STRUCTURE-PROPERTY RELATIONS FOR POLYMERIC MATERIALS
Research Topics
Rate Dependent Mechanisms of Deformation and Toughening in Amorphous and Semicrystalline Thermoplastics Modified with Micro and Nano-scale Fillers (R. Aronow, E. Kopesky, S. Dachanel, and S. Soong)
Polymers are known to exhibit strong rate-dependent mechanical behavior. For thermorheologically complex materials in different temperature and/or frequency regimes, the rate sensitivities of polymers change significantly as various primary (α) secondary (β) molecular mobility mechanisms are accessed. The incorporation of nanoparticles into the polymer matrix can potentially alter the local molecular level structure and thus offers an opportunity to tailor the rate-dependent mechanical deformation and failure behavior of the polymer. In this study, methacryl-POSS is incorporated into poly(vinyl chloride), producing a range in weight fraction of well- dispersed POSS. Dioctyl phthalate (DOP) plasticized PVC is also prepared with a range in DOP content using the same method. Both methacryl-POSS and DOP plasticize the PVC. Dynamic mechanical analysis (DMA) revealed that the incorporation of POSS in PVC introduced reductions in both the primary (α) and secondary (β) transition temperatures. DOP reduced the α-transition temperature in the blends, whereas a pronounced suppression of the relaxation peak was observed at higher DOP content. The rate-dependent yield and postyield behavior are characterized in compression testing over a wide range of strain rates (10-4-3000/s). A clear rate-dependent transition associated with the strain-rate-dependent relaxation of motions was observed in the compression yield data of PVC/POSS blends. This transition was much milder in the case of PVC/DOP due to the suppression of the (β) transition in these blends. The constitutive model for the rate-dependent elastic-plastic behavior of amorphous polymer proposed by Mulliken and Boyce was used to predict the strain rate dependence of the yield stress of the PVC/POSS and PVC/DOP blends in uniaxial compression tests. The rate-dependent yield model follows the essence of the multiple process Ree-Eyring model as utilized by Bauwens-Crowet.
We have also investigated the toughenability of poly(methyl methacrylate) (PMMA) using polyhedral oligomeric silsesquioxane (POSS) nanocages at loadings between 0 and 15 wt%. Three distinct POSS species were used: a crystallizable type that did not disperse on a molecular scale within the PMMA matrix (cyclohexyl-POSS), and two types of POSS that formed homogeneous mixtures over the loadings we have investigated (methacryl-POSS and trisilanol-phenyl-POSS). Each of the three types of POSS was able to toughen PMMA in slow-speed tension tests at loadings ≤5 wt%; however, the reproducibility was poor due to the high flaw sensitivity of these binary blends. Ternary blends containing both cyclohexyl-POSS and methacryl-POSS showed the greatest increase in tensile toughness and also excellent reproducibility of toughening.
A blend containing 2.5 wt% of both cyclohexyl-POSS and methacryl-POSS maintained the same modulus as the unfilled PMMA while increasing the toughness by a factor of 4. Electron micrographs showed extensive particle–matrix debonding of the PMMA from the cyclohexyl-POSS crystallites and some evidence of plastic deformation on the fracture surface. In high rate (1000 s-1 ) split-Hopkinson pressure bar (SHPB) tests, binary blends of POSS and PMMA were able to improve the impact toughness of PMMA; however, once again the combined addition of both cyclohexyl-POSS and methacryl- POSS led to the greatest reproducibility of toughening. Comparison with previous results suggests that in order to toughen PMMA with rigid fillers, weakly-adhering particles with sizes on the order of 100 nm are required.
