Reports: ND751622-ND7: Dynamic and Morphological Characterization of Reverse Osmosis Membranes

Sungsool Wi, PhD, Virginia Tech

The fund was used for the following items:

  1. Support for a graduate student, Ying Chen, who was involved in the development of the project; Fall 2011, Spring 2012, and Summer 2012.
  2. The summer salary (2012) and conference travel (ENC 2012) of the PI.
  3. User fee of 300 MHz and 600 MHz solid-state NMR spectrometers.

Molecular dynamics study of polymeric materials by solid-state NMR (Sungsool Wi and James McGrath): Solid-state NMR spectroscopic methods have been used to understand the physico-chemical properties of polymeric membranes applicable for the purification of water by reverse osmosis. The internal molecular dynamics controls the mechanical properties of materials, thus insight into the nature of motional processes governing local segmental motion may provide better understanding and control of the molecular parameters for optimizing the desired physico-chemical properties of materials. One major factor in designing polymer membrane materials is an understanding of the basic correlations between the chain dynamics of polymers and the transport of permeant molecules. The transport of permeant molecules within a dense membrane is determined by the diffusivity and solubility of permeants in the membrane. In a dense membrane, very small, temporary pores or cavities (3-5 Å in diameter) formed between the strands of polymeric chains act as free volume elements and determine the diffusivity and solubility of permeant molecules. Permeant molecules occupied in these cavities diffuse through the membrane down a concentration or pressure gradient. These cavities are dynamic because their sizes are within the range of the thermal motions of the polymer chains. Thus, the transport of permeant molecules through the membrane is largely coupled to the thermal motions of polymer chains. The thermal motion of polymeric chains can lift a few strands of polymeric chains by a statistical chance, forming a temporary channel connecting two adjacent cavities. This then allows permeant molecules to make a jump into the adjacent cavity where it remains until another channel is formed–giving it the opportunity to jump into another cavity.

One approach incorporated in our experiments is to change the thermal history of candidate polymers to increase the diffusion of water. Diffusion of permeant molecules through an ionomeric membrane is more complicated. The presence of hydrophilic sulfonate groups and hydrophobic polymer chains provides nanophase-separated hydrophilic and hydrophobic domains. The hydrophilic domains act as a water pocket and are scattered around in the matrix. Both the shape of hydrophilic domain and the cavities therein are dynamic and influenced by the thermal motions of the polymer chains. Permeant molecules in both hydrophilic and hydrophobic domains undergo nanoscopic, dynamic fluctuations in response to the segmental reorientations of the polymer chains. For instance, water molecules captured in hydrophilic domains thermally diffuse in the polymer matrix via temporarily formed tortuous channels connecting hydrophilic domains. Polymeric backbone structures undergo slow rotational reorientations. The Centerband-Only Detection Exchange (CODEX) spectroscopy was carried out on disulfonated poly(arylene ether sulfone] (PAES) random block copolymers (BisA-X; X is the degree of sulfonation [DS]) with and without thermal annealing effect. An example is an evidence demonstrating a correlation between the water permeability and slow reorientational chain motions. Thermal annealing effects modify both the rate of the chain reorientational motions and macroscopic water permeability when the DS value is low. For BisA-20, the motional correlation time obtained from the sample casted on a hot plate at 60 oC (without thermal annealing) is faster than that of the sample casted at 150 oC (with thermal annealing) throughout a temperature range of 22-100 oC. The sample state obtained on the 60 oC plate approaches that of the sample obtained at 150 oC when the temperature of NMR experiment approaches 80 oC. This means that BisA-20 undergoes a thermal annealing process around 80 oC and a membrane film must be obtained at a temperature below 80 oC to maintain its intrinsic water permeability property. The thermal annealing process modifies the polymer matrix by increasing the packing order. The chain reorientational motion in a less densely-packed polymeric matrix without thermal annealing would be faster. Faster reorientational chain motion, in turn, leads to a faster diffusion of water molecules in the polymer matrix, resulting in improved water permeability.

Crystallinity and Motional Dynamics Study of a Series of Poly(Arylene Ether Sulfone) Segmented Copolymer Analogues. Solid-state NMR spectroscopy was utilized to study the crystallinity and its correlation to the motional dynamics of a series of biphenol based poly(arylene ether sulfone) (PAES) copolymer analogues obtained by incorporating flexible aliphatic blocks. Introduction of a series of conformationally flexible aliphatic blocks into the rigid aromatic PAES blocks in the copolymer sequence had increased the crystallinity of the polymer matrix because the copolymer system with aliphatic blocks provided a decrease in the glass transition temperature (Tg) while maintaining a non-variant melting temperature (Tm). Modified PAES copolymer systems with aliphatic blocks had yielded shorter 1H T1 relaxation times and longer 1H T1rho relaxation times relative to the neat aromatic PAES copolymer. Trends observed in 1H T1rho and T1 data had demonstrated direct correlations to the observed temperature difference (Tm – Tg), thus to the amount of crystallinity in the polymer matrix. Slow segmental reorientations of PAES blocks in a few milliseconds range also became slightly faster as the size of an aliphatic, segmented block became larger. Additionally, the local electronic environments of aromatic PAES blocks were invariant to the incorporation of aliphatic segments in the copolymer sequence. This work was published in the Journal of Physical Chemistry B.