Session R03: Polymer Crystals and Crystallization I

Live

Tammann’s nuclei development method allows for a detailed study of crystal nucleation by letting the nuclei formed at some nucleation temperature grow to measurable sizes at a higher development temperature. In this method, however, the problem occurs to what extent the crystal nuclei formed in nucleation retain their stability in the course of transfer to the development temperature. By fast scanning calorimetry (FSC), the influence of the transfer of nuclei from the nucleation stage at low temperature to the growth stage at higher temperature was systematically studied. Poly(L-lactic acid) (PLLA), a slowly crystallizing polymer, was chosen for the experiments performed in a wide transfer-heating-rate range. At heating rates between 1 K s^-1 and 20,000 K s^-1, there occurs stabilization/growth of crystal nuclei at low heating rates and their dissolution/melting at high heating rates. Heating rates above 1000 K s^-1 are sufficient to prevent growth of crystal nuclei at the transfer from 60 °C to 125 °C and to higher temperatures. The critical heating rate for preventing nuclei growth is about 1000 times higher than the critical heating rate to prevent crystal growth in a nucleated sample on heating.   

Live

Flow- or stretch-induced crystallization (FIC or SIC) is believed to be mainly responsible for the excellent mechanical properties of polymers during real service conditions. With the development of synchrotron radiation X-ray scattering, time resolution has been increased from minutes to millisecond, the in-situ tracing of the structural evolution of polymers under complex external fields becomes possible. Recently, the FIC behaviors of polymers (i.e. natural rubber and polyethylene) during multi-axial deformation like biaxial stretching, film blowing and balloon blowing have been systematically studied by our group with a series of custom-built experimental devices combined with synchrotron radiation X-ray scattering techniques. Considering the phenomenon of frustrating SIC for natural rubber during biaxial stretching, here we proposed a new model for SIC based on the results of theoretical calculation, which decouples the free energy contributions of chain orientation from that of conformational entropy reduction. Furthermore, based on molecular dynamics simulation techniques which provide great advantages in exploring molecular level information, the nucleation and growth mechanism during biaxial deformation were acquired, which not only verifies the above model, endows it with molecular origin but also proposes the probability of application of this model for the crystallization of different polymer systems and other flow fields.   

Live

Prefreezing refers to the abrupt formation of a crystalline layer at the melt-solid interface above the melting temperature, and is an equilibrium phenomenon.¹ Phenomenological theory of prefreezing² predicts that the transition temperature T_max depends primarily on the difference of the interfacial energies Δγ = γ_sub,melt – (γ_sub,cry + γ_cry,melt), whereas the minimum jump of thickness l_min at T_max is controlled by the ratio (γ_sub,cry + γ_cry,melt)/γ_sub,melt. To test these predictions, we performed in situ AFM experiments on various polymer-substrate systems. The results for polyethylene (PE) on a molybdenum disulfide (MoS₂) substrate evidence a much higher T_max than on graphite, caused by a larger value of Δγ of PE-MoS₂. In case of poly(ε-caprolactone) (PCL), where direct measurements of the prefrozen layer thickness are possible, T_max of the prefrozen PCL on MoS₂ remains the same as on graphite, whereas l_min decreases to a smaller value. With these findings, we confirm that, firstly, Δγ is the driving force for prefreezing and, secondly, T_max and l_min are independent.

1. Tariq et al. Macromolecules 2019, 52, 9140-9148
2. Dolynchuk et al. J. Phys. Chem. Lett. 2019, 10, 1942-1946   

Live

Poly(ε-caprolactone) (PCL) is a semicrystalline, biodegradable polyester predominantly used in biomedical engineering applications as a tissue scaffold and a drug delivery medium. Degree of crystallinity, crystal morphology, and crystallite size are known to affect degradation rates of PCL fibers and films, so morphological control is important in designing PCL materials for its applications. Inspired by the use of solvent vapor annealing (SVA) to control block copolymer morphology and solvent-induced crystallization in semicrystalline polymers, we are studying how SVA impacts PCL crystallization and its morphology. Prior studies have focused on the roles of solvent choice, extent of film swelling, and solvent evaporation rate on the final film morphology. Herein, we apply grazing incidence X-ray scattering techniques in situ to probe the mechanisms of polymer dissolution during solvent uptake and recrystallization behavior during solvent removal aiming to understand why we observe new morphologies in semicrystalline PCL thin films.   

