Polymer Morphology

Enhancements Chapter 12

Enhancement 12.1.       Polymer Materials

Polymers are used in a wide range of everyday applications, in clothing, housing materials, appliances, communication devices, and for transportation. Feedstocks for synthetic polymers are petroleum, coal and natural gas, which are sources of ethylene, methane, alkenes and aromatics. Polymers have advantages over other types of materials such as metals and ceramics, because of their low process costs, low weight, high specific strength and stiffness, and because they are often transparent and tough. The unique combination of strength, modulus, elongation and impact strength make plastics cost effective versus metals for many applications.

Polymers are macromolecules formed by joining a large number of small molecules, or monomers, in a chain. These monomers, small repeat units, react chemically to form long molecules. The repetition of monomer units can be linear, branched or interconnected to form three-dimensional networks. Homopolymers are composed of a single repeating monomer whereas copolymers are often formed from a sequence of two types of monomers. A wide variety of heteropolymers and copolymers may be formed which have very different structures and properties. Polymers may be amorphous or crystalline in nature.

There are three major polymer classes: thermoplastics, thermosets and rubbers or elastomers. Thermoplastics are among the most common polymers and these materials are commonly termed ‘plastics’. Linear or branched thermoplastics can be reversibly melted or can be dissolved in a suitable solvent. Thermoplastics may be crosslinked during processing so as to provide heat stability and limit flow and melting during use. Typical crystallizable thermoplastics include polyacetal, polyamide, polycarbonate, polyethylene and poly(ethylene terephthalate) or polyesters. Glassy thermoplastics include polystyrene and poly(vinyl chloride). 

In thermosets there is a three dimensional network structure, a single highly connected molecule, which imparts rigidity and intractability. Thermosets are heated to form rigid structures but once set they do not melt or dissolve. Typical thermosets include epoxies and phenolics.  Polymers with long flexible chains between crosslinks are rubbers and elastomers which, like the thermosets, cannot be melted. A three-dimensional crosslinked network that can be stretched and spring back to its original form characterizes elastomers. Crosslinks are chemical bonds between molecules, such as the vulcanization of rubber. Examples of elastomers include polybutadiene and styrene-butadiene rubber. Multiphase polymers, combinations of thermoplastics and elastomers, take advantage of the ease of fabrication of thermoplastics and the increased toughness of elastomers, providing toughened engineering thermoplastics.

The background material is organized in two parts. The first part is a discussion of Polymer Morphology, including especially definition of terms. The second part is an overview of some Polymer Processes that need to be understood as they play a major role in formation of polymer structure and morphology.

Enhancement 12.2        Polymer Morphology

In polymer science the term structure refers to the local atomic and molecular details whereas morphology generally refers to larger forms and organization. Often these terms are interchanged although the characterization techniques are complementary and differ for these levels of details. X-ray, electron and optical scattering techniques are commonly applied to determine the structure of polymers. They permit analysis of the interatomic ordering and chain packing. The morphology of polymers is determined by a wide range of optical and electron microscopy techniques, which are the subject of this chapter. The general morphology of polymers will be summarized below to aid interpretation of images formed by microscopy techniques.

12.2.1 Amorphous Polymers

Polymers that are glassy or rubbery at room temperature are typically amorphous polymers – they are noncrystalline. They can form brittle glasses when cooled rapidly from the melt. The glass transition temperature, Tg or glass-rubber transition is the temperature above which the polymer is rubbery and can be elongated and below which the polymer behaves as a glass. Plastic deformation in glassy polymers and in rubber toughened polymers is due to crazing and shear banding. Crazing is the formation of thin sheets perpendicular to the tensile stress direction, which contain fibrils and voids. The fibrils and the molecular chains in them are aligned parallel to the tensile stress direction. Crazes scatter light and often can be seen by eye as whitened areas. Shear banding is a local deformation, generally at about 45° to the stress direction, which results in a high degree of chain orientation. The material in the shear band is more highly oriented than in the adjacent regions.

12.2.2              Semicrystalline Polymers

Semicrystalline polymers exhibit a melting transition temperature (Tm), a glass transition temperature (Tg), and crystalline order, as shown by X-ray and electron scattering. The fraction of the crystalline material is determined by X-ray diffraction, heat of fusion and density measurements. Major structural units of semicrystalline polymers are the platelet-like crystallites, or lamellae. The dominant feature of melt crystallized specimens is the spherulite. Keith and Padden (1963), Ward (1975), Bassett (1981, 1984) among many others have described the formation of polymer crystals and spherulites.

