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Mechanicals, Morphology and Molecular Weight

Polymer materials have a wide range of process and performance characteristics that are influenced by many factors—mechanical properties, morphology and molecular weight are three of the most common.

By Jim Rancourt

Published December 7, 2010
When I reflect on the many reasons why companies submit samples for independent analysis it occurs to me that there are three principal ones. Of high importance to engineers and researchers who use polymer materials in medical products and devices are mechanical properties, morphology and molecular weight.

Mechanical Properties
A true understanding of the mechanical properties of raw materials and manufactured products is of obvious relevance to manufacturers. Well-established ASTM and ISO tests provide protocols for evaluating raw materials in standard formats. So, one can take a portion of compounded plastic pellets and produce well-defined standard shapes for mechanical testing, with tensile (ASTM D638 and ISO 527-1), flexural (ASTM D6395), and compression (ASTM D695 and ISO 604) properties being most common.

For product design and development purposes there are many well-established standard test methods to compare and contrast materials. These standard test methods also can be used for lot acceptance testing and for certifying an alternate supplier.

Other standard test methods are product specific. ASTM F2150, for example, is a characterization guide for biomaterial scaffolds. And, packaging is also a critical component that influences the life cycle of medical devices and products. ASTM F2097, for example, is a standard guide for the design and evaluation of primary flexible packaging for medical products.

Another important variant of mechanical testing is custom testing to predict, verify, evaluate, or explain product performance. Root cause analysis of product failures often is supported by mechanical test data. The sophistication of the tests and the required experience level of the laboratory staff shifts significantly for standard tests compared with product-specific failure analysis investigations.

The properties of polymer materials are strongly influenced by the morphology of the plastic material. This is true because virtually every polymer is semi-crystalline. Although the phrase “semi-crystalline” could be inferred to mean “half crystalline” that is not true. Practically speaking, the phrase simply means “partially crystalline,” and that’s where the technical consequences of morphology begin.

In general, polymers have crystalline regions and amorphous regions. The amorphous region is characterized by the glass transition temperature, is a softening temperature for the polymer, imparting impact resistance, flexibility, optical clarity and higher gas permeability. In contrast, the crystalline phase of a polymer is characterized by the melting temperature, sets the upper use limit of the polymer, and increases stiffness, can cause opacity, and lowers gas permeability.

A specific polymer does not have a unique pre-defined level of amorphous and crystalline content. The percent crystallinity is influenced by the polymer, additives, process parameters, storage conditions and end-use conditions (exposure to mechanical stress, elevated temperature, or solvents, for example). Mechanical properties and product performance are dependent on the crystallinity that may change over time. Post-processing steps can be taken to produce a more stable polymer system with annealing being a common approach.

Determining the morphology of polymers is typically done using either differential scanning calorimetry (DSC) or X-ray diffraction (XRD) analysis. The DSC method determines heat capacity as a function of temperature and reveals thermal transitions such as the glass transition temperature and the crystalline melting temperature. Polymers actually have broad melting ranges, very different from the sharp melting peaks for pure organic compounds. As a result, it is important for the polymer engineer to get the full details of the melting transition that includes the onset and termination of melting, not just the peak temperature. ASTM D3418 and ISO 11357-3 describe the application of DSC to plastic materials. The more sophisticated XRD method often is used to calibrate the DSC method by directly providing the percent crystallinity.

Molecular Weight
The molecular weight of a polymer is second in importance only to the chemical structure of the polymer itself. The molecular weight of a plastic material directly correlates with the “size” of the molecules that form the plastic. Higher molecular weight will general provide a stronger material but may cause process issues. So, tradeoffs are made between processing characteristics and product performance.

Another component of “molecular weight” is the molecular weight distribution. Just as people are not all the same height, the molecules of a polymer also have a distribution of sizes. Polymer distributions can be narrow, broad, or polydisperse. The breadth of the molecular weight distribution curve is described as "polydispersity." The decision as to whether a distribution is broad or narrow is often process and performance based. An injection molding grade of polymer may be said to have a broad molecular weight distribution whereas the same resin in a blow molding application may be considered narrow.

Some molecular weight distributions are actually multimodal.  That is, the molecular weight distribution is a combination of two or more distinct distributions, akin to the partial overlap of two or more normal distributions. Multi-modal distributions are easy to objectively identify because multi-modal polymer compounds are composed of two or more distinct molecular weight distributions. Polymer blends can be poly-modal and this type of blend is often prepared to enhance process characteristics while retaining required performance properties.

The breadth of molecular weight (MW) distribution curves and the potential for polydispersity cause gel permeation chromatography (GPC) or size exclusion chromatography (SEC) to be the most powerful analysis methods more molecular weight characterization. This is true because the GPC and SEC methods provide the concentration of each molecular size. The analysis requires expert analysts, moderately expensive instrumentation, and frequently very aggressive solvent systems.

Simpler MW analysis techniques are intrinsic viscosity (IV), dilute solution viscosity (DSV), and melt flow index (MFI, or melt flow rate, MFR) tests. Although operator experience and skill, and properly calibrated and validated methods and equipment are still required, the DSV and MFI equipment is low-cost compared with the GPC and SEC methods. The IV and DSV techniques use dilute polymer solutions. Both techniques are excellent for determining average molecular weight. Technologically, these methods are often used to track raw materials and molecular weight shifts in response to processing, sterilization, and shelf life. The IV and DSV methods provide only “a single data point”, not the molecular weight distribution.

Like IV and DSV, the melt flow index value is also a single data point. In this case, however, the polymer is evaluated in the molten state, not as a dilute solution. The most common reasons for performing MFI tests are to verify that raw materials conform to a specification or certificate of analysis and for processors to have starting-point values for setting process parameters. Comparison of MFI values for the same polymer purchased from several suppliers, for example, gives polymer processors an idea as to whether process difficulties are likely.

Polymer materials have a wide range of process and performance characteristics that are influenced by many factors. Of those many factors, mechanicals, morphology and molecular weight are three of the most common. A large percentage of the thousands of industrial-related projects I have worked on involved these three factors. Of course there are other factors that influence process and performance properties of plastics, additives, contamination, and regrind, for example.

Jim Rancourt, Ph.D., is founder and CEO of Polymer Solutions Incorporated (PSI) in Blacksburg, Va. PSI is an independent laboratory and resource for chemical analysis, physical testing, research and development services, and litigation services. The company is ISO-17025 accredited, cGMP compliant, FDA registered and DEA licensed.

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