Live

Asymmetric PEO-b-PCL diblock copolymers exhibit sequential crystallization at high weight fractions. Sequential crystallization opens the door to evaluate the crystallization behavior of the minority block under varying thermal histories. Here, we have isothermally crystallized the majority block at varying conditions to evaluate changes in the crystallization temperature and crystallinity in the minority block using DSC. Surprisingly, microphase separation, or diluent effects, seem to be the controlling factor instead of confinement.   

Live

We apply molecular dynamics (MD) simulations to investigate the roles of phase separation and interfaces in the crystallization of incompatible polymer blends. We use binary blends of polyethylene (PE) and isotactic polypropylene (iPP) oligomers with pre-constructed planar interfaces as the model systems. By varying the simulation temperatures, we adjust the incompatibility and the interface-induced orientational and conformational order of polymer segments. However, PE and iPP oligomers are mildly incompatible so that the interface-induced order is nevertheless weak. To access a broader range of interface-induced order, we also simulate iPP with "artificial" PE oligomers. We weaken the intra-species attraction between the "artificial" PE and iPP to enhance the incompatibility. We also modify the dihedral potential to increase the stiffness of the "artificial" PE. The enhanced incompatibility and stiffness give rise to the increased and longer-ranged interface-induced order in the phase-separated blends. By simulating crystal nucleation and tracking the distributions of crystal nuclei in the natural and "artificial" blends of PE and iPP, we demonstrate the effects of phase separation and interfaces on the crystallization of incompatible polymers.   

Live

Entanglement effect on chain-folding structure of semicrystalline polymers is not clearly understood over the past decades. Very recently, we demonstrated that Poly(Lactic Acids) chains with three different molecular weights of 46K, 90K, and 320K g/mol do not change adjacent re-entry structure under different supercoolings. It was suggested that entanglement of polymer chains limit adjacent re-entry number in the melt-grown crystals. In this study, we isothermally crystallize polymer blend samples consisting of long and short Poly(Lactic acid) chains from the melt state. Only long polymer chains are labeled by ¹³C. ¹³C-¹³C double quantum (DQ) spectroscopy is applying to study local chain-folding structure of PLLA as a function of blending ratio and as a function of molecular weight of short PLLA chains.   

Live

Spherulites, shish-kebab, cylindrites and other morphological features of bulk semicrystalline polymers have been studied for decades using methods such as polarized optical microscopy, TEM or AFM. For these studies either thin films or thin sections were used, giving a 2D but not a 3D picture. The organization in the 3^rd dimension has been implied rather than directly observed. Attempts at 3D imaging have been made with varying success using approaches, notably electron tomography (TEMT) and some other methods. Most studies focussed on polymer blends and block copolymers, relying on contrast produced by the chemically different components, although some remarkable images of crystal lamellae were obtained by TEMT. However 3D images of microstructure on the crucial 10-1000 micron scale, especially of single-component polymers or their nanocomposites, are still missing. Here we present first such images obtained by confocal microscopy using appropriate sample preparation and image processing. Already rather unsuspected features of 3D morphology are emerging, previously being hidden by the limitations of the conventional techniques.   

Live

Polymeric micro/nanoparticles have attracted significant interest in the past few decades due to their relevance in a number of fields. The methods commonly employed tend to provide particles that have simple morphology of spheres or cylinders that are inherently fluidic and dynamic due to the long chain nature of macromolecules. As more complex particles, in function and structure, are emerging as solutions to a variety of problems, simple and versatile methods to obtain them are a necessity. Towards this aim, our group has developed a miniemulsion crystallization process where polymer single crystal-like growth is confined to a dynamic liquid/liquid interface. This process produces unique hollow polymer capsules termed “crystalsomes” that have excellent mechanical properties compared to their amorphous counterpart the polymersome. This talk will focus upon controlling the crystalsome structure and morphology by programing the growth condition as well as co-crystallization of carefully selected amphiphilic block co-polymers in the miniemulsion process. It will be shown by altering the hydrophobic monomeric unit, the crystallization temperature, and the particle size a variety of crystalsomes with tunable porosity can be obtained. Detailed formation mechanisms will be discussed.   

