Thermo-Switchable Pressure-Sensitive Adhesives Based on Poly(N

Thermo-Switchable Pressure-Sensitive Adhesives Based on Poly(N-vinyl ... D. I. Mendeleyev University of Chemical Technology of Russia, 9 Miusskaya Squ...
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Thermo-Switchable Pressure-Sensitive Adhesives Based on Poly(N‑vinyl caprolactam) Non-Covalently Cross-Linked by Poly(ethylene glycol) Mikhail M. Feldstein,†,‡,§,* Kermen A. Bovaldinova,§ Eugenia V. Bermesheva,∥ Alexander P. Moscalets,§ Elena E. Dormidontova,⊥ Valery Y. Grinberg,§ and Alexei R. Khokhlov†,§ †

Faculty of Physics, M. V. Lomonosov Moscow State University, Leninskie Gory, Moscow 119991, Russia D. I. Mendeleyev University of Chemical Technology of Russia, 9 Miusskaya Square, Moscow 125047, Russia § A. N. Nesmeyanov Institute of Organoelement Compounds (INEOS), Russian Academy of Sciences, 28 Vavilova Street, 119991 Moscow, Russia ∥ A. V. Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, 29 Leninsky Prospect, 119991 Moscow, Russia ⊥ Institute of Materials Science and Physics Department, University of Connecticut, 97 North Eagleville Road, Storrs, Connecticut 06269-3136, United States ‡

S Supporting Information *

ABSTRACT: The properties of new hydrophilic pressure-sensitive adhesives (PSA) obtained by blending poly(N-vinyl caprolactam) (PVCL) with short-molecular weight poly(ethylene glycol) (PEG) were studied in aqueous media by a combination of several calorimetric and adhesion testing techniques. We found that the adhesive properties of the blends are the result of an extensive hydrogen bonding network formed between PVCL and PEG similar to the poly(N-vinylpyrrolidone) (PVP)/PEG blends, except the extent of cross-linking is nearly 3 times higher in PVCL−PEG networks. Accordingly, we observed substantially higher peel adhesion in PVCL−PEG blends, which depends strongly on the amount of adsorbed water and the temperature. The adhesive properties of PVCL−PEG gels are considerably diminished when the amount of absorbed water exceeds 30% or at elevated temperature but can be easily recovered by drying or cooling the sample. The observed responsiveness of PVCL−PEG hydrogels in physiologically relevant temperature range makes them interesting candidates for industrial and biomedical applications as smart PSAs.

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the feature article by Kamperman and Synytska.2 Polymers which demonstrate low critical solution temperature (LCST) behavior in an aqueous environment, including polyacrylamides, polyvinylcaprolactone, poly(ethylene glycol)s, and polypeptides,3 have been employed to design reversibly switchable adhesives. In this case, reversible formation of hydrogen bonds is responsible for switching. At room temperature, for example, polyacrylamide chains form hydrogen bonds with surrounding water molecules and adhesion is poor. An increase of temperature induces a phase transition (LCST) and leads to formation of hydrogen bonds between neighboring polymer chains and an increase in adhesion.4 Hydrophilic adhesives are used in a wide variety of commercially significant products, particularly designed for medical applications. A general distinctive feature of hydrophilic

INTRODUCTION Pressure-sensitive adhesives (PSAs) constitute a special class of viscoelastic polymers that form strong adhesive joints with substrates of various chemical nature under application of slight external pressures (1−10 Pa) over very short periods of time (1−5 s).1 To be a PSA, a polymer should possess both high fluidity under applied bonding pressure, to form good adhesive contact, high-cohesive strength, and elasticity, which are necessary for resistance to debonding stresses and for dissipation of large mechanical energy at the stage of adhesive bond failure under detaching force. These generally conflicting properties are difficult to consolidate in a single polymer material. Despite evident progress in adhesion technology over the last few decades, it remains challenging to produce materials that are sticky on demand. Recent efforts in developing reversibly switchable adhesives, that exhibit the ability to trigger adhesion in response to environmental stimulipH, solvent, temperature, mechanics and electromagnetic fieldare reviewed in © XXXX American Chemical Society

