chem 7010 macromolecular synthesis 2 nd introduction to polymer synthesis: general aspects of...
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CHEM 7010Macromolecular Synthesis
2nd Introduction to Polymer Synthesis: General Aspects of Polymer Structures
onPolymer Properties
2011
Multidimensional Order of Structure-Property Relationships in Polymers
Molecular
*Chemical Composition
*MW/MWD
*Branching and/or
Cross-linking
Microscopic
*Crystallinity
*Phases *Orientation
Synthesis and Processing
Macroscopic
*Strength
*Modulus
*Impact Resistance
Permeability
Clarity
Microscopic
Macroscopic
Molecular
IMPACT OF MOLECULAR WEIGHT ON MATERIAL PROPERTIES
Increasing Degree of Polymerization, DP
Oligomers
Tough
Wax
Brittle Wax
meltingpoint
density
crystallinity
Tensile
Strength
Elongation
Properties
Low Density PE
High Density PE
Molecular Weight
Vinyl Monomers, CH2=CH-X
• X Polymer Abbreviation• H Polyethylene PE • CH3 Polypropylene PP • Cl Poly(vinyl chloride) PVC • Phenyl Polystyrene PSt • CN Polyacrylonitrile PAN • COOCH3 Poly(methyl acrylate) PMA• O-COCH3 Poly(vinyl acetate) PVAc
Vinylidene Monomers, CH2=C(X)Y
• X Y Polymer Abbreviation• CH3 CH3 Polyisobutylene PIB
• Cl Cl Poly(vinylidene chloride) PVDC• F F Poly(vinylidene fluoride) PVDF
• Phenyl CH3 Poly(-methyl styrene)
• CH3 COOCH3 Poly(methyl methacrylate) PMMA
• CN COOR Poly(alkyl -cyanoacrylate
Structural Complexity of Polymers
• Homopolymers• Head to Tail vs. Head to Head Adducts e.g. a-olefins
• 1,2- vs 1,4 Adducts; e.g. butadiene
• Tacticity of Enchainments
• Branching
Tacticity• Isotactic
• All asymmetric carbons have same configuration• Methylene hydrogens are meso• Polymer forms helix to minimize substituent interaction•
Syndiotactic
• Asymmetric carbons have alternate configuration• Methylene hydrogens are racemic• Polymer stays in planar zig-zag conformation• Heterotactic (Atactic)• Asymmetric carbons have statistical variation of configuration
X X X X XH
H H
X
X XH X X XX
Structural Complexity of Polymers
• Copolymers• Identity and Number of Comonomers
• Ratio and Distribution of Comonomers
• Statistical Alternating
Gradient
• Blocks Grafts
Structural Complexity of Polymers
• Molecular Weight – Molecular Weight Distribution, MWD
– Polydispersity Index, PDI
– Mn, Mw, Mz, Mv Averages
• Crosslinking Density
– Length of Crosslinks
Structural Complexity of Polymers
Time Dependent Changes
• Chemical Reactions• Hydrolysis• Dehydrohalogenation• Photodegradation• Oxidation
Structural Complexity of Polymers• Thermal Degradation• Processing• Aging• Crystallization • Changes in Polymorphism
• Weathering-- Combination of Above
• Plasticizer Loss -- Imbrittlement
Microscopic Properties (Intermolecular Interactions)
• Morphology
• Chain entanglement –amorphous
• Chain ordering--liquid crystalline
• Crystallinity
• Phase separations (microdomains)
Types of Intermolecular Forces • Type of Force Relative Strength Low Molecular Analog Polymer
• Dispersion or Weak Methane Polyethylene • Van der Waals Hexane Polypropylene
• Dipole-Dipole Medium CH3Cl PVCCH3CO2CH3
PMMA
Hydrogen bonding Strong H2O Cellulose
CH3CONH2 Proteins Electrostatic Very Strong CH3CO2
-Na+ Ionomers
GLASS TRANSITION, Tg
• Definition: The onset of seqmental motion of seqments with 40-50 carbons atoms
• Physical Change Expansion of volume • Free volume required to allow segmental motion• Tg is an approximation• Depends upon measurement technique• Depends upon molecular weight• Polystyrene MW = 4000 Tg = 40C• = 300,000 = 100
GLASS TRANSITION, Tg
• Properties Affected
• Specific Volume / Density
• Specific Heat, Cp
• Refractive Index
• Modulus
• Dielectric Constant
• Permeability
FACTORS INFLUENCING Tg
• Tg is proportional to Rotational Freedom
• For symmetrical polymers Tg, / Tm in K 1/2
• unsymmetical polymers 2/3
• 1. Chain flexibility
• Silicone Ether Hydrocarbon Cyclic HC Aromatics
FACTORS INFLUENCING Tg
– Tg = -55C 88C
•
2. Steric Bulk of Substituents
Tg = -120C 5C -24C -50C
•Long side chains may act as plasticizers (C 6)
O O
FACTORS INFLUENCING Tg
• 3. Molecular Symmetry
• Asymmetry increases chain stiffness.
