Initial Transition Research Articles

Development of a Strain-Based Model to Predict Eviscerated Thoracic Response From Dynamic Individual Rib Tests.

The objective of this study was to develop an analytical model using strain-force relationships from individual rib and eviscerated thorax impacts to predict bony thoracic response. Experimental eviscerated thorax forces were assumed to have two distinct responses: an initial inertial response and subsequently, the main response. A second-order mass-spring-damper model was used to characterize the initial inertial response of eviscerated thorax force using impactor kinematics. For the main response, equivalent strains in rib levels 4-7 were mapped at each time point and a strain-based summed force model was constructed using individual rib tests and the same ribs in the eviscerated thorax test. A piecewise approach was developed to join the two components of the curve and solve for mass, damping, stiffness parameters in the initial response, transition point, and scale factor of the strain-based summed force model. The final piecewise model was compared to the overall experimental eviscerated thorax forces for each postmortem human subjects (PMHS) (n = 5) and resulted in R2 values of 0.87-0.96. A bootstrapping approach was utilized to validate the model. Final model predictions for the validation subjects were compared with the corridors constructed for the eviscerated thorax tests. Biofidelity ranking system score (BRSS) values were approximately 0.71 indicating that this approach can predict eviscerated responses within one standard deviation from the mean response. This model can be expanded to other tissue states by quantifying soft tissue and visceral contributions, therefore successfully establishing a link between individual rib tests and whole thoracic response.

Does Social Class Matter Equally for the Timely Transition Into and Out of College? Evidence from the NLSY97

Although the positive relationship between social determinants and college attainment is well established, less is known about how social class specifically relates to the linear and timely completion of postsecondary degrees. In this paper, we empirically examine on-time completion of bachelor’s degrees using social class proxies for a national sample of U.S. high school graduates, using the life course perspective and social selection hypothesis to contextualize social effects on the two key transitions—timely full-time enrollment and timely degree completion—that bound the traditional 4-year college pathway. We find strongly positive associations between several social indicators and attainment of both transition events, although effects are larger and more numerous for the initial transition, indicating social selection may be more influential in launching the 4-year college pathway than in completing it. Gradients of social advantage also appear more complexly gendered and racialized at the start of the college pathway than at the end. Finally, we confirm that parenthood is highly incompatible with a 4-year path to a degree regardless of social class and conspicuously more likely to interfere with the timely completion of a bachelor’s degree than other major life transitions.

Comparative Acceptability of Weightbearing Physical Activity in Sporting Footwear Versus Barefoot in Habitually Shod Individuals.

Perceived acceptability of barefoot use has largely been ignored in the literature despite its importance to long-term implementation and behavior change. This study aimed to compare the acceptability of undertaking weightbearing physical activities in regular running shoes versus barefoot in habitually shod individuals. Healthy young men and women were recruited from the Gold Coast. Participants completed six activities (ie, lunges, walking, jogging, sidestep, vertical jump, and hop) in shod and barefoot conditions then answered questions pertaining to level and source of discomfort, ease of performance, and acceptability. Indices of bone quality were measured from their dominant calcaneus by quantitative ultrasound. Seventeen healthy male (n = 8) and female (n = 9) university students participated in the study (age, 26.59 ± 7.26 years; body mass index, 23.08 ± 3.58 kg/m2). Men were taller, heavier, and had higher broadband ultrasound attenuation than women (P < .05). For "no" discomfort, "very easy" ease of performance, and a "good amount" or "very good amount" of acceptability, the shod condition demonstrated response rates of 87.25%, 55.88%, and 72.55%, respectively. The barefoot condition demonstrated rates of those responses of 62.75%, 39.22%, and 48.03%, respectively, and reported more ball-of-foot, forefoot, heel, and plantar skin locations as sources of discomfort during activity than in the shod condition. The group vertical jump height was higher barefoot than shod (44.88 ± 8.44 cm and 43.25 ± 8.76 cm, respectively; P < .05), but no difference was seen for the hop. Men jumped and hopped higher than women under both footwear conditions (P < .05). Participants initiating barefoot weightbearing exercise may experience slightly greater discomfort and less ease of performance in the initial transition from the shod condition, but may perform better in vertical jump. Whether those differences in experience persist over the long term will require longitudinal studies.

