Average Glass Transition Temperature Research Articles

Comparative Studies for Cryopreservation of Agave Shoot Tips by Droplet-Vitrification.

The objective of this work was to assess the suitability of the Droplet-vitrification protocol previously developed with Agave peacockii shoot tips for the cryopreservation of six Agave species. Shoot tips were precultured for 1 day on a medium with 0.3 M sucrose in the dark, loaded in a solution with 1.6 M glycerol and 0.4 M sucrose for 20 min, and dehydrated by exposure to Plant Vitrification Solution 2 (PVS2) at 0 °C for 20 min. Complementary studies using histological analysis, Differential scanning calorimetry (DSC), and evaluation of morphological characteristics in cryo-derived plants were performed. Survival rates ranged from 84% to 100% and from 76% to 97% before and after cryopreservation regardless of the Agave species belonging to two taxonomic subgenera. Thermal analysis of shoot tips subjected to the successive steps of the Droplet-vitrification protocol identified ice crystal formation after loading treatment and glass transition after osmotic dehydration with PVS2. The average glass transition temperature (Tg) was -55.44 °C based on the results of four Agave species. The histological studies showed the anatomical differences that could be found in the meristematic structures depending on the loss of apical dominance. This is the most advanced research on cryopreservation of Agave shoot tips.

Влияние кардового полисульфона на вязкоупругие свойства эпоксидных полимеров

In this work, the viscoelastic properties of epoxy polymers modified with cardo copolymer of polysulfone were studied. The dependence of the modulus of elasticity on temperature for epoxy polymers with different content of thermoplastic is shown. The effect of a thermoplastic modifier on the average glass transition temperature of cured epoxyamine binders has been studied.

The viscoelasticity and deformation mechanism of Taxodium hybrid ‘Zhongshanshan’ wood by dynamic mechanical analysis

The viscoelasticity of Taxodium hybrid ‘Zhongshanshan’ wood, while undergoing hydrothermal processing, was investigated via dynamic thermomechanical analysis. The results showed that the elastic deformation and viscous deformation of the Taxodium hybrid ‘Zhongshanshan’ heartwood were greater than the sapwood. The heartwood average storage modulus and average loss modulus were greater than the sapwood. The difference between the heartwood and sapwood had little effect on the average glass transition temperature of their hemicellulose, which was approximately 74 °C. The radial average storage modulus was greater than the tangential, and the difference between the average loss modulus in the radial and tangential directions was negligible. The average glass transition temperature in the radial direction was slightly lower than the tangential direction. As the moisture content increased, the average storage modulus and its average hemicellulose glass transition temperature decreased. The average glass transition temperature tended to be lower as the moisture content increased. This study revealed the structural deformation and molecular movement of Taxodium hybrid ‘Zhongshanshan’ wood, while undergoing hydrothermal processing; this has important theoretical value for understanding its characteristics as well as its rational and efficient usage.

Curing behavior, chain dynamics, and microstructure of high Tg thiol-acrylate networks with systematically varied network heterogeneity

A series of networks is introduced with systematically varied network heterogeneity and high overall values of average glass transition temperature (Tg), based on polymerization of rigid acrylate and aromatic thiol monomers. The curing behavior, chain dynamics, and microstructure of these networks were investigated through a combination of dynamic mechanical analysis and infrared spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, and x-ray scattering, respectively. The maximum Tg achieved during cure can be related to the breadth of the mechanical loss tangent, as others have previously suggested, as well as the temperature dependence of the chain dynamics in the network as monitored by 1H NMR. In addition, the microstructures of the networks are characterized by periodic, fractal microgels with characteristic length scales of ca. 20–40 nm. Intriguingly, this structural motif persists in the more homogeneous networks exhibiting comparatively narrow glass transitions and chain dynamics, indicating that dynamically homogeneous networks can still exhibit significant compositional heterogeneity at the mesoscale.

Review and reproducibility of forming adsorbed layers from solvent washing of melt annealed films.

