Fig. 3: Chemical characterization of PEI binder.
From: Additive manufacturing of strong silica sand structures enabled by polyethyleneimine binder
a 13C NMR of PEI cured at 180 °C for 2 h between 30 and 0 ppm and 170 and 125 ppm. b Comparison of 13C NMR from PEI and PEI cured at 2 h between 60 and 30 ppm. The alteration in structures due to curing can be seen for carbons attached to eight types of amines: (1) N-CH2-CH2NH2, (2) N-CH2-CH2NH, (3) N-CH2-CH2-N, (4) NH-CH2-CH2NH2, (5) NH-CH2-CH2-NH, (6) NH-CH2-CH2N, (7) NH2CH2-CH2N, and (8) NH2CH2-CH2N. c FT-IR spectra of PEI as a function of curing time at 180 °C. d Sum frequency generation spectra from uncured PEI, cured PEI and isotopically labeled (ND) cured PEI. The molecular conformation at the PEI-quartz interface with uncured PEI is sketched in e, where few H-bonds are formed, and the PEI species lay on the surface with hydrophobic groups oriented away from the surface. Upon curing the molecular ordering changes to promote H-bonding with the surface: this drives PEI chains to pack tightly such that the CH2 groups are pointed parallel to the surface plane, thereby suppressing sum-frequency generation signals from the -CH2 groups. In panel e the colored balls represent Si (blue), O (red), N (green), C (black), and H (white), and solid orange lines denote H-bonding between the quartz surface and PEI.
