Mikhail B. Novikov

Pressure-sensitive adhesion in the blends of poly(N-vinyl pyrrolidone) and poly(ethylene glycol) of disparate chain lengths

Adhesive behavior in blends of high molecular weight poly(N-vinyl pyrrolidone (PVP) with a short-chain, liquid poly(ethylene glycol) (PEG) has been studied using a 180° peel test as a function of PVP-PEG composition and water vapor sorption. Hydrophilic pressure-sensitive adhesives are keenly needed in various fields of contemporary industry and medicine, and the PVP-PEG blends, pressure-sensitive adhesion has been established to appear within a narrow composition range, in the vicinity of 36 wt% PEG, and it is affected by the blend hydration. Both plasticizers, PEG and water, behave as tackifiers (enhancers of adhesion) in the blends with glassy PVP. However, PEP alone is shown to account for the occurrence of adhesion, and the tackifying effect of PEG is appreciably stronger than that of sorbed water. Blend hydration enhances adhesion for the systems that exhibit an apparently adhesive type of debonding from a standard substrate (at PEG content less than 36 wt%), but the same amounts of sorbed water are also capable of depressign adhesion in the PEG-overloaded blends, where a cohesive mechanism of adhesive joint failure is typical. The PVP-PEG blend with 36% PEG couples both the adhesive and cohesive mechanisms of bond rupture (i.e., the fibrillation of adhesive polymer under debonding force and predominantly adhesive locus of failure). Blend hydration effect on adhesion has been found to be reversible. The micromechanics of adhesive joint failure for PVP-PEG hydrogels involves the fibrillation of adhesive polymer, followed by fibrils stretching and fracturing as their elongation attains 1000-1500%. Peel force to rupture the adhesive bond of PVP-PEG blends increases with increasing size of the tensile deformation zone, increasing cohesive strength of the material, and increasing tensile compliance of the material, obeying the well-known Kaelble equation, derived originally for conventional rubbery pressure-sensitive adhesives. The major deformation mode upon peeling the PVP-PEG adhesive from a standard substrate is extension, and direct correlations have been established between the composition behaviour of peel strength and that of the total work of viscoelastic strain to break the PVP-PEG films under uniaxial drawing. As a result of strong interfacial interaction with the PET backing film, the PVP-PEG adhesive has a heterogeneous two-layer structure, where different layers demonstrate dissimilar adhesive characteristics.

Dynamic mechanical and tensile properties of poly(N-vinyl pyrrolidone)-poly(ethylene glycol) blends

Abstract Mechanical properties of miscible blends of high molecular weight poly(N-vinyl pyrrolidone) (PVP) with a short-chain, liquid poly(ethylene glycol) (PEG) of molecular weight 400 g/mol have been examined as a function of PVP–PEG composition and degree of hydration. The small-strain behavior in the linear elastic region has been evaluated with the dynamic mechanical analysis and compared with the viscoelastic behavior of PVP–PEG blends under large strains in the course of uniaxial drawing to fracture and under cyclic extension. A strong decoupling between the small-strain and the large strain properties of the blends has been observed, indicative of a pronounced deviation from rubber elasticity in the behavior of the blends. This deviation, also seen on tensile tests under fast drawing, is attributed to the peculiar phase behavior of the blends and the molecular mechanism of PVP–PEG interaction. Nevertheless, for the PVP blend with 36% PEG, under comparatively low extension rates, the reversible contribution to the total work of deformation up to e=300% has been found to be maximum at around 70%, while the blends containing 31 and 41% PEG behave rather as an elastic–plastic solid and a viscoelastic liquid, respectively.

