FLOW INDUCED MECHANOCHEMICAL ACTIVATION

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1 FLOW INDUCED MECHANOCHEMICAL ACTIVATION H Magnus Andersson 1, Charles R Hickenboth 2, Stephanie L Potisek 2, Jeffrey S Moore 2, Nancy R Sottos 3 and Scott R White 4 1 Beckman Institute, 2 Department of Chemistry, 3 Department of Materials Science and Engineering, 4 Department of Aerospace Engineering University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA magand@uiuc.edu (H Magnus Andersson) The focus of this paper is an experimental method for systematic studies of stress-activated reactions using probe macromolecules and elongational flow fields in an extensional flow cell where strain rate provides a continuously variable input of mechanical energy. Polymer chain elongation is confirmed by flow-induced birefringence and chain scission by gel permeation chromatography. 1 Introduction Materials that alter their properties or form in response to a change in their environment are known as stimuli-responsive materials. Unlike conventional polymers that undergo chain degradation through bond scission when subjected to high stresses, mechanically-responsive polymers would productively utilize mechanical energy to initiate chemical reactions prior to bond scission. For example, self-reinforcing polymers could be realized if cross-linking reactions are induced in the vicinity of high stress concentration. As another example, a fluorescence unit generated by a stress-induced reaction would signal that a self-assessing polymer was loaded above a threshold value. The focus of this paper is an experimental method for systematic studies of stress-activated reactions using probe macromolecules and elongational flow fields in an extensional flow cell where strain rate provides a continuously variable input of mechanical energy. 1.1 Mechanochemistry via link-functionalized polymers Recently, structural polymers with the ability to autonomically heal cracks have been developed at the University of Illinois (White et al. 2001). In the design of self-assessing and self-reinforcing polymers, we now seek to apply this self-healing concept at the molecular level via mechanochemical triggering. By incorporating mechanochemical linking groups in high molecular weight polymer chains, we are aiming at a stress-induced reaction that is coupled directly and tailored to the mechanical field. Mechanochemical reactions can then be used to trigger other functions, e.g. fluorescence, healing, repair, toughening, etc. Mechanochemical activation studies require polymers with a broader range of molecular weights, including ultrahigh molecular weight polymers. Simple coupling reactions are insufficient to obtain ultrahigh molecular weight materials and single electron transfer (SET) living radical polymerization is being used to produce a variety of link-functionalized polymers up to a molecular weight of 10 6 Da. 1 Springer 2007

2 2 Dilute polymer solutions in stagnation point extensional flow 2.1 Flow-induced chain extension and fracture The most common example of stress-induced reactions in polymers is homolytic bond scission. Staudinger (1930) was the first to recognize the possibility of flow-induced destruction of chain molecules. de Gennes (1974), predicted a sudden transition from a coil to an extended state (coil-to-stretch transformation) as the strain rate is increased above a critical level in an elongational flow field. Confirming the prediction of de Gennes, Odell and Keller (1986) experimentally showed that dilute solutions of high molecular weight macromolecules in extensional flow fields undergo chain extension above a critical strain rate,. ε c, and if the strain rate is increased further, chain scission occurs above a fracture strain rate,. ε f. Importantly, Odell and Keller observed that the polymer chains usually break at the midpoint, which would be expected for a fully stretched chain. Various methods have been developed to create extensional flow fields, including rotating rollers (Taylor, 1934), opposed jets (Frank et al. 1971) and cross-slots (Scrivener et al. 1979). These methods are all based on stagnation point flow to provide a sufficient strain rate that is maintained for a long enough time to achieve full chain extension. 2.2 Extensional flow cell The apparatus used in this study is based on opposed jets, where two glass tubes are immersed in solution and aligned facing each other. A stagnation point is created in the center of the flow-cell by extracting fluid through both tubes simultaneously (see figure 1). The stagnation point provides a sufficiently long residency time for the chains to become fully stretched. To a first approximation, the flow field created by sucking fluid into two opposed jets can be considered uniaxial extension. The nominal strain rate is determined from the macroscopic geometry of the flow cell and the volume flow rate, Q, and can be calculated as an average across the region between the jets as. Q ε = (1) Ad where A is the area of each jet and d is the jet separation. The flow rate is controlled by a continuously proportionating solenoid valve with a pressure reservoir after the flow-cell to minimize pressure and flow-rate pulsations. 2 Springer 2007

