Session P55: Polymer Crystallization I: Structure and Morphology

Microscopic Observation of Interface-Induced Crystallization via Prefreezing from Polymers Melts

The microscopic ordering process that a liquid undergoes during crystallization is often initiated at an interface to a solid. Different processes have been suggested by theory to occur at this interface. Of special interest is prefreezing - the formation of a thin crystalline layer at the interface already at temperatures above the melting temperature. Because of the difficult accessibility of the buried interface, experimental proof of crystallization by prefreezing has been elusive in molecular systems. We here present in situ AFM-observations of such a process in two polymeric model systems and show that prefreezing is a first-order prewetting transition. A quantitative description of the phenomena is possible with a phenomenological theory. The results not only contribute to our fundamental understanding of crystallization but might also be useful for the preparation of well-ordered oriented thin films of crystalline organic materials.
References
1. A.-K. Löhmann, T. Henze, T. Thurn-Albrecht,
Direct observation of prefreezing at the melt-solid in polymer crystallization
PNAS, 111, 17368-17372 (2014)
2. A.-K. Flieger, M. Schulz, T. Thurn-Albrecht,
Interface-Induced Crystallization of Polycaprolactone on Graphite via First-Order Prewetting of the Crystalline Phase
Macromolecules 51, 189-194 (2018)
  

Supramolecular Crystals and Crystallization

Supramolecular crystal and crystallization is a field that is an extension of polymer crystals and crystallization. This topic associates with non-equilibrium thermodynamic features via analyzing and understanding structures and morphologies of supramolecular crystals. These crystals possess semi-ordered structures which are caused by crystallization and phase transformation kinetics controlled by nucleation processes and growth. We will focus on a few examples of the formation of Frank-Kasper phase and quasicrystal kinetics in the melt and transition between these phases. Specific mechanisms of phase stricture changes will be revealed in thse supramolecular crystals.   

Crystallization of conjugated polymers

The final semicrystalline morphology of a polymer strongly depends on the processing conditions under which crystallization occurs. Even small variations in the degree of crystallinity, size/orientation of crystallites, and connectivity between crystalline aggregates can lead to significant changes in final properties. For conjugated polymers, the basic interplay between molecular structure, processing parameters and structure development remains particularly elusive because, typically, the crystallization process occurs under ill-defined flow and thermal conditions while a solvent evaporates relatively quickly. Here, we explore the crystallization behavior of poly-3-hexylthiophene of various molecular weights under well-defined conditions. The results suggest that memory and self-nucleation can play an important role in the way crystallization proceeds.
  

Tuning the Phase Behavior of Hydrogenated Polynorbornene via Epimerization

Hydrogenated polynorbornene (hPN) synthesized by ring-opening metathesis polymerization exhibits a thermotropic polymorphic transition at a temperature T_cc below the melting point T_m. Though atactic, hPN is able to crystallize, and the ability of hPN to accommodate defects within the crystal allows the influence of a variety of defects on T_cc to be studied. Polynorbornene (PN) can be saturated via multiple pathways, and the choice of hydrogenation route influences both the crystal structure and T_cc. Three hydrogenation routes for complete PN saturation are compared: a homogeneous route that generates a diimide molecule in situ, a Ni/Al catalyst complex, and a supported Pd catalyst. The resulting polymer chains show zero, minor (< 4 %), and major (> 8 %) amounts of epimerization of the cyclopentylene ring (from cis to trans) for the diimide, Ni/Al, and Pd-hydrogenated hPNs respectively, but all show similar degrees of crystallinity. The window of stability of the rotationally-disordered polymorph (T_m - T_cc) increases with increasing epimerization. The crystal structure of the rotationally-ordered polymorph – observed below T_cc – changes sharply with low levels of epimerized units along the chain, but is weakly influenced by further epimerization.   