Received: June 9, 2014 Revised: August 1, 2014

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PSAs is that they typically adhere to wet substrates, while conventional hydrophobic (rubber-based) PSAs typically lose their adhesive capability when moistened.5 A difficult challenge that arises in designing a skin contact PSA for the medical industry is to balance the needs for achieving high bond strength to skin along with the ease of removal from skin. A medical adhesive that could be optimized for performance on skin, but could be subsequently removed without the potential for skin damage and adhesive trauma, would be highly desirable. Our present work on poly(N-vinyl caprolactam)− (PVCL−) PEG pressure-sensitive adhesives contributes to this aim, as PVCL is a thermoresponsive and biocompatible polymer that possesses a LCST near physiological temperature. As discussed in the literature it is possible to produce pressure sensitive adhesive compositions based on poly(Nisopropylacrylamide) PNIPAM, which can be made to swell and deswell reacting to an outside stimulus, such as a temperature change or shift in pH.4 A PSA composition employed in that study was based on a series of NIPAM− acrylic acid or NIPAM and n-tert-butylacrylamide (NTBA) copolymers. Upon immersion into water at ambient temperature, the adhesive composition has swollen into a gel and has lost its pressure-sensitive adhesion properties. On consecutive immersion in water at 70 °C, the composition has regained its tack as most of the absorbed water is expelled. This procedure can be repeated several times, and the composition will always regain its tack on immersing above its LCST. To the best of our knowledge, none of thermoswitchable PSAs, described to date, loses the tack completely and reversibly in aqueous media under temperature elevation above the LCST. Thus, aside from its compelling scientific significance, the research described below is of explicit practical value. As was recently shown, the strength of PSA adhesives is controlled by a combination of diffusion, viscoelastic, and relaxation mechanisms.6 At the molecular level, strong adhesion is the result of a narrow balance between high-cohesive strength and large free volume. Individually, high-cohesive interaction energy and large free volume are necessary but insufficient prerequisites for PSA strength. In a series of recent studies Feldstein et al. have shown that blends of poly(N-vinylpyrrolidone) (PVP) and poly(ethylene glycol) (PEG) which individually do not possess adhesive properties, form strong adhesives in a narrow range of PVP−PEG composition, which is affected by relative humidity (RH).7,8 Although short-chain PEG fractions of Mw = 200−600 g/mol are completely miscible with PVP, higher molecular weight PEG fractions are found to be immiscible,9 suggesting a significant role played by PEG proton-donating terminal hydroxyl groups in interaction with PVP. Fourier transform infrared spectroscopy studies have shown that miscibility in PVP−PEG blends containing various amounts of absorbed water is due to hydrogen bonding of hydroxyl groups at both ends of PEG short chains (see Figure 1) to carbonyl groups in monomer units of PVP macromolecules.10 In the infrared spectrum, the stretching vibration of PEG hydroxyl groups appear in the region of 3600−3200 cm−1, whereas the bands of interest for PVP carbonyls range between 1720 and 1650 cm−1.11,12 The PVP−PEG blending leads to the shift in the hydrated PVP carbonyls peak from 1679 to 1655 cm−1 (Figure 3 in ref 12) and to the corresponding displacement of the PEG hydroxyls band from 3455 to 3332 cm−1 (Figure 4 in ref 12), indicating strong specific interaction between these groups (ΔH = 21.4 kJ/ mol). 10 The spectra exhibit strong evidence for the

Figure 1. Chemical structures of poly(N-vinylpyrrolidone), poly(Nvinyl caprolactam), and schematic presentation of the corresponding networks formed with poly(ethylene glycol) oligomer, PEG-400.

participation of water molecules bound to PVP carbonyls in PVP−PEG H-bonding. When both terminal groups of PEG are capped by inert methyl groups, the resulting dimethyl ether of PEG-400 (DMPEG) does not form H-bonded complexes with PVP and produces a crystalline phase upon cooling below its melting temperature.13 Quantum chemical calculations have demonstrated that the most stable and energetically favorable network complexes arise when both PEG terminal OH-groups form H-bonds with PVP carbonyls, acting simultaneously as comparatively long and flexible reversible noncovalent crosslinks and spacers between longer PVP macromolecules (Figure 1 left).6 The PVP−PEG PSAs have been prepared by simple mixing of two nonadhesive components, glassy high MW PVP and liquid, short chain PEG. This finding is of critical industrial importance. Until the present time, novel PSAs were typically produced by chemical synthesis or modification of initially tacky polymers. Now it is evident that innovative PSAs can be also obtained by physical mixing of nonadhesive polymers. Thus, insights gained into the molecular structures responsible for the occurrence of pressure-sensitive adhesion have opened the door to the molecular design of new PSAs with optimized performance properties by blending nonadhesive polymers bearing complementary functional groups capable of forming hydrogen or electrostatic bonds to one another. Taking into account that PVCL is a close homologue of PVP (containing respectively seven- and five-membered lactam rings in side-chains of their backbones, Figure 1), it is logical to expect that blends of PVCL with PEG will also exhibit adhesive properties. Thus, the aim of the present research is to investigate the adhesive properties of PVCL composites with oligomeric PEG and to analyze the influence of the LCST of PVCL on the adhesive properties of the composites. Comparative studies of the PVP and PVCL properties in aqueous solutions are relatively rare.14−19 Since the PVP is a much more hydrophilic polymer than PVCL, the former exhibits LCST around 170 °C that was evaluated theoretically from the data on the temperature relationship of PVP swelling degree in water15,16 and confirmed experimentally. Tager et al. have studied thermodynamics of PVCL mixing with water and obtained the corresponding phase diagrams.20 They have shown that hydrophobic PVCL hydration dominates at temperatures close to the binodal curve. As a result, the mutual PVCL mixing with water decreases under temperature elevation and phase separation occurs. Meessen et al.21 followed by many other authors14,22−29 have performed cloud point measurements in PVCL−water system and discussed effects of various factors such as solution concentration,30 B

