End Of Block Research Articles

Improvement of the self‐assembly of low χABA triblock copolymers with the addition of an ionic liquid

AbstractSelf‐assembly of high molecular weight commercially available low χ ABA triblock copolymers with ionic liquids (ILs) was observed after solvent casting without the implementation of an additional annealing step. Here, the addition of an IL to a triblock copolymer poly(methyl methacrylate‐b‐butyl acrylate‐b‐methyl methacrylate) (MAM) with an elastomeric central block and rigid end blocks is proposed to tune the self‐assembled morphology. Two different compositions of this copolymer with different initial microstructures (ill‐defined morphology without long‐range order and a lamellar morphology) were selected. The selectivity of the IL addition in one of the block phases was studied through the evolution of the glass transition of both blocks. The impact of the IL weight concentration on the morphology as well as the domain spacing is presented. Adding IL to the first copolymer detected a lyotropic disorder–order transition towards a gyroid morphology, observed even for a high molecular weight copolymer due to the low χ parameter, and for higher IL content a second transition to a lamellar morphology was observed. For the second copolymer, only an order–order transition (lamellae to cylinders) could be detected at high IL content. IL also enhanced the long‐range ordering of the MAM morphology obtained through solvent casting without additional annealing steps, compared with the same composition without IL. © 2022 Society of Industrial Chemistry.

Dynamic Mechanical Properties of Self-Assembled Bottlebrush Polymer Networks

We systematically investigate the effects of composition on the dynamic mechanical properties of bottlebrush polymer networks self-assembled by linear–bottlebrush–linear triblock copolymers. We fix the molecular architecture of the bottlebrush, which consists of 51 poly(dimethyl siloxane) (PDMS) side chains of 5 kg/mol and has a molecular weight of 255 kg/mol, and increase only the volume fraction f of the linear poly(benzyl methacrylate) (PBnMA) blocks. As f increases from 0.05 to 0.41, the network shear modulus G at room temperature increases from ∼4 to ∼100 kPa. Yet, depending on the network morphology, the relation between G and f exhibits two regimes. (i) For sphere morphology, G is nearly a constant; yet, because of a large fraction of loops, the absolute value of G is about 40% of the stiffness Gm of the PDMS bottlebrush matrix. (ii) For cylinder morphology, G increases slowly with f but remains nearly 4 orders of magnitude lower than 109 Pa for the glassy cylinders formed by the end PBnMA blocks. We explain this remarkable behavior by modeling the polymer as a polycrystalline material consisting of randomly oriented grains, and each grain is a fiber-reinforced composite. We propose a modified Halpin–Tsai model to describe the shear modulus of such a polycrystalline material: G = Gm(1+ζf)/(1–f), in which ζ is an adjustable parameter that describes the grain size relative to the fiber diameter. Above the glass-transition temperature of end blocks, the reinforcement to network modulus from the glassy fibers diminishes, such that G becomes a constant of the matrix stiffness. Our results not only reveal previously unexplored molecule–structure–property relations of self-assembled bottlebrush polymer networks but also provide a new class of soft, solvent-free, and reprocessable polymeric materials with a wide range of controllable stiffness.

Comparative Analysis of Student’s Academic Performance in Online and Face to Face Teaching of Biochemistry at HITEC-IMS Taxila

Background: During Covid pandemic the teaching/learning shifted from face to face to online. All institutions around the world developed learning environment for the students to facilitate distant learning. Subsequently assessments also followed online. After opening of the institutions on campus learning and assessments were carried out as usual. Objective: To see the effectiveness of teaching methods (online versus on campus) and to suggest improvement in both methods of teaching. Study Design: Retrospective comparative study Place and Duration of Study: Hitec-IMS, Taxila Pakistan from 1st September 2021 to 29th February 2022. Methodology: One hundred and ninety six academic performances of students in both the methods of learning were enrolled. Using purposive sampling technique, the EOB results of online and on campus were analyzed. Results: The better performance of student in term of summative assessments during online learning. Conclusion: Students found online environment better for learning and performing during examinations. Keywords: HITEC-IMS (Hitec Institute of Medical Sciences), NUMS (National University of Medical Sciences), EOB (end of block)

