Chain length effect on the crystal thermal stability of oligoesters crystallized in thin layers

Abstract

Specific heat of two fractions of oligomeric ethylene adipates (EA-2 with molecular weight <M n>=1700 and EA-4 <M n>=4000) containing up to 85 weight parts of untreated Aerosil-175, was measured in the temperature range 150–400 K. Detailed analysis of experimental data in terms of equations (1) and (2) led to the conclusion that significantly higher rate of decrease of both crystalline melting point as well as “true” (i. e., corrected for crystallinity) enthalpy of fusion with inverse value of distance between filler particles for EA-2 reflects higher affinity of end hydroxyl groups of oligomer chains in extended conformation towards polar Aerosil surface, as compared to that for middle-chain segments of EA-4 molecules which presumably are in a coiled conformation in the melt, and fold back on themselves on crystallization. On the other hand, sudden drop of experimental heats of fusion and crystallinities for both oligomers at filler contents above 50 weight parts is attributed to drastic jump of nucleation barrier for crystallization due to severe loss of conformational entropy of oligomer chains which either form “ties” between filler particles, with end groups anchored on the latter (as in the case of EA-2), or are squeezed in coiled conformation, with middle-chain segments on the periphery of coils interacting with neighboring filler particles (EA-4).