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molecular weight31,32 and synthesis conditions17 on phase behavior. Many authors20,33,34 have evaluated aqueous PVCL solutions with the microcalorimetry technique. In general, it would be desirable to investigate the impact of hydrogen bonding between PVCL (or other hydrophilic thermoresponsive polymers) and PEG end-groups (or similar proton-donating polymers) on the LCST behavior of the blends in aqueous solutions. Even more important is to understand the general features of the performance of solid composite materials based on hydrophilic polymers demonstrating LCST behavior. The present research represents the first step in this direction. Comparison of the properties of PVCL−PEG blends with that of the well-studied PVP−PEG system will allow us to gain a deeper insight into the characteristic features of the PVCL−PEG composites in the solid state and in aqueous media.

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was used to avoid moisture condensation at the sensor. In the DSC apparatus the samples were first quench cooled with liquid nitrogen from ambient temperature to −100 °C over 2−3 min and then heated to 220 °C at a rate of 20 °C/min. In the DSC experiments the content of absorbed water in the blends was determined by weighing the samples before and after DSC scans using a Mettler Analytical Balance, AE 240, with an accuracy of ±0.01 mg. Weight loss of the sample after scanning was compared to the amount of desorbed water evaluated from the enthalpy change associated with water evaporation from the sample by DSC. Microcalorimetric Measurements. These measurements were performed using a differential adiabatic scanning microcalorimeter (DASM-4) (NPO Biophyzpribor, Pushchino, Russia) with an automatic system for data acquisition and processing. Measurements were performed under an excess pressure of 5.0 bar over the temperature range 2−130 °C. The employed heating rate of 1.0 Ko/ min is optimal for instruments of the DASM-4 type. The polymer concentration in the calorimetric samples was varied in the range 1.5− 10 mg/mL. The specific partial heat capacity of PNVCL was calculated, based on the calorimetric data using its specific partial volume (0.788 cm3/ g).33 For calculations of the transition excess heat capacity, the transition baseline was approximated by the Takashi−Sturtevant progress function as is earlier described.35 Output parameters of the calorimetric experiments were the transition temperature, Tt, specific enthalpy, and specific heat capacity increment.33 Peel Adhesion Testing. The adhesive joint strength of PVP−PEG and PVCL−PEG hydrogels was evaluated by 180° peel testing using an Instron 1221 Tensile Strength Tester at a peeling rate of 10 mm/ min. A Multiplex polyethyleneterephtalate (PET) film 12 μm in thickness (see Experimental Section) was employed as a standard substrate. The time to attain a maximum strength of adhesive contact with the substrate was 20 min. This dwell time on the PET substrate prior to peeling has been found to be sufficient to provide the maximum strength of the adhesive bond for all the polymer blends examined in this work. Probe Tack Adhesion. Adhesive properties were studied with probe tack tests using a TA.XT.plus texture analyzer from Stable Micro Systems (Goldalming, Surrey, U.K.) equipped with a thermal cabinet for measurements at temperatures above and below ambient. The probe tack test can be divided into two stages. The first stage is compression where the flat stainless steel probe of 4 mm diameter penetrates into the adhesive film at a constant rate of 0.1 mm/s and stops when compressive bonding stress achieves a value of 0.8 MPa. After 1 s of contact, the probe is withdrawn from the adhesive layer at a constant rate of 0.1 mm/s. All probe tack curves reported in this research have been corrected for the compliance of the probe tack tester (9.79 μm/N). The probe used in this test was a standard, cylindrical, polished stainless steel probe obtained from Stable Micro Systems. The probe was cleaned with acetone after each test. Such a cleaning procedure was adequate to obtain meaningful and reproducible results. Force vs time and displacement vs time curves were thus directly obtained from this test. For each experimental condition, we carried out three to ten probe tests. The specific stress− strain curves shown in this paper are representative of one of these individual tests while the mechanical parameters, such as the maximum stress, σmax, the maximum extension, εmax, and the practical work of adhesion W, defined as the area under probe tack curve, are average values.