3 Figure 1: A schematic view of the opposed jets and the flow-field created between them. The dotted line is the symmetry axis and the stagnation point is marked with a cross 2.3 Assessment of orientation and stretching by birefringence Birefringent materials refract light depending on the load state and act as temporary wave plates with a slow and a fast axis. The magnitude of the birefringence is the difference in refractive index between light polarized parallel and perpendicular to the elongation axis. The coil-to-stretch transformation of polymer chains in elongational flow can be monitored by an optical rheometer utilizing the flow birefringence resulting from the corresponding molecular orientation in the almost ideal uniaxial flow field (Fuller 1990). A quarter-wave plate is used as a Senarmont compensator between crossed polarizers (figure 2) to correct for residual cell birefringence and to achieve complete extinction for dark-mode registration of the molecular chain extension (Odell and Keller 1986). Photodetector Lens Analyzer Flow-Cell Condenser Lens λ/4 Plate Polarizer Laser Figure 2: A schematic representation of the optical components 3 Springer 2007

4 3 Measurements of flow birefringence and chain scission Initial experiments were performed with high molecular weight poly(ethylene oxide) (PEO) and polystyrene (PS). Polymer chain elongation was confirmed by recording the intensity of flow-induced birefringence. Figure 3 shows the intensity of birefringence as a function of increasing strain rate for 900,000 Da PEO in water. To demonstrate chain scission, a solution of high molecular weight PS in Decalin was circulated at some exceedingly high strain rate, well above the predicted fracture strain rate. Samples of the solution were collected at regular intervals and analyzed by gel permeation chromatography (GPC) to determine the molecular weight distribution. Figure 4 clearly shows the progress of chain fracture as a function of passes through the flow-cell Strain Rate (10 3 s - Figure 3: Intensity of flow-induced birefringence vs. strain rate for PEO in water (900,000 Da, 0.1 wt%) 4 Springer 2007

5 3,900,000 MW Passes: Retention Volume (ml) Figure 4: Molecular weight distribution curves obtained by GPC for the polymer after multiple passes through the flow cell at 40,000 s -1 (3,900,000 Da 0.01% PS in Decalin) 4 Concluding remarks Polymer chain elongation and scission has been demonstrated in an extensional flow cell with continuously variable strain rate. Chain elongation was confirmed by flow-induced birefringence and chain scission by GPC. Recent work in our group reports on mechanochemical activation triggered by ultrasound. Extensional flow experiments with linkfunctionalized polymers will now be conducted as a function of finely tuned strain rate to demonstrate flow-induced mechanochemical activation. ACKNOWLEDGEMENTS This work is sponsored by the AFOSR Multidisciplinary University Research Initiative; Grant No. FA REFERENCES 1. De Gennes PG (1974) Coil-stretch transition of dilute flexible polymers under ultrahigh velocity gradients. Journal of Chemical Physics, vol 60, 12: Frank FC, Keller A, Mackley MR (1971) Polymer chain extension produced by impinging jets and its effect on polyethylene solution. Polymer 12: Fuller GG (1990) Optical rheometry. Annual Reviews in Fluid Mechanics 22: Odell JA, Keller A (1986) Flow-induced chain fracture of isolated linear macromolecules in solution. Journal of Polymer Science: Part B: Polymer Physics 24: Scrivener O, Berner C, Cressley R, Hocquart R, Sellin R, Vlachos NS (1979) Dynamical behaviour of drag-reducing polymer solutions. Journal of Non-Newtonian Fluid Mechanics 5: Staudinger H (1930) Highly polymerized compounds, XLVIII, Molecular size of cellulose. Berichte der Deutschen Chemischen Gesellschaft 63: Taylor GI (1934) The formation of emulsions in definable fields of flow. Proceeding of the Royal Society: Series A, vol 146, 858: White SR, Sottos NR, Geubelle PH, Moore JS, Kessler MR, Sriram SR, Brown EN, Viswanathan S (2001) Autonomic healing of polymer composites. Nature 409: Springer 2007