Orientation Of Molecular Chains Research Articles

Comparative study on the molecular chain orientation and strain-induced crystallization behaviors of HNBR with different acrylonitrile content under uniaxial stretching

Molecular chain orientation and strain-induced crystallization (SIC) behaviors of hydrogenated nitrile butadiene rubber (HNBR) with acrylonitrile content of 19%, 33%, 41%, 44%, i.e., HNBR-19 to HNBR-44, were characterized by using polarized FTIR spectroscopy and synchrotron radiation WAXD. Results demonstrated that the hydrogenated butadiene and acrylonitrile segments showed different orientation behaviors in these HNBRs. Orientation decreased with increasing acrylonitrile content, except for HNBR-44. HNBR-19 had the highest SIC ability and the crystals were composed of only hydrogenated butadiene homopolymer segments. HNBR-33 could not crystallize. As for HNBR-41 and HNBR-44 with high acrylonitrile content, HNBR-44 showed the better SIC ability and hydrogenated butadiene-acrylonitrile alternating copolymer segments were proved to participate in the crystallization. Relationship between microstructure and stress-strain characteristics was also discussed and results revealed that the relatively homogeneous crystal structure, higher crystal orientation and rapidly increasing molecular chain orientation were attributed to the highest stress value of HNBR-44 after SIC occurred.

The effect of cellulose molecular weight on internal structure and properties of regenerated cellulose fibers as spun from the alkali/urea aqueous system

The low-temperature alkali/urea aqueous system has received wide public attention since it is a better method to reduce pollution by preparing the regenerated cellulose fiber through the green dissolution system. Since molecular weight has a significant impact on the solubility and mechanical property of regenerated cellulose fiber; thus, we designed this experiment that cellulose materials with molecular weight ranging between 6.8 × 104 to 13.5 × 104 were employed for wet-spinning in alkali/urea aqueous system. It is found that as molecular weight of cellulose increases, cellulose solution decreases in solubility but cellulose molecular chain aggregates increase. However, bath the stability and viscosity of cellulose solutions almost remain unchanged with the change of molecular weight. As for the spun fibers, it is found that the orientation and crystallinity are relatively high when the molecular weight is low (6.8 × 104–9 × 104). These are excellent mechanical properties However, unlike conventional polymers, when molecular weight further increases to 9 × 104–13.5 × 104, the degree of orientation and crystallinity gradually decreases. This is due to a large number of entanglements between adjacent molecular chains, thereby hindering the molecular chain orientation along with reduced mechanical property. Our work demonstrates that the considerable molecular weight of cellulose is neither good for dissolution nor spun fibers' mechanical property. The choice of cellulose with relatively low molecular weight is essential not only for the excellent dissolution but also for fiber property.

Spinning Regenerated Silk Fibers with Improved Toughness by Plasticizing with Low Molecular Weight Silk.

Low-molecular weight (LMW) silk was utilized as a LMW silk plasticizer for regenerated silk, generating weak physical crosslinks between high-molecular weight (HMW) silk chains in the amorphous regions of a mixed solution of HMW/LMW silk. The plasticization effect of LMW silk was investigated using mechanical testing, Raman spectroscopy, and wide-angle X-ray scattering (WAXS). Small amounts (10%) of LMW silk resulted in a 19.4% enhancement in fiber extensibility and 37.8% increase in toughness. The addition of the LMW silk facilitated the movement of HMW silk chains during drawing, resulting in an increase in molecular chain orientation when compared with silk spun from 100% HMW silk solution. The best regenerated silk fibers produced in this work had an orientation factor of 0.94 and crystallinity of 47.82%, close to the values of natural degummedBombyx mori silk fiber. The approach and mechanism elucidated here can facilitate artificial silk systems with enhanced properties.

