Catheryn L. Jackson

The melting behavior of organic materials confined in porous solids

The solid–liquidphase transition temperatures and heats of fusion ΔH f of nonpolar organic solids confined in the pores of controlled pore glasses were measured by differential scanning calorimetry. The pore diameters d were in the range of 40–730 A and the organics studied were cis‐decalin, trans‐decalin, cyclohexane, benzene, chlorobenzene, naphthalene, and heptane. In accordance with previous reports on studies of primarily inorganic materials, the melting point of the pore solidT(d) decreased with decreasing pore diameter. In addition, a large reduction in the bulk enthalpy of fusion ΔH f of the pore solid was measured, which apparently has not been studied in detail by other workers. A linear correlation was found between the melting point depression (ΔT m ) and the reciprocal diameter, as predicted by theories of solidification in a capillary. The calculated values of the solid–liquid interfacial energy σsl were in reasonable agreement with values reported in the literature based on other methods of measurement.

The glass transition of organic liquids confined to small pores

Abstract The glass-transition temperatures, T g , of organic liquids confined to small pores were studied by differential scanning calorimetry (DSC). The T g was measured as a function of pore size in controlled pore glasses (CPG) having pore diameters in the range of 40–730 A. The surface of the glass was treated with hexamethyldisilazane to promote wetting by the organic liquids studied ( o -terphenyl and benzyl alcohol). Glasses formed in the pores had a lower T g than in the bulk and the reduction in T g increased as the pore size decreased. For example, the depression of the glass transition temperature, ΔT g , of benzyl alcohol in 40 A and 85 A pores was 7.2 K and 3.1 K, respectively. The magnitude of ΔT g also depends on the material; e.g. for o -terphenyl in the 85 A pores, ΔT g was 8.8 K versus 3.1 K for benzyl alcohol. In general, it was noted that ΔT g was considerably less than for the depression of the crystalline melting point, ΔT m , studied in related work. For example, for benzyl alcohol in the 85 A pores, ΔT m was ∼ 25 K and ΔT g was ∼ 3 K.

Epoxy/SiO2 interpenetrating polymer networks

We demonstrate a potentially useful method of generating an SiO2 morphology, in situ, with interpenetrating polymer networks (IPN) chemistry. Organic/inorganic IPNs were synthesized with an organic phase made of epoxy resin and an SiO2 phase made by sol—gel chemistry. The two types of polymerization used were sequential and simultaneous with SiO2 content ranging from 0.02 to 0.43 g SiO2/g total weight. The resultant morphologies were examined by small angle X-ray scattering and transmission electron microscopy. The sequential IPNs were strongly phase separated into a finely divided SiO2 phase of ∼10 nm size scale. The simultaneous IPNs were weakly phase separated with considerable mixing in the phases. Thermal studies showed increased thermal stability for the IPNs, compared with unfilled epoxies or physically mixed silica filled epoxies.

On the Anomalous Freezing and Melting of Solvent Crystals in Swollen Gels of Natural Rubber

Abstract The anomalously large solvent freezing point depression, ΔTf, observed in crosslinked rubbers swollen in solvent has been a subject of study for over thirty years without clear resolution. While a sizeable ΔTf is accounted for by the lowering of the thermodynamic potential of solvent molecules in a polymer solution derived from the Flory theory, the additional ΔTf observed for crosslinked rubbers has been attributed to various physical effects such as restriction of solvent crystals to small size by the network mesh or difficulty in nucleation of the solvent crystals. In this paper, we identify two points of misunderstanding in the literature on this problem, and attempt to clarify the analysis of ΔTf for solvent swollen rubbers. The first point relates to the application of the Flory calculation to solvent freezing, where nucleation is a concern, rather than for solvent melting, for which it was intended. We present new calorimetric data on both the freezing and melting of solvent crystals in cr...

