dataesr/scientific-paragraphs-categorization huggingface dataset:
E. Comments on the Previous Work At this point, it seems imperative for us to comment on the previous work of Blanchard et al. [32] , which is the only paper in the literature that claims direct observation of chain retraction by SANS. Their report contradicts not only the current work, but also at least two other independent studies  [25, 26]. Rather than speculating what might have gone wrong in the work of Blanchard et al. , we would instead emphasize the steps we have taken to improve the execution of the experiments and data analysis. First, our stretching experiments were conducted in the Forced Convection Oven (FCO) of the RSA-G2 Solids Analyzer (TA Instruments), which is a well-tested commercial sample environment. We further verified the uniformity and stability of the temperature by monitoring the built-in upper and lower resistance temperature detectors (RTD) of the oven, as well as an additional RTD that we attached to the lower sample clamp.
The consistency of our stretching experiments on RSA-G2 is demonstrated by the recorded stress response (Fig. 4b ) -the stress rise and relaxation data of different runs essentially collapse onto the same envelope. Fourth, the width and thickness of our quenched samples were carefully measured. We verified that the dimensions of the samples were consistent with the applied macroscopic strain. It is important to note that accurate sample thickness is critical for determination of the absolute scattering intensity. The work of Blanchard et al. did not describe how the sample thickness was obtained -this is a non-trivial issue for their soft, compressible polyisoprene sample. In contrast, the thickness measurement for the high-T g polystyrene is rather straightfor-ward. Fifth, we confirmed that our quenched samples has uniform stress distribution by performing birefringence measurements.
ACKNOWLEDGMENTS This research was sponsored by the  Laboratory Directed Research and Development Program of Oak Ridge National Laboratory , managed by  UT Battelle, LLC , for the  U. S. Department of Energy. W. -R. Chen  acknowledges the support by the  U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division. J. Liu  and  Z. Zhao  thank the financial support by the  NSF   Polymer Program  ( DMR-1105135 ). The polymer synthesis and characterization were carried out at the  Center for Nanophase Materials Sciences , which is a  DOE Office of Science User Facility. We acknowledge the support of the  National Institute of Standards and Technology, U. S. Department of Commerce , for providing the neutron research facilities used in this work.
In particular, very high efficiencies, low emissions have been demonstrated by the use of low cetane fuels at intermediate compression ratios between those typical of CI and SI engines  [19, 20]. Further, dual fuel concepts such as reactivity controlled compression ignition employ a combination of low and high cetane fuels, delivered through separately timed injection events, to achieve similar fuel economy and emissions performance. Numerous studies have recently explored the very wide operating space including fuel, engine parameters and operating conditions  [21, 22]. The need to understand and control autoignition has been strongly motivated by a growing interest in HCCI and LTC concepts in recent years. Additionally, parallel development in highly boosted, downsized direct injected, spark-ignited gasoline engines has highlighted some limitations of the current autoignition database.
Autoignition chemistry A significant database for autoignition chemistry of various fuels at engine-relevant conditions has been compiled ever since IC engines have existed, where early interest targeted gasoline engine knock, and to some extent diesel fuel autoignition. Recent efforts have also targeted jet fuels. Chemical kinetic models are typically formulated based on data from a variety of sources, including rapid compression machines (RCMs)  [39] , motored or skip-fired engines  [40] , flow/jet reactors  [41, 42] , and shock tubes  [43]. Typical ranges of operation for these apparatuses are summarized in Fig. 1  and Table  1 ; though some facilities operate outside of these ranges. Figure  1  is presented to highlight the thermodynamic conditions of these devices, but the reactor, as well as chemical time scale of the mixture, which is discussed in Section 1.
