Melt Memory of Crystallization and Melt Structure of LLDPEs

Transcription

1 Melt Memory of Crystallization and Melt Structure of LLDPEs R. G. Alamo Chemical and Biomedical Engineering Department FAMU-FSU College of Engineering Tallahassee, Florida Work done by: B. Reid M. Vadlamudi A. Mamun X. Chen Y. Sang M. Ren

2 Polymer Crystallization: Nucleation and Growth G n = H T S The nucleation barrier is lowered in melts with foreign nuclei (nucleants) or with residual segmental order (self nucleation or melt memory). Crystallization is faster due to a depression in free energy. Sources of self nuclei may be: Traces of previous crystallites Remains of less entangled polymer chains at temperatures above melting Sheared melts that did not relax toward the equilibrated random-coil state. Homopolymers T > T m o Slow Crystallization Homopolymers T m T m o Tm< T <T m o Self-Nucleation Fast Crystallization

3 Crystallization of Homopolymers ( Experimental data for Linear polyethylene fractions) SNPA 316K Table 1. Characterization of Linear Polyethylene Fractions (SNPA Samples) Sample ID (a) (a) M w T Pl c T m (g/mol) ( C) ( C) SNPA4k SNPA1k 1 ~ SNPA16.5k SNPA69k SNPA12k SNPA189k SNPA316k 316 ~ SNPA8k T m = C SNPA 316K In the whole range of T melt (above T mo ), crystallization and melting are independent of T melt. Homopolymer melts above equilibrium don t have crystallization memory. B. Reid, R.G. Alamo et al., Macromolecules,46, 6485, 213 Random Copolymers?

4 Crystallization of Random Ethylene Copolymers Tm ( o C) Branches of 1-alkene comonomers longer than methyl are excluded from the crystal lattice Their melting behavior is explained on the basis of Flory s equilibrium theory: 1 T o m T 1 o m. copo The experimentally observed copolymer melting is well below the equilibrium value, as expected. R H Crystallites may survive at any temperature between the observed melting and the equilibrium value. Crystallization from temperatures above the equilibrium melting should be independent of the temperature of the initial melt. u ln p Equilibrium Hydrogenated polybutadienes Ethylene-butenes Ethylene-hexenes Ethylene-octenes Mole Percent Branch Points Copolymer melt Semicrystalline Copolymer Alamo et al.. J. Phys. Chem., 88, 6587, 1984 Alamo et al. Thermochim. Acta, 238, 155, 1994

5 T melt ( C) Crystallization in Random Copolymers Crystallization from temperatures above the equilibrium melting should be independent of the temperature of the initial melt Experimental evidence (Crystallization followed by DSC) HPBD (P24) Ethylene 1-butene Heat Flow (J/s) T m copo = C Heat Flow (J/s) HPBD (P24) Temperature ( C) Temperature ( C) T m copo = C T c peak ( C) First evidence of a strong crystallization memory in copolymer melts at T > equilibrium value Some type of seeds remain in the melt enhancing crystallization rate. Is this effect specific to the unique crystallization path of random ethylene copolymers?

6 Model Random Ethylene 1-Butene Copolymers Hydrogenated Polybutadienes Table 2. Characterization of HPBD Samples with ~2.2 mol% ethyl branching Sample ID M w BP T o (a) Pl m copo (g/mol) (mol%) ( C) P.8 8 ~ 1.1 ~ P P P P P P P P P12* 12 ~ 1.1 ~ P P HPBD 2.2 mol% Low molar mass No melt memory for HPBDs with Mw < ~ 7K

7 T onset ( C) T melt ( C) HPBD 2.2 mol% Mw > 1K P12* P18 P42 P49 P24 P16 T onset Very strong melt memory of crystallization above the equilibrium melting for copolymers with Mw >1K. Dramatic effect in nucleation density, as seen in POM. 14 T m copo = C T c ( C) P The strength of melt memory (highest T melt above T m o copo to observe MM) is molar mass dependent The extrapolated Mw where melt memory vanishes (13 g/mol) is the critical entanglement molar mass (Me) of polyethylene M w (g/mol)

8 T melt ( C) T melt ( C) T melt ( C) 2 Approaching T melt from below Model Ethylene / 1-butene, 2.2 mol% 2 Approaching T melt from above (open triangles) Model Ethylene / 1-butene, 2.2 mol% 2 Holding time in the melt from 5 to 6 min (red squares) Model Ethylene / 1-butene, 2.2 mol% T m copo = C T m copo = C T m copo = C T c,peak ( C) T c,peak ( C) T c,peak ( C) The MM effect is absent when approaching T melt from above, therefore the MM effect must be associated with the formation of copolymer crystallites Holding time up to 6 minutes, makes little difference in the crystallization in HPBDs with Mw/Mn ~ 1.1, hence, melt diffusion of any self-seeds is slow. B. Reid, R.G. Alamo et al., Macromolecules,46, 6485, 213

9 Structure of the self-seeds DSC and POM give no indication of any remaining crystallites above Tom copo and crystallographic reflections are absent in the WAXD patterns at T > 11 C. Crystallites do not survive at T > Tom copo 85 P18 Tmocopo.5 W/g C 4 C 15 C 13 C 14 C 18 C 11 C Intensity, a.u. 83 Tc (oc) Heat Flow (W/g) Temperature (oc) θ (degrees) I (q) No long range periodicity by SAXS. 4 C 14 C 2 C C 15 C.3 q (Å-1) 13 C 18 C.4 Clusters of long ethylene sequences with little or no order remain above Tom copo as long-live transients

