Laboratory procedures in polymer chemistry

Transcription

1 Polymer Chemistry Course, KTE080, 2015 Patric Jannasch Laboratory procedures in polymer chemistry CAS - Polymer & Materials Chemistry

2 1. Free Radical Bulk Polymerization of Styrene This protocol describes the bulk polymerization of styrene at 60 o C using 2,2 azobisisobutyronitril (AIBN) as an initiator. The monomer styrene first has to be purified before it can be used. The purification is performed in order to remove inhibitors and impurities, which can have a detrimental effect on the reaction. Styrene is purified by passing the monomer through a column of activated basic aluminum oxide and by this treatment the inhibitors and impurities are adsorbed. The styrene used in this laboratory experiment has already been purified in this way, and has then been kept in a refrigerator to prevent thermal auto-initiation of the monomer. In total 8 ml of the desired mixtures of initiator and styrene and pure styrene are added to reaction test tubes provided with glass stopcocks. A typical initiator concentration is around 1-8 g/l. The test tubes are shaken in order to properly homogenize the mixtures. Next, the tubes with the glass stopcock are secured with springs. In order to purge (remove) the oxygen, which will otherwise interfere with the polymerization, the tubes are evacuated using a vacuum connection, and are then filled with pure nitrogen gas. This process is repeated three more times to make sure that all the oxygen has been removed. The polymerization reactions are performed in a heated water bath at 60 C for 2 hours. Note the exact reaction time in seconds! After 2 hours the polymerization is stopped by immersing the test tubes in an ice bath. The contents in the tubes are dissolved in 40 ml of toluene and the different polymers are then precipitated by adding the solutions slowly, drop-by-drop, into 400 ml of methanol under vigorous stirring. It is very important that the solution is added dropwise slowly to the methanol so that a fine polymer precipitate is formed! Styrene is soluble in methanol, while polystyrene is not. If the solution is added too quickly, monomeric styrene will be captured in the polystyrene, which will lead to the formation of a monomer-swollen lump of polymer, which in turn will cause much trouble in the next step. In order to obtain an appropriate drop rate, a separation funnel should be used. The precipitate is first collected by filtering off the precipitate using a glass filter crucible (weighed) and thoroughly washed with methanol and after that dried in an oven at 50 C until a constant weight is achieved. Note the final weight of the samples. The dried samples are placed in marked vials and are kept for further analysis. Polymer solutions for analysis by size-exclusion chromatography should be prepared at least 24 hours prior to the analysis. 1

3 2. Free Radical Copolymerization of Styrene and Butyl Methacrylate This protocol describes the bulk copolymerization of styrene and butyl methacrylate at 60 o C using 2,2 -azobisisobutyronitril (AIBN) as an initiator. The monomers first have to be purified before they can be used. The purification is performed in order to remove inhibitors and impurities, which can have a detrimental effect on the reaction. The monomers are purified by passing them through columns of activated basic aluminum oxide and by this treatment the inhibitors and impurities are adsorbed. The monomers used in this laboratory experiment have already been purified in this way, and have then been kept in a refrigerator to prevent thermal auto-initiation. In total 8 ml of the desired mixtures of initiator, butyl methacrylate, and styrene are added to marked reaction test tubes provided with glass stopcocks. It is also suitable to homopolymerize styrene in one of the tubes. The initiator concentration should be kept constant in these experiments, and a suitable concentration is around 4 g/l. The test tubes are shaken in order to properly homogenize the mixtures. Next, the tubes with the glass stopcock are secured with springs. In order to purge (remove) the oxygen, which will otherwise interfere with the polymerization, the tubes are evacuated using a vacuum connection, and are then filled with pure nitrogen gas. This process is repeated three more times to make sure that all the oxygen has been removed. The polymerization reactions are performed in a heated water bath at 60 C for 2 hours. Note the exact reaction time in seconds! After 2 hours the polymerization is stopped by immersing the test tubes in an ice bath. The contents in the tubes are dissolved in 40 ml of toluene and the different polymers are then precipitated by adding the solutions slowly, drop-by-drop, into 400 ml of methanol under vigorous stirring. It is very important that the solution is added dropwise slowly to the methanol so that a fine polymer precipitate is formed! The monomers are soluble in methanol, while the polymers are not. If the solution is added too quickly, the monomers will be captured in the polymers, which will lead to the formation of a monomerswollen lump of polymer, which in turn will cause much trouble in the next step. Copolymers rich in butul methacrylate are more liable to form monomer-swollen lumps, and cooling the methanol with ice might be necessary. In order to obtain an appropriate drop rate, a separation funnel should be used. The precipitate is first collected by filtering off the precipitate using a glass filter crucible (weighed) and thoroughly washed with methanol and after that dried in an oven at 50 C until a constant weight is achieved. Note the final weight of the samples. The dried samples are placed in marked vials and are kept for further analysis. 2

