Influence of Branch Incorporation into the Lamella Crystal on the Crystallization Behavior of Polyethylene with Precisely Spaced Branches

Abstract

Depending on the degree of short chain branch (SCB) incorporation, the crystallization behavior and resultant crystalline structure drastically change in polyethylene with precisely spaced branches. In polyethylene with hexyl branches precisely spaced on every 21st carbon (HB21), only crystallization mediated by a transient hexagonal phase without incorporation of the SCB was observed. On the other hand, in polyethylene with ethyl branches precisely spaced on every 21st carbon (EB21), crystallization behavior was strongly dependent on the crystallization temperature. A thin lamella was formed through crystallization mediated by a hexagonal phase and no thickening occurred at 5–8 °C, while thickening of the transient hexagonal lamellae occurred at 10–15 °C, and one SCB seemed to be incorporated into a crystal stem. At 17 °C, no thickening of the hexagonal phase occurred and a hexagonal phase with sufficient lamella thickness was directly formed from the melt. At 21–28 °C, crystallization mediated by hexago..

Systematic Studies of Morphological Changes of Precision Polyethylene

“…At slightly higher crystallization temperatures (10–15 °C), the transient hexagonal mesophase forms and subsequently thins or thickens to include zero or one short‐chain branch (SCB). The folded chains simultaneously adopt the trans‐conformation and slide past one another in the mesophase to reach one of two new equilibrium lamellar thicknesses, the thicker of these measuring about 56 Å (approximately the length of two –(CH 2 ) 20 – units) and the thinner one of about 25 Å (approximately the length of one –(CH 2 ) 20 – unit) . Thus, Et‐21 crystallized isothermally between 10 and 15 °C exhibits lamellae of two thicknesses.…”

“…In metallocene PE, which contains random branching, Mirabella found that the longer ethylene sequences crystallize before the shorter sequences, forming thick orthorhombic crystals composed of the longer ethylene sequences, with the shorter run lengths remaining in the amorphous phase. Conversely, in precisely branched PE, all run lengths are the same without long sequences to crystallize rapidly at higher temperatures, thereby allowing a transient hexagonal mesophase to form and persist long enough for the formation of energetically stable crystals …”

“…To illustrate this point, we first consider the isothermal crystallization from the melt of precision polyethylene‐bearing ethyl groups on every 21st backbone carbon (abbreviated Et‐21 ), which was studied in detail by wide‐angle X‐ray scattering (WAXS), small‐angle X‐ray scattering (SAXS), and infrared (IR) spectroscopy by Matsui et al The crystallization behavior of Et‐21 depends on temperature and illustrates the opposing kinetic and thermodynamically favored structures (see Figure ) …”

“…When Et‐21 is isothermally crystallized at low temperatures (5–8 °C), kinetically favored thin lamellae form without branch inclusion (i.e., lamellar thickness ≈ distance between ethyl groups) through a hexagonal phase without thickening …”

“…With isothermal crystallization at 17 °C, a hexagonal phase comparable to the most stable lamellar thickness is formed from the melt . Once the mesophase is formed, there is no thinning or thickening observed, and the resulting lamellae measure 53 Å, which translates to the inclusion of one ethyl branch in the stem …”