The drying strategy of atmospheric freeze drying apple cubes based on glass transition

Freeze drying (FD) yields the best quality of dried apple cubes but at the cost of long drying time and also the overall cost. To achieve economical freeze drying along with a high quality product an atmospheric freeze drying (AFD) technique was developed to dry apple cubes. The effect of different air temperature loading scheme on product quality and drying process was studied during AFD. According to the glass transition temperature of apple, a step-up temperature loading strategy for AFD process was developed to reduce the drying time by almost half and provided a similar good product quality.     

Correlation of the glass transition temperature of plasticized PVC using a lattice fluid model

A model has been developed to estimate the glass transition temperature of polymer+plasticizer mixtures (up to 30 wt% plasticizer). The model is based on the Sanchez-Lacombe equation of state and the Gibbs-DiMarzio criterion, which states that the entropy of a mixture is zero at the glass transition. The polymers studied included polystyrene and poly(vinyl chloride). The plasticizers studied included a wide range of chemicals from methyl acetate to di-undecyl phthalate. The model qualitatively accounted for the effect of different plasticizers on the mixture glass transition temperature.     

The effect of confinement in nanoporous polymers on the glass transition temperature

The glass transition temperature (T_g) of nanoporous polyetherimide (PEI) and PEI thin films was investigated. The T_g decreased from its bulk value in both of these confined systems. Monte Carlo simulations were performed to calculate the nearest neighbor pore-to-pore distances in the nanoporous PEI. A quantitative analogy between the nanoporous PEI and PEI thin films is proposed through an equivalence of nearest neighbor pore-to-pore distances and thin film thickness. The effect of confinement is believed to be due to the interface regions, which possess higher chain mobility than the bulk. When these high mobility interface regions are sufficiently close together, the excess mobility at the interface region affects the dynamics of the system by restraining percolation of the slow domains resulting in the observed decrease in T_g.     

The effect of the glass transition temperature on the toughness of photopolymerizable (meth)acrylate networks under physiological conditions

The purpose of this study is to evaluate how the toughness of photopolymerizable (meth)acrylate networks is influenced by physiological conditions. By utilizing two ternary (meth)acrylate networks, MA-co-MMA-co-PEGDMA and 2HEMA-co-BMA-co-PEGDMA, relationships between glass transition temperature (T_g), water content and state, and toughness were studied by varying the weight ratio of the linear monomers (MA to MMA or 2HEMA to BMA). Differential scanning calorimetry and thermogravimetric analysis were performed to evaluate the thermal behavior and water content as a function of either MA or 2HEMA concentration while tensile strain-to-failure tests were performed at 37 °C to determine network toughness. Both networks exhibited a maximum in toughness in PBS in the composition corresponding to a T_g close to the testing temperature. This toughness maximum was achieved by adjusting the glass transition temperature and/or hydrophilicity through changes in chemistry. These relationships may be utilized to design tough photopolymerizable networks for use in mechanically rigorous biomedical applications.     

Factors determining glass transition temperature and relaxation time of poly(vinyl chloride)

Glass transition properties were obtained by measurements of the dilatometric glass transition temperature T_g at elevated pressure by isobaric cooling at a constant rate. The properties include the pressure-volume-temperature (P-V-T) relations of the liquid, the specific heat capacity and the dielectric properties for a single sample of poly(vinyl chloride). From the experimental results, various thermodynamic excess quantities such as the free volumes and the configurational entropy and energies were evaluated as factors determining T_g and the relaxation time τ. Some empirical rules concerning T_g, which have been proposed by Simha and Boyer, and Wunderlich, are also examined. The main conclusions are as follows: (1) the free volume is not an essential factor determining T_g and τ; (2) the Adam-Gibbs parameter is slightly superior to the other excess entropy and energies defined as the difference of the entropy and energies between the liquid and the hypothetical crystal whose thermal expansivity and compressibility are the same as those of the corresponding glasses; and (3) the glass transition may not be treated as a quasi-equilibrium thermodynamic transition to which one of the Ehrenfest relations applies.     

Effects of freeze-drying on the glass temperature of cyclic polystyrenes

The calorimetric glass temperature was measured for three cyclic polystyrenes with apparent molecular weights ranging from 4.0×10³ to 195.5×10³ g/mol for both bulk material and for samples freeze-dried from dilute solution. Freeze-drying from dilute solution was found to reduce the glass temperature by 7–14 K depending on the sample. These T_g depressions are 5–12 K greater than those found previously for freeze-dried linear polystyrene. Annealing at 403.2 and 443.2 K (130 and 170 °C) resulted in recovery of the T_g back to the bulk value with the time scales depending on both temperature and the magnitude of the T_g reduction; the low apparent activation energy dependence of the recovery of T_g precludes its being due to viscous flow.     

