Inside the Ionic Aggregates Constrained by Covalently Attached Polymer Chain Segments: Order or Disorder?

Area of Science:

• Materials Science
• Polymer Chemistry
• Crystallography

Background:

• Small-molecule ionic crystals exhibit ordered structures.
• Ionomers are formed by group-substituting ionic crystals with polymer segments.
• The internal structure of ionic aggregates in ionomers remains an open question.

Purpose of the Study:

• To investigate the internal structural order of ionic aggregates in ionomers.
• To determine if constrained ionic aggregates maintain ordered internal structures.
• To characterize the ordering process and its dependence on time.

Main Methods:

• Transmission Electron Microscopy (TEM) electron diffraction to analyze schlieren textures.
• Wide-Angle X-ray Scattering (WAXS) to obtain Bragg-type reflections.
• Differential Scanning Calorimetry (DSC) to study thermal transitions.

Main Results:

• TEM and WAXS data provide solid proof of the internal monocrystalline order within ionic aggregates.
• DSC reveals an order-disorder transition within the aggregates.
• Physical aging at room temperature over 3 months shows a gradual enhancement of the DSC endotherm, indicating slow ordering.

Conclusions:

• Ionic aggregates in Na-salt sulfonated-polystyrene ionomers possess internal crystalline order.
• The ordering within aggregates is a slow relaxation process that increases with time.
• This study corroborates the existence of polymer-bound small-molecule ionic (quasi)crystals, a novel form of order.