Switchable Surfaces

EVALUATION OF AN AMPHIPHILIC COPOLYMER FOR SWITCHABLE “SMART” SURFACE COATING

Part A: A Review Of Current State of Switchable Surfaces

1. ABSTRACT

Switchable surfaces, also named “smart” surfaces, are a type of surfaces capable to be responsive to external stimuli by rearranging themselves. The switchable surfaces can be made of many materials, which are almost utterly comprised of metal oxides (e.g. TiO2 and ZrO) and polymers. Among these materials, polymers are the most intensively studied. In this review, switchable surfaces made by several homopolymers and copolymers, and their responses, in both microscopic (contact angle) and microscopic (Transmission Electron Micrographs and Atomic Force Microscopy) scales, to different solvents are introduced. As a preparation for further study in an amphiphilic graft copolymer poly(isoprene-alt-maleic anhydride)-graft-poly(ethylene glycol) methyl ether, this review introduces more details in previous researches in amphiphilic graft copolymers used as switchable surfaces. In addition, some polymer-based surfaces or polymer brushes with responsive behaviours to pH-value and temperature changes are also briefly described. The materials for making switchable surfaces are quite promising to be applied in many fields such as anti-fogging and anti-fouling coatings, filtration membranes, also biocompatible components and drug delivery devices. Some of these materials have already found their commercial applications.

2. INTRODUCTION

Switchable surfaces, also named as smart surfaces, are widely considered as a kind of surfaces which are capable to rearrange their morphology or composition in response to the change of the ambient environment. This restructuring mainly derives from surface energy, entropy of system, and interactions of chain segments, among which the surface energy plays the dominant role, because the surfaces are primarily prone to minimize their interfacial energy in different environments. [1] Due to the amphiphilic property, switchable surfaces have many potential applications such as anti-fogging and anti-fouling coatings, implants and tissue engineering scaffolds, drug delivery devices and filtration membranes. [2] The Switchable surfaces can be classified, based on chemical composition, into two categories: metal oxide-based and polymer-based.

Before talking about the surfaces, the methods for surface characterization need to be introduced. Contact angle measurement and X-ray Photoelectron Spectroscopy (XPS) are widely used, respectively, to test the surface wettability and the composition of surfaces, and they were applied in many previous studies to monitor the surface rearrangement. XPS is a surface analysis technique developed in mid-1960s to investigate the elemental composition, chemical stoichiometry, chemical state, and electronic state of the elements within a material by capturing the x-ray photoelectrons exited by x-ray and calculating the bonding energy of electrons. [3]

Contact angle analysis is usually applied to investigate the wettability of surfaces by measuring the angle made by the edge of a solvent droplet on a homogeneous surface. [4] Three interfacial tensions, solid-liquid, γsl; liquid-vapour, γlv; and solid-vapour, γsv, see 1, define the contact angle of the liquid droplet on a solid surface, and Young discovered the relation of the interfacial tensions and the contact angle and described it in the equation as follow:

(1)

Static and dynamic sessile drop methods are the most widely applied to measure the contact angle. In this method, the contact angle forms between liquid-solid interface and liquid-vapour interface, and it is captured by a high-resolution camera (although measurements can also be taken by hand).

In a static contact angle measurement, the volume of the droplet does not change during the whole process. This measurement is usually chosen to study the change of the contact angle over a prolonged time. Since a needle does not stay in the droplet, the static contact angle prevents the droplet from being distorted. Surfaces with relatively low rigidity, like rubbers, are recommended to be tested by static method, because the contact angle is much easier to reproduce in this way. The static contact measurement is better to be taken on different areas of the surface to avoid the influence of any local irregularities such as dirt and inhomogeneous areas.

In terms of dynamic contact angle, different from static sessile drop method, the size of the droplets in this dynamic method is changing, by steadily increasing or decreasing the amount of liquid during the process. The contact angle is recorded when the value reaches a steady state. Normally, the advancing angle θa (the liquid increases) is larger than static contact angle and the receding angle θr (the liquid decreases) is smaller than static contact angle. The former one shows the contact angle of the surface when being wetted, and is usually applied in the measurement of surface free energy of solids. The different between advancing angle and receding angle is termed contact angle hysteresis, H, which reflects the surface roughness or the chemical inhomogeneties of the surface. High hysteresis is also caused by rearrangement of the surface molecules, and swelling or dissolving at the surface.

