Degree Of Molecular Orientation Research Articles

Study on the influence of in-mold sequential injection molding process parameters on mechanical properties of self-reinforced single composites

Abstract In this paper, the self-reinforced single polymer composites (SR-SPCs) with different mechanical properties were obtained by compound injection molding technology, and the micro-morphology of these samples was observed. Then, using structured statistical methods, analysis of variance, and response surface methodology, study the effects of various molding variables on material morphology and properties and determine the most important molding variables and their interactions. Finally, the associated experimental data are fitted by the least squares minimization program, and the relevant dimensionless equations are obtained. The purpose is to objectively analyze the influence mechanism of molding parameters on SR-SPCs and establish a mechanism model. It was found that temperature change was the most important factor affecting the morphology and mechanical properties. The degree of molecular orientation is the most important factor to determine the tensile strength and elastic modulus of the sample. The change of crystallinity is the most important factor related to the elongation at break. By establishment relevant dimensionless equations, the influence of molding parameters on the mechanical properties of SR-SPCs, such as tensile strength and elastic modulus, was preliminarily studied.

Marine biodegradation of poly[(R)-3-hydroxybutyrate-co-4-hydroxybutyrate] elastic fibers in seawater: dependence of decomposition rate on highly ordered structure.

Here, we report the marine degradability of polymers with highly ordered structures in natural environmental water using microbial degradation and biochemical oxygen demand (BOD) tests. Three types of elastic fibers (non-porous as-spun, non-porous drawn, and porous drawn) with different highly ordered structures were prepared using poly[(R)-3-hydroxybutyrate-co-16mol%-4-hydroxybutyrate] [P(3HB-co-16mol%-4HB)], a well-known polyhydroxyalkanoate. Scanning electron microscopy (SEM) images indicated that microorganisms attached to the fiber surface within several days of testing and degraded the fiber without causing physical disintegration. The results of BOD tests revealed that more than 80% of P(3HB-co-16mol%-4HB) was degraded by microorganisms in the ocean. The plastisphere was composed of a wide variety of microorganisms, and the microorganisms accumulated on the fiber surfaces differed from those in the biofilms. The microbial degradation rate increased as the degree of molecular orientation and porosity of the fiber increased: as-spun fiber < non-porous drawn fiber < porous drawn fiber. The drawing process induced significant changes in the highly ordered structure of the fiber, such as molecular orientation and porosity, without affecting the crystallinity. The results of SEM observations and X-ray measurements indicated that drawing the fibers oriented the amorphous chains, which promoted enzymatic degradation by microorganisms.

Structural Heterogeneity Dependence of the Fracture Feature Distribution in the Tensile Elongation of Microinjection Molded Polyethylene

The microscopic structure and tensile statistical fracture behavior of microinjection molded polyethylene samples with and without a weldline were investigated as a function of mold temperature using small-angle X-ray scattering (SAXS), wide-angle X-ray diffraction (WAXD), and stress–strain measurements. Probability density functions were adopted to describe the statistical nature of bulk mechanical parameters. The distributions of the elongation at break and fracture toughness follow a Gaussian function for the samples molded at 27 and 50 °C, whereas the fracture feature distributions deviate evidently from Gaussian statistics in the case of the samples molded at 75 °C. This discrepancy is due to the different extent of structural heterogeneity along the shear flow direction as evidenced by the distributions of crystallinity and degree of molecular orientation. Being composed of a more uniform structural distribution on both the lamellar and molecular length scales across the length, a fraction of the samples molded at 75 °C can surpass the barrier of localized homogeneous deformation as observed in the ones produced at low mold temperatures upon tensile stretching and thus are globally elongated to a large strain before ultimate failure, resulting finally in a broad non-Gaussian distribution of the fracture parameters. The fracture toughness–yield stress relationships revealed that the degree of orientation of the polymeric chains is of more importance than the crystallinity with regard to the fracture toughness.

Anomalous Postprocessing Dimensional Change of Injection-Molded Products Composed of Poly(lactic acid) and Poly(vinyl alcohol)

The present study revealed that an injection-molded product comprising poly(lactic acid) (PLA) and poly(vinyl alcohol) (PVAL) exhibited an anomalous dimensional change, i.e., expansion in the flow direction, after postprocessing annealing (around 2.9% expansion by annealing at 80–100 °C for 3 h). Although the crystallization of the PLA during cooling was not accelerated by the addition of PVAL, the PLA showed transcrystallization from the dispersed PVAL droplets, which were deformed in the flow direction, during postprocessing annealing. Therefore, after annealing, the injection-molded products exhibited a high degree of molecular orientation, in contrast to the original unannealed products, which were not oriented. This anomalous behavior should be noted as a means of controlling dimensional change after processing.