Relaxation Properties of Pressure-sensitive Adhesives upon Withdrawal of Bonding Pressure

ABSTRACT Relaxation properties of pressure-sensitive adhesives (PSA) have been studied with the squeeze-recoil tester used in the regime of parallel-plate dilatometer under conditions imitating the removal of compressive force in the course of adhesive bond formation. The relaxation properties of PSAs are compared with their adhesive behavior measured using the 180-Deg Peel Test. Two classes of PSAs are considered: 1) conventional rubbery adhesives based on the mixtures of styrene-isoprene-styrene (SIS) block copolymer with a tackifier resin and a plasticizer, and butyl rubber plasticized with low-molecular-weight polyisobutylene, and 2) hydrophilic PSAs composed of the blends of high-molecular-weight poly(N-vinyl pyrrolidone) (PVP) with oligomeric polyethylene glycol (PEG). By comparing the adhesive and relaxation behaviors of different PSAs, the relaxation criteria for pressure-sensitive adhesion have been stated. Relaxation behavior of the examined PSAs demonstrates two values of retardation time: the shorter retardation time of 10–70 sec and the longer time of 300–660 sec. These times can be associated, respectively, with small- and large-scale mechanisms of strain recovery. By comparing the relaxation and adhesive properties of PVP-PEG blend (which involves the formation of a hydrogen-bonded network through both terminal hydroxyl groups in PEG short chains) with the properties of covalently crosslinked copolymers of vinyl pyrrolidone (VP) with PEG-diacrylate and comb-like VP copolymers with PEG-monomethacrylate, the contributions of covalent crosslinking and H-bonding network have been characterized.

A new class of pressure-sensitive adhesives based on interpolymer and polymer-oligomer complexes

On the basis of previous concepts concerning the molecular nature of pressure-sensitive adhesion, a simple method of preparing new adhesives with the desired mechanical and adhesive behavior and water-absorbability via mixing of nonadhesive polymers has been developed. Pressure-sensitive adhesion is related to the combination of a high energy of cohesion and a large free volume, which leads to a high molecular mobility. This method is based on the formation of interpolymer or polymer-oligomer complexes during mixing of macromolecules capable of hydrogen, electrostatic, or ionic bonding. In interpolymer complexes, a high cohesion results from the formation of bonds between macromolecules carrying complementary groups in main chains, whereas free volume is related to defectiveness of the resulting network and formation of loops. In complexes formed by a high-molecular-mass polymer and an oligomer carrying complementary reactive groups at ends of short chains, a high energy of cohesion is related to their interaction with mainchain functional groups of the polymer, whereas a relatively large free volume is associated with the length and flexibility of intermacromolecular crosslinks via oligomer chains. The adhesive and viscoelastic properties of adhesives and their water absorbability are regulated by changes in the composition of mixtures of a film-forming polymer with a polymer or oligomer crosslinker and a plasticizer. In this case, an increase in cohesive strength is achieved owing to an increase in the crosslinker concentration, while the enhancement of free volume is ensured by the increasing plasticizer content in the blend. Adhesive materials capable of adherence to wet substrates, hydroactivated adhesives, and adhesion moisture sorbents have been prepared for the first time.

Stress Relaxation During Bond Formation and Adhesion of Pressure-Sensitive Adhesives

Relaxation properties and adhesion of pressure-sensitive adhesives (PSAs) have been studied with the Probe Tack method under the conditions corresponding to the adhesive bond formation. Typical representatives of various PSA classes are examined: adhesives based on the styrene-isoprene-styrene (SIS) block copolymer, polyisobutylene of two molecular weights, acrylic and silicone PSAs. By comparison of the adhesive and relaxation behaviors of different PSAs it has been established that the PSA relaxation contributes appreciably to the strength of the adhesive bond and underlies the impact of contact time on adhesion. Direct correlation has been established between the compressive stress relaxation in the course of bond formation and the mechanism of debonding. All the examined PSAs can be classified into two groups: 1) the fluid-like PSAs that are capable of relaxing fully under compression (PIB, silicone adhesives) and 2) the PSAs, which reveal a residual unrelaxed stress. Physically crosslinked SIS and chemically crosslinked acrylic adhesives exemplify the PSAs of the second group. The occurrence of two peaks on the debonding stress–strain curves is typical of the PSAs of the second group. High adhesive strength requires the contribution of the longer relaxation times that vary for different PSAs in the range from 150 to 800 s. Minimum values of the longer relaxation times are featured for fluid adhesives, whereas the maximum values are found for crosslinked, network, and entangled adhesives. The adhesive strength achieves its maximum when the slow relaxation processes become dominating. Relative contributions of viscous and elastic deformations to relaxation properties of PSAs are assessed in terms of the Deborah number.

