Segmental Interaction Parameters Research Articles

Phase Behavior of Charged Star Block Copolymers at Fluids Interface

AbstractThe phase behavior of block copolymers (BCPs) at the water‐oil interface is influenced by the segmental interaction parameter ( ) and chain architecture. We synthesized a series of star block copolymers (s‐BCPs) having polystyrene (PS) as core and poly(2‐vinylpyridine) (P2VP) as corona. The interaction parameters of block‐block ( PS‐P2VP) and block‐solvent ( P2VP‐solvent) were varied by adjusting the pH of the aqueous solution. Lowering pH increased the fraction of quaternized‐P2VP (Q‐P2VP) with enhanced hydrophilicity. By transferring the equilibrated interfacial assemblies, morphologies ranging from bicontinuous films at pH of 7 and 3.1 to nanoporous and nanotubular structure at pH of 0.65 were observed. The nanoporous films formed hexagonally packed pores in s‐BCP matrix, while nanotubes comprised Q‐P2VP as corona and PS as core. Control over pore size, d‐spacing between pores, and nanotube diameters was achieved by varying polymer concentration, molecular weight, volume fraction and arm number of s‐BCPs. Large‐scale nanoporous films were obtained by freeze‐drying emulsions. Remarkably, the morphologies of linear BCPs were inverted, forming hexagonal‐packed rigid spherical micelles with Q‐P2VP as core and PS as corona in multilayer. This work provides insights of phase behaviors of BCP at fluids interface and offer a facile approach to prepare nanoporous film with well‐controlled pore structure.

Phase Behavior of Charged Star Block Copolymers at Fluids Interface.

The phase behavior of block copolymers (BCPs) at the water-oil interface is influenced by the segmental interaction parameter ( ) and chain architecture. We synthesized a series of star block copolymers (s-BCPs) having polystyrene (PS) as core and poly(2-vinylpyridine) (P2VP) as corona. The interaction parameters of block-block ( PS-P2VP ) and block-solvent ( P2VP-solvent ) were varied by adjusting the pH of the aqueous solution. Lowering pH increased the fraction of quaternized-P2VP (Q-P2VP) with enhanced hydrophilicity. By transferring the equilibrated interfacial assemblies, morphologies ranging from bicontinuous films at pH of 7 and 3.1 to nanoporous and nanotubular structure at pH of 0.65 were observed. The nanoporous films formed hexagonally packed pores in s-BCP matrix, while nanotubes comprised Q-P2VP as corona and PS as core. Control over pore size, d-spacing between pores, and nanotube diameters was achieved by varying polymer concentration, molecular weight, volume fraction and arm number of s-BCPs. Large-scale nanoporous films were obtained by freeze-drying emulsions. Remarkably, the morphologies of linear BCPs were inverted, forming hexagonal-packed rigid spherical micelles with Q-P2VP as core and PS as corona in multilayer. This work provides insights of phase behaviors of BCP at fluids interface and offer a facile approach to prepare nanoporous film with well-controlled pore structure.

Using Grazing-Incidence Small-Angle Neutron Scattering to Study the Orientation of Block Copolymer Morphologies in Thin Films

We used grazing-incidence small-angle neutron scattering (GISANS) to probe the depth-dependent orientation of self-assembled morphologies of block copolymers. A hydrophobic–hydrophobic block copolymer, poly(solketal methacrylate)-block-polystyrene (PSM-b-PS), was synthesized and spin-coated into films tens of nanometers in thickness where the polymers adopted a disordered state. PSM-b-PS was then hydrolyzed by exposing the polymer thin film to trifluoroacetic acid vapor or embedding a photoacid generator in the film during spin coating, followed by exposure to UV light. The hydrolysis converted the block copolymer into a hydrophilic–hydrophobic block copolymer, poly(glycerol monomethacrylate)-block-polystyrene (PGM-b-PS), significantly increasing the segmental interaction parameter (χ) and causing microphase separation of the copolymers. GISANS was used to study the depth-dependent structure of the films and revealed that the orientation of the microphase-separated structure differed at the polymer–air and polymer–substrate interfaces. In the regions where the lamellae were parallel to the interfaces, the orientation propagated into the film from the interfaces. However, in the areas where the lamellae were perpendicular to the interfaces, the orientation did not persist with increasing distance from the interfaces.