Correlation between ordering and shear thinning in confined OMCTS liquids.

Despite decades of extensive research, the behavior of confined liquids, particularly in the mixed/boundary lubrication regime, remains unelucidated. This can be attributed to several factors, including the difficulty to make direct experimental observations of the behavior of lubricant molecules under nonequilibrium conditions, the high computational cost of molecular simulations to reach steady state, and the low signal-to-noise ratio at extremely low shear rates corresponding to actual operating conditions. In this regard, we studied the correlation between the structure formation and shear viscosity of octamethylcyclotetrasiloxane confined between two mica surfaces in a mixed/boundary lubrication regime. Three different surface separations-corresponding to two-, three-, and five-layered structures-were considered to analyze the effect of confinement. The orientational distributions with one specific peak for n = 2 and two distributions, including a parallel orientation with the surface normal for n > 2, were observed at rest. The confined liquids exhibited a distinct shear-thinning behavior independent of surface separations for a relatively low shear rate, γ̇≲108s-1. However, the shear viscosities at γ̇≲108s-1 depended on the number of layered structures. Newtonian behavior was observed with further increase in the shear rate. Furthermore, we found a strong correlation between the degree of molecular orientation and the shear viscosity of the confined liquids. The magnitude of the shear viscosity of the confined liquids can primarily be determined by the degree of molecular orientation, and shear thinning originates from the vanishing of specific orientational distributions with increasing shear rate.

Effect of molecular orientation on predissociation of NaI molecules

Based on the time-depend quantum wave packet method, a scheme is proposed to investigate the effect of the molecular orientation on the predissociation of NaI molecules. The molecular orientation in the ground state is steered by THz half-cycle pulse. Then the oriented molecule is excited by femtosecond pump pulse. Molecular orientation has a significant effect on the angular distribution of wave packets. The excited wave packet is distributed in a more narrow region with the increase of molecular orientation degree. The angular distribution of dissociated products is essentially consistent with the angular distribution of excited wave packet. The dissociation probability is proportional to the excitation probability of pump pulse, which is related to the molecular orientations. Compared with the result of predissociation without molecular orientation, the maximal dissociation probability can increase by 6.5% in our work. The electric field intensity of the pump pulse also has a significant influence on dissociation probability.

Role of Molecular Layering in the Enhanced Mechanical Properties of Stable Glasses.

The density, degree of molecular orientation, and molecular layering of vapor-deposited stable glasses (SGs) vary with substrate temperature (Tdep) below the glass-transition temperature (Tg). Density and orientation have been suggested to be factors influencing the mechanical properties of SGs. We perform nanoindentation on two molecules which differ by only a single substituent, allowing one molecule to adopt an in-plane orientation at low Tdep. The reduced elastic modulus and hardness of both molecules show similar Tdep dependences, with enhancements of 15-20% in reduced modulus and 30-45% in hardness at Tdep ≈ 0.8Tg, where the density of vapor-deposited films is enhanced by ∼1.4% compared to that of the liquid-quenched glass. At Tdep < 0.8Tg, one of the molecules produces highly unstable glasses with in-plane orientation. However, both systems show enhanced mechanics. Both the modulus and hardness correlate with the degree of layering, which is similar in both systems despite their variable stability. We suggest that nanoindentation performed normal to the film's surface is influenced by the tighter packing of the molecules in this direction.

Design and Analysis of a Mandrel Process to Control the Properties of Poly(p-Phenylene Terephthalamide) Films