EXPERIMENTAL SECTION

Materials. All chemicals are used as received from the suppliers, unless stated otherwise. Three grades of poly(vinyl caprolactam) (PVCL) have been employed in this research. High molecular weight (MW) PVCL Mw ≈ 2 500 000 (HMW PVCL), was synthesized by Prof. Y. E. Kirsh, at the L.Y. Karpov Institute of Physical Chemistry, Moscow, Russia, as earlier described.15 Low molecular weight PVCL 100 000 g/mol (PVCL BASF), was obtained from BASF, Germany, as Luviscol-Plus. The PVCL ISP, Mw = 118 000 g/mol, polydispertisity index Mw/Mn = 5, K value 46.4 was obtained from the International Specialty Products (ISP) Inc., currently Ashland, USA. Poly(Nvinylpyrrolidone) (PVP), Mw = 1 000 000 g/mol, and PEG-400 were supplied by BASF as Kollidon K-90 and Lutrol E-400, respectively. The contents of absorbed water in the parent polymer components and their blends were determined by thermogravimetry. Weight versus temperature curves of PVCL and PVCL−PEG blends were registered using TGA/DSC1 thermobalance (Mettler Toledo, Switzerland). The samples of 5−10 mg in weight were placed in aluminum oxide pans, 70 μL in volume, and heated with the heating rate of 10 °C/min within temperature range from 30 to 1000 °C in an argon atmosphere (gas flow 10 mL/min). Measurement accuracy was ±0.3 K for the temperature determination and 0.1 μg for the weight control. Solutions in ethyl alcohol were prepared by the dissolution of the polymer components for 12 h at room temperature. The PEG content varied from 30 to 60 wt %. Adhesive films (250−300 μm thick) were then prepared by the casting of the solutions onto microscope glass slides previously cleaned with ethyl alcohol for Probe Tack tests and on the Multiplex poly(ethylene terephtalate) (PET) backing film 12 μm in thickness (Vladimir film material plant, Russia) for Peel adhesion testing. The films were dried first at room temperature for 24 h and then for 2 h in vacuo at 65 °C. Unsupported adhesive films 700 μm in thickness were produced by casting the solution onto a PEBAX600 release linear (0.6 mm in thickness) and drying for 3 days at ambient temperature. A uniform thickness of polymer films was produced using a BYK-Gardner film casting knife. After the blend dried the release linear was removed and unsupported films were used in cloud point measurements. Cloud Point Determination. Cloud points of the polymer solutions and hydrogels at various temperatures were visually estimated as the temperatures and the compositions at which a sharp appearance of solution turbidity or gel opaqueness was observed. This coincided within 1 °C of the temperature at which the system became transparent upon cooling. Differential Scanning Calorimetry. The glass transition temperatures (Tg) were recorded based on the half-height position of the relevant heat capacity jumps in DSC heating thermograms using a Mettler TA 4000/DSC 30 thermoanalyser, calibrated with indium and gallium. All reported values are the average of replicate experiments varying less than 1−2%. Samples of 5−15 mg in weight were sealed in standard aluminum pans supplied with pierced lids so that absorbed moisture could evaporate upon heating. An argon purge (50 mL/min)

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RESULTS AND DISCUSSION Network Structure and Stoichiometry of PVCL−PEG Complexes. As discussed in the Introduction, pressure sensitive adhesives can be prepared simply by mixing of appropriate polymers, such as PVP or PVCL, which is of interest in our current study, with short-chain PEG. Despite the simplicity of the approach, the properties of the obtained material are strongly affected by blend composition, compatibility and related to the fractional free volume in the system. C

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involved in the H-bonded cross-links. Assuming that every OH group forms an H-bond only with a single PVCL or PVP carbonyl, this product yields the mole percent of repeat units of polymer cross-linked through PEG chains, M+H:13,37

Thus, measurements of glass transition (Tg) of the blend can provide valuable insights on system behavior and properties. Composition dependences of glass transition temperature in blends of PVCL (or PVP) with PEG-400 are illustrated in Figure 2. The lines show the expected Tg values calculated

MH+ =

w*PEG [OH] × × 100% wPEG [PVCL]

(2)

M+H,

shown in Figure 3 for PVCL−PEG and PVP−PEG blends, is remarkably independent of PEG content within a

Figure 3. Percentage of poly(N-vinyl lactams) (PNVL, i.e. PVP or PVCL) repeat units noncovalently cross-linked through PEG chains in hydrogen-bonded network complex. The data for PVP−PEG stoichiometric complex are adopted from refs 13 and 37. Error bars for PVCL−PEG complex are within symbol size.

Figure 2. Glass transition temperature of PVP and HMW PVCL blends with PEG-400 as a function of PEG weight fraction. Lines are results of the Fox equation. Points are the data of experimental measurements.

wide composition range, which is direct evidence of the nonequimolar stoichiometry of the PVP and PVCL complexes with PEG-400. Indeed, approximately 60% of the PVCL monomer units form H-bonded network junctions with PEG400 (Figure 3), while in PVP−PEG blends the amount of noncovalently cross-linked polymer repeat units is only about 20%,13,37 i.e., three times lower than in PVCL−PEG blends (see Figure 3). The accuracy of the stoichiometry evaluation is controlled by the precision of Tg measurement by DSC technique, which is within 5 °C. The negative Tg deviations from the values predicted by the Fox equation36 range between 54 to 98 °C and from 20 to 71 °C for the PVCL and PVP blends with the PEG400, respectively. The higher the PEG concentration, the smaller the negative Tg deviations. Adhesive Properties as a Function of Composition of PVCL−PEG Blends. Mixing the PVCL with PEG results in tacky, rubber-like blends which look like the PVP−PEG PSAs. The probe tack test is an illustrative and informative tool for characterizing adhesive joint strength and the types of adhesive bond failure and for gaining qualitative insight into relative contributions of solid-like and liquid-like debonding mechanisms. The probe tack test imitates the process of touching the surface of a PSA film with a finger and sensing the force required to detach it. When the contribution of the energy of intermolecular cohesion (Ec) dominates that of free volume ( f v), the probe tack stress−strain curve demonstrates a sharp maximum at rather low strains and a comparatively small area under the stress−strain curve that is defined as the practical work of adhesion (W, J/m2).6 Such type of debonding is typical of solid-like PSAs. Adhesive joint failure in this case proceeds