Vapor-liquid and liquid-liquid equilibria in the water + poly(propylene glycol) system

The aim of this study was to investigate phase equilibria in a water + poly(propylene glycol) [Mn = 425 g mol−1] system. Vapor-liquid equilibria (VLE) were determined ebulliometrically in the completely miscible region with higher water concentrations at isotherms 323.15, 333.15, 343.15, and 353.15 K. These data were correlated with the UNIQUAC free-volume (FV) model to be consistent with our previous studies. Liquid-liquid equilibrium (LLE) was determined using the cloud-point method at 320–350 K, and lower critical solution temperature was found at 320.2 K. LLE was correlated with the entropic FV-UNIQUAC segmental interaction model developed earlier, which is suited for correlation and calculation of LLE of polymer solutions. We estimated new binary segmental interaction parameters for interactions between propylene glycol and water segments.

Blends of Polyisoprene with Model Styrene–Olefin Copolymers: Mixing Energetics in Blends versus Block Copolymers

Most theoretical treatments of polymer mixing thermodynamics assume that the phase behavior of both polymer blends and block copolymers is controlled by the same Flory segmental interaction parameter, χ—a quantity which is proportional to the interaction energy density, X. However, there are few comparative experimental studies on this point. This work presents a systematic experimental comparison between the interaction energy densities measured on model polydiene–polyolefin blends (Xblend) and analogous block copolymers (XBC). Experimental cloud point temperature (Tcl) curves for blends of polyisoprene (PI) with selectively saturated poly(butadiene-r-styrene) (hSBR) random copolymers are fit to the Flory–Huggins theory, and the resultant values of Xblend are corrected for critical fluctuations and for end-group contributions, wherever significant. The temperature-dependent PI/hSBR Xblend(T) and previously reported PI–hSBR XBC values show satisfactory agreement at all levels of styrene incorporation, reinforcing the conclusion that random incorporation of styrene units into the saturated polybutadiene chain significantly enhances its compatibility with PI.

Morphological Evolution of Poly(solketal methacrylate)-block-polystyrene Copolymers in Thin Films

The morphological evolution of the lamellar microdomains in the thin films of symmetric poly(solketal methacrylate-b-styrene) (PSM-b-PS) copolymers that can be converted into poly(glycerol monomethacrylate-b-styrene) (PGM-b-PS) copolymers through acid hydrolysis reaction was investigated. This simple chemical transformation was performed in the solid state using trifluoroacetic acid vapor, markedly increasing the segmental interaction parameter (χ) from 0.035 to 0.438 at 25 °C and a 118 Å3 reference volume by changing the hydrophobic PSM block to the hydrophilic PGM block. To control the orientation of the lamellar microdomains using this responsive block copolymer (BCP), a protected random copolymer (PSM-r-PS) with 31 mol % SM, which was concurrently transformed into PGM-r-PS with the BCP, was used to tune the interfacial energies at the substrate. Atomic force microscopy and grazing-incidence small-angle X-ray scattering measurements as a function of exposure time to an acid vapor were performed to characterize the transition from the disordered state into the ordered state and to assess the orientation of the microdomains. As a result, a 9.4 nm full pitch lamellar microdomain morphology in the thin films was achieved after full conversion and thermal annealing, indicating that the modified substrate exhibited the surface neutrality toward the two blocks and successfully induced vertical orientation without any additional layers.

