H. P. Schreiber

Aspects of acid-base interactions and use of inverse gas chromatography

Abstract Acid-base forces are known to contribute significantly to interactions in macromolecular systems. The evolution of acid-base concepts is traced from their early stages, as represented by the work of Lewis, Bronsted and others, to their current, complex state, with particular reference to their application to polymers. The requirement to express acid-base interactions quantitatively draws attention to the techniques of inverse gas chromatography. The ability of this technique to furnish information on acid-base interaction potentials of polymer surfaces is considered critically, as is its use in evaluations of polymer surface energetics. The large and rapidly expanding amount of literature on the capability of acid-base interactions to rationalize various industrially important properties of complex polymer systems has not been reviewed in detail. In this paper, a series of illustrations is used to indicate the power of acid-base concepts in effecting this rationalization. These illustrations suggest the need to take into account acid-base interactions if polymer systems are to be formulated, so as to satisfy steadily increasing demands on their performance and durability.

Advances in basic and applied aspects of microwave plasma polymerization

Abstract Microwave plasma polymerization of organosilicone compounds has several features of basic as well as applied importance. One of these is the attainment of high deposition rates, in the range of several hundred angstroms per second. Another is the ability to “tailor” (that is, to vary predictably and reproducibly) film structure and properties by controlled variation of substrate temperature T s or power density P in the plasma or both: it is shown that the composition of films can be controllably shifted from primarily “organic” characteristics, obtained at low T s or P , to predominantly “inorganic” character at high T s or P . Together with chemical composition, morphological and physicochemical properties also undergo substantial changes: films deposited at high T s or P are free of defects and adhere strongly to their substrate; they are dense, have low permeability to gases or vapours and display a high optical refractive index. The potential applied value of organosilicone plasma polymers is illustrated by two examples: the inhibition of corrosion on coated metal surfaces exposed to aggressive media and the surface modification of aromatic polyamide fibres, used to reinforce polymer-based composites. Apparatus is being developed for pilot- or industrial-scale implementation of microwave plasma processes.

Transesterification, morphology and some mechanical properties of thermotropic liquid crystal polymers/polycarbonate blends

Abstract Blends of polycarbonate (PC) and thermotropic liquid crystal polymer Vectra-A (VA), with up to 20 wt.% VA were prepared at temperatures up to 320°C. Transesterification (TE) was found to take place notably at the highest blending temperature. At this temperature, it increased linearly with the VA content. TE products, located at PC/VA interfaces led to reductions in the size of dispersed phase droplets, as shown by scanning electron microscopy. The inference of increased compatibility was consistent with improvements in mechanical property parameters of the blends. These also showed that mixing time was a factor in defining interfacial states. Acid–base interaction data gave further evidence of the presence of TE products, notably when blending occurred at 320°C, and suggested that ∼10 wt.% of VA was needed to saturate interfaces with PC. The absolute values of acid and base interaction constants were found to be very low, confirming earlier evidence that near 320°C dispersion forces are dominant at PV/VA interfaces, contributing to enhanced compatibility in blends prepared at these high temperatures.

Surface contributions to the two-layer structure in the plasma deposition of a-Si : H

Abstract Recent studies in several laboratories have shown that some thicknessdependent physical properties of a-Si : H may be modelled on a thin, constant-thickness surface layer adjoining a variable-thickness bulk layer. Water permeation, Auger electron spectroscopy and elastic-recoil detection results from our laboratory all point to contamination of the a-Si : H surface as a source of the thin layer.

Polymer diffusion and the evolution of adhesive bond strength

An attempt has been made to establish the role of diffusion across the polymer interface as a contributing factor to lap-shear bond strength. The assemblies studied were polypropylene/linear low-density polyethylene (PP/LLDPE); polystyrene/polyvinyl chloride (PS/PVC); polystyrene/polymethyl methacrylate (PS/PMMA); and polyvinyl chloride/polyvinylidene fluoride (PVC/PVDF). Initial bond strength measurements were followed by measurements on joints that had been annealed for up to 72 h at temperatures in the range of 60-160°C. Following induction times of tens of minutes where bond characteristics remained constant, substantial increments in bond strength were observed for PP/LLDPE and for PS/PVC but not for PS/PMMA or PVC/PVDF. Results point to a significant contribution to bond strength arising from diffusion when dispersion forces and favorable acid/base interactions act at the interface. The times required to establish the apparent diffusion effects far exceed normal bonding times and may account for the failure of diffusion to be recognized as a significant mechanism in the formation and maintenance of adhesive bonds.

