Tailoring the Attraction of Polymers toward Surfaces | Macromolecules

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Cite This: Macromolecules 2019, 52, 4787−4802

Tailoring the Attraction of Polymers toward Surfaces Gila E. Stein,*,† Travis S. Laws,† and Rafael Verduzco*,‡,§ †

Department of Chemical and Biomolecular Engineering, University of Tennessee, Knoxville, Tennessee 37996, United States Department of Chemical and Biomolecular Engineering and §Materials Science and NanoEngineering, Rice University, Houston, Texas 77005, United States

Downloaded via 5.189.202.129 on July 18, 2019 at 08:16:53 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

ABSTRACT: In polymer blends and block copolymers, one constituent (or segment type) is often enriched at the surface. This enrichment has important consequences for a variety of surface functions, including wettability, adhesive interactions, and fouling resistance, and can also influence the structure that forms deeper into the bulk. Herein, we review the thermodynamic principles that control the attraction of polymers toward surfaces, emphasizing cases where entropic effects associated with molecular weight or architecture can compete with enthalpic preferences. While models and simulations have guided our understanding of this interplay, we show that it remains difficult to anticipate the outcomes when using chemically complex materials or nonequilibrium processing conditions. Nevertheless, it is possible to leverage established principles to tailor the wetting of polymers at surfaces, which is important for the design of membranes, coatings, lithographic materials, and thin film electronics.

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Table 1. Surface Energies of Linear Homopolymers at 20 °C

INTRODUCTION The surface composition of polymeric materials, including polydisperse homopolymers, polymer blends, and block copolymers, is usually different from the bulk composition. This effect is controlled by a combination of enthalpic and entropic driving forces, and it can be leveraged to engineer desired properties or functions at a surface. For example, surface-active polymer additives are used to control interactions with a surrounding medium,1−5 thereby decoupling attributes such as adhesive strength and wettability from the bulk polymer properties. Furthermore, a balance of enthalpic interactions and entropic effects will control the orientation of block copolymer domains near surfaces,6−9 and this knowledge can be used to design new materials for nanoscale patterning and thin film electronics. A starting point for predicting the surface composition is measuring the surface energy or surface tension of each constituent. The surface energy refers to the free energy per unit area of creating an interface with air or vacuum, and the surface tension is the force per unit length along the surface opposing the creation of an interface with air or vacuum. These quantities are directly related through the concept of virtual work needed to create an interface,10 and the terms are often used interchangeably. Surface energies can be estimated at ambient temperature by measuring the contact angle of probe liquids on the polymer surface.11−13 Melt surface tensions are measured by the Wilhemy technique14,15 or analysis of pendant drop profiles,16 and these data reflect both enthalpic and entropic effects. Enthalpic contributions arise due to energetically unfavorable interactions between the materials on either side of the interface. For example, the interaction energy of a polymer in contact with air will generally increase with its polarity (Table 1),17 although differences in chain packing or © 2019 American Chemical Society

polymer poly(tetrafluoroethylene) poly(3-dodecylthiophene) poly(3-hexylthiophene) poly(dimethylsiloxane) poly(2-(2-ethoxy)ethoxyethyl vinyl ether) poly(1,2-butadiene) poly(vinylidene fluoride) poly(vinyl methyl ether) cis-Polyisoprene polyisobutylene polyethylene, low density polyethylene, high density poly(vinyl fluoride) poly(N-isopropylacrylamide) poly(d8-styrene) polystyrene poly(methyl methacrylate) poly(lactic acid) poly(ethylene oxide) polyacrylonitrile poly(urea urethane) (TDI based) poly(2-vinylpyridine) poly(acrylic acid), 0.01 M base

abbrev

γ (mJ/m2) 13

Tg (°C)

PTFE P3DDT P3HT PDMS PEOEOVE

19.1 19.837 2137 22.813 23.240

11736 −4337 1038 −12539 −6040

PB PVDF PVME PI PIB LDPE HDPE PVF PNIPAAM dPS PS PMMA PLA PEO PAN PUU P2VP PAA

2541 30.342 30.436 3241 33.642 35.313 35.742 36.713 38.944 40.630 40.746 41.146 4247 42.942 43.348 43.450 5051 ca.7053

−2839 −3843 −2836 −6339 −7039 −13039 −12039 6439 14045 10039 10039 10539 6039 −6739 9749 15250 10052 10339

Received: March 11, 2019 Revised: June 6, 2019 Published: June 24, 2019 4787

DOI: 10.1021/acs.macromol.9b00492 Macromolecules 2019, 52, 4787−4802

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Figure 1. (A) Schematic and molecular structure for brush polymers with fluorinated end blocks resulting in an end-on orientation on a substrate. Reproduced with permission from ref 21. Copyright 2015 Wiley. (B) Schematic for the end-on orientation of a semiconductive polymer modified with a terminal perfluoroundecyl group. Reproduced with permission from ref 22.

flexibility can lead to deviations from this trend. Entropic contributions to the surface energy are controlled by the molecular weight, stiffness, and architecture of the polymer and arise from a loss of configurational entropy near an interface.18,19 As an example, the effects of molecular weight alone can account for 5−10% variation in the melt surface tension of linear polymers.20 The enthalpic interactions that control surface attraction are often intuitively understood and much stronger than any entropic effects. As an example, “low-energy” chain segments based on fluorinated chemistries are strongly attracted to an air surface (Table 1). These moieties can enable the design of surface-active polymer additives that resist fouling in marine environments,23,24 provide “neutral” interactions for block copolymer lithography,2−4 and produce barrier layers for immersion lithography.5 Precisely positioned fluoroalkyl moieties can also induce an end-on chain orientation in polymer semiconductors25 and bottlebrush polymers,21 as illustrated in Figure 1, and drag “high-energy” chain segments toward surfaces.22,26 However, in blends of polymers with similar surface energies and/or complex architectures, the segregation of polymers toward surfaces is strongly influenced by entropic effects. As an example, in polydisperse homopolymers, the entropic attraction of chain ends toward surfaces can lead to enrichment of end segments27 and shorter chains28,29 at the interface with air. This chain-end attraction can compete with weak enthalpic preferences at a surface, such as those encountered in isotopic blends.30,31 Therefore, the observed surface composition reflects a delicate balance between enthalpic and entropic driving forces, and this interplay can be revealed by carefully designed experiments that are interpreted with the aid of analytical models or simulations. In systems with complex polymer architectures, the range of conditions where entropic effects will influence surface composition is much broader than in linear polymer blends. For example, highly branched polymer architectures

can lead to conditions where a low-energy surface is wet by the polymer with the highest cohesive energy density32 as well as cases where a high-energy surface is wet by the polymer with the lowest cohesive energy density.9 The limits of this wettingreversal behavior, which was first predicted 20 years ago,33 could be tested by using modern synthetic protocols for branched materials that are tolerant of a variety of monomer chemistries.34,35 This Perspective describes approaches to tailor the attraction of polymers toward surfaces, emphasizing those that leverage entropic effects to reverse or amplify the wetting behavior at an air surface. The reversal behavior is somewhat underdeveloped but could provide a route to drive polymers with polar or hydrogen-bonding moieties toward an air surface. We discuss applications that benefit from consideration of these entropic effects, including antifouling and fouling-release surfaces, polymer-based thin film electronics, block copolymer lithography, and additive manufacturing. Finally, we highlight challenges and opportunities that relate to predicting surface enrichment in materials with complex chemistries, rigid chains, and/or high dispersities as well as questions regarding the role of nonequilibrium processing on structure formation.

