CHEM 7010 Macromolecular Synthesis 2 nd Introduction to Polymer Synthesis: General Aspects of Polymer Structures on Polymer Properties 2011

Multidimensional Order of Structure-Property Relationships in Polymers Molecular Microscopic Microscopic *Crystallinity *Phases *Orientation Synthesis and Processing Molecular *Chemical Composition *MW/MWD *Branching and/or Cross-linking Macroscopic Macroscopi c *Strength *Modulus *Impact Resistance Permeability Clarity

IMPACT OF MOLECULAR WEIGHT ON MATERIAL PROPERTIES Increasing Degree of Polymerization, DP High Density PE Properties

Brittle Wax melting point density crystallinity Tensile Strength Elongation Oligomers

   Molecular Weight

Tough Wax

Low Density PE 

Vinyl Monomers, CH

2

=CH-

X

• X • H • CH3 • Cl • Phenyl • CN • COOCH3 • O-COCH3

Polymer Polyethylene Abbreviation PE Polypropylene PP Poly(vinyl chloride) PVC Polystyrene PSt Polyacrylonitrile PAN Poly(methyl acrylate) PMA Poly(vinyl acetate) PVAc

Vinylidene Monomers, CH 2 =C( X ) Y

• X

Y Polymer Abbreviation • CH 3 • Cl CH Cl 3 Polyisobutylene Poly(vinylidene chloride) PIB PVDC • F F • Phenyl • CH 3 Poly(vinylidene fluoride) CH 3 Poly(  -methyl styrene) COOCH 3 Poly(methyl methacrylate) PVDF 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

H H

Tacticity

• Isotactic • • • • H X X X X All asymmetric carbons have same configuration X Methylene hydrogens are meso Polymer forms helix to minimize substituent interaction Syndiotactic X • • • • • H X X X X X X 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

• • •

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 • Van der Waals • Dipole-Dipole Weak Medium Methane Hexane Polyethylene Polypropylene CH 3 Cl CH 3 CO 2 CH 3 PVC PMMA Hydrogen bonding Strong

Electrostatic Very Strong

H 2 O CH 3 CONH 2

CH 3 CO 2 Na +

Cellulose Proteins

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 = 300,000 Tg = 40  C = 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  HC  Ether Aromatics  Hydrocarbon  Cyclic

FACTORS INFLUENCING Tg

2. Steric Bulk of Substituents

• Tg = -120  C 5  C -24  C -50  C •Long side chains may act as plasticizers (C  6)

O

– Tg = -55  C

O

88  C

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  HC  Ether Aromatics  Hydrocarbon  Cyclic

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 tacticity cis or trans -C=C-; cyclic ring

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 (Polycondensation)

After many repetitions:

Polyethyleneterephthalate (PET)

Step Polymerization Two Routes: A-A + B-B; or A-B 1 on 39 1 on 40 2 on 39 Either way, need: • high conversion • stoichiometric amount 2 on 40

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 (2-1a): 3 on 40 4 on 40 5 on 40 6 on 40

Basis for Kinetics: Step Polymerization 1on 41 2 on 41

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 H 2 O HO O Mechanism (acid catalyzed): 2 on 44 1 on 45 O O OH 2 on 45

Chain Polymerization Concepts General Mechanism Initiation I R * * could be radical, cation, or anion Propagation

Chain Polymerization Concepts: Nature of Chain Polymerization; 3-1 3-1a. Comparison of Chain and Step Polymerizations 12 10 8 6 4 chain 2 0 0 2 Chain: a. Rapid high mw b. Snapshot: monomer, high poly, growing chains c. MW does not change with time 4 step 6 Step: 8 10 a. Monomer gone fast, get dimer, trimer, etc.

b. MW increases with time c. No high mw until end

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 C. Donating Groups i.

Inductance ii. Resonance e.g. Polym of vinyl ether Good for cationic Example of resonance stabilization of cation by Delocalization of of positive charge. If oxygen not there, no stabilization

Chain Polymerization Concepts: Nature of Chain Polymerization; 3-1 3-1b-2. Effects of Substituents on C=C monomers for Ionic Polymerization C. Donating Groups ii. Resonance e.g. Polym of Styrene

Chain Polymerization Concepts: Nature of Chain Polymerization; 3-1 3-1b-2. Effects of Substituents on C=C monomers for Ionic Polymerization D. Withdrawing Groups i. inductance Good for anionic ii. resonance

Chain Polymerization Concepts: Nature of Chain Polymerization; 3-1 3-1b-2. Effects of Substituents on C=C monomers for radical polymerization A. Radicals not affected by charge, but can have resonance stabilization B. Also stabilized by primary

Chain Polymerization Concepts: Structural Arrangement of Monomer Units; 3-2 3-1a. Possible Modes of Propagation A. Two possible points of attachment i. 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]

Chain Polymerization Concepts: Structural Arrangement of Monomer Units; 3-2 3-1a. Possible Modes of Propagation Usually only 1-2% H-H. How do we know? A. Chemical Methods B. NMR Methods (we’ll check this out in the Spectroscopy section)