Evaporation-Induced Polyelectrolyte Complexation: The Role of Base Volatility and Cosolvents

Film formation is a vital step for coating applications where a homogeneous, defect-free solid phase should be obtained, starting from a liquid casting formulation. Recently, an alternative waterborne-coating approach was proposed, based on the formation of a polyelectrolyte complex film. In this approach, an evaporating base induces a pH change during drying that initiates the complexation of oppositely charged polyelectrolytes, followed by further densification. In previous studies, ammonia was used as the evaporative base, leading to relatively fast evaporation and resulting in films showing significant brittleness, which tended to crack at low relative humidity or larger thicknesses. We hypothesize that slower complexation and/or evaporation can reduce the problematic stress build-up in the prepared polyelectrolyte complex coatings. For this reason, we studied the changes in the film formation process when there are different bases and cosolvents. We found that reducing the evaporation rate by changing ammonia to the slower evaporating dimethylamine or by adding DMSO as a cosolvent, led to less internal stress build-up during film formation, which could be beneficial for film application. Indeed, films prepared with ammonia showed cracking after 1 h, while films prepared with dimethylamine only showed cracking after one month. The fast evaporation of ammonia was also found to cause a temporary turbid phase, indicating phase separation, while for the slower evaporating bases, this did not occur. All prepared films remained sensitive to humidity, which poses the next challenge for these promising coatings.

Since we only need the turbidity (whiteness) information, we focused on the K value which determines how black the color is.K = 100 means black, while K = 0 means white.The turbidity of films can be calculated basing on Equation S1: where K 0 is the initial K value of the film at 0 s and K t is the K value at a certain time t.When turbidity equals to 100%, it suggests the blackness of the substrate is fully covered.Similarly, CMYK (cyan, magenta, yellow, key (black)) values can be obtained from the colors.
Here, we focused on the Y/C ratio which determines how green the color is.From Figure S1c, we can obtain roughly the ratio trend for pH 7-13 as a reference (Figure S10).As shown in Figure S1a, the yellow color from PSS itself became stronger with the pH increase.In principle, as pH increases, a more blue color should be obtained.However, due to this increase of yellow, an increase in Y/C was observed from pH 9 to 12. Thus instead of one equation, the Y/C ratio was first calculated for each point and from the value, it was assigned to a region then a certain equation was used for calculation.For example, if Y/C value = 0.5, it can be either pH 9-10 or pH 11-12.
According to previous points, it already reached pH 10, so this point should fall into the region pH 9-10 and the pH would be calculated based on equation 3.      Table S1.The thickness (µm) of tensile samples vs time (* Cracked samples that could not be measured).The higher value for ammonia indicates a higher molar rate of evaporation per gram of applied coating than for dimethylamine.This supports our hypothesis that ammonia evaporates faster, and pH decreases more rapidly in these samples.
Notably, both mass curves also converge to a roughly linear decrease, as observed in the original, longer timescale plots.This seems to indicate that most of the volatile base evaporates initially, and the water evaporation is the main contributor after a certain time.This is further supported by the fact that after 1000 s, the mass decrease is roughly the same as the initial added weight of volatile base.We do stress that the last portion of volatile base that evaporates might still significantly change the pH, even though it is not as clearly observable in a mass decrease.To visualize this further, we add the following Figure S21, where it is visible the weight decrease is roughly the same as initially added volatile base (6.5 wt% and 17.3 wt% added for ammonia and dimethylamine respectively):

Figure
Figure S2.a) When keeping the SS:NaOH to 1:4, the basicity was so high that PSS was

Figure S7 .
Figure S7.PEI:PSS-NH 3 at a ratio of 2:1 film turned turbid.a) At this stage, a continuous film

Figure S8 .
Figure S8.Screenshot pictures of different films with thymol blue drying vs time.Same amount of thymol blue was added into the

Figure S9 .
Figure S9.Color change vs time of different samples.The color of the same point was tracked

Figure S10 .
Figure S10.Y/C value vs pH obtained from the reference color system.According to the Y/C

Figure S11 .
Figure S11.Example g 2 intensity autocorrelation functions for the dimethylamine sample shown

Figure S12 .
Figure S12.Example g 1 normalized field correlation functions for the dimethylamine sample

Figure S15 .
Figure S15.SEM images of different samples.Due to vacuum, cracks could be generated for

Figure S19 .
Figure S19.Representative stress-strain curves of PEI:PSS samples at a ratio of 3:1 with: a) PSS-

Figure S21 .
Figure S21.Lost paint mass and  0 over time during initial drying stages.Note that the lost paint mass at t = 1000 s is roughly equal to the amount of added ammonia (6.5 wt%) and

Table S2 .
Tensile measurements (Young's modulus/Tensile strength/Elongation) of PEI:PSS at a ratio of 2:1 samples vs time (* Cracked samples that could not be measured).