Amorphous and semicrystalline thermoplastics, and their melts

Schematic illustration.  Typical amorphous random coil (left) and semicrystalline (right) morphologies of thermoplastic polymers.  Atactic polystyrene and bisphenol-A polycarbonate are examples of amorphous thermoplastics.  High-density polyethylene and poly(ethylene terephthalate) are examples of semicrystalline thermoplastics.  The amorphous and crystalline domains are connected to each other by covalent chemical bonds in a semicrystalline polymer so that a chain may traverse both amorphous and crystalline regions.  Chain segments linking two crystalline domains are often called "tie chains".  The boundaries between amorphous and crystalline domains are usually diffuse.  They are sometimes called "interphase" regions.

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Conventional (thermoset) rubbers/elastomers

Covalent crosslinks provide a three-dimensional elastic network with "junctions".  When the specimen is heated, these junctions survive until their thermal or thermooxidative degradation temperatures are reached.  The material is hence a "thermoset", in the sense that once its network structure is formed it can no longer be processed by using melt processing techniques.  However, it is also an elastomer (a "rubbery" material) because its glass transition temperature is low and so it has a low stiffness (low elastic moduli, and a lot of "bounce") at typical end use temperatures.

Schematic illustration.  The chains of natural rubber (cis-polyisoprene) are crosslinked slightly with sulfur (-S-) linkages by using the "vulcanization" process.  There are many other types of elastomers as well, differing from this example by the use of different polymer structures, crosslink types, and preparation processes.  

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Thermoplastic elastomers

A three-dimensional elastic network is provided by "physical crosslinks", such as crystalline and/or rigid glassy amorphous domains in a soft amorphous matrix.  Unlike conventional (thermoset) elastomers, thermoplastic elastomers can be processed by using melt processing techniques, since their crystalline and/or rigid glassy amorphous domains can be melted during processing and can then reestablish themselves upon cooling.

Schematic illustration.  Evolution of thermodynamic equilibrium morphology of a typical AB-diblock copolymer [such as poly(styrene-block-butadiene)] with immiscible A and B components as the volume fraction of the B component (black) is changed.  From left to right: spheres of B domains in a matrix of A, cylinders of B domains in a matrix of A, ordered bicontinuous gyroid, lamellar bicontinuous, ordered bicontinuous gyroid, cylinders of A domains in a matrix of B, and spheres of A domains in a matrix of B.

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Rigid thermosets

These materials are, in general, amorphous polymers where covalent crosslinks provide a three-dimensional network.  However, unlike conventional (thermoset) elastomers, rigid thermosets are stiff (have high elastic moduli) at their use temperatures (and often up to much higher temperatures) because their particular combinations of chain stiffness and crosslink density characteristics result in a high glass transition temperature.  The most familiar examples are the cured epoxy thermosets.

Schematic illustration.  Note the similarity to the example of a conventional (thermoset) elastomeric network that was shown earlier.  The main practical difference between these two classes of polymers is that rigid thermosets have much higher glass transition temperatures than conventional (thermoset) elastomers.

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Gels

A gel is a semi-solid jelly-like state of a material, similar to gelatin in its consistency.  There are many different types of polymeric gels.  For example, during the synthesis of a crosslinked thermoset from liquid oligomer and curing agent reactants, it increases in stiffness (modulus) rapidly when the "gel point" is reached so that a three-dimensionally percolating network of crosslinks is established.  On the other hand, a covalently or physically crosslinked "solid" polymer can form a gel by absorbing a sufficient amount of an appropriately chosen solvent.  A hydrogel is a gel where water is the solvent.   A smart gel or stimuli-responsive gel (such as certain synthetic polyacrylamide gels) can change its volume (expand or contract), its elastic properties (soften or stiffen), and/or its optical properties (color) rapidly and drastically as functions of small changes in solvent composition, pH, temperature, lighting, electric field, magnetic field, etc.

A gel.

