Thermomechanical Transitions Research Articles

Quadruple Stimuli-Responsive Behaviour of Acylhydrazone Crystals: Mechanical, Photomechanical, Thermomechanical, and Optical Waveguide Properties.

As society advances, the demand for high-performance crystal materials with versatile stimuli-responsive capabilities has increased. However, there remains a notable shortage of simple molecular crystals that exhibit stable and swift responses to various stimuli. In this study, we synthesized a novel acylhydrazone crystal by integrating benzene-fused heterocycles and halogens, achieving a crystal material with multiple stimuli-responsive behaviours. The crystal demonstrates good mechanical flexibility, capable of with standing a strain of 2.3 % under applying mechanical force and recovering its original shape after force removal. Crystal structure analysis reveals this property is due to the strong and flexible intermolecular interactions. It also exhibits rapid photomechanical bending away from light source under UV irradiation, which could be attributed to the facile photoisomerization. Moreover, the crystal undergoes significant thermomechanical transitions, including cracking and jumping, upon heating. Additionally, the prepared crystal exhibits passive optical waveguide of red light at 640 nm, which, combining the photomechanical bending, can achieve controllable light output direction. These multi-stimuli-responsive behaviors make the acylhydrazone crystal a promising candidate for applications in multifunctional sensors, optoelectronic devices, and thermal switches. This work provides a helpful reference for the study of potential crystal material design and functionality of advanced responsive systems.

Thermomechanical transitions of meat-analog based fried foods batter coating

This study aimed to characterize the thermomechanical transitions of meat-analog (MA) based coated fried foods. Wheat and rice flour-based batters were used to coat the MA and fried at 180 °C in canola oil for 2, 4 and 6 min. Glass-transition-temperature (Tg) of the coatings were assessed by differential scanning calorimetry, directly after frying or after post-fry holding. Mechanical texture analyzer and X-ray microtomography were employed to assess textural attributes and internal microstructure, respectively. Batter-formulation substantially impacted the Tg of fried foods coating i.e., crust. Tg of fried foods crust were ranged between −20 °C to −24 °C. Tg was positively correlated with frying time and internal microporosity (%), whereas negatively correlated with moisture content. Internal microstructure greatly influenced the textural attributes (hardness, brittleness, crispiness). Post-fry textural stability considerably impacted by Tg. Negative Tg value explains post-fry textural changes (hard-to-soft, brittle-to-ductile, crispy-to-soggy) of MA-based coated products at room-temperature (25 °C) and under IR-heating (65 °C).

A Combined Rheological and Thermomechanical Analysis Approach for the Assessment of Pharmaceutical Polymer Blends

The viscoelastic nature of polymeric formulations utilised in drug products imparts unique thermomechanical attributes during manufacturing and over the shelf life of the product. Nevertheless, it adds to the challenge of understanding the precise mechanistic behaviour of the product at the microscopic and macroscopic level during each step of the process. Current thermomechanical and rheological characterisation techniques are limited to assessing polymer performance to a single phase and are especially hindered when the polymers are undergoing thermomechanical transitions. Since pharmaceutical processing can occur at these transition conditions, this study successfully proposes a thermomechanical characterisation approach combining both mechanical and rheological data to construct a comprehensive profiling of polymeric materials spanning both glassy and rubbery phases. This approach has been used in this study to assess the mechanical and rheological behaviour of heterogenous polymer blends of hydroxypropyl cellulose (HPC) and hydroxypropyl methylcellulose (HPMC) over a shearing rate range of 0.1–100 s−1 and a temperature range of 30–200 °C. The results indicate that HPC and HPMC do not appear to interact when mixing and that their mixture exhibits the mechanistic properties of the two individual polymers in accordance with their ratio in the mixture. The ability to characterise the behaviour of the polymers and their mixtures before, throughout, and after the glassy to rubbery phase transition by application of the combined techniques provides a unique insight towards a quality-by-design approach to this and other polymer-based solid dosage forms, designed with the potential to accelerate their formulation process through obviating the need for multiple formulation trials.

Collective and cooperative dynamics in transition domains of amorphous polymers with multi-shape memory effect

The multi-shape memory effect (multi-SME) in amorphous shape memory polymers (SMPs) linked with collective and cooperative rearrangements and accommodation of monomeric segments leads to the generation of complex thermodynamic modes. In this study, an extended domain size model is initially formulated to describe variations in temperature-dependent relaxation behavior and domain transitions in amorphous SMPs. According to the Adam–Gibbs theory, a cooperative model is employed to identify the principle role of domain size in the collective dynamics of multi-SMEs in amorphous SMPs. The phase transition theory is then combined with the multi-branch Kelvin model to describe the collective and cooperative relaxation behavior of the SMPs with multiple transition domains. It is shown that the proposed model is able to characterize the thermomechanical transitions and multiple shape recovery processes. Finally, the model is applied to predict the shape recovery behavior of SMPs with triple- and quadruple-SMEs, respectively, and the theoretical results are well validated by the experimental results.

Anomalous Appearance of a Distinct Glass Transition in Perfluoroimide Acid Ionomers

Perfluorosulfonic acid ionomers (PFSAs) are the benchmark materials used as proton exchange membranes in fuel cells due to their exceptional proton conductivity and chemical and mechanical stability. Typical PFSAs consist of a PTFE backbone with perfluoroether sidechains bearing pendant sulfonic acid groups. PFSAs of different ion contents and sidechain compositions have been developed in an effort to optimize functionality while maintaining mechanical stability. One such ionomer is 3M’s perfluoroimide acid (PFIA) containing a sulfonimide acid group in addition to the sulfonic acid on each sidechain to increase proton conductivity while preserving TFE backbone segments to maintain sufficient crystallinity. In this study, we investigate the effect of this additional acidic site on the thermomechanical properties and morphology of PFIA compared to the single acid PFSA. Utilizing alkylammonium counterions we are able to systematically manipulate the strength of the physically crosslinked network in the perfluorinated ionomers to probe the origins of thermomechanical transitions. The addition of the sulfonimide in PFIA leads to interesting differences in the morphology and thermomechanical properties including the appearance of an anomalous glass transition that is not observed in single acid PFSAs. Techniques including dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), and variable temperature small angle x-ray scattering (SAXS) are utilized to probe the anomalous glass transition and assign it to the true glass transition temperature for PFIA. Figure 1

Determination of thermal transitions of gluten-free chestnut flour doughs enriched with brown seaweed powders and antioxidant properties of baked cookies

A protocol for determining the characteristic temperatures of thermomechanical transitions on gluten-free flour doughs is proposed. This protocol is based on the mathematical analysis of experimental curve of storage modulus (G′) vs temperature obtained by means of Dynamic Mechanical Thermal Analysis (DMTA) technique. Doughs at constant consistency of chestnut flour with different levels (3, 6 and 9% flour basis, f.b.) of brown seaweed (Bifurcaria bifurcata, Fucus vesiculosus and Ascophyllum nodosum) powders addition, 2% f.b. of guar gum and 1.8% f.b. of salt with different water absorption were used to test the proposed protocol. The ranges of temperatures corresponding to starch gelatinization (59–97 °C), amylopectin crystallites melting (82–101 °C), reversible dissociation of lipid-amylose complexes (107–128 °C) and amylose melting (133–171 °C) showed a strong dependence with water absorption of samples. Doughs with the same water absorption submitted to starch gelatinization during mixing were also analysed to corroborate the protocol suitability. Total polyphenols content and radical scavenging activity of extracts from chestnut flour-seaweed powder blends and seaweed-enriched chestnut cookies baked at 180 °C were determined. Extraction assisted with ultrasounds was carried out employing acetone-water (70:30 v/v) solution as solvent during 4 min with a liquid/solid ratio of 30 w/w. Seaweed powder addition had a positive effect on antioxidant properties of doughs before baking. However, the seaweed powder addition effect on baked products (cookies) is not clear due to antioxidant activity is overlapped by Maillard's products generated during baking.

Compressive cyclic ratcheting and fatigue of synthetic, soft biomedical polymers in solution

The use of soft, synthetic materials for the replacement of soft, load-bearing tissues has been largely unsuccessful due to a lack of materials with sufficient fatigue and wear properties, as well as a lack of fundamental understanding on the relationship between material structure and behavior under cyclic loads. In this study, we investigated the response of several soft, biomedical polymers to cyclic compressive stresses under aqueous conditions and utilized dynamic mechanical analysis and differential scanning calorimetry to evaluate the role of thermo-mechanical transitions on such behavior. Studied materials include: polycarbonate urethane, polydimethylsiloxane, four acrylate copolymers with systematically varied thermo-mechanical transitions, as well as bovine meniscal tissue for comparison. Materials showed compressive moduli between 2.3 and 1900MPa, with polycarbonate urethane (27.3MPa) matching closest to meniscal tissue (37.0MPa), and also demonstrated a variety of thermo-mechanical transition behaviors. Cyclic testing resulted in distinct fatigue-life curves, with failure defined as either classic fatigue fracture or a defined increased in maximum strain due to ratcheting. Our study found that polymers with sufficient dissipation mechanisms at the testing temperature, as evidenced by tan delta values, were generally tougher than those with less dissipation and exhibited ratcheting rather than fatigue fracture much like meniscal tissue. Strain recovery tests indicated that, for some toughened polymers, the residual strain following our cyclic loading protocol could be fully recovered. The similarity in ratcheting behavior, and lack of fatigue fracture, between the meniscal tissue and toughened polymers indicates that such polymers may have potential as artificial soft tissue.

Thermomechanical transitions of egg-ceramide monolayers.

Ceramides have unique biophysical properties. Their high melting temperature and their ability to form lateral domains have converted ceramides into the paradigm of rigid lipids. Here, using shear surface rheology of egg-ceramide Langmuir monolayers, a solid to fluid transition was evidenced as a vanishing shear rigidity at lower temperatures than the lipid melting temperature. Such a mechanical transition, which depends on the lipid lateral pressure, was found in a broad range temperature (40-50 °C). The solid to fluid transition was correlated to a LC to LC+LE phase transition, as confirmed by BAM experiments. Interestingly, together with the softening transition, a supercooling process compatible with a glassy behavior was found upon freezing. A new phase scenario is then depicted that broadens the mechanical behavior of natural ceramides. The phase diversity of ceramides might have important implications in their physiological roles.

Physicomechanical characterization of monodisperse multivesiculated polyester particles

This work describes physicomechanical properties of low-density polyester particles with multi-alveolar inner morphology, obtained via a water/oil/water double emulsion process. Monodisperse particle diameters were produced thanks to the use of a microchannel T-junction device for droplet generation. The drying conditions tested showed that rapid water evaporation at 120°C, combined with slow diffusion towards the exterior, causes internal fracture and particle deformation. Interestingly, all particles present virtually identical internal damage and external deformation features under these drying conditions, demonstrating the uniformity in internal structures. Drying at 70°C allows for efficient water removal with no damage. Thermomechanical transitions and thermal stability were analyzed by dynamic mechanical analysis (DMA in single-particle compression mode), dynamic scanning calorimetry (DSC) and thermogravimetry (TG). The effectiveness of curing conditions was evaluated by FTIR and DSC, allowing to identify the need of a thermal post-treatment for consumption of residual styrene. The consequent increase in degree of crosslinking produced a positive shift in glass transition temperature measured by DMA. Finally, mechanical reinforcement of the multivesiculated polyester particles was obtained by loading the polymer with surface-modified fumed silica, yielding 75% increase in storage modulus.

Microfluidic Thermally Activated Materials for Rapid Control of Macroscopic Compliance.

Macroscopic structures that undergo rapid and reversible stiffness transitions can serve as functional polymeric materials for many applications in robotics and medical devices. Thermomechanical phase transitions can provide a suitable mechanism for transient control of mechanical properties. However, the characteristic time scale for actuation is large and dictated by the dimensions of the structure. Embedding vascular networks within bulk polymers can reduce the characteristic length scale of the material and permit rapid and reversible thermomechanical transitions. Here we report perfusable bulk materials with embedded microvascular networks that can undergo rapid and reversible stiffness transitions. Acrylate-based thermoplastic structures exhibit storage moduli with a dynamic range between E' = 1.02 ± 0.07 GPa and E' = 13.5 ± 0.7 MPa over time scales as small as 2.4 ± 0.5 s using an aqueous thermal perfusate. The spatiotemporal evolutions of temperature profiles were accurately predicted using finite element simulation and compared to experimental values. Rigid-compliant transitions were leveraged in a demonstration in which a microvascularized device was used to grasp an external object without the aid of moving parts.

Solvent-free copolymerization of rigid and flexible bis-1,3-benzoxazines: Facile tunability of polybenzoxazine network properties

We report the copolymerization of a flexible aliphatic-bridged bisphenol-based benzoxazine monomer comprising ten methylene units (BZ(10)BA) with two rigid benzoxazine monomers (commercially available Araldite 35600 and 35900) via a solvent-free cationic ring-opening polymerization process. The effects of monomer feed composition on polymerization behavior, thermomechanical transitions, and thermal degradation properties are reported. DSC of the ring-opening copolymerizations showed that the copolymerization behavior – in terms of polymerization onset temperature and total exothermic transition – depend greatly on the composition of the monomer feed. Samples containing larger concentrations of BZ(10)BA exhibited higher onset temperatures with lower polymerization enthalpies. The thermomechanical properties of the copolybenzoxazine networks, as evaluated by DMA, show a strong dependence on the monomer feed ratio, where higher Araldite content resulted in a higher Tg of the network. The most salient feature of benzoxazine copolymerization was revealed in the tailorability in thermomechanical properties, which were varied by 149 °C simply by changing the monomer feed ratio and the Tg was observed to be accurately predicted using the Fox equation.

Flexible aliphatic-bridged bisphenol-based polybenzoxazines

In this paper, we report the synthesis of a series of novel aliphatic-bridged bisphenol-based benzoxazine monomers comprising four to ten methylene unit spacers (BZ(n)BA). Cationic ring-opening polymerization of these monomers provides flexible polybenzoxazine thermosets with good film forming characteristics under solvent-free processing conditions. The effects of aliphatic bisphenol chain length on polymerization behavior, thermomechanical transitions, and mechanical properties of the polybenzoxazine thermosets are reported. Fourier transform-infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) show the ring-opening polymerization proceeds to high conversion with minimal dependence on spacer chain length. Thermomechanical properties of the pBZ(n)BA thermosets, such as rubbery storage modulus and glass transition temperature (Tg), show a strong dependence on the length of the aliphatic-bridged bisphenol linker where both properties decrease with increasing linker length. In particular, changing the length of the aliphatic-bridged bisphenol linker enables tailoring the Tg of the pBZ(n)BA series from 67 °C to 101 °C, as determined by dynamic mechanical analysis (DMA). Tensile properties of the pBZ(n)BA series exhibit similar trends with Young's modulus decreasing and elongation at break increasing with increasing aliphatic-bridged bisphenol linker length. The pBZ(n)BA materials all show a similar three mode degradation process by thermogravimetric analysis (TGA) consistent with other bisphenol based polybenzoxazines, and additionally exhibit a decrease in char yield with increasing aliphatic chain length owing to a decrease in aromatic content in the thermoset network.

Seawater degradable thermoplastic polyurethanes

AbstractDegradable 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

Thermomechanical transitions in doubly-clamped micro-oscillators

The small size and low damping of MEMS oscillators give rise to phenomena that are not observed routinely at the macroscopic scale. In this work we document and explain an experimentally observed transition in the response of a doubly clamped micromechanical oscillator with pretension. The transition from softening to hardening is repeatedly observed upon increasing the power of an incident sensing laser beam, a procedure routinely used to improve signal strength during optical detection of resonant motion of microstructures. At intermediate laser power, a novel resonant response that displays characteristics of both softening and hardening in the same sweep, is observed experimentally. Increased laser heating of a structure in tension may be expected to increase softening behavior. Using tools from non-linear dynamics and continuum mechanics, we show that the observed counter-intuitive behavior can be explained by a competition between the opposing responses of linear and non-linear stiffnesses to a change in temperature.

Thermomechanical Transitions of Rice Kernels

ABSTRACTThermomechanical analysis (TMA) and differential scanning calorimetry (DSC) were used to investigate the thermal transitions of long‐grain rice kernels. Three distinct thermomechanical transitions were identified as rice kernels were heated from 0 to 200°C. The identified transitions were a low temperature transition with onset at ≈45°C, an intermediate temperature transition at ≈80°C, and a high temperature transition at ≈180°C. Low temperature transition with onset from ≈60°C at 5% moisture content (MC) to 30°C at 20% MC was identified as the glass transition of the rice kernels. Intermediate temperature transition from 60 to 100°C, depending on MC, may be caused by rapid evaporation of moisture in the rice kernels. High temperature transition was associated with melting of the crystalline structure of rice starch. The temperatures of all three transitions decreased as MC increased, confirming that moisture acted as a plasticizer in rice kernels.

Spectromechanical analysis of the structure and oxidation of PMR thermoset thermally-stable resins

The long-term ageing of high temperature polyimide thermoset polymers such as the commercial PMR15 resin and model molecules which compose the PMR15 chemical structure, was studied. The amount of each oligomer directly influences the physico-chemical and mechanical properties. The aim was to understand the oxidation mechanisms of these thermoset polyimide resins and the consequences for the mechanical properties. Nuclear magnetic resonance analysis has been associated with the thermo-mechanical transitions.

Moisture Redistribution and Phase Transitions During Bread Staling

ABSTRACTStandard white breads were stored with or without crust at 25°C in hermetic pouches. During two weeks of storage, the crumb moisture content and water activity (aw) decreased significantly when stored with crust. When stored without crust, moisture content and aw remained relatively unchanged. The causes of the initial firming of both breads over zero to seven days were not conclusive. But when stored beyond seven days, bread stored with crust was significantly firmer in texture and higher in amylopectin recrystallization than bread stored without crust. Moisture redistribution from crumb to crust played a significant role. This was accompanied by a decrease in freezable water in the bread crumb stored with crust. This loss in freezable water coincided with changes in the thermomechanical profile only in the case of sample stored with crust intact (and with a significant total and freezable water loss). Bread crumb stored without crust did not change in total and freezable water and showed less change in thermomechanical transitions. The transition occurring at ≈60°C (T2) correlated with amylopectin recrystallization but it could also have been caused by moisture loss during the analysis. Moisture migration from crumb to crust greatly reduced the total and freezable water in the crumb region, resulting in a significant reduction in the magnitude of the mechanical transition at ≈0°C (T1) as well as an increase in the storage modulus.

Optimal control of thermomechanical phase transitions in shape memory alloys: Necessary conditions of optimality

The necessary conditions for the optimality of an optimal control problem associated with soiid–solid phase transitions in shape memory alloys are established.

Convergent numerical approximations of the thermomechanical phase transitions in shape memory alloys

Discrete approximations are constructed to a nonlinear evolutionary system of partial differential equations arising from modelling the dynamics of solid-state phase transitions of thermomechenical nature in the case of one space dimension. The class of problems considered includes the so-called shape memory alloys, in particular. It is shown that the obtained discrete solutions converge to the solution of the original problem, and numerical simulations for the shape memory alloy Au23Cu30Zn47 demonstrate the quality of the discrete model.

Application of a new structural theory to polymers. ii. transition states for amorphous polymers and copolymers

Abstract A recently developed nonequilibrium thermodynamic theory of continuum rheology is combined with a generalized definition of thermomechanical transitions, to produce a single equation for interrelating the basic variables (stress o, strain rate ε, pressure p, temperature T, and structure ℊ) at a transition. Specialization of ℊ to represent uncrosslinked polymers leads to incorporation of molecular weight M as a variable. New predictions are thus made for the glass transition [Tg(M), Tg(p), Tg(ε) and others] and compared successfully with data. Particularly remarkable are the results that 1/Tg is a piecewise linear function of In M, and T is piecewise linear with p. Comparable results and confirmation with data arise when applying the theory to the liquid-liquid transition, T ll (M). For random copolymers, application of a single mixing rule to the transition equation leads to a prediction of Tg as a function of composition and the Tgi for the homopolymers (components i). This relationship reduces, in various cases, to several familiar equations in which the parameters were simply empirical, thus providing an interpretation of those parameters and defining restrictions applicable to each case. Finally, an alternative interpretation of ℊ in terms of free volume allows the theory to be extended to other systems, including those with small molecules.