Microstructure and orientation evolution of microinjection molded β‐nucleated isotactic polypropylene/poly(ethylene terephthalate) blends

AbstractThe influence of different shear stresses induced by changing injection molding speeds on molecular chain orientation and lamellar branching of β‐nucleated iPP/poly(ethylene terephthalate) (PET) microparts was investigated using two‐dimensional wide‐angle X‐ray diffraction and 2D‐small‐angle X‐ray scattering. Results indicated that the prevailing shear stress can promote the formation of parent–daughter α‐crystal structure and twisted shish–kebab structure in subsequent microparts. The diffraction of (300) plane of β‐crystals was also observed at varying injection speeds. Increasing injection speeds can significantly enhance the content of β‐crystals from 24 to 41% for β‐nucleated iPP microparts. Additionally, the content of β‐crystals was further enhanced in β‐nucleated iPP/PET microparts with in situ formation of PET microfibrils under intensive shearing conditions. The addition of both PET and β‐nucleation agents coupling with high shearing conditions exerts a synergetic effect on the development of β‐crystals. However, the orientation degree of crystal lattice decreased with increasing injection speeds for both β‐nucleated iPP and iPP/PET microparts.

High Thermal Stability, High Tensile Strength, and Good Water Barrier Property of Terpolyester Containing Biobased Monomer for Next-Generation Smart Film Application: Synthesis and Characterization.

This research synthesizes novel copolyester (PCITN) containing biobased isosorbide, 1,4-cyclohexandimethanol, terephthalic acid, and 2,6-naphthalene dicarboxylic acid and characterize its properties. The PCITN copolyester was extruded into film, and its performance properties including: tensile strength, Young’s modulus, thermal, dimensional stability, barrier (water barrier), and optical (birefringence and transmittance) were analyzed after uniaxial stretching. The films have higher Tg, Tm, dimensional stability, and mechanical properties than other polyester-type polymers, and these performance properties are significantly increased with increasing stretching. This is due to the increased orientation of molecular chains inside the films, which was confirmed by differential scanning calorimetry (DSC), X-ray diffraction (XRD), and birefringence results. Good water barrier (0.54%) and lower birefringence (△n: 0.09) of PCITN film compared to poly(ethylene terephthalate) (PET), poly(ethylene 2,6-naphthalate) (PEN), and polyimide (PI) films, used as conventional substrate materials for optical devices, make it an ideal candidate as performance material for next-generation flexible devices.

Edge-On Self-Assembly of Tetra-bromoanthracenyl-porphyrin on Silver Surfaces

Molecular self-assembly on surfaces is driven by the range of interactions between the molecules themselves and the substrate. Generally, a face-on structure is favored for aromatic molecules lying flat on the surface. Here we report on the supramolecular self-assembly of 5,10,15,20-tetrakis(10-bromoanthracen-9-yl)porphyrin on the Ag(111) and Ag(110) surfaces. Well-ordered molecular chains were observed by room-temperature scanning tunneling microscopy on both surfaces. The relatively small size of the unit cell revealed an edge-on configuration of the porphyrin macrocycles, i.e. perpendicular to the surface plane, as confirmed by molecular mechanics calculations. Distinct intermolecular interactions were found on the two surfaces, providing different molecular chain orientations on Ag(111) and on Ag(110).

Effect of Electron Beam Irradiation on Gas-barrier Property of Biaxially Drawn Nylon/Montmorillonite Nanocomposite Films

This study aimed to improve the properties, such as tensile strength and gas barrier properties, of nylon/montmorillonite nanocomposite films for them to be better suited for packaging applications. In this study, organic/inorganic nanocomposite films were fabricated by melt compounding with a twin-screw extruder using a polyamide (m-xylene diamine 6 nylon; MXD6) as the polymer matrix and organic nanoclay (montmorillonite; MMT) as a filler. An electron beam (EB) was irradiated onto the fabricated nanocomposite film and its effects on the film properties were examined. Biaxial drawing and annealing were conducted after EB irradiation to investigate the orientation of molecular chains and crystallization. Transmission electron microscopy revealed that the clay nanoparticles were finely dispersed in the polymer matrix. Furthermore, the EB irradiation-induced polymer crosslinking was verified by measuring the gel content in the films. Finally, the thermal properties of the film were verified by differential scanning colorimetry (DSC), and the light transmittance was examined using a UV-visible spectrophotometer. Application of the sequential process of EB irradiation, biaxial drawing, and annealing on the MXD6/MMT-5 wt% nanocomposite films increased their tensile strength by 56% and decreased thermal shrinkage by 26%, with a minimal loss of the optical transparency. Increasing both nanoclay contents and draw ratios resulted in improved oxygen-gas barrier properties. In particular, the gas barrier property of the MXD6/MMT-5 wt% film approximately doubled compared with that of the untreated specimen. The thermal analysis by DSC revealed that the crystallinity decreased because of the addition of MMT and the EB irradiation.

