The Role of Polymer Crystallizability on the Formation of Polymer-Urea-Inclusion Compounds

Abstract

Polymer-urea inclusion compounds (P-U-ICs) were formed using a series of linear aliphatic polyesters with varying crystallizabilities: from highly crystalline to wholly amorphous. The traditional hexagonal P-U-ICs were obtained irrespective of the crystallinities of the neat guest polyesters. Two distinct co-crystallization mechanisms were evident based on the observation of the change in thermal stabilities of the ICs using DSC and the crystal morphologies by SEM; one involves polymer chain folding back and forth in a lamella-like crystal structure and the other grows much like short chain molecule U-ICs absent of chain reentering different channels. For polymers with sufficient chain length, their inherent flexibility is the key factor determining the co-crystallization mechanism while their crystallizability affects the kinetics, the consequences of which are more pronounced during recrystallization from melt. The amorphicity induced by random ester group placement is an interchain property, which does not play a role in affecting IC thermal stability. Rather, increasing the average ester group content can be understood as introducing more defects in the IC crystal and therefore reduces its thermal stability. The understanding gleaned in this study provides a new avenue for designing P-U-ICs to be used in both theoretical modeling and engineering of high-performance materials.

Controlling Polymeric Supramolecular Microstructures through Host–Guest Interactions Assisted with Ultrasound Oscillation

“…The feature bands of PCL and urea are shown in the spectra of all IC samples. The N–H stretching bands of pure urea at 3428 and 3333 cm –1 shifted to lower wavenumbers, indicating the formation of the hexagonal urea crystal structure in ICs. ,, …”

“…The N−H stretching bands of pure urea at 3428 and 3333 cm −1 shifted to lower wavenumbers, indicating the formation of the hexagonal urea crystal structure in ICs. 24,28,43 In the spectrum of pure PCL, the blue shift of the carbonyl band normally indicates a conversion from crystalline phases to amorphous phases. 24,44 The guest PCL can be considered as an amorphous state when isolated and separately included in a host canal, according to a previous report.…”

“…Simply put, with more guest molecules protruding in the canal axis, the IC crystal will grow in a longitudinal way; with less protruding, a flat platelet will form. Shen et al 43 have come up with two co-crystallization mechanisms for polymer ICs, polymer-crystallization-driven and urea-crystallization-driven mechanisms, depending on the flexibility of the guest polymer with sufficient chain length. Flexible linear guest polymers follow a polymer-crystallizationdriven mechanism when forming ICs: polymer chain folding back and forth in a lamellar-like crystal structure.…”

A New Two‐Step Strategy for Encapsulating Amorphous Polymer Chains in Thiourea Crystals

“…An IC with canal-shape compact structure typically leads to a higher melting temperature than the neat host crystal. [10,11,15] However, when the host structure is non-canal, such as a layered structure, the complex is normally metastable with loose packing and lower melting temperatures. For example, the reported layered PEO-TU complexes had lower melting temperatures than neat TU.…”

“…[8][9][10] Polymer chains are isolated in the nanoscale canals formed by small molecular hosts and adopt extended states. [11][12][13][14] More interestingly, it has been reported that the extended chain conformations of guest polymer chains can be largely retained after removing the host molecules. [13,[15][16][17] Distinct to most methods, including stretching, rubbing, or chemical reactioncontrolled methods, like topochemical polymerizations, which are mainly applicable to semicrystallize polymers, forming ICs is a "bottom-up" approach to control chain alignment of polymeric materials regardless of polymer crystallization capacity.…”