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PKG 829 Packaging Plastics Laboratory

General Objectives. To get familiar with chromatographic procedure, especially, gel permeation chromatography (GPC).To get familiar with the test procedure for determining molecular weight distribution of packaging material using GPC.To calculate the different average molecular weight (Mn, Mw, Mz)

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PKG 829 Packaging Plastics Laboratory

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    1. PKG 829 Packaging Plastics Laboratory Lab 4 Determination of Molecular Weight and Molecular Weight Distribution of Polymers by Gel Permeation Chromatography Li Xiong Fall 2004

    2. General Objectives To get familiar with chromatographic procedure, especially, gel permeation chromatography (GPC). To get familiar with the test procedure for determining molecular weight distribution of packaging material using GPC. To calculate the different average molecular weight (Mn, Mw, Mz), and dispersity index (DI).

    3. Lab 4 Procedure A dataset from analyzing an unknown plastic resin with GPC will be given, from which we calculate and plot: MW and DI the diagraph of MWD The diagraph of cumulative molecular weight versus degree of polymerization The average MWs for your PE samples from lab 1-3 will be given to you.

    4. Some Background Info. Molecular Weight and Its Distribution A polymer is a mixture of different-sized molecules (different degree of polymerization) and each of them has a unique molecular weight. Average molecular weight is used to denote the overall molecular weight of a polymer. Number average molecular weight (Mn) Weight average molecular weight (Mw) Z average molecular weight (Mz) Viscosity average molecular weight (Mv)

    5. Significance of average molecular weight Different average MW indicates different physical properties of a polymer. For example, Mn is related to osmotic pressure and MZ is related to viscosity. DI (ratio of Mw to Mn) is the measurement of MWD. Higher DI indicates wider molecular weight distribution. Wider the distribution, wider the heat sealing temperature range, higher the MFI would be.

    6. Principles of Chromatography Adsorption chromatography (gas-solid, liquid-solid). Partition chromatography (gas-liquid, liquid-liquid). Ion exchange chromatography Size exclusion (or gel permeation) chromatography

    8. GPC (Gel Permeation Chromatography) Mechanism: separation occurs by movement of solute molecules having different sizes and shapes through a solid phase of controlled-porosity particles. Bigger molecules cannot enter pores, thus travel through the column faster. Smaller molecules can enter pores, thus come out of the column slower. GPC produces a separation based on molecular size (correspondingly, molecular weight).

    9. GPC System Mobile phase Requirements (see lab manual) Trichlorobenzene for PE Tetrahydofuran for PS Column exclusion range A broad range of pore sizes to accommodate the test specimen molecular weight range. A normal distribution of chromatograph will be obtained.

    10. Column packing material Hydrophilic dextran (Sephadex) beads Lypophilic polystyrene beads Porous glass beads Swollen, cross-linked polymer beads Detector Refractometer (Refractive Index (RI) detector) IR (Infrared) detector Variable wavelength UV detector

    11. Use GPC to Determine MW and MWD Briefly stated, it involves two stages Construct a calibration curve using mono-disperse polystyrene standards (DI < 1.1) with different molecular weights. Analyze the sample and calculate the average MW and MWD

    12. Calibration

    13. Ostensibly universal calibration The separation is assumed dependent on the molecular weight M only. Most popular model is a linear relation between Log M and Tr.

    14. This model does not apply to all polymers over the whole range of molecular weight. Instead, some empirical correlation between Log M and Tr has been developed for some specific polymer and a specific set of conditions (solvent, column, temperature, etc.)

    15. Universal calibration Hydrodynamic volume (? M [?]) is taken as the controlling parameter in the separation process: For linear polymers, the Mark-Houwink-Sakurada equation gives us: K, a = constants, specific for different polymer composition and architecture Mv = viscosity average molecular weight

    16. For branched polymers, the effect of LCB (long-chain branching) on the hydrodynamic volume must be taken into consideration. MSH does not hold anymore. Instead, more complicated translation from M to [?] is required. For any two polymer 1 and 2, the following relationship always holds:

    17. The universal model is expressed as a relationship between M [?] and Tr

    18. Sample Analysis The recorded data is detector signal (?n) as a function of elution volume (Ve). With the help of GPC computer software and the calibration curve, we can transform: Where Mi, Ni, Ai and wi are molecular weight, number of molecules having Mi, area of the peak in the chromatograph, and weight fraction of fraction i in a polymer.

    20. Calculation

    22. Testing procedures Select column for GPC analys. Choose test conditions. Calibrate column Sample analysis. Calculate average molecular weight’s, DI and plot MWD.

    23. Calibration data

    24. Data of GPC Analysis of PE

    25. Average MWs of Your Two Unknown PE Samples The Mn, Mw and Mz of your unknown PE samples from lab 1- 3 will be given to you. Calculate DI for your two uknown PE samples.

    26. Report and Discussion Part 1: based on the GPC data Calibration table, curve and fitted linear relationship between Log M and retention time Tr. Calculate Mn, Mw, and Mz (show the detailed calculation in a spreadsheet). Calculate DI and determine whether the plastic resin has wide or narrow MWD. Show the detailed calculation (in spreadsheet) to determine MWD profile and plot the profile (wi vs. Mi)

    27. Lable Mn, Mw, Mz on the MWD plot. Show the detailed calculation of cumulative molecular weight Wi and its first-oder differential value (dWi/dx) in a spreadsheet x = X/1000, where X is the degree of polymerization Wi = (?wi-1) + wi/2 Plot the cumulative molecular weight Wi and its first-order differential value (dWi/dx) versus x.

    28. Part 2: for your two unknown PE samples List the average MWs and DI in a table. Compare the average MWs of the two PE resins. Compare the mechanical properties of the two PE resins. Do the results agree with the results from lab 1, 2 and 3? Use specific data to demonstrate your point (use table please!) Discuss the expected heat sealing properties of the two PE resins (temperature and range) based on their average MWs and MWD.

    29. Some Suggestions Start your work early. Read the related chapter in the textbook of PKG 827. Show me the calculations in spreadsheet, clearly and precisely. Do NOT forget table and figure titles. No NOT forget to denote the scales of x-axis and y-axis of all your figures. Pay attention to your notations of variables (lower case vs. capital case), follow the lab manual to avoid confusion.

    30. Questions? My Office Hour: Mondays 4-5 pm or by appointment Lab report is not a group project!!!

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