Evolution of the long period of oriented polypropylene film in the temperature interval 25 – 150 °C
The structural morphology of polymers can be subdivided into several levels. On the primary level the architecture of an individual polymer chain is characterized. The secondary level is concerned with the assemblage of primary chains into crystalline phases. The tertiary level describes how secondary-level elements can be ordered into larger assemblies, e.g. lamellae. Finally on the quaternary level even large assemblies (e.g. spherulites) formed by lamellae are characterized. In this case study we discuss the characterization of the tertiary (lamellae) level of polymer morphology using the example of isotactic α-polypropylene (α-iPP). Polypropylene, as any other polymer crystallized from a melt, exhibits an ordered lamellae structure (Figure 1). It presents itself as a regular stack of alternating crystalline and amorphous layers with a narrow transition zone between each two adjacent layers. One of the main parameters characterizing the lamellae stack is the distance between two crystalline layers (the long period). Generally the long period is in the order of tens of nanometers and therefore can be easily measured using small- angle X-ray scattering (SAXS). Here we present an in situ study of the long period of α-polypropylene as a function of temperature.
SAXS measurements were performed using the Empyrean X-ray diffraction platform configured with a Cu tube, an X-ray focusing mirror and PIXcel3D detector. The Anton Paar Multi-Sample Humidity Chamber for transmission geometry (MHC-trans) was used to create a sample environment with variable temperature(T) conditions. A series of SAXS curves was measured in the interval from 25 to 150 ºC from a sample of α-iPP  and an empty sample holder with Mylar foil was measured for background correction. Malvern Panalytical EasySAXS and HighScore software packages were used for data treatment and analysis.
Figure 1. Schematic drawing of the simplified ordered lamellae structure of polypropylene formed by a regular stack of crystalline lamellae and amorphous region
Results and discussion
In order to extract the average long period from the SAXS signal, a number of corrections should be applied: (i) background correction to account for the contribution of the sample holder and air scattering, (ii) desmearing correction to account for the slit smearing effect and (iii) Lorentz correction to account for the 2D dimensionality of lamellae. All three steps can be done using the EasySAXS software package. Figure 2 presents the set of corrected SAXS curves measured during the heating cycle of α-iPP from 25 ºC to 150 ºC. The first Bragg peak, marked as q1 corresponds to the average spacing between crystalline lamellae (the long period). The peak is very well pronounced and its position can be easily determined. The second maximum (marked as q2) is quite broad and weak, especially below 50 ºC. However, at higher temperatures the ratio of the angles of the second and first maxima (q2/q1) is approximately 2, suggesting that the second maximum is related to the second order reflection.
The position of q1 as a function of temperature was extracted using profile fit (HighScore) and the result is given in Figure 3. At room temperature the average value of the long period of α-iPP is ~ 23.0 nm. The increase of temperature causes a gradual increase of the long period to the value of ~ 26.3 nm at 150 ºC. Upon cooling the long period reversibly decreases, however a certain hysteresis is observed. After the temperature cycle the average long period of α-iPP reaches the value of ~ 23.9 nm at 25ºC.
In the temperature range from 25 ºC to 150 ºC not only the position of q1 reversibly changes with temperature but also its intensity (Figure 2). Upon heating from 25 ºC to 150 ºC the intensity of q1 increases by a factor of 4.5 (Figure 2). Cooling of the sample down to room temperature results in the reversed behavior, the intensity of q1 decreases by a factor of 4 (data are not shown here).
The presented temperature dependence of the long period is observed not only for α-iPP, it reflects the general temperature behavior of any polymer system with lamellae morphology. In an attempt to explain the experimentally observed behavior a number of molecular mechanisms has been proposed. The model by Yeh et al.  seems to account for all effects observed upon heating and cooling of polymers. In the framework of this model lamellas are subdivided into microparacrystallites. Temperature-induced changes in the lateral arrangements and the mobility of these structural units explain the temperature trends observed in polymers, including the evolution of a long period.
Figure 2. SAXS curves measured from the α-iPP sample at 25 – 150 ºC (background, slit smearing and Lorentz correction were applied)
Figure 3. The long period of α-iPP as a function of temperature
By combining the instrument setup for SAXS in air with the non-ambient chamber for measurements in transmission geometry (Anton Paar MHC-trans) we characterize the long period of α-polypropylene as a function of temperature.