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BLENDS OF LINEAR HOMOPOLYMERS In blends of linear homopolymers, the surface is often enriched by one type of polymer. This behavior originates from a combination of enthalpic and entropic driving forces. When considering energetics alone, the polymer with the lowest cohesive energy density is attracted to a low-energy surface (i.e., air or vacuum).17 The cohesive energy density is set by the choice of monomer chemistry, so the strength of this attraction is highly system-specific. When considering entropic factors alone, and assuming all polymers have linear architectures and comparable Kuhn lengths, the polymer with the lowest molecular weight is enriched at the surface. This effect is driven by an entropic attraction of chain ends to 4788


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Macromolecules surfaces,18,28,54,55 the same phenomenon that leads to a reduction in surface tension with decreasing molecular weight.14,55 The first study of entropy-controlled surface attraction in polymer blends used a mean-field lattice model to examine chemically identical homopolymers (i.e., athermal system) with a bimodal distribution of molecular weights.54 In this analysis, the long chains suffer a greater entropy loss at a surface than the short chains, so the short chains are enriched at the surface. Similar behavior is predicted by the analytical linear response theory, which assigns an attractive surface potential to chain ends and a repulsive surface potential to “joints”,18,56 and also by recent calculations based on an offlattice implementation of the self-consistent-field theory.19,28 These modeling approaches are different in many aspects, including their ability to describe melt compressibility and the discreteness of the polymer chain, so they do not predict identical density profiles near a surface. However, the key trends predicted by these models are qualitatively consistent, such as greater surface enrichment by the short chains with increasing molecular weight of the long chains. It is difficult to test the predictions for athermal systems through experiments, as most depth-profiling techniques are sensitive to chemistry rather than molecular weight. Therefore, one of the polymers in a blend is usually “tagged” with a chemical label that allows for its detection, and these labels will introduce enthalpic interactions that compete with the subtle entropic effects. However, a new experimental technique called surface layer matrix-assisted laser desorption ionization timeof-flight mass spectrometry (SL-MALDI-TOF-MS) can resolve the distribution of molecular weights at a surface and in bulk, enabling a “label-free” measurement of surface enrichment by short chains (Figure 2).29,57,58 SL-MALDI-TOF-MS is usually

introduce enthalpic interactions that impact both bulk miscibility and surface attraction,30,66−69 so it is challenging to distinguish between entropic and enthalpic driving forces. However, experiments using blends of miscible polymers can offer insight into these behaviors, particularly when the average molecular weights of each constituent are systematically varied. As an example, thin film blends of polystyrene (PS) and deuterated polystyrene (dPS) have served as a model system for numerous studies of surface attraction. PS and dPS have a weak enthalpic incompatibility (χ ∼ 10−4)66 but are miscible over a broad range of molecular weights (≲106 g/mol), so experiments can be performed without the added complication of macroscale phase separation. The surface energy of dPS is slightly lower than that of PS, so dPS is enthalpically preferred at the surface of a film.30 When the dPS chains are shorter than or similar in length to the PS chains, both enthalpic and entropic effects will favor surface enrichment by dPS,31,60,70 as shown in Figure 3. On the other hand, when the PS chains are

Figure 3. Dynamic SIMS measurements of the concentration of dPS in thin film blends of dPS and PS, where dPS and PS have similar molecular weights. Reproduced with permission from ref 60. Copyright 1993 AIP.

much shorter than dPS chains, the surface is enriched with the higher energy PS.31,61 Therefore, in blends of linear dPS and linear PS, the balance between weak enthalpic and entropic driving forces can lead to surface enrichment by either polymer.55,71,72 It is important to note that the entropic attraction of short chains toward surfaces cannot balance a strong enthalpic preference for long chains. For example, in blends of PS and poly(vinyl methyl ether) (PVME), the surface is always enriched by the lower energy PVME (Table 1), even when the PVME chain length is 20 times longer than that of PS.73 End-group chemistry is another attribute that can enhance or suppress the attraction of chain ends to a surface.1 When PS is end-functionalized with low-energy fluoroalkanes, both entropic and enthalpic effects will favor the segregation of chain ends to the surface.62,63,74,75 Under certain conditions, this strong chain-end attraction can be used to drive a higher energy homopolymer toward the surface of a blend. The enthalpic chain-end attraction can be differentiated from entropic effects by examining blends of miscible linear homopolymers with similar average chain lengths (i.e., symmetric) but different end-group chemistries. As an example, in symmetric blends of dPS and fluoroalkyl-

Figure 2. Experimental distribution of PS chain lengths (N) at the surface and in the bulk, measured by SL-MALDI-TOF-MS, compared with SCFT prediction for the surface based on the bulk measurement. Reproduced with permission from ref 29.

restricted to fairly low polymer molecular weights, so it cannot be used to characterize every blend system of interest. Nevertheless, the quantitative feedback from SL-MALDITOF-MS can be used to validate SCFT simulations28 or predict surface properties that are sensitive to molecular weight, such as the glass transition temperature.59 For other depth-profiling techniques, including neutron reflectivity (NR), 26,59−63 dynamic secondary ion mass spectrometry (DSIMS),60 time-of-flight secondary ion mass spectrometry (TOF-SIMS),59,63 forward recoil spectroscopy,30 surface-enhanced Raman scattering,31 X-ray photoelectron spectroscopy,26,63 and nuclear-reaction analysis,64,65 chemical “tags” provide the signal for the measurement. These tags 4789