Examination of the influence of cooperative segmental dynamics on the glass transition and coefficient of thermal expansion in thin films probed using poly(n-alkyl methacrylate)s

The thermal behavior of thin films of a homologous series of poly(n-alkyl methacrylate)s supported on silicon substrates was probed using spectroscopic ellipsometry. Deviations from bulk behavior for the glass transition temperature (T_g) and coefficient of thermal expansion (CTE) were observed for films thinner than approximately 60 nm, consistent with most observations for confinement effects in polymer films. However, the extent of the decrease of CTE and the deviation in T_g are significantly influenced by the alkyl chain length. As the alkyl chain length is increased from methyl to n-octyl, the deviation from bulk behavior is significantly suppressed. This behavior is similar to that observed by Torkelson and coworkers (Physical Review Letters 2004; 92: 095702) for polystyrene films as small molecule diluents were added; this behavior was attributed to the decrease in size of the cooperative segmental dynamics, ξ_i(CRR), with addition of solvent. ξ_i(CRR) decreases as the alkyl chain length is increased; this is consistent with the hypothesis for the relationship between ξ_i(CRR) and confinement effects in thin polymer films. However, a two order of magnitude difference between the thickness of nanoconfinement onset and ξ_i(CRR) for poly(n-octyl methacrylate) results in uncertainty for ξ_i(CRR) as the origin of the nanoconfinement effect.     

Investigation of the glass transition temperature and damping of an acrylic/epoxy bonding tape using the peak damping method

The dynamic properties of a structural tape used for adhesive bonding applications have been measured at different temperatures to determine its glass transition temperature and damping properties. For this purpose, free layer beams consisting of a base layer steel and the tape layer were vibrated using a resonant beam technique with free-free end conditions. To measure the dynamic values (elastic modulus and loss factor) of the tape, the necessary equations were derived and the frequency dependence of the beams was investigated from –55°C to +60°C. Three beams with different layers were tested. Results have shown that as the temperature increases, the elastic modulus of the tape decreases, while the loss factor of the tape increases up to 20°C and then decreases to a constant level. The results from the three beams are in agreement, showing that the glass transition temperature of the tape is about 20°C, which implies viscoelastic properties at room temperature.     

Self-Reinforcing Composite Based on Ethylene Acrylic Elastomer (AEM)/Liquid Crystalline Polymer (LCP) Blend

Abstract

Blends of ethylene acrylic elastomer (AEM) and thermotropic liquid crystalline polymer (TLCP) have been prepared by melt mixing technique. Processing studies indicated a decrease in the viscosity of the blends with the addition of liquid crystalline polymer (LCP). At lower level of LCP the tensile strength and tear strength increased. However, at higher level of LCP tensile strength values decreased due to insufficient adhesion between two phases. The modulus of the samples increased with the LCP content. The degree of crystallinity increased with increasing LCP content. This improvement in crystallinity is associated with the increase in crystallite size. For the blends, thermal studies indicated, the endothermic signals which were more prominent at all the peak temperatures. The heat of degradation values increased with the LCP content. Scanning electron microscope (SEM) study suggested the fibril formation, which affected the failure mechanism under DMA studies. Storage modulus and loss modulus of the blends increased with increasing LCP content. At above glass transition temperature (Tg) improvement in storage modulus is nearly five times higher than that of the pure AEM.     

Photo-induced deformation behavior depending on the glass transition temperature on the surface of urethane copolymers containing a push-pull type azobenzene moiety

Urethane copolymers containing a push-pull type azobenzene moiety with the same dye content were synthesized to investigate the relationship between photo-induced deformation and molecular mobility. The copolymers exhibit different glass transition temperatures (T_g), from 46 to 143 °C, due to their different main chain structures. An indented nanostructure induced by the optical near field around the polystyrene microspheres and a surface relief grating (SRG) induced by exposure to a two-beam interference pattern were examined using films of copolymers. We found the dependency of the deformation efficiency on T_g was inverted depending on the irradiation power. The deformation depth increased with T_g under high power irradiation in both the indented nanostructure and the SRG forming experiments. In contrast, the deformation depth of the SRG decreased with increasing T_g under low power irradiation. The discovery of this inverted tendency suggests that, in addition to the molecular mobility, we should consider other factors in the deformation mechanism, such as the recovery of deformation, the degree of plasticization, and the thermal effects.     