Physical-chemical interactions A significant database for physical-chemical interactions covering a range of engine-relevant conditions has been developed through the use of steady flow / constant pressure devices like burners and  [49, 50]. These configurations have the ability to generate statistically stationary conditions (more so than motored engines) and are able to provide wide diagnostic access for the implementation of opticallybased configurations, as well as physical sampling based ones. These steady flow devices are able to cover a wide range of flow rates and power levels, but can be limited in, for instance pressure ratings. Achieving energy densities that replicate reciprocating and gas turbine engines is also challenging. Single-shot devices like combustion bombs  [51, 52]  and spray chambers  [53]  have also been used to achieve IC enginerelevant conditions, but the diagnostic access is reduced, while shot-to-shot variability complicates analysis of the data, and comparison with models.
Some datasets relevant to physical-chemical interactions have also been acquired via controlled-turbulence RCMs, and these unique contributions are discussed here, with suggestions for possible future work in this area. Computational databases for physical-chemical interaction studies are also growing covering the finest scales through direct numerical simulation  [54] [55] [56] [57] [58] , as well as larger scales via large eddy simulation  [59, 60]. Extremeley beneficial insight has been drawn, but the applicable Reynolds and Mach numbers, and other regimes, e. g. , Damköhler, can be limited.
The chemical kinetic processes, and observed behavior depend on the temporal evolution of temperature and pressure throughout the experimental tests, and thus can complicate interpretations of the measurements, as well as comparisons between different RCM, shock tube, and reciprocating engine datasets where different operating characteristics affect the state preparation and induction period. For example, ignition delay time measurements in the NTC region can vary by ±25 % or more for hydrocarbons displaying this behavior, and this can be seen in Fig. 2  [62]. The data presented here include results from several RCM facilities in the 2 nd International RCM Workshop, and highlight the need for rigorous specification of the initial and boundary conditions for each test, e. g. , heat loss characteristics.
Piston crevice containment The development of creviced piston configurations has been a significant advancement in the design of RCMs towards studying autoignition chemistry and fuel reactivity, and has substantially improved the quality of data recorded and reduced uncertainties in datasets from these machines. Improved piston designs have enabled better descriptions of the gas mixture during and post compression via the adiabatic core model which is presented and discussed in Section 4. For many reactive systems this approximation is sufficiently accurate, as long as the thermal boundary layer represents only a small portion of the reaction chamber volume, isolated at the walls of the chamber. There are many ways to conduct chemical kinetic simulations using this hypothesis which are reviewed in Section 4.
Desgroux et al. [144, 145]  were the first to use Rayleigh scattering measurements for temperature measurements. The repetition rate of their laser (10 Hz, a pulsed frequency doubled Nd:YAG laser providing emission at 532 nm) limited the measurements to discrete intervals after the end of compression in their RCM (nominally one data point each, taken 100 ms after the end of compression), and high interference from reflections limited measurements near the wall  [144, 145]. The authors quoted a temperature accuracy of 3-4%, or ±30 K, at the experimental conditions studied. The path-averaged measurements provided some corroboration with the thermocouple data, though questions arose regarding discrepancies between the datasets, including differences in fidelity and tradeoffs between the techniques.
exp 1 K Q dT T dp dt p V dt g g é ù - = + ê ú ê ú ë û (2) Here, the total number of moles in the reactor is approximated as constant throughout the test. Park and Keck  [270]  suggested using an isochoric assumption, facilitated by setting dp/dt| exp = 0, beyond the point of noticeable exothermicity in the test, defined as dp/dt| exp > 0. The adiabatic core approach has been the basis of most HRMs utilized over the past two decades, and is also frequently employed to estimate the thermodynamic state conditions for experimental tests, i. e. , T c , especially for comparison against other datasets, through Arrhenius diagrams. This is achieved by rearranging Eq. (  2 ) with the heat release term set to zero and integrating from the initial, pre-compressed conditions at t = 0, yielding ln 1 c i T c T i p dT T p g g = - ò (3) where the subscripts i and c denote the initial and compressed conditions, respectively.
