Session A33: Liquid Crystalline and Semicrystalline Polymers

Decoupling how structure and processing affects the properties of Liquid Crystal Elastomers.

The apparently simple and facile addition of liquid crystalline order to a crosslinked polymer network elevates a material from a conventional entropic elastomer to a mechanically complex and stimuli responsive system. The mechanical properties evolved - anisotropy, mechanical dissipation, and negative Poisson ratios - are intrinsic properties which are reminiscent of those found in natural materials, and which typically are only seen in composite materials featuring a micro- or macro-scopic structure. The emergent shape actuation behaviours are responsive to a wide range of thermal, optical, chemical, and electrical stimuli and again the behaviour of example devices mimics those seen in living organisms.

Despite all these celebrated phenomena and example devices, much remains to be understood in terms of how molecular structure, monomer compositions, and processing influences the properties and behaviours of LCEs, particularly when understanding their dynamical behaviours.

In this talk, I will focus on our work to decouple the effects of structure, composition, and processing on the properties of liquid crystal elastomers. I will consider three examples: The compositional effects on the dynamic and quasi-static mechanical behaviours of LCEs, tuning and understanding the processing parameters of 3D printed liquid crystal elastomers, and the properties of chemically identical liquid crystal elastomers templated with different liquid crystalline phases.   

Mesoscale Simulations of Liquid Crystalline Diblock Copolymers

Side-chain liquid crystal (SCLC) diblock copolymers possess the characteristics of both block copolymers and liquid crystalline polymers. This combination makes SCLC block copolymers useful for membrane separation applications and more responsive to external stimuli, enabling one to drive nanoscale assembly over large length scales. While particle-based and continuum methods have long been established for liquid crystalline materials, mesoscale methods that can capture structure on the 10-100 nm length scale have been relatively lacking. We have developed a mesoscale model that we are using to study both isotropic-to-nematic and smectic phases as well as the order-disorder phase transitions of SCLC diblock copolymers. We have studied the interference between the order to disorder transition of the block copolymer phases and the liquid crystal transitions, finding that the nematic phase suppresses the formation of phases without at least one direction of symmetry.   

Self-assembly and liquid crystal behavior of computationally designed peptide coiled-coil bundlemers with parallel symmetry

Computationally designed α-helical peptides formed by 29 amino acids can assemble into the homotetrameric, parallel coiled coils in the aqueous solution, which are also called ‘bundlemers’. Bundlemers have a hydrophobic interior core and multiple amino acid side-chain residues on the exterior as possible modification positions for targeted solution behavior. The exposed N-termini additionally provide potential sites for ‘click’ chemistry functional pairs to form multibundlemer chains by covalent linkages. Parallel folding coiled coil bundlemers offer the unique possibility for monodisperse, rigid, dimer bundlemer rods when linked together in a head-to-head fashion. Short rods are observed with transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS).  Results show the monodisperse rod length and diameter consistent with the computationally designed parallel, homotetrameric bundlemer building block connected into dimer rods. Optical birefringence phenomenon is observed in concentrated rod solution under the polarized optical microscopy (POM). Different exterior surface modification strategies will be discussed to produce liquid crystals formed by different mechanisms.   

Liquid Crystalline Nonconjugated Open-Shell Organic Molecules

Recently, nonconjugated radical polymers, organic radicals have shown specific conductive, electrochemical, and magnetic properties that make them potential candidates to supplement, or replace, more established electronic materials. Moreover, liquid crystals (LCs) and single crystals are already well-known as suitable materials for optoelectrical devices due to their intriguing nanostructural features. However, radical-containing LCs have been infrequently reported. This work focuses on understanding the crystalline and LCs structures shown by a class of 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) derivatives. Depending on the length of the carbon chain on the tail of TEMPO LCs, various crystalline phases and liquid crystalline (e.g., the nematic and smectic phases) were observed. For the crystalline materials, the structure was calculated from the unit cell of TEMPO-C_S and the nearest TEMPO-TEMPO distance were 6.14 Å and 6.12 Å. Specific phases and the growth of the LCs were determined using polarized optical microscopy (POM). One radical crystal shows electrical conductivity of ~ 16.97 S m^-1, which is the highest value reported for nonconjugated radical conductors over a 1 mm scale. However, the other material shows 10,000 times lower conductivity despite the similar molecular structure. These results present a structure-property relationship, the design rules, and the charge transport mechanism for a new set of radical-based small-molecule materials.   