When a surface contacts with water, the terms hydrophobicity and hydrophilicity are employed to describe the interaction between the surface and water. Surfaces with a very high hydrophilicity can display a zero-degree contact angle, and the water contact angle increases up to 90° with the lessening of surface hydrophilicity. If a surface has a very high hydrophobicity, the water contact angle can be as high as 150° or even 180°.

3. METAL OXIDE-BASED SWITCHABLE SURFACES

Due to unique chemical properties, titanium oxide (TiO2) and Zirconium oxide (ZnO) are considered as the typical types of metal oxides, which have the ability to convert the surface property under external stimuli. [5] Sun et al. [6] discovered the changes in the wettability of the surfaces of TiO2 and ZnO with the exposure under ultra-violet light. Wang et al. [7, 8] revealed an interesting property that the single-crystal surface of titanium oxide became compatible to both water and oil under UV-light, and the surface reverted to hydrophobic via long-term storage in darkness, and this property can be applied to make TiO2-coated antifogging glass. TiO2 is widely-known for its properties like chemical inertness, nontoxicity and oxidizing power, and recent research [9] found that a water-splitting reaction can occur on the photo-induced surface of TiO2-based electrodes, thus the material has been marked promising for converting solar energy. [5]

4. POLYMER-BASED SWITCHABLE SURFACES

The other type of switchable surface is called polymer-based switchable surfaces, which is going to be the focus of this review. Polymers have a fundamental nature of changing their conformations, demonstrating various changes on their surface properties according to their different chain configurations. [10] With respect to thermodynamics, the changes of the molecular conformations are based on surface energy, entropy of the system and the interactions between segments. Among these driving forces, Luzinov et al. [1] noted that the surface energy is drastically important, because a surface rearranges its conformation to minimize interfacial energy in different environments. Many surfaces of polymers have this ability, and they can be the surfaces of homopolymers, copolymers and modified polymers, or directly grafted layers.

4.1 Homopolymers

Holly and Refojo, [11] studied the wettability of poly(2-hydroxyethyl methacrylate) (PHEMA) hydrogel with water and air, which can be considered as a hydrophobic medium. They found that water was incompatible with the surface of the hydrogel in the air for a short time, but the surface reorganized the conformation of the molecular chains so as to reduce the interfacial energy. For further study, Ruckensten and Lee [12] investigated the surface of hydrophobic polysiloxane and hydrophilic PHEMA homopolymers, finding that they both reconstruct their surfaces to adapt to the surrounding medium. For instance, they found that polar solvent triggered higher polarity of the surface of PHEMA, while a non-polar solvent reduced the polarity.

2, Transmission electron micrographs of the poly(HEMA-block-polyisoprene) at the cross sections or the extremely thin areas, casted film from DMF/THF (9/1, v/v) under different conditions, which indicated the surface rearrangement, the dark area represents the polyisoprene component, and the bright area refers to the pHEMA component. Image a) shows a newly casted surface was mainly covered by PI component. Image b) shows the surface with increased pHEMA component, because of a 30-minute immersion in water, pHEMA block rearranged to surface. Image c) shows the surface with more PI domains recovered to surface, because the surface was dried for 1 hour in air, which is an hydrophobic media. In image d), the surface was almost utterly occupied by PI domains, because the surface had been dried in air for 24 hours, during which more PI moved to the surface. Image e) shows the surface annealed at annealed at 90°C for 30 minutes after the immersion in water, the pHEMA domains significantly decreased than that in the image b). The image f) shows the surface mophology of the specimen immersed again in water again for 30 minutes after annealing, pHEMA obviously increased for the contact with hydrophilic solvent. This diagram is adapted from the reference [16].

3, water contact angle (cosθ) of the surface of poly(HEMA-block-polyisoprene) casted film vs. dry and wet treating periods. a) the film was casted from DMF/THF (9/1, v/v); b) the film was casted from DMF/methanol (5/1, v/v). Both films showed similar reversible behaviours over the treatments, but the film casted form DMF/THF (9/1, v/v) indicated a slightly quicker in contact angle change. The contact angle is presented by cosθ, thus higher value in this means lower water contact angle θ, for example, in image a), the water contact angle become very low after being in water, which is hydrophilic for 30 minutes, but it increases for being positioned in hydrophobic air and soars to about 90° after being annealed at 90 °C, and the water contact angle changes reversibly via altering its surrounding environment. This diagram is adapted from the reference [16].

