Structure and Properties of Plastics

Molecular Structure of Plastics

Plastics are synthetic organic polymers whose macroscopic properties are determined by their molecular structure. While the previous chapter focuses on how polymer chains are formed, here the focus is on how the architecture and interactions of these chains generate the characteristic properties of plastics.

Chain Architecture

The way repeat units are connected and arranged along and between the chains is crucial.

Linear, Branched, and Crosslinked Polymers

• Linear polymers
  Chains consist of long, unbranched macromolecules that can lie close to one another.
• Tend to have higher density and can crystallize more easily.
• Often show good flexibility and can be melt-processed (e.g. many thermoplastics).
• Branched polymers
  Side chains are attached to the main chain.
• Branching reduces packing efficiency and crystallinity.
• Often lower density and softer materials (e.g. low-density polyethylene).
• Crosslinked (network) polymers
  Chains are connected via covalent bonds to form 2D or 3D networks.
• Crosslink density strongly affects hardness, elasticity, and solvent resistance.
• Typical of thermosets (e.g. epoxy resins) and elastomer vulcanizates (e.g. vulcanized rubber).

Copolymer Structure

When two or more different monomers are incorporated into one polymer:

• Random copolymers: the different repeat units are statistically distributed along the chain.
• Block copolymers: longer segments (“blocks”) of one monomer alternate with blocks of another.
• Graft copolymers: side chains of one polymer are grafted onto a backbone of another.

Copolymers allow tuning of properties, for example improving impact resistance, flexibility, or compatibility with other materials.

Degree of Polymerization and Molar Mass

The degree of polymerization $n$ is the average number of repeat units per chain. The molar mass $M$ is then approximately:
$$
M \approx n \cdot M_\text{repeat unit}
$$

• Higher molar mass usually leads to:
• Higher tensile strength and toughness.
• Higher viscosity in the melt or solution.
• Higher softening or flow temperatures.
• Lower molar mass:
• Easier processing due to lower viscosity.
• Often more brittle materials if too low.

In real plastics there is a distribution of chain lengths; this molar mass distribution also influences mechanical behavior and processing.

Conformation, Configuration, and Tacticity

Chain Conformation

Even with the same connectivity, chains can adopt many shapes (coils, extended chains) due to rotation about single bonds.

• In the melt or in amorphous regions, chains form random coils.
• Under stress (stretching), chains can be oriented and partially straightened, which changes the mechanical properties and optical appearance (e.g. stress whitening, birefringence).

Configuration and Tacticity

For polymers with stereocenters in the backbone (e.g. polypropylene), the spatial arrangement (configuration) of substituents along the chain is key:

• Isotactic: all substituents arranged on the same side.
• Syndiotactic: substituents alternating regularly from side to side.
• Atactic: random arrangement.

High regularity (isotactic or syndiotactic) facilitates crystallization, giving stiffer and often more heat-resistant plastics than atactic variants.

Crystalline and Amorphous Regions

Most plastics are semi-crystalline, containing both ordered (crystalline) and disordered (amorphous) regions.

Crystalline Regions

• Chain segments pack in ordered structures, often forming lamellae and spherulites.
• Properties:
• Higher stiffness and strength.
• Higher chemical resistance.
• Often opaque or translucent due to light scattering.
• More defined melting transition with a melting temperature $T_m$.

Amorphous Regions

• Chains are disordered and entangled without long-range order.
• Properties:
• More flexible and impact-resistant.
• Often transparent (e.g. polystyrene, PMMA).
• Do not have a sharp melting point; instead they soften over a range and exhibit a glass transition temperature $T_g$.

Degree of Crystallinity

The crystallinity (fraction of the material that is crystalline) can be influenced by:

• Cooling rate from the melt (slow cooling → more crystallinity).
• Chain regularity and tacticity.
• Presence of nucleating agents or fillers.

Higher crystallinity usually leads to higher stiffness and heat resistance but lower transparency and impact toughness.

Thermal Properties of Plastics

Glass Transition and Melting

• Glass transition temperature $T_g$
  At $T_g$, amorphous regions change from a glassy, brittle state (below $T_g$) to a rubbery, more flexible state (above $T_g$).
• Below $T_g$: material is rigid and brittle.
• Above $T_g$: material softens and can deform more easily.
• Melting temperature $T_m$
  For crystalline plastics, crystalline regions melt at $T_m$.
• Below $T_m$: solid with crystalline domains in a matrix.
• Above $T_m$: polymer behaves as a viscous melt.

Amorphous plastics have a $T_g$ but no well-defined $T_m$; semi-crystalline plastics display both.

Thermoplastics, Thermosets, and Elastomers (Property View)

Although classification is introduced elsewhere, from a property perspective:

• Thermoplastics
• Soften reversibly upon heating (above $T_g$ or $T_m$).
• Can be melted and reshaped multiple times (within limits).
• Usually linear or lightly branched chains.
• Thermosets
• Formed by curing reactions that create a dense crosslinked network.
• Do not melt upon reheating; instead they degrade.
• Hard and dimensionally stable at elevated temperatures.
• Elastomers
• Lightly crosslinked polymers with $T_g$ well below room temperature.
• Exhibit high reversible elasticity: large deformation with recovery when stress is removed.

Thermal Stability and Decomposition

Polymers degrade at high temperature via:

• Chain scission (breaking of the backbone).
• Side-group elimination or oxidation.
• Crosslinking reactions (in some systems).

