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Synthetic aromatic polymers are ubiquitous and indispensable to modern life, industry, and the global economy. The direct functionalization of these materials remains a considerable challenge on account of their unreactive aromatic C-H bonds and robust physical properties. Here, we demonstrate that homogeneous gold catalysis offers a mild, chemoselective, and practical approach to functionalize high-volume commodity aromatic polymers. Utilizing a gold-catalyzed intermolecular hydroarylation between a methyl ester functionalized alkyne, methyl propiolate, and nucleophilic arenes within polystyrene (PS) results in direct functionalization of phenyl rings with 1,2-substituted methyl acrylate functional groups. The reactivity and functionalization depend on the steric and electronic environment of the catalyst, counterion pairing, and method of activation. The reactivity is broad in scope, enabling the functionalization of arenes within commercial polysulfone (PSU) and waste polyethylene terephthalate (PET). These reactions open new opportunities to chemically transform aromatic polymers and modify their physical properties.

Atomic oxygen (AO) attacks carbon-fiber reinforcing polymers on the surfaces of spacecraft in low Earth orbit and threatens safe spacecraft operation and service life. Incorporating phenylphosphine...

The objective of this work was to computationally predict the melting temperature and melt properties of thermosetting monomers used in aerospace applications. In this study, we applied an existing voids method by Solca. to examine four cyanate ester monomers with a wide range of melting temperatures. Voids were introduced into some simulations by removal of molecules from lattice positions to lower the free-energy barrier to melting to directly simulate the transition from a stable crystal to amorphous solid and capture the melting temperature. We validated model predictions by comparing melting temperature against previously reported literature values. Additionally, the torsion and orientational order parameters were used to examine the monomers' freedom of motion to investigate structure-property relationships. Ultimately, the voids method provided reasonable estimates of melting temperature while the torsion and order parameter analysis provided insight into sources of the differing melt properties between the thermosetting monomers. As a whole, the results shed light on how freedom of molecular motions in the monomer melt state may affect melting temperature and can be utilized to inspire the development of thermosetting monomers with optimal monomer melt properties for demanding applications.

Polymer science seeks to explain the properties of polymer nanocomposites in terms of nanostructure formation within the polymeric matrix. In this chapter, we consider how the POSS molecules are assembled to form the bulk nanocomposite materials, which must be understood to develop relations between the structure and properties of these materials. In broad terms, we can predict the nanostructure evolution based on the molecular architecture of the POSS molecule; however, several levels of structural arrangement may exist depending on the interactions between POSS-polymer or POSS-POSS molecules. This may result in a complex nanostructure in the electron microscope. The properties of nanocomposites are defined significantly by these interactions operating at every level of the nanocomposite manufacturing process. Here we discuss the influence of the incorporation of POSS into polymer matrices, as affected by surface functional group, and the POSS concentration, on the structure of nanocomposites.

Epoxy hybrid networks containing up to 50 wt.% pendant polyhedral oligomeric silsesquioxane (POSS) dispersed at molecular level are prepared by polymerizing DGEBA-POSS precursors with 4,4′-diaminodiphenylmethane (DDM). A high molecular weight control precursor was prepared without pendant POSS cage to quantify the effect of pendant POSS cage on network free volume, number of crosslinks per unit volume and glass transition temperature. Differential scanning calorimetry (DSC) shows drastic decrease in T g with increasing pendant POSS content. The T g value of hybrid network containing 50wt.% POSS is 100 °C lower than the DGEBA-DDM network compare to 77 °C for epoxy network prepared by high molecular weight control precursor (without POSS cage). In order to establish structure-property correlations, these variations have been correlated with free volume, crosslinking density, and chain packing in the network. For the epoxy hybrid networks containing up to 50wt% pendant POSS, the storage modulus of the glassy state (−50 °C) are lower than that of the DGEBA-DDM network, indicating that incorporation of pendant POSS at molecular level increases the spacing and free volume between the chains, and provides them freedom to promote motions. Also, lower storage modulus values in the rubbery state suggest there is no nano-reinforcement effect of POSS cages even at a very high loading (50wt%). Thermogravimetric analysis (TGA) and cone calorimetry show the POSS containing epoxy nanocomposites display high ceramic yields, suggesting the flame retardant behavior of the materials is improved by the molecular level dispersion of pendant POSS. A total of 62% increase in char yield and 53% decrease in peak heat release rate (PHRR) during combustion were observed in hybrid network containing 50wt% POSS compared to DGEBA-DDM network. This study gives new insights into the effects of structural changes in POSS containing thermosets on their performance properties.