4.2 Block Copolymers

In fact, similar surface reorganization also occurred in copolymers, Lukas et al. [13] investigated the surface reconstructions, in both dry and hydrated states, of random methacrylate copolymers, poly(2-hydroxyethyl methacrylate-co-butyl methacrylate) and poly{2-[2-(2-ethoxy)ethoxy]ethyl methacrylate-co-butyl methacrylate} by XPS. They found composition differences in the surfaces in both dry and hydrated states, which indicated the reorganization of the surface configuration.

In the family of copolymers, block and graft copolymers (the latter will be introduced in detail later) are the typical types of polymers which have surface responsive/adaptive properties. This is because different blocks can form various immiscible domains, which can segregate to the surface according to external stimuli. [1] Ratner et al. [14, 15] found the surface of poly(HEMA-block-polystyrene) was able to rearrange itself from a hydrated state back to a dry state under the condition of vacuum by XPS. They concluded that the movement of the PHEMA block to the surface in the hydrated state and the movement of the PS block to the surfaces in the dry state caused the rearrangement of the surface. Another PHEMA-based block copolymer was synthesised and investigated by Nakahama and co-workers. [16] They applied transmission electron microscopy (TEM), contact angle and angle-dependent XPS measurements to study the rearrangement of the surfaces of poly(HEMA-block-polyisoprene), of which the specimens were casted from DMF/THF and THF/methanol solvents. They found the surfaces reversible. As newly casted, the surface was mainly occupied by polyisoprene component, forming tiny areas of PI phase, when the surface was soaked in water for some time, it restructured to have more PVA component, and if the surface could be annealed, the proportion of PI component on the surface would increase again. 2 demonstrates this reversion of component of the surface in a direct way using TEM pictures, and 3 adopted the value changes of contact angle to indicate this reversion in a quantitative way.

4.3 Modified Surfaces

In order to make a surface to have such a responsive characteristic, the modification of polymer surface is usually an option. Whitesides and co-workers [17, 18] made an oxidatively functionalized polyethylene by treating the surface with chromic acid, which introduced hydrophilic carboxylic acid and ketone groups. They studied the wettability of the surface under different condition by measuring the contact angle, finding that the functionalized surface still showed a hydrophobic behaviour after annealing on heating under vacuum, which was believed to be the movement of the hydrophilic groups back into the bulk PE, and these groups could migrate back to surface by being heated in water as well.

More interestingly, a pH-dependent behaviour of the wettability of the functionally modified PE surface was reported by Randall Holmes-Farly et al. [19] When the pH was no more than 4, the surface demonstrated a relatively hydrophobic characteristic, which for higher pH, no less than 10, the surface became hydrophilic, the contact angle dropped from 55° in former situation to 20° in the latter one.

4.4 Heat-Responsive Surfaces

The surfaces do not only response to the surrounding solvent, but also react to the other external stimuli. Interestingly, some surfaces composed by polymers with a lower critical solution temperature (LCST), show adaptive/responsive behaviours towards temperature changes. The LCST is a characteristic temperature in a two-phase system, which is homogeneous when real temperature is above LCST, while the two-phase system segregates into two independent phases when the real temperature is below LCST. [20] Because of an easily accessible LCST point about 32°C, poly-n-isopropylacrylamide (PNIPAAM) was mostly studied. [21] Research [22] showed that the densely grafted and partially cross-linked polymer films made by PNIPAAM still preserve a property of the bulk materials, the LCST is still around 32°C. When the temperature of the system goes beyond the LCST at the particular composition, the polymer will be immiscible to water, which means hydrophobic.

4.5 Polyelectrolytes

Polyelectrolytes' surfaces also respond to surrounding environments, because of the high concentration of ionic groups in the polymer chain, external stimuli like pH, temperatur ionic concentration, solvent components, electric fields and chemical reactants can lead to a reversible response on the surface. [23] Ito and co-workers [24] demonstrated the pH-dependent behaviour of polyelectrolyte layers. They grafted poly(acrylic acid) onto a porous glass filter, and then measured the water-permeation under different pH-value solution. They observed that the water permeation was decreased under low-pH condition, and vice versa. This could be explained that in low-pH condition, the polyelectrolyte (PAA) was in a non-ionic state, thus the polymer chain shrunk into a coil like state and set enough space aside for water molecules to reach the pores on the glass. This pH-dependent behaviour was also observed in the mixed PAA/P2VP brush, which is a surface densely grafted with PAA and PAA-P2VP charged in different pH.gifP2VP chains. 4 [25] indicates the hydrophilicity by the swelling of the surface, when being in both low- and high-pH states, the swelling of the surface was much stronger than that of a medium-pH state. This layer showed a unique wettability to pH changes, the layer of the mixed brush was hydrophilic in both low- and high-pH states. 5 explains the reason of this unique trait of PAA/P2VP mixed brush, when the surface is involved in a low-pH condition, the nitrogen atoms in PAA are ionized or positively charged, and when the surface is immersed in a high-pH condition, the carboxyl groups in P2VP are anionized, thus these materials are polyelectrolytes in both low- and high-pH conditions and shows strong hydrophilicity, while when the pH value is in medium, both the PAA and P2VP cannot be ionized, the material is not a polyelectrolyte and shows relatively high hydrophobicity.

