Session V11: Polymer Crystallization

Model-guided experimental design of flow-induced crystallization of poly(1-butene) under uniaxial extensional flow as measured by small angle x-ray scattering

A key assumption in most current flow-induced crystallization models is that flow-induced molecular orientation/stretching leads to enhanced nucleation rate, which in turn leads to accelerated crystallization kinetics. This underlying hypothesis was directly tested by subjecting poly(1-butene) to various extension rates and Hencky strains that resulted in similar number density of flow-induced nuclei, which was calculated from the degree of stretch during extensional flow and relaxation, as predicted by a simplified Rolie-Poly model. Uniaxial extensional flow was produced using an SER housed in a custom-built oven designed to facilitate \textit{in situ }synchrotron x-ray experiments. Samples were first heated well into the melt, and then cooled to a crystallization temperature selected such that negligible quiescent crystallization would occur on reasonable time scales. A short burst of extensional flow was then applied, and crystallization as well as the degree of crystallite orientation were monitored using small-angle x-ray scattering. For experimental conditions that are expected to result in similar degree and kinetics of crystallization, SAXS invariants generally do not show agreement, while the degrees of orientation are in general agreement.   

Simultaneous rheology and crystallinity measures of the shear-assisted crystallization of polycaprolactones

Polycaprolactone is employed in a variety of applications including additive manufacturing, tissue engineering, and drug delivery. Often, non-destructive optical techniques are employed to characterize the degree of crystallinity and chain morphology in polymeric biomaterials, which are key parameters that dictate the mechanical response of these systems. In this talk, we utilize a rheo-Raman microscope to simultaneously measure the Raman spectra, rheology, and birefringent structure evolution of polycaprolactones crystallizing isothermally. We first isolate and identify the quantitative crystalline indicators in the Raman spectra from single chain effects to calculate a crystallinity mass fraction, then apply this analysis to our measurements to correlate crystallinity, small amplitude oscillatory modulus, and polarized optical microscopy. The crystallization kinetics are significantly enhanced when more shear (or specific work) is applied at temperatures between the equilibrium melting point and the temperatures where we perform isothermal crystallization measurements. We compare both the quiescent and shear-assisted crystallization measurements with the wide variety of models describing the modulus evolution as a function of crystallinity to find that a relatively simple suspension-based model works surprisingly well.   

Chain Trajectory of Semicrystalline Polymers As Revealed by Solid-State NMR Spectroscopy

Over the last half century, chain-folding structure of semicrystalline polymers is debatable of matter in polymer science. Recently, 13C-13C double quantum (DQ) NMR spectroscopy combined with 13C selective isotope labeling has been devel-oped to investigate re-entrance sites of the folded chains, mean values of adjacent re-entry number \textless n\textgreater and fraction \textless F\textgreater of semi-crystalline polymers. This viewpoint highlights the versatile approaches using NMR and 13C isotope labeling for revealing i) chain trajectory in melt- and solution-grown crystals, ii) conformation of the folded chains in single crystals, iii) self-folding in the early stage of crystallization, and iv) unfolding of folded chains under stretching.   

``Plunger" Method for Simulating Polymer Crystal-Melt Interfacial Tensions

Crystal-melt interfacial free energies are important ingredients in predicting the nucleation barrier for polymers to crystallize. Experimental measurement of polymer crystal-melt interfacial tensions is extremely challenging. We propose a simple way to obtain the interfacial free energy for any polymer crystal surface and melt using molecular dynamics simulation. We measure the force on a simulated nanoscale ``plunger", that restrains a melt from flowing into the gap between two crystals. This gives the difference between the crystal-vacuum and crystal-melt interfacial free energies. Separately, the crystal-vacuum interfacial free energy is obtained by measuring the force required to hold two crystals apart and integrating the force with respect to distance. We obtain the crystal-melt interfacial free energy by subtracting the above values. Results from this method applied to n-alkanes can be compared to measurements of the homogeneous nucleation barrier.   

Tammann's Nuclei Development Approach in Crystallization of Macromolecules.