Abstract Kinematic and rheological considerations have been used to design and analyze the processing steps for obtaining biaxial chain orientation in poly(p-phenylene terephthalamide) (PPTA) films. A semi-quantitative approach to the mandrel design allows one to predict the type and degree of molecular orientation obtained. For the case of PPTA dissolved in H2SO4, the basic steps are extrusion of the liquid crystalline solution through an annular die followed by expansion and elongational flow over a mandrel. The mandrels used include conical, hyperbolic and ogival shapes. Molecular orientation as well as mechanical properties were found to be dependent on mandrel shape and presence. Wide angle x-ray diffraction (WAXS) techniques using White-Spruiell orientation factors and pole figures were used to characterize the films. Mechanical properties as well as the above techniques indicate a high degree of biaxial orientation can be obtained with a bias in certain film directions depending on the mandrel used. Tensile strength and Young's modulus values were found to be on the order of 30 000 psi and one million psi respectively. They also can be equally distributed in the film plane depending on mandrel shape. White-Spruiell orientation factors were as high as 0.49 and 0.5 for f 1 B a n d f 2 B $f_{1}^{B}\text{ }and\text{ }f_{2}^{B}$ respectively. Scanning electron microscopy was used to observe the film superstructure as a function or processing conditions. All films exhibited a skin-core structure. Uniaxially drawn films are fibillar while the biaxial films appear to be more homogeneous.

Stronger orientation of state-selected OCS molecules with relative-delay-adjusted nanosecond two-color laser pulses.

Using the all-optical molecular orientation technique with intense nonresonant two-color laser pulses, stronger molecular orientation |⟨cos θ2D⟩| ∼ 0.34 is achieved by employing the following two strategies: (1) carbonyl sulfide molecules lying in the lower rotational states are selected using a home-built molecular deflector and (2) the rising parts of the two wavelengths of the pump pulse are adjusted by introducing a Michelson-type delay line in the optical path. The achieved degree of molecular orientation is higher than that observed in the proof-of-principle experiment [Oda et al., Phys. Rev. Lett. 104, 213901 (2010)] by about an order of magnitude and the highest ever characterized directly by Coulomb explosion imaging with appropriate probe polarization.

Controlling molecular orientation by laser pulses with two different envelope shapes

AbstractWe use density matrix theory to numerically simulate the orientation of LiH molecules induced by laser pulses with two different envelope shapes. The results show that the envelope shape of the terahertz single‐cycle laser pulse (SCP) has significant effects on molecular orientation degree and duration. When a SCP with adjusted laser field amplitude and carrier envelope phase is used to steer the molecular orientation, a larger orientation degree can be obtained by the Gaussian‐shaped SCP, while a longer orientation duration can be realized by the sin2‐shaped SCP. When combined with the two‐color femtosecond laser pulse, the sin2‐shaped SCP is better than Gaussian‐shaped SCP, either in improving orientation degree and duration or in stability and controllability shown with increasing temperature. The optimal molecular orientation can be realized under the combination of the two‐color femtosecond laser pulse and sin2‐shaped SCP which has the appropriate delay time: the maximal orientation degree reaches 0.801 at 0 K and 0.450 at 20 K, the maximal orientation duration is 31.3% of the rotational period at 0 K.

Formation and Distribution of the Mesophase in Ultrasonic Micro-Injection-Molded Isotactic Polypropylene

The influence of mold temperature and filling velocity on the formation and distribution of the mesophase in ultrasonic micro-injection-molded isotactic polypropylene (iPP) was explored using a wide-angle X-ray diffraction (WAXD) technique. A Gaussian fitting method was used to decompose one-dimensional WAXD profiles into quantitative fractions of the crystalline lamellae, mesophase, and amorphous phase. It was found that the mesophase formation can only be detected up to a mold temperature of 60 °C due to its low thermal stability. Moreover, the content of the mesophase decreases distinctly as the filling velocity is increased. The component is mainly located in the core layer after examining the distribution of each crystal form along the thickness direction, irrespective of mold temperature and filling velocity. All these findings can be attributed to the correlation between the degree of molecular orientation and the cooling rate. A high cooling rate occurred during the micro-injection molding process produces the result that there is less time for the polymeric chains with a low level of orientation to crystallize into a well-ordered structure, thus yielding many mesophase crystals with a paracrystalline smectic arrangement especially in the core layer. Finally, a diagram for mesophase formation was constructed in mold temperature–filling velocity space, which offers a detailed road map for regulating mesophase fraction in ultrasonic micro-injection molding operations.

On the Structural Peculiarities of Self-Reinforced Composite Materials Based on UHMWPE Fibers.