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repulsion of a single joint and can lead to a reduction in surface tension on the order of 5 mJ/m2.15 It follows that branched polymer additives can tune the surface tension79 and surface composition33,80 of a linear polymer melt. Here, we focus on methods to tailor surface composition and related functions by leveraging the entropic attraction of branched polymers to surfaces. For athermal blends of linear and branched polymers, the linear response theory80 and SCFT lattice simulations33 both predict a broad window of conditions (i.e., architectures and molecular weights) where branched polymers are strongly segregated at a surface. When comb polymer additives are smaller than the linear polymer host, their surface excess increases with the number and density of branches on the comb architecture.33,80 Similarly, when star polymer additives are smaller than the linear polymer host, their surface excess increases when the star architecture has a large number of short arms.80 However, in cases where the linear polymer host is smaller than the branched additive, the branched additive can be depleted from the surface. This effect is consistent with the relatively higher concentration of chain ends on a short linear chain compared with a large star or comb polymer. As noted in the previous section, most measurement techniques are sensitive to chemistry rather than polymer architecture or molecular weight, so it is difficult to test predictions for surface enrichment in athermal blends. For example, in NR measurements of star dPS and linear PS, both enthalpic and entropic effects favor the accumulation of the star polymer at a surface.80,81 If the isotopic label is applied to the linear polymer instead, then a weak enthalpic preference for the linear polymer at a surface could overcome the entropic preference for the branched polymer, as demonstrated in models and simulations.33,71 Furthermore, the end-group and joint chemistries introduce additional enthalpic interactions with a surface that are difficult to predict a priori. Therefore, a concerted experimental and modeling effort is critical to the successful design of surface-active branched polymer additives. Anionic polymerization can produce complex polymer architectures with low dispersity in chain lengths,82 offering a route to synthesize model materials for fundamental studies of surface attraction in polymer blends. As an example, anionic polymerization was used to prepare a series of branched PS architectures where the numbers of chain ends and joints were independently varied.83,84 In blends with linear dPS of comparable molecular weight, the branched architectures were always enriched at the surface. The surface potentials for chain ends and joints were calculated by comparing the measured surface excesses with predictions based on the linear response theory. The outcomes demonstrate that the attraction of chain ends to a surface is much stronger than attraction or repulsion of joints. As a result, the surface excess increases with the number of chain ends but is far less sensitive to changes in the number of joints. Interestingly, the analysis suggested that hydrogen-terminated PS chains (i.e., styrenic end unit) were more strongly attracted to the surface than butyl-terminated chains, even though alkanes have lower cohesive energy densities than aromatics. The analysis also suggested that joint chemistries (flexible moieties) were weakly attracted to the surface. In our own works, we examined surface segregation in thin films of bottlebrush and linear PS.85,86 The bottlebrush polymers were synthesized by a grafting-through ring-opening metathesis polymerization (ROMP) of norbornenyl-termi-

terminated PS, the surface is enriched by the end-functional PS,63 even though dPS has a slightly lower surface energy than PS. The amount of the surface enrichment is reduced as the average chain length increases, demonstrating that surface attraction of fluoroalkyl-terminated PS is partly controlled by the concentration of chain ends. In symmetric blends of PVME and fluoroalkyl-terminated dPS, the enthalpic preference for low-energy PVME at the surface is suppressed as the average chain lengths are reduced,26 leading to a condition where PVME and fluoroalkyl-terminated PS are equally preferred at the surface. Furthermore, in blends where the fluoroalkylterminated dPS chains are much shorter than the PVME chains, fluoroalkyl-terminated dPS is enriched at the surface.26 PS can also be functionalized with high-energy carboxyl or hydroxyl end groups,58,62,63,76 introducing enthalpic interactions that compete with the entropic attraction of chain ends. In symmetric blends of carboxyl-terminated PS and poly(methyl methacrylate) (PMMA), where both polymers have ≈100 repeat units, the surface is enriched by the (slightly) higher energy PMMA.76 The carboxyl-terminated PS can undergo interchain association, effectively doubling its molecular weight, so it is unclear whether PMMA is driven toward the surface because of end-group repulsion on PS, asymmetry in the lengths of PS and PMMA chains, or a combination of both effects.63,76 A recent study used SLMALDI-TOF-MS to examine symmetric blends of hydroxylterminated PS and PS, where both polymers had short chains of ≈60 repeat units and showed that the surface layer was fully depleted of the end-functional chains.58 The entropic effects associated with rubber elasticity can also compete with enthalpic interactions at a surface, leading to dynamic changes in the surface composition of polymer networks.77 While this effect is distinct from those associated with molecular weight or end-group chemistry, it is relevant to understanding the surface properties in plasma-treated or chemically oxidized polyolefins. As an example, in surfaceoxidized poly(1,2-butadiene) (PB) networks, the surface becomes hydrophobic when heated in contact with water, which means the hydroxyl and carboxyl moieties are migrating away from the interface with water. This counterintuitive behavior is attributed to an elastic restoring force that opposes the enthalpic preference for solvation of the oxidized segments.78

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BLENDS OF LINEAR HOMOPOLYMERS AND BRANCHED ADDITIVES The entropic effects that contribute to surface attraction can be enhanced with changes to polymer architecture. As an example, experiments have shown that the melt surface tension of star PS will decrease with an increasing number of arms.15,55 In the limit of high molecular weights, these measurements are consistent with predictions based on the linear response theory:55 γ ≃ γ∞ + ρb RT (neU e + n jU j)/M w

Here, Mw is molecular weight, ρb is bulk density, R is the gas constant, T is temperature, ne and nj are the number of chain ends and joints, respectively, and Ue and Uj are the corresponding surface potentials for chain ends and joints. The surface potentials of chain ends and joints are usually described as attractive and repulsive, respectively. In a star polymer, the attraction of ne chain ends will dominate over the 4790


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Macromolecules nated macromonomers, yielding a high density of side chains but a broad distribution of molecular weights. We used DSIMS and TOF-SIMS to map the combinations of architectural parameters that produce an enrichment of the bottlebrush additive at the surface. The parameter space included the average number of backbone units (Nb) and average side-chain length (Nsc) of the bottlebrush polymer as well as the average chain length (Nm) of the linear host. The signal for these measurements was derived from deuteration of either the linear or bottlebrush architecture, so there was a weak enthalpic preference for one constituent at the surface. Under most conditions, the enthalpic interactions were secondary to entropic effects: When Nm/Nsc ∼ 1, the small linear polymer host had approximately double the concentration of chain ends compared with a large bottlebrush polymer additive, so the bottlebrush was depleted from the surface. When Nm/Nsc and Nb/Nsc were both large, the bottlebrush had a higher concentration of chain ends than the linear polymer host, and the bulk entropy of mixing was low. These effects drove enrichment of the bottlebrush at the surface. In addition, we found an intermediate range of conditions where the bottlebrush and linear polymers were equally attracted to the surface. The equal attraction was observed when the bottlebrush polymers had partially deuterated PS side chains, so one might expect an enthalpic preference for bottlebrush at the surface. However, calculations based on SCFT demonstrated that the condition of equal attraction reflects an enthalpic preference for the linear polymer at the free surface, which is inconsistent with the isotopic effect. This outcome implies that the chemistries of chain ends and/or backbone joints, which were dodecyl trithiocarbonate and norbornenyl moieties, respectively, were contributing to the observed phase behavior. Our interest in bottlebrush polymers stems from early SCFT lattice simulations that examined surface enrichment in blends of chemically distinct comb and linear polymers, where the comb polymer was repelled by the surface.33 Below a critical molecular weight for the comb polymer, the SCFT analysis predicts surface enrichment by the “high energy” additive. This critical molecular weight depends on the strength of the surface repulsion as well as the magnitude of χ. In a related study, the same authors tested this concept using blends of linear PMMA and comb poly(ethylene oxide) (PEO).32 PMMA and PEO are weakly miscible in bulk, and PEO has a slightly higher surface energy than PMMA. After thermal annealing, NR measurements revealed an enrichment of comb PEO at the surface, while control studies with linear PEO additive showed depletion. The properties of the combenriched surface were examined with contact angle measurements and protein adsorption assays, which demonstrated that the additive increased the hydrophilicity and fouling resistance, respectively (Figure 4). To our knowledge, the limits to this wetting-reversal behavior have not been examined. However, the SCFT calculations predict that bulk immiscibility works in concert with entropic factors to drive high-energy comb additives toward a surface. We note that two-dimensional SCFT calculations of surface segregation in confined blends cannot predict all phenomena that may be observed in experiments, such as lateral phase separation and dewetting that is typical in thin films of immiscible polymers.87 Therefore, three-dimensional measurements and simulations are needed to explore this parameter space.