Formation Mechanism of Residual Stresses in Micro-Injection Molding of PMMA: A Molecular Dynamics Simulation.

Injection molding is an economical and effective method for manufacturing polymer parts with nanostructures and residual stress in the parts is an important factor affecting the quality of molding. In this paper, taking the injection molding of polymethyl methacrylate (PMMA) polymer in a nano-cavity with an aspect ratio of 2.0 as an example, the formation mechanism of residual stresses in the injection molding process was studied, using a molecular dynamics simulation. The changes in dynamic stress in the process were compared and analyzed, and the morphological and structural evolution of molecular chains in the process of flow were observed and explained. The effects of different aspect ratios of nano-cavities on the stress distribution and deformation in the nanostructures were studied. The potential energy, radius of gyration and elastic recovery percentage of the polymer was calculated. The results showed that the essence of stress formation was that the molecular chains compressed and entangled under the flow pressure and the restriction of the cavity wall. In addition, the orientation of molecular chains changed from isotropic to anisotropic, resulting in the stress concentration. At the same time, with the increase in aspect ratio, the overall stress and deformation of the nanostructures after demolding also increased.

Effect of Multi-Level Microstructure on Local and Bulk Mechanical Properties in Micro-Injection Molded PC/PET Blend

This study introduces a method to investigate the relationship between the multi-level microstructures and mechanical properties of polymer blends prepared by micro-injection molding (µIM). Special morphological features were systematically researched. Polycarbonate (PC), poly(ethylene terephthalate) (PET), and PC/PET microparts all exhibit typical “skin-core” morphologies. The thickness of the core layer is much greater than that of the skin layer, and the thickness of the skin layer gradually decreases along the flow direction. Photoacoustic Fourier transform infrared spectroscopy records reveal that the PC molecular chain has the biggest orientation degree, followed by PC/PET and PET chains under the same µIM processing conditions. Moreover, the molecular chains orientation in the skin layer is more than 50% that in the core layer. Nanoindentation tests are conducted to study local mechanical properties. The higher modulus in the shear layer is affected to a greater extent by high shear action in comparison with the frozen and core layers. Uniaxial tensile testing demonstrates that the tensile strength of PC/PET micropart is 15.5% higher than that of the PET micropart, while the toughness is 16% higher than that of the PC microparts. In-situ, high- speed tensile imaging, combined with scanning electron microscopy micrographs of the fracture section, are used to study the fracture behaviors of the microparts. The results gathered in this paper may provide a theoretical basis and data to support the feasibility and efficiency of micro-injection molded polymer blends.

On the nucleation of polylactide by melt-soluble oxalamide based organic compounds

In this work, a series of oxalamide based organic compounds (OBOC) are synthesized and their capability to enhance the nucleation of polylactide is reported. The OBOCs are soluble in the polylactide melt and crystallize upon cooling, providing surface for heterogeneous nucleation to the polylactide matrix. One interesting observation is that the nucleation efficiency of the OBOCs increases at high cooling rates, making the use of OBOCs as a nucleating agent attractive for industrial processing conditions. The paper addresses the mechanism involved in the cooling rate dependence on the nucleation efficiency of the OBOC: The nucleation efficiency of polylactide is significantly enhanced in the presence of OBOC crystals, as a transcrystalline PLA morphology grows from the OBOC crystal surface (at relatively low supercooling from the equilibrium melting temperature of PLA). However, such crystallization of PLA occurs only when the OBOC crystals are formed at or below 145 °C while cooling at 10 °C/min. In contrast, when the OBOC crystals are formed above 145 °C (i.e. from a lower supersaturated state), the OBOC crystals display a significantly reduced capability for PLA nucleation. These findings override the possibility of both epitaxy and soft-epitaxy as a plausible nucleation mechanism. Supported by polarized optical microscopy, differential scanning calorimetry, plate-plate rheology and molecular modelling we evaluate the possibility of nucleation resulting from local stresses imposed on the polylactide melt invoking stress-enhanced nucleation. The imposed local shear rates, facilitated by the rapid growth of the OBOC crystals, are found high enough to facilitate contour orientation of the high molecular weight PLA chains next to the growing OBOC crystals, confirming the possibility for stress-enhanced nucleation. In addition, we identify surface roughness of OBOC crystals as a second parameter that influences the PLA nucleation process; the OBOC crystallization at high supersaturation is expected to yield smaller/defected OBOC crystals, which are presumed to provide high surface area and surface roughness. In contrast, when the OBOC crystals are grown at lowered supersaturation (>150 °C), they are bound to anneal during crystallization, providing a smoother surface and lower surface area – retrospectively exhibiting a decreased capability to promote the PLA nucleation, irrespective of the PLA supercooling. Interestingly, both the surface roughness of OBOC crystals and the local stresses they impose on the PLA melt increase when the OBOC crystal growth proceeds from a highly supersaturated state, providing an explanation to the cause of the favored crystallization of PLA at the high cooling rates in the presence of the chosen OBOCs.

