P. Sajkiewicz

Phase transitions during stretching of poly(vinylidene fluoride)

Abstract The effect of temperature and deformation rate on the structure of drawn poly(vinylidene fluoride) samples was investigated by means of wide angle X-ray scattering. It was shown that drawing in the range of 50 and 145°C results in transition of non-polar α phase into polar β crystallites. The analysis of the reflection from the planes (111) and (201) of the β phase indicates that the content and orientation of β crystallites are affected by the temperature of drawing. The highest content of well oriented β crystallites was achieved during drawing at 87°C. This temperature is close to the above temperature of αc transition/relaxation and corresponds with the temperature of deformation applied in industrial processes in the formation of piezoelectric poly(vinylidene fluoride) (PVDF) films.

Crystallization behaviour of poly(vinylidene fluoride)

Crystallization of poly(vinylidene fluoride) is investigated by calorimetry and wide-angle X-ray scattering. It is shown that in addition to α and γ crystallization at high temperatures, α crystallization occurs at low temperatures. It is probable that low temperature α crystallization occurs from the segments with high concentration of head-to-head defects which are unable to crystallize at high temperatures. The Hoffman–Weeks plot of α crystallites formed at high temperatures is affected by additional low temperature α crystallization. The change in slope of a Hoffman–Weeks plot for α crystallites, reported in several papers, occurs only in the structure additionally crystallized during cooling to room temperature. The equilibrium melting temperature of the α and γ form is determined from the Hoffman–Weeks plot.

Application of the Ozawa model to non-isothermal crystallization of poly(ethylene terephthalate)

Abstract The non-isothermal crystallization of poly(ethylene terephthalate) (PET) during cooling with constant rates is investigated by differential scanning calorimetry (DSC). The analysis is aided by microscopic investigation of the final morphology. The results are analyzed using the Ozawa model. It is shown that this model describes the non-isothermal process only at relatively low cooling rates. At rates exceeding 20°/min, crystallization progress becomes higher, indicating higher crystallization rates than those resulting from the Ozawa approach. Additional deviation from the Ozawa model observed at the very beginning and the end of crystallization can be attributed to spatial constraints of spherulitic growth. In the first case, the spherulitic growth is impeded by the dense instantaneous nucleation on the polymer surface (transcrystallization) and in the second one by impingements of bulk growing spherulites.

Effects of Cooling Rate on Crystallinity of i-Polypropylene and Polyethylene Terephthalate Crystallized in Nonisothermal Conditions

Crystallization of polyethylene terephthalate and i-polypropylene in nonisothermal conditions is studied by means of differential scanning calorimetry. Measurements, carried out at several constant cooling rates, are interpreted in terms of a new theory 1,2 that takes into account effects related to a transient, nonsteady-state course of the process as well as athermal nucleation, which may occur under such circumstances. This article gives preliminary results based on analysis of final crystal- linity reached at the end of cooling. Results indicate that the classical isokinetic approach is not adequate to describe crystallization kinetics at high cooling rates. A parameter quantizing the magnitude of deviations from isokinetic law is evaluated.

Electrospinning and Structure of Bicomponent Polycaprolactone/Gelatin Nanofibers Obtained Using Alternative Solvent System

Bicomponent polycaprolactone/gelatin (PCL/Gt) nanofibers were successfully formed for the first time by electrospinning using a novel polymer–solvent system with solvents being alternative to the commonly used toxic solvents like fluorinated alcohols. The mixture of acetic acid (AA) with formic acid (FA; 90:10) was applied. Stable electrospinning was possible despite the fact the mixture of PCL and gelatin in AA/FA solvent showed emulsive structure. From the practical perspective, there is no doubt that it is possible to obtain PCL/Gt fibers using AA/FA mixture with morphology similar to that for fibers spun from hexafluoroisopropanol (HFIP) solutions. GRAPHICAL ABSTRACT

Optimization of melting conditions for the analysis of crystallization kinetics of poly(3-hydroxybutyrate)

Abstract Studies of kinetics of polymer crystallization are generally performed by heating the material above the melting point, in order to erase previous thermal and mechanical history, followed by rapid cooling to the desired crystallization temperature or by cooling at a constant rate. For poly(3-hydroxybutyrate) this procedure implies some degradation of the polymer chain, which starts below the onset of melting. In this article the effects of melting conditions on the subsequent crystallization kinetics are discussed. It is shown that in order to sufficiently cancel memories of previous crystalline order of the analyzed PHB, it is necessary to bring the material at a temperature higher than 192 °C. Thermal treatments conducted at lower temperatures are not sufficient to destroy all solid aggregates, and crystallization of PHB has an anticipated onset of crystallization due to nucleation occurring via self-seeding. The chain degradation attained upon exposure at high temperatures has much lesser influence on crystallization kinetics than incomplete melting, with some effects detectable on the spherulitic morphology and on the final degree of crystallinity.

