3.1. Swelling and Dissolution Behaviour of PA 6.6 Fibres in CaCl2/H2O/EtOH Solvents
For the qualitative observation of swelling and dissolution behaviour, 26 different compositions of CaCl2
O were prepared and their effects on PA 6.6 fibres were investigated (Table 1
When PA 6.6 is added to the alcoholic solution, the carbonyl group approaches the calcium–alcohol complex. As the affinity of calcium is higher towards the oxygen of the carbonyl group than the hydroxyl, a transfer occurs. Thus, a complex between calcium and PA 6.6 forms and the alcohol becomes free again (Figure 1
a). Due to the complex formation, hydrogen bonds are disrupted (Figure 1
b) and dissolution of PA 6.6 takes place. The mechanism shown in Figure 1
has been proposed by Sun based on methanol/CaCl2
]. The concept of the disruption of the hydrogen bonds between the polyamide chains by Lewis acids has also been reported by other authors [13
In this case, hydrolysis due to alkaline pH medium can be excluded because the formal pH measured in the concentrated solution was observed at 3.2, which is in agreement with the Lewis acid character of CaCl2
. However, it must also be taken into consideration that the concept of pH is only valid in diluted aqueous solutions. In our case, we have concentrated ethanolic solutions, hence the pH value cannot be utilised to characterize the solution system. Essentially, the dissolution process in polymers is based on two different transport processes; diffusion and chain disentanglement [25
]. The dissolution starts when the polymer comes into contact with a suitable solvent. Then the solvent starts to penetrate into the polymer and interactions between the polymer chains are broken; this is the diffusion process, which induces swelling. In the swollen polymer, the chains are still entangled, and the chain disentanglement proceeds at the interface between the swollen polymer and the solvent. Depending on the velocities of these two processes, either swelling or dissolution occurs. The photomicrographs in Figure 2
show the different behaviours: (a) no effect of the solvent on the fibres; (b) dissolution of the fibre surface leading to fibre thinning; and (c) formation of a swollen shell.
The observed results were plotted in a ternary phase diagram (Figure 3
) with different symbols representing the behaviours of polyamide fibres in the different solvents.
It is reported in the literature that adding small amounts of non-solvent to a solvent changes the dissolving and swelling behaviour of a polymer [25
]. In the case of the mixture of calcium chloride and ethanol, the addition of water caused an increase of the swelling rate of PA 6.6 fibres. It was found that not only the calcium chloride content plays an important role, but also does the amount of water, and therefore the ratio between water and ethanol has an impact. As a result, it was possible to separate the ternary phase diagram into three areas: I, II, and III; and they can be assigned to the different effects of the solvents (Figure 3
, Table 2
Solvents with CaCl2
content above 6 mol % and below 10 mol %, ethanol content lower than or equal to 25 mol %, and H2
O/EtOH mole ratios above 2.5 did not dissolve the fibres. The same was observed for solvents with CaCl2
content below 6 mol %, ethanol content above 25 mol %, and H2
O/EtOH mole ratio below 2.5. These values describe area I in Figure 3
. Solvents with CaCl2
amount above 6 mol %, ethanol content above 25 mol %, and H2
O/EtOH mole ratios below or equal to 2.5 dissolved the fibres. The solvents in area II caused a reduction of the fibre diameter and later complete dissolution. Solvents that caused swelling had CaCl2
amounts greater than 10 mol %, ethanol content below 25 mol %, and H2
O/EtOH mole ratios greater than 2.5. In these swelling solvents, the outer diameter of the fibre increased. Under the microscope, a swollen outer shell and a core region were observed. The size of the shell region increased with time, while that of the core region decreased. Finally, the whole fibre was swollen and no core region was observed. This effect is referred to as area III.
In area II (dissolving), the diffusion velocity of the solvent into the polymer is the same or slower than the polymer chain disentanglement at the surface. In this case, no gel-like layer was observed because of the high velocity of the disentanglement.
In area III (swelling), the mechanism is different. The amounts of deionised water, the non-solvent in this case, were much higher. Water molecules are very small and mobile, thus diffuse easily into the polymer and cause swelling. However, the transport of polymer molecules from the surface into solution is slow. This is the reason that swelling was observed. Furthermore, the phenomena of swelling or dissolving depended on the water/ethanol mole ratio and the amount of CaCl2
in the solution. Although this effect had not been reported for PA 6.6 fibres, it has been observed for ramie fibres, cotton, kraft pulp, and rayon in N
-oxide containing various amounts of water [26
3.2. Microscopic Investigation of Swelling and Dissolving of PA 6.6 Fibres
To quantify the previously described observations of swelling and dissolution, diameter measurements were conducted under the microscope. Two solutions were chosen to be investigated more thoroughly: one of the solutions with a H2
O/EtOH mole ratio below or equal to 2.5 (dissolving solution 17 in Table 1
); and the other with a H2
O/EtOH mole ratio higher than 2.5 (swelling solution 2 in Table 1
3.2.1. In Situ Swelling and Dissolution Experiments
At first, the fibre diameters were examined in situ according to the experimental procedure described above; therefore, no mechanical forces were affecting the fibres. In Figure 4
, swelling of PA 6.6 fibres in SW is shown as a function of time.