Figure 4. (A) Cell adhesion of PMMA films. (B) Cell adhesion on PMMA films with 20% branched PEO additive. (C) Volume fraction of PEO additive (ϕA) as a function of reduced film thickness (z/L) and additive loading, calculated from NR measurements. Adapted from ref 32.

The chemistry and architecture of bottlebrush polymers are easily manipulated, offering a convenient platform to manipulate the entropic and enthalpic effects that control their attraction to surfaces. As already discussed, the entropic effects are tunable through changes in Nb, Nm, and Nsc.85,86 Furthermore, the grafting-through ROMP reaction is tolerant of a range of macromonomer chemistries, so a variety of bottlebrush random copolymers can be synthesized. This attribute provides a simple route to tune enthalpic interactions in the bulk and at a surface. As an example, we showed that bottlebrush poly(dimethylsiloxane)-ran-poly(lactic acid) (PDMS-r-PLA) additives in linear PLA hosts will spontaneously accumulate at the surface, producing a hydrophobic and low-energy barrier.88 Furthermore, Figure 5 shows surface segregation of bottlebrush PS-r-PMMA from blends with linear PMMA: In these experiments, thin film blends were cast on silicon and analyzed by TOF-SIMS to determine the location of the bottlebrush throughout the film thickness. Surface segregation is observed only when the linear PMMA has a much higher molecular weight than the bottlebrush side chains, which is consistent with an entropy-mediated mechanism.

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APPLICATIONS OF BLENDS Adhesion. Surface segregation in polymer blends can impact many properties and functions that are important to end-use applications. One example is adhesion to external surfaces. In some cases, surface-active additives are used to promote binding of a polymer with a surface, such as acrylatebased copolymers in dental resins.89 In other cases, migration of surface-active components toward the surface can disrupt adhesive interactions. As an example, in pressure-sensitive adhesives, high concentrations of small tackifier additives are used to modulate the bulk glass transition temperature and 4791


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Figure 6. Atomic force microscopy measurement of a thin film with 30 wt % oligomeric isobutylene (IB) in PI. Strong incompatibility drives lateral phase separation in addition to surface enrichment of oligo-IB, which leads to lateral variation in the adhesion strength. Reproduced with permission from ref 91. Copyright 2017 Royal Society of Chemistry.

Figure 5. Time-of-flight secondary ion mass spectroscopy analysis of bottlebrush PS-r-PMMA (4 kg/mol side chains, overall molecular weight of 180 kg/mol) blended with linear PMMA. The films were annealed at 150 °C for 2 days in a nitrogen-filled glovebox. The same bottlebrush polymer was mixed with linear PMMA of two different molecular weights, and segregation is only observed for the highmolecular-weight PMMA. The red and blue colors reflect the measured secondary ion intensities for fragments corresponding to PS and PMMA, respectively. The green color reflects the silicon substrate.

(holes) and negative (electrons) charge carriers are transported across the interface between the active layer and the electrodes. Segregation of donor and acceptor materials toward electron and hole transport layers, respectively, can therefore enhance performance by preventing recombination and other loss processes. Similar effects can improve charge injection in polymer light-emitting diodes.93 The vast majority of studies that examine surface segregation in organic semiconductors have focused on polymer−fullerene blends. We briefly discuss strategies for controlling surface segregation in polymer−fullerene OPV blends because many of the insights and strategies that were developed with these systems are applicable to polymer−polymer semiconductor blends. In polymer−fullerene blends, segregation occurs due to surface energy differences, preferential wetting at an interface with an electrode, repulsive interactions between donor and acceptor, kinetic effects during solution casting, and postdeposition annealing procedures.94 These devices are usually spin-cast and then thermally annealed, although processing conditions have been widely varied to understand the effect on morphology and device performance. A number of studies on blends of poly(3-hexylthiophene) (P3HT) and fullerene derivative phenyl-C61-butyric acid methyl ester (PCBM) have shown that P3HT, which has a lower surface energy than PCBM, segregates to the film−air interface after casting, consistent with the enthalpic preference at the surface.95−100 This processing-induced stratification profile can change after depositing an electrode followed by thermal annealing. In the case of P3HT/PCBM blends, deposition of an aluminum electrode followed by thermal annealing results in an enrichment of the higher energy PCBM near the aluminum surface and improved device performance relative to unannealed films.98 Strategies for driving one component toward an air or electrode surface include end-functionalizing P3HT with high- or low-energy end groups,101 tethering fluorocarbon chains to PCBM,102 irradiating films with light during deposition,103 soaking blend films in mixed solvents,104 and exposing the film to high-pressure carbon dioxide.105 Some of these strategies have also been applied to other polymer−

shear modulus.90 These additives can also segregate near the surface of the adhesive, which leads to undesirable consequences such as contamination of the bonded material and decreased performance.91 Many of the commercial PSAs are random or block copolymers, and the additives are oligomeric rather than polymeric, so the underlying thermodynamics that control surface attraction are distinct from those in miscible polymer blends. These effects were recently studied in polyolefin-based hot melt adhesives with oligomeric tackifiers, where the extent of surface enrichment was largely controlled by bulk miscibility rather than differences in molecular weight (Figure 6).91 Another example where surface segregation can cause problems is plasma-modified PDMS. This material is used in microelectronics fabrication, and its adhesion to metals is controlled by the high-energy polar groups at the surface. However, low-molecular-weight PDMS in the bulk will migrate toward the surface, leading to a “hydrophobic recovery” that impedes bonding.92 Thin Film Electronics. Another example of an application where surface segregation can impact performance is electronic devices based on blends of polymer semiconductors, including organic photovoltaics (OPVs), thin-film transistors (TFTs), and light-emitting diodes (LEDs). In contrast to the linear polymer blends described above, semiconductive polymers are typically rigid, semicrystalline, and immiscible, which presents additional challenges in understanding the factors that control the thin film morphology. There is significant interest in controlling segregation processes in these systems because enrichment of one constituent near the electrode interface can either enhance or suppress charge transport in electronic devices. During the operation of OPVs, for example, positive 4792