Live

Under isothermal crystallization (T_c) from the melt, polyacetals spaced by 12, 18, 19 or 23 methylenes develop two or three layered polymorphs. Crystals formed in the lowest T_c range are kinetically favored (hexagonal and Form I) and characterized by highly nucleated small axialites. In the higher range of T_c, thermodynamically more stable spherulitic Form II crystals develop. While the overall crystallization kinetics of Form I display the usual negative temperature coefficient, an inversion of the dependence of the rate of Form II with temperature occurs when approaching from above the narrow T_c range where Form I and Form II coexist. The inversion is attributed to a competition in nucleation between Forms I and II. Just before inception of Form II, the crystallization rate is so low that it becomes basically extinguished. The degree of crystallinity recovers when pure Form II develops with a small increase in T_c. From analysis of linear growth rate data, the 2-6 times higher energy barrier found for nucleation of Form II explains the large differences in primary nucleation. The differences in energy barriers, and a possible preferential self-poisoning during nucleation of Form II explain the extinguished crystallization at the overlap T_c between two polymorphs.   

Live

The flow-induced crystallization of polymers is a pervasive aspect of processing where the effect of flow can dramatically alter end-use properties. Here we will describe development of a new methodology whereby the effect of flow on crystallization kinetics in polyethylene can be determined after an article is formed. The approach for rapidly crystallizing polyethylene was different than what has been done for other materials. Critically, an understanding of the self-nucleation and melt memory behavior as well as using thermal contact fluids with low kinematic viscosity was needed. With an optimized protocol, linear low-density polyethylene, LLDPE, can be heated to temperatures just above the melting point of the crystals using fast-scanning calorimetry and memory of the flow history can be maintained. As a result, the polymer can be melted many times without significantly changing the flow-induced crystallization kinetics. We demonstrate the ability to measure the flow-induced enhancement of crystallization kinetics on samples sheared using a parallel-plate rheometer or fabricated as blown film. Differences in crystallization rates between different classes of LLDPE blended with LDPE will also be discussed.   

Live

Polymer blends provide a diverse platform for adjusting polymer film properties. In this study, we probed the amorphous phase of thin film immiscible blends of poly(methyl methacrylate) (PMMA) and semi-crystalline poly(ethylene oxide) (PEO) and investigated how blend composition and film thickness influence film swelling under methyl ethyl ketone (MEK) vapor atmosphere. As the PEO content increases, the swelling decreases as the crystalline phase does not swell. Generally, the thinnest films (ca. 20 nm) swell more than thicker films (ca. 50 nm to 200 nm), but transitions in swelling behavior as a function of film thickness depend on the composition of the blend and morphology of the film. These investigations are relevant to applications that utilize blends of semicrystalline polymers, such as polymer electronics.   

On Demand

Recently, terahertz spectroscopy has attracted attention as a powerful tool for investigating the evolution of physical bonds in polymers, because most macromolecular vibrational modes that sensitively depend on the physical bond formation, lie in the terahertz frequency range. Here, we report on the terahertz spectroscopic investigation of the hydrogen bond formation between the two enantiomeric molecular chains in the stereocomplex (sc) PLA [1]. A remarkable red-shift of the macromolecular librational mode of the sc-PLA crystals at 46 cm^-1 was observed as the crystallinity increased. It has been pointed out that the librational motion was induced by the hydrogen bond and its vibrational potential was anharmonic [2]. Thus, we considered that the hydrogen bond formation associated with the crystallization increased the anharmonicity of the librational potential, resulting in the spectral red-shift. This result suggests that the crystallization process of sc-PLA can be sensitively probed by terahertz spectroscopy.

[1] J. Hirata et al., Macromolecules 53, 7171 (2020).
[2] F. Kollipost et al., J. Chem. Phys. 141, 174314 (2014).