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Knoxville, TN, United States

Ma Q.,Tufts University | Pyda M.,Tufts University | Pyda M.,Rzeszow University of Technology | Pyda M.,ATHAS MP Company | And 2 more authors.
Polymer (United Kingdom)

We investigated the molecular orientation, crystallization mechanism, phase structure and transitions of aligned electrospun fibers, including the constrained amorphous phase and its relationship to the mesophase. Aligned poly(lactic acid) (PLA) fibers were successfully electrospun (ES) by adopting a high-speed rotating wheel as the counter electrode. Using thermal analysis and wide angle X-ray scattering (WAXS), we evaluated the confinement that exists in as-spun amorphous, and heat-treated semicrystalline, fibers. Differential scanning calorimetry confirmed the existence of a constrained amorphous phase in as-spun aligned fibers, without the presence of crystals or fillers to serve as fixed physical constraints. Then, using WAXS, for the first time the mesophase fraction, consisting of oriented non-crystalline PLA chains, was quantitatively characterized in PLA nanofibers. The formation of oriented crystals during subsequent heating, and the evolution of the phase fractions (crystal, mobile amorphous, and constrained amorphous) with temperature, were investigated as to their dependence upon the method of fixing the ends of the fibers. In free-end ES nanofibers, the mesophase does not directly transition into the crystal phase of higher packing order. Instead, at temperatures above Tg, the free-end fiber undergoes residual stress relaxation, accompanied by the devitrification of the mesophase from a confined solid state to a mobile liquid state. The mesophase, which possesses some degree of medium-range order, behaves very similarly to the rigid amorphous phase, and a correlation between the two is established in free-end fibers. On the other hand, for fixed-end as-spun fibers, the mesophase forms a more ordered structure when heated. By fixing the fiber ends, relaxation of the mesophase above Tg is partially disabled. The more ordered mesophase that survives above Tg undergoes a mesophase-to-crystal phase transition upon further heating, making an additional contribution to the heat of fusion. A new phase structure model is proposed to describe aligned electrospun PLA nanofibers. © 2013 Elsevier Ltd. All rights reserved. Source

Magon A.,Rzeszow University of Technology | Pyda M.,Rzeszow University of Technology | Pyda M.,Poznan University of Medical Sciences | Pyda M.,ATHAS MP Company
Journal of Chemical Thermodynamics

The qualitative and quantitative thermal analyses of crystalline and amorphous D(-)-fructose were studied utilising methods of standard differential scanning calorimetry (DSC), quasi-isothermal temperature- modulated differential scanning calorimetry (quasi-isothermal TMDSC), and thermogravimetric analysis (TGA). Advanced thermal analysis of fructose was performed based on heat capacity. The apparent total and apparent reversing heat capacities, as well as phase transition parameters were examined on heating and cooling. The melting temperature,Tm, of crystalline D(-)-fructose shows a heating rate dependency, which increases with raising the heating rate and leads to superheating. The equilibrium melting temperatures: Tm o(onset) = 370 K and Tm o(peak) = 372 K, and the equilibrium enthalpy of fusion ΔfusHo = 30.30 kJ - mol-1, of crystalline D(-)-fructose were estimated on heating for the results at zero heating rate. Anomalies in the heat capacity in the liquid state of D(-)-fructose, assigned as possible tautomerisation equilibrium, were analysed by DSC and quasi-isothermal TMDSC, both on heating and cooling. Thermal stability of crystals in the region of the melting temperature was examined by TGA and quasi-isothermal TMDSC. Melting, mutarotation, and degradation processes occur simultaneously and there are differences in values of the liquid heat capacity of D(-)-fructose with varied thermal history, measured by quasi-isothermal TMDSC. Annealing of amorphous D(-)-fructose between the glass transition temperature, Tg,and the melting temperature,Tm, also leads to crystallization of the sample and shows changes in the total apparent heat capacity. The experimental, apparent heat capacity of fully crystalline and fully or partially amorphous D(-)-fructose was analysed with reference to the solid (vibrational) and liquid heat capacities based on the ATHAS scheme. © 2012 Elsevier Ltd. All rights reserved. Source

Magon A.,Rzeszow University of Technology | Pyda M.,Rzeszow University of Technology | Pyda M.,Poznan University of Medical Sciences | Pyda M.,ATHAS MP Company
Carbohydrate Research