Deformation-Induced Crystallization Behavior of Isotactic Polypropylene Sheets Containing a β-Nucleating Agent under Solid-State Stretching.

The deformation-induced crystallization of an isotactic polypropylene (iPP) sheet containing a β-nucleating agent was evaluated. The phase transformation of the β-modifications was investigated and the crystal morphology was observed at room temperature after stretching at different temperatures. The results showed that the crystallinity increased after solid-state stretching. When the stretching temperature was below the initial crystallization temperature, stretching deformation promoted the orientation of amorphous molecular chains. When the deformation temperature exceeded the crystallization temperature, part of the β-modifications underwent a phase transformation process and was stretched into a shish-kebab structure. However, once the stretching temperature was close to the melting point, the β-modifications melted and recrystallized, and the shish-kebab structure underwent stress relaxation due to poor thermal stability, transforming into α-modifications. It was revealed that the crystal phase transformation mechanism of the β-modifications was based on the orientation of the molecular chains between the adjacent lamellae. In addition, the shish-kebab cylindrite structure played an important role in modifying the tensile and impact properties of the iPP sheet. The tensile and impact strengths increased by as much as 34% and 126%, respectively.

Structures and Properties of Polyimide with Different Pre-imidization Degrees

A series of polyimide (PI) films derived from pyromellitic dianhydride (PMDA) and 4,4′-oxydianiline (ODA) were prepared with the employment of chemical pre-imidization, and the pre-imidization degree (pre-ID) was found influential on structures and properties of the films obtained. Specifically, a certain degree of chemical imidization could promote the in-plane orientation of molecular chains inside the film, which then enhanced the mechanical strength and reduced the coefficient of thermal expansion (CTE) of the films. Further, such pre-imidization process could expand the internal space gap inside the films, thereby lowering their dielectric constant and glass transition temperature. Our study provides a new approach for preparing high-performance PI films through chemical imidization.

Micro-Mechanism Research into Molecular Chains Orientation Synergistically Induced by Carbon Nanotube and Shear Flow in Injection Molding

For the degree of orderly arrangement of the molecular chains at the interface of nanocomposites, the static and sheared polyethylene (PE)/carbon nanotube (CNT) models and the sheared pure PE model were constructed, and molecular simulation experiments were carried out in comparison. The micro-mechanism of molecular chains orientation, synergistically induced by the carbon nanotube and shear flow in injection molding, was discussed by analyzing the radius of gyration, molecular chain motion, conformation evolution of molecular chains, bond orientation parameter, interface binding energy and atom distribution. The results show that, for the static composite system, the conformation adjustment of PE molecular chains induced by CNT is limited due to the hindrance from the surrounding chains. Thus, the orientation and radius of gyration of molecular chains increase slightly. For the sheared pure PE system, the orientation induced by shear flow is unstable. After the cessation of shear, the molecular chains undergo intense thermal movement and relax quickly. The disorientation is obvious, and the radius of gyration decreases considerably. It is worth noting that for the sheared composite system, shear flow and the CNT have a synergistic effect on the orientation of the molecular chains, which is due to the adsorption effect of the CNT on shear-induced oriented chains and the inhibition effect of CNT on the relaxation of shear-induced oriented chains. Thus, the orientation stability of PE chains is greatly improved, and interface crystallization is promoted. Moreover, because of the more regular arrangement of molecular chains in the sheared composite system, more H atoms and C atoms are close to the surface of the CNT, which increases the van der Waals force, and correspondingly increases the interface binding energy.