When a polymer is melted and then cooled it can recrystallize, with process variables such as temperature, rate of cooling, pressure and additives affecting the nature of the structures formed. Bulk crystallized material is composed of microscopic units called spherulites, formed during crystallization under quiescent conditions. The structures exhibit radially symmetric growth of the lamellae from a central nucleus with the molecular chain direction perpendicular to the growth direction. The plates branch as they grow. The molecular chains run perpendicular to the spherulite radius with the thickness dependent on the molecular weight of the polymer, crystallization conditions and thermal history. In commercial processes additives are used to control nucleation density and the resulting properties of the plastic. 

A schematic of the spherulite structure is shown in Figure 12.1 (Ward, 1975). The structure consists of radiating fibrils with amorphous material, additives and impurities between the fibrils and between individual spherulites. The shape of the growing spherulite is round; however when spherulites impinge upon one another the resulting shape tends to be polyhedral. A polarized light micrograph of a thin section of bulk crystallized nylon, in Figure 12.2, shows the size and shape typical of spherulites.  Spherulites appear bright when viewed in crossed polarizers in a transmitted light microscope because they are anisotropic and crystalline in nature. Isotropic materials exhibit the same properties in all directions whereas anisotropic materials exhibit a variation with direction.

Enhancement 12.3        Polymer Processes

The term plastic is used for commodity polymers whereas polymers with high performance properties are typically called engineering resins. Commercial resins are often filled with organic or inorganic fibers, minerals and additives. Plastics and engineering resins are processed into a wide range of fabricated forms, such as fibers, films, membranes and filters, moldings and extrudates. New technologies have emerged resulting in novel polymers with highly oriented structures. High modulus fibers and films can be produced from extended chain polymers, notably polyethylene, and liquid crystalline polymers (LCPs). Polymers that exhibit liquid crystallinity in the melt or in solution often can be processed into materials with ultrahigh performance characteristics.

Commercial processes used to manufacture polymer materials include fiber spinning, drawing and annealing; film extrusion and stretching; rod extrusion, compounding and injection and compression molding. Development of relationships between the structure and properties of the polymers requires an understanding of the effect of these processes on the morphology and properties. A short summary of two process classes, extrusion of fibers and films and molding of fabricated plastics follows.

12.3.1              Extrusion of Fibers and Films

Polymer fibers are found in textiles for clothing and household items such as sheeting and also for industrial uses, such as cords, rope and belts. A melt or solution spinning process that results in oriented materials can produce fibers. Spun fibers taken up on a bobbin may be further oriented by drawing on-line or by a post-treatment process. Polymer films are used in many applications, including packaging and electronic recording and in membranes for separation applications. Deformation processes that impart orientation to polymers can result in anisotropic mechanical properties often with increased stiffness and strength only in one dimension. Textile fibers formed by melt extrusion or solution spinning, as shown in the schematic in Figure 12.3 are crystallized and then drawn, creating a highly oriented structure similar to that described for the drawing of bulk polymers. Films are also formed by extrusion and drawing processes with uniaxial or biaxial structures. Increasing the draw ratio is known to increase the stiffness and the breaking strength by improving the degree of molecular alignment or extension. The diameter of the microfibrillar texture is also affected by the draw ratio. 

Liquid crystalline melts (thermotropic) or solutions (lyotropic) are composed of sequences of monomers with long rigid molecules. Aromatic polyaramides form liquid crystal solutions whereas aromatic copolyesters form nematic LCP melts at elevated temperature. Melt or solution spinning of anisotropic LCPs results in an extended chain structure in the fiber or film (Figure 12.3). Heat treatment generally improves the orientation and thus the stiffness and strength properties.

12.3.2              Extrusion and Molding

Polymer morphology in extrudates and moldings is affected by process variables such as melt and mold temperature, pressure, shear and elongational flow. The resulting morphology in turn influences the performance and mechanical properties. Pressure increases, for instance, can increase both the melting temperature and the glass transition temperature of a polymer, with the result that the polymer solidifies more quickly. In a crystalline polymer the nucleation density can increase, resulting in a decrease in spherulite size with increased pressure in injection molding.