Evaluation of the Interaction Parameter for Poly(solketal methacrylate)-block-polystyrene Copolymers

A series of symmetric poly(solketal methacrylate-b-styrene) (PSM-b-PS) copolymers with varying molecular weights that can transform a hydrophobic PSM block to a hydrophilic poly(glycerol monomethacrylate) (PGM) block through an acid hydrolysis were investigated. This simple chemical transformation significantly enhances the segmental interaction parameter (χ), enabling a phase-mixed block copolymer (BCP) to microphase separate without any additives. Temperature-dependent small-angle X-ray scattering (SAXS) measurements as a function of the degree of polymerization (16 ≤ N ≤ 316) and PSM hydrolysis conversion were conducted to characterize the order-to-disorder transition (ODT) behavior as well as the lamellar microdomain features. Using a mean-field correlation-hole analysis of the scattering, the χ value for PSM and PS was determined as a function of the conversion of PSM to PGM. For 100% conversion of PSM to PGM, the χ with PS was found to be given by χ = 0.3144 + 36.91/T, with χ = 0.438 at 25 °C, which...

Locally Favored Two-Dimensional Structures of Block Copolymer Melts on Nonneutral Surfaces

Self-assembly of block copolymers (BCPs) into arrays of well-defined nanoscopic structures has attracted extensive academic and industrial interests over the past several decades. In contrast to the bulk where phase behavior is controlled by the segmental interaction parameter, the total number of segments in BCPs and volume fraction, the morphologies and orientations of BCP thin films can also be strongly influenced by the substrate surface energy/chemistry effect (considered as a “substrate field”). Here, we report the formation of locally favored structures where all constituent blocks coexist side-by-side on nonneutral solid surfaces irrespective of their chain architectures, microdomain structures, and interfacial energetics. The experimental results using a suite of surface-sensitive techniques intriguingly demonstrate that individual preferred blocks and nonpreferred blocks lie flat on the substrate surface and form a two-dimensional percolating network structure as a whole. The large numbers of so...

Realizing 5.4 nm Full Pitch Lamellar Microdomains by a Solid-State Transformation

A simple method was developed to realize a self-assembled block copolymer (BCP) lamellar microdomain morphology with a full pitch of 5.4 nm through an acid hydrolysis of poly(solketal methacrylate-b-styrene) (PSM-b-PS). The acid hydrolysis transforms PSM-b-PS, having two hydrophobic blocks, into poly(glycerol monomethacrylate-b-styrene) (PGM-b-PS), having one hydrophilic and one hydrophobic block. This transformation markedly increases the segmental interaction parameter such that a phase-mixed PSM-b-PS can be transformed in the solid state into a microphase-separated BCP without the use of any additives. Correlation hole scattering of the PSM-b-PS in the phase mixed state yields a segmental interaction parameter (χ) given by χ = 0.0196 + 4.694/T, where T is the absolute temperature. With the symmetric BCPs used in this study, the ordered lamellar microdomain pitch was determined by small-angle X-ray scattering (SAXS) as a function of the degree of polymerization (16 ≤ N ≤ 1246). For the lamellar microdom...

Toughened Isotactic Polypropylene: Phase Behavior and Mechanical Properties of Blends with Strategically Designed Random Copolymer Modifiers

A series of poly(cyclohexylethylene-ran-ethylene) (CE) copolymers were synthesized with statistical segment lengths designed to match that of isotactic poly(propylene) (iPP). Melt blending the CE compounds containing 50–70 wt % cyclohexylethylene repeat units with iPP at loadings between 5 and 20 wt % resulted in a dispersion of CE droplets with number-average diameters between 150 and 500 nm as determined by scanning electron microscopy. Linear dynamic mechanical elastic (G′) and loss (G″) moduli data were fit to the Palierne model providing estimates for the interfacial tension between iPP and CE, which was interpreted based on enthalpic (χH) and entropic (χS) contributions to the overall segment–segment interaction parameter, χ = χH + χS, attributed to density and statistical segment length differences, respectively. The small particle sizes are explained by a reduced interfacial tension, driven by minimization of χS (≈0), counterbalanced by χH > 0, establishing an optimal balance between phase separat...

