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Mays, Jimmy

Publications and source records attributed to Mays, Jimmy.

At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Carbon nanofibers based carbon–carbon composite fibers

Textile grade polyacrylonitrile (PAN) was used as a precursor material for carbon fiber preparation. E-beam irradiated polyacrylonitrile grafted carbon nanofibers were dispersed in polyacrylonitrile solution (dissolved in dimethyl formamide). Carbon nanofibers (CNF) infused polyacrylonitrile solution was wet spun on a lab-scale wet-spinning setup to form 50 to 70 µm diameter fibers with 3.2 wt.% CNF-PAN, 6.4 wt.% CNF-PAN, and neat PAN. Precursor fibers were characterized for thermal, mechanical and morphological properties using various techniques. Drawing the precursor fibers further enhanced polymer chain orientation and coalesced the voids, enhancing tensile strength and modulus by more than 150% compared to those of the undrawn fibers. Precursor composite fibers on carbonization showed enhanced strength, compared to that of pristine PAN fibers, by four times and stiffness by 14 times. The carbon–carbon composite fibers were further characterized with SEM/FIB, XRD and tensile strength. The property improvements were dependent on the uniform distribution of carbon nanofibers, and surface modification of carbon nanofibers further enabled their dispersion in the composite fibers. Furthermore, 3.2 wt.% CNFs in PAN fibers showed maximum improvement in properties compared to 6.4 wt.% CNF in PAN fibers, indicating that the property enhancements go through a maximum and then drop off due to challenge in getting uniform distribution of nanofibers.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Effect of Microstructure on Chain Flexibility and Glass Transition Temperature of Polybenzofulvene

Polybenzofulvene (PBF) is a polydiene with a very high glass transition temperature (T g ), which makes it a potential candidate for use as the hard block for high-temperature thermoplastic elastomer applications. The T g s of polydienes are known to be related to chain flexibility. However, no studies have been reported that correlate the chain flexibility to the microstructure of PBF. Herein, we present a study of solution properties of linear PBFs with narrow molecular weight dispersity and having 1,2-addition ranging from 23% to 99%. The materials were prepared by living anionic polymerization under different conditions. Specifically, the chain flexibility as defined by the Flory's characteristic ratio, , and dependence of chain flexibility on microstructure by combined measurements of intrinsic viscosity and molecular weight using triple-detector size exclusion chromatography (SEC) is studied. The persistence lengths (l p ) and chain diameters (d B ) were also estimated using the touched-bead wormlike chain model. To the best of our knowledge, the characteristic ratio of PBF is found to be the highest of all polydienes that have been reported so far.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Role of tunable polymer flexibility in controlling wetting behavior and thermal properties of poly(1,3-cyclohexadiene)-silica nanocomposites

We present how altering the chain flexibility affects the nanoscale organization of polymer-grafted nanoparticles (PGNPs) and its ultimate impact on macroscale thermal properties. To isolate the role of chain flexibility on wetting behavior in athermal polymer nanocomposites (PNC), the graft and matrix chemistry is kept identical by utilizing 1,3-cyclohexadiene-based polymer materials. Increasing the rigidity and molecular weight of both the graft and matrix is found to favor mixing of poly(1,3-cyclohexadiene) PCHD-grafted silica NPs with the matrix, supported by a concomitant increase in glass transition temperatures of the PNCs. Further, the associated entropic factors that drive wetting behavior and dispersion of PGNPs are discussed, emphasizing the dominant role-played by chain flexibility. Alterations in graft flexibility had the strongest impact on dispersion and Tg values of the PNC, while molecular weight (MW) plays a secondary role. This investigation is a unique demonstration of how chain flexibility alteration in athermal semiflexible systems can be used to alter NP organization by altering filler-matrix wettability which also impacts thermal properties.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Effects of Asymmetric Molecular Architecture on Chain Stretching and Dynamics in Miktoarm Star Copolymers

