Session V42: Polymer Architecture, Control of Structure and Dynamics in Polyolefins

Melt structure and self-nucleation of ethylene copolymers

A strong memory effect of crystallization has been observed in melts of random ethylene copolymers well above the equilibrium melting temperature. These studies have been carried out by DSC, x-ray, TEM and optical microscopy on a large number of model, narrow, and broad copolymers with different comonomer types and contents. Melt memory is correlated with self-seeds that increase the crystallization rate of ethylene copolymers. The seeds are associated with molten ethylene sequences from the initial crystals that remain in close proximity and lower the nucleation barrier. Diffusion of all sequences to a randomized melt state is a slow process, restricted by topological chain constraints (loops, knots, and other entanglements) that build in the intercrystalline region during crystallization. Self-seeds dissolve above a critical melt temperature that demarcates homogeneity of the copolymer melt. There is a critical threshold level of crystallinity to observe the effect of melt memory on crystallization rate, thus supporting the correlation between melt memory and the change in melt structure during copolymer crystallization. Unlike binary blends, commercial ethylene-1-alkene copolymers with a range in inter-chain comonomer composition between 1 and about 15 mol {\%} display an inversion of the crystallization rate in a range of melt temperatures where narrow copolymers show a continuous acceleration of the rate. With decreasing the initial melt temperature, broadly distributed copolymers show enhanced crystallization followed by a decrease of crystallization rate. The inversion demarcates the onset of liquid-liquid phase separation (LLPS) and a reduction of self-nuclei due to the strong thermodynamic drive for molecular segregation inside the binodal. The strong effect of melt memory on crystallization rate can be used to identify liquid-liquid phase separation in broadly distributed copolymers, and offers strategies to control the state of copolymer melts in ways of technological relevance for melt processing of LLDPE and other random olefin copolymers. References: B. O. Reid, et al, Macromolecules 46, 6485-6497, 2013 H. Gao, et al, Macromolecules 46, 6498-6506, 2013 A. Mamun et al, Macromolecules 47, 7958-7970, 2014 X. Chen et al, Macromol. Chem. Phys. 216, 1220$-$1226, 2015 M. Ren et al, Macromol. Symp. 356, 131--141, 2015   

Crystal Structures of Precise Functional Copolymers: Atomistic Molecular Dynamics Simulations and Comparisons with Experiments

Layered crystal structures have been observed in linear poly(ethylene-co-acrylic acid) in which the carboxylic acid groups are placed precisely every 21 carbon atoms along the backbone. The alkane segments form structures resembling orthorhombic polyethylene crystals, while the acid groups form continuous domains that may act as pathways for ion conduction. Further details of the crystal structure have been difficult to elucidate experimentally, but could be important for understanding structure-property relationships. Here, two classes of crystal structures are evaluated via atomistic molecular dynamics: extended chain structures, wherein the polymer backbones are highly extended in near-trans conformations, and adjacent reentry structures, wherein the polymer backbones conform in adjacent reentry loops near the site of each covalently-bonded acid group. Energies of relaxed structures and hydrogen bonding states are compared, and X-ray scattering and other experimental data is compared with the simulation results.   

Modeling neutron scattering in disperse, nonuniformly labeled commercial polyolefins

In spite of their chemically simple monomer elements, understanding of many structural, thermodynamic, and other aspects of polyolefins has remained elusive. Scattering studies on polyolefins are challenged by their nearly identical density in the melt, requiring the use of deuterium-labeling to provide contrast for small-angle neutron scattering (SANS). Until recently, labeling of commercial polyolefins has been prohibitively costly, leading SANS investigations on polyolefins to focus on non-disperse model systems. Commercial polyolefins often have broad molecular weight and composition distributions, and such dispersity plays an important role in their rheology, crystallization, and mechanical properties. Recent reports have described facile hydrogen-deuterium exchange reactions that preserve the chain architecture of polyolefins. However, such exchange is not uniformly distributed across the chain population. Here, we report a generalized application of the Random Phase Approximation prediction for SANS from homogeneous polymer blends to account for such dispersity. A Monte-Carlo method is used to calculate the deuterium distribution that corresponds to SANS measurements. These methods provide powerful tools for probing the structure of disperse polymer architectures.   

Time-resolved WAXD studies on the crystallization of semicrystalline/graphene nanocomposites

Graphene is one layer of carbon atoms, which has good electronic, thermal conductivity, and mechanical properties. By adding graphene to semicrystalline polymers such as polyethylene (PE) and isotactic polypropylene (iPP), the mechanical and electrical properties of the polymers are significantly improved. To further achieve high performance of semicrystalline polymer/graphene nanocomposites, it is important to investigate the relationship between the crystalline structure of the polymer and the mechanical property of the nanocomposites. In this study, the effect of graphene on the crystallization behavior of PE and iPP was investigated by using time-resolved wide angle X-ray diffraction (WAXD). The in situ WAXD results on the melt-crystallization process showed that the crystalline structures of all the samples remained the same as the pure bulk, while both the rate and degree of the crystallinity increased. We will discuss the detailed structure information along with DSC and mechanical test results.   