An attempt to probe the size distribution of homogeneously formed crystal nuclei in polymers was realized employing Tammann's two-stage crystal nuclei development method and fast scanning calorimetry [1]. A transfer heating rate of 500 000 K/s prevents nuclei growth on heating in poly(epsilon-caprolactone) (PCL). Data collected in a wide range of crystallization temperatures allow, in combination with a theoretical model based on classical nucleation theory, for an estimate of the nuclei size distribution and the growth rate. The employed temperature profile includes formation of nuclei at large undercooling and following their isothermal growth at higher temperatures. Fast scanning calorimetry allowed us to reach the deep supercooling of the melt at 100 000 K/s avoiding homogeneous nuclei formation and heterogeneous nuclei growth. Then crystal nuclei were allowed to form isothermally at the temperature corresponding to the maximum of the steady-state nucleation rate for homogeneous nucleation (210 K for PCL, Tg $=$ 209 K), where both the effect of heterogeneous nucleation and the growth rate are low. The presence of these crystal nuclei and its effect on crystallization were probed at higher temperatures. The transfer heating rate up to 500 000 K/s were applied in order to minimize growth on heating. The study focuses on early stages of nucleation and growth. [1] E. Zhuravlev, et al., Experimental test of Tammann's nuclei development approach in crystallization of macromolecules, Crystal Growth {\&} Design (2015), DOI: 10.1021/cg501600s.   

Chain Trajectory of Poly (l-Lactic Acid) in Solution- and Melt-Grown Crystals As Studied by $^{\mathrm{13}}$C - $^{\mathrm{13}}$C Double-Quantum NMR

Crystallization of long polymer chains induces drastically structural change from random coils to folded chains in thin crystals. Various factors including kinetics, polymer concentration, entanglements of polymers, etc. in complexity influence crystallization process and folded chain structures. However, chain-folding structures and mechanisms of polymer chains have been debated in the past decades. Very recently, our group developed a novel strategy using $^{\mathrm{13}}$C -- $^{\mathrm{13}}$C double-quantum (DQ) NMR in combination with selective isotope labeling, to investigate the chain-folding structures in the crystallization. In this presentation, we investigate chain-folding structure of poly (l-lactide) (PLLA) solution-grown single crystals and melt-grown crystals formed in a wide crystallization temperature range. It will be demonstrated that how kinetics, polymer concentration and entanglements influence chain-folding structures of PLLA in solution and melt-grown crystals.   

New insights on crystallization in a benchmark organic photovoltaic system by fast scanning chip calorimetry

Using the advanced thermal analysis technique of Fast Scanning Chip Calorimetry, which relies on thin membrane chips, a methodology was developed which allows for a ‘true’ isothermal study, i.e. avoiding non-isothermal effects which may alter metastable structures, by employing scanning rates of 30000 K.s$^{-1}$. Isothermally formed structures, which were not observable before, were now conserved and analyzed in the subsequent heating. This methodology was used to investigate the P3HT/PC$_{61}$BM (poly(3-hexylthiophene/[6,6] – phenyl C$_{61}$ – butyric acid methyl ester) benchmark system used in organic photovoltaics, as well as its pure components. By applying the methodology to P3HT, the bell-shaped curve of isothermal crystallization rate was constructed for a P3HT layer with a thickness of ca. 550. Surprisingly, the PC$_{61}$BM acceptor is capable of crystallizing significantly below its glass transition, a type of behavior seen before for several non-polymeric organic glasses.   

Nanostructures of Oligo- and Polythiophenes by Chain-Growth Polymerization.

Conjugated polymers represent an important platform for designing electronic and optoelectronic devices with tunable characteristics. Typically, processing such materials requires them to be functionalized with alkyl substituents directly attached to conjugated polymer backbone, but such substituents tend to reduce polymer fluorescent quantum yields by introducing low lying vibrational and rotational energy levels. In addition, solubilizing alkyl side chains play a key role in the oxidative degradation of these materials. Thus, conjugated polymers without alkyl substituents should display higher quantum yield and better stability. In this presentation, we will discuss a novel method for preparing unsubstituted polythiophene nanostructures by controlled chain-growth polymerization. In contrast to previously developed methods, nanostructures prepared by this method exhibit diverse shapes, including nanoparticles, nanorods, and nanofibers, and display tunable photophysical properties such as near-infrared fluorescence, related to efficient excitation energy transfer within the particles. X-ray and neutron scattering studies revealed hierarchical organization of the polythiophene nanostructures. A two-stage mechanism for the formation of these nanostructures will also be discussed.   

