Functional Membranes with Thermo-responsive Hydrogel Gates
5.2 Functional Membranes with Thermo-responsive Hydrogel Gates Fabricated by Plasma-Induced
5.2.2 Effect of Grafting Degree on the Thermo-responsive Gating Characteristics
The grafting degree of thermo-responsive membranes could be represented by either grafting yield or pore-filling ratio. The grafting yield of the membrane is defined as the weight increase of the membrane after the grafting per unit mass or per unit area
and could be calculated according to the Eqs.5.1and5.2, respectively. However, the pore-filling ratio of the membrane is defined as the volume ratio of grafted polymeric gates to membrane pores (Eq.5.3):
YWD WgW0
W0 100% (5.1)
YAD WgW0
A 100% (5.2)
where YWand YAare the grafting yields calculated by unit mass and unit area of substrate membrane [wt% or mgcm2], respectively. Wgand W0represent the mass [mg] of the membrane after and before grafting, respectively. A stands for the area of the substrate membrane [cm2];
YFD Vg
Vm 100% (5.3)
where YFis the pore-filling ratio [%] and Vgand Vmare the total volume [cm3] of the grafted PNIPAM polymeric gates in the membrane and the membrane pores before grafting, respectively.
The thermo-responsive gating characteristics of the membranes are always investigated by conducting pressure-driven filtration experiment and concentration- driven diffusion experiment at different operation temperatures. To quantitatively describe the thermo-responsive gating characteristics of the membranes, a parameter known as the thermo-responsive gating coefficient is defined. It is the ratio of hydraulic permeability, diffusional permeability, or membrane pore sizes at tem- perature above the LCST of PNIPAM (usually 40 ıC) to that below the LCST (usually 25 ıC). These thermo-responsive gating coefficients are called the flux thermo-responsive coefficient RJ(Eq.5.4), diffusion thermo-responsive coefficient RD(Eq.5.5), and thermo-responsive coefficient of membrane pore size Rd(Eq.5.6), respectively. The higher the thermo-responsive gating coefficients are, the better the thermo-responsive gating characteristics of the membranes are.
RJD J40
J25
(5.4) RDD D40
D25
(5.5)
RdD d40 d25 D 4
s J4040
J4025 (5.6)
where J40 and J25 are the water fluxes or hydraulic permeabilities [mlcm2s1] of thermo-responsive membrane at 40 ıC and 25 ıC, respectively. D40 and D25
5.2 Functional Membranes with Thermo-responsive Hydrogel Gates. . . 115
are the diffusional coefficients or permeability coefficients [cm2s1] of model drug molecules across membrane at 40ıC and 25ıC, respectively. d40 and d25 are the effective pore diameters [cm] of grafted membranes at 40 ıC and 25 ıC under the same operation pressures, respectively. The effective pore diameters could be calculated by Hagen-Poiseuille’s law (Eq.5.7):
J D nd4P
128l (5.7)
where J is the water flux or hydraulic permeability [mlcm2s1] of membrane, n for the number of pores per unit membrane area [cm2], d for the membrane pore mean diameter [cm], P the transmembrane pressure [Pa],for the liquid viscosity [Pas], and l the thickness of membrane [cm].
The architectures and relatively physicochemical properties of the grafted PNI- PAM linear chains include the length and density of the PNIPAM chains. They will change with the variation of the grafting degree and have remarkable effect on the thermo-responsive gating characteristics of thermo-responsive membranes [8]. The effect of grafting degree on the thermo-responsive gating characteristics of mem- branes is investigated systematically by thermo-responsive filtration experiment and diffusion experiment.
Thermo-responsive hydraulic permeability: A series of thermo-responsive mem- branes, with a wide range of grafting yields and pore-filling ratios, are prepared by grafting PNIPAM via plasma-induced grafting polymerization method [8–12]. The membrane substrates are either hydrophilic N6 and PE membranes or hydrophobic PVDF and PC track-etched substrate membranes. For all of the investigated thermo- responsive membranes, the grafted PNIPAM polymers are found homogeneously distributed on the inner surfaces of the pores throughout the entire membrane thickness. The hydraulic permeability and diffusional permeability across these thermo-responsive membranes are dependent on the grafting degree which in turn heavily affects the thermo-responsive gating characteristics.
