Versatile Synthesis of Amine-Reactive Microgels by Self-Assembly of Azlactone-Containing Block Copolymers

Relations between molecular design, chemical functionality, and stimulus-triggered response are important for a variety of applications of polymeric systems. Here, reactive amphiphilic block copolymers (BCPs) of poly­(2-vinylpyridine)-<i>block</i>-poly­(2-vinyl-4,4-dimethyl­azlactone) (PVP-<i>b</i>-PVDMA) were synthesized and assembled into microgels capable of incorporating functional amines. The composition of the PVP-<i>b</i>-PVDMA BCPs was varied to control the number of reactive sites in the spherical aggregates created by self-assembly of PVP-<i>b</i>-PVDMA BCPs in a 2-propanol/THF (v:v = 19:1) solvent mixture, which is selective for PVP. PVDMA and PVP segments were selectively cross-linked by 1,4-diaminobutane (DAB) or 1,4-diiodobutane (DIB) to fabricate core- and corona-cross-linked azlactone-containing microgels, respectively. Non-cross-linked aggregates of PVP-<i>b</i>-PVDMA and DIB-cross-linked microgels dissociate when exposed to THF, which is a good solvent for both blocks. However, the DAB-cross-linked BCP microgels swell in THF, suggesting the formation of a stable, three-dimensional network structure. Because of their ability to be reactively modified in ways that allows their stability or disassembly characteristics to be tailored, these azlactone-containing BCP microgels provide an attractive platform for applications in a wide range of fields, including catalysis, imaging, molecule separation, and guest loading for targeted delivery

Microstructured Block Copolymer Surfaces for Control of Microbe Adhesion and Aggregation

The attachment and arrangement of microbes onto a substrate is influenced by both the biochemical and physical surface properties. In this report, we develop lectin-functionalized substrates containing patterned, three-dimensional polymeric structures of varied shapes and densities and use these to investigate the effects of topology and spatial confinement on lectin-mediated microbe immobilization. Films of poly(glycidyl methacrylate)-block-4,4-dimethyl-2-vinylazlactone (PGMA-b-PVDMA) were patterned on silicon surfaces into line arrays or square grid patterns with 5 μm wide features and varied pitch. The patterned films had three-dimensional geometries with 900 nm film thickness. After surface functionalization with wheat germ agglutinin, the size of Pseudomonas fluorescens aggregates immobilized was dependent on the pattern dimensions. Films patterned as parallel lines or square grids with a pitch of 10 μm or less led to the immobilization of individual microbes with minimal formation of aggregates. Both geometries allowed for incremental increases in aggregate size distribution with each increase in pitch. These engineered surfaces combine spatial confinement with affinity-based capture to control the extent of microbe adhesion and aggregation, and can also be used as a platform to investigate intercellular interactions and biofilm formation in microbial populations of controlled sizes

Tailoring Surface Properties through in Situ Functionality Gradients in Reactively Modified Poly(2-vinyl-4,4-dimethyl azlactone) Thin Films

Generating physical or chemical gradients in thin-film scaffolds is an efficient approach for screening and optimizing an interfacial structure or chemical functionality to create tailored surfaces that are useful because of their wetting, antifouling, or barrier properties. The relationship between the structure of poly­(2-vinyl-4,4-dimethyl azlactone) (PVDMA) brushes created by the preferential assembly of poly­(glycidyl methacrylate)-<i>block</i>-PVDMA diblock copolymers and the ability to chemically modify the PVDMA chains in situ to create a gradient in functionality are examined to investigate how the extent of functionalization affects the interfacial and surface properties. The introduction of a chemical gradient by controlled immersion allows reactive modification to generate position-dependent properties that are assessed by ellipsometry, attenuated total reflectance-Fourier transform infrared spectroscopy, contact angle measurements, and atomic force microscopy imaging. After functionalization of the azlactone rings with <i>n</i>-alkyl amines, ellipsometry confirms an increase in thickness and contact angle measurements support an increase in hydrophobicity along the substrate. These results are used to establish relationships between layer thickness, reaction time, position, and the extent of functionalization and demonstrate that gradual immersion into the functionalizing solution results in a linear change in chemical functionality along the surface. These findings broadly support efforts to produce tailored surfaces by in situ chemical modification, having application as tailored membranes, protein resistant surfaces, or sensors