Ultrathin Multilayer Films as Nanoreactors for Inorganic Cluster Synthesis and Controlled Porosity Development(A. Nolte, Z. Li, Z. Wu, D. Lee, and A. Kunz)
Heterostructured magnetic tubes with sub-micron dimensions were assembled by the layer-by-layer deposition of polyelectrolytes and nanoparticles in the pores of track- etched polycarbonate membranes . Multilayers comprised of poly(allylamine hydrochloride) and poly(styrene sulfonate) assembled at a high pH condition (pH > 9.0) were first assembled into the pores of track-etched polycarbonate membranes, and then multilayers of magnetite (Fe3O4) nanoparticles and PAH were deposited. Transmission electron microscopy (TEM) confirmed the formation of multilayer nanotubes with an inner shell of magnetite nanoparticles. These tubes exhibited superparamagnetic characteristics at room temperature (300K) as determined by a SQUID magnetometer. The surface of the magnetic nanotubes could be further functionalized by adsorbing poly(ethylene oxide)-b-poly(methacrylic acid) block copolymers. The separation and release behavior of low molecular weight anionic molecules (i.e., ibuprofen, rose bengal, and acid red 8) by/from the multilayer nanotubes were studied as these tubes could potentially be used as separation or targeted delivery vehicles. The magnetic tubes could be successfully used to separate (or remove) a high concentration of dye molecules (i.e., rose bengal) from solution by activating the nanotubes in acidic solution. The release of the anionic molecules in physiologically relevant buffer solution showed that while bulky molecules (e.g., rose bengal) release slowly, small molecules (i.e., ibuprofen) release rapidly from the multilayers. The combination of the template method and layer-by-layer deposition of polyelectrolytes and nanoparticles provides a versatile means to create functional nanotubes with heterostructures that can be used for separation as well as targeted delivery.
We have developed a route for synthesizing iron oxide nanoclusters that utilizes monolayer films of micelles of the amphiphilic block copolymer, poly(styrene-b-acrylic acid) (PS-b-PAA). This system has significant value because it enables the creation of nanocluster arrays of a chosen metal species, with independent control of the nanocluster diameter and areal density. In previous work, nanocluster diameters were varied between 5 nm and 16 nm and the areal density was varied from 6.0 x 1010 cm-2 to 1 x 109 cm-2, although variation outside of these ranges is easily accessible. At higher areal densities, the nanoclusters are hexagonally ordered. Further, as we have presented separately, the nanoclusters arrays can be patterned on the micron length scale using microcontact printing. We have exploited this system to create arrays of uniform-diameter iron oxide nanoclusters, having quantifiable areal densities that can be varied over more than an order of magnitude. We achieve vertical CNT growth from our catalyst system through appropriate selection of the substrate, catalyst preparation procedure, and reaction conditions. Because this catalyst system allows for precise quantification of the nanocluster areal density, we can estimate the percentage of nanoclusters that nucleate the growth of a CNT. By uniformly varying the areal density of iron oxide nanoclusters on the substrate surface, we manipulate the morphology of the CNT film from a tangled and sparse arrangement of individual CNTs, through a transition region with locally bunched and self-aligned CNTs, to rapid growth of thick vertical CNT films.
Nanoporous thin films have been fabricated from layer-by-layer assembled silica nanoparticles and a polycation. The resultant multilayer films were found to exhibit both antifogging and antireflection properties. The antifogging properties are a direct result of the development of superhydrophilic wetting characteristics (water droplet contact angle <5° within 0.5 s or less). The nearly instantaneous sheetlike wetting promoted by the superhydrophilic multilayer prevents light scattering water droplets from forming on a surface. The low refractive index of the multilayer film (as low as 1.22) resulting from the presence of nanopores was found to impart excellent antireflection properties. Glass slides coated on both sides with a nanoporous multilayer film exhibited transmission levels as high as 99.8%. Stable superhydrophilic wetting characteristics were obtained only after a critical number of bilayers were deposited onto a surface. The assembly conditions (solution pH and nanoparticle concentration), as well as the choice of nanoparticle size, were found to strongly influence film properties. It is suggested that the superhydrophilic behavior is driven by the rapid infiltration of water into a 3D nanoporous network created under specific assembly conditions.
We have also demonstrated a surface structure that mimics the water harvesting wing surface of the Namib Desert beetle. Hydrophilic patterns on superhydrophobic surfaces were created with water/2-propanol solutions of a polyelectrolyte to produce surfaces with extreme hydrophobic contrast. Selective deposition of multilayer films onto the hydrophilic patterns introduces different properties to the area including superhydrophilicity. Potential applications of such surfaces include water harvesting surfaces, controlled drug release coatings, open-air microchannel devices, and lab-on-chip devices.