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Macromolecules fullerene systems, as described in a recent review on the topic.94 Entropic effects would preferentially drive PCBM to the film interfaces, but the impact of entropy appears to be much weaker than enthalpic effects in these systems100 and has not been studied in detail. Segregation toward air and electrode surfaces has also been observed in OPVs based on semiconductive polymer blends.106 However, rather than encouraging processinginduced stratification or segregation during thermal annealing, strategies for improving the performance of polymer−polymer blend systems have primarily focused on reducing or inhibiting large-scale phase separation by increasing the polymer molecular weight or modifying side chains to improve compatibility.107−110 Processing conditions are also typically chosen to limit large-scale phase separation. For example, highperforming polymer−polymer blend OPV films are often not thermally annealed after deposition,109,111 are aged at room temperature,112 or are thermally annealed for short periods of time to encourage polymer crystallization while limiting largescale phase separation.107 As a result, the final morphology is likely to be dictated primarily by kinetic factors during casting and processing. The role of entropic effects in polymer− polymer blends for OPVs has not been studied in detail, although we note that large differences in polymer molecular weights may be responsible or partially responsible for surface segregation observed in prior studies.106 As discussed in the Challenges and Opportunities section, taking advantage of entropic effects in polymer−polymer blends may offer opportunities for further tailoring the enrichment of one component at a surface and improving device performance. For example, the broad molecular weight distribution of many semiconductive polymers will likely result in the segregation of lower molecular weight polymers to the surface in many of these devices, which may impact performance or inform processing and materials design approaches. In polymeric TFTs, blends of semiconductive and insulating polymers have been shown to have superior stability and performance compared with a pure semiconductor polymer film97 due to crystallization and segregation of the semiconductor to the interface with the dielectric. In one study, blends of P3HT with either high-density polyethylene (HDPE) or atactic PS (a-PS) were used to fabricate bottomgate TFTs. P3HT was the minority component in the blends, so functioning TFT devices could only be produced if P3HT segregated to the top of the blend film. P3HT has a lower surface energy compared with both polymers, but segregation toward the gate electrode was only observed in blends with HDPE. This was attributed to strong crystallization-induced phase separation in P3HT/HDPE since both P3HT and HDPE crystallized during casting, while a-PS is amorphous.97 Other studies showed that segregation toward the surface and gate electrode during deposition of blends of insulating and semiconductive polymers could be leveraged to simultaneously cast channel and dielectric layers.113−118 This approach takes advantage of the immiscibility of the two components and the differences in surface energies to produce phase-separated bilayers. In a study of P3HT/PMMA blends, pure P3HT and PMMA bilayers were formed during deposition of a 20:80 blend of P3HT:PMMA. Segregation of PMMA toward the SiO2 dielectric was attributed to its preferential interaction with the SiO2 substrate113 (Figure 7A). In another example, a blend of a semiconductive polymer poly(3,3‴-didodecylquarterthiophene) (PQT12) and PMMA was cast onto a

Figure 7. (A) Schematic and mobility measurements of TFT devices produced by deposition and segregation of a poly(3-hexylthiophene) (P3HT)/PMMA polymer blend toward surface and dielectric. Reproduced with permission from ref 113. Copyright 2008 Wiley. (B) Volume fraction of deuterium-labeled poly(9,9′-dioctylfluorene) (PFO) in an immiscible blend with poly(9,9′-dioctylfluorene-altbenzothiadiazole) (F8BT) determined by nuclear reaction analysis. Reproduced with permission from ref 65. Copyright 2003 Nature Publishing Group.

surface with a hydrophobically defined TFT array pattern. The PQT12 segregated to the patterned array while the PMMA formed a film that encapsulated the device, resulting in a functional TFT array.118 In blends of a small molecular organic semiconductor with poly(α-methylstyrene) (PαMS), both enthalpic and entropic effects were important in driving segregation toward the air surface and substrate. Blends with high-molecular-weight PαMS (Mn ≈ 400 kg/mol) showed segregation of the semiconductor to the top and bottom surfaces of the film during casting. Blends with low-molecularweight PαMS (Mn ≈ 1.5 kg/mol) showed no segregation after casting, but thermal annealing resulted in crystallization and segregation of the semiconductor toward the air surface.119 The nonpolar semiconductor has an enthalpic preference for the air surface rather than the polar SiO2 substrate. Therefore, in blends with the high-molecular-weight PαMS, segregation of the semiconductor to the bottom interface was attributed to entropic effects.120 Segregation has also been used to improve performance in polymer-blend LEDs (Figure 7B).65,93,121,122 Similar to several examples described above for TFTs, the final blend film is macroscopically phase separated, and segregation toward electrodes is driven primarily by surface energy differences between the polymers. Control of Surface Fouling. A third applications example is the design of surface-active polymeric additives that suppress fouling of surfaces by proteins, oils, and marine organisms or, alternatively, additives that promote adhesion of biomolecules 4793


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polymers are annealed in water, both entropic and enthalpic effects will favor surface enrichment of the comb polymers. This enrichment will enhance surface wettability and improve resistance to protein fouling. A related example, albeit more complex, is hydrophilic branched polymer additives for poly(vinylidene fluoride) (PVDF)-based membranes. These membranes are prepared by non-solvent-induced phase separation (NIPS): the PVDF and comb polymer additive are dissolved in a mutual nonvolatile solvent, spread into a wet film, and then immersed in a water bath to drive precipitation of the polymer. The NIPS process involves large gradients in water concentration that favor an accumulation of hydrophilic additives at the interface with water (Figure 8). This effect has been demonstrated with

