H. R. Brown

Macroscopic Evidence of the Effect of Interfacial Slippage on Adhesion

The adhesion strengths of a viscoelastic adhesive were measured on various substrates that were prepared by grafting silanes bearing organic functional groups to silicon wafers. Conventional theories predict that adhesion should be proportional to the surface free energy of the substrate; but adhesion on a fluorocarbon surface was significantly greater than on some of the hydrocarbon surfaces, although the fluorocarbon surface has the lowest surface free energy. This result could be explained by invoking a model of adhesion based on the slippage of the adhesive at the interface.

Mixed-mode effects on the toughness of polymer interfaces

Normal, symmetric fracture toughness tests can give high values for the toughness of the joint between the immiscible polymers polystyrene and polymethylmethacrylate. These high values, which are caused by crazes growing away from the interface into the polymer with lower craze resistance, are not a fair characterization of the toughness of the joint. Much lower, and more realistic, toughness values are obtained by the use of an asymmetric test that tends to drive the crack and crazes more along the interface.

Spreading characteristics of thin liquid films of perfluoropolyalkylethers on solid surfaces. Effects of chain-end functionality and humidity

Spreading characteristics of thin liquid films of perfluoropolyalkyl ethers (PFPE) on silica surfaces and thermally bonded PFPE surfaces have been measured by scanning microellipsometer as function of molecular weight, chain-end functionality, and humidity. The effects of molecular weight are much smaller than those of chain-end functionality. Changes in spreading profiles with time show that interactions of chain-end groups with solid surfaces have dramatic effects on the structure and mobility of thin polymer films on solid surfaces. Wetting autophobicity is clearly manifested by PFPEs with functional chain-end groups on silica surfaces.

Modes of deformation in rubber-modified thermoplastics during tensile impact

Real-time small-angle X-ray scattering (RTSAXS) studies were performed on a series of rubber-modified thermoplastics. Scattering patterns were measured at successive time intervals as short as 1.8 ms and were analysed to determine the plastic strain due to crazing. Simultaneous measurements of the absorption of the primary beam by the sample allowed the total plastic strain to be computed. The plastic strain due to other deformation mechanisms, e.g. particle cavitation and macroscopic shear deformation was determined by the difference. Samples of commercial thicknesses can be studied at high rates of deformation without the inherent limitations of microscopy and its requirement of thin samples (i.e., plane strain constraint is maintained on sample morphology).Contrary to the conclusions drawn from many previous dilatation-based studies, it has been demonstrated that the strain due to non-crazing mechanisms, such as rubber particle cavitation, and deformation of the glassy ligaments between rubber particles, occurs before that due to crazing mechanisms. Crazing accounts for at most only half of the total plastic strain in HIPS (high impact polystyrene) and ABS (rubber-modified styrene-acrylonitrile copolymer) materials. The proportion of strain attributable to crazing can be much less than half the total in thermoplastic systems with considerable shear yield during plastic deformation.The predominant deformation mechanism in polycarbonate-ABS blends is shear in the PC (polycarbonate) with associated rubber gel particle cavitation in the ABS. This cavitation means that there appears to be a direct relationship between gel particle rubber content in the ABS and toughness of the blend. The mechanism is the same whether the tensile stress is in the direction parallel or perpendicular to the injection-moulded orientation, with simply less total strain being reached before fracture in the weaker perpendicular direction. Crazing, although the precursor to final fracture, occurs after the predominant mechanism and contributes only a few per cent to the total plastic deformation.

Diffusion and self-adhesion of the polyimide PMDA-ODA

Abstract The relationship between the interdiffusion of two layers of the polyimide PMDA-ODA and the adhesion between the two layers has been examined. The polyimide layers were made by successively depositing a polyamic acid layer from solution and then curing the layers at elevated temperature to the polyimide. Diffusion occurred during the curing process of the second layer and was controlled by the cure schedule. The interdiffusion was measured using forward recoil spectrometry (FRES) and the adhesion measured by peel tests. Good correlation was found between the interdiffusion distance and the adhesion. It was found that a large diffusion distance, at least 200 nm, was required to obtain a bond whose strength was equal to that of bulk material.

