Microphase separation temperature transition

For most BC the phase diagram is characterized by the presence of an upper critical solution temperature, UCST, also known as an order-disorder transition temperature or a microphase separation temperature. Below UCST the block copolymers phase separate, while above it, an isotropic melt is obtained. Owing to the chemical... 

Figure 2 Temperature dependent characteristic parameters ofP(S-b-B) during an upward and downward temperature scan, inverse of the normalized scattering peak maximum, I o/l (i o is the peak intensity at the transition), maximum position, q, and peak width, Aq, as obtained from fits of a Lorentz function to the q dependent scattering intensity. The solid lines are guides to the eyes. The data point in brackets represents the peak intensity of the sample directly q/ter the preparation. The dotted line indicates the position of the microphase separation temperature Tmst-...

Fig. 3.44. SAXS curves measured for a polystyrene-6/ocfc-polyisoprene ((PS) = 0.44, M = 1.64 10 ) in the temperature range of the microphase-separation. The transition occurs at Tt = 362 K. Data from Stiihn et al.[28]...

Whereas random copolymers exhibit one T described by equation 38, block copolymers, because of this microphase separation, exhibit two glass-transition temperatures. The T of each block is close to, if not the same as, the homopolymer from which it was formed. Polymer properties are also affected by the arrangement of the blocks. This is shown for high styrene-containing or high molecular-weight styrene resias of various block arrangements ia Table 3. 

Microdomain stmcture is a consequence of microphase separation. It is associated with processability and performance of block copolymer as TPE, pressure sensitive adhesive, etc. The size of the domain decreases as temperature increases [184,185]. At processing temperature they are in a disordered state, melt viscosity becomes low with great advantage in processability. At service temperamre, they are in ordered state and the dispersed domain of plastic blocks acts as reinforcing filler for the matrix polymer [186]. This transition is a thermodynamic transition and is controlled by counterbalanced physical factors, e.g., energetics and entropy. 

In microphase-separated systems, ESR spectra may consist of a superposition of two contributions, from nitroxides in both fast and slow-tumbling regimes. Such spectra provide evidence for the presence of two types of domains with different dynamics and transition temperatures. This case was detected for a HAS-derived nitroxide radical in heterophasic polyfacrylonitrile-butadiene-styrene) (ABS) as shown in Figure 5, the fast and slow components in the ESR spectrum measured represent nitroxide radicals located in butadiene-rich (B-rich) and styrene/acrylonitrile-rich (SAN-rich) domains, respectively [40]. These two components were determined by deconvoluting the ESR spectrum of HAS-NO measured at 300 K. 

Calorimetric measurements and morphological observations showed that PS-fr-PCL, PB-fr-PCL and PS-fc-PB-fc-PCL copolymers exhibit microphase separation and crystallization if the molecular weight is high enough. Only in PS-fc-PCL diblock copolymers, a shift of the PS glass transition to lower temperatures has been observed. In PS-fo-PB-fo-PCL, the crystallizable block (i.e., PCL) is covalently linked to a rubbery block and it has a free end. For these reasons there is no significant reduction in the melting temperature and... 

In a blend of immiscible homopolymers, macrophase separation is favoured on decreasing the temperature in a blend with an upper critical solution temperature (UCST) or on increasing the temperature in a blend with a lower critical solution temperature (LCST). Addition of a block copolymer leads to competition between this macrophase separation and microphase separation of the copolymer. From a practical viewpoint, addition of a block copolymer can be used to suppress phase separation or to compatibilize the homopolymers. Indeed, this is one of the main applications of block copolymers. The compatibilization results from the reduction of interfacial tension that accompanies the segregation of block copolymers to the interface. From a more fundamental viewpoint, the competing effects of macrophase and microphase separation lead to a rich critical phenomenology. In addition to the ordinary critical points of macrophase separation, tricritical points exist where critical lines for the ternary system meet. A Lifshitz point is defined along the line of critical transitions, at the crossover between regimes of macrophase separation and microphase separation. This critical behaviour is discussed in more depth in Chapter 6. 

The resulting phase diagram for diblock copolymers is shown in Fig. 2.40.The theory predicts that microphase separation occurs to a body-centred cubic structure for all compositions except where a direct second-order transition to a lamellar structure is predicted. First-order transitions to hex and lam phases are expected on further lowering the temperature for asymmetric diblocks. 

Fig. 6.3 Schematic phase diagram for lamellar PS-PB diblocks in PS homopolymer (volume fraction 0h). where the homopolymer Mv is comparable to that of the PS block (Jeon and Roe 1994). L is a lamellar phase, I, and I2 are disordered phases, M may correspond to microphase-separated copolymer micelles in a homopolymer matrix. Point A is the order-disorder transition.The horizontal lines BCD and EFG are lines where three phases coexist at a fixed temperature and are lines of peritectic points. The lines BE and EH denote the limit of solubility of the PS in the copolymer as a function of temperature.

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