Session Z19: Semi-Crystalline Polymers

Pseudobrush theory of amorphous interphase in semicrystalline polymers.

The conformation of polymer chains emerging from the face of a crystalline lamella has long been a matter of dispute. Long ago, arguments pitted ``adjacent reentry'' versus the ``switchboard model'' as extreme limits of possible behavior. Later, two theoretical approaches were attempted, but one (the Gambler's Ruin model) did not properly account for the constraint of melt density, and the other (heuristic configuration counting of Flory et al.) did not account for chain connectivity. These shortcomings are resolved by a new ``pseudobrush'' theory of the amorphous interphase, which represents the reentrant chains as a polydisperse brush of loops in a self-consistent hydrostatic pressure field. This theory predicts the fraction of adjacent reentry, shows how the anisotropy of the interphase dies away with distance, and how the Gambler's Ruin model is recovered far from the interface. Extension to the case of a finite slab between two crystal-melt interfaces predicts the frequency of tie chains, a key parameter for nonlinear deformation and ductile failure of semicrystalline polymers.   

The impact of chain folding on the structure and morphology of polymer lamellae

The major structure and morphology features of polymer lamellae (and single crystals) are usually defined by the characteristics of the crystal lattice (cell geometry and symmetry, stem length). However, the fold surfaces that sandwich the crystalline core may have an impact on that crystalline core. In isotactic polyolefins, restrictions on the relative (up/down) orientation of stems linked by a fold may result in lowered unit-cell symmetry. Also, surface stresses linked with the presence of folds determine to a large extent the non-planar shape of polymer crystals. In bulk crystallization, they may induce twisted and/or scrolled lamellae. However, this impact can be inferred only for specific polymers and/or crystal structures and/or unit-cell symmetries. (Work performed with Dr. A. Thierry and J. Ruan.)   

Unexpected Observation of 2.5 Dimensional Growth of Polymer Spherulite

Preparation of integrated polymer spherulite from both solution and melt has been a challengeable subject. In this letter, micro-sized spherulites of ultrahigh molecular weight polyethylene (UHMWPE) have been successfully prepared from supercritical ethanol. The spherulite grows to 2.5 but not 3 dimensions with only one nucleation site on the surface. The 2.5 dimensional growth makes it possible to observe both the nucleation site and sheaf-like structure on the surface of a spherulite and to obtain one global final spherulite as a single particle. A possible mechanism for the particular morphology of spherulites is proposed based on the contributions of surface nucleation on polymer droplet, high molecular weight of UHMWPE as well as the soft confinement of supercritical fluid.   

Structural evolution under uniaxial drawing of Poly(D, L-lactide) Films

Aliphatic polyesters are an important class of biodegradable polymers. They have drawn particular attention in the last few years as food packaging materials because they can be derived from renewable resources. Among this family, polylactide (PLA) is considered as one of the most promising ``green'' polymer for use as a substitute to petroleum-based polymers. In the present work, we investigate the mechanical behaviour of amorphous poly(D, L-lactide) films in relation to the structural evolution upon stretching at various draw temperatures (Td) above the glass transition temperature. Examination of the drawing behaviour shows that PLA initially behaves like a rubbery material until a true strain of the order of 1. Strain hardening occurs beyond this strain level, up to film fracture. Such strain hardening is generally ascribed to a strain induced crystallization phenomenon. In the present case, it is clearly more pronounced for Td = 90\r{ }C than for Td = 70\r{ }C. The corresponding structural evolutions are investigated by means of WAXS. The diffraction patterns reveal the marked influence of draw temperature. Indeed for Td = 70\r{ }C a mesophase is induced whereas strain-induced crystallisation takes place at Td = 90\r{ }C. Further work is in progress, in order to elucidate mesophase development and mechanical response.   