Chain pullout and mobility effects in friction and lubrication.

The interfacial shear stress that occurs when a network of a polymer that is highly mobile at the segment level (an elastomer) is slid over a smooth surface of an immobile (glassy) polymer has been measured. The glassy material is covered by a thin layer of end-attached chains of the mobile material. The experiment was designed so that there were no free chains at the interface; the slip occurred between network chains on the one side and rigid material plus end-attached mobile chains on the other side. Two main results were obtained. (i) The interfacial shear stress is strongly affected by the segment mobility of the materials on both sides of the slip plane, and considerably lower stress is observed when the materials on both sides of the interface are highly mobile. (ii) Very thin layers of tethered chains can increase the interfacial friction. Both results are relevant to the understanding of a number of practical situations that range from the operation of thin layers of lubricants, such as those found in magnetic storage devices, to the problem of wall slip and melt fracture in polymer processing.

Creating Smart Polymer Surfaces with Selective Adhesion Properties

Abstract A new concept for polymer surface modification is described that employs surface-active ω-functional block copolymers as additives to create polymers with smart sufaces. The block copolymers are composed of three components: a low surface energy block that causes the copolymer to segregate to the surface of the matrix homopolymer to which it is added, an anchor block that tethers the copolymer into that matrix, and a functional group located at the terminus of the surface-active block. The functional end group is selected to interact selectively with a complementary receptor on the target substrate. When the modified polymer surface and a substrate are placed in contact, adhesion is enhanced only if the functional end group senses an appropriate receptor on that substrate with which it can form the specific interaction. If a receptor is not present, the modified surface exhibits release properties. This class of copolymer additives can thereby be employed to create smart polymer surfaces with sel...

Impact-modified epoxy resin with glassy second component

A resorcinol-based epoxy resin was modified by incorporating a glassy second component. The mixture showed a heterogeneous morphology with two clearly defined phases, one phase rich in oligomer, the other phase composed mainly of resorcinol epoxy resin. The fracture toughness measured asG1c andK1c values showed an increase from 174J m−2 and 0.89 MN m−1.5 S in pure epoxy resin to 431 J m−2 and 1.36 MN m−1.5 in 30% oligomer modified resins. The scanning electron micrographs showed that the oligomer-rich phase exhibited ductile failure behaviour and formed the dispersed phase at low concentrations while it was the continuous matrix when the concentration was 30%. Optical observations on the failure mode of thin films of the oligomer-modified epoxy resin showed the existence of both inter face failure and considerable distortion in both phase.

Adhesion between polymers

This paper is concerned with recent work that relates to the adhesion between nonreacting polymers. Advances in the under- standing of cracks at bimaterial interfaces are considered, with particular emphasis on their implications in the interpretation of adhesion tests. An interpretation of the peel and blister tests is then discussed. Consideration is given to mechanisms of polymer failure as they relate to adhesion, with an emphasis on the distinctions between the properties of glassy and elastomeric materials. Polymer self-adhesion and its relation to interdiffusion are reviewed and compared with the adhesion of miscible polymers. In considering the adhesion between immiscible polymers, emphasis is given to the use of copolymers as coupling agents at the interface.

Effect of a polystyrene-polyisoprene diblock layer on the adhesion between polystyrene and polyisoprene

Abstract The adhesion between polystyrene and crosslinked polyisoprene homopolymers was found to be increased by the presence of a thin layer of polystyrene-polyisoprene block copolymer at the interface. The interfacial toughness was a function of both the layer thickness and molecular weight of the copolymer. If the homopolyisoprene was crosslinked before the joint was made, the joint failed at a relatively low toughness by the polyisoprene of the diblock pulling out of the polyisoprene homopolymer. Joints that failed by pull-out could be reformed by contact at room temperature. A high degree of interfacial toughness was found when the homopolymer and copolymer polyisoprene were crosslinked together.