Recent studies suggest chain adsorption in the melt may be responsible for a number of property changes in thin films by making correlations between the residual adsorbed layer thickness hads(t) measured after a given solvent washing procedure as a function of annealing time t of the film at an elevated temperature prior to this solvent rinse. This procedure, frequently called "Guiselin's experiment", refers to the thought experiment proposed in a 1992 theoretical treatment by Guiselin that assumed chain segments in contact with the surface are irreversibly adsorbed whereby unadsorbed chains could be washed away by solvent without disturbing the adsorbed substrate contact points in the melt. In the present work, we review this recent literature, identifying and experimentally testing a common protocol for forming adsorbed layers hads(t) from solvent washing melt films. We find hads(t) curves to be far less reproducible and reliable than implied in the literature, strongly dependent on solvent washing and substrate cleaning conditions, and annealing at elevated temperatures is unnecessary as densification of films sitting at room temperature makes the glassy film harder to wash off, leaving behind hads of comparable thickness. This review also summarizes literature understanding developed over several decades of study on polymer adsorption in solution, which experimentally demonstrated that polymer chains in solution are highly mobile, diffusing and exchanging on the surface even in the limit of strong adsorption, contradicting Guiselin's assumption. Preformed adsorbed layers of different thicknesses hads are shown to not affect the average glass transition temperature or physical aging of 30 nm thick films. In summary, a number of open questions and implications are discussed related to thin films and polymer nanocomposites.

High power short time microwave finish drying of paprika (Capsicum annuum L.): Development of models for moisture diffusion and color degradation

The commercially available paprika at 16.25% (db) moisture was quickly finish-dried using microwaves at higher power density (5–25 W g−1). The moisture diffusivity was estimated using Fick’s second law of diffusion and the generalized kinetic model was used to estimate the color degradation rates. The moisture diffusivity and color degradation showed a close correlation with the difference between the average product temperature (T) achieved due to microwave heating, and average glass transition temperature (Tg) of paprika. Acceleration in moisture diffusion and color degradation was observed with the rise in the difference between the T and Tg. Further, the color degradation rate showed correlation with monolayer moisture content, average moisture content, T, and Tg of paprika during finish drying. The constants of the Gordon and Taylor model showed the less plasticization effect of water. Also, Tg showed a good correlation between water activity and moisture content. The activation energies for moisture diffusion and color degradation were found to be 92.53 and 11.03 kJ mol−1, respectively. The microstructural analysis of finish-dried paprika showed the expanded and newly formed intercellular spaces. The developed correlations can be used to simulate heat and mass transfer operations such as drying and sterilization.

Using poly(l-lactic acid) and poly(ɛ-caprolactone) blends to fabricate self-expanding, watertight and biodegradable surgical patches for potential fetoscopic myelomeningocele repair.

Our study focuses on the development and characterization of a self-expanding, watertight and biodegradable patch for fetoscopic myelomeningocele (MMC) prenatal repair. We fabricated poly(l-lactic acid) (PLA) and poly(ɛ-caprolactone) (PCL) blend films by solution casting. Formulation c with average glass transition temperature of 37.6 ± 1.2°C was chosen for temporospatial recovery. Favorable results from surface studies reflected homogeneous dispersion of polymers in the blend. The cytotoxicity was studied in human foreskin fibroblasts. The blend film was cytocompatible, evidenced by matching percentage of live cells in exposed and control solutions. Subsequently, liquid water permeability experiments confirmed watertight nature of films. Finally, in vitro degradation was investigated in phosphate buffered saline (PBS) and amniotic fluid (AF) separately for 16 weeks. Similar weight loss (n = 6, p = 0.912) and significantly different (n = 3, p = 0.025) surface roughness was observed in PBS and AF, respectively, at 16 weeks. Functional group analysis displayed increasing carbonyl and hydroxyl bonds in PBS and AF, respectively, over time, indicating progression of hydrolytic degradation. Favorable characterization results provide strong evidence to employ PLA-PCL blend films as surgical patches in fetoscopic MMC repair. Designed patch serves as standalone system to successfully tackle impending hurdles of MMC repair and proves to be a superior alternative compared to existing patches. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 107B: 295-305, 2019.