Sensitive detection of abasic sites in double-stranded DNA based on the selective reaction of enzymes

The apurinic/apyrimidinic (AP) site is one of the most common DNA lesions and a critical intermediate during the base excision repair pathway. Therefore, AP sites are essential in clinical diagnosis, treatment and detection. However, the existing detection methods are complicated in design and synthesis and have high instrument requirements, limiting their wide application. Therefore, there is an urgent need for a sensitive and straightforward detection method without time-consuming and heterogeneous reactions. Herein, we developed two compatible detection methods for AP sites in long and short dsDNA. For long and short dsDNA, the background signal was successfully suppressed by the affinity difference of Terminal deoxynucleotidyl transferase (TdT) and 3’ -end blocking, respectively, thus achieving high detectability and specificity. The detection limit was 13 pM in 20 μL, meaning that the LOD was 0.26 fmol for AP site amount and 0.05% for AP site abundance. The method has been successfully applied to detect AP sites in various biological samples quickly. Therefore, it has broad clinical application prospects, catering for the need for a point of care.

Glued Floor Coverings Based on Wooden End Blocks

Glued Floor Coverings Based on Wooden End Blocks

Academic Performance Improves with Emotional Intelligence Awareness and Physical Exercise among Medical Students

Background and objective: The importance of exercise and knowledge of emotional intelligence (EI) is gaining pace in academia, which is a positive development. Only a few single-blind research studies on the benefits of exercise and Emotional intelligence have indicated their effects on educational attainment among primary school children, but there are very few reports from young adult students. This longitudinal study aims to develop an intervention protocol to investigate the differential association between induced moderate physical exercise combined with emotional intelligence awareness on academic performance among 18-24-year-old private medical university students. Methods: Students are voluntarily enrolled in two groups: TEST (n = 180) and CONTROL (n = 180) based on inclusion criteria along with applicable consent’s and ethical clearance. The groups' baseline data mapping was completed in three phases. For the TEST group, baseline data was obtained in three stages during Phase-1; -, which is the pre-intervention phase: Anthropometric data, haemoglobin concentration, and cardiorespiratory measurements were all measured in Stage 1. (Lab-based data). Stage 2: two questionnaires were used, one to measure EI awareness and the other to assess learning styles (VARK). Stage 3 entailed the simultaneous recording of pre-intervention academic grades, of a said end block examination for both TEST and CONTROL groups. Phase 2: This is the intervention phase, which included only two components for the TEST group: the first was a moderate exercise regime (BRISK WALKING) of 2-3 sessions per week for about 30 minutes per session, and the second was addressing EI awareness. Which refers to knowing the current status of EI level, and fostering EI knowledge through various teaching-learning methods. Following the end of Phase -1 baseline data mapping, the two intervention components for the TEST group run concurrently for about 6months/24weeks, for the TEST group. Phase-3: is the post-intervention phase which included repeating of Phase-1; Stages 1, 2, and 3 for the TEST group, as well as a concurrent record and analysis of post-intervention academic grades, of the final professional examination for both TEST and CONTROL groups was done after Phase-2. The outcomes of Phase-1 and Phase-3 differential association compared. Results: The Statistical Package for Social Science (SPSS) Version 21.0 software was used to analyse all data from phases 1 and 3. A statistically significant correlation was found between the TEST group (p = 0.0001) and the CONTROL group (p = 0.406) when the pre-intervention period of multifactorial components subjected to intervention was compared to the post-intervention period. Conclusion: Intervention protocol with combined effect of emotional intelligence awareness and induced moderate physical exercise (BRISK WALKING) has shown significant improvement in academic grades.

Effect of Molecular Architecture and Symmetry on Self-Assembly: A Quantitative Revisit Using Discrete ABA Triblock Copolymers.