Vitrification and Devitrification of Rigid Amorphous Fraction of PET during Quasi-isothermal Cooling and Heating

Poly(ethylene terephthalate), PET, was studied by quasi-isothermal (QI) Temperature Modulated Differential Scanning Calorimetry (TMDSC). For the first time, both the temperature dependent crystalline fraction and rigid amorphous fraction (RAF) were quantitatively analyzed during QI cooling and reheating. Specific reversing heat capacity measurements show that most RAF vitrifies step by step during QI cooling after completion of crystallization. Upon subsequent QI reheating, the RAF devitrifies also step by step and only a small RAF of 0.04 remains at 470K, while melting starts above 473K. To obtain the exact temperature of the start of melting, heat capacity measurements were made using subsequent standard DSC heating, after QI cooling. By combining this method with the QI results, the temperature dependent phase fractions were obtained during standard DSC heating. We conclude that RAF completely devitrifies before the temperature reaches the crystal melting endotherm under the conditions used in this work.   

Crystallization of Model Long Chain Branched Polyethylenes with Different Branching Architectures

While the impact of long chain branching (LCB) of different architectures (stars, $\alpha -\omega $ H type, pom-pom, combs) on rheology has been studied extensively, the effect on crystallization is less known. This work analyses the influence of LCB architecture on crystallization from quiescent melts using models based on hydrogenated polybutadienes, all with a constant 2.1 mol{\%} of ethyl branches (LCB PEs). Crystallization rates measured by DCS, the phase structure, and morphology of the LCB PEs are studied in reference to the linear chain. At a fixed undercooling the crystallization rates of all LCB PEs are 30 to 40{\%} lower than the rate of the linear as expected from transport limitations to the nucleation rate of the LCB systems. Smaller differences in the rate are found within the various LCB architectures. The components of the phase structure are controlled by the content of short chain branching with a negligible effect from the LCB architectures. For all LCB PEs the crystalline component is $\sim $30{\%} and the interphase region $\sim $15{\%} as determined by WAXD, RAMAN and DSC. A major impact of LCB is found in the supermolecular morphology. Restrictions from the LCB melt topology to propagate long organized arrays bring about a change from spherulites (linear) to poorly organized crystallites (LCB PEs). Long range dynamics (NMR T$_{2H})$ and lamellar structures (AFM) are presently investigated.   

An anomaly in the crystallisation rate of bimodal poly(ethylene oxide) weight distributions

We present results on the crystallisation of blends of poly(ethylene oxide) (PEO) of differing molecular weight, $M_{w}$. Previous studies probing PEO blends have mainly focused on blends of low $M_{w},$ for which integral chain folding is important, with very high $M_{w}$. The PEO samples used in this study consist of a blend with both $M_{w}$'s well above the integral chain folding limit. In general, one would expect that such blends should show a monotonic decrease in spherulite growth rate, $G$, as the blend contains more high-$M_{w }$component. Our results however show a clear non-monotonic $G$, with a minimum in a plot of $G$ as a function of the volume fraction. In short, blending a small amount of the low- $M_{w}$ into the High- $M_{w}$ PEO slows the growth kinetics. These results along with a possible mechanism will be discussed.   

Effect of ppm Levels of Long Chain Branching on the Crystallization of Isotactic Poly(propylenes) from the Melt and from Solution

Small concentrations of long chain branching (LCB) added via copolymerization with a diene to a linear poly(propylene) chain have a dramatic effect on crystallization. LCB iPPs with diene levels between 100 and 400 ppm show greatly enhanced nucleation density, increasing with diene content. The increased nucleation observed in optical micrographs, is also evident in the crystallization half-time observed by DSC in isothermal crystallization as a function of crystallization temperature. Isothermal crystallization studies from dilute solution measured by light scattering gave the same trend with diene content and rule out any effect of gel structures affecting the nucleation rate. The linear growth rates are unchanged. The increase in nucleation rate is not linear with the increase of diene as revealed from studies of blends of LCBiPPs with the linear matrix, suggesting that length of molecules and connectivity of the LCB structure are more important than the chemical nature of the junction. The enhanced nucleation levels off at concentrations of diene that approach the critical concentration for coil overlapping (c*) as calculated for chains generated by a Monte Carlo simulation of the LCB molecules having the experimental molecular weight distribution.   

