12 research outputs found
Thermal Control of Nanostructure and Molecular Network Development in Epoxy-Amine Thermosets
Epoxy-amine resins find wide application as the matrix material of high performance polymer composites because of their favorable mechanical properties, thermal properties and solvent stability. These properties result from the complicated, highly cross-linked molecular network that is characteristic of epoxy amine thermoset polymers. The connectivity of the molecular network has a strong influence on the physical performance of the finished part. Nonhomogeneity in the network structure can degrade these favorable properties through the introduction of low energy pathways for solvent penetration or fracture propagation. This work examines the influence Of Cure temperature on the network building cross linking reaction and the subsequent effect On the homogeneity of the cross linked molecular network. Specific attention is paid to nanoscale variation in the distribution of cross-link density. Thermal, rheological, and spectroscopic techniques are used to monitor key chemical and structural changes during network growth. Atomic force microscopy is used to understand nanoscale fracture behavior in terms of the low energy pathways that result from a nonhomogeneous distribution of cross-link density. The influence of processing-induced changes in molecular connectivity is discussed in terms of observed nanoscale morphology and fracture properties of the cured material
Molecular Network Development and Evolution of Nanoscale Morphology In an Epoxy-Amine Thermoset Polymer
Epoxy-amine thermoset polymers exhibit a complicated, highly crosslinked network structure. The connectivity of this network drives material parameters such as mechanical properties and solvent permeation. Understanding the molecular network architecture is also an important aspect of the developing realistic network topologies for use in molecular dynamic simulations. Here, the evolution of network connectivity in a typical crosslinked epoxy-amine network (Epon 828/3-aminophenyl sulfone) is monitored as a function of cure time. Special attention is paid to nanoscale variation in the crosslink density of the network. Submicron atomic force microscope images of sample fracture surfaces revealed three distinct types of crack tip propagation. Near-infrared spectroscopy, rheological and thermal characterization were used to correlate each type of fracture propagation behavior to a different stage of network development. Monitoring changes in the nanoscale fracture behavior reveals information regarding changes in the network architecture during cure and provides insight into the final structure of the epoxy-amine network. © 2012 Wiley Periodicals, Inc
Electrically Stimulated Gradients in Water and Counterion Concentrations Within Electroactive Polymer Actuators
While ionic polymer metal composites (IPMCs) have been studied for more than 10 years, the specific actuation mechanism is still unclear. In this work, neutron imaging, applied potential atomic force microscopy (APAFM) and current sensing atomic force microscopy (CSAFM) methods are employed to fundamentally investigate the actuation mechanism of this electroactive polymer system. Direct neutron imaging allowed a mapping of the water-counterion concentration gradient profile (i.e., a non-flat optical density profile sloping from the cathode to the anode) across an IPMC cross-section. While the neutron imaging method was capable of visualizing inside an operating IPMC, APAFM-CSAFM characterized changes in the nanoscale morphology and local surface properties due to redistribution of water-counterions under electrical stimulation. In APAFM, the darker, more energy dissipative features disappeared as the applied bias was varied from 0 V to 3 V, indicating that the surface became dehydrated. Surface dehydration undoubtedly supports the concept of proton and water migration to the negatively charged substrate. Water-counterion redistribution was further evidenced by CSAFM. With a negatively charged substrate (a 2 V bias), 2.8 pA of the average current were detected over the perfluorosulfonate ionomer (PFSI) surface in contact with AFM tip, which suggests the depletion of positively charged cations on the surface. On the contrary, a positively charged substrate (a -2 V bias) led to the average current of -90 pA over the PFSI surface in contact with the AFM tip, which indicates the formation of a cation-rich fluid on the top surface of the PFSI membranes. The observed water-counterion redistribution upon electrical stimulation directly supports a hydraulic contribution to the overall mechanism of actuation in IPMCs
Effect of Nanoparticle Functionalization on the Performance of Polycyanurate/Silica Nanocomposites
The
impact of silica functionalization in determining the performance
of polycyanurate networks polymerized from 1,1-bisÂ(4-cyanatophenyl)Âethane,
known commercially as Primaset LECy, reinforced with modified fumed
silica, was elucidated through systematic comparison of the properties
of nanocomposite networks in which the silica surface treatment was
altered. Three types of surfaces were investigated: moderately acidic
(unmodified silanol), neutral (alkylsilane modified), and slightly
basic (3-aminopropylsilane modified). In terms of cyanate ester cure,
the acidic surface proved to be moderately catalytic, the neutral
surface mildly catalytic due to slight residual silanol content, and
the basic amino-functional surface mildly inhibitory. In terms of
network performance, the amino-functional surface led to significant
degradation of the network at elevated temperatures, while the silanol-functional
surface outperformed the alkyl-functional surface in terms of protection
against hydrolytic degradation. In agreement with expectations, the
addition of 2–5 wt% of relatively well-dispersed silica nanoparticles
had negligible impact on the fracture toughness of the cyanurate networks.
