Controlled surface grafting of poly (Gamma-benzyl L-Glutamate) via surface-initiated vapor deposition polymerization [electronic resource] : instrumentation, mechanistic study, and orientation study
Keywords: Chemistry

Because of their capability of assembling hierarchically into stable ordered conformations, polypeptides grafted to a surface can demonstrate strikingly high electromechanical and electrooptical efficiency as well as reversible response to an external trigger, and thus show promise for applications such as biosensors and optical storage and display devices. Because of the intrinsic advantages of SI-VDP and the significant progress on SI-VDP optimization, this method has become the most effective approach to synthesize grafted polypeptides. However, high vacuum has been required to achieve high surface-grafting efficiency. Moreover, little quantitative study has been done to understand the mechanistic details of an SI-VDP process, more specifically, of its five major sub-processes: monomer vaporization and reservoir polymerization in the monomer reservoir and NCA condensation and physisorbed and chemisorbed polymerization on the substrate surface. Moreover, the resulting thick surface-grafted PBLGs could contain a significant amount of secondary structures other than helices, making a conventional method like ER-FTIR improper for orientation study. In this work, we developed an SI-VDP system with improved pressure and temperature control to achieve comparable high grafting efficiency under rough vacuum. For example, we synthesized 167 nm chemisorbed PBLG film under 0.75 mbar at substrate temperature of 90 °C and monomer heating temperature of 110°C with nitrogen purge. Most importantly, we monitored the amount of vaporized NCAs and developed a VDP reaction profile (VDPRP) method to study the major monomer reservoir processes. Meanwhile, we also developed a quantitative FTIR analysis of both as-deposited PBLG and chemisorbed PBLG films in addition to ellipsometric data to evaluate the major substrate surface processes. We observed the monomer-heating-temperature-determined unstable and stable paths of the reservoir processes, which were characterized by unstable VDPRPs with random pulses and by stable VDPRPs with two peaks, respectively, and proposed possible mechanisms. We also found that two peaks of stable VDPRPs can selectively track both reservoir processes in real time. For surface processes, we proposed possible mechanisms to obtain the surface-grafted PBLGs that have either high packing density with mostly [alpha]-helix segments or low packing density with a significant amount of both random coil and [alpha]-helix segments. Wang and Chang developed a method that they termed "solvent quenching", in which surface-grafted PBLG films are treated by a good solvent to stretch out the molecular chains followed by a poor solvent to "freeze" the orientation. However, based on their external reflection-FTIR measurement, Wang and Chang reported that their solvent quenching method could decrease the average tilt angle of a thick surface-grafted film from 49° to 3°. In this study, we select Wang and Chang's solvent quenching process as our model system to demonstrate how to apply the extended variable angle linear polarized transmission FTIR (VALP-TFTIR) method to study the change of complex secondary structures and orientation of a thick chemisorbed PBLG film by monitoring their change during solvent quenching. After the quenching, the apparent PBLG film thickness increased from 151 [plus-minus] 5 to 308 [plus-minus] 16 nm, while the refractive index decreased from 1.59 [plus-minus] 0.01 to 1.17 [plus-minus] 0.02, as measured by ellipsometry. Based on the VALP-TFTIR measurements, we found that a significant amount of amine acid repeating units in random coils coexisted with those in [alpha]-helices in the film before solvent quenching. After solvent quenching, the amount of amino acid repeating units in random coils became undetectable, while those in [alpha]-helices decreased 11%. However, the average tilt angle ‹[theta]› of [alpha]-helices remained at 40° during solvent quenching. In contrast to Wang and Chang, we propose that the significant increase of the quenched film thickness is due to flexible random coils transforming to much less flexible structures, such as aggregated strands, instead of the PBLG [alpha]-helix orientation changing.