We report the effect of molecular weight and comonomer content on melt crystallization of model random ethylene 1-butene copolymers. A large set of narrowly distributed linear polyethylenes (PE) was used as reference of unbranched molecules. The samples were crystallized from a melt state above the equilibrium melting temperature and cooled at a constant rate. The exothermic peaks of the melt-solid transition are reported as the crystallization temperatures (T c ). Following expectations, the T c of unbranched PE samples was constant and independent of the initial melt temperature. The same independence was observed for copolymers (2.2 mol % ethyl branches) with molar mass below 4500 g/mol. Moreover, the T c of copolymers with higher molar mass depends on the temperature of the initial melt, T c increases as the temperature of the melt decreases. We attribute the increase in T c to a strong crystallization memory in the melt above the equilibrium melting, and correlate this phenomenon with remains in the melt of the copolymer’s crystallizable sequence partitioning. Albeit molten, long crystallizable sequences remain in the copolymer’s melt at a close proximity, lowering the change in free energy barrier for nucleation. The residual sequence segregation in the melt is attributed to restrictions of the copolymer crystalline sequences to diffuse upon melting and to reach the initial random topology of the copolymer melt. Erasing memory of the prior sequence selection in copolymer melts requires much higher temperatures than the theoretical equilibrium value. The critical melt temperature to reach homogeneous copolymer melts (T onset ), and the comonomer content at which melt memory above the equilibrium melting vanishes are established. The observed correlation between melt memory, copolymer crystallinity and melt topology offers strategies to control the state of copolymer melts in ways of technological relevance for melt processing of LLDPE and other random olefin copolymers.

Narrow metallocene-made random ethylene copolymers display a strong memory effect of crystallization above their equilibrium melting point akin to the melt memory effect observed in model ethylene−1-butene copolymers. The onset temperature for self-nucleation or surviving self-seeds displays a bell shape with increasing comonomer content with a maximum at ∼2 mol % branches. Self-seeds do not survive at temperatures above the equilibrium melting point for homopolymers and copolymers either with very low branching or with a branching content >4.5 mol %. The self-seeds are associated with clusters of ethylene sequences that remain in the melt in close proximity and accelerate a subsequent crystallization, as observed by higher crystallization peak temperatures and higher nucleation density. Contrasting this behavior, commercial ethylene−1-alkene copolymers with a broad, bimodal comonomer distribution display an inversion of the crystallization rate in a range of melt temperatures where narrow copolymers show a continuous acceleration of the rate. The inversion demarcates the onset of a self-seed assisted liquid−liquid phase separation (LLPS) between comonomer-rich and comonomer-poor molecules. The interplay between number of self-seeds and chain diffusion during LLPS causes a decrease in the crystallization rate with decreasing melt temperature. When crystallites remain in the melt at temperatures <123°C, the crystallization rate again accelerates quickly. The effect in nucleation density and in overall crystalline morphology of crystallization from one-phase homogeneous melts (region A), one-phase heterogeneous melts (region B), and two-liquid-phase melts (region C) was followed by polarized optical microscopy, transmission electron microscopy, and atomic force microscopy. INTRODUCTIONIn classical "self-nucleation" studies a standard crystalline state is brought to a melt temperature where clusters of molecular segments retain a more ordered conformation than the state that corresponds to the equilibrated random coil. These clusters are self-seeds that effectively lower the free energy barrier for nucleation and henceforth increase the overall crystallization rate. Studies of self-nucleation are relevant first to understand the effect of crystallization on melt chain topology upon fusion and second to understand the effect of the polymer melt topology on a subsequent crystallization. Furthermore, since most commercial crystalline polymers are processed from the melt, self-nucleation can shorten molding or solidification cycles and improve optical properties by reducing the size of the spherulites due to enhanced nucleation density. Self-seeds may originate from traces of previous crystallites via incomplete melting, from oriented molecular segments in sheared melts that did not relax after cessation of shear, from chains with a different entangled topology than that corresponding to the equilibrated random coil conformation, or, in the case of random copolymers with a noncrystallizable comonomer, from crystalline s...

