nonwovens of different grades of soy proteins in their native as well as thermally-denatured states, including purified glycinin and beta-conglycinin as well as commercial soy flour and isolate was investigated at 25 °C in PBS buffer (pH 7.4). It was found that application of a primer layer of a cationic surfactant, dioctadecyldimethylammonium bromide (DODA) dramatically enhanced protein adsorption, which resulted in fully wettable systems. Fluorescence imaging experiments with tagged proteins confirmed the contribution of a fully-covering layer facilitated by the cationic surfactant pre-treatment. Furthermore, complementary wicking tests indicated that the nonwoven fabrics absorbed a significant amount of water (≈25 times their weight) when the fibers carried pre-adsorbed proteins.