Phase behavior and interactions in poly(dl-lactide)/poly(styrene-co-vinylphenol) blends

Miscibility of blends of poly(dl-lactide) (PDLLA) with poly(styrene) (PS) and poly(styrene-co-vinylphenol) (PSVPh) copolymers prepared by solution/precipitation is investigated. Differential Scanning Calorimetry (DSC) analysis reveals phase separation during the first heating scan for the as precipitated samples, but a single intermediate glass transition is observed in the subsequent heating scan, indicating that mixing at the molecular level occurs during the first heating step. An exotherm is also observed in the first heating scans, attributed to the enthalpy of mixing. PSVPh copolymers with vinylphenol contents starting at 16mol% are miscible with PDLLA. Segmental interaction parameters (Bij) corresponding to contacts between the different repeat units studied in this work have been calculated and the mean field Flory–Huggins lattice model has been applied to the PDLLA/PSVPh blends to obtain the critical VPh content necessary to achieve miscibility. The critical value calculated using the Hansen approach is in better agreement with the experimental result than the critical value calculated based on the Hildebrand approach. The lack of specific interactions observed by Fourier Transform Infrared (FTIR) spectroscopy for the as-precipitated blends confirms phase separation for the initial samples. FTIR spectra recorded during heating reveals the development of hydrogen bonding interactions between OH groups of VPh and CO groups of PDLLA at temperatures around those of the exotherm observed by DSC. Observed spectral shifts suggest weaker specific interactions than those occurring in blends of PSVPh with other polyesters.

Sub-5 nm Domains in Ordered Poly(cyclohexylethylene)-block-poly(methyl methacrylate) Block Polymers for Lithography

A series of poly(cyclohexylethylene)-block-poly(methyl methacrylate) (PCHE–PMMA) diblock copolymers with varying molar mass (4.9 kg/mol ≤ Mn ≤ 30.6 kg/mol) and narrow molar mass distribution were synthesized through a combination of anionic and atom transfer radical polymerization (ATRP) techniques. Heterogeneous catalytic hydrogenation of α-(hydroxy)polystyrene (PS-OH) yielded α-(hydroxy)poly(cyclohexylethylene) (PCHE-OH) with little loss of hydroxyl functionality. PCHE-OH was reacted with α-bromoisobutyryl bromide (BiBB) to produce an ATRP macroinitiator used for the polymerization of methyl methacrylate. PCHE–PMMA is a glassy, thermally stable material with a large effective segment–segment interaction parameter, χeff = (144.4 ± 6.2)/T – (0.162 ± 0.013), determined by mean-field analysis of order-to-disorder transition temperatures (TODT) measured by dynamic mechanical analysis and differential scanning calorimetry. Ordered lamellar domain pitches (9 ≤ D ≤ 33 nm) were identified by small-angle X-ray sc...

Attempts to form the 10-nm-order pitch of self-assembled nanodots using PS-PDMS block copolymer

Block copolymer (BCP) self-assembly exhibits its ability to form various nanostructures with a size down to 3 nm, which is particularly attractive for emerging technologies such as bit patterned media (BPM). Poly(styrene-b-dimethyl siloxane) (PS-PDMS) has been acknowledged as the most promising block copolymer for self-assembly fabrication due to its possibility to form nanopattern with a fine pitch, high etching selectivity, and its characteristic of robustness for pattern transfer. Here, we attempt to form PS-PDMS self-assembled nanodot array with a 10-nm-order pitch in order to satisfy the demand of low cost technique to form ultrahigh density nanodot array for BPM, which is regarded as the next-generation magnetic recording media, and for quantum devices such as the third-generation quantum dot photovoltaic cell and future battery cell. In this study, self-assembled nanodot array on a large area with pitches of 11 nm ( σ = 2.08 nm) and 10 nm ( σ = 1.61 nm) was formed using PS-PDMS with molecular weights of 5600–1300 g/mol (minority block volume fraction f PDMS = 19.8%) and 4700–1200 g/mol (f PDMS = 21.4%), respectively. In experiments, it was shown that some critical parameters, such as the thickness of PS-PDMS film, the treatment of the substrate surface, and annealing condition, played crucial roles in forming 10-nm-order size of self-assembled nanodot array using PS-PDMS. The relationships between pitches of PS-PDMS nanodot array and the product of the total number of segments and the Flory-Huggins segmental interaction parameter (χN) obtained by calculations and experiments will be discussed. Based on the position of PS-PDMS on mean-field phase diagram for BCP melts, the possibility to form nanodot array with a pitch of less than 10 nm is described. This study promises to open way toward the fabrication of more than 7.45 Tbit/in. 2 storage devices and the application of quantum devices.

