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Mobile-amorphous-free crystals of a semiconducting polymer, poly(3-hexyl thiophene) (P3HT), were made via a flow induced crystallization technique to yield crystals that were microns long while the...

Optical technologies in the midwave and long wave infrared spectrum (MWIR, LWIR) are important systems for high resolution thermal imaging in near, or complete darkness. While IR thermal imaging has been extensively utilized in the defense sector, application of this technology is being driven toward emerging consumer markets and transportation. In this viewpoint, we review the field of IR thermal imaging and discuss the emerging use of synthetic organic and hybrid polymers as novel IR transmissive materials for this application. In particular, we review the critical role of elemental sulfur as a novel feedstock to prepare high refractive index polymers via inverse vulcanization and discuss the fundamental chemical insights required to impart improved IR transparency into these polymeric materials.

Extensive efforts have been employed to improve the power conversion efficiency of organic solar cells. One of the most successful approaches is the morphological control of the solar cells’ active...

The morphology of poly (3-hexylthiophene) (P3HT) in its liquid phase and its manipulation via flow-induced solution crystallization and its crystallization kinetics was studied to determine its mechanism. Shear flow-induced ordering of semiconducting P3HT, which generates more perfect crystal structures than quiescent methods, is elucidated using in situ rheo-SANS and rheo-SALS measurements, and an Avrami analysis is performed. Characteristic lengths of P3HT crystals were measured as a function of time, and 3-D networks of percolated P3HT fibril crystals were determined by measuring the apparent fractal, ∼2.6, by fitting the rheo-SANS data with a power law function. Additionally, UV–vis and DSC results revealed a process of P3HT crystal perfection determined by following the evolution of absorption peak characteristics of pi–pi stacking at 600 nm and the melting peaks as they shifted and narrowed with respect to increasing shear time. The Avrami exponent, m, reached a maximum value of 2 indicating homogeneous nucleation of P3HT macromolecules that allowed one-dimensional fibril crystal growth and was limited by contact time between the P3HT molecules rather than the diffusion of P3HT chains and this is attributed to the highly directional pi–pi stacking attractions of electron pi in the thiophene rings.

We report on a new strategy for preparing polymer–nanoparticle composite Faraday rotators for use in magnetic sensing and optical isolation. While most applications of Faraday rotators make use of inorganic garnet crystals, these are generally limited by low magneto-optical activity (low Verdet constants), high cost, and/or limited processing options. This has led to an interest in new materials with improved activity and processing characteristics. We have developed a new type of magneto-optical material based on polymer–nanoparticle composites that can be completely prepared by solution processing methods with tunable Verdet constants and device sensitivity. By exchanging native surface ligands on magneto-optically active CoFe2O4 nanocrystals with polymer compatible ligands, enhanced nanoparticle dispersion in processible polymer matrices was observed at up to 15 wt% inorganic loading. Employing a multilayer polymer film construct, functional Faraday rotator devices were prepared by simple sequential spin-coating of active nanocomposite and protective, barrier cellulose acetate layers. For these assemblies, magneto-optic activity and sensitivity are easily tuned through variation of nanoparticle feed and control of polymer film layers, respectively. These multilayered Faraday rotators show up to a 10-fold enhancement in Verdet constant compared to reference terbium gallium garnets at 1310 nm, opening new possibilities for the fabrication of “plastic garnets” as low cost alternatives to existing inorganic materials for use in the near-IR.

Polymeric structures fabricated using Fused Filament Fabrication (FFF) suffer from poor inter-laminar fracture toughness. As a result, these materials exhibit only a fraction of the mechanical performance of those manufactured through more traditional means. Here we show that thermal annealing of confined structures manufactured using the FFF technique dramatically increases their inter-laminar toughness. Single Edge Notch Bend (SENB) fracture specimens made from acrylonitrile-butadiene-styrene (ABS) feedstock were isothermally heated in a supporting fixture, post-manufacture, across a range of times and temperatures. Fracture testing was then used to quantify the changes in inter-laminar toughness offered by annealing through measurements of the Mode I critical elastic-plastic strain energy release rate, JIc. Under the most aggressive annealing conditions, the inter-laminar toughness increased by more than 2700% over the non-annealed baseline material. Void migration and aggregation during the annealing process was analyzed using X-ray tomography and provides insight into the toughening mechanisms. Time-scales of reptation and polymer mobility at the interface during annealing are also modeled and agree with fracture experiments.

