Functional Membranes with Thermo-responsive Hydrogel Gates
5.2 Functional Membranes with Thermo-responsive Hydrogel Gates Fabricated by Plasma-Induced
5.2.3 Gating Characteristics of Thermo-responsive Membranes with Grafted Linear and Cross-linked Hydrogel Gates
Therefore, it is easier for the solute to find water-filled regions in the membranes with hydrophilic PNIPAM gates (below the LCST) rather than in the membranes with hydrophobic PNIPAM gates (above the LCST) under the high grafting yield.
The grafting degree of PNIPAM-grafted PE membranes also has an effect on the thermo-responsive models [12]. When the pore-filling ratio of PNIPAM-grafted PE membrane is below 30 %, the diffusional coefficients of the solutes across the PNIPAM-grafted membranes are higher at temperatures above the LCST than those below the LCST. In contrast, when the pore-filling ratio is higher than 30 %, the diffusional coefficients are lower at temperatures above the LCST than those below the LCST. That is, the PNIPAM-grafted membranes change from positive thermo-responsive to negative thermo-responsive types with increasing pore-filling ratios at around 30 %. Phenomenological models for predicting the diffusion coefficient of a solute across PNIPAM-grafted membranes at temperatures both above and below the LCST are developed. The predicted diffusional coefficients of solutes across PNIPAM-grafted flat membranes are shown to fit with experimental values. To obtain ideal results for diffusional thermo-responsive controlled release through PNIPAM-grafted membranes, those substrates strong enough to prevent any conformation changing should be used in the preparation of the thermo-responsive membranes rather than weak substrates.
These results verify that it is also very important to choose or design a proper grafting yield of PNIPAM for obtaining a desired thermo-responsive diffusional permeability.
5.2.3 Gating Characteristics of Thermo-responsive Membranes
5.2 Functional Membranes with Thermo-responsive Hydrogel Gates. . . 121
0.5 1 1.5 2 2.5 3 3.5 4 4.5
0 0.06 0.12 0.18 0.24 0.3
LM-LT CM-LT LM-HT CM-HT UM
Operation pressure [MPa]
LM-LT CM-LT LM-HT CM-HT UM
Rd [-]
Fig. 5.7 Thermo-responsive gating coefficients of different membranes operated under different pressures
(Reproduced with permission from Ref. [13], Copyright (2009), Wiley-VCH Verlag GmbH & Co. KGaA)
membrane are more stable at an environmental temperature of 25 ıC and under high operation pressure. When the operation pressure increases to above 0.12 MPa, the effective pore size of the CM-LT membrane does not change. However, the pores of the LM-LT membrane change from the “fully closed” state at pressures lower than 0.12 MPa to the “slightly opened” state at operation pressures higher than 0.12 MPa. It results from the collapse of the linear-grafted PNIPAM chains under high operation pressure.
For all the PNIPAM-grafted membranes, the thermo-responsive gating coeffi- cients under different operation pressures remain unchanged when each operation pressure is higher than a certain critical pressure value (Fig. 5.7). The critical pressures for the LM-LT, LM-HT, CM-LT, and CM-HT membranes are 0.14 MPa, 0.06 MPa, 0.06 MPa, and 0.04 MPa, respectively. However, at operation pressures lower than the critical pressure, the thermo-responsive gating coefficients are heavily affected by the thermo-responsive hydrophilicity/hydrophobicity change of pore surface. In order to get a stable thermo-responsive gating property of the PNIPAM-grafted membrane, the operation pressure should be higher than a critical value.
When the operation pressure is above 0.12 MPa, the thermo-responsive coeffi- cient of membrane pore size Rdof the LM-LT membrane under a fixed operation pressure is the highest among the four kinds of grafted membranes, which reaches 3.7 (Fig.5.7). It means that the effective pore diameter of the LM-LT membrane at 40ıC is nearly 4 times as large as that at 25ıC. The thermo-responsive coefficient of membrane pore size of the LM-HT and CM-LT membranes are 2.3 and 2.8, respectively. It also can be seen that the Rd value of the CM-HT membrane is the lowest among the four PNIPAM-grafted membranes, which is only 1.3.
For both membranes with grafted linear and cross-linked PNIPAM gates, the membranes prepared at 25 ıC show larger thermo-responsive gating coefficients
d25
LM-HT: T<LCST
T>LCST d40
d25
d25
LM-LT:
a
b
c
d
T>LCST T<LCST
d40
CM-LT:
T>LCST T<LCST
d40
CM-HT:
T<LCST T>LCST
d25
d40
Fig. 5.8 Schematic illustrations of thermo-responsive microstructural changes of linear and cross- linked PNIPAM-grafted N6 membranes prepared at 25ıC and 40ıC, respectively. (a) LM-LT, (b) LM-HT, (c) CM-LT, and (d) CM-HT (Reproduced with permission from Ref. [13], Copyright (2009), Wiley-VCH Verlag GmbH & Co. KGaA)
than those prepared at 40ıC. It is attributable to the effect of grafting temperature on the microstructures of the grafted polymeric gates in the membrane pores (Fig.5.8).
The distributions of grafted polymeric gates in the PNIPAM-grafted membranes with the same grafting yield but prepared at different temperatures are also verified by SEM images. At the grafting temperature of 25 ıC, the grafted layer on the membrane surface and at the pore entrance near the membrane surface is thicker than that in the middle of the membrane pores. On the other hand, at the grafting temperature of 40ıC, more homogeneous grafted layer is fabricated throughout the whole membrane thickness. Therefore, with the same grafting yield, the effective
5.2 Functional Membranes with Thermo-responsive Hydrogel Gates. . . 123
pore sizes of PNIPAM-grafted membranes, with either linear or cross-linked gates prepared at 25 ıC, are smaller than those prepared at 40 ıC. In addition, the cross-linked network structures of grafted PNIPAM layers of CM-LT and CM- HT membranes should be more compact than the linear PNIPAM chains with free ends of LM-LT and LM-HT membranes. Consequently, the effective pore sizes of cross-linked PNIPAM-grafted membranes are slightly larger than those of linear PNIPAM-grafted membranes when the grafting yields and grafting temperature are the same (CM-LT vs. LM-LT, and CM-HT vs. LM-HT). Therefore, the effective pore sizes of the LM-LT and CM-HT membranes at 25ıC are the smallest and largest among the four grafted membranes, respectively. Because the effective pore sizes of the membranes at 40ıC are almost the same, the thermo-responsive gating coefficients of membranes is mainly determined by the effective pore sizes at 25ıC.
That’s why the thermo-responsive gating coefficients of the LM-LT and CM-HT membranes are the highest and lowest one, respectively.
Both linear and cross-linked PNIPAM gates in the grafted membranes exhibit stable and repeatable thermo-responsive “open-close” switch performance under operation pressure of 0.26 MPa even the membranes have been tested for 20 runs. To get desired and satisfactory thermo-responsive gating characteristics of PNIPAM- grafted membranes, it is quite important and essential to design the grafted gates with proper structures (linear chains or cross-linked networks), to fabricate the PNIPAM-grafted gates at a proper temperature (higher or lower than the LCST of PNIPAM), and to operate the membrane under a proper pressure (should be higher than a critical value). The results in this study provide valuable guidance for designing, fabricating, and operating thermo-responsive gating membranes with desirable performances.