The influence of shear on the ordering temperature of a triblock copolymer melt

The effect of shear on the ordering temperature of a triblock copolymer melt of polystyrene‐polybutadiene‐polystyrene (SBS) is examined by in situ small angle neutron scattering (SANS). Results obtained by SANS are compared to the rheologically determined order–disorder transition temperature, TRODT=115±5 °C. The SANS measurements from a Couette geometry shear cell are then used to construct a ‘‘dynamical phase diagram’’ based on characteristic changes in the scattering with temperature and shear rate, γ. A shear rate dependent ordering temperature, Tord(γ), is identified as the system is sheared isothermally from the disordered state. The scattering behavior is shown to be highly strain dependent. We compare our findings on the shear rate dependence of the ordering transition in triblock materials with previous observations on diblock copolymer materials and theoretical expectations for the shear rate dependence of the order–disorder transition temperature. A simple scaling argument leads to a good des...

Phase separation kinetics and morphology in a polymer blend with diblock copolymer additive

The phase diagram for a low molecular weight blend of deuterated polystyrene (PSD) and polybutadiene (PB) was determined by temperature jump light scattering (TJLS) measurements and phase contrast optical microscopy (PCOM). The PSD/PB blend exhibited upper critical solution temperature behavior, and the critical temperatures measured by these two techniques were consistent. Upon addition of 0 to 0.12 mass fraction of a comparable molecular weight PSD-PB symmetric diblock copolymer, a linear decrease in the phase transition temperature was observed with increasing diblock copolymer content. At a constant, shallow quench depth, the kinetics of phase separation via spinodal decomposition as measured by TJLS were greatly retarded by the presence of the copolymer. Additionally, the time dependence of the concentration fluctuation growth did not seem to follow a universal functional form anywhere in the accessible q range when the diblock was present. The results of morphology study of the blends in the late stage of phase separation by PCOM also indicated that the phase separated domain sizes did not grow to the same size for a given annealing time as diblock content increased.

Shear-induced changes in the order-disorder transition temperature and the morphology of a triblock copolymer

A summary of our work on a triblock copolymer under steady shear is presented. The experiments were conducted using small-angle neutron scattering (SANS), as a function of shear rate and temperature, and transmission electron microscopy (TEM) on quenched specimens. The triblock copolymer is composed of polystyrene-d 8 /polybutadiene/polystyrene-d 8 , and the ordered microstructure normally consists of hexagonally packed cylinders. Two temperature-dependent, characteristic shear rates, γ C1 and γ C2 , are identified from the scattering results. The first characteristic shear rate identifies the shear rate required to go from a disordered state to an ordered state, while the second characteristic shear rate is interpreted as the shear rate necessary to produce a different ordered morphology. This latter transition was previously identified as being analogous to a martensitic transition in metal alloys. The supporting SANS and TEM evidence for the new ordered phase is presented. Dynamical aspects of the structural transition between different ordered triblock morphologies are discussed using the model of a martensitic-like transformation.

The Significance of Percolation on the Dynamics of Polymer Chains Bound to Carbon Black

A critical need in the fundamental understanding of reinforcement in filled polymers is the characterization of the polymer-filler interface and the dynamics of the polymer in this interfacial regime. In carbon black filled polymers, one of the central themes in the mechanism of reinforcement is that of “bound” polymer. Understanding the dynamics of this bound polymer may be key to arriving at an understanding of reinforcement mechanisms in filled polymers. The interactions between polymers and filler surfaces are also key in the development of more advanced nanocomposite materials. We have previously utilized inelastic neutron scattering methods to examine the variation of bound polymer dynamics as a function of carbon black type for a single, initial carbon black concentration. An apparent change in the distribution in backbone motions was observed in the bound polymer compared with the pure polymer. In this study, we extend our prior work to examine the bound polymer dynamics as a function of the type of carbon black and the initial concentration of carbon black. The results suggest that two types of dynamic behavior are observed as a function of the initial carbon black concentration. This critical cutoff concentration may be related to the percolation threshold of the carbon black and suggests that quantifying the amount of bound polymer is insufficient for understanding the relationship between mechanical behavior and bound polymer content.