We report on the preparation of ultrahigh refractive index polymers via the inverse vulcanization of elemental sulfur, selenium, and 1,3-diisopropenylbenzene for use as novel transmissive materials for mid-infrared (IR) imaging applications. Poly(sulfur-random-selenium-random-(1,3-diisopropenylbenzene)) (poly(S-r-Se-r-DIB) terpolymer materials from this process exhibit the highest refractive index of any synthetic polymer (n > 2.0) and excellent IR transparency, which can be directly tuned by terpolymer composition. Sulfur or selenium containing (co)polymers prepared via inverse vulcanization can be described as Chalcogenide Hybrid Inorganic/Organic Polymers (CHIPs) and are polymeric analogues to wholly inorganic Chalcogenide Glasses (ChGs), which are commonly used as transmissive materials in mid-IR imaging. Finally, we demonstrate that CHIPs composed of (poly(S-r-Se-r-DIB) can be melt processed into windows that enabled high quality mid-IR thermal imaging of human subjects and highly resolved imaging of...

The synthesis of a novel high sulfur content material possessing improved thermomechanical properties is reported via the inverse vulcanization of elemental sulfur (S8) and 1,3,5-triisopropenylbenzene (TIB). A key feature of this system was the ability to afford highly cross-linked, thermosetting materials, where the use of TIB as a comonomer enabled facile control of the network structure and dramatically improved the glass transition temperature (relative to our earlier sulfur copolymers) of poly(sulfur-random-(1,3,5-triisopropenylbenzene)) (poly(S-r-TIB)) materials over a range from T = 68 to 130 °C. This approach allowed for the incorporation of a high content of sulfur–sulfur (S–S) units in the copolymer that enabled thermomechanical scission of these dynamic covalent bonds and thermal reprocessing of the material, which we confirmed via dynamic rheological characterization. Furthermore, the high sulfur content also imparted high refractive index (n > 1.75) and IR transparency to poly(S-r-TIB) copoly...

High sulfur content copolymers were prepared via the inverse vulcanization of elemental sulfur with styrene. This reaction was carried out at a relatively low temperature and invokes a new chain transfer mechanism of abstraction of benzylic protons to form stable copolymers. The use of styrene as a comonomer for inverse vulcanization was attractive due to the low cost and wide spread industrial use of styrenics in free radical processes. The copolymers were used as the active cathode material in Li-S batteries that exhibited outstanding device performance, maintaining 489 mAh/g capacity after 1000 cycles. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 107–116

Tapered block polymers are an emerging class of macromolecules with unique and diverse self-assembly behavior and properties. Herein, we directly examine the manipulation of self-assembled interfaces in poly(isoprene-b-styrene) (I-S)-based block polymers (BPs) by synthesizing non-tapered (I-S), normal tapered (I-IS-S), and inverse tapered (I-SI-S) BPs with controlled monomer segment distributions. We provide the first direct measurements of interfacial mixing for these tapered polymers through X-ray reflectivity (XRR). The density profiles from XRR are compared to results from fluids density functional theory (fDFT) with good agreement. We find that our normal tapered BPs (30 vol % tapering) have similar interfacial mixing to diblock polymers, while our inverse tapered BPs (30 vol % tapering) have much wider interfaces. Additionally, differential scanning calorimetry (DSC) studies elucidate the influence of tapering on the glass transition temperature (Tg) and change of heat capacity (ΔCP) for each BP pha...