Tremendous effort is focused on improving epoxy toughness, but common approaches diminish processability and mechanical properties. This work presents hybrid additives which simultaneously enhance toughness, modulus, and other mechanical properties without reducing the network's processability. The hybrid herein was molecularly designed to contain “hard” nanoparticles and “soft” organic segments which both covalently bond and interact to form strong interfaces with epoxy networks. It is shown that a hyperbranched epoxy (HBE) reacted with a molecular silica, polyhedral oligomeric silsesquioxane (POSS), can provide the desired simultaneous reinforcement and toughness enhancements while maintaining processability. Epoxy network incorporation of the hybrid toughener led to a 220% increase in fracture toughness while also bringing about an increase in Young's modulus of over 20%. These significant improvements are described by the cured network's multi-scale morphologies and toughener dispersion states, and are related to strong interactions which occur between POSS's pendants and the HBE.

A novel high shear continuous reactor method was developed to obtain molecular level incorporation of polyhedral oligomeric silsesquioxane (POSS) containing one amine group and seven isobutyl groups into epoxy network as pendant cage. The method consisted of optimizing reaction time and temperature for the synthesis of POSS-epoxy precursor in a high shear continuous reactor. A full conversion of POSS into POSS-epoxy precursor was achieved within 30–40 s of reaction time in a continuous reactor for all molar compositions from 1:140 to 1:1.05 in comparison to a batch process with the reaction time of 18 h. The structure of this precursor at the end of the reaction, determined by silicon Nuclear Magnetic Resonance spectroscopy (29Si NMR), was close to the ideal one. The distribution of reaction product was determined by size exclusion chromatography (SEC). The reaction product containing up to 1:3-mol ratio of POSS: epoxy molecules showed ‘epoxy-POSS-epoxy’ precursor as a primary product in comparison to 1:2 and 1:1.05-mol ratios with higher molecular weight precursor as the primary product. Hybrid networks containing up to 50 wt % POSS were prepared by curing these precursors with an aromatic amine curative 4, 4'diamino diphenyl methane (DDM). The resulting networks are completely transparent and no phase separation was observed by SEM and TEM in the course of polymerization despite the incompatibility of the isobutyl groups attached to the POSS with the aromatic epoxy-amine networks. In addition to that, DMA and TGA results prove that the introduction of high shear continuous reactor promotes the dispersion of pendant POSS to a molecular level into epoxy networks.

Digital Image Correlation (DIC), which exploits non-contact advantages and full-field analysis, provides more data in-situ that are not possible with traditional techniques. In this work, elevated temperature digital image correlation techniques were applied to a glassy polymer network via thermal expansion and contraction experiments to study volumetric behavior during the curing process. A glassy epoxy network was tested in both cured and under-cured states and heated to the ultimate cure temperature. Matrix volume changes due to both thermal expansion and cure shrinkage were quantified. Concurrently, the thermal expansion of aerospace-grade composite laminates was also observed in matrix and fiber-dominant directions. Additionally, the strain-free temperature of a non-symmetric composite laminate was identified through thermal compensation of process-induced curvatures. Finally, laminate dimension changes were related to the strain-free temperature as means to probe process-induced strains within composite laminates. Thermal properties of the neat matrix and composite laminates were compared to traditional techniques, validating the benefit of elevated temperature digital image correlation for composite matrix qualification.

The Jonscher universal power law for ac conductivity versus frequency (f = ω/2π) in the dispersion region was tested for a multiwall carbon nanotube/epoxy nanocomposite. The effect of changes in agglomerate morphology on the fitting parameters A and n in the equation σac = Aωn was investigated. Changing nanotube agglomerate morphology was tracked by optical microscopy through curing. Evolving morphology was compared alongside ac conductivity obtained via a broadband dielectric spectrometer to elucidate possible physical meaning of the universal power law in the context of this system. The −logA/n was unaffected by changes in agglomerate morphology affected during cure, yet connected with each other in their dependence on temperature. For this system, the relationship between the fitting parameters in the universal dynamic response equation remains empirical at this stage with regard to biphasic “texture” or morphology within such a network. Electrical conductivity σ versus frequency ω for a composite consisting of agglomerated multiwalled carbon nanotubes dispersed throughout a cured epoxy matrix was discovered to follow the empirical universal dynamic response equation of Jonscher. The frequency behavior of the exponent n is discussed in terms of underlying morphology throughout which charge carriers migrate. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 1918–1923

Careful control of polyacrylonitrile (PAN) precursor properties and, in particular, the polymerization method and ensuing molecular weight and molecular weight distribution are important considerations for mitigating defects and enhancing processability of carbon fibers. Herein, a comprehensive study was performed to understand the influence of molecular weight, molecular weight distribution, and polymerization method between reversible addition-fragmentation chain transfer (RAFT) and conventional free radical (FR) solution polymerization on the cyclization behavior and structural evolution of stabilized PAN. The kinetic parameters of activation energy (Ea) and pre-exponential factor (A) were determined along with the cyclization index (CI) by differential scanning calorimetry (DSC) and the extent of stabilization (Es) was measured via fourier transform infrared spectroscopy (FTIR). Thermogravimetric analysis (TGA) was used to determine the degradation differences in the polymers. Structural characterization was performed by wide-angle X-ray scattering (WAXS). DSC and FTIR analysis indicate that cyclization initiates at lower temperature for FR polymers. Significantly, PAN-based RAFT copolymers show greater mass retention post thermo-oxidative degradation as compared to analogous FR copolymers.