Cross-sectional Study of College Students’ Depression, Coping Techniques, and Health Risk Behaviors during the Initial Transition to Remote Learning: COVID-19 Pandemic

Objective: COVID-19 has brought many challenges to college campuses. In this cross-sectional study, we assessed depression, coping, and past 30-day health risk behaviors of college students attending a mid-sized liberal arts university in Florida. Methods: We sent a survey link to all undergraduates. The data, which included depression scores (PHQ-9), coping techniques (Brief COPE), health risk behaviors (alcohol and vape use) and demographics were collected approximately 40 days after the university transitioned from face-to-face to online learning during the COVID-19 pandemic. We used descriptive and regression analyses using SPSS to analyze data. Results: A total 45.6% of the respondents (N = 759) self-reported moderate to severe depression (PHQ-9 mean= 9.96, SD = 6.2). Students indicated positive and negative coping strategies. Alcohol and vape use increased with higher depressive symptoms scores (p &lt; 0.01). Seniors had a greater odds of alcohol consumption (p &lt; .01); however, freshmen and juniors had greater odds of vape use. Our data revealed some opportunities for positive behavior change. Conclusion: The online transition did modify students’ behaviors. Students who self-reported depressive symptoms were most vulnerable to adopting greater risk behaviors. These students require ongoing monitoring and resources. Colleges should implement rapid assessment of students’ depressive status and risk behaviors during volatile times.

Qualitative Research with Former International Students: Reflections on Conceptualization, Planning and Relational Engagement

The number of international students seeking a foreign education, particularly in Westernized countries, has grown dramatically over the past decade, and is predicted to continue to increase, despite a period of disruption due to COVID-19. Given this growth, there is a significant body of research on key insights into the initial transition experiences, both academic and personal, of international students to the host country, with a developing body of research exploring their post-study transition. Understanding these post-study transitions is important in creating policy and services that appropriately support international students. Due to the diverse and sometimes complex post-study pathways of former international students, accessing this population to conduct qualitative research can create challenges for researchers. To help address these challenges, the authors highlight three critical considerations based on their qualitative research experiences in Westernized countries with former international students, including conceptual understandings, logistical planning, and relational engagement. Moreover, the authors share examples of pragmatic solutions related to challenges with conceptual understandings, logistical planning, and relational engagement in qualitative research with former international students. The purpose of this article is to start and invite discussion around how best to reach, access, and work with former international students to expand qualitative research on the post-study experience.

Metastatic Transition of Pancreatic Ductal Cell Adenocarcinoma Is Accompanied by the Emergence of Pro-Invasive Cancer-Associated Fibroblasts.

