AFM/ESEM Lab

Two common techniques for observation of 2D surface structure in polymer samples are atomic force microscopy (AFM) and environmental scanning electron microscopy (SEM).  AFM has two main advantages over SEM, samples do not need to be coated and there is no electron charge buildup on non-conducting surfaces.  AFM has the potential for much higher resolution compared to SEM but is difficult to use as a screening technique to find a region of interest.  For polymers both techniques are invasive in that there is some potential for sample modification during analysis.  We will use the ESEM instrument which is an SEM with an elaborate vacuum system that allows about 20 Torr of pressure of an arbitrary gas at the sample position while maintaining high vacuum at the electron gun.  Different detectors are used in the ESEM.  For polymers ESEM is of importance due to the breakdown of polymers in the electron beam leading to significant offgassing which can corrupt conventional SEM and TEM instruments.  Localized heating can lead to melting of polymer crystals and other thermally induced modifications of morphology. 

Several samples will be imaged using the ESEM and AFM. The lab highlights some applications of ESEM and AFM to scientific issues in polymer science.

  1. Calculation of dispersion and correlation in phase separated domains by calculation of the correlation function and the particle size distribution.
  2. Observation and quantification of disorganized particulate filler in polymers.
  3. Observation and description of polymer crystal structure
  4. Observation and description of fibrillation in semi-crystalline polymers.
  5. Observation and description of mass fractal structure in two common polymer additives, organic pigments and inorganic filler aggregates.

Please read the linked web pages for more detailed information on these instruments.

Experimental:

The following samples will be prepared prior to lab, a Jimble will assist you in obtaining images from these samples.

Samples: (Other samples may be substituted)

  1. HIPS Phase Separated Domains
  2. PE, PP, PHB Crystals
  3. Fiber PP from mechanical testing
  4. Organic Pigment, Fractal Structure
  5. Titania, Fractal Structure

Analysis

  1. Phase separated domains: Measure the diameter, aspect ratio of a number of rubber domains in HIPS. Also measure the size of a number of inclusions within the rubber domains. Calculate the mean and standard deviation for these two structures. Plot a particle size distribution curve for both of these structures. (Link to Needle Throwing Method for Pairwise Correlation Function)
  2. Measure the average fiber diameter and standard deviation for fibrils in the PP fibers.
  3. Measure the mean primary particle size, mean aspect ratios and standard deviations for the organic pigment and the titania samples.
  4. Measure the mean aggregate size and standard deviation.
  5. Estimate the mass fractal dimension, df, for both of these samples. (M ~ Rdf, M is mass of the aggregate and R is the aggregate size.  Here M ~ N where N is the number of primary particles and R ~ R/dpp where dpp is the size of a primary particle, so df = ln(M)/ln(R/dpp)).
  6. For several images use ImageJ shareware to determine the autocorrelation function p(r) using an ImageJ plugin available from their web page. 
    p(r) = 1 -S/(4V) + ... for very small r near 0.  You need to scale your image in ImageJ to the micron scale bar.  Determine the Sauter mean diameter from S/V, dp = V/(6S).
  7. Describe other features of p(r).  If there are peaks in p(r) explain what feature in the micrograph corresponds to the peaks.
  8. Use ImageJ to determine the average particle size using thresh holding in several micrographs. 

Questions

  1. Comment on the problems you encountered in quantifying sizes from micrographs. Especially consider the usefulness of 2d depictions to describe 3d structure.
  2. Are the domains (particles) you observed randomly distributed? Give support from your data and analysis using the correlation function.
  3. Describe the differences between the mass fractal structures observed.
  4. Do the mass fractal structures reflect reaction or transport limited growth? Give a possible aggregate growth mechanism for both systems and support it with your data. (web link for this question.)