Effects of Spacer Length and Terminal Group on the Crystallization and Morphology of Biscarbamates: A Longer Spacer Does Not Reduce the Melting Temperature
Bibliographic record
Abstract
The effects of alkyl side chain and spacer lengths and the type of terminal group on the morphology and crystallization of a homologous series of biscarbamates (model compounds for polyurethanes) were investigated. Biscarbamates were synthesized with alkyl side chains of various lengths ranging from C4 to C18 and an alkyl spacer group with 12 CH2 units (C12 spacer) between the two hydrogen bonding motifs. The crystallization and morphological features are compared with the previously studied biscarbamates with a C6 spacer. As a token example, we also studied a biscarbamate molecule in which the terminal methyl group was replaced by a phenyl group. We stress four important conclusions of the study: (1) A number of studies in the literature found that the longer alkyl spacers reduced the thermal transition temperatures of the molecules, and such behavior was attributed to an increase in the flexibility of the alkyl spacer. However, the results of the present study are to the contrary. With the biscarbamates studied here, the hydrogen-bonding groups on both sides of the C12 spacer act as "anchors", and the longer spacer does not reduce the melting temperatures compared with those with the C6 spacer. (2) The melt viscosity measurements show shear-thinning behavior, which has been mostly observed with polysaccharides and hydrogen-bonded polymers. (3) Avrami analysis shows a two-stage crystallization, which is not commonly observed in organic small molecule systems. (4) The phenyl end group does not add another self-assembly code in terms of π-stacking but acts as a defect. While formation of crystals was observed for biscarbamates with short alkyl side chains with a C6 spacer, an increase in spacer length to C12 induces spherulitic morphology. Although the overall sizes of the spherulites are the same for both spacers, the rate of spherulite growth was higher and the crystallization rate was lower with the C12 spacer compared with the C6 spacer. In contrast with the biscarbamates containing C6 spacer previously studied, we find that among the biscarbamates with 12 CH2 units in the spacer the C12-C12 molecule shows the minimum spherulite size, spherulite growth rate, and rate of crystallization. The infrared frequency shifts of the N-H group due to hydrogen bonding were used to calculate the N···O hydrogen-bonding distance and found to be close to the value observed in the crystal structure of the biscarbamate with a C6 spacer and C10 alkyl side chain.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one teacher head, not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".