We use broad-band dielectric spectroscopy (BDS) and small-angle X-ray scattering (SAXS) to investigate the impact of architectural asymmetry in the miktoarm star copolymer on the dielectric polymer relaxations. The miktoarm copolymers studied contain one or two polystyrene (PS) chains of constant molecular weight and two identical poly(cis-1,4-isoprene) (PI) chains with varied molecular weights. Using the chains in the PI block as dielectric probes, we find that the architecturally asymmetric miktoarm star copolymer systems (PSPI 2 ) feature distributions in chain relaxation times and dielectric relaxation strengths that are not dependent on molecular weight or morphology, in stark contrast to the effects of morphological confinement observed for symmetric diblock systems (PS-b-PI or PS 2 PI 2 ). Along with evidence from the SAXS measurements regarding phase separation, these results are attributed to influences from chain stretching within the framework of the Gaussian chain model for block copolymer systems. As such, the molecular architecture in block copolymers happens to be a versatile handle to control polymer chain dynamics and, ultimately, the macroscopic physicochemical properties in architecturally complex polymers.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Chain flexibility and glass transition temperatures of poly(n-alkyl (meth)acrylate)s: Implications of tacticity and chain dynamics

The relationship between chain microstructure and glass transition temperature (T g ) is complicated for poly(alkyl methacrylate)s and poly(alkyl acrylate)s. Despite intensive studies, relationships between the structures of these polymers and their properties, including solution characteristics and T g , are still controversial. Solution properties, chain conformations, including Flory's characteristic ratio (C ∞ ), persistence length (l p ), and chain diameters, and T g are reported for series of poly(n-alkyl acrylate)s and poly(n-alkyl methacrylate)s having alkyl side chain lengths (n) ranging from 1 to 10 carbons in length, with uniform and well-characterized tacticities. Chain flexibilities of both series of polymers decrease as n increases, reflecting increased hindrances to rotation about backbone bonds as side chains become longer. Conversely, the T g s for both series of polymers decrease substantially as n increases, reflecting the greater side chain mobilities of long alkyl substituents. For shorter alkyl chain lengths, T g s for the poly(n-alkyl acrylate)s are much lower than for the corresponding poly(n-alkyl methacrylate)s, a difference which has been attributed in the past to the presumed reduced chain flexibility of polymethacrylates due to the presence of the α-methyl substituent. However, contrary to such expectations, C ∞ and l p values for these two series of polymers are nearly identical at a given n value, except for the longest n-alkyl substituents. Instead, the differences in T g may be attributed to the differences in tacticity of the two series – almost ideally atactic for the poly(n-alkyl acrylate)s but high in syndiotacticity for the poly(n-alkyl methacrylate)s. The T g s for the two series of polymers approach that of polyethylene at longer alkyl chain lengths. In addition, the effects of tacticity and chain dynamics on C ∞ , l p , and T g of poly(methyl methacrylate) is discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Single-step process to improve the mechanical properties of carbon nanotube yarn

Carbon nanotube (CNT) yarns exhibit low tensile strength compared to conventional high-performance carbon fibers due to the facile sliding of CNTs past one another. Electron beam (e-beam) irradiation was employed for in a single-step surface modification of CNTs to improve the mechanical properties of this material. To this end, CNT yarns were simultaneously functionalized and crosslinked using acrylic acid (AA) and acrylonitrile (AN) in an e-beam irradiation process. The chemical modification of CNT yarns was confirmed by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and scanning electron microscopy (SEM). The best improvement in mechanical properties was achieved on a sample treated with an aqueous solution of AA and subsequent irradiation. CNT yarn treatment with AA enhanced the strength (444.5 ± 68.4 MPa) by more than 75% and the modulus (21.5 ± 0.6 GPa) by more than 144% as compared to untreated CNT yarn (strength 251 ± 26.5 MPa and modulus 8.8 ± 1.2 GPa).