For PNIPAM-grafted PVDF membranes with moderate grafting yields (e.g., 0.5–
3.0 %), the water flux across membranes changes dramatically at 32ıC (around the LCST of PNIPAM) [8]. With an increase in the grafting yield, the hydraulic permeability decreases rapidly at both 25ıC and 40ıC because of the decrease in the pore size of membranes. When the grafting yield is smaller than 2.81 %, both the RJand Rdvalues increase with an increase in the grafting yield; however, when the grafting yield is higher than 6.38 %, both coefficients are always equal to 1.
When the grafting yield exceeds 6.38 %, the length of grafted PNIPAM chains is long enough to “choke” the membrane pores and thus there is no obvious change of the pore size at temperatures across the LCST (Range II in Fig. 5.2). As a result, the thermo-responsive coefficients of grafted membranes with the grafting yields higher than 6.38 % are equal to unity which indicates there are no thermo- responsive gating characteristics anymore. However, when the grafting yield is smaller than 6.38 %, the length of PNIPAM chains grafted in the membrane pores
Fig. 5.2 Effect of the grafting yield on the thermo-responsive gating characteristics and schematic illustration of the
thermo-responsive pore size (Reproduced with permission from Ref. [8], Copyright (2004), American Chemical Society)
is too short to “choke” the membrane pores (Range I in Fig. 5.2). There is a distinct difference between the lengths of PNIPAM chains at temperature below and above the LCST. As a result, the obvious change of membrane pore and hydraulic permeability at temperatures across the LCST is obtained. That is, the good thermo-responsive gating characteristics are achieved at the grafting yields of 0.5–3.0 %. The maximum flux thermo-responsive coefficient RJ and thermo- responsive coefficient of membrane pore size Rdare 2.54 and 1.16 when the grafting yield is 2.81 %.
Similar to the trend of the PNIPAM-grafted PVDF membranes, the water flux through the PNIPAM-grafted hydrophilic N6 membranes with proper grafting yields also has a dramatic change at 32ıC. For the PNIPAM-grafted N6 membranes, the flux thermo-responsive coefficient RJ reaches the maximum value of 15.41 when the grafting yield is 7.47 % [9]. When the grafting yield of PNIPAM-grafted N6 membranes is less than 7.47 %, the RJ value increases with increasing grafting yield. However, the RJ value tends to be 1.0 as the grafting yield approaches 12.84 %. When the grafting yield is larger than 12.84 %, the length and density of the PNIPAM-grafted chains are large enough to choke the membrane pores.
As a result, no water flux could permeate through the membranes at both 25ıC and 40ıC and therefore no thermo-responsive gating characteristics exist anymore.
Interestingly, the PNIPAM-grafted N6 membranes exhibit much larger maximum thermo-responsive gating coefficient RJ than that of the PNIPAM-grafted PVDF membranes (i.e., 15.41 and 2.54). Moreover, the optimum grafting yield of PNIPAM
5.2 Functional Membranes with Thermo-responsive Hydrogel Gates. . . 117
1 3 5 7 9 11 13 15 17
0 2 4 6 8 10 12 14 16
Grafting yield [%]
R J [-]
N6 PVDF Substrates Fig. 5.3 Effect of grafting
yield on the thermo- responsive gating
characteristics of PNIPAM- grafted membranes with different substrates
(Reproduced with permission from Ref. [9], Copyright (2006), Wiley-VCH Verlag GmbH & Co. KGaA)
(7.47 %) for the PNIPAM-grafted N6 membranes corresponding to the maximum RJvalue is also higher than that (2.81 %) of the PNIPAM-grafted PVDF membranes (Fig. 5.3). These phenomena should result from the differences in the physical and chemical properties of the porous substrates, such as the hydrophilicity and microstructure. During the design of thermo-responsive gating membranes, it is quite important to choose appropriate porous membrane substrates, with different physical and chemical properties to achieve different gating membrane responses.
