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The temperature assigned to a glass transition under specified measurement conditions.

A sufficiently phase-separated block copolymer can show two glass transitions, associated approximately with its two microdomains. Simultaneous feeding instead tends to distribute the monomers along each chain, giving a statistical copolymer and usually one composition-dependent glass transition. Different monomer reactivities could produce a gradient rather than a perfectly random sequence.

For a homogeneous mixture an approximate interpolation is the Fox equation, $T_g^{-1}=w_1/T_{g1}+w_2/T_{g2}$, with temperatures in kelvin. Two transitions are not guaranteed merely by block architecture: short blocks, miscibility or insufficient experimental resolution can give one broad transition. The contrast is between domain-specific segmental environments and a more uniform mixed environment.

One explanation is block-copolymer kinetic trapping. A solvent-swollen transient sheet structure can be immobilized when a block vitrifies during drying; accurately measured block fractions do not guarantee thermodynamic equilibrium. Thermal annealing above the relevant glass transition, or a controlled nonselective solvent-vapour anneal followed by structural characterization, can test whether the sheets relax toward the bulk composition-appropriate morphology.

A second is surface or confinement selection of lamellar microphase. The diagram describes a bulk reference model. A thin film, preferential wetting at the substrate/free surface or a commensurate confining thickness adds interfacial and geometrical free energy terms that can favour layered arrangements even at an asymmetric global fraction. Unequal layer thicknesses can accommodate $f_A=0.15$; equal block fractions are not required for every layered metastable or confined structure. Varying film thickness and surface chemistry, and comparing bulk-like and confined samples after adequate annealing, distinguishes this explanation from kinetic arrest. Strong conformational asymmetry is another possible departure from the reference model, but an incorrect measured composition need not be invoked.