Quantitative morphological analysis is essential to the fundamental understanding of semiconducting polymers. Temperature modulated differential scanning calorimetry is used to determine the amount of crystalline and noncrystalline phases within regioregular poly(3-hexylthiophene) (rrP3HT). Careful optimization of the experimental conditions shows that the glass transition of rrP3HT consists of three parts corresponding to the devitrification of the side chains, mobile amorphous fraction (MAF), and rigid amorphous fraction (RAF), consecutively. Measurements taken from this, as well as from the melting transition, allows the first calculation of the degree of crystallinity, MAF and RAF, to be achieved in a single experiment for rrP3HT. This technique thus enables the morphological phases to be determined and potentially related to the performance of electronic devices made from semiconducting polymers.

We report on dynamic covalent polymers derived from elemental sulfur that can be used as thermally healable optical polymers for mid-IR thermal imaging applications. By accessing dynamic S–S bonds in these sulfur copolymers, surface scratches and defects of free-standing films of poly(sulfur-random-1,3-diisopropenylbenzene) (poly(S-r-DIB) can be thermally healed, which enables damaged lenses and windows from these materials to be reprocessed to recover their IR imaging performance. Correlation of the mechanical properties of these sulfur copolymers with different curing methods provided insights to reprocess damaged samples of these materials. Mid-IR thermal imaging experiments with windows before and after healing of surface defects demonstrated successful application of these materials to create a new class of “scratch and heal” optical polymers. The use of dynamic covalent polymers as healable materials for IR applications offers a unique advantage over the current state of the art (e.g., germanium or ...

The synthesis of dynamic covalent polymers with controllable amounts of sulfur–sulfur (S–S) bonds in the polymer backbone via inverse vulcanization of elemental sulfur (S8) and 1,3-diisopropenylbenzene (DIB) is reported. An attractive feature of the inverse vulcanization process is the ability to control the number and dynamic nature of S–S bonds in poly(sulfur-random-(1,3-diisopropenylbenzene)) (poly(S-r-DIB) copolymers by simple variation of S8/DIB feed ratios in the copolymerization. S–S bonds in poly(S-r-DIB) copolymers of high sulfur content and sulfur rank were found to be more dynamic upon exposure to either heat, or mechanical stimuli. Interrogation of dynamic S–S bonds was conducted in the solid-state utilizing electron paramagnetic resonance spectroscopy and in situ rheological measurements. Time-dependent rheological property behavior demonstrated a compositional dependence of the healing behavior in the copolymers, with the highest sulfur (80 wt % sulfur) content affording the most rapid dynam...

The enthalpy of fusion for a perfect, infinite poly(3-hexylthiophene) (P3HT) crystal (ΔHm∞) must be known to evaluate the absolute crystallinity of P3HT. This value, however, is still ambiguous as different values have been reported using various experimental techniques. Here, we extrapolate the enthalpy of fusion for extended chain crystals of oligomeric P3HT to infinite molecular weight and obtain a value of ΔHm∞≈ 42.9 ± 2 J/g employing differential scanning calorimetry with a correction based on grazing incidence small angle X-ray scattering data. Also, we define the onset of chain folding within P3HT crystallites at a chain length of 5 Kuhn segments. Knowledge of ΔHm∞ allows calculation of P3HT percent crystallinity in thin films for applications such as organic field effect transistors and solar cells. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 1469–1475

The rheological properties of poly(3-hexylthiophene) (P3HT) solutions were investigated in a nonvolatile solvent, 2-ethylnaphthalene, at various P3HT concentrations. A mild viscosity increase was noted during shear for lower concentration P3HT solutions over 24 h, yet the viscosity increase was greater than if the sample was not sheared at all over the same time period. In this case, crystalline fibrils with large aspect ratios were formed during shear. Crystallization was determined to be dictated by Brownian motion and mediated by shear, or in other words Brownian motion brought the molecules together while shear changed their conformation to allow crystallization. Higher concentration P3HT solutions produced a significant viscosity increase, up to 2 orders of magnitude, during shear. Again, if a similar solution was merely aged without shear, a much lower viscosity increase was noted. Simple calculations show that the fibril concentration was above the percolation threshold at the higher concentration ...