Potential exists for their use in various RCM studies, including autoignition chemistry investigations to address and reduce uncertainties under NTC conditions and long ignition delay times where the measurements can be significantly affected by boundary layer gradients  [267,283][140]. Furthermore, this approach may be beneficial for tests employing advanced diagnostics (e. g. , line-of-sight absorption spectroscopy), where boundary layer gradients have the potential to influence the measurements so that undertaking representative simulations could be important for interpreting these datasets. Stochastic reactor models (SRMs) have recently been utilized to simulate conditions where turbulence effects are influential. Section 2 discussed ways that turbulence can be generated in RCMs for TCI studies.
Mild ignition in RCMs, shock tubes and flow reactors has been observed for many years, and is associated with convolved deflagrative/autoignitive processes. It is identified as undesirable in studies of autoignition chemistry since it complicates the interpretation of datasets, and thus the evaluation and validation of chemical kinetic models  [287, 288]. The fundamental causes of mild ignition and its mitigation are of continued interest. Finally, knock can lead to unwanted engine noise, and result in structural damage and engine failure if not properly eliminated via operating conditions, fuel selection and/or combustion chamber design  [289]. In this section, highlights of RCM studies of chemical-physical interactions are presented, starting with autoignition under stratified thermal and/or compositional conditions, followed by results of work focused on turbulence-chemistry interactions.
For most of their dataset they identified deflagrative phenomena, where these transitioned to volumetric ignition and detonation under some conditions. Shortened ignition times relative to purely chemical kinetic processes were observed. Some times were estimated to be a factor of 10 faster due to the non-uniform, multi-dimensional deflagration processes. These phenomena can challenge the interpretation of data from a chemical kinetic perspective  [305]. Fieweger et al. [304]  argued however, that for tests that transition to detonation, the only interactions between the gas at the endwall and the deflagrating front were due to pressure couplings, so by assuming adiabatic compression of the end gas during the induction period to account for exothermicity in other portions of the reacting mixture, shifts or advancements in ignition timing could be taken into account.
Formulating comparisons in this manner however, as discussed earlier, requires (at a minimum) access to pressure-time histories for such datasets, e. g. ,  [306] , and while it may improve comparisons with homogeneous reactor model calculations, it introduces additional experimental and modeling uncertainties. As such, a constrained reaction volume concept for shock tubes has been developed  [307]  in an attempt to eliminate mild ignition phenomena, whereby fuel is loaded only near the endwall of the driven section to minimize exothermic effects. These shock tube investigations identified complex physical and chemical dynamics that influence mild ignition phenomena, where sensitivities to inhomogeneities in the mixture, impurities, wall properties, and the generation of local hot spots were highlighted.
Again, theoretical analyses based on Zeldovich  [248] , Gu et al. [249], and Sankaran et al. [250]  were used to explain the transition to mild ignition at low temperatures and high pressures. The thermal gradients required to facilitate mild ignition were similarly estimated to be near dT/dx = 5-10 K/mm, and this gave good agreement between the three datasets. Grogan et al. [257]  and Im et al. [319]  have proposed similar regime diagrams based on Damköhler and Reynolds number scaling that can be used as guides when conducting tests under conditions where mild ignition is possible. Figure  35  from Im et al. [319]  utilizes two ignition Damköhler numbers, namely integral scale Da l and mixing scale Da λ , the turbulent Reynolds number, and includes a sensitivity parameter, K, for the ignition delay which is based on the prior work by Zeldovich  [248] , Gu et al.
They found significant non-linear behavior where, for hydrogen addition to methane at hydrogen mole percentages below 20% there was only a slight decrease in reactivity but at mole percentages above 50% there was a substantial decrease in ignition delay times. Results for fuel-lean mixtures showed a slight effect of f on reactivity. All experiments were simulated using the detailed chemical kinetic model of Petersen et al. [360] , where generally good agreement was observed between the simulations and the experimental results. Donohoe et al. [353]  measured ignition delay times in both a shock tube and an RCM to determine the increase in reactivity due to the addition of hydrogen to mixtures of methane. Experiments in this dataset were performed over a very wide range of T c , p c , f and dilution.
