1 / 27

Polymer Mixtures (Blends/Alloys)

Polymer Mixtures (Blends/Alloys). Of great technological importance ~30% of all ‘polymers’ sold in mixed form Binary Mixtures: -- Immiscible: multiphase, no mixing between components -- Miscible: single phase, soluble -- ‘Partially miscible’: multiphase but some mixing

colm
Download Presentation

Polymer Mixtures (Blends/Alloys)

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Polymer Mixtures (Blends/Alloys) • Of great technological importance ~30% of all ‘polymers’ sold in mixed form • Binary Mixtures: -- Immiscible: multiphase, no mixing between components • -- Miscible: single phase, soluble • -- ‘Partially miscible’: multiphase but some mixing • -- ‘Incompatible’: from mechanical perspective; weak, brittle • -- ‘Compatible’: multiphase, but good to excellent mechanical properties

  2. What’s So Special About Polymer Blends? • Multiphase • Economics • Enhanced mechanical properties • eg. PS/PB (5-20%) • -- Mechanical blend: no increase in impact properties (‘incompatible’) • -- PB dispersed in styrene then polymerize  PS, PB, • PS-g-PB (‘compatiblizer’ (surfactant) , few %) [HIPS] • - ‘phase within a phase within a phase’ morphology •  mechanically toughen • -- ABS similar

  3. Why Should We Care About Blends? (con’d) ‘Added’ graft or block Notched Izod PS 0.25 – 0.4 (ft-lb/in) HIPS 0.5 – 4 but E and b  as rubber  Impact strength Blend Elastomer % ~5 - 15% • Why enhancement of impact strength? •  rubber particles act as stress concentrators to initiate matrix yielding (crazing &/or shear yielding)

  4. Mechanism of Rubber Toughening in Thermoplastic Polymers • Mechanism of energy absorption: initially brittle polymers (e.g. PS, PMMA) • a. Rubber particles act to initiate very large number (~106) of crazes in glassy matrix • b. Crazes grow until they encounter another rubber particle or craze •  Energy consumed principally in plastic deformation of matrix • c. Rubber particles act as craze termination points and obstacles to craze or crack propagation

  5. HIPS TEM OsO4 staining for contrast Dark = PB PS inclusions in ‘rubber’ particles Wagner and Robeson (1970)

  6.  direction HIPS (con’d) • Crazing in thin HIPS film Rubber particle: PS inclusions What are crazes?

  7.  direction ABS: Acrylonitrile - Styrene - Butadiene TEM Synthesis? Dispersed phase particle size ~0.1 - 0.5 m

  8. Rubber Toughening (con’d) •  Important features required to produce good impact properties: • a. elastomer must phase separate and be dispersed randomly as small discrete particles • b. elastomer must have low Tg (Tg << Ttest) • c. good interfacial adhesion ('compatibilizers') • d. low shear modulus in relation to matrix • What toughening mechanism often operates to toughen ‘quasi-ductile’ polymers (e.g. that lead to ‘super tough’ nylons or polyacetals)?

  9. Why Should We Care AboutMiscible Blends? • Miscible mixtures: • - No interfaces to contend with • - Cost/property combination (e.g. Noryl [PPO/PS]) • - Macromolecular plasticizer or other additive (non-migrating) • - In some cases, modest ‘synergism’ in some mechanical properties • - Model systems for fundamental studies

  10. Phase Diagrams Phase Behavior  f (T, P, Vi) Upper Critical Soln Temp. (UCST) - binodal (coexistence curve) boundary between stable & unstable - spinodal: boundary between metastable & unstable Critical point -- Quench into 2 phase region  compositions along binodal define composition of phases (lever rule)

  11. Phase Diagrams (con’d) • Lower Critical Soln Temp. (LCST) • -- Binodal • -- Spinodal • Difference between metastable & unstable regions? • -- mechanism of phase decomposition (spinodal decomposition vs. nucleation and growth)

  12. Phase Diagrams (con’d) ‘Hourglass’- merging of LCST & UCST Both LCST & UCST Closed loop behavior

  13. Gmix (= Hmix – T Smix) as f(T,P,V1) • Theory of polymer solutions: • Flory-Huggins (polymer + solvent)  lattice model; emphasis on calculation of Sm • Extension to polymer-polymer mixtures [Scott, (1949)] • Improved theories (e.g. eqn of state theories [Flory (1970)]; inclusion of strong intermolecular interactions: lattice fluid; group contributions, etc) Understanding Miscibility andPhase Behavior

  14. Simplified Theory of Polymer Solutions* 1. Polymer/solvent  UCST & LCST 2. Polymer/polymer  a) dispersive forces (Hm > 0)  single phase only if MW < few 1000 (UCST) b) strong interactions (Hm < 0)  miscible at high MW (LCST) 3. Two miscible polymers + solvent (possible to get phase separation in ternary mix even if all 3 binaries are miscible, if large difference in affinities of polymers for solvent (“effect”) 4. Two strongly interacting polymers (miscible) + solvent (possible to get phase separation in ternary mixture if polymer - polymer interactions become very large) *Patterson & Robard, Macromolecules (1978); Patterson, PES (1982)