Another example of responsive brushes was presented by Howarter and Youngblood [26], they studied the rate of the surface rearrangement from the aspect of steric constraint, which, in this case, is the density of the polymer chains grafted on the substrate. They grafted polyethylene glycol with short perfluorinated end caps (f-PEGs) onto the surface pre-treated with isocyanated functionalized silane. Interestingly, they found that the polymer brush with lower f-PEG density could exhibit a more rapid response to external stimuli than the polymer brush with a higher graft density, because of the spatial confinement of the high-densely grafted polymer brush hindered the process of rearrangement.

5 GRAFT COPOLYMERS AS SWITCHABLE SURFACES

Another big group of polymers, graft copolymers, are also of great potential to be applied as switchable surface materials. A graft copolymer, as defined, is a kind of copolymer which contains a linear chain as the backbone with side branches attached. [27]A graft copolymer can be designed to have a hydrophilic backbone with hydrophobic branches, and vice versa, thus this allows the graft copolymer to respond to the external environment. Although graft copolymers have been widely applied in many fields, still the researches in their applications as switchable surfaces are a few.

Some of the amphiphilic graft copolymers have already found their potential applications. In recent years, amphiphilic graft copolymers were investigated as additives to optimize the hydrophilicity of porous membranes. [28, 29] It is believed that the hydrophilic chains of amphiphilic graft copolymers can segregate to the surface the membranes, enhancing the hydrophilicity so as to prevent the protein and other organisms, which severely reduce the flux of membranes, from adsorption. Zhu et al. [30] demonstrated this idea. They blended polyethersulfone (PES) with amphiphilic poly(styrene-alt-maleic anhydride)-graft-methoxyl poly(ethylene glycol) (SMA-g-MPEG), and investigated the improvement in surface hydrophilicity and anti-fouling ability by water contact angle and change in specimen weight with water uptake and protein absorption. They found that the water contact angle and the protein absorption of the surface significantly decreased due to the blend of the graft copolymer, conversely the water absorption was greatly enhanced. They explained that this hydrophilization of the surface derived from the preferentially segregation of the hydrophilic branches of the copolymer to the membrane surface.

For the details of the surface rearrangement, Tezuka and Araki [31] adopted XPS and contact angle measurements to study the surfaces of poly(vinyl alcohol)-polystyrene grafted copolymer with different PS side chain length and PS content. They found, in the dry state, the surfaces of the graft copolymers demonstrated the same wetting property as PS homopolymer, thus they argued that the PS component utterly covered the surfaces of the newly casted graft copolymer films. They reported in their previous paper [32] that, as bulk material, PS domains became small spherical phases dispersed in the continuous phase formed by PVA component, and during the film casting process, the PS component segregates to the surface to lower the interfacial energy with air.

6, atomic force micrography of the surface of PVA-PS graft copolymer with a PS content of 12.9 mol %. These images demonstrate the changes of the surface morphologies of the specimens immersed in water for a) 20, b) 425, and c) 1320 min. The “hole-with-rim” structures can be observed, and these structures increase slowing in their size, while significantly in their quantity. This is adapted from the reference [33].

After immersion in water, the films were found to show an appreciable decrease in water contact angle. Tezuka and Araki explained this process was that the surface restructured from hydrophobic PS to hydrophilic PVA. Having the idea that the surface was utterly covered by the PS component in the dry state, they thought that, after immersing into water, there should be a long-distance attraction between water and PVA component to induce the surface rearrangement. This was thought to be the case because water penetration was too slow to trigger such a rapid transformation on surface. But Tezuka et al. [33] modified their previous understanding of the surface arrangement by investigating the surfaces by atomic force microscopy (AFM).