Effect of drying treatments on texture and color of vegetables (pumpkin and green pepper)

The present work evaluates the effect of different drying treatments on the color and textural attributes of green bell peppers and pumpkin, which were dried using two different methods: air drying and freeze-drying. The treatments in air drying were carried out at 30 °C and 70 °C.From the results it is possible to conclude that the increase in drying temperature reduced drastically the hardness of green peppers and the freeze drying had an intermediate effect between vegetables dried at 30 °C and 70 °C. Moreover, the springiness was higher in dried green peppers but an opposite effect was observed on chewiness. With respect to pumpkin, any dependence between the fiber orientation and the hardness of the fresh vegetable was not found. In addition, increasing temperature from 30 °C to 70 °C particularly reduced the hardness and the chewiness of dried product and maintained cohesiveness and springiness approximately constant.Regarding the color, it was possible to conclude that air drying at 30 °C produced small changes in color of green pepper whereas air drying at 70 °C and freeze drying originated more intense color changes. The increase of temperature on air drying augmented the color saturation of dried pumpkin while decreased the hue angle by a linear relationship. In addition, the chroma of dried pumpkin decreased significantly with the freeze drying, while the hue angle was maintained constant as compared with the fresh vegetable.     

The Effect of Glass Transition Temperature on the Procedure of Microwave–Freeze Drying of Mushrooms (Agaricus bisporus)

Freeze drying (FD) yields the best quality of dried mushroom but at the cost of a long drying time and high overall cost. Air drying (AD) gives an unacceptably poor quality product. To achieve faster drying along with a high-quality product, a microwave–freeze drying (MFD) technique was developed to dry mushrooms. The relationship between dried mushroom quality and the glass transition temperature during the MFD process was studied to optimize the MFD process. According to the change tendency of the glass transition temperature of mushroom during MFD, a step-down microwave loading scheme for the MFD process was developed to obtain good product quality.     

Study of crystalline and amorphous phases of biodegradable poly(lactic acid) by advanced thermal analysis

The qualitative and quantitative thermal analysis of biodegradable poly(lactic acid) PLA is presented. The glass transition, melting process, and heat capacity of a semi-crystalline poly(lactic acid) are studied utilizing the differential scanning calorimetry and temperature-modulated DSC. The mobile amorphous fraction, W_a degree of crystallinity, W_c and rigid-amorphous fraction, W_RAF were estimated depending on the thermal history of semi-crystalline PLA. From qualitative thermal analysis, the glass transition of rigid-amorphous phase was observed as a broadening from the changes of heat-flow-rate between mobile glass transition temperature and melting temperature. The amount of the rigid-amorphous fraction (RAF) was evaluated from W_RAF = 1−W_c − W_a and graphically was presented as the result of a deflection from the linearity of the dependence of the change of degree of mobile amorphous phase (W_a) vs. the degree of crystalline fraction (W_c) for semi-crystalline PLA with different thermal history. The degree of crystallinity of semi-crystalline samples of PLA can be discussed in terms of a two- or three-phase model. In contrast, the quantitative thermal analysis of the experimental apparent heat capacity of semi-crystalline PLA did not show any appearance of RAF in the examples of analyzed samples. The experimental heat capacity of PLA was analyzed in reference to the solid and liquid equilibrium heat capacities of poly(lactic acid) found in the ATHAS Data Bank.     

The glass transition temperature of poly(N-vinyl pyrrolidone) by differential scanning calorimetry

Previously a wide range of values have been reported for the glass transition temperature, T_g, of poly(N-vinyl pyrrolidone), PVP, and it was suggested that lower values are due to variable uptakes of water caused by the hygroscopic nature of the polymer. Now it has been found that there are large variations in T_g, even in carefully dried specimens of PVP. Other factors found to influence T_g are residual monomer and the molecular weight of PVP. Polymers prepared by bulk polymerization, either by γ-irradiation or by heating with 2-azobisisobutyronitrile, have much lower values of T_g than dried ones prepared containing 30% water. The difference is mainly due to depression of T_g by residual monomer which, in the absence of water during polymerization, fails to react completely because of conversion to a glassy state. An unexplained observation is that even when all residual monomer has been removed, polymers prepared by bulk polymerization still have a lower T_g than would be expected from their molecular weight.     