10 T melt ( C) Crystallization of Narrow Random Copolymers Two melt structures: Homogeneous : Above T onset, long sequences randomize. Heterogeneous: Below T onset, long crystallizable sequences retain memory of their partitioning during crystallization P18 T onset T m copo = C Copolymer semicrystalline structure Heterogeneous melt Homogeneous melt T c ( C) We attribute the memory effect to Self- Nucleation or seeds made of molten clusters of ethylene sequences with little or no branching that remain as a memory of the crystallizable sequence length partitioning in the evolution of the initial crystallites. B. Reid, R.G. Alamo et al., Macromolecules, 46, 6485, 213 H. Gao, W. Hu et al. Macromolecules, 46, 6498, 213

11 Effect of Crystallinity on Melt Memory 2 Temperature ( o C) T c, peak ( C) Isothermal crystallization HPBD 2.2 mol% ethyl br. 2 o C 89 Isothermal Crystallization Non-isothermal Crystallization o C t c : -12 min 14 o C X c,threshold ~6% 5 4 o C X c,threshold ~12% X c (%) Time (min) Only when sufficient Xc develops, is the topology of the melt sufficiently constrained (Knots, ties,..) to prevent fast diffusion of the sequences in the initial crystallites. Chen, Mamun, Alamo, Macromol. Chem. Phys. 215, 216, 122

12 T onset - T m o copo ( C) Effect of Comonomer Content on Melt Memory Table 3. Characterization of HPBD samples with varying ethyl branching content. Sample ID M n M w BP Pl (g/mol) (mol%) ( C) SNPA12k P18K ArgP P P T m copo (a) Inter-crystalline topological constraints decrease with increasing comonomer content due to a reduced level of crystallinity % 4.14% 3.6% 2.2% T melt ( C) T c ( C) Branching Content (mol %)

13 T melt ( o C) Intensity (a.u) Commercial Broadly Distributed Ethylene 1-Alkene Copolymers Many commercial LLDPEs with enhanced impact and tear properties have a broad, bimodal comonomer content distribution. They resemble blends of copolymer chains with different comonomer contents. The melt topology and phase structure are drastically affected by the distribution TREF ~1 mol % br Ethylene 1-hexene 5 ~8 mol% br Elution Temperature ( o C) 2 18 A 16 B M. Vadlamudi, R.G. Alamo et al., Macromol. Symp., 282, 1, 29 M. Vadlamudi, R.G. Alamo et al., JTAC,, 96, 697, Issue 3, C D Inversion of crystallization rate with decreasing temperature of the initial melt from 15ºC to 123ºC T c,peak ( o C) A. Mamun, X. Chen, R.G. Alamo, Macromolecules, 47, 7958, 214

14 T melt ( o C).2 µm The inversion of the rate demarcates the onset of Liquid-Liquid Phase Separation between comonomer rich and comonomer poor molecules. LLPS materials with enhanced impact Basis: Data on binary blends Wignall, Alamo et al., Macromolecules, 34, 816, 21 C.C. Han et al., Macromolecules, 35, 172, 22 Phase Diagram of EH (1 mol%) + EB (8 mol%) 2 18 A ~5% by mass ~8 mol% br ~1 mol % br Temperature ( o C) 16 B 14 C T c,peak ( o C) D.5 µm TEM provides some evidence for LLPS Interplay Between Self-Nucleation and Liquid-Liquid Phase Separation: The strong thermodynamic drive during LLPS drives molecular diffusion and effectively dissolves self-seeds. Phase separated melt Region A: Homogeneous one-phase melt Region B: Heterogeneous one-phase melt Region C: Two liquid-liquid phase melt Region D: Region of partial melting

15 T melt ( o C) T c,peak ( o C) Broadly Distributed Ethylene 1-Alkene Copolymers. Diffusion a 5 min at 135 C 15 o C o C o C 2 h at 135 C T c,peak ( o C) time (min) Segmental dynamics to fully dissipate self-seeds are very slow. Molecular diffusion inside the binodal accelerates seeds dissipation.

16 Conclusions The strong melt-memory observed in LLDPEs is caused by seeds made of molten clusters of ethylene sequences with no branching that remain in the melt as a memory of the crystallizable sequence length partitioning to form the initial crystallites. The onset of melt memory depends on molar mass, comonomer content, and comonomer type. Strong MM in copolymers requires a relatively high degree of crystallinity. Explained by the concept of Trapped knots and other entanglements : Seeds remain in the melt due to increased restrictions of the copolymer crystalline sequences to diffuse upon melting and to reach the initial random topology of the copolymer melt. In broadly distributed copolymers, the self-seeds assist LLPS reversing (retarding) a subsequent crystallization from the two liquid-phase region. 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.

17 Acknowledgments W. Hu, H. Gao, Nanjing University, China B. Monrabal (PolymerChar for EO samples) B. Goderis, Catholic University of Leuven D. Fiscus, ExxonMobil Funding from NSF