Investigation of polymers in solid state through DFT-based analysis of FT-IR spectra

Methodology Steps:

1. Structure Preparation:
o Simulation boxes for various polymers (acrylic acid, styrene, polystyrene, polypropylene, vinyl alcohol, and butyl acrylate) were constructed using crystallographic data, including space group and atomic coordinates.
o The crystallographic structures were built using VESTA software.

2. Density Functional Theory (DFT) Setup:
o Gaussian Plane Wave (GPW) DFT simulations were performed using the CP2K package with an energy cutoff of 300 Ry.
o The electronic structure was modeled using the BLYP exchange-correlation functional with dispersion corrections, GTH pseudopotentials, and a single-zeta basis set (SZV-MOLOPT-GTH).

3. Pre-optimization (Molecular Dynamics - MD):
o Structures were minimized with the steep descent algorithm (2000 steps).
o Annealing was performed in an NVT ensemble, heating from 0 K to 300 K over 1 ns with a 0.5 fs timestep.

4. Geometry Optimization:
o Post-annealing, structures were optimized using the limited-memory Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithm.
o Optimization continued until the forces were reduced below 2.0 x 10⁻⁶ Eh/Bohr.
o Orbital Transformation (OT) method with direct inversion in the iterative subspace was applied for faster convergence.

5. Vibrational Analysis:
o DFT-based vibrational analyses were conducted at the same level of theory.
o The Hessian matrix was built using a step size of 10⁻² Bohr radii with a three-point central difference approximation.
o The matrix was diagonalized to obtain eigenvalues and eigenvectors, determining normal mode frequencies.

6. IR Spectra Calculation:
o Infrared spectra were generated from normal mode analysis within the Born-Oppenheimer approximation.
o Spectral frequencies were scaled with a factor between 0.88 and 0.95, based on a Pseudo-Voigt profile fit in the 500-3800 cm⁻¹ range.
o Spectra were broadened using a Gaussian line-shape with a full width at half maximum (FWHM) of 10 cm⁻¹.

7. Vibrational Mode Assignment:
o Mode assignments were made through visual inspection of animated normal modes using the Molden program, considering the symmetry of the modes in regions of interest.

Current Spectra:
acrylic acid

polystyrene

styrene

polypropylene