ADVANCED MATERIALS

Membranes for Clean Water

Objective

This project provides measurement solutions that probe the surface 

and internal structure of polymer membranes used in water purification, 

and correlate that structure to the transport of water and other species 

through the membrane. Our methods are focused on elucidating the 

role of roughness, surface charge, surface chemistry, crosslink density 

and monomer chemistry on the interfacial and internal dynamics of the 

membrane. With this knowledge, industry will better understand the 

performance of these membrane materials as well as identify essential 

features to enable next-generation, energy-efficient, high-flux membranes.

Approach

Our team is developing measurement methods to correlate membrane 

structure and dynamics to performance (flux and selectivity) and improve 

reproducibility and comparability of measurements on these materials. 

We have developed an approach for synthesizing model crosslinked polyamide membranes based on molecular layer-by-layer (mLbL) deposition, 

which enables precise control over network structure, surface chemistry, 

residual charge, roughness and film thickness as well as placement of 

specific functional groups at defined locations within the network.

Armed with these model membranes as a reference, we apply our expertise in advanced surface analytical tools such as X-ray photoelectron 

spectroscopy, near-edge X-ray adsorption spectroscopy fine structure 

and atomic force microscopy to assess the impact of surface structure on 

permeability and transport of water and solutes across the water-membrane interface. We also employ scattering techniques such as X-ray/neutron reflectivity and high-flux backscattering to measure the molecular 

topology and chain motions of the membrane network, which will be correlated to the diffusion and transport of water and solutes through the 

membrane.

Impact and Customers

Access to affordable, clean water is vital to the nation’s economic growth 

and security. Polymer-based membrane separation technologies based 

on reverse osmosis, forward osmosis and nanofiltration will play an increasingly critical role in the production of clean, safe water. In order to 

remain competitive, these membrane technologies must demonstrate 

improvements in water flux and reductions in energy consumption.

However, the membrane community lacks a “quantitative understanding 

of the connections between the molecular structure of the membrane 

and water permeability.”1 Such an understanding will guide innovative 

designs of next-generation water purification membranes with improved 

performance and reduced energy demands.

Our team works closely with leaders in the field of membrane science and 

water purification including General Electric, Dow Filmtec, Millipore, Pall 

Corporation and the University of Pennsylvania.

Model 

Membranes

Fundamentals

Membrane

Performance/Degradation

1Society News, MRS Bulletin 37, 163, (2012).
Molecular Layer-by-Layer Deposition of 

Model Polyamide Active Layers

To form the polyamide active layer used 

in reverse osmosis membranes, interfacial 

polymerization of trimesoyl chloride (TMC) 

and m-phenylene diamine (MPD) occurs at 

an organic-water interface. Although effective, the rapid polymerization rate and 

unstable reaction conditions produce films 

with rough surface structures and chemical 

heterogeneity. Therefore, quantification of 

membrane structure-property relationships 

is difficult, since changes in synthesis conditions alter multiple properties, and film 

roughness prevents depth profiling analysis 

techniques. 

To address this measurement need, we developed an approach based on a molecular 

layer-by-layer (mLbL) assembly process, alternating single reactions of TMC and MPD, 

for producing model polyamide active layers (Figure 1).

This process affords exquisite control of 

membrane topology, monomer placement, 

surface chemistry and film thickness. Furthermore, the resulting chemistry at the 

mLbL surface provides a facile route for subsequent chemical modification of the membrane surface, which allows for systematic 

studies of the effect of surface chemistry 

and morphology on permeability and transport of molecules across the membrane 

interface. We control film thickness directly 

by the number of sequential reaction steps 

performed. The resulting films exhibit more 

than an order of magnitude reduction in 

surface roughness as compared to conventional interfacial polymerization, thus enabling advanced measurements of molecular topology and transport (Figure 2).

Mechanical Properties of Commercial 

Polyamide Active Layers

During use, the polyamide active layer 

can undergo chemical degradation due to 

operating at high pressures and temperatures, as well as the introduction of chlorine 

to prevent membrane fouling. This chemical degradation can lead to changes in the 

mechanical properties of the membrane 

active layer that eventually lead to membrane failure. There is, however, a dearth of 

information concerning the full mechanical 

spectrum of these types of thin layers due 

to the challenge of measuring materials at 

the nanometer scale.

In response to this need, we established 

and validated a measurement method 

based on wrinkling-cracking phenomena 

that allows unambiguous measurements of 

three fundamental mechanical properties 

in nanoscale thin film geometries, including elastic modulus, strength and fracture 

strain. In this approach, we transfer the 

active layer from commercial membranes 

onto a soft, elastomeric substrate and apply an increasing uniaxial tensile strain (ε). 

At relatively low strains, a critical point is 

reached where a periodic wrinkling pattern appears parallel to the applied strain 

with a well-defined wavelength (λ). As described previously and extensively investigated, this wavelength can be related to 

the elastic modulus or stiffness of the film. 

At higher strains, the film begins to crack 

orthogonal to the strain direction, showing 

multiple cracks with roughly equal spacing 

(d) that decreases with increasing strain. By 

monitoring the average crack spacing as a 

function of strain, we can deduce both the 

fracture strength and onset fracture strain 

of the film. The ability to measure these 

three material properties provides key figures of merit for assessing the mechanical 

characteristics of thin films and membranes 

(Figure 3).

Learn More

Christopher M. Stafford 

Project Leader

301-975-4368

chris.stafford@nist.gov

http://www.nist.gov/mml/polymers/

Publications

PM Johnson, J Yoon, JY Kelly, JA Howarter, CM Stafford, Molecular layer-by-layer deposition of highly crosslinked polyamide films, J Polym Sci B: Polym Phys 50 (3), 168-173 (2012).

JY Chung, J-H Lee, KL Beers, CM Stafford, Stiffness, strength, and ductility of nanoscale thin films 

and membranes: a combined wrinkling-cracking methodology, Nano Lett 11 (11), 3361-3365 (2011).

EP Chan, Y Hu, PM Johnson, Z Suo, CM Stafford, Spherical indentation testing of poroelastic relaxations in thin hydrogel layers, Soft Matter 8 (5), 1492-1498 (2012).

JY Chung, AJ Nolte, CM Stafford, Surface wrinkling: a versatile platform for measuring thin-film properties, Adv Mater 23 (3), 349-368 (2011).

ADVANCED MATERIALS

Figure 3. Optical image of wrinkling-cracking of a thin 

membrane active layer.

Figure 2. Atomic force microscopy images comparing 

mLbL film to its interfacially polymerized counterpart.

Figure 1. Film growth rate of mLbL assembly as a function 

of number of bilayers deposited.

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Accomplishments