Melting, Recrystallization and Superheating of Polymer Crystals Studied by Fast Calorimetry (1 MK/s)

For polymers the origin of the multiple melting peaks observed in DSC curves is still controversially discussed. This is due to the difficulty to investigate the melting of the originally formed crystals exclusively. Recrystallization is a fast process and most experimental techniques applied so far do not allow fast heating in order to prevent recrystallization totally. We developed a thin-film (chip) calorimeter allowing scanning rates as high as one million Kelvin per second. The calorimeter was used to study the melting of isothermally crystallized polymers like isotactic polystyrene (iPS), isotactic polypropylene (iPP), poly(ethylene therephthalate) (PET) and others. Our results on melting at rates as high as 1,000,000~K/s support the validity of a melting-recrystallization-remelting process at low scanning rates (DSC) for all studied polymers. At isothermal conditions they form crystals, which all melt within a few dozens of K slightly above the isothermal crystallization temperature. There is no evidence for the formation of different populations of crystals with significantly different stability (melting temperatures) under isothermal conditions.   

Nano-Scale Confinement Effects on Poly($\varepsilon $-caprolactone) Crystallization at the Air/Water Interface

Poly($\varepsilon $-caprolactone)s (PCL) with different mole mass were synthesized by using tri-ester primary amine as an initiator, tin(II) 2-ethylhexanoate as the catalyst, anhydrous tetrahydrofuran as a solvent at 80 \r{ }C. The three triester groups were further hydrolyzed to render PCL linear polymers with triacid end groups (PCL Triacid). The as-synthesized PCL triacids were attached to 9 nm magnetic nanoparticles (MNP) by ligand exchange with oleic acid in refluxing chloroform for 24 h. The crystallization behavior of PCL trihead and PCL MNPs at the air/water interface were compared with reported linear PCL crystallization. Regular crystalline morphologies observed for linear PCL are not observed for PCL-trihead and PCL-MNPs.   

Microphase Separation Controlled Beta Sheet Crystallization Kinetics in Silk Fibroin Protein.

We investigate the mechanism of isothermal crystallization kinetics of beta-sheet crystals in silk multiblock fibrous proteins. The Avrami analysis kinetic theory, for studies of synthetic polymer crystal growth, is for the first time extended to investigate protein self-assembly in beta-sheet rich \textit{Bombyx mori} silk fibroin samples, using time-resolved Fourier transform infrared spectroscopy, differential scanning calorimetry and synchrotron real-time wide-angle X-ray scattering. Results indicate formation of beta sheet crystals in silk proteins is different from the 3-D spherulitic crystal growth found in synthetic homopolymers. Observations by scanning electron microscopy support the view that the protein structures vary during the different stages of crystal growth, and show a microphase separation pattern after chymotrypsin enzyme biodegradation. We present a model to explain the crystallization of the multiblock silk fibroin protein, by analogy to synthetic block copolymers. This model could be widely applicable in other proteins with multiblock (i.e., crystallizable and non-crystallizable) domains.   

Correlation between Structure and Vapor Sorption in Semi-crystalline Polymers: One Dimensional Nano-Swelling Measured using iVSANS

Changes in the nanoscale structure of semi-crystalline polyethylene (PE) resulting from the sorption/diffusion of n-hexane vapor have been observed and quantified by the \textit{in-situ }Vapor Sorption Small-Angle Neutron Scattering (iVSANS). We found a linear correlation between vapor sorption/diffusion and nano-expansion of the amorphous phase within the lamellae confined in the spherulite domains. The diffusion coefficient measurements using (iVASNS) revealed that the molecular packing in amorphous phase is different between low and high crystalline PE. Furthermore, the one dimensional expansion is reversible during cyclic sorption and desorption processes, suggesting that the crystals were not destroyed unlike the inference of a previous study. These results are relevant to processing methods for tailoring the barrier and sorptive properties of semicrystalline polymer films. * This work utilized facilities supported in part by the National Science Foundation under Agreement No. DMR-0454672. The author, MHK, acknowledges support by the KIST (2E20844).