Those studies highlighted the fact that non-ideal effects in shock tubes, including incident shock wave attenuation due to boundary layer growth, as well as other physical interactions may lead to pressure increases during the induction period, resulting in shorter ignition delays than if at a constant internal energy/specific volume condition. Moreover, in the experiments performed by Herzler et al. [376, 377]  this type of pressure rise was identified and highlighted by Petersen et al. [379] , though the cause of it could not be exactly determined. As discussed in Sections 2 and 4, RCMs generally experience a pressure decrease during the induction period due to heat loss, and this extends ignition delay times relative to a constant internal energy/specific volume condition. The different characteristic effects of heat loss in RCMs and in shock tubes can explain some discrepancies in literature data, though further experimental datasets may help to clarify these issues further.
Significantly, the discrepancies highlight the need for careful selection of the simulation framework, at the minimum including heat loss and boundary layer effects, as described in Section 5, when comparing simulation results to many experimental datasets. Recently, Dames et al. [364]  developed a binary fuel model for propane and DME under enginerelevant conditions where they studied pure propane over a range of conditions. Both t 1 and t ign were measured experimentally for these mixtures and a detailed chemical kinetic model compared favorably to the data. It was found that the reactions of H-atom abstraction from the fuel by ȮH radicals (R1) and also the reactions n-C 3 H 7 OȮ=C 3 H 6 +HȮ 2 (R3c) and ȮOCH 2 CH 2 CH 2 OOH=ketohydroperoxides+ȮH (R6a, R7) all controlled the low-temperature autoignition of propane.
One important point of their work was that, in some cases, LTHR was predicted to occur during the compression stroke. This feature, as discussed in Section 4, can complicate the interpretation of experimental datasets since the 'compression' limit is reached and it is difficult to ascertain a baseline for defining the ignition delay time. Modifications to the test gas conditions, e. g. , p c , or the machine configuration, e. g. , t 50 , can be used to overcome this, while simulations that include the full compression stroke are necessary to compare with experimental measurements. Cox et al. [77]  studied the autoignition of n-pentane in stoichiometric mixtures with air. In the lower temperature range, two-stage ignition occurred and the first-stage of ignition consumed approximately 15% of the fuel, based on physical gas sampling and subsequent GC analysis.
[411]  recently conducted a series of t ign measurements in six different shock tubes and two RCMs covering a very wide range of pressure and temperature. This study provided a basis for extensive facility cross-comparisons for experimental t ign , while a large database was generated for model validation. Experimental results were also compared to predictions of the chemical kinetic mechanism developed in this study and good agreement was observed. Recently, Zhou et al. [412]  carried out a detailed experimental and kinetic modeling investigation of the oxidation of iso-butene. This study included measurements of t ign in shock tubes and RCMs, laminar flame speeds, and speciation data measured in flow and jet-stirred reactors. Based on chemical kinetic modeling it was found that the reactivity of the alkene species at very low temperatures (< 800 K) is dominated by HȮ 2 addition reactions to the double bond of the alkene followed by addition of O 2 to the resulting radical.
Alkyl substituted benzenes Mittal and Sung  [414]  reported the ignition delay times of toluene/O 2 /N 2 /Ar. Experiments were conducted to understand the effects of f and oxygen concentration on ignition delays. It was noted that toluene was much more reactive than benzene under the same conditions. The experimental results were compared to shock tube data  [424]  to specifically analyze the ITHR. Under similar state conditions and overall stoichiometry, but with 2. 5 times greater fuel loading, i. e. , fuel mole fraction of 0. 0228 compared to 0. 00962, the shock tube pressure records indicated substantially larger ITHR compared to the RCM data. Mittal and Sung  [414]  argued that this could be due to surface contamination in the shock tube experiments leading to non-uniform, or mild ignition events, and presented evidence for this by conducting tests where the RCM reaction chamber was intentionally vitiated.