The order-disorder phenomenon of local packing structures, space heterogeneity, and molecular dynamics and average lamellar thickness, , of the alpha form of isotactic polypropylene (iPP) crystallized at various supercooling temperatures, DeltaT, are investigated by solid-state (SS) NMR and SAXS, respectively. increases with lowering DeltaT, and extrapolations of (-1) versus averaged melting point, , gives an equilibrium melting temperature, T(m)(0) = 457 +/- 4 K. High-power TPPM decoupling with a field strength of 110 kHz extremely improves (13)C high-resolution SS-NMR spectral resolution of the ordered crystalline signals at various DeltaT. A high-resolution (13)C SS-NMR spectrum combined with a conventional spin-lattice relaxation time in the rotating frame (T(1rhoH)) filter easily accesses an order-disorder phenomenon for upward and downward orientations of stems and their packing in the crystalline region. It is found that ordered packing fraction, f(order), increases with lowering DeltaT and reaches a maximum value of 62% at DeltaT = 34 K. The ordering phenomenon of stem packing indicates that chain-folding direction changes from random in the disordered packing to order in the ordered packing along the a sin theta axis under a hypothesis of adjacent re-entry structures. It is also found that f(order) significantly increases prior to enhancement of lamellar thickness. Additionally, annealing experiments indicate that is significantly enhanced after a simultaneous process of partial melting and recrystallization/reorganization into the ordered packing at annealing temperature >/=423 K. Furthermore, the center-bands only detection of exchange (CODEX) NMR method demonstrates that time-kinetic parameters of helical jump motions are highly influenced by DeltaT. These dynamic constraints are interpreted in terms of increment of and packing ordering. Through these new results related to molecular structures and dynamics, roles of polymer chain trajectory and molecular dynamics for the lamellar thickening process are discussed.

Primary nucleation behavior of isotactic poly(styrene) was studied in a wide range of crystallization temperatures (T c ), times, and melt temperatures (T f ). Samples were melted at several melt temperatures (T f ) from 225 to 250 °C (or vice versa) and then isothermally crystallized. Time dependence on the nucleation behavior showed a sigmoidal curve with an induction time (τ 0 ). The spherulites sporadically appeared (the steady state of nucleation, I) until they reached a limiting number of nuclei (the saturated nucleation density, N s ). The number of nuclei reached saturation much earlier than the induction time (τ over ) in the overall crystallization. N s was not associated with impingement of spherulites, since about 90% of the melt region remained. It was found that there was a critical spherulite size, in which induced nucleation sites began to appear within the outline of the original spherulite in the subsequent crystallization. When the crystallization was stopped before reaching the critical spherulite size, there was no melt temperature dependence on the nucleation behavior. On the other hand, when spherulites were grown beyond the critical spherulite size, a large number of small spherulites (granular structure) appeared within the outline of the original spherulite in the subsequent crystallization. The number of additional nucleation sites increased with an increase in the original spherulite size, and the increment of nucleation sites decreased with an increase in T f . These granular structures could be due to insufficient melting of the crystal structure in the original spherulite, which will induce nucleation sites as a self-seeding effect upon subsequent crystallization. N s increased by a factor of several thousand with a decrease in T f from 250 to 225 °C. The crystallization temperature dependence on the nucleation rate has a bell-shaped curve with a maximum nucleation rate (I max at T imax ). Both I max and T imax decreased with an increase in T f up to about 240 °C and then remained constant values. I max is a function of nucleation sites and is expressed by a power law of N s 0.5 . The nucleation density was found to be a function of the amount of 3 1 -helix composed with 5 monomeric units remaining in the melt.

Isotactic-poly(1-butene) (iPB1) shows superior mechanical properties after crystal-crystal transitions. Recently, Miyoshi et al. found that crystalline stems in metastable tetragonal crystal perform uniaxial rotational diffusions accompanying side-chain conformational transitions in the fast motional limit (correlation time, /s c S o10 À7 s; Macromolecules 2010, 43, 3986-3989.). In this study, molecular dynamics in stable trigonal crystal is investigated by solid-state nuclear magnetic resonance, which indicates that crystalline stems and side-chain conformations are completely fixed up to melting points (/s c S 410 s). In addition, lamellar thickness, /lS of iPB1 and a low isotacticity iPB1 (low_iPB1) with /mmmmS¼78%, respectively, were investigated by small-angle X-ray scattering. The low_iPB1 sample shows very week supercooling dependence of /lS (B5 nm), whereas iPB1 shows strong supercooling dependence of /lS (10-28 nm). On the basis of molecular dynamics and /lS results, molecular dynamics effects on structures and unique mechanical property of iPB1 are discussed.