Structure of Hydrated Poly(d,l-lactic acid) Studied with X-ray Diffraction and Molecular Simulation Methods

Abstract

The effect of hydration on the molecular structure of amorphous poly (D, L-lactic acid) (PDLLA) with 50:50 L-to-D ratio has been studied by combining experiments with molecular simulations. X-ray diffraction measurements revealed significant changes upon hydration in the structure functions of the copolymer. Large changes in the structure functions at ~ 10 days of incubation coincided with the large increase in the water uptake from ~1 to ~40% and the formation of voids in the film. Computer modeling based on the recently developed TIGER2/TIGER3 mixed sampling scheme was used to interpret these changes by efficiently equilibrating both dry and hydrated models of PDLLA. Realistic models of bulk amorphous PDLLA structure were generated as demonstrated by close agreement between the calculated and the experimental structure functions. These molecular simulations were used to identify the interactions between water and the polymer at the atomic level including the change of positional order between atoms in the polymer due to hydration. Changes in the partial O-O structure functions, about 95% of which were due to water-polymer interactions, were apparent in the radial distribution functions. These changes, and somewhat smaller changes in the C-C and C-O partial structure functions, clearly demonstrated the ability of the model to capture the hydrogen bonding interactions between water and the polymer, with the probability of water forming hydrogen bonds with the carbonyl oxygen of the ester group being about four times higher than with its ether oxygen.

Multiscale approach for the construction of equilibrated all-atom models of a poly(ethylene glycol)-based hydrogel

“…[54][55][56][57][58] Finally, the complete atomistic structure of the PEG-based hydrogel network was obtained by reverse mapping the equilibrated CG model to an all-atom model followed by adding explicit hydration and final further equilibration using molecular dynamics (MD), thus providing an all-atom representation of an equilibrated PEG-based hydrogel network.…”

“…58 The structure functions were obtained from the x-ray scattering curves measured from q ¼ 0.3-20 Å…”

Synthesis of supramolecular precision polymers: Crystallization under conformational constraints

“…S15-S18), which can be assigned to the newly developed internal double bonds. 30,56,57 The obtained molecular weights and associated polydispersity indices are listed in Table 1. The polymers without protection group (6a-c and 8) could not be characterized by SEC and NMR spectroscopy due to their insolubility in any tested solvent presumably caused by the formation of hydrogen bonds and the segregation from the saturated alkyl chains.…”

“…The hydrogenation of the polymers was achieved by the reaction with TsNHNH 2 and N,N-diisopropylethylamine in DMF at 150 8C for 6h, which already turned out to be a successful method, 51,[57][58][59] proving the completeness to the fully saturated polymers (7d-i) by the disappearance of all olefinic signals (from 4.90 to 5.84 ppm) and the formation of terminal methyl groups (0.85-0.88 ppm) in the 1 H NMR spectra (see Supporting Information Fig. S20).…”

“…Only the fully hydrogenated polymer with the longest methylene spacer length (7i, solid green curve) shows a small endotherm in the heating curve indicating a 12% crystallinity and a melting temperature of 15 8C, explainable by the size of the benzyl protection group, which is too big to be incorporated into the crystal and thus hinders crystallization, compensated only by an increasing chain length of the alkyl chain in between the functional groups, in turn enabling the formation of alkyl-crystals. 57 If an Nmethyl protection group is used instead, only the unsaturated polymer with the shortest methylene spacer length (6d) shows no crystallinity, whereas all of the other methyl protected polymers (6e-f) show crystallinities in the range of 17-35%. Removing the methyl protection group (polymers 6a-c) leads to an increase in melting temperature from 123 up to 138 8C, certainly by the hydrogen bonding between the DAP units, thus enhancing the thermal stability of this polymers.…”