Polymer (2) (1)
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Title: Polymers
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Definition of Polymers
A polymer is a material made of large molecules with many atoms linked by covalent bonds, classifying them as macromolecules.
Typically consist of identical or similar units called monomers.
The arrangement of atoms in polymers influences their properties.
Polymers may have different dimensional structures: 1-D, 2-D, or 3-D, affecting their properties.
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Examples of Polymers
Long chain molecular structures can be illustrated as follows:
Poly(ethylene): CH2-CH2-CH2...
Poly(styrene): CH2-CH-CH2...
Poly(vinyl chloride): CH2-CH-CH2... Cl Cl
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Importance of Polymers
Naturally occurring polymers:
Proteins
Cellulose (in plants)
Starch (in food)
Natural rubber
Engineering polymers are usually synthetic, representing a fast-growing materials industry.
Synthetic polymers boast properties such as manufacturability, recyclability, mechanical strength, and cost-effectiveness compared to metals and ceramics.
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Degree of Polymerization (DP)
Degree of polymerization refers to the number of monomer units in a polymer chain.
For synthetic macromolecules, typically: N = 10^2 to 10^4
For natural macromolecules, like DNA: N = 10^9 to 10^{10}.
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Average Degree of Polymerization
The degree of polymerization (DP) is the number of repeating units in the polymer chain.
The molecular weight of a polymer is calculated as:
Molecular Weight = (DP) x (molecular weight of repeating unit)
Polyethylene example:
DP = 1000 -> Molecular Weight = 28,000.
Polymer samples consist of mixtures with varying DP, leading to average molecular weights.
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Physical Properties Governing Polymers
Monomer units are linked, limiting independent motion.
This characteristic results in a low number of degrees of freedom compared to gases or liquids.
Polymers exhibit low entropy and are generally flexible due to their elongation.
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Factors Influencing Polymer Molecular Structure
Chemistry (Monomer composition)
Size (Molecular Weight)
Shape (chain twisting, entanglement)
Structure types:
Linear, branched, cross-linked, ring networks (isomers include stereo and geometrical)
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Types of Polymer Structures
**Homopolymers: ** consist of identical monomer units.
Copolymers: contain two or more different types of monomer units, e.g., proteins (20 types), DNA (4 types).
The arrangement is referred to as primary structure.
Bifunctional (2-D): Ethylene chain structure.
Trifunctional (3-D): Network structures, e.g., Bakelite.
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Types of Copolymers
Mixtures of two reactive monomers can polymerize to form copolymers categorized as:
Random copolymer: Mer units enter the chain randomly.
Alternating copolymer: Mer units alternate in sequence.
Block copolymer: One homopolymer's chains are linked to another's chains.
Graft copolymer: Combinations of different structures within one chain.
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Illustrative Examples of Copolymers
Random Copolymer: A-B-A-A-B-B-B
Alternating Copolymer: A-B-A-B-A-B
Block Copolymer: A-A-A-A-A-B-B-B-B
Graft Polymer: A-A-A-A-A | B | B | B
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Branched Macromolecule Structures
Types include:
Comb-like
Randomly branched
Star-like
Polymer networks
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Ring Macromolecules
Types:
Unknotted ring
Knotted ring
Tangled pairing of rings
Olympic gel forms
Double helix (pairing of strands)
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Classification of Polymers
Traditional classification methods (gases, liquids, crystals) do not capture polymer characteristics accurately.
Distinctions based on crystallinity vs. partial crystallinity are more applicable.
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Physical States of Polymers
Partially Crystalline State
Viscoelastic State (Polymer Melt)
Highly Elastic State (Rubbers)
Glassy State (e.g., styrene, methyl methacrylate, vinyl chloride).
Polymers lack a vapor phase; they decompose instead when heated.
Solid polymers display dual-phase characteristics: crystalline and amorphous phases.
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Crystalline vs. Amorphous Polymers
Crystalline Polymers:
Rigid, high melting points, less solvent penetration.
High crystallinity (95-99%) results in brittleness.
Amorphous Polymers:
Inability to pack regularly due to branches; they are non-crystalline.
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Molecular Weight Considerations for Polymers
Actual samples exhibit a distribution of molecular weights rather than uniform molecular weight.
Characterized by a bell curve of molecular weights, necessitating discussion of average molecular weights.
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Types of Average Molecular Weights
Number Average Molecular Weight (Mn): Total weight divided by total number of molecules.
Weight Average Molecular Weight (Mw): Heavier molecules contribute more due to their mass.
Viscosity Average Molecular Weight (Mv): Based on solution viscosity, correlates more with Mw.
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Number Average Molecular Weight Calculation
Formula for calculating the number average is:
Number-average molecular weight = Total weight divided by total molecules (Ni associated with Mi).
Relevant for thermodynamic measurements such as osmotic pressure and freezing point depression.
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Weight Average Molecular Weight Calculation
Weight-average focuses on the mass distribution of polymer molecules.
Techniques like light scattering and ultracentrifugation yield this measurement.
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Measure of Polydiversity
Weight-average molecular weight (Mw) is always equal to or greater than the number-average (Mn).
Polydispersity indicates width in the range of molecular weights; a ratio around 1.0 indicates narrow distribution, while a high ratio indicates wide distribution.
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Important Note on Molecular Weight Plots
Molecular weight typically increases from right to left on plots, not from left to right.