Isomeric tri-aryl ketone amines, 1,3-bis(3-aminobenzoyl)benzene (133 BABB), 1,3-bis(4-aminobenzoyl)benzene (134 BABB), and 1,4-bis(4-aminobenzoyl)benzene (144 BABB) are synthesized and cured with diglycidyl ether of bisphenol A and diglycidyl ether of bisphenol F in this work. Differential scanning calorimetry and near-infrared spectroscopy reveal higher rate constants and enhanced secondary amine conversion with increasing para substitution attributed to resonance effects and the electron withdrawing nature of the carbonyl linkages. Glass transition temperatures increase from 133 BABB to 134 BABB, but decrease modestly for the 144 BABB hardener. With increasing para substitution, the flexural modulus and strength both decrease while the strain to failure increases but all BABB amines displaying higher mechanical properties than the corresponding 4,4-diaminodiphenyl sulfone (44 DDS) networks. The thermal stability of the BABB networks is found to be modestly lower than 44 DDS, but char yields are significantly higher. Changes in thermal and mechanical properties are described in terms of molecular structure and equilibrium packing density.

The physical, cognitive, emotional, and financial costs associated with head injuries in sport can be great. Head injuries in sport can take different forms that range from obvious structural damage of the skull, mandible, and soft tissue to the subtler mild traumatic brain injury (mTBI). Regardless of injury type, most head injuries in sport typically result from biomechanical forces generated from either a head-to-object collision (e.g., playing surface or another player) or projectile-to-head collision (e.g., ball or stick). Over the modern era severe sport-related head injuries (e.g., skull fracture) have declined, but sport-related mTBIs—also referred to as sport-related concussions (SRCs)—have increased. This trend is highly problematic because of the serious nature of SRC’s symptoms, inherent sequelae (e.g., postconcussion syndrome and second impact syndrome), and potential for long-term cognitive decline (e.g., chronic traumatic encephalopathy). Since 2001 the International Symposium on Concussion in Sport has defined concussion as a “complex pathophysiological process affecting the brain, induced by traumatic biomechanical forces.” The definition continues to include several features that can be used to define the nature of concussion. The most recent definition of SRC developed at the 5th International Symposium on Concussion in Sport held in Berlin in 2016 defines SRC as “a traumatic brain injury induced by biomechanical forces.” The definition goes on to include several common features that could be used to define the nature of a concussion. These biomechanical forces can be generated via direct insult to the head or neck region or by forces encountered elsewhere on the body that are transmitted to the head. Unfortunately, uncertainty still exists for healthcare medical providers regarding the recognition of concussion, particularly for on-field providers and the best methods to prevent SRC. The primary objective of this chapter is to provide a broad overview of protective helmets in sport, their composition, and manufacturing processes.

Utilizing reversible addition-fragmentation chain transfer (RAFT) polymerization, the composition profile of PAN-based copolymers becomes a tunable parameter in preparing carbon fiber precursors. In this work, poly(acrylonitrile-co-N-isopropylacrylamide) copolymers were prepared via a semibatch process mediated by 2-cyano-2-propyl dodecyl trithiocarbonate (CPDT). The monomer reactivity ratios were determined by the Fineman-Ross (FR), Kelen-Tudos (KT), and non-linear least squares (NLLS) models. The number average sequence length of acrylonitrile (AN) was found to be highly dependent on the content of N-isopropylacrylamide (NIPAM). NIPAM was introduced at controlled rates into a reaction vessel in attempt to facilitate its distribution in the polymer backbone. These semibatch copolymers were evaluated based on their ring-closing cyclization efficiency, which was characterized by differential scanning calorimetry (DSC), fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA). Significantly, DSC exotherms were reduced and cyclization occurred much faster reaching a greater thermal stability when NIPAM was introduced at slow feed rates.

Digital image correlation techniques were applied to compressive strain recovery experiments of glassy network isomers to study architectural contributions to deformational response. Strain recovery experiments provided a direct comparison of network isomer structure on viscoelastic deformation. Diglycidyl ether of bisphenol-A (DGEBA) was cured with 3,3′- and 4,4′-diaminodiphenyl sulfone (33DDS and 44DDS). The comparison revealed the networks have similar elastic and plastic strain components, but differ in anelastic strain response. DGEBA cured with 44DDS developed pre-yield anelastic strain at a significantly higher rate attributed to increase in free volume, molecular motions and energy dissipation. In contrast, DGEBA cured with 33DDS displayed lower pre-yield energy dissipation attributed to lower free volume and molecular motions, but exhibited a higher degree of post-yield strain-softening due to larger segmental rearrangements.