Simple SummaryHow stromal cells control tumor progression is an active area of investigation. Cancer associated fibroblasts (CAFs) have been suggested to both limit, or promote cancer dissemination in various contexts, but a systemic understanding of stromal influence in determining cancer metastasis is lacking. This study takes a closer look at the diversity of fibroblasts that emerge in different metastatic stages of PDAC (Pancreatic Ductal Cell Adenocarcinoma), a cancer type with high stromal infiltration. Specifically, we found that the CAFs respond to the growing (but yet not metastasized) tumor by transition to a myofibroblast like stage, potentially resisting cancer invasion. As the cancer disseminates, these fibroblasts are nearly entirely replaced by highly diverse set of subpopulations with distinct functions. We have previously advanced a framework, termed Evolved Levels of Invasability (ELI), explaining the vast differences in cancer metastasis and placental invasion among mammals. We found that PDAC’s metastatic transition and stromal trespass is accompanied by the downregulation of pro-resistive and the upregulation of pro-invasability ELI signatures in CAF subpopulations. Cancer-associated fibroblasts (CAFs) are now appreciated as key regulators of cancer metastasis, particularly in cancers with high stromal content, e.g., pancreatic ductal cell carcinoma (PDAC). However, it is not yet well understood if fibroblasts are always primed to be cooperative in PDAC transition to metastasis, if they undergo transformation which ensures their cooperativity, and if such transformations are cancer-driven or intrinsic to fibroblasts. We performed a fibroblast-centric analysis of PDAC cancer, as it transitioned from the primary site to trespass stromal compartment reaching the lymph node using published single-cell RNA sequencing data by Peng et al. We have characterized the change in fibroblast response to cancer from a normal wound healing response in the initial stages to the emergence of subclasses with myofibroblast and inflammatory fibroblasts such as signatures. We have previously posited “Evolved Levels of Invasibility (ELI)”, a framework describing the evolution of stromal invasability as a selected phenotype, which explains the large and correlated reduction in stromal invasion by placental trophoblasts and cancer cells in certain mammals. Within PDAC samples, we found large changes in fibroblast subclasses at succeeding stages of PDAC progression, with the emergence of specific subclasses when cancer trespasses stroma to metastasize to proximal lymph nodes (stage IIA to IIB). Surprisingly, we found that the initial metastatic transition is accompanied by downregulation of ELI-predicted pro-resistive genes, and the emergence of a subclass of fibroblasts with ELI-predicted increased invasibility. Interestingly, this trend was also observed in stellate cells. Using a larger cohort of bulk RNAseq data from The Cancer Genome Atlas for PDAC cancers, we confirmed that genes describing this emergent fibroblast subclass are also correlated with lymph node metastasis of cancer cells. Experimental testing of selected genes characterizing pro-resistive and pro-invasive fibroblast clusters confirmed their contribution in regulating stromal invasability as a phenotype. Our data confirm that the complexity of stromal response to cancer is really a function of stage-wise emergence of distinct fibroblast clusters, characterized by distinct gene sets which confer initially a predominantly pro-resistive and then a pro-invasive property to the stroma. Stromal response therefore transitions from being tumor-limiting to a pro-metastatic state, facilitating stromal trespass and the onset of metastasis.

An Insight into Students’ Transition to Tertiary or Academic Pathways Programs

ABSTRACT There is a lot of research on pathways students’ transition to universities, however, little is known about those students’ initial transition to the Academic Pathway Program (APP). This study investigated students’ transition to an Academic Pathway Program (APP) offered at a College affiliated to a metropolitan university in Sydney, Australia. Data collected from a sample of students using the Transition, Wellbeing, Help-seeking, and Adjustments Survey (TWHAS) were analysed both descriptively and inferentially. Most students reported positive moods, emotional coping, and were able to seek help without many barriers. Also, most students reported adjusting positively to the College, despite facing a few personal and social challenges such as finding time to exercise, balance study with family commitments and socialise with friends outside the College. Further, a possible association between students’ adjustments and their initial feelings when beginning the College, proficiency in written and spoken English, and help-seeking was noted. Additionally, differences in the students’ transitional challenges, wellbeing, help-seeking behaviors, adjustments across gender, age, and type of enrollment were evident. The implications of these findings for supporting students’ transition to the Academic Pathway Programs and recommendations for future research addressing the limited sample size are discussed.

Using the TUG Test for the Functional Assessment of Patients with Selected Disorders.