Shape-Persistent, Thermoresponsive Polypeptide Brushes Prepared by Vapor Deposition Surface-Initiated Ring-Opening Polymerization of alpha-Amino Acid N-Carboxyanhydrides

Surface-grafting thermoresponsive polymers allows the preparation of thin polymer brush coatings with surface properties that can be manipulated by variation of temperature. In most instances, thermoresponsive polymer brushes are produced using polymers that dehydrate and collapse above a certain temperature. This report presents the preparation and properties of polymer brushes that show thermoresponsive surface properties, yet are shape-persistent in that they do not undergo main chain collapse. The polymer brushes presented here are obtained via vapor deposition surface-initiated ring-opening polymerization (SI-ROP) of gamma-di- or tri(ethylene glycol)-modified glutamic acid N-carboxyanhydrides. Vapor deposition SI-ROP of gamma-di- or tri(ethylene glycol)-modified l- or d-glutamic acid N-carboxyanhydrides affords helical surface-tethered polymer chains that do not show any changes in secondary structure between 10 and 70 degrees C. QCM-D experiments, however, revealed significant dehydration of poly(gamma-(2-(2-methoxyethoxy)ethyl)-l-glutamate) (poly(L-EG(2)-Glu)) brushes upon heating from 10 to 40 degrees C. At the same time, AFM and ellipsometry studies did not reveal significant variations in film thickness over this temperature range, which is consistent with the shape-persistent nature of these polypeptide brushes and indicates that the thermoresponsiveness of the films is primarily due to hydration and dehydration of the oligo(ethylene glycol) side chains. The results presented here illustrate the potential of surface-initiated NCA ring-opening polymerization to generate densely grafted assemblies of polymer chains that possess well-defined secondary structures and tunable surface properties. These polypeptide brushes complement their conformationally unordered counterparts that can be generated via surface-initiated polymerization of vinyl-type monomers and represent another step forward to biomimetic surfaces and interfaces

Assessing Chemical Transformation of Reactive, Interfacial Thin Films Made of End-Tethered Poly(2-vinyl-4,4-dimethyl azlactone) (PVDMA) Chains

Designing thin films or surface scaffolds with an appropriate display of chemical functionality is useful for biomedical applications, sensing platforms, adhesives, and barrier coatings. Relationships between the structural characteristics of model thin films based on reactive poly­(2-vinyl-4,4-dimethyl azlactone) (PVDMA) brushes and the amount and distribution of primary amines used to chemically functionalize these layers <i>in situ</i> are quantitatively detailed via neutron reflectometry and compared with results from ellipsometry. After functionalization, the PVDMA brush thickness increases as a result of the primary amines reacting with the azlactone rings. Both techniques show that the extent of functionalization by small-molecule amines depends on the size of the amine, the grafting density of brush chains, and their molecular weight. However, constrained analysis of neutron reflectivity data predicated on that technique’s sensitivity to isotopic substitution and its ability to resolve structure at the nanoscale shows that the extent of functionalization is not accurately represented by the average extent of functionalization determined from ellipsometric thickness: reactive modification is not uniform, even in modestly dense brushes, except when the penetrant is small. In addition, there appears to be a loss of PVDMA chains during functionalization, attributed to chain scission resulting from additional stretching brought about by functionalization. These findings provide unprecedented insight into the alteration of surface properties by reactive modification and broadly support efforts to produce tailored surfaces in which properties such as friction, colloidal stability, adhesion, wettability, and biocompatibility can be modulated <i>in situ</i> by chemical modification

Buckling Instabilities in Polymer Brush Surfaces via Postpolymerization Modification

We report a simple route to engineer ultrathin polymer brush surfaces with wrinkled morphologies using postpolymerization modification (PPM), where the length scale of the buckled features can be tuned from hundreds of nanometers to one micrometer using PPM reaction time. We show that partial cross-linking of the outer layer of the polymer brush under poor solvent conditions is critical to obtain wrinkled morphologies upon swelling. Characterization of the PPM kinetics and swelling behavior via ellipsometry and the through thickness composition profile via time-of-flight secondary ion mass spectroscopy (ToF-SIMS) provided key insight into parameters influencing the buckling behavior