Lower critical solution temperature Morphology

Miscible blends of poly(vinyl methyl ether) and polystyrene exhibit phase separation at temperatures above 100 C as a result of a lower critical solution temperature and have a well defined phase diagram ( ). This system has become a model blend for studying thermodynamics of mixing, and phase separation kinetics and resultant morphologies obtained by nucleation and growth and spinodal decomposition mechanisms. As a result of its accessible lower critical solution temperature, the PVME/PS system was selected to examine the effects of phase separation and morphology on the damping behavior of the blends and IPNs. 

To achieve desirable macroscopic properties, one would like to control the degree of miscibility of a blend. Heat treatment/annealing is a simple technique to modify a phase structure of a blend. Morphological changes induced by heat treatment can also affect NMR observable (see Section 10.3.2.1). Furthermore, as shown in Section 10.2.1, several blends exhibit a lower critical solution temperature (LCST) phase diagram. Such a blend phase-separates at temperatures above its LCST temperature. The compositional fluctuation during the phase-separation process is examined in Section... 

Owing to the presence of specific interactions, most blends have a phase separation diagram with a lower critical solution temperature, LOST, i.e. phase separation occurs upon heating. Two separation mechanisms are known spinodal decomposition (SD), and nucleation-and-growth (NG). The morphology generated in NG is dispersed, whereas that in SD is co-continuous. Cahn and HiUiard s theory describes well the SD kinetics [1, 2]. 

Matyjaszewski et al. [2] patented a novel and flexible method for the preparation of CNTs with predetermined morphology. Phase-separated copolymers/stabilized blends of polymers can be pyrolyzed to form the carbon tubular morphology. These materials are referred to as precursor materials. One of the comonomers that form the copolymers can be acrylonitrile, for example. Another material added along with the precursor material is called the sacrificial material. The sacrificial material is used to control the morphology, self-assembly, and distribution of the precursor phase. The primary source of carbon in the product is the precursor. The polymer blocks in the copolymers are immiscible at the micro scale. Free energy and entropic considerations can be used to derive the conditions for phase separation. Lower critical solution temperatures and upper critical solution temperatures (LCST and UCST) are also important considerations in the phase separation of polymers. But the polymers are covalently attached, thus preventing separation at the macro scale. Phase separation is limited to the nanoscale. The nanoscale dimensions typical of these structures range from 5-100 nm. The precursor phase pyrolyzes to form carbon nanostructures. The sacrificial phase is removed after pyrolysis. 

When polymers undergo phase separation in thin films, the kinetic and thermodynamic effects are expected to be pronounced. The phase separation process can be controlled to effect desired morphologies. Under suitable conditions a film deposition process can lead to pattern replication. Demixing of polymer blends can lead to structure formation. The phase separation process can be characterized by the binodal and spinodal curves. UCST is the upper critical solution temperature, which is the temperature above which the blend constituents are completely miscible in each other in all proportions. LUST behavior is not found as often in systems other than among polymers. LUST is the lower critical solution temperature. This is the... 

The TTT Cure Diagram Evidence to date suggests that interpenetrating polymer networks, like most polymer blends, exhibit lower critical solution temperatures (42). However, the development of morphology with polymerization of one or more of its components is complicated by the presence of cross-linking. Thus the materials cannot be stirred beyond a certain point, as can the HIPS materials, and cannot be made to fiow at elevated temperatures they are thermoset materials. 

Out of a number of known polymers known to exhibit this behavior, water-soluble poly(N-isopropylacrylamide) (PNIPAAM) is a very common and extensively studied material. It has a lower critical solution temperature (LCST) of about 32°C [1-6], and below this temperature, the chains exhibit chain-extended conformations and random coil structure. The intermolecu-lar hydrogen bonding with the water molecules due to the chain extended morphology generates the hydrophilic nature of the chains. The chains transform into a more collapsed globular form above the lower critical... 

Figure 19.1 Schematic representations of (a) temperature-composition diagram for a partially miscible blend with a lower critical solution temperature and most common blend morphologies including (b) droplet-matrix structure, (c) fibrillar morphology, (d) cocontinuous...

Tagit, O., Tomczak, N., and Vancso, G.J. 2008. Probing the morphology and nanoscale mechanics of single poly(V-isopropylacrylamide) microgels across the lower-critical-solution temperature by atomic... 

Two-component block copolymers commonly display upper critical solution temperature (UCST) behavior [7]. They form ordered, microphase-separated morphologies at lower temperatures but ean be heated to temperatures where the discrete heterogeneity is lost. The transition point from a heterogeneous microstructure to a compositionally homogeneous state is termed the order-to-disorder transition (Eqdt)- At any given degree of polymerization N, the highest Eqdt exists in systems with equal volumes of the two components, at... 

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