The Influence of Processing-History on the Main Melting Temperature and Secondary Melting Temperature Using DSC Abstract: Stephen Sansoterra Canfield High School, 100 Cardinal Drive, Canfield Ohio Polymers are in a majority of commercial products, and are synthesized in various ways. DSC was used to analyze the melting temperatures of Low Density Polyethylene (LDPE), cellulose acetate, and polypropylene (PP). Two melting cycles were completed in order to look for differences which allude to a processing-history of a commercial polymer. In every sample, there was a processing-history that was indicated by differences in heat absorption between the two melting cycles in each sample. This allows further research in how various processes affect the Differential Scanning Calorimetry (DSC) graphs when compared to a specific sample. Introduction: Polymers are used globally for the packaging of food and commercial goods. Some common polymers used are polyolefin, polyester, polyamide, polyvinyl chloride, polyvinylidene chloride, and polystyrene. Polymer research and development is very important because we come into contact everyday with polymers in products such as grocery store bags to water bottles, to plastic wrappers. Every polymer has unique properties that revolve around its own specific structure. Even tiny changes in structure can affect a large variety of properties, decreasing functionality. An endothermic reaction is a reaction in which the reactants absorb energy whereas an exothermic reaction involves the release of energy into the system. Semi crystalline polymers are molecules that have interspersed amorphous regions (linear, non- crystalline regions). Processing-history is how a polymer was synthesized and handled during production. Hypothesis- Can a difference in Tm1 values and Tm2 values indicate a processing-history based on the amount of energy absorbed by the sample? 1
Experiments: Several polymer samples from everyday items were prepared. A sample was created from a Ziploc bag, a popcorn bag wrapper, a name badge, and a bag labelled PE-LD Ziploc Bag PE-LD Bag Popcorn Bag Wrapper Name Badge Differential Scanning Calorimetry (DSC): DSC is a tool used to measure the glass transition, melting, and crystallization temperatures of a polymer while it is heated or cooled. The samples examined show specific melting and crystallization peaks based on the polymer within the sample that is being examined. Two heating cycles were done on each sample, with one cooling cycle done in between. The TA instruments DSC (Model Q2000) was used for each of the DSC measurements. Low mass Aluminum sample pans were used for analysis, and the sample was prepared to be between 2 and 3 milligrams. Samples were heated and cooled at a rate of 10 degrees Celsius per minute. TA Universal Analysis was then used for deeper analysis of the peaks to prepare the DSC data found in this report. Fourier Transform Infrared Spectroscopy (FTIR): FTIR is a technique that creates a graph that indicates specific functional groups within a molecule based on the absorption of infrared light and the vibration of the molecules that is caused by the absorption of energy. A specific peak found in many FTIR graphs is a carbon- hydrogen bond that occurs at about 3000 cm -1. Each molecule has its own fingerprint region of the graph which is used for the identification of specific molecules and polymers. The Perkin- Elmer (Model Spectrum Two) was used to take FTIR spectrum for the wavelength range of 4000 to 700 cm -1 for each sample. The samples were prepared by taping polymer film over a paper business card with a 3 cm in diameter circular aperture. 2
Results: FTIR was done on every sample in order to determine what polymer was within each of the four samples. The FTIR was then compared to data from the European Commission to better understand what polymer was being examined. Ziploc Bag FTIR PE-LD Bag FTIR Both the Ziploc Bag and the PE-LD Bag had a very similar FTIR graph as an FTIR graph of LDPE created and analyzed by the European Commission. The only difference highlighted on the two graphs was within the fingerprint region, and was a small peak. This along with DSC data 3
discussed later in the paper indicates that the LPDE in the PE-LD bag had less branching than the LPDE within the Ziploc bag. The next graphs show the similarity between the results from our FTIR graphs compared to FTIR graphs of strictly the polymer thought to make up each sample. The peaks are nearly identical, but as in the case in polypropylene, the peaks are out of proportion. Also noted in the popcorn bag wrapper from the FTIR of polypropylene is the presence of large waves that just were caused by the denseness of our samples. Peaks were even more out of proportion compared to the 4
cellulose acetate. However, the two graphs look quite similar, with some small differences within the fingerprint region. Popcorn Bag Wrapper (Polypropylene) FTIR Spectrum of Name Badge FTIR Spectrum of Cellulose Acetate After a major component of our samples was determined, DSC was then used to investigate the differences between the main and secondary melting temperatures of each sample. Tm1 of the PE-LD 5
Main Melting Temp. Secondary Melting Temp. 121.78 o C 106.35 o C Tm2 of PE-LD Bag 6
Main Melting Temp. 121.30 o C 7
