Changes In Storage Modulus Research Articles

Phase-Changing Bistable Electroactive Polymer Exhibiting Sharp Rigid-to-Rubbery Transition

A phase-changing polymer comprising stearyl acrylate and a long-chain urethane diacrylate was studied as a new bistable electroactive polymer. The abrupt and reversible phase transition of the crystalline aggregates of the stearyl moieties results in a rapid shift between the rigid and rubbery states of the polymers during temperature cycles. The transition temperature is tunable between 34–46 °C. A storage modulus change of ∼1000 fold can be obtained within a narrow temperature range of 10 °C. The polymer shows excellent shape memory properties with both fixation rate and recovery rate close to 100%. Diaphragm actuators based on the polymer thin films were electrically actuated up to 70% strain at 50 °C. The actuated shape can be “frozen” after the films were allowed to cool below the transition temperature. This rigid-to-rigid deformation is refreshable and repeatable via the rigid-to-rubbery transition and electrical actuation in the rubbery state.

Ester-free thiol–ene dental restoratives—Part B: Composite development

ObjectivesTo assess the performance of thiol–ene dental composites based on selected ester-free thiol–ene formulations. Further, to point out the benefits/drawback of having a hydrolytically stable thiol–ene matrix within a glass filled composite. MethodsComposite samples containing 50–65wt% of functionalized glass microparticles were prepared and photopolymerized in the presence of a suitable visible light photoinitiator. Shrinkage stress measurements were conducted as a function of the irradiation time. Degrees of conversion were measured by FT-IR analysis by comparing the double bond signals before and after photopolymerization. Mechanical tests were carried out on specimens after curing as well as after extended aging in water. Dynamic mechanical analysis was employed to track the changes in storage modulus near body temperature. The properties of the thiol–ene composites were compared with those of the BisGMA/TEGDMA control. ResultsDepending on the resin type, similar or higher conversions were achieved in thiol–ene composites when compared to the dimethacrylate controls. At comparable conversions, lower shrinkage stress values were achieved. Although exhibiting lower initial elastic moduli, the thiol–ene composites’ flexural strengths were found to be comparable with the controls. Contrary to the control, the mechanical properties of the ester-free thiol–ene composites were shown to be unaffected by extensive aging in water and at least equaled that of the control after aging in water for just five weeks. SignificanceEmploying non-degradable step-growth networks as organic matrices in dental composites will provide structurally uniform, tough materials with extended service time.

Thermal stimuli‐responsive behavior of pyrene end‐functionalized PDMS through tunable Π–Π interactions

ABSTRACTPyrene end‐functionalized, telechelic poly(dimethyl siloxane) (PDMS) materials were synthesized and their response to different thermal stimuli was evaluated. The incorporation of pyrene end groups introduces strong π–π interactions that facilitated a broad range of thermally responsive properties, in some circumstances forming pyrene nanocrystals that serve as physical crosslinks leading to elastic materials. By synthesizing different chain lengths, samples exhibiting a 7 orders of magnitude change in storage modulus in response to thermal stimuli were produced by modifying only the end‐groups (0.6 wt % of all polymer segments). Repeated thermal cycling during rheological experiments revealed that π–π interaction and crystallization/melting kinetics of pyrene chain‐ends plays a key role in their thermal responsiveness. The properties of these materials were tuned by adding free pyrene, neat PDMS, or graphene oxide (GO) nanoparticles, making them attractive for many applications (e.g., tunable damping materials, heat/light sensors, conductive gels, or light repositionable adhesives). For example, nanocomposites containing 1 wt % GO caused the melting temperature for pyrene crystal domains to more than double, and even induced pyrene end‐group crystallization in samples that did not exhibit crystals in neat form. It is hypothesized that these features originate from π–π interactions between pyrene ends and GO surfaces. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 159–168

Processability of PEEK, a new polymer for High Temperature Laser Sintering (HT-LS)