Tensile properties and adhesion of water-absorbing hydrogels based on ternary poly(N-vinyl pyrrolidone)/poly(ethylene glycol)/poly(methacrylic acid-co-ethylacrylate) blends

Rubber-like elasticity and pressure-sensitive adhesion of ternary blends of high-molecularweight poly(N-vinyl pyrrolidone) (PVP) with oligomeric poly(ethylene glycol) (PEG) of molecular weight 400 g/mol and a co-polymer of methacrylic acid with ethylacrylate (PMAA-co-EA) is due to formation of a interpolymer hydrogen bonded complex. In the ternary blend, PVP is present in a greater amount and acts as a film-forming polymer (FFP). As each short chain of PEG bears two terminal hydroxyl groups, which are capable of forming hydrogen bonds with the carbonyl groups in PVP repeat units, PEG behaves as a reversible carcass-like cross-linker (CLC) between long PVP macromolecules. In addition, the PMAA-co-EA forms a ladder-like interpolymer complex with PVP via hydrogen bonding of carboxyl groups and serves as a ladder-like cross-linker (LLC) of PVP. Adhesion behavior, mechanical properties and water-absorbing capacity of PVP/PEG/PMAA-co-EA blends are functions of blend composition. The CLC (PEG) endows pressure-sensitive adhesion to PVP blends, acting simultaneously as PVP plasticizer, as well as enhancer of cohesive strength. While the PVP/PEG inter-polymer complex is soluble in water, the LLC (PMAA-co-EA) provides insolubility and swellability, increasing further the cohesive strength of the blend composition. At high LLC concentrations, the blends lose their initial tack but become tacky upon water uptake. In this way, ternary PVP/PEG/PMAA-co-EA blends combine tack, typical of conventional hydrophobic pressure-sensitive adhesives, with the capability to adhere to highly hydrated substrates, typical of bioadhesives.

Temperature dependence of electro-optic effect and natural linear birefringence in quartz measured by low-coherence interferometry

Temperature dependencies of the half-wave voltage and natural linear birefringence in mechanically free quartz are measured at 1560 nm in the temperature range of 85-310 K. Measurements are carried out using a low-coherence interferometric scheme with a pair of identical quartz crystals independently linked with a scanning Michelson interferometer. Half-wave voltage V11 driven by an electro-optic coefficient r11 grows from 868 kV at 85 K to 923 kV at 310 K. The temperature derivative of natural linear birefringence Δn has nearly linear temperature dependence: ∂Δn/∂T=-7.260×10(-7)-9.93×10(-10)  T K(-1). Moderate temperature dependence of the electro-optic effect, along with other properties, makes quartz an appropriate sensing medium for electro-optic voltage sensors for the electric power industry.

RELAXATION TIMES FEATURED FOR POLYMERIC PRESSURE SENSITIVE ADHESIVES

Relaxation properties and adhesion of pressure‐sensitive adhesives (PSA) have been studied with the Probe Tack and Squeeze Recoil methods under the conditions corresponding to the adhesive bond formation under compressive stress and upon the removal of bonding pressure. Direct correlation has been established between the compressive stress relaxation in the course of adhesive bond formation and the mechanism of debonding. High adhesive strength requires the contribution of the longer relaxation times that vary for different PSAs in the range from 150 to 800 seconds. Relative contributions of viscous end elastic deformations into relaxation properties of pressure sensitive adhesives are assessed in the terms of Deborah number. At the stage of adhesive polymer relaxation after withdrawing of bonding pressure, the relaxation properties have been characterized in terms of retardation times. By comparison of the adhesive and relaxation behaviors of different PSAs, the relaxation criteria for pressure‐sensitive...