Abstract
Delta (δ) and epsilon (ε) co-crystalline forms of syndiotactic polystyrene with a carboxylic acid guest were obtained by sorption of liquid hexanoic acid in syndiotactic polystyrene films exhibiting delta and epsilon nanoporous-crystalline forms. The characterization study is facilitated by axially stretched syndiotactic polystyrene films, used both for polarized FTIR spectra and for WAXD fiber patterns. Particularly informative are two carbonyl-stretching FTIR peaks, attributed to monomeric and dimeric hexanoic acid. The dichroism of these carbonyl peaks indicates that both delta and epsilon phases are able to include hexanoic acid as isolated guest molecules, while only the epsilon phase is also able to include dimeric hexanoic acid molecules in its crystalline channels. The inclusion of both isolated and dimeric hexanoic acid species in the epsilon form crystalline channels produces extremely fast hexanoic acid uptakes by syndiotactic polystyrene epsilon form films.
Highlights
Co-crystalline (CC) forms between a polymeric host and low-molecular-mass guest molecules are known for many polymers [1]
Forms and nanoporous-crystalline (NC) forms (δ [14,15,16,17,18] and ε [19] for syndiotactic polystyrene (s-PS) and α and β for PPO [20,21]), i.e., crystalline forms whose density is lower than the corresponding amorphous phases
By using suitable procedures, stable CC forms of s-PS and PPO can be prepared with a large number of organic compounds, provided that their molecular volume is smaller than 0.2−0.3 nm3 [1,16]
Summary
Co-crystalline (CC) forms between a polymeric host and low-molecular-mass guest molecules are known for many polymers [1]. The behavior of two polymers, syndiotactic polystyrene (s-PS) and poly(2,6-dimethyl1,4-phenylene ether) (PPO), is completely different: they are able to form CC forms and nanoporous-crystalline (NC) forms (δ [14,15,16,17,18] and ε [19] for s-PS and α and β for PPO [20,21]), i.e., crystalline forms whose density is lower than the corresponding amorphous phases These kinetically stable NC forms are able to absorb large amounts of many different kinds of guest molecules, leading to corresponding CC forms. Informative are the carbonyl stretching FTIR peaks, whose position depends on the presence or absence of hydrogen bonds between acid guest molecules [34,37,38,39,40,41,42]
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