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Effect of Solvent Quality and Monomer Water Solubility on Soft Nanoparticle Morphology

Previous investigations have identified the correlation between synthetic conditions and soft nanoparticle morphology, including their molecular weight, internal crosslinked structure, total nanoparticle radius, and topology, or degree of “fuzziness” of each nanoparticle. To provide further insight to these guidelines, we examine the effect of surfactant concentration and monomer water solubility on the topology, size, and polydispersity of soft poly(butyl acrylate) nanoparticles. Small-angle neutron scattering studies confirm that increased monomer water solubility and polymer chain flexibility decreases the control of nanoparticle topology and size in a nano-emulsion polymerization. Further studies document the morphological change of polystyrene soft nanoparticles with a decrease in solvent quality. Small-angle neutron scattering was also used to characterize dilute solutions of soft polystyrene nanoparticles in a good and theta solvent, including the analysis of their internal structure. When the soft nanoparticles are dispersed in a thermodynamically good solvent they exhibit a swollen morphology, however, when dispersed in a theta solvent, the nanoparticle core contracts due to unfavorable thermodynamic interactions. Moreover, the structure of soft nanoparticles in a theta solvent provides insight that will guide the interpretation of the behavior of soft nanoparticles dispersed into a bulk polymer matrix.

Martin, Halie J.↗
DOE OSTI2017

Block Copolymers: Synthesis, Self-Assembly, and Applications

Research on block copolymers (BCPs) has played a critical role in the development of polymer chemistry, with numerous pivotal contributions that have advanced our ability to prepare, characterize, theoretically model, and technologically exploit this class of materials in a myriad of ways in the fields of chemistry, physics, material sciences, and biological and medical sciences. The breathtaking progress has been driven by the advancement in experimental techniques enabling the synthesis and characterization of a wide range of block copolymers with tailored composition, architectures, and properties. In this review, we briefly discussed the recent progress in BCP synthesis, followed by a discussion of the fundamentals of self-assembly of BCPs along with their applications.

applications↗
DOE OSTI2017

All acrylic-based thermoplastic elastomers with high upper service temperature and superior mechanical properties

All acrylic-based thermoplastic elastomers (TPEs) offer potential alternatives to the widely-used styrenic TPEs. However, the high entanglement molecular weight ( M e ) of polyacrylates, as compared to polydienes, leads to “disappointing” mechanical performance as compared to styrenic TPEs. In this study, triblock copolymers composed of alkyl acrylates with different pendant groups and different glass transition temperatures ( T g s), i.e. 1-adamatyl acrylate (AdA) and tetrahydrofurfuryl acrylate (THFA), were synthesized via reversible addition–fragmentation chain transfer (RAFT) polymerization. Thermal characterization of the resulting polymers was performed using differential scanning calorimetry (DSC), and the T g s of both segments were observed for the block copolymers. This indication of microphase separation behavior was further demonstrated using atomic-force microscopy (AFM) and small angle X-ray scattering (SAXS). Dynamic mechanical analysis (DMA) showed that the softening temperature of the PAdA domains is 123 °C, which is higher than that of both styrenic TPEs and commercial acrylic based TPEs with poly(methyl methacrylate) (PMMA) hard block. Here, the resulting triblock copolymers also exhibited stress–strain behavior superior to that of conventional all acrylic-based TPEs composed of PMMA and poly( n -butyl acrylate) (PBA) made by controlled radical processes, while the tensile strength was lower than for products made by living anionic polymerization.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Polyacrylonitrile nanocomposite fibers from acrylonitrile-grafted carbon nanofibers

For the first time, uniform distribution of surface functionalized carbon nanofibers (CNFs) has been achieved in low molecular weight (≈120,000 g/mol) textile grade-polyacrylonitrile (PAN)-based composite filaments. Furthermore, surface grafting of CNFs with acrylonitrile enhances the dispersion of nanofibers in PAN fiber matrix. XPS study reveals high atomic nitrogen content (7%) on the CNF surface due to the grafting reaction. The solution-spun filaments have been characterized for distribution of CNFs in the PAN matrix by electron microscopy. PAN composite filaments containing 3.2 wt.% CNF and processed at draw ratio of ≈6.3 exhibit enhanced tensile strength and modulus by more than three folds compared to the control PAN filament. Because of chemically compatible surface modification of the nanofibers, better dispersion and improved mechanical properties were accomplished in the reinforced PAN fibers. This should then allow the production of CNF reinforced carbon fibers with improved tensile properties. An increase in CNF loading (6.4 wt.%), however, reduced performance due to inefficient alignment of CNF along the fiber axis. Nevertheless, hot stretching (at draw ratio ≈ 10) of the filaments enhanced tensile strength and elastic modulus of PAN composite filaments by 20–30% compared to the control hot stretched PAN filaments.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Effect of solvent/polymer infiltration and irradiation on microstructure and tensile properties of carbon nanotube yarns