Gap Dependent Percolation of Spherulites during Crystallization: Rheology, Microscopy and Simulation

We employ simultaneous mechanical rheology and optical microscopy, with augmentation by deterministic reconstruction and simple simulations to develop a geometrical model of spherulitic percolation during polymer crystallization. We observe nucleation of surface and bulk spherulites of isotactic polypropylene which are initially isolated and then impinge on each other to form clusters that eventually span the gap, and correlate this with rheology. There is a strong gap dependence to our observations, which is explained by two distinct mechanisms that both work to enhance kinetics at lower gaps. First, the time required for a cluster to span the gap decreases with decreasing gap width, attributable to finite size effects in percolation theory. Second surface nucleation enhances the overall crystallization rate in a fashion which increases in relative importance as the gap width decreases. The modulus-crystallinity relationship can be described through general effective medium theory which indicates a transformation from percolating behavior at large gap towards a linear behavior at small. We describe our results in terms of dimensionless parameters that indicate when gap dependent effects can be anticipated.   

Miniemulsions as Dynamic Confinement Environments for Polymer Crystallization

Introducing interfaces and/or confinement to the polymer crystallization process is unique as the resulting kinetics, structure, and morphology can be vastly different compared to the bulk. Manipulating the confinement size and interfacial energy can provide control over the crystallization process resulting in tunable morphologies and properties. Miniemulsions where polymer/poor solvent is used as the dispersed phase present an intriguing case where the confinement is introduced by the liquid/liquid (L/L) interface and the confinement length scale, droplet size, can be readily tuned. A unique aspect of this system is the (L/L) interface that is dynamic when compared to solid/liquid, solid/solid, or vapor/liquid and that has a tunable interfacial energy by choice of emulsifier, solvent, and polymer. The nanoscale curvature of the interface is also interesting in regard to polymer crystallization where the curvature is incommensurate with the crystal’s translational symmetry. Our group has recently utilized a miniemulsion system to study polymer crystallization at curved (L/L) interface for a variety of polymers and hybrid materials. This talk will focus on our findings of the unique nanoscale morphologies obtained along with the unique thermal and mechanical properties observed.   

Solvent Vapor Annealing to Control Polymer Crystal Morphology

Poly(ε-caprolactone) (PCL) is a semi-crystalline, hydrophobic, biodegradable polymer that has found uses in the packaging industry and in biomedical engineering as an anti-adhesion biomaterial film and drug delivery medium among other applications. Degree of crystallinity, crystal morphology, and crystallite size are known to affect the biodegradation profile of PCL fibers and films by enzymes, so morphological control is important to designing PCL coatings and films for these applications. Inspired by the literature related to solvent vapor annealing (SVA) in block copolymer films and solvent-induced crystallization in semi-crystalline polymers, we are studying how SVA treatments impact crystal morphology. We use in-situ grazing incidence wide-angle X-ray scattering to determine when/if polymer crystals dissolve during solvent uptake and at what solvent concentrations recrystallization occurs during solvent removal. Additionally, we examine polymer morphology across multiple length scales using a combination of optical microscopy, profilometry, and atomic force microscopy.   

Crystallite Dissolution in Poly(ethylene oxide) Polymers Caused by Water

Poly(styrene)-block-poly(ethylene oxide) (PS-b-PEO) is a semicrystalline block copolymer (BCP) with interesting properties. It is mechanically tough and amphiphilic. The mechanical toughness is due to the crystallinity of PEO and the glassiness of PS, as well as the morphological structure of the BCP. These polymers have high CO₂, water, and salt solubility that derive from the polar PEO component. Potential applications include CO₂ separation, water purification, and lithium air batteries. In all of the aforementioned applications, water transport is an important parameter. The presence of water can also affect thermal and mechanical properties. Water transport and thermal and mechanical properties of a lamellar PS-b-PEO copolymer, as well as PS and PEO homopolymers have been measured as a function of water activity. Water was found to dissolve PEO crystallites and plasticize PS, which in turn affects the transport. Fourier transform infrared-attenuated total reflectance (FTIR-ATR) spectroscopy was employed to examine crystallite dissolution and water diffusion. A model accounting for both processes captured the experimental observations.   

