X-ray Analysis and Molecular Modeling of Poly(vinyl alcohol)s with Different Stereoregularities J. D. Cho, W. S. Lyoo, † S. N. Chvalun, ‡ and J. Blackwell* Department of Macromolecular Science, Case Western Reserve University, Cleveland, Ohio 44106-7202 Received May 28, 1999; Revised Manuscript Received July 29, 1999 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  ) 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  ) 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. Introduction Atactic poly(vinyl alcohol) (PVA) prepared by saponi- fication of poly(vinyl acetate) is distinctly crystalline despite its lack of stereoregularity. 1-3 X-ray fiber dia- grams point to a planar zigzag backbone conformation with an advance per monomer in the region of 2.52 Å. Bunn 1,3 indexed the observed Bragg reflections with a monoclinic unit cell with the following dimensions: a ) 7.81 Å, b ) 2.52 Å (chain axis), c ) 5.51 Å, and  ) 91.42°, containing single monomer units of two chains. These dimensions have been confirmed in several later analyses. 4-9 The ac projection of the model proposed by Bunn is shown in Figure 1a, and this illustrates the possible hydrogen bonding for the two monomer con- figurations. The unit cell describes an average structure (copolymer) in which each monomer has half hydroxyls at the two possible positions. An alternative packing arrangement has been proposed by Sakurada et al. 2 in which the chains have a different setting angle and the hydrogen bonds point along the a axis, as shown in Figure 1b. Recently there has been interest in the production of high performance fibers based on high molecular weight (HMW) syndiotacticity-rich PVA, which have mechan- ical and physical properties that are superior to those of preparations of the atactic polymer. 10-15 This im- provement is ascribed to the higher crystallinity due to the increased stereoregularity. In our laboratories, it was found that oriented microfibrils of PVA were formed during saponification of poly(vinyl pivalate) (PVPi). 16,17 Control of the molecular weight and stereoregularity was achieved by saponification of copolymers of vinyl pivalate (VPi) and vinyl acetate (VAc). Depending on the comonomer ratio and the concentration of initiator (2,2′-azobis(2,4-dimethylvaleronitrile)), the PVAs ob- tained had number-average molecular weights in the range (2.47-7.26) × 10 5 (DP ) (5.6-16.5) × 10 3 ) and syndiotactic diad (S-diad) contents of 52.8-61.5%. The highest molecular weight and S-diad content were obtained for specimens derived from the VPi homopoly- mer prepared at low initiator concentration, and these parameters declined progressively with increasing VAc content in the parent copolymer. Interestingly, the structures of these PVAs show a consistent transition between ∼55 and 57% S-diad contents. Specimens with lower S-diad contents have shapeless morphologies, but above 56% S-diad content, the polymers have a fibrous * To whom correspondence should be addressed. Telephone: (216) 368-6370. Fax: (216) 368-4202. E-mail: jxb6@po.cwru.edu. † Present address: School of Textiles, Yeungnam University, Kyongsan 712-749, Republic of Korea. ‡ Permanent Address: Polymer Structure Laboratory, Karpov Institute of Physical Chemistry, Ul. Vorontsovo Pole 10, Moscow, 103064, Russia. Figure 1. ac projection of the crystal structure of atactic PVA proposed by (a) Bunn, 1 and (b) Sakurada. 2 (dark circle, carbon; white circle, oxygen; dashed line, hydrogen bond). 6236 Macromolecules 1999, 32, 6236-6241 10.1021/ma9908402 CCC: $18.00 © 1999 American Chemical Society Published on Web 09/03/1999