Currently, the HT-LS sector is predominantly based around one commercial Poly Ether Ketone (PEK) polymer. Although the combination of polymer and process works well, a lower melting temperature polymeric material, part of the same Poly Aryl Ether Ketone (PAEK) family would be preferable in certain applications. This study presents the optimisation and characterisation of Poly Ether Ether Ketone (PEEK), a polymer which is part of the PAEK family with a 30°C lower melting temperature than PEK. The systematic characterisation of laser sintered samples of PEEK revealed a very good overall performance in comparison with the HP3 PEK material, with no change in storage modulus and only 25% drop in tensile strength. The possibility of variable building configurations available within the HT-LS system, i.e. reduced, half and full chamber building modes, is examined in relation to the mechanical performances of the components. The effect of the post sintering time, an additional heating phase supplied to the powder bed at every layer, found only in the HT-LS system EOSINT P 800, is also examined.

Rheological Properties and Stability of Highly Concentrated Water-In-Oil Emulsions with Different Emulsifiers

An investigation was performed into the effect of emulsifiers on the rheological properties and stability of highly concentrated water-in-oil emulsions (HCEs). The emulsifiers were oligomers of the polyisobutylene succinic anhydride (PIBSA) based on different head-group and relatively low molecular weight sorbitan monooleate (SMO). The viscosity and storage modulus of HCE depend on the nature of head-groups of PIBSA-based emulsifiers and the ratio of SMO. It was shown that an increase in the SMO concentration results in the increase of viscosity and storage modulus. The stability of HCE with aging is also influenced by the nature of emulsifiers. The results of the precipitation of ammonium nitrate and the change of storage modulus with time show that the increase in the ratio of SMO decreases the stability of HCE. It was found that the phase transition of ammonium nitrate is important for the stability of HCE.

Structure and Properties of Polybenzimidazole/Silica Nanocomposite Electrolyte Membrane: Influence of Organic/Inorganic Interface

Although increased number of reports in recent years on proton exchange membrane (PEM) developed from nanocomposites of polybenzimidazole (PBI) with inorganic fillers brought hope to end the saga of contradiction between proton conductivity and variety of stabilities, such as mechanical, thermal,chemical, etc.; it still remains a prime challenge to develop a highly conducting PEM with superior aforementioned stabilities. In fact the very limited understanding of the interactions especially interfacial interaction between PBI and inorganic filler leads to confusion over the choice of inorganic filler type and their surface functionalities. Taking clue from our earlier study based on poly(4,4'-diphenylether-5,5'-bibenzimidazole) (OPBI)/silica nanocomposites, where silica nanoparticles modified with short chain amine showed interfacial interaction-dependent properties, in this work we explored the possibility of enhanced interfacial interaction and control over the interface by optimizing the chemistry of the silica surface. We functionalized the surface of silica nanoparticles with a longer aliphatic chain having multiple amine groups (named as long chain amine modified silica and abbreviated as LAMS). FTIR and (13)C solid-state NMR provided proof of hydrogen bonding interactions between the amine groups of modifier and those of OPBI. LAMS nanoparticles yielded a more distinguished self-assembly extending all over the OPBI matrix with increasing concentrations. The crystalline nature of these self-assembled clusters was probed by wide-angle X-ray diffraction (WAXD) studies and the morphological features were captured by transmission electron microscope (TEM). We demonstrated the changes in storage modulus and glass transition temperature (Tg) of the membranes, the fundamental parameters that are more sensitive to interfacial structure using temperature dependent dynamic mechanical analysis (DMA). All the nanocomposite membranes displayed enhanced mechanical, thermal and chemical stability than neat OPBI. The lower water uptake and swelling ratio and volume in both acid and water reflected the more hydrophobic characteristic of the nanocomposites. All the nanocomposite membranes showed phosphoric acid (PA) values to be higher than OPBI but the levels showed decreasing trend with increasing silica content; the reason attributed to the interparticle interaction. The self-assembled clusters of LAMS nanoparticles in the matrix created more sites for proton hopping as a result of which the proton conductivity of all the nanocomposites displayed an increasing trend.