One of the tests used for quantitative diagnostics is Timed Up-and-Go (TUG), however, no reports were found regarding the percentage share of individual test components, which seems to have a greater diagnostic value in differentiating the functional status of the patients. The aim of the study was to analyze the percentage of the individual components of the TUG test in functional assessment in a population of healthy children and in clinical trials patients with various diseases. Material and Methodology. The material consisted of patients with orthopedic (n = 165), metabolic (n = 116) and neurological dysfunctions (n = 96). Results. The components of the TUG test that differentiated the studied groups of patients to the greatest extent were in the order: relapse tug3%, initial transition tug2%, sitting tug5% and standing up tug1%, while during the final transition tug4% statistically significant differences were found only between healthy children and the studied groups of patients. Conclusions. The TUG test turned out to be a good diagnostic tool, differentiating the studied groups of patients. The analysis of the percentage of the components of the TUG test can help in assessing the mobility of children and adolescents, monitor the effects of physiotherapy or the effects of surgical procedures.

The effect of kenaf loading on kenaf/ABS composites structure and thermal properties

Many manufacturers have recently become interested in using fiber-reinforced polymer composites (FRPs) in structural applications. Synthetic fibres, such as carbon and glass fibres, have been commercialised internationally for decades, but they cause environmental issues because synthetic fibres are non-biodegradable and difficult to recycle once they have served their purpose, potentially polluting the environment. Thus, natural fibre composites like kenaf is a possible replacement for synthetic fibre due to their superior physical and mechanical properties. Kenaf appears to be the best candidate for replacing synthetic fibres in order to accomplish the goal of environmental preservation while also displaying excellent properties such as equivalent specific strength, low density, and renewable resources. The kenaf fiber was treated in KOH and added to ABS matrix to produce new composites at different loading (10, 15, 20 and 25 wt.%) by using Two Roll Mill machine. The influence of the fiber on the composites properties was evaluated. The produced material was subjected to SEM, MFI, TGA and DSC analysis. The incorporation of the treated kenaf fiber has an influence on the properties of kenaf/ABS composites. The addition of 10 wt.% kenaf was found to be the best loading with MFI value, initial degradation temperature and glass transition temperature at 0.8208 g/10 min, 322.63°C and 130°C respectively. The fiber was well dispersed in the matrix and shown good adhesion to the ABS. The addition of treated fiber contribute to a reduction in the MFI, improved the thermal stability of the composites and typical effects of Tg of the composite compare to pure ABS. The results suggest the need to continue the study in order to further analyse higher kenaf loading and shed more light on the properties of the composites to improve understanding of kenaf/ABS composites. Obtained results are a solution to alternative of synthetic fibers, which may contribute to the sustainable development of composites materials industry through the utilization of kenaf fiber with ABS matrix.

Intelligent recognition of radar emitters with agile waveform based on deep reinforcement learning

AbstractIn the traditional radar emitter recognition task, the number of emitter category tags is fixed and relatively small. We input the pulse characteristic parameters into the machine learning model to model the radar emitter. The radar emitters with agile waveform involves four different waveform categories. The number of specific emitter categories contained in different waveform categories is extremely unbalanced. Their parameters vary widely, so only using the same model or strategy to model each waveform category cannot effectively extract the core features of the radar emitters with agile waveform. The above is the main reason for the low recognition accuracy of the recognition task of radar emitters with agile waveform. In this paper, a deep reinforcement learning framework is designed to be used in the intelligent recognition task of radar emitters with agile waveform. The task is regarded as a two‐step decision game. We use CNN and Bi‐LSTM to model the radar emitter, and calculate the initial state and transition state of the game. At the same time, we design the penalty function in reinforcement learning and increase the penalty for the wrong decision in the first step to deal with the imbalance of the number of emitters between different waveform categories. Finally, the Q‐Learning algorithm with approximate values is used to learn the control strategy of the game, that is, the modeling and recognition strategy adopted for emitters of different waveform categories. The simulation experiment results show that the deep reinforcement learning framework constructed in the article can improve the recognition accuracy of 1.2% compared with the state‐of‐the‐art method in the intelligent recognition task of radar emitters with agile waveform.

