Amphiphilic Polymer Conetworks Based on End-Linked “Core-First” Star Block Copolymers: Structure Formation with Long-Range Order

Abstract

Amphiphilic polymer conetworks are cross-linked polymers that swell both in water and in organic solvents and can phase separate on the nanoscale in the bulk or in selective solvents. To date, however, this phase separation has only been reported with short-range order, characterized by disordered morphologies. We now report the first example of amphiphilic polymer conetworks, based on end-linked "core-first" star block copolymers, that form a lamellar phase with long-range order. These mesoscopically ordered systems can be produced in a simple fashion and exhibit significantly improved mechanical properties. P olymeric hydrogels consist mostly of water held in place by a hydrophilic polymer matrix. They possess both liquidand solid-like characteristics. While these materials do not flow, they are very soft and allow the free diffusion of solutes. Furthermore, the high water content usually leads to biocompatibility. Extensively studied over the past 50 years, 1 hydrogels find uses as superabsorbents 2,3 (packing material in hygienic and baby diapers, media for water retention in agriculture and wastewater management), in medicine 4 (vehicles for controlled drug delivery 5 and as scaffolds for tissue engineering 6 ), in biotechnology 7 (matrices for enzyme immobilization and media for electrophoresis), as soft contact lenses, 8 and in sensors and actuators. 9 Amphiphilic polymer conetworks (APCNs), first reported in 1988 by Kennedy 10 and Stadler, 11 are related to hydrogels and have received increased attention in the past 10−15 years. 12 APCNs are composed of covalently interconnected hydrophilic and hydrophobic segments, swell in both polar and nonpolar solvents, and can solubilize both polar and nonpolar solutes. Furthermore, the arrangement of each type of units in relatively long sequences results in APCN self-assembly in selective solvents, very much like the micellization of conventional lowmolecular-weight surfactants and linear amphiphilic block copolymers, and the creation of a large interfacial area. 13 This internal self-organization differentiates APCNs from simple hydrogels and enables them for use in niche applications, e.g., as materials in antifouling coatings, 14 modern soft contact lenses, 15 matrices for phase transfer reactions used in bio-and organocatalysis, and the fabrication of gas and optical sensors. 16 APCNs may also have advantages over conventional hydrogels when used as matrices for controlled drug delivery. 17 The cross-linked nature of APCNs implies some special characteristics for their microphase separation: First, chain relaxation and equilibration are slow compared to those of nonnetwork polymer systems (linear and star copolymers), and second, unavoidable loops and chain entanglements 18 lead to the entrapment of individual segments in domains of opposite philicity, compromising nanodomain purity. These explain why all experimentally studied APCNs exhibit incompletely separated nanophases with blurred interfaces, 19 in spite of theoretical studies that predict APCN phase ...

Amphiphilic Polymer Conetworks Based on Interconnected Hydrophobic Star Block Copolymers: Synthesis and Characterization

“…In four of the APCNs, the molar ratio of the hydrophobic MMA to the hydrophilic DMAEMA units in the arms of the stars was fixed at 3–1, with the overall (nominal) degrees of polymerization (DP) of the arms being 10, 20, 30 and 40. This particular composition is expected to result in materials less swollen in water, leading to better‐ordered aqueous nanophases . The remaining two APCNs comprised star block copolymers whose arms were less hydrophobic, with MMA to DMAEMA molar ratios of 1–1, and 1–3, and constant overall (nominal) arm DP equal to 20.…”

“…The more compact nature and the usually large molecular weight of star polymers are expected to introduce fewer network defects related to chain entanglements, and, therefore, allow for more regular organization of the nanophases in the phase separated state. Although most of the APCNs already prepared from amphiphilic star block copolymers presented nanophase separation with only short‐range order, some recent reports revealed morphologies with longer‐range order . In these latter reports, a rather low amount of cross‐linker was used, so as to allow more freedom to the polymer segments to reach a lowest‐energy configuration, resulting in the formation of better‐ordered morphologies within the APCNs.…”

Novel Anti‐Biofouling Soft Contact Lens: l‐Cysteine Conjugated Amphiphilic Conetworks via RAFT and Thiol–Ene Click Chemistry

“…The silicone hydrogels with amphiphilic conetwork (APCN) structure have been specifically developed for soft contact lenses with dramatically improvement of oxygen permeability. The cocontinuous microphase morphology in APCNs is achieved by skillful combination of the hydrophilic phase with the hydrophobic polydimethylsiloxane phase, which perfectly matches the need of extended‐wear contact lenses …”

Networks Based on “Core‐First” Star Polymers End‐Linked Using a Degradable Ketal Cross‐Linker: Synthesis, Characterization, and Cleavage

“…In general, the GPC traces of the EGDMA-based cores and "core-first" star polymers in the present investigation were similar to those in our previous work on (nondegradable) amphiphilic polymer conetworks based on interconnected "core-first" star block copolymers. [36,37] The GPC traces of all the extractables (shown in black continuous lines) from the networks were monomodal and much narrower than those corresponding to the network precursors, but displayed a long tailing toward shorter elution times, particularly the ones corresponding to networks with a DMOEPbased (primary) core. The GPC traces of the extractables from all polymer networks, but particularly those from networks with EGDMA-based (primary) cores, largely overlapped with the lower molecular weight peak of the "core-first" star polymer precursor.…”