Wide- and Small-angle X-ray Scattering Study on Poly(ethylene furanoate): Crystal Structure and Time-resolved Experiment

A new type of bio-based polyester, poly(ethylene furanoate) (PEF), was studied using synchrotron wide- and small-angle X-ray scattering techniques (WAXS/SAXS). Crystal structure of PEF was semi-quantitatively determined by fiber diffraction method. A monoclinic cell, with a = 5.784 \AA, b = 6.780 \AA, c = 20.296 \AA, and $\gamma$ = 103.3$^{\circ}$ was adopted, with two chains contained in one unit cell. Space group was $P2_1$; each repeating unit contained 2 monomers. Inter-chain staggering was identified based on extinction of diffraction peaks along meridian. Time-resolved study on uniaxial deformation was carried out by setting up a stretching unit at synchrotron X-ray scattering beamline, where WAXS/SAXS data were precisely mapped to stress-strain curve. Orientation of amorphous polymer chains was decoupled from that of crystals, which provides insights into deformation behavior at molecular level.   

Evidence of LLPS in melts of broadly distributed ethylene copolymers via deuterium labeling and effect on self-nucleation and crystallization

Ethylene copolymers with a broad comonomer composition range (0.5 -- 13 mol {\%}) display unusual self-nucleation behavior. Cooling from different, progressively lower melt temperatures, these materials display the expected accelerated crystallization kinetics and a range of lower melt temperatures from which the crystallization rate is observed to decrease. This unusual inversion of the crystallization rate was postulated to arise from the onset of liquid-liquid phase separation (LLPS) between comonomer-rich molecules and molecules with less comonomer. We have now reproduced the broad distribution by blending multiple narrow components covering the same range of composition with partial deuteration in the components with low comonomer content. SANS investigations confirm that these broad copolymers display an UCST behavior.   

Heat Capacity, Crystallization, and Nucleation in Poly(vinyl alcohol) Thin Films

Polyvinyl alcohol (PVA) is hydrophilic, biodegradable, semi-crystalline polymer with a wide array of applications ranging from textiles and packaging to medicine. Despite possessing favorable properties, PVA thermally degrades at temperatures just in excess of 200 \textdegree C which occurs slightly below the observed peak endothermic melting peak at 203 \textdegree C. Utilizing fast scanning calorimetry it is possible to minimize sample degradation allowing measurements of the liquid phase heat capacity as well as study nucleation and crystallization from the amorphous melt state. Samples cut from parent films 2-3 $\mu $m thick were placed on UFSC1 sensors and brought between -80 and 270 \textdegree C at rates of 2000 \textdegree C/s under a nitrogen atmosphere. After five complete cycles samples did not show any signs of degradation. By fitting the symmetry corrected glassy phase heat capacity with literature values for the specific heat capacity from the ATHAS databank sample masses were determined to vary between 15-50 ng. Homogeneous nucleation was observed for all samples cooled from the melt with peak temperature 123 \textdegree C. Fitting linear heat capacity baselines in the melt and glassy states it was possible to obtain an experimental measurement of the heat capacity increment 44.5 J/mol K at the glass transition 85 \textdegree C.   

Nucleation, crystallization, and melting of atactic polystyrene.

Here we present the study of using low molecular weight atactic polystyrene (aPS) as the model system to understand the nucleation, crystallization, and meting behaviors of the stereo-regular polymer chains in aPS. The result is consistent with the theoretical calculation proposed by Semenov. In addition, both the crystallization and melting experiments indicate that all crystals are on or near the surface. Finally, the nucleation experiment below the glass transition temperature provides another piece of evidence of the enhanced surface dynamics in glassy polymers.