Photocrosslinkable and degradable polymers are finding a broad range of applications as drug-delivery vehicles, tissueengineering scaffolds, and in the fabrication of microdevices. [1][2][3] However, the synthesis of multifunctional macromers that form these degradable networks commonly involves multiple functionalization and purification steps, which makes the development of large numbers of polymers with diverse properties difficult. Here, we develop the first combinatorial library of degradable photocrosslinked materials. A library of acrylate-terminated poly(b-amino ester)s was synthesized in parallel via a condensation reaction that combines primary or secondary amines with diacrylates. This library of macromers was then photopolymerized to form degradable networks, with a wide range of degradation times (< 1 day to minimal mass loss after three months), mass-loss profiles, and mechanical properties (∼ 4 to 350 MPa). We believe this library approach will allow for the rapid screening and design of degradable polymers for a variety of applications. The spatial and temporal control afforded during photoinitiated polymerizations has motivated their wide application in the general field of biomaterials. [1,2] For example, photocrosslinkable hydrogels are used for the delivery of cells to injured tissues, [4][5][6][7][8] for the encapsulation and controlled delivery of biological molecules, [9][10][11] and for controlled fluid flow and cell confinement in microfluidics. [12,13] Additionally, highly crosslinked photopolymers are currently used in dentistry [14] and are being developed as bone-replacement materials [15,16] and for the fabrication of microdevices.[17] Many of these applications are only possible owing to the controlled nature of this type of polymerization. For example, photoinitiated control of polymerization allows for their application as injectable biomaterials [18,19] with a non-cytotoxic polymerization process.[20]Additionally, through use of masks and lasers, the spatial control of the polymerization process allows for unique patterning and construction of complex materials.[21]Numerous photopolymerizable and degradable materials have been developed, including polyanhydrides, poly(propylene fumarates), poly(ethylene glycol), and polysaccharides, [8,15,16,18] all utilizing multiple reaction and purification steps for synthesis of the photopolymerizable precursors. Despite this work, it has proven challenging to predict specific desirable properties (e.g., degradation and mechanics) from known chemical and structural details of the network precursors. These properties are essential in the design of degradable polymers. For instance, it may be desirable to synthesize a very hard material for some applications (e.g., orthopaedics), whereas a soft material is advantageous for other applications (e.g., tissue adhesive). [22,23] One potential solution to the inability to predict physical behavior is the generation of a higher-throughput approach to rapidly synthesize and screen photopolymerizable li...

Although there is an increasing appreciation that physical properties of amorphous (glassy) polymer surfaces and interfaces can differ substantially from those of the bulk, the mechanisms and implications for mechanical performance of thin films, surfaces of bulk polymers, and nanocomposites are unclear. For example, several natural and synthetic nanocomposites exhibit markedly enhanced stiffness and strength that cannot be explained via two-phase composite rules-of-mixtures. Here we apply recent advances in contact deformation to determine the apparent elastic (or storage) moduli over 5 to 200 nanometers from the free surface of amorphous polystyrene, poly(methyl methacrylate), and polycarbonate. We observe that the apparent stiffness of the surface under contact can exceed that of the bulk by up to 200%, independent of processing scheme, macromolecular structural characteristics, and relative humidity. We attribute this enhanced apparent stiffness at the surface to the contact stress-induced formation of a mechanically confined phase at the probepolymer interface. These observations are consistent with the increased macromolecular mobility of glassy polymer free surfaces, and relate directly to the material physics of the interphase in synthetic and biological polymer nanocomposites.Most experimental investigations of amorphous polymer surfaces have focused on thermally activated behavior such as the glass transition temperature T g [1][2][3] and structural relaxation. [4,5] However, few overarching conclusions exist regarding surface and interface properties, [6] in large part because experimental and sample preparation capabilities have not yet been optimized for the nanometer-length scales over which these surface-specific phenomena are observed. There are two generally accepted conclusions regarding amorphous polymer surface behavior: that T g is a function of polymer film thickness t f for t f < 100 nm, and that the magnitude and direction of the T g shift depends on the polymer and/or substrate [7] . For example, the T g of amorphous polystyrene (PS) films has been found to be depressed by 35°C in spin-coated films of t f < 20 nm on Si substrates [1] and by 70°C for free standing films of t f < 30 nm, [2] while amorphous poly(2-vinylpyridine) has demonstrated a 35°C elevation in T g for t f = 10 nm that is attributed to secondary bonding with the Si substrate.[8]Here, we sought to consider the consequences of such a physical property variation on the resistance of amorphous polymer surfaces to localized contact deformation. Depression of T g in polymers such as PS and PMMA suggests that, over distances < 100 nm from the free surface of these socalled glassy polymers, the macromolecular chains are more mobile than those located within the bulk. This conceptualization is consistent with computational simulations of molecular mobility of free surfaces and confined volumes, [9][10][11] as well as recent experimental observations for PS thin films of t f < 40 nm, including broadened structural relaxation times...

