Order-Order Transition in Statistical Copolymer Thin Film Induced by LCST-Type Behavior

In this paper, we describe the formation of an ordered structure in a copolymer thin film through hydration, which subsequently transitions to a different ordered structure upon dehydration. A statistical copolymer of poly(N-octadecyl acrylamide-stat-hydroxymethyl acrylamide) with a comonomer content ratio of 1:1, denoted as p(ODA50/HEAm50), was synthesized via free radical copolymerization. We prepared a thin film of this copolymer on a solid substrate and annealed it at 60 °C under humid conditions. This treatment formed a side-chain segregated lamellar (SCSegL) structure, in which the ODA and HEAm units are oriented perpendicularly to the polymer backbone and opposite each other. Increasing the annealing temperature to 90 °C led to a transition to a side-chain mixed lamellar (SCMixL) structure, where the ODA and HEAm units are also oriented perpendicularly to the polymer backbone but in both directions. The quartz crystal microbalance (QCM) data indicate that p(ODA50/HEAm50) exhibits LCST-like behavior with a transition temperature of approximately 50 °C. We conclude that the formation of the SCSegL structure at 60 °C is due to pronounced segregation between the water-adsorbed HEAm groups and the hydrophobic ODA. Conversely, dehydration at 90 °C reduces the segregation forces, forming the SCMixL structure, which exhibits lower strain. These results demonstrate that the p(ODA50/HEAm50) film undergoes an order-to-order transition driven by the hydration-dehydration process. Additionally, we found that changes in the lamellar structure significantly alter the swelling properties of the film.

Universal Access to Water-Compatible and Nanostructured Materials via the Self-Assembly of Cationic Alternating Copolymers

Herein, we report the water-assisted self-assembly of alternating copolymers bearing imidazolium cations and hydrophobic groups to create water-compatible and nanostructured materials. The copolymers efficiently absorbed water into the cationic segments from the outer environments, depending on the relative humidity. The absorbed water serves as hydrophilic molecules to modulate the weight fraction of hydrophilic/hydrophobic units in the samples. Thus, the morphologies and domain spacing of the nanostructures can be controlled by not only the side chains, but also the amount of absorbed water. The self-assembly of the cationic copolymers, developed herein, afforded universal access to various morphologies, including lamella, gyroid, and cylinder, in addition to the precision control of the domain spacing at the 0.01 nm level.

Lamellar Microphase Separation and Phase Transition of Hydrogen-Bonding/Crystalline Statistical Copolymers: Amide Functionalization at the Interface

Microphase separation of random copolymers, as well as that of high χ-low N block copolymers, is promising to construct sub-10-nm structures into materials. Herein, we designed statistical copolymers consisting of 2-hydroxyethyl acrylate (HEA) and N-octadecylacrylamide (ODAAm) to produce crystallization and hydrogen bond-assisted lamellar structure materials. The copolymers not only formed a crystalline lamellar structure with 3-4 nm domain spacing but also maintained an amorphous lamellar structure via phase transition above the melting temperature up to approximately 100 °C. The key is to introduce hydrogen-bonding amide junctions between the octadecyl groups and the polymer backbones, by which the polymer chains are physically fixed at the interface of lamellar structures even above the melting temperature. The stabilization of the lamellar structure by the amide units is also supported by the fact that the lamellar structure of all-acrylate random copolymers bearing hydroxyethyl and crystalline octadecyl groups is disordered above the melting temperature. By spin-coating on a silicon substrate, the HEA/ODAAm copolymer formed a multilayered lamellar thin film consisting of a hydrophilic hydroxyethyl/main chain phase and a hydrophobic octadecyl phase. The structure and order-disorder transition were analyzed by neutron reflectivity.

