Melting of solvents nanoconfined by polymers and networks

Abstract

Thermoporosimetry is becoming increasingly used to study nanoscale heterogeneity and structure in polymer networks. The starting point for thermoporosimetry is the Gibbs-Thomson (GT) relation between melting point and inverse crystal size. In the case of polymers, the Flory-Huggins (FH) model also predicts that there is a depression of the melting point because of the mixing of the polymer and the solvent molecules, and this needs to be taken into account. The first step in analysis of size heterogeneity using thermoporosimetry and the GT equation requires that there be quantitative agreement between the FH theory and the melting points of the diluent in the uncrosslinked rubber. We find that both benzene and hexadecane exhibit excessive melting point depressions in uncrosslinked polyisoprene. This may imply that the uncrosslinked polymer is divided into 'nanoheterogeneities'. We further find that the heat of fusion decreases as polymer concentration increases for the benzene, but not for the hexadecane. Finally, we compare pore size distributions obtained for a crosslinked polyisoprene as determined from the melting behavior of n-hexadecane as a diluent, using different references for how the uncrosslinked polymer behaves. While number average distribution is not very different between the different analyses, the weight average distribution is.

Evolution of EPDM networks aged by gamma irradiation – Consequences on the mechanical properties

“…This experiment is based on the fact that solvent molecules constrained to small volumes form only relatively small crystallites upon crystallization and therefore exhibit lower crystallization temperatures [25]. Broad distributions of cavity sizes should give rise to broad distributions of crystallites sizes and thus crystallites melting points [26]. Therefore, differential scanning calorimetry measurements performed on swollen sample with solvent molecules constrained in the pores can give evidence of several crystallization temperatures, which are indicative of the pore sizes and of the distributions of these sizes.…”

Anomalous melting behavior of cyclohexane and cyclooctane in poly(dimethyl siloxane) precursors and model networks

“…24,25 In addition to the plasticization and potential polymersolvent interaction effects, more complications can arise because small size has been observed to decrease the heat of fusion of organic liquids confined in nanopores. 23 As a continuation to previous work from Texas Tech, 10 the present work further explores the validity of the FH model prediction and the method of thermoporosimetry. Differential scanning calorimetry (DSC) is used to characterize the melting behavior of cyclohexane and cyclooctane in poly (dimethyl siloxane) (PDMS) precursors and model networks.…”

“…The magnitude of the melting point depression for cyclohexane in the PDMS precursors varies from 0 to 70 C, which is much larger than what is observed for many other systems, such as benzene and hexadecane in polyisoprene or natural rubber. 9,10,20,19 In these systems, the melting point depression of the solvents generally does not exceed 40 C. Honiball et al 20 also examined a number of solvents and found that cyclohexane exhibits large melting point depressions in natural rubber. The heat of fusion decreases almost linearly with the increase of polymer content, as indicated in Figure 3.…”

“…mckenna@ttu.edu) developed by some researchers to deduce the size and size distribution of nanoscale heterogeneities in polymer networks by using melting (or freezing) point depression of the solvents. [9][10][11][12][13][14][15] In this method, the Gibbs-Thomson (GT) equation 16 is used in conjunction with the Flory-Huggins (FH) model 17,18 to obtain the size and size distribution of solvent crystals, which are comparable with the nanoscale mesh size and distribution in polymer networks. Nevertheless, previous results on the melting (or freezing) behavior of swelling agents confined in polymer networks are complicated and contradictory, and the mechanisms of melting (or freezing) point depression are not well understood.…”

Structural evolution of a constrained epoxy functional polyethylene network crosslinked by a bio-based reactant

“…10. From this plot, the width of the distribution provides an idea of the network homogeneity in terms of molar mass distribution between two crosslinks [29,31,41,49]. Furthermore, the pore size weight average value Rp m can be determined by using Eq.…”