The creep compliance of viscoelastic materials such as synthetic polymers is an established metric of the rate at which strain increases for a constant applied stress and can, in principle, be implemented at the nanoscale to compare quantitatively bulk or thin film polymers of different structures or processing histories. Here, we outline the evolution of contact creep compliance analysis and application for both conical and spherical indenter geometries. Through systematic experiments on four amorphous (glassy) polymers, two semi-crystalline polymers and two epoxies, we show that assumptions of linear viscoelasticity are not maintained for any of these polymers when creep compliance is measured via conical indentation at the nanoscale, regardless of the rate of stress application (step or ramp). Further, we show that these assumptions can be maintained to evaluate the contact creep complianceJc(t) of these bulk polymers, regardless of the rate of stress application, provided that the contact strains are reduced sufficiently through spherical indentation. Finally, we consider the structural and physical properties of these polymers in relation toJc(t), and demonstrate thatJc(t) correlates positively with molecular weight between entanglements or crosslinks of bulk, glassy polymers.

relation functional proposed by Perdew, Burke, and Ernzerhof (PBE) [20] was adopted. To account for the valence-core interaction, ultrasoft pseudopotentials [21] were chosen for Nb 4p and 4d states and norm-conserving pseudopotentials [22] were chosen for other states. The wavefunctions were expanded in the plane-wave basis set with an energy cut-off of 25 Ry; the energy cut-off for the augmentation charge was 400 Ry. The Brillouin zone of the supercell was sampled only at the C point. In the energy optimization, the Davidson method and the Broyden charge-density mixing method were used. The structure optimization was quenched using molecular dynamics.Synthesis [1] Application of highthroughput syntheses toward the rapid discovery and optimization of functional materials has required parallel advances in materials characterization. [2] In the context of polymer design for applications ranging from biomaterials to microelectronic insulators, combinatorial approaches can enable systematic, high-throughput surveying of structure-processing-property relationships as a function of composition and operating conditions, in nanoliter to microliter volumes. [3][4][5] Here, we develop a high-throughput synthesis/nanomechanical-profiling approach capable of accurately screening the mechanical properties of a large, discrete polymer library comprising nanoliter-scale material volumes. Within just a few days, a library of over 1700 photopolymerizable materials was synthesized and then assayed for mechanical properties using COMMUNICATIONS

The purpose of this study was to synthesize copolymers of different molecular architecture, i.e., monomer distribution over the polymer chain, and to compare their physical and mechanical properties. A series of random copolymers of 2-ethyl-2-oxazoline (EtOx) and 2-nonyl-2-oxazoline (NonOx) were synthesized via a cationic ring-opening polymerization procedure in acetonitrile under microwave irradiation. The polymerization kinetics for EtOx and NonOx were studied in refluxing butyronitrile using thermal heating. The resulting kinetic data were applied to synthesize a series of block copolymers with the same chemical composition as the random copolymers. The random and block copolymers exhibited the desired composition, molecular weight, and narrow molecular weight distribution. The surface energies of the random copolymers with 65−85 wt % NonOx were higher than the surface energy of their block copolymer counterparts as the random distribution of EtOx units hindered the segregation of the NonOx units to the surface. The variation in polymer architecture also resulted in different phase segregation behavior and different transition temperatures, as shown by differential scanning calorimetry (DSC). The observed elastic moduli, which differed considerably between the random and the block series, were well explained by the phases identified through DSC.

Depth‐sensing or instrumented indentation is an experimental characterization approach well‐suited for high‐throughput investigation of mechanical properties of polymeric materials. This is due to both the precision of force and displacement, and to the small material volumes required for quantitative analysis. Recently, considerable progress in the throughput (number of distinct material samples analyzed per unit time) of indentation experiments has been achieved, particularly for studies of elastic properties. Future challenges include improving the agreement between various macroscopic properties (elastic modulus, creep compliance, loss tangent, onset of nonlinear elasticity, energy dissipation, etc.) and their counterpart properties obtained by indentation. Sample preparation constitutes a major factor for both the accuracy of the results and the speed and efficiency of experimental throughput. It is important to appreciate how this processing step may influence the mechanical properties, in particular the onset of nonlinear elastic or plastic deformation, and how the processing may affect the agreement between the indentation results and their macroscopic analogues.

Impact testing with nanoscale spatial, force, and temporal resolution has been developed to address quantitatively the response of surfaces to impingement of local contact at elevated velocities. Here, an impact is generated by imparting energy to a pendulum carrying an indenter, which then swings towards a specimen surface. The pendulum displacement as a function of time x(t) is recorded, from which one can extract the maximum material penetration x max , residual deformation x r , and indentation durations t in and t out . In an inverse application one can use the x(t) response to extract material constants characterizing the impact deformation and extent of energy absorption, including material specific resistance coefficient C in , coefficient of restitution e, and dynamic hardness H imp . This approach also enables direct access to the ratio H/E, or resilience of the deformed material volume, at impact velocities of interest. The impact response of aluminum was studied for different contact velocities, and the mechanical response was found to correlate well with our one-dimensional contact model. Further experiments on annealed and work hardened gold showed that dynamic hardness H imp scales with contact velocity and highlighted the importance of rate-dependent energy absorption mechanisms that can be captured by the proposed experimental approach.

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