Presentation on theme: "Micro Structures in Polymers Chapter 3"— Presentation transcript:
1 Micro Structures in Polymers Chapter 3 Professor Joe GreeneCSU, CHICOMFGT 041September 20, 1999
2 Chapter 3 Objectives Objectives Polymer length, molecular weight, molecular weight distribution (MWD)Physical and mechanical property implications of molecular weight and MWDMelt IndexAmorphous and crystalline structures in polymersThermal transitions in plastics (thermoplastics and thermosetsSteric (shape) effects
3 Polymer Length Polymer Length Molecular Weight Polymer notation represents the repeating groupExample, -[A]-n where A is the repeating monomer and n represents the number of repeating units.Molecular WeightWay to measure the average chain length of the polymerDefined as sum of the atomic weights of each of the atoms in the molecule.Example,Water (H2O) is 2 H (1g) and one O (16g) = 2*(1) + 1*(16)= 18g/moleMethane CH4 is 1 C (12g) and 4 H (1g)= 1*(12) + 4 *(1) = 16g/molePolyethylene -(C2H4)-1000 = 2 C (12g) + 4H (1g) = 28g/mole * 1000 = 28,000 g/mole
4 Molecular Weight Average Molecular Weight Polymers are made up of many molecular weights or a distribution of chain lengths.The polymer is comprised of a bag of worms of the same repeating unit, ethylene (C2H4) with different lengths; some longer than others.Example,Polyethylene -(C2H4)-1000 has some chains (worms) with 1001 repeating ethylene units, some with 1010 ethylene units, some with 999 repeating units, and some with 990 repeating units.The average number of repeating units or chain length is 1000 repeating ethylene units for a molecular weight of 28*1000 or 28,000 g/mole .
5 Molecular Weight Average Molecular Weight Distribution of values is useful statistical way to characterize polymers.For example,Value could be the heights of students in a room.Distribution is determined by counting the number of students in the class of each height.The distribution can be visualized by plotting the number of students on the x-axis and the various heights on the y-axis.
6 Molecular Weight Molecular Weight Distribution Count the number of molecules of each molecular weightThe molecular weights are counted in values or groups that have similar lengths, e.g., between 100,000 and 110,000For example,Group the heights of students between 65 and 70 inches in one group, 70 to 75 inches in another group, 75 and 80 inches in another group.The groups are on the x-axis and the frequency on the y-axis.The counting cells are rectangles with the width the spread of the cells and the height is the frequency or number of moleculesFigure 3.1A curve is drawn representing the overall shape of the plot by connecting the tops of each of the cells at their midpoints.The curve is called the Molecular Weight Distribution (MWD)
7 Molecular Weight Average Molecular Weight Determined by summing the weights of all of the chains and then dividing by the total number of chains.Average molecular weight is an important method of characterizing polymers.3 ways to represent Average molecular weightNumber average molecular weightWeight average molecular weightZ-average molecular weight
8 Gel Permeation Chromatography GPC Used to measure Molecular Weightsform of size-exclusion chromatographysmallest molecules pass through bead pores, resulting in a relatively long flow pathlargest molecules flow around beads, resulting in a relatively short flow pathchromatogram obtained shows intensity vs. elution volumecorrect pore sizes and solvent critical
10 Number Average Molecular Weight, Mn where Mi is the molecular weight of that species (on the x-axis)where Ni is the number of molecules of a particular molecular species I (on the y-axis).Number Average Molecular Weight gives the same weight to all polymer lengths, long and short.Example, What is the molecular weight of a polymer sample in which the polymers molecules are divided into 5 categories.Group Frequency50,000 1100,000 4200,000 5500,000 3700,000 1
11 Molecular Weight Number Average Molecular Weight. Figure 3.2 The data yields a nonsymmetrical curve (common)The curve is skewed with a tail towards the high MWThe Mn is determined experimentally by analyzing the number of end groups (which permit the determination of the number of chains)The number of repeating units, n, can be found by the ratio of the Mn and the molecualr weight of the repeating unit, M0, for example for polyethylene, M0 = 28 g/moleThe number of repeating units, n, is often called the degree of polymerization, DP.DP relates the amount ofmonomer that has been converted to polymer.
12 Weight Average Molecular Weight, Mw Favors large molecules versus small onesUseful for understanding polymer properties that relate to the weight of the polymer, e.g., penetration through a membrane or light scattering.Example,Same data as before would give a higher value for the Molecular Weight. Or, Mw = 420,000 g/mole
13 Z- Average Molecular Weight Emphasizes large molecules even more than MwUseful for some calculations involving mechanical properties.Method uses a centrifuge to separate the polymer
14 Molecular Weight Distribution Molecular Weight Distribution represents the frequency of the polymer lengthsThe frequency can be Narrow or Broad, Fig 3.3Narrow distribution represents polymers of about the same length.Broad distribution represents polymers with varying lengthsMW distribution is controlled by the conditions during polymerizationMW distributions can be symmetrical or skewed.
15 Physical and Mechanical Property Implications of MW and MWD Higher MW increasesTensile Strength, impact toughness, creep resistance, and melting temperature.Due to entanglement, which is wrapping of polymer chains around each other.Higher MW implies higher entanglement which yields higher mechanical properties.Entanglement results in similar forces as secondary or hydrogen bonding, which require lower energy to break than crosslinks.