  15. Sm (S-S) > Sm (S-P) > Sm (P-P) Important Contributions to Gmix • Combinatorial Smix;intermolecular interactions (contact energy); ‘free volume’ effects • 1. Combinatorial Smix Sm  related to # of permutations of molecules of liquid among sites on soln) ‘quasi-lattice’, i.e., positional disorder Sm =k ln  always + entropic stabilization

  16. Regular Solution Theory for Small Molecules -Sm/R = N1 lnX1 + N2 lnX2 Not appropriate for polymers:  Equivalent to assuming solvent & entire polymer occupy 1 lattice site Rather, define: Vsolv = Ns Vs / (Ns Vs + MNpVs) Polymer defined = chain of segments which have same vol as solvent M = vol. occupied by poly vol. occupied by solvent M = not necessarily = DP Mole fraction Vol. fraction (s) # segments in poly # molecules or moles

  17. Entropic stabilization: P - P  0, in the limit of  MW for both Entropic stabilization: P - S (V2 0) Soln more stable at high [P]?? Entropic stabilization: S - S composition Flory – Huggins Sm/R = -[N1 lnV1 + N2 lnV2] Mix N1 moles of #1 with N2 moles #2: On per volume of soln basis (V): Assume V1 = V2 = 1/2, & for solvent/solvent V1 ~ V2 ~ 100 cc/mol:  Molar volumes

  18. + Gm 0  > Free Energy and Phase Separation Mixture can develop lower Gm by separating into 2 phases V2 At a given T and P • Phase separation occurs if Gm becomes concave downward i.e., if • Thermodynamic criteria for miscibility: • Gmand

  19. Related to weakness or strength of 1-2 contacts relative to 1-1 & 2-2 • Flory: • Hm/RTV = (z W12  kTVs) V1V2 = (12 V1) V1V2 • characterizes the interaction energy lattice coordination number Exchange energy for breaking 1/1& 2/2 contacts & forming 1/2 2. Interactional Contribution = 1/2(W11 +W22) -W12 Vs = molar vol of segment (of S in P/S) Interaction parameter # segments in 1 (= S in P/S) Dispersive 1-2 contacts  + Hm Strong Interactions  - Hm important in poly/poly mixing

  20. combinatorialinteractional for polymer/polymer (r1 )  12  ! Important quality  12 / r1 or12 / V1  independent of MW Total Free Energy of Mixing Gm/RTV = [V1 lnV1 V1 + V2 lnV2 V2] +(12 V1) V1V2 Another common form: Gm/RT = [V1 lnV1 M1 + V2 lnV2 M2] + 12 V1V2 “DP” Defined as per segment - normally used in the literature Gm per mole of lattice sites

  21. Typical Polymer Solution (in LMW solvent) (12 V1) V1V2 for 12 > 0 Total Gm for polymer/solvent Total Gm for polymer/polymer with dispersive forces Concave downward throughout entire composition range (phase separate into pure 1&2)

  22. 12 V1 + 0  Temperature Dependence of 12 12 ~ T-1 For dispersive forces 12 r1 = z W12  kT For strong interactions For a given composition Temperature 

  23. Arises as a consequence of –Vmix & related to difference in coefficient of thermal expansion of components •  Negative contribution to Hm and Smpositive contribution to Gm (destabilizing) •  It’s the non-combinational Sm that derives phase separation at high T in polymer-polymer (LCST has hence been termed an entropy driven process) •  Free volume effects get collected in 12 along with interactions •  As T , Vfree difference between solvent & polymer (or 2 polymers) increases, as does the positive contribution to 12 3. Free Volume Contribution Vmix  0 Soln has free vol closer to polymer

  24. If 12 independent of concentration: •  (12 V1)crit = 1/2 [V1-0.5 + V2-0.5]2 • Both low MW; from regular soln theory of small molecule mixs:    Critical Points  Also equivalent expression where molar volumes replaced with M’s i.e., 12> 2  phase separated

  25. Polymer/solvent • V2  (i.e., MW )  (12 V1)crit  • Polymer/polymer  V1 &V2  0; (12 V1)crit  0 3. Critical Points (con’d)

  26. Phase Behavior critdecreases as MW increases 0 c = a + b (dispersive forces)  LCST and UCST d = a + b (strong interactions dominate) For given composition

  27. Interplay of combinational Sm, interactions, and Vfree contributions, as function of T and composition, determine whether system will be single or multiphase + allows qualitative description of phase behavior of polymer mixtures (P-S and P-P) For high MW P-P mixtures, simple theory suggests that strong interactions required to obtain single phase state at given T (and these mixtures will phase separate on increasing T) Initial Summary

More Related