Polymers and plastics are used in nearly everything we touch every day. These materials deliver a huge diversity of performance properties that are required for many different market areas. Polymer chemists are continually developing new polymer materials to address these performance needs. One relatively new area for polymer development is to use renewable resources as feedstock to build the polymer backbone. The use of renewable resources reduces the dependence on petroleum-based feedstocks and results in a ‘green’ material. The polymers discussed here are based on the polymerization of lactides, a diester formed from lactic acid.
As new polymers are developed, the polymer chemist relies on data from a variety of different analytical tools to make decisions about the chemistry in the new polymer. Analytical data can be produced that describe both the chemical structure of the polymer, and the physical properties of the polymer. Combining these data streams enables the development of structure-property relationships. Chemists can utilize these structure-property relationships to understand how changes to one portion of the chemical structure impact the performance attributes of the ensuing material.
For these experiments, we will measure both the chemical structure of the polymers, and the material properties of the polymer. Key chemical structure analyses will include:
- Gas Chromatography/Mass Spectrometry (GC/MS) to measure residual monomer in the polymer
- Nuclear Magnetic Resonance (NMR) Spectroscopy to measure the connections that make up the polymer backbone and end groups (both 1H and 13C)
- Size Exclusion Chromatography (SEC) to measure the molecular mass distributions of the polymer chains
Key material properties of the polymers will be analyzed using:
- Modulated Differential Scanning Calorimetry (MDSC) to measure the thermal properties of the polymer
- Water Uptake to measure the amount of moisture taken up by the polymer under different conditions
- Dynamic Mechanical Analysis (DMA) to measure the flow characteristics of the polymers, including viscosity
All of these experiments were conducted at Intertek Allentown in Allentown, PA.
Results and Discussion
These polymers were synthesized by ring-opening polymerization of D,L-lactide, the cyclic dimer of lactic acid. The polymer chemists synthesized a variety of samples to explore an experimental design with three different axes: Molecular Mass, End Group (acid or ester), and Glycolide content.
Residual monomer is a very important quality control measure for the polymer chemist. Knowledge of the residual monomer levels helps the chemist understand the efficiency of the polymerization and is an early indicator of the effectiveness of the polymer synthesis method. GC/MS analyses of these polymers showed that they contained some residual L-lactide monomer levels ranging between 0.01 – 0.1 wt. %.
Polymer backbone chemistry is the key to the polymer synthesis. The polymer chemist is trying to create a specific arrangement of monomer units connected together to generate the desired polymer. NMR analysis of the polymer samples confirmed the overall composition of the copolymer compositions, a combination of lactide and glycolide run lengths. The NMR was also able to show that these samples have low blockiness, but that some samples show relatively subtle differences in blockiness from lot-to-lot. The blockiness is a measure of how many consecutive lactide or glycolide units are connected together. The NMR data shows that the average length of the isotactic runs ranged from 2.1 – 2.5 and the average length of the syndiotactic runs ranged from 1.0 - 1.2.
NMR also analyzed the relative concentrations of the two monomers used to create the copolymers, lactide and glycolide. For the samples included in this study the copolymers ranged from 50:50 lactide:glycolide to 85:15 lactide:glycolide.
Another key element of the chemical structure is the end group of the polymer chain. Especially for relatively low molecular weight polymers, the end groups have a strong impact on how the different polymer chains interact, and may introduce a different chemical functionality to the polymer from the backbone units. Typical industrial polymers have too high a molecular mass for NMR to detect the end groups. These polymers were sufficiently low in molecular mass for NMR to identify the end groups. NMR was able to identify two different end groups in this set of materials, either an acid or an ester end group.
The third important component of polymer chemical structure is the molecular mass distribution. This measurement provides an understanding of the average polymer chain length and the range of chain lengths present in the material. Chain length contributes directly to a variety of polymer properties, including toughness, durability, and ability to flow. SEC was used to measure the molecular mass distributions of these polymers. SEC data are reported as the moments of the distribution of chain lengths observed, usually denoted as MN, MW, and MZ. The range of chain lengths observed, also termed the polydispersity index (PDI = MW/MN), is another important parameter for characterizing the molecular weight distribution. For these polymers, SEC analysis shows polymers with the following characteristics:
- MN ranging from 5,000 D to 27,000 D
- MW ranging from 11,000 D to 174,000 D
- MZ ranging from 16,000 D to 320,000 D
- PDI ranging from 1.7 to 9.6
Figure 1 shows a typical SEC chromatogram for one of these polymer samples.
Polymer Material Properties
The thermal properties of the polymers were explored with MDSC. Compared to standard DSC measurements, MDSC offers the advantage of being able to separate thermal features into reversing and non-reversing events. MDSC showed that the thermal properties were relatively comparable across the set of samples. One important polymer property measured by both MDSC and DMA is the glass transition temperature, Tg. Tg shows the polymer chemist at what temperature the polymer transitions from a hard, glass-like material to a softer, more rubber-like material. This temperature is a key performance property of polymers. MDSC shows that the Tg’s for these polymers range from 38 – 52 °C. MDSC also showed that the heat of relaxation of these polymers ranged from 2.8 – 6.7 J/g on the first heating, and from 1.5 – 2.1 J/g on the second heating. An example DSC thermogram is shown in Figure 2. Specifically the total heat flow curve is the upper trace in Figure 2. The Tg is denoted by the step change in the reversing heat flow curve (middle trace). The peak in the non-reversing heat flow curve (lower trace) is a relaxation endotherm.
Water uptake is an indication of how much environmental conditions, such as relative humidity, will impact the material properties of the polymer. This is important as small amounts of residual moisture can adversely impact molded articles. At 74% relative humidity, these polymers absorbed between 0.5 – 1.4 wt. % water. Samples with low molecular masses and acid end groups tended to pick up more water. Water adsorption and desorption results were in very good agreement.
The polymer rheological or flow properties were measured using DMA. The DMA data shows that the polymers exhibit a single Tg, and the Tg values measured by DMA were consistent with those measured by MDSC. The largest property differences observed were in the complex viscosity above the Tg. Viscosities ranged from 1,000 to 1,000,000 P at 95°C. The lower molecular weight polymers exhibited a significant decrease in viscosity with increasing temperature. Likely because of molecular entanglements, the highest molecular mass polymers appeared more network-like.
By comparing the molecular mass results with the rheology results, the average entanglement mass can be estimated. Figure 3 shows a plot of viscosity (from the DMA data) vs. MZ (from the SEC data). The data in Figure 3 show the classic break in viscosity vs. MZ. The break point is indicative of the entanglement point. For these polymers it occurs at an MZ of about 30,000 D.
As polymer chemists develop new materials, they require a wide variety of analytical data to probe the chemical structure and material properties of the new polymer. These analytical data are critical to determine if the new material meets the requirements of the application, and to examine the effectiveness of the new polymer synthesis method. Collaboration between polymer synthetic chemists and analytical chemists can provide the rich insight provided by complex data to enable real innovation in polymer and plastic development.
We would like to thank our customers who provide challenging and important polymers for us to characterize. We would also like to acknowledge our colleagues at Intertek Allentown, especially John Zielinski, Ann Kotz, Scott Voth, Chanell Brown, and Steve Deppen, who helped design and execute these complex analytical experiments.