The onset of decomposition typically occurs above processing temperatures but limits maximum use temperature. Stabilizers (antioxidants, UV stabilizers) are often added to improve resistance against heat and light.

Mechanical Properties of Plastics

Mechanical behavior reflects the molecular mobility, chain interactions, and morphology.

Elasticity, Plasticity, and Viscoelasticity

• Elastic deformation: reversible; material returns to original shape when load is removed.
• Plastic deformation: irreversible; permanent shape change.
• Viscoelasticity: time-dependent combination of viscous flow and elastic response, typical for plastics.

Viscoelastic behavior means:

• Deformation depends on loading rate and duration.
• Creep (slow deformation under constant load) and stress relaxation (stress decreases under constant strain) occur.

Key Mechanical Parameters

Important measurable quantities include:

• Elastic modulus (Young’s modulus): stiffness; higher values = stiffer materials.
• Tensile strength: maximum stress before fracture.
• Elongation at break: measure of ductility (how far a material can be stretched).
• Impact strength: resistance to sudden impact or shock loading.
• Hardness: resistance to surface indentation or scratching.

These parameters vary strongly with:

• Temperature (especially around $T_g$).
• Degree of crystallinity.
• Presence of fillers, plasticizers, or reinforcing fibers.
• Orientation of chains (e.g. in drawn fibers or films).

Influence of Structure on Mechanical Properties

• Long, entangled chains, high molar mass → higher strength and toughness.
• Strong intermolecular forces (e.g. polar groups, hydrogen bonds) → higher stiffness and often higher $T_g$.
• Higher crystallinity → higher stiffness and strength but reduced elongation and impact toughness.
• Crosslinking:
• Low crosslink density: elastic, rubber-like behavior.
• High crosslink density: rigid, brittle materials; little or no flow.

Chemical and Physical Interactions

Intermolecular Forces and Solubility

Interactions between chains and between chains and other substances are dominated by:

• Dispersion (van der Waals) forces.
• Dipole–dipole interactions and hydrogen bonding (if polar or H-bonding groups are present).

These determine:

• Solubility and swelling in solvents:
• “Like dissolves like”: nonpolar plastics in nonpolar solvents; polar plastics in polar solvents.
• Some plastics swell (absorb solvent and expand) without dissolving.
• Barrier properties:
• Resistance to permeation by gases or liquids depends on chain packing, crystallinity, and polarity.

Chemical Resistance

Chemical stability relates to the bond types in the backbone and side groups:

• Hydrocarbon backbones (e.g. polyethylene) are generally resistant to water and many chemicals but sensitive to oxidation and UV without protection.
• Polar or hydrolyzable groups (esters, amides, carbonates) can be sensitive to hydrolysis, especially at elevated temperatures or extreme pH.
• Fluorinated polymers (e.g. PTFE) have very high chemical resistance due to strong C–F bonds.

Additives (stabilizers, antioxidants, UV absorbers) improve resistance to weathering and aging.

Optical and Electrical Properties

Optical Properties

• Transparency vs. opacity:
• Amorphous plastics with homogeneous structure are often transparent (e.g. PMMA, polycarbonate).
• Semi-crystalline plastics tend to be opaque or translucent due to light scattering at crystalline–amorphous interfaces.
• Pigments and fillers increase scattering and absorption, changing color and opacity.
• Refractive index and birefringence:
• Chain orientation can lead to anisotropic optical behavior (birefringence), visible under polarized light.
• Stress-induced orientation may cause stress patterns in transparent parts.

Electrical Properties

Most plastics are good electrical insulators:

• Low electrical conductivity due to localized, saturated bonds.
• High volume resistivity and dielectric strength, useful for cable insulation and electronic components.

Additives and structural features can modify behavior:

• Conductive fillers (carbon black, metal particles) → conductive or antistatic plastics.
• Polar groups → higher dielectric constant, relevant for capacitors and insulating materials.

Influence of Additives and Composites

Commercial plastics almost always contain additives that modify structure and properties.

Plasticizers

Plasticizers are small, often polar molecules that insert between chains:

• Reduce intermolecular attractions.
• Increase chain mobility.
• Lower $T_g$ and soften the material.
• Increase flexibility but often reduce strength and chemical resistance.

Common in flexible PVC and certain elastomer blends.

Fillers and Reinforcements

• Fillers (e.g. chalk, talc, silica):
• Change stiffness, density, thermal conductivity.
• Often reduce cost.
• Reinforcing fibers (glass, carbon, aramid):
• Dramatically increase strength and stiffness (fiber-reinforced plastics).
• Create anisotropic properties depending on fiber orientation.

Stabilizers, Flame Retardants, and Colorants

• Stabilizers: protect against heat, oxygen, and UV radiation.
• Flame retardants: reduce flammability or slow burning.
• Pigments and dyes: define color and can affect heat absorption and light stability.

These additives interact with the polymer matrix and can influence processing and end-use performance.

Processing–Structure–Property Relationships

The manufacturing process of a plastic part is not just shaping; it also controls internal structure:

• Cooling rate in injection molding or extrusion affects:
• Crystallinity (rapid cooling → lower crystallinity).
• Size and distribution of crystalline regions.
• Flow and stretching of melt induce:
• Chain orientation and anisotropy in mechanical and optical properties.
• Post-treatments (annealing, stretching, crosslinking) can:
• Increase crystallinity and dimensional stability.
• Improve mechanical strength or create oriented films and fibers.

Understanding these relationships is essential for designing plastics with tailored properties for specific applications.