Structure and rheology of mixed suspensions of montmorillonite and silica nanoparticles.

We present a study of the structure and rheology of mixed suspensions of montmorillonite clay platelets and Ludox TMA silica spheres at pH 5, 7, and 9. Using cryogenic transmission electron microscopy (cryo-TEM), we probe the changes in the structure of the montmorillonite suspensions induced by changing the pH and by adding silica particles. Using oscillatory and transient rheological measurements, we examine the changes in storage modulus and yield stress of the montmorillonite suspensions upon changing the pH and adding silica particles. Cryo-TEM images reveal that changes in pH have a significant effect on the structure of the suspensions, which can be related to the change in charge of the edges from positive at pH 5 to negative at higher pH. Furthermore, at pH 7, the cryo-TEM images show indications of a microphase separation between clay and silica particles. The addition of silica leads to lowering of the storage modulus and yield stress, which we connect to the structural changes of the suspension.

Rheological Properties and Stability of Emulsion Explosive Matrix

An investigation was performed into the rheological properties and stability of emulsion explosive matrix with different surfactants. The surfactants were oligomers of the polyisobutylene succinic anhydride (PIBSA) based on different head group and relative low molecular weight sorbitan monooleate (Span 80). The viscosity, shear stress, and storage modulus of emulsion explosive matrix depend on the nature of surfactants. It was shown that the matrix formed by Span 80 has a lower viscosity, shear stress, and storage modulus, compared with the PIBSA-based surfactants, especially the blended surfactant of Span 80 and PIBSA-TEA. The result of the precipitation of ammonium nitrate (AN) and the change of storage modulus with time show that the strength of interfacial film formed by Span 80 is weak. This allows the crystal of AN to pass through the interfacial film more easily; thus, the stability of the matrix is decreased. However, the storage modulus in aging was approximately constant and the ratio of the precipitation of AN is the lowest when the matrix was stabilized by blended surfactant. It shows that the matrix formed by blended surfactant has a better stability than single surfactants.

Dynamic mechanical properties and correlation with dynamic fragility of sisal reinforced composites

In this study, the reinforcement effect in sisal/polyester composites containing distinct reinforcement content was studied ranging from fragile to strong classification. The results indicate that the reduced storage modulus changes steadily, and the loss modulus and the tan δ peak are broader for composites containing more fiber (dynamically strong composites). As more fiber was incorporated in the resin, lower peak height for the tan delta curve was obtained, which may be indicative of lower energy dissipation due to a greater relative amount of interface. Also, the use of reduced dynamic mechanical curves (similar to dynamic fragility) is an alternative to facilitate the study of the material behavior. POLYM. COMPOS., 36:161–166, 2015. © 2014 Society of Plastics Engineers

Tung oil‐based thermosetting polymers for self‐healing applications

ABSTRACTSeveral bio‐renewable thermosetting polymers were successfully prepared from tung oil through cationic polymerization for the use as the healing agent in self‐healing microencapsulated applications. The tung oil triglyceride was blended with its methyl ester, which was produced by saponification followed by esterification. The changes in storage modulus, loss modulus, and glass transition temperature as functions of the methyl ester content were measured using dynamic mechanical analysis. In addition, the fraction of cross‐linked material in the polymer was calculated by Soxhlet extraction, while proton nuclear magnetic resonance, Fourier transform infrared spectroscopy and TEM were used to investigate the structure of the copolymer networks. The thermal stability of the thermosets as a function of their methyl ester blend contents was determined by thermogravimetric analysis. Finally, the adhesive properties of the thermosets were studied using compressive lap shear and the fracture surfaces were analyzed using SEM. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40406.

