RHEOLOGICAL MEASUREMENTS BY AFM OF THE FORMATION OF POLYMER NANOFIBERS M. M. Yazdanpanah, M. Hosseini, S. Pabba, S. M. Berry, V. V. Dobrokhotov, A. Safir, R. S. Keynton and R. W. Cohn ElectroOptics Research Institute & Nanotechnology Center University of Louisville

The Processing Parameter P : Key material parameter that predicts nanofiber dimensions Measurements of material constants that comprise P : Surface tension, evaporation rate, viscosity Using constant diameter nanoneedles for measuring P and drawing of nanofibers RHEOLOGICAL MEASUREMENTS BY AFM OF THE FORMATION OF POLYMER NANOFIBERS

PROCESSING PARAMETER AND FIBER FORMATION Fiber diameter depends on as well as droplet diameter (Tripathi and McKinley) Evaporation rate x viscosity P = Surface tension Harfenist et al. Nanoletter 2004

CONSTANT DIAMETER NANONEEDLE AFM TIPS FOR PULLING POLYMERIC NANOFIBER Thinnest fibers are drawn with thinnest tips Needles on AFM tips point to interactive measurement of processing parameter while drawing fibers 5  m 100 nm Needle Polymer fiber Yazdanpanah et. al., JAP 2005

AFM MEASUREMENT OF Surface tension Evaporation rate Viscosity To get processing parameter using constant diameter nanoneedles

Circumference needed to extract surface tension On tapered tips, wetting force grows unstably with depth Viscous drag force is directly related to insertion depth ADVANTAGES OF CONSTANT DIAMETER NEEDLES FOR AFM ANALYSIS OF LIQUIDS

10  m SELECTIVE GROWTH OF LONG NEEDLES Time lapse images show growth of a 70 micron long needle

MEASUREMENT OF SURFACE TENSION PLUS CONTACT ANGLE Surface tension is found from Contact angle is found from SEM images of needle being retracted from vacuum oil Fr Fe

TYPICAL F-D CURVES FOR MEASURING SURFACE TENSION AND CONTACT ANGLE 5  m 100 nm 5  m 300 nm Ag 2 Ga alloy, dual-diameter needle Parylene coated, single-diameter needle Vertical axis scaled by circumference to be in units of surface tension

Surface tension (mN/m) Surface tension published (mN/m) Contact angle (degrees) BareCoatedBareCoated Water ± 262 ± 2 Chlorobenzene ± 321 ± 3 Dibasic ester ±122 ± 1 Vacuum oil ± 127 ± 1 Isopropanol ±319 ± 3 Toluene ± 2 SURFACE TENSION AND CONTACT ANGLE Measured and published values of surface tension in agreement Contact angles differ by up to 11 o for bare and coated needles Each measurement is the average of at least 20 measurements. The deviation reported is peak-to-peak.

MEASURING EVAPORATION RATE AFM is sensitive to sub-nanometer changes in surface height between repetitive scans AFM data is in good agreement with TGA data l AFM (  m/s) TGA (  m/s) Acetone1.45 ± IPA0.32 ± CLB0.11 ± DI0.027 ± DMF ± DBE ± Vacuum oil~ 0

VISCOUS DRAG FORCE ON NEEDLE LOWERS Q OF THE VIBRATING CANTILEVER Measurements of Q-damping vs. needle insertion depth are fit to a model of drag force to give one value of viscosity Measurements of glycerol-water track literature and our own shear plate viscometer measurements over three orders of magnitude Needle is rugged: Same needle used for every measurement!! 99.5 wt % 80 wt % 13.4 wt % Glycerol in water

Fiber length increase over an order of magnitude closely following a log-log trend Liquid with these low value of P does not form stable fibers or strings, but were used to enable cleaner, more ideal first-time measurements These results point towards the controlled, interactive drawing of nanofibers with these needles and higher P liquids STRONG CORRELATION BETWEEN FIBER LENGTH AND PROCESSING PARAMETER AFM F-T curves of breakup length Processing Parameter P Length (  m) lblb

Constant diameter nanoneedles were used to measure the Processing Parameter P as well as to show the correlation between P and fiber length For simple liquids, surface tension, evaporation rate and viscosity were measured with reasonable to high accuracy For random chain polymeric liquids viscosity was very low, but this may accurately reflect viscosity for the nanoscale displacements of the thermally-actuated AFM cantilevers The needles are very rugged and reliable for liquid studies— Only 4 needles were used for the entire study! SUMMARY RHEOLOGICAL MEASUREMENTS BY AFM