[82]  which was validated based on ignition delays and intermediate species concentrations, where the kinetic model predictions were found to agree well with the experimental values. Darcy et al. [417]  studied the autoignition of n-propylbenzene. These RCM data were compared with and complemented shock tube ignition delay data which had previously been recorded under similar conditions of pressure and f  [426]. Where overlapping data existed, very good agreement was observed between t ign measured in the shock tube and the RCM facilities. The experimental data were simulated using an updated detailed chemical kinetic model, which included reactions in the low-temperature oxidation regime with special attention given to ROȮ isomerizations (R3a) and HȮ 2 elimination (R3c) increased complexity.
Ethanol In the RCM study of Lee et al. [428] , there were five data points reported for stoichiometric ethanol mixtures. The measured activation energy for ethanol autoignition was in good agreement with previous shock tube results of 130 kJ/mol, despite t ign predicted by the shock tube correlation being a factor of three lower than the measured values. Lee et al. [431]  followed this with more expansive tests, again using ethanol at stoichiometric conditions. The measured ignition delays in this study were used to develop an updated kinetic model for the combustion of ethanol, with good agreement shown between the data and the model. Mittal et al. [432]  studied the autoignition of ethanol over a wide range of stoichiometry, where f was changed by changing the oxygen mole fraction at constant fuel mole fraction.
Two-stage ignition was observed within the RCM at temperatures up to 810 K, and the evolution of t ign with temperature demonstrated a clear deviation from Arrhenius behavior in the temperature range 680-750 K. The JSR measurements, which were conducted at 1 bar and highly diluted conditions, corroborated the RCM data in that an NTC zone was observed in the temperature range of 600-750 K. Reactive mixtures were sampled from the RCM indicating the formation of C 1 -C 4 aldehydes and alkenes, a variety of oxygenated heterocycles, including oxirane, methyloxirane, oxetane, furan, both isomers of dihydrofuran, 1,4-dioxene and butyrolactone, as well as cyclopropane carboxaldehyde, formic acid, 2-propenyl ester and butanedial. Speciated samples from the JSR indicated discrepancies with the RCM measurements where many of the aforementioned oxygenated intermediates were not detected, while methylvinylether and dihydrofuranols were observed.
The differences between the results for the two experimental platforms indicate that the relative significance of reaction pathways may differ between RCM and JSR devices, and this highlights the importance of conducting autoignition chemistry studies across a wide range of conditions, including at high pressures and undiluted concentrations that are representative of IC engine operation. Furthermore, they emphasize the need for complementary datasets that can provide insight into fuel reactivity, such as the importance of cross reactions between fuel components. Nakamura et al. [443]  measured t ign for diethyl carbonate (DEC) using both an RCM and a shock tube. The data were complemented by speciation data measured in a JSR. The RCM results showed near-NTC behavior and relatively low reactivity in the low-temperature regime (660-900 K) and these results were interpreted using a chemical kinetic model.
5, 0. 69, and 1. 02 for diesel/O 2 /N 2 mixtures. The equivalence ratio was varied by changing the oxygen mole percentage while keeping the fuel mole percentage fixed at 0. 514%. The results demonstrated that diesel blends with similar cetane ratings but different compositions can exhibit varying ignition propensities across different temperature regimes. In particular, differences were observed at temperatures where low temperature branching reactions are important, as illustrated in Fig. 37. Furthermore, the RCM measurements were found to complement existing shock tube data in the literature  [467]   [468]  and thus provided an addition to the experimental database of diesel blends needed for development and validation of diesel surrogate models. The collection of studies shows that the still imperfect state of knowledge in kinetic modeling of individual hydrocarbons, added to non-linear interaction effects between different components means that the expected uncertainty in the predicted ignition delays of full boiling fuels relatively to surrogate mixtures under autoignition conditions is still no better than within a factor of two, particularly at high pressures.