Entropic Barrier Theory of Polymer Melting and Energy Cascade

We will present a theory of melting kinetics of semicrystalline polymers at temperatures above the equilibrium melting temperature, by accounting for conformational entropy of chains during melting and the characteristics of lamellae. We show that melting of lamellae is always accompanied by a free energy barrier which is entirely entropic in origin. In terms of the parameters characterizing the lamellae and the extent of superheating, closed-form formulas will be presented for the equilibrium melting temperature, driving force for crystallization, free energy barrier height, average expulsion time of a single chain from a lamella, and the melting velocity of lamellae. The predicted dependence of melting velocity on superheating is nonlinear and non-Arrhenius, in qualitative agreement with experimental observations reported in the literature. In addition, the cascade of dissipation of injected energy into semicrystalline polymers will be addressed.   

Molecular Dynamics Simulations of Flow Induced Crystallization of a Simple Polymer Melt Model

Flow induced crystallization is a process with significant theoretical and industrial interest in the field of polymer processing and applications that involve semicrystalline materials. Nucleation is the first step of this complex, nonequilibrium process, and though it has been extensively studied, there are still fundamental questions about its mechanism that remain unanswered.

Our study focuses on the role of shear flow in the orientation and alignment of model polymer melts without chemical detail, and the nematic order developed in these melts. Using a dissipative particle dynamics (DPD)-type model, we conduct molecular dynamics simulations and investigate the effect of polydispersity and entanglements in the crystallization of polymer melts. We quantify the extent of crystallization under different flow strength and attempt to gain insights in the rate of crystallization, pre-cursor nuclei, and the overall morphology of polydisperse semi-crystalline melts.   

Effects of entanglements on the crystallization and morphology of semicrystalline polymers

Crystallization of polymers from entangled melts generally leads to the formation of semicrystalline materials with a nanoscopic morphology consisting of stacks of alternating crystalline and amorphous layers. The factors controlling the thickness of the crystalline layers were extensively studied; however, there is no quantitative understanding of the thickness of the amorphous layers. We elucidate the effect of entanglements on the semicrystalline morphology by the use of a series of model blends of high-molecular weight polymers with unentangled oligomers leading to a reduced entanglement density in the melt as characterized by rheological measurements. Small-angle X-ray scattering experiments after isothermal crystallization reveal a reduced thickness of the amorphous layers, while the crystal thickness remains largely unaffected. We introduce a simple, yet quantitative model without adjustable parameters, according to which the measured thickness of the amorphous layers adjusts itself in such a way that the entanglement concentration reaches a specific maximum value. Furthermore, our model suggests an explanation for the large supercooling that is typically required for crystallization of polymers if entanglements cannot be dissolved during crystallization.   

Rheological hysteresis in semicrystalline polymers during crystallization and melting

Semicrystalline polymers, like polypropylene, are known to exhibit history dependent properties. This history dependence puts additional emphasis on the polymer processing pathways used in manufacturing; processing conditions must be designed with final product properties in mind. Here we show that semicrystalline polymers do not melt in the same way that they crystallize, and this difference has direct effects on the rheological properties of a material; it is possible to have the same material at the same crystallinity behave differently, simply due to the direction that the semicrystalline state was approached (heating or cooling). We simultaneously measure small-amplitude rheological behavior and crystallinity of polypropylene during temperature sweeps using rheo-Raman spectroscopy. We observe that for a given temperature rheological properties are dependent on if the polymer is crystallizing or melting; the polymer exhibits thermal-rheological hysteresis. Interestingly, and counterintuitively, we also observe that a semicrystalline polymer can exhibit different rheological properties at the same crystallinity depending on whether the polymer is crystallizing or melting; the polymer exhibits crystallinity-rheological hysteresis as well. We then apply a generalized effective medium model for crystallization processes, finding that the percolation process from a spanning semicrystalline network contributes to the observed hysteresis.   

Specific Work as a Useful Way to Describe Flow of Polyethylene During Injection Molding

The flow-induced polymer chain orientation and subsequent crystallization of polyethylene (PE) is a ubiquitous aspect of processing materials into final articles. Better understanding of how polymer flow affects morphology and crystallization will allow industry to better design materials for downgauging and recycling. While specific work has been shown to be a useful way to describe polymer flow using rheometers in a lab setting, little effort has been put into applying these ideas to commercial scale processing of polyethylene. Here we will describe how specific work can be used to describe the observed polymer orientation, morphology, and crystallization behavior in an injection molded bar. The position dependent flow, both through the thickness and along the length of the molded bar, can be described by modeling the specific work experienced by the material during the entire process.   