16 Physical and Mechanical Property Implications of MW and MWD Higher MW increases tensile strengthResistance to an applied load pulling in opposite directionsTension forces cause the polymers to align and reduce the number of entanglements. If the polymer has many entanglements, the force would be greater.Broader MW Distribution decreases tensile strengthBroad MW distribution represents polymer with many shorter molecules which are not as entangled and slide easily.Higher MW increases impact strengthImpact toughness or impact strength are increased with longer polymer chains because the energy is transmitted down chain.Broader MW Distribution decreases impact strengthShorter chains do not transmit as much energy during impact
17 Thermal Property Implications of MW & MWD Higher MW increases Melting PointMelting point is a measure of the amount of energy necessary to have molecules slide freely past one another.If the polymer has many entanglements, the energy required would be greater.Low molecular weights reduce melting point and increase ease of processing.Broader MW Distribution decreases Melting PointBroad MW distribution represents polymer with many shorter molecules which are not as entangled and melt sooner.Broad MW distribution yields an easier processed polymerMechanicalPropertiesMeltingPoint* DecompositionMWMW
18 Example of High Molecular Weight Ultra High Molecular Weight Polyethylene (UHWMPE)Modifying the MWD of Polyethylene yields a polymer withExtremely long polymer chains with narrow distributionExcellent strengthExcellent toughness and high melting point.Material works well in injection molding (though high melt T)Does not work well in extrusion or blow molding, which require high melt strength.Melt temperature range is narrow and tough to process.Properties improved if lower MW polyethyleneActs as a low-melting lubricantProvides bimodal distributions, Figure 3.5Provides a hybrid material with hybrid properties
19 Melt Index Melt index test measure the ease of flow for material Procedure (Figure 3.6)Heat cylinder to desired temperature (melt temp)Add plastic pellets to cylinder and pack with rodAdd test weight or mass to end of rod (5kg)Wait for plastic extrudate to flow at constant rateStart stop watch (10 minute duration)Record amount of resin flowing on pan during time limitRepeat as necessary at different temperatures and weights
20 Melt Index and Viscosity Melt index is similar to viscosityViscosity is a measure of the materials resistance to flow.Viscosity is measured at several temperatures and shear ratesMelt index is measured at one temperature and one weight.High melt index = high flow = low viscosityLow melt index = slow flow = high viscosityExample, (flow in 10 minutes)Polymer Temp MassHDPE 190C kgNylon 235C kgPS 200C Kg
21 Melt Index and Molecular Weight Melt index is related closely with average molecular weightHigh melt index = high flow = small chain lengths = low MnLow melt index = slow flow = long chain lengths = high MnTable 3.1 Melt Index and Average Molecular WeightMn Melt Index* (g/10min)100,150,250,* Note: PS at T= 200C and mass= 5.0Kg
22 States of Thermoplastic Polymers Amorphous- Molecular structure is incapable of forming regular order (crystallizing) with molecules or portions of molecules regularly stacked in crystal-like fashion.A - morphous (with-out shape)Molecular arrangement is randomly twisted, kinked, and coiled
24 States of Thermoplastic Polymers Crystalline- Molecular structure forms regular order (crystals) with molecules or portions of molecules regularly stacked in crystal-like fashion.Very high crystallinity is rarely achieved in bulk polymersMost crystalline polymers are semi-crystalline because regions are crystalline and regions are amorphousMolecular arrangement is arranged in a ordered state
26 Factors Affecting Crystallinity Cooling Rate from mold temperaturesBarrel temperaturesInjection PressuresDrawing rate and fiber spinning: Manufacturing of thermoplastic fibers causes CrystallinityApplication of tensile stress for crystallization of rubber
27 Form of PolymersThermoplastic Material: A material that is solid, that possesses significant elasticity at room temperature and turns into a viscous liquid-like material at some higher temperature. The process is reversiblePolymer Form as a function of temperatureGlassy: Solid-like form, rigid, and hardRubbery: Soft solid form, flexible, and elasticMelt: Liquid-like form, fluid, and elasticTempGlassyRubberyMeltPolymerFormIncreasing TempTmTg
28 Glass Transition Temperature, Tg Glass Transition Temperature, Tg: The temperature by which:Below the temperature the material is in an immobile (rigid) configurationAbove the temperature the material is in a mobile (flexible) configurationTransition is called “Glass Transition” because the properties below it are similar to ordinary glass.Transition range is not one temperature but a range over a relatively narrow range (10 degrees). Tg is not precisely measured, but is a very important characteristic.Tg applies to all polymers (amorphous, crystalline, rubbers, thermosets, fibers, etc.)
29 Glass Transition Temperature, Tg Glass Transition Temperature, Tg: Defined asthe temperature wherein a significant the loss of modulus (or stiffness) occursthe temperature at which significant loss of volume occursModulus(Pa)or(psi)Temperature-50C50C100C150C200C250CTgTemperature-50C50C100C150C200C250CAmorphousCrystallineTgVol.
30 Crystalline Polymers: Tm MeltTm: Melting TemperatureT > Tm, The order of the molecules is random (amorphous)T < Tm >Tg, Crystallization begins at various nuclei and the order of the molecules is a mixture of crystals and random polymers (amorphous). Crystallization continues as T drops until maximum crystallinity is achieved. The amorphous regions are rubbery and don’t contribute to the stiffness. The crystalline regions are unaffected by temperature and are glassy and rigid.T < Tg, The amorphous regions gain stiffness and become glassyTmTempRubberyDecreasing TempTgGlassyPolymer Form
32 Temperature Effects on Specific Volume T > Tm, The amorphous polymer’s volume decreases linearly with T.T < Tm >Tg, As crystals form the volume drops since the crystals are significantly denser than the amorphous material.T < Tg, the amorphous regions contracts linearly and causes a change in slope-50C50C100C150C200C250CAmorphousCrystallineTgSpecificVolumeTemperature
33 Thermal PropertiesTable 3.2 Thermal Properties of Selected Plastics