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The crystallization and self-assembly behavior of an amphiphilic starlike macromolecule (ASM) with a lipophilic core and four poly(ethylene oxide) (PEO) arms (Mn = 2000 g/mol) were characterized by synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction at different crystallization temperatures (Tc). The overall d-spacing of the alternating amorphous core and crystalline PEO lamellae almost doubled from 11.4 nm for samples quenched into liquid nitrogen to 21.0 nm for Tc = 42 °C, indicating gradual transitions from nearly once-folded to fractionally folded and finally to extended chain crystals with increasing the Tc. On the basis of SAXS, transmission electron microscopy, and computer simulation results, it was suggested that the lamellae contained an interdigitated, single-layered PEO crystal, and an amorphous layer consisted of a double-layered lipophilic core and amorphous PEO. A folded (four PEO arms at one side of the core) rather than an extended (two PEO arms on each side of the core) overall molecular conformation (OMC) of the ASM was deduced in the solid state. These results implied that chain-folding and OMC of the ASM closely depended on its star-block-like architecture.

The nature of strain-induced crystallization in vulcanized polyisoprene rubbers was investigated by in situ synchrotron wide-angle X-ray diffraction under varying thermomechanical conditions. In particular, the methods of constrained cooling and constrained heating under constant strain were used to explore the relationships between the strain-induced crystalline network and corresponding stress. It was found that constrained cooling resulted in a large reduction of stress and a notable increase in the amount of strain-induced crystallites, while constrained heating led to an initial increase in stress and subsequent melting of strain-induced crystallites. Results indicate that strain-induced crystallization induces a new network structure, which reinforces the chemical network points. However, the sulfur bridges in the chemical network points would break down at high temperatures under high strains, despite the existence of strain-induced crystalline network.

Structural changes during constrained melting of blown polyethylene (PE) films were followed by in-situ synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques. Results indicated that thermally stable flow-induced crystallization precursor structures (i.e., shish-kebabs) can be revealed above the nominal melting point determined by differential scanning calorimetry (DSC). Two samples were investigated:  (1) linear low-density polyethylene (LLDPE) with &Mmacr;w of 116 kg/mol and (2) a binary blend of 95 wt % LLDPE and 5 wt % high-density polyethylene (HDPE). HDPE had a bimodal molecular weight distribution, containing 80 wt % of low molecular weight (LMW) component (&Mmacr;w = 99 kg/mol) and high molecular weight (HMW) component (&Mmacr;w = 1100 kg/mol). Even though the blend contained only a low concentration of HMW−HDPE chains (c = 1 wt %), above its overlap concentration (c* = 0.5 wt %), the initial film showed significantly higher lamellar orientation, and the high-temperature film showed a much more intense shish-kebab scaffold than LLDPE. The Ruland method for meridional streak analysis was used to analyze the kebab diameter and orientation during melting; the Vonk procedure for single lamella scattering was used to analyze the corresponding kebab thickness change. The temperature dependence of kebab thickness for both LLDPE and HDPE/LLDPE blend was used to determine the equilibrium melting temperature for PE (Tm° = 142.6 °C), which showed a very reasonable agreement with the literature value.

Semicrystalline polymer/layered silicate nanocomposites were prepared by solution blending of a low molecular weight poly(ethylene oxide) (PEO) with an organically modified montmorillonite, Cloisite 10A (C10A). The intercalation morphology was studied by temperature-dependent synchrotron wide-angle X-ray diffraction (WAXD). Unlike PEO homopolymers, significant secondary crystallization was observed in the PEO/C10A nanocomposites. Reversible de-intercalation and intercalation processes were detected during secondary crystallization and subsequent melting of secondary crystals. On the basis of two-dimensional WAXD results on oriented samples, an interphase layer between the silicate primary particles and PEO lamellar crystals was proposed. Secondary PEO crystallization in the interphase regions was inferred to be the primary driving force for polymer chains to diffuse out of the silicate gallery. This study provided a useful method to investigate polymer diffusion in nanoconfined spaces, which can be controlled by PEO secondary crystallization and melting outside the silicate gallery.

