Hydrogels as mucoadhesive and bioadhesive materials: a review

Kinetics of polymer crystallisation

Publisher Summary This chapter provides an overview on the fibers from polymer blends. In fiber technology, two or more polymers may be incorporated into a product to obtain combinations of individual material characteristics or new ones by either of two routes, both of which are known as blending. The first is the older and well-developed concept of fiber–fiber blending in which conventional synthetic or natural fibers are mixed and are referred to as fiber blends. Fiber blending can be done in different ways. This chapter discusses an approach in which the polymer–polymer blends comprise the individual filaments that is known as the blend fibers. It is limited to synthetic fibers as the polymer blending occurs prior to fiber formation rather than in later steps of textile processing. The chapter also discusses the classification of blend fibers by phase morphology. The chapter further highlights the new products that lead to solving problems faced while using the old blend fiber. One polymer simply serves as a convenient fiber additive that has advantages over other potential additives because its high molecular weight makes it less prone to loss by evaporation or leaching and thereby more permanent. The chapter also discusses the improved hot–wet mechanical properties, flame retardance, and dyeability among other improved properties.

Fibers from Polymer Blends

Publisher Summary Most small-molecule organic liquids are mutually miscible and their mixtures do not form stable interfaces. Polymers are, however, usually immiscible and their mixtures form multiphase structures with stable interfaces. The dispersion, morphology, and adhesion of the component phases are affected by the interfacial energies, which thereby play an important role in determining the mechanical properties of a multiphase polymer blend. This chapter discusses interfacial energy, structure, and adhesion between polymers as related to the properties of polymer blends. It provides an overview of surface tension and interfacial tension, and discusses various theories of interfacial tension such as Antonoff's rule, theory of Good and Girifalco, theory of fractional polarity, and lattice theories. The interfacial tension varies more slowly with temperature than the surface tension and arises mainly from the disparity between the polarities of two phases. The extent of non-polar (dispersion) interaction between the two phases does not vary greatly from system to system but that of polar interactions can vary greatly. The polar interactions largely determine the magnitude of the interfacial tension. The polarity of a polymer can be calculated from the interfacial tension between the polymer and a non-polar polymer such as polyethylene, which can be regarded to be completely nonpolar. Thus, if the interfacial tension of a polar polymer against polyethylene is known, the polarity of the polar polymer can be calculated. Adhesion is the bonding or joining of dissimilar bodies, while autohesion or cohesion refers to joining of identical bodies. Several adhesion theories have been proposed. They have been a subject of much controversy. Most theories deal with the formation of the adhesive bond. The chapter reviews various factors affecting the formation and the fracture of adhesive bonds.

Interfacial Energy, Structure, and Adhesion between Polymers

Young’s modulus, yield (break) strength, and elongation to yield (break) have been measured for blends of poly(2,6‐dimethyl‐1,4‐phenylene oxide) (PPO) with polystyrene (PS), poly(p‐chlorostyrene) (PpClS), and random copolymers of styrene and p‐chlorostyrene (pClS). The significant difference between blend compositions is the compatibility of PPO with each styrene polymer. Blends of PPO with PS or copolymers with 67.1 mole% or less pClS are compatible (i.e., one Tg) and show small synergistic maxima in modulus, strength, and elongation as a function of PPO composition. These maxima correspond to observed maxima in packing density as a result of specific interactions contributing to blend compatibility. A rule of mixtures for one‐phase systems with an adjustable compatibility parameter gives adequate fit to the observed composition dependence of the modulus. In a narrow composition range between 67.8 and 68.6 mole% pClS, copolymers exhibit partial miscibility with PPO. Two mixed composition phases are present. Moduli of these transitional blends follow the same form of synergistic dependence on blend composition as do the compatible blends but strength and elongation exhibit a sigmoidal relation to blend PPO content. At about 20% PPO, strength (and elongation) reaches a minimum as predicted by a simple composite model for a dispersed phase with good adhesion to the matrix. A maximum is reached at ∼80% PPO at which composition blend test specimens yield prior to failure. Blends of PPO with PpClS and with copolymers of ≳68.6 mole% pClS exhibit a broader minimum in strength (and elongation) but a similar maximum at 80% PPO. Unlike the compatible and transitional blends, moduli follow a nonsynergistic composition dependence adequately represented by the series model for two‐phase systems.

Modeling of tensile properties of polymer blends: PPO/poly(styrene-co-p-chlorostyrene)

The thickness of the interphase layer as a function of time at 160°C has been determined directly in a microheterogeneous polymer mixture, and the layer thickness between two bonded sheets has also been measured by phase-contrast microscopy. Two mixtures of polymers, isotatic polypropylene with polyethylene and poly(vinyl chloride) with polyethylene, were prepared by mixing size-sorted powders in suspension, followed by evaporating the suspension medium and compacting the powdered mixtures. The mutual penetration initially obeyed Fick's second law; then diffusion was retarded and stopped after 15–20 min. An equilibrium state was reached which did not correspond to the classical thermodynamic equilibrium, at which the so-called equilibrium thickness of the interphase layer for the pair isotactic polypropylene-polyethylene was about 28,000 A and for the pair poly(vinyl chloride)-polyethylene was about 89,600 A. The equilibrium values can be used as a quantitative criterion for the compatibility of the polymer pairs.

Diffuse interphase layer in microheterogeneous polymer mixtures

The mutual diffusion of pairs of polymers has been studied at 160°C for two systems: isotactic polypropylene with polyethylene and poly(vinyl chloride) with polyethylene. From the assumption that Fick's law is valid and with a rhombohedral lattice model for the microheterogeneous system, it was possible to obtain from volume changes the thickness of the interphase layer after various diffusion times. It was found that the values so calculated differ considerably from the experimental values, the greatest deviations occuring after 20 min diffusion. From this it follows that non-Fickian diffusion strongly influences the distribution of concentrations in the diffuse interface, and thus affects the calculated value of the interphase layer thickness. For both polymer pairs Fickian behavior was confirmed for up to about 10 min diffusion time, in good agreement with experimental data.

Ján Letz

Papers

Diffuse interphase layer in microheterogeneous polymer mixtures

Thickness of the interphase layer determined from volume changes produced by mutual diffusion in binary microheterogeneous polymer mixtures