Recently carbon nanotube (CNT) yarns have been gaining importance as an approach to harvest the excellent properties of the CNTs. However, the properties of CNT yarns at this stage are well below the expected value. Investigation of the structure of CNT yarns and possible approaches to enhance the strength and modulus are reported. Scanning electron microscopy and focused ion beam imaging reveal the inherently porous structure and poor orientation, emphasizing the need to enhance packing of CNT bundles in the yarns for increased strength and modulus. Densification of CNT yarn by toluene or polystyrene increases the strength by 140 or 172 % and modulus by 79 or 218 %, respectively, as compared to that of the pristine yarn. E-beam irradiation was investigated as a means to introduce crosslinking and enhanced internanotubes bonding to increase strength and modulus. However, the irradiation resulted in generation of defects and damages to the yarn contributing to reduction in strength and modulus. Raman spectroscopy studies on the irradiated samples reveal the change in bonding characteristics resulting in poor mechanical properties. As a result, denser packing of nanotubes and increased interaction without any damage is the key to improve the properties of CNT yarns.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Effect of Molecular Weight on the Ion Transport Mechanism in Polymerized Ionic Liquids

The unique properties of ionic liquids (ILs) have made them promising candidates for electrochemical applications. Polymerization of the corresponding ILs results in a new class of materials called polymerized ionic liquids (PolyILs). Though PolyILs offer the possibility to combine the high conductivity of ILs and the high mechanical strength of polymers, their conductivities are typically much lower than that of the corresponding small molecule ILs. In this study, seven PolyILs were synthesized having degrees of polymerization ranging from 1 to 333, corresponding to molecular weights (MW) from 482 to 160 400 g/mol. Depolarized dynamic light scattering, broadband dielectric spectroscopy, rheology, and differential scanning calorimetry were employed to systematically study the influence of MW on the mechanism of ionic transport and segmental dynamics in these materials. Finally, the modified Walden plot analysis reveals that the ion conductivity transforms from being closely coupled with structural relaxation to being strongly decoupled from it as MW increases.

36 MATERIALS SCIENCE↗
DOE OSTI2016

High temperature thermoplastic elastomers synthesized by living anionic polymerization in hydrocarbon solvent at room temperature

We present the synthesis and characterization of a new class of high temperature thermoplastic elastomers composed of polybenzofulvene–polyisoprene–polybenzofulvene (FIF) triblock copolymers. All copolymers were prepared by living anionic polymerization in benzene at room temperature. Homopolymerization and effects of additives on the glass transition temperature (T g ) of polybenzofulvene (PBF) were also investigated. Among all triblock copolymers studied, FIF with 14 vol % of PBF exhibited a maximum stress of 14.3 ± 1.3 MPa and strain at break of 1390 ± 66% from tensile tests. The stress–strain curves of FIF-10 and 14 were analyzed by a statistical molecular approach using a nonaffine tube model to estimate the thermoplastic elastomer behavior. Dynamic mechanical analysis showed that the softening temperature of PBF in FIF was 145 °C, much higher than that of thermoplastic elastomers with polystyrene hard blocks. Microphase separation of FIF triblock copolymers was observed by small-angle X-ray scattering, even though long-range order was not achieved under the annealing conditions employed. Additionally, the microphase separation of the resulting triblock copolymers was examined by atomic force microscopy.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Effect of cross-link density on carbon dioxide separation in polydimethylsiloxane-norbornene membranes