Mechanical Properties of Ultrathin Polymer Films (A. Nolte)
We have developed a modified version of a strain-induced buckling instability technique that relies on the analysis of a two-beam composite film deposited on an elastomeric poly(dimethylsiloxane) (PDMS) substrate. This method broadens the applicability of previous techniques to a wider array of thin film materials without significantly affecting the convenience and ease of use that gives the buckling instability technique many advantages in the field of thin film mechanical characterization. We have previously shown that the “strain-induced elastic buckling instability for mechanical measurements” (SIEBIMM) technique is suitable for testing polyelectrolyte multilayers (PEMs) that are amenable to deposition directly on the testing substrate. Here we demonstrate how PEMs that would not normally be amenable to deposition on PDMS can be assembled onto a thin layer of polystyrene (PS) that has been transferred to the PDMS surface and treated to promote adhesion. Multilayers assembled onto the PS- coated PDMS substrates yielded thin two-beam PS-PEM composite films on the surface of the PDMS substrates. We demonstrate how these two-beam composite films buckled similarly to their homogeneous counterparts, and how by proper analysis of the mechanics it was possible to de-convolute the contribution of the PS to arrive at a Young’s modulus value for the PEM part of the two-beam film. Accuracy of the new method was confirmed by comparing results from two systems evaluated with both conventional SIEBIMM and the two-beam technique. Following this, we demonstrate modulus measurements on two PEM assemblies of poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA), one deposited at pH 3.5 and one at pH 4.0. Both of these systems could not be measured by the conventional SIEBIMM approach. Thus, measurements were possible only by employing a two-beam analysis.
Patterned Surfaces for Biomaterial Functionality(H. Kim, R. Bennett, and A. Miller)
Systems designed to detect whole pathogen particles, their components, or other proteins diagnostic of infection or disease are of interest as sensors for biodefense and clinical diagnostics. We examined a sensing strategy based on live T cell – B cell interactions in a biosensor chip format. An approach to fabricate patterned hydrogel microwells functionalized at their bases with antibodies to promote specific immobilization of lymphocytes was developed and used to array single T cells in a regular pattern on a substrate. A sensing platform was created by overlaying the arrayed T cells with a confluent layer of antigen-capturing B cells.
In this system, a peptide analyte added to the chip was captured by B cells and physically presented to arrayed T cells, thereby triggering the T cells through their T cell receptors. T cell recognition of the target peptide was detected by fluorescence measurements of T cell intracellular calcium levels, a biochemical read-out of T cell receptor triggering. We demonstrated that this approach allows rapid, sensitive detection of a model peptide analyte, and that T cell arrays allow for maximal T cell-B cell contacts while simultaneously optimizing single cell fluorescence detection for analysis of the sensor response. This approach could be of interest for the design of sensing platforms that can detect both peptide fragments and whole intact pathogens, irrespective of surface mutations that might be induced naturally or during ‘weaponization’.
Rheological Studies of Polymeric Nanocomposites(B. Wang and E. Kopesky)
Two distinct oligomeric species of similar mass and chemical functionality (Mw≈2,000 g/mol), one a linear methyl methacrylate oligomer (radius of gyration Rg≈1.1 nm) and the other a hybrid organic–inorganic polyhedral silsesquioxane nanocage (methacryl-POSS, r≈1.0 nm), were subjected to thermal and rheological tests to compare the behaviors of these geometrically dissimilar molecules over the entire composition range. The glass transition temperatures of the blends varied monotonically between the glass transition temperatures of the pure oligomer (Tg = 47.3°C) and the pure POSS (Tg = 61.0°C). Blends containing high POSS contents (with volume fraction φPOSS≥0.90) exhibited enhanced enthalpy relaxation in differential scanning calorimetry (DSC) measurements, and the degree of enthalpy relaxation was used to calculate the kinetic fragility indices m of the oligomeric MMA (m=59) and the POSS (m=74). The temperature dependences of the viscosities were fitted by the free-volume based Williams–Landel–Ferry (WLF) and Vogel–Fulcher–Tammann (VFT) framework and a dynamic scaling relation. The calculated values of the fragility from the WLF–VFT fits were similar for the POSS (m=82) and for the oligomer (m=76), and the dynamic scaling exponent was similar for the oligomeric MMA and the POSS. Within the range of known fragilities for glass-forming liquids, the temperature dependence of the viscosity was found to be similarly fragile for the two species. The difference in shape of the nanocages and oligomer chains is unimportant in controlling the glass-forming properties of the blends at low volume fractions (φPOSS<0.20). However, at higher volume fractions, adjacent POSS cages begin to crowd each other, leading to an increase in the fractional free volume at the glass transition temperature and the observed enhanced enthalpy relaxation in DSC.
