8.4 Thermal analysis techniques for studying transitions

Thermal analysis techniques are crucial for understanding polymer behavior. They measure how polymers react to heat, revealing important properties like glass transition, crystallization, and melting temperatures. These methods help scientists and engineers design better materials for various applications.

Differential Scanning Calorimetry, Thermomechanical Analysis, and Dynamic Mechanical Analysis are key tools in this field. They provide insights into polymer structure, thermal properties, and mechanical behavior. Understanding these techniques is essential for anyone working with polymers.

Thermal Analysis Techniques

Thermal analysis techniques for polymers

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• Differential Scanning Calorimetry (DSC) measures heat flow into or out of a sample as a function of temperature to detect thermal transitions such as glass transition ($T_g$), crystallization ($T_c$), and melting ($T_m$) which provides information on the thermal properties and morphology of polymers (polyethylene, polypropylene)
• Thermomechanical Analysis (TMA) measures dimensional changes in a sample as a function of temperature to detect softening behavior and thermal expansion of polymers which provides information on the dimensional stability and thermal properties of polymers (polyvinyl chloride, polyethylene terephthalate)
• Dynamic Mechanical Analysis (DMA) measures the viscoelastic properties of polymers as a function of temperature by applying a sinusoidal stress to a sample and measuring the resulting strain to provide information on the storage modulus ($E'$), loss modulus ($E''$), and damping behavior ($\tan \delta$) of polymers (polycarbonate, polyurethane)

Interpretation of DSC thermograms

• Glass Transition ($T_g$) appears as a step change in the heat flow signal representing the temperature range where the polymer transitions from a glassy to a rubbery state characterized by a change in heat capacity ($\Delta C_p$)
• Crystallization Peak ($T_c$) appears as an exothermic peak in the heat flow signal representing the temperature at which the polymer chains organize into crystalline structures with the area under the peak proportional to the degree of crystallinity
• Melting Peak ($T_m$) appears as an endothermic peak in the heat flow signal representing the temperature at which the crystalline regions of the polymer melt with the area under the peak proportional to the heat of fusion ($\Delta H_f$)

Principles of thermomechanical analysis

• TMA measures the change in sample dimensions (length, width, or thickness) as a function of temperature while applying a constant load to the sample during heating or cooling and recording the dimensional change using a linear variable differential transformer (LVDT)
• TMA determines the coefficient of thermal expansion (CTE) which quantifies the change in sample dimensions per unit change in temperature and is important for assessing the dimensional stability of polymers in various applications (automotive parts, packaging materials)
• TMA detects softening behavior by identifying the temperature at which the polymer starts to soften and deform under load relating to the glass transition temperature and the onset of molecular mobility

Dynamic mechanical analysis applications

• DMA applies a sinusoidal stress to a sample and measures the resulting strain with the stress and strain out of phase due to the viscoelastic nature of polymers and the phase lag ($\delta$) between stress and strain measured
• Viscoelastic properties measured by DMA include:
  1. Storage Modulus ($E'$) representing the elastic component of the material's response and relating to the ability of the polymer to store energy during deformation
  2. Loss Modulus ($E''$) representing the viscous component of the material's response and relating to the ability of the polymer to dissipate energy during deformation
  3. Tan Delta ($\tan \delta$) as the ratio of the loss modulus to the storage modulus ($E''/E'$) providing a measure of the damping behavior of the polymer
• DMA determines the glass transition temperature ($T_g$) which appears as a peak in the tan delta curve or a drop in the storage modulus and provides information on the molecular mobility and viscoelastic behavior of polymers (acrylic polymers, epoxy resins)
• DMA characterizes the viscoelastic properties as a function of temperature helping understand the temperature-dependent behavior of polymers which is important for designing polymers for specific applications and processing conditions (high-temperature resistant polymers, viscoelastic damping materials)