The structure of self-reinforced composites (SRCs) based on ultra-high molecular weight polyethylene (UHMWPE) was studied by means of Wide-Angle X-ray Scattering (WAXS), X-ray tomography, Raman spectroscopy, Scanning Electron Microscopy (SEM) and in situ tensile testing in combination with advanced processing tools to determine the correlation between the processing conditions, on one hand, and the molecular structure and mechanical properties, on the other. SRCs were fabricated by hot compaction of UHMWPE fibers at different pressure and temperature combinations without addition of polymer matrix or softener. It was found by WAXS that higher compaction temperatures led to more extensive melting of fibers with the corresponding reduction of the Herman’s factor reflecting the degree of molecular orientation, while the increase of hot compaction pressure suppressed the melting of fibers within SRCs at a given temperature. X-ray tomography proved the absence of porosity while polarized light Raman spectroscopy measurements for both longitudinal and perpendicular fiber orientations showed qualitatively the anisotropy of SRC samples. SEM revealed that the matrix was formed by interlayers of molten polymer entrapped between fibers in SRCs. Moreover, in situ tensile tests demonstrated the increase of Young’s modulus and tensile strength with increasing temperature.

Peeling back the layers: Investigating the effects of polyelectrolyte layering on surface structure and stability of oil-in-water nanoemulsions

HypothesisLayer-by-layer deposition of polyelectrolytes is a useful technique for modifying surface functionalities. For drug delivery systems, alternating layers of biopolymers coat nanoemulsions, which house and protect the cargo until the time and destination of delivery. Here, we investigate molecular factors contributing to the stability and interfacial properties of nanoemulsions prepared by a co-adsorption of polymers poly(styrene sulfonate) and polyethylenimine (PEI), and surfactant dodecyltrimethylammonium bromide. We hypothesize the interplay between electrosteric and hydrophobic effects upon multi-polymer co-adsorption contributes to both macroscopic and molecular-level interfacial properties of nanoemulsions. ExperimentsTo probe interfacial layering properties, we use vibrational sum frequency scattering spectroscopy with ζ-potential measurements to determine the adsorptive behavior and molecular conformational arrangement of the polymer layers. Complementing these interfacial studies are dynamic light scattering experiments measuring the nanoemulsion size distribution and polydispersity index over a 30-day period. FindingsOur light scattering, ζ-potential, and spectroscopic results of the nanoemulsion surface show that the duration of droplet stability and the degree of molecular orientation of adsorbed polymers can be tuned by surfactant concentration, PEI concentration, and pH. These results illustrate how molecular surface properties of multi-polymer coated nanoemulsions contribute to synergistic effects and droplet stability, enabling advancements in applications surrounding biopharmaceuticals, cosmetics, and food sciences.

Field-free molecular orientation induced by a four-color laser pulse

The mechanisms of four-color (ω ± 2 ω ± 3 ω ± 4 ω) laser-induced field-free molecular orientation based on the polarizability and hyperpolarizability are explored. The CO molecule is used as a computational example. It is shown that the degree and direction of the molecular orientation can be coherently manipulated by changing the polarization direction of the odd- and even-order harmonics of the four-color laser pulse. When relative amplitude σ1 = σ2 = σ3 = σ4 = 1, the molecular orientation reaches the maximum. By modulating the phase of the odd- and even-order harmonics of the four-color laser pulse, enhancement and suppression of molecular orientation can be effectively controlled. The laser field peak amplitude is an important parameter to control the molecular orientation. Moreover, the combination of two four-color pulses can significantly increase the degree of molecular orientation.

Molecular orientation induced by H+ collision and modulated with combined fields

We take H++CO as a prototype to analyze the effect of ion or proton collision on molecular orientation modulated by a two-color shaped pulse combined with the time-delayed terahertz (THz) pulse. Through examining the effect of ion collision on the molecular orientation, we found that when the impact parameter and collisional velocity have weak inverse influences on the maximal orientation degree, the appropriate two-color and THz field intensity employed can improve the molecular orientation degree. The carrier envelope phase and frequency of the THz laser pulse as well as the temperature also have certain influence on the collision-induced molecular orientation.

Optimal field-free molecular orientation with nonresonant two-color adiabatic-turn-on and sudden-turn-off laser pulses

Field-free molecular orientation, achieved by using two-color slow turn-on and rapid turn-off laser pulses, is theoretically investigated for linear molecules. We derive analytical results for the maximum degree of field-free orientation for a given number of populated rotational states. We show that numerical results with adiabatic-turn-on and sudden-turn-off laser pulses attain the optimal orientation. Based on the adiabatic theorem, the effects of laser and molecular parameters on both the adiabatic process and the degree of molecular orientation are analyzed. The adiabaticity is well reflected through the temporal evolution of the phase difference between two adjacent rotational states.

