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The molecular weight distribution of polymers strongly influence their properties, such as tensile strength, crack resistance, and solubility. Gel permeation chromatography is a commonly used technique to measure molecular weight distribution, but relies on the ability of the polymers to be dissolved in a solvent that is readily usable in a GPC column. Additionally, the molecular weight distribution is inferred from polymer standards, which should have a similar, if not identical, repeat unit to the unknown sample for the most reliable results.

Melt rheometry can be be used to determine molecular weight distributions, using the fact that the viscosity of a polymer is strongly dependent on the molecular weight and distribution. For instance, at low shear rates, the zero shear viscosity of a polymer melt scales with the weight average molecular weight to the 3.4 power. Similarly, the dynamic viscoelastic properties of polymer melts can be used to determine the overall molecular weight distribution. To do so, a frequency sweep is conducted on the sample, providing the G' and G" as a function of frequency. Using theoretical models by Mead or Thimm, the molecular weight distribution can then be determined, assuming that the model parameters are known for the polymer in question.

Contact CPG for more information on this technique.

CPG recently purchased a new impact tester, a CEAST 9050 (Instron). With both V-notching and blade notching capabilities, we can perform Izod impact testing on materials in compliance with ASTM D256, as well as impact testing on UHMWPE per ASTM F648. The pneumatic release option on the impact tester allows very reproducible results.

CPG researcher Dr. Stephen Spiegelberg was recently elected to the post of Recording Secretary for ASTM Committee F04 “Medical and Surgical Materials and Devices”.  Click here for more information on the committee. The committee's 160 members meet biannually, attending over two days of technical meetings capped by a symposium or workshop on relevant topics in the medical/surgical materials and device industry.

Polarized light microscopy is an effective tool to examine the crystalline structure of materials. In this technique, a sample is placed between two polarizers which are oriented 90 degrees to each other, or are "crossed". Light is transmitted through the polarizers and samples into an objective. Light travelling through the first polarizer becomes polarized in the plane of the polarizer. When it hits the second polarizer, no light will be transmitted as the second polarizer (also called the analyzer) is oriented 90 degrees to the first polarizer, unless the sample is optically anisotropic. Optically anisotropic materials are ones where the optical properties are different when probed in different directions. They have a different refractive index, or speed of light, in different orientations normally due to molecular alignment. Crystalline materials will show birefringence, as will oriented materials (e.g. stretched in one direction) if there is a notable chemical dissimilarity along one axis of the molecule relative to the cross direction (such as polystyrene). In this case, the light will be slowed down along one axis of its path, causing it to rotate as it passes through the sample. As such, a portion of the light will be transmitted through the analyzer, showing up as a bright spot. The amount of rotation depends on the thickness of the sample, the amount of orientation, the wavelength of the light, and the chemical nature of the material. Crystalline samples will often show the classic "Maltese Cross" pattern, such as those seen in the image of polyethylene oxide above. The dark sections in the crystalline structure are the portions where the orientation is either normal or parallel to the polarization axis of the transmitted light, so that the light is not rotated as it passes through the sample. The multi-colored ares in the image above are thicker sections of PEO, which causes the light to undergo multiple rotations, and will separate the white light into various colors, like a prism.

For more information on the theory of birefringence, go to this application note.

AUGUST 2013 - CAMBRIDGE POLYMER GROUP ANNOUNCES THE OPENING OF A NEW WEST COAST OFFICE

Cambridge Polymer Group has expanded its operation with the opening of a West Coast office to support growing demand for materials consultation.  The new office will be run by Ayyana Chakravartula, PhD.  (617) 629-4400 Ext. 23, and is located in Oakland, CA. This office has been established to assist our west coast and mountain plains customers.