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Threshold Molecular Weight
Refers to a minimum molecular weight at which an oligomer exhibits necessary properties.
The threshold depends on the application, e.g., adhesives may require lower weights than materials for trash barrels.
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Commercial Polymer Range
Beyond a critical degree of polymerization, molecular weight enhances polymer strength but eventually reaches a limit for cost-effective processing.
Practical molecular weight typically lies between degrees of polymerization 200 to 2000, corresponding to molecular weights from 20,000 to 200,000.
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One-Dimensional Polymers
Most common structure, formed when two reacting chains combine.
Regularly packed chains lead to crystalline polymers, while irregularly tangled chains result in amorphous polymers.
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Two-Dimensional Polymers
Rare but exemplified by the planar structure of graphite, which provides significant lubricating capabilities.
Its unique structure allows for low shear strength.
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Three-Dimensional Polymers
Illustrated by crystalline diamond with tetrahedral carbon bonds forming a lattice.
Exhibits mechanical behaviors similar to ceramics, showcasing high melting point, hardness, and strength.
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Classification by Thermal Response
Thermoplastic Polymers
Linear, one-dimensional structures with strong intramolecular bonds and weak intermolecular bonds. Easily melted and remolded at elevated temperatures.
Thermosetting Polymers
Highly crosslinked, three-dimensional polymers that become hard and infusible post polymerization.
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Rubber and Elastomers
Rubber can stretch significantly and return to its original shape due to its high polymer structure.
Rubber-like elasticity arises from molecular coiling/uncoiling. It must remain amorphous for optimal performance.
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Polymer Synthesis Mechanisms
Addition Polymerization: Chains grow through sequential addition to a reactive site.
Linear chain growth; high molecular weight reached early.
Step-Growth Polymerization: Monomers combine to form larger oligomers, leading to polymers after several reactions.
Exponential growth; higher molecular weight achieved later in the process.
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Addition Polymerization Diagram
[Illustrative diagram of initiation, propagation, and termination stages of addition polymerization]
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Types of Addition Polymerization
Anionic
Radical
Cationic
Ziegler-Natta
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Step-Growth Polymerization Stages
Consumption of monomer.
Combination of small fragments.
Formation of high molecular weight polymer.
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Characteristics of Step-Growth Polymerization
High molecular weight forms near the end of the reaction; efficiency dependent on high conversion of monomers.
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Step-Growth Polymerization Overview
Involves the combination of bifunctional monomers to produce branched or network polymers.
Requires multifunctional monomers to create network structures.
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Chain Growth and Addition Polymerization
Involves active chain sites reacting with unsaturated monomers for continuous growth.
First synthesized through vinyl polymer preparations.
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Chain Growth Techniques Overview
Free Radical Polymerization
Anionic Polymerization
Cationic Polymerization
Techniques focus on different active sites changing the outcome of synthesis.
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Glass Transition Temperature
Amorphous polymers exhibit two types of mechanical behavior: rigid or rubbery, depending on temperature.
Glass transition temperature (Tg) is specific to each polymer.
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Molecular Motions in Amorphous Polymers
Key motions contributing to Tg include:
Entire molecule translation (flow)
Segment wriggling (flexibility)
Atomic motion (vibration).
Tg is where these movements transition states.
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Factors Influencing Tg
Free volume of the polymer.
Attractive forces among molecules.
Internal chain mobility.
Chain stiffness.
Chain length dependency on Tg.
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Description of Crystallinity
Refers to the arrangement of polymer molecules in repeating patterns.
Degree of crystallinity affects material density due to closer packing in the crystalline state.
Influenced by cooling rates during solidification and chain configurations.
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Characteristics of Degree of Crystallinity Table
Lists polymer types comparing their densities in crystalline versus amorphous states.
Crystallinity significantly affects mechanical attributes like hardness or stiffness.
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Electrical Properties of Polymers
Most polymers act as electrical insulators in pure form.
Conductivity can be engineered using doping for achieving ionic or electronic conducting properties.
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Understanding Conductivity Basics
Defined using Ohm's Law: V=IR.
Conductivity relies on charge carrier quantity and mobility.
Conductors require free-moving electrons, unlike insulators with tightly bound electrons.
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Doping as a Method to Create Conductivity
Doping involves inserting or removing electrons to induce conductivity.
Types include:
Oxidation with halogens (p-doping)
Reduction with alkali metals (n-doping).
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Conditions to Achieve Conductivity
Requires conjugated double bonds in the polymer structure.
Must allow free electron movement to achieve conductive properties.
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Factors Affecting Conductivity
Density of charge carriers.
Mobility of carriers.
Direction of electron flow.
Presence of dopants.
Temperature influence.
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Polymer Conducting Applications
Notable applications include:
Anti-static substances.
Corrosion inhibitors.
Electronic components like transistors, LEDs, and displays.
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Conclusion on Conductivity
Conductance requires free electrons; conjugated polymers are semiconductors.
Conductivity trends inversely with temperature for conducting polymers, unlike metals.
Innovation is ongoing in developing conductive plastics.
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Classification of Polymers
Key classifications based on chemical composition, structure, arrangement, thermal response, mode of formation, application, tacticity, and crystallinity.
Differences between addition and condensation polymerization highlight their distinct formation mechanisms.
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Example Calculations for Average Molecular Weights
Given a polyethene sample:
Calculate both number and weight-average molecular weights based on provided chain distributions.