Session Y04: Polymer Crystals and Crystallization II

Live

One of the fundamental laws in crystallization is translational symmetry, which accounts for the profound shapes observed in natural mineral crystals and snowflakes. The translational symmetry, however, is broken in a class of polymer crystals defined as shape-symmetry incommensurate crystals (SSICs) which include helical, helicoidal, scrolled, tubular crystals, and the newly discovered crystalsomes. In this talk, we discuss the crystallization behavior of crystalline molecular bottlebrush (mBB) polymers. Spherical hollow crystalline shells are formed via solution crystallization. The unique structure is named as mBB crystalsome (mBBC), highlighting its similarity to the classical molecular vesicles and previously reported crystalsomes. The spherical morphology of mBBCs suggests the spontaneous translational symmetry breaking during crystal growth. Fluorescence resonance energy transfer (FRET) experiments demonstrated that the mBBC formation is driven by local chain overcrowding-induced asymmetrical lamella bending.   

Live

Crystallization is an essential step in the processing of most polymers. It takes place under conditions of rapid cooling and high strain rate, and is strongly accelerated relative to quiescent conditions. The initial step in this process is flow-enhanced nucleation (FEN), which occurs on time and length scales that are hard to capture experimentally. To resolve this problem, we use nonequilibrium molecular dynamics simulations to characterize FEN from a melt of linear polyethylene-like chains. Both short and long (entangled) chains are simulated. First, methods are described for identifying the critical nucleation event using mean first-passage times (MPFT). Fitting of the data to a master equation is used to extract important thermodynamic and kinetic quantities. Results for nucleation kinetics accelerated under different modes of deformation, e.g. simple shear or uniaxial extension, and rates of strain are used to assess several of the existing models in the literature for flow-enhanced nucleation, based on their abilities to describe the data accurately, and new models based on the orientational ordering of Kuhn segments induced by flow are proposed. Evidence is presented for a breakdown of classical nucleation theory for entangled polymer melts at high strain rates, which in turn is traced to the flow-induced formation of nematic domains in the melt. The appearance of such domains suggests a different perspective on the underlying physics of flow-enhanced nucleation of long chain molecules.   

Live

We employ Monte Carlo simulations [1] to study the phase behavior of linear, freely jointed chains whose spherical monomers interact through the square well potential [2]. We investigate in detail the effect of attraction intensity and range on the ability of chains to crystallize. Depending on the simulation conditions the chain cluster is glassy or highly ordered. By varying the attraction range distinct crystals are obtained: from close packed crystals of pure face centered cubic or hexagonal close packed character to more dilute hexagonal and body centered cubic ones. A simple geometric argument explains the expected dominance of each different crystal. Present results allow a further comparison between chain and monomeric analogs [3]. Such insights can help the design of colloidal and granular polymers with short-range attractions.
[1] P. Ramos, N. C. Karayiannis and M. Laso, J. Comput. Phys. 375, 918 (2018).
[2] M.Herranz et al. Polymers 12, 1111 (2020).
[3] N. C. Karayiannis, K. Foteinopoulou and M. Laso, Phys. Rev. Lett. 103, 045703 (2009).   

Live

While semicrystalline polymers are prevalent in today’s markets and homes, many facets of the crystallization process remain mysterious. For example, connectivity and stiffness in polymers complicate the nucleation process greatly, introducing nematically-ordered phases. There has been recent interest in understanding the relative stability of these phases, and their role in mediating the melt-to-crystal transition in polymers. To investigate this claim, we have investigated the possibility of using a GPU-accelerated Wang-Landau Monte Carlo algorithm with an efficient polymer move set to sample the full density of states (DOS) of a crystallizing polymer melt. In principle, the DOS contains full thermodynamic information, making it possible to construct a free energy surface based on crystallization-relevant order parameters. We will discuss the challenges with this approach and our recent progress.   