to a surface. The use of these additives is particularly valuable for porous scaffolds or membranes, as internal surfaces are difficult to modify with postfabrication chemistry.123 The same underlying principle guides the design of both fouling-resistant and adhesion-promoting polymer surfaces: a cheap commodity polymer is used for the bulk material, and the surface-active polymer incorporates a chemistry that modulates interactions with the surrounding medium. This means that enthalpic interactions will play a role in determining the surface composition, and in many cases, they are much stronger than any entropic effects. However, it is difficult to understand what driving forces are relevant to the surface segregation of fouling-resistant or adhesion-promoting additives without the context of processing. Polymer coatings, membranes, and scaffolds are often thermally annealed in air (or vacuum), so the design of surface-active polymeric additives can draw on the previously described balance of enthalpic and entropic effects. Enthalpic effects are dominant in linear polymer additives, so the surface attraction is modulated with well-chosen comonomers and/or end groups. As an example, in surface-active amphiphilic copolymers, the inclusion of a few fluoroalkyl moieties can drag a high-energy hydrophilic end block to the interface with air.22,124 The surface will reconstruct when placed in water, as it is more favorable to enrich the interface with hydrophilic groups and tuck the fluorinated segments into the film. These materials provide both antifouling and fouling-release properties, so they are promising as additives for thermoplastic elastomer coatings in marine environments. With comb polymer additives, the highly branched architecture can either balance or enhance an enthalpic attraction toward a surface. We already reviewed the “balancing” scenario and noted that PEO comb polymers can spontaneously accumulate at the surface of a PMMA film, even though PEO has a higher surface energy than PMMA, which increases hydrophilicity and reduces protein fouling.32 A related example where architecture and chemistry both contribute to surface attraction is poly(2-(2-ethoxy)ethoxyethyl vinyl ether) (PEOEOVE) comb polymer additives for PMMA.125 PEOEOVE is a hydrophilic polymer that has a lower surface tension than PMMA at ambient temperature, despite its higher polarity. It is speculated that this low surface tension is associated with high segmental mobility.40 Like PEO-based combs, this material enhances the hydrophilicity of a surface and reduces fouling by proteins. Another approach for tailoring interactions at a surface is to change the surrounding environment. This can be accomplished by processing films that contain high-energy, hydrophilic additives in water rather than air. As an example, PEOsilane amphiphiles can be dispersed in a silicone elastomer, but they will migrate into the bulk elastomer when the system is used in air. However, upon exposure to water, the amphiphiles migrate to the elastomer−water interface due to the hydrophilicity of the PEO block.126−130 In another example, PEObased comb polymers were studied as additives for linear PLA. PLA has a lower surface energy than PEO and is therefore enthalpically preferred at an air surface. However, PEO is more hydrophilic than PLA, so both entropic and enthalpic effects will favor surface enrichment of the PEO-based comb additive when annealed in water.131 Furthermore, these comb additives can be conjugated to peptides that promote cell adhesion, which is useful for tissue engineering scaffolds. Similarly, when blends of polyacrylonitrile (PAN) and PEO-based comb

Figure 8. Schematic for a branched polymer additive used to reduce fouling of a membrane surface. Reproduced with permission from ref 132. Copyright 2019 Royal Society of Chemistry.

comb polymer additives based on short oligo(ethylene glycol) side chains133 and zwitterionic side chains132 as well as star polymer additives with 12 PEO arms.134 All of these additives improve wettability, which is important for high permeance, and they also resist fouling by proteins. To better address the multiple desired surface functions, branched additives can be synthesized with multiple functional chemistries. For example, comb polymers that have a mixture of hydrophobic (fluorinated) and hydrophilic side chains can combine fouling release and fouling resistance in a single platform.135 These additives will segregate on the membrane surface during NIPS, which suggests that the preference for hydrophilic arms at the polymer/water interface is strong enough to drag the hydrophobic arms along.

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BLOCK COPOLYMER FILMS The preceding sections described methods to control the surface composition in polymer blends. Here, we discuss approaches that leverage entropic effects to control interactions of block copolymers with surfaces. We note that several reviews on block copolymer lithography136−138 provide a comprehensive discussion on ways to engineer the enthalpic interactions at thin film boundaries. In thin films of cylindrical and lamellar block copolymers, domain orientations throughout the film thickness are partly controlled by interactions with the surface and substrate. For example, the block with the lowest cohesive energy density is usually favored at the air surface, producing a wetting layer at the top of the film. Similarly, one block will typically have a stronger interaction with the substrate, leading to selective adsorption and the formation of a brush-like wetting layer at the bottom interface. Wetting layers will induce layering of domains normal to the surface, which is problematic for applications that require a perpendicular orientation of the 4794


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of each arm to form wetting layers at the film boundaries. In thin films of (PS-b-PDMS)3 and (PS-b-PDMS)4, where the PDMS blocks are anchored to the core, lamellar and cylindrical domains will orient normal to the substrate but adopt a parallel orientation near the air surface (Figure 9).9 This material also

domains relative to the surfaces, such as block copolymer lithography.136−139 However, as with blends of linear or architecturally complex homopolymers, entropic effects can compete with preferential enthalpic interactions. This competition leads to unusual behaviors, such as selective wetting at the polymer/air interface when the blocks have equal surface energies, and conditions where blocks with distinct surface energies are both attracted to the surface. The case of selective wetting at an energetically neutral surface has been studied through NR experiments and dissipative particle dynamics simulations.8,140 In these examples, AB diblock copolymers that form lamellar phases were confined to thin films, and the film thicknesses were commensurate with the equilibrium periodicity of the lamellar domains. In experiments, each block was a saturated hydrocarbon, so the melt surface energies were nearly identical, but the A block had a smaller statistical segment length than the B block (i.e., conformationally asymmetric). These studies showed that the more flexible A block will wet the air surface,8 even when the A block has a slightly higher melt surface energy than the B block, which drives a parallel orientation of the lamellar domains at this boundary. Recent simulations demonstrate that this outcome may reflect a competition between enthalpic interactions and entropic effects: A perpendicular domain orientation is enthalpically favored at the air surface because it is more favorable to place the block junctions near a repulsive boundary than in the interior of a film.141 On the other hand, entropic effects favor chains that are oriented normal to the surface, with the more flexible block near the boundary.140 Tailoring the block copolymer architecture can lead to situations where two blocks with distinct surface energies are both driven to an air surface. This concept was first investigated in ABA triblock copolymers,6,7,51 where the A end-blocks had higher melt surface tensions than the B midblock. SCFT calculations show that entropic effects favor the placement of A end-blocks near the air surface, as chain ends lose less entropy near a surface when compared with the midsegments,7 while the difference in melt surface energies (γA − γB > 0) favors wetting by the B mid-blocks.6 In experiments when γA − γB is less than ≈4 mJ/m2,6 lamellar and cylindrical ABA domains can orient normal to the surface. This demonstrates that entropic effects can offset the energy gain from placing A blocks at the surface. However, as the difference in melt surface tension is increased, the low-energy mid-block will form a looped wetting layer at the air surface,6,51,142 which drives layering of domains parallel to the surface. Large differences in melt surface energy are typical of popular “high-χ” block copolymer chemistries for lithographic applications, such as PS and poly(2-vinylpyridine) (P2VP)51 or PS and silicon-containing blocks,9,142 so an ABA architecture cannot stabilize a perpendicular domain orientation in such systems. Star block copolymers with the general structure (AB)n, having n arms of an AB diblock copolymer, also exhibit unusual interactions with surfaces. In thin films of (PS-bPMMA)18 star block copolymers, where the PMMA blocks are tethered to the core, lamellar and cylindrical domains will adopt a perpendicular orientation near both the air surface and silicon substrate.143 PS has a slightly lower melt surface energy that PMMA, while PMMA is energetically preferred over PS at the native oxide on silicon, so the perpendicular orientation reflects the high entropic cost of perturbing the conformations

Figure 9. In (PS-b-PDMS)n block copolymers, entropic effects associated with architecture can suppress the formation of a PS wetting layer at the high-energy substrate but cannot prevent the formation of a PDMS wetting layer at the low-energy air surface. Adapted with permission from ref 9.