Adding metallic nanoparticles into bulk-heterojunction, polymer-based solar cells has been proven an effective strategy to enhance light absorption of the active layer and device performance. However, the high-energy surfaces on the nanoparticles may also affect the morphology of the active layer by influencing phase-separation, which has not been studied in detail. Here, we show that silica particles embedded in the active layer will affect the aggregation behavior of [6,6]-phenyl C61-butyric acid methyl ester (PCBM) in the bulk-heterojunction of poly(3-hexylthiophene) (P3HT):PCBM. Using a novel graphical technique to analyze the absolute scattering intensity of small angle neutron scattering data, we conclusively demonstrate that some PCBM will migrate away from the bulk solution to the surface of the silica upon annealing and improve the device performance. The overall effect is to decrease the device series resistance and improve the power conversion efficiency by 10 to 20% relative to the control group. In contrast to metallic nanoparticles that utilize the surface plasmon resonance, our results indicate that, even with optically inert particles, the induced phase separation of PCBM may also result in an improved device.

We present a dual length morphological model for the active layer of bulk-heterojunction, polymer-based solar cells using results from neutron and X-ray scattering techniques. Two critical characteristic lengths are found in the mixtures composed of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). A characteristic length at 15 nm is the local characteristic of the P3HT crystals and PCBM agglomerations, which is independent of the bulk composition upon relaxation by thermal annealing. Conversely, a larger bicontinuous structure described by Teubner–Strey model with phase distances between 23 and 35 nm forms only after thermal annealing, which is highly correlated to the bulk compositions. These results suggest phase separation between the polymer and fullerene can only be partially manipulated by simple processing techniques such as coating conditions and annealing, and a more rigorous design of the morphology should be implemented in the future. © 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym. Phys. 2014, 52, 387–396

The synthesis and thermomechanical properties of a novel class of self-healing perfluoropolyethers (PFPEs) is reported. By decoration of 2-ureido-4[1H]-pyrimidone end groups on the termini of low molar mass PFPE, the formation of supramolecular polymers and networks held together via hydrogen bonding associations was achieved. These novel supramolecular polymer materials exhibit a combination of enhanced modulus and elasticity, along with self-healing properties, where rapid self-healing time was demonstrated using dynamic rheological measurements. These types of supramolecular PFPEs are anticipated to be useful for a number of emerging areas in lubrication. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3598–3606

A series of organic/inorganic hybrid block and random copolymers were prepared by reversible addition–fragmentation chain transfer (RAFT) polymerization using poly(ethylene glycol) methyl ether methacrylate (PEGMA) and 3-(3,5,7,9,11,13,15-heptaisobutylpentacyclo[9.5.1.13,9.15,15.17,13]octasiloxane-1-yl)propyl methacrylate (MA-POSS) as monomers in order to study the effect of polymer morphology and POSS content on the properties of polymer electrolytes. Flexible and dimensionally stable free-standing films were made from the hybrid block and random copolymers mixed with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) when the contents of MA-POSS unit were larger than 31 and 16 mol %, respectively. The ionic conductivity of the solid-state block copolymer (PBP) electrolyte was found to be 1 order of magnitude higher than that of the random copolymer (PRP) electrolyte when they had similar MA-POSS content, although their glass transition temperature values of their ion-conducting segments were quite clos...