Mechanical and thermal properties of glass fiber–vinyl ester resin composite for pipeline repair exposed to hot-wet conditioning

Fiber-reinforced composites are a well-recognized option for repair and rehabilitation of the pipelines for the oil and gas industry. Infilled composite sleeve system provides an effective rehabilitation solution, where the sleeve acts as prime reinforcement without any direct contact with steel. However, the long-term performance of the repair is dependent, in part, on the effect of hygrothermal ageing of the composites. In this publication, glass transition temperature and mechanical properties are compared for glass-fiber reinforced vinyl ester composite, both as-manufactured and after hot-wet conditioning at 80℃. The tensile and shear strength reduced substantially during conditioning, whilst the elastic modulus was relatively stable. The average glass transition temperature of the composite dropped from the as-manufactured value of 110℃ to 97℃ and 101℃, after 1000 and 3000 h of conditioning, respectively, indicating that it is stable and that the composite is suitable for use as a pipeline repair material operating at 80℃. The results indicate that a 1000 h conditioning period, specified as a minimum period in ISO/TS 24817 is suitable for representing long-term properties for stiffness-based designs for the composite material and conditioning temperature investigated.

Effect of Adjacent Rubbery Layers on the Physical Aging of Glassy Polymers

We investigate the effect of glassy–rubbery interfaces between neighboring polymer domains on the local stability and physical aging of confined glassy layers. We present results demonstrating how ellipsometry can be used to measure the physical aging rate of glassy polystyrene (PS) layers atop rubbery poly(n-butyl methacrylate) (PnBMA) layers. With decreasing PS layer thickness, down to 84 nm, we observe no change in PS aging rate despite a 25–30 °C reduction in the average glass transition temperature Tg of this layer. These results are in strong contrast to single layer PS films, which show a decrease in aging rate with decreasing thickness due to the local Tg reduction at the free surface. The aging rate of the glassy PS layers atop rubbery PnBMA cannot be explained by any shift in local Tg, suggesting some additional factor is affecting the structural relaxation occurring near the glassy–rubbery interface.

Ovalbumin-BasedPorous Scaffolds for Bone Tissue Regeneration

Cell differentiation on glutaraldehyde cross-linked ovalbumin scaffolds was the main focus of this research. Salt leaching and freeze drying were used to create a three-dimensional porous structure. Average pore size was 147.84 ± 40.36 μm and 111.79 ± 30.71 μm for surface and cross sectional area, respectively. Wet compressive strength and elastic modulus were 6.8 ± 3.6 kPa. Average glass transition temperature was 320.1 ± 1.4°C. Scaffolds were sterilized with ethylene oxide prior to seeding MC3T3-E1 cells. Cells were stained with DAPI and Texas red to determine morphology and proliferation. Average cell numbers increased between 4-hour- and 96-hour-cultured scaffolds. Alkaline phosphatase and osteocalcin levels were measured at 3, 7, 14, and 21 days. Differentiation studies showed an increase in osteocalcin at 21 days and alkaline phosphatase levels at 14 days, both indicating differentiation occurred. This work demonstrated the use of ovalbumin scaffolds for a bone tissue engineering application.

Role of Diblock Copolymers toward Controlling the Glass Transition of Thin Polymer Films

The physical properties of thin polymer films are often thickness, h, dependent, influenced by confinement and by interfacial interactions between the chains and the external interfaces. We show that the magnitude and film thickness dependence of the average glass transition temperature, Tg, of the polystyrene−silicon oxide (PS/SiOx/Si) system are influenced appreciably with the addition of polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymers. The Tg can be “tailored” to increase, or decrease, with decreasing h or to remain independent of h. Tg shifts of as much of 35 °C are obtained for films of h ≈ 20 nm. Additionally, we report that the critical micelle concentration, φcmc, of the copolymer in thin films is considerably larger than for the bulk; specifically, micelles form only beyond a critical film thickness, determined by the size of the chains and by the number of chains in the system. The h dependence of Tg is not influenced by the ϕcmc or by the number of micelles in this system.