Hyperbranched polymers have been shown to improve epoxy fracture toughness at the cost of other mechanical properties. In this study an aliphatic, epoxide-functional hyperbranched polymer was synthesized and incorporated at different concentrations into an aromatic epoxy-amine network without cure-induced phase separation. The resulting homogeneous network architecture and thermomechanical properties were studied using DSC, DMA and FTIR, while the mechanical properties were investigated using uniaxial compression and single edge notch beam 3-point bending. It was found that although the flexible hyperbranched structure decreased the glass transition temperature of the network, it also allowed for simultaneous enhancement in both fracture toughness and Young's modulus. These findings were attributed to relationships between chemical structure, crosslink density and non-bond interactions, and indicate the potential for further glassy network fracture toughness improvement without loss of critical mechanical properties by tailoring of the hyperbranched structure.

High molecular weight polyacrylonitrile (PAN) with low dispersity has been successfully synthesized utilizing reversible addition-fragmentation chain transfer (RAFT) polymerization. A comprehensive study was performed to understand the influence of reaction temperature, RAFT agent structure, and [M]0:[CTA]0[I]0 on the polymerization kinetics, molecular weight, and dispersity. Enhanced control is attributed to reduction of side reactions by conducting the polymerization at lower temperature, and optimizing the radical exchange between active and dormant states via appropriate selection of RAFT agent and initiator. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015

Cross-linked epoxies prepared from diglycidyl ether of bisphenol A (DGEBA) exhibit substantial property differences due to isomeric structures of the diamine curatives, 3,3′- and 4,4′-diaminodiphenyl sulfone (DDS). Here, the motions of phenylene rings on both the bisphenol A (BPA) and DDS structures of the networks were investigated through solid-state NMR 2H line shape analysis to trace the molecular origins. Ring deuterated monomers, namely, DGEBA-d8, 33DDS-d8, and 44DDS-d8, were synthesized, and four deuterated networks were prepared. The line shapes of the ring motions were interpreted using a motional model that described both ring π-flips and main-chain fluctuations. The contributions of different ring motions to mechanical relaxations are elucidated, and the molecular origins of property differences are identified.

We report the design and synthesis of a polymer structure from a cross-linkable epoxy–ionic liquid system which behaves like a hard and brittle epoxy thermoset, perfectly ductile thermoplastic and an elastomer, all depending on controllable network compositions.

Surfaces and interfaces of polyethersulfone (PESU) materials play a crucial role in the overall performance of the polymer in applications such as filtration membranes, medical devices, and protective coatings. This work investigates the chain end localization to the polymer-air interface of solution cast films of semi-rigid rod PESU polymers end-capped with phenol (OH) and fluorescein isothiocyanate (FITC) groups. PESU solutions of varying molecular weight and low distribution were characterized via gel permeation chromatography (GPC) as well as static and dynamic light scattering. X-ray photoelectron spectroscopy (XPS) and confocal laser microscopy revealed a preferential localization of both –OH and –FITC chain ends to the PESU-air interface. Scaling of the chain end concentration as a function of polymer dimensions was determined to differ from that of flexible chain polymers. Surface analysis demonstrated that chain end chemistry could be used to tailor the nano- and macro-scale properties of the resulting PESU surface.

A novel 3D printing formulation based on a multifunctional benzoxazine (BOX) monomer possessing both photo and thermally polymerizable functional groups is reported. Printing formulation viscosity is readily tuned using a monofunctional-acrylate reactive diluent to enable Stereolithography (SLA) 3D printing. In the primary curing step, the printing formulation is UV-cured by SLA 3D printing to prepare accurate parts on the millimeter size scale. The 3D printed parts are then heated in the secondary curing step to activate a thermally initiated BOX ring opening polymerization. Dynamic mechanical analysis demonstrated that the 3D printed parts exhibit a single Tan δ peak after both the primary UV-cure and secondary thermal cure steps, suggesting the two polymerizations behave as one crosslinked network. The unique dual-cure strategy demonstrated in this research utilizes both photo and thermally initiated polymerizations to expand the library of materials available for 3D printing applications.

Nanocomposites of multi-walled carbon nanotubes and polypropylene were fabricated using twin screw extrusion and subjected to heating rates on the order of 100,000 degrees per second using laser pulse heating. Raman and infrared spectroscopy, as well as electron microscopy observations, showed that the nanotubes retain their structure and that a nanotube-rich layer formed at the surface of the composites during laser heating. By varying the weight fraction of filler in the composites, a 54% reduction in mass loss per laser pulse was achieved, compared to the pure polymer. The nanotube composites also displayed lower mass loss than nanoclay composites under the same conditions, despite having a significantly lower weight fraction of additive. Further reduction in mass loss per laser pulse was achieved in solvent-processed samples, which appear to have better dispersion than the extrusion-processed composites. We attribute these observations to the formation of a nanotube network formed on the surface, which acts as a protective barrier, slowing the degradation of the underlying polymer. Several observations made in this study are consistent with those reported in previously published slow heating rate studies, showing that while some phenomena are unique to transient heating, there are some commonalities between the two regimes.