Previously we characterized the active layer in polymer-based solar cells containing Poly(3-hexylthiophene) with the electron acceptor Phenyl-C61-butyric acid methyl ester (PCBM) to find a thin, pure polymer layer at the air interface just after spin coating. In this study, we find that when the aluminum back electrode was thermally evaporated onto the active layer, at high enough rate, craters were found in the pure polymer layer. This was determined by dissolving the aluminum and characterizing the active layer with an atomic force microscope. Poor device performance was noted under this condition. However, if the aluminum was evaporated at a slower rate, resulting in a flat active layer surface and no crater formation, the efficiency more than doubled. A similar result is found if lithium fluoride (LiF) is deposited before aluminum evaporation and no craters were found even for the higher aluminum evaporation rate. So, it appears that LiF acts as a momentum shield to crater formation allowing superior device performance. If the active layer is annealed before deposition of the back electrode then, regardless of deposition rate, similar device performance is found. Again, in our previous study, it was found that annealing the active layer forced PCBM to the air interface which apparently also acts as a momentum shield. Annealing the device after aluminum deposition produces poorer performance than annealing before deposition. However, these devices have a better fill factor. Examining the active layer shows that it undergoes a buckling transition due to differences in the aluminum and active layers' thermal expansivities which reduces overall contact with the electrodes. However, whatever contact is made, is superior, accounting for the improved fill factor. If the buckling instability could be avoided then this processing procedure may be used in the future to manufacture even better devices than with any of the other annealing procedures. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011

Diels−Alder (DA) chemistry is increasing popular due to its simplicity and efficiency; however, one concept that has yet to be thoroughly explored is incorporation of DA linkages within materials f...

Polymeric nanoparticles can be manufactured by intramolecular cross-linking of benzocyclobutene (BCB)/styrene copolymers. We study this process by extensive molecular dynamics simulations of both atomistic and coarse-grained models. It is found that the rate of intramolecular cross-linking is close to quadratic in the number of unlinked cross-linkers, namely BCB monomeric units, while the completeness of the reaction is affected by the steric hindrance of the cross-linked polymer. An equation incorporating both of these processes yields a good agreement with simulation data. A rigidity effect is identified when the cross-linkers account for more than about 60% of a precursor BCB/styrene copolymer. Above this threshold, the radii of gyration of intramolecularly cross-linked BCB/styrene copolymers show little dependence on their cross-linker concentrations, and a relatively large number of BCB monomeric units are left unreacted in each polymer. In addition, a temperature series study is performed on the int...

A new synthetic approach to the preparation of intramolecularly collapsed nanoparticles under mild, room temperature conditions has been developed from commercially available vinyl monomers. Reaction of isocyanate functionalized linear copolymers with a diamine in dilute solution leads to the efficient formation of nanoparticles where the diameter of the nanoparticle can be varied by controlling both the molecular weight and mole percentage of isocyanate repeat units. Physical properties for the intramolecularly collapsed nanoparticles were fully consistent with a three-dimensional structure and analysis of the collapse reaction revealed that approximately 75% of the isocyanate groups along the backbone underwent crosslinking with 25% being available for further reaction with mono-functional amines. This stepwise consumption of the isocyanates allows the chemical and physical properties of the nanoparticles to be further tuned and significantly opens up the range of nanoparticles that can be prepared using this mild and highly efficient chemistry.

It is demonstrated that nanocomposites exhibiting reduced viscosity and multifunctional performance enhancements may be fabricated using simple processing procedures. These behaviors are elucidated by analysis of the effects of dispersed organic (fullerene) nanoparticles and inorganic (magnetite) nanoparticles on the behavior of polystyrene, demonstrating that simple spherical nanoparticles can induce a range of unexpected behavior due to nanoscale effects. In general, multifunctional performance improvements including enhanced mechanical, electrical, magnetic and thermal degradation properties as well as reduced viscosity are promoted when simple design guidelines are followed. These guidelines are tabulated.

Polymandelide, an aryl analogue of polylactide, was synthesized by the ring-opening polymerization of mandelide, the cyclic dimer of mandelic acid. The poor solubility of rac-mandelide limited the synthesis of polymandelide via solution polymerization, but polymerization of mandelide at 70 °C as a heterogeneous slurry in acetonitrile yielded first-order kinetic plots and polydispersities 150 °C exhibit properties that mimic those of polystyrene. Polymandelide is a glassy amorphous polymer with a Tg of 100 °C, with rheological properties comparable to polystyrene, and thermal gravimetric analyses under nitrogen show that the polymer is stable to ∼300 °C. Racemization during polymerization precluded formation of a crystalline polymer. Degradation of polymandelide in pH 7.4 buffer at 55 °C is consistent with a bulk erosion model and, due to its high Tg, proceeds at ∼1/100 the rate of polylactide under similar conditions.

