Characterizing Polymers in Terms of Molecular Weight and Structure

Polymer scientists are making continuous efforts to manufacture products with appropriate characteristics for specific applications. The polymer's properties can be altered by tweaking the molecular weight, chemistry and branching for different applications, ranging from containers and fittings to viscosity modifiers and drug delivery vehicles. Here, the key challenge is that the behavior and physical properties of polymer products are strongly based on the properties of the polymer molecules themselves.

This is applicable even for finished products consisting of or made from polymers. For instance, Table 1 shows the effect of changing properties like branching, polydispersity and molecular weight on the bulk properties of a polymer. These are common trends, and the typical effect is based on the specific polymer.

Table 1. Effect of molecular weight, polydispersity and structure on the bulk properties of a polymer.

Where ↑ represents an increase, ↓ represents a decrease, ↕ represents an increase or decrease, and ↔ represents little change.

Instrumentation

Gel-permeation, or size-exclusion chromatography (GPC/SEC), is an ideal tool for studying parameters like structure, intrinsic viscosity, molecular size, and molecular weight, and molecular weight distribution of synthetic polymers, all of which influence the behavior of the polymer material.

In GPC/SEC, the sample is separated as it passes through a porous yet inert chromatography column matrix. The smaller molecules more deeply penetrate the pores, while the larger molecules cannot and so more rapidly pass through the column. As a result, the sample is separated based on hydrodynamic volume. A traditional GPC/SEC system setup includes a refractive index (RI) or ultraviolet (UV) detector and an isocratic pump, and provides a concentration profile of the size-separated sample. Under such conditions, any molecular weight measured will only be relative, incorrect with different sample, and standard. They will also be relative to difference between site and columns, and system and setup. Additional useful information related to a sample can be simultaneously determined with multiple detectors, including light scattering and intrinsic viscosity.

A Mark-Houwink plot can be generated by plotting the molecular weight (MW), directly measured from the light scattering detector, against the intrinsic viscosity (IV), directly measured from the viscometer detector, in order to interpret the relationship between molecular structure and molecular weight. This article discusses the comparisons of the structural distributions of some common polymers made using the Mark-Houwink plot. Figure 1 shows the Malvern Panalytical OMNISEC system from which the data is generated.

Figure 1. OMNISEC system, comprising OMNISEC RESOLVE (left) and OMNISEC REVEAL.

Materials and Methods

Two Viscotek T6000M columns were used to separate the samples. Using 300 ppm BHT, the mobile phase was THF stabilized. In order to ensure full dissolution of the samples, they were left overnight. The operating conditions of the OMNISEC system set-up include the following:

• Column oven temperature: 35 °C
• Detectors temperature: 35 °C
• Flow rate: 1.0 mL/min
• Autosampler temperature: 15 °C

The OMNISEC software (v10 or later) was used to perform all system control, data acquisition, data analysis and data reporting. Synthetic polymers, including polyvinylchloride (PVC), polycarbonate (PC), polymethylmethacrylate (PMMA) and polystyrene (PS), were the four samples used for analysis.

Results

Figure 2 shows overlaid duplicate RI chromatograms for each sample, along with their corresponding measured molecular weights. Figure 3 shows the overlays of the molecular weight distributions. From these figures, the differences in elution volumes and in the molecular weights at the same retention volumes can be observed, thus yielding accurate measurement irrespective of the structural differences. These differences cannot be observed in molecular weight measurements based on light scattering. This clearly demonstrates how different samples with different structures elute at different points in time for any given molecular weight. The data obtained for the samples, displaying the absolute molecular weights calculated directly from the RALS/LALS detector, are shown in Table 2.

Figure 2. Overlaid duplicate RI chromatograms of the four polymers with their measured molecular weights.

Figure 3. Overlaid duplicate molecular weight distributions of the four polymers.

Table 2. Molecular weight, size and structure results for the four different polymers.

It is evident that the values for the measured IV and hydrodynamic radius (Rh), as well as other parameters, including the average MH a and log k, are provided by the system. The slope and the intercept on the Mark-Houwink plot are defined by the M-H a and log k parameters. The difference in the polymer types can be clearly seen from this, with the PVC, for instance, having the highest IVw value and second highest Mw value. However, it is impossible to obtain a complete picture based on the molecular weight distributions and average values for broad polymers. Hence, it is better to consider the structure across the whole MW distribution with the help of the MH plot.

The structural differences (Figure 4) between all the four samples can be clearly observed from the Mark-Houwink plot, which indicates that lowest sample on the plot, PMMA, has the highest density in solution. The densities of both PVC and PC are lower than that of polystyrene. Even with similar average Mark-Houwink a values for both PVC and PC, some subtle differences can be observed. This indicates that PC has the most open structure over most of its distribution.

Another aspect that can be interpreted from the Mark-Houwink plot is that the higher molecular weight material in the PVC sample possesses a different slope on the plot. This suggests some probably unidentified branched material. These types of minor difference or contaminants present in the polymer samples are exactly what the multi-detector GPC can identify, despite being unidentifiable in conventional systems. As a result, finer control of polymer production, and hence, its final properties, can be achieved.

In conventional GPC, polystyrene standard-based calibrations would result in completely wrong molecular weight values for all three of the other polymers. Further, even with the application of a linear 'Mark-Houwink' correction, the non-linear nature of the MH relationships observed in Figure 4 would lead to erroneous results.

Figure 4. Mark-Houwink overlay of all four polymer samples showing the structural differences.

Conclusion

The OMNISEC system is capable of providing high quality, information rich polymer GPC data. Using this system, the characterization of all these four common polymers has been performed. With the combination of IV data from the new viscometer design, and absolute MW values from the high sensitivity LS, the powerful MH plot can be accessed in more detail than ever before. The data obtained through this plot is of major interest to all polymer chemists who wish to learn more about the polymer differences or trends with respect to molecular weight or structural changes independently of one other.

Polymers that are optimally placed on the Mark-Houwink plot for a suitable application can be produced through the control of level of branching, or substitution of a polymer. By doing this, it is possible to control the physical properties of their materials with high precision. The improved control ensures high quality and a high value product with fewer production failures. This helps to achieve successful syntheses with improved data quality and faster publication of research results.

This information has been sourced, reviewed and adapted from materials provided by Malvern Panalytical.

For more information on this source, please visit Malvern Panalytical.