Strong Memory Effect of Crystallization above the Equilibrium Melting Point of Random Copolymers

Abstract

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.

Communication: Theory of melt-memory in polymer crystallization

“…Fitting the derived expression for the nucleation rate for specific systems would lead to an understanding of the newly defined transition temperature and the effective dissolution temperature of the heterogeneous melt state. It would be of further interest to extend the present theory to account for additional concomitant mechanisms such as microphase separation, 17 liquid-liquid phase separation, 18,20,33 and entanglement effects.…”

“…Such a state can arise from local orientational ordering of chain segments, analogous to the smectic pearl-like baby nuclei which are strung together topologically by chain connectivity [26][27][28][29][30] or from local clustering, 17 as sketched in Fig. 2(a).…”

“…5 The same feature has been observed in other polymers as well. 17 In another set of experiments, Strobl and collaborators 13 have shown that the shapes of isotherms and characteristic crystallization times depend on the temperature of the melt T m prior to cooling to the crystallization temperature T c . They have identified two bounds of the melt temperature, one upper (T and T ℓ m , a crossover kinetics between the above mentioned two universal kinetics is observed with the nucleation time progressively decreasing with lowering of the melt temperature.…”

“…[6][7][8][9][10][11]15,16 However, there have been several investigations where any possible memory from such remnant crystalline domains is deliberately wiped out, but still observing melt-memory effects during second crystallization. 5,[12][13][14][17][18][19][20][21] As a classic example, Schultz observed that the maximum crystallization rate of linear polyethylene can be significantly lower if the sample was prepared at higher melt temperatures. 5 The same feature has been observed in other polymers as well.…”

“…As plausible explanation of this effect, existence of a precursor state, local clustering of chain segments in the molten state, and entanglement effects have been suggested in the literature. 13,17,19 Sometimes a distinction is made between two kinds of memory effects. 21 If the melt temperature is lower than the equilibrium melting temperature, some remnant crystalline domains could linger around and become responsible for melt-memory.…”