Characterization of degradation behaviors of PLA biodegradable plastics by infrared spectroscopy.

In this paper, the degradation behavior of two kinds of polylactic acid (PLA) biodegradable material products (pure PLA cup cover and modified PLA straw) was studied. It was found that under the composting environment specified in the International Standard, in the first 35 days, the degradation rate of the straw (with 50%-60% poly butylenes succinate (PBS)) was faster than that of the pure PLA cup cover, but in the later stage, the PLA cup cover exceeded the straw and disintegrated preferentially, and both could be degraded in about 70 days. After further analyzing the far-infrared (FIR, can also be called THz) and mid infrared (MIR) spectra of cup cover and straw, we observed that the material structure had not changed until disintegration, only the ester bond was hydrolyzed, the polymers became oligomers, which could be reflected in the change of the effective area of the characteristic peak at 7.15 THz (cup cover, labeled 1921) and 6.99 THz (straw, labeled 4386) in the THz spectrum. With the degradation, the effective area decreased continuously. Due to the strong absorption of the material in MIR band, most characteristic peaks were flattened and lost analytical value. The bivariate correlation of degradation time, biodegradation rate, total carbon dioxide release and the effective area of the characteristic peak at 7.15 THz (1921) and 6.99 THz (4386) in THz spectrum was analyzed by SPSS software. We discovered that the degradation time was significantly positively correlated with biodegradation rate and carbon dioxide release at the level of 0.01 and negatively correlated with the effective area of characteristic peak at the level of 0.05. The biodegradation rate was significantly negatively correlated with the effective area of characteristic peak at the level of 0.01. Taking the degradation time as the independent variable and the biodegradation rate, carbon dioxide release and effective area of characteristic peak as the dependent variables, we got that the THz spectrum could be used to describe the degradation behavior of PLA products as long as appropriate coefficient correction was made. In this way, we could separate from the laboratory environment, study the impact of environmental diversification on material degradation performance, and reduce the cost of material degradation performance identification. Using density functional theory (DFT), reduced density gradient (RDG) method and visualization software, the changes of weak interaction position and intensity in the molecule during the polymerization of lactic acid into PLA were further analyzed. We found that the vibration of ester bond corresponded to the characteristic peak with weak intensity in the spectrum, and the peak with large intensity mainly originated from the out-of-plane swing of O-H bond in the molecule.

Terahertz spectra and weak intermolecular interactions of nucleosides or nucleoside drugs.

In this paper, terahertz (THz) spectra of four DNA nucleosides (Adenosine, Thymidine, Cytidine and Guanosine) and two nucleoside derivatives (Ribavirin and Entecavir, first time reported) in the solid phase were studied experimentally by Fourier Transform Infrared Spectroscopy (FTIR) in the frequency of 1-10 THz. The lattice energy, geometric structure, vibration spectrum of them were analyzed theoretically by the generalized energy-based fragmentation approach under periodic boundary conditions (denoted as PBC-GEBF) and the density functional theory (DFT). The intra- and inter-molecular weak interactions corresponding to the vibrational modes of the crystal, polymer and monomer were obtained, with the help of the potential energy distribution (PED) and reduced density gradient (RDG) methods. It was found that the sum of electronic and thermal free energies increased from the monomer to polymer, and from the polymer to crystal. For example, the inter-molecular interaction energy from the monomer to dimer of adenosine increased 6.969 kcal/mol, and that from the dimer to crystal (the periodic boundary conditions were considered) increased 666.792 kcal/mol. Therefore, only the crystal structure constrained the periodic boundary conditions could well describe the experimental results, although the former scholars chose the monomer or polymer as the initial configuration due to the limitation of computing resources and methods. In THz band, the vibrational modes were generally originated from the collective vibration (more than 99% of them were vibration, only less than 1% of them were rotation and translation) of all molecules involved, which could reflect the molecular structure and spatial distribution of different substances. In order to accurately identify the spectra, we studied the location, type and contribution of all weak interactions, and found that the strong characteristic peaks corresponding to the strong hydrogen bonds came from inter-molecular, while the weak hydrogen bonds mainly originated from intra- and inter-molecular, the out-of-plane bending made the largest contribution, accounting for more than 90%. Furthermore, taking guanine, guanosine and two guanosine derivatives (Ribavirin and Entecavir) as examples, the differences of weak interaction among them caused by different molecular configuration, arrangement and substituent position were studied, and the fundamental reason of THz spectrum change was found. This research can lay a foundation for crystal engineering, supramolecular chemistry, molecular recognition and self-assembly, protein-ligand interaction, etc.