This paper describes a new continuous reactor method to prepare thermoplastic modified epoxy prepolymers for aerospace prepregs with the aim of replacing traditional batch reactors. Compared with batch reactors, the continuous reactor is capable of producing epoxy prepolymers through simultaneous dissolution of polyethersulfone (PES) and 4,4′-diaminodiphenylsulfone in tetraglycidyl-4,4′- diaminodiphenylmethane (TGDDM). In addition, concurrent chain extension reactions advance prepolymer molecular weights to desired viscosities in less than 2 min of mean residence time. Optical micrographs were used to define how process temperature influences PES dissolution in TGDDM in a continuous reactor. Kinetic studies confirmed that the chain extension reaction in a continuous reactor is similar to that in a batch reactor, and the molecular weights and viscosities of prepolymers were readily controlled through reaction kinetics. Atomic force microscopy was used to confirm similar cured network morphologies for formulations prepared from batch and continuous reactors. Additionally tensile strength, tensile modulus and fracture toughness analyses concluded that mechanical properties of cured epoxy matrices produced from the two reactors were equivalent. © 2014 Society of Chemical Industry

A new family of poly(ethylene glycol) (PEG) based membranes for CO2 separation was developed using thiol–ene photopolymerization. Compared to photopolymerized PEG-containing acrylate membranes, these new thiol–ene based membranes offer improved mechanical properties and processing advantages. The starting material, a combination of a trithiol cross-linker and a PEG diene, was gradually modified with a PEG dithiol while maintaining 1:1 thiol:ene stoichiometry. This approach made it possible to decrease the network cross-link density, resulting in simultaneous increases in free volume and PEG content. Materials with high concentrations of dithiol were very stretchable, with largely, up to 500%, improved elongation at break, yet they exhibited commendable CO2/N2, O2, H2, and CH4 permeability-selectivity performance. The average molecular weight of polymer chains between cross-links, Mc, was determined experimentally by fitting the classic network affine model to stress–strain data obtained via tensile testin...

Stoichiometry and cure temperature were evaluated for epoxy systems based on the diglycidyl ethers of bisphenol-A and bisphenol-F and cured with 3,3′- or 4,4′-diaminodiphenylsulfone. The materials were formulated as stoichiometric benchmarks and with an excess of epoxide and cured in two steps (125°C/200°C) or one step (180°C). Dynamic mechanical analysis and free volume testing indicated decreased crosslink density and increased chain packing in the excess-epoxy materials, as well as a narrowing gap in properties between 33- and 44-cured networks with excess epoxy. The narrowing gap was less pronounced in materials cured at 180°C. The excess-epoxy materials were more resistant to water ingress, exhibiting reduced equilibrium water uptake. The excess-epoxy materials were also more resistant to methyl ethyl ketone ingress, which occurred at a slower rate in most excess-epoxy materials. The improvement in fluid resistance was attributed to enhanced chain packing in the materials with lower crosslink densities. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Thermoplastic polyethersulfone (PES) modified multifunctional tetraglycidyl-4,4′-diaminodiphenylmethane (TGDDM) and triglycidyl para-aminophenol (TGAP) epoxy prepolymers cured with 4,4′-diaminodiphenylsulfone (44DDS) were prepared using a continuous reactor method and their reaction-induced phase separated morphologies and mechanical properties were measured and correlated with chemical compositions. 1H nuclear magnetic resonance (1H NMR) and near-infrared spectroscopy (NIR) were used to quantify the chemical network formation. Atomic force microscopy (AFM) with nanomechanical mapping was employed to resolve the nanoscale phase-separated morphologies. The extent of phase separation in cured networks and resultant domain sizes were determined to be controllable depending upon the multifunctional epoxy compositions. The results obtained from mechanical studies further indicated that tensile modulus was not largely affected by multifunctional epoxy compositions while fracture toughness increased with increase of TGAP content. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 133, 44775.

