Fast Scanning Calorimetry of Semicrystalline Polymers: From Fundamental Research to Industrial Applications

Abstract

The global production of polymer products currently exceeds 400 megatons annually. To ensure effective and environmentally responsible use of this vast resource, optimizing the properties of the products is essential. Achieving this requires precise control over the internal structure of the polymers. Depending on the materials used, polymers can exist in either amorphous or semicrystalline states. Processing is often performed from the melt state, and the cooling rate plays a critical role in determining whether amorphous or semicrystalline products are formed alongside other process parameters such as the pressure and shear rates.

To understand the structure formation during processing, knowledge of the cooling rate dependence is therefore essential. As all of these processes are associated with thermal effects, calorimetry is universally applicable here. Achieving cooling rates that are comparable to those during processing has therefore long been a challenge for calorimetric measurement methods. With the introduction of MEMS-based chip sensors for calorimetry, significant progress has been made in reproducing conditions, such as those that occur during injection molding. These special calorimetric techniques are often summarized under the terms Fast Scanning Calorimetry (FSC) or Nanocalorimetry, alluding to nanogram samples.

Investigations with controlled cooling rates of up to 1 × 10⁶ K/s are now possible with special chip sensors and allow the study of material properties under extreme conditions. Technological issues such as crystallization and nucleation processes under process-relevant conditions can be investigated in most cases with commercial devices that achieve cooling rates of 10⁴ K/s. The cooling rates to be considered in relation to various manufacturing processes are discussed here, and the functionality of corresponding chip calorimeters is briefly presented.

Since calorimetry only provides general information on the processes taking place in the material, but not directly on the resulting structures, combinations of FSC and methods for structure elucidation, e.g., microscopy, are also presented. The main part of this Account deals with contributions of FSC to the understanding of crystallization processes under conditions as they occur in different manufacturing processes. Not only the influence of the cooling rate during injection molding but also the multistage cooling by chill rolls during film production is considered.

Thanks to the high scanning rate of FSC, needed to bypass crystallization in the low-supercooling temperature range where heterogeneous nucleation dominates, an important aspect of polymer structure formation─homogeneous crystal nucleation─has become accessible for direct observation. Homogeneous nucleation can occur not only during cooling but also during storage at temperatures close to or even below the glass transition temperature in the amorphous state. The possibilities of FSC for the generation and investigation of amorphous states are illustrated by an example. Finally, possible further developments of FSC and expected further applications of this fascinating technology are considered.