Substrate interaction effects on order to disorder transition behavior in block copolymer films

AbstractWe present an overview of the recent progress on the phase transition in the block copolymer (BCP) films in terms of the interfacial interactions effects of the substrates and the χ (Flory‐Huggins segmental interaction parameter) effects between the two blocks. For the BCP films thinner than a critical thickness (Lc) above which the transition is independent of film thickness, the order‐to‐disorder transition (ODT) increased or decreased with decreasing film thickness depending on the interfacial interaction types. The rapid and slow changes in the ODT were attributed to the relative magnitude of enthalpic contribution to χ between two blocks. Interestingly, a periodic amplification in the block composition for the BCP films suppressed the compositional fluctuation in the film geometry, resulting in the ODT shifts from the bulk ODTs above Lc. This effect of the BCP films was more illustrated by the ODT shift effects depending on the strength of the preferential interactions on the substrates. © 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013

Fluctuations, Order, and Disorder in Short Diblock Copolymers

The phase behavior for a series of poly(1,4‐isoprene‐b‐DL‐lactide) diblock copolymers characterized by a relatively large Flory–Huggins segment–segment interaction parameter (χ) and low degrees of polymerization (N) over a range of compositions was reported. Ordered‐state morphologies were deduced from small‐angle x‐ray scattering (SAXS) measurements, and χ(T) was determined from order‐disorder transition temperatures (TODT's) associated with the compositionally symmetric specimens and assuming the mean‐field theory, that is, (χN)ODT=10.5. The ODT was determined by differential scanning calorimetry, SAXS, and dynamic mechanical spectroscopy, and shown to be weakly first‐order, with latent heats of transition that vary strongly with composition. We interpret this behavior in terms of the mean and Gauss interfacial curvature of the ordered‐state morphologies and with respect to composition fluctuations in the disordered state. These results offer a fresh strategy for investigating weakly first‐order phase transitions within the Brazovskii universality class. © 2013 American Institute of Chemical EngineersAIChE J, 59: 3502–3513, 2013

Orienting Block Copolymer Microdomains with Block Copolymer Brushes

A simple, rapid, and robust technique for controlling the self-assembly of block copolymers (BCPs) with a large segmental interaction parameter, χ, is described using a surface modified with anchored BCP brushes. End-functionalized poly(styrene-b-ethylene oxide)s (PS-b-PEOs), where the fraction of PS (f(PS)) was varied, end-functionalized neat PS, and end-functionalized neat PEO were end-grafted onto Si substrates modifying the surface with polymer brushes. Thin films of cylinder-forming PS-b-PEO were prepared on modified Si substrates and thermally annealed. When neat PS and PEO were used as the anchored brushes, the microdomains of the PS-b-PEO oriented parallel to the substrate upon thermal annealing due to the preferential interactions of one block to the anchored brushes. However, when end-functionalized PS-b-PEOs were used to modify the substrate, hexagonally packed cylindrical PEO microdomains oriented normal to the substrate, having long-range lateral ordering, were obtained over a very wide range of f(PS) (0.32 to 0.77).

Thin Film Self-Assembly of Poly(trimethylsilylstyrene-b-d,l-lactide) with Sub-10 nm Domains

Integrating block copolymer self-assembly with existing lithography processes to enhance their patterning capability is a promising approach for manufacturing a variety of semiconductor devices and next-generation magnetic storage media. Sub-10 nm block copolymer domains are specifically targeted in many of these applications, yet there are relatively few block copolymers that can achieve these dimensions. Here the synthesis and self-assembly characteristics of a new block copolymer poly(trimethylsilylstyrene-b-d,l-lactide) (PTMSS-b-PLA) capable of forming domains as small as ∼5 nm are described. Several lamellar and cylinder forming diblocks were synthesized with bulk domain periodicities of 12–15 nm which are among the smallest domains yet reported for any neat block copolymer. Such small domains are possible because this new material has a large segment–segment interaction parameter which is an order of magnitude higher than poly(styrene-b-methyl methacrylate) (PS-b-PMMA) and twice as large as poly(styrene-b-dimethylsiloxane) (PS-b-PDMS), two commonly studied polymers for these applications. Furthermore, the PTMSS-b-PLA blocks have glass transitions well above room temperature with a large reactive ion etch rate contrast between them (∼28) which is at least 4 times greater than PS-b-PMMA due to incorporation of a trimethylsilyl group into the styrene monomer.

