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Micro Structures in Polymers Chapter 3

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1 Micro Structures in Polymers Chapter 3
Professor Joe Greene CSU, CHICO MFGT 041 September 20, 1999

2 Chapter 3 Objectives Objectives
Polymer length, molecular weight, molecular weight distribution (MWD) Physical and mechanical property implications of molecular weight and MWD Melt Index Amorphous and crystalline structures in polymers Thermal transitions in plastics (thermoplastics and thermosets Steric (shape) effects

3 Polymer Length Polymer Length Molecular Weight
Polymer notation represents the repeating group Example, -[A]-n where A is the repeating monomer and n represents the number of repeating units. Molecular Weight Way to measure the average chain length of the polymer Defined 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/mole Methane CH4 is 1 C (12g) and 4 H (1g)= 1*(12) + 4 *(1) = 16g/mole Polyethylene -(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 weight The molecular weights are counted in values or groups that have similar lengths, e.g., between 100,000 and 110,000 For 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 molecules Figure 3.1 A 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 weight Number average molecular weight Weight average molecular weight Z-average molecular weight

8 Gel Permeation Chromatography
GPC Used to measure Molecular Weights form of size-exclusion chromatography smallest molecules pass through bead pores, resulting in a relatively long flow path largest molecules flow around beads, resulting in a relatively short flow path chromatogram obtained shows intensity vs. elution volume correct pore sizes and solvent critical

9 Gel Permeation Chromatography

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 Frequency 50,000 1 100,000 4 200,000 5 500,000 3 700,000 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 MW The 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/mole The number of repeating units, n, is often called the degree of polymerization, DP. DP relates the amount of monomer that has been converted to polymer.

12 Weight Average Molecular Weight, Mw
Favors large molecules versus small ones Useful 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 Mw Useful 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 lengths The frequency can be Narrow or Broad, Fig 3.3 Narrow distribution represents polymers of about the same length. Broad distribution represents polymers with varying lengths MW distribution is controlled by the conditions during polymerization MW distributions can be symmetrical or skewed.

15 Physical and Mechanical Property Implications of MW and MWD
Higher MW increases Tensile 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 strength Resistance to an applied load pulling in opposite directions Tension 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 strength Broad MW distribution represents polymer with many shorter molecules which are not as entangled and slide easily. Higher MW increases impact strength Impact toughness or impact strength are increased with longer polymer chains because the energy is transmitted down chain. Broader MW Distribution decreases impact strength Shorter chains do not transmit as much energy during impact

17 Thermal Property Implications of MW & MWD
Higher MW increases Melting Point Melting 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 Point Broad MW distribution represents polymer with many shorter molecules which are not as entangled and melt sooner. Broad MW distribution yields an easier processed polymer Mechanical Properties Melting Point * Decomposition MW MW

18 Example of High Molecular Weight
Ultra High Molecular Weight Polyethylene (UHWMPE) Modifying the MWD of Polyethylene yields a polymer with Extremely long polymer chains with narrow distribution Excellent strength Excellent 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 polyethylene Acts as a low-melting lubricant Provides bimodal distributions, Figure 3.5 Provides 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 rod Add test weight or mass to end of rod (5kg) Wait for plastic extrudate to flow at constant rate Start stop watch (10 minute duration) Record amount of resin flowing on pan during time limit Repeat as necessary at different temperatures and weights

20 Melt Index and Viscosity
Melt index is similar to viscosity Viscosity is a measure of the materials resistance to flow. Viscosity is measured at several temperatures and shear rates Melt index is measured at one temperature and one weight. High melt index = high flow = low viscosity Low melt index = slow flow = high viscosity Example, (flow in 10 minutes) Polymer Temp Mass HDPE 190C kg Nylon 235C kg PS 200C Kg

21 Melt Index and Molecular Weight
Melt index is related closely with average molecular weight High melt index = high flow = small chain lengths = low Mn Low melt index = slow flow = long chain lengths = high Mn Table 3.1 Melt Index and Average Molecular Weight Mn 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

23 Amorphous Materials PVC Amorphous PS Amorphous Acrylics Amorphous
ABS Amorphous Polycarbonate Amorphous Phenoxy Amorphous PPO Amorphous SAN Amorphous Polyacrylates Amorphous

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 polymers Most crystalline polymers are semi-crystalline because regions are crystalline and regions are amorphous Molecular arrangement is arranged in a ordered state

25 Crystalline Materials
LDPE Crystalline HDPE Crystalline PP Crystalline PET Crystalline PBT Crystalline Polyamides Crystalline PMO Crystalline PEEK Crystalline PPS Crystalline PTFE Crystalline LCP (Kevlar) Crystalline

26 Factors Affecting Crystallinity
Cooling Rate from mold temperatures Barrel temperatures Injection Pressures Drawing rate and fiber spinning: Manufacturing of thermoplastic fibers causes Crystallinity Application of tensile stress for crystallization of rubber

27 Form of Polymers Thermoplastic 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 reversible Polymer Form as a function of temperature Glassy: Solid-like form, rigid, and hard Rubbery: Soft solid form, flexible, and elastic Melt: Liquid-like form, fluid, and elastic Temp Glassy Rubbery Melt Polymer Form Increasing Temp Tm Tg

28 Glass Transition Temperature, Tg
Glass Transition Temperature, Tg: The temperature by which: Below the temperature the material is in an immobile (rigid) configuration Above the temperature the material is in a mobile (flexible) configuration Transition 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 as the temperature wherein a significant the loss of modulus (or stiffness) occurs the temperature at which significant loss of volume occurs Modulus (Pa) or (psi) Temperature -50C 50C 100C 150C 200C 250C Tg Temperature -50C 50C 100C 150C 200C 250C Amorphous Crystalline Tg Vol.

30 Crystalline Polymers: Tm
Melt Tm: Melting Temperature T > 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 glassy Tm Temp Rubbery Decreasing Temp Tg Glassy Polymer Form

31 Crystalline Polymers Tg
Tg: Affected by Crystallinity level High Crystallinity Level = high Tg Low Crystallinity Level = low Tg Modulus (Pa) or (psi) High Crystallinity Medium Crystallinity Low Crystallinity Tg Temperature -50C 50C 100C 150C 200C 250C

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 -50C 50C 100C 150C 200C 250C Amorphous Crystalline Tg Specific Volume Temperature

33 Thermal Properties Table 3.2 Thermal Properties of Selected Plastics

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