Epitaxial growth of polyethylene oxide atop muscovite mica

Semi-crystalline polymers confined to nanoscale dimensions often crystallize in unexpected ways vastly different from that of bulk crystallization. The crystallization of ultrathin (< 100 nm) films atop solid surfaces is one example, in which the interaction of polymer chains with the underlying substrate is vital in determining the crystallization kinetics and film morphology. Additionally, geometric matching at the molecular scale between the polymer and substrate may induce epitaxial growth. In this work, we deposit ultrathin films of polyethylene oxide atop muscovite mica by matrix-assisted pulsed laser evaporation. We demonstrate an epitaxial relationship between needle-like polymer crystals and the substrate. Mechanisms of nucleation and growth of the epitaxial crystals are discussed.   

Effect of zone annealing on anisotropic nanoparticle reordering in polymer nanocomposites

Improving mechanical properties of polymeric systems is a primary research focus within the soft matter community. Isothermal crystallization studies have been performed on systems composed of polymer-grafted nanoparticles dispersed within a polymeric matrix, where the crystallized samples exhibit an increase in the tensile modulus by almost an order of magnitude as compared with the well-dispersed samples. Recently, a technique called zone annealing has been developed in which samples are rotated or translated across a fixed temperature gradient. This technique reduces processing times from days to hours. Initial X-ray scattering experiments have demonstrated that the nanoparticles seem to order in an anisotropic fashion along the crystallizing polymer lamellae after zone annealing.   

Utilizing Mixed Nanofillers to Control Crystallization Induced Ordering

The technique of controllably tuning nanoparticle ordering in semicrystalline polymers by crystallizing at very slow growth velocities (i.e. high temperatures) represents a very interesting physical situation. While organized PMMA-g-silica fillers have been shown to further enhance the mechanical properties, the particles’ inherent effect on the crystallization process leads to significantly longer processing times and lower degrees of crystallinity than is desirable. To combat this, a mixture of fillers of different sizes (specifically a bimodal distribution) is used to simultaneously increase the rate of crystallization, through nucleation, while allowing smaller fillers to continue to organize. This provides the simultaneous benefit of adding another level to the hierarchically ordered system. By tuning the relative size of either particle or aggregate one can optimize this nucleation effect while benefiting from the structural reinforcement of the secondary filler.   

Crystallization Behavior of Poly(ε-caprolactone) in Spin-coating Film-forming processes

Crystallization behavior in spin-coating film-forming processes has been investigated for Poly(e-caprolactone) (PCL) having the weight-average molecular weight (M_w) of 129×10³, 39×10³ and 16.8×10³ by time-resolved measurements of grazing-incidence small-angle and wide-angle X-ray scattering (GISWAXS) using synchrotron radiation in SPring-8 (RIKEN, Japan). Effect on the rotational speed, the PCL concentration in solutions and the molecular weight of PCL on crystallization kinetics and lamellar orientation during spin-coating will be explained in my presentation.
  

Regenerated cellulose fibers: Relating mechanical response to semicrystalline microstructure

We compare regenerated cellulose fibers manufactured using Viscose and Lyocell processes. These fibers show qualitatively similar mechanical response in different experiments e.g. stretching, stress relaxation and stress recovery. We show that the linear viscoelastic response of these fibers is accurately captured using a phenomenological model. The model parameters correlate well to the structural features and thus provide a robust structure-property relation for the linear region response. When stretched beyond the linear region, regenerated cellulose fibers show a transition in the slope of stress strain curve at a critical strain. This transition has been attributed in the literature to yielding of fibers. We demonstrate that this is incorrect and that the critical strain corresponds to an apparent yield. When subjected to strains higher than this apparent yield point, the fibers retain a memory of the mechanical deformation, that decays logarithmically with time and is lost over about a day as the fiber structure transitions back to the original as spun fiber.