using the Fox equation,36 whereas the points represent the data of DSC measurements. As is seen, both systems exhibit large negative deviations of the measured Tg values from those predicted by the Fox equation. The negative Tg deviations for the PVCL−PEG blends are about 25−30 K greater than those for the PVP−PEG system at comparative contents of PEG-400 (Table S1 in the Supporting Information). As has been discussed in our earlier paper, large negative deviations of Tg from the Fox relationship observed in a single-phase blend can be related to the weight fraction of PEG-400 molecules forming hydrogen bonds with polymer through both terminal hydroxyl groups, w*PEG.37 The w*PEG quantity can be evaluated by fitting the data shown in Figure 2 by the Fox equation in the following modified form: wH2O w + w*PEG w 1 = PNVL + + PEG Tg Tg Tg Tg PNVL

H2O

PEG

(1)

where Tg refers to the glass transition temperatures and w to the weight fractions of poly(N-vinyl lactams) (PNVL. i.e. PVP or PVCL), water, and PEG respectively. As follows from the data shown in Figure 2 and Table S1 (Supporting Information), w*PEG is greater for PVCL−PEG mixtures than for PVP−PEG blends. With an increase of PEG content the w*PEG value tends to decrease implying that more extensive cross-linking occurs in the blends with a low PEG fraction. Taking the product of the ratios of the total number of PEG OH-groups per PNVL repeat unit ([OH]/[PNVL]) and the molar fraction of PEG chains cross-linking the PNVL units (w*PEG/wPEG), we obtain the mole percent of OH groups D

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like and liquid-like properties which are typical of viscoelastic PSAs. The HMW PVCL blend with 60% PEG is a tacky liquid. The impact of PEG concentration on the practical work of adhesion, W, and the maximum debonding stress of the PVCL−PEG PSAs, σmax, is shown in the inset of Figure 4. Both curves go through a maximum at 40 wt % of PEG in the blend. Because this blend demonstrates a transitional type of deformation from solid-like to more ductile, in following description we use PVCL blends with 45 wt % of PEG-400 as a model platform for development of thermo-switchable PSAs. Temperature Relationship of Probe Tack Adhesion. Besides the composition of the blend, temperature can also impact the adhesive properties, as it affects free volume and influences the degree of hydrogen bonding between the polymer and PEG. Figure 5 illustrates most typical probe

through interfacial crack propagation between the probe and the adhesive film surface and is called “adhesive debonding”. At the other extreme, when f v prevails, the probe tack curve is characteristic of fluid PSAs, which demonstrate comparatively low cohesion strength, indicated by a shallow peak of debonding stress, σ, coupled with a relatively high value of elongation, ε. In this case, the adhesive joint breaks by cohesive fracture in the bulk of the adhesive layer and the debonding process is governed by viscous flow. This type of debonding is called ‘‘cohesive debonding,’’ where some residues of adhesive are left on the probe at the end of the test. In between these two cases, when a high Ec is accompanied by large f v, the area under the probe tack curve achieves its maximum value. Debonding proceeds via cavitation and fibrillation of the adhesive layer, which is typical for PSAs with optimized adhesion. The curve shows a peak of debonding stress followed by a more or less pronounced plateau, and then a gradual or sharp decrease of detaching force to zero. Detachment in that case occurs at the interface between the probe and the adhesive layer, with no macroscopic residue left on the probe. While a gradual approach of the debonding curve to zero stress is a distinctive feature of cohesive failure and the break within the bulk of adhesive material, an abrupt fall of detaching stress is a characteristic feature of the adhesive type of debonding process.6 Probe tack curves of PVCL blends with various amounts of PEG-400 are presented in Figure 4. All PSAs demonstrate

Figure 5. Effect of temperature on probe tack adhesion of PVCL ISP blend with 45 wt % PEG. RH = 45−55%. The initial content of absorbed water is 3−5 wt %. Inset: Temperature dependence of the practical work of adhesion, W, and the maximum debonding stress, σmax, for the PVCL ISP blend with 45 wt % PEG.

