This research is aimed at generating functional polymer surfaces with compositional economy, new or improved properties, and facile, conventional processing. One research thrust focuses on P[AB] (co-polyoxetane) soft block polyurethanes as minor constituents (2 wt% or less) in blends such that the co-polyoxetane soft block defines surface properties while the base polymer defines bulk properties. A unique feature is synergistic interactions of A and B co-repeats that produce new surface phenomena. Prior research points to opportunities via increasing the mole fraction charged (quaternary) B co-repeat units in P[AB]-soft block polyurethanes. This strategy is aimed at facilitating physical studies of surface concentration and accelerating kinetics of antimicrobial action. Newly discovered "seeding" of layer by layer (LbL) processing via polycationic modifiers will be explored as a method to quickly and radically change surface properties. Other P[AB]-soft block polyurethanes that confer "contraphilic" wetting will be studied as these surfaces may have unique biocompatibility. The structure and surface dynamics of new semicrystalline fluorous polyoxetanes will be evaluated as promising candidates for environmentally responsible hydrophobic/oleophobic coatings. Finally, modifying condensation cured polydimethylsiloxane networks by a recently discovered approach is aimed at introducing oleophobicity and surface concentration of quaternary charge.
NON-TECHNICAL SUMMARY:
Polymer surfaces are encountered in a myriad of everyday things such as painted or coated surfaces and molded objects. This research is aimed at economically introducing special functions into such surfaces. One subject is generation of durable, intrinsically sterile surfaces in an economical way. The special function of sterility (with economy) is needed for healthcare facilities to thwart the spread of disease. To achieve these and other desirable surface characteristics, special additives are designed for conventional coatings such that a very small amount spontaneously concentrates at the surface during conventional application. Another surface function being developed is a novel wetting characteristic that is counterintuitive: a surface that is initially "water loving" but becomes resistant to wetting after the surface is initially wetted. Such a wetting property looks promising for medical device coatings that require compatibility with body tissue or fluids. Broader impacts of this research include interdisciplinary training in chemistry, surface science and microbiology, which results in high demand for students and postdoctoral scientists by companies such as contact lens producers and companies employing biotechnology for drug manufacture. This research is aimed at effecting broader impacts locally, through undergraduate and graduate course development (e.g., the course "Introduction to Polymers in Medicine") and through underrepresented minority participation in research. On a national level, support for this research facilitates leadership activities such as co-organization of the biennial American Chemical Society K-12 workshops / symposia "Polymer Science of Everyday Things". International collaborative work and student exchange with groups in Japan, Italy, and Sweden play a vital role in understanding how the targeted surfaces are modified.
A challenging goal for this research project has been the development of polymer modifiers that, in small amounts (1) "bloom" to the surface during a conventional coating process and (2) deliver desired surface function. The objective is illustrated in Figure 1 that shows a polymer coating cross-section with one weight percent modifier. The thickness of a typical polymer coating is about three times the thickness of a human hair. It follows, then, that the modifier thickness is about 1 micron or 0.00008 inch or 8 mils. Background. An initial target has been the introduction of non-leaching function for quickly killing bacteria in contact with the surface. Applications for modified coatings or molded objects include items in daily life or especially in hospitals and care facilities that are frequently touched, such as keyboards, cell phones, and doorknobs. Touching such surfaces can pass along pathogenic bacteria. Contact kill surfaces are aimed at blocking this transmission. We chose widely used polyurethanes and set an objective to use just one percent modifier. The purpose of using such a small amount was to make sure that the excellent properties of the substrate polymer (99%) were not changed. Simultaneously, this approach minimizes cost. In 2007, a strategy for modification used a polymer chain having one "chaperone" segment for surface concentration and another segment for antimicrobial function. In collaboration with the VCU School of Medicine, Department of Microbiology and Immunology, a test was developed that showed 100% kill of sprayed-on Gram positive and Gram negative bacteria in 30 minutes. Excitement about these results turned to dismay when in 2009 further research showed that the biocidal activity went away after 2-3 weeks. What was happening? A powerful surface imaging technique (Atomic Force Microscopy or AFM) was used to reveal that the initially smooth surface spontaneously became amazingly rough as shown in Figure 2. We concluded that the "chaperone" was misbehaving and sequestering the antimicrobial segment. The separation of modifier and base polymer was basically like that of oil and water but much s-l-o-w-e-r! Results from DMR 0802452. In 2010, guided by theory and experiment, a polymer chain modifier was made with a new chaperone. The results from biotesting on a modified polyurethane are shown in Figure 3. Two days after coating, 1 wt% of the new modifier effected 99% kill of sprayed on Gram negative P. aeruginosa. Thirty days after coating, kill was reduced to 55%. This was much better stability than prior results for noted above and AFM images did not change with time. Clear progress was made but we realized we had to do better. In 2011, toward the end of the grant period, a new "hybrid" modifier described as "bottle-brush nanoglass, or BB-NG" was developed. Preliminary results showed 100% kill against P. aeruginosa in the spray challenge at 7 and 30 days. We are excited about these results as the BB-NG process is simpler than that used initially. Because bio-testing takes days and sometimes weeks, we simultaneously developed a "zeta potential" test to (1) correlate results from bio-testing and (2) predict stability. In theory, zeta potentials (in millivolts) are proportional to surface biocidal charge. Results show that the new BB-NG modified surfaces have constant and high zeta potentials. These results validate the hypothesis that zeta potentials offer a fast method to predict stability of surface charge. In contrast, tests on previous generations of modified surfaces show decreasing charge with time, which correlates with decreasing biocidal effectiveness. Exciting new results on BB-NG modified antimicrobial surfaces with long term stability in air and water form part of current research under a new NSF grant. We believe this new surface modification technology will provide a platform for broader impacts by introduction of other desirable functions such as resistance to staining (no more coffee rings!?). Broader Impacts. VCU is an urban East Coast university with a >50% non-white male student body. Reflecting this diversity, the Wynne group is 45% non-white male. Students (REU) and/or volunteers join the group for varying periods. Showing the linkage between education and research, junior Daniel Henke was sparked by the PI’s Engineering Instrumentation course, volunteered for summer 2011, then happily switched to employee with "Dean’s match" for NSF-REU funding. Daniel presented his results "Purification of fluorous polyoxetane diols by liquid-liquid extraction," at the 2011 AIChE meeting. These results are being used in breakthrough BB-NG research on ice-release coatings. The PI was Chair of the ACS Polymer Division (2003), became an ACS Fellow in the "first class" of 2009, was an editorial board member (2009 – 2011) of Langmuir and ACS Applied Materials and Interfaces, was appointed Commonwealth Professor in 2011, has given many invited talks at national and international meetings, and has taken leadership in organizing or co-organizing symposia and workshops such as Macromex 2011 held last December.
