X-ray Analysis and Molecular Modeling of Poly(vinyl alcohol)s with Different Stereoregularities

Abstract

Wide-angle X-ray methods have been used to compare the structures of poly(vinyl alcohol) (PVA) with syndiotactic diad (S-diad) contents in the range 51-63%. The fiber diagram of a PVA with 51.2% S-diad content (essentially atactic) is indexed by a monoclinic unit cell with dimensions a ) 7.82 ( 0.03 Å, b ) 2.53 ( 0.01 Å (chain axis), c ) 5.52 ( 0.01 Å, and β ) 91.5 ( 0.2°. These dimensions are very similar to those proposed by Bunn (Nature 1948, 161, 929) and Sakurada (Bull. Inst. Chem. Res., Kyoto Univ. 1950, 23, 78). As the S-diad content is increased, the crystallites become larger and less distorted and there is lateral contraction of the crystal structure, suggesting that the chains can pack more efficiently. For a syndiotacticity-rich PVA with a S-diad content of 63.1%, the unit cell dimensions are as follows: a ) 7.63 ( 0.02 Å, b ) 2.54 ( 0.01 Å, c ) 5.41 ( 0.01 Å, and β ) 91.2 ( 0.1°. The latter structure has a theoretical density of 1.40 g/mL, which is ∼5% higher than the value of 1.34 g/mL predicted for the atactic polymer. The efficiency of chain packing in the two unit cells has been compared by molecular dynamics modeling of arrays of chain segments of different random sequences, with 50% and 65% S-diad contents. For both structures, the results favor hydrogen bonding similar to that proposed by Bunn rather than that due to Sakurada. NVT calculations show that the larger unit cell is favored for the atactic polymer, while the smaller unit cell is favored for the 65% syndiotactic polymer. A NPT assembly for the atactic model starting in the contracted unit cell expanded and equilibrated close to the observed, lower density structure. Likewise, a starting model for the 65% syndiotactic polymer packed with the dimensions for the atactic polymer was found to adjust to the observed contracted structure. The changes are to be understood in terms of the balance between hydrogen bonds (electrostatic) and van der Waals forces, which vary depending on the tacticity.

Nanostructured poly(vinyl alcohol)/bioactive glass and poly(vinyl alcohol)/chitosan/bioactive glass hybrid scaffolds for biomedical applications

“…XRD analyses were conducted in the 2θ range from 3.03 to 89.91 • with steps of 0.06 • . Narrow peaks identified within the scan range were confirmed using previously published literature [30,31,34,45,46]. …”

Synthesis and characterization of novel, highly crystalline poly(vinyl alcohol) microspheres for chemoembolization therapy

“…PVA is a semicrystalline synthetic polymer whose structure is characterized by chains in transplanar conformation packed in a monoclinic unit cell with a ¼ 7. ; these reflections correspond to the superposition of the equatorial (101) and (101) reflections, respectively. [22][23][24] On the basis of the unit cell proposed by Bunn 22 and other more recent crystallographic studies on the structures of PVA hydrogels, 25,26 PVA microfibrils, 27,28 and PVA films, 29 it was possible to assign the diffraction peaks of sample 1 observed at 2y values of 11.5, 19.5, 23.0, and 40.5 to the (100), (101), (200), and (201) reflections, respectively. Figure 3 shows the degree of crystallinity of the PVA particles calculated from the DSC data with the following correlation:…”

Hydrogen-Bonding Assembly of Rigid-Rod Poly(p-sulfophenylene terephthalamide) and Flexible-Chain Poly(vinyl alcohol) for Transparent, Strong, and Tough Molecular Composites

“…This is because PVA crystals do not have adequate birefringence. 44 As shown in Figure 5A, the PVA/sPPTA composite films remain nonbirefringent even when the content of sPPTA is as high as 8 wt %. This demonstrates that sPPTA disperses very uniformly in PVA matrix and does not form birefringent sPPTA supramolecular assemblies in PVA matrix.…”