Structures that are typically observed in molded parts and extrudates tend to be anisotropic. Elongation or shear flow in extrusion can result in highly oriented rods at higher draw ratios and/or smaller diameters. Commonly observed structures with an oriented skin and less oriented central core – skin-core texture—is often observed due to temperature variations between the bulk and the core. For instance, extensional flow along the melt front causes orientation and solidification of the polymer on the colder mold surface. The skin-core texture results from a rapidly cooled, well-oriented skin and a slowly cooled randomly oriented core as shown in the schematic in Figure 12.4. Typically in a semicrystalline polymer there are three zones within the molded part: an oriented, nonspherulitic skin; a subsurface region with high shear orientation; and a randomly oriented spherulitic core. Thickness of each zone is a function of factors such as melt and mold temperatures. Amorphous polymers show a thin surface oriented skin on injection molding.

Many major chemical industries are based on toughened plastics, such as ABS (acrylonitrile-butadiene-styrene) and HIPS (high impact polystyrene). Important issues are the design of fracture resistant polymers such as compatibility, deformation, toughening mechanisms and characterization. Particle size distribution and adhesion to the matrix must be determined by microscopy to develop structure-property relationships. Rubber toughened polymers are usually either copolymers or polymer blends. Polymer blends are typically made via compounding, the process used to introduce fillers, particles or additives to a resin prior to molding. Composites are engineering resins with particle and/or fibrous fillers, such as mica, talc, glass, carbon or polymer fibers. Specialty composites, such as those reinforced with carbon fibers, are used in aerospace applications while short or long glass fiber reinforced resins are used in automobiles, appliances, housing, etc. Composite properties depend upon the size, shape, agglomeration and distribution of the filler and its adhesion to the resin matrix. In addition to extrusion and molding, other processes, such as thermoforming and blow molding are used to form plastic articles. The process parameters used in these various manufacturing processes have a major affect on the physical and mechanical properties of the final engineering resin.

                                                        
Table 12.1  Polymer functional groups and stains

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Polymers                                                         Stains
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Unsaturated hydrocarbons, alcohols,              Osmium tetroxide
ethers, amines

Acids or esters                                                (a) Hydrazine
(b) Osmium tetroxide

Unsaturated rubber                                         Ebonite
(resorcinol-formaldehyde-latex)

Saturated hydrocarbons (PE, PP)                   Chlorosulfonic acid/
uranyl acetate

Amides, esters and PP                                    Phosphotungstic acid

Ethers, alcohols, aromatics,                            Ruthenium tetroxide
amines, rubber,
bisphenol A and styrene

Esters, aromatic polyamides                           Silver sulfide

Acids, esters                                                    Uranyl acetate

                      
Table 12.2  Specific functional groups, examples and stains

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Functional group       Examples                                Stains

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--CH—CH--              Saturated hydrocarbons          Chlorosulfonic acid
(PE, PP)                                 Phosphotungstic acid
(HDPE)                                  Ruthenium tetroxide

--C=C--                      Unsaturated hydrocarbons      Osmium tetroxide
(Polybutadiene, rubber)         Ebonite
Ruthenium tetroxide

--OH, --COH             Alcohols, aldehydes                Osmium tetroxide
(Polyvinyl alcohol)                 Ruthenium tetroxide
Silver sulfide

--O--                          Ethers                                      Osmium tetroxide
Ruthenium tetroxide

--NH2                         Amines                                    Osmium tetroxide
Ruthenium tetroxide

--COOH                     Acids                                       Hydrazine, then
Osmium tetroxide

--COOR                     Esters                                      Hydrazine, then
(butyl acrylate)                      Osmium tetroxide
(polyesters)                            Phosphotungstic acid
(ethylene-vinyl acetate)         Silver sulfide
Methanolic NaOH

--CONH                     Amides                                    Phosphotungstic acid
--CONH--                    (nylon)                                  Tin chloride

Aromatics                  Aromatics                               Ruthenium tetroxide
Aromatic polyamides              Silver sulfide
Polyphenylene oxide              Mercury trifluoroacetate

Bisphenol A               Epoxy resin                             Ruthenium tetroxide
based epoxies

Figure Titles, Enhancement

Figure 12.1         A schematic of the spherulite structure (Ward, 1975).

Figure 12.2         A thin section of bulk crystallized nylon, in polarized light, reveals bright, birefringent spherulitic texture (Sawyer and Grubb, 1996).

Figure 12.3         A schematic depicting fiber extrusion of conventional and liquid crystalline polymers (Sawyer and Grubb, 1996).

Figure 12.4         A schematic diagram of molten resin entering the gate of an injection molding machine and filling the mold (Sawyer and Grubb, 1996).