The effect of periodicity in the elastic turbulence regime

In this work, we show that the double-periodic boundary conditions typically applied in numerical simulations of elastic turbulence can lead to significantly incorrect results if not treated properly. This is demonstrated by simulating elastic turbulence using the popular four-roll mill benchmark at different levels of periodicity, namely, 16, 36 and 64 rolls using the popular Oldroyd-B model with added artificial diffusivity. We find that the initial onset of elastic turbulence causes a breakdown in symmetry independent of periodicity, which is characterised by a leading vortex and is known to be attributed to artificial diffusivity. Beyond this initial transition, the standard four-roll mill case transitions into a periodic state, a well-known characteristic from the literature. On the other hand, the cases with higher levels of periodicity quickly overcome the effects of a leading vortex and experience purely chaotic flow fluctuations, characterised by a broadband spectrum and steep power law behaviour. Certain qualities of the flow at higher levels of periodicity are reminiscent of the true solutions of elastic turbulence obtained numerically without any artificial diffusivity (Gupta &amp; Vincenzi, J. Fluid Mech., vol. 870, 2019). These results suggest that the well-known periodic states observed for the four-roll mill are due to insufficient periodicity as the problem transitions into the elastic turbulence regime, leading to a dominant vortex cycling around all four quadrants of the unit cell throughout time unable to recover the initial symmetry. This work demonstrates the importance and caution required when applying periodic boundary conditions in numerical experiments of the elastic turbulence regime and further emphasises the impact and care required for artificial diffusivity.

Errors in using fixed flexion facet centers to determine tibiofemoral kinematics increase fourfold for multi-radius femoral component designs with early versus late decreases in the radius of curvature

BackgroundOne method to determine tibiofemoral joint kinematics following total knee arthroplasty (TKA) is to quantify movement of the anterior-posterior (AP) position of the flexion facet center (FFC) on each femoral condyle relative to the tibia during knee flexion. The primary objective was to determine how closely AP positions of fixed FFCs approximate AP positions of variable FFCs of multi-radius femoral component designs with early versus late initial transition angles (i.e. earliest flexion angle where the radius of curvature decreases markedly). MethodsVariable FFCs were determined for each femoral condyle as centers of best-fit circles to 20° segments of the sagittal profile from 0° to 120° of flexion in 15° increments. The fixed FFC of each condyle was the center of the best-fit circle from 0° to 120° of flexion. Errors in AP positions were differences between AP positions of fixed FFCs and variable FFCs. ResultsFor profiles with a late initial transition angle of 120° of flexion, the root mean square error (RMSE) was limited to 0.7 mm. For profiles with an early initial transition angle of 60° of flexion, the RMSE was 2.7 mm, nearly a fourfold increase. ConclusionsTo determine whether fixed FFCs can be used to indicate AP positions of femoral condyles with minimal RMSE < 1 mm, the initial transition angle should be found as an important first step. Condylar AP positions for designs with an early initial transition angle should not be approximated by AP positions of fixed FFCs when determining tibiofemoral kinematics.

Carbon Emission Efficiency Network: Evolutionary Game and Sensitivity Analysis between Differentiated Efficiency Groups and Local Governments

With its proposal of the “double carbon” (peak carbon dioxide emissions and carbon neutralization) goal, China has entered a new stage in creating an ecological civilization and achieving sustainable development. Based on the formation and evolution mechanism of the carbon emission efficiency network, in this study, a trilateral evolutionary game model—including efficiency groups (high- and low-efficiency groups) and local governments—was constructed, in an attempt to discuss the conditions needed for different players and trilateral interconnected systems to implement balanced and stable strategies. Furthermore, the sensitivity of the participants’ evolutionary trajectories toward factors such as the initial strategy ratio, transition cost, and network capital were tested via a system simulation. The main conclusions were as follows: (1) Efficiency groups form a virtuous circle when the initial proportion of the participants’ strategies reaches a certain threshold, and converge into a stable “win–win” state. Under these circumstances, high-efficiency groups tend to give full play to their efficiency advantages in terms of carbon emission reduction and green development, while low-efficiency groups tend to choose green transformation and accept the spillover effect from high-efficiency groups. (2) When efficiency groups achieve a “win–win” state or form good self-management, local governments move from active supervision to a passive supervision strategy in order to reduce supervision costs. (3) While different initial strategy proportions do not affect the stable convergence point of the evolutionary system, they have a differentiated impact on the convergence speed of the players. Under the condition of a low initial strategy ratio, transformation costs can reduce the green transformation enthusiasm of inefficient groups, while network capital can enhance the green transformation willingness of inefficient groups.