The toughening mechanism of nanocomposite films comprising ultrahigh molecular weight polyethylene (UHMWPE) and modified carbon nanofiber (MCNF) was investigated by in-situ synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) techniques during uniaxial stretching. Surface modification of carbon nanofibers included oxidation and subsequent chemical reaction with octadecylamine. At room temperature, the toughness of melt-pressed nanocomposite films was found to increase over 10 times by addition of 0.2 and 5 wt % of MCNF compared to that of pure UHMWPE. WAXD and SAXS results indicated that MCNF acted as a solvent carrier in the stiff UHMWPE matrix, whereby the grafted short hydrocarbon chains (n = 18) plasticized the surrounding UHMWPE chains in the nanoscale vicinity (10−20 nm) and induced interfacial flow under stretching, resulting in a large elongation-to-break ratio (>500%). A martensitic crystal transformation in UHMWPE was detected in all samples during deformation, where the transformation mode could be assigned as T12. At high temperature (118 °C), the toughness of the MCNF/UHMWPE composite films was still about 2 times higher than that of pure UHMWPE. The mobile hydrocarbon layers at the UHMWPE/MCNF interface appeared to be the key to overcome the barrier of high chain entanglements in the solid UHMWPE matrix and to induce the significantly toughened performance.

A series of nanocomposites prepared by melt-blending of Cloisite organoclays with ethylene-co-vinyl acetate (EVA) and ethylene-co-methyl acrylate (EMA) copolymers were investigated by using small-angle X-ray scattering (SAXS), wide-angle X-ray diffraction (WAXD), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), and rheological techniques. SAXS and TEM results confirmed mixed clay intercalation and exfoliation in all tested nanocomposites. The melting temperature, Tm, and crystalline structure (orthorhombic) in EMA and EVA were not significantly affected by the presence of organoclays, indicating that the clay particles were predominantly confined to the amorphous phase. Rheological properties above Tm were very similar in EVA and EMA nanocomposites. Both systems exhibited pseudo-solid rheological behavior in small-strain oscillatory shear experiments, yet they could yield and flow under a steady shear, which is characteristic of physical gelation. The pseudo-solid rheological behavior in EVA and EMA nanocomposites becomes more pronounced at higher contents of organoclay and at higher temperatures. SAXS results indicated that the silicate gallery spacings (d), intercalated by EVA and EMA chains, decreased with increasing temperature. This can be attributed to the reduced compatibility between organoclay and polymer (i.e., a LCST-type phase behavior). The unusual rheological properties of the nanocomposites at high temperatures were probably due to the formation of a 3D network of clay tactoids. Novel analytic models were proposed to describe rheological data from meltlike to gellike behaviors in EVA− and EMA−organoclay nanocomposites.

An asymmetric ABC coil−coil−disk triblock oligomer, based on polyethylene-block-poly(ethylene oxide)-block-pentakis(pentyloxy)triphenylene (PE-b-PEO-b-P5T or EEO−P5T), was successfully synthesized by coupling a hydroxyl-terminated PE-b-PEO diblock oligomer and 2-hydroxy-3,6,7,10,11-pentakis(pentyloxy)triphenylene using oxalyl chloride. The structure and morphology of the supramolecular self-assembly in bulk EEO−P5T were studied by differential scanning calorimetry, small- and wide-angle X-ray scatterings, polarized light microscopy, and transmission electron microscopy. The PE block crystallized into interdigitated, extended chain crystals with a chain-tilting angle of 23° from the lamellar normal. The peak melting temperature (Tm) of the PE crystals was at 93.8 °C. The PEO block remained in the amorphous state because of its both ends being tethered to other two blocks and the mismatch of the cross-section areas at the PE−PEO and PEO−P5T interfaces. Bilayer P5Ts, sandwiched between amorphous PEO layers, exhibited a nematic columnar (Ncol) to nematic discotic (ND) transition at ca. 23 °C. Intriguingly, the ND to isotropic (or vice versa) transition was observed to coincide with the melting (or crystallization) of the PE block.