Live

Dynamic polymer networks with topology conserving exchange reactions (vitrimers) have emerged as a promising platform for sustainable and reprocessable materials. While the role of dynamic bonds on stress relaxation and viscosity is well documented, their role on crystallization kinetics is not well understood. Precise ethylene vitrimers with 8, 10, or 12 methylene units between boronic ester junctions were studied to understand the impact of bond exchange on crystallization kinetics and morphology. Compared to linear polyethylene which has been heavily investigated for decades, a long induction period for crystallization is seen in the vitrimers (100 – 1000 min) and increases with increasing crosslink density, ultimately taking weeks in the densest networks. An increase in melting temperatures (Tm) of 25-30 K is observed with isothermal crystallization over 43200 min. Wide angle x-ray scattering (WAXS) shows an unexpected crystal – crystal transition from the canonical orthorhombic to a proposed monoclinic phase, which is also tracked with optical microscopy (OM). Control experiments on a precise permanent network indicate that dynamic bonds are facilitating long-time rearrangements of the crystals, which is critical to understand for applications of semi-crystalline vitrimers.   

Live

Polyamide 66 (PA66) blends offer a good balance between material properties and cost. A good understanding of the processing of PA66 and its blends at conditions including fast cooling or high undercooling serves immediate industry interest. In order to study crystallization at such conditions, flash scanning chip calorimetry (FSC) has been utilized to study crystallization kinetics of PA66 and its blends with polycaprolactam (PA6). Subsequent experiments using WAXS, Micro-IR, and POM have been carried out on samples mounted on FSC chips. Isothermal analysis of the crystallization rate indicates crystallization in blends becomes slower with a higher blend content. Interestingly, bimodal temperature-dependent kinetics, observed in PA66 homopolymer, gradually merges into a unimodal crystallization kinetics profile as PA66 blend fraction decreases. The mechanism that leads to the changes in isothermal crystallization kinetics will be discussed.   

Live

Crystallization of liquids often starts at the interface to a solid. The underlying process can be either heterogeneous nucleation or the recently observed process of prefreezing. The latter is the reversible abrupt formation of a crystalline layer at the melt-solid interface at temperatures T_max above the bulk melting point. We present a phenomenological theory of prefreezing and derive such equilibrium properties as the temperature dependent thickness of the prefrozen layer, the prefreezing temperature T_max, and the mesoscopic jump of thickness l_min at T_max.¹ The theory provides a clear thermodynamic explanation of the abrupt formation of a crystalline layer resulting from the interplay of the interfacial energies of the substrate-crystal γ_sub,cry, crystal-melt γ_cry,melt, and substrate-melt γ_sub,melt interfaces. It is found that while the prefreezing temperature T_max depends primarily on the difference of the interfacial energies Δγ = γ_sub,melt – (γ_sub,cry + γ_cry,melt), the minimum jump of thickness l_min is controlled by the ratio (γ_sub,cry + γ_cry,melt) / γ_sub,melt. Thus, T_max and l_min can vary independently.

[1] Dolynchuk et al. J. Phys. Chem. Lett. 2019, 10 (8), 1942-1946.   

Live

Fast and anisotropic chain dynamics coupled with structural disorder significantly contribute to structural evolution of several semicrystalline polymers. We report a new class of structural disorder, “configurational disorder coupled with conformational flexibility (CDCF)” in polymer crystal. It is revealed that atactic-hydrogenated poly(norbornene) (hPNB) conducts slow conformational dynamics below crystal-crystal transition temperature (T_cc) and fast uniaxial chain dynamics above T_cc while isotactic (i)- and syndiotactic (s)-hPNBs do not perform such fast chain dynamics. It is for the first time demonstrated that the presence of fast chain dynamics leads to much higher crystallinity (80 %) and much longer long-period (L = 80 nm) in a-hPNB than those of s- and i-hPNBs (crystallinity of 50-55 % and L of 17-21 nm). It is concluded that CDCF plays an important role for phase transition, fast chain dynamics, and unique structural evolution in stereo-irregular polymer. .   

Live

A crystal structure in which functional groups are present in layers is characteristic of many linear polymers with regularly spaced moieties. We report the first known instance where layered crystal formation is bypassed by rapid quenching. Long-spaced aliphatic polyesters with CH₂ spacer lengths between 18 and 48 form orthorhombic, highly symmetric, layered crystals on slow crystallization. Though crystals are still lamellar and orthorhombic on rapid quenching, ester layer periodicity disappears from X-ray patterns, indicating formation of unlayered crystals. Development of unlayered crystals requires a larger quenching depth with decreasing CH₂ spacer length. Unlayered crystals transform to the layered type on heating. We posit that on fast crystallization, lamellar crystals form via random chain folding and fast staggering of polyester segments, while on slow crystallization ester layering is facilitated by maximizing packing of the full length of CH₂ units and intermolecular dipole interactions of esters. At the same undercooling, enhanced contribution of esters as defects in unlayered crystals limits long-range packing symmetry, leading to crystals which are thinner and metastable with respect to their layered counterparts.   