Empirical research methods department: Mission, learnings, and future plans

Empirical research methods department: Mission, learnings, and future plans

Transition from pore fouling to cake filtration in high level waste dead end filtration

ABSTRACT Treatment of radioactive waste at the Hanford site is dependent upon dead end filtration to prepare feed for ion exchange. Recent testing has indicated that the filter media that will be deployed for this treatment process is susceptible to depth fouling. A model was developed to understand the transition from pore fouling to cake filtration to provide insight into the planned operations of this facility. These results indicate that an initial transition takes place during which the filter is depth fouled. However, after the majority of the pores have been plugged the filter flux is governed by cake filtration on the surface of the fouled filter.

A Long Journey before Cycling: Regulation of Quiescence Exit in Adult Muscle Satellite Cells.

Skeletal muscle harbors a pool of stem cells called muscle satellite cells (MuSCs) that are mainly responsible for its robust regenerative capacities. Adult satellite cells are mitotically quiescent in uninjured muscles under homeostasis, but they exit quiescence upon injury to re-enter the cell cycle to proliferate. While most of the expanded satellites cells differentiate and fuse to form new myofibers, some undergo self-renewal to replenish the stem cell pool. Specifically, quiescence exit describes the initial transition of MuSCs from quiescence to the first cell cycle, which takes much longer than the time required for subsequent cell cycles and involves drastic changes in cell size, epigenetic and transcriptomic profiles, and metabolic status. It is, therefore, an essential period indispensable for the success of muscle regeneration. Diverse mechanisms exist in MuSCs to regulate quiescence exit. In this review, we summarize key events that occur during quiescence exit in MuSCs and discuss the molecular regulation of this process with an emphasis on multiple levels of intrinsic regulatory mechanisms. A comprehensive understanding of how quiescence exit is regulated will facilitate satellite cell-based muscle regenerative therapies and advance their applications in various disease and aging conditions.

Application of ant colony and immune combined optimization algorithm in path planning of unmanned craft

The ant colony optimization (ACO) algorithm is improved and further integrated with the immune algorithm (IA) to address its problems, such as slow convergence, local optimum, and premature convergence in the path planning. An algorithm integrating IA and improved ant colony optimization (IACO) is, therefore, put forward to realize the optimal planning of global path for an unmanned surface vehicle (USV). First, the ACO algorithm was improved in three aspects, that is, generation of initial pheromones, transition probability, and update of pheromones. The proposed IA-IACO algorithm combined the advantages of IA and IACO, sped up the convergence, and enhanced the optimization capability and operational efficiency. Second, the IA-IACO algorithm was designed and applied in the global path planning of an unmanned surface vehicle, achieving great global optimization and convergence. Finally, a path smoothing algorithm was devised to achieve the implementable, economic, and stable path while guaranteeing the safe navigation of the USV. A simulation test was carried out to prove the effectiveness and superiority of the designed global path planning algorithm in the practical engineering.

"Drunk People Are on a Different Level": A Qualitative Study of Reflections From Students About Transitioning and Adapting to United Kingdom University as a Person Who Drinks Little or No Alcohol.

