Onsets of Tethered Chain Overcrowding and Highly Stretched Brush Regime via Crystalline−Amorphous Diblock Copolymers

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Abstract

Lamellar single crystals grown in dilute solutions can be used as templates for tethered chain analysis. Two series of diblock copolymers, poly(ethylene oxide)-block-polystyrene (PEO-b-PS) and poly(l-lactic acid)-block-polystyrene (PLLA-b-PS), were used as model templates to generate tethered PS blocks on the single-crystal basal surfaces. Controlled and tunable reduced tethering density, σ, defined by σπRg2 (where σ is the tethered chain density and is equal to the reciprocal of the covered area of the chain and Rg is the radius of gyration of this tethered chain in its end-free state at the same conditions), could be achieved in a broad range (up to 24) by changing the molecular weights (MW's) of the crystalline and amorphous blocks and by varying the crystallization temperature (Tx) of different PEO-b-PS and PLLA-b-PS solutions. For PEO and PLLA homopolymers crystallized in dilute solutions, the lamellar crystal thicknesses (dCRYST) were observed to be proportional to the reciprocal undercooling ΔT (where ΔT = Td − Tx and Td is the equilibrium dissolution temperature of the crystals). The σ of the tethered PS chains on the crystal surface increased with decreasing ΔT because at a fixed MW of the PEO or PLLA block, an increase in the dCRYST was evidence of a decrease in the number of folds. When we plotted the relationships between 1/dCRYST and Tx for these two series of diblock copolymers, sudden and discontinuous changes of the slopes in some of these were observed at σ = 3.7 (σ*). This was as a result of the drastic interaction change of the neighboring PS tethered chains. An average reduced surface free energy of the tethered PS chains (ΓPS) was defined and used as a parameter to characterize the PS tethered chain interactions. The relationship between ΓPS and σ showed a discontinuous transition at σ*, which had a close similarity to the hard-sphere-like interaction model. This could be identified as the onset of the tethered PS chain overcrowding in solution. This transition indicates that the extra entropic surface free energy created by the repulsion of tethered PS chains started to affect the nucleation barrier of the PEO or PLLA block crystallization. On the basis of the scaling laws, the onset of highly stretched brush regime could be identified at σ = 14.3 (σ**). In the ΓPS vs σ plot, the transition appears to be continuous. Thus, a crossover regime in the tethered PS chains exists between σ* = 3.7 and σ** = 14.3. It is defined as the regime where the interaction of the tethered PS chains undergoes changes from being noninteracting toward penetration to, finally, chain stretching normal to the surface.

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