Synthesis, Thermodynamics, and Dynamics of Poly(4-tert-butylstyrene-b-methyl methacrylate)

A series of symmetric poly(4-tert-butylstyrene-block-methyl methacrylate) (PtBS-b-PMMA) diblock copolymers with varying molar masses and narrow molar mass distributions were prepared using sequential anionic polymerization. Order-to-disorder transition (ODT) temperatures were determined as a function of the overall degree of polymerization, N, using a combination of low-frequency dynamic mechanical spectroscopy (DMS) and variable temperature small-angle X-ray scattering (SAXS), leading to a mean-field expression for the segment–segment interaction parameter, χ = (41.2 ± 0.9)/T – (0.044 ± 0.002). This material is characterized by a larger value of χ, and much greater temperature sensitivity, than polystyrene-b-PMMA, providing access to tunable lamellar periods (pitch) down to 14 nm at technologically relevant temperatures. Nucleation and growth of lamellae, following cooling from the disordered state, was characterized by SAXS well above the glass transition temperature (Tg ≈ 130 °C) and extrapolated to lower processing temperatures based on polymer chain relaxation times extracted from time–temperature superposed DMS data. These results qualify PtBS-b-PMMA as an attractive candidate for development as a new lithographic material.

Decoupling Bulk Thermodynamics and Wetting Characteristics of Block Copolymer Thin Films.

The consequences on certain physical properties of controlled levels of epoxidation of the poly(isoprene) blocks in poly(styrene-b-isoprene) (PS-PI) diblock copolymers and poly(isoprene) (hPI) homopolymers have been studied, where the products after epoxidation are denoted PS-PIxn and hPIxn, respectively. The effective interaction parameters χeff between the PS and the PIxn blocks were estimated by applying mean-field theory to the lamellar periodicities identified by small-angle X-ray scattering and to the order-to-disorder transition temperatures determined by dynamic mechanical spectroscopy. These results were fit to a binary segment-segment interaction parameter model indicating a nonlinear change in χeff with percent epoxidation. In contrast, contact angle measurement on hPIxn and lamellar orientations of thin-film PS-PIxn suggest that the surface energy of PIxn increases linearly with epoxidation. This decoupling of bulk and thin-film thermodynamic behaviors is attributed to the different roles that a random copolymer architecture plays in establishing three-dimensional order versus wetting at a two-dimensional surface.

Localization of spherical nanoparticles within lamellar AB diblock copolymer melts through self-consistent field theory

Here we report the results of a three dimensional hybrid self-consistent field theoretic (HSCFT) model describing the equilibrium particle distribution of spherical nanoparticles within symmetric AB diblock copolymer melts. Holding the polymer composition and morphology fixed, we consider a comprehensive parameter space comprised of the Flory interaction parameter describing interactions between B segments and the particle surface compared to the segment–segment interaction parameter ( χ B P / χ A B ) , the particle volume fraction ( ɸ P ), and the ratio of the particle diameter to block copolymer domain spacing ( d P / d A B ) . Analysis of the free energy over this parameter space yields phase diagrams showing the conditions under which particles segregate to the intermaterial diving surface (IMDS) or the center of the domain. Interestingly, w e predict a particle concentration-dependent “reentrant” phase transition in which particles move from the domain interior, to the IMDS, and back as ɸ P increases. These results are interpreted as a subtle consequence of the competition between enthalpic polymer–particle interactions and the chain packing frustration imposed by the particulate inclusion. These results are consistent with recent experiments on block copolymer nanocomposites.