POLARIZED LIGHT MICROSCOPE EXAMINATIONS OF ORIENTED POLYMER FILMS

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1 POLARIZED LIGHT MICROSCOPE EXAMINATIONS OF ORIENTED POLYMER FILMS Jenny M. Smith, Criminalist, Missouri State Highway Patrol Crime Lab, Jefferson City, MO Introduction: Much can be gained in the examination of transparent or semi-transparent plastic tape films using the polarized light microscope (PLM). In the analysis and comparison of different tape products, determination of the polymer class and film thickness is an easy task but not very discriminating since most of these products fall into just a few different polymer classes. Polarized light microscopy (PLM) examinations of clear polymer films offer additional discriminating power within a class of polymers even where they have the same thickness. These exams are particularly useful in the differential analysis of clear packaging tapes where regardless of the manufacturer most are constructed of polypropylene and their adhesives typically falls in one of only three classes. Subtle differences in the manufacturing process of the polymer film produces crystalline behavior that can be observed with the PLM. Polymer Processing: A polymer is a macromolecule formed by a chain of repeating monomers. The type of flexible polymers that are used as films in the above mentioned products are typically linear or branched thermoplastics. These include polypropylene, polyethylene and polyester. The normal state of a polymer chain is amorphous. That is, the chain is in a tangled, unordered state. (Figure 1) Figure 1 Amorphous, unordered polymer chain Through a process of controlled heating and cooling, drawing and stretching the chains within a polymer can disentangle and become more ordered as well as oriented. This order comes from the formation of crystallites; where areas of the chain fold on itself in an orderly fashion. Spherulites are aggregates of crystallites and impart a polymer with crystalline behavior. A crystalline polymer will have both amorphous and crystalline areas in varying degrees. (Figure 2) Figure 2 Ordered polymer chains showing both amorphous and crystalline areas achieved with controlled heating and cooling.

2 Drawing and stretching a polymer as it is slowly cooled causes the spherulites to line-up, or orient themselves in the direction of the stretch. In this process a mono-axially oriented polymer is produced. (Figure 3) If the polymer is stretched in two directions as it is cooled, a bi-axially oriented polymer is produced. In general, as crystallinity increases polymer rigidity increases. Figure 3 Ordered polymer chains as in figure 2 but with orientation in one direction achieved by stretching during the cooling process. This orienting process gives a film greater toughness and tensile strength. It may be either mono-axially oriented polypropylene (MOPP) or bi-axially oriented polypropylene (BOPP). A MOPP tape can be torn by hand (along the direction of the one orientation), BOPP tape cannot be torn by hand. An unoriented film like that of a plastic garment cover is flimsy and tears in both directions. This processing of the polymer does not change its organic composition but does alter its optical properties. A polymer with crystallinity behaves like a crystal. Therefore, PLM skills are particularly useful for these clear polymer films. There are 3 principle refractive indices (RI) in these crystalline films; n md, RI in the machine direction (along the length), n cd, RI in the cross direction, n fm, RI normal to the plane. (Figure 4) This gives a polymer that behaves like a biaxial crystal. (Figure 5) Note that the term biaxial has a different meaning to a polymer chemist (bidirectional orientation of spherulites) than it does for a microscopist (a crystal with 3 refractive indices). Figure 4. Three principle refractive indices

3 Sample Preparation: Figure 5 Biaxial interference figures of polyester sheet protector. Transparent or semi-transparent adhesive tape films may be stuck to the slide adhesive side down without media or coverslip. The adhesive does not need to be removed since it is isotropic. Be sure to include the machine edge. If your sample has no adhesive, cut a piece (1-2 cm 2 ) and mount it in a mounting media with a refractive index between with a cover slip. PLM Observations: 1. Crystalline or Amorphous Under 100X magnification view the focused sample first under plane polarized light, then under crossed polarized light. If the sample is completely dark under the crossed polars, it is isotropic. This tells you that the polymer is non-cyrstalline. On the other hand, if you can see the film under crossed polars your sample is anisotropic and crystalline and you can proceed to other observations. (There are isotropic crystals, but not in these polymer films.) 2. Monoaxial or Biaxial orientation Under 100X magnification and crossed polars, rotate the stage just off extinction about 5 degrees. Observe the patterns in the film. If there are a series of Xs such as in a chain link fence, this represents the bidirectional stretching of a biaxial film (BO). If the interference colors run in only one direction, this is a monoaxially oriented film (MO). If the interference colors seen do not show any orientation, this is a unoriented crystalline film. (Figure 6,7)

