Switchable Asymmetric Water Transport in Dense Nanocomposite Membranes

Directional water transport is technologically relevant in separation processes, functional clothing, and other applications. While asymmetric water transport characteristics are a vital feature of leaf cuticles, examples of artificial membranes that display this effect are limited. Here, we report compositionally asymmetric membranes that are based on hydrophobic poly(styrene)-block-poly(butadiene)-block-poly(styrene) (SBS) and hydrophilic poly(vinyl alcohol) (PVA) nanofibers and display directional water transport when a high relative humidity (RH) gradient is applied. This effect is caused by the asymmetric structure of the membrane and the fact that the water permeability of PVA depends on the water pressure applied and the extent of plasticization that it causes. The transport characteristics can be tuned by varying the composition of the membranes. Such materials with switchable asymmetric water transport may be useful for smart packaging applications in which the take-up or release of water is regulated as needed.

The total resistance of the composite membrane R M is expressed by Equation S1: where l M and A are the total thickness and the transport area of the membrane.WP is the water permeability of the membrane, which assumes two different values, depending on the transport direction investigated with the wet cup method (RH D = 100%).
The experimentally determined data are: PVA → SBS WP = 3.97 × 10 -14 kg m m -2 s -1 Pa -1 Using Equation S1, we obtain: Using a series-resistance model, we can express the total resistance of the membranes R M as the sum of the resistances of the SBS layer (R SBS ) and the PVA-rich side (R PVA-rich ), as shown in

Equation S2
: Similarly, as for the total resistance R M , we can express the resistance of the SBS layer R SBS with

Equation S3
: where l SBS is the thickness of the SBS layer in the composite membrane and WP SBS is the water permeability of neat SBS measured with the wet cup method (RH D = 100%).
The experimentally determined data are: l SBS = 48 µm WP SBS = 2.13 × 10 -14 kg m m -2 s -1 Pa -1 Using Equation S3 we obtain: After evaluating R M and R SBS , using Equation S2 we can derive the resistance of the PVA-rich side of the composite membrane R PVA-rich in the two transport directions.We obtain: These calculations were made using the average thickness value of the total membrane l M and the SBS layer l SBS in the SBS-PVA 23 composite membrane reported in Table 1 of the main manuscript.Similarly, for the WP of the SBS-PVA 23 composite in the two transport directions, we assumed the average values measured with the wet cup method and reported in Table 3 of the main manuscript.The resistances reported in Table 3 of the main manuscript are the mean ± standard deviation of analogous calculations made using the actual values of l M , l SBS and WP of n = 4 different membranes.

S10
From these values we can evaluate the volume fraction of PVA in the PVA-rich side v PVA as follows: = 18 18 + 32 = 0.36 All the arrows indicated in the calculation (→) represent a conversion step between weight and volume (or vice versa) in which we used the densities reported in the specifications of the supplier (Merck).

Water permeability and resistance of the PVA-rich side
We can express the water permeability of the PVA-rich side of the composite membrane (WP PVA-rich ) as the combination of the water permeability of PVA (WP PVA ) and SBS (WP SBS ) reported in Equation S4:

Figure S1 :
Figure S1: (a) Effect of the processing conditions on the WP of reference films of neat SBS; data

Figure S2 :
Figure S2: SEM images of the SBS-PVA 20 composite membranes prepared with the film applier.

Figure S3 :
Figure S3: Top-view SEM images of the surfaces of the SBS-PVA composite membranes; (a)

Figure S4 :
Figure S4: Thermogravimetric analysis (TGA) curves of the neat SBS, the electrospun PVA mat, 108 µm A = 31.67cm 2 and Transport direction: SBS → PVA WP = 1.93 × 10 -14 kg m m -2 s -1 Pa -1 S4)where v PVA is the volume fraction of PVA in the PVA-rich side of the membrane evaluated before, while for the water permeability of SBS WP SBS we use the value measured with the wet cup method on the reference films of neat SBS.The water permeability of PVA WP PVA changes from a dry (unplasticized) to a wet (plasticized) value.If for the unplasticized value we use the water permeability measured with the dry cup method at RH D = 60%, and for the plasticized value we use the water permeability measured with the wet cup method (RH D = 100%), we obtain: v PVA = 0.36