7, Schematic picture of the response of the surface of PVA-PS graft copolymers to different environments (air and water). The upper area and the lower area show the rearrangement of the specimens with respectively a lower and higher PS content. This is adapted from the reference [33].

They examined the surfaces of PVA and PS homopolymers, and then tested PVA-g-PS copolymer surfaces in both the dry state and water. In the dry state, they found PS component was in the form of isolated spherical domains randomly dispersed in the PVA continuous phase for the specimens with 4.2 mol% PS content, while for the specimens with 26.5 mol% PS content, the surface demonstrated a “wormlike” morphology. For the specimens immersed in water, they observed a “hole-with-rim” morphology formed on the surface of the specimens with 12-23 mol% of PS content, see 6, and the number of the structure increased slowly and the size of them also remained almost the same with the prolonged time. They argued that this “hole-with-rim” structure formed because the uppermost PS domains ruptured themselves to reduce their interaction with water so as to reduce interfacial energy with water, while the PVA domains swelled, though still geometrically confined by PS domains, to cover all the surface of the “hole-with-rim” structures to maximize the contact with water, see 7. They also believed that there was a surface rearrangement in the specimens with higher PS content (26-41 mol %), because they noticed the formation of PS droplets on the surface within five minutes after the immersion these specimens into water.

Tezuka et al. [31] also investigated the surface mobility according to difference polystyrene content, length of the side chains and temperature. They observed that the surface mobility of the specimens with the same length of PS chains decreased with the increase of the polystyrene content at the same temperature, see 8, the air contact angles of the specimen with PS content of 9.8%, 29.1%, and 44.3% showed slight different in their increasing rate, and all of them reached above 160° after 120 minutes, only the air contact angle of the specimen with a PS content of 62.7% showed a significantly slower mobility and only increased to about 120° after 120 minutes. In terms of the influence of the length of the PS chains, see 9, when the PS contents of the specimens were at a similar level, the influence from the chain length was slightly different, and in general, the surface with the copolymer having longer PS side chains demonstrated a higher mobility. In addition, the influence of the temperature on the surface mobility is shown in 10, which depicts the variations of the air contact angle of the graft copolymer specimen at different temperatures form 10 °C to 57 °C, and only at the temperature of 10 °C the mobility of the surface was detected to be suppressed, and more interestingly this temperature was independent from the PS content and the PS chain length, in other words, the specimens with any PS content and PS chain length show very poor surface mobility at 10 °C. More interestingly in the temperature, this PVA-g-PS copolymer showed mobility at a temperature lower than the glass transition temperatures of both its components, which could be a consequence of the plasticization of PVA by water, and Tezuka et al. also explained that the mobility of PS components at the temperature low than its Tg was derived from that the molecular weight of the PS chains on the surface didn't reach a critical value of about 30000, above which PS chains starts to entangle.

Recently, Oh et al. [34] reported the switchability of the surface made of polypropylene-graft-poly(ethylene glycol) (PP-g-PEG). They characterized the surface switchability of the material via calculating its work of adhesion Wa, which can be depicted as the energy per unit area required to separate two phases, which originally form an interface. [35] The work of adhesion also can be approximately described by two components, as below:

(2)

Where Wad stands for the work of adhesion contributed by the London dispersion forces, and for most of the interactions this value is very similar, Waab represents the proportion offered by acid-base interactions, which is considered able to happen between Lewis acids and Lewis bases. [36] In this case, the interface was the polymer/solvents interfaces, and the acid-base interaction happened only between the PEG grafted chains and solvents, while the backbone was polypropylene which does not participate in the acid-base interaction. According to Dupré's equation of the work of adhesion for liquid-solid interface in the presence of vapour phase [36]:

(3)

And by introducing the equation (1), this expression can be converted into the form, as below:

(4)

In this case, contact angles were measured as raw data, which was computed into the work of adhesion by the equation (4). In their research, they tactfully adopted methylene iodide and water as solvents, of which the former one has no acid-based interaction, and the latter one has both dispersive and acid-base interaction. Measured with water, the work of adhesion Wa was constituted by both Wad and Waab, while the measurement with methylene iodide only presented Wad. Therefore, the Waab of a surface with water can be obtained simply by doing some subtracting. This technique is more subtle than conventional contact angle measurement, which only indicates the surface wettability in a microscopic way and reflects complex contributing factors. In the work of Oh et al, the concentration and its changes of the PEG component on the surface of the graft copolymer were demonstrated more precisely.

