Lecture 04: Mechanical and Thermophysical Properties of Polymers

Crystalline and Amorphous Solids

• Crystalline Solids: Atoms/molecules arranged in a highly ordered, repeating 3D pattern (crystal lattice).
• Amorphous Solids: Lack a well-defined, repeating crystal structure; arrangement is more random.
• Semi-Crystalline Solids: Combination of crystalline (crystallites) and amorphous structures.

Properties of Crystalline, Amorphous, and Semi-Crystalline Materials

• Crystalline Solids: Distinct melting points, sharp X-ray diffraction patterns, well-defined optical properties.
• Amorphous Materials: Gradual transition from solid to liquid, no distinct melting point.
• Semi-Crystalline Materials: Properties of both crystalline and amorphous materials; may have a distinct melting point.

Polymer Crystallinity

• Crystallinity occurs when molecular chains are aligned and packed in an ordered arrangement.
• Polymers can be amorphous, crystalline, or semi-crystalline.
• Crystallinity is facilitated by simple, regular, and symmetrical chain structures.
• Degree of crystallinity ranges from completely amorphous to almost entirely (up to ~95%) crystalline.

Induction of Crystallinity

• Cooling of molten polymer.
• Evaporation of polymer solution.
• Heating of polymer at a specific temperature.
• Drawing.

Effects of Crystallinity

• Increased Density.
• Increased Stiffness (modulus).
• Reduced permeability.
• Increased chemical resistance.
• Reduced toughness.

Stress-Strain Curve of Polymers

• Brittle response: aligned, crosslinked & networked polymer
• Plastic response: semi-crystalline polymers
• Increasing temperature:
  • Decrease in elastic modulus.
  • Reduction in tensile strength.
  • Enhancement of ductility.

Macroscopic Deformation

• At the upper yield point, a small neck forms where chains become oriented.
• Resistance to continued deformation occurs, and specimen elongation proceeds by the propagation of this neck region.

Melting of Polymers

• Transformation from ordered solid to viscous liquid at the melting temperature (T_m).

Features of Melting Temperature of Polymers

1. Melting occurs over a range of temperatures.
2. Melting behavior depends on specimen history, especially crystallization temperature.
3. Melting temperature depends on crystallization temperature.
4. Impurities and imperfections decrease the melting temperature.
5. Apparent melting behavior is a function of heating rate; increasing rate elevates the melting temperature.

Glass Transition of Polymers

• Occurs in amorphous and semicrystalline polymers; no glass transition for crystalline polymers.
• Upon cooling, gradual transformation from liquid to rubbery material and finally to a rigid solid.
• Glass transition temperature (T_g): temperature at which polymer transitions from rubbery to rigid states.
• Abrupt changes in stiffness, heat capacity, and coefficient of thermal expansion.

Why are Tm and Tg important?

• Define upper and lower temperature limits for applications, especially for semicrystalline polymers.
• T_g may define the upper use temperature for glassy amorphous materials.
• Influence fabrication and processing procedures for polymers and polymer-matrix composites.

Factors Affecting Melting Temperature (T_m)

• Molecular chemistry and structure influence rearrangement ability.
• Double bonds and aromatic groups increase T_m by lowering chain flexibility.
• Increasing molecular weight raises T_m (at relatively low molecular weights).
• Side branches introduce defects, lowering T_m.

Factors Affecting Glass Transition Temperature (T_g)

• Depends on molecular characteristics affecting chain stiffness.
• Increased by bulky side groups, chain length, double bonds, and aromatic groups.
• Small amount of branching lowers T_g, while a high density of branches elevates it.

Polymers Rheology

• Rheology is the study of deformation and flow.
• Viscosity represents the resistance to flow.
• In viscous flow, a material continues to deform as long as a stress is applied.

Viscosity

• Shear stress: \tau = \frac{F}{A}, where \tau is shear stress in Pascal (Pa), F is force in N, and A is the surface area in m^2.
• Shear strain: \gamma = \frac{dx}{dy}
• Shear strain rate: \dot{\gamma} = \frac{d\gamma}{dt} = \frac{d}{dt} \frac{dx}{dy} = \frac{dx}{dt} \frac{d}{dy} = \frac{du}{dy}, where u is velocity and t is time.
• Viscosity: \mu = \frac{\tau}{\dot{\gamma}}. Polymer melts have viscosities in the range 2-300 Pa.S
• Viscosity decreases with temperature.

Viscoelasticity

• Combination of viscosity and elasticity.
• Example: die swell in extrusion

Rheometry

• Experimental techniques and tools used to determine the rheological properties of materials.
• Rotational or shear rheometers: control the applied shear stress or shear strain.
• Extensional rheometers: apply extensional stress or extensional strain rate.