A Key Characteristic of Medical Plastics: Molecular Weight

3 December 2013 15:46

Source: Polymer Solutions.

James “Jim” Rancourt, PhD, Founder & CEO of Polymer Solutions, an independent chemical analysis and physical testing lab, has written exclusively for Medical Plastics News about molecular weight as it relates to medical polymers.

Introduction

Plastic materials are a critical component to the supply chain for the manufacture of safe medical devices. The incorrect use of plastics can lead to frustration, lost time, wasted money, and product failures. The correct use of plastics allows for the implementation of innovative product designs, unique device features, and cost-effective manufacturing processes. How well the plastic performs throughout its lifecycle, and the mechanisms through which a plastic can fail, are both strongly influenced by polymer molecular weight. The molecular weight of polymers is critical to know and understand.

In end-use applications numerous issues can arise with components of medical devices or even with the medical device packaging. Implants can fail prematurely, bioabsorbable polymers may degrade at the wrong rate, medical tubing may become embrittled, packaging materials may become yellowed, and medical devices may crack. The cause of failure can be determined using a variety of analytical methods. A common starting point is to determine the molecular weight of the plastic material.

Simply put, molecular weight is the size of the molecule. For example, if 2,000 styrene chemical repeat units are linked together, the molecular weight of the polymer is 208,000 because the formula weight of the styrene repeat unit is 104. As molecular weight increases, the impact resistance, abrasion resistance, tensile strength, and melt viscosity also increase. Higher molecular weight polymers can be more difficult to manufacture, more expensive, and difficult to process into finished parts. Therefore, from a practical perspective, there is a range of molecular weight values that are most desirable for a specific product and process.

Molecular Weight

When confronted with product failures, those who are tasked with the root cause analysis often want to rule molecular weight in or out as a contributor to the failure. That is best accomplished by comparing the molecular weight of the failed portion of the product with a non-failed control sample. This is a cost-effective approach because there is not a “correct molecular weight value” for any plastic part. The comparison is a rapid assessment that determines if the molecular weight has decreased significantly or not. If the molecular weight has not decreased significantly, molecular weight has quickly been ruled out as a primary factor in the failure mechanism. Alternatively, if the molecular weight has decreased significantly, the team members next look to the cause of the molecular weight reduction to initiate corrective actions.

Changes to Molecular Weight

Every step in the life of a plastic can alter the molecular weight of the polymer chains and thereby influence the physical properties and performance of the product. An example of the molecular weight of a plastic declining is provided by considering a bioabsorbable implant device. The plastic starts as raw polymer whose molecular weight is dropped as a result of thermal and hydrolytic degradation that occurs during the molding operations. Next, sterilization significantly degrades the molecular weight further as a result of molecular chains being broken by the sterilization process. During storage for modest periods of time, in properly designed storage containers, the polymer molecular weight remains stable. Then, as designed, the polymer degrades substantially as a result of being implanted. An explanted bioabsorbable polymer may retain virtually no polymer chains. The progression of molecular weight for this scenario is provided in the table below.

Stage of Life: DSV

Raw material: 5.5 dL/g

Moulded component: 3.5 dL/g

Sterilised product: 1.8 dL/g

Stored product: 1.7 dL/g

Explanted device: <0.1 dL/g

Two points to be understood from the previous example are the following. First, for the implant to operate properly and be degraded to low molecular weight by-products in the desired time frame requires a specific molecular weight. Second, in order to achieve the starting molecular weight for the implanted bioabsorbable polymer, the polymer must start with a higher molecular weight. The higher starting molecular weight is critical because it is known that processing will lower the molecular weight and that sterilization will lower the molecular weight even further. The molecular weight attained after molding and sterilization must be repeatable and must attain the target value.

The Methods and the Numbers

There are two primary methods that are used to determine molecular weight, dilute solution viscosity (DSV) and gel permeation chromatography (GPC). Both methods can be used for decision-making purposes by the team members.

Dilute Solution Viscosity

Dilution solution viscosity measurements are relatively inexpensive, can be performed quickly with a small amount of sample, and provide a single numerical value that is related to the molecular weight of the portion of the sample tested. The basic principle of the method is a comparison between the flow rate of a solvent through a capillary tube and the same solvent containing dissolved polymer. The dissolved polymer decreases the flow rate of the polymer in proportion to the molecular weight of the polymer. If a failed sample is compared with a control sample, the following scenarios may occur:

Measured Dilute Solution Viscosity Values, dL/g

Control sample

Scenario 1: 3.5 dL/g

Scenario 2: 3.5 dL/g

Scenario 3: no control available

Failed part sample

Scenario 1: 3.4 dL/g

Scenario 2—1.8 dL/g

Scenario 3—1.7 dL/g

Conclusion

Scenario 1: no difference

Scenario 2: significant difference

Scenario 3: experience-based decision or more analysis

In the first example, the team can conclude that the molecular weight of the failed sample and the control sample are not significantly different. As a result, other contributors to the failure need to be identified and investigated.
In the second example, the team can conclude that the molecular weight of the failed sample and control sample are significantly different. The team will then work to determine the cause of the molecular weight reduction.
The third example is the weakest position to be in when working to determine a root cause analysis: there is no control sample and no baseline data with which to compare the dilute solution viscosity values. Now, the investigating team needs to assess the reasonableness of the value to make a determination as to whether or not molecular weight is a plausible cause of the failure. This decision may be based on the experience of a team member who knows the DSV value that represents and appropriate molecular weight for the polymer. It is wise to proactively acquire DSV values for critical polymers so that scenario 3 does not occur.

Gel Permeation Chromatography

Understanding that molecular weight is the “size” of the molecules that comprise the plastic leads to the next component of molecular weights and plastic materials. The actual molecular weight of plastic materials is always a distribution, not a single value. For example, the polystyrene described above, having a molecular weight of 208,000 is present as part of a collection of molecules that have some polystyrene molecules that are smaller and others that are larger than 208,000. Scientists and engineers refer to this as “the molecular weight distribution.”

The average molecular weight can be determined quickly and relatively inexpensively with simple DSV measurement equipment. However, the complete distribution of molecular weights that comprise a sample can be assessed by using a gel permeation chromatograph (GPC). This instrument separates the molecules based on their size and the detectors are able to “count” the number of molecules of each size. The resulting data set includes the average molecular weight as well as the full distribution of the molecular weights that comprise the sample. Simply knowing the average is not always enough information, a molecular weight distribution graph is information-rich.

Summary

A common starting point in a failure analysis investigation is to determine the molecular weight of the plastic material. Two common methods of analysis are used for this purpose, dilute solution viscosity (DSV) and gel permeation chromatography (GPC). The DSV provides a single number that is representative of the entire molecular weight distribution. In contrast, the GPC method provides the complete molecular weight distribution curve. It is wise, and a best practice, to proactively benchmark the molecular weight of critical polymers by obtaining DSV or GPC data sets. This ensures a baseline data set is available during a failure analysis investigation.

About the author

James “Jim” Rancourt, Ph.D., is the Founder & CEO of Polymer Solutions, an independent chemical analysis and physical testing lab. He has decades of experience with analytical chemistry and is recognized as an authority in the field of polymer analysis.