Profound changes in network architecture from blending trifunctional (m-triglycidylaminophenol, mTGAP) or tetrafunctional (tetraglycidyldiaminodiphenylmethane, TGDDM) epoxides with diglycidyl ether of bisphenol-A (DGEBA) and a curative amine (3,3′-diaminodiphenylsulfone, 3,3′-DDS) were observed using PVT, DMA, and PALS analyses. Increasing multifunctional content, which increased the crosslink density (with the expected increase in Tg), produced a decrease in the average free volume hole size (Vh) accompanied by a counterintuitive increase in fractional free volume (FFV). This unusual inverse relationship between FFV and Vh allowed clear resolution of their respective roles in equilibrium moisture uptake vs. the rate of uptake (diffusivity). Equilibrium water uptake increased with increasing multifunctional content, concomitant with the increase in FFV. Water diffusivity, on the other hand, decreased with increasing multifunctional content, concomitant with the decrease in Vh. The decreasing Vh in the epoxy blends also had interesting consequences for organic solvent sensitivity. MEK ingress was substantial in the binary DGEBA/DDS epoxy and completely inhibited for most of the blends, implying hole size selectivity was responsible for the MEK uptake inhibition. MEK uptake was precluded in epoxies whose Vh was below a critical threshold value of ∼68 A3. A small amount of mTGAP or TGDDM was sufficient to reduce the Vh of DGEBA/DDS epoxy below the threshold and prevent MEK uptake.

Epoxy networks of the diglycidyl ether of bisphenol A (DGEBA) were prepared using 3,3′- and 4,4′-diaminodiphenyl sulfone isomer crosslinkers. Secondary relaxations and the glass transitions of resultant networks were probed using broadband dielectric spectroscopy (BDS). A sub-Tgγ relaxation peak for both networks shifts to higher frequencies (f) with increasing temperature in Arrhenius fashion, both processes having the same activation energy and being assigned to phenyl ring flipping in DGEBA chains. A β relaxation is assigned to local motions of dipoles that were created during crosslinking reactions. 4,4′-based networks exhibited higher Tg relative to 3,3′-based networks as per dynamic mechanical as well as BDS analyses. The Vogel–Fulcher–Tammann–Hesse equation fitted well to relaxation time vs. temperature data and comparison of Vogel temperatures suggests lower free volume per mass for the 3,3′-based network. The Kramers–Krönig transformation was used to directly calculate dc-free ɛ″ vs. f data from experimental ɛ′ vs. f data. Distribution of relaxation times (DRT) curves are bi-modal for the 3,3′-crosslinked resin suggesting large-scale microstructural heterogeneity as opposed to homogeneity for the 4,4′-based network whose DRT consists of a single peak. U.S. Office of Naval Research, Award N00014-07-1-1057 and fellowship support from the Department of Education Graduate Assistance in Areas of National Need Award P200A090066. Qatar University's Center for Advanced Materials' Start-Up grant.

A digital image correlation (DIC) technique to quantify complex strain behavior resulting from solvent ingress into glassy polymer networks has been developed. Our results clearly show the strain behavior for Case II diffusion solvent ingress. The utilization of DIC to study solvent ingress has shown a new method for quantifying the advancing flow front of solvent penetration into polymer networks. This non-contact technique is advantageous for quantifying the kinetic rate of solvent ingress and provides a method for studying the amount of strain developing in polymer samples during solvent swelling. DIC provides an ability to track strain variations between swollen and unperturbed states of polymers, and quantifies strain variance across the sample between fully swollen and non-swollen states. To our knowledge, a method for measuring strain for solvent ingress simultaneously across all states of the sample does not exist, and this method offers new opportunities for studying the ingress behavior for a broad range of solvent penetrants and polymers.

Dispersion of monoamine-functionalized polyhedral oligomeric silsesquioxane (POSS) in an epoxy network was improved by pre-reacting the POSS with excess epoxide and employing a high-temperature cure. DGEBA/DDS networks were formulated with 2.5 and 10 wt% POSS. In some samples, POSS was pre-reacted with DGEBA. The hybrid materials were characterized via SEM, TEM, and DMA. The microscopy and DMA results evinced a multiscale morphology with POSS-rich glassy domains, nano- and microcrystallites, and crystallite agglomerations. For a loading level of 2.5 wt% POSS, the sample with unmodified POSS cured at 125 °C had inorganic crystallites on the order of 1–5 μm and agglomerations on the order of 10–20 μm, whereas the sample with pre-reacted POSS cured at 180 °C had near-perfect dispersion with no agglomerations and very few POSS crystallites. The 10 wt% POSS epoxies also showed improved dispersion with pre-reaction and increasing cure temperature. The dispersion improvements were attributed to the enhanced miscibility of the pre-reacted POSS and the increased rate of POSS reaction into the growing epoxy network at a higher cure temperature.

A novel fluorescent co-polymer with an organoboron quinolate and an E -BODIPY (BODIPY: 4,4-difluoro-4-bora-3a-4a-diaza- s -indacene) moiety was synthesized with the aim of producing a donor-acceptor polymeric system where the organoboron quinolate acts as the donor and the E -BODIPY moiety is the acceptor. The polymer has three prominent absorption bands: 264 nm (corresponding to the organoboron quinolate), 397 nm (corresponding to the organoboron quinolate and BODIPY) and 516 nm (corresponding to the E -BODIPY moiety). Excitation of the organoboron quinolate at 264 nm resulted in emission at 525 nm, giving a 261 nm Stokes shift. Energy transfer from the donor (organoboron quinolate) unit to the acceptor (BODIPY) explains the origin of this large Stokes shift.

