Polymers

Historically, polymer materials were characterized solely by their bulk properties including stiffness, elongation, gas permeability, impact and modulus. Because the bulk polymers were essentially homogeneous, characterization of substructure was not important. Recent advances in polymer science require advanced methods for polymer micro and nano structure characterization.

Polymer nano-composite materials are now created having clusters, layers, rods, and networks of nanometer sized materials blended with a bulk polymer. These new materials, such as Nylon-6/clay nano-composites, can have desirable properties such as a high modulus and low gas permeability. Ideally, nano-composite polymer materials show a dramatic increase in a physical property with a small amount of nano-particle additive.

A unique aspect of the new class of nano-composite polymer materials is the high interfacial volume of the materials, just the opposite of traditional bulk polymers. The chemical and physical properties of the interfaces are what give rise to the unique and often desirable properties of nano-composite polymer materials.

Additionally, polymer materials are being developed for a host of new applications in the high technology and industrial sectors. As an example, low K materials derived from polymers are under development and may replace SiO₂ in semiconductor devices.

Polymer coatings are routinely used in packaging applications in the food industry. Such applications require characterization of surface structure and surface properties.

Traditional methods for surface structure characterization are optical and electron microscopes. Besides giving a 2-D projection of the sample topography, the optical microscope can give contrast associated with polarization. However, the optical microscope is limited to resolutions of greater than 0.5 µm . Electron microscopes such as SEM and TEM give high-resolution images but require sample coating to make the polymer electrically conductive. The Atomic Force Microscope is well suited for the characterization of nano-composite polymer materials when compared to traditional microscopes such as the electron and optical microscopes.

Atomic force microscopes give direct 3-D measurements of the surface structure of polymers. Further, using materials sensing modes such as lateral force and phase contrast, it is possible to differentiate the types of materials at a polymer surface.

Two imaging modes, continuous and vibrating, may be used for imaging polymer samples. If the sample is relatively "hard", then "contact" mode may be used. However if the sample is relatively "soft", then it is necessary to use "close contact" mode. The forces placed on the sample surface with "close contact " mode are about 10 - 100 times less than in "contact" mode.

Direct visualization of 2-D and 3-D images of polymer surfaces is helpful for understanding the effects in changes in process conditions. Because the AFM does not require conductive samples, there is little or no sample preparation required. Below are three polymer samples prepared from the introduction of polypeptides. In figures 2a, 2b, and 2c, the density of peptides is 1%, 10%, and 100%.

Many methods are available for measuring the size of particles suspended in a liquid. However, such techniques give only a statistical average of particle size. Detailed visualization and metrology of nano-particles is possible with traditional electron microscopy. However, SEM and TEM require sample preparation such as coating and microtoning. AFM allows the direct measurement of particle metrology. Sample preparation requires finding a substrate to which the particles naturally adhere.

Below are images of two sizes of latex spheres measured with the AFM. The size of the particles can be directly measured with a line profile.

The AFM makes it possible to measure not only the topography of the sample but also changes in the mechanical properties of the surface. The two most common types of material sensing modes are Lateral Force Microscopy (LFM) and Phase Contrast Mode (PCM).

The torsion of the cantilever is monitored with the AFM scanning head in lateral force microscopy. As the cantilever is scanned across the surface, the amount of torsion changes depending on the types of materials at the surface. It is possible to acquire both an LFM image and a topographic image simultaneously with an AFM.

Although this technique is not quantitative, it provides a contrast mechanism for differentiating chemical materials at a sample surface. LFM is best applied to samples with small topographic features because changes in topography can also cause changes in the cantilever torsion.

It is possible to measure changes in the mechanical properties at a polymer surface with PCM. Close contact mode operates by maintaining the amplitude of a vibrating cantilever constant while scanning over a polymer surface. At the same time, monitoring changes in the phase of the cantilever gives an image that is related to the localized "hardness" of the surface. A harder section of a sample will give more (or less) phase shift of the vibrating cantilever as it scans across a sample.