They studied the influence of the material used as the mould for film making, finding that the material with a higher work of adhesion Waab with water induced higher polarity on the film, which as they explained was caused by the segregation of the PEG component to the interface with the mould material to reduce the interfacial energy. They also studied the restructuring of the surface in non-polar and polar environments. Oh et al. also investigated the conversion of the surface. They annealed some specimens in vacuum at 90°C for different durations of time, and then found that the surfaces of the specimens had decreasing polarity over a prolonged time. This reflected a surface restructuring to a more hydrophobic state. When they immersed some specimens in water for 24 hours, they found the Waab increased significantly, and reporting it as the consequence of the accumulation of PEG component to the surface.

6 MATERIAL FOR FURTHER STUDY

In the present work, a new type of graft copolymer, (polyisoprene-graft-maleic anhydride)-graft-poly(ethylene glycol), will be studied. This polymer has been synthesised by grafting polyethylene glycol methyl ether (PEGME) onto a polyisoprene backbone containing grafted maleic anhydrides groups, see 11. The PI backbone is designed to have a weight average molecular weight of 25000, the PEGME branches are of an Mw of 2000, and in every 100 isoprene units, there is one maleic anhydride grafted. If all the maleic anhydrides are reacted with PEGME, the index l in the 11 equals to zero. Since the MAHs are randomly grafted on the PI backbone, PEGME side chains are also randomly located along the whole backbone. Interestingly, the glass transition temperature of this graft copolymer is much lower than the graft copolymers introduced in the previous part of this literature review, and the Tg of PI is -60 ~ -70 °C, which is greatly below the room temperature, therefore it is promising for this material to have a more rapid rate in conformational changes. Recently, a 11, the structure of (polyisoprene-graft-maleic anhydride)-graft-poly(ethylene glycol methyl ether)

big family of amphiphilic graft copolymers with low Tg backbones, including the material used in the present research, have been patented by a company name Revolymer. [37, 38] In one of these patents, the static contact angle value of several amphipilic graft copolymers against prolonged time has been studied, see 12, but the change in these static contact angle values were very slight, which indicated a very weak surface rearrangement of these amphipilic graft copolymer coatings. This insignificant phenomena might derive from the insensitivity of the static contact angle measurement, thus in order to study the conformational rearrangement of the amphiphilic graft copolymers, more parameters are required, therefore it is preferential to measure the advancing angle and the receding angle, and also calculate the hysteresis. However, these materials still haven't been applied to the topic of switchable surfaces. In the present research, the PEGME is studied for that the price of the material is relatively cheap, and it is also easy to make, moreover the application of this material as switchable surfaces will be interesting and also an important technology.

12, The static contact angle of a series of amphiphilic graft copolymers over a prolonged time. The value of the contact angle only had a slight change. IB = isobutylene, MA = maleic anhydride, O = octadecene, BA = butyl acrylate, VA = vinyl acetate, E = ethylene, and MPEG = poly(ethylene glycol) methyl ether. This is adapted from the reference [34].

7 CONCLUSION

This review mainly introduces the switchable surfaces made by polymers, which rearrange their conformations to response to external stimuli. Amphiphilic copolymers are very suited to make switchable surfaces, because of their nature of possessing both hydrophilic and hydrophobic components in one macromolecule, and when the amphiphilic copolymer surface is treated with hydrophobic solvents, the hydrophobic components of the copolymer restructure to the surface to increase the contact with the solvent to decrease the interfacial energy. Similar with the material for further study, the amphiphilic graft copolymer PP-g-PEG show apparent surface rearrangement according to solvent alternation, and this was precisely proven by work of adhesion contributed by the acid-base interaction. In the previous researches, most of the amphiphilic graft copolymers were synthesised with high glass transition temperature polymers as their backbones, but the graft copolymer like PVA-g-PEG also showed notable mobility at the temperatures lower than the Tgs of both components. The backbone of the material for future further study is of a very low Tg, thus it is rational to predict that this material may be able to show switchablity at a low temperature or response more rapidly at a relatively higher temperature. In terms of the methods for monitoring the surface reorganization, TEM and ATM are quite effective to directly observe the morphological changes of the surfaces, and water contact angle measurement and XPS are ways to indirectly characterize the rearrangement of the surface by measuring the changes of the interaction between water and the surface and the composition of a target component respectively. Especially, based on contact angle, measuring the work of acid-base interaction Waab of a component with the acid-base interaction copolymerized with a component without such an interaction is method quite precise and efficient, and it can be considered for the next step of this project.

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