The development of high performance materials for CO 2 separation and capture will significantly contribute to a solution for climate change. In this work, (bicycloheptenyl) ethyl terminated polydimethylsiloxane (PDMSPNB) membranes with varied cross-link densities were synthesized via ring-opening metathesis polymerization. The developed polymer membranes show higher permeability and better selectivity than those of conventional cross-linked PDMS membrane. The achieved performance (CO 2 permeability ~ 6800 Barrer and CO 2 /N 2 selectivity ~ 14) is very promising for practical applications. The key to achieving this high performance is the use of an in-situ cross-linking method of the difunctional PDMS macromonomers, which provides lightly cross-linked membranes. By combining positron annihilation lifetime spectroscopy, broadband dielectric spectroscopy and gas solubility measurements, we have elucidated the key parameters necessary for achieving their excellent performance.

36 MATERIALS SCIENCE↗
DOE OSTI2015

All-acrylic multigraft copolymers: Effect of side chain molecular weight and volume fraction on mechanical behavior

We present in this paper the synthesis of poly(n-butyl acrylate)-g-poly(methyl methacrylate) (PnBA-g-PMMA) multigraft copolymers via a grafting-through (macromonomer) approach. The synthesis was performed using two controlled polymerization techniques. The PMMA macromonomer was obtained by high-vacuum anionic polymerization followed by the copolymerization of n-butyl acrylate and PMMA macromonomer using reversible addition–fragmentation chain transfer (RAFT) polymerization to yield the desired all-acrylic multigraft structures. The PnBA-g-PMMA multigraft structures exhibit randomly spaced branch points with various PMMA contents, ranging from 15 to 40 vol %, allowing an investigation into how physical properties vary with differences in the number of branch points and molecular weight of grafted side chains. The determination of molecular weight and polydispersity indices of both the PMMA macromonomer and the graft copolymers was carried out using size exclusion chromatography with triple detection, and the structural characteristics of both the macromonomer and PnBA-g-PMMA graft materials were characterized by 1 H and 13 C NMR. Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry was employed for monitoring the macromonomer synthesis. Thermal characteristics of the materials were analyzed using differential scanning calorimetry and thermogravimetric analysis. The mechanical performance of the graft materials was characterized by rheology and dynamic mechanical analysis, revealing that samples with PMMA content of 25–40 vol % exhibit superior elastomeric properties as compared to materials containing short PMMA side chains or <25 vol % PMMA. In conclusion, atomic force microscopy showed a varying degree of microphase separation between the glassy and rubbery components that is strongly dependent on PMMA side chain molecular weight.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Precise synthesis of thermoreversible block copolymers containing reactive furfuryl groups via living anionic polymerization: the countercation effect on block copolymerization behavior

The anionic block copolymerization of 4,4' -vinylphenyl-N,N-bis(4-tert-butylphenyl)benzenamine (A) with furfuryl isocyanate (B) was carried out using potassium naphthalenide (K-Naph) in tetrahydrofuran at -78 and -98 °C to prepare well-defined block copolymers containing furan groups for the formation of thermoreversible networks via a Diels Alder (DA) reaction. While no block copolymerization was observed in the absence of sodium tetraphenylborate (NaBPh 4 ) due to side reactions, well-defined poly-(B-b-A-b-B) (PBAB) copolymers with controlled molecular weights (M n = 18 700 19 500 g mol -1 ) and narrow molecular weight distributions (M w /M n = 1.08 -1.17) were successfully synthesized in the presence of excess NaBPh 4 . We prevented the occurrence of the undesirable side reactions during polymerization of B of NaBPh 4 , which results in the change in the countercation from K + to Na + for further polymerization of B. Moreover, the cross-linking via the DA reaction between the furan groups of PBAB and bismaleimide was proved by FT-IR and differential scanning calorimetry (DSC), and the thermoreversible properties of the cross-linked polymer were subsequently investigated using DSC and solubility testing.

36 MATERIALS SCIENCE↗