tack curves for the PVCL blends with 45 wt % PEG-400 and 5% water obtained at different temperatures. The curve corresponding to 25 °C shows a symmetric peak which is typical for solid-like adhesives and the adhesive type of debonding (stress curve approaches zero more or less abruptly). At 35 °C, the probe tack profile of the same adhesive is typical of the PSAs and exhibits a cavitation peak followed by a well pronounced, continuous fibrillation plateau. The mechanism of adhesive joint failure in this instance is predominantly adhesive.6,38,39 The curve at 45 °C is characteristic of tacky liquids with the cohesive mechanism of debonding. The comparison of probe tack curves shown in Figures 4 and 5 demonstrates that the temperature elevation produces an effect similar to the addition of a plasticizer (PEG). As follows from the inset of Figure 5, the maximum of adhesion is observed at 35 °C. Upon the transition from a glassy polymer to viscous liquid, e.g., with a rise of molecular mobility induced by the increase of plasticizer content or temperature, the adhesion strength always passes through a maximum as a result of specific balance between two mutually conflicting properties: high energy of intermolecular cohesion and large free volume.6 Water is a good plasticizer for the PVCL−PEG blend. With an increase of the amount of absorbed water the cohesive strength tends to diminish, whereas the molecular mobility grows. In the case of the PVCL−PEG (45%) PSAs these opposing factors counterbalance each other at 35 °C.

Figure 4. Effect of PEG content on probe tack curves of HMW PVCL blends with PEG-400 at the ambient temperature 20−23 °C and the relative humidity of surrounding atmosphere RH = 45−55%. The weight percents of PEG in blends are indicated next to the curves. Inset: Dependence of the practical work of adhesion, W, and maximum debonding stress, σmax, on the PEG content in the blends with HMW PVCL.

dualistic behavior, combining the properties of liquid and solid elastic materials. Liquid like properties of the PSAs are necessary to wet the substrate surface when external bonding pressure is applied to form good adhesive contact. At the same time, solid-like behavior of the PSAs is required to make a strong adhesive bond and dissipate large amount of mechanical energy as a detaching force is applied. As follows from the probe tack curves shown in Figure 4, the PVCL blend containing 30% PEG-400 is solid-like and is in essence nontacky. The blend with 40% PEG is a typical solidlike adhesive, where contribution of cohesive strength dominates that of the free volume. Further increase of the PEG content to 45 and 50 wt % results in well-balanced solidE

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The error bars for the maximum debonding stress σmax are usually larger than that for the area under probe tack curve, defined as the practical work of adhesion W.39−42 As has been shown by Creton et al. for rubber-like triblock copolymers, the practical work of adhesion is a much more reliable property of an adhesive material than the actual peak stress. Indeed, the peak of the debonding stress is the boundary between linear and nonlinear tensile strain behaviors. The onset of the following plateau (at higher PEG content) on a probe tack curve signifies the transition to a supramolecular structure of the PSAs and a predominantly plastic deformation mode. Effects of Temperature and Water Absorption on Phase Separation in PVCL−PEG Hydrogels. The effects of temperature and solution composition on cloud point behavior in PVCL−water and PVCL−PEG−water system are demonstrated in Figure 6. The shape of the curve for the binary PVCL

heat capacity for the PVCL−PEG−H2O systems superimpose on one another and exhibit a peak at 37 °C. The onset of phase separation is observed at 33 °C, whereas no phase separation occurs at temperatures above 70 °C. Within this high temperature range both hydrogen bonding and hydrophobic interactions become rather weak and do not contribute to the enthalpy and entropy of the phase separation. In the connection with reported phase behavior the question arises: how does LCST affect the adhesion behavior of PVCL− PEG hydrogels in the course of their swelling (upon water addition) and temperature elevation? To answer this question we performed 180° peel tests, which enable the control of water content captured by the hydrogel in the course of heating if a water impermeable film is employed as a substrate. Temperature Relationship of Peel Adhesion. The effects of temperature and the amount of absorbed water on the 180° peel adhesion of the HMW PVCL with 45 wt % PEG400 are illustrated in Figure 7. As follows from these data, in the

Figure 6. Effects of temperature and the content of absorbed water on cloud point behavior of HMW PVCL and its blend with 45 wt % PEG400.

Figure 7. Temperature dependence of 180° peel adhesion force for the HMW PVCL−PEG (45 wt %) hydrogels containing 10, 20, and 30 wt % of absorbed water. Peel rate is 10 cm/min.

− water system is in good agreement with the literature data of Meeussen et al.21 The LCST is observed at 36 °C, and its position is shifted toward dilute solutions (10 wt % of polymer). The LCST is slightly higher than the value reported by Meeussen et al. (∼30 °C) for a somewhat similar molecular weight polymer, but agrees perfectly with the data by Kirsh, measured for the same HMW PVCL sample.15 With an increase of PVCL concentration the cloud point temperature climbs smoothly, achieving 60 °C at 26 wt % of water in polymer. In general, the behavior of ternary PVCL−PEG(45 wt %)− water system follows the pattern shown by the PVCL solution in water. At high water content (80−90%) the curves are practically superimposed. This behavior is as expected, because at such high concentrations of water, it forms H-bonds with both film-forming polymer (PVCL) and its oligomeric crosslinker (PEG), so the formation of a stoichiometric PVCL−PEG complex can hardly be possible. At higher PVCL concentrations the curves become parallel to each other, with the polymer−oligomer complex curve running 3−5 °C below that of pure PVCL. To be certain that PEG has no effect on the phase separation temperature of the PVCL in very dilute aqueous solutions, we have performed microcalorimetric studies of the solutions with various PVCL:PEG ratios. As follows from Figure S1 in the Supporting Information, the temperature profiles of the excess