4 Biaxially oriented polypropylene Monoaxially oriented polypropylene BOPP MOPP Figure 6 Two different brands of Clear Packing Tape viewed just off extinction under crossed polars at 100X. The BOPP tape on the left shows bi-directional stretching. The MOPP tape on the right shows stretching in one direction. It is sold as hand tearable. Figure 7 Polyester film in a filament tape. 100X crossed polars just off extinction, shows the bidirectional stretching. Blue areas on each side are the nylon fiber reinforcement. 3. Extinction angle relative to machine edge. The two refractive indices in the plane of the film do not necessarily run N-S-E-W with respect to the machine edge. Under crossed polars align the machine edge of the film with the vertical line of the eyepiece graticule. Note the stage position in degrees. Rotate the stage to the extinction position. Again note the stage position and find the difference from the first measurement. (Figure 8)

5 machine edge rotate 13 0 to extinction air tape air tape A B Figure 8 In Figure A under crossed polars at 100X the machine edge of film is lined up to the verticle eye piece graticule. Stage position is noted, then rotated to extinction, B. Difference in stage position is the extinction angle relative to machine edge. 4. Determination of retardation of film Retardation (color under cross polars) is a function of the refractive indices (R.I) (more specifically, the birefringence) of the polymer and the thickness. Where two films are being compared, both belonging to the same class of polymers, the R.I. of the films will be essentially the same. However, even small difference in the thickness of the films will give different retardations (color). When comparing two samples the best way to assess their relative retardation is to mount them on the slide, machine edge to machine edge. Under crossed polars and rotated to maximum brightness, slight differences in film thickness will then be obvious. (figure 9) K K Q Q Figure 9 Questioned and known tape edges are mounted edge to edge and viewed in same 100X field under cross polars and rotated to maximum brightness. A very slight difference in the thickness of the films is seen in the view on the left. This difference is more obvious in the same field on the right with the quarter wave plate in.

6 Compensation in tape film 280nm Figure X, quartz wedge in. The compensation (black) in this tape sample is shown at about 280nm (edge of 1st order yellow). The machine edge of the film runs across center to allow a view of the wedge colors. 5. Other observations. Polymer films may have additives that are visible in transmitted or polarized light. While the identification of these additives is not always possible, their presence and optical properties are useful for comparison purposes and should be noted. (Figure 11) Figure 11 Additives to polymer films: On the left is unidentified flower shaped objects in a polyethlene zip lock bag under 400X transmitted light. On the right are high birefringent particles in a packaging tape at 100X crossed polars. Irregularities in the thickness of the film may be observed under crossed polars as multiple interference colors. Some films may not totally extinguish under cross polars having a mottled appearance. While these methods work best for clear polymers, PLM exams can yield useful information on matte and other semi-opaque films as well.

7 Conclusions: In the analysis and comparisons of polymer tape films, FTIR spectroscopy can classify the polymer to a group but is insensitive to other optical difference imparted on the film in the manufacturing process. PLM offers a great discrimination power where slight differences exist, such as in thickness, stretch and extinction angles relative to the machine edge. Jenny M. Smith Missouri State Highway Patrol Crime Lab Jefferson City, MO ex 282 [email protected] REFERENCES 1. Rappe, R. G. (1991), Microscopical Examination of Polymer Films, Presentation at INTER- MICRO-91, Aug 19-22, Chicago, Ill. 2. Rappe, R.G. (1987), Measurement of the Principal Refractive Indices of Oriented Polymer Films, Microscope 35, Carraher, C.E. Jr. (1996), Seymour/ Carraher's Polymer Chemistry - an Introduction, 4th Edition, Marcel Dekker, Inc. 4. Hemsley, D.A. (1984), The Light Microscopy of Synthetic Polymers - Microscopy Handbook 07, Oxford University Press, Royal Microscopical Society 5. Menold, Ron (1999), Materials Analysis Unit - FBI Laboratory, Washington D.C., Personal Communication Acknowledgements The author wishes to acknowledge the peer review of this paper by John Johnston, Will Randle, Missouri State Highway Patrol Crime Laboratory and Thom Hopen, ATF Forensic Laboratory, Atlanta, GA

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