III. Molecular Structure Chapter 3. 3. Molecular Size Size & Shape
Molecular Structure (1)Molecular Size & Shape Size : molecular weight molecular weight distribution Shape : branching (2) Molecular Flexibility freedom of rotation
3. Molecular Size and Shape Introduction temperature motions of molecules what happens at absolute zero? if there is no motion of the molecules, they are all? States of matters gas : low MW - molecular interaction liquid : MW - molecular interaction solid : higher MW polymer : extremely high MW mechanical strength structural materials entanglement attraction (intermolecular)
MW processability, mechanical, thermal, chemical, electrical, optical,,, some properties change linearly up to infinity as MW changes (Fig. 3-1 (a)) P = km ( when a = 1, in P = km a ) if they depend on molecular disentanglement: melt processability, solvent resistance most properties behaves similar to (b) in fig 3-1 MW property then, level off when property depends on end groups, or intermolecular interaction P = a + b/m (a, b constant, M : molecular weight (what kind?))
Most polymers do not have uniform MW because of the polymerization reactions addition polymerization : chain transfer, termination occur at any time (so, if there are no chain transfer, & termination & fast initiation MW can be uniform called living polymerization ) condensation polymerization : stepwise increase of MW occurs at any molecular size (ex, dimer + monomer, trimer + pentamer ) broad spectrum ; Molecular Weight Distribution (MWD) so, there are several definitions of MW Types of molecular weights in polymers Number average MW (Mn) : useful for properties depend on smaller molecules Weight average MW (Mw) : useful for propetiesdepend on larger molecules - when property depends on entanglement/disentanglement Viscosity average MW (Mv)* Z average MW (Mz) : some properties correlate better with these higher order average MW general equation
Therefore, it is important to determine which type of molecular weight is most appropriate and will correlate better - see Fig 3-3
3-1. Pocessability general rule w/ many exceptions - easy processability with low MW - better final properties with high MW (1) Thermoplastic : processing at constant MW - compromise between processability and property (2) Thermosetting : - low MW (processability) -process >high MW (property)
(1) Thermopastic processing at constant MW melt processing : common plastic processing : cheap compared w/ solution p low MW - low melt viscosity - rapid economical processing general relationship melt viscosity, η = KM w q (note M =M w ) at low molecular weight : q =1 - similar to dilute solution viscosity with a good solvent at higher MW : q = 3.4 - when it passes a critical molecular weigh, Mc, beyond which entanglement of molecules important Log η q=3.4 q=1 Mc Log Mw
Complex relationships becomes more complex under certain cases, such as, at high shearrate, or at broad MWD - for ex, long chain PE (low density PE) pseudoplastic melt cf. Newtonian flow
Process requiring high MW - when it requires melt strength extrusion generally requires higher MW (higher melt strength) than injection molding Plasticizer effect lowering MW : increase low MW fraction broadening MWD lower melt viscosity, processing temp, increase tackiness and wetting or increase lubricity w/ semicompatable plasticizer materials -waxes,, In case of crystalline polymers I) low MW fraction increase No. of end groups do not fit into crystalline lattice lower perfection of crystallinity melting point - same effect when plasticizers are added melting point (for that purpose, comonomer is more effective than plasticizer) II) On the other hand, low MW fraction lower melt viscosity crystallize more easily so, plasticizer speed crystallization higher crystallinity antiplasticizer
(2) Conversion to high MW during processing (thermosetting) 1) 100 % conversion (W/o solvent) thermosetting resins : unsaturated esters, urea, melamine, phenolic resins, epoxy, polyurethane, silicone,, polymerized to low MW (A or B-stage) : easy handling then, process, (polymerize, cure) monomer casting : MMA, nylon6, good for delicate specimens 2) solution process only when the thickness of the final product is low enough for solvent to evaporate evaporation in dilute solutions polymer molecules is a swollen sphere travelling independently in concentrated solution, these spheres entangle much higher viscosity choice of solvent is critical if evaporation is too fast, not enough intercoil (entanglement) weak, grainy product if solvent mixtures (good and poor solvents) are used evaporation of the good solvent before poor one gel, drying orientation strength if good one evaporates too fast precipitation grainy, weak product
3-2. Mechanical properties mechanical properties come from the forces which holds the molecules or atoms together - intramolecular forces : bondings - intermolecular forces : secondary forces think about gas, liquid, solid -Fig 3-7 gas : intermolecular forces are already overcame by thermal energy liquid : intermolecular forces are stronger, so, low Mw solids : fixed position, but, when the stress applied, the molecules slide over each other and separate from each other. High Mw solids : i) molecules entangled ii) intermolecular forces > bonding so, it may easier to break primary covalent bonding than to separate one molecules from another beyond this, the mechanical prop. level off.
