Grantee Research Project Results
1997 Progress Report: Design and Synthesis of Dendrimer Block Copolymers as Nanoporous Membranes for Environmental Separation Applications
EPA Grant Number: R825224Title: Design and Synthesis of Dendrimer Block Copolymers as Nanoporous Membranes for Environmental Separation Applications
Investigators: Hammond, Paula T.
Institution: University of Colorado at Boulder
EPA Project Officer: Hahn, Intaek
Project Period: November 1, 1996 through September 30, 2001
Project Period Covered by this Report: November 1, 1996 through September 30, 1997
Project Amount: $472,548
RFA: Exploratory Research - Early Career Awards (1996) RFA Text | Recipients Lists
Research Category: Early Career Awards
Objective:
The development of ultrathin membranes designed with a molecular level of control provides the potential to greatly broaden the range of applications for synthetic membranes. In particular, monolayer microporous films that may be designed with a specific pore size or range of pore sizes are attractive. The monolayer chemistry could be altered to attain selective adsorption, or even reaction, with a specific permeant. These features make ultrathin polymer membranes attractive as pollution control agents on exhausts, as protective coatings on clothing, as highly selective membranes on polymer supports, and as microporous selective or reactive layers in gas or aqueous purifications or conversions.The porous interiors of hyperbranched polymers called dendrimers have led to investigations of their use as unimolecular micelles and as hosts for smaller molecular species. Dendrimer chemistry has been shown to be extremely versatile; their chemical structures may be varied to produce corresponding changes in the internal porosity of the dendrimer, and the inherent pore size. It is the objective of this study to design and synthesize nanoporous dendrimeric block copolymers specifically for membrane applications. Ultrathin films will be formed from the most promising dendrimeric systems. Because these are new materials, the amphiphilic nature of these materials, and the nature of the resulting films, will be of great interest, and must be investigated to determine the morphology of the resulting films. Polymer membranes will be formed as skin layers in ultrathin film/substrate composite membranes using self-assembly techniques, and the permeation properties and selectivity of the dendrimeric films as gas or liquid separations membranes will be determined as a function of the dendrimer molecular structure and film characteristics.
Progress Summary:
Our research group has successfully completed the synthesis of two new series of dendrimeric diblock copolymers as part of an ongoing program investigating the use of dendrimer copolymer thin films for gas separations membranes and solution separations applications. Dendrimer block copolymers, consisting of a linear polymer block attached to a dendrimeric block, have been synthesized such that the dendrimeric block has functional end groups (i.e., amino or ester groups). These structures include a dendron branched structure and a dendrimeric rod-like structure, both shown schematically in Figure 1. The structure on the left is a classical dendron attached to a linear polymer at the initial branch point. The dendrimer rod on the right is formed from the functionalization of a polyethylene imine backbone with dendritic branches along a main chain polymer backbone, to form a densely packed rod structure due to the steric effects of the branches. Current versions of these materials have been synthesized for the purpose of forming bilayer and multilayer thin films for selective membrane applications. The existing materials consist of a polyethylene oxide linear block and polyamidoamine dendritic blocks which have been end functionalized with nonpolar stearic groups to yield an amphiphilic polymer. Characterization of these block copolymers, and an examination of their solution and diblock copolymer properties is described below.
PAMAM-Dendron Block
Two series of linear-dendritic diblock copolymers with a linear polyethyleneoxide block and a dendritic polyamidoamine block have been successfully synthesized according to the synthetic sheme . In the first series, the polyethyleneoxide tail had a molecular weight of 2000 with the dendrimer generations going up to 4.0 [ PEO2k- 0.5G,1.0G,1.5G 4.0G]. In the second series, the polyethyleneoxide tail had a molecular weight of 5000 with dendrimer generation going up to 4.5 [PEO5k- 0.5G,1.0G,1.5G .4.5G]. The chemical structure of the copolymers was ascertained using 1H NMR. The peak positions obtained were in good agreement with values found by other researchers for a similar class of materials.
