Crystallization, Melting and Morphology of Syndiotactic Polypropylene Fractions. 4. In Situ Lamellar Single Crystal Growth and Melting in Different Sectors

Abstract

Over a wide crystallization temperature (Tc) range, elongated rectangular lamellar single crystals of syndiotactic polypropylene (s-PP) grown from the melt of thin films exhibit sectorization along both diagonal directions to form the (010) [with the (010) free edge] and the (100) [with the (100) free edge] sectors. They can be observed by transmission electron and atomic force microscopy (TEM and AFM). Electron diffraction experiments show that the crystals in both sectors are form III orthorhombic structure, as recently proposed. 1-4 The observation of this sectorization is due to the thickness difference between these two sectors with different fold planes. In situ observations of the single-crystal growth are monitored using an AFM coupled with a hot stage. The Tc-dependent crystal growth rates along both the (100) and (010) normal can be measured, and they are anisotropic. This leads to a large aspect ratio of the rectangular single crystal. AFM results also show that the thickness difference between the two sectors is dependent upon crystallization time (tc). In the initial stage of single-crystal development, the ratio between the thickness difference and the (100) sector thickness is large [∼35% of the thickness of the (100) thick sectors]. With increasing tc, this ratio of the thickness difference gradually decreases to ∼15%. Multiple-step cooling and isothermal experiments in a successive Tc-decreasing sequence reveal that each Tc produces a unique lamellar thickness. With reducing Tc, the thickness decreases. Room-temperature TEM observations show that the (010) thin sectors are molten while the (100) thick sectors remain after the single crystals are annealed on a temperature-gradient bar for a prolonged time. In situ single crystal melting monitored by AFM reveals different melting pathways of these two sectors. When the isothermally grown single crystals are heated to 5°C above T c (120°C), the melting first appears in the (100) thick sectors in the form of molten domains. These domains can be repaired via recrystallization if the samples are annealed at this temperature for a long period of time. As further increasing the temperature, the melting of the (100) thick sectors again starts in molten domains along the (100) free edges of the single crystals. In the (010) thin sectors the melting takes place on the free edges of the single crystals, and progresses toward the inner area of the sectors.

Model experiments for a molecular understanding of polymer crystallization

“…In a simplified picture, polymer crystallization can be described as the transition from an amorphous, randomly coiled state to a crystalline, ordered lamellar state. However, as simple as this seems, central questions concerning the formation of initial nuclei, [5][6][7][8][9][10][11][12][13][14][15] the growth kinetics resulting in folded intermediate metastable states, 3,16 -20 and subsequent morphological evolution during annealing [21][22][23][24] are still under intensive debate. To improve our understanding, it is necessary to analyze at a high resolution the molecular pathways polymers take when forming a crystalline state, a question that has been discussed for more than 50 years.…”

“…[21][22][23][24][25][26] However, the corresponding relaxations processes in the crystalline state are usually slow and rather localized because the long sequence of connected segments is not very mobile. The morphology of a polymer crystal does not only depend on the rate at which it was created (i.e., the crystallization temperature) but also, and sometimes most importantly, on the thermal treatment (e.g., the storage conditions) the sample experienced after the initial crystallization process.…”

Micro‐ and nanomorphologies of isotactic polystyrene revealed by PLM, AFM, and TEM

“…As illustrated in Figure 4(B), three main features can be observed: (1) The residual granular lamellae between the dominant ones completely disappear, indicating the complete melting of subsidiary lamellae; (2) the lengths of the remaining lamellae, to some extent, decrease compared with those in Figure 4(A) (1 min), illustrating the occurrence of lamellar fragmentation 26,27 ; and (3) under the used conditions (210°C for 10 min), recrystallization (not melting) dominates the whole reorganization process, being reflected by the formation of many recrystallized lamellae [ Fig. 4(B)].…”

The impact of polypropylene‐graft‐maleic anhydride on the crystallization and dynamic mechanical properties of isotactic polypropylene

“…A double melting endotherm for isotactic PP is consistent with the existence of two crystal forms with distinct lamellar thickness and thermal stability. [18][19][20][21][22] Neat MAPP melts at 142 C, $ 20 C below PP. In the MAPP/PP blends, only one melting point around 165 C, characteristic of PP, remains.…”