The inherent statistical heterogeneities associated with chain length, composition, and architecture of synthetic block copolymers compromise the quantitative interpretation of their self-assembly process. This study scrutinizes the contribution of molecular architecture on phase behaviors using discrete ABA triblock copolymers with precise chemical structure and uniform chain length. A group of discrete triblock copolymers with varying composition and symmetry were modularly synthesized through a combination of iterative growth methods and efficient coupling reactions. The symmetric ABA triblock copolymers self-assemble into long-range ordered structures with expanded domain spacings and enhanced phase stability, compared with the diblock counterparts snipped at the middle point. By tuning the relative chain length of two end blocks, the molecular asymmetry reduces the packing frustration, and thus increases the order-to-disorder transition temperature and enlarges the domain sizes. This study would serve as a quantitative model system to correlate the experimental observations with the theoretical assessments and to provide quantitative understandings for the relationship between molecular architecture and self-assembly.

An experimental study on the torsional stiffness and limit torque of a jaw coupling with consideration of spacer's hardness and installation methods

Abstract The jaw coupling with a flexible spacer is frequently used in the torque transmission between shafts with misalignment for machinery. Its torsional stiffness and limit torque closely determine the operational capacity and the dynamic characteristics of the system because the coupling is usually the most flexible link in the driving chain. In this study, the optimal design of a jaw coupling with an elastomeric spacer was investigated using the Taguchi method by considering four design factors: the tightening method of the clamping bolts, the tightening torque of the clamping bolts, the hardness of the thermoplastic polyether ester elastomer spacer and the installation angle between the two end blocks of the coupling. All specimens were tested by using an in-house torsion tester to record the torque-angular deformation responses. The results showed good reliability and repeatability, with a coefficient of variance within 5%. Spacer's hardness was found to be the most significant factor regarding the torsional stiffness, while the magnitude of the clamping torque had the most critical role in the limit torque. The estimation formulae for the torsional stiffness and limit torque of the jaw coupling were obtained by using the statistical regression of the measured data, respectively. Both formulae predicted the performance of the optimal designs within 5% of error compared to the confirmation tests.

Curving and Twisting in Self-Assembly of Triblock Terpolymers Driven by a Chiral End Block

A series of triblock terpolymers composed of a chiral end block, poly(l-lactide) (PLLA) (designated as chiral triblock terpolymers), are designed and synthesized for self-assembly. The chiral PLLA block can contribute the chirality effect for the twisting of microphase-separated domains. By attaching an asymmetric amount of the chiral block to a symmetric diblock of polyisoprene-b-polystyrene (PI-PS), compositional asymmetry can be created for the curving of microphase-separated domains. Moreover, the difference in persistence lengths of PS and PLLA blocks can give conformational asymmetry to the curving. With the combined effects of twisting and curving, the curvature at the saddle surface of microphase-separated domains might become significant to give the formation of a network phase. Consequently, a transformation from two-domain lamellae to core–shell network phases, then to a core–shell cylinder phase, and finally to a three-domain lamellar phase with undulation due to the rod–rod interaction with the chirality effect from the semiflexible PLLA, can be found in the isopleths of the triblocks. Interestingly, a core–shell double diamond phase could be found near the center of the ternary phase diagram; we speculate that the alleviation of packing frustration in the core of the self-assembled microdomain is attributed to the character of semiflexible PLLA. Accordingly, the phase window of network formation can be enlarged by the combined effects of compositional asymmetry, conformational asymmetry, and chirality.

Effect of hydrophilic block end groups and block junction on block copolymer self-assembly in solution.

Recent research suggests that the end groups of polymers can affect their self-assembly. However, the effect of end groups on the self-assembly of block copolymers in solution remains unclear, and thus far, only micelle–vesicle transformations have been achieved via end-group modification. Herein, we report that hydrophilic block end groups and the junction between two blocks can affect the solution self-assembly of block copolymers, leading to the formation of different morphologies, including vesicles, cubosomes, and hexosomes. Poly(ethylene glycol)-b-polystyrene (PEG-b-PS) with hydroxyl, methoxy, azido, or amino groups at the PEG chain ends was synthesized and self-assembled in solution via the cosolvent method. As a result, the morphology of the block copolymers transformed from vesicles to hexosomes upon increasing the end-group hydrophobicity. In addition, a morphological transition from cubosomes to vesicles was observed upon changing the junction from a triazole to an amide, and the interaction between the solvent and end groups significantly affected the self-assembly behavior.