Live

To effectively transition to green polymers, it is essential that such polymers are able to meet or surpass mechanical properties of existing petroleum polymers such as PE, PP, and PET. In a prior publication ¹ we have shown how brittle PLA can be made ductile by uniaxial melt extension and resistant to heat well above its T_g of 60 ^oC. The present work further explores the molecular physics operative to create nano-confined crystallization (NCC) in PLA. For example, shear deformation is applied to show the emergence of NCC. On the other hand, NCC is insufficient to afford PLA ductility at room temperature in absence of the geometric condensation effect ².

1. Razavi, M.; Wang, S.-Q. Why Is Crystalline Poly(lactic acid) Brittle at Room Temperature? Macromolecules 2019, 52 (14), 5429-5441 DOI: 10.1021/acs.macromol.9b00595.
2. Zartman, G. D.; Cheng, S. W.; Li, X.; Lin, F.; Becker, M. L.; Wang, S. Q. How Melt-Stretching Affects Mechanical Behavior of Polymer Glasses. Macromolecules 2012, 45 (16), 6719-6732 DOI: 10.1021/ma300955h.   

On Demand

The microscopic mechanism of lamellar crystal growth of polymers has raised controversial arguments and hot debates in the field of polymer crystallization. So far, there are three theories that have derived their growth rate equations, i.e. Lauritzen-Hoffman theory, Sadler-Gilmer theory and intramolecular nucleation theory. One can however recognize their similarities in the equation formulas as they are all constituted by a barrier term multiplied with a driving-force term. Although three theories made different interpretations on the microscopic mechanism of their barrier terms, their driving-force terms appeared as the same. The details of three microscopic models on the growth barriers are introduced and discussed.   

On Demand

Perovskite is a promising material candidate for solar energy harvest and extensively studied. To make uniform, large-area, high-quality deposition of perovskite films, inkjet printing is expected to be an appropriate versatile fabrication process. To prevent the coffee-ring effect and to improve the evaporation control, we examine a feasibility of perovskite multisolvent nanoinks that consist of perovskite nanoparticles and two solvents with large and slow evaporation rates. Particularly, we observe a variety of evaporation dynamics and deposit patterns of perovskite multisolvent nanoinks, which deviate from classical models for evaporation dynamics. To understand complexity in evaporation and deposition of perovskite multisolvent nanoinks, we suggest a new physical model that quite well works in perovskite multisolvent nanoinks. Based on high-resolution, high-speed microscopic observations with X-ray nanotomography and light holographic microscopy, we develop an idea to control evaporation and deposition of perovskite multisolvent nanoinks.

Keyword:multisolvent,evaporation,deposition   

Not Participating

We present results on the spontaneous crystallization of a very extensive system (54 chains of 1000 hard sphere monomers) by means of a suite of Monte Carlo (MC) moves. Starting from an isotropic amorphous melt the system reaches a final crystalline state of unprecented perfection. We use the wealth of information gleaned from this simulation to assess the applicability of two classical crystallization paradigms introduced by Ostwald more than 100 years ago. In spite of the lack of dynamic information, the sequence of microstates joining the initially amorphous system with the final, stable face centered cubic chain crystal satisfactorily complies with both Ostwald's ripening and step rule. Given that there is neither non-bonded interaction (beyond the hard-sphere core) nor bending or torsion potentials in the polymer chains, our results strongly indicate that Ostwald's step rule and ripening can be purely entropy-driven.

W. Ostwald, Z Phys. Chem. 22, 289 (1897)
I. M. Lifshitz and V. V. Slyozov, J. Phys. Chem. Solids. 19, 35 (1961)
N. C. Karayiannis, K. Foteinopoulou and M. Laso, Phys. Rev. Lett. 103, 045703 (2009)