4 3. Condensation Polymerization to Prepare an Unsaturated Polyester The following protocol describes a condensation polymerization to produce an unsaturated linear aliphatic polyester through a reaction between equimolar amounts of maleic acid, and 1,4- butanediol using a strong acid as catalyst. The polymerization will be performed in a five-neck glass reactor setup equipped with an electrical heater, a mechanical stirrer, a nitrogen gas inlet, a thermometer, a water trap, and a condenser. Add 0.5 mol 1,4-butanediol, 0.5 mol maleic acid, and p-toluene sulfonic acid ( weight %) to the reaction vessel. The catalyst should be weighed using an analytic balance since the exact mass must be known in order to be able to calculate the average degree of polymerization of the product after titration with KOH. Next, add 50 ml of toluene to the reaction mixture and close the reactor. Fill the water trap with water up to the first line of the scale and then fill the rest of the water trap with toluene. Start slowly the nitrogen flow and the stirrer, and start to heat the reaction vessel until the toluene begins to boil. The reaction temperature, given by the boiling point of toluene, will be between C. The reaction can continuously be followed by measuring of the amount of water produced. Note the amount of water every second minute until the conversion has reached at least 97-98% (what is the maximum amount of water produced?). Quickly extract small samples from the reaction mixture at approx. 20, 40, 60, and 80% for the subsequent analysis by infrared spectroscopy. The toluene should be evaporated from the samples before the analysis. When the reaction mixture has cooled down, the concentration of carboxylic groups is determined the following way: extract three 1 g (note the exact weight) liquid samples of the final reaction mixture. Next, weigh the samples in E-flasks on an analytic balance. Dissolve the samples in 20 ml acetone and titrate with 0.1 M alcoholic KOH using phenolphftalein as indicator. Titrate also a blind sample of 20 ml acetone to ensure that the acetone does not consume any KOH. Do not forget to correct for the p-toluene sulfonic acid when calculating the concentration of acid groups! 3

5 4. Procedure for Measurement of Molecular weights and Molecular Weight Distributions using Size-Exclusion Chromatography (SEC) This protocol describes the analysis of chain lengths of polymers by SEC using conventional calibration. This technique is applicable to a wide variety of polymers, of both low and high molecular weights, dissolved in solvents of varying polarity. The selection of column type, column pore size, solvent, and temperature must be appropriately made for each polymer. The setup available comprises of: A Viscotek VE 1122 delivery system (1 ml chloroform per minute) A dual detector (refractive index/viscosity) model 250 from Viscotek equipped with the TriSec software A three-column package (Shodex SEC KF-805, 804 and 802.5) coupled in series A degassed solvent reservoir of chloroform solvent under a blanket of nitrogen. You also have available 0.45 m Acrodisc CR PTFE filters to remove particles, etc, from your samples, and a 500- L syringe to inject the samples. The polymer samples to be analyzed should be prepared at least 24 hours before analysis as follows. Accurately weigh about mg of the polymer into a 10 ml volumetric flask. Take note of the exact weight. Add 6-7 ml of chloroform solvent to the volumetric flask using a clean and dry pipette and close the flask. Let the polymer to dissolve overnight. On the day of the SEC experiment, add chloroform up to the mark of the volumetric flask. Be sure to shake the flasks several times to obtain homogeneous solutions. Label and uncap an appropriate number of clean SEC vials. Rinse a 10-mL syringe and plunger twice with pure chloroform. Next, place a 0.45 m Acrodisc CR PTFE filter onto the tip of the syringe. Extract the sample solution into the syringe. Insert the plunger and press until liquid passes through the filter into the vial. Immediately cap the vials containing filtered sample. Finally, rinse the syringe with the filter twice between each sample using chloroform. To launch an experiment, use the following procedure: - Open the programme Data Acquisition - To set-up an experiment, go first to System File > System set-up. There, select the correct group number in the operator field. No other change should be made. Once you are in System File > Data Acq Direcotry, check that you are in the following directory c:\gpc2005\lab. Finally, you need to enter a run queue by clicking on New in the Run queue menu. There, type in the name of the sample that you will run. The file name has to start with gr and your group number. It is a good idea to enter all the names of the samples that will be run during the day to speed up the work (each sample being alternatively from one or the other group). - To launch the first experiment in the queue, click on Start Acquisition in the menu Acquisition. A message Waiting for trigger will appear. The run will not start before the trigger on the Data Manager is pushed. The trigger has to be pushed at the same time that the sample is injected into the system. Before injecting the sample, make sure that the temperature of the detectors is stabilized and that the SEC set-up is not recycling the eluent. 4

