PKG 829 Packaging Plastics Laboratory

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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

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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).

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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.

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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)

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• 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.

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

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• 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).

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• 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.

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• 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

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

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• Calibration

With those transformations, we can construct a
calibration curve as a plot of molecular weight M
vs. retention time Tr, or molecular weight M vs.
elution volume Ve
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• 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.

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• 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.)

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• 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

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• 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

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• The universal model is expressed as a
  relationship between M ? and Tr

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• 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.

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• Calculation

Fig. Molecular weight distribution curve (weight
fraction wi versus molecular weight Mi)
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Testing procedures

• Select column for GPC analys.
• Choose test conditions.
• Calibrate column
• Sample analysis.
• Calculate average molecular weights, DI and plot
  MWD.

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Calibration data
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Data of GPC Analysis of PE
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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.

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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)

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• 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.

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• 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.

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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.

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Questions?

• My Office Hour
• Mondays 4-5 pm or by appointment
• Lab report is not a group project!!!