classes (1) reversible deformation - modulus : (rigidity/flexibility) (2) permanent deformation - ultimate failure i) under low rate of test : conventional ultimate tests ii) under high rate : impact iii) under multiple cycling stress (3) complex (1) reversible rigidity/flexibility (modulus) modulus : slope of stress/strain curve (at the origin or the seant to any standard elongation) theoretically, modulus is related to segment of polymer molecules so, not a function of MW, little effects - Fig3-8
1) amorphous glassy polymer (under Tg) : - no effect of MW on modulus (stensile modulus ~ 500,000psi) 2) flexible polymers : Tg near (leathery) or below room temp.(rubbery) segment motions are affected by the temp. and MW there is a effect - Fig 3-10 i) low speed of testing permit molecules to untangle and separate a significant effect of MW on modulus low MW : chain ends, mobility, free volume greater mobility of other segments of the polymer chain high MW : entanglement reducing the mobility ii) high speed of testing : little effects on MW (broader plateau of rubbery properties) high rate of test low rate of test MW
therefore, the main effects of MW on modulus long term permanent deformation (little effect on modulus itself) Plasticizer effects : in general, lower hardness, stiffness, modulus high mobility of plasticizer free volume free volume of neighboring polymers mobility of segments lower modulus other consideration : plasticizers reduce intermolecular forces plasticizer : more effective in lowering Tg than comonomer (Fig. 3-16) (compare with Fig. 3-5)
(2) Mechanical failure (permanent deformation) excessive mechanical stress deform permanently (break) irreversible at low MW : disentangle, slipping separate at higher MW (above critical MW of entanglement) : rupture of primary covalent bonding 1) permanent deformation measured as yield strength, stress relaxation chain disentangle slipping of chains permanent deformation : should be rate dependent creep ~ 1/Mw (weight average) - beyond Mc : intermolecular entanglement act as X-link preventing creep glassy state : permanent deformation is negligible flexible and rubbery : Mw rate of chain disentangle so, rate of permanent deformation
2) ultimate strength (stress) and elongation (strain) measured at time of break tensile, flexural, compressive, torsional, shear : the same fundamental mechanism ultimate tensile strength (UTS) : depend on MW, (Fig 3-18, 19) ultimate elongation : more complex, but similar, (Fig 3-20)
Molecular Weight Distribution ultimate tensile strength : a -b/ M (could be either Mn, or Mw) plasticization : plasticizer, ultimate tensile strength, ultimate elongation table 3-2, fig 3-22 3) Impact strength breakage under high speed impact : I.S. as MW
plasticizer : I.S. (Fig 3-25) water absorption : act as plasticizer (fig 3-26) 4) multiple cycling stress fatigue resistance : directly correlate with MW
(3) Complex Mechanical Properties stress cracking resistance : in chemical reagent under stress correlate with Mn (3-32) because extraction or leaching of low MW fractions adhesion : low MW are favored : better wetting friction : high MW ( UHDPE, ski bottoms )
3-3. Thermal properties thermal properties (1) thermal mechanical properties (2) thermodynamic properties (3) thermal stability (1) Thermal Mechanical Properties effects of MW on thermal mechanical prop. is best understood by examining the relationships between modulus and temperature (fig.3-33) five states: Glassy : rigid Glass transition (leathery transition) : stiffly flexible, leathery Rubbery pleateau : yield easily, recover readily Rubbery flow : disentangle, elastic under low stress, flow under high stress liquid flow : flow easily, permanently