The swelling started immediately after adding the solvent to the fibres. After 10 min, the outer diameter was already increased by 77.0%, and a further increase was observed until the core region disappeared. Note that the average diameter of the PA 6.6 fibres was initially around 15.6 µm. During the soaking in SW, the outer shell diameter increased while the inner core diameter decreased. After 20 min, the outer shell diameter had increased by 100.4%. Thereafter the swelling rate decreased. A table with the detailed values listed can be found in the Supplementary Information (Table S1)
. The picture after 50 min shows that the shape of the fibre was starting to become more irregular, which indicates that the fibre was losing its integrity with increased swelling. This can be explained by the decrease of interactions between the polymer chains, because the hydrogen bonds are severed as the complex between the carbonyl groups and the CaCl2
is formed (Figure 4
). To obtain a reliable value of fibre diameter, it was measured once at the thicker site and at the thinner site and the mean value was calculated.
For comparison, pictures of fibres in DISS were also taken as a function of treatment time (Figure 5
In this solvent, no gel-like layer was visible, but an attenuation was observed. After 20 min, the diameter was already decreased by 25.2% of the initial diameter. After 50 min, the diameter reached a width of around 4.3 µm. Afterwards, the fibre was not visible anymore and had dissolved completely. A table with the detailed values listed can be found in the Supplementary Information (Table S2)
. Thus, no swelling was occurring in this solvent, and instead the fibre was thinning until the whole fibre had dissolved.
In Figure 6
, the diameter changes of fibres in SW and DISS are compared. In DISS, it was observed that the diameter was decreasing linearly over time. In the case of SW, a non-linear increase of the outer fibre diameter was noticed, accompanied by a decrease of the core diameter. The high swelling rate at the beginning was decreasing over time. When the fibre was completely swollen and no core was left, the swelling stopped. In Figure 7
, the comparison of four solvents with different H2
O/EtOH mole ratios for a treatment time of 10 min is shown.
An increased water amount and therefore higher H2
O/EtOH mole ratios resulted in a higher swelling rate. The critical value of the H2
O/EtOH mole ratio where swelling starts is 2.5. Below that value, no swelling was observed, but a higher dissolution rate was detected. These observations support the dissolution mechanism of polymers as described above. As seen in Figure 7
, the swelling rate did not increase continuously with an increasing H2
O/EtOH ratio, and it decreased again at high water amounts. The explanation for this phenomenon could be visualised using the ternary phase diagram (Figure 3
). With increasing H2
O/EtOH ratios, the solvent composition was shifted along the ethanol axes, from area II (dissolving leading to smaller fibre diameter) to area III (swelling leading to bigger fibre diameter). However, with further increase of the water content, the solvent composition was further shifted close to area I (no effect). In this case, water started to act as a coagulating agent and thus prevented further diffusion of PA 6.6 chains into the solvent medium. Therefore, a smaller increase in fibre diameter was observed (Figure 7
3.2.2. Ex Situ Swelling and Dissolution Experiments with a Washing Step
In further experiments, the fibres were treated for definite time periods and then washed with deionised water to remove the solvent. In order to identify possible chemical changes, the fibres were characterised by infrared spectroscopy.
In Figure 8
, FTIR spectra of virgin fibres and modified fibres using SW and DISS after the washing step with deionised water are shown. It can be observed that all spectra are very similar, showing characteristic peaks of PA 6.6. When CaCl2
was removed by washing, decomplexation of the PA 6.6–CaCl2
complex took place and the dissolved polymer was precipitated [23
]. FTIR spectra in Figure 8
indicated that there were no changes in the chemical structure of PA 6.6. fibres. In other words, the decomplexation led to an intact PA 6.6 structure, comparable to the virgin one before the treatment. Sun reported a shift of the amide I band to lower frequencies in the FTIR spectra after forming a complex between polyamide and the CaCl2
]. They evaporated the solvent after treatment and thus CaCl2
residues were expected to remain on the substrate. In our study, the fibres were extensively washed with deionised water to remove solvent residues. Representative specimens analysed for calcium with titrimetric determination showed no detectable amounts (data not given). Hence, the FTIR spectra in this study indicate the complete decomplexation of the polyamide after precipitation and washing with deionised water.
In the case of the fibre in SW, a swollen shell was observed under the light microscope, which appeared to be in an equilibrium state of neither dissolved nor undissolved PA 6.6 adhering to the surface of the bulk PA 6.6 of the core region. This shell was returning to an undissolved state after removing the solvent by washing with deionised water. Observations under the microscope after washing still showed the swollen shell and the core region. The completely dissolved proportion of PA 6.6 was precipitating in the form of small particles, which caused turbidity of the solution in the washing step. Furthermore, the treated and washed fibres exhibited a rougher surface all over the fibre compared to the untreated fibres. In the case of the fibres in DISS, a swollen shell was not observable under the light microscope and thus it appears that dissolved PA 6.6 was dispersed in the solvent. With the addition of water, the dissolved PA 6.6 also precipitated in the form of small particles. The turbidity of the solution was much higher because of the precipitation. The fibres still had a smooth surface but some of the precipitated particles were retained at the surface of the treated fibres.
In Figure 9
, the diameter changes vs time are illustrated for the fibres in DISS and in SW after the washing step. For the fibres in SW, measurements were carried out until a treatment time of 15 min. After longer treatment times, the fibre strength was too low to withstand the washing procedure and the fibres broke or deformed, and therefore no diameter measurements were possible. In DISS, measurements were only possible until a treatment time of 35 min. Tables with the detailed values listed can be found in the Supplementary Information (Tables S3 and S4)