B. Mannfors

Amorphous cell studies of polyglycolic, poly(l-lactic), poly(l,d-lactic) and poly(glycolic/l-lactic) acids

Abstract In this paper static amorphous state properties (solubility parameter, free volume (using the Voorintholt method and the Voronoi tessellations) and pair correlation functions, the last ones also by including water molecules in the cells), which can be related to the probability for water uptake, have been studied for polyglycolic (PGA), poly( l -lactic) (PLLA), poly( l , d -lactic) (PLLA/PDLA) and poly(glycolic/ l -lactic) (PGA/PLLA) acids, known to be biodegradable polymers. The polymer consistent force field, as modified by the authors, has been used in the calculations. The main purpose of this paper is to investigate, which of the amorphous state properties would be relevant for water uptake. We also discuss the validity of th6e methods used for these kinds of studies, and the related reliability of the computed results. Chain flexibilities of the studied polyesters in the amorphous phase have been analyzed, and the intermolecular interactions are found to cause the most significant variations in the distributions of the adjacent chain dihedral angle pairs and in the related populations of the low-energy regions of the comonomers. The solubility parameters, as calculated from the cohesion energy densities of the constructed models, suggest PGA being most compatible with water, in agreement with experiments. On the other hand, the quantitative structure–property relationships method ‘Synthia’ suggests a very similar solubility in water for all particular polyesters. In the PLAs and PGA/PLLA, however, a larger number of hydrogen bonds is formed between the water molecules and the carbonyl oxygen atoms of the chains showing a better possibility of PLLA and its copolymers to break into shorter chains. As an explanation, the hydrophobic methyl groups of the lactide units are suggested to push the water molecules closer to the carbonyl groups than in homo-PGA.

Raman scattering from p-benzoquinone

Abstract The Raman spectrum of powdered p -benzoquinone and p -benzoquinone- d 4 has been investigated at liquid nitrogen temperature using Kr + -laser excitation. Several new lines were observed. To facilitate the assignment, depolarization ratios have been measured in the melt and in acetone solution. Overlapping peaks were resolved with the aid of a computer program.

An in-plane force field for p-benzoquinone and some structurally related molecules

Abstract A simplified out-of-plane valence force field in the harmonic approximation has been constructed for ethene, butadiene, glyoxal, acrolein, and p-benzoquinone using the overlay technique. p-Benzoquinone is considered to consist of two conjugated OCCHCHCO systems and force constants from the first four mentioned molecules are shown to be successfully transferable to p-benzoquinone. The force field has 21 independent parameters and gives a root-mean-square frequency error of 7.61 cm−1.

RIS Metropolis Monte Carlo studies of some aliphatic main chain and side group polyesters

Structure related single chain properties of polyesters with isolated carboxyl groups have been studied to test the performance of the RIS Metropolis Monte Carlo (RMMC) method. The mean square end-to-end distances, radii of gyration, characteristic ratios and persistence lengths have been calculated for poly(methyl acrylate), poly(methyl metacrylate), poly(vinyl acetate) and for some aliphatic main chain polyesters having alkyl chains of various lengths between the carboxyl groups. The PCFF force field, modified by the authors in a recent study on some model molecules for the polyesters in question, has been used in the generation of chain conformations. The effect of the choice of run parameters (i.e. the cut-off limit for nonbonded interactions and chain length) on the results is discussed in more detail. The characteristic ratios calculated with the RMMC method using the PCFF force field with the refined torsion potentials agree well with the experimental values. The dependence of the calculated characteristic ratios on the tacticity of the chain in poly(methyl metacrylate) is found to follow experimental behavior.

Molecular mechanics force field for some vinyl and methyl substituted benzenes

Abstract A molecular mechanics force field comprising 28 diagonal and 46 non-diagonal force constants has been optimized using data from ethene, trans -1,3-butadiene, benzene, styrene, toluene, m - and p -xylene and mesitylene. The force field is based on a spectroscopic overlay force field for the same set of molecules, previously derived in this laboratory. Molecular symmetry has been utilized in the vibrational analysis in order to preserve the vibrational assignments during the optimization. As expected the transferability of the derived molecular mechanics force field is better than that of our corresponding spectroscopic force field, the vibrational frequencies being reproduced with about the same accuracy by both force fields. Special attention is paid to the torsional potential for the C (sp 2 )C (sp 2 ) bond. An expression is introduced which reproduces the structure as well as the torsional frequency of both the trans and gauche conformers of butadiene.

Torsional potential for the “weak” C(sp2)C(sp2) bond in butadiene, biphenyl and styrene

Abstract The transferability of the torsional potential of the C(sp 2 )C(sp 2 ) weak double bond in the molecular mechanics method was investigated using a set of simple model molecules: biphenyl, styrene, α-methylstyrene, cis -β-methylstyrene, 1,3-butadiene, isoprene, and 2,3-dimethylbutadiene. Individual torsional potentials for these molecules as well as different general potentials were derived. In the optimization of the torsional parameters, conformational energies from ab initio calculations at the Hartree—Fock level, and well-assigned torsional vibrational frequencies, were used.

A transferable force field for a series of molecules containing CC groups

Abstract A simplified valence force field has been developed for some molecules containing the CC group by using the overlay technique. In constructing the force field special attention was paid to the transferability of the force constants since they are intended to be used in large molecules. The force field contains 57 parameters out of which 22 are transferred from other overlay force fields and 35 are optimized using 200 experimental frequencies. The root-mean-square frequency error is 10.5 cm −1 which includes the CH strecthing vibrations.

Molecular mechanics potential energy functions from vibrational spectroscopy and quantum chemistry

Abstract The construction of molecular mechanics potential energy functions is discussed from the viewpoint of vibrational spectroscopy. The rapid growth in computer capacity has made it possible to use increasingly higher level quantum chemical methods to derive information about molecular potential energy surfaces and electrostatic properties. If such information is fully utilized it could lead to a breakthrough in the ability of molecular mechanics and dynamics simulations to predict vibrational spectra. Two methods that make extensive use of quantum chemically calculated data are compared. It is concluded that the Spectroscopically Determined Force Field technique, in which scaled ab initio force constants and structures are mathematically transformed into molecular mechanics energy parameters, has several advantages compared to least squares fitting procedures. Torsion potentials are discussed separately, with special attention given to C(sp2)C(sp2) bonds. Finally, a few existing force fields are compared in their ability to reproduce vibrational frequencies.

A comparison of methods for decomposing overlapping bands in Raman spectra

Abstract Curve fitting and different deconvolution procedures for analysing overlapping bands in Raman spectra are compared. The deconvolution methods considered are direct Fourier deconvolution, iterative Fourier deconvolution and spectral estimate according to the principle of maximum entropy. Both advantages and disadvantages of these processes are discussed. The reliability of decomposed peaks has been investigated using simulated spectra which were composed of linear combinations of Voigt profiles. As expected, deconvolution and curve fitting are advantageous to use in combination.