Brittle-to-ductile transition of PLA induced by macromolecular orientation

This work deals with the influence of biaxial orientation on the mechanical behavior of Polylactide (PLA) films. Comparisons between a crystallizable grade and a non-crystallizable one have been made in order to separate the effect of macromolecular orientation from the potential influence of the crystalline phase induced during the thermomechanical treatment. While unstretched PLAs exhibited brittle behavior at room temperature, it was observed for both types of PLA the strains at break for biaxially drawn films were remarkably enhanced to values over 100%. This study highlights for the first time, in the case of PLA, that a critical molecular chain orientation of the amorphous phase is the necessary condition to induce a ductile behavior. In-situ small-angle X-ray scattering (SAXS) experiments and post-mortem microscopic observations have revealed that it is a change of elementary plastic deformation mechanisms that is at the origin of this Brittle-to-Ductile (B-D) transition.

Thermal Expansion Behavior of Poly(amide-imide) Films with Ultrahigh Tensile Strength and Ultralow CTE

A series of novel poly(amide-imide) (PAI) films with different amide contents were prepared from pyromellitic dianhydride and four amide-containing diamines. These PAI films exhibited excellent mechanical and thermal properties with tensile strength of 203.7–297.4 MPa and Tg above 407 °C. The rigid backbone structures combined with strong intermolecular interactions provided PAI films with ultralow in-plane CTE values from −4.17 ppm/°C to −0.39 ppm/°C in the temperature range of 30–300 °C. The correlation between thermal expansion behavior and aggregation structures of PAI film was investigated. The results suggested that hydrogen bonding interactions could be maintained even at high temperature, thus resulting in good dimension reversibility of films in multiple heating-cooling cycles. It is demonstrated that dimensional stabilities of PAI films are determined by the rigidity, orientation, and packing of molecular chains. Heat-resistant PAI films with ultralow CTE can be developed as flexible substrates by regulating backbones and aggregation structures for optoelectronic application.

Molecular weight dependency of β phase formation in injection‐molded isotactic polypropylene

ABSTRACTThe influence of β‐nucleating agents and the molecular weight of isotactic polypropylene (iPP) on the formation of β phase in the injection‐molded iPP were explored using wide‐angle X‐ray diffraction technique. Without the β‐nucleating agents, the molecular weight of iPP was the dominant factor controlling the production of β phase during injection. A higher molecular weight could promote the orientation of molecular chains in the skin layer of injected sample, bringing out a larger content of β phase in the corresponding region. Although the β‐nucleating agents had a profound effect on the formation of β phase, it was only efficient for the iPP with moderate molecular weights. The systems of the lowest and highest molecular weight iPP samples compounded with β‐nucleating agents still performed the same with the corresponding ones without β‐nucleating agents. Such performances can be interpreted as a result of the competition between the β‐nucleating agents and the orientation degree of molecular chains. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48555.

Strain-optical behavior of polyethylene terephthalate film during uniaxial stretching investigated by Mueller matrix ellipsometry

Polyethylene terephthalate (PET) has gained widespread applications in various flexible devices where its optical properties after drawing process in manufacturing play a great importance in the performance of devices. In this work, the strain-optical behavior of pre-oriented polyethylene terephthalate (PET) film during uniaxial stretching is studied utilizing a proposed anisotropic model and Mueller matrix ellipsometry (MME). In the biaxial anisotropic model, the molecular chain orientation is correlated to the change of refractive indices and strain-induced birefringence, while the rotation of molecular chains is directly characterized by the rotation of principal optical axis. According to this model, the refractive indices, extinction coefficients and Euler angles extracted from measured Mueller matrix spectra can be used to interpret the micro-structural response of PET. A set of tensile tests have been carried out, with loading direction being varied. The results show that the strain-induced birefringence is linearly correlated to strain, while loading direction significantly affects the slope of achieved birefringence-strain curve. Meanwhile, the birefringence exhibits a positive linear relation with the orientation estimated by extinction coefficients. Through the analysis of the Euler angle ϕE, the principal optical axis rotating towards the tensile direction is accurately observed. Our results show that MME can provide a deep and systematic understanding of strain-optical behavior which can give accurate control of the optical properties of polymer films during deformation.

Homocrystallization and Stereocomplex Crystallization Behaviors of As-Spun and Hot-Drawn Poly(l-lactide)/Poly(d-lactide) Blended Fibers During Heating.