Order-order transitions in poly(N-octadecyl acrylamide-co-hydroxyethyl acrylamide) statistical copolymer films

This study demonstrates that a simple statistical copolymer can form self-assembled lamellae, whose structures depend on both the comonomer composition and the annealing temperature. Statistical copolymers of octadecyl acrylamide and hydroxyethyl acrylamide [p(ODA/HEAm)] were prepared via free-radical copolymerization, and their thermal properties were studied by differential scanning calorimetry. Thin films of p(ODA/HEAm) were prepared via spin-coating, and their structures were analyzed using X-ray diffraction. It was found that copolymers with HEAm contents between 28 and 50% formed self-assembled lamellae upon annealing at a temperature ∼10 °C above the glass-transition temperature. The self-assembled form was found to possess a "side-chain-mixed" lamellar structure, in which the ODA and HEAm side chains are oriented perpendicular to the lamellar plane composed of the polymer main chain. Interestingly, a copolymer with a HEAm content between 36 and 50% transformed from the side-chain-mixed lamellar structure to generate a "side-chain-segregated" lamellar structure upon annealing at a significantly higher temperature (∼50 °C above T_g). In this structure, the ODA and HEAm side chains were found to be oriented in opposite directions but perpendicular to the lamellar plane. The packing of the side chains in the lamellar structures was studied using Fourier-transform infrared spectroscopy. It was concluded that the structures of the self-assembled lamellae are determined by the strain forces generated during self-assembly, and by the segregation forces existing between the comonomers.

Synthesis of poly(n-alkyl acrylamides) and evaluation of nanophase separation effects by temperature-dependent infrared spectroscopy

Common linear polymers are known to undergo phase changes at the glass-transition temperature (Tg) and the melting point (Tm). In recent years, it has also been shown that molecules with long aliphatic side chains can give rise to a backbone-independent melting phenomenon, known as nanophase separation. This effect describes the self-assembly — independent of the polymer backbone — of alkyl side chains into semi-crystalline nanostructures. This work presents optimized, gram scale synthesis routes for dodecyl and octadecyl acrylamide and their respective homopolymers. Differential scanning calorimetry (DSC) experiments detected a broad endothermal signal for poly(n-dodecyl acrylamide) at − 29 °C and a narrower, more intense signal for poly(n-octadecyl acrylamide) at 34 °C. These signals indicate the nanophase separation TM of the alkyl side chains. We undertook the first temperature-controlled infrared spectroscopy investigations of these materials revealing a clear hypsochromic shift of the C–H stretching signals above TM and the amide I signal shifts that occurred only above and below Tg. These results provide further evidence, that the side chains act independently of the polymer backbone and show that infrared spectroscopy is a powerful tool for monitoring conformational changes in polymer side chains.

Graphical abstract

Self-Assembly and -Cross-Linking Lamellar Films by Nanophase Separation with Solvent-Induced Anisotropic Structural Changes

In this study, we have prepared thermally and chemically stable lamellar polymer films via humid annealing. The amphiphilic polymer poly(N-dodecyl acrylamide-stat-3-(trimethoxysilyl)propyl acrylate) [p(DDA/TMSPA)] forms a self-assembled lamellar structure via annealing at 60 °C under 98% relative humidity (humid annealing) due to nanophase separation between the hydrophobic dodecyl side and main chains with the amide groups that contain adsorbed water. Moreover, a self-cross-linking reaction of TMSPA proceeds during the humid annealing. As a result, the lamellar films maintain their structure even when annealed above their glass-transition temperature. On the other hand, the films swell when immersed in toluene. The highly ordered lamellar structure collapses due to the swelling but can be re-established by subsequent humid annealing. A multilayer freestanding film can be exfoliated via sonication in toluene. The exfoliated multilayer films initially form a dome-shaped structure, which is converted to a plate-shaped structure upon humid annealing. In their entirety, these results reveal that the molecular-scale movement associated with the formation of the lamellar structure induces a macroscopic structural change. Consequently, p(DDA/TMSPA) can be considered as a new stimulus-responsive polymer.