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Solutions

Depending on the chemical structure of the solvent and the chemical structure and chain architecture of the polymer, one can find a wide range of behaviors and morphologies, including homogeneous solution, precipitation of the polymer, and a wealth of ordered phases similar to those shown above for block copolymers.  An example is shown below.

Schematic illustration.  Morphologies in mixtures of polyol-based triblock copolymer surfactants with water.  Going clockwise from the top left, micellar cubic, hexagonal, bicontinuous cubic, lamellar, reverse bicontinuous cubic, reverse hexagonal, and reverse micellar cubic.  By definition, the hydrophobic (“oil”) phase domains are enclosed in the continuous water phase in the micellar cubic and hexagonal morphologies, while the water domains are enclosed in the continuous hydrophobic (“oil”) phase in the reverse micellar cubic and reverse hexagonal morphologies.

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Blends

Blends span the entire range from fully miscible to completely immiscible.  The thermodynamic drive towards phase separation increases with increasing inherent incompatibility and as with increasing average molecular weights of polymer chains.  Unlike block copolymers where highly ordered morphologies (as shown above) are found, one does not normally find ordered arrangements of regularly-shaped domains in a blend since the polymer chains of different blend components are not bonded to each other.  The blend morphology can be affected significantly by many factors.  These factors include the incorporation of compatibilizers, the kinetic "freezing in" of nonequilibrium morphologies by the application of shear during processing, and annealing at an elevated temperature in order to release the kinetically frozen-in morphological features and thus approach thermodynamic equilibrium.

Schematic illustration.  Types of possible polymer blend phase diagrams, for binary blends where additional complications that can be introduced by competing processes (such as crystallization of a component) are absent.  The coefficients d₁ and d₂ refer to a general functional form (as a function of temperature and component volume fractions) of the binary interaction parameter that quantifies deviations from ideal mixing.

Illustration.  Models of slabs of typical open-cell foams (left) and closed-cell foams (right).  The foam cells adopt polyhedral shapes which result from the combination of the physical factors governing the foaming process and the geometrical constraints of packing.

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Composites

The term composite is often used to describe matrix polymers containing conventional fillers (such as glass spheres, short fibers, continuous fibers, or talc particles).  Composites where (most often highly anisotropic) nanofillers (such as carbon nanotubes or "exfoliated" clay platelets) have been dispersed in a polymer are commonly called nanocomposites.

Schematic illustration.  A composite consisting of stiff (high-modulus, darkly-shaded) matrix polymer containing flexible (low-modulus, lightly-shaded) discontinuous fillers (left), and a flexible matrix polymer containing stiff discontinuous fillers (right).  The properties of a composite depend on the material properties and volume fractions of each component; the shape, orientation and size distributions of each type of filler present in the composite; the strengths of the matrix-filler interfaces, and the sample preparation conditions.

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Suspensions

Often also referred to as dispersions, the systems of interest most often consist of various types of particles in ordinary or polymeric fluids.  The particles can be spherical, fibrous or discoidal in shape.  Particles of more than one type of material and/or different shapes may be present in the same suspension.  The morphology and the rheological properties of a suspension depend on the same factors as those listed in the caption of the figure shown above as affecting the properties of composites.

A suspension of monodisperse spherical particles.

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Liquid crystalline polymers

From left to right, the key differences between crystalline order, liquid crystalline order, and liquidlike disorder are shown below. The molecular arrangement manifests both translational order and orientational order in a crystalline arrangement.  Orientational order along a preferred direction is also observed in liquid crystalline arrangements, while the full three-dimensional translational periodicity is lost.  There are several types of liquid crystalline order (nematic, smectic, cholesteric and columnar phases), differing in the extent and the details of the order manifested by them. 

Schematic illustration.  Polymers manifesting liquid crystalline order include various "rigid rodlike" aromatic polyamides and polyaromatic heterocyclics.  These polymers have very stiff chains.  These chains do not readily adopt the random coil conformation found in typical amorphous polymers.  However, they also do not pack well into a fully three-dimensionally periodic crystalline lattice because of their molecular shapes.  The result is a preference for liquid crystalline order.

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