Novel polystyrene nanoparticles were synthesized by the controlled intra- molecular crosslinking of linear polymer chains to produce well-defined single-mole- cule nanoparticles of varying molecular mass, corresponding directly to the original linear precursor chain. These nanoparticles are ideal to study the relaxation dynam- ics/processes of high molecular mass polymer melts, as the high degree of intramolec- ular crosslinking potentially inhibits entanglements. Both the nanoparticles and their linear analogs were characterized by measuring their intrinsic viscosity, hydrody- namic radius (Rh), and radius of gyration (Rg). The ratio Rg/Rh was computed to char- acterize the molecular architecture of the nanoparticles in solution, revealing a shift toward the constant density sphere limit with increasing crosslink density and molec- ular mass. Further, confirming particulate behavior, Kratky plots obtained from neu- tron scattering data show a shift toward particle-like nature. The rheological behav- ior of the particles was found to be strongly dependent on both the extent of intramo- lecular crosslinking and molecular mass, with a minimal viscosity change at low crosslinking levels and a gel-like behavior evident for a large degree of crosslinking. These and other results suggest the presence of a secondary mode of polymer relaxa- tion/movement besides reptation, which in this case, is influenced by the total num- ber of crosslinked loops present in the nanoparticle. V C 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1930-1947, 2006

The conformation of linear−dendrimer hybrid diblock copolymers in solution has been studied using both small-angle neutron scattering and dynamic light scattering. The diblock consisted of a fourth-generation benzyl ether dendrimer with different molecular weight polystyrene bonded to the focal point of the dendrimer; the total molecular weight ranged from 20 to 100 kDa. In agreement with previous studies, it was found that this dendron, without a linear chain attached, alters its size upon solvent change. The addition of a polystyrene chain to the focal point of a dendrimer was also found to have an effect on the dimensions and shape of the dendrimer block. A low-molecular-weight polystyrene chain swelled the dendrimer without perturbing its native spherelike conformation. However, at larger polystyrene molecular weights, the linear block manipulates a transition of the dendrimers' morphology form spherelike to an extended conformation. Control of this shape and size change has potential for these unique...

The effect of nanoscopic, shape persistent polystyrene (PS) nanoparticles on the rheological properties of linear PS is studied and a dramatic viscosity reduction is observed. This is an ideal blend which simplifies enthalpic interactions between the components and can be used to delineate the effect of particle size on the properties of the blends. Homogeneous blends are assured through small-angle neutron scattering (SANS) experiments which establish the absence of phase segregation (depletion flocculation) in the nanoparticle-polymer blend. We previously found that nanoparticles reduce the viscosity of high molecular mass linear PS when the interparticle gap is smaller than the linear polymer size. In the present study, we find that such confinement of entangled polymers is necessary for the viscosity reduction since lower concentrations provide a viscosity increase. Furthermore, the behavior is found to be dependent on the presence or absence of entanglements and confinement is seemingly not important for unentangled polymers. It is proposed that constraint release caused by the addition of nanoparticles is responsible for some of the observed changes in viscosity although it is suspected this is a very complicated phenomenon and introduction of free volume by the nanoparticles is certain to play a key role.

Hybrid copolymers with dendritic−linear blocks are shown to exhibit many of the classic microphase-separated structures of linear−linear block copolymers. Transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), and small-angle neutron scattering (SANS) were used to evaluate the morphology of a sixth generation (G6) poly(benzyl ether) dendron covalently bonded to linear polystyrene (PS) at the dendron focal point. Increasing the fraction of the linear block, φPS, through an increase in the molecular weight of the PS block revealed morphologies evolving from disordered to ordered lamellar to hexagonally close-packed dendron cylinders. Significantly, the observed morphologies are distinct from those expected for analogous linear−linear blocks at equivalent volume fraction, although the direction of progression follows expectation. Quantitative analysis suggests substantial molecular deformation or shape change in the dendritic phase. The possible role of conformational entropy in determi...

