Ionic Compatibilization of Polymers

Abstract

The small specific entropy of mixing of high molecular weight polymers implies that most blends of dissimilar polymers are immiscible with poor physical properties. Historically, a wide range of compatibilization strategies have been pursued, including the addition of copolymers or emulsifiers or installing complementary reactive groups that can promote the in situ formation of block or graft copolymers during blending operations. Typically, such reactive blending exploits reversible or irreversible covalent or hydrogen bonds to produce the desired copolymer, but there are other options. Here, we argue that ionic bonds and electrostatic correlations represent an underutilized tool for polymer compatibilization and in tailoring materials for applications ranging from sustainable polymer alloys to organic electronics and solid polymer electrolytes. The theoretical basis for ionic compatibilization is surveyed and placed in the context of existing experimental literature and emerging classes of functional polymer materials. We conclude with a perspective on how electrostatic interactions might be exploited in plastic waste upcycling.

Figure 1

Vast design space of ion-containing polymers spans materials bearing “hard” compact ions or “soft” ionic-liquid-type ions with delocalized charge and variations in both charge density and polymer architecture.

Figure 2

An ionically cross-linked blend with the degree of polymerization between cross-links of N_x has similar microphase separation behavior as an A₂B₂ star block polymer with A and B arms of length N_x/2.

Figure 3

Schematic representation of five types of ionic supramolecular block copolymer structures.

Figure 4

Phase diagram of a symmetric blend of monofunctional A and B polymers (1:1) of length N_A = N_B ≡ N/2. The inverse segregation strength on the y-axis is a pseudo temperature variable, while h/χN is an approximately temperature-independent ratio of bond to segregation strengths. The indicated phases are Dis (disordered homogeneous phase), 2 phase (coexistence of two homogeneous liquid phases), and Lam (lamellar mesophase). The points labeled LS are Lifshitz tricritical points. Reproduced from Feng, E. H.; Lee, W. B.; Fredrickson, G. H. Macromolecules2007, 40, 693–702 (ref (21)). Copyright 2007 American Chemical Society.

Figure 5

(top) RPA stability analysis of a symmetric blend of chain length N = 100 with counterions included for each charged residue. The solid black curves are the stability limits of the homogeneous disordered phase. The segments of the black curves to the left of the points of intersection with the blue curve represent a spinodal instability to a LAM microphase, while the segments to the right describe a critical demixing transition to the coexisting macrophases. The critical segregation strength, (χN)_c, to either micro- or macrophase separation is seen to grow rapidly with the total charge per chain Q and to saturate beyond an electrostatic strength γ of about 1.5. (bottom) Stability curves for three values of N and two values of Q. The chain length dependence is relatively weak and saturates at large N’s.

Figure 6

(a, b) Chemical structures of pristine and chain-end functionalized PS/PDMS blends. The reactive PS-acid/PDMS-base blend demonstrates improved optical clarity. (c, d) SAXS and TEM patterns reveal the formation of a LAM microphase.

Figure 7

Optical images, TEM, and SAXS analyses of three different SBCPs. (a) Triblock-type SBCPs (1:2 motif) demonstrate an increase in lamellar domain spacing with increasing temperature due to the reversal of proton transfer resulting in nonionic homopolymers that swell the mesophase. Reproduced with permission from Huh, J.; Park, H.; Kim, K.; Kim, K.; Park, C.; Jo, W. Adv. Mater.2006, 18, 624–629 (ref (11)). Copyright 2006 John Wiley and Sons. (b) Miktoarm star SBCPs (1:3 motif) display defective LAM mesophases where steric constraints might limit proton transfer/ionic bond formation. Reproduced from Pispas, S.; Floudas, G.; Pakula, T.; Lieser, G.; Sakellariou, S.; Hadjichristidis, N. Macromolecules2003, 36, 759–763 (ref (12)). Copyright 2003 American Chemical Society. (c) Multiblock ionic SBCPs (2:2 motif) produced from PS and poly(isobutylene) telechelics are clear, flexible materials with LAM order. However, the incompletely bonded SBCPs cannot support as much stress as covalently bonded multiblocks, leading to relatively low elongation and stress at break. Reproduced from Zhang, L.; Kucera, L. R.; Ummadisetty, S.; Nykaza, J. R.; Elabd, Y. A.; Storey, R. F.; Cavicchi, K. A.; Weiss, R. A. Macromolecules2014, 47, 4387–4396 (ref (9)). Copyright 2014 American Chemical Society.

Figure 8

(a) Chemical structures of the PDMS-Im/PBA-TFSI blend, which formed a coacervate (b) in a common solvent. (c) The blend after solvent removal was optically transparent and globally disordered (DIS) but locally segregated with a correlation length of ca. 5 nm.

Figure 9

Coacervation between CPE and PIL results in the formation of a concentrated phase with 50% w/v polymer. This paste-like phase can be blade-coated to form a μm-thick homogeneous film with improved electrical conductivity relative to a cast film of the pure CPE component. Adapted from Le, M. L.; Rawlings, D.; Danielsen, S. P.; Kennard, R. M.; Chabinyc, M. L.; Segalman, R. A. ACS Macro Lett.2021, 10, 1008–1014 (ref (73)). Copyright 2021 American Chemical Society.

Figure 10

Schematic representation of ionic compatibilization by reactive blending. Scenario 1: a waste stream of prefunctionalized PE-acid and iPP-base is melted and passed through an extruder to affect proton transfer and compatibilization. Scenario 2: PE and iPP chains are selectively functionalized by two regents to install acid and base groups, respectively; the reactions, blending, and compatibilization occurring in a single extruder. Scenario 3: waste streams rich in PE and iPP are functionalized separately in two extruders before being combined in a third extruder.