6 To inject the sample in the system, use the following procedure: - Using the 500 L syringe, take up to 450 L of your previously filtered sample. Make sure that no air bubbles are present in the syringe. Next, place the syringe into the injector in the LOAD position and inject your entire sample (the excess is collected with a beaker). - While keeping the syringe in the injector, turn the injector to the INJECT position and push the trigger of the Data Manager at the same time. - Take out the syringe and rinse it three times with previously filtered chloroform. At the end of the experiment, the next sample name in the queue list is loaded and the message waiting for trigger will appear. Follow the same procedure to run the next sample. Print the chromatogram after analyzing your first sample. You will then be given a chromatogram of narrow standards with known molecular weights to prepare the calibration curve. - Read the retention volume, V r, for each of the narrow distribution polymer standards. - Using the retention volumes, V r, for the narrow distribution polymers and their molecular weight, M, a calibration curve is constructed with log M versus V e. - Use the chromatogram of the polymer sample with unknown molecular weight to calculate its M n, M w, and M w /M n. Please note that the amount of polymer at a specific V e, or corresponding molecular weight, is proportional to the height of the curve, H i, at this specific V e : - H i ~ n i M i, where n i is the moles of the ith polymer and M i is the molecular weight of that polymer. A sufficient number of divisions should be taken to have a good accuracy. The interpretation of the all the chromatograms will then also be carried out using the programme Conventional calibration. Please refer to the manual for more information. 5

7 5. Structural Analysis of Polymers by Fourier Transform Infrared (FTIR) Spectroscopy FTIR spectroscopy may be used to determine if any characteristic absorption peaks appear or disappear in the spectra, or if the relative intensities between two peaks change, for example, during a polymerization. If the polymers to be analyzed are flowing melts at room-temperature, spectra can be measured in transmission mode with the polymer spread out on a KBr tablet. Make sure that the sample is properly dried before analysis. First, a ca mm thick tablet is prepared by compressing a suitable amount of KBr powder for ca. 3 min. The tablet is gently taken out of the mold. Next, a very thin film of the polymer is formed by first placing a small droplet of the polymer melt on top of the tablet, and then scraping off the excess polymer with a razor blade. The sample is now ready for analysis. If the polymers solid at room-temperature, spectra can be measured in transmission mode with the polymer in the form of a micrometer thick films. An appropriate amount of polymer is first dissolved in a suitable solvent. The solution is poured into a Petri dish. After evaporation of the solvent, the polymer films are carefully dried under vacuum at 50 C. Alternatively, a small amount of carefully dried polymer precipitate is ground together with KBr powder. Subsequently, a ca mm thick tablet is prepared by compression. The tablet is gently taken out of the mold and is ready for analysis. Make sure that the settings of the spectrometer are correct. You should measure spectra in the range cm -1 in transmission mode. The results should be presented as absorption spectra. Designate your recorded spectra so that you can easily identify it afterwards. In order to subtract the contributions from oxygen, carbon dioxide, etc, from the atmosphere, a background spectrum is first recorded. The same background spectrum is then used for all the subsequent analyses. The lid of the spectrometer is opened for a short time to let in air. Make sure that there is no sample in the sample holder. Gently close the lid. After waiting for 15 min., the background spectrum is recorded. Now place the sample in the sample holder and place it in the spectrometer. Wait 15 min. and then record the spectrum. The spectrum is directly visible on the screen after the measurement is finished. Try to identify as many of the peaks and bands in the spectra as possible using tables and charts of the absorption wavenumbers of known chemical entities. 6