This manuscript demonstrates the synthesis of glassy polymer network isomers to control morphological variations and study solvent ingress behavior independent of chemical affinity. Well-controlled network architectures with varying free volume average hole-sizes have been shown to substantially influence solvent ingress within glassy polymer networks. Bisphenol-A diglycidyl ether (DGEBA), bisphenol-F diglycidyl ether (DGEBF) and tetraglydicyl-4,4′-diamino-diphenyl methane (TGDDM) were cured with 3,3′- and 4,4′-diaminodiphenyl sulfone (DDS) at a stoichiometric ratio of 1:1 oxirane to amine active hydrogen to generate a series of network architectures with an average free volume hole-size (Vh) ranging between 59 and 82 A3. Polymer networks were exposed to water and a broad range of organic solvents ranging in van der Waals (vdW) volumes from 18 to 88 A3 for up to 10,000 h time. A clear relationship between glassy polymer network Vh and fluid penetration has been established. As penetrant vdW volume approached Vh uptake kinetics significantly decreased, and as penetrant vdW volume exceeded Vh a blocking mechanism dominated ingress and prevented penetrant transport. These results suggest that reducing the free volume hole-size is a reasonable approach to control solvent properties for glassy polymer networks.

The utility of incorporating only 20 mol% of multifunctional thiols into UV-cured poly(ethyleneglycol)diacrylate (PEGDA) membranes for CO2 separation is presented. Five different multifunctional thiols were investigated, spanning a range of functionalities from 2 to 4 and also different rigidities. All thiol-modified networks exhibited lower modulus values according to both small-strain and large-strain mechanical testing. This decrease in modulus was primarily attributed to a decrease in crosslink density via the step-growth thiol–acrylate reaction. Decreased crosslink density was more pronounced for dithiol additives which formed a “train-like” topology of connected PEGDA segments. Because of this more linear topology, dithiol-modified networks exhibited increases in gas permeability and strain at break. CO2 selectivity of each thiol-modified network was nearly comparable to the unmodified PEGDA network. Each network rapidly reached 100% conversion under a nitrogen atmosphere. However in air, the pure acrylate network reached negligible conversion while the networks modified with tri and tetrathiols reached noticeably higher conversions. Outdoor sunlight curing revealed that all thiol-modified networks were tack-free after only several minutes depending on thiol functionality. In contrast, the pure acrylate membrane remained tacky and uncured on the surface indefinitely. This study emphasizes the importance of thiol monomer selection in diminishing oxygen inhibition in UV-initiated acrylate polymerization as well as marked improvement in permeability for this specific PEGDA network via the incorporation of dithiols.

Analytical methods which apply digital image correlation (DIC) to study the strain behavior of glassy polymer networks in uniaxial compression are presented. This non-contact technique generates accurate and reproducible data for analyzing compression modulus, Poisson’s ratio and yield strain. DIC eliminates the need for extensometers or strain gauges, and provides a substantial increase in accuracy for measuring strain compared to a LVDT. DIC allows for simultaneous longitudinal and transverse strain measurements to calculate Poisson’s ratio, and is particularly sensitive for measuring small strain compression modulus of glassy solids. DIC techniques are useful for studying the pre-yield, yield and post-yield behavior of glassy polymer networks and provide insight of the non-uniform strain behavior of these materials prior to yielding. An accurate yield strain was obtained using DIC which clearly eliminated any erroneous strain data associated with fixture and equipment compliance. Finally, DIC allowed for the calculation of true compression stress and true compression strain of glassy polymers.

Our manuscript demonstrates a new “click-type” polymerization reaction. We present the synthesis of a new polymer poly(1,4-diphenyl-1,4-dihydro-1,2,4,5-tetrazine-3,6-diyl-2,6-pyridinediyl) through bis-nitrilimine intermediates which undergo bis-1,3-dipolar cycloaddition polymerization reactions. We synthesized N,N′-diphenyl-2,6-pyridindioic(bis-hydrazide) and halogenated this precursor to form N,N′-diphenyl-2,6-pyridindioic (bis-hydrazonoyl dichloride) as a new monomer. The resulting polymeric structure is a high molecular weight alternating copolymer of pyridine and diphenyltetrazine soluble in a variety of organic solvents. Synthetic preparation and spectral analysis of the synthesis is presented. In addition, we demonstrate coordination of the polymer with cobalt metal salts indicating high complexation capability for these polymers. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