Overall, these results demonstrate that the functionalization of nanoparticle
additives for polycyanurate networks is an important determinant of
performance and must be taken into consideration in the development
of polycyanurate nanocomposites, even at levels that are too low to
strongly affect mechanical properties
Di(cyanate Ester) Networks Based on Alternative Fluorinated Bisphenols with Extremely Low Water Uptake
A new polycyanurate network exhibiting extremely low moisture uptake has been produced via the treatment of perfluorocyclobutane-containing Bisphenol T with cyanogen bromide and subsequent thermal cyclotrimerization. The water uptake, at 0.56 ± 0.10% after immersion in water at 85 °C for 96 h, represents some of the most promising moisture resistance observed to date in polycyanurate networks. This excellent performance derives from a near optimal value of the glass transition at 190 °C at full cure. Superior dielectric loss characteristics compared to commercial polycyanurate networks based on Bisphenol E were also observed. Polycyanurate networks derived from this new monomer appear particularly well-suited for applications such as radomes and spacecrafts where polycyanurates are already widely recognized as providing outstanding properties
Curing of a Bisphenol E Based Cyanate Ester Using Magnetic Nanoparticles as an Internal Heat Source through Induction Heating
We report on the control of cyclotrimerization
forming a polycyanurate
polymer using magnetic iron oxide nanoparticles in an alternating-current
(ac) field as an internal heat source, starting from a commercially
available monomer. Magnetic nanoparticles were dispersed in the monomer
and catalytic system using sonication, and the mixture was subjected
to an alternating magnetic field, causing the magnetic nanoparticles
to dissipate the energy of the magnetic field in the form of heat.
Internal heating of the particle/monomer/catalyst system was sufficient
to start and sustain the polymerization reaction, producing a cyanate
ester network with conversion that compared favorably to polymerization
through heating in a conventional laboratory oven. The two heating
methods gave similar differential scanning calorimetry temperature
profiles, conversion rates, and glass transition temperatures when
using the same temperature profile. The ability of magnetic nanoparticles
in an ac field to drive the curing reaction should allow for other
reactions forming high-temperature thermosetting polymers and for
innovative ways to process such polymers
Synergistic Physical Properties of Cocured Networks Formed from Di- and Tricyanate Esters
The co-cyclotrimerization of two
tricyanate ester monomers, Primaset
PT-30 and 1,2,3-trisÂ(4-cyanato)Âpropane (FlexCy) in equal parts by
weight with Primaset LECy, a liquid dicyanate ester, was investigated
for the purpose of exploring synergistic performance benefits. The
monomer mixtures formed stable, homogeneous blends that remained in
the supercooled liquid state for long periods at room temperature,
thereby providing many of the processing advantages of LECy in combination
with significantly higher glass transition temperatures (315–360
°C at full cure) due to the presence of the tricyanate-derived
segments in the conetwork. Interestingly, the glass transition temperatures
of the conetworks after cure at 210 °C, at full cure, and after
immersion in 85 °C water for 96 h were all higher than predicted
by the Flory–Fox equation, most significantly for the samples
immersed in hot water. Conetworks comprising equal parts by weight
of PT-30 and LECy retained a “wet” glass transition
temperature near 270 °C. The onset of thermochemical degradation
for conetworks was dominated by that of the thermally less stable
component, while char yields after the initial degradation step were
close to values predicted by a linear rule of mixtures. Values for
moisture uptake and density in the conetworks also showed behavior
that was not clearly different from a linear rule of mixtures. An
analysis of the flexural properties of catalyzed versions of these
conetworks revealed that, when cured under the same conditions, conetworks
containing 50 wt % PT-30 and 50 wt % LECy exhibited higher modulus
than networks containing only LECy while conetworks containing 50
wt % FlexCy and 50 wt % LECy exhibited a lower modulus but significantly
higher flexural strength and strain to failure. Thus, in the case
of “FlexCy”, LECy was copolymerized with a tricyanate
that provided both improved toughness and a higher glass transition
temperature
Di(cyanate Ester) Networks Based on Alternative Fluorinated Bisphenols with Extremely Low Water Uptake
A new
polycyanurate network exhibiting extremely low moisture uptake
has been produced via the treatment of perfluorocyclobutane-containing
Bisphenol T with cyanogen bromide and subsequent thermal cyclotrimerization.
The water uptake, at 0.56 ± 0.10% after immersion in water at
85 °C for 96 h, represents some of the most promising moisture
resistance observed to date in polycyanurate networks. This excellent
performance derives from a near optimal value of the glass transition
at 190 °C at full cure. Superior dielectric loss characteristics
compared to commercial polycyanurate networks based on Bisphenol E
were also observed. Polycyanurate networks derived from this new monomer
appear particularly well-suited for applications such as radomes and
spacecrafts where polycyanurates are already widely recognized as
providing outstanding properties