Masters Theses

Jeongihm Pak

Date of Award

5-2000

Degree Type

Thesis

Degree Name

Master of Science

Major

Chemistry

Major Professor

Bernhard Wunderlich

Committee Members

Alexander van Hook, Mark Dadmun

Abstract

Paraffins of sufficient chain length can serve as model compounds for flexible macro molecules. They crystallize almost completely and do not suffer from chain folding as long as their chain length is less than about 37 nm. The melting and crystallization of n-paraffins, C50H102 (n pentacontane, C50), C44H90 (n-tetratetracontane, C44), and C26H54 (n-hexacontane, C26) was analyzed as such model compounds with both a standard differential scanning calorimetry (DSC) and a temperature-modulated DSC (TMDSC) using sawtooth modulation and quasi-isothermal modulation with very small temperature amplitude (0.05 K). Melting and crystallization of C50 and C44 showed practically no superheating or supercooling, respectively, i e, with heating and cooling rates up to 12 K min'' the onsets of the corresponding transitions occurred at the same temperature. Similarly, the isotropization of the condis crystals of C26 and the ordering of the melt to the condis crystals are detected at practically the same temperature without superheating and supercooling. The observation of no supercooling for crystallization was confirmed by visual inspection using hot-stage microscopy and a melting-point apparatus. Only the transition on cooling of the C26 condis crystal, which results in fully-ordered crystals, shows a supercooling of 4.0 K when nuclei are not present

To make the link to linear macromolecules, polyethylene of molar mass 15520 Da (PE15520) and oligomers of 2150 Da (PE2150) and 560 Da molar mass (PE560) were analyzed in same manner, using standard DSC only. The PE560 has a molar mass close to the paraffin C40 and also shows almost no supercooling for crystallization from the melt, while PE2150 (»C153) and PE15520 (=C1106) need atypical degree of supercooling for polymer, namely «10 K, assumed to be due to molecular nucleation. Due to the well-known chain folding, which starts with C294 (37 nm molecular length), the melting temperature of PE15520 is lower than that expected for equilibrium, extended-chain crystals of this molar mass (411.9 K), and even lower than that for PE2150 which grows as an extended chain crystal and melts at a temperature close to the equilibrium melting temperature of 397.2 K.

The apparent heat capacity measurements using quasi-isothermal TMDSC with 0.05 K amplitude, revealed that melting of C50 was completed within 1.0 K, isotropization of C26 -within less than 0.6 K, and melting of C44 within 4.3 K. But 62-78% of total transitions of C26 and C50 occurred over a much narrower temperature range of 0.1 K or less. A "reversing" melting in the paraffins was detected by TMDSC and integral analysis was proven to be a useful tool for quantitative analysis of the thermal transition, specially of samples exhibiting multiple transitions like C26.

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