A room-temperature cure epoxy consisting of diglycidyl ether of bisphenol-F (DGEBF) and diethylene triamine (DETA) was modified with 26.5 wt.% and 63.5 wt.% octaglycidyl polyhedral oligomeric silsesquioxane (POSS) to investigate elevated temperature thermomechanical performance. Composites fabricated using vacuum assist resin transfer molding (VARTM) were compared to vinylester and epoxy standards. POSS modified matrices were low viscosity of 0.25–0.40 Pa s. Although Tg was 20% lower than vinylester, we observed an increase of >300% in 150 °C storage modulus, >50% in tensile modulus, >35% in flexural modulus, and the complete elimination of a heat distortion temperature (HDT) up to 200 °C. The matrices demonstrated an excellent balance of flow, wetting, and pot-life behaviors making them attractive alternatives for ambient cure marine applications.

Degradable thermoplastic polyurethane (TPU) elastomers incorporating poly(D,L-lactide-co-glycolide) (PLGA) were synthesized and characterized. The soft segments consisted of a mixture of poly(butylene adipate) (PBA) and PLGA with PBA/PLGA ratios of 100/0, 75/25, and 50/50 wt %. Two PLGA polyesters were used. BD-PLGA was initiated from butanediol; whereas BHMBA-PLGA was initiated from 2,2-bis-(hydroxymethyl)butanoic acid. The hard segments consisted of dicyclohexylmethane-4,4′-diisocyanate (H12MDI) and 1,4-butanediol (BD). The hard segment content, expressed as the weight ratio of BD to polyol used in the TPU formulation, was set either at 8 or 12% (31.2 or 38.1% hard segment by weight, respectively). In all cases initial [NCO]/[OH] ratio was 1.03. The tensile modulus of the TPUs ranged from 9 to 131 MPa and ultimate strains ranged from 100 to 750%. DMA was used to probe the thermomechanical transitions of the TPUs and indicated useful application temperatures from well below zero up to 60–80°C depending on the formulation. Hydrolytic degradation of the TPUs was tested in seawater at 37°C. All of the PLGA-containing TPUs showed enhanced degradation compared to those with only PBA as the soft segment. The latter compositions remained essentially unchanged throughout the test while the PLGA-containing TPUs lost as much as 45% of their initial mass in 153 days. Molecular weights of TPUs containing degradable polyols were lower than those derived from 100% PBA polyol. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

We report the synthesis of polyurethane-urea elastomers containing 1,2,4,5-tetrazine covalently reacted within the main chains of the polymers. Our study investigates the synthesis of 3,6-diamino-1,2,4,5-tetrazine (DAT), the polymerization reaction conditions for reacting DAT into the backbone of segmented polyurethane elastomers, and the metal-complexation capabilities of tetrazine-containing elastomers with cobalt (II) chloride. Tetrazines are highly colored and electro-active heterocyclic moieties, which have a very high electron affinity which make them reducible at high to very high potentials. Upon complexation with metals, we observed a strong color shift of the polymers from deep red to blue indicating the binding efficacy for the polymers. We quantified the metal-complexation capability of the tetrazine elastomers and determined a molar ratio of approximately two metal atoms per tetrazine allowing us to provide a plausible complexation mechanism for the active polymers. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

We report the activity for a new tin-polyhedral oligomeric metal silsesquioxane (POMS) catalyst in 1-butanol and 2-butanol model reactions with 4,4'-methylenebis(cyclohexylisocyanate) (H 12 MDI) in toluene and N,N-dimethylformamide (DMF). Kinetic rate constants for varying levels of tin-POMS ranging between 100 ppm and 1000 ppm tin are reported. We observed urethane reactions in toluene to follow second order reaction kinetics, whereas similar reactions in DMF followed first order reaction kinetics. We determined tin-POMS is an efficient catalyst system for urethane reactions and found the new catalyst to be easy to handle, soluble, and very effective for catalyzing urethane reactions. By direct comparison of a model reaction between tin-POMS and dibutyltin dilaurate (DBTDL), tin-POMS was found to be quite similar to DBTDL for urethane catalytic activity. In addition, we show the efficacy for tin-POMS to be an excellent polyurethane reaction catalyst through a model reaction of H 12 MDI with 2000 g/mol poly(e-caprolactone) diol.

We report the synthesis of sym-1,4-diphenyl-1,4-dihydro-1,2,4,5-polytetrazine through 1,3-dipolar cycloaddition polymerization reactions where bis-hydrazonoyl chloride was converted to a tetrazine based polymer through bis-nitrilimine intermediates. Polymer molecular weights approached 90,000 g/mol under optimized reaction conditions with low polydispersity indices of approximately 1.05. The polymers are soluble in a variety of organic solvents and the reactions were characterized through a series of spectral, thermal and chromatographic techniques. The tetrazine based polymers display high complexation potential with cobalt chloride demonstrating metal complexation capability.