Differential Scanning Calorimetry (DSC) is a critical analytical technique for quantifying the crystallinity of electrospun nanofibers, particularly in polymers such as polycaprolactone (PCL) and polyvinylidene fluoride (PVDF). The method relies on measuring the heat flow associated with phase transitions, primarily melting, to determine the degree of crystallinity within the nanofibrous structure. The quantification process involves comparing the melting enthalpy of the sample to that of a fully crystalline reference, accounting for nucleation effects, processing conditions, and post-treatment modifications like annealing.
The crystallinity of electrospun nanofibers is a key determinant of their mechanical, thermal, and functional properties. Unlike bulk polymers, nanofibers exhibit unique crystallization behavior due to their high surface-to-volume ratio and confinement effects during electrospinning. DSC provides a direct measurement of the melting endotherm, which correlates with the crystalline content. For semi-crystalline polymers like PCL and PVDF, the melting enthalpy (ΔHm) is obtained by integrating the area under the endothermic peak in the DSC thermogram. The percent crystallinity (Xc) is then calculated using the formula:
Xc (%) = (ΔHm / ΔHm°) × 100
where ΔHm° is the melting enthalpy of a 100% crystalline polymer. For PCL, ΔHm° is typically 139.5 J/g, while for PVDF, it is 104.7 J/g. Experimental ΔHm values for electrospun PCL nanofibers often range between 40–70 J/g, corresponding to crystallinities of 30–50%, depending on processing parameters. PVDF nanofibers, known for their polymorphic behavior, exhibit ΔHm values between 30–50 J/g, translating to crystallinities of 30–50% for the α-phase.
Electrospinning parameters significantly influence crystallinity. The rapid solvent evaporation and elongational forces during fiber formation can lead to molecular alignment and strain-induced crystallization. However, the fast kinetics may also result in incomplete crystallization or metastable phases. For instance, PVDF nanofibers often show a mixture of α- and β-phases, with the latter being favored under high stretching forces or electrical poling. DSC thermograms can distinguish these phases by their distinct melting temperatures (Tm): α-phase melts near 170°C, while β-phase melts at approximately 175°C. The relative enthalpic contributions of each phase provide insights into the polymorphic composition.
Nucleation effects play a crucial role in the crystallization of electrospun nanofibers. The high surface area of nanofibers promotes heterogeneous nucleation, often leading to higher crystallinity compared to solvent-cast films. However, the confined geometry can also restrict crystal growth, resulting in smaller crystallites. DSC heating rates are critical in such analyses; slower rates (e.g., 5°C/min) allow for better resolution of overlapping transitions, while faster rates (e.g., 20°C/min) may reveal kinetically trapped states. Cold crystallization, observed as an exothermic peak in some DSC scans, indicates reorganization of amorphous regions upon heating, further complicating crystallinity assessments.
Annealing studies are frequently conducted to modify the crystallinity and phase composition of electrospun nanofibers. Post-treatment at temperatures below the melting point but above the glass transition temperature (Tg) enables chain relaxation and crystal perfection. For PCL nanofibers, annealing at 50–60°C (near Tg of −60°C but below Tm of 60°C) can increase crystallinity by 10–20% due to secondary crystallization. PVDF nanofibers annealed at 140–150°C (below Tm but above Tg of −40°C) often show a shift from α- to β-phase, accompanied by an increase in melting enthalpy. DSC thermograms of annealed samples typically exhibit sharper melting peaks and higher ΔHm values, reflecting improved crystal quality.
The following table summarizes typical DSC-derived crystallinity data for electrospun PCL and PVDF nanofibers under varying conditions:
| Polymer | As-spun Xc (%) | Annealed Xc (%) | Annealing Conditions |
|---------|----------------|------------------|-----------------------|
| PCL | 30–50 | 40–70 | 50–60°C, 1–2 h |
| PVDF | 30–50 | 50–70 | 140–150°C, 1–2 h |
Quantitative DSC analysis must account for sample mass accuracy, baseline correction, and calibration standards. Indium is commonly used for temperature and enthalpy calibration due to its sharp melting transition at 156.6°C with ΔHm of 28.4 J/g. Baseline subtraction is essential to isolate the melting endotherm from instrumental artifacts or overlapping thermal events. For nanofibers, the low mass of individual fibers necessitates careful sample preparation to ensure representative measurements. Bulk samples comprising aligned or random fiber mats are typically analyzed, with corrections for any residual solvent or additives that may influence the thermogram.
The cooling cycle in DSC further elucidates crystallization kinetics. The crystallization temperature (Tc) and enthalpy (ΔHc) provide information on nucleation density and crystal growth rates. Electrospun nanofibers often exhibit higher Tc values than bulk polymers due to enhanced nucleation sites. For example, PCL nanofibers may show Tc at 20–25°C, compared to 15–20°C for solvent-cast films, indicating faster crystallization kinetics in the confined fiber geometry.
In summary, DSC is an indispensable tool for quantifying crystallinity in electrospun nanofibers, offering insights into melting behavior, polymorphic composition, and the effects of processing and post-treatment. By correlating enthalpic measurements with structural analyses, researchers can tailor the properties of PCL, PVDF, and other polymeric nanofibers for applications ranging from tissue engineering to energy harvesting. The technique’s sensitivity to thermal transitions makes it ideal for probing the complex crystallization dynamics inherent to nanofibrous systems.