(1) Rigidity/Flexibility and Tg MW : not much effects on modulus in glassy state, but MW & Tg : distinctive effects (Fig. 3-34) at low MW (below 20,000) at high MW Mechanism this effect is due to : -mobility of segments ( > 5 atoms) within the polymer molecules is limited by their attachments at both ends the chain ends : more flexible, mobile, larger free volume in turn, this free volume provides free volume to the all the segments of the polymer molecules greater flexibility as a whole since chain ends are the reasons, function of M_ Tg = (fig. 3-35)
Practical effects impact strength : often function of Tg (Mn)
but, may deviate at low MW (fig 3-36) Plasticizer effects on Tg : very effective to reduce Tg -improve low temperature impact strength the same free volume theory can be applied : even more efficient general effects of plasticizer
plasticizer : 1) lower Tg, 2) broaden the transition directly proportional to volume fraction (Fig. 3-39) Tg = on the other hand, plasticizer lower heat distortion temp & increase creep Residual monomers and solvents: similar effects as plasticizers (2) Leathery region as temp increases size of the mobile segments frequency polymers are stiffly flexible or leathery plasticizer : broaden the transition
(3) Rubbery Plateau as temp increase large mobile segment soft, fast, rubbery (elastic) response MW : no. of entanglements rubbery plateau broadens Fig. (4) Melting Point Transition MW : m.p.? Due to polymer chain ends M_ eq.
Tg vs Tm? (fig. 3-47) Tg = 1/2 Tm (when ) = 2/3 Tm (when )
(2) Thermodynamic Properties thermal conductivity polymers : generally poor thermal conductor (why?) good thermal insulator thermal energy (atomic vibrations) transmitted through covalent bonds so, ~ Mw 1/2 (up to a certain MW, 100,000), above it : constant Coefficient of thermal expansion (CTE) polymers : generally much higher thermal expansion (why?) end groups are contributing to the high CTE (a part of reason) so, CTE = A + k / Mn
(3) Thermal stability MW thermal stability why? 1. End groups : more reactive than the polymer chains inside double bonds in vinyl polymers polar groups in condensation polymers 2. Mobility 3. Asymptoticity (I hope you remember the figure) Brittle point (~Tg), then why? Side chain (packing) crystallization free volume, restriction of rotation
Branched side chains : side chain branching bulkier steric hindrance flexibility Tg, Tm ex. Poly(4-methylpentene-1) vs ( Fig 4-12 ) (Table 4-6) Tg poly-1-hexene -50 poly(4-methylpentene-1) 29 when side groups are too bulky unstable polymer conformation ceiling temp. (depolymerization)
Cyclic side groups : cyclics bulkier restrict rotation especially when they are close to the backbone styrene derivatives : Tg (flexibility) strongly depends on position o-substituted - steric hindrance p-substituted alkyl chain - tent-pole effect longer alkyl chain - side chain crystalization bulky halogen sub. - steric effect α-substituted - steric effect
(ii) Polarity - polar side groups : repel each other restrict rotation stiffening PTFE (-fluorine) - larger, highly polar repel stiff rod-like PVC (-chlorine) - even larger, quite polar fairly rigid PAN (-CN) - steric, dipole repulsion, stiffening rigid rod polyelectrolyte (polyacrylate ions, poly quartenary ammonium ions) strong electric repulsion rod-like extended conformation solution viscosity
Copolymerization - random copolymers : average, intermediate properties between individual homopolymers
- homogeneous copolymers : sharp transition - heterogeneous copolymers : broader transition Random copolymers 1/Tg = w1/tg1+ w2/ Tg2 Block and graft copolymers two individual Tgs
Plasticizer structure - Molecular flexibility is also important in the plasticizer structures as well - same flexibility rule applies - linear plasticizers are more efficient than cyclic plasticizer dioctyl adipate diphenyl octyl phosphate dioctyl phtalate butyl benzyl phthalate tricresyl phosphate