Studies of dilute polymer solutions in shear flow suggest that the mean fractional extension of molecules increases gradually with the Weissenberg number (Wi = shear rate × longest relaxation time) and approaches an asymptotic value of 0.4−0.5, while in elongational flow it approaches full contour length above a certain critical strain rate. In an entangled polymer melt, this behavior is more complex due to inter- and intramolecular interactions. In situ rheo-SAXS (small-angle X-ray scattering) and -WAXD (wide-angle X-ray diffraction) experiments were performed to investigate the effects of shear rate, shear duration, and Wi on the extent of molecular orientation/extension and crystal orientation in an isotactic polypropylene (iPP) melt. Two series of experiments were designed:  (1) variation of shear rate (30, 45, and 60 s^-1) at a constant shear duration (5 s) and (2) variation of shear duration (1.3, 3, and 5 s) at a constant rate (60 s^-1). The degree of crystal orientation (Herman's orientation function, f) observed at 165 °C and fraction of oriented crystals (Xo) observed in a fully crystallized sample at room temperature increased with both shear rate and shear duration. Interestingly, at a constant strain (rate × duration), short-duration shear at a high rate was found to be more effective (i.e., higher f and Xo) than long-duration shear at a low rate. The longest relaxation time for the iPP sample and Wi were estimated from the dynamic moduli data. Both f and Xo were found to gradually increase with Wi and approached plateau values at high values of Wi. Results indicated that, even under a very intense shear field (or high Wi values), molecules do not extend to full contour length, and there is a limiting value for mean orientation/extension and subsequent crystal orientation in a polymer matrix. Characteristic dimensions of the shish-kebab entity formed in a sheared iPP melt at 165 °C were determined from the rheo-SAXS data. It was found that the average shish length was 700−750 nm and the average spacing between adjacent kebabs was 60−70 nm.

Deformation-induced phase transitions and superstructure formation in poly(ethylene terephthalate) (PET) were studied by means of in-situ synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) as well as Raman spectroscopy. The deformation conditions involved uniaxial stretching of quenched PET films at a temperature just below its glass transition temperature (Tg), where a notable “plastic deformation” stage was observed. WAXD results indicated that the initial sample contained a “slush” structure (amorphous + nematic), whereby deformation induced oriented amorphous, nematic, smectic (C and quasi-A), and stable triclinic crystalline phases. SAXS results indicated that the fibrillar superstructure was formed upon the formation of oriented slush. In-situ Raman spectroscopic data revealed the orientation information on ethylene glycol and benzene ring as well as the gauche−trans transition in deformation of PET chains, which are in good agreement with X-ray results. A mechanism for deformation-induced phase transitions and for hierarchical structure formation has been proposed to correlate the structural information with the mechanical properties.

The morphology and order-to-disorder transition (ODT) of solutions of a low molecular weight poly(styrene-b-ethylene-alt-propylene) in squalane were studied by four different methods:  DSC, modulated DSC, rheology, and small-angle X-ray scattering (SAXS). The ordering from disordered micelles to the BCC morphology was traced by both rheological and SAXS measurements. The ordering mechanism was found to follow the “nucleation and growth” pathway under both shallow and deep temperature quenches. The ordering process resembles the crystallization of semicrystalline polymers. The ordering half-time (t0.5) showed a clear minimum at an intermediate temperature. At temperatures far below the ODT temperature (TODT) but well above the glass transition temperatures of the solvent and the corona block of the micelles, t0.5 increased with concentration. The temperature dependence of the ordering process was analyzed using the Fredrickson−Binder theory. Following a shallow quench (TODT − T < 15 °C), t0.5 was found to vary with the quenching depth as ln t0.5 ∝ (TODT − T)^-2. Following a deep quench (TODT − T > 50 °C), the ordering time followed a simple Arrhenius relationship, and the activation energy showed negligible dependence on concentration.

In-situ synchtrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) are used to follow the isothermal crystallization (lamellar thickness, crystallinity, orientation, and parent-to-daughter ratio) of a polydisperse isotactic polypropylene subjected to “short term shearing” as a function of imposed shear stress, shearing duration, and crystallization temperature. The X-ray data are interpreted in view of the real-space morphological information from ex-situ microscopy. Under “mild” shearing conditions (shear stress less than a critical value and shearing duration less than a critical time), needlelike nuclei are induced during shear but are so far apart that crystallites splay substantially as they grow to form somewhat distorted spherulites; the X-ray results show weakly oriented growth on a time scale that is rapid compared to quiescent crystallization and show that the orientation distribution broadens as crystallization progresses. Stronger shearing leads to the elaboration of these nuclei into threadlike structures that template the formation of highly oriented crystals with fiberlike orientation. The parent-to-daughter ratio is influenced by both temperature and flow. As expected, increasing the crystallization temperature leads to fewer daughter crystals relative to the parents. Shear also enhances the formation of parents relative to daughters:  as parent crystals form with their chain axis along the flow direction, the epitaxial daughter crystals have their chain axis in an unfavorable direction, perpendicular to the flow.

