Polymers Division

NIST Combinatorial Methods Center Focused Project

The NIST Combinatorial Methods Center is pleased to announce the initiation of a two year focused project to develop an integrated measurement platform for polymer solutions and blends for the coatings and surface-active solutions industries.  The project focuses on the adaptation of in-situ millifludics for integrated characterization of solution rheology, light scattering, and film forming characteristics.  These properties will be measured in a model system using composition and temperature as processing variables.  We are inviting NCMC member organizations and others to join the project.  The members of the focused project will help the NCMC prioritize the requirements for selecting a model system and optimize the corresponding integrated measurement platform.  Research results will be available to focused project members throughout the project period and member organizations will be able to participate directly with NIST researchers in development of new measurement systems.

E-mail:  kathryn.beers@nist.gov 

Development of advanced formulations in the coatings and surfactant industries requires the exploration of a multi-parameter space that includes varying amounts of key components and appropriate processing conditions, and correlating these to final product properties.^1,2  Finding new formulations and processing conditions for an application, or fine-tuning an existing system, requires extensive sample preparation and testing.  For polymer solutions and polymer blends to realize their full market potential in a cost effective manner, new high-throughput technologies in sample preparation and analysis are needed.  The thin film technologies developed by the NCMC, along with new initiatives in millifluidics for polymer formulations, are promising routes for development of new integrated measurement technologies that provide rapid development and cost effectiveness despite the complex, multi-parameter space of the polymeric formulations.

Polymer solutions and polymer blends are widely used by the coatings, detergent and personal care industries in order to obtain a wide range of final properties.  This fine-tuning is obtainable due to the dispersion and synergy of different component polymers and stabilizers (e.g., surfactants) and other property modifier additives.  Polymer blending provides a convenient combination of individual homopolymer characteristics, which is often non-linear in composition, and therefore yields a wide range of enhanced properties without the averaging effects seen in statistical copolymers.  The ability to tune a material for a particular application using polymer solutions and/or blends comes at the cost of substantial experimental effort and reliance on empirical models and experience.  Application of combinatorial methods to formulation science is a facile way to address these problems.

Recently the NCMC has undertaken initiatives to develop modular, fluidics-based techniques for polymer formulations.  These new techniques include: 1) Multi component mixing and generation of combinatorial solution and blend libraries; 2) Measurement of viscosity using a capillary fluidics-based viscometer and rheology (including complex viscosity) using a dynamic mini-rheometer; 3) Light scattering on a fluidics chip.  Additionally, the NCMC has already developed and published techniques in thin film formation and metrology.^2,3  These methodologies will be used in an integrated form and tailored to select model-solution studies of relevance to the NCMC partners.

Each of these developing technologies will be used and tailored considering the model system chosen and then integrated into a measurement platform for model coatings and surface-active agents.  The resulting data will be processed and managed using the NCMC informatics database.  This medium for processing and visualizing results will be used in collaboration with member companies to further optimize the measurement capabilities for robust, quantitative analysis methods.

Thin film libraries and metrology

            The NCMC has demonstrated expertise in the creation of thickness, composition, and thermal gradient libraries of thin films.³  Additionally, we have demonstrated combinatorial measurements of morphology, surface energy, modulus, and adhesion of these thin-film libraries.  More information and relevant publications are available through the NCMC web page.⁶

Rapid prototyping of fluidic devices and multi-component mixing

We have recently developed a prototyping technique for the fabrication of fluidic devices in a polymeric matrix.⁷  The method employs contact lithography and a UV-curable adhesive and is capable of producing features with accurately controlled vertical (from 100s mm to millimeters) and lateral dimensions (from 10s mm and above).  These features challenge conventional photolithography capabilities and are particularly suited for millifluidic research.  Moreover, the devices can be made impervious to a range of organic solvents, a requirement for most polymer formulations studies.  In addition, the technique is inherently fast and devices can be produced and rapidly, allowing for multiple iterations in design.  An example of a ternary active mixer is given in Figure 2a.

We have demonstrated and validated this application of microfluidics with a study of polymer blend phase behavior.  Preparation of a combinatorial library of a binary mixture was performed using several syringe pumps, XYZ motion stages and controllers, and a desktop computer with home-developed software to control the system.  Figure 2b shows an example of such a PS/PB library isothermally annealed through the thermodynamic miscibility gap.  The entire phase diagram is obtained by varying temperature with time, in a high throughput manner with ultrahigh composition resolution (Df=0.01).  Alternatively, liquid formulations of continuously varying compositions with time can be produced with a modular mixer (as shown in 2a).

Millifluidics viscosity chip

            A simple capillary viscometer has been fabricated using our rapid lithography technique mentioned above.  The principle of operation is simple and works well with Newtonian fluids.  The fluid velocity through a channel of known dimensions is measured for a given pressure drop and the viscosity is then calculated.  For a cylindrical channel, the well-known equation for a capillary viscometer is used:

where R is the radius of the capillary, DP is the pressure drop, Q is the volumetric flow rate, and L is the length of the capillary.  Although this device works best with Newtonian fluids, shear rate dependence may be explored along with potential wall slip effects by varying the channel dimensions and pressure drop.  The advantage of the capillary viscometer is its ease of use and the straightforward calculation of the apparent viscosity.  Figure 3 displays one such device constructed in our laboratory where an array of channels are used to access different shear rates.  Given the inexpensive design and fabrication of these devices, they could be used in a semi-disposable manner.