\textbf{Porous Polyolefin Films via Polymer Blends}

Porous polymer films have broad application including battery separators, membrane supports and filters. Polyolefins are attractive for these applications because of their solvent resistance, low electrical and thermal conductivity, easy fabrication and cost. We will describe fabrication of porous films using cocontinuous blends of a polyolefin with another polymer which can be readily removed with a solvent. Methods to image and control the cocontinuous morphology will be presented.\\ \\Bell, J. R., K. Chang, C. R. Lopez-Barron, C. W. Macosko, and D. C. Morse, "Annealing of cocontinuous polymer blends: effect of block copolymer molecular weight and architecture," Macromolecules \textbf{43}, 5024-5032 (2010).\\ Lopez-Barron, C. R., and C. W. Macosko, "Direct measurement of interface anisotropy of bicontinuous structures via 3D image analysis," Langmuir \textbf{26}, 14284-14293 (2010).\\ Trifkovic, M., A. T. Hedegaard, K. Huston, M. Sheikhzadeh, and C. W. Macosko, "Porous films via PE/PEO cocontinuous blends," Macromolecules \textbf{45}, 6036-6044 (2012).\\ Hedegaard, A.T., L.L. Gu and C. W. Macosko, ``Effect of Extensional Viscosity on Cocontinuity of Immiscible Polymer Blends'' J. Rheol. 59, 1397-1417 (2015).   

Quantifying tie-molecule content in semicrystalline polymers

Tie molecules bridging adjacent crystalline lamellae in semicrystalline polymers strongly impact mechanical properties, but they remain difficult to characterize. We demonstrate a new method of measuring tie-chain content: applying equilibrium swelling theory to small-angle neutron scattering patterns from semicrystalline polyethylene films whose interlamellar amorphous regions are swollen with deuterated organic solvent in a vapor-flow sample environment. To aid in validating the measurement, measured tie-chain content is compared with a primary structural parameter (PSP2) that is calculated from molecular architecture and correlates with slow crack growth behavior. Agreement is favorable for a linear polyethylene and a series of ethylene-hexene copolymers. Recent applications of the technique are also discussed.   

Understanding the Evolution in Meso/Nanostructure in UHMWPE Fibers

Ultra-high-molecular-weight polyethylene (UHMWPE) fibers are increasingly used in composite armor applications. Understanding the complex sub-filament structure which ultimately dictates macroscopic mechanical performance is important as a materials by design approach is taken. In this work, the meso/nanostructure of fibers is studied through a series of atomic force microscopy, X-ray diffraction, and tomography techniques. Fibers with varying thermomechanical processing histories and macroscopic mechanical properties are examined to correlate the evolution of structure with fiber mechanical response in both tension and transverse compression. This work spotlights the sub-filament structural hierarchy in the fiber. The study of nanoscale fibril geometry and crystal structure provides some insight into the load pathways within the fiber, while the identification of a three-dimensional fibrillar network indicates the presence of complex mechanical interactions throughout. The presence and geometry of mesoscale voids in highly drawn fibers is discussed, and tomography results are presented to further understand their distribution throughout the fiber. Finally, the presence of these features are explored in the context of their influence on the energy dissipative capabilities at the fiber level.   

Unfolding of Isotactic Polypropylene under Uniaxial Stretching

Despite numerous investigations on polymer processing, understanding the deformation mechanisms of semicrystalline polymer under uniaxial stretching is still challenging. In this work, $^{\mathrm{13}}$C-$^{\mathrm{13}}$C Double Quantum (DQ) NMR was applied to trace the structural evolution of $^{\mathrm{13}}$C-labeled isotactic polypropylene ($i$PP) chains inside the crystallites stretched to engineering strain (e) of 21 at 100 \textdegree C. DQ NMR based on spatial proximity of $^{\mathrm{13}}$C labeled nuclei proved conformational changes from the folded chains to the locally extended chains induced by stretching. By combining experimental findings with literature results on molecular dynamics, it was concluded that transportation of the crystalline chains plays a critical role to achieve large deformability of $i$PP.   

Deformation across length scales in polyolefines: effect of the chain microstructure on the polymorphism, phase transitions and morphological changes.

The transformations related to phase changes of the crystals, and at lamellar length scales by effect of tensile deformation are studied in the case of some isotactic polypropylene samples having high molecular mass, polydispersity index $\approx $2, and stereodefects at different concentrations and with a uniform distribution, The stress induced transformations are followed in real time during stretching through wide and small angle X-ray scattering measurements. The data analysis evidences that during the transformations of the spherulitic into the fibrillar morphology, stress-induced phase transitions occurring during plastic deformation are regulated by the same factors that govern the textural and morphological changes, that is the ability of the entangled amorphous chains to transmit the stress and the intrinsic stability of the lamellar crystals. Since the relative stability of the different polymorphic forms involved in the structural transformations and the intrinsic flexibility of the chains depend on the stereoregularity, precise correlations between the stereoregularity of the chains, and the deformation behavior are outlined, paving the way for understanding the material properties at molecular level.   

Topological Constraints on Chain-Folding Structure of Semicrystalline Polymer as Studied by $^{13}$C-$^{13}$C Double Quantum NMR

Chain-folding process is a prominent feature of long polymer chains during crystallization. Over the last half century, much effort has been paid to reveal the chain trajectory. Even though various chain-folding models as well as theories of crystallization at molecule levels have been proposed, they could be not reconciled due to the limited experimental evidences. Recent development of double quantum NMR with selective isotope labeling identified the chain-trajectory of $^{13}$C labeled \textit{isotacti}c poly(1-butene). The systematic experiments covered a wide range of parameters, i.e. kinetics, concentration, and molecular weight ($M_{w})$. It was demonstrated that i) adjacent re-entry site was invariant as a function of crystallization temperature ($T_{c})$, concentration, and$ M_{w}$, ii) long-range order of adjacent re-entry sequence is independence of kinetics at a given concentration while it decreased with increasing the polymer concentration at a given $T_{c}$ due to the increased interruption between the chains, and iii) high $M_{w}$ chains led to the multilayer folded structures in single crystals, but the melt state induced the identical short adjacent sequences of long and short polymer over a wide range of $T_{c}$ due to the entanglements. The behaviors indicated that the topological restriction plays significant roles in the chain-folding process rather than the kinetics. The proposed framework to control the chain-folding structure presents a new perspective into the chain organization by either the intra- or inter-chain interaction.