The crystallization of the PLLA block within the microphase-separated lamellar microdomain in a poly(styrene)-b-poly(l-lactide) (PS-PLLA) diblock copolymer was carried out from hard confinement (i.e., the crystallization temperature of PLLA (Tc,PLLA) < the glass transition temperature of PS (Tg,PS)) to soft confinement (i.e., Tc,PLLA ≥ Tg,PS), where interesting morphological evolution was observed. At Tc,PLLA < Tg,PS, a typical confined morphology was obtained, while at Tc,PLLA ≥ Tg,PS, a unique undulated morphology was seen for the first time. The amplitude and periodicity of the undulation instability are dependent upon the orientation of microphase-separated lamellae. The PLLA crystals under confinement formed perpendicular morphology having crystalline chains normal to the lamellae as evidenced by 2D small-angle X-ray scattering and wide-angle X-ray scattering. On the basis of morphological observations and crystallization kinetics studies, the undulation instability can be attributed to the change of crystallization mechanism from hard to soft confinement, which coincides with the transition from homogeneous to heterogeneous nucleation.

In this study, hyaluronic acid (HA) was electro-spun and electro-blown to prepare nonwoven nanofibrous membranes. Critical parameters for processing and corresponding effects on the membrane morphology were investigated using the methods of rheology and scanning electron microscopy (SEM). During electro-spinning, the optimal HA concentration window for nanofibrous formation was determined within a narrow range of 1.3−1.5 w/v %, corresponding to a solution viscosity range of 3−30 Pa s at a shear rate of 1 s^-1. SEM results indicated that, with increases in (1) the total concentration by blending of low molecular weight HA, (2) the evaporation rate by the addition of ethanol, and/or (3) the feeding rate of solution, the electro-spinning performance for creating nanofibers was improved. However, the improvement was not sufficient to achieve a consistent production of high quality nonwoven nanofiber membranes. This problem was overcome by a new electro-blowing process using the combination of air flow and electro-spinning. Although air blowing at room temperature around the spinneret orifice did not exhibit a remarkable enhancement of nanofiber formation of HA, the performance was significantly improved with an increase in the air blowing rate. SEM results showed that the temperature of air-blowing was the most effective parameter in ensuring HA nanofiber formation. As the temperature of the blown air increased from 25 to 57 °C, the nanofiber formation became consistent and uniform. A high quality HA nonwoven membrane of nanofibers was successfully produced by blowing air at 57 °C with a 70 ft³/hr flow rate.

Development of shear-induced crystallization precursor structure was studied by in-situ rheo-SAXS (small-angle X-ray scattering) and rheo-WAXD (wide-angle X-ray diffraction) techniques using binary polymer blends of high and low molecular weight polyethylenes near their nominal melting temperatures (120 °C). Two low molecular weight polyethylene copolymers, containing 2 mol % hexene, with weight-average molecular weights (Mw) of 50 000 (MB-50K) and 100 000 (MB-100K), and polydispersity of about 2, were used as the noncrystallizing matrices. A high molecular weight polyethylene homopolymer with Mw of 250 000 (MB-250K) and polydispersity of about 2 was used as the crystallizing minor component. Two series of model blends, MB-50K/MB-250K and MB-100K/MB-250K, each containing weight ratios of 100/0, 97/3, 95/5, and 90/10, were prepared by solution blending to ensure thorough mixing at the molecular level. At the chosen shear conditions (rate = 60 s^-1, duration = 5 s, T = 120 °C), while no flow-induced structures were seen in pure MB-50K and MB-100K melts, the blends in both series showed distinct but different shear-induced structures. Results indicate that the high molecular weight component dominates the formation of crystallization precursor structures in the blend under shear, which can act as a template for further crystallization. A “shish-kebab” structure, detected by both SAXS and WAXD, was observed in the MB-100K/MB-250K (90/10) blend, while only a twisted lamellar structure (kebab) was seen in the rest of the blends under the same shear conditions. These findings suggest that the matrix viscosity plays an important role to influence the formation of crystallization precursor structure of the high molecular component under flow. In the MB-100K/MB-250K (90/10) blend, the length of the shish was estimated from the equatorial streak in SAXS, which showed a noticeable decrease with time, while the corresponding scattering intensity was found to increase. The evolution of the shish-kebab structure from SAXS is consistent with the appearance of the (110) peak in WAXD, which can be explained by the coil−stretch transition induced by flow.