The hydrodynamic volume for a series of hyperbranched polymers was studied to determine the volume change in a variety of solvents. The chemically different interior core and branching units were found to readily expand and contract by a factor of 2 creating large or small free volume, respectively. Furthermore, a solvent that maximally swells the polymer created a viscosimetric (hydrodynamic) radius which changed linearly with molecular mass. This is contrary to what is expected for dendrimers where the radius has been shown to scale with ln(M). A model was developed to account for the effect of molecular mass polydispersity on the intrinsic viscosity (viscosimetric volume), since hyperbranched polymers are polydisperse in nature, and it was found that this did not affect the observation. Solvents that contracted the hyperbranched polymers showed a complicated hydrodynamic radius scaling with mass. It was generally concluded that these polymers readily change volume with solvent effects important in infl...

The intrinsic viscosity of narrow molecular mass distribution polystyrene standards, poly(benzyl) ether dendrimers, and their hybrid block copolymers was measured in a variety of solvents. The relative change in the intrinsic viscosity for all these polymers was almost equivalent, indicating that poly(benzyl ether) dendrimers expand and contract quite readily with a change in solvent. The intrinsic viscosity maximum for all polymers was found to occur at essentially the same solvent solubility parameter demonstrating that the thermodynamic interaction with the selected range of solvents was also equivalent. The intrinsic viscosity for hybrid copolymers in a good solvent for both components was found to fall well below the intrinsic viscosity for neat polystyrene at intermediate molecular masses. Below and above this mass range the intrinsic viscosity was quite similar to polystyrene. Yet, the transition for intermediate to high molecular mass behavior was quite sharp as seen through a rapid intrinsic visc...

The pressure-volume-temperature and thermal properties of dendrimers based on benzyl ether were measured and compared with literature values for monodisperse, linear polystyrenes. In addition, property measurements are presented for an exact linear analogue to the fifth-generation dendrimer. The thermodynamic properties' molecular weight behavior for the dendrimers is unique when compared with that of linear polystyrene. All of the evidence presented in this work suggests that some form of structural transition occurs in the bulk at a molecular mass near that for the fourth-generation dendrimer. No such transition is seen for polystyrene. Dendrimers exhibit an increased packing efficiency as evidenced by a decreased specific volume (increased density) as compared with an exact linear analogue of the fifth-generation dendrimer analogue, and the dendrimer highlights the entropic differences between the two molecules. In addition, differences in the change in heat capacity with temperature for the two systems further allude to their entropic differences. A crystalline state can be formed for the lower generation dendrimer and linear analogue. This crystalline state is not seen in dendrimers above the third generation. These behaviors compiled with the difference in the glass-transition temperature for the linear analogue suggest that the dendrimers' microstructure has a significant influence on the bulk thermodynamic state of the material. The Tait equation was fitted to the volume data for comparative purposes; the Tait equation has known limitations but was selected because of its widespread application. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1766–1777, 2001

A supramolecular elastomer comprising a blend of a pyrenyl-tweezer-ended polyamide intercalating with a chain-folding polyimide shows near-quantitative thermal healability. It also exhibits greatly enhanced toughness relative to an analogous system in which the polyamide end-groups are simple pyrenyl residues, reflecting the increased association constant between the two complementary binding motifs.