Mini, magnetic field driven combi-rheometer

            Polymer formulations and blends are complicated, often non-Newtonian, fluids.  This requires the ability to make measurements of viscosity at different shear rates, and preferably to measure dynamic properties such as relaxation/compliance, complex viscosity, and complex modulus.  These requirements have led to our development of a dynamic rheometer capable of measuring the properties of multiple samples.  A prototype rheometer has been developed which uses magnetic fields in a two axis Helmholtz coil to provide a uniform shear stress across four samples.  The device is a stress-controlled rheometer using either a Couette or disk geometry.

Figure 4. Combi Rheometer. A multi sample, stress controlled rheometer using magnetic fields from a two axis Helmholtz coil to apply uniform force across the samples.

Our new millifluidics approach to investigating the issues relevant to polymer solutions will be modified/adapted to an integrated modular platform suitable for flow coating capabilities for polymer blend coatings.  The technical approach will involve the following steps:

1.       Optimize existing library generation methods for identified parameters. These include utilizing conventional pumps for inducting pre-mixed solutions with varying polymer composition, and controlling formulation components and temperature. The basic approach will involve flowing polymer solutions through millifluidic channels custom designed by rapid prototyping methods for solution property measurements. Adaptation of knife blade draw techniques on motorized stages and open channel architecture at the exit of the millifluidic channels can be used to generate polymer blend coatings.

3.    Demonstrate informatics feedback with respect to each aspect of the overall system (e.g., composition, mixing, rheology, light scattering, and select end properties).

¹R. Dagani, “Tapping into NIST’s combi expertise,” Chem. & Eng. News 80, 58-60 (2002).

²“Special Issue: High-Throughput and Combinatorial Methods in Polymer Science,” Macromol. Rapid Comm. 24, (2003); especially: R. Hoogenboom, M. A. R. Meier, U. S. Schubert, “Combinatorial methods, automated synthesis and high-throughput screening in polymer research: past and present,” Macromol. Rapid Commun. 24, 16-32 (2003).

³J. C. Meredith, A. Karim, E. J. Amis, “High-throughput measurement of polymer blend phase behavior,” Macromolecules 33, 5760-5762 (2000); and J. C. Meredith, A. P. Smith, A. Karim, E. J. Amis, “High-throughput measurement of polymer blend phase behavior,” Macromolecules 33, 9747- (2000)

⁴C.M. Stafford, C. Harrison, A. Karim, E. J. Amis, “Measuring modulus of gradient polymer films by strain-induced buckling instabilities,” ACS Polymer Preprints 43, 1335 (2002).

⁵A. J. Crosby, A. Karim, E. J. Amis, “Combinatorial investigations of interfacial failure,” J. Polym. Sci. Polm. Phys. 41, 883-891 (2003).

⁶http://polymers.msel.nist.gov/combi/index.html

⁷C. Harrison, J. T. Cabral, C. M. Stafford, E. J. Amis, A. Karim, “A rapid prototyping technique for fabrication of solvent-resistant structures,” Lab on a Chip J., to be submitted (2003); preliminary results available to NCMC members.

Figure 5.  Proposed integration of fluidics modules with existing methods in gradient thin film preparation and analysis. Films generated by this focus project could be subjected to further study such as adhesion testing (not a part of this focus project).  Composition and temperature gradients exist on each chip.

A meeting will be held approximately six weeks after the formal launch of the project as well as at six-month intervals for the duration of the project.  Quarterly reports will be submitted to the members, with updates more frequently via conference calls and other postings.  In order to facilitate the collaboration, specifications for methods, instruments, programs, data analysis, and other aspects of this work will be available to members during the course of the project.  A summary report will be provided within two months of the end of the project.  The NCMC labs will be open to prearranged visits from member scientists interested in hands-on participation in method development.

As with base level membership in the NCMC, all of the research carried out in the Focused Project is non-proprietary and is intended for publication in the public domain. No proprietary information or materials will be solicited or accepted by NIST from member organizations. The scope of the work by NIST included in this focused project is limited as described in Appendix A of the focused project agreement.

• Select suitable model polymer solution and polymer blend system and define parameters, variables, etc. of interest to the project team.
• Develop strategy to tailor investigation of the model systems using millifluidic and blend film coating techniques including prototype development, library generation, and analysis.
• Conduct preliminary tests on a selected model polymer formulation and blend film system.

• Test model systems using the tailored, integrated prototype measurement platform. Issues in library generation, high-throughput measurement, and analysis will be considered.
• Integrate the above system into the NCMC informatics database.
• Allow focused project members to investigate a suitable non-proprietary commercial solution or blend system to study cause effect relationships between parameters and performance using the integrated system developed and provide feedback for optimization.

NIST
Attn: Administrative Officer, Polymers Division
100 Bureau Drive
Mailstop 8540
Gaithersburg, MD USA
20899