BackgroundThough sobriety in young people is on the rise, students who drink little or no alcohol may experience social exclusion at University, impacting well-being. We aim to understand the social experiences of United Kingdom (UK) undergraduate students who drink little or no alcohol.MethodsA mixed-methods study using semi-structured, one-to-one interviews and the 24-Item Social Provisions Scale and Flourishing Scale with 15 undergraduate students who drink little or no alcohol. Descriptive statistics are presented for quantitative data and thematic analysis for qualitative.ResultsEight main themes and four subthemes were generated from thematic analysis summarised in two sections ‘views of drinkers from non-drinkers’ and ‘how peer pressure feels and how people deal with it.’ The initial transition to University represented a challenge, where participants struggled to find their ‘true’ friends. However, students generally had high levels of social provision, well-being and enjoyed close friendships with fewer casual acquaintances. All students experienced some kind of peer pressure (of a varying extremity) and developed coping strategies when in social situations involving alcohol. Fear of missing out on the ‘typical’ University experience heightened self-imposed expectations to drink. Despite participants acknowledging their counter-normative behaviour, some felt they were subject to stigmatisation by drinkers, doubting their non-drinker status, causing feelings of exclusion or being ‘boring.’ Their desire to ‘be like everyone else’ exposed some insight into the negative stereotypes of sobriety, including frustration behind alcohol’s status elevation.ConclusionStudents adopt strategies to minimise peer pressure and to fit in. Future research should interrogate drinkers’ perceptions of their sober peers to deepen understanding, better break down ‘us and them,’ and mitigate future expectations within the University drinking culture.

Highly Oriented {010} Crystal Plane Induced by Boron in Cobalt-Free Li- and Mn-Rich Layered Oxide.

Li- and Mn-rich layered oxide (LMR) materials are a promising candidates for next-generation Li-ion battery (LIB) anode materials because of their high specific capacity. However, their low initial Coulombic efficiency, voltage decay, and irreversible phase transition during cycling are the fatal drawbacks of LMR materials. This work reports on a cobalt-free LMR material composed of primary particles with a boron-induced exposed long- strip-like {010} plane. Because of this unique structure, the long strip-like cathode exhibits excellent electrochemical performance with a discharge capacity of 202 mAh g-1 at 1 C and a retention rate of 95.2% after 200 cycles. In addition, it is found that this long strip-like structure can modulate the redox of oxygen and enhance the reversibility. The irreversible phase transition process from the layered to a spinel and then to a rock-salt phase during cycling is also significantly suppressed. This work provides a feasible method for regulating the exposed {010} plane and a new idea for the structural design of LMR materials.

Exploring the "Living" Growth of Block Copolymer Nanofibers from Surface-Confined Seeds by In Situ Solution-Phase Atomic Force Microscopy.

Living crystallization-driven self-assembly of polymeric and molecular amphiphiles is of growing interest as a seeded growth route to uniform 1D, 2D, and more complex micellar nanoparticles with controlled dimensions and a range of potential applications. Although most studies have been performed using colloidally stable seeds in bulk solution, growth of block copolymer (BCP) nanofibers from seeds confined to a surface is attracting increased attention. Herein, we have used atomic force microscopy (AFM) to undertake detailed studies of the growth of BCP nanofibers from immobilized seeds located on a Si surface. Through initial ex situ AFM studies and in situ AFM video analysis in solution, we determined that growth occurred in four stages, whereby an initial surface-bound growth regime transitions to surface-limited growth. As the nanofiber length increases, surface influence is diminished as the newly grown micelle segment is no longer bound to the Si substrate. Finally, a surface-independent regime occurs where nanofiber growth continues into bulk solution. In addition to the anticipated nanofiber elongation, our studies revealed occasional examples of AFM tip-induced core fragmentation. In these cases, the termini of the newly formed fragments were also active to further growth. Furthermore, unidirectional growth was detected in cases where the seed was oriented at a significant angle with respect to the surface, thereby restricting unimer access to one terminus.