Selective dissolution method: core-shell fibre structure

i) Background

As stated earlier, transmission electron microscopy (TEM) and light microscopy could not be used in this study to validate the core-shell fibre structure of the fibre prepared by coaxial electrospinning. The various approaches evaluated for direct visualisation

of the core shell structure using TEM and other methods are documented in Appendix B. A new selective dissolution method was used to validate the core-shell fibre structure of the fibres.

ii) Overview

PLLA-core/PDLLA-shell solutions were coaxially electrospun into fibre mats (electrospinning parameters Table 3.2). Twisted core-shell fibre yarns were washed in ethyl acetate which is a solvent for PDLLA and not PLLA. Fibre mass before and after washing in ethyl acetate was measured and compared with theoretical mass yield calculations. The fibre diameters before and after exposure to ethyl acetate were measured using scanning electron microscopy. The remaining fibres and the polymer dissolved in ethyl acetate were dried. The separated materials analysed with differential scanning calorimetry (DSC) and optical rotation measurements (after each component was re-dissolved in 5ml chloroform).

Table 3.2 Coaxial electrospinning parameters

Designation Polymer Solutions Polymer Conc. (wt%) Flow Rate (ml/h) Applied Voltage (kV) Distance (cm) Collector Speed (m/s) Temp. & Humdity Core PLLA 70/30 DCM/DMF 8 0.3 Shell PDLLA 70/30 DCM/DMF 10 1.2 15.5 15 1.23 20-24°C / 40- 50%rH

It was postulated that if,

• coaxial electrospinning occurs with a stable coaxial Taylor cone and jet,

• and the mass and fibre diameters of the core-shell fibres decrease after washing in ethyl acetate,

• so that one polymer is dissolved in ethyl acetate and the other remains in fibrous form,

• and the polymer selectively dissolved displays neither optical rotation nor a melt endotherm,

• and the fibres remaining on the yarn holders rotate light and display a melt endotherm,

then the polymer that was selectively dissolved is PDLLA and the remaining fibres are PLLA. It is then reasonable to propose that the initial fibres have a core-shell structure with a continuous PLLA core entrained in a PDLLA shell.

iii) Procedure

The twisted yarns were secured under tension in a clamping device (herein referred to as the yarn holder). The yarn holders were immersed into glass poly-tops filled with ethyl acetate (Figure 3.8). The poly-tops were subjected to two washing steps to dissolve the PDLLA shell: (1) slow rocking action (Figure 3.9) at room temperature for 2hrs and (2) replacing initial ethyl acetate with clean ethyl acetate followed by sonication for 30mins in an ultra-sonic bath (50Hz) and rocking for another 2hrs. Steps 1 and 2 were repeated three times. The yarns before and after washing with ethyl acetate as well as the ethyl acetate solutions were placed in a vacuum oven at room temperature (vacuum of 100mBar) over night to evaporate the ethyl acetate. The dried polymer samples were stored in a desiccator prior to analysis

Figure 3.8 Yarn holder with yarn in poly top filled with ethyl acetate

Yarn holder Yarn

Figure 3.9 Rocking action administered to poly tops

iv) Analyses

Gravimetry

The mass of the coaxial electrospun yarns before and after selective dissolution with ethyl acetate were measured. The theoretical mass of the cores and core-shell fibres were calculated as follows:

Theoretical Mass (g) = Flow rate (ml/h)*Spinning duration(h)*concentration of the

polymer solution(g/ml)

Scanning Electron Microscopy (SEM)

The electrospun fibre mats were collected on a wire drum collector and were twisted into yarns. Three yarns for each experimental condition were prepared and therefore approximately 900 fibre diameter measurements per sample were made (100 measurements per image X 3 images per specimen X 3 yarn repeats = 900). Only fibres with clearly defined boundaries were measured. The fibre diameter measurements before and after washing in ethyl acetate were carried out as described in Section 3.2.2.

Optical rotation

Optical rotation measurements were performed for the separated fibre components (the remaining fibres on the yarn holders and the polymer extract recovered from ethyl acetate) after re-dissolution in chloroform, using a polarimeter (Bellingham & Stanley Ltd ADP 220). The average of three optical rotation measurements was taken for the each separated component for each yarn sample. A 100mm sample tube was

used and measurements were recorded at room temperature (RT) at a nominal wavelength 589nm. The specific optical rotation was calculated as follows,

where α is the angle of rotation in rad, γ is the mass concentration in kg/m3, and ‘l’ is the length of the sample tube in m.

Differential scanning calorimetry (DSC)

Differential scanning calorimetry (DSC) measurements (TA Instruments Q100) were conducted under nitrogen atmosphere. 5-10mg samples were sealed in aluminum pans for the measurements. The samples were heated from 10 to 200°C at a rate of 10°C/min, held at 200°C for 1 min, and cooled at the same rate to 10°C. This cycle was repeated twice. The percentage crystallinity was determined using TA Universal Analysis software: by calculating the area under the melt endotherm of the thermogram and using 93.7J/g as the heat of fusion for a theoretical 100% crystalline PLLA material.10 The percentage crystallinity was determined using equation

χ% = ∆Hf/∆H0f X100%

where χ% is the percentage crystallinity, ∆Hf area under the melt endotherm (J/g) from DSC thermograms and ∆H0f is the theoretical heat of fusion value of 100% crystalline PLLA polymer material which is taken to be 93.7J/g.10 The area of the melt enotherm was calculated for the 1st heating cycle in DSC of the samples as the areas obtainable from the second heating cycle, (i.e. for the study of parameter effects on crystallisable material present in the yarn) were too small as the result of dilution effects of PDLLA on PLLA crystallisation.11

3.3.5. Parameter Effects

3.3.5.1. Effect of solution concentration and flow rate on core-shell fibre