In addition, PNIPAM is also successfully grafted on the surfaces and in the pores of PC track-etched membranes by using plasma-induced pore-filling graft polymerization method. Because of the cylindrical and straight pores with narrow pore size distribution, the track-etched membranes are considered as excellent substrates to study the microstructures of gating membranes. The PNIPAM chains are grafted inside the pores throughout the entire membrane thickness, and there is no dense PNIPAM layer formed on the membrane surface even when the pore- filling ratio is as high as 76.1 % [10]. For the grafted membranes with pore-filling ratio smaller than 44.2 %, the water flux across the PNIPAM-grafted PC membranes at 40ıC is always larger than that at 25ıC, and the water fluxes at both 25ıC and 40ıC decrease with increasing the pore-filling ratio (Fig.5.4). With the pore- filling ratio increasing, the pore diameters of PNIPAM-grafted membranes become smaller. When the pore-filling ratio is smaller than 44.2 %, the pores of PNIPAM- grafted PC membranes show thermo-responsive gating characteristics because of the conformational change of grafted PNIPAM in the pores. However, when the pore-filling ratio is larger than 44.2 %, the pores of membranes immersed in water are choked by the volume expansion of the grafted PNIPAM polymers, and the membranes does not show thermo-responsive gating characteristics any longer. The critical pore-filling ratio for choking the membrane pores is in the range from 30 % to 40 %. The pore diameter of the PNIPAM-grafted PC membrane with pore-filling ratio of 23.9 % increases dramatically when the temperature changes from 28ıC
0 0.05 0.1 0.15
0 20 40 60 80
Pore filling ratio [%]
J [ml·cm-2·s-1]
T = 40 οC T = 25 οC Fig. 5.4 Effect of pore-filling
ratios on water flux across the PNIPAM-grafted PC membranes (Reproduced with permission from Ref. [10], Copyright (2005), Elsevier)
to 34ıC but keep unvaried at the temperatures lower than 28ıC and/or higher than 34ıC. The pore diameter of the grafted membrane at 40ıC is almost twofold of that at 25ıC according to Hagen-Poiseuille’s law. The contact angle of PNIPAM- grafted PC membrane increases from 58.5ıto 87.9ıwhen the temperature changes from 25ıC to 40ıC. The thermo-responsive gating characteristics of the water flux of PNIPAM-grafted PC membranes are mainly dependent on the pore size change rather than the variation of membrane/pore surface hydrophilicity/hydrophobicity.
At last, the effect of grafting yield on the thermo-responsive water flux through porous PE membranes with PNIPAM gates is investigated [11]. At 25ıC, the grafted PNIPAM chains in the membrane pores are in the swollen state, and therefore the pore size decrease rapidly with increasing the grafting yield of PNIPAM. As a result, the water flux decreases rapidly at 25ıC with increasing the grafting yield.
At environmental temperature of 40ıC, however, the water flux across the grafted membrane increases firstly to a peak, and then decreases and finally tends to zero because of the pore size becoming smaller and smaller (Fig.5.5). It is suggested to set the grafting yield in the range 0.40.8 mgcm2for the membrane to get an effective thermo-responsive gating water flux.
To draw a conclusion, the PNIPAM-grafted membranes must be designed and prepared with a proper grafting yield to obtain a desired or satisfactory thermo- responsive gating performance.
Thermo-responsive diffusional permeability: During the diffusional permeation experiments, the thermo-responsive membranes show positive and negative thermo- responsive models depending on the grafting yield. In detail, the diffusional coefficient of solute molecules across membranes with low grafting yields increase with temperature, while that across membranes with high grafting yields decrease with temperature [8]. Figure5.6shows the thermo-responsive diffusional perme- ability through PNIPAM-grafted PVDF membranes with different grafting yields.
The diffusional coefficient of sodium chloride (NaCl) across the grafted membrane changes dramatically at temperatures around the LCST of PNIPAM, which is due
5.2 Functional Membranes with Thermo-responsive Hydrogel Gates. . . 119
Fig. 5.5 Effect of grafting yield on the thermo-responsive water flux through the PNIPAM-grafted PE membranes
Fig. 5.6 Effect of the grafting yield on
thermo-responsive diffusional permeation through
PNIPAM-grafted PVDF membranes (Reproduced with permission from Ref. [8], Copyright (2004), American Chemical Society)
to the conformational change of the PNIPAM chains grafted in membrane pores.
When the grafting yield of PNIPAM is low, the diffusional coefficient D of NaCl across the membrane is higher at temperature above the LCST than that below the LCST. It is attributable to the pores of the membrane being controlled open/closed by the shrinking/swelling mechanism of the grafted PNIPAM gates. However, when the grafting yield is high, the diffusional coefficient is lower at temperature above the LCST than that below the LCST owing to the hydrophilic/hydrophobic phase transition of the grafted PNIPAM gates. Because the solute used in the experiments is water soluble, any solute diffusion within the membranes occurs primarily within the water-filled regions in the spaces delineated by the grafted PNIPAM chains.
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.