Study on Seismic Performance of Connecting Prefabricated Coupling Beam and Shear Wall with Equivalent Bars

This study proposes a new type of method for the connection of the prefabricated coupling beam and shear wall to solve the problems in the construction of assembled frame‐shear wall structures. To investigate the seismic properties of this new type of assembled coupling beam, experiments with 2 groups of test pieces under low‐period cycling loads were completed. Furthermore, this study studied some important influence factors. Results show that this novel coupling beam has a similar performance to the cast‐in‐place coupling beam. The bearing capacity and energy dissipation capacity of cast‐in‐place structures increase with the increase in end block thickness and the ductility inversely, yet the bearing capacity, seismic energy dissipation capacity, and ductility of assembled coupling beam structure decrease with the increase in end block thickness. Numerical simulation shows that increasing the area of equivalent steel bars will increase its yield load but will also reduce the seismic energy dissipation capacity and ductility. The thicker the shear wall thickness, the higher specimen’s bearing capacity, the stronger the seismic energy dissipation capacity, and the worse the ductility is.

Peer Irrigators and the Choice of Field Management and Water Control Practices for Irrigation in Arkansas

We examined how irrigation techniques in use by family and friends influence the use and share of land utilizing different irrigation techniques by Arkansas producers. A bivariate sample selection model simultaneously estimated how farm characteristics determine the use and explain the share of a farm that utilizes an irrigation technique. We found that the irrigation techniques in use by family and friends do affect the irrigation techniques a producer uses and the share of acres utilizing different irrigation techniques. A producer with a family or friend that uses end-blocking irrigation is 41% more likely to use end-blocking themselves. Having a family or friend who uses pivot irrigation technology tends to decrease the share of irrigated acres that utilizes end block irrigation by 0.211. We also found that when the irrigation techniques in use by family and friends interact with variables such as location and participation in a regional conservation partnership program, the effects on the producer’s decision vary. The share of irrigated acres that use cutback irrigation decreases by 0.21 for a producer who has a peer that uses irrigation scheduling. However, if the producer lives along Crowley’s Ridge and has a peer that uses irrigation scheduling, the share of irrigated acres that use cutback irrigation decreases by an additional 0.54.

Thermoreversible poly(trimethylene carbonate)-based block copolymers containing reactive furfuryl groups via sequential ROP and ATRP

Block copolymers containing poly(trimethylene carbonate) (PTMC) middle block and poly(furfuryl methacrylate) PFMA end blocks were synthesized through sequential organocatalytic ring opening polymerization (ROP) of trimethylene carbonate and thermal and photoinitiated metal-free atom transfer radical polymerizations (ATRP) of furfuryl methacrylate. The thermal and mechanical properties of the triblock copolymers were studied by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and dynamical mechanical analysis (DMA). The crosslinking via Diels-Alder (DA) reaction between the furan groups of the block copolymers and bismaleimide and bisitaconimide was proved by FTIR and DSC and the thermoreversibility properties of the crosslinked polymer were subsequently investigated using DSC and solubility testing.

Thermoresponsive Self-Assembly of Twofold Fluorescently Labeled Block Copolymers in Aqueous Solution and Microemulsions.