Molecular dynamics simulations of polyethylene inter-crystalline phase formation

Semicrystalline microstructures consisting of both the crystalline and amorphous regions, govern the properties of many polymer materials. The formation of the semicrystalline morphology, especially the structural evolution in the amorphous regions, is still not well understood. Here we apply atomistic molecular dynamics simulations to crystal growth in model polyethylene (PE) samples. We placed two crystalline seeds into the PE melt and quenched the melt to 350K to trigger instantaneous crystallization. We quantitatively analyze the crystal growth and formation of the inter-crystalline region for samples with seeds of various inter-seed distances and relative orientations. Using the  Z1+ method, we show that lamellar thickening is rapid until the lamellar thickness is about the size of the entanglement strands in the melt precursor state. Further crystallization requires entanglement relaxation, which leads to impeded crystal growth. Over time, a layer of “trapped” entanglements formed among loops and tie chains accumulate near the crystal surface and hinder further stem lengthening. Using simulations, we also quantitatively investigated the stress transmitters, which are the entangled loops and tie molecules, in the inter-crystalline region. We show a modified Huang-Brown (HB) model can estimate the tie-chain fraction between two crystalline slabs.   

Particle Concentration Promotes Flow-Induced Crystallization of High Molecular Weight Isotactic Polypropylene

Flow-induced crystallization occurs when semicrystalline polymer melts are subjected to large deformations prior to supercooling. The entropy reduction and chain orientation involved in this process can form flow-induced precursors, leading to faster crystallization kinetics and changes in crystalline morphology. With increasing levels of shear stress, isotactic polypropylene can form highly oriented structures, which contribute to greatly improved material properties. Herein, the effect of particle concentration (specifically, catalyst residue) on the flow-induced crystallization of two samples of isotactic polypropylene was investigated through shear rheology and ex-situ simultaneous small/wide angle X-ray scattering (SAXS/WAXS). Upon the application of flow, the sample with higher particle concentration crystallized at faster rates relative to the sample with fewer heterogenous impurities. The nucleation ability of these particles was particularly pronounced at lower levels of deformation, while flow effects became prominent as larger deformations were applied. For sufficiently strong flows, a lower critical shear stress was observed for the formation of shish-kebab structures in the sample with higher concentrations of particles. At equivalent levels of specific work within both flow regimes, the morphology of these anisotropic structures were found to be characteristically distinct from one another.   

Temperature-Dependent FTIR (TD-FTIR) Analysis of PEO-b-PCL Self-Nucleation

Controlling crystallization in biomedically-relevant block copolymers such as PEO-b-PCL has potential to tailor degradation and drug release properties. Our previous work has demonstrated that the crystallization sequence in symmetrical PEO-b-PCL copolymers can be switched using selective solvent casting. The crystals' metastabilities are dependent on their nucleation and growth processes. These processes are altered when the crystallization sequence is inverted. We used self-nucleation studies to evaluate these changes in metastability. Because the polymer weight fraction of each block is nearly identical, DSC studies cannot easily identify the block undergoing melting and crystallization events during thermal processing. TD-FTIR provides a unique opportunity to look at how the individual blocks of PEO and PCL are exhibiting thermal transitions during the self-nucleation processes.    

Electrostriction-enhanced piezoelectric property of poly(vinylidene fluoride) via high-power ultrasonication

Despite decades of research, the structure-piezoelectric property relationship is still unclear for ferroelectric polymers, such as poly(vinylidene fluoride) (PVDF); therefore, their piezoelectric coefficients (-d₃₃ and d₃₁) are often limited to ~30 pC/N. Recently, we report electrostriction-enhanced piezoelectricity in PVDF, taking advantage of the head-to-head and tail-to-tail (HHTT) defects (usually 3-6 mol.%). After high-power ultrasonication, the piezoelectric performance of PVDF can be significantly enhanced. Two PVDF homopolymers with different HHTT contents were studied. The PVDF with a lower HHTT content (4.3%) exhibited a higher melting temperature (T_m, denoted

as HMT), whereas that with a higher HHTT content (5.9%) exhibited a lower T_m (denoted as LMT). In addition to the primary crystals (PCs) and the isotropic amorphous fraction, X-ray diffraction also suggested the presence of the oriented amorphous fraction (OAF) and secondary crystals (SCs), which are important in enhancing the piezoelectricity for PVDF. Intriguingly, the LMT PVDF exhibited higher piezoelectric performance than the HMT PVDF, because it had a higher OAF/SC content. In addition, high-power ultrasonication was shown to effectively break relaxor-like SCs off from the PCs, further enhancing the piezoelectric performance. That is, the inverse piezoelectric coefficient d₃₁ reached as high as 76.2 pm/V at 65 ^◦C for the ultrasonicated LMT PVDF. The insight from this study will enable us to design better piezoelectric PVDF polymers for practical electromechanical applications.