has asymmetric interactions with the boundaries: PDMS has a much lower melt surface energy than PS, and PS is energetically preferred over PDMS at the silicon substrate. SCFT calculations for stars with 2−4 arms show qualitative agreement with these experiments and confirm that entropic effects associated with the complex topology will inhibit the formation of wetting layers by either block. However, when the energy gain from placing the “wrong block” at a surface is large,9 such as PS instead of PDMS at an air surface, the star architecture will deform to produce a wetting layer at the boundary. Similar effects can be used to control domain orientations in block copolymers of PS and poly(3-dodecylthiophene) (P3DDT). P3DDT is polymer semiconductor that is widely studied for applications in thin film electronics. P3DDT has much lower surface energy than PS, which is a consequence of the long alkyl substituents. Therefore, in thin films of lamellar PS-b-P3DDT, the domains will orient parallel to both boundaries to minimize the free energy at the polymer/air and polymer/substrate interfaces. Star block copolymers of lamellar (P3DDT-PS)18, where the PS blocks are anchored to the core, can eliminate the wetting layers at both boundaries and produce vertical domains throughout the thickness of a film.144 This outcome is consistent with the behaviors of previously described (AB)n materials, suggesting that similar entropic effects play a role. As an alternative to changing the block copolymer architecture, polymer additives can be used to tailor interactions with a surface. These studies aim to stabilize a perpendicular orientation of lamellar or cylindrical domains with respect to the air surface, which is accomplished by driving an energetically neutral polymer to the surface. The design of these surface-active neutral polymers can leverage enthalpic interactions or entropic effects. For example, neutral polymer additives with a low-energy “anchor” will spontaneously accumulate at the air surface during casting and/or annealing steps. This has been demonstrated in thin films of PS-b-PMMA with fluoroalkyl-terminated P(S-co-MMA) additive2 and in thin films of PS-b-P2VP with poly(hexafluoro4795


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Macromolecules isopropyl methacrylate)-b-PMMA additive.3 Alternatively, highly branched polymer additives will spontaneously accumulate at the surface of a block copolymer film through an entropy-mediated process. This effect has been demonstrated in blends of PS-b-PMMA with star polymer additives having neutral P(S-co-MMA) arms (Figure 10):145,146 With an

understood yet critical for optimizing the active layers in thin film electronics based on polymer blends. Finally, many engineering applications use rapid solution- or melt-based processing. The fields that develop during these protocols can strongly influence the surface composition, so it is unclear if the thermodynamic principles that typically guide the design of surface-active polymers are relevant. We elaborate on each of these points in the following paragraphs. Predictive Materials Design. When complex polymer architectures are used, the chemistry of chain ends and joints can have a surprisingly large impact on bulk thermodynamics, surface attraction, and the ultimate surface properties. As an example, in thermoresponsive poly(N-isopropylacrylamide) (PNIPAAM) bottlebrush polymers, a hydrophobic end group on each PNIPAAM chain will reduce solubility in water and depress the lower critical solution temperature (LCST).147 In this case, the hydrophobic end group was a fragment of the chain transfer agent (CTA) rather than a deliberate choice, and conversion to a terminal thiol largely eliminated any effect on solution thermodynamics. However, the challenge with postsynthesis modifications of end group (or joint) chemistry is that the observed impacts on bulk thermodynamics and surface interactions are frequently inconsistent with expectations.84,86 For example, we measured water contact angles on thin films of bottlebrush PS as a function of end group chemistry.148 When the PS side chains were terminated with dodecyl trithiocarbonate, i.e., a fragment of the CTA, the water contact angle was 91.1 ± 0.8°. When the CTA was converted to an 8-mer oligo(ethylene glycol), the water contact angle was increased to 96.3 ± 1.5°. This demonstrates that a hydrophilic oligomeric end group can apparently increase the hydrophobicity of bottlebrush PS. Additionally, in hyperbranched poly(urea urethane)s, a phenyl end group produces a lower surface energy than an n-butyl end group,50 even though an aromatic has a higher cohesive energy density than an alkane. This effect is similar to the previously discussed blends of linear and branched polystyrenes, where analysis of experimental data using the linear response theory suggested that styrene-terminated chains are more strongly attracted to surfaces than butyl-terminated chains.84 These examples show that certain outcomes are difficult to predict from data for the individual chemical units, such as tabulated cohesive energy densities, solubility parameters, or surface tensions, which may lead to iterative material syntheses. Blends of Polymer Semiconductors. While segregation toward electrodes or the air surface has been studied in a number of organic semiconductor blends, as described previously, entropic effects that may contribute to these processes have been largely overlooked. Semiconductive polymers are commonly synthesized using condensation polymerization chemistries that produce polymers with broad molecular weight dispersities, including those used in highperformance OPV and TFT devices. Furthermore, the molecular weights of semiconductive polymers in blends are oftentimes widely mismatched.106 Prior studies have examined segregation toward electrodes in blends of polymer semiconductors, but such works have not considered the role of molecular weight on these composition profiles. In many cases, the semiconductive polymers used in blends are designed in a way that minimizes enthalpic repulsion, as improved compatibility will limit large-scale phase separation.108,109 As a result, the entropic effects due to molecular weight dispersity of a constituent, and mismatch among constituents, should

Figure 10. Star polymer additives induce a perpendicular orientation of lamellar PS-b-PMMA domains. (A) Top-down scanning electron microscopy. (B) Scattering length density profile of star polymer additives from NR, which shows enrichment of the additive at the top and bottom of the film. Scale bar = 500 nm. Adapted with permission from ref 145.

increasing number of arms, the bulk miscibility declines and surface attraction increases, and both of these effects work to drive the star additives to the surface. Bulk miscibility is partly controlled by excluded volume effects, as the entropic penalty for solubilizing an additive increases with its effective size, while surface attraction reflects the entropic preference for highly branched additives at surfaces. Most of the examples we reviewed in this section were motivated by applications in semiconductor lithography, as stabilizing the perpendicular domain orientation is critical for successful pattern transfer. It is important to note that “neutral coatings” and/or solvent annealing can be used to tailor enthalpic interactions at the top and bottom of the film, and these approaches are far more prevalent in the lithography literature than those based on entropic effects.136−139 However, there are other applications that may benefit from leveraging entropic effects to control the interactions of block copolymers with surfaces. For example, in thin film electronics based on block copolymer semiconductors, wetting layers at an electrode can facilitate or block charge transfer processes. Additionally, the surface composition of a block copolymer film will control wettability and fouling resistance, both of which are important in the design of membranes for water purification.