A nonionic double hydrophilic block copolymer with a long permanently hydrophilic and a small thermoresponsive block is synthesized by reversible addition-fragmentation chain-transfer polymerization (RAFT). By employing a specifically designed chain-transfer agent, the polymer is functionalized with complementary end groups which are suited for Förster resonance energy transfer (FRET). The end group attached to the permanently hydrophilic block of poly(N,N-dimethylacrylamide) pDMAm is designed as a permanently hydrophobic segment ("sticker") comprising a long alkyl chain and the 4-aminonaphthalimide fluorophore. The other end attached to the thermoresponsive block of poly(N-isopropylacrylamide) pNiPAm incorporates a coumarin fluorophore. The temperature-dependent self-assembly of the twofold fluorescently labeled copolymer is studied in pure aqueous solution as well as in an o/w microemulsion by several techniques including turbidimetry, dynamic light scattering (DLS), and fluorescence spectroscopy. It is compared to the behaviors of the analogous twofold-labeled pDMAm and pNiPAm homopolymer references. The findings indicate that the block copolymer behaves as a polymeric surfactant at low temperatures, with one relatively small hydrophobic end block and an extended hydrophilic chain forming "hairy micelles". At elevated temperatures above the LCST phase transition of the pNiPAm block, however, the copolymer behaves as an associative telechelic polymer with two nonsymmetrical hydrophobic end blocks, which do not mix. Thus, instead of a network of bridged "flower micelles", large dynamic aggregates are formed. These are connected alternatingly by the original micellar cores as well as by clusters of the collapsed pNiPAm blocks. This type of structure is even more favored in the o/w microemulsion than in pure aqueous solution, as the microemulsion droplets constitute an attractive anchoring point for the hydrophobic dodecyl sticker but not for the collapsed pNiPAm chains.

Synthesis of High Etch Contrast Poly(3-hydroxystyrene)-Based Triblock Copolymers and Self-Assembly of Sub-5 nm Features

We demonstrate the successful synthesis of a series of poly(3-hydroxystyrene)-block-poly(dimethylsiloxane)-block-poly(3-hydroxystyrene) (P3HS-b-PDMS-b-P3HS) triblock copolymers by atom transfer radical polymerization (ATRP). This system is a promising candidate for pattern transfer of single-digit nanometer features, due to its intrinsic high etch contrast, etch versatility, and the triblock architecture. Hydroxy-terminated PDMS polymers were directly functionalized to initiate the ATRP of an acetal-protected 3-hydroxystyrene monomer. The resulting triblocks have dispersity ranging from 1.10 to 1.26, and the synthesis provides robust control over molecular weights and volume fractions. The large chemical incompatibility between the blocks enabled the formation of ordered structures at low molecular weights, which yielded lamellar periodicities as small as 9.3 nm. This triblock hence permits access to sub-5 nm feature sizes. The phase diagram is asymmetric, with lamellar morphologies observed at lower volume fractions of the P3HS block, relative to the diblocks. This asymmetry is ascribed to the dispersity of the middle block relative to the end blocks.

Criteria Governing Rod Formation and Growth in Nonionic Polymer Micelles.

Self-assembled wormlike micelles (WLMs) are widely studied in small-molecule surfactants due to their unique ability to break and recombine; however, less is known about the structure and dynamics of nonionic polymer WLMs. Here, solutions of seven triblock poloxamers, composed of poly(propylene oxide) (PPO) midblocks and poly(ethylene oxide) (PEO) end blocks, are comprehensively examined to determine the role of poloxamer composition, temperature, and inorganic salt type and concentration on rod formation and subsequent elongation into WLMs. Phase separation and sphere-to-rod transition temperatures were quantified via cloud point measurements and shear rheology, respectively, and corroborated with small-angle neutron scattering (SANS). The local microstructure of resulting rodlike micelles is remarkably similar across poloxamer type and sodium fluoride (NaF) or sodium chloride (NaCl) content. Salt addition reduces transition temperatures, with the most pronounced effects for poloxamers with high PEO molecular weights and PEO fractions. Between these two temperatures, several poloxamers elongate into WLMs, where shear rheology detects increases in viscosity up to 6 orders of magnitude. Despite similar local microstructures, poloxamer identity and salt content impact micelle growth substantially, where large poloxamers with lower PEO fractions exhibit the highest viscosities and longest relaxation times. While sodium fluoride has little impact on micelle growth, increasing NaCl concentration dramatically increases the WLM viscosity and relaxation time. This result is explained by different interactions of each salt with the micelle: whereas NaF interacts primarily with PEO chains, NaCl may also partition to the PPO/PEO interface in low levels, increasing micelle surface tension, scission energy, and contour length.