Electrospinning of gelatin for tissue engineering – molecular conformation as one of the overlooked problems

Gelatin is one of the most promising materials in tissue engineering as a scaffold component. This biopolymer indicates biocompatibility and bioactivity caused by the existence of specific amino acid sequences, being preferred sites for interactions with cells, with high similarity to natural extracellular matrix. The present paper does not aspire to be a full review of electrospinning of gelatin and gelatin containing nanofibers as scaffolds in tissue engineering. It is focused on the still open question of the role of the higher order structures of gelatin in scaffold’s bioactivity/functionality. Gelatin molecules can adopt various conformations depending on temperature, solvent, pH, etc. Our review indicates the potential ways for formation of α-helix conformation during electrospinning and the methods of further structure stabilization. It is intuitively expected that the native α-helix conformation appearing as a result of partial renaturation of gelatin can be beneficial from the viewpoint of bioactivity of scaffolds, providing thus a much cheaper alternative approach as opposed to expensive electrospinning of native collagen.

Fabrication and characterization of electrospun bionanocomposites of poly (vinyl alcohol)/nanohydroxyapatite/cellulose nanofibers

ABSTRACT The aim of the present study was preparation, optimization, and systematic characterization of electrospun bionanocomposite fibers based on polyvinyl alcohol (PVA) as matrix and nanohydroxy apatite (nHAp) and cellulose nanofibers (CNF) as nanoreinforcements. The presence of nHAp and nHAp-CNF affected the morphology of electrospun mats and reduced fiber diameter, particularly at a higher content of nanofillers. The obtained results of FTIR, DSC, and WAXS proved the crystallinity reduction of electrospun nancomposites. Both nHAp and nHAp-CNF addition led to a significant increase of Young modulus with the highest stiffness for nanocomposite fibers at 10 wt% of nHAp and 3 wt% of CNF. GRAPHICAL ABSTRACT

The Effect of Selected Electrospinning Parameters on Molecular Structure of Polycaprolactone Nanofibers

The effect of electrospinning parameters on morphology, molecular, and supermolecular structure of polycaprolactone (PCL) fibers was analyzed, with respect to tissue engineering applications. Fibers morphology and structure are mainly determined by solution concentration and collector type. Applied voltage does not significantly influence supermolecular structure (crystallinity) and mechanical stiffness. There is correlation between changes in structure and proliferation of 3T3 cells as evidenced by in vitro study. Processing window of optimal scaffolds is relatively wide, however, variation of electrospinning parameters do not significantly affect their biological functionality. GRAPHICAL ABSTRACT

Development of electrospun poly (vinyl alcohol)-based bionanocomposite scaffolds for bone tissue engineering

The article is focused on the role of nanohydroxy apatite (nHAp) and cellulose nanofibers (CNFs) as fillers in the electrospun poly (vinyl alcohol) (ES-PVA) nanofibers for bone tissue engineering (TE). Fibrous scaffolds of PVA, PVA/nHAp (10 wt.%), and PVA/nHAp(10 wt.%)/CNF(3 wt.%) were successfully fabricated and characterized. Tensile test on electrospun PVA/nHAp10 and PVA/nHAp10/CNF3 revealed a three-fold and seven-fold increase in modulus compared with pure ES-PVA (45.45 ± 4.77). Although, nanofiller loading slightly reduced the porosity percentage, all scaffolds had porosity higher than 70%. In addition, contact angle test proved the great hydrophilicity of scaffolds. The presence of fillers reduced in vitro biodegradation rate in PBS while accelerates biomineralization in simulated body fluid (SBF). Furthermore, cell viability, cell attachment, and functional activity of osteoblast MG-63 cells were studied on scaffolds showing higher cellular activity for scaffolds with nanofillers. Generally, the obtained results confirm that the 3-componemnt fibrous scaffold of PVA/nHAp/CNF has promising potential in hard TE.