We have found that the PEO(2000)-dendrimer diblocks behave like linear polymers with the [h] increasing with increasing generation number. The relation between [h] and generation number of the dendrimer for the PEO(5000)-dendrimer diblock series is very different. In both ester and amine terminated PEO(5000)- dendrimer diblocks, the PEO chain probably forms a corona around the hydrophobic dendrimer repeat units shielding them from the solvent.
The dendron-block copolymers become amphiphilic when a stearyl group is attached to the amino end functional groups by reaction with stearic acid in the presence of dicyclocarbodiimide (DCC). The hybrid linear-dendritic diblocks with stearate end groups form condensed phases at the air-water interface as indicated by the high surface pressures achieved in the pressure-area isotherm before collapse. We have found that the relative size of the PEO and dendron blocks is important to the arrangement of the block copolymers at the air-water interface; large generations form more stable films. Uniform films have been formed from bilayers of the third and fourth generation dendrimer block copolymers.
.3.2 Dendritic Rod Diblock Copolymers
We have introduced a new architecture, the linear-dendritic rod diblock copolymer which is a combination of the hybrid linear-dendrimer diblock copolymer and the dendritic rod. We expect that the solution properties of this linear-dendritic rod diblock copolymer will be dependent on the surface functionality, the generation number, and the relative length of the two blocks.
We have successfully synthesized polymers consisting of a polyethylene oxide- polyethylene imine diblock copolymer backbone around which polyamido amine (PAMAM) type branches were constructed. The chemical structure of these diblock copolymers has been verified by 1H NMR and FTIR. At lower generations, generation 1.5 and less, branch addition appeared to be almost quantitative. However, at higher generation, branch addition was not 100% complete, and in the amine terminated generations, a small amount of branch crosslinking was observed in generation 3.0. Nonetheless, we were still been able to continue adding higher generations to the diblock copolymers.
The solution properties of these dendritic rod diblock copolymers were studied by intrinsic viscosity in methanol. In general, the intrinsic viscosity of both the amine and the ester generations decreased with increasing generation. Dynamic light scattering was also used to study the solution properties of these dendritic rod diblock copolymers in methanol. In somewhat dilute solution, the polyethylene oxide-polyethylene imine diblock copolymer backbone and the generation 0.5 ester terminated diblock copolymer both appeared to form micelles, while at similar concentrations, the higher generation dendritic diblock copolymers did not show this behavior. The thermal transitions of these dendritic rod diblock copolymers have also been studied by differential scanning calorimetry (DSC). Preliminary characterization results suggest that these materials are phase segregated, and should arrange in ordered block copolymer domains under appropriate annealing conditions.
Future Activities:
Future work will include further investigation of the PAMAM-dendron thin films, with an emphasis placed on the build up of multiple layers. Bilayer films will be formed on porous supports for simple transport measurements. PAMAM-Dendritic Rod systems will be synthesized to higher generation, and placed in the LB trough to determine the stabiilty of the films at the air-water interface. Final films will be examined and compared to the dendron polymers for morphology and structure. Finally, micron thick block copolymer films will be cast and annealed to make oriented block copolymer thin films.Journal Articles on this Report : 2 Displayed | Download in RIS Format
| Other project views: | All 5 publications | 2 publications in selected types | All 2 journal articles |
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Iyer J, Hammond PT. Langmuir behavior and ultrathin films of new linear-dendritic diblock copolymers. Langmuir 1999;15(4):1299-1306. |
R825224 (1997) |
not available |
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Johnson MA, Santini CMB, Iyer J, Satija S, Ivkov R, Hammond PT. Neutron reflectivity of linear-dendritic diblock copolymer monolayers. Macromolecules 2002;35(1):231-238. |
R825224 (1997) |
not available |
Supplemental Keywords:
polymers, engineering, chemistry, chemical transport, filters, contaminant removal, water, air., Scientific Discipline, Water, Chemical Engineering, Engineering, Chemistry, & Physics, Analytical Chemistry, chemical transport modeling, dendrimer block copolymers, separation applications, polymer chemistry, transport model, nanoporous membranes, chemical composition, chemical detection techniques, ultrathin organic films, chemical kinetics, waste reductionProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.