The dynamic viscoelastic behavior of Poly(l-lactic acid) (PLLA), with molecular weights ranging from 2,000 to 360,000, have been studied over a broad range of reduced frequencies (approximately 1 × 10-3 s-1 to 1 × 103 s-1), using time-temperature superposition principle. Melts are shown to have a critical molecular weight, Mc, of approximately 16,000 g/mol, and an entanglement density of 0.16 mmol/cm3 (at 25°C). PLLA polymers are noted to require substantially larger molecular weights in order to display similar melt viscoelastic behavior, at a given temperature, as that for conventional non-biodegradable polymers such as polystyrene. The reason for this deviation is suspected to be due to steric hindrance, resulting from excessive coil expansion or other tertiary chain interactions. PLLA melts show a dependence of 0 on chain length to the 4.0 power (M[stack 4.0W ]), whilst J[stack 0e ] is independent of MW in the terminal region. Low molecular weight PLLA ( 40,000) shows Newtonian-like behavior at shear rates typical of those achieved during film extrusion. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1803-1814, 1999

The zero shear melt viscosity, eta0, for a variety of dendritic poly(benzyl ether) macromolecules has been measured. The viscosity of low generation dendrimers exhibits a strong molecular mass, M, dependence, yet, direct proportionality between eta0 and M is observed at higher generations. The viscosity scales with M for mono- and tridendrons rather than generation number. Hypercore dendrimers continue this scaling although the viscosity is slightly greater. A large deviation from the master curve for the mono- and tridendrons is found for an end-substituted monodendron. The viscosity for the end-substituted monodendron is also much more temperature dependent than virgin dendrimers, and it is concluded that end groups have a much larger effect on the viscosity than the core. Reexamination of literature data for intrinsic viscosity and glass transition temperature is performed, and these quantities also scale with molecular mass. Comparison with free volume theory indicates that the theory is extremely robust although some question of its validity is made when applied to dendrimers.

A review of current chemorheological techniques and measurement systems is presented for unfilled and filled thermoset resins. The specific measurement techniques for the kinetics, chemorheology, and modeling of these systems are presented, with particular emphasis on the chemorheological techniques and measuring systems. These techniques and measurement systems provide a greater understanding of traditionally complex thermoset processes, provide effective quality control measures for these processes, and will reduce design and operating costs in associated industries.

Linear low-density polyethylene (LLDPE) was irradiated with electron beam radiation at room temperature and in the melt. It was found that the gel point, as measured by an insoluble fraction in boiling xylene, occurred at a slightly lower radiation dose for melt irradiated samples. The melt radiation-crosslinked samples showed different melt rheological properties from the room-temperature radiation-crosslinked samples, as exemplified by the stress relaxing more slowly in a step strain experiment (relaxation modulus) for the former at a given radiation dose. The relaxation modulus could be fitted to a power-law expression consisting of two parameters, which, when plotted against each other, resulted in a single master curve for both irradiation temperatures. The curve showed two distinct regions of differing slope, which intersect at a point argued to represent a pseudogel point.

The impurities present in the commercial (Ciba-Geigy) N,N,N',N'-tetraglycidyl-4,4'-diaminodiphenylmethane (TGDDM) resins MY720 and MY72I and a purified TGDDM resin were studied using high-performance liquid chromatography and infrared spectroscopy. The cure kinetics for the three resins cured with 27% 4,4'-diaminodiphenylsulfone (DDS) were described using a previously developed kinetic model and near-infrared spectroscopy with the catalysis of epoxide reactions by impurity hydroxyl groups being seen to have a large effect on the initial rate of epoxide reaction. The gel times were determined using the crossover point between storage and loss moduli (i.e. tan (= 1) as measured using dynamic rheometry. An impurity effect on the-degree of epoxide reaction necessary for gelation to occur was identified and an empirical relationship was developed to quantify this effect. An approach was proposed for the prediction of gel times for TGDDM resins of any purity cured with DDS combining the use of the kinetic model and empirical gel time function.

A novel supramolecular polymer system, in which the terminal pyrenyl groups of a polyamide intercalate into the chain-folds of a polyimide via electronically-complementary pi-pi stacking, shows both enhanced mechanical properties relative to those of its individual components and facile healing characteristics as a result of the thermoreversibility of non-covalent interactions.