temperature range from 20 to 90 °C, the PVCL−PEG hydrogels containing 10 and 20 wt % of water exhibit gradual reduction of adhesion with the increase in temperature. In contrast to this behavior, the hydrogels containing 30 wt % of water and more demonstrate the loss of adhesion in rather narrow temperature ranges. Thus, the PVCL blend with 45 wt % of PEG-400, containing 30 wt % of absorbed water, loses its adhesion sharply between 55 and 70 °C. The higher the content of absorbed water, the lower the temperature of spontaneous detachment of the adhesive film. Thus, the temperature behavior of adhesion in the PVCL−PEG hydrogels correlates fairly reasonably with the temperature dependence of the cloud point, shown in Figure 6. The temperature transitions of the mixing−demixing behavior and the change of adhesion, presented in Figures 6 and 7 respectively, are fully reversible (see recorded movie in Supporting Information). As an opaque detached adhesive film is removed from warm aqueous solution, it becomes transparent and tacky within 1−1.5 min as the result of both cooling and partial evaporation of absorbed water. The fact that the PVCL−PEG-400 blends containing 10 and 20 wt % of water demonstrate only a smooth decrease of adhesion with increasing temperature implies that the amount of absorbed water in these hydrogels is too low to inhibit adhesion. Indeed, the PVCL is a hydrophilic water-absorbing F

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temperatures 20−50 °C, are presented in Figure 8(top). The curve registered at 25 °C exhibits a mixed character, typical of the beginning of the transition from an adhesive to cohesive debonding mechanism that is evidenced by well-separated sharp peaks in the curve. With an increase of temperature up to 50 °C the adhesion decreases and becomes typical of cohesive failure. However, a further increase of temperature to 70 °C results in a radical transformation of the peel test run. The average peel adhesion increases again and the curve demonstrates a sawtooth, notched character, which is usually observed for peeling solid adhesives with 100% adhesive joint failure, when no visible remainder of adhesive material has been left on substrate surface. Such temperature behavior is absolutely unique and has been never observed and reported earlier. As one can recall, 70 °C is close to the critical solution (phase separation binodal) temperature of PVCL−PEG blend at 10% of adsorbed water (Figure 6), so the difference in the pattern of adhesive behavior is related to phase changes in the blend, i.e. dehydration of polymer. Figure 9 shows a photograph of the adhesive layer of the PVCL−PEG (45%) − H2O (20%) hydrogel upon peeling the

polymer and the fraction of absorbed water, toughly bound to polymer (via H-bonding within the first hydrate shell) is too low to induce demixing and initiate the process of detaching. As follows from Figure 7 and Figure S2 in the Supporting Information, in thermo-switchable PVCL−PEG PSA hydrogels the peel adhesion passes through a maximum at 20 wt % of water and is ordered as follows: PVCL−PEG + 20% water > PVCL−PEG + 10% water > PVCL−PEG + 30% water. The absolute values of peel force are extremely high, ranging from 2280 to 570 N/m at 20 °C. For comparison, the maximum value of peel adhesion for PVP−PEG blends was found to be 370 N/m. These data, obtained with HMW PVCL, are in good agreement with the results for LMW PVCL BASF−PEG complexes. The reasons for the higher peel adhesion of PVCL−PEG blends as compared to PVP−PEG PSAs can be explained in the following manner. Disregarding the interfacial interactions, adhesion strength is governed by the cohesive strength and fluidity of the adhesive material. As Figures 1 and 3 illustrate, the PVCL−PEG network is much denser than the PVP−PEG one, resulting in stronger adhesion. Nevertheless, a question remains: why is the peel adhesion of the PVP−PEG and PVCL−PEG PSAs so different whereas the probe tack adhesion is similar? In our opinion the answer resides in the fact that the major type of adhesive polymer deformation during the probe tack test is in extension,6 while under peeling the contribution of shear strain is more important.43 In support of this conjecture we consider the 180° peel force (from the PET substrate) versus displacement traces for the PVCL−PEG thermo-switchable PSA, presented in Figure 8. Representative 180° peel test runs for PVCL−PEG (45%) PSA hydrogels, containing 10% of absorbed water, at

Figure 9. Snapshot of PVCL ISP−PEG (45 wt %) adhesive hydrogels containing 10 wt % of absorbed water upon 180° peel test performed at temperature 70 °C.