• 4. Polar Interactions increase Tg
• Hydrogen bonding
• 5. Molecular Weight up to Critical Limit
• 6. Crosslinking
• Reduces Segment Mobility
FACTORS INFLUENCING Tm
• 1. Chain flexibility
• Silicone Ether Hydrocarbon Cyclic HC Aromatics
• 2. Substituents Producing Lateral Dipoles
• Hydrogen bonding
• 3. Molecular Symmetry
• Symmetry allows close packing
FACTORS INFLUENCING Tm
• 4. No Bulky Substituents to Disrupt Lattice if placement is Random
• 5. Structural Regularity• monomer placement• head to tail • 1,2- vs 1,4- • 1,2- vs 1,3- vs 1,4- aromatic substitution• geometric isomers of enchainments• cis or trans -C=C-; cyclic ring• tacticity
FACTORS REQUIRED TO PROMOTE CRYSTALLIZATION
• Thermodynamic
• 1. Symmetrical chains which allow regular close packing in crystallite
• 2. Functional groups which encourage strong intermolecular attraction to stabilize ordered alignment.
FACTORS REQUIRED TO PROMOTE CRYSTALLIZATION
• Kinetic• 1. Sufficient mobility to allow chain
disentanglement and ultimate alignment• Optimum range for mobility• Tm -10 Tg + 30• at Tm segmental motion too high• at Tg viscosity too high• 2. Concentration of nuclei• concentration of nucleating agents• thermal history of sample
Macroscopic Properties (Physical Behavior)
• Tensile and/or Compressive Strength• Elasticity • Toughness• Thermal Stability• Flammability and Flame Resistance• Degradability• Solvent Resistance• Permeability• Ductility (Melt Flow)
Step Polymerization
Two Routes: A-A + B-B; or A-B
1 on 39
2 on 39
1 on 40
2 on 40
Either way, need:• high conversion• stoichiometric amount
Step vs. Chain Polymerization
Step Polymerization
• Any two molecular species can react. • Monomer disappears early. • Polymer MW rises throughout. • Growth of chains is usually slow (minutes to days). • Long reaction times increase MW, but yield hardly changes. • All molecular species are present throughout. •Usually (but not always) polymer repeat unit has fewer atoms than had the monomer.
Step vs. Chain, cont.Chain Polymerization
• Growth occurs only by addition of monomer to active chain end. • Monomer is present throughout, but its concentration decreases. • High polymer forms immediately. • MW and yield depend on mechanism details. •Chain growth is usually very rapid (seconds to microseconds). • Only monomer and polymer are present during reaction. • Usually (but not always) polymer repeat unit has the same atoms as had the monomer.
Step Polymerization
Concepts
Comparison of Step and Chain Polymerization
1. Step: any 2 molecules in the system can react with each other Chain: chain growth occurs on end of growing polymer
2. Step: loss of monomer at early stage (dimers, tetramers, etc.) Chain: monomer concentration decreases steadily
3. Step: broad molecular weight distribution in late stages Chain: narrower distribution; just polymer and monomer
Step Polymerization
Basis for Kinetics (2-1b): NOTE: Assume equal reactivity of functional groups
table 2.1on 42
Thus, assumption looks ok
Step Polymerization
Kinetics (2.2)
Example: diacid + diol
HO
O O
OH HO OH+
-n H2O
HO
O O
O OH
Mechanism (acid catalyzed):
2 on 44
1 on 45
2 on 45
Chain Polymerization
Concepts
General Mechanism
Initiation
I R* * could be radical, cation, or anion
Propagation
Chain PolymerizationConcepts: Nature of Chain Polymerization; 3-1
3-1a. Comparison of Chain and Step Polymerizations
Chain: a. Rapid high mw b. Snapshot: monomer, high poly, growing chains c. MW does not change with time
Step: a. Monomer gone fast, get dimer, trimer, etc. b. MW increases with time c. No high mw until end
0
2
4
6
8
10
12
0 2 4 6 8 10
chain
step
Chain Polymerization
Concepts: Nature of Chain Polymerization; 3-1
3-1b-1. General Considerations of Polymerizability
Thermodynamics
(Chain t-fer)
Chain Polymerization
Concepts: Nature of Chain Polymerization; 3-1
3-1b-2. Effects of Substituents on C=C monomers for Ionic Polymerization
A. Initiation opportunities: 2 types of bond cleavage/resonance
B. Radical, Anionic, Cationic: Depends on: i. Inductance
ii. Resonance
Chain Polymerization
Concepts: Nature of Chain Polymerization; 3-1
3-1b-2. Effects of Substituents on C=C monomers for Ionic Polymerization
i. Inductanceii. Resonancee.g. Polym of vinyl ether
Example of resonance stabilization of cation byDelocalization of of positive charge.
If oxygen not there, no stabilization
C. Donating GroupsGood for cationic
Chain Polymerization
Concepts: Nature of Chain Polymerization; 3-1
3-1b-2. Effects of Substituents on C=C monomers for Ionic Polymerization
ii. Resonance
e.g. Polym of Styrene
C. Donating Groups
Chain Polymerization
3-1b-2. Effects of Substituents on C=C monomers for Ionic Polymerization
D. Withdrawing Groups
Concepts: Nature of Chain Polymerization; 3-1
i. inductance
ii. resonance
Good for anionic
Chain Polymerization
3-1b-2. Effects of Substituents on C=C monomers for radical polymerization
Concepts: Nature of Chain Polymerization; 3-1
A. Radicals not affected by charge, but can have resonance stabilization
B. Also stabilized by primary<secondary<tertiary
Draw your own pic
Chain Polymerization
Concepts: Structural Arrangement of Monomer Units; 3-2
3-1a. Possible Modes of Propagation
A. Two possible points of attachmenti. On carbon 1:
ii. On carbon 2:
Chain Polymerization
Concepts: Structural Arrangement of Monomer Units; 3-2
3-1a. Possible Modes of Propagation
B. If attachment is regular, get head-to-tail (H-T)
C. If attachment is irregular, get head-to-head (H-H) [and T-T]