Extraction of intrinsic effects of glassy domain cross-linking on the tensile properties of ABA block copolymer elastomers via photo cross-linking approach

Chemical or physical cross-links into the A glassy domains have been reported to enhance mechanical properties for glassy-b-soft-b-glassy ABA triblock copolymer-based elastomers. However, no reports dealt with the pure effects of the domain cross-linking, since the cross-linking processes inevitably altered the morphological characteristics. This study extracts the intrinsic effects of domain cross-linking on the tensile properties based on the ABA triblock copolymer with the A blocks cross-linked via photo cross-linking without heat and cross-linker molecules. Differential scanning calorimetry, viscoelasticity measurements, and small-angle X-ray scattering (SAXS) with a model function analyses were used to confirm preservation of the segmental dynamics of the B block and the morphology of the self-assembled structure after cross-linking, which are all important for a fair discussion. Tensile properties were then investigated by simple elongation and cyclic tests, revealing that the domain cross-linking enhanced the stress in the small deformation region and the recovery property primarily. These changes of the tensile properties were discussed in terms of the deformation resistance of the micro-phase separated structure based on the SAXS data under elongation. The superiority or inferiority of cross-linking the end block domains or middle block strands for mechanical property enhancement was interpreted based on the observed difference between the neat and cross-linked samples. In the last section, effects of domain photo cross-linking for the different morphologies were also examined. Throughout the article, this study can provide new and essential knowledge of the mechanical properties of block copolymer-based materials, and some valuable hints for further modification of the physical properties of these materials.

Seismic Resistance Properties of Improved Dry-Type Beam-Column Joint: An Experimental Research

Dry-type joints are an advanced type of sustainable beam-column connection mode used in the prefabricated concrete frame structural system. This paper proposed an improvement scheme for high-strength bolt dry-type joints and designed a new type of common bolt dry-type joints. A pseudo test involving low-cycle repeated loading is conducted to assess the seismic resistance properties of new joints including damage mode, hysteretic curve, skeleton curve, and ductility factor. Numerical simulation is applied to validate the rationality of experimental results. It is found that when the bending capacity of the end block of the beam is consistent with that of the bolt, the deformation of the bolt will no longer increase greatly after a period of large deformation; at this period, the bolt does not fully enter the plastic stage, but at this time, the end block of the beam begins to appear large cracks and enter the plastic deformation and has good energy dissipation performance.

One-Pot Synthesis of Supertough, Sustainable Polyester Thermoplastic Elastomers Using Block-Like, Gradient Copolymer as Soft Midblock

It remains challenging to synthesize supertough thermoplastic elastomers (TPEs) since the stretchability and tensile strength are mutually exclusive. Here, we report a one-pot strategy for the prep...

Transition molecular structures between block copolymers and hyperbranched copolymers suitable for toughening epoxy thermosets

Various toughening agents had been explored to overcome the intrinsic brittleness of epoxy thermosets. While they effectively toughened epoxy thermosets, however, usually produced plasticizing effect, leading to compromised thermal stability, modulus and glass transition temperature (Tg). Herein dumbbell-shaped SEBS-g-PEG copolymers, owning transitional molecular structures between block copolymers and hyperbranched copolymers, were synthesized via grafting methoxypolyethylene glycol (PEG) onto both end blocks of polystyrene-b-poly(ethylene-ran-butylene)-b-polystyrene (SEBS). They not only had good compatibility with epoxy thermosets without plasticizing effect, but also could form micro-phase separation morphology suitable for toughening epoxy thermosets via combining the advantages of block copolymers and hyperbranched copolymers. At higher branching density, their 10% load could increase ~230% KIC and impact strength without sacrificing thermal stability, Tg, flexural strength and modulus. The morphology results showed that their soft nanostructures’ size increased and the phase interfaces became blurred as increasing the branching density in SEBS-g-PEG. Thus, the toughening mechanism was attributed to the strengthened bonding between the interphases and the enlarged size of soft nanostructures.