A series of poly(l-lactide)/poly(d-lactide) blended chips (LDC), as-spun LD fibers (LDA) and hot-drawn LD fibers (LDH) were prepared for investigating the homocrystallization and stereocomplex crystallization behaviors of LDA and LDH fibers during heating. Modulated differential scanning calorimetry (MDSC), hot stage polarized microscopy (HSPM), and real-time wide-angle X-ray diffraction (WAXD) were used for studying the crystallization and melting behaviors, fiber morphology, and crystalline structure evolution of the LDA and LDH fibers’ homocrystals and stereocomplex crystals during heating. The molecular chain orientations of the LDA and LDH fibers were obtained through spinning and improved through the hot drawing processes. When the molecular chain was oriented on the fiber axis, the homocrystals and stereocomplex crystals of the fibers began to form in turn as the heating temperature exceeded the glass transition temperature of the fiber. The side-by-side packing of the molecular chains was promoted by mixing the molecular chains with the extrusion screw during the spinning process, facilitating stereocomplex crystallization. When the LDA fiber was heated above the glass transition temperature of the fiber, movement of the fiber molecular chain—including molecular chain orientation and relaxation, as well as crystallization, melting, and recrystallization of homocrystals and stereocomplex crystals—were investigated through HSPM. MDSC and real-time WAXD were used to observe the molecular chains of the melted poly(l-lactide) and poly(d-lactide) homocrystals of the fibers rearranging and transiting to form stereocomplex crystals during heating.

Temperature and strain rate dependences on hardening and softening behaviours in semi-crystalline polymers: Application to PEEK

Semi-crystalline polymers often present a complex non-linear behaviour that combines thermo-viscoelastic and thermo-viscoplastic contributions associated to different deformation mechanisms. During the initial deformation stages, the process is influenced by the rupture and reorientation of crystalline phases while, during the final deformation stages, the process is mainly governed by the mobility and orientation of the amorphous molecular chains. Moreover, the level of reorientation of crystalline and amorphous phases is strongly affected by variables such as temperature and strain rate. This work focusses on the role of such mechanisms in the mechanical behaviour of polyether-ether-ketone (PEEK) within its different thermal-behaviour regions: initial glassy region, glass transition and final rubbery region. To this end, samples of PEEK are subjected to large deformations under uniaxial tension at temperatures from 20 to 240 °C, and strain rates from 0.0001 to 0.1 s−1 (covering both isothermal and adiabatic conditions). In addition, a constitutive model is proposed to complementarily explain the experimental observations by means of entropic strain hardening due to reorientation of polymer chains influenced by thermo-viscoelastic effects, as well as thermo-viscoplastic behaviours defining the material yielding by means of crystallites deformation and breaking. These results provide new insights into the deformation mechanisms of semi-crystalline polymers below and above glass transition, which are significantly relevant for thermoforming processes of biomedical prosthesis.

Influence of the Mold Temperature and Part Thickness on the Replication Quality and Molecular Orientation in Compression Injection Molding of Polystyrene

Abstract It is well known that the process of injection molding with dynamic mold temperature control leads to a good replication quality of high aspect ratio microstructures. However, the inhomogeneous pressure distribution during the holding pressure phase results in an anisotropy of the component properties, low dimensional accuracy and, especially with optical polymers, in undesired stress birefringence. The anisotropy is based on the orientation of the molecular chains in the flow direction, which can be reduced by an injection-compression molding (ICM) process. In order to use the synergy from both processes, an injection-compression molding process with dynamic mold temperature control can be utilized. Within the scope of this investigation, the new process was reproduced by an ICM process at elevated mold temperature (ICM_EMT) and compared with injection molding (IM) with regard to molding accuracy and optical properties in dependence of component thickness and mold temperature. In order to evaluate the molding accuracy, the roughness of a wire-eroded microstructure on the cavity surface was measured. To determine the degree of orientation, the optical properties considered were the transmission and the path difference. It was shown that the adapted ICM process was able to achieve a high degree of replication accuracy with a low degree of orientation, especially for thin-walled components. ICM at elevated mold temperature reduced the path difference in the components with the lowest wall thickness by a factor of two while at the same time optimizing the replication of the microstructure. This could also be confirmed by transmission measurements.