Promoted Formation of α Crystals in the Polymorph Selection of Syndiotatic Polystyrene under the Coupling of Pressure, Flow, and Temperature

Abstract

It remains elusive about the modification selection of polymers under coupled fields prevalent in actual polymer processing. In this work, we investigated the effect of pressure, flow, and temperature (the three most essential parameters in polymer processing) on the polymorphic behavior of syndiotactic polystyrene (sPS) that is a representative model polymer due to its complicated yet diverse modifications. Unlike the β crystal usually observed under pressure or flow alone, the α crystal was induced with the coupling of pressure and flow. The critical shear rate for the formation of α crystals showed a negative dependence on the imposed pressure. Such a promotion became more pronounced at lower crystallization temperature. The phase diagram for the formation of α/β crystals was drawn with respect to three coupled variables for the first time. There was a strong synergy between pressure and shear on the formation of α crystals, where lowering the temperature intensified this effect. The exclusive formation of α crystals was attributed to the enhanced nucleation competitiveness under high pressure and at low T c and the orientation of chains induced by flow, where pressure and T c played an essential part by slowing down the orientation relaxation. Our current effort provides a deep insight into polymorphism selection of sPS under melt processing-related conditions by which the structure manipulation can be directed.

Flow-Induced Polymer Crystallization under Pressure and Its Engineering Application in “Structuring” Polymer Processing

“…For polymorphic polymers, the situation is more complicated because each crystalline phase shows a distinct pressure dependence on the equilibrium melting temperature, so that thermodynamic conditions for different crystalline phases vary with pressure. 30,67,68 iPP is a good example, as the thermodynamic stable crystalline phase changes from the α-phase to γ-phase with increasing the pressure. 67,69 How shear flow affects the crystallization kinetics is dependent on the shear rate.…”

“…98,99 However, in contrast to the β-phase induced by pressure or shear alone, the α-phase is preferentially formed under the coupling of pressure and flow (Figure 6b). 68 The resultant crystal phase composition of polymorphic polymers is a result of different nucleation rates of varied crystalline phases, followed by their growth competition. As for FICP, the combined pressure and flow affect the nucleation and growth of different crystalline phases in different ways.…”

“…Though how the separated pressure and flow affect polymer crystallization has been widely studied, it remains a challenge to predict the crystal phase composition when processing fields coexist. To date, the crystal phase compositions of iPP, polylactide (PLA), and syndiotactic polystyrene ( s PS) crystallization under the coexistence of temperature, pressure, and flow have been reported.…”

“…The β-phase is a thermodynamically stable one. The individual pressure and flow are shown to be favorable for β-phase crystallization. , However, in contrast to the β-phase induced by pressure or shear alone, the α-phase is preferentially formed under the coupling of pressure and flow (Figure b) . The resultant crystal phase composition of polymorphic polymers is a result of different nucleation rates of varied crystalline phases, followed by their growth competition.…”

“…Pressure is thus able to change thermodynamic conditions for both nucleation and growth of polymer crystal. For polymorphic polymers, the situation is more complicated because each crystalline phase shows a distinct pressure dependence on the equilibrium melting temperature, so that thermodynamic conditions for different crystalline phases vary with pressure. ,, iPP is a good example, as the thermodynamic stable crystalline phase changes from the α-phase to γ-phase with increasing the pressure. , How shear flow affects the crystallization kinetics is dependent on the shear rate. For a shear flow that is unable to induce chain orientation ( W i ,rep = γ̇τ rep < 1, where γ̇ and τ rep are the shear rate and reptation time, respectively) and chain stretch ( W i ,R = γ̇τ R < 1, where τ R is the Rouse time), it will not speed up the crystallization kinetics.…”