Effect of polymer structure on the molecular dynamics and thermal behavior of poly(allyl acetoacetate) and copolymers

In this paper, dielectric and calorimetric studies of the small-molecule glass former allyl acetoacetate monomers as well as its newly synthetized homopolymer and copolymers with different styrene composition were performed in both the liquid and glassy states. The molecular dynamics studies by the broadband dielectric spectroscopy and the stochastic temperature modulated differential scanning calorimetry enabled us to explore relaxation processes of examined materials in the wide frequency range. We found that the copolymers reveal two co-existing glass transitions characterized by the glass transition temperatures, which are very close to those of the corresponding homopolymers. These results suggest that the copolymers exhibited some sequences of acetoacetate units with a microphase-separated morphology in agreement with the value of reactivity ratio previously determined. We investigated effects of copolymerization compositions on the glass transition temperature, the isobaric fragility index, the dielectric and calorimetric intensity, and the dynamic heterogeneity on the glass transitions of the materials.     

Study on thermal transitions of toluene diisocyanate-based polyurethane elastomers with poly (tetramethylene oxide) as the soft segment by TSC/RMA, DSC and DMA thermal analyzers

Thermal transitions of TDI-based polyurethane elastomers with PTMO as the soft segment were characterized by the depolarization technique in TSC and by using with the thermal windowing technique on selected specimens in the RMA measurements. Results indicate that the broadened thermal transition in the glass transition region as observed in the DSC thermogram is related to the combined T_g transition and the T_global transition in the TSC spectrum. This T_global transition is associated with the macromolecular property as detected by tan δ in DMA measurement. The increase in the T_g with a high NCO content may be explained by the structural modification found on the urethanic chain with the additional linkage of the hard segment that affects the cooperative motion of the molecular chain. Data measured from DSC, TSC/RMA and DMA with simulated DEA and wide angle X-ray data are presented for the characterization of the polyurethanes. The RMA measurement leads to a compensation search on T_g transition and provides pertinent thermokinetic data that correlates the NCO content with changes in enthalpy and entropy on the relaxation behaviors in the T_g transition of polyurethane elastomers.     

Relation of glass transition temperature to the hydrogen bonding degree and energy in poly(N-vinyl pyrrolidone) blends with hydroxyl-containing plasticizers: 3. Analysis of two glass transition temperatures featured for PVP solutions in liquid poly(ethylene glycol)

The phase behaviour of poly(N-vinyl pyrrolidone)-poly(ethylene glycol) (PVP-PEG) blends has been examined in the entire composition range using Temperature Modulated Differential Scanning Calorimetry (TM-DSC) and conventional DSC techniques. Despite the unlimited solubility of PVP in oligomers of ethylene glycol, the PVP-PEG system under consideration demonstrates two distinct and mutually consistent glass transition temperatures (T_g) within a certain concentration region. The dissolution of PVP in oligomeric PEG has been shown earlier (by FTIR spectroscopy) to be due to hydrogen bonding between carbonyl groups in PVP repeat units and complementary hydroxyl end-groups of PEG chains. Forming two H-bonds through both terminal OH-groups, PEG acts as a reversible crosslinker of PVP macromolecules. To characterise the hydrogen bonded complex formation between PVP (M_w=10⁶) and PEG (M_w=400) we employed an approach described in the first two papers of this series that is based on the modified Fox equation. We evaluated the fraction of crosslinked PVP units and PEG chains participating to the complex formation, the H-bonded network density, the equilibrium constant of complex formation, etc. Based on the established molecular details of self-organisation in PVP-PEG solutions, we propose a three-stage mechanism of PVP-PEG H-bonded complex formation/breakdown with increase of PEG content. The two observed T_gs are assigned to a coexisting PVP-PEG network (formed via multiple hydrogen bonding between a PEG and PVP) and a homogeneous PVP-PEG blend (involving a single hydrogen bond formation only). Based on the strong influence of coexisting regions on each other and the absence of signs of phase separation (evidenced by Optical Wedge Microinterferometry) we conclude that the PVP-PEG blend is fully miscible on a molecular scale.     

Study of the molecular weight dependence of glass transition temperature for amorphous poly(l-lactide) by molecular dynamics simulation

Molecular dynamics simulation has been used to investigate the molecular weight dependence of glass transition temperature for amorphous poly(l-lactide). Amorphous PLLA systems were created using molecular modeling and NPT ensemble MD simulations were carried out using the modified OPLS-AA force field. The fractal dimension of the PLA systems was 1.62. The molecular weight dependence of glass transition temperature, self-diffusion coefficient and shear viscosity were studied and the good agreement between the simulation results and experiments was obtained.