PET film at 70 °C. The hydrogel surface displays regular wrinkles located in the places corresponding to the notches in the peel test run (Figure 8 bottom). Such wrinkles most likely arise from remarkably strong shear stress developed within the bulk of the hydrogel in the direction of peeling. This observed unusual phenomenon undoubtedly deserves further investigation. As is evident from above presented data, the adhesion switching temperature is controlled by the PVCL:PEG ratio and the amount of absorbed water. While the former value is easily controllable, the adjustment of the latter is a challenge due to hydrophilicity and high water-absorbing capability of PSA material. The larger the amount of water captured by the PVCL−PEG PSA hydrogel, the lower the temperature of phase separation (Figure 6) and thus the expected adhesion switching temperature. To exert control over the amount of absorbed water, we should take into our account that the sample of adhesive hydrogel during in the peel test consists of an adhesive layer that is supported by a backing film. The latter is a polymer

Figure 8. Typical load versus displacement traces of 180° peeling the PET film from PVCL ISP−PEG (45 wt %) hydrogels containing 10 wt % of water. Top: Temperature 20 and 50 °C. Bottom: Temperature 75 °C. G

dx.doi.org/10.1021/ma501191k | Macromolecules XXXX, XXX, XXX−XXX

Macromolecules

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PVCL−PEG and PVP−PEG aqueous blends, figures showing excess heat capacity as a function of temperature for PVCL− PEG blends and peel force as a function of the amount of absorbed water for PVCL−PEG hydrogels at temperatures 25, 50, and 70 °C, and a movie illustrating the reversibility of the adhesive properties of PVCL−PEG blends. This material is available free of charge via the Internet at http://pubs.acs.org.

membrane possessing a certain (easily measurable) permeability for water. Thus, the backing films of different permeability for liquid water can be employed as controlling elements over the water content in hydrogels.

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CONCLUSIONS In this paper, we show that similarly to its closest homologue, PVP, PVCL blends with oligomeric PEG form a rubber-like stoichiometric H-bonded network and possesses pressuresensitive adhesive properties. The network structure and phase behavior of the PVCL−PEG complexes are qualitatively similar to those of their PVP−PEG counterparts. However, the PVP− PEG PSAs in an aqueous environment demonstrate neither phase separation nor a switching temperature. In the stoichiometric PVCL−PEG complexes about 60% of the PVCL repeat units are cross-linked by hydrogen bonds through both terminal hydroxyl groups of PEG-400, whereas in PVP stoichiometric complexes with the same telechelic oligomer only 20% are cross-linked. These quantitative distinctions result in stronger peel adhesion of the PVCL−PEG PSAs as compared with that of PVP−PEG adhesives. The observed distinct features in the PVP−PEG and PVCL−PEG complex structure, stoichiometry and properties most likely originate from the additional contribution of sufficiently strong hydrophobic interactions in the latter, leading to a lower critical solution and adhesion switching temperatures for PVCL−PEG blends. In contrast to PVP, PVCL possesses a LCST in dilute aqueous solutions in the range of 32−37 °C. In concentrated aqueous solutions both pure PVCL and its blends with PEG exhibit rather similar behavior of phase separation (binodal) temperature. As the amount of absorbed water in PVCL−PEG hydrogels increases, the phase separation temperature gradually decreases. In moderately swollen PVCL−PEG hydrogels, containing 30−50% water, the phase separation occurs several degrees Celsius below than that for PVCL − H2O system. At the point of phase separation the PVCL−PEG hydrogels reversibly lose their adhesion and behave as thermo-switchable PSAs. The temperature of adhesion switching can be tuned within the range of 60−40 °C by an increase or decrease of the content of absorbed water. The comparison of PVP−PEG and PVCL−PEG stoichiometric complexes shows that their remarkable viscoelastic and adhesive properties is a common feature of the whole class of polyvinyl lactam or even polyvinylamide polymers. The PVP− PEG and PVCL−PEG blends are not unique examples of high molecular weight hydrophilic polymers and short-chain telechelics that form adhesive interpolymer complexes. As has been shown earlier, the PVP may be replaced by other polymers, bearing H-bonding capable recurring units. In particular, as has been recently demonstrated by Takemoto et al., poly(N-vinyl acetamide) forms H-bonded complexes with PEGs of varying molecular weight (from 200 to 600 g/mol) and with glycerol.44 These complexes have excellent compressive strength and can be used for biomaterial applications. In a similar manner, we believe that adhesives based on PVP−PEG and PVCL−PEG complexes have a great potential to become an important part of hydrophilic water-absorbing adhesive materials worldwide.

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AUTHOR INFORMATION

Corresponding Author

*(M.M.F.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was in part supported by the Ministry of Education and Science of the Russian Federation, State Contract No. 14.574.21.0073. We thank Prof. Y. Kirsh, Prof. A. Chalykh, and Dr. A. Shcherbina for the fruitful discussion of the research results. We also appreciate the stimulating interest of Dr. P. Heederik of the Avery Dennison Corporation in our research. We are grateful to Dr. O. Gerasimova and Dr. E. Baylis of the ISP (now Ashland Inc.) for their courtesy with the synthesis of PVCL on our request. Generous financial support of the International Innovation Nanocenter “Dubna” has been also acknowledged.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Table with values of measured glass transition values and values calculated based on the Fox equation including fitting for H

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