Evaluation of highly compliant magneto‐active elastomers with colossal magnetorheological response

ABSTRACTHighly compliant elastomers with a shear storage modulus as low as 25 Pa are prepared using commercially available silicone, plasticizer, and tactile mutator silicone additive. They are used as matrix material for magneto‐active elastomers (MAEs) with carbonyl iron contents between 0 and 85 wt %. In the absence of an external magnetic field, the storage modulus of MAEs based on two selected mixtures ranges between ∼100 Pa and ∼2000 Pa. Addition of a mutator to the matrix mixture results in a long post‐cure period depending on the curing temperature and the initial mixture. In the presence of a magnetic field, the presented MAEs exhibit a strong magneto‐induced change in storage modulus resulting in a colossal magnetorheological effect of >106 % which is ∼30 times higher than previously reported values. The results are of interest in applications using such elastomers as cell substrates with magnetically tunable rigidity. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 39793.

Dynamic rheological and dynamic thermomechanical properties of poly(trimethylene terephthalate)/short carbon fibre composites

The dynamic rheology and thermomechanical properties of poly(trimethylene terephthalate) (PTT)/short carbon fibre (CF) composites at different mechanical states were investigated by a rotational rheometer and a dynamic mechanical analyzer (DMA). At molten state, the composite melts were pseudo-plastic fluids, and the complex viscosity of the composite melts decreased much with increasing CF content because of the poor adhesion at the fiber/matrix interface. The viscous behavior was predominant rather than elastic behavior in the composites melt and viscous behavior was increased with increasing CF at low shearing frequency. An apparent slope change in storage modulus and loss modulus plot suggested that a structure change occurred in the melt that was dependent on shearing frequency. At glassy state, the storage modulus increased with increasing CF content, suggesting that CFs had good reinforcing effect on PTT. At glass transition region, the increasing loss modulus indicated a better toughness of the composites, and the elastic behavior was predominant rather than viscous behavior. Moreover, the glass-transition temperatures of the composites increased with 10% CF content. The composites have larger cold-crystallization rate than pure PTT.

Viscoelastic hydrogels from poly(vinyl alcohol)-Fe(iii) complex.

Hydrogels are three dimensional scaffolds of hydrophilic polymers with inter-connected water channels. In this investigation, an interesting approach for the fabrication of a highly viscoelastic hydrogel derived from a polyvinyl alcohol (PVA) based macromer is reported. PVA was crosslinked using Fe3+ ions under basic conditions to obtain a stable hydrogel with a highly porous network structure. The swelling ratio of the designed hydrogels was evaluated and correlated to the network structure of the metal coordinated crosslinked macromers. Compressive modulus and dynamic viscoelastic measurements were carried out using a dynamic mechanical analyzer and no significant changes in storage modulus were observed by increasing the temperature up to 45 °C. A creep study revealed that the elastic recovery was enhanced with an increase in the degree of crosslinking. The effect of crosslinking influenced properties such as the glass transition, melting point and melting enthalpy of the crosslinked network. The cytocompatibility of the gels was studied using human lung fibroblasts (IMR-90) and no toxic effects or significant cell attachments were observed on the surface of the gel after 5 days. The designed hydrogels may be useful for biomedical procedures which require highly viscoelastic, biocompatible, thermally and mechanically stable biomaterials.

Magnetically Tunable Elasticity for Magnetic Hydrogels Consisting of Carrageenan and Carbonyl Iron Particles

A new class of magnetoelastic gel that demonstrates drastic and reversible changes in storage modulus without using strong magnetic fields was obtained. The magnetic gel consists of carrageenan and carbonyl iron particles. The magnetic gel with a volume fraction of magnetic particles of 0.30 exhibited a reversible increase by a factor of 1400 of the storage modulus upon a magnetic field of 500 mT, which is the highest value in the past for magnetorheological soft materials. It is considered that the giant magnetoelastic behavior is caused by both high dispersibility and high mobility of magnetic particles in the carrageenan gel. The off-field storage modulus of the magnetic gel at volume fractions below 0.30 obeyed the Krieger-Dougherty equation, indicating random dispersion of magnetic particles. At 500 mT, the storage modulus was higher than 4.0 MPa, which is equal to that of magnetic fluids, indicating that the magnetic particles move and form a chain structure by magnetic fields. Morphological study revealed the evidence that the magnetic particles embedded in the gel were aligned in the direction of magnetic fields, accompanied by stretching of the gel network. We conclude that the giant magnetoelastic phenomenon originates from the chain structure consisting of magnetic particles similar to magnetic fluids.