Surface modification strategies for multicomponent polymer systems,VI:acid–base interactions as a strategy for interfacial modification in immiscible polymer blends

Abstract Three copolymers with different emulsification efficacies were used as potential interface modifiers for an immiscible blend of linear low density polyethylene (LLDPE) and poly(vinyl)chloride (PVC). The specific interaction between blend components and the modifying copolymers was determined by measurements of interfacial tension and by inverse gas chromatographic (IGC) data, characterizing the acid–base properties of the polymers. Additional information on modifier effectiveness was obtained from blend morphology data. Acid–base pair interaction parameters, computed from IGC results, predicted the modifying potential of the two copolymers which emulsified the system, and were consistent with the morphology and interfacial tension results. IGC, however, did not distinguish between the emulsification qualities of the two good modifiers. The best of the three modifiers (P4VP-PIP) also increased the impact strength of the modified LLDPE/PVC system. The results indicate that specific (acid–base) interactions at component interfaces may represent a promising strategy for the choice of emulsifying agents for some immiscible polymer blends.

Adhesion at partially restructured polymer surfaces

Abstract The controlled modification of the surface structure of styrene-4-vinyl pyridene (SVP) and of ethylene-acrylic acid (EAA) copolymers was achieved by the timed exposure of the polymers to non-orienting (n-alkane) and orienting (water) fluids. The degree of surface modification was monitored by periodic measurements of non-dispersive surface energies from static contact angle data. Assemblies of rigid PVC/SVP, of poly(methyl methacrylate) (PMMA)/EAA and of polypropylene (PP)/EAA were prepared with the SVP and EAA in various stages of restructuring. Bond characteristics of these assemblies were evaluated from single lap-shear measurements. Bond strengths responded strongly to the orientational states of the copolymer adhesives. In PVC/SVP and PMMA/EAA, significant increases in bond strength were observed when orientational changes raised the nondispersive contribution, γ nd , to the surface energies of these copolymers. Surprisingly, full increments in bond strengths were realized at only about 25–40% of the total increment in γ nd . Surface restructuring had little effect on adhesion with the non-polar PP substrate. Residual bond strengths, following controlled aging, varied directly with the initial bond strength. The residual bond strengths may be used to specify ideal restructuring events in the adhesives which would render the assemblies fully resistant to the chosen aging process. Acid/base interaction concepts have been used to rationalize the observations. These suggest that only relatively few polar elements need to migrate into the surface region of the adhesives to maximize initial bond strength, but larger numbers are needed to ensure adequate resistance to aging.

Specific interactions and the rheology of pigmented polymer solutions

Rheological properties have been measured for solutions of polyesters containing quantities of inorganic and organic pigments. Complex behavior patterns were simplified by computing the effective volume fraction of dispersed solids, taking in account the thickness of polymer layers adsorbed on pigment surfaces. The dimension of interphases created by the adsorbed polymers was rationalized by applying principles of specific interaction between polymers and pigment surfaces. The parameters of specific interaction were obtained from inverse gas chromatographic data. Pigment sedimentation data were used to identify a range of specific interactions best suited to stabilize dispersions of the solids.

Component interactions and the stability of some pigment/polymer dispersions

The sedimentation behavior of rutile and selected organic pigments dispersed in polymer solutions has been interpreted in terms of acid-base interactions at the polymer/pigment interface. The acid-base information was obtained from inverse gas chromatography. Moderate strengths of acid-base interaction were beneficial to the stability of dispersions, whereas both the absence of and excessive interaction resulted in the rapid deposition of solids. The effect is associated with the orientation of adsorbed polymer molecules, which must provide an adequate steric (entropic) barrier for the aggregation and deposition of solids. Implicated is the balance of interaction forces at pigment/polymer and polymer/solvent contacts. Rheological data for the dispersions have been used to obtain a parameter related to the thickness of the adsorbed polymer layer, by way of amplifying on the state of the pigment/polymer interface.

Surface modification strategies for multicomponent polymer systems. V. Interaction states and mechanical properties of LLDPE/PVC blends with corona-modified rutile pigment

Rutile pigment with preadsorbed monomers of acrylamide (AM) or acrylic acid (AA) has been treated in air corona discharges at various input power levels for times from 30–120 s. Inverse gas chromatographic data showed that the treatments reduced dispersive surface energies and significantly altered the acid/base interaction potential of the surfaces. Inferred is the corona-activated synthesis of oligomeric or polymeric structures anchored to the pigment surface. XPS analyses report modifications in the chemical structure of pigment surfaces, which are consistent with the suggested consequence of corona treatment. When incorporated into LLDPE and PVC host polymers, compounds with the corona-modified rutiles have better mechanical properties than analogues with untreated pigment, notable being improved elastic moduli, yield stresses, and stress/strain relationships at break. AM-modified rutiles were preferable to AA-modified versions in this regard. The addition of treated pigments to immiscible LLDPE/PVC (75/25) blends resulted in similar benefits to mechanical properties, AM pretreatment again being preferred. Stronger acid-base interaction at contacts between corona-modified rutiles and the PVC component is an apparent reason for improved mechanical properties. Speculatively also, AM pretreatments lead to attached chains of sufficient length to entangle with the LLDPE, further strengthening the interphase and relevant bulk properties.