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CHALLENGES AND OPPORTUNITIES A combination of experiments, models, and simulations has provided a foundation to tailor the attraction of polymers toward surfaces. Recent progress in this field has benefited from advances in depth-profiling techniques, new synthetic approaches to achieve sophisticated polymer architectures, and rapid improvements in computational power. However, it remains challenging to predict the outcomes when the materials are “chemically complex”. Such complexity is often a characteristic of the synthetic protocol, such as backbones, joints, or chain ends which are chemically distinct from the chosen monomers but can also be a deliberate choice. Furthermore, the roles of dispersity, stiffness, and crystallinity/liquid crystallinity on surface attraction are poorly 4796


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Macromolecules play a role in surface segregation. Understanding these effects and whether they are beneficial or detrimental to device performance would help in the further development of organic electronic devices. Recent work has also demonstrated that through incorporation of random side chains in conjugated polymers the surface energies of semiconductive polymers can be decoupled from electronic properties.149−152 To date, this has primarily been applied to examine organic alloying and open-circuit voltage changes in bulk heterojunction OPV blends, but this strategy could also be used to tune segregation toward electrodes in a variety of organic electronic devices. Semiconductive polymer blends also present a number of challenges that complicate the quantitative analysis of segregation using existing theories. Semiconductive polymers are rigid or semiflexible, and this influences the chain conformational entropy near an interface.153,154 Most semiconductive polymers are semicrystalline or exhibit strong interchain interactions, which can lead to phase separation and segregation of semiconductive polymers to the surface of cast films.97 Finally, the effects of processing conditions on thin film structure cannot be neglected: Organic electronic devices, especially blends of polymeric semiconductors, are typically processed under conditions that limit large-scale phase separation and produce a nonequilibrium device morphology. This may involve avoiding thermal annealing after casting, using processing additives that improve compatibility during processing, and annealing in the presence of mixtures of solvents and/or heating for short periods of time. Processing Effects. In fundamental investigations of surface attraction, solution-cast thin films are thermally annealed above the glass transition for days (or weeks) to reach an equilibrated structure. However, the as-cast structure may be more relevant to commercial applications, as a process that requires prolonged annealing above the glass transition is not practical. The effects of solvent evaporation on the structure of solution-cast homopolymer blends and polymer nanocomposites were examined in recent simulations.155,156 When the Péclet (Pe) number is Pe ≫1, corresponding with fast solvent evaporation relative to diffusive time scales, vertical stratification of constituents is observed in both types of blends. The stratification profiles depend on the relative sizes of each constituent as well as the strength of interactions between them: In athermal homopolymer blends and weakly interacting polymer/particle blends, the simulations show accumulation of the smaller constituent at the surface.156 In strongly interacting polymer/particle blends, where the particle is roughly double the size of the polymer, the particles accumulate at the surface.155 We recently examined the structure of solution-cast films with linear PS and bottlebrush PS additives using TOF-SIMS measurements and found that surface composition is largely determined by the ratio of linear to side-chain lengths η = Nm/ Nsc (Figure 11).86 This ratio describes the relative concentration of chain ends in each architecture as well as the entropic contribution to bulk miscibility and is frequently used to characterize wetting transitions at brush/linear interfaces. When η was low, the surface composition was approximately equal to the bulk composition, which indicates that bottlebrush and linear homopolymers were equally attracted to the surface. When η was large, the bottlebrush polymer was enriched at the surface. This enrichment for large η was also observed in ascast blends of linear homopolymers with bottlebrush random copolymers (χ > 0), such as linear PLA with bottlebrush

Figure 11. Surface characteristics in solution-cast blends of linear and bottlebrush polymer. Adapted from ref 86.

PDMS-r-PLA,88 linear PS with bottlebrush PS-r-PMMA,148 and linear PMMA with bottlebrush PS-r-PMMA (unpublished). These outcomes demonstrate that the stratification profiles are not entirely controlled by the tendency to minimize the surface free energy, which means it may be possible to drive high-energy branched additives to a surface through directional solvent evaporation. Presumably, these effects could be captured in simulations that account for the unique bulk and surface interactions of highly branched architectures. The use of surface-active polymer additives in meltprocessable resins is not widely studied, despite the commercial relevance of injection molding and extrusion processes. However, several works have shown that branched polymer additives will reduce power requirements, increase rates, and eliminate “sharkskin” effects in film blowing and extrusion processes of polyolefins.157−159 These improvements are seen because branched additives can reduce the melt viscosity and also migrate to the surface to form a lubricating layer. The formation of this lubricating layer has been attributed to bulk incompatibility of branched additives and the host polymer. More recent efforts with fused deposition modeling, a type of 3D printing process, demonstrate that polymeric additives can improve the strength of interfacial welds.160−162 In one example,161 the authors used highmolecular-weight linear PLA as the major component of the resin and included low-molecular-weight linear PLA, 3-arm star PLA, and 4-arm star PLA additives. All types of additives reduced the melt viscosity, which promoted interdiffusion and entanglements across adjacent layers. This effect improved tensile stress and modulus and also reduced anisotropy in mechanical properties. However, at high loadings of additive, the interfacial welds were very poor, which was attributed to extensive surface segregation of the additives. 3D printing is a popular tool for the manufacture of custom plastic parts, and the ability to improve mechanical properties by changing the polymer formulation is important for many applications. More generally, 3D printing and the related filament extrusion processes can be implemented with smaller material quantities than conventional melt-processing tools, and these instruments are now available at many universities. Therefore, without scaling-up syntheses or investing in new equipment, one can examine the effects of large temperature gradients and short time scales on surface segregation processes. 4797


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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Gila E. Stein: 0000-0002-3973-4496 Rafael Verduzco: 0000-0002-3649-3455 Notes

The authors declare no competing financial interest. Biographies

Rafael Verduzco is an Associate Professor in Chemical and Biomolecular Engineering and Materials Sciences and NanoEngineering at Rice University. Verduzco received his Ph.D. in Chemical Engineering from the California Institute of Technology in 2007 and his BS in Chemical Engineering from Rice University in 2001. The Verduzco laboratory uses a combination of materials chemistry and multiscale characterization tools to address challenges in polymer science.

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ACKNOWLEDGMENTS The authors thank the National Science Foundation for financial support under Awards CMMI-1727517 (G.E.S., T.S.L.) and CMMI-1563008 (R.V.). Time-of-flight secondary ion mass spectroscopy measurements were supported by the National Science Foundation under Award CBET-1626418.

Gila E. Stein received her BS in Chemical Engineering from Drexel University in 2002 and obtained a PhD in Chemical Engineering from the University of California, Santa Barbara, in 2006. She then joined the National Institute of Standards and Technology, where she was a National Research Council Postdoctoral Fellow in the Center for Nanoscale Science and Technology from 2007−2008. Dr. Stein moved to the University of Houston as an Assistant Professor of Chemical and Biomolecular Engineering in 2009 and was promoted to Associate Professor in 2015. She joined the University of Tennessee as the Prados Associate Professor of Chemical and Biomolecular Engineering in 2016. Her research is focused on the design of polymer films with unique structures, properties and functions.

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REFERENCES

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Travis S. Laws received his B.S. in Chemistry from Appalachian State University in 2016, where he did undergraduate research with Dr. Brett Taubman developing brewing methods to produce gluten-free sour beer through yeast/bacteria cofermentation. He is currently working toward his doctorate in chemical engineering at the University of Tennessee, Knoxville, under the guidance of Dr. Gila Stein. His current research projects are focused on the design of surface-active polymer additives. 4798


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