Secondary Melting Temp. Minor Shoulder Peak 117.27 o C 103.59 o C 8
When looking at the first cycle (Tm1), there are significant differences from the third cycle (Tm2) such as the presence of a small third peak. The peaks are indicative of several different regions of various levels of crystallinity within the polymer. This means that one region of crystallinity had changed when melting for the second time after recrystallizing. It also means that the processing-history behind the molecule required a different amount of energy the first time it was melted, resulting in a different peak structure from when the sample was melted a second time during cycle three of the DSC. A similar graph was constructed from a DSC of the same polymer from a different source. T m1 of Ziploc Bag Main Melting Temp. Secondary Melting Temp. 121.78 o C 106.35 o C 9
T m2 of Ziploc Bag Main Melting Temp. Secondary Melting Temp. 119.64 o C 105.34 o C Both the PE-LD bag and the Ziploc bag exhibit melting peaks in roughly the same spot. However the main peak is at a temperature of 119.54 degrees Celsius, which contrasts the main peak temperature difference of the PE-LD bag (121.78 degrees Celsius). The lower melting peak temperature in the Ziploc bag is indicative of longer, branched chains. This is consistent with the differences found between the FTIR graphs. An interesting feature of the Tm1 graph of the Ziploc bag is that, just like in the graphs of the PE-LD bag, there is a more unstructured line leading up to the melting point when moving from left to right. This also shows that there is a processinghistory of the polymer noted by the different amounts of energy that are needed to perform the first melting of the sample when compared to the second. T m1 of Popcorn Bag Wrapper Main Melting Temp. 163.42 o C 10
T m2 of Popcorn Bag Wrapper Main Melting Temp. 159.34 o C The DSC graphs for the popcorn bag wrapper have a single peak as opposed to the double peaks of the Ziploc and PE-LD bag. This indicates that there is one region of crystallization. Another interesting feature is the significant difference between the two melting temperature graphs in the popcorn bag wrapper. The marked regions show the much more extensive processing-history that this polymer had over the Ziploc bag and the PE-LD bag. T m1 of Name Badge Main Melting Temp. 121.78 o C 11
T m2 of Name Badge No Melting Peak For the Name Badge, a crosslink polymer was formed from the degradation of the cellulose acetate within the name badge. In between 250 and 300 degrees Celsius, the heat was too much for the polymer and a completely new product was formed. This explains why in the second heating of the sample, there was no melting peak because there was a completely different molecule within the DSC machine. Conclusion: The FTIR graphs were created and used to compare the samples to pure polymers. The comparisons were similar, except for small differences within the fingerprint region due to the samples not being single pure polymers. Another small difference was the presence in our FTIR graphs of large waves along the top of the graph due to the high density of our samples, however using DSC served as a powerful method to identify different plastics. The DSC results showed that there was a processing-history of each sample that affected the Tm1 graph. This was shown in each sample as a significant difference between both times the samples were melted; one example being the 163.42 degrees Celsius Tm1 compared to the 159.34 degrees Celsius Tm2 of the Popcorn bag wrapper. The DSC also yielded results about the thermal degradation of polymers as shown by the name badge. It had become a completely new molecule because it was over heated. This was shown through the unique Tm1 graph, and the second heating graph that had no melting peak because the structure of the cellulose acetate was thermally altered. DSC allowed us to see the difference between LDPE and HDPE while FTIR did not, however FTIR is still a powerful tool to quickly scan the sample and allow us to identify different plastics. After examining the DSC results, the hypothesis was deemed correct as a processing-history was observed in the melting temperature graphs of every sample. In the future it would be interesting to pursue how the various processes used for commercial polymer production alter the DSC graphs as compared to a pure sample. 12
Acknowledgement: I would like to thank both of my parents, my fellow high school colleagues, Dr. Ma, Dr. Ryu, Sara, Casey, Miriam, Ben, Louis, Theodore, the Science Department at Canfield High School, Ms. Cannel, Mr. Pavlansky, Research in Polymers Program at Rensselaer Polytechnic Institute, and the National Science Foundation (Award Number 1308617) References: 1. M. A. Peltzer and C. Simoneau OLC002 2013 Identification of Polymeric Materials European Commission, Joint Research Centre, Institute for Health and Consumer Protection (ISBN 978-92-79-35267-6), Publications Office of the European Union, Luxembourg (2013). 2. Thomas, L.C. Characterization of Melting Phenomena in Linear Low Density Polyethylene by Modulated DSC. TA Instruments Inc. 3. Chordiya MA, Gangurde HH, Senthilkumaran K, Kothari LP. Formulation development and in vitro evaluation of gastrorelentive hollow microspheres of famotidine. Int J Pharma Investig 2011;1:105-11 13