High Strength Electrospun Single Copolyacrylonitrile (coPAN) Nanofibers with Improved Molecular Orientation by Drawing

High-performance carbon nanofibers are highly dependent on the performance of their precursors, especially polyacrylonitrile (PAN). In this work, the copolymer of PAN (coPAN) was synthesized for electrospinning. A self-assembling set-up was used for the stretching of single coPAN nanofibers. FTIR and Raman spectroscopies were used to characterize the chemical structure of coPAN nanofibers. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to monitor the morphology of single coPAN nanofibers under different drawing times. Micro-tensile test was used to determine the mechanical properties of single coPAN nanofibers. The results indicated that the drawing led to an increase in degree of molecular orientation along the fiber axis from 0.656 to 0.808, tensile strength from 304 MPa to 595 MPa, and modulus from 3.1 GPa to 12.4 GPa. This research would provide fundamental information of high-performance electrospun coPAN nanofibers and offer opportunities for the preparation of high-performance carbon nanofibers.

Modification of poly(vinyl alcohol) fibers with lithium bromide

The effect of lithium bromide (LiBr) on the structure and properties of poly (vinyl alcohol) (PVA) fibers obtained by wet-spinning was studied. The tensile modulus and strength of a PVA fiber were greatly enhanced irrespective of the post-spinning hot-stretching process. The high degree of molecular orientation arose from the reduction in the crystallinity and hydrogen bonding between PVA chains in the solution resulting from the addition of LiBr, which was revealed by the rheological properties of aqueous solutions. The LiBr was removed from the fiber during the spinning process in the coagulation bath, and during the washing process. This ensures strong hydrogen bonding in the fibers, and results in excellent mechanical properties.

Effect of molecular weight on molecular orientation and morphology of polypropylene sheets containing a β‐nucleating agent

AbstractIn this study, polypropylene (PP) films containing the β nucleating agent,N,N′‐dicyclohexyl‐2,6‐naphthalenedicarboxamide, were prepared using PP with three different molecular weights (low, medium, and high) by extrusion process with T‐shaped die. The structure and morphology of the films were studied after stretching. It was found that a unique molecular orientation, in which both thec‐axis and crystalline lamellae were oriented perpendicular to the flow direction, was formed in all undrawn film samples, irrespective of the molecular weights of the PP. In the PP sheets stretched in the machine direction, the low‐molecular‐weight sample containing the nucleating agent exhibited brittle properties owing to a lack of tie chains in the stretching direction. In contrast, cavitation was prominent in the medium (M‐PP)‐ and high (H‐PP)‐molecular‐weight samples. Notably, M‐PP containing the nucleating agent, with a high degree of molecular orientation, promoted the formation of a large number of voids. In H‐PP containing the nucleating agent, the presence of numerous tie chains inhibited cavitation, resulting in fewer voids. The experimental results demonstrated the influence of the molecular weight on the void structure, which will be useful in the field of microporous membranes.

Origin of electric field response signals in gate modulation spectroscopy for organic field effect transistors

Although gate modulation (GM) spectroscopy performed by combining optical measurements with gate bias application in a field effect transistor (FET) is one of the most straightforward techniques for electronic state evaluation of operating FETs, the origin of the spectral response to the gate bias application has not been fully elucidated. We here address the origin of the GM signals from variety of GM spectroscopic techniques performed for pentacene-based organic FETs. The GM spectrum under the negative gate bias in the condition that the source and drain electrodes are short-circuited exhibits a complicated lineshape with many unresolved peaks. We demonstrate that the GM signal under FET driving condition with drain bias applied is dominated by second-derivative (SD) Stark signals. From the comparison of the first and second harmonic response GM signals, the SD-Stark signal is found to be given by local electric fields from hole carriers injected from the electrodes, rather than given by an external electric field from the gate-bias. The SD signal intensity is enhanced when a high degree of molecular orientation exists around the carriers, thus the SD signal can be used to investigate molecular orientation around gate-induced carriers. • Gate bias application causes Stark signals as well as carrier signals in OFETs. • Gate modulation spectroscopy under drain bias applied is performed. • Stark signals arise from local electric field by injected hole carriers. • Molecular orientation around carriers can be examined by gate-induced Stark signals.