The characterization of polymer fibers and films with respect to their phase composition as well as polymer orientation is of vital importance to understand the effect of different processes (heating, stretching, etc.) on the properties of the of the final production. X-ray diffraction plays an important role in this characterization process as it provides insight into the phase composition and the structure of the polymers.
Polyvinylidene fluoride (PVDF) is a highly non-reactive thermoplastic fluoropolymer that is produced by polymerization of vinylidene difluoride. Due to its unique mechanical and physical properties (strength, flexibility, biocompability, high resistance against heat, abrasion and chemical corrosion, strong piezoelectricity) PVDF is used for various applications in chemical, medical, semiconductor and defense industries. PVDF exists in four modifications (α-, β-, γ-, and δ-phase) depending on temperature and mechanical deformation with α- and β-PVDF (Figure 1) being present in this study.
Figure 1: (a) image of PVDF fibers. (b) structures of α- and β-PVDF. α-PVDF transforms to the β-phase upon mechanical deformation/strain.
Here we present results of 2D wide angle X-ray scattering measurements (WAXS) obtained on two PVDF fiber series synthesized by electrospinning from (a) water and (b) Limanol LB25 (15%) at a spinning speed of 195 m/s and a temperature of 120 °C that were stretched to different ratios ranging from 1.42 to 4.3. The goal was to study the effect of the applied stretching of the phase composition and the alignment of polymer chains. A total of 22 individual fiber samples were analyzed. Both series independently of used solvent behave very similarly and are hence not discussed separately during the discussion of the results.
- The use of the Empyrean diffractometer in combination with the dedicated 2D WAXS setup enables the collection of high-quality 2D diffraction patterns
- Simple and reproducible sample mounting (no sample alignment necessary)
- Suitable for fibers, composites, films, etc.
- Short measurement times due to the newest detector technology
- XRD2DScan SW package provides the optimal tools for 2D data evaluation.
Figure 2. Experimental setup used in this study
Measurements were performed on an Empyrean multipurpose diffraction platform equipped with a Mo LFF HR X-ray tube (λ=0.709319 nm), a focusing X-ray mirror, a dedicated 2D WAXS setup and a GaliPIX3D detector, pixel size 60 μm (hexagonal arrangement, >95% detection efficiency for Cu-, Mo-, and Ag-radiation).
Static 2D measurements with a 2θ coverage of ca. 62°x 52° (±31° x ±26° 2θ) and a collection time of 60 minutes per sample were performed. Subsequent data evaluation was done using the XRD2DScan 6.1 software package.
Malvern Panalytical’s 2D-WAXS solution
- Complete solution for powders, fibers, films/foils, sheets, composites, liquids/gels
- Available for Cu-, Mo-, and Ag-radiation
- Easy and reproducible sample mounting
- Beam diameter ~100 μm
- Accessible range of 1 to 30 °2θ (Detector coverage of ± 30 °2θ per 2D frame
- GaliPIX3D solid state detector for the highest 2D data quality.
XRD2DScan 6.1 software package provides a complete 2D data analysis solution (visualization, 2θ- and azimuthal integrations, fitting, orientation parameters).
The obtained 2D diffraction patterns from both fiber series show significant changes with increasing, total stretching applied after/during electrospinning (Figure 3).
At low stretching ratios of 1.4 to 1.6 the 2D WAXS measurements show more or less homogeneous diffraction rings at 8.4 (4.82 Å), 9.2 (4.42 Å), 15.9 (2.56 Å), and 18.5 °2θ (2.20 Å). Comparison of the corresponding 2θ integration of this 2D pattern to a reference database shows that the observed signals are the (020), (110), (021), (200)/(131) of α-PVDF (Figure 3). Of those only the α-(110) reflection at 20 °2θ shows a slight intensity increase along its diffraction ring parallel to the fiber axis.
Figure 3: characteristic 2D WAXS patterns (shown in square root scale) collected in this study showing the influence of the increased stretching ratio on the PVDF fibers synthesized in Limanol LB25 (15%).
Figure 4: 2θ-integrations corresponding to the 2D WAXS patterns presented in figure 3.
With increasing stretching factors (1.6 to 2.5) this intensity variation becomes significantly more pronounced. At stretching ratios exceeding 2.8 two distinct intensity maxima at 9.42 °2θ (4.32 Å) along the fiber axis develop at stretching ratios exceeding 2.8 (Figure 2).
This is caused by uniaxial alignment of the polymer chains along the stretching direction (parallel to the fiber axis). In addition to this preferential orientation the data shows that the stretching is accompanied by a transition from α- to β-PVDF with the simultaneous formation of disordered (amorphous) meso-PVDF. In the respective 2D WAXS patterns the highly oriented signal at 4.32 Å corresponds to the (110)/(200) reflection of the β-phase whereas the broad continuous halo at 8.2 °2θ belongs to the disordered/amorphous meso-phase (Figure 3c). In addition, also the other dominant signals of the β-phase (001) and (111)/(201) show distinct orientation features with increasing stretching.
Figure 5: collected 2D-WAXS image as well as the azimuthal integration of the predomintant β (110) reflection and its respective fit from which the FWHMaz was determined.
The effect of the applied stretching on the fiber structure is best seen in azimuthal integrations along the (110) signal of the α- and β-phase (Figure 5). The FWHM of the azimuthal signal (FWHMaz) changes with increasing stretching ratio and can therefore be used as a measure for the uniaxial alignment of the polymer chains within the fibers.
In order to quantify this alignment an orientation parameter fc is calculated using the FWHMaz and the equation seen below:
In case of completely random orientation of the polymer chains fc would be equal/close to 0, whereas in case of an ideally uniaxial oriented sample fc would be equal to 1.
The FWHMaz significantly decreases with increasing stretching ratio which can already be clearly observed when plotting the azimuthal integrations in an isoline plot (figure 6a). Fitting of the FWHMaz shows that it changes from almost 80 °γ at a stretching ratio of 1.4 to ~11 °γ at a stretching ratio exceeding 3.5.
The corresponding fc values show a significant increase in orientation from 0.55 to 0.9 between stretching factors of 1.4 to 2.5. Above 2.5 the orientation factor only slightly increases with further stretching until it reaches a final value of 0.94 (representing almost ideal uniaxial alignment within the fiber) at a total applied stretching ratio of 4.3 (see Figure 6b).
Figure 6: (a) Azimuthal integrations and corresponding isoline plot derived from the Limanol LB25 sample series and (b) plot of the determined FWHMaz and the calculated orientation parameter, respectively, as a function of the total applied stretching.
Malvern Panalytical’s new 2D WAXS solution in combination with the XRD2DScan 6.1 software package offers a complete solution for the characterization of polymers. Here we studied the effect of stretching with respect to phase transitions as well as the orientation of the polymer chains within PVDF fibers. We demonstrate that the data is easy to collect and analyze. Fine differences can be observed and the structural ordering can be quantified.