The thermal behaviors of α-d-glucose in the melting and glass transition regions were examined utilizing the calorimetric methods of standard differential scanning calorimetry (DSC), standard temperature-modulated differential scanning calorimetry (TMDSC), quasi-isothermal temperature- modulated differential scanning calorimetry (quasi-TMDSC), and thermogravimetric analysis (TGA). The quantitative thermal analyses of experimental data of crystalline and amorphous α-d-glucose were performed based on heat capacities. The total, apparent and reversing heat capacities, and phase transitions were evaluated on heating and cooling. The melting temperature (T m) of a crystalline carbohydrate such as α-d-glucose, shows a heating rate dependence, with the melting peak shifted to lower temperature for a lower heating rate, and with superheating of around 25 K. The superheating of crystalline α-d-glucose is observed as shifting the melting peak for higher heating rates, above the equilibrium melting temperature due to of the slow melting process. The equilibrium melting temperature and heat of fusion of crystalline α-d-glucose were estimated. Changes of reversing heat capacity evaluated by TMDSC at glass transition (T g) of amorphous and melting process at T m of fully crystalline α-d-glucose are similar. In both, the amorphous and crystalline phases, the same origin of heat capacity changes, in the T g and T m area, are attributable to molecular rotational motion. Degradation occurs simultaneously with the melting process of the crystalline phase. The stability of crystalline α-d-glucose was examined by TGA and TMDSC in the melting region, with the degradation shown to be resulting from changes of mass with temperature and time. The experimental heat capacities of fully crystalline and amorphous α-d-glucose were analyzed in reference to the solid, vibrational, and liquid heat capacities, which were approximated based on the ATHAS scheme and Data Bank. © 2011 Elsevier Ltd. All rights reserved. Source

Zarzyka I.,Rzeszow University of Technology | Di Lorenzo M.L.,CNR Institute of Chemistry and Technology of Polymers | Pyda M.,Rzeszow University of Technology | Pyda M.,ATHAS MP Company
Scientific World Journal

The phase behavior of linear poly(N-isopropylacrylamide) (PNIPA), linear copolymer poly(N-isopropylacrylamide) and poly(sodium acrylate) (PNIPA-SA), and chemically cross-linked PNIPA in water has been determined by temperature modulated differential scanning calorimetry (TM-DSC). Experiments related to linear polymers (PNIPA and PNIPA-SA) indicated nontypical demixing/mixing behavior with a lower critical solution temperature (LCST), which do not correspond to the three classical types of limiting critical behavior. Some similarities and differences are observed in comparison to our literature data using standard TM-DSC for PNIPA/water. Furthermore no influence of composition cross-linked PNIPA/water system on demixing/mixing temperature was observed. © 2014 Iwona Zarzyka et al. Source

Czerniecka A.,Rzeszow University of Technology | Magon A.,Rzeszow University of Technology | Schliesser J.,Brigham Young University | Woodfield B.F.,Brigham Young University | And 2 more authors.
Journal of Chemical Thermodynamics

The heat capacity of poly(3-hydroxybutyrate) (P3HB) has been measured using a quantum design Physical Property Measurement System (PPMS), differential scanning calorimetry (DSC), and temperature-modulated differential scanning calorimetry (TMDSC) over the temperature range of (1.9 to 460) K. The results within the range of (1.9 to 250) K were obtained using the quantum design PPMS, and established the baseline of the solid heat capacity. This experimental low-temperature heat capacity was linked to the vibrational molecular motion of P3HB. The solid heat capacity of P3HB was computed based approximately on groups of vibration and skeletal vibration spectra. The skeletal vibration heat capacity contribution was estimated by a general Tarasov equation with three Debye characteristic temperatures Θ1 = 549.1 K and Θ2 = Θ3 = 71.8 K, and ten skeletal modes, Nskeletal = 10. The experimental and calculated solid heat capacities agree with an error of ±0.2% over the temperature range from (5 to 250) K. The vibrational, solid heat capacity was extended to higher temperatures to judge additional contributions to the experimental heat capacity from other large-amplitude motion or latent heat during the quantitative thermal analysis of semi-crystalline P3HB. The liquid heat capacity of semi-crystalline P3HB above its melting temperature and of fully amorphous P3HB above the glass transition temperature was approximated by a linear regression and expressed as Cpliquid(exp)= 0.1791 T + 94.722 in units of J · K-1 · mol-1. The calculated solid and liquid heat capacities can serve as equilibrium baselines for the quantitative thermal analysis of semi-crystalline P3HB. Also, the integral thermodynamic functions of enthalpy, entropy and free enthalpy for the equilibrium condition were calculated using estimated parameters of transitions. © 2014 Elsevier Ltd. All rights reserved. Source

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