Self-Assembled Lamellar Films of Comb-Shaped Copolymers by Segregation between Hydrophobic Side Chains and the Main Chain with Hydrophilic Comonomers

Self-assembled lamellar films of poly(N-dodecyl acrylamide-stat-vinyl phosphonic acid) [p(DDA/VPA)] were formed via the segregation between the hydrophilic main chain and VPA and dodecyl side chains. p(DDA/VPA) copolymers were synthesized by free-radical copolymerization of DDA and VPA with VPA molar concentrations of 19% [p(DDA/VPA19)] and 64% [p(DDA/VPA64)]. Both copolymers exhibited a glass-transition temperature (T_g) and melting temperature for p(DDA/VPA19), but no crystalline or liquid-crystalline phase-transition temperatures, which suggests that both copolymers are amorphous. Thin films of the copolymers were prepared by spin coating, and the structure of the films was studied by X-ray diffraction (XRD) measurements. The as-cast films of the copolymers showed broad diffraction patterns, which suggested the formation of alkyl nanodomains similar to that observed in the pDDA homopolymers. On the other hand, the XRD patterns for both copolymer films showed a sharp Bragg diffraction in the low-q region after annealing at 60 °C. Furthermore, the p(DDA/VPA19) film showed first- and second-order Bragg diffractions with a ratio of 1:2. These XRD patterns suggest that the copolymer films form an ordered lamellar structure. We concluded that the main chain became more hydrophilic by the introduction of VPA, resulting in an increased segregation force relative to the hydrophobic dodecyl side chains, which induces the formation of lamellae. Moreover, doping a p(DDA/VPA64) film with imidazole increased the ordering and uniformity of the lamellar structures due to the increased segregation force by the formation of ion pairs in the hydrophilic comonomer. In their entirety, the results show that statistical copolymerization can be used as a new method to create self-assembled structures.

Amphiphilic random and random block terpolymers with PEG, octadecyl, and oleyl pendants for controlled crystallization and microphase separation

Amphiphilic random and random block terpolymers bearing PEG chains, crystalline octadecyl groups, and amorphous oleyl groups were designed to control crystallization and microphase separation in the solid state.

Design guide of amphiphilic crystalline random copolymers for sub-10 nm microphase separation

Sub-10 nm lamellar structures are efficiently constructed by the pendant microphase separation of amphiphilic crystalline random copolymers with broad molecular weight distribution that are obtained from free radical copolymerization.

Amphiphilic Nanogels: Fuzzy Spheres with a Pseudo-Periodic Internal Structure

Amphiphilic polymer nanogels (NGs) are promising drug delivery vehicles that extend the application of conventional hydrophilic NGs to hydrophobic cargoes. By randomly introducing hydrophobic groups into a hydrophilic polymer network, loading and release profiles as well as surface characteristics of these colloids can be tuned. However, very little is known about the underlying internal structure of such complex colloidal architectures. Of special interest is the question how the amphiphilic network composition influences the internal morphology and the "fuzzy" surface structure. To shine light into the influence of varying network amphiphilicity on these structural features, we investigated a small library of water-swollen amphiphilic NGs using small-angle X-ray scattering (SAXS). It was found that overall hydrophilic NGs, consisting of pure poly(N-(2-hydroxypropyl)methacrylamide) (PHPMA), display a disordered internal structure as indicated by the absence of a SAXS peak. In contrast, a SAXS peak is present for amphiphilic NGs with various amounts of incorporated hydrophobic groups such as cholesteryl (CHOLA) or dodecyl (DODA). The internal composition of the NGs is considered structurally homologous to microgels. Application of the Teubner-Strey model reveals that hydrophilic PHPMA NGs have a disordered internal structure (positive amphiphilicity factor) while CHOLA and DODA samples have an ordered internal structure (negative amphiphilicity factor). From the SAXS data it can be derived that the internal structure of the amphiphilic NGs consists of regularly alternating hydrophilic and hydrophobic domains with repeat distances of 3.45-5.83 nm.