Temperature dependence of dynamic moduli of alumina suspensions and alumina powder compacts

The complex moduli of 10–30vol% aqueous suspensions of alumina particles of median size of 310nm were measured at 1Hz of applied frequency in a temperature range of −100 to 100°C at a heating rate of 1°C/min. The storage modulus of distilled water as a reference material showed a drastic decrease at 0°C, where a phase transition occurred from solid ice to liquid water. The loss tangent of water showed a sharp peak associated with the phase transition at 0°C. In 10vol% alumina suspensions at pH 3, 5 and 7, more additional peaks of storage modulus and loss tangent appeared at 7−51°C. The peaks at 30–51°C represent the phase transition of colloidal suspension from a dispersed state to a flocculated state. When the alumina content of the suspension was increased to 30vol%, little change of storage modulus and loss tangent were measured in the wide temperature range of −100 to 100°C, reflecting little change of dense packing structure of alumina particles after the melt of solid ice with increasing temperature. A similar dependence of storage modulus and loss tangent on heating temperature was measured in the calcined alumina porous compact infiltrated with water in the open pores.

Mechanical Properties Of Multifunctional Epoxy resin/glass Fiber Reinforced Composites Modified with Poly(ether Imide)

In the present paper effect of thermoplastic on various mechanical and thermal properties of multifunctional epoxy resin have been studied. Epoxy phenol novolac resin has been cured with hardner diamino, diphenyl sulfone. Changes in mechanical and thermal properties of epoxy phenol novolac resin with engineering thermoplastic poly (ether imide) have been investigated. Specimens were prepared using different mixing orders for multifunctional epoxy resin with poly (ether imide). Effect on glass transition temperature (Tg) were observed by using DSC measurements. Variation in mechanical properties viz. Tensile strength, flexural strength, flexural modulus, interlaminar shear strength and impact strength have been observed. With the thermoplastic modification of thermoset matrix material, improvement in mechanical properties of epoxy-glass fiber reinforced composites have been expected. Changes in storage modulus and loss modulus of all specimens were also evaluated by dynamic mechanical analysis (DMA). Scanning electron microscopy (SEM) was used to investigate the relationship between the morphological study of the fractured epoxy resins and mechanical properties of the modified epoxy resins and glass fiber reinforced composites. If the polymer matrix is fairly brittle (unmodified epoxy), there may be a corresponding reduction in mechanical properties. Incorporation of engineering thermoplastic Poly (ether-imide) has resulted in improvement of above stated mechanical properties. All results indicated that thermoplastic modified multifunctional epoxy resin proved to be a good matrix material which enhances the mechanical properties of glass fiber reinforced composites.

Mechanical and tribological properties of epoxy modified by liquid carboxyl terminated poly(butadiene‐co‐acrylonitrile) rubber

AbstractDiglycidyl ether of bisphenol A (DGEBA)‐based epoxy resin was modified using liquid carboxyl‐terminated poly(butadiene‐co‐acrylonitrile) (CTBN) rubber. The liquid CTBN contents used ranged from 2.5 to 20 parts per hundred parts of resin (phr). Mechanical properties of the modified resins were evaluated and the microstructures of the fracture surfaces were examined using SEM technique. The changes in storage modulus and the glass transition temperature were also evaluated using dynamic mechanical analysis (DMA). The tribological tests were performed using a ball‐on‐disc tribometer. The worn surfaces and the ball counter‐mates after tribological tests were investigated using optical microscope technique. The results revealed the influence of liquid CTBN content on mechanical and tribological properties, and also microstructure of the modified epoxy resins. Impact resistance increased whereas the storage modulus and the hardness decreased when the CTBN rubber was introduced to the epoxy network. The coefficient of friction of the CTBN‐modified epoxy was lower than that of the neat epoxy. The CTBN content of lower than 10 phr was recommended for improving the wear resistance of epoxy resin. Changes in tribological properties of the CTBN‐modified epoxy correspond well to those in mechanical changes, especially the toughness properties. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Correlations between alkyl side chain length and dynamic mechanical properties of poly(n-alkyl acrylates) and poly(n-alkyl methacrylates)

In the present paper, dynamic mechanical properties of poly(n-alkyl acrylates) (PnAA) and poly(n-alkyl methacrylates) (PnAMA) with different alkyl side chain length were studied. The results show that with the increase of alkyl side chain length, the storage modulus changes more steadily, and the loss modulus peak and the tan δ peak become broader for PnAA and PnAMA. At the same time, the tan δ peak is more and more apart from the loss modulus peak and the point where the storage modulus begins to drop. For quantitative discussion, three variables, the steepness index ( S), the transition wideness ( W) of storage modulus and the integration area ( A) of tan δ were defined to investigate the potential correlation between the dynamic mechanical properties and alkyl side chain length. It can be observed that S decreases while W and A increase with increasing alkyl side chain length. Moreover, the relaxation spectra of the two series of polymers are calculated from the corresponding mechanical spectra. The shapes of the relaxation spectra are broader and broader with the increase of the alkyl side chain length. These phenomena are interpreted by the perspective of fragility, molecular packing efficiency and intermolecular coupling.

Investigation of the rheological, dynamic mechanical, and tensile properties of single‐walled carbon nanotubes reinforced poly(vinyl chloride)

AbstractPolymer nanocomposites consisting of single‐walled carbon nanotubes (SWCNTs) and poly(vinyl chloride) were prepared by casting technique. The complex viscosity increased with increasing SWCNTs content, and it had a percolation concentration threshold equal to 0.45 wt % of SWCNTs. The storage modulus, G′, increased with increasing either SWCNTs content or frequency. A gradual decrease in the terminal zone slope of G′ for the nanocomposites with increasing SWCNTs content may be explained by the fact that the nanotube–nanotube interactions will be dominant at higher CNTs content, and lead to the formation of the interconnected or network‐like structures of SWCNTs in the polymer nanocomposites. The rheological loss factor indicates two relaxation peaks at frequencies of 0.11 and 12.8 Hz due to the interaction between SWCNTs and polymer chains and glass transition, respectively. Dynamic mechanical properties were measured for the prepared composites. The results indicate that the storage modulus changes steadily, and the tanδ peaks are less intense for high SWCNTs content. Tensile tests were measured and depicted by an increase in the elastic modulus with increasing SWCNTs content, but it decreases for all composites as the testing temperature increased. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012

Study of Humidity and UV Wavelength Effects on Degradation of Photo-Irradiated Polyethylene Films Using DMA

Plastic bags, mostly made of polyethylene, cause pollution as solid waste due to their nondegradable nature. Accelerated degradation, as a solution to mitigate the menace, can be achieved through moisture enhanced photolysis. This study evaluated the effect of three relative humidity environments, i.e., 25%, 40%, and 60% RH, at a constant temperature of 55°C. The effect was studied for ultraviolet (UV) irradiated and nonirradiated samples of polyethylene (PE) films processed under conventional ways. Photodegradation was initiated using ultraviolet irradiation in the ranges (200–300) nm and (300–400) nm for two hours and the effects of subsequent humidity treatment analyzed. Dynamic mechanical analysis was used to measure the dynamic storage modulus to monitor degradation. For nonirradiated samples, there was essentially no change in storage modulus at the three relative humidity environments after 550 hrs. Irradiation in the (300–400) nm range showed faster degradation than for the (200–300)nm range with the highest drop in storage modulus being 67% after 550 hrs. Raising the humidity from 25% to 40% and 60% RH resulted in 41%, 62%, and 67% drop of storage modulus, respectively, at the 550 hrs.