Direct Detection of Effective Glass Transitions in Miscible Polymer Blends by Temperature-Modulated Differential Scanning Calorimetry

Individual effective glass transitions of components in miscible blends of polystyrene/poly(vinyl methyl ether), poly(o-chlorostyrene)/poly(vinyl methyl ether), and poly(styrene-block-n-butyl acrylate)/polystyrene were detected by temperature-modulated differential scanning calorimetry. These blends apparently showed broad single glass transitions in their heat capacity (Cp) curves. This result has been widely accepted as a criterion of miscible mixing of both components. However, the derivative Cp curves of some blends showed bimodal peaks. This indicates the coexistence of two relaxations in the miscible blend. Moreover, the ratio of the two peaks was related to the compositions of the blends. The temperatures at the peaks of the derivative Cp curve were taken to be effective glass transition temperatures (Tg,A and Tg,B) of the components in the blend. Tg,A and Tg,B were much different from the average glass transition temperature (Tg,av) at which the endothermic shift was half in the Cp curve. On the other hand, Tg,av and Tg,A of homopolymers and poly(styrene-random-n-butyl acrylate)s were almost coincident with each other. Tg,A and Tg,B of the blends were in excellent agreement with the effective glass transition temperatures predicted by the model, taking into account the self-concentration effect.

Curing characteristics of carboxyl functionalized glucose resin and epoxy resin

The curing characteristics of carboxylic functionalized glucose resin (glucose maleic acid ester vinyl resin: GMAEV) and epoxy resin have been studied using DSC and FTIR methods. Exothermic reactions attributed to esterification and etherification reactions of the hydroxyl and carboxyl functionalities of GMAEV with the epoxy groups were identified. Exothermic reactions showed very different patterns according to the degree of carboxyl group substituent of GMAEV. The results showed that esterification reaction occurs in the early stage of cure and then etherification followed after completion of the esterification. A cured matrix containing epoxy resin and 50 wt.% of GMAEV was prepared and characterized. The cured matrix showed thermal stability up to 300 °C. The average glass transition temperature and storage modulus of the matrix were as high as 95 °C and 2700 MPa, respectively. The cured matrix of epoxy resin and GMAEV with higher degree of carboxyl group was found to have a lower density due to the formation of bulky groups in the crosslinks.

GLASS TRANSITION TEMPERATURE MAPPING USING MAGNETIC RESONANCE IMAGING

Representation of a system using an average glass transition temperature (Tg) value will become inadequate if the system is not sufficiently homogeneous in terms of physical structure and chemical composition. Many food systems are heterogeneous, and therefore a nonuniform distribution of Tg values is expected. In this exploratory article, the basis for determination of Tg using nuclear magnetic resonance (NMR) techniques is discussed, and a magnetic resonance imaging (MRI) method is proposed for determining the spatial distribution of Tg in food systems. The Tg maps for maltodextrin and bread samples, obtained using this method, are presented.

Segmental dynamics and incompatibility in hard/soft polymer blends

We report on the segmental dynamics of the binary polymer blend polystyrene (PS)/poly (methylphenylsiloxane) (PMPS) in the two-phase region using dielectric spectroscopy that essentially probes the PMPS component. Based on the experimental orientation relaxation functions, the average glass transition temperature Tg controls phase separation. When the spinodal temperature Ts exceeds Tg, the PMPS segmental relaxation displays two distinct decays characteristic of a merely pure and a mixed, roughly at the initial composition, PMPS regions. On the contrary, when Ts falls in the proximity of Tg, the PMPS relaxation is strongly nonexponential and its average time reflects mixed regions rich in PMPS due to incomplete phase separation, which drives only the glassy